Evelyn Walenta

A Gpr120-selective agonist improves insulin resistance and chronic inflammation in obese mice

It is well known that the ω–3 fatty acids (ω–3-FAs; also known as n–3 fatty acids) can exert potent anti-inflammatory effects. Commonly consumed as fish products, dietary supplements and pharmaceuticals, ω–3-FAs have a number of health benefits ascribed to them, including reduced plasma triglyceride levels, amelioration of atherosclerosis and increased insulin sensitivity. We reported that Gpr120 is the functional receptor for these fatty acids and that ω–3-FAs produce robust anti-inflammatory, insulin-sensitizing effects, both in vivo and in vitro, in a Gpr120-dependent manner. Indeed, genetic variants that predispose to obesity and diabetes have been described in the gene encoding GPR120 in humans (FFAR4). However, the amount of fish oils that would have to be consumed to sustain chronic agonism of Gpr120 is too high to be practical, and, thus, a high-affinity small-molecule Gpr120 agonist would be of potential clinical benefit. Accordingly, Gpr120 is a widely studied drug discovery target within the pharmaceutical industry. Gpr40 is another lipid-sensing G protein–coupled receptor, and it has been difficult to identify compounds with a high degree of selectivity for Gpr120 over Gpr40 (ref. 11). Here we report that a selective high-affinity, orally available, small-molecule Gpr120 agonist (cpdA) exerts potent anti-inflammatory effects on macrophages in vitro and in obese mice in vivo. Gpr120 agonist treatment of high-fat diet–fed obese mice causes improved glucose tolerance, decreased hyperinsulinemia, increased insulin sensitivity and decreased hepatic steatosis. This suggests that Gpr120 agonists could become new insulin-sensitizing drugs for the treatment of type 2 diabetes and other human insulin-resistant states in the future.

Characterization of Distinct Subpopulations of Hepatic Macrophages in HFD/Obese Mice

The current dogma is that obesity-associated hepatic inflammation is due to increased Kupffer cell (KC) activation. However, recruited hepatic macrophages (RHMs) were recently shown to represent a sizable liver macrophage population in the context of obesity. Therefore, we assessed whether KCs and RHMs, or both, represent the major liver inflammatory cell type in obesity. We used a combination of in vivo macrophage tracking methodologies and adoptive transfer techniques in which KCs and RHMs are differentially labeled with fluorescent markers. With these approaches, the inflammatory phenotype of these distinct macrophage populations was determined under lean and obese conditions. In vivo macrophage tracking revealed an approximately sixfold higher number of RHMs in obese mice than in lean mice, whereas the number of KCs was comparable. In addition, RHMs comprised smaller size and immature, monocyte-derived cells compared with KCs. Furthermore, RHMs from obese mice were more inflamed and expressed higher levels of tumor necrosis factor-α and interleukin-6 than RHMs from lean mice. A comparison of the MCP-1/C-C chemokine receptor type 2 (CCR2) chemokine system between the two cell types showed that the ligand (MCP-1) is more highly expressed in KCs than in RHMs, whereas CCR2 expression is approximately fivefold greater in RHMs. We conclude that KCs can participate in obesity-induced inflammation by causing the recruitment of RHMs, which are distinct from KCs and are not precursors to KCs. These RHMs then enhance the severity of obesity-induced inflammation and hepatic insulin resistance.

NAT8L (N-Acetyltransferase 8-Like) Accelerates Lipid Turnover and Increases Energy Expenditure in Brown Adipocytes

Background: NAT8L (N-acetyltransferase 8-like) synthesizes N-acetylaspartate and is required for myelination in the brain. Its function in other tissues was undefined. Results: Nat8l is highly expressed in adipose tissues and impacts adipogenic marker gene expression, lipid turnover, and energy metabolism in brown adipocytes. Conclusion: Nat8l expression influences cellular bioenergetics in adipocytes. Significance: These findings establish a novel pathway in brown adipocyte metabolism. NAT8L (N-acetyltransferase 8-like) catalyzes the formation of N-acetylaspartate (NAA) from acetyl-CoA and aspartate. In the brain, NAA delivers the acetate moiety for synthesis of acetyl-CoA that is further used for fatty acid generation. However, its function in other tissues remained elusive. Here, we show for the first time that Nat8l is highly expressed in adipose tissues and murine and human adipogenic cell lines and is localized in the mitochondria of brown adipocytes. Stable overexpression of Nat8l in immortalized brown adipogenic cells strongly increases glucose incorporation into neutral lipids, accompanied by increased lipolysis, indicating an accelerated lipid turnover. Additionally, mitochondrial mass and number as well as oxygen consumption are elevated upon Nat8l overexpression. Concordantly, expression levels of brown marker genes, such as Prdm16, Cidea, Pgc1α, Pparα, and particularly UCP1, are markedly elevated in these cells. Treatment with a PPARα antagonist indicates that the increase in UCP1 expression and oxygen consumption is PPARα-dependent. Nat8l knockdown in brown adipocytes has no impact on cellular triglyceride content, lipogenesis, or oxygen consumption, but lipolysis and brown marker gene expression are increased; the latter is also observed in BAT of Nat8l-KO mice. Interestingly, the expression of ATP-citrate lyase is increased in Nat8l-silenced adipocytes and BAT of Nat8l-KO mice, indicating a compensatory mechanism to sustain the acetyl-CoA pool once Nat8l levels are reduced. Taken together, our data show that Nat8l impacts on the brown adipogenic phenotype and suggests the existence of the NAT8L-driven NAA metabolism as a novel pathway to provide cytosolic acetyl-CoA for lipid synthesis in adipocytes.

