Parathyroid hormone-related protein down-regulates bone sialoprotein gene expression in cementoblasts: role of the protein kinase A pathway

Attenuation of orthodontically induced inflammatory root resorption by using low-intensity pulsed ultrasound as a therapeutic modality- a systematic review

Ultrasound is an effective tool for both diagnostic and therapeutic applications. As an imaging tool, ultrasound has mostly been used for real-time noninvasive diagnostic imaging. As ultrasound propagates through a material, a reflected radio-frequency (RF) signal is generated when encountering a mismatch in acoustic impedance. While traditionally recognized for its diagnostic imaging capabilities, the application of ultrasound has broadened to encompass therapeutic interventions, most notably in the form of Low-Intensity Pulsed Ultrasound (LIPUS). Low-Intensity Pulsed Ultrasound (LIPUS) is a form of mechanical energy transmitted transcutaneously by high-frequency acoustic pressure waves. The intensity of LIPUS (30 mW/cm2) is within the range of ultrasound intensities used for diagnostic purposes (1-50 mW/cm2) and is regarded as non-thermal, non-destructive, permeating living tissues and triggering a cascade of biochemical responses at the cellular level. The LIPUS device produces a 200 µs burst of 1.5 MHz acoustic sine waves, that repeats at a modulation frequency of 1 kHz and provides a peak pressure of 30 mW/cm2. Low-intensity pulsed ultrasound (LIPUS) forms one of the currently available non-invasive healing-enhancing devices besides electro-stimulation (pulsed electro-magnetic field, PEMF). This modality has been leveraged to enhance drug delivery, expedite injury recovery, improve muscle mobility, alleviate joint stiffness and muscle pain, and enhance bone fracture healing. Although LIPUS has been embraced within various medical disciplines, its integration into standard dental practices is still in its nascent stages, signifying an unexplored frontier with potentially transformative implications. Low-intensity pulsed ultrasound (LIPUS) has emerged as an attractive adjuvant therapy in various dental procedures, such as orthodontic treatment and maxillary sinus augmentation. Its appeal lies in its simplicity and non-invasive nature, positioning LIPUS as a promising avenue for clinical innovation. One particular area of interest is orthodontically induced inflammatory root resorption (OIIRR), an oftenunavoidable outcome of the orthodontic intervention, resulting in the permanent loss of root structure. Notably, OIIRR is the second most common form of root resorption (RR), surpassed only by root resorption related to pulpal infection. Given the high prevalence and potential long-term consequences of OIIRR, this literature review seeks to evaluate the efficacy of LIPUS as a therapeutic approach, with an emphasis on assessing its capacity to reduce the severity of OIIRR to a level of clinical significance. To conduct this systematic review, a comprehensive automated literature search was executed across multiple databases, including MEDLINE, Embase, PsycINFO, Web of Knowledge, Scopus, CINAHL, LILACS, SciELO, Cochrane, PubMed, trials registries, 3ie, and Google Scholar. Both forward and backward citation tracking was employed, encompassing studies published from database inception through January 2009 to April 2023. The review focused on randomized controlled trials (RCTs) that specifically evaluated the effects of low-intensity pulsed ultrasound therapy on orthodontically induced inflammatory root resorption (OIIRR), without restrictions of publication date. A stringent selection criterion was applied, and only studies demonstrating high levels of statistical significance were included. Ultimately, fourteen studies met the inclusion criteria and were subjected to further analysis. The overall quality of the included randomized controlled trials (RCTs) was rigorously assessed utilizing the Grading of Recommendations Assessment, Development, and Evaluation (GRADE) approach. This analysis revealed certain methodological limitations that posed challenges in drawing definitive conclusions from the available evidence. Despite these constraints, the review offers invaluable insights that can inform and guide future research. Specifically, it delineates recommendations for targeted populations, necessary interventions, appropriate outcome measures, suitable study designs, and essential infrastructure to facilitate further investigations. The synthesis of these insights aims to enhance the development and application of low-intensity pulsed ultrasound therapy within the field of dentistry, thereby contributing to improved patient outcomes.

