Metabolic dysregulation and adipose tissue fibrosis: role of collagen VI. Mol Cell Biol. 2009;29:1575-1591. [PMID: 19114551 DOI: 10.1128/mcb.01300-08]

Insulin resistance in the defense against obesity

In the face of the current obesity epidemic, the nature of the relationship between overnutrition and type 2 diabetes is of great importance. Obesity can be considered a state of excessive insulin action that elicits a series of cellular homeostatic responses, producing systemic insulin resistance. These responses occur in four steps: homologous desensitization to insulin action, leptin secretion, inflammation, and, finally, a counter-inflammatory phase that serves to conserve energy storage. The molecular mechanisms underlying these steps are discussed in the context of potential new therapeutic approaches.

Structural and biochemical characteristics of various white adipose tissue depots

It is now widely accepted that white adipose tissue (WAT) is not merely a fuel storage organ, but also a key component of metabolic homoeostatic mechanisms. Apart from its major role in lipid and glucose metabolism, adipose tissue is also involved in a wide array of other biological processes. The hormones and adipokines, as well as other biologically active agents released from fat cells, affect many physiological and pathological processes. WAT is neither uniform nor inflexible because it undergoes constant remodelling, adapting the size and number of adipocytes to changes in nutrients' availability and hormonal milieu. Fat depots from different areas of the body display distinct structural and functional properties and have disparate roles in pathology. The two major types of WAT are visceral fat, localized within the abdominal cavity and mediastinum, and subcutaneous fat in the hypodermis. Visceral obesity correlates with increased risk of insulin resistance and cardiovascular diseases, while increase of subcutaneous fat is associated with favourable plasma lipid profiles. Visceral adipocytes show higher lipogenic and lipolytic activities and produce more pro-inflammatory cytokines, while subcutaneous adipocytes are the main source of leptin and adiponectin. Moreover, adipose tissue associated with skeletal muscles (intramyocellular and intermuscular fat) and with the epicardium is believed to provide fuels for skeletal and cardiac muscle contraction. However, increased mass of either epicardial or intermuscular adipose tissue correlates with cardiovascular risk, while the presence of the intramyocellular fat is a risk factor for the development of insulin resistance. This review summarizes results of mainly human studies related to the differential characteristics of various WAT depots.

Adipose tissue biglycan as a potential anti-inflammatory target of sodium salicylate in mice fed a high fat diet

Background

Inflammation in adipose tissue (AT) during obesity causes impaired AT function. Although multiple extracellular matrix (ECM) proteins are expressed in AT their potential role in adipose tissue inflammation is unclear. Biglycan, a pro-inflammatory ECM gene, is highly enriched in adipose tissue. However, whether it is correlated with adipose tissue inflammation is unknown. We provide evidence in support of a strong association between biglycan expression and inflammatory status of adipose tissue.

Methods

C57BL6 mice were fed either a control (10% fat calories) or a high fat diet (HFD) (60% fat calories) for 8 weeks. Adipose tissue was analyzed for the expression of biglycan, IL-6 and TNFα. Biglycan knockout or wild type were also fed a high fat diet for 8 weeks and the expression of inflammatory genes in the mesenteric adipose tissue was examined. To test anti-inflammatory treatment on biglycan expression, a group of mice were fed either the low fat or high fat diet for eight weeks supplemented with either saline or sodium salicylate @ 25mg/100ml in their drinking water.

Results

Mice on HFD had an increase in ECM genes (BGN and COL1A1), inflammatory genes (IL-6 and TNFα) in both the subcutaneous and epididymal depots. However, correlation analysis only shows a positive correlation between biglycan, IL-6 and TNFα expression. In addition, lower expression of IL-6 and CD68 was found in the mesenteric adipose tissue of biglycan knockout mice compared to the wild type. Sodium salicylate treatment reduced subcutaneous adipose tissue expression of BGN, COL1A1, and COL6A1 and a concurrent downregulation of TNFα and IL-6 and TLR4 expression. Salicylate also lowered the serum TGFβ1 levels.

Conclusion

Biglycan expression correlates with adipose tissue inflammation, especially in the subcutaneous depot compared to the epididymal depot. This is supported by the greater effect of sodium salicylate in attenuating both inflammatory and ECM gene expression the subcutaneous adipose depot compared to the epididymal depot. These results show that inflammatory state may explain the induction of biglycan, and perhaps, other ECM genes in adipose tissue.

Peri-adipocyte ECM remodeling in obesity and adipose tissue fibrosis

Adipocytes differentiate and function in environments rich in extracellular matrix (ECM) proteins. The phenotypes of genetically modified mice have aided in recognizing the importance of ECM proteins and their modifiers, e.g., proteinases, in the regulation of obesity and metabolism. Most of the molecular mechanisms through which ECM proteins and modifiers regulate adipogenesis or adipocyte function have not been fully defined. Adipose tissue fibrosis may be a factor that links obesity to diabetes or cardiovascular disease risk in conjunction with tissue inflammation. Defining the molecular mechanisms through which the ECM environment regulates adipogenesis and adipocyte function should provide us with a better understanding of the disease link between obesity and diabetes or cardiovascular diseases.

ITIH-5 expression in human adipose tissue is increased in obesity

Adipocytes secrete many proteins that regulate metabolic functions. The gene inter-α (globulin) inhibitor H5 (ITIH-5) encodes a secreted protein and is known to be expressed abundantly in the placenta. However, using gene expression profiles data we observed high expression of ITIH-5 in adipose tissue. The aim of this study was to test the hypothesis that ITIH-5 is strongly expressed in human adipocytes and adipose tissue, and is related to obesity and clinical metabolic variables. ITIH-5 adipose tissue mRNA expression was analyzed with DNA microarray and real-time PCR, and its association with clinical variables was examined. ITIH-5 protein expression was analyzed using western blot. ITIH-5 mRNA expression was abundant in human adipose tissue, adipocytes, and placenta, and higher in subcutaneous (sc) compared to omental adipose tissue (P < 0.0001). ITIH-5 mRNA and protein expression in sc adipose tissue were higher in obese compared to lean subjects (P < 0.0001 and P < 0.001, respectively). ITIH-5 mRNA expression was reduced after diet-induced weight loss (P < 0.0001). ITIH-5 mRNA expression was associated with anthropometry and clinical metabolic variables. In conclusion, ITIH-5 is highly expressed in sc adipose tissue, increased in obesity, down regulated after weight loss, and associated with measures of body size and metabolism. Together, this indicates that ITIH-5 merits further investigation as a regulator of human metabolism.

Absence of type VI collagen paradoxically improves cardiac function, structure, and remodeling after myocardial infarction

RATIONALE

We previously reported that type VI collagen deposition increases in the infarcted myocardium in vivo. To date, a specific role for this nonfibrillar collagen has not been explored in the setting of myocardial infarction (MI).

OBJECTIVE

To determine whether deletion of type VI collagen in an in vivo model of post-MI wound healing would alter cardiac function and remodeling in the days to weeks after injury.

METHODS AND RESULTS

Wild-type and Col6a1(-/-) mice were subjected to MI, followed by serial echocardiographic and histological assessments. At 8 weeks after MI, infarct size was significantly reduced, ejection fraction was significantly preserved (43.9% ± 3.3% versus 29.1% ± 4.3% for wild-type), and left ventricular chamber dilation was attenuated in the Col6a1(-/-) MI group (25.8% ± 7.9% increase versus 62.6% ± 16.5% for wild-type). The improvement in cardiac remodeling was evident as early as 10 days after MI in the Col6a1(-/-) mice. Myocyte apoptosis within the infarcted zones was initially greater in the Col6a1(-/-) group 3 days after MI, but by day 14 this was significantly reduced. Collagen deposition also was reduced in the infarcted and remote areas of the Col6a1(-/-) hearts. The reductions in chronic myocyte apoptosis and fibrosis are critical events leading to improved long-term remodeling and functional outcomes.

