Characterization of the stromal osteogenic cell line MN7: mRNA steady-state level of selected osteogenic markers depends on cell density and is influenced by 17 beta-estradiol

Bone morphogenetic protein 2- and estradiol-17β-induced changes in ovarian transcriptome during primordial follicle formation†

Estradiol-17β has been shown to promote primordial follicle formation and to involve bone morphogenetic protein 2 (BMP2) as a downstream effector to promote primordial follicle in hamsters. However, the molecular mechanism whereby these factors regulate ovarian somatic cells to pre-granulosa cells transition leading to primordial follicle formation remains unclear. The objective of this study was to determine whether BMP2 and/or estradiol-17β would regulate the expression of specific ovarian transcriptome during pre-granulosa cells transition and primordial follicle formation in the mouse ovary. BMP2 mRNA level increased during the period of primordial follicle formation with the concurrent presence of BMP2 protein in ovarian somatic cells. Estradiol-17β but not BMP2 exposure led to increased expression of ovarian BMP2 messenger RNA (mRNA), and the effect of estradiol-17β could not be suppressed by 4-[6-[4-(1-Piperazinyl)phenyl]pyrazolo[1,5-a]pyrimidin-3-yl]quinoline dihydrochloride (LDN) 193189. BMP2 or estradiol-17β stimulated primordial follicle formation without inducing apoptosis. Ribonucleic acid-sequence analysis (RNA-seq) of ovaries exposed to exogenous BMP2 or estradiol-17β revealed differential expression of several thousand genes. Most of the differentially expressed genes, which were common between BMP2 or estradiol-17β treatment demonstrated concordant changes, suggesting that estradiol-17β and BMP2 affected the same set of genes during primordial follicle formation. Further, we have identified that estradiol-17β, in cooperation with BMP2, could affect the expression of three major transcription factors, GATA binding protein 2, GATA binding protein 4 and Early growth response 2, and one serine protease, hepsin, in pre-granulosa cells during primordial follicle formation. Taken together, results of this study suggest that estradiol-17β and BMP2 may regulate ovarian gene expression that promote somatic cells to pre-granulosa cells transition and primordial follicle formation in the mouse ovary.

Effect of pinocembrin isolated from Alpinia zerumbet on osteoblast differentiation

Bone mass is regulated by osteoblast-mediated bone formation and osteoclast-mediated bone resorption. Osteoporosis is a bone metabolism disorder in which bone mass decreases due to increased bone resorption rather than bone formation. We focused on the traditional plant Alpinia zerumbet in Okinawa, Japan, and searched for promising compounds for the prevention and treatment of osteoporosis. Pinocembrin isolated from the leaves of A. zerumbet showed enhanced alkaline phosphatase (ALP) activity and mineralization and increased mRNA expression of osteoblast-related genes Alp and Osteocalcin (Ocn) in MC3T3-E1 cells. Pinocembrin increased the mRNA expression of Runx2 and Osterix, which are important transcription factors in osteoblast differentiation, and the mRNA expression of Dlx5 and Msx2, which are enhancers of these transcription factors. The bone morphogenetic protein (BMP) antagonist noggin, its receptor kinase inhibitor LDN-193189 and p38 MAPK inhibitor SB203580 attenuated pinocembrin-promoted ALP activity. Pinocembrin increased the mRNA of Bmp-2 and its target gene Id1. In addition, the estrogen receptor (ER) inhibitor ICI182780 suppressed pinocembrin-stimulated ALP activity. Pinocembrin may increase BMP-2 expression via ER. Then, the BMP-2 promotes osteoblast specific genes expression and mineralization through both Smad-dependent and independent pathway following Runx2 and Osterix induction. Our findings suggest that pinocembrin has bone anabolic effects and may be useful for the prevention and treatment of bone metabolic diseases such as osteoporosis.

