MMPs and ADAMs/ADAMTS Inhibition Therapy of Abdominal Aortic Aneurysm

Abstract

Abdominal aortic aneurysm (AAA) is a chronic vascular degenerative disease featured by progressive dilation and remodeling of the vascular wall, which may lead to aortic rupture and high mortality. The occurrence and development of AAA involve multiple mechanisms, including extracellular matrix degradation, chronic inflammation, oxidative stress, apoptosis of vascular smooth muscle cells and innate immunity. Extracellular matrix degradation is considered as the most important mechanism causing AAA. Matrix metalloproteinases (MMPs) are key factors in this process, contributing greatly to the occurrence and development of AAA. But whether the zinc-dependent endopeptidases (ADAM/ADAMTS) are involved in this process is very little known. This study is a review about the role of MMPs and ADAM/ADAMT as well as the existing MMP inhibitors in abdominal aortic aneurysm, with the purpose of providing reference for the clinical treatment of abdominal aortic aneurysm.

Graphical abstract

1. Introduction

Abdominal aortic aneurysm (AAA), the most common true arterial aneurysm, is defined as a segmental dilation of abdominal aorta that is 50% greater than the normal aortic diameter at the level of the renal arteries. In most adults, abdominal aorta with diameter above 30mm is considered as aneurysm[1]. This disease is usually asymptomatic in clinic, with permanent dilation of all layers of the abdominal aortic wall. AAA has complex etiology and hidden course of disease, and its incidence is higher in the elderly people. The mortality following aneurysm rupture can be as high as 50%−80%[2]. Surgical treatment is the preferred choice for AAA with a diameter of 55mm and above or dilation speed >10 mm/y; but for those with a diameter below 55mm, no internal medicine treatment is available yet. The occurrence of AAA is closely related to the extracellular matrix (ECM) remodeling, apoptosis of vascular smooth muscle cells (VSMCs), inflammation, oxidative stress, and innate immunity[3–5]. ECM remodeling is one of the major pathological changes of AAA, where extracellular proteases degrading the elastin and collagen play a crucial role. Matrix metalloproteinases (MMPs) and ADAM / ADAMTS are two types of proteases degrading ECM. In physiological condition, ECM remodeling contributed by MMPs and ADAM / ADAMTS is required in angiogenesis to allow endothelial cells migrate, whereas in AAA, both MMPs and ADAM / ADAMTS promote the occurrence and development of AAA by degrading the elastic and collagen in the aortic media[6, 7]. Starting from this perspective, efforts have been underway to develop drugs that can inhibit MMPs and ADAM / ADAMTS generation or activity, so as to block the occurrence and development of AAA and to provide more medication options for clinic. This study is a review about the MMPs and ADAM/ADAMTS inhibitors, with the purpose of providing reference for clinical medication and research.

2. Literature screening

Literature reports were searched in the PubMed, Web of Science and other databases. The search words used were “abdominal aortic aneurysm”, “MMP/drug/inhibitor”, “ADAMS”, “ADAMTS/drug/inhibitor”, “ADAMs/drug/inhibitor”, “traditional Chinese medicine”, “herbal medicine” and “herbal formulation”, either searched alone or in combination. Then a review was performed for the researched literature.

