Asha Sarah Eapen

Calcium-mediated Stress Kinase Activation by DMP1 Promotes Osteoblast Differentiation

Calcium signaling and calcium transport play a key role during osteoblast differentiation and bone formation. Here, we demonstrate that DMP1 mediated calcium signaling, and its downstream effectors play an essential role in the differentiation of preosteoblasts to fully functional osteoblasts. DMP1, a key regulatory bone matrix protein, can be endocytosed by preosteoblasts, triggering a rise in cytosolic levels of calcium that initiates a series of downstream events leading to cellular stress. These events include release of store-operated calcium that facilitates the activation of stress-induced p38 MAPK leading to osteoblast differentiation. However, chelation of intracellular calcium and inhibition of the p38 signaling pathway by specific pharmacological inhibitors and dominant negative plasmid suppressed this activation. Interestingly, activated p38 MAPK can translocate to the nucleus to phosphorylate transcription factors that coordinate the expression of downstream target genes such as Runx 2, a key modulator of osteoblast differentiation. These studies suggest a novel paradigm by which DMP1-mediated release of intracellular calcium activates p38 MAPK signaling cascade to regulate gene expression and osteoblast differentiation.

Endoplasmic Reticulum Chaperone Protein GRP-78 Mediates Endocytosis of Dentin Matrix Protein 1

Dentin matrix protein 1 (DMP1), a phosphorylated protein present in the mineral phase of both vertebrates and invertebrates, is a key regulatory protein during biogenic formation of mineral deposits. Previously we showed that DMP1 is localized in the nuclear compartment of preosteoblasts and preodontoblasts. In the nucleus DMP1 might play an important role in the regulation of genes that control osteoblast or odontoblast differentiation. Here, we show that cellular uptake of DMP1 occurs through endocytosis. Interestingly, this process is initiated by DMP1 binding to the glucose-regulated protein-78 (GRP-78) localized on the plasma membrane of preodontoblast cells. Binding of DMP1 to GRP-78 receptor was determined to be specific and saturable with a binding dissociation constant KD = 85 nm. We further depict a road map for the endocytosed DMP1 and demonstrate that the internalization is mediated primarily by caveolae and that the vesicles containing DMP1 are routed to the nucleus along microtubules. Immunohistochemical analysis and binding studies performed with biotin-labeled DMP1 confirm spatial co-localization of DMP1 and GRP-78 in the preodontoblasts of a developing mouse molar. Co-localization of DMP1 with GRP-78 was also observed in T4-4 preodontoblast cells, dental pulp stem cells, and primary preodontoblasts. By small interfering RNA techniques, we demonstrate that the receptor for DMP1 is GRP-78. Therefore, binding of DMP1 with GRP-78 receptor might be an important mechanism by which DMP1 is internalized and transported to the nucleus during bone and tooth development.

Dentin phosphoprotein (DPP) activates integrin-mediated anchorage-dependent signals in undifferentiated mesenchymal cells.

Background: DPP mediates activation of anchorage-dependent signals. Results: DPP activates focal adhesion complexes and MAPK signaling in undifferentiated mesenchymal cells and primary pulp cells, leading to their terminal differentiation into odontoblast-like cells. Conclusion: DPP on the substrate provides a tight association between the structural and signaling elements in undifferentiated mesenchymal cells. Significance: DPP promotes adhesion-based odontogenic cell differentiation. Dentin phosphoprotein (DPP), a major noncollagenous protein of the dentin matrix, is a highly acidic protein that binds Ca2+ avidly and is thus linked to matrix mineralization. Here, we demonstrate that the RGD domain in DPP can bind to integrins on the cell surface of undifferentiated mesenchymal stem cells and pulp cells. This coupling generates intracellular signals that are channeled along cytoskeletal filaments and activate the non-receptor tyrosine kinase focal adhesion kinase, which plays a key role in signaling at sites of cellular adhesion. The putative focal adhesion kinase autophosphorylation site Tyr397 is phosphorylated during focal adhesion assembly induced by DPP on the substrate. We further demonstrate that these intracellular signals propagate through the cytoplasm and activate anchorage-dependent ERK signaling. Activated ERK translocates to the nucleus and phosphorylates the transcription factor ELK-1, which in turn coordinates the expression of downstream target genes such as DMP1 and dentin sialoprotein (DSP). These studies suggest a novel paradigm demonstrating that extracellular DPP can induce intracellular signaling that can be propagated to the nucleus and thus alter gene activities.

