Effects of InsulinThe Insulin receptor is composed of two subunits; alpha and beta attached via disulfide bridges- The alpha subunit is extracellular and binds insulin- The beta subunit is a single transmembrane unit with tyrosine kinase activity - Insulin binding will activate the tyrosine kinase domain; un-phosphorylated at rest- Phosphotyrosine residues attract IRS proteins (Insulin Receptor Substrates) that get phosphorylated/activated and activate other proteinsPage 1 of 8BICD 150Sp’14Fortes85/20/141 of 8Insulin binds to a growth factor receptor; activates a variety of downstream events Insulin is a growth factor:- Mitogenic pathway stimulates cell growth and mitosis- Metabolic pathway: Anabolic events and inhibits catabolismThe Ras MAP/ERK kinase pathway is responsible for the mitogenic effects of insulin Metabolic pathway includes the activation of PI3K and other kinases that trigger the exocytosis of GLUT-4 vesicles in skeletal muscle and adipose tissue. Metabolic effects of Insulin:- Allows for an increase in glucose uptake and glycogen synthesis via activation of the enzyme glycogen synthase - Increase in Glycolysis- Increase protein synthesis - Inhibits glycogenolysis - Inhibits gluconeogenesis - Stimulation of the enzyme lipoprotein lipase in endothelial cells of blood vessels; Lipoproteins are a means of transporting lipids in the blood and include chylomicrons and LDLs. This lipase hydrolyzes circulating triglycerides that make up lipoproteins to release free fatty acids (FFA) andglycerol, which can then be taken up by tissues like adipocytes and the liver and used to reform lipids inside of cells . - Increase in lipid synthesis (lipogenesis); Insulin is the hormone “that makes you fat” o An increase in dietary sugar/carbohydrate consumption will trigger an increased release of insulin, which will stimulate lipogenesis and make you fat - Inhibits lipolysis via inhibiting the enzyme hormone sensitive lipase. (Glucagon activates this enzyme as we’ll see) Effects on gene expression:Foxo1 is a very important gene that regulates metabolism that has several effects including formation of VLDLs and Gluconeogenesis. Foxo1 also has inhibitory effects on pancreatic beta cell survival.Insulin stimulates AKT, a protein kinase, which in turn will inhibit the expression of Foxo1 (see figure above)- The inhibition of Foxo1 is the mechanism by which insulin inhibits gluconeogenesis - Insulin increases beta cell survival and proliferation by inhibiting Foxo1Insulin’s action as a modulator of metabolism:Page 2 of 8The enzyme Adenylate Kinase is a monitor of the energy level of cells, which is related to the ratio of ADP to ATP.- When ADP is high and ATP is low cells are in a low energy state - Adenylate kinases increases levels of ATP when its levels drop- Catalyzes the reaction ADP + ADP -> AMP + ATP Adenylate kinase generates large amounts of AMP when ATP is low Another metabolic enzyme known as AMP-Kinase (AMPK) is activated when levels of AMP increaseWhen AMPK is stimulated (low energy levels of cell) has several effects:- Increase production of mitochondria to increase ATP production- Stimulate glycolysis - Stimulate lipid oxidation - Stimulate glucose utilization- Inhibits protein synthesis and growth- Stimulate fatty acid oxidation As you can see, AMPK stimulates Catabolic activity of cells in order to increase the availability of substrates for metabolismAs mentioned earlier, Insulin generally has Anabolic effects. It achieves this by inhibiting activity of AMPKvia AKT just as it inhibits expression of Foxo1Metformin is a drug used to treat Type II diabetes (insulin resistance) - Stimulates AMPK and inhibits gluconeogenesis (a good thing as you generally have too much circulating blood glucose in diabetics w/out insulin) Adipocytes also secrete signaling molecules as they grow. Known as Adipokines: - Leptin: Increases insulin resistance - Resistin: Increases insulin resistance and inhibits AMPK- TNF-a: Inflammatory compound that increases insulin resistance - IL-6: Inflammatory compound that increases insulin resistance- Adiponectin: Decreases insulin resistance and activates AMPKAdipokines and their effects will be important when we begin our discussion of diabetes. Termination of insulin signaling: - Remove insulin from receptor- Tyrosine Phosphatases of the receptor and IRSPage 3 of 8- Protein Phosphatases of downstream kinases Insulin resistance can be caused by increased expression of phosphatases and down-regulation of insulinreceptors Glucagon:A gene codes for pro-glucagon, a large protein containing several peptides including Glucagon and GLP- Recall that GLP are peptides called “Incretins” that increase insulin secretion Glucagon is the hormone “of the fasted state”- Secretion increases in between meals- Catabolic effects: provides cells with substrates needed for metabolism when one is not eating Stimuli for Glucagon secretion:- Decrease in circulating glucose - Increase in Amino Acids - Stimulation by both the sympathetic and parasympathetic Inhibition of glucagon:- Increased circulating glucose; stimulates the Beta and delta cells of the pancreas to secrete insulin and Somatostatin, both of which decrease glucagon secretion. They will also release GABA, an inhibitory NT that prevents the release of Glucagon from the alpha cells. Effects of glucagon are mainly on the liver but also on adipocytes and skeletal muscle Glucagon receptors are Gas coupled - Increase AC and [cAMP] -> Increase PKA - PKA in the liver will phosphorylate and activate Glycogen Phosphorylase Kinase (GPK)- GPK will increase the activity of Glycogen Phosphorylase (GP)- GP is the enzyme that breaks down glycogen into individual Glucose-6 Phosphate molecules The Liver has a phosphatase that will remove the phosphate from glucose, allowing free glucose to be released into the blood stream via GLUT 2 transporters. This is how the liver provides glucose to the rest of the body during fasting periods. Muscle cells do not contain the glucose phosphatase enzyme. When stimulated by glucagon, they will break down glucose from glycogen. They are “selfish” tissues because they cannot share this glucose. They instead use it in glycolysis for their own energy needs. In prolonged periods of fasting there will be an increase in protein breakdown into amino acids for energy. This is also seen in diabetes, when there is no insulin or insulin signaling so glucose cannot enter cellsPage