3. MMPs

3.1. An introduction to MMPs

MMPs are a group of zinc- and calcium-dependent endopeptidases, which can degrade ECM and basement membrane components and participate in such pathophysiological processes as inflammatory response, cardiovascular diseases and tumor infiltration and metastases[8]. MMPs are composed of lyophobic signal peptide sequence, propeptide domain, catalytic domain, hinge region and carboxyl terminal region (see Fig. 1). The newly formed peptide enters the cytoplasm via the signal peptide sequence. The cysteine in the propeptide binds to the zinc ion, thus maintaining the stability of MMP zymogen; histidine at the active site of the catalytic domain binds to the zinc ion; the catalytic region in the hinge region binds to the carboxyl terminal region, which is related to the specificity of enzymatic substrate and binds to the endogenous inhibitor to promote activation or inhibition of various MMPs. So far 28 MMPs have been identified. Based on substrate specificity and sequence similarity of MMPs, MMPs are divided into 6 types: collagenases, gelatinases, Matrilysins, stromelysins, membrane type MMPs (MT-MMP) and other MMPs (Table 1). Given the important role of MMPs in ECM remodeling, MMPs exist in large abundance in the majority of connective tissues. MMPs in the aortic wall are mainly derived from the macrophages, monocytes and medial SMCs. MMPs plays the key role in tissue remodeling by promoting the degradation of various ECMs (including collagen, gelatin, proteoglycan and other matrix glycoproteins. Collagen and elastin play a key role in the structural integrity of vascular wall and serve as important substrates for MMPs. MMP can degrade type I—X and type XIV collagen and other ECM substrates (e.g., fibronectin, laminin and elastin) with different efficiency. This further leads to disruption of dynamic balance between ECM synthesis and degradation, aortic wall remodeling, aortic wall dilation and finally the formation of aneurysm. MMPs cause damage to the cell-cell adhesion and cell-ECM adhesion, degrading ECM, promoting angiogenesis and thereby promoting cancer invasion and metastases[9, 10]. Apparently, MMPs play an important role in the occurrence, development and metastasis of AAA. Existing studies have shown that MMP-2, MMP-3, MMP-9 and MMP-12 are closely related to AAA.

Fig. 1.

Structural diagram of MMPs. MMPs are composed of signal peptide sequence, propeptide domain, catalytic domain, hinge region and carboxyl terminal region. The propeptide domain contains a cysteine switch, and the cysteine chelates with the active site Zn²⁺ to maintain stability of the MMP zymogen. At the active core of the catalytic domain, three histidines bind to Zn²⁺. Some MMPs have special structures. For example, gelatinases contain 3 types of II fibronectin repeat sequences in the catalytic domain. Matrilysins contain neither the hinge region, nor the carboxyl terminal region. Stromelysins contain a furin recognition region in propeptide domain. MT-MMPs have a membrane anchor region and also a furin recognition region.

Table 1.

Members of MMP family and corresponding substrates

Type	MMP	Collagen substrate	Non-collagen ECM substrate
Collagenases	MMP-1, MMP-8, MMP-13 and MMP-18	Type I, II and III gelatin	Fibronectin, laminin
Gelatinases	MMP-2, MMP-9	Type IV, V and VII gelatin, type X collagen	Fibronectin, laminin, elastin
Matrilysins	MMP-7, MMP-26	Type IV collagen, gelatin	Fibronectin
Stromelysins	MMP-3, MMP-10 and MMP-11	Type II, III, IV and IX collagen, gelatin	Fibronectin, laminin
MT-MMPs	MMP-14, MMP-15, MMP-16, MM-17, MMP-24, MMP-25	Gelatin, type I, II and III collagen	Fibronectin, laminin, elastin
Other	MMP-12, MMP-19, MMP-20, MMP-21, MMP-22, MMP-23, MMP-27, MMP-28

3.1.1. The role of MMP-2 and MMP-9 in AAA

MMP-2 and −9 share a similar structure with other proteases in the MMP family, except for a unique collagen-binding domain. For a long time, MMP-2 and −9 were considered as the two most important MMPs in ECM degradation and also key factors in the formation and development of AAA.

MMP-2 is mainly expressed by the mesenchymal stromal cells. Fibroblasts can also produce a small amount of MMP-2. The major sources of the fibroblasts are found in the media and extima of the aortic wall, promoting the degradation of elastin and collagen and leading to structural damage of the aortic wall with AAA[11]. VSMCs are key factors damaging ECM in the arterial tissues. The apoptosis of VSMCs and the release of proteolytic enzyme MMP-2 can accelerate the degradation of ECM, aggravating inflammation and promoting AAA. proMMP-2 accumulates on cell surface, and under the action of MT1-MMP/TIMP-2 complex, proMMP-2 will undergo autocatalysis. As a result, the proMMP-2 accumulating on cell surface promotes local dissolution of collagen. TIMP-2 inhibits MMP-2, while MT1-MMP activates MMP-2. Thus, the synergism between the two regulates the expression and activity of MMP-2[12, 13]. It has been shown that MMP-2 is not expressed in the normal abdominal aorta, but it is upregulated in the AAA tissues. Moreover, the MMP-2 expressions in the small- and medium-sized AAA are higher than those in the larger-size and ruptured AAA. This indicates that the formation and early expansion of AAA are closely related to MMP-2^[13].

