Insulin

INSULIN

Insulin is a peptide hormone produced by the β cells of the islets of Langerhans, which are clusters of cells that are embedded in the endocrine portion of the pancreas (Figure 23.2). [Note: “Insulin” is from the Latin for island.] The islets make up only about 1%– 2% of the total cells of the pancreas. Insulin is the most important hormone coordinating the use of fuels by tissues. Its metabolic effects are anabolic, favoring, for example, synthesis of glycogen, triacylglycerols (TAGs), and protein.

Figure 23.2 Islets of Langerhans.

A. Structure of insulin

Insulin is composed of 51 amino acids arranged in two polypeptide chains, designated A (21 amino acids) and B, which are linked together by two disulfide bridges (Figure 23.3A). The insulin molecule also contains an intramolecular disulfide bridge between amino acid residues of the A chain. [Note: Insulin was the first peptide for which the primary structure was determined, and the first therapeutic molecule made by recombinant DNA technology.]

Figure 23.3 A. Structure of insulin. B. Formation of human insulin from preproinsulin.

B. Synthesis of insulin

The processing and transport of intermediates that occur during the synthesis of insulin are shown in Figures 23.3B and 23.4. Biosynthesis involves production of two inactive precursors, preproinsulin and proinsulin, that are sequentially cleaved to form the active hormone plus the connecting or C-peptide (see Figure 23.4). [Note: The C-peptide is essential for proper insulin folding. Also, because its half-life in the plasma is longer than that of insulin, the C-peptide is a good indicator of insulin production and secretion.] Insulin is stored in the cytosol in granules that, given the proper stimulus (see below), are released by exocytosis. Insulin is degraded by insulin-degrading enzyme, which is present in the liver and, to a lesser extent, in the kidneys. Insulin has a plasma half-life of approximately 6 minutes. This short duration of action permits rapid changes in circulating levels of the hormone.

Figure 23.4 ntracellular movements of insulin and its precursors. mRNA = messenger RNA; RER = rough endoplasmic reticulum.

C. Regulation of insulin secretion

Secretion of insulin is regulated by bloodborne fuels and hormones.

1. Stimulation of insulin secretion: Insulin secretion by the pancreatic β cells is closely coordinated with the release of glucagon by pancreatic α cells (Figure 23.5). The relative amounts of insulin and glucagon released by the pancreas are regulated so that the rate of hepatic glucose production is kept equal to the use of glucose by peripheral tissues. In view of its coordinating role, it is not surprising that the β cell responds to a variety of stimuli. In particular, insulin secretion is increased by glucose, amino acids, and gastrointestinal peptide hormones.

Figure 23.5 Changes in blood levels of glucose, insulin, and glucagon after ingestion of a carbohydrate-rich meal.

a. Glucose: Ingestion of a carbohydrate-rich meal leads to a rise in blood glucose, the primary stimulus for insulin secretion (see Figure 23.5). The β cells are the most important glucose-sensing cells in the body. Like the liver, β cells contain GLUT-2 transporters and express glucokinase (hexokinase IV;). At blood glucose levels above 45 mg/dl, glucokinase phosphorylates glucose in amounts proportional to the glucose concentration. Proportionality results from the lack of direct inhibition of glucokinase by glucose 6-phosphate, its product. Additionally, the sigmoidal relationship between the velocity of the reaction and substrate concentration maximizes the enzyme’s responsiveness to changes in blood glucose level. Metabolism of glucose 6-phosphate generates adenosine triphosphate (ATP), leading to insulin secretion (see blue box below).

b. Amino acids: Ingestion of protein causes a transient rise in plasma amino acid levels (for example, arginine) that enhances the glucose-stimulated secretion of insulin. [Note: Fatty acids have a similar effect.]

c. Gastrointestinal peptide hormones: The intestinal peptides glucagon-like protein-1 (GLP-1) and gastric-inhibitory polypeptide ([GIP]; also called glucose-dependent insulinotropic peptide) increase the sensitivity of β cells to glucose. They are released from the small intestine after the ingestion of food, causing an anticipatory rise in insulin levels and, thus, are referred to as “incretins.” Their action may account for the fact that the same amount of glucose given orally induces a much greater secretion of insulin than if given intravenously (IV).

Glucose-dependent release of insulin into blood is mediated through a rise in calcium (Ca²⁺) concentration in the β cell. Glucose taken into β cells is phosphorylated and metabolized, with subsequent production of ATP. ATP-sensitive potassium (K+) channels close, causing depolarization of the plasma membrane, opening of voltage-gated Ca²⁺ channels, and influx of Ca²⁺ into the cell. Ca²⁺ causes vesicles containing insulin to be exocytosed from the β cell. Sulfonylureas, oral agents used to treat type 2 diabetes, increase insulin secretion by closing ATP-sensitive K⁺ channels.

2. Inhibition of insulin secretion: The synthesis and release of insulin are decreased when there is a scarcity of dietary fuels and also during periods of physiologic stress (for example, infection, hypoxia, and vigorous exercise). These effects are mediated primarily by the catecholamines norepinephrine and epinephrine, which are made from tyrosine in the sympathetic nervous system and the adrenal medulla and secreted. Secretion is largely controlled by the nervous system. The catecholamines (primarily epinephrine) have a direct effect on energy metabolism, causing a rapid mobilization of energy-yielding fuels, including glucose from the liver (produced by glycogenolysis or gluconeogenesis;) and fatty acids from adipose tissue (produced by lipolysis;). In addition, these biogenic amines can override the normal glucose-stimulated release of insulin. Thus, in emergency situations, the sympathetic nervous system largely replaces the plasma glucose concentration as the controlling influence over β-cell secretion. The regulation of insulin secretion is summarized in Figure 23.6.