Stimulation of primordial follicle assembly by estradiol-17β requires the action of bone morphogenetic protein-2 (BMP2)

Primordial follicle (PF) pool determines the availability of follicles for ovulation in all mammals. Premature depletion of the PF reserve leads to subfertility or infertility. Bone morphogenetic protein 2 (BMP2) promotes PF formation by facilitating oocyte and granulosa cell development. Estradiol-17β (E2) upregulates PF formation in developing hamster ovaries. However, if BMP2 mediates E2 effect is not known. We hypothesize that E2 facilitates the effect of BMP2 on somatic to granulosa cell transition. BMP2 and E2 together significantly upregulated the percentage of PFs in hamster fetal ovaries in vitro compared with either of the treatments alone. E2 also promoted BMP2 expression in vivo. Inhibition of BMP2 receptors suppressed E2-stimulation of PF formation while knockdown of BMP2 in vitro significantly suppressed the E2 effect. In contrast, estrogen receptor blocker did not affect BMP2 action. Inhibition of the activity of E2 or BMP2 receptors, either alone or combined during the last two days of the culture (C6-C8) resulted in a significant decrease in PF formation by C8, suggesting that both BMP2 and E2 action is essential for somatic cell differentiation for PF formation. Together, the results suggest that E2 activates BMP2-BMPR system leading to the formation of primordial follicles.

Estrogen facilitates osteoblast differentiation by upregulating bone morphogenetic protein-4 signaling

Imbalanced functions of osteoclasts and osteoblasts are involved in various types of bone damage including postmenopausal osteoporosis. In the present study, we investigated the cellular mechanism by which estrogen interacts in the process of osteoblastic differentiation regulated by BMP-4 using mouse MC3T3-E1 cells that express estrogen receptors (ER) and BMP-4. Estradiol enhanced BMP-4-induced Runx2, osterix, ALP and osteocalcin expression in MC3T3-E1 cells. BMP-4-induced mineralization shown by Alizarin red staining was also facilitated by estrogen treatment. It was revealed that estrogen upregulated BMP-4-induced Smad1/5/8 phosphorylation, BRE-Luc activity and Id-1 mRNA expression. The expression of BMPRII was increased by estrogen in MC3T3-E1 cells, and inhibition of BMPRII or ALK-2/3 signaling impaired the effect of estrogen on BMP-4 signaling. Of note, the enhanced expression of osterix, ALP and osteocalcin mRNAs induced by BMP-4 and estrogen was reversed in the presence of an ER antagonist. Given that membrane-impermeable estrogen also upregulated BMP-4-induced expression of osteoblastic markers and Id-1 mRNA, non-genomic ER activity is involved in the mechanism by which estrogen enhances BMP-4-induced osteoblast differentiation in MC3T3-E1 cells. On the other hand, the expression of ERα and endogenous BMP-4 was suppressed by BMP-4 treatment regardless of the presence of estrogen, implying the presence of a negative feedback loop for osteoblast differentiation. Thus, estrogen is functionally involved in the process of osteoblast differentiation regulated by BMP-4 through upregulating BMP sensitivity of MC3T3-E1 cells.

Estrogen and glucocorticoid regulate osteoblast differentiation through the interaction of bone morphogenetic protein-2 and tumor necrosis factor-alpha in C2C12 cells

Imbalanced functions between osteoclasts and osteoblasts are involved in inflammatory bone damage. The clinical effectiveness of blocking TNF-alpha in treatment of active rheumatoid arthritis established the significance of TNF-alpha in the pathogenesis. In the present study, we investigated the cellular mechanism by which estrogen and glucocorticoid interact in osteoblastic differentiation regulated by BMP and TNF-alpha using mouse myoblastic C2C12 cells. The expression of estrogen receptors, (ER)alpha and ERbeta, and glucocorticoid receptor (GCR) was significantly increased by BMP-2 treatment regardless of the presence of estradiol and dexamethasone. Estradiol, but not dexamethasone, enhanced BMP-induced Runx2 and osteocalcin expression in C2C12 cells. In addition, TNF-alpha suppressed BMP-2-induced Runx2 and osteocalcin expression, and estradiol and dexamethasone reversed the TNF-alpha effects on BMP-2-induced Runx2 expression. Dexamethasone also abolished osteocalcin expression induced by BMP-2. Interestingly, BMP-2-induced Smad1/5/8 phosphorylation and Id-1 promoter activity were enhanced by estradiol pretreatment. On the other hand, dexamethasone suppressed BMP-2-induced Smad1/5/8 activation. TNF-alpha-induced SAPK/JNK activity was suppressed by estradiol, while NFkappaB phosphorylation was inhibited by dexamethasone. Of note, the inhibitory effects of TNF- on BMP-2-induced Runx2 and osteocalcin expression were reversed by SAPK/JNK inhibition regardless of the presence of estradiol. The estradiol effects that enhance BMP-2-induced Runx2 and osteocalcin mRNA expression were restored by antagonizing ER, and moreover, membrane-impermeable estradiol-BSA failed to enhance the BMP-2-induced osteoblastic differentiation. Thus, estrogen and glucocorticoid are functionally involved in the process of osteoblast differentiation regulated by BMPs and TNF-alpha. BMP-2 increases the sensitivities of ERs and GCR, whereas estrogen and glucocorticoid differentially regulate BMP-Smad and TNF-alpha signaling.

