A novel kinase inhibitor establishes a predominant role for protein kinase D as a cardiac class IIa histone deacetylase kinase

Class IIa histone deacetylases (HDACs) repress genes involved in pathological cardiac hypertrophy. The anti-hypertrophic action of class IIa HDACs is overcome by signals that promote their phosphorylation-dependent nuclear export. Several kinases have been shown to phosphorylate class IIa HDACs, including calcium/calmodulin-dependent protein kinase (CaMK), protein kinase D (PKD) and G protein-coupled receptor kinase (GRK). However, the identity of the kinase(s) responsible for phosphorylating class IIa HDACs during cardiac hypertrophy has remained controversial. We describe a novel and selective small molecule inhibitor of PKD, bipyridyl PKD inhibitor (BPKDi). BPKDi blocks signal-dependent phosphorylation and nuclear export of class IIa HDACs in cardiomyocytes and concomitantly suppresses hypertrophy of these cells. These studies define PKD as a principal cardiac class IIa HDAC kinase.     

Novel Histone Deacetylase Class IIa Selective Substrate Radiotracers for PET Imaging of Epigenetic Regulation in the Brain

Histone deacetylases (HDAC’s) became increasingly important targets for therapy of various diseases, resulting in a pressing need to develop HDAC class- and isoform-selective inhibitors. Class IIa deacetylases possess only minimal deacetylase activity against acetylated histones, but have several other client proteins as substrates through which they participate in epigenetic regulation. Herein, we report the radiosyntheses of the second generation of HDAC class IIa–specific radiotracers: 6-(di-fluoroacetamido)-1-hexanoicanilide (DFAHA) and 6-(tri-fluoroacetamido)-1-hexanoicanilide ([¹⁸F]-TFAHA). The selectivity of these radiotracer substrates to HDAC class IIa enzymes was assessed in vitro, in a panel of recombinant HDACs, and in vivo using PET/CT imaging in rats. [¹⁸F]TFAHA showed significantly higher selectivity for HDAC class IIa enzymes, as compared to [¹⁸F]DFAHA and previously reported [¹⁸F]FAHA. PET imaging with [¹⁸F]TFAHA can be used to visualize and quantify spatial distribution and magnitude of HDAC class IIa expression-activity in different organs and tissues in vivo. Furthermore, PET imaging with [¹⁸F]TFAHA may advance the understanding of HDACs class IIa mediated epigenetic regulation of normal and pathophysiological processes, and facilitate the development of novel HDAC class IIa-specific inhibitors for therapy of different diseases.     

The Development Process: from SAHA to Hydroxamate HDAC Inhibitors with Branched CAP Region and Linear Linker

Histone deacetylases (HDACs) belong to a group of epigenetic regulatory enzymes that participate in modulating the acetylation level of histone lysine residues as well as non‐histone proteins, and they play a key role in the regulation of gene expression. HDACs are potential anticancer drug targets highly expressed in various kinds of cancer cells. So far, five small molecules targeting HDACs have been approved for the therapy of cancer, and over 20 inhibitors of HDACs are under different phases of clinical trials. Among them, hydroxamate‐based HDAC inhibitors (HDACis) represent a well‐investigated series of chemical entities. The current review covers the recent progress in the discovery process, form SAHA to hydroxamate HDAC inhibitors with branched CAP region and linear linker. At the same time, the pharmacological and structure‐activity relationship (SAR) studies of the specific derivatives from SAHA and the HDACis with branched CAP region and linear linker are also introduced.     

New role for hPar-1 kinases EMK and C-TAK1 in regulating localization and activity of class IIa histone deacetylases

Class IIa histone deacetylases (HDACs) are found both in the cytoplasm and in the nucleus where they repress genes involved in several major developmental programs. In response to specific signals, the repressive activity of class IIa HDACs is neutralized through their phosphorylation on multiple N-terminal serine residues and 14-3-3-mediated nuclear exclusion. Here, we demonstrate that class IIa HDACs are subjected to signal-independent nuclear export that relies on their constitutive phosphorylation. We identify EMK and C-TAK1, two members of the microtubule affinity-regulating kinase (MARK)/Par-1 family, as regulators of this process. We further show that EMK and C-TAK1 phosphorylate class IIa HDACs on one of their multiple 14-3-3 binding sites and alter their subcellular localization and repressive function. Using HDAC7 as a paradigm, we extend these findings by demonstrating that signal-independent phosphorylation of the most N-terminal serine residue by the MARK/Par-1 kinases, i.e., Ser155, is a prerequisite for the phosphorylation of the nearby 14-3-3 site, Ser181. We propose that this multisite hierarchical phosphorylation by a variety of kinases allows for sophisticated regulation of class IIa HDACs function.