Figure 23.6 Regulation of insulin release from pancreatic β cells. [Note: Gastrointestinal peptide hormones also stimulate insulin release.]

D. Metabolic effects of insulin

Insulin promotes the storage of nutrients as glycogen, TAG, and protein and inhibits their mobilization.

1. Effects on carbohydrate metabolism: The effects of insulin on glucose metabolism promote its storage and are most prominent in three tissues: liver, muscle, and adipose. In the liver and muscle, insulin increases glycogen synthesis. In the muscle and adipose, insulin increases glucose uptake by increasing the number of glucose transporters (GLUT-4;) in the cell membrane. The IV administration of insulin, thus, causes an immediate decrease in blood glucose level. In the liver, insulin decreases the production of glucose through the inhibition of glycogenolysis and gluconeogenesis. [Note: The effects of insulin are due not just to changes in enzyme activity, but also in enzyme amount insofar as insulin affects gene transcription.]

2. Effects on lipid metabolism: Adipose tissue responds rapidly to a rise in insulin, which causes a significant reduction in the release of fatty acids by inhibiting the activity of hormone-sensitive lipase, which degrades lipids in adipose tissue. Insulin acts by promoting the dephosphorylation and, hence, inactivation of the enzyme. Insulin also increases the transport and metabolism of glucose into adipocytes, providing the glycerol 3-phosphate substrate for TAG synthesis. Expression of the gene for lipoprotein lipase is increased by insulin in adipose tissue, thereby providing fatty acids for esterification to the glycerol. [Note: Insulin also promotes the conversion of glucose to TAG in the liver. The TAGs are secreted in very-low-density lipoproteins (VLDLs).]

3. Effects on protein synthesis: In most tissues, insulin stimulates the entry of amino acids into cells and protein synthesis. [Note: Insulin stimulates protein synthesis through activation of factors required for translation initiation.]

E. Mechanism of insulin action

Insulin binds to specific, high-affinity receptors in the cell membrane of most tissues, including liver, muscle, and adipose. This is the first step in a cascade of reactions ultimately leading to a diverse array of biologic actions (Figure 23.7).

Figure 23.7 Insulin receptor. P = phosphate; Tyr = tyrosine.

1. Insulin receptor: The insulin receptor is synthesized as a single polypeptide that is glycosylated and cleaved into α and β subunits, which are then assembled into a tetramer linked by disulfide bonds (see Figure 23.7). The extracellular α subunit contains the insulin-binding site. A hydrophobic domain in each β subunit spans the plasma membrane. The cytosolic domain of the β subunit is a tyrosine kinase, which is activated by insulin. As a result, the insulin receptor is classified as a tyrosine-kinase receptor.

2. Signal transduction: The binding of insulin to the α subunits of the insulin receptor induces conformational changes that are transmitted to the β subunits. This promotes a rapid autophosphorylation of specific tyrosine residues on each β subunit ( s e e Figure 23.7). Autophosphorylation initiates a cascade of cell-signaling responses, including phosphorylation of a family of proteins called insulin receptor substrates (IRSs). At least four IRSs have been identified that show similar structures but different tissue distributions. Phosphorylated IRS proteins interact with other signaling molecules through specific domains, activating a number of pathways that affect gene expression, cell metabolism, and growth. The actions of insulin are terminated by dephosphorylation of the receptor.

3. Membrane effects of insulin: Glucose transport in some tissues, such as muscle and adipose, increases in the presence of insulin (Figure 23.8). Insulin promotes movement of insulin-sensitive glucose transporters (GLUT-4s) from a pool located in intracellular vesicles to the cell membrane. [Note: Movement is the result of a signaling cascade in which an IRS binds to and activates a kinase (phosphoinositide 3-kinase), leading to phosphorylation of the membrane phospholipid phosphatidylinositol 4,5-bisphosphate to the 3,4,5-trisphosphate form that binds to and activates phosphoinositide kinase 1. This kinase, in turn, activates Akt (protein kinase B), resulting in GLUT-4 movement.] In contrast, other tissues have insulin-insensitive systems for glucose transport (Figure 23.9). For example, hepatocytes; erythrocytes; and cells of the nervous system, intestinal mucosa, renal tubules, and cornea do not require insulin for glucose uptake.

Figure 23.8 Insulin-mediated recruitment of glucose transporters (GLUT-4s) from intracellular stores in skeletal and cardiac muscle and adipose tissue.

Figure 23.9 Characteristics of glucose transport in various tissues.

4. Receptor regulation: Binding of insulin is followed by internalization of the hormone–receptor complex. Once inside the cell, insulin is degraded in the lysosomes. The receptors may be degraded, but most are recycled to the cell surface. [Note: Elevated levels of insulin promote the degradation of receptors, therby decreasing the number of surface receptors. This is one type of “downregulation.”]

5. Time course of insulin actions: The binding of insulin provokes a wide range of actions. The most immediate response is an increase in glucose transport into adipocytes and skeletal and cardiac muscle cells that occurs within seconds of insulin binding to its membrane receptor. Insulin-induced changes in enzymic activity in many cell types occur over minutes to hours and reflect changes in the phosphorylation states of existing proteins. Insulin-induced increase in the amount of many enzymes, such as glucokinase, liver pyruvate kinase, acetyl coenzyme A (CoA) carboxylase (ACC), and fatty acid synthase, requires hours to days. These changes reflect an increase in gene expression through increased transcription (mediated by sterol regulatory element–binding protein-1;) and translation.