Lipid lowering and imaging protease activation in atherosclerosis

Abstract

Background

Lipid lowering is a mainstay of modern therapeutic approach to atherosclerosis. We sought to evaluate matrix metalloproteinase (MMP)-targeted microSPECT imaging for tracking of the effect of lipid-lowering interventions on plaque biology in atherosclerotic mice in vivo.

Methods and Results

ApoE^−/− mice fed on a high fat diet (HFD) for 2 months were randomly assigned to continuation of HFD, HFD plus simvastatin, HFD plus fenofibrate and high fat withdrawal (HFW). The animals underwent serial microSPECT/CT imaging using RP805, a ^99mTc-labeled MMP-targeted tracer at 1 and 4 weeks after randomization. All three interventions reduced total blood cholesterol by 4 weeks. In animals on HFD, aortic arch RP805 uptake significantly increased from 1 week to 4 weeks. Tracer uptake in fenofibrate and HFW groups was significantly lower than uptake in the HFD group at 4 weeks. Similarly, CD 68 gene expression, reflecting plaque inflammation, was significantly lower in fenofibrate and HFW groups compared to HFD group. MMP tracer uptake significantly correlated with aortic CD68, but not VE-cadherin or smooth muscle α-actin expression.

Conclusions

MMP tracer uptake paralleled the effect of lipid-lowering interventions on plaque inflammation in atherosclerotic mice. MMP-targeted imaging may be used to track the effect of therapeutic interventions in atherosclerosis.

Introduction

Lipid-lowering, in conjunction with anti-platelet and anti-hypertensive therapies are the mainstay of modern therapeutic approach to atherosclerosis.¹ Part of the beneficial effects of lipid-lowering therapies, such as statins, may be linked to stabilizing effects on plaque biology rather than plaque regression.² Inflammation is a key determinant of plaque vulnerability. Lipids, such as modified LDL, promote the recruitment and activation of inflammatory cells and trigger vessel wall inflammation in atherosclerosis.³ Macrophages (as well as other vascular cells) produce a number of proteases, including members of the matrix metalloproteinase (MMP) family.⁴ Activation of these proteases promotes further recruitment of inflammatory cells and directly contributes to plaque rupture through weakening of the fibrous cap.⁵ Some lipid lowering interventions, especially using statins, reduce vascular events linked to plaque rupture. However, there is little direct information on the effect of these interventions on vessel wall inflammation. More specifically, it is unclear whether lipid lowering, independent of the agent used, is associated with changes in MMP activation in atherosclerosis.

Molecular imaging provides an opportunity to evaluate regional and focal changes in vessel wall biology in vivo. As MMPs mediate vessel wall inflammation in atherosclerosis, MMP-targeted imaging may be a useful tool for assessing inflammation in atherosclerosis.6, 7, 8 Indeed, there is a correlation between macrophage marker expression and MMP activation detected by molecular imaging in atherosclerosis.^9,10 Whether this relation persists in the presence of various therapeutic interventions, and hence, MMP-imaging can be used for tracking the effect of specific therapeutic interventions remains to be determined. Here, we investigate whether lipid-lowering, regardless of the agent used, leads to a reduction in plaque inflammation and MMP activation in parallel with changes in blood cholesterol level; and whether the effect of various lipid lowering interventions on plaque inflammation in atherosclerotic mice can be tracked through non-invasive imaging of MMP activation.