Metabolic Response to Starvation - YR1 Lecture 1H 2021 - PDF

Metabolic response to starvation Anand. A. Hardikar, PhD Associate Professor and Head, Diabetes and Islet Biology JDRF Australia T1D Clinical Research Network Fellow (CDA), School of Medicine | Western Sydney University Visiting Professor, Danish Diabetes Academy, Roskilde University and Steno Diabe...

Metabolic response to starvation Anand. A. Hardikar, PhD Associate Professor and Head, Diabetes and Islet Biology JDRF Australia T1D Clinical Research Network Fellow (CDA), School of Medicine | Western Sydney University Visiting Professor, Danish Diabetes Academy, Roskilde University and Steno Diabetes Centre Copenhagen Vice‐President, Islet Society, Sweden www.isletbiology.info | E‐mail: [email protected] @ AnandHardikar Learning objectives 1. Describe the major metabolic features of the 24 hour starved‐ fed cycle 2. Discuss the metabolic adaptations that occur during starvation of varying duration including changes in carbohydrate, fat and protein metabolism by different tissues 3. Explain the role of ketone bodies as a metabolic fuel 4. Explain the Barker hypothesis 2 Feeding and starvation cycle The human body attempts to maintain a “normal” glucose concentration (4‐6mmol/L) mmol/L = mg/dL 18 1. What happens when we eat 2. What happens during periods of fasting (at night/intermittent fasting/IF and longer periods of starvation) 3. What are the players involved? 3 Key organs and players in maintaining glucose homeostasis The pancreatic islets of Langherhans, contain alpha and beta cells that secrete glucagon and insulin respectively. Insulin and glucagon exert antagonistic effects on peripheral organs to control blood glucose levels. Insulin exerts its glucose lowering effects by stimulating glucose uptake in skeletal muscle, through inhibiting hepatic glucose production and by blunting lipolysis. Glucagon raises circulating glucose levels by increasing gluconeogenesis and lipolysis. Ruud, J., et al (2017) Nat Commun 8, 15259 https://doi.org/10.1038/ncomms15259 4 Dietary composition and fuel Food gives us energy (measured kilocalories/kcal or kilojoules/kJ). The amount of energy contained in each gram is the energy density. Most meals are based on the three macronutrients (along with vitamins, minerals, fiber and water): 1. Carb Carb: produce glucose (incorporated into glycogen or TAGs) 2. Prot Prot: broken down to amino acids /incorporated into proteins 3. Fats: Broken down to Fatty acid and glycerol Every gram of carb, protein and fat provides energy (kcal) 5 1kcal=4.18 kilojoules (4.18 kJ). Glucose uptake by cells and insulin release Fura2 calcium signalling following glucose exposure (Hardikar A et al – unpublished)  [glucose]  Activation of (GLUT)  glucose metabolism occurs inside β‐cells through glycolysis, the Krebs cycle and the electron transport chain, generating adenosine triphosphate (ATP) ATP  closes ATP‐sensitive potassium channels (KATP),  reduces resting membrane potential  membrane depolarization  voltage gated Ca2+ channels open and Ca2+ concentration increases  triggers insulin vesicle fusion with cell membrane and exocytosis. Castiello FR et al (2016) Lab Chip, 16, 409–431 Hardikar A et al (unpublished) 6 Glucose transporters Name Tissue location GLUT1 GLUT2 All mammalian tissues Liver and pancreatic β cells GLUT3 GLUT4 All mammalian tissues Muscle and fat cells GLUT5 Small intestine/enterocytes Km Comments 1 mM Basal glucose uptake 15 – 20 mM In the pancreas, plays a role in regulation of insulin In the liver, removes excess glucose from the blood 1 mM Basal glucose uptake 5 mM Amount in muscle plasma membrane increases with endurance training — Primarily a fructose transporter (now classified class II glucose transporter) Most members of class II (GLUT5, GLUT7, GLUT9, GLUT11) and class III (GLUT6, GLUT8, GLUT10, GLUT12, GLUT13) have been identified recently in homology searches of expressed sequence tag (EST) databases with sequence information provided through various genome projects. Berg JM, Tymoczko JL, Stryer L (Biochemistry) 