Topoisomerase inhibitor-induced apoptosis accompanied by down-regulation of Bcl-2 in human lung cancer cells.

BACKGROUND: Anticancer drug-induced apoptosis is one of the important mechanisms in the effectiveness of chemotherapy. In this study, we investigated apoptosis induced by topoisomerase inhibitors and its relationship with chemosensitivity, the expression of Bcl-2 family proteins and p53 status in human lung cancer cells.
MATERIALS AND METHODS: A total of six human lung cancer cell lines, i.e., two small cell lung cancer (SCLC) and four non-small cell lung cancer (NSCLC) cell lines, were used. For induction of apoptosis, these cell lines were treated with SN-38 (DNA topoisomerase I inhibitor) or etoposide (DNA topoisomerase II inhibitor). Drug sensitivity was determined using a microculture tetrazolium assay. The rates of apoptosis and alterations of Bcl-2 and Bax expression were analyzed by flow cytometry.
RESULTS: Apoptotic cells increased in a time-dependent manner after exposure to topoisomerase inhibitors. Induction of apoptosis was accompanied by the down-regulation of Bcl-2 expression, but there was little alteration of Bax expression. These events were significantly more extensive in SCLC cell lines, which are more sensitive to topoisomerase inhibitors, than in NSCLC cell lines, which are more resistant to these inhibitors. However, neither induction of apoptosis nor chemosensitivity correlated with p53 status in the lung cancer cell lines studied.
CONCLUSION: The more extensive induction of apoptosis with Bcl-2 down-regulation in SCLC than in NSCLC might explain, at least in part, the higher clinical sensitivity to topoisomerase inhibitors in the former disease.