Suppression of smooth-muscle alpha-actin expression by platelet-derived growth factor in vascular smooth-muscle cells involves Ras and cytosolic phospholipase A2.

Platelet-derived growth factor (PDGF), which is a potent mitogen for vascular smooth-muscle cells (VSMC), also inhibits the expression of specific smooth-muscle proteins, including smooth-muscle alpha-actin (SM-alpha-actin), in these cells. The goal of this study was to identify signalling pathways mediating these distinct effects. In rat aortic VSMC, PDGF caused a rapid activation of Ras and Raf, leading to the activation of mitogen-activated protein kinases (ERKs). Cells stably transfected with constitutively active Ras (H-Ras) expressed low levels of SM-alpha-actin protein. Arginine vasopressin, which stimulated SM-alpha-actin promoter activity in wild-type cells or controls (Neo; transfected with a plasmid lacking an insert), failed to do so in cells transiently expressing H-Ras. The effects of Ras on suppression of SM-alpha-actin expression were not mediated by the Raf/ERK pathway, since cells stably expressing constitutively active Raf (BxB-Raf) had normal levels of SM-alpha-actin protein, and stimulation of SM-alpha-actin promoter activity by vasopressin was unaffected in cells transiently expressing BxB-Raf. Furthermore a specific inhibitor of ERK activation had no effect on SM-alpha-actin expression. Exposure of wild-type VSMC to PDGF, or stable expression of Ras but not Raf, also resulted in constitutive increases in prostaglandin E2 production and cytosolic phospholipase A2 (cPLA2) activity, which was mediated by an increased expression of cPLA2 protein. Transient expression of cPLA2 in wild-type VSMC inhibited the stimulation of SM-alpha-actin promoter activity by vasopressin. These results suggest that PDGF-induced inhibition of SM-alpha-actin expression is mediated through a Ras-dependent/Raf independent pathway involving the induction of cPLA2 and eicosanoid production.