Abstract 5471: Peroxisome Proliferator-Activated Receptor-γ Activation Reduces Inflammation in Atherosclerosis as Assessed by Serial In Vivo Fluorescence Molecular Imaging
Background: Although indirect evidence suggests that peroxisome proliferator-activated receptor (PPAR)-γ agonists modulate atherosclerosis, their effects have not been directly imaged, tracked, or quantified in vivo. To address this issue, we developed a serial near-infrared fluorescence (NIRF) molecular imaging approach to study pioglitazone (PIO)-induced alterations in matrix metalloproteinase (MMP) activity and macrophage content in apoE−/− mice.
Methods: Cholesterol fed apoE−/− mice (HCD, n=19, 26 weeks) underwent survival multi-wavelength intravital fluorescence microscopy (IVFM#1, resolution 13x13x10 μm) of the right carotid artery. At 24h prior to IVFM#1, mice (n=16) were i.v. co-injected with a MMP-activatable probe (MMPsense680, 200 nmol/kg, ex/em 680/700nm, VisEn Medical) and a macrophage-avid nanoparticle (CLIO-Cy7, 10 mg Fe/kg, ex/em 750/800nm, MGH-CMIR), or saline (n=3). Mice were next randomized to 8 weeks of HCD+0.012% w/w PIO or HCD (n=8 in each group). Mice then underwent IVFM#2 of the same carotid plaque imaged in IVFM#1. Multichannel 3D IVFM stacks were analyzed (ImageJ), and plaque MMP/macrophage target-to-background ratios (TBRs) and plaque areas were measured. The change in TBR (ΔTBR) was = TBR(IVFM#2) -TBR(IVFM#1). Atherosclerotic aortas underwent immunoblotting for MMP-9 and macrophages.
Results: IVFM#1 revealed abundant carotid plaque MMP activity and macrophage NIRF signals (p<0.0001 vs. saline-injected animals). In contrast to HCD, PIO treatment reduced plaque MMP activity (ΔTBR, -0.4±0.4 vs. 1.0±0.5 HCD, p=0.0003) and reduced macrophage content (ΔTBR, −0.5±0.9 vs. 1.1±0.9 HCD, p=0.01). PIO therapy further limited plaque expansion (Δplaque area, 13.6±19.7% vs. 72.6±13.0% HCD, p=0.03). Immunoblots of PIO-treated mice revealed reduced MMP-9 and macrophage aortic signal compared to HCD mice.
Conclusion: PPAR-γ agonist therapy reduces plaque inflammation and limits plaque expansion as assessed by serial in vivo fluorescence molecular imaging. These results support the potential of pioglitazone as an anti-atherosclerotic agent, and provide a foundation for translational fluorescence imaging studies to study unresolved issues regarding PPAR-γ agonists effects on atherosclerosis.