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

The p210 tyrosine kinase appears to be responsible for initiating and maintaining the leukemic phenotype in chronic myelogenous leukemia (CML) patients. p21-p120GAP interactions play a central role in transducing mitogenic signals. Therefore, we investigated whether p21 and p120GAP are regulated by p210, and whether this activation is functionally significant for CML cell proliferation. We report that transient expression of p210 in fibroblast-like cells induces simultaneous activation of p21 and inhibition of GTPase-promoting activity of p120GAP, and confirm these data showing that downregulation of p210 expression in CML cells with bcr/abl antisense oligodeoxynucleotides induces both inhibition of p21 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 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 expression revealed a requirement for functional p21 in the proliferation of Philadelphia chromosome-positive CML primary cells. Thus, the p210-dependent regulation of p120GAP activity is responsible, in part, for the maintenance of p21 in the active GTP-bound form, a crucial requirement for CML cell proliferation.