Cyclic AMP-dependent activation of the proenkephalin gene requires phosphorylation of CREB at serine-133 and a Src-related kinase

The transcription factor CREB [cyclic AMP response element (CRE)-binding pr otein] is activated by several kinase pathways on phosphorylation of serine -133. Phosphorylation of CREB at serine-133 is required for the induction o f target gene expression. The proenkephalin gene is a target of cyclic AMP- dependent agonists like forskolin, and its expression is driven by the enha ncer element CRE-2, It has been shown that CREB binds CRE-2 in extracts fro m striatum and hypothalamus, However, these studies did not show a function al requirement for CREB serine-133 phosphorylation in CRE-2 function. We de monstrate that CREB binds CRE-2 in primary astrocyte cultures and that tran scriptional activation of CRE-2 requires CREB phosphorylation at serine-133 . In addition, it has recently been shown that, at least in some contexts, CREB phosphorylation is not sufficient to activate target gene expression a nd that another intracellular signal seems to be required. Therefore, we al so sought to determine if another signaling event, in addition to CREB phos phorylation, might be involved in cyclic AMP-mediated induction of the proe nkephalin gene. We have found that the inhibition of src related nonrecepto r tyrosine kinases blocks forskolin-induced proenkephalin gene expression w ithout having any effect on serine-133-phosphorylated CREB revels and that constitutively activated src kinase can activate the proenkephalin promoter .