Histone deacetylase 6 (HDAC6) is a cytoplasmic HDAC that is unusual in having two adjacent catalytic domains. Kinetic data and X-ray crystallographic analyses of human and zebrafish HDAC6 enzymes provide insight into HDAC6 catalysis and its inhibition by small molecules. Histone deacetylase 6 (HDAC6) is a critical target for drug design because of its role in oncogenic transformation and cancer metastasis, and is unique among all histone deacetylases in that it contains tandem catalytic domains designated CD1 and CD2. We now report the crystal structures of CD2 from Homo sapiens HDAC6 and of CD1 and CD2 from Danio rerio HDAC6. We correlated these structures with activity measurements using 13 different substrates. The catalytic activity of CD2 from both species exhibited broad substrate specificity, whereas that of CD1 was highly specific for substrates bearing C-terminal acetyllysine residues. Crystal structures of substrate complexes yielded unprecedented snapshots of the catalytic mechanism. Additionally, crystal structures of complexes with eight different inhibitors, including belinostat and panobinostat (currently used in cancer chemotherapy), the macrocyclic tetrapeptide HC toxin, and the HDAC6-specific inhibitor N-hydroxy-4-(2-((2-hydroxyethyl)(phenyl)amino)-2-oxoethyl)benzamide, revealed surprising new insight regarding changes in Zn2+ coordination and isozyme-specific inhibition.

ARID1A, encoding a subunit of the SWI/SNF chromatin-remodelling complex, is the most frequently mutated epigenetic regulator across all human cancers. ARID1A and TP53 mutations are typically mutually exclusive. Therapeutic approaches that correlate with this genetic characteristic remain to be explored. Here, we show that HDAC6 activity is essential in ARID1A-mutated ovarian cancers. Inhibition of HDAC6 activity using a clinically applicable small-molecule inhibitor significantly improved the survival of mice bearing ARID1A-mutated tumours. This correlated with the suppression of growth and dissemination of ARID1A-mutated, but not wild-type, tumours. The dependence on HDAC6 activity in ARID1A-mutated cells correlated with a direct transcriptional repression of HDAC6 by ARID1A. HDAC6 inhibition selectively promoted apoptosis of ARID1A-mutated cells. HDAC6 directly deacetylates Lys120 of p53, a pro-apoptotic post-translational modification. Thus, ARID1A mutation inactivates the apoptosis-promoting function of p53 by upregulating HDAC6. Together, these results indicate that pharmacological inhibition of HDAC6 is a therapeutic strategy for ARID1A-mutated cancers.

Cationic polyamines such as spermidine and spermine are critical in all forms of life, as they regulate the function of biological macromolecules. Intracellular polyamine metabolism is regulated by reversible acetylation and dysregulated polyamine metabolism is associated with neoplastic diseases such as colon cancer, prostate cancer and neuroblastoma. Here we report that histone deacetylase 10 (HDAC10) is a robust polyamine deacetylase, using recombinant enzymes from Homo sapiens (human) and Danio rerio (zebrafish). The 2.85 Å-resolution crystal structure of zebrafish HDAC10 complexed with a transition-state analogue inhibitor reveals that a glutamate gatekeeper and a sterically constricted active site confer specificity for N⁸-acetylspermidine hydrolysis and disfavour acetyllysine hydrolysis. Both HDAC10 and spermidine are known to promote cellular survival through autophagy. Accordingly, this work sets a foundation for studying the chemical biology of autophagy through the structure-based design of inhibitors that may also serve as new leads for cancer chemotherapy.

A novel class of ZnSalens (ZnL₁1_–10–10) with lipophilic and cationic conjugates as optical probes in single and two-photon fluorescence microscopy images of living cells were prepared, which exhibited chemo- and photostability, low cytotoxicity and high subcellular selectivity.