Osmotic stimuli attenuate vasoactive intestinal peptide gene expression in the rat anterior pituitary gland

Intercellular communication in the anterior pituitary

In addition to hypothalamic and feedback inputs, the secretory cells of the anterior pituitary are influenced by the activity of factors secreted within the gland. The list of putative intrapituitary factors has been expanding steadily over the past decade, although until recently much of the work was limited to descriptions of potential interactions. This took the form of evidence of production within the pituitary of factors already known to influence activity of secretory cells, or further descriptions of actions on pituitary cells by such factors when added exogenously. A new phase of discovery has been entered, with extensive efforts being made to delineate the control of the synthesis and secretion of the pituitary factors within the gland, regulation of the receptors and response mechanisms for the factors in pituitary cells, and measurements of the endogenous actions of the factors through the use of specific immunoneutralization, receptor blockade, tissue from transgenic animals, and other means. Taken together, these findings are producing blueprints of the intrapituitary interactions that influence each of the individual types of secretory cells, leading toward an understanding of the physiological significance of the interactions. The purpose of this article is to review the recent literature on many of the factors acting as intrapituitary signals and to present such finding in the context of the physiology of the secretory cells.

Transcription of the vasoactive intestinal peptide gene in response to glucocorticoids: differential regulation of alternative transcripts is modulated by a labile protein in rat anterior pituitary

Expression of the vasoactive intestinal peptide (VIP) gene is controlled by glucocorticoids in a tissue- and endocrine status-specific manner. We have investigated the molecular mechanisms that determine glucocorticoid regulation of VIP gene expression in the rat pituitary. In initial experiments, using explant cultures of rat pituitary glands, we have demonstrated that treatment with the glucocorticoid agonist dexamethasone leads to a marked increase in VIP mRNA levels. This effect was found to be selective for the larger of two alternatively polyadenylated VIP transcripts, and in addition, protein synthesis inhibitors markedly 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 demonstrated that the effects of dexamethasone in vitro are mediated largely, if not completely, at the level of transcription. In order to investigate the role of VIP promoter sequence in the glucocorticoid response, we then demonstrated that the activity of rat VIP gene promoter/reporter constructs in GH3 pituitary cells are up-regulated by dexamethasone. This up-regulation is virtually abolished following removal of promoter sequence between -162 and -89 of the start of transcription. Using an in vitro electrophoretic mobility shift assay, we have also demonstrated that this region of the promoter binds recombinant glucocorticoid receptor protein. The results of our study therefore indicate a direct 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 transcripts.