Development of a histone deacetylase 6 inhibitor and its biological effects

Abstract

Development of isoform-selective histone deacetylase (HDAC) inhibitors is important in elucidating the function of individual HDAC enzymes and their potential as therapeutic agents. Among the eleven zinc-dependent HDACs in humans, HDAC6 is structurally and functionally unique. Here, we show that a hydroxamic acid-based small-molecule N-hydroxy-4-(2-[(2-hydroxyethyl)(phenyl)amino]-2-oxoethyl)benzamide (HPOB) selectively inhibits HDAC6 catalytic activity in vivo and in vitro. HPOB causes growth inhibition of normal and transformed cells but does not induce cell death. HPOB enhances the effectiveness of DNA-damaging anticancer drugs in transformed cells but not normal cells. HPOB does not block the ubiquitin-binding activity of HDAC6. The HDAC6-selective inhibitor HPOB has therapeutic potential in combination therapy to enhance the potency of anticancer drugs.anticancer agents | epigenetics-based chemotherapy | drug discovery H istone deacetylase 6 (HDAC6) is unique among the eleven zinc-dependent HDACs in humans. HDAC6 is located in the cytoplasm, and it has two catalytic domains and an ubiquitinbinding domain at the C-terminal region (1-3). This study focused on the development of a HDAC6-selective inhibitor and its biological effects. The substrates of HDAC6 include nonhistone proteins such as α-tubulin, peroxiredoxin (PRX), cortactin, and heat shock protein 90 (Hsp90) but not histones (4-7). HDAC6 plays a key role in the regulation of microtubule dynamics including cell migration and cell-cell interactions. The reversible acetylation of Hsp90, a substrate of HDAC6, modulates its chaperone activity and, accordingly, the stability of survival and antiapoptotic factors, including epidermal growth factor receptor (EGFR), protein kinase AKT, proto-oncogene C-RAF, survivin, and other factors. HDAC6, through its ubiquitin-binding activity and interaction with other partner proteins, plays a role in the degradation of misfolded proteins by binding polyubiquitinated proteins and delivering them to the dynein and motor proteins for transport into aggresomes which are degraded by lysosomes (8-10). Thus, HDAC6 has multiple biological functions through deacetylasedependent and -independent mechanisms modulating many cellular pathways relevant to normal and tumor cell growth, migration, and death. HDAC6 is an attractive target for potential cancer treatment.There are several previous reports on the development of HDAC6-selective inhibitors (11)(12)(13)(14)(15). The most extensively studied is tubacin (16,17). Tubacin has non-drug-like qualities, high lipophilicity, and difficult synthesis and has proved to be more useful as a research tool rather than as a potential drug (18). We and others (12)(13)(14)(15)19) have developed HDAC6-selective inhibitors whose pharmacokinetics, toxicity, and efficacy make them potentially more useful than tubacin as therapeutic agents. ACY-1215, 2-(Diphenylamino)-N-(7-(hydroxyamino)-7-oxoheptyl)pyrimidine-5-carboxamide, a HDAC6-selective inhibitor, is currently being evaluated in clinical trial...

Limited efficacy of specific HDAC6 inhibition in urothelial cancer cells

“…13 While confirming the activity of 17-AAG as such, we observed only minor synergistic effects of Tubacin and 17-AAG on UC cell lines, independent of their HDAC6 expression levels. Similarly, treatment with Tubacin did not enhance the sensitivity of UC cells to the DNA-damaging anticancer drugs doxorubicin and etoposide, unlike glioblastoma, prostate, mammary, and lung cancer cell lines 23,24 . The minor effects would not allow to reduce concentrations to a minimum that could be administered in the clinic.…”

“…Newly developed compound as such as HPOB or C1A are however reported to be selective for HDAC6 compared with HDAC10. 24,51 Subsequently, we compared the three compounds regarding their effects on the viability of urothelial cancer cell lines. In parallel with the data on the molecular effects, ST-80 appeared as the least potent compound with little efficacy even at high micromolar concentrations (100 μM).…”

“…As selective inhibition of HDAC6 has been reported to sensitize cancer cell lines from glioblastoma, breast, prostate, and lung cancer to treatment with DNA damaging compounds, 23,24 we investigated combined treatment of Tubacin with doxorubicin or etoposide compared with SAHA for synergistic effects on urothelial cancer cell lines. However, we did not observe any significant synergistic effects (Fig.…”

Rational design and diversity-oriented synthesis of peptoid-based selective HDAC6 inhibitors

“…3,4 HDAC6, a class IIb enzyme, is structurally and functionally unique among the eleven human zinc-dependent HDACs. 6 It is the only HDAC with two independent functional catalytic domains and a C-terminal zinc finger motif responsible for binding ubiquitinated proteins. 5b Interestingly, it has been demonstrated that the second catalytic site is the major functional domain of HDAC6.…”

Creation of a histone deacetylase 6 inhibitor and its biological effects

“…Histone deacetylase enzymatic activity was assayed as previously described (3). In vitro activities of the 11 recombinant human zinc-dependent HDAC enzymes (BPS Biosciences) were detected by fluorigenic release of 7-amino-4-methylcoumarin from substrate upon deacetylase enzymatic activity.…”