Adipose Tissue Fibrosis Research Articles

Osteoarthritis: Inflammation and fibrosis in adipose tissue of osteoarthritic joints.

The infrapatellar fat pad and other adipose tissues in close proximity to the joint have been linked with osteoarthritis (OA), but the nature of this relationship is unclear. Two new studies elucidate the features of intra-articular adipose tissues in OA and raise important questions for future investigations.

Adipose Tissue VEGF‐D Overexpression Increases Lymphatic Vessel Density and Protects Against Inflammation and Insulin Resistance in Obesity

Lymphatic vessels modulate tissue fluid balance and inflammation and provide a conduit for endocrine and lipid transport: all mechanisms that could potentially regulate adipose tissue physiology in obesity. The growth of new lymphatic vessels in is mediated through vascular endothelial growth factor receptor‐3 (VEGFR‐3) signaling. We took advantage of the unique binding of murine VEGF‐D to VEGFR‐3 and generated mice capable of inducible, tissue‐specific expression of murine VEGF‐D under a tightly‐controlled tetracycline response element (TRE) promoter to stimulate lymphangiogenesis. Crossed with adipocyte‐specific adiponectin‐rtTA mice (Adipo‐VD), VEGF‐D overexpression by adipocytes induced de novo lymphangiogenesis in murine white adipose tissue and a massive expansion of brown adipose tissue lymphatics. Upon removal of the doxycycline stimulus, VEGF‐D expression returned to normal and the expanded adipose tissue lymphatics regressed. The new white adipose lymphatic network provided a route for increased glycerol flux during lipolysis. As a chemokine, VEGF‐D expression recruited macrophages and caused fibrosis in white adipose tissue on chow diet, but no negative systemic metabolic consequences were identified. On high fat diet feeding, Adipo‐VD gained equivalent weight and adiposity as littermate controls, but were markedly more insulin sensitive. White adipose tissues in Adipo‐VD mice were full of lymphatics, but exhibited less fibrosis, contained fewer crown‐like structure macrophages, and displayed increased UCP‐1 RNA and protein. Infusion of CD45.1+ leukocytes into CD45.2+ Adipo‐VD mice identified fewer immune cells accumulating in the Adipo‐VD subcutaneous adipose with more trafficking out and into the inguinal lymph node. The increased lymphatic density thus reduces local inflammation in adipose tissue, manifesting as decreased liver triglyceride, increased insulin sensitivity, and overall healthier obesity highlighting the roles of lymphatic in regulating inflammatory pathologies and VEGFR‐3 signaling as a potential target in the metabolic syndrome.

Skeletal muscle integrin‐linked kinase contributes to adipose tissue fibrosis and adipogenesis in diet‐induced obese mice

