ANTITUMOR EFFECTS OF BUTYROLACTONE-I, A SELECTIVE CDC2 KINASE INHIBITOR, ON HUMAN LUNG-CANCER CELL-LINES

Butyrolactone I, which is a naturally occurring specific inhibitor of the cdc2 kinase family, showed antitumor effects on several non-small- and small-cell-lung cancer cell lines with IC50 values the order of 5 0 mu g/ml on the former. No cross-resistance of several drug-resistant cell lines, including those with the multidrug-resistant phenotype an d five cisplatin-resistant cell lines to butyrolactone I was observed. The cdc2 kinase activity of PC-14 cells was inhibited by treatment wi th 20 mu g/ml butyrolactone I, a concentration comparable to the IC50 value, for 2 hours. Longer exposure to butyrolactone I (>24 hours) red uced the cdc2 kinase protein level. Butyrolactone I arrested the cells at the G2/M phase in a concentration dependent manner These results s uggest that butyrolactone I actually acts on cdc2 kinase, rather than other cdk kinases, in PC-14 cells. Inhibition of DNA synthesis, determ ined by measuring thymidine uptake, occurred earlier (2 hours) after i nitiating exposure than the decrease in the cdc2 protein level and was concentration dependent, suggesting that butyrolactone I inhibited DN A synthesis. Cell permeabilization by digitonin enhanced DNA synthesis inhibition by butyrolactone I, suggesting that the permeability of th e membrane to this agent was the limiting factor for its growth inhibi tory effect. Many anticancer agents, such as alkylating agents and cis platin, cause cells to accumulate at the G2/M phase of the cell cycle. We investigated whether butyrolactone I had any modulatory effect on the antitumor effects of several anticancer drugs in vitro. Butyrolact one I showed no modulatory effects on vindesine, paclitaxel, or etopos ide, but exposure of PC-9 and PC-14 cells to butyrolactone I together with or prior to treatment with cisplatin reduced the cytotoxicity of the latter. Thin-layer chromatographic analysis revealed that butyrola ctone I bound to cisplatin, which was a possible cause of the reduced cisplatin cytotoxicity in the presence of bytyrolactone I.