Hormonal regulation of the production of collagenase and a collagenase inhibitor activity by rat osteogenic sarcoma cells

Abaloparatide Has the Same Catabolic Effects on Bones of Mice When Infused as PTH (1-34)

Abaloparatide is a peptide analog of parathyroid hormone-related protein (PTHrP 1-34) and was approved in 2017 as the second osteoanabolic peptide for treating osteoporosis. We previously showed that intermittent abaloparatide is equally as effective as PTH (1-34). This study was designed to compare the catabolic effects of PTH (1-34) and abaloparatide on bone in young female wild-type mice. Two-month-old C57Bl/6J female mice were continuously infused with human PTH (1-34) or abaloparatide at 80 μg/kg BW/day or vehicle for 2 weeks. At euthanasia, DEXA-PIXImus was performed to assess bone mineral density (BMD) in the whole body, femurs, tibiae, and vertebrae. Bone turnover marker levels were measured in sera, femurs were harvested for micro-computer tomography (μCT) analyses and histomorphometry, and tibiae were separated into cortical and trabecular fractions for gene expression analyses. Our results demonstrated that the infusion of abaloparatide resulted in a similar decrease in BMD as infused PTH (1-34) at all sites. μCT and histomorphometry analyses showed similar decreases in cortical bone thickness and BMD associated with an increase in bone turnover from the increased bone formation rate found by in vivo double labeling and serum P1NP and increased bone resorption as shown by osteoclast numbers and serum cross-linked C-telopeptide. Trabecular bone did not show major changes with either treatment. Osteoblastic gene expression analyses of trabecular and cortical bone revealed that infusion of PTH (1-34) or abaloparatide led to similar and different actions in genes of osteoblast differentiation and activity. As with intermittent and in vitro treatment, both infused PTH (1-34) and abaloparatide similarly regulated downstream genes of the PTHR1/SIK/HDAC4 pathway such as Sost and Mmp13 but differed for those of the PTHR1/SIK/CRTC pathway. Taken together, at the same dose, infused abaloparatide causes the same high bone turnover as infused PTH (1-34) with a net resorption in female wild-type mice. © 2023 The Authors. JBMR Plus published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research.

The phytochemical plumbagin reciprocally modulates osteoblasts and osteoclasts

Bone metabolism is essential for maintaining bone mineral density and bone strength through a balance between bone formation and bone resorption. Bone formation is associated with osteoblast activity whereas bone resorption is linked to osteoclast differentiation. Osteoblast progenitors give rise to the formation of mature osteoblasts whereas monocytes are the precursors for multi-nucleated osteoclasts. Chronic inflammation, auto-inflammation, hormonal changes or adiposity have the potential to disturb the balance between bone formation and bone loss. Several plant-derived components are described to modulate bone metabolism and alleviate osteoporosis by enhancing bone formation and inhibiting bone resorption. The plant-derived naphthoquinone plumbagin is a bioactive compound that can be isolated from the roots of the Plumbago genus. It has been used as traditional medicine for treating infectious diseases, rheumatoid arthritis and dermatological diseases. Reportedly, plumbagin exerts its biological activities primarily through induction of reactive oxygen species and triggers osteoblast-mediated bone formation. It is plausible that plumbagin's reciprocal actions - inhibiting or inducing death in osteoclasts but promoting survival or growth of osteoblasts - are a function of the synergy with bone-metabolizing hormones calcitonin, Parathormone and vitamin D. Herein, we develop a framework for plausible molecular modus operandi of plumbagin in bone metabolism.

Abaloparatide at the Same Dose Has the Same Effects on Bone as PTH (1-34) in Mice

Abaloparatide, a novel analog of parathyroid hormone-related protein (PTHrP 1-34), became in 2017 the second osteoanabolic therapy for the treatment of osteoporosis. This study aims to compare the effects of PTH (1-34), PTHrP (1-36), and abaloparatide on bone remodeling in male mice. Intermittent daily subcutaneous injections of 80 μg/kg/d were administered to 4-month-old C57Bl/6J male mice for 6 weeks. During treatment, mice were followed by DXA-Piximus to assess changes in bone mineral density (BMD) in the whole body, femur, and tibia. At either 4 or 18 hours after the final injection, femurs were harvested for μCT analyses and histomorphometry, sera were assayed for bone turnover marker levels, and tibias were separated into cortical, trabecular, and bone marrow fractions for gene expression analyses. Our results showed that, compared with PTH (1-34), abaloparatide resulted in a similar increase in BMD at all sites, whereas no changes were found with PTHrP (1-36). With both PTH (1-34) and abaloparatide, μCT and histomorphometry analyses revealed similar increases in bone volume associated with an increased trabecular thickness, in bone formation rate as shown by P1NP serum level and in vivo double labeling, and in bone resorption as shown by CTX levels and osteoclast number. Gene expression analyses of trabecular and cortical bone showed that PTH (1-34) and abaloparatide led to different actions in osteoblast differentiation and activity, with increased Runx2, Col1A1, Alpl, Bsp, Ocn, Sost, Rankl/Opg, and c-fos at different time points. Abaloparatide seems to generate a faster response on osteoblastic gene expression than PTH (1-34). Taken together, abaloparatide at the same dose is as effective as PTH (1-34) as an osteoanabolic, with an increase in bone formation but also an increase in bone resorption in male mice. © 2019 American Society for Bone and Mineral Research.

Matrix Metalloproteinases in Bone Resorption, Remodeling, and Repair

Matrix metalloproteinases (MMPs) are the major protease family responsible for the cleavage of the matrisome (global composition of the extracellular matrix (ECM) proteome) and proteins unrelated to the ECM, generating bioactive molecules. These proteins drive ECM remodeling, in association with tissue-specific and cell-anchored inhibitors (TIMPs and RECK, respectively). In the bone, the ECM mediates cell adhesion, mechanotransduction, nucleation of mineralization, and the immobilization of growth factors to protect them from damage or degradation. Since the first description of an MMP in bone tissue, many other MMPs have been identified, as well as their inhibitors. Numerous functions have been assigned to these proteins, including osteoblast/osteocyte differentiation, bone formation, solubilization of the osteoid during bone resorption, osteoclast recruitment and migration, and as a coupling factor in bone remodeling under physiological conditions. In turn, a number of pathologies, associated with imbalanced bone remodeling, arise mainly from MMP overexpression and abnormalities of the ECM, leading to bone osteolysis or bone formation. In this review, we will discuss the functions of MMPs and their inhibitors in bone cells, during bone remodeling, pathological bone resorption (osteoporosis and bone metastasis), bone repair/regeneration, and emergent roles in bone bioengineering.

Excessive dietary intake of vitamin A reduces skull bone thickness in mice

Calvarial thinning and skull bone defects have been reported in infants with hypervitaminosis A. These findings have also been described in humans, mice and zebrafish with loss-of-function mutations in the enzyme CYP26B1 that degrades retinoic acid (RA), the active metabolite of vitamin A, indicating that these effects are indeed caused by too high levels of vitamin A and that evolutionary conserved mechanisms are involved. To explore these mechanisms, we have fed young mice excessive doses of vitamin A for one week and then analyzed the skull bones using micro computed tomography, histomorphometry, histology and immunohistochemistry. In addition, we have examined the effect of RA on gene expression in osteoblasts in vitro. Compared to a standard diet, a high dietary intake of vitamin A resulted in a rapid and significant reduction in calvarial bone density and suture diastasis. The bone formation rate was almost halved. There was also increased staining of tartrate resistant acid phosphatase in osteocytes and an increased perilacunar matrix area, indicating osteocytic osteolysis. Consistent with this, RA induced genes associated with bone degradation in osteoblasts in vitro. Moreover, and in contrast to other known bone resorption stimulators, vitamin A induced osteoclastic bone resorption on the endocranial surfaces.

