Abstract
Adipogenesis plays an important role in adipose tissue formation. Several transcriptional factors are involved in regulation of this complex process. Nutritional molecules, such as vitamin D and retinoic acid, have been reported as regulators of adipogenesis. The major aims of the present study were to investigate the mechanisms of vitamin D or retinoic acid regulation of adipogenesis and also the interaction between vitamin D or retinoic acid and adipogenic factors, such as peroxisome proliferator-activated receptor γ (PPARγ) and CCAAT-enhancer-binding protein α (C/EBPα). The bioactive form of vitamin D, 1,25-dihydroxyvitamin D (1,25-(OH)2D3) has been reported as a potential inhibitor of adipogenesis, and retinoic acid has also been shown as an inhibitor of adipogenesis. The inhibitory effect of both 1,25-(OH)2D3 and retinoic acid on adipogenesis in 3T3-L1 cells was detected. Gene expression of the adipogenic key transcription factors PPARγ and C/EBPα were inhibited by both high concentrations of 1,25-(OH)2D3 (10 and 100 nM)and retinoic acid (100 and 1000 nM), and in contrast, gene expression of the other two C/EBP family members, C/EBPβ and γ, were not influenced by any concentration of 1,25-(OH)2D3 or retinoic acid. Fatty acid binding protein 4 (FABP4) gene expression showed a marked response to both 1,25-(OH)2D3 and retinoic acid, even at the lower concentrations studied (0.1 and 1 nM of 1,25-(OH)2D3 treatments, and 1 and 10 nM of retinoic acid treatments). Unlike 1,25-(OH)2D3, retinoic acid had greater inhibitory impact on C/EBPα gene expression compared to PPARγ. Both 1,25-(OH)2D3 and retinoic acid had gradual inhibitory effects on the gene expression of stearoyl-coenzyme A desaturase 1 (SCD-1) compared to FABP4. C/EBPα promoter activity in response to 1,25-(OH)2D3 (100 nM) or retinoic acid (1000 nM) treatments were tested in 3T3-L1 cells. The results showed that 1,25-(OH)2D3 had little impact on the activity of the C/EBPα promoter, while retinoic acid appeared to induce activation of the promoter, despite an overall inhibitory effect on C/EBPα mRNA concentration. This observation suggests that the actions of retinoic acid may be mediated through an mRNA degradative pathway. Gold-nanoconjugates linked with KDEL peptide was used to deliver siRNA against C/EBPα into 3T3-L1 cells. Transfection of gold-nanoconjugates into preadipocytes and mature adipocytes was observed by confocal microscopy. This result suggests that gold-nanoconjugates can be used as a delivery vector into mature adipocytes. Unfortunately, C/EBPα siRNA silencing was not detected on all the three time points measured, suggesting that further studies will be focused on optimizing time points, and cellular uptake trafficking and co-localization of gold-nanoconjugates/siRNA in adipocytes. Overall, adipogenesis was inhibited by both 1,25-(OH)2D3 and retinoic acid treatments, and the gene expression of adipogenic transcription factors were inhibited in response to 1,25-(OH)2D3 and retinoic acid treatments, suggesting that the mechanisms of 1,25-(OH)2D3 and retinoic acid regulation of adipogenesis involved transcriptional regulation.