Crosstalk of CREB and Fos/Jun on a single cis-element: transcriptional repression of the steroidogenic acute regulatory protein gene.

Transcriptional regulation of the steroidogenic acute regulatory (StAR) protein gene by cAMP-dependent mechanisms occurs in the absence of a consensus cAMP-response element (CRE; TGACGTCA) and is mediated by several sequence-specific transcription factors. We previously identified three CRE-like sites (within the -151/-1 bp cAMP-responsive region of the mouse StAR gene), of which the CRE2 site overlaps with an activator protein-1 (AP-1) motif (TGACTGA, designated as CRE2/AP-1) that can bind both CRE and AP-1 DNA-binding proteins. The present studies were aimed at exploring the functional crosstalk between CREB (CRE-binding protein) and cFos/cJun (AP-1 family members) on the CRE2/AP-1 element and its role in regulating transcription of the StAR gene. Using MA-10 mouse Leydig tumor cells, we demonstrate that the CRE and AP-1 families of proteins interact with the CRE2/AP-1 sequence. CREB, cFos, and cJun proteins were found to bind to the CRE2/AP-1 motif but not the CRE1 and CRE3 sites. Treatment with the cAMP analog (Bu)(2)cAMP augmented phosphorylation of CREB (Ser(133)), cFos (Thr(325)), and cJun (ser(73)). Chromatin immunoprecipitation studies revealed that the induction of CREB, cFos, and cJun by (Bu)(2)cAMP was correlated with protein-DNA interactions and recruitment of the coactivator CREB-binding protein (CBP) to the StAR promoter. EMSA studies employing CREB and cFos/cJun proteins demonstrated competition between these factors for binding to the CRE2/AP-1 motif. Transfection of cells containing the -151/-1 StAR reporter with CREB and cFos/cJun resulted in trans-repression of the StAR gene, an event tightly associated with CBP, demonstrating that both CREB and Fos/Jun compete with each other for binding with limited amounts of intracellular CBP. Overexpression of adenovirus E1A, which binds and inactivates CBP, markedly suppressed StAR gene expression. Ectopic expression of CBP eliminated the repression of the StAR gene by E1A and potentiated the activity of CREB and cFos/cJun on StAR promoter responsiveness. These findings identify molecular events involved in crosstalk between CREB and cFos/cJun, which confer both gain and loss of function on a single cis-element in fine-tuning of the regulatory events involved in transcription of the StAR gene.