SUPPRESSION OF SMOOTH-MUSCLE ALPHA-ACTIN EXPRESSION BY PLATELET-DERIVED GROWTH-FACTOR IN VASCULAR SMOOTH-MUSCLE CELLS INVOLVES RAS AND CYTOSOLIC PHOSPHOLIPASE A(2)

Platelet-derived growth factor (PDGF), which is a potent mitogen for v ascular smooth-muscle cells (VSMC), also inhibits the expression of sp ecific 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 VS MC, PDGF caused a rapid activation of Ras and Raf, leading to the acti vation of mitogen-activated protein kinases (ERKs). Cells stably trans fected with constitutively active Ras (H-Ras) expressed low levels of SM-alpha-actin protein, Arginine vasopressin, which stimulated SM-alph a-actin promoter activity in wild-type cells or controls (Neo; transfe cted with a plasmid lacking an insert), failed to do so in cells trans iently expressing H-Ras. The effects of Ras on suppression of SM-alpha -actin expression were not mediated by the Raf/ERK pathway, since cell s stably expressing constitutively active Raf (BxB-Raf) had normal lev els of SM-alpha-actin protein, and stimulation of SM-alpha-actin promo ter activity by vasopressin was unaffected in cells transiently expres sing BxB-Raf, Furthermore a specific inhibitor of ERK activation had n o effect on SM-alpha-actin expression. Exposure of wild-type VSMC to P DGF, or stable expression of Ras but not Raf, also resulted in constit utive increases in prostaglandin E-2 production and cytosolic phosphol ipase A(2) (cPLA(2)) activity, which was mediated by an increased expr ession of cPLA, protein. Transient expression of cPLA(2) 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 indepen dent pathway involving the induction of cPLA(2) and eicosanoid product ion.