PTH Signaling and Epigenetic Control of Bone Remodeling

As our understanding of the mechanisms that govern bone development advance, the role of epigenetic modifications in these processes become increasingly evident. Interestingly, in parathyroid hormone (PTH)-induced bone metabolism and remodeling, recent evidence shows that PTH signaling employs a particular facet of the epigenetic machinery to elicit its desired effects. In this review, we briefly discuss the known epigenetic events occurring in cells of the osteoblast lineage. More specifically, we elaborate on current findings that reveal the utilization of histone deacetylating enzymes (HDACs) in PTH-regulated modulation of gene expression in bone.

The dynamic skeleton

Adding to its well-known roles in locomotion and calcium balance, the skeleton has recently been appreciated as a true endocrine organ. Bone remodeling, a highly dynamic process, requires synchronized activities and crosstalk between bone cells. Discovery and characterization of the Wnt/β catenin pathway in bone formation, FGF23 regulation of phosphate homeostasis and osteocalcin in energy and glucose homeostasis have reframed our view of the skeleton from simply a target tissue of the endocrine system to an endocrine tissue itself. This comprehensive review provides an overview of these complex pathways, their application to human bone disorders and implications for developing diagnostic and therapeutic targets.

Triclosan blocks MMP-13 expression in hormone-stimulated osteoblasts

BACKGROUND

Matrix metalloproteinase-13 (MMP-13) is an important enzyme for the modulation of bone turnover and gingival recession. Elevated levels of MMP-13 are associated with alveolar bone resorption, periodontal ligament breakdown, and gingival attachment loss, which are the clinical symptoms of periodontal disease. Evidence continues to suggest that periodontal disease contributes to oral tissue breakdown and is linked to numerous systemic conditions. Triclosan (TCN) is a long-standing, proven antibacterial and anti-inflammatory agent found in the only Food and Drug Administration-approved dentifrice for the treatment of plaque and gingivitis.

METHODS

This study examines the inhibitory effects of TCN on lipopolysaccharide-, parathyroid hormone (PTH)-, and prostaglandin E2 (PGE2)-induced expression of MMP-13 in UMR 106-01 cells, an osteoblastic osteosarcoma cell line. The cells were stimulated with PTH or PGE2 to induce MMP-13 mRNA expression, and real-time reverse transcription-polymerase chain reaction was performed to determine gene expression levels. Western blot analysis assessed the presence or absence of protein degradation or inhibition of protein synthesis. MMP-13 promoter reporter assay was used to explore possible direct effects of TCN on the MMP-13 promoter.

RESULTS

TCN significantly reduced PTH or PGE2 elevated expression of MMP-13 in osteoblastic cells without affecting basal levels of the mRNA. Surprisingly, TCN enhanced the expression of c-fos and amphiregulin mRNA. A promoter assay indicated that TCN directly inhibits the activation of the PTH-responsive minimal promoter of MMP-13.

CONCLUSION

The present study appears to have identified a nuclear mechanism of action of TCN that accounts for the ability of TCN to inhibit PTH- or PGE2-induced MMP-13 expression in osteoblastic cells.

A new insight into the dissociating effect of strontium on bone resorption and formation

Calcium phosphates are widely used as biomaterials and strontium (Sr) is known to have the ability to modify the bone balance towards osteosynthesis. In the present study we investigated the capacity of Sr-substituted sol-gel calcium phosphate to modify the expression of genes and proteins involved in extracellular matrix synthesis by primary bone cells. We first determined the most effective concentration of strontium using human primary bone cells. Sol-gel biphasic calcium phosphate (BCP) powders were then synthesised to obtain release of the optimal concentration of strontium. Finally, human osteoblasts obtained from explant cultures were cultured in the presence of sol-gel BCP, Sr-substituted BCP (5% Sr-substituted BCP, corresponding to a release of 5×10(-5)M [Sr(2+)] under the culture conditions (BCP(5%))) and medium containing strontium chloride (SrCl(2)). Viability, proliferation, cell morphology, protein production and protein activity were studied. We demonstrated that 5×10(-5)M SrCl(2) and BCP(5%) increased the expression of type I collagen and SERPINH1 mRNA and reduced the production of matrix metalloproteinases (MMP-1 and MMP-2) without modifying the levels of the tissue inhibitors of MMPs (TIMPs). Thus strontium has a positive effect on bone formation.

Cellular and molecular mechanisms of bone remodeling

Physiological bone remodeling is a highly coordinated process responsible for bone resorption and formation and is necessary to repair damaged bone and to maintain mineral homeostasis. In addition to the traditional bone cells (osteoclasts, osteoblasts, and osteocytes) that are necessary for bone remodeling, several immune cells have also been implicated in bone disease. This minireview discusses physiological bone remodeling, outlining the traditional bone biology dogma in light of emerging osteoimmunology data. Specifically discussed in detail are the cellular and molecular mechanisms of bone remodeling, including events that orchestrate the five sequential phases of bone remodeling: activation, resorption, reversal, formation, and termination.

HDAC4 represses matrix metalloproteinase-13 transcription in osteoblastic cells, and parathyroid hormone controls this repression

Parathyroid hormone (PTH) is a hormone regulating bone remodeling through its actions on both bone formation and bone resorption. Previously we reported that PTH induces matrix metalloproteinase-13 (MMP-13) transcription in osteoblastic cells. Here, we show that histone deacetylase 4 (HDAC4) interacts with Runx2, binds the MMP-13 promoter, and suppresses MMP-13 gene transcription in the rat osteoblastic cell line, UMR 106-01. PTH induces the rapid cAMP-dependent protein kinase-dependent release of HDAC4 from the MMP-13 promoter and subsequent transcription of MMP-13. Knock-out of HDAC4 either by siRNA in vitro or by gene deletion in vivo leads to an increase in MMP-13 expression, and overexpression of HDAC4 decreases the PTH induction of MMP-13. All of these observations indicate that HDAC4 represses MMP-13 gene transcription in bone. Moreover, PTH stimulates HDAC4 gene expression and enzymatic activity at times corresponding to the reassociation of HDAC4 with the MMP-13 promoter and a decline in its transcription. Thus, HDAC4 is a basal repressor of MMP-13 transcription, and PTH regulates HDAC4 to control MMP-13 promoter activity. These data identify a novel and discrete mechanism of regulating HDAC4 levels and, subsequently, gene expression.

Identification and characterization of Runx2 phosphorylation sites involved in matrix metalloproteinase-13 promoter activation

Matrix metalloproteinase-13 (MMP-13) plays a critical role in parathyroid hormone (PTH)-induced bone resorption. PTH acts via protein kinase A (PKA) to phosphorylate and stimulate the transactivation of Runx2 for MMP-13 promoter activation. We show here that PTH stimulated Runx2 phosphorylation in rat osteoblastic cells. Runx2 was phosphorylated on serine 28 and threonine 340 after 8-bromo cyclic adenosine mono phosphate (8-Br-cAMP) treatment. We further demonstrate that in the presence of 8-Br-cAMP, the wild-type Runx2 construct stimulated MMP-13 promoter activity, while the Runx2 construct having mutations at three phosphorylation sites (S28, S347 and T340) was unable to stimulate MMP-13 promoter activity. Thus, we have identified the Runx2 phosphorylation sites necessary for PKA stimulated MMP-13 promoter activation and this event may be critical for bone remodeling.

