| Biomineralization is a complex process involving cells, mineral and several key matrix molecules. Biglycan (BGN), a member of the small leucine-rich repeats proteoglycans (SLRPs) family, has been implicated in bone formation; however, the underlying mechanisms are not fully understood. To investigate the potential roles of BGN in matrix mineralization, MC3T3-E1-derived cell clones stably expressing higher and lower levels of BGN (S- and AS clones) were generated and partially characterized. In comparison with controls, MC3T3-E1 cells and those transfected with an empty vector (EV), the proliferation rate of S- and AS clones were unchanged, and the matrix organization and maturation were not significantly altered. However, osteoblast differentiation, determined by mRNA expression pattern of osteogenic markers, i.e., Cbfa1/Runx2, Osterix, type I collagen alpha 2 chain (Col1A2) and Bone Sialoprotein, was significantly affected by the levels of BGN. In S clones, the expression levels of all markers were markedly up-regulated, whereas they were suppressed in the case of AS clones when compared to those of controls. Furthermore, in comparison with the controls, the onset and extent of in vitro matrix mineralization in S clones were significantly accelerated and greater while these processes were markedly delayed in AS clones. In addition, when transplanted into immunodeficient mice, although all cell types possessed the capability to form bone-like matrices, the bone-like matrix area in S clones was significantly larger than either controls or AS clones. These results clearly indicate that BGN regulates osteoblast differentiation and, as a consequence, matrix mineralization. In an attempt to explore a potential mechanism of BGN regulation of osteoblast differentiation, the interaction of BGN to bone morphogenic proteins (BMPs) and the effects of BGN on the BMP-induced transdifferentiaton from myogenic to osteoblastic lineage were investigated. The results demonstrated that BGN directly binds to BMP-2, and possibly BMP-4 as well, and the binding significantly enhanced the BMP-2-induced osteoblast differentiation. These results indicate that BGN modulates osteoblast differentiation through, at least in part, its interaction with BMP-2. |
