The ability to generate, store, and mobilize energy is fundamental to life, and disruptions in these processes underlie metabolic disease. This review examines how these mechanisms evolved to rely profoundly on lipids, a large family of molecules that serve an unusually wide range of biological functions. From early evolutionary times, lipids have enabled chemiosmotic energy transduction, organized the endomembrane systems that synthesize and package neutral lipids, served as concentrated reservoirs of metabolic energy, and functioned as signaling molecules that coordinate cellular and systemic physiology. Adipocytes are specialized cells that can accumulate large amounts of lipid within specialized lipid droplet organelles. Over evolutionary time, mammals developed distinct adipocyte subtypes with tailored physiological roles. White adipocytes, characterized by a single large unilocular lipid droplet, coordinate energy storage and release in response to systemic cues. In contrast, the role of brown and beige adipocytes is to protect the organism from cold exposure by generating heat as a primary output of UCP1-mediated mitochondrial uncoupling. Crucially, this thermogenic process requires extensive systemic coordination: sympathetic neural input triggers lipolysis, vascular networks deliver fuel from multiple adipose depots, and hormonal signals integrate metabolic demand across organs. These requirements position thermogenic adipocytes as metabolic integration “nodes” that orchestrate whole-body fuel allocation and energy homeostasis. The presence of functional thermogenic adipocytes is strongly associated with improved cardiometabolic health, protecting against obesity, type 2 diabetes, and cardiovascular disease. Understanding how these specialized cells sense and respond to systemic signals offers a powerful entry point for developing strategies to counteract metabolic disease.

Whether a pharmacological strategy can replicate the broad improvement of human cardiometabolic health associated with brown fat (BAT) remains an active area of investigation. Here, we show that adipokine Adissp activates both glucose disposal and energy expenditure within white fat, delivering pleiotropic metabolic benefits. Endogenous Adissp is essential for glucose homeostasis. Administration of recombinant Adissp (rAdissp) protein sustainably normalizes hyperglycemia in type 1 and type 2 diabetic mice by activating insulin-independent Akt signaling. Furthermore, rAdissp robustly induces a comprehensive thermogenic program, which not only reduces body weight but also independently ameliorates a wide range of cardiometabolic diseases. Thus, a single adipokine, Adissp, recapitulates the systemic metabolic benefits of BAT and essentially functions as a cold mimetic. These findings reveal an unanticipated insulin-independent glucose uptake pathway and offer mechanistic insights into the cardiometabolic protection linked to human BAT. Adissp and its analogs represent a promising class of therapeutic agents to concurrently and synergistically treat diabetes and cardiometabolic diseases.

The ability to generate, store, and mobilize energy is fundamental to life, and disruptions in these processes underlie metabolic disease. This review examines how these mechanisms evolved to rely profoundly on lipids, a large family of molecules that serve an unusually wide range of biological functions. From early evolutionary times, lipids have enabled chemiosmotic energy transduction, organized the endomembrane systems that synthesize and package neutral lipids, served as concentrated reservoirs of metabolic energy, and functioned as signaling molecules that coordinate cellular and systemic physiology. Adipocytes are specialized cells that can accumulate large amounts of lipid within specialized lipid droplet organelles. Over evolutionary time, mammals developed distinct adipocyte subtypes with tailored physiological roles. White adipocytes, characterized by a single large unilocular lipid droplet, coordinate energy storage and release in response to systemic cues. In contrast, the role of brown and beige adipocytes is to protect the organism from cold exposure by generating heat as a primary output of UCP1-mediated mitochondrial uncoupling. Crucially, this thermogenic process requires extensive systemic coordination: sympathetic neural input triggers lipolysis, vascular networks deliver fuel from multiple adipose depots, and hormonal signals integrate metabolic demand across organs. These requirements position thermogenic adipocytes as metabolic integration "nodes" that orchestrate whole-body fuel allocation and energy homeostasis. The presence of functional thermogenic adipocytes is strongly associated with improved cardiometabolic health, protecting against obesity, type 2 diabetes, and cardiovascular disease. Understanding how these specialized cells sense and respond to systemic signals offers a powerful entry point for developing strategies to counteract metabolic disease.

