Brown adipose tissue (BAT) plays a key role in energy expenditure through its specialized thermogenic function. Therefore, BAT activation may help prevent and/or treat obesity. Interestingly, subcutaneous white adipose tissue (WAT) also has the ability to differentiate into brown-like adipocytes and may potentially contribute to increased thermogenesis. We have previously reported that eicosapentaenoic acid (EPA) reduces high-fat (HF)-diet-induced obesity and insulin resistance in mice. Whether BAT mediates some of these beneficial effects of EPA has not been determined. We hypothesized that EPA activates BAT thermogenic program, contributing to its antiobesity effects. BAT and WAT were harvested from B6 male mice fed HF diets supplemented with or without EPA. HIB 1B clonal brown adipocytes treated with or without EPA were also used. Gene and protein expressions were measured in adipose tissues and H1B 1B cells by quantitative polymerase chain reaction and immunoblotting, respectively. Our results show that BAT from EPA-supplemented mice expressed significantly higher levels of thermogenic genes such as PRDM16 and PGC1α and higher levels of uncoupling protein 1 compared to HF-fed mice. By contrast, both WATs (subcutaneous and visceral) had undetectable levels of these markers with no up regulation by EPA. HIB 1B cells treated with EPA showed significantly higher mRNA expression of PGC1α and SIRT2. EPA treatment significantly increased maximum oxidative and peak glycolytic metabolism in H1B 1B cells. Our results demonstrate a novel and promising role for EPA in preventing obesity via activation of BAT, adding to its known beneficial anti-inflammatory effects.

Uncoupling protein 1 (UCP1) plays a central role in thermogenic tissues by uncoupling cellular respiration to dissipate energy. Beige adipocytes, an inducible form of thermogenic cells in subcutaneous adipose tissue (SAT), have become a major focus in obesity research. We have previously shown that eicosapentaenoic acid (EPA) ameliorated high-fat diet (HFD)-induced obesity by activating brown fat in C57BL/6J (B6) mice at thermoneutrality (30 °C), independently of UCP1. Here, we investigated whether ambient temperature (22 °C) impacts EPA effects on SAT browning in wild-type (WT) and UCP1 knockout (KO) male mice and dissected underlying mechanisms using a cell model. We observed resistance to diet-induced obesity in UCP1 KO mice fed HFD at ambient temperature, with significantly higher expression of UCP1-independent thermogenic markers, compared to WT mice. These markers included the fibroblast growth factor 21 (FGF21) and sarco/endoplasmic reticulum Ca²⁺-ATPase 2b (SERCA2b), suggesting the indispensable role of temperature in beige fat reprogramming. Surprisingly, although EPA induced thermogenic effects in SAT-derived adipocytes harvested from both KO and WT mice, EPA only increased thermogenic gene and protein expression in the SAT of UCP1 KO mice housed at ambient temperature. Collectively, our findings indicate that the thermogenic effects of EPA, which are independent of UCP1, occur in a temperature-dependent manner.

Visceral obesity may be a driving factor in nonalcoholic fatty liver disease (NAFLD) development. Previous studies have shown that the omega-3 polyunsaturated fatty acid, eicosapentaenoic acid (EPA), ameliorates obesity in high-fat (HF) fed male, C57Bl/6 mice at thermoneutral conditions, independent of uncoupling protein 1 (UCP1). Our goals herein were to investigate sex-dependent mechanisms of EPA in the livers of wild type (WT) and UCP1 knockout (KO) male and female mice fed a HF diet (45% kcal fat; WT-HF, KO-HF) with or without supplementation of 36 g/kg EPA (WT-EPA, KO-EPA). KO significantly increased body weight in males, with no significant reductions with EPA in the WT or KO groups. In females, there were no significant differences in body weight among KO groups and no effects of EPA. In males, liver TGs were significantly higher in the KO-HF group and reduced with EPA, which was not observed in females. Accordingly, gene and protein markers of mitochondrial oxidation, peroxisomal biogenesis and oxidation, as well as metabolic futile cycles were sex-dependently impacted by KO and EPA supplementation. These findings suggest a genotypic difference in response to dietary EPA supplementation on the livers of male and female mice with diet-induced obesity and housed at thermoneutrality.