An inhibitor-resistant mutant of Hck protects CML cells against the antiproliferative and apoptotic effects of the broad-spectrum Src family kinase inhibitor A-419259

Chronic Myelogenous Leukemia (CML) is driven by Bcr-Abl, a constitutively active protein-tyrosine kinase that stimulates proliferation and survival of myeloid progenitors. Global inhibition of myeloid Src family kinase (SFK) activity with the broad-spectrum pyrrolo-pyrimidine inhibitor, A-419259, blocks proliferation and induces Apoptosis in CML cell lines, suggesting that transformation by Bcr-Abl requires SFK activity. However, the contribution of HCK and other individual SFKs to Bcr-Abl signaling is less clear. Here, we developed an A-419259-resistant mutant of HCK by replacing the gatekeeper residue (Thr-338; c-Src numbering) in the inhibitor-binding site with a bulkier methionine residue (Hck-T338M). This substitution reduced HCK sensitivity to A-419259 by more than 30-fold without significantly affecting kinase activity in vitro. Expression of Hck-T338M protected K-562 CML cells and Bcr-Abl-transformed TF-1 myeloid cells from the apoptotic and antiproliferative effects of A-419259. These effects correlated with persistence of Hck-T338M kinase activity in the presence of the compound, and were accompanied by sustained ERK and STAT5 activation. In contrast, control cells expressing equivalent levels of wild-type HCK retained sensitivity to the inhibitor. We also show for the first time that A-419259 induces cell-cycle arrest and Apoptosis in primary CD34(+) CML cells with equal potency to imatinib. These data suggest that HCK has a nonredundant function as a key downstream signaling partner for Bcr-Abl and may represent a potential drug target in CML.