Metabolic dysfunction-associated steatohepatitis (MASH) is a progressive liver disease driven by hepatocellular lipid overload, immune activation, and hepatic stellate cell (HSC)-driven fibrogenesis. Human genetic studies reveal that loss-of-function (LoF) variants in 17 beta-hydroxysteroid dehydrogenase 13 ( HSD17B13 ) confer robust protection against advanced fibrosis and cirrhosis, establishing HSD17B13 as a critical genetic modifier of MASH severity. Yet the mechanisms linking HSD17B13 activity to fibrogenic progression remain poorly understood. Here, we show that both wild-type and catalytically deficient HSD17B13 (mHSD) localize to lipid droplets (LDs) in cultured human hepatocytes, but only catalytically active HSD17B13 enhances hepatocellular lipid accumulation and markedly upregulates the lipogenic transcriptional regulator carbohydrate-responsive element-binding protein (ChREBP). This HSD17B13-driven lipogenic axis elicits potent paracrine activation of LX2 stellate cells, both in hepatocyte-HSC co-culture and in response to hepatocyte-conditioned medium (CM). Screening of candidate signaling mediators revealed that transforming growth factor beta-1 (TGFb-1) is uniquely and strongly upregulated by active HSD17B13, with minimal induction by mHSD. Remarkably, siRNA-mediated knockdown of TGFB1 or neutralization of active TGFb-1 protein abolishes CM-induced LX2 activation and collagen synthesis. Collectively, these findings identify HSD17B13 as a dual metabolic and profibrotic effector that drives TGFb-1-dependent HSC activation, thereby linking hepatocellular lipid dysregulation to fibrogenic progression and providing a mechanistic framework for understanding how HSD17B13 contributes to MASH pathogenesis.

Adipose tissue dysfunction in obesity is a major global public health risk, contributing to insulin resistance and chronic diseases such as diabetes and cardiovascular disorders. Here, we identify a dominant c.37A>G p.(Arg13Gly) variant in the long isoform of CIDEC (CIDEC-L), a key regulator of lipid droplet (LD) size, as the underlying cause of familial obesity. Affected individuals display marked subcutaneous fat accumulation in white adipose tissue (WAT), elevated fat content in brown adipose tissue (BAT) and insulin resistance. Accordingly, patient-derived iPSCs differentiated into white adipocytes exhibit accelerated LD growth, a phenotype mirrored by CIDEC-LR13G overexpression. Mechanistically, we find that the p.Arg13Gly variant disrupts the N-terminal structural order of CIDEC-L, shifting its phase separation properties to enable, rather than restrict, lipid exchange through condensation plates between LDs. Notably, knock-in mice with the analogous Cidec-L p.(Arg10Gly) mutation recapitulate the human BAT hypertrophy and exhibit impaired thermogenesis. These findings establish the CIDEC-LR13G variant as the first example of a dominantly inherited monogenic obesity driven by a dysfunctional adipocyte LD protein, revealing a critical role for CIDEC-L in restraining fat accumulation and maintaining metabolic health. ### Competing Interest Statement The authors have declared no competing interest. ### Funding Statement F. P. is a recipient of a long-term European Molecular Biology Organization (EMBO) postdoc fellowship and a short-term EMBO travel fellowship. Her research is supported by the Singapore Ministry of Health National Medical Research Council under its Young Individual Research Grant scheme (Project ID MOH-000549-01) and A*STAR under its Career Development Award (Project number C210112002). L. J. T. is supported by the A*STAR Career Development Fund (CDF C243512024). We thank Prof. Patrick TAN and the Genome Institute of Singapore (GIS) for supporting the costs associated with the travel and clinical phenotyping of two patients in Singapore. Research in L.P. laboratory was supported by the National Key R&D Program of China (2024YFA1802802) and the National Natural Science Foundation of China (92357302). Funding by the National Institutes of Health (DK130852 and DK116056 to M.P.C) and by the Isadore and Fannie Foxman endowed Chair in Medical Research at the University of Massachusetts Chan Medical School to M.P.C. is gratefully acknowledged. (SR/MED/GENT/16/01). B.R. is a fellow of the Branco Weiss Foundation (Switzerland) and an EMBO Young Investigator (Europe). The research reported in this publication was supported by funding from King Abdullah University of Science and Technology (KAUST) and from GIS at A*STAR (Singapore). ### Author Declarations I confirm all relevant ethical guidelines have been followed, and any necessary IRB and/or ethics committee approvals have been obtained. Yes The details of the IRB/oversight body that provided approval or exemption for the research described are given below: Institutional Review Boards of the Agency for Science, Technology and Research in Singapore and King Abdullah University of Science and Technology in Saudi Arabia; as well as the Domain Specific Review Board of the Singapore National Healthcare Group gave ethical approval for this work I confirm that all necessary patient/participant consent has been obtained and the appropriate institutional forms have been archived, and that any patient/participant/sample identifiers included were not known to anyone (e.g., hospital staff, patients or participants themselves) outside the research group so cannot be used to identify individuals. Yes I understand that all clinical trials and any other prospective interventional studies must be registered with an ICMJE-approved registry, such as ClinicalTrials.gov. I confirm that any such study reported in the manuscript has been registered and the trial registration ID is provided (note: if posting a prospective study registered retrospectively, please provide a statement in the trial ID field explaining why the study was not registered in advance). Yes I have followed all appropriate research reporting guidelines, such as any relevant EQUATOR Network research reporting checklist(s) and other pertinent material, if applicable. Yes All data produced in the present study are available upon reasonable request to the authors

