1,25-Dihydroxyvitamin D3 receptor-like macromolecule in rat osteogenic sarcoma cell lines

Commonly used mouse models of osteosarcoma

Osteosarcoma is the commonest primary tumour of bone and the second highest cause of cancer-related death in the paediatric age group. Little is known of the aetiology of human osteosarcoma and lesser still of the various interactions that occur between host and tumour cells that govern growth and progression of osteosarcoma in vivo. Although numerous osteosarcoma cell lines have been established and characterized in vitro, some as far back as in the 1960s, there is a scarcity of reliable and reproducible in vivo animal models that mimics all aspects of the human condition at the temporal, physiological and histopathological level, hence, making the accurate testing of therapeutic strategies difficult. Given that osteosarcoma is a disease that affects young people and better disease management strategies are essential, development of a robust human osteosarcoma model is long overdue.

A new vitamin D analog, 2MD, restores trabecular and cortical bone mass and strength in ovariectomized rats with established osteopenia

An orally active, highly potent analog of 1alpha,25-dihydroxyvitamin D3, 2MD, restores trabecular and cortical bone mass and strength by stimulating periosteal bone formation and decreasing trabecular bone resorption in OVX rats with established osteopenia.

INTRODUCTION

The purposes of this study were to determine the effects of long-term treatment with 2-methylene-19-nor-(20S)-1alpha,25(OH)2D3 (2MD) on restoring bone mass and bone strength in ovariectomized (OVX) rats with established osteopenia and 2MD effects on bone formation and bone resorption on trabecular and cortical bone surfaces.

MATERIALS AND METHODS

Sprague-Dawley female rats were sham-operated (sham) or OVX at 4 months of age. Beginning at 8 weeks after OVX, OVX rats were orally dosed with 2MD at 0.5, 1, 2.5, 5, or 10 ng/kg/day for 16 weeks. Serum calcium was measured at 6, 13, and 16 weeks after treatment, and bone mass and structure, bone formation, bone resorption, and bone strength were determined at the end of the study.

RESULTS

Serum calcium did not change significantly with 2MD at 0.5 or 1 ng/kg/day, whereas it significantly increased at 2.5, 5, or 10 ng/kg/day. 2MD significantly and dose-dependently increased total body BMD, total BMC, and stiffness of femoral shaft (FS), maximal load and stiffness of femoral neck, and toughness of the fifth lumbar vertebral body (L5) at all doses compared with OVX controls. In 2MD-treated OVX rats, there was a dose-dependent increase in total BMD and total BMC of the distal femoral metaphysis (DFM), trabecular bone volume of L3, ultimate strength and stiffness of L5, and maximal load of FS compared with OVX controls at dosages>or=1 ng/kg/day. At dosages>2.5 ng/kg/day, most of the bone mass and bone strength related parameters were significantly higher in 2MD-treated OVX rats compared with sham controls. Bone histomorphometric analysis of L3 showed dose-dependent decreases in osteoclast number and osteoclast surface on trabecular bone surface and a dose-dependent increase in periosteal bone formation associated with 2MD treatment.

CONCLUSIONS

2MD not only restored both trabecular and cortical bone mass but also added bone to the osteopenic OVX rats beyond that of sham controls by stimulating bone formation on the periosteal surface and decreasing bone resorption on the trabecular surface. 2MD increased bone mass and strength at doses that did not induced hypercalcemia.

Calbindin-D28k is expressed in osteoblastic cells and suppresses their apoptosis by inhibiting caspase-3 activity

The rate of osteoblast apoptosis is a critical determinant of the rate of bone formation. Because the calcium-binding protein calbindin-D(28k) has anti-apoptotic properties in neuronal cells and lymphocytes, we searched for the presence of this protein in osteoblastic cells and investigated whether it can modify their response to proapoptotic signals. Calbindin-D(28K) was expressed at low levels in several osteoblastic cell lines and at high levels in primary cultures of murine osteoblastic cells. Transient transfection of rat calbindin-D(28k) cDNA blocked tumor necrosis factor alpha (TNFalpha)-induced apoptosis in osteoblastic MC3T3-E1 cells, as determined by cell viability and nuclear morphology of cells cotransfected with the green fluorescent protein targeted to the nucleus, whereas transfection of the empty vector had no effect. Calbindin-D(28k) levels in several stably transfected MC3T3-E1 lines were directly related to protection from TNFalpha-induced apoptosis. Purified rat calbindin-D(28k) markedly reduced the activity of caspase-3, a critical molecule for the degradation phase of apoptosis, in a cell-free assay. In addition, cell extracts from MC3T3-E1 cells expressing high levels of calbindin-D(28k) decreased caspase-3 activity, compared with extracts from vector-transfected cells. This effect was apparently unrelated to the calcium binding properties of calbindin, as chelation of calcium by EGTA or addition of other calcium-binding proteins such as calbindin-D(9k), S100, calmodulin, and osteocalcin, did not affect caspase-3 activity. Last, calbindin-D(28k) interacts with the active form of caspase-3 as demonstrated by a GST pull-down assay. These results demonstrate that calbindin-D(28k) is a biosynthetic product of osteoblasts with a role in the regulation of apoptosis. They also reveal that the antiapoptotic properties of calbindin-D(28k) may result not only from calcium buffering but also from the ability of the protein to interact with and to inhibit caspase-3 activity, a property that is independent of its calcium binding capability.

The mouse vitamin D receptor is mainly expressed through an Sp1-driven promoter in vivo

The availability of the mouse vitamin D receptor (mVDR) gene has allowed a characterization of a TATA-less promoter containing a cluster of four Sp1 sites named Sp1-1, Sp1-2, Sp1-3, and Sp1-4 (F. Jehan and H. F. DeLuca, 1997, Proc. Natl. Acad. Sci. USA 94, 10138-10143). By means of primer extension analysis, S1 nuclease mapping and ribonuclease protection assay, the start site has been deduced, as has the existence of other minor transcription start sites. Initiation of transcription at the major site is located 4 bp upstream of the 5' end of the mVDR cDNA sequence and very close to the putative Sp1 sites. A second minor promoter might exist between exon 1 and exon 2 of the mVDR gene. The nucleotide sequence of the Sp1 region is well conserved between the mouse, the human, and the chicken VDR genes, suggesting an important role for these Sp1 sites. Gel shift analysis of the four Sp1 sites of the mVDR promoter has confirmed specific binding complexes to Sp1-1, Sp1-2, and Sp1-4 (Sp1-3 rather binds an unknown complex that is unable to bind the canonical Sp1 GGGGCGGGGC). Deletion or mutation of all the Sp1 sites eliminates promoter activity. However, mutation or deletion of individual Sp1 sites did not dramatically change the promoter activity, except for mutation of Sp1-3 that increases promoter activity. We, therefore, conclude that the mVDR promoter is controlled by the Sp1 sites and is the main VDR promoter in intestine and kidney.

Structure-function studies of new C-20 epimer pairs of vitamin D3 analogs

A growing number of calcitriol (1alpha,25-dihydroxyvitamin D3) analogs have become available in recent years. Many of these analogs exhibit lower calcemic effects than calcitriol and inhibit cell proliferation and enhance cell differentation more efficiently than calcitriol. We have compared structure-function relationships of a series of new C-20 epimer (20-epi) vitamin D3 analogs with their natural C-20 counterparts. In human MG-63 osteosarcoma cells, quantification of cellular osteocalcin mRNA levels by Northern blot analysis and osteocalcin biosynthesis by radioimmunoassay indicated that most studied analogs at a concentration of 10 nm induced osteocalcin gene expression more efficiently than the parent compound, calcitriol. Interestingly, when the biological responses were compared with the binding affinities of the analogs to in-vitro translated human vitamin D receptor and with their ability to protect the receptor against partial proteolytic digestion, significant correlations were not observed. Further, molecular modelling of the compounds by energy minimization did not reveal marked differences in the three-dimensional structures of the analogs. These results suggest that higher than normal ligand binding affinity or 'natural' conformation of the ligand-receptor complex are not necessarily required for the 'superagonist' transactivation activity. The mechanism of action of the efficient analogs may involve stabilization and/or differential binding of transcriptional coactivators or transcription intermediary factors to the hVDR during transactivation.

