Methoxyestradiols mediate the antimitogenic effects of estradiol on vascular smooth muscle cells via estrogen receptor-independent mechanisms.

Estrogen receptors (ERs) are widely held to mediate the ability of 17 beta-estradiol (estradiol) to attenuate injury-induced proliferation of vascular smooth muscle cells (VSMCs) leading to vascular lesions. However, recent findings that estradiol prevents injury-induced vascular lesion formation in knock-out mice lacking either ER alpha or ER beta seriously challenge this concept. Here we report that the local metabolism of estradiol to methoxyestradiols, endogenous metabolites of estradiol with no affinity for ERs, is responsible for the ER-independent inhibitory effects of locally applied estradiol on rat VSMC growth. These finding imply that local vascular estradiol metabolism may be an important determinant of the cardiovascular protective effects of circulating estradiol. Thus, interindividual differences, either genetic or acquired, in the vascular metabolism of estradiol may define a given female's risk of cardiovascular disease and influence the cardiovascular benefit she receives from estradiol replacement therapy in the postmenopausal state. These findings also imply that nonfeminizing estradiol metabolites may confer cardiovascular protection in both women and men.

Cholera toxin treatment of vascular smooth muscle cells decreases smooth muscle alpha-actin content and abolishes the platelet-derived growth factor-BB-stimulated DNA synthesis.

The second messenger cyclic AMP regulates diverse biological processes such as cell morphology and cell growth. We examined the role of the second messenger cyclic AMP on rat aortic vascular smooth muscle cell (VSMC) morphology and the intracellular transduction pathway mediated by platelet-derived growth factor beta-receptor (PDGF-Rbeta). The effect of PDGF-BB on VSMCs growth was assessed by [(3)H]-thymidine incorporation. Tyrosine phosphorylation of PDGF-Rbeta, PLC-gamma1, ERK1 and ERK2, p125(FAK) and paxillin as well as Sm alpha-actin was examined by the chemiluminescence Western blotting method. Actin mRNA level was quantitated by Northern blotting. Visualization of Sm alpha-actin filaments, paxillin and PDGF-Rbeta was performed by immunfluorescence microscopy. Cholera toxin (CTX; 10 nM) treatment lead to a large and sustained increase in the cyclic AMP concentration after 2 h which correlated with change of VSMC morphology including complete disruption of the Sm alpha-actin filament array and loss of focal adhesions. Treatment of VSMCs with CTX did not influence tyrosine phosphorylation of p125(FAK) and paxillin but decreased the content of a Sm alpha-actin protein. Maximal decrease of 70% was observed after 24 h of treatment. CTX also caused a 90% decrease of the actin mRNA level. CTX treatment completely abolished PDGF-BB stimulated DNA-synthesis although PDGF-Rbeta level and subcellular distribution and translocation was not altered. Furthermore CTX attenuated the PDGF-BB-induced tyrosine phosphorylation of the PDGF-Rbeta, PI 3'-K, PLC-gamma1 and ERK1/2 indicating an action of cyclic AMP on PDGF-beta receptor. We conclude that although cyclic AMP attenuates the PDGF-Rbeta mediated intracellular transduction pathway, an intact actin filament may be required for the PDGF-BB-induced DNA synthesis in VSMCs.

Angiotensin II-induced progression of neointimal thickening in the balloon-injured rat carotid artery is AT1 receptor mediated.

