Clinical and molecular profiling of human visceral adipose tissue reveals impairment of vascular architecture and remodeling as an early hallmark of dysfunction

Adipose tissue dysfunction is more related to insulin resistance than body mass index itself and an alteration in adipose tissue function is thought to underlie the shift from metabolically healthy to unhealthy obesity. Herein, we performed a clustering analysis that revealed distinct visceral adipose tissue gene expression patterns in patients with obesity at distinct stages of metabolic dysregulation. We have built a cross-sectional cohort that aims at reflecting the evolution of the metabolic sequelae of obesity with the main objective to map the sequential events that play a role in adipose tissue dysfunction from the metabolically healthy (insulin-sensitive) state to several incremental degrees of metabolic dysregulation, encompassing insulin resistance establishment, pre-diabetes, and type 2 diabetes. We found that insulin resistance is mainly marked by the downregulation of adipose tissue vasculature remodeling-associated gene expression, suggesting that processes like angiogenesis and adaptative expansion/retraction ability suffer early dysregulation. Prediabetes was characterized by compensatory growth factor-dependent signaling and increased response to hypoxia, while type 2 diabetes was associated with loss of cellular response to insulin and hypoxia and concomitant upregulation of inflammatory markers. Our findings suggest a putative sequence of dysregulation of biological processes that is not linear and has multiple distinct phases across the metabolic dysregulation process, ultimately culminating in the climax of adipose tissue dysfunction in type 2 diabetes. Several studies have addressed the transcriptomic changes in adipose tissue of patients with obesity. However, to the best of our knowledge, this is the first study unraveling the potential molecular mechanisms associated with the multi-step evolution of adipose tissue dysfunction along the metabolic sequelae of obesity.

Adipose Tissue Analysis Toolkit (ATAT) for automated analysis of adipocyte size and extracellular matrix in white adipose tissue

OBJECTIVE

The pathological expansion of white adipose tissue (WAT) in obesity involves adipocyte hypertrophy accompanied by expansion of the collagen-rich pericellular extracellular matrix (ECM) and development of crown-like structures (CLS). Traditionally, WAT morphology is assessed through immunohistochemical analysis of WAT sections. However, manual analysis of large histological sections is time-consuming, and the available digital tools for analyzing adipocyte size and pericellular ECM are limited. To address this gap, the authors developed the Adipose Tissue Analysis Toolkit (ATAT), an ImageJ plugin facilitating analysis of adipocyte size, WAT ECM, and CLS.

METHODS AND RESULTS

ATAT utilizes local and image-level differentials in pixel intensity to independently threshold image background, distinguishing adipocyte-free tissue without user input. It accurately captures adipocytes in histological sections stained with common dyes and automates the analysis of adipocyte cross-sectional area, total-field, and localized region-of-interest ECM. ATAT allows fully automated batch analysis of histological images using default or user-defined adipocyte detection parameters.

CONCLUSIONS

ATAT provides several advantages over existing WAT image analysis tools, enabling high-throughput analyses of adipocyte-specific parameters and facilitating the assessment of ECM changes associated with WAT remodeling due to weight changes and other pathophysiological alterations that affect WAT function.

Downregulation of the kidney glucagon receptor, essential for renal function and systemic homeostasis, contributes to chronic kidney disease

The glucagon receptor (GCGR) in the kidney is expressed in nephron tubules. In humans and animal models with chronic kidney disease, renal GCGR expression is reduced. However, the role of kidney GCGR in normal renal function and in disease development has not been addressed. Here, we examined its role by analyzing mice with constitutive or conditional kidney-specific loss of the Gcgr. Adult renal Gcgr knockout mice exhibit metabolic dysregulation and a functional impairment of the kidneys. These mice exhibit hyperaminoacidemia associated with reduced kidney glucose output, oxidative stress, enhanced inflammasome activity, and excess lipid accumulation in the kidney. Upon a lipid challenge, they display maladaptive responses with acute hypertriglyceridemia and chronic proinflammatory and profibrotic activation. In aged mice, kidney Gcgr ablation elicits widespread renal deposition of collagen and fibronectin, indicative of fibrosis. Taken together, our findings demonstrate an essential role of the renal GCGR in normal kidney metabolic and homeostatic functions. Importantly, mice deficient for kidney Gcgr recapitulate some of the key pathophysiological features of chronic kidney disease.

GIPR Agonism Enhances TZD-Induced Insulin Sensitivity in Obese IR Mice

Recent studies have found that glucose-dependent insulinotropic polypeptide receptor (GIPR) agonism can enhance the metabolic efficacy of glucagon-like peptide-1 receptor agonist treatment by promoting both weight-dependent and -independent improvements on systemic insulin sensitivity. These findings have prompted new investigations aimed at better understanding the broad metabolic benefit of GIPR activation. Herein, we determined whether GIPR agonism favorably influenced the pharmacologic efficacy of the insulin-sensitizing thiazolidinedione (TZD) rosiglitazone in obese insulin-resistant (IR) mice. Genetic and pharmacological approaches were used to examine the role of GIPR signaling on rosiglitazone-induced weight gain, hyperphagia, and glycemic control. RNA sequencing was conducted to uncover potential mechanisms by which GIPR activation influences energy balance and insulin sensitivity. In line with previous findings, treatment with rosiglitazone induced the mRNA expression of the GIPR in white and brown fat. However, obese GIPR-null mice dosed with rosiglitazone had equivalent weight gain to that of wild-type (WT) animals. Strikingly, chronic treatment of obese IR WT animals with a long-acting GIPR agonist prevented rosiglitazone-induced weight-gain and hyperphagia, and it enhanced the insulin-sensitivity effect of this TZD. The systemic insulin sensitization was accompanied by increased glucose disposal in brown adipose tissue, which was underlined by the recruitment of metabolic and thermogenic genes. These findings suggest that GIPR agonism can counter the negative consequences of rosiglitazone treatment on body weight and adiposity, while improving its insulin-sensitizing efficacy at the same time.

ARTICLE HIGHLIGHTS

Leptin Reduction as a Required Component for Weight Loss

Partial leptin reduction can induce significant weight loss, while weight loss contributes to partial leptin reduction. The cause-and-effect relationship between leptin reduction and weight loss remains to be further elucidated. Here, we show that FGF21 and the glucagon-like peptide 1 receptor (GLP-1R) agonist liraglutide rapidly induced a reduction in leptin. This leptin reduction contributed to the beneficial effects of GLP-1R agonism in metabolic health, as transgenically maintaining leptin levels during treatment partially curtailed the beneficial effects seen with these agonists. Moreover, a higher degree of leptin reduction during treatment, induced by including a leptin neutralizing antibody with either FGF21 or liraglutide, synergistically induced greater weight loss and better glucose tolerance in diet-induced obese mice. Furthermore, upon cessation of either liraglutide or FGF21 treatment, the expected immediate weight regain was observed, associated with a rapid increase in circulating leptin levels. Prevention of this leptin surge with leptin neutralizing antibodies slowed down weight gain and preserved better glucose tolerance. Mechanistically, a significant reduction in leptin induced a higher degree of leptin sensitivity in hypothalamic neurons. Our observations support a model that postulates that a reduction of leptin levels is a necessary prerequisite for substantial weight loss, and partial leptin reduction is a viable strategy to treat obesity and its associated insulin resistance.

ARTICLE HIGHLIGHTS

CRISPR-Cas9 base editing of pathogenic CaMKIIδ improves cardiac function in a humanized mouse model

Cardiovascular diseases are the most common cause of worldwide morbidity and mortality, highlighting the necessity for advanced therapeutic strategies. Ca2+/calmodulin-dependent protein kinase IIδ (CaMKIIδ) is a prominent inducer of various cardiac disorders, which is mediated by 2 oxidation-sensitive methionine residues within the regulatory domain. We have previously shown that ablation of CaMKIIδ oxidation by CRISPR-Cas9 base editing enables the heart to recover function from otherwise severe damage following ischemia/reperfusion (IR) injury. Here, we extended this therapeutic concept toward potential clinical translation. We generated a humanized CAMK2D knockin mouse model in which the genomic sequence encoding the entire regulatory domain was replaced with the human sequence. This enabled comparison and optimization of two different editing strategies for the human genome in mice. To edit CAMK2D in vivo, we packaged the optimized editing components into an engineered myotropic adeno-associated virus (MyoAAV 2A), which enabled efficient delivery at a very low AAV dose into the humanized mice at the time of IR injury. CAMK2D-edited mice recovered cardiac function, showed improved exercise performance, and were protected from myocardial fibrosis, which was otherwise observed in injured control mice after IR. Our findings identify a potentially effective strategy for cardioprotection in response to oxidative damage.

Endotrophin, a Key Marker and Driver for Fibroinflammatory Disease

Our overview covers several key areas related to recent results obtained for collagen type VI and endotrophin (ETP): i) An introduction to the history of ETP, including how it was identified, how it is released and its function and potential receptors. ii) An introduction to the collagen family, with a focus on what differentiates collagen type VI from an evolutionary standpoint. iii) An overview of collagen type VI, the six individual chains (COL6A1, A2, A3, A4, A5 and A6), their differences and similarities, as well as their expression profiles and function. iv) A detailed analysis of COL6A3, including the cleaved product endotrophin, and what separates it from the other five collagen 6 molecules, including its suggested function based on insights gained from knockout and gain of function mouse models. v) An introduction to the history of ETP, including how it was identified, how it is released and its function and potential receptors. vi) The pathology of ETP. What leads to its presence and release and what are the consequences thereof? vii) Functional implications of circulating ETP. Here we review the data with the functional roles of ETP in mind. viii) We propose that ETP is a mediator for fibrotic (or fibro-inflammatory? ) disorders. Based on what we know about ETP, we have to consider it as a target for the treatment of fibrotic (or fibro-inflammatory) disorders. What segment(s) of the patient population would most dramatically respond to an ETP-targeted intervention? How can we find the population that would profit most from an intervention? We aim to present a broad overview over the ETP field at large, providing an assessment of where the future research efforts need to be placed to tap into the vast potential of ETP, both as a marker and as a target in different diseases.

Adipose Tissue Analysis Toolkit (ATAT) for Automated Analysis of Adipocyte Size and Extracellular Matrix in White Adipose Tissue

Objective

The pathological expansion of white adipose tissue (WAT) in obesity involves adipocyte hypertrophy accompanied by expansion of collagen-rich pericellular extracellular matrix (ECM) and the development of crown-like structures (CLS). Traditionally, WAT morphology is assessed through immunohistochemical analysis of WAT sections. However, manual analysis of large histological sections is time-consuming, and available digital tools for analyzing adipocyte size and pericellular ECM are limited. To address this gap, we developed the Adipose Tissue Analysis Toolkit (ATAT), an ImageJ plugin facilitating analysis of adipocyte size, WAT ECM and CLS.

Methods and Results

ATAT utilizes local and image-level differentials in pixel intensity to independently threshold background, distinguishing adipocyte-free tissue without user input. It accurately captures adipocytes in histological sections stained with common dyes and automates the analysis of adipocyte cross-sectional area, total-field, and localized region-of-interest ECM. ATAT allows fully automated batch analysis of histological images using default or user-defined adipocyte detection parameters.

Conclusions

ATAT provides several advantages over existing WAT image analysis tools, enabling high-throughput analyses of adipocyte-specific parameters and facilitating the assessment of ECM changes associated with WAT remodeling due to weight changes and other pathophysiological alterations that affect WAT function.

Study Importance Questions

What is already known about this subject?: The manual analysis of large WAT histological sections is very time-consuming, while digital tools for the analysis of WAT are limited.What are the new findings in your manuscript?: - ATAT enables fully automated analysis of batches of histological images using either default or user-defined adipocyte detection parameters- ATAT allows high-throughput analyses of adipocyte-specific parameters and pericellular extracellular matrix- ATAT enables the assessment of fibrotic changes associated with WAT remodeling and crown-like structuresHow might your results change the direction of research or the focus of clinical practice?: - ATAT is designed to work with histological sections and digital images obtained using a slide scanner or a microscope.- This tool will help basic and clinical researchers to conduct automated analyses of adipose tissue histological sections.

Protective roles of adiponectin and molecular signatures of HNF4α and PPARα as downstream targets of adiponectin in pancreatic β cells

The disease progression of the metabolic syndrome is associated with prolonged hyperlipidemia and insulin resistance, eventually giving rise to impaired insulin secretion, often concomitant with hypoadiponectinemia. As an adipose tissue derived hormone, adiponectin is beneficial for insulin secretion and β cell health and differentiation. However, the down-stream pathway of adiponectin in the pancreatic islets has not been studied extensively. Here, along with the overall reduction of endocrine pancreatic function in islets from adiponectin KO mice, we examine PPARα and HNF4α as additional down-regulated transcription factors during a prolonged metabolic challenge. To elucidate the function of β cell-specific PPARα and HNF4α expression, we developed doxycycline inducible pancreatic β cell-specific PPARα (β-PPARα) and HNF4α (β-HNF4α) overexpression mice. β-PPARα mice exhibited improved protection from lipotoxicity, but elevated β-oxidative damage in the islets, and also displayed lowered phospholipid levels and impaired glucose-stimulated insulin secretion. β-HNF4α mice showed a more severe phenotype when compared to β-PPARα mice, characterized by lower body weight, small islet mass and impaired insulin secretion. RNA-sequencing of the islets of these models highlights overlapping yet unique roles of β-PPARα and β-HNF4α. Given that β-HNF4α potently induces PPARα expression, we define a novel adiponectin-HNF4α-PPARα cascade. We further analyzed downstream genes consistently regulated by this axis. Among them, the islet amyloid polypeptide (IAPP) gene is an important target and accumulates in adiponectin KO mice. We propose a new mechanism of IAPP aggregation in type 2 diabetes through reduced adiponectin action.

Adipose tissue fibrosis: the unwanted houseguest invited by obesity

The prevalence of obesity is increasing exponentially across the globe. The lack of effective treatment options for long-term weight loss has magnified the enormity of this problem. Studies continue to demonstrate that adipose tissue holds a biological memory, one of the most important determinant of long-term weight maintenance. This phenomenon is consistent with the metabolically dynamic role of adipose tissue: it adapts and expands to store for excess energy and serves as an endocrine organ capable of synthesizing a number of biologically active molecules that regulate metabolic homeostasis. An important component of the plasticity of adipose tissue is the extracellular matrix, essential for structural support, mechanical stability, cell signaling and function. Chronic obesity upends a delicate balance of extracellular matrix synthesis and degradation, and the ECM accumulates in such a way that prevents the plasticity and function of the diverse cell types in adipose tissue. A series of maladaptive responses among the cells in adipose tissue leads to inflammation and fibrosis, major mechanisms that explain the link between obesity and insulin resistance, risk of type 2 diabetes, cardiovascular disease, and nonalcoholic fatty liver disease. Adipose tissue fibrosis persists after weight loss and further enhances adipose tissue dysfunction if weight is regained. Here, we highlight the current knowledge of the cellular events governing adipose tissue ECM remodeling during the development of obesity. Our goal is to delineate the relationship more clearly between adipose tissue ECM and metabolic disease, an important step toward better defining the pathophysiology of dysfunctional adipose tissue.