Pancreatic cancer patients have the highest rates and most severe forms of cancer cachexia, yet cachexia etiologies remain largely elusive, leading to a lack of effective intervening therapies. Parathyroid hormone-related protein (PTHrP) has been clinically implicated as a putative regulator of cachexia, with serum PTHrP levels correlating with increased weight loss in PDAC patients. Here we show that cachectic PDAC patients have high expression of tumor PTHrP and use a genetically engineered mouse model to functionally demonstrate that loss of PTHrP blocks cachectic wasting, dramatically extending overall survival. The re-expression of PTHrP in lowly cachectic models is sufficient to induce wasting and reduce survival in mice, which is reversed by the conditional deletion of the PTHrP receptor, Pth1r, in adipocytes. Mechanistically, tumor-derived PTHrP suppresses de novo lipogenesis in adipocytes, leading to a molecular rewiring of adipose depots to promote wasting in the cachectic state. Finally, the pharmacological disruption of the PTHrP-PTH1R signaling axis abrogates wasting, highlighting that a targeted disruption of tumor-adipose crosstalk is an effective means to limit cachexia.

Bone homeostasis within the skeletal system is predominantly maintained by bone formation and resorption, where formation of new bone involves maturation of stromal cells to mineral and matrix secreting mature osteoblasts, which requires cellular energy or adenosine triphosphate. Alterations in systemic metabolism can influence osteoblast function. In line with this, type 2 diabetes mellitus (T2DM), a common metabolic disorder is also associated with reduced bone formation and increased risk of fracture. Impairment in lipid metabolism is one of the key features associated with T2DM-related pathologies in multiple tissues. Therefore, we tested the hypothesis that the reduced bone formation reported in obese murine models of impaired glucose tolerance is a function of disrupted lipid metabolism in osteoblasts. We first confirmed that mice fed a high-fat diet (HFD) have reduced bone microarchitecture along with lower bone formation rates. Interestingly, osteoblasts from obese mice harbor higher numbers of cytosolic lipid droplets along with decreased bioenergetic profiles compared to control cells. Further supporting this observation, bone cortex demonstrated higher total lipid content in HFD fed mice compared to control-fed mice. As a further proof of principle, we generated a novel murine model to conditionally delete Plin2 in osteoblast-progenitor cells using Prrx1-Cre, to enhance lipid droplet breakdown. Our data demonstrate that knocking down Plin2 in an osteoprogenitor specific manner protects from HFD induced osteoblast dysfunction. Furthermore, the mechanism of action involves enhanced osteoblast fatty acid oxidation. In conclusion, the current studies establish that HFD induced glucose intolerance leads to perturbations in osteoblast lipid metabolism, thus causing lower bone formation, which can be protected against by increasing fatty acid oxidation. Obesity is a chronic condition that results from excess fat, which can result in diseases such as type 2 diabetes mellitus. A striking consequence of diabetes is weaker bones leading to the increased risk of fracture; however, factors contributing to this remain unknown. Due to the connection between altered fat metabolism during diabetes, along with weaker bones, we sought to investigate how bone cell fat metabolism was altered during diabetes. First, we demonstrate that high-fat diet mouse models of diabetes resulted in lower bone formation as a function of fat accumulation in the bone, with lower energy production. We next used a genetic mouse model to promote fat metabolism in bone cells responsible for bone formation. Interestingly, these mice were protected from diabetes-associated bone loss. We were able to demonstrate this protection was through changes in bone cell fat metabolism. Collectively, these data establish that bone cell fat metabolism is critical for bone formation and bone quality, and that conditions such as diabetes result in altered fat metabolism and contribute to bone fragility.