To investigate the relative importance of AT1 and AT2 receptors in angiotensin II (Ang II)-induced restimulation of neointimal smooth muscle cell (SMC) DNA synthesis and increased neointimal cross-sectional area (CSA), male Wistar rats were subcutaneously infused for 2 weeks with Ang II and losartan, an AT1 receptor antagonist, or Ang II and PD123319, an AT2 receptor antagonist, during the third and fourth week after balloon injury of the left common carotid artery. Concomitantly, all rats received 5-bromo-2'-deoxyuridine to label DNA-synthesizing SMCs. Neointimal CSAs and SMC DNA synthesis were compared with control groups that received Ang II, 0.9% NaCl, losartan, or PD123319. Systolic blood pressure (SBP) was measured at different times during the infusion. Ang II induced an increase in SBP that was significantly different from the SBP in the NaCl group. Infusion of Ang II together with losartan reduced the Ang II-induced increase in SBP to levels comparable with those obtained in the NaCl group. Infusion of Ang II+PD123319 caused an increase in SBP that was comparable with the increase in SBP of the Ang II group and significantly different from the SBP of the NaCl group. Infusion of losartan or PD123319 alone did not affect SBP. Ang II significantly enhanced neointimal CSA (47%, P < .05) compared with the control group infused with NaCl. Losartan significantly reduced Ang II-induced neointimal thickening (neointimal CSA, -37%, P < .05). Infusion of PD123319 together with Ang II did not affect Ang II-induced neointimal thickening. Losartan or PD123319 alone did not reduce neointimal thickening, since the neointimal CSAs in these groups did not differ from the neointimal CSA of the NaCl group. Comparable effects were found for SMC DNA synthesis in the neointima. Ang II infusion increased neointimal SMC DNA synthesis. Addition of losartan reduced the fraction of DNA-synthesizing neointimal SMCs from 23.7 +/- 2.1% in the Ang II group to 12.8 +/- 1.8% in the Ang II+losartan group, whereas the labeling fraction in the neointima remained 26.6 +/- 3.1% in the Ang II+PD123319 group. The labeling fractions in the neointimas of the groups that received losartan or PD123319 alone did not differ from the labeling fraction in the NaCl group. These data indicate that AT1 but not AT2 receptors mediate the progression of neointimal thickening induced by delayed application of Ang II in the injured left carotid artery in the rat. Furthermore, these data suggest that AT1 and AT2 receptors are not involved in the regulation of normal growth of a neointima in the third and fourth week after balloon injury.

Mononuclear leukocytes invade rabbit arterial intima during thickening formation via CD18-and VLA-4-dependent mechanisms and stimulate smooth muscle migration.

The role of mononuclear leukocytes for the migration of smooth muscle cells (SMCs) during intimal thickening was investigated in the rabbit model of electrically stimulated carotid artery. The approach was to inhibit leukocyte entry into the arterial intima with antibodies against the adhesion molecules very late activation antigen-4 (VLA-4) and CD11/CD18. In electrically stimulated control rabbits treated either with saline or a nonspecific antibody, all types of granulocytes, monocytes, and lymphocytes migrated across an intact endothelium into the acellular subendothelial space, followed by the movement of SMCs from the media into the intima within 36 hours of applying electrical current. Treatment of the rabbits with monoclonal antibody (mAb) HP1/2 directed toward the alpha 4 subunit (CD49d) of VLA-4 inhibited mononuclear leukocyte invasion (consisting of monocytes and lymphocytes) by approximately 70% compared with the IgG-treated control rabbits and completely abolished the minimal influx of basophils and eosinophils after 36 hours. Neutrophil infiltration, however, remained unaffected by anti-VLA-alpha 4 treatment. Under these conditions, SMC migration across the internal elastic lamina was reduced by 50%. The use of mAb HP1/2 together with mAb 60.3 (directed to the beta 2 chain of CD11/CD18) completely abolished the influx of monocytes, lymphocytes, and all types of granulocytes into the arterial intima. This complete blockade of leukocyte infiltration resulted in a 70% reduction of intimal SMC accumulation. Together with our previous findings excluding neutrophils as stimulators of SMC migration, the present results indicate that mononuclear leukocyte promote lesion development by stimulating SMC migration.

Ca2+ entry and vasoconstriction during osmotic swelling of vascular smooth muscle cells.

Exposure of aortic strips from guinea-pigs to hypotonic extracellular fluid is followed by marked vasoconstriction, which is inhibited by D-600 (3 microM), a blocker of voltage-sensitive Ca2+ channels. Conventional electrophysiology, patch-clamp studies, pH determination with 2',7' bis(2-carboxyethyl)-5,6-carboxyfluorescein (BCECF) and Ca2+ measurements with Fura-2 have been performed on smooth muscle cells cultured either from rat or human aorta to further elucidate the underlying mechanisms. Exposure of the cells to a 25% hypotonic extracellular fluid leads to a rapid and fully reversible depolarization, paralleled by an increase of the selectivity and conductance of the cell membrane to Cl-, an acidification of the cytoplasm and an increase of intracellular Ca2+ concentration ([Ca2+]i). The latter is inhibited by the Ca2+ channel blocker D-600 (1-3 microM). It is concluded that osmotic cell swelling leads to the activation of an anion channel. The subsequent depolarization of the cell membrane activates voltage-sensitive Ca2+ channels which increases [Ca2+]i, thus stimulating the contraction of vascular smooth muscle cells.

Mechanism of inhibition of neointimal formation by the angiotensin-converting enzyme inhibitor cilazapril. A study in balloon catheter-injured rat carotid arteries.