CONCLUSIONS

These unexpected results demonstrate for the first time that deletion of type VI collagen in this knockout model plays a critical protective role after MI by limiting infarct size, chronic apoptosis, aberrant remodeling, and fibrosis, leading to preservation of cardiac function.

TGFbeta family members are key mediators in the induction of myofibroblast phenotype of human adipose tissue progenitor cells by macrophages

Objective

The present study was undertaken to characterize the remodeling phenotype of human adipose tissue (AT) macrophages (ATM) and to analyze their paracrine effects on AT progenitor cells.

Research Design and Methods

The phenotype of ATM, immunoselected from subcutaneous (Sc) AT originating from subjects with wide range of body mass index and from paired biopsies of Sc and omental (Om) AT from obese subjects, was studied by gene expression analysis in the native and activated states. The paracrine effects of ScATM on the phenotype of human ScAT progenitor cells (CD34⁺CD31⁻) were investigated.

Results

Two main ATM phenotypes were distinguished based on gene expression profiles. For ScAT-derived ATM, obesity and adipocyte-derived factors favored a pro-fibrotic/remodeling phenotype whereas the OmAT location and hypoxic culture conditions favored a pro-angiogenic phenotype. Treatment of native human ScAT progenitor cells with ScATM-conditioned media induced the appearance of myofibroblast-like cells as shown by expression of both α-SMA and the transcription factor SNAIL, an effect mimicked by TGFβ1 and activinA. Immunohistochemical analyses showed the presence of double positive α-SMA and CD34 cells in the stroma of human ScAT. Moreover, the mRNA levels of SNAIL and SLUG in ScAT progenitor cells were higher in obese compared with lean subjects.

Conclusions

Human ATM exhibit distinct pro-angiogenic and matrix remodeling/fibrotic phenotypes according to the adiposity and the location of AT, that may be related to AT microenvironment including hypoxia and adipokines. Moreover, human ScAT progenitor cells have been identified as target cells for ScATM-derived TGFβ and as a potential source of fibrosis through their induction of myofibroblast-like cells.

Inflammasome activation in obesity-related inflammatory diseases and autoimmunity

The inflammasome is a highly regulated protein complex that triggers caspase-1 activation and subsequent secretion of IL-1β and IL-18. Recognition of microbial components and danger signals by NOD-like receptor (NLR) family members in the cytosol promotes inflammasome activation and downstream inflammatory cytokine production. Pathogen recognition by NLRs and downstream release of inflammasome-derived cytokines are important in host defense against numerous infections. Recent studies have also identified a unique role for inflammasome regulation in the induction and pathogenesis of multiple autoimmune and inflammatory disorders. We now know that obesity-related factors and endogenous markers of cellular stress can lead to unchecked activation of the inflammasome and provoke inflammation and subsequent destruction of vital organs. This review will highlight recent findings that link inflammasome signaling to the progression of autoinflammatory and autoimmune diseases. We will focus on the contribution of inflammasome activation to the pathogenesis of autoinflammatory and autoimmune diseases that are of major significance to human health including type 2 diabetes, atherosclerosis, multiple sclerosis, and type 1 diabetes.

ECM roles in the function of metabolic tissues

All metazoan cells produce and/or interact with tissue-specific extracellular matrices (ECMs). Such ECMs play important structural roles not only in connective tissues, but in all tissues in which they provide support and anchorage for cells. However, in addition to such structural roles it has become increasingly clear that the tissue-specific microenvironments formed by the ECM play instructional roles that inform the proper phenotypes and functional behaviors of specialized cell types, and recent in vivo and in vitro studies suggest that ECM components also affect metabolic function. This review summarizes data that provide insights into the roles of the ECM in informing the proper development and functioning of highly specialized cells of metabolic tissues, such as adipocytes and islet β cells.

Proteomics in the characterization of adipose dysfunction in obesity

Adipose tissue plays a central role in body weight homeostasis, inflammation, and insulin resistance via serving as a fat-buffering system, regulating lipid storage and mobilization and releasing a large range of adipokines and cytokines. Adipose tissue is also the major inflammation-initiated site in obesity. Adipose-derived adipokines and cytokines are known to be involved in the modulation of a wide range of important physiological processes, particularly immune response, glucose and lipid homeostasis and insulin resistance. Adipose tissue dysfunction, characterized by an imbalanced secretion of pro- and anti-inflammatory adipokines and cytokines, decreased insulin-stimulated glucose uptake, dysregulation of lipid storage and release and mitochondrial dysfunction, has been linked to obesity and its associated metabolic disorders. Proteomic technology has been a powerful tool for identifying key components of the adipose proteome, which may contribute to the pathogenesis of adipose tissue dysfunction in obesity. In this review, we summarized the recent advances in the proteomic characterization of adipose tissue and discussed the identified proteins that potentially play important roles in insulin resistance and lipid homeostasis.

Adipose tissue extracellular matrix and vascular abnormalities in obesity and insulin resistance

CONTEXT

Insulin resistance is associated with inflammation, fibrosis, and hypoxia in adipose tissue.

OBJECTIVE

This study was intended to better characterize the extracellular matrix (ECM) and vascularity of insulin-resistant adipose tissue.

DESIGN

Adipose expression of collagens, elastin, and angiogenic factors was assessed using real-time RT-PCR and immunohistochemistry (IHC) in abdominal sc adipose tissue. Adipocyte-macrophage coculture experiments examined the effects of polarized macrophages on adipose ECM gene expression, and the effects of collagens were measured in an angiogenesis assay.

PARTICIPANTS AND SETTING

A total of 74 nondiabetic subjects participated at a University Clinical Research Center.

INTERVENTIONS

Interventions included baseline adipose biopsy and measurement of insulin sensitivity.

MAIN OUTCOME MEASURES

Outcome measures included characterization of vascularity and ECM in adipose tissue.

RESULTS

CD31 (an endothelial marker) mRNA showed no significant correlation with body mass index or insulin sensitivity. In a subgroup of 17 subjects (nine obese, eight lean), CD31-positive capillary number in obese was decreased by 58%, whereas larger vessels were increased by 70%, accounting for the lack of change in CD31 expression with obesity. Using IHC, obese (compared with lean) subjects had decreased elastin and increased collagen V expression, and adipocytes cocultured with M2 macrophages had reduced elastin and increased collagen V expression. In obese subjects, collagen V was colocalized with large blood vessels, and the addition of collagen V to an angiogenesis assay inhibited endothelial budding.

CONCLUSIONS

The adipose tissue from obese/insulin-resistant subjects has fewer capillaries and more large vessels as compared with lean subjects. The ECM of adipose tissue may play an important role in regulating the expandability as well as angiogenesis of adipose tissue.

Macrophage-induced expression and release of matrix metalloproteinase 1 and 3 by human preadipocytes is mediated by IL-1β via activation of MAPK signaling

Obesity is associated with a chronic low-grade inflammation and increased macrophage infiltration in adipose tissue. Matrix metalloproteinases (MMPs) are involved in adipose tissue remodeling and inflammatory responses in obesity. This study investigated whether macrophage-derived factors modulate expression and secretion of MMP1 and MMP3 in human preadipocytes. The potential mediators and signaling pathways were also explored. MMP1 and MMP3 were primarily expressed and secreted by preadipocytes and dramatically reduced post-differentiation. Preadipocytes were incubated with RPMI 1640 medium (control) or THP-1 macrophage-conditioned (MC) medium (25% and 100%) for 24 h. MC medium markedly increased mRNA levels of MMP1 (up to 122-fold) and MMP3 (up to 59-fold), as well as protein release of MMP1 (up to 378-fold) and MMP3 (up to 10-fold) in a dose-dependent manner. Treatment with IL-1β or TNFα, the major products of macrophages, also induced MMP1 and MMP3 secretion by preadipocytes. Neutralizing IL-1β abolished the induction of MMP1 and MMP3 in preadipocytes by MC medium while the effects of TNFα neutralization were modest. Furthermore, MC medium or IL-1β led to the phosphorylation of p38, ERK and JNK MAPKs. Inhibition of p38, ERK and JNK reversed the stimulatory effects of MC or IL-1β on MMP1 and MMP3 production. MC medium and IL-1β also activated NF-κB p65 whereas reduced IκBα protein expression in preadipocytes. These results suggest that macrophage accumulation in adipose tissue has a central role in stimulating MMP1 and MMP3 production by preadipocytes, and this is partially mediated by IL-1β via activation of the MAPK and NF-κB signaling pathways.

