Interleukin-1β suppresses retinoid transactivation of two hepatic transporter genes involved in bile formation

Cytokines have been implicated in the pathogenesis of inflammatory cholestasis. This is due to transcriptional down-regulation of hepatic transporters including the Na+/bile acid cotransporter, ntcp, and the multispecific organic anion exporter, mrp2. We have recently shown that ntcp suppression by lipopolysaccharide in vivo is caused by down-regulation of transactivators including the previously uncharacterized Footprint B-binding protein. Both the ntcp FpB element and the mrp2 promoter contain potential retinoid-response elements. We hypothesized that retinoic acid receptor (RAR) and retinoid X receptor (RXR) heterodimers would activate these two genes and that cytokines that reduce bile flow might do so by suppressing nuclear levels of these transactivators. Retinoid transactivation and interleukin-1 beta down-regulation of the ntcp and mrp2 promoters were mapped to RXR alpha:RAR alpha-response elements. Gel mobility shift assays demonstrated specific binding of RXR alpha: RAR alpha heterodimers to the ntcp and mrp2 retinoid-response elements. The RXR alpha:RAR alpha complex was downregulated by IL-1 beta in HepG2 cells. An unexpected finding was that an adjacent CAAT-enhancer-binding protein element was required for maximal transactivation of the ntcp promoter by RXR alpha:RAR alpha. Taken together, these studies demonstrate regulation of two hepatobiliary transporter genes by RXR alpha:RAR alpha and describe a mechanism which likely contributes to their down-regulation during inflammation.