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Distinct Regulation of Mitogen-activated Protein Kinases and p27Kip1 in Smooth Muscle Cells from Different Vascular Beds

Excessive proliferation and migration of vascular smooth muscle cells (SMCs) participate in atherosclerotic plaque growth. In this study, we investigated whether SMCs from vessels with different atherogenicity exhibit distinct growth and migratory potential and investigated the underlying mechanisms. In fat-fed rabbits, we found increased cell proliferation and atheroma formation in the aortic arch versus the femoral artery. When examined in culture, SMCs isolated from the aortic arch (ASMCs) displayed a greater capacity for inducible proliferation and migration than paired cultures of femoral artery SMCs. Two lines of evidence suggested that distinct regulation of the growth suppressor p27Kip1(p27) contributes to establishing these phenotypic dissimilarities. First, p27 expression was comparably lower in ASMCs, which exhibited a higher fraction of p27 phosphorylated on Thr-187 and ubiquitinated. Second, forced p27 overexpression in ASMCs impaired their proliferative and migratory potential. We found that platelet-derived growth factor-BB-dependent induction of the mitogen-activated protein kinase (MAPK)/extracellular signal-regulated kinase (ERK) pathway was comparably higher in ASMCs. Importantly, pharmacological inhibition of MAPKs increased p27 expression and attenuated ASMC proliferation and migration. In contrast, forced MAPK activation diminished p27 expression and markedly augmented femoral artery SMC proliferation and migration. We propose that intrinsic differences in the regulation of MAPKs and p27 play an important role in creating variance in the proliferative and migratory capacity of vascular SMCs, which might in turn contribute to establishing regional variability in atherogenicity.

cutaneous transluminal angioplasty has become a well established technique for revascularization of patients with arterial occlusive disease. The occurrence of restenosis at the site of angioplasty remains the major limitation despite the successful procedure. The molecular basis of atherosclerosis and restenosis involves dedifferentiation of vascular smooth muscle cells (SMCs) 1 to a so-called "synthetic state" characterized by abundant production of matrix components and excessive proliferative and migratory activities (1)(2)(3). Therefore, a better understanding of the molecular mechanisms underlying these processes should help develop novel therapeutic approaches for the treatment of cardiovascular disease. Cellular proliferation is regulated by the balance between multiple cyclin-dependent kinase (CDK)/cyclin holoenzymes and members of the Cip/Kip and INK4 families of CDK inhibitors (4,5). Active CDK/cyclin complexes promote cell cycle progression by phosphorylating the retinoblastoma gene product, pRb, and the related pocket proteins p107 and p130 from mid-G 1 to mitosis. CDK inhibitors associate with and inhibit the activity of CDK/cyclin holoenzymes. Studies arguing for a role of the Cip/Kip protein p27 Kip1 (p27) in the pathophysiology of the cardiovascular system include the following. 1) p27 may contribute to the reestablishment of the quiescent phenotype after the initial proliferative response to balloon angioplasty in rat and porcine arteries, and adenovirus-mediated overexpression of p27 inhibited neointimal growth in these experimental models (6 -8). 2) p27 may function as a molecular switch that regulates the phenotypic response of vascular SMCs to both hyperplastic and hypertrophic stimuli (9,10). 3) p27 is a negative regulator of endothelial cell proliferation and migration in vitro, and adenovirus-mediated overexpression of p27 inhibited angiogenesis in vivo (11,12). 4) p27 may contribute to integrin-mediated control of vascular SMC proliferation (13). 5) p27 may limit cardiomyocyte proliferation during early postnatal development and after injury in adult mice (14,15). 6) Changes in p27 expression might regulate human vascular cell proliferation within atherosclerotic lesions (7,16), and a causal link between reduced p27 expression and atherosclerosis has been established in apolipoprotein E-deficient mice (17). It has been established that the expression of p27 is regulated mainly at the level of translation and protein turnover (18).
Multiple growth factors and cytokines interact with specific receptors located in the cytoplasmic membrane of vascular cells in response to a variety of pathological stimuli, thus triggering a complex signal transduction cascade, which culminates in changes in gene expression that execute a proliferative and migratory response (2,3). The activation of the mitogen-activated protein kinase (MAPK) signal transduction pathway is thought to play an important role during cardiovascular disease (19 -23).
