ABSTRACT
<p><b>BACKGROUND</b>Adipocytes behave like a rich source of pro-inflammatory cytokines including monocyte chemoattractant protein-1 (MCP-1). Oxidized low-density lipoprotein (oxLDL) participates in the local chronic inflammatory response, and high-density lipoprotein could counterbalance the proinflammatory function of oxLDL, but the underlying mechanism is not completely understood. This study aimed to evaluate the effect of apolipoprotein A-I mimetic peptide L-4F on the secretion and expression of MCP-1 in fully differentiated 3T3-L1 adipocytes induced by oxLDL and to elucidate the possible mechanisms.</p><p><b>METHODS</b>Fully differentiated 3T3-L1 adipocytes were incubated in the medium containing various concentration of L-4F (0-50 μg/ml) with oxLDL (50 μg/ml) stimulated, with/without protein kinase A (PKA) inhibitor H-89 (10 μmol/L) preincubated. The concentrations of MCP-1 in the supernatant, the mRNA expression of MCP-1, the levels of CCAAT/enhancer binding protein α (C/EBPα), and CCAAT/enhancer binding protein β (C/EBPβ) were evaluated. The monocyte chemotaxis assay was performed by micropore filter method using a modified Boyden chamber.</p><p><b>RESULTS</b>OxLDL stimulation induced a significant increase of MCP-1 expression and secretion in 3T3-L1 adipocytes, which were inhibited by L-4F preincubation in a dose-dependent manner. PKA inhibitor H-89 markedly reduced the oxLDL-induced MCP-1 expression, but no further decrease was observed when H-89 was used in combination with L-4F (50 μg/ml) (P > 0.05). OxLDL stimulation showed no significant effect on C/EBPα protein level but increased C/EBPβ protein level in a time-dependent manner. H-89 and L-4F both attenuated C/EBPβ protein level in oxLDL-induced 3T3-L1 adipocytes.</p><p><b>CONCLUSIONS</b>OxLDL induces C/EBPβ protein synthesis in a time-dependent manner and enhances MCP-1 secretion and expression in 3T3-L1 adipocytes. L-4F dose-dependently counterbalances the pro-inflammatory effect of oxLDL, and cyclic AMP/PKA-C/EBPβ signaling pathway may participate in it.</p>
Subject(s)
Animals , Humans , Mice , 3T3-L1 Cells , CCAAT-Enhancer-Binding Protein-beta , Physiology , Chemokine CCL2 , Genetics , Bodily Secretions , Cyclic AMP , Physiology , Cyclic AMP-Dependent Protein Kinases , Physiology , Lipoproteins, LDL , Pharmacology , Peptides , Pharmacology , Signal Transduction , PhysiologyABSTRACT
OBJECTIVE@#To explore whether oxidized low-density lipoprotein (ox-LDL) can stimulate the cholesterol efflux in fully differentiated 3T3-L1 cells and the possible mechanism.@*METHODS@#Fully differentiated 3T3-L1 cells were incubated in the medium containing various concentrations of ox-LDL ( 0 to 50 microg/mL) for 8 or 24 hours. 22(R)-Hydroxycholesterol (10 micromol/L) was exposed to preconditioned adipocytes with 25 microg/mL ox-LDL for 24 hours. Reverse transcription polymerase chain reaction (RT-PCR) was used to evaluate ATP binding cassette transporter A1 (ABCA1), scavenger receptor class B type I (SR-BI), and liver X receptor alpha (LXRalpha) mRNA expression. Cholesterol efflux mediated by apolipoprotein A-I (apoA-I) was determined using liquid scintillator.@*RESULTS@#Low levels (12.5-25 microg/mL) of ox-LDL could increase cholesterol efflux via the enhancement of ABCA1 pathway and SR-BI expression, whereas the higher concentration (50 microg/mL) could not. In adipocytes preincubated with 25 microg/mL ox-LDL for 24 hours, 22(R)-hydroxycholesterol could increase ABCA1 and LXRalpha mRNA and apoA-I-mediated cholesterol efflux, but had no effect on the SR-BI mRNA expression.@*CONCLUSION@#Low levels of ox-LDL may enhance the LXRalpha-ABCA1-apoA-I pathway in adipocytes, up-regulate SR-BI mRNA expression, and then increase the cholesterol efflux. This new effect of ox-LDL will not only make contribution to cholesterol homeostasis in adipocytes, but also be potentially atheroprotective.