Lipid-Induced ER Stress: Synergistic Effects of Sterols and Saturated Fatty Acids

Dysregulation of ceramide metabolism causes phytoceramide-dependent induction of the unfolded protein response

The unfolded protein response (UPR) detects and mitigates the harmful effects of dysregulated endoplasmic reticulum (ER) function. The UPR has been best characterized as a protein quality control response, and the sole UPR sensor in yeast, Ire1, is known to detect misfolded ER proteins. However, recent work suggests the UPR can also sense diverse defects within the ER membrane, including increased fatty acid saturation and altered phospholipid abundance. These and other lipid-related stimuli have been referred to as lipid bilayer stress and may be sensed independently through Ire1's transmembrane domain. Here, we show that the loss of Isc1, a phospholipase that catabolizes complex ceramides, causes UPR induction, even in the absence of exogenous stress. A series of chemical and genetic approaches identified a requirement for very long-chain fatty acid (VLCFA)-containing phytoceramides for UPR induction. In parallel, comprehensive lipidomics analyses identified large increases in the abundance of specific VLCFA-containing phytoceramides in the isc1Δ mutant. We failed to identify evidence of an accompanying defect in protein quality control or ER-associated protein degradation. These results extend our understanding of lipid bilayer stress in the UPR and provide a foundation for mechanistic investigation of this fascinating intersection between ceramide metabolism, membrane homeostasis, and the UPR.

Endoplasmic Reticulum Membrane Homeostasis and the Unfolded Protein Response

The endoplasmic reticulum (ER) is the key organelle for membrane biogenesis. Most lipids are synthesized in the ER, and most membrane proteins are first inserted into the ER membrane before they are transported to their target organelle. The composition and properties of the ER membrane must be carefully controlled to provide a suitable environment for the insertion and folding of membrane proteins. The unfolded protein response (UPR) is a powerful signaling pathway that balances protein and lipid production in the ER. Here, we summarize our current knowledge of how aberrant compositions of the ER membrane, referred to as lipid bilayer stress, trigger the UPR.

Stearoyl-CoA desaturase 1 inhibition induces ER stress-mediated apoptosis in ovarian cancer cells

Ovarian cancer is a leading cause of death among gynecologic tumors, often detected at advanced stages. Metabolic reprogramming and increased lipid biosynthesis are key factors driving cancer cell growth. Stearoyl-CoA desaturase 1 (SCD1) is a crucial enzyme involved in de novo lipid synthesis, producing mono-unsaturated fatty acids (MUFAs). Here, we aimed to investigate the expression and significance of SCD1 in epithelial ovarian cancer (EOC). Comparative analysis of normal ovarian surface epithelial (NOSE) tissues and cell lines revealed elevated SCD1 expression in EOC tissues and cells. Inhibition of SCD1 significantly reduced the proliferation of EOC cells and patient-derived organoids and induced apoptotic cell death. Interestingly, SCD1 inhibition did not affect the viability of non-cancer cells, indicating selective cytotoxicity against EOC cells. SCD1 inhibition on EOC cells induced endoplasmic reticulum (ER) stress by activating the unfolded protein response (UPR) sensors and resulted in apoptosis. The addition of exogenous oleic acid, a product of SCD1, rescued EOC cells from ER stress-mediated apoptosis induced by SCD1 inhibition, underscoring the importance of lipid desaturation for cancer cell survival. Taken together, our findings suggest that the inhibition of SCD1 is a promising biomarker as well as a novel therapeutic target for ovarian cancer by regulating ER stress and inducing cancer cell apoptosis.

MemPrep, a new technology for isolating organellar membranes provides fingerprints of lipid bilayer stress

Biological membranes have a stunning ability to adapt their composition in response to physiological stress and metabolic challenges. Little is known how such perturbations affect individual organelles in eukaryotic cells. Pioneering work has provided insights into the subcellular distribution of lipids in the yeast Saccharomyces cerevisiae, but the composition of the endoplasmic reticulum (ER) membrane, which also crucially regulates lipid metabolism and the unfolded protein response, remains insufficiently characterized. Here, we describe a method for purifying organelle membranes from yeast, MemPrep. We demonstrate the purity of our ER membrane preparations by proteomics, and document the general utility of MemPrep by isolating vacuolar membranes. Quantitative lipidomics establishes the lipid composition of the ER and the vacuolar membrane. Our findings provide a baseline for studying membrane protein biogenesis and have important implications for understanding the role of lipids in regulating the unfolded protein response (UPR). The combined preparative and analytical MemPrep approach uncovers dynamic remodeling of ER membranes in stressed cells and establishes distinct molecular fingerprints of lipid bilayer stress.

Targeting Fatty Acid Desaturase I Inhibits Renal Cancer Growth Via ATF3-mediated ER Stress Response

Monounsaturated fatty acids (MUFAs) play a pivotal role in maintaining endoplasmic reticulum (ER) homeostasis, an emerging hallmark of cancer. However, the role of polyunsaturated fatty acid (PUFAs) desaturation in persistent ER stress driven by oncogenic abnormalities remains elusive. Fatty Acid Desaturase 1 (FADS1) is a rate-limiting enzyme controlling the bioproduction of long-chain PUFAs. Our previous research has demonstrated the significant role of FADS1 in cancer survival, especially in kidney cancers. We explored the underlying mechanism in this study. We found that pharmacological inhibition or knockdown of the expression of FADS1 effectively inhibits renal cancer cell proliferation and induces cell cycle arrest. The stable knockdown of FADS1 also significantly inhibits tumor formation in vivo. Mechanistically, we show that while FADS1 inhibition induces ER stress, its expression is also augmented by ER-stress inducers. Notably, FADS1-inhibition sensitized cellular response to ER stress inducers, providing evidence of FADS1's role in modulating the ER stress response in cancer cells. We show that, while FADS1 inhibition-induced ER stress leads to activation of ATF3, ATF3-knockdown rescues the FADS1 inhibition-induced ER stress and cell growth suppression. In addition, FADS1 inhibition results in the impaired biosynthesis of nucleotides and decreases the level of UPD-N-Acetylglucosamine, a critical mediator of the unfolded protein response. Our findings suggest that PUFA desaturation is crucial for rescuing cancer cells from persistent ER stress, supporting FADS1 as a new therapeutic target.

The spatially resolved transcriptome signatures of glomeruli in chronic kidney disease

Here, we used digital spatial profiling (DSP) to describe the glomerular transcriptomic signatures that may characterize the complex molecular mechanisms underlying progressive kidney disease in Alport syndrome, focal segmental glomerulosclerosis, and membranous nephropathy. Our results revealed significant transcriptional heterogeneity among diseased glomeruli, and this analysis showed that histologically similar glomeruli manifested different transcriptional profiles. Using glomerular pathology scores to establish an axis of progression, we identified molecular pathways with progressively decreased expression in response to increasing pathology scores, including signal recognition particle-dependent cotranslational protein targeting to membrane and selenocysteine synthesis pathways. We also identified a distinct signature of upregulated and downregulated genes common to all the diseases investigated when compared with nondiseased tissue from nephrectomies. These analyses using DSP at the single-glomerulus level could help to increase insight into the pathophysiology of kidney disease and possibly the identification of biomarkers of disease progression in glomerulopathies.

