A New Action of Parathyroid Hormone

Abstract

The major physiological function of parathyroid hormone (PTH) is the maintenance of Ca 2؉ /P i homeostasis via the parathyroid hormone/parathyroid hormone-related protein receptor (PTHR) in kidney and bone. An important consequence of PTHR activation in bone is enhanced local acidification of the extracellular space. Agonist activation of some seven transmembrane-domain receptors increases the extracellular acidification rate (ECAR). We utilized microphysiometry to investigate PTH-stimulated, receptor-mediated increases in ECAR in human osteoblast-like SaOS-2 cells. PTH-(1-34) elicited a large, acute, dose-dependent increase in ECAR with an EC 50 of about 2 nM. The PTH-induced increase in ECAR was specific to cells expressing the PTHR and was inhibited by PTHR antagonists. Rapid, partial, homologous desensitization of the PTH-induced increase in ECAR was observed. Incubation of SaOS-2 cells with 8-bromo-cyclic AMP neither mimicked nor abrogated the PTH effect, and PTH stimulated an acute increase in ECAR in cAMP-resistant SaOS-2 Ca#4A cells. Stimulation of ECAR by PTH was independent of transient increases in cytosolic free calcium. Both inhibition and down-regulation of PKC reduced the PTH-induced increase in ECAR. Inhibition of Na ؉ /H ؉ exchange did not affect the PTH-induced ECAR response. We conclude that PTH caused a receptor-mediated, concentration-dependent, increase in ECAR, which was not dependent on the cAMP/PKA signaling pathway or the Na ؉ /H ؉ exchanger but involved the action of PKC. Thus, acid production in bone, a physiologically important action of PTH, is not confined to osteoclasts as previously considered but is also mediated by osteoblasts.The parathyroid hormone/parathyroid hormone-related protein receptor (PTHR) 1 is a member of the G protein-coupled, seven-transmembrane domain superfamily of receptors (1). The principal physiological action of PTH is on the regulation of Ca 2ϩ /P i homeostasis, which is mediated by the PTHR in kidney and bone. The biochemical and molecular mechanisms of the receptor-mediated actions of PTH are not completely understood. PTH stimulates osteoclast-mediated bone resorption indirectly via the activation of the PTHR on osteoblasts (2); however, the mechanism coupling these events is not known. Additionally, PTH has an anabolic effect on bone when administered intermittently (3). It has been proposed that the diverse actions of PTH may be a consequence of activation of both the cAMP/PKA and inositol lipid/Ca 2ϩ /PKC signaling pathways by the PTHR in osteoblasts (4). Receptor regulation is also important in the action of PTH on osteoblasts (4 -9). Homologous desensitization of PTH-stimulated increases in intracellular cAMP accumulation ([cAMP] i ) and in cytosolic free calcium concentration ([Ca 2ϩ ] i ) and down-regulation of the PTHR have been described in several target cell systems (4 -9).Previous investigations of the physiological actions of PTH have largely used animal and whole bone experimental systems (10 -14), while studies of PTHR signaling have co...

Profiling Insulin Like Factor 3 (INSL3) Signaling in Human Osteoblasts

“…Only the latter is positively regulated by INSL3 and these data again well agree with our previous experiments on Rxfp2 −/− mice [4], as PTH stimulates osteoclast-mediated bone resorption indirectly via the activation of the PTHR on osteoblasts [42].…”

Effects of PTH on osteoblast bioenergetics in response to glucose

“…Furthermore, Esen et al, provided evidence that the majority of the increase in glucose uptake with PTH treatment is mediated through PKA signaling. Conversely, other studies in human SaOS-2 and rat osteoblast like UMR-106 cells have shown that the increased glycolysis with PTH (1–34) treatment is mostly mediated by PKC signaling, independent of cAMP/PKA and Na+/H+ exchanger pathways ( Barrett et al, 1997 ; Belinsky et al, 1999 ; Belinsky and Tashjian Jr., 2000 ). Alternatively, another study using MC3T3E1C4 cells showed that PTH treatment enhanced osteoblast differentiation through GPR81-Gβγ-PLC-PKC-Akt signaling coupled with increased lactate production ( Wu et al, 2018 ).…”

Deorphanization of G Protein Coupled Receptors (GPCRs): a historical perspective.

“…The Cytosensor microphysiometer (Hafner, 2000) was widely adopted in the late 90s to measure metabolic changes and was particularly relevant for APJ deorphanization. This technology provides a quantitative measurement of extracellular acidification rate elicited by small molecules, neuropeptides, and hormones in response to GPCR activation but also to ligand-gated ion channels and receptor tyrosine kinases (Barrett et al, 1997;Lang et al, 2001;Merkouris et al, 1997;Ng et al, 2001;Pitchford et al, 1995;Pitchford et al, 1997).…”