Parathyroid Hormone Receptor Research Articles

THU346 Repairing Skeletal Deterioration In Diabetes With Bone Anabolic Therapies

Abstract Disclosure: S. Marino: None. N. Akel: None. S. Li: None. M. Cregor: None. B. Perez: None. G. Troncoso: None. J. Meeks: None. S. Stuart: None. A. Sato: None. I. Nookaew: None. T. Bellido: None. Treatments for skeletal fragility in diabetes are sorely needed as antiresorptive agents, the current standard of care, stop bone loss but do not rebuild bone nor restore the deteriorated bone microarchitecture. Herein, we demonstrate bone restoration in a preclinical model of established diabetes by two bone anabolic pathways: activation of the PTH1 receptor, with equimolar doses of either PTH (1-34) or Abaloparatide/ABL (100/95 µg/kg/day), and neutralization of the osteocyte-derived inhibitor of bone formation sclerostin with romosozumab/Scl-Ab (100mg/kg/w). Type 2 diabetes (T2D) was induced in skeletally mature male C57BL/6J mice by high fat diet combined with streptozotocin; and, after established diabetes, mice were treated for 4 weeks with the bone anabolic agents. At the end of the study, T2D mice exhibited decreased total (−5%), femoral (−7%) and spinal (−11%) BMD vs nondiabetic mice. PTH/ABL restored the bone lost and increased BMD to a similar extent: total 8%, femoral 11% and spinal 9% vs vehicle-treated T2D mice. Scl-Ab also increased BMD at all sites, but the effects were superior: total 24%, femoral 26%, and spinal 40%. T2D mice exhibited low bone formation (BF) and high bone resorption (BR) circulating markers throughout the disease progression. However, at the tissue level BF and BR were suppressed as evidenced by histomorphometric indexes (BFR −70%; Oc.S/BS -34%) and bone transcriptome GO profiles. PTH/ABL increased BF (BFR ∼700% and P1NP 130/244%) as well as BR (Oc.S/BS unchanged/34%; TRAP5b ∼200%) and corrected the diabetic low bone remodeling signature towards high bone remodeling with bone gain. In contrast, Scl-Ab increased formation (BFR 500% and P1NP 50%) but drastically decreased resorption (TRAP5b −190% and Oc.S/BS −500%). T2D mice exhibited damaged bone microarchitecture. The reduced femoral and spinal cancellous bone BV/TV (−22 and −10%) was corrected by ABL (40 and 42%), Scl-Ab (130 and 83%), and PTH only in the spine (43%). Further, femoral and spinal trabecular thickness were increased equally by PTH/ABL (35 and 24%) and also by Scl-Ab (60 and 66%). Moreover, the decreased cortical thickness (−7%) and BA/TA (−5%) at the femoral mid-diaphysis of T2D mice were corrected and further increased by PTH/ABL (10 and 11%) and Scl-Ab (23 and 19%). Yet, whereas PTH reduced marrow area, ABL increased tissue area TA, and Scl-Ab exerted both effects, resulting in advantageous bone distribution by ABL and Scl-Ab leading to increased energy to ultimate load (60 and 90%) and toughness (50 and 82%). Nevertheless, all agents corrected the low stiffness of T2D bone (-19%) and also increased ultimate load. These findings demonstrate the therapeutic value of bone anabolic agents to treat diabetes-induced bone disease by increasing bone formation and improving bone microarchitecture and show the benefit of simultaneously inhibiting resorption to achieve superior bone gain and strength. Presentation: Thursday, June 15, 2023

Harnessing Aromatic-Histidine Interactions through Synergistic Backbone Extension and Side Chain Modification.

Peptide engineering efforts have delivered drugs for diverse human diseases. Side chain alteration is among the most common approaches to designing new peptides for specific applications. The peptide backbone can be modified as well, but this strategy has received relatively little attention. Here we show that new and favorable contacts between a His side chain on a target protein and an aromatic side chain on a synthetic peptide ligand can be engineered by rational and coordinated side chain modification and backbone extension. Side chain modification alone was unsuccessful. Binding measurements, high-resolution structural studies and pharmacological outcomes all support the synergy between backbone and side chain modification in engineered ligands of the parathyroid hormone receptor-1, which is targeted by osteoporosis drugs. These results should motivate other structure-based designs featuring coordinated side chain modification and backbone extension to enhance the engagement of peptide ligands with target proteins.