MMP-9 is a multi-functional protease, involved in ECM degradation and aortic wall weakening[14, 15]. MMP-9 is mainly produced by SMCs and macrophages in the media. Under normal condition, SMCs will not produce MMP-9; but when the aortic wall is challenged by inflammatory stimuli, MMP-9 will be expressed excessively[16, 17]. Consequently, the elastin in the media of elastic wall is damaged, which further leads to AAA. The degradation products of elastic promote leukocyte chemotaxis, and therefore, the leukocytes will continuously infiltrate the tissues. If the MMP-9 expression persists, the elastin will be further damaged, which finally results in the rupture of AAA. Given the above facts, MMP-9 is closely related to the continuous expansion and rupture of AAA. Studies have also shown that as the MMP-9 zymogen level in AAA increases, the activity of proteolytic enzyme also increases, and so does the risk of AAA rupture[18, 19]. After surgery, a persistently high MMP-9 level in AAA may predict continuous development of the repaired AAA or the risk of leakage[20, 21]. Changes in the MMP-9 level can be used as an important basis for early diagnosis and postoperative efficacy assessment in AAA.

Synergism between MMP-9 and MMP-2 plays a key role in the dilation and rupture of AAA[22]. Other studies have shown that MMP-2 and MMP-9 can activate the tumor necrosis factor precursor and the transforming growth factor, and participate in tumor cell growth, invasion and metastases[23]. Both are candidate targets for AAA treatment.

3.1.2. The role of MMP-3 in AAA

MMP-3 is a kind of stromelysin with some unique structural features. MMP-3 still maintains the function of a protease after the zinc and nickle ions are replaced. MMP-3 is an ECM-degrading secretory endopeptidase that can degrade type II, IV and IX collagen, fibronectin and laminin, while activating other MMPs (e.g., MMP-1, −7 and −9) and enhancing elastin degradation[24, 25]. It has been found that the MMP-3 expression is significantly upregulated in AAA as compared with the normal arterial tissues. In particular, the MMP-3 expression is enhanced in the region infiltrated by the inflammatory cells, suggesting that the inflammatory cells can produce MMP-3 or increase the MMP-3 expression^[26]. MMP-3 plays a role in the occurrence and development of AAA as an initiating factor, thus promoting inflammatory cell infiltration and release of inflammatory factors. Moreover, MMP-3 can activate other MMPs, but few studies have been conducted on the working mechanism of MMP-3 in AAA.

3.1.3. The role of MMP-12 in AAA

MMP-12, or macrophage elastase, is highly expressed by macrophages and other stromal cells. It can degrade elastin and plays an important role in the migration of macrophages[8]. MMP-12 is upregulated in AAA and mainly found in the residual fragment of elastin in the media of adjacent, non-dilated aortic segment. It plays a key role in the continuous expansion of AAA. It has been shown that MMP-12 downregulation can inhibit the development of AAA, which may be achieved through the inhibition of macrophage aggregation[26].

4. MMP inhibitors

MMP inhibitors (MMPIs) have been developed in recent years, and some have even entered the stage of clinical trial. By origin, MMPIs are divided into 3 categories: (1) Endogenous inhibitors, such as tissue inhibitors of MMPs (TIMPs) and α2-macroglobulin; (2) synthetic inhibitors, such as batimastat; (3) inhibitors isolated from natural products, such as tetracyclines.

