Effects Of SIRT1on Differentiation Of Visceral Adipocytes And Underlying Mechanisms

Background and purposeObesity is a popular health problem with increasing morbidity all over the world, which always results from abnormalities in lipid metabolism and in adipocytes differentiation. Compared with peripheral obesity, excessive lipid deposition in visceral adipose tissue is more closely associated with diabetes mellitus, coronary artery disease and so on. Therefore, it is important to explore the mechanisms responsible for visceral obesity.There are two major kinds of adipose tissues existing in human, which is called white adipose tissue and brown/beige adipose tissue, respectively. Increasing amount of white adipose tissue is the direct cause of obesity, while activation of brown/beige adipose tissue can expend energy by up-regulating mitochondrial uncoupling protein (UCP1) expression and promoting thermogenic effects.It has been reported that PR domain-containing16(PRDM16) is the key factor determining the formation of brown/beige adipose from multiple precursor cells. There are two main isoforms including full-length PRDM16and short PRDM16(sPRDM16) after alternative splicing, but the role of the two isoforms in the differentiation of brown/beige adipose remains unknown.Studies demonstrate that Sirtuin-1gene (Sirtl)can recruit PRDM16to induce the formation of beige adipose in subcutaneous white adipose tissue, but no similar phenomena is observed in visceral white adipose tissue. Since clinical studies reveal that expression of Sirtl is closely associated with the amount of visceral fat, we mainly disclose the role of Sirtl in the differentiation of visceral adipocytes, and examine the effects of a new Sirtl agonist BTM-0512on treatment of high fat diet-induced obesity.MethodsClinical study:visceral adipose tissue was collected from patients who received abdominal surgery. We use BMI to identify control group (BMI18.5-24.9) and over-weight/obesity group (BMI>25). Immunohistochemical staining was performed to examine protein expressions of Sirtl and total PRDM16(full PRDM16+sPRDM16), and real-time PCR was used to detect mRNA expressions of Sirtl, full length PRDM16as well as total PRDM16.In vitro study:Visceral adipose tissue from control group was collected to culture stromal vascular fraction (SVF), and flow cytometry was used to detect cell surface molecular markers. Human Sirtl over-expression was performed to observe effects of Sirtl on expressions of PRDM16, UCP1, C/EBPβ, Resistin as well as lipid deposition in SVF cells.In vivo study:C57BL/6J mice were fed with high fat diet for8weeks, and then divided into six groups:control, model, model treated with BTM-0512at different doses (5,10,20mg/kg) for2,4,6weeks, and traditional Sirtl agonist Resveratrol (10mg/kg) was also used as positive control. Then body weight, body length, visceral fat wet weight as well as gene expressions of Sirtl, PRDM16, UCP1, C/EBP, Resistin, TMEM26were examined.Results(1) Compared with control, the expressions of Sirtl and total PRDM16were decreased in visceral adipose tissue of over-weight/obesity patients, but no significant changes of full PRDM16expression were observed.(2) Compared with control, Sirtl gene over-expression in SVF significantly up-regulated mRNA expressions of Sirtl, UCP1, C/EBP, but down-regulated resistin, full PRDM16expressions and reduced lipid deposition. Total PRDM16level did not changed.(3) Compared with control, obesity mice showed high levels of blood glucose, TC, TG, LDL, HDL/TC, Lee’s index, visceral fat index, which can be significantly reversed by BTM-0512and Resveratrol. Moreover, compared with Resveratrol, the effects of BTM-0512were more outstanding.(4) Compared with non-treated obesity model, both BTM-0512and Resveratrol treatment up-regulated Sirtl and total PRDM16level, but down-regulated full PRDM16, UCP1, C/EBPβ, TMEM26, Resistin expressions in visceral fat of obesity mice.(5) Compared with non-treated obesity mice, BTM-0512and Resveratrol significantly up-regulated the expressions of Sirtl, UCP1, full PRDM16and total PRDM16in brown adipose tissue of obesity mice. But no significance was seen in mRNA expressions of TMEM26and C/EBPβ.Conclusions(1) Sirtl over-expression depressed differentiation of visceral fat precursor to both white and beige adipocytes;(2) The mechanisms for inhibition of visceral white/Beige adipose conversion by Sirtl over-expression might at least partly due to reduced full PRDM16level;(3) BTM-0512significantly inhibited high fat diet-induced obesity.