Interaction of parathyroid hormone-related peptide-responsive dual signal transduction systems in osteoblastic osteosarcoma cells: role in PTHrP-induced homologous desensitization

Role of the Parathyroid Hormone Type 1 Receptor (PTH1R) as a Mechanosensor in Osteocyte Survival

Osteocytes have a major role in the control of bone remodeling. Mechanical stimulation decreases osteocyte apoptosis and promotes bone accrual, whereas skeletal unloading is deleterious in both respects. PTH1R ablation or overexpression in osteocytes in mice produces trabecular bone loss or increases bone mass, respectively. The latter effect was related to a decreased osteocyte apoptosis. Here, the putative role of PTH1R activation in osteocyte protection conferred by mechanical stimulation was assessed. Osteocytic MLO-Y4 cells were subjected to mechanical stimuli represented by hypotonic shock (216 mOsm/kg) or pulsatile fluid flow (8 Hz, 10 dynes/cm(2)) for a short pulse (10 min), with or without PTH1R antagonists or after transfection with specific PTHrP or PTH1R siRNA. These mechanical stimuli prevented cell death induced within 6 hours by etoposide (50 μM), related to PTHrP overexpression; and this effect was abolished by the calcium antagonist verapamil (1 μM), a phospholipase C (PLC) inhibitor (U73122; 10 μM), and a PKA activation inhibitor, Rp-cAMPS (25 μM), in these cells. Each mechanical stimulus also rapidly induced β-catenin stabilization and nuclear ERK translocation, which were inhibited by the PTH1R antagonist PTHrP(7-34) (1 μM), or PTH1R siRNA, and mimicked by PTHrP(1-36) (100 nM). Mechanical stretching by hypotonic shock did not affect cAMP production but rapidly (<1 min) stimulated Ca(i)(2+) transients in PTH1R-overexpressing HEK-293 cells and in MLO-Y4 cells, in which calcium signaling was unaffected by the presence of a PTHrP antiserum or PTHrP siRNA but inhibited by knocking down PTH1R. These novel findings indicate that PTH1R is an important component of mechanical signal transduction in osteocytic MLO-Y4 cells, and that PTH1R activation by PTHrP-independent and dependent mechanisms has a relevant role in the prosurvival action of mechanical stimulus in these cells.

Both N- and C-terminal domains of parathyroid hormone-related protein increase interleukin-6 by nuclear factor-kappa B activation in osteoblastic cells

Parathyroid hormone (PTH)-related protein (PTHrP) seems to affect bone resorption by interaction with bone cytokines, among them interleukin-6 (IL-6). Recent studies suggest that nuclear factor (NF)-kappaB activation has an important role in bone resorption. We assessed whether the N-terminal fragment of PTHrP, and its C-terminal region, unrelated to PTH, can activate NF-kappaB, and its relationship with IL-6 gene induction in different rat and human osteoblastic cell preparations. Here we present molecular data demonstrating that both PTHrP (1-36) and PTHrP (107-139) activate NF-kappaB, leading to an increase in IL-6 mRNA, in these cells. Using anti-p65 and anti-p50 antibodies, we detected the presence of both proteins in the activated NF-kappaB complex. This effect induced by either the N- or C-terminal PTHrP domain in osteoblastic cells appears to occur by different intracellular mechanisms, involving protein kinase A or intracellular Ca(2+)/protein kinase C activation, respectively. However, the effect of each peptide alone did not increase further when added together. Our findings lend support to the hypothesis that the C-terminal domain of PTHrP, in a manner similar to its N-terminal fragment, might stimulate bone resorption. These studies also provide further insights into the putative role of PTHrP as a modulator of bone remodeling.

Inducible RGS2 is a cross-talk regulator for parathyroid hormone signaling in rat osteoblast-like UMR106 cells