It has been well established that different segments of the arterial tree display significant differences in their susceptibility to atherosclerosis, both in animal models and humans. In this regard, it is notable that vascular SMCs display regional phenotypic variance both when comparing cells obtained from different compartments of the same vessel or cells isolated from vessels from different vascular beds (24 -30). The findings of this study demonstrate that p27 and MAPKs are critical regulators of vascular SMC proliferation and migration. Our results suggest that intrinsic differences in the regulation of p27 and MAPKs may contribute to the establishment of regional variance in the proliferative and migratory capacity of SMCs from distinct regions of the vascular system.
Rabbit Studies-Male white New Zealand rabbits (4 -5-month-old) were fed either control chow (n ϭ 5) or received for 2 months a high fat diet (n ϭ 10) containing 10 g of cholesterol (Sigma) and 60 ml of peanut oil/kg control chow (1% cholesterol). Animals received four intraperitoneal injections of 5-BrdUrd (20 mg/kg each, Sigma) at 12-h intervals starting 48 h before sacrifice. Rabbits were killed with an overdose of pentobarbital. A cut was made in the cava vein, and the systemic circulation was thoroughly perfused with saline through the heart. The aortic arch and the right femoral artery were fixed in situ with 100% methanol. Arteries were removed, fixation was continued overnight, and tissues were paraffin-embedded and cut in 5-m crosssections. Immunohistochemistry using mouse monoclonal anti-BrdUrd antibody (1:50) was done with a biotin/streptavidinperoxidase detection system (Signet Laboratories) and 3,3Јdiaminobenzidine tetrahydrochloride substrate (Sigma).
Cell Culture and Retroviral Infection-The aortic arch, the common carotid artery, and the femoral artery of 4-month-old male New Zealand White rabbits were extracted to prepare primary cultures (SMCs isolated from the aortic arch (ASMCs), carotid artery SMCs (CSMCs), and femoral artery SMCs (FSMCs), respectively). Arteries were dissected free from surrounding tissue and adventitia and cut into small pieces. Aortic tissue was digested with collagenase (2 mg/ml, Worthington) in Dulbecco's modified Eagle's medium-F12 supplemented with 5% fetal bovine serum (FBS) for 3 h in a shaking bath at 37°C. Cells were incubated at 37°C in a humidified 5% CO 2 , 95% O 2 atmosphere in Dulbecco's modified Eagle's medium-F12 supplemented with 10% FBS, 100 units/ml penicillin, 0.1 mg/ml streptomycin, and 2 mmol/liter Lglutamine. All studies were carried out with primary cultures between passages 2 and 8. Pharmacological inhibition of MAPK kinase (MEK) was achieved by exposing ASMC cultures to PD98059 (Tocris) as indicated in the figure legends.
Recombinant retrovirus were generated using the retroviral vectors pBabePuro-p27 wt (31) and pBabePuro-MEKE, which encode for wildtype p27 and a constitutively active MEK1 mutant (32), respectively. pBabePuro-MEKE was generated by digesting pcDNAIII-MEKE (a gift of C. Caelles) with BamHI and XhoI and subcloning the MEKE cDNA into pBabePuro. The infection of asynchronously growing cells was performed as suggested by the supplier of the PT67-packaging cells (Clontech). Infected cells were selected in the presence of puromycin (2.5 g/ml, Sigma).
Immunofluorescence Labeling of Vascular SMC Differentiation Markers and TUNEL Assay-Cells were plated onto glass coverslips. To examine the expression of differentiation markers, cells were grown until reaching confluence and then were maintained in mitogen-free insulin-transferrin-selenium (Invitrogen) supplemented with 250 mol/ liter ascorbic acid (ITC, Sigma) medium (33) for 2 days. Cells were fixed with 4% paraformaldehyde in phosphate-buffered saline (PBS) at room temperature for 1 h and permeabilized with 0.1% Triton X-100/PBS. Cells were blocked with 1% bovine serum albumin/PBS, and expression of smooth muscle ␣-actin (SM ␣-actin) and desmin was examined by indirect immunofluorescence. Microscopic images were digitally recorded on an Axioscope II microscope (Zeiss).
For TUNEL assays, cells were grown to ϳ60% confluence and were maintained in mitogen-free ITC medium for 2 days. For UV light irradiation, cell culture medium was removed and the cells were washed twice with PBS. The cultures then were placed in the tissue culture hood and exposed to UV light for 45 min (UV G-30-watt lamp, Sylvania, Japan). Control (not irradiated) and UV-irradiated cells were fixed and permeabilized as indicated above, and TUNEL assay was performed using an in situ cell death detection kit as suggested by the manufacturer (Roche Molecular Biochemicals).