4.1. Endogenous inhibitors

4.1.1. MMP inhibitors

TIMPs are endogenous specific inhibitors of MMPs with a potent inhibitory effect. So far, 4 types of TIMPs (TIMP-1, −2, −3 and −4) have been identified. TIMP-1, 2 and 4 are soluble secretory proteins, and TIMP-3 is ECM-bound insoluble protein. Although TIMPs are wide-spectrum inhibitors of MMPs, different TIMPs selectively inhibit different MMPs. TIMP-1 exhibits a good inhibitory effect on most MMPs except for the activated MMP-1, 3 and 9 and TM-MMP (e.g., MMP-14, MMP-16 and MMP-24). TIMP-2 mainly inhibits the activity of MMP-2 and can block the hydrolase activity of all activated MMPs. TIMP-3 can inhibit all MMPs, and it is considered as a specific inhibitor of MMP-9[27]. Moreover, TIMP-3 can also inhibit the A disintegrin and metalloprotease (ADAM). TIMP-4 has a good inhibitory effect on both MMP-14 and MMP-2, competitively binds to MMP-2 with TIMP-2 and inhibits angiogenesis. The inhibitory effect of TIMPs on MMPs is mainly achieved through two pathways: (1) At the stage of zymogenesis, TIMPs can form stable complexes with the precursor of MMPs (pro-MMP), thereby blocking zymogenesis of pro-MMP; (2) At the stage of MMP activation, TIMPs form complexes with the activated MMPs in a 1:1 ratio, which further blocks MMP activity and inhibits MMP-medicated ECM degradation.

Although TIMPs can inhibit the activity of MMPs, their half-life is short, and inactivation can easily occur. TIMPs not only inhibit excessively expressed MMPs, but also the normally expressed MMPs. That means the inhibitory effect of TIMPs lacks selectivity, which restricts its clinical application. For TIMPs to be applied clinically, its inhibitory effect on the normally expressed MMPs must be altered through modification. And this represents a research focus in the future.

4.1.2. α2-macroglobulin

α2-macroglobulin is a wide-spectrum proteolytic enzyme inhibitor in human, and also has an inhibitory effect on cysteine, aspartic and serine proteinases in addition to MMPs[28]. It is a very important scavenging agent for MMP and can non-specifically inhibit and clear plasma MMPs[29]. α2-macroglobulin can tightly envelop the proteases through its snap-trap mechanism, thus blocking the reaction between MMPs and substrate. However, due to its large molecular weight, α2-macroglobulin only has a limited effect in ECM. Its bioavailability and medicinal value are low.

4.2. Synthetic inhibitors

In recent years, several synthetic MMPIs have been developed, including peptide based MMPs, which are divided into peptidomimetic and non-peptide MMPIs. Peptidomimetic MMPIs mainly act on the pseudopeptide derivatives of the active site of MMPs. The zinc ions usually bind to hydroxamic acid, carboxylic acid and sulfodiimides, and the activity of hydroxamic acids is the highest. At present, some hydroxamic acid MMPIs have entered the stage of clinical trial. Non-peptide MMPIs refer to a group of compounds mimicking the structure of active peptides. They share a similar structure but not the features of peptides, and they can mimic the reaction with certain enzymes, thereby activating or blocking the biological functions of some enzymes.

After literature search, synthetic MMP inhibitors already studied include marimastat, solimastat, prinomastat, cipemastat and batimastat (BB-94)[30].(Table 2) However, prinomastat is not found to be applied to AAA, while solimastat and cipemastat are not yet studied. The use of other MMP inhibitors such as Ro-28–2653, IW449, PD166793, BAY12–9566, AG-3340, AE-941BPHA, MMI-166 and ONO-4817 has not been reported in AAA. Thus, potential application of these MMP inhibitors in AAA remains to be explored in the future.

Table 2.