Hyperleptinemia contributes to antipsychotic drug-associated obesity and metabolic disorders

Despite their high degree of effectiveness in the management of psychiatric conditions, exposure to antipsychotic drugs, including olanzapine and risperidone, is frequently associated with substantial weight gain and the development of diabetes. Even before weight gain, a rapid rise in circulating leptin concentrations can be observed in most patients taking antipsychotic drugs. To date, the contribution of this hyperleptinemia to weight gain and metabolic deterioration has not been defined. Here, with an established mouse model that recapitulates antipsychotic drug-induced obesity and insulin resistance, we not only confirm that hyperleptinemia occurs before weight gain but also demonstrate that hyperleptinemia contributes directly to the development of obesity and associated metabolic disorders. By suppressing the rise in leptin through the use of a monoclonal leptin-neutralizing antibody, we effectively prevented weight gain, restored glucose tolerance, and preserved adipose tissue and liver function in antipsychotic drug-treated mice. Mechanistically, suppressing excess leptin resolved local tissue and systemic inflammation typically associated with antipsychotic drug treatment. We conclude that hyperleptinemia is a key contributor to antipsychotic drug-associated weight gain and metabolic deterioration. Leptin suppression may be an effective approach to reducing the undesirable side effects of antipsychotic drugs.

Update on Adipose Tissue and Cancer

Adipose tissue is the largest endocrine organ and an accepted contributor to overall energy homeostasis. There is strong evidence linking increased adiposity to the development of 13 types of cancer. With increased adiposity comes metabolic dysfunction and insulin resistance, and increased systemic insulin and glucose support the growth of many cancers, including those of the colon and endometrium. There is also an important direct crosstalk between adipose tissue and various organs. For instance, the healthy development and function of the mammary gland, as well as the development, growth, and progression of breast cancer, are heavily impacted by the breast adipose tissue in which breast epithelial cells are embedded. Cells of the adipose tissue are responsive to external stimuli, including overfeeding, leading to remodeling and important changes in the secretion of factors known to drive the development and growth of cancers. Loss of factors like adiponectin and increased production of leptin, endotrophin, steroid hormones, and inflammatory mediators have been determined to be important mediators of the obesity-cancer link. Obesity is also associated with a structural remodeling of the adipose tissue, including increased localized fibrosis and disrupted angiogenesis that contribute to the development and progression of cancers. Furthermore, tumor cells feed off the adipose tissue, where increased lipolysis within adipocytes leads to the release of fatty acids and stromal cell aerobic glycolysis leading to the increased production of lactate. Both have been hypothesized to support the higher energetic demands of cancer cells. Here, we aim to provide an update on the state of the literature revolving around the role of the adipose tissue in cancer initiation and progression.

Regulated adipose tissue-specific expression of human AGPAT2 in lipodystrophic Agpat2-null mice results in regeneration of adipose tissue

Genetic loss of Agpat2 in humans and mice results in congenital generalized lipodystrophy with near-total loss of adipose tissue and predisposition to develop insulin resistance, diabetes mellitus, hepatic steatosis, and hypertriglyceridemia. The mechanism by which Agpat2 deficiency results in loss of adipose tissue remains unknown. We studied this by re-expressing human AGPAT2 (hAGPAT2) in Agpat2-null mice, regulated by doxycycline. In both sexes of Agpat2-null mice, adipose-tissue-specific re-expression of hAGPAT2 resulted in partial regeneration of both white and brown adipose tissue (but only 30%-50% compared with wild-type mice), which had molecular signatures of adipocytes, including leptin secretion. Furthermore, the stromal vascular fraction cells of regenerated adipose depots differentiated ex vivo only with doxycycline, suggesting the essential role of Agpat2 in adipocyte differentiation. Turning off expression of hAGPAT2 in vivo resulted in total loss of regenerated adipose tissue, clear evidence that Agpat2 is essential for adipocyte differentiation in vivo.

Endogenous renal adiponectin drives gluconeogenesis through enhancing pyruvate and fatty acid utilization

Adiponectin is a secretory protein, primarily produced in adipocytes. However, low but detectable expression of adiponectin can be observed in cell types beyond adipocytes, particularly in kidney tubular cells, but its local renal role is unknown. We assessed the impact of renal adiponectin by utilizing male inducible kidney tubular cell-specific adiponectin overexpression or knockout mice. Kidney-specific adiponectin overexpression induces a doubling of phosphoenolpyruvate carboxylase expression and enhanced pyruvate-mediated glucose production, tricarboxylic acid cycle intermediates and an upregulation of fatty acid oxidation (FAO). Inhibition of FAO reduces the adiponectin-induced enhancement of glucose production, highlighting the role of FAO in the induction of renal gluconeogenesis. In contrast, mice lacking adiponectin in the kidney exhibit enhanced glucose tolerance, lower utilization and greater accumulation of lipid species. Hence, renal adiponectin is an inducer of gluconeogenesis by driving enhanced local FAO and further underlines the important systemic contribution of renal gluconeogenesis.

Proteolytic rewiring of mitochondria by LONP1 directs cell identity switching of adipocytes

Mitochondrial proteases are emerging as key regulators of mitochondrial plasticity and acting as both protein quality surveillance and regulatory enzymes by performing highly regulated proteolytic reactions. However, it remains unclear whether the regulated mitochondrial proteolysis is mechanistically linked to cell identity switching. Here we report that cold-responsive mitochondrial proteolysis is a prerequisite for white-to-beige adipocyte cell fate programming during adipocyte thermogenic remodelling. Thermogenic stimulation selectively promotes mitochondrial proteostasis in mature white adipocytes via the mitochondrial protease LONP1. Disruption of LONP1-dependent proteolysis substantially impairs cold- or β3 adrenergic agonist-induced white-to-beige identity switching of mature adipocytes. Mechanistically, LONP1 selectively degrades succinate dehydrogenase complex iron sulfur subunit B and ensures adequate intracellular succinate levels. This alters the histone methylation status on thermogenic genes and thereby enables adipocyte cell fate programming. Finally, augmented LONP1 expression raises succinate levels and corrects ageing-related impairments in white-to-beige adipocyte conversion and adipocyte thermogenic capacity. Together, these findings reveal that LONP1 links proteolytic surveillance to mitochondrial metabolic rewiring and directs cell identity conversion during adipocyte thermogenic remodelling.

Endotrophin neutralization through targeted antibody treatment protects from renal fibrosis in a podocyte ablation model

OBJECTIVE

Renal fibrosis is a hallmark for chronic kidney disease (CKD), and often leads to end stage renal disease (ESRD). However, limited interventions are available clinically to ameliorate or reverse renal fibrosis.

METHODS

Herein, we evaluated whether blockade of endotrophin through neutralizing antibodies protects from renal fibrosis in the podocyte insult model (the "POD-ATTAC" mouse). We determined the therapeutic effects of endotrophin targeted antibody through assessing renal function, renal inflammation and fibrosis at histological and transcriptional levels, and podocyte regeneration.

