Inhibition of Apoptosome Formation by Suppression of Hsp90β Phosphorylation in Tyrosine Kinase-Induced Leukemias

ABSTRACT Constitutively active tyrosine kinases promote leukemogenesis by increasing cell proliferation and inhibiting apoptosis. However, mechanisms underlying apoptotic inhibition have not been fully elucidated. In many settings, apoptosis occurs by mitochondrial cytochrome c release, which nucleates the Apaf-1/caspase-9 apoptosome. Here we report that the leukemogenic kinases, Bcr-Abl, FLT3/D835Y, and Tel-PDGFRβ, all can inhibit apoptosome function. In cells expressing these kinases, the previously reported apoptosome inhibitor, Hsp90β, bound strongly to Apaf-1, preventing cytochrome c-induced Apaf-1 oligomerization and caspase-9 recruitment. Hsp90β interacted weakly with the apoptosome in untransformed cells. While Hsp90β was phosphorylated at Ser 226/Ser 255 in untransformed cells, phosphorylation was absent in leukemic cells. Expression of mutant Hsp90β (S226A/S255A), which mimics the hypophosphorylated form in leukemic cells, conferred resistance to cytochrome c-induced apoptosome activation in normal cells, reflecting enhanced binding of nonphosphorylatable Hsp90β to Apaf-1. In Bcr-Abl-positive mouse bone marrow cells, nonphosphorylatable Hsp90β expression conferred imatinib (Gleevec) resistance. These data provide an explanation for apoptosome inhibition by activated leukemogenic tyrosine kinases and suggest that alterations in Hsp90β-apoptosome interactions may contribute to chemoresistance in leukemias.