Differential Lipotoxic Effects of Palmitate and Oleate in Activated Human Hepatic Stellate Cells and Epithelial Hepatoma Cells.

BACKGROUND/AIMS: Nonalcoholic fatty liver disease (NAFLD) progression to fibrosis, cirrhosis and hepatocellular carcinoma, alters the cellular composition of this organ. During late-stage NAFLD, fibrotic and possibly cancerous cells can proliferate and, like normal hepatocytes, are exposed to high concentrations of fatty acids from both surrounding tissue and circulating lipid sources. We hypothesized that primary human activated hepatic stellate cells and epithelial hepatoma (HepG2) cells respond differently to lipotoxic conditions, and investigated the mechanisms involved. METHODS: Primary activated hepatic stellate cells and HepG2 cells were exposed to pathophysiological concentrations of fatty acids and comparative studies of lipid metabolic and stress response pathways were performed. RESULTS: Both cell types remained proliferative during exposure to a combination of palmitate plus oleate reflective of the general saturated versus unsaturated fatty acid composition of western diets. However, exposure to either high palmitate or high oleate alone induced cytotoxicity in activated stellate cells, while only palmitate caused cytotoxicity in HepG2 cells. mRNA microarray and biochemical comparisons revealed that stellate cells stored markedly less fatty acids as neutral lipids, and had reduced capacity for beta-oxidation. Similar to previous observations in HepG2 cells, palmitate, but not oleate, induced ER stress and actin stress fiber formation in activated stellate cells. In contrast, oleate, but not palmitate, induced the inflammatory signal TXNIP, decreased cytoskeleton proteins, and decreased cell polarity preceding cell death in activated stellate cells. CONCLUSIONS: Palmitate-induced lipotoxicity was associated with ER stress pathways in both primary activated hepatic stellate cells and epithelial hepatoma cells, whereas high oleate caused lipotoxicity only in activated stellate cells, possibly through a distinct mechanism involving disruption of cytoskeleton components. This may have implications for optimal dietary fatty acid compositions during various stages of NAFLD.

Elongation Factor 1A-1 Is a Mediator of Hepatocyte Lipotoxicity Partly through Its Canonical Function in Protein Synthesis.

Elongation factor 1A-1 (eEF1A-1) has non-canonical functions in regulation of the actin cytoskeleton and apoptosis. It was previously identified through a promoter-trap screen as a mediator of fatty acid-induced cell death (lipotoxicity), and was found to participate in this process downstream of ER stress. Since ER stress is implicated in the pathogenesis of nonalcoholic fatty liver disease (NAFLD), we investigated the mechanism of action of eEF1A-1 in hepatocyte lipotoxicity. HepG2 cells were exposed to excess fatty acids, followed by assessments of ER stress, subcellular localization of eEF1A-1, and cell death. A specific inhibitor of eEF1A-1 elongation activity, didemnin B, was used to determine whether its function in protein synthesis is involved in lipotoxicity. Within 6 h, eEF1A-1 protein was modestly induced by high palmitate, and partially re-localized from its predominant location at the ER to polymerized actin at the cell periphery. This early induction and subcellular redistribution of eEF1A-1 coincided with the onset of ER stress, and was later followed by cell death. Didemnin B did not prevent the initiation of ER stress by high palmitate, as indicated by eIF2α phosphorylation. However, consistent with sustained inhibition of eEF1A-1-dependent elongation activity, didemnin B prevented the recovery of protein synthesis and increase in GRP78 protein that are normally associated with later phases of the response to ongoing ER stress. This resulted in decreased palmitate-induced cell death. Our data implicate eEF1A-1, and its function in protein synthesis, in hepatocyte lipotoxicity.