Papp-a Negatively Regulates ABCA1, ABCG1and SR-B1Expression By Inhibiting LXRα Through The ICF-I-mediated Signaling Pathway

Objective: PAPP-A has been involved in the atherosclerotic process through hroughregulation of local expression of IGF-1that mediates the activation of PI3-K and Aktsignaling cascades. In addition, IGF-1in development and progression ofatherosclerosis also stimulates mitogen-activated protein kinase (MAPK) cascades.This study was to investigate the changes of cholesterol efflux, ABCA1, ABCG1,SR-B1mRNA and protein expression in THP-1macrophage derived foam cells aftertreated by PAPP-A and to discover the combined effects and mechanisms in foamcells.Methods: We first induced THP-1cells to become macrophages by PMA. ThenTHP-1macrophages were induced to be foam cells by ox-LDL. The foam cells wereexposed to PAPP-A with different levels（0，10，50，250ng/mL）for24hours orexposed to250ng/mL for increasing periods of time（0，3，6，12，24h）. Ourexperiment also used250ng/mL or together with LXRα agonist22(R)-Hch orLY294002or Raf1kinase inhibitor I (10μM each) to pretreat (3hours) THP-1macrophage derived foam cells for24hours. Cellular lipid accumulation wasdetermined by high performance liquid chromatography analysis. Cholesterol effluxwas determined by FJ-2107P type liquid scintillator. ABCA1, ABCG1, SR-B1, LXRα,IGF-1and IGFBP-5mRNA were measured by real-time quantitative PCR(RT-PCR).Western blotting was used to determine protein levels for ABCA1, ABCG1, LXRαIGF-1R, IGFBP-5, phosphorylated Akt and PI3-K p85subunit. Free IGF-Iconcentrations in the culture medium was estimated with ELISA. Results: PAPP-A inhibits ABCA1, ABCG1, SR-BI expression and reduces cholesterolefflux in THP-1macrophage-derived foam cells; LXRα is involved inPAPP-A-induced down-regulation of ABCA1, ABCG1and SR-BI in THP-1macrophage-derived foam cells; IGF-1/PI3-K/Akt signaling pathway is related todown-regulated expression of LXRα, ABCA1, ABCG1and SR-BI by PAPP-A inTHP-1macrophage-derived foam cells; PAPP-A promotes the IGF-1/PI3-K/Aktsignaling pathway in THP-1macrophage-derived foam cells.Conclusions: PAPP-A may first down-regulate the expression of LXRα through theIGF-1/PI3-K/Akt signaling pathway and then inhibit cholesterol efflux andphospholipids efflux by decrease the expression of ABCA1, ABCG1and SR-BI inTHP-1macrophage-derived foam cells.