Impaired cyclic AMP-dependent phosphorylation renders CREB a repressor of C/EBP-induced transcription of the somatostatin gene in an insulinoma cell line.
Transcription factor CREB regulates cyclic AMP (cAMP)-dependent gene expression by binding to and activating transcription from cAMP response elements (CREs) in the promoters of target genes. The transcriptional transactivation functions of CREB are activated by its phosphorylation by cAMP-dependent protein kinase A (PKA). In studies of many different phenotypically distinct cells, the CRE of the somatostatin gene promoter is a prototype of a highly cAMP-responsive element regulated by CREB. We now report on a somatostatin-producing rat insulinoma cell line, RIN-1027-B2, in which transcription from the somatostatin gene promoter is paradoxically repressed by CREB. We find that CREB fails to transactivate a CRE-containing somatostatin-chloramphenicol acetyltransferase reporter even when coexpressed with the catalytic subunit of PKA. CAAT box/enhancer-binding protein beta (C/EBP beta) and C/EBP-related activating transcription factor bind to the CRE in the promoter of the somatostatin gene and transactivate transcription. CREB binds competitively with C/EBP beta to the somatostatin CRE in vitro and represses C/EBP beta-induced transcription of the CRE-containing somatostatin-chloramphenicol acetyltransferase reporter. The lack of CREB-mediated transcriptional stimulation is due to the presence of a heat-stable inhibitor of PKA that prevents activation of PKA and subsequent CREB phosphorylation in the nucleus. These findings indicate that dephosphorylated CREB is a negative regulator of C/EBP-activated transcription of the somatostatin gene promoter in RIN-1027-B2 cells.
Vallejo
M
M
Laboratory of Molecular Endocrinology, Massachusetts General Hospital, Howard Hughes Medical Institute, Boston 02114.
Gosse
M E
ME
Beckman
W
W
Habener
J F
JF
D051701:Activating Transcription Factor 4; D001483:Base Sequence; D022762:CCAAT-Enhancer-Binding Proteins; D000242:Cyclic AMP / Q000502:physiology; D017362:Cyclic AMP Response Element-Binding Protein / Q000378:metabolism*; D017868:Cyclic AMP-Dependent Protein Kinases / Q000378:metabolism*; D017931:DNA Primers / Q000737:chemistry; D004268:DNA-Binding Proteins / Q000378:metabolism*; D004742:Enhancer Elements, Genetic; D066298:In Vitro Techniques; D007340:Insulinoma; D008969:Molecular Sequence Data; D009687:Nuclear Proteins / Q000378:metabolism*; D010766:Phosphorylation; D012097:Repressor Proteins / Q000378:metabolism*; D015398:Signal Transduction; D013004:Somatostatin / Q000235:genetics*; D015534:Trans-Activators / Q000378:metabolism; D014407:Tumor Cells, Cultured
C081452:Atf4 protein, mouse; D022762:CCAAT-Enhancer-Binding Proteins; D017362:Cyclic AMP Response Element-Binding Protein; D017931:DNA Primers; D004268:DNA-Binding Proteins; D009687:Nuclear Proteins; D012097:Repressor Proteins; D015534:Trans-Activators; D051701:Activating Transcription Factor 4; D013004:Somatostatin; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases