THE CONSTITUTIVELY ACTIVE MUTANT G(ALPHA-13) TRANSFORMS MOUSE FIBROBLAST CELLS DEFICIENT IN INSULIN-LIKE GROWTH-FACTOR-I RECEPTOR

Insulin-like growth factor-I (IGF-I) receptor plays an important role in normal cell cycle progression and tumor growth, and it is thought t o be essential for cellular transformation, To test this hypothesis, w e stably transfected a GTPase-deficient mutant human G(alpha 13), whic h is highly oncogenic when overexpressed in vitro, into R-fibroblasts derived from IGF-I receptor-deficient mice, Northern blots of multiple clones revealed the expression of a 1.8-kilobase pair mutant G(alpha 13) transcript in transfected cells, in addition to the 6-kilobase pai r endogenous mRNA, The transfection resulted in a doubling of the expr ession of G(alpha 13) protein in these cells as assessed by Western bl ot analysis, The transforming ability of the mutant G(alpha 13) was te sted using the soft agar assay, Nontransfected R-cells cultured with 1 0% fetal bovine serum failed to form colonies after 3 weeks, Most of t he mutant G(alpha 13)-expressing clones formed significant numbers of colonies (11-50 colonies/1000 cells plated), Overexpression of the IGF -I receptor enabled R-cells to form colonies (27 colonies), and co tra nsfection of the mutant G(alpha 13) caused a further increase in colon y formation (117-153 colonies) in three of five clones analyzed, Appar ently G(alpha 13) works through pathways other than mitogen-activated protein kinase and c-Jun N-terminal kinase in transforming R-cells, be cause their activities were not significantly altered by the mutant G( alpha 13) expression, These results demonstrate that G(alpha 13) can i nduce cellular transformation through pathways apparently independent of the IGF-I receptor and that activation of the IGF-I receptor signal ing pathways, although not essential for the transforming phenotype, e nhances the effect of other pathways.