Week 1 MC Questions PDF

Liver Pathology Part III MAFLD – e-learning P1 BMS 200 Week 1 Non-Alcoholic Fatty Liver Disease (NAFLD) – Recall from BMS 150 Steatosis in the absence of significant alcohol consumption ○ Most common cause of liver disease in US ○ Estimated prevalence of u...

Liver Pathology Part III MAFLD – e-learning P1 BMS 200 Week 1 Non-Alcoholic Fatty Liver Disease (NAFLD) – Recall from BMS 150 Steatosis in the absence of significant alcohol consumption ○ Most common cause of liver disease in US ○ Estimated prevalence of up to 40% of the population Forms include: ○ Simple hepatic steatosis and steatosis complicated by inflammation These show fewer long-term complications unless they progress to NASH Progression to NASH is uncommon, and it is unclear why some progress ○ Non-alcoholic steatohepatitis (NASH) Progresses to cirrhosis in 10 – 20% of cases In those that progress to cirrhosis, the incidence of liver cancer can be as high as 1-2% per year Non-Alcoholic Fatty Liver Disease (NAFLD) – Recall from BMS 150 Pathologic findings: ○ Initially hepatocyte ballooning, lobular inflammation, and steatosis (fat accumulation in hepatocytes) ○ With progressive disease there is steadily more fibrosis, eventually leading to cirrhosis ○ Strongly associated with obesity and the metabolic syndrome Pathophysiology: ○ “two-hit” model, involving 1) hepatic fat accumulation and 2) increased oxidative stress Free radicals cause lipid peroxidation of the accumulated intracellular fat ○ Obesity seems to be associated with reduced intestinal barrier function 🡪 increased inflammation in the liver Movement of microbes from the gut into the portal circulation  General cirrhosis pathogenesis – Recall from BMS 150 Cirrhosis definition: Diffuse remodeling of the liver into parenchymal nodules surrounded by fibrous bands and variable degree of vascular shunting Pathogenesis (most likely theory): Stellate cells become activated & differentiate into highly fibrogenic myofibroblasts ▪ Activated by inflammatory cytokines (eg. TNF-alpha), toxins, reactive oxygen species ▪ Signals implicated include PDGF, TGF-beta, IL-17 ▪ Deposit ECM into the space of Disse 🡪 fibrous septae formation in regions of hepatocyte loss Kumar et. al., Robbins and Cotran Pathologic Basis of Disease 9th ed.  Non-Alcoholic Fatty Liver Disease (NAFLD) – Recall from BMS 150 > 80% < 20% ~ 11% of NASH (2%?) NAFLD – a dynamic spectrum of liver pathology: A. Healthy liver. B. Simple steatosis (arrow shows fatty hepatocyte) C. Nonalcoholic steatohepatitis (NASH); ballooned hepatocyte (arrow) near central vein with adjacent blue-stained pericellular fibrosis (arrowheads). Non-Alcoholic Fatty Liver Disease (NAFLD) – Recall from BMS 150 Clinical features: ○ Usually asymptomatic until hepatic failure (due to cirrhosis) clinical findings usually due to accompanying atherosclerotic disease/diabetes Cardiovascular disease a frequent cause of death ○ Fatigue and right-sided abdominal pain can occur in some ○ Increased risk of hepatocellular carcinoma Diagnosis: ○ Liver enzymes alone are poorly reliable ○ Scores calculated from age, BMI, fasting glucose, AST, ALT are helpful for gauging degree of inflammation and fibrosis ○ Detection of fibrosis via imaging or biopsy for definitive diagnosis Kumar et. al., Robbins and Cotran Pathologic Basis of Disease 9th ed. MAFLD vs NAFLD MAFLD = “metabolic dysfunction associated fatty liver disease” How does MAFLD differ from NAFLD in terms of diagnostic criteria? Both require a 5% “level” of hepatic steatosis ○How is this diagnosed? For discussion later NAFLD diagnosis requires exclusion of other causes of liver disease (i.e. significant alcohol use, hemochromatosis, etc.) ○negative criterion MAFLD diagnosis requires the presence of metabolic drivers of hepatic steatosis and inflammation, which can be any one of the following positive criteria: ○T2DM ○Obesity (this can be a tricky thing to define) ○A “metabolic dysfunction” composite score (see next slide) Gofton et. Al., “MAFLD: How is it different from NAFLD?”, Clin Mol Hepatol, 2023; 29 (Supp) MAFLD and NAFLD Diagnostic Criteria * Metabolic risk abnormalities for MAFLD – 2 out of 7 (these are FYI) Elevated waist circumference Blood pressure > 130/85 (or BP meds) Elevated plasma triglycerides (> 1.7 mmol/L) MAFL Decreased HDL cholesterol Prediabetes (to cover in future diabetes lecture) D Elevated HOMA insulin resistance score (to cover in future diabetes lecture) Elevation in CRP Looks a lot like the metabolic syndrome NAFL criteria D Adapted from Gofton et. Al., “MAFLD: How is it different from NAFLD?”, Clin Mol Hepatol, 2023; Why do the diagnostic differences between NAFLD and MAFLD matter? Clearer diagnoses: Patients could have non-alcoholic steatohepatitis AND have a chronic viral hepatitis… with the viral hepatitis complicating NASH ○ What do you call that? Someone with MAFLD who also has hep B or hep C just has MAFLD complicated with a viral hepatitis ○ No negative criteria 🡪 diagnostic clarity Better identification of patients who need more aggressive/targeted management Studies seem to indicate that a MAFLD diagnosis is more closely linked to the development of worsened liver fibrosis Gofton et. Al., “MAFLD: How is it different from NAFLD?”, Clin Mol Hepatol, 2023; 29 (Supp) Why do the diagnostic differences between NAFLD and MAFLD matter? Identification of those with higher “wear and tear” due to “metabolic dysregulation” A MAFLD diagnosis increases the risk of the following conditions over a NAFLD diagnosis (although sometimes only slightly) ○Subsequent development of diabetes (in those who are not already diabetics) ○Chronic kidney disease ○Worsened lung function (and worsened complications after COVID infection) Gofton et. Al., “MAFLD: How is it different from NAFLD?”, Clin Mol Hepatol, 2023; 29 (Supp) Why do the diagnostic differences between NAFLD and MAFLD matter? Under-reporting of alcohol ingestion It is estimated in some cohorts that up to 30% of those diagnosed with NAFLD actually do consume alcohol regularly and in clinically significant quantities (as assessed with fancy biochemistry tests) ○ There can be serious stigma in some cultures about frequent alcohol consumption (and a regular stigma in most cultures) ○ This leads to patient under-reporting Not an issue with MAFLD diagnostic criteria A small caveat: A small proportion of those with a fatty liver do not fulfill ANY of the other criteria for MAFLD… and therefore would have NAFLD, but not MAFLD ○ Approximately 5% of those with fatty livers Gofton et. Al., “MAFLD: How is it different from NAFLD?”, Clin Mol Hepatol, 2023; 29 (Supp) Is a MAFLD diagnosis “redundant”? There is little argument that MAFLD results in worsened mortality compared to NAFLD… but is that just because those with MAFLD also have metabolic syndrome, obesity, or diabetes on top of steatosis? ○Why not just say someone has NAFLD as well as metabolic syndrome or obesity or diabetes? This is a good question, but perhaps not a very clinically- relevant one ○Simply put, MAFLD is a more straightforward diagnosis that helps guide the clinician to more comprehensive treatment sooner ○Studies will likely address this question in the future Gofton et. Al., “MAFLD: How is it different from NAFLD?”, Clin Mol Hepatol, 2023; MAFLD – some extra pathophysiology Why do obesity, insulin resistance, and fatty liver tend to cluster together? Insulin resistance 🡪 increased FFA liberation from adipocytes 🡪 conversion into triglycerides and storage in hepatocytes Elevated glucose and insulin levels 🡪 hepatic triglyceride synthesis Greenberger’s Current Diagnosis & Treatment in Gastroenterology, Hepatology, &  MAFLD – some extra pathophysiology Why do obesity, insulin resistance, and fatty liver tend to cluster together? As intra-abdominal fat increases, adiponectin