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Endocrinology Vol. 144, No. 12 5511-5520
Copyright © 2003 by The Endocrine Society

Parathyroid Hormone-Related Peptide Interacts with Bone Morphogenetic Protein 2 to Increase Osteoblastogenesis and Decrease Adipogenesis in Pluripotent C3H10T1/2 Mesenchymal Cells

George K. Chan, Dengshun Miao, Ron Deckelbaum, Isabel Bolivar, Andrew Karaplis and David Goltzman

Calcium Research Laboratory, McGill University Health Center (G.K.C., D.M., I.B., D.G.), Lady Davis Research Institute-Jewish General Hospital (R.D., A.K.), and Department of Medicine, McGill University (G.K.C., D.M., R.D., I.B., A.K., D.G.), Montréal, Québec, Canada H3A-1A1

Address all correspondence and requests for reprints to: Dr. David Goltzman, Calcium Research Laboratory, Room H 4.67, Royal Victoria Hospital/McGill University Health Center, 687 Pine Avenue, Montréal, Québec, Canada H3A 1A1. E-mail: david.goltzman{at}mcgill.ca.


   Abstract
Top
 Abstract
Introduction
 Materials and Methods
 Results
 Discussion
 References
 
We examined the effect of PTH-related peptide (PTHrP) on modulating adipogenesis and osteoblastogenesis in the pluripotent mesenchymal cell line C3H10T1/2. These cells express the type 1 PTH/PTHrP receptor, thereby allowing PTHrP to inhibit bone morphogenetic protein 2 (BMP2) from enhancing gene expression of peroxisome proliferator-activated receptor {gamma} and the adipocyte-specific protein aP2 and from augmenting the accumulation of lipid. In the presence of BMP2, PTHrP or a protein kinase C (PKC) stimulator (phorbol ester) increased the expression of indexes of the osteoblast phenotype, including alkaline phosphatase, type I collagen, and osteocalcin, whereas a PKC inhibitor (chelerythrin chloride) inhibited PTHrP action. PTHrP and a phorbol ester increased gene expression of the BMP IA receptor, and both enhanced BMP2-dependent increases in promoter activity of the signaling molecule SMAD6. Overexpression of the BMP IA receptor facilitated the capacity of BMP2 to increase osteoblastogenesis in the absence of PTHrP and a dominant negative BMP IA receptor variant inhibited this effect of BMP2. These results demonstrate that PTHrP can direct osteoblastic, rather then adipogenic, commitment of mesenchymal cells, implicate PKC signaling in this activity, and show that PTHrP action involves enhanced gene expression of the BMP IA receptor, which facilitates BMP2 action in enhancing osteoblastogenesis in pluripotent mesenchymal cells.


   Introduction
Top
 Abstract
 Introduction
Materials and Methods
 Results
 Discussion
 References
 
OSTEOBLASTS, THE PRINICIPAL bone-forming cells, and adipocytes, the main fat-storing cells, are believed to originate from the same pluripotent mesenchymal stem cells (1). Both cell types also contribute to the architectural structure of bone, and a decrease in bone volume is generally accompanied by an increase in adipocyte number within the bone marrow (2). This reciprocal relationship is exemplified in the development of age-related osteopenia. Thus, bone marrow at a very early age is virtually devoid of adipocytes; however as aging progresses, a decrease in bone volume occurs with a reciprocal increase in fat deposits within the marrow (2, 3). One hypothesis to explain these phenomena is that the reduced number of osteoblasts and the increased number of adipocytes are the result of increasing commitment of pluripotent mesenchymal cells along the adipocytic, as opposed to the osteoblastic, lineage (4). Regulation of this process may involve bone morphogenetic proteins (BMPs). BMPs are expressed within the bone marrow stroma and are the only known morphogens that are able to induce both adipogenesis and osteogenesis of pluripotent mesenchymal cells (5). BMP2, BMP4, and BMP7 have all been shown to be effective inducers of both osteoblast and adipocyte commitment in vitro; however, the cell lineage that is specified is often determined by the concentration of applied BMP. BMP family members may therefore serve to activate transcription of distinct target genes in a dose-dependent manner, leading to distinct cellular phenotypes. BMPs act by binding to a type I and a type II receptor. BMP binding to the type II receptor enhances the affinity for the type I receptor, and heterodimerization of the two receptors follows. Once heterodimerization takes place, the type II receptor trans-phosphorylates and activates the type I receptor. It is the type I receptor that contains the functional kinase domain that is essential for serine phosphorylation and activation of downstream SMAD transcription factors (6).

PTH and PTH-related peptide (PTHrP) analogs have been shown to effectively induce bone formation both in vivo and in vitro (7). Both peptides interact at a common G protein-coupled receptor, termed the type I PTH/PTHrP receptor (PTHR), which is linked to the adenylyl cyclase/protein kinase A (PKA) signaling system and the phospholipase C/protein kinase C (PKC) systems (8). Several mechanisms have been suggested for the actions of PTH and PTHrP on increasing bone formation. These include proliferation of osteogenic progenitor cells (9), enhancing osteoblast differentiation (10), and inhibiting apoptosis of osteoblastic cells (11). PTH has also been shown to enhance the transcriptional activity of the osteoblast differentiation factor core binding factor A1 (CBFA1) via phosphorylation through an adenylyl cyclase-PKA-dependent mechanism (12).