Bone remodelling: its local regulation and the emergence of bone fragility

Bone modelling prevents the occurrence of damage by adapting bone structure - and hence bone strength - to its loading circumstances. Bone remodelling removes damage, when it inevitably occurs, in order to maintain bone strength. This cellular machinery is successful during growth, but fails during advancing age because of the development of a negative balance between the volumes of bone resorbed and formed during remodelling by the basic multicellular unit (BMU), high rates of remodelling during midlife in women and late in life in both sexes, and a decline in periosteal bone formation. together resulting in bone loss and structural decay each time a remodelling event occurs. The two steps in remodelling - resorption of a volume of bone by osteoclasts and formation of a comparable volume by osteoblasts - are sequential, but the regulatory events leading to these two fully differentiated functions are not. Reparative remodelling is initiated by damage producing osteocyte apoptosis, which signals the location of damage via the osteocyte canalicular system to endosteal lining cells which forms the canopy of a bone-remodelling compartment (BRC). Within the BRC, local recruitment of osteoblast precursors from the lining cells, the marrow and circulation, direct contact with osteoclast precursors, osteoclastogenesis and molecular cross-talk between precursors, mature cells, cells of the immune system, and products of the resorbed matrix, titrate the birth, work and lifespan of the cells of this multicellular remodelling machinery to either remove or form a net volume of bone appropriate to the mechanical requirements.

Matrix metalloproteinases and bone

Matrix metalloproteinases (MMPs) are members of a family of zinc-dependent proteolytic enzymes. Several of the MMPs are expressed at high levels in bone and cartilage in mammals including humans and mice and are capable of cleaving native, undenatured collagens with long uninterrupted triple helices; these MMPs therefore potentially function as collagenases in vivo. Several MMPs expressed in the skeleton appear to function in endochondral ossification during embryonic development and in modeling and remodeling of bone postnatally and later in life. Different functions of MMPs have been elucidated through observations of spontaneous mutations in MMP genes in humans and of targeted mutations in Mmp genes and collagen (substrate) genes in mice. Potential mechanisms to account for effects of these mutations are considered in this review.

The roles of proteolytic enzymes in the development of tumour-induced bone disease in breast and prostate cancer

Tumour-induced bone disease is a common clinical feature of advanced breast and prostate cancer and is associated with considerable morbidity for the affected patients. Our understanding of the molecular mechanisms underlying the development of bone metastases is incomplete, but proteolytic enzymes are implicated in a number of processes involved in both bone metastasis and in normal bone turnover, including matrix degradation, cell migration, angiogenesis, tumour promotion and growth factor activation. Malignant as well as non-malignant cells in the primary and secondary sites express these enzymes, the activity of which may be regulated by soluble factors, cell- or matrix-associated components, as well as a number of cell signalling pathways. A number of secreted and cell surface-associated proteolytic enzymes are implicated in tumour-induced bone disease, including the matrix metalloproteinases, lysosomal cysteine proteinases and plasminogen activators. This review will introduce the role of proteolytic enzymes in normal bone turnover and give an overview of the studies in which their involvement and regulation in the development of bone metastases in breast and prostate cancer has been described. The results from trials involving protease inhibitors in clinical development will also be briefly discussed.

Stretch-induced modulation of matrix metalloproteinases in mineralizing osteoblasts via extracellular signal-regulated kinase-1/2

Matrix metalloproteinases (MMPs) and tissue inhibitors of metalloproteinases (TIMPs) produced by osteoblasts play an essential role in bone remodeling. Hence, these proteins could provide an interesting means by which mechanical loading leads to adaptation of bone. Here, we examined the effect of stretch on MMP-1, -2, -3, -8, -9, -13, and -14, as well as TIMP-1 and -2 gene expression in differentiating, mineralizing, and nonmineralizing human SV-40 immortalized preosteoblast cells. In the mineralizing osteoblast culture, but not in the nonmineralizing cultures, cyclic stretch for only 15 min resulted in an increase of MMP-1 (fourfold) and -3 (depending on differentiation stage up to 25-fold) transcript abundance. No clear effect was observed for other MMPs, TIMP-1 or -2. The increase of MMP-1 and -3 was confirmed on the protein level. Stretching experiments performed in the presence of a specific inhibitor of extracellular signal-regulated kinase (ERK) showed a strong suppression of the stretch-induced increase in MMP-1 and -3. In conclusion, we show that MMP-1 and MMP-3 are mechanosensitive genes in mineralizing the human osteoblast, and that the mechano-induction of these genes is mediated via the ERK pathway. Our findings implicate that these MMPs are important factors in the mechanoregulation of bone turnover. With the ability to generate MMPs at highly stretched sites, osteoblasts can potantially direct osteoclasts to specific bone surface areas prepared for resorption.

Regulation of matrix metalloproteinase (MMP) activity by the low-density lipoprotein receptor-related protein (LRP). A new function for an “old friend”

Matrix metalloproteinases (MMPs) are essential contributors to a microenvironment that promotes tumour progression. During the two last decades, inhibition of MMPs has become the focus of considerable interest for cancer therapy, and numerous synthetic metalloproteinase inhibitors have been developed by the pharmaceutical industry. However, clinical trials have shown disappointing efficacy or unexpected toxicity and new targets are thus eagerly awaited. The identification of endocytic clearance of several MMPs by the low-density lipoprotein receptor-related protein (LRP) might provide insight into novel strategies for controlling MMP level during malignant processes. This review attempts to summarize recent aspects on the cellular and molecular basis of LRP-mediated endocytic disposal of MMPs.

PGE2 induces the gene expression of bone matrix metalloproteinase-1 in mouse osteoblasts by cAMP-PKA signaling pathway

Prostaglandin E2 (PGE2), an abundant eicosanoid in bone, has been implicated in a number of pathological states associated with bone loss, and is also known to stimulate matrix metalloproteinase (MMP)-1 synthesis and secretion in rat and human osteoblast cells, although the nature of the intracellular reaction remains unclear. Although MMP-1 plays a critical role in bone-remodeling, it would be of interest to examine whether PGE2 regulates MMP-1 expression by mouse osteoblasts or not. Here we demonstrate that PGE2 is a potent inducer of MMP-1 production in fetal osteoblasts and show that PGE2 stimulates the activity of the MMP-1 promoter in osteoblasts, suggesting that PGE2 controls MMP-1 gene expression at least at the transcriptional level. PGE2 induced MMP-1 messenger RNA (mRNA) expression in the cells within 4 h, and this expression was maintained for 36 h. The increase in MMP-1 production with 0.1-2.0 microM PGE2 was dose-dependent. We also found that PGE2 (1.5 microM) up-regulated MMP-1 protein levels in cultured mouse osteoblasts, as evidenced by ELISA. To examine whether PGE2 mediated response and signal pathway are involved in the intracellular action, the PGE2-mediated expression of the MMP-1 gene was investigated in mouse osteoblast cells. A Northern blot analysis showed that PGE2 and PGE1 were potent stimulators of MMP-1 transcription, and the presence of thromboxane B2 had no effect. The increase in MMP-1 transcript after PGE2 treatment was observed at 4h, reaching a maximum at 6h, and persisted for 24h. This response was dose-dependent. Cycloheximide, an inhibitor of protein synthesis, completely blocked this effect by PGE2, indicating that the expression of other genes is also required. The second messenger analog, 8-bromo-cAMP, mimicked the effects of PGE2 by stimulating a dose-dependent increase in MMP-1 mRNA levels, with a maximal effect that was quantitatively similar to that observed with PGE2. Thus, the present results strongly suggest that the PGE2 stimulation of MMP-1 synthesis is due to the activation of MMP-1 gene transcription and a subsequent marked increase in MMP-1 transcription. This effect is dependent on de novo protein synthesis and is mimicked by protein kinase A activation. The findings suggest that PGE2 is involved in the cAMP-PKA signaling pathway in regulating MMP-1 gene expression in osteoblasts.

