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, Gi1/2alpha was undetectable in six and barely detectable in nine tumors. A similar pattern of expression was observed by probing with the antibody to Gi3alpha, which detected a very weak band in 11 tumors and no protein in four. In contrast, using large amounts of membrane proteins (40 microg), both Gi1/2alpha and Gi3alpha 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 Gi1/2alpha and Gi3alpha 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 microg loaded proteins were used, whereas Gi1/2alpha and Gi3alpha were barely detected with 5 microg. 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 Gi1/2alpha and Gi3alpha. 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.
Ballaré
E
E
Institute of Endocrine Sciences, Ospedale Maggiore, Milan, Italy.
Mantovani
S
S
Bassetti
M
M
Lania
A
A
Spada
A
A
D000236:Adenoma / Q000378:metabolism*; D000328:Adult; D000368:Aged; D005260:Female; D019084:Fluorescent Antibody Technique, Indirect; D019204:GTP-Binding Proteins / Q000378:metabolism*; D019382:Human Growth Hormone / Q000378:metabolism; D006801:Humans; D015151:Immunoblotting; D008297:Male; D008875:Middle Aged; D010911:Pituitary Neoplasms / Q000378:metabolism*; D011388:Prolactin / Q000378:metabolism; D013972:Thyrotropin / Q000378:metabolism
D019382:Human Growth Hormone; D011388:Prolactin; D013972:Thyrotropin; D019204:GTP-Binding Proteins