Summary of Synthetic MMPIs

Drug name	Type	MMPs targeted	Stage of development	Existing problems
Batimastat	Peptidomimetic (hydroxamic acid)	MMP-1, −2, −3, −7, −9, −14	Phase III clinical trial	Low bioavailability, musculoskeleta 1 side-effects
Marimastat	Peptidomimetic (hydroxamic acid)	MMP-1, −2, −3, −7, −9,−12	Phase III clinical trial	Musculoskeletal side-effects
Disulfiram	Non-peptide (sulfanilamides)	MMP-2, −9	Phase II clinical trial	Neurotoxicity
XL784	Non-peptide	MMP-2, elastin	Phase II clinical trial	Unknown
Doxycycline hydrochloride	Tetracyclines	Wide spectrum	Phase II/III clinical trial	Gastrointestinal Side effects
Statins	Statins	MMP-1, −2, −3 and −9	Convention al drugs	Increased risk of Diabetes Mellitus

4.2.1. Batimastat

Batimastat (BB-94) is a hydroxamic acid MMPI with a similar structure as the collagenases. It can inhibit the activity of MMPs by binding to the zinc ion in the active site of MMPs. As a wide-spectrum MMPI, batimastat exerts an inhibitory effect on a variety of MMPs, including MMP-1, MMP-2, MMP-3, MMP-7 MMP-9 and MMP-14. Batimastat is originally intended as an anti-tumor drug. And it is the first MMPI to be studied in clinical trials. Phase I and Phase II trials showed that about half of the evaluable patients with malignant disease responded to the treatment. However, due to low solubility and could not be administered orally, the clinical trials of Batimastat was finally stopped even though it reached Phase III[31, 32].To increase its solubility and utilize it as a potential drug for treating AAA, a group of researchers conjugated Batimastat on a nanoparticle based delivery system and targeted AAA in rat models. It has been shown that the use of nanoparticles can enhance the target ability of Batimastat and inhibits aneurysmal development of aorta[33]. If further development is successful, nanoparticle delivered Batimastat will benefit AAA patients. (Figure 2)

Fig. 2.

Chemical Structure of Batimastat, Marimastat, Disulfiram, and Tetracycline.

4.2.2. Marimastat

Marimastat (BB-2516) is an analog of Batimastat. It’s a low-molecular-weight hydroxamic acid MMPI and also a wide-spectrum MMPI[34]. By binding to the zinc ion at the active site of MMPs, it can inhibit MMP-1, MMP-2, MMP-3, MMP-7, MMP-9 and MMP-12, while there is no activity on other MMPs such as the angiotensin converting enzyme. Oral marimastat has high bioavailability, but it may lead to many adverse events. For example, a phase Ⅲ clinical trial has shown that marimastat induced apparent adverse events such as skeletal muscle and joint pain and inflammatory response[35]. Effect of Marimastat on aneurysm was assessed in a study in which Marimastat was treated on aortic aneurysm organ cultures, showing that Marimastat effectively inhibited elastin degradation and MMP-2 production[36].(Figure 2)

4.2.3. Disulfiram

Disulfiram (DSF) is a kind of FDA-approved synthetic sulfonamides for treating chronic alcohol intoxication. It has the benefits of good tolerance, low cost and anti-tumor effect. DSF has a strong ability in chelating with the metals. Its anti-tumor effect relies on the chelating ability with Cu on one hand, and relies on the inhibitory effect of DSF/Cu on several tumor-related pathways on the other hand. The tumor cell growth can be inhibited by DSF in these two pathways[37]. It has been reported that DSF can inhibit tumor metastasis and invasion. DSF can inhibit the activity of type IV collagenase. This effect further leads to the blocking of tumor invasion and angiogenesis through the cell-mediated pathway, instead of the non-cell-mediated pathway. DSF can directly act on MMP-2 and MMP9, and chelate with zinc to inhibit its hydrolase activity[38]. MMP-2 and MMP-9 may be the inhibitory target for the DSF treatment against tumors.(Figure 2)

4.2.4. XL784

XL784, as a novel narrow-spectrum synthetic MMP inhibitor, was found effectively inhibiting the animal model of AAA in a dose-dependent manner[39]. XL784 can significantly inhibit the elastic fibers and damage related to aortectasia, though the mitigating effect on aortic inflammation is small. It has been shown that XL784 is a narrow-spectrum inhibitor with high specificity on MMP-2. Nevertheless, XL784 can still effectively inhibit AAA in the animal model, while avoiding the musculoskeletal syndrome that may be otherwise caused by the wide-spectrum MMP inhibitors.

