Parathyroid hormone stimulates hyaluronan synthesis in an osteoblast-like cell line

High-molecular-weight Hyaluronan Administration Inhibits Bone Resorption and Promotes Bone Formation in Young-age Osteoporosis Rats

Osteoporosis poses a significant global health concern, affecting both the elderly and young individuals, including athletes. Despite the development of numerous antiosteoporotic drugs, addressing the unique needs of young osteoporosis patients remains challenging. This study focuses on young rats subjected to ovariectomy (OVX) to explore the impact of high-molecular-weight hyaluronan (HA) on preventing OVX-induced osteoporosis. Twenty-four rats underwent OVX, while 12 underwent sham procedures (sham control group). Among the OVX rats, half received subcutaneous injections of HA (MW: 2700 kDa) at 10 mg/kg/week into their backs (OVX-HA group), whereas the other half received saline injections (0.5 ml/week) at the same site (OVX-saline group). OVX-HA group exhibited significantly higher percentages of osteoclast surface (Oc. S/BS), osteoblast surface per bone surface (Ob. S/BS), and bone volume/tissue volume (BV/TV) compared with OVX-saline group at the same age. The proportions of Ob. S/BS and BV/TV in the OVX-HA group closely resembled those of the sham control group, whereas the proportion of Oc. S/BS in the OVX-HA group was notably higher than that in the sham control group. In summary, the administration of HA significantly mitigated bone resorption and enhanced bone formation, suggesting a crucial role for HA in the treatment of young adult osteoporosis.

Roles of Parathyroid Hormone-Related Protein (PTHrP) and Its Receptor (PTHR1) in Normal and Tumor Tissues: Focus on Their Roles in Osteosarcoma

Osteosarcoma (OS) is the most common primary bone tumor and originates from bone forming mesenchymal cells and primarily affects children and adolescents. The 5-year survival rate for OS is 60 to 65%, with little improvement in prognosis during the last four decades. Studies have demonstrated the evolving roles of parathyroid hormone-related protein (PTHrP) and its receptor (PTHR1) in bone formation, bone remodeling, regulation of calcium transport from blood to milk, regulation of maternal calcium transport to the fetus and reabsorption of calcium in kidneys. These two molecules also play critical roles in the development, progression and metastasis of several tumors such as breast cancer, lung carcinoma, chondrosarcoma, squamous cell carcinoma, melanoma and OS. The protein expression of both PTHrP and PTHR1 have been demonstrated in OS, and their functions and proposed signaling pathways have been investigated yet their roles in OS have not been fully elucidated. This review aims to discuss the latest research with PTHrP and PTHR1 in OS tumorigenesis and possible mechanistic pathways.

This review is dedicated to Professor Michael Day who died in May 2020 and was a very generous collaborator.

Parathyroid hormone/parathyroid hormone-related peptide regulate osteosarcoma cell functions: Focus on the extracellular matrix (Review)

Osteosarcoma (OS) is a primary bone tumor of mesenchymal origin mostly affecting children and adolescents. The OS extracellular matrix (ECM) is extensively altered as compared to physiological bone tissue. Indeed, the main characteristic of the most common osteoblastic subtype of OS is non-mineralized osteoid production. Parathyroid hormone (PTH) is a polypeptide hormone secreted by the chief cells of the parathyroid glands. The PTH-related peptide (PTHrP) may be comprised of 139, 141 or 173 amino acids and exhibits considerate N-terminal amino acid sequence homology with PTH. The function of PTH/PTHrP is executed through the activation of the PTH receptor 1 (PTHR1) and respective downstream intracellular pathways which regulate skeletal development, bone turnover and mineral ion homeostasis. Both PTHR1 and its PTH/PTHrP ligands have been shown to be expressed in OS and to affect the functions of these tumor cells. This review aims to highlight the less well known aspects of PTH/PTHrP functions in the progression of OS by focusing on ECM-dependent signaling.