Proliferation Assays-Cells for [ 3 H]thymidine incorporation assays were plated in 10% FBS/Dulbecco's modified Eagle's medium-F12 at a density of 4 ϫ 10 4 cells/well in 12-well plates. When ϳ80% confluence was reached, cells were rendered quiescent by maintaining cultures for 48 -72 h in mitogen-free ITC medium (33). Starvation-synchronized cultures were stimulated with platelet-derived growth factor-BB (PDGF-BB) (10 ng/ml) to induce cell cycle reentry, and cells were pulsed with 1 mCi/liter [ 3 H]thymidine (Amersham Biosciences) during the last 4 h of incubation. After washes with cold PBS, DNA was precipitated with 15% trichloroacetic acid and solubilized with 0.2 mol/liter NaOH. Radioactivity incorporated into DNA was measured in a scintillation counter (Wallac).
Migration Assays-Migration of cultured cells labeled with the fluorescent dye calcein-AM (Molecular Probes) was assessed with the FAL-CON HTS FluoroBlock system as suggested by the manufacturer (BD Biosciences). Labeled cells were placed in the inserts (8.0-m pore size, 5 ϫ 10 4 cells/insert) in serum-free media. The lower chamber contained either serum-free media (unstimulated cells) or the chemotactic agent (10% FBS or 10 ng/ml PDGF-BB) (induced cells). Serum-free medium was supplemented with 0.1% bovine serum albumin. Chemotaxis at different times after plating the cells was assessed by detecting the fluorescence in the lower chamber using a Victor 4120 multilabel counter (Wallac). Results represent the average fluorescence of induced cells (n ϭ 3) after subtracting the fluorescence of unstimulated cells (n ϭ 2-3).
CDK activity in cell lysates (100 g of protein) was determined as described previously (6) with the exception that CDK/cyclin holoenzymes were immunoprecipitated with 0.2 g of each of the anti-cyclin E and anti-cyclin A antibodies.
Statistical Analysis-Results are reported as the mean Ϯ S.E. Differences were evaluated using either two-tail unpaired Student's t test or ANOVA and Fisher's post hoc test (Statview, SAS Institute).

Arterial Cell Proliferation and Atherogenesis in Different
Vascular Beds of Hypercholesterolemic Rabbits-We investigated atherogenesis in fat-fed New Zealand White rabbits, which rapidly develop atheromas in response to dietary manipulation (34). To examine arterial cell proliferation, animals received four injections of BrdUrd prior to sacrifice. Although aortic atherosclerosis and BrdUrd immunoreactivity were essentially undetectable in rabbits fed with control chow (n ϭ 5, data not shown), all of the fat-fed rabbits included in our study displayed atheromatous lesions in the aortic arch and exhibited abundant BrdUrd immunoreactivity in both intimal and me- dial cells (n ϭ 10, Fig. 1A). In marked contrast, only 3 of 10 fat-fed rabbits displayed small atherosclerotic lesions in the femoral artery (Fig. 1B). Moreover, the number of BrdUrdpositive cells in femoral arteries was negligible in the media and was lower within the lesions as compared with the aortic arch (Fig. 1B). These findings are consistent with previous rabbit studies demonstrating that the aortic arch is highly susceptible to diet-induced atherosclerosis (34 -37).
ASMCs and FSMCs Display Dissimilar Migratory and Proliferative Activity in Vitro-Having demonstrated distinct proliferative response and atherogenicity in the aortic arch and femoral artery, we isolated SMCs from these vessels (ASMCs and FSMCs, respectively) to ascertain whether their phenotypic dissimilarities were maintained in vitro. In primary cultures grown to confluence in serum-free media, ASMCs exhibited an epithelioid shape ( Fig. 2A), whereas FSMCs disclosed a bipolar, spindle-shaped morphology (Fig. 2B). We next performed indirect immunofluorescence experiments in passage 2 cultures to examine the expression of SMC differentiation markers. Both ASMCs and FSMCs revealed abundant SM ␣-actin immunoreactivity in a prominent stress fiber pattern (Fig. 2, C and D). In contrast, desmin expression appeared more abundant in FSMCs (Fig. 2, E and F). These phenotypes were stable at least up to passage 8 (data not shown).
