Abstract
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Objectives Matrix metalloproteinases (MMPs) are expressed in atherosclerosis and play an important role in plaque instability. We established and validated an in vivo imaging approach for monitoring MMP activation in atherosclerotic mouse aorta and used it to assess the response to dietary modification.
Methods Apo E-/- mice (n=38) were placed on a high fat diet (HFD) for 1, 2 or 3 months, or 2 months of HFD followed by 1 month of normal chow. In vivo micro-SPECT/CT imaging using RP782 (an 111In-labeled tracer targeting activated MMPs) was followed by autoradiography and histological analysis.
Results After 3 months of HFD, histological analysis demonstrated considerable plaque development along the aorta. In vivo imaging showed a heterogeneous pattern of RP782 uptake in the aorta, which increased from 1 to 3 months of HFD. Tracer uptake was confirmed by autoradiography, and specificity was demonstrated using 50-fold unlabeled precursor. In-vivo quantification of RP782 uptake strongly correlated with ex-vivo uptake quantified by autoradiography (r=0.68 and p<0.001). Oil red O staining demonstrated the presence of plaques in areas with high or low tracer uptake. There was a significant reduction in MMP activation detected by RP782 imaging 1 month after withdrawal of the HFD (0.36±0.07 versus 0.14±0.05 cpv/MBq injected for 3 months HFD versus withdrawal group, respectively, n=4, p<0.03). MMP expression and macrophage infiltration quantified by QRT-PCR in the aorta was significantly reduced following withdrawal from HFD. Finally, RP782 uptake significantly correlated with regional CD68 (macrophages, r=0.65, p=0.04) but not CD31 (endothelial cells) and smooth muscle α-actin expression in the aorta.
Conclusions Molecular imaging of MMP activation is a useful experimental, and potentially clinical, tool to track plaque biology and response to therapy in atherosclerosis