9. Starve-Feed Cycle.pdf

MODULE 4 9 STARVE/FEED CYCLE 10 DIABETES 1  MODULE 4 9 STARVE/FEED CYCLE 10 DIABETES 2 9 THE STARVE-FEED CYCLE 3  METABOLIC INTERRELATIONSHIPS — CONTENTS — I. METABOLIC INTERRELATIONSHIPS...

MODULE 4 9 STARVE/FEED CYCLE 10 DIABETES 1  MODULE 4 9 STARVE/FEED CYCLE 10 DIABETES 2 9 THE STARVE-FEED CYCLE 3  METABOLIC INTERRELATIONSHIPS — CONTENTS — I. METABOLIC INTERRELATIONSHIPS 1. The concept of malnutrition 2. Metabolic pathways and tissues II. THE STARVE-FEED CYCLE 1. The well-fed state 2. The early fasting state 3. Fasting state 4. Early refed state III. MALNUTRITION IV. ENERGY- AND NUTRIENT SENSORS 1. AMPK 2. mTOR 3. The ULK1 complex 4. The kinase triad as a sensor mTOR and cancer 4  9. STARVE-FEED CYCLE – LEARNING OBJECTIVES – Discuss how liver coordinates metabolic pathways in the well-fed state, early fasting state, and fasting state Explain the mechanism of liver ketogenesis in the fasting state Apply the knowledge on fasting state to starvation, listing the active metabolic pathways to furnish energy to the brain Apply the main concepts of mTOR signaling to chronic over- feeding as in obesity Discuss how AMPK establishes neural control on feeding and which metabolic pathways are activates Explain why inhibition of mTOR can be useful for cancer treatment Discuss the significance of mitophagy in quality control 5 I. METABOLIC INTER-RELATIONSHIPS 6  I. METABOLIC INTER-RELATIONSHIPS Malnutrition Consumption of food at rates greater than the basal caloric requirements leads to the storage of calories in the form of glycogen and triacylycerol. The unlimited capacity to store calories as triacylglycerol results in the commonest form of malnutrition in affluent countries (e.g., USA): obesity. Other forms of malnutrition, such as protein malnutrition and starvation, are prevalent in developing countries. Obesity together with insulin resistance, dyslipidemia, and hypertension constitute the metabolic syndrome and contributes enormously to the high rate of cardiovascular death in Western countries. Obesity Insulin resistance Metabolic Syndrome Cardiovascular Disease Dyslipidemia Hypertension 7  I. METABOLIC INTERRELATIONSHIPS Metabolic Pathways and Tissues The central organ involved in these metabolic interrelationships is the liver and the peripheral tissues are brain, muscle, adipose tissue, kidney, and red blood cells. Brain: glucose is virtually the sole fuel for the human brain, except during prolonged starvation. Muscle: the major fuels are glucose, fatty acids, and ketone bodies. Adipose tissue: synthesis and degradation of triacylglycerols. Adipose tissue needs glucose (from liver, via glycolysis) to synthesize triacylglycerols. Liver: the metabolic activities of the liver are essential for providing fuel to the brain, muscle, adipose tissue, and other peripheral organs. 8 II. THE STARVE-FEED CYCLE 9  II. STARVE-FEED CYCLE Stages of the Starve-Feed Cycle A major concept preceding a detailed analysis of the starve – feed cycle is established by the control of glucose level by the liver: GLUCOSE GLUCOSE (from blood) (released to blood) glycogen glucose-6P glucose glycogen glucose-6P glucose fatty acid used as fuel fatty acid used as fuel synthesis synthesis fatty acid-containing fatty acids VLDL from (to adipose tissue) adipose tissue WELL-FED STATE FASTING STATE (after a meal) (after an overnight fast) 10  II. STARVE-FEED CYCLE 1. Stages of the Starve-Feed Cycle The starve-feed cycle has four stages: 1. The well-fed state in which the diet provides the energy requirements 2. The early fasting state in which hepatic glycogenolysis is the most important source of blood glucose 3. The fasting state in which amino acids are used to synthesize glucose (gluconeogenesis) 4. The early-refed state is characterized by a normal metabolism of fat and re-establishment of a normal glucose metabolism 11  II. STARVE-FEED CYCLE 1. Stages of