Premed · Premed · Biochemistry
Lecture 24: Integration and Hormonal Regulation of Metabolism
Biochemistry
Learning Objectives
By the end of this lecture, students will be able to:
- Describe the metabolic profiles of major organs (liver, muscle, adipose, brain, kidney) in fed and fasted states
- Explain the roles of insulin, glucagon, epinephrine, and cortisol in regulating metabolism
- Describe the signaling mechanisms of insulin and glucagon at the molecular level
- Explain the metabolic transitions between the fed state, early fasting, prolonged fasting, and starvation
- Integrate carbohydrate, lipid, and amino acid metabolism into a coherent whole-body framework
- Discuss metabolic dysregulation in diabetes mellitus and metabolic syndrome
Lecture Content
I. Organ-Specific Metabolic Profiles
A. Liver — The Metabolic Hub
The liver occupies a unique position as the body's central metabolic coordinator. In the fed state, the liver performs glycogenesis, glycolysis, fatty acid synthesis, lipogenesis, VLDL secretion, amino acid catabolism, and urea synthesis. In the fasted state, it switches to glycogenolysis, gluconeogenesis, beta-oxidation, and ketogenesis. The liver can use most fuels except ketone bodies, because it lacks succinyl-CoA:acetoacetate-CoA transferase (thiophorase). It is the only organ capable of exporting glucose into the blood (thanks to glucose-6-phosphatase) and exports ketone bodies for other tissues. The liver also synthesizes plasma proteins, bile acids, and cholesterol.
B. Skeletal Muscle
Skeletal muscle is the largest tissue mass in the body and the major consumer of metabolic fuel. At rest, it primarily oxidizes fatty acids via beta-oxidation. During active exercise, it initially draws on muscle glycogen and blood glucose; with prolonged exercise, it shifts progressively to fatty acids and eventually ketone bodies. During fasting, muscle uses fatty acids and ketone bodies, and proteolysis releases amino acids (primarily alanine and glutamine) that travel to the liver for gluconeogenesis. Muscle possesses hexokinase (not glucokinase), which traps glucose efficiently at low concentrations, but lacks glucose-6-phosphatase and therefore cannot export glucose into the blood. Creatine phosphate provides a rapid ATP regeneration system for short bursts of activity.
C. Adipose Tissue
In the fed state, adipose tissue takes up glucose via GLUT4 (insulin-dependent), takes up fatty acids from chylomicrons and VLDL via lipoprotein lipase (LPL), and esterifies fatty acids to TAG for storage. In the fasted state, hormone-sensitive lipase (HSL) is activated by glucagon and epinephrine, driving lipolysis and releasing free fatty acids and glycerol. Adipose tissue is highly insulin-sensitive: insulin promotes glucose uptake, LPL expression, and lipogenesis while inhibiting HSL.
D. Brain
The brain accounts for roughly 20% of basal oxygen consumption despite representing only about 2% of body weight. Under normal conditions, glucose is the brain's primary fuel, requiring approximately 120 g per day. Glucose enters via GLUT1 and GLUT3 transporters, which are insulin-independent, ensuring a constant supply regardless of insulin status. The brain cannot use fatty acids because they do not cross the blood-brain barrier efficiently. During prolonged fasting and starvation, however, the brain adapts to use ketone bodies for up to 60 to 70% of its energy needs. This adaptation is critically important because it spares muscle protein by reducing the demand for gluconeogenesis. The brain cannot store significant glycogen reserves.
E. Kidney
The kidneys have a high metabolic rate and use fatty acids and glucose as primary fuels. During prolonged fasting, the kidney becomes a major site of gluconeogenesis, contributing up to 40% of glucose production alongside the liver. The kidneys excrete urea and uric acid and produce NH4+ from glutamine via glutaminase for acid-base balance. In metabolic acidosis, increased glutamine catabolism generates NH4+ to buffer hydrogen ion excretion.
