Differential Mitochondrial Gene Expression in Adipose Tissue Following Weight Loss Induced by Diet or Bariatric Surgery

Single nucleus RNA-sequencing integrated into risk variant colocalization discovers 17 cell-type-specific abdominal obesity genes for metabolic dysfunction-associated steatotic liver disease

BACKGROUND

Abdominal obesity increases the risk for non-alcoholic fatty liver disease (NAFLD), now known as metabolic dysfunction-associated steatotic liver disease (MASLD).

METHODS

To elucidate the directional cell-type level biological mechanisms underlying the association between abdominal obesity and MASLD, we integrated adipose and liver single nucleus RNA-sequencing and bulk cis-expression quantitative trait locus (eQTL) data with the UK Biobank genome-wide association study (GWAS) data using colocalization. Then we used colocalized cis-eQTL variants as instrumental variables in Mendelian randomization (MR) analyses, followed by functional validation experiments on the target genes of the cis-eQTL variants.

FINDINGS

We identified 17 colocalized abdominal obesity GWAS variants, regulating 17 adipose cell-type marker genes. Incorporating these 17 variants into MR discovers a putative tissue-of-origin, cell-type-aware causal effect of abdominal obesity on MASLD consistently with multiple MR methods without significant evidence for pleiotropy or heterogeneity. Single cell data confirm the adipocyte-enriched mean expression of the 17 genes. Our cellular experiments across human adipogenesis identify risk variant -specific epigenetic and transcriptional mechanisms. Knocking down two of the 17 genes, PPP2R5A and SH3PXD2B, shows a marked decrease in adipocyte lipidation and significantly alters adipocyte function and adipogenesis regulator genes, including DGAT2, LPL, ADIPOQ, PPARG, and SREBF1. Furthermore, the 17 genes capture a characteristic MASLD expression signature in subcutaneous adipose tissue.

INTERPRETATION

Overall, we discover a significant cell-type level effect of abdominal obesity on MASLD and trace its biological effect to adipogenesis.

FUNDING

NIH grants R01HG010505, R01DK132775, and R01HL170604; the European Research Council (ERC) under the European Union's Horizon 2020 research and innovation program (Grant No. 802825), Academy of Finland (Grants Nos. 333021), the Finnish Foundation for Cardiovascular Research the Sigrid Jusélius Foundation and the Jane and Aatos Erkko Foundation; American Association for the Study of Liver Diseases (AASLD) Advanced Transplant Hepatology award and NIH/NIDDK (P30DK41301) Pilot and Feasibility award; NIH/NIEHS F32 award (F32ES034668); Finnish Diabetes Research Foundation, Kuopio University Hospital Project grant (EVO/VTR grants 2005-2021), the Academy of Finland grant (Contract no. 138006); Academy of Finland (Grant Nos 335443, 314383, 272376 and 266286), Sigrid Jusélius Foundation, Finnish Medical Foundation, Finnish Diabetes Research Foundation, Novo Nordisk Foundation (#NNF20OC0060547, NNF17OC0027232, NNF10OC1013354) and Government Research Funds to Helsinki University Hospital; Orion Research Foundation, Maud Kuistila Foundation, Finish Medical Foundation, and University of Helsinki.

Increased secretion of adipocyte-derived extracellular vesicles is associated with adipose tissue inflammation and the mobilization of excess lipid in human obesity

BACKGROUND

Obesity is a worldwide epidemic characterized by adipose tissue (AT) inflammation. AT is also a source of extracellular vesicles (EVs) that have recently been implicated in disorders related to metabolic syndrome. However, our understanding of mechanistic aspect of obesity's impact on EV secretion from human AT remains limited.

METHODS

We investigated EVs from human Simpson Golabi Behmel Syndrome (SGBS) adipocytes, and from AT as well as plasma of subjects undergoing bariatric surgery. SGBS cells were treated with TNFα, palmitic acid, and eicosapentaenoic acid. Various analyses, including nanoparticle tracking analysis, electron microscopy, high-resolution confocal microscopy, and gas chromatography-mass spectrometry, were utilized to study EVs. Plasma EVs were analyzed with imaging flow cytometry.

RESULTS

EVs from mature SGBS cells differed significantly in size and quantity compared to preadipocytes, disagreeing with previous findings in mouse adipocytes and indicating that adipogenesis promotes EV secretion in human adipocytes. Inflammatory stimuli also induced EV secretion, and altered EV fatty acid (FA) profiles more than those of cells, suggesting the role of EVs as rapid responders to metabolic shifts. Visceral AT (VAT) exhibited higher EV secretion compared to subcutaneous AT (SAT), with VAT EV counts positively correlating with plasma triacylglycerol (TAG) levels. Notably, the plasma EVs of subjects with obesity contained a higher number of adiponectin-positive EVs than those of lean subjects, further demonstrating higher AT EV secretion in obesity. Moreover, plasma EV counts of people with obesity positively correlated with body mass index and TNF expression in SAT, connecting increased EV secretion with AT expansion and inflammation. Finally, EVs from SGBS adipocytes and AT contained TAGs, and EV secretion increased despite signs of less active lipolytic pathways, indicating that AT EVs could be involved in the mobilization of excess lipids into circulation.

CONCLUSIONS

We are the first to provide detailed FA profiles of human AT EVs. We report that AT EV secretion increases in human obesity, implicating their role in TAG transport and association with adverse metabolic parameters, thereby emphasizing their role in metabolic disorders. These findings promote our understanding of the roles that EVs play in human AT biology and metabolic disorders.

The expression of NFAT family genes in subcutaneous adipose tissue before and after weight loss in obese individuals

BACKGROUND AND AIMS

Adipose tissue (AT) serves as a vital energy storage site and plays a pivotal role in metabolic regulation, exhibiting a high response to insulin. Impairment in this response may closely associate with obesity, and NFAT (nuclear factor of activated T cells) family genes may be involved in the process. However, human data linking NFAT and AT remains elusive. The aim of this study was to assess the expression of NFAT family genes and markers of adipogenesis in subcutaneous adipose tissue (SAT) among normal-weight and overweight/obese individuals before and after weight loss, in relation to insulin sensitivity.

METHODS AND RESULTS

The study included 45 participants, 15 normal-weight (control group) and 30 overweight or obese, who underwent a 12-week dietary intervention (DI) program. Before and after the program hyperinsulinemic-euglycemic clamp and SAT biopsy were conducted. Before DI, a positive correlations was observed in the expression of NFATc1, NFATc4, and NFAT5 with insulin sensitivity. The expression of NFAT family genes and markers of adipogenesis in SAT was lower in individuals with overweight or obesity compared to normal-weight. Additionally, a positive correlation was noted between NFAT family genes and adipogenesis markers both before and after weight loss. Following the DI program, there was an increase in the expression of NFATc3, NFATc4, and NFAT5 in SAT.

CONCLUSION

Decreased SAT expression of NFAT genes in obesity is partly reversed in response to weight loss. NFAT genes in SAT are associated with insulin sensitivity and adipogenesis. Registration number for clinical trial: NCT01393210.

Surgical Tumor Resection Deregulates Hallmarks of Cancer in Resected Tissue and the Surrounding Microenvironment

