title: DNA repair enzyme, O6-methylguanine DNA methyltransferase, modulates cytotoxicity of camptothecin-derived topoisomerase I inhibitors.
abstract: Two camptothecin-resistant cell lines, CPT30 and KB100, were established and characterized previously in our laboratory. Because enhanced sensitivity to 1,3-bis(2-chloroethyl)-1-nitrosourea (BCNU) and decreased expression of O(6)-methylguanine-DNA methyltransferase (MGMT) protein were observed in these lines, we hypothesized that MGMT may be a determinant of cytotoxicity associated with camptothecin-derived DNA topoisomerase I inhibitors (CPTs). We used the Tet-On system to induce expression of MGMT in Chinese hamster ovary (CHO) cells and RNA interference to knock down MGMT expression in human nasopharyngeal carcinoma HONE-1 cells in order to identify any correlations between MGMT expression and CPTs cytotoxicity. CHO-derived Tet-On-inducible cells (S12+) showed MGMT overexpression and statistically significant more resistance to BCNU, camptothecin, 7-ethyl-10-hydrocamptothecin (SN38), and topotecan than parental CHO cells (p < 0.05), but there was less resistance to CPTs than to BCNU. Knockdown of MGMT expression with small interfering RNA in HONE-1 cells conferred increased sensitivity to BCNU and CPTs compared with mock control. Furthermore, alteration of MGMT expression coincides with CPT-induced cell death and poly(ADP-ribose) polymerase cleavage. There were no differences in protein levels and catalytic activity of topoisomerase I between MGMT-proficient and MGMT-deficient cells from the Tet-On-inducible and small interfering RNA (siRNA) systems. Resistance to CPTs coincided with decreased amounts of protein-linked DNA breaks generated by CPTs in MGMT-proficient cells and vice versa in MGMT-deficient cells. Our data indicate that MGMT can modulate cytotoxicity of CPT-derived topoisomerase I inhibitors.
authors.affiliation: Division of Cancer Research, National Health Research Institutes, Taipei, Taiwan, Republic of China.
authors.forename: Ching-Chuan
authors.initials: CC
authors.lastname: Kuo
authors.forename: Jin-Fen
authors.initials: JF
authors.lastname: Liu
authors.forename: Jang-Yang
authors.initials: JY
authors.lastname: Chang
mesh_terms: D000818:Animals; D000972:Antineoplastic Agents, Phytogenic / Q000494:pharmacology*; D016466:CHO Cells; D002166:Camptothecin / Q000031:analogs & derivatives / Q000494:pharmacology*; D002384:Catalysis; D045744:Cell Line, Tumor; D002470:Cell Survival / Q000187:drug effects; D003001:Cloning, Molecular; D006224:Cricetinae; D003412:Cricetulus; D045643:DNA Repair Enzymes / Q000096:biosynthesis / Q000235:genetics / Q000502:physiology*; D019008:Drug Resistance, Neoplasm; D020868:Gene Silencing; D006801:Humans; D019853:O(6)-Methylguanine-DNA Methyltransferase / Q000096:biosynthesis / Q000235:genetics / Q000502:physiology*; D034741:RNA, Small Interfering / Q000235:genetics; D059004:Topoisomerase I Inhibitors; D014162:Transfection
chemical_list: D000972:Antineoplastic Agents, Phytogenic; D034741:RNA, Small Interfering; D059004:Topoisomerase I Inhibitors; D019853:O(6)-Methylguanine-DNA Methyltransferase; D045643:DNA Repair Enzymes; D002166:Camptothecin