the Starve-Feed Cycle The starve-feed cycle has four stages: 1. The well-fed state in which the diet provides the energy requirements 2. The early fasting state in which hepatic glycogenolysis is the most important source of blood glucose 3. The fasting state in which amino acids are used to synthesize glucose (gluconeogenesis) 4. The early-refed state is characterized by a normal metabolism of fat and re-establishment of a normal glucose metabolism 12  II. STARVE-FEED CYCLE 1. The well-fed state Glucose, amino acids, and fats obtained from the food are metabolized as follows: Glucose passes from the intestinal epithelial cells to the liver by way of the portal vein. Amino acids are also released into portal blood, but there is a small metabolism in the gut. Fats (triacylglycerols) in the form of chylomicrons, are secreted into lymphatics and by way of the subclavian vein into blood. 13  II. STARVE-FEED CYCLE fats ingested in the diet 8 fatty acids Glycogen are oxidized as fuel or esterified for CO2, H2O a. storage gallbladder gallbladder GLUCOSE GLUCOSE adipocyte GLUCOSE Portal Vein Pyruvate Lactate TG b. small Gut intestine fat 7 fatty acids enterTCA cells 1 bile salts emulsify lipoprotein lipase c. dietary fats in the 6 lipoprotein lipase, activated 1. The well-fed state small intestine, forming by ApoC-II in the capillary mixed micelles converts triacylglycerols to capillary fatty acids and glycerol Glucose 2 intestinal lipases – Liver distributes most of glucose intestinal degrade triacylglycerols mucosa 5 chylomicrons move through the lymphatic e. Lactate to other organs: system to tissues chylomicron d. a.brain 3 fatty acids are takendepends up by the intestinal mucosa and almost 4solely triacylglycerolson glucose are incorporated with cholesterol and apoproteins glycogen converted into triacylglycerols into chylomicrons where it is metabolized to CO2 andLehninger, H2O;2nd Edition b.adipose tissue converts most of glucose into triacylglycerides; c. red blood cells metabolize glucose via glycolysis to lactate. 14  II. STARVE-FEED CYCLE fats ingested in the diet 8 fatty acids Glycogen are oxidized as fuel or esterified for CO2, H2O a. storage gallbladder gallbladder GLUCOSE GLUCOSE adipocyte GLUCOSE Portal Vein Pyruvate Lactate TG b. small Gut intestine fat 7 fatty acids enterTCA cells 1 bile salts emulsify lipoprotein lipase c. 6 lipoprotein lipase, activated d. muscle stores glucose as glycogen dietary fats in the small intestine, forming by ApoC-II in theand capillary mixed micelles converts triacylglycerols to capillary metabolizes 2 intestinal lipases it to lactate via glycolysis. intestinal fatty acids and glycerol degrade triacylglycerols mucosa 5 chylomicrons move through the lymphatic e. Lactate e. lactate, originating from erythrocytes and system to tissues chylomicron d. 3 fattymyocytes acids are taken up by is taken up by4 triacylglycerols liver inare incorporated the well- glycogen the intestinal mucosa and with cholesterol and apoproteins converted into triacylglycerols into chylomicrons fed state, where it is used for the synthesis Lehninger, 2nd Edition of fat. The Cori cycle (the conversion of glucose to lactate in peripheral tissues followed by conversion of lactate back to glucose in liver) is interrupted in the well- fed state. 15  II. STARVE-FEED CYCLE 1. The well-fed state Amino acids from dietary protein are transported to liver by portal blood. (a) A normal concentration of amino acids usually passes through the liver without metabolism and is distributed to peripheral tissues mainly for protein synthesis. (b) When the concentration of amino acids is unusually high (high protein diet), liver retains some amino acids for protein synthesis, metabolism to CO2 and H2O, and urea synthesis. Some intermediates of amino acid metabolism are used for fatty acid synthesis (lipogenesis) in liver. protein fats ingested synthesis in the