Heparin binding, internalization, and metabolism in vascular smooth muscle cells: II. Degradation and secretion in sensitive and resistant cells

Low-molecular-weight heparin and unfractionated heparin decrease Th-1, 2, and 17 expressions

Background

We evaluated the effects of T helper cell differentiation in a mite-allergic animal model treated with inhaled heparins of different molecular weight.

Method

BALB/c mice were divided into four groups: 1. Control, 2. Mite intratracheal (mIT), 3. Inhaled heparin (hIN), 4. Inhaled low-molecular-weight heparin (lmwhIN). Groups 2, 3, and 4 were sensitized twice with Der p allergen subcutaneously on day 1 and day 8. Der p allergen was administered intratracheally on day 15. Groups 3 and 4 were treated with heparin or low-molecular-weight (lmw) heparin intranasally from day 1 to 22. Splenocytes from sacrificed mice stimulated with 16 µg/ml of Der p were cultured for 72 hours. Supernatants of splenocyte were collected to analyze the effect of Interleukin (IL)17-A/F, Interferon(IFN)-γ, IL-4, IL-13, and IL-10. Serum was also collected for Der P-specific IgE level on day 23. Total RNA was extracted from spleen tissue for mRNA expression. Gene expression of Foxp3, IL-10 IFN-γ, GATA3, IL-5, and RORγt were analyzed.

Results

Both hIN and lmwhIN groups had lower serum IgE level than that of the mIT group (both p<0.0001). Both hIN and lmwhIN groups showed significantly decreased transcripts of GATA-3, IFN-γ, IL-5, and RORγt mRNA in their spleen. Regarding the supernatant of splenocyte culture stimulated with Der p, compared with the mIT group, there were significant decreases in IL-17A/F, IFN-γ, IL-4, IL-13, and IL-10 secretion in inhaled hIN and lmwhIN groups.

Conclusions

From this balb/c mice study, the analyses of mRNA and cytokines revealed that both intranasal heparin and lmw heparin treatment decreased the expression of Th1, Th2, and Th17 in spleen. The underlying mechanism(s) warrant further studies.

Surface modification with dopamine and heparin/poly-L-lysine nanoparticles provides a favorable release behavior for the healing of vascular stent lesions

Surface biofunctional modification of coronary artery stents to prevent thrombosis and restenosis formation, as well as accelerate endothelialization, has become a new hot spot. However, bioactive coatings on implants are not yet sufficiently developed for long-term activity, as they quickly lose efficiency in vivo and finally fail. On the basis of a novel time-ordered concept of biofunctionality for vascular stents, heparin/poly l-lysine nanoparticle (NP) was developed and immobilized on a polydopamine-coated titanium surface, with the aim of regulating and maintaining the intravascular biological response within the normal range after biomaterial implantation. An in vitro dynamic release model was established to mimic the blood flow condition in vivo with three phases: (1) An early phase (1-7 days) with release of predominantly anticoagulant and anti-inflammatory substances and to a minor degree antiproliferative effects against smooth muscle cells (SMCs); (2) this is followed by a phase (7-14 days) of supported endothelial cell (ECs) proliferation and suppressed SMC proliferation with persisting high antithrombogenicity and anti-inflammatory properties of the surface. (3) Finally, a stable stage (14-28 days) with adequate biomolecules on the surface that maintain hemocompatibility and anti inflammation as well as inhibit SMCs proliferation and promote ECs growth. In vivo animal tests further confirmed that the NP-modified surface provides a favorable release behavior to apply a stage-adjusted remedy. We suggested that these observations provide important guidance and potential means for reasonable and suitable platform construction on a stent surface.

Low-concentration heparin suppresses ionomycin-activated CAMK-II/EGF receptor- and ERK-mediated signaling in mesangial cells

