CELLULAR VARIATIONS IN HETEROTRIMERIC G-PROTEIN LOCALIZATION AND EXPRESSION IN RAT PITUITARY

The secretory cells of the anterior pituitary are regulated by hypothalamic and target endocrine hormones. In many cases, intracellular signaling after ligand binding to cell surface receptors is mediated by heterotrimeric G proteins. In this study, cells from the rat pituitary and two pituitary cell lines (GH(3) and AtT-20 cells) were doubly labeled by immunofluorescence with specific rabbit antibodies to the alpha-subunits of G proteins (G(s), G(i1-3) or G(q)) and monoclonal antibodies against pituitary hormones (GH, PRL, beta LH, beta TSH, or beta FSH) to identify specific cell types. G(s), G(i), and G(q) were detected in all secretory types, but differences were found in their levels of expression and cellular distribution. The cell periphery was the predominant site of localization of an these G proteins. In addition, both G(i3) and at least one member of the G(q) family were seen by immunofluorescence on intracellular sites typical of the Golgi region. By immunogold labeling, alpha(i3) was localized to membranes of Golgi cisternae in sections of rat pituitary and GH(3) cells, where it was concentrated on the cis-side of the Golgi stack. These findings demonstrate that G proteins are widely expressed in anterior pituitary cells and further support a role for G(i alpha 3,), and possibly G(a) in intracellular membrane trafficking.