Adipose tissue dysfunction leads to a variety of deleterious systemic consequences including ectopic lipid deposition and impaired insulin sensitivity. PPARγ is a major regulator of adipocyte differentiation and functionality and is thus a determinant of systemic metabolic health. We recently reported that deletion of adipocyte fatty acid synthase (AdFasnKO) impairs autophagy in association with a striking upregulation of genes controlled by PPARγ, including thermogenic uncoupling protein 1 (Ucp1). In this present study, screening for PPARγ coactivators regulated by autophagy revealed a protein denoted as Nuclear receptor coactivator 4 (Ncoa4), known to mediate ferritinophagy and interact with PPARγ and other nuclear receptors. Indeed, we found Ncoa4 is upregulated in the early phase of adipocyte differentiation and is required for adipogenesis. Ncoa4 is also elevated in FasnKO adipocytes and necessary for full upregulation of Ucp1 expression in vitro , even in response to norepinephrine. Consistent with these findings, adipose-selective knockout of Ncoa4 (AdNcoa4KO mice) impairs Ucp1 expression in brown adipose tissue and cold-induced thermogenesis. Adipose-selective double KO of Fasn plus Ncoa4 (AdFasnNcoa4DKO mice) prevents the upregulation of classic PPARγ target genes normally observed in the white adipose tissue of AdFasnKO mice, but not thermogenic Ucp1 expression. These findings reveal Ncoa4 is a novel determinant of adipocyte PPARγ activity and regulator of white and brown adipocyte biology and suggest that manipulation of autophagy flux modulates PPARγ activity and key adipocyte functions via Ncoa4 actions.

Circulating lactate is a fuel source for liver metabolism but may exacerbate metabolic diseases such as nonalcoholic steatohepatitis (NASH). Indeed, haploinsufficiency of lactate transporter monocarboxylate transporter 1 (MCT1) in mice reportedly promotes resistance to hepatic steatosis and inflammation. Here, we used adeno-associated virus (AAV) vectors to deliver thyroxin binding globulin (TBG)-Cre or lecithin-retinol acyltransferase (Lrat)-Cre to MCT1fl/fl mice on a choline deficient, high fat NASH diet to deplete hepatocyte or stellate cell MCT1, respectively. Stellate cell MCT1KO (AAV-Lrat-Cre) attenuated liver type 1 collagen protein expression and caused a downward trend in trichrome staining. MCT1 depletion in cultured human LX2 stellate cells also diminished collagen 1 protein expression. Tetra-ethylenglycol-cholesterol (Chol)-conjugated siRNAs, which enter all hepatic cell types, and hepatocyte-selective tri-N-acetyl galactosamine (GN)-conjugated siRNAs were then used to evaluate MCT1 function in a genetically obese NASH mouse model. MCT1 silencing by Chol-siRNA decreased liver collagen 1 levels, while hepatocyte-selective MCT1 depletion by AAV-TBG-Cre or by GN-siRNA unexpectedly increased collagen 1 and total fibrosis without effect on triglyceride accumulation. These findings demonstrate that stellate cell lactate transporter MCT1 significantly contributes to liver fibrosis through increased collagen 1 protein expression in vitro and in vivo, while hepatocyte MCT1 appears not to be an attractive therapeutic target for NASH.