Surgery exposes tumor tissue to severe hypoxia and mechanical stress leading to rapid gene expression changes in the tumor and its microenvironment, which remain poorly characterized. We biopsied tumor and adjacent normal tissues from patients with breast (n = 81) and head/neck squamous cancers (HNSC; n = 10) at the beginning (A), during (B), and end of surgery (C). Tumor/normal RNA from 46/81 patients with breast cancer was subjected to mRNA-Seq using Illumina short-read technology, and from nine patients with HNSC to whole-transcriptome microarray with Illumina BeadArray. Pathways and genes involved in 7 of 10 known cancer hallmarks, namely, tumor-promoting inflammation (TNF-A, NFK-B, IL18 pathways), activation of invasion and migration (various extracellular matrix-related pathways, cell migration), sustained proliferative signaling (K-Ras Signaling), evasion of growth suppressors (P53 signaling, regulation of cell death), deregulating cellular energetics (response to lipid, secreted factors, and adipogenesis), inducing angiogenesis (hypoxia signaling, myogenesis), and avoiding immune destruction (CTLA4 and PDL1) were significantly deregulated during surgical resection (time points A vs. B vs. C). These findings were validated using NanoString assays in independent pre/intra/post-operative breast cancer samples from 48 patients. In a comparison of gene expression data from biopsy (analogous to time point A) with surgical resection samples (analogous to time point C) from The Cancer Genome Atlas study, the top deregulated genes were the same as identified in our analysis, in five of the seven studied cancer types. This study suggests that surgical extirpation deregulates the hallmarks of cancer in primary tumors and adjacent normal tissue across different cancers.

IMPLICATIONS

Surgery deregulates hallmarks of cancer in human tissue.

Age-dependent genes in adipose stem and precursor cells affect regulation of fat cell differentiation and link aging to obesity via cellular and genetic interactions

BACKGROUND

Age and obesity are dominant risk factors for several common cardiometabolic disorders, and both are known to impair adipose tissue function. However, the underlying cellular and genetic factors linking aging and obesity on adipose tissue function have remained elusive. Adipose stem and precursor cells (ASPCs) are an understudied, yet crucial adipose cell type due to their deterministic adipocyte differentiation potential, which impacts the capacity to store fat in a metabolically healthy manner.

METHODS

We integrated subcutaneous adipose tissue (SAT) bulk (n=435) and large single-nucleus RNA sequencing (n=105) data with the UK Biobank (UKB) (n=391,701) data to study age-obesity interactions originating from ASPCs by performing cell-type decomposition, differential expression testing, cell-cell communication analyses, and construction of polygenic risk scores for body mass index (BMI).

RESULTS

We found that the SAT ASPC proportions significantly decrease with age in an obesity-dependent way consistently in two independent cohorts, both showing that the age dependency of ASPC proportions is abolished by obesity. We further identified 76 genes (72 SAT ASPC marker genes and 4 transcription factors regulating ASPC marker genes) that are differentially expressed by age in SAT and functionally enriched for developmental processes and adipocyte differentiation (i.e., adipogenesis). The 76 age-perturbed ASPC genes include multiple negative regulators of adipogenesis, such as RORA, SMAD3, TWIST2, and ZNF521, form tight clusters of longitudinally co-expressed genes during human adipogenesis, and show age-based differences in cellular interactions between ASPCs and adipose cell types. Finally, our genetic data demonstrate that cis-regional variants of these genes interact with age as predictors of BMI in an obesity-dependent way in the large UKB, while no such gene-age interaction on BMI is observed with non-age-dependent ASPC marker genes, thus independently confirming our cellular ASPC results at the biobank level.

CONCLUSIONS

Overall, we discover that obesity prematurely induces a decrease in ASPC proportions and identify 76 developmentally important ASPC genes that implicate altered negative regulation of fat cell differentiation as a mechanism for aging and directly link aging to obesity via significant cellular and genetic interactions.

White adipose tissue mitochondrial bioenergetics in metabolic diseases

White adipose tissue (WAT) is an important endocrine organ that regulates systemic energy metabolism. In metabolically unhealthy obesity, adipocytes become dysfunctional through hypertrophic mechanisms associated with a reduced endocrine function, reduced mitochondrial function, but increased inflammation, fibrosis, and extracellular remodelling. A pathologic WAT remodelling promotes systemic lipotoxicity characterized by fat accumulation in tissues such as muscle and liver, leading to systemic insulin resistance and type 2 diabetes. Several lines of evidence from human and animal studies suggest a link between unhealthy obesity and adipocyte mitochondrial dysfunction, and interventions that improve mitochondrial function may reduce the risk of obesity-associated diseases. This review discusses the importance of mitochondrial function and metabolism in human adipocyte biology and intercellular communication mechanisms within WAT. Moreover, a selected interventional approach for better adipocyte mitochondrial metabolism in humans is reviewed. A greater understanding of mitochondrial bioenergetics in WAT might provide novel therapeutic opportunities to prevent or restore dysfunctional adipose tissue in obesity-associated diseases.