Quantitative proteomic analysis of the adipocyte plasma membrane

The adipocyte is a key regulator of mammalian metabolism. To advance our understanding of this important cell, we have used quantitative proteomics to define the protein composition of the adipocyte plasma membrane (PM) in the presence and absence of insulin. Using this approach, we have identified a high confidence list of 486 PM proteins, 52 of which potentially represent novel cell surface proteins, including a member of the adiponectin receptor family and an unusually high number of hydrolases with no known function. Several novel insulin-responsive proteins including the sodium/hydrogen exchanger, NHE6 and the collagens III and VI were also identified, and we provide evidence of PM-ER association suggestive of a unique functional association between these two organelles in the adipocyte. Together these studies provide a wealth of potential therapeutic targets for the manipulation of adipocyte function and a valuable resource for metabolic research and PM biology.

Adiponectin regulation of stellate cell activation via PPARγ-dependent and -independent mechanisms

In this study, we elucidated the mechanism by which adiponectin modulates hepatic stellate cell activation and fibrogenesis. Adiponectin-overexpressing transgenic mice receiving thioacetamide were resistant to fibrosis, compared with controls. In contrast, adiponectin-null animals developed severe fibrosis. Expression of collagen α1(I) and α-smooth muscle actin (α-SMA) mRNAs were significantly lower in adiponectin-overexpressing mice, compared with controls. In wild-type stellate cells exposed to a lentivirus encoding adiponectin, expression of peroxisome proliferator-activated receptor-γ (PPARγ), SREBP1c, and CEBPα mRNAs was significantly increased (3.2-, 4.1-, and 2.2-fold, respectively; n = 3; P < 0.05, adiponectin virus versus control), consistent with possible activation of an adipogenic transcriptional program. Troglitazone, a PPARγ agonist, strongly suppressed up-regulation of collagen α1(I) and α-SMA mRNA in stellate cells isolated from wild-type mice; however, stellate cells from adiponectin-null animals failed to respond to troglitazone. Furthermore, in isolated stellate cells in which PPARγ was depleted using an adenovirus-Cre-recombinase system and in which adiponectin was also overexpressed, collagen α1(I) and α-SMA were significantly inhibited. We conclude that the PPARγ effect on stellate cell activation and the fibrogenic cascade appears to be adiponectin-dependent; however, the inhibitory effect of adiponectin on stellate cell activation was not dependent on PPARγ, suggesting the presence of PPARγ-dependent as well as independent pathways in stellate cells.

Melanocortin 4 receptor-deficient mice as a novel mouse model of nonalcoholic steatohepatitis

Obesity may be viewed as a state of chronic low-grade inflammation that participates in the development of the metabolic syndrome. Nonalcoholic steatohepatitis (NASH) is considered a hepatic phenotype of the metabolic syndrome and a high risk for progression to cirrhosis and hepatocellular carcinoma. Although the "two hit" hypothesis suggests involvement of excessive hepatic lipid accumulation and chronic inflammation, the molecular mechanisms underlying the development of NASH remain unclear, in part because of lack of appropriate animal models. Herein we report that melanocortin 4 receptor-deficient mice (MC4R-KO) develop steatohepatitis when fed a high-fat diet, which is associated with obesity, insulin resistance, and dyslipidemia. Histologic analysis reveals inflammatory cell infiltration, hepatocyte ballooning, and pericellular fibrosis in the liver in MC4R-KO mice. Of note, all of the MC4R-KO mice examined developed well-differentiated hepatocellular carcinoma after being fed a high-fat diet for 1 year. They also demonstrated enhanced adipose tissue inflammation, ie, increased macrophage infiltration and fibrotic changes, which may contribute to excessive lipid accumulation and enhanced fibrosis in the liver. Thus, MC4R-KO mice provide a novel mouse model of NASH with which to investigate the sequence of events that make up diet-induced hepatic steatosis, liver fibrosis, and hepatocellular carcinoma and to aid in understanding the pathogenesis of NASH, pursuing specific biomarkers, and evaluating potential therapeutic strategies.

Regulation of pre-adipocyte proliferation and apoptosis by the small leucine-rich proteoglycans, biglycan and decorin

Evidence for a functional role for extracellular matrix (ECM) proteins in adipose tissue is demonstrated in dynamic changes in expression of ECM genes during adipocyte differentiation and in obesity. Components of the ECM may regulate adipose cell number expansion by restricting pre-adipocyte proliferation, regulating apoptosis and inhibiting adipogenesis. Although pre-adipocytes express multiple proteoglycans, their role in pre-adipocyte proliferation up to now has remained unknown. The study described here was conducted to characterize roles of small leucine-rich proteoglycans (SLRPs) in adipocyte proliferation. Pre-adipocytes were seeded on plates coated with biglycan and decorin and were allowed to differentiate. In addition, pre-adipocytes were incubated on plates coated with biglycan, decorin, or fibronectin and measurements were made of cell proliferation and apoptosis. We are able to report that SLRPs decorin and biglycan did not affect differentiation of our 3T3-L1 cells; however, biglycan and decorin did reduce proliferation of pre-adipocytes, partly by induction of apoptosis. Furthermore, anti-proliferative capabilities of decorin and biglycan were nullified with removal of GAG side-chains suggesting that the chains played key roles in anti-proliferative effects of the SLRPs. We also found that co-treatment of decorin or biglycan with the proteoglycan fibronectin restored normal proliferation, an indication that multiple ECM proteins may act in concert to regulate overall proliferation rates of pre-adipocytes. These studies indicate that SLRPs may compose a regulatory factor in adipose tissue expansion, through hyperplasia.

Association analysis of polymorphisms in lumican gene and systemic lupus erythematosus in a Taiwan Chinese Han population

OBJECTIVE

Lumican (LUM) is predominantly localized in areas of pathological fibrosis. To determine whether polymorphisms in LUM gene are associated with development of systemic lupus erythematosus (SLE), we analyzed 2 single-nucleotide polymorphisms (SNP) of LUM in a Taiwan Chinese Han population.

METHODS

Participants included 168 patients with SLE and 192 age-matched controls in whom examinations had excluded SLE. Genotyping of -628 A/-(rs17018757) and c.1567 T/C polymorphisms in LUM were carried out in each patient and control using the polymerase chain reaction-restriction fragment-length polymorphism method, and validated by Taqman SNP genotyping assay. Data were correlated with the development of SLE and various clinical symptoms by chi-square analysis.

RESULTS

Frequencies of C/C genotype and the C allele at c.1567 T/C were significantly higher in patients than controls. Polymorphism at c.1567 C/T was found to be associated with arthritis and photosensitivity in patients with SLE, which are both connective tissue-related symptoms.

CONCLUSION

The c.1567 T/C polymorphism of LUM is related to the development and clinical symptoms of SLE.