Nucleofection-BasedEx VivoNonviral Gene Delivery to Human Stem Cells as a Platform for Tissue Regeneration

There are several gene therapy approaches to tissue regeneration. Although usually efficient, virusbased approaches may elicit an immune response against the viral proteins. An alternative approach, nonviral transfer, is safer, and can be controlled and reproduced. We hypothesized that in vivo bone formation could be achieved using human mesenchymal stem cells (hMSCs) nonvirally transfected with the human bone morphogenetic protein-2 (hBMP-2) or -9 (hBMP-9) gene. Human MSCs were transfected using nucleofection, a unique electropermeabilization-based technique. Postnucleofection, cell viability was 53.6 +/- 2.5% and gene delivery efficiency was 51% to 88% (mean 68.2 +/- 4.1%), as demonstrated by flow cytometry in enhanced green fluorescent protein (EGFP)-nucleofected hMSCs. Transgene expression lasted longer than 14 days and was very low 21 days postnucleofection. Both hBMP-2- and hBMP-9-nucleofected hMSCs in culture demonstrated a significant increase in calcium deposition compared with EGFP-nucleofected hMSCs. Human BMP-2- and hBMP-9-nucleofected hMSCs transplanted in ectopic sites in NOD/SCID mice induced bone formation 4 weeks postinjection. We conclude that in vivo bone formation can be achieved by using nonvirally nucleofected hMSCs. This could lead to a breakthrough in the field of regenerative medicine, in which safer, nonviral therapeutic strategies present a very attractive alternative.

Comparison of osteoblast responses to hydroxyapatite and hydroxyapatite/soluble calcium phosphate composites

Hydroxyapatite/soluble calcium phosphate composites (HAp/SCaP) are novel HAp-based materials with enhanced solubility that have been developed by annealing HAp in a vacuum. This study compared the effects of HAp and HAp/SCaP on osteoblast proliferation, differentiation, and mineralization using an MC3T3-E1 cell culture system. MC3T3-E1 cells were cultured on HAp or HAp/SCaP, and the number of attached cells and their morphology were examined. The influence of the extract from HAp/SCaP on osteoblast differentiation was determined by the measurement of alkaline phosphatase activity and reverse transcriptase-polymerase chain reaction analysis of the expression of osteoblastic markers. In addition, mineralization was evaluated by the staining of calcium deposits with Alizarin red. Attachment of a greater number of cells exhibiting no degeneration in their morphology was observed on HAp/SCaP compared with HAp after incubation for 7 days or more. Culturing cells with the extract from HAp/SCaP resulted in promotion of alkaline phosphatase activity, the expression of type I collagen, and bone-like tissue formation. The results of the present study indicate that HAp/SCaP shows greater ability in osteogenesis than HAp by increasing collagen synthesis and calcification of the extracellular matrix.

Estrogens activate bone morphogenetic protein-2 gene transcription in mouse mesenchymal stem cells

