Insulin Secretion
The major function of insulin is to counter the concerted action of a number of hyperglycemia-generating hormones and to maintain low blood glucose levels. Because there are numerous hyperglycemic hormones, untreated disorders associated with insulin generally lead to severe hyperglycemia and shortened life span.
In addition to its role in regulating glucose metabolism, insulin stimulates lipogenesis, diminishes lipolysis, and increases amino acid transport into cells. Insulin also modulates transcription, altering the cell content of numerous mRNAs. It stimulates growth, DNA synthesis, and cell replication, effects that it holds in common with the insulin-like growth factors (IGFs) and relaxin.
Insulin is synthesized as a preprohormone in the β-cells of the islets of Langerhans. Its signal peptide is removed in the cisternae of the endoplasmic reticulum and it is packaged into secretory vesicles in the Golgi, folded to its native structure, and locked in this conformation by the formation of 2 disulfide bonds. Specific protease activity cleaves the center third of the molecule, which dissociates as C peptide, leaving the amino terminal B peptidedisulfide bonded to the carboxy terminal A peptide.
Insulin secretion from β-cells is principally regulated by plasma glucose levels. Increased uptake of glucose by pancreatic β-cells leads to a concomitant increase in metabolism. The increase in metabolism leads to an elevation in the ATP/ADP ratio. This in turn leads to the inhibition of an ATP-sensitive potassium channel (KATP channel). The net result is a depolarization of the cell leading to Ca2+ influx and insulin secretion.
The KATP channel is a complex of 8 polypeptides comprising four copies of the protein encoded by the ABCC8 (ATP-binding cassette, sub-family C, member 8) gene and four copies of the protein encoded by the KCNJ11 (potassium inwardly-rectifying channel, subfamily J, member 11) gene. The ABCC8 encoded protein is also known as the sulfonylurea receptor (SUR). The KCNJ11 encoded protein forms the core of the KATP channel and is called Kir6.2. As might be expected, the role of KATP channels in insulin secretion presents a viable therapeutic target for treating hyperglycemia due to insulin insufficiency as is typical in type 2 diabetes. Chronic increases in numerous other hormones, such as growth hormone, placental lactogen, estrogens, and progestins, up-regulate insulin secretion, probably by increasing the preproinsulin mRNA and enzymes involved in processing the increased preprohormone.
Actions of Insulin
Insulin, secreted by the β-cells of the pancreas, is directly infused via the portal vein to the liver, where it exerts profound metabolic effects. These effects are the response of the activation of the insulin receptor which belongs to the class of cell surface receptors that exhibit intrinsic tyrosine kinase activity (see Signal Transduction). The insulin receptor is a heterotetramer of 2 extracellular α-subunits disulfide bonded to 2 transmembrane β-subunits. With respect to hepatic glucose homeostasis, the effects of insulin receptor activation are specific phosphorylation events that lead to an increase in the storage of glucose with a concomitant decrease in hepatic glucose release to the circulation as diagrammed below (only those responses at the level of glycogen synthase and glycogen phosphorylase are represented). Actions of insulin-insulin receptor interactions at the level of insulin receptor substrate-1 (IRS1) and activation of the kinase cascade leading to altered activities of glycogen phosphorylase and glycogen synthase. PI3K = phosphatidylinositol-3-kinase; PIP2 = phosphatidylinositol-4,5-bisphosphate; PIP3 = phosphatidylinositol-3,4,5-bisphosphate; PDK1 = PIP3-dependent protein kinase; Tsc1 and Tsc2 = Tuberous sclerosis tumor suppressors 1 (hamartin) and 2 (tuberin); Rheb = Ras homolog enriched in brain; mTOR = mammalian target of rapamycin. PKB/Akt = protein kinase B/Akt2; GSK3 = glycogen synthase kinase-3; S6K = 70kDa ribosomal protein S6 kinase, also called p70S6K. The insulin-mediated activation of mTOR also leads to changes in protein synthesis (see below).
Insulin-insulin receptor actions on glycogen homeostasis showing the role of protein targeting glycogen (PTG) in complexing many of the enzymes and substrates together. PTG is a subunit of PP1. Also diagrammed is the response to insulin at the level of glucose transport into cells via GLUT4 translocation to the plasma membrane. GS/GP kinase = glycogen synthase: gycogen phosphorylase kinase. PP1 = protein phosphatase-1. Arrows denote either direction of flow or positive effects, T lines represent inhibitory effects.