Microfibrillar-associated protein 5 is linked with markers of obesity-related extracellular matrix remodeling and inflammation

Objective:

Microfibrillar-associated protein 5 (MFAP5) is an extracellular matrix (ECM) glycoprotein, which is colocalized with microfibrils in elastin networks. Its function in adipose tissue (AT) is not known. We have recently shown that the expression of MFAP5 is downregulated in AT along with weight reduction (WR) in persons with metabolic syndrome (MetS). The aim of this work was to study whether the change of MFAP5 mRNA expression in response to WR is associated with markers of adiposity, glucose metabolism and insulin resistance in human AT.

Design:

Weight reduction intervention study in parallel study design (The Genobin study). Altogether 46 obese subjects with impaired glucose tolerance and features of MetS were randomized to a WR (n=28) or a control group (n=18) lasting for 33 weeks.

Measurements:

Circulating glucose and insulin concentrations were measured and subcutaneous AT biopsies were performed before and after the intervention. The mRNA expression was studied by quantitative real-time PCR (QPCR).

Results:

QPCR of human AT biopsy samples confirmed that MFAP5 is highly expressed in AT and its expression is decreased during WR. The mRNA expression of MFAP5 correlated positively with body mass index, and the change in MFAP5 mRNA expression during WR correlated positively with the change of body fat mass. Furthermore, the MFAP5 mRNA expression correlated negatively with circulating fasting concentrations of adiponectin and interleukin (IL)-1β and positively with leptin, insulin and IL-1Ra levels. In addition, the MFAP5 mRNA expression correlated positively with the mRNA expressions of peroxisome proliferator-activated receptor gamma, cyclin D2 and A disintegrin and metalloproteinase domain 12, genes involved in AT remodeling.

Conclusion:

This study demonstrates that MFAP5 is highly expressed in human AT and is correlated with markers of insulin resistance. Furthermore, it is possible that MFAP5 is related to ECM remodeling during development of obesity.

Paracrine and endocrine effects of adipose tissue on cancer development and progression

The past few years have provided substantial evidence for the vital role of the local tumor microenvironment for various aspects of tumor progression. With obesity and its pathophysiological sequelae still on the rise, the adipocyte is increasingly moving center stage in the context of tumor stroma-related studies. To date, we have limited insight into how the systemic metabolic changes associated with obesity and the concomitant modification of the paracrine and endocrine panel of stromal adipocyte-derived secretory products ("adipokines") influence the incidence and progression of obesity-related cancers. Here, we discuss the role of adipocyte dysfunction associated with obesity and its potential impact on cancer biology.

Adipose tissue remodeling as homeostatic inflammation

Evidence has accumulated indicating that obesity is associated with a state of chronic, low-grade inflammation. Obese adipose tissue is characterized by dynamic changes in cellular composition and function, which may be referred to as “adipose tissue remodeling”. Among stromal cells in the adipose tissue, infiltrated macrophages play an important role in adipose tissue inflammation and systemic insulin resistance. We have demonstrated that a paracrine loop involving saturated fatty acids and tumor necrosis factor-α derived from adipocytes and macrophages, respectively, aggravates obesity-induced adipose tissue inflammation. Notably, saturated fatty acids, which are released from hypertrophied adipocytes via the macrophage-induced lipolysis, serve as a naturally occurring ligand for Toll-like receptor 4 complex, thereby activating macrophages. Such a sustained interaction between endogenous ligands derived from parenchymal cells and pathogen sensors expressed in stromal immune cells should lead to chronic inflammatory responses ranging from the basal homeostatic state to diseased tissue remodeling, which may be referred to as “homeostatic inflammation”. We, therefore, postulate that adipose tissue remodeling may represent a prototypic example of homeostatic inflammation. Understanding the molecular mechanism underlying homeostatic inflammation may lead to the identification of novel therapeutic strategies to prevent or treat obesity-related complications.

Adipose tissue macrophages: phenotypic plasticity and diversity in lean and obese states

PURPOSE OF REVIEW

Proinflammatory adipose tissue macrophages (ATMs) contribute to obesity-associated disease morbidity. We will provide an update of the current state of knowledge regarding the phenotypic and functional diversity of ATMs in lean and obese mice and humans.

RECENT FINDINGS

The phenotypic diversity of ATMs is now known to include more than two types requiring an expansion of the simple concept of an M2 to M1 shift with obesity. Potential functions for ATMs now include the regulation of fibrosis and response to acute lipolysis in states of caloric restriction. Novel pathways that can potentiate ATM action have been identified, which include inflammasome activation and the response to lipodystrophic adipose tissue. Studies provide a new appreciation for the ability of ATMs to respond dynamically to the adipose tissue microenvironment.

SUMMARY

ATMs play a key role in shaping the inflammatory milieu within adipose tissue, and it is now apparent that ATM heterogeneity is acutely shaped by the adipose tissue environment. To account for the new findings, we propose a new nomenclature for ATM subtypes that takes into account their diversity.

Growth hormone and adipose tissue: beyond the adipocyte

The last two decades have seen resurgence in research focused on adipose tissue. In part, the enhanced interest stems from an alarming increase in obesity rates worldwide. However, an understanding that this once simple tissue is significantly more intricate and interactive than previously realized has fostered additional attention. While few would argue that growth hormone (GH) radically alters fat mass, newer findings revealing the complexity of adipose tissue requires that GH's influence on this tissue be reexamined. Therefore, the objective of this review is to describe the more recent understanding of adipose tissue and to summarize our current knowledge of how GH may influence and contribute to these newer complexities of this tissue with special focus on the available data from mice with altered GH action.

Effect of collagen I and aortic carboxypeptidase-like protein on 3T3-L1 adipocyte differentiation

Aortic carboxypeptidase-like protein (ACLP) is a secreted protein expressed in preadipocytes and down-regulated during adipogenesis. Results from previous studies on the influence of ACLP overexpression on adipogenesis vary from no effect to complete inhibition. We hypothesized that ACLP may modulate adipogenesis in the presence of collagen I, a protein to which it binds. We compared control (pLXSN) 3T3-L1 preadipocytes with 3T3-L1 preadipocytes stably overexpressing ACLP (pLXSN-ACLP) that were grown in standard vs collagen I-coated dishes. Aortic carboxypeptidase-like protein overexpression, via retroviral transduction, resulted in a 3.2-fold increase in ACLP cellular levels and a 2.1-fold increase in ACLP levels released into medium. Aortic carboxypeptidase-like protein overexpression did not inhibit differentiation in standard dishes. In collagen I-coated dishes compared with standard dishes, control preadipocytes, when induced to differentiate, exhibited the same increase in triacylglycerol accumulation, but showed a significantly higher induction of fatty acid synthase (1.6-fold more), peroxisome proliferator-activated receptor γ (1.4-fold more), and CCAAT/enhancer-binding protein α (1.4-fold more). Aortic carboxypeptidase-like protein overexpression significantly reduced this enhanced induction of fatty acid synthase, peroxisome proliferator-activated receptor γ, and CCAAT/enhancer-binding protein α by 65%, 59%, and 66%, respectively, but had no effect on the accumulation of triacylglycerol during differentiation. Finally, studies on proadipogenic insulin signaling in ACLP-overexpressing preadipocytes demonstrated that insulin-stimulated Akt phosphorylation was significantly decreased by 27% in cells cultured in collagen I-coated dishes vs standard dishes. Our data suggest that ACLP inhibits certain aspects of 3T3-L1 adipogenesis in a collagen I-rich environment.

Adipose tissue remodeling and obesity

To fulfill its role as the major energy-storing tissue, adipose has several unique properties that cannot be seen in any other organ, including an almost unlimited capacity to expand in a non-transformed state. As such, the tissue requires potent mechanisms to remodel, acutely and chronically. Adipocytes can rapidly reach the diffusional limit of oxygen during growth; hypoxia is therefore an early determinant that limits healthy expansion. Proper expansion requires a highly coordinated response among many different cell types, including endothelial precursor cells, immune cells, and preadipocytes. There are therefore remarkable similarities between adipose expansion and growth of solid tumors, a phenomenon that presents both an opportunity and a challenge, since pharmacological interventions supporting healthy adipose tissue adaptation can also facilitate tumor growth.

