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

Abstract

Expression of tumor necrosis factor-α (TNFα) 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 TNFα. BRL 49653 binds and activates the peroxisome proliferator-activated receptor (PPARγ2), an important nuclear transcription factor in adipocyte differentiation; however, its regulation of PPARγ2 in differentiated adipocytes is unknown. In this paper, we find that BRL 49653 blocked the ability of TNFα to down-regulate the expression and transcription of several adipocyte genes, but BRL 49653 did not prevent TNFα from down-regulating PPARγ2. Moreover, BRL 49653 alone initially decreased the expression of PPARγ2 mRNA and protein greatly. After 24 h of treatment in 3T3-L1 adipocytes, BRL 49653 down-regulated PPARγ2 by greater than 90% and potentiated the decrease of PPARγ2 mRNA by TNFα 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 PPARγ2. With prolonged BRL 49653 treatment, PPARγ2 mRNA expression was not decreased as greatly, and the protein levels were decreased 20-30% below control at 72 h compared to 90% at 24 h. Although BRL 49653 continued to prevent the inhibitory effects of TNFα on perilipin and aP2 mRNA, by 72 h, BRL 49653 was not as potent an inhibitor of TNFα's down-regulation of perilipin protein. Since PPARγ2 protein was more abundant at this time, these results suggest that the level of PPARγ2 protein is not the sole factor that regulates the transcriptional control by BRL 49653.