Stearoyl-CoA desaturase 1 activity is required for autophagosome formation

Stearoyl-CoA desaturase 1 is targeted by EBV-encoded miR-BART20-5p and regulates cell autophagy, proliferation, and migration in EBV-associated gastric cancer

Epstein-Barr virus (EBV) is the first human oncogenic virus known to express microRNAs (miRNAs), which are closely associated with the development of various tumors, including nasopharyngeal and gastric cancers. Stearoyl-CoA Desaturase 1 (SCD1) is a key enzyme in fatty acid synthesis, highly expressed in numerous tumors, promoting tumor growth and metastasis, making it a potential therapeutic target. In this study, we found that SCD1 expression in EBV-associated gastric cancer (EBVaGC) was significantly lower than in EBV-negative gastric cancer (EBVnGC) at both cellular and tissue levels. In addition, EBV-miR-BART20-5p targets the 3'-UTR of SCD1, downregulating its expression. Moreover, overexpression of SCD1 in EBVaGC cells promoted cell migration and proliferation while inhibiting autophagy. These results suggest that EBV-encoded miRNA-BART20-5p may contribute to EBVaGC progression by targeting SCD1.

Death and the desaturase: Implication of Stearoyl-CoA desaturase-1 in the mechanisms of cell stress, apoptosis, and ferroptosis

Growth and proliferation of normal and cancerous cells necessitate a finely-tuned regulation of lipid metabolic pathways to ensure the timely supply of structural, energetic, and signaling lipid molecules. The synthesis and remodeling of lipids containing fatty acids with an appropriate carbon length and insaturation level are required for supporting each phase of the mechanisms of cell replication and survival. Mammalian Stearoyl-CoA desaturases (SCD), particularly SCD1, play a crucial role in modulating the fatty acid composition of cellular lipids, converting saturated fatty acids (SFA) into monounsaturated fatty acids (MUFA) in the endoplasmic reticulum (ER). Extensive research has elucidated in great detail the participation of SCD1 in the molecular mechanisms that govern cell replication in normal and cancer cells. More recently, investigations have shed new light on the functional and regulatory role of the Δ9-desaturase in the processes of cell stress and cell death. This review will examine the latest findings on the involvement of SCD1 in the molecular pathways of cell survival, particularly on the mechanisms of ER stress and autophagy, as well in apoptotic and non-apoptotic cell death.

Scd1 and monounsaturated lipids are required for autophagy and survival of adipocytes

OBJECTIVE

Exposure of adipocytes to 'cool' temperatures often found in the periphery of the body induces expression of Stearoyl-CoA Desaturase-1 (Scd1), an enzyme that converts saturated fatty acids to monounsaturated fatty acids. The goal of this study is to further investigate the roles of Scd in adipocytes.

METHOD

In this study, we employed Scd1 knockout cells and mouse models, along with pharmacological Scd1 inhibition to dissect the enzyme's function in adipocyte physiology.

RESULTS

Our study reveals that production of monounsaturated lipids by Scd1 is necessary for fusion of autophagosomes to lysosomes and that with a Scd1-deficiency, autophagosomes accumulate. In addition, Scd1-deficiency impairs lysosomal and autolysosomal acidification resulting in vacuole accumulation and eventual cell death. Blocking autophagosome formation or supplementation with monounsaturated fatty acids maintains vitality of Scd1-deficient adipocytes.

CONCLUSION

This study demonstrates the indispensable role of Scd1 in adipocyte survival, with its inhibition in vivo triggering autophagy-dependent cell death and its depletion in vivo leading to the loss of bone marrow adipocytes.

SCD1 is the critical signaling hub to mediate metabolic diseases: Mechanism and the development of its inhibitors

Metabolic diseases, featured with dysregulated energy homeostasis, have become major global health challenges. Patients with metabolic diseases have high probability to manifest multiple complications in lipid metabolism, e.g. obesity, insulin resistance and fatty liver. Therefore, targeting the hub genes in lipid metabolism may systemically ameliorate the metabolic diseases, along with the complications. Stearoyl-CoA desaturase 1(SCD1) is a key enzyme that desaturates the saturated fatty acids (SFAs) derived from de novo lipogenesis or diet to generate monounsaturated fatty acids (MUFAs). SCD1 maintains the metabolic and tissue homeostasis by responding to, and integrating the multiple layers of endogenous stimuli, which is mediated by the synthesized MUFAs. It critically regulates a myriad of physiological processes, including energy homeostasis, development, autophagy, tumorigenesis and inflammation. Aberrant transcriptional and epigenetic activation of SCD1 regulates AMPK/ACC, SIRT1/PGC1α, NcDase/Wnt, etc, and causes aberrant lipid accumulation, thereby promoting the progression of obesity, non-alcoholic fatty liver, diabetes and cancer. This review critically assesses the integrative mechanisms of the (patho)physiological functions of SCD1 in metabolic homeostasis, inflammation and autophagy. For translational perspective, potent SCD1 inhibitors have been developed to treat various types of cancer. We thus discuss the multidisciplinary advances that greatly accelerate the development of SCD1 new inhibitors. In conclusion, besides cancer treatment, SCD1 may serve as the promising target to combat multiple metabolic complications simultaneously.

Identification of Chemicals That Abrogate Folate-Dependent Inhibition of Starch Accumulation in Non-Photosynthetic Plastids of Arabidopsis

Folate, also known as vitamin B9, is an essential cofactor for a variety of enzymes and plays a crucial role in many biological processes. We previously reported that plastidial folate prevents starch biosynthesis triggered by the influx of sugar into non-starch-accumulating plastids, such as etioplasts, and chloroplasts under darkness; hence the loss of plastidial folate induces the accumulation of starch in plastids. To understand the molecular mechanism underlying this phenomenon, we screened our in-house chemical library and searched their derivatives to identify chemicals capable of inducing starch accumulation in etioplasts. The results revealed four chemicals, compounds #120 and #375 and their derivatives, compounds #120d and #375d, respectively. The derivative compounds induced starch accumulation in etioplasts and suppressed hypocotyl elongation in dark-grown Arabidopsis seedlings. They also inhibited the post-germinative growth of seedlings under illumination. All four chemicals contained the sulfonamide group as a consensus structure. The sulfonamide group is also found in sulfa drugs, which exhibit antifolate activity, and in sulfonylurea herbicides. Further analyses revealed that compound #375d induces starch accumulation by inhibiting folate biosynthesis. By contrast, compound #120d neither inhibited folate biosynthesis nor exhibited the herbicide activity. Protein and metabolite analyses suggest that compound #120d abrogates folate-dependent inhibition of starch accumulation in etioplasts by enhancing starch biosynthesis.

SCD1 and monounsaturated lipids are required for autophagy and survival of adipocytes

Exposure of adipocytes to 'cool' temperatures often found in the periphery of the body induces expression of Stearoyl-CoA Desaturase-1 (SCD1), an enzyme that converts saturated fatty acids to monounsaturated fatty acids. In this study, we employed Scd1 knockout cells and mouse models, along with pharmacological SCD1 inhibition, to investigate further the roles of SCD1 in adipocytes. Our study reveals that production of monounsaturated lipids by SCD1 is necessary for fusion of autophagosomes to lysosomes and that with a SCD1-deficiency, autophagosomes accumulate. In addition, SCD1-deficiency impairs lysosomal and autolysosomal acidification resulting in vacuole accumulation and eventual cell death. Blocking autophagosome formation or supplementation with monounsaturated fatty acids maintains vitality of SCD1-deficient adipocytes. Taken together, our results demonstrate that in vitro inhibition of SCD1 in adipocytes leads to autophagy-dependent cell death, and in vivo depletion leads to loss of bone marrow adipocytes.

