Tumour necrosis factor alpha down-regulates the expression of peroxisome proliferator activated receptor alpha (PPARα) in human hepatocarcinoma HepG2 cells by activation of NF-κB pathway

Peroxisome proliferator activated receptor-alpha (PPAR alpha) plays a major role in the regulation of lipid and glucose homeostasis, and inflammatory responses. The objectives of the study were to systematically investigate the effects of TNF-alpha and its regulatory pathway on PPAR alpha expression in HepG2 cells using Real-Time RT-PCR and western blot analysis. Here, TNF-alpha suppressed PPAR alpha mRNA expression in a dose- and time-dependent manner at the level of gene transcription. Pre-treatment of cells with 10 mu M of Wedelolactone for 2 h was sufficient to restore PPAR alpha expression to basal levels and also affected the expression of PPAR alpha-regulated genes. This study also demonstrated that TNF-alpha represses PPAR alpha expression by augmenting the activity of canonical NF-kappa B signalling pathway. This was shown by the abrogation of TNF-alpha-mediated PPAR alpha down-regulation, after both p65 and p50 were knocked down via siRNA. The IKK contributes to I kappa B alpha degradation and mediates inducible phosphorylation of p105 at Ser933. Surprisingly, phosphorylation of p65 at Ser468 and Ser536 were severely abrogated with Wedelolactone inhibition, suggesting that Ser468 and Ser536, but not Ser276, may mediate the TNF-alpha inhibitory action on PPAR alpha gene expression. These results suggest that TNF-alpha might, at least in part, suppress PPAR alpha expression through activation of IKK/p50/p105/p65 pathway. Furthermore, phosphorylation of p65 at Ser468 and Ser536 may play a crucial role in the mechanism that limits PPAR alpha production in the human HepG2 cells. (C) 2012 Elsevier Ltd. All rights reserved.