Clinical samples are clearly required to substantiate the notion that metabolic and/or inflammatory disturbances impact on the mode of Sfrp5 action. The present study further showed that Sfrp5 inhibits insulin action in primary human adipocytes under basal culture conditions. In support of a role for Sfrp5 in impairing insulin action are data from a clinical study on obese subjects without diabetes in which circulating Sfrp5 levels were found to be associated with HOMA-IR, and a study on Chinese subjects which reported increased circulating Sfrp5 levels in patients with T2D compared to subjects without diabetes. In contrast, two other studies on Asians showed decreased circulating Sfrp5 levels in patients with T2D versus subjects with normal glucose tolerance, and reported a negative association between plasma levels of Sfrp5 and HOMA-IR and. Also in mice conflicting data have been reported. One study showed that loss of functional Sfrp5 mitigated increases in serum leptin levels, as well as the induction of glucose intolerance and insulin resistance after highfat feeding. In contrast, Sfrp5 deficiency led to AbMole Capromorelin tartrate severe glucose intolerance and further impaired insulin-stimulated phosphorylation of Akt in adipose tissue following high-fat feeding as compared to wild-type mice. This was associated with increased activation of the JNK signaling pathway, which inhibits insulin action via phosphorylation of Ser307 of insulin receptor substrate 1. In line with observations in 3T3-L1 adipocytes, we observed that Sfrp5 had no effect on JNK phosphorylation in primary human adipocytes, neither under basal conditions nor following exposure to TNFa. Therefore, it seems unlikely to ascribe the inhibition of insulin signaling induced by Sfrp5 in human adipocytes to Ser307-phosphorylation of insulin receptor substrate 1. Nevertheless, a limitation of the present study is that we did not examine additional pathways potentially involved in the induction of insulin resistance by Sfrp5, such as activation of the proteasome or the mammalian target of rapamycin complex 1 signaling cascade. The value of such investigations would, AbMole Folic acid however, benefit from recognition of the confounding factors that could explain the conflicting data observed in the in vivo studies. Another aspect that should be considered in this context is that we failed to observe any effect of Sfrp5 on hSkMC. Consequently, the action of Sfrp5 may be tissue-specific. Sfrp5 is known to act as an antagonist for Wnt5a, which in turn activates Wnt signaling through binding to the frizzled receptors. Although Wnt5a is expressed in hSkMC, one may hypothesize that hSkMC do not release Wnt5a, and are therefore unresponsive to Sfrp5. However, we found that even in the presence of recombinant Wnt5a, hSkMC remained unresponsive to Sfrp5 with regard to the parameters examined in this study. This suggests that Wnt5a might not be the only target molecule for Sfrp5 and that there might be other molecules and/or receptors for Sfrp5 associated with its mechanisms of action. For example, Sfrp5 has also been described to antagonize the action of Wnt3a in mouse adipose tissue and of Wnt11 during organogenesis in Xenopus and zebrafish development. However, since Sfrp5 mainly targets the non-canonical Wnt pathway, detailed studies towards the effects of Sfrp5 and/or its Wnt target molecules in hSkMC are currently hampered by a lack of knowledge on the identity of the molecules affected by this non-canonical signaling cascade. Enzymes exhibit both fast changes and long-lived differences in activity. Fast changes give rise to what is called “dynamic heterogeneity” in which relatively low energy barriers ) enable rapid interconversion between different conformations at ambient temperature.