Family with sequence similarity 13, member A modulates adipocyte insulin signaling and preserves systemic metabolic homeostasis

Cancer-associated fibroblasts promote the progression and chemoresistance of HCC by inducing IGF-1

Crosstalk between cancer-associated fibroblasts (CAFs) and tumour cells plays a critical role in multiple cancers, including hepatocellular carcinoma (HCC). CAFs contribute to tumorigenesis by secreting growth factors, modifying the extracellular matrix, supporting angiogenesis, and suppressing antitumor immune responses. However, effect and mechanism of CAF-mediated promotion of hepatocellular carcinoma cells are still unclear. In study, we demonstrated CAFs promoted the proliferation and inhibited the apoptosis of HCC cells by secreting interleukin-6 (IL-6), which induced autocrine insulin-like growth factor-1 (IGF-1) in HCC. IGF-1 promoted the progression and chemoresistance of HCC. IGF-1 receptor (IGF-1R) inhibitor NT157 abrogated the effect of CAF-derived IL-6 and autocrine IGF-1 on HCC. Mechanistic studies revealed that NT157 decreased IL-6-induced IGF-1 expression by inhibiting STAT3 phosphorylation and led to IRS-1 degradation, which mediated the proliferation of tumour by activating AKT signalling in ERK-dependent manner. Inhibition of IGF-1R also enhanced the therapeutic effect of sorafenib on HCC, especially chemoresistant tumours. STATEMENT OF SIGNIFICANCE: Our study showed IL-6-IGF-1 axis played crucial roles in the crosstalk between HCC and CAFs, providing NT157 inhibited of STAT3 and IGF-1R as a new targeted therapy in combination with sorafenib.

Thromboxane A2-TP axis promotes adipose tissue macrophages M1 polarization leading to insulin resistance in obesity

Aberrant arachidonic acid metabolism has been implicated in multiple pathophysiological conditions, and the downstream prostanoids levels are associated with adipocyte dysfunction in obesity. However, the role of thromboxane A₂ (TXA₂) in obesity remains unclear. We observed that TXA₂, through its receptor TP, is a candidate mediator in obesity and metabolic disorders. Obese mice with upregulated TXA₂ biosynthesis (TBXAS1) and TXA₂ receptor (TP) expression in caused insulin resistance and macrophage M1 polarization in white adipose tissue (WAT), which can be prevented by treatment with aspirin. Mechanistically, the activation of TXA₂-TP signaling axis leads to accumulation of protein kinase Cɛ (PKCɛ), thereby enhancing free fat acid (FFA) induced Toll-like receptor4 (TLR4) proinflammatory macrophage activation and the tumor necrosis factor-a (TNF-a) production in adipose tissues. Importantly, TP knockout mice reduced the accumulation of proinflammatory macrophages and adipocyte hypertrophy in WAT. Thus, our findings demonstrate that TXA₂-TP axis plays a crucial role in obesity-induced adipose macrophage dysfunction, and rational targeting TXA₂ pathway may improve obesity and its associated metabolic disorders in future. In this work, we establish previously unknown role of TXA₂-TP axis in WAT. These findings might provide new insight into the molecular pathogenesis of insulin resistance, and indicate rational targeting TXA₂ pathway to improve obesity and its associated metabolic disorders in future.

Exploratory integrated analysis of circulating exosomal miRNA and tissue mRNA related to long-term physical activity for more than 25 years: a bioinformatics study