Sex differences during the course of diet-induced obesity in mice: adipose tissue expandability and glycemic control

OBJECTIVE

Adverse effects of obesity on glucose homeostasis are linked to low-grade adipose tissue inflammation and accumulation of lipids in non-adipose tissues. The goal of this study was to evaluate the role of adipose tissue plasticity in a less severe deterioration of glucose homeostasis in females compared with males during the course of high-fat (HF) feeding in mice.

DESIGN

Mice of the C57BL/6N strain were fed either a chow or obesogenic HF diet for up to 35 weeks after weaning. Metabolic markers and hormones in plasma, glucose homeostasis, adipocyte size and inflammatory status of gonadal (gWAT) and subcutaneous (scWAT) adipose depots and liver steatosis were evaluated at 15 and 35 weeks of HF feeding.

RESULTS

HF-fed males were heavier than females until week ∼20, after which the body weights stabilized at a similar level (55-58 g) in both sexes. Greater weight gain and fat accumulation in females were associated with larger adipocytes in gWAT and scWAT at week 35. Although adipose tissue macrophage infiltration was in general less frequent in scWAT, it was reduced in both fat depots of female as compared with male mice; however, the expression of inflammatory markers in gWAT was similar in both sexes at week 35. In females, later onset of the impairment of glucose homeostasis and better insulin sensitivity were associated with higher plasma levels of adiponectin (weeks 0, 15 and 35) and reduced hepatosteatosis (weeks 15 and 35).

CONCLUSIONS

Compared with males, female mice demonstrate increased capacity for adipocyte enlargement in response to a long-term HF feeding, which is associated with reduced adipose tissue macrophage infiltration and lower fat deposition in the liver, and with better insulin sensitivity. Our data suggest that adipose tissue expandability linked to adiponectin secretion might have a role in the sex differences observed in obesity-associated metabolic disorders.

Architecture and the extracellular matrix: the still unappreciated components of the adipose tissue

Fibrosis is usually characterized by the modification of both the amount and composition of a wide panel of extracellular matrix (ECM) proteins. In the liver, pancreas, kidney and lung the accumulation of fibrosis disrupts cellular processes and appears detrimental for organ function. This review highlights the available evidence supporting an important ECM remodelling in adipose tissue (AT) and, in particular, during the development of obesity. The modifications and occurrence of new adipose ECM components leads to an abnormal accumulation of fibrosis in this tissue. This phenomenon was well described in rodent models and evidence is beginning to emerge in humans; however, the origin and potential impact of these depots in AT biology are unclear. Two animal models with disruptions in ECM components (secreted proteins acidic in nature rich in cysteine null mice and ob/ob collagen VI null mice) suggest that fibrosis limits adipocyte hypertrophy and may cause the metabolic disorders associated with obesity. Over-expression of Hypoxia-inducible factor 1 leading to an increase in collagen expression suggests a role for hypoxia in fibrosis development. We conclude this review with possible hypotheses regarding the cellular and molecular contributors of fibrosis initiation.

Dicer is required for the formation of white but not brown adipose tissue

Dicer, an enzyme involved in microRNA maturation, is required for proper embryo gastrulation and tissue morphogenesis during mammalian development. Using primary cultures of fibroblasts and pre-adipocytes, we have previously shown that Dicer is essential for early stages of adipogenic cell differentiation. In this study, we have utilized Dicer-conditional mice to explore a role for Dicer and microRNA biogenesis in the terminal differentiation of adipocytes in vivo and in the formation of white and brown adipose tissue. Deletion of Dicer in differentiated adipocytes in Dicer-conditional, aP2-Cre transgenic mice reduced the level of various adipogenic-associated transcripts and inhibited lipogenesis in white adipocytes, resulting in a severe depletion of white adipose tissue in mice. In contrast, Dicer was not required in vivo for lipogenesis in brown adipose or for brown fat formation. However, Dicer deletion in brown adipose did decrease the expression of genes involved in thermoregulation. The results of our study provide genetic evidence of a role for microRNA molecules in regulating adipogenesis and reveal distinct requirements for Dicer in the formation of white and brown adipose tissue.

Childhood-Adolescent Obesity in the Cardiorenal Syndrome: Lessons from Animal Models

BACKGROUND/AIMS

Childhood-adolescent overweight and obesity have grown to pandemic proportions during the past decade. The onset of obesity in younger adults will likely be manifested as earlier onset of myocardial and renal end-organ disease in younger adults. For the first time, it is estimated that the current generation may not live to be as old as their parents. Thus, it is important to develop animal models of childhood obesity to understand fundamental pathological organ changes.

METHODS

In this regard, we utilize transmission electron microscopy evaluation to evaluate early remodeling changes of two adolescent rodent obesity models: the Zucker obese (fa/fa) rat and the db/db mouse models of obesity. We have concentrated on the initial ultrastructural remodeling (obese adipose tissue, skeletal muscle, and islet remodeling) and the associated changes in target end organs (including the myocardium and kidney) in young rodent models of obesity and insulin resistance, collectively manifesting as the cardiorenal metabolic syndrome (CRS).

RESULTS

Briefly, tissues revealed the following ultrastructural remodeling abnormalities: inflammation, hypertrophy, and early fibrosis in adipose tissue; loss of mitochondria in skeletal muscles, hyperplasia, fibrosis, and depletion of insulin-secretory granules in pancreatic islets; increased intramyocardial lipid accumulation, fibrosis, and mitochondrial deposition in the myocardium, and obesity-related glomerulopathy and tubulopathy in the kidney.

CONCLUSION

Based on the current knowledge and ultrastructural observations of organ pathology, we propose mechanisms whereby obesity appears to be the driving force behind the development of the CRS.

Hepatocyte-derived Snail1 propagates liver fibrosis progression

Chronic exposure of the liver to hepatotoxic agents initiates an aberrant wound healing response marked by proinflammatory, as well as fibrotic, changes, leading to compromised organ structure and function. In a variety of pathological states, correlative links have been established between tissue fibrosis and the expression of transcription factors associated with the induction of epithelial-mesenchymal cell transition (EMT) programs similar to those engaged during development. However, the role played by endogenously derived, EMT-associated transcription factors in fibrotic states in vivo remains undefined. Using a mouse model of acute liver fibrosis, we demonstrate that hepatocytes upregulate the expression of the zinc finger transcriptional repressor, Snail1, during tissue remodeling. Hepatocyte-specific ablation of Snail1 demonstrates that this transcription factor plays a key role in liver fibrosis progression in vivo by triggering the proximal genetic programs that control multiple aspects of fibrogenesis, ranging from growth factor expression and extracellular matrix biosynthesis to the ensuing chronic inflammatory responses that characterize this class of pathological disorders.

Collagen VI is a basement membrane component that regulates epithelial cell-fibronectin interactions

Collagen VI is a heterotrimer composed of three α chains (α1, α2, α3) widely expressed throughout various interstitial matrices. Collagen VI is also found near the basement membranes of many tissues where it serves as an anchoring meshwork. The aim of this study was to investigate the distribution and role of collagen VI at the epithelial-stromal interface in the intestine. Results showed that collagen VI is a bona fide epithelial basal lamina component and constitutes the major collagen type of epithelial origin in this organ. In vitro, collagen VI co-distributes with fibronectin. Targeted knockdown of collagen VI expression in intestinal epithelial cells was used to investigate its function. Depletion of collagen VI from the matrix led to a significant increase in cell spreading and fibrillar adhesion formation coinciding with an upregulation of fibronectin expression, deposition and organization as well as activation of myosin light chain phosphorylation by the myosin light chain kinase and Rho kinase dependent mechanisms. Plating cells deficient for collagen VI on collagen VI rescued the phenotype. Taken together, these data demonstrate that collagen VI is an important basal lamina component involved in the regulation of epithelial cell behavior most notably as a regulator of epithelial cell-fibronectin interactions.