Protein-rich foods, sea foods, and gut microbiota amplify immune responses in chronic diseases and cancers - Targeting PERK as a novel therapeutic strategy for chronic inflammatory diseases, neurodegenerative disorders, and cancer

The endoplasmic reticulum (ER) is a cellular organelle that is physiologically responsible for protein folding, calcium homeostasis, and lipid biosynthesis. Pathological stimuli such as oxidative stress, ischemia, disruptions in calcium homeostasis, and increased production of normal and/or folding-defective proteins all contribute to the accumulation of misfolded proteins in the ER, causing ER stress. The adaptive response to ER stress is the activation of unfolded protein response (UPR), which affect a wide variety of cellular functions to maintain ER homeostasis or lead to apoptosis. Three different ER transmembrane sensors, including PKR-like ER kinase (PERK), activating transcription factor 6 (ATF6), and inositol-requiring enzyme-1 (IRE1), are responsible for initiating UPR. The UPR involves a variety of signal transduction pathways that reduce unfolded protein accumulation by boosting ER-resident chaperones, limiting protein translation, and accelerating unfolded protein degradation. ER is now acknowledged as a critical organelle in sensing dangers and determining cell life and death. On the other hand, UPR plays a critical role in the development and progression of several diseases such as cardiovascular diseases (CVD), metabolic disorders, chronic kidney diseases, neurological disorders, and cancer. Here, we critically analyze the most current knowledge of the master regulatory roles of ER stress particularly the PERK pathway as a conditional danger receptor, an organelle crosstalk regulator, and a regulator of protein translation. We highlighted that PERK is not only ER stress regulator by sensing UPR and ER stress but also a frontier sensor and direct senses for gut microbiota-generated metabolites. Our work also further highlighted the function of PERK as a central hub that leads to metabolic reprogramming and epigenetic modification which further enhanced inflammatory response and promoted trained immunity. Moreover, we highlighted the contribution of ER stress and PERK in the pathogenesis of several diseases such as cancer, CVD, kidney diseases, and neurodegenerative disorders. Finally, we discuss the therapeutic target of ER stress and PERK for cancer treatment and the potential novel therapeutic targets for CVD, metabolic disorders, and neurodegenerative disorders. Inhibition of ER stress, by the development of small molecules that target the PERK and UPR, represents a promising therapeutic strategy.

Endoplasmic Reticulum Stress and Obesity

In recent years, the world has seen an alarming increase in obesity and is closely associated with insulin resistance, which is a state of low-grade inflammation, the latter characterized by elevated levels of proinflammatory cytokines in blood and tissues. A shift in energy balance alters systemic metabolic regulation and the important role that chronic inflammation, endoplasmic reticulum (ER) dysfunction, and activation of the unfolded protein response (UPR) plays in this process.Why obesity is so closely associated with insulin resistance and inflammation is not understood well. This suggests that there are probably many causes for obesity-related insulin resistance and inflammation. One of the faulty mechanisms is protein homeostasis, protein quality control system included protein folding, chaperone activity, and ER-associated degradation leading to endoplasmic reticulum (ER) stress.The ER is a vast membranous network responsible for the trafficking of a wide range of proteins and plays a central role in integrating multiple metabolic signals critical in cellular homeostasis. Conditions that may trigger unfolded protein response activation include increased protein synthesis, the presence of mutant or misfolded proteins, inhibition of protein glycosylation, imbalance of ER calcium levels, glucose and energy deprivation, hypoxia, pathogens, or pathogen-associated components and toxins. Thus, characterizing the mechanisms contributing to obesity and identifying potential targets for its prevention and treatment will have a great impact on the control of associated conditions, particularly T2D.

The leucine zipper domain of the transcriptional repressor Opi1 underlies a signal transduction mechanism regulating lipid synthesis

In Saccharomyces cerevisiae, the transcriptional repressor Opi1 regulates the expression of genes involved in phospholipid synthesis responding to the abundance of the phospholipid precursor phosphatidic acid at the endoplasmic reticulum. We report here the identification of the conserved leucine zipper (LZ) domain of Opi1 as a hot spot for gain of function mutations and the characterization of the strongest variant identified, Opi1N150D. LZ modeling posits asparagine 150 embedded on the hydrophobic surface of the zipper and specifying dynamic parallel homodimerization by allowing electrostatic bonding across the hydrophobic dimerization interface. Opi1 variants carrying any of the other three ionic residues at amino acid 150 were also repressing. Genetic analyses showed that Opi1N150D variant is dominant, and its phenotype is attenuated when loss of function mutations identified in the other two conserved domains are present in cis. We build on the notion that membrane binding facilitates LZ dimerization to antagonize an intramolecular interaction of the zipper necessary for repression. Dissecting Opi1 protein in three polypeptides containing each conserved region, we performed in vitro analyses to explore interdomain interactions. An Opi11-190 probe interacted with Opi1291-404, the C terminus that bears the activator interacting domain (AID). LZ or AID loss of function mutations attenuated the interaction of the probes but was unaffected by the N150D mutation. We propose a model for Opi1 signal transduction whereby synergy between membrane-binding events and LZ dimerization antagonizes intramolecular LZ-AID interaction and transcriptional repression.

The Role of Endoplasmic Reticulum in Lipotoxicity during Metabolic Dysfunction-Associated Steatotic Liver Disease (MASLD) Pathogenesis

Perturbations in lipid and protein homeostasis induce endoplasmic reticulum (ER) stress in metabolic dysfunction-associated steatotic liver disease (MASLD), formerly known as nonalcoholic fatty liver disease. Lipotoxic and proteotoxic stress can activate the unfolded protein response (UPR) transducers: inositol requiring enzyme1α, PKR-like ER kinase, and activating transcription factor 6α. Collectively, these pathways induce expression of genes that encode functions to resolve the protein folding defect and ER stress by increasing the protein folding capacity of the ER and degradation of misfolded proteins. The ER is also intimately connected with lipid metabolism, including de novo ceramide synthesis, phospholipid and cholesterol synthesis, and lipid droplet formation. Following their activation, the UPR transducers also regulate lipogenic pathways in the liver. With persistent ER stress, cellular adaptation fails, resulting in hepatocyte apoptosis, a pathological marker of liver disease. In addition to the ER-nucleus signaling activated by the UPR, the ER can interact with other organelles via membrane contact sites. Modulating intracellular communication between ER and endosomes, lipid droplets, and mitochondria to restore ER homeostasis could have therapeutic efficacy in ameliorating liver disease. Recent studies have also demonstrated that cells can convey ER stress by the release of extracellular vesicles. This review discusses lipotoxic ER stress and the central role of the ER in communicating ER stress to other intracellular organelles in MASLD pathogenesis.

Fundamental and Applicative Aspects of the Unfolded Protein Response in Yeasts

Upon the dysfunction or functional shortage of the endoplasmic reticulum (ER), namely, ER stress, eukaryotic cells commonly provoke a protective gene expression program called the unfolded protein response (UPR). The molecular mechanism of UPR has been uncovered through frontier genetic studies using Saccharomyces cerevisiae as a model organism. Ire1 is an ER-located transmembrane protein that directly senses ER stress and is activated as an RNase. During ER stress, Ire1 promotes the splicing of HAC1 mRNA, which is then translated into a transcription factor that induces the expression of various genes, including those encoding ER-located molecular chaperones and protein modification enzymes. While this mainstream intracellular UPR signaling pathway was elucidated in the 1990s, new intriguing insights have been gained up to now. For instance, various additional factors allow UPR evocation strictly in response to ER stress. The UPR machineries in other yeasts and fungi, including pathogenic species, are another important research topic. Moreover, industrially beneficial yeast strains carrying an enforced and enlarged ER have been produced through the artificial and constitutive induction of the UPR. In this article, we review canonical and up-to-date insights concerning the yeast UPR, mainly from the viewpoint of the functions and regulation of Ire1 and HAC1.

Elevated Adipocyte Membrane Phospholipid Saturation Does Not Compromise Insulin Signaling

Increased saturated fatty acid (SFA) levels in membrane phospholipids have been implicated in the development of metabolic disease. Here, we tested the hypothesis that increased SFA content in cell membranes negatively impacts adipocyte insulin signaling. Preadipocyte cell models with elevated SFA levels in phospholipids were generated by disrupting the ADIPOR2 locus, which resulted in a striking twofold increase in SFA-containing phosphatidylcholines and phosphatidylethanolamines, which persisted in differentiated adipocytes. Similar changes in phospholipid composition were observed in white adipose tissues isolated from the ADIPOR2-knockout mice. The SFA levels in phospholipids could be further increased by treating ADIPOR2-deficient cells with palmitic acid and resulted in reduced membrane fluidity and endoplasmic reticulum stress in mouse and human preadipocytes. Strikingly, increased SFA levels in differentiated adipocyte phospholipids had no effect on adipocyte gene expression or insulin signaling in vitro. Similarly, increased adipocyte phospholipid saturation did not impair white adipose tissue function in vivo, even in mice fed a high-saturated fat diet at thermoneutrality. We conclude that increasing SFA levels in adipocyte phospholipids is well tolerated and does not affect adipocyte insulin signaling in vitro and in vivo.