Physical activity exerts various positive effects on both physical and mental health. Although the comprehensive expression profiles of each microRNA (miRNA) or messenger RNA (mRNA) related to physical activity have already been reported, the association between miRNA and mRNA remains unclear. Here, the integrated study was conducted to comprehensively explore the potential miRNA-mRNA relationships related to long-term physical activity over 25 years. Genome-wide public deposited mRNA expression data of adipose tissue (GSE20536) from six same-sex twin pairs (no information regarding gender) and of skeletal muscle tissue (GSE20319) from ten same-sex twin pairs (four female twin pairs) were used, and differentially expressed mRNAs (DEMs) related to discordant leisure-time physical activity for 30 years were identified using GEO2R. Overlapped mRNAs between DEMs and predicted possible target mRNAs, based on a previous study and TargetScan tool, were then identified and used as long-term physical activity-related mRNAs targeted by miRNAs. In adipose tissue, 36 mRNAs and 42 mRNAs were identified as upregulated or downregulated DEMs, respectively. Based on the results of the overlapped analysis between DEMs and predicted possible target mRNAs targeted by miRNAs, 15 upregulated mRNAs, including NDRG4, FAM13A, ST3GAL6, and AFF1, and 10 downregulated mRNAs, including RPL14, LBP, and GLRX, were identified. In muscle tissue, three downregulated mRNAs overlapped with the predicted target mRNAs targeted by miRNAs. Fifteen upregulated mRNAs in adipose tissue showed a tendency to enrich in "Cardiovascular" in GAD_DISEASE_CLASS category. Potential miRNA-mRNA relationships related to long-term physical activity over 25 years were identified through bioinformatics analysis.

The impact of SNP density on quantitative genetic analyses of body size traits in a wild population of Soay sheep

Understanding the genetic architecture underpinning quantitative traits in wild populations is pivotal to understanding the processes behind trait evolution. The 'animal model' is a popular method for estimating quantitative genetic parameters such as heritability and genetic correlation and involves fitting an estimate of relatedness between individuals in the study population. Genotypes at genome-wide markers can be used to estimate relatedness; however, relatedness estimates vary with marker density, potentially affecting results. Increasing density of markers is also expected to increase the power to detect quantitative trait loci (QTL). In order to understand how the density of genetic markers affects the results of quantitative genetic analyses, we estimated heritability and performed genome-wide association studies (GWAS) on five body size traits in an unmanaged population of Soay sheep using two different SNP densities: a dataset of 37,037 genotyped SNPs and an imputed dataset of 417,373 SNPs. Heritability estimates did not differ between the two SNP densities, but the high-density imputed SNP dataset revealed four new SNP-trait associations that were not found with the lower density dataset, as well as confirming all previously-found QTL. We also demonstrated that fitting fixed and random effects in the same step as performing GWAS is a more powerful approach than pre-correcting for covariates in a separate model.

Epigenomic and Transcriptomic Prioritization of Candidate Obesity-Risk Regulatory GWAS SNPs

Concern about rising rates of obesity has prompted searches for obesity-related single nucleotide polymorphisms (SNPs) in genome-wide association studies (GWAS). Identifying plausible regulatory SNPs is very difficult partially because of linkage disequilibrium. We used an unusual epigenomic and transcriptomic analysis of obesity GWAS-derived SNPs in adipose versus heterologous tissues. From 50 GWAS and 121,064 expanded SNPs, we prioritized 47 potential causal regulatory SNPs (Tier-1 SNPs) for 14 gene loci. A detailed examination of seven loci revealed that four (CABLES1, PC, PEMT, and FAM13A) had Tier-1 SNPs positioned so that they could regulate use of alternative transcription start sites, resulting in different polypeptides being generated or different amounts of an intronic microRNA gene being expressed. HOXA11 and long noncoding RNA gene RP11-392O17.1 had Tier-1 SNPs in their 3' or promoter region, respectively, and strong preferences for expression in subcutaneous versus visceral adipose tissue. ZBED3-AS1 had two intragenic Tier-1 SNPs, each of which could contribute to mediating obesity risk through modulating long-distance chromatin interactions. Our approach not only revealed especially credible novel regulatory SNPs, but also helped evaluate previously highlighted obesity GWAS SNPs that were candidates for transcription regulation.