Hypertrophy-driven adipocyte death overwhelms recruitment under prolonged weight gain

Fat pads dynamically regulate energy storage capacity under energy excess and deficit. This remodeling process is not completely understood, with controversies regarding differences between fat depots and plasticity of adipose cell number. We examined changes of mouse adipose cell-size distributions in epididymal, inguinal, retroperitoneal, and mesenteric fat under both weight gain and loss. With mathematical modeling, we specifically analyzed the recruitment, growth/shrinkage, and loss of adipose cells, including the size dependence of these processes. We found a qualitatively universal adipose tissue remodeling process in all four fat depots: 1), There is continuous recruitment of new cells under weight gain; 2), the growth and shrinkage of larger cells (diameter >50 μm) is proportional to cell surface area; and 3), cell loss occurs under prolonged weight gain, with larger cells more susceptible. The mathematical model gives a predictive integrative picture of adipose tissue remodeling in obesity.

Differential lipid partitioning between adipocytes and tissue macrophages modulates macrophage lipotoxicity and M2/M1 polarization in obese mice

OBJECTIVE

Obesity-associated insulin resistance is characterized by a state of chronic, low-grade inflammation that is associated with the accumulation of M1 proinflammatory macrophages in adipose tissue. Although different evidence explains the mechanisms linking the expansion of adipose tissue and adipose tissue macrophage (ATM) polarization, in the current study we investigated the concept of lipid-induced toxicity as the pathogenic link that could explain the trigger of this response.

RESEARCH DESIGN AND METHODS

We addressed this question using isolated ATMs and adipocytes from genetic and diet-induced murine models of obesity. Through transcriptomic and lipidomic analysis, we created a model integrating transcript and lipid species networks simultaneously occurring in adipocytes and ATMs and their reversibility by thiazolidinedione treatment.

RESULTS

We show that polarization of ATMs is associated with lipid accumulation and the consequent formation of foam cell-like cells in adipose tissue. Our study reveals that early stages of adipose tissue expansion are characterized by M2-polarized ATMs and that progressive lipid accumulation within ATMs heralds the M1 polarization, a macrophage phenotype associated with severe obesity and insulin resistance. Furthermore, rosiglitazone treatment, which promotes redistribution of lipids toward adipocytes and extends the M2 ATM polarization state, prevents the lipid alterations associated with M1 ATM polarization.

CONCLUSIONS

Our data indicate that the M1 ATM polarization in obesity might be a macrophage-specific manifestation of a more general lipotoxic pathogenic mechanism. This indicates that strategies to optimize fat deposition and repartitioning toward adipocytes might improve insulin sensitivity by preventing ATM lipotoxicity and M1 polarization.

Caveolin-1 and mitochondrial SOD2 (MnSOD) function as tumor suppressors in the stromal microenvironment: a new genetically tractable model for human cancer associated fibroblasts

We have recently proposed a new model for understanding tumor metabolism, termed: "The Autophagic Tumor Stroma Model of Cancer Metabolism". In this new paradigm, catabolism (autophagy) in the tumor stroma fuels the anabolic growth of aggressive cancer cells. Mechanistically, tumor cells induce autophagy in adjacent cancer-associated fibroblasts via the loss of caveolin-1 (Cav-1), which is sufficient to promote oxidative stress in stromal fibroblasts. To further test this hypothesis, here we created human Cav-1 deficient immortalized fibroblasts using a targeted sh-RNA knock-down approach. Relative to control fibroblasts, Cav-1 deficient fibroblasts dramatically promoted tumor growth in xenograft assays employing an aggressive human breast cancer cell line, namely MDA-MB-231 cells. Co-injection of Cav-1 deficient fibroblasts, with MDA-MB-231 cells, increased both tumor mass and tumor volume by ~4-fold. Immuno-staining with CD31 indicated that this paracrine tumor promoting effect was clearly independent of angiogenesis. Mechanistically, proteomic analysis of these human Cav-1 deficient fibroblasts identified > 40 protein biomarkers that were upregulated, most of which were associated with i) myofibroblast differentiation, or ii) oxidative stress/hypoxia. In direct support of these findings, the tumor promoting effects of Cav-1 deficient fibroblasts could be functionally suppressed (nearly 2-fold) by the recombinant over-expression of SOD2 (superoxide dismutase 2), a known mitochondrial enzyme that de-activates superoxide, thereby reducing mitochondrial oxidative stress. In contrast, cytoplasmic soluble SOD1 had no effect, further highlighting a specific role for mitochondrial oxidative stress in this process. In summary, here we provide new evidence directly supporting a key role for a loss of stromal Cav-1 expression and oxidative stress in cancer-associated fibroblasts, in promoting tumor growth, which is consistent with "The Autophagic Tumor Stroma Model of Cancer". The human Cav-1 deficient fibroblasts that we have generated are a new genetically tractable model system for identifying other suppressors of the cancer-associated fibroblast phenotype, via a genetic "complementation" approach. This has important implications for understanding the pathogenesis of triple negative and basal breasts cancers, as well as tamoxifen-resistance in ER+ breast cancers, which are all associated with a Cav-1 deficient "lethal" tumor micro-environment, driving poor clinical outcome.

Adipokines in inflammation and metabolic disease

The worldwide epidemic of obesity has brought considerable attention to research aimed at understanding the biology of adipocytes (fat cells) and the events occurring in adipose tissue (fat) and in the bodies of obese individuals. Accumulating evidence indicates that obesity causes chronic low-grade inflammation and that this contributes to systemic metabolic dysfunction that is associated with obesity-linked disorders. Adipose tissue functions as a key endocrine organ by releasing multiple bioactive substances, known as adipose-derived secreted factors or adipokines, that have pro-inflammatory or anti-inflammatory activities. Dysregulated production or secretion of these adipokines owing to adipose tissue dysfunction can contribute to the pathogenesis of obesity-linked complications. In this Review, we focus on the role of adipokines in inflammatory responses and discuss their potential as regulators of metabolic function.

Proliferative and molecular effects of the dual PPARalpha/gamma agonist tesaglitazar in rat adipose tissues: relevance for induction of fibrosarcoma

The dual peroxisome-proliferator-activated receptor (PPAR) α/γ agonist tesaglitazar has been shown to produce fibrosarcomas in rats. Here, the authors studied morphology, proliferation, differentiation, and inflammation markers in adipose tissue from rats exposed to 1, 3, or 10 µmol/kg tesaglitazar for 2 or 12 weeks, including recovery groups (12 weeks treatment followed by 12 weeks recovery), and 3 or 10 µmol/kg tesaglitazar for 24 weeks. Subcutaneous white and brown fat revealed reversible dose-related histopathological alterations and after 12 and 24 weeks developed areas of thickened skin (fatty lumps). There was a dose-dependent increase in proliferation of interstitial cells in white and brown fat as shown by increased mitotic index in all dose groups after 2 weeks. This was limited to the high dose after 12 and 24 weeks in white fat. Gene expression analyses showed that while tesaglitazar induced differentiation of adipose tissue characterized with a switch in cyclin D1 and D3 mRNA by 12 weeks, longer exposure at high doses reversed this differentiation concurrent with a reappearance of early adipocyte and inflammatory markers. These data suggest that sustained increased turnover of mesenchymal cells in adipose tissues, concomitant with onset of inflammation and fibrosis, drives development of fibrosarcomas in rats treated with tesaglitazar.