The ultrastructural function of MLN64 in the late endosome-mitochondria membrane contact sites in placental cells

The close apposition between two different organelles is critical in essential processes such as ion homeostasis, signaling, and lipid transition. However, information related to the structural features of membrane contact sites (MCSs) is limited. This study used immuno-electron microscopy and immuno-electron tomography (I-ET) to analyze the two- and three-dimensional structures of the late endosome-mitochondria contact sites in placental cells. Filamentous structures or tethers were identified that connected the late endosomes and mitochondria. Lamp1 antibody-labeled I-ET revealed enrichment of tethers in the MCSs. The cholesterol-binding endosomal protein metastatic lymph node 64 (MLN64) encoded by STARD3 was required for the formation of this apposition. The distance of the late endosome-mitochondria contact sites was <20 nm, shorter than that in STARD3-knockdown cells (<150 nm). The perturbation of cholesterol egress from the endosomes induced by U18666A treatment produced a longer distance in the contact sites than that in knockdown cells. The late endosome-mitochondria tethers failed to form correctly in STARD3-knockdown cells. Our results unravel the role of MLN64 involved in MCSs between late endosomes and mitochondria in placental cells.

De novo lipogenesis fuels adipocyte autophagosome and lysosome membrane dynamics

Adipocytes robustly synthesize fatty acids (FA) from carbohydrate through the de novo lipogenesis (DNL) pathway, yet surprisingly DNL contributes little to their abundant triglyceride stored in lipid droplets. This conundrum raises the hypothesis that adipocyte DNL instead enables membrane expansions to occur in processes like autophagy, which requires an abundant supply of phospholipids. We report here that adipocyte Fasn deficiency in vitro and in vivo markedly impairs autophagy, evident by autophagosome accumulation and severely compromised degradation of the autophagic substrate p62. Our data indicate the impairment occurs at the level of autophagosome-lysosome fusion, and indeed, loss of Fasn decreases certain membrane phosphoinositides necessary for autophagosome and lysosome maturation and fusion. Autophagy dependence on FA produced by Fasn is not fully alleviated by exogenous FA in cultured adipocytes, and interestingly, imaging studies reveal that Fasn colocalizes with nascent autophagosomes. Together, our studies identify DNL as a critical source of FAs to fuel autophagosome and lysosome maturation and fusion in adipocytes.

SNORD88C guided 2'-O-methylation of 28S rRNA regulates SCD1 translation to inhibit autophagy and promote growth and metastasis in non-small cell lung cancer

Small nucleolar RNAs (snoRNAs) have been shown to play critical regulatory roles in cancer development. SNORD88C, which located at the intronic region of C19orf48 in chromosome 19q.33 with a 97-nt length was screened through database and snoRNA-sequencing. We firstly verified this snoRNA was up-regulated in tissue and plasma and served as a non-invasive diagnostic biomarker; then confirmed that SNORD88C promoted proliferation and metastasis of NSCLC in vitro and in vivo. Mechanistically, SNORD88C promoted 2'-O-methylation modification at the C3680 site on 28S rRNA and in turn enhanced downstream SCD1 translation, a central lipogenic enzyme for the synthesis of MUFA that can inhibit autophagy by regulating lipid peroxidation and mTOR, providing the novel insight into the regulation of SNORD88C in NSCLC.

Oleic acid from cancer-associated fibroblast promotes cancer cell stemness by stearoyl-CoA desaturase under glucose-deficient condition

BACKGROUND

Cancer-associated fibroblasts (CAFs) coordinate the malignancy of cancer cells via secretory materials. Reprogrammed lipid metabolism and signaling play critical roles in cancer biology. Oleic acid (OA) serves as a source of energy under glucose-deficient conditions, but its function in cancer progression remains unclear. The present study investigated that CAFs in xenografted tumors had higher amounts of fatty acids, particularly OA, compared to normal fibroblasts, and promoted the cancer cell stemness in lung adenocarcinoma cells under glucose-deficient condition.

METHODS

Xenografts were established in immunodeficient mice by injection of NCI-H460 (H460) cells. Lipids and fatty acids were evaluated using the BODIPY staining and fatty-acid methyl esters analysis. The expression levels of markers for lipid metabolism and cancer stemness were determined by western blot, flow cytometry, and real-time PCR. Cancer cell subclones against stearoyl-CoA desaturase (SCD) were produced by lentiviral vector and CRISPR/cas9 systems. The expression of SCD was examined immunochemically in human adenocarcinoma tissues, and its clinical relevance to survival rate in lung adenocarcinoma patients was assessed by Kaplan-Meier analysis.

RESULTS

Transferred CAF-derived OA through lipid transporter upregulated SCD in cancer cells under glucose-deficient conditions, resulting in enhanced lipid metabolism and autophagosome maturation. By OA treatment under glucose deficient condition, cancer cell stemness was significantly enhanced through sequential activation of SCD, F-actin polymerization and nuclear translocation of yes-associated protein. These findings were confirmed by experiments using chemical inhibitors, SCD-overexpressing cells and SCD-knockout (KO) cells. When xenografted, SCD-overexpressing cells produced larger tumors compared with parental cells, while SCD-KO cells generated much smaller tumors. Analysis of tumor tissue microarray from lung adenocarcinoma patients revealed that SCD expression was the marker for poor prognosis involving tumor grade, clinical stage and survival rate.

CONCLUSION

Our data indicate that CAFs-derived OA activated lipid metabolism in lung adenocarcinoma cells under glucose-deficient conditions, subsequently enhancing stemness and progression toward malignancy.

The cholesterol transport protein GRAMD1C regulates autophagy initiation and mitochondrial bioenergetics

During autophagy, cytosolic cargo is sequestered into double-membrane vesicles called autophagosomes. The contributions of specific lipids, such as cholesterol, to the membranes that form the autophagosome, remain to be fully characterized. Here, we demonstrate that short term cholesterol depletion leads to a rapid induction of autophagy and a corresponding increase in autophagy initiation events. We further show that the ER-localized cholesterol transport protein GRAMD1C functions as a negative regulator of starvation-induced autophagy and that both its cholesterol transport VASt domain and membrane binding GRAM domain are required for GRAMD1C-mediated suppression of autophagy initiation. Similar to its yeast orthologue, GRAMD1C associates with mitochondria through its GRAM domain. Cells lacking GRAMD1C or its VASt domain show increased mitochondrial cholesterol levels and mitochondrial oxidative phosphorylation, suggesting that GRAMD1C may facilitate cholesterol transfer at ER-mitochondria contact sites. Finally, we demonstrate that expression of GRAMD family proteins is linked to clear cell renal carcinoma survival, highlighting the pathophysiological relevance of cholesterol transport proteins.

Acyl-CoA binding protein regulates nutrient-dependent autophagy

BACKGROUND

Homeostasis of autophagy under normal conditions and nutrient stress is maintained by adaptive activation of regulatory proteins. However, the protein-lipid crosstalk that modulates the switch from suppression to activation of autophagy initiation is largely unknown.

RESULTS

Here, we show that human diazepam-binding inhibitor (DBI), also known as acyl-CoA binding protein (ACBP), binds to phosphatidylethanolamine of the phagophore membrane under nutrient-rich growth conditions, leading to inhibition of LC3 lipidation and suppression of autophagy initiation. Specific residues, including the conserved tyrosine residues of DBI, interact with phosphatidylethanolamine to stabilize the later molecule in the acyl-CoA binding cavity of the protein. Under starvation, phosphorylation of serine-21 of DBI mediated by the AMP-activated protein kinase results in a drastic reduction in the affinity of the protein for phosphatidylethanolamine. The release of serine-21 phosphorylated DBI from the phagophore upon nutrient starvation restores the high LC3 lipidation flux and maturation of the phagophore to autophagosome.

CONCLUSION

DBI acts as a strategic barrier against overactivation of phagophore maturation under nutrient-rich conditions, while triggering autophagy under nutrient-deficient conditions.

