A differential role of CREB phosphorylation in cAMP-inducible gene expression in the rat pineal

In the rat pineal gland cAMP mediates nocturnal induction of the enzyme arylalkylamine N-acetyltransferase (AA-NAT) as well as of transcription factors such as inducible cAMP early repressor (ICER), Fos-related antigen-2 (Fra-2) and JunB. Cyclic AMP stimulates the phosphorylation of the DNA binding protein cAMP response element binding protein (CREB). While cAMP-induced CREB phosphorylation appears to be a prerequisite for AA-NAT and ICER gene expression, it is not known whether CREB phosphorylation accounts for the full cAMP response of the two genes. Furthermore, the significance of CREB phosphorylation in cAMP-activated Fra-2 and JunB transcription is unknown. In the present in vitro study we used the serine/threonine protein phosphatase inhibitor okadaic acid (OA) to phosphorylate CREB without altering intrapineal cAMP concentration. It was observed that OA (10(-7) M) was less effective than dibutyryl cAMP (dbcAMP; 10(-3) M) in inducing AA-NAT mRNA and ICER mRNA, respectively. On the basis of this finding, it is concluded that CREB phosphorylation alone is apparently not sufficient for the full cAMP response of the two genes. By contrast, OA and dbcAMP equally stimulated the accumulation of the mRNAs of Fra-2 and JunB. Therefore cAMP may induce Fra-2 and JunB transcripts via CREB phosphorylation. Our observations suggest that CREB phosphorylation plays a critical role in diversification of cAMP-dependent gene induction in the rat pineal.