Fatty acids in cancer chemoresistance

Despite the remarkable clinical success of immunotherapy and molecular targeted therapy in patients with advanced tumors, chemotherapy remains the most commonly used treatment for most tumor patients. Chemotherapy drugs effectively inhibit tumor cell proliferation and survival through their remarkable mechanisms. However, tumor cells often develop severe intrinsic and acquired chemoresistance under chemotherapy stress, limiting the effectiveness of chemotherapy and leading to treatment failure. Growing evidence suggests that alterations in lipid metabolism may be implicated in the development of chemoresistance in tumors. Therefore, in this review, we provide a comprehensive overview of fatty acid metabolism and its impact on chemoresistance mechanisms. Additionally, we discuss the potential of targeting fatty acid metabolism as a therapeutic strategy to overcome drug resistance.

The Unfolded Protein Response: A Double-Edged Sword for Brain Health

Efficient brain function requires as much as 20% of the total oxygen intake to support normal neuronal cell function. This level of oxygen usage, however, leads to the generation of free radicals, and thus can lead to oxidative stress and potentially to age-related cognitive decay and even neurodegenerative diseases. The regulation of this system requires a complex monitoring network to maintain proper oxygen homeostasis. Furthermore, the high content of mitochondria in the brain has elevated glucose demands, and thus requires a normal redox balance. Maintaining this is mediated by adaptive stress response pathways that permit cells to survive oxidative stress and to minimize cellular damage. These stress pathways rely on the proper function of the endoplasmic reticulum (ER) and the activation of the unfolded protein response (UPR), a cellular pathway responsible for normal ER function and cell survival. Interestingly, the UPR has two opposing signaling pathways, one that promotes cell survival and one that induces apoptosis. In this narrative review, we discuss the opposing roles of the UPR signaling pathways and how a better understanding of these stress pathways could potentially allow for the development of effective strategies to prevent age-related cognitive decay as well as treat neurodegenerative diseases.

The Unfolded Protein Response and Its Implications for Novel Therapeutic Strategies in Inflammatory Bowel Disease

The endoplasmic reticulum (ER) is a multifunctional organelle playing a vital role in maintaining cell homeostasis, and disruptions to its functions can have detrimental effects on cells. Dysregulated ER stress and the unfolded protein response (UPR) have been linked to various human diseases. For example, ER stress and the activation of the UPR signaling pathways in intestinal epithelial cells can either exacerbate or alleviate the severity of inflammatory bowel disease (IBD), contingent on the degree and conditions of activation. Our recent studies have shown that EPICERTIN, a recombinant variant of the cholera toxin B subunit containing an ER retention motif, can induce a protective UPR in colon epithelial cells, subsequently promoting epithelial restitution and mucosal healing in IBD models. These findings support the idea that compounds modulating UPR may be promising pharmaceutical candidates for the treatment of the disease. In this review, we summarize our current understanding of the ER stress and UPR in IBD, focusing on their roles in maintaining cell homeostasis, dysregulation, and disease pathogenesis. Additionally, we discuss therapeutic strategies that promote the cytoprotection of colon epithelial cells and reduce inflammation via pharmacological manipulation of the UPR.

The acyltransferase Gpc1 is both a target and an effector of the unfolded protein response in Saccharomyces cerevisiae

The unfolded protein response (UPR) is sensitive to proteotoxic and membrane bilayer stress, both of which are sensed by the ER protein Ire1. When activated, Ire1 splices HAC1 mRNA, producing a transcription factor that targets genes involved in proteostasis and lipid metabolism, among others. The major membrane lipid phosphatidylcholine (PC) is subject to phospholipase-mediated deacylation, producing glycerophosphocholine (GPC), followed by reacylation of GPC through the PC deacylation/reacylation pathway (PC-DRP). The reacylation events occur via a two-step process catalyzed first by the GPC acyltransferase Gpc1, followed by acylation of the lyso-PC molecule by Ale1. However, whether Gpc1 is critical for ER bilayer homeostasis is unclear. Using an improved method for C14-choline-GPC radiolabeling, we first show that loss of Gpc1 results in abrogation of PC synthesis through PC-DRP and that Gpc1 colocalizes with the ER. We then probe the role of Gpc1 as both a target and an effector of the UPR. Exposure to the UPR-inducing compounds tunicamycin, DTT, and canavanine results in a Hac1-dependent increase in GPC1 message. Further, cells lacking Gpc1 exhibit increased sensitivity to those proteotoxic stressors. Inositol limitation, known to induce the UPR via bilayer stress, also induces GPC1 expression. Finally, we show that loss of GPC1 induces the UPR. A gpc1Δ mutant displays upregulation of the UPR in strains expressing a mutant form of Ire1 that is unresponsive to unfolded proteins, indicating that bilayer stress is responsible for the observed upregulation. Collectively, our data indicate an important role for Gpc1 in yeast ER bilayer homeostasis.

Molecular Mechanisms of Lipid-Based Metabolic Adaptation Strategies in Response to Cold

Temperature changes and periods of detrimental cold occur frequently for many organisms in their natural habitats. Homeothermic animals have evolved metabolic adaptation strategies to increase mitochondrial-based energy expenditure and heat production, largely relying on fat as a fuel source. Alternatively, certain species are able to repress their metabolism during cold periods and enter a state of decreased physiological activity known as torpor. By contrast, poikilotherms, which are unable to maintain their internal temperature, predominantly increase membrane fluidity to diminish cold-related damage from low-temperature stress. However, alterations of molecular pathways and the regulation of lipid-metabolic reprogramming during cold exposure are poorly understood. Here, we review organismal responses that adjust fat metabolism during detrimental cold stress. Cold-related changes in membranes are detected by membrane-bound sensors, which signal to downstream transcriptional effectors, including nuclear hormone receptors of the PPAR (peroxisome proliferator-activated receptor) subfamily. PPARs control lipid metabolic processes, such as fatty acid desaturation, lipid catabolism and mitochondrial-based thermogenesis. Elucidating the underlying molecular mechanisms of cold adaptation may improve beneficial therapeutic cold treatments and could have important implications for medical applications of hypothermia in humans. This includes treatment strategies for hemorrhagic shock, stroke, obesity and cancer.

Lipid metabolism and Alzheimer's disease: clinical evidence, mechanistic link and therapeutic promise

Alzheimer's disease (AD) is an age-associated neurodegenerative disorder with multifactorial etiology, intersecting genetic and environmental risk factors, and a lack of disease-modifying therapeutics. While the abnormal accumulation of lipids was described in the very first report of AD neuropathology, it was not until recent decades that lipid dyshomeostasis became a focus of AD research. Clinically, lipidomic and metabolomic studies have consistently shown alterations in the levels of various lipid classes emerging in early stages of AD brains. Mechanistically, decades of discovery research have revealed multifaceted interactions between lipid metabolism and key AD pathogenic mechanisms including amyloidogenesis, bioenergetic deficit, oxidative stress, neuroinflammation, and myelin degeneration. In the present review, converging evidence defining lipid dyshomeostasis in AD is summarized, followed by discussions on mechanisms by which lipid metabolism contributes to pathogenesis and modifies disease risk. Furthermore, lipid-targeting therapeutic strategies, and the modification of their efficacy by disease stage, ApoE status, and metabolic and vascular profiles, are reviewed.