Estrogens exert their physiological effects on target tissues by interacting with the estrogen receptors, ERalpha and ERbeta. Estrogen replacement is one the most common and effective strategies used to prevent osteoporosis in postmenopausal women. Whereas it was thought that estrogens work exclusively by inhibiting bone resorption, our previous results show that 17beta-estradiol (E2) increases mouse bone morphogenetic protein (BMP)-2 mRNA, suggesting that estrogens may also enhance bone formation. In this study, we used quantitative real-time RT-PCR analysis to demonstrate that estrogens increase BMP-2 mRNA in mouse mesenchymal stem cells. The selective ER modulators, tamoxifen, raloxifene, and ICI-182,780 (ICI), failed to enhance BMP-2 mRNA, whereas ICI inhibited E2 stimulation of expression. To investigate if estrogens increase BMP-2 expression by transcriptional mechanisms and if the response is mediated by ERalpha and/or ERbeta, we studied the effects of estrogens on BMP-2 promoter activity in transient transfected C3H10T1/2 cells. E2 produced a dose-dependent induction of the mouse -2712 BMP-2 promoter activity in cells cotransfected with ERalpha and ERbeta. At a dose of 10 nM E2, ERalpha induced mouse BMP-2 promoter activity 9-fold, whereas a 3-fold increase was observed in cells cotransfected with ERbeta. Tamoxifen and raloxifene were weak activators of the mouse BMP-2 promoter via ERalpha, but not via ERbeta. ICI blocked the activation of BMP-2 promoter activity by E2 acting via both ERalpha and ERbeta, indicating that mouse BMP-2 promoter activation is ER dependent. In contrast to E2 and selective ER modulators, the phytoestrogen, genistein was more effective at activating the mouse BMP-2 promoter with ERbeta, compared with ERalpha. Using a deletion series of the BMP-2 promoter, we determined that AP-1 or Sp1 sites are not required for E2 activation. A mutation in a sequence at -415 to -402 (5'-GGGCCActcTGACCC-3') that resembles the classical estrogen-responsive element abolished the activation of the BMP-2 promoter in response to E2. Our studies demonstrate that E2 activation of mouse BMP-2 gene transcription requires ERalpha or ERbeta acting via a variant estrogen-responsive element binding site in the promoter, with ERalpha being the more efficacious regulator. Estrogenic compounds may enhance bone formation by increasing the transcription of the BMP-2 gene.

Selective delivery of estradiol to bone by aspartic acid oligopeptide and its effects on ovariectomized mice

We have developed a novel osteotropic prodrug of estradiol (E(2)) conjugated with L-Asp-hexapeptide (E(2).3D(6)), which has very low affinity for estrogen receptors, and in this study, we examined its pharmacokinetic behavior and pharmacological potential. After a single iv injection of E(2) x 3D(6) to mice, the half-time for elimination from plasma was about 100 min; however, E(2) was selectively delivered to the bone and eliminated very slowly, declining to the endogenous level at about 7 days. After a single iv injection of E(2), the half-time in plasma was about 70 min, whereas E(2) was highly distributed to the uterus, and the bone concentration of E(2) was only slightly increased at 6 h. When E(2) (0.37 micromol/kg, sc, every third day) or E(2) x 3D(6) (0.11 to 1.1 micromol/kg, sc, every seventh day) was administered to OVX mice for 4 weeks, E(2) increased the bone mineral density (BMD) together with weights of liver and uterus, whereas E(2) x 3D(6) increased only the BMD, in a dose-dependent manner. E(2) x 3D(6) enhanced the expression of messenger RNAs of bone matrix proteins (osteopontin, bone sialoprotein, type I collagen alpha) of OVX mice at 4 h after administration, but E(2) did very slightly. These results indicate that the E(2) prodrug was delivered to the bone, where it gradually released E(2), thereby ameliorating bone loss. This acidic oligopeptide appears to be a good candidate for selective drug delivery to bone.

Nitric oxide mediates 17beta-estradiol-stimulated human and rodent osteoblast proliferation and differentiation

Oestradiol can stimulate osteoblast activity. Osteoblast function is thought to be regulated by nitric oxide (NO). We hypothesised that the effect of 17beta-oestradiol (17beta-E(2)) on osteoblast activity is mediated by NO. This hypothesis was tested using osteoblasts isolated from human trabecular bone, calvariae of rats, endothelial NO synthase (eNOS) gene-deficient mice, and their wild-type counterparts. Our results show that 17beta-E(2) dose-dependently stimulated proliferation and differentiation of primary human, rat and wild-typeosteoblasts. The presence of N(G)-monomethyl-l-arginine (10(-3) M), an inhibitor of NOS activity, blocked the 17beta-E(2)-(10(-7) M)-induced increases in thymidine incorporation (P < 0.01), alkaline phosphatase activity (P < 0.01) and bone nodule formation (P < 0.01) of wild-type, human and rat osteoblasts, respectively. Moreover, 17beta-E(2) did not induce a response in eNOS gene-deficient osteoblasts. 17beta-E(2) also increased total eNOS enzyme expression in rat osteoblasts. These findings indicate 17beta-E(2) modulates osteoblast function by NO-dependent mechanisms mediated via the eNOS isoform.

