Immunodetection of G proteins in human pituitary adenomas: evidence for a low expression of proteins of the Gi subfamily

G proteins mediate signal transduction in a variety of cell systems. As the expression of these proteins has not yet been investigated in detail in human pituitary tumors, we evaluated the presence of G proteins in a series of tumors including six non-functioning adenomas, five GH-secreting adenomas, three prolactinomas and one TSH-secreting adenoma, using immunoblotting and immunohistochemistry, By immunoblotting, Gi1/2 alpha was undetectable in six and barely detectable in nine tumors. A similar pattern of expression was observed by probing with the antibody to Gi3 alpha, which detected a very weak band in 11 tumors and no protein in four, In contrast, using large amounts of membrane proteins (40 mu g), both Gi1/2 alpha and Gi3 alpha were detected, although at very low levels, in the negative tumors. The low expression of these proteins appeared to be specific to tumoral tissues, as both Gi1/2 alpha and Gi3 alpha were abundant in normal human and rat pituitary. In all tumors, Go alpha, the two Gs alpha forms, Gq/11 and G beta were present in significant amounts. Semiquantitative analysis indicated that Gs alpha was clearly detected when 2.5 mu g loaded proteins were used, whereas Gi1/2 alpha and Gi3 alpha were barely detected with 5 mu g. By immunofluorescence, all tumors studied were markedly positive for Gs alpha that was immunolocalized at the cell periphery, whereas they showed a weak positivity for Gi1/2 alpha and Gi3 alpha. The study is the first to provide evidence for a low expression of Gi proteins, which are involved in the transduction of inhibitory signals, in pituitary adenomas.