Intermittent parathyroid hormone promotes cementogenesis in a PKA- and ERK1/2-dependent manner

BACKGROUND

Intermittent parathyroid hormone (PTH) promotes cementogenesis and provides a promising biotherapeutic to rehabilitate resorbed roots. However, the underlying mechanisms remain inconclusive. Cyclic aenosine monophosphate (AMP)-dependent protein kinases A (PKA) and extracellular signal-regulated MAP kinases 1/2 (ERK1/2) are key regulators of bone remodeling. The present study aims to investigate whether PKA and ERK1/2 are involved in the process of intermittent PTH-promoted cementogenesis.

METHODS

Sprague-Dawley rats in experimental group (n = 30) received a daily subcutaneous injection of PTH and the control (n = 30) received placebo vehicle for 1, 2, 3, 4, and 5 weeks. Results were evaluated by hematoxylin and eosin and immunohistochemistry staining. In vitro, OCCM-30 cells were incubated with intermittent PTH. H89 and U0126 were used to determine the role of PKA and ERK1/2, respectively. The cementogenic results were analyzed by reverse transcription-polymerase chain reaction (RT-PCR), western blotting, alkaline phosphatase activity assay and Alizarin Red S staining. The interaction of PKA and p-ERK1/2 was determined by co-immunoprecipitation (Co-IP).

RESULTS

Intermittent PTH exerted anabolic effect on cellular cementum in developing teeth with elevated expression of osteocalcin, osteopontin, and PKA (catalytic subunit) in PTH injection group. The promoting effects of intermittent PTH on cementogenesis and osteogenic differentiation were abrogated by H89 and U0126 in vitro, respectively. Blocking of PKA pathway downregulated intermittent PTH-induced ERK1/2 phosphorylation.

CONCLUSIONS

Intermittent PTH promotes cementogenesis in a PKA- and ERK1/2-dependent manner. In this process, PKA and p-ERK1/2 interact with each other. These results support the future biotherapeutic applications of PTH in cementum resorption.

Cementoblast Gene Expression is Regulated by Porphyromonas gingivalis Lipopolysaccharide Partially via Toll-like Receptor-4/MD-2

Lipopolysaccharides are potent inflammatory mediators considered to contribute to destruction of periodontal tissues. Here, we hypothesized that Porphyromonas gingivalis lipopolysaccharide (P-LPS) treatment would regulate gene expression in murine cementoblasts through Toll-like receptor 4. Real-time (RT)-PCR and Northern blot analysis indicated that P-LPS decreased expression of transcripts for osteocalcin (OCN) and receptor activator of nuclear factor κB ligand (RANKL). In contrast, a dose-dependent up-regulation in mRNA levels for osteopontin (OPN) and osteoprotegerin (OPG) was observed. Similarly, ELISA demonstrated decreased RANKL and increased OPG levels. A monoclonal antibody specific for mouse TLR-4/MD-2 partially neutralized the P-LPS effect on cementoblasts. These results indicate that exposure of cementoblasts to P-LPS can alter cell function by regulating markers of osteoclastic activity ( e.g., RANKL/OPG), thereby potentially affecting the inflammation-associated resorption of mineralized tissues.

Intermittent PTHrP(1-34) exposure augments chondrogenesis and reduces hypertrophy of mesenchymal stromal cells

