DISSOCIATION OF PHOSPHATIDYLINOSITOL-3 KINASE-ACTIVITY AND MITOGENIC INHIBITION IN VASCULAR SMOOTH-MUSCLE CELLS

Phosphatidylinositol-3 kinase becomes activated upon association with stimulated tyrosine kinase coupled receptors, but it is also catalytic ally active in platelets incubated with the G-protein coupled growth f actor receptor agonist, thrombin. Furthermore, phorbol esters have bee n shown to be growth inhibitory when added to vascular smooth muscle c ells simultaneously with thrombin. In order to clarify the role of pho sphatidylinositol-3 (PI-3) kinase in thrombin-induced mitogenesis, we asked whether PI-3 kinase activity is decreased in parallel to mitogen esis in cells stimulated with phorbol-12-myristate-13-acetate (PMA) an d thrombin. Although PMA inhibits thrombin-stimulated growth by 92% wh en the two compounds are added simultaneously, the level of PI-3 kinas e activity under similar conditions is not decreased. This phenomenon is independent of protein kinase C, since there is no difference in PI -3 kinase activity when similar experiments are performed after protei n kinase C is down-regulated by 24 h pre-incubation with PMA. We concl ude that either (i) PI-3 kinase is not required for the mitogenic sign alling of thrombin, or (ii) PMA is acting downstream of PI-3 kinase in thrombin's signalling pathway.