ELEVATION OF TOPOISOMERASE-I MESSENGER-RNA, PROTEIN, AND CATALYTIC ACTIVITY IN HUMAN TUMORS - DEMONSTRATION OF TUMOR-TYPE SPECIFICITY AND IMPLICATIONS FOR CANCER-CHEMOTHERAPY

Topoisomerase I has been identified as an intracellular target of camp tothecin, a plant alkaloid with anticancer activity. Various lines of evidence suggest that the sensitivity of cells to this drug is directl y related to the topoisomerase I content. In humans, the levels of top oisomerase I have been shown to be elevated in colorectal tumors, comp ared to normal colon mucosa. The aim of our studv was to determine whe ther (a) topoisomerase I levels are elevated in other solid tumors, (b ) the elevated enzyme is catalytically active in these tumors, and (c) the increase in topoisomerase I levels in colorectal tumors is a resu lt of increased transcription or translation. Topoisomerase I levels w ere quantitated in crude extracts from colorectal, prostate, and kidne y tumors and their matched normal counterparts by Western blotting and by direct determination of catalytic activity, and mRNA levels were d etermined by Northern blotting. By Western blotting, colorectal tumors showed 5-35-fold increases in topoisomerase I levels, compared to the ir normal colon mucosa. In the case of prostate tumors, the increase w as 2-10-fold, compared with benign hyperplastic prostate tissue from t he same patients. However, no difference was observed in topoisomerase I levels in kidney tumors, compared to their normal counterparts. The catalytic activity of topoisomerase I was determined by a quantitativ e P-32-transfer assay in crude homogenates, without isolating nuclei. Colorectal and prostate tumors exhibited 11-40- and 4-26-fold increase s, respectively, in catalytic activity. However, kidney tumors did not show any alteration in catalytic activity, compared to their normal m atched samples. Thus, for all three tumor types there was a good corre lation between enzyme levels and catalytic activity. Finally, colorect al tumors were analyzed for steady state mRNA levels. A 2-33-fold incr ease in mRNA levels was found in colorectal tumors, compared to normal colon mucosa. These results suggest that alterations in topoisomerase I expression in humans are tumor type specific and that the increase in topoisomerase I levels results from either increased transcription of the topoisomerase I gene or increased mRNA stability.