Cross-talk of parathyroid hormone-responsive dual signal transduction systems in osteoblastic osteosarcoma cells: its role in PTH-induced homologous desensitization of intracellular calcium response

Functional characterization of the parathyroid hormone 1 receptor in human periodontal ligament cells

OBJECTIVES

Intermittent parathyroid hormone (PTH) exerts anabolic effects on bone and has been approved for osteoporosis therapy. The dual actions of PTH are mediated primarily through the parathyroid hormone 1 receptor (PTH1R). Upon ligand binding, PTH1R activates diverse signaling pathways, including cAMP/protein kinase A (PKA)- and phospholipase C/protein kinase C (PLC/PKC)-dependent pathways. PTH1R has been abundantly studied in bone cells. Knowledge on PTH1R characteristics and physiology in periodontal ligament (PDL) cells is still in its infancy.

MATERIALS AND METHODS

We characterized PTH1R in PDL cells in terms of its cellular localization, binding affinity, and signal transduction and compared these characteristics to those of MG63 osteoblast-like cells.

RESULTS

PTH1R mRNA/protein was identified in PDL and MG63 cells. PTH1R was mainly localized on the plasma membrane, in vesicular structures inside the cell, and, to some extent, in the nucleus of both cell types. Binding characteristics of PTH1R were cell type specific, with PDL cells demonstrating a lower binding affinity. The response of cAMP and active PKC production in MG63 cells was dose dependent with increasing PTH(1-34) concentration, whereas in PDL cells, it was regulated biphasically. However, we observed a cross talk between the cAMP/PKA and PLC/PKC signaling pathways, which were regulated diametrically opposed at a given concentration of PTH(1-34).

CONCLUSION

These data indicate that, albeit the similarity in its subcellular distribution, PTH1R in PDL cells exhibits characteristics different from those in MG63 cells, pointing to the cell type specificity of this receptor.

CLINICAL RELEVANCE

The findings further elucidate the characteristics of PTH action in dental tissues and widen the theoretical basis for the development of anabolic treatment strategies.

Studies on the mechanisms of the skeletal anabolic action of endogenous and exogenous parathyroid hormone

Parathyroid hormone (PTH) has been viewed as catabolic for bone. Nevertheless, exogenous PTH is anabolic when administered intermittently, at a frequency that permits complete clearance between doses. In the fetus and neonate, endogenous PTH is required for normal trabecular bone formation. In older animals PTH produces net bone loss in fulfilling its calcium homeostatic role, whereas PTH-related peptide (PTHrP), acting in a paracrine/autocrine mode, is anabolic. The proliferative, differentiating, and anti-apoptotic effects of PTH on cells of the osteoblast lineage leading to anabolism can be direct, or indirect via release of local growth factors. The anabolic effect of PTH is also influenced by osteoclastic activity such that suppression of osteoclasts with anti-resorptive agents, concomitant to administering PTH, may enhance the anabolic effect by delaying a reactive osteoclastic response. In contrast, prolonged suppression of osteoclast activity prior to administering PTH appears to diminish molecular signals that increase the osteoblast pool and thereby reduces the anabolic efficacy of PTH. These observations may define the proper timing of the use of PTH as a therapeutic in diseases of bone loss. Finally, the capacity of exogenous PTH to modulate extra-osseous factors such as 1,25 dihydroxyvitamin D may also modulate its potency as an anabolic agent.