Stearoyl-CoA desaturase 1 as a therapeutic target for cancer: a focus on hepatocellular carcinoma

One of the main characteristics of cancer cells is the alteration in lipid composition, which is associated with a significant monounsaturated fatty acids (MUFAs) enrichment. In addition to their structural functions in newly synthesized membranes in proliferating cancer cells, these fatty acids are involved in tumorigenic signaling. Increased expression and activity of stearoyl CoA desaturase (SCD1), i.e., an enzyme converting saturated fatty acids to Δ9-monounsaturated fatty acids, has been observed in various cancer cells. This increase in expression and activity has also been associated with cancer aggressiveness and poor patient outcome. Previous studies have also indicated the SCD1 involvement in increased cancer cells proliferation, growth, migration, epithelial to mesenchymal transition, metastasis, chemoresistance, and maintenance of cancer stem cells properties. Hence, SCD1 seems to be a player in malignancy development and may be considered a novel therapeutic target in cancers, including hepatocellular carcinoma (HCC). This review study aims to discuss the impact of SCD1 as a major component in lipid signaling in HCC.

Complex Changes in Membrane Lipids Associated with the Modification of Autophagy in Arabidopsis

Autophagy is a conserved mechanism among eukaryotes that degrades and recycles cytoplasmic components. Autophagy is known to influence the plant metabolome, including lipid content; however, its impact on the plant lipidome is not fully understood, and most studies have analyzed a single or few mutants defective in autophagy. To gain more insight into the effect of autophagy on lipid concentrations and composition, we quantitatively profiled glycerolipids from multiple Arabidopsis thaliana mutants altered in autophagy and compared them with wild-type seedlings under nitrogen replete (+N; normal growth) and nitrogen starvation (-N; autophagy inducing) conditions. Mutants include those in genes of the core autophagy pathway, together with other genes that have been reported to affect autophagy. Using Matrix-Assisted Laser Desorption/Ionization-Mass Spectrometry (MALDI-MS), we imaged the cellular distribution of specific lipids in situ and demonstrated that autophagy and nitrogen treatment did not affect their spatial distribution within Arabidopsis seedling leaves. We observed changes, both increases and decreases, in the relative amounts of different lipid species in the mutants compared to WT both in +N and -N conditions, although more changes were seen in -N conditions. The relative amounts of polyunsaturated and very long chain lipids were significantly reduced in autophagy-disrupted mutants compared to WT plants. Collectively, our results provide additional evidence that autophagy affects plant lipid content and that autophagy likely affects lipid properties such as chain length and unsaturation.

Inhibition of stearoyl-CoA desaturase 1 potentiates anti-tumor activity of amodiaquine in non-small cell lung cancer

Non-small cell lung cancer (NSCLC) is one of the leading causes of cancer related death with few therapeutic treatment options. Under adverse tumor microenvironment, autophagy is an important mechanism of metabolic adaptations to sustain the survival and proliferation of tumor cells. Therefore, targeting autophagic activity represents a promising opportunity for NSCLC treatment. Here, we found that amodiaquine (AQ) increased autophagosome numbers and LC3BII and p62 at protein levels in A549 lung cancer cells suggesting the blockade of autophagic flux by AQ. To identify the key metabolic vulnerability associated with autophagy inhibition by AQ treatment, we then performed transcriptomics analysis in the presence or absence of AQ in A549 lung cancer cells and found stearoyl-CoA desaturase 1 (SCD) 1 was one of the most highly upregulated with AQ exposure. The induction of SCD1 by AQ exposure at both protein and mRNA level suggests that SCD1 could represent a potential therapeutic target of AQ treatment. Treatment of AQ in combination with SCD1 inhibition by A939572 demonstrated robust synergistic anti-cancer efficacy in cell proliferation assay and a lung cancer mouse xenograft model. Taken together, our study identified SCD1 could be a new therapeutic target upon autophagy inhibition by AQ exposure. Combinational treatment of autophagy inhibition and SCD1 inhibition achieves synergistic anti-tumor effect both in vitro and in vivo. This combinational approach could be a promising strategy for NSCLC treatment.

Autophagy and cancer metabolism-The two-way interplay

Autophagy is an intracellular catabolic process that degrades cytoplasmic components for recycling in response to stressed conditions, such as nutrient deprivation. Dysregulation of autophagy is associated with various diseases, including cancer. Although autophagy plays dichotomous and context-dependent roles in cancer, evidence has emerged that cancer cells exploit autophagy for metabolic adaptation. Autophagy is upregulated in many cancer types through tumor cell-intrinsic proliferation demands and the hypoxic and nutrient-limited tumor microenvironment (TME). Autophagy-induced breakdown products then fuel into various metabolic pathways to supply tumor cells with energy and building blocks for biosynthesis and survival. This bidirectional regulation between autophagy and tumor constitutes a vicious cycle to potentiate tumor growth and therapy resistance. In addition, the pro-tumor functions of autophagy are expanded to host, including cells in TME and distant organs. Thus, inhibition of autophagy or autophagy-mediated metabolic reprogramming may be a promising strategy for anticancer therapy. Better understanding the metabolic rewiring mechanisms of autophagy for its pro-tumor effects will provide insights into patient treatment.

SCD1, autophagy and cancer: implications for therapy

BACKGROUND

Autophagy is an intracellular degradation system that removes unnecessary or dysfunctional components and recycles them for other cellular functions. Over the years, a mutual regulation between lipid metabolism and autophagy has been uncovered.

METHODS

This is a narrative review discussing the connection between SCD1 and the autophagic process, along with the modality through which this crosstalk can be exploited for therapeutic purposes.

RESULTS

Fatty acids, depending on the species, can have either activating or inhibitory roles on autophagy. In turn, autophagy regulates the mobilization of fat from cellular deposits, such as lipid droplets, and removes unnecessary lipids to prevent cellular lipotoxicity. This review describes the regulation of autophagy by lipid metabolism in cancer cells, focusing on the role of stearoyl-CoA desaturase 1 (SCD1), the key enzyme involved in the synthesis of monounsaturated fatty acids. SCD1 plays an important role in cancer, promoting cell proliferation and metastasis. The role of autophagy in cancer is more complex since it can act either by protecting against the onset of cancer or by promoting tumor growth. Mounting evidence indicates that autophagy and lipid metabolism are tightly interconnected.

CONCLUSION

Here, we discuss controversial findings of SCD1 as an autophagy inducer or inhibitor in cancer, highlighting how these activities may result in cancer promotion or inhibition depending upon the degree of cancer heterogeneity and plasticity.

RNAseq and quantitative proteomic analysis of Dictyostelium knock-out cells lacking the core autophagy proteins ATG9 and/or ATG16

BACKGROUND

Autophagy is an evolutionary ancient mechanism that sequesters substrates for degradation within autolysosomes. The process is driven by many autophagy-related (ATG) proteins, including the core members ATG9 and ATG16. However, the functions of these two core ATG proteins still need further elucidation. Here, we applied RNA_seq and tandem mass tag (TMT) proteomic approaches to identify differentially expressed genes (DEGs) and proteins (DEPs) in Dictyostelium discoideum ATG9‾, ATG16‾ and ATG9‾/16‾ strains in comparison to AX2 wild-type cells.

RESULT

In total, we identified 332 (279 up and 53 down), 639 (487 up and 152 down) and 260 (114 up and 146 down) DEGs and 124 (83 up and 41 down), 431 (238 up and 193 down) and 677 (347 up and 330 down) DEPs in ATG9‾, ATG16‾ and ATG9‾/16‾ strains, respectively. Thus, in the single knock-out strains, the number of DEGs was higher than the number of DEPs while in the double knock-out strain the number of DEPs was higher. Comparison of RNA_seq and proteomic data further revealed, that only a small proportion of the transcriptional changes were reflected on the protein level. Gene ontology (GO) analysis revealed an enrichment of DEPs involved in lipid metabolism and oxidative phosphorylation. Furthermore, we found increased expression of the anti-oxidant enzymes glutathione reductase (gsr) and catalase A (catA) in ATG16‾ and ATG9‾/16‾ cells, respectively, indicating adaptation to excess reactive oxygen species (ROS).

