Macrophage Content in Subcutaneous Adipose Tissue: Associations With Adiposity, Age, Inflammatory Markers, and Whole-Body Insulin Action in Healthy Pima Indians

OBJECTIVE— In severely obese individuals and patients with diabetes, accumulation and activation of macrophages in adipose tissue has been implicated in the development of obesity-associated complications, including insulin resistance. We sought to determine whether in a healthy population, adiposity, sex, age, or insulin action is associated with adipose tissue macrophage content (ATMc) and/or markers of macrophage activation.RESEARCH DESIGN AND METHODS— Subcutaneous ATMc from young adult Pima Indians with a wide range of adiposity (13–46% body fat, by whole-body dual-energy X-ray absorptiometry) and insulin action (glucose disposal rate 1.6–9 mg/kg estimated metabolic body size/min, by glucose clamp) were measured. We also measured expression in adipose tissue of factors implicated in macrophage recruitment and activation to determine any association with ATMc and insulin action.RESULTS— ATMc, as assessed by immunohistochemistry (Mphi) and by macrophage-specific gene expression (CD68, CD11b, and CSF1R), were correlated with percent body fat, age, and female sex. Gene expression of CD68, CD11b, and CSF1R but not Mphi was correlated negatively with glucose disposal rate but not after adjustment for percent body fat, age, and sex. However, adipose tissue expression of plasminogen activator inhibitor type-1 (PAI-1) and CD11 antigen-like family member C (CD11c), markers produced by macrophages, were negatively correlated with adjusted glucose disposal rate (r = −0.28, P = 0.05 and r = −0.31, P = 0.03).CONCLUSIONS— ATMc is correlated with age and adiposity but not with insulin action independent of adiposity in healthy human subjects. However, PAI-1 and CD11c expression are independent predictors of insulin action, indicating a possible role for adipose tissue macrophage activation.Obesity is an inflammatory condition leading to chronic activation of an innate immune response (1). This inflammatory response has been implicated in the pathogenesis of obesity-associated complications, including atherosclerosis (2), nonalcoholic fatty liver disease (3), and insulin resistance (4). Adipose tissue is a primary site of obesity-induced inflammation and a complex organ containing adipocytes as well as connective tissue matrix, nerve tissue, stromal vascular cells, and immune cells. A cardinal feature of obesity-induced inflammation in adipose tissue is the recruitment of immune cells, specifically macrophages (5,6). Although the adipocyte is the defining cell of adipose tissue and does contribute to the production of inflammatory molecules (7), it appears that macrophages contribute substantially to the inflammatory signals that are induced by obesity (5,8–11).Among the inflammatory factors whose expression is upregulated in adipose tissue with the onset of obesity, some have been implicated in recruitment of macrophages to adipose tissue, including chemokines, while others appear to be derived primarily from adipose tissue macrophages (ATMs). Studies in rodents indicate that ATMs are bone marrow–derived cells recruited to adipose tissue during periods of positive energy balance and increasing adiposity (5). However, the physiology of macrophage recruitment remains largely unknown. It has been hypothesized that a metabolic signal(s) or stress(es) leads to activation of endothelial cells, production of chemoattractants with subsequent transendothelial migration of monocytes (12), monocyte differentiation into mature macrophages, and ultimately macrophage activation. A few studies have also suggested that differentiation of adipocyte precursors into macrophage-like cells (6,13) can occur, although this remains controversial. Studies have implicated monocyte chemoattractant proteins, hypoxia, and angiogenesis in ATM recruitment. In particular, the adhesion molecule intercellular adhesion molecule 1 (ICAM1) is important in the recruitment of monocytes to sites of inflammation (14), its soluble plasma concentrations have been found to be positively associated with adiposity (15,16), and in previous microarray studies in mice adipose tissue Icam1 expression was correlated with body mass (5).Animal and human studies of obese and diabetic subjects indicate that adipose tissue macrophage content (ATMc) correlates with degree of adiposity (5,6,8,9,12). In a small, interventional study, the subcutaneous expression of CD68, a macrophage marker, correlated with insulin resistance (10). In obese individuals, the degree of hepatic fibroinflammatory lesions or fat liver content is associated with omental or subcutaneous ATM infiltration (17–19). The association of ATMc with insulin resistance and nonalcoholic fatty liver disease indicates a role for ATMc in obesity-related complications. However, it is not clear yet whether ATMc or activation in healthy adults affects insulin action beyond their association with adiposity. In rodents, genetic manipulation of the activation of myeloid cells, including macrophages, alters insulin sensitivity (20–22).In the present study, we examined in healthy nondiabetic individuals the association of subcutaneous ATMc and activation with direct measurements of both adiposity and whole-body insulin sensitivity. In addition, we investigated the relationship of ATMc and subcutaneous adipose tissue expression of genes potentially involved in attraction of macrophages into adipose tissue.