Mitochondrial Function in Healthy Human White Adipose Tissue: A Narrative Review

As ¾ of the global population either have excess or insufficient fat, it has become increasingly critical to understand the functions and dysfunctions of adipose tissue (AT). AT serves as a key organ in energy metabolism, and recently, attention has been focused on white AT, particularly its mitochondria, as the literature evidence links their functions to adiposity. This narrative review provides an overview of mitochondrial functionality in human white AT. Firstly, it is noteworthy that the two primary AT depots, subcutaneous AT (scAT) and visceral AT (vAT), exhibit differences in mitochondrial density and activity. Notably, vAT tends to have a higher mitochondrial activity compared to scAT. Subsequently, studies have unveiled a negative correlation between mitochondrial activity and body mass index (BMI), indicating that obesity is associated with a lower mitochondrial function. While the impact of exercise on AT mitochondria remains uncertain, dietary interventions have demonstrated varying effects on AT mitochondria. This variability holds promise for the modulation of AT mitochondrial activity. In summary, AT mitochondria exert a significant influence on health outcomes and can be influenced by factors such as obesity and dietary interventions. Understanding the mechanisms underlying these responses can offer potential insights into managing conditions related to AT and overall health.

Cross-Tissue Single-Nucleus RNA Sequencing Discovers Tissue-Resident Adipocytes Involved in Propanoate Metabolism in the Human Heart

BACKGROUND

Adipocytes are crucial regulators of cardiovascular health. However, not much is known about gene expression profiles of adipocytes residing in nonfat cardiovascular tissues, their genetic regulation, and contribution to coronary artery disease. Here, we investigated whether and how the gene expression profiles of adipocytes in the subcutaneous adipose tissue differ from adipocytes residing in the heart.

METHODS

We used single-nucleus RNA-sequencing data sets of subcutaneous adipose tissue and heart and performed in-depth analysis of tissue-resident adipocytes and their cell-cell interactions.

RESULTS

We first discovered tissue-specific features of tissue-resident adipocytes, identified functional pathways involved in their tissue specificity, and found genes with cell type-specific expression enrichment in tissue-resident adipocytes. By following up these results, we discovered the propanoate metabolism pathway as a novel distinct characteristic of the heart-resident adipocytes and found a significant enrichment of coronary artery disease genome-wide association study risk variants among the right atrium-specific adipocyte marker genes. Our cell-cell communication analysis identified 22 specific heart adipocyte-associated ligand-receptor pairs and signaling pathways, including THBS (thrombospondin) and EPHA (ephrin type-A), further supporting the distinct tissue-resident role of heart adipocytes. Our results also suggest chamber-level coordination of heart adipocyte expression profiles as we observed a consistently larger number of adipocyte-associated ligand-receptor interactions and functional pathways in the atriums than ventricles.

CONCLUSIONS

Overall, we introduce a new function and genetic link to coronary artery disease for the previously unexplored heart-resident adipocytes.

Cross-tissue omics analysis discovers ten adipose genes encoding secreted proteins in obesity-related non-alcoholic fatty liver disease

BACKGROUND

Non-alcoholic fatty liver disease (NAFLD) is a fast-growing, underdiagnosed, epidemic. We hypothesise that obesity-related inflammation compromises adipose tissue functions, preventing efficient fat storage, and thus driving ectopic fat accumulation into the liver.

METHODS

To identify adipose-based mechanisms and potential serum biomarker candidates (SBCs) for NAFLD, we utilise dual-tissue RNA-sequencing (RNA-seq) data in adipose tissue and liver, paired with histology-based NAFLD diagnosis, from the same individuals in a cohort of obese individuals. We first scan for genes that are differentially expressed (DE) for NAFLD in obese individuals' subcutaneous adipose tissue but not in their liver; encode proteins secreted to serum; and show preferential adipose expression. Then the identified genes are filtered to key adipose-origin NAFLD genes by best subset analysis, knockdown experiments during human preadipocyte differentiation, recombinant protein treatment experiments in human liver HepG2 cells, and genetic analysis.

