Sensitization of Human Liver Cells Toward Fas-Mediated Apoptosis by the Metabolically Activated Pyrrolizidine Alkaloid Lasiocarpine.

Abstract

Pyrrolizidine alkaloids (PAs) are common phytotoxins. Intoxication can lead to liver damage. Previous studies showed PA-induced apoptosis in liver cells. However, the exact role of the extrinsic apoptotic pathway has not been investigated yet. This study aims to analyze whether the PA representative lasiocarpine sensitizes human liver cells toward extrinsic Fas-mediated apoptosis. HepG2 cells with limited xenobiotic metabolic activity are used to analyze metabolism-dependent effects. External in vitro metabolism is simulated using rat or human liver enzymes. Additionally, metabolically competent HepaRG cells are used to confirm the observed effects in a human liver cell system with internal xenobiotic metabolism. Metabolized lasiocarpine decreases cell viability and induces Fas receptor gene expression in both cell lines. Increased Fas receptor protein expression on the cell surface is demonstrated by flow cytometry. The addition of a Fas ligand-simulating antibody induces apoptosis. Induction of extrinsic Fas-mediated apoptosis is verified by Western blotting for cleaved caspase 8, the initiator caspase of extrinsic apoptosis. All effects are dependent on lasiocarpine metabolism. The results demonstrate that metabolically metabolized lasiocarpine sensitizes human liver cells toward Fas-mediated apoptosis. They broaden our knowledge on the hepatotoxic molecular mechanisms of PA as widely distributed food contaminants.