MICROCYSTINS, NATURAL ENVIRONMENTAL TOXINS

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The microcystins, a family of related heptapeptides produced by the freshwater bloom-forming cyanobacterium Microcystis aeruginosa are naturally occurring environmental toxins. They are quite potent, (LD(50)ip in mice <O.l mg/kg) and chemically stable; their hepatotoxicity is not lost on boiling, nor are they inactivated by ordinary potable water treatment, thereby posing a threat to public health. An epidemiological study has shown that the presence of toxic peptides in the water supply correlated with increased liver damage in the population. Although the liver lesions caused by microcystin have been well described, the mechanism by which these peptide toxins act at the molecular or cellular level is not known. The overall purpose of this proposal is to elucidate the mechanism by which the microcystins cause liver damage. The specific work planned follows from the work done by the PI in Australia. It is known that microcystin is taken up by isolated hepatocytes, and that when administered to animals, it rapidly accumulates in the liver. (1) It is planned here to look at the transport, distribution (particularly intrahepatically), and metabolic fate of the toxin, using the easily prepared and well characterized (125)I-labeled derivative of microcystin. This will be correlated with the pathological changes found in animals following administration of microcystin. (2) The possibility that microcystin toxicity in vivo is mediated by an effect on non-parenchymal liver cells will be studied. Previous work has shown that microcystin causes changes in isolated hepatocytes. From this, two specific aims arise. These are: (3) to study the interaction of microcystin with the cytoskeleton, and (4) to characterize metabolic changes, especially changes in glutathione and calcium homeostasis. A variety of models will be employed: whole animals, isolated perfused livers, freshly isolated and cultured hepatocytes, endothelial cells. The work will be done in the laboratory of the co-PI, where all these models are in use for other liver studies. The results obtained from all these approaches will reconcile the hepatotoxicity in animals of these toxins with the changes the toxins cause in isolated hepatocytes.