Ubiquitin/26S proteasome-mediated degradation of topoisomerase I as a resistance mechanism to camptothecin in tumor cells

Camptothecin (CPT) induces down-regulation of topoisomerase I (TOPI) via an ubiquitin/26S proteasome pathway. Studies using a panel of breast and colo rectal cancer cell lines as well as primary nontransformed and oncogene-tra nsformed cells have demonstrated that CPT-induced down-regulation exhibits a high degree of heterogeneity. In general, nontransformed cells are much m ore proficient in CPT-induced TOPI downregulation than their transformed co unterparts. Among the breast and colorectal cancer cell lines, there was a general correlation between the extent of CPT-induced TOP1 down-regulation and CPT resistance. The breast cancer cell line ZR-75-1, the most sensitive to CPT, was completely defective in CPT-induced TOP1 down-regulation, wher eas the breast cancer cell line BT474, the least sensitive to CPT, exhibite d effective CPT-induced TOPI down-regulation. The 26S proteasome inhibitor MG132 was shown to inhibit CPT-induced down-regulation of TOPI in BT474 cel ls and selectively sensitized BT474 but not ZR-75-1 cells to CPT-induced cy totoxicity and apoptosis. In the aggregate, these results suggest that CPT- induced down-regulation of TOPI could be an important parameter for determi ning CPT sensitivity/resistance in tumor cells. Analysis of the levels of T OP1 cleavable complexes, SUMO-1-TOP1 conjugates, and ubiquitin-TOPI conjuga tes in ZR-75-1 and BT474 cells has suggested that the heterogeneity of CPT- induced down-regulation of TOPI in tumor cells is at least in part attribut able to altered regulation of a process(es) downstream from the TOPI cleava ble complex.