The monoclonal antibodies 18d7/91f2 recognize a receptor regulatory protein on mouse bone marrow stromal cells

Two monoclonal antibodies 18D7 and 91F2 were developed by immunizing rats with the mouse bone marrow-derived osteogenic cell line MN7. Hybridomas secreting rat antibodies against MN7 cell surface markers were selected by flow cytometry analysis. Both the monoclonal antibody 18D7 and the monoclonal antibody 91F2 are directed against the same cell surface antigen present on MN7 cells. Here, we report on the immunopurification of the 18D7/91F2 antigen and its identification as the prostaglandin F2 alpha receptor regulatory protein (FPRP). FPRP is expressed as a single messenger RNA (mRNA) of approximately 6 kilobases (kb) in MN7 cells and is differentially expressed in developing osteogenic cultures of bone marrow cells of the mouse. However, addition of the monoclonal antibodies 18D7 and 91F2 to these cultures did not inhibit bone formation in vitro. Both monoclonal antibodies reacted with mouse stromal cell lines established from bone marrow, thymus, spleen, and mandibular condyles. Immunohistochemical analysis of mature tibia of mice using the monoclonal antibody 18D7 revealed the presence of a distinct population of bone marrow cells close to trabecular and endosteal bone surfaces. In the central bone marrow, hardly any positive cells were found. In 17-day-old fetal mouse radius 18D7 immunoreactivity was restricted to cells in the periosteum in close vicinity to the bone collar. Mature osteoblasts, osteoclasts, osteocytes, growth plate chondrocytes, and mature macrophages were all negative. Taken together, these results suggest that FPRP plays a role in the osteogenic differentiation process.

Effect of hypophysectomy on the proliferation and differentiation of rat bone marrow stromal cells

Conditions such as estrogen deficiency, skeletal unloading, and aging have all been demonstrated to have various effects on the proliferation and differentiation of bone marrow stroma-derived osteoprogenitor cells. Here we have sought to examine the effects of pituitary hormone deficiency on the proliferation and the differentiation of these osteoprogenitor cells using the hypophysectomized (HX) rat as a model. In the present study, we use an in vitro culture system to examine the effects of HX on the osteogenic potential of rat bone marrow stroma. With the intact animal as a control, we used [3H]thymidine incorporation and cell number as indexes of proliferation. We also measured alkaline phosphatase enzyme activity, relative levels of osteocalcin expression with RT-PCR, and osteopontin and bone sialoprotein steady-state levels by Northern blot to delineate the effect on differentiation. Our results indicate that osteoprogenitor cells exposed to a pituitary hormone-deficient environment in vivo demonstrate an enhanced proliferative capacity and also exhibit an augmented expression of differentiation markers when exposed to an optimal environment in vitro.

drp, a novel protein expressed at high cell density but not during growth arrest

Contact is a vital mechanism used by cells to interact with their environment. Contact with living and nonliving elements adjacent to a cell is the basis for many common biological events ranging from growth regulation to metastasis to embryonic pattern formation. We describe the cloning and characterization of a novel density-regulated protein (drp) whose expression is increased in cultured cells at high density compared with cells at low density. A drp cDNA was isolated from the human teratocarcinoma cell line PA-1. Northern analysis with a drp probe revealed transcripts of 2.8 and 3.2 kb. The drp RNA was expressed in a variety of tissues, with the highest amounts in skeletal and cardiac muscle. Using antipeptide antisera, increasing amounts of a 70-kDa protein were detected using several experimental approaches in several cells lines as cell density is increased. Conditioned medium from high-density cells was unable to induce expression of drp in cells growing at low density. Similarly, growth arrest by serum starvation or transforming growth factor-beta (TGF-beta) treatment failed to elicit drp expression. We conclude that drp is a novel protein whose expression is increased at high cell density but not growth arrest.