Activation of the ERK1/2 Mitogen-Activated Protein Kinase Cascade by Dentin Matrix Protein 1 Promotes Osteoblast Differentiation

DMP1 has been shown to play many roles in osteogenesis. We recently demonstrated that calcium-mediated stress kinase activation by DMP1 leads to osteoblast differentiation. In this study we demonstrate that DMP1 can also activate the extracellular signal-regulated kinase (ERK)-MAPK pathway. This activation was mediated through the RGD integrin-binding domain in DMP1. Further, we demonstrate that Runx2, an essential transcription factor, is stimulated by the ERK-MAPK pathway.

Dentin Phosphophoryn Activates Smad Protein Signaling through Ca2+-Calmodulin-dependent Protein Kinase II in Undifferentiated Mesenchymal Cells

Background: DPP stimulates differentiation of mesenchymal cells to osteogenic lineage. Results: DPP promotes differentiation by activating the intracellular Ca2+ influx and CaMKII-Smad1 signaling. Conclusion: DPP activates Ca2+-mediated signaling events, thereby playing a crucial role in osteoblast differentiation. Significance: DPP stimulates osteogenic phenotypic alterations in pluripotent stem cells by activating CaMKII-Smad1 signaling. Dentin phosphophoryn (DPP) is a major noncollagenous protein in the dentin matrix. In this study, we demonstrate that pluripotent stem cells such as C3H10T1/2 and human bone marrow cells can be committed to the osteogenic lineage by DPP. Treatment with DPP can stimulate the release of intracellular Ca2+. This calcium flux triggered the activation of Ca2+-calmodulin-dependent protein kinase II (CaMKII). Activated CaMKII induced the phosphorylation of Smad1 and promoted nuclear translocation of p-Smad1. Inhibition of store Ca2+ depletion by 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid tetrakis(acetoxymethyl ester) or down-regulation of CaMKII by KN-62, a selective cell-permeable pharmacological inhibitor or a dominant negative plasmid of CaMKII, blocked DPP-mediated Smad1 phosphorylation. Activation of Smad1 resulted in the expression of osteogenic markers such as Runx2, Osterix, DMP1, Bone sialoprotein, Osteocalcin, NFATc1, and Schnurri-2, which have been implicated in osteoblast differentiation. These findings suggest that DPP is capable of triggering commitment of pluripotent stem cells to the osteogenic lineage.

DPP in the matrix mediates cell adhesion but is not restricted to stickiness: a tale of signaling.

Cell adhesion to DPP substrate is an integrin-mediated event and involves integrin binding, clustering, assembly of focal adhesion complexes and cytoskeletal organization. Cells perceive the DPP substrate through the integrin receptor αvβ1 and bind the actin cytoskeleton to the membrane via focal adhesion sites. The cells respond to this proteinaceous rigid substrate by activating the mechano-chemical signaling events leading to cell spreading and formation of focal adhesions. Focal adhesions, which are sites of integrin binding to the extracellular matrix, form in the leading edge during cell migration. These sites are dynamic and the supramolecular assemblies contain structural and signaling components regulating cell functions. In our study, we present a scenario that integrins utilize the actin network to permit activation of the mitogen-activated kinase modules to transduce signals through the cytoplasm to the nucleus in the presence of DPP. We specifically demonstrate that ERK-mediated transcriptional events impinge on activation of transcription factors leading to cell differentiation.

