The role of orthosilicic acid-induced autophagy on promoting differentiation and mineralization of osteoblastic cells.

Silicon, an element, has been reported to possess various osteogenic biological functions. However, the precise cellular action mechanism of silicon in osteogenesis is elusive. Here, we examine whether autophagy is associated with the osteogenic differentiation and mineralization of preosteoblasts stimulated by orthosilicic acid (unique soluble form of silicon). The autophagy induced by silicon was detected via autophagy-related protein expression. The correlation between orthosilicic acid-induced autophagy and the osteogenic process was verified by the autophagy inhibitor 3-methyladenine. We also investigated its applicability in vivo through bone mineral density, collagen staining, hydroxyproline, and immunohistochemistry on osteoporosis rats. Concentrations above 10 µM increased alkaline phosphatase activity, whereas those up to 50 µM decreased alkaline phosphatase activity. Orthosilicic acid at concentrations up to 20 µM increased the osteogenetic indicators, the expression of the autophagy-related factors SQSTM1/p62, and the conversion of microtubule-associated protein 1 light chain 3-II (LC3-II) from microtubule-associated protein 1 light chain 3-I (LC3-I) according to Western Blot analyses. The application of 3-methyladenine decreased the expression of Runt-related transcription factor 2 (RUNX-2), bone morphogenetic protein-2, and collagen I in the Western Blot analysis. Low dose of orthosilicic acid supplemented rats (OVX-1, 2) showed improvements in such parameters as the expression rate of collagenous fiber in bone, bone mineral density, femoral hydroxyproline content, and bone morphogenetic protein-2; while high dose groups (OVX-3, 4) presented no better outcome. The present study showed that orthosilicic acid stimulated the osteogenic differentiation and mineralization of cultured preosteoblasts by inducing autophagy. In addition, it also can enhance bone mineral density of osteoporosis rats.