Fibroblast growth factor 21 (FGF21) plays critical roles in regulating glucose and lipid metabolism. beta-Klotho is the co-receptor for mediating FGF21 signaling, and the mRNA levels of this receptor are increased in the liver of human subjects with obesity. However, the molecular mechanisms underlying the regulation of beta-klotho expression remain poorly defined. Here, we report that elevation of beta-klotho protein expression in diet-induced obese mice and human patients is associated with increased endoplasmic reticulum (ER) stress. In vivo study indicates that administration of the ER stressor tunicamycin in mice led to increased expression of beta-klotho in the liver. In addition, we show that ER stress is sufficient to potentiate FGF21 signaling in HepG2 cell and ATF4 signaling pathway is essential for mediating the effect of ER stress on beta-klotho expression. These findings demonstrate a link of ER stress with up-regulation of hepatic beta-klotho expression and the molecular mechanism underlying ER stress-regulated FGF21 signaling. (C) 2015 Elsevier Inc. All rights reserved.