levels tend to decrease ○Adiponectin is an adipokine released by visceral fat ○It increases glucose utilization and fatty acid oxidation ○Studies show adiponectin levels tend to be lower in patients with fatty liver and steatohepatitis Greenberger’s Current Diagnosis & Treatment in Gastroenterology, Hepatology, &  MAFLD – counteracting pathophysiology What works in MAFLD? ○ Weight loss through lifestyle modification – the best therapeutic option so far! Those that are obese or have type II diabetes can reduce steatosis with weight reduction Exercise and weight loss can improve fibrosis and steatosis ○ No approved meds yet, though some do improve inflammation and steatosis in patients Thiazolidenedione medications (i.e. pioglitazone) improve insulin sensitivity and seem to increase adiponectin secretion Incretin agonists improve insulin secretion and help with weight loss in diabetes ○ Vitamin E seems to reduce oxidative stress in hepatocytes, and is linked to reduced steatosis and inflammation in patients with steatohepatitis Greenberger’s Current Diagnosis & Treatment in Gastroenterology, Hepatology, & An Approach to Hepatic Investigations Blood Labs: Liver Enzymes, Liver Function Tests, and others Application of Selected Tests to Liver Pathology E-learning P2 BMS 200 Laboratory Analysis of the Liver Learning outcome: Compare and contrast how the different transaminases (ALT, AST), ALP, bilirubin, albumin, and PT/iNR can be used to assess etiology and extent of liver damage Liver enzyme tests: ○ ALT & AST ○ GGT, ALP, & 5’ nucleotidase Liver function tests: ○ Albumin & PT/iNR ○ Bilirubin Direct (conjugated) and indirect (unconjugated) Selected disease-specific tests General Patterns and Concepts Damage to hepatocytes or to the biliary tree causes enzymes that are normally found within the cell or attached to cell membranes to “leak” into the bloodstream ○ When serum levels of these enzymes increase (liver enzyme tests), it indicates damage, but says nothing about liver function The function of the liver can be crudely evaluated by considering three major parameters in the blood: ○ Serum albumin (maintains oncotic pressure, helps transport hydrophobic substances) – decreases as hepatic function is impaired ○ Bilirubin (breakdown product of heme, excreted into the biliary tree) – increases as hepatocytes are damaged or sometimes when hepatic function is impaired ○ “Time to clot”, known as the PT/INR (the liver produces coagulation proteins) – increases as hepatic function is impaired Elevated PT/INR = longer time to form a clot = less coagulation proteins General Patterns and Concepts In situations where hepatocytes suffer damage but the biliary tree does not experience obstruction or damage: ○ ↑↑ AST and ALT ○ normal or mildly elevated ALP or GGT ○ Known as a hepatocellular pattern In situations where the primary problem is obstruction/inflammation of the biliary tree (intrahepatic or extrahepatic bile ducts or the wall of the gallbladder): ○ ↑↑ ALP and GGT ○ Normal or ↑ AST and ALT ○ Known as a cholestatic pattern Liver Enzyme tests - ALT ALT = alanine aminotransferase Found mostly in the cytosol of hepatocytes ○ few other cells express significant amounts, main other source is the kidney (but far more in hepatocytes) ○ Therefore elevations in ALT are relatively specific for hepatocyte damage Small amounts of ALT are released into the bloodstream in healthy individuals, but significant elevations usually indicate hepatocyte damage ○ FYI – normal level varies from lab to lab, but usually ranges from 10 to no more than 40 IU (a little higher for men) ○ Half life of a few days Liver Enzyme tests - AST AST = aspartate aminotransferase Found in the cytosol and mitochondria of hepatocytes ○ Many other cells containg AST – elevations in the serum could be due to hepatic, skeletal myocyte, cardiomyocyte, renal, or pancreatic injury ○Therefore elevations in AST are less specific for hepatocyte damage As with ALT, low levels of AST are present in the blood of healthy individuals, but significant elevations usually indicate hepatocyte damage FYI factoids: ○ Normal level similar to ALT, 10 to no more than 40 IU, half life is a few days ○Less AST than ALT in the cytosol 🡪 if hepatocyte membrane is disrupted, usually more ALT is released than AST Liver Enzyme tests - ALP ALP = alkaline phosphatase Found at/around the canalicular membrane of hepatocytes ○ ALP is also expressed by bone (during growth or fracture or other changes to bone) and by the placenta Therefore, levels are increased during pregnancy or childhood/adolescence ○ During damage to the biliary apparatus ALP is released to the bloodstream Includes cholestasis in the intra- and extrahepatic biliary tree, and sometimes during cholecystitis (25%) Assorted ALP FYI info ○ Normal level ranges between 35 – 100 IU (varies widely in normal population) Liver Enzyme tests – GGT & 5’ nucleotidase GGT = gamma-glutamyl transpeptidase GGT is found in the cell membranes of a wide variety of cells (hepatocytes & cholangiocytes, kidney, pancreas, spleen, heart… many) ○ Although sensitive, it is not specific – many people have an isolated elevated GGT and no significant pathology ○ Alcohol-use disorder can result in disproportionate increases in GGT vs ALP… one has to be aware of the non-specific nature of GGT, though, before stigmatizing the patient 5-NT is also found in a wide variety of cells, but increases usually mean hepatobiliary disease (more specific than GGT) Both of these labs are meant to “double-check” whether an elevation of ALP means hepatobiliary disease ○ GGT normal levels: 9 – 85 U/L 🡪 this is the preferred “double check” test for ALP elevations ○ 5’ NT normal levels: 0 – 18 U/L (varies with age) Liver Function tests - bilirubin Where does bilirubin come from? (preview for hematology) : Hemoglobin is degraded to the “heme” portion and the “globin” portion ○ “globin” portion is just a protein – it gets metabolized by macrophages ○ “heme” portion is a porphyrin ring structure… tougher to break down Macrophage degrades heme to biliverdin 🡪 unconjugated bilirubin (hydrophobic) ○ Unconjugated bilirubin is carried (via albumin) to the hepatocyte… The hepatocyte conjugates bilirubin to a soluble form (bilirubin glucuronide) Rubin’s Pathology: Mechanisms of th Liver Function tests - bilirubin Two forms of bilirubin measured: ○ Unconjugated bilirubin (indirect) – bilirubin in the serum that has not yet been complexed to glucuronide Due to large hematomas, disorders where large numbers of RBCs are damaged Due to disorders that affect bilirubin conjugation Some increase can be seen with hepatocyte damage ○ Conjugated bilirubin (direct) – the hepatocyte has conjugated bilirubin, but it has “leaked back” into the bloodstream because of: Blockage within the biliary system Inability to transport conjugated bilirubin into canaliculi (rare) Damage to hepatocytes Liver Function tests - bilirubin Assorted bilirubin facts: Texts/papers differ on whether this is a “function” test or an “enzyme” test ○ Most of the time, an elevation in serum bilirubin is due to hepatocyte damage or impaired bile flow… so it’s similar to AST/ALT/GGT/ALP in that way ○ However, sometimes elevated serum bilirubin is a deficit in liver function (i.e. Gilbert’s syndrome) ○ Most texts group bilirubin with the “function” tests or group it independently from “function” or “enzyme” tests Hepatocytes are able to rapidly conjugate bilirubin – the rate-limiting step is transport of bilirubin into the canaliculi ○ This means when a hepatocyte is damaged, conjugated bilirubin increases in the serum more than unconjugated bilirubin ○ Elevations in only unconjugated bilirubin is almost always an RBC problem or an enzyme deficit in conjugation Liver Function tests - albumin Albumin is a protein synthesized by the liver – major functions include: ○ Carrier for hydrophobic molecules (like bilirubin) ○ Establishes oncotic pressure in the capillary Decreases in serum albumin are usually caused by: ○Long-term deficiency in production – chronic liver diseases or serious protein malnutrition (can’t build protein if you don’t eat it) ○ Loss of albumin from a variety of organs (usually the kidney, but there are others) ○ Accumulation of more extracellular fluid than usual – “dilutes” albumin in the blood Albumin has a pretty long half life (almost 3 weeks), so liver dysfunction has to be present for a while before it drops Liver function tests – PT/INR Why such a weird name? ○ PT refers to the prothrombin time – the time it takes to clot using a specific “branch” of the coagulation cascade (you’ll learn about this in hematology) ○ INR = international normalized ratio… it’s a mathematical correction that standardizes the result between labs (often this test is just known as the INR) Increased PT/INR means that it takes longer to form a clot due to a deficiency of coagulation factors in the blood ○ Ratio value – the normal PT/INR is 1 ○ 1.5 roughly means it takes 50% longer for blood to clot than the lab’s reference “normal” The liver is the source of almost all serum coagulation factors, and many depend on the presence of vitamin K ○ Therefore liver dysfunction or vitamin K deficiency can increase clotting times Liver function tests – PT/INR PT/INR FYI factoids INR increases fairly quickly after the liver’s synthetic capabilities are compromised ○Half life of most coagulation factors range from hours to a few days ○Therefore a better way to assess liver function acutely than albumin Bringing it all together – liver labs *Most types of acute hepatocellular injury: ○ Both ALT and AST are very elevated, but ALT more than AST ○ ALP and GGT may be normal, but often modestly increased (elevated less than 3X normal) ○ Not enough time for albumin to change, but both direct and indirect bilirubin are often increased If PT/INR increased, indicates severe damage *Most types of chronic hepatocellular injury: ○ ALT and AST modestly increased, usually ALT > AST ○ ALP and GGT normal or modestly increased ○ Albumin often decreased, PT/INR and both unconjugated/conjugated bilirubin increased Decreased liver function is often a result of late-stage disease – early disease often exhibits normal bilirubin, PT/INR, and albumin Bringing it all together – liver labs *Cholestasis: ○ AST and ALT are normal to moderately elevated ○ ALP and GGT are very elevated ○ Usually large increase in serum bilirubin, especially conjugated , but PT/INR and albumin are normal Other liver test patterns (FYI): ○ Cirrhosis – looks like chronic hepatocellular injury, except the AST:ALT ratio is often 2 or greater ○ Alcoholic liver disease – can resemble acute or chronic hepatocellular injury (depending on stage and situation, except AST:ALT ratio is often 2 or greater and often GGT is very elevated ○ Hepatocellular carcinoma, liver metastases – most tests are normal, but ALP is very elevated ○ Gilbert’s syndrome – all liver tests are normal except for an elevated unconjugated bilirubin (conjugated normal) Bringing it all together – liver labs Assorted tests beyond general liver labs that are useful in diagnosing hepatobiliary disease covered in BMS thus far Disorder Liver Lab Pattern *Other labs Hepatitis A Acute hepatocellular Anti-HepA IgM Hepatitis B Acute or chronic hepatocellular 🡪 HBeAg, HBV DNA, HBsAg cirrhosis Anti-HBe, anti-HBs, anti-HBc Hepatitis C Chronic hepatocellular 🡪 cirrhosis HCV RNA Autoimmune hepatitis Usually chronic hepatocellular, Anti-smooth muscle antibodies, ANA sometimes flares with acute Rarely anti-cytochrome antibodies hepatocellular, LFTs often N Primary biliary ALP, GGT most elevated 🡪 cirrhosis Anti-mitochondrial antibodies cirrhosis/cholangitis pattern Primary sclerosing ALP, GGT and AST most elevated early p-ANCA (associated with ulcerative colitis) cholagnitis (looks like cirrhosis due to cholestasis) Other labs FYI Liver imaging – FYI review Simple ultrasound is the best way to examine the larger biliary ducts, gallbladder initially MRI/CT can assess masses and MRI can give some information about fibrosis ○MRI can also provide more detailed views of the biliary system (known as MRCP) Liver biopsy is necessary to evaluate and stage fibrosis or cirrhosis, and is the definitive test for liver disease that is difficult to diagnose non-invasively Liver investigations in MAFLD - FYI Most common liver disease – but expensive to diagnose definitively How is steatosis > 5% diagnosed? Tough question ○ Elastography assesses fibrosis well, but can have false positives can be done via ultrasound (cheaper, accurate) and MRI (not as much data, emerging) ○ MRI is best at diagnosing steatosis precisely ○ Rarely biopsy – biopsy definitively diagnosis steatosis and fibrosis Since we can’t screen everyone using ultrasound elastography (Fibroscan) or MRI, patients are referred for imaging when: ○ Fulfill the metabolic criteria of MAFLD ○ Elevation in liver enzymes – elevations in ALT most common, early chronic hepatocellular pattern ○ They are scored using (usually simple) blood tests and other clinical data – the scoring systems are used to predict presence of fibrosis MAFLD screening tests (FYI) FIB-4 score – use the following to calculate: ○Patient age, AST, ALT, platelet count NAFLD fibrosis score ○Patient age, BMI, presence of insulin resistance/diabetes ○AST, ALT, platelet count There are others ○Some, such as the ELF, assess molecular markers that are not part of the “typical” blood tests that most labs run frequently (but Lifelabs carries it) Etiology of Eating Disorders Anorexia nervosa and bulimia are complex eating disorders with multiple factors contributing to their etiology Psychological factors: OCD traits, cognitive rigidity, emotion sensitivity and impulsivity as well as history of developmental stressors/ trauma and challenging interpersonal relationships Body dissatisfaction is important as well as some behavioural factors like engaging in diets, and particular athletics Accurso 2019 Etiology of Eating Disorders Anorexia nervosa and bulimia are complex eating disorders with multiple factors contributing to their etiology Biological factors: 50% due to multiple genetic effects Dysfunction in serotonin, dopamine, norepinephrine, opioid and cholecystokinin systems Hypothalamic regulation (amenorrhea can precede weight loss!) Some research suggests changes within peripheral satiety network Vicious cycle as malnourishment can exacerbate the comorbid psychiatric conditions and further the behaviours Accurso 2019 Etiology of Eating Disorders Anorexia nervosa and bulimia are complex eating disorders with multiple factors contributing to their etiology Sociocultural factors: Idealization of thinness Triggers: Dieting is often a precipitating factor for triggering ED Illness leading to weight loss – especially true for AN Accurso 2019 Complications of Eating Disorders All-cause mortality 2-10x higher then general population Independent of weight: Vitamin and mineral deficiencies Stunted growth (if across lifespan) Reduced gastric motility (more discomfort and more food avoidance) Consequences of malnutrition: Bradycardia, hypotension, orthostasis, hypothermia, Metabolic alkalosis, hypochloremia, increased bicarbonate Osteopenia Myopathies Consequences of purging: Esophageal tears, intractable vomiting, hematemesis Metabolic acidosis (abuse of laxatives), hypokalemia cardiomyopathies (d/t specific vomit inducers) Accurso 2019 Pathogenesis of Obesity, Part 1 In-person Class 1 BMS 200 Week 1 References: REFERENCE: Oussaada SM, van Galen KA, Cooiman, MI, Kleinendorst L, Hazebroek EJ, van Haelst MM, Ter Horst KW, Serlie MJ. The pathogenesis of obesity. 