We recently demonstrated that PTHrP is able in vitro to inhibit the terminal differentiation of committed preadipocytes by stimulating MAPK, which, in turn, can phosphorylate and down-regulate the adipogenic determining factor peroxisome proliferator-activated receptor {gamma} (PPAR{gamma}) (13). We also reported that in vivo PTHrP heterozygous null mice that are haplo-insufficient for PTHrP develop a premature form of osteopenia, characterized by reduced trabecular bone volume and increased bone marrow adiposity (14). In the current studies we investigated whether we could demonstrate in vitro that PTHrP might play a role in directing mesenchymal cell differentiation toward the adipocytic or osteoblastic lineage. For these studies we employed the pluripotent mesenchymal cell line C3H10T1/2. These cells are derived from mouse embryo connective tissue and can be induced to differentiate along several mesenchymal cell lineages (5, 15). When treated with low concentrations of BMP2, C3H10T1/2 cells differentiate along the adipocytic lineage; however, with higher concentrations of BMP2, osteogenesis is enhanced (16).

We therefore employed C3H10T1/2 cells to study the effect of PTHrP on inducing the osteoblast phenotype in these cells. Our studies show that in this system PTHrP can inhibit the capacity of BMP to increase adipogenesis and can facilitate the action of BMP to direct differentiation toward the osteoblastic lineage. The PKC pathway is implicated as a signaling mechanism in this process, and the PTHrP effect involves up-regulation of the BMP IA receptor, which enhances sensitivity to BMP and favors development of the osteoblastic, rather than the adipocytic, lineage.


   Materials and Methods
Top
 Abstract
 Introduction
 Materials and Methods
Results
 Discussion
 References
 
Cell culture and cell transfections
C3H10T1/2 clone 8 cells were obtained from American Type Culture Collection (Manassas, VA) and maintained in DMEM containing 10% heat-inactivated fetal calf serum. Fresh medium was applied every second day. To induce osteogenesis, cells were grown to confluence then maintained in {alpha}MEM supplemented with ascorbic acid (100 µg/ml), ß-glycerophosphate (5 mM), and BMP2 (Research Diagnostics, Inc., Flanders, NJ; 6 x 10-9 M) unless stated otherwise. Fresh medium was applied every 2 d. When required, PTHrP-(1–34) was added at a concentration of 1 x 10-7 M unless otherwise stated. The PKC inhibitor chelerythrin chloride (17, 18) was assessed at concentrations of 1.0 x 10-5, 5.0 x 10-6, 2.5 x 10-6, 1.25 x 10-6, and 5.0 x 10-7 M. The maximal concentration permitting cell viability, as determined by trypan blue exclusion, was 1.25 x 10-6 M. This concentration was therefore employed in all subsequent experiments.

C3H10T1/2 cells expressing the BMP IA receptor were generated after cloning cDNA encoding the rat BMP IA receptor (19) (provided by Dr. Gideon Rodan, Merck & Co., West Point, PA) into the pcDNA3 expression plasmid. The expression plasmid was then used for stable transfection as previously described (13). C3H10T1/2 cells expressing a BMP IA dominant negative receptor (DNBMP IA) lacking kinase activity were generated after cloning the cDNA (provided by Dr. Gideon Rodan) into the pcDNA3 expression plasmid. Stably transfected cells were generated in a similar manner, and control cells were generated by stable transfection with empty pcDNA3 plasmid. Selection was performed using 400 µg geneticin/ml (Life Technologies, Inc., Gaithersburg, MD). All transfections were carried out using FUGENE6 reagent (Roche, Indianapolis, IN). SMAD6 luciferase reporter transformants were generated by cotransfection of C3H10T1/2 cells with 5 µg of a SMAD6 promoter fused to a luciferase reporter as previously described (20) and 0.5 µg pCDNA3, due to the lack of a selection marker for the SMAD6 luciferase reporter construct. Serial dilutions were performed, and selection of individual clones was carried out using 400 µg geneticin/ml medium.

Northern blot analysis
Total RNA was isolated using TRIzol reagent (Life Technologies, Inc.). For Northern blot analysis, 10 µg total RNA/sample were electrophoresed through a formaldehyde/agarose gel. The RNA was transferred to a Bio-Trans nitrocellulose membrane (Amersham Pharmacia Biotech, Arlington Heights, IL). For detection of adipocytic genes, the membrane was probed with an internal EcoRI fragment of PPAR{gamma} mouse cDNA (13) and the full-length cDNA of mouse aP2 (13). For detection of osteoblastic genes, the membrane was probed with a 1135-bp fragment encoding the amino terminus of mouse alkaline phosphatase (ALP) cDNA, a 1600-bp fragment of rat {alpha}1R1 collagen 1 cDNA (21), and the entire 467-bp coding region of rat osteocalcin cDNA. For detection of BMP IA and DNBMP IA mRNA, a 233-bp fragment of the mouse BMP IA receptor cDNA was generated by PCR (5'-CTT GGA CCA GAA GAA GCC AG-3' and 5'-CTT TCG GTG AAT CCT TGC AT-3'). Northern analysis detection was performed using a Cyclone Storage Phosphor System (Packard Instrument Co., Inc., Meriden, CT). Levels of mRNA were quantified using Image (Scion Corp., Frederick, MD) and were standardized by comparison with 18S RNA levels that were detected simultaneously, as previously described (13).

BMP2-responsive luciferase assays
SMAD6 promoter-luciferase reporter assays were performed by a modification of a previously described method (20). Briefly, C3H10T1/2 cells stably transfected with the SMAD6 promoter-luciferase reporter were pretreated with BMP2 (6 x 10-9 M) alone for 5 d. To examine the PTHrP effects, in some culture plates PTHrP (10-7 M) was added without or with chelerythrin chloride (1.25 x 10-6 M) and with BMP2. To examine the effects of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA), in some plates, TPA (1 µM) was added with or without chelerythrin chloride (1.25 x 10-6 M) and with BMP2 on the fourth day for 24 h. After an additional 24 h of serum deprivation, all cells were stimulated with PBS vehicle or BMP2 (6 x 10-9 M) for 24 h, and luciferase activities were measured.

PKA and PKC activity assay
C3H10T1/2 cells were incubated with BMP2 (6 x 10-9 M) for 4 d. The day before assays were to be performed, the cells were serum-deprived overnight. The following day, the PKA and PKC responses to PTHrP were measured intermittently after treatment of cells for 15 min with PTHrP-(1–34), followed by lysis. No readings were taken for d 1, as cells were treated for a minimum of 24 h with BMP2 before serum deprivation. PKA and PKC activities were assessed with a PKA assay kit (Upstate Biotechnology, Inc., Lake Placid, NY) and a PKC assay kit (Upstate Biotechnology, Inc.), respectively. For the PKA assay, kemptide (S6 kinase substrate; Upstate Biotechnology, Inc.) was used in place of the provided substrate. Six samples were prepared for each time point.

Oil Red O staining
After 14 d of incubation with BMP-2 (6 x 10-9 M), C3H10T1/2 cells were washed twice with PBS, then fixed for 30 min with 10% formalin. Oil Red O stain (5% Oil Red O in 70% pyridine) was applied to the cells for 30 min, and then cells were washed three times with PBS.

ALP staining
Cytochemical staining for ALP was performed by incubating the cells for 15 min at room temperature in 100 mM Tris-maleate buffer containing 0.2 mg/ml naphthol AS-MX phosphate (Sigma-Aldrich Corp., St. Louis, MO) dissolved in ethylene glycol monomethyl ether (Sigma-Aldrich Corp.) as a substrate, and Fast Red TR (0.4 mg/ml; Sigma-Aldrich Corp.) as a stain for the reaction product.

Immunocytochemistry
Cultured cells were stained for type I collagen, osteopontin, and osteocalcin using the avidin-biotin-peroxidase complex technique as described previously (9). The cells were first treated with 0.5% bovine testicular hyaluronidase (Sigma-Aldrich Corp.) for 30 min at 37 C, followed by application of primary antibodies, affinity-purified goat antihuman type I collagen antibody (Southern Biotechnology Associates, Inc., Birmingham, AL), goat antimouse osteocalcin (Biomedical Technologies, Inc., Stoughton, MA), and PPAR{gamma} (Research Diagnostics Inc., Flanders, NJ), overnight at room temperature. As a negative control, the preimmune serum was substituted for the primary antibody. After washing with high salt buffer (50 mM Tris-HCl, 2.5% NaCl, and 0.05% Tween 20, pH 7.6) for 10 min at room temperature, followed by two 10-min washes with 50 mM Tris-HCl, 150 mM NaCl, and 0.01% Tween 20, pH 7.6, the cells were incubated with a secondary antibody (biotinylated rabbit antigoat IgG, Sigma-Aldrich Corp.). Cells were then washed as before and incubated with the Vectastain ABC-AP kit (Vector Laboratories, Inc., Ontario, Canada) for 45 min. After washing as before, red pigmentation to identify regions of immunostaining was produced by a 10- to 15-min treatment with Fast Red TR/Naphthol AS-MX phosphate (Sigma-Aldrich Corp.) containing 1 mM levamisole as endogenous ALP inhibitor.

Quantification for both cytochemistry and immunocytochemistry by image analysis was performed as previously described (9).

Computer-assisted image analysis
Computer-assisted image analysis was performed as described previously (9). Briefly, images of stained culture dishes were photographed with transmitted light over a light box. All images were processed using Northern Eclipse image analysis software (version 5.0, Empix Imaging, Inc., Mississauga, Canada). For determining the area of positive colonies in cultured cells, thresholds were set using green and red channels. The thresholds were determined interactively and empirically on the basis of three different images. Subsequently, this set threshold was used to automatically analyze all recorded images of all sections that were stained in the same staining session under identical conditions.

Statistical analysis
Statistical analysis of cell cultures is based on three random fields. Statistical analysis was performed using a t test, Fisher’s test, or ANOVA, followed by Bonferroni adjustment as appropriate. P <= 0.05 was taken as significant.


   Results
Top
 Abstract
 Introduction
 Materials and Methods
 Results
Discussion
 References
 
Evidence for functional type I PTH/PTHrP receptors (PTHR) in C3H10T1/2 cells
Northern analysis revealed PTHR mRNA expression in C3H10T1/2 cells, which remained constant throughout a 5-d incubation with BMP2 (Fig. 1Go, A and B). PTHrP is known to stimulate both PKA and PKC after binding to its G protein-coupled receptor (PTHR). To determine the functionality of the PTHR expressed in C3H10T1/2 cells, we determined basal and PTHrP-stimulated PKA and PKC activity over a 5-d incubation with BMP2. In the absence of PTHrP, PKA and PKC levels increased and then stabilized by d 4 (Fig. 1Go, C and E). In the presence of PTHrP, significant increases in both PKA and PKC activity were observed at each time point (Fig. 1Go, C and E). Furthermore, when assessed on d 4, PTHrP elicited a concentration-dependent increase in both PKA and PKC activities (Fig. 1Go, D and F).