Phenotype instability and premature hypertrophy prevent the use of human mesenchymal stromal cells (MSCs) for cartilage regeneration. Aim of this study was to investigate whether intermittent supplementation of parathyroid hormone-related protein (PTHrP), as opposed to constant treatment, can beneficially influence MSC chondrogenesis and to explore molecular mechanisms below catabolic and anabolic responses. Human MSCs subjected to chondrogenic induction in high-density culture received PTHrP(1-34), forskolin, dbcAMP, or PTHrP(7-34) either constantly or via 6-h pulses (three times weekly), before proteoglycan, collagen type II, and X deposition; gene expression; and alkaline phosphatase (ALP) activity were assessed. While constant application of PTHrP(1-34) suppressed chondrogenesis of MSCs, pulsed application significantly increased collagen type 2 (COL2A1) gene expression and the collagen type II, proteoglycan, and DNA content of pellets after 6 weeks. Collagen type 10 (COL10A1) gene expression was little affected but Indian hedgehog (IHH) expression and ALP activity were significantly downregulated by pulsed PTHrP. A faster response to PTHrP exposure was recorded for ALP activity over COL2A1 regulation, suggesting that signal duration is critical for catabolic versus anabolic reactions. Stimulation of cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA) signaling by forskolin reproduced major effects of both treatment modes, whereas application of PTHrP(7-34) capable of protein kinase C (PKC) signaling was ineffective. Pulsed PTHrP exposure of MSCs stimulated chondrogenesis and reduced endochondral differentiation apparently uncoupling chondrogenic matrix deposition from hypertrophic marker expression. cAMP/PKA was the major signaling pathway triggering the opposing effects of both treatment modes. Intermittent application of PTHrP represents an important novel means to improve chondrogenesis of MSCs and may be considered as a supporting clinical-treatment mode for MSC-based cartilage defect regeneration.

PKA regulatory subunit expression in tooth development

Protein kinase A (PKA) plays critical roles in many biological processes including cell proliferation, cell differentiation, cellular metabolism and gene regulation. Mutation in PKA regulatory subunit, PRKAR1A has previously been identified in odontogenic myxomas, but it is unclear whether PKA is involved in tooth development. The aim of the present study was to assess the expression of alpha isoforms of PKA regulatory subunit (Prkar1a and Prkar2a) in mouse and human odontogenesis by in situ hybridization. PRKAR1A and PRKAR2A mRNA transcription was further confirmed in a human deciduous germ by qRT-PCR. Mouse Prkar1a and human PRKAR2A exhibited a dynamic spatio-temporal expression in tooth development, whereas neither human PRKAR1A nor mouse Prkar2a showed their expression in odontogenesis. These isoforms thus showed different expression pattern between human and mouse tooth germs.

Continuous and intermittent exposure of neonatal rat calvarial cells to PTHrP (1-36) inhibits bone nodule mineralization in vitro by downregulating bone sialoprotein expression via the cAMP signaling pathway

The development and growth of the skeleton in the absence of parathyroid-hormone-related protein (PTHrP) is abnormal.  The shortening of appendicular bones in PTHrP gene null mice is explained by an effect of PTHrP on endochondral bone growth.  Whether or not PTHrP influences intramembranous ossification is less clear.  The purpose of this study was to determine the effect of exogenous PTHrP on intramembranous ossification in vitro.  Neonatal rat calvarial cells maintained in primary cell culture conditions that permit spontaneous formation of woven bone nodules by intramembranous ossification were studied. The expression of PTHrP, parathyroid hormone 1 receptor (PTH1R), and alkaline phosphatase (AP) by osteogenic cells in developing nodules and the effects of PTHrP (1-36) on nodule development was determined over 3-18 days. PTHrP and PTH1R were detected colonies of osteogenic cells on culture day three, and AP was detected on day six. PTHrP and its receptor were localized in pre-osteoblasts, osteoblasts, and osteocytes, and AP activity was detected in pre-osteoblasts and osteoblasts but not osteocytes. Continuous and intermittent exposure to PTHrP (1-36) decreased the number of mineralized bone nodules and bone sialoprotein (BSP) mRNA and protein, but had no effect on the number of AP-positive osteogenic cell colonies, cell proliferation, apoptosis, or osteopontin (OPN) mRNA. These results demonstrate that osteogenic cells that participate in the formation of woven bone nodules in vitro exhibit PTHrP and PTH1R before they demonstrate AP activity. Exogenous PTHrP (1-36) inhibits the mineralization of woven bone deposited during bone nodule formation in vitro, possibly by reducing the expression of BSP.

Response of immortalized murine cementoblast cells to hypoxia in vitro

OBJECTIVES

The aim of the study was to investigate the impact of hypoxia on proliferation, apoptosis and mineralization of cementoblast-like cells (OCCM-30) in vitro.