In most nonhepatic tissues, insulin increases glucose uptake by increasing the number of plasma membrane glucose transporters: GLUTs. Glucose transporters are in a continuous state of turnover. Increases in the plasma membrane content of GLUTs stem from an increase in the rate of recruitment of the transporters into the plasma membrane, deriving from a special pool of preformed transporters localized in the cytoplasm. GLUT1 is present in most tissues, GLUT2 is found primarily in intestine, pancreatic β-cells, kidney and liver, GLUT3 is found primarily in neurons but also found in the intestine, GLUT4 is found in insulin-responsive tissues such as heart, adipose tissue and skeletal muscle and GLUT5 is expressed in intestine, kidney, testes, skeletal muscle, adipose tissue and brain. In liver glucose uptake is dramatically increased because of increased activity of the enzymes glucokinase, phosphofructokinase-1 (PFK-1), and pyruvate kinase (PK), the key regulatory enzymes of glycolysis. The latter effects are induced by insulin-dependent activation of phosphodiesterase, with decreased PKA activity and diminished phosphorylation of pyruvate kinase and phosphofructokinase-2, PFK-2. Dephosphorylation of pyruvate kinase increases its' activity while dephosphorylation of PFK-2 renders it active as a kinase. The kinase activity of PFK-2 converts fructose-6-phosphate into fructose-2,6-bisphosphate (F2,6BP). F2,6BP is a potent allosteric activator of the rate limiting enzyme of glycolysis, PFK-1, and an inhibitor of the gluconeogenic enzyme, fructose-1,6-bisphosphatase. In addition, phosphatases specific for the phosphorylated forms of the glycolytic enzymes increase in activity under the influence of insulin. All these events lead to conversion of the glycolytic enzymes to their active forms and consequently a significant increase in glycolysis. In addition, glucose-6-phosphatase activity is down-regulated. The net effect is an increase in the content of hepatocyte glucose and its phosphorylated derivatives, with diminished blood glucose. In addition to the above described events, diminished cAMP and elevated protein phosphatase activity combine to convert glycogen phosphorylase to its inactive form and glycogen synthase to its active form, with the result that not only is glucose funneled to glycolytic products, but glycogen content is increased as well.
All of the post-receptor responses initiated by insulin binding to its receptor are mediated as a consequence of the activation of several signal transduction pathways. These include receptor activation of phosphatidylinositol-3-kinase, PI3K. Activation of PI3K involves a linkage to receptor activation of insulin receptor substrates (of which there are four: IRS1, IRS2, IRS3 and IRS4). Activated PI3K phosphorylates membrane phospholipids, the major product being phosphatidylinositol-3,4,5-trisphosphate, (PIP3). PIP3 in turn activates the enzyme protein kinase B, PKB (also called Akt). There are three members of the PKB/Akt family of serine/threonine kinases identified as Akt1, Akt2, and Akt3. It is Akt2 that is important in insulin-mediated glucose homeostasis. Insulin-mediated activation of AKt also results in inhibition of lipolysis and gluconeogenesis and activation of protein synthesis and glycogen synthesis.
Additional enzymes activated by insulin receptor signaling are PIP3-dependent kinase, (PDK), some isoforms of protein kinase C, PKC (principally PKC-λ) and small ribosomal subunit protein 6 (p70) kinase, (p70S6K). The MAP kinase (MAPK) pathway is also activated either through insulin receptor phosphorylation of SRC homology 2 containing protein (Shc) which then interacts with growth factor receptor binding protein-2 (GRB2) or via IRS1 activation.
With respect to insulin responses, activation of PKB and PKC-λ lead to translocation of GLUT4 molecules to the cell surface resulting in increased glucose uptake which is significant in skeletal muscle. Activation of PKB also leads to the phosphorylation and inhibition of glycogen synthase kinase-3 (GSK3), which is a major regulatory kinase of glycogen homeostasis. In addition, PKB phosphorylates and inhibits the activity of a transcription factor (FKHRL1), now called FoxO3a) that has pro-apoptotic activity. This results in reduced apoptosis in response to insulin action. The role of insulin in the stimulation of protein synthesis occurs at the level of translational initiation and elongation and is exerted primarily via a cascade leading to the activation of mammalian target of rapamycin,mTOR, a protein with homology to a family of proteins first identified in yeast that bind to the immunosuppressant drug, rapamycin. mTOR is a kinase whose catalytic domain shares significant homology with lipid kinases of the PI3K family.