2019;92:26-27. https://doi-org.ccnm.idm.oclc.org/10.1016/j.metabol.2018.12.012 von Loeffelholz C, Birkenfeld AL. Non-Exercise Activity Thermogenesis in Human Energy Homeostasis. Endotext [Internet]. MA. 2022 https://www.ncbi.nlm.nih.gov/books/NBK279077/ O’Rahilly S, Farooqi I. Pathobiology of Obesity. In: Loscalzo J, Fauci A, Kasper D, Hauser S, Longo D, Jameson J. eds. Harrison's Principles of Internal Medicine, 21e. McGraw Hill; 2022. Accessed July 13, 2023. https://accessmedicine-mhmedical-com.ccnm.idm.oclc.org/content.aspx?bookid=3095&sectionid=2654 45670 Review from 150: Insulin Resistance & Obesity Most important environmental risk factor for insulin resistance is obesity ○ Central obesity seems to be more crucial ○ A separate risk factor from obesity is lack of exercise Most cases of insulin resistance are caused by a combination of genetic and environmental risk factors… ○ … but don’t forget that T2DM is highly heritable ○ Thought that about 5% of population-wide variance in body weight is due to genetic causes, but environment-gene interactions make this very difficult to study Review from 150: Obesity FAQs Do obese people eat more? ○ Often, but not always In studies that do not record exact caloric intake, often there is a poor association between body weight and questionnaire-reported caloric intake In studies that record caloric intake more closely, the association is better Obese people and those with glucose intolerance often have impaired satiety mechanisms – i.e. poorly-characterized leptin resistance Do obese people “burn less energy”? ○ This is a complicated question: In states where weight loss is not occurring, the obese person seems to use more calories than someone who is lean but may use less calories than someone who is lean during weight loss states Many obese people do have reduced BMR, though most don’t Literature still developing around this area Review from 150: Regulation of body weight and appetite Satiety signals: Leptin, GLP1, CCK, PYY, vagal afferents “Hunger” signals: Ghrelin (released by the stomach during fasting) All of these act on different nuclei in the hypothalamus Review from 150: Controllers of appetite Review from 150: Insulin Resistance and visceral fat Non-esterified fatty acids (NEFA) increase insulin resistance ○ More released from central fat than peripheral fat ○ Increased intracellular concentrations of NEFA cause serine phosphorylation of insulin receptor – which inactivates it (tyrosine phosphorylation activates it) Adipokines modify sensitivity of insulin receptor ○ Adipose tissue is endocrine tissue – protein hormones from fat cells (adipokines) increase sensitivity of insulin receptor and increase activity of enzymes that oxidize NEFA Increased NEFA oxidation mediated by AMP-K, a protein kinase activated by metformin Anti-hyperglycemic adipokines: leptin, adiponectin (drops in T2DM) Hyperglycemic adipokines: resistin, retinol-binding-protein 4 Pro-inflammatory cytokines also secreted by fat cells, and decrease insulin receptor sensitivity Visceral obesity, insulin resistance, and inflammation This was briefly discussed in BMS 150 (next slide) Visceral adipocytes: ○ Recruit macrophages and activate them 🡪 production of pro- inflammatory cytokines (TNF-alpha, IL-6) Expression of MCP-1 (a chemokine) brings monocytes into visceral fat as well as production of pro-inflammatory cytokines by the adipocyte Elevated systemic levels of IL-6 🡪 increased production of CRP by the liver ○ Insulin resistance in visceral adipocytes 🡪 increased concentrations of free fatty acids 🡪 activation of DAMPs in many cells increased production of pro-inflammatory cytokines ○ Pro-inflammatory cytokines cross-talk with intracellular signaling cascades that lead to insulin resistance (serine phosphorylation of the insulin receptor) Kahn et. Al., J Clin Invest. 2019 Oct 1; 129(10): 3990–4000; doi: Review from 150: Chronic Inflammation and obesity Systemic inflammatory effects of obesity ○ excessive lipid build-up can stress the adipocyte (ROS) ○ free fatty acids in high concentrations may bind to PAMP-R within the adipocyte both of the above can lead to As shown above, pro- the production of IL-6 and inflammatory TNF-alpha by the adipocyte cytokines lead to insulin resistance, and Review from 150: Insulin Resistance and visceral fat 4 9 Obesity - Definitions What is overweight? What is obesity? ○ BMI definition (most used): Overweight 🡪 BMI ≥ 25 kg/m 2 Obesity 🡪 BMI ≥ 30 kg/m2 ○ Waist:hip ratio for obesity (sometimes used): Men 🡪 ratio > 0.90 Women 🡪 ratio > 0.85 Advantages to using each definition? Obesity - Definitions EE = Energy expenditure The total amount of energy we expend, measured in kcal/day Consists of: ○ Resting metabolic rate (RMR) – metabolism of an individual at rest Energy requirements of respiration, circulation, etc. ○Activity-related energy expenditure (AEE) – exactly what it sounds like Exercise activity thermogenesis (EAT) – energy used during “dedicated exercise” Non-exercise activity thermogenesis (NEAT) – energy used when an individual is moving, but “not exercising” 🡪 much larger component of AEE ○Diet-induced thermogenesis (DIT) – increase in metabolic rate associated with ingestion of food and post-absorptive heat production Ousaada et. al., Metab Clin Exp. 92: 26 Energy expenditure breakdown Model of human energy expenditure components ○ Exercise-related physical activity is comparable to exercise-related activity thermogenesis (EAT), while spontaneous physical activity is comparable to non- exercise activity thermogenesis (NEAT) ○ In most people, EAT is much, much less than NEAT In those who train regularly, it can be 15 – 30% of EE (very regular, dedicated exercise) 2 hours of training/week – 1-2 % of inter-individual variance ○ Note that parts of spontaneous physical activity are beyond voluntary control, also called “fidgeting.” fidgeting can consume between 100 – 800 kcal/day Ousaada et. al., Metab Clin Exp. 92: 26 https://www.ncbi.nlm.nih.gov/books/ RMR RMR = energy expenditure in an individual that is at rest and has not recently eaten Proportion of EE due to RMR varies between individuals ○ If sedentary, 60 – 75% of total daily EE Main determinant of RMR is fat-free mass (FFM) ○ Main component of FFM is weight of skeletal muscle ○ also includes bone, visceral organs, extra-cellular fluid RMR varies from 2 – 10% in the same individual ○ time of day, temperature season, etc. as well as errors in measurement RMR varies much more between individuals, between 7.5 and 18% ○ Amount of FFM is the main determinant of inter-individual variability – about 62% ○ Over 25% of inter-individual variation in RMR is not well understood – assumed to be genetic/molecular differences Ousaada et. al., Metab Clin Exp. 92: 26 Components of daily EE a) total EE expenditure b) EE per kg of FFM Ousaada et. al., Metab Clin Exp. 92: 26 RMR vs. BMR RMR has less stringent requirements than BMR Basal metabolic rate (BMR): ○ Completely rested subjects in the morning, after 8 hours of sleep, fasting for 12 hours, and at a room temperature of between 22 – 26 Celsius ○ 80% of variations in BMR are due to FFM variations (same as RMR) RMR: ○ post-absorptive (i.e. not right after a meal) state at any time of day, at rest ○ can vary from BMR by 10% https://www.ncbi.nlm.nih.gov/books/ NEAT - generalities NEAT = “portion of daily energy expenditure resulting from spontaneous physical activity that is not specifically the result of voluntary exercise” ○ variation can be up to 2000 kcal/day in two similar-sized individuals ○ differences in occupations, leisure activities, molecular/genetic factors, seasonal effects ○ The most variable aspect of energy expenditure on a population basis 6-10% of EE in individuals with a sedentary lifestyle up to 50% in highly active individuals (often those that are standing