Cell lines and primary cell cultures in the study of bone cell biology

Bone is a metabolically active and highly organized tissue consisting of a mineral phase of hydroxyapatite and amorphous calcium phosphate crystals deposited in an organic matrix. Bone has two main functions. It forms a rigid skeleton and has a central role in calcium and phosphate homeostasis. The major cell types of bone are osteoblasts, osteoclasts and chondrocytes. In the laboratory, primary cultures or cell lines established from each of these different cell types provide valuable information about the processes of skeletal development, bone formation and bone resorption, leading ultimately, to the formulation of new forms of treatment for common bone diseases such as osteoporosis.

Parathyroid hormone: a double-edged sword for bone metabolism

Parathyroid hormone (PTH) is the major hormone regulating calcium metabolism. It is also the only FDA-approved drug for osteoporosis treatment that stimulates bone formation when injected daily. However, continuous infusion of PTH causes severe bone loss in line with its known catabolic effects. Many studies to understand the dual effects of PTH have been carried out, and in recent years a growing number of molecular and cellular mechanisms underlying these effects have emerged. Here, we outline the present knowledge and conclude that the kinetics of administration and subsequent signaling probably account for the divergent actions of the hormone.

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.

Transcriptional regulation of collagenase-3 by interleukin-1 alpha in osteoblasts

Interleukin-1 (IL-1)alpha is an autocrine/paracrine agent of the skeletal tissue and it regulates bone remodeling. Collagenase-3 or matrix metalloproteinase (MMP)-13 is expressed in osteoblasts and its expression is modulated by several cytokines including IL-1alpha. Because the molecular mechanism of increased synthesis of collagenase-3 in bone cells by IL-1alpha is not known, we investigated if collagenase-3 expression by IL-1alpha in osteoblasts is mediated by transcriptional or post-transcriptional mechanisms. Exposure of rat osteoblastic cultures (Ob cells) to IL-1alpha at concentrations higher than 0.5 nM increased the synthesis of collagenase-3 mRNA up to eightfold and the secretion of immunoreactive protein up to 21-fold. The effects of IL-1alpha on collagenase-3 were time- and dose-dependent. Although prostaglandins stimulate collagenase-3 expression, stimulation of collagenase-3 in Ob cells by IL-1alpha was not mediated through increased biosynthesis of prostaglandins. The half-life of collagenase-3 mRNA from control and IL-1alpha-treated Ob cells was similar suggesting that the stabilization of collagenase-3 mRNA did not contribute to the increase in collagenase-3. However, IL-1alpha stimulated the rate of transcription of the collagenase-3 gene by twofold to fourfold indicating regulation of collagenase-3 expression in Ob cells at the transcriptional level. Stimulation of collagenase-3 by IL-1alpha in osteoblasts may in part mediate the effects of IL-1alpha in bone metabolism.

Intracellular membrane trafficking in bone resorbing osteoclasts

There is ample evidence now that the two major events in bone resorption, namely dissolution of hydroxyapatite and degradation of the organic matrix, are performed by osteoclasts. The resorption cycle involves several specific cellular activities, where intracellular vesicular trafficking plays a crucial role. Although details of these processes started to open up only recently, it is clear that vesicular trafficking is needed in several specific steps of osteoclast functioning. Several plasma membrane domains are formed during the polarization of the resorbing cells. Multinucleated osteoclasts create a tight sealing to the extracellular matrix as a first indicator of their resorption activity. Initial steps of the sealing zone formation are alpha(v)beta(3)-integrin mediated, but the final molecular interaction(s) between the plasma membrane and mineralized bone matrix is still unknown. A large number of acidic intracellular vesicles then fuse with the bone-facing plasma membrane to form a ruffled border membrane, which is the actual resorbing organelle. The formation of a ruffled border is regulated by a small GTP-binding protein, rab7, which indicates the late endosomal character of the ruffled border membrane. Details of specific membrane transport processes in the osteoclasts, e.g., the formation of the sealing zone and transcytosis of bone degradation products from the resorption lacuna to the functional secretory domain remain to be clarified. It is tempting to speculate that specific features of vesicular trafficking may offer several potential new targets for drug therapy of bone diseases.

Macrophage migration inhibitory factor up-regulates matrix metalloproteinase-9 and -13 in rat osteoblasts. Relevance to intracellular signaling pathways

Neutral matrix metalloproteinases (MMPs) play an important role in bone matrix degradation accompanied by bone remodeling. We herein show for the first time that macrophage migration inhibitory factor (MIF) up-regulates MMP-13 (collagenase-3) mRNA of rat calvaria-derived osteoblasts. The mRNA up-regulation was seen at 3 h in response to MIF (10 microg/ml), reached the maximum level at 6-12 h, and returned to the basal level at 36 h. MMP-13 mRNA up-regulation was preceded by up-regulation of c-jun and c-fos mRNA. Tissue inhibitor of metalloproteinase (TIMP)-1 and MMP-9 (92-kDa type IV collagenase) were also up-regulated, but to a lesser extent. The MMP-13 mRNA up-regulation was significantly suppressed by genistein, herbimycin A and 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine. Similarly, a selective mitogen-activated protein kinase (MAPK) kinase (MEK)1/2 inhibitor (PD98059) and c-jun/activator protein (AP)-1 inhibitor (curcumin) suppressed MMP-13 mRNA up-regulation induced by MIF. The mRNA levels of c-jun and c-fos in response to MIF were also inhibited by PD98059. Consistent with these results, MIF stimulated phosphorylation of tyrosine, autophosphorylation of Src, activation of Ras, activation of extracellular signal-regulated kinases (ERK) 1/2, a MAPK, but not c-Jun N-terminal kinase or p38, and phosphorylation of c-Jun. Osteoblasts obtained from calvariae of newborn JunAA mice, defective in phosphorylation of c-Jun, or newborn c-Fos knockout (Fos -/- ) mice, showed much less induction of MMP-13 with the addition of MIF than osteoblasts obtained from wild-type or littermate control mice. Taken together, these results suggest that MIF increases the MMP-13 mRNA level of rat osteoblasts via the Src-related tyrosine kinase-, Ras-, ERK1/2-, and AP-1-dependent pathway.

