Daniel L. Kaufman Abstract Oral Treatment with c-Aminobutyric Acid Improves Glucose Tolerance and Insulin Sensitivity by Inhibiting Inflammation in High Fat Diet-Fed Mice Jide Tian*, Hoa N. Dang, Jing Yong, Wing-Sheung Chui, Matthew P. G. Dizon, Catherine K. Y. Yaw, Department of Molecular and Medical Pharmacology, University of California Los Angeles, Los Angeles, California, United States of America Adipocyte and b-cell dysfunction and macrophage-related chronic inflammation are critical for the development of obesity- related insulin resistance and type 2 diabetes mellitus (T2DM), which can be negatively regulated by Tregs. Our previous studies and those of others have shown that activation of c-aminobutyric acid (GABA) receptors inhibits inflammation in mice. However, whether GABA could modulate high fat diet (HFD)-induced obesity, glucose intolerance and insulin resistance has not been explored. Here, we show that although oral treatment with GABA does not affect water and food consumption it inhibits the HFD-induced gain in body weights in C57BL/6 mice. Furthermore, oral treatment with GABA significantly reduced the concentrations of fasting blood glucose, and improved glucose tolerance and insulin sensitivity in the HFD-fed mice. More importantly, after the onset of obesity and T2DM, oral treatment with GABA inhibited the continual HFD-induced gain in body weights, reduced the concentrations of fasting blood glucose and improved glucose tolerance and insulin sensitivity in mice. In addition, oral treatment with GABA reduced the epididymal fat mass, adipocyte size, and the frequency of macrophage infiltrates in the adipose tissues of HFD-fed mice. Notably, oral treatment with GABA significantly increased the frequency of CD4+Foxp3+ Tregs in mice. Collectively, our data indicated that activation of peripheral GABA receptors inhibited the HFD-induced glucose intolerance, insulin resistance, and obesity by inhibiting obesity-related inflammation and up-regulating Treg responses in vivo. Given that GABA is safe for human consumption, activators of GABA receptors may be valuable for the prevention of obesity and intervention of T2DM in the clinic. Citation: Tian J, Dang HN, Yong J, Chui W-S, Dizon MPG, et al. (2011) Oral Treatment with c-Aminobutyric Acid Improves Glucose Tolerance and Insulin Sensitivity by Inhibiting Inflammation in High Fat Diet-Fed Mice. PLoS ONE 6(9): e25338. doi:10.1371/journal.pone.0025338 Editor: Jacques Zimmer, Centre de Recherche Public de la Sante´ (CRP-Sante´), Luxembourg Received August 24, 2011; Accepted September 1, 2011; Published September 22, 2011 Copyright: 2011 Tian et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: These authors have no support or funding to report. Competing Interests: Drs. Jide Tian and Daniel L. Kaufman are co-inventors of a patent titled ‘‘GABA Modulates Inflammation’’, but both had no patent-related activity of employment, consultancy, products in development or modified products. This does not alter the authors’ adherence to all the PLoS ONE policies on sharing data and materials. The authors declare no other conflicts of interest. * E-mail: jtian@mednet.ucla.edu Introduction Obesity is associated with the development of type 2 diabetes mellitus (T2DM) and stems from the imbalance between calorie intake and expenditure, as well as genetic factors, leading to the accumulation of excess fat in the body. T2DM is characterized by impaired glucose tolerance, insulin resistance and insufficient insulin production by the pancreatic islet b-cells [1,2]. The incidence of obesity-related T2DM is dramatically increasing worldwide. Individuals with obesity and T2DM are at risk of developing micro- and macrovascular diseases, such as hyperten- sion, cardiovascular diseases and cerebovascular diseases [3,4]. Although many medications are available for the management of hyperlipidemia and hyperglycemia, they fail to completely restore glucose homeostasis and/or have adverse effects. Therefore, the discovery and development of new reagents that can safely inhibit obesity development and improve glucose metabolism will be of great benefit for slowing the development of T2DM and limiting its long-term complications. Previous studies have shown that adipocyte and b-cell dysfunction along with low-grade macrophage-related chronic inflammation are critical for the development of obesity-related insulin resistance and T2DM [2,5,6,7,8]. During the development of obesity and T2DM, adipocytes can produce adipokines and other factors, which recruit the infiltration of macrophages and other immunocompetent cells into the adipose tissues and affect insulin sensitivity in other organs, leading to low grade inflammation [9,10,11]. Apparently, inhibition of chronic inflam- mation and macrophage infiltration may inhibit adipocyte hypertrophy and improve glucose tolerance and insulin sensitivity. Indeed, regulatory T cells (Tregs) have been shown to inhibit the high fat diet (HFD)-induced adipocyte dysfunction, glucose intolerance, and insulin resistance in mice [12]. Notably, GABAA receptors (GABAA-R) are expressed by adipose tissues and immunocompetent cells, such as macrophages and T cells [13,14,15]. Our previous studies and those of others have shown that activation of GABAA-R inhibits inflammatory diseases, such as type 1 diabetes (T1D), experimental autoimmune encephalo- myelitis (EAE), collagen-induced rheumatoid arthritis (RA) in mice and prolongs the survival of syngenic islet grafts in diabetic NOD mice [16,17,18,19,20]. A recent study as well as our unpublished observations show that GABA promotes regeneration of the pancreatic b-cells and reverses hyperglycemia in the mouse model of T1D [20]. Accordingly, it is possible that activation of GABA PLoS ONE | www.plosone.org 1 September 2011 | Volume 6 | Issue 9 | e25338