Parathyroid hormone (PTH) activates dual signal transduction systems via Galphas and Galphaq proteins. We now report a novel mechanism by which "cross-talk" may occur between the Galphas and Galphaq signaling pathways. RGS2 (Regulator of G protein Signaling 2) mRNA was rapidly and transiently increased only by PTH analogs (PTH1-84, 1-34, 1-31, and PTHrP) that activated the Galphas-mediated cAMP/PKA signaling pathway, whereas activation of the Galphaq-mediated Ca(2+)/PKC signaling pathway by PTH3-34 had no effect on RGS2 expression. Treatment of UMR106 cells with nonPTH activators of the cAMP/PKA signaling pathway such as cholera toxin, forskolin, 8-Br-cAMP, and dibutyryl-cAMP also significantly elevated RGS2 mRNA levels, while activator of the Galphaq pathway PMA did not. Pretreatment using the Galphas signaling pathway inhibitors SQ22536 and H89 significantly blocked PTH-induced RGS2 expression, but the Galphaq signaling pathway inhibitor bisindolylmaleimide I had no effect. Therefore, RGS2 expression is governed solely by the Galphas signaling pathway. Additionally, we demonstrate for the first time that RGS2 binds to both Galphas and Galphaq subunits in their transition state (GDP/AlF(-4)-bound) forms, suggesting that RGS2 has the potential to act as a bridge between the cAMP/PKA and Ca(2+)/PKC pathways, and that it may act as a cross-talk regulator for these two PTH signaling pathways.

C-terminal parathyroid hormone-related protein (PTHrP) (107-139) stimulates intracellular Ca(2+) through a receptor different from the type 1 PTH/PTHrP receptor in osteoblastic osteosarcoma UMR 106 cells

Studies were undertaken to determine whether PTH-related protein (PTHrP) (107-139) mobilizes [Ca(2+)](i) in osteoblastic osteosarcoma UMR 106 cells. PTHrP (107-139), in a manner similar to PTHrP (107-111), induced a rapid [Ca(2+)](i) response in these cells that was dose dependent (EC(50) of approximately 0.1 pM) and more efficient than that of PTHrP (1-36) (EC(50) of approximately 1 nM). This effect of PTHrP (107-139) was abrogated by micromolar doses of verapamil or nifedipine. However, it was unaffected by 10 microM U73122 (a phospholipase C inhibitor), 100 microg/ml heparin (an inositol 1,4,5-trisphosphate receptor inhibitor), or 400 ng/ml pertussis toxin (a G(i) inhibitor), which inhibited the [Ca(2+)](i) response to PTHrP (1-36), or by either 25 nM bisindolylmaleimide I (BIM), a protein kinase (PK) C inhibitor, or 1 microM phorbol-12-myristate-13-acetate preincubation (22 h). PTHrP (107-139) and PTHrP (1-36), at 100 nM, desensitized the [Ca(2+)](i) response to a second challenge with the same peptide, but not with the other peptide in these cells. PTHrP (7-34), a type 1 PTH/PTHrP receptor (PTH1R) antagonist, decreased the effect of PTHrP (1-36) on [Ca(2+)](i). In contrast, PTHrP (107-111), but neither PTHrP (109-138) nor PTHrP (7-34), abolished this effect of PTHrP (107-139). Both PTHrP (107-139) and PTHrP (1-36), added together at submaximal doses, induced a higher [Ca(2+)](i) response. Moreover, PTHrP (107-139) increased the efficacy of PTHrP (1-36) on [Ca(2+)](i), but decreased its induced increase in PKA activity in these cells. Verapamil or nifedipine (at 50 microM) or 25 nM BIM, but not 25 microM adenosine 3',5'-cyclic monophosphorothioate, Rp-isomer, a PKA inhibitor, abolished the PTHrP (107-139)-induced increase in interleukin 6 messenger RNA (assessed by RT, followed by PCR) in UMR 106 cells. This peptide also increased c-fos messenger RNA in these cells; an effect inhibited by BIM, but unaffected by either verapamil or EGTA. These findings support the existence of high-affinity receptors for PTHrP (107-139), associated with an induced Ca(2+) influx, different from the PTH1R in UMR 106 cells. The present results suggest that PTHrP could affect bone turnover by interacting with the PTH1R and other yet unknown receptors in bone cells through complex mechanisms.

Phosphorylation of the cytoplasmic tail of the PTH/PTHrP receptor

Activation of the G protein-coupled receptor for parathyroid hormone (PTH)/PTH-related protein (PTHrP) produces homologous desensitization of receptor signaling. We have shown recently that the opossum PTH/PTHrP receptor stably expressed in human embryonic kidney (HEK) 293 cells is phosphorylated upon agonist binding and upon activation of serine/threonine protein kinases (PKA and PKC), an event which for some G protein-coupled receptors has been linked to desensitization. To locate the sites of phosphorylation, mutated forms of the opossum PTH/PTHrP receptor were stably expressed in HEK 293 cells, and ligand-stimulated receptor phosphorylation was evaluated. The five serine and threonine residues of the third cytoplasmic loop of the receptor were not required for receptor phosphorylation. Basal and ligand-induced phosphorylation were, however, completely abolished upon deletion of all but the 16 juxtamembrane residues of the cytoplasmic C-terminal tail of the receptor, even though this truncated receptor resembled the wild-type receptor in its level of expression based on Western blotting and radioligand binding. To identify further the phosphorylation sites, the 129 amino acid C-terminal tail of the rat PTH/PTHrP receptor was expressed in E. coli as a recombinant glutathione S-transferase fusion protein. Elimination of a single PKA consensus site in the tail (serine 491) resulted in > or = 90% loss of PKA-mediated phosphorylation, identifying this as the preferential site for PKA, with two other sites (serine 473 and/or 475) being minor sites. Phosphorylation by PKC occurred largely in the proximal portion of the tail, whereas beta-adrenergic receptor kinase 1 (beta ARK1) phosphorylated more distally in the tail. The ability of these kinases to phosphorylate the PTH/PTHrP receptor at distinct sites on the cytoplasmic tail may allow differential regulation of receptor signaling and trafficking.

Serpentine receptors for parathyroid hormone, calcitonin and extracellular calcium ions

The cloning of the receptors for PTH, CT and extracellular calcium ions represents a significant advance in the elucidation of the mechanisms through which extracellular calcium ions are regulated. All are members of the superfamily of GPCR, and the inclusion of the Ca2+o-sensing receptor in this superfamily documents that extracellular calcium ions can serve as an extracellular first messenger, in addition to subserving their better known role as a key intracellular second messenger. Furthermore, it has proved possible to identify several human diseases that result from inactivating or activating mutations in the PTH or Ca2+o-sensing receptor. Finally, the availability of these cloned receptors will enable many more studies on structure-function relationships for these receptors as well as clarifying their tissue distribution, regulation and roles in health and disease. It may also be possible to design novel therapeutic agents that permit manipulation of the receptors when their function is abnormal.

Second messenger signaling of c-fos gene induction by parathyroid hormone (PTH) and PTH-related peptide in osteoblastic osteosarcoma cells: its role in osteoblast proliferation and osteoclast-like cell formation

The present study was performed to clarify second messenger signaling in parathyroid hormone (PTH)-induced c-fos gene expression, to characterize the participation of the c-fos gene in the regulation of osteoblast proliferation and function as well as osteoclast-like cell formation by PTH and to compare these effects of PTH with those of PTH-related peptide (PTHrP). Both human (h) PTH-(1-34) and hPTHrP-(1-34) at 10(-8) M induced a transient c-fos gene expression to a similar degree in osteoblastic osteosarcoma cells, UMR-106. N6,O2'-dibutyryl adenosine 3',5'-cyclic monophosphate (dbcAMP) as well as Sp-diastereoisomer of adenosine cyclic 3',5'-phosphorothioate (Sp-cAMPS), an activator of cAMP-dependent protein kinase (PKA), induced a weak c-fos gene expression. Although Rp-diastereoisomer of adenosine cyclic 3',5'-phosphorothioate (Rp-cAMPS), an inhibitor of PKA, almost completely antagonized dbCAMP- and Sp-cAMPS-induced expression of c-fos gene, it did not cause an obvious inhibition of PTH- or PTHrP-induced expression. Phorbol 12-myristate 13-acetate (PMA), an activator of protein kinase C (PKC), induced an intense expression of the c-fos gene, while 4 alpha-phorbol 12,13-didecanoate (4 alpha PDD), incapable of activating PKC, and calcium ionophores (A23187 and ionomycin) did not. Protein kinase C inhibitor (H-7, 50 microM) completely blocked the expression of the c-fos gene by PTH as well as by PTHrP). Antisense oligodeoxynucleotides (as-ODN) complementary to c-fos mRNA, which have been shown to inhibit its mRNA translation, at 1 microM significantly antagonized PTH- and PTHrP-induced inhibition of [3H] thymidine incorporation and stimulation of osteoclast-like cell formation in the presence of osteoblasts, but not an increase in alkaline phosphatase activity, compared to control oligodeoxynucleotides with same nucleotides as as-ODN but with a random sequence. The present study indicates the involvement of PKC system in c-fos gene expression by PTH as well as PTHrP and also indicates the involvement of the c-fos gene in the regulation of bone cell physiology by PTH and PTHrP.