Sublethal Hepatic Pathology and Evidence of Intranuclear Effects of the Cyanobacterial Toxin Microcystin -Lr

127 p.Thesis (Ph.D.)--University of Illinois at Urbana-Champaign, 2001.Microcystin-LR (MCLR) is a hepatotoxin and phosphatase inhibitor produced by Microcystis aeruginosa. The pathology of sublethal exposure is not well characterized. MCLR immunohistochemistry (MCLR-IHC) studies indicate that the nucleus may be a target of MCLR. Balb/c mice were given one sublethal dose at 45 mug/kg or repeated daily doses for 2, 4 and 7 days. At 2 hours post-dosing there was centrilobular hepatocellular hypertrophy, glycogen depletion, apoptosis and eosinophilic cytoplasmic condensation. Repeated dosing induced hepatocytomegaly and karyomegaly with multinucleation. Hepatocyte nuclei were strongly labeled by MCLR-IHC, 4 hours after one sublethal dose. Western blotting confirmed the presence of toxin in the nucleus bound to protein phosphatases 1 and 2A. Nuclear phosphatase activity of mice given an acutely lethal dose was profoundly inhibited. Nuclear phosphatase activity was unchanged in mice given sublethal doses despite detection of MCLR by ELISA. Using phosphoserine specific antibodies it was shown that MCLR exposure increases the phosphorylation of p53. I postulated that MCLR-induced hepatocyte apoptosis was a p53 dependent event involving postranslational phosphorylation. Mice homozygous for a defective p53 gene and mice with normal p53 were given doses of MCLR. Also, pifithrin, a chemical inhibitor of p53, was used to determine if it could prevent MCLR induced hepatocyte apoptosis. In both experiments, p53 did not play an essential role in the induction of apoptosis, indicating that other pathways are involved. Mice given a similar daily sublethal dose for 7 days, had decreased amounts of p53 after challenge with a DNA-damaging agent. It was concluded that sublethal doses of MCLR induce hepatic lesions that in conjunction with MCLR-IHC could assist in the diagnosis of field cases. MCLR accumulates in the hepatocyte nucleus, an event that results in increased phosphorylation of p53, but other pathways are likely involved in the execution of hepatocyte apoptosis after MCLR exposure. Reduced production of this tumor suppressor protein induced by prolonged exposure to MCLR would favor the formation of hepatic neoplasia.U of I OnlyRestricted to the U of I community idenfinitely during batch ingest of legacy ETD