Parathyroid hormone versus phorbol ester stimulation of activator protein-1 gene family members in rat osteosarcoma cells

N-cadherin restrains PTH activation of Lrp6/β-catenin signaling and osteoanabolic action

Interaction between parathyroid hormone/parathyroid hormone-related peptide receptor 1 (PTHR1) and low-density lipoprotein receptor-related protein 6 (Lrp6) is important for parathyroid hormone (PTH) signaling and anabolic action. Because N-cadherin has been shown to negatively regulate canonical Wnt/β-catenin signaling, we asked whether N-cadherin alters PTH signaling and stimulation of bone formation. Ablation of the N-cadherin gene (Cdh2) in primary osteogenic lineage cells resulted in increased Lrp6/PTHR1 interaction in response to PTH1-34 , associated with enhanced PTH-induced PKA signaling and PKA-dependent β-catenin C-terminus phosphorylation, which promotes β-catenin transcriptional activity. β-catenin C-terminus phosphorylation was abolished by Lrp6 knockdown. Accordingly, PTH1-34 stimulation of Tcf/Lef target genes, Lef1 and Axin2, was also significantly enhanced in Cdh2-deficient cells. This enhanced responsiveness to PTH extends to the osteo-anabolic effect of PTH, as mice with a conditional Cdh2 deletion in Osx+ cells treated with intermittent doses of PTH1-34 exhibited significantly larger gains in trabecular bone mass relative to control mice, the result of accentuated osteoblast activity. Therefore, N-cadherin modulates Lrp6/PTHR1 interaction, restraining the intensity of PTH-induced β-catenin signaling, and ultimately influencing bone formation in response to intermittent PTH administration.

Parathyroid hormone (1-34) counteracts the suppression of interleukin-11 expression by glucocorticoid in murine osteoblasts: a possible mechanism for stimulating osteoblast differentiation against glucocorticoid excess

Glucocorticoid (GC) excess causes a rapid loss of bone with a reduction in bone formation. Intermittent PTH (1-34) administration stimulates bone formation and counteracts the inhibition of bone formation by GC excess. We have previously demonstrated that mechanical strain enhances interleukin (IL)-11 gene transcription by a rapid induction of ΔFosB expression and protein kinase C (PKC)-δ-mediated phosphorylation of phosphorylated mothers against decapentaplegic (Smad)-1. Because IL-11 suppresses the expression of dickkopf-1 and -2 and stimulates Wnt signaling, IL-11 appears to mediate at least a part of the effect of mechanical strain on osteoblast differentiation and bone formation. The present study was undertaken to examine the effect of PTH(1-34) and GCs on IL-11 expression in murine primary osteoblasts (mPOBs). PTH(1-34) treatment of mPOBs enhanced IL-11 expression in a time- and dose-dependent manner. PTH(1-34) also stimulated ΔFosB expression and Smad1 phosphorylation, which cooperatively stimulated IL-11 gene transcription. PTH(1-34)-induced Smad1 phosphorylation was mediated via PKCδ and was abrogated in mPOBs from PKCδ knockout mice. Dexamethasone suppressed IL-11 gene transcription enhanced by PTH(1-34) without affecting ΔFosB expression or Smad1 phosphorylation, and dexamethasone-GC receptor complex was bound to JunD, which forms heterodimers with ΔFosB. High doses of PTH(1-34) counteracted the effect of dexamethasone on apoptosis of mPOBs, which was blunted by neutralizing anti-IL-11 antibody or IL-11 small interfering RNA. These results demonstrate that PTH(1-34) and GCs interact to regulate IL-11 expression in parallel with osteoblast differentiation and apoptosis and suggest that PTH(1-34) and dexamethasone may regulate osteoblast differentiation and apoptosis via their effect on IL-11 expression.

