In this study, we clearly demonstrate a significant association between AGE/RAGE and aneurysm formation in humans as demonstrated by the presence of these two proteins in human aneurysmal tissues. To determine cause and effect, we employed a mouse model of AAA produced by the infusion of angiotensin II in ApoE deficient mice. We were able to demonstrate a markedly reduced incidence of AAA in mice that were lacking the gene for RAGE. To our knowledge, this is the first study to report an association between AGE/RAGE and aneurysms. Our findings suggest a novel association between AGE and RAGE and the pathogenesis of this important and lethal condition.
It is well known that the degradation of AGE is diminished with aging which in turn results in its tissue accumulation25
. Indeed, several studies have reported that AGE and RAGE are involved in age related diseases such as Alzheimer’s, diabetes and atherosclerosis26, 27
. AGE has a number of actions that contribute to its provocative effects on disease including effects on extracellular matrix as well as its ability to induce inflammation and the generation of reactive oxygen species.
Likely the most prevalent risk factor for aneurysms is age. Aneurysms become increasingly prevalent with age with the incidence in the 9th
decade being almost 10 times that in the 6th
decade. Thus, it is a plausible hypothesis that AGE and RAGE might play a role in the development of AAA. Previous investigators have reported that RAGE and AGE are involved in other vascular diseases specifically restenosis and atherosclerosis. In fact our laboratory has demonstrated an important role for RAGE in restenosis17, 28, 29
. However, over the past several years it has become clear that aneurysms once thought to be a form of atherosclerosis, have a pathogenesis that is distinct from that of occlusive vascular disease.
We chose in our initials studies of RAGE to employ a model that uses angiotensin-II to induce the development of aneurysms. This is a unique model that is characterized by the initial formation of dissection followed by dilatation of the supera renal aorta. Interestingly, this model may more closely parallel in humans, aortic dissection which produces aneurysms secondary to a weakened dissected wall; a process that may differ from the one that predisposes to the traditional infrarenal aneurysm in humans. There are two additional animal models of AAA, one resulting from the external application of calcium chloride and a second that is produced by the intraluminal infusion of elastase. These models are similar in that in all three, inflammation is essential. However, the differences are sufficiently great that the role of RAGE in the formation of AAA should confirmed using conditions that more precisely model those that produce the traditional infrarenal aneurysm.
The accumulation of natural RAGE ligands, such as AGE as well as the HMGB-1 and S100A12 polypeptides, has been associated with the activation of inflammatory cells and the production of proinflammatory mediators30
. Amongst these ligand-RAGE effects are the cellular activation of signal transduction cascades that generate reactive oxygen species which in turn trigger inflamation31
. Multiple intracellular pathways including the MAP kinases and PI3 kinase have been reported to be activated by RAGE32
. Activation of these signal transduction pathways in turn turns on the redox-sensitive transcription nuclear factor NK-κB31
, which in turn leads to gene expression of pro-inflammatory cytokines and leukocyte adherence molecules33
Extracellular matrix (ECM) proteins, such as collagens, elastin and proteoglycans, are important structural components in arteries34
. Degradation of these structural components by matrix metalloproteinases (MMPs), such as MMP-2 and MMP-9, has been reported as important contributor to the pathogenesis of AAA in various animal models of AAA35, 36
and in humans2, 37
. Moreover, systemic administration of MMP antagonists can decrease aneurysmal dilatation in the elastase-induced rodent model of AAA38
. There is preliminary data in humans that that treatment of patients with small aneurysms with the MMP inhibitor, doxycycline, may decrease the growth rate of aneurysms39
. In the current study, we demonstrate that AGE is a potent inducer of MMP-9 production in macrophages in vitro. Moreover, the formation of aneurysms in the angiotensin model is associated with enhanced expression of MMP-9 and this enhanced expression is eliminated in the RAGE knockout model. It is likely that AGE/RAGE at least in part affects the formation of aneurysms via the induction of MMP-9. As a possible mechanism for AGEs’ effect on MMP-9, it has been demonstrated that AGE/RAGE induces NF-κB activation through a variety of signal transduction pathways40–42
. The NF-κB in turn, through activation of a protein 1 (AP-1) binding site in the MMP-9 promoter, is essential for activation of MMP-9 gene43
. Nevertheless, the precise mechanism of AGE/RAGE induced MMP-9 needs to be further studied.
The soluble form of RAGE (sRAGE) can potentially bind to an AGE ligand thereby acting as a decoy, preventing the AGE–RAGE interaction and RAGE activation44
. Administration of a recombinant soluble form of RAGE (sRAGE) consisting of the extracellular ligand-binding domain has been shown not only to suppress the development of atherosclerosis, but also to stabilize established atherosclerosis in diabetic apolipoprotein E null mice45, 46
. Inhibition of the AGE/RAGE interaction opens new possibilities for the treatment of small aneurysms. An unanswered question is the timing of therapy. Animal models allow institution of preventative therapies at the inception of the processes that lead to aortic dilatation. However, most aneurysms are discovered after aortic dilatation has already occurred and presumably many years after the inciting events. Whether RAGE inhibitors will be useful at the later stages of AAA development remains to be determined. Encouraging is the fact that increased levels of RAGE remain present in the tissue of the relatively large aneurysms sampled in this study and inflammation also appears to play an important role in aneurysms of all sizes.
In summary, our results suggest that AGE and RAGE are involved in the development of AAAs. These findings contribute to a better understanding of the mechanisms underlying the pathogenesis of AAAs. Manipulation of the interaction between AGE and RAGE may ultimately lead to novel therapies to treat and prevent AAA progression.