Modifications in bone matrix of estrogen-deficient rats treated with intermittent PTH

Bone matrix dictates strength, elasticity, and stiffness to the bone. Intermittent parathyroid hormone (iPTH), a bone-forming treatment, is widely used as a therapy for osteoporosis. We investigate whether low doses of intermittent PTH (1-34) change the profile of organic components in the bone matrix after 30 days of treatment. Forty 6-month-old female Wistar rats underwent ovariectomy and after 3 months received low doses of iPTH administered for 30 days: daily at 0.3 µg/kg/day (PTH03) or 5 µg/kg/day (PTH5); or 3 times per week at 0.25 µg/kg/day (PTH025). After euthanasia, distal femora were processed for bone histomorphometry, histochemistry for collagen and glycosaminoglycans, biochemical quantification of sulfated glycosaminoglycans, and hyaluronan by ELISA and TUNEL staining. Whole tibiae were used to estimate the bone mineral density (BMD). Histomorphometric analysis showed that PTH5 increased cancellous bone volume by 6% over vehicle-treated rats. In addition, PTH5 and PTH03 increased cortical thickness by 21% and 20%, respectively. Tibial BMD increased in PTH5-treated rats and this group exhibited lower levels of chondroitin sulfate; on the other hand, hyaluronan expression was increased. Hormonal administration in the PTH5 group led to decreased collagen maturity. Further, TUNEL-positive osteocytes were decreased in the cortical compartment of PTH5 whereas administration of PTH025 increased the osteocyte death. Our findings suggest that daily injections of PTH at low doses alter the pattern of organic components from the bone matrix, favoring the increase of bone mass.

Parathyroid hormone (PTH) peptides through the regulation of hyaluronan metabolism affect osteosarcoma cell migration

Parathyroid hormone (PTH) strongly stimulates hyaluronan (HA) synthesis and secretion of both normal and carcinogenic cells of the osteoblastic lineage and improves skeletal microarchitecture. HA, a glycosaminoglycan component of the extracellular matrix (ECM), is capable of transmitting ECM-derived signals to regulate cellular function. In this study, we investigated whether the changes of HA metabolism induced by PTH (1-34) and PTH (7-84) peptides in moderately MG-63 and well-differentiated Saos 2 osteosarcoma cell lines, are correlated to their migration capabilities. Our results demonstrate that intermittent PTH (1-34) treatment significantly (P < or = 0.01) supported the migration of MG-63 cells, increased their HA-synthase-2 (HAS2) expression (P < or = 0.001), and enhanced their high-molecular size HA deposition in the pericellular matrix. Both increased endogenous HA production (P < or = 0.01) and treatment with exogenous high-molecular weight HA (P < or = 0.05) correlated to a significant increase of MG-63 cell migration capacity. Transfection with siHAS2 showed that PTH (1-34), mainly through HAS2, enhanced HA and regulated MG-63 cell motility. Interestingly, continuous PTH (1-34) treatment stimulated both Saos 2 cell HAS2 (P < or = 0.001) and HAS1 (P < or = 0.001) isoform expression inhibited their HYAL2 expression (P < or = 0.001) and modestly (P < or = 0.05) enhanced their migration. Therefore, the PTH (1-34) administration mode appears to distinctly modulate the migratory responses of the MG-63 moderately and Saos 2 well-differentiated osteosarcoma cell lines. Conclusively, the obtained data suggest that there is a regulatory effect of PTH (1-34), in an administration mode-dependent manner, on HA metabolism that is essential for osteosarcoma cell migration.

Effect of hyaluronan on osteogenic differentiation of porcine bone marrow stromal cells in vitro

Hyaluronan (HA) plays a predominant role in tissue morphogenesis, cell migration, proliferation, and cell differentiation. The aims of the present study were to investigate whether (i) prolonged presence of high concentration (4.0 mg/mL) 800 KDa HA and (ii) pretreatment with HA can modify osteogenic differentiation of pig bone marrow stromal cells (pBMSC). Cell proliferation and mineralization were measured. Expression of differentiation-related genes was evaluated by means of real-time reverse transcription polymerase chain reaction (RT-PCR). HA increased cell proliferation on day 7. HA decreased the basal level of bone-related gene expression and increased the basal level of sox9 marginally during 7-day pretreatment with HA. HA increased calcium deposit on day 21. cbfa1, ALP, and type 1 alpha collagen (Col1) expression was increased when pBMSC were cultivated in osteogenic medium, whereas their expression was decreased in the presence of HA on day 7. On day 14, the addition of HA upregulated cbfa1 and ALP expression compared to osteogenic medium group; there was no significant difference in Col1 expression. At day 21, osteocalcin (OC) expression showed 2.5-fold upregulation over osteogenic medium. These results suggest that exogenous HA stimulates endogenous HA, which together may play a synergetic role in osteogenic differentiation under osteoinducing conditions although gene expression was inhibited at the early stage.

