Inhibition of Cyclin D1 Expression and Induction of Apoptosis by Inostamycin in Small Cell Lung Carcinoma Cells

Previously, we demonstrated that inostamycin, an inhibitor of phosphatidylinositol turnover, caused cell cycle arrest at the G1 phase, inhibiting the expression of cyclins D1 and E in normal cells. In the present study, we examined the effects of inostamycin on cell cycle progression and apoptosis in human small cell lung carcinoma Ms‐1 cells. Treatment of exponentially proliferating Ms‐1 cells with low concentrations of inostamycin caused cells to accumulate in the G1 phase. We found that inostamycin decreased cyclin D1, and increased cyclin‐dependent kinase inhibitors such as p21(WAF1) and p27(KIP1) in Ms‐1 cells. On the other hand, higher concentrations of inostamycin induced morphological apoptosis and DNA fragmentation in Ms‐1 cells without affecting the expression of p53, Bcl‐2 and Bax. Inostamycin‐induced apoptosis was suppressed by an inhibitor of caspase‐3, and a 17 kDa fragment of activated caspase‐3 was detected following inostamycin treatment. Therefore, caspase‐3(‐like) would appear to be involved in inostamycin‐induced apoptosis. On the other hand, an inhibitor of caspase‐3(‐like) proteases did not affect the inhibitory effect of inostamycin on cyclin D1 expression, suggesting that caspase‐3(‐like) proteases were not responsible for inostamycin‐induced G1 arrest.