protein kinase C

PKC is activated downstream of Gq protein-coupled receptors (e.g., alpha-1 adrenergic, M1/M3 muscarinic, V1 vasopressin, angiotensin II receptors) via the phospholipase C (PLC) pathway. PLC cleaves PIP2 into IP3 and DAG: IP3 triggers calcium release from the endoplasmic reticulum, while DAG remains in the membrane and, together with calcium, activates conventional PKC isoforms. Activated PKC translocates to the membrane and phosphorylates serine/threonine residues on target proteins, altering their activity, and thereby modulates numerous cellular processes including cell growth, differentiation, apoptosis, gene expression, and secretion.

Clinical significance: Chronic hyperglycemia in diabetes mellitus increases DAG synthesis, leading to sustained PKC activation (particularly the beta isoform), which contributes to microvascular complications—diabetic retinopathy, nephropathy, and neuropathy—via increased vascular permeability, extracellular matrix production, and cytokine release. Ruboxistaurin, a PKC-beta inhibitor, was studied for diabetic retinopathy. Phorbol esters (tumor promoters) are exogenous activators of PKC that mimic DAG, contributing to their role in carcinogenesis by promoting uncontrolled cell proliferation. PKC also plays roles in platelet activation, smooth muscle contraction, and cardiac hypertrophy signaling. Understanding this pathway is essential for grasping the mechanism of drugs acting on Gq-coupled receptors and the pathogenesis of diabetic vascular disease—a frequently tested concept on USMLE Step 1.