Hyaluronan synthesis induces microvillus-like cell surface protrusions

Hyaluronan synthases (HASs) are plasma membrane enzymes that simultaneously elongate, bind, and extrude the growing hyaluronan chain directly into extracellular space. In cells transfected with green fluorescent protein (GFP)-tagged Has3, the dorsal surface was decorated by up to 150 slender, 3-20-microm-long microvillus-type plasma membrane protrusions, which also contained filamentous actin, the hyaluronan receptor CD44, and lipid raft microdomains. Enzymatic activity of HAS was required for the growth of the microvilli, which were not present in cells transfected with other GFP proteins or inactive GFP-Has3 mutants or in cells incubated with exogenous soluble hyaluronan. The microvilli induced by HAS3 were gradually withered by introduction of an inhibitor of hyaluronan synthesis and rapidly retracted by hyaluronidase digestion, whereas they were not affected by competition with hyaluronan oligosaccharides and disruption of the CD44 gene, suggesting independence of hyaluronan receptors. The data bring out the novel concept that the glycocalyx created by dense arrays of hyaluronan chains, tethered to HAS during biosynthesis, can induce and maintain prominent microvilli.

Hyaluronan increases RANKL expression in bone marrow stromal cells through CD44

HA activates CD44 to stimulate RANKL expression in bone marrow stromal cells. HA stimulation of RANKL is blocked by anti-CD44 antibody and is absent in cells from CD44(-/-) mice. CD44(-/-) mice exhibit thicker cortical bone and a smaller medullary cavity, but indices of bone resorption are not affected.

INTRODUCTION

Hyaluronan (HA), the major nonprotein glycosaminoglycan component of the extracellular matrix in mammalian bone marrow, functions in part through its receptor, CD44, to stimulate a series of intracellular signaling events that lead to cell migration, adhesion, and activation. To determine whether HA activation of CD44 influences RANKL and osteoprotegerin (OPG) expression and whether CD44 is functionally important in bone metabolism, we studied whole bone and bone marrow stromal cells (BMSCs) from wildtype and CD44(-/-) mice.

MATERIALS AND METHODS

BMSCs from wildtype and CD44(-/-) mice at 7 weeks of age were cultured and treated with either HA or anti-CD44 antibody. The levels of mRNA of RANKL, OPG, CD44, alkaline phosphatase (ALP), osteocalcin (OC), and alphaI collagen (COLL) were determined by quantitative real-time RT-PCR. Levels of RANKL and CD44 protein were measured by immunoblotting, and expression of CD44 in whole bone was determined by immunohistochemical staining. Double immunofluorescence staining and confocal microscopy were used to study colocalization of Cbfa1, CD44, and HA. Tibias were imaged using muCT, and cancellous and cortical parameters were measured. Osteoblast and osteoclast surface in the distal femoral metaphysis and osteoclast on the endocortical surface at the tibio-fibular junction were measured using quantitative histomorphometry. Differences were analyzed using ANOVA and the Newman-Keuls test.

RESULTS

Addition of HA dose-dependently increased RANKL mRNA (3.6-fold) and protein (3-fold) levels in BMSCs. Stimulation of RANKL by HA could be blocked with anti-CD44 antibody. Treatment of cells with HA or anti-CD44 antibody had no significant effect on OPG mRNA levels. Both CD44 and HA localized on the plasma membrane in cells expressing Cbfa1. HA localization on the cell membrane disappeared when cells were preincubated with anti-CD44 antibody. Compared with control mice, cortical bone of CD44(-/-) was thicker, and medullary area was smaller at both 7 and 17 weeks, but at 7 weeks, indices of bone resorption were normal. At 17 weeks of age, tibial mass of CD44(-/-) mice was higher than control mice. CD44(-/-) animals expressed less RANKL in whole bone (-30%) and in BMSCs (-50%). Cells from CD44(-/-) animals failed to respond to either HA or CD44 antibody treatment.