7 Pancreatic hormones maintain glucose homeostasis Wong WKM, Joglekar MV et al (2021) iScience 1. Stimulates glycogen formation 2. Stimulates glucose uptake from blood Fed state Glycogen Glucose Fasted state 1. Stimulates glycogen breakdown 2. Raises blood glucose Actions of insulin and glucagon 1. Liver  glucose uptake, glycolysis and glycogen synthesis  gluconeogenesis, glycogenolysis and ketogenesis (Glucagon  gluconeogenesis, glycogenolysis) 2. Muscle  Glucose & AA uptake (Val, Leu, Ile), protein and glycogen synthesis  protein breakdown 3. Fat  Glucose uptake, fat synthesis  lipolysis (Glucagon  lipolysis and  fat synthesis) 9 Cellular metabolism: converting energy in food to cellular energy Fat Carbohydrates Protein Fatty acids Glucose Amino acids ATP Beta oxidation Glycolysis Anaerobic Oxidative deamination or transamination Acetyl Coenzyme A ATP Krebs cycle aka CAC/TCA cycle Aerobic ATP Electron transport and oxidative phosphorylation All of these processes are important as they allow energy from food to be released gradually. 10 Early fasting (Day 1) state has three stages: 1) Post‐absorptive stage after a meal Glucose, amino acids: transported from intestine to blood Lipids: packed into chylomicrons, transported to blood (via lymphatic sys) 2) Early fasting stage during night Glucagon levels start to  as glucose and insulin ↓. The main target organ for glucagon is the liver. Glucagon stimulates glycogen breakdown and inhibits glycogen synthesis Glucagon also inhibits fatty acid synthesis by diminishing the production of pyruvate Glucagon promotes the release of fatty acids by adipose tissue, Shift in fuel from glucose to fatty acids by muscle and liver 3) Refed stage after breakfast The liver does not initially absorb glucose from blood, but leaves for peripheral tissues. The liver remains in a gluconeogenic mode and the newly synthesized glucose is used to replenish the liver’s glycogen stores. Once replenished, liver processes excess glucose for fatty acid synthesis. 11 Glycolysis and gluconeogenesis 1. Glycolysis is the sequence of reactions that metabolizes one molecule of glucose to two molecules of pyruvate with the concomitant net production of two molecules of ATP. The anaerobic metabolism of glucose (glycolysis) provides such a source of energy for short, intense bouts of exercise. Pyruvate can be further processed anaerobically (fermented) to lactate (lactic acid fermentation) or ethanol (alcoholic fermentation). Under aerobic conditions, pyruvate can be completely oxidized to CO2, generating much more ATP. 2. Gluconeogenesis: the synthesis of glucose from non‐carbohydrate precursors. Brain and RBCs depend on glucose as primary fuel. Daily glucose requirement of whole body is 160g; and for the brain is about 120g. Glucose present in body fluids is about 20g, and that readily available from glycogen is ~190g. Thus, the direct glucose reserves are sufficient to meet glucose needs for about a day. During a longer period of starvation, glucose must be formed from noncarbohydrate sources. 12 Glycolysis and Gluconeogenesis In glycolysis, glucosepyruvate; C₆H₁₂O₆ (glucose) Gluconeogenesis is not a reversal of glycolysis. Most of the decrease in free energy in glycolysis takes place in the three essentially irreversible steps catalyzed by hexokinase, phosphofructokinase, and pyruvate kinase. In gluconeogenesis, Pyruvate  glucose. In gluconeogenesis, the following new steps bypass these virtually irreversible reactions of glycolysis: Phosphoenolpyruvate (PEP) is formed from pyruvate by way of oxaloacetate through the action of pyruvate carboxylase and PEP carboxykinase Fructose 6‐phosphate is formed from fructose 1,6‐bisphosphate by hydrolysis of the phosphate ester at carbon 1. Fructose 1,6‐bisphosphatase catalyzes this exergonic hydrolysis. Glucose is formed by hydrolysis of glucose 6‐phosphate in a reaction catalyzed by glucose 6‐phosphatase. CH₃COCOO⁻ (pyruvate) 13 Prolonged Starvation (1‐3 days): Adaptations to minimize protein degradation The first priority of metabolism in starvation is to