Influence of skeletal site of origin and donor age on 1,25(OH)2D3-induced response of various osteoblastic markers in human osteoblastic cells

Age-related bone loss may be a consequence of a lack of osteoblastic formation and/or function. In vitro, the osteoblastic response to 1,25(OH)2D3, an important regulator of osteoblastic function, appears to depend on the stage of osteoblastic maturation. In this study, we examined the response to 1,25(OH)2D3 of C-terminal type I procollagen (PICP), alkaline phosphatase (ALP), and osteocalcin (OC) secretion in primary cultures of osteoblastic cells from human trabecular bone (hOB). Forty-four bone samples were obtained from subjects undergoing knee arthroplastia, 20 aged 50-70 (64 +/- 5), and 24 >70 (73 +/- 2) years. Another 33 bone samples were obtained from subjects undergoing hip arthroplastia, 21 were aged 50-70 (64 +/- 4) and 12 >70 (75 +/- 5) years. Pooling knee and hip hOB cell cultures, we found that PICP secretion decreased after 1,25(OH)2D3 in hOB cells from the older group (>70 years). Treatment with 1,25(OH)2D3 increased ALP secretion in these cells only in the younger group (50-70 years), whereas it increased OC secretion in hOB cells in both age groups. By pooling hOB cell cultures from both age groups we found that knee hOB cells increased OC secretion, and decreased PICP secretion, after 1,25(OH)2D3. This metabolite also increased OC secretion in hip hOB cells. Considering the influence of donor age at the same skeletal site, 1,25(OH)2D3 was found to stimulate ALP secretion only in knee hOB cells in the younger group. In contrast, this metabolite decreased ALP secretion in hip hOB cells in the older group. PICP secretion decreased after 1,25(OH)2D3 only in hOB cells in the older group, at both skeletal sites. In age-matched cultures, OC secretion was lower in hip hOB cells compared with those from the knee in the older group, but was similar in these cell cultures from both skeletal sites in the younger group. OC secretion after 1,25(OH)2D3 stimulation did not show age differences in knee hOB cells, but was lower in hip hOB in the older group. In summary, our results demonstrate that the response of various osteoblastic markers to 1,25(OH)2D3 in primary cultures of hOB cells depends on the donor age and skeletal site of origin.

Characterization of aromatase and 17 beta-hydroxysteroid dehydrogenase expression in rat osteoblastic cells

Postmenopausal loss of 17 beta-estradiol (E2) in women is associated with decreased bone mineral density and increased susceptibility to osteoporotic bone fracture. These changes in bone status are assumed to be due to circulating levels of the hormone; therapeutic replacement of E2 can alleviate the bone disease. However, recent reports have shown that human osteoblastic (OB) cells are able to synthesize estrogens locally, via expression of the enzyme aromatase. In this study, we have characterized the expression and activity of aromatase and 17 beta-hydroxysteroid dehydrogenase (17 beta-HSD) in rat OB cell lines. Aromatase activity in ROS 17/2.8, ROS 25/1, and UMR 106 cells was similar to that shown in human OB cells, with the highest levels of activity observed in the more differentiated ROS 17/2.8 cells (Vmax = 45 pmol/h/mg of protein). The rat OB cells also showed 17 beta-HSD activity, with the predominant metabolism in all three cell lines being estrone (E1) to E2. As with aromatase, the highest activity was observed in ROS 17/2.8 cells (Vmax = 800 pmol/h/mg of protein). Northern analyses indicated the variable presence of transcripts corresponding to the type 1, 2, 3, and 4 isoforms of 17 beta-HSD. Further analysis of androstenedione metabolism indicated that the net effect of aromatase and 17 beta-HSD activity varied with cell type and culture treatment. All three OB cell lines were able to synthesize E1, E2, and testosterone from androstenedione, although activity varied between OB cell types. Regulatory effects were observed with 1,25-dihydroxyvitamin D3 (positive) and dexamethasone (negative). These data suggest that local synthesis of sex hormones is an important function of OB cells and may play a key role in the modulation of bone turnover independent of circulating hormone concentrations.

Expression of Indian hedgehog in osteoblasts and its posttranscriptional regulation by transforming growth factor-beta

Indian hedgehog (Ihh) was recently reported to be expressed in chondrocytes and to regulate chondrocyte differentiation. This report examined the expression of Ihh in osteoblastic cells and its regulation by calcitropic cytokines. We found that Ihh messenger RNA (mRNA) was expressed as a single 2.5-kilobase band at a modest level in rat osteoblastic osteosarcoma ROS17/2.8 cells. In sharp contrast to the previous observation of dpp regulation of hedgehog expression in Drosophila embryos, bone morphogenetic protein-2 did not affect Ihh expression in these cells. On the other hand, treatment with 2 ng/ml transforming growth factor-beta1 (TGFbeta1) increased the steady state level of Ihh mRNA 2- to 4-fold. Western blot analysis of the cell lysates using antisera also showed enhancement of the Ihh protein level by TGFbeta1 treatment. The effect of TGFbeta1 on Ihh mRNA abundance started within 3 h, peaked at 24 h and lasted at least 48 h after the initiation of the treatment. The effect of TGFbeta1 on the increase in Ihh mRNA was dose dependent, starting at 0.2 ng/ml and saturating at 2 ng/ml. Neither actinomycin D nor cycloheximide blocked this effect. Experiments using 5,6-dichloro-1-beta-D-ribofuranosylbenzimidazole showed an enhancement of Ihh mRNA stability by TGFbeta1, indicating the presence of posttranscriptional regulation. We then examined the effects of TGFbeta1 on Ihh mRNA in osteoblast-enriched cells isolated from neonatal rat calvariae. TGFbeta1 also enhanced Ihh mRNA expression in these cells. Our data indicate for the first time that Ihh is one of the members of the cytokines produced by osteoblastic cells and that the expression of Ihh is regulated posttranscriptionally by TGFbeta.

Vitamin D receptor content and transcriptional activity do not fully predict antiproliferative effects of vitamin D in human prostate cancer cell lines

Prostate cancer cell lines exhibit variable growth suppression by the hormonal form of vitamin D3, 1,25-Dihydroxyvitamin D3 [1,25 (OH)2D] (1,25 D3). To understand the molecular basis for this differential sensitivity to 1,25 D3, we compared growth response to 1,25 d3, vitamin D receptor (VDR) content and VDR transcriptional activity in four well-characterized human prostate cancer cell lines: LNCaP, DU145, PC-3 and ALVA-31. In PC-3 and DU145 cells, relative lack of growth inhibition by 1,25 D3 (< 10% inhibition) correlates with very low levels of VDR (9-15 fmol/mg protein) compared to classical vitamin D3 target tissues (approximately 75-200 fmol/mg protein). Transfection of DU145 and PC-3 cells with a VDR cDNA expression vector is sufficient to establish growth sensitivity to 1,25 D3, suggesting that low VDR levels are responsible for the failure of these cell lines to respond to 1,24 D3. LNCaP cells are highly sensitive to growth inhibition by 1.25 D3 (approximately 55% inhibition) and contain approximately 2-3-fold more VDR (25 fmol/mg) than the relatively 1,25 D3-insensitive PC-3 and DU145 cell lines. However, ALVA-31 cells display less than 20% growth inhibition to 1.25 D3 although they contain the highest levels of VDR (45 fmol/mg) of the four cell lines. Thus, sensitivity to growth inhibition by 1,25 D3 does not correlate with VDR content in ALVA-31 and LNCaP cells. This lack of correlation between VDR density and growth responses to 1,25 D3 led us to investigate VDR-mediated gene transcription in these cell lines. We employed two different naturally occurring vitamin D response elements (VDREs) linked to a reporter gene. Reporter gene activation by 1,25 D3 correlated well with VDR content in all four cell lines. Therefore, compared to LNCaP cells, decreased sensitivity of ALVA-31 to growth inhibition by 1,25 D3 is not due to a decrease in the general transcriptional activity of VDR. We conclude that growth inhibition by 1,25 D3 in prostate cancer cells requires VDR but that this response is modulated by non-receptor factors that are cell line-specific.

