Radiosensitization of calreticulin-overexpressing human glioma cell line by the polyphenolic acetate 7, 8-diacetoxy-4-methylcoumarin.

BACKGROUND: Calreticulin (CRT), an endoplasmic reticulum-resident protein generally overexpressed in cancer cells, is associated with radiation resistance. CRT shows higher transacetylase activity, as shown by us earlier, in the presence of the polyphenolic acetates (like 7, 8-diacetoxy-4-methylcoumarin, DAMC) and modifies the activity of a number of proteins, thereby influencing cell signaling. AIM: To investigate the relationship between CRT expression and radiation response in a human glioma cell line and to evaluate the radiomodifying effects of DAMC. METHODS AND RESULTS: Studies were carried out in an established human glioma cell line (BMG-1) and its isogenic clone overexpressing CRT (CROE, CRT-overexpressing cells) by analyzing clonogenic survival, cell proliferation, micronuclei analysis, and protein levels by Western blotting as parameters of responses. CRT overexpression conferred resistance against radiation-induced cell death in CROE cells (D37  = 7.35 Gy, D10  = 12.6 Gy and D0  = 7.25 Gy) as compared to BMG-1 cells (D37  = 5.70 Gy, D10  = 9.2 Gy and D0  = 5.6 Gy). A lower level of radiation-induced micronuclei formation observed in CROE cells suggested that reduced induction and/or enhanced DNA repair partly contributed to the enhanced radioresistance. Consistent with this suggestion, we noted that CRT-mediated radioresistance was coupled with enhanced grp78 level and reduced P53 activation-mediated prodeath signaling, while no changes were noted in acetylation of histone H4. DAMC-enhanced radiation-induced delayed (secondary) apoptosis, which was higher in CROE cells. CONCLUSION: CRT overexpression confers resistance against radiation-induced death of human glioma cells, which can be overcome by the polyphenolic acetate DAMC.

Cytotoxic and radio-sensitizing effects of polyphenolic acetates in a human glioma cell line (BMG-1).

BACKGROUND AND AIMS: Polyphenolic acetates (PAs) have antioxidant/ pro-oxidant properties and can also acetylate proteins (enzymes) by a novel acetoxy drug: calreticulin transacetylase acetylation system. While PAs have been investigated as pharmacological agents for the treatment of various diseases, their potential as anti-cancer agents or their efficacy as an adjuvant in anti-cancer therapeutics remains to be explored. In the present study we investigated the cytotoxic and radio-sensitizing effects of 7, 8- diacetoxy-4-methyl coumarin (DAMC) and 7- acetoxy-4-methyl coumarin (7-AMC) in a human glioma cell line (BMG-1). METHODS: Cytotoxic and radio-sensitizing effects were investigated by studying reactive oxygen species (ROS) levels, metabolic viability, clonogenic survival, growth inhibition, cell cycle perturbation, DNA repair and cytogenetic damage, besides analyzing the histone (H3) acetylation. RESULTS: Exposure of cells to DAMC and 7-AMC for 24 h showed a dose dependent increase in toxicity as indicated by reduced metabolic viability, clonogenic survival and cell proliferation, with DAMC being more toxic than 7-AMC. The degree of radiosensitization by DAMC was also higher as compared to 7-AMC as reflected by a decrease in the clonogenicity, enhanced radiation-induced cell cycle perturbation and apoptosis. DAMC impaired the removal of radiation-induced DNA double stranded breaks (measured by γH2AX immuno- fluorescence) and enhanced the cytogenetic damage (micronuclei formation), leading to an increase in mitotic death. While DAMC alone induced pan nuclear γH2AX fluorescence, both pan nuclear and spatially restricted foci was observed with the combined treatment (DAMC + Radiation) suggesting a complex nature of DNA damage and repair. DAMC- induced cytotoxic and radio-sensitizing effects were independent of its pro-oxidant activity, whereas histone H3 lysine (9/14) hyperacetylation appeared to be a potential target, regulating cellular responses to ionizing radiation (IR). CONCLUSIONS: The polyphenolic acetate 7, 8- diacetoxy-4-methyl coumarin (DAMC) demonstrates both anticancer effects and radiosensitizing potential under in vitro conditions.

7, 8-diacetoxy-4-methylcoumarin induced cell death in human tumor cells is influenced by calreticulin.

Calreticulin (CRT), an endoplasmic reticulum resident protein demonstrates transacetylase activity in presence of 7, 8 diacetoxy-4-methyl coumarin (DAMC) in vitro. To investigate the possible role of CRT and DAMC mediated protein acetylation in cells, we investigated the effects of DAMC in tumor cells with different levels of CRT. DAMC was more toxic (clonogenicity, metabolic viability and proliferation) to human glioma cells (BMG-1) expressing low endogenous CRT level as compared to head and neck carcinoma cells (KB) with a high CRT level. The cytotoxicity was accompanied by loss of mitochondrial membrane potential in both the cells, which correlated with corresponding changes in the levels of pro-apoptotic (Bax) and anti-apoptotic (NFkB) regulators. Manipulation of CRT protein level in KB cells by application of small RNA interference enhanced the sensitivity by four folds while over expression of CRT in BMG-1 cells reduced their sensitivity to DAMC by ~20% strongly suggesting the influence of CRT on DAMC induced cytotoxicity. The partial rescue of CROE cells from DAMC induced toxicity was accompanied by changes in NFkB levels and over all protein acetylation status, besides increase in the NADPH-cytochrome c reductase activity related to its well known antioxidant property. Since CRT is over-expressed in cancer cells, which are generally resistant to radio- and chemotherapy; targeting CRT transacetylase system, may be an attractive approach for increasing the efficacy of anticancer therapies.