Heparin and endogenous heparinoids inhibit the proliferation of smooth muscle cells, including renal mesangial cells; multiple effects on signaling pathways are well established, including effects on PKC, Erk, and CaMK-II. Many studies have used heparin at concentrations of 100 microg/ml or higher, whereas endogenous concentrations of heparinoids are much lower. Here we report the effects of low-concentration (1 microg/ml) heparin on activation of several kinases and subsequent induction of the c-fos gene in mesangial cells in response to the calcium ionophore, ionomycin, in the absence of serum factors. Ionomycin rapidly increases the phosphorylation of CaMK-II (by 30 s), and subsequently of the EGF receptor (EGFR), c-Src, and Erk 1/2. Low-dose heparin suppresses the ionomycin-dependent phosphorylation of EGFR, c-Src, and Erk 1/2, but not of CaMK-II, whereas inhibition of activated CaMK-II reduces phosphorylation of EGFR, c-Src, and Erk. Our data support a mechanism whereby heparin acts at the cell surface to suppress downstream targets of CaMK-II, including EGFR, leading in turn to a decrease in Erk- (but not c-Src-) dependent induction of c-fos.

The anti-inflammatory effects of heparin and related compounds

Heparin is a glycosaminoglycan well known for its anticoagulant properties. In addition, heparin possesses anti-inflammatory effects. Although the mechanisms responsible for the anticoagulant effects of heparin are well understood, those underlying its anti-inflammatory effects are not. This review presents some of the evidence from clinical and animal studies supporting an anti-inflammatory role for heparin and heparin-related derivatives. Potential mechanisms by which heparin can exert its anti-inflammatory effects are discussed. The clinical use of heparin as an anti-inflammatory agent has been held back by the fear of bleeding. Development of nonanticoagulant heparins or heparin derivatives should mitigate this concern.

Heparin suppresses lipid raft-mediated signaling and ligand-independent EGF receptor activation

Heparin is well known to suppress vascular smooth muscle cell (VSMC) proliferation, and attempts to exploit this therapeutically have led to recognition of multiple pathways for heparin's anti-mitogenic actions. At low concentrations (ca. 1 microg.ml(-1)), these suppressive effects may reflect physiological activities of endogenous heparan sulfates, and appear to be rapid responses to extracellular or cell surface-associated heparin. Because heparin has been shown to influence expression of caveolin proteins, and caveolae/lipid rafts are critical structures modulating cell signaling, we examined the effect of heparin on signaling involving cholesterol-rich membrane microdomains. The VSMC line PAC-1 activates the MAP kinase Erk in response to the cholesterol-sequestering agents methyl-beta-cyclodextrin and nystatin. This follows a temporal sequence that involves Ras-GTP activation of MEK, and is independent of PKC, Src, and PI3 kinase. However, ligand-independent phosphorylation of the EGF receptor (EGFR) by removal of cholesterol precedes Ras activation, and the EGFR kinase inhibitor AG1478 blocks Erk phosphorylation, supporting occurrence of the signaling sequence EGFR-Ras-MEK-Erk. Phosphorylation of EGFR occurs predominantly in caveolin-rich microdomains as identified by Western blotting of fractions from density gradient centrifugation of membranes prepared under detergent-free conditions. In these situations, heparin inhibits phosphorylation of EGFR on the Src-dependent site Tyr(845), but not the autophosphorylation of Tyr(1173), and decreases Ras activation and Erk phosphorylation. We conclude that heparin can suppress Erk signaling in VSMC with effects on site-specific phosphorylation of EGFR localized in caveolin-enriched lipid rafts.

Suppression of mitogen-activated protein kinase phosphatase-1 (MKP-1) by heparin in vascular smooth muscle cells

Heparin inhibits vascular smooth muscle cell (VSMC) proliferation, but mechanisms remain elusive. Because heparin inhibits signaling through multiple kinase cascades, we investigated the possibility that phosphatases could be involved. Mitogen-activated protein kinase phosphatase-1 (MKP-1) was the predominant MKP detected in VSMC lines. MKP-1 protein was increased by serum stimulation of quiescent cells, and this increase was diminished by heparin (1 microg/mL). Increased MKP-1 expression was dependent on the mitogen-activated protein kinase, Erk. Decreased Erk activity in the presence of heparin preceded, and may account for, decreased MKP-1. The antimitogenic effects of heparin are therefore unlikely to act through a shift in the kinase/phosphatase balance, but rather through direct kinase suppression. However, because MKP-1 is known to cause an increase in activity of kinases upstream of Erk, that may signal through additional pathways, the decrease in MKP-1 activity may paradoxically enhance heparin's antiproliferative effects. VSMC selected to grow in the presence of heparin express decreased levels of MKP-1 that are unresponsive to heparin, and Erk activity becomes unresponsive to heparin in one cell line. We conclude that phosphatase activation is not a direct mechanism of suppression of multiple kinase cascades by heparin.