METHODS

The effects of different periods of hypoxia (2% O2) on proliferation, apoptosis, cementoblastic potential and root cementum resorption capability of OCCM-30 were evaluated, by using MTT, flow cytometry, alkaline phosphatase (ALP) activity assay, reverse transcription-polymerase chain reaction measurement, enzyme-linked immunosorbent assay and mineralization nodule formation assay.

RESULTS

OCCM-30 viability was significantly inhibited by hypoxia while the apoptosis ratio was enhanced in a time-dependent manner; hypoxia inducible factor-1α and vascular endothelial growth factor mRNA were induced by hypoxia in different manners; temporary hypoxia (<24 h) stimulated cementoblastic function of OCCM-30, while long-term hypoxia inhibited it, manifested by decreased mRNA level or release of ALP, osteocalcin, bone sialoprotein, osteopontin and osteoprotegerin. In addition, hypoxia affected mineralized nodule formation of OCCM-30 in a time-dependent fashion; moreover, root cementum resorption function was also induced by hypoxia, manifested by increased receptor activator of nuclear factor kappa B ligand mRNA and protein expression.

CONCLUSION

Temporary exposure of OCCM-30 to hypoxia inhibited proliferation, promoted apoptosis and mineralization, while longer duration of hypoxia could inhibit the cementoblast function. The findings may provide theoretical basis for developing novel therapeutics to prevent root resorption during orthodontic treatment.

Intermittent PTH(1-34) signals through protein kinase A to regulate osteoprotegerin production in human periodontal ligament cells in vitro

Periodontal ligament (PDL) cells have been associated with the regulation of periodontal repair processes by the differential expression of osteoprotegerin and RANKL in response to intermittent parathyroid hormone (PTH) resulting in a modified activity of bone-resorbing osteoclasts. Here, we examined the intracellular signaling pathways that PDL cells use to mediate the PTH(1-34) effect on osteoprotegerin production and hypothesized that those would be dependent on the cellular maturation stage. Two stages of confluence served as a model for cellular maturation of 5th passage human PDL cells from six donors. Intermittent PTH(1-34) (10(-12) M) and PTH(1-31), the latter lacking the protein kinase C (PKC) activating domain, induced a significant decrease of osteoprotegerin production in confluent cultures, whereas the signal-specific fragments PTH(3-34) and PTH(7-34), which both are unable to activate protein kinase A (PKA), had no effect. The addition of the PKA inhibitor H8 antagonized the PTH(1-34) effect, whereas the PKC inhibitor RO-32-0432 did not. In pre-confluent, less mature cultures, intermittent PTH(1-34) resulted in a significant increase of osteoprotegerin. Similar results were obtained when PTH(1-31) substituted for PTH(1-34) as opposed to a lack of an effect of PTH(3-34) and PTH(7-34). Likewise, in confluent cultures, H8 inhibited the PTH(1-34) effect in pre-confluent cultures contrasted by RO-32-0432 which had no effect. These findings indicate that PTH(1-34) signaling targeting osteoprotegerin production in PDL cells involves a PKA-dependent pathway. The PTH(1-34) effect is dependent on cell status, whereas intracellular signal transduction is not. Clinical trials will have to prove whether those in vitro data are of physiological relevance for interference strategies.

Parathyroid hormone increases activating transcription factor 4 expression and activity in osteoblasts: requirement for osteocalcin gene expression

PTH is an important peptide hormone regulator of calcium homeostasis and osteoblast function. However, its mechanism of action in osteoblasts is poorly understood. Our previous study demonstrated that PTH activates mouse osteocalcin (Ocn) gene 2 promoter through the osteoblast-specific element 1 site, a recently identified activating transcription factor-4 (ATF4) -binding element. In the present study, we examined effects of PTH on ATF4 expression and activity as well as the requirement for ATF4 in the regulation of Ocn by PTH. Results show that PTH elevated levels of ATF4 mRNA and protein in a dose- and time-dependent manner. This PTH regulation requires transcriptional activity but not de novo protein synthesis. PTH also increased binding of nuclear extracts to osteoblast-specific element 1 DNA. PTH stimulated ATF4-dependent transcriptional activity mainly through protein kinase A with a lesser requirement for protein kinase C and MAPK/ERK pathways. Lastly, PTH stimulation of Ocn expression was lost by small interfering RNA down-regulation of ATF4 in MC-4 cells and Atf4(-/-) bone marrow stromal cells. Collectively, these studies for the first time demonstrate that PTH increases ATF4 expression and activity and that ATF4 is required for PTH induction of Ocn expression in osteoblasts.

