Acidosis Activates Endoplasmic Reticulum Stress Pathways through GPR4 in Human Vascular Endothelial Cells

Abstract

Acidosis commonly exists in the tissue microenvironment of various pathophysiological conditions such as tumors, inflammation, ischemia, metabolic disease, and respiratory disease. For instance, the tumor microenvironment is characterized by acidosis and hypoxia due to tumor heterogeneity, aerobic glycolysis (the “Warburg effect”), and the defective vasculature that cannot efficiently deliver oxygen and nutrients or remove metabolic acid byproduct. How the acidic microenvironment affects the function of blood vessels, however, is not well defined. GPR4 (G protein-coupled receptor 4) is a member of the proton-sensing G protein-coupled receptors and it has high expression in endothelial cells (ECs). We have previously reported that acidosis induces a broad inflammatory response in ECs. Acidosis also increases the expression of several endoplasmic reticulum (ER) stress response genes such as CHOP (C/EBP homologous protein) and ATF3 (activating transcription factor 3). In the current study, we have examined acidosis/GPR4-induced ER stress pathways in human umbilical vein endothelial cells (HUVEC) and other types of ECs. All three arms of the ER stress/unfolded protein response (UPR) pathways were activated by acidosis in ECs as an increased expression of phosphorylated eIF2α (eukaryotic initiation factor 2α), phosphorylated IRE1α (inositol-requiring enzyme 1α), and cleaved ATF6 upon acidic pH treatment was observed. The expression of other downstream mediators of the UPR, such as ATF4, ATF3, and spliced XBP-1 (X box-binding protein 1), was also induced by acidosis. Through genetic and pharmacological approaches to modulate the expression level or activity of GPR4 in HUVEC, we found that GPR4 plays an important role in mediating the ER stress response induced by acidosis. As ER stress/UPR can cause inflammation and cell apoptosis, acidosis/GPR4-induced ER stress pathways in ECs may regulate vascular growth and inflammatory response in the acidic microenvironment.

Mechanisms of Metabolic Acidosis–Induced Kidney Injury in Chronic Kidney Disease

“…[97][98][99][100] Other examples include acidosis activation of GPR4 in the vascular endothelial cell inflammatory response. 101,102 Pyk2 regulates coupling of receptor proteins to MAPK signaling pathways, such as integrins, GPRs, vascular endothelial growth factor receptor, and EGF receptor. 103 Additionally, Pyk2 is involved in signaling pathways entailing both ET and angiotensin II, and the control of vasoconstriction and BP.…”

Acidosis promotes cell apoptosis through the G protein‑coupled receptor 4/CCAAT/enhancer‑binding protein homologous protein pathway

“…It has been reported that GPR4 is involved in the regulation of acid-base balance and kidney acid secretion (6). Acidosis may activate the expression of GPR4 in order to regulate the expression of the genes involved in different biological pathways, including the ERS response and cell metabolism (3,21). A previous study demonstrated that acidosis/GPR4 regulated endothelial cell adhesion through cAMP production and served a role in the inflammatory response of vascular endothelial cells (3).…”

Association between G‑protein coupled receptor 4 expression and microvessel density, clinicopathological characteristics and survival in hepatocellular carcinoma

“…Moreover, the ischemic and hypoxic conditions of the tumor microenvironment activates GPR4 and thereby promotes angiogenesis. Another study with ECs demonstrated that GPR4 promotes angiogenesis via the regulation of C/EBP homologous protein (CHOP) in an acidic environment (45). Although these studies have shown that GPR4 promotes angiogenesis, the role of GPR4 in liver cancer was not investigated.…”