cDNA cloning of alkaline phosphatase from rat osteosarcoma (ROS 17/2.8) cells

Effect of safflower seeds supplementation on stimulation of the proliferation, differentiation and mineralization of osteoblastic MC3T3-E1 cells

Anti-bone resorption properties of the Korean herbal formulation, Gami-Honghwain (HJ), which comprises Carthamus tinctorius L. seed and hominis placenta, were investigated. We demonstrate that the production of PGE2 is inhibited by 20-100 microg/ml HJ in nontransformed osteoblastic cells (MC3T3-E1 cells), indicating that HJ inhibits PGE2 production. The effect of HJ on the proliferation and osteoblastic differentiation in MC3T3-E1 was also studied. HJ dose-dependently increased DNA synthesis (significant at 20-100 microg/ml), and increased alkaline phosphatase (ALP) and prolyl hydroxylase activities of MC3T3-E1 cells (20-100 microg/ml), while anti-estrogen tamoxifen eliminated the stimulation of proliferation and ALP activity of MC3T3-E1 which was induced by HJ. These results indicate that HJ directly stimulates cell proliferation and differentiation of osteoblasts. Also, when we assessed the effects of HJ on osteoblastic differentiation in MC3T3-E1, HJ enhanced ALP activity and mineralization in a dose- and time-dependent fashion. This stimulatory effect of the HJ was observed at relatively low doses (significant at 20-100 microg/ml and maximal at 100 microg/ml). Northern blot analysis showed that the HJ (60 microg/ml) increased in bone morphogenetic protein-2 as well as ALP mRNA concentrations in MC3T3-E1 cells. HJ (100 microg/ml) slightly increased in type I collagen mRNA abundance throughout the culture period, whereas it markedly inhibited the gene expression of collagenase-1 between days 15 and 20 of culture. These results indicate that HJ has anabolic effect on bone through the promotion of osteoblastic differentiation, suggesting that it could be used for the treatment of common metabolic bone diseases.

Icaritin and its glycosides enhance osteoblastic, but suppress osteoclastic, differentiation and activity in vitro

Icariin, a principal flavonoid glycoside in Herba Epimedii, is hypothesized to possess beneficial effects on bone mass. Icariin is metabolized to icariside II and then to icaritin in vivo. In the present study, we investigated the in vitro effects of icariin, icariside II and icaritin on both osteoblasts and osteoclasts. After treatment with these compounds at concentrations 10(-5)-10(-8) mol/l, osteoblasts were examined for proliferation, alkaline phosphatase activity, osteocalcin secretion and matrix mineralization, as well as expression levels of bone-related proteins. The formation of osteoclasts was assessed by counting the number of multinucleated TRAP-positive cells. The activity of isolated rat osteoclasts was evaluated by measuring pit area, actin rings and superoxide generation. Icariside II and icaritin increased the mRNA expression of ALP, OC, COL-1 and OPG, but suppressed that of RANKL. In addition, these compounds reduced the number of multinucleated TRAP-positive cells and the osteoclastic resorption area. Also decreases were observed in superoxide generation and actin ring formation that are required for osteoclast survival and bone resorption activity. These findings suggest that icaritin, which was more potent than icariin and icariside II, enhanced the differentiation and proliferation of osteoblasts, and facilitated matrix calcification; meanwhile it inhibited osteoclastic differentiation in both osteoblast-preosteoclast coculture and osteoclast progenitor cell culture, and reduced the motility and bone resorption activity of isolated osteoclasts.

Effects of pulse frequency of low-level laser therapy (LLLT) on bone nodule formation in rat calvarial cells

OBJECTIVE

The purpose of this study was to determine the effect of pulse frequencies of low-level laser therapy (LLLT) on bone nodule formation in rat calvarial cells in vitro.

BACKGROUND DATA

Various photo-biostimulatory effects of LLLT, including bone formation, were affected by some irradiation factors such as total energy dose, irradiation phase, laser spectrum, and power density. However, the effects of pulse frequencies used during laser irradiation on bone formation have not been elucidated.

MATERIALS AND METHODS

Osteoblast-like cells isolated from fetal rat calvariae were irradiated once with a low-energy Ga-Al-As laser (830 nm, 500 mW, 0.48-3.84 J/cm2) in four different irradiation modes: continuous irradiation (CI), and 1-, 2-, and 8-Hz pulsed irradiation (PI-1, PI-2, PI-8). We then investigated the effects on cellular proliferation, bone nodule formation, alkaline phosphatase (ALP) activity, and ALP gene expression.

RESULTS

Laser irradiation in all four groups significantly stimulated cellular proliferation, bone nodule formation, ALP activity, and ALP gene expression, as compared with the non-irradiation group. Notably, PI-1 and -2 irradiation markedly stimulated these factors, when compared with the CI and PI-8 groups, and PI-2 irradiation was the best approach for bone nodule formation in the present experimental conditions.

CONCLUSION

Since low-frequency pulsed laser irradiation significantly stimulates bone formation in vitro, it is most likely that the pulse frequency of LLLT an important factor affecting biological responses in bone formation.

Absence of transcription factor c-maf causes abnormal terminal differentiation of hypertrophic chondrocytes during endochondral bone development

In this study, we report that the transcription factor c-Maf is required for normal chondrocyte differentiation during endochondral bone development. c-maf is expressed in hypertrophic chondrocytes during fetal development (E14.5-E18.5), with maximal expression in the tibia occurring at E15.5 and E16.5, in terminally differentiated chondrocytes. In c-maf-null mice, fetal bone length is decreased approximately 10%, and hypertrophic chondrocyte differentiation is perturbed. There is an initial decrease in the number of mature hypertrophic chondrocytes at E15.5 in c-maf-null tibiae, with decreased expression domains of collagen X and osteopontin, markers of hypertrophic and terminal hypertrophic chondrocytes, respectively. By E16.5, there is an expanded domain of late hypertrophic, osteopontin-positive chondrocytes in the c-maf-/-. This accumulation of hypertrophic chondrocytes persists and is still observed at 4 weeks of age. These data suggest that c-Maf facilitates the initial chondrocyte terminal differentiation and influences the disappearance of hypertrophic chondrocytes. BrdU and TUNEL analyses show normal proliferation rate and apoptosis in the c-maf-null. There is a specific decrease in MMP-13 expression at E15.5 in the c-maf-null. MMP-13 is known to be regulated by AP-1 and may also be a target of c-Maf. Thus, cartilage is a novel system in which c-Maf acts during development, where c-Maf is required for normal chondrocyte differentiation.

An immunohistochemical study on hard tissue formation in a subcutaneously transplanted rat molar

While dental pulp undergoes calcification following tooth replantation or transplantation, we actually know little about these mechanisms. We therefore conducted histological and immunohistochemical evaluations of mineralized tissue that formed in the pulp of rat maxillary molar transplanted into abdominal subcutaneous tissue. One, 2, 3, and 4 weeks post-transplantation, the teeth were investigated immunohistochemically using antibodies to osteocalcin (OCN), osteopontin (OPN), bone sialoprotein (BSP), dentin sialoprotein (DSP), and tissue non-specific alkaline phosphatase (TNAP). In the 1st week after transplantation, cell-rich hard tissue was formed at the root apex. At 2 weeks, formations of hard tissue, with few cells in the root canals and bone-like tissue in the coronal pulp chamber, were noted. After 3 and 4 weeks, the amounts of these hard tissues were increased. The immunolocalization of OCN, OPN, and BSP was seen strongly in coronal and apical hard tissues, but weakly in the root hard tissue. Conversely, DSP localized in the root hard tissue, but not in other newly formed hard tissues. At 1 week, TNAP localized along the periphery of the apical hard tissue and the lower surfaces of root predentin. These results demonstrate that the newly formed hard tissues in the pulp cavity of subcutaneously transplanted molars could be classified into three types, suggesting that these might be formed by type-specific cells.

Role of cartilage-derived anti-angiogenic factor, chondromodulin-I, during endochondral bone formation

OBJECTIVE

Cartilage is a typical avasclar tissue that exhibits powerful resistance to angiogenesis or vascular invasion. We previously identified a cartilage-specific 25 kDa glycosylated protein, chondromodulin-I (ChM-I), as anti-angiogenic factor. Taking advantage of ectopic bone formation and xenograft tumour model by human chondrosarcoma cell line OUMS-27, we examined how ChM-I is involved in switching of angiogenesis in cartilage.

DESIGN

Gene expression pattern of ChM-I was examined in 4-week-old mice and mouse embryos by northern blot analysis and in situ hybridization. To evaluate the effect of ChM-I on ectopic bone formation, guanidine extracts of demineralized bone matrix were mixed with the ChM-I-bound heparin-Sepharose beads and were implanted onto the fasciae of back muscle of 6-week old nude mice. To analyse the effect of ChM-I on tumour angiogenesis, the level of ChM-I mRNA in cartilaginous tumours was assessed by competitive PCR, and compared with that of articular cartilage. Then, human chondrosarcoma OUMS-27 cells were inoculated into the back of nude mice to form a tumour about 45 mm3 in size. Recombinant ChM-I protein was administrated into OUMS-27 xenograft tumours for the initial 5 days to study its effect against tumour-angiogenesis.

RESULTS

ChM-I gene was specifically expressed in cartilage of 4-week-old mice. Eye and thymus were also identified as minor expression sites. However, during endochondral bone development, cartilage changes its character from anti-angiogenic into angiogenic prior to the replacement of calcified cartilage by bone. In embryos, ChM-I mRNA was expressed in proliferative and upper hypertrophic cartilage zones in the developing cartilaginous bone rudiments, but completely abolished in lower hypertrophic and calcified cartilage zones. Purified ChM-I protein apparently inhibited vascular invasion into cartilage induced by the implantation of demineralized bone matrix in nude mice, leading to the inhibition of replacement of cartilage. The level of ChM-I transcripts in the lower-grade chondrosarcomas was substantially reduced to several hundreds or less in the lower-grade chondrosarcomas, compared with that of articular cartilage or other benign cartilage tumours. The local administration of recombinant human ChM-I almost completely blocked tumour angiogenesis and growth in the human chondrosarcoma xenografts in mice.