Demonstration of vitamin D receptor transcripts in actively resorbing osteoclasts in bone sections

The effects of the active metabolite of vitamin D, 1,25 dihydroxyvitamin D3 (1,25D), are mediated via the vitamin D receptor (VDR). 1,25D is known to have profound effects on bone resorption, but proof that the human osteoclast expresses VDR in vivo is absent. Receptors have been demonstrated in osteoblasts, and it has been generally accepted that the effects of 1,25D on formed osteoclasts are mediated via osteoblasts. Using conventional riboprobe in situ hybridization, VDR transcripts were readily detectable in osteoblasts within sections taken from normal bone and several actively remodelling bone tissues, namely, Paget's disease, renal hyperparathyroidism, and healing fracture callus. However, VDR transcripts also appeared to be present at low levels within osteoclasts from two pagetic samples and two hyperparathyroid samples. To examine this latter finding further, we have used the novel technique of in situ-reverse transcriptase-polymerase chain reaction (IS-RT-PCR) for specific amplification and detection of VDR mRNA within sections taken from the same conditions described above, and also from osteoclastoma samples. As expected, VDR transcripts were amplified and detected in osteoblasts and marrow cells, but were also prominently found in osteoclasts at approximately 50% of the level detected in osteoblasts in normal bone and at 60% in the active bone tissues. This suggests that in addition to effects on osteoclast precursors and those mediated via osteoblasts, 1,25D could exert direct effects on the active bone resorbing cells in vivo.

Transcriptional control of the osteocalcin gene by 1,25-dihydroxyvitamin D-2 and its 24-epimer in rat osteosarcoma cells

The effects of two vitamin D analogs, 1,25-dihydroxyvitamin D-2 and 24-epi-1,25-dihydroxyvitamin D-2, were examined on osteocalcin gene expression in the rat osteosarcoma cell line ROS 17/28. Our results indicate that these analogs are more transcriptionally active than 1,25-dihydroxyvitamin D-3, particularly the 24-epimer. Assessment of reporter gene chloramphenicol acetyltransferase (CAT) activity, using the vitamin D responsive element (VDRE) derived from the human osteocalcin gene promoter. revealed that both analogs stimulated CAT activity 5- to 10-fold. 1,25-Dihydroxyvitamin D-2 was slightly more active than 1,25-dihydroxyvitamin D-3, while the 24-epimer was twice as effective. 1,25-Dihydroxyvitamin D-3 also stimulated osteocalcin mRNA accumulation by 2-fold over vehicle-treated cells, 1,25-dihydroxyvitamin D-2 by 2.5-fold, and 24-epi-1,25-dihydroxyvitamin D-2 by 4-fold. Electrophoretic mobility shift assays using the osteocalcin vitamin D responsive element revealed no increase in DNA binding with either analog when compared to 1,25-(OH)2D3. Examination of CAT activity using the rat 24-hydroxylase VDRE indicated no significant difference in transcription with these compounds, suggesting that the vitamin D-2 analogs preferentially activate osteocalcin gene expression.

Attachment and proliferation of osteoblasts and fibroblasts on biomaterials for orthopaedic use

Using a variety of cell types, cell attachment and growth was studied on prospective (polyethersulphone (PES) and polyetheretherketone) and currently used (titanium 318 alloy, cobalt chrome molybdenum alloy and ultra-high molecular weight polyethylene (UHMWPE)) orthopaedic biomaterials. Proliferation of fibroblasts and osteoblasts was measured using incorporation of tritiated thymidine into total DNA. Attachment of cells was assessed by indirect immunofluorescent labelling of vinculin, a component of the cell's focal adhesion plaque. The degree of cell attachment was quantified on the materials by determining the mean number of adhesion plaques and using an image analysis system to determine the mean total area of plaques per cell. Fibroblasts and osteoblasts responded differently to the materials tested. When grown on PES surfaces, rat tail fibroblasts synthesized significantly greater amounts of DNA than cells on all other surfaces, whilst fibroblasts on UHMWPE synthesized significantly less DNA than cells on all other materials. Interestingly, there was no significant difference between the amounts of DNA synthesized by osteoblasts grown on the various materials. Determination of the number of vinculin adhesion plaques per cell and the mean total area of the plaques per cell showed that the attachment of fibroblasts to UHMWPE was significantly reduced compared with other materials. In contrast there was no significant difference in the adhesion of osteoblasts to different materials. Scanning electron microscope (SEM) observations of cells on the materials correlated with the morphometric data. Cells with the greatest number and area of adhesion plaques were well spread and flattened whilst those with the least number of adhesion plaques were more rounded and less spread.

Differentiation of Dunn osteosarcoma cells in response to dibutyryl cyclic 3',5'-adenosine monophosphate

We investigated the effects of dibutyryl cyclic adenosine 3',5'-monophosphate (Bt2cAMP) on the differentiation of Dunn osteosarcoma cells. Flow-cytometric analysis and DNA synthesis assay showed that Bt2cAMP decreased the cell population in the S phase in a time- and dose-dependent manner. Also, the cells showed distinct morphological and functional alterations; the cell morphology changed to a fibroblast-like appearance with long and thin protoplasmic processes, the knobs or blebs on both the cell membrane and nuclear membrane disappeared and the intracellular alkaline phosphatase activity increased. Moreover, Bt2cAMP-treated cells secreted a large quantity of fibronectin, which was deposited on the extended cell surface in the culture medium. Thus, Dunn osteosarcoma cells are differentiated morphologically and functionally by Bt2cAMP, and might be transformed to benign precursor cells.

Cellular biology of bone resorption

Past knowledge and the recent developments on the formation, activation and mode of action of osteoclasts, with particular reference to the regulation of each individual step, have been reviewed. The following conclusions of consensus have emerged. 1. The resorption of bone is the result of successive steps that can be regulated individually. 2. Osteoclast progenitors are formed in bone marrow. This is followed by their vascular dissemination and the generation of resting preosteoclasts and osteoclasts in bone. 3. The exact pathways of differentiation of the osteoclast progenators to mature osteoclasts are debatable, but there is clear evidence that stromal cells support osteoclast generation. 4. Osteoclasts are activated following contact with mineralized bone. This appears to be controlled by osteoblasts that expose mineral to osteoclasts and/or release a factor that activates these cells. 5. Activated osteoclasts dissolve the bone mineral and digest the organic matter of bone by the action of agents secreted in the segregated microcompartments underlying their ruffled borders. The mineral is solubilized by protons generated from CO2 by carbonic anhydrase and secreted by an ATP-driven vacuolar H(+)-K(+)-ATPase located at the ruffled border. The organic matrix of the bone is removed by acid proteinases, particularly cysteine-proteinases that are secreted together with other lysosomal enzymes in the acid environment of the resorption zone. 6. Osteoclastic bone resorption is directly regulated by a polypeptide hormone, calcitonin (CT), and locally, by ionized calcium (Ca2+) generated as a result of osteoclastic bone resorption. 7. There is new evidence that osteoclast activity may also be influenced by the endothelial cells via generation of products including PG, NO and endothelin.

