title: CCAAT/enhancer-binding protein-β participates in oxidized LDL-enhanced proliferation in 3T3-L1 cells.
abstract: Increased circulating oxidized LDL (oxLDL) have been found in obese subjects. Obesity is characterized by an excess of fat mass resulting from an increase in adipocyte number and size. The generation of new adipocytes is a tightly controlled process where multiple factors acting in a signaling cascade follow a precise temporal expression pattern; oxLDL appear to have a role in the impairment of this process. The purpose of this study was to examine the effects of oxLDL on the mechanisms involved in the proliferative stage of the differentiation process in 3T3-L1 cells. After hormonal induction, 3T3-L1 cells undergo approximately two rounds of mitotic clonal expansion (MCE), a process required for adipogenesis. CCAAT/enhancer-binding protein β (C/EBPβ) is immediately expressed after induction, and plays a crucial role in MCE, but its expression must decrease to allow preadipocytes to mature into adipocytes. We found that, in the presence of stimuli to differentiate, oxLDL induced a higher proliferation rate in this cell line, associated with a sustained up-regulation of C/EBPβ, which remained activated inside the nucleus for several days. RNAi-mediated knockdown of C/EBPβ 24 h after oxLDL treatment counteracted the increase in proliferation rate. Both C/EBPβ expression and proliferation processes appear to be influenced by cAMP/protein kinase A (PKA) and extracellular signal-regulated kinases1/2 (ERK1/2) pathways. OxLDL treatment led to increased levels of cAMP, and to a strong, prolonged phosphorylation of ERK1/2 and C/EBPβ. The addition of cAMP and PKA inhibitors, SQ22536 and H-89, respectively, reduced proliferation only in oxLDL-treated cells, whereas the addition of ERK1/2 inhibitor U0126 blocked proliferation in both control and oxLDL-treated cells. C/EBPβ nuclear expression and DNA-binding activity were reduced by U0126, under all tested conditions. These findings show that the altered expression pattern of C/EBPβ is involved in the increase in the number of proliferating cells induced by oxLDL, in hormone-stimulated 3T3-L1 cells.
authors.affiliation: Department of Veterinary Public Health and Food Safety, Italian National Institute of Health, Istituto Superiore di Sanità, Viale Regina Elena 299, 00161 Rome, Italy. c.santan@iss.it
authors.forename: Carmela
authors.initials: C
authors.lastname: Santangelo
authors.forename: Rosaria
authors.initials: R
authors.lastname: Varì
authors.forename: Beatrice
authors.initials: B
authors.lastname: Scazzocchio
authors.forename: Carmelina
authors.initials: C
authors.lastname: Filesi
authors.forename: Massimo
authors.initials: M
authors.lastname: D'Archivio
authors.forename: Claudio
authors.initials: C
authors.lastname: Giovannini
authors.forename: Roberta
authors.initials: R
authors.lastname: Masella
mesh_terms: D041721:3T3-L1 Cells; D017667:Adipocytes / Q000166:cytology / Q000378:metabolism; D000818:Animals; D022782:CCAAT-Enhancer-Binding Protein-beta / Q000378:metabolism*; D049109:Cell Proliferation; D000242:Cyclic AMP / Q000378:metabolism; D017868:Cyclic AMP-Dependent Protein Kinases / Q000235:genetics / Q000378:metabolism; D006801:Humans; D008077:Lipoproteins, LDL / Q000097:blood / Q000378:metabolism*; D051379:Mice; D048052:Mitogen-Activated Protein Kinase 3 / Q000235:genetics / Q000378:metabolism; D008938:Mitosis; D034622:RNA Interference
chemical_list: D022782:CCAAT-Enhancer-Binding Protein-beta; D008077:Lipoproteins, LDL; C099252:oxidized low density lipoprotein; D000242:Cyclic AMP; D017868:Cyclic AMP-Dependent Protein Kinases; D048052:Mitogen-Activated Protein Kinase 3