We investigated the mechanism of inhibition of neointima formation by the angiotensin-covering enzyme the carotid artery. We looked for the effects of cilazapril on all phases of the response to injury, ie, on proliferation of smooth muscle cells (SMCs) in the media, their migration, their proliferation in the neointima, and their disposition of extracellular matrix in the neointima. Although treatment was discontinued after 2 weeks, the inhibitory effect of cilazapril on neointimal formation was evident even 52 weeks after injury. The amount of extracellular matrix deposited in the intima during cilazapril treatment was decreased by 20% 2 weeks after injury, but no effect was seen if tissues were analyzed at 4 or 52 weeks. [3H]Thymidine-labeled cells (pulse labeling as well as 14-day continuous labeling) showed a decrease in SMC labeling in the tunica medica by 50%, but no inhibition in the labeling indices was seen in the neointima. The fraction of unlabeled neointimal cells in the cilazapril-treated rats as judged from continuous labeling experiments was inhibited by 86%. Taken together, these data suggest an antiproliferative effect on medial SMCs and an inhibition of SMC migration into the intima by cilazapril. Since intimal extracellular matrix deposition was only delayed, the decrease in medial SMC proliferation and subsequent migration seems to be the main reason for the reduction of neointima formation.

Effects of H2O2 on membrane potential and [Ca2+]i of cultured rat arterial smooth muscle cells.

The effect of 1 mmol/l H2O2 was studied on the membrane potential and [Ca2+]i with microelectrodes and the fura-2 technique, respectively. H2O2 induced a biphasic increase in [Ca2+]i with a fast transient peak and a subsequent plateau. H2O2 also led to a biphasic hyperpolarization of the cells with a similar time course. This was followed by a slight depolarization after wash-out of H2O2. External Ca2+ free solutions and treatment with the Ca2+ ionophore A23187 (1 mumol/l) abolished the effect of H2O2 on [Ca2+]i and almost entirely reduced the effect on the membrane potential. Phenylephrine (10 mumol/l) or A23187 also induced very similar biphasic hyperpolarizations of the membrane as H2O2 which were fully reversible after wash-out. It is concluded that H2O2 hyperpolarizes the membrane by opening of Ca2+ dependent K+ channels.

Renal proliferative and phenotypic changes in rats with two-kidney, one-clip Goldblatt hypertension.

Angiotensin II (AII) is a vasoconstrictive peptide with hypertrophic and mitogenic effects on many cell types. Previous studies have shown that in vivo administration of AII in rats results in proliferation of, and phenotypic changes in, many renal cell populations, but in doses also causing hypertension. Thus, it was not possible to differentiate nonhemodynamic from hypertensive effects of AII. Therefore, we studied rats with renin-dependent, AII-mediated hypertension (the two-kidney, one-clip Goldblatt model; mean systolic blood pressure 238 +/- 48 v 140 +/- 6 mm Hg in sham-operated controls). The unclipped kidneys, which were exposed to high blood pressure, developed significant glomerular and tubulointerstitial injury, tubulointerstitial cell proliferation, dense focal interstitial monocyte-macrophage influx, increased deposition of types I and IV collagen, as well as increased cellular expression of desmin and actin, in tubulointerstitial areas when examined at 11 weeks. In contrast, clipped kidneys, protected from hypertension but with high local renin expression, had minimal abnormalities. These studies suggest that in this model increased renin, and presumably AII, does not mediate significant proliferative or phenotypic changes in the kidney in the absence of hypertension at 11 weeks.

Mouse model of arterial injury.

In the present study, we established an injury model of the mouse carotid artery. Complete removal of the endothelium was achieved with a flexible wire. A platelet monolayer covered the denuded surface, and damage to underlying medial smooth muscle cells (SMCs) was detected. Injection of [3H]thymidine was used to determine the replication index for medial SMCs, which was found to be 1.6% at 2 days after denudation and 9.8% at 5 days. SMCs were observed in the intima by day 8 (replication index, 66%), and by 2 weeks the intimal lesion had a similar cell content as the media. In most animals, repair of the endothelial lining was complete 3 weeks after injury. The present model will allow us to use transgenic animals to address questions relevant to vascular biology and atherosclerosis.

Pituitary factors in blood plasma are necessary for smooth muscle cell proliferation in response to injury in vivo.

