Sodium Butyrate Enhances the Cytotoxic Effect of Etoposide in HDACi-Sensitive and HDACi-Resistant Transformed Cells

To overcome the problem of antitumor agent toxicity for normal cells, a combined therapy using drugs with synergistic effects seems to be more effective. We investigated the molecular mechanisms of the sensitization of tumor cells resistant and sensitive to histone deacetylase inhibitors (HDACis) upon etoposide treatment together with the HDACi sodium butyrate (NaBut). We showed that NaBut enhances the cytotoxic effect of etoposide in both HDACi-sensitive and HDACi-resistant cells due to the accumulation of the Bax protein and the dissociation of Ku70-Bax inhibitory complexes. In HDACi-resistant cells, NaBut causes the cytoplasmic accumulation of Bax dissociated from mitochondria in complexes with Ku70 proteins. The increased phosphorylation of the pro-apoptotic Bad protein due to the NaBut-induced activation of Erk and Akt kinases is one of the possible reasons for the accumulation of Bax in the cytoplasm. Despite the inactivation of Bax in HDACi-resistant cells, its accumulation in the cytoplasm upon NaBut treatment makes it possible to enhance the apoptotic response against agents activating the intrinsic pathway of apoptosis. Thus, HDACis involved in combined therapy mediate the sensitization of tumor cells to genotoxic drugs, regardless of the cells' resistance to HDACis.

HDAC Inhibitor Sodium Butyrate Attenuates the DNA Repair in Transformed but Not in Normal Fibroblasts

Many cancer therapy strategies cause DNA damage leading to the death of tumor cells. The DNA damage response (DDR) modulators are considered as promising candidates for use in combination therapy to enhance the efficacy of DNA-damage-mediated cancer treatment. The inhibitors of histone deacetylases (HDACis) exhibit selective antiproliferative effects against transformed and tumor cells and could enhance tumor cell sensitivity to genotoxic agents, which is partly attributed to their ability to interfere with DDR. Using the comet assay and host-cell reactivation of transcription, as well as γH2AX staining, we have shown that sodium butyrate inhibited DNA double-strand break (DSB) repair of both endo- and exogenous DNA in transformed but not in normal cells. According to our data, the dysregulation of the key repair proteins, especially the phosphorylated Mre11 pool decrease, is the cause of DNA repair impairment in transformed cells. The inability of HDACis to obstruct DSB repair in normal cells shown in this work demonstrates the advantages of HDACis in combination therapy with genotoxic agents to selectively enhance their cytotoxic activity in cancer cells.

SODIUM BUTYRATE ENHANCES THE ANTIPROLIFERATIVE EFFECT OF HIGH CONCENTRATIONS OF ACTINOMYCIN D

In the present article we have studied the combined effect of actinomycin D (AMD) with HDI sodium butyrate (NaBut) on proliferation of transformed cells. It has been shown that NaBut increased the cytotoxic effect of low concentrations of AMD (8 nM). However, the viability of the cells transformed with E1A + Ras was slightly increased under the joint action of high concentrations of AMD and NaBut. Simultaneous action of NaBut and low concentration of AMD sharply reduced the clonogenic survival of transformed cells, in parallel with increased apoptotic cell death.The study of the mechanisms of cell death induced by the combined action of NaBut and low concentrations of AMD showed that the joint treatment resulted in activation of proapoptotic transcription factor p53 and the suppression of the activity of anti-apoptotic factor NF-kB. Thus, NaBut increased cytotoxic effect of low concentrations of AMD in oncogene-transformed cells under the joint action, increasing their apoptotic death. These results can be used in the selection of the correct dosage of drugs for the effective combined anti-cancer therapy using HDIs and AMD.

[EFFECT OF HDAC INHIBITORS ON THE ROLE OF Gadd45 IN THE DNA REPAIR]

Histone deacetylase inhibitors (HDIs) causes irreversible cell cycle arrest and senescence of E1A+Ras transformed cells. HDI sodium butyrate (NaB) affects the DNA repair and consequently modulates the phosphorylation of DNA breaks marker histone H2AX. For Gadd45 family proteins shown the role in regulation of cell proliferation, apoptosis and DNA repair. In the present paper, we studied the effect of NaB on the DNA repair through Gadd45a protein modulation. Our results indicate that suppression of Gadd45α reduces the DNA repair efficiency, in both basal, and NaB-dependent. However NaB-induced supression of DNA repair is reduced in Gadd45-expressing transformed cells. This is the result of Gadd45α accumulation in the nucleus and redistribution of oligomeric forms of Gadd45 under NaB treatment. This results in changing of Gadd45α interaction with proteins involved in the cell cycle progression and DNA repair regulation. NaB amplified the interaction of Gadd45 with p21/Waf1 protein, thereby reducing the amount of the p21/Waf1 in complex with PCNA. Thus, in the absence Gadd45α the amount of DNA breaks accumulates due to the reduced efficiency of repair, while HDIs dependent induction of Gadd45α promotes the DNA repair. DNA damages induced with HDIs or genotoxic agents could not be repaired under the lack of Gadd45α. As a result Gadd45 deficient transformed cells initiate apoptotic death program.