Proteomic Analysis of Hepatic Tissue of Zebrafish (Danio rerio) Experimentally Exposed to Chronic Microcystin-LR

Microcystin-LR (MCLR) is the most toxic and most frequently encountered hepatotoxin in the aquatic environment. This study investigated the protein profiles of zebrafish (Danio rerio) livers chronically exposed to MCLR concentrations (2 or 20 mu g/l) using the proteomic approach as well as cell ultrastructure, protein phosphatase (PP) activity, protein phosphatase 2A (PP2A) abundance, and toxin content analysis of the hepatic tissue. The results showed that, after 30-day exposure, the presence of MCLR strikingly enhanced toxin accumulation and the PP activity in zebrafish livers. However, the PP2A amounts were independent of toxin treatments. MCLR caused a noticeable damage to liver ultrastructure, a widespread swelling in the rough endoplasmatic reticulum and mitochondria was observed in the MCLR-exposed hepatocytes, and a honeycomb-like structure was formed in the treated nucleoli. Comparison of two-dimensional electrophoresis (2-DE) protein profiles of MCLR-exposed and nonexposed zebrafish livers revealed that the abundance of 22 proteins, measured by 2-DE, was remarkably altered in response to toxin exposure. These proteins were involved in cytoskeleton assembly, macromolecule metabolism, oxidative stress, and signal transduction, indicating that MCLR toxicity in fish liver is complex and diverse. Thus, proteomics provides a new insight into MCLR toxicity, that chronic toxicity of MCLR is different from acute toxicity, and we speculate that the reactive oxygen species pathway might be the main toxic pathway instead of the PP one. Moreover, even a low concentration of MCLR in water could significantly interrupt cellular processes, and more care should be taken in determining the criterion for MCLR content in drinking water.