Biochemical indicators of bone metabolic activity in bovine periparturient hypocalcemia

Twenty-one paretic and 30 nonparetic periparturient ambulatory cows were studied together with 13 non-pregnant control cows. Blood samples were collected during a time period of 6 hours before or after parturition and from nonpregnant cows. A severe hypocalcemia (-44%) and hypophosphatemia (-69%) were found in the paretic vs. nonparetic periparturient animals, whereas serum magnesium concentrations were not altered. Serum alkaline phosphatase activity was modestly (-12%) but not significantly decreased in the paretic vs. nonparetic periparturient cows. Serum osteocalcin concentrations, which reflect osteoblast activity, were strikingly low during the periparturient period and more in the paretic cows but the difference between the paretic (-36%) and nonparetic animals was not statistically significant. In the paretic cows, serum 1,25-dihydroxyvitamin D concentrations were significantly increased (+53%) and those of hydroxyproline decreased (-18%) compared with the nonparetic animals. Serum estradiol levels were markedly increased in the periparturient vs. nonpregnant cows and more in the paretic animals, but, again, the difference between the paretic (+47%) and nonparetic animals was not statistically significant. Serum retinol concentrations were significantly lower (-18%) in the paretic vs. nonparetic periparturient cows probably reflecting increased lactation in the paretic animals. Collectively, the results indicate very low bone metabolic activity in the periparturient vs. nonpregnant cows and support the hypothesis of both osteoblast and osteoclast functions being impaired in the paretic animals.

Differentiating and antitumor activities of 1 alpha,25-dihydroxyvitamin D3 in vitro and 1 alpha-hydroxyvitamin D3 in vivo on human osteosarcoma

The differentiating and antitumor activities of 1 alpha,25-dihydroxyvitamin D3 (1 alpha,25(OH)2D3) in vitro and 1 alpha-hydroxyvitamin D3 (1 alpha(OH)D3) in vivo were studied with a human osteosarcoma cell line (OST strain). Anti-tumor activity was estimated with the use of 3-(4,5-dimethylthiazol)-2, 5-diphenyltetrazolium bromide (MTT) assay, colony-forming assay, and athymic mouse assay. The intracellular alkaline phosphatase (ALP) activity of tumor cells and production of bone Gla protein (BGP) in culture media were measured to mark osteoblastic differentiation. In addition, the combination of 1 alpha,25(OH)2D3 and cis-dichlorodiammineplatinum(II) (CDDP) was tested by the colony-forming assay and the measurement of ALP activity and BGP production for differentiating and antitumor effects. The assays revealed that 1 alpha,25(OH)2D3 exerted a dose-related, growth-inhibitory influence. In the colony-forming assay, the 1 alpha,25(OH)2D3-treated colonies were smaller than the untreated colonies. The ALP activity and the BGP production also increased in relation to dose. In the assay in athymic mice, the relative weight of tumors treated with 1 alpha(OH)D3 at 2.5 nmol/kg was significantly smaller than that of the controls, and no side effects were observed in the 1 alpha(OH)D3-treated mice. Marked tumor chondrogenesis was observed in human osteosarcoma treated with 1 alpha(OH)D3 in athymic mice. The combination of 1 alpha,25(OH)2D3 at 10(-8) M and CDDP at 2 micrograms/ml significantly enhanced both the differentiation and the growth inhibition in vitro. Our study apparently is the first demonstration that vitamin D3 metabolites have an antitumor and differentiating effect on human osteosarcoma cells in vitro and in athymic mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Verapamil decreases cyclic load-induced calcium incorporation in ROS 17/2.8 osteosarcoma cell cultures

Bone is a tissue that responds to mechanical load by changing its internal architecture. However, the mode of transmission of mechanical stimuli into biological signals and the effect of load at the cellular level are still not clear. An in vitro system, a Flexercell Strain Unit, was used to apply cyclic load to osteoblast-like cells in culture. In the first series of experiments, ROS 17/2.8 rat osteosarcoma cells, cultured on Flex I, flexible bottomed culture plates, were subjected to a 0.05 Hz, 0.24 STRAIN cyclic load regime for 3 and 7 days, in vitro. One group subjected to load received verapamil, a calcium channel blocker, throughout the experimental period. A second group was exposed to load but received no verapamil. A third group had no drug or load and a fourth group had no load but received verapamil. Cultures were incubated for 24 hours prior to collection with 10 microCi of 45CaCl in the medium, then well bottoms were divided to yield outer (maximum) and inner (minimum) load zones for assay of radioactivity. The effect of verapamil during a 7-day loading period was studied by adding the drug to individual cultures at daily intervals. Results indicated that mechanical loading stimulates calcium incorporation in ROS 17/2.8 cell cultures by day 7 but not by day 3. Only early verapamil addition decreased load-induced calcium incorporation when drug was added prior to day 4. If verapamil was added after 4 days, the channel blocker did not diminish load-induced calcium incorporation.

Effect of age on the rate of tooth movement in combination with local use of 1,25(OH)2D3 and mechanical force in the rat

The purpose of this study was to compare the amount and rate of tooth movement in young and mature rats administered 1,25(OH)2D3 simultaneous with application of mechanical force. In 30 seven-week-old and 30 28-week-old male Wistar rats, the right maxillary first molar was moved buccally with a fixed appliance. The appliances delivered forces ranging from 5 to 20 g. Twenty microL of 1,25(OH)2D3 (10(-10) and 10(-8) mol/L) was injected locally into the submucosal palatal area of the root bifurcation of the right first molar. The left side was injected with phosphate-buffered saline (PBS). In young rats receiving 10(-10) mol/L 1,25(OH)2D3 every three days, tooth movement significantly increased to 126% of that in PBS-injected control rats on day 20. In 1,25(OH)2D3-injected mature rats, tooth movement was stimulated markedly and increased with 10(-10) mol/L to 245% and with 10(-8) mol/L to 154% of the amount of tooth movement seen in the PBS-injected controls by the end of the experiment. PBS-injected rats had a plateau stage where tooth movement did not occur at all, while there was no such lag-time in the 1,25(OH)2D3-injected group which showed continuous tooth movement. The local injection of 1,25(OH)2D3 did not change serum calcium, phosphate, and alkaline phosphatase activity, and there were no apparent clinical or microscopic side-effects.

TPA decreases 1,25(OH)2D3 binding and calbindin D-28K in renal (MDBK) cells

The effect of the phorbol ester TPA (12-O-tetradecanoylphorbol 13-acetate) on vitamin D receptors (VDRs) was studied in MDBK cells, a normal bovine renal epithelial cell line. 24 h treatment of MDBK cells with TPA resulted in down-regulation of VDR number, with no change in the binding affinity for 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) or approximate molecular weight determined by fast protein liquid chromatography (FPLC). TPA treatment also reduced the level of calbindin D-28K, a vitamin D-dependent renal protein. 4 alpha-Phorbol 12,13-didecanoate (4 alpha-PDD), an inactive phorbol ester, did not affect either 1,25(OH)2D3 binding or calbindin D-28K levels. TPA elicited a significant decrease in membrane-associated protein kinase C (PKC) activity which coincided with the reduction in VDR number and calbindin D-28K. These data support a link between TPA, PKC activity and vitamin D actions in kidney.