Diet‐induced obesity (DIO) elicits a complex phenotype where insulin resistance (IR) is induced, at least in part, by tissue‐specific accumulation of lipids in metabolically active organs (e.g. skeletal muscle (SkM)). Recent efforts by a number of groups have demonstrated a causal role of extracellular matrix expansion and integrin receptor signaling in the etiology of insulin resistance, but their impact on lipid metabolism is poorly understood. We have recently demonstrated that mice lacking integrin‐linked kinase (ILK), an intracellular scaffolding protein involved in integrin signaling, specifically in SkM (mILK‐KO mice) prevents IR and fat accumulation in SkM independent of changes in body composition or energy expenditure in DIO mice. Here, we tested the hypothesis that excess lipid in DIO mILK‐KO mice preferentially accumulates in adipose tissue and liver.Wild‐type (WT) and SkM ILK deficient (mILK‐KO) mice fed a chow or high fat (HF) diet for 16 (histology) or 27 (all other measurements) weeks were used for these studies. Mice were fasted for five hours prior to collection of plasma or tissue. Free fatty acids (FFAs), diacylglycerol (DAG), triacylglycerol (TAG) and phospholipid (PL) content was measured with gas chromatography (n=6–8/group). Adipocyte size was measured using H&E stained sections and fibrosis was assessed as percent area stained with Masson's Trichrome using unbiased ImageJ scripts (n=10–14/group). Plasma adiponectin was determined using a commercially available ELISA kit (ALPCO) (n=4–6/group). Total acetyl‐CoA carboxylase (ACC), perilipin, fatty acid synthase (FAS) and peroxisome proliferator activated receptor gamma (PPARγ) were measured via western blot (n=8/group).Epididymal adipose fat pads were larger in DIO mILK‐KO mice than WT mice. Adipose tissue of DIO mILK‐KO mice contained more TG compared to WT mice with no differences in chow‐fed mice. No differences in TGs were observed in livers of mILK‐KO mice. Adipocyte size was greater in HF‐fed mILK‐KO mice compared to HF‐fed WT mice. Adipogenesis markers (ACC, Perilipin, FAS and PPARγ) are all decreased in adipose tissue of mILK‐KO mice compared to HF‐fed WT mice. Extracellular matrix accumulation was significantly reduced in adipose tissue of HF‐fed mILK‐KO mice compared to HF‐fed WT mice. Plasma adiponectin was elevated in HF‐fed mILK‐KO mice compared to WT mice.Limited adipose tissue expansion is a major contributor to multiple aspects of the metabolic syndrome. In obesity, the capacity to store lipid is restricted by fibrosis that establishes a structural barrier to adipocyte expansion. Here, we demonstrate that loss of SkM ILK improves adipose tissue health by reducing fibrosis, increasing adipocyte size, decreasing expression of adipogenesis markers and enhancing TG storage. Collectively, these findings suggest that SkM ILK, through a currently undefined mechanism, is a novel contributor to adipose tissue fibrosis and may be a viable approach to treat the metabolic syndrome.Support or Funding InformationThis research was supported by NIH/NIDDK funding to DHW (DK 054902) and an American Heart Association postdoctoral fellowship to DSL (16POST29910001).

The role of curcumin in disruption of HIF-1α accumulation to alleviate adipose fibrosis via AMPK-mediated mTOR pathway in high-fat diet fed mice

Adipose hypoxia and hypoxia-inducible factor 1α (HIF-1α) accumulation are tightly associated with adipose dysfunction. Curcumin is a polyphenol with positive effects on inflammation and fibrosis. This study aims to investigate the effects of curcumin on HIF-1α and fibrosis in hypoxic adipose tissue. Curcumin attenuated high-fat diet (HFD) induced adipose hypoxia and HIF-1α overexpression. Moreover, curcumin effectively reduced inflammatory cytokine and extracellular matrix deposition in adipose tissue, demonstrating its inhibitory effects on adipose inflammation and fibrosis. Curcumin increased AMP-activated protein kinase (AMPK) phosphorylation and suppressed mammalian target of rapamycin (mTOR) activation, while knockdown of AMPK diminished the suppressive effects on mTOR activation and HIF-1α induction, further indicating that curcumin blocked mTOR/HIF-1α signaling via regulation of AMPK. In conclusion, curcumin prevented adipose hypoxia and decreased HIF-1α-associated adipose fibrosis via suppression of AMPK-mediated mTOR pathway, suggesting that HIF-1α might be a therapeutical target for the prevention of fibrosis in adipose tissue.

