Reversal of inflammation-induced impairment of glucose uptake in adipocytes by direct effect of CB1 antagonism on adipose tissue macrophages.

Macrophage infiltration into adipose tissue (AT-MP) is thought to induce insulin resistance and diabetes in obesity. Here, we investigated the effect of the antiobesity drug SR141716 (a CB1 antagonist) on macrophage-mediated inhibition of insulin signaling in adipocytes. THP1 macrophages (THP1) were stimulated in vitro with lipopolysaccharide (LPS) and SR141716 or vehicle. The resulting conditioned medium (CM) was analyzed and incubated on human adipocytes. CM from LPS-stimulated THP1 inhibited insulin-induced AKT phosphorylation in adipocytes, in contrast to CM from nonactivated THP1. Moreover, it contained higher concentrations of tumor necrosis factor-α (TNFα) and lower levels of the anti-inflammatory cytokine IL-10. SR141716 reduced TNFα production and increased IL-10 secretion, resulting in a rescue of insulin signaling in adipocytes. To confirm these findings in vivo, AT-MP CM from cafeteria diet-fed or Zucker diabetic fatty (ZDF) rats that had received SR141716 for 3 weeks were isolated, analyzed, and incubated with adipocytes. Cafeteria diet induced macrophage-mediated inhibition of insulin signaling in adipocytes. Interestingly, SR141716 rescued insulin-induced glucose uptake in adipocytes. Finally, AT-MP CM from obese ZDF rats inhibited insulin-stimulated glucose uptake in adipocytes in contrast to AT-MP CM from lean ZDF rats. After treatment with SR141716, AT-MP CM rescued insulin-induced glucose uptake in adipocytes. In summary, our data indicate that CB1 receptor antagonism in macrophages modified their cytokine production and improved the insulin responsiveness of adipocytes that had been incubated with macrophage CM. Thus, SR141716 ameliorated adipose tissue insulin resistance by direct action on AT-MP demonstrating a novel peripheral mode of action of CB1 antagonism.

Chemotaxis and differentiation of human adipose tissue CD34+/CD31- progenitor cells: role of stromal derived factor-1 released by adipose tissue capillary endothelial cells.

The native CD34+/CD31- cell population present in the stroma-vascular fraction of human adipose tissue (hAT) displays progenitor cell properties since they exhibit adipocyte- and endothelial cell-like phenotypes under appropriate stimuli. To analyze the signals within hAT regulating their phenotypes, the influence of hAT-derived capillary endothelial cells (CECs) was studied on the chemotaxis and differentiation of the hAT-CD34+/CD31- cells. Conditioned medium from hAT-CECs led to a strong chemotaxis of the hAT-CD34+/CD31- cells that was inhibited with pretreatments with pertussis toxin, CXCR-4 antagonist, or neutralizing antibodies. Furthermore, hAT-CECs produced and secreted the CXCR-4 ligand, that is, the stromal derived factor-1 (SDF-1). Finally, hAT-CECs induced the differentiation of hAT-CD34+/CD31- cells toward an endothelial cell (EC) phenotype. Indeed, hAT-CECs and -CD34+/CD31- cell coculture stimulated in a two-dimensional system the expression of the EC CD31 marker by the hAT-progenitor cells and, in a three-dimensional approach, the formation of capillary-like structures via a SDF-1/CXCR-4 dependent pathway. Thus, the migration and differentiation of hAT progenitor cells are modulated by hAT-CEC-derived factors. SDF-1, which is secreted by hAT-derived CECs, and its receptor CXCR-4, expressed by hAT-derived progenitor cells, may promote chemotaxis and differentiation of hAT-derived progenitor cells and thus contribute to the formation of the vascular network during the development of hAT.

Macrophage-secreted factors impair human adipogenesis: involvement of proinflammatory state in preadipocytes.

Obesity is considered a chronic low-grade inflammatory state. The white adipose tissue produces a variety of inflammation-related proteins whose expression is increased in obese subjects. The nonadipose cell fraction, which includes infiltrated macrophages, is a determinant source of inflammation-related molecules within the adipose tissue. Our working hypothesis is that macrophage infiltration affects fat expansion through a paracrine action on adipocyte differentiation. Human primary preadipocytes were then differentiated in the presence of conditioned media obtained from macrophages differentiated from blood monocytes. Preadipocytes treated by macrophage-conditioned medium displayed marked reduction of adipogenesis as assessed by decreased cellular lipid accumulation and reduced gene expression of adipogenic and lipogenic markers. In addition to this effect, the activation of macrophages by lipopolysaccharides stimulated nuclear factor kappaB signaling, increased gene expression and release of proinflammatory cytokines and chemokines, and induced preadipocyte proliferation. This phenomenon was associated with increased cyclin D1 gene expression and maintenance of the fibronectin-rich matrix. Anti-TNFalpha neutralizing antibody inhibits the inflammatory state of preadipocytes positioning TNFalpha as an important mediator of inflammation in preadipocytes. Strikingly, conditioned media produced by macrophages isolated from human adipose tissue exerted comparable effects with activated macrophages, i.e. decreased adipogenesis and increased inflammatory state in the preadipocytes. These data show that macrophage-secreted factors inhibit the formation of mature adipocytes, suggesting possible role in limiting adipose tissue expansion in humans.

Role of macrophage tissue infiltration in metabolic diseases.

PURPOSE OF REVIEW: White adipose tissue is necessary for optimal energy homeostasis and the excessive development of fat mass is clearly associated with the metabolic syndrome. The fact that adipocytes secrete a number of specific factors or 'adipokines' has forced a reassessment of the involvement of adipose tissue in a wide range of physiological and pathophysiological processes. Obesity has recently been described as a 'low-grade' inflammatory condition, a state proposed to represent a common determinator in the genesis of obesity-associated pathologies, i.e. diabetes and atherosclerosis. RECENT FINDINGS: Recent reports of an increase in the number of macrophages that infiltrate the fat mass in obese individuals led to the suggestion that adipose tissue itself is a source and site of inflammation. SUMMARY: This review summarizes recent data on the characterization of the macrophage population in fat tissue. Their origin, fate and activation will be considered. The potential involvement of adipose tissue macrophages in the development of insulin resistance and vascular pathologies, as well as in the control of adipose tissue growth and metabolism, will be examined.

From blood monocytes to adipose tissue-resident macrophages: induction of diapedesis by human mature adipocytes.

Obesity has been suggested to be a low-grade systemic inflammatory state, therefore we studied the interaction between human adipocytes and monocytes via adipose tissue (AT)-derived capillary endothelium. Cells composing the stroma-vascular fraction (SVF) of human ATs were characterized by fluorescence-activated cell sorter (FACS) analysis and two cell subsets (resident macrophages and endothelial cells [ECs]) were isolated using antibody-coupled microbeads. Media conditioned by mature adipocytes maintained in fibrin gels were applied to AT-derived ECs. Thereafter, the expression of endothelial adhesion molecules was analyzed as well as the adhesion and transmigration of human monocytes. FACS analysis showed that 11% of the SVF is composed of CD14(+)/CD31(+) cells, characterized as resident macrophages. A positive correlation was found between the BMI and the percentage of resident macrophages, suggesting that fat tissue growth is associated with a recruitment of blood monocytes. Incubation of AT-derived ECs with adipocyte-conditioned medium resulted in the upregulation of EC adhesion molecules and the increased chemotaxis of blood monocytes, an effect mimicked by recombinant human leptin. These results indicate that adipokines, such as leptin, activate ECs, leading to an enhanced diapedesis of blood monocytes, and suggesting that fat mass growth might be linked to inflammatory processes.