FAM13A promotes proliferation of bovine preadipocytes by targeting Hypoxia-Inducible factor-1 signaling pathway

The family with sequence similarity 13 member A (FAM13A) gene has been discovered in recent years and is related to metabolism. In this study, the function of FAM13A in precursor adipocyte proliferation in Qinchuan cattle was investigated using fluorescence quantitative polymerase chain reaction (PCR), western blotting, 5-ethynyl-2'-deoxyuridine staining, and other tests. FAM13A promoted precursor adipocyte proliferation. To determine the pathway FAM13A was involved in, transcriptome sequencing, fluorescence quantitative PCR, western blotting, and other tests were used, which identified the hypoxia inducible factor-1 (HIF-1) signalling pathway. Finally, cobalt chloride, a chemical mimic of hypoxia, was used to treat precursor adipocytes. mRNA and protein levels of FAM13A were significantly increased after hypoxia. Thus, FAM13A promoted bovine precursor adipocyte proliferation by inhibiting the HIF-1 signalling pathway, whereas chemically induced hypoxia negatively regulated FAM13A expression, regulating cell proliferation.

miR‑30a‑5p induces the adipogenic differentiation of bone marrow mesenchymal stem cells by targeting FAM13A/Wnt/β‑catenin signaling in aplastic anemia

Aplastic anemia (AA) is a bone marrow failure syndrome with high morbidity and mortality. Bone marrow (BM)-mesenchymal stem cells (MSCs) are the main components of the BM microenvironment, and dysregulation of BM-MSC adipogenic differentiation is a pathologic hallmark of AA. MicroRNAs (miRNAs/miRs) are crucial regulators of multiple pathological processes such as AA. However, the role of miR-30a-5p in the modulation of BM-MSC adipogenic differentiation in AA remains unclear. The present study aimed to explore the effect of miR-30a-5p on AA BM-MSC adipogenic differentiation and the underlying mechanism. The levels of miR-30a-5p expression and family with sequence similarity 13, member A (FAM13A) mRNA expression in BM-MSCs were quantified using reverse transcription-quantitative (RT-q) PCR. The mRNA expression levels of adipogenesis-associated factors [fatty acid-binding protein 4 (FABP4), lipoprotein lipase (LPL), perilipin-1 (PLIN1), peroxisome proliferator-activated receptor γ (PPARγ) and CCAAT/enhancer binding protein α (C/EBPα)] were analyzed using RT-qPCR. Lipid droplet accumulation was evaluated using Oil Red O staining in BM-MSCs. The interaction between miR-30a-5p and the FAM13A 3′-untranslated region was identified by TargetScan, and a dual-luciferase reporter assay was used to confirm the interaction. The expression levels of FAM13A and Wnt/β-catenin pathway-related proteins were examined via western blotting. The results showed that miR-30a-5p expression levels were significantly elevated in BM-MSCs from patients with AA compared with those in control subjects (iron deficiency anemia). miR-30a-5p expression levels were also significantly increased in adipose-induced BM-MSCs in a time-dependent manner. miR-30a-5p significantly promoted AA BM-MSC adipogenic differentiation, and significantly enhanced the mRNA expression levels of FABP4, LPL, PLIN1, PPARγ and C/EBPα as well as lipid droplet accumulation. miR-30a-5p was also demonstrated to target FAM13A in AA BM-MSCs. FAM13A significantly reduced BM-MSC adipogenic differentiation by activating the Wnt/β-catenin signaling pathway. In conclusion, miR-30a-5p was demonstrated to serve a role in AA BM-MSC adipogenic differentiation by targeting the FAM13A/Wnt/β-catenin signaling pathway. These findings suggest that miR-30a-5p may be a therapeutic target for AA.

Dysmetabolic adipose tissue in obesity: morphological and functional characteristics of adipose stem cells and mature adipocytes in healthy and unhealthy obese subjects