or constantly moving around in their occupation) https://www.ncbi.nlm.nih.gov/books/ NEAT – impact of diet and exercise Measurement of NEAT is extremely complex, and studies vary in terms of the impact of dietary changes With overfeeding: ○ a significant minority of people will increase NEAT, to the point where increased activity compensates for increased calories consumed ○ the majority do not increase NEAT to compensate for over-feeding, but total EE does still increase somewhat With underfeeding: ○ RMR and NEAT both decrease in those that are inactive 20% weight loss 🡪 320 – 500 kcal/day reduction in EE Due mostly to losses in FFM ○ Studies suggest that those that undergo exercise regimens with underfeeding will not suffer as large a decrease in NEAT https://www.ncbi.nlm.nih.gov/books/ Models of energy expenditure Independent model of energy expenditure: ○ changes in EE are independent of the energy you “budget” for a behaviour (NEAT, EAT) ○ Therefore, if you increase your NEAT & EAT, your total EE goes up… and it’s “easier” to lose weight Compensation/allocation model of energy expenditure ○ if you increase the energy expenditure in one area (EAT for example), you decrease the expenditure in another (RMR or NEAT) There is evidence for both models Literature on weight loss also identifies two types of populations: ○ Compensator – if a compensator is overfed, then spontaneous physical activity (NEAT) increases ○ Non-compensator – with overfeeding, less increase in EE, mostly due to less of an increase in NEAT 57% of variability of spontaneous physical activity is believed to be a result of inheritance/ genetics https://www.ncbi.nlm.nih.gov/books/  Exercise, diet, and insulin resistance With weight loss due to caloric restriction, skeletal muscles seem to become more efficient ○ not sure why – in rats, the following was found: decreases SNS activity 🡪 decreased cardiovascular energy expenditure and skeletal muscle energy expenditure a molecular “switch” to isoforms of the myosin heavy chain that expend less ATP Reductions in the activity of the SNS as well as reductions in the release of thyroid hormone with underfeeding also contribute to decreases in EE (RMR) Exercise causes translocation of GLUT-4 transporters to the sarcolemma 🡪 improved glucose transport from blood to “busy” muscle… impact on insulin resistance? https:// www.ncbi.nlm.nih.g ov/books/ Small Group Activity Dive into the following paper: ○ https://www-sciencedirect-com.ccnm.idm.oclc.org/science/article/pii/S0026049519300071?via%3Dihub ○ It’s in your folder for today Each group look specifically for the following information: Group 1 – definition of diet-induced thermogenesis Group 2 – do some macronutrients have different degrees of diet- induced thermogenesis? Which ones have the highest? Group 3 – does the temperature of food impact diet-induced thermogenesis? How do we regulate energy intake? Homeostatic pathway – basically, stimulates eating behaviour when energy stores are low, and suppresses it in ○ “Major peripheral players” – sense when nutrients are present or absent: Adipose tissue, stomach, intestines, special senses, pancreas, liver These areas release the many, many endocrine signals that modulate appetite and energy expenditure ○ “Major central players” – nerves and central nervous system nuclei/connections that integrate this information and translate it to a “sense of hunger” Vagus nerve, brainstem, hypothalamic nuclei, cortex, aspects of the limbic system Ousaada et. al., Metab. Clin. Exp. 92, 26-36, 2019;  Simple “CNS portion” of the homeostatic model The arcuate nucleus of the hypothalamus (ACN) and the paraventricular nucleus (PVN) seem important in integrating signals from the periphery and the CNS Note: AGRP When nutrients are neurons also secrete NP Y present, POMC neurons in the ACN release MSH 🡪 NP Y stimulates PVN drives behaviour to When nutrients are present, AGRP neurons are inhibited 🡪 less food intake reduce eating and AGRP (separate ▪ AGRP blocks the MSH receptors (MC4R) mechanism) increase energy expenditure Therefore, satiety is mediated by increased MSH signaling, either directly (MSH release) or indirectly (inhibition of AGRP release) Serotonin signaling and the homeostatic pathway Serotonergic neurons in the midbrain (the raphe nucleus) project to the arcuate nucleus as well as nuclei involved in the hedonic pathway ○Increased serotonin signaling 🡪 activation of MSH neurons, inhibition of AGRP/NPY neurons… Impact on satiety? ○ Lorcarserin is a 5-HTc receptor agonist which can induce weight loss in obese subjects ○ Signaling mechanisms are complicated and can involve changes in serotonin release, modulation of serotonin receptors, and upregulation/downregulation of serotonin transporters Quite a bit of human data that suggests that it is important in satiety and weight regulation Ousaada et. al., Metab. Clin. Exp. 92, 26-36, 2019; How do we regulate energy intake? Hedonic model – food intake is driven by reward pathways in the brain, less by nutrient availability ○ Therefore, “less reliance” on feedback from peripheral sites that are exposed to nutrients However, this is likely an oversimplification, as nutrients and hormones associated with nutrient intake have recently been found to directly influence these brain areas ○ “Major central players” – many of these brain areas are important areas of the CNS that mediate reward Lateral hypothalamus, ventral tegmental area, nucleus accumbens (part of ventral striatum), limbic system nuclei Major neurotransmitters implicated are dopamine as well as the endogenous opioids (enkephalins, endorphins, etc.) ○ Reward = pleasant sensations associated with eating, and that can often drive eating in the absence of hunger Ousaada et. al., Metab. Clin. Exp. 92, 26-36, 2019;  Simple “CNS portion” of the hedonic model Basic pathway: The lateral hypothalamus projects to a midbrain area, the ventral tegmental area Dopaminergic neurons in the ventral tegmental area project diffusely to the: ○ Nucleus accumbens ○ Amygdala ○ Prefrontal and orbitofrontal cortex The VTA and NA also “talk back” to the hypothalamus Stogios et. Al., Nutrients 2020, 12, 3883; How do we regulate energy intake? Are the hedonic and homeostatic pathways separate? ○ Not at all… the hedonic pathway can impact the arcuate nucleus ○ As well, activity (or lack of it) in the homeostatic pathway will modulate the reward pathway ○ It seems like there is a disruption in the “balance” between these two pathways in obese persons ○In the end, it appears that the “master integrator” is the hypothalamus 🡪 it weighs information from both pathways to drive eating behaviour Ousaada et. al., Metab. Clin. Exp. 92, 26-36, 2019; Energy intake and reward deficiency Reward deficiency hypothesis ○ In all of us, delicious (often fatty, often sweet) food rewards us… the reward is that adjective “delicious”, and all the positive feelings that accompany eating something you like Viewing it simply 🡪 hedonic pathway activation ○ Human evidence seems to indicate that lean subjects have better activation of the reward pathway than obese subjects… therefore it’s possible obese subjects are “reward deprived”… Striatal dopamine release seems to be impaired in those who are obese, with decreased D2/D3 receptor activation ○ In addition, those who are obese may anticipate reward more when expecting a meal In those who are obese, visual palatable food stimuli elicit greater activation of the corticolimbic system Increased reward expectation + decreased reward upon eating 🡪 increased eating behaviour Ousaada et. al., Metab. Clin. Exp. 92, 26-36, 2019;  The “peripheral” homeostatic players Adipose-derived: Leptin, adiponectin, resistin, retinol-binding protein 4 (RBP-4), FGF- 21 GI-derived: Stomach – ghrelin (orexigenic) Rest of the GI tract: GLP- 