Phosphorylation of the cAMP response element-binding protein and activation of transcription by adrenergic receptors

Activation of α₁ adrenergic receptors not only stimulates smooth muscle contraction but also modifies gene expression. We wondered if α₁ adrenergic receptors could activate transcription of genes regulated by the cAMP response element-binding protein (CREB). Using Rat1 cells stably transfected with each of the three cloned human α₁ adrenergic receptor subtypes, norepinephrine strongly stimulated CREB phosphorylation in α(1A) and α(1B) but more weakly in α(1D)-transfected cells. Norepinephrine increased the activity of a somatostatin cAMP-regulated enhancer- chloramphenicol acetyltransferase reporter in these cells. α₁ adrenergic receptors are known to activate protein kinase C (PKC) and increase [Ca²⁺ ](i). Nonetheless, neither GF109203X, a PKC inhibitor, nor BAPTA-AM, a calcium chelator, blocked phosphorylation of CREB induced by norepinephrine. In addition, α₁ adrenergic receptor-induced CREB phosphorylation was not mediated via the mitogen-activated protein kinase pathway because norepinephrine did not stimulate mitogen-activated protein kinase activity in these cells. Activation of α₁ adrenergic receptors increased cAMP accumulation in these cells. Norepinephrine-induced cAMP-regulated enhancer- chloramphenicol acetyltransferase activity was inhibited either by expression of the PKA inhibitory peptide or a dominant negative PKA regulatory subunit mutant. These results demonstrate that α₁ adrenergic receptors activate the transcription factor CREB by a PKA-dependent pathway.