Fatty Acyl Coenzyme A Synthetase Fat1p Regulates Vacuolar Structure and Stationary-Phase Lipophagy in Saccharomyces cerevisiae

During yeast stationary phase, a single spherical vacuole (lysosome) is created by the fusion of several small ones. Moreover, the vacuolar membrane is reconstructed into two distinct microdomains. Little is known, however, about how cells maintain vacuolar shape or regulate their microdomains. Here, we show that Fat1p, a fatty acyl coenzyme A (acyl-CoA) synthetase and fatty acid transporter, and not the synthetases Faa1p and Faa4p, is essential for vacuolar shape preservation, the development of vacuolar microdomains, and cell survival in stationary phase of the yeast Saccharomyces cerevisiae. Furthermore, Fat1p negatively regulates general autophagy in both log- and stationary-phase cells. In contrast, Fat1p promotes lipophagy, as the absence of FAT1 limits the entry of lipid droplets into the vacuole and reduces the degradation of liquid droplet (LD) surface proteins. Notably, supplementing with unsaturated fatty acids or overexpressing the desaturase Ole1p can reverse all aberrant phenotypes caused by FAT1 deficiency. We propose that Fat1p regulates stationary phase vacuolar morphology, microdomain differentiation, general autophagy, and lipophagy by controlling the degree of fatty acid saturation in membrane lipids. IMPORTANCE The ability to sense environmental changes and adjust the levels of cellular metabolism is critical for cell viability. Autophagy is a recycling process that makes the most of already-existing energy resources, and the vacuole/lysosome is the ultimate autophagic processing site in cells. Lipophagy is an autophagic process to select degrading lipid droplets. In yeast cells in stationary phase, vacuoles fuse and remodel their membranes to create a single spherical vacuole with two distinct membrane microdomains, which are required for yeast lipophagy. In this study, we discovered that Fat1p was capable of rapidly responding to changes in nutritional status and preserving cell survival by regulating membrane lipid saturation to maintain proper vacuolar morphology and the level of lipophagy in the yeast S. cerevisiae. Our findings shed light on how cells maintain vacuolar structure and promote the differentiation of vacuole surface microdomains for stationary-phase lipophagy.

Mitochondrial remodelling is essential for female germ cell differentiation and survival

Stem cells often possess immature mitochondria with few inner membrane invaginations, which increase as stem cells differentiate. Despite this being a conserved feature across many stem cell types in numerous organisms, how and why mitochondria undergo such remodelling during stem cell differentiation has remained unclear. Here, using Drosophila germline stem cells (GSCs), we show that Complex V drives mitochondrial remodelling during the early stages of GSC differentiation, prior to terminal differentiation. This endows germline mitochondria with the capacity to generate large amounts of ATP required for later egg growth and development. Interestingly, impairing mitochondrial remodelling prior to terminal differentiation results in endoplasmic reticulum (ER) lipid bilayer stress, Protein kinase R-like ER kinase (PERK)-mediated activation of the Integrated Stress Response (ISR) and germ cell death. Taken together, our data suggest that mitochondrial remodelling is an essential and tightly integrated aspect of stem cell differentiation. This work sheds light on the potential impact of mitochondrial dysfunction on stem and germ cell function, highlighting ER lipid bilayer stress as a potential major driver of phenotypes caused by mitochondrial dysfunction.

Endoplasmic reticulum stress and lipids in health and diseases

The endoplasmic reticulum (ER) is a complex and dynamic organelle that regulates many cellular pathways, including protein synthesis, protein quality control, and lipid synthesis. When one or multiple ER roles are dysregulated and saturated, the ER enters a stress state, which, in turn, activates the highly conserved unfolded protein response (UPR). By sensing the accumulation of unfolded proteins or lipid bilayer stress (LBS) at the ER, the UPR triggers pathways to restore ER homeostasis and eventually induces apoptosis if the stress remains unresolved. In recent years, it has emerged that the UPR works intimately with other cellular pathways to maintain lipid homeostasis at the ER, and so does at cellular levels. Lipid distribution, along with lipid anabolism and catabolism, are tightly regulated, in part, by the ER. Dysfunctional and overwhelmed lipid-related pathways, independently or in combination with ER stress, can have reciprocal effects on other cellular functions, contributing to the development of diseases. In this review, we summarize the current understanding of the UPR in response to proteotoxic stress and LBS and the breadth of the functions mitigated by the UPR in different tissues and in the context of diseases.

Lipid saturation induces degradation of squalene epoxidase for sterol homeostasis and cell survival

A fluid membrane containing a mix of unsaturated and saturated lipids is essential for life. However, it is unclear how lipid saturation might affect lipid homeostasis, membrane-associated proteins, and membrane organelles. Here, we generate temperature-sensitive mutants of the sole fatty acid desaturase gene OLE1 in the budding yeast Saccharomyces cerevisiae Using these mutants, we show that lipid saturation triggers the endoplasmic reticulum-associated degradation (ERAD) of squalene epoxidase Erg1, a rate-limiting enzyme in sterol biosynthesis, via the E3 ligase Doa10-Ubc7 complex. We identify the P469L mutation that abolishes the lipid saturation-induced ERAD of Erg1. Overexpressed WT or stable Erg1 mutants all mislocalize into foci in the ole1 mutant, whereas the stable Erg1 causes aberrant ER and severely compromises the growth of ole1, which are recapitulated by doa10 deletion. The toxicity of the stable Erg1 and doa10 deletion is due to the accumulation of lanosterol and misfolded proteins in ole1 Our study identifies Erg1 as a novel lipid saturation-regulated ERAD target, manifesting a close link between lipid homeostasis and proteostasis that maintains sterol homeostasis under the lipid saturation condition for cell survival.

Regulation of membrane fluidity by RNF145-triggered degradation of the lipid hydrolase ADIPOR2

The regulation of membrane lipid composition is critical for cellular homeostasis. Cells are particularly sensitive to phospholipid saturation, with increased saturation causing membrane rigidification and lipotoxicity. How mammalian cells sense membrane lipid composition and reverse fatty acid (FA)-induced membrane rigidification is poorly understood. Here we systematically identify proteins that differ between mammalian cells fed saturated versus unsaturated FAs. The most differentially expressed proteins were two ER-resident polytopic membrane proteins: the E3 ubiquitin ligase RNF145 and the lipid hydrolase ADIPOR2. In unsaturated lipid membranes, RNF145 is stable, promoting its lipid-sensitive interaction, ubiquitination and degradation of ADIPOR2. When membranes become enriched in saturated FAs, RNF145 is rapidly auto-ubiquitinated and degraded, stabilising ADIPOR2, whose hydrolase activity restores lipid homeostasis and prevents lipotoxicity. We therefore identify RNF145 as a FA-responsive ubiquitin ligase which, together with ADIPOR2, defines an autoregulatory pathway that controls cellular membrane lipid homeostasis and prevents acute lipotoxic stress.

Selective inhibition of protein secretion by abrogating receptor-coat interactions during ER export

Protein secretion is an essential process that drives cell growth, movement, and communication. Protein traffic within the secretory pathway occurs via transport intermediates that bud from one compartment and fuse with a downstream compartment to deliver their contents. Here, we explore the possibility that protein secretion can be selectively inhibited by perturbing protein-protein interactions that drive capture into transport vesicles. Human proprotein convertase subtilisin/kexin type 9 (PCSK9) is a determinant of cholesterol metabolism whose secretion is mediated by a specific cargo adaptor protein, SEC24A. We map a series of protein-protein interactions between PCSK9, its endoplasmic reticulum (ER) export receptor SURF4, and SEC24A that mediate secretion of PCSK9. We show that the interaction between SURF4 and SEC24A can be inhibited by 4-phenylbutyrate (4-PBA), a small molecule that occludes a cargo-binding domain of SEC24. This inhibition reduces secretion of PCSK9 and additional SURF4 clients that we identify by mass spectrometry, leaving other secreted cargoes unaffected. We propose that selective small-molecule inhibition of cargo recognition by SEC24 is a potential therapeutic intervention for atherosclerosis and other diseases that are modulated by secreted proteins.

