Experimental Studies
Because the predominance of AngII effects in mice appears to be mediated via AT1a receptors, most of the genetic studies have used mice that are depleted of this receptor (Table
2). There has been a great consistency in the marked reduction in atherosclerosis in AT1a receptor -/- mice that are hypercholesterolemic due to deficiency of either apoE or LDL receptors combined with diet enriched in saturated fat [
11,
27‐
30]. AT1a receptor deficiency also completely attenuated atherosclerosis in LDL receptor -/- mice augmented by AngII infusion [
31,
32]. Furthermore, AT1a receptor deficiency also reduced atherosclerosis in diabetic apoE -/- mice [
33]. The reduced atherosclerosis in AT1a receptor deficient mice has been consistent across all reports, in both genders, at all ages studied, and in different vascular beds that have been studied. The atherosclerosis literature is replete with conflicting data for the effect of genetic manipulations. Therefore, the consistency of the literature on AT1a receptor deficiency reducing atherosclerosis is both unusual and noteworthy.
Table 2
Effects of genetic manipulations of angiotensin II receptors on development of atherosclerosis in mice
Whole body deficiency |
AT1a | LDL receptor -/- | Male and female | Decrease | |
| apoE -/- | Male | Decrease | |
| | | Decrease | |
| | | Decrease | |
| | | Decrease | |
| Diabetic apoE-/- | | Decrease | |
AT2 | LDL receptor -/- | Male and female | No effect | |
| apoE-/- | Male | Increase | |
| | | No effect on size, but composition change | |
| Diabetic apoE-/- | | Decrease | |
Bone marrow transplantation |
AT1a | LDL receptor -/-; AngII infused | Male | No effect in AT1aR -/-; modest decrease in +/+ | |
| apoE -/-; AngII infused | | No effect in AT1aR -/-; modest decrease in +/+ | |
| LDL receptor -/- | | No effect | |
| apoE -/- | | Decrease | |
| | | Decrease | |
| | | Increase | |
Because most of the atherosclerosis mouse models are generated by dysfunctional cholesterol metabolism, it is surprising that deletion of AT1a receptors has such a large and consistent effect on lesion reduction. The resolution of this apparent paradox may be that the hypercholesterolemic state is associated with a marked augmentation of the RAS in terms of circulating proteins involved in the synthesis and secretion of angiotensin peptides [
11]. The protective effect of AT1a receptor deficiency has been associated with changes in several measurements, including decreased oxidant stress and metalloproteinase expression and enhanced endothelial function. However, it has not been defined if these changes are a cause or consequence of the decreased atherosclerosis [
27,
29]. Blood pressure changes have also been inconsistent in hypercholesterolemic AT1a receptor–deficient mice [
11,
27]. Nevertheless, blood pressure per se does not appear to be a major factor in the development of atherosclerosis in mouse models of the disease [
13].
The effect of AT1a receptors on atherosclerosis has also been studied using bone marrow transplantation. It has primarily been used to define the role of AT1a receptor expression on leukocytes that infiltrate during lesion development, although this technique has the potential to also influence other cell types that originate from bone marrow. Unlike the uniform demonstration of whole body AT1a receptor deficiency decreasing atherosclerosis, the literature using bone marrow transplantation has produced inconsistent results. AT1a receptor deficiency in bone marrow–derived cells has been demonstrated to attenuate [
32,
34], have no effect [
7,
31,
35], or increase atherosclerosis [
36] in hypercholesterolemic mice, either induced by hypercholesterolemia alone or augmented with AngII infusion. The basis for this inconsistency is not readily apparent. Another approach to determine the role of the AT1a receptor expression in specific cell types is the use of Cre-lox technology to develop deficiencies that can be both cell specific and inducible at specific time intervals. However, there are currently no reports that have used conditional genetic deficiencies of AT1a receptors to define the cellular components that are stimulated by AngII or hypercholesterolemia to augment atherosclerosis.
As described above, AngII stimulation of AT2 receptors is commonly considered to exert opposite effects to AngII stimulation of AT1 receptors. Contrary to the consistent reductions of atherosclerosis in AT1a receptor–deficient mice, genetically engineered deficiency of AT2 receptors has generated a range of responses. The changes of atherosclerosis in hypercholesterolemic AT2 receptor–deficient mice cover the entire spectrum of decreases [
37], no effect [
11], no effect on size but changes in composition [
38], or increases [
39] in lesion formation.
Overall, the effects of the genetically engineered whole body deficiency of AT1a receptors have been both striking in the magnitude of the effects and consistent in reports from many laboratories. This contrasts sharply with the literature to define the cellular location of the AT1a receptors involved in the development of atherosclerosis. It also contrasts sharply with genetic studies on AT2 receptors. There is no obvious explanation for this inconsistent literature.
Human Studies
The relevance of angiotensin receptors in atherosclerosis has been observed in humans in a limited number of gene association human studies. As noted above, the AT1 receptor located on chromosome 3 is thought to mediate most of the effects of AngII [
40]. One of the most common genetic variants is the A1166C SNP located in the 3’ UTR region of the AT1 receptor gene. The CC mutant genotype has been associated with increased risk of myocardial infarction, coronary artery stenosis, and carotid atherosclerosis [
41,
42]. This polymorphism was also associated with increased total plasma cholesterol concentrations and influenced the risk for essential hypertension [
43]. In Chinese men, the combined effects of all functional genetic variants present in this gene were associated with increased risk of myocardial infarction [
44]. In addition, combination of genetic variants of ACE I/D along with AT1 receptor A1166C polymorphism have been predicted as powerful determinants for atherosclerosis. Conversely, numerous studies have also demonstrated no association of this polymorphism with atherosclerosis [
45‐
47]. Another polymorphism in exon 5 of the AT1 receptor gene, which is conversion of C to T at 537 position, has been found to be not associated with atherosclerosis [
48].
The AT2 receptor gene is located on the X chromosome. A common polymorphism of AT2 receptors, -1332 G/A, is present in the promoter region but has not been found to be associated with premature coronary artery disease [
49]. Recently, another polymorphism in the AT2 receptor gene has been reported, which is A1675G located in intron 1. This genetic variant has been associated with the increased severity of atherosclerosis, especially in hypertensive males [
50]. Collectively, there have been inconsistent reports obtained from population-based studies. As with the gene association studies in proteins of angiotensin peptide synthesis, they have yielded relatively little to providing insight into the pathology of atherosclerosis.