Signaling involved in osteoblastic cell differentiation remains largely unknown. This study further investigates mechanisms involved in BMP-2-induced osteoblastic cell differentiation. We report that BMP-2 can activate JNK and p38 in osteoblastic cells and provide evidences that these MAP kinases have distinct roles in regulating alkaline phosphatase and osteocalcin expression.Introduction: Bone morphogenetic protein (BMP)-2 exerts many of its biological effects through activation of the Smad pathway. Cooperative interactions between the Smads and the stress-activated protein kinase (SAPK) p38 and c-Jun-NH2-terminal kinase (JNK) pathways have recently been observed in TGF-␤ signaling. Materials and Methods: Activation of mitogen-activated protein (MAP) kinases by BMP-2 and the role of these signaling pathways for cell differentiation induced by BMP-2 was investigated in mouse MC3T3-E1 and primary cultured calvaria-derived osteoblastic cells using immunoprecipitation, in vitro kinase assay and Western blot analysis, as well as specific MAP kinase inhibitors. Results: Associated with the rapid activation of Smads, BMP-2 barely affected extracellular-signal regulated kinase (ERK) activity, whereas it induced a transient activation of p38 and JNK. The role of p38 and JNK in mediating BMP-2-induced stimulation of osteoblastic cell differentiation was evaluated using the respective specific inhibitors SB203580 and SP600125. Inhibition of p38 by SB203580 was mainly associated with decreased alkaline phosphatase (ALP) activity, whereas inhibition of JNK by SP600125 was associated with a marked reduction in osteocalcin (OC) production induced by BMP-2. Corresponding alterations in ALP and OC mRNA levels were found in cells treated with BMP-2 and inhibitors, suggesting an implication of p38 and JNK pathways in BMP-2-induced osteoblastic cell differentiation at a transcriptional level. Conclusion: Data presented in this study describe p38 and JNK as new signaling pathways involved in BMP-2-induced osteoblastic cell differentiation with evidences for a distinct role of each MAP kinase in the control of alkaline phosphatase and osteocalcin expression.

Sp1 and Sp3 effects on the transcription of the human ␣1(II) procollagen gene (COL2A1) were investigated in both differentiated and de-differentiated rabbit articular chondrocytes. Transient transfection with constructs of deleted COL2A1 promoter sequences driving the luciferase reporter gene revealed that the region spanning ؊266 to ؉121 base pairs showed Sp1-enhancing effects, whatever the differentiation state. In contrast, Sp3 did not influence COL2A1 gene transcription. Concomitant overexpression of the two Sp proteins demonstrated that Sp3 blocked the Sp1 induction of COL2A1 promoter activity. Moreover, inhibition of Sp1/ Sp3 binding to their target DNA sequence decreased both COL2A1 gene transcription and Sp1-enhancing effects. DNase I footprinting and gel retardation assays revealed that Sp1 and Sp3 bind specifically to cis-sequences of the COL2A1 gene promoter whereby they exert their transcriptional effects. Sp1 and Sp3 levels were found to be reduced in de-differentiated chondrocytes, as revealed by DNA-binding and immunochemical study. Sp1 specifically activated collagen neosynthesis whatever the differentiation state of chondrocytes, suggesting that this factor exerts a major role in the expression of collagen type II. However, our data indicate that type II collagen-specific expression in chondrocytes depend on both the Sp1/Sp3 ratio and cooperation of Sp1 with other transcription factors, the amounts of which are also modulated by phenotype alteration.Differentiation of mesenchymal cells into chondrocytes results in the synthesis and secretion of a series of proteins characteristic of the cartilage matrix, including type II, IX, XI, and X collagens, the proteoglycan aggrecan, link protein, and cartilage matrix protein (1, 2). Type II collagen is considered as a critical phenotypic marker gene for analysis of molecular events involved in chondrogenesis process as well as in chondrocyte phenotype maintenance. Alteration of type II collagen expression in cartilage may be due to a variety of genetic, inflammatory, or degenerative circumstances and may lead to a variety of chondrodysplasias and joint diseases such as osteoarthritis (3-8). In osteoarthritis, chondrocytes undergo dedifferentiation and synthesize types I and III collagens at the expense of type II (9 -11). Similarly, when chondrocytes are subcultured in vitro as monolayers, they progressively reduce their synthesis of type II collagen (12-14), mimicking the behavior of osteoarthritic chondrocytes. However, they can recover the chondrocytic phenotype by transfer to three-dimensional culture systems (14 -16). Therefore, in vitro analysis of the molecular mechanisms that regulate COL2A1 gene expression can be an approach to understand the process of phenotype alteration in chondrocytes, and its impact on joint diseases.A 48-bp 1 minimal DNA element has been identified as an enhancer that directs chondrocyte-specific expression of the COL2A1 gene in transgenic mice (17)(18)(19). Such an element was also found in the rat COL2A1 gene (20, 21). ...

