THROMBIN, PHORBOL ESTER, AND CAMP REGULATE THROMBIN RECEPTOR PROTEIN AND MESSENGER-RNA EXPRESSION BY DIFFERENT PATHWAYS

Human mesangial cells have been used to study the regulation of thromb in receptor protein and mRNA expression during cross-talk between diff erent signal transduction pathways. Persistent activation of thrombin receptor by thrombin led to homologous down-regulation of thrombin rec eptor protein, However, thrombin receptor mRNA expression was not affe cted, suggesting that increased receptor degradation is responsible fo r homologous down-regulation. Chronic activation of protein kinase C b y phorbol 12-myristate 13-acetate (PMA) and of adenylylcyclase by pros taglandin E1 (PGE1) resulted in heterologous down-regulation of thromb in receptor protein. In contrast to thrombin, PMA and PGE1 reduced in parallel thrombin receptor mRNA levels to 51% and 24% of control, resp ectively, indicating that heterologous downregulation of thrombin rece ptor protein is, at least in part, due to inhibition of receptor mRNA expression. The mechanisms of heterologous down-regulation of thrombin receptor protein have been studied in detail and compared to homologo us down-regulation. PMA induced down-regulation was completely blocked by GF 109 203 X, an inhibitor of protein kinase C. However, the loss of thrombin receptor induced by thrombin was not prevented by GF 109 2 03 X, indicating that homologous regulation is not dependent on protei n kinase C activation. The heterologous effect of PGE1 was mimicked by 8-bromo-cAMP, isobutylmethylxanthine, and forskolin, suggesting that an increase in intracellular cAMP level is involved in heterologous re gulation, Interestingly, heterologous down regulation induced by PGE1 seems not to require previous internalization of thrombin receptor. Th ese data indicate that thrombin receptor protein and mRNA expression c an be regulated in homologous and heterologous ways by different mecha nisms.