Function And Mechanism Of IL-6 In Browning Of White Fat

Obesity has become a global public health concern with changes in lifestyle and dietary structure.Obesity is a disease caused by excessive accumulation of fat in weight,which is medically manifested as an increase in the volume or number of white fat cells.Obesity is closely related to diabetes,fatty liver and other diseases.There are three kinds of adipose tissue in mammals:white,brown,and beige.While white adipose tissue（WAT）stores excess energy.Brown adipose tissue,by contrast,is designed to burn energy.Beige fat is converted from white adipose tissue and functions similarly to brown adipose tissue.Studies have shown that Interleukin-6（IL-6）plays an important role in the occurrence of obesity and other chronic inflammation.However,whether peripheral IL-6 signaling regulates the browning of white fat,and whether it regulates glucose metabolism and fatty liver is rarely reported.In this study,stromal vascur fraction cells（SVFs）,wild-type（WT）mice and IL-6 knockout（IL-6 KO）mice derived from white fat were used as experimental materials to investigate the effects of IL-6 on browning of white fat,glucose metabolism and liver fat deposition.The main results are as follows:（1）Exogenous IL-6 activates STAT3 signaling pathway to enhance browning of white fat.We collected SVFs from WT and IL-6 KO mice in vitro.Through Mito Tracker staining and UCP1 immunofluorescence staining,we found that white adipocytes derived from IL-6 KO mice showed lower heat production than WT mouse cells,accompanied by decreased phosphorylation of STAT3 at Tyr705.Exogenous IL-6 increased the protein expression level of p-STAT3^Tyr705,and at the same time promoted the browning of white fat.Nic,an inhibitor of STAT3 signaling,attenuated the promoting effect of IL-6.The above experiments confirmed that IL-6 accelerated the browning of white fat by activating the STAT3 signaling pathway.（2）IL-6 promoted the binding of p-STAT3 to the promoter regions of Pparγand Ucp1 to enhance the browning of white fat.Ch IP experiments showed that IL-6 can facilitate the binding of p-STAT3 to the promoter regions of Pparγand Ucp1,thereby enhanced the transcription level of Pparγand Ucp1.In addition,GW9662,an inhibitor of PPARγ,can reverse the promotion effect of IL-6 on the browning of white fat,suggesting that PPARγis a key regulatory factor of IL-6 promoting the browning of white fat.Subsequent studies further confirmed that IL-6 enhanced the protein interaction between PPARγand PGC-1α,thereby significantly enhancing the binding of Pparγto the Ucp1 promoter.This part of the experiment showed that IL-6 was dependent on the interaction between PGC-1αand PPARγto activate the expression of UCP1 and promote the browning of white fat.（3）Knockout of the IL-6 gene was associated with a decrease in the browning of white fat.By collecting the white adipose tissue of WT and IL-6 KO mice,it was found that the knockout of IL-6 gene increased lipid droplets and fat proportion,inhibited the expression of browning gene and related proteins,and decreased the copy number and volume of mitochondria in white adipose cells.These changes were accompanied by a decrease in the phosphorylation level of STAT3.（4）IL-6 gene deletion disrupts glucose homeostasis.Further research found that the knockout of IL-6 significantly increased the serum glucose level of mice.The glucose tolerance test（GTT）showed that the glucose tolerance of IL-6 KO mice was significantly reduced.And the insulin tolerance test（ITT）confirmed that IL-6 knockout damaged insulin sensitivity of mice,though not significantly.To further verify the effect of IL-6 gene knockout on insulin sensitivity,we found that the knockout of IL-6 reduced the response of mice to p-Akt after insulin injection in two groups of mice,thus proving that the knockout of IL-6 reduced the insulin sensitivity of mice.（5）IL-6 gene deletion mice develop fatty liver by high fat diet.Livers of the two groups of mice were collected,and the results of H&E staining and Oil Red O staining showed that the fat deposition in the livers of IL-6 KO mice was higher than that of WT mice.The levels of Triglyceride（TG）and Total cholesterol（TC）in the liver of mice were measured.The results showed that the levels of TG and TCH in the liver of mice were increased by IL-6 gene deletion.Subsequently,we fed two groups of mice with a high-fat diet,which was consistent with the results of a normal diet.Knockout of IL-6 exacerbated the symptoms of fatty liver induced by the high-fat diet.In summary,this study found that peripheral IL-6 regulated the browning of white fat by activating the STAT3 signaling pathway,thereby maintaining the balance of fat metabolism and glucose homeostasis.The results of this study provided a theoretical basis for targeting IL-6/STAT3 pathway to regulate fat metabolism,and then resisted obesity and metabolic-related diseases.