Reactive oxygen species (ROS) are crucial in the pathogenesis of diabetic nephropathy (DN).X-box binding protein 1 (XBP1), a key mediator of endoplasmic reticulum (ER) stress, has been proved having the capability of preventing oxidative stress.In this study, the effects of spliced XBP1 (XBPlS) on high glucose (HG)-induced ROS production and extracellular matrix (ECM) synthesis were investigated in cultured renal mesangial cells (MCs) and STZ-induced diabetic rats.Real time PCR and Western blot were used to evaluate the XBP1 S, collagen Ⅳ, fibronectin and p47phox mRNA and protein levels respectively.Transfection of recombinant adenovirus vector carrying XBP 1S gene (Ad-XBP 1 S) was used to upregulate XBP 1 S expression.XBP1S siRNA was used to knockdown the XBP1S expression.ROS level was detected by dihydroethidium (DHE) fluorescent probe assay.The results showed that HG treatment significantly reduced XBP1S protein and mRNA levels in the cultured MCs, in the mean time of increasing ROS, collagen Ⅳ and fibronectin levels;Diphenylene-chloride iodonium (DPI) prevented HG-induced increases in ROS as well as collagen Ⅳ and fibronectin expressions.Transfection of Ad-XBP1S reversed HG-induced ROS and ECM productions.Knockdown intrinsic XBP1 S expression induced an increase in ROS production.P47phox was increased in HG-treated MCs.Ad-XBP1 S transfection reversed HG-induced p47phox increase while XBP1S knockdown upregulated p47phox expression.In diabetic rats, the expression of XBP1S was reduced while p47phox was elevated.These results suggested that suppression in XBP 1S pathway of ER stress was involved in HG-induced oxidative stress.A downstream target of XBP1S in regulating ROS formation might be NADPH oxidase.