Dietary vitamin D3 requirement of magang goslings from 1–21 days of age

The present study was conducted to investigate the effect of dietary vitamin D3 levels (0, 200, 400, 800, 1600, and 3200 IU/kg) on growth performance, calcium-phosphorus metabolism, and tibia development of goslings from 1 to 21 days of age. A total of 720 one-day-old Magang male goslings with similar initial body weight were randomly divided into 6 treatments with 6 replicates per treatment and 20 geese per replicate. The goslings fed with 400 IU vitamin D3/kg of diet obtained the maximum average daily gain, while vitamin D3 supplementation significantly improved the feed/gain of goslings aged from 1 to 21 days (P < 0.05). The vitamin D3 requirement of gosling aged from 1 to 21 days for optimal feed/gain ratio and average daily gain were estimated from three regression models (one-slope straight, two-slope straight, and quadratic broken-line model) to be 246–352 IU/kg diet and 400–518 IU/kg diet, respectively. Serum calcium and phosphorus contents, parathyroid hormone level, and 25-hydroxycholecalciferol level responded to increasing dietary vitamin D3 levels (P < 0.05) and reached a plateau at 800, 0, and 200 IU vitamin D3/kg diet, respectively. The mRNA expressions of target genes related to calcium homeostasis such as vitamin D3 receptor, Na+/Ca2+ exchanger 1, and plasma membrane calcium ATPase 1b in the duodenum and kidney (P < 0.05) and parathyroid hormone receptor mRNA expression in the kidney (P < 0.05) were increased linearly and quadratically with the increasing dietary vitamin D3 levels, respectively. Graded doses of dietary vitamin D3 from 0 to 3200 IU/kg produced linear responses in tibial density (P < 0.05). The vitamin D3 requirement of gosling aged from 1 to 21 days ranged from 519 to 698 IU/kg diet when tibial density as an evaluation criterion. In conclusion, the vitamin D3 requirement of gosling aged from 1 to 21 days for optimal growth performance (feed/gain and average daily gain) and tibial density was estimated to be 246–352 IU/kg of diet, 400–518 IU/kg of diet, and 519–698 IU/kg of diet based on three regression models (one-slope straight, two-slope straight, and quadratic broken-line model), respectively. The most appropriate vitamin D3 requirement was recommended at 602 IU/kg of diet of young geese based on the mean estimated values of three regression models for the tibial density with the highest coefficient of determination and lowest mean square error.

Role of Parathyroid Hormone and Parathyroid Hormone-Related Protein in Protein-Energy Malnutrition.

Parathyroid hormone (PTH) is an endocrine peptide found exclusively in the parathyroid glands, whereas parathyroid hormone-related protein (PTHrP) is expressed in a wide range of tissues and organs and exerts endocrine, paracrine, and autocrine actions. PTH and PTHrP have a similar homology, sharing the initial 13 amino acid residues at the N-terminus and binding to the same type 1 PTH receptor (PTH1R), which regulates calcium homeostasis. An abnormal increase in PTH production can occur in primary and secondary hyperparathyroidism, whereas PTHrP can be produced in large quantities by malignant cancer cells from solid organs. In addition to increased bone resorption and hypercalcemia, recent evidence suggests that excess PTH and PTHrP can result in protein-energy wasting, malnutrition, and cachexia. Through binding to PTH1R and activation of cyclic adenosine monophosphate (cAMP)-dependent protein kinase A in white adipose tissue, PTH and PTHrP can stimulate the expression of thermogenic genes causing adipose tissue browning. This change results in an increase in resting energy expenditure, loss of muscle and fat mass, and weight loss. These findings provide a mechanistic link for the long-established relationship between hyperparathyroidism and myopathy, as well as cancer and cachexia. The purpose of this review is to provide a summary of the emerging evidence from both experimental and clinical studies on the role of PTH and PTHrP in protein-energy malnutrition.

Conserved class B GPCR activation by a biased intracellular agonist.