We next compared the migratory and proliferative capacity of cultured ASMCs and FSMCs. Although FSMCs did not migrate in response to 6 h of stimulation with either PDGF-BB or FBS, both agents elicited a robust migratory response in paired Confluent cultures were maintained for 72 h in mitogen-free ITC medium and then exposed to 10 ng/ml PDGF-BB as indicated. Cell extracts were prepared in lysis buffer A containing phosphatase inhibitors (A, B, D, and E) or buffer B (C), which did not contain phosphatase inhibitors. The analysis of lysates included cyclin A/cyclin E-associated CDK activity using histone H1 and [␥-32 P]ATP substrates (A), Western blot with the indicated antibodies (B-D), and immunoprecipitation with an anti-ubiquitin antibody followed by Western blot of the immunoprecipitated material using anti-p27 antibodies (E). A, kinase reactions were analyzed by SDS-PAGE and autoradiography. Relative activity was estimated after densitometric analysis (0 h is set as 1 for each cell type). B, densitometric analysis was performed to estimate the relative level of cyclin D1 and A. Each cyclin value was divided by its corresponding CDK2-loading control. Shown below is the PVDF membrane stained with Ponceau prior to incubation with antibodies. C, densitometric analysis was performed to estimate the relative p27 level. Each p27 value was divided by its corresponding tubulin-loading control (ASMC at 0 h ϭ 1; nd, not detected). D and E, the phospho-specific anti-p27 antibody only recognizes p27 phosphorylated on Thr-187. Open and closed arrowheads point to the slow and faster migrating p27-immunoreactive band, respectively. Note that the slow migrating band that undergoes phosphorylation on Thr-187 and ubiquitinilation prevailed in ASMCs. By contrast, the faster migrating p27 band that does not contain protein phosphorylated on Thr-187 and does not undergo ubiquitination predominated in FSMCs.
cultures of ASMCs (Fig. 2G). Likewise, [ 3 H]thymidine incorporation in starvation-synchronized cultures restimulated with PDGF-BB was lower in FSMCs (Fig. 2H). For example, compared with starved cultures, maximum [ 3 H]thymidine incorporation at 24 h post-stimulation increased by 16-and 42-fold in FSMCs and ASMCs, respectively. The proliferative response toward 10% FBS was also stronger in ASMCs (data not shown). In contrast, as determined by the TUNEL assay, apoptosis was similar in ASMCs and FSMCs, both under control conditions and after UV irradiation (Fig. 2I).
Lineage analysis experiments have suggested that neural crest-derived (ectoderm) SMCs prevail in arterial segments proximal to the heart (i.e. aortic arch and great vessels of the head and neck), whereas arteries located more distally to the heart contain mainly mesoderm-derived SMCs (i.e. abdominal aorta and hind limb arteries) (1,27,38). Thus, dissimilar behavior and morphology of ASMCs and FSMCs raised the possibility that adult SMC phenotypic properties are related, at least in part, to their primary embryonic lineage. Consistent with this notion, we found that CSMCs (also of neural crest origin) behaved in a similar fashion as the ASMCs in proliferation and migration assays (Fig. 3). Fig. 2H showing greater PDGF-BB-dependent proliferation in ASMCs than in FSMCs, CDK activity was higher in PDGF-BB-stimulated ASMCs (Fig. 4A). Likewise, the up-regulation of the positive cell cycle regulators cyclin D1 and cyclin A, whose expression is induced as starvation-synchronized cells resume progression through G 1 and S-phase upon mitogen restimulation (4, 5), occurred earlier and was more prominent in PDGF-BB-stimulated ASMCs versus FSMCs (Fig. 4B). The expression of the PDGFR-␤ was similar in ASMCs and FSMCs, both under mitogen-free conditions and upon PDGF-BB stimulation (Fig. 4C), suggesting that dissimilar PDGF-BB-dependent proliferation and migration in ASMCs and FSMCs were not a consequence of distinct regulation of PDGFR-␤ expression. Down-regulation of PDGFR-␤ 9 h after PDGF-BB stimulation is consistent with the notion that binding of PDGF to its receptor leads to internalization and degradation of the ligand-receptor complex in endosomes (39).

