Caspase-mediated degradation of PPARgamma proteins in adipocytes.
OBJECTIVE
Peroxisome proliferator-activated receptor gamma (PPARgamma) is a transcription factor that plays an important role in adipocyte gene expression. Previous studies from our laboratory and others have shown that PPARgamma 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 alpha (TNFalpha)-induced degradation of PPARgamma proteins in 3T3-L1 adipocytes.


METHODS
In these studies, 3T3-L1 cells were studied by performing subcellular fractionations and western blotting after various TNFalpha treatments. Whole tissue extracts from rat adipose tissue were also used to examine PPARgamma degradation.


RESULTS
We observed that TNFalpha can induce a caspase-mediated degradation of PPARgamma proteins in the presence of cycloheximide. The caspase-mediated degradation of both PPARgamma1 and PPARgamma2 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 TNFalpha and cycloheximide (CH), and inhibition of caspase-1 blocked the cleavage of PPARgamma proteins in cultured adipocytes. In addition, a similar PPARgamma degradation product was observed in rodent adipose tissue.


CONCLUSIONS
In summary, this is the first study to demonstrate that PPARgamma levels can be modulated by caspase activity.
He
Fang
F
Department of Biological Sciences, Louisiana State University, Baton Rouge, LA, USA.
Doucet
Julie A
JA
Stephens
Jacqueline M
JM
D041721:3T3-L1 Cells; D017667:Adipocytes / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D000818:Animals; D020170:Caspase 1 / Q000378:metabolism*; D003513:Cycloheximide / Q000494:pharmacology; D004305:Dose-Response Relationship, Drug; D051379:Mice; D047495:PPAR gamma / Q000378:metabolism*; D014409:Tumor Necrosis Factor-alpha / Q000494:pharmacology
D047495:PPAR gamma; D014409:Tumor Necrosis Factor-alpha; D003513:Cycloheximide; D020170:Caspase 1