Timing, rather than the concentration of cyclic AMP, correlates to osteogenic differentiation of human mesenchymal stem cells

Previously, we demonstrated that protein kinase A (PKA) activation using dibutyryl-cAMP in human mesenchymal stem cells (hMSCs) induces in vitro osteogenesis and bone formation in vivo. To further investigate the physiological role of PKA in hMSC osteogenesis, we tested a selection of G-protein-coupled receptor ligands which signal via intracellular cAMP production and PKA activation. Treatment of hMSCs with parathyroid hormone, parathyroid hormone-related peptide, melatonin, epinephrine, calcitonin or calcitonin gene-related peptide did not result in accumulation of cAMP or induction of alkaline phosphatase (ALP) expression. The only ligand that did induce cAMP, prostaglandin E2, even inhibited ALP expression and mineralization, suggesting that physiological levels of cAMP may inhibit osteogenesis. Furthermore, intermittent exposure of hMSCs to dibutyryl-cAMP inhibited ALP expression, whereas we did not observe an inhibitive effect at low dibutyryl-cAMP concentrations. Taken together, our results demonstrate that cAMP can either stimulate or inhibit osteogenesis in hMSCs, depending on the duration, rather than the strength, of the signal provided.

PTH and PTHrP signaling in osteoblasts

The striking clinical benefit of PTH in osteoporosis began a new era of skeletal anabolic agents. Several studies have been performed, new studies are emerging out and yet controversies remain on PTH anabolic action in bone. This review focuses on the molecular aspects of PTH and PTHrP signaling in light of old players and recent advances in understanding the control of osteoblast proliferation, differentiation and function.

Sp proteins and Runx2 mediate regulation of matrix gla protein (MGP) expression by parathyroid hormone

As part of its catabolic action in bone, parathyroid hormone (PTH) inhibits extracellular matrix mineralization. We previously showed that PTH dose-dependently induces matrix gla protein (MGP) expression in osteoblasts and this induction is at least partially responsible for PTH-mediated inhibition of mineralization. Recently, we identified PKA and ERK/MAPK as the key signaling pathways involved in PTH regulation of MGP expression. The goal of this study was to further characterize the mechanism by which PTH stimulates expression of MGP. Deletion analysis of the murine Mgp gene promoter identified a PTH-responsive region between -173 bp and-49 bp. Using gel-mobility shift assays we found that Sp1/Sp3, and Runx2 bind to distinct sites within this region. Mutation of either the Sp or the Runx2 site reduced MGP induction by PTH, while mutation of both sites completely abolished PTH responsiveness. Overexpression of Runx2 or Sp1 activated the Mgp reporter, while Sp3 was a dose-dependent repressor of Sp1 and PTH-induced MGP expression. Collectively, these data show that PTH regulates MGP gene transcription in osteoblasts through altered activities of Sp and Runx2 transcription factors.

Parathyroid hormone increases activating transcription factor 4 expression and activity in osteoblasts: requirement for osteocalcin gene expression

PTH is an important peptide hormone regulator of calcium homeostasis and osteoblast function. However, its mechanism of action in osteoblasts is poorly understood. Our previous study demonstrated that PTH activates mouse osteocalcin (Ocn) gene 2 promoter through the osteoblast-specific element 1 site, a recently identified activating transcription factor-4 (ATF4) -binding element. In the present study, we examined effects of PTH on ATF4 expression and activity as well as the requirement for ATF4 in the regulation of Ocn by PTH. Results show that PTH elevated levels of ATF4 mRNA and protein in a dose- and time-dependent manner. This PTH regulation requires transcriptional activity but not de novo protein synthesis. PTH also increased binding of nuclear extracts to osteoblast-specific element 1 DNA. PTH stimulated ATF4-dependent transcriptional activity mainly through protein kinase A with a lesser requirement for protein kinase C and MAPK/ERK pathways. Lastly, PTH stimulation of Ocn expression was lost by small interfering RNA down-regulation of ATF4 in MC-4 cells and Atf4(-/-) bone marrow stromal cells. Collectively, these studies for the first time demonstrate that PTH increases ATF4 expression and activity and that ATF4 is required for PTH induction of Ocn expression in osteoblasts.