Reconstruction of gene association network reveals a transmembrane protein required for adipogenesis and targeted by PPARγ.

We have developed a method for reconstructing gene association networks and have applied this method to gene profiles from 3T3-L1 cells. Priorization of the candidate genes pinpointed a transcript annotated as APMAP (adipocyte plasma membrane-associated protein). Functional studies showed that APMAP is upregulated in murine and human adipogenic cell models as well as in a genetic mouse model of obesity. Silencing APMAP in 3T3-L1 cells strongly impaired the differentiation into adipocytes. Moreover, APMAP expression was strongly induced by the PPARγ ligand rosiglitazone in adipocytes in vitro and in vivo in adipose tissue. Using ChIP-qPCR and luciferase reporter assays, we show a functional PPARγ binding site. In addition, we provide evidence that the extracellular C-terminal domain of APMAP is required for the function of APMAP in adipocyte differentiation. Finally, we demonstrate that APMAP translocates from the endoplasmatic reticulum to the plasma membrane during adipocyte differentiation.

Arxes: retrotransposed genes required for adipogenesis

Retrotransposed sequences arise from messenger RNAs (mRNAs) that have been reinserted into genomic DNA by reverse transcription. Usually, these sequences are embedded in dormant regions, collect missense mutations over time and constitute processed, nonfunctional pseudogenes. There are thousands of processed pseudogenes in the mouse and human genome. Here, we report evidence for two paralog genes (termed Arxes1 and Arxes2), which arose by retrotransposition of the signal peptidase Spcs3 followed by a segmental duplication event. They gained a functional promoter that we show to be transactivated by adipogenic transcription factors. We further show that the Arxes mRNAs are highly expressed in adipose tissue and strongly upregulated during adipogenesis in different cell models. Additionally, their expression is elevated by an anti-diabetic agent in vitro and in vivo. Importantly, we provide evidence that the Arxes genes are translated and that the proteins are located in the endoplasmic reticulum. Although the sequence similarity and subcellular location are reminiscent of their parental gene, our data suggest that the Arxes have developed a different function, since their expression is required for adipogenesis, whereas Spcs3 is dispensable. In summary, we report retrotransposed-duplicated genes that evolved from a parental gene to function in a tissue and adipogenesis-specific context.

Enzymatic synthesis of antibody-human serum albumin conjugate for targeted drug delivery using tyrosinase from Agaricus bisporus

Highly specific targeted drug delivery devices can be obtained with antibody-human serum albumin (mAb-HSA) conjugates. However, their conventional production involves several reaction steps including chemical modification and activation of both proteins followed by cross-linking often involving toxic chemicals. Here, we describe the enzymatic synthesis of mAb-HSA conjugates for targeted drug delivery devices using tyrosinase from Agaricus bisporus under mild reaction conditions (pH 6.8, 25 °C). Reaction conditions were optimized by using fluorescence labeled HSA to facilitate SDS-PAGE analysis with fluorescence scanning. Enzymatic cross-linking in the presence of natural low molecular weight phenolic compounds (e.g. caffeic acid) resulted in reaction products in the molecular weight range of ∼216 kDa, corresponding to mAb-HSA conjugates. The composition of the conjugates was confirmed with tryptic digestion followed by LC-MS/MS analysis of the resulting peptide fragments. Successful binding of mAb-HSA conjugates (in contrast to free HSA) to MHC II molecules, located on antigen-presenting cells, was demonstrated by both ELISA and flow cytometry analysis.

Omega-3 fatty acids and FFAR4

The beneficial roles of omega-3 fatty acids (ω3-FAs) on obesity, type 2 diabetes, and other metabolic diseases are well known. Most of these effects can be explained by their anti-inflammatory effects triggered through their receptor, free fatty acid receptor 4 (FFAR4) activation. Although the whole mechanism of action is not fully described yet, it has been shown that stimulation of ω3-FA to FFAR4 is followed by receptor phosphorylation. This makes FFAR4 to be capable of interacting with β-arrestin-2, which in turn, results in association of β-arrestin-2 with TAB1. This stealing of an important partaker of the inflammatory cascade leads to interruption of the pathway, resulting in reduced inflammation. Besides this regulation of the anti-inflammatory response, FFAR4 signaling also has been shown to regulate glucose homeostasis, adiposity, gastrointestinal peptide secretion, and taste preference. In this review, we summarize the current knowledge about the interaction of ω3-FAs with FFAR4 and the consequent opportunities for the application of ω3-FAs and possible FFAR4 targets.