Role of p27 in the Establishment of Phenotypic Variance between ASMCs and FSMCs-Differences in proliferation and migration between ASMCs and FSMCs prompted us to investigate the underlying molecular mechanisms. Consistent with the results of
We next investigated the expression of the growth suppressor p27 in the same confluent cultures of ASMC and FSMC used for the PDGFR-␤ immunoblot. Of note, the lysis buffer used in these assays did not contain phosphatase inhibitors (buffer B). Both under mitogen-free conditions and at different time points after PDGF-BB stimulation, p27 was detected as a single band that was more abundant in confluent cultures of FSMCs versus ASMCs (Fig. 4C). For example, whereas p27 was not detected in ASMC after 9 h of stimulation, FSMCs expressed more p27 at this time point than did unstimulated ASMCs. An analysis of subconfluent cultures also disclosed higher level of p27 expression in FSMCs (data not shown). We next examined cell lysates prepared in the presence of phosphatase inhibitors (buffer A), which also disclosed higher p27 expression in FSMCs versus ASMC (Fig. 4D, top blot). Notably, these experiments demonstrated the presence of two p27-immunoreactive bands of different electrophoretic mobility and FIG. 5. Inhibition of ASMC proliferation and migration by retrovirus-mediated p27 overexpression. ASMCs were infected with control retrovirus (Rev-LacZ) or a retrovirus encoding for p27 (Rev-p27). Infected cells were selected with puromycin. A, cells were maintained in 10% FBS/Dulbecco's modified Eagle's medium and pulsed for 4 h with [ 3 H]thymidine, and radioactivity incorporated into DNA was quantified. Differences were evaluated using two-tail unpaired Student's t test (*, p Ͻ 0.0015; n ϭ 5). Puromycin-resistant cells were also lysed in buffer A to perform Western blot analysis using anti-p27 and anti-␣-tubulin antibodies. Densitometric analysis was performed to estimate the relative level of p27. Each p27 value was divided by its corresponding tubulin-loading control (Rev-LacZ; n ϭ 1). B, migration of ASMCs infected with Rev-LacZ or Rev-p27 was measured as indicated in Fig. 2G using 10% FBS as the chemotactic agent. Differences were evaluated using ANOVA and Fisher's post hoc test. Comparisons versus t ϭ 0: *, p Ͻ 0.005; **, p Ͻ 0.0001; comparisons between Rev-LacZ and Rev-p27 at each time point: †, p Ͻ 0.02; † †, p Ͻ 0.0001.

FIG. 6. ASMCs and FSMCs display dissimilar MAPK regulation.
Western blot analysis of cells maintained for 72 h in mitogen-free ITC medium and then exposed to 10 ng/ml PDGF-BB for the indicated time. Cell lysates were prepared in lysis buffer A, P-ERK1/2 and ERK1/2 indicate phosphorylated (active) and total ERK1/2, respectively. Densitometric analysis was performed to estimate the relative level of P-ERK1/2. Each P-ERK value was divided by its corresponding ERK-loading control (ASMC at 0 h ϭ 1). distinct relative abundance in these cells. Averaged over four experiments, the slower migrating band (open arrowhead) predominated in ASMCs (89.7% Ϯ 8.0), whereas the faster migrating band (closed arrowhead) prevailed in FSMCs (95.7% Ϯ 1.5). Western blot analysis using a phospho-specific antibody identified the slower migrating band as p27 phosphorylated on Thr-187 (Fig. 4D, middle blot). This phosphorylation event is thought to initiate the major pathway for p27 proteolysis via a mechanism involving its ubiquitination and subsequent turnover in the proteasome (18). Consistent with this notion, immunoprecipitation experiments using an anti-ubiquitin antibody followed by Western blot analysis revealed the presence of ubiquitinated p27 in the slower migrating p27-immunoreactive band in both ASMCs and FSMCs (Fig. 4E). It is noteworthy that the faster migrating p27-immnuoreactive band in ASMCs but not in FSMCs also contained ubiquitinated p27 (see "Discussion"). Collectively, these results suggest that the majority of p27 in ASMCs undergoes phosphorylation on Thr-187 and ubiquitination, whereas these post-translational modifications are detected only in a small fraction of p27 in FSMCs.
We next investigated the effect of p27 overexpression on ASMC proliferation and migration by infecting these cells with retroviral vectors encoding for p27 (Rev-p27). Rev-p27-infected ASMCs disclosed a 3-fold increase in p27 expression, which caused a reduction in [ 3 H]thymidine incorporation (Fig. 5A) and migration (Fig. 5B) as compared with control cultures infected with Rev-LacZ. These findings demonstrate that increased p27 expression is sufficient to attenuate the growth and migratory capacity of ASMCs. Thus, distinct regulation of p27 expression might contribute to establishing differences in the proliferative and migratory capacity of ASMCs and FSMCs.