JunB as a downstream mediator of PTHrP actions in cementoblasts

The role of AP-1 family members in the action of PTHrP was examined in cementoblasts. PTHrP increased mRNA and protein levels of all Fos members, but only one Jun member (JunB) was increased. Overexpression of JunB in cementoblasts mimicked actions of PTHrP to support osteoclastogenesis and inhibit cementoblast differentiation, suggesting that the actions of PTHrP on mesenchymal cells operate through JunB.

INTRODUCTION

Cementoblasts are mesenchymal cells that share phenotypic features with osteoblasts in vitro; however, unlike osteoblasts, cementoblasts rarely support osteoclastogenesis in vivo. The osteoblast-mediated support of osteoclastogenesis involves PTH-induced reduction in osteoprotegerin (OPG) expression. PTH acts on osteoblastic cells through specific signaling pathways and transcription factors such as activator protein 1 (AP-1). The purpose of this study was to determine the impact of PTH-related protein (PTHrP) on AP-1 transcription factors in cementoblasts and the role of JunB in the actions of PTHrP.

MATERIALS AND METHODS

Cementoblastic cells were treated with PTHrP and evaluated for mRNA and protein levels of AP-1 family members. Stable transfectants of OCCM cells overexpressing JunB were evaluated for OPG production, ability to support osteoclastogenesis, and measures of proliferation and differentiation.

RESULTS

PTHrP treatment in vitro resulted in a time-dependent upregulation of mRNA and proteins for the Fos family members, but only JunB of the Jun family. OPG mRNA and protein levels were reduced by PTHrP in OCCM and were lower in JunB overexpressing cells than controls. In co-culture experiments, TRACP+ cells were increased with RANKL treatment in JunB overexpressing cells compared with controls. Cementoblast differentiation was reduced with overexpression of JunB as measured by a decrease in mineralized nodule formation and gene expression for bone sialoprotein and osterix. Measures of proliferation including cell number and cyclin D1 levels were increased in JunB overexpressing clones. In vivo, cementoblast implants exhibited a cementoblastoid nature with copious mineral-like matrix, whereas JunB-overexpressing implants were densely cellular with little mineralized matrix.

CONCLUSIONS

JunB was the only Jun family member increased by PTHrP, and its overexpression showed similar patterns of gene expression and OPG production as PTHrP treatment of controls. These data suggest that JunB may be a key mediator of PTHrP actions in cementoblasts.

Increased expression of G11α in osteoblastic cells enhances parathyroid hormone activation of phospholipase C and AP-1 regulation of matrix metalloproteinase-13 mRNA

In osteoblasts parathyroid hormone (PTH) stimulates the PTH/PTH-related peptide (PTHrP) receptor (PTH1R) that couples via G(s) to adenylyl cyclase stimulation and via G(11) to phospholipase C (PLC) stimulation. We have investigated the effect of increasing G(11)alpha levels in UMR 106-01 osteoblastic cells by transient transfection with cDNA encoding G(11)alpha on PTH stimulation of PLC and protein kinase C (PKC) as well as PTH regulation of mRNA encoding matrix metalloproteinase-13 (MMP-13). Transfection with G(11)alpha cDNA resulted in a 5-fold increase in PTH-stimulated PLC activity with no change in PTH-stimulated adenylyl cyclase. PTH-induced translocation of PKC-betaI, -delta, and -zeta to the cell membrane and PKC-zeta to the nucleus was also increased. Increased G(11)alpha protein resulted in increased stimulation of MMP-13 mRNA levels at all doses of PTH. There was a 2.5 +/- 0.35 fold increase in maximal PTH-stimulation of c-jun mRNA and smaller but significant increases in c-fos accompanied by increased basal and PTH-stimulated AP-1 binding in cells expressing increased G(11)alpha. Runx-2 mRNA and protein levels were not significantly increased by increased G(11)alpha expression. The increase in PTH stimulation of c-jun, c-fos, and MMP-13 in G(11)alpha-transfected cells were all blocked by bisindolylmaleimide I, a selective inhibitor of PKC. These results demonstrate that regulation of the PLC pathway through the PTH1R is significantly increased by elevating expression of G(11)alpha in osteoblastic cells. This leads to increased PTH stimulation of MMP-13 expression by increased stimulation of AP-1 factors c-jun and c-fos.