Regulation of matrix Gla protein by parathyroid hormone in MC3T3-E1 osteoblast-like cells involves protein kinase A and extracellular signal-regulated kinase pathways

Inhibition of osteoblast-mediated mineralization is one of the major catabolic effects of parathyroid hormone (PTH) on bone. Previously, we showed that PTH induces matrix gamma-carboxyglutamic acid (Gla) protein (MGP) expression and established that this induction is critical for PTH-mediated inhibition of osteoblast mineralization. In the present study, we focus on the mechanism through which PTH regulates MGP expression in osteoblastic MC3T3-E1 cells. Following transient transfection of these cells with a -748 bp murine MGP promoter-luciferase construct (pMGP-luc), PTH (10 (-7) M) induced promoter activity in a time-dependent manner with a maximal four- to six fold induction seen 6 h after PTH treatment. Both H-89 (PKA inhibitor) and U0126 (MEK inhibitor), suppressed PTH induction of MGP promoter activity as well as the MGP mRNA level. In addition, forskolin (PKA activator) stimulated MGP promoter activity and mRNA levels confirming that PKA is one of the signaling molecules required for regulation of MGP by PTH. Co-transfection of MC3T3-E1 cells with pMGP-luc and MEK(SP), a plasmid encoding the constitutively active form of MEK, led to a dose-dependent increase in MGP promoter activity. Both MGP promoter activity and MGP mRNA level were not affected by the protein kinase C (PKC) inhibitor, GF109203X. However, phorbol 12-myristate 13-acetate (PMA), a selective PKC activator induced MGP mRNA expression through activation of extracellular signal-regulated kinase (ERK). Taken together, these results indicate that PTH regulates MGP via both PKA- and ERK-dependent pathways.

Contributions of parathyroid hormone (PTH)/PTH-related peptide receptor signaling pathways to the anabolic effect of PTH on bone

PTH regulates osteoblastic function by activating PTH/PTHrP receptors (PTH1Rs), which trigger several signaling pathways in parallel, including cAMP/protein kinase A (PKA) and, via both phospholipase-C (PLC)-dependent and PLC-independent mechanisms, protein kinase C (PKC). These signaling functions have been mapped to distinct domains within PTH(1-34), but their roles in mediating the anabolic effect of intermittent PTH in vivo are unclear. We compared the anabolic effects in mice of hPTH(1-34) with those of two analogs having restricted patterns of PTH1R signaling. [G(1),R(19)]hPTH(1-28) lacks the 29-34 domain of hPTH(1-34) needed for PLC-independent PKC activation, incorporates a Gly(1) mutation that prevents PLC activation, and stimulates only cAMP/PKA signaling. [G(1),R(19)]hPTH(1-34) retains the 29-34 domain and activates both cAMP/PKA and PLC-independent PKC. Human PTH(1-34) (40 microg/kg), [G(1),R(19)]hPTH(1-34) (120 microg/kg), and [G(1),R(19)]hPTH(1-28) (800 microg/kg), at doses equipotent in elevating blood cAMP at 10 min and cAMP-dependent gene expression in bone at 6 h after s.c. injection, were administered to 10-week-old female C57BL/6J mice 5 days/week for 4 weeks. Acute blood cAMP responses, retested after 4 weeks, were not reduced by the preceding PTH treatment. The three PTH peptides induced equivalent increases in distal femoral bone mineral density (BMD), and, by microCT analysis, distal femoral and vertebral bone volume and trabecular thickness and mid-femoral cortical endosteal apposition. [G(1),R(19)]hPTH(1-34) and hPTH(1-34) increased distal femoral BMD more rapidly and augmented total-body BMD and bone volume of proximal tibial trabeculi to a greater extent than did [G(1),R(19)]hPTH(1-28). We conclude that cAMP/PKA signaling is the dominant mechanism for the anabolic actions of PTH in trabecular bone and that PLC-independent PKC signaling, attributable to the PTH(29-34) sequence, appears to accelerate the trabecular response and augment BMD at some skeletal sites. PTH1R PLC signaling pathway is not required for an anabolic effect of intermittent PTH(1-34) on bone.

Regulation of PTH1 receptor expression by uremic ultrafiltrate in UMR 106-01 osteoblast-like cells

BACKGROUND

Homologous down-regulation/desensitization of the parathyroid hormone receptor (PTH1R)/adenylate cyclase system has been demonstrated in uremia, and may contribute to parathyroid hormone (PTH) resistance; however, additional studies have shown that parathyroidectomy fails to normalize the down-regulation of the PTH1R. The present studies were designed to test directly, in vitro, the hypothesis that factors circulating in the uremic environment, other than PTH, decrease the response of osteoblastic cells to PTH.