Intimal thickening in response to vascular injury is inhibited in animals previously subjected to hypophysectomy. We have investigated the nature and cell kinetics of this effect in a balloon catheter model of injury to the rat carotid artery. The ability of injury to stimulate [3H]thymidine labeling 48 hours after injury was almost completely eliminated in hypophysectomized (hypox) compared with control animals (0.1% versus 32.1%). Total DNA content of the developing neointima 14 days after injury was only 30% of the values found in ballooned carotid arteries of normal rats. If hypox rats were treated with recombinant human growth hormone, the proliferative response was not restored. There are two possible general explanations for the reduction of proliferative response in hypox animals: 1) that smooth muscle cells in the hypox animals have lost the ability to respond to the stimulus of injury or 2) that the ability of the smooth muscle cells to respond has not been reduced by prior hypophysectomy, but that the response itself requires the presence of pituitary-dependent factors. Transplantation experiments were performed in vivo to distinguish between these possibilities. Carotid arteries in inbred Lewis rats were excised 1 hour after balloon injury to give platelets the opportunity to adhere. These vessels were then transplanted from hypox into control animals and vice versa. At 48 hours, proliferation of smooth muscle cells in "control-to-hypox" transplants was 0.3% compared with 14.3% in "control-to-control" transplants, whereas vessels from hypox rats increased their indices to 4.8% if transplanted into control animals.(ABSTRACT TRUNCATED AT 250 WORDS)

Factors controlling the development of arterial lesions after injury.

This review discusses the role of growth factors on the proliferation of smooth muscle cells (SMCs) in injured arteries. We have used a variety of procedures to injure rat carotid arteries and noted that removal of endothelium followed by platelet adherence does not always initiate SMC replication. Furthermore, thrombocytopenia did not reduce the early SMC replication induced by balloon catheter injury. Platelet-derived growth factor (PDGF) has been found to exert little effect on SMC replication but markedly influences the ability of SMCs to migrate to the intima. Basic fibroblast growth factor (bFGF) is a potent mitogen for SMCs in denuded arteries while having no effect on cells in control uninjured arteries. We have hypothesized that arterial injury leads to release of bFGF from injured SMCs and so stimulates cell replication. Rats were treated with antibodies to bFGF immediately before balloon injury, and this significantly reduced the SMC replication. These findings suggest that in vivo bFGF is an important mitogen for initiating SMC replication and that PDGF is important as a chemotactic for SMCs.

Inhibition of leukocyte extravasation with a monoclonal antibody to CD18 during formation of experimental intimal thickening in rabbit carotid arteries.

In the rabbit model of electrically induced intimal thickening, the adherence processes of different leukocyte subsets as well as the functional significance of leukocyte invasion in the initial migration of smooth muscle cells (SMCs) into the intima were studied by using monoclonal antibody (MAb) 60.3 (directed to the leukocyte adherence glycoprotein CD18), a known potent inhibitor of leukocyte adhesive functions. In control carotid arteries exposed to two periods of electrical stimulation within 36 hours, leukocytes, including all granulocyte subsets, monocytes, and lymphocytes, invaded the cell-free subendothelium. Concomitantly, SMCs were observed to migrate from the media into the intima. In the MAb 60.3-treated rabbits, however, neutrophil emigration into the stimulated arteries was abolished, whereas mononuclear leukocyte accumulation in the intima was only partially inhibited, indicating a complete CD18-dependent mechanism for neutrophil extravasation and additional receptor-ligand systems for the emigration of mononuclear leukocytes. SMCs moved into the intima despite complete blockage of neutrophils and the reduced accumulation of mononuclear cells within the subendothelium after MAb administration. These results preclude neutrophils as initiators of SMC migration into the intima. The influence of mononuclear cells on the migratory behavior of SMCs in intimal thickening formation, however, needs further elucidation.

T lymphocytes inhibit the vascular response to injury.

The proliferation of vascular smooth muscle cells is controlled by specific growth factors and cytokines acting in paracrine networks. Macrophage products such as the platelet-derived growth factor and interleukin 1 promote smooth muscle proliferation and are released in the arterial wall during atherosclerosis and repair processes. T lymphocytes are also present in vascular tissue, but their role in vascular growth control in vivo has been unclear. We now demonstrate that rats in which T lymphocytes have been eliminated by a monoclonal antibody develop larger proliferative arterial lesions after balloon-catheter injury. Larger lesions also develop in athymic rnu/rnu rats that lack T lymphocytes, when compared with rnu/+ littermates with normal T-cell levels. Finally, injection of the lymphokine interferon gamma inhibits smooth muscle proliferation and results in smaller lesions compared with controls injected with buffer alone. These results indicate that T lymphocytes modulate smooth muscle proliferation during vascular repair. We propose that T lymphocytes may play an important, immunologically nonspecific role in tissue repair processes.

Alpha 1-receptor antagonists urapidil and prazosin inhibit neointima formation in rat carotid artery induced by balloon catheter injury.