CONCLUSIONS

ChM-I is involved in the anti-angiogenic property of cartilage and its absence creates a permissive microenvironment for vascular invasion into cartilage under physiological and pathological conditions.

Differential regulation of the two principal Runx2/Cbfa1 n-terminal isoforms in response to bone morphogenetic protein-2 during development of the osteoblast phenotype

Cbfa1/Runx2 is a transcription factor essential for bone formation and osteoblast differentiation. Two major N-terminal isoforms of Cbfa1, designated type I/p56 (PEBP2aA1, starting with the sequence MRIPV) and type II/p57 (til-1, starting with the sequence MASNS), each regulated by distinct promoters, are known. Here, we show that the type I transcript is constitutively expressed in nonosseous mesenchymal tissues and in osteoblast progenitor cells. Cbfa1 type I isoform expression does not change with the differentiation status of the cells. In contrast, the type II transcript is increased during differentiation of primary osteoblasts and is induced in osteoprogenitors and in premyoblast C2C12 cells in response to bone morphogenetic protein-2. The functional equivalence of the two isoforms in activation and repression of bone-specific genes indicates overlapping functional roles. The presence of the ubiquitous type I isoform in nonosseous cells and before bone morphogenetic protein-2 induced expression of the type II isoform suggests a regulatory role for Cbfa1 type I in early stages of mesenchymal cell development, whereas type II is necessary for osteogenesis and maintenance of the osteoblast phenotype. Our data indicate that Cbfa1 function is regulated by transcription, cellular protein levels, and DNA binding activity during osteoblast differentiation. Taken together, our studies suggest that developmental timing and cell type- specific expression of type I and type II Cbfa isoforms, and not necessarily molecular properties or sequences that reside in the N-terminus of Cbfa1, are the principal determinants of the osteogenic activity of Cbfa1.

Increased mRNA expression and protein secretion of interleukin-6 in primary human osteoblasts differentiated in vitro from rheumatoid and osteoarthritic bone

We have investigated the expression and synthesis of potential bone-resorbing cytokines, interleukin-6 (IL-6), interleukin-1 (IL-1), and tumor necrosis factor (TNF) in rheumatoid arthritic (RA) and osteoarthritic (OA) bone, two common diseases which are associated with bone loss. Primary human osteoblast (hOB) cultures were established to determine the temporal mRNA expression of IL-6, IL-1 (alpha and beta), and TNF (alpha and beta) in relation to osteoblast growth and phenotypic genes. IL-6 mRNA levels were found to be significantly higher (P < 0.04) in both OA hOB (17 patients) and RA hOB (10 patients) compared to normal (NO) hOB (9 patients) and reached five-fold increases in OA hOB and 13-fold increases in RA hOB. Maximal levels of IL-6 are expressed at Day 21 which corresponds to the mineralization stage reflected by decreasing collagen I (alpha(1)), osteopontin, bone sialoprotein, alkaline phosphatase mRNA levels, while osteocalcin (OC) mRNA levels increased. IL-6 protein levels also were significantly higher (P < 0.05) in OA hOB and RA hOB compared to NO hOB. These increases were not attributable to sex or age of the donor bone. Neither the mRNA encoding IL-1(alpha and beta) and TNF(alpha and beta) nor the related proteins were detectable. These results indicate that differentiated OA hOB and RA hOB within a bone tissue-like matrix constitutively express and secrete high levels of IL-6. This inherent property suggests that these osteoblasts, independent of local inflammatory parameters, can contribute to enhanced recruitment of osteoclast progenitors and thereby bone resorption.

The orphan nuclear estrogen receptor-related receptor alpha (ERRalpha) is expressed throughout osteoblast differentiation and regulates bone formation in vitro

The orphan nuclear estrogen receptor-related receptor alpha (ERRalpha), is expressed by many cell types, but is very highly expressed by osteoblastic cells in which it transactivates at least one osteoblast-associated gene, osteopontin. To study the putative involvement of ERRalpha in bone, we first assessed its expression in rat calvaria (RC) in vivo and in RC cells in vitro. ERRalpha mRNA and protein were expressed at all developmental stages from early osteoprogenitors to bone-forming osteoblasts, but protein was most abundant in mature cuboidal osteoblasts. To assess a functional role for ERRalpha in osteoblast differentiation and bone formation, we blocked its expression by antisense oligonucleotides in either proliferating or differentiating RC cell cultures and found inhibition of cell growth and a proliferation-independent inhibition of differentiation. On the other hand, ERRalpha overexpression in RC cells increased differentiation and maturation of progenitors to mature bone-forming cells. Our findings show that ERRalpha is highly expressed throughout the osteoblast developmental sequence and plays a physiological role in differentiation and bone formation at both proliferation and differentiation stages. In addition, we found that manipulation of receptor levels in the absence of known ligand is a fruitful approach for functional analysis of this orphan receptor and identification of potential target genes.

The function of alkaline phosphatase in the liver: regulation of intrahepatic biliary epithelium secretory activities in the rat

We studied the effects of alkaline phosphatase (AP) on the secretory processes of the rat intrahepatic biliary epithelium as well as the role of the intrahepatic biliary epithelium in the uptake and biliary secretion of exogenous AP. The effects of acute and chronic administration of AP on bile secretory parameters were investigated in vivo in normal and bile duct ligated (BDL) rats and in vitro in isolated rat bile duct units (IBDU). In vivo, acute AP administration decreased bile flow and biliary bicarbonate excretion and abolished secretin choleresis in BDL rats but not in normal rats. On the contrary, the AP inhibitor, levamisole, increased in BDL rat bile flow and biliary bicarbonate excretion. In vitro, basal and secretin-stimulated Cl(-)/HCO(3)(-) exchanger activity in IBDU was immediately inhibited by AP intraluminal microinjection (apical exposure) but only after a prolonged exposure to the basolateral pole. Levamisole increased the Cl(-)/HCO(3)(-) exchanger activity of IBDU. A significant basolateral uptake of AP occurs in IBDU with a progressive transport to the apical domain. AP chronic treatment increased AP and gamma-glutamyltranspeptidase (gamma-GT) activities in the intrahepatic bile ducts and hepatocyte canalicular pole, promoted enlargement of bile canaliculi, and decreased bile flow and biliary bicarbonate excretion. In conclusion, the intrahepatic biliary epithelium plays a role in the uptake and biliary secretion of serum AP. AP inhibits the secretory processes of the intrahepatic biliary epithelium and induces features of intrahepatic cholestasis after chronic administration. These findings indicate that AP plays an active role in down-regulating the secretory activities of the intrahepatic biliary epithelium.

SOX9 enhances aggrecan gene promoter/enhancer activity and is up-regulated by retinoic acid in a cartilage-derived cell line, TC6

SOX9 is a transcription factor that plays a key role in chondrogenesis. Aggrecan is one of the major structural components in cartilage; however, the molecular mechanism of aggrecan gene regulation has not yet been fully elucidated. TC6 is a clonal chondrocytic cell line derived from articular cartilage. The purpose of this study was to examine whether SOX9 modulates aggrecan gene expression and to further identify molecules that regulate Sox9 expression in TC6 cells. SOX9 overexpression in TC6 cells enhanced by approximately 3-fold the transcriptional activity of the AgCAT-8 construct containing 8-kilobase (kb) promoter/first exon/first intron fragments of the aggrecan gene. SOX9 enhancement of aggrecan promoter activity was lost when we deleted a 4.5-kb fragment from the 3'-end of the 8-kb fragment corresponding to the region including the first intron. In TC6 cells, SOX9 enhanced the transcriptional activity of a reporter construct containing the Sry/Sox consensus sequence >10-fold. SOX9 enhancement of aggrecan gene promoter activity and SOX9 transactivation through the Sry/Sox consensus sequence were not observed in osteoblastic osteosarcoma cells (ROS17/2.8), indicating the dependence on the cellular background. Northern blot analysis indicated that TC6 cells constitutively express Sox9 mRNA at relatively low levels. To examine regulation of Sox9 gene expression, we investigated the effects of calciotropic hormones and cytokines. Among these, retinoic acid (RA) specifically enhanced Sox9 mRNA expression in TC6 cells. The basal levels of Sox9 expression and its enhancement by RA were observed similarly at both permissive (33 degrees C) and nonpermissive (39 degrees C) temperatures. Furthermore, RA treatment enhanced the transcriptional activity of a reporter construct containing the Sry/Sox consensus sequence in TC6 cells. Moreover, RA treatment also enhanced the transcriptional activity of another reporter construct containing the enhancer region of the type II procollagen gene in TC6 cells. These observations indicate that SOX9 enhances aggrecan promoter activity and that its expression is up-regulated by RA in TC6 cells.

Cloning the full-length cDNA for rat connective tissue growth factor: implications for skeletal development

The mammalian osteopetroses represent a pathogenetically diverse group of skeletal disorders characterized by excess bone mass resulting from reduced osteoclastic bone resorption. Abnormalities involving osteoblast function and skeletal development have also been reported in many forms of the disease. In this study, we used the rat mutation, osteopetrosis (op), to examine differences in skeletal gene expression between op mutants and their normal littermates. RNA isolated from calvaria and long bones was used as a template for mRNA-differential display. Sequence information for one of the many cDNA that were selectively expressed in either normal or mutant bone suggested that it is the rat homologue of connective tissue growth factor (CTGF) previously cloned in the human, mouse, and other species. A consensus sequence was assembled from overlapping 5'-RACE clones and used to confirm the rat CTGF cDNA protein coding region. Northern blot analysis confirmed that this message was highly (8- to 10-fold) over-expressed in op versus normal bone; it was also upregulated in op kidney but none of the other tissues (brain, liver, spleen, thymus) examined. In primary rat osteoblast cultures, the CTGF message exhibits a temporal pattern of expression dependent on their state of differentiation. Furthermore, CTGF expression is regulated by prostaglandin E(2), a factor known to modulate osteoblast differentiation. Since members of the CTGF family regulate the expression of specific genes, such as collagen and fibronectin, we propose that CTGF may play a previously unreported role in normal skeletal modeling/remodeling. Its dramatic over-expression in the op mutant skeleton may be secondary to the uncoupling of bone resorption and bone formation resulting in dysregulation of osteoblast gene expression and function.

