Induction Of Mdr1b Expression by Tumor Necrosis Factor–α in Rat Liver Cells Is Independent of P53 But Requires Nf–κB Signaling

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Abstract
The multidrug resistance protein Mdr1b in rats is up–regulated during liver regeneration after partial hepatectomy or after endotoxin treatment. We hypothesize that up–regulation of Mdr1b in these models is TNF–α-dependent. The mechanism of Mdr1b activation by TNF–α is unknown as TNF–α can signal through various pathways, including NF–κB and p53, transcription factors for which binding sites in the Mdr1b promoter have been identified. We aimed to elucidate the mechanism of up–regulation of Mdr1b by TNF–α. We selectively used constructs expressing dominant negative Fas–associated death domain protein (FADD), TNF receptor associated factor–2 (TRAF2) or IκB to inhibit pathways downstream of the TNF receptor. Further, the proteasome inhibitor MG–132 was used, which prevents the breakdown of IκB. We show a critical role for NF–κB in activation of Mdr1b gene expression both in primary rat hepatocytes and in rat hepatoma H–4–II–E cells. Because p53 is up–regulated by TNF–α in an NF–κB-dependent manner and the Mdr1b promoter contains a p53 binding site, we used liver cells expressing a dominant negative p53 to show that TNF–α up–regulation of Mdr1b is independent of functional p53. Using transient transfection assays, we show that Mdr1b up–regulation correlates with activation of the promoter. Mutation of the NF–κB site in the Mdr1b promoter prevents its induction by TNF–α. In conclusion our results show that activation of the rat Mdr1b gene by TNF–α is a result of NF–κB signaling and independent of p53. (Hepatology 2001;33:1425–1431.)

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