Regulation of somatostatin gene transcription by cyclic adenosine monophosphate☆

Abstract 

Cyclic adenosine monophosphate (cAMP) stimulates transcription of somatostatin and other target genes with burstattenuation kinetics. The kinetics of protein kinase (PK-A)—dependent cAMP response element binding protein (CREB) phosphorylation closely parallel the changes in transcription of cAMP-responsive genes by run-on assay. Nuclear translocation of PK-A, visualized by microinjection of fluorescently labeled PK-A holoenzyme, appears to represent the rate-limiting step in CREB phosphorylation and transcriptional activation. We and others have recently characterized a CREB-binding protein (CBP), which specifically recognizes sequences within the Ser133 phosphorylated form of CREB. CBP does not regulate the DNA binding, dimerization, or nuclear targeting properties of CREB, but binds selectively to the kinase-inducible 60 amino acid trans-activation domain (KID) of CREB, critical for PK-A—inducible transcription. We developed an antiserum directed against amino acid 634–648 within the CREB-binding domain of CBP. We detected a 265-kd polypeptide by Western blot as predicted from the cDNA, which coincided with the predominant phospho-CREB—binding activity in Hela nuclear extracts by “Far Western” blot assay. An identical phospho-CREB—binding activity was also found in NIH-3T3 cells. This phospho-CREB—binding protein appeared to be specific for Ser133-phosphorylated CREB, because no such band was detected with CREB labeled to the same specific activity at a nonregulatory phosphoacceptor site (Ser156) by casein kinase II (CKII). Following microinjection into nuclei of NIH-3T3 cells, a cAMP response element (CRE)-lacZ reporter was markedly induced by treatment with 8-Br cAMP plus isobutyl methyl xanthine (IBMX). Coinjection of CBP antiserum with the CRE-lacZ plasmid inhibited cAMP-dependent activity in a dose-dependent manner, but control immunoglobulin G (IgG) had no effect on this response. We can now begin reconstituting PK-A—dependent transcription in vitro, using well-characterized proteins such as CREB, TAF 110, and CBP. The assembly of such factors on cAMP-regulated promoters like somatostatin may enable responsiveness to a variety of hormonal stimuli that employ cAMP as their second messenger.

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