Insulin-mediated cascade leading to enhanced translation (not intended to be a complete description of all of the targets of insulin action that affect translation rates). Also shown is the effect of an increase in the AMP to ATP ratio which activates AMP-activated kinase, AMPK. STK11-LKB1-PJS = serine-threonine kinase 11, Peutz-Jeghers syndrome gene. IRS1 = insulin receptor substrate-1; PI3K = phosphatidylinositol-3-kinase; PIP2 = phosphatidylinositol-4,5-bisphosphate; PTEN = phosphatase and tensin homolog deleted on chromosome 10; PDK1 = PIP3-dependent protein kinase; Tsc1 and Tsc2 = Tuberous sclerosis tumor suppressors 1 (hamartin) and 2 (tuberin); Rheb = Ras homolog enriched in brain; mTOR = mammalian target of rapamycin. PKB/Akt = protein kinase B; GSK3 = glycogen synthase kinase-3; 4EBP1 = eIF-4E binding protein; p70S6K = 70kDa ribosomal protein S6 kinase, also called S6K. Insulin action leads to an increase in the activity of PI3K which in turn phosphorylates membrane phospholipids generating phosphatidylinositol-3,4,5-trisphophate (PIP3) from phosphatidylinositol-4,5-bisphosphate (PIP2). PIP3then activates the kinase PDK1 which in turn phosphorylates and activates PKB/Akt. Activated PKB/Akt will phosphorylate TSC2 (tuberin) of the TSC1/TSC2 complex on two residues (S939 and T1462) resulting in altered activity of the complex. The TSC1/TSC2 complex functions as a GTPase-activating protein (GAP) which increases GTP hydrolyzing activity of Rheb. The GAP activity resides in the C-terminal portion of tuberin. The faster the GTPase action of Rheb the faster will be the reduction in Rheb activation of mTOR. When TSC1/TSC2 is phosphorylated by PKB it is less effective at stimulating the GTPase activity of Rheb and therefore Rheb activation of mTOR will remain high as is the case in response to insulin action. AMPK phosphorylates TSC2 at two sites (T1271 and S1387) that are distinct from the sites that are the PKB/Akt targets for phosphorylation. Evidence indicates that the AMPK-mediated phosphorylation of TSC2 promotes the GTPase activity of Rheb resulting in inhibition of mTOR and thus a decrease in protein synthesis. Recent evidence has shown that PKB/Akt actually phosphorylates tuberin at 4 sites (S939, S1130, S1132, T1462) all of which result in inhibition of the Rheb-GAP activity of the TSC1/TSC2 complex.
The ultimate activation of mTOR leads to phosphorylation and activation of p70S6K which in turn leads to increased phosphorylation of eEF2 kinase. eEF2 kinase normally phosphorylates eEF2 leading to a decrease in its’ role in translation elongation. When phosphorylated by p70S6K, eEF2 kinase is less active at phosphorylating eEF2, thus eEF2 is much more active in response to insulin action. In addition, insulin action leads to a rapid dephosphorylation of eEF-2 via activation of protein phosphatase 2A (PP2A). Taken together, reduced eEF2K-mediated phosphorylation and increased eEF-2 dephosphorylation lead to increased protein synthesis.
Both mTOR and p70S6K have been shown to phosphorylate the regulator of translation initiation, eIF-4E binding protein, 4EBP1. Phosphorylation of 4EBP1 prevents it from binding to eIF-4E. Binding of 4EBP1 to eIF-4E prevents eIF-4E from interaction with the cap structure of mRNAs which is necessary for translational initiation. Thus, the consequences of 4EBP1:eIF-4E interaction is a reduction in translation initiation. As a consequence of the concerted actions of mTOR and p70S6K, insulin results in increased protein synthesis. PKB activation will also lead to phosphorylation and inhibition of glycogen synthase kinase-3 (GSK3). One of the targets of GSK3, relative to translation, is eIF2B. Phosphorylation of eIF2B prevents it from performing its GTPase activating (GAP) function in association with eIF2 (see the Protein Synthesis page for more details) and as a consequence results in reduced translational initiation. However, when GSK3 is inhibited by PKB phosphorylation the GAP activity of eIF2B remains high and consequently the rate of translational initiation by eIF2 remains high so protein synthesis is favored. Insulin also has profound effects on the transcription of numerous genes, effects that are primarily mediated by regulated function of sterol-regulated element binding protein, SREBP. These transcriptional effects include (but are not limited to) increases in glucokinase, pyruvate kinase, lipoprotein lipase (LPL), fatty acid synthase (FAS) and acetylCoA carboxylase (ACC) and decreases in glucose 6-phosphatase, fructose 1,6-bisphosphatase and phosphoenolpyruvate carboxykinase (PEPCK). In contrast, epinephrine diminishes insulin secretion by a cAMP-coupled regulatory path. In addition, epinephrine counters the effect of insulin in liver and peripheral tissue, where it binds to β-adrenergic receptors, induces adenylate cycles activity, increases cAMP, and activates PKA similarly to that of glucagon. The latter events induce glycogenolysis and gluconeogenesis, both of which are hyperglycemic and which thus counter insulin's effect on blood glucose levels. In addition, epinephrine influences glucose homeostasis through interaction with α-adrenergic receptors.