Class B G-protein-coupled receptors (GPCRs), including glucagon-like peptide 1 receptor (GLP1R) and parathyroid hormone 1 receptor (PTH1R), are important drug targets1-5. Injectable peptide drugs targeting these receptors have been developed, but orally available small-molecule drugs remain under development6,7. Here we report the high-resolution structure of human PTH1R in complex with the stimulatory G protein (Gs) and a small-molecule agonist, PCO371, which reveals an unexpected binding mode of PCO371 at the cytoplasmic interface of PTH1R with Gs. The PCO371-binding site is totally different from all binding sites previously reported for small molecules or peptide ligands in GPCRs. The residues that make up the PCO371-binding pocket are conserved in class B GPCRs, and a single alteration in PTH2R and two residue alterations in GLP1R convert these receptors to respond to PCO371. Functional assays reveal that PCO371 is a G-protein-biased agonist that is defective in promoting PTH1R-mediated arrestin signalling. Together, these results uncover a distinct binding site for designing small-molecule agonists for PTH1R and possibly other members of the class B GPCRs and define a receptor conformation that is specific only for G-protein activation but not arrestin signalling. These insights should facilitate the design of distinct types of class B GPCR small-molecule agonist for various therapeutic indications.

Generation of Hypoparathyroid Rats via Carbon-Nanoparticle-Assisted Parathyroidectomy.

Hypoparathyroidism (HypoPT) is a rare disease involving the parathyroid glands that is characterized by a reduced secretion or potency of the parathyroid hormone (PTH), which leads to high serum phosphorus levels and low serum calcium levels. HypoPT most commonly results from accidental damage to the glands or their removal during thyroid or other anterior neck surgery. Parathyroid/thyroid surgery has become more common in recent years, with a corresponding rise in the occurrence of HypoPT as a postoperative complication. There is a critical need for a HypoPT animal model to better understand the mechanisms underlying the effects of HypoPT on mineral ion homeostasis and to verify the therapeutic effectiveness of novel treatments. Here, a technique is reported to create acquired HypoPT in male rats by performing parathyroidectomy (PTX) using carbon nanoparticles. The rat model shows great promise over the mouse models of hypoparathyroidism.Importantly, the human PTH receptor binding region has an 84.2% sequence similarity with that of the rat, which is higher than the 73.7% similarity shared with mice. Moreover, the effects of estrogen, which can affect the PTH/PTHrP receptor signaling pathway, have not been fully investigated in male rats. Carbon nanoparticles are lymphatic tracers that stain the thyroid lymph nodes black without affecting their function, but they do not stain the parathyroid glands, which makes them easy to identify and remove. In this study, serum PTH levels were undetectable after PTX, and this resulted in significant hypocalcemia and hyperphosphatemia. Thus, the clinical state of postoperative HypoPT can be remarkably represented in the rat model. Carbon-nanoparticle-assisted PTX can, therefore, serve as an extraordinarily effective and readily implementable model for studying the pathogenesis, treatment, and prognosis of HypoPT.

Altered Signaling and Desensitization Responses in PTH1R Mutants Associated with Eiken Syndrome

The parathyroid hormone receptor type 1 (PTH1R) is a G protein-coupled receptor that plays key roles in regulating calcium homeostasis and skeletal development via binding the ligands, PTH and PTH-related protein (PTHrP), respectively. Eiken syndrome is a rare disease of delayed bone mineralization caused by homozygous PTH1R mutations. Of the three mutations identified so far, R485X, truncates the PTH1R C-terminal tail, while E35K and Y134S alter residues in the receptor’s amino-terminal extracellular domain. Here, using a variety of cell-based assays, we show that R485X increases the receptor’s basal rate of cAMP signaling and decreases its capacity to recruit β-arrestin2 upon ligand stimulation. The E35K and Y134S mutations each weaken the binding of PTHrP leading to impaired β-arrestin2 recruitment and desensitization of cAMP signaling response to PTHrP but not PTH. Our findings support a critical role for interaction with β-arrestin in the mechanism by which the PTH1R regulates bone formation.