1, CCK, peptide YY, oxyntomodulin Pancreas: Insulin Interaction of peripheral and central players in the homeostatic system Importance of the hypothalamus Surrounds the 3rd ventricle and the CSF has some “crossover” with molecules from the peripheral circulation Parts of the hypothalamus have a “leakier” BBB However, many peripheral signals can impact other brain areas (transported across BBB, i.e. insulin) As well, the vagus synapses with the hypothalamus, and is important in relaying peripheral messages  Segue - Types of fat White fat – predominant form of adipose tissue, serves as a store of triglycerides, and visceral adipose tissue (within the peritoneum, especially omental fat) is an important endocrine organ ○ Storage vacuole – specialized phospholipid monolayer with local adaptations to limit lipid peroxidation in the presence of free radicals Brown fat – steadily decreases as we age, most found in infants in particular areas of the body, main role is thermogenesis… and energy balance? ○ Uncoupling protein in mitochondria “burns fat” (beta oxidation) without generating ATP ○ Allows leakage of protons across inner mitochondrial membrane 🡪 heat production ○ Regulated by catecholamines (activated by beta-3 receptors) ○ Small fat vacuoles, many more mitochondria White fat can “brown” with exercise, cold, sympathetic stimulation ○ Known as beige, or brite (brown-like) adipose tissue, looks like brown-fat cells interspersed with white fat cells Kahn et. Al., J Clin Invest. 2019 Oct 1; 129(10): 3990–4000; doi: Location/characteristics of different types of adipose tissue Kahn et. Al., J Clin Invest. 2019 Oct 1; 129(10): 3990–4000; doi: Homeostatic pathway mediators Leptin: Secreted by white adipocytes in the presence of insulin, inhibited by catecholamines (so increased post-prandially) Anorexigenic – suppresses NPY and AGRP, increases MSH secretion from the arcuate nucleus of the hypothalamus ○ Those with congenital leptin deficiencies (very rare) become obese due to hyperphagia Discovered in 1994 and generated lots of excitement ○ The weight loss pill/molecule/injection! ○ Alas, it was not to be… Obese subjects tend to have elevated leptin levels and the hypothalamus is resistant to leptin ○ May be mediated by gliosis/inflammation in the hypothalamus, but yet to be conclusively proven Kahn et. Al., J Clin Invest. 2019 Oct 1; 129(10): 3990–4000; doi: Homeostatic pathway mediators Insulin: Secreted by pancreatic beta-cells in response to elevated blood glucose and some amino acids Receptors for insulin are present in the ventral striatum, and are linked to increased dopamine signaling in that area ○ Increased hedonic pathway signaling can amplify homeostatic satiety signaling in the hypothalamus Kahn et. Al., J Clin Invest. 2019 Oct 1; 129(10): 3990–4000; doi: GI hormones and homeostatic appetite signaling Ghrelin – released by cells in the gastric fundus in response to fasting ○ Stimulates hunger pathways (i.e. amplification of AGRP and NP Y, inhibition of MSH) likely by stimulating the vagus nerve There are receptors in the brain for ghrelin, though ○ Only orexigenic hormone ○ Obesity: Fasting levels of ghrelin are negatively correlated with BMI Obese patients might not suppress ghrelin as effectively after a meal GI hormones and homeostatic appetite signaling We’ve seen all of these before in BMS 150 – secreted by enteroendocrine cells in various parts of the GI tract (next slide) CCK is released proximally in the small intestine (duodenum) ○ Slows gastric emptying, and increases sensations of satiety GLP-1 and PYY are released more distally in the intestine ○ also slow gastric emptying and increase satiety The brain expresses receptors for all of these enteroendocrine hormones ○ However, in those with transection of the vagus, their satiety- inducing effect is blunted or abrogated Vagal afferents have receptors for all of these molecules ○Likely that much of the satiety effect is through vagus 🡪 NTS of the brainstem 🡪 hypothalamus Longo et. Al., Acta Diabetologica (2023) 60:1007–1017; doi: 10.1007/s00592 BMS 150 review Cell Location Hormone (Stimulus) Main Hormonal Functions Stomach, Somatostatin Generally “turns down” the release of hormones D duodenum, (many different stimuli cause from nearby cells pancreas release) ECL – stomach ECL – histamine (stimulated by ECL – stimulates acid production EC – stomach, vagus) EC – increased motility EC, ECL small and large EC – serotonin, substance P intestines (mechanical, neural, endo) Gastrin (amino acids in the Increases secretion of stomach acid G Stomach stomach, vagal stimulation, gastrin-releasing peptide) Small Intestine CCK (fats and proteins in the Pancreatic enzyme secretion, gallbladder I* (especially duodenum) contraction, satiety duodenum) Inhibits gastric acid secretion Glucagon-like peptide (amino GLP-1 - Insulin secretion, satiety acids & carbs) Inhibits gastric acid secretion L* Small intestine Peptide YY (distal small intestine) Peptide YY – inhibits gastric secretion & motility – slows gastric emptying Motilin (fasting) Migrating motor complex Mo Small intestine Secretin (acid in small intestine, Bicarbonate and water secretion from pancreas S Small intestine especially duodenum) Inhibits gastric acid secretion, emptying Cause and effect in regulation of appetite Most human data shows association between a component of the homeostatic/hedonic pathway and obesity ○ correlation does not necessarily imply causation For example: ○ high-calorie snacking in lean subjects can change serotonin transporter activity 🡪 impaired serotonin signaling ○ striatal dopamine signaling can be increased after bariatric surgery- induced weight loss ○In these situations, weight loss or gain 🡪 changes in the pathways Ousaada et. al., Metab. Clin. Exp. 92, 26-36, 2019; Adipokines, insulin resistance, and obesity Is adiponectin part of the homeostatic pathway? produced mostly by white adipose tissue (particularly subcutaneous white fat), but other tissues (muscle, bone, liver) can secrete it As visceral fat and insulin resistance increase, adiponectin tends to decrease Not enough evidence to say that it stimulates or inhibits appetite in humans However, adiponectin does: ○ increase insulin sensitivity (our body will need less insulin secreted to act) ○ decrease fat accumulation in the liver and hepatic glucose output Other adipokines can increase insulin resistance – in particular resistin (perhaps more important in mice) and retinol-binding protein 4 (RBP-4) Kahn et. Al., J Clin Invest. 2019 Oct 1; 129(10): 3990–4000; doi: Summary of Obesity Complications System Description Dyslipidemia Increased TG and LDL, systemic inflammation 🡪 atherosclerosis Fatty Liver Disease Ectopic fat in hepatocytes 🡪 fibrosis 🡪 cirrhosis Type 2 Diabetes & insulin Visceral fat has a large impact on overall insulin sensitivity resistance PCOS/ Hypogonadism reduced testosterone in hypogonadism, likely due to related to insulin resistance Hormonal abnormalities in PCOS 🡪 increased androgen production (likely due to insulin resistance) which is partially converted to estrogens by visceral fat 🡪 dysregulated cycles Skin Skin folds 🡪 increased risk of fungal infection acanthosis nigricans Cardiovascular Increase atherothrombotic vascular disease independent of diabetes type 2, but still suspected to be related to insulin resistance Harrison’s Principles of Internal Medicine 2022 Summary of Obesity Complications System Description Respiratory Dyspnea due to increased mass and increased pressure on thoracic cage, increased adipose tissue around upper airways GI Increased intraabdominal pressure can result in reflux esophagitis, also increased risk of gallstones Rheumatic Increased OA of knees/hips, but NO increase in RA or other inflammatory joint disease Cancer As BMI increases by 5 kg/m2 🡪 cancer