Adipose tissue transcriptome changes during obesity development in female dogs

During the development of obesity, adipose tissue undergoes major expansion and remodeling, but the biological processes involved in this transition are not well understood. The objective of this study was to analyze global gene expression profiles of adipose tissue in dogs, fed a high-fat diet, during the transition from a lean to obese phenotype. Nine female beagles (4.09 ± 0.64 yr; 8.48 ± 0.35 kg) were randomized to ad libitum feeding or body weight maintenance. Subcutaneous adipose tissue biopsy, blood, and dual x-ray absorptiometry measurements were collected at 0, 4, 8, 12, and 24 wk of feeding. Serum was analyzed for glucose, insulin, fructosamine, triglycerides, free fatty acids, adiponectin, and leptin. Formalin-fixed adipose tissue was used for determination of adipocyte size. Adipose RNA samples were hybridized to Affymetrix Canine 2.0 microarrays. Statistical analysis, using repeated-measures ANOVA, showed ad libitum feeding increased (P < 0.05) body weight (0 wk, 8.36 ± 0.34 kg; 24 wk, 14.64 ± 0.34 kg), body fat mass (0 wk, 1.36 ± 0.24 kg; 24 wk, 6.52 ± 0.24 kg), adipocyte size (0 wk, 114.66 ± 17.38 μm(2); 24 wk, 320.97 ± 0.18.17 μm(2)), and leptin (0 wk, 0.8 ± 1.0 ng/ml; 24 wk, 12.9 ± 1.0 ng/ml). Microarrays displayed 1,665 differentially expressed genes in adipose tissue as weight increased. Alterations were seen in adipose tissue homeostatic processes including metabolism, oxidative stress, mitochondrial homeostasis, and extracellular matrix. Adipose transcriptome changes highlight the dynamic and adaptive response to ad libitum feeding and obesity development.

The NLRP3 inflammasome instigates obesity-induced inflammation and insulin resistance

Emergence of chronic ‘sterile’ inflammation during obesity in absence of overt infection or autoimmune process is a puzzling phenomenon. The Nod Like Receptor (NLR) family of innate immune cell sensors like the Nlrp3 inflammasome are implicated in recognizing certain non-microbial origin ‘danger–signals’ leading to caspase-1 activation and subsequent IL-1β and IL-18 secretion. We show that reduction in adipose tissue expression of Nlrp3 is coupled with decreased inflammation and improved insulin–sensitivity in obese type-2 diabetic patients. The Nlrp3 inflammasome senses the lipotoxicity–associated ceramide to induce caspase-1 cleavage in macrophages and adipose tissue. Ablation of Nlrp3 prevented the obesity–induced inflammasome activation in fat depots and liver together with enhanced insulin–signalling. Furthermore, elimination of Nlrp3 in obesity reduced IL-18 and adipose tissue IFNγ along with an increase in naïve and reduction in effector adipose tissue T cells. Collectively, these data establish that Nlrp3 inflammasome senses obesity–associated ‘danger–signals’ and contributes to obesity–induced inflammation and insulin–resistance.

New insights into adipose tissue dysfunction in insulin resistance

In a state of caloric excess, adipose tissue plays an essential role by storing lipids. Its expandability determines the onset of metabolic syndrome (central obesity, dyslipidemia, glucose intolerance and hypertension). When the adipocyte endoplasmic reticulum is no longer capable of processing the excess nutrients, the so-called “endoplasmic reticulum stress” develops. This triggers efflux of free fatty acids from adipocytes into the circulation and causes triglyceride overload in skeletal muscle, liver and pancreas. Adipose tissue hypoxia then develops, due to the failure of vasculature to expand with adipocyte hypertrophy. Increased catabolism in mitochondria leads there to oxidative stress. Both phenomena cause deranged adipokine secretion and low-grade inflammation. Inflammatory cytokines, reactive oxygen species and ectopic lipid deposition are the main mediators of insulin resistance and vascular impairment, which both lead finally to diabetes type 2 and cardiovascular disease. Recently, fibrosis of adipose tissue was also demonstrated in obesity, contributing to the interplay of deleterious factors forcing inflammation. The present paper reviews recent evidence for adipose tissue dysfunction, trying to define causes and consequences. In conclusion, insulin resistance and associated complications originate from excess lipids, which cannot be stored without limit in adipose tissue, thus affecting its integrity and adipokine secretion.

Adipose tissue dysfunction signals progression of hepatic steatosis towards nonalcoholic steatohepatitis in C57BL/6 mice

OBJECTIVE

Nonalcoholic fatty liver disease (NAFLD) is linked to obesity and diabetes, suggesting an important role of adipose tissue in the pathogenesis of NAFLD. Here, we aimed to investigate the interaction between adipose tissue and liver in NAFLD and identify potential early plasma markers that predict nonalcoholic steatohepatitis (NASH).

RESEARCH DESIGN AND METHODS

C57Bl/6 mice were chronically fed a high-fat diet to induce NAFLD and compared with mice fed a low-fat diet. Extensive histological and phenotypical analyses coupled with a time course study of plasma proteins using multiplex assay were performed.

RESULTS

Mice exhibited pronounced heterogeneity in liver histological scoring, leading to classification into four subgroups: low-fat low (LFL) responders displaying normal liver morphology, low-fat high (LFH) responders showing benign hepatic steatosis, high-fat low (HFL) responders displaying pre-NASH with macrovesicular lipid droplets, and high fat high (HFH) responders exhibiting overt NASH characterized by ballooning of hepatocytes, presence of Mallory bodies, and activated inflammatory cells. Compared with HFL responders, HFH mice gained weight more rapidly and exhibited adipose tissue dysfunction characterized by decreased final fat mass, enhanced macrophage infiltration and inflammation, and adipose tissue remodeling. Plasma haptoglobin, IL-1β, TIMP-1, adiponectin, and leptin were significantly changed in HFH mice. Multivariate analysis indicated that in addition to leptin, plasma CRP, haptoglobin, eotaxin, and MIP-1α early in the intervention were positively associated with liver triglycerides. Intermediate prognostic markers of liver triglycerides included IL-18, IL-1β, MIP-1γ, and MIP-2, whereas insulin, TIMP-1, granulocyte chemotactic protein 2, and myeloperoxidase emerged as late markers.

CONCLUSIONS

Our data support the existence of a tight relationship between adipose tissue dysfunction and NASH pathogenesis and point to several novel potential predictive biomarkers for NASH.

Adipose tissue macrophages in insulin-resistant subjects are associated with collagen VI and fibrosis and demonstrate alternative activation

Adipose tissue macrophages are associated with insulin resistance and are linked to changes in the extracellular matrix. To better characterize adipose macrophages, the extracellular matrix, and adipocyte-macrophage interactions, gene expression from adipose tissue and the stromal vascular fraction was assessed for markers of inflammation and fibrosis, and macrophages from obese and lean subjects were counted and characterized immunohistochemically. Coculture experiments examined the effects of adipocyte-macrophage interaction. Collagen VI gene expression was associated with insulin sensitivity and CD68 (r = -0.56 and 0.60, P < 0.0001) and with other markers of inflammation and fibrosis. Compared with adipose tissue from lean subjects, adipose tissue from obese subjects contained increased areas of fibrosis, which correlated inversely with insulin sensitivity (r = -0.58, P < 0.02) and positively with macrophage number (r = 0.70, P < 0.01). Although macrophages in crownlike structures (CLS) were more abundant in obese adipose tissue, the majority of macrophages were associated with fibrosis and were not organized in CLS. Macrophages in CLS were predominantly M1, but most other macrophages, particularly those in fibrotic areas, were M2 and also expressed CD150, a marker of M2c macrophages. Coculture of THP-1 macrophages with adipocytes promoted the M2 phenotype, with a lower level of IL-1 expression and a higher ratio of IL-10 to IL-12. Transforming growth factor-β (TGF-β) was more abundant in M2 macrophages and was further increased by coculture with adipocytes. Downstream effectors of TGF-β, such as plasminogen activator inhibitor-1, collagen VI, and phosphorylated Smad, were increased in macrophages and adipocytes. Thus adipose tissue of insulin-resistant humans demonstrated increased fibrosis, M2 macrophage abundance, and TGF-β activity.