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  		FIG. 1. Influence of BMP2 on PTHR expression and function in C3H10T1/2 cells. A, Northern analysis was performed to determine the expression of PTHR over a 5-d incubation with BMP2 (6 x 10-9 M). 18S RNA levels were assessed as a control. B, Computer-assisted quantification of Northern analysis was performed as described in Materials and Methods. Relative levels of PTHR expression was standardized vs. relative levels of 18S. C, PKA activity was measured as described in Materials and Methods. Cells were incubated over 5 d with BMP-2 (6 x 10-9 M), and enzyme activity was determined after treatment with BMP2 plus vehicle (control) or with BMP2 plus PTHrP (10-7 M). D, After 4 d of incubation with BMP-2, increasing concentrations of PTHrP were added, and PKA activity was assessed. E, PKC activity was measured as described in Materials and Methods and was determined under the same conditions as those described for PKA. F, The effect of increasing concentrations of PTHrP on PKC activity was determined under the same conditions as those described for PKA. For C, D, E, and F, each bar represents the mean ± SE of six replicates. *, P < 0.05 vs. control.

 
PTHrP inhibits adipogenesis of pluripotent C3H10T1/2 cells
We incubated C3H10T1/2 cells over a 2-wk period either with a concentration of BMP2 known to induce adipocyte differentiation (16) or with the same concentration of BMP2 in combination with PTHrP. BMP2-treated cultures demonstrated extensive adipogenesis as assessed by Oil Red O staining of the cells (Fig. 2Go, B and D). In contrast, markedly reduced adipogenesis was observed in the presence of BMP2 and PTHrP (Fig. 2Go, C and D).



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  		FIG. 2. Influence of BMP2 and PTHrP on adipogenesis in C3H10T1/2 cells. A, C3H10T1/2 cells were cultured for 14 d and then stained with Oil Red O. B, Cells were incubated with BMP2 (6 x 10-9 M) alone for 14 d and then stained with Oil Red O as described in Materials and Methods. C, Cells were incubated with BMP2 (6 x 10-9 M) and PTHrP (10-7 M) for 14 d and then processed as described in A. Each photomicrograph in A–C is representative of three random fields from three different cultures. D, Quantification of Oil Red O staining per field was performed with Northern Eclipse software as described in Materials and Methods. Cells were untreated, treated with BMP2 alone (6 x 10-9 M; left bar), or treated with BMP2 and PTHrP (10-7 M right bar). Each bar represents the mean ± SE of triplicate determinations using three random fields from three different cultures. *, P < 0.05. E, Northern blots of PPAR{gamma} and aP2 were performed as described in Materials and Methods and are shown in the upper panel. RNA was extracted from cells before treatment (d 0) and on d 7 with and without treatment. Cells were treated with BMP2 (6 x 10-9 M) alone (control) or with BMP2 plus PTHrP (10-7 M PTHrP). Numbers represent two different experiments. Levels of mRNA encoding 18S were determined as a loading control (lower panel). F, Computer-assisted quantification of Northern analysis was performed, whereby relative levels of the adipocytic markers PPAR{gamma} and aP2 were standardized vs. relative levels of 18S. G, Untreated C3H10T1/2 cultured cells after a 14-d incubation following analysis for PPAR{gamma} expression. H, Cells were treated with BMP2 (6 x 10-9 M) alone for 14 d and then analyzed for PPAR{gamma} protein expression as described in Materials and Methods. I, Cells were incubated with BMP2 (6 x 10-9 M) and PTHrP (10-7 M) for 14 d, followed by analysis for PPAR{gamma} protein expression. Photomicrographs in G–I are each representative of three random fields from three different cultures. J, Quantitation of PPAR{gamma} protein expression was performed using Northern Eclipse software as described in Materials and Methods. C3H10T1/2 cells were untreated (left bar), treated with BMP2 (6 x 10-9 M) alone (center bar), or treated with BMP2 and PTHrP (10-7 M right bar). Each bar represents the mean ± SE of triplicate determinations using three random fields from three different cultures. *, P < 0.05.

 
To examine the mechanism of this effect, mRNA was isolated before cell treatment (d 0) and 7 d later from untreated cells or cells treated with either BMP2 alone or BMP2 and PTHrP. Northern blots were performed to examine expression of the adipogenic transcription factor PPAR{gamma} and the fat-specific gene aP2. BMP2 alone increased both PPAR{gamma} and aP2 expression by d 7, whereas with PTHrP treatment, a reduction in both PPAR{gamma} and aP2 mRNA levels was observed (Fig. 2Go, E and F).

The expression of PPAR{gamma} was further confirmed by immunocytochemistry of cells that were untreated (Fig. 2GGo) or were incubated with either BMP2 (Fig. 2HGo) or BMP2 in combination with PTHrP (Fig. 2IGo). Untreated cells demonstrated sparse expression of PPAR{gamma} protein (Fig. 2Go, G and J), whereas cells treated with BMP2 expressed abundant PPAR{gamma} protein (Fig. 2Go, H and J). Cells treated with both BMP2 and PTHrP showed limited expression of PPAR{gamma} (Fig. 2Go, G and J). Consequently, PTHrP appeared capable of curtailing the adipogenic program induced by BMP2 in these cells.

PTHrP enhances osteoblastogenesis in pluripotent C3H10T1/2
C3H10T1/2 cells were incubated over a 2-wk period with medium alone or with either an adipogenic dose of BMP2 alone or an adipogenic dose of BMP2 in combination with PTHrP. Indexes of osteoblastic differentiation were then examined. Evidence of expression of ALP, type I collagen, and osteocalcin protein and mRNA were observed with PTHrP treatment (Fig. 3Go, A–D), whereas only minimal expression of these markers was observed in cultures treated with BMP2 alone (Fig. 3Go, A–D). Consequently, PTHrP appeared to stimulate osteoblastogenesis while inhibiting adipogenesis.



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  		FIG. 3. Effects of BMP2 and of PTHrP on the osteoblast phenotype. A, C3H10T1/2 cells were either cultured untreated or were incubated with BMP2 (6 x 10-9 M) alone or with BMP2 plus PTHrP (10-7 M) and after 14 d were examined for the expression of ALP, type I collagen (Col I), and osteocalcin (OCN) protein as described in Materials and Methods. Photomicrographs are representative of three different fields from four different cultures. B, Expression of ALP, type I collagen, and osteocalcin of untreated cells or cells after treatment with BMP2 (6 x 10-9 M) alone or with BMP2 and PTHrP was quantified using Northern Eclipse imaging software as described in Materials and Methods and is represented as the percentage of the field that stained positively for each osteoblastic marker (% staining/field). Each bar represents the mean ± SE of three random fields per culture from four different cultures. *, P < 0.05 vs. control (BMP2 alone). C, Expression of mRNA encoding ALP, type I collagen, and osteocalcin was determined in untreated cells, cells treated with BMP2 alone, or cells treated with BMP2 and PTHrP, as described in Materials and Methods. mRNA loading was assessed by probing with 18S. D, Quantification of Northern analysis was performed to determine the expression of osteoblastic markers in untreated cells, cells treated with BMP2 (6 x 10-9 M), and cells treated with BMP2 and PTHrP. Computer-assisted quantification of Northern analysis was performed as described in Materials and Methods. Relative levels of ALP, type I collagen, and osteocalcin expression were standardized vs. relative levels of 18S.

 
PTHrP acts by a PKC mechanism to increase osteoblastogenesis and BMP IA receptor expression and signaling
To assess whether the PKA pathway, the PKC pathway, or both mediates the effects of PTHrP, C3H10T1/2 cells were treated with low levels of BMP2 (Fig. 4Go). On d 4, vehicle alone (control), the PKA activator forskolin, the PKC activator TPA, or both forskolin and TPA were added for 24 h. After an additional 10 d in culture, cells were examined for expression of the osteoblast markers ALP, type I collagen, and osteocalcin. Treatment with forskolin had only a slight effect on the expression of these markers, whereas treatment with TPA produced dramatic increases (Fig. 4Go, A and B). Cells treated with both forskolin and TPA did not show enhanced expression as did those treated with TPA alone (Fig. 4AGo and B). Consequently, the PTHrP effect on osteoblastogenesis appeared to be predominantly due to a PKC-mediated response. Furthermore, there seemed to be no synergy between the PKC and PKA pathways in mediating this action.



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  		FIG. 4. Effects of PKA and PKC signaling pathways on the osteoblast phenotype. A, C3H10T1/2 cells were incubated for 14 d with BMP2 (6 x 10-9 M). On d 4 of this incubation, vehicle (control), forskolin (100 µM), TPA (1 µM), or both forskolin (100 µM) and TPA (1 µM) were added for 24 h. After the 14-d incubation period, cells were examined for the expression of ALP, type I collagen (Col 1), and osteocalcin (OCN) as described in Materials and Methods. Photomicrographs are representative of three random fields from three different experiments. B, Expression of the markers shown in A was quantitated using Northern Eclipse imaging software as described in Materials and Methods. The results are represented as the percentage of the field that stained positively for each osteoblastic marker (% staining/field). Each bar represents the mean ± SE of triplicate fields from three different experiments. *, P < 0.05 vs. control (BMP2 alone); **, P < 0.05 vs. BMP2 and forskolin treatment. C, C3H10T1/2 cells were incubated for 14 d with vehicle only (Control) or with BMP2 (6 x 10-9 M) alone or in combination with PTHrP (10-7 M) or PTHrP and chelerythrin chloride (1.25 x 10-6 M). After the 14-d incubation period, the expression of osteocalcin was determined by immunocytochemistry. Photomicrographs are representative of three random fields from three different experiments. D, Quantitation of osteocalcin staining was performed as described in Materials and Methods. Each bar represents the mean ± SE of three fields per culture from three separate cultures. * and **, P < 0.05 vs. BMP alone (control) and vs. BMP plus PTHrP, respectively.

 
To further determine whether PTHrP enhances osteoblastogenesis as a result of enhanced PKC signaling, C3H10T1/2 cells were induced to differentiate with BMP2 alone or with BMP2 and either PTHrP alone or PTHrP and the PKC inhibitor chelerythrin chloride. After culturing, cells were stained for the osteoblast-specific protein osteocalcin. Cells treated with PTHrP and chelerythrin demonstrated significantly lower levels of osteocalcin expression compared with cells treated with PTHrP alone (Fig. 4Go, C and D). Consequently, the PTHrP effect on osteoblastogenesis appeared to be due at least in part to a PKC-mediated response.

In view of the fact that even in the absence of PTHrP, higher concentrations of BMP2 have been reported to enhance differentiation along the osteoblast lineage (16), we next assessed whether PTHrP might augment the sensitivity of the cells to BMP2 by amplifying its signaling. We therefore first examined the effect of PTHrP on expression of the BMP I receptors that transduce the BMP signal via its serine/threonine kinase activity. Cells were incubated with low (adipogenic) concentrations of BMP2 alone, with BMP2 plus PTHrP, or with BMP2 plus forskolin or TPA (added on d 4). On d 5 RNA was isolated and examined for expression of the BMP I receptors. The BMP IB receptor was undetectable by Northern blot or RT-PCR (data not shown). The BMP IA receptor was detected in all four samples, and expression was standardized against the 18S levels of each respective lane. Equivalent expression was observed in control cells and forskolin-treated cells. BMP IA receptor expression was markedly increased, however, in both PTHrP-treated and TPA-treated cells (Fig. 5Go, A and B).