The energy-burning capability of beige adipose tissue is a potential therapeutic tool for reducing obesity and metabolic disease, but this capacity is decreased by aging. Here, we evaluate the impact of aging on the profile and activity of adipocyte stem and progenitor cells (ASPCs) and adipocytes during the beiging process. We found that aging increases the expression of Cd9 and other fibro-inflammatory genes in fibroblastic ASPCs and blocks their differentiation into beige adipocytes. Fibroblastic ASPC populations from young and aged mice were equally competent for beige differentiation in vitro, suggesting that environmental factors suppress adipogenesis in vivo. Examination of adipocytes by single nucleus RNA-sequencing identified compositional and transcriptional differences in adipocyte populations with age and cold exposure. Notably, cold exposure induced an adipocyte population expressing high levels of de novo lipogenesis (DNL) genes, and this response was severely blunted in aged animals. We further identified natriuretic peptide clearance receptor Npr3, a beige fat repressor, as a marker gene for a subset of white adipocytes and an aging-upregulated gene in adipocytes. In summary, this study indicates that aging blocks beige adipogenesis and dysregulates adipocyte responses to cold exposure and provides a unique resource for identifying cold and aging-regulated pathways in adipose tissue.

Disruption of adipocyte de novo lipogenesis (DNL) by deletion of fatty acid synthase (FASN) in mice induces browning in inguinal white adipose tissue (iWAT). However, adipocyte FASN knockout (KO) increases acetyl-coenzyme A (CoA) and malonyl-CoA in addition to depletion of palmitate. We explore which of these metabolite changes triggers adipose browning by generating eight adipose-selective KO mouse models with loss of ATP-citrate lyase (ACLY), acetyl-CoA carboxylase 1 (ACC1), ACC2, malonyl-CoA decarboxylase (MCD) or FASN, or dual KOs ACLY/FASN, ACC1/FASN, and ACC2/FASN. Preventing elevation of acetyl-CoA and malonyl-CoA by depletion of adipocyte ACLY or ACC1 in combination with FASN KO does not block the browning of iWAT. Conversely, elevating malonyl-CoA levels in MCD KO mice does not induce browning. Strikingly, adipose ACC1 KO induces a strong iWAT thermogenic response similar to FASN KO while also blocking malonyl-CoA and palmitate synthesis. Thus, ACC1 and FASN are strong suppressors of adipocyte thermogenesis through promoting lipid synthesis rather than modulating the DNL intermediates acetyl-CoA or malonyl-CoA.

Bone formation is a highly energy-demanding process that can be impacted by metabolic disorders. Glucose has been considered the principal substrate for osteoblasts, although fatty acids are also important for osteoblast function. Here, we report that osteoblasts can derive energy from endogenous fatty acids stored in lipid droplets via lipolysis and that this process is critical for bone formation. As such, we demonstrate that osteoblasts accumulate lipid droplets that are highly dynamic and provide the molecular mechanism by which they serve as a fuel source for energy generation during osteoblast maturation. Inhibiting cytoplasmic lipolysis leads to both an increase in lipid droplet size in osteoblasts and an impairment in osteoblast function. The fatty acids released by lipolysis from these lipid droplets become critical for cellular energy production as cellular energetics shifts towards oxidative phosphorylation during nutrient-depleted conditions. In vivo, conditional deletion of the ATGL-encoding gene Pnpla2 in osteoblast progenitor cells reduces cortical and trabecular bone parameters and alters skeletal lipid metabolism. Collectively, our data demonstrate that osteoblasts store fatty acids in the form of lipid droplets, which are released via lipolysis to support cellular bioenergetic status when nutrients are limited. Perturbations in this process result in impairment of bone formation, specifically reducing ATP production and overall osteoblast function.

