The expression of P-glycoproteins encoded by the mdr gene family is associated with the emergence of multidrug resistance phenotype in animal cells. However, the mechanisms controlling the expression of these genes have not been well elucidated. Here, we report that the expression of rat mdr1b gene in cultured H-4-II-E hepatoma cells can be induced by insulin. Transient transfection assays using reporter gene constructs containing various 5 mdr1b sequences showed that the sequence located

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... n base pairs ؊243 and ؊163 is important for insulin's induction of mdr1b promoter activity. Further analyses revealed that a NF-B-binding site (located between base pairs ؊167 and ؊158) is required for insulin-induced promoter activity. Gel mobility shift assay demonstrated that insulin stimulates the binding of nuclear p50/p65 subunits to the mdr1b NF-B sequence. Cotransfection of plasmids expressing either the p50/p65 NF-B subunits or Raf-1 kinase or both resulted in increased expression of the gene containing wild-type but not NF-B site-mutated mdr1b promoter. Finally, expression of either the antisense p65 subunit of NF-B or dominant negative Raf-1 kinase blocked insulin's induction of the mdr1b promoter activity. Taken together, our results suggest that the insulin-induced mdr1b expression is mediated by transcription factor NF-B via the Raf-1 kinase signaling pathway.