Modulation of smooth muscle phenotype in vitro by homologous cell substrate

We have developed a novel cell culture system that supports the shortening of smooth muscle cells. Primary rat airway smooth muscle cells were plated on an ethanol-fixed, confluent monolayer of homologous smooth muscle cells (homologous cell substrate, HCS). Cells grown on HCS exhibited morphological and functional characteristics consistent with a differentiated phenotype. Cells on HCS were spindle shaped with a well-defined long axis, whereas cells grown on glass were larger and irregularly shaped. Smooth muscle-specific alpha-actin immunostained diffusely in cells on HCS, whereas it appeared as stress fibers in cells on glass. Agonists recruited a greater fraction of HCS cells to contract, resulting in greater changes in cell area or length on average, but the maximal capacity of shortening of individual cells was similar between the groups. Unlike cells on glass, cells on HCS shortened to methacholine. HCS was reversible and persisted over several passages. Agonists stimulated intracellular Ca(2+) oscillations in cells on HCS, whereas they elicited biphasic peak and plateau transients in cells on glass. HCS modulates smooth muscle cell phenotype in vitro.

Heparin inhibits phosphorylation and autonomous activity of Ca(2+)/calmodulin-dependent protein kinase II in vascular smooth muscle cells

Vascular smooth muscle cell (VSMC) hyperproliferation is a characteristic feature of both atherosclerosis and restenosis seen after vascular surgery. A number of studies have shown that heparin inhibits VSMC proliferation in vivo and in culture. To test our hypothesis that heparin mediates its antiproliferative effect by altering Ca(2+) regulated pathways involved in mitogenic signaling in VSMC, we analyzed the effect of heparin on multifunctional Ca(2+)/calmodulin dependent protein kinase II (CaM kinase II) which is abundantly expressed in VSMC. Using activity assays, radioactive labeling, and immunoprecipitation it was found that heparin inhibits the overall phosphorylation of the delta-subunit of CaM kinase II which is consistent with inhibition of autophosphorylation-dependent, Ca(2+)/calmodulin-independent CaM kinase II activity. This effect was less evident in heparin-resistant cells, consistent with a role for CaM kinase II in mediating the antiproliferative effect of heparin. Finally, the effects of pharmacological inhibitors of phosphatases like okadaic acid, calyculin, and tautomycin suggest that heparin inhibits CaM kinase II phosphorylation by activating protein phosphatases 1 and 2A. These findings support the hypothesis that alterations in calcium-mediated mitogenic signaling pathways may be involved in the antiproliferative mechanism of action of heparin.

Low molecular weight fucoidan prevents neointimal hyperplasia in rabbit iliac artery in-stent restenosis model

OBJECTIVE

Smooth muscle cell (SMC) proliferation within the intima is regulated by heparan sulfates. We studied a low molecular weight (LMW) fucoidan (sulfated polysaccharide from brown seaweed) on SMC proliferation in vitro and intimal hyperplasia in vivo.

METHODS AND RESULTS

In vitro study revealed that LMW fucoidan reduces rabbit SMC proliferation and is internalized in SMC perinuclear vesicles. On rabbit iliac arteries perfused in vivo with fluorolabeled LMW fucoidan after angioplasty, the labeling was mainly located on sites of injury. Pharmacokinetic studies showed that LMW fucoidan exhibited in rats an elimination half-life of 56+/-25 minutes (n=8) after intravenous administration and a constant plasma rate for > or =6 hours after intramuscular administration. After stent implantation in their iliac arteries, rabbits were also treated with LMW fucoidan (5 mg/kg IM twice a day). Histomorphometric analysis at day 14 indicated that LMW fucoidan reduced intimal hyperplasia by 59% (1.79+/-0.4 versus 0.73+/-0.2 mm2, P<0.0001) and luminal cross-sectional area narrowing by 58% (0.38+/-0.08 versus 0.16+/-0.04, P<0.0001). Blood samples showed no anticoagulant activity due to LMW fucoidan.

CONCLUSIONS

This natural polysaccharide with high affinity for SMCs and sustained plasma concentration markedly reduced intimal hyperplasia, suggesting its use for the prevention of human in-stent restenosis.