FINDINGS

We discover a set of genes, including 10 SBCs, that may modulate NAFLD pathogenesis by impacting adipose tissue function. Based on best subset analysis, we further follow-up on two SBCs CCDC80 and SOD3 by knockdown in human preadipocytes and subsequent differentiation experiments, which show that they modulate crucial adipogenesis genes, LPL, SREBPF1, and LEP. We also show that treatment of the liver HepG2 cells with the CCDC80 and SOD3 recombinant proteins impacts genes related to steatosis and lipid processing, including PPARA, NFE2L2, and RNF128. Finally, utilizing the adipose NAFLD DE gene cis-regulatory variants associated with serum triglycerides (TGs) in extensive genome-wide association studies (GWASs), we demonstrate a unidirectional effect of serum TGs on NAFLD with Mendelian Randomization (MR) analysis. We also demonstrate that a single SNP regulating one of the SBC genes, rs2845885, produces a significant MR result by itself. This supports the conclusion that genetically regulated adipose expression of the NAFLD DE genes may contribute to NAFLD through changes in serum TG levels.

INTERPRETATION

Our results from the dual-tissue transcriptomics screening improve the understanding of obesity-related NAFLD by providing a targeted set of 10 adipose tissue-active genes as new serum biomarker candidates for the currently grossly underdiagnosed fatty liver disease.

FUNDING

The work was supported by NIH grants R01HG010505 and R01DK132775. The Genotype-Tissue Expression (GTEx) Project was supported by the Common Fund of the Office of the Director of the National Institutes of Health, and by NCI, NHGRI, NHLBI, NIDA, NIMH, and NINDS. The KOBS study (J. P.) was supported by the Finnish Diabetes Research Foundation, Kuopio University Hospital Project grant (EVO/VTR grants 2005-2019), and the Academy of Finland grant (Contract no. 138006). This study was funded by the European Research Council under the European Union's Horizon 2020 research and innovation program (Grant No. 802825 to M. U. K.). K. H. P. was funded by the Academy of Finland (grant numbers 272376, 266286, 314383, and 335443), the Finnish Medical Foundation, Gyllenberg Foundation, Novo Nordisk Foundation (grant numbers NNF10OC1013354, NNF17OC0027232, and NNF20OC0060547), Finnish Diabetes Research Foundation, Finnish Foundation for Cardiovascular Research, University of Helsinki, and Helsinki University Hospital and Government Research Funds. I. S. was funded by the Instrumentarium Science Foundation. Personal grants to U. T. A. were received from the Matti and Vappu Maukonen Foundation, Ella och Georg Ehrnrooths Stiftelse and the Finnish Foundation for Cardiovascular Research.

Liraglutide demonstrates a therapeutic effect on mitochondrial dysfunction in human SGBS adipocytes in vitro

AIMS

Liraglutide (LG), a glucagon-like peptide-1 receptor (GLP-1R) agonist, has been shown to improve white adipose tissue mitochondrial metabolism in mice but not in human adipocytes. Therefore, we explored whether LG has therapeutic efficacy in mitochondrial dysfunction in human adipocytes in vitro.

METHODS

We tested the effects of short-term (ST-LG: 24 h) and long-term (LT-LG: D0-15 days) treatments in human SGBS adipocytes on mitochondrial respiration, mRNA and protein expression. GLP-1R inhibition was investigated by the co-treatment of GLP-1R inhibitor, exendin 9-39 (Ex9-39) and ST-LG treatment. We also explored the ability of ST-LG to alleviate mitochondrial dysfunction induced by tumor necrosis factor-alpha (TNFα).

RESULTS

LT-LG treatment induced the formation of smaller lipid droplets and increased the expression of genes related to lipolysis. Both ST-LG and LT-LG treatments promoted mitochondrial respiration. Additionally, LT-LG treatment increased the expression of a brown adipocyte marker, uncoupling protein 1 (UCP-1), and the markers of mitochondrial biogenesis. Interestingly, ST-LG rescued TNFα-induced defects in mitochondrial energy metabolism and inflammation in SGBS adipocytes.

CONCLUSION

LG stimulates mitochondrial respiration and biogenesis in human adipocytes, potentially via UCP-1-mediated adipocyte browning. Importantly, our study demonstrates for the first time that LG has a therapeutic potential on mitochondrial activity in human adipocytes.

The antigen CD300e drives T cell inflammation in adipose tissue and elicits an antibody response predictive of the insulin sensitivity recovery in obese patients

Obesity and insulin resistance (IR), the key features of metabolic syndrome, are closely associated with a state of chronic, low-grade inflammation. Bariatric surgery leads to a considerable reduction in the adipose tissue mass and systemic inflammation along with a reduction of IR, with a whole-body metabolic improvement. However, a sizable portion of people experience an IR relapse within few years of remission.Numerous studies have attempted to explore the best clinical predictors of the improvement of insulin sensitivity and the maintenance of glucose homeostasis after bariatric surgery, but no simple fasting blood test has been found to be effective in predicting the short and long-term beneficial effects on glycaemia.With the present study, we investigated T-cell and antibody responses against CD300e, an antigen highly expressed in the adipose tissue of patients with obesity before the bariatric surgery-induced weight loss. We found both in fat tissue and in peripheral blood anti-CD300e-specific T helper 1 responses. Moreover, we evidenced in the sera of individuals with obesity an antibody response towards CD300e and revealed the existence of a significant correlation between the level of antibodies before surgery and the maintenance of glucose control after the intervention.

Metabolic Slowing Vanished 5 Years After Sleeve Gastrectomy in Patients With Obesity and Prediabetes/Diabetes

BACKGROUND

Resting energy expenditure (REE) decreases after weight loss more than expected according to body composition changes. Metabolic adaptation (MA) or metabolic slowing represents the difference between measured (m) and predicted (p) REE, and it is not clear whether it persists in the long-term. The aim of this study is to evaluate MA occurring 1 year (V1) and 5 years (V5) after laparoscopic sleeve gastrectomy (LSG) in patients with obesity and normal glucose tolerance, prediabetes (preDM) and type 2 diabetes (T2DM).

METHODS

We reassessed 37 patients (14 males/23 females) of 44.8 ± 10 years old, since they registered all the biochemical, body composition, and REE assessments at baseline (V0), V1, and V5. Physical activity (PA) was assessed by interview and questionnaire.

RESULTS

Patients displayed a percentage of weight loss of 31.5 ± 7.4% at V1 and a weight regain of 8.9 ± 7.5% at V5. Comparing V1 and V5, fat mass showed a slight increase (P = 0.011), while free fat mass remained unchanged (P = 0.304). PA improved at V1 (P < 0.001), remaining stable at V5 (P = 0.9). Measured REE (mREE) displayed a 31.2% reduction with a corresponding decrease of predicted REE (pREE) of 21.4% at V1, compared with V0 (P = 0.005), confirming a significant MA at V1. Conversely, no difference between mREE and pREE was observed at V5 (P = 0.112).

CONCLUSION

Our results suggested that only patients with preDM and T2DM displayed MA at V1, which vanished 5 years after LSG. Patients who practiced more PA prevent MA after surgery-induced wight loss.

