Activation of Protein Kinase C by Tumor Necrosis Factor-α in Human Non-Pigmented Ciliary Epithelium

Abstract

Previously, we have shown that tumor necrosis factor-α (TNF-α), a proinflammatory cytokine, increases the synthesis and release of endothelin-1 (ET-1), a potent vasoactive peptide from human non-pigmented ciliary epithelial (HNPE) cells, in a protein kinase C (PKC)-dependent manner. Diacylglycerol (DAG) and intracellular calcium ([Ca²⁺]_i) are well known activators of PKC. Some cytokines induce PKC activation by stimulating phospholipase C that hydrolyzes phosphatidylinositol bisphosphate (PIP₂) into IP₃ (intracellular calcium mobilizer) and DAG. In this study, the existence of a similar pathway was evaluated in HNPE cells treated with TNF-α, using intracellular calcium ([Ca²⁺]_i) measurements, PKC translocation assays and thin-layer chromatography (TLC) for quantification of DAG. Incubation times for agonists and inhibitors ranged from 1-30 minutes. The increase in DAG levels with TNF-α treatment was consistent with the observed translocation of the calcium-dependent PKC α isoform from the cytosol to the plasma membrane. However, these observations were not accompanied by a concomitant increase in [Ca²⁺]_i. Similar translocation responses were observed with phorbol ester (phorbol 12-myristate 13-acetate) treatment. Our results indicate that TNF-α-induced PKC activation in HNPE cells occurs as a result of elevated DAG levels and is not due to an increase in intracellular calcium. Activated PKC, could enhance the pro-inflammatory responses of TNF-α in part by increasing the production of endothelins in the eye.