Parathyroid hormone-Smad3 axis exerts anti-apoptotic action and augments anabolic action of transforming growth factor beta in osteoblasts

Although several studies indicated that parathyroid hormone (PTH) exerted anabolic action on bone, its precise mechanisms have been unknown. On the other hand, transforming growth factor beta (TGF-beta), abundantly stored in bone matrix, stimulates bone formation with a local injection in rodents. Although our previous study suggested that Smad3 is an important molecule for the stimulation of bone formation, no reports have been available about the effects of PTH on Smad3. In this present study, we examined the effects of PTH on Smad3 and the physiological significance in mouse osteoblastic cells. PTH promoted the expression of Smad3 mRNA within 10 min and the protein level in a dose-dependent manner in MC3T3-E1 and rat osteoblastic UMR-106 cells. Protein kinase A (PKA) activator as well as protein kinase C (PKC) activators increased Smad3 protein level, and both PKA and PKC inhibitors antagonized PTH-induced Smad3, indicating that PTH promotes the production of Smad3 through both PKA and PKC pathways. Next, we examined anti-apoptotic effects of PTH and Smad3 in these cells, employing trypan blue, transferase-mediated nick end labeling, and Hoechst staining. Pretreatment with PTH or overexpression of Smad3 decreased the number of apoptotic cells induced by dexamethasone and etoposide. Moreover, a dominant negative mutant, Smad3DeltaC, abrogated PTH-induced anti-apoptotic effects. On the other hand, PTH augmented TGF-beta-induced transcriptional activity. Furthermore, PTH enhanced TGF-beta-induced production of type I collagen, whereas it did not affect TGF-beta-reduced proliferation in MC3T3-E1 cells. These observations indicated that PTH amplified the anabolic effects of TGF-beta by accelerating the transcriptional activity of Smad3. In conclusion, we first demonstrated that PTH-Smad3 axis exerts anti-apoptotic effects in osteoblasts and reinforces the anabolic action by TGF-beta in osteoblasts. Hence, PTH-Smad3 axis might be involved in the bone anabolic action of PTH.

The role of protein kinase C-delta in PTH stimulation of IGF-binding protein-5 mRNA in UMR-106-01 cells

We have investigated the role of protein kinase C (PKC) signal transduction pathways in parathyroid hormone (PTH) regulation of insulin-like growth factor-binding protein-5 (IGFBP-5) gene expression in the rat osteoblast-like cell line UMR-106-01. Involvement of the PKC pathway was determined by the findings that bisindolylmaleimide I inhibited 40% of the PTH effect, and 1 microM bovine PTH-(3-34) stimulated a 10-fold induction of IGFBP-5 mRNA. PTH-(1-34) and PTH-(3-34) (100 nM) both stimulated PKC-delta translocation from the membrane to the nuclear fraction. Rottlerin, a PKC-delta-specific inhibitor, and a dominant negative mutant of PKC-delta were both able to significantly inhibit PTH-(1-34) and PTH-(3-34) induction of IGFBP-5 mRNA, suggesting a stimulatory role for PKC-delta in the effects of PTH. Phorbol 12-myristate 13-acetate (PMA) stimulated PKC-alpha translocation from the cytosol to the membrane and inhibited approximately 50% of the PTH-(1-34), forskolin, and 8-bromoadenosine 3',5'-cyclic monophosphate-stimulated IGFBP-5 mRNA levels, suggesting that PKC-alpha negatively regulates protein kinase A (PKA)-mediated induction of IGFBP-5 mRNA. These results suggest that the induction of IGFBP-5 by PTH is both PKA and PKC dependent and PKC-delta is the primary mediator of the effects of PTH via the PKC pathway.

