Corticotropin-releasing hormone stimulates proopiomelanocortin transcription by cFos-dependent and -independent pathways: characterization of an AP1 site in exon 1.

Abstract

The POMC gene, encoding a hormonal precursor protein, is primarily expressed in the pituitary in a tissue-specific manner. The POMC gene is transcriptionally regulated by a variety of hormones and neuropeptides and the second messengers cAMP and Ca++. Using the corticotrope-derived AtT20 cell line, we have previously shown that overexpression of cFos stimulates POMC transcription. The aim of this work was to analyze whether cFos directly interacts with the POMC gene in basal and corticotropin-releasing hormone (CRH) stimulated cells. Using progressively deleted POMC promoter sequences or heterologous promoter constructs coupled to the chloramphenicol acetyl transferase reporter gene, we demonstrate the existence of a major cFos- responsive sequence within the first exon of the POMC gene. This sequence, TGACTAA, appears functionally indistinguishable from the canonical AP1 binding site. When fused to a minimal promoter, this sequence confers inducibility by cFos and CRH. Gel shift analyses with CRH-stimulated AtT20 nuclear extracts or in vitro synthesized proteins revealed that this sequence efficiently binds Fos and Jun. Expression of c-fos anti-sense mRNA reduced CRH-stimulated POMC transcription, thus indicating that, at least in part, cFos mediates the effect of CRH on POMC transcription. However, deletion of this major exonic AP1 site from the POMC constructs greatly reduced the effect of c-fos overexpression but did not suppress POMC stimulation by CRH, indicating that CRH stimulates POMC transcription by one or more cFos-independent mechanism(s).

Corticotropin-Releasing Factor and the Urocortins Induce the Expression of TLR4 in Macrophages via Activation of the Transcription Factors PU.1 and AP-1

“…CRF has been reported to activate the AP-1 transcriptional complex in keratinocytes and the AtT-20 corticotrope-derived cell line (29,31). The composition of AP-1 triggered by CRF appears to be a complex phenomenon consisting of alternation of c-Fos, JunB, Fra2, and JunD proteins (30).…”

Corticotropin-releasing hormone or dexamethasone regulates rat proopiomelanocortin transcription through Tpit/Pitx-responsive element in its promoter

“…Hypothalamic corticotropin-releasing hormone (CRH) stimulates transcription of POMC and secretion of ACTH in vivo (Bruhn et al 1984), in cultured anterior pituitary cells (Loeffler et al 1986, Gagner & Drouin 1987 and ACTHproducing pituitary tumor AtT-20 cells , Boutillier et al 1995. CRH binds to its specific receptor, CRH-R1 (Vita et al 1993, Timpl et al 1998, stimulating the accumulation of intracellular cAMP levels (Giguere et al 1982, Aguilera et al 1983, Litvin et al 1984, Vita et al 1993, Grammatopoulos & Chrousos 2002, which is followed by the activation of protein kinase A (PKA) in corticotrophs , Kovalovsky et al 2002.…”

60 YEARS OF POMC: Transcriptional and epigenetic regulation of POMC gene expression

“…Indeed, CRH action on its receptor leads to activation of the cAMP/PKA pathway and of the MAPK pathway (Kovalovsky et al 2002, Maira et al 2003a, the pituitary corticotrophs being one of the unusual tissues where these two pathways are positively linked rather than antagonizing each other. CRH signaling has been associated with activation of AP1 TFs constituted of heterodimers between jun and fos and putative targets of AP1 were identified on the POMC promoter (Boutillier et al 1991(Boutillier et al , 1995. Growth and serum stimulation also activates these TFs and their role in hormonal control of POMC transcription by CRH remains ill defined.…”