The effects of short-chain fatty acid acetate on brown adipocytes differentiation and metabolism

Abstract

Short-chain fatty acids (SCFA) are a sub-group of fatty acids including formic acid, acetic acid, propionic acid, isobutyric acid, butyric acid, isovaleric acid and valeric acid. Acetate, propionate and butyrate are three major shortchain fatty acids, which are mainly formed in the gastrointestinal tract via colonic bacteria fermentation of carbohydrates, especially resistant starches and dietary fibre. There has been increasing interest in the idea that the short-chain fatty acids play crucial roles in a range of physiological functions. Recently, increasing evidence suggested there is a strong link between short-chain fatty acids and energy homeostasis. Several studies highlighted the protective effects of the short-chain fatty acids on high-fat diet induced obesity and other harmful metabolic disorders in mice. However, the coherent understanding of the multi-level network in which short-chain fatty acids exert their effects still needs to be elucidated. Up to date, it has been demonstrated that short-chain fatty acids can mediate energy balance via affecting appetite control in brain, increasing adipogenesis in white adipocyte, and regulating insulin sensitivities in white adipose tissue and muscle, etc. However, the effects of short-chain fatty acids on brown adipocytes have not been fully investigated. In this study, we examined the roles of short-chain fatty acid acetate and its receptor(s) in the regulation of brown adipocyte differentiation and metabolism. Firstly, we identified the expression of short-chain fatty acids sensing GPR43 in brown adipose tissue and immortalized brown adipocytes by real-time PCR, Western blots and immunohistochemistry. Moreover, GPR43 expression was found to increase during the adipogenesis of cultured brown adipocytes. Pro-adipogenic reagent PPARγ agonist stimulation led to a further augment of GPR43 expression while antiadipogenic reagents such as PPARγ antagonist, RXR antagonist and STAT5 inhibitor played the opposite role on GPR43 expression. Transcription factors such as XBP1 and STAT5 were identified to be involved in GPR43 expression regulation in brown adipocytes. Furthermore, we also examined the role of acetate in the regulation of brown adipogenesis. Our results showed that acetate treatment during adipogenesis up-regulated AP2, PGC-1α and UCP1 expression and affected the morphological changes of brown adipocytes. Moreover, an increase in mitochondrial biogenesis was observed after acetate treatment. Acetate also elicited the activation of ERK and CREB, and these responses were sensitive to G(i/o)-type G-protein inactivator, Gβγ-subunit inhibitor, PLC inhibitor and MEK inhibitor, indicating a role for the G(i/o)βγ/PLC/PKC/MEK signalling pathway in these responses. These effects of acetate were mimicked by treatment with 4-CMTB, a synthetic GPR43 agonist, and were impaired in GPR43 knock-down cells, further supported the hypothesis that GPR43 mediates the pro-adipogenic effects of acetate in brown adipocytes. Furthermore, the effects of acetate treatment on brown adipose tissue were also measured in vivo. Mice fed with acetate demonstrated increased PGC-1α in brown adipose tissue, which was in agreement with the results obtained from immortalized brown adipocytes. In addition, we also measured the effects of acetate on lipid metabolism in differentiated brown adipocytes. The results showed effects of acetate treatment on lipolysis were different in white adipocytes and brown adipocytes. Acetate treatment significantly decreased the lipolysis in white adipocytes while had little effects on lipolysis in brown adipocytes. Besides, acetate treatment was also found to decrease TF2-C12 fatty acid uptake in differentiated IM-BAT cells, suggesting acetate may affect many aspects of lipid metabolism in brown adipocytes. Collectively, our results indicated that acetate might have important physiological roles in brown adipocytes. Short-chain fatty acids may serve to regulate brown adipose tissue functions and therefore improve metabolic health