Inhibition of histone deacetylase 6 as a therapeutic strategy for acute lymphocytic leukemia

There is emerging interest in targeting histone deacetylase 6 (HDAC6) as a therapeutic approach to hematological and non-hematological malignancies. HDAC6 has a number of substrates in addition to histones, and many of these approaches are based on inhibiting deacetylation of such non-histone substrates. In various experimental models, HDAC6-inhibition induced pleiotropic effects, including a selective decrease in proliferation and a relative increase in apoptosis in malignant cells as compared to non-transformed cells [1–3]. Such effects of HDAC6-inhibition were shown to be independent of histone acetylation state, but were dependent upon acetylation of a-tubulin, one of the alternative HDAC6 substrates. HDAC6 is a class II histone deacetylase with two putative catalytic domains. The C-terminal domain is responsible for deacetylation of a-tubulin, while the second domain functions as an acetyl lysine binding site to facilitate interaction with the C-terminal domain [4]. Tubulin deacetylation is involved in a number of cellular processes which are essential for cell proliferation and homeostasis. For example, deacetylation of a-tubulin plays an important role in degradation of unfolded and ubiquitinated proteins that are stored in microtubule associated aggresomes. Interaction of a-tubulin with HDAC6 results in a-tubulin deacetylation and assembly of a multi-protein complex which includes the motor protein dynein and the aggresome. After assembly, this complex transports the aggresome to lysosomes or proteasomes for protein degradation (reviewed in [5]). De-acetylation of a-tubulin is also necessary for the assembly of microtubule complexes involved in mitotic cell division. Therefore the acetylation status of a-tubulin influences cell proliferation [5]. In addition, a-tubulin acetylation state influences the activity of some proteins, such as the KþNaþadenosine triphosphatase (ATPase), which are activated by association with the cytoskeleton in an atubulin dependent manner [6]. HDAC6 has other substrates that may also impact the anti-neoplastic role of HDAC6 inhibition, including the chaperone protein Hsp90 and redox-related peroxiredoxin proteins. Tubacin is a HDAC6-specific inhibitor that was identified by screening a chemical library of 1,3dioxane derivatives for molecules which increase atubulin acetylation, but not histone acetylation [4]. Subsequent work demonstrated that tubacin treatment results in rapid a-tubulin acetylation in multiple cell types, but does not alter histone acetylation or gene expression [4]. Based on the functions of acetylated a-tubulin, it was anticipated that tubacin would impair the degradation of aggresome proteins thereby causing stress-induced apoptosis; decrease microtubule function during mitosis thereby impairing proliferation; and inhibit KþNaþ-ATPase activity thereby disrupting intercellular ion handling. Consistent with the anti-neoplastic potential of tubacininduced HDAC6-inhibition, tubacin was found to synergize with proteasome inhibitors to increase apoptosis in multiple myeloma cells and prostate cancer cell lines [1,2]. Apoptosis in these cells