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ANIMAL GROWTH, PHYSIOLOGY, AND REPRODUCTION |
,1
* Department of Animal Sciences and
and
College of Agricultural Sciences, Oregon State University, Corvallis 97331
1 Correspondence: 138 Strand Ag Hall (phone: 541-737-1915; fax: 541-737-4574; e-mail: ChingYuan.Hu{at}orst.edu).
All-trans retinoic acid (ATRA) potently inhibits the differentiation of porcine preadipocytes in primary culture; however, the mechanism by which ATRA exerts this effect in pigs is poorly understood. The objective of this study was to use retinoid receptor-specific ligands to investigate the mechanism underlying the antiadipogenic action of retinoids in cultured pig preadipocytes by identifying the retinoid receptor mediating this action and examining the effect of retinoids on the expression of key adipogenic transcription factors. Stromal-vascular cells were harvested from porcine adipose tissue and cultured in serum-free medium. Glycerol-3-phoshphate dehydrogenase (GPDH) activity, a late marker of preadipocyte differentiation, was decreased (P < 0.01) by the addition of 0 to 10 µM of either ATRA, a nonspecific agonist for both the retinoic acid receptor (RAR) and the retinoid X receptor (RXR) or the selective RAR agonist, 4-(E-2-[5,6,7,8-tet-rahydro-5,5,8,8-tetramethyl-2-naphthalenyl]-1-propenyl) benzoic acid (TTNPB). Addition of increasing amounts of Ro-61, a RAR-specific antagonist (0 to 10 µM) prevented ATRA and TTNBP from decreasing GPDH activity. Addition of methoprene acid, an RXR-specific agonist, increased (P < 0.01) GPDH activity. Preadipocytes were then continuously treated with 10 nM of TTNPB in the presence or absence of 1 µM Ro-61, and mRNA was isolated on d 2 and 8. Addition of TTNPB decreased (P < 0.001) the expression of peroxisome proliferator-activated receptor 
(PPAR), sterol regulatory element-binding protein-1c (SREBP-1c), retinoid X receptor 
(RXR), and adipocyte fatty acid binding protein (aP2) mRNA transcripts, whereas these effects were prevented by the presence of Ro-61. Interestingly, TTNBP increased (P < 0.001) the mRNA abundance of the orphan nuclear receptor chicken ovalbumin upstream promoter-transcription factor 1 (COUP-TF1), whereas Ro-61 prevented this increase. These changes were independent of alterations in the mRNA abundances of the retinoic acid receptor , and CCAAT/enhancer binding protein and ß (C/EBPß; C/EBP) genes. These results indicate that retinoic acid inhibits porcine preadipocyte differentiation by a mechanism that involves activation of the RAR and downregulation of PPAR, RXR, and SREBP-1C mRNA. This mechanism is independent of changes in C/EBPß and C/EBP mRNA abundance and may involve COUP-TF.
Key Words: Adipogenesis • Chicken Ovalbumin Upstream Promoter Transcription Factor 1 • Primary Culture • Retinoic Acid • Swine
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