Parathyroid hormone-dependent signaling pathways regulating genes in bone cells

Parathyroid hormone (PTH) is an 84-amino-acid polypeptide hormone functioning as a major mediator of bone remodeling and as an essential regulator of calcium homeostasis. PTH and PTH-related protein (PTHrP) indirectly activate osteoclasts resulting in increased bone resorption. During this process, PTH changes the phenotype of the osteoblast from a cell involved in bone formation to one directing bone resorption. In addition to these catabolic effects, PTH has been demonstrated to be an anabolic factor in skeletal tissue and in vitro. As a result, PTH has potential medical application to the treatment of osteoporosis, since intermittent administration of PTH stimulates bone formation. Activation of osteoblasts by PTH results in expression of genes important for the degradation of the extracellular matrix, production of growth factors, and stimulation and recruitment of osteoclasts. The ability of PTH to drive changes in gene expression is dependent upon activation of transcription factors such as the activator protein-1 family, RUNX2, and cAMP response element binding protein (CREB). Much of the regulation of these processes by PTH is protein kinase A (PKA)-dependent. However, while PKA is linked to many of the changes in gene expression directed by PTH, PKA activation has been shown to inhibit mitogen-activated protein kinase (MAPK) and proliferation of osteoblasts. It is now known that stimulation of MAPK and proliferation by PTH at low concentrations is protein kinase C (PKC)-dependent in both osteoblastic and kidney cells. Furthermore, PTH has been demonstrated to regulate components of the cell cycle. However, whether this regulation requires PKC and/or extracellular signal-regulated kinases or whether PTH is able to stimulate other components of the cell cycle is unknown. It is possible that stimulation of this signaling pathway by PTH mediates a unique pattern of gene expression resulting in proliferation in osteoblastic and kidney cells; however, specific examples of this are still unknown. This review will focus on what is known about PTH-mediated cell signaling, and discuss the established or putative PTH-regulated pattern of gene expression in osteoblastic cells following treatment with catabolic (high) or anabolic (low) concentrations of the hormone.

Parathyroid hormone stimulates fra-2 expression in osteoblastic cells in vitro and in vivo

PTH and PTH-related protein (PTHrP) are key mediators of skeletal development and homeostasis through their activation of the PTH-1 receptor. Previous studies have found that several AP-1 family members are regulated by PTH, such as c-fos, fra-1, and c-jun. There are numerous genes in the bone microenvironment that contain AP-1 sites, and different Fos family members are reported to have opposing transcriptional activities at AP-1 sites. The purpose of this study was to identify the effects of PTH on expression of the AP-1 protein complex member, fra-2, to extend our understanding of transcriptional regulators of PTH action. PTH induction of fra-2 messenger RNA (mRNA) levels in MC3T3-E1 preosteoblastic cells was maximal with 0.1 microM PTH (1-34). The expression in vitro was greatest 1 h after treatment and was present with N-terminal PTH but not PTH (7-34) or (53-84). Cycloheximide treatment induced fra-2 expression, and actinomycin D inhibited basal and PTHrP-induced expression. AP-1 protein in nuclear extracts of MC3T3-E1 cells was increased with PTH treatment at 3 h and consisted of high levels of Fra-2 protein, as evidenced by a supershift in an electrophoretic mobility shift assay and Western blot analysis. Up-regulation of steady-state fra-2 mRNA was also noted in vivo, where injection of PTH (1-34) (20 microgram) resulted in a more-than-7-fold maximal increase in fra-2 mRNA expression in the calvaria of mice, after 1 h of treatment. These data add to the transcriptional mediators induced by PTH and suggest that the interplay of AP-1 family members will provide insight into regulatory pathways of PTH and PTHrP for their anabolic and catabolic actions in bone.