Partial Replacement of High-Fat Diet with Beef Tallow Attenuates Dyslipidemia and Endoplasmic Reticulum Stress in db/db Mice

High-fat diet (HFD) consumption is closely associated with an increased risk of metabolic syndromes (MetS), such as obesity, type 2 diabetes, and cardiovascular diseases (CVDs). Therefore, the consumption of alternative and functional fatty acids to replace saturated fatty acids and/or trans-fatty acids with polyunsaturated fatty acids has become an important dietary strategy for the prevention of MetS. Consumption of omega-3 fatty acids (n-3) reduces various physiological complications, including CVDs, nonalcoholic fatty liver disease, and insulin resistance, related to inflammatory responses. In this study, we investigated the partial replacement effects of HFD with beef tallow (BT) on dyslipidemia and endoplasmic reticulum (ER) stress in male db/db mice. The animals were grouped to one of four dietary intervention groups (n = 16 per group): (1) normal diet, (2) HFD, (3) HFD partially replaced with regular beef tallow (HFD+BT1), or (4) HFD partially replaced with beef tallow containing a relatively reduced omega-6 fatty acid (n-6)/n-3 ratio (HFD+BT2) than HFD+BT1. After 6 weeks of dietary intervention, 1 mg/kg of phosphate-buffered saline or tunicamycin (TM) was injected intraperitoneally. HFD+BT2 significantly suppressed the serum total cholesterol and non-high-density lipoprotein cholesterol levels more than HFD and HFD+BT1, and triglyceride levels in the epididymal adipose tissue (EAT) were remarkably decreased. Mice that received HFD+BT2 had elevated protein expressions of phospho-AMP-activated protein kinase (p-AMPK). Moreover, HFD+BT2 effectively inhibited ER stress in the liver and EAT. Consistent with our hypothesis, HFD+BT2 remarkably alleviated dyslipidemia and TM-inducible ER stress, while activating p-AMPK.

A lipid viewpoint on the plant endoplasmic reticulum stress response

Organisms, including humans, seem to be constantly exposed to various changes, which often have undesirable effects, referred to as stress. To keep up with these changes, eukaryotic cells may have evolved a number of relevant cellular processes, such as the endoplasmic reticulum (ER) stress response. Owing to presumably intimate links between human diseases and the ER function, the ER stress response has been extensively investigated in various organisms for a few decades. Based on these studies, we now have a picture of the molecular mechanisms of the ER stress response, one of which, the unfolded protein response (UPR), is highly conserved among yeasts, mammals, higher plants, and green algae. In this review, we attempt to highlight the plant UPR from the perspective of lipids, especially membrane phospholipids. Phosphatidylcholine (PtdCho) and phosphatidylethanolamine (PtdEtn) are the most abundant membrane phospholipids in eukaryotic cells. The ratio of PtdCho to PtdEtn and the unsaturation of fatty acyl tails in both phospholipids may be critical factors for the UPR, but the pathways responsible for PtdCho and PtdEtn biosynthesis are distinct in animals and plants. We discuss the plant UPR in comparison with the system in yeasts and animals in the context of membrane phospholipids.

THEM6-mediated reprogramming of lipid metabolism supports treatment resistance in prostate cancer

Despite the clinical benefit of androgen-deprivation therapy (ADT), the majority of patients with advanced prostate cancer (PCa) ultimately develop lethal castration-resistant prostate cancer (CRPC). In this study, we identified thioesterase superfamily member 6 (THEM6) as a marker of ADT resistance in PCa. THEM6 deletion reduces in vivo tumour growth and restores castration sensitivity in orthograft models of CRPC. Mechanistically, we show that the ER membrane-associated protein THEM6 regulates intracellular levels of ether lipids and is essential to trigger the induction of the ER stress response (UPR). Consequently, THEM6 loss in CRPC cells significantly alters ER function, reducing de novo sterol biosynthesis and preventing lipid-mediated activation of ATF4. Finally, we demonstrate that high THEM6 expression is associated with poor survival and correlates with high levels of UPR activation in PCa patients. Altogether, our results highlight THEM6 as a novel driver of therapy resistance in PCa as well as a promising target for the treatment of CRPC.

Induction and Aggravation of the Endoplasmic-Reticulum Stress by Membrane-Lipid Metabolic Intermediate Phosphatidyl-N-Monomethylethanolamine

Phosphatidylcholine (PC) is produced via two distinct pathways in both hepatocytes and yeast, Saccharomyces cerevisiae. One of these pathways involves the sequential methylation of phosphatidylethanolamine (PE). In yeast cells, the methyltransferase, Cho2, converts PE to phosphatidylmonomethylethanolamine (PMME), which is further modified to PC by another methyltransferase, Opi3. On the other hand, free choline is utilized for PC production via the Kennedy pathway. The blockage of PC production is well known to cause endoplasmic reticulum (ER) stress and activate the ER-stress sensor, Ire1, to induce unfolded protein response (UPR). Here, we demonstrate that even when free choline is sufficiently supplied, the opi3Δ mutation, but not the cho2 Δ mutation, induces the UPR. The UPR was also found to be induced by CHO2 overexpression. Further, monomethylethanolamine, which is converted to PMME probably through the Kennedy pathway, caused or potentiated ER stress in both mammalian and yeast cells. We thus deduce that PMME per se is an ER-stressing molecule. Interestingly, spontaneously accumulated PMME seemed to aggravate ER stress in yeast cells. Collectively, our findings demonstrate the multiple detrimental effects of the low-abundance phospholipid species, PMME.

Disruption of the Snf1 Gene Enhances Cell Growth and Reduces the Metabolic Burden in Cellulase-Expressing and Lipid-Accumulating Yarrowia lipolytica

Yarrowia lipolytica is known to be capable of metabolizing glucose and accumulating lipids intracellularly; however, it lacks the cellulolytic enzymes needed to break down cellulosic biomass directly. To develop Y. lipolytica as a consolidated bioprocessing (CBP) microorganism, we previously expressed the heterologous CBH I, CBH II, and EG II cellulase enzymes both individually and collectively in this microorganism. We concluded that the coexpression of these cellulases resulted in a metabolic drain on the host cells leading to reduced cell growth and lipid accumulation. The current study aims to build a new cellulase coexpressing platform to overcome these hinderances by (1) knocking out the sucrose non-fermenting 1 (Snf1) gene that represses the energetically expensive lipid and protein biosynthesis processes, and (2) knocking in the cellulase cassette fused with the recyclable selection marker URA3 gene in the background of a lipid-accumulating Y. lipolytica strain overexpressing ATP citrate lyase (ACL) and diacylglycerol acyltransferase 1 (DGA1) genes. We have achieved a homologous recombination insertion rate of 58% for integrating the cellulases-URA3 construct at the disrupted Snf1 site in the genome of host cells. Importantly, we observed that the disruption of the Snf1 gene promoted cell growth and lipid accumulation and lowered the cellular saturated fatty acid level and the saturated to unsaturated fatty acid ratio significantly in the transformant YL163t that coexpresses cellulases. The result suggests a lower endoplasmic reticulum stress in YL163t, in comparison with its parent strain Po1g ACL-DGA1. Furthermore, transformant YL163t increased in vitro cellulolytic activity by 30%, whereas the “total in vivo newly formed FAME (fatty acid methyl esters)” increased by 16% in comparison with a random integrative cellulase-expressing Y. lipolytica mutant in the same YNB-Avicel medium. The Snf1 disruption platform demonstrated in this study provides a potent tool for the further development of Y. lipolytica as a robust host for the expression of cellulases and other commercially important proteins.