Adipose tissue fibrosis in human cancer cachexia: the role of TGF\u03b2 pathway

BackgroundCancer cachexia is a multifactorial syndrome that dramatically decreases survival. Loss of white adipose tissue (WAT) is one of the key characteristics of cachexia. WAT wasting is paralleled by microarchitectural remodeling in cachectic cancer patients. Fibrosis results from uncontrolled ECM synthesis, a process in which, transforming growth factor-beta (TGFβ) plays a pivotal role. So far, the mechanisms involved in adipose tissue (AT) re-arrangement, and the role of TGFβ in inducing AT remodeling in weight-losing cancer patients are poorly understood. This study examined the modulation of ECM components mediated by TGFβ pathway in fibrotic AT obtained from cachectic gastrointestinal cancer patients.MethodsAfter signing the informed consent form, patients were enrolled into the following groups: cancer cachexia (CC, n = 21), weight-stable cancer (WSC, n = 17), and control (n = 21). The total amount of collagen and elastic fibers in the subcutaneous AT was assessed by histological analysis and by immunohistochemistry. TGFβ isoforms expression was analyzed by Multiplex assay and by immunohistochemistry. Alpha-smooth muscle actin (αSMA), fibroblast-specific protein (FSP1), Smad3 and 4 were quantified by qPCR and/or by immunohistochemistry. Interleukin (IL) 2, IL5, IL8, IL13 and IL17 content, cytokines known to be associated with fibrosis, was measured by Multiplex assay.ResultsThere was an accumulation of collagen and elastic fibers in the AT of CC, as compared with WSC and controls. Collagens type I, III, VI, and fibronectin expression was enhanced in the tissue of CC, compared with both WSC and control. The pronounced expression of αSMA in the surrounding of adipocytes, and the increased mRNA content for FSP1 (20-fold) indicate the presence of activated myofibroblasts; particularly in CC. TGFβ1 and TGFβ3 levels were up-regulated by cachexia in AT, as well in the isolated adipocytes. Smad3 and Smad4 labeling was found to be more evident in the fibrotic areas of CC adipose tissue.ConclusionsCancer cachexia promotes the development of AT fibrosis, in association with altered TGFβ signaling, compromising AT organization and function.

Forecasting Fat Fibrosis

Excess ECM and fibrosis of white adipose tissue (WAT) is associated with tissue dysfunction and type 2 diabetes. In this issue of Cell Metabolism, Marcelin etal. (2017) elucidate a key mechanism behind WAT fibrosis in which the activation of PDGFRα on adipocyte precursors drives this population toward a fibrotic phenotype.

A PDGFRα-Mediated Switch toward CD9high Adipocyte Progenitors Controls Obesity-Induced Adipose Tissue Fibrosis

Obesity-induced white adipose tissue (WAT) fibrosis is believed to accelerate WAT dysfunction. However, the cellular origin of WAT fibrosis remains unclear. Here, we show that adipocyte platelet-derived growth factor receptor-α-positive (PDGFRα+) progenitors adopt a fibrogenic phenotype in obese mice prone to visceral WAT fibrosis. More specifically, a subset of PDGFRα+ cells with high CD9 expression (CD9high) originates pro-fibrotic cells whereas their CD9low counterparts, committed to adipogenesis, are almost completely lost in the fibrotic WAT. PDGFRα pathway activation promotes a phenotypic shift toward PDGFRα+CD9high fibrogenic cells, driving pathological remodeling and altering WAT function in obesity. These findings translated to human obesity as the frequency of CD9high progenitors in omental WAT (oWAT) correlates with oWAT fibrosis level, insulin-resistance severity, and type 2 diabetes. Collectively, our data demonstrate that in addition to representing a WAT adipogenic niche, different PDGFRα+ cell subsets modulate obesity-induced WAT fibrogenesis and are associated with loss of metabolic fitness.

Increased fibrosis and angiogenesis in subcutaneous gluteal adipose tissue in nascent metabolic syndrome

AimsMetabolic syndrome (MetS) is globally a common disorder that predisposes to both diabetes and cardiovascular disease (CVD). There is a paucity of data on fibrosis and angiogenesis in adipose tissue (AT) in patients with nascent MetS uncomplicated by diabetes or CVD. Hence, we assayed various indices of fibrosis and angiogenesis in subcutaneous AT (SAT). MethodsIn both patients with MetS and matched controls, we determined fibrosis and the densities of CD31, VEGF and Angiopoietin (Angio) 2 and 1 by immunohistochemistry in gluteal SAT. ResultsThe fibrosis score was significantly increased in SAT of Met S. Also, both CD31 and VEGF densities were significantly increased. Surprisingly, Angio-2 was not increased and the ratio of Angio2:1 was decreased. Both indices of fibrosis and angiogenesis correlated with biomediators of inflammation. ConclusionsIn conclusion, we report increased fibrosis and paradoxical increased angiogenesis in gluteal SAT and speculate that the increased angiogenesis is a protective mechanism in mitigating further adipose tissue dysregulation in this depot.