CONCLUSIONS

HA can increase RANKL expression in BMSCs through CD44.

Parathyroid Hormone Rapidly Stimulates Hyaluronan Synthesis by Periosteal Osteoblasts in the Tibial Diaphysis of the Growing Rat

Short term treatment (3-24 h) with parathyroid hormone (PTH) stimulated the synthesis and accumulation of hyaluronan (HyA) in explant cultures of tibial diaphyses from young rats. PTH increased the overall HyA content of periosteum 5-fold, with the basal cambium layer exhibiting the greatest enhancement ( approximately 8-fold). PTH increased the HyA content of cortical bone by 2-fold while not affecting the HyA content of bone marrow. PTH treatment greatly enhanced HyA staining throughout all layers of the periosteum, although its most dramatic effect occurred in the basal cambium layer. Here, unlike in the control tissue sections, nearly all cambium-lining osteoblasts stained intensely positive for HyA. PTH treatment enhanced the HyA staining of osteocytes in cortical bone tissue sections to the extent that the lacunocanalicular system became visualized. Three significant findings were revealed in this study. First, mature periosteal osteoblasts, under natural conditions, do not contain much HyA in their surrounding extracellular matrix but dramatically enhance their matrix HyA content when treated with PTH. Second, pre-osteocytes and osteocytes contain more HyA in their natural matrix than mature lining osteoblasts, and they appear to have functional PTH receptors because they responded to PTH treatment with an enhancement of HyA content. Finally, it was observed that the lining cells along the endosteal surface of the diaphysis did not stain strongly positive for HyA either naturally or when exposed to PTH treatment. This indicates that periosteal and endosteal osteoblastic cell populations exhibit metabolic differences in their extracellular matrix responses to PTH.

Hyaluronic acid reverses the abnormal synthetic activity of human osteoarthritic subchondral bone osteoblasts

The underlying mechanisms responsible for both cartilage loss and subchondral bone changes in osteoarthritis (OA) remain unknown. It is becoming recognized that the extracellular matrix influences the metabolism of cells both in vivo and in vitro and can modify their responses to external stimuli. Indeed, the glycosaminoglycan/proteoglycan matrix is of major importance for the proliferation and/or differentiation of a number of cells. Here, we determined the potential role of hyaluronic acid (HA) of increasing molecular weight (MW) to alter the expression of metabolic markers and cytokine production by human osteoarthritic (OA) subchondral osteoblasts (Ob). Both 1,25(OH)(2)D(3)-induced alkaline phosphatase activity (ALPase) and osteocalcin release were increased in OA Ob when compared to normal. HA reduced osteocalcin release in OA Ob at MW of 300 and above, whereas HA failed to significantly modify ALPase. Parathyroid hormone (PTH) stimulated cyclic AMP (cAMP) formation by OA Ob. HA had a biphasic effect on this PTH-dependent activity, totally inhibiting cAMP formation at MW of 300 and 800. HA of increasing MW progressively reduced the levels of Prostaglandin E(2) (PGE(2)) and interleukin-6 (IL-6) produced by OA Ob. Interestingly, urokinase plasminogen activator (uPA) and and PA inhibitor-1 (PAI-1) levels were not significantly affected by HA of increasing MW; however, the PAI-1 to uPA ratio showed a slight, yet nonsignificant increase. Surprisingly, uPA activity was increased in OA Ob under the same conditions. Last, HA had no effect on the production of insulin-like growth factor-1 by these cells. Our data suggest that high MW HA can modify cellular parameters in OA Ob that are increased when compared to normal. The effect of HA on inflammatory mediators, such as PGE(2) and IL-6, and on uPA activity is more striking at higher MW as well. Taken together, these results could suggest that HA of increasing MW has positive effects on OA Ob by modifying their biological synthetic capacities.