TSC2 integrates Wnt and energy signals via a coordinated phosphorylation by AMPK and GSK3 to regulate cell growth

Mutation in the TSC2 tumor suppressor causes tuberous sclerosis complex, a disease characterized by hamartoma formation in multiple tissues. TSC2 inhibits cell growth by acting as a GTPase-activating protein toward Rheb, thereby inhibiting mTOR, a central controller of cell growth. Here, we show that Wnt activates mTOR via inhibiting GSK3 without involving beta-catenin-dependent transcription. GSK3 inhibits the mTOR pathway by phosphorylating TSC2 in a manner dependent on AMPK-priming phosphorylation. Inhibition of mTOR by rapamycin blocks Wnt-induced cell growth and tumor development, suggesting a potential therapeutic value of rapamycin for cancers with activated Wnt signaling. Our results show that, in addition to transcriptional activation, Wnt stimulates translation and cell growth by activating the TSC-mTOR pathway. Furthermore, the sequential phosphorylation of TSC2 by AMPK and GSK3 reveals a molecular mechanism of signal integration in cell growth regulation.

Effects of ultrasound on cementoblast metabolism in vitro

Ultrasound (US) has been shown to alter cell/tissue functions. However the effects of US on cementoblasts are not known. The aim of the present study was to evaluate the effect of US exposure on proliferation and metabolism of cementoblast (murine cementoblastic cell line [OCCM-30]) in vitro. Cultured cementoblasts received US exposure (frequency = 1 MHz; pulsed 1:4; spatial-average intensities (I(SA)) = 100, 150 or 400mW/cm(2)) or sham exposure for 15 min per d for 4 d. They were then assayed for calcium content and cell proliferation. Furthermore, expression levels of osteocalcin, bone-sialoprotein, alkaline phosphatase and osteopontin were analyzed by real time polymerase chain reaction. Calcium content was statistically increased (p < 0.05) after US exposure at 100 or 150 mW/cm2. Alkaline phosphatase mRNA levels were (p < 0.05) increased significantly by US stimulation with 150 mW/cm2. These results demonstrate that US affects cementoblasts by regulation of some genes-related protein in vitro and, although more studies are necessary, it may be important to consider in designing in vivo US therapies targeted at the oral cavity.

JunB as a downstream mediator of PTHrP actions in cementoblasts

The role of AP-1 family members in the action of PTHrP was examined in cementoblasts. PTHrP increased mRNA and protein levels of all Fos members, but only one Jun member (JunB) was increased. Overexpression of JunB in cementoblasts mimicked actions of PTHrP to support osteoclastogenesis and inhibit cementoblast differentiation, suggesting that the actions of PTHrP on mesenchymal cells operate through JunB.

INTRODUCTION

Cementoblasts are mesenchymal cells that share phenotypic features with osteoblasts in vitro; however, unlike osteoblasts, cementoblasts rarely support osteoclastogenesis in vivo. The osteoblast-mediated support of osteoclastogenesis involves PTH-induced reduction in osteoprotegerin (OPG) expression. PTH acts on osteoblastic cells through specific signaling pathways and transcription factors such as activator protein 1 (AP-1). The purpose of this study was to determine the impact of PTH-related protein (PTHrP) on AP-1 transcription factors in cementoblasts and the role of JunB in the actions of PTHrP.

MATERIALS AND METHODS

Cementoblastic cells were treated with PTHrP and evaluated for mRNA and protein levels of AP-1 family members. Stable transfectants of OCCM cells overexpressing JunB were evaluated for OPG production, ability to support osteoclastogenesis, and measures of proliferation and differentiation.