Differential expression and activity of tissue-nonspecific alkaline phosphatase (TNAP) in rat odontogenic cells in vivo

Among the four existing isoforms of alkaline phosphatase (AP), the present study is devoted to tissue-nonspecific alkaline phosphatase (TNAP) in mineralized dental tissues. Northern blot analysis and measurements of phosphohydrolase activity on microdissected epithelium and ectomesenchyme, in situ hybridization, and immunolabeling on incisors confirmed that the AP active in rodent teeth is TNAP. Whereas the developmental pattern of TNAP mRNA and protein and the previously described activity were similar in supra-ameloblastic and mesenchymal cells, they differed in enamel-secreting cells, the ameloblasts. As previously shown for other proteins involved in calcium and phosphate handling in ameloblasts, a biphasic pattern of steady-state TNAP mRNA levels was associated with additional variations in ameloblast TNAP protein levels during the cyclic modulation process. Although the association of TNAP upregulation and the initial phase of biomineralization appeared to be a basic feature of all mineralized tissues, ameloblasts (and to a lesser extent, odontoblasts) showed a second selectively prominent upregulation of TNAP mRNA/protein/activity during terminal growth of large enamel crystals only, i.e., the maturation stage. This differential expression/activity for TNAP in teeth vs bone may explain the striking dental phenotype vs bone reported in hypophosphatasia, a hereditary disorder related to TNAP mutation. (J Histochem Cytochem 47:1541-1552, 1999)

In vitro analysis of the stimulation of bone formation by highly bioactive apatite- and wollastonite-containing glass-ceramic: released calcium ions promote osteogenic differentiation in osteoblastic ROS17/2.8 cells

We analyzed the mechanisms of the efficient bone formation on the osteoconductive surface of apatite- and wollastonite-containing glass-ceramic (AW) by using an in vitro system. AW releases Ca ions and bonds to bone via a submicron-thick hydroxycarbonate apatite (HCA) layer. AW disks were conditioned with simulated body fluid (SBF) to grow HCA layers, and the amount of released Ca ion was regulated by modulating the conditioning time from 24 to 240 h. Surface-transformed AW disks increased alkaline phosphatase (AP) activity in osteoblastic ROS17/2.8 cells by 1.5- to threefold over unconditioned disks. AW disks conditioned for 24 h [AW(24)], which had a homogeneous, submicron-thick apatite layer and increased extracellular ionized Ca concentration ([Ca(2+)](e)) in the culture medium to the greatest extent, enhanced the AP activity the most. High [Ca(2+)](e) promoted osteogenic differentiation in ROS17/2.8 cells: It increased AP activity in a dose-dependent manner by up to 1.6-fold, and up-regulated the expression of AP, osteocalcin (OC), and transforming growth factor-beta1 mRNAs in dose- and time-dependent manners. AW(24) enhanced AP activity in ROS17/2.8 cells as much as AW disks conditioned with SBF containing serum to exhibit in vivo surface-structure changes. AW(24) increased AP activity in ROS17/2.8 cells by 1.6-fold and enhanced the expression of AP and OC mRNAs significantly, compared with sintered hydroxyapatite (HA). After implantation of AW and HA in the distal metaphyses of rabbit femurs, thin, newly formed bone lined with cuboidal, osteoblast-like cells was characteristically observed adjacent to the AW surface within 8 days. These results provide evidence for the hypothesis that AW stimulates bone formation on its surface by increasing [Ca(2+)](e) to promote the HCA layer formation and the differentiation of osteogenic cells.

N-cadherin expression during periodontal ligament cell differentiation in vitro

BACKGROUND

When confluent periodontal ligament (PDL) cells were cultured in the presence of dexamethasone (Dex), ascorbic acid (AA), and beta-glycerophosphate (GP), they underwent sequential differentiation, demonstrating distinct morphological characteristics. At 1 week, localized cell proliferation led to the formation of multilayers of cells. As cell differentiation progressed, they formed nodules by deposition of matrix in the clusters of cells at 2 weeks, and mineralized the nodules at 3 weeks. These changes implicate extensive cell-to-cell interactions. Cadherins are known to play an important role in establishing cell contacts during tissue formation.

METHODS

To determine whether cadherins are involved in PDL cell differentiation, and the formation and mineralization of nodules by the cells in vitro, we investigated the expression of N-cadherin using immunofluorescence labeling and Northern blot analysis.

RESULTS

Immunolabeling showed that N-cadherin was expressed in PDL cells in the stages of nodule formation and mineralization. Northern blot analysis demonstrated a 3-fold increase in the expression of N-cadherin mRNA in the stages. However, neither E-cadherin nor P-cadherin was expressed.

CONCLUSIONS

Our data suggest that N-cadherin may play an important role in PDL cell differentiation and the formation of mineralized nodules by PDL cells.

Expression of TGF-beta isoforms and their receptors during mineralized nodule formation by rat periodontal ligament cells in vitro

Transforming growth factor-betas (TGF-beta s) and bone morphogenetic proteins (BMPs), members of a TGF-beta superfamily, are known to play an important role in osteogenic cell differentiation and consequently bone formation. We have reported previously that periodontal ligament (PDL) cells differentiate and form mineralized nodules when cultured in the presence of dexamethasone (Dex), beta-glycerophosphate (GP) and ascorbic acid (AA). To understand the roles of TGF-beta isoforms (TGF-beta 1, 2 and 3) and TGF-beta type I receptors (activin receptor-like kinase (ALK)-2, -3, -5 and -6) in PDL cell differentiation, their expression was investigated using Northern blot analysis. Rat PDL cells, derived from coagulum in the tooth socket, were cultured in the presence of Dex (5 microM), GP (10 mM) and AA (50 micrograms/ml) for up to 21 d. Total RNA was isolated from PDL cells after 0, 7, 14 and 21 d and used for northern blot analysis of mRNAs for matrix proteins, TGF-beta isoforms and their receptors using 32P-labeled cDNAs as probes. Four stages showing distinct morphological characteristics and matrix expression during development of mineralized nodules were identified. Type I collagen (Col I) and SPARC (secreted protein, acidic and rich in cysteine) mRNAs were expressed at the confluent stage, but decreased during the mineralization stage. Osteopontin (OPN) and alkaline phosphatase (ALP) transcripts were initially observed at multilayer stage, while bone sialoprotein (BSP) and osteocalcin (OC) at the nodule stage and all 4 were expressed thereafter. TGF-beta 1 mRNA expression increased with the progression of PDL cell differentiation, while a relatively high level of TGF-beta 3 transcript decreased slightly during their differentiation. TGF-beta 2 mRNA was not expressed. The expression of TGF beta-RI mRNA decreased, whereas that of TGF beta-RIII increased dramatically with PDL cell differentiation. TGF beta-RII gene activities remained high throughout all stages. ALK-2, ALK-3 and ALK-6 mRNA expression increased with the progression of PDL cell differentiation, suggesting that these receptors may play important roles in Dex-induced PDL cell differentiation and mineralized nodule formation.

Inhibition of osteoblastic cell differentiation by lipopolysaccharide extract from Porphyromonas gingivalis

Lipopolysaccharide from Porphyromonas gingivalis (P-LPS), an important pathogenic bacterium, is closely associated with inflammatory destruction of periodontal tissues. P-LPS induces the release of cytokines and local factors from inflammatory cells, stimulates osteoclastic-cell differentiation, and causes alveolar bone resorption. However, the effect of P-LPS on osteoblastic-cell differentiation remains unclear. In this study, we investigated the effect of P-LPS extract prepared by the hot-phenol-water method, on the differentiation of primary fetal rat calvaria (RC) cells, which contain a subpopulation of osteoprogenitor cells, into osteoblastic cells. P-LPS extract significantly inhibited bone nodule (BN) formation and the activity of alkaline phosphatase (ALPase), an osteoblastic marker, in a dose-dependent manner (0 to 100 ng of P-LPS extract per ml). P-LPS extract (100 ng/ml) significantly decreased BN formation to 27% of the control value and inhibited ALPase activity to approximately 60% of the control level on days 10 to 21 but did not affect RC cell proliferation and viability. P-LPS extract time-dependently suppressed the expression of ALPase mRNA, with an inhibitory pattern similar to that of enzyme activity. The expression of mRNAs for osteocalcin and osteopontin, matrix proteins related to bone metabolism, was markedly suppressed by P-LPS extract. Furthermore, P-LPS extract increased the expression of mRNAs for CD14, LPS receptor, and interleukin-1beta in RC cells. These results indicate that P-LPS inhibits osteoblastic-cell differentiation and suggest that LPS-induced bone resorption in periodontal disease may be mediated by effects on osteoblastic as well as osteoclastic cells.