mortality increases by 10% Infections Increased susceptibility to bacterial wound infections and SARS-COV2 complications CNS Increased risk of dementia & stroke (atherosclerosis) Increased incidence of idiopathic intracranial hypertension Harrison’s Principles of Internal Medicine 2022 How can the gut microbiota influence our food intake? Introducing - Microbiota-Gut-Brain Axis (MGBA): Composed of ANS, ENS, Spinal nerves, HPA axis, Immune system, Enteroendocrine cells (EEC’s) and Microbiome CNS receives and directs input via spinal nerves that connect with ENS ENS receives and directs input via EEC’s which in terms can be modified by and can influence the composition of the microbiome HPA: it’s unclear the specific role that it plays but appears to be implicated in MGBA; chronic elevations of cortisol are correlated with changes in gut microbiota function Immune system facilitates a symbiotic relationship with commensal gut microbiota Vagus nerve; directly detects intestinal distention via mechanoreceptors AND vagal chemoreceptors connect to EEC’s; appear to be activated by changes in microbiota (animal studies) Von Son 2021, Longo 2023 How can the gut microbiota influence our food intake? Possible mechanisms: Changes in gut microbiota seen in obesity result in increased gut permeability that allow LPS to enter circulation and trigger proinflammatory state within adipocytes and systematically, which can then promote insulin insensitivity Obese humans have increase in plasma LPS post high-fat meal versus lean humans Enteroendocrine cells modify their secretions (serotonin, ghrelin, CCK, GLP-1, PYY) in response to microbial metabolites like SCFA’s, these can then influence insulin, gastric acid and bile acid secretion Vagus nerve connects gut to nucleus of solitary tract which connects to hypothalamic arcuate nucleus that is involved in energy balance Vagotomy has been associated with changes in body weight Von Son 2021, Longo 2023 How can the gut microbiota influence our food intake? Possible mechanisms: Gut microbiome produces SCFA’s, GABA, dopamine and serotonin SCFA’s have been implicated in regulating satiety: Increase PYY and GLP-1 secretion Stimulating vagus nerve Passing through BBB and inducing anorexigenic signals Reducing fat accumulation in adipocytes Inducing thermogenesis and increased energy expenditure Increase leptin production HOWEVER, intervention studies failed to show benefit of supplementary SCFA’s on weight in metabolic syndrome Von Son 2021, Longo 2023 How can the gut microbiota influence our food intake? Possible mechanisms: Homeostatic Pathway Observations: Bifidobacterium and Lactobacillus positively correlate with leptin (promotes satiety) and negatively with ghrelin (promotes hunger) Following H. pylori eradication increase in Bacteroidetes/ Firmicutes ratio correlates with reduced ghrelin concentrations However, CCK promotes reduced food intake via vagus nerve… Human studies with CCK agonists fail to produce weight loss Animal dysbiosis studies demonstrate reduced vagus nerve input to brain resulting reduced CCK-induced satiety Hedonistic Pathways Observations: No direct connection between mesolimbic dopaminergic system (reward system) and gut microbiota has been established, hypothesize that gut dysbiosis may influence via causing generalized neuroinflammation Von Son 2021, Longo 2023 Microbiota-gut-brain axis (MGBA) and GLP-1 Glucagon-like peptide 1 (GLP-1) Released by intestinal epithelial enteroendocrine cells Within the small intestine, this release is induced by presence of specific nutrients Within the large intestine, this release is induced by the microbiota Functions: Increase insulin and reduce glucagon Delay gastric emptying and promote pyloric contractions Regulate appetite (suspected via vagus nerve) Giving butyrate (SCFA) via IV 🡪 no influence on appetite (animal study) BUT via diet 🡪 decreases food intake Longo 2023 Microbiota: Glucose and Insulin Metabolism Human observations: Transplant of microbiota from healthy patients into patients with metabolic syndrome increases insulin sensitivity Intestinal dysbiosis correlates with low grade inflammation in obesity and in insulin resistance Possible mechanism: Gut microbiota induces increase intestinal permeability resulting in LPS and fatty acid leakage and activation of TLR4 🡪 systemic inflammation Longo 2023 Microbiota: Energy Harvesting Observations – what do we know so far? Obese humans seem to have reduced counts of Bacteroidetes compared to control (though debated in some studies) Germ-free mice are leaner, but once allowed to be colonized increase body fat by 50% and reduce insulin sensitivity Gut microbiome genes present in obese mice and obese humans appear to be involved in energy harvesting: digesting polysaccharides, transport and intracellular metabolism Fecal transplant from obese mice to germ-free mice results in transfer of obese phenotype Greiner 2011 Microbiota: Lipogenesis Additional observations: Increased energy harvest 🡪 increased fermentation 🡪 increased production of short chain fatty acids (SCFA’s) Obese humans have elevated SCFA production Mice deficient in SCFA receptors are leaner than controls Greiner 2011 Pathogenesis of Diabetes, Part 2 BMS 200 Week 1 The twin cycle hypothesis and T2DM What is the general model? What are the two cycles? Al-Mrabeh, Card. Endo. Metab 2020, 9:132-142; doi: 10.1097/XCE.0000000000000201 Background to the twin cycle hypothesis T2DM is thought to develop after many years of insulin resistance ▪ Key aspect of the transition from early insulin resistance to loss of beta cell mass is dysregulated lipid and glucose metabolism across multiple different organs Liver, muscle, pancreas, skeletal muscle, visceral adipose tissue, subcutaneous adipose tissue The twin cycle begins with early insulin resistance ▪ Easier to understand in a setting with: Excess caloric intake (exacerbated by leptin resistance, biopsychosocial factors… etc.) Impaired ability to “clear” nutrients by skeletal The twin cycle hypothesis and T2DM Start here! Al-Mrabeh, Card. Endo. Metab 2020, 9:132-142; doi: 10.1097/XCE.0000000000000201 The “first” cycle – the liver Studies in humans seem to indicate that diabetic patients have a “tipping point”, where normal energy storage options are saturated Where should we store our energy? ▪ Lots in subcutaneous fat ▪ Some in muscle (what is the energy storage form?) ▪ Some in the liver (what is the energy storage form?) ▪ A bit in visceral fat Sensitive, effective insulin receptors aid “normal” energy storage in these “healthy” energy depots Al-Mrabeh, Card. Endo. Metab 2020, 9:132-142; doi: 10.1097/XCE.000000 The “first” cycle – the liver In the setting of positive energy/calorie balance and constant, “longer-term” insulin secretion, skeletal muscle and subcutaneous fat begins to experience insulin resistance ▪ This is the beginning of the first cycle Circulating nutrients – in particular glucose – remain in the bloodstream longer AND adipose tissue loses some of its ability to convert these calories into stored triglycerides (due to insulin resistance) ▪ The result is an increase in circulating free fatty acids (FFA) for long periods of time throughout the day Elevated FFAs then have to be dealt with by the liver Al-Mrabeh, Card. Endo. Metab 2020, 9:132-142; doi: 10.1097/XCE.000000  The “first” cycle – the liver The liver seems to have 3 major options when it is “drowning” in these elevated levels of FFAs ▪ Burn the energy – (beta oxidation) ▪ Store the energy – hepatic steatosis (not good) ▪ Export the energy – VLDL production Recall – what are the characteristics of VLDL? What is its goal? What is its life cycle? Al-Mrabeh, Card. Endo. Metab 2020, 9:132-142; doi: 10.1097/XCE.0000000000000201 Summary –Endogenous Pathway Endogenous HDL Lipoprotein Liver synthesis