RESULTS

PTHrP treatment in vitro resulted in a time-dependent upregulation of mRNA and proteins for the Fos family members, but only JunB of the Jun family. OPG mRNA and protein levels were reduced by PTHrP in OCCM and were lower in JunB overexpressing cells than controls. In co-culture experiments, TRACP+ cells were increased with RANKL treatment in JunB overexpressing cells compared with controls. Cementoblast differentiation was reduced with overexpression of JunB as measured by a decrease in mineralized nodule formation and gene expression for bone sialoprotein and osterix. Measures of proliferation including cell number and cyclin D1 levels were increased in JunB overexpressing clones. In vivo, cementoblast implants exhibited a cementoblastoid nature with copious mineral-like matrix, whereas JunB-overexpressing implants were densely cellular with little mineralized matrix.

CONCLUSIONS

JunB was the only Jun family member increased by PTHrP, and its overexpression showed similar patterns of gene expression and OPG production as PTHrP treatment of controls. These data suggest that JunB may be a key mediator of PTHrP actions in cementoblasts.

Cementum engineering with three-dimensional polymer scaffolds

Cloned cementoblasts (OCCMs), periodontal ligament fibroblasts (SV-PDLs), and dental follicle (SV-F) cells obtained from mice were used as a tool to study periodontal tissue engineering. OCCM, SV-PDL, and SV-F cells were seeded onto three-dimensional poly lactic-co-glycolic acid (PLGA) scaffolds and cultured with the use of bioreactors or implanted subcutaneously in severe combined immune deficiency (SCID) mice for up to 6 weeks. We explored the behavior of these cells in porous PLGA sponges by cell growth, expression of mineral-associated genes using reverse transcriptase polymerase chain reaction, and mineralization by histologic analysis in vitro and in vivo. Results indicated that cells attached to PLGA scaffolds under either static or dynamic conditions in vitro. Only OCCM implants, retrieved from both in vitro bioreactors and SCID mice at 3-and 6-weeks post-cell implantation exhibited mineral formation. Types I and XII collagens, osteocalcin, and bone sialoprotein genes were detected in all implants retrieved from SCID mice. These results suggest that delivery of selected cells via PLGA scaffolds may serve as a viable approach for promoting periodontal tissue regeneration.

Parathyroid hormone induction of the osteocalcin gene. Requirement for an osteoblast-specific element 1 sequence in the promoter and involvement of multiple-signaling pathways

Parathyroid hormone (PTH) is an important peptide hormone regulator of bone formation and osteoblast activity. However, its mechanism of action in bone cells is largely unknown. This study examined the effect of PTH on mouse osteocalcin gene expression in MC3T3-E1 preosteoblastic cells and primary cultures of bone marrow stromal cells. PTH increased the levels of osteocalcin mRNA 4-5-fold in both cell types. PTH also stimulated transcriptional activity of a 1.3-kb fragment of the mouse osteocalcin gene 2 (mOG2) promoter. Inhibitor studies revealed a requirement for protein kinase A, protein kinase C, and mitogen-activated protein kinase pathways in the PTH response. Deletion of the mOG2 promoter sequence from -1316 to -116 caused no loss in PTH responsiveness whereas deletion from -116 to -34 completely prevented PTH stimulation. Interestingly, this promoter region does not contain the RUNX2 binding site shown to be necessary for PTH responsiveness in other systems. Nuclear extracts from PTH-treated MC3T3-E1 cells exhibited increased binding to OSE1, a previously described osteoblast-specific enhancer in the mOG2 promoter. Furthermore, mutation of OSE1 in DNA transfection assays established the requirement for this element in the PTH response. Collectively, these studies establish that actions of PTH on the osteocalcin gene are mediated by multiple signaling pathways and require OSE1 and associated nuclear proteins.