RESULTS

We demonstrated that neutralizing endotrophin antibody treatment significantly ameliorates renal fibrosis at the transcriptional, morphological, and functional levels. In the antibody treatment group, expression of pro-inflammatory and pro-fibrotic genes was significantly reduced, normal renal structures were restored, collagen deposition was decreased, and proteinuria and renal function were improved. We further performed a lineage tracing study confirming that podocytes regenerate as de novo podocytes upon injury and loss, and blockade of endotrophin efficiently enhances podocyte-specific marker expressions.

CONCLUSION

Combined, we provide pre-clinical evidence supporting neutralizing endotrophin as a promising therapy for intervening with renal fibrosis in CKD, and potentially in other chronic fibro-inflammatory diseases.

Extracellular Matrix (ECM) and Fibrosis in Adipose Tissue: Overview and Perspectives

Fibrosis in adipose tissue is a major driver of obesity-related metabolic dysregulation. It is characterized by an overaccumulation of extracellular matrix (ECM) during unhealthy expansion of adipose tissue in response to over nutrition. In obese adipose-depots, hypoxia stimulates multiple pro-fibrotic signaling pathways in different cell populations, thereby inducing the overproduction of the ECM components, including collagens, noncollagenous proteins, and additional enzymatic components of ECM synthesis. As a consequence, local fibrosis develops. The result of fibrosis-induced mechanical stress not only triggers cell necrosis and inflammation locally in adipose tissue but also leads to system-wide lipotoxicity and insulin resistance. A better understanding of the mechanisms underlying the obesity-induced fibrosis will help design therapeutic approaches to reduce or reverse the pathological changes associated with obese adipose tissue. Here, we aim to summarize the major advances in the field, which include newly identified fibrotic factors, cell populations that contribute to the fibrosis in adipose tissue, as well as novel mechanisms underlying the development of fibrosis. We further discuss the potential therapeutic strategies to target fibrosis in adipose tissue for the treatment of obesity-linked metabolic diseases and cancer. © 2023 American Physiological Society. Compr Physiol 13:4387-4407, 2023.

Pathophysiology of cellulite: Possible involvement of selective endotoxemia

The most relevant hallmarks of cellulite include a massive protrusion of superficial adipose tissue into the dermis, reduced expression of the extracellular glycoprotein fibulin-3, and an unusually high presence of MUSE cells in gluteofemoral white adipose tissue (gfWAT) that displays cellulite. Also typical for this condition is the hypertrophic nature of the underlying adipose tissue, the interaction of adipocytes with sweat glands, and dysfunctional lymph and blood circulation as well as a low-grade inflammation in the areas of gfWAT affected by cellulite. Here, we propose a new pathophysiology of cellulite, which connects this skin condition with selective accumulation of endogenous lipopolysaccharides (LPS) in gfWAT. The accumulation of LPS within a specific WAT depot has so far not been considered as a possible pathophysiological mechanism triggering localized WAT modifications, but may very well be involved in conditions such as cellulite and, secondary to that, lipedema.

Deficient Caveolin-1 Synthesis in Adipocytes Stimulates Systemic Insulin-Independent Glucose Uptake via Extracellular Vesicles

Caveolin-1 (cav1) is an important structural and signaling component of plasma membrane invaginations called caveolae and is abundant in adipocytes. As previously reported, adipocyte-specific ablation of the cav1 gene (ad-cav1 knockout [KO] mouse) does not result in elimination of the protein, as cav1 protein traffics to adipocytes from neighboring endothelial cells. However, this mouse is a functional KO because adipocyte caveolar structures are depleted. Compared with controls, ad-cav1KO mice on a high-fat diet (HFD) display improved whole-body glucose clearance despite complete loss of glucose-stimulated insulin secretion, blunted insulin-stimulated AKT activation in metabolic tissues, and partial lipodystrophy. The cause is increased insulin-independent glucose uptake by white adipose tissue (AT) and reduced hepatic gluconeogenesis. Furthermore, HFD-fed ad-cav1KO mice display significant AT inflammation, fibrosis, mitochondrial dysfunction, and dysregulated lipid metabolism. The glucose clearance phenotype of the ad-cav1KO mice is at least partially mediated by AT small extracellular vesicles (AT-sEVs). Injection of control mice with AT-sEVs from ad-cav1KO mice phenocopies ad-cav1KO characteristics. Interestingly, AT-sEVs from ad-cav1KO mice propagate the phenotype of the AT to the liver. These data indicate that ad-cav1 is essential for healthy adaptation of the AT to overnutrition and prevents aberrant propagation of negative phenotypes to other organs by EVs.

PMON280 Effect of Gender Affirming Hormone Therapy (GAHT) on Adipose Tissue Morphology and Metabolism in Transgender Individuals

Background

Circulating sex hormones exert unique effects on metabolism, leading to sexual dimorphism in white adipose tissue distribution and function. The gender affirming hormone therapy (GAHT) or cross-sex hormone therapy (CSHT) for transgender patients lower the endogenous sex hormones while raising the opposite sex hormones. Sex hormones are known to have a significant impact on white adipose tissue and metabolism. However, long-term metabolic changes with GAHT in transgender individuals have not been studied, nor are the underlying mechanisms properly elucidated.

Hypothesis

We test the hypothesis that GAHT will result in sexual dimorphic changes in morphology and function of white adipose tissue according to the type of sex hormones used and the native sex of the patients.

Aim 1

To study the effect of cross-sex hormones on clinical parameters such as blood pressure, weight, waist-hip circumference ratio, serum lipids, insulin sensitivity

Aim 2

To study the changes in human subcutaneous white adipose tissue (SWAT) distribution and expansion with GAHT

Aim 3

To study changes in circulating metabolites and adipokines, and pro-inflammatory and pro-fibrotic gene expression in SWAT with GAHT.

Methodology

31 hormone-naive patients (19 Male-to-Female; MtF and 12 Female-to-Male; FtM) were enrolled in the longitudinal study and followed up for 3 years. Clinical parameters and blood samples were gathered at baseline and at 6 month intervals. DXA scans and fat biopsies (peri-umbilical SWAT) were performed at baseline and at 1 year intervals.

Results

The results presented here compare baseline and at 1 year of GAHT. There was an increase in total cholesterol in the MtF group. HOMA-IR, total fat mass and visceral fat mass increased in MtF group, while they decreased in the FtM group. In MtF group, lean mass and bone mineral content (BMC) decreased at 1 year, but no change was observed in the FtM group. On histological assessment, fat cell size increased in MtF group, while it decreased in the FtM group.

Conclusion

In MtF transgender patients, androgen deprivation and estrogen treatment resulted in reduced insulin sensitivity together with a rise in total cholesterol possibly via a decrease in lean mass and adipose tissue expansion through significant adipocyte hypertrophy. In FtM transgender patients, testosterone treatment caused improved insulin sensitivity possibly via a decrease in total and visceral fat mass. The greatest strengths of our project include its longitudinal nature over 3 years to capture long-term effects in the same patients as well as deeper mechanistic insights into the underlying metabolite, adipokine and gene expression changes. The limitations of our study include the small sample size with heterogeneity of study population in terms of demographics and lifestyles.

Presentation: Monday, June 13, 2022 12:30 p.m. - 2:30 p.m.

Adipose tissue macrophages exert systemic metabolic control by manipulating local iron concentrations

Iron is essential to many fundamental biological processes, but its cellular compartmentalization and concentration must be tightly controlled. Although iron overload can contribute to obesity-associated metabolic deterioration, the subcellular localization and accumulation of iron in adipose tissue macrophages is largely unknown. Here, we show that macrophage mitochondrial iron levels control systemic metabolism in male mice by altering adipocyte iron concentrations. Using various transgenic mouse models to manipulate the macrophage mitochondrial matrix iron content in an inducible fashion, we demonstrate that lowering macrophage mitochondrial matrix iron increases numbers of M2-like macrophages in adipose tissue, lowers iron levels in adipocytes, attenuates inflammation and protects from high-fat-diet-induced metabolic deterioration. Conversely, elevating macrophage mitochondrial matrix iron increases M1-like macrophages and iron levels in adipocytes, exacerbates inflammation and worsens high-fat-diet-induced metabolic dysfunction. These phenotypes are robustly reproduced by transplantation of a small amount of fat from transgenic to wild-type mice. Taken together, we identify macrophage mitochondrial iron levels as a crucial determinant of systemic metabolic homeostasis in mice.

Adipocyte mesenchymal transition contributes to mammary tumor progression

Obesity is associated with increased cancer incidence and progression. However, the relationship between adiposity and cancer remains poorly understood at the mechanistic level. Here, we report that adipocytes from tumor-invasive mammary fat undergo de-differentiation to fibroblast-like precursor cells during tumor progression and integrate into the tumor microenvironment. Single-cell sequencing reveals that these de-differentiated adipocytes lose their original identities and transform into multiple cell types, including myofibroblast- and macrophage-like cells, with their characteristic features involved in immune response, inflammation, and extracellular matrix remodeling. The de-differentiated cells are metabolically distinct from tumor-associated fibroblasts but exhibit comparable effects on tumor cell proliferation. Inducing de-differentiation by Xbp1s overexpression promotes tumor progression despite lower adiposity. In contrast, promoting lipid-storage capacity in adipocytes through MitoNEET overexpression curbs tumor growth despite greater adiposity. Collectively, the metabolic interplay between tumor cells and adipocytes induces adipocyte mesenchymal transition and contributes to reconfigure the stroma into a more tumor-friendly microenvironment.

Zhu et al. report that in mammary tumors, adipocytes undergo metabolic reprograming and de-differentiation. They acquire myofibroblast- and macrophage-like features, a process referred to as “adipocyte mesenchymal transition,” which modifies the tumor microenvironment via ECM remodeling and activation of the immune response and likely contributes to tumor progression.

Activating Connexin43 gap junctions primes adipose tissue for therapeutic intervention

Adipose tissue is a promising target for treating obesity and metabolic diseases. However, pharmacological agents usually fail to effectively engage adipocytes due to their extraordinarily large size and insufficient vascularization, especially in obese subjects. We have previously shown that during cold exposure, connexin43 (Cx43) gap junctions are induced and activated to connect neighboring adipocytes to share limited sympathetic neuronal input amongst multiple cells. We reason the same mechanism may be leveraged to improve the efficacy of various pharmacological agents that target adipose tissue. Using an adipose tissue-specific Cx43 overexpression mouse model, we demonstrate effectiveness in connecting adipocytes to augment metabolic efficacy of the β 3-adrenergic receptor agonist Mirabegron and FGF21. Additionally, combing those molecules with the Cx43 gap junction channel activator danegaptide shows a similar enhanced efficacy. In light of these findings, we propose a model in which connecting adipocytes via Cx43 gap junction channels primes adipose tissue to pharmacological agents designed to engage it. Thus, Cx43 gap junction activators hold great potential for combination with additional agents targeting adipose tissue.

ATF4 Protects the Heart From Failure by Antagonizing Oxidative Stress

BACKGROUND

Cellular redox control is maintained by generation of reactive oxygen/nitrogen species balanced by activation of antioxidative pathways. Disruption of redox balance leads to oxidative stress, a central causative event in numerous diseases including heart failure. Redox control in the heart exposed to hemodynamic stress, however, remains to be fully elucidated.

METHODS

Pressure overload was triggered by transverse aortic constriction in mice. Transcriptomic and metabolomic regulations were evaluated by RNA-sequencing and metabolomics, respectively. Stable isotope tracer labeling experiments were conducted to determine metabolic flux in vitro. Neonatal rat ventricular myocytes and H9c2 cells were used to examine molecular mechanisms.

RESULTS

We show that production of cardiomyocyte NADPH, a key factor in redox regulation, is decreased in pressure overload-induced heart failure. As a consequence, the level of reduced glutathione is downregulated, a change associated with fibrosis and cardiomyopathy. We report that the pentose phosphate pathway and mitochondrial serine/glycine/folate metabolic signaling, 2 NADPH-generating pathways in the cytosol and mitochondria, respectively, are induced by transverse aortic constriction. We identify ATF4 (activating transcription factor 4) as an upstream transcription factor controlling the expression of multiple enzymes in these 2 pathways. Consistently, joint pathway analysis of transcriptomic and metabolomic data reveal that ATF4 preferably controls oxidative stress and redox-related pathways. Overexpression of ATF4 in neonatal rat ventricular myocytes increases NADPH-producing enzymes' whereas silencing of ATF4 decreases their expression. Further, stable isotope tracer experiments reveal that ATF4 overexpression augments metabolic flux within these 2 pathways. In vivo, cardiomyocyte-specific deletion of ATF4 exacerbates cardiomyopathy in the setting of transverse aortic constriction and accelerates heart failure development, attributable, at least in part, to an inability to increase the expression of NADPH-generating enzymes.

CONCLUSIONS

Our findings reveal that ATF4 plays a critical role in the heart under conditions of hemodynamic stress by governing both cytosolic and mitochondrial production of NADPH.

Ceramides are early responders in metabolic syndrome development in rhesus monkeys

Metabolic syndrome increases risk of complicating co-morbidities. Current clinical indicators reflect established metabolic impairment, preventing earlier intervention strategies. Here we show that circulating sphingolipids are altered in the very early stages of insulin resistance development. The study involved 16 paired overweight but healthy monkeys, one-half of which spontaneously developed metabolic syndrome over the course of 2 years. Importantly, animals did not differ in adiposity and were euglycemic throughout the study period. Using mass spectrometry, circulating sphingolipids, including ceramides and sphingomyelins, were detected and quantified for healthy and impaired animals at both time points. At time of diagnosis, several ceramides were significantly different between healthy and impaired animals. Correlation analysis revealed differences in the interactions among ceramides in impaired animals at diagnosis and pre-diagnosis when animals were clinically indistinguishable from controls. Furthermore, correlations between ceramides and early-stage markers of insulin resistance, diacylglycerols and non-esterified fatty acids, were distinct for healthy and impaired states. Regression analysis identifies coordinated changes in lipid handling across lipid classes as animals progress from healthy to insulin resistant. Correlations between ceramides and the adipose-derived adipokine adiponectin were apparent in healthy animals but not in the metabolically impaired animals, even in advance of loss in insulin sensitivity. These data suggest that circulating ceramides are clinically relevant in identifying disease risk independent of differences in adiposity, and may be important in devising preventative strategies.

