Abstract 5276: Insulin-like Growth Factor I Downregulates Vascular Smooth Muscle and Macrophage 12/15-Lipoxygenase Expression: Potential Mechanism for Igf-1-induced Atheroprotection.
We have shown that IGF-1 infusion in ApoE−/− mice reduces atherosclerosis. To define mechanisms we performed RT-PCR gene arrays and found that IGF-1 (1.8 mg/kg/d, i.p., 1w, n=6) markedly reduces aortic 12/15-lipoxygenase (LOX) expression (64±6% decrease, P<0.05). LOX plays a major role in oxidation of native LDL to its pro-atherogenic form (OxLDL) and is expressed in vascular smooth muscle cells (SMC) and macrophages (MΦ). DHE staining in situ revealed increased superoxides in atherosclerotic plaque of ApoE−/− mice (77±6% increase vs. adjacent media, P<0.05) correlating with increased MΦ infiltration and LOX expression. A LOX-specific inhibitor baicalin (50 μM) dramatically suppressed plaque superoxides (59±8% inhibition, P<0.01), consistent with LOX being a major source of superoxides. IGF-1 infusion in ApoE−/− mice (1.5 mg/kg/d, 12 w, western diet, n=12) markedly reduced aortic plaque LOX immunopositivity (74±9% decrease, P<0.05), superoxide levels (84±7%, P<0.05) and atherosclerotic lesion size (IGF-1: 0.259±0.018 μm2 saline: 0.356±0.331 μm2, 28% decrease, P<0.05). To obtain insight into the relative role of SMC vs. MΦ LOX in IGF-1 induced atheroprotection we generated SMP8/ApoE−/− mice in which IGF-1 is overexpressed in SMC. SMP8/ApoE−/− mice (western diet, 12w) had decreased aortic LOX expression (71% decrease vs. ApoE−/− mice, P<0.05, n=5) but no reduction in atherosclerosis suggesting that the ability of increased circulating IGF-1 to reduce atherosclerosis was primarily mediated via its effect on MΦ LOX. To further define mechanisms and to model in vivo conditions, SMC or THP-1 derived MΦ were exposed to OxLDL (80ug/mL, 16h) with/without IGF-1. OxLDL markedly increased LOX activity in MΦ and SMC (18-fold vs. nLDL), however, pre-treatment with IGF-1 completely abolished OxLDL-induced LOX activation only in MΦ. In summary, IGF-1 infusion markedly reduces plaque LOX levels and decreases atherosclerotic plaque burden, but SMC-specific IGF-1 over expression reduces LOX but does not alter atherosclerosis. Taken together with our finding that IGF-1 markedly inhibits OxLDL-induced LOX activation in MΦ in vitro our data suggests that IGF-1 downregulation of MΦ LOX activity plays an important role in reducing atherosclerotic plaque burden.