Molecular cloning, characterization, and genetic mapping of the cDNA coding for a novel secretory protein of mouse. Demonstration of alternative splicing in skin and cartilage

A novel 85-kDa protein secreted by the mouse stromal osteogenic cell line MN7 was identified using two-dimensional polyacrylamide gel electrophoresis (Mathieu, E., Meheus, L., Raymackers, J., and Merregaert, J. (1994) J. Bone Miner. Res. 9, 903-913). Degenerate primers were used to isolate the cDNA coding for this protein. The full-length cDNA clone is 1.9 kilobases (kb) and codes for a protein of 559 amino acid residues. The DNA and deduced amino acid sequences have no counterparts in public data bases, but a structural similarity involving typical cysteine doublets can be observed to serum albumin family proteins and to Endo16 (a calcium-binding protein of sea urchin). Northern blot analysis revealed the presence of a 1.9-kb transcript in various tissues, and a shorter transcript of 1.5 kb, derived by alternative splicing in tail, front paw and skin of embryonic mice. The gene for the p85 protein, termed Ecm1 (for extracellular matrix protein 1), is a single-copy gene, which was localized to the region on mouse chromosome 3 known to contain at least one locus associated with developmental disorders of the skin, soft coat (soc). Alternative splicing may serve as a mechanism for generating functional diversity in the Ecm1 gene.

Importance of 1,25-dihydroxyvitamin D3 and the nonadherent cells of marrow for osteoblast differentiation from rat marrow stromal cells

Although steroid hormones regulate mature osteoblast function, much less is known about their actions on osteoprogenitor cells. The possibility of steroid hormone regulation of early stages in osteoblast differentiation was investigated by measuring the growth and induction of the osteoblast marker enzyme alkaline phosphatase (AP) in rat bone marrow stromal cell cultures. Experiments were performed in charcoal-stripped serum; conditions which markedly impaired stromal cell growth. However, growth could be stimulated by nonadherent marrow cell-derived conditioned medium. 1,25(OH)2D3, but not dexamethasone, 17 beta-estradiol, or retinoic acid, increased both stromal cell proliferation and AP activity. The increased proliferation with 1,25(OH)2D3 was nonadherent cell-dependent. BMP-2 also increased AP levels and acted in synergy with 1,25(OH)2D3. These results suggest that (i) nonadherent marrow cells may support stromal cell development, and (ii) 1,25(OH)2D3 as well as glucocorticoids may regulate osteogenesis from the bone marrow but a similar role for estrogen is not supported.

Characterization of the osteogenic stromal cell line MN7: identification of secreted MN7 proteins using two-dimensional polyacrylamide gel electrophoresis, western blotting, and microsequencing

Proteins secreted by the osteogenic stromal cell line MN7 were analyzed using two-dimensional polyacrylamide gel electrophoresis (PAGE), western blotting, immunodetection, and microsequencing. Trichloroacetic acid-precipitated proteins from the conditioned medium of MN7 cell cultures, harvested at different times of growth, were dissolved in denaturing and reducing sample buffer and separated in the first dimension according to isoelectric point and in the second dimension according to molecular weight. Protein patterns were visualized using silver staining. Among the 350 separated protein spots, we identified type I collagen, bone sialoprotein, osteonectin, and cathepsin B by western blotting and immunodetection using polyclonal antibodies. Osteocalcin could not be detected in the conditioned medium of MN7 cells. Furthermore, 15 MN7-specific protein spots were localized after comparison with two-dimensional PAGE patterns from the conditioned medium of the nonosteogenic stromal cell lines MM1 and MV1. Microsequencing of the internal peptides of five selected spots revealed three known proteins, namely the carboxyl-terminal propeptide of the alpha 2 chain of collagen type I, cathepsin L, and the tissue inhibitor of metalloproteinases-2, an 18 kilodalton peptide fragment from osteopontin that has not previously been described, and a novel glycosylated 85 kD protein with an average isoelectric point of 5.7. All identified proteins did not vary in presence between the different time points analyzed by two-dimensional PAGE. The use of two-dimensional PAGE to investigate the secreted proteins of MN7 cells will enable us to establish a complete protein data base of extracellular osteoblast-specific proteins. Furthermore, two-dimensional PAGE in combination with other techniques is a fast and accurate method for the identification of novel proteins that could function as markers in osteoblast differentiation and/or bone formation.