Toxicity and uptake mechanism of cylindrospermopsin and lophyrotomin in primary rat hepatocytes

The toxicities and uptake mechanisms of two hepatotoxins, namely cylindrosp ermopsin and lophyrotomin, were investigated on primary rat hepatocytes by using microcystin-LIZ (a well-known hepatotoxin produced by cyanobacteria) as a comparison. Isolated rat hepatocytes were incubated with different con centrations of hepatotoxins for 0, 24, 48 and 72 h. The cell viability was assayed by the tetrazolium-based (MTT) assay. Microcystin-LR, cylindrosperm opsin and lophyrotomin all exhibited toxic effects on the primary rat hepat ocytes with 72-h LC50 of 8, 40 and 560 ng/ml, respectively. The involvement of the bile acid transport system in the hepatotoxin-induced toxicities wa s tested in the presence of two bile acids, cholate and taurocholate. Resul ts showed that the bile acid transport system was responsible for the uptak e, and facilitated the subsequent toxicities of lophyrotomin on hepatocytes . This occurred to a much lesser extent with cylindrospermopsin. With its s maller molecular weight, passive diffusion might be one of the possible mec hanisms for cylindrospermopsin uptake into hepatocytes. This was supported by incubating a permanent cell line, KB (devoid of bile acid transport syst em), with cylindrospermopsin which showed cytotoxic effects. No inhibition of protein phosphatase 2A by cylindrospermopsin or lophyrotomin was found. This indicated that other toxic mechanisms besides protein phosphatase inhi bition were producing the toxicities of cylindrospermopsin and lophyrotomin , and that they were unlikely to be potential tumor promoters. (C) 2001 Els evier Science Ltd. All rights reserved.