The effects of d-δ-tocotrienol and trans, trans-farnesol on the differentiation of murine 3T3-F442A preadipocytes
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Suppression of adipogenesis is one of the potential approaches in obesity prevention. The statins, competitive inhibitors of 3-hydroxy-3-methylglutaryl coenzyme A (HMG CoA) reductase, suppress the differentiation of adipocytes, suggesting the role of mevalonate in adipocyte differentiation. We investigated the effect of d-δ-tocotrienol, a suppressor of HMG CoA reductase, and trans, trans-farnesol (t,t-farnesol or farnesol), a mevalonate-derived sesquiterpene, on differentiation of murine 3T3-F442A preadipocytes. With AdipoRed assay and Oil Red O staining, we showed that an 8 day incubation with 2.5 - 10 μmol/L d-δ-tocotrienol dose-dependently reduced the intracellular triglyceride content of murine 3T3-F442A adipocytes, reminiscent of the lovastatin (1.25 μmol/L) effect. Concomitantly, d-δ-tocotrienol dose-dependently inhibited glucose uptake by 3T3-F442A cells and the expression of GLUT4, pAKT, and HMG CoA reductase proteins. Rosiglitazone (1 µmol/L), an agonist of peroxisome proliferator-activated receptor γ (PPARγ), reversed the effects of d-δ-tocotrienol on cellular lipid content and glucose uptake. d-δ-Tocopherol, a vitamin E molecule with no effect on HMG CoA reductase, at 10 µmol/L did not have any impact on 3T3-F442A differentiation. Trypan blue staining showed no changes in cell viability following 48-h incubation of 3T3-F442A cells with d-δ-tocotrienol (0-80 µmol/L), suggesting that the adipogenesis-suppressive activity of d-δ-tocotrienol was independent of cytotoxicity. Farnesol dose-dependently (25 - 75 μmol/L) increased glucose uptake and intracellular triglyceride content of 3T3-F442A cells without affecting cell viability. GW9662 (10 µmol/L), an antagonist of PPARγ, and Ly294002 (10 µmol/L), a phosphatidyl-inositol 3 kinase (PI3K) inhibitor, reversed the effects of farnesol on cellular lipid content. Real-time qPCR and western-blot showed that farnesol increased the mRNA and protein levels of GLUT4 and PPARγ. The mRNA levels for fatty acid binding protein 4, lipoprotein lipase, and adiponectin were also upregulated. Farnesol did not affect the protein or mRNA level of HMG CoA reductase. Farnesol may induce adipocyte differentiation via upregulation of PPARγ and PI3K, which may lead to the increased expression of GLUT4 and adipogenic genes. d-δ-Tocotrienol, a mevalonate pathway suppressor, and trans, trans-farnesol, a mevalonate-derived sesquiterpene, imposed opposite effects on the differentiation of 3T3-F442A preadipocytes. Their effects were mediated by PI3K-PKB/Akt signaling pathway that is responsible for the translocation of GLUT4 protein to plasma membrane and glucose uptake. Future studies may further delineate the role of mevalonate pathway in adipocyte differentiation and the value of the mevalonate pathway as a target for obesity intervention.