PTHR1 in osteosarcoma: Specific molecular mechanisms and comprehensive functional perspective

Abstract Osteosarcoma occurs largely in children and adolescents and is the most common primary malignant tumour of bone. Although surgical advances and neoadjuvant chemotherapy have made great strides in recent years, rates of local recurrence and lung metastasis remain high, with a plateau in overall survival during the past decade. It is thus urgent to explore the pathogenesis of osteosarcoma and identify potential therapeutic targets. Parathyroid hormone receptor 1 (PTHR1) belongs to the broad family of G protein–coupled receptors, binding both parathyroid hormone (PTH) and parathyroid hormone–related peptide (PTHrP, a paracrine factor). Previous studies have shown that in tissues and cells of osteosarcoma, expression of PTHR1 is markedly increased, correlating with aggressive biologic behaviour and a poor prognosis. PTHR1 expression also correlates closely with epigenetic regulation, transcriptional regulation, post‐translational modification and protein interaction. Herein, we have summarized the latest research on the role played by PTHR1 in progression of osteosarcoma, assessing its clinical utility as a novel biomarker and its therapeutic ramifications.

precise pathways contributing to occurrences and metastasis of osteosarcoma remain elusive and must be clarified in our search for better preventive and therapeutic measures.
Parathyroid hormone (PTH) is a polypeptide hormone secreted by parathyroid master cells. Through its action on osteoblasts and osteoclasts, via cyclic adenosine monophosphate (cAMP) and phospholipase C pathways, it serves to modulate blood calcium levels.
Parathyroid hormone receptor 1 (PTHR1) belongs to the G proteincoupled cell membrane receptor family, widely distributed and of greatest import in canine and rat osteosarcoma. PTHR1 binds both PTH and PTH-related peptide (PTHrP) and is primarily expressed in bone, kidney and cartilage. The vascular system, certain developmental organs and human MCF7 breast cancer cells also show high levels of expression. 14,15 It is encoded by 14 exon genes on chromosome 3 and plays a critical role in regulating serum and endochondral bone concentrations of calcium. 16 Along with other specifics (ie target cell type, molecular structure of binding ligand and homeostatic bodily conditions), PTHR1 has been implicated in a number of intracellular signalling pathways, the nature, degree and duration of which are decisive in the biologic responses induced. 17 Biochemical and cellular responses to PTHR1 activation may thus differ according to cell type. In primary failure of eruption (PFE), clinical and radiographic characteristics are highly specific for PTHR1 effects 18 ; and compared with responses in wildtype (WT) mice, physiological responses to injected PTH ligands are acutely and severely disrupted in mice bearing the phosphorylationdeficient (PD) PTHR1 knock-in mutation. 19 In both WT and PD animals, PTH administration increases the volume and trabecular thickness of vertebral and distal femoral bones, but PTHR1 phosphorylation is not a major factor in anabolic actions of PTH. 20  In this review, we have summarized present knowledge surrounding regulatory and functional aspects of PTHR1 in osteosarcoma. We have also probed the clinical implications of altering PTHR1 expression and the latest therapeutic strategies for targeting PTHR1 in this setting (Table 2, Figure 2).

| B I OLOGY OF P THR1
PTHR1 is a class B G protein-coupled receptor that binds PTH and PTHrP. Although present in other tissues, it is primarily expressed in bone, kidney and cartilage. 23,24 Despite their shared signalling mechanisms, the biologic functions of PTH and PTHrP are actually quite different. 25,26 PTH modulates serum calcium through endocrine effects on bone and kidney cells, whereas PTHrP is a paracrine modulator of cell proliferation and differentiation at developmental sites, such as bony growth plates. 22 The biologic responses induced by PTHR1 activation generally reflect the nature, intensity and duration of signalling, in conjunction with other variables (ie target cell type, ligand structure and primary homeostatic conditions). Activation of PTHR1 thus triggers unique biochemical and cellular responses, depending on cell type.
In osteoblasts and chondrocytes, PTHR1 activation modulates proliferative and apoptotic efficiency and contributes to production of various signalling factors involved in bone and cartilage metabolism. 22 In renal tubules, PTHR1 activation modulates transmembrane transport of mineral ions through expression levels and functional activities of related proteins. The global response to PTHR1 activation TA B L E 1 Potential therapeutic approaches of osteosarcoma

| E X TENT AND CLINI C AL RELE VAN CE OF P THR1 E XPRE SS I ON IN OS TEOSARCOMA
In patients with osteosarcoma, PTHR1 overexpression has been linked to greater risk of metastasis and a poor prognosis. Using quantitative  foci. The precise regulatory mechanisms and their potential for new therapeutic targets must be further studied and fully explored.

| Matrix metalloproteinases and extracellular matrix regulation by PTHR1
The Broader study is essential in this area to assess their potential as clinical biomarkers and therapeutic targets.
The microenvironment, including bone, stroma, vascular elements and immune cells, is also critical in regulating the growth and metastasis of osteosarcoma. 44

| FUN C TI ON OF P THR1 IN OS TEOSARCOMA
As a crucial oncogenic gene, oncogenic properties of PTHR1 pertaining to osteosarcoma have been adequately chronicled. In this section, we describe the functions of PTHR1 as a pro-oncogenic gene in this setting, serving to regulate PTHR1 expression.

| Promoting cellular proliferation and growth
In the absence of added exogenous PTHrP, overexpression of PHTR1 intensifies cellular proliferation, motility and Matrigel invasion, as presumptive autocrine effects. PTHR1 overexpression is also associated with deferred osteoblastic differentiation and upregulation of genes involved in ECM production, inclusive of TGF-b1 and connective tissue growth factor. 30  Mechanistic studies further indicate that LINC01278 is a competing endogenous RNA of PTHR1 (by sponging miR-133a-3p) and a likely point of tumour inhibition. Down-regulation of PTHR1 serves to restore the inhibitory impact of miR-133a-3p. This carcinogenic effect of LINC01278, as a consequence of miR-133a-3p/PTHR1 signalling, represents a viable opportunity for therapeutic targeting. 29

| CON CLUS I ON AND FUTURE PER S PEC TIVE S
Patients with advanced osteosarcoma are currently confined to systemic chemotherapy as treatment, although the limitations of present-day regimens have become increasingly evident. 51,52 Finding new treatments is therefore a matter of urgency. The mTOR pathway inhibitor, rapamycin, mitigates mRNA translation and inhibits metastasis of osteosarcoma cells, 53

ACK N OWLED G EM ENTS
We thank the generous support by Liaoning Cancer Hospital & Institute (Shenyang).

CO N FLI C T O F I NTE R E S T
The authors declare no competing interests.