An alpha-helical domain within the carboxyl terminus of herpes simplex virus type 1 (HSV-1) glycoprotein B (gB) is associated with cell fusion and resistance to heparin inhibition of cell fusion

Previous studies from our laboratory indicated that a 28-amino-acid carboxyl-terminal truncation of gB caused extensive virus-induced cell fusion (Baghian et al., 1993, J Virol 67, 2396-2401). We tested the ability of additional truncations and mutations within gB to cause cell fusion in the recently established virus-free cell fusion assay (Turner et al., 1998, J. Virol. 72, 873-875). Deletion of the carboxyl-terminal 28 amino acids of gB (gBDelta28), which removed part of the predicted alpha-helical structure H17b, caused extensive cell fusion. A gB truncation specified by gBDelta36, which removed the entire H17b domain, caused as much cell fusion as the gBDelta28 truncation. Similarly, gB(A874P) containing a substitution of an Ala with Pro within H17b caused cell fusion. Heparin, a gB-specific inhibitor of virus-induced cell fusion, inhibited both wild-type gB and gB(syn3)-mediated cell fusion. In contrast, fusion of cells transfected with gB(Delta28), gB(Delta36), or gB(A874P) was resistant to heparin inhibition of cell fusion. We concluded the following: (1) The predicted alpha-helical structure of H17b within the carboxyl terminus of gB is involved in both virus-induced and virus-free cell fusion. (2) Heparin is a specific inhibitor of gB-mediated fusion in both systems. (3) Resistance to heparin inhibition of gB-mediated cell fusion is associated with the predicted alpha-helical structure H17b within the carboxyl terminus of gB.

Molecular mechanisms of phenotypic plasticity in smooth muscle cells

Morphological, functional, molecular and cell biology studies have revealed a striking multifunctional nature of individual smooth muscle cells (SMC). SMCs manifest phenotypic plasticity in response to changes in environment and functional requirements, acquiring a range of structural and functional properties bounded by two extremes, called "synthetic" and "contractile." Each phenotypic state is characterized by expression of a unique set of structural, contractile, and receptor proteins and isoforms that correlate with differing patterns of gene expression. Recent studies have identified signaling pathways and transcription factors (e.g., RhoA GTPase/ROCK, also known as Rho kinase, and serum response factor) that regulate the transcriptional activities of genes encoding proteins associated with the contractile apparatus. Mechanical plasticity of contractile-state smooth muscle further extends SMC functional diversity. This may also be regulated, in part, by the RhoA GTPase/ROCK pathway, via reorganization of cytoskeletal and contractile proteins. Future studies that define transcriptional and posttranscriptional mechanisms of SMC plasticity are necessary to fully understand the role of SMC in the pathogenesis and morbidity of human diseases of the airways, vasculature, and gastrointestinal tract.

Chondroitin sulphate inhibits connective tissue mast cells

1. Mast cells derive from the bone marrow and are responsible for the development of allergic and possibly inflammatory reactions. Mast cells are stimulated by immunoglobulin E (IgE) and specific antigen, but also by a number of neuropeptides such as neurotensin (NT), somatostatin or substance P (SP), to secrete numerous pro-inflammatory molecules that include histamine, cytokines and proteolytic enzymes. 2. Chondroitin sulphate, a major constituent of connective tissues and of mast cell secretory granules, had a dose-dependent inhibitory effect on rat peritoneal mast cell release of histamine induced by the mast cell secretagogue compound 48/80 (48/80). This inhibition was stronger than that of the clinically available mast cell 'stabilizer' disodium cromoglycate (cromolyn). Inhibition by chondroitin sulphate increased with the length of preincubation and persisted after the drug was washed off, while the effect of cromolyn was limited by rapid tachyphylaxis. 3. Immunologic stimulation of histamine secretion from rat connective tissue mast cells (CTMC) was also inhibited, but this effect was weaker in umbilical cord-derived human mast cells and was absent in rat basophilic leukemia (RBL) cells which are considered homologous to mucosal mast cells (MMC). Oligo- and monosaccharides were not as effective as the polysaccharides. 4. Inhibition, documented by light and electron microscopy, involved a decrease of intracellular calcium ion levels shown by confocal microscopy and image analysis. Autoradiography at the ultrastructural level showed that chondroitin sulphate was mostly associated with plasma and perigranular membranes. 5. Chondroitin sulphate appears to be a potent mast cell inhibitor of allergic and nonimmune stimulation with potential clinical implications.

