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

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          Abstract

          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.

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          Most cited references31

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          Acidic extracellular microenvironment and cancer

          Acidic extracellular pH is a major feature of tumor tissue, extracellular acidification being primarily considered to be due to lactate secretion from anaerobic glycolysis. Clinicopathological evidence shows that transporters and pumps contribute to H+ secretion, such as the Na+/H+ exchanger, the H+-lactate co-transporter, monocarboxylate transporters, and the proton pump (H+-ATPase); these may also be associated with tumor metastasis. An acidic extracellular pH not only activates secreted lysosomal enzymes that have an optimal pH in the acidic range, but induces the expression of certain genes of pro-metastatic factors through an intracellular signaling cascade that is different from hypoxia. In addition to lactate, CO2 from the pentose phosphate pathway is an alternative source of acidity, showing that hypoxia and extracellular acidity are, while being independent from each other, deeply associated with the cellular microenvironment. In this article, the importance of an acidic extracellular pH as a microenvironmental factor participating in tumor progression is reviewed.
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            Proton-sensing G-protein-coupled receptors.

            Blood pH is maintained in a narrow range around pH 7.4 mainly through regulation of respiration and renal acid extrusion. The molecular mechanisms involved in pH homeostasis are not completely understood. Here we show that ovarian cancer G-protein-coupled receptor 1 (OGR1), previously described as a receptor for sphingosylphosphorylcholine, acts as a proton-sensing receptor stimulating inositol phosphate formation. The receptor is inactive at pH 7.8, and fully activated at pH 6.8-site-directed mutagenesis shows that histidines at the extracellular surface are involved in pH sensing. We find that GPR4, a close relative of OGR1, also responds to pH changes, but elicits cyclic AMP formation. It is known that the skeleton participates in pH homeostasis as a buffering organ, and that osteoblasts respond to pH changes in the physiological range, but the pH-sensing mechanism operating in these cells was hitherto not known. We detect expression of OGR1 in osteosarcoma cells and primary human osteoblast precursors, and show that these cells exhibit strong pH-dependent inositol phosphate formation. Immunohistochemistry on rat tissue sections confirms the presence of OGR1 in osteoblasts and osteocytes. We propose that OGR1 and GPR4 are proton-sensing receptors involved in pH homeostasis.
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              Pathogenesis of pituitary tumors.

              Pituitary adenomas may hypersecrete hormones (including prolactin, growth hormone and adrenocorticotropic hormone, and rarely follicle-stimulating hormone, luteinizing hormone or TSH) or may be nonfunctional. Despite their high prevalence in the general population, these tumors are invariably benign and exhibit features of differentiated pituitary cell function as well as premature proliferative arrest. Pathogenesis of dysregulated pituitary cell proliferation and unrestrained hormone hypersecretion may be mediated by hypothalamic, intrapituitary and/or peripheral factors. Altered expression of pituitary cell cycle genes, activation of pituitary selective oncoproteins or loss of pituitary suppressor factors may be associated with aberrant growth factor signaling. Considerable information on the etiology of these tumors has been derived from transgenic animal models, which may not accurately and universally reflect human tumor pathophysiology. Understanding subcellular mechanisms that underlie pituitary tumorigenesis will enable development of tumor aggression markers as well as novel targeted therapies.
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                Author and article information

                Journal
                J Reprod Dev
                J. Reprod. Dev
                JRD
                The Journal of Reproduction and Development
                The Society for Reproduction and Development
                0916-8818
                1348-4400
                19 January 2020
                April 2020
                : 66
                : 2
                : 175-180
                Affiliations
                [1) ]Laboratory of Cell Signaling Regulation, Department of Life Sciences, School of Agriculture, Meiji University, Kawasaki 214-8571, Japan
                [2) ]Department of Biochemistry, Jikei University School of Medicine, Tokyo 105-8461, Japan
                [3) ]Laboratory of Integrated Signaling Systems, Institute for Molecular and Cellular Regulation, Gunma University, Maebashi 371-8512, Japan
                [4) ]Laboratory of Medical Neuroscience, Institute for Molecular and Cellular Regulation, Gunma University, Maebashi 371-8512, Japan
                [5) ]Laboratory of Pathophysiology, Faculty of Pharmacy, Aomori University, Aomori 030-0943, Japan
                [6) ]Institute of Endocrinology, Meiji University, Kawasaki 214-8571, Japan
                Author notes
                Correspondence: H Tomura (e-mail: tomurah@ 123456meiji.ac.jp )
                Article
                2019-159
                10.1262/jrd.2019-159
                7175386
                31956173
                1f2cf01f-012f-4867-856b-b615cc87ef0d
                ©2020 Society for Reproduction and Development

                This is an open-access article distributed under the terms of the Creative Commons Attribution Non-Commercial No Derivatives (by-nc-nd) License. (CC-BY-NC-ND 4.0: https://creativecommons.org/licenses/by-nc-nd/4.0/)

                History
                : 17 December 2019
                : 08 January 2020
                Categories
                Original Article

                extracellular acidification,growth hormone,gpr4,mtt/s,prolactin

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