Comparison of BMIPP-SPECT/CT to 18FDG-PET/CT for Imaging Brown or Browning Fat in a Preclinical Model

Obesity is a leading cause of preventable death and morbidity. To elucidate the mechanisms connecting metabolically active brown adipose tissue (BAT) and metabolic health may provide insights into methods of treatment for obesity-related conditions. 18F-fluorodeoxyglucose positron emission tomography/computed tomography (18FDG-PET/CT) is traditionally used to image human BAT activity. However, the primary energy source of BAT is derived from intracellular fatty acids and not glucose. Beta-methyl-p-iodophenylpentadecanoic acid (BMIPP) is a fatty acid analogue amenable to in vivo imaging by single photon emission computed tomography/CT (SPECT/CT) when radiolabeled with iodine isotopes. In this study, we compare the use of 18FDG-PET/CT and 125I-BMIPP-SPECT/CT for fat imaging to ascertain whether BMIPP is a more robust candidate for the non-invasive evaluation of metabolically active adipose depots. Interscapular BAT, inguinal white adipose tissue (iWAT), and gonadal white adipose tissue (gWAT) uptake of 18FDG and 125I-BMIPP was quantified in mice following treatment with the BAT-stimulating drug CL-316,243 or saline vehicle control. After CL-316,243 treatment, uptake of both radiotracers increased in BAT and iWAT. The standard uptake value (SUVmean) for 18FDG and 125I-BMIPP significantly correlated in these depots, although uptake of 125I-BMIPP in BAT and iWAT more closely mimicked the fold-change in metabolic rate as measured by an extracellular flux analyzer. Herein, we find that imaging BAT with the radioiodinated fatty acid analogue BMIPP yields more physiologically relevant data than 18FDG-PET/CT, and its conventional use may be a pivotal tool for evaluating BAT in both mice and humans.

Post-acute sequelae of COVID-19: A metabolic perspective

The SARS-CoV-2 pandemic continues to rage around the world. At the same time, despite strong public health measures and high vaccination rates in some countries, a post-COVID-19 syndrome has emerged which lacks a clear definition, prevalence, or etiology. However, fatigue, dyspnea, brain fog, and lack of smell and/or taste are often characteristic of patients with this syndrome. These are evident more than a month after infection, and are labeled as Post-Acute Sequelae of CoV-2 (PASC) or commonly referred to as long-COVID. Metabolic dysfunction (i.e., obesity, insulin resistance, and diabetes mellitus) is a predisposing risk factor for severe acute COVID-19, and there is emerging evidence that this factor plus a chronic inflammatory state may predispose to PASC. In this article, we explore the potential pathogenic metabolic mechanisms that could underly both severe acute COVID-19 and PASC, and then consider how these might be targeted for future therapeutic approaches.

A new signal that shrinks fat

Healthy white adipose tissue is dependent on proliferation of endothelial cells to maintain homeostasis or undergo expansion. A new study shows that endothelial cells communicate with adipocytes via polyamines to promote vascularization of adipose tissue, thereby reversing the metabolic effects of obesity.

Adult pancreatic islet endocrine cells emerge as fetal hormone-expressing cells

The precise developmental dynamics of the pancreatic islet endocrine cell types, and their interrelation, are unknown. Some authors claim the persistence of islet cell differentiation from precursor cells after birth (“neogenesis”). Here, using four conditional cell lineage tracing (“pulse-and-chase”) murine models, we describe the natural history of pancreatic islet cells, once they express a hormone gene, until late in life. Concerning the contribution of early-appearing embryonic hormone-expressing cells to the formation of islets, we report that adult islet cells emerge from embryonic hormone-expressing cells arising at different time points during development, without any evidence of postnatal neogenesis. We observe specific patterns of hormone gene activation and switching during islet morphogenesis, revealing that, within each cell type, cells have heterogeneous developmental trajectories. This likely applies to most maturating cells in the body, and explains the observed phenotypic variability within differentiated cell types. Such knowledge should help devising novel regenerative therapies.

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Adult pancreatic islet endocrine cells arise as embryonic hormone-expressing cells

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No detectable islet cell differentiation from putative precursor cells after birth

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Some embryonic hormone-producing cells display a switch in hormone gene expression

Intercellular and interorgan crosstalk through adipocyte extracellular vesicles

Functional adipose tissue is essential for homeostatic maintenance of systemic metabolism. As such, adipose tissue dysfunction, like that seen in the obese state, directly contributes to system-wide pathological metabolism, leading to the development of type 2 diabetes and other obesity-associated comorbidities. In addition to the storage function of adipocytes, they also secrete numerous factors that robustly regulate metabolism-related pathways throughout the body. Many of these factors, in addition to other signaling proteins, RNA species and lipids, are found in extracellular vesicles (EVs) released from adipocytes. EVs are vesicles with a lipid bilayer, known to carry signaling proteins and lipids, mRNAs and miRNAs. Because of this diverse cargo, EVs can have robust and pleotropic signaling effects depending on the receiving target cells. We are only now starting to understand how adipocyte EVs can modulate metabolism within adipose tissue and beyond. Here, we highlight the current literature that demonstrates EV-mediated crosstalk between adipocytes and other tissues or distal cells. We become increasingly aware of the importance of these adipocyte-derived EV signals that establish a so far underappreciated endocrine system. Adipocyte EVs offer a new avenue for pharmacological manipulation of metabolism to treat obesity-related disease.

Why does obesity cause diabetes?

The accumulation of an excessive amount of body fat can cause type 2 diabetes, and the risk of type 2 diabetes increases linearly with an increase in body mass index. Accordingly, the worldwide increase in the prevalence of obesity has led to a concomitant increase in the prevalence of type 2 diabetes. The cellular and physiological mechanisms responsible for the link between obesity and type 2 diabetes are complex and involve adiposity-induced alterations in β cell function, adipose tissue biology, and multi-organ insulin resistance, which are often ameliorated and can even be normalized with adequate weight loss.

The metabolic syndrome, thiazolidinediones, and implications for intersection of chronic and inflammatory disease

BACKGROUND

Chronic disease appears connected to obesity. However, evidence suggests that chronic metabolic diseases are more specifically related to adipose dysfunction rather than to body weight itself.

SCOPE OF REVIEW

Further study of the first generation "insulin sensitizer" pioglitazone and molecules based on its structure suggests that is possible to decouple body weight from the metabolic dysfunction that drives adverse outcomes. The growing understanding of the mechanism of action of these agents together with advances in the pathophysiology of chronic metabolic disease offers a new approach to treat chronic conditions, such as type 2 diabetes, fatty liver disease, and their common organ and vascular sequelae.

MAJOR CONCLUSIONS

We hypothesize that treating adipocyte dysfunction with new insulin sensitizers might significantly impact the interface of infectious disease and chronic metabolic disease.

Skin aging: Dermal adipocytes metabolically reprogram dermal fibroblasts

Emerging data connects the aging process in dermal fibroblasts with metabolic reprogramming, provided by enhanced fatty acid oxidation and reduced glycolysis. This switch may be caused by a significant expansion of the dermal white adipose tissue (dWAT) layer in aged, hair-covered skin. Dermal adipocytes cycle through de-differentiation and re-differentiation. As a result, there is a strongly enhanced release of free fatty acids into the extracellular space during the de-differentiation of dermal adipocytes in the catagen phase of the hair follicle cycle. Both caveolin-1 and adiponectin are critical factors influencing these processes. Controlling the expression levels of these two factors also offers the ability to manipulate the metabolic preferences of the different cell types within the microenvironment of the skin, including dermal fibroblasts. Differential expression of adiponectin and caveolin-1 in the various cell types may also be responsible for the cellular metabolic heterogeneity within the cells of the skin.

