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Obesity is associated with infiltration of white adipose tissue (WAT) by macrophages, which contributes to the development of insulin resistance. In this issue of the JCI, Kosteli and colleagues demonstrate that weight loss is unexpectedly also associated with rapid, albeit transient, recruitment of macrophages to WAT and that this appears to be related to lipolysis.
Macrophages are derived from monocytes and comprise a heterogenous population of cells found in nearly all tissues (1). In addition to their pivotal role in host defense, inflammation, and tissue repair, studies over the last decade have focused on their role in chronic metabolic diseases, such as obesity and insulin resistance (2). The initial descriptions of macrophage infiltration of white adipose tissue (WAT) during obesity, which were published in the JCI in 2003 (3, 4), garnered a great deal of attention, as evidenced by over 1,000 subsequent reports that confirmed the link between adipose tissue macrophages (ATMs) and inflammation and insulin resistance. In contrast, the role of innate and adaptive immunity in weight loss, the ultimate translational goal of research on obesity, is less clear. In this issue of the JCI, Kosteli and colleagues report that acute weight loss results in recruitment of macrophages to WAT (5). However, in this case, the recruited macrophages do not promote inflammation but rather regulate lipolysis. Since stimuli that enhance adipocyte lipolysis increase macrophage recruitment to WAT, the authors suggest that release of FFAs is a general signal for macrophage recruitment.
The chronic, low-grade inflammation that is characteristic of obesity has long been suspected to contribute to the development of insulin resistance (6). Almost two decades ago, Spiegelman and colleagues demonstrated that TNF-α, which is induced in the adipose tissue of obese animals, inhibits glucose disposal by promoting insulin resistance in peripheral tissues (7). Although adipocytes were identified as the source of TNF-α, expression of TNF-α was also observed in the stromovascular fraction rich in immune cells. This observation fueled a new direction in metabolic research and led to the discovery that macrophage infiltration of WAT is responsible for obesity-associated inflammation (3, 4).
While WAT from lean animals contains a resident population of alternatively activated macrophages (also known as M2 macrophages), which are characterized by expression of F4/80, CD301, and arginase 1 (Arg1), obesity is associated with recruitment of classically activated macrophages (also known as M1 macrophages), which are characterized by expression of F4/80, CD11c, and iNOS (Table (Table1).1). This observation led Lumeng and colleagues to conclude that obesity induces a switch in macrophage activation in WAT (8). Since alternatively activated macrophages can suppress inflammation in an autocrine and/or paracrine manner, in part via secretion of IL-10, the net effect of this shift in macrophage polarization is an increase in adipose tissue inflammation. However, this is likely to be an oversimplification because macrophages in vivo exhibit a dynamic range of gene expression patterns along the entire continuum of classical to alternative activation. Nonetheless, impairment in alternative macrophage activation, as in mice lacking either PPARγ or PPARδ specifically in macrophages (9–11), leads to insulin resistance, whereas disruption of inflammatory pathways in classically activated macrophages confers protection against insulin resistance (12). Taken together, these findings suggest that a balance between alternatively and classically activated macrophages is required for normal adipocyte function and insulin action. Furthermore, in addition to macrophages, there is a growing list of immune cells, including CD4+ and CD8+ T cells, Tregs, and mast cells, that participate in the orchestration of inflammation in obese WAT (13–15).
Based on the preferential recruitment of classically activated macrophages during obesity (Figure (Figure1),1), weight loss would be predicted to have the opposite effect; i.e., reduction in adiposity should lead to reduction in ATM content and inflammation. This is indeed the case in patients that have undergone gastric bypass surgery for weight loss (16). However, these measurements were made three months after weight loss and only provide a snapshot of macrophage phenotypes in adipose tissue. To better define the role of macrophages in weight loss, Kosteli and colleagues established a model of gradual weight loss by reducing caloric intake to 70% of normal in mice made obese by high-fat diet (5). While chronic weight loss (>21 days after initiation of caloric restriction) led to a progressive decrease in ATM content in the WAT, there was a surprising increase in the number of ATMs during the first week of weight loss (Figure (Figure1).1). Interestingly, despite the transient increase in ATMs during weight loss, the aggregate expression of markers of inflammation (Tnf, Nos2, Itgax [which encodes CD11c], Serpine 1) or alternative activation (Arg1, chitinase 3-like 3 [Chi3l3]) was not markedly different. However, definitive confirmation of this would require flow cytometric analysis of WAT macrophage subsets during weight loss.