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  		FIG. 5. Effects of PTHrP and of PKA and PKC signaling pathways on the BMP IA receptor. A, Northern blot of the BMP IA receptor (upper panel). RNA was extracted from cells treated with BMP2 (6 x 10-9 M) alone (control), BMP2 plus PTHrP (10-7 M PTHrP), BMP2 plus forskolin (100 µM Forskolin), or BMP2 plus TPA (1 µM; TPA) under conditions preparing them for a luciferase reporter assay as described in Materials and Methods. Levels of 18S mRNA were concomitantly determined as a loading control (lower panel). Incubations and blots were performed as described in Materials and Methods. B, Quantification of BMP IA expression from three independent Northern blots. Analysis was performed using Scion image, with values obtained by determination of relative BMP IA expression divided by their respective 18S mRNA levels. Fsk, Forskolin treatment. C, Luciferase reporter activity in C3H10T1/2 cells stably transfected with a SMAD6 promoter-luciferase reporter construct as described in Materials and Methods. Cells were pretreated with vehicle, BMP2 (6 x 10-9 M) alone, BMP2 plus PTHrP (10-7 M), BMP2 plus PTHrP and chelerythrin chloride (1.25 x 10-6 M), BMP2 plus TPA (1 µM), or BMP2, TPA, and chelerythrin chloride as described in Materials and Methods. Cells were then treated with vehicle (PBS) or BMP2 (6 x 10-9 M) alone. Each bar represents the mean ± SE of quadruplicate determinations. *, P < 0.05 vs. treatment with vehicle; **, P < 0.05 vs. pretreatment with BMP2 alone, followed by treatment with BMP2; ***, P < 0.05 vs. pretreatment with BMP2 plus PTHrP, followed by treatment with BMP2; ****, P < 0.05 vs. pretreatment with BMP2 plus TPA, followed by treatment with BMP2.

 
To determine whether the increased BMP IA receptor expression was associated with increased postreceptor signaling, we stably transfected C3H10T1/2 cells with a SMAD6 promoter-luciferase reporter construct that has been demonstrated to be responsive to BMP2-induced signaling (20). Cells were pretreated with an adipogenic dose of BMP-2 alone (control) or with BMP-2 plus PTHrP for 5 d. A third set was pretreated with an adipogenic dose of BMP-2 alone for 5 d, with TPA added to the pretreatment mixture for 24 h on d 4 to mimic PTHrP stimulation of PKC. Pretreatment with PTHrP or TPA in combination with BMP-2 significantly increased BMP-2-stimulated SMAD6 promoter-reporter activity (Fig. 5CGo) compared with the control. Furthermore, the effects of PTHrP or TPA in combination BMP2 were abolished with the addition of the PKC inhibitor chelerythrin chloride (Fig. 5CGo). Consequently, BMP IA receptor signaling appeared to be enhanced by PTHrP and PKC activation.

BMP IA receptor mediates BMP2-induced osteoblastogenesis
To determine whether increased BMP IA receptor expression can induce differentiation along the osteoblast lineage, we stably transfected pluripotent C3H10T1/2 cells with the cDNA encoding the BMP IA receptor or with the empty vector (pcDNA3) as a control (Fig. 6Go, A and B). When cells overexpressing the BMP IA receptor were treated with BMP2 (6 x 10-9 M) in the absence of PTHrP, these cells expressed considerably higher levels of ALP, collagen type I, and osteocalcin than the pcDNA3-transfected cells (Fig. 6AGo). To further confirm the role of the BMP IA receptor in this process, we also stably transfected wild-type C3H10T1/2 cells with a dominant negative form of the BMP IA receptor (DNBMP IA). These cells, BMP IA-overexpressing cells, and empty-vector transfected cells (pcDNA3) were then treated with a higher dose (1 x 10-8 M) of BMP2 alone or with PTHrP (Fig. 6CGo). Control cells readily expressed ALP when treated with the higher dose of BMP2 even in the absence of PTHrP, but PTHrP produced a further increase in activity (Fig. 6Go, C and E). Cells overexpressing the functional BMP IA receptor when treated with the higher concentration of BMP2 demonstrated even greater expression of the osteoblastic marker, but PTHrP added no further enhancement (Fig. 6Go, C and E). In contrast, cells transfected with DNBMP IA failed to express the osteoblast marker ALP even in the presence of the higher dose of BMP2 or of BMP2 plus PTHrP (Fig. 6Go, C and E), confirming the critical role of the BMP IA receptor in modulating differentiation of C3H10T1/2 cells along the osteoblastic lineage. Stable expression of the BMP IA and DNBMP IA mRNA was confirmed by Northern analysis (Fig. 6DGo).