Pathways involved in the regulation of glycogen phosphorylase by epinephrine activation of α-adrenergic receptors. See Glycogen Metabolism for details of the epinephrine action. PLC-γ is phospholipase C-γ. The substrate for PLC-γ is phosphatidylinositol-4,5-bisphosphate, (PIP2) and the products are inositol trisphosphate, IP3and diacylglycerol, DAG. Similar calmodulin-mediated phosphorylations lead to inhibition of glycogen synthase.
Nutrient Intake and Hormonal Control of Insulin Action
Two of the many gastrointestinal hormones have significant effects on insulin secretion and glucose regulation. These hormones are the glucagon-like peptides (principally glucagon-like peptide-1, GLP-1) and glucose-dependent insulinotropic peptide (GIP). Both of these gut hormones constitute the class of molecules referred to as theincretins. Incretins are molecules associated with food intake-stimulation of insulin secretion from the pancreas. GLP-1 is derived from the product of the proglucagon gene. This gene encodes a preproprotein that is differentially cleaved dependent upon the tissue in which it is synthesized. For example, in pancreatic α-cells prohormone convertase 2 action leads to the release of glucagon. In the gut prohormone convertase 1/3 action leads to release of several peptides including GLP-1. Upon nutrient ingestion GLP-1 is secreted from intestinal enteroendocrine L-cells that are found predominantly in the ileum and colon with some production from these cell types in the duodenum and jejunum. Bioactive GLP-1 consists of 2 forms; GLP-1(7-37) and GLP-1(7-36)amide, where the latter form constitutes the majority (80%) of the circulating hormone.
Structure of the mammalian preproglucagon product. GRPP=glicentin-related pancreatic peptide. IP=intervening peptide. GLP-2=glucagon-related peptide-2. Additional peptides are derived from the preproprotein including glicentin which is composed of amino acids 1–69, oxyntomodulin is composed of amino acids 30–69 and the major proglucagon fragment (MPGF) comprises amino acids 72–158.
The primary physiological responses to GLP-1 are glucose-dependent insulin secretion, inhibition of glucagon secretion and inhibition of gastric acid secretion and gastric emptying. The latter effect will lead to increased satiety with reduced food intake along with a reduced desire to ingest food. The action of GLP-1 at the level of insulin and glucagon secretion results in significant reduction in circulating levels of glucose following nutrient intake. This activity has obvious significance in the context of diabetes, in particular the hyperglycemia associated with poorly controlled type 2 diabetes. The glucose lowering activity of GLP-1 is highly transient as the half-life of this hormone in the circulation is less than 2 minutes. Removal of bioactive GLP-1 is a consequence of N-terminal proteolysis catalyzed by dipeptidylpeptidase IV (DPP IV or DPP4). DPP4 was originally identified as the lymphocyte cell surface antigen CD26. In humans CD26 functions in many pathways that are not directly related to its peptidase activity nor to its role in incretin inactivation. DPP4 harbors adenosine deaminase-binding (ADA-binding) properties and is involved in extracellular matrix binding. Of importance to the immune system, CD26 expression and activity are enhanced upon T-cell activation. CD26 interacts with other lymphocyte cell surface antigens including ADA, CD45 and the chemokine receptor CXCR4 (notable is the fact that CXCR4 is a T-cell attachment site for HIV). As yet it has not been clearly delineated as to whether the enzymatic activity of DPP4 is essential for the T-cell activating and co-stimulatory functions assigned to CD26. Of significance, however, is that in gene knock-out mice lacking CD26 there is enhanced insulin secretion and improved glucose tolerance indicating that inhibition of DPP4 activity is a potential therapeutic target in thetreatment of type 2 diabetes. All of the effects of GLP-1 are mediated following activation of the GLP-1 receptor (GLP-1R). The GLP-1R is a typical seven-transmembrane spanning receptor coupled to G-protein activation, increased cAMP production and activation of PKA. However, there are also PKA-independent responses initiated through the GLP-1R. Other major responses to the actions of GLP-1 include pancreatic β-cell proliferation and expansion concomitant with a reduction of β-cell apoptosis (death). In addition, GLP-1 activity results in increased expression of the glucose transporter-2 (GLUT2) and glucokinase genes in pancreatic cells.