So far there is a lack of intact pharmacodynamic study. In mice, the peak plasma level of XL784 is reached in an absence of clinical side effects, and the corresponding dose is lower than the maximum dose assessed by the phase 1 trial in human subjects. Its half-life at 3h in mice is apparently shorter than that in human subjects, where the half-life is observed to be about 7–9h. Therefore, it seems to be plausible to use this novel type of inhibitor for clinical treatment of AAA.

Whether XL784 is indeed effective in inhibiting the growth of small AAA depends on several factors, including the long-term side effects and its effect in preventing progressive expansion of the existing AAA. As shown by the short-term studies, relative specificity of XL784 on elastase is conducive to lowering the risk of side effects, thus improving the tolerance to its long-term use. Moreover, because of its high specificity on MMP-2, XL784 can at least be as effective as the broad-spectrum MMP inhibitors in inhibiting AAA growth in the mouse model. Further studies should be conducted on XL784, so as to determine its clinical application in AAA treatment.

4.3. Natural MMPIs

Given the disadvantages of synthetic MMPIs such as low bioavailability and adverse events, the researchers have conducted an in-depth investigation into the natural products derived from plants, fungi and marine organisms. Some effective natural MMPIs have been developed so far[40].

Tetracyclines are first derived from the bacteria and a series of effective MMPIs have been obtained after modification of their chemical structure, such as doxycycline hyclate and novel tetracycline analogue COL-3. Tetracycline derivatives not only inhibit MMPs activity but also suppress their production.

Doxycycline hydrochloride is a classical, long-acting tetracycline antibiotics, which has a non-selective inhibitory effect on MMPs, reducing the secretion of MMP-2 and MMP-9[41]. It has been proven in most of the animal experiments that doxycycline hydrochloride can prevent the formation of AAA by inhibiting MMPs[42–44]. In the calcium chloride-induced aortic injury in mice, doxycycline hydrochloride can inhibit AAA expansion in a dose-dependent manner[45]. It has been found in the mouse model of Marfan syndrome that doxycycline hydrochloride can delay the rupture of AAA[46]. Some clinical trials have been performed on doxycycline in human AAA treatment[47–49]. But in a randomized, double-blind, placebo-controlled trial, 18 months of doxycycline hydrochloride did not inhibit AAA progression and induce AAA repair[50]. This indicates that the initiation and formation of AAA are different from the progression of AAA. Therefore, a well-designed animal experiment is necessary to evaluate the efficacy of doxycycline hydrochloride in inhibiting the growth of AAA but not in the initiation of AAA.

Oral doxycycline has been proven to induce systemic adverse events in a dose-dependent manner, such as gastrointestinal tract dysfunction and photosensitive skin diseases[51]. MMPs are also involved in the remodeling of healthy tissues, and systemic inhibition by doxycycline may produce an adverse impact. Sivaraman B et al[52]. studied the nanoparticle-based drug delivery system for AAA treatment, which was the doxycycline-loaded poly(lactic-co-glycolic acid) (PLGA) nanoparticles. This system could achieve long-term targetability, control and sustained release of doxycycline (delivery within AAA tissues for over 3 weeks), thus avoiding the systemic side effects of doxycycline. The successful development of doxycycline nanoparticles sheds new light on the targeting and controlled- and sustained-release formulations.

4.4. Statins

Statins are recommended by the guidelines as conventional drugs for AAA[53], as they not only lower the blood lipid level and reduce the risk of adverse cardiovascular events, but also slow down atherosclerosis in AAA patients. However, the working mechanism of statins in human AAA is not fully understood yet. Several studies has shown that statins can act on AAA by inhibiting the activity of several MMPs and immune cells and downregulating various inflammatory molecules[54–56]. Both animal studies and in vitro cultures have demonstrated the inhibitory effect of statins on MMPs. In vitro cultures have shown that statins can reduce the secretion of pro-inflammatory proteins in human AAA, including MMP-9 and monocyte chemotactic protein-1 (MCP-1)[57, 58]. Statins can prevent the progression of AAA in animal models by inhibiting MMP-9 secretion[59]. Some clinical studies have indicated that statins are associated with the slowed growth of AAA. Periard et al. found through a retrospective analysis that statins could reduce the expansion rate of AAA by 1.5mm/y[60]. In Karrowni et al.’ study, statins reduced the expansion rate of AAA by 2.3mm/y[61]. One meta-analysis on statins treatment of AAA indicated that for AAA smaller than 55m, statins could effectively prevent its expansion and slow down the progression of AAA[62].