PDGFRα controls the balance of stromal and adipogenic cells during adipose tissue organogenesis.

Adipose tissue is distributed in depots throughout the body with specialized roles in energy storage and thermogenesis. PDGFRα is a marker of adipocyte precursors, and increased PDGFRα activity causes adipose tissue fibrosis in adult mice. However, the function of PDGFRα during adipose tissue organogenesis is unknown. Here, by analyzing mice with juxtamembrane or kinase domain point mutations that increase PDGFRα activity (V561D or D842V), we found that PDGFRα activation inhibits embryonic white adipose tissue organogenesis in a tissue-autonomous manner. By lineage tracing analysis, we also found that collagen-expressing precursor fibroblasts differentiate into white adipocytes in the embryo. PDGFRα inhibited the formation of adipocytes from these precursors while favoring the formation of stromal fibroblasts. This imbalance between adipocytes and stromal cells was accompanied by overexpression of the cell fate regulator Zfp521. PDGFRα activation also inhibited the formation of juvenile beige adipocytes in the inguinal fat pad. Our data highlight the importance of balancing stromal versus adipogenic cell expansion during white adipose tissue development, with PDGFRα activity coordinating this crucial process in the embryo.

Adipose Tissue Hypoxia in Obesity and Its Impact on Preadipocytes and Macrophages: Hypoxia Hypothesis.

Obese subjects exhibit lower adipose tissue oxygen consumption in accordance with the lower adipose tissue blood flow. Thus, compared with lean subjects, obese subjects have 44% lower capillary density and 58% lower vascular endothelial growth factor (VEGF). The VEGF expression together with hypoxia-inducible transcription factor-1 (HIF-1) activity also requires phosphatidylinositol 3-kinase (PI3K)- and target of rapamycin (TOR)-mediated signaling. HIF-1alpha is an important signaling molecule for hypoxia to induce the inflammatory responses. Hypoxia affects a number of biological functions, such as angiogenesis, cell proliferation, apoptosis, inflammation and insulin resistance. Additionally, reactive oxygen radical (ROS) generation at mitochondria is responsible for propagation of the hypoxic signal. Actually mitochondrial ROS (mtROS) production, but not oxygen consumption is required for hypoxic HIF-1alpha protein stabilization. Adipocyte mitochondrial oxidative capacity is reduced in obese compared with non-obese adults. In this respect, mitochondrial dysfunction of adipocyte is associated with the overall adiposity. Furthermore, hypoxia also inhibits macrophage migration from the hypoxic adipose tissue. Alterations in oxygen availability of adipose tissue directly affect the macrophage polarization and are responsible from dysregulated adipocytokines production in obesity. Hypoxia also inhibits adipocyte differentiation from preadipocytes. In addition to stressed adipocytes, hypoxia contributes to immune cell immigration and activation which further aggravates adipose tissue fibrosis. Fibrosis is initiated in response to adipocyte hypertrophy in obesity.