Stimulation of Periodontal Ligament Stem Cells by Dentin Matrix Protein 1 Activates Mitogen-Activated Protein Kinase and Osteoblast Differentiation

BACKGROUND Periodontitis can ultimately result in tooth loss. Many natural and synthetic materials have been tried to achieve periodontal regeneration, but the results remain variable and unpredictable. We hypothesized that exogenous treatment with dentin matrix protein 1 (DMP1) activates specific genes and results in phenotypic and functional changes in human periodontal ligament stem cells (hPDLSCs). METHODS hPDLSCs were isolated from extracted teeth and cultured in the presence or absence of DMP1. Quantitative polymerase chain reactions were performed to analyze the expression of several genes involved in periodontal regeneration. hPDLSCs were also processed for immunocytochemical and Western blot analysis using phosphorylated extracellular signal-regulated kinase (pERK) and ERK antibodies. Alkaline phosphatase and von Kossa staining were performed to characterize the differentiation of hPDLSCs into osteoblasts. Field emission scanning electron microscopic analysis of the treated and control cell cultures were also performed. RESULTS Treatment with DMP1 resulted in the upregulation of genes, such as matrix metalloproteinase-2, alkaline phosphatase, and transforming growth factor β1. Activation of ERK mitogen-activated protein kinase signaling pathway and translocation of pERK from the cytoplasm to the nucleus was observed. Overall, DMP1-treated cells showed increased expression of alkaline phosphatase, increased matrix, and mineralized nodule formation when compared with untreated controls. CONCLUSION DMP1 can orchestrate a coordinated expression of genes and phenotypic changes in hPDLSCs by activation of the ERK signaling pathway, which may provide a valuable strategy for tissue engineering approaches in periodontal regeneration.

Abstract 500: DMP1 activates osteolytic cycle in a tumor environment

Proceedings: AACR 103rd Annual Meeting 2012‐‐ Mar 31‐Apr 4, 2012; Chicago, IL Bone metastasis is one of the skeletal malignancies that could be caused by breast cancer. Dentin matrix protein 1 (DMP1) is an acidic noncollagenous protein localized specifically in the mineralized matrix of bone and dentin. It is a multifunctional protein, involved in gene regulation (within the cell) and HAP nucleation (in the ECM). DMP1 is endocytosed by preosteoblasts, triggering the calcium dependent and calcium independent signaling pathways resulting in a series of downstream events leading to osteoblast differentiation. These signaling events facilitate the activation of a calcium dependent stress-induced p38 MAP kinase and a calcium independent integrin mediated ERK1/2 MAP kinase pathway resulting in the expression of downstream target genes such as Runx2 and osteocalcin. Results from this study have shown that the osteoblast cells upon differentiation secrete receptor activator of nuclear factor kappa B ligand (RANKL) that stimulates the differentiation of precursor monocytes to osteoclasts. The study also focuses on identifying the role of DMP1 in breast cancer cells that could enhance osteoclast formation thereby accelerating bone resportion. Thus, the study reveals a new mechanism by which DMP1 can activate the vicious cycle leading to the aggressive growth and behavior of the cancer cells. To address the mechanism by which DMP1 might contribute to the osteolytic process, DMP1 was overexpressed in the metastatic cancer cell line MDA MB231. Upregulation of markers like Runx2, MMP2, MMP9, RANKL, OPN, BSP, VEGF and the activation of Smad2/3 and MAP kinase pathways (p38 and ERK1/2 pathways) were observed in overexpressed cells. Results from this study have shown that DMP1 secreted by the breast cancer cells into the extracellular environment could stimulate the differentiation of osteoblasts, leading to the secretion of factors that signal monocytes to differentiate to osteoclast thus resulting in bone metastasis. Overall, the results obtained from this study identify a new role for DMP1 in the differentiation of breast cancer tumor cells that may be directly related to their metastatic potential. Supported by NIH DE 115657 and the Brodie Endowment Fund Citation Format: {Authors}. {Abstract title} [abstract]. In: Proceedings of the 103rd Annual Meeting of the American Association for Cancer Research; 2012 Mar 31-Apr 4; Chicago, IL. Philadelphia (PA): AACR; Cancer Res 2012;72(8 Suppl):Abstract nr 500. doi:1538-7445.AM2012-500