The effect of local application of 1,25-dihydroxycholecalciferol on osteoclast numbers in orthodontically treated rats

Orthodontic tooth movement requires remodeling of periodontal tissues, especially alveolar bone. 1,25-(OH)2D3, the active form of vitamin D3, is known to be a potent stimulator of osteoclastic bone resorption. The purpose of this study was to investigate the effect of local application of 1,25-(OH)2D3 on osteoclast numbers induced by experimental tooth movement. A piece of orthodontic elastic band was inserted between the first and second upper molars of male Wistar rats weighing about 200 g each. Twenty microL of 1,25-(OH)2D3 (10(-12)-10(-7) mol/L) was injected locally into the submucosal palatal area of the root bifurcation of the right first molar. The left side was injected with phosphate-buffered saline (PBS). The number of osteoclasts was counted in a 700 x 1050 microns 2 area of the interradicular septum. The local injection of 1,25-(OH)2D3 caused a dose-dependent increase in osteoclast number. The effect of 1,25-(OH)2D3 reached a response plateau at 10(-10) mol/L when greater than a three-fold rise in osteoclast number was attained compared with the PBS-treated controls. While the insertion of a piece of elastic band for three days induced a significant increase in osteoclasts in the alveolar bone, daily injections of 20 microL of 10(-10) mol/M 1,25-(OH)2D3 for three days markedly stimulated the numbers of osteoclasts induced by the insertion of an elastic band. 1,25-(OH)2D3 was apparently synergistic with mechanical stimuli, resulting in enhancement of the numbers of osteoclasts induced by mechanical stimuli alone.(ABSTRACT TRUNCATED AT 250 WORDS)

Actions of calcipotriol (MC 903), a novel vitamin D3 analog, on human bone-derived cells: comparison with 1,25-dihydroxyvitamin D3

The actions of a novel vitamin D3 analog calcipotriol (MC 903), on human bone-derived cells were compared to those of 1,25-dihydroxyvitamin D3 [1,25-(OH)2D3]. Both calcipotriol and 1,25-(OH)2D3 inhibited the proliferation of human osteoblast-like cells in a dose-dependent manner (10(-10)-10(-6) M), an effect observed at different cell densities. Lower concentrations of either agent exerted no marked effect on the growth of the cells compared to untreated cultures. Calcipotriol and 1,25-(OH)2D3 were equipotent in stimulating the activity of alkaline phosphatase and the synthesis of osteocalcin in human osteoblast-like cells. The stimulation of alkaline phosphatase activity and osteocalcin synthesis by both compounds was evident by 24 h and was increased progressively up to 96 h in a dose-dependent manner over the concentration range of 10(-10)-10(-6) M. The increment in both proteins was dependent on cell density and was attenuated at higher cell densities. In contrast to these actions, neither calcipotriol nor 1,25-(OH)2D3 (10(-14)-10(-6) M) affected the synthesis of prostaglandin E2. These studies indicate that calcipotriol and 1,25-(OH)2D3 exhibit a similar spectrum of activity on human osteoblast-like cells in vitro.

The specific production of the third component of complement by osteoblastic cells treated with 1 alpha,25-dihydroxyvitamin D3

A 190 kDa protein was purified from conditioned media of mouse marrow-derived stromal cell (ST2) cultures treated with 1 alpha,25-dihydroxyvitamin D3 (1 alpha,25(OH)2D3) and identified as the third component of mouse complement (C3). Northern and Western blot analysis revealed that the production of C3 by ST2 and primary osteoblastic cells was strictly dependent on 1 alpha,25(OH)2D3, but the production by hepatocytes was not. Adding 1 alpha,25(OH)2D3 together with mouse C3 antibody to bone marrow cultures greatly inhibited the formation of tartrate-resistant acid phosphatase (TRAP)-positive osteoclast-like multinucleated cells. Adding C3 alone induced no TRAP-positive cell formation. These results suggest that, in bone tissues, C3 is specifically produced by osteoblasts in response to 1 alpha,25(OH)2D3 and somehow involved in inducing differentiation of bone marrow cells into osteoclasts in concert with other factors produced by osteoblasts in response to 1 alpha,25(OH)2D3.

Differentiation-related regulation of 1,25-dihydroxyvitamin D3 receptor mRNA in human leukaemia cells HL-60

Vitamin D receptor (VDR) is a nuclear protein which mediates the physiological actions of its hormone ligand, 1,25-dihydroxyvitamin D3 (1,25(OH)2D3). While it appears that the receptor-hormone complex regulates the expression of hormone-dependent genes involved in mineral homeostasis, its role in induction of differentiation of leukaemic cells is less clear. We have studied the expression of the VDR gene in several sublines of HL-60 leukaemic cells with varying responsiveness to 1,25(OH)2D3. Sublines which rapidly differentiated to monocytic forms were shown to contain elevated steady-state levels of VDR mRNA within 1 h of exposure to high concentration of 1,25(OH)2D3. This up-regulation of the expression of VDR was not apparent in sublines in which monocytic differentiation occurred after a delay of several days. Beginning at approximately 3 h after exposure to 1,25(OH)2D3 in most cases, there was a gradual decline in VDR mRNA levels. Measurement of steady-state levels of mRNA for c-myc and c-fos showed that in sublines of HL-60 cells which respond rapidly to 1,25(OH)2D3, elevation of VDR mRNA is evident prior to the changes in proto-oncogene expression. These data are consistent with the hypothesis that a change in VDR gene expression is one of the steps that promote monocytic differentiation.

Use of non-collagen markers in osteoporosis studies

Clinical and research laboratories routinely measure various hormonal and nonhormonal parameters of calcium and phosphorus metabolism, and markers of bone turnover. Such measurements may help clinical decision-making relating to metabolic bone disease and osteoporosis. Molecular biological and cell-culture techniques are being used in basic biochemical research on bone-cell metabolism. Results may aid understanding of normal and abnormal regulation of the bone-cell metabolism, and thus provide further insights relating to the diagnosis and prevention of osteoporosis.

Interleukin 1 regulates the expression of osteopontin mRNA by osteoblasts

Osteopontin is a matrix protein which belongs to the integrin superfamily and is involved in cell adhesion. In the present study, we examined the regulation of the mRNA expression of osteopontin by interleukin 1 alpha (IL-1 alpha) in osteoblasts. IL-1 alpha greatly increased the steady-state level of osteopontin mRNA in both a mouse osteoblastic cell line (MC3T3-E1) and mouse primary osteoblast-like cells. The increase in the osteopontin mRNA expression by IL-1 alpha was dose-dependent at a range of 0.004-0.2 nM. This was most likely due to an increase in the transcriptional rate, not to an increase in the stability of osteopontin mRNA. The in vitro nuclear transcription experiment showed that IL-1 alpha-treated MC3T3-E1 cells increased the synthesis of osteopontin mRNA. Besides IL-1 alpha, tumor necrosis factor alpha (TNF-alpha), lipopolysaccharides (LPS) and 1 alpha,25-dihydroxyvitamin D3 (1 alpha,25(OH)2D3) increased the osteopontin mRNA expression in both the clonal osteoblasts (MC3T3-E1) and the primary osteoblast-like cells. In response to such bone-resorbing agents, primary osteoblast-like cells expressed osteopontin mRNA much more strongly than primary fibroblast-like cells isolated from mouse calvaria. Both IL-1 alpha and 1 alpha,25(OH)2D3 greatly increased the production of 68 and 62 kDa phosphoproteins in conditioned media of MC3T3-E1 cell cultures, which probably correspond to osteopontin. These results suggest that osteopontin plays an important role in bone remodeling, in particular bone resorption.

