Excess fatty acid accumulation in non-adipose tissues is a hallmark of metabolic disease. When elevated lipid levels exceed the cell’s capacity to store or utilize fatty acids, a lipotoxic-response is elicited, characterized by destruction of organelle membranes, activation of stress pathways, and apoptosis. This review focuses on the mechanisms by which lipid overload causes non-adipose cell death and contributes to the pathogenesis of obesity and diabetes.

SUMMARY Lipotoxicity is a metabolic stress response implicated in the pathogenesis of diabetes complications and has been shown to involve lipid-induced oxidative stress. To elucidate the molecular mechanisms of lipotoxicity, we used retroviral promoter trap mutagenesis to isolate a cell line that is resistant to lipotoxic and oxidative stress. We show that loss of three box C/D small nucleolar RNAs (snoRNAs) encoded in the ribosomal protein L13a (rpL13a) locus is sufficient to confer resistance to lipotoxic and oxidative stress in vitro and prevents the propagation of oxidative stress in vivo. Our results provide evidence for a previously unappreciated, non-canonical role for box C/D snoRNAs as regulators of metabolic stress response pathways in mammalian cells.

In obesity and diabetes, an imbalance in fatty acid uptake and fatty acid utilization leads to excess accumulation of lipid in non-adipose tissues. This lipid overload is associated with cellular dysfunction and cell death, which contribute to organ failure, a phenomenon termed lipotoxicity. To elucidate the molecular mechanism of lipid-mediated cell death, we generated and characterized a mutant Chinese hamster ovary cell line that is resistant to palmitate-induced cell death. In this mutant, random insertion of a retroviral promoter trap has disrupted the gene for the non-coding RNA, growth arrested DNA-damage inducible gene 7 (gadd7). Here we report that gadd7 is induced by lipotoxic stress in a reactive oxygen species (ROS)-dependent fashion and is necessary for both lipid-and general oxidative stressmediated cell death. Depletion of gadd7 by mutagenesis or short hairpin RNA knockdown significantly reduces lipid and nonlipid induced ROS. Furthermore, depletion of gadd7 delays and diminishes ROS-induced endoplasmic reticulum stress. Together these data are the first to implicate a non-coding RNA in a feed-forward loop with oxidative stress and its induction of the endoplasmic reticulum stress response.Cellular homeostasis can be perturbed by a myriad of stimuli, including metabolic imbalance, oxidative stress, and aberrant protein folding. In response to such stressors, cells induce specific molecular pathways that commonly involve activation of signaling cascades or alterations in gene expression (1, 2). These responses enable cells to adapt to relatively modest stress and regain homeostasis. However, if the stress is extreme or prolonged, cells are unable to re-establish homeostasis and in turn, activate pathways that result in cell death.In obesity and diabetes, high serum triglycerides and free fatty acids (FFAs) 2 lead to excess accumulation of lipid in nonadipose tissues. This lipid accumulation is associated with cellular dysfunction and cell death, which contribute to organ failure, a phenomenon termed lipotoxicity (3). Evidence from human studies implicates lipotoxicity in heart failure associated with obesity and diabetes by showing a link between cardiomyocyte lipid accumulation and heart muscle dysfunction (4 -6). In rodent models of diabetes and in several transgenic mouse models, increased cardiac fatty acid uptake and oxidation and/or cardiomyocyte lipid accumulation is associated with heart failure (7-12). Similarly, lipid accumulation in the pancreas, kidney, and liver in obesity and diabetes is associated with organ dysfunction (13-15). Furthermore, end-organ damage in diabetes and obesity is associated with oxidative and endoplasmic reticulum (ER) stress that may be related in part to lipotoxicity, because perturbation of lipid metabolism alone can lead to these responses (11, 16 -22).Studies from our laboratory and others show that lipotoxicity can be modeled in established cell lines by supplementation of culture media with pathophysiological concentrations of the saturated FFA, palmitate...

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