Mitochondrial toxicity of microcystin-LR on cultured cells: Application to the analysis of contaminated water samples

Microcystes (MC) are potent hepatic toxins delivered into the cells by organic anion transporting peptides (OATP) where they target protein phosphatases and mitochondria. We analyzed the effects of MC-LR on primary hepatocytes, HepG2, and Jurkat T cells, and isolated rat liver mitochondria by measuring changes in O₂ consumption by optical oxygen sensing technique. Respiration of fresh primary hepatocytes was inhibited by MC-LR with EC50 = 2.74 ± 0.65 nM, whereas an uncoupling effect on mitochondrial state 2 and state 3 respiration was observed with glutamate/malate as a substrate. HepG2 and Jurkat T cells lacking OATP showed no sensitivity to MCLR; however, facilitated delivery of MC-LR resulted in a marked enhancement of HepG2 O₂ consumption and inhibition of Jurkat O₂ consumption at ≥0.1 nM. The respiratory response did not coincide with changes in viability, total cellular ATP, extracellular acidification, ROS formation, or protein phosphorylation, which were detectable at higher MC-LR doses. Such prominent effect on cellular respiration was therefore used for the detection of MC-LR in environmental samples. A simple and sensitive screening assay for MC-LR toxicity was developed, which uses Jurkat cells, facilitated delivery of the toxin(s) and measurement on a fluorescent reader. The assay was applied to a panel of environmental samples suspected to contain MC and benchmarked againstthe ELISA test. It allowed identification of toxic samples and quantification of both nonspecific and MC-LR type of toxicity.