Comparative studies on the combined cytotoxic effect of forskolin with mitomycin C and responsiveness to forskolin in rat ascites hepatoma AH66 cells and AH66F cells.

Abstract

The combined cytotoxic effect of forskolin with mitomycin C (MMC) was investigated using two cell lines, AH66 and AH66F. Forskolin significantly enhanced the cytotoxicity of MMC and increased the uptake of MMC and the intracellular adenosine 3',5'-cyclic monophosphate (cyclic AMP) level in AH66 cells but similar results were not obtained with AH66F cells. Dibutyryl cyclic AMP enhanced the effect and uptake of MMC in both cell lines. These results confirm that elevated cyclic AMP in the cells increases the cellular uptake of MMC and enhances the cytotoxicity of MMC as reported in our previous papers. Therefore, forskolin may be suitable for antitumor combination therapy. On the other hand, the functional components associated with the cyclic AMP generating system were thought to be present in membranes of both AH66 cells and AH66F cells from results that showed that cholera toxin, islet-activating protein and prostaglandin E1 increased the level of cyclic AMP in both cells. The elevation of cyclic AMP by these adenylate cyclase activators in AH66 cells was augmented by forskolin. However, the cyclic AMP level in AH66F cells was scarcely affected by forskolin and the elevation of cyclic AMP by the activators was inhibited by this diterpene. From these results, it appears that the AH66F cell line may be useful for the elucidation of the action mechanism of forskolin and of the transmission system from hormone receptors to adenylate cyclase.