CONCLUSIONS

Our study provides the first combined transcriptome and proteome analysis of ATG9‾, ATG16‾ and ATG9‾/16‾ cells. Our results suggest, that most changes in protein abundance were not caused by transcriptional changes, but were rather due to changes in protein homeostasis. In particular, knock-out of atg9 and/or atg16 appears to cause dysregulation of lipid metabolism and oxidative phosphorylation.

Stearoyl-CoA Desaturase-Mediated Monounsaturated Fatty Acid Availability Supports Humoral Immunity

Immune cells can metabolize glucose, amino acids, and fatty acids (FAs) to generate energy. The roles of different FA species and their impacts on humoral immunity remain poorly understood. Here, we report that proliferating B cells require monounsaturated FAs (MUFAs) to maintain mitochondrial metabolism and mTOR activity and to prevent excessive autophagy and endoplasmic reticulum (ER) stress. Furthermore, B cell-extrinsic stearoyl-CoA desaturase (SCD) activity generates MUFA to support early B cell development and germinal center (GC) formation in vivo during immunization and influenza infection. Thus, SCD-mediated MUFA production is critical for humoral immunity.

Zhou et al. show that monounsaturated fatty acids (MUFAs), generated by stearoyl-CoA desaturase (SCD), support B cell mitochondrial metabolism and mTOR activity and promote B cell development and humoral immune responses. These data establish MUFA availability as a key regulator for humoral immunity and a potential therapeutic target.

Membrane characteristics tune activities of endosomal and autophagic human VPS34 complexes

The lipid kinase VPS34 orchestrates diverse processes, including autophagy, endocytic sorting, phagocytosis, anabolic responses and cell division. VPS34 forms various complexes that help adapt it to specific pathways, with complexes I and II being the most prominent ones. We found that physicochemical properties of membranes strongly modulate VPS34 activity. Greater unsaturation of both substrate and non-substrate lipids, negative charge and curvature activate VPS34 complexes, adapting them to their cellular compartments. Hydrogen/deuterium exchange mass spectrometry (HDX-MS) of complexes I and II on membranes elucidated structural determinants that enable them to bind membranes. Among these are the Barkor/ATG14L autophagosome targeting sequence (BATS), which makes autophagy-specific complex I more active than the endocytic complex II, and the Beclin1 BARA domain. Interestingly, even though Beclin1 BARA is common to both complexes, its membrane-interacting loops are critical for complex II, but have only a minor role for complex I.

De novo phosphatidylcholine synthesis is required for autophagosome membrane formation and maintenance during autophagy

Macroautophagy/autophagy can enable cancer cells to withstand cellular stress and maintain bioenergetic homeostasis by sequestering cellular components into newly formed double-membrane vesicles destined for lysosomal degradation, potentially affecting the efficacy of anti-cancer treatments. Using 13C-labeled choline and 13C-magnetic resonance spectroscopy and western blotting, we show increased de novo choline phospholipid (ChoPL) production and activation of PCYT1A (phosphate cytidylyltransferase 1, choline, alpha), the rate-limiting enzyme of phosphatidylcholine (PtdCho) synthesis, during autophagy. We also discovered that the loss of PCYT1A activity results in compromised autophagosome formation and maintenance in autophagic cells. Direct tracing of ChoPLs with fluorescence and immunogold labeling imaging revealed the incorporation of newly synthesized ChoPLs into autophagosomal membranes, endoplasmic reticulum (ER) and mitochondria during anticancer drug-induced autophagy. Significant increase in the colocalization of fluorescence signals from the newly synthesized ChoPLs and mCherry-MAP1LC3/LC3 (microtubule-associated protein 1 light chain 3) was also found on autophagosomes accumulating in cells treated with autophagy-modulating compounds. Interestingly, cells undergoing active autophagy had an altered ChoPL profile, with longer and more unsaturated fatty acid/alcohol chains detected. Our data suggest that de novo synthesis may be required to increase autophagosomal ChoPL content and alter its composition, together with replacing phospholipids consumed from other organelles during autophagosome formation and turnover. This addiction to de novo ChoPL synthesis and the critical role of PCYT1A may lead to development of agents targeting autophagy-induced drug resistance. In addition, fluorescence imaging of choline phospholipids could provide a useful way to visualize autophagosomes in cells and tissues.

ABBREVIATIONS

AKT: AKT serine/threonine kinase; BAX: BCL2 associated X, apoptosis regulator; BECN1: beclin 1; ChoPL: choline phospholipid; CHKA: choline kinase alpha; CHPT1: choline phosphotransferase 1; CTCF: corrected total cell fluorescence; CTP: cytidine-5'-triphosphate; DCA: dichloroacetate; DMEM: dulbeccos modified Eagles medium; DMSO: dimethyl sulfoxide; EDTA: ethylenediaminetetraacetic acid; ER: endoplasmic reticulum; GDPD5: glycerophosphodiester phosphodiesterase domain containing 5; GFP: green fluorescent protein; GPC: glycerophosphorylcholine; HBSS: hanks balances salt solution; MAP1LC3/LC3: microtubule associated protein 1 light chain 3; LPCAT1: lysophosphatidylcholine acyltransferase 1; LysoPtdCho: lysophosphatidylcholine; MRS: magnetic resonance spectroscopy; MTORC1: mechanistic target of rapamycin kinase complex 1; PCho: phosphocholine; PCYT: choline phosphate cytidylyltransferase; PLA2: phospholipase A2; PLB: phospholipase B; PLC: phospholipase C; PLD: phospholipase D; PCYT1A: phosphate cytidylyltransferase 1, choline, alpha; PI3K: phosphoinositide-3-kinase; pMAFs: pancreatic mouse adult fibroblasts; PNPLA6: patatin like phospholipase domain containing 6; Pro-Cho: propargylcholine; Pro-ChoPLs: propargylcholine phospholipids; PtdCho: phosphatidylcholine; PtdEth: phosphatidylethanolamine; PtdIns3P: phosphatidylinositol-3-phosphate; RPS6: ribosomal protein S6; SCD: stearoyl-CoA desaturase; SEM: standard error of the mean; SM: sphingomyelin; SMPD1/SMase: sphingomyelin phosphodiesterase 1, acid lysosomal; SGMS: sphingomyelin synthase; WT: wild-type.

Emerging roles of ATG proteins and membrane lipids in autophagosome formation

Autophagosome biogenesis is a dynamic membrane event, which is executed by the sequential function of autophagy-related (ATG) proteins. Upon autophagy induction, a cup-shaped membrane structure appears in the cytoplasm, then elongates sequestering cytoplasmic materials, and finally forms a closed double membrane autophagosome. However, how this complex vesicle formation event is strictly controlled and achieved is still enigmatic. Recently, there is accumulating evidence showing that some ATG proteins have the ability to directly interact with membranes, transfer lipids between membranes and regulate lipid metabolism. A novel role for various membrane lipids in autophagosome formation is also emerging. Here, we highlight past and recent key findings on the function of ATG proteins related to autophagosome biogenesis and consider how ATG proteins control this dynamic membrane formation event to organize the autophagosome by collaborating with membrane lipids.

Mechanisms governing autophagosome biogenesis

Autophagosomes are double-membrane vesicles newly formed during autophagy to engulf a wide range of intracellular material and transport this autophagic cargo to lysosomes (or vacuoles in yeasts and plants) for subsequent degradation. Autophagosome biogenesis responds to a plethora of signals and involves unique and dynamic membrane processes. Autophagy is an important cellular mechanism allowing the cell to meet various demands, and its disruption compromises homeostasis and leads to various diseases, including metabolic disorders, neurodegeneration and cancer. Thus, not surprisingly, the elucidation of the molecular mechanisms governing autophagosome biogenesis has attracted considerable interest. Key molecules and organelles involved in autophagosome biogenesis, including autophagy-related (ATG) proteins and the endoplasmic reticulum, have been discovered, and their roles and relationships have been investigated intensely. However, several fundamental questions, such as what supplies membranes/lipids to build the autophagosome and how the membrane nucleates, expands, bends into a spherical shape and finally closes, have proven difficult to address. Nonetheless, owing to recent studies with new approaches and technologies, we have begun to unveil the mechanisms underlying these processes on a molecular level. We now know that autophagosome biogenesis is a highly complex process, in which multiple proteins and lipids from various membrane sources, supported by the formation of membrane contact sites, cooperate with biophysical phenomena, including membrane shaping and liquid-liquid phase separation, to ensure seamless segregation of the autophagic cargo. Together, these studies pave the way to obtaining a holistic view of autophagosome biogenesis.