4.5. Extracts or active ingredients of Chinese herbal medicines

Given the complex etiology and uncertain pathogenesis of AAA, Chinese herbal medicines, which is featured by multiplicity of components and targets, may provide an alternative approach to suppress AAA. By far, the vasoprotective effect of some extracts or active ingredients in herbal medicines has been demonstrated in several animal experiments (Table 3). Among those chemical ingredients, quercetin and resveratrol are famous for alleviating oxidative stress and have been shown cardiovascular benefits in various studies. In vivo studies with mice model revealed that both quercetin and resveratrol can eliminate MMP 2 and MMP 9 activation during AAA formation. However, it’s still challenging to fairly evaluate efficacy and safety of Chinese herbal medicines so that we are unable to predict the clinical effect in the treatment of AAA. Lack of standardization in herbal products and multifunction of numerous compounds in herbs increase the difficulty in quality control and the herbal preparation for clinical trials. Well-designed controlled trials are needed to verify the clinical effect of the Chinese herbal medicines. Combination with the western medicine may be also a good strategy, though a large amount of studies are needed to prove the benefits of Chinese herbal medicines in AAA.

Table 3.

Effects of active ingredients in Chinese herbal medicines in AAA inanimal models

Active ingredients of Chinese herbal medicines	Working mechanism	References
Ginsenoside Rb1	Produced by MMP, degrading ECM, inflammatory cell infiltration, VSMC dysfunction, inhibiting the JNK and p38 signaling pathways	[63]
Aalvianolic acid C and A	Inhibiting the activity of MMP-2 and MMP-9, inhibiting macrophage infiltration, protecting elastin	[64, 65]
EGCG-3"-O-Me. (green tea polyphenols)	Inhibiting the MMP-9 expression and MMP-1 activity	[66]
Quercetin	Inhibiting MMP-2 and MMP-9 activities	[67]
Ginkgo leaf extract	Inhibiting the activity of MMP-2 and MMP-9, protecting elastin	[68]
Baicalein	Inhibiting MMP-2 and MMP-9 activities	[69]
Resveratrol	Inhibiting MMP-2 and MMP-9 activities	[70]
Ursolic acid	Inhibiting the expressions of MMP-2, MMP-9 and ADAM18, protecting elastin	[71]
Daidzein	Inhibiting MMP-2 and TIMP-1	[72]

5. ADAM/ADAMTS

ADAM and ADAMTS are both zinc-dependent endopeptidases and members of proteolytic enzyme family, and are closely related to other MMPs. Both play a key role in maintaining normal tissues and tissue homeostasis. ADAM and ADAMTS share similar structural domains and protein sequence, and both have highly conservative structure and protein homology[72]. Both have pro-domain, MMP structural domain, disintegrin domain and cysteine-rich structural domain. However, ADAMTS has the characteristic thrombospondin type 1 repeat (TSR), but lacks epidermal growth factor-like region (EGF), functional sites and cytoplasmic tail[72] (Fig. 3). Both have the functions of sheddase and regulate the shedding of membrane-binding proteins, growth factors, cytokines, ligands and receptors. They play a variety of biological roles, including cell-matrix interaction, zymogen activation (shedding) and cell adhesion. These effects are conducive to cell migration, protein decomposition and angiogenesis[73]. Their activities are the same as MMPs and are regulated by the endogenous inhibitors such as TIMPs; they can be also inhibited by the synthetic small-molecular inhibitors. At present, 22 ADAMs have been identified in the human genome, including 13 with hydrolase activity^[77].

