Tubacin, An Inhibitor of HDAC6, Induces Apoptosis of Acute Lymphoblastic Leukemia Cells in Vitro and in Vivo through a Na+/K+ATPase-Dependent Pathway.

Acute lymphoblastic leukemia (ALL) is the most common form of childhood cancer. Despite effective chemotherapy, 25 to 30% of children will relapse. In adults, less than 30% of patients with ALL are cured. Therefore, it is critical that we identify novel therapies to treat ALL. We are studying the effects of a small molecule compound known as tubacin (tubulin acetylation inducer) that selectively inhibits histone deacetylase 6 (HDAC6) resulting in increased acetylation of alpha-tubulin by inhibiting one of the two catalytic domains of HDAC6. We found that treatment of both pre-B and T-ALL cell lines with tubacin inhibits growth at very low micromolar concentrations (Jurkat IC50=1μM, Loucy IC50=3μM, REH IC50=2μM, Nalm6 IC50=5μM). We also determined that there is a therapeutic window, since tubacin inhibits the growth of normal bone marrow progenitor cells in methylcellulose colony assays at 20μM and normal human lymphocytes cultured in IL-2 at an IC50 of 16μM. We next tested the effects of tubacin in vivo. SCID mice injected with pre-B ALL Nalm-6 cells were treated with tubacin intraperitoneally at 50 mg/kg/day. Preliminary data using bioluminescence imaging in SCID mouse models showed that tubacin inhibited leukemic progression in vivo. To understand the mechanism of tubacin in ALL cells, we examined both apoptosis and cell cycle regulation by PARP cleavage, activation of caspases, and propidium iodide staining with FACs analysis. Tubacin induced apoptosis of pre-B and T-ALL cells within 12 hours of treatment. There was no effect on cell cycle progression, Retinoblastoma protein phosphorylation, or p21 upregulation, which have been observed with other HDAC inhibitors. Unlike in myeloma cells, tubacin did not increase JunK/SAPK activation or accumulation of acetylated HSP90 in ALL cells. Tubacin treatment resulted in accumulation of acetylated alpha-tubulin after 1 hour and an increase in polyubiquitinated proteins after 7 hours. To address potential mechanisms of tubacin in ALL, we tested whether Na+/K+ ATPase could be contributing to apoptosis. Previous work has shown that treatment with L-glutamate dissociates the Na+/K+ ATPase complex from acetylated tubulin and restores ATPase enzymatic activity. We hypothesized that the accumulation of acetylated tubulin could potentially inhibit the activity of the cytosolic Na/K ATPase pump, which could be reversed by treatment with 1mM sodium glutamate. Preliminary data demonstrate that we can partially rescue the effects of tubacin on PARP cleavage with sodium glutamate. These results suggest that tubacin induces apoptosis through a novel pathway in ALL cells and provide rationale for targeting the aggresome pathway to treat ALL in the future.