Sympathetic nerves and catecholamines seem to have a trophic influence on vascular smooth muscle cells in vivo. We therefore tested whether alpha 1-antagonists would inhibit proliferation in arterial smooth muscle in vivo. Smooth muscle cells were stimulated to form a neointima in rat carotid arteries after deendothelialization by means of a 2F embolectomy catheter. The development of intimal lesions was determined 14 days after injury. The size of the neointima was measured using two parameters. Intimal DNA content was estimated from 5-mm segments of carotid arteries after DNA extraction and staining with Hoechst-stain. The size of the neointima was determined morphometrically as intimal area in histological cross-sections. Prazosin and urapidil were given orally once per day. Urapidil-treated rats showed significant inhibition of neointima formation for both parameters in a dose-dependent fashion. For prazosin a significant reduction could only be observed if DNA content was considered.

Intimal lesion formation in rat carotid arteries after endothelial denudation in absence of medial injury.

Injury of an artery by passage of a balloon catheter causes both endothelial denudation and medial damage and produces a marked smooth muscle cell (SMC) proliferative response. In this study, the endothelium from rat carotid arteries was removed by use of a rotating loop of 5/0 monofilament suture (gentle denudation technique), which did not cause any detectable damage to the underlying medial cells but did cause platelet adherence. Expression of platelet-derived growth factor (PDGF) A-chain and PDGF receptor mRNA was comparable to that seen in ballooned carotids, but the medial SMC proliferative response to gentle denudation was markedly reduced when compared to that observed after balloon denudation (1.4% vs. 13.6%). Intimal lesions were only observed in those zones that remained denuded for more than 7 days. These results demonstrate that a denuding injury with no medial trauma is sufficient to induce intimal lesions and that the significantly higher proliferation seen in ballooned vessels might reflect a response of the medial cells to trauma that occurred during denudation.

Role of platelets in smooth muscle cell proliferation and migration after vascular injury in rat carotid artery.

Intimal lesion formation was investigated in rats made thrombocytopenic by a single i.p. injection of a polyclonal antibody made against rat platelets that reduced circulating platelet counts to less than 1% of normal. The carotid artery was then denuded of endothelium with a 2 French balloon catheter, after which no platelets were found adhering to the exposed subendothelium. In control animals, platelets adhered instantly to the denuded artery. Six hours after denudation mRNA for ornithine decarboxylase, a marker for early G1 events, was found to be elevated in both thrombocytopenic and control arteries. Two days after injury the smooth muscle cell replication rate in thrombocytopenic rats was found to be significantly elevated as compared with that in uninjured carotids (13.7% +/- 8.4% vs. 0.65% +/- 0.23%) but was similar to the replication rate observed in denuded carotid arteries from animals treated with nonimmune IgG. One important difference between these animals was that no intimal thickening was observed in thrombocytopenic animals at day 4, and by day 7 the intimas were still significantly smaller than those from control rats. In a separate group of animals which were thrombocytopenic for the entire experiment, no intimal lesions were observed 7 days after injury by balloon catheter. From these results, we conclude that platelets do not play a role in the initiation of smooth muscle cell proliferation after injury by balloon catheter but may regulate their movement into the intima.

Regrowth of arterial endothelium. Denudation with minimal trauma leads to complete endothelial cell regrowth.

Endothelial regeneration in the rat carotid artery was investigated using two different techniques of denudation. With balloon catheter denudation, medial cell death occurred, and endothelial regrowth stopped after several weeks, leaving a large area devoid of endothelium. After denudation with a new technique that removed the endothelium without damaging the media complete endothelial regrowth was achieved. Acutely after this denudation, large platelet thrombi were present on the subendothelial surface of vessels denuded with the filament loop. In contrast, balloon catheter denuded arteries showed only a platelet monolayer on their luminal surface. Within the first few weeks after denudation with either technique the regenerating endothelial cells stained strongly for basic fibroblast growth factor. At later times when replication of endothelium had stopped, the balloon catheter denuded vessels did not stain with this antibody. After filament denudation endothelial cell replication remained high until regrowth was complete and intensive staining was observed in the regenerating endothelial cells at all times. No differences were seen in staining of smooth muscle cells for transforming growth factor-beta and fibronectin in either set of denuded vessels. Both groups showed transforming growth factor-beta to be located in the developing intima and especially on the apical surface of luminal smooth muscle cells. The surface of these luminal smooth muscle cells also stained with antibody to fibronectin. These data demonstrate that total regrowth of endothelium can occur over large denuded areas despite the presence of transforming growth factor-beta and fibronectin on these surfaces. Furthermore the ability of these endothelial cells to proliferate would appear to be dependent on the presence of basic fibroblast growth factor and on the severity of the trauma induced by denudation.