The way by which subcutaneous adipose tissue (SAT) expands and undergoes remodeling by storing excess lipids through expansion of adipocytes (hypertrophy) or recruitment of new precursor cells (hyperplasia) impacts the risk of developing cardiometabolic and respiratory diseases. In unhealthy obese subjects, insulin resistance, type 2 diabetes, hypertension, and obstructive sleep apnoea are typically associated with pathologic SAT remodeling characterized by adipocyte hypertrophy, as well as chronic inflammation, hypoxia, increased visceral adipose tissue (VAT), and fatty liver. In contrast, metabolically healthy obese individuals are generally associated with SAT development characterized by the presence of smaller and numerous mature adipocytes, and a lower degree of VAT inflammation and ectopic fat accumulation. The remodeling of SAT and VAT is under genetic regulation and influenced by inherent depot-specific differences of adipose tissue-derived stem cells (ASCs). ASCs have multiple functions such as cell renewal, adipogenic capacity, and angiogenic properties, and secrete a variety of bioactive molecules involved in vascular and extracellular matrix remodeling. Understanding the mechanisms regulating the proliferative and adipogenic capacity of ASCs from SAT and VAT in response to excess calorie intake has become a focus of interest over recent decades. Here, we summarize current knowledge about the biological mechanisms able to foster or impair the recruitment and adipogenic differentiation of ASCs during SAT and VAT development, which regulate body fat distribution and favorable or unfavorable metabolic responses.

Genome-wide discovery of genetic loci that uncouple excess adiposity from its comorbidities

Obesity is a major risk factor for cardiometabolic diseases. Nevertheless, a substantial proportion of individuals with obesity do not suffer cardiometabolic comorbidities. The mechanisms that uncouple adiposity from its cardiometabolic complications are not fully understood. Here, we identify 62 loci of which the same allele is significantly associated with both higher adiposity and lower cardiometabolic risk. Functional analyses show that the 62 loci are enriched for genes expressed in adipose tissue, and for regulatory variants that influence nearby genes that affect adipocyte differentiation. Genes prioritized in each locus support a key role of fat distribution (FAM13A, IRS1 and PPARG) and adipocyte function (ALDH2, CCDC92, DNAH10, ESR1, FAM13A, MTOR, PIK3R1 and VEGFB). Several additional mechanisms are involved as well, such as insulin-glucose signalling (ADCY5, ARAP1, CREBBP, FAM13A, MTOR, PEPD, RAC1 and SH2B3), energy expenditure and fatty acid oxidation (IGF2BP2), browning of white adipose tissue (CSK, VEGFA, VEGFB and SLC22A3) and inflammation (SH2B3, DAGLB and ADCY9). Some of these genes may represent therapeutic targets to reduce cardiometabolic risk linked to excess adiposity.

Chondroitin Sulfate N-acetylgalactosaminyltransferase-2 Impacts Foam Cell Formation and Atherosclerosis by Altering Macrophage Glycosaminoglycan Chain

OBJECTIVE

Chondroitin sulfate proteoglycans are the primary constituents of the macrophage glycosaminoglycan and extracellular microenvironment. To examine their potential role in atherogenesis, we investigated the biological importance of one of the chondroitin sulfate glycosaminoglycan biosynthesis gene, ChGn-2 (chondroitin sulfate N-acetylgalactosaminyltransferase-2), in macrophage foam cell formation. Approach and Results: ChGn-2-deficient mice showed decreased and shortened glycosaminoglycans. ChGn-2^-/-/LDLr^-/- (low-density lipoprotein receptor) mice generated less atherosclerotic plaque after being fed with Western diet despite exhibiting a metabolic phenotype similar to that of the ChGn-2^+/+/LDLr^-/- littermates. We demonstrated that in macrophages, ChGn-2 expression was upregulated in the presence of oxLDL (oxidized LDL), and glycosaminoglycan was substantially increased. Foam cell formation was significantly altered by ChGn-2 in both mouse peritoneal macrophages and the RAW264.7 macrophage cell line. Mechanistically, ChGn-2 enhanced oxLDL binding on the cell surface, and as a consequence, CD36-an important macrophage membrane scavenger receptor-was differentially regulated.

CONCLUSIONS

ChGn-2 alteration on macrophages conceivably influences LDL accumulation and subsequently accelerates plaque formation. These results collectively suggest that ChGn-2 is a novel therapeutic target amenable to clinical translation in the future. Graphic Abstract: A graphic abstract is available for this article.

