Expression of G protein α subunits in normal rat colon and in azoxymethane-induced colonic neoplasms

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Background & Aims: Heterotrimeric G proteins are important in growth-regulating signal transduction. The aim of this study was to characterize the relative expression of G protein α subunits in rat colonocytes, colonocyte antipodal plasma membranes, and colonic neoplasms. Methods: Antipodal plasma membranes were prepared from isolated colonocytes. Azoxymethane was administered to rats to induce colonic neoplasms. K- ras mutations in the neoplasms were determined by oligonucleotide hybridization and confirmed by primer mediated–restriction fragment length polymorphism. Colonocyte and tumor homogenates or membranes were probed for Gα subunits by Western blotting with isoform-specific antibodies. Results: The expressions of Gαi2, αi3, and αq/11 were significantly enriched in the basolateral compared with brush border fraction of colonic antipodal plasma membranes. In neoplasms without K- ras mutations, the expression of Gαi2 increased 4-fold, Gαs(long) increased 2.5-fold, and Gαi3 increased 1.5–2-fold. Expression did not differ among tumor grades. K- ras mutations were associated with lowered expression of G proteins, especially Gαo. Conclusions: In colonocytes, Gα subunits are localized primarily in basolateral plasma membranes. The increased expressions of Gαi2 and, to a lesser degree, Gαi3 and Gαs(long) in tumors was independent of tumor grade but was modulated by the presence of K- ras mutations. GASTROENTEROLOGY 1998;115:1494-1503