Reduced pathological angiogenesis and tumor growth in mice lacking GPR4, a proton sensing receptor

Abstract

The G protein-coupled receptor GPR4 is activated by acidic pH and recent evidence indicates that it is expressed in endothelial cells. In agreement with these reports, we observe a high correlation of GPR4 mRNA expression with endothelial marker genes, and we confirm expression and acidic pH dependent function of GPR4 in primary human vascular endothelial cells. GPR4-deficient mice were generated; these are viable and fertile and show no gross abnormalities. However, these animals show a significantly reduced angiogenic response to VEGF (vascular endothelial growth factor), but not to bFGF (basic fibroblast growth factor), in a growth factor implant model. Accordingly, in two different orthotopic models, tumor growth is strongly reduced in mice lacking GPR4. Histological analysis of tumors indicates reduced tumor cell proliferation as well as altered vessel morphology, length and density. Moreover, GPR4 deficiency results in reduced VEGFR2 (VEGF Receptor 2) levels in endothelial cells, accounting, at least in part, for the observed phenotype. Our data suggest that endothelial cells sense local tissue acidosis via GPR4 and that this signal is required to generate a full angiogenic response to VEGF.

Emerging roles for the pH-sensing G protein-coupled receptors in response to acidotic stress

“…47 Studies on another GPR4 knockout mouse strain observed that the blood vessels of tumors formed in the GPR4-deficient mice were fragmented and fragile, also suggesting that GPR4 may regulate blood vessel integrity. 44 Similar to the previous study, 47 the phenotype of this GPR4 knockout mouse strain was also grossly normal. 44 However no perinatal mortality was observed in this knockout strain, which might be related to differences in mouse strain genetic backgrounds, knockout constructs, and breeding conditions.…”

“…44 Similar to the previous study, 47 the phenotype of this GPR4 knockout mouse strain was also grossly normal. 44 However no perinatal mortality was observed in this knockout strain, which might be related to differences in mouse strain genetic backgrounds, knockout constructs, and breeding conditions. Wyder et al also noted that the GPR4-deficient mice showed a reduced angiogenic response to VEGF, implicating the involvement of GPR4 in VEGF-driving angiogenesis.…”

“…The GPR4 receptor is highly expressed in vascular endothelial cells. 33,44 GPR4 is activated by extracellular acidic pH to initiate signaling cascades which regulate cellular function. 19,33,45,46 It has been reported that GPR4 is a functional pH sensor in vascular …”

“…Wyder et al also noted that the GPR4-deficient mice showed a reduced angiogenic response to VEGF, implicating the involvement of GPR4 in VEGF-driving angiogenesis. 44 It has been shown that GPR4 activation by acidosis stimulated the expression of endoplasmic reticulum stress response genes such as ATF3 and CHOP (DDIT3) in vascular endothelial cells. 32,48 Moreover, a recent study demonstrated that acidosis/GPR4-induced CHOP expression was involved in endothelial cell apoptosis in a renal ischemia/reperfusion mouse model.…”

“…GPR4, GPR65, and GPR68 play roles in tumor cell apoptosis, proliferation, metastasis, angiogenesis, and immune cell function. 19,27,32,33,44,45,96,97 GPR4 has had conflicting reports in terms of tumor suppressing or promoting activities. One study demonstrated that GPR4 could act as a tumor metastasis suppressor, when overexpressed and activated by acidic pH in B16F10 melanoma cells, by impeding migration and invasion of tumor cells.…”