Actions of bFGF on mitogenic activity and lineage expression in rat osteoprogenitor cells: effect of age

Rat osteoprogenitor cells were used to examine the effects of bFGF on DNA synthesis and the expression of osteoblast (OB)-related genes. bFGF, as low as 0.1 ng/ml, stimulated DNA synthesis. bFGF also increased the mRNA level of osteopontin (OP) and decreased that of type I collagen (COL I). When cultures were grown in dexamethasone (DEX) to induce OB lineage commitment, the expression of COL I, alkaline phosphatase (AP) and OP was greatly enhanced. Subsequent incubation with bFGF partially negated the stimulatory effect of DEX on AP and COL I mRNAs. bFGF also inhibited the expression of osteocalcin mRNA in cells grown in 1,25(OH)2D3 and DEX. Combined effects of bFGF with IGF-I or PDGF on DNA synthesis and OP expression were examined. bFGF + IGF-I, but not bFGF + PDGF, was more effective than PDGF alone. By comparing cells from adult and old animals, we found that bFGF-induced mitogenic activity was reduced significantly with age. In contrast, the effect of bFGF on the expression of OB genes was not significantly altered by age. These findings suggest that bFGF plays a dual role as a local positive and negative regulator on proliferation and osteogenic lineage expression, respectively, in osteoprogenitor cells, and that the mitogenic activity in response to bFGF was impaired in aging.

Expression of dentin sialoprotein (DSP) and other molecular determinants by a new cell line from dental papillae, MDPC-23

The purpose of this study was to characterize the molecular expression of a spontaneously immortalized and cloned cell line (MDPC-23) derived from 18-19 day CD-I fetal mouse molar dental papillae to determine if these cells were odontoblast-like. Western blots showed that a protein band, at approximately 105 kDa, reacting positively with anti-DSP antibodies and co-migrating with mouse DSP, was present in lysates of cells from passages 7, 37 and 77, in serum-free conditioned medium from passage 37 cells, and in mouse dentin extract. A minor band at 55 kDa was also apparent in cell lysates. Using a cDNA probe for a 486bp mouse DSP coding sequence, DSP or DSP-PP mRNA expression was detected by Northern analysis as well as Southern analysis after RT-PCR in all three passages. It was also shown that in these cells 1,25 (OH)2 vitamin D3 upregulated both osteopontin and osteocalcin mRNA, and dexamethasone downregulated alkaline phosphatase and alpha2(I) collagen mRNA. Thus, MDPC-23 cells express proteins which are common to mineralizing tissue. The expression of DSP and DSP-PP strongly suggests that this cell line is from the odontoblast lineage.

Behavior of bone marrow cells cultured on three different coatings of gel-derived bioactive glass-ceramics at early stages of cell differentiation

Previous studies have shown different macrophage responses to three compact pellets (with slightly different chemical composition) of gel-derived bioactive glass-ceramics of the CaO-P2O5-SiO2 system. In the present study primary bone marrow cells directed in vitro to form osteoblastic or osteoclastic cells were cultivated on glass slides coated by these three glass-ceramics. Glass slides were used as controls. In osteoblastic cultures alkaline phosphatase activity varied, depending on the type of coatings. Northern analysis showed high mRNA expressions of bone-related proteins on both the glass-ceramics and control glass. Mineralized nodules were not formed on the control glass, but coating glass slides with the glass-ceramics promoted mineralization without any substantial differences between the types of coatings. In osteoclastic cultures, the normal morphology of tartrate resistant acid phosphatase-positive multinucleated cells on standard culture plates was altered on the control glass and the glass-ceramics. The number of these cells differed, depending on the type of coatings, with no particular differences in the arrangement of F-actin by these cells. These analyses demonstrate complete biocompatibility of the glass-ceramic coatings but not the control glass, on which the cells failed to form mineralized nodules. The phenotype expression of the cells appeared to be influenced by microstructure, surface roughness, and the general character of the coatings rather than their surface reactivity.

Alkaline phosphatase and type I collagen gene expressions were reduced by hydroxyl radical-treated fibronectin substratum

The influence of fibronectin (FN) substratum treated with hydroxyl radicals (.OH) generated by the H2O2-Cu2+ systems on osteoblast cells was studied. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis analysis revealed that FN was degradated and/or modified by .OH treatment. The calcified bone nodule formation of osteoblast cells cultured on .OH treated FN-coated wells was significantly lower compared to those on intact FN-coated wells. Alkaline phosphatase (ALP) activity and the secretion of type I collagen were reduced by .OH-treated FN. By RT-PCR mRNA levels of ALP and type I collagen genes were also diminished. These findings suggested that the .OH damaged FN molecules and reduced the bone formation of osteoblast cells via inhibition of proliferation and/or differentiation of osteoblast progenitors and the calcification process.

Sequential progression of the differentiation program by bone morphogenetic protein-2 in chondrogenic cell line ATDC5

During embryonic development of long bones, chondroprogenitor cells exhibit the transitions of phenotype, i.e., from type I collagen-expressing cells to type II collagen-expressing chondrocytes through cellular condensation (early-phase differentiation) and then to type X collagen-expressing mineralizing chondrocytes (late-phase differentiation). The chondrogenic cell line ATDC5 displays the sequential transitions of phenotype in a synchronous manner in vitro. Taking advantage of the sequential differentiation, the effects of growth factors were evaluated at each differentiation step of ATDC5 cells. Among the factors examined, bone morphogenetic protein-2 (BMP-2) specifically stimulated a progression of the early-phase differentiation. Rounded chondrocytic cells were formed all over the culture plates by skipping out a cellular condensation stage. Fibroblast growth factor-2 stimulated growth of undifferentiated ATDC5 cells, but failed to stimulate overt chondrogenesis. The proliferation of differentiated cells ceased as cartilage nodules became maturated. At this stage, BMP-2 markedly up-regulated expression of type X collagen mRNA (a 9.1-fold increase) and alkaline phosphatase mRNA (a 7.5-fold increase) within 48 h. On the other hand, it down-regulated expression of type II collagen and parathyroid hormone (PTH)/PTH-related peptide (PTHrP) receptor mRNAs, markers of the early differentiation. BMP-2 stimulated the formation of calcified matrix, an end product of terminally differentiated chondrocytes. These results indicated that BMP stimulated the sequential progression of early- and late-phase differentiation of ATDC5 cells.

Estrogen Receptor-α Is Developmentally Regulated during Osteoblast Differentiation and Contributes to Selective Responsiveness of Gene Expression

Estrogen responsiveness of bone is a fundamental regulatory mechanism operative in skeletal homeostasis. We examined the expression of estrogen receptor-alpha (ER) messenger RNA (mRNA) in cultured rat calvarial-derived osteoblasts during progressive development of the osteoblast phenotype. Levels of ER message were compared with the expression of traditional osteoblastic markers that have been mapped throughout the differentiation process of these cells. ER transcripts, measured using semiquantitative RT-PCR analysis, were expressed at low levels in early stage proliferating osteoblasts and increased at confluence upon initial expression of bone cell phenotypic genes. A 23-fold up-regulation of ER mRNA expression coincided with the initiation of alkaline phosphatase activity (day 8). ER mRNA levels progressively increased 70-fold, reaching a maximum level on days 22-25 in fully differentiated osteoblasts when osteocalcin expression peaked, but declined precipitously by day 32 in osteocytic cells. Analysis of RNA isolated directly from rat calvaria confirmed these in vitro results and demonstrated that ER message levels become more abundant postnatally as bone becomes more mineralized. We also examined the responsiveness of osteoblasts to 17beta-estradiol (17beta-E2) at two periods of maturation: the nodule-forming stage (day 14) and the late mineralization stage (day 30). Estradiol suppressed the levels of alkaline phosphatase, osteocalcin, osteonectin, and ER mRNAs on day 14, but up-regulated these messages on day 30. In contrast, 17beta-E2 treatment regulated the steady state levels of transforming growth factor-beta1 and type I procollagen mRNAs only in the late mineralization stage, whereas histone H4 message was unaffected by the steroid at either stage of differentiation. Thus, the observed developmental expression of ER mRNA correlates with progressive osteoblast differentiation and may be a contributing factor to differential regulation of bone cell gene expression by 17beta-E2.

Temporal changes of mRNA expression of matrix proteins and parathyroid hormone and parathyroid hormone-related protein (PTH/PTHrP) receptor in bone development

Expression of parathyroid hormone and parathyroid hormone-related protein (PTH/PTHrP) receptor is one of the osteoblastic phenotypes; however, it has not been clear whether this phenotype expression is a marker of immature or mature osteoblasts. We examined the temporal expression pattern of PTH/PTHrP receptor in bone development in vivo and in vitro compared with the expression of other osteoblastic phenotypes: osteopontin (OPN), bone sialoprotein (BSP) and osteocalcin (OC), alkaline phosphatase (ALP), and mineralization. Total RNA was extracted from rat calvariae, and cell culture of rat bone marrow at different developmental stages and then Northern blot hybridization were performed. Mineralization was detected with contact microradiography (CMR) in calvaria or with Alizarin Red S staining in bone marrow cell culture. Both in calvaria and in marrow cell culture, extensive expression of OPN, BSP, type I collagen (COL I), and ALP coincided with the onset of mineralization, and OC expression was observed after mineralized tissue formation. Notably, PTH/PTHrP receptor was expressed at an early developmental stage (prenatal day 14 in calvaria, day 5 in culture) when mineralized tissue was not formed and other osteoblastic phenotypes were scarcely detected. Further study in cell culture revealed that the fold increase in cyclic adenosine monophosphate (cAMP) in response to PTH was elevated with the advance in the culture stage. These results indicate that mRNA expression of PTH/PTHrP receptor could be the early differentiation marker in osteoblastic lineage and that the levels of cAMP production in response to PTH represent the stage of osteoblastic differentiation.