Chondrocyte-derived transglutaminase promotes maturation of preosteoblasts in periosteal bone

During endochondral development, elongation of the bone collar occurs coordinately with growth of the underlying cartilaginous growth plate. Transglutaminases (TGases) are upregulated in hypertrophic chondrocytes, and correlative evidence suggests a relationship between these enzymes and mineralization. To examine whether TGases are involved in regulating mineralization/osteogenesis during bone development, we devised a coculture system in which one cellular component (characterized as preosteoblastic) is derived from the nonmineralized region of the bone, and the other cellular component is hypertrophic chondrocytes. In these cocultures, mineralization is extensive, with the preosteoblasts producing the mineralized matrix, and the chondrocytes regulating this process. Secreted regulators are involved, as conditioned medium from chondrocytes induces mineralization in preosteoblasts, but not vice versa. One factor is TGase. In the cocultures, inhibition of TGase reduces mineralization, and addition of the enzyme enhances it. Exogenous TGase also induces markers of osteoblastic differentiation (i.e., bone sialoprotein and osteocalcin) in the preosteoblasts, suggesting their differentiation into osteoblasts. Two possible signaling pathways may be affected by TGase and result in increased mineralization (i.e., TGF-beta and protein kinase A pathways). Addition of exogenous TGF-beta2 to the cocultures increases mineralization; though, when mineralization is induced by TGase, there is no detectible elevation of TGF-beta, suggesting that these two factors stimulate osteogenesis by different pathways. However, an interrelationship seems to exist between TGase and PKA-dependent signaling. When mineralization of the cocultures is stimulated through the addition of TGase, a concomitant reduction (50%) in PKA activity occurs. Consistent with this observation, addition of an activator of PKA (cyclic AMP) to the cultures inhibits matrix mineralization, while known inhibitors of PKA (H-89 and a peptide inhibitor) cause an increase in mineralization. Thus, at least one mechanism of TGase stimulation probably involves inhibition of the PKA-mediated signaling.

Bone morphogenetic protein-2 inhibits differentiation and mineralization of cementoblasts in vitro

As an approach for improving the outcome and predictability of periodontal regenerative therapies, we have focused on determining the responses of cells within the local environment to putative regenerative factors. This study examined the effects of bone morphogenetic protein-2 (BMP-2) on murine cementoblasts in vitro. Northern blot analysis indicated that BMP-2 decreased mRNA levels of bone sialoprotein and type I collagen dose-dependently (10-300 ng/mL). At low doses, up to 100 ng/mL, BMP-2 had no effect on transcripts for osteocalcin and osteopontin, whereas at 300 ng/mL, BMP-2 greatly increased expression of these two genes. BMP-2 also inhibited cementoblast-mediated mineral nodule formation in a dose-dependent manner (inhibition was noted at 10 ng/mL). Noggin reversed the effects of BMP-2 on gene expression and on mineralization. These findings reflect the diverse responses of periodontal cells to BMP-2 and highlight the need to consider the complexity of factors involved in designing predictable regenerative therapies.

Cementum matrix formation in vivo by cultured dental follicle cells

Dental follicle is the fibrous tissue that surrounds the developing tooth germ, and it is believed to contain progenitors for cementoblasts, periodontal ligament cells, and osteoblasts. In this study, we report the presence of cementoblast progenitors in cultures of bovine dental follicle cells and demonstrate their differentiation capacity. Bovine dental follicle cells (BDFC) obtained from tooth germs by collagenase digestion were compared with bovine alveolar bone osteoblasts (BAOB) and bovine periodontal ligament cells (BPDL) in vitro and in vivo. In culture, BDFC exhibited low levels of alkaline phosphatase activity and expressed mRNA for osteopontin (OP) and type I collagen (COLI), as well as low levels of osteocalcin (OC) mRNA. In contrast, cultured BAOB exhibited high alkaline phosphatase activity levels and expressed mRNA for OC, OP, COLI, and bone sialoprotein (BSP). To elucidate the differentiation capacity of BDFC in vivo, cells were transplanted into severe combined immunodeficiency (SCID) mice and analyzed after 4 weeks. Transplanted BDFC formed fibrous tissue and cementum-like matrix, which stained positive for anti-cementum attachment protein (CAP) monoclonal antibody (3G9), and expressed mRNA for OC, OP, COLI, and BSP. On the other hand, transplanted BAOB formed bone-like matrix, but were negative for anti-CAP monoclonal antibody. The BPDL transplants formed fibrous tissue that contained a few cells expressing CAP. These results indicate that cementoblast progenitors are present in BDFC, which can provide a useful model for investigating the molecular mechanisms of cementogenesis.

