Histone deacetylase 2 selective inhibitors: A versatile therapeutic strategy as next generation drug target in cancer therapy

Acetylation and deacetylation of histone and several non-histone proteins are the two important processes amongst the different modes of epigenetic modulation that are involved in regulating cancer initiation and development. Abnormal expression of histone deacetylases (HDACs) is often reported in various types of cancers. Few pan HDAC inhibitors have been approved for use as therapeutic interventions for cancer treatment including vorinostat, belinostat and panobinostat. However, not all the HDAC isoforms are abnormally expressed in certain cancers, such as in the case of, ovarian cancer where overexpression of HDAC1-3, lung cancer where overexpression of HDAC 1 and 3 and gastric cancer where overexpression of HDAC2 is seen. Therefore, pan-inhibition of HDAC is not an efficient way to combat cancer via HDAC inhibition. Hence, isoform-selective HDAC inhibition can be one of the best therapeutic strategies in the treatment of cancer. In this context since aberrant expression of HDAC2 largely contributes to cancer progression by silencing pro-apoptotic protein expressions such as NOXA and APAF1 (caspase 9-activating proteins) and inactivation of tumor suppressor p53, HDAC2 specific inhibitors may help to develop not only the direct targets but also indirect targets that are crucial for tumor development. However, to develop a HDAC2 specific and potent inhibitor, extensive knowledge of its structure and specific functions is essential. The present review updates details on the structural features, physiological functions, and roles of HDAC2 in different types of cancer, emphasizing the challenges and status of the development of HDAC2 selective inhibitors against various types of cancer.

Targeted release of transcription factors for cell reprogramming by a natural micro-syringe

Ectopic expression of defined transcription factors (TFs) for cell fate handling has proven high potential interest in reprogramming differentiated cells, in particular for regenerative medicine, ontogenesis study and cell based modelling. Pluripotency or transdifferentiation induction as TF mediated differentiation is commonly produced by transfer of genetic information with safety concerns. The direct delivery of proteins could represent a safer alternative but still needs significant advances to be efficient. We have successfully developed the direct delivery of proteins by an attenuated bacterium with a type 3 secretion system that does not require challenging and laborious steps for production and purification of recombinant molecules. Here we show that this natural micro-syringe is able to inject TFs to primary human fibroblasts and cord blood CD34⁺ hematopoietic stem cells. The signal sequence for vectorization of the TF Oct4 has no effect on DNA binding to its nucleic target. As soon as one hour after injection, vectorized TFs are detectable in the nucleus. The injection process is not associated with toxicity and the bacteria can be completely removed from cell cultures. A three days targeted release of Oct4 or Sox2 embryonic TFs results in the induction of the core pluripotency genes expression in fibroblasts and CD34⁺ hematopoietic stem cells. This micro-syringe vectorization represents a new strategy for TF delivery and has potential applications for cell fate reprogramming.

When barriers ignore the "rule-of-five"

Why are a few drugs with properties beyond the rule of 5 (bRo5) absorbed across the intestinal mucosa while most other bRo5 compounds are not? Are such exceptional bRo5 compounds exclusively taken up by carrier-mediated transport or are they able to permeate the lipid bilayer (passive lipoidal diffusion)? Our experimental data with liposomes indicate that tetracycline, which violates one rule of the Ro5, and rifampicin, violating three of the rules, significantly permeate a phospholipid bilayer with kinetics similar to labetalol and metoprolol, respectively. Published data from experimental work and molecular dynamics simulations suggest that the formation of intramolecular H-bonds and the possibility to adopt an elongated shape besides the presence of a significant fraction of net neutral species facilitate lipid bilayer permeation. As an alternative to lipid bilayer permeation, carrier proteins can be targeted to improve absorption, with the potential drawbacks of drug-drug interactions and non-linear pharmacokinetics.

