Caspase-mediated degradation of PPARγ proteins in adipocytes
Objective: Peroxisome proliferator-activated receptor γ (PPARγ) is a transcription factor that plays an important role in adipocyte gene expression. Previous studies from our laboratory and others have shown that PPARγ can be ubiquitin modified and targeted to the proteasome for degradation in response to transcriptional activation. In this study, we determined whether other degradation pathways contributed to the tumor necrosis factor α (TNFα)-induced degradation of PPARγ proteins in 3T3-L1 adipocytes. Methods and Procedures: In these studies, 3T3-L1 cells were studied by performing subcellular fractionations and western blotting after various TNFα treatments. Whole tissue extracts from rat adipose tissue were also used to examine PPARγ degradation. Results: We observed that TNFα can induce a caspase-mediated degradation of PPARγ proteins in the presence of cycloheximide. The caspase-mediated degradation of both PPARγ1 and PPARγ2 resulted in the generation of a specific 44-kd cleavage product. The specific cleavage product was unaffected by proteasome inhibitors, but was repressed by a general caspase inhibitor. Use of several specific caspase inhibitors revealed that caspase-1 was activated following treatment with TNFα and cycloheximide (CH), and inhibition of caspase-1 blocked the cleavage of PPARγ proteins in cultured adipocytes. In addition, a similar PPARγ degradation product was observed in rodent adipose tissue. Discussion: In summary, this is the first study to demonstrate that PPARγ levels can be modulated by caspase activity. © 2008 The Obesity Society.