Metals Differentially Activate Ovarian Cancer G Protein-Coupled Receptor 1 in Various Species

Metal-Stimulated Interleukin-6 Production Through a Proton-Sensing Receptor, Ovarian Cancer G Protein-Coupled Receptor 1, in Human Bronchial Smooth Muscle Cells: A Response Inhibited by Dexamethasone

Purpose

Human bronchial smooth muscle cells (BSMCs) contribute to airway obstruction and hyperresponsiveness in patients with bronchial asthma. BSMCs also generate cytokines and matricellular proteins in response to extracellular acidification through the ovarian cancer G protein-coupled receptor 1 (OGR1). Cobalt (Co) and nickel (Ni) are occupational agents, which cause occupational asthma. We examined the effects of Co and Ni on interleukin-6 (IL-6) secretion by human BSMCs because these metals may act as ligands of OGR1.

Methods

Human BSMCs were incubated in Dulbecco's Modified Eagle Medium (DMEM) containing 0.1% bovine serum albumin (BSA) (0.1% BSA-DMEM) for 16 hours and stimulated for the indicated time by exchanging the medium with 0.1% BSA-DMEM containing any of the metals or pH-adjusted 0.1% BSA-DMEM. IL-6 mRNA expression was quantified via reverse transcription polymerase chain reaction (RT-PCR) using the real-time TaqMan technology. IL-6 was measured using an enzyme-linked immunosorbent assay. Dexamethasone (DEX) was added 30 minutes before each stimulation. To knock down the expression of OGR1 in BSMCs, small interfering RNA (siRNA) targeting OGR1 (OGR1-siRNA) was transfected to the cells and non-targeting siRNA (NT-siRNA) was used as a control.

Results

Co and Ni both significantly increased IL-6 secretion in human BSMCs at 300 μM. This significant increase in IL-6 mRNA expression was observed 5 hours after stimulation. BSMCs transfected with OGR1-siRNA produced less IL-6 than BSMCs transfected with NT-siRNA in response to either Co or Ni stimulation. DEX inhibited Co- and Ni-stimulated IL-6 secretion by human BSMCs as well as pH 6.3-stimulated IL-6 secretion in a dose-dependent manner. DEX did not decrease phosphorylation of ERK1/2, p38 MAP kinase, and NF-κB p65 induced by either Co or Ni stimulation.

Conclusion

Co and Ni induce secretion of IL-6 in human BSMCs through activation of OGR1. Co- and Ni-stimulated IL-6 secretion is inhibited by DEX.

Differential Roles of Extracellular Histidine Residues of GPR68 for Proton-Sensing and Allosteric Modulation by Divalent Metal Ions

GPR68, an orphan G-protein coupled receptor, senses protons, couples to multiple G-proteins, and is also activated or inhibited by divalent metal ions. It has seven extracellular histidine residues, although it is not clear how these histidine residues play a role in both proton-sensing and metal ion modulation. Here we demonstrate that divalent metal ions are allosteric modulators that can activate or inhibit proton activity in a concentration- and pH-dependent manner. We then show that single histidine mutants have differential and varying degrees of effects on proton-sensing and metal ion modulation. Some histidine residues play dual roles in proton-sensing and metal ion modulation, while others are important in one or the other but not both. Two extracellular disulfide bonds are predicted to constrain histidine residues to be spatially close to each other. Combining histidine mutations leads to reduced proton activity and resistance to metal ion modulation, while breaking the less conserved disulfide bond results in a more severe reduction in proton-sensing over metal modulation. The small-molecule positive allosteric modulators (PAMs) ogerin and lorazepam are not affected by these mutations and remain active at mutants with severely reduced proton activity or are resistant to metal ion modulation. These results suggest GPR68 possesses two independent allosteric modulation systems, one through interaction with divalent metal ions at the extracellular surface and another through small-molecule PAMs in the transmembrane domains. A new GPR68 model is developed to accommodate the findings which could serve as a template for further studies and ligand discovery by virtual ligand docking.