The endoplasmic reticulum stress/unfolded protein response in gliomagenesis, tumor progression and as a therapeutic target in glioblastoma

Endoplasmic reticulum (ER) stress disrupts among others protein homeostasis in cells leading to the activation of the unfolded protein response (UPR) that is crucial for restoring this balance and cell survival. Hypoxia, reactive oxygen species and nutrient deprivation, conditions commonly present in the tumor microenvironment, are well-known triggers of the UPR. Apart from being an adaptive response, recently the UPR has been implicated in oncogenesis. Here we review the current understanding of the UPR in the most life threatening brain tumor in adults, glioblastoma multiforme (GBM). The UPR is controlled by BiP/GRP78 and three different sensors, PERK, IRE1 and ATF6. In orthotopic GBM mouse models IRE1 was reported to control angiogenesis, invasion and mesenchymal differentiation. Furthermore, PERK also was found to stimulate GBM growth. However, a direct role of the UPR in gliomagenesis remains to be demonstrated. Patient samples display chronic activation of the UPR and in vitro standard chemo- and radiotherapy partially act by aggravating ER stress leading to cell death. The UPR has been linked to enhanced sensitivity for apoptosis-inducing agents such as TRAIL and MDA-7. A number of agents such as proteasome inhibitors and several natural products were reported to exert cytotoxicity by enhancing ER stress in GBM cells, and some demonstrated activity in clinical studies. Finally, ER stress was suggested to be implicated in the maintenance of homeostasis in GBM stem cells. Taken together, the UPR appears to play an important role in GBM tumor progression and is a promising target for developing novel therapeutic interventions.

Inhibition of DNA methyltransferase or histone deacetylase protects retinal pigment epithelial cells from DNA damage induced by oxidative stress by the stimulation of antioxidant enzymes

Epigenetic modifications influence DNA damage response (DDR). In this study we explored the role of DNA methylation and histone acetylation in DDR in cells challenged with acute or chronic oxidative stress. We used retinal pigment epithelial cells (ARPE-19), which natively are exposed to oxidative stress due to permanent exposure to light and high blood flow. We employed a DNA methyltransferase inhibitor - RG108 (RG), or a histone deacetylase inhibitor - valproic acid (VA). ARPE-19 cells were exposed to tert-butyl hydroperoxide, an acute oxidative stress inducer, or glucose oxidase, which slowly liberates low-doses of hydrogen peroxide in the presence of glucose, creating chronic conditions. VA and RG reduced level of intracellular reactive oxygen species and DNA damage in ARPE-19 cells in normal condition and in oxidative stress. This protective effect of VA and RG was associated with the up-regulated expression of antioxidant enzyme genes: CAT, GPx1, GPx4, SOD1 and SOD2. RG decreased the number of cells in G2/M checkpoint in response to chronic oxidative stress. Neither RG nor VA changed the DNA repair or apoptosis induced by oxidative stress. Therefore, certain epigenetic manipulations may protect ARPE-19 cells from detrimental effects of oxidative stress by modulation of antioxidative enzyme gene expression, which may be further explored in pharmacological studies on oxidative stress-related eye diseases.

Epigenetic modulators mitigate angiogenesis through a complex transcriptomic network

In this review, we summarize the knowledge pertaining to the role of epigenetics in the regulation of angiogenesis. In particular, we show that lysine acetylation and cytosine methylation are important transcriptional regulators of angiogenic genes in endothelial cells. Lysine acetylation and cytosine methylation inhibitors idiosyncratically tune the transcriptome and affect expression of key modulators of angiogenesis such as VEGF and eNOS. Transcriptomic profiling also reveals a series of novel genes that are concomitantly affected by epigenetic modulators. The reversibility and overall tolerability of currently available epigenetic inhibitors open up the prospect of therapeutic intervention in pathologies where angiogenesis is exacerbated. This type of multitargeted strategy has the major advantage of overcoming the compensatory feedback mechanisms that characterize single anti-angiogenic factors.