Activation and inactivation of cAMP-response element-mediated gene transcription in cardiac myocytes.

Chronic beta-adrenergic stimulation of the cAMP-dependent signalling pathway is implicated in functionally relevant expressional changes in congestive heart failure. We studied activation and inactivation of the cardiac gene transcription mediated by the cAMP-response element (CRE) and the CRE-binding protein (CREB) as an important mechanism of a cAMP-dependent gene regulation. We investigated the transcriptional activation by forskolin, an activator of the adenylyl cyclase, in chick embryonic cardiomyocytes transfected with a CRE-controlled luciferase construct in comparison to the phosphorylation and expression of CREB determined on immunoblots. Forskolin (10 micromol/l; 8 h) increased CRE-mediated transcription and phosphorylation of CREB 13- and 1.5-fold, respectively. The phosphorylation was further elevated in combination with cantharidin, an inhibitor of type 1+2A protein phosphatases. The transcriptional response to forskolin was desensitized by pretreatment with forskolin (1 micromol/l; 24 h) while CREB phosphorylation was increased. In forskolin-pretreated cells, total CREB protein levels were decreased. Cantharidin did not restore the attenuated transcriptional response. In cardiomyocytes, there is an activation of the CRE-mediated gene transcription by forskolin that is attenuated after prolonged stimulation, and this attenuation is not dependent from a dephosphorylation of CREB. We suggest that attenuation of the CRE-mediated transcription through chronic stimulation of the cAMP-pathway, e.g. by elevated catecholamines, contributes to the altered expressional regulation in congestive heart failure.