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. Gil/2α was undetectable in six and barely detectable in nine tumors. A similar pattern of expression was observed by probing with the antibody to Gi3α, which detected a very weak band in 11 tumors and no protein in four. In contrast, using large amounts of membrane proteins (4() μg), both Gil/2α and Gi3α 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 Gi1/2α and Gi3α were abundant in normal human and rat pituitary. In all tumors, Goα, the two Gsα forms. Gq/11 and Gβ were present in significant amounts. Semiquantitative analysis indicated that Gsα was clearly detected when 2.5 μg loaded proteins were used, whereas Gi1/2α and Gi3α were barely detected with 5 μg. By immunofluorescence, all tumors studied were markedly positive for Gsα that was immunolocalized at the cell periphery, whereas they showed a weak positivity for Gil/2α and Gi3α. 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.