Cells were then mounted with Gel/Mount containing DAPI (Molecular Probes). promotes lipolysis by enhancing the activity and translocation of hormone sensitive lipase (HSL) in mature adipocytes, which suggests that IL-4 plays a pro-lipolytic role in lipid metabolism by boosting HSL activity. Our results demonstrate that IL-4 harbors pro-lipolysis capacity by inhibiting adipocyte differentiation and lipid accumulation as well as by promoting lipolysis in mature adipocytes to decrease lipid deposits. The above findings uncover the novel roles of IL-4 in lipid metabolism and provide new insights into the interactions among cytokine/immune Torin 1 responses, insulin sensitivity, and metabolism. Keywords:peroxisome proliferator-activated receptor-, CCAAT/enhancer-binding protein-, hormone sensitive lipase, perilipin Obesity is characterized by an expansion of white adipose tissue (WAT) mass resulting from increased adipocyte number and/or size (1). It is a key risk factor leading to type 2 diabetes mellitus (T2DM) and hyperlipidemia, and has become a pan-endemic health problem with rapidly growing global incidence (2,3). Obesity is associated with systemic chronic inflammation characterized by altered cytokine production and activation of inflammatory signaling (4,5). Abundant studies have linked the increased production of inflammatory cytokines, such as tumor necrosis factor- (TNF-), interleukin (IL)-6, and certain adipokines, during the inflammatory process to obesity, as well as to the development of insulin resistance (68). However, the effect of anti-inflammatory cytokines, such as IL-4, in the development of insulin resistance or obesity is less understood. IL-4, secreted by activated Th2 lymphocytes, basophils, and mast cells, executes pleiotropic functions, Torin 1 such as induction of Th2 differentiation, immunoglobulin class switching, and B cell proliferation (9). The production of IL-4 by splenic lymphocytes from diet-induced obese mice is increased (10) and the amount of serum IL-4 is reduced in Sprague-Dawley rats after receiving visceral fat removal surgery (11). IL-4, secreted from adipocytes and hepatocytes, shows the capacity of modulating local immune response and insulin sensitivity (12,13). These results suggest IL-4 may participate in the processes of diet-induced obesity and metabolism. In support of the abovementioned studies, significant associations between IL-4 genotypes and T2DM, as well as between IL-4 genotypes and circulatory levels of high density lipoprotein-cholesterol (HDL-C), are identified in our previous study (14). Our most recent report also demonstrates that polymorphisms of the IL-4 receptor gene are significantly associated with HDL-C (15). In addition, results from our animal study reveal that IL-4 improves insulin sensitivity and glucose tolerance while inhibiting lipid accumulation in fat tissues, which leads to decreased weight gain and fat mass (16). Taken together, these studies suggest that IL-4 may participate in lipid metabolism and diabetic susceptibility. To further explore the roles of IL-4 in lipid metabolism, the pathogenesis of obesity, and T2DM, the present study aimed to elucidate the effects of IL-4 on adipogenesis and lipid metabolism by using adipocytes as a study model. Our data shows that IL-4 not only inhibits adipogenesis at the early phase of adipocyte differentiation through the signal transducer and activator of transcription 6 (STAT6) signaling pathway, but also promotes lipolysis by upregulating the activity of hormone sensitive lipase (HSL), the key enzyme for triacylglyceride degradation. == MATERIALS AND METHODS == == Reagents == Mouse recombinant IL-4 was purchased from Millipore (Temecula, CA). Tyrphostin AG 490 (AG490) and ECL reagent were purchased from Calbiochem (Merck Millipore, Billerica, MA). 3-isobutyl-methylxanthine (IBMX), dexamethasone (Dex), insulin, isoproterenol (ISO), thiazolidinedione (TZD), and the free glycerol determination kit were purchased from Sigma (St. Louis, MO). Protein A/G beads, TRIzol reagent, and Applied Biosystems SYBR Green Realtime PCR Master Mix were purchased from Life Technologies (Carlsbad, CA). Small interfering RNA (siRNA) and DharmaFECT 1 reagent were purchased form Dharmacon (Lafayette, CO) == Cell culture, adipogenesis of 3T3-L1 cells, and cell counting == 3T3-L1 preadipocytes were maintained in DMEM containing 10% calf serum (Hyclone Laboratories, South Logan, UT). Two day postconfluent 3T3-L1 cells (designated day 0) were induced to differentiate by addition of Torin 1 a standard cocktail composed of 0.5 mM IBMX, 1 M Dex, and 10 g/ml insulin in 10% FBS for 2 days (designated as MDI cocktail). The cells were then cultured in DMEM supplemented with 10% FBS and 5 g/ml insulin. The medium was replaced with fresh medium every 2 days. For the AG490 experiments, cells were preincubated with 10 M AG490 for 1 h and then treated with 10 ng/ml IL-4. For cell counting, cells were harvested and stained with trypan blue (0.5%; Biological Industries, Kibbutz Beit Haemek, Israel), and TRIM13 viable cells were counted with the use of a hemocytometer (Lauda-Knigshofen, Marienfeld-Superior, Germany). == Oil Red O staining == Oil Red O (ORO) staining was performed as.