Scribble scrambles parathyroid hormone receptor interactions to regulate phosphate and vitamin D homeostasis

G protein-coupled receptors, including PTHR, are pivotal for controlling metabolic processes ranging from serum phosphate and vitamin D levels to glucose uptake, and cytoplasmic interactors may modulate their signaling, trafficking, and function. We now show that direct interaction with Scribble, a cell polarity-regulating adaptor protein, modulates PTHR activity. Scribble is a crucial regulator for establishing and developing tissue architecture, and its dysregulation is involved in various disease conditions, including tumor expansion and viral infections. Scribble co-localizes with PTHR at basal and lateral surfaces in polarized cells. Using X-ray crystallography, we show that colocalization is mediated by engaging a short sequence motif at the PTHR C-terminus using Scribble PDZ1 and PDZ3 domain, with binding affinities of 31.7 and 13.4 μM, respectively. Since PTHR controls metabolic functions by actions on renal proximal tubules, we engineered mice to selectively knockout Scribble in proximal tubules. The loss of Scribble impacted serum phosphate and vitamin D levels and caused significant plasma phosphate elevation and increased aggregate vitamin D3 levels, whereas blood glucose levels remained unchanged. Collectively these results identify Scribble as a vital regulator of PTHR-mediated signaling and function. Our findings reveal an unexpected link between renal metabolism and cell polarity signaling.

Re-Evaluating the Role of PTHrP in Breast Cancer.

Parathyroid-hormone-related protein (PTHrP) is a protein with a long history of association with bone metastatic cancers. The paracrine signaling of PTHrP through the parathyroid hormone receptor (PTHR1) facilitates tumor-induced bone destruction, and PTHrP is known as the primary driver of humoral hypercalcemia of malignancy. In addition to paracrine signaling, PTHrP is capable of intracrine signaling independent of PTHR1 binding, which is essential for cytokine-like functions in normal physiological conditions in a variety of tissue types. Pre-clinical and clinical studies evaluating the role of PTHrP in breast cancer have yielded contradictory conclusions, in some cases indicating the protein is tumor suppressive, and in other studies, pro-growth. This review discusses the possible molecular basis for the disharmonious prognostic indications of these studies and highlights the implications of the paracrine, intracrine, and nuclear functions of the protein. This review also examines the current understanding of the functional domains of PTHrP and re-evaluates their role in the unique context of the breast cancer environment. This review will expand on the current understanding of PTHrP by attempting to reconcile the functional domains of the protein with its intracrine signaling in cancer.

Molecular insights into peptide agonist engagement with the PTH receptor

The parathyroid hormone (PTH) 1 receptor (PTH1R) is a G protein-coupled receptor (GPCR) that regulates skeletal development and calcium homeostasis. Here, we describe cryo-EM structures of the PTH1R in complex with fragments of the two hormones, PTH and PTH-related protein, the drug abaloparatide, as well as the engineered tool compounds, long-acting PTH (LA-PTH) and the truncated peptide, M-PTH(1-14). We found that the critical N terminus of each agonist engages the transmembrane bundle in a topologically similar fashion, reflecting similarities in measures of Gαs activation. The full-length peptides induce subtly different extracellular domain (ECD) orientations relative to the transmembrane domain. In the structure bound to M-PTH, the ECD is unresolved, demonstrating that the ECD is highly dynamic when unconstrained by a peptide. High resolutions enabled identification of water molecules near peptide and G protein binding sites. Our results illuminate the action of orthosteric agonists of the PTH1R.

Fracture risk reduction and safety by osteoporosis treatment compared with placebo or active comparator in postmenopausal women: systematic review, network meta-analysis, and meta-regression analysis of randomised clinical trials

ObjectiveTo review the comparative effectiveness of osteoporosis treatments, including the bone anabolic agents, abaloparatide and romosozumab, on reducing the risk of fractures in postmenopausal women, and to characterise the effect...

Treatment of chronic hypoparathyroidism by Eneboparatide, a novel PTH receptor-1 agonist: Results from a phase 2a study

Searchable abstracts of presentations at key conferences in endocrinology ISSN 1470-3947 (print) | ISSN 1479-6848 (online)

Eneboparatide, a Novel PTH-1 Receptor Agonist, Has No Impact on Bone Parameters Following Chronic Treatment of Non-Human Primates

Searchable abstracts of presentations at key conferences in endocrinology ISSN 1470-3947 (print) | ISSN 1479-6848 (online)

Renal proximal tubule cells: power and finesse

The proximal tubule is the high-capacity reabsorptive powerhouse of the kidney. Two papers in recent issues of the JCI highlight mechanisms of more delicate effects of the proximal tubule. Yoon et al. demonstrated the intracellular mechanism by which parathyroid hormone (PTH) increases production of 1,25-vitamin D. Activation of PTH receptor 1/cAMP/PKA signaling inhibited salt-inducible kinase 2 (SIK2) and SIK3, which increased CYB27B1 transcription and 1,25-vitamin D production. Replication of these effects with small-molecule SIK inhibitors suggests possible therapeutic applications for patients with disorders characterized by 1,25-vitamin D deficiency. Zhou et al. discovered that proximal tubule glycolysis acts as a phosphate sensor that regulates fibroblast growth factor 23 production in bone. They described several kidney-specific metabolic modifications that enabled glycolysis to be deployed as a phosphate sensor. The provocative results raise intriguing questions with implications for patients with disorders of phosphate homeostasis, including chronic kidney disease.

