Study On The Function Of Transketolase (TKT) In Adipose Tissues

Transketolase(TKT),an important enzyme involved in the non-oxidative branch of the pentose phosphate pathway(PPP),catalyzes two reversible reactions.One is the conversion from xylulose-5-phosphate(Xu5P)and ribose-5-phosphate(R5P)to glyceraldehyde-3-phosphate(G3P)and sedoheptulose-7-phosphate(S7P).The other is to produce G3 P and fructose-6-phosphate(F6P)from Xu5 P and erythrose-4-phosphate(E4P).Therefore,TKT serves as a bridge linking the PPP and glycolysis.TKT is ubiquitously expressed in mammals and highly abundant in adipose tissues.However,the role of TKT in adipose tissues is rarely reported.The adipose tissue is critical for energy balance and nutritional homeostasis,whose dysfunction usually causes obesity and obesity-associated metabolic disorders including type 2 diabetes,atherosclerosis and non-alcoholic fatty liver disease.In recent years,although emerging evidence has suggested a strong link between the PPP and obesity,the role of TKT in adipose tissues remains obscure.Therefore,we specifically delete TKT in mouse mature adipocytes by using the Cre-lox P strategy in order to explore the role of TKT in adipose tissues.We find no obvious phenotype upon normal diet feeding.However,adipocyte TKT abrogation attenuates high-fat diet(HFD)-induced obesity,reduces hepatic steatosis,improves glucose tolerance,alleviates insulin resistance and increases energy expenditure.We further perform metabonomics profiling with TKT-deficient and wild-type white adipose tissues from HFD-fed mice.TKT deficiency accumulates non-oxidative PPP metabolites,whereas levels of glycolytic metabolites are decreased.However,metabolites of TCA cycle remain stable,suggesting an alternative carbon source feeding into the TCA cycle.RNA-seq analysis reveals that lipolysis pathway is most significantly upregulated in TKT-deficient adipocytes.TKT abrogation leads to upregulation of lipolytic and fatty acid oxidation(FAO)enzyme genes,promoting lipolysis and FAO.In order to prove that insufficiency of glucose into TCA cycle due to TKT-deficiency causes a compensatory increase in lipolysis,we performed ex vivo lipolysis assay in the absence or presence of pyruvate.Interestingly,pyruvate supplementation not only inhibits lipolysis,with reduced production of glycerol and fatty acids,but also minimizes the difference in lipolysis due to TKT deficiency.In addition,TKT-deficiency upregulates many genes encoding mitochondrial component including the electron transport chain(ETC)and uncoupling protein 1(UCP1)in the white adipose tissue.However,loss of TKT does not alter the expression of UCP1 in the brown adipose tissue.In summary,our data reveals that adipocyte TKT ablation disturbs the crosstalk between the PPP and glycolysis,accumulating the non-oxidative PPP metabolites and decreasing pyruvate input into the mitochondria,which leads to increased lipolytic enzyme gene expression and enhanced lipolysis,FAO and mitochondrial respiration.Thus,loss of TKT in adipose tissues protects mice from diet-induced obesity.Our findings not only identify a novel role of TKT in adipose tissues,but also suggest limiting glucose-derived carbon into the mitochondria induces lipid catabolism.