II. Hormonal Regulation of Metabolism
A. Insulin — The Hormone of the Fed State
Insulin is secreted by pancreatic beta-cells in response to elevated blood glucose (and also amino acids and GLP-1). It promotes anabolic pathways: glucose uptake via GLUT4 translocation in muscle and adipose, glycogenesis through activation of glycogen synthase via PP1, glycolysis through transcriptional induction of glucokinase, PFK-1, and pyruvate kinase, fatty acid synthesis through dephosphorylation and activation of ACC and transcriptional induction of FAS, ACC, and ATP-citrate lyase via SREBP-1c, protein synthesis through mTOR activation, and amino acid uptake. Insulin simultaneously inhibits catabolic pathways: glycogenolysis (by inactivating glycogen phosphorylase), gluconeogenesis (by repressing PEPCK, FBPase-1, and G6Pase gene expression), lipolysis (by activating phosphodiesterase to degrade cAMP, thereby inactivating PKA and HSL), ketogenesis, and proteolysis.
Insulin Signaling Pathway: Insulin binds the insulin receptor, a receptor tyrosine kinase consisting of an alpha2-beta2 tetramer. Binding triggers autophosphorylation of beta-subunit tyrosine residues, which recruits and phosphorylates IRS (insulin receptor substrate) proteins. IRS activates PI3K (phosphoinositide 3-kinase), which converts PIP2 to PIP3. PIP3 recruits Akt/PKB (protein kinase B), which is activated by PDK1. Akt mediates most of insulin's metabolic effects: GLUT4 vesicle translocation to the plasma membrane, glycogen synthase activation (by inactivating GSK3), mTOR activation (promoting protein synthesis), and FOXO inhibition (suppressing gluconeogenic gene expression). Insulin signaling also activates the Ras-MAPK pathway for cell growth and proliferation.
B. Glucagon — The Hormone of the Fasted State
Glucagon is secreted by pancreatic alpha-cells in response to low blood glucose (and also amino acids, especially arginine and alanine). It acts primarily on the liver -- importantly, muscle does NOT respond to glucagon because muscle cells lack glucagon receptors. Glucagon promotes catabolic pathways: glycogenolysis (by activating glycogen phosphorylase via the cAMP/PKA cascade), gluconeogenesis (by inducing PEPCK, FBPase-1, and G6Pase, and by inactivating PFK-2 to decrease F-2,6-BP and reduce PFK-1 activity), fatty acid oxidation (by inactivating ACC, decreasing malonyl-CoA, and thereby relieving CPT-I inhibition), and ketogenesis. Glucagon inhibits anabolic pathways: glycogenesis (by inactivating glycogen synthase via PKA), glycolysis, and fatty acid synthesis.
Glucagon Signaling Pathway: Glucagon binds a Gs-protein-coupled receptor (GPCR) on hepatocytes, activating adenylyl cyclase and increasing cAMP levels. cAMP activates PKA (protein kinase A), which phosphorylates key target proteins: phosphorylase kinase (activating glycogen phosphorylase and thereby glycogenolysis), glycogen synthase (inactivating it and inhibiting glycogenesis), PFK-2/FBPase-2 (shifting the bifunctional enzyme toward FBPase-2 activity, decreasing F-2,6-BP, and thereby decreasing glycolysis while increasing gluconeogenesis), and CREB (a transcription factor that induces PEPCK and G6Pase for gluconeogenesis). PKA also phosphorylates and inactivates ACC, promoting fatty acid oxidation.
C. Epinephrine — The Fight-or-Flight Hormone
Epinephrine is secreted by the adrenal medulla in response to stress, exercise, and hypoglycemia. It acts on liver, muscle, and adipose tissue. In the liver, its effects parallel those of glucagon: beta-2 adrenergic signaling activates the cAMP/PKA pathway, promoting glycogenolysis and gluconeogenesis. In muscle, beta-2 adrenergic signaling drives glycogenolysis via cAMP/PKA, and alpha-1 adrenergic signaling activates the IP3/DAG pathway to release Ca2+ and activate phosphorylase kinase. In adipose tissue, beta-adrenergic signaling through cAMP/PKA activates HSL, driving lipolysis. Notably, muscle glycogen breakdown produces glucose-6-phosphate for glycolysis rather than blood glucose, because muscle lacks glucose-6-phosphatase.