diet 8 fatty acids are oxidized as fuel or esterified for storage AMINO (a) ACIDS gallbladder AMINO protein AMINO adipocyte ACIDS synthesis ACIDS Portal AMINO (b) ACIDS Vein small fat urea intestine protein Gut 7 fatty acids enter cells synthesis 1 bile salts emulsify protein lipoprotein lipase dietary fats in the synthesis 6 lipoprotein lipase, activated small intestine, forming mixed micelles by ApoC-II in the capillary 16 converts triacylglycerols to capillary fatty acids and glycerol 2 intestinal lipases intestinal  II. STARVE-FEED CYCLE 1. The well-fed state Fats present in the diet are delivered to peripheral tissues via chylomicrons; fats synthesized in liver are released from this organ in the form of very low density lipoproteins (VLDL). Both chylomicrons (from gut) and VLDL (from liver) circulate in blood and are acted upon by lipoprotein lipase. The synthesis of triacylglycerols in adipose fats ingested tissue requires glucose (as a source of glycerol-P). in the diet 8 fatty acids are oxidized glucose as fuel or esterified for Lymphatics storage gallbladder amino adipocyte pyruvate lactate FAT CM acids FAT small intestine Gut 7 fatty acids enter cells VLDL 1 bile salts emulsify lipoprotein lipase dietary fats in the 6 lipoprotein lipase, activated small intestine, forming Chylomicrons VLDL by ApoC-II in the capillary mixed micelles converts triacylglycerols to capillary fatty acids and glycerol 2 intestinal lipases intestinal 5 chylomicrons move degrade triacylglycerols mucosa through the lymphatic system to tissues chylomicron 3 fatty acids are taken up by 4 triacylglycerols are incorporated 17 the intestinal mucosa and with cholesterol and apoproteins converted into triacylglycerols into chylomicrons  II. STARVE-FEED CYCLE 1. Stages of the Starve-Feed Cycle The starve-feed cycle has four stages: 1. The well-fed state in which the diet provides the energy requirements 2. The early fasting state in which hepatic glycogenolysis is the most important source of blood glucose 3. The fasting state in which amino acids are used to synthesize glucose (gluconeogenesis) 4. The early-refed state is characterized by a normal metabolism of fat and re-establishment of a normal glucose metabolism 18  II. STARVE-FEED CYCLE 2. The early fasting state In the early stages of fasting, hepatic glycogenolysis is crucial for maintenance of blood glucose. Lactate, pyruvate, and amino acids are used for the formation of glucose (gluconeogenesis). The Cori cycle and the Alanine cycle become important in maintaining plasma glucose levels and represent important energy sources for muscle and erythrocytes. glucose CO2, H2O glycogen glucose glycogenolysis glucose gluconeogenesis Cori cycle amino Pyruvate Alanine cycle acids lactate alanine lactate alanine 19  II. STARVE-FEED CYCLE 1. Stages of the Starve-Feed Cycle The starve-feed cycle has four stages: 1. The well-fed state in which the diet provides the energy requirements 2. The early fasting state in which hepatic glycogenolysis is the most important source of blood glucose 3. The fasting state in which amino acids are used to synthesize glucose (gluconeogenesis) 4. The early-refed state is characterized by a normal metabolism of fat and re-establishment of a normal glucose metabolism 20  II. STARVE-FEED CYCLE 3. Fasting state Hepatic gluconeogenesis occurs at expense of protein degradation in muscle (glutaminolysis) and triglyceride degradation in adipose tissue. The synthesis of glucose from alanine in liver is closely linked to the urea cycle. Hepatic ketogenesis occurs at expense of the fatty acids released from triglyceride degradation in adipose tissue. protein aa glutamine glutamate pyruvate α-ketoglutarate gluconeogenesisglucose glutaminolysis alanine alanine glutaminolysis urea glucose ketone CO2, H2O glycerol-P glycerol-P bodies lipolysis fatty acids fatty acids acetyl-CoA ketone ketogenesis bodies 21  