provide sufficient glucose to the brain and other tissues (eg RBCs) that are absolutely dependent on this fuel. Precursors of glucose are not abundant. Most energy is stored in the fatty acyl moieties of triacylglycerols. Fatty acids cannot be converted into glucose, because acetyl CoA cannot be transformed into pyruvate The second priority of metabolism in starvation is to preserve protein, which is accomplished by shifting fuel used from glucose to fatty acids and ketone bodies The dominant metabolic processes are the mobilization of triacylglycerols in adipose tissue and gluconeogenesis by the liver Muscle shifts almost entirely from glucose to fatty acids for fuel. β‐oxidation of fatty acids by muscle halts conversion of pyruvate into acetyl CoA (as acetyl CoA stimulates phosphorylation of pyruvate dehydrogenase complex rendering it inactive). Pyruvate, lactate, and alanine are therefore exported to liver for conversion into glucose. Glycerol derived from cleavage of triacylglycerols is another raw material for glucose synthesis in14liver. Days 1‐3 of starvation Glucose glycolysis Alanine ALT (Alanine Aminotransferase) Vitamin B6 (cofactor) PC (Pyruvate Carboxylase) Biotin (cofactor) Oxaloacetate Cytosol Gluconeogenesis LDH (Lactate dehydrogenase) Pyruvate Citric Acid Cycle or TCA cycle Vitamin B3 (cofactor) Lactic acid PDH (Pyruvate dehydrogenase) B1, B2, B3, B5, Lipoic acid (cofactors) Acetyl‐CoA Mitochondria 15 Day 3‐7 starvation AcCoA cannot leave liver cells and must be converted to Ketone Bodies (KBs). KBs are thus “portable” forms of AcCoA that are produced (mainly) in the mitochondria of liver cells when oxaloacetate levels are limiting After about 3 days of starvation, the liver forms large amounts of KBs; acetoacetate and d‐3‐ hydroxybutyrate. Acetone is the third and least abundant KB;  in diabetes (T1D). “Useful” KBs to generate energy Carbohydrates, Amino acids and FA, all contribute to AcCoA. Useful as a biomarker (in Diabetes) but not to generate energy Acetoacetate and Beta‐hydroxybutyrate do not need any transporter to get them out of the mitochondrial/cellular membranes. Ketone bodies are the primary fuels for heart and skeletal muscle as well as by all organs (even brain) in starvation Stryer (5th edition) and Essigmann J, MIT lectures: https://www.youtube.com/watch?v=qmqiF0YJ4LM 16 Fuel Choice During Starvation The plasma levels of fatty acids and ketone bodies increase in starvation, whereas that of glucose decreases. Glucose Time Fuel Driven by. 24h Glycogen Glucagon Epinephrine 1‐3d Hepatic gluconeogenesis Alanine Lactate (for the brain) 3‐7d Free fatty acids Ketone body degradation for the brain 1W+ protein Absence/depletion of lipid stores Usual energy stores last 1‐3 months in adults and death results from loss of function of heart, liver, kidney From figure 30.16 Stryer (Biochemistry) 17 Starvation in humans Examples of starvation on the human body Dutch famine (Nov 1944‐ Apr 1945) Minnesota starvation study (Nov 1944 – Oct 1945) Irish hunger strike (Oct 1980‐ Oct 1981) The siege of Leningrad (Sept 8, 1941 – Jan 27, 1944) The great Chinese famine (1958‐1961) 18 Dutch famine (Nov 1944‐ Apr 1945) A perfectly designed, although tragic, human study on the effects of intrauterine deprivation and subsequent adult health. 19 Dutch famine (Nov 1944‐ Apr 1945) The Dutch Famine Birth Cohort allowed to investigate the effects of in utero undernutrition on health in later life. In utero undernutrition lead to low birth weight, and children of these also had low birth weight epigenetics changes Children of women exposed to famine during late, mid or early pregnancy had different effects: In late gestation: Glucose intolerance In mid gestation: Glucose intolerance Microalbuminuria Obstructive airway disease In early gestation: Glucose intolerance Atherogenic lipid profile Obesity (women) Coronary heart disease Stress sensitivity 20 From T. Roseboom et al. Early Human Development (2006) 82, 485—491 Minnesota starvation experiment (Nov 1944 – Oct 1945) Video: https://www.youtube.com/watch?v=s5mujgdy8qo