Camptothecin (
CPT), a
topoisomerase I poison, is an important
drug for the treatment of solid
tumors in the clinic.
Nitric oxide (·NO), a physiological signaling molecule, is involved in many cellular functions, including cell proliferation, survival and death. We have previously shown that ·NO plays a significant role in the detoxification of
etoposide (VP-16), a
topoisomerase II poison in vitro and in human
melanoma cells. ·NO/·NO-derived species are reported to modulate activity of several important cellular
proteins. As topoisomerases contain a number of free sulfhydryl groups which may be targets of ·NO/·NO-derived species, we have investigated the roles of ·NO/·NO-derived species in the stability and activity of
topo I. Here we show that ·NO/·NO-derived species induces a significant down-regulation of
topoisomerase I protein via the
ubiquitin/
26S proteasome pathway in human colon (HT-29) and breast (MCF-7)
cancer cell lines. Importantly, ·NO treatment induced a significant resistance to
CPT only in MCF-7 cells. This resistance to
CPT did not result from loss of
topoisomerase I activity as there were no differences in
topoisomerase I-induced DNA cleavage in vitro or in
tumor cells, but resulted from the stabilization/induction of bcl2
protein. This up-regulation of bcl2
protein in MCF-7 cells was wtp53 dependent as pifithrine-α, a small molecule inhibitor of wtp53 function, completely reversed
CPT resistance, suggesting that wtp53 and
bcl2 proteins played important roles in
CPT resistance. Because
tumors in vivo are heterogeneous and contaminated by infiltrating macrophages, ·NO-induced down-regulation of
topoisomerase I protein combined with bcl2
protein stabilization could render certain
tumors highly resistant to
CPT and drugs derived from it in the clinic.