The stimulatory impact of d-delta-tocotrienol on the differentiation of murine MC3T3-E1 preosteoblasts
Osteoblasts and osteoclasts play essential and opposite roles in maintaining bone homeostasis. Osteoblasts fill cavities excavated by osteoclasts. The mevalonate pathway provides essential prenyl pyrophosphates for the activities of GTPases that promote differentiation of osteoclasts but suppress that of osteoblasts. Preclinical and clinical studies suggest that mevalonate suppressors such as statins increase bone mineral density and reduce risk for bone fracture. Tocotrienols down-regulate 3-hydroxy-3- methylglutaryl coenzyme A (HMG CoA) reductase, the rate-limiting enzyme in the mevalonate pathway. In vivo studies have shown the bone-protective activity of tocotrienols. We hypothesize that d-δ-tocotrienol, a mevalonate suppressor, induces differentiation of murine MC3T3-E1 pre-osteoblasts. Alizarin staining showed that d-δ- tocotrienol (0-25 μmol/L) induced mineralized nodule formation in a concentrationdependent manner in MC3T3-E1 preosteoblasts. D-δ-Tocotrienol (0-25 μmol/L), but not d-α-tocopherol (25 μmol/L), significantly induced alkaline phosphatase activity, an indicator of preosteoblast differentiation. The expression of differentiation marker genes including BMP-2 and VEGFα were stimulated dose dependently by d-δ-tocotrienol (0-25 μmol/L). Concomitantly, western-blot analysis showed that d-δ-tocotrienol downregulated HMG-CoA reductase. D-δ-Tocotrienol (0-25 μmol/L) had no impact on the viability of MC3T3-E1 preosteoblasts following 48-h incubations, suggesting lack of cytotoxicity at these doses. Tocotrienols and other mevalonate suppressors have potential in maintaining bone health.