Enhancing effect of troglitazone on porcine adipocyte differentiation in primary culture: A comparison with dexamethasone

Objective: This study compares the effects of the thiazolidinedione, trogli tazone (TGZ), dexamethasone (DEX), and DEX plus TGZ on preadipocyte differe ntiation and the expression of CCAAT/enhancer-binding protein-alpha (C/EBP alpha) and peroxisome proliferator-activated receptor-gamma (PPAR gamma). Research Methods and Procedures: Adipose tissue was obtained from postnatal pigs to isolate stromal-vascular cells. First, we applied 1, 5, or 10 muM TGZ and 10% fetal bovine serum for 3 days and counted the number of recruit ed preadipocytes. Next, we used either 10 muM TGZ, 80 nM DEX, or DEX plus T GZ with 10% fetal bovine serum for 3 days and then switched to serum-free m edium with insulin for 6 days. On day 3 of culture, we counted preadipocyte s, and on days 3 and 6 of culture, we performed immunostaining and Western blot analysis to determine the expression of C/EBP alpha and PPAR gamma pro teins. On day 9 of culture, we stained for lipids with oil red-O and measur ed the activity of glycerol phosphate dehydrogenase. Results: DEX and TGZ equally enhanced recruitment of preadipocytes and late differentiation, but these effects were not additive with DEX plus TGZ tre atment. However, TGZ and DEX had a differential effect on morphogenesis; DE X-treated adipocytes were larger and organized in loose clusters, whereas T GZ-treated cells were smaller and formed compact clusters. Both agents incr eased C/EBP alpha expression but in a temporally distinct manner. DEX was a better inducer than TGZ, and its effect was early and temporary. However, treatment with either TGZ or DEX did not change PPAR gamma protein expressi on as evaluated by a Western blotting, but immunocytochemistry showed a ten dency for increased numbers of PPAR gamma positive cells. Discussion: TGZ and DEX equally enhance early and late adipocyte differenti ation, possibly by using some common pathways for preadipocyte recruitment. The differential effect on morphogenesis implies a potential differential effect on the expression of extracellular matrix components. C/EBP alpha ma y be the critical transcription factor involved in TGZ- and DEX-induced adi pogenesis.