METHODS

Studies were conducted in confluent cultures of UMR 106-01 osteoblast-like cells. Uremic ultrafiltrate (UUF) was obtained from patients on hemodialysis. Cells were exposed to media containing 50% uremic ultrafiltrate for periods of up to 72 hours. Control cultures were exposed to a buffered salt solution containing a comparable ionic composition to that of the UUF. PTH-stimulated cyclic adenosine monophosphate (cAMP) generation was determined by radioimmunoassay (RIA), PTH binding and PTH1R mRNA levels were determined by radioligand binding and Northern analysis, respectively.

RESULTS

PTH-stimulated cAMP generation from cultures treated with uremic ultrafiltrate for 48 hours was 1385.8 +/- 183.2 pmol/culture/5 minutes, whereas control cultures generated 2389.5 +/- 271 pmol cAMP/culture/5 minutes (P < 0.05). PTH binding was decreased by 30% in cultures incubated with UUF as compared to controls. The decrease in binding induced by UUF was accompanied by a decrease in PTH1R mRNA levels.

CONCLUSION

These findings demonstrate that factors present in UUF decrease PTH-stimulated cAMP generation by a mechanism that involves a decrease in the levels of PTH1R mRNA levels. Thus, the skeletal resistance to PTH in the setting of chronic kidney disease, may be explained, at least in part, by circulating factors other than PTH.

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.

Parathyroid hormone responses of cyclic AMP-, serum- and phorbol ester-responsive reporter genes in osteoblast-like UMR-106 cells

Parathyroid hormone (PTH) and PTH-related protein interact with a G protein-coupled receptor linked to the activation of adenylyl cyclase and phospholipase C signaling pathways. Regulation by PTH of the expression of three distinct, stably transfected luciferase reporter genes responsive to cAMP (CRE-luc), serum (SRE-luc) and phorbol ester (TRE-luc) has been studied in rat osteoblast-like UMR-106 cells. Maximal 43-fold induction of CRE-luc expression occurred in response to 100 nM rat (r)PTH(1-34) (EC50=0.44 nM), but SRE-luc and TRE-luc remained unaffected. Maximal 2.8- and 3.4-fold inductions of SRE-luc by 10 ng/ml EGF and 100 nM phorbol ester (PMA) were suppressed with 100 nM rPTH(1-34) (IC50=0.04 and 0.15 nM, respectively). Similarly, 7.3-fold induction of TRE-luc by 100 nM PMA was inhibited to 50% with 100 nM rPTH(1-34) (IC50=0.5 nM). Activation of mitogen-activated protein kinase by EGF and PMA was also suppressed by rPTH(1-34). 1 mM 8-Br-cAMP and 0.1 mM forskolin mimicked all the effects of rPTH(1-34). In conclusion, the regulation of target genes by PTH in osteoblast-like UMR-106 cells is mediated by the activation of the cAMP/protein kinase A signaling pathway.

Mechanisms of homologous and heterologous desensitization of PTH/PTHrP receptor signaling in LLC-PK1 cells

Parathyroid hormone (PTH) activates multiple intracellular effectors, including adenylyl cyclase (AC) and phospholipase C (PLC), via a single receptor [PTH/parathyroid hormone-related protein receptor (PTHR)] expressed in bone and kidney. Homologous desensitization of PTHR signaling occurs, but the relative importance of reduced receptor expression vs. impaired receptor-effector coupling in this process remains unclear. It also is not known if AC and PLC responses to PTH are desensitized independently or interdependently. In LLC-PK1 cells that expressed transfected wild-type PTHRs, PTH caused dose- and time-dependent desensitization of both the AC and PLC-responses to PTH without altering PTHR expression. Desensitization of AC was blocked in mutant cells resistant to adenosine 3',5'-cyclic monophosphate but not when cells expressed mutant PTHRs with defective PLC coupling. Desensitization of PLC was unaffected by PKA blockade, partially mimicked by phorbol ester, and not reproduced by agents that selectively activated AC. The finding that homologous PTHR desensitization in LLC-PK1 cells is signal specific suggests that prior exposure of other cells to PTH also may induce discordant regulation of subsequent PTHR signaling, altering the character as well as the intensity of the hormonal response.