An important role for JNK* and p38 has recently been discovered in the differentiating effect of bone morphogenetic protein 2 (BMP-2) on osteoblastic cells. In this study, we investigated the molecular mechanism by which BMP-2 activates JNK and p38 in MC3T3

Interleukin-1␤ (IL-1␤) is a pleiotropic cytokine that was shown to inhibit the biosynthesis of articular cartilage components. Here we demonstrate that IL-1␤ inhibits the production of newly synthesized collagens in proliferating rabbit articular chondrocytes and that this effect is accompanied by a decrease in the steady-state levels of type II collagen mRNA. IL-1␤ down-regulates COL2A1 gene transcription through a ؊41/؊33 bp sequence that binds a multimeric complex including Sp1 and Sp3 transcription factors. Specificity of IL-1␤ effects on COL2A1 promoter activity was demonstrated in experiments in which transfection of a wild type ؊50/؉1 sequence of COL2A1 promoter as a decoy oligonucleotide abolished the IL-1␤ inhibition of a ؊63/؉47 COL2A1-mediated transcription. By contrast, transfection of the related oligonucleotide harboring a targeted mutation in the ؊41/؊33 sequence did not modify the negative effect the cytokine. Because we demonstrated previously that Sp1 was a strong activator of COL2A1 gene expression via the ؊63/؉1 promoter region, whereas Sp3 overexpression blocked Sp1-induced promoter activity and inhibited COL2A1 gene transcription, we conclude that IL-1␤ down-regulation of that gene, as we found previously for transforming growth factor-␤1, is mediated by an increase in the Sp3/Sp1 ratio. Moreover, IL-1␤ increased steady-state levels of Sp1 and Sp3 mRNAs, whereas it enhanced Sp3 protein expression and inhibited Sp1 protein biosynthesis. Nevertheless, IL-1␤ decreased the binding activity of both Sp1 and Sp3 to the 63-bp short COL2A1 promoter, suggesting that the cytokine exerts a post-transcriptional regulatory mechanism on Sp1 and Sp3 gene expressions. Altogether, these data indicate that modulation of Sp3/Sp1 ratio in cartilage could be a potential target to prevent or limit the tissue degradation.Articular cartilage is a highly specialized tissue composed of a complex extracellular matrix of proteoglycans, collagens, and noncollagenous glycoproteins. Cartilage collagens include type II as the major form and types VI, IX, and XI as minor components (1). Type II collagen is an homotrimer composed of ␣1(II) chains encoded by the COL2A1 gene. Previous studies have delineated minimal sequences in the first intron of human, mouse, and rat COL2A1 genes which are sufficient to direct chondrocyte-specific expression in cultured chondrocytes and transgenic mice (2-5). Several binding sites of the intronic enhancer sequences were shown to interact with transcription factors that form chondrocyte-specific complexes, such as SOX9, L-SOX5, and SOX6 (6, 7), and also with factors having less tissue-specific expression, such as Sp1, Sp3, and C-KROX (5, 8). Indeed, promoter sequences are also implicated through interaction with the intronic enhancer sequence, for tissuespecific expression during in vivo and in vitro chondrogenesis (7, 9, 10). In a 266-bp promoter of the human COL2A1 gene mediating enhanced transcription activity, we identified several binding sites for Sp1, Sp3, and C-KROX (5, 8, 11). Sp1 w...

In medicine, N-methyl pyrrolidone (NMP) has a long track record as a constituent in medical devices approved by the Food and Drug Administration and thus can be considered as a safe and biologically inactive small chemical. In the present study, we report on the newly discovered pharmaceutical property of NMP in enhancing bone regeneration in a rabbit calvarial defect model in vivo. At the cellular level, the pharmaceutical effect of NMP was confirmed, in particular, in combination with bone morphogenetic protein (BMP)-2, because NMP increased early and late markers for maturation of preosteoblasts and human bone marrow-derived stem cells in vitro. When we used the multipotent cell line C2C12 without autologous BMP expression, NMP alone had no effect on alkaline phosphatase activity, a marker for osteogenic transdifferentiation. Nevertheless, in combination with low BMP-2 doses, alkaline phosphatase activity was more than eight times as great. Thus, the pharmaceutical NMP mode of action is that of an enhancer of BMP activity. The dependency of the effects of NMP on BMP was confirmed in preosteoblasts because noggin, an extracellular BMP inhibitor, suppressed NMP-induced increases in early markers for osteoblast maturation in vitro. At the molecular level, NMP was shown to have no effect on the binding of BMP-2 to the ectodomain of the high-affinity BMP receptor IA. However, NMP further increased the phosphorylation of p38 and Smad1,5,8 induced by BMP-2. Thus, the small chemical NMP enhances BMP activity by increasing the kinase activity of the BMP receptor complex for Smad1,5,8 and p38 and could be employed as a potent drug for bone tissue regeneration and engineering.