Interleukin-1β-mediated suppression of RXR:RAR transactivation of the Ntcp promoter is JNK-dependent

Bile flow is rapidly and markedly reduced in hepatic inflammation, correlating with suppression of critical hepatic bile acid transporter gene expression including the principal hepatic bile acid importer, the Na₊/taurocholate cotransporting polypeptide (Ntcp, SlclOal). Endotoxin treatment of rats and interleukin-1β (IL-β) treatment of liver-derived HepG2 cells leads to a marked decline in the nuclear binding activity of a main Ntcp gene regulator, the nuclear receptor heterodimer retinoid X receptor:retinoic acid receptor (RXR:RAR). How IL-lβ signaling leads to reduced RXR:RAR nuclear binding activity is unknown, and we sought to determine whether mitogen-activated protein kinase (MAPK) pathways were involved. IL-Iβ treatment of cultured primary rat hepatocytes markedly reduced Ntcp RNA levels and Ntcp promoter activity in transiently transfected HepG2 cells. Pretreatment with inhibitors of extracellular signal-regulated kinase (ERK, PD98059) orp38 MAPK (SB203580) did not affect IL- lβmediated suppression of Ntcp gene expression, whereas curcumin, a derivative of the spice turmeric and a recently described inhibitor of c-Jun N-terminal kinase (JNK), completely ameliorated the effects of IL.lβ. Co-transfection of a JNK expression plasmid inhibited RXR:RAR-mediated activation of the Ntcp promoter, while a dominant negative JNK expression plasmid completely blocked IL-lβ-mediated suppression. Curcumin, but not PD98059 or SB203580, inhibited IL-lβ-mediated suppression of nuclear RXR:RAR binding activity, which correlated with inhibition of JNK phosphorylation and phospho-JNK-mediated phosphorylation of RXR. Taken together, these data provide evidence supporting a novel player (JNK), as well as its inhibitor (curcumin), in inflammation-mediated regulation of hepatobiliary transporters and correlate JNK-dependent RXR phosphorylation with reduced RXR-dependent hepatic gene expression.