Characterization of [3H]-heparin binding in human vascular smooth muscle cells and its relationship to the inhibition of DNA synthesis

1. The glycosaminoglycan heparin inhibits vascular smooth muscle cell (VSMC) proliferation and migration, but the mechanism of its antiproliferative action remains unclear. Heparin has been reported to bind to high affinity cell surface sites on animal VSMC before undergoing receptor mediated endocytosis resulting in signal transduction into the cytoplasm and modulation of genes involved in proliferation. In this study, we have characterized the binding of [3H]-heparin to human saphenous vein-derived VSMC and examined whether there is any relationship between the affinity of [3H]-heparin binding and the inhibitory effect of heparin and its structural analogues on DNA synthesis. 2. At 4 degrees C [3H]-heparin binding to human VSMC occurred in a specific, time and concentration-dependent manner and was not influenced by the removal of calcium ions. Binding of the ligand appeared to occur to the cell surface and was both saturable and reversible. Kinetic and steady state data indicated a single class of binding sites. 3. The pharmacology of [3H]-heparin binding was examined in displacement studies using unlabelled heparin and structural analogues. A comparison of the rank potencies of heparin, heparan sulphate fraction II, low molecular weight heparin and trehalose octasulphate showed that there was a marked discrepancy between their estimated affinities in the binding assays and their effect on DNA synthesis. 4. In summary, we have characterized the heparin binding site on human saphenous vein-derived VSMC. Our findings suggest that the action of heparin and its analogues on DNA synthesis does not simply reflect an interaction with the cell-associated heparin binding site defined in these studies, but may also be determined by the internalization and metabolism of the glycosaminoglycan(s).

Heparin proteoglycans released from rat serosal mast cells inhibit proliferation of rat aortic smooth muscle cells in culture

-Mast cells are present in the human arterial intima. To study whether mast-cell degranulation influences the rate of proliferation of smooth muscle cells, we cocultured sensitized (IgE-bearing) rat serosal mast cells and rat aortic smooth muscle cells (SMCs). When sensitized mast cells were stimulated to degranulate with antigen, the rate of proliferation of the cocultured SMCs decreased sharply. This inhibitory effect was found to be due mainly to the very high molecular weight (Mr) heparin proteoglycans (average Mr 750 000) released from the stimulated mast cells. When the heparin proteoglycans were purified from mast-cell granule remnants and added to the SMC culture, they were found to block the cell cycle at the G0-->S transition and the exit from the G2/M phase, their inhibitory effect resembling that of commercial heparin. However, in contrast to the reported dependence of the inhibitory effect of commercial heparin on the release of transforming growth factor-beta from serum, the inhibitory effect of the mast cell-derived heparin proteoglycans in the presence of serum was not transforming growth factor-beta dependent. Moreover, the effect of the mast cell-derived heparin proteoglycans was more efficient than that of commercial heparins of high (average Mr 15 000) and low (average Mr 5000) molecular weight. We also purified heparin glycosaminoglycans (average Mr 75 000) from the mast cell-derived heparin proteoglycans and found that they also inhibited SMC growth efficiently, although less strongly than their parent heparin proteoglycans. These results reveal, for the first time, that mast cells are able to regulate SMC growth. Thus, activated mast cells, by releasing heparin proteoglycans, possibly participate in the regulation of SMC growth in the human arterial intima, the site of atherogenesis.

Heparin sensitive and resistant vascular smooth muscle cells: biology and role in restenosis

Vascular smooth muscle cells (VSMC)s are characterized by their acute growth inhibition by heparin and heparan sulfates; however, recently the isolation of VSMCs which display greatly diminished sensitivity to the antiproliferative action of heparin have been reported. These heparin resistant (HR) VSMCs have been derived through multiple passage of normal rat VSMCs in culture media containing high heparin doses, by transformation of VSMCs with oncogene-containing vectors, or have been isolated from vascular tissues of spontaneously hypertensive rats, healthy humans, or humans with restenosis where their presence is not limited to sites of injury. Initial characterizations of HR VSMCs are reviewed, and here we propose a definition of HR VSMCs. To date the mechanisms underlying heparin insensitivity remain elusive. Further study of HR VSMCs may expand our understanding of cell growth regulation by heparin, establish whether HR VSMCs contribute to the reported failure of heparin to combat restenosis in humans, and identify cellular mechanisms driving certain vascular proliferative diseases.

