Insulin: proinsulin, IRS-1, GLUT4, insulin resistance

Biological effects of insulin

Insulin is the primary anabolic hormone regulating carbohydrate, lipid, and protein metabolism. Its action shifts the metabolic balance toward the synthesis of fuel molecules and inhibits their breakdown.

Anabolic effects of insulin:

• stimulation of the synthesis of glycogen, triacylglycerols, and fatty acids;
• activation of cholesterol synthesis;
• enhancement of amino acid uptake and protein synthesis;
• inhibition of gluconeogenesis.

Effects on catabolism:

• enhancement of glycolysis — the only catabolic process activated by insulin;
• inhibition of glycogenolysis, lipolysis, and proteolysis.

Levels of metabolic regulation by insulin

Insulin alters enzyme activity at two levels:

• Qualitative — dephosphorylation of key enzymes (rapid response);
• Quantitative — induction or repression of enzyme gene transcription (slow response).

Enzymes regulated by dephosphorylation

Activated by dephosphorylation:

• Phosphofructokinase-1 — key enzyme of glycolysis;
• Pyruvate kinase — catalyzes the conversion of phosphoenolpyruvate to pyruvate;
• Kinase activity of the bifunctional enzyme that forms fructose-2,6-bisphosphate, an allosteric activator of phosphofructokinase-1;
• Glycogen synthase — attaches UDP-glucose to the glycogen primer;
• Acetyl-CoA carboxylase — converts acetyl-CoA to malonyl-CoA (the first reaction of fatty acid synthesis);
• HMG-CoA reductase — rate-limiting enzyme of cholesterol synthesis.

Inactivated by dephosphorylation:

• Glycogen phosphorylase — cleaves α-1,4-glycosidic bonds of glycogen to form glucose-1-phosphate;
• TAG lipase — cleaves fatty acids from triacylglycerols, fat mobilization is inhibited.

Regulation at the transcription level

Insulin induces the synthesis of enzymes that ensure glucose utilization and lipogenesis:

• Glucokinase — phosphorylates glucose to glucose-6-phosphate in hepatocytes;
• Glucose-6-phosphate dehydrogenase — initiates the pentose phosphate pathway, supplying NADPH for fatty acid synthesis;
• Acetyl-CoA carboxylase and fatty acid synthase are the key enzymes of lipogenesis.

The key enzyme of gluconeogenesis, phosphoenolpyruvate carboxykinase (PEPCK), which converts oxaloacetate to phosphoenolpyruvate, is repressed.

Insulin receptor and signaling cascade

The insulin receptor belongs to catalytic receptors with intrinsic enzymatic activity. Structure: heterotetramer consisting of 2 α-subunits (extracellular, bind the hormone) and 2 β-subunits (transmembrane, possess tyrosine kinase activity).

Stages of signal transduction:

1. binding of insulin to α-subunits;
2. autophosphorylation of β-subunits at tyrosine residues;
3. phosphorylation of the IRS-1 (Insulin Receptor Substrate-1) protein at tyrosine residues;
4. activation of small G proteins and the protein kinase cascade;
5. protein kinases phosphorylate and activate protein phosphatases, which dephosphorylate target enzymes;
6. transcription factors are phosphorylated in parallel, altering the mRNA levels of enzymes.

One of the most important effects of the cascade is the insertion of the GLUT4 transporter into the plasma membrane of myocytes and adipocytes, ensuring glucose uptake by insulin-dependent tissues. Decreased tissue sensitivity to insulin with preserved or increased insulin secretion is termed insulin resistance and underlies type 2 diabetes mellitus.

Structure and biosynthesis of insulin

Insulin is a polypeptide of 51 amino acid residues consisting of two chains:

• chain A — 21 residues, contains an intrachain disulfide bridge;
• chain B — 30 residues;
• the chains are connected by two interchain disulfide bonds.

Biosynthesis occurs in the β-cells of the islets of Langerhans in the pancreas and proceeds via inactive precursors:

1. Preproinsulin with an N-terminal signal peptide is synthesized on the polyribosomes of the rough ER.
2. The signal peptide directs the chain into the ER lumen and is cleaved off to form proinsulin.
3. Proinsulin is transported to the Golgi apparatus, where specific proteases excise the central fragment — C-peptide (about 31 amino acids).
4. Mature insulin and C-peptide are packaged into secretory vesicles in an equimolar 1:1 ratio and secreted together.

Regulation of secretion and insulin metabolism

Stimulators of insulin secretion:

• elevation of blood glucose concentration (main stimulus);
• certain amino acids — arginine, lysine;
• gastrointestinal hormones (incretins).

Inhibitors of secretion include counter-regulatory hormones, primarily epinephrine.

The half-life of insulin is 3–10 minutes; it is degraded by hepatic insulinase. The C-peptide circulates significantly longer — up to 30 minutes, is not degraded in the liver, and is used in clinical diagnostics to assess endogenous β-cell secretory function (especially during exogenous insulin therapy).

Key points

• Insulin is an anabolic hormone that activates the synthesis of glycogen, fats, cholesterol, and proteins; of the catabolic pathways, it only enhances glycolysis.
• Metabolic regulation occurs qualitatively (enzyme dephosphorylation) and quantitatively (transcription induction/repression).
• The insulin receptor is an α₂β₂ tyrosine kinase; the key substrate of the cascade is IRS-1.
• In muscle and adipose tissue, insulin promotes the insertion of GLUT4 into the plasma membrane.
• Insulin is synthesized in the β-cells of the islets of Langerhans via preproinsulin and proinsulin.
• In the Golgi apparatus, C-peptide is cleaved from proinsulin and secreted together with insulin in a 1:1 ratio.
• The half-life of insulin is 3–10 min (hepatic insulinase), and that of C-peptide is up to 30 min, which is used in diagnostics.
• Decreased tissue response to insulin—insulin resistance—is the basis of type 2 diabetes mellitus.