GPAT4-Generated Saturated LPAs Induce Lipotoxicity through Inhibition of Autophagy by Abnormal Formation of Omegasomes

Excessive levels of saturated fatty acids are toxic to vascular smooth muscle cells (VSMCs). We previously reported that mice lacking VSMC-stearoyl-CoA desaturase (SCD), a major enzyme catalyzing the detoxification of saturated fatty acids, develop severe vascular calcification from the massive accumulation of lipid metabolites containing saturated fatty acids. However, the mechanism by which SCD deficiency causes vascular calcification is not completely understood. Here, we demonstrate that saturated fatty acids significantly inhibit autophagic flux in VSMCs, contributing to vascular calcification and apoptosis. Mechanistically, saturated fatty acids are accumulated as saturated lysophosphatidic acids (LPAs) (i.e. 1-stearoyl-LPA) possibly synthesized through the reaction of GPAT4 at the contact site between omegasomes and the MAM. The accumulation of saturated LPAs at the contact site causes abnormal formation of omegasomes, resulting in accumulation of autophagosomal precursor isolation membranes, leading to inhibition of autophagic flux. Thus, saturated LPAs are major metabolites mediating autophagy inhibition and vascular calcification.

Inhibition of stearoyl-coenzyme A desaturase 1 ameliorates hepatic steatosis by inducing AMPK-mediated lipophagy

SCD1 is a key enzyme controlling lipid metabolism and a link between its activity and NAFLD has been proposed. Lipophagy is a novel regulatory approach to lipid metabolism regulation, which is involved in the development of NAFLD. However, the possible functional connection between SCD1 and lipophagy in NAFLD remains unknown. To investigate the molecular mechanisms through which SCD1 regulates lipophagy in hepatic steatosis, the model of hepatic steatosis was established by inducing mouse primary hepatocytes with sodium palmitate and feeding C57BL/6 mice with HFD. Our results indicated that sodium palmitate-treated hepatocytes exhibited increased SCD1 expression, AMPK inactivation and defective lipophagy. Inhibition of SCD1 expression in hepatocytes resulted in enhanced AMPK activity and lipophagy, and reduced lipid deposition. Although SCD1 overexpression led to decreased AMPK activity and lipophagy, lipid deposition was increased in hepatocytes. SCD1 regulated lipophagy through AMPK to affect lipid metabolism in mouse primary hepatocytes. Additionally, compared to HFD-fed mice, CAY10566(an SCD1-specific inhibitor)-treated mice exhibited significantly decreased hepatic steatosis and hepatic lipid droplet accumulation, as well as enhanced AMPK activity and lipophagy. This study elucidated that SCD1 inhibition ameliorates hepatic steatosis by inducing AMPK-mediated lipophagy, suggesting that the SCD1-AMPK-lipophagy pathway is a potential therapeutic target for NAFLD.

Local Fatty Acid Channeling into Phospholipid Synthesis Drives Phagophore Expansion during Autophagy

Autophagy is a conserved catabolic homeostasis process central for cellular and organismal health. During autophagy, small single-membrane phagophores rapidly expand into large double-membrane autophagosomes to encapsulate diverse cargoes for degradation. It is thought that autophagic membranes are mainly derived from preformed organelle membranes. Instead, here we delineate a pathway that expands the phagophore membrane by localized phospholipid synthesis. Specifically, we find that the conserved acyl-CoA synthetase Faa1 accumulates on nucleated phagophores and locally activates fatty acids (FAs) required for phagophore elongation and autophagy. Strikingly, using isotopic FA tracing, we directly show that Faa1 channels activated FAs into the synthesis of phospholipids and promotes their assembly into autophagic membranes. Indeed, the first committed steps of de novo phospholipid synthesis at the ER, which forms stable contacts with nascent autophagosomes, are essential for autophagy. Together, our work illuminates how cells spatially tune synthesis and flux of phospholipids for autophagosome biogenesis during autophagy.

Lineage-Restricted Regulation of SCD and Fatty Acid Saturation by MITF Controls Melanoma Phenotypic Plasticity

Phenotypic and metabolic heterogeneity within tumors is a major barrier to effective cancer therapy. How metabolism is implicated in specific phenotypes and whether lineage-restricted mechanisms control key metabolic vulnerabilities remain poorly understood. In melanoma, downregulation of the lineage addiction oncogene microphthalmia-associated transcription factor (MITF) is a hallmark of the proliferative-to-invasive phenotype switch, although how MITF promotes proliferation and suppresses invasion is poorly defined. Here, we show that MITF is a lineage-restricted activator of the key lipogenic enzyme stearoyl-CoA desaturase (SCD) and that SCD is required for MITF^High melanoma cell proliferation. By contrast MITF^Low cells are insensitive to SCD inhibition. Significantly, the MITF-SCD axis suppresses metastasis, inflammatory signaling, and an ATF4-mediated feedback loop that maintains de-differentiation. Our results reveal that MITF is a lineage-specific regulator of metabolic reprogramming, whereby fatty acid composition is a driver of melanoma phenotype switching, and highlight that cell phenotype dictates the response to drugs targeting lipid metabolism.

Recent Insights into the Mitochondrial Role in Autophagy and Its Regulation by Oxidative Stress

Autophagy is a self-digestive process that degrades intracellular components, including damaged organelles, to maintain energy homeostasis and to cope with cellular stress. Autophagy plays a key role during development and adult tissue homeostasis, and growing evidence indicates that this catalytic process also has a direct role in modulating aging. Although autophagy is essentially protective, depending on the cellular context and stimuli, autophagy outcome can lead to either abnormal cell growth or cell death. The autophagic process requires a tight regulation, with cellular events following distinct stages and governed by a wide molecular machinery. Reactive oxygen species (ROS) have been involved in autophagy regulation through multiple signaling pathways, and mitochondria, the main source of endogenous ROS, have emerged as essential signal transducers that mediate autophagy. In the present review, we aim to summarize the regulatory function of mitochondria in the autophagic process, particularly regarding the mitochondrial role as the coordination node in the autophagy signaling pathway, involving mitochondrial oxidative stress, and their participation as membrane donors in the initial steps of autophagosome assembly.

Hepatic Stearoyl-CoA desaturase-1 deficiency-mediated activation of mTORC1- PGC-1α axis regulates ER stress during high-carbohydrate feeding

Stearoyl CoA desaturase 1 (SCD1) is a key enzyme in lipogenesis as it catalyzes the synthesis of monounsaturated fatty acids (MUFAs), mainly oleate (18:1n9) and palmitoleate (16:1n7) from saturated fatty acids (SFA), stearate (18:0) and palmitate (16:0), respectively. Studies on SCD1 deficiency in mouse models demonstrated beneficial metabolic phenotypes such as reduced adiposity and improved glucose tolerance. Even though, SCD1 represents a potential target to resolve obesity related metabolic diseases; SCD1 deficiency causes endoplasmic reticulum (ER) stress and activates unfolded protein response (UPR). The induction of ER stress in response to SCD1 deficiency is governed by the cofactor, PGC-1α. However, the mechanism by which SCD1 deficiency increases PGC-1α and subsequently induces ER stress still remains elusive. The present study demonstrates that despite reduced lipogenesis, liver specific SCD1 deficiency activates the mechanistic target of rapamycin complex 1 (mTORC1) along with induction of PGC-1α and ER stress. Further, mTORC1 inhibition attenuates SCD1 deficiency-mediated induction of both PGC-1α and ER stress. Similar observations were seen by restoring endogenously synthesized oleate, but not palmitoleate, suggesting a clear mTORC1-mediated regulation of ER stress during SCD1 deficiency. Overall, our results suggest a model whereby maintaining adequate levels of hepatic oleate is required to suppress mTORC1-mediated ER stress. In addition, the activation of mTORC1 by SCD1 deficiency reveals an important function of fatty acids in regulating different cellular processes through mTORC1 signaling.

