E3 Ligase FBXW7 Inhibits Browning Of White Fat And Promotes Obesity

Obesity is a metabolic syndrome caused by imbalance of energy intake and expenditure.Nonetheless,despite years of progress in identifying energy metabolism regulated by diverse transcription factors,researches on protein post-translational modification is limited,especially ubiquitin-mediated modification in fat metabolism.In this study,we demonstrate that F-box and WD repeat domain-containing 7(FBXW7),an E3 ubiquitin protein ligase,were highly expressed in adipose tissue and up-regulated in brown and beige fat of mice that were fed an HFD(high fat diet),leptin-deficient ob/ob mice or leptin receptor-deficient db/db mice,suggesting that FBXW7 may be involved in fat energy metabolism and the occurrence and development of obesity.We next obtain FBXW7 fat-conditional overexpression mice(FBXW7-FKI).The FBXW7-FKI mice were similar to their wild-type(WT)littermates fed NCD for 8 weeks in appearance and comparable body weights and fat mass,while reduced insulin resistance,decreased energy metabolism and ability to maintain body temperature in cold environment.FBXW7-FKI mice also promoted obesity,decreased energy metabolism and impaired glucose metabolism under HFD.Meanwhile,The large,unilocular lipid droplets were more distributed in BAT,i WAT and e WAT,with increased lipid accumulation and decreased i WAT browning in FBXW7-FKI mice.We then obtain FBXW7 fat-conditional knockout mice(FBXW7-FKO mice)by crossing FBXW7-Lox P mice with Adiponectin-Cre mice.The FBXW7-FKO mice performed part of metabolic characteristics opposite to FBXW7-FKI mice including stronger ability to maintain body temperature after cold stress.FBXW7-FKO mice were resistant to HFD induced obesity with reduced fat mass and increased energy metabolism.Importantly,the gross appearance of BAT was much larger in FBXW7 FKO mice,while i WAT and e WAT were smaller.Histological analysis and cell counting showed that the lipid droplets of BAT were comparable with no significant difference in the numbers of nuclei per unit area,while the DNA content measurement showed that the total number of brown adipocytes increased significantly,which was manifested as hyperplasia.Meanwhile,multiple uneven smaller fat droplets and increased nuclear density and total cell number per unit area of beige adipocytes were observed in i WAT of FBXW7-FKO mice,suggesting enhanced browning of i WAT.Indeed,the protein levels of UCP1 were upregulated in fat tissues of FBXW7-FKO mice under HFD.To further illuminate the mechanism of FBXW7 in the regulation of fat biology and energy homeostasis,mass spectrometry was used to examine proteomics of brown adipocytes from wild type and FBXW7-FKO mice under HFD.We found that the ribosomal proteins increased significantly in BAT of FBXW7-FKO mice,suggesting promoted ribosome protein synthesis.Correspondingly,relative protein levels of S6K1 protein,a key effector kinase downstream of m TOR signaling pathway,and pho-S6K1(Ser371)were greatly improved.Western blot also verified that S6K1 protein and its phosphorylation level increased,while S6K1 m RNA level remained unchanged,indicating that FBXW7 might regulate S6K1 protein stability to control m TOR signaling transduction and protein synthesis.Further study showed that FBXW7 targets S6K1 for ubiquitination and subsequent proteasome degradation.In conclusion,we provide evidence showing adipose FBXW7 act as a major regulator for energy homeostasis and potential therapeutic target for obesity and metabolic diseases.