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 (D  = 7.35 Gy, D  = 12.6 Gy and D  = 7.25 Gy) as compared to BMG-1 cells (D  = 5.70 Gy, D  = 9.2 Gy and D  = 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.