Sfrp5 expression and secretion in adipocytes are up-regulated during differentiation and are negatively correlated with insulin resistance

We have examined the patterns of Sfrp5 (secreted frizzled-related protein 5) mRNA expression and protein secretion during adipocyte differentiation, and investigated the potential role of Sfrp5 in IR (insulin resistance) in adipocytes. 3T3-L1 pre-adipocytes were induced for differentiation, and RT-PCR (reverse transcription-PCR) and ELISA assays were used to determine Sfrp5 mRNA expression and protein secretion. The results showed that with the differentiation and maturity of pre-adipocytes, transcription and protein secretion of Sfrp5 gradually increased, peaking on the 9th day of differentiation. Sfrp5 mRNA expression in mature adipocytes was decreased by 20, 22 and 32 upon treatment with dexamethasone, insulin and TNF (tumour necrosis factor) respectively, whereas Sfrp5 protein secretion was decreased by 15, 17 and 30%, correspondingly. In contrast, Sfrp5 mRNA expression in mature adipose was increased by 34 and 19% upon treatment with rosiglitazone and metformin respectively, whereas Sfrp5 protein secretion was increased by 10 and 6%, correspondingly. In conclusion, Sfrp5 mRNA expression and protein secretion depend on the differentiation of adipocytes. The dysregulation of Sfrp5 expression and secretion is directly correlated with IR. Up-regulation of Sfrp5 expression and secretion in adipocytes may be one crucial mechanism by which rosiglitazone and metformin improve insulin sensitivity.