Histone deacetylase inhibitors induce proteolysis of activated CDC42-associated kinase-1 in leukemic cells.

PURPOSE: Activated CDC42-associated kinase-1 (ACK1/TNK2) and epigenetic regulators of the histone deacetylase (HDAC) family regulate the proliferation and survival of leukemic cells. 18 HDACs fall into four classes (I-IV). We tested the impact of clinically relevant histone deacetylase inhibitors (HDACi) on ACK1 and if such drugs combine favorably with the therapeutically used ACK1 inhibitor Dasatinib.
METHODS: We applied the broad-range HDACi Panobinostat/LBH589 and the class I HDAC-specific inhibitor Entinostat/MS-275 to various acute and chronic myeloid leukemia cells (AML/CML). We also used the replicative stress inducer Hydroxyurea (HU), a standard drug for leukemic patients, and the apoptosis inducer Staurosporine (STS). To assess cytotoxic effects of HDACi, we measured cell cycle profiles and DNA fragmentation by flow cytometry. Western blot was employed to analyze protein expression and phosphorylation.
RESULTS: LBH589 and MS-275 induce proteolysis of ACK1 in CML and AML cells. Panobinostat more strongly induces apoptosis than Entinostat, and this correlates with a significantly pronounced loss of ACK1. STS and HU also propel the degradation of ACK1 in leukemic cells. Moreover, the caspase inhibitor z-VAD-FMK reduces ACK1 degradation in the presence of HDACi. Concomitant with the attenuation of ACK1, we noticed decreased phosphorylation of STAT3. Direct inhibition of ACK1 with Dasatinib also suppresses STAT3 phosphorylation. Furthermore, Dasatinib and HDACi combinations are effective against CML cells.
CONCLUSION: HDACs sustain the ACK1-STAT3 signaling node and leukemic cell growth. Consistent with their different effects on ACK1 stability or auto-phosphorylation, Dasatinib and HDACi combinations produce beneficial antileukemic effects.