PKC α regulates thrombin-induced PDGF-B chain gene expression in mesangial cells

Thrombin is a potent mitogen for mesangial cells and stimulates PDGF B-chain gene expression in these cells. It also activates phospholipase C (PLC) resulting in an increase in cytosolic Ca²⁺ and diacylglycerol (DAG) that are the physiological activators of protein kinase C (PKC). Immunoprecipitation of specific PKC isotypes from thrombin-stimulated mesangial cells with subsequent measurement of their enzymatic activity shows activation of Ca²⁺-dependent PKC α and Ca²⁺-independent PKC Σ in a time dependent manner. Optimum activation of both of these isozymes was obtained at 60 minutes. PKC α activity increased 83% over basal while activity of PKC Σ increased 104%. Prolonged exposure of mesangial cells to phorbol myristate acetic acid (PMA) inhibited the enzymatic activity of PKC α but not PKC Σ. This inhibition of PKC α had no effect on thrombin-induced DNA synthesis but abolished PDGF B-chain gene expression induced by thrombin. These data provide the first evidence that PKC α activation is necessary for thrombin-induced PDGF B-chain gene expression but not for thrombin-induced DNA synthesis.