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  		FIG. 6. Effect of overexpression and inhibition of the BMP IA receptor on PTHrP effects on the osteoblastic phenotype. A, C3H10T1/2 cells were transfected with the empty pcDNA3 vector (top three panels) or with the vector expressing the BMP IA receptor (bottom three panels), incubated with a low concentration of BMP2 (6 x 10-9 M), and then stained for ALP, type I collagen, and osteocalcin as described in Materials and Methods. Photomicrographs are representative of four random fields from four different experiments. B, Quantitation of ALP, collagen type I (Col I), and osteocalcin (OCN) staining as described in Materials and Methods after treatment with BMP2 (6 x 10-9 M) in cells transfected with the empty pcDNA3 vector or with the vector expressing the BMP IA receptor. Each bar represents the mean ± SE of four random fields per culture from four separate cultures. *, P < 0.05. C, C3H10T1/2 cells were transfected as described in Materials and Methods with the empty pcDNA3 vector (left panels), with the vector expressing the BMP IA receptor, BMP IA (middle panels), or with the vector expressing a dominant negative BMP IA receptor, DNBMP IA (right panels). Cells were then incubated with a higher concentration of BMP2 (1 x 10-8 M) or with BMP2 and PTHrP (10-7 M) and stained for ALP. Photomicrographs are representative of four random fields from four different experiments. D, Gene expression of endogenous BMP IA receptor (pcDNA3), the expressed BMP IA receptor (BMP IA), and the dominant negative BMP IA receptor (DNBMP IA). RNA was extracted from cells, and Northern blots were performed as described in Materials and Methods. The blot was probed with the cDNA encoding the amino terminus of the BMP IA receptor. Levels of the 18S mRNA were concomitantly determined as a loading control. E, Quantitation of ALP staining, as described in Materials and Methods, after treatment with BMP2 (1 x 10-8 M) or with the combination of BMP2 (1 x 10-8 M) and PTHrP (10-7 M) of cells transfected with the empty pcDNA3 vector, the vector expressing the BMP IA receptor, or the vector expressing the DNBMP IA receptor. Each bar represents the mean ± SE of measurements of four random fields per culture from four separate cultures. * and **, P < 0.05 vs. pcDNA3 and BMP IA, respectively.

 

   Discussion
Top
 Abstract
 Introduction
 Materials and Methods
 Results
 Discussion
References
 
Several in vitro systems have been employed to enhance the regulation of osteoblastic cell differentiation by autocrine/paracrine or endocrine factors. These systems include primary cultures derived from fetal rat calvaria or long bone, or established clonal multipotential and osteoblastic cell lines. Each system presents both advantages and disadvantages. Isolated clonal lines may exhibit heterogeneity of expressed genes and of regulation by hormones and growth factors. This may reflect true functional heterogeneity, that is, in vivo heterogeneity, or may reflect aberrant behavior or instability resulting from immortalization or culture manipulations. Primary cultures, on the other hand, are limited by the fact that they contain a diverse mixture of osteoblastic cells at different stages of differentiation and cells of other lineages, including fibroblastic cells. Among the most commonly used multipotential cell lines are 2T3 cells derived from the calvaria of a transgenic mouse expressing simian virus-40 T antigen driven by the BMP2 promoter (22), C2C12 cells, a subclone of a mouse myoblastic cell line (23) and C3H10T1/2 cells. In our studies we employed C3H10T1/2 clone 8 cells isolated from a line of C3H mouse embryo cells (24). C3H10T1/2 clone 8 cells are an established mesenchymal stem cell line that can be induced in both the parent line and subclones to differentiate into multiple mesenchymal phenotypes (15). Other than the PTHrP receptor, which can be expressed in multiple cell types, markers of the osteoblast phenotype were not observed in our studies before the addition of BMP2 and PTHrP and have not previously been reported in C3H10T1/2 cells before the addition of exogenous regulatory factors. Nevertheless, as limiting dilutions to prepare isolated subclones were not performed in our studies, it is quite possible that committed early mesenchymal progenitors, including osteoprogenitors, existed in our cultures in addition to multipotential mesenchymal stem cells.

Previous studies involving pluripotent mesenchymal cell lines have shown that BMP2 can induce the adipocytic phenotype (16, 25, 26, 27, 28, 29, 30, 31). We showed increased activity of PKA and PKC as a result of PTHrP stimulation of the PTHR in the mesenchymal cell line C3H10T1/2 cells and that this responsiveness to PTHrP appeared to manifest itself in part by inhibiting BMP2-induced adipogenesis. The inhibition by PTHrP involved a reduction in BMP2-induced increases in mRNA encoding PPAR{gamma} and aP2 as well as a decrease in cytological staining of lipid. We also found that in the presence of PTHrP, concentrations of BMP2 that normally induce an adipocytic phenotype now induced markers of the osteoblast lineage. Consequently, the effect may involve the specification of multipotent precursor cells to the osteoblast lineage, trans-differentiation of adipocytes to osteoblastic cells, or the simultaneous inhibition of committed adipocyte progenitors and stimulation of early committed osteoprogenitor cells within the C3H10T1/2 cell cultures.

To explore the signaling pathway involved in the PTHrP-induced effect, we employed activators of both the PKA and PKC pathways and found that the PKC pathway predominantly mediated commitment to the osteoblast phenotype. To further support the role of PTHrP stimulation of PKC as the mechanism for enhanced osteogenesis, the PKC inhibitor chelerythrin chloride was found to limit the osteogenic potential of PTHrP. Our finding that PTHrP enhances the development of the osteoblastic lineage by a PKC-dependent mechanism differs from a recent report that the effect of PTH-(1–34) to increase ALP positivity in C3H10T1/2 cells transfected with both BMP2 and PTHR appeared to be mediated by forskolin rather than TPA (32). However, those studies employed C3H10T1/2 cells constitutively expressing BMP2, making comparison of the two systems difficult.