D. Cortisol — The Stress Hormone
Cortisol is a glucocorticoid from the adrenal cortex that acts over hours to days through transcriptional regulation. It promotes gluconeogenesis (by inducing PEPCK and G6Pase), proteolysis in muscle (providing amino acid substrates for gluconeogenesis), and lipolysis in adipose tissue (permissive for catecholamine action). It inhibits glucose uptake in peripheral tissues (contributing to insulin resistance) and protein synthesis in muscle. Chronic cortisol excess causes Cushing syndrome, characterized by hyperglycemia, central obesity, muscle wasting, and osteoporosis.
<image>A comprehensive diagram comparing insulin and glucagon signaling in the hepatocyte. Left panel (Fed state / Insulin): Insulin binds the receptor tyrosine kinase, triggering IRS phosphorylation, PI3K activation, PIP3 generation, and Akt activation. Downstream effects are shown: GLUT4 translocation, glycogen synthase activation (via GSK3 inhibition), SREBP-1c activation (lipogenesis), mTOR activation (protein synthesis), FOXO inhibition (suppresses gluconeogenesis). Right panel (Fasted state / Glucagon): Glucagon binds a GPCR, activating Gs, adenylyl cyclase, and cAMP production. PKA is activated and phosphorylates: phosphorylase kinase (activates glycogenolysis), glycogen synthase (inactivates), PFK-2/FBPase-2 (shifts to FBPase-2 activity, lowering F2,6-BP), ACC (inactivates, promoting fat oxidation), CREB (induces PEPCK and G6Pase for gluconeogenesis). A central bar highlights the insulin:glucagon ratio as the master metabolic switch.</image>
III. Metabolic Transitions: Fed to Fasted to Starvation
A. Fed State (Absorptive; 0-4 hours after a meal)
The insulin:glucagon ratio is high. Blood glucose is elevated and insulin secretion is robust. The liver takes up glucose via glucokinase, stores glycogen, converts excess carbon to fatty acids via lipogenesis, and packages fat as VLDL. Muscle takes up glucose via GLUT4, stores glycogen, uses glucose for energy, and takes up amino acids for protein synthesis. Adipose tissue takes up glucose via GLUT4 and synthesizes TAG from fatty acids delivered by LPL action on VLDL and chylomicrons. The brain uses glucose.
B. Early Fasting / Post-absorptive State (4-12 hours)
The insulin:glucagon ratio declines as glucagon rises. Liver glycogenolysis maintains blood glucose, and hepatic gluconeogenesis begins from lactate, alanine, and glycerol. Adipose lipolysis begins, releasing fatty acids. Muscle starts to shift toward fatty acid oxidation.
C. Fasting (12-72 hours)
The insulin:glucagon ratio is low. Liver glycogen is depleted by approximately 24 hours. Gluconeogenesis becomes the primary source of blood glucose, performed by both liver and kidney, using amino acids (especially alanine from muscle), glycerol (from lipolysis), and lactate (from the Cori cycle) as substrates. Fatty acid oxidation is the primary energy source for most tissues. Ketogenesis in the liver increases as acetyl-CoA from beta-oxidation exceeds TCA cycle capacity because OAA is being diverted to gluconeogenesis. Blood ketone body levels rise, and ketones are used by muscle, heart, and increasingly by the brain.
D. Prolonged Starvation (> 3 days to weeks)
The critical metabolic adaptation of prolonged starvation is that the brain adapts to use ketone bodies for up to 60 to 70% of its energy needs. This dramatically reduces the need for gluconeogenesis from amino acids and thereby spares muscle protein, slowing the rate of proteolysis. The kidneys become a significant site of gluconeogenesis. Metabolic rate decreases due to reduced T3 thyroid hormone levels. Blood glucose is maintained at approximately 60 to 65 mg/dL -- lower than normal but sufficient. Fat stores provide roughly 85% of total energy. Death from starvation usually occurs when fat stores are exhausted and protein catabolism accelerates.