In vivo comparison of activated protein-1 gene activation in response to human parathyroid hormone (hPTH)(1-34) and hPTH(1-84) in the distal femur metaphyses of young mice

Intermittent parathyroid hormone (PTH) treatment increases bone mass in humans and animals. Although intact human PTH has 84 amino acids, the N-terminal 31 to 38 amino acids are sufficient for bone anabolic activity in vivo. Prior studies have evaluated hPTH(1-34) and hPTH(1-84) with respect to bone mass increase and quality, but there have been no in vivo comparisons of dose-dependent molecular responses. After confirming that young male BALB/c mice respond to daily PTH with increased bone mass, we profiled the steady-state mRNA levels of activating protein-1 (AP-1) genes regulated by hPTH(1-34) and hPTH(1-84) at doses ranging from 0 to 19.4 nmol/kg in the distal femur metaphyses. We selected AP-1 genes, which include jun and fos, as they play a fundamental role mediating signals for proliferation, differentiation, and apoptosis in cells of different origins, including bone, and are known to be regulated by PTH. Human PTH(1-34) and hPTH(1-84) increased steady-state mRNA expression of c-jun, junB, c-fos, and fra-2 in an equivalent dose- and time-dependent manner. Expression of fosB or fra-1 was not detected with either peptide. When averaged across dose and time, responses to hPTH(1-34) and hPTH(1-84) were not significantly different from each other. Expression of c-jun, junB, and c-fos peaked 30 minutes after the injection while fra-2 expression peaked 30 minutes later. All AP-1 genes stimulated by PTH returned to the levels of vehicle treated controls by 3 h after injection. The expression level of junD, which was abundant in the distal metaphysis, was not altered by either peptide. No change in magnitude was observed after 1, 3, or 7 days of once-daily subcutaneous treatment of either peptide. When individual comparisons for each dose between peptides were made, the minimum effective dose necessary to stimulate a significant increase in c-fos and junB expression was equivalent for both peptides. The minimum effective dose for hPTH(1-34) was at least tenfold lower than hPTH(1-84) in stimulating c-jun and fra-2 expression. Area under the curve for the highest dose (19.4 nmol/kg) of either peptide showed no significant differences in the expression of any of the genes. In conclusion, in young mice given once-daily subcutaneous injections up to 7 days, hPTH(1-34) and hPTH(1-84) induced equivalent responses by time and dose in the selected AP-1 genes. These data on molecular regulation in mouse bone confirm and extend prior data from rat studies showing equivalence on bone mass at equimolar doses.

Extracellular glucose influences osteoblast differentiation and c-Jun expression

Insulin dependent diabetes mellitus, marked by high blood glucose levels and no insulin secretion, is associated with decreased bone mass and increased fracture rates. Analysis of bone histology suggests that osteoblast phenotype and function are influenced by diabetes. To determine if elevated extracellular glucose levels could directly influence osteoblast phenotype we treated mouse osteoblasts, MC3T3-E1 cells, with 22 mM glucose and analyzed osteoblast gene expression. Collagen I mRNA levels significantly increased while osteocalcin mRNA levels decreased 24 h after the addition of glucose. Expression of other genes, actin, osteopontin, and histone H4, was unaffected. Effects on collagen I expression were seen as early as 1 h after treatment. c-Jun, an AP-1 transcription factor involved in the regulation of osteoblast gene expression and growth, was also modulated by glucose. Specifically, an increase in c-jun expression was found at 1 h and maintained for 24 h following glucose treatment. Treatment of osteoblasts with an equal concentration of mannitol completely mimicked glucose treatment effects on collagen I and c-jun expression, demonstrating that osmotic stress rather than glucose metabolism is responsible for the effects on osteoblast gene expression and phenotype. Additional studies using staurosporine and Ro-31-8220 demonstrate that protein kinase C is required for the glucose up regulation of collagen I and c-jun. Taken together, our results demonstrate that osteoblasts respond to increasing extracellular glucose concentration through an osmotic response pathway that is dependent upon protein kinase C activity and results in upregulation of c-jun and modulation of collagen I and osteocalcin expression.