Other mechanisms for enhanced osteogenesis have also been ascribed to the capacity of PTHrP to stimulate PKA. The osteoblast differentiation transcription factor CBFA1 is essential for osteogenesis in vivo (33, 34, 35) and has been shown to be a target of PKA. Posttranslational phosphorylation of CBFA1 via PTHrP signaling enhances the transcriptional activity of CBFA1 (12). It is possible that in our system enhanced osteoblastic commitment by PTHrP is independent of CBFA1 activation by PKA, or that PKC may also activate CBFA1. Alternatively, CBFA1 function may occur downstream of PTHrP action. Thus, BMP2-induced osteoblastic differentiation of C3H10T1/2 cells appears to involve CBFA1 mRNA and protein expression (26, 36, 37). CBFA1 can enhance the transcriptional activation of Smad proteins (38, 39), and our studies show that PTHrP-stimulated osteoblastic commitment requires a BMP2-dependent response. Consequently, CBFA1 stimulation may still converge on the pathway of PTHrP-induced osteogenesis, although further downstream in the signaling pathway.

The mechanism of PTHrP action in the C3H10T1/2 cells appears to involve PKC-induced gene expression of the BMP IA receptor. This differs from results obtained in the T antigen immortalized clonal cell line 2T3, in which the expression of a constitutively active BMP IA receptor induced adipocyte differentiation, whereas the expression of a constitutively active BMP IB receptor induced formation of mineralized bone matrix. However, both receptor subtypes are expressed in developing bone, and targeted deletion of the BMP IB receptor gene has revealed no significant change in osteoblast differentiation (40). Additionally, gene array analysis of differentiating osteoprogenitor cells demonstrates that increased BMP IA expression correlates with the terminal differentiation of an osteoprogenitor cell (41). Consequently, an increase in BMP IA receptor expression may well regulate specification of osteoblast development.

The receptor-regulated SMADs, SMAD1, SMAD5, and SMAD8, are directly activated by the BMP type I receptor, and both SMAD1 (42) and SMAD5 (43) have been implicated in BMP2-induced osteoblastic differentiation. SMAD4 may then associate with the activated SMADs, forming an activated SMAD complex that can translocate to the nucleus and participate in the regulation of target genes. SMAD complexes can also increase transcriptional regulation of inhibitory SMADs that include SMAD6 (20). To examine the functional integrity of the BMP IA receptor that was increased by PTHrP, we assessed the capacity of BMP2 to increase SMAD6 promoter activity after pretreatment of target cells with either PTHrP or a PKC agonist. The results demonstrate the enhanced promoter activity induced by BMP2 after pretreatment with PTHrP or a PKC activator and illustrate the functional capacity of the receptor.

We demonstrated that ectopic overexpression of BMP IA receptors in C3H10T1/2 cells enhances osteoblastic differentiation in the presence of concentrations of BMP2 that are generally ineffective in the absence of PTHrP. Furthermore, the higher concentrations of BMP2 that are effective in inducing osteoblast commitment even in the absence of PTHrP became ineffective in the presence of a dominant negative form of the BMP IA receptor, and PTHrP was ineffective in the presence of the dominant negative BMP IA receptor. These findings support the view that PTHrP acts to increase sensitivity to BMP2 by enhancing the expression of functional BMP IA receptors, which then signals the initiation or progression of a genetic osteogenic program.

Sonic hedgehog (Shh) is a member of the hedgehog family of morphogens that includes Indian hedgehog (Ihh), an important regulator of skeletal development. Both Shh and Ihh share substantial amino acid sequence homology, and Shh has been employed in vitro to mimic the effects of Ihh. During endochondral bone formation, Ihh can stimulate PTHrP production, which then mediates the inhibitory effect of Ihh on chondrocytic differentiation. Shh has recently been shown to inhibit BMP2-induced adipogenesis in C3H10T1/2 cells (44, 45). Furthermore, recombinant N-terminal Shh (N-Shh) has also been reported to enhance osteoblastic commitment in the presence of BMP2. This synergistic effect was mediated at least in part by BMP-stimulated SMAD signaling to increase gene transcription. Although it would be tempting to hypothesize that the effects of hedgehog analogs on enhancing BMP2-induced osteoblastic commitment are, in fact, mediated by PTHrP, it appears that N-Shh has no effect on PTHrP or PTHR expression in C3H10T1/2 cells (45), nor does exogenous PTHrP appear to affect N-Shh-induced endochondral bone formation (46). Consequently, these effects may independently converge on the BMP pathway.

Our studies therefore demonstrate that PTHrP plays a critical role in regulating an inverse relationship between adipocytes and osteoblasts by inhibiting cell differentiation toward the adipocytic lineage and synergizing with BMP2 to enhance differentiation toward the osteogenic lineage. This supports a role for PTHrP in cell fate determination that may prove to be an important component of its anabolic effect on the skeleton.


   Acknowledgments
 
We gratefully acknowledge Dr. Jean Jacques Lebrun for his invaluable assistance. We also thank Dr. Wataru Ishida for providing the SMAD6 promoter luciferase construct.


   Footnotes
 
This work was supported by grants (to D.G. and A.K.) from the Canadian Institute for Health Research and the National Cancer Institute of Canada and a scientist award (to A.K.) from the Canadian Institute for Health Research.

Abbreviations: ALP, Alkaline phosphatase; BMP2, bone morphogenetic protein 2; CBFA1, core binding factor A1; Ihh, Indian hedgehog; N-Shh, N-terminal sonic hedgehog; PKA, protein kinase A; PKC, protein kinase C; PPAR{gamma}, peroxisome proliferator-activated receptor {gamma}; PTHR, PTH receptor; PTHrP, PTH-related peptide; Shh, sonic hedgehog; TPA, 12-O-tetradecanoylphorbol-13-acetate.

Received March 3, 2003.

Accepted for publication August 20, 2003.


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 Discussion
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