Tuo Deng

Medium Chain Fatty Acids Are Selective Peroxisome Proliferator Activated Receptor (PPAR) γ Activators and Pan-PPAR Partial Agonists

Thiazolidinediones (TZDs) act through peroxisome proliferator activated receptor (PPAR) γ to increase insulin sensitivity in type 2 diabetes (T2DM), but deleterious effects of these ligands mean that selective modulators with improved clinical profiles are needed. We obtained a crystal structure of PPARγ ligand binding domain (LBD) and found that the ligand binding pocket (LBP) is occupied by bacterial medium chain fatty acids (MCFAs). We verified that MCFAs (C8–C10) bind the PPARγ LBD in vitro and showed that they are low-potency partial agonists that display assay-specific actions relative to TZDs; they act as very weak partial agonists in transfections with PPARγ LBD, stronger partial agonists with full length PPARγ and exhibit full blockade of PPARγ phosphorylation by cyclin-dependent kinase 5 (cdk5), linked to reversal of adipose tissue insulin resistance. MCFAs that bind PPARγ also antagonize TZD-dependent adipogenesis in vitro. X-ray structure B-factor analysis and molecular dynamics (MD) simulations suggest that MCFAs weakly stabilize C-terminal activation helix (H) 12 relative to TZDs and this effect is highly dependent on chain length. By contrast, MCFAs preferentially stabilize the H2-H3/β-sheet region and the helix (H) 11-H12 loop relative to TZDs and we propose that MCFA assay-specific actions are linked to their unique binding mode and suggest that it may be possible to identify selective PPARγ modulators with useful clinical profiles among natural products.

Class II Major Histocompatibility Complex Plays an Essential Role in Obesity-Induced Adipose Inflammation

Adipose-resident T cells (ARTs) regulate metabolic and inflammatory responses in obesity, but ART activation signals are poorly understood. Here, we describe class II major histocompatibility complex (MHCII) as an important component of high-fat-diet (HFD)-induced obesity. Microarray analysis of primary adipocytes revealed that multiple genes involved in MHCII antigen processing and presentation increased in obese women. In mice, adipocyte MHCII increased within 2 weeks on HFD, paralleling increases in proinflammatory ART markers and decreases in anti-inflammatory ART markers, and preceding adipose tissue macrophage (ATM) accumulation and proinflammatory M1 polarization. Mouse 3T3-L1 and primary adipocytes activated T cells in an antigen-specific, contact-dependent manner, indicating that adipocyte MHCII is functional. HFD-fed MHCII(-/-) mice developed less adipose inflammation and insulin resistance than did wild-type mice, despite developing similar adiposity. These investigations uncover a mechanism whereby a HFD-induced adipocyte/ART dialog involving MHCII instigates adipose inflammation and, together with ATM MHCII, escalates its progression.

Obesity, Inflammation, and Cancer.

Obesity, a worldwide epidemic, confers increased risk for multiple serious conditions, including cancer, and is increasingly recognized as a growing cause of preventable cancer risk. Chronic inflammation, a well-known mediator of cancer, is a central characteristic of obesity, leading to many of its complications, and obesity-induced inflammation confers additional cancer risk beyond obesity itself. Multiple mechanisms facilitate this strong association between cancer and obesity. Adipose tissue is an important endocrine organ, secreting several hormones, including leptin and adiponectin, and chemokines that can regulate tumor behavior, inflammation, and the tumor microenvironment. Excessive adipose expansion during obesity causes adipose dysfunction and inflammation to increase systemic levels of proinflammatory factors. Cells from adipose tissue, such as cancer-associated adipocytes and adipose-derived stem cells, enter the cancer microenvironment to enhance protumoral effects. Dysregulated metabolism that stems from obesity, including insulin resistance, hyperglycemia, and dyslipidemia, can further impact tumor growth and development. This review describes how adipose tissue becomes inflamed in obesity, summarizes ways these mechanisms impact cancer development, and discusses their role in four adipose-associated cancers that demonstrate elevated incidence or mortality in obesity.

Transcriptome analysis of human adipocytes implicates the NOD-like receptor pathway in obesity-induced adipose inflammation

Adipose tissue inflammation increases with obesity, but adipocyte vs. immune cell contributions are unclear. In the present study, transcriptome analyses were performed on highly-purified subcutaneous adipocytes from lean and obese women, and differentially expressed genes/pathways were determined in both adipocyte and stromal vascular fraction (SVF) samples. Adipocyte but not SVF expression of NOD-like receptor pathway genes, including NLRP3 and PYCARD, which regulate caspase-1-mediated IL-1β secretion, correlated with adiposity phenotypes and adipocyte class II major histocompatibility complex (MHCII) gene expression, but only MHCII remained after adjusting for age and body mass index. IFNγ stimulated adipocyte MHCII, NLRP3 and caspase-1 expression, while adipocyte MHCII-mediated CD4(+) T cell activation, an important factor in adipose inflammation, induced IFNγ-dependent adipocyte IL-1β secretion. These results uncover a dialogue regulated by interactions among T cell IFNγ and adipocyte MHCII and NLRP3 inflammasome activity that appears to initiate and escalate adipose tissue inflammation during obesity.