Regulation of collagenase-3 gene expression in osteoblastic and non-osteoblastic cell lines

Collagenase-3 expression in osteoblastic (UMR 106-01, ROS 17/2.8) and non-osteoblastic cell lines (BC1, NIH3T3) was examined. We observed that parathyroid hormone (PTH) induces collagenase-3 expression only in UMR cells but not in BC1 (which express collagenase-3 constitutively) or ROS and NIH3T3 cells. Since we know from UMR cells that the AP-1 factors and Cbfa1 are required for collagenase-3 expression, we analyzed the expression and PTH regulation of these factors by gel shift and Northern blot analysis in all cell lines. Gel mobility shift with a [(32)P]-labeled collagenase-3 AP-1 site probe indicated the induction of c-Fos in osteoblastic cells upon PTH treatment. While c-fos was induced in UMR cells, both c-fos and jun B were induced in ROS cells. Since Jun B is inhibitory of Fos and Jun in the regulation of the rat collagenase-3 gene in UMR cells, it is likely that high levels of Jun B prevent PTH stimulation of collagenase-3 in ROS cells. When we carried out gel shift analysis with a [(32)P]-labeled collagenase-3 RD (runt domain) site probe and Northern blot analysis with a Cbfa1 specific probe, we have observed the presence of Cbfa1 in both osteoblastic and non-osteoblastic cell lines, but there was no change in the levels of Cbfa1 RNA or protein in these cells under either control conditions or PTH treatment. From our studies above, it is evident that the expression of collagenase-3 and its regulation by PTH in osteoblastic and non-osteoblastic cells may be influenced by differential temporal stimulation of the AP-1 family members, especially c-Fos and Jun B along with the potential for posttranslational modification(s) of Cbfa1.

Parathyroid hormone regulates transforming growth factor beta1 and beta2 synthesis in osteoblasts via divergent signaling pathways

Parathyroid hormone 1-34 [PTH(1-34)] was shown to increase transforming growth factor beta1 (TGF-beta1) and TGF-beta2 concentrations in supernatants of cultured human osteoblasts and to increase TGF-beta1 and TGF-beta2 messenger RNA (mRNA) concentrations and gene transcription in these cells. Because PTH(1-34) activates both protein kinase C (PKC) and protein kinase A (PKA) pathways in osteoblasts, we investigated the role of each kinase pathway in activation of TGF-beta isoforms. PTH(29-32), which activates the PKC pathway in rat osteoblasts, increased TGF-beta1 but not TGF-beta2 concentrations in supernatants of osteoblasts. Phorbol myristate acetate (PMA), a PKC agonist, increased TGF-beta1 but not TGF-beta2 concentrations. Specific PKC antagonists safingol and Gö6976 attenuated PTH(1-34)-mediated increases in TGF-beta1 but not TGF-beta2 synthesis. PTH(1-31), which increases PKA activity in several cell culture systems, increased TGF-beta2 but not TGF-beta1 concentrations in human osteoblast supernatants. Forskolin, a PKA agonist, increased TGF-beta2 but not TGF-beta1 concentrations in supernatants of human osteoblasts. The PKA antagonist H-89 blunted PTH(1-34)-mediated increases in TGF-beta2 but not TGF-beta1 synthesis. Our results are consistent with the concept that PTH increases TGF-beta1 expression and secretion by pathways that involve the PKC pathway, whereas it increases TGF-beta2 expression and secretion via the PKA pathway.

Constitutive expression and regulation of collagenase-3 in human breast cancer cells

Matrix metalloproteinases (MMPs) are a family of secreted or transmembrane proteins that have been implicated in multiple physiological and pathological processes related to extracellular matrix turnover. Recent evidence strongly suggests a role for collagenase-3 (MMP-13) in tumor metastasis and invasion. We report here that collagenase-3 is constitutively expressed in the breast cancer cell line MDA-MB231 (MDA) and outline the molecular mechanism regulating its expression. Functional analysis of the collagenase-3 promoter showed that both the activator protein-1 (AP-1) site and the runt domain (RD) binding site were required for maximal constitutive expression of collagenase-3 in MDA cells. Determination of factors binding to those sites by Northern analysis and transient transfections identified the requirement of Fra-1, c-Jun, and Cbfa1 for basal collagenase-3 promoter activity in MDA cells.

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

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