Induction of Apoptosis and Cell Cycle Arrest in Mouse Colon 26 Cells by Benastatin A

Benastatin A, isolated from Streptomyces bacteria, is reported to inhibit mammalian glutathione transferases (GSTs). Since GST inhibitors such as ethacrynic acid are suggested to induce apoptosis in some cell lines, the effect of benastatin A on the survival of mouse colon 26 adenocarcinoma cells was compared with that of ethacrynic acid. When cells in stationary phase were treated with benastatin A, viable cells were found to be dose‐dependently decreased after 3 days. In the case of ethacrynic acid, this became apparent within 24 h. Electrophoretic analysis revealed DNA fragmentation, indicating that cell loss was due to apoptosis in both cases. The dominant GST in colon 26 cells was identified as the class Pi‐form (GST‐II), and the activities in crude extracts as well as purified GST‐II were almost completely inhibited by 50 μM ethacrynic acid. Immunoblot and northern blot analyses revealed increased GST‐II protein and mRNA levels in cells treated with ethacrynic acid. Benastatin A did not significantly affect the activity in the crude extract even at 20 μM, a 10‐fold higher concentration than that which almost completely inhibited the activity of purified GST‐II. However, GST activity and GST‐II protein were decreased in colon 26 cells treated with benastatin A for 5 days, no significant activity being detected in the range of 16–20 μM. In addition, β‐actin and bax mRNAs were also decreased in a dose‐dependent manner. Furthermore, flow cytometric analysis of colon 26 cells revealed that benastatin A blocked the cell cycle at the G1/G0 phase. Thus, benastatin A also induces apoptosis of colon 26 cells, but this is unlikely to be due to inhibition of GST activity.