Among the variables examined, increase in the breakdown of stored triglycerides in WAT was tightly correlated with recruitment of ATMs during weight loss (Figure (Figure1).1). This observation led Kosteli and colleagues to postulate that fasting, which increases FFA release from adipocytes, might promote recruitment of macrophages to WAT (5). Indeed, administration of the highly selective β3-agonist CL316,243, which induces lipolysis in adipocytes, or subjection of mice to a 24-hour fast led to an increase in ATMs. Conversely, inhibition of lipolysis by feeding of high-carbohydrate diet or genetic deletion of adipose triglyceride lipase (Pnpla2) prevented macrophage infiltration of WAT. Interestingly, macrophage chemotaxis was enhanced by adipose tissue explants from fasted mice, suggesting the release of a chemotactic factor. However, CCR2 ligands, which promote macrophage recruitment during obesity, were not differentially expressed, implicating the release of an unknown chemoattractant during fasting.
While ATMs in obesity contribute to adipose tissue inflammation and insulin resistance, the functions of ATMs during weight loss are not known. To address this question, Kosteli et al. examined the morphological and molecular characteristics of ATMs during weight loss (5). These studies revealed remarkable similarities between weight loss ATMs and cholesterol-laden macrophages in atheromatous plaques. For instance, both sets of recruited macrophages exhibit higher expression of scavenger receptors (Cd36 and macrophage scavenger receptor 1 [Msr1]) and lipid-handling genes (adipose differentiation–related protein [Adfp], fatty acid-binding protein 4 [Fabp4], Apoe, and Abca1) and increased accumulation of oil red O–positive neutral lipids. Despite these similarities, it is likely that the composition of lipid droplets found in ATMs during weight loss is distinct from that of atheroma macrophages; the latter are known to contain primarily cholesterol (17), whereas the former are likely engorged with triglycerides.
Based on the recruitment and gene expression characteristics of weight loss ATMs, one would predict that these cells facilitate the trafficking of lipids from adipose tissue to liver during fasting or acute weight loss. However, depletion of ATMs by clodronate-containing liposomes gave the opposite result (5). The acute depletion of macrophages increased expression of Pnpla2, resulting in increased release of FFAs in fasted mice. Together, these findings suggest that the recruited ATMs function in a negative feedback loop to dampen the release of FFAs from WAT. How ATMs regulate adipose tissue lipolysis remains unknown and will be an important subject for future investigations.
The report by Kosteli et al. (5) is the first to describe the rapid recruitment of macrophages to adipose tissue during weight loss and their involvement in lipolysis (Figure (Figure1).1). While broadening our understanding of the importance of the immune system in regulating metabolism, these initial studies also raise many new questions, specifically, the why, what, and how: why are macrophages recruited to WAT during fasting, what are the molecular signals that regulate their trafficking in WAT during weight loss, and how do the recruited ATMs regulate adipocyte triglyceride metabolism? We suggest that potential answers to these questions may reside in the known functions of macrophages in metabolism and host defense.
Our understanding of chronic metabolic diseases has been greatly informed by previous studies on macrophage involvement in the reverse cholesterol transport pathway, which promotes net movement of cholesterol from the periphery to the liver for its eventual degradation into bile acids (17). One can envision a similar role for ATMs during fasting or weight loss, in which they facilitate the net movement of FFAs from adipose tissue to liver for their eventual use in energy production. Thus, it will be important to understand the role of ATMs in metabolic adaptations to fasting and weight loss. In addition, the intracellular fate of FFAs taken up by fasting ATMs and the transcriptional response of macrophages to their influx needs to be thoroughly examined. Since fatty acids are endogenous activators of PPARs (2), the involvement of these nuclear receptors in weight loss ATMs deserves further exploration. Finally, the nature of chemoattractant signals (chemokines versus lipids) that promote recruitment of ATMs during fasting or weight loss needs to be investigated.
Because macrophages function as sentinels of host defense, functional consideration of weight loss ATMs in immunity is warranted. In this regard, recent studies have demonstrated dramatic tropism of dormant bacilli and parasites, such as Mycobacterium tuberculosis, Rickettsia prowazekii, and Trypanosoma cruzi, for adipose tissue, which allows these pathogens to evade host immunity (18–20). Since these pathogens cluster around adipocyte lipid droplets, lipolysis might release pathogen-associated lipids, providing an opportune moment for detection of these evasive pathogens. While many questions remain unanswered, the report by Kosteli and colleagues (5) suggests a crucial function for ATMs in both obesity and weight loss. Thus, similar to the road to obesity and insulin resistance, the road to better health might also involve macrophages.
We thank A. Loh for critique on this commentary. A. Chawla was supported by the NIH (DK076760, HL076746, DK081405), a Larry L. Hillblom Foundation Network grant, and the NIH Director’s Pioneer Award (1DP1OD6415-1). A. Red Eagle was supported by the Stanford Medical Scientist Training Program and a Howard Hughes Medical Institute Gilliam fellowship.
Conflict of interest: The authors have declared that no conflict of interest exists.
Citation for this article: J Clin Invest. 2010;120(10):3437–3440. doi:10.1172/JCI44721.
See the related article beginning on page 3466.