Ethylenediaminetetraacetic acid (EDTA) enhances cAMP production in human TDAG8-expressing cells

Ethylenediaminetetraacetic acid (EDTA) is a chelating agent that binds tightly to metal ions. We found that cAMP response element (CRE)-driven promoter activity by protons was enhanced by EDTA in human T-cell death-associated gene 8 (TDAG8)-overexpressed HEK293T cells. The enhancing action by EDTA was also detected by proton-induced cAMP production that is located upstream from the CRE-driven promoter activity even at physiological proton concentration pH7.4. The proton-induced CRE-driven promoter activity was not enhanced by other chelating agents, ethylene glycol tetraacetic acid (EGTA) and sodium citrate. The enhanced CRE-driven promoter activity by EDTA was not attenuated by increasing the extracellular calcium ion concentration. These results indicate that the EDTA-enhancing action may not be due to its chelating action but might rather be another EDTA-specific effect. Enhanced cAMP production by EDTA was also detected in a human leukemia cell line HL-60, in which TDAG8 and OGR1 (ovarian cancer G-protein-coupled receptor 1) were endogenously expressed, suggesting that the medical use of EDTA would influence the physiological and pathophysiological functions of hematopoietic cells.

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.

Involvement of GPR4 in increased growth hormone and prolactin expressions by extracellular acidification in MtT/S cells

Hormone-secreting pituitary adenomas show unregulated hormonal hypersecretion and cause hyperpituitarism. However, the mechanism of the unregulated hormone production and secretion has not yet been fully elucidated. Solid tumors show reduced extracellular pH, partly due to lactate secretion from anaerobic glycolysis. It is known that extracellular acidification affects hormone secretion. However, whether and how the extracellular acidification influences the unregulated hormone production and secretion remain unknown. In the present study, we found that GPR4, a proton-sensing G protein-coupled receptor, was highly expressed in MtT/S cells, a growth hormone-producing and prolactin-producing pituitary tumor cell line. When we reduced the extracellular pH, growth hormone and prolactin mRNA expressions increased in the cells. Both increased expressions were partially suppressed by a GPR4 antagonist. We also found that extracellular acidification enhanced growth hormone-releasing factor-induced growth hormone secretion from MtT/S cells. These results suggest that GPR4 may play a role in hypersecretion of the hormone from hormone-producing pituitary tumors. A GPR4 antagonist will be a useful tool for preventing the hypersecretion.

Characterization of molecular mechanisms of extracellular acidification-induced intracellular Ca2+ increase in LβT2 cells

Extracellular acidification regulates endocrine cell functions. Ovarian cancer G protein-coupled receptor 1 (OGR1), also known as GPR68, is a proton-sensing G protein-coupled receptor and is activated by extracellular acidification, resulting in the activation of multiple intracellular signaling pathways. In the present study, we found that OGR1 was expressed in some gonadotropic cells in rat anterior pituitary and in LβΤ2 cells, which are used as a model of gonadotropic cells. When we reduced extracellular pH, a transient intracellular Ca²⁺ increase was detected in LβT2 cells. The Ca²⁺ increase was inhibited by a G_q/11 inhibitor and Cu²⁺, which is known as an OGR1 antagonist. We also found that extracellular acidification enhanced GnRH-induced Gaussia luciferase secretion from LβT2 cells. These results suggest that OGR1 may play a role in the regulation of gonadotropic cell function such as its hormone secretion.

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.