Chondroitin sulfate mediates liver responses to injury induced by dual endothelin receptor inhibition

Although dual endothelin receptor antagonists (ERAs) show great promise for treating various conditions, their propensity to induce liver injury limits their clinical usage. Inflammation and fibrosis are important processes in liver responses to injury and it has been suggested that they and dual ERA-induced liver injury are mediated by the proteoglycan component chondroitin sulfate (CS), which is synthesized by CHST3 and CHST13. In this study, we investigated whether dual ER inhibition in the liver could alter CHST3 and CHST13 expression and thus CS production and whether liver CS content could prevent inflammatory and fibrosis responses after liver injury. We observed increased CHST3 and CHST13 expression after liver injury in bile duct ligated mice and histologically confirmed abundant CS deposition in the injured liver. Moreover, treating Hep3B cells with a dual ERA mimic significantly increased CHST3 and CHST13 expression, inflammatory cytokine levels, and glycosaminoglycan deposition. Furthermore, pro-inflammatory and pro-fibrotic markers were observed after dual ERA treatment, while treatment with CS-degrading chondroitinase ABC was able to successfully reverse these phenotypes. These observations suggest that CHST3- and CHST13-induced CS production can mediate liver injury responses caused by dual ER inhibition and thus could be an alternative pathway for treating ERA-induced liver injury.

FAM13A Represses AMPK Activity and Regulates Hepatic Glucose and Lipid Metabolism

Obesity commonly co-exists with fatty liver disease with increasing health burden worldwide. Family with Sequence Similarity 13, Member A (FAM13A) has been associated with lipid levels and fat mass by genome-wide association studies (GWAS). However, the function of FAM13A in maintaining metabolic homeostasis in vivo remains unclear. Here, we demonstrated that rs2276936 in this locus has allelic-enhancer activity in massively parallel reporter assays (MPRA) and reporter assay. The DNA region containing rs2276936 regulates expression of endogenous FAM13A in HepG2 cells. In vivo, Fam13a-/- mice are protected from high-fat diet (HFD)-induced fatty liver accompanied by increased insulin sensitivity and reduced glucose production in liver. Mechanistically, loss of Fam13a led to the activation of AMP-activated protein kinase (AMPK) and increased mitochondrial respiration in primary hepatocytes. These findings demonstrate that FAM13A mediates obesity-related dysregulation of lipid and glucose homeostasis. Targeting FAM13A might be a promising treatment of obesity and fatty liver disease.

FAM13A affects body fat distribution and adipocyte function

Genetic variation in the FAM13A (Family with Sequence Similarity 13 Member A) locus has been associated with several glycemic and metabolic traits in genome-wide association studies (GWAS). Here, we demonstrate that in humans, FAM13A alleles are associated with increased FAM13A expression in subcutaneous adipose tissue (SAT) and an insulin resistance-related phenotype (e.g. higher waist-to-hip ratio and fasting insulin levels, but lower body fat). In human adipocyte models, knockdown of FAM13A in preadipocytes accelerates adipocyte differentiation. In mice, Fam13a knockout (KO) have a lower visceral to subcutaneous fat (VAT/SAT) ratio after high-fat diet challenge, in comparison to their wild-type counterparts. Subcutaneous adipocytes in KO mice show a size distribution shift toward an increased number of smaller adipocytes, along with an improved adipogenic potential. Our results indicate that GWAS-associated variants within the FAM13A locus alter adipose FAM13A expression, which in turn, regulates adipocyte differentiation and contribute to changes in body fat distribution.

Loss of family with sequence similarity 13, member A exacerbates pulmonary hypertension through accelerating endothelial-to-mesenchymal transition