Effect of 1,25(OH)2D3 on its receptor mRNA levels and osteocalcin synthesis in human osteosarcoma cells

Hormone-dependent accumulation of specific binding sites for 1,25(OH)2D3 and changes in human 1,25-dihydroxy-vitamin D receptor (hVDR) mRNA levels were examined in cell lines (MG-63, SaOs-2 and U2-Os) derived from human bone. Osteocalcin synthesis and secretion as well as alkaline phosphatase activity were also characterized as biochemical markers of the osteoblastic phenotype. Specific binding sites for 1,25(OH)2D3 were quantified by incubating cultured intact cells with [3H]1,25(OH)2D3 at 37 degrees C. Based on the uptake of 1,25(OH)2D3, there were about 3000 to 4000 receptor molecules per cell with apparent dissociation constants varying between 0.02 to 0.03 nM. The binding was saturated with 1,25(OH)2D3 in 3 to 6 h after the hormone addition and further exposure to the hormone resulted in an upregulation of the bindings sites. The levels were elevated by as little as 10 to 200 pM 1,25(OH)2D3, and maximal binding was achieved with 0.2-0.7 nM 1,25(OH)2D3. Treatment with 1,25(OH)2D3 also resulted in a clear increase (about 3-fold) in hVDR mRNA by 24 h in all three cell lines. The increase in hVDR mRNA level was time- and dose-dependent. MG-63 cells responded with 2- and 15-fold increases, respectively, in intracellular and secreted levels of osteocalcin after the 1,25(OH)2D3-treatment. In dot-blot hybridization assay, MG-63 cells expressed osteocalcin mRNA which was inducible with 1,25(OH)2D3 while, in SaOs-2 and U2-Os cells, osteocalcin mRNA was not detected under the same circumstances. Also, no secretion of osteocalcin was detected in SaOs-2 and U2-Os cells with or without addition of 1,25(OH)2D3.

Rapid, simple identification of individual osteoblastic cells and their specific products by cell blotting assay

Biochemical and molecular biological studies of osteoblastic cell function and hormonal regulation are frequently confounded by the inherent cellular heterogeneity and phenotypic instability of existing in vitro and in vivo model systems. A new technique (derived from Western blotting or antibody-based detection of protein molecules bound to nitrocellulose paper) is described for identification of individual cells which synthesize osteoblast-specific gene products (bone Gla-protein, type I collagen, and alkaline phosphatase) or produce cAMP in response to parathyroid hormone (PTH) or isoproterenol. Dispersed primary neonatal rat calvariae or osteogenic sarcoma cells were "plated" on Immobilon-P (a hydrophobic transfer membrane with very high protein-binding capacity) for 30 minutes to several hours, followed by agonist treatment, formalin fixation, hematoxylin staining, and immunostaining with a battery of antibodies specific for osteoblastic products. Individual cells and their secretory zones were visualized by light microscopy and counted. Treatment with PTH with or without isoproterenol resulted in increases in the percentages of osteoblastic cells elaborating cAMP, as well as the intensity of immunostaining, but had no effects on MCF-7 cells, a nonosteoblastic breast carcinoma control line. The percentage of cells within each primary osteoblastic cell population isolated or rat osteogenic sarcoma cell clone (G2 or C12) that elaborated bone-specific proteins or that generated cAMP in response to PTH varied with time and the individual cellular preparation, reconfirming the cellular heterogeneity of these systems. This method, in conjunction with techniques such as in vitro hybridization, should prove useful in characterizing discrete osteoblastic bone cell subpopulations and in clarifying mechanisms of hormonal regulation by local and systemic agents.

Type I collagen degradation by mouse calvarial osteoblasts stimulated with 1,25-dihydroxyvitamin D-3: evidence for a plasminogen-plasmin-metalloproteinase activation cascade

To understand the mechanisms regulating osteoid removal by osteoblasts, mouse calvarial osteoblasts were grown on 14C-labelled type I collagen films and stimulated with 1,25-dihydroxyvitamin D-3 (2.5.10(-8) M) for 48-72 h. In the presence of 5% non-inhibitory rabbit serum this resulted in a 2-3-fold increase in collagen degradation and a dramatic change in osteoblast morphology, when compared with untreated osteoblasts. Collagenolysis was accompanied by increased synthesis and release of latent collagenase, gelatinase and stromelysin and a concomitant decrease in their specific inhibitor, TIMP (tissue inhibitor of metalloproteinases). In serum-free medium, osteoblasts failed to degrade collagen, but their ability to lyse collagen could be restored by adding plasminogen (5 micrograms/ml) to the cultures. Plasminogen-dependent collagenolysis was inhibited by human recombinant TIMP (5 units/ml), demonstrating that plasmin, derived from plasminogen, activated latent collagenase and did not itself degrade collagen. Plasminogen activator production was confirmed by culturing osteoblasts on 125I-labelled fibrin plates. Comparison with urokinase-type and tissue-type plasminogen activator standards suggested that osteoblast plasminogen activator was predominantly cell-associated and likely to be of the urokinase type. Immunocytochemistry indicated that osteoblasts also constitutively produce plasminogen activator inhibitor-1. These findings provide evidence for the involvement of a plasminogen-plasmin-latent metalloproteinase activation cascade in type I collagen degradation by osteoblasts, and for its regulation by TIMP and plasminogen activator inhibitor-1.

1,25-Dihydroxyvitamin D3 prevents the decrease of bone mineral appositional rate in rats with inflammation-mediated osteopenia (IMO)

We have studied the effects of 1,25-dihydroxyvitamin D3 (1,25(OH)2D3) on bone mass and bone mineral appositional rate in intact rats and rats with inflammation-mediated osteopenia (IMO), where osteoblast number and mineral appositional rate are decreased. 1,25(OH)2D3 prevents IMO-specific bone loss when given in a daily dose of 25 ng per rat, but does not when given in higher doses. The hormone was effective, when given over the complete duration of the experiment (21 days), but not when given over shorter time periods (7 and 14 days, respectively). 1,25(OH)2D3 prevents IMO-dependent reduction in mineral appositional rate and leads to an only moderate increase in intact rats. We conclude, that 1,25(OH)2D3 is more effective in stimulating mineral appositional rate in rats with IMO where mineral apposition is impaired.

Regulation of bone cell metabolism

Bone formation and resorption are normal physiologic processes. In pathologic states such as in periodontal disease or osteoporosis a shift in the balance of these two processes occurs, resulting in a net loss of mineralized tissue. Osteoclasts have historically been considered to be the primary bone resorbing cells, but current research has lead to the hypothesis that osteoblastic cells play an integral role in bone resorption as well. It appears that osteoblasts respond to bone resorbing agents via a series of intracellular responses after interactions with specific surface receptors. Two basic pathways involving different "second messengers" have been identified. The first pathway involves cyclic 3',5' adenosine monophosphate (cAMP) production and the second involves membrane phospholipids, diacylglycerol and calcium. A cytosolic enzyme, protein kinase C (PKC), has been shown to affect both cAMP as well as calcium fluxes and may act to regulate both these pathways. It is the purpose of this paper to discuss current studies and hypotheses concerning the nature of mechanisms involved in regulation of bone metabolism with emphasis on second messenger systems. Information of this nature is critical to the development of rationale regarding diagnosis, treatment and management of systemic and local pathoses of bone.

