Mouse models of atherosclerosis

Blockade of platelet-derived growth factor or its receptors transiently delays but does not prevent fibrous cap formation in ApoE null mice

Platelet-derived growth factor (PDGF) is a potent stimulant of smooth muscle cell migration and proliferation in culture. To test the role of PDGF in the accumulation of smooth muscle cells in vivo, we evaluated ApoE -/- mice that develop complex lesions of atherosclerosis. Fetal liver cells from PDGF-B-deficient embryos were used to replace the circulating cells of lethally irradiated ApoE -/- mice. One month after transplant, all monocytes in PDGF-B -/- chimeras are of donor origin (lack PDGF), and no PDGF-BB is detected in circulating platelets, primary sources of PDGF in lesions. Although lesion volumes are comparable in the PDGF-B +/+ and -/- chimeras at 35 weeks, lesions in PDGF-B -/- chimeras contain mostly macrophages, appear less mature, and have a reduced frequency of fibrous cap formation as compared with PDGF-B +/+ chimeras. However, after 45 weeks, smooth muscle cell accumulation in fibrous caps is indistinguishable in the two groups. Comparison of elicited peritoneal macrophages by RNase protection assay shows an altered cytokine and cytokine receptor profile in PDGF-B -/- chimeras. ApoE -/- mice were also treated for up to 50 weeks with a PDGF receptor antagonist that blocks all three PDGF receptor dimers. Blockade of the PDGF receptors similarly delays, but does not prevent, accumulation of smooth muscle and fibrous cap formation. Thus, elimination of PDGF-B from circulating cells or blockade of PDGF receptors does not appear sufficient to prevent smooth muscle accumulation in advanced lesions of atherosclerosis.

Peroxisome proliferator-activated receptor-gamma in macrophage lipid homeostasis

Peroxisome proliferator-activated receptor-gamma (PPARgamma), a fatty acid receptor, has received particular attention as the molecular target of insulin-sensitizing drugs, and as a regulator of lipid accumulation by the coronary artery macrophages known as foam cells. Controversial results have been reported regarding the consequences of PPARgamma activation in the inflammatory response, the progression or improvement of the atherosclerotic lesion, and the identity of target tissues (muscle or fat) for PPARgamma-specific antidiabetic drugs. A clear understanding of how PPARgamma functions in each of these processes is therefore necessary to advance its utility as a therapeutic target. Receptor-dependent and -independent actions of PPARgamma agonists have been carefully examined with a combination of Pparg-knockout mice, PPARgamma-null embryonic stem cells, PPARgamma-specific drugs, and mouse models of atherosclerosis. Through those combined studies, a physiological and therapeutic role for PPARgamma in lipid management by the macrophage has emerged.

Repeated Three-Dimensional Magnetic Resonance Imaging of Atherosclerosis Development in Innominate Arteries of Low-Density Lipoprotein Receptor-Knockout Mice

Background— In vivo methods to evaluate the size and composition of atherosclerotic lesions in animal models of atherosclerosis would assist in the testing of antiatherosclerotic drugs. We have developed an MRI method of detecting atherosclerotic plaque in the major vessels at the base of the heart in low-density lipoprotein (LDL) receptor-knockout (LDLR −/− ) mice on a high-fat diet.

Methods and Results— Three-dimensional fast spin-echo magnetic resonance images were acquired at 7 T by use of cardiac and respiratory triggering, with ≈140-μm isotropic resolution, over 30 minutes. Comparison of normal and fat-suppressed images from female LDLR −/− mice 1 week before and 8 and 12 weeks after the transfer to a high-fat diet allowed visualization and quantification of plaque development in the innominate artery in vivo. Plaque mean cross-sectional area was significantly greater at week 12 in the LDLR −/− mice (0.14±0.086 mm 2 [mean±SD]) than in wild-type control mice on a normal diet (0.017±0.031 mm 2 , P <0.01). In the LDLR −/− mice, but not control mice, increase in plaque burden at week 12 relative to week 1 was also highly significant ( P =0.001). Lumen cross section was not significantly different between time points or groups. MRI and histological assessments of plaque size were closely correlated ( R =0.8). The lumen of proximal coronary arteries could also be visualized.

Conclusions— This is the first report of in vivo detection of aortic arch atherosclerosis in any animal model. The method could significantly assist rapid evaluation of experimental antiatherosclerotic therapies.

Understanding atherosclerosis through mouse genetics

During the past year studies with mouse models have significantly clarified our understanding of atherosclerosis. Noteworthy achievements include: the discovery of a number of novel genes and pathways; new evidence emphasizing the role of lymphocytes in atherogenesis; the development of mouse models exhibiting advanced lesions with evidence of thrombosis; and new results indicating an anti-atherogenic effect of testosterone.

Inflammatory pathways in atherosclerosis and acute coronary syndromes

Evidence from a broad range of studies demonstrates that atherosclerosis is a chronic disease that, from its origins to its ultimate complications, involves inflammatory cells (T cells, monocytes, macrophages), inflammatory proteins (cytokines, chemokines), and inflammatory responses from vascular cells (endothelial cell expression of adhesion molecules). Investigators have identified a variety of proteins whose levels might predict cardiovascular risk. Of these candidates, C-reactive protein, tumor necrosis factor-alpha, and interleukin-6 have been most widely studied. There is also the prospect of inflammation as a therapeutic target, with investigators currently debating to what extent the decrease in cardiovascular risk seen with statins, angiotensin-converting enzyme inhibitors, and peroxisome proliferator-activated receptor ligands derives from changes in inflammatory parameters. These advances in basic and clinical science have placed us on a threshold of a new era in cardiovascular medicine.