GPR174 suppression attenuates retinopathy in angiotensin II (Ang II)-treated mice by reducing inflammation via PI3K/AKT signaling

Hypertension is closely associated with eye diseases, including hypertensive retinopathy. However, the molecular mechanism controlling the progression of hypertensive retinopathy remains poorly understood. G-Protein Coupled Receptor 174 (GPR174) is expressed in various tissues, and plays a critical role in regulating immune and inflammatory responses. However, if GPR174 is involved in angiotensin II (Ang II)-induced retinopathy is still unclear. In this study, hypertensive retinopathy was induced by Ang II infusion in the wild type (GPR174^+/+) and GPR174 knockout (GPR174^-/-) mice. Clinical data indicated that GPR174 levels were markedly up-regulated in serum of patients with hypertension (HP) or hypertensive retinopathy (HR), along with increased trypsin-like activity. Similar changes of GRP174 and trypsin-like activity were observed in Ang II-infused mice. Furthermore, Ang IIsignificantly increased the central retinal thickness, vascular permeability and inflammatory response in GPR174^+/+ micewhen compared with the saline GPR174^+/+ mice. Of note, these effects were markedly alleviated by the knockout ofGPR174 in Ang II-treated mice. Consistently, vascular endothelial growth factor (VEGF) expression levels in retinal tissue were also stimulated by Ang II, which were clearly attenuated by GPR174^-/-. In addition, phosphatidylinositol 3-kinase (PI3 K)/AKT and nuclear factor-κB (NF-κB) signaling pathways were markedly activated in retinas of Ang II-infused GPR174^+/+ mice, whereas being greatly ameliorated by GPR174^-/-. The in vitro data showed that pre-treatment of PI3 K/AKT specific inhibitor LY294002 remarkably abrogated GPR174 over-expression-accelerated expression levels of Iba-1, tumor necrosis factor-α (TNF-α) and interleukin (IL)-6 in lipopolysaccharide (LPS)-incubated retinal microglial cells.Furthermore, in LPS-exposed retinal microglial cells, PI3 K/AKT and NF-κB pathways were further promoted by GPR174 over-expression, which were significantlyabolished by LY294002. Thus, GPR174 might be a positive meditator of inflammation, contributing to Ang II-induced retinopathy by activating PI3 K/AKT signaling, and could be considered as a novel therapeutic target for the treatment of hypertensive retinopathy.