The stimulatory impact of d-delta-tocotrienol on the differentiation of murine MC3T3-E1 preosteoblasts
Abstract
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.