title: Inhibition of apolipoprotein A-I expression by TNF-alpha in HepG2 cells: requirement for c-jun.
abstract: Tumor necrosis factor alpha (TNF α) signals in part through the mitogen activated protein (MAP) kinase c-jun-N-terminal kinase (JNK). Activation of JNK has been shown to promote insulin resistance and dyslipidemia, including reductions in plasma high-density lipoprotein (HDL) and apolipoprotein A-I (apo A-I). To examine how TNF α-mediated JNK activation inhibits hepatic apo A-I production, the effects of c-jun activation on apo A-I gene expression were examined in HepG2 cells. Apo A-I gene expression and promoter activity were measured by Northern and Western blotting and transient transfection. Transient transfection and siRNA were used to specifically over-express or knockout c-jun, c-jun-N-terminal kinase-1 and -2 (JNK1 and JNK2, respectively) and mitogen-activated protein kinase-4 (MKK4). TNF α-treatment of HepG2 cells induced rapid phosphorylation of c-jun on serine 63. In cells treated with phorbol-12-myristate-13-acetate (PMA), apo A-I gene promoter activity was inhibited and apo A-I mRNA content and apo A-I protein secretion decreased. Likewise, over-expression of JNK1 and JNK2 inhibited apo A-I promoter activity. Over-expression of constitutively active MKK4, an upstream protein kinase that directly activates JNK, also inhibited apo A-I promoter activity, while over-expression of a dominant-negative MKK4 de-repressed apo A-I promoter activity in TNF α-treated cells. Inhibition of c-jun synthesis using siRNA but not a control siRNA prevented TNF α-mediated inhibition of apo A-I. These results suggest that the MKK4/JNK/c-jun signaling pathway mediates TNF α-dependent inhibition of apo A-I synthesis.
authors.affiliation: Division of Endocrinology, Diabetes, and Metabolism, Department of Internal Medicine, Saint Louis University, Saint Louis, Missouri, 63104.
authors.forename: Shant
authors.initials: S
authors.lastname: Parseghian
authors.forename: Luisa M
authors.initials: LM
authors.lastname: Onstead-Haas
authors.forename: Norman C W
authors.initials: NC
authors.lastname: Wong
authors.forename: Arshag D
authors.initials: AD
authors.lastname: Mooradian
authors.forename: Michael J
authors.initials: MJ
authors.lastname: Haas
mesh_terms: D016632:Apolipoprotein A-I / Q000037:antagonists & inhibitors / Q000096:biosynthesis*; D050171:Dyslipidemias / Q000235:genetics; D015972:Gene Expression Regulation, Neoplastic; D055786:Gene Knockout Techniques; D056945:Hep G2 Cells; D006801:Humans; D048055:Mitogen-Activated Protein Kinase 8 / Q000096:biosynthesis*; D048056:Mitogen-Activated Protein Kinase 9 / Q000096:biosynthesis / Q000235:genetics*; D034741:RNA, Small Interfering; D015398:Signal Transduction / Q000235:genetics; D014409:Tumor Necrosis Factor-alpha / Q000096:biosynthesis* / Q000235:genetics
chemical_list: D016632:Apolipoprotein A-I; D034741:RNA, Small Interfering; D014409:Tumor Necrosis Factor-alpha; D048056:Mitogen-Activated Protein Kinase 9; D048055:Mitogen-Activated Protein Kinase 8
keywords: APOLIPOPROTEIN A-I; CARDIOVASCULAR DISEASE; HIGH-DENSITY LIPOPROTEIN-CHOLESTEROL; PRO-INFLAMMATORY CYTOKINES; TRANSCRIPTIONAL REGULATION; c-jun