Bone morphogenetic protein 2 induces dental follicle cells to differentiate toward a cementoblast/osteoblast phenotype

When triggered appropriately, dental follicle cells are considered to be able to differentiate toward a cementoblast/osteoblast phenotype. However, factors and mechanisms regulating follicle cell differentiation remain undefined. This study focused on determining the ability of bone morphogenetic protein (BMP) 2 to promote the differentiation of follicle cells and periodontal ligament (PDL) cells along a cementoblast/ osteoblast pathway. Follicle cells and PDL cells were isolated from the first molar region of CD-1 mice and immortalized with SV40. Both cell types expressed BMP-4 and BMP receptors (BMPR) IA and II, but only follicle cells expressed BMP-2 mRNA. Cells were exposed to recombinant human BMP (rhBMP)-2 (0-100 ng/ml) and Northern blots were used to determine the expression of mineral-associated markers. BMP-2, in a dose- and time-dependent manner, induced cementoblast/osteoblast differentiation of follicle cells, as reflected by enhanced core binding factor alpha (Cbfal), bone sialoprotein (BSP), and osteocalcin (OCN) mRNA expression and enhanced mineral formation. U0126, a specific inhibitor of MEK-1/2 members of the MAPK family, abolished BMP-2-mediated expression of BSP and OCN. In contrast, exposure of PDL cells to BMP-2 resulted in modest expression of OCN and minimal promotion of mineralization. These results suggest that BMP-2 triggers follicle cells to differentiate toward a cementoblast/osteoblast phenotype and that the MAPK pathway is involved.

Matrix gamma-carboxyglutamic acid protein is a key regulator of PTH-mediated inhibition of mineralization in MC3T3-E1 osteoblast-like cells

As part of its overall function as a major regulator of calcium homeostasis, PTH stimulates bone resorption and inhibits osteoblast-mediated biomineralization. To determine the basis for the inhibitory actions of this hormone, we compared the time course of PTH-dependent inhibition of mineralization in MC3T3-E1 osteoblast-like cells with changes in mRNA levels for several extracellular matrix proteins previously associated either with induction or inhibition of mineralization. Mineralizing activity was rapidly lost in PTH-treated cells ( approximately 30% inhibition after 3 h, 50% inhibition at 6 h). Of the proteins examined, changes in matrix gamma-carboxyglutamic acid protein were best correlated with PTH-dependent inhibition of mineralization. Matrix gamma-carboxyglutamic acid protein mRNA was rapidly induced 3 h after PTH treatment, with a 6- to 8-fold induction seen after 6 h. Local in vivo injection of PTH over the calvaria of mice also induced a 2-fold increase in matrix gamma-carboxyglutamic acid protein mRNA. Warfarin, an inhibitor of matrix gamma-carboxyglutamic acid protein gamma-carboxylation, reversed the effects of PTH on mineralization in MC3T3-E1 cells, whereas vitamin K enhanced PTH activity, as would be expected if a gamma-carboxyglutamic acid-containing protein were required for PTH activity. Levels of the other mRNAs examined were not well correlated with the observed changes in mineralization. Osteopontin, an in vitro inhibitor of mineralization, was induced approximately 4-fold 12 h after PTH addition. Bone sialoprotein mRNA, which encodes an extracellular matrix component most frequently associated with mineral induction, was inhibited by 50% after 12 h of PTH treatment. Osteocalcin mRNA, encoding the other known gamma-carboxyglutamic acid protein in bone, was also inhibited by PTH, but, again, with a significantly slower time course than was seen for mineral inhibition. Taken together, these results show that the rapid inhibition of osteoblast mineralization induced by in vitro PTH treatment is at least in part explained by induction of matrix gamma-carboxyglutamic acid protein.