Mitochondrial regulation and white adipose tissue homeostasis

The important role of mitochondria in the regulation of white adipose tissue (WAT) remodeling and energy balance is increasingly appreciated. The remarkable heterogeneity of the adipose tissue stroma provides a cellular basis to enable adipose tissue plasticity in response to various metabolic stimuli. Regulating mitochondrial function at the cellular level in adipocytes, in adipose progenitor cells (APCs), and in adipose tissue macrophages (ATMs) has a profound impact on adipose homeostasis. Moreover, mitochondria facilitate the cell-to-cell communication within WAT, as well as the crosstalk with other organs, such as the liver, the heart, and the pancreas. A better understanding of mitochondrial regulation in the diverse adipose tissue cell types allows us to develop more specific and efficient approaches to improve adipose function and achieve improvements in overall metabolic health.

Integrated Stress Response Couples Mitochondrial Protein Translation With Oxidative Stress Control

BACKGROUND

The integrated stress response (ISR) is an evolutionarily conserved process to cope with intracellular and extracellular disturbances. Myocardial infarction is a leading cause of death worldwide. Coronary artery reperfusion, the most effective means to mitigate cardiac damage of myocardial infarction, causes additional reperfusion injury. This study aimed to investigate the role of the ISR in myocardial ischemia/reperfusion (I/R).

METHODS

Cardiac-specific gain- and loss-of-function approaches for the ISR were used in vivo. Myocardial I/R was achieved by ligation of the cardiac left anterior descending artery for 45 minutes followed by reperfusion for different times. Cardiac function was assessed by echocardiography. Cultured H9c2 cells, primary rat cardiomyocytes, and mouse embryonic fibroblasts were used to dissect underlying molecular mechanisms. Tandem mass tag labeling and mass spectrometry was conducted to identify protein targets of the ISR. Pharmacologic means were tested to manipulate the ISR for therapeutic exploration.

RESULTS

We show that the PERK (PKR-like endoplasmic reticulum resident kinase)/eIF2α (α subunit of eukaryotic initiation factor 2) axis of the ISR is strongly induced by I/R in cardiomyocytes in vitro and in vivo. We further reveal a physiologic role of PERK/eIF2α signaling by showing that acute activation of PERK in the heart confers robust cardioprotection against reperfusion injury. In contrast, cardiac-specific deletion of PERK aggravates cardiac responses to reperfusion. Mechanistically, the ISR directly targets mitochondrial complexes through translational suppression. We identify NDUFAF2 (NADH:ubiquinone oxidoreductase complex assembly factor 2), an assembly factor of mitochondrial complex I, as a selective target of PERK. Overexpression of PERK suppresses the protein expression of NDUFAF2 and PERK inhibition causes an increase of NDUFAF2. Silencing of NDUFAF2 significantly rescues cardiac cell survival from PERK knockdown under I/R. We show that activation of PERK/eIF2α signaling reduces mitochondrial complex-derived reactive oxygen species and improves cardiac cell survival in response to I/R. Moreover, pharmacologic stimulation of the ISR protects the heart against reperfusion damage, even after the restoration of occluded coronary artery, highlighting clinical relevance for myocardial infarction treatment.

CONCLUSIONS

These results suggest that the ISR improves cell survival and mitigates reperfusion damage by selectively suppressing mitochondrial protein synthesis and reducing oxidative stress in the heart.

Obesity, Diabetes, and Increased Cancer Progression

Rates of obesity and diabetes have increased significantly over the past decades and the prevalence is expected to continue to rise further in the coming years. Many observations suggest that obesity and diabetes are associated with an increased risk of developing several types of cancers, including liver, pancreatic, endometrial, colorectal, and post-menopausal breast cancer. The path towards developing obesity and diabetes is affected by multiple factors, including adipokines, inflammatory cytokines, growth hormones, insulin resistance, and hyperlipidemia. The metabolic abnormalities associated with changes in the levels of these factors in obesity and diabetes have the potential to significantly contribute to the development and progression of cancer through the regulation of distinct signaling pathways. Here, we highlight the cellular and molecular pathways that constitute the links between obesity, diabetes, cancer risk and mortality. This includes a description of the existing evidence supporting the obesity-driven morphological and functional alternations of cancer cells and adipocytes through complex interactions within the tumor microenvironment.

Cannabinoid receptor-1 signaling in hepatocytes and stellate cells does not contribute to NAFLD

The endocannabinoid system regulates appetite and energy expenditure and inhibitors of the cannabinoid receptor-1 (CB-1) induce weight loss with improvement in components of the metabolic syndrome. While CB-1 blockage in brain is responsible for weight loss, many of the metabolic benefits associated with CB-1 blockade have been attributed to inhibition of CB-1 signaling in the periphery. As a result, there has been interest in developing a peripherally restricted CB-1 inhibitor for the treatment of nonalcoholic fatty liver disease (NAFLD) that would lack the unwanted centrally mediated side effects. Here, we produced mice that lacked CB-1 receptors in hepatocytes or stellate cells to determine if CB-1 signaling contributes to the development of NAFLD or liver fibrosis. Deletion of CB-1 receptors in hepatocytes did not alter the development of NAFLD in mice fed a high sucrose high fat diet or high fat diet (HFD). Similarly, deletion of CB-1 deletion specifically in stellate cells also did not prevent the development of NAFLD in mice fed the HFD nor did it protect mice for carbon tetrachloride (CCl4)-induced fibrosis. Combined, these studies do not support a direct role for hepatocyte or stellate cell CB-1 signaling in the development of NAFLD or liver fibrosis.

Extracellular vesicle-based interorgan transport of mitochondria from energetically stressed adipocytes

Adipocytes undergo intense energetic stress in obesity resulting in loss of mitochondrial mass and function. We have found that adipocytes respond to mitochondrial stress by rapidly and robustly releasing small extracellular vesicles (sEVs). These sEVs contain respiration-competent, but oxidatively damaged mitochondrial particles, which enter circulation and are taken up by cardiomyocytes, where they trigger a burst of ROS. The result is compensatory antioxidant signaling in the heart that protects cardiomyocytes from acute oxidative stress, consistent with a preconditioning paradigm. As such, a single injection of sEVs from energetically stressed adipocytes limits cardiac ischemia/reperfusion injury in mice. This study provides the first description of functional mitochondrial transfer between tissues and the first vertebrate example of "inter-organ mitohormesis." Thus, these seemingly toxic adipocyte sEVs may provide a physiological avenue of potent cardio-protection against the inevitable lipotoxic or ischemic stresses elicited by obesity.

PKM1 Exerts Critical Roles in Cardiac Remodeling Under Pressure Overload in the Heart

BACKGROUND

Metabolic remodeling precedes most alterations during cardiac hypertrophic growth under hemodynamic stress. The elevation of glucose utilization has been recognized as a hallmark of metabolic remodeling. However, its role in cardiac hypertrophic growth and heart failure in response to pressure overload remains to be fully illustrated. Here, we aimed to dissect the role of cardiac PKM1 (pyruvate kinase muscle isozyme 1) in glucose metabolic regulation and cardiac response under pressure overload.

METHODS

Cardiac-specific deletion of PKM1 was achieved by crossing the floxed PKM1 mouse model with the cardiomyocyte-specific Cre transgenic mouse. PKM1 transgenic mice were generated under the control of tetracycline response elements, and cardiac-specific overexpression of PKM1 was induced by doxycycline administration in adult mice. Pressure overload was triggered by transverse aortic constriction. Primary neonatal rat ventricular myocytes were used to dissect molecular mechanisms. Moreover, metabolomics and nuclear magnetic resonance spectroscopy analyses were conducted to determine cardiac metabolic flux in response to pressure overload.

