Differential Regulation of Phospholipase A2 Activity and Prostacyclin Synthesis in Rat Vascular Smooth Muscle Cells and Cardiomyocytes

Abstract

The respective roles of protein kinase C (PKC), cytosolic free calcium ([Ca++]i) and of 5’, 3’-cyclic adenosine monophosphate (cAMP) in phospholipase A2 (PLA2) activation and prostacyclin (PGI2) generation were studied in cultured rat aortic smooth muscle cells (VSMCs) and cardiomyocytes (CMs). Cells were stimulated with the PKC activator 4β phorbol 12-myristate 13-acetate (PMA), the Ca++ ionophore A23187, the adenylyl cyclase activator forskolin and the vasoactive hormone angiotensin II (Ang II). Effects were measured in terms of PKC activation, PLA2 activity, PGI2 release and cAMP formation. Exposure of VSMCs and CMs to PMA (0.1.µM) and/or Ang II (0.05 µM) lead to significant increases in membranous PKC activity, a response that was associated with enhanced cellular PLA2 activity and PGI2 production. PMA proved to be the more potent stimulus for PGI2 production in CMs, while its effect in VSMCs was small as compared to that of Ang II. Furthermore, addition of the PKC inhibitor staurosporine (1 µM) totally abolished both PMA and Ang II-induced PGI2 formation in CMs while it only partially affected Ang II-induced PGI2 production in VSMCs. These results indicate that [Ca++]I plays an important role in the activation of vascular smooth muscle PLA2, a hypothesis that was confirmed by incubating cells with A23187. As expected, A23187 (1 µM) induced an important (16-fold) increase in PGI2 production in VSMCs while it was relatively ineffective in generating PGI2 production in CMs. Interestingly, A23187-induced PGI2 production was potentiated by 0.1.tM PMA in VSMCs but not in CMs, suggesting that the two cell types express different PLA2s. This was further confirmed by exposing cells to forskolin (10 µM) and dibutyryl-cAMP (dBcAMP, 50011M); while both agents failed to increase PGI2 production in vascular smooth muscle cells, forskolin and dBcAMP elicited potent PGI2 responses in cardiomyocytes. Taken together, these results indicate that PLA2 activation and PGI2 production are PKC mediated processes in both VSMCs and CMs and further suggest that the VSMC PLA2 is a Ca++-dependent enzyme while the cardiac phospholipase is a Ca++-independent enzyme endogenously activated by a cAMP-dependent pathway.