Adipose Triglyceride Lipase and Hormone-Sensitive Lipase Are Involved in Fat Loss in JunB-Deficient Mice

Proteins of the activator protein-1 family are known to have roles in many physiological processes such as proliferation, apoptosis, and inflammation. However, their role in fat metabolism has yet to be defined in more detail. Here we study the impact of JunB deficiency on the metabolic state of mice. JunB knockout (JunB-KO) mice show markedly decreased weight gain, reduced fat mass, and a low survival rate compared with control mice. If fed a high-fat diet, the weight gain of JunB-KO mice is comparable to control mice and the survival rate improves dramatically. Along with normal expression of adipogenic marker genes in white adipose tissue (WAT) of JunB-KO mice, this suggests that adipogenesis per se is not affected by JunB deficiency. This is supported by in vitro data, because neither JunB-silenced 3T3-L1 cells nor mouse embryonic fibroblasts from JunB-KO mice show a change in adipogenic potential. Interestingly, the key enzymes of lipolysis, adipose triglyceride lipase and hormone-sensitive lipase, were significantly increased in WAT of fasted JunB-KO mice. Concomitantly, the ratio of plasma free fatty acids per gram fat mass was increased, suggesting an elevated lipolytic rate under fasting conditions. Furthermore, up-regulation of TNFα and reduced expression of perilipin indicate that this pathway is also involved in increased lipolytic rate in these mice. Additionally, JunB-KO mice are more insulin sensitive than controls and show up-regulation of lipogenic genes in skeletal muscle, indicating a shuttling of energy substrates from WAT to skeletal muscle. In summary, this study provides valuable insights into the impact of JunB deficiency on the metabolic state of mice.

α/β-Hydrolase Domain Containing Protein 15 (ABHD15) – an Adipogenic Protein Protecting from Apoptosis

Our knowledge about adipocyte metabolism and development is steadily growing, yet many players are still undefined. Here, we show that α/β-hydrolase domain containing protein 15 (Abhd15) is a direct and functional target gene of peroxisome proliferator-activated receptor gamma (PPARγ), the master regulator of adipogenesis. In line, Abhd15 is mainly expressed in brown and white adipose tissue and strongly upregulated during adipogenesis in various murine and human cell lines. Stable knockdown of Abhd15 in 3T3-L1 cells evokes a striking differentiation defect, as evidenced by low lipid accumulation and decreased expression of adipocyte marker genes. In preconfluent cells, knockdown of Abhd15 leads to impaired proliferation, which is caused by apoptosis, as we see an increased SubG1 peak, caspase 3/7 activity, and BAX protein expression as well as a reduction in anti-apoptotic BCL-2 protein. Furthermore, apoptosis-inducing amounts of palmitic acid evoke a massive increase of Abhd15 expression, proposing an apoptosis-protecting role for ABHD15. On the other hand, in mature adipocytes physiological (i.e. non-apoptotic) concentrations of palmitic acid down-regulate Abhd15 expression. Accordingly, we found that the expression of Abhd15 in adipose tissue is reduced in physiological situations with high free fatty acid levels, like high-fat diet, fasting, and aging as well as in genetically obese mice. Collectively, our results position ABHD15 as an essential component in the development of adipocytes as well as in apoptosis, thereby connecting two substantial factors in the regulation of adipocyte number and size. Together with its intricate regulation by free fatty acids, ABHD15 might be an intriguing new target in obesity and diabetes research.

Loss of ABHD15 Impairs the Anti-lipolytic Action of Insulin by Altering PDE3B Stability and Contributes to Insulin Resistance

Summary Elevated circulating fatty acids (FAs) contribute to obesity-associated metabolic complications, but the mechanisms by which insulin suppresses lipolysis are poorly understood. We show that α/β-hydrolase domain-containing 15 (ABHD15) is required for the anti-lipolytic action of insulin in white adipose tissue (WAT). Neither insulin nor glucose treatments can suppress FA mobilization in global and conditional Abhd15-knockout (KO) mice. Accordingly, insulin signaling is impaired in Abhd15-KO adipocytes, as indicated by reduced AKT phosphorylation, glucose uptake, and de novo lipogenesis. In vitro data reveal that ABHD15 associates with and stabilizes phosphodiesterase 3B (PDE3B). Accordingly, PDE3B expression is decreased in the WAT of Abhd15-KO mice, mechanistically explaining increased protein kinase A (PKA) activity, hormone-sensitive lipase (HSL) phosphorylation, and undiminished FA release upon insulin signaling. Ultimately, Abhd15-KO mice develop insulin resistance. Notably, ABHD15 expression is decreased in humans with obesity and diabetes compared to humans with obesity and normal glucose tolerance, identifying ABHD15 as a potential therapeutic target to mitigate insulin resistance.