Pulmonary hypertension is a progressive lung disease with poor prognosis due to the consequent right heart ventricular failure. Pulmonary artery remodeling and dysfunction are culprits for pathologically increased pulmonary arterial pressure, but their underlying molecular mechanisms remain to be elucidated. Previous genome-wide association studies revealed a significant correlation between the genetic locus of family with sequence similarity 13, member A (FAM13A) and various lung diseases such as chronic obstructive pulmonary disease and pulmonary fibrosis; however whether FAM13A is also involved in the pathogenesis of pulmonary hypertension remained unknown. Here, we identified a significant role of FAM13A in the development of pulmonary hypertension. FAM13A expression was reduced in the lungs of mice with hypoxia-induced pulmonary hypertension. We identified that FAM13A was expressed in lung vasculatures, especially in endothelial cells. Genetic loss of FAM13A exacerbated pulmonary hypertension in mice exposed to chronic hypoxia in association with deteriorated pulmonary artery remodeling. Mechanistically, FAM13A decelerated endothelial-to-mesenchymal transition potentially by inhibiting β-catenin signaling in pulmonary artery endothelial cells. Our data revealed a protective role of FAM13A in the development of pulmonary hypertension, and therefore increasing and/or preserving FAM13A expression in pulmonary artery endothelial cells is an attractive therapeutic strategy for the treatment of pulmonary hypertension.

Endothelial progeria induces adipose tissue senescence and impairs insulin sensitivity through senescence associated secretory phenotype

Vascular senescence is thought to play a crucial role in an ageing-associated decline of organ functions; however, whether vascular senescence is causally implicated in age-related disease remains unclear. Here we show that endothelial cell (EC) senescence induces metabolic disorders through the senescence-associated secretory phenotype. Senescence-messaging secretomes from senescent ECs induced a senescence-like state and reduced insulin receptor substrate-1 in adipocytes, which thereby impaired insulin signaling. We generated EC-specific progeroid mice that overexpressed the dominant negative form of telomeric repeat-binding factor 2 under the control of the Tie2 promoter. EC-specific progeria impaired systemic metabolic health in mice in association with adipose tissue dysfunction even while consuming normal chow. Notably, shared circulation with EC-specific progeroid mice by parabiosis sufficiently transmitted the metabolic disorders into wild-type recipient mice. Our data provides direct evidence that EC senescence impairs systemic metabolic health, and thus establishes EC senescence as a bona fide risk for age-related metabolic disease.

Loss of Family with Sequence Similarity 13, Member A Exacerbates Pulmonary Fibrosis Potentially by Promoting Epithelial to Mesenchymal Transition

Idiopathic pulmonary fibrosis (IPF) is a devastating disease with poor prognosis due to limited clinical treatment options. IPF is characterized by the augmented deposition of extracellular matrix driven by myofibroblasts, and the epithelial-mesenchymal transition (EMT) has been known to play an essential role in the mechanism of pulmonary fibrosis. Previous genome-wide association study identified Fam13a as one of genes that showed genetic link with IPF and chronic obstructive pulmonary disease. Here, we analyzed the role of Fam13a in the pathogenesis of pulmonary fibrosis using Fam13a-deficient mice. We found that Fam13a was down-regulated in mouse lungs of bleomycin-induced pulmonary fibrosis model. Of note, genetic deletion of Fam13a exacerbated the lung fibrosis induced by bleomycin in association with enhanced EMT in mice. Moreover, silencing of Fam13a accelerated EMT induced by TGF-β and TNF-α in alveolar epithelial cells, accompanied by increased active β-catenin and its nuclear accumulation. Our data revealed a crucial role of Fam13a in the development of pulmonary fibrosis potentially through inhibiting EMT, and thus Fam13a has a therapeutic potential in the treatment of IPF.

The Molecular Characteristics of the FAM13A Gene and the Role of Transcription Factors ACSL1 and ASCL2 in Its Core Promoter Region

The gene family with sequence similarity 13 member A (FAM13A) has recently been identified as a marker gene in insulin sensitivity and lipolysis. In this study, we first analyzed the expression patterns of this gene in different tissues of adult cattle and then constructed a phylogenetic tree based on the FAM13A amino acid sequence. This showed that subcutaneous adipose tissue had the highest expression in all tissues except lung tissue. Then we summarized the gene structure. The promoter region sequence of the gene was successfully amplified, and the -241/+54 region has been identified as the core promoter region. The core promoter region was determined by the unidirectional deletion of the 5' flanking promoter region of the FAM13A gene. Based on the bioinformatics analysis, we examined the dual luciferase activity of the vector constructed by the mutation site, and the transcription factors ACSL1 and ASCL2 were found as transcriptional regulators of FAM13A. Moreover, electrophoretic mobility shift assay (EMSA) further validated the regulatory role of ACSL1 and ASCL2 in the regulation of FAM13A. ACSL1 and ASCL2 were finally identified as activating transcription factors. Our results provide a basis for the function of the FAM13A gene in bovine adipocytes in order to improve the deposition of fat deposition in beef cattle muscle.