Cellular hypertrophy and calcification of embryonal carcinoma-derived chondrogenic cell line ATDC5 in vitro

During the process of endochondral bone formation, proliferating chondrocytes give rise to hypertrophic cells, which then deposit a mineralized matrix to form calcified cartilage prior to replacement by bone. Previously, we reported that a clonal cell line, ATDC5, undergoes efficient chondrogenic differentiation through a cellular condensation stage. Here we report that the differentiated ATDC5 cells became hypertrophic at the center of cartilage nodules, when the cells ceased to grow. Formation of hypertrophic chondrocytes took place in association with type X collagen gene expression and a dramatic elevation of alkaline phosphate (ALPase) activity. After 5 weeks of culture, mineralization of the culture could be discerned as Alizarin red-positive spots, which spread throughout the nodules even in the absence of beta-glycerophosphate. Electron microscopy and electron probe microanalysis revealed that calcification was first initiated at matrix vesicles in the territorial matrix and that it advanced progressively along the collagen fibers in a manner similar to that which occurs in vivo. The infrared spectrum of the mineralized nodules indicated two absorption doublets around 1030 cm-1 and 600 cm-1, which are characteristic of apatitic mineral. Calcifying cultures of ATDC5 cells retained responsiveness to parathyroid hormone (PTH): PTH markedly inhibited elevation of ALPase activity and calcification in the culture in a dose-dependent manner. Thus, we demonstrated that ATDC5 cells keep track of the multistep differentiation process encompassing the stages from mesenchymal condensation to calcification in vitro. ATDC5 cells provide an excellent model to study the molecular mechanism underlying regulation of cartilage differentiation during endochondral bone formation.

Direct action of nitric oxide on osteoblastic differentiation

The effect of nitric oxide (NO) on osteoblastic differentiation was examined in cultured mouse osteoblasts. Interleukin-1beta and tumor necrosis factor-alpha expressed inducible NO synthase gene with little effect on constitutive NO synthase gene. These cytokines increased NO production, which was inhibited by L-NMMA pretreatment, and decreased alkaline phosphatase (AIPase) activity, which was not restored by L-NMMA. Furthermore, NO donors, sodium nitroprusside and NONOate dose-dependently elevated AIPase activity and expression of osteocalcin gene. These results suggest that NO directly facilitates osteoblastic differentiation and the cytokine-induced inhibition of AIPase activity is mediated via mechanism other than NO.

Targeted disruption of Cbfa1 results in a complete lack of bone formation owing to maturational arrest of osteoblasts

A transcription factor, Cbfa1, which belongs to the runt-domain gene family, is expressed restrictively in fetal development. To elucidate the function of Cbfa1, we generated mice with a mutated Cbfa1 locus. Mice with a homozygous mutation in Cbfa1 died just after birth without breathing. Examination of their skeletal systems showed a complete lack of ossification. Although immature osteoblasts, which expressed alkaline phophatase weakly but not Osteopontin and Osteocalcin, and a few immature osteoclasts appeared at the perichondrial region, neither vascular nor mesenchymal cell invasion was observed in the cartilage. Therefore, our data suggest that both intramembranous and endochondral ossification were completely blocked, owing to the maturational arrest of osteoblasts in the mutant mice, and demonstrate that Cbfa1 plays an essential role in osteogenesis.

The mature osteoblast phenotype is characterized by extensive plasticity

While both morphological and biochemical-molecular attributes demarcate differentiation stages in specific cell and tissue types, what constitutes necessary and sufficient expression to define particular cell types is not always known. For example, mature osteoblasts (OBs) are defined morphologically as the cuboidal, biosynthetically active, basophilic cells residing on bone surfaces and responsible for the deposition of osteoid matrix. However, several recent observations suggest that not all mature OBs are identical. To explore further the validity of the hypothesis that heterogeneity of phenotype exists among mature OBs, we grew fetal rat calvaria cells in vitro at low density under conditions in which bone nodules form and mineralize in isolation of other contaminating cell and colony types. Cells resident in mature OB colonies, i.e., those comprising mainly cuboidal cells associated with an osteoid matrix that had begun to mineralize, were analyzed in situ for protein expression by immunocytochemistry with antibodies against collagen type I, alkaline phosphatase, osteopontin, bone sialoprotein, and osteocalcin. Consistent with the expected phenotype of mature OBs, many OBs expressed high levels of all of these markers, but strikingly even adjacent morphologically indistinguishable cuboidal OBs had differences in protein expression, especially in relation to osteopontin, bone sialoprotein, and osteocalcin expression. Double-labeling with Hoechst 33258 and osteocalcin indicated that the variation in antibody labeling intensity/protein expression appeared independent of a variation in cell cycle. To further ascertain the extent of this heterogeneity, 20 single cells were micromanipulated from colonies and subjected to poly(A)-PCR to analyze the simultaneous coexpression profiles of the same five markers analyzed by immunocytochemistry and two other markers, the OB-osteocyte transition marker E11 and the parathyroid hormone/parathyroid hormone-related protein receptor. Notably, the repertoire of genes expressed and their levels of expression varied markedly in individual OBs. The observed heterogeneity suggests that the mature OB phenotype is not a single unique phenotype but rather encompasses a flexible pattern of expression from the repertoire of OB-associated markers.

Site selectivity of osteoblast gene expression response to thyroid hormone localized by in situ hybridization

We have previously reported that thyroid-stimulating hormone (TSH)-suppressive doses of L-thyroxine (L-T4) decrease femoral, but not vertebral, bone mineral density (BMD) in rats. L-T4-induced decreases in BMD were associated with increased expression of genes, reflecting osteoblast activity in mRNA extracted from whole femurs but not from vertebrae. To document that this skeletal selectivity reflected altered osteoblast activity, we studied gene expression by in situ hybridization in 8-wk-old rats treated with L-T4 (20 microg x 100 g body wt(-1) x day(-1)) for 4 wk. TSH-suppressive doses of L-T4 were associated with decreased femoral (0.299 +/- 0.005 vs. 0.273 +/- 0.005 g/cm2, P < 0.01), but not vertebral (0.222 +/- 0.004 vs. 0.218 +/- 0.003 g/cm2), BMD. In situ hybridization documented that L-T4 administration for 4 wk increased expression of osteocalcin and alkaline phosphatase mRNA in femoral, but not vertebral, osteoblasts. This study demonstrates a differential gene expression response of vertebral and femoral osteoblasts to L-T4. This altered degree of gene expression markers of osteoblast activity documented by in situ hybridization may in part explain the apparent clinical differences in the effect of L-T4 on femoral and vertebral BMD.

Alkaline phosphatase in osteoblasts is down-regulated by pulsatile fluid flow

It is our hypothesis that interstitial fluid flow plays a role in the bone remodeling response to mechanical loading. The fluid flow-induced expression of three proteins (collagen, osteopontin, and alkaline phosphatase) involved in bone remodeling was investigated. Rat calvarial osteoblasts subjected to pulsatile fluid flow at an average shear stress of 5 dyne/cm2 showed decreased alkaline phosphatase (AP) mRNA expression after only 1 hour of flow. After 3 hours of flow, AP mRNA levels had decreased to 30% of stationary control levels and remained at this level for an additional 5 hours of flow. Steady flow (4 dyne/cm2 fluid shear stress), in contrast, resulted in a delayed and less dramatic decrease in AP mRNA expression to 63% of control levels after 8 hours of flow. The reduced AP mRNA expression under pulsatile flow conditions was followed by reduced AP enzyme activity after 24 hours. No changes in collagen or osteopontin mRNA expression were detected over 8 hours of pulsatile flow. This is the first time fluid flow has been shown to affect gene expression in osteoblasts.

Detection of a proliferation specific gene during development of the osteoblast phenotype by mRNA differential display

Fetal rat calvarial-derived osteoblasts in vitro (ROB) reinitiate a developmental program from growth to differentiation concomitant with production of a bone tissue-like organized extracellular matrix. To identify novel genes which may mediate this sequence, we isolated total RNA from three stages of the cellular differentiation process (proliferation, extracellular matrix maturation, and mineralization), for screening gene expression by the differential mRNA display technique. Of 15 differentially displayed bands that were analyzed by Northern blot analysis, one prominent 310 nucleotide band was confirmed to be proliferation-stage specific. Northern blot analysis showed a 600-650 nt transcript which was highly expressed in proliferating cells and decreased to trace levels after confluency and throughout the differentiation process. We have designated this transcript PROM-1 (for proliferating cell marker). A full length PROM-1 cDNA of 607 bp was obtained by 5' RACE. A short open reading frame encoded a putative 37 amino acid peptide with no significant similarity to known sequences. Expression of PROM-1 in the ROS 17/2.8 osteosarcoma cell line was several fold greater than in normal diploid cells and was not downregulated when ROS 17/2.8 cells reached confluency. The relationship of PROM-1 expression to cell growth was also observed in diploid fetal rat lung fibroblasts. Hydroxyurea treatment of proliferating osteoblasts blocked PROM-1 expression; however, its expression was not cell cycle regulated. Upregulation of PROM-1 in response to TGF-beta paralleled the stimulatory effects on growth as quantitated by histone gene expression. In conclusion, PROM-1 represents a small cytoplasmic polyA containing RNA whose expression is restricted to the exponential growth period of normal diploid cells; the gene appears to be deregulated in tumor derived cell lines.

