c-Abl is an effector of Src for growth factor-induced c-myc expression and DNA synthesis

The mechanism by which the ubiquitously expressed Src family kinases regulate mitogenesis is not well understood. Here we report that cytoplasmic tyrosine kinase c-Abl is an important effector of c-Src for PDGF- and serum-induced DNA synthesis. Inactivation of cytoplasmic c-Abl by the kinase- inactive Abl-PP-K(–) (AblP242E/P249E/K290M) or by microinjection of Abl neutralizing antibodies inhibited mitogenesis. The kinase-inactive SrcK295M induced a G(1) block that was overcome by the constitutively active Abl-PP (AblP242E/P249E). Conversely, the inhibitory effect of Abl-PP-K(–) was not compensated by Src. c-Src-induced c-Abl activation involves phosphorylation of Y245 and Y412, two residues required for c-Abl mitogenic function. Finally, we found that p53 inactivation and c-myc expression, two cell cycle events regulated by Src during mitogenesis, also implied c-Abl: c-Abl function was dispensable in cells deficient in active p53 and inhibition of c-Abl reduced mitogen-induced c-myc expression. These data identify a novel function of cytoplasmic c-Abl in the signalling pathways regulating growth factor-induced c-myc expression and we propose the existence of a tyro sine kinase signalling cascade (PDGFR/c-Src/c-Abl) important for mitogenesis.