Secreted isoform of endoplasmic reticulum membrane complex subunit 10 (scEMC10) is a poorly characterized secreted protein of largely unknown physiological function. Here we demonstrate that scEMC10 is upregulated in people with obesity and is positively associated with insulin resistance. Consistent with a causal role for scEMC10 in obesity, Emc10-/- mice are resistant to diet-induced obesity due to an increase in energy expenditure, while scEMC10 overexpression decreases energy expenditure, thus promoting obesity in mouse. Furthermore, neutralization of circulating scEMC10 using a monoclonal antibody reduces body weight and enhances insulin sensitivity in obese mice. Mechanistically, we provide evidence that scEMC10 can be transported into cells where it binds to the catalytic subunit of PKA and inhibits its stimulatory action on CREB while ablation of EMC10 promotes thermogenesis in adipocytes via activation of the PKA signalling pathway and its downstream targets. Taken together, our data identify scEMC10 as a circulating inhibitor of thermogenesis and a potential therapeutic target for obesity and its cardiometabolic complications.
Endoplasmic reticulum membrane protein complex subunit 10 (EMC10) is an evolutionarily conserved and multifunctional factor across species. We previously reported that Emc10 knockout (KO) leads to mouse male infertility. Emc10-null spermatozoa exhibit multiple aspects of dysfunction, including reduced sperm motility. Two subunits of a Na/K-ATPase, ATP1A4 and ATP1B3, are nearly absent in Emc10 KO spermatozoa. Here, two isoforms of EMC10 were characterized in the mouse testis and epididymis: the membrane-bound (mEMC10) and secreted (scEMC10) isoforms. We present evidence that mEMC10, rather than scEMC10, is required for cytoplasm sodium homeostasis by positively regulating ATP1B3 expression in germ cells. Intra-testis mEMC10 overexpression rescued the sperm motility defect caused by Emc10 KO, while exogenous recombinant scEMC10 protein could not improve the motility of spermatozoa from either Emc10 KO mouse or asthenospermic subjects. Clinically, there is a positive association between ATP1B3 and EMC10 protein levels in human spermatozoa, whereas no correlation was proven between seminal plasma scEMC10 levels and sperm motility. These results highlight the important role of the membrane-bound EMC10 isoform in maintaining cytoplasm sodium homeostasis and sperm motility. Based on the present results, the mEMC10-Na, K/ATPase α4β3 axis is proposed as a novel mechanism underlying the regulation of cytoplasmic sodium and sperm motility, and its components seem to have therapeutic potential for asthenospermia.
Endothelial cell insulin resistance contributes to the development of vascular complications in diabetes. Hypoxia-inducible factors (HIF) modulate insulin sensitivity, and we have previously shown that a negative regulator of HIF activity, CBP/p300 interacting transactivator-2 (CITED2), is increased in the vasculature of people with type 2 diabetes. Therefore, we examined whether CITED2 regulates endothelial insulin sensitivity. In endothelial cells isolated from mice with a "floxed" mutation in the Cited2 gene, loss of CITED2 markedly enhanced insulin-stimulated Akt phosphorylation without altering ERK1/2 phosphorylation. Similarly, insulin-stimulated Akt phosphorylation was increased in aortas of mice with endothelial-specific deletion of CITED2. Consistent with these observations, loss of CITED2 in endothelial cells increased insulin-stimulated endothelial nitric oxide synthase phosphorylation, Vegfa expression, and cell proliferation. Endothelial cells lacking CITED2 exhibited an increase in insulin receptor substrate (IRS)-2 protein, a key mediator of the insulin signaling cascade, while IRS-1 was unchanged. Conversely, overexpression of CITED2 in endothelial cells decreased IRS-2 protein by 55% without altering IRS-1, resulting in impaired insulin-stimulated Akt phosphorylation and Vegfa expression. Overexpression of HIF-2α significantly increased activity of the Irs2 promoter and co-expression of CITED2 abolished this increase. Moreover, ChIP showed that loss of CITED2 increased occupancy of p300, a key component of the HIF transcriptional complex, on the Irs2 promoter. Together, these results show that CITED2 selectively inhibits endothelial insulin signaling and action through the PI3K/Akt pathway via repression of HIF-dependent IRS-2 expression. CITED2 is thus a promising target to improve endothelial insulin sensitivity and prevent the vascular complications of diabetes.
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