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Tuesday 06 November 2007

Reduced expression of DNA topoisomerase I in SF295 human glioblastoma cells selected for resistance to homocamptothecin and diflomotecan.

By: Liao Z, Robey RW, Guirouilh-Barbat J, To KK, Polgar O, Bates SE, Pommier Y.

Mol Pharmacol 2007 Nov;(): [Epub ahead of print]

Homocamptothecins (hCPTs) are a novel class of topoisomerase I (Top1) inhibitors with enhanced chemical stability compared to the currently used camptothecin (CPT) analogues, irinotecan and topotecan. The hCPT derivative, diflomotecan (BN80915), is currently in clinical trials. We established two resistant human glioblastoma cell lines, SF295/hCPT50 and SF295/BN50 by stepwise exposure of the parental SF295 line to increasing concentrations of hCPT and BN80915, respectively. The two resistant cell lines were 15- to 22-fold resistant to hCPT and BN80915 as well as 7- to 27-fold cross resistant to other Top1 inhibitors, including CPT, topotecan, and the indenoisoquinolines MJ-III-65 (NSC 706744) and NSC 724998, but sensitive to the topoisomerase II inhibitors mitoxantrone and etoposide. Neither of the resistant cell lines displayed any detectable expression of the three major drug transporters Pgp, MRP1 or ABCG2 as assessed by immunoblot or flow cytometry. Reduced expression of Top1 protein occurred at the transcriptional level in both of the resistant cell lines, consistent with the reduction of Top1 enzyme level as the major contribution to the resistance phenotype in SF295/hCPT50 and SF295/BN50 cells. Treatment of the resistant cell lines with the histone deacetylase inhibitor depsipeptide or the DNA methyltransferase inhibitor 5-aza-2'-deoxycytidine alone or concomitantly did not result in re-expression of Top1. Our studies suggest that selection for resistance to hCPT or BN80915 is primarily related to reduced Top1 expression at the transcriptional level, resulting in reduced enzyme levels.

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