Fibrosis in adipose tissue is correlated to liver injury in NAFLD

Fibrosis in adipose tissue is correlated to liver injury in NAFLD

Evolution of subcutaneous adipose tissue fibrosis after bariatric surgery

AimObesity is associated with the development of metabolic complications such as insulin resistance (IR). The mechanisms leading to IR remain unclear. This study aimed to investigate the relationship between adipose tissue fibrosis and IR in obese patients before and after bariatric surgery. MethodsThirty-five obese patients awaiting bariatric surgery (12 with type 2 diabetes) were included in the study. Non-diabetic patients were classified as either insulin-sensitive (n=11) or insulin-resistant (n=12), based on the Matsuda insulin sensitivity index (ISIMatsuda). Homoeostasis model assessment (HOMA-IR) was used for longitudinal evaluation of insulin resistance. Fibrosis was quantified by Masson's trichrome staining on microscopy, and mRNA levels of fibrosis-related genes were examined in subcutaneous (SAT) and visceral adipose tissue (VAT) biopsies collected during and 6 months after bariatric surgery (SAT only). ResultsDespite their similar age, body mass index and fat mass, SAT fibrosis was significantly higher in diabetic vs insulin-sensitive patients (P<0.05), and associated with IR as assessed by both ISIMatsuda (r=−0.417, P=0.038) and HOMA-IR (r=0.464, P=0.007) at baseline, whereas VAT fibrosis was not. Six months after surgery and significant weight loss, fibrosis levels remained unchanged in SAT, although IR was significantly reduced in all groups (P<0.0001). No correlation was found between SAT fibrosis and IR after surgery. ConclusionOverall, these results show a significant but, most likely, transient association between SAT fibrosis and IR in obese humans.

Macrophage Proliferation Sustains Adipose Tissue Inflammation in Formerly Obese Mice.

Obesity causes dramatic proinflammatory changes in the adipose tissue immune environment, but relatively little is known regarding how this inflammation responds to weight loss (WL). To understand the mechanisms by which meta-inflammation resolves during WL, we examined adipose tissue leukocytes in mice after withdrawal of a high-fat diet. After 8 weeks of WL, mice achieved similar weights and glucose tolerance values as age-matched lean controls but showed abnormal insulin tolerance. Despite fat mass normalization, total and CD11c+ adipose tissue macrophage (ATM) content remained elevated in WL mice for up to 6 months and was associated with persistent fibrosis in adipose tissue. ATMs in formerly obese mice demonstrated a proinflammatory profile, including elevated expression of interferon-γ, tumor necrosis factor-α, and interleukin-1β. T-cell–deficient Rag1−/− mice showed a degree of ATM persistence similar to that in WT mice, but with reduced inflammatory gene expression. ATM proliferation was identified as the predominant mechanism by which ATMs are retained in adipose tissue with WL. Our study suggests that WL does not completely resolve obesity-induced ATM activation, which may contribute to the persistent adipose tissue damage and reduced insulin sensitivity observed in formerly obese mice.

Relevance of omental pericellular adipose tissue collagen in the pathophysiology of human abdominal obesity and related cardiometabolic risk

Adipose tissue fibrosis is a relatively new notion and its relationship with visceral obesity and cardiometabolic alterations remains unclear, particularly in moderate obesity. Our objective was to examine if total and pericellular collagen accumulation are relevant for the pathophysiology of visceral obesity and related cardiometabolic risk. Surgical omental (OM) and subcutaneous (SC) fat samples were obtained in 56 women (age: 47.2±5.8 years; body mass index (BMI): 27.1±4.4 kg/m2). Body composition and fat distribution were measured by dual-energy X-ray absorptiometry and computed tomography, respectively. Total and pericellular collagen were measured using picrosirius red staining. CD68+ cells (total macrophages) and CD163+ cells (M2-macrophages) were identified using immunohistochemistry. We found that only pericellular collagen percentage, especially in OM fat, was associated with higher BMI, body fat mass and adipose tissue areas as well as lower radiologic attenuation of visceral adipose tissue and altered cardiometabolic risk variables. Strong correlations between peri-adipocyte collagen percentage and total or M2-macrophage percentages were observed in both depots. Total collagen percentage in either compartment was not related to adiposity, fat distribution or cardiometabolic risk. As opposed to whole tissue-based assessments of adipose tissue fibrosis, collagen deposition around the adipocyte, especially in the OM fat compartment is related to total and regional adiposity as well as altered cardiometabolic risk profile.

Advanced non-alcoholic fatty liver disease and adipose tissue fibrosis in patients with Alström syndrome.

