The importance of α-thrombin in the clotting cascade is well-known, but it is also a potent mitogen1-3. Like many other mitogens, thrombin causes receptor-mediated activation of a phosphatidy-linositol-specific phospholipase C (PLC), leading to the release of diacylglycerol and the subsequent activation of protein kinase C (refs 3-6). Protein kinase C is probably important in cell proliferation, as activation of this enzyme by phorbol esters promotes growth in many systems4-6. Some growth factors have tyrosine kinase activity and function without activation of PLC or protein kinase C7-9 . In this report we show that α-thrombin retains its mitogenicity in vascular smooth muscle cells depleted of protein kinase C. Phorbol-12-myristate-13-acetate (PMA) is found to be a potent growth inhibitor when added to vascular smooth muscle cells with α-thrombin. Moreover, growth inhibition is maximal when protein kinase C is activated 4 hours after exposure to thrombin, long after the completion of 'early events' induced by thrombin. Thus, PMA probes an event late in the G1 phase of the cell cyle or at the G1-S transition.
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