The expression of metalloproteinase-2, -9, and -14 and of tissue inhibitors-1 and -2 is developmentally modulated during osteogenesis in vitro, the mature osteoblastic phenotype expressing metalloproteinase-14

During osteogenesis, in vitro, of tibial-derived rat osteoblasts (ROB) and derived clones, changes occur in the interactions of mature osteoblasts with the endogenous extracellular matrix (ECM) and these culminate in the formation of tridimensional nodules, which become sites of mineral deposition. We investigated if these changes might be mediated by remodeling of ECM, and we focused our study on the neutral metalloproteinases (MMPs), known agents of matrix remodeling, and on their tissue inhibitors (TIMPs). We report that during in vitro differentiation, osteoblasts express the secreted MMP-2 and -9 and the membrane gelatinase MMP-14. These, along with the tissue inhibitors TIMP-1 and -2, are developmentally regulated according to the maturation stage of osteoblasts. Their levels change in a similar association with osteoblast phenotypic maturation in different populations of ROB, which take different times to complete osteogenesis in vitro. MMP-14 expression coincides in both cell populations with the mature osteoblastic phenotype and is localized in the cells forming nodules. MMP-2 and -9 are expressed diffusely in the osteoblast population. Developmentally associated changes in the activation of MMP-2 are detected, associated in their timing with the expression of MMP-14 in both populations of ROB, and MMP-14 activates pro-MMP-2 in vitro. Expression of messenger RNAs (mRNAs) for the three MMPs increases up to the time of nodule formation. At this stage, TIMP-1 mRNA levels are lowest. TIMP-2 mRNA decreases throughout osteogenesis. In situ hybridization in 7-day-old rat tibias shows the strongest expression of MMP-14 among osteogenic cells, in lining osteoblasts on the newly formed trabeculae under the growth plate, and on the endosteal surface of cortical bone. Our data support the concept that the developmentally regulated expression of MMP-14 triggers localized proteolysis within the osteogenic population, concomitant in vitro to nodule formation.

Osteocyte and osteoblast apoptosis and excessive bone deposition accompany failure of collagenase cleavage of collagen

Mice carrying a targeted mutation (r) in Col1a1, encoding a collagenase-resistant form of type I collagen, have altered skeletal remodeling. In hematoxylin and eosin-stained paraffin sections, we detect empty lacunae in osteocytes in calvariae from Col1a1(r/r) mice at age 2 weeks, increasing through age 10-12 months. Empty lacunae appear to result from osteocyte apoptosis, since staining of osteocytes/periosteal osteoblasts with terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling is increased in Col1a1(r/r) relative to wild-type bones. Osteocyte perilacunar matrices stained with Ab that recognizes collagenase collagen alpha1(I) chain cleavage ends in wild-type but not Col1a1(r/r) calvariae. Increased calvarial periosteal and tibial/femoral endosteal bone deposition was found in Col1a1(r/r) mice from ages 3-12 months. Calcein labeling of calvarial surfaces was increased in Col1a1(r/r) relative to wild-type mice. Daily injections of synthetic parathyroid hormone for 30 days increased calcein-surface labeling in wild-type but caused no further increase in the already high calcein staining of Col1a1(r/r) bones. Thus, failure of collagenase cleavage of type I collagen in Col1a1(r/r) mice is associated with osteocyte/osteoblast death but increases bone deposition in a manner that mimics the parathyroid hormone-induced bone surface activation seen in wild-type mice.

Effects of dioxin and estrogen on collagenase-3 in UMR 106-01 osteosarcoma cells

Since estrogen is important in preventing osteoporosis in postmenopausal women and 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) is an estrogen antagonist in reproductive tissues, we investigated the effects of 17beta-estradiol (E2) and TCDD on collagenase-3 secretion using parathyroid hormone (PTH)-stimulated UMR 106-01 cells, a rat osteoblastic osteosarcoma cell line. Whereas E2 or TCDD had no effect on UMR cells in the absence of PTH, cells grown in the presence of 10(-7) M PTH, which induces a dramatic 30-fold increase in collagenase-3 secretion, surprisingly demonstrated a further stimulation of collagenase-3 secretion in the presence of TCDD or E2. However, the potentiating response was biphasic; i.e., at higher concentrations of E2 or TCDD, there was no enhancement of the PTH effect. PTH induces multiple effects on UMR cells, including inducing collagenase-3 mRNA transcription and regulating its extracellular abundance through a specific receptor and endocytosis. Thus, we investigated the ability of TCDD or E2 to stimulate the induction of collagenase-3 mRNA using Northern analysis. As previously reported, PTH dose dependently induced collagenase-3 mRNA after 4 h of treatment. There was little effect of TCDD or E2 on PTH-induced levels of collagenase-3 mRNA. These data could not account for the final effects on secreted collagenase-3. We postulated that low concentrations of E2 and TCDD may downregulate the collagenase-3 endocytotic two-step receptor-mediated process that includes the LDL-receptor-related protein to enhance the effects of PTH. However, this was not the case. Therefore, we conclude that low concentrations of TCDD and estrogen alter translation or secretion of PTH-stimulated collagenase-3.

Mesenchymal stem cells from osteoporotic patients produce a type I collagen-deficient extracellular matrix favoring adipogenic differentiation

Mesenchymal stem cells (MSCs), precursor cells resident in the bone marrow, have the capacity to differentiate into bone, cartilage, fat, and connective tissue. We have recently reported that MSCs from "healthy" donors differ from cells obtained from osteoporotic postmenopausal women in their proliferation rate, mitogenic response to osteogenic growth factors, and potential to mineralize. The purpose of this study was to examine the factors that explain the differential capacity of MSCs derived from "healthy" control and osteoporotic postmenopausal women to support mineralization. In addition, we examined the factors that regulate the differentiation of osteoporotic cells into adipocytes. For this purpose, we isolated MSCs from bone marrow of donors and analyzed the synthesis and deposition of type I collagen, the main component of bone extracellular matrix, the time course of gelatinolytic activity expression, the deposition of transforming growth factor beta (TGF-beta), and the ability of cells to differentiate into adipocytes. Our results indicate that cells derived from osteoporotic donors synthesized 50% less type I collagen than normal cells and maintained higher levels of gelatinolytic activity under differentiation conditions (70% versus 15% after 14 days in culture). MSCs derived from osteoporotic women produced 60-65% less TGF-beta and expressed higher adipogenic capacity. We conclude that the capacity of MSCs derived from osteoporotic postmenopausal women to generate and maintain type I collagen-rich extracellular matrix is decreased, favoring their adipogenic differentiation. These observations may explain the decreased mineralization previously observed in these types of cells.

Developmental regulation of collagenase-3 mRNA in normal, differentiating osteoblasts through the activator protein-1 and the runt domain binding sites

Collagenase-3 mRNA is initially detectable when osteoblasts cease proliferation, increasing during differentiation and mineralization. We showed that this developmental expression is due to an increase in collagenase-3 gene transcription. Mutation of either the activator protein-1 or the runt domain binding site decreased collagenase-3 promoter activity, demonstrating that these sites are responsible for collagenase-3 gene transcription. The activator protein-1 and runt domain binding sites bind members of the activator protein-1 and core-binding factor family of transcription factors, respectively. We identified core-binding factor a1 binding to the runt domain binding site and JunD in addition to a Fos-related antigen binding to the activator protein-1 site. Overexpression of both c-Fos and c-Jun in osteoblasts or core-binding factor a1 increased collagenase-3 promoter activity. Furthermore, overexpression of c-Fos, c-Jun, and core-binding factor a1 synergistically increased collagenase-3 promoter activity. Mutation of either the activator protein-1 or the runt domain binding site resulted in the inability of c-Fos and c-Jun or core-binding factor a1 to increase collagenase-3 promoter activity, suggesting that there is cooperative interaction between the sites and the proteins. Overexpression of Fra-2 and JunD repressed core-binding factor a1-induced collagenase-3 promoter activity. Our results suggest that members of the activator protein-1 and core-binding factor families, binding to the activator protein-1 and runt domain binding sites are responsible for the developmental regulation of collagenase-3 gene expression in osteoblasts.

