No abstract

Enzymes from the histone deacetylase (HDAC) family are highly regulated by different mechanisms. However, only very limited knowledge exists about the regulation of HDAC8, an established target in multiple types of cancer. A previous dedicated study of HDAC class I enzymes identified no redox-sensitive cysteinyl thiol in HDAC8. This is in contrast to the observation that HDAC8 preparations show different enzyme activities depending on the addition of reducing agents. In the light of the importance of HDAC8 in tumorigenesis a possible regulation by redox signaling was investigated using biochemical and biophysical methods combined with site directed mutagenesis. The occurrence of a characteristic disulfide bond under oxidizing conditions is associated with a complete but reversible loss of enzyme activity. Cysteines 102 and 153 are the integral components of the redox-switch. A possible regulation of HDAC8 by redox signal transduction is suggested by the observed relationship between inhibition of reactive oxygen species generating NOX and concomitant increased HDAC8 activity in neuroblastoma tumor cells. The slow kinetics for direct oxidation of HDAC8 by hydrogen peroxide suggests that transmitters of oxidative equivalents are required to transfer the H2O2 signal to HDAC8.

Epigenetics plays a fundamental role in cancer progression, and developing agents that regulate epigenetics is crucial for cancer management. Among Class I and Class II HDACs, HDAC8 is one of the essential epigenetic players in cancer progression. Therefore, we designed, synthesized, purified, and structurally characterized novel compounds containing N-substituted TZD (P1-P25). Cell viability assay of all compounds on leukemic cell lines (CEM, K-562, and KCL22) showed the cytotoxic potential of P8, P9, P10, P12, P19, and P25. In-vitro screening of different HDACs isoforms revealed that P19 was the most potent and selective inhibitor for HDAC8 (IC 50-9.3 μM). Thermal shift analysis (TSA) confirmed the binding of P19 to HDAC8. In-vitro screening of all compounds on the transport activity of GLUT1, GLUT4, and GLUT5 indicated that P19 inhibited GLUT1 (IC 50-28.2 μM). P10 and P19 induced apoptotic cell death in CEM cells (55.19% and 60.97% respectively) and P19 was less cytotoxic on normal WBCs (CC 50-104.2 μM) and human fibroblasts (HS27) (CC 50-105.0 μM). Thus, among this novel series of TZD *

Human histoned eacetylase 8i saw ell-recognized target forT-cell lymphomaa nd particularly childhoodn euroblastoma. PD-404,182 was shown to be as elective covalent inhibitor of HDAC8t hat formsm ixed disulfides with several cysteine residues and is also able to transform thiol groups to thiocyanates. Moreover,H DAC8 was shown to be regulated by ar edox switch based on the reversible formation of a disulfideb ond between cysteines Cys 102 andC ys 153 .T his study on the distinct effects of PD-404,182 on HDAC8 reveals that this compound induces the dose-dependent formationo fi ntramolecular disulfide bridges. Therefore,t he inhibition mechanism of HDAC8b yP D-404,182 involves both, covalentmodification of thiols as well as ligand mediated disulfide formation. Moreover,t his study provides ad eep moleculari nsighti ntot he regulation mechanismo fH DAC8i nvolvings everal cysteines with graduated capability to form reversible disulfide bridges.

Targeted covalent inhibition and the use of irreversible chemical probes are important strategies in chemical biology and drug discovery. To date, the availability and reactivity of cysteine residues amenable for covalent targeting have been evaluated by proteomic and computational tools. Herein, we present a toolbox of fragments containing a 3,5‐bis(trifluoromethyl)phenyl core that was equipped with chemically diverse electrophilic warheads showing a range of reactivities. We characterized the library members for their reactivity, aqueous stability and specificity for nucleophilic amino acids. By screening this library against a set of enzymes amenable for covalent inhibition, we showed that this approach experimentally characterized the accessibility and reactivity of targeted cysteines. Interesting covalent fragment hits were obtained for all investigated cysteine‐containing enzymes.