Molecular structure of the rat bone Gla protein gene and identification of putative regulatory elements

The rat bone Gla protein (BGP, osteocalcin) gene was isolated from a rat genomic library and sequenced. BGP is a 5.8-kD noncollagenous protein secreted by calcified tissues whose expression is regulated by 1,25-dihydroxyvitamin D3 [1,25(OH)2D3]. The BGP gene consists of 4 exons separated by 3 introns and spans almost 1 kb of DNA. The 5'-flanking region of the gene contains CCAAT and TATAAA elements common to eukaryotic genes. An analysis of approximately 600 bp of 5'-flanking sequence of this gene revealed sequences homologous to regulatory elements for glucocorticoids, metal ions, and cAMP. The latter is especially significant since recent evidence suggests that the rat BGP gene can be regulated by cAMP. This region of the gene also contains numerous pairs of inverted repeat sequences (imperfect palindromes). The sequence of the rat BGP gene was compared to that of the recently published human BGP gene (Celeste et al., EMBO J. 5, 1885, 1986). The coding regions of these two genes share 77% sequence identity, and several regions of the 5'-flanking sequences are also well conserved. Knowledge of the sequence of the rat BGP gene will allow studies of its regulation by 1,25(OH)2D3, cAMP, and other trans-acting transcriptional factors, and identification of the regulatory sequence elements involved.

Calbindin-D9K immunolocalization and vitamin D-dependence in the bone of growing and adult rats

This report presents evidence for the presence of the vitamin D-dependent calcium-binding protein, calbindin-D9K, in bone cells and matrix. In undecalcified frozen sections of growing and adult rat bone, calbindin-D9K was immunohistochemically localized in trabecular bone of the epiphysis and metaphysis and in cortical bone of the diaphysis. It was found within the cytoplasm of osteocytes, of osteoblasts lining the osteoid, and osteoblasts inside the osteoid seams. It was also found in the osteoblast processes and the anastomosed reticulum of the processes connecting the osteocytes with each other. Extracellularly, calbindin-D9K immunoreactivity was present in compact cortical bone in the areas of the mineralized matrix surrounding the osteocyte lacunae, and in the pericanalicular walls containing the cell processes. Calbindin-D9K immunoreactivity was low or absent from the cytoplasm of osteocytes in trabecular bone from severely vitamin D-deficient rats and restored in vitamin D-deficient rats given a single dose of 1,25(OH)2-VitD3. Thus, the synthesis of immunoreactive calbindin-D9K by osteoblasts and osteocytes in trabecular bone is vitamin D-dependent. The presence of immunoreactive calbindin-D9K in the osteocytes and their cell processes suggests that this calcium-binding protein is involved in the calcium fluxes regulating bone calcium homeostasis. Its localization in osteoblasts involved in bone formation and in their cell processes suggests that it has a role in the calcium transport from these cells towards the sites of active bone mineralization.(ABSTRACT TRUNCATED AT 250 WORDS)

A differentiation-inducing factor produced by the osteoblastic cell line MC3T3-E1 stimulates bone resorption by promoting osteoclast formation

We have reported that the differentiation-inducing factor (DIF) is present in conditioned medium of mouse osteoblast-like cell (MC3T3-E1) cultures. In the present study, the DIF from conditioned medium of MC3T3-E1 cells was partially purified and its biologic activity was examined. The DIF was purified by monitoring the induction of phagocytic activity of mouse myeloblastic leukemia cells (M1). The DIF induced differentiation of not only M1 cells but also mouse myelomonocytic cells (WEHI-3). Furthermore, the DIF increased the in vitro bone-resorbing activity and the osteoclast number in mouse calvaria. The increases were inhibited by the addition of either salmon calcitonin or indomethacin. When mouse bone marrow cells were cultured with the DIF for 8 days, formation of osteoclast-like multinucleated cells was stimulated dose dependently. The DIF from MC3T3-E1 cells appeared to be different from interleukin-1 (IL-1), tumor necrosis factor (TNF), and transforming growth factor beta (TGF-beta). These results suggest that the DIF partially purified from osteoblast-like cell cultures stimulates osteoclastic bone resorption by promoting differentiation and fusion of osteoclast progenitors to form multinucleated osteoclasts.

Bone-resorbing agents promote and interferon-gamma inhibits bone cell collagenase production

Parathyroid hormone, prostaglandin E2, 1 alpha,25-dihydroxyvitamin D3, interleukin-1, tumor necrosis factor alpha, and epidermal growth factor, all known stimulators of bone resorption, markedly enhanced collagenase secretion by rat fetus osteoblastlike cells in primary culture as judged by enzyme-linked immunosorbent assay. Untreated cells contained no immunostainable or extractable collagenase. Collagenase was detected in the treated cells and media only after 1-3 h of treatment, and there was no increment in collagenase activity when cells were treated in the presence of actinomycin D or cycloheximide. Cells secreted collagenase in a latent form and also elaborated collagenase inhibitor; chromatographic separation of collagenase from collagenase inhibitor and subsequent activation of the collagenase with trypsin yielded the active species in stimulated but not in unstimulated cells. The ability of individual prostanoids, among seven tested, to promote collagenase production correlated positively with their reported capacity to promote bone resorption. Interferon-gamma (IFN-gamma), a known resorption inhibitor, blocked the increment in collagenase production caused by all agents tested. These results indicate a close linkage between stimulation of bone resorption and collagenase production by osteoblastlike cells. Various resorption stimulators, including some not previously tested for effects on collagenase, augment the de novo synthesis and secretion of collagenase and act by an IFN-gamma-inhibitable mechanism.

Growth of rat osteoblast-like cells in a lipid-enriched culture medium and regulation of function by parathyroid hormone and 1,25-dihydroxyvitamin D

To examine the role of lipid metabolism in the growth and function of osteoblast-like cells, we studied ROS 17/2.8 osteosarcoma cells and primary cultures of rat calvarial osteoblasts during growth in a serum-free medium supplemented by purified human lipoproteins or by liposomes. Increase in ROS cell number was measured in sparse (1-5 X 10(3)/cm2) cultures over 6-8 days. Liposomes (0-300 micrograms/ml) and high (HDL), low (LDL), and very low density (VLDL) lipoprotein fractions (0-300 micrograms apoprotein) markedly stimulated cell growth. Cells plated at 5 X 10(3)/cm2 achieved growth rates in the presence of LDL or HDL comparable to 10% fetal bovine serum. Serum-free culture with exogenous lipid maintained the response of cell cyclic AMP accumulation to parathyroid hormone. Cyclic AMP response to parathyroid hormone was enhanced by glucocorticosteroid, and was attenuated by 1,25-dihydroxyvitamin D (1,25(OH)2D) with an EC50 (10(-10) M) comparable to that previously observed in serum-cultured cells (J. Biol. Chem. 258:736, 1985). 1,25(OH)2D also increased the alkaline phosphatase activity in ROS cells cultured in lipid-supplemented serum-free culture. Lipoproteins or liposomes also markedly enhanced the proliferative response of sparse cultures of normal rat osteoblasts to polypeptide mitogens.

