Butyrolactone I induces cyclin B1 and causes G2/M arrest and skipping of mitosis in human prostate cell lines

Several naturally occurring cyclin-dependent kinase (CDK) inhibitors have been isolated from different lower organisms. In this report, we examined the effect of one of the CDK inhibitors, butyrolactone I (BL), on the expression of cyclins D2, A and B1 in three human prostatic cancer cell lines (DU145, PC-3, LNCaP) using two colored flow cytometric analysis. The percentage of DU145 cells in the 4C phase of the cell cycle were increased significantly at both 70 μM and 100 μM BL. Furthermore, an additional 8C peak was observed which had double the DNA content of the 4C phase at these concentrations of BL. The appearance of the 8C peak increased gradually and was more evident in DU145 and PC-3 than LNCaP. Cells in the 8C peak had either two nuclei or abnormal nuclei as observed by Papanicolaou stain. BL also increased the amount of cyclin B1 positive cells in the 4C phase. This increase was apparent on day 1 and returned to normal by day 3. Since BL selectively inhibits cyclin-dependent kinase, cyclin B1 might accumulate without being degraded. Other cyclins were not significantly changed by BL. The data demonstrate that BL inhibited Cdc2 of unsynchronized cultured prostate cancer cells, and interrupted the cell cycle progression toward cell division. The BL inhibition of Cdc2 led to the accumulation of cells in the 4C phase without mitosis resulting in an accumulation of cyclin B1. The appearance of cells in the 8C phase may be due to the progression of cells in the 4C phase through the cell cycle skipping mitosis. Cyclin B1 decreased in correlation with the progression through a new cell cycle. These results suggest that BL does not cause a complete arrest of the cell cycle in G2/M but that BL occasionally allows for the skipping of mitosis and subsequent progression through the cell cycle to occur.