TRANSCRIPTION OF THE VASOACTIVE-INTESTINAL-PEPTIDE GENE IN RESPONSE TO GLUCOCORTICOIDS - DIFFERENTIAL REGULATION OF ALTERNATIVE TRANSCRIPTSIS MODULATED BY A LABILE PROTEIN IN RAT ANTERIOR-PITUITARY

Expression of the vasoactive intestinal peptide (VIP) gene is controle d by glucocorticoids in a tissue- and endocrine status-specific manner . We have investigated the molecular mechanisms that determine glucoco rticoid regulation of VIP gene expression in the rat pituitary. In ini tial experiments, using explant cultures of rat pituitary glands, we h ave demonstrated that treatment with the glucocorticoid agonist dexame thasone leads to a marked increase in VIP mRNA levels. This effect was found to be selective for the larger of two alternatively polyadenyla ted VIP transcripts, and in addition, protein synthesis inhibitors mar kedly enhanced the magnitude of this response indicating that a labile pituitary protein acts to attenuate the transcript-selective response to glucocorticoids. Nuclear run-on analysis of transcription demonstr ated that the effects of dexamethasone in vitro are mediated largely, if not completely, at the level of transcription. In order to investig ate the role of VIP promoter sequence in the glucocorticoid response, we then demonstrated that the activity of rat VIP gene promoter/report er constructs in GH3 pituitary cells are up-regulated by dexamethasone . This up-regulation is virtually abolished following removal of promo ter sequence between - 162 and - 89 of the start of transcription. Usi ng an in vitro electrophoretic mobility shift assay, we have also demo nstrated that this region of the promoter binds recombinant glucocorti coid receptor protein. The results of our Study therefore indicate a d irect mechanism of action for the modulation of VIP gene expression by glucocorticoids, and furthermore provide evidence of a mechanism that permits selective glucocorticoid regulation of alternative VIP transc ripts. (C) 1997 Elsevier Science Ireland Ltd.