Remodeling of the vessel wall after copper-induced injury is highly attenuated in mice with a total deficiency of plasminogen activator inhibitor-1

Steroid receptor coactivator-3 is required for inhibition of neointima formation by estrogen

BACKGROUND

The vasoprotective effects of estrogen are mediated by estrogen receptors (ERs). ERs are transcription factors that require coactivators to exert transcriptional activity. The steroid receptor coactivator-3 (SRC-3, also known as pCIP, AIB1, ACTR, and TRAM-1) interacts with estrogen-bound ERs and strongly coactivates the transcription of target genes in cultured cells. This study has characterized the expression of SRC-3 in cardiovascular tissue and the role of SRC-3 in estrogen-dependent vasoprotection from vascular injury.

METHODS AND RESULTS

Phenotypically normal SRC-3(+/-) mice with a knock-in LacZ reporter were used to characterize SRC-3 expression by X-gal staining within the cardiovascular system. Staining signals were specifically detected in vascular smooth muscle cells and endothelial cells but not in myocardial cells. The role of SRC-3 during vascular remodeling was analyzed using a unilateral carotid ligation model. The extent of neointima formation in SRC-3(-/-) mice was significantly higher than in wild-type mice, and this difference was diminished after depletion of estrogen by ovariectomy. After ovariectomy, neointimal growth in wild-type mice was almost completely inhibited by estrogen treatment but only partially inhibited in SRC-3(-/-) mice. Furthermore, estrogen treatment resulted in reduced inhibition of intimal cell proliferation in SRC-3(-/-) mice.

CONCLUSIONS

SRC-3 is highly expressed in vascular smooth muscle cells and endothelial cells. The loss of SRC-3 function causes a decrease in sensitivity of estrogen-mediated inhibition of neointimal growth, which may be attributable to an insufficient suppression of vascular cell proliferation. These results indicate that SRC-3 largely facilitates ER-dependent vasoprotective effects under conditions of vascular trauma.

Endogenous vitronectin and plasminogen activator inhibitor-1 promote neointima formation in murine carotid arteries

We examined the roles of vitronectin and plasminogen activator inhibitor-1 (PAI-1) in neointima development. Neointima formation after carotid artery ligation or chemical injury was significantly greater in wild-type mice than in vitronectin-deficient (Vn(-/-)) mice. Vascular smooth muscle cell (VSMC) proliferation did not differ between groups, suggesting that vitronectin promoted neointima development by enhancing VSMC migration. Neointima formation was significantly attenuated in PAI-1-deficient (PAI-1(-/-)) mice compared with control mice. Because intravascular fibrin may function as a provisional matrix for invading VSMCs, we examined potential mechanisms by which vitronectin and PAI-1 regulate fibrin stability and fibrin-VSMC interactions. Inhibition of activated protein C by PAI-1 was markedly attenuated in vitronectin-deficient plasma. The capacity of PAI-1 to inhibit clot lysis was significantly attenuated in vitronectin-deficient plasma, and this effect was not explained simply by the PAI-1-stabilizing properties of vitronectin. The adhesion and spreading of VSMCs were significantly greater on wild-type plasma clots and PAI-1-deficient plasma clots than on vitronectin-deficient plasma clots. We conclude that endogenous levels of vitronectin and PAI-1 enhance neointima formation in response to vascular occlusion or injury. Their effects may be mediated to a significant extent by their capacity to promote intravascular fibrin deposition and by the capacity of vitronectin to enhance VSMC-fibrin interactions.

Lack of plasminogen activator inhibitor-1 promotes growth and abnormal matrix remodeling of advanced atherosclerotic plaques in apolipoprotein E-deficient mice

Epidemiological studies suggest that elevated plasma levels of plasminogen activator inhibitor-1 (PAI-1) predispose an individual to ischemic heart disease or promote plaque progression by inhibiting fibrinolysis. In the present study, loss of PAI-1 in apolipoprotein E (apoE)-deficient (apoE(-/-):PAI-1(-/-)) mice promoted the growth of advanced atherosclerotic plaques, which was due to enhanced extracellular matrix deposition. ApoE(-/-):PAI-1(-/-) plaques also exhibited collagen fiber disorganization and degradation. Immunostaining and bone marrow transplantation revealed that smooth muscle cells, not macrophages, primarily expressed PAI-1 in plaques. Thus, although PAI-1 may promote plaque growth because of its antifibrinolytic properties, the present study reveals a protective role for PAI-1 by limiting plaque growth and preventing abnormal matrix remodeling.

