Targeting Histone Deacetylase 6 and the Aggresome Pathway in Acute Lymphoblastic Leukemia Cells.

Acute lymphoblastic leukemia (ALL) is the most common form of childhood cancer. Despite effective chemotherapy, 20% of patients will relapse. Therefore, it is critical that we identify novel therapies to treat ALL. We are studying a new small molecule compound known as tubacin (tubulin acetylation inducer) that selectively inhibits histone deacetylase 6 (HDAC6). HDAC6 binds to polyubiquitinated misfolded proteins and to dynein motor proteins, including alpha-tubulin, thereby recruiting misfolded or unwanted proteins to aggresomes and subsequent degradation by the lysosome. Tubacin was discovered through a chemical genetic screen of a 7,392 small molecule library and was found to induce acetylation of alpha-tubulin by inhibiting one of the two catalytic domains of HDAC6. This inhibition disrupts the interaction of HDAC6 with dynein resulting in marked accumulation of ubiquitinated proteins. Previous work demonstrated that treatment of multiple myeloma cells with tubacin inhibited growth at low micromolar concentrations. Tubacin does not appear to affect global histone acetylation, gene expression, or cell cycle regulation. To determine the effects of tubacin in human ALL cells, we treated both B- and T-cell ALL cell lines with varying concentrations of the drug and performed MTT assays. In T-ALL cells (Jurkat, Loucy), the IC50 of tubacin was found to be 1 to 3 uM, while in B-cell ALL cells (REH, Nalm-6), the IC50 was 2 to 5uM. Tubacin induced apoptosis of Jurkat and Loucy cells stained with Annexin V and propidium iodide. Within 12 hours, we observed increased protein polyubiquitination and PARP cleavage, but no difference in Rb phosphorylation in cells treated with tubacin. Furthermore, tubacin treatment increased acetylation of alpha-tubulin in Loucy and Jurkat cells within 3 hours. To study whether tubacin was toxic to normal hematopoietic cells, we treated human bone marrow cells cultured in methylcellulose containing IL-3, IL-6, and Stem Cell Factor with varying concentrations of tubacin. The IC50 of 20uM was determined from numbers of colonies plated in triplicate. Similarly, treatment of normal human lymphocytes cultured in IL-2, demonstrated an IC50 of 16uM. Finally, we examined the effects of tubacin on growth of primary ALL cells. Bone marrow cells from three patients with B-cell ALL at diagnosis were cultured in tubacin at varying concentrations and MTT assays were performed. In two of the three ALL samples, the IC50 was less than 5uM. Experiments to study the effects of tubacin in mouse models of leukemia are in progress. Our results suggest that inhibition of HDAC6 and the aggresome pathway provides a novel approach to treat ALL.