The Mitochondria-Endoplasmic Reticulum Contacts and Their Critical Role in Aging and Age-Associated Diseases

The recent discovery of interconnections between the endoplasmic reticulum (ER) membrane and those of almost all the cell compartments is providing novel perspectives for the understanding of the molecular events underlying cellular mechanisms in both physiological and pathological conditions. In particular, growing evidence strongly supports the idea that the molecular interactions occurring between ER and mitochondrial membranes, referred as the mitochondria (MT)-ER contacts (MERCs), may play a crucial role in aging and in the development of age-associated diseases. As emerged in the last decade, MERCs behave as signaling hubs composed by structural components that act as critical players in different age-associated disorders, such as neurodegenerative diseases and motor disorders, cancer, metabolic syndrome, as well as cardiovascular diseases. Age-associated disorders often derive from mitochondrial or ER dysfunction as consequences of oxidative stress, mitochondrial DNA mutations, accumulation of misfolded proteins, and defective organelle turnover. In this review, we discuss the recent advances associating MERCs to aging in the context of ER-MT crosstalk regulating redox signaling, ER-to MT lipid transfer, mitochondrial dynamics, and autophagy.

Stearoyl-CoA Desaturase 1 as a Therapeutic Target for the Treatment of Cancer

A distinctive feature of cancer cells of various origins involves alterations of the composition of lipids, with significant enrichment in monounsaturated fatty acids. These molecules, in addition to being structural components of newly formed cell membranes of intensely proliferating cancer cells, support tumorigenic signaling. An increase in the expression of stearoyl-CoA desaturase 1 (SCD1), the enzyme that converts saturated fatty acids to ∆9-monounsaturated fatty acids, has been observed in a wide range of cancer cells, and this increase is correlated with cancer aggressiveness and poor outcomes for patients. Studies have demonstrated the involvement of SCD1 in the promotion of cancer cell proliferation, migration, metastasis, and tumor growth. Many studies have reported a role for this lipogenic factor in maintaining the characteristics of cancer stem cells (i.e., the population of cells that contributes to cancer progression and resistance to chemotherapy). Importantly, both the products of SCD1 activity and its direct impact on tumorigenic pathways have been demonstrated. Based on these findings, SCD1 appears to be a significant player in the development of malignant disease and may be a promising target for anticancer therapy. Numerous chemical compounds that exert inhibitory effects on SCD1 have been developed and preclinically tested. The present review summarizes our current knowledge of the ways in which SCD1 contributes to the progression of cancer and discusses opportunities and challenges of using SCD1 inhibitors for the treatment of cancer.

Identification and characterization of stearoyl-CoA desaturase in Toxoplasma gondii

Few information of the function of stearoyl-coenzyme A (CoA) desaturase (SCD) in apicomplaxan parasite has been obtained. In this study, we retrieved a putative fatty acyl-CoA desaturase (TGGT1_238950) by a protein alignment with Plasmodium falciparum SCD in ToxoDB. A typical Δ9-desaturase domain was revealed in this protein. The putative desaturase was tagged with HA endogenously in Toxoplasma gondii, and the endoplasmic reticulum localization of the putative desaturase was revealed, which was consistent with the fatty acid desaturases in other organisms. Therefore, the TGGT1_238950 was designated T. gondii SCD. Based on CRISPR/Cas9 gene editing technology, SCD conditional knockout mutants in the T. gondii TATi strain were obtained. The growth in vitro and pathogenicity in mice of the mutants suggested that SCD might be dispensable for tachyzoite growth and proliferation. The SCD-overexpressing line was constructed to further explore SCD function. The portion of palmitoleic acid and oleic acid were increased in SCD-overexpressing parasites, compared with the RH parental strain, indicating that T. gondii indeed is competent for unsaturated fatty acid synthesis. The SCD-overexpressing tachyzoites propagated slower than the parental strain, with a decreased invasion capability and weaker pathogenicity in mice. The TgIF2α phosphorylation and the expression changes of several genes demonstrated that ER stress was triggered in the SCD-overexpressing parasites, which were more apt toward autophagy and apoptosis. The function of unsaturated fatty acid synthesis of TgSCD was consistent with our hypothesis. On the other hand, SCD might also be involved in tachyzoite autophagy and apoptosis.

Ascorbyl stearate stimulates cell death by oxidative stress-mediated apoptosis and autophagy in HeLa cervical cancer cell line in vitro

In this study, Asc-s was evaluated for anti-cancer effect using cervical cancer cells (HeLa). Results determine that Asc-s treatment-induced dose-dependent inhibition of proliferation of HeLa cells and induced apoptosis. Flow-cytometry analysis shows Asc-s treatment-induced accumulation of cells at sub-G0/G1 stage of cell cycle and induced apoptosis as confirmed by DAPI, propodium iodide, and acridine staining in HeLa cells. Asc-s entered the cells and metabolized to ascorbate and stearate moieties, increased membrane permeability, and decreased membrane fluidity in HeLa cells. Asc-s treatment-induced dose-dependent increase in autophagy protein LC3-II, mRNA levels and decreased Nrf-2 levels in HeLa cells. It is hypothesized that both ascorbyl radical and stearoyl moieties of Asc-s induced cytotoxicity by generating reactive oxygen species (ROS) and modulating membrane fluidity/permeability leading to apoptosis/autophagy of HeLa cells. Thus, our findings demonstrate that Asc-s as anti-proliferative and apoptosis inducing compound in cervical cancer cells.

Autophagy as an emerging target in cardiorenal metabolic disease: From pathophysiology to management

Although advances in medical technology and health care have improved the early diagnosis and management for cardiorenal metabolic disorders, the prevalence of obesity, insulin resistance, diabetes, hypertension, dyslipidemia, and kidney disease remains high. Findings from numerous population-based studies, clinical trials, and experimental evidence have consolidated a number of theories for the pathogenesis of cardiorenal metabolic anomalies including resistance to the metabolic action of insulin, abnormal glucose and lipid metabolism, oxidative and nitrosative stress, endoplasmic reticulum (ER) stress, apoptosis, mitochondrial damage, and inflammation. Accumulating evidence has recently suggested a pivotal role for proteotoxicity, the unfavorable effects of poor protein quality control, in the pathophysiology of metabolic dysregulation and related cardiovascular complications. The ubiquitin-proteasome system (UPS) and autophagy-lysosomal pathways, two major although distinct cellular clearance machineries, govern protein quality control by degradation and clearance of long-lived or damaged proteins and organelles. Ample evidence has depicted an important role for protein quality control, particularly autophagy, in the maintenance of metabolic homeostasis. To this end, autophagy offers promising targets for novel strategies to prevent and treat cardiorenal metabolic diseases. Targeting autophagy using pharmacological or natural agents exhibits exciting new strategies for the growing problem of cardiorenal metabolic disorders.

Inactivation of autophagy leads to changes in sebaceous gland morphology and function

We have reported recently that inactivation of the essential autophagy-related gene 7 (Atg7) in keratinocytes has little or no impact on morphology and function of the epidermal barrier in experimental animals. When these mice aged, mutant males, (Atg7 ΔKC), developed an oily coat. As the keratin 14 promoter driven cre/LoxP system inactivates floxed Atg7 in all keratin 14 (K14) expressing cells, including sebocytes, we investigated whether the oily hair phenotype was the consequence of changes in function of the skin sebaceous glands. Using an antibody to the GFP-LC3 fusion protein, autophagosomes were detected at the border of sebocyte disintegration in control but not in mutant animals, suggesting that autophagy was (a) active in normal sebaceous glands and (b) was inactivated in the mutant mice. Detailed analysis established that dorsal sebaceous glands were about twice as large in all Atg7 ΔKC mice compared to those of controls (Atg7 F/F), and their rate of sebocyte proliferation was increased. In addition, male mutant mice yielded twice as much lipid per unit hair as age-matched controls. Analysis of sebum lipids by thin layer chromatography revealed a 40% reduction in the proportion of free fatty acids (FFA) and cholesterol, and a 5-fold increase in the proportion of fatty acid methyl esters (FAME). In addition, the most common diester wax species (58-60 carbon atoms) were increased, while shorter species (54-55 carbon atoms) were under-represented in mutant sebum. Our data show that autophagy contributes to sebaceous gland function and to the control of sebum composition.

