Mechanism of action of Insulin ?

Case details

A 16-year-old type 1 diabetic female checks her finger stick blood glucose before a meal. She injects herself subcutaneously with exogenous insulin and then starts to eat. As Insulin is absorbed in to her blood, it binds to insulin receptors that activate:

A. Tyrosine kinase

B. Adenylate cyclase

C. Cyclic AMP

D. Protein kinase C

E. Phospholipase C

The correct answer is- A- Tyrosine kinase.

Insulin binding to its receptor stimulates intrinsic tyrosine kinase activity (figure -1) leading to receptor auto phosphorylation and the recruitment of intracellular signaling molecules, such as insulin receptor substrates (IRS). IRS and other adaptor proteins initiate a complex cascade of phosphorylation and dephosphorylation reactions, resulting in the widespread metabolic and mitogenic effects of insulin.

Insulin receptor

Figure -1- showing the structure of Insulin receptor. The receptor is composed of two extracellular α-subunits that are each linked to a ß-subunit and to each other by disulfide bonds.

As an example, activation of the phosphatidylinositol-3′-kinase (PI-3-kinase) pathway stimulates translocation of glucose transporters (e.g., GLUT4) to the cell surface, an event that is crucial for glucose uptake by skeletal muscle and fat (Figure-2), induce glycogen synthesis, protein synthesis, and lipogenesis. Activation of other insulin receptor signaling pathway (MAP kinase) induces cellular proliferation and growth, and regulation of  expression of various genes in insulin-responsive cells.

Mechanism of insulin action

Figure-2- showing the mechanism of action of Insulin

As regards other options-

Adenylate cyclase is an enzyme that catalyzes the conversion of ATP to c- AMP. Hormones such as glucagon and epinephrine activate Adenylate cyclase , the released  c AMP acts as a second messenger for their action.

Oxytocin and Thyrotropin- releasing hormone (TRH) activate protein kinase C.

Phospholipase C  cleaves PIP2 into Diacyl glycerol and Inositol triphosphate. DAG further activated protein kinase C(figure-3).

Phospholipase C

Figure-3- Mechanism of action of phospholipase C

For further details of insulin action follow the link-

http://www.namrata.co/insulin-biosynthesis-secretion-and-action/

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