DIFFERENTIAL TOXICITY OF THE PROTEIN PHOSPHATASE INHIBITORS MICROCYSTIN AND CALYCULIN A

Microcystin (Mcyst) and calyculin A (CalA) in vitro inhibit protein ph osphatases (PP)1 and 2A activity (IC50 0.1-2.0 nM). This study was aim ed at determining the contribution of PP inhibition to Mcyst hepatotox icity by comparing the effect of these two chemically different inhibi tors in perfused rat livers, Both compounds (60 mu g Mcyst and 6 mu g CalA/150 ml perfusate) caused cessation of bile flow and inhibition of PP activity after 20 min of perfusion to 8% and 37% of control activi ty for Mcyst and CalA treatments, respectively. Histopathological find ings included loss of cord sinusoidal pattern and of normal liver arch itecture. There also was hepatocyte swelling, pyknotic changes and nec rosis. Mcyst caused a modest increase in perfusion pressure of 1.2 cm of water, whereas CalA caused a 3-fold increase. The most likely expla nation for this hemodynamic effect is direct action of CalA on the vas cular endothelium and/or sinusoidal and perisinusoidal cells. This pos sibility was explored with hepatocytes and sinusoidal endothelial cell s. PP activity of both cell types was inhibited by 10 to 100 nM CalA f ollowed later by cell lysis, whereas Mcyst (500 nM-2 mu M) had no effe ct on sinusoidal endothelial cells, but inhibited PP activity and caus ed later lysis in hepatocytes (Mcyst 20-160 nM). Mcyst hepatotoxicity is therefore a direct consequence of PP inhibition in hepatocytes, the loss of sinusoidal integrity following from the primary toxic insult to the hepatocyte. Inhibition of PP activity of the cells of the presi nusoidal vasculature and/or nonparenchymal cells results in hepatic hy pertension.