Topoisomerase I has been identified as an intracellular target of
camptothecin, a
plant alkaloid with anticancer activity. Various lines of evidence suggest that the sensitivity of cells to this
drug is directly related to the
topoisomerase I content. In humans, the levels of
topoisomerase I have been shown to be elevated in
colorectal tumors, compared to normal colon mucosa. The aim of our study was to determine whether (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 result of increased transcription or translation.
Topoisomerase I levels were quantitated in
crude extracts from colorectal, prostate, and kidney
tumors and their matched normal counterparts by Western blotting and by direct determination of catalytic activity, and
mRNA levels were determined by Northern blotting. By Western blotting,
colorectal tumors showed 5-35-fold increases in
topoisomerase I levels, compared to their normal colon mucosa. In the case of prostate
tumors, the increase was 2-10-fold, compared with benign hyperplastic prostate tissue from the 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 quantitative 32P-transfer assay in crude homogenates, without isolating nuclei. Colorectal and prostate
tumors exhibited 11-40- and 4-26-fold increases, respectively, in catalytic activity. However, kidney
tumors did not show any alteration in catalytic activity, compared to their normal matched samples. Thus, for all three
tumor types there was a good correlation between
enzyme levels and catalytic activity. Finally,
colorectal tumors were analyzed for steady state
mRNA levels. A 2-33-fold increase 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.