Insulin resistance in type 2 diabetes involves impaired post-receptor signaling. At the molecular level, which specific kinase is the earliest critically impaired step in the insulin receptor signal transduction cascade in insulin-resistant adipocytes?

• A AMP-activated protein kinase (AMPK) — reduced activation prevents glucose transporter translocation
• B Phosphoinositide 3-kinase (PI3K) — impaired due to reduced IRS-1 tyrosine phosphorylation, blocking downstream Akt/PKB activation and GLUT4 translocation
• C Protein kinase C (PKC) epsilon isoform — activated by diacylglycerol from lipid accumulation, phosphorylates IRS-1 on threonine/serine residues impairing downstream signaling ✓
• D Glycogen synthase kinase-3 (GSK-3) — hyperactive in insulin resistance, phosphorylating and inactivating glycogen synthase

Explanation

In ectopic lipid accumulation (as occurs with obesity and type 2 diabetes), intracellular diacylglycerol (DAG) and ceramides accumulate. DAG activates novel PKC isoforms (particularly PKC-epsilon in liver and PKC-theta in skeletal muscle). These activated PKC isoforms phosphorylate IRS-1 and IRS-2 on serine/threonine residues (as opposed to the tyrosine phosphorylation by the insulin receptor), which inhibits insulin receptor tyrosine kinase activity, reduces IRS-1 tyrosine phosphorylation, and impairs downstream PI3K/Akt signaling. This prevents GLUT4 vesicle translocation to the plasma membrane, reducing glucose uptake. Although GSK-3 hyperactivity and reduced AMPK are downstream consequences, the initial 'lipid-induced PKC' step is the primary molecular lesion.

Reference: KD Tripathi, Essentials of Medical Pharmacology, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.