Remission of obesity and insulin resistance is not sufficient to restore mitochondrial homeostasis in visceral adipose tissue

Metabolic plasticity is the ability of a biological system to adapt its metabolic phenotype to different environmental stressors. We used a whole-body and tissue-specific phenotypic, functional, proteomic, metabolomic and transcriptomic approach to systematically assess metabolic plasticity in diet-induced obese mice after a combined nutritional and exercise intervention. Although most obesity and overnutrition-related pathological features were successfully reverted, we observed a high degree of metabolic dysfunction in visceral white adipose tissue, characterized by abnormal mitochondrial morphology and functionality. Despite two sequential therapeutic interventions and an apparent global healthy phenotype, obesity triggered a cascade of events in visceral adipose tissue progressing from mitochondrial metabolic and proteostatic alterations to widespread cellular stress, which compromises its biosynthetic and recycling capacity. In humans, weight loss after bariatric surgery showed a transcriptional signature in visceral adipose tissue similar to our mouse model of obesity reversion. Overall, our data indicate that obesity prompts a lasting metabolic fingerprint that leads to a progressive breakdown of metabolic plasticity in visceral adipose tissue.

Insulin-inducible THRSP maintains mitochondrial function and regulates sphingolipid metabolism in human adipocytes

BACKGROUND

Thyroid hormone responsive protein (THRSP) is a lipogenic nuclear protein that is highly expressed in murine adipose tissue, but its role in humans remains unknown.

METHODS

We characterized the insulin regulation of THRSP in vivo in human adipose tissue biopsies and in vitro in Simpson-Golabi-Behmel syndrome (SGBS) adipocytes. To this end, we measured whole-body insulin sensitivity using the euglycemic insulin clamp technique in 36 subjects [age 40 ± 9 years, body mass index (BMI) 27.3 ± 5.0 kg/m²]. Adipose tissue biopsies were obtained at baseline and after 180 and 360 min of euglycemic hyperinsulinemia for measurement of THRSP mRNA concentrations. To identify functions affected by THRSP, we performed a transcriptomic analysis of THRSP-silenced SGBS adipocytes. Mitochondrial function was assessed by measuring mitochondrial respiration as well as oxidation and uptake of radiolabeled oleate and glucose. Lipid composition in THRSP silencing was studied by lipidomic analysis.

RESULTS

We found insulin to increase THRSP mRNA expression 5- and 8-fold after 180 and 360 min of in vivo euglycemic hyperinsulinemia. This induction was impaired in insulin-resistant subjects, and THRSP expression was closely correlated with whole-body insulin sensitivity. In vitro, insulin increased both THRSP mRNA and protein concentrations in SGBS adipocytes in a phosphoinositide 3-kinase (PI3K)-dependent manner. A transcriptomic analysis of THRSP-silenced adipocytes showed alterations in mitochondrial functions and pathways of lipid metabolism, which were corroborated by significantly impaired mitochondrial respiration and fatty acid oxidation. A lipidomic analysis revealed decreased hexosylceramide concentrations, supported by the transcript concentrations of enzymes regulating sphingolipid metabolism.

CONCLUSIONS

THRSP is regulated by insulin both in vivo in human adipose tissue and in vitro in adipocytes, and its expression is downregulated by insulin resistance. As THRSP silencing decreases mitochondrial respiration and fatty acid oxidation, its downregulation in human adipose tissue could contribute to mitochondrial dysfunction. Furthermore, disturbed sphingolipid metabolism could add to metabolic dysfunction in obese adipose tissue.

Flying on empty: Reduced mitochondrial function and flight capacity in food-deprived monarch butterflies

Mitochondrial function is fundamental to organismal performance, health, and fitness - especially during energetically challenging events, such as migration. With this investigation, we evaluated mitochondrial sensitivity to ecologically relevant stressors. We focused on an iconic migrant, the North American monarch butterfly (Danaus plexippus), and examined the effects of two stressors: seven days of food deprivation, and infection by the protozoan parasite Ophryocystis elektroscirrha (known to reduce survival and flight performance). We measured whole-animal resting (RMR) and peak flight metabolic rate, and mitochondrial respiration of isolated mitochondria from the flight muscles. Food deprivation reduced mass-independent RMR and peak flight metabolic rate, whereas infection did not. Fed monarchs used mainly lipids in flight (respiratory quotient 0.73), but the respiratory quotient dropped in food-deprived individuals, possibly indicating switching to alternative energy sources, such as ketone bodies. Food deprivation decreased mitochondrial maximum oxygen consumption but not basal respiration, resulting in lower respiratory control ratio (RCR). Furthermore, food deprivation decreased mitochondrial complex III activity, but increased complex IV activity. Infection did not result in any changes in these mitochondrial variables. Mitochondrial maximum respiration rate correlated positively with mass-independent RMR and flight metabolic rate, suggesting a link between mitochondria and whole-animal performance. In conclusion, low food availability negatively affects mitochondrial function and flight performance, with potential implications on migration, fitness, and population dynamics. Although previous studies have reported poor flight performance in infected monarchs, we found no differences in physiological performance, suggesting that reduced flight capacity may be due to structural differences or low energy stores.

