The steady rise in Western obesity rates has been closely linked to significant increases in a multitude of accompanying health problems including Non-Alcoholic Fatty Liver Disease (NAFLD). NAFLD severity ranges from simple steatosis to acute steatohepatitis, but the molecular mechanisms controlling progression of this disease are poorly understood. Recent literature suggests that elevated free fatty acids (FFAs), especially saturated FFAs, may play an important role in lipotoxic mechanisms, both in experimental models and in NAFLD patients. This review highlights important cellular pathways involved in hepatic lipotoxicity and how the degree of intrahepatic lipid saturation controls cell fate in response to an elevated FFA load. Relevant cellular processes that have been causally linked to lipid-induced apoptosis, known as lipoapoptosis, include endoplasmic reticulum (ER) stress, oxidative stress, mitochondrial dysfunction, and Jun N-terminal kinase (JNK) signaling. In contrast, increased triglyceride synthesis has been shown to have a protective effect against lipotoxicity, despite being one of the hallmark traits of NAFLD. Developing a more nuanced understanding of the molecular mechanisms underlying NAFLD progression will lead to more targeted and effective therapeutics for this increasingly prevalent disease, which to date has no proven pharmacologic treatment to prevent or reverse its course.

Palmitate overload induces hepatic cell dysfunction characterized by enhanced apoptosis and altered citric acid cycle (CAC) metabolism; however, the mechanism of how this occurs is incompletely understood. We hypothesize that elevated doses of palmitate disrupt intracellular calcium homeostasis resulting in a net flux of calcium from the ER to mitochondria, activating aberrant oxidative metabolism. We treated primary hepatocytes and H4IIEC3 cells with palmitate and calcium chelators to identify the roles of intracellular calcium flux in lipotoxicity. We then applied 13C metabolic flux analysis (MFA) to determine the impact of calcium in promoting palmitate-stimulated mitochondrial alterations. Co-treatment with the calcium-specific chelator BAPTA resulted in a suppression of markers for apoptosis and oxygen consumption. Additionally, 13C MFA revealed that BAPTA co-treated cells had reduced CAC fluxes compared to cells treated with palmitate alone. Our results demonstrate that palmitate-induced lipoapoptosis is dependent on calcium-stimulated mitochondrial activation, which induces oxidative stress.

Objective Hepatic lipotoxicity is characterized by reactive oxygen species (ROS) accumulation, mitochondrial dysfunction, and excessive apoptosis, but the precise sequence of biochemical events leading to oxidative damage and cell death remain unclear. The goal of this study was to delineate the role of mitochondrial metabolism in mediating hepatocyte lipotoxicity. Materials/Methods We treated H4IIEC3 rat hepatoma cells with free fatty acids in combination with antioxidants and mitochondrial inhibitors designed to block key events in the progression toward apoptosis. We then applied 13C metabolic flux analysis (MFA) to quantify mitochondrial pathway alterations associated with these treatments. Results Treatment with palmitate alone led to a doubling in oxygen uptake rate and in most mitochondrial fluxes. Supplementing culture media with the antioxidant N-acetyl-cysteine (NAC) reduced ROS accumulation and caspase activation and partially restored cell viability. However, 13C MFA revealed that treatment with NAC did not normalize palmitate-induced metabolic alterations, indicating that neither elevated ROS nor downstream apoptotic events contributed to mitochondrial activation. To directly limit mitochondrial metabolism, the complex I inhibitor phenformin was added to cells treated with palmitate. Phenformin addition eliminated abnormal ROS accumulation, prevented the appearance of apoptotic markers, and normalized mitochondrial carbon flow. Further studies revealed that glutamine provided the primary fuel for elevated mitochondrial metabolism in the presence of palmitate, rather than fatty acid beta-oxidation, and that glutamine consumption could be reduced through co-treatment with phenformin but not NAC. Conclusion Our results indicate that ROS accumulation in palmitate-treated H4IIEC3 cells occurs downstream of altered mitochondrial oxidative metabolism, which is independent of beta-oxidation and precedes apoptosis initiation.

FFAs are involved in a diverse range of functions within hepatic cells, including esterifi cation into triacylglycerols (TAGs); oxidation to fuel mitochondrial metabolism, synthesis, and remodeling of phospholipids (PLs); and conversion to signaling molecules such as prostaglandins or leukotrienes. The effects of elevated FFAs have been previously studied in cultured hepatic cell lines as a model for recapitulating the lipotoxicity that has been observed in obese type 2 diabetes and nonalcoholic fatty liver disease (NAFLD) ( 1-5 ). In these disease states, ectopic accumulation of FFAs in nonadipose tissues such as liver, pancreas, and skeletal muscle can interfere with normal cellular function and induce apoptotic cell death. These lipotoxic effects have shown dependence on FA chain length and saturation. Exposure to long-chain saturated FAs (SFAs), such as palmitate (PA) or stearate, leads to lipoapoptosis in many cell types including hepatocytes ( 2, 6, 7 ). In contrast, MUFAs, such as oleate (OA) , are not acutely cytotoxic to hepatic cells and have been shown to exert a protective effect when combined with toxic loads of SFAs ( 8-10 ).The mechanism by which metabolism of specifi c lipid species results in apoptosis has not been fully elucidated.

Experiments in a variety of cell types, including hepatocytes, consistently demonstrate the acutely lipotoxic effects of saturated fatty acids, such as palmitate (PA), but not unsaturated fatty acids, such as oleate (OA). PA+OA co-treatment fully prevents PA lipotoxicity through mechanisms that are not well defined but which have been previously attributed to more efficient esterification and sequestration of PA into triglycerides (TGs) when OA is abundant. However, this hypothesis has never been directly tested by experimentally modulating the relative partitioning of PA/OA between TGs and other lipid fates in hepatocytes. In this study, we found that addition of OA to PA-treated hepatocytes enhanced TG synthesis, reduced total PA uptake and PA lipid incorporation, decreased phospholipid saturation and rescued PA-induced ER stress and lipoapotosis. Knockdown of diacylglycerol acyltransferase (DGAT), the rate-limiting step in TG synthesis, significantly reduced TG accumulation without impairing OAmediated rescue of PA lipotoxicity. In both wild-type and DGAT-knockdown hepatocytes, OA cotreatment significantly reduced PA lipid incorporation and overall phospholipid saturation compared to PA-treated hepatocytes. These data indicate that OA’s protective effects do not require increased conversion of PA into inert TGs, but instead may be due to OA’s ability to compete against PA for cellular uptake and/or esterification and, thereby, normalize the composition of cellular lipids in the presence of a toxic PA load.