Negative regulation of p120GAP GTPase promoting activity by p210^bcr/abl: Implication for RAS-dependent Philadelphia chromosome positive cell growth

The p210^bcr/abl tyrosine kinase appears to be responsible for initiating and maintaining the leukemic phenotype in chronic myelogenous leukemia (CML) patients. p21^ras-p120GAP interactions play a central role in transducing mitogenic signals. Therefore, we investigated whether p21^ras and p120GAP are regulated by p210^bcr/abl, and whether this activation is functionally significant for CML cell proliferation. We report that transient expression of p210^bcr/abl in fibroblast-like cells induces simultaneous activation of p21^ras and inhibition of GTPase-promoting activity of p120GAP, and confirm these data showing that downregulation of p210^bcr/abl expression in CML cells with bcr/abl antisense oligodeoxynucleotides induces both inhibition of p21^ras activation and stimulation of GTPase-promoting activity of p120GAP. Tyrosine phosphorylation of two p120GAP-associated proteins, p190 and p62, which may affect p120GAP activity, also depends on p210^bcr/abl tyrosine kinase expression. Direct dependence of these effects on the kinase activity is proven in experiments in which expression of c-MYB protein in fibroblast-like cells or downregulation of c-MYB expression resulting in analogous inhibition of CML cell proliferation does not result in the same changes. Use of specific antisense oligodeoxynucleotides to downregulate p21^ras expression revealed a requirement for functional p21^ras in the proliferation of Philadelphia chromosome-positive CML primary cells. Thus, the p210^brc/abl-dependent regulation of p120GAP activity is responsible, in part, for the maintenance of p21^ras in the active GTP-bound form, a crucial requirement for CML cell proliferation.