Differential requirement for Rho family GTPases in an oncogenic insulin-like growth factor-I receptor-induced cell transformation

Insulin-like growth factor I receptor (IGFR) plays an important role in cel l growth and transformation. We dissected the downstream signaling pathways of an oncogenic variant of IGFR, Gag-IGFR, called NM1, Loss of function mu tants of NM1, Phe-1136 and dS2, that retain kinase activity but are attenua ted in their transforming ability were used to identify signaling pathways that are important for transformation of NIH 3T3 cells. MAPK, phospholipase C gamma, and Stat3 were activated to the same extent by NM1 and its two mu tants, suggesting that activation of these pathways, individually or in com bination, was not sufficient for NM1-induced cell transformation. The mutan t dS2 has decreased IRS-1 phosphorylation levels and IRS-l-associated phosp hatidylinositol 3 ' -kinase activity, suggesting that this impairment may b e in part responsible for the defectiveness of dS2, We show that Rho family members, RhoA, Rad, and Cdc42 are activated by NM1, and this activation, p articularly RhoA and Cdc42, is attenuated in both mutants of NM1, Dominant negative mutants of Rho, Rac, and Cdc42 inhibited NM1-induced cell transfor mation, as measured by focus and colony forming ability. Dominant negative Rho most potently inhibited the focus forming activity, whereas Cdc42 was m ost effective in inhibiting the colony forming ability of NM1-expressing ce lls. Conversely, constitutively activated (ca) Rho is more effective than c a Rac or ca Cdc42 in rescuing the focus forming ability of the mutants. By contrast, ca Cdc42 is most effective in rescuing the colony forming ability of both mutants.