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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.)