II. STARVE-FEED CYCLE 3. Fasting state Hepatic ketogenic in the fasting state is determined by the following features: Liver can synthesize ketone bodies but cannot utilize ketone bodies as a source of ketogenesis O energy, for it lacks succinyl-CoA transferase || CH3—C—CH2—COOH (SCOT) required for the activation of acetoacetate. Ketone bodies as secondary energy source O || CH3—C—CH 2—COOH in brain during fasting. During fasting (or succinyl-SCoA succinyl-CoA transferase (SCOT) starvation), the brain utilizes ketone bodies succinate ketolysis as a energy source. Brain has succinyl-CoA O || CH3—C—CH2—CO-SCoA transferase (SCOT) activity. HSCoA thiolase O O || || CH3—C—SCoA CH3—C—SCoA tricarboxylic acid cycle 22  II. STARVE-FEED CYCLE 1. Stages of the Starve-Feed Cycle The starve-feed cycle has four stages: 1. The well-fed state in which the diet provides the energy requirements 2. The early fasting state in which hepatic glycogenolysis is the most important source of blood glucose 3. The fasting state in which amino acids are used to synthesize glucose (gluconeogenesis) 4. The early-refed state is characterized by a normal metabolism of fat and re-establishment of a normal glucose metabolism 23  II. STARVE-FEED CYCLE 3. Early refed state Shortly after fuel is absorbed from the gut, triacylglycerol (in chylomicrons) is metabolized by peripheral tissues (adipose tissue and muscle) as in the well-fed state. The liver, however, remains in the gluconeogenic mode a few hours after feeding; glucose-6-P formed by gluconeogenesis in liver is stored as glycogen instead of being hydrolyzed to free glucose and released into blood. Therefore, fats ingested during the early-refed state, the liver aims at restoring the glycogen stores. in the diet 8 fatty acids are oxidized glucose as fuel or esterifiedglucose for storage gallbladder amino amino gluconeogenesis acids glycogen acids adipocyte lactate fat CM urea protein glucose CO2, H2O small synthesis lactate gut intestine 7 fatty acids enter cells 1 bile salts emulsify chylomicrons lipoprotein lipase dietary fats in the 6 lipoprotein lipase, activated small intestine, forming by ApoC-II in the capillary mixed micelles converts triacylglycerols to capillary fatty acids and glycerol 2 intestinal lipases intestinal 5 chylomicrons move degrade triacylglycerols mucosa through the lymphatic fat glycogen system to tissues chylomicron fat 3 fatty acids are taken up by 4 triacylglycerols are incorporated the intestinal mucosa and with cholesterol and apoproteins 24 converted into triacylglycerols into chylomicrons Lehninger, 2nd Edition  fats ingested in the diet 8 fatty acids are oxidized as fuel orGlycogen storage esterified for CO2, H2O a. gallbladder adipocyte GLUCOSE GLUCOSE GLUCOSE Portal Vein Pyruvate Lactate TG b. small intestine Gut 7 fatty acids enter cells after a meal fat TCA 1 bile salts emulsify dietary fats in the lipoprotein lipase 6 lipoprotein lipase, activated c. small intestine, forming by ApoC-II in the capillary mixed micelles converts triacylglycerols to capillary fatty acids and glycerol 2 intestinal lipases intestinal 5 chylomicrons move degrade triacylglycerols mucosa through the lymphatic system to tissues Lactate chylomicron 3 fatty acids are taken up by 4 triacylglycerols are incorporated d. the intestinal mucosa and with cholesterol and apoproteins glycogen converted into triacylglycerols into chylomicrons Lehninger, 2nd Edition glucose CO2, H2O glycogen glucose glycogenolysis early fasting glucose gluconeogenesis amino acids Pyruvate lactate alanine lactate alanine protein glucose aa gluconeogenesis glutamine alanine glutaminolysis urea glucose fasting ketone CO2, H2O glycerol-P glycerol-P bodies lipolysis ketogenesis fatty acids fatty acids acetyl-CoA ketone bodies 25 III. MALNUTRITION 26  III. MALNUTRITION Fasting and Starvation Hepatic gluconeogenesis occurs at expense of protein degradation in muscle