Endothelin-1 and angiotensin II act as progression but not competence growth factors in vascular smooth muscle cells

The direct effects of endothelin-1 and angiotensin II on cell cycle progression were investigated in rat aorta smooth muscle cells in primary culture. The phase of the cell cycle was determined by an immunocytochemical analysis of cell cycle-specific nuclear antigens. The primary cultured cells were synchronized in the G(0) phase (100%) by serum deprivation for 24 h. Endothelin-1 (0.1 mu M) or angiotensin II (1 mu M) had no effect on the cell cycle of G(0) cells, whereas platelet-derived growth factor (PDGF) stimulated the entry of the G(0) cells into the G(1) phase (100%) without a further progression to the S and M phases. Endothelin-1 or angiotensin II stimulated the progression of the PDGF-pretreated G(1) cells to the S and M phases. Fura-2 microfluorometry revealed that, between the G(0) and G(1) cells, there were no differences in the extent and time course of cytosolic Ca2+ elevations induced by endothelin-1 or angiotensin II, which suggested that endothelin-1 and angiotensin II receptors and their signaling pathways regulating cytosolic Ca2+ remained intact in these cell phases. We thus conclude that endothelin-1 and angiotensin II require the prior G(0)/G(1) transition induced by a competence growth factor such as PDGF to exert their mitogenic effects. These results suggest the important role of endothelin-1 and angiotensin II in atherosclerosis as promoters (progression growth factors), but not as initiators.