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Objective. The aim of this study was to test the possible involvement, relevance and significance of dentin matrix protein 1 (DMP1) in chondrocyte redifferentiation and OA.Methods. To examine the function of DMP1 in vitro, bone marrow stromal cells (BMSCs) and articular chondrocytes (ACs) were isolated and differentiated in micromasses in the presence or absence of DMP1 small interfering RNA and analysed for chondrogenic phenotype. The association of DMP1 expression with OA progression was analysed time dependently in the OA menisectomy rat model and in grade-specific OA human samples.Results. It was found that DMP1 was strongly related to chondrogenesis, which was evidenced by the strong expression of DMP1 in the 14.5-day mouse embryonic cartilage development stage and in femoral heads of post-natal days 0 and 4. In vitro chondrogenesis in BMSCs and ACs was accompanied by a gradual increase in DMP1 expression at both the gene and protein levels. In addition, knockdown of DMP1 expression led to decreased chondrocyte marker genes, such as COL2A1, ACAN and SOX9, and an increase in the expression of COL10A and MMP13 in ACs. Moreover, treatment with IL-1β, a well-known catabolic culprit of proteoglycan matrix loss, significantly reduced the expression of DMP1. Furthermore, we also observed the suppression of DMP1 protein in a grade-specific manner in knee joint samples from patients with OA. In the menisectomy-induced OA model, an increase in the Mankin score was accompanied by the gradual loss of DMP1 expression.Conclusion. Observations from this study suggest that DMP1 may play an important role in maintaining the chondrogenic phenotype and its possible involvement in altered cartilage matrix remodelling and degradation in disease conditions like OA. [ABSTRACT FROM PUBLISHER]

Objectives. The p38 mitogen-activated protein kinase (MAPK) signal transduction pathway is involved in a variety of inflammatory responses, including cytokine generation, cell differentiation proliferation and apoptosis. Here, we examined the effects of systemic p38 MAPK inhibition on cartilage cells and OA disease progression by both in vitro and in vivo approaches.Methods. p38 kinase activity was evaluated in normal and OA cartilage cells by measuring the amount of phosphorylated protein. To examine the function of p38 signalling pathway in vitro, normal chondrocytes were isolated and differentiated in the presence or absence of p38 inhibitor SB203580 and analysed for chondrogenic phenotype. Effect of systemic p38 MAPK inhibition in normal and OA (induced by menisectomy) rats were analysed by treating animals with vehicle alone [dimethylsulphoxide (DMSO)] or p38 inhibitor (SB203580). Damage to the femur and tibial plateau was evaluated by modified Mankin score, histology and immunohistochemistry.Results. Our in vitro studies have revealed that a down-regulation of chondrogenic and an increase of hypertrophic gene expression occurs in the normal chondrocytes when p38 is neutralized by a pharmacological inhibitor. We further observed that the basal levels of p38 phosphorylation were decreased in OA chondrocytes compared with normal chondrocytes. These findings together indicate the importance of this pathway in the regulation of cartilage physiology and its relevance to OA pathogenesis. At the in vivo level, systematic administration of a specific p38 MAPK inhibitor, SB203580, continuously for more than a month led to a significant loss of proteoglycan, aggrecan and cartilage thickness. On the other hand, SB203580-treated normal rats showed a significant increase in Terminal dUTP nick end-labelling (TUNEL)-positive cells, cartilage hypertrophy markers such as Type 10 collagen, Runt-related transcription factor and MMP-13 and substantially induced OA-like phenotypic changes in the normal rats. In addition, menisectomy-induced OA rat models that were treated with p38 inhibitor showed aggravation of cartilage damage.Conclusions. In summary, this study has provided evidence that the component of the p38 MAPK pathway is important to maintain cartilage health, and its inhibition can lead to severe cartilage degenerative changes. The observations in this study highlight the possibility of using activators of the p38 pathway as an alternative approach in the treatment of OA. [ABSTRACT FROM PUBLISHER]