Angiotensin II-stimulated protein synthesis in cultured vascular smooth muscle cells

Abstract

To investigate the role of vasoconstrictor hormones in vascular smooth muscle cell growth we have studied the effects of the potent vasoconstrictor angiotensin II on cell growth in a cultured rat aortic cell model. Angiotensin II was not mitogenic for these cells, as assessed by determining cell number, nor was it synergistic in this regard with 10% calf serum. However, 24-hour exposure to 100 nM angiotensin II caused an 80% increase in protein synthesis (compared with 0.4% increase with serum control) as measured by tritiated leucine incorporation. This was a "hypertrophic" response as indicated by a 30% increase in protein content and a 45% increase in cell volume. Angiotensin II-induced smooth muscle cell hypertrophy was maximal at 100 nM, had an ED50 of 1 nM, and was inhibited by the competitive antagonist [Sar1, Ile8]angiotensin II. The increase in protein synthesis required continuous presence of angiotensin II for 6 hours and required messenger RNA (mRNA) synthesis as suggested by complete inhibition after exposure to actinomycin D. Angiotensin II-stimulated protein synthesis was dependent on a rise in intracellular Ca2+ concentration evidenced by a 70% decrease in tritiated leucine incorporation after chelation of Ca2+ with 25 microM quin 2-AM. This treatment did not alter protein synthesis induced by 10% calf serum. Decreasing extracellular Na+ to prevent Na+/H+ exchange and intracellular alkalinization did not inhibit the angiotensin II response but decreased the 10% calf serum-stimulated protein synthesis by 35%. Downregulation of protein kinase C by 24-hour treatment with phorbol 12,13-dibutyrate did not inhibit angiotensin II-induced protein synthesis, while phorbol 12-myristate 13-acetate-stimulated protein synthesis was abolished. These findings suggest that angiotensin II-induced hypertrophy, acting via a Ca2+ mechanism, may play an important role in abnormal vascular smooth muscle cell growth in certain forms of hypertension.