Effects of the vitamin D3 analog 1 alpha, 25-dihydroxyvitamin D3-3 beta-bromoacetate on rat osteosarcoma cells: comparison with 1 alpha, 25-dihydroxyvitamin D3

The actions of the hormonal form of vitamin D, 1 alpha, 25-dihydroxyvitamin D3 [1 alpha, 25-(OH)2 D3], are mediated by both genomic and nongenomic mechanisms. Several vitamin D synthetic analogs have been developed in order to identify and characterize the site(s) of action of 1 alpha, 25-(OH)2D3 in many cell types including osteoblastic cells. We have compared the effects of 1 alpha, 25-(OH)2D3 and a novel 1 alpha, 25-(OH)2D3 bromoester analog (1,25-(OH)2-BE) that covalently binds to vitamin D receptors. Rat osteosarcoma cells that possess (ROS 17/2.8) or lack (ROS 24/1) the classic intracellular vitamin D receptor were studied to investigate genomic and nongenomic actions. In ROS 17/2.8 cells plated at low density, the two vitamin D compounds (1 x 10(-8) M) caused increased cell proliferation, as assessed by DNA synthesis and total cell counts. Northern blot analysis revealed that the mitogenic effect of both agents was accompanied by an increase in steady-state osteocalcin mRNA levels, but neither agent altered alkaline phosphatase mRNA levels in ROS 17/2.8 cells. ROS 17/2.8 cells responded to 1,25-(OH)2-BE but not the natural ligand with a significant increase in osteocalcin secretion after 72, 96, 120, and 144 hr of treatment. Treatment of ROS 17/2.8 cells with the bromoester analog also resulted in a significant decrease in alkaline phosphatase-specific activity. To compare the nongenomic effects of 1 alpha, 25-(OH)2D3 and 1,25-(OH)2-BE intracellular calcium was measured in ROS 24/1 cells loaded with the fluorescent calcium indicator Quin 2. At 2 x 10(-8) M, both 1 alpha,25-(OH)2D3 and 1, 25-(OH)2-BE increased intracellular calcium within 5 min. Both the genomic and nongenomic actions of 1,25-(OH)2-BE are similar to those of 1 alpha,25-(OH)2D3, and since 1,25-(OH)2-BE has more potent effects on osteoblast function than the naturally occurring ligand due to more stable binding, this novel vitamin D analog may be useful in elucidating the structure and function of cellular vitamin D receptors.

Expression and regulation of galectin 3 in rat osteoblastic cells

Galectin 3 is an endogenous soluble beta-galactoside-specific lectin originally identified and termed epsilon BP or IgE-binding protein in rat basophilic leukemia cells, but its wide tissue distribution and the multiple contexts in which it has been isolated have suggested that its function may not be limited to IgE binding but may include a role in cell growth regulation and differentiation, neoplastic transformation, and cell adhesion (Liu, 1990, Crit. Rev. Immunol., 10:289-306; Barondes et al., 1994, J. Biol. Chem., 269:20807-20810). After immunoscreening of a lambda gt11 cDNA expression library made from bone-nodule forming cultures of fetal rat calvaria (RC) cells with an antibody raised against osteoblastic cells (Turksen et al., 1992, J. Histochem. Cytochem., 40:1339-1352), three cDNA clones were isolated and sequenced; the sequence matched that of rat galectin 3. Galectin 3 mRNA was detected in various fetal and adult rat tissues, including calvaria and cultured RC cells. In RC cells and the rat osteosarcoma cell line ROS 17/2.8, galectin 3 mRNA expression increased with time in culture, in contrast to its behavior in fetal rat skin fibroblasts (RSF) in which its expression decreased with time in culture. In a second rat osteosarcoma line, UMR 106.01, galectin 3 mRNA was almost nondetectable. The synthetic glucocorticoid dexamethasone (Dex) enhanced galectin 3 expression in RSF cell cultures, while 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) had no significant effect. In contrast, Dex downregulated and 1,25(OH)2D3 upregulated galectin 3 expression in RC and ROS 17/2.8 cells, especially at later time points in culture when expression of osteoblast-associated differentiation markers by these cell types is most marked. Immunolabeling with an antibody against rat galectin 3 to identify galectin 3 protein showed that cells labelled within both the ROS 17/2.8 and RC populations but with marked intercellular heterogeneity of intensity. Our data support the conclusion that galectin 3 is a previously unrecognized product of osteoblastic cells, that galectin 3 mRNA and protein expression increases with time in vitro concomitant with other markers of osteogenesis, including formation of bone nodules and expression of osteoblast-associated markers such as alkaline phosphatase, bone sialo-protein, and osteocalcin, and that its expression is regulated by hormones such as glucocorticoids and 1,25(OH)2D3 that modulate other aspects of the osteoblast phenotype.

Parathyroid hormone regulates osteoblast differentiation positively or negatively depending on the differentiation stages

The effects of parathyroid hormone (1-34) (PTH (1-34) on osteoblast differentiation were investigated using primary osteoblast-like cells isolated from newborn mouse calvaria. The osteoblast-like cells cultured at low cell densities, in which the cells remained in a subconfluent state at the end of culture, were exposed for 7 days to PTH. This stimulated alkaline phosphatase (ALP) activity in a dose-dependent manner. In contrast, PTH dose-dependently inhibited both ALP activity and osteocalcin production in cells inoculated at high cell densities, in which they had reached a confluent state before the end of culture. The changes of ALP activity by PTH were accompanied with the expression of ALP messenger RNA. PTH induced no changes of the hydroxyproline content in the cell layer when the cells were exposed to the hormone at a subconfluent state, but reduced the content at a postconfluent state. The stimulation of ALP activity by PTH at a preconfluent state was retained even after the removal of PTH from the culture media. The opposite effect of PTH, observed between the preconfluent and the postconfluent state, was reproduced by adding dibutyryl cyclic adenosine monophosphate (cAMP) or forskolin, but not by adding phorbol myristate acetate. In a colony-forming unit fibroblastic (CFU-F) assay, using bone marrow cells isolated from tibiae of 10-week-old mice, PTH induced no changes in the total number of CFU-Fs, but increased the proportion of ALP-positive colonies. These results indicate that PTH exerts opposite effects on the phenotypic expression of osteoblasts, depending on their differentiation stages of osteoblasts. PTH may preferentially stimulate osteoblast differentiation in immature osteoblasts but inhibit it in more mature cells.

Fibronectin regulates calvarial osteoblast differentiation

The secretion of fibronectin by differentiating osteoblasts and its accumulation at sites of osteogenesis suggest that fibronectin participates in bone formation. To test this directly, we determined whether fibronectin-cell interactions regulate progressive differentiation of cultured fetal rat calvarial osteoblasts. Spatial distributions of alpha 5 integrin subunit, fibronectin, osteopontin (bone sialoprotein I) and osteocalcin (bone Gla-protein) were similar in fetal rat calvaria and mineralized, bone-like nodules formed by cultured osteoblasts. Addition of anti-fibronectin antibodies to cultures at confluence reduced subsequent formation of nodules to less than 10% of control values, showing that fibronectin is required for normal nodule morphogenesis. Anti-fibronectin antibodies selectively inhibited steady-state expression of mRNA for genes associated with osteoblast differentiation; mRNA levels for alkaline phosphatase and osteocalcin were suppressed, whereas fibronectin, type I collagen and osteopontin were unaffected. To identify functionally relevant domains of fibronectin, we treated cells with soluble fibronectin fragments and peptides. Cell-binding fibronectin fragments (type III repeats 6–10) containing the Arg-Gly-Asp (RGD) sequence blocked both nodule initiation and maturation, whether or not they contained a functional synergy site. In contrast, addition of the RGD-containing peptide GRGDSPK alone did not inhibit nodule initiation, although it did block nodule maturation. Thus, in addition to the RGD sequence, other features of the large cell-binding fragments contribute to the full osteogenic effects of fibronectin. Nodule formation and osteoblast differentiation resumed after anti-fibronectin antibodies or GRGDSPK peptides were omitted from the media, showing that the inhibition was reversible and the treatments were not cytotoxic. Outside the central cell-binding domain, peptides from the IIICS region and antibodies to the N terminus did not inhibit nodule formation. We conclude that osteoblasts interact with the central cell-binding domain of endogenously produced fibronectin during early stages of differentiation, and that these interactions regulate both normal morphogenesis and gene expression.

Effect of age on the expression of insulin-like growth factor-I, interleukin-6, and transforming growth factor-beta mRNAs in rat femurs following marrow ablation

The expression of insulin-like growth factor-I (IGF-I), interleukin-6 (IL-6), and transforming growth factor-beta 1 (TGF-beta 1) mRNA in rat femurs was examined following marrow ablation. Northern blot analysis showed multiple transcripts of IGF-I, a major transcript of 1.3 kb and a minor one of 2.4 kb for IL-6 and a single band of 2.5 kb for TGF-beta 1, respectively. Examination of the temporal activation pattern showed IGF-I expression peaked at day 3 (150% over the basal level) after injury and preceded the maximal expression of procollagen alpha 1(I), osteopontin, alkaline phosphatase, and osteocalcin mRNAs. This suggests that IGF-I is involved mainly in osteoblast development and bone formation. In contrast, IL-6 expression was elevated between days 3 and 9 (45-60% over the basal level). The sustained elevation of IL-6 expression at day 9 is consistent with the role for this cytokine in the development of osteoclasts and bone resorption. The expression of TGF-beta 1 was not altered up to day 9 after marrow ablation. While the temporal expression patterns of IGF-I and IL-6 mRNA did not differ between adult and old rats, the maximal level of IGF-I mRNA at day 3 was 72% higher in adult as compared to old bones. In contrast, the peak level of IL-6 mRNA at days 6-9 was 45% higher in old as compared to adult bones. Although the level of TGF-beta 1 mRNA did not change following marrow ablation, levels of TGF-beta 1 were consistently higher in old rats. Our results suggest that the impaired bone formation and elevated bone resorption in aged animals may be due in part to the reduced expression of IGF-I and an overexpression of IL-6 in old bone.

cGMP produced in response to ANP and CNP regulates proliferation and differentiation of osteoblastic cells

The effects of natriuretic peptides on the proliferation and differentiation of osteoblast-like cells from rat calvariae were examined. Natriuretic peptides are physiological agonists that activate receptor guanylate cyclases, namely, natriuretic peptide receptor (NPR)-A and NPR-B. Exposure of cells to atrial natriuretic peptide (ANP) and C-type natriuretic peptide (CNP) resulted in large increases in the rate of intracellular production of guanosine 3',5'-cyclic monophosphate (cGMP). Moreover, CNP-like immunoreactivity was detected in the conditioned medium from osteoblast-like cells, while ANP was undetectable. In cells exposed to natriuretic peptides, a dose-dependent reduction in the rate of DNA synthesis was observed. Natriuretic peptides also stimulated the activity of alkaline phosphatase (ALPase) and the expression of mRNA for ALPase and osteocalcin and the mineralization of nodules by the cultured cells. These results could be reproduced by treating cells with 8-bromo-cGMP. Endothelin-1, whose physiological functions are the opposite of those of natriuretic peptides, decreased the ALPase activity and the mineralization of nodules. In the present study, natriuretic peptides were demonstrated to promote bone formation via the action of cGMP in a signal-transduction pathway mediated by specific receptors in osteoblast-like cells.

