Small-molecule inhibition of proteasome and aggresome function induces synergistic antitumor activity in multiple myeloma

Abstract

We have shown that the proteasome inhibitor bortezomib (formerly known as PS-341) triggers significant antitumor activity in multiple myeloma (MM) in both preclinical models and patients with relapsed refractory disease. Recent studies have shown that unfolded and misfolded ubiquitinated proteins are degraded not only by proteasomes, but also by aggresomes, dependent on histone deacetylase 6 (HDAC6) activity. We therefore hypothesized that inhibition of both mechanisms of protein catabolism could induce accumulation of ubiquitinated proteins followed by significant cell stress and cytotoxicity in MM cells. To prove this hypothesis, we used bortezomib and tubacin to inhibit the proteasome and HDAC6, respectively. Tubacin specifically triggers acetylation of ␣-tubulin as a result of HDAC6 inhibition in a dose-and time-dependent fashion. It induces cytotoxicity in MM cells at 72 h with an IC 50 of 5-20 M, which is mediated by caspase-dependent apoptosis; no toxicity is observed in normal peripheral blood mononuclear cells. Tubacin inhibits the interaction of HDAC6 with dynein and induces marked accumulation of ubiquitinated proteins. It synergistically augments bortezomib-induced cytotoxicity by c-Jun NH 2-terminal kinase͞caspase activation. Importantly, this combination also induces significant cytotoxicity in plasma cells isolated from MM patient bone marrow. Finally, adherence of MM cells to bone marrow stromal cells confers growth and resistance to conventional treatments; in contrast, the combination of tubacin and bortezomib triggers toxicity even in adherent MM cells. Our studies therefore demonstrate that tubacin combined with bortezomib mediates significant anti-MM activity, providing the framework for clinical evaluation of combined therapy to improve patient outcome in MM.histone deacetylase

Histone deacetylase inhibitors and the promise of epigenetic (and more) treatments for cancer

“…As discussed earlier, hyper-acetylation of cytoplasmic Ku70 causes the activation of BAX and apoptosis 97 . HDACi also cause hyper-acetylation of HSP90 and its inactivation, leading to the degradation of proteins that require the chaperone function of HSP90 (including some oncoproteins 98,99 100 . A limitation of these observations is their lack of systematic analysis, the use of cell lines as model systems -which do not necessarily reflect the biology of primary tumour cells -and the focus on a limited number of HDACi.…”

HDAC6, at the crossroads between cytoskeleton and cell signaling by acetylation and ubiquitination

“…The specific inhibition of HDAC6 catalytic activity was also shown to synergistically increase the cytotoxicity induced by the proteasome inhibitor bortezomib, therefore potentiating its anti-tumor activity in multiple myeloma (Hideshima et al, 2005). HDAC6 TDAC activity also contributes to the antiproliferative and anti-mitotic effect of a treatment combining farnesyltransferase inhibitor and taxanes, mainly in taxaneresistant cancer patients (Marcus et al, 2005(Marcus et al, , 2006).…”

Chemistry of acetyl transfer by histone modifying enzymes: structure, mechanism and implications for effector design

“…Taken together, although it may be possible to exploit the slight differences in the electrostatic properties and loop structures of the active site entrance of the different HDACs, it would appear that designing HDAC-specific inhibitors for the class I and II HDAC members might be particularly challenging. Although some HDAC-specific inhibitors have been identified, such as tubacin, the mode of HDAC specificity exploited by this inhibitor is unknown (Hideshima et al, 2005).…”