Induction of rat interstitial collagenase (MMP-13) mRNA in a development-dependent manner by parathyroid hormone in osteoblastic cells

The purpose of this study was to determine whether the production of interstitial collagenase mRNA in response to parathyroid hormone (PTH) changes with osteoblast phenotypic development. To accomplish this, cells derived from fetal rat calvaria were examined. The calvarial osteoblasts, which proliferate when placed in culture, can be made to differentiate after confluence. Studies were performed on cells while they were proliferating, at confluence, and during the differentiation process. The cells were treated with PTH for various times, and interstitial collagenase mRNA was quantified by RNase protection assay. We concluded that the ability of PTH to induce interstitial collagenase mRNA in these cells increased with osteoblast phenotypic development. We also determined that the response could be mimicked by combining the effect of 8-bromo-cAMP and 12-O-tetradecanoyl-phorbol-13-acetate, stimulators of the protein kinase A and protein kinase C pathways, respectively, both known to be activated by PTH. The binding of nuclear factors to two regions previously reported to be important for PTH induction of the gene in UMR 106-01 cells was also examined. These data indicated that the binding of nuclear factors to oligonucleotides encompassing the TRE (-51) or the PEA3 (-80) elements changed with development of the osteoblast phenotype. The latter was also shown to be PTH responsive.

Proteinases in bone resorption: obvious and less obvious roles

Bone resorption is critical for the development and the maintenance of the skeleton, and improper regulation of bone resorption leads to pathological situations. Proteinases are necessary for this process. In this review, we show that this need of proteinases is not only because they are required for the solubilization of bone matrix, but also because they are key components of the mechanism that determines where and when bone resorption will be initiated. Moreover, there are indications that proteinases may also determine whether resorption will be followed by bone formation. Some of the proteinases involved in these different steps of the resorption processes were recently identified, as for instance cathepsin K, MMP-9 (gelatinase B), and interstitial collagenase. However, there is also increasing evidence showing that the critical proteinase(s) may vary depending on the bone type or on other factors.

Regulation of expression of collagenase-3 in normal, differentiating rat osteoblasts

We investigated the regulation of collagenase-3 expression in normal, differentiating rat osteoblasts. Fetal rat calvarial cell cultures showed an increase in alkaline phosphatase activity reaching maximal levels between 7-14 days post-confluence, then declining with the onset of mineralization. Collagenase-3 mRNA was just detectable after proliferation ceased at day 7, increased up to day 21, and declined at later ages. Postconfluent cells maintained in non-mineralizing medium expressed collagenase-3 but did not show the developmental increase exhibited by cells switched to mineralization medium. Cells maintained in non-mineralizing medium continued to proliferate; cells in mineralization medium ceased proliferation. In addition, collagenase-3 mRNA was not detected in subcultured cells allowed to remineralize. These results suggest that enhanced accumulation of collagenase-3 mRNA is triggered by cessation of proliferation or acquisition of a mineralized extracellular matrix and that other factors may also be required. After initiation of basal expression, parathyroid hormone (PTH) caused a dose-dependent increase in collagenase-3 mRNA. Both the cyclic adenosine monophosphate (cAMP) analogue, 8-bromo-cAMP (8-Br-cAMP), and the protein kinase C (PKC) activator, phorbol myristate acetate, increased collagenase-3 expression, while the calcium ionophore, ionomycin, did not, suggesting that PTH was acting through the protein kinase A (PKA) and PKC pathways. Inhibition of protein synthesis with cycloheximide caused an increase in basal collagenase-3 expression but blocked the effect of PTH, suggesting that an inhibitory factor prevents basal expression while an inductive factor is involved with PTH action. In summary, collagenase-3 is expressed in mineralized osteoblasts and cessation of proliferation and initiation of mineralization are triggers for collagenase-3 expression. PTH also stimulates expression of the enzyme through both PKA and PKC pathways in the mineralizing osteoblast.

Collagenase-3 binds to a specific receptor and requires the low density lipoprotein receptor-related protein for internalization

We have previously identified a specific receptor for collagenase-3 that mediates the binding, internalization, and degradation of this ligand in UMR 106-01 rat osteoblastic osteosarcoma cells. In the present study, we show that collagenase-3 binding is calcium-dependent and occurs in a variety of cell types, including osteoblastic and fibroblastic cells. We also present evidence supporting a two-step mechanism of collagenase-3 binding and internalization involving both a specific collagenase-3 receptor and the low density lipoprotein receptor-related protein. Ligand blot analysis shows that (125)I-collagenase-3 binds specifically to two proteins ( approximately 170 kDa and approximately 600 kDa) present in UMR 106-01 cells. Western blotting identified the 600-kDa protein as the low density lipoprotein receptor-related protein. Our data suggest that the 170-kDa protein is a specific collagenase-3 receptor. Low density lipoprotein receptor-related protein-null mouse embryo fibroblasts bind but fail to internalize collagenase-3, whereas UMR 106-01 and wild-type mouse embryo fibroblasts bind and internalize collagenase-3. Internalization, but not binding, is inhibited by the 39-kDa receptor-associated protein. We conclude that the internalization of collagenase-3 requires the participation of the low density lipoprotein receptor-related protein and propose a model in which the cell surface interaction of this ligand requires a sequential contribution from two receptors, with the collagenase-3 receptor acting as a high affinity primary binding site and the low density lipoprotein receptor-related protein mediating internalization.

Triiodothyronine induces collagenase-3 and gelatinase B expression in murine osteoblasts

Triiodothyronine (T3) increases bone resorption, but its effects on matrix metalloprotease (MMP) expression in bone are unknown. We tested the effects of T3 on collagenase-3 and gelatinase A and B expression in MC3T3 osteoblastic cells. T3 at 1 nM to 1 microM for 24-72 h increased collagenase-3 and gelatinase B mRNA levels, but it did not increase gelatinase A transcripts. In addition, T3 increased immunoreactive collagenase and gelatinase activity. Cycloheximide prevented the stimulatory effect of T3 on collagenase-3 but not on gelatinase B transcripts. Indomethacin did not prevent the effect of T3 on either MMP. T3 did not alter the decay of collagenase-3 or gelatinase B mRNA in transcriptionally arrested MC3T3 cells, and it increased the rate of collagenase-3 and gelatinase B gene transcription. Although T3 enhanced the expression of the tissue inhibitor of metalloproteinase-1 in MC3T3 cells, it increased collagen degradation in cultured intact rat calvariae. In conclusion, T3 increases collagenase-3 and gelatinase B synthesis in osteoblasts by transcriptional mechanisms. This effect may contribute to the actions of T3 on bone matrix remodeling.

Intracellular measurement of prostaglandin E2: effect of anti-inflammatory drugs on cyclooxygenase activity and prostanoid expression

Cyclooxygenase (COX) converts arachidonic acid to prostaglandin (PG) H2, which is further metabolized to various prostaglandins, prostacyclin and thromboxane A2. COX exists in at least two different isoforms. COX-1 is constitutively expressed, whereas COX-2 is induced by proinflammatory stimuli. Prostaglandin E2 is a major metabolite of COX activation. In order to compare the activity of target ligands and COX inhibitors on PGE2 synthesis and release, the responsiveness of several cell lines to the calcium ionophore A23187, bacterial lipopolysaccharide (LPS), nonsteroidal anti-inflammatory drugs (NSAIDs), and the glucocorticoid, dexamethasone, were investigated. For intracellular measurements, the culture supernatant was aspirated, and the cells were thoroughly washed and lysed with dodecyltrimethylammonium bromide. Intracellular and secreted PGE2 were measured with an enzyme immunoassay. A23187 and LPS increased intracellular PGE2 in a dose-dependent manner. Kinetic experiments with A23187-stimulated mouse 3T3 fibroblast cells revealed a distinct biphasic response in COX activity. In the presence of NSAIDs or dexamethasone, there was a dose-dependent inhibition in intracellular PGE2 with A23187-stimulated 3T3 cells. Inhibitory studies demonstrated an apparent increased sensitivity of COX activity to the action of inhibitors when measuring intracellular PGE2 compared with using cell culture supernatants. Indeed, intracellular PGE2 levels were comprehensively reduced in the presence of low concentrations of inhibitor. The utilization of cell culture lysates and, in particular, measurement of intracellular PGE2 should prove useful for identifying new COX inhibitors.

