Abstract 367: Macrophage 12/15-lipoxygenase Mediates Insulin-like Growth Factor I (IGF-1)-induced Suppression of Foam Cells: Potential Mechanism Responsible for IGF-1-induced Atheroprotection

Abstract

We have shown that IGF-1 infusion into Apoe-null mice suppressed atherosclerosis. To define mechanisms we performed RT-PCR array and found that IGF-1 (0.8 mg/kg, i.p, 7d) reduced aortic 12/15-lipoxygenase (LOX) expression (64±6% decrease vs. saline control, P<0.05). Macrophage LOX is a major pro-atherogenic enzyme that mediates oxidation of native low density lipoprotein (nLDL) and via this mechanism LOX promotes lipid uptake by macrophages (MΦ) and formation of foam cells. IGF-1 infusion into Apoe-null mice (1.5 mg/kg/d, 12 wks on a Western diet) reduced LOX plaque immunopositivity (74±9% decrease, P<0.05), suppressed plaque oxidative stress (64±7% decrease, IHC with 8-oxodG a/b, P<0.05), reduced plaque lipids (35%±6 decrease, Oil Red staining, P<0.05) and decreased atherosclerotic lesion area in aortic valve (IGF-1: 0.259 ± 0.018 mm2, saline: 0.356 ± 0.031 mm2; P<0.05). To further define mechanisms THP-1-derived MΦ were exposed to IGF-1. IGF-1 (50 ng/ml, 16h) reduced LOX mRNA (49±6% decrease, RT-PCR, P<0.05), decreased LOX protein (42±6% decrease, immunoblotting with LOX a/b, P<0.05) and reduced LOX activity (32±5% decrease, 15S-HETE ELISA, P<0.05) and IGF-1 suppressed STAT6 transcription factor binding to LOX gene promoter region (CHIP assay with STAT6 a/b). IGF-1 markedly reduced oxidation of nLDL mediated by THP-1 or mouse peritoneal MΦ (85±6% and 67±5% decrease, respectively, MDA assay). IGF-1 decreased subsequent LDL uptake (i.e. formation of foam cells) mediated by THP-1 or WT peritoneal MΦ (45±8% and 51±3% decrease, respectively, Oil Red staining); however both lipid uptake and IGF-1 effect were decreased in peritoneal MΦ isolated from LOX-null mice (P>0.05). These data indicate that specific LOX downregulation mediates IGF-1-induced suppression of nLDL oxidation and formation of foam cells. In summary, IGF-1 reduced LOX expression in atherosclerotic plaque and this effect correlated with reduction in plaque lipids and in total atherosclerotic burden. IGF-1 downregulated LOX in cultured MΦ via inhibition of STAT6-dependent transcription and IGF-1 decreased lipid oxidation and uptake by MΦ. Taken together these data strongly suggest that MΦ LOX downregulation mediates IGF-1-induced reduction in foam cells and in atherosclerosis.