Cyanobacterial toxin cylindrospermopsin (CYN) is an emerging freshwater contaminant, whose expanding environmental occurrence might result into increased human health risks. CYN is potent hepatotoxin, with cytotoxicity and genotoxicity documented in primary hepatocytes or hepatoma cell lines. However, there is only limited information about CYN effects on adult human liver stem cells (LSCs), which play an important role in liver tissue development, regeneration and repair. In our study with human liver cell line HL1-hT1 which expresses characteristics of LSCs, CYN was found to be cytotoxic and increasing cell death after 24-48 h exposure to concentrations >1 μM. Subcytotoxic 1 μM concentration did not induce cell death or membrane damage, but inhibited cellular processes related to energy production, leading to a growth stagnation after >72 h. Interestingly, these effects were not associated with increased DNA damage, reactive oxygen species production, or endoplasmic reticulum stress. However, CYN induced a sustained (24-48 h) activation of mitogen-activated protein kinases ERK1/2 and p38, and increased expression of stress-related transcription factor ATF3. Thus, LSCs were not primarily affected by CYN-induced genotoxicity and oxidative stress, but via activation of signaling and transcriptional pathways critical for regulation of cell proliferation, stress responses, cell survival and inflammation. Alterations of LSCs during CYN-induced liver injury, including the role of nongenotoxic mechanisms, should be therefore considered in mechanistic assessments of chronic CYN hepatotoxicity and hepatocarcinogenicity.

RECETOX Faculty of Science MasarykUniversity Kamenice 753 5 625 00 Brno Czech Republic

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