Apoprote Initial VLDL ins from lipoprotein (ApoB-100, ApoA-V, (key apoproteins) ApoC-II) HDL LD Intermediate IDL (ApoE, ApoB-100) VLD IDL lipoproteins ↓ L L (key apoproteins) LDL (ApoB-100) VLD L Cleared by Liver LD (clearance (LDL receptor for B-100, mechanism) ApoE clearance) L IDL Function Carry liver-synthesized lipids to other cells The twin cycle hypothesis and T2DM Step 2 Start here! Al-Mrabeh, Card. Endo. Metab 2020, 9:132-142; doi: 10.1097/XCE.0000000000000201 The “first” cycle – the liver Insulin resistance isn’t just “shutting insulin off” ▪ It’s an imbalance in the see-saw between glucagon and insulin signaling – which is especially important when considering liver physiology In those with longer-term insulin resistance/T2DM, the liver accumulates lipid (steatosis) and this may exacerbate insulin resistance in hepatocytes ▪ Steatosis 🡪 Inappropriate gluconeogenesis 🡪 increased blood glucose 🡪 increased pancreatic insulin secretion… ▪ You end up with the strange metabolic situation of fat accumulation in hepatocytes, increased VLDL export (both “insulin-driven” functions) and decreased ability of the liver to shut off gluconeogenesis (“glucagon-driven” function) Al-Mrabeh, Card. Endo. Metab 2020, 9:132-142; doi: 10.1097/XCE.0000000000000201 The “first” cycle – the liver Don’t forget the role of elevated circulating FFAs (from insulin-resistant adipocytes) ▪ The body has no choice but to deal with these – we don’t have an “energy excretion” mechanism ▪ The hepatocytes tend to take these FFAs out of circulation and “repackage” them as triglycerides in VLDL or incorporate them as lipid droplets – this activity is enhanced in T2DM ▪ Elevation of FFAs (in particular palmitic acid) activates TLRs and seems to exacerbate insulin resistance and “stress” the cell out Many of those with the metabolic syndrome/T2DM have “ectopic” deposition of fat ▪ Within skeletal muscle ▪ Within the pancreas 🡪 next cycle Al-Mrabeh, Card. Endo. Metab 2020, 9:132-142; doi: 10.1097/XCE.0000000000000201 The twin cycle hypothesis and T2DM Step 2 Start here! Step 3 Al-Mrabeh, Card. Endo. Metab 2020, 9:132-142; doi: 10.1097/XCE.0000000000000201 The “second” cycle – the pancreas The changes to the pancreas in T2DM have been the most poorly characterized… until recently ▪ Pancreas is very “deep” in the body, hard to assess, hard to biopsy, and there has always been a conceptual “block” 🡪 The acinar cells and > 95% of pancreatic tissue Bad don’t matter in T2DM… it’s all about the islets? assum ▪ Studies that examine the degree of adiposity and p-tion? fibrosis in the islets AND the surrounding acinar cells suggest that the pancreas is an important player – “the second cycle” Intrapancreatic fat accumulation is common in those with T2DM, and reversal of T2DM is associated with large decreases in pancreatic fat Al-Mrabeh, Card. Endo. Metab 2020, 9:132-142; doi: 10.1097/XCE.0000000000000201 The “second” cycle – the pancreas The liver delivers excess TGs (via VLDL) to the pancreas ▪ Linked to fibrosis and fatty accumulation in the pancreas in general (around acinar cells) ▪ Linked to accumulation of palmitic acid (FFA) in islet cells, specifically pancreatic beta- cells What are the consequences of “fatty pancreas”? ▪ Initially hypothesized to be apoptosis of beta-cells over time Increased ER stress, ROS production, apoptosis of beta cells “beta cell burnout” due to continuous insulin production This may be true Al-Mrabeh, Card. Endo. Metab 2020, 9:132-142; doi: 10.1097/XCE.0000000000000201 The “second” cycle – the pancreas What are the consequences of “fatty pancreas”? ▪ There are other possibilities beyond apoptosis – beta cell de-differentiation ▪ In animal and in vitro studies, excess fat and highly excessive glucose lead to beta cells that act more like alpha cells What does an alpha cell secrete? ▪ This is being intensely studied right now in humans Whatever the cause of beta cell dysfunction, resolution of fat deposition and fibrosis in the pancreas (as assessed by MRI) seems to be improved with dietary restriction and the resolution of T2DM, and is linked with resolution of hepatic steatosis Al-Mrabeh, Card. Endo. Metab 2020, 9:132-142; doi: 10.1097/XCE.0000000000000201 The twin cycle hypothesis and T2DM Step 2 Start here! Step 3 Al-Mrabeh, Card. Endo. Metab 2020, 9:132-142; doi: 10.1097/XCE.0000000000000201 Note the positive feedback loop Insulin resistance 🡪 increased hepatic steatosis and VLDL output 🡪 TG and FFA accumulation in the pancreas 🡪 compromised insulin secretion (impaired ability to secrete large, immediate “pulses of insulin) 🡪 hyperglycemia 🡪 conversion to fat, increased circulating FFAs… Elevations in VLDL and the general inflammation associated with T2DM… ▪ Describe the likely impact on atherosclerosis  Endogenous pathway (review slide) Initial lipoprotein: VLDL is synthesized by the liver – contains mostly TGs but also contains phospholipids, cholesteryl esters, vitamin E Contain ApoC-II and ApoA-V – these are important for the activity of LPL in peripheral tissues 🡪 as LPL “drains” triglycerides from VLDL, it becomes IDL and then LDL Intermediate As Also forms: TGscontains ApoB-100 are removed from – major VLDL itstructural becomesprotein and IDL IDL – most allows later by is cleared clearance the liver via byApoE the liver binding to the LDL receptor (ApoE was transferred via HDL particles) IDL that loses TGs and becomes more and more cholesterol-rich becomes Cleared by: LDL Liver LDL clears IDL and is cleared byLDL thevia LDLthe LDL receptor receptor, on hepatocytes and seems to have no useful physiologic LDL receptor can bind to either ApoE or ApoB-100 role If LDL LDL is receptor not cleared, can itbind cantobecome oxidized both ApoE and becomes a major risk and ApoB-100 factor for the development of atherosclerosis  Subcutaneous fat “saturation” Adipose tissue cannot take all the fat (VLDL) out of circulation – everyone has a limit ▪ Determined by genetics, sex, age In the setting of insulin resistance, the adipocytes are less able to build triglycerides, and instead release FFAs Our subcutaneous fat mass seems to be “good at” secreting adiponectin ▪ For reasons that are not clear, the subcutaneous fat store secretion of adiponectin cannot “keep up” with the secretion of leptin (from visceral and subcutaneous fat) ▪ In insulin resistance/T2DM, the ratio of leptin:adiponectin is increased… but many are resistant to leptin, so food intake becomes more poorly regulated The twin cycle hypothesis and T2DM That is a very detailed… and therefore hard-to-prove… hypothesis ▪ What is the evidence? ▪ How would we test it? With successful T2DM treatment do we observe: ▪ Improvement in fatty liver? ▪ Improvement in fatty pancreas? ▪ Weight loss? ▪ A drop in serum triglycerides? You would think that in long-term T2DM – when the vast majority of the pancreatic beta cell mass has been exhausted – the “twin cycles” would be difficult to stop Al-Mrabeh, Card. Endo. Metab 2020, 9:132-142; doi: DiRECT trial in the UK 306 T2DM patients subjected to intense weight-loss program, supervised by dieticians and nurses ▪ All antihypertensive and antidiabetic meds stopped on first day of the trial, plus: 825 – 853 kcal/day for 3 months Structured food “reintroduction” to a more sustainable caloric content over the next 1 – 2 months ▪ Had to have been diagnosed with T2DM within the last 5 years (no long-term T2DM) ▪ Open-label study… but it got published in the Lancet (high-impact journal) because of the impressive results Al-Mrabeh, Card. Endo. Metab 2020, 9:132-142; doi: https://doi.org/10.1016/S0140- 10.1097/XCE.0000000000000201  DiRECT trial in the UK Results: 149 with dietary intervention, 149 control At 12 months, weight loss of 15 kg or more in 36 (24%) participants in the intervention group and no participants in the control group (p