and triglyceride degradation in adipose tissue. The synthesis of glucose from alanine in liver is closely linked to the urea cycle. Hepatic ketogenesis occurs at expense of the fatty acids released from triglyceride degradation in adipose tissue. protein aa glutamine glutamate pyruvate glucose α-ketoglutarate gluconeogenesis alanine alanine glutaminolysis glutaminolysis urea glucose ketone CO2, H2O glycerol-P glycerol-P bodies lipolysis fatty acids fatty acids acetyl-CoA ketone ketogenesis bodies 27  III. MALNUTRITION Starvation The breakdown of fatty acids via b-oxidation to acetyl-CoA is followed by the entrance of the latter into the tricarboxylic acid cycle, where it is oxidized to CO2. This is dependent upon the presence of oxaloacetate, the acceptor of acetyl-CoA, which condenses with acetyl-CoA to form citrate. During starvation, metabolism shifts to provide fuel for the brain, i.e, hepatic glucose metabolism glucose is shifted towards gluconeogenesis making oxaloacetate unavailable to condense with acetyl-CoA. The concentration of acetyl CoA increases and its fate is diverted to the formation of ketone bodies. BALANCED DIET STARVATION / DIABETES glucose gluconeogenesis fatty acyl-CoA fatty acyl-CoA β-oxidation β-oxidation acetyl-CoA acetyl-CoA ketone bodies oxaloacetate citrate || citrate oxaloacetate malate isocitrate malate isocitrate fumarate α-ketoglutarate fumarate α-ketoglutarate succinate succinyl-CoA succinate succinyl-CoA 28  III. MALNUTRITION Starvation Starvation leads to a syndrome known as marasmus, which is not restricted to a particular age group but is common in children under 1 year of age in developing countries. In marasmus, fat is mobilized as an energy source to the liver for ketogenesis. Muscle temporarily provides amino acids (glutaminolysis) to the liver for the synthesis of glucose (gluconeogenesis). Glucose and ketone bodies are metabolized by brain. Ultimately, energy and protein reserves are exhausted, and the child starves to death. Adults can suffer marasmus as a result of diseases that prevent swallowing (cancer of the throat or esophagus) or interfere with access to food (dementia or stroke). ketogenesis TG fatty acids fatty acids ketone bodies ketone bodies glucose CO2 + H2O amino glutaminolysis aminogluconeogenesis Proteins acids acids glucose urea 29 IV. ENERGY- AND NUTRIENT SENSORS 30  Regulation of the Nutrient ⇔ Energy Axis REGULATION OF THE NUTRIENT Û ENERGY Implications for AXIS Insulin Resistance, Obesity, and Implications for Diabetes Insulin Resistance, Obesity, and Diabetes AMPK Kinase Triad mTOR ULK1 Leptin Hormones Adiponectin WAT Stomach Ghrelin 31  IV. ENERGY- AND NUTRIENT SENSORS Overview Cell growth requires an abundance of energy and biosynthetic precursors such as lipids and amino acids. The energy and nutrient status of the cell are monitored and maintained by a kinase triad that entails: AMPK (AMP-activated protein Kinase) mTOR (mammalian Target Of Rapamycin) ULK1 (Unc-51-Like Kinase 1) Energy Insufficiency Energy Production AMPK Nutrient Nutrient Sufficiency Starvation mTOR ULK1 Cell Growth Autophagy ! 32  IV. ENERGY- AND NUTRIENT SENSORS 1 AMPK Energy AMPK = a nutrient and energy sensor that maintains Insufficiency Energy Production energy homeostasis – AMPK monitors cellular energy status by sensing increases in the ratios of AMP/ATP AMPK and ADP/ATP. AMPK regulates energy balance by activating catabolic pathways that generate ATP while conserving ATP by downregulating anabolic pathways. mTOR ULK1 AMPK regulates metabolism and energy balance at the whole-body level via effects on the hypothalamus. 