Li Tiangang, Jahan Asmeen, Chiang John Y L
Department of Microbiology, Immunology, and Biochemistry, Northeastern Ohio Universities College of Medicine, Rootstown, OH 44272, USA.
Hepatology. 2006 Jun;43(6):1202-10. doi: 10.1002/hep.21183.
Cholesterol 7 alpha-hydroxylase (CYP7A1) of the bile acid biosynthesis pathway is suppressed by bile acids and inflammatory cytokines. Bile acids are known to induce inflammatory cytokines to activate the mitogen-activated protein kinase/c-Jun N-terminal kinase (JNK) signaling pathway that inhibits CYP7A1 gene transcription. c-Jun has been postulated to mediate bile acid inhibition of CYP7A1. However, the c-Jun target involved in the regulation of CYP7A1 is unknown. Human primary hepatocytes and HepG2 cells were used as models to study chenodeoxycholic acid (CDCA) and interleukin-1 beta (IL-1 beta) regulation of human CYP7A1 gene expression via real-time polymerase chain reaction, reporter assays, co-immunoprecipitation and chromatin immunocipitation (ChIP) assays. IL-1 beta and CDCA reduced CYP7A1 but induced c-Jun messenger RNA expression in human primary hepatocytes. IL-1beta inhibited human CYP7A1 reporter activity via the HNF4 alpha binding site. A JNK-specific inhibitor blocked the inhibitory effect of IL-1 beta on HNF4 alpha expression and CYP7A1 reporter activity. c-Jun inhibited HNF4 alpha and PPARgamma coactivator-1 alpha (PGC-1 alpha) coactivation of CYP7A1 reporter activity, whereas a dominant negative c-Jun did not. Co-immunoprecipitation and ChIP assays revealed that IL-1 beta and CDCA reduced HNF4 alpha bound to the CYP7A1 chromatin, and that c-Jun interacted with HNF4 alpha and blocked HNF4 alpha recruitment of PGC-1 alpha to the CYP7A1 chromatin. In conclusion, IL-1 beta and CDCA inhibit HNF4 alpha but induce c-Jun, which in turn blocks HNF 4 alpha recruitment of PGC-1 alpha to the CYP7A1 chromatin and results in inhibition of CYP7A1 gene transcription. The JNK/c-Jun signaling pathway inhibits bile acid synthesis and protects hepatocytes against the toxic effect of inflammatory agents.
胆汁酸生物合成途径中的胆固醇7α-羟化酶（CYP7A1）受到胆汁酸和炎性细胞因子的抑制。已知胆汁酸可诱导炎性细胞因子激活丝裂原活化蛋白激酶/c-Jun氨基末端激酶（JNK）信号通路，该通路会抑制CYP7A1基因转录。据推测，c-Jun介导胆汁酸对CYP7A1的抑制作用。然而，参与CYP7A1调控的c-Jun靶点尚不清楚。以人原代肝细胞和HepG2细胞为模型，通过实时聚合酶链反应、报告基因检测、免疫共沉淀和染色质免疫沉淀（ChIP）检测，研究鹅去氧胆酸（CDCA）和白细胞介素-1β（IL-1β）对人CYP7A1基因表达的调控作用。IL-1β和CDCA可降低人原代肝细胞中CYP7A1的表达，但可诱导c-Jun信使核糖核酸的表达。IL-1β通过肝细胞核因子4α（HNF4α）结合位点抑制人CYP7A1报告基因活性。一种JNK特异性抑制剂可阻断IL-1β对HNF4α表达和CYP7A1报告基因活性的抑制作用。c-Jun可抑制HNF4α和过氧化物酶体增殖物激活受体γ共激活因子-1α（PGC-1α）对CYP7A1报告基因活性的共激活作用，而显性负性c-Jun则无此作用。免疫共沉淀和ChIP检测显示，IL-1β和CDCA可减少与CYP7A1染色质结合的HNF4α，且c-Jun与HNF4α相互作用，阻止HNF4α将PGC-1α募集至CYP7A1染色质。总之，IL-1β和CDCA可抑制HNF4α，但可诱导c-Jun，进而阻止HNF4α将PGC-1α募集至CYP7A1染色质，导致CYP7A1基因转录受到抑制。JNK/c-Jun信号通路可抑制胆汁酸合成，并保护肝细胞免受炎性介质的毒性作用。