Regulation and functions of membrane lipids: Insights from Caenorhabditis elegans

The Caenorhabditis elegans plasma membrane is composed of glycerophospholipids and sphingolipids with a small cholesterol. The C. elegans obtain the majority of the membrane lipids by modifying fatty acids present in the bacterial diet. The metabolic pathways of membrane lipid biosynthesis are well conserved across the animal kingdom. In C. elegans CDP-DAG and Kennedy pathway produce glycerophospholipids. Meanwhile, the sphingolipids are synthesized through a different pathway. They have evolved remarkably diverse mechanisms to maintain membrane lipid homeostasis. For instance, the lipid bilayer stress operates to accomplish homeostasis during any perturbance in the lipid composition. Meanwhile, the PAQR-2/IGLR-2 complex works with FLD-1 to balance unsaturated to saturated fatty acids to maintain membrane fluidity. The loss of membrane lipid homeostasis is observed in many human genetic and metabolic disorders. Since C. elegans conserved such genes and pathways, it can be used as a model organism.

The Unfolded Protein Response as a Guardian of the Secretory Pathway

The endoplasmic reticulum (ER) is the major site of membrane biogenesis in most eukaryotic cells. As the entry point to the secretory pathway, it handles more than 10,000 different secretory and membrane proteins. The insertion of proteins into the membrane, their folding, and ER exit are affected by the lipid composition of the ER membrane and its collective membrane stiffness. The ER is also a hotspot of lipid biosynthesis including sterols, glycerophospholipids, ceramides and neural storage lipids. The unfolded protein response (UPR) bears an evolutionary conserved, dual sensitivity to both protein-folding imbalances in the ER lumen and aberrant compositions of the ER membrane, referred to as lipid bilayer stress (LBS). Through transcriptional and non-transcriptional mechanisms, the UPR upregulates the protein folding capacity of the ER and balances the production of proteins and lipids to maintain a functional secretory pathway. In this review, we discuss how UPR transducers sense unfolded proteins and LBS with a particular focus on their role as guardians of the secretory pathway.

Induction of Endoplasmic Reticulum Stress-Mediated Apoptosis by Aminosteroid RM-581 Efficiently Blocks the Growth of PC-3 Cancer Cells and Tumors Resistant or Not to Docetaxel

Aminosteroid derivative RM-581 was previously identified as an endoplasmic-reticulum (ER) stress inducer with potent in vitro and in vivo anticancer activities. We report its evaluation in androgen-independent prostate cancer (PC-3) cells. RM-581 efficiently blocks PC-3 cell proliferation with stronger activity than that of a selection of known antineoplastic agents. This later also showed a synergistic effect with docetaxel, able to block the proliferation of docetaxel-resistant PC-3 cells and, contrary to docetaxel, did not induce cell resistance. RM-581 induced an increase in the expression level of ER stress-related markers of apoptosis, potentially triggered by the presence of RM-581 in the ER of PC-3 cells. These in vitro results were then successfully translated in vivo in a PC-3 xenograft tumor model in nude mice, showing superior blockade than that of docetaxel. RM-581 was also able to stop the progression of PC-3 cells when they had become resistant to docetaxel treatment. Concomitantly, we observed a decrease in gene markers of mevalonate and fatty acid pathways, and intratumoral levels of cholesterol by 19% and fatty acids by 22%. Overall, this work demonstrates the potential of an ER stress inducer as an anticancer agent for the treatment of prostate cancers that are refractory to commonly used chemotherapy treatments.

Protein Quality Control and Lipid Droplet Metabolism

Lipid droplets (LDs) are endoplasmic reticulum-derived organelles that consist of a core of neutral lipids encircled by a phospholipid monolayer decorated with proteins. As hubs of cellular lipid and energy metabolism, LDs are inherently involved in the etiology of prevalent metabolic diseases such as obesity and nonalcoholic fatty liver disease. The functions of LDs are regulated by a unique set of associated proteins, the LD proteome, which includes integral membrane and peripheral proteins. These proteins control key activities of LDs such as triacylglycerol synthesis and breakdown, nutrient sensing and signal integration, and interactions with other organelles. Here we review the mechanisms that regulate the composition of the LD proteome, such as pathways that mediate selective and bulk LD protein degradation and potential connections between LDs and cellular protein quality control.

Adipocyte-driven unfolded protein response is a shared transcriptomic signature of metastatic prostate carcinoma cells

A critical unknown in the field of skeletal metastases is how cancer cells find a way to thrive under harsh conditions, as exemplified by metastatic colonization of adipocyte-rich bone marrow by prostate carcinoma cells. To begin understanding molecular processes that enable tumor cells to survive and progress in difficult microenvironments such as bone, we performed unbiased examination of the transcriptome of two different prostate cancer cell lines in the absence or presence of bone marrow adipocytes. Our RNAseq analyses and subsequent quantitative PCR and protein-based assays reveal that upregulation of endoplasmic reticulum (ER) stress and unfolded protein response (UPR) genes is a shared signature between metastatic prostate carcinoma cell lines of different origin. Pathway analyses and pharmacological examinations highlight the ER chaperone BIP as an upstream coordinator of this transcriptomic signature. Additional patient-based data support our overall conclusion that ER stress and UPR induction are shared, important factors in the response and adaptation of metastatic tumor cells to their micro-environment. Our studies pave the way for additional mechanistic investigations and offer new clues towards effective therapeutic interventions in metastatic disease.

How the brain fights fatty acids' toxicity

Neurons spurn hydrogen-rich fatty acids for energizing oxidative ATP synthesis, contrary to other cells. This feature has been mainly attributed to a lower yield of ATP per reduced oxygen, as compared to glucose. Moreover, the use of fatty acids as hydrogen donor is accompanied by severe β-oxidation-associated ROS generation. Neurons are especially susceptible to detrimental activities of ROS due to their poor antioxidative equipment. It is also important to note that free fatty acids (FFA) initiate multiple harmful activities inside the cells, particularly on phosphorylating mitochondria. Several processes enhance FFA-linked lipotoxicity in the cerebral tissue. Thus, an uptake of FFA from the circulation into the brain tissue takes place during an imbalance between energy intake and energy expenditure in the body, a situation similar to that during metabolic syndrome and fat-rich diet. Traumatic or hypoxic brain injuries increase hydrolytic degradation of membrane phospholipids and, thereby elevate the level of FFA in neural cells. Accumulation of FFA in brain tissue is markedly associated with some inherited neurological disorders, such as Refsum disease or X-linked adrenoleukodystrophy (X-ALD). What are strategies protecting neurons against FFA-linked lipotoxicity? Firstly, spurning the β-oxidation pathway in mitochondria of neurons. Secondly, based on a tight metabolic communication between neurons and astrocytes, astrocytes donate metabolites to neurons for synthesis of antioxidants. Further, neuronal autophagy of ROS-emitting mitochondria combined with the transfer of degradation-committed FFA for their disposal in astrocytes, is a potent protective strategy against ROS and harmful activities of FFA. Finally, estrogens and neurosteroids are protective as triggers of ERK and PKB signaling pathways, consequently initiating the expression of various neuronal survival genes via the formation of cAMP response element-binding protein (CREB).

Beyond Proteostasis: Lipid Metabolism as a New Player in ER Homeostasis

Biological membranes are not only essential barriers that separate cellular and subcellular structures, but also perform other critical functions such as the initiation and propagation of intra- and intercellular signals. Each membrane-delineated organelle has a tightly regulated and custom-made membrane lipid composition that is critical for its normal function. The endoplasmic reticulum (ER) consists of a dynamic membrane network that is required for the synthesis and modification of proteins and lipids. The accumulation of unfolded proteins in the ER lumen activates an adaptive stress response known as the unfolded protein response (UPR-ER). Interestingly, recent findings show that lipid perturbation is also a direct activator of the UPR-ER, independent of protein misfolding. Here, we review proteostasis-independent UPR-ER activation in the genetically tractable model organism Caenorhabditis elegans. We review the current knowledge on the membrane lipid composition of the ER, its impact on organelle function and UPR-ER activation, and its potential role in human metabolic diseases. Further, we summarize the bi-directional interplay between lipid metabolism and the UPR-ER. We discuss recent progress identifying the different respective mechanisms by which disturbed proteostasis and lipid bilayer stress activate the UPR-ER. Finally, we consider how genetic and metabolic disturbances may disrupt ER homeostasis and activate the UPR and discuss how using -omics-type analyses will lead to more comprehensive insights into these processes.

