Induction of endosomal/lysosomal pathways in differentiating osteoblasts as revealed by combined proteomic and transcriptomic analyses

We have analyzed proteome changes associated with bone-forming osteoblast differentiation by quantitative differential proteomic and transcriptomic analyses using in vitro differentiation model. Sixty nine proteins were found up-regulated (>2-fold) and 18 were down-regulated (<0.5-fold) at protein level. The mRNA levels of these proteins were then analyzed by quantitative real-time PCR combined with clustering analysis. The most prominent cluster with increased protein and mRNA levels contains endosomal and lysosomal proteins, demonstrating the drastic induction of degradative endosomal/lysosomal pathways in osteoblasts. Osteoblasts, therefore, are involved not only in the synthesis but also in the turnover of the extracellular matrix proteins such as collagens.