Differential Regulation of MAPKs in ASMCs and FSMCs and Role in the Regulation of Vascular SMC Proliferation and
Migration-Because the MAPK pathway plays a pivotal role in transducing environmental signals required for both cellular proliferation and migration (40), we examined the kinetics of expression and activation of individual MAPKs in ASMCs and FSMCs. Western blot analysis using an antibody specific for the phosphorylated (active) form of the MAPK isoforms of 44 and 42 kDa (dubbed ERK1 and ERK2, respectively) revealed a rapid activation of these proteins upon PDGF-BB stimulation of mitogen-depleted ASMCs and FSMCs (Fig. 6, top blot). However, the maximum level of ERK1/2 activation was higher in ASMCs than in FSMCs. Moreover, ERK1/2 activation was more prolonged in ASMCs. These differences occurred despite similar level of total ERK1/2 in ASMCs and FSMCs (Fig. 6,  bottom blot).
To determine whether dissimilar MAPK regulation might contribute to phenotypic differences between ASMCs and FSMCs, we performed loss-and gain-of-function experiments. Treatment of ASMCs with PD98059, a selective inhibitor of MEK, impaired PDGF-BB-dependent ERK1/2 activation (Fig.  7A) and up-regulated p27 expression (Fig. 7B). Importantly, the exposure of asynchronously growing ASMCs to PD98059 inhibited [ 3 H]thymidine incorporation in a dose-dependent manner (Fig. 7C), and preincubation of starvation-synchronized ASMCs with PD98059 blocked de novo DNA synthesis upon mitogen stimulation (Fig. 7D). Moreover, the exposure of ASMCs to PD98059 inhibited migration (Fig. 8A).
We also examined the effect of forced ERK1/2 activation on FIG. 7. MAPK inhibition up-regulates p27 expression and inhibits PDGF-BB-dependent ASMC proliferation. Differences were evaluated using ANOVA and Fisher's post hoc test. A and B, ASMCs were maintained for 72 h in ITC medium and then were exposed to 10 ng/ml PDGF-BB for short (A) or long (B) periods of time (8 h). Cell lysates were prepared in lysis buffer A to perform immunoblot analysis with the indicated antibodies. Treatment with 50 M PD98059 was initiated 1 h before the addition of PDGF-BB. P-ERK1/2 and ERK1/2 indicate phosphorylated (active) and total ERK1/2, respectively. Densitometric analysis was performed to estimate the relative level of P-ERK1/2 and p27. Each P-ERK or p27 value was divided by its corresponding loading control (total ERK or tubulin, respectively; nd, not detected). For p27, results are shown relative to control (set as 1). C, asynchronously growing ASMCs were treated for 1 h in mitogen-free ITC medium supplemented with PD98059 or vehicle, and then cells were incubated for 24 h with 10 ng/ml PDGF-BB. Cultures were pulsed with FSMC proliferation and migration by infecting cultures with a retroviral vector encoding for a constitutively active MEK1 mutant (Rev-MEKE). As compared with control cultures, Rev-MEKE-infected FSMCs disclosed constitutive activation of ERK1/2 (Fig. 9A), which markedly reduced p27 expression (Fig.  9B), increased [ 3 H]thymidine incorporation (Fig. 9C), and aug-mented cell migration (Fig. 8B). Collectively, the above studies suggest that differential regulation of ERK1/2 in ASMCs and FSMCs plays an important role in the establishment of intrinsic differences in the proliferative and migratory potential of these cells.

DISCUSSION
Vascular SMCs undergo dedifferentiation and excessive proliferation and migration during atherosclerosis and restenosis post- angioplasty (1-3). Up-regulation of the growth suppressor p27 in the arterial wall might limit SMC proliferation at late time points after balloon angioplasty in rat and porcine arteries (6,7), and adenovirus-mediated overexpression of p27 inhibited neointimal thickening in these animal models (8,41). Regarding the role of p27 on atherosclerosis, genetic disruption of p27 increased arterial cell proliferation and accelerated atheroma formation in hypercholesterolemic apolipoprotein E-deficient mice (17). Moreover, p27 might mediate transforming growth factor-␤-dependent inhibition of cell growth in human atheromas (16), and proliferating cells within human coronary atheromas appear to express a low level of p27 (7). Consistent with the observation that p27 overexpression attenuated human vascular endothelial cell migration in vitro (12) and that p27 inactivation reduced rapamycin-dependent inhibition of vascular SMC migration (42), we found that retrovirus-mediated overexpression of p27 inhibