| Background and objective:Increased incidences of obesity have unfortunately become concurrent with rapideconomic development and improved living standards in the modern society. Obesity, acommon disease of nutritional disturbance, leads to metabolic diseases such as type2diabetes mellitus(1). Obesity is also related to impairments in the endocrine functions ofadipose tissues, such as low inflammatory conditions(2-4).The main cause of human obesityis the secretion imbalance of pro-and anti-inflammatory cytokines in adipose tissues(5).Secreted frizzled-related protein (SFRP)-5(Sfrp5), a newly identifiedanti-inflammatory adipokine, can inhibit chronic inflammation and consequently improveinsulin sensitivity. Sfrp5expression is abundant in mouse adipocytes, but is decreased invarious rodent models, such as obesity and type2diabetes(6). Studies show that one of thecommon features of SFRPs is a cysteine-rich domain (CRD), which has a high homologywith the CRD of the wingless-type (Wnt) receptor frizzled protein. Therefore, SFRPs couldcompete with the frizzled protein receptor in combining with the Wnt ligand, and Sfrp5plays a negative regulatory role in the Wnt pathway(7,8). The anti-inflammatory function ofSfrp5is enabled by its combination with Wnt5a expressed in adipose tissues, therebyinhibiting the activation of the downstream target c-Jun N-terminal kinase (JNK) ofnon-canonical Wnt signaling. Consequently, the secretion of pro-inflammatory cytokines islimited, and the insulin receptor substrate-1serine307phosphorylation is antagonized(6,9).Adipocyte differentiation is closely related to glucose and lipid metabolism, insulinresistance (IR), type2diabetes mellitus, hyperlipoidemia, etc(10). Therefore, changes inSfrp5expression and secretion during the dynamic process of adipocyte differentiation needto be evaluated.The present study aimed to investigate the expression of Sfrp5mRNA and changes inthe protein secretion during the dynamic process of adipocyte differentiation. Another aimwas to examine the influence of pathophysiological conditions related to insulin resistance on the expression and secretion of adipocyte Sfrp5. The regulation of Sfrp5expression byrosiglitazone and metformin was also assessed. To investigate the relationship of serumSfrp5with obesity and insulin resistance in non-diabetic individuals.Methods:(1) Cells of3T3-L1pre-adipocytes were cultivated via differentiation induced by1-methyl-3-isobutyl xanthenes, dexamethasone, and insulin.(2) To interfere with the differentiation of6d-old3T3-L1cells,1μM dexamethasone,100nM insulin,10ng/l TNF-a,10μM rosiglitazone, and1mM metformin were applied for24h.(3)Reverse-transcriptase polymerase chain reaction and enzyme-linked immunosorbentassay were performed to examine the expression and protein secretion level of Sfrp5mRNAadipocytes under different differentiation periods and regulated culture component.(4) On the basis of body mass index (BMI),244subjects were divided withnon-obese group(77cases,BMI<24kg/m2),obese group(170cases,BMI≥24kg/m2).(5) Serum Sfrp5was detected by ELISA.(6) Experimental data were analyzed by SPSS17.0. The differences between groupswere tested using T-tset. The correlation of variables was determined by Pearson'scorrelation and multiple linear regression was used to correct the effects of the covariatesand test independent factors. P <0.05was considered statistically significant for allanalysis.Result:(1) Sfrp5was not expressed in3T3-L1pre-adipocytes. With the differentiation andmaturity of pre-adipocytes, the gene transcription level and Sfrp5protein secretiongradually became more highly regulated. mRNA expression peaked on the9th day (about45±6times, P <0.001). Protein secretion in the supernatant had a delayed expressioncompared with that of mRNA, and was observable on the2nd and3rd day of differentiation.(2) mRNA expression in mature adipocytes decreased by20%(P <0.01),22%(P <0.01), and32%(P <0.01) using1μM dexamethasone,100nM insulin, and10ng/l TNF,respectively. The corresponding values for protein secretion levels in the supernatant were15%(P <0.01),17%(P <0.01), and30%(P <0.01),,respectively.(3) Sfrp5mRNA expression in mature adipose increased by34%(P <0.01) and19% (P <0.01) using10μM rosiglitazone and1mM metformin, respectively. The correspondingvalues for protein secretion quantity in the supernatant increased by10%(P <0.01) and6%(P <0.05), respectively.(4) In investigated subjects, serum Sfrp5levels were lower in obese group than innon-obese group.(5)Serum Sfrp5levels did not exhibit evident correlation with age,BMI, systolic bloodpressure, diastolic pressure, mean arterial pressure, uric acid(P>0.05). Serum SFRP5levelswere negatively correlated with gender,waist-hip ratio, total cholesterol, triglycerides, lowdensity lipoprotein cholesterol,fasting plasma glucose, FINS, HOMA2-%B, HOMA2-IR(r=-0.168,-0.134,-0.132,-0.154,-0.146,-0.161,-0.312,-0.228,-0.281,P<0.05). Serum Sfrp5levels were positively correlated with high density lipoprotein cholesterol(r=0.171,P<0.05).(6)After adjustment for age, gender, BMI, WHR, Serum Sfrp5levels was stillnegatively correlated with HOMA2-IR,(β=-0.425,P<0.001).Conclusion:(1) Sfrp5plays an important role in the differentiation of pre-adipocytes.(2) The dysregulation of Sfrp5expression and secretion directly correlates with insulinresistance.(3) Rosiglitazone-and metformin-induced resistances to organ inflammation andinsulin sensitivity are improved by enhancing Sfrp5mRNA expression and protein secretionin mature adipocytes.(4) Serum Sfrp5levels was negatively correlated with HOMA2-IR,which showed thatthis novel adipokine may play a role in the pathophysiology of obesity and insulinresistance.
|
PDF Full Text Request