Endothelial proteoglycans inhibit bFGF binding and mitogenesis

Basic fibroblast growth factor (bFGF) is a known mitogen for vascular smooth muscle cells and has been implicated as having a role in a number of proliferative vascular disorders. Binding of bFGF to heparin or heparan sulfate has been demonstrated to both stimulate and inhibit growth factor activity. The activity, towards bFGF, of heparan sulfate proteoglycans present within the vascular system is likely related to the chemical characteristics of the glycosaminoglycan as well as the structure and pericellular location of the intact proteoglycans. We have previously shown that endothelial conditioned medium inhibits both bFGF binding to vascular smooth muscle cells and bFGF stimulated cell proliferation in vitro. In the present study, we have isolated proteoglycans from endothelial cell conditioned medium and demonstrated that they are responsible for the bFGF inhibitory activity. We further separated endothelial secreted proteoglycans into two fractions, PG-A and PG-B. The large sized fraction (PG-A) had greater inhibitory activity than did PG-B for both bFGF binding and bFGF stimulation of vascular smooth muscle cell proliferation. The increased relative activity of PG-A was attributed, in part, to larger heparan sulfate chains which were more potent inhibitors of bFGF binding than the smaller heparan sulfate chains on PG-B. Both proteoglycan fractions contained perlecan-like core proteins; however, PG-A contained an additional core protein (approximately 190 kDa) that was not observed in PG-B. Both proteoglycan fractions bound bFGF directly, and PG-A bound a significantly greater relative amount of bFGF than did PG-B. Thus the ability of endothelial heparan sulfate proteoglycans to bind bFGF and prevent its association with vascular smooth muscle cells appears essential for inhibition of bFGF-induced mitogenesis. The production of potent bFGF inhibitory heparan sulfate proteoglycans by endothelial cells might contribute to the maintenance of vascular homeostasis.

Diverse effects of heparin on mitogen-activated protein kinase-dependent signal transduction in vascular smooth muscle cells

Proliferation of vascular smooth muscle cells (SMCs) is implicated in pathological events, including atherosclerosis and intimal hyperplasia following angioplasty. The glycosaminoglycan heparin is a growth inhibitor of SMCs in vitro and in vivo. The underlying mechanism, however, is still poorly understood. In the present study, we report that heparin inhibited the activation of the mitogen-activated protein kinase (MAPK) in baboon SMCs by serum but not by platelet-derived growth factor (PDGF). When fibroblast growth factor was used, heparin had a stimulatory effect on MAPK. The only MAPK-activating kinase found in SMCs was MAPK kinase (MAPKK)-1, although MAPKK-2 was present in comparable amounts. Activation of MAPKK-1 and DNA synthesis were affected by heparin in a similar fashion. Heparin does not appear to exert its effects through members of the protein kinase C family, which are downregulated by phorbol esters, because it was still capable of inhibiting MAPK/MAPKK-1 stimulation by FCS in phorbol ester-pretreated cells. The present findings support the conclusions that the effects of heparin depend on the nature of the mitogen and that heparin inhibits SMC proliferation by preventing activation of MAPKK-1.

Nitric oxide degradation of heparin and heparan sulphate

NO is a bioactive free radical produced by NO synthase in various tissues including vascular endothelium. One of the degradation products of NO is HNO2, an agent known to degrade heparin and heparan sulphate. This report documents degradation of heparin by cultured endothelial-cell-derived as well as exogenous NO. An exogenous narrow molecular-mass preparation of heparin was recovered from the medium of cultured endothelial cells using strong-anion exchange. In addition, another narrow molecular-mass preparation of heparin was gassed with exogenous NO under argon. Degradation was evaluated by gel-filtration chromatography. Since HNO2 degrades heparin under acidic conditions, the reaction with NO gas was studied under various pH conditions. The results show that the degradation of exogenous heparin by endothelial cells is inhibited by NO synthase inhibitors. Exogenous NO gas at concentrations as low as 400 p.p.m. degrades heparin and heparan sulphate. Exogenous NO degrades heparin at neutral as well as acidic pH. Endothelial-cell-derived NO, as well as exogenous NO gas, did not degrade hyaluronan, an unrelated glycosaminoglycan that resists HNO2 degradation. Peroxynitrite, a metabolic product of the reaction of NO with superoxide, is an agent that degrades hyaluronan; however, peroxynitrite did not degrade heparin. Thus endothelial-cell-derived NO is capable of degrading heparin and heparan sulphate via HNO2 rather than peroxynitrite. These observations may be relevant to various pathophysiological processes in which extracellular matrix is degraded, such as bone development, apoptosis, tissue damage from inflammatory responses and possible release of growth factors and cytokines.

