The proto-oncogene c-fos has been implicated in the modulation of various cell functions. We have found that thrombin, a pleiotropic activator of endothelial cells, induced c-fos mRNA in human umbilical vein endothelial cells (HEC). This effect was dose-related (0.05 to 1.0 U/mL) and transient (maximal after 1 hour and negligible within 4 hours). Since thrombin activates phosphoinositide (PI) turnover through a pertussis toxin (PT)-sensitive guanosine triphosphate-binding regulatory protein(s) (G-protein) with subsequent stimulation of protein kinase C (PKC) and Ca2+ movements, we investigated whether these intracellular pathways are also responsible for c-fos induction. PT inhibited thrombin's effect on c-fos expression, but had no effect on c-fos expression by phorbol myristate acetate (PMA). Down regulation of PKC by prolonged exposure to PMA had no effect on thrombin and ionomycin stimulation of c-fos, but inhibited PMA activation of this gene. Quenching of the Ca1(2+) increase in response to quin2 loading in the absence of external Ca2+ suppressed thrombin activity on c-fos transcription. Under the same conditions PMA activity was not inhibited or only partially inhibited. Interleukin-1 beta (IL-1 beta) and basic fibroblast growth factor (bFGF) stimulation of c-fos mRNA level were not inhibited by quin2; on the contrary, ionomycin effect was blocked by this agent. These results indicate that thrombin-induced c-fos expression in HEC does not require a fully active PKC but is dependent on normal intracellular Ca2+ availability.