Regulation of osteopontin expression by type I collagen in preosteoblastic UMR201 cells

When UMR201 cells, phenotypically preosteoblastic, were placed onto a type I collagen gel, expression of osteopontin (OP) mRNA and protein were strongly upregulated, compared to cells plated onto plastic. This upregulation was dose-dependent, with respect to the concentration of collagen gel, and was observable within hours of cells having attached and spread on the substrate. Retinoic acid (RA) acted synergistically with type I collagen at each concentration to induce a much greater increase in OP mRNA than in cells on plastic. In addition, RA increased the phosphorylation of secreted OP. The exogenous collagen substrate inhibited the growth of UMR201 cells, with the extent and duration of inhibition dependent on the collagen concentration. The effect of type I collagen was specific; plating cells on fibronectin, laminin or vitronectin did not upregulate OP expression. In contrast to the effects on OP expression, the strong RA induction of alkaline phosphatase (ALP) mRNA in cells on plastic was attenuated in cells plated on type I collagen. Growth on type I collagen did not change OP mRNA stability or transcription rate, although there was decreased stability of the ALP mRNA in cells on collagen.

Systemic administration of transforming growth factor-beta 2 prevents the impaired bone formation and osteopenia induced by unloading in rats

We investigated the effect of recombinant human transforming growth factor beta 2 (rhTGF-beta 2) administration on trabecular bone loss induced by unloading in rats. Hind limb suspension for 14 d inhibited bone formation and induced osteopenia as shown by decreased bone volume, calcium and protein contents in long bone metaphysis. Systemic infusion of rhTFG-beta 2 (2 micrograms/kg per day) maintained normal bone formation rate, and prevented the decrease in bone volume, bone mineral content, trabecular thickness and number induced by unloading. In vitro analysis of tibial marrow stromal cells showed that rhTGF-beta 2 infusion in unloaded rats increased the proliferation of osteoblast precursor cells, but did not affect alkaline phosphatase activity or osteocalcin production. Northern blot analysis of RNA extracted from the femoral metaphysis showed that rhTGF-beta 2 infusion in unloaded rats increased steady-state levels of type I collagen mRNA but not alkaline phosphatase mRNA levels. rhTGF-beta 2 infusion at the dose used had no effect on metaphyseal bone volume and formation, osteoblast proliferation or collagen expression in control rats. The results show that systemic administration of rhTGF-beta 2 enhances osteoblast precursor cell proliferation and type I collagen expression by osteoblasts, and prevents the impaired bone formation and osteopenia induced by unloading.

Tissue-nonspecific alkaline phosphatase participates in the establishment and growth of feather germs in embryonic chick skin cultures

Alkaline phosphatase activity is present in the mesoderm of embryonic chick skin and becomes spatially restricted to the dermal condensation of the developing feather germs. Inhibitors to tissue-nonspecific (liver/bone/kidney), but not intestinal alkaline phosphatase inhibit the establishment and growth of feather germs in cultured skins. A window of maximum sensitivity to the inhibitor was observed to be the first day of culture when early development and establishment of pattern takes place. The cDNA for the avian tissue-nonspecific alkaline phosphatase was cloned and sequenced, and Southern analysis revealed a single copy of this gene in the avian genome. Northern analysis revealed that a 2.8 kb transcript for this form of alkaline phosphatase is present in developing skin.

Effect of platelet-derived growth factor on DNA synthesis and gene expression in bone marrow stromal cells derived from adult and old rats

The effects of platelet-derived growth factor (PDGF) on DNA synthesis and mRNA expression of osteoblast markers in marrow stromal cells derived from adult (6 months) and old (24 months) rats were examined. Treatment of stromal cells from adult rats with dexamethasone induced the appearance of osteoblast-like cells. PDGF partially also inhibited the differentiation of stromal cells induced by dexamethasone. In cultures of serum-starved stromal cells, PDGF stimulated [3H]-thymidine incorporation into DNA in a dose-dependent manner with a maximum stimulation of 15-fold at 500 ng/ml. By comparison, insulin-like growth factor (IGF-I) has a small effect on [3H]-thymidine incorporation. The effect of PDGF and IGF-I on DNA synthesis was additive. Treatment of the confluent stromal cells from adult rats with PDGF increased the mRNA level of osteopontin fourfold without any significant effect on alkaline phosphatase and type I collagen mRNAs. In contrast, dexamethasone stimulated the mRNA expression of alkaline phosphatase, type I collagen, and osteopontin 2.1-, 2.3-, and 14-fold, respectively. Addition of PDGF to dexamethasone-treated cells failed to induce any further increase in osteopontin expression whereas the expression of alkaline phosphatase and type I collagen was partially reduced. The expression of osteocalcin mRNA was negligible in stromal cells but stimulated several fold by dexamethasone and 1,25(OH)2D3. PDGF inhibited drastically the elevation of osteocalcin mRNA. In contrast, IGF-I stimulated type I collagen expression 100% without any appreciable effect on the expression of osteopontin and alkaline phosphatase. The stimulatory effect of PDGF on osteopontin expression was augmented by IGF-I. Furthermore, PDGF attenuated the stimulatory effect of IGF-I on type I collagen expression. The responses of cultured cells from old rats to growth factors were also examined. PDGF or PDGF plus IGF-I increased [3H]-thymidine incorporation in stromal cells from old rats but to a lesser extent. However, PDGF was equally effective in stimulating osteopontin expression in cells from both adult and old rats. We concluded that PDGF is a potent mitogen but that the response of stromal cells from old rats is impaired. In addition, PDGF stimulates osteopontin expression in stromal cells and this effect is not age dependent.

Administration of colony stimulating factor-1 to toothless osteopetrotic rats normalizes osteoblast, but not osteoclast, gene expression

The toothless (tl) osteopetrotic mutation in the rat is characterized by generalized skeletal sclerosis, a severe reduction in the numbers of osteoclasts, monocytes, and macrophages, and absence of tooth eruption. Studies examining gene expression in bone-derived cells of tl rats and their normal littermates have shown that genes related to osteoblast function are aberrantly expressed in tl rats compared to normal littemates. We have previously shown that exogenous administration of colony stimulating factor-1 (CSF-1) to tl rats results in a dramatic reduction of the skeletal sclerosis and significant increases in the number of osteoclasts. Thus, we examined the effects of CSF-1 on osteoblast and osteoclast gene expression in tl rats as demonstrated by Northern blot analysis. While osteoblast-related gene expression as reflected by mRNA levels of alkaline phosphatase, osteocalcin, osteopontin, and type I collagen was normalized, osteoclast-related gene expression, as reflected by mRNA levels of carbonic anhydrase II and tartrate-resistant adenosine triphosphatase, remained significantly lower in CSF-1-treated tl rats compared to untreated normal littermates. Since previous studies have not demonstrated the CSF-1 receptor on osteoblasts, these results suggest that osteoblast abnormalities in tl rats are an effect of the osteopetrotic condition rather than the cause of the disease.

Dexamethasone promotes von kossa-positive nodule formation and increased alkaline phosphatase activity in costochondral chondrocyte cultures

This study examined the effect of dexamethasone on von Kossa-positive nodule formation and alkaline phosphate specific activity of costochondral chondrocytes at two distinct stages of maturation. The nodules formed by the more mature growth zone chondrocyte cultures contained von Kossa-positive deposits in the extracellular matrix that had a punctate morphology. The nodules formed by the less mature resting zone cells also contained von Kossa-positive deposits, but differentiation was delayed by three-to-five days compared to the growth zone cell cultures. Dexamethasone stimulated the number of nodules formed and shortened the length of time required for von Kossa-positive nodule formation in both types of cultures. During the first 48 h of exposure to dexamethasone, alkaline phosphatase specific activity in the cell layer of both resting zone and growth zone cultures was increased in a dose-dependent manner. At 12 days post-confluence and thereafter, enzyme activity was inhibited in the dexamethasone-treated cultures. Changes in matrix vesicle alkaline phosphatase specific activity reflected those changes seen in the cell layer after dexamethasone treatment, but with higher magnitude, suggesting that one effect of dexamethasone might be to regulate matrix vesicle function. With the exception of one culture, the chondrocytes did not synthesize type X collagen under any of the experimental conditions used. Fourier transform infrared spectroscopy (FTIR) failed to detect the presence of calcium phosphates in any of the cultures exposed to dexamethasone except one. These results demonstrate that dexamethasone promotes early differentiation events, including nodule formation and increased alkaline phosphatase activity, in costochondral chondrocyte cultures. The failure to detect type X collagen synthesis and mineralization in both dexamamethasone-treated and control cultures suggests that these cultures lack the factors necessary for terminal differentiation and mineralization.

Bone progenitor cell deficits and the age-associated decline in bone repair capacity

Aging bone shows a progressive decline in mass and strength. Previous studies have suggested that bone marrow stem cells are reduced with aging and that this could be responsible, in part, for age-associated bone deficits. We measured the number of osteoprogenitor cells present in the bone marrow from adult and aged rats as well as their ability to differentiate in vitro and to form bone in vivo. We found that the number of adherent colony-forming cells was significantly lower (65%) in marrow cells isolated from aged compared with adult rats. Furthermore, 88% of the colonies obtained from aged rats were alkaline phosphatase (AP) positive, whereas virtually all the colonies from adult rats were positive. The addition of dexamethasone to the culture medium decreased the proliferation of the adherent cells and reduced the number of colonies obtained from both adult and aged bone marrow, all of which were AP positive. No significant differences were found in the expression of certain major bone cell marker genes as a function of donor age. However, dexamethasone treatment increased expression of osteopontin (OP) by fivefold. Adult stromal cells not treated with dexamethasone and implanted subcutaneously in recipient rats exhibited about 10-fold greater formation of bone compared with cells from aged rats. In contrast, dexamethasone-treated cells exhibited high levels of bone formation, irregardless of donor age or the age of the recipient into which the cells were grafted. These studies are consistent with a deficit of osteoprogenitor cells in the bone marrow site as a contributing, perhaps correctable factor in the decline in bone repair and bone mass with age.

