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

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 a 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 G alpha subunits by Western blotting with isoform-specific antibodies. Results: The expressions of G alpha i2, alpha i3, and alpha q/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 G alpha i2 increased 4-fold, G alpha s(long) increased 2.5-fold, and G alpha i3 increased 1.5-2-fold. Expression did not differ among tumor grades. K-ras mutations were associated with lowered expression of G proteins, especially G alpha o. Conclusions: In colonocytes, G alpha subunits are localized primarily in basolateral plasma membranes. The increased expressions of G alpha i2 and, to a lesser degree, G alpha i3 and G alpha s(long) in tumors was independent of tumor grade but was modulated by the presence of K-ras mutations.