RESULTS

We found that PKM1 expression is reduced in failing human and mouse hearts. It is important to note that cardiomyocyte-specific deletion of PKM1 exacerbates cardiac dysfunction and fibrosis in response to pressure overload. Inducible overexpression of PKM1 in cardiomyocytes protects the heart against transverse aortic constriction-induced cardiomyopathy and heart failure. At the mechanistic level, PKM1 is required for the augmentation of glycolytic flux, mitochondrial respiration, and ATP production under pressure overload. Furthermore, deficiency of PKM1 causes a defect in cardiomyocyte growth and a decrease in pyruvate dehydrogenase complex activity at both in vitro and in vivo levels.

CONCLUSIONS

These findings suggest that PKM1 plays an essential role in maintaining a homeostatic response in the heart under hemodynamic stress.

Adipocyte iron levels impinge on a fat-gut crosstalk to regulate intestinal lipid absorption and mediate protection from obesity

Iron overload is positively associated with diabetes risk. However, the role of iron in adipose tissue remains incompletely understood. Here, we report that transferrin-receptor-1-mediated iron uptake is differentially required for distinct subtypes of adipocytes. Notably, adipocyte-specific transferrin receptor 1 deficiency substantially protects mice from high-fat-diet-induced metabolic disorders. Mechanistically, low cellular iron levels have a positive impact on the health of the white adipose tissue and can restrict lipid absorption from the intestine through modulation of vesicular transport in enterocytes following high-fat diet feeding. Specific reduction of adipocyte iron by AAV-mediated overexpression of the iron exporter Ferroportin1 in adult mice effectively mimics these protective effects. In summary, our studies highlight an important role of adipocyte iron in the maintenance of systemic metabolism through an adipocyte-enterocyte axis, offering an additional level of control over caloric influx into the system after feeding by regulating intestinal lipid absorption.

The mitochondrial dicarboxylate carrier prevents hepatic lipotoxicity by inhibiting white adipocyte lipolysis

BACKGROUND & AIMS

We have previously reported that the mitochondrial dicarboxylate carrier (mDIC [SLC25A10]) is predominantly expressed in the white adipose tissue (WAT) and subject to regulation by metabolic cues. However, the specific physiological functions of mDIC and the reasons for its abundant presence in adipocytes are poorly understood.

METHODS

To systemically investigate the impact of mDIC function in adipocytes in vivo, we generated loss- and gain-of-function mouse models, selectively eliminating or overexpressing mDIC in mature adipocytes, respectively.

RESULTS

In in vitro differentiated white adipocytes, mDIC is responsible for succinate transport from the mitochondrial matrix to the cytosol, from where succinate can act on the succinate receptor SUCNR1 and inhibit lipolysis by dampening the cAMP- phosphorylated hormone-sensitive lipase (pHSL) pathway. We eliminated mDIC expression in adipocytes in a doxycycline (dox)-inducible manner (mDIC^iKO) and demonstrated that such a deletion results in enhanced adipocyte lipolysis and promotes high-fat diet (HFD)-induced adipocyte dysfunction, liver lipotoxicity, and systemic insulin resistance. Conversely, in a mouse model with dox-inducible, adipocyte-specific overexpression of mDIC (mDIC^iOE), we observed suppression of adipocyte lipolysis both in vivo and ex vivo. mDIC^iOE mice are potently protected from liver lipotoxicity upon HFD feeding. Furthermore, they show resistance to HFD-induced weight gain and adipose tissue expansion with concomitant improvements in glucose tolerance and insulin sensitivity. Beyond our data in rodents, we found that human WAT SLC25A10 mRNA levels are positively correlated with insulin sensitivity and negatively correlated with intrahepatic triglyceride levels, suggesting a critical role of mDIC in regulating overall metabolic homeostasis in humans as well.

CONCLUSIONS

In summary, we highlight that mDIC plays an essential role in governing adipocyte lipolysis and preventing liver lipotoxicity in response to a HFD.

LAY SUMMARY

Dysfunctional fat tissue plays an important role in the development of fatty liver disease and liver injury. Our present study identifies a mitochondrial transporter, mDIC, which tightly controls the release of free fatty acids from adipocytes to the liver through the export of succinate from mitochondria. We believe this mDIC-succinate axis could be targeted for the treatment of fatty liver disease.

Adiponectin and cardiometabolic trait and mortality: where do we go?

Adiponectin is an adipocyte-derived cytokine known for its cardioprotective effects in preclinical studies. Early epidemiologic studies replicated these findings and drew great interest. Subsequent large-scale prospective cohorts, however, showed that adiponectin levels seemed not to relate to incident coronary artery disease (CAD). Even more surprisingly, a paradoxical increase of all-cause and cardiovascular (CV) mortality with increased adiponectin levels was reported. The adiponectin-mortality paradox has been explained by some groups asserting that adiponectin secretion is promoted by elevated natriuretic peptides (NP). Other groups have proposed that adiponectin is elevated due to adiponectin resistance in subjects with metabolic syndrome or heart failure (HF). However, there is no unifying theory that can clearly explain this paradox. In patients with HF with reduced ejection fraction (HFrEF), stretched cardiomyocytes secrete NPs, which further promote release of adiponectin from adipose tissue, leading to adiponectin resistance. On the other hand, adiponectin biology may differ in patients with heart failure with preserved ejection fraction (HFpEF), which constitutes 50% of all of HF. Most HFpEF patients are obese, which exerts inflammation and myocardial stiffness, that is likely to prevent myocardial stretch and subsequent NP release. This segment of the patient population may display a different adiponectin biology from its HFrEF counterpart. Dissecting the adiponectin-mortality relation in terms of different HF subtypes may help to comprehensively understand this paradox. Mendelian Randomization (MR) analyses claimed that adiponectin levels are not causally related to CAD or metabolic syndrome. Results from MR studies, however, should be interpreted with great caution because the underlying history of CAD or CHF were not taken into account in these analyses, an issue that may substantially confound the results. Here, we discuss many aspects of adiponectin; cardiometabolic traits, therapeutic interventions, and the ongoing debate about the adiponectin paradox, which were recently described in basic, epidemiologic, and clinical studies.

Characterization of ALTO-encoding circular RNAs expressed by Merkel cell polyomavirus and trichodysplasia spinulosa polyomavirus

Circular RNAs (circRNAs) are a conserved class of RNAs with diverse functions, including serving as messenger RNAs that are translated into peptides. Here we describe circular RNAs generated by human polyomaviruses (HPyVs), some of which encode variants of the previously described alternative large T antigen open reading frame (ALTO) protein. Circular ALTO RNAs (circALTOs) can be detected in virus positive Merkel cell carcinoma (VP-MCC) cell lines and tumor samples. CircALTOs are stable, predominantly located in the cytoplasm, and N6-methyladenosine (m6A) modified. The translation of MCPyV circALTOs into ALTO protein is negatively regulated by MCPyV-generated miRNAs in cultured cells. MCPyV ALTO expression increases transcription from some recombinant promoters in vitro and upregulates the expression of multiple genes previously implicated in MCPyV pathogenesis. MCPyV circALTOs are enriched in exosomes derived from VP-MCC lines and circALTO-transfected 293T cells, and purified exosomes can mediate ALTO expression and transcriptional activation in MCPyV-negative cells. The related trichodysplasia spinulosa polyomavirus (TSPyV) also expresses a circALTO that can be detected in infected tissues and produces ALTO protein in cultured cells. Thus, human polyomavirus circRNAs are expressed in human tumors and infected tissues and express proteins that have the potential to modulate the infectious and tumorigenic properties of these viruses.