33  IV. ENERGY- AND NUTRIENT SENSORS 1 AMPK AMPK occurs as a complex comprising a catalytic a-subunit and regulatory b- and g-subunits. AMP (allosteric regulator) binds to the g- subunit that acts as an energy sensor. Binding of AMP promotes phospho- Ca2+ catalytic subunit β and phosphorylation site α regulatory subunits LKB1 CaMKKβ γ allosteric site P binding of AMP β β α α rylation of the a-subunit by upstream γ γ kinases: liver kinase B1 (LKB1) (thus AMP introducing a link between AMPK and cancer) and Ca/calmodulin-dependent kinase kinases (CaMKKs), especially protein CaMKKb. phosphatase PP2C, PP2A 34  IV. ENERGY- AND NUTRIENT SENSORS 1 AMPK Some of the metabolic effects that ensue following AMPK activation are particularly relevant to treatment of type 2 diabetes: Agonists → Ca2+ → CaMKK ANABOLISM Catabolic pathways are activated by AMPK: glucose uptake via GLUT4 and LKB1 AMPK GLUT1, glycolysis, fatty acid uptake, Metabolic → AMP CATABOLISM stress fatty acid oxidation, mitochondrial bio- inhibition of anabolism activation of catabolism genesis, and autophagy. fatty acid synthesis glucose uptake Agonists Anabolic Capathways 2+ CaMKKare inhibited by mTOR activation anabolism protein synthesis glycolysis AMPK: fatty acid, triglyceride, LKB1 AMPK chole- sterol, glycogen, protein, and rRNA. AMPK catabolism Metabolic synthesis; stress transcription AMP of lipogenic cholesterol synthesis fatty acid oxidation enzymes, transcription of gluconeogenic gluconeogenesis mitochondrial enzymes. biogenesis 35 Factors determining the energy balance and storage hunger saciety signal Hunger signal signal Satiety signal The neural control of caloric intake ­Appetite, Feeding ¯Appetite, Feeding to balance energy expenditure is controlled by two hormones, NPY/AgRP released by stomach (ghrelin) and neurons POMC adipose tissue (leptin). Ghrelin acts neurons ghrelin receptor on Agouti-Related Protein-express- leptin receptor ing neurons that generate the neuro- peptide Y (NPY/AgRP neurons). Ghrelin Leptin Ghrelin Leptin Leptin acts on Pro-Opio-Melano Cortin-expressing neurons (POMC neurons). AgRP neurons induce adipose stomach tissue feeding whereas POMC neurons stomach adipose inhibit feeding. stomach adipose tissue tissue 36  IV. ENERGY- AND NUTRIENT SENSORS 1 AMPK AMPK-regulated control of feeding behavior The primary appetite control center is in hypothalamus NPY/AgRP (arcuate nucleus), in which neuropeptide Y and neurons Agouti-Related Protein-expressing neurons induce feeding, whereas Pro-OpioMelanoCortin-expressing neurons (POMC neurons) inhibit feeding. The AgRP system contains a receptor for the Presynaptic neurons hormone ghrelin, released by the stomach. The POMC Ghrelin neurons contain a receptor for the hormone leptin, released by the adipose tissue. Ghrelin Leptin Ghrelin receptor POMC neuron Leptin receptor Leptin hypothalamus stomach adipose tissue 37  IV. ENERGY- AND NUTRIENT SENSORS 1 AMPK ↑!Appetite FEEDING AMPK-regulated control of feeding behavior – In the fasting state, ghrelin, a 'hunger signal', activates AMPK in the presynaptic neurons acting upstream of NPY/AgRP NPY/AgRP neurons via the Ca2+/calmodulin-activated protein kinase-b (CaMKKb). This causes release of Ca2+ from intracellular stores and the continuous release of neurotransmitter onto the NPY/AgRP neuron. The NPYAgRP neurons promote Ca2+ ghrelin AMPK receptor feeding (and inhibit the POMC neurons, which inhibit ghrelin CaMKKβ feeding). Ghrelin hypothalamus stomach stomach 38  IV. ENERGY- AND NUTRIENT SENSORS 1 AMPK ↑! Appetite FEEDING AMPK-regulated control of feeding behavior – The POMC neurons are stimulated by the 'satiety signal', NPY/AgRP leptin, derived from adipose tissue. Binding of leptin to the leptin receptor in POMC neurons promotes the release of opioids that inhibit AMPK in the presynaptic neurons upstream of the NPY/AgRP neurons, switching them back AMPK an inactive state. Ghrelin Leptin opiod receptor ↑! opiod leptin receptor hypothalamus adipose adipose tissue leptin stomach tissue 39  IV. ENERGY- AND NUTRIENT SENSORS 2 mTOR mTOR links nutrient abundance with growth and the accumulation of energy stores in anticipation of future nutrient shortage. When nutrients are available, mTOR is activated and AMPK drives anabolism as well as energy storage and consumption. Conversely, during fasting, mTOR mTOR ULK1 Nutrient must be suppressed to avoid the insurgence of Sufficiency conflicting metabolic signals. Chronic over- Cell Growth feeding can lead to an excess of mTOR activa- tion and metabolic derangements (as observed in obesity). 