Uric Acid Impairs Insulin Signaling by Promoting Enpp1 Binding to Insulin Receptor in Human Umbilical Vein Endothelial Cells

High levels of uric acid (UA) are associated with type-2 diabetes and cardiovascular disease. Recent pieces of evidence attributed to UA a causative role in the appearance of diabetes and vascular damage. However, the molecular mechanisms by which UA induces these alterations have not been completely elucidated so far. Among the mechanisms underlying insulin resistance, it was reported the role of a transmembrane glycoprotein, named either ectonucleotide pyrophosphatase/phosphodiesterase 1 (ENPP1) or plasma cell antigen 1, which is able to inhibit the function of insulin receptor (I_R) and it is overexpressed in insulin-resistant subjects. In keeping with this, we stimulated human umbilical vein endothelial cells (HUVECs) with insulin and UA to investigate the effects of UA on insulin signaling pathway, testing the hypothesis that UA can interfere with insulin signaling by the activation of ENPP1. Cultures of HUVECs were stimulated with insulin, UA and the urate transporter SLC22A12 (URAT1) inhibitor probenecid. Akt and endothelial nitric oxide synthase (eNOS) phosphorylation levels were investigated by immunoblotting. ENPP1 binding to I_R and its tyrosine phosphorylation levels were tested by immunoprecipitation and immunoblotting. UA inhibited insulin-induced Akt/eNOS axis. Moreover, UA induced ENPP1 binding to I_R that resulted in an impairment of insulin signaling cascade. Probenecid reverted UA effects, suggesting that UA intracellular uptake is required for its action. In endothelial cells, UA directly interferes with insulin signaling pathway at receptor level, through ENPP1 recruitment. This evidence suggests a new molecular model of UA-induced insulin resistance and vascular damage.

Maternal hypervitaminosis D reduces fetal bone mass and mineral acquisition and leads to neonatal lethality

Pregnancy challenges maternal calcium handling because sufficient calcium has to be transferred to the fetus to ensure fetal bone mass acquisition. 1,25(OH)2 vitamin D [1,25(OH)2D] is an important regulator of calcium homeostasis during adulthood, yet its role seems redundant for the maternal adaptations to pregnancy as well as during fetal development. However, not only deficiency but also excess of 1,25(OH)2D can be harmful and we therefore questioned whether high maternal 1,25(OH)2D levels may injure fetal development or neonatal outcome, as maternal-fetal transport of 1,25(OH)2D has been largely disputed. To this end, vitamin D receptor (VDR) null (Vdr(-/-)) females, displaying high 1,25(OH)2D levels, were mated with Vdr(+/-) males to obtain pregnancies with fetuses that are responsive (Vdr(+/-)) or resistant (Vdr(-/-)) to 1,25(OH)2D. Surprisingly, most of the Vdr(+/-) neonates died shortly after birth, whereas none of the Vdr(-/-). Mechanistically, we noticed that in Vdr(+/-) embryos, serum calcium levels were normal, but that skeletal calcium storage was reduced as evidenced by decreased mineralized bone mass as well as bone mineral content. More precisely, bone formation was decreased and the level of bone mineralization inhibitors was increased. This decreased fetal skeletal calcium storage may severely compromise calcium balance and survival at birth. In conclusion, these data indicate that high maternal 1,25(OH)2D levels are transferred across the placental barrier and adversely affect the total amount of calcium stored in fetal bones which is accompanied by neonatal death.