α₁-Adrenergic receptor activation of c-fos expression in transfected rat-1 fibroblasts: Role of Ca^{2+ 1}

α₁-Adrenergic receptors mediate mitogenic responses and increase intracellular free Ca²⁺ ([Ca²⁺](i)) in vascular smooth muscle cells. Induction of c-fos is a critical early event in cell growth; expression of this gene is regulated by a number of signaling pathways including Ca²⁺. We wondered whether Ca²⁺ signaling plays a critical role in the induction of c-fos gene by α₁- adrenergic receptors. Using stably transfected rat-1 fibroblasts, we confirmed that PE induced c-fos mRNA expression in a time- and dose-dependent manner, and also increased [Ca²⁺](i) (measured with Fura-2 AM). These responses were blocked by the α₁-adrenergic receptor antagonist doxazosin. Both intracellular Ca²⁺ chelation (using BAPTA/AM) and extracellular Ca²⁺ depletion (using EGTA) significantly inhibited PE- induced c-fos expression by α(1A) and α(1B) receptors. Brief (1-min) stimulation of α(1A) and α(1B) receptors with PE did not maximally induce c-fos expression, suggesting that a sustained increase in [Ca²⁺](i) due to Ca²⁺ influx is required. The calmodulin (CAM) antagonists, R24571, W7, and trifluoperazine, but not the CaM-dependent protein kinases inhibitor KN-62, significantly inhibited c-fos induction by α(1A) and α(1B) receptors. Neither inhibition of protein kinase C nor inhibition of adenylyl cyclase modified c-fos induction by PE. These results suggest that α₁-adrenergic receptor-induced c-fos expression in rat-1 cells is dependent on a Ca²⁺/CaM-associated pathway.