PROLONGED SUBLETHAL EXPOSURE TO THE PROTEIN PHOSPHATASE INHIBITOR MICROCYSTIN-LR RESULTS IN MULTIPLE DOSE-DEPENDENT HEPATOTOXIC EFFECTS

The purpose of this study was to relate dose-dependent hepatotoxicity stemming from prolonged exposure to sublethal concentrations of the cy clic heptapeptide microcystin-LR (Mcyst) to hepatic Mcyst concentratio ns and protein phosphatase activity. Mcyst is a potent inhibitor of pr otein phosphatase types 1 and 2A (PP1 and PP2A). Twenty male Sprague-D awley rats were infused continuously with 0, 3, 6, or 9 mu g Mcyst/day for 28 days using intraperitoneal mini-osmotic pumps containing highl y purified toxin or saline. At the end of 28 days, dose-dependent incr eases in several serum biochemical tests including sorbitol dehydrogen ase, aspartate aminotransferase, gamma-glutamyl transferase, alkaline phosphatase, and bile acids had occurred. Serum albumin decreased in a dose-dependent fashion. Liver activity of both PP1 and PP2A decreased in a dose-dependent manner, but with a relatively greater effect on P P2A than PP1. Liver cytosol Mcyst concentrations, measured by direct c ompetitive ELISA, also increased in a dose-dependent manner, although at a higher rate than would be predicted from the incremental increase in dose given. This disproportional increase is suggestive of the bio accumulation of Mcyst with increasing dose. Histopathological abnormal ities included hepatocellular apoptosis and cytosolic vacuolation of p rincipally zone 3 hepatocytes. Immunohistochemical stains revealed Mcy st predominantly within pericanalicular regions of zone 3 hepatocytes. It was concluded that prolonged exposure to sublethal concentrations of Mcyst results in multiple dose-dependent hepatotoxic effects that c orrespond to decreased hepatic serine/threonine protein phosphatase ac tivity and increasing cytosolic Mcyst concentrations. The disproportio nal increase of hepatic Mcyst concentrations observed may suggest the bioaccumulation of toxin and an increasing relative risk of hepatotoxi city with increasing dose. (C) 1998 Society of Toxicology.