We also revealed that Scd is a functional

We also revealed that Scd1 is a functional target of miR-378 that promotes lipolysis in adipose tissues of miR-378 Tg mice. SCD1 critically regulates lipid metabolism, and dysregulation of SCD1 activity results in various metabolic disorders, including diabetes, cardiovascular disease, and obesity (Dobrzyn et al., 2015; Sampath and Ntambi, 2011, 2014; Stamatikos and Paton, 2013). Similar to our miR-378 Tg mice, Scd1-knockout mice are also resistant to HFD-induced obesity (Cohen et al., 2002; Ntambi et al., 2002). Therefore, in miR-378 Tg mice, we propose a model in which miR-378 activates the Akt1-FoxO1-PEPCK pathway in skeletal muscle to induce the pyruvate-PEP futile cycle, which impairs glucose metabolism and causes Aminoallyl-dUTP - Cy3 deficiency. Then, at the same time in adipose tissues, miR-378 directly targets Scd1 to enhance lipolysis (Fig. S7). Because of miR-378 induced glucose futile cycle, the Tg mice utilize fatty acids to meet energy demand. As a consequence, FFA in the circulation gets low despite of elevated lipolysis in adipose tissues. To validate the primary effect of miR-378-targeted Akt-FoxO1-PEPCK, we used constitutively active Akt1 (ca-Akt1) to rescue the metabolic phenotype of miR-378 Tg mice and observed significantly attenuated lipolysis in ca-Akt1 treated Tg mice. These findings suggest that miR-378 mediates the metabolic communication between muscle and fat by targeting Akt1 and Scd1 to regulate systemic energy homeostasis. It would be very intriguing to identify signals or molecules in mediating the metabolic communications between muscle and fat tissues in miR-378 transgenic mice. Previous publications demonstrate that some metabolites may mediate the regulation of the whole body metabolic homeostasis. For example, insufficient alanine supply mediates a muscle-liver-fat signaling by upregulating FGF21 expression in liver (Shimizu et al., 2015). We observed significantly lower level of pyruvate, lactate, actyl-CoA and FFA in Tg mice than those in wild type control. All of those metabolites are indicators for energy supply, thus the data indicating miR-378 Tg mice suffer severe energy insufficiency. Identifications of the molecules or signals which convey such energy deficiency and mediate inter-organ crosstalk will help us to understand the mechanism of miR-378 in maintaining whole body metabolic homeostasis. Clinically, our findings support a critical role for miR-378 in human metabolic disorders. Similarly, Kulyté A et al. recently reported that miR-378 was significantly upregulated in cachectic cancer patients, and its expression was strongly and positively associated with catecholamine-stimulated lipolysis in human adipocytes (Kulyte et al., 2014). Their findings suggest that increased miR-378 expression could be an etiological factor in the disease-associated loss of adipose tissue by affecting adipocyte lipolysis and inhibiting disease-induced upregulation of miR-378. Thus, miR-378 may represent a promising target for ameliorating the severity of human diseases caused by impaired lipid metabolism.
Author contributions Y.Z. and C.L. contributed equally to this work. Y.Z. and C.L. conceived of the study, performed experiments, and drafted the manuscript. H.L. carried out the Western-blot analysis. Y.S. performed fatty acids analysis. Y.Z. conducted tail-vein injections of the Adeno-Akt1 and AgomiR-378 on Tg mice and HFD-fed mice, respectively. L.Z. carried out the luciferase-activity assay of the Scd1 experiment. H.W. participated in discussion of the experimental design and helped to draft the manuscript. R.Z. served as technical support. H.T. participated in discussion of the experimental design. D.Z. conceived of and supervised the project and wrote the manuscript. The authors declare no competing financial interests.
Acknowledgments This work was supported by grants from the National Basic Research Program of China (2011CBA01104, 2015CB943103) and the National Natural Science Foundation of China (31271470, 91019010). The funding agencies do not involve in any research activities in the present study.