Accelerated skin wound healing in plasminogen activator inhibitor-1-deficient mice

Components of the fibrinolytic system have been implicated in cell migratory events associated with tissue remodeling. Studies in plasminogen-deficient mice (PG(-/-)) indicated that skin wound healing is impaired, but is resolved with an additional fibrinogen deficiency. Plasminogen activator inhibitor-1 (PAI-1) expression by keratinocytes has been identified shortly after wound injury. PAI-1 expression could affect wound healing by regulating the fibrinolytic environment of the wounded area, as well as influencing events associated with cell attachment and detachment through interactions with matrix proteins. The present study directly assesses PAI-1 involvement in skin wound healing through analyses of a dermal biopsy punch model in PAI-1-deficient (PAI-1(-/-) mice. While the cellular events associated with the healing process are similar between wild-type (WT) and PAI-1(-/-) mice, the rate of wound closure is significantly accelerated in PAI-1(-/-) mice.

Attenuation of neointima formation following arterial injury in PAI-1 deficient mice

Atherosclerosis is a chronic inflammatory disease in which the fibrinolytic system has been implicated as playing a major role. In order to directly assess the physiological impact an imbalanced fibrinolytic system has on both early and late stages of this disease, mice deficient for PAI-1 (PAI-1-/-) were used in a model of vascular injury/repair and compared to wildtype mice (WT). Copper-containing cuffs were placed around the carotid arteries of these mice and the injured arteries were removed at either 7 or 21 days for histological analyses. At both times after injury, fibrin was prevalent in WT arteries, whereas only diffuse in PAI-1-/- arteries. At 21 days after injury, a prominent, multilayered neointima was evident in WT arteries, with no evidence of a neointima in PAI-1-/- arteries. Results from this study directly confirm the involvement of the fibrinolytic system in vascular repair processes following injury and indicate that fibrin could potentially play a role in lesion formation by stimulating smooth muscle cell proliferation, collagen synthesis, and intracellular cholesterol accumulation.

Plasminogen activator inhibitor type 1 enhances neointima formation after oxidative vascular injury in atherosclerosis-prone mice

BACKGROUND

Plasminogen activator inhibitor type 1 (PAI-1) inhibits neointima formation after vascular injury. Hyperlipidemia modulates the expression of multiple genes, however, and the effects of PAI-1 on the arterial response to injury under hyperlipidemic conditions are unknown. The purpose of this study was to examine the impact of PAI-1 on intimal hyperplasia and other vascular changes that develop after arterial injury in apolipoprotein E-deficient (apoE(-/-)) mice.

METHODS AND RESULTS

Ferric chloride injury of the midportion of the common carotid arteries of apoE(-/-) mice (n=22) induced formation of a neointima that contained smooth muscle cells, foam cells, neutral lipid, tissue factor, and von Willebrand factor. Interactions between vascular injury and apolipoprotein E deficiency were strongly synergistic; either stimulus alone was insufficient to induce significant neointima formation. Mean intima/media ratios were significantly greater (P<0.03) in apoE(-/-), PAI-1(+/+) mice (5.6+/-1.8, n=12) than in apoE(-/-), PAI-1(-/-) mice (1.2+/-0.55, n=12), as were the percentages of bromodeoxyuridine-positive cells in the intima and media (P<0.03). Transiently occlusive (<48 hours) and nonocclusive mural thrombi persisted longer in apoE(-/-), PAI-1(+/+) mice than in apoE(-/-), PAI-1(-/-) mice.

CONCLUSIONS

In atherosclerosis-prone mice, PAI-1 promotes neointima formation after oxidative vascular injury. The apparent hyperlipidemia-dependent effect of PAI-1 may be mediated by its capacity to inhibit the clearance of platelet-fibrin thrombi that can deliver growth factors to the blood vessel wall or be incorporated into developing vascular lesions. Alternatively, hyperlipidemia may alter the pattern of gene expression in the blood vessel wall to enhance potential effects of PAI-1 on antiproliferative processes, such as transforming growth factor-beta activation and apoptosis.