Fibrosis in human adipose tissue: composition, distribution, and link with lipid metabolism and fat mass loss

OBJECTIVE

Fibrosis is a newly appreciated hallmark of the pathological alteration of human white adipose tissue (WAT). We investigated the composition of subcutaneous (scWAT) and omental WAT (oWAT) fibrosis in obesity and its relationship with metabolic alterations and surgery-induced weight loss.

RESEARCH DESIGN AND METHODS

Surgical biopsies for scWAT and oWAT were obtained in 65 obese (BMI 48.2 ± 0.8 kg/m(2)) and 9 lean subjects (BMI 22.8 ± 0.7 kg/m(2)). Obese subjects who were candidates for bariatric surgery were clinically characterized before, 3, 6, and 12 months after surgery, including fat mass evaluation by dual energy X-ray absorptiometry. WAT fibrosis was quantified and characterized using quantitative PCR, microscopic observation, and immunohistochemistry.

RESULTS

Fibrosis amount, distribution and collagen types (I, III, and VI) present distinct characteristics in lean and obese subjects and with WAT depots localization (subcutaneous or omental). Obese subjects had more total fibrosis in oWAT and had more pericellular fibrosis around adipocytes than lean subjects in both depots. Macrophages and mastocytes were highly represented in fibrotic bundles in oWAT, whereas scWAT was more frequently characterized by hypocellular fibrosis. The oWAT fibrosis negatively correlated with omental adipocyte diameters (R = -0.30, P = 0.02), and with triglyceride levels (R = -0.42, P < 0.01), and positively with apoA1 (R = 0.25, P = 0.05). Importantly, scWAT fibrosis correlated negatively with fat mass loss measured at the three time points after surgery.

CONCLUSIONS

Our data suggest differential clinical consequences of fibrosis in human WAT. In oWAT, fibrosis could contribute to limit adipocyte hypertrophy and is associated with a better lipid profile, whereas scWAT fibrosis may hamper fat mass loss induced by surgery.

Temporal profiling of the secretome during adipogenesis in humans

Adipose tissue plays a key role as a fat-storage depot and as an endocrine organ. Although mouse adipogenesis has been studied extensively, limited studies have been conducted to characterize this process in humans. We carried out a temporal proteomic analysis to interrogate the dynamic changes in the secretome of primary human preadipocytes as they differentiate into mature adipocytes. Using iTRAQ-based quantitative proteomics, we identified and quantified 420 proteins from the secretome of differentiated human adipocytes. Our results revealed that the majority of proteins showed differential expression during the course of differentiation. In addition to adipokines known to be differentially secreted in the course of adipocyte differentiation, we identified a number of proteins whose dynamic expression in this process has not been previously documented. They include collagen triple helix repeat containing 1, cytokine receptor-like factor 1, glypican-1, hepatoma-derived growth factor, SPARC related modular calcium binding protein 1, SPOCK 1, and sushi repeat-containing protein. A bioinformatics analysis using Human Protein Reference Database and Human Proteinpedia revealed that of the 420 proteins identified, 164 proteins possess signal peptides and 148 proteins are localized to the extracellular compartment. Additionally, we employed antibody arrays to quantify changes in the levels of 182 adipokines during human adipogenesis. This is the first large-scale quantitative proteomic study that combines two platforms, mass spectrometry and antibody arrays, to analyze the changes in the secretome during the course of adipogenesis in humans.

The secretome of adipocytes is regulated during the differentiation of preadipocytes into adipocytes. Using iTRAQ-based quantitative proteomics, we identified and quantified 420 proteins. Additionally, we employed antibody arrays to quantify changes in the levels of 182 adipokines. This is the first large-scale quantitative proteomic study that combines two platforms, mass spectrometry and antibody arrays, to analyze the changes in the secretome during the course of adipogenesis in humans.

Regulation of energy metabolism by inflammation: a feedback response in obesity and calorie restriction

Caloric restriction (CR), in the absence of malnutrition, delays aging and prevents aging-related diseases through multiple mechanisms. A reduction in chronic inflammation is widely observed in experimental models of caloric restriction. The low inflammation status may contribute to the reduced incidence of osteoporosis, Alzheimer's disease, cardiovascular diseases and cancer in the aging subjects. The association of caloric restriction with low inflammation suggests a role of energy accumulation in the origin of the chronic inflammation. This point is enforced by recent advances in obesity research. Abundant literature on obesity suggests that chronic inflammation is a consequence of energy accumulation in the body. The emerging evidence strongly supports that the inflammatory response induces energy expenditure in a feedback manner to fight against energy surplus in obesity. If this feedback system is deficient (Inflammation Resistance), energy expenditure will be reduced and energy accumulation will lead to obesity. In this perspective, we propose that an increase in inflammation in obesity promotes energy expenditure with a goal to get rid of energy surplus. A decrease in inflammation under caloric restriction contributes to energy saving. Inflammation is a mechanism for energy balance in the body. Inflammation resistance will lead to obesity. We will review the recent literature in support of the viewpoints.

Genetic link between obesity and MMP14-dependent adipogenic collagen turnover

OBJECTIVE

In white adipose tissue, adipocytes and adipocyte precursor cells are enmeshed in a dense network of type I collagen fibrils. The fate of this pericellular collagenous web in diet-induced obesity, however, is unknown. This study seeks to identify the genetic underpinnings of proteolytic collagen turnover and their association with obesity progression in mice and humans.

RESEARCH DESIGN AND METHODS

The hydrolysis and degradation of type I collagen at early stages of high-fat diet feeding was assessed in wild-type or MMP14 (MT1-MMP)-haploinsufficient mice using immunofluorescent staining and scanning electron microscopy. The impact of MMP14-dependent collagenolysis on adipose tissue function was interrogated by transcriptome profiling with cDNA microarrays. Genetic associations between MMP14 gene common variants and obesity or diabetes traits were examined in a Japanese cohort (n = 3,653).

RESULTS

In adult mice, type I collagen fibers were cleaved rapidly in situ during a high-fat diet challenge. By contrast, in MMP14 haploinsufficient mice, animals placed on a high-fat diet were unable to remodel fat pad collagen architecture and display blunted weight gain. Moreover, transcriptional programs linking type I collagen turnover with adipogenesis or lipogenesis were disrupted by the associated decrease in collagen turnover. Consistent with a key role played by MMP14 in regulating high-fat diet-induced metabolic programs, human MMP14 gene polymorphisms located in proximity to the enzyme's catalytic domain were closely associated with human obesity and diabetes traits.

CONCLUSIONS

Together, these findings demonstrate that the MMP14 gene, encoding the dominant pericellular collagenase operative in vivo, directs obesogenic collagen turnover and is linked to human obesity traits.

The density of extracellular matrix proteins regulates inflammation and insulin signaling in adipocytes

Cells can not only sense the type of extracellular matrix (ECM) protein that is present in the microenvironment, but they can also sense its density. Here, we investigated the effects of ECM protein density on adipokine secretion and insulin signaling in adipocytes. To this end, 3T3-L1 adipocytes were cultured on the surface of polyacrylamide gels that were coated with gradient densities of a collagen type I and fibronectin mixture. We found that high density ECM causes a decrease in insulin signaling and adiponectin secretion, whereas the secretion of monocyte chemoattractant protein-1 (MCP-1) was increased via the activation of nuclear factor-κB (NF-κB). These results indicate that the density of the ECM directly regulates the inflammatory response and insulin sensitivity of adipocytes.