Differential expression of DNA topoisomerases in non-small cell lung cancer and normal lung

Abstract

DNA topoisomerases are ubiquitous nuclear enzymes, and important targets of cancer chemotherapy. Expression of topoisomerase genes is often correlated with in vitro chemosensitivity. We investigated the expression of the topoisomerase genes in normal lung and non-small cell lung cancer. Expression of topoisomerase II-alpha, topoisomerase II-beta, and topoisomerase I genes has been assessed in tumor samples of 60 patients who underwent operation for a non-small cell lung carcinoma, by RNase protection assay, and by immunohistochemistry. The expression of topoisomerase II-alpha gene was either undetectable or very low in normal lung, while most NSCLC expressed readily quantifiable levels of this gene. No alteration of the topoisomerase II-alpha gene was found by Southern blotting in the NSCLC samples. In contrast to topoisomerase II-alpha, topoisomerase II-beta was expressed in most normal as well as in tumor tissue samples, at a similar level. The levels of expression of both topoisomerase II isoforms was lower than that of human lung cancer cell lines. The results of the topoisomerase II mRNA expression were confirmed by immunohistochemistry. Whereas topoisomerase II-alpha staining was mainly limited to the nucleus, staining with topoisomerase II-beta antibody was exclusively observed in nucleoli. Topoisomerase I was localized in the nuclei and expression was mainly limited to tumor cells. By RNase protection, topoisomerase I expression in NSCLC samples was in the range of that of human lung cancer cell lines. The expression of the topoisomerase genes did not seem to be coordinated. In tumor cells, there was a positive association between expression of topoisomerase II-alpha and Ki-67, a marker of cell proliferation, as assessed by immunohistochemistry, but not with topoisomerase II-beta or topoisomerase I. Clinical characteristics of the patients, and their survival did not appear to be correlated to the level of expression of any of the topoisomerase genes, although a trend towards a shorter survival was observed in patients whose tumors expressed relatively high topoisomerase II-alpha mRNA levels.

In conclusion: (1) the two isoforms of topoisomerase II are differentially expressed in normal lung and NSCLC cells; (2) higher topoisomerase II-alpha expression is associated with higher cell proliferation in NSCLC; (3) the expression of topoisomerase II-alpha and topoisomerase I, but not of topoisomerase II-beta, was higher in tumor cells compared to normal lung. Given the differential expression of topoisomerases in normal lung and tumors, research of more potent and specific topoisomerase inhibitors might prove beneficial in non-small cell lung cancer. Immunohistochemistry may be indicated in prospectively investigating the correlation between expression of topoisomerases and results of chemotherapy treatment.