Metal loproteases in remodeling of vascular extracellular matrix

Matrix metalloproteinase 9 facilitates collagen remodeling and angiogenesis for vascular constructs

Degradation of the extracellular matrix, facilitated by matrix metalloproteinases (MMPs), can lead to mechanical failure of vascular constructs, suggesting that MMP inhibition could improve survival of constructs. Therefore, we investigated the role of MMP-9 in collagen remodeling in vitro, focusing on the three major steps of production, degradation, and organization. Because an adequate blood supply is essential for survival of tissue-engineered constructs, we also evaluated the influence of MMP-9 deficiency on angiogenesis in vivo by implantation of thin biodegradable polymer scaffolds. Using aortic smooth muscle cells (SMCs) from wild-type and genetically deficient (9KO) mice, we examined the role of MMP-9 in collagen mRNA expression and protein accumulation, both with and without ascorbic acid treatment. We measured collagen assembly in a fibrillogenesis assay. We investigated in vivo angiogenesis and cell invasion, using fluorescence microangiography and histology. MMP-9 deficiency did not affect collagen mRNA production or polymer scaffold degradation, but collagen accumulation was greater in cultures of 9KO SMCs than in wild-type SMCs. Both MMP-9 deficiency and chemical inhibition impaired collagen degradation. Ascorbic acid treatment enhanced collagen production by 9KO SMCs compared with wild-type SMCs at 3 days, but by 7 days this effect was reversed. MMP-9 improved fibrillogenesis of collagen, significantly more on ascorbic acid treatment. MMP-9 deficiency dramatically decreased inflammatory cell invasion, but also capillary formation within biodegradable polymer scaffolds in vivo. Our data suggest that MMP inhibition, by impairing collagen organization and angiogenesis, might have detrimental effects on the survival of vascular constructs.

Stimulated release of tissue plasminogen activator from artery wall sympathetic nerves: implications for stress-associated wall damage

Recurrent stress is clinically associated with early onset hypertension and coronary artery disease. A mechanism linking emotion to pathogenic remodeling of the artery wall has not been identified. Stress stimulates acute regulated release of tissue plasminogen activator (t-PA) into the circulation, which is presently attributed to the vascular endothelium. Sympathetic neurons also synthesize t-PA and axonally transport it to the arterial smooth muscle. Unlike release by the endothelium, a stress-stimulated sympathetic discharge would potentially accelerate degradation of the wall matrix by plasmin. To assess whether sympathetic axons are the principal source of acute stress-induced arterial release of t-PA, we compared the output from small densely innervated and large sparsely innervated isolated artery segments before and after sympathetic stimulation, and after ablations. Following phenylephrine infusion densely-innervated microvessels in uveal eyecups were released over 60-fold greater amounts of active t-PA per milligram than the sparsely innervated aorta; and ten-fold more than carotid artery segments. Mesenteric artery release was 4.8-fold greater than release by the carotid artery. In vivo, uveal release of t-PA increased more than three-fold within one minute following superior cervical sympathetic ganglion electrical stimulation, and after phenylephrine, or nicotine infusions of the anterior chamber. Circulating levels of t-PA fell 70% following chemical sympathectomy. We propose that sympathetic nerves are the primary source of stress-induced release of t-PA into and from the densely innervated resistance arteries and arterioles, where dysregulated plasmin-induced proteolysis could damage the wall matrix.

Elastinolytic matrix metalloproteinases and their inhibitors as therapeutic targets in atherosclerotic plaque instability

Atherosclerosis is a dynamic pathologic process involving interactions between many cell types and chemical mediators. There is increased evidence in the literature that matrix metalloproteinases, especially those with elastolytic activity, are associated with atherosclerotic plaque instability. Results of recent studies also suggest that the balance between matrix metalloproteinases and their inhibitors contributes to the extracellular matrix integrity, and an imbalance could be a predeterminate of both cerebral and cardiac ischemic events. Significant evidence demonstrates that the balance between elastolytic matrix metalloproteinases and their inhibitors are involved in the atherosclerotic process. Studies investigating pharmacologic therapies that inhibit matrix metalloproteinases or increase their natural inhibitor levels suggest an antiatherosclerotic and potential plaque-stabilizing benefit. Carefully designed clinical trials must be completed to better understand the functions and interactions of these enzymes with the goal of developing selective therapies to prevent the progression and complications of atherosclerosis.

Enhanced tissue plasminogen activator synthesis by the sympathetic neurons that innervate aging vessels

We investigated the source of the increased release of tissue plasminogen activator (t-PA) into the circulation that occurs during natural aging. Both the basal release and the acute stress-associated release induced by sympathetic stimulations are greater in older subjects. It is widely assumed that the source of these increases is vascular endothelium. However, the sympathetic neurons that densely innervate resistance vessel walls were recently shown to synthesize and transport active t-PA to axon terminals in vascular smooth muscle, suggesting an alternative source. These fine t-PA-bearing axons lie in the seldom-studied deep adventitia of vessel walls, where they are less visible than endothelium in tissue sections. Using Northern blot analysis, we observed that t-PAmRNA synthesis is increased 54% in the ganglion parent neuron cell bodies that innervate aged vessels. The t-PA release from isolated, aged ganglia in cultures was twofold greater than that from younger controls. In addition, aged whole-artery explants showed a 20% greater basal and a 50% greater acute release of stored t-PA in vitro. In vivo levels of active t-PA were 33% greater in the blood and 40% greater in the aqueous humor. These results are consistent with an increased infusion of the active t-PA protease from sympathetic axon terminals into the vessel wall extracellular matrix and the blood during natural aging, in addition to the basal endothelial release. We suggest that the cumulative impact of an accelerated plasmin production and matrix degradation within vessel walls, especially during repetitive stress, may play an unrecognized role in the pathogenesis of vascular aging. The possibility that increased sympathetic nervous system plasminogenesis influences the aging process in nonvascular tissues also deserves further investigation.

