Melatonin Repairs the Lipidome of Human Hepatocytes Exposed to Cd and Free Fatty Acid-Induced Lipotoxicity

Hepatocyte lipotoxicity is central to the aetiology of nonalcoholic fatty liver disease (NAFLD), a leading cause of liver failure and transplantation worldwide. Long-lasting toxic pollutants have increasingly been considered as environmental risk factors of NAFLD. These include cadmium (Cd), a metal that synergizes with other cellular toxicants and metabolic stimuli to induce fat build-up and lipotoxicity. Recent studies demonstrated that melatonin (MLT) holds great potential as repairing agent in this form of hepatocyte lipotoxicity. In this study, the molecular hints of this MLT effect were investigated by lipidomics analysis in undifferentiated HepaRG cells, a human pre-hepatocyte cell line, exposed to Cd toxicity either alone or combined with prototypical free fatty acids (FFA), namely the saturated species palmitic acid and the monounsaturated oleic acid (OA and PA, respectively), to simulate the cellular lipotoxicity conditions of fatty liver disease. Cd exposure synergized with FFAs to induce cellular steatosis, and PA produced higher levels of lipotoxicity compared to OA by leading to increased levels of H₂O₂ production and apoptotic death. These effects were associated with changes of the cellular lipidome, which approximate those of NAFLD liver, with differentially expressed lipids in different classes that included triacylglycerols (TG), di- and mono-acylglycerols, phospholipids (PL), sphingolipids, acylcarnitines and FA; characteristic differences were observed in all these classes comparing the combinations of Cd exposure with PA or OA treatments. MLT significantly reduced the effects of either individual or combinatorial treatments of Cd and FFAs on lipotoxicity hallmarks, also repairing most of the alterations of the cellular lipidome, including those of the chain length and number of double bonds of acyl residues esterified to TG and PL classes. These findings and their bioinformatics interpretation suggest a role for the earliest acyl elongase and desaturase steps of FA metabolism in this repairing effect of MLT; biochemistry studies validated such interpretation identifying a specific role for SCD1 activity. This lipidomics study shed light on the cytoprotective mechanism of MLT in Cd and FFA-induced hepatocyte lipotoxicity, highlighting a repairing effect of this molecule on the cellular lipidome, which may hold therapeutic potential in fatty liver diseases.