Microcystin-LR and liver tumor promotion: Effects on cytokinesis, ploidy, and apoptosis in cultured hepatocytes

There is mounting evidence that the cyanobacterial toxins, the microcystins, can act as tumor promoters. However, due to their requirement for active uptake by the cell, there have been few in vitro studies of the mechanism by which this might occur. Most of our understanding of this process has been deduced from experiments using the cell permeant okadaic acid, despite differences in the effects of these toxins. A cell culture system was developed in which nonmitogenically stimulated proliferating primary mouse hepatocytes could be exposed to microcystin-LR and the effects on various cell cycle parameters could be determined. It was found that cytokinesis was stimulated and the rate of apoptosis reduced by picomolar concentrations of microcystin-LR, whereas at higher (nanomolar) concentrations, cytokinesis appeared to be inhibited and cell death was induced. Cell killing was selective at these higher concentrations, favoring retention of a proliferatively active cohort of cells. The differences in effect between okadaic acid and microcystin-LR found by other researchers were confirmed, although in this system the effective concentrations of the toxins were approximately 100-fold lower than those reported previously. The mechanistic implications of these findings with regard to tumor promotion are discussed. (C) 1999 by John Wiley & Sons, Inc.