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

Background and 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 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, Gas(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.