Impact of Bariatric Surgery on Adipose Tissue Biology

Bariatric surgery (BS) procedures are actually the most effective intervention to help subjects with severe obesity achieve significant and sustained weight loss. White adipose tissue (WAT) is increasingly recognized as the largest endocrine organ. Unhealthy WAT expansion through adipocyte hypertrophy has pleiotropic effects on adipocyte function and promotes obesity-associated metabolic complications. WAT dysfunction in obesity encompasses an altered adipokine secretome, unresolved inflammation, dysregulated autophagy, inappropriate extracellular matrix remodeling and insufficient angiogenic potential. In the last 10 years, accumulating evidence suggests that BS can improve the WAT function beyond reducing the fat depot sizes. The causal relationships between improved WAT function and the health benefits of BS merits further investigation. This review summarizes the current knowledge on the short-, medium- and long-term outcomes of BS on the WAT composition and function.

Identification of TBX15 as an adipose master trans regulator of abdominal obesity genes

Background

Obesity predisposes individuals to multiple cardiometabolic disorders, including type 2 diabetes (T2D). As body mass index (BMI) cannot reliably differentiate fat from lean mass, the metabolically detrimental abdominal obesity has been estimated using waist-hip ratio (WHR). Waist-hip ratio adjusted for body mass index (WHRadjBMI) in turn is a well-established sex-specific marker for abdominal fat and adiposity, and a predictor of adverse metabolic outcomes, such as T2D. However, the underlying genes and regulatory mechanisms orchestrating the sex differences in obesity and body fat distribution in humans are not well understood.

Methods

We searched for genetic master regulators of WHRadjBMI by employing integrative genomics approaches on human subcutaneous adipose RNA sequencing (RNA-seq) data (n ~ 1400) and WHRadjBMI GWAS data (n ~ 700,000) from the WHRadjBMI GWAS cohorts and the UK Biobank (UKB), using co-expression network, transcriptome-wide association study (TWAS), and polygenic risk score (PRS) approaches. Finally, we functionally verified our genomic results using gene knockdown experiments in a human primary cell type that is critical for adipose tissue function.

Results

Here, we identified an adipose gene co-expression network that contains 35 obesity GWAS genes and explains a significant amount of polygenic risk for abdominal obesity and T2D in the UKB (n = 392,551) in a sex-dependent way. We showed that this network is preserved in the adipose tissue data from the Finnish Kuopio Obesity Study and Mexican Obesity Study. The network is controlled by a novel adipose master transcription factor (TF), TBX15, a WHRadjBMI GWAS gene that regulates the network in trans. Knockdown of TBX15 in human primary preadipocytes resulted in changes in expression of 130 network genes, including the key adipose TFs, PPARG and KLF15, which were significantly impacted (FDR < 0.05), thus functionally verifying the trans regulatory effect of TBX15 on the WHRadjBMI co-expression network.

Conclusions

Our study discovers a novel key function for the TBX15 TF in trans regulating an adipose co-expression network of 347 adipose, mitochondrial, and metabolically important genes, including PPARG, KLF15, PPARA, ADIPOQ, and 35 obesity GWAS genes. Thus, based on our converging genomic, transcriptional, and functional evidence, we interpret the role of TBX15 to be a main transcriptional regulator in the adipose tissue and discover its importance in human abdominal obesity.

Supplementary Information

The online version contains supplementary material available at 10.1186/s13073-021-00939-2.