40  IV. ENERGY- AND NUTRIENT SENSORS 2 mTOR mTOR (mammalian Target Of Rapamycin) is a kinase consisting of two distinct protein complexes: mTOR complex 1 (mTOR C1) contains the protein raptor; this complex is sensitive to rapamycin. mTOR C1 promotes cell growth and proliferation by stimulating nutrient uptake and metabolism and integrating inputs from various sources, such as growth factors and nutrients (amino acids): both help activate mTOR and promote cell AMPK growth. mTOR complex 2 (mTOR C2) contains the mTOR ULK1 protein rictor and is insensitive to rapa- Nutrient Sufficiency mycin. mTOR C2 helps activate Akt (insulin signaling) and it regulates the Cell Growth raptor rictor actin cytoskeleton via Rho family mTOR mTOR GTPases. Akt (insulin signaling) activates mTOR C1 mTOR C2 ________________ ____________ mTOR C1 indirectly by phosphorylating Cellular growth Cell survival other intermediates. Proliferation mRNA translation 41  IV. ENERGY- AND NUTRIENT SENSORS 2 mTOR Physiological activation of mTOR mTOR links nutrient abundance with growth and the accumulation of energy stores in anticipation of future nutrient shortage. The ultimate effects of mTOR are to promote mRNA translation and to inhibit autophagy: this is carried out by integrating nutrient signals generated by (a) growth factors (such as insulin and insulin growth factor (IGF)), (b) amino acids, and (c) energy signals (through AMPK) NUTRIENT GROWTH ABUNDANCE ACCUMULATION ENERGY STORES mTOR AMPK Growth Amino Energy Factors Acids Signals (a) (b) (c) mTOR ULK1 Nutrient Sufficiency Cell Growth 42  IV. ENERGY- AND NUTRIENT SENSORS 2 mTOR Physiological activation of mTOR glucose insulin IR IRS1 PI3K mTOR2 amino Rag acids GDP Akt Rag GTP mTOR1 AUTOPHAGY AUTOPHAGY GLUCONEOGENESIS ADIPOGENESIS AMPK LIPOGENESIS GLYCOGENESIS mTOR ULK1 Nutrient Sufficiency Cell Growth 43  insulin mTOR AND CANCER IR mTOR is ubiquitously expressed within cells and a validated target in the treatment of cancer insulin insulin IR IRS1 rictor IR PI3K mTOR2 Rag GDP Akt IRS1 rictor PI3K mTOR2 amino Rag IRS1 Rag rictor raptor acids GDP mTOR1 PI3K Akt mTOR2 GTP amino Rag am ac ino in s acids id ul s mTOR integrates signals from GDP Rag raptor Akt in mTOR1 mTOR2 GTP mTOR1 I R growth factors cell andgrowth nutrients to cell survival Ra proliferation Rag raptorGmTOR1 g mTOR2 promote cell mRNA growth, proliferation, GTP translation DP growth cell mTOR1 (–) mTOR1/2 cell inhibitors survival Ra g proliferation and survival. GT mRNA translation P IR mTOR1 S1 mTOR2 mTOR signaling is upregulated in cell growth ra cell survival pt proliferation o PI benign and malignant neoplastic m r mRNA translation TO 3K AMPK R 1 disorders c m m pr ell TO Ak (–) RN oRapalogs l gr R A ifer ow 1 t m T ric ULK1 Drugs targeting mTOR activity are tra ati th (Rapamycin Nutrient mTORO R2 to r ns on lat Sufficiency anticipated to be useful for the analogs) io n Cell Growth treatment of different cancers ce mT ll O su R rv 2 44 iv al mTOR AND CANCER A derivative of rapamycin (temsirolimus), administered as single agent, is associated with substantial improvements in patients with advanced renal-cell carcinoma. It is also approved for use in mantle-cell lymphoma, where it has shown a notable improvement in progression-free survival Rapamycin, Rapamune (1999) Prevention of sirolimus allograft rejection ________________________________________________ Cypher rapamycin- Anti-restenosis Wyeth-Ayerst Eluting stent (2002 Europe / 2003 USA) CCI-779, Torisel (2007) Advanced kidney Temsirolimus cancer ________________________________________________ Torisel (2008) Mantle cell lymphoma Wyeth-Ayerst/ Pfizer 45  IV. ENERGY- AND NUTRIENT SENSORS 3 ULK1 AMPK The ULK1 complex senses nutrient signals for autophagy activation. Autophagy is mTOR ULK1 active under conditions of energy and Nutrient nutrient deprivation; hence, it plays a central Starvation role in starvation. It functions to degrade and Autophagy recycle damaged and redundant organelles and macromolecules in order to provide a source of building blocks and energy to allow cell survival under stress conditions. Autophagy is regulated by the ULK-1 ULK1 ULK1 The ULK1 Complex AT G1 FIP200 complex 3 complex, which consists of the ULK1 protein, Atg13 (AuTophaGy related gene 13), and focal adhesion kinase interacting protein of 200 kD (FIP200). ULK1 is activated upon phosphorylation by AMPK. 46  AUTOPHAGY Autophagy is triggered by ULK1 complex and includes several other factors, such as PI3K class III, Beclin 1, and importantly, LC3-II, which anchors to the membrane and interacts with the cargo (orga- nelles, cytosolic proteins, etc) through receptors LC3-II LC3-II that recognize the cargo. receptors receptors Once the autophagosome is formed, it fuses with lysosomes, whose proteolytic enzymes degrade ➁ ➁ ➂ ➂ phagophore phagophore nucleation expansion the cargo. nucleation expansion AMPK ULK1 ULK1 PI3K PI3K class class III III Beclin Beclin 11 ① ① initiation autophagosome autophagosome initiation mTOR ➃ ➃ fusion fusion ➅ ➅ release release autolysosome lysosome lysosome clearance autolysosome clearance ➄➄ degradation degradation 47  AUTOPHAGY AND MITOPHAGY MITOPHAGY IS A QUALITY CONTROL MECHANISM Non-selective autophagy degrades Mitophagy selectively cytosolic proteins and organelles degrades mitochondria Pathology Parkinson’s disease, Isolation Alzheimer’s disease, and/or ALS membrane Retinopathy Pulmonary hypertension Pink1 Cardiomyocyte senescence Fatty liver disease HEALTHY Cytosolic Damaged Parkin Mitochondrial damage proteins and Mitochondrion organelles Removal of damaged mitochondria PARKINSON’S DISEASE Autophagosome Mitochondrial damage Removal of damaged Ubiquitin mitochondria Lysosome Lysosome 48  IV. ENERGY- AND NUTRIENT SENSORS The kinase triad that senses energy and nutrients Energy Insufficiency As a kinase triad, AMPK, mTOR, and ULK1 control Energy Production energy and nutrient homeostasis through feedback mechanisms. AMPK is activated when the cellular and organismal AMPK energy levels decrease, resulting in Nutrient Nutrient Sufficiency Starvation stimulation of catabolism and inhi- mTOR ULK1 bition of anabolism for energy Cell Growth Autophagy production. Nutrient sufficiency (amino acids) ! _________________________________________ leads to the activation of mTOR; Signal Effector Outcome these nutrients are used for cell _______________________________________________________________________ growth. Energy insufficiency AMPK Energy production During nutrient starvation, the cell Nutrient sufficiency mTOR Cell growth degrades macromolecules and orga- Nutrient insufficiency ULK1 Autophagy _______________________________________________________________________ nelles (autophagy) to yield energy; autophagy is regulated by the ULK1 complex. 49  ACTIVATION OF AUTOPHAGY – PHARMACOLOGY Autophagy is triggered by ULK1 complex and includes several factors, such as PI3K class III and Beclin 1. Drugs that inhibit mTOR or activate AMPK result in the activation of autophagy. LC3-II LC3-II receptors receptors Metformin ➁ ➁ ➂ ➂ phagophore phagophore nucleation nucleation expansion expansion AMPK ULK1 ULK1 PI3K PI3K class class III III Beclin Beclin 11 ① ① initiation autophagosome autophagosome initiation mTOR ➃ ➃ fusion fusion Promote Rapamycin formation of Sirolamus ➅ ➅ autophagosome Temsirolimus release release autolysosome lysosome lysosome clearance autolysosome clearance ➄➄ degradation degradation 50