A Peroxisome Proliferator-activated Receptor γ (PPARγ)/PPARγ Coactivator 1β Autoregulatory Loop in Adipocyte Mitochondrial Function

Peroxisome proliferator-activated receptor γ (PPARγ) activation induces adipogenesis and also enhances lipogenesis, mitochondrial activity, and insulin sensitivity in adipocytes. Whereas some studies implicate PPARγ coactivator 1α (PGC-1α) in the mitochondrial effect, the mechanisms involved in PPARγ regulation of adipocyte mitochondrial function are not resolved. PPARγ-activating ligands (thiazolidinediones (TZDs)) are important insulin sensitizers and were recently shown to indirectly induce PGC-1β transcription in osteoclasts. Here, we asked whether similar effects occur in adipocytes and show that TZDs also strongly induce PGC-1β in cultured 3T3-L1 cells. This effect, however, differs from the indirect effect proposed for bone and is rapid and direct and involves PPARγ interactions with an intronic PPARγ response element cluster in the PGC-1β locus. TZD treatment of cultured adipocytes results in up-regulation of mitochondrial marker genes, and increased mitochondrial activity and use of short interfering RNA confirms that these effects require PGC-1β. PGC-1β did not participate in PPARγ effects on adipogenesis or lipogenesis, and PGC-1β knockdown did not alter insulin-responsive glucose uptake into 3T3-L1 cells. Similar effects on PGC-1β and mitochondrial gene expression are seen in vivo; fractionation of obese mouse adipose tissue reveals that PPARγ and PGC-1β, but not PGC-1α, are coordinately up-regulated in adipocytes relative to preadipocytes and that TZD treatment induces PGC-1β and mitochondrial marker genes in adipose tissue of obese mice. We propose that PPARγ directly induces PGC-1β expression in adipocytes and that this effect regulates adipocyte mitochondrial activity.

Mechanisms of peroxisome proliferator activated receptor γ regulation by non-steroidal anti-inflammatory drugs

Non-steroidal anti-inflammatory drugs (NSAIDs) display anti-inflammatory, antipyretic and analgesic properties by inhibiting cyclooxygenases and blocking prostaglandin production. Previous studies, however, suggested that some NSAIDs also modulate peroxisome proliferator activated receptors (PPARs), raising the possibility that such off target effects contribute to the spectrum of clinically relevant NSAID actions. In this study, we set out to understand how peroxisome proliferator activated receptor-γ (PPARγ/PPARG) interacts with NSAIDs using X-ray crystallography and to relate ligand binding modes to effects on receptor activity. We find that several NSAIDs (sulindac sulfide, diclofenac, indomethacin and ibuprofen) bind PPARγ and modulate PPARγ activity at pharmacologically relevant concentrations. Diclofenac acts as a partial agonist and binds to the PPARγ ligand binding pocket (LBP) in typical partial agonist mode, near the β-sheets and helix 3. By contrast, two copies of indomethacin and sulindac sulfide bind the LBP and, in aggregate, these ligands engage in LBP contacts that resemble agonists. Accordingly, both compounds, and ibuprofen, act as strong partial agonists. Assessment of NSAID activities in PPARγ-dependent 3T3-L1 cells reveals that NSAIDs display adipogenic activities and exclusively regulate PPARγ-dependent target genes in a manner that is consistent with their observed binding modes. Further, PPARγ knockdown eliminates indomethacin activities at selected endogenous genes, confirming receptor-dependence of observed effects. We propose that it is important to consider how individual NSAIDs interact with PPARγ to understand their activities, and that it will be interesting to determine whether high dose NSAID therapies result in PPAR activation.

Epicardial Fat in the Maintenance of Cardiovascular Health

Epicardial fat is a unique adipose tissue located between the myocardium and the visceral layer of pericardium. This tissue is characterized by highly active fatty acid metabolism and highly expressed thermogenic genes. Epicardial fat and the underlying myocardium share the same microcirculation, suggesting a close and strong interaction between these two structures. Under physiological conditions, epicardial fat protects and supports the heart to exert its normal function. Many clinical studies have shown significant associations between increased amounts of epicardial fat and coronary artery disease (CAD). In patients with CAD, increased epicardial fat becomes inflammatory and may promote plaque development through secretion of proinflammatory cytokines and other mechanisms. Therefore, epicardial fat is a biomarker of cardiovascular risk and a potential therapeutic target for cardiovascular disease. Weight loss and pharmaceuticals can reduce epicardial fat and improve its protective physiological functions.

Inflammasomes and Obesity

Inflammasomes are a family of protein complexes that recognize diverse microbial and endogenous danger signals to promote innate immune responses, tissue inflammation and injury, or cell death via pyroptosis. Inflammasome activation results in the recruitment and activation of caspase-1, which is required for the production of the proinflammatory cytokines interleukin-1β (IL-1β) and IL-18 that can modulate both adaptive and innate immune responses through effects on leukocyte survival, proliferation, differentiation, and migration. Recent studies suggest that inflammasome activity may also play important roles in several nonmicrobial disease states associated with chronic inflammation. For example, NLRP3 inflammasome expression and IL-1β production are both increased in obesity, and recent work has implicated NLRP3 inflammasome activation in a variety of obesity-linked conditions including gout, type 2 diabetes mellitus, metabolic liver disease, atherosclerosis, Alzheimer’s disease, cancer and rheumatoid arthritis. Further, many of the factors associated with these conditions, including elevated plasma glucose, fatty acids, uric acid and cholesterol crystals, and β-amyloid, have been shown to be elevated during obesity and to stimulate NLRP3 inflammasome expression or activation. Since chronic NLRP3 activation appears to play important roles in several common disease states, better understanding of inflammasome regulation and function may lead to better therapeutic approaches. Several agents that attenuate NLRP3 inflammasome activity or inhibit its primary effector, IL-1β, are currently under development or in early clinical trials as therapeutics to treat these common disease conditions. This chapter will review new research on inflammasome activation, its role in obesity and other chronic inflammatory states, and the status of approaches to attenuate NLRP3 inflammasome activity.