Saturated fatty acids alter the late secretory pathway by modulating membrane properties.

Saturated fatty acids (SFA) have been reported to alter organelle integrity and function in many cell types, including muscle and pancreatic ß-cells, adipocytes, hepatocytes and cardiomyocytes. SFA accumulation results in increased amounts of ceramides/sphingolipids and saturated phospholipids (PL). In this study, using a yeast-based model that recapitulates most of the trademarks of SFA-induced lipotoxicity in mammalian cells, we demonstrate that these lipid species act at different levels of the secretory pathway. Ceramides mostly appear to modulate the induction of the unfolded protein response and the transcription of nutrient transporters destined to the cell surface. On the other hand, saturated PL, by altering membrane properties, directly impact vesicular budding at later steps in the secretory pathway, i.e. at the trans-Golgi Network level. They appear to do so by increasing lipid order within intracellular membranes which, in turn, alters the recruitment of loose lipid packing-sensing proteins, required for optimal budding, to nascent vesicles. We propose that this latter general mechanism could account for the well-documented deleterious impacts of fatty acids on the last steps of the secretory pathway in several cell types.

Modulation of lipid-induced ER stress by fatty acid shape.

Exposure of pancreatic ß cells to long-chain saturated fatty acids (SFA) induces a so-called endoplasmic reticulum (ER) stress that can ultimately lead to cell death. This process is believed to participate in insulin deficiency associated with type 2 diabetes, via a decrease in ß-cell mass. By contrast, some unsaturated fatty acid species appear less toxic to the cells and can even alleviate SFA-induced ER stress. In the present study, we took advantage of a simple yeast-based model, which brings together most of the trademarks of lipotoxicity in human cells, to screen fatty acids of various structures for their capacity to counter ER stress. Here we demonstrate that the tendency of a free fatty acid (FFA) to reduce SFA toxicity depends on a complex conjunction of parameters, including chain length, level of unsaturation, position of the double bonds and nature of the isomers (cis or trans). Interestingly, potent FFA act as building blocks for phospholipid synthesis and help to restore an optimal membrane organization, compatible with ER function and normal protein trafficking.

Chemical biology approaches to membrane homeostasis and function.

The study of membranes is at a turning point. New theories about membrane structure and function have recently been proposed, however, new technologies, combining chemical, physical, and biochemical approaches are necessary to test these hypotheses. In particular, the NCCR in chemical biology aims to visualize and characterize membrane microdomains and determine their function during hormone signaling.

Lipid-induced ER stress in yeast and ß cells: parallel trails to a common fate.

Exposure to long-chain saturated fatty acids (SFAs; e.g. palmitate) induces apoptosis in pancreatic ß cells, a process that may contribute to the development of type 2 diabetes. Under palmitate treatment, ß cells undergo a so-called endoplasmic reticulum (ER) stress that can be counteracted by the unfolded protein response (UPR). The UPR is a coordinated response, which is primarily devoted to helping the ER to cope with the accumulation of misfolded proteins. Sustained SFA exposure may ultimately overwhelm the UPR, resulting in cell death. By contrast, unsaturated fatty acids (e.g. oleate) are much less harmful to the cells and can even alleviate palmitate toxicity. Surprisingly, recent evidences indicate that a simple unicellular eukaryote, the budding yeast Saccharomyces cerevisiae, which is not routinely exposed to high-fat diets, also undergoes ER stress under lipotoxic conditions. This suggests that the mechanisms of SFA toxicity are largely conserved throughout eukaryotes and are not specific of a given cell type. The present review discusses the mechanisms of SFA toxicity in yeast and ß cells, with a main emphasis on their potential impacts on ER-membrane organization/function and ER-based processes.

Lipid-induced ER stress: synergistic effects of sterols and saturated fatty acids.

Stress within the endoplasmic reticulum (ER) induces a coordinated response, namely the unfolded protein response (UPR), devoted to helping the ER cope with the accumulation of misfolded proteins. Failure of the UPR plays an important role in several human diseases. Recent studies report that intracellular accumulation of saturated fatty acids (SFAs) and cholesterol, seen in diseases of high incidence, such as obesity or atherosclerosis, results in ER stress. In the present study, we evaluated the effects of perturbations to lipid homeostasis on ER stress/UPR induction in the model eukaryote Saccharomyces cerevisiae. We show that SFA originating from either endogenous(preclusion of fatty acid desaturation) or exogenous (feeding with extracellular SFA) sources trigger ER stress and that ergosterol, the major sterol in yeast, acts synergistically with SFA in this process. This latter effect is connected to ergosterol accumulation within microsomal fractions from SFA-accumulating cells, which display highly saturated phospholipid content. Moreover, treating the cells with the molecular chaperone 4-phenyl butyrate abolishes UPR induction, suggesting that lipid-induced ER stress leads to an overload of misfolded protein that acts, in turn, as the molecular signal for induction of the UPR. The present data are discussed in the context of human diseases that involve lipid deregulation.

A lipid-mediated quality control process in the Golgi apparatus in yeast.

When heme biosynthesis is disrupted, the yeast Saccharomyces cerevisiae becomes unable to synthesize its major sterol, ergosterol, and desaturate fatty acids. We took advantage of this physiological peculiarity to evaluate the consequences of ergosterol and/or unsaturated fatty acid (UFA) depletions on the biogenesis of a model polytopic plasma membrane protein, the uracil permease Fur4p. We show that under UFA shortage, which results in low amounts of diunsaturated phospholipid species, and under ergosterol depletion, Fur4p is prematurely routed from the Golgi apparatus to the vacuolar lumen in a process that requires the ubiquitin ligase Rsp5p. Interestingly, this diversion is not correlated to Fur4p exclusion from detergent-resistant membranes. In an independent set of experiments, we show that Fur4p targeting to the plasma membrane depends on phosphatidylethanolamine amounts and more specifically on the propensity of this phospholipid to form a hexagonal phase. In light of recent literature, we propose a model in which ergosterol and diunsaturated phospholipid species maintain optimal membrane curvature for Fur4p to evade the Golgi quality control process and to be properly delivered to its normal destination.