Disease relevance of GPR4

• We present the cloning and sequencing of the human gene for a novel G-protein coupled receptor (GPR4), from the critical myotonic dystrophy (DM) region on chromosome 19q13 [1].
• Endothelial cells infected with a retrovirus containing small interference RNA (siRNA) to GPR4 resulted in 40-50% decreased GPR4 expression, which corresponded with partial prevention of the LPC-induced 1) decrease in transendothelial resistance, 2) stress fiber formation, and 3) activation of RhoA [2].
• Leucine-rich repeat-containing, G protein-coupled receptor 4 null mice exhibit intrauterine growth retardation associated with embryonic and perinatal lethality [3].

High impact information on GPR4

• The analysis of intracellular cAMP levels showed that gpr4 mutants still respond to glucose but not to certain amino acids, such as methionine [4].
• Unexpectedly, the acid-induced cAMP accumulation was also largely inhibited by OGR1 siRNA but only slightly by GPR4 siRNA [5].
• SPC and LPC stimulate DNA synthesis in GPR4-expressing Swiss 3T3 cells [6].
• SPC and LPC bind to GPR4 in GPR4-transfected CHO cells with K(d)/SPC = 36 nm, and K(d)/LPC = 159 nm, respectively [6].
• Both SPC and LPC activate GPR4-dependent activation of serum response element reporter and receptor internalization [6].

Biological context of GPR4

• Overexpression of GPR4 or TDAG8 in HEK293 cells led to transcriptional activation from SRE- and CRE-driven promoters, independent of exogenously added ligand [7].
• The GPR4 subfamily consists of four G protein-coupled receptors that share significant sequence homology [7].
• Gene GPR4 shared identity (40% in the transmembrane regions) with the human platelet-activating factor receptor and was localized to chromosome 19 (q13.2-q13.3) [8].
• In human, two isoforms of GPR4 are expressed, differing in their 3' untranslated region due to the use of alternate polyadenylation signals and measuring approximately 2.8 and 1.8 kb, respectively [1].
• Screening of a human lung cDNA library with GPR12A as a probe also identified a closely-related cDNA (GPR6C.1) that has been previously reported as GPR4 [13] [9].

Anatomical context of GPR4

• Here we show that GPR4 also malignantly transforms NIH3T3 cells and that TDAG8 malignantly transforms the normal mammary epithelial cell line NMuMG [7].
• However, in a functional assay to examine the specificity of ligand binding, we found that neither SPC nor LPC, or other related lysophospholipids, induced internalization of GPR4 from the plasma membrane [10].
• In addition, neither SPC nor LPC stimulated the binding of GTPgammaS to membranes prepared from GPR4 expressing cells and did not activate ERK1/2 [10].
• Overall, we report that endothelial cells selectively expressed GPR4, a specific LPC receptor [11].
• We developed a peptide Ab against GPR4 that detected GPR4 expression in transfected COS 7 cells and endogenous GPR4 expression in endothelial cells by Western blot [2].

Associations of GPR4 with chemical compounds

• Surprisingly, enforced expression of GPR4 inhibited activation of ERK1/2 induced by several stimuli, including SPC, sphingosine-1-phosphate, and even EGF [10].
• It has been reported that ovarian cancer G protein-coupled receptor 1 (OGR1) is a high affinity receptor for SPC, and its closely related homologue GPR4 is a high affinity receptor for SPC with low affinity for lysophosphatidylcholine (LPC) [10].
• The inhibitory psychosine effect was also observed for pH-dependent actions in OGR1- and GPR4-expressing cells but not for prostaglandin E(2)- and sphingosine 1-phosphate-induced actions in any pH in native and sphingosine 1-phosphate receptor-expressing cells [12].
• Sequence analysis suggests that GPR4 is a peptide receptor and shares strongest homologies with purinergic receptors and receptors for angiotensin II, platelet activating factor, thrombin, and bradykinin [1].
• We find that GPR4, a close relative of OGR1, also responds to pH changes, but elicits cyclic AMP formation [13].

Other interactions of GPR4

• We conclude that endogenous GPR4 in endothelial cells may be a potential G protein-coupled receptor by which LPC signals proinflammatory activities [11].
• Human brain microvascular (HBMEC) and dermal microvascular endothelial cells (HMEC) were prepared for RT-PCR analysis for possible expression of the G protein-coupled receptors, GPR4 and G2A, which are believed to be specific LPC receptors [11].
• In contrast, HMEC expressed high basal GPR4 mRNA, which was not further increased by either TNF-alpha or H2O2 stimulation [11].
• In human umbilical vein EC, down-regulation of GPR4 specifically inhibits SPC-, but not sphingosine-1-phosphate-, or vascular endothelial growth factor (VEGF)-induced tube formation [14].
• Using LacZ complementation assays, we have shown that LPA receptors form homo- and hetero-dimers within the LPA receptor subgroup and hetero-dimers with other receptors (S1P(1-3) and GPR4) [15].

Analytical, diagnostic and therapeutic context of GPR4

• Northern blot analysis showed that GPR4 is widely expressed, with higher levels in kidney, heart, and especially lung, where it is at least fivefold greater than in other tissues [1].

References