Role And Underlying Mechanism Of Osteopontin In The Browning Of White Adipocyte Tissues In Balb/c Mice

Objective: Our previous researches have shown that overexpression of Osteopontin(OPN)could induce the browning of white adipocyte(WA).The aim of the study is to investigate the effects of overexpression OPN on the differentiation from white adipocytes to mature brown adipocytes by constructing recombinant adenoviruses(Ad-aP2-OPN)and infecting obese mice.Moreover,it was further confirmed that OPN could promote the transformation of white adipocyte into brown adipocyte in vivo.Methods:Briefly,40 Balb/c mice(weighing16 –21g,male ande female in half)were allocated into four groups randomly,that is,Ctrl group,Obese group,Vehicle group and OPN group.All mice were fed with standard chow for one week,and then,Obese group,Vehicle group and OPN group were fed with high fat diet(HFD)for twelve weeks.The weights of all mice were measured every week.Twelve weeks later,these mice were infected with recombinant adenovirus according to the above groups.Obese group mice were not infected with adenovirus.Four weeks later,blood was collected and then killed after 12 hours of fasting.The following indicators were detected:(1)weight,length,body mass index(BMI)and fat index(FI);(2)fasting plasma glucose(FPG),serum insulin(FINS),triglyceride(TG),total cholesterol(TCH)and insulin resistance index(HOMA-IR);(3)the levels of sOPN in serum;(4)the lipid accumulation and morphological of white adipose tissue(WAT)after Oil red O staining;(5)western blotting was used to measure the expression levels of iOPN,UCP-1,PRDM16,PPARγ,PGC-1α of WAT;(6)western blotting was used to measure the expression levels of AKT-pS473,AKT,PI3 K of WAT;(7)The expression and localization of UCP-1 and PRDM16 in brown adipose by immunohistochemical methods.Results:(1)The weight,length,BMI and FI in HFD group were significantly higher than those in Ctrl group;the levels of fasting plasma glucose,insulin,triglyceride,total cholesterol,HOMA-IR in HFD group were also significantly higher than those in Ctrl group,suggesting establishment of the obese and IR model wassuccessful.(2)The expression levels of iOPN and sOPN in WAT in OPN group were significantly higher than those in Obese group,Vehicle group and Ctrl group,suggesting that the adenovirus infection was successful.(3)The weight,length,BMI and FI in OPN group had no significant difference than those in Vehicle group and Ctrl group;The levels of fasting plasma glucose,insulin,triglyceride,total cholesterol,HOMA-IR in OPN group were significantly higher than those in Vehicle group and Ctrl group,suggesting OPN could improve IR in obese mice.(4)The results of oil red O staining showed that in Obese group and Vehicle group,the volume of oil droplets in WAT of obese group was larger than that in the Ctrl group;a large number of small oil droplets were found in OPN group WAT,and cell morphology was similar to BA,suggesting that OPN could promote the transformation of white adipocytes into brown adipocytes.(5)The expression levels of UCP-1 and PRDM16 were enhanced significantly in OPN group compared with those in Obese and Vehicle group.Similar results were obtained by immunohistochemical analysis.Further immunohistochemistry showed that UCP-1 was mainly located in cytoplasm,while PRDM16 located in the nucleus.These results confirm that OPN has the capability to promote WAT browning.(6)Mechanism study showed the expression levels of PI3 K and AKT-pS473 in OPN group were enhanced significantly compared with those in Obese and Vehicle group.Moreover,the expression of PPAR and PGC-1α were significantly increased,and the expression of BA specific Markers UCP-1 and PRDM16 were also increased.Conclusion: The overexpression of OPN can induce white adipocytes to brown adipocytes in obese and insulin resistant mice and then reduce insulin resistance.The regulation mechanism may be through the pathway of PI3K-AKT-PPARγ to promote the expression of UCP-1 and PRDM16.