Triglycerides (TG) in adipocytes provide the major stores of metabolic energy in the body. Optimal amounts of TG stores are desirable as insufficient capacity to store TG, as in lipodystrophy, or exceeding the capacity for storage, as in obesity, results in metabolic disease. We hypothesized that mice lacking TG storage in adipocytes would result in excess TG storage in cell types other than adipocytes and severe lipotoxicity accompanied by metabolic disease. To test this hypothesis, we selectively deleted both TG-synthesis enzymes, DGAT1 and DGAT2, in adipocytes (ADGAT DKO mice). As expected with depleted energy stores, ADGAT DKO mice did not tolerate fasting well and, with prolonged fasting, entered torpor. However, ADGAT DKO mice were unexpectedly otherwise metabolically healthy and did not accumulate TGs ectopically or develop associated metabolic perturbations, even when fed a high-fat diet. The favorable metabolic phenotype resulted from activation of energy expenditure, in part via BAT activation and beiging of white adipose tissue. Thus, the ADGAT DKO mice provide a fascinating new model to study the coupling of metabolic energy storage to energy expenditure.

Objectives: Nuclear receptor interacting protein 1 (NRIP1) suppresses energy expenditure via repression of nuclear receptors, and its depletion markedly elevates uncoupled respiration in mouse and human adipocytes. We tested whether NRIP1 deficient adipocytes implanted into obese mice would enhance whole body metabolism. Since b-adrenergic signaling through cAMP strongly promotes adipocyte thermogenesis, we tested whether the effects of NRIP1 knock-out (NRIP1KO) require the cAMP pathway.Methods: NRIP1KO adipocytes were implanted in recipient high-fat diet (HFD) fed mice and metabolic cage studies conducted. The Nrip1 gene was disrupted by CRISPR in primary preadipocytes isolated from control vs adipose selective GsaKO (cAdGsaKO) mice prior to differentiation to adipocytes. Protein kinase A inhibitor was also used.Results: Implanting NRIP1KO adipocytes into HFD fed mice enhanced whole-body glucose tolerance by increasing insulin sensitivity, reducing adiposity, and enhancing energy expenditure in the recipients. NRIP1 depletion in both control and GsaKO adipocytes was equally effective in upregulating uncoupling protein 1 (UCP1) and adipocyte beiging, while b-adrenergic signaling by CL 316,243 was abolished in GsaKO adi-pocytes. Combining NRIP1KO with CL 316,243 treatment synergistically increased Ucp1 gene expression and increased the adipocyte sub-population responsive to beiging. Estrogen-related receptor a (ERRa) was dispensable for UCP1 upregulation by NRIPKO.Conclusions: The thermogenic effect of NRIP1 depletion in adipocytes causes systemic enhancement of energy expenditure when such adi-pocytes are implanted into obese mice. Furthermore, NRIP1KO acts independently but cooperatively with the cAMP pathway in mediating its effect on adipocyte beiging.& COPY; 2023 The Author(s). Published by Elsevier GmbH. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/).

Adipose tissue signals to brain, liver, and muscles to control whole body metabolism through secreted lipid and protein factors as well as neurotransmission, but the mechanisms involved are incompletely understood. Adipocytes sequester triglyceride (TG) in fed conditions stimulated by insulin, while in fasting catecholamines trigger TG hydrolysis, releasing glycerol and fatty acids (FAs). These antagonistic hormone actions result in part from insulin's ability to inhibit cAMP levels generated through such G-protein-coupled receptors as catecholamine-activated β-adrenergic receptors. Consistent with these antagonistic signaling modes, acute actions of catecholamines cause insulin resistance. Yet, paradoxically, chronically activating adipocytes by catecholamines cause increased glucose tolerance, as does insulin. Recent results have helped to unravel this conundrum by revealing enhanced complexities of these hormones' signaling networks, including identification of unexpected common signaling nodes between these canonically antagonistic hormones.