Ancel Keys and Josef Brozek assessed physiological and psychological effects in 36 healthy volunteers (all men) during normal (baseline) conditions, semistarvation, and then under conditions of restricted, followed by ad libitum rehabilitation. Ancel Keys (1904‐2004) Semi‐starvation Restricted Ad libitum From Keys A et al 1950, p117 Abnormal eating behaviour: food rituals, keeping food in mouth for longer, binge eating disorder Social & Psychological effects: compulsive behaviours, anxiety, depression, nail, bitting, irritability, social withdrawal, loss of sexual drive http://jn.nutrition.org/content/135/6/1347.full.pdf+html https://www.nature.com/articles/s41430‐018‐0138‐6 Foetal origins of adult disease “The fetal origins hypothesis states that fetal undernutrition in middle to late gestation, which leads to disproportionate fetal growth, programmes later coronary heart disease. 22 The Thrifty phenotype hypothesis The Hales‐Barker thrifty phenotype hypothesis identified that early‐life trade‐offs are not reversible, and that poor fetal as well as early post‐natal nutrition imposes mechanisms of nutritional thrift upon the growing individual. 23 The human genome is complex A single human cell is one‐hundredth of a milimeter in diameter Our DNA is 2 meters long Only 2% of our DNA produces all the proteins we need Non‐coding RNAs, which are encoded from the remaining 98% DNA, can regulate expression of our genes Epigenetics (DNA/Histone modifications) Histone – DNA interactions regulate gene expression 1. Text Monomethyl Dimethyl Acetyl Phos H3K4 ? Euchromatic - - H3K9 Heterochromatic Heterochromatic Euchromatic - H3K27 Heterochromatic Eu/Heterochromatic - - H3K36 Euchromatic Euchromatic - - H4K20 Heterochromatic Euchromatic ? - 5-MeC Heterochromatic - - - H3S10 - - - Heterochromatic https://www.cell.com/trends/genetics/comments/S0168‐9525(04)00045‐9 26 Multigenerational undernutrition, programming, obesity and diabetes 27 Summary 1. 2. 3. 4. 5. 6. 7. 8. Starve – fed cycle Duration of starvation and the switch from carb to fat metabolism Glycogen stores are used up in ~ one day During first 24h‐48h, there is increased gluconeogenesis from precursors (amino acids, glycerol) Lipolysis and ketogenesis ensure that fuel needs are met by fatty acids and ketone bodies (brain switches to KB) Protein breakdown begins on prolonged starvation to provide precursors for gluconeogenesis and ultimately, organs fail to function with degradation of structural protein Dutch famine, Minnesota semi‐starvation and examples of human starvation Foetal origins of adult disease (Barker hypothesis) and epigenetic regulation of gene expression 28 Further reading Starvation‐induced metabolic changes https://www.ncbi.nlm.nih.gov/books/NBK22414/ Minnesota semi‐starvation study http://jn.nutrition.org/content/135/6/1347.full.pdf+html http://www.possibility.com/wiki/index.php?title=EffectsOfSemiStarvation Dutch famine https://www.nytimes.com/2018/01/31/science/dutch‐famine‐genes.html Barker hypothesis https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2550226/ https://www.sciencedirect.com/science/article/pii/S0140673686913401?via%3Dihub http://www.oxfordreference.com/view/10.1093/oi/authority.20110803095447459 Would like to hear more on genes and eating choices? Check/read ‐ Is Obesity a Choice? ‐ Giles Yeo (Youtube: https://www.youtube.com/watch?v=88tWJ1p5d4o) and work from S Farooqi/S O'Rahilly 29 Contact Anand Hardikar Associate Professor and Head, Diabetes and Islet Biology JDRF Australia T1D Clinical Research Network Fellow (CDA), 30.2.27 | School of Medicine | Western Sydney University www.isletbiology.info | E‐mail: [email protected] @ AnandHardikar 30 ATP Yield One movement through glycolysis down to the electron transport chain can generate anywhere between 30‐32 ATP molecules. It is important to remember that it is the oxidative phosphorylation step that produces the most amount of ATP, which is only possible due to shuffling NADH and FADH2 through the electron transport chain. https://www.youtube.com/watch?v=LsRQ5_EmxJA 31