Storage and release of tissue plasminogen activator by sympathetic axons in resistance vessel walls

We studied the immunolocalization of tissue plasminogen activator (t-PA) in rat precapillary arteries, arterioles, and terminal arterioles. Lack of information about the precise location of t-PA within small vessel walls has contributed to uncertainty about its cellular source. The presumed origin has been an endothelial phenotype largely restricted to certain small vessels. However, vessel wall sympathetic axons were recently also shown to store significant amounts of a neuron-generated t-PA in secretory vesicles. Using immunolocalizations we determined the extension of t-PA-bearing axons into the resistance vasculature. Light and confocal images revealed the persistence of t-PA-bearing sympathetic nerve filaments down to the level of 15-microm-diameter terminal arterioles in vasa vasora and the choroidal microvasculature. Immunoelectron localizations confirmed the confinement of t-PA within individual nerve filaments in the deep adventitia. A complete plasminogen activator system (t-PA, plasminogen, and plasmin) was localized in the arteriolar wall matrix. Isolated iris-choroid and mesenteric artery explants from sympathectomized animals released 65 and 43% less t-PA, respectively, than controls. These data support the hypothesis that resistance vessel sympathetic axons release neural t-PA into the wall matrix and the microvascular plasma.

Matrix Metalloproteinases in Vascular Remodeling and Atherogenesis

Vascular remodeling, defined as any enduring change in the size and/or composition of an adult blood vessel, allows adaptation and repair. On the other hand, inappropriate remodeling, including its absence, underlies the pathogenesis of major cardiovascular diseases, such as atherosclerosis and restenosis. Since degradation of the extracellular matrix scaffold enables reshaping of tissue, participation of specialized enzymes called matrix metalloproteinases (MMPs) has become the object of intense recent interest in relation to physiological (“good”) and pathological (“bad”) vascular remodeling. Experimental evidence acquired in vitro and in vivo suggests that the major drivers of vascular remodeling, hemodynamics, injury, inflammation, and oxidative stress, regulate MMP expression and activity. Alternatively, nonspecific MMP inhibition seems to oppose remodeling, as suggested by the inhibition of intimal thickening and outward arterial remodeling. An emerging concept is that MMP-related genetic variations may contribute to heterogeneity in the presentation and natural history of atherosclerosis. The hypothesis that MMPs contribute to weakening of atherosclerotic plaques is especially attractive for the potential development of therapeutic interventions aimed at preventing plaque disruption (“the ugly”), a major cause of acute cardiovascular events. However, the current lack of appropriate experimental tools, including availability of specific MMP inhibitors and pertinent animal models, still limits our understanding of the many actions and relative contributions of specific MMPs. Our future potential ability to control vascular remodeling via regulation of MMPs will also depend on reaching a consensus of what is indeed “good” or “bad” vascular remodeling, concepts that have continued to evolve and change.

Matrix metalloproteinases in cancer invasion, metastasis and angiogenesis

Matrix metalloproteinases (MMPs) are a family of proteinases that play an important role in cancer as well as in numerous other diseases. In this article, we summarize the current views on the role of MMPs in cancer with respect to invasion, metastasis and angiogenesis. A positive correlation between tumor progression and the expression of multiple MMP family members in tumor tissues has been demonstrated in numerous human and animal studies. It has been assumed that cancer cells are responsible for producing the MMPs in human tumors. However, recent evidence suggests that tumor cells have docking sites that bind stromal-cell-secreted MMPs. Furthermore, the role of MMPs produced by endothelial cells, especially MMP-2 and MT1-MMP, appear to be crucial for tumor angiogenesis, which is a requirement for cancer growth and dissemination.

Plasminogen is a critical determinant of vascular remodeling in mice

Extracellular proteolysis is likely to be a feature of vascular remodeling associated with atherosclerotic and restenotic arteries. To investigate the role of plasminogen-mediated proteolysis in remodeling, polyethylene cuffs were placed around the femoral arteries of mice with single and combined deficiencies in plasminogen and fibrinogen. Neointimal development occurred in all mice and was unaffected by genotype. Significant compensatory medial remodeling occurred in the cuffed arteries of control mice but not in plasminogen-deficient mice. Furthermore, focal areas of medial atrophy were frequently observed in plasminogen-deficient mice but not in control animals. A simultaneous deficit of fibrinogen restored the potential of the arteries of plasminogen-deficient mice to enlarge in association with neointimal development but did not eliminate the focal medial atrophy. An intense inflammatory infiltrate occurred in the adventitia of cuffed arteries, which was associated with enhanced matrix deposition. Adventitial collagen deposition was apparent after 28 days in control and fibrinogen-deficient arteries but not in plasminogen-deficient arteries, which contained persistent fibrin. These studies demonstrate that plasmin(ogen) contributes to favorable arterial remodeling and adventitial collagen deposition via a mechanism that is related to fibrinogen, presumably fibrinolysis. In addition, these studies reveal a fibrin-independent role of plasminogen in preventing medial atrophy in challenged vessels.