Nonalcoholic steatohepatitis (NASH) is a malady of multiple cell types associated with hepatocyte triglyceride (TG) accumulation, macrophage inflammation, and stellate cell-induced fibrosis, with no approved therapeutics yet available. Here, we report that stellate cell fatty acid synthase (FASN) in de novo lipogenesis drives the autophagic flux that is required for stellate cell activation and fibrotic collagen production. Further, we employ a dual targeting approach to NASH that selectively depletes collagen through selective stellate cell knockout of FASN (using AAV9-LRAT Cre in FASN fl/fl mice), while lowering hepatocyte triglyceride by depleting DGAT2 with a GalNac-conjugated, fully chemically modified siRNA. DGAT2 silencing in hepatocytes alone or in combination with stellate cell FASNKO reduced liver TG accumulation in a choline-deficient NASH mouse model, while FASNKO in hepatocytes alone (using AAV8-TBG Cre in FASN fl/fl mice) did not. Neither hepatocyte DGAT2 silencing alone nor FASNKO in stellate cells alone decreased fibrosis (total collagen), while loss of both DGAT2 plus FASN caused a highly significant attenuation of NASH. These data establish proof of concept that dual targeting of DGAT2 plus FASN alleviates NASH progression in mice far greater than targeting either gene product alone.

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.

Macroautophagy/autophagy requires enormous membrane expansions during concerted actions of transient autophagic vesicles and lysosomes, yet the source of the membrane lipids is poorly understood. Recent work in adipocytes has now pinpointed the de novo lipogenesis pathway as the preferred source of fatty acids for phospholipid in autophagic membrane synthesis, as loss of FASN (fatty acid synthase) disrupts autophagic flux and lysosome function in vivo and in vitro. These data indicate fatty acid synthesis channels lipid for membrane expansions, whereas fatty acids from circulating lipoproteins provide for adipose lipid storage. Importantly, autophagy blockade upon loss of fatty acids promotes a strong thermogenic phenotype in adipocytes, another striking example whereby autophagy controls cell behavior.

Hepatic steatosis associated with high-fat diet, obesity, and type 2 diabetes is thought to be the major driver of severe liver inflammation, fibrosis, and cirrhosis. Cytosolic acetyl CoA (AcCoA), a central metabolite and substrate for de novo lipo-genesis (DNL), is produced from citrate by ATP-citrate lyase (ACLY) and from acetate through AcCoA synthase short chain family member 2 (ACSS2). However, the relative contributions of these two enzymes to hepatic AcCoA pools and DNL rates in response to high-fat feeding are unknown. We report here that hepatocyte-selective depletion of either ACSS2 or ACLY caused similar 50% decreases in liver AcCoA levels in obese mice, showing that both pathways contribute to the generation of this DNL substrate. Unexpectedly however, the hepatocyte ACLY depletion in obese mice paradoxically increased total DNL flux measured by D2O incorporation into palmitate, whereas in contrast, ACSS2 depletion had no effect. The in-crease in liver DNL upon ACLY depletion was associated with increased expression of nuclear sterol regulatory element- binding protein 1c and of its target DNL enzymes. This upre-gulated DNL enzyme expression explains the increased rate of palmitate synthesis in ACLY-depleted livers. Furthermore, this increased flux through DNL may also contribute to the observed depletion of AcCoA levels because of its increased conversion to malonyl CoA and palmitate. Together, these data indicate that in fat diet-fed obese mice, hepatic DNL is not limited by its immediate substrates AcCoA or malonyl CoA but rather by activities of DNL enzymes.