Bone resorption induced by parathyroid hormone is strikingly diminished in collagenase-resistant mutant mice

Parathyroid hormone (PTH) stimulates bone resorption by acting directly on osteoblasts/stromal cells and then indirectly to increase differentiation and function of osteoclasts. PTH acting on osteoblasts/stromal cells increases collagenase gene transcription and synthesis. To assess the role of collagenase in the bone resorptive actions of PTH, we used mice homozygous (r/r) for a targeted mutation (r) in Col1a1 that are resistant to collagenase cleavage of type I collagen. Human PTH(1-34) was injected subcutaneously over the hemicalvariae in wild-type (+/+) or r/r mice four times daily for three days. Osteoclast numbers, the size of the bone marrow spaces and periosteal proliferation were increased in calvariae from PTH-treated +/+ mice, whereas in r/r mice, PTH-induced bone resorption responses were minimal. The r/r mice were not resistant to other skeletal effects of PTH because abundant interstitial collagenase mRNA was detected in the calvarial periosteum of PTH-treated, but not vehicle-treated, r/r and +/+ mice. Calcemic responses, 0.5-10 hours after intraperitoneal injection of PTH, were blunted in r/r mice versus +/+ mice. Thus, collagenase cleavage of type I collagen is necessary for PTH induction of osteoclastic bone resorption.

Skeletal bone morphogenetic proteins suppress the expression of collagenase-3 by rat osteoblasts

Bone morphogenetic proteins (BMPs) are secreted by skeletal cells, induce the differentiation of mesenchymal cells into cells of the osteoblastic lineage, and increase their differentiated function. BMPs also decrease collagenase-3 expression by the osteoblast. We tested the autocrine role of BMPs on collagenase-3 expression in osteoblast-enriched cells from fetal rat calvariae (Ob cells) by examining the effects of noggin, a specific inhibitor of BMP binding and function. Although collagenase-3 transcript expression declined in untreated Ob cells in culture over a 24-h period, BMP-2, -4, and -6 decreased collagenase-3 messenger RNA levels in cells treated for 2-24 h. The addition of noggin prevented the decrease of collagenase-3 transcripts in control cultures, opposed the inhibitory actions of BMP-2, and increased the levels of the protease in the culture medium. Noggin did not alter the decay of collagenase-3 messenger RNA in transcriptionally arrested cells, and it increased the levels of collagenase-3 heterogeneous nuclear RNA in Ob cells. In conclusion, noggin enhances the synthesis of collagenase-3 in osteoblasts, supporting the notion that BMPs act as autocrine suppressors of collagenase-3 in skeletal cells, an effect that may contribute to the maintenance of the bone matrix.

Hydroxyapatite induces autolytic degradation and inactivation of matrix metalloproteinase-1 and -3

In the course of studies to identify a protease capable of producing a long-lived 50 kDa fragment of bone acidic glycoprotein-75 (BAG-75), it was observed that incubation of matrix metalloproteinase (MMP)-3 (stromelysin 1) with preparations of BAG-75 led to inactivation of proteolytic function, e.g., an inability to fragment 125I-labeled BAG-75 added subsequently. MMP-1 (interstitial collagenase) was also inactivated by exposure to BAG-75 preparations. Investigation of the mechanism revealed that BAG-75 preparations contained millimolar levels of inorganic phosphate which formed hydroxyapatite crystals under digestion conditions. Hydroxyapatite crystals alone and in BAG-75-hydroxyapatite complexes induced the autolytic degradation of both active and precursor forms of MMP-1 and MMP-3. Autolytic degradation in the presence of hydroxyapatite was demonstrated by a loss in catalytic function assayed with peptide and/or protein substrates, and, by fragmentation into polypeptides of <10 kDa. The fate of MMP-3 incubated with hydroxyapatite depends upon the time of incubation, the free calcium concentration, and the concentration of crystals. Specifically, hydroxyapatite-induced autolysis requires a near physiological free calcium concentration of 0.5-1.0 mM. Autolysis was maximal in the presence of 150 microg/ml hydroxyapatite where MMP-3 was only partially bound to crystals. However, autolysis also occurred at higher crystal concentrations where all input MMP-3 was bound (>1000 microg/ml), suggesting that autolysis may be mediated by bound enzyme. The effect of hydroxyapatite appears to be specific for MMP-1 and MMP-3 since the catalytic activity of chymotrypsin, trypsin, papain, and thermolysin remained unchanged after exposure to hydroxyapatite. These results document for the first time a novel catalytic role for hydroxyapatite crystals in vitro and provide an initial biochemical characterization of the intermolecular, autolytic, calcium ion-dependent, matrix metalloproteinase-specific degradative mechanism.

Dual regulation of stromelysin-3 by fibroblast growth factor-2 in murine osteoblasts

Osteoblasts express stromelysin-3, a matrix metalloproteinase associated with normal remodeling processes and with stromal fibroblasts surrounding many invasive carcinomas. Fibroblast growth factors (FGFs) play an important role in skeletal development, fracture repair, and osteoblast function. The osteoblastic cell line MC3T3 was used to study the regulation of stromelysin-3 by FGF-2. Acutely, FGF-2 decreased stromelysin-3 mRNA levels, whereas prolonged treatment caused an induction of stromelysin-3 mRNA. RNA stability studies and nuclear run-off assays indicated that acute treatment with FGF-2 decreased stromelysin-3 mRNA stability but did not alter gene transcription. However, the induction of stromelysin-3 after prolonged treatment with FGF-2 resulted from increased gene transcription, with no effect on RNA stability. The stimulatory effect was protein synthesis-dependent, whereas the inhibitory effect was not. This study demonstrates dual regulation of stromelysin-3 by FGF-2: acute destabilization of stromelysin-3 mRNA, followed by induction of gene transcription. This complex regulation may be important in the function of stromelysin-3 in bone and in remodeling processes, such as wound and fracture repair.

Parathyroid hormone regulates the rat collagenase-3 promoter in osteoblastic cells through the cooperative interaction of the activator protein-1 site and the runt domain binding sequence

Parathyroid hormone induces collagenase-3 gene transcription in rat osteoblastic cells. Here, we characterized the basal, parathyroid hormone regulatory regions of the rat collagenase-3 gene and the proteins involved in this regulation. The minimal parathyroid hormone-responsive region was observed to be between base pairs -38 and -148. Deleted and mutated constructs showed that the activator protein-1 and the runt domain binding sites are both required for basal expression and parathyroid hormone activation of this gene. The runt domain site is identical to an osteoblast-specific element-2 or acute myelogenous leukemia binding sequence in the mouse and rat osteocalcin genes, respectively. Overexpression of an acute myelogenous leukemia-1 repressor protein inhibited parathyroid hormone activation of the promoter, indicating a requirement of acute myelogenous leukemia-related factor(s) for this activity. Overexpression of c-Fos, c-Jun, osteoblast-specific factor-2, and core binding factor-beta increased the response to parathyroid hormone of the wild type (-148) promoter but not with mutation of either or both the activator protein-1 and runt domain binding sites. In summary, we conclude that there is a cooperative interaction of acute myelogenous leukemia/polyomavirus enhancer-binding protein-2-related factor(s) binding to the runt domain binding site with members of the activator protein-1 transcription factor family binding to the activator protein-1 site in the rat collagenase-3 gene in response to parathyroid hormone in osteoblastic cells.