“Whither flows the fluid in bone?” An osteocyte's perspective

Bone represents a porous tissue containing a fluid phase, a solid matrix, and cells. Movement of the fluid phase within the pores or spaces of the solid matrix translates endogenous and exogenous mechanobiological, biochemical and electromechanical signals from the system that is exposed to the dynamic external environment to the cells that have the machinery to remodel the tissue from within. Hence, bone fluid serves as a coupling medium, providing an elegant feedback mechanism for functional adaptation. Until recently relatively little has been known about bone fluid per se or the influences governing the characteristics of its flow. This work is designed to review the current state of this emerging field. The structure of bone, as an environment for fluid flow, is discussed in terms of the properties of the spaces and channel walls through which the fluid flows and the influences on flow under physiological conditions. In particular, the development of the bone cell syncytium and lacunocanalicular system are presented, and pathways for fluid flow are described from the systemic to the organ, tissue, cellular and subcellular levels. Finally, exogenous and endogenous mechanisms for pressure-induced fluid movement through bone, including mechanical loading, vascular derived pressure gradients, and osmotic pressure gradients are discussed. The objective of this review is to survey the current understanding of the means by which fluid flow in bone is regulated, from the level of the skeletal system down to the level of osteocyte, and to provide impetus for future research in this area of signal transduction and coupling. An understanding of this important aspect of bone physiology has profound implications for restoration of function through innovative treatment modalities on Earth and in space, as well as for engineering of biomimetic replacement tissue.

Crosslinking of CD44 on human osteoblastic cells upregulates ICAM-1 and VCAM-1

Cell adhesion of osteoblasts and osteoclastic precursors of hematopoietic origin is a prerequisite for osteoclast maturation. We have investigated the relevance of osteoblast-matrix binding and regulation of adhesion molecules to this process. Human osteoblastic cells highly expressed CD44, a major receptor for hyaluronan present in the surrounding bone matrix. Crosslinking of CD44 on osteoblastic cells with specific antibodies augmented the expression of intercellular adhesion molecule (ICAM)-1 and vascular cell adhesion molecule (VCAM)-1. Hyaluronan, the major ligand of CD44, also up-regulated ICAM-1 expression. Stimulation of CD44 on osteoblastic cells amplified their adhesion to monocytic cells through ICAM-1 and VCAM-1. These results suggest that such crosstalk among distinct adhesion molecules may be relevant to bone metabolism, including osteoclastogenesis.

Reversible suppression of in vitro biomineralization by activation of protein kinase A

Parathyroid hormone (PTH-(1-34)) potently suppresses apatite deposition in osteoblastic cultures. These inhibitory effects are mediated through signaling events following PTH receptor binding. Using both selective inhibitors and activators of protein kinase A (PKA), this study shows that a transient activation of PKA is sufficient to account for PTH's inhibition of apatite deposition. This inhibition is not a result of reduced cell proliferation, reduced alkaline phosphatase activity, increased collagenase production, or lowering medium pH. Rather, data suggest a functional relationship between matrix assembly and apatite deposition in vitro. Bone sialoprotein (BSP) and apatite co-localize in the extracellular matrix of mineralizing cultures, with matrix deposition of BSP temporally preceding that of apatite. Transient activation of PKA by either PTH-(1-34) or short term cAMP analog treatment blocks the deposition of BSP in the extracellular matrix without a significant reduction in the total amount of BSP synthesized and secreted. This effect is reversible after allowing the cultures to recover in the absence of PKA activators for several days. Thus, a transient activation of PKA may suppress mineral deposition in vitro as a consequence of altering the assembly of an extracellular matrix permissive for apatite formation.

Signal transduction pathways mediating parathyroid hormone stimulation of bone sialoprotein gene expression in osteoblasts