Alström syndrome (AS) is a recessive monogenic syndrome characterized by obesity, extreme insulin resistance and multi-organ fibrosis. Despite phenotypically being high risk of non-alcoholic fatty liver disease (NAFLD), there is a lack of data on the extent of fibrosis in the liver and its close links to adipose in patients with AS. Our aim was to characterize the hepatic and adipose phenotype in patients with AS. Observational cohort study with comprehensive assessment of metabolic liver phenotype including liver elastography (Fibroscan® ), serum Enhanced Liver Fibrosis (ELF) Panel and liver histology. In addition, abdominal adipose histology and gene expression was assessed. We recruited 30 patients from the UK national AS clinic. A subset of six patients underwent adipose biopsies which was compared with control tissue from nine healthy participants. Patients were overweight/obese (BMI 29.3 (25.95-34.05) kg/m2 ). A total of 80% (24/30) were diabetic; 74% (20/27) had liver ultrasound scanning suggestive of NAFLD. As judged by the ELF panel, 96% (24/25) were categorized as having fibrosis and 10/21 (48%) had liver elastography consistent with advanced liver fibrosis/cirrhosis. In 7/8 selected cases, there was evidence of advanced NAFLD on liver histology. Adipose tissue histology showed marked fibrosis as well as disordered pro-inflammatory and fibrotic gene expression profiles. NAFLD and adipose dysfunction are common in patients with AS. The severity of liver disease in our cohort supports the need for screening of liver fibrosis in AS.

Nitric Oxide Produced by Macrophages Inhibits Adipocyte Differentiation and Promotes Profibrogenic Responses in Preadipocytes to Induce Adipose Tissue Fibrosis.

Fibrosis of adipose tissue induces ectopic fat accumulation and insulin resistance by inhibiting adipose tissue expandability. Mechanisms responsible for the induction of adipose tissue fibrosis may provide therapeutic targets but are poorly understood. In this study, high-fat diet (HFD)-fed wild-type (WT) and iNOS(-/-) mice were used to examine the relationship between nitric oxide (NO) produced by macrophages and adipose tissue fibrosis. In contrast to WT mice, iNOS(-/-) mice fed an HFD were protected from infiltration of proinflammatory macrophages and adipose tissue fibrosis. Hypoxia-inducible factor 1α (HIF-1α) protein level was increased in adipose tissue of HFD-fed WT mice, but not iNOS(-/-) mice. In contrast, the expression of mitochondrial biogenesis factors was decreased in HFD-fed WT mice, but not iNOS(-/-) mice. In studies with cultured cells, macrophage-derived NO decreased the expression of mitochondrial biogenesis factors, and increased HIF-1α protein level, DNA damage, and phosphorylated p53 in preadipocytes. By activating p53 signaling, NO suppressed peroxisome proliferator-activated receptor γ coactivator 1α expression, which induced mitochondrial dysfunction and inhibited preadipocyte differentiation in adipocytes. The effects of NO were blocked by rosiglitazone. The findings suggest that NO produced by macrophages induces mitochondrial dysfunction in preadipocytes by activating p53 signaling, which in turn increases HIF-1α protein level and promotes a profibrogenic response in preadipocytes that results in adipose tissue fibrosis.

Necroptosis DAMPens anti-tumor immunity.

Necroptosis DAMPens anti-tumor immunity.

The role of metformin and resveratrol in the prevention of hypoxia-inducible factor 1α accumulation and fibrosis in hypoxic adipose tissue.

Hypoxic activation of hypoxia-inducible factor 1α (HIF-1α) and fibrosis in adipose tissue contribute to adipose dysfunction. This study was designed to investigate the effects of metformin and resveratrol on the regulation of HIF-1α and fibrosis in hypoxic adipose tissue. Mice were fed a high-fat diet to induce hypoxia and fibrosis in adipose tissue; adipose tissue incubated in vitro in 1% O2 showed a similar change. The effects of metformin and resveratrol on hypoxia, HIF-1α accumulation, endoplasmic reticulum stress and gene expressions of extracellular matrix components and pro-inflammatory cytokines were examined. Oral administration of metformin or resveratrol prevented hypoxia and reduced HIF-1α accumulation with dephosphorylation of inositol-requiring enzyme 1α and eukaryotic initiation factor 2α, indicative of suppression of hypoxic HIF-1α activation and endoplasmic reticulum stress. Metformin and resveratrol down-regulated gene expressions of Col3α, Col6α, elastin and lysyl oxidase and thereby reduced collagen deposition in adipose tissue. The increased gene expressions of TNF-α, IL-6, monocyte chemoattractant protein 1 and F4/80 were also down-regulated by metformin and resveratrol. Metformin and resveratrol had similar effects in adipose tissue exposed to 1% O2 . Metformin reduced ATP production and prevented the reduction in oxygen tension in 3T3-L1 cells, suggesting that it prevented hypoxia by limiting oxygen consumption, whereas resveratrol reduced HIF-1α accumulation by promoting its proteasomal degradation via the regulation of AMPK/SIRT1. Hypoxia and fibrosis are early causes of adipose dysfunction in obesity. Both metformin and resveratrol effectively inhibited HIF-1α activation-induced fibrosis and inflammation in adipose tissue, although by different mechanisms.

AMPK Activation by Metformin Suppresses Abnormal Extracellular Matrix Remodeling in Adipose Tissue and Ameliorates Insulin Resistance in Obesity.

Fibrosis is emerging as a hallmark of metabolically dysregulated white adipose tissue (WAT) in obesity. Although adipose tissue fibrosis impairs adipocyte plasticity, little is known about how aberrant extracellular matrix (ECM) remodeling of WAT is initiated during the development of obesity. Here we show that treatment with the antidiabetic drug metformin inhibits excessive ECM deposition in WAT of ob/ob mice and mice with diet-induced obesity, as evidenced by decreased collagen deposition surrounding adipocytes and expression of fibrotic genes including the collagen cross-linking regulator LOX. Inhibition of interstitial fibrosis by metformin is likely attributable to the activation of AMPK and the suppression of transforming growth factor-β1 (TGF-β1)/Smad3 signaling, leading to enhanced systemic insulin sensitivity. The ability of metformin to repress TGF-β1-induced fibrogenesis is abolished by the dominant negative AMPK in primary cells from the stromal vascular fraction. TGF-β1-induced insulin resistance is suppressed by AMPK agonists and the constitutively active AMPK in 3T3L1 adipocytes. In omental fat depots of obese humans, interstitial fibrosis is also associated with AMPK inactivation, TGF-β1/Smad3 induction, aberrant ECM production, myofibroblast activation, and adipocyte apoptosis. Collectively, integrated AMPK activation and TGF-β1/Smad3 inhibition may provide a potential therapeutic approach to maintain ECM flexibility and combat chronically uncontrolled adipose tissue expansion in obesity.

Persistence of inflammatory adipose tissue immune cells despite weight loss in obese mice

Abstract Obesity causes dramatic changes to the adipose tissue immune environment including the recruitment and activation of macrophages. The inflammatory polarization of these cells is thought to play a critical role in the severity of insulin resistance associated with obesity. Weight loss can improve insulin resistance, but little is known concerning the effects of weight loss on immune activity in adipose. Diet switch from a 60% high-fat diet to 13% normal-fat diet was used to induce weight loss in diet-induced obese C57Bl/6 mice. Adipose tissue structural changes were evaluated by histology. Leukocyte populations were analyzed by flow cytometry, including sorting and comparing cytokine expression of adipose tissue macrophages before and after weight loss. Weight loss improved glucose tolerance and reduced serum insulin. However, extensive adipose fibrosis developed during weight loss and was associated with macrophage accumulation. Analysis of adipose macrophages after weight loss showed they retained an inflammatory gene expression profile similar to those found during obesity. These macrophages were derived primarily from older cells present during obesity. These macrophages were maintained though high proliferation and low apoptosis. The data indicate that inflammatory activation of adipose macrophages is not sufficient for whole body metabolic dysfunction, but the retained macrophages may promote adipose tissue fibrosis while impacting adipocyte health and function.