The short- and long-term effects of tumor necrosis factor-α and BRL 49653 on peroxisome proliferator-activated receptor (PPAR)γ2 gene expression and other adipocyte genes

Expression of turner necrosis factor-alpha (TNF alpha) in adipocytes has been reported to correlate with insulin resistance associated with obesity, The thiazolidinediones such as BRL 49653 have been reported to improve insulin sensitivity in obese animals and humans. Although its exact mechanism of action is not known, BRL 49653 has been shown to antagonize some of the inhibitory actions of TNF alpha. BRL 49653 binds and activates the peroxisome proliferator-activated receptor (PPAR gamma 2), an important nuclear transcription factor in adipocyte differentiation; however, its regulation of PPAR gamma 2 in differentiated adipocytes is unknown. In this paper, we find that BRL 49653 blocked the ability of TNF alpha to down-regulate the expression and transcription of several adipocyte genes, but BRL 49653 did not prevent TNF alpha from down-regulating PPAR gamma 2. Moreover, BRL 49653 alone initially decreased the expression of PPAR gamma 2 mRNA and protein greatly. After 24 h of treatment in 3T3-L1 adipocytes, BRL 49653 down-regulated PPAR gamma 2 by greater than 90% and potentiated the decrease of PPAR gamma 2 mRNA by TNF alpha at this time. These unexpected results prompted us to repeat the experiments for a longer time to determine whether BRL 49653 would continue to down-regulate PPAR gamma 2. With prolonged BRL 49653 treatment, PPAR gamma 2 mRNA expression was not decreased as greatly, and the protein levels were decreased 20-30% below control at 72 h compared Po 90% at 24 h. Although BRL 49653 continued to prevent the inhibitory effects of TNF alpha on perilipin and aP2 mRNA, by 72 h, BRL 49653 was not as potent an inhibitor of TNF alpha's down-regulation of perilipin protein. Since PPAR gamma 2 protein was more abundant at this time, these results suggest that the level of PPAR gamma 2 protein is not the sole factor that regulates the transcriptional control by BRL 49653.