Parathyroid hormone exerts disparate effects on osteoblast differentiation depending on exposure time in rat osteoblastic cells

It has been reported that PTH exerts bone-forming effects in vivo when administered intermittently. In the present study, the anabolic effects of PTH(1-34) on osteoblast differentiation were examined in vitro. Osteoblastic cells isolated from newborn rat calvaria were cyclically treated with PTH(1-34) for the first few hours of each 48-h incubation cycle. When osteoblastic cells were intermittently exposed to PTH only for the first hour of each 48-h incubation cycle and cultured for the remainder of the cycle without the hormone, osteoblast differentiation was inhibited by suppressing alkaline phosphatase activity, bone nodule formation, and mRNA expression of alkaline phosphatase, osteocalcin, and PTH/PTHrP receptor. Experiments using inhibitors and stimulators of cAMP/protein kinase A (PKA) and Ca2+/PKC demonstrated that cAMP/PKA was the major signal transduction system in the inhibitory action of PTH. In contrast, the intermittent exposure to PTH for the first 6 h of each 48-h cycle stimulated osteoblast differentiation. Both cAMP/ PKA and Ca2+/PKC systems appeared to be involved cooperatively in this anabolic effect. Continuous exposure to PTH during the 48-h incubation cycle strongly inhibited osteoblast differentiation. Although both cAMP/PKA and Ca2+/PKC were involved in the effect of continuous exposure to PTH, they appeared to act independently. A neutralizing antibody against IGF-I blocked the stimulatory effect on alkaline phosphatase activity and the expression of osteocalcin mRNA induced by the 6-h intermittent exposure. The inhibitory effect induced by the 1-h intermittent exposure was not affected by anti-IGF-I antibody. These results suggest that PTH has diverse effects on osteoblast differentiation depending on the exposure time in vitro mediated through different signal transduction systems. These in vitro findings explain at least in part the in vivo action of PTH that varies with the mode of administration.

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.

Homologous desensitization of ATP-stimulated mitogenesis: mechanism involves desensitization of arachidonic acid release and cAMP elevation but not the activation of protein kinase A

Prolonged incubation of quiescent 3T3, 3T6, and A431 cells with the P2Y purinoceptor agonists ATP, ADP, or AMPPNP reduced the mitogenic responses of target cells to a further challenge by these agonists, as measured by [3H]thymidine incorporation. The mitogenic desensitization was agonist-specific, for no effect was seen on DNA synthesis stimulated by epidermal growth factor, insulin, bombesin, 12-O-tetradecanoyl-phorbol-12 acetate (TPA), or adenosine. The desensitization was completely reversible, since after a 24 hr incubation in the absence of ATP, the cells responded fully to the mitogenic action of ATP. The presence of a low level of cycloheximide blocked recovery, suggesting that down-regulation of the P2Y receptor may have occurred during desensitization. In Swiss 3T3 cells, stimulation of DNA synthesis occurs predominantly by activation of arachidonic acid release, followed by its oxidation to prostaglandin E2 and stimulation of adenylyl cyclase. Interestingly, prolonged preincubation with ATP produced a similar degree of desensitization of DNA synthesis and of ATP-dependent arachidonic acid release and cAMP accumulation. Furthermore, this was true for both wild type cells and mutants with a defective cAMP-dependent protein kinase (PKA). We conclude that homologous desensitization is likely due to uncoupling of the P2Y purinoceptor from phospholipase A2, and this process does not require activation of protein kinase A.