Histone deacetylase 6 (HDAC6) is a multidomain cytosolic hydrolase acting mostly on nonhistone protein substrates. Investigations of the substrate specificity of HDAC6 are confounded by the presence of 2 catalytically active deacetylase domains (DD1 and DD2). In this study, acetylome peptide microarrays and peptide libraries were used to map the substrate specificity of DD1 and DD2 of human HDAC6. The results show that DD1 is solely responsible for the deacetylation of substrates harboring the acetyllysine at their C terminus, whereas DD2 exclusively deacetylates peptides with an internal acetyllysine residue. Also, statistical analysis of the deacetylation data revealed amino acid preferences at individual positions flanking the acetyllysine, where glycine and arginine residues are favored at positions N‐terminal to the central acetyllysine; negatively charged glutamate is strongly disfavored throughout the sequence. Finally, the deacylation activity of HDAC6 was profiled by using a panel of acyl derivatives of the optimized peptide substrate and showed that HDAC6 acts as a proficient deformylase. Our data thus offer a detailed insight into the substrate preferences of the individual HDAC6 domains at the peptide level, and these findings can in turn help in elucidating the biologic roles of the enzyme and facilitate the development of new domain‐specific inhibitors as research tools or therapeutic agents.—Kutil, Z., Skultetyova, L., Rauh, D., Meleshin, M., Snajdr, I., Novakova, Z., Mikesova, J., Pavlicek, J., Hadzima, M., Baranova, P., Havlinova, B., Majer, P., Schutkowski, M., Barinka, C. The unraveling of substrate specificity of histone deacetylase 6 domains using acetylome peptide microarrays and peptide libraries. FASEB J. 33,4035–4045 (2019). http://www.fasebj.org

Histone deacetylase 6 (HDAC6) is a unique member of the HDAC family of enzymes due to its complex domain organization and cytosolic localization. Experimental data point toward the therapeutic use of HDAC6-selective inhibitors (HDAC6is) for use in both neurological and psychiatric disorders. In this article, we provide side-by-side comparisons of hydroxamate-based HDAC6is frequently used in the field and a novel HDAC6 inhibitor containing the difluoromethyl-1,3,4-oxadiazole function as an alternative zinc-binding group (compound 7). In vitro isotype selectivity screening uncovered HDAC10 as a primary off-target for the hydroxamate-based HDAC6is, while compound 7 features exquisite 10,000-fold selectivity over all other HDAC isoforms. Complementary cell-based assays using tubulin acetylation as a surrogate readout revealed approximately 100-fold lower apparent potency for all compounds. Finally, the limited selectivity of a number of these HDAC6is is shown to be linked to cytotoxicity in RPMI-8226 cells. Our results clearly show that off-target effects of HDAC6is must be considered before attributing observed physiological readouts solely to HDAC6 inhibition. Moreover, given their unparalleled specificity, the oxadiazole-based inhibitors would best be employed either as research tools in further probing HDAC6 biology or as leads in the development of truly HDAC6-specific compounds in the treatment of human disease states.

Bis(dimethylsulfido)decaborane, 6,9‐(Me2S)2‐arachno‐B10H12, reacts smoothly with ferrocenyl alkynes FcC≡CR [1a–h; Fc = ferrocenyl, R = H (1a), CH3 (1b), Ph (1c), 4‐MeO2CC6H4 (1d), Fc (1e), C≡CFc (1f), C(O)CH3 (1g), and CO2CH2CH3 (1h)] to afford the corresponding 1‐ferrocenyl‐1,2‐dicarba‐closo‐dodecaboranes 2a–h in good yields. Ester 2h was further reduced to the respective hydroxymethyl derivative, 1‐Fc‐2‐CH2OH‐1,2‐closo‐C2B10H10 (3). The reaction of 6,9‐(Me2S)2‐B10H12 with FcC≡CSiMe3 proceeded in a different manner to produce (among other products) an SMe2 adduct of an opened decaborane substituted with a 2‐ferrocenyl‐2‐(trimethylsilyl)ethen‐1‐yl group (4). This compound probably results through hydroboration of the starting alkyne and migration of the SiMe3 group. All prepared compounds were characterized by spectroscopic methods (1H, 13C, and 11B NMR spectroscopy, IR spectroscopy, and mass spectrometry), and their molecular structures were determined by single‐crystal X‐ray diffraction analysis. In addition, the compounds were studied by cyclic and differential pulse voltammetry on a platinum disc electrode to reveal simple ferrocenyl‐centered oxidations for the singly ferrocenylated carboranes and two consecutive oxidation waves for compounds 2e and 2f, which possess two ferrocenyl substituents.

Thebaine was converted to oripavine in three steps by employing two different modes of protection of the diene moiety; as an iron tricarbonyl complex and as a Diels-Alder adduct with thioformyl cyanide. The two C-ring-protected thebaine derivatives were subjected to 3-O-demethylation by four different protocols, providing oripavine derivatives, which yielded oripavine after deprotection. Oripavine was then converted to hydromorphone by a three-step process of ketalization, hydrogenation, and deprotection, without the isolation of intermediates.

Juvenile hormones (JHs) control insect metamorphosis and reproduction. JHs act through a receptor complex consisting of methoprene-tolerant (Met) and taiman (Tai) proteins to induce transcription of specific genes. Among chemically diverse synthetic JH mimics (juvenoids), some of which serve as insecticides, unique peptidic juvenoids stand out as being highly potent yet exquisitely selective to a specific family of true bugs. Their mode of action is unknown. Here we demonstrate that, like established JH receptor agonists, peptidic juvenoids act upon the JHR Met to halt metamorphosis in larvae of the linden bug, Pyrrhocoris apterus . Peptidic juvenoids induced ligand-dependent dimerization between Met and Tai proteins from P. apterus but, consistent with their selectivity, not from other insects. A cell-based split-luciferase system revealed that the Met–Tai complex assembled within minutes of agonist presence. To explore the potential of juvenoid peptides, we synthesized 120 new derivatives and tested them in Met–Tai interaction assays. While many substituents led to loss of activity, improved derivatives active at sub-nanomolar range outperformed hitherto existing peptidic and classical juvenoids including fenoxycarb. Their potency in inducing Met–Tai interaction corresponded with the capacity to block metamorphosis in P. apterus larvae and to stimulate oogenesis in reproductively arrested adult females. Molecular modeling demonstrated that the high potency correlates with high affinity. This is a result of malleability of the ligand-binding pocket of P. apterus Met that allows larger peptidic ligands to maximize their contact surface. Our data establish peptidic juvenoids as highly potent and species-selective novel JHR agonists.