Bone sialoprotein is a major noncollagenous protein of bone. Parathyroid hormone (PTH) was shown to cause a 2-4-fold increase in the steady-state levels of bone sialoprotein mRNAs within primary cultures of embryonic osteoblasts. The induction could be mimicked by both forskolin and phorbol 12-myristate 13-acetate and was not inhibited by cycloheximide. Transient expression of a approximately 1200-base pair avian 5' bsp promoter/reporter construct demonstrated similar inductions as mRNA levels. Co-transfection of an expression plasmid encoding heat-stable inhibitor of cAMP-dependent protein kinase, a peptide inhibitor of PKA, decreased both the basal and PTH-induced bsp transcription, while co-expression of the catalytic subunit of PKA-induced bsp expression 3-fold. Protein kinase C activation, on the other hand, did not appear to work through its activation of c-fos, since co-transfection of an expression clone for c-fos had no effect. Interestingly, heat-stable inhibitor of cAMP-dependent protein kinase also inhibited the phorbol 12-myristate 13-acetate induction, suggesting that the protein kinase C acts through some form of interaction with the cAMP/PKA pathway. A half-cAMP response element site in the bsp promoter was identified as the cis-acting element that mediated the PTH response by the transient transfections with reporter constructs containing nested deletions of the promoter or a heterologous promoter containing the cAMP response element. In conclusion, these data indicate that PTH stimulation of bsp gene expression is specific to osteoblasts and mediated by changing cellular cAMP/PKA levels. They further suggest that although protein kinase C is capable of stimulating the gene by itself, it plays a minimal role in mediating the PTH induction of bone sialoprotein.

Influence of vitamin D status on hyaluronan localization in bone

The distribution of hyaluronan was investigated in the proximal tibiotarsal bones of normal (vitamin D-treated) chicks, and chicks with rachitic lesions induced by vitamin D deficiency. Localization studies using a biotinylated hyaluronan-binding probe revealed that in vitamin D-treated chicks, a high level of hyaluronan staining was present in upper proliferative zone cartilage and upper hypertrophic zone cartilage. Hyaluronan staining was greatly reduced in the zone of provisional calcification. In the metaphyses and diaphyses of normal chicks hyaluronan was predominantly localized to the non-bone-forming surfaces of osteoblasts but was also present on the basolateral surfaces of lining cells and osteoclasts. Marked changes in hyaluronan distribution were observed in vitamin D-deficient chicks. The amount of hyaluronan present in proliferative zone growth cartilage was similar to control chicks, although with a more widespread distribution, extending into lower proliferative zone cartilage. In the zone of hypertrophy/calcification, biochemical analyses revealed that hyaluronan levels in rachitic chicks were about 3.6 times greater than in vitamin D-treated chicks; localization studies demonstrated that this increase was associated with the presence of hyaluronan-positive spindle-shaped cells in the metaphyseal vascular spaces. Intense hyaluronan staining was also associated with abundant spindle-shaped cells occupying the marrow spaces of rachitic diaphyseal bone. The distribution of hyaluronan in vitamin D-treated chick bone, and the alterations observed in rachitic tissue suggests a role for hyaluronan in endochondral bone formation.

Rapidly forming apatitic mineral in an osteoblastic cell line (UMR 106-01 BSP)

This study evaluated a rapid biomineralization phenomenon exhibited by an osteoblastic cell line, UMR 106-01 BSP, when treated with either organic phosphates [beta-glycerophosphate (beta-GP), Ser-P, or Thr-P], inorganic phosphate (P(i)), or calcium. In a dose-dependent manner, these agents (2-10 mM) stimulated confluent cultures to deposit mineral in the cell layer (ED50 of approximately 4.6 mM for beta-GP (30 +/- 2 nmol Ca2+/microgram DNA) and approximately 3.8 mM (29 +/- 2 nmol Ca2+/microgram DNA) for P(i)) with a plateau in mineral formation by 20 h (ET50 approximately 12-15 h). beta-GP or P(i) treatment yielded mineral crystals having an x-ray diffraction pattern similar to normal human bone. Alizarin red-S histology demonstrated calcium mineral deposition in the extracellular matrix and what appeared to be intracellular paranuclear staining. Electron microscopy revealed small, needle-like crystals associated with fibrillar, extracellular matrix deposits and intracellular spherical structures. Mineral formation was inhibited by levamisole (ED50 approximately 250 microM), pyrophosphate (ED50 approximately 1-10 microM), actinomycin C1 (500 ng/ml), cycloheximide (50 micrograms/ml), or brefeldin A (1 microgram/ml). These results indicate that UMR 106-01 BSP cells form a bio-apatitic mineralized matrix upon addition of supplemental phosphate. This process involves alkaline phosphatase activity, ongoing RNA and protein synthesis, as well as Golgi-mediated processing and secretion.