Dolastatin 15 induces apoptosis and BCL-2 phosphorylation in small cell lung cancer cell lines

Background: The dolastatins are natural peptides which inhibit microtubule assembly and induce apoptosis in a number of malignant cell types. In small cell lung cancer (SCLC), bcl-2 overexpression is very common and appears to be a mediator of cell growth and treatment-resistance, suggesting that bcl-2 is a rational target for novel anti-SCLC strategies. Since several other-tubulin-binding agents have been reported to induce apoptosis in association with phosphorylation and inactivation of bcl-2 we evaluated the effects of dolastatin 10 and 15 in SCLC cells. Methods: The growth inhibitor activity of dolastatin 10 and 15 was evaluated in four SCLC cell lines with an MTT assay. Cell cycle and the induction of apoptosis were evaluated by flow cytometry and fluorescent microscopy. Immunoblot analysis was used to determine bcl-2 expression and phosphorylation. Results: Dolastatin 15 displayed growth inhibitory activity against all four SCLC cell lines (NCI-H69, NCI-H82, NCI-H345, NCI-H446) with IC50 values ranging from 0.039-28.8 nM, which were 2.7-9.2-fold higher than the values for dolastatin 10. All four SCLC cell lines underwent G(2)/M arrest within 24 hours of exposure to dolastatin 15, and three had morphologic evidence of apoptosis after 48 hours. Immunoblot analysis of dolastatin 15-treated cells which overexpressed bcl-2 revealed a pattern consistent with bcl-2 phosphorylation. Conclusion: Dolastatin 15 has in vitro activity against SCLC cells, but is less potent than dolastatin 10. This activity is associated with the induction of apoptosis and bcl-2 phosphorylation. These findings suggest that further evaluation of the activity of the dolastatins in SCLC is reasonable and that the pharmacologic modulation of apoptotic pathways deserves further study as a potential anticancer strategy.