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Prolonged sublethal exposure to the protein phosphatase inhibitor microcystin-LR results in multiple dose-dependent
P F Solter1, G K Wollenberg, X Huang
1College of Veterinary Medicine, University of Illinois, Urbana 61802, USA.
Abstract:
The purpose of this study was to relate dose-dependent hepatotoxicity stemming from prolonged exposure to sublethal concentrations of the cyclic heptapeptide microcystin-LR (Mcyst) to hepatic Mcyst concentrations and protein phosphatase activity. Mcyst is a potent inhibitor of protein phosphatase types 1 and 2A (PP1 and PP2A). Twenty male Sprague-Dawley rats were infused continuously with 0, 3, 6, or 9 micrograms Mcyst/day for 28 days using intraperitoneal mini-osmotic pumps containing highly purified toxin or saline. At the end of 28 days, dose-dependent increases in several serum biochemical tests including sorbitol dehydrogenase, 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 PP2A than PP1. Liver cytosol Mcyst concentrations, measured by direct competitive 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 bioaccumulation of Mcyst with increasing dose. Histopathological abnormalities included hepatocellular apoptosis and cytosolic vacuolation of principally zone 3 hepatocytes. Immunohistochemical stains revealed Mcyst 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 correspond to decreased hepatic serine/threonine protein phosphatase activity and increasing cytosolic Mcyst concentrations. The disproportional increase of hepatic Mcyst concentrations observed may suggest the bioaccumulation of toxin and an increasing relative risk of hepatotoxicity with increasing dose.
Insights
Prolonged exposure to microcystin-LR (Mcyst) causes liver damage in rats, linked to toxin accumulation and reduced protein phosphatase activity. Higher doses increase Mcyst levels and hepatotoxicity risk.
Area of Science:
- Toxicology
- Hepatology
- Environmental Health
Background:
- Microcystin-LR (Mcyst) is a cyclic heptapeptide and potent inhibitor of protein phosphatase types 1 and 2A (PP1 and PP2A).
- Understanding the dose-dependent hepatotoxicity of sublethal Mcyst exposure is crucial for risk assessment.
Purpose of the Study:
- To correlate dose-dependent hepatotoxicity from prolonged, sublethal microcystin-LR exposure with hepatic Mcyst concentrations and protein phosphatase activity.
- To investigate the bioaccumulation potential of Mcyst in liver tissue.
Main Methods:
- Twenty male Sprague-Dawley rats received continuous intraperitoneal infusion of 0, 3, 6, or 9 micrograms Mcyst/day for 28 days.
- Serum biochemical tests, liver protein phosphatase activity assays (PP1 and PP2A), and direct competitive ELISA for hepatic Mcyst concentrations were performed.
- Histopathological examination and immunohistochemical staining were used to assess liver damage and Mcyst localization.
Main Results:
- Dose-dependent increases in serum liver enzymes (SDH, AST, GGT, ALP) and bile acids, with a dose-dependent decrease in serum albumin.
- Significant, dose-dependent reduction in hepatic PP1 and PP2A activity, with a greater impact on PP2A.
- Increased hepatic Mcyst concentrations correlated with dose, showing a disproportionate rise suggestive of bioaccumulation.
- Histopathology revealed hepatocellular apoptosis and vacuolation, primarily in zone 3 hepatocytes, with Mcyst localized in pericanalicular regions.
Conclusions:
- Prolonged exposure to sublethal microcystin-LR induces dose-dependent hepatotoxicity in rats.
- Hepatotoxicity is associated with decreased hepatic serine/threonine protein phosphatase activity and increased intracellular Mcyst concentrations.
- The observed disproportionate increase in hepatic Mcyst suggests bioaccumulation, potentially increasing hepatotoxicity risk with higher doses.