Regulation of lipid metabolism by the unfolded protein response

The endoplasmic reticulum (ER) is the site of protein folding and secretion, Ca²⁺ storage and lipid synthesis in eukaryotic cells. Disruption to protein folding or Ca²⁺ homeostasis in the ER leads to the accumulation of unfolded proteins, a condition known as ER stress. This leads to activation of the unfolded protein response (UPR) pathway in order to restore protein homeostasis. Three ER membrane proteins, namely inositol‐requiring enzyme 1 (IRE1), protein kinase RNA‐like ER kinase (PERK) and activating transcription factor 6 (ATF6), sense the accumulation of unfolded/misfolded proteins and are activated, initiating an integrated transcriptional programme. Recent literature demonstrates that activation of these sensors can alter lipid enzymes, thus implicating the UPR in the regulation of lipid metabolism. Given the presence of ER stress and UPR activation in several diseases including cancer and neurodegenerative diseases, as well as the growing recognition of altered lipid metabolism in disease, it is timely to consider the role of the UPR in the regulation of lipid metabolism. This review provides an overview of the current knowledge on the impact of the three arms of the UPR on the synthesis, function and regulation of fatty acids, triglycerides, phospholipids and cholesterol.

Modulation of de Novo Lipogenesis Improves Response to Enzalutamide Treatment in Prostate Cancer

Simple Summary

Prostate cancer cells produce lipids via the activation of a specific pathway called fatty acid synthesis, also known as De novo lipogenesis. This pathway is essential for the survival and growth of most types of cancer cells, including prostate cancer. In our study, we showed that prostate cancer cells activate this lipid synthesis pathway to become more aggressive and develop resistance to commonly used therapeutic agents for advanced prostate cancer such as enzalutamide, an effective and commonly used androgen receptor (AR) targeted agent. Interestingly, by combining enzalutamide with a lipid synthesis pathway inhibitor, we were able to show that growth of prostate cancer tumors was more effectively reduced than with either agent alone. We also showed that this combination led to cell stress and death by changing the lipid content in the cell. These important findings could lead to new therapeutic strategies combining effective AR targeted therapies with lipid synthesis inhibitors for the treatment of advanced prostate cancer.

Abstract

De novo lipogenesis (DNL) is now considered as a hallmark of cancer. The overexpression of key enzymes of DNL is characteristic of both primary and advanced disease and may play an important role in resistance to therapies. Here, we showed that DNL is highly enhanced in castrate resistant prostate cancer (CRPC) cells compared to hormone sensitive and enzalutamide resistant cells. This observation suggests that this pathway plays an important role in the initiation of aggressive prostate cancer and in the development of enzalutamide resistance. Importantly, here we show that both prostate cancer cells sensitive and resistant to enzalutamide are dependent on DNL to proliferate. We next combined enzalutamide with an inhibitor of Stearoyl CoA Desaturase 1 (SCD1), an important enzyme in DNL, and observed significantly reduced tumor growth caused by the important change in tumoral lipid desaturation. Our findings suggest that the equilibrium between monounsaturated fatty acids and saturated fatty acids is essential in the establishment of the more aggressive prostate cancer phenotype and that the combination therapy induces a disruption of this equilibrium leading to an important decrease of cell proliferation. These findings provide new insights into the role of DNL in the progression of prostate cancer cells. The study also provides the rationale for the use of an inhibitor of SCD1 in combination with enzalutamide to improve response, delay enzalutamide resistance and improve disease free progression.

Glued in lipids: Lipointoxication in cystic fibrosis

Cystic Fibrosis (CF) is an autosomal recessive disease caused by mutations in the CF transmembrane regulator (CFTR) gene, which encodes a chloride channel located at the apical surface of epithelial cells. Unsaturated Fatty Acid (UFA) deficiency has been a persistent observation in tissues from patients with CF. However, the impacts of such deficiencies on the etiology of the disease have been the object of intense debates. The aim of the present review is first to highlight the general consensus on fatty acid dysregulations that emerges from, sometimes apparently contradictory, studies. In a second step, a unifying mechanism for the potential impacts of these fatty acid dysregulations in CF cells, based on alterations of membrane biophysical properties (known as lipointoxication), is proposed. Finally, the contribution of lipointoxication to the progression of the CF disease and how it could affect the efficacy of current treatments is also discussed.

Metabolism of Storage Lipids and the Role of Lipid Droplets in the Yeast Schizosaccharomyces pombe

Storage lipids, triacylglycerols (TAG), and steryl esters (SE), are predominant constituents of lipid droplets (LD) in fungi. In several yeast species, metabolism of TAG and SE is linked to various cellular processes, including cell division, sporulation, apoptosis, response to stress, and lipotoxicity. In addition, TAG are an important source for the generation of value-added lipids for industrial and biomedical applications. The fission yeast Schizosaccharomyces pombe is a widely used unicellular eukaryotic model organism. It is a powerful tractable system used to study various aspects of eukaryotic cellular and molecular biology. However, the knowledge of S. pombe neutral lipids metabolism is quite limited. In this review, we summarize and discuss the current knowledge of the homeostasis of storage lipids and of the role of LD in the fission yeast S. pombe with the aim to stimulate research of lipid metabolism and its connection with other essential cellular processes. We also discuss the advantages and disadvantages of fission yeast in lipid biotechnology and recent achievements in the use of S. pombe in the biotechnological production of valuable lipid compounds.

A comprehensive review about SARS-CoV-2

The coronavirus disease (COVID-19) was first identified in China, December 2019. Since then, it has spread the length and breadth of the world at an unprecedented, alarming rate. Severe acute respiratory syndrome coronavirus (SARS-CoV)-2, which causes COVID-19, has much in common with its closest homologs, SARS-CoV and Middle East respiratory syndrome-CoV. The virus-host interaction of SARS-CoV-2 uses the same receptor, ACE2, which is similar to that of SARS-CoV, which spreads through the respiratory tract. Patients with COVID-19 report symptoms including mild-to-severe fever, cough and fatigue; very few patients report gastrointestinal infections. There are no specific antiviral strategies. A few strong medications are under investigation, so we have to focus on proposals which ought to be taken to forestall this infection in a living host.

A Quantitative Analysis of Cellular Lipid Compositions During Acute Proteotoxic ER Stress Reveals Specificity in the Production of Asymmetric Lipids

The unfolded protein response (UPR) is central to endoplasmic reticulum (ER) homeostasis by controlling its size and protein folding capacity. When activated by unfolded proteins in the ER-lumen or aberrant lipid compositions, the UPR adjusts the expression of hundreds of target genes to counteract ER stress. The proteotoxic drugs dithiothreitol (DTT) and tunicamycin (TM) are commonly used to induce misfolding of proteins in the ER and to study the UPR. However, their potential impact on the cellular lipid composition has never been systematically addressed. Here, we report the quantitative, cellular lipid composition of Saccharomyces cerevisiae during acute, proteotoxic stress in both rich and synthetic media. We show that DTT causes rapid remodeling of the lipidome when used in rich medium at growth-inhibitory concentrations, while TM has only a marginal impact on the lipidome under our conditions of cultivation. We formulate recommendations on how to study UPR activation by proteotoxic stress without interferences from a perturbed lipid metabolism. Furthermore, our data suggest an intricate connection between the cellular growth rate, the abundance of the ER, and the metabolism of fatty acids. We show that Saccharomyces cerevisiae can produce asymmetric lipids with two saturated fatty acyl chains differing substantially in length. These observations indicate that the pairing of saturated fatty acyl chains is tightly controlled and suggest an evolutionary conservation of asymmetric lipids and their biosynthetic machineries.

Stress sensor Ire1 deploys a divergent transcriptional program in response to lipid bilayer stress

Membrane integrity at the endoplasmic reticulum (ER) is tightly regulated, and its disturbance is implicated in metabolic diseases. Using an engineered sensor that activates the unfolded protein response (UPR) exclusively when normal ER membrane lipid composition is compromised, we identified pathways beyond lipid metabolism that are necessary to maintain ER integrity in yeast and in C. elegans. To systematically validate yeast mutants that disrupt ER membrane homeostasis, we identified a lipid bilayer stress (LBS) sensor in the UPR transducer protein Ire1, located at the interface of the amphipathic and transmembrane helices. Furthermore, transcriptome and chromatin immunoprecipitation analyses pinpoint the UPR as a broad-spectrum compensatory response wherein LBS and proteotoxic stress deploy divergent transcriptional UPR programs. Together, these findings reveal the UPR program as the sum of two independent stress responses, an insight that could be exploited for future therapeutic intervention.

The Roles of Endoplasmic Reticulum in NLRP3 Inflammasome Activation

The NLRP3 (nucleotide-binding domain, leucine-rich-repeat-containing family, pyrin domain-containing 3) inflammasome senses pathogen-associated molecular patterns (PAMPs) and danger-associated molecular patterns (DAMPs), and activates caspase-1, which provokes release of proinflammatory cytokines such as interleukin-1β (IL-1β) and IL-18 as well as pyroptosis to engage in innate immune defense. The endoplasmic reticulum (ER) is a large and dynamic endomembrane compartment, critical to cellular function of organelle networks. Recent studies have unveiled the pivotal roles of the ER in NLRP3 inflammasome activation. ER–mitochondria contact sites provide a location for NLRP3 activation, its association with ligands released from or residing in mitochondria, and rapid Ca²⁺ mobilization from ER stores to mitochondria. ER-stress signaling plays a critical role in NLRP3 inflammasome activation. Lipid perturbation and cholesterol trafficking to the ER activate the NLRP3 inflammasome. These findings emphasize the importance of the ER in initiation and regulation of the NLRP3 inflammasome.

Tunicamycin induced endoplasmic reticulum changes in endothelial cells investigated in vitro by confocal Raman imaging

This paper describes how tunicamycin (Tu), the most widely used pharmacological agent for inducing endoplasmic reticulum (ER) stress, interacts with endothelial cells. Our results show that tunicamycin enters the cells and accumulates within the ER area. ER stress takes place when improperly folded or damaged proteins begin to accumulate; however, spectroscopic markers of these changes have not been identified as yet. In this work, Raman spectroscopy and scanning electron microscopy imaging of individual endothelial cells treated with Tu were performed. The changes in the biochemical composition of endothelial cells induced by Tu attributed to ER stress were studied in detail. A main feature of the Tu impact on the cells was a decrease of the phospholipid content in the area of ER, and the most abundant lipid with phosphorus groups found there, was identified as sphingomyelin.

CrABCA2 Facilitates Triacylglycerol Accumulation in Chlamydomonas reinhardtii under Nitrogen Starvation

The microalga Chlamydomonas reinhardtii accumulates triacylglycerols (TAGs) in lipid droplets under stress conditions, such as nitrogen starvation. TAG biosynthesis occurs mainly at the endoplasmic reticulum (ER) and requires fatty acid (FA) substrates supplied from chloroplasts. How FAs are transferred from chloroplast to ER in microalgae was unknown. We previously reported that an Arabidopsis thaliana ATP-binding cassette (ABC) transporter, AtABCA9, facilitates FA transport at the ER during seed development. Here we identified a gene homologous to AtABCA9 in the C. reinhardtii genome, which we named CrABCA2. Under nitrogen deprivation conditions, CrABCA2 expression was upregulated, and the CrABCA2 protein level also increased. CrABCA2 knockdown lines accumulated less TAGs and CrABCA2 overexpression lines accumulated more TAGs than their untransformed parental lines. Transmission electron microscopy showed that CrABCA2 was localized in swollen ER. These results suggest that CrABCA2 transports substrates for TAG biosynthesis to the ER during nitrogen starvation . Our study provides a potential tool for increasing lipid production in microalgae.

Protein misfolding in endoplasmic reticulum stress with applications to renal diseases

Protein misfolding may be the result of a variety of different processes that disrupt the ability of a protein to form a thermodynamically stable tertiary structure that allows it to perform its proper function. In this chapter, we explore the nature of a protein's form that allows it to have a stable tertiary structure, and examine specific mutation that are known to occur in the coding regions of DNA that disrupt a protein's ability to be folded into a thermodynamically stable tertiary structure. We examine the consequences of these protein misfoldings in terms of the endoplasmic reticulum stress response and resulting unfolded protein response. These conditions are specifically related to renal diseases. Further, we explore novel therapeutics, pharmacological chaperones, that are being developed to alleviate the disease burden associated with protein misfolding caused by mutations. These interventions aim to stabilize protein folding intermediates and allow proper folding to occur as well as prevent protein aggregation and the resulting pathophysiological consequences.

Dysregulation of very-long-chain fatty acid metabolism causes membrane saturation and induction of the unfolded protein response

The unfolded protein response (UPR) senses defects in the endoplasmic reticulum (ER) and orchestrates a complex program of adaptive cellular remodeling. Increasing evidence suggests an important relationship between lipid homeostasis and the UPR. Defects in the ER membrane induce the UPR, and the UPR in turn controls the expression of some lipid metabolic genes. Among lipid species, the very-long-chain fatty acids (VLCFAs) are relatively rare and poorly understood. Here, we show that loss of the VLCFA-coenzyme A synthetase Fat1, which is essential for VLCFA utilization, results in ER stress with compensatory UPR induction. Comprehensive lipidomic analyses revealed a dramatic increase in membrane saturation in the fat1Δ mutant, likely accounting for UPR induction. In principle, this increased membrane saturation could reflect adaptive membrane remodeling or an adverse effect of VLCFA dysfunction. We provide evidence supporting the latter, as the fat1Δ mutant showed defects in the function of Ole1, the sole fatty acyl desaturase in yeast. These results indicate that VLCFAs play essential roles in protein quality control and membrane homeostasis and suggest an unexpected requirement for VLCFAs in Ole1 function.

The unfolded protein response and hepatic lipid metabolism in non alcoholic fatty liver disease

Nonalcoholic fatty liver disease is a major public health burden. Although many features of nonalcoholic fatty liver disease pathogenesis are known, the specific mechanisms and susceptibilities that determine an individual's risk of developing nonalcoholic steatohepatitis versus isolated steatosis are not well delineated. The predominant and defining histologic and imaging characteristic of nonalcoholic fatty liver disease is the accumulation of lipids. Dysregulation of lipid homeostasis in hepatocytes leads to transient generation or accumulation of toxic lipids that result in endoplasmic reticulum (ER) stress with inflammation, hepatocellular damage, and apoptosis. ER stress activates the unfolded protein response (UPR) which is classically viewed as an adaptive pathway to maintain protein folding homeostasis. Recent studies have uncovered the contribution of the UPR sensors in the regulation of hepatic steatosis and in the cellular response to lipotoxic stress. Interestingly, the UPR sensors can be directly activated by toxic lipids, independently of the accumulation of misfolded proteins, termed lipotoxic and proteotoxic stress, respectively. The dual function of the UPR sensors in protein and lipid homeostasis suggests that these two types of stress are interconnected likely due to the central role of the ER in protein folding and trafficking and lipid biosynthesis and trafficking, such that perturbations in either impact the function of the ER and activate the UPR sensors in an effort to restore homeostasis. The precise molecular similarities and differences between proteotoxic and lipotoxic ER stress are beginning to be understood. Herein, we provide an overview of the mechanisms involved in the activation and cross-talk between the UPR sensors, hepatic lipid metabolism, and lipotoxic stress, and discuss the possible therapeutic potential of targeting the UPR in nonalcoholic fatty liver disease.