CCL5 promotes macrophage recruitment and survival in human adipose tissue

Adipokines identified as new downstream targets for adiponectin: lessons from adiponectin-overexpressing or -deficient mice

Adipokines play a central role in the pathogenesis of the metabolic syndrome. Among them, adiponectin (ApN), a master regulator of immune and fuel homeostasis, is decreased. Identifying downstream adipokines targeted by ApN may help in deciphering this syndrome. We have generated transgenic mice, allowing persistent and moderate overexpression of ApN (ApN-Overex) specifically in white adipose tissue (AT). We took advantage of this model to unravel the adipokine secretion profile triggered by ApN. AT was fractionated into adipocytes and stromal-vascular cells (SVC), which were cultured for 8 h. Profiling of secretory products by antibody arrays and subsequent ELISAs showed that the secretion of three proinflammatory factors (IL-17B, IL-21, TNFα) and three hematopoietic growth factors [GF; thrombopoietin and granulocyte (macrophage) colony-stimulating-factors] was reduced in adipocytes of ApN-Overex mice compared with wild-type mice. In the SVC of these mice, besides the hematopoietic GFs, the secretion of another GF (vascular endothelial GF receptor 1), two chemokines (RANTES and ICAM-1), and two proinflammatory factors (IL-6 and IL-12p70) was reduced as well. Only one cytokine, IL-1 receptor 4, was oversecreted by SVC of ApN-Overex mice, which may exhibit anti-inflammatory properties. Most of these changes in secretion were due to corresponding changes in mRNAs. A reverse profile of adipokine expression was observed in ApN-KO mice. In conclusion, ApN regulates in vivo the secretion of downstream adipokines, thereby inducing a shift of the immune balance in both adipocytes and SVC toward a less inflammatory phenotype. These downstream adipokines may be new therapeutic targets for the management of the metabolic syndrome.

Apoptosis inhibitor of macrophage (AIM) is required for obesity-associated recruitment of inflammatory macrophages into adipose tissue

Infiltration of inflammatory macrophages into adipose tissues with the progression of obesity triggers insulin resistance and obesity-related metabolic diseases. We recently reported that macrophage-derived apoptosis inhibitor of macrophage (AIM) protein is increased in blood in line with obesity progression and is incorporated into adipocytes, thereby inducing lipolysis in adipose tissue. Here we show that such a response is required for the recruitment of adipose tissue macrophages. In vitro, AIM-dependent lipolysis induced an efflux of palmitic and stearic acids from 3T3-L1 adipocytes, thereby stimulating chemokine production in adipocytes via activation of toll-like receptor 4 (TLR4). In vivo administration of recombinant AIM to TLR4-deficient (TLR4(-/-)) mice resulted in induction of lipolysis without chemokine production in adipose tissues. Consistently, mRNA levels for the chemokines that affect macrophages were far lower in AIM-deficient (AIM(-/-)) than in wild-type (AIM(+/+)) obese adipose tissue. This reduction in chemokine production resulted in a marked prevention of inflammatory macrophage infiltration into adipose tissue in obese AIM(-/-) mice, although these mice showed more advanced obesity than AIM(+/+) mice on a high-fat diet. Diminished macrophage infiltration resulted in decreased inflammation locally and systemically in obese AIM(-/-) mice, thereby protecting them from insulin resistance and glucose intolerance. These results indicate that the increase in blood AIM is a critical event for the initiation of macrophage recruitment into adipose tissue, which is followed by insulin resistance. Thus, AIM suppression might be therapeutically applicable for the prevention of obesity-related metabolic disorders.

Defining macrophage phenotype and function in adipose tissue

In obesity, chronic low-grade inflammation is thought to mediate the effects of increased adipose tissue mass on metabolic comorbidity. Of the different cell types that contribute to obesity-induced inflammation in adipose tissue, this review focuses on macrophages and their monocytes precursors. Mechanisms for monocyte recruitment to adipose tissue, and how both monocytes and macrophages are phenotypically modified in this environment in response to increasing fat mass, are considered. The versatile phenotype of adipose tissue macrophages might contribute not only to inflammatory and metabolic alterations, but could also help to maintain adipose tissue homeostasis in the setting of obesity.

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.

Adipocyte-macrophage interaction may mediate LPS-induced low-grade inflammation: Potential link with metabolic complications

Chronic low-grade infection has been suggested to be associated with metabolic disorder such as diabetes. However, the molecular mechanism underlying this important association is largely unknown. The only clue established so far is that many subjects exhibit elevated levels of C-reactive protein as measured by highly sensitive assay. Here, we hypothesized that adipocyte–macrophage interaction plays a key role in amplifying such low grade infection to the level of influencing metabolic disorders. The presence of macrophages in abdominal adipose tissues was investigated by immunohistochemistry. To see whether molecules associated with acute phase protein, LPS signaling, and persistent recruitment of monocytes, are produced at higher amounts in adipocytes co-cultured with macrophages stimulated with low concentration of LPS (1 ng/ml), we measured serum amyloid A (SAA), LPS binding protein (LBP), soluble CD14 (sCD14), and RANTES levels in culture supernatant of co-cultures. Lastly, we investigated in vivo effect of low-grade LPS infusion on the production of these molecules using obese model mice. The macrophages were certainly identified in abdominal adipose tissues. Investigated molecules, especially LBP, SAA, and RANTES were produced at higher amounts in co-cultures stimulated with LPS compared with the cells without LPS. The ob/ob, and high-fat diet-induced obesity mice produced higher amounts of LBP, SAA, and RANTES one day after LPS infusion (1 ng/ml/g body weight) compared with ob/– and normal-fat fed control mice. Thus, adipocytes and infiltrated macrophages, and their interaction with low endotoxin stimulation appear to play an important role in amplifying and maintaining LPS-induced low-grade inflammation.

Urokinase plasminogen activator receptor in adipose tissue macrophages of morbidly obese subjects

OBJECTIVE

At present, circulating markers characterizing the inflammatory infiltration of white adipose tissue (WAT) in human obesity are not well known. We previously identified, by a pangenomic approach (microarrays), the urokinase plasminogen activator receptor (PLAUR or CD87) as a potential marker of subcutaneous adipose tissue macrophage infiltration (ATM).

METHOD

We studied i) the presence of PLAUR protein in WAT; ii) the PLAUR amount in plasma of obese patients; iii) the circulating variations during surgery-induced weight loss, and iv) the correlations between PLAUR circulating levels and bioclinical parameters.

RESULTS

We observed that PLAUR is preferentially expressed by infiltrating ATMs, with a typical localization on macrophage membrane. Circulating soluble PLAUR levels were significantly elevated in obese patients compared to lean controls. However, despite a trend towards a decrease 3 months after weight loss, PLAUR plasma levels were not modulated during a 1-year weight loss follow-up, suggesting the contribution of secretion sites other than subcutaneous WAT in obese patients.

CONCLUSIONS

These findings indicate that PLAUR mRNA expression could be used for the estimation of local subcutaneous ATMs infiltration in obese patients, but it cannot be used as a systemic marker of this inflammatory infiltration in dynamic phases of weight loss.

Absence of macrophage inflammatory protein-1{alpha} does not impact macrophage accumulation in adipose tissue of diet-induced obese mice

Macrophages and T-lymphocytes are known to accumulate in the white adipose tissue (WAT) of obese mice and humans, but the factors that cause this infiltration are not yet determined. Chemokines, which attract leukocytes to inflammatory sites, are candidates for this process. Macrophage inflammatory protein-1alpha (MIP-1alpha) expression is significantly elevated in WAT of obese mice and humans and positively correlates with fasting plasma insulin, but its potential role in leukocyte recruitment to WAT is unknown. MIP-1alpha-deficient, heterozygous, and wild-type mice were fed a Western diet (WD) for 16 wk. Plasma lipids, adipose tissue mass, energy expenditure, food intake, liver triglyceride content, and inflammatory cytokine expression were not different among genotypes. Gene expression of macrophage markers F4/80 and CD68, as well as T-lymphocyte marker CD3epsilon was increased in perigonadal WAT of obese WD-fed mice but was not influenced by MIP-1alpha expression level. Immunohistochemical analysis of WAT also showed no effect of MIP-1alpha on macrophage content. Two related chemokines, MIP-1beta and RANTES, had reduced expression in obese male MIP-1alpha-deficient mice compared with wild-type controls (P < or = 0.05). In mice fed the WD for 6 wk, WAT macrophage content was unchanged; however, CD8+ T-lymphocytes accumulated to a lesser extent in the MIP-1alpha-null mice. Therefore, expression of MIP-1alpha, as well as that of MIP-1beta and RANTES, increases as a consequence of weight gain, but these chemokines may not be required for the recruitment of monocytes to WAT during diet-induced obesity in mice and may impact T-lymphocyte recruitment only at early time points after WD feeding.

Adipose tissue inflammation: novel insight into the role of macrophages and lymphocytes

PURPOSE OF REVIEW

Obesity is associated with low-grade chronic inflammation in adipose tissue. This review presents an update on human and rodent studies analyzing the nature of fat-infiltrating immune cells, the time course of adipose tissue infiltration and underlying mechanisms.

RECENT FINDINGS

Intensive studies in rodents have shown that not only cells of the innate immune system traffic into adipose tissue but also various lymphocytes of the adaptive immunity are involved in inflammatory processes in fat. Several studies also provide insight in the order of appearance of macrophages and lymphocytes during the onset of obesity. Adipocytes and preadipocytes are also active players by their secretion of chemotactic adipokines.

SUMMARY

This review summarizes strong evidence for a link between the action of innate and adaptive immune systems in adipose tissue in the context of obesity and metabolism in rodents, but more studies in humans are necessary to relate this topic to human physiology. Targeting different immune cells at different stages of obesity may eventually lead to novel therapeutic approaches for the metabolic syndrome.

Adipose tissue recruitment of leukocytes

PURPOSE OF REVIEW

In December of 2003, two seminal articles describing the presence of macrophages in obese adipose tissue were published. These adipose tissue macrophages (ATMs) are inflammatory and promote local and systemic insulin resistance. Due to the continuing rise in obesity around the world, understanding how these ATMs contribute to metabolic disorders is of much interest.

RECENT FINDINGS

Chemokines have been extensively studied for their role in ATM recruitment. Deficiency or antagonism of chemokine receptors that interact with multiple chemokine ligands reduces ATM accumulation. ATMs are now defined as either classically (M1) or alternatively (M2) activated. Peroxisome proliferator-activated receptor activation and adiponectin promote an M2-polarized state resulting in improved insulin sensitivity. Finally, recent studies have provided evidence that T lymphocytes, natural killer T cells, mast cells, and B cells also enter adipose tissue and may interact with macrophages and adipocytes.

SUMMARY

Literature published during the past year has shown that macrophage recruitment to adipose tissue is only one of the important mediators of obesity-related insulin resistance. The phenotype of ATMs and recruitment of other immune cells to the adipose tissue play key roles in the overall contribution of adipose tissue to systemic metabolic outcomes of obesity.