Characterisation of Fumonisin B1 toxicity in a cancerous liver cell line- induction of tissue transglutaminase and the endoplasmic reticulum stress pathway.
Fumonisin B1 (FB1) is a mycotoxin which is well characterised as a contaminant of maize and maize-based products worldwide, especially in South Africa. Its toxic effects have been associated with hepatotoxicity, nephrotoxicity and carcinogenicity. Tissue transglutaminase (TG2) is a unique and ubiquitous enzyme that catalyses the post-translational modification of proteins and has GTPase activity. Tissue transglutaminase is an important enzyme in a number of biological processes such as cell differentiation and proliferation, extracellular matrix organisation, cell signalling and apoptosis. This study investigated the possible role of TG2 induction by FB1 and the effect FB1 toxicity has on the endoplasmic reticulum (ER) stress pathway in HepG2 cells. A SDS-PAGE adaption of the TG2 activity assay confirmed TG2 crosslinking activity by FB1 incorporation into fibronectin in the presence of calcium and TG2. This interaction was validated using fluorescent microscopy where FB1 incorporated into the HepG2 cell’s cytoplasmic vesicles and plasma membrane. The up-regulation of TG2 in HepG2 cells treated with FB1 was further investigated using western blotting and showed increased TG2 up-regulation. Fumonisin B1 disrupts membrane-bound sphingolipids as a mechanism of toxicity; FB1 was shown to cause cytoskeletal damage and disrupted the cell’s membranes leading to cell stress. The protein kinase RNA-like endoplasmic reticulum kinase (PERK) ER stress pathway was induced as a result of FB1 exposure and investigated using western blotting and quantitative polymerase chain reaction. After 72hours with 50μM and 100μM FB1 total PERK decreased, phosphorylated eukaryotic initiating factor α remained activated with a significant increase in messenger RNA (mRNA) expression (p<0.05) and transcription factor CCAAT-enhancer-binding protein homologous protein mRNA was significantly induced (p<0.05). The involvement of nuclear factor kappa B (NFkB) and TG2 in ER stress induced apoptosis was investigated through western blotting and quantitative polymerase chain reaction. After 72hours, an up-regulation of both nuclear NFkB and nuclear TG2 was observed; with a corresponding significant increase in nuclear TG2 mRNA expression (p<0.05). A significant increase in transcription factor, Sp1 mRNA expression (p<0.05) was observed after 72hours. Data suggests PERK activation leads to NFkB induction and nuclear translocation; which promoted nuclear TG2 transcription. The activation of TG2 resulted in Sp1 crosslinks that could act as potential inducers of FB1 induced apoptosis. Flow cytometry was used to measure apoptosis and mitochondrial depolarisation. Caspase activity was measured using the Caspase-Glo® assays and ATP concentration was measured using CellTitre-GloTM assay. After 72hours caspases 3/7 and 8 showed a significant decrease in activity at 100μM FB1 (p<0.05) and a decrease in caspase 9. After 72hours with 10μM FB1 treatment a significant increase in phosphatidylserine externalisation (p<0.05), a significant decrease in healthy/live cells (p<0.05) and a significant increase in depolarised mitochondria (p<0.05) were observed. There was also a significant increase in Sp1 mRNA expression (p<0.05). However, at 50μM FB1 treatment there was a decrease in phosphatidylserine externalisation, a significant increase in live cells (p<0.05) and a significant decrease in depolarised mitochondria (p<0.05). Data suggests that ER stress persisted in HepG2 cells with no apoptosis or cell recovery occurring at high chronic doses of FB1 whilst ER stress induced apoptosis at low chronic doses of FB1 in HepG2 cells. Fumonisin B1 may be a possible substrate for TG2 crosslinking activity due to its primary amine group, since this mycotoxin has the potential to induce TG2 expression and activation. Further studies are required to determine the role of FB1 in the inositol-requiring protein 1α and activating transcription factor 6 arms of the ER stress pathway.