Type X collagen gene expression in mouse chondrocytes immortalized by a temperature-sensitive simian virus 40 large tumor antigen

Mouse endochondral chondrocytes were immortalized with a temperature-sensitive simian virus 40 large tumor antigen. Several clonal isolates as well as pools of immortalized cells were characterized. In monolayer cultures at the temperature permissive for the activity of the large tumor antigen (32 degrees C), the cells grew continuously with a doubling time of approximately 2 d, whereas they stopped growing at nonpermissive temperatures (37 degrees C-39 degrees C). The cells from all pools and from most clones expressed the genes for several markers of hypertrophic chondrocytes, such as type X collagen, matrix Gla protein, and osteopontin, but had lost expression of type II collagen mRNA and failed to be stained by alcian blue which detects cartilage-specific proteoglycans. The cells also contained mRNAs for type I collagen and bone Gla protein, consistent with acquisition of osteoblastic-like properties. Higher levels of mRNAs for type X collagen, bone Gla protein, and osteopontin were found at nonpermissive temperatures, suggesting that the expression of these genes was upregulated upon growth arrest, as is the case in vivo during chondrocyte hypertrophy. Cells also retained their ability to respond to retinoic acid, as indicated by retinoic acid dose-dependent and time-dependent increases in type X collagen mRNA levels. These cell lines, the first to express characteristic features of hypertrophic chondrocytes, should be very useful to study the regulation of the type X collagen gene and other genes activated during the last stages of chondrocyte differentiation.

Bone morphogenetic protein-2 converts the differentiation pathway of C2C12 myoblasts into the osteoblast lineage

The implantation of bone morphogenetic protein (BMP) into muscular tissues induces ectopic bone formation at the site of implantation. To investigate the mechanism underlying this process, we examined whether recombinant bone morphogenetic protein-2 (BMP-2) converts the differentiation pathway of the clonal myoblastic cell line, C2C12, into that of osteoblast lineage. Incubating the cells with 300 ng/ml of BMP-2 for 6 d almost completely inhibited the formation of the multinucleated myotubes expressing troponin T and myosin heavy chain, and induced the appearance of numerous alkaline phosphatase (ALP)-positive cells. BMP-2 dose dependently induced ALP activity, parathyroid hormone (PTH)-dependent 3',5'-cAMP production, and osteocalcin production at concentrations above 100 ng/ml. The concentration of BMP-2 required to induce these osteoblastic phenotypes was the same as that required to almost completely inhibit myotube formation. Incubating primary muscle cells with 300 ng/ml of BMP-2 for 6 d also inhibited myotube formation, whereas induced ALP activity and osteocalcin production. Incubation with 300 ng/ml of BMP-2 suppressed the expression of mRNA for muscle creatine kinase within 6 h, whereas it induced mRNA expression for ALP, PTH/PTH-related protein (PTHrP) receptors, and osteocalcin within 24-48 h. BMP-2 completely inhibited the expression of myogenin mRNA by day 3. By day 3, BMP-2 also inhibited the expression of MyoD mRNA, but it was transiently stimulated 12 h after exposure to BMP-2. Expression of Id-1 mRNA was greatly stimulated by BMP-2. When C2C12 cells pretreated with BMP-2 for 6 d were transferred to a colony assay system in the absence of BMP-2, more than 84% of the colonies generated became troponin T-positive and ALP activity disappeared. TGF-beta 1 also inhibited myotube formation in C2C12 cells, and suppressed the expression of myogenin and MyoD mRNAs without inducing that of Id-1 mRNA. However, no osteoblastic phenotype was induced by TGF-beta 1 in C2C12 cells. TGF-beta 1 potentiated the inhibitory effect of BMP-2 on myotube formation, whereas TGF-beta 1 reduced ALP activity and osteocalcin production induced by BMP-2 in C2C12 cells. These results indicate that BMP-2 specifically converts the differentiation pathway of C2C12 myoblasts into that of osteoblast lineage cells, but that the conversion is not heritable.

In vivo and in vitro effects of insulin-like growth factor-I (IGF-I) on femoral mRNA expression in old rats

The in vivo response of bone to IGF-I infusion in a marrow ablation model and the effect of IGF-I on bone marrow stromal cells in vitro was evaluated. IGF-I (25 ng/day), infused directly into femur, stimulated the expression of alkaline phosphatase, procollagen alpha 1 (I) and osteopontin mRNA, while osteocalcin mRNA was not affected. The dose dependency to IGF-I was bi-phasic, with stimulation at 25 and 50 ng but not at 150 ng/day. The effect of IGF-I was observed in the aged but not in the adult rat femur. However, the elevated mRNA levels in old bones with IGF-I treatment were still below those observed in adult bones. The effect of IGF-I was also examined in cultured stromal cells. IGF-I (50 ng/ml) stimulates the expression of alkaline phosphatase, procollagen alpha 1 (I), osteopontin and osteocalcin mRNA in stromal cells from both adult and old rats. These results suggest that the lack of response of adult bone to IGF-I in vivo was not due to the impaired response of the stromal cells to IGF-I. Differences in the responses of stromal cells from adult and old animals were noted. In the presence of serum (10%), stromal cells from adult rats were stimulated to synthesize DNA at lower levels of IGF-I than stromal cells from old animals. Our results show that IGF-I can stimulate mRNA expression of osteoblast markers in vivo in aged rats in a marrow ablation model and enhance DNA synthesis and gene expression in cultured marrow stromal cells from old rats. Thus, it is possible that exogenous IGF-I could be beneficial in treating age-associated osteopenia.

Differential effects of warfarin on mRNA levels of developmentally regulated vitamin K dependent proteins, osteocalcin, and matrix GLA protein in vitro

The role of the vitamin K dependent proteins, osteocalcin which is bone specific and matrix Gla protein (MGP) found in many tissues, has been studied by inhibition of synthesis of their characteristic amino acid, gamma-carboxyglutamic acid (Gla) with the anticoagulant sodium warfarin. The effect of sodium warfarin on expression of these proteins, and other phenotypic markers of bone and cartilage during cellular differentiation and development of tissue extracellular matrix, was examined in several model systems. Parameters assayed include cell growth (reflected by histone gene expression) and collagen types I and II, osteopontin, alkaline phosphatase, and mineralization. Studies were carried out in calvarial bone organ cultures, normal diploid rat osteoblast and chondrocyte cultures, and rat osteosarcoma cell lines ROS 17/2.8 and 25/1. In normal diploid cells, warfarin consistently stimulated cell proliferation (twofold). In osteoblast cultures, MGP mRNA levels were generally increased (three to tenfold). Notably, MGP mRNA levels were not affected in chondrocyte cultures, either with chronic or acute warfarin treatments. Osteocalcin mRNA levels and synthesis were decreased up to 50% in ROS 17/2.8 cells and in chronically treated (1 and 5 micrograms/ml sodium warfarin) rat osteoblast cultures after 22 days. Early stages of osteoblast phenotype development from the proliferation period to initial tissue formation (nodules) appeared unaffected; while after day 14, further growth and mineralization of the nodule areas were significantly decreased in warfarin-treated cultures. In summary, warfarin has opposing effects on the expression of two vitamin K dependent proteins, MGP and osteocalcin, in osteoblast cultures and MGP is regulated differently between cartilage and bone as reflected by cellular mRNA levels. Additionally, warfarin effects expression of nonvitamin K dependent proteins which may reflect the influence of warfarin on endoplasmic reticulum associated enzymes.

TGF beta alters growth and differentiation related gene expression in proliferating osteoblasts in vitro, preventing development of the mature bone phenotype

This study examines the mechanism by which TGF-beta 1, an important mediator of cell growth and differentiation, blocks the differentiation of normal rat diploid fetal osteoblasts in vitro. We have established that the inability for pre-osteoblasts to differentiate is associated with changes in the expression of cell growth, matrix forming, and bone related genes. These include histone, jun B, c-fos, collagen, fibronectin, osteocalcin, alkaline phosphatase, and osteopontin. Morphologically, the TGF-beta 1-treated osteoblasts exhibit an elongated, spread shape as opposed to the characteristic cuboidal appearance during the early stages of growth. This is followed by a decrease in the number of bone nodules formed and the amount of calcium deposition. These effects on differentiation can occur without dramatic changes in cell growth if TGF-beta 1 is given for a short time early in the proliferative phase. However, continuous exposure to TGF-beta 1 leads to a bifunctional growth response from a negative effect during the proliferative phase to a positive growth effect during the later matrix maturation and mineralization phases of the osteoblast developmental sequence. Extracellular matrix genes, fibronectin, osteopontin and alpha 1(I) collagen, are altered in their expression pattern which may provide an aberrant matrix environment for mineralization and osteoblast maturation and potentiate the TGF-beta 1 response throughout the course of osteoblast differentiation. The initiation of a TGF-beta 1 effect on cell growth and differentiation is restricted to the proliferative phase of the culture before the cells express the mature osteoblastic phenotype. Second passage cells that are accelerated to differentiate by the addition of dexamethasone or by seeding cultures at a high density are refractory to TGF-beta 1. These in vitro results indicate that TGF-beta 1 exerts irreversible effects at a specific stage of osteoblast phenotype development resulting in a potent inhibition of osteoblast differentiation at concentrations from 0.1 ng/ml.