The influence of vitamin D metabolites on the calcification of cartilage matrix and the C-propeptide of type II collagen (chondrocalcin)

The influence of vitamin D metabolites (at 1 X 10(-10) M) on the calcification of cartilage matrix (measured by 45Ca2+ uptake) and the C-propeptide of type II collagen (measured by radioimmunoassay) has been studied using organ cultures and chondrocytes isolated from growth plates of vitamin D-deficient and -sufficient 11-day-old rats. Vitamin D-deficient rats had reduced amounts of C-propeptide in their serum and freshly isolated growth plate chondrocytes. In all chondrocytes cultured from vitamin D-deficient animals, the C-propeptide content was maximal at 24 hr whereas calcification continued to increase for up to 72 hr. In organ and chondrocyte cultures of tissue from vitamin D-sufficient rats, both 1,25-dihydroxycholecalciferol (1,25(OH)2D3) and 24,25-dihydroxycholecalciferol (24,25(OH)2D3) were required for maximal stimulation of calcification and maximal increases in C-propeptide content. In these D-replete tissues, 24,25-(OH)2D3 had a less stimulatory effect on both calcification (organ and cell cultures) and C-propeptide (organ cultures only), while 1,25(OH)2D3 alone had no effect in cell cultures but an inhibitory effect in organ cultures. Studies of cells or tissue from growth plates of vitamin D-deficient rats demonstrated that 24,25(OH)2D3 alone produced maximal calcification and maximal increases in the C-propeptide content. 1,25(OH)2D3 generally had an inhibitory effect on both calcification and C-propeptide when used alone. In the presence of 1,25(OH)2D3, the stimulatory effect of 24,25(OH)2D3 was partly abrogated. Maximal stimulation of calcification and increases in C-propeptide by 24,25(OH)2D3 were observed at 1 X 10(-9) M and 1 X 10(-10) M. In none of these studies was there any effect on proteoglycan content.(ABSTRACT TRUNCATED AT 250 WORDS)

Application of the rat thymus receptor in a specific, sensitive and simplified assay of 1,25-dihydroxyvitamin D in blood serum

A simple method has been employed to prepare crude nuclear extract from rat thymus, using hypertonic buffer after previous treatment with hypotonic buffer. The preparation is free from serum vitamin D-binding protein and contains a 3.7 S receptor molecule, which specifically binds 1,25-dihydroxyvitamin D-3 (1,25-(OH)2D3). The receptor is of high affinity (KD = 0.85 X 10(-11) M at O degrees C) and low capacity (260-460 fmol/g tissue). The Scatchard analysis of ligand binding results in a concave downward curve. The Hill analysis of the same data gives good linear fitting (r = +0.971) with the Hill coefficient nH = 1.63. These facts indicate positive cooperativity between two ligand binding sites of the rat thymus 1,25-(OH)2D3 receptor. The preparation was used in a competitive protein binding assay of 1,25-(OH)2D in serum extracts, purified on Sep-Pak C18 followed by silica Sep-Pak cartridges. The method was sensitive to 0.5 pg/tube (2.0 ng/l) when 1 ml of serum was extracted. Intra- and interassay coefficients of variation were 9% and 14%, respectively. The serum 1,25-(OH)2D concentration estimated in 33 children (mean age 6.5 +/- 3 years) was 46.6 +/- 18.4 ng/l (mean +/- S.D.).

Effects of diastereoisomers of 1,25-dihydroxyvitamin D3-26,23-lactone on alkaline phosphatase and collagen synthesis in osteoblastic cells

The effects of the four diastereoisomers of 1,25-dihydroxyvitamin D3-26,23-lactone (1,25-(OH)2D3-26,23-lactone) on alkaline phosphatase (AP) activity and collagen and noncollagen protein synthesis were examined in cultures of the osteoblastic clone MC3T3-E1 cell line. The four lactone diastereoisomers had little effect on the protein and DNA content of the cells. The 23(S),25(S)- and 23(R),25(R)-1,25-(OH)2D3-26,23-lactones increased AP activity in a linear dose-dependent fashion. Maximal effects were observed at 100 and 1000 pg/ml, respectively. In contrast, the naturally occurring 23(S),25(R)-, 1,25-(OH)2D3-26,23-lactone and the 23(R),25(S)-1,25-(OH)2D3-26,23-lactone showed biphasic stimulatory effects on AP activity. At both 80 and 10,000 pg/ml, they stimulated maximum increases in alkaline phosphatase activity. At 80 pg/ml the 23(S),25(R)- and 23(R),25(S)-isomers stimulated an increase in collagen synthesis, while at 10,000 pg/ml these isomers and 1,25-dihydroxyvitamin D3 (1,25-(OH)2D3) did not. Moreover, these two isomers (at 10,000 pg/ml) plus insulin or dexamethasone had an additive effect on AP activity, but not at 80 pg/ml. At 80 pg/ml but not at 10,000 pg/ml, the 23(S),25(R)-isomer had an additive effect on AP activity with the simultaneous addition of 25-hydroxyvitamin D3. Relative to 1,25-(OH)2D3, the binding affinities of 23(S),25(S)-, 23(R),25(R)-, 23(S),25(R)- and 23(R),25(S)-1,25-(OH)2D3-26,23-lactones were calculated to be 1/13.0, 1/131.8, 1/805.2, and 1/1083.3, respectively. No metabolites could be detected in the medium when [1-3H]23(S),25(R)-1,25-(OH)2D3-26,23-lactone (the naturally occurring diastereoisomer) was added to the cultures. However, the stimulative effects of 1,25-(OH)2D3 and the 23(S),25(R)-isomer at both concentrations were completely abolished by L-1-tosyl-amido-2-phenylethyl chloromethyl ketone. These results indicate that 1,25-(OH)2D3-lactone has a stimulative effect on osteoblastic cell functions in vitro. The naturally occurring 23(S),25(R)-1,25-(OH)2D3-lactone acts biphasically and may act on bone metabolism in vivo, possibly through a 1,25-(OH)2D3-receptor-mediated pathway.

Induction of monocytic differentiation by calcitriol (1,25-dihydroxyvitamin D3) in the human promyelocytic leukemic cell line (HL-60) in serum-free medium

The effect of calcitriol on the induction of differentiation in human promyelocytic leukemic cell line (HL-60) cultured in serum-free chemically defined medium (SFM) was investigated. The utilization of SFM containing RPMI-1640 basal medium supplemented with insulin (5 micrograms/ml), transferrin (5 micrograms/ml), sodium selenite (5 ng/ml), and bovine serum albumin (0.5 micrograms/ml), transferrin examination of the cellular/molecular mechanism of calcitriol's action in HL-60 cell differentiation without interference of components present in serum. HL-60 cells grown in SFM were induced to differentiate into monocytes/macrophages by calcitriol as indicated by induction of differentiation-associated biological and biochemical parameters: chemiluminescent (CL) responsiveness, lysozyme activity, nonspecific esterase, expression of cell surface antigens, and reduced proliferation. The exposure of HL-60 cells in SFM to calcitriol (from 10(-10) to 10(-8)M) resulted in dose-dependent induction of these parameters, which was similar to those obtained with cells grown in 10% fetal calf serum containing medium (10% SCM). However, calcitriol was 5-fold more potent for HL-60 cells cultured in SFM than those cultured in 10% SCM as indicated by shifts in dose-response curves for induction of CL responsiveness and lysozyme activity. The effect of calcitriol on the proliferation and acquisition of several monocyte-associated cell surface antigens was also more sensitive for HL-60 cells cultured in SFM than for cells grown in 10% SCM. We characterized and quantitated calcitriol receptors in HL-60 cells cultured in SFM in comparison to those in 10% SCM after exposing intact cells to radiolabeled calcitriol. Cells cultured in either SFM or 10% SCM exhibited calcitriol receptors that migrated at 3.4S as a single peak on sucrose gradients and elicited inherent DNA binding ability. There was essentially no difference in the apparent dissociation constants (Kd) nor in the number of calcitriol binding sites per HL-60 cell, that is approximately 6.0 X 10(-11) M and approximately 3000 binding sites/cell respectively. It is concluded that culturing HL-60 cells in SFM results in full expression of calcitriol-induced phenotypic changes excluding the possibility that such changes result from the indirect effect of calcitriol mediated by identified and/or unidentified components present in serum.