Parathyroid hormone receptors in GtoPdb v.2023.1

The parathyroid hormone receptors (nomenclature as agreed by the NC-IUPHAR Subcommittee on Parathyroid Hormone Receptors [50]) are class B G protein-coupled receptors. The parathyroid hormone (PTH)/parathyroid hormone-related peptide (PTHrP) receptor (PTH1 receptor) is activated by precursor-derived peptides: PTH (84 amino acids), and PTHrP (141 amino-acids) and related peptides (PTH-(1-34), PTHrP-(1-36)). The parathyroid hormone 2 receptor (PTH2 receptor) is activated by the precursor-derived peptide TIP39 (39 amino acids). [125I]PTH may be used to label both PTH1 and PTH2 receptors. The structure of a long-active PTH analogue (LA-PTH, an hybrid of PTH-(1-13) and PTHrP-(14-36)) bound to the PTH1 receptor-Gs complex has been resolved by cryo-electron microscopy [148]. Another structure of a PTH-(1-34) analog bound to a thermostabilized inactive PTH1 receptor has been obtained with X-ray crytallography [35].

Bidirectional control of parathyroid hormone and bone mass by subfornical organ

Parathyroid hormone (PTH) is one of the most important hormones for bone turnover and calcium homeostasis. It is unclear how the central nervous system regulates PTH. The subfornical organ (SFO) lies above the third ventricle and modulates body fluid homeostasis. Through retrograde tracing, electrophysiology, and invivo calcium imaging, we identified the SFO as an important brain nucleus that responds to serum PTH changes in mice. Chemogenetic stimulation of GABAergic neurons in SFO induces decreased serum PTH followed by a decrease in trabecular bone mass. Conversely, stimulation of glutamatergic neurons in the SFO promoted serum PTH and bone mass. Moreover, we found that the blockage of different PTH receptors in the SFO affects peripheral PTH levels and the PTH's response to calcium stimulation. Furthermore, we identified a GABAergic projection from the SFO to the paraventricular nucleus, which modulates PTH and bone mass. These findings advance our understanding of the central neural regulation of PTH at cellular and circuit level.

β-Arrestin 2 knockout prevents bone loss in response to continuous parathyroid hormone stimulation in male and female mice

ABSTRACT Background β-Arrestin 2 (β-arr2) binds activated parathyroid hormone (PTH) receptors stimulating internalization. PTH stimulates both anabolic and catabolic effect on bone depending on the way it is administered. Intermittent PTH stimulation increases trabecular bone formation in mice, but this is decreased in mice lacking β-arr 2, suggesting a role for β-arr 2 in the anabolic effects of PTH. The role of β-arr 2 in the catabolic effects of continuous PTH (cPTH) treatment is not known. Objective To assess the effects of cPTH administration on bone in mice lacking β-arr 2 compared to wild-type (WT). Methods Groups of male and female WT or β-arr2 knockout (KO) mice were administered either PTH or phosphate-buffered saline by osmotic pumps for 2 weeks. Following treatment, serum calcium and phosphate levels were measured, bone structure and mineral density were measured by microcomputed tomography, and bone cells measured by static and dynamic histomorphometry. Results β-arr2 KO had no effects on skeletal development in mice of either sex. PTH treatment caused hypercalcemia and hypophosphatemia and decreased trabecular and cortical bone only in male WT mice. β-arr2 KO in male mice completely abrogated the effects of PTH on bone, while in female β-arr2 KO mice, PTH treatment increased trabecular bone with no effects on cortical bone. Conclusions These results demonstrate a profound sex effect on skeletal responses to cPTH treatment, suggesting a protective effect of estrogen on bone loss. β-arr2 plays a role in restraining the anabolic effects of PTH in both male and female mice.