Regulation of matrix metalloproteinases (MMP-2, -3, -9, and -13) by interleukin-1 and interleukin-6 in mouse calvaria: association of MMP induction with bone resorption

Interleukin-1 (IL-1) greatly induces osteoclast formation and stimulates bone resorption of mouse calvaria in culture. In the presence of soluble IL-6 receptor (sIL-6R), IL-6 similarly induces osteoclast formation, but the potency of IL-6 in inducing bone resorption in organ culture is weaker than that of IL-1. To study the differences in bone-resorbing activity between IL-1 and IL-6, we examined the effects of the two cytokines on the induction of matrix metalloproteinases (MMPs). In mouse calvarial cultures, IL-1 markedly enhanced the messenger RNA (mRNA) expression of MMP-13 (collagenase 3), MMP-2 (gelatinase A), MMP-9 (gelatinase B), and MMP-3 (stromelysin 1), which associated with increases in bone matrix degradation. A hydroxamate inhibitor of MMPs significantly suppressed bone-resorbing activity induced by IL-1. Gelatin zymography showed that both pro- and active-forms of MMP-2 and MMP-9 were detected in the conditioned medium collected from calvarial cultures, and IL-1 markedly stimulated both pro- and active-forms of the two gelatinases. IL-6 with sIL-6R also stimulated mRNA expression and biological activities of these MMPs, but the potency was much weaker than that of IL-1. Conditioned medium collected from IL-1-treated calvariae degraded native type I collagen, but 3/4- and 1/4-length collagen fragments were not detected, suggesting that both collagenases and gelatinases synergistically degraded type I collagen into smaller fragments. In mouse osteoblastic cells, the expression ofMMP-2, MMP-3, and MMP-13 mRNAs could be detected, and they were markedly enhanced by IL-1alpha on days 2 and 5. IL-6 with sIL-6R also induced expression of MMP-13 and MMP-2 mRNAs on day 2, but the expression was rather transient. These results demonstrate that the potency of induction of MMPs by IL-1 and IL-6 is closely linked to the respective bone-resorbing activity, suggesting that MMP-dependent degradation of bone matrix plays a key role in bone resorption induced by these cytokines.

Cytokines stimulate matrix metalloproteinase production by human pulp cells during long-term culture

Interleukin-1 and tumor necrosis factor-alpha are inflammatory cytokines that are known to be potent stimulators of mineralized tissue resorption. One of the mechanisms by which these cytokines induce this loss is through the stimulation of matrix metalloproteinase (MMP) production and secretion by the host cells present at the inflammatory site. We have previously shown that these cytokines have little effect on MMP production by human pulp cells in short-term culture (24 to 48 h). In this study, we examined the production of MMPs by human pulp cells in the presence and absence of interleukin-1 and tumor necrosis factor-alpha in long-term cultures (2 to 16 days) using substrate gel zymography. The major band present in all samples examined migrated at 68 kDa, corresponding to the migration pattern of MMP-2, whereas a minor band migrated at 90 kDa, corresponding to the migration pattern of MMP-9. In the presence of cytokines, elevated levels of MMP-2 and MMP-9 were apparent at days 9 through 16. In addition, a band migrating at 110 kDa was present. This study demonstrates that cytokines stimulate the production of elevated levels of MMPs by human pulp cells in long-term cultures and that these MMPs may play a role in pulpal inflammation.

The migration of purified osteoclasts through collagen is inhibited by matrix metalloproteinase inhibitors

The most obvious proteolytic event controlled by the osteoclast is bone matrix removal in the resorption compartment. Here, however, we investigated whether matrix metalloproteinase (MMP) activity of the osteoclast might be involved in its migration to its future bone resorption site. We seeded either nonpurified or purified osteoclasts onto either uncoated or collagen-coated dentine slices and cultured them in the presence or absence of specific MMP inhibitors. When nonpurified osteoclasts were cultured on uncoated dentine, MMP inhibitors did not prevent pit formation, as previously reported. However, when collagen-coated dentine was used, pit formation was strongly inhibited by MMP inhibitors. The same results were obtained when performing these experiments with purified osteoclasts, thus demonstrating the ability of osteoclasts by themselves to migrate through collagen via an MMP-dependent pathway. This demonstration was confirmed by using collagen-coated invasion chambers. In addition, the invasions were not, or only slightly, inhibited by inhibitors of serine proteinases, cysteine proteinases, and carbonic anhydrase, though the latter two are well established bone resorption inhibitors that strongly inhibited pit formation. It is concluded that osteoclasts can migrate through collagen in the absence of other cells and that this migration relies on MMP activity, whereas other enzymes typically required for bone removal in the resorption compartment are not essential for migration. Some of the osteoclast MMPs might thus be relevant to the migratory/invasive activity of the osteoclast, rather than to its bone resorptive activity itself.

Role of type X collagen on experimental mineralization of eggshell membranes

Type X collagen is a transient and developmentally regulated collagen that has been postulated to be involved in controlling the later stages of endochondral bone formation. However, the role of this collagen in these events is not yet known. In order to understand the function of type X collagen, if any, in the process of biomineralization, the properties of type X collagen in eggshell membranes were further investigated. Specifically, calvaria-derived osteogenic cells were tested for their ability to mineralize eggshell membranes in vitro. Immunohistochemistry with specific monoclonal antibodies was used to correlate the presence or absence of type X collagen or its propeptide domains with the ability of shell membranes to be mineralized. The extent of mineralization was assessed by Von Kossa staining, scanning electron microscopy and energy-dispersive spectroscopy. The results indicate that the non-helical domains of type X collagen must be removed to facilitate the cell-mediated mineralization of eggshell membranes. In this tissue, intact type X collagen does not appear to stimulate or support cell-mediated mineralization. We postulate that the non-helical domains of type X collagen function in vivo to inhibit mineralization and thereby establish boundaries which are protected from mineral deposition.

Initiation of osteoclast bone resorption by interstitial collagenase

Osteoclasts form an acidic compartment at their attachment site in which bone demineralization and matrix degradation occur. Although both the cysteine proteinases and neutral collagenases participate in bone resorption, their roles have remained unclear. Here we show that interstitial collagenase has an essential role in initiating bone resorption, distinct from that of the cysteine proteinases. Treatment of osteoclasts with cysteine proteinase inhibitors did not affect the number of resorption lacunae ("pits") formed on the surface of dentine slices, but it generated abnormal pits that were demineralized but filled with undegraded matrix. Treatment with metalloproteinase inhibitors did not alter the qualitative features of lacunae, but it greatly reduced the number of pits and surface area resorbed. Treatment of bone cells with an inhibitory anti-rat interstitial collagenase antiserum reduced bone resorption markedly. In the presence of collagenase inhibitors, resorption was restored by pretreatment of dentine slices with rat interstitial collagenase or by precoating the dentine slices with collagenase-derived gelatin peptides or heat-gelatinized collagen. Immunostaining revealed that interstitial collagenase is produced at high levels by stromal cells and osteoblasts adjacent to osteoclasts. These results indicate that interstitial collagenase can function as a "coupling factor," allowing osteoblasts to initiate bone resorption by generating collagen fragments that activate osteoclasts.