Apelin is transcriptionally regulated by ER stress-induced ATF4 expression via a p38 MAPK-dependent pathway

Apelin, which is an endogenous ligand for the orphan G-protein-coupled receptor APJ, was reported to be up-regulated by hypoxia-inducible factor 1-alpha (HIF1-alpha) in hypoxia- and insulin-treated cell systems. However, a negative transcriptional regulator of apelin has not yet been identified. In this study, we showed that apelin is down-regulated by ATF4 via the pro-apoptotic p38 MAPK pathway under endoplasmic reticulum (ER) stress. First, we analyzed the human apelin promoter to characterize the effects of ER stress on apelin expression in hepatocytes. Treatment with thapsigargin, an inducer of ER stress, and over-expression of ATF4 decreased apelin expression in hepatocytes. This work identified an ATF4-responsive region within the apelin promoter. Interestingly, ATF4-mediated repression of apelin was dependent upon the N-terminal domain of ATF4. C/EBP-beta knockdown experiments suggest that C/EBP-beta, which acts as an ATF4 binding partner, is critical for the ER stress-induced down-regulation of apelin. We also demonstrated that ATF4 regulates apelin gene expression via p38 pathways. Ectopic expression of constitutively active MKK6, an upstream kinase of p38, suggested that activation of the p38 pathway is sufficient to induce ATF4-mediated repression of apelin. Moreover, apelin enhanced cell migration in a wound healing assay in a p38 MAPK-dependent manner. Furthermore, analysis of caspase-3 activation indicated that ATF4 knockdown up-regulated apelin expression, leading to the inability of MKK6 (CA) to exert pro-apoptotic effects. Taken together, our results suggest that ATF4-mediated repression of apelin contributes substantially to the pro-apoptotic effects of p38.