<image>A timeline diagram showing the metabolic fuel transitions from the fed state through starvation. The x-axis shows time (0 hours to weeks). Stacked area curves show the relative contribution of each fuel source to total body energy: glucose (from glycogen, then from gluconeogenesis), fatty acids, ketone bodies, and amino acids. Key phases are marked: Fed (0-4 h), Post-absorptive (4-12 h), Early fasting (12-24 h), Fasting (1-3 days), and Prolonged starvation (>3 days). Annotations show: glycogen depletion by ~24 h, peak gluconeogenesis, rising ketone bodies, brain adaptation to ketones, and protein-sparing effect. Hormone levels (insulin declining, glucagon rising, cortisol rising) are shown as line graphs above the main diagram.</image>
IV. The Insulin:Glucagon Ratio as a Metabolic Switch
The insulin:glucagon ratio determines the overall metabolic direction more powerfully than either hormone alone. A high ratio (approximately 10:1, in the fed state) drives anabolic processes -- fuel storage and biosynthesis. A low ratio (approximately 0.5:1, in the fasted state) drives catabolic processes -- fuel mobilization and oxidation. This ratio controls key regulatory enzymes through phosphorylation and dephosphorylation cascades governing glycogen metabolism (synthase versus phosphorylase), fat metabolism (ACC and HSL), glycolysis versus gluconeogenesis (PFK-2/FBPase-2), and transcriptional effects on glucokinase, PEPCK, G6Pase, SREBP-1c, and FOXO.
V. Clinical Correlations
A. Type 1 Diabetes Mellitus
Type 1 diabetes results from autoimmune destruction of pancreatic beta-cells, leading to absolute insulin deficiency. Metabolically, the body behaves as though it is in a fasted or starved state even after eating. Uncontrolled gluconeogenesis and glycogenolysis cause hyperglycemia. Uncontrolled lipolysis elevates free fatty acids, driving uncontrolled ketogenesis and diabetic ketoacidosis (DKA). Proteolysis leads to muscle wasting and weight loss. DKA is characterized by metabolic acidosis, ketonemia, ketonuria, Kussmaul breathing, fruity breath from acetone, and dehydration. Treatment requires exogenous insulin.
B. Type 2 Diabetes Mellitus
Type 2 diabetes involves insulin resistance (target tissues fail to respond adequately) combined with a relative insulin deficiency as beta-cell function deteriorates over time. Hyperglycemia occurs despite hyperinsulinemia in early stages. The disease is strongly associated with obesity and metabolic syndrome (central obesity, hypertension, dyslipidemia, and hyperglycemia). Patients are less prone to DKA because residual insulin suppresses ketogenesis, but they can develop hyperosmolar hyperglycemic state (HHS). Treatment includes lifestyle modification, metformin (which activates AMPK and reduces hepatic glucose output), sulfonylureas, GLP-1 agonists, SGLT2 inhibitors, and insulin when needed.
C. Metabolic Syndrome
Metabolic syndrome is a cluster of risk factors including central obesity, insulin resistance, hyperglycemia, dyslipidemia (high TAG, low HDL), and hypertension. It greatly increases cardiovascular disease risk. The underlying pathophysiology involves excess adipose tissue, especially visceral fat, which releases increased free fatty acids, inflammatory cytokines (TNF-alpha, IL-6), and dysregulated adipokines, all of which impair insulin signaling.
<image>A whole-body integration diagram showing inter-organ metabolic communication in the fasting state. The liver is central, with arrows showing: glucose export to brain and RBCs, ketone body export to brain and muscle, urea export to kidney. Adipose tissue releases free fatty acids and glycerol to liver. Muscle releases alanine and glutamine to liver (glucose-alanine cycle) and lactate to liver (Cori cycle). The kidney is shown performing gluconeogenesis and excreting urea, uric acid, and NH4+. The brain is shown using glucose and ketone bodies. Hormonal signals are overlaid: glucagon acting on liver, epinephrine acting on liver, muscle, and adipose, cortisol acting on muscle and liver. Blood glucose level is shown being maintained by the combined hepatic and renal gluconeogenesis. Key metabolic pathways active in each organ are listed in boxes.</image>
