ANGIOTENSIN-II ACTIVATES THE BETA-1 ISOFORM OF PHOSPHOLIPASE-C IN VASCULAR SMOOTH-MUSCLE CELLS

Vascular smooth muscle cells (VSMC) contribute to the pathophysiology of hypertension through cell growth and contraction, and phospholipase C (PLC) is a critical effector enzyme in growth factor and vasoconstr ictor signaling. There is indirect evidence that angiotensin II (ANG I I) receptors are Linked to the PLC-beta isoform signaling pathways. Ho wever, recent studies suggest that PLC-beta isoforms may not be expres sed in VSMC. Our data demonstrate that in human aortic VSMC, PLC-beta 1 and PLC-gamma 1 proteins were detected by immunoblot analysis, and P LC-beta 1 mRNA was identified by reverse transcriptase-polymerase chai n reaction in rat aortic VSMC. Incubation of permeabilized VSMC with a nti-PLC-beta 1 or anti-G(q) alpha antibodies inhibited ANG II-dependen t inositol polyphosphate (IP) formation, while anti-PLC-gamma 1 antibo dies did not inhibit ANG II-regulated IP formation. Conversely, anti-P LC-gamma 1 antibodies completely abolished platelet-derived growth fac tor (PDGF)-dependent IP generation, whereas anti-PLC-beta 1 antibodies had no effect on PDGF-induced PLC activation. Inhibition of tyrosine phosphorylation with genistein or herbimycin A did not diminish ANG II -stimulated IP formation or cytosolic free Ca2+ concentration transien ts, thereby confirming that ANG II signals via a PLC-gamma 1-independe nt mechanism. In summary, PLC-beta 1 and PLC-gamma 1 are expressed in human aortic VSMC, and PLC-beta 1 is the isoform that is critical for ANG II-regulated PLC signaling in these cells.