Obesity is associated with inflammation, which may play an important role in fatty liver disease and insulin resistance (1
). In this study, we showed that in a group of obese Hispanics and African Americans, 44% of the participants had adipose tissue inflammation, which was associated with higher amounts of VAT and liver fat, hyperinsulinemia, and reduced β-cell function. At the molecular level, individuals with CLS showed upregulation of several genes belonging to the NF-κB stress pathway.
In obese individuals, inflammatory processes are thought to originate from the excessive accumulation of fat in the adipose tissue, where it translates into recruitment of macrophages around dead adipocytes, forming ring patterns known as CLS (12
). Consistent with a role for CLS in adipose tissue inflammation, we demonstrated the presence of a subclass of proinflammatory macrophages, CD11c+ dendritic cells, which have been linked to systemic insulin resistance, only in adipose tissue of subjects with CLS (10
). Moreover, we demonstrated that in obese Hispanics and African Americans, VAT, liver fat, and circulating TNF-α were significantly higher in individuals with CLS in fat biopsies, independent of ethnicity, sex, total body fat, and SAT. Such accumulation of fat in the visceral compartment and the liver markedly increases the risk for type 2 diabetes, and these metabolic abnormalities have been recognized as independent features of the metabolic syndrome (19
). Indeed, inflammation is also closely associated with insulin resistance, both in the liver and the adipose tissue (21
). This may be because of decreased insulin signaling by inflammatory mediators such as IL-1 and TNF-α (24
). Consistent with these previous observations, we show that individuals with CLS (CLS+) had increased fasting glucose and insulin, as well as decreased DI, reflecting altered glucose homeostasis and β-cell function.
We further investigated the effect of inflammation separately in Hispanics and African Americans. The prevalence of individuals with inflammation was equally distributed among ethnicities, suggesting that there is no existing ethnic predisposition to adipose tissue inflammation. Previous reports have shown that Hispanics are more prone to accumulation of lipids in the VAT and the liver (11
). Given the tight link between ectopic fat and inflammation, we therefore expected Hispanics to have a higher degree of inflammation. However, our results show that inflammation may occur in Hispanics regardless of VAT amount. Moreover, presence of CLS was associated with lower SI and increased plasma TNF-α concentrations. In contrast, African Americans usually display lower amounts of VAT; in this ethnic group, we found that VAT was significantly increased only by the presence of adipose tissue inflammation. Although we cannot establish any causal relationship at this point, these ethnic discrepancies suggest that in Hispanics, presence of inflammation is not a major player in these adipose tissue depots accumulation, but is instead associated with the development of systemic inflammation and insulin resistance. By contrast, in African Americans, who are usually protected against VAT accumulation, presence of adipose tissue inflammation may reflect a generalized increase in adipocyte depot mass and activation of inflammatory pathways. This suggests that presence of adipose tissue inflammation may be linked to distinct metabolic outcomes, depending on ethnicity.
Based on these clinical observations, we investigated how different gene expression patterns between CLS+ and CLS− individuals could provide a functional link between adipose tissue inflammation, hepatic fat accumulation, and insulin resistance. As a result, several molecules related to the NF-κB pathway were upregulated in CLS+ individuals. NF-κB pathway mediates important stress responses and activates several proinflammatory cascades (25
). Classically, its activation requires the binding of proinflammatory stimuli such as TNF-α, IL-1, or bacterial lipopolysaccharide to their appropriate membrane receptors, which subsequently trigger the activation cascade (26
). Activation of NF-κB results in enhanced transcription of IL-6, IL-8, and TNF-α, which trigger stress and inflammatory pathways and further stimulate the NF-κB pathway (21
). NF-κB also leads to increased MCP-1, which locally recruits macrophages, and MMP9, which plays a role in inflammation-mediated tissue remodeling, such as adipocyte size expansion (27
) and fibrosis in the liver (28
). Notably, all of these molecules were upregulated in CLS+ individuals in the current study, reflecting a higher degree of inflammation and macrophage activation (). In mononuclear cells from obese individuals, the NF-κB pathway is also activated, contributing to higher blood concentrations of proinflammatory mediators such IL-6, TNF-α, and MMP9 (29
). This suggests that CLS-associated macrophages may have similar roles in adipose tissue, thus contributing to local release of proinflammatory cytokines (27
). Using pathway analysis, we also observed that the most differentially regulated pathways were related to liver functions/diseases, such as liver X receptor/retinoid X receptor activation, hepatic cholestasis, and hepatic fibrosis pathways, with activation of several genes related to inflammatory processes, and more specifically to the NF-κB pathway. These pathways play pivotal roles in liver injury and may lead to increased hepatic lipid synthesis (30
Our present results show that CLS may be found in some but not all obese individuals, independently of their ethnic background or sex, and is associated with increased VAT, hepatic fat content, and insulin resistance (33
). Although our results are based on associations, when evaluated together with existing literature, the findings collectively suggest that with increasing obesity, adipocytes enlarge, until reaching a threshold (33
). This may promote adipocyte death, macrophage aggregation, and CLS formation (8
). The ensuing inflammatory activity may trigger the NF-κB stress pathway and additional remodeling mechanism, characterized by increased MMP9 protease expression (34
). Excess TG accumulation in adipose tissue may subsequently spill over to the systemic circulation and in turn accumulate in ectopic tissues, such as VAT and the liver (7
), which may both contribute to decreased SI. The fact that in Hispanics high amounts of VAT are not necessarily associated with adipose tissue inflammation suggests that such relationships may be more subtle and possibly depend on other environmental and/or genetic factors.
One limitation of our study is the absence of nutritional data. Both high-fat or high-fructose diets induce insulin resistance and inflammation (37
). It remains therefore possible that differences in dietary intakes may contribute to the inflammatory status of adipose tissue. Whether the nutritional status may affect adipose tissue inflammation and the presence of CLS over the long run remains to be investigated. Moreover, this study was limited to African Americans and Hispanics, and obesity duration was not collected. It remains therefore to determine how the onset and duration of obesity may affect adipose tissue inflammation in these populations. Finally, we did not separate adipocytes from other cell types. Further studies using cell sorting will be required to discriminate the expression profile between cell types within the adipose tissue.
In conclusion, this study demonstrates that macrophage infiltration in SAT from obese individuals is equally distributed between sexes and ethnicities. Presence of inflammation is associated with higher VAT and hepatic fat content, as well as higher fasting glucose and insulin and reduced β-cell function, independent of total fat. These phenotypes may be attributed to upregulation of major gene pathways involved in proinflammatory cascades, such as the NF-κB stress pathway. Further intervention studies will be required to assess the time course of such metabolic alterations and establish causal relationship between adipose tissue inflammation, hepatic fat accumulation, and development of insulin resistance.