Mitochondrial β-oxidation of adipose-derived fatty acids by osteoblasts fuels parathyroid hormone-induced bone formation.

The energetic costs of bone formation require osteoblasts to coordinate their activities with tissues, like adipose, that can supply energy-dense macronutrients. In the case of intermittent parathyroid hormone (PTH) treatment, a strategy used to reduce fracture risk, bone formation is preceded by a change in systemic lipid homeostasis. To investigate the requirement for fatty acid oxidation by osteoblasts during PTH-induced bone formation, we subjected mice with osteoblast-specific deficiency of mitochondrial long-chain β-oxidation as well as mice with adipocyte-specific deficiency for the PTH receptor or adipose triglyceride lipase to an anabolic treatment regimen. PTH increased the release of fatty acids from adipocytes and β-oxidation by osteoblasts, while the genetic mouse models were resistant to the hormone's anabolic effect. Collectively, these data suggest that PTH's anabolic actions require coordinated signaling between bone and adipose, wherein a lipolytic response liberates fatty acids that are oxidized by osteoblasts to fuel bone formation.

Parathyroid hormone receptor-1 signaling aggravates hepatic fibrosis through upregulating cAMP response element-binding protein-like 2.

Parathyroid hormone receptor-1 (PTH1R) is a class B G protein-coupled receptor central to skeletal development, bone turnover, and calcium homeostasis. However, the role of PTH1R signaling in liver fibrosis is largely unknown. Here, the role of PTH1R signaling in the activation of HSCs and hepatic fibrosis was examined. PTH1R was highly expressed in activated HSCs and fibrotic liver by using human liver specimens or carbon tetrachloride (CCl 4 )-treated or methionine and choline-deficient diet (MCD)-fed C57/BL6 mice. The mRNA level of hepatic PTH1R was positively correlated to α-smooth muscle actin in patients with liver cirrhosis. Mice with HSCs-specific PTH1R deletion were protected from CCl 4 , MCD, or western diet, plus low-dose CCl 4 -induced liver fibrosis. Conversely, parathyroid hormone (PTH) aggravated liver fibrosis in CCl 4 -treated mice. Mouse primary HSCs and LX2 cell lines were used for in vitro experiments. Molecular analyses by luciferase reporter assays and chromatin immunoprecipitation assays in combination with mRNA sequencing in HSCs revealed that cAMP response element-binding protein-like 2 (Crebl2), a novel regulator in HSCs treated by PTH that interacted with mothers against decapentaplegic homolog 3 (SMAD3) and increased the transcription of TGFβ in activating HSCs and collagen deposition. In agreement, HSCs-specific Crebl2 deletion ameliorated PTH-induced liver fibrosis in CCl 4 -treated mice. In both mouse and human models, we found that PTH1R was highly expressed in activated HSCs and fibrotic liver. PTH1R signaling regulated collagen production in the HSCs through Crebl2/SMAD3/TGFβ regulatory circuits. Blockade of PTH1R signaling in HSCs might help mitigate the development of liver fibrosis.

Parathyroid hormone 1 receptor signaling mediates breast cancer metastasis to bone in mice.

Bone metastases are a common complication of breast cancer. We have demonstrated that intermittent administration of parathyroid hormone (PTH[1-34]) reduces the incidence of bone metastases in murine models of breast cancer by acting on osteoblasts to alter the bone microenvironment. Here, we examined the role of signaling mediated by PTH 1 receptor (PTH1R) in both osteoblasts and breast cancer cells in influencing bone metastases. In mice with impaired PTH1R signaling in osteoblasts, intermittent PTH did not reduce bone metastasis. Intermittent PTH also did not reduce bone metastasis when expression of PTH1R was knocked down in 4T1 murine breast cancer cells by shRNA. In 4T1 breast cancer cells, PTH decreased expression of PTH-related protein (PTHrP), implicated in the vicious cycle of bone metastases. Knockdown of PTHrP in 4T1 cells significantly reduced migration toward MC3T3-E1 osteoblasts, and migration was further inhibited by treatment with intermittent PTH. Conversely, overexpression of PTHrP in 4T1 cells increased migration toward MC3T3-E1 osteoblasts, and this was not inhibited by PTH. In conclusion, PTH1R expression is crucial in both osteoblasts and breast cancer cells for PTH to reduce bone metastases, and in breast cancer cells, this may be mediated in part by suppression of PTHrP.