Genome-Wide and Abdominal MRI Data Provide Evidence That a Genetically Determined Favorable Adiposity Phenotype Is Characterized by Lower Ectopic Liver Fat and Lower Risk of Type 2 Diabetes, Heart Disease, and Hypertension

Recent genetic studies have identified alleles associated with opposite effects on adiposity and risk of type 2 diabetes. We aimed to identify more of these variants and test the hypothesis that such favorable adiposity alleles are associated with higher subcutaneous fat and lower ectopic fat. We combined MRI data with genome-wide association studies of body fat percentage (%) and metabolic traits. We report 14 alleles, including 7 newly characterized alleles, associated with higher adiposity but a favorable metabolic profile. Consistent with previous studies, individuals carrying more favorable adiposity alleles had higher body fat % and higher BMI but lower risk of type 2 diabetes, heart disease, and hypertension. These individuals also had higher subcutaneous fat but lower liver fat and a lower visceral-to-subcutaneous adipose tissue ratio. Individual alleles associated with higher body fat % but lower liver fat and lower risk of type 2 diabetes included those in PPARG, GRB14, and IRS1, whereas the allele in ANKRD55 was paradoxically associated with higher visceral fat but lower risk of type 2 diabetes. Most identified favorable adiposity alleles are associated with higher subcutaneous and lower liver fat, a mechanism consistent with the beneficial effects of storing excess triglycerides in metabolically low-risk depots.

Obesity-associated family with sequence similarity 13, member A (FAM13A) is dispensable for adipose development and insulin sensitivity

Background

Obesity and its associated morbidities represent the major and most rapidly expanding world-wide health epidemic. Recent genome-wide association studies (GWAS) reveal that single nucleotide polymorphism (SNP) variant in the Family with Sequence Similarity 13, Member A (FAM13A) gene is strongly associated with waist–hip ratio (WHR) with adjustment for body mass index (BMI) (WHRadjBMI). However, the function of FAM13A in adipose development and obesity remains largely uncharacterized.

Methods

The expression of FAM13A in adipose tissue depots were investigated using lean, genetic obese and high fat diet-induced obese (DIO) animal models and during adipocyte differentiation. Stromal vascular cells (SVCs) or 3T3-L1 cells with gain and loss of function of FAM13A were used to determine the involvement of FAM13A in regulating adipocyte differentiation. Adipose development and metabolic homeostasis in Fam13a^−/− mice were characterized under normal chow and high fat diet feeding.

Results

Murine FAM13A expression was nutritionally regulated and dramatically reduced in epididymal and subcutaneous fat in genetic and diet-induced obesity. Its expression was enriched in mature adipocytes and significantly upregulated during murine and human adipogenesis potentially through a peroxisome proliferator-activated receptor-gamma (PPARγ)-dependent mechanism. However, Fam13a^−/− mice only exhibited a tendency of higher adiposity and were not protected from DIO and insulin resistance. While Fam13a^−/− SVCs maintained normal adipogenesis, overexpression of FAM13A in 3T3-L1 preadipocytes downregulated β-catenin signaling and rendered preadipocytes more susceptible to apoptosis. Moreover, FAM13A overexpression largely blocked adipogenesis induced by a standard hormone cocktail, but adipogenesis can be partially rescued by the addition of PPARγ agonist pioglitazone at an early stage of differentiation.

Conclusions

Our results suggest that FAM13A is dispensable for adipose development and insulin sensitivity. Yet the expression of FAM13A needs to be tightly controlled in adipose precursor cells for their proper survival and downstream adipogenesis. These data provide novel insights into the link between FAM13A and obesity.