Fig. 3.

Structure of ADAM and ADAMTS. In addition to the signal peptide, pro-domain and MMP structural domain in MMP, ADAM also contains the disintegrin-like functional domain, hypervariable region (HVR) in the cysteine-rich structural domain, EGF-like functional domain, transmembrane structural domain and cytoplasmic tail. Members of the ADAMTS family resemble ADAM, but lack the EGF-like functional domain, transmembrane domain and cytoplasmic tail. Instead, they have TSR and interval region.

Whether ADAMS is involved in AAA and what may be the possible working mechanism are little known. Except for ADAM-17, the relationship between ADAMS and AAA has not been studied. Similarly, there has been no report on the role of ADAMTS in AAA and none of the members of the ADAMTS family is known to be associated with AAA.

Emina Vorkapic et al.[74] showed that as compared with the control group, the mRNA expression of ADAMTS decreased in AAA. ADAMT-1 was overexpressed in angiotensin Ⅱ-induced AAA in mice, and the severity of AAA was similar to that in wild-type mice. It was thus concluded that ADAMTS-1 was a secondary factor promoting AAA progression.

Lipp C et al. [75] showed that ADAM −8, −9, −10, −12, −15 and −17 were expressed in the aortic vascular wall, especially in SMCs. Their expressions may be necessary in the normal cells, for example, for cell-cell and cell-ECM interactions, ECM degradation, migration and activation of the proteolytic enzymes and various signaling molecules. However, their study did not prove whether ADAM was upregulated in AAA.

Tatsuo Kawai et al.[76] showed that ADAM-17 was upregulated in the endothelium and extima of AAA. This was consistent with the results by Takayanagi T et al., but the role of ADAM-17 in the vascular endothelium and extima remains to be further investigated[77].

6. Conclusion

The occurrence and development of AAA are closely related to the changes in the connective tissues of the aortic wall, especially ECM degradation. ECM degradation requires the activity of MMPs. At present, some remarkable progress has been made in the animal experiments on some MMPIs. But except for doxycycline hydrochloride and statins that have been used in patients, most of other MMPIs are currently at the clinical trial stage or the clinical trials seem have failed for MMPIs. Nearly no other MMPIs have been successfully developed yet. The reasons may include the followings: (1) Endogenous MMPIs (TIMPs) not only inhibit overexpressed MMPs, but also the normally expressed MMPs. The inhibitory effect of TIMPs is not selective and therefore TIMPs are not the preferred MMPIs. However, the dynamic balance between TIMPs and MMP in vivo may block the occurrence and development of AAA. Restoring the dynamic balance between TIMPs and MMPs may be the direction of future research. (2) Most of the synthetic MMPIs fail in vivo because of low bioavailability and apparent adverse events. The occurrence and development of AAA are highly complex and involve much more than the MMPs. Therefore, we should fully consider the selectivity of MMP and bioavailability of the drugs. (3) By analyzing the MMPIs under study, we have found that some of the animal models and clinical trials on these MMPIs are not properly designed. It is necessary that we consider all influence factors and develop reasonable test scheme and clinical trial so as to ensure the development of effective MMPIs.

Although ADAM/ADAMTS are involved in ECM degradation, their role in AAA has not been little reported. It should be noted that none of the members of the ADAMTS family is related to AAA. ADAM/ADAMTS may be the secondary factors promoting the progression of AAA, and they may be also necessary for the functioning of normal cells. Therefore, no consensus has been reached as to the role of ADAM/ADAMTS in AAA.

We have clarified the role of extracts or active ingredients of Chinese herbal medicines in the animal models of AAA, but the clinical effect is still unpredictable. Neither can we know about the benefits of combined treatment with western medicine. We believe that safe and effective Chinese herbal monomers or compound preparations may be developed as supplement to the existing western medicine treatment for AAA.

Taken together, improving the level of TIMPs through preclinical and clinical studies so as to inhibit the activity of MMPs will be the major direction for further research. Moreover, developing MMPIs based on extracts or active ingredients from the Chinese herbal medicine is another strategy worthy of attention.