Differentiated properties and proliferation of arterial smooth muscle cells in culture

The smooth muscle cell is the sole cell type normally found in the media of mammalian arteries. In the adult, it is a terminally differentiated cell that expresses cytoskeletal marker proteins like smooth muscle alpha-actin and smooth muscle myosin heavy chains, and contracts in response to chemical and mechanical stimuli. However, it is able to revert to a proliferative and secretory active state equivalent to that seen during vasculogenesis in the fetus, and this is a prerequisite for the involvement of the smooth muscle cell in the formation of atherosclerotic and restenotic lesions. A similar transition from a contractile to a synthetic phenotype occurs when smooth muscle cells are established in culture. Accordingly, an in vitro system has been used extensively to study the regulation of differentiated properties and proliferation of these cells. During the first few days after seeding, the cells are reorganized structurally with a loss of myofilaments and formation of a widespread endoplasmic reticulum and a prominent Golgi complex. In parallel, they lose their contractility and instead become competent to divide in response to a large variety of mitogens, including platelet-derived growth factor (PDGF) and basic fibroblast growth factor (bFGF). After entering the cell cycle, they start to produce these and other mitogens on their own, and continue to replicate in the absence of exogenous stimuli for a restricted number of generations. Furthermore, they start to secrete extracellular matrix components such as collagen, elastin, and proteoglycans. The mechanisms that control this change in morphology and function of the smooth muscle cells are still poorly understood. Adhesive proteins such as fibronectin and laminin apparently have an important role in determining the basic phenotypic state of the cells and exert their effects via integrin receptors. The proliferative and secretory activities of the cells are influenced by a multitude of growth factors, cytokines, and other molecules. Although much work remains before an integrated view of this regulatory machinery can be achieved, there is no doubt that the cell culture technique has contributed substantially to our knowledge of smooth muscle differentiation and growth. At the same time, it has been crucial in exploring the role of these cells in vascular disease and developing new therapeutic strategies to cope with major causes of human death and disability.

Heparin binding, internalization, and metabolism in vascular smooth muscle cells: I. Upregulation of heparin binding correlates with antiproliferative activity

Vascular smooth muscle cell (SMC) hyperplasia is an important component in the pathogenesis of arteriosclerotic lesions and is responsible for the failure of many vascular surgical procedures. SMC proliferation is inhibited by the glycosaminoglycan heparin; however, the precise mechanisms of action are still not understood. One important question in this regard is whether binding, internalization, and metabolism of heparin are necessary for the antiproliferative activity. In this study, we have analyzed SMC rendered resistant to the antiproliferative effect of heparin by drug selection and retroviral infection of SMC. We first examined the ability of heparin to bind to SMC. Experiments using [3H]heparin indicate the presence of saturable, heparin-displaceable, protease-sensitive binding sites on both sensitive and resistant SMC. The affinity of heparin binding does not correlate with the antiproliferative response. Using fluorescent and radiolabeled heparin probes, we observed that early heparin internalization kinetics in both sensitive and resistant SMC is similar, indicating that resistance to heparin is not due to changes in the ability of cells to take up heparin. In contrast, we observed during the continuous incubation with heparin that binding to resistant SMC is rapidly downregulated, whereas sensitive cells continue to bind and internalize heparin. These results suggest that upregulation of heparin binding to the SMC surface is required for an antiproliferative response. In an accompanying paper (Letourneur et al. [1995] J. Cell Physiol., 165:687-695, this issue), we describe the degradation and secretion of internalized heparin in both sensitive and resistant SMC.