Species-Dependent Enhancement of Ovarian Cancer G Protein-Coupled Receptor 1 Activation by Ogerin

Ogerin is a positive allosteric modulator of human and mouse ovarian cancer G protein-coupled receptors (OGR1s). In the present study, we found that ogerin differentially enhances the activation of OGR1 in various animal species. Amino acid residues of OGR1 that are associated with ogerin are conserved among the species. This suggests that other amino acid residues may be involved in the action of ogerin. Chimeric receptors between human and zebrafish OGR1s showed that the amino acid residues that determine the species specificity of ogerin-induced enhancement reside in the transmembrane and/or intracellular regions of OGR1. This result highlights the importance of first verifying the effectiveness of ogerin to the OGR1 of the species of interest at the cellular level prior to analyzing the physiological and pathophysiological roles of OGR1 in the species.

Evolution of acid nociception: ion channels and receptors for detecting acid

Nociceptors, i.e. sensory neurons tuned to detect noxious stimuli, are found in numerous phyla of the Animalia kingdom and are often polymodal, responding to a variety of stimuli, e.g. heat, cold, pressure and chemicals, such as acid. Owing to the ability of protons to have a profound effect on ionic homeostasis and damage macromolecular structures, it is no wonder that the ability to detect acid is conserved across many species. To detect changes in pH, nociceptors are equipped with an assortment of different acid sensors, some of which can detect mild changes in pH, such as the acid-sensing ion channels, proton-sensing G protein-coupled receptors and several two-pore potassium channels, whereas others, such as the transient receptor potential vanilloid 1 ion channel, require larger shifts in pH. This review will discuss the evolution of acid sensation and the different mechanisms by which nociceptors can detect acid. This article is part of the Theo Murphy meeting issue 'Evolution of mechanisms and behaviour important for pain'.

Design, Synthesis, and Characterization of Ogerin-Based Positive Allosteric Modulators for G Protein-Coupled Receptor 68 (GPR68)

G protein-coupled receptor 68 (GPR68) is an understudied orphan G protein-coupled receptor (GPCR). It is expressed most abundantly in the brain, potentially playing important roles in learning and memory. Pharmacological studies with GPR68 have been hindered by lack of chemical tools that can selectively modulate its activity. We previously reported the first small-molecule positive allosteric modulator (PAM), ogerin (1), and showed that 1 can potentiate proton activity at the GPR68-G_s pathway. Here, we report the first comprehensive structure-activity relationship (SAR) study on the scaffold of 1. Our lead compound resulted from this study, MS48107 (71), displayed 33-fold increased allosteric activity compared to 1. Compound 71 demonstrated high selectivity over closely related proton GPCRs and 48 common drug targets, and was bioavailable and brain-penetrant in mice. Thus, our SAR study has resulted in an improved GPR68 PAM for investigating the physiological and pathophysiological roles of GPR68 in vitro and in vivo.

Protons Differentially Activate TDAG8 Homologs from Various Species

Mammalian T cell death-associated gene 8 (TDAG8)s are activated by extracellular protons. In the present study, we examined whether the TDAG8 homologs of other species are activated by protons as they are in mammals. We found that Xenopus TDAG8 also stimulated cAMP response element (CRE)-driven promoter activities reflecting the activation of Gs/cAMP signaling pathways when they are stimulated by protons. On the other hand, the activities of chicken and zebrafish TDAG8s are hardly affected by protons. Results using chimeric receptors of human and zebrafish TDAG8s indicate that the specificity of the proton-induced activation lies in the extracellular region. These results suggest that protons are not an evolutionarily conserved agonist of TDAG8.

GPR68: An Emerging Drug Target in Cancer

GPR68 (or ovarian cancer G protein-coupled receptor 1, OGR1) is a proton-sensing G-protein-coupled receptor (GPCR) that responds to extracellular acidity and regulates a variety of cellular functions. Acidosis is considered a defining hallmark of the tumor microenvironment (TME). GPR68 expression is highly upregulated in numerous types of cancer. Emerging evidence has revealed that GPR68 may play crucial roles in tumor biology, including tumorigenesis, tumor growth, and metastasis. This review summarizes current knowledge regarding GPR68-its expression, regulation, signaling pathways, physiological roles, and functions it regulates in human cancers (including prostate, colon and pancreatic cancer, melanoma, medulloblastoma, and myelodysplastic syndrome). The findings provide evidence for GPR68 as a potentially novel therapeutic target but in addition, we note challenges in developing drugs that target GPR68.