Cytotoxic and morphological effects of microcystin-LR in in vitro models - a comparision between HepG2, Vero-E6, MDCK and CaCo-2 cell lines

Microcystin-LR (MCLR), a potent hepatotoxin, is transported selectively into the cells throught specific membrane polypeptides mostly present in the liver. These transporters are also expressed in the brain, kidneys and intestine, although the toxicity of MCLR in these cell types is still poorly understood. In this study, morphological, ultrastructural and biochemical analyses were performed in hepatic, renal and intestinal cell lines in order to evaluate the toxicity of MCLR obtained from semi-purified cyanobacterial extracts. Our results show that after 24h of exposure, MCLR induces the viability decrease of all cell lines, in a concentration-dependent manner. HepG2 cells are the most susceptible, followed by Vero, MDCK and CaCo-2. Ultrastructural analyses show that subcytotoxic concentrations of MCLR induce the formation of large cytoplasmic vacuoles, particularly in the HepG2 cell line. Immunofluorescence microscopy reveals that these vacuoles are enriched in LC3B protein, suggesting autophagy as an early cellular response of HepG2 and Vero cells to MCLR. At cytotoxic MCLR concentrations, lysossomal dysfunction in both cell lines occurs prior to mitochondrial disruption, as demonstrated by the specific labeling with Acridine Orange and Rhodamine-123. This suggests that besides mitochondria, lysossomes may also be an MCLR-early target. Immunolocalization and western blot analysis of the endoplasmic reticulum anti-apoptotic protein GRP94 show distinct MCLR-induced effects in Vero and HepG2 cells: re-localization of GRP94 within Vero cells and decrease of GRP94 expression in the HepG2 cell line. These results demonstrate that all the studied cell lines are susceptible to MCLR although with cell type specificity and differential organelle targeting.