Adipose Dysfunction, Interaction of Reactive Oxygen Species, and Inflammation

As the obesity epidemic in the United States continues to increase, understanding the mechanisms by which intrinsic and extrinsic factors can contribute to dysfunctional adipose becomes very important. Our symposium brought together several experts in the field to delineate the significance of the very complex world of obesity-associated reactive oxygen species (ROS), inflammation, and adipose tissue dysfunction. Thus, the content of this symposium, the first ASN symposium devoted to this emerging paradigm, was both timely and cutting edge. Because many nutrients and food components have anti-inflammatory and antioxidant activities, we gained valuable insights into the mechanisms by which dietary factors may modulate adipose tissue dysfunction. Recent data indicate that adipose dysfunction in obesity involves interactions between inflammatory pathways and ROS signaling. Although numerous studies indicate that oxidative stress is elevated overall in the body as a result of obesity, only now is there emerging literature detailing the role of oxidant stress and related signaling pathways in the regulation and dysfunction of adipose tissue. However, there is dearth of data and an incomplete understanding of adipose tissue cell types in relation to oxidative stress associated with obesity. Furthermore, the inflammatory processes that are present in obese adipose tissue modulate cellular oxidant stress/antioxidant responses to oxidative stress. As a result, the objectives of this symposium were to 1) describe the cell types that contribute to normal and pathological function of adipose tissue, 2) understand the process of adipocyte differentiation in normal and pathological conditions, 3) describe the pathways that lead to changes in oxidant signaling and oxidant stress in obesity, 4) describe cellular responses to elevated oxidant signaling and how they influence overall adipocyte metabolism, and 5) understand the inflammatory signaling processes that influence oxidant signaling in adipose tissues. These objectives were presented and discussed by 4 invited speakers in this symposium. Their presentations are summarized in the following. Professor Martin Obin’s presentation focused on the functions and metabolic impacts of leukocyte recruitment to adipose tissue in obesity. Data presented in his talk included information regarding adipocyte turnover and the accompanying formation of crown-like structures (CLS)5. CLS formation is a seminal event associated with macrophage localization, polarization, and activation in adipose tissue. CLS develop into sites of fibrosis (excessive deposition of collagen and other extracellular matrix components) that ultimately impair the ability of adipocytes to expand and safely store triglycerides. The data emphasized an important role for recruited macrophages and macrophage-derived inflammatory mediators in obesity-associated adipose tissue remodeling. Blockade of adipocyte hypertrophy and lipid storage is linked to the development of peripheral and systemic insulin resistance and hepatic steatosis. Other work indicated that different classes of macrophages are present in adipose tissue under lean and obese conditions. M1 macrophages, having a proinflammatory phenotype, are present in obese adipose tissue and secrete TNF-α, whereas M2 macrophages, having a more anti-inflammatory effect, are present in lean adipose tissue and secrete IL-10. Subsequent work is focused on the signaling mechanisms, including the impact of oxidative stress, by which macrophages modulate CLS formation and adipose tissue dysfunction. The presentation of Professor Alexander Sorisky focused on cell-surface receptor signal transduction in the context of macrophage-adipocyte interactions in healthy adipose tissue and during adipose dysfunction. His research group uses primary human adipose cells and established cell lines to understand how disturbances in signal transduction networks, regulated by cell-surface tyrosine kinase or G protein–coupled receptors, result in metabolic malfunction such as insulin resistance and inflammation. Data presented showed that factors present in macrophage-conditioned medium inhibit adipocyte differentiation. On the other hand, macrophage-secreted platelet-derived growth factor protects preadipocytes from apoptosis following serum withdrawal. Other data from the Sorisky laboratory indicate that the activation status (LPS-treated vs. IL-4 treated) of the macrophage further modulates these effects. This work clearly demonstrates that the inflammatory condition of adipose tissue governs “healthy” versus “dysfunctional” adipose tissue in obesity. Data presented by Professor David Bernlohr demonstrated a mechanistic link between inflammatory cytokines, oxidative damage, and aberrant respiration that centers on the metabolism and chemical reactivity of carbonyl species. Protein-bound carbonyls are widely used as markers of oxidative stress, but this modification of proteins has functional consequences for protein function. Seminal proteomic studies by the Bernlohr laboratory identified elevated levels of several carbonylated proteins in adipose tissue of obese individuals and in animal models of obesity. Expression of glutathione S-transferase (GST) A4, an enzyme with a high activity toward detoxifying reactive carbonyls, is reduced in adipose tissue from obese individuals and obese mice. In vitro studies with 3T3-L1 adipocytes demonstrate that treatment with the inflammatory cytokine TNF-α reduces GSTA4 expression. Knockdown of GSTA4 reduces mitochondrial respiration in 3T3-L1 adipocytes, an effect recapitulated in the adipocytes of GSTA4 null mice. In both systems, there is an increase in superoxide anion generation in the adipocytes lacking GSTA4 as well as an increase in Krebs cycle intermediates and acyl-carnitines. Further studies are under way to identify the mitochondrial proteins affected by GSTA4 knockout, if they are carbonylated, and in what manner. Although oxidative damage and oxidative stress are viewed as pathological mediators of multiple diseases, ROS and oxidative modifications to proteins have nonpathological signaling properties. The transcription factor nuclear factor E2-related factor 2 (Nrf2) resides in the cytosol until it is released during oxidative/electrophilic stress. Nrf2 then travels to the nucleus and binds to the antioxidant response element present in the promoters of multiple oxidant stress defense genes such as those for glutathione synthesis and GSTs. However, groundbreaking studies presented by Professor Jefferson Chan indicate that the Nrf2/antioxidant response element signaling pathway regulates energy metabolism. Mice lacking Nrf2 are leaner and are not able to accrue adipose tissue compared with wild-type mice. In addition, Nrf2 null mice do not gain adipose tissue on a high-fat diet compared with wild-type animals. In vitro studies demonstrate that the lack of Nrf2 inhibits adipogenesis in part by blocking PPAR-γ expression. On the other hand, it is likely that this mechanism is not the only reason for a lack of adipose tissue in vivo. Dr. Chan’s preliminary data suggest that lack of Nrf2 increases energy metabolism because Nrf2 null mice have similar food intake and fat absorption. Subsequent studies are under way to define the extent to which Nrf2 regulates energy utilization. From the 4 presentations, the audience gained a greater understanding of the interactions that inflammatory stimuli and ROS have on adipose tissue function and dysfunction. In conclusion, it was also clear that adipose dysfunction is a complex phenomenon, which includes the interplay of multiple cell types and several signaling pathways.