Emerging role of lipid metabolism alterations in Cancer stem cells

BACKGROUND

Cancer stem cells (CSCs) or tumor-initiating cells (TICs) represent a small population of cancer cells with self-renewal and tumor-initiating properties. Unlike the bulk of tumor cells, CSCs or TICs are refractory to traditional therapy and are responsible for relapse or disease recurrence in cancer patients. Stem cells have distinct metabolic properties compared to differentiated cells, and metabolic rewiring contributes to self-renewal and stemness maintenance in CSCs.

MAIN BODY

Recent advances in metabolomic detection, particularly in hyperspectral-stimulated raman scattering microscopy, have expanded our knowledge of the contribution of lipid metabolism to the generation and maintenance of CSCs. Alterations in lipid uptake, de novo lipogenesis, lipid droplets, lipid desaturation, and fatty acid oxidation are all clearly implicated in CSCs regulation. Alterations on lipid metabolism not only satisfies the energy demands and biomass production of CSCs, but also contributes to the activation of several important oncogenic signaling pathways, including Wnt/β-catenin and Hippo/YAP signaling. In this review, we summarize the current progress in this attractive field and describe some recent therapeutic agents specifically targeting CSCs based on their modulation of lipid metabolism.

CONCLUSION

Increased reliance on lipid metabolism makes it a promising therapeutic strategy to eliminate CSCs. Targeting key players of fatty acids metabolism shows promising to anti-CSCs and tumor prevention effects.

Mitochondria-associated membranes in aging and senescence: structure, function, and dynamics

Sites of close contact between mitochondria and the endoplasmic reticulum (ER) are known as mitochondria-associated membranes (MAM) or mitochondria-ER contacts (MERCs), and play an important role in both cell physiology and pathology. A growing body of evidence indicates that changes observed in the molecular composition of MAM and in the number of MERCs predisposes MAM to be considered a dynamic structure. Its involvement in processes such as lipid biosynthesis and trafficking, calcium homeostasis, reactive oxygen species production, and autophagy has been experimentally confirmed. Recently, MAM have also been studied in the context of different pathologies, including Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, type 2 diabetes mellitus and GM1-gangliosidosis. An underappreciated amount of data links MAM with aging or senescence processes. In the present review, we summarize the current knowledge of basic MAM biology, composition and action, and discuss the potential connections supporting the idea that MAM are significant players in longevity.

A large-scale RNA interference screen identifies genes that regulate autophagy at different stages

Dysregulated autophagy is central to the pathogenesis and therapeutic development of cancer. However, how autophagy is regulated in cancer is not well understood and genes that modulate cancer autophagy are not fully defined. To gain more insights into autophagy regulation in cancer, we performed a large-scale RNA interference screen in K562 human chronic myeloid leukemia cells using monodansylcadaverine staining, an autophagy-detecting approach equivalent to immunoblotting of the autophagy marker LC3B or fluorescence microscopy of GFP-LC3B. By coupling monodansylcadaverine staining with fluorescence-activated cell sorting, we successfully isolated autophagic K562 cells where we identified 336 short hairpin RNAs. After candidate validation using Cyto-ID fluorescence spectrophotometry, LC3B immunoblotting, and quantitative RT-PCR, 82 genes were identified as autophagy-regulating genes. 20 genes have been reported previously and the remaining 62 candidates are novel autophagy mediators. Bioinformatic analyses revealed that most candidate genes were involved in molecular pathways regulating autophagy, rather than directly participating in the autophagy process. Further autophagy flux assays revealed that 57 autophagy-regulating genes suppressed autophagy initiation, whereas 21 candidates promoted autophagy maturation. Our RNA interference screen identified genes that regulate autophagy at different stages, which helps decode autophagy regulation in cancer and offers novel avenues to develop autophagy-related therapies for cancer.

Autophagy in the context of the cellular membrane-trafficking system: the enigma of Atg9 vesicles

Macroautophagy is an intracellular degradation system that involves the de novo formation of membrane structures called autophagosomes, although the detailed process by which membrane lipids are supplied during autophagosome formation is yet to be elucidated. Macroautophagy is thought to be associated with canonical membrane trafficking, but several mechanistic details are still missing. In this review, the current understanding and potential mechanisms by which membrane trafficking participates in macroautophagy are described, with a focus on the enigma of the membrane protein Atg9, for which the proximal mechanisms determining its movement are disputable, despite its key role in autophagosome formation.

Feedback activation of AMPK-mediated autophagy acceleration is a key resistance mechanism against SCD1 inhibitor-induced cell growth inhibition

Elucidating the bioactive compound modes of action is crucial for increasing success rates in drug development. For anticancer drugs, defining effective drug combinations that overcome resistance improves therapeutic efficacy. Herein, by using a biologically annotated compound library, we performed a large-scale combination screening with Stearoyl-CoA desaturase-1 (SCD1) inhibitor, T-3764518, which partially inhibits colorectal cancer cell proliferation. T-3764518 induced phosphorylation and activation of AMPK in HCT-116 cells, which led to blockade of downstream fatty acid synthesis and acceleration of autophagy. Attenuation of fatty acid synthesis by small molecules suppressed the growth inhibitory effect of T-3764518. In contrast, combination of T-3764518 with autophagy flux inhibitors synergistically inhibited cellular proliferation. Experiments using SCD1 knock-out cells validated the results obtained with T-3764518. The results of our study indicated that activation of autophagy serves as a survival signal when SCD1 is inhibited in HCT-116 cells. Furthermore, these findings suggest that combining SCD1 inhibitor with autophagy inhibitors is a promising anticancer therapy.

Ole1, fatty acid desaturase, is required for Atg9 delivery and isolation membrane expansion during autophagy in Saccharomyces cerevisiae

Macroautophagy, a major degradation pathway of cytoplasmic components, is carried out through formation of a double-membrane structure, the autophagosome. Although the involvement of specific lipid species in the formation process remains largely obscure, we recently showed that mono-unsaturated fatty acids (MUFA) generated by stearoyl-CoA desaturase 1 (SCD1) are required for autophagosome formation in mammalian cells. To obtain further insight into the role of MUFA in autophagy, in this study we analyzed the autophagic phenotypes of the yeast mutant of OLE1, an orthologue of SCD1. Δole1 cells were defective in nitrogen starvation-induced autophagy, and the Cvt pathway, when oleic acid was not supplied. Defects in elongation of the isolation membrane led to a defect in autophagosome formation. In the absence of Ole1, the transmembrane protein Atg9 was not able to reach the pre-autophagosomal structure (PAS), the site of autophagosome formation. Thus, autophagosome formation requires Ole1 during the delivery of Atg9 to the PAS/autophagosome from its cellular reservoir.

Loss of ULK1 increases RPS6KB1-NCOR1 repression of NR1H/LXR-mediated Scd1 transcription and augments lipotoxicity in hepatic cells

Lipotoxicity caused by saturated fatty acids (SFAs) induces tissue damage and inflammation in metabolic disorders. SCD1 (stearoyl-coenzyme A desaturase 1) converts SFAs to mono-unsaturated fatty acids (MUFAs) that are incorporated into triglycerides and stored in lipid droplets. SCD1 thus helps protect hepatocytes from lipotoxicity and its reduced expression is associated with increased lipotoxic injury in cultured hepatic cells and mouse models. To further understand the role of SCD1 in lipotoxicity, we examined the regulation of Scd1 in hepatic cells treated with palmitate, and found that NR1H/LXR (nuclear receptor subfamily 1 group H) ligand, GW3965, induced Scd1 expression and lipid droplet formation to improve cell survival. Surprisingly, ULK1/ATG1 (unc-51 like kinase) played a critical role in protecting hepatic cells from SFA-induced lipotoxicity via a novel mechanism that did not involve macroautophagy/autophagy. Specific loss of Ulk1 blocked the induction of Scd1 gene transcription by GW3965, decreased lipid droplet formation, and increased apoptosis in hepatic cells exposed to palmitate. Knockdown of ULK1 increased RPS6KB1 (ribosomal protein S6 kinase, polypeptide 1) signaling that, in turn, induced NCOR1 (nuclear receptor co-repressor 1) nuclear uptake, interaction with NR1H/LXR, and recruitment to the Scd1 promoter. These events abrogated the stimulation of Scd1 gene expression by GW3965, and increased lipotoxicity in hepatic cells. In summary, we have identified a novel autophagy-independent role of ULK1 that regulates NR1H/LXR signaling, Scd1 expression, and intracellular lipid homeostasis in hepatic cells exposed to a lipotoxic environment.

Gestational food restriction decreases placental interleukin-10 expression and markers of autophagy and endoplasmic reticulum stress in murine intrauterine growth restriction

Intrauterine growth restriction (IUGR) affects up to 10% of pregnancies and often results in short- and long-term sequelae for offspring. The mechanisms underlying IUGR are poorly understood, but it is known that healthy placentation is essential for nutrient provision to fuel fetal growth, and is regulated by immunologic inputs. We hypothesized that in pregnancy, maternal food restriction (FR) resulting in IUGR would decrease the overall immunotolerant milieu in the placenta, leading to increased cellular stress and death. Our specific objectives were to evaluate (1) key cytokines (eg, IL-10) that regulate maternal-fetal tolerance, (2) cellular processes (autophagy and endoplasmic reticulum [ER] stress) that are immunologically mediated and important for cellular survival and functioning, and (3) the resulting IUGR phenotype and placental histopathology in this animal model. After subjecting pregnant mice to mild and moderate FR from gestational day 10 to 19, we collected placentas and embryos at gestational day 19. We examined RNA sequencing data to identify immunologic pathways affected in IUGR-associated placentas and validated messenger RNA expression changes of genes important in cellular integrity. We also evaluated histopathologic changes in vascular and trophoblastic structures as well as protein expression changes in autophagy, ER stress, and apoptosis in the mouse placentas. Several differentially expressed genes were identified in FR compared with control mice, including a considerable subset that regulates immune tolerance, inflammation, and cellular integrity. In summary, maternal FR decreases the anti-inflammatory effect of IL-10 and suppresses placental autophagic and ER stress responses, despite evidence of dysregulated vascular and trophoblast structures leading to IUGR.

Stearoyl CoA desaturase-1: New insights into a central regulator of cancer metabolism

The processes of cell proliferation, cell death and differentiation involve an intricate array of biochemical and morphological changes that require a finely tuned modulation of metabolic pathways, chiefly among them is fatty acid metabolism. The critical participation of stearoyl CoA desaturase-1 (SCD1), the fatty acyl Δ9-desaturing enzyme that converts saturated fatty acids (SFA) into monounsaturated fatty acids (MUFA), in the mechanisms of replication and survival of mammalian cells, as well as their implication in the biological alterations of cancer have been actively investigated in recent years. This review examines the growing body of evidence that argues for a role of SCD1 as a central regulator of the complex synchronization of metabolic and signaling events that control cellular metabolism, cell cycle progression, survival, differentiation and transformation to cancer.

Regulation of autophagy by mitochondrial phospholipids in health and diseases

Autophagy is an evolutionarily conserved mechanism that maintains nutrient homeostasis by degrading protein aggregates and damaged organelles. Autophagy is reduced in aging, which is implicated in the pathogenesis of aging-related diseases, including cancers, obesity, type 2 diabetes, cardiovascular diseases, and neurodegenerative diseases. Mitochondria-derived phospholipids cardiolipin, phosphatidylethanolamine, and phosphatidylglycerol are critical throughout the autophagic process, from initiation and phagophore formation to elongation and fusion with endolysosomal vesicles. Cardiolipin is also required for mitochondrial fusion and fission, an important step in isolating dysfunctional mitochondria for mitophagy. Furthermore, genetic screen in yeast has identified a surprising role for cardiolipin in regulating lysosomal function. Phosphatidylethanolamine plays a pivotal role in supporting the autophagic process, including autophagosome elongation as part of lipidated Atg8/LC3. An emerging role for phosphatidylglycerol in AMPK and mTORC1 signaling as well as mitochondrial fission may provide the first glimpse into the function of phosphatidylglycerol apart from being a precursor for cardiolipin. This review examines the effects of manipulating phospholipids on autophagy and mitophagy in health and diseases, as well as current limitations in the field. This article is part of a Special Issue entitled: Lipids of Mitochondria edited by Guenther Daum.

Cyp1b1 affects external control of mouse hepatocytes, fatty acid homeostasis and signaling involving HNF4α and PPARα

Cytochrome P450 1b1 (Cyp1b1) is expressed in endothelia, stellate cells and pre-adipocytes, but not hepatocytes. Deletion alters liver fatty acid metabolism and prevents obesity and hepatic steatosis. This suggests a novel extra-hepatocyte regulation directed from cells that express Cyp1b1. To characterize these mechanisms, microarray gene expression was analyzed in livers of normal and congenic Cyp1b1-ko C57BL/6 J mice fed either low or high fat diets. Cyp1b1-ko gene responses indicate suppression of endogenous PPARα activity, a switch from triglyceride storage to mitochondrial fatty acid oxidation and decreased oxidative stress. Many gene responses in Cyp1b1-ko are sexually dimorphic and correspond to increased activity of growth hormone mediated by HNF4α. Male responses stimulated by GH pulses are enhanced, whereas responses that decline exhibit further suppression, including Cyp regulation by PPARα, CAR and PXR. These effects of Cyp1b1 deletion overlap with effects caused by deletion of the small heterodimeric partner, a suppressor of these nuclear factors. Redirection of gene expression associated with liver fat homeostasis in Cyp1b1-ko mice that directs hypothalamic control of GH and leptin. Cyp1b1-ko suppresses neonatal Scd1 and delays adult maturation of dimorphic GH/HNF4α signaling. Alternatively, deletion may diminish hypothalamic metabolism of estradiol, which establishes adult GH regulation.

Membrane dynamics in autophagosome biogenesis

Bilayered phospholipid membranes are vital to the organization of the living cell. Based on fundamental principles of polarity, membranes create borders allowing defined spaces to be encapsulated. This compartmentalization is a prerequisite for the complex functional design of the eukaryotic cell, yielding localities that can differ in composition and operation. During macroautophagy, cytoplasmic components become enclosed by a growing double bilayered membrane, which upon closure creates a separate compartment, the autophagosome. The autophagosome is then primed for fusion with endosomal and lysosomal compartments, leading to degradation of the captured material. A large number of proteins have been found to be essential for autophagy, but little is known about the specific lipids that constitute the autophagic membranes and the membrane modeling events that are responsible for regulation of autophagosome shape and size. In this Commentary, we review the recent progress in our understanding of the membrane shaping and remodeling events that are required at different steps of the autophagy pathway. This article is part of a Focus on Autophagosome biogenesis. For further reading, please see related articles: 'ERES: sites for autophagosome biogenesis and maturation?' by Jana Sanchez-Wandelmer et al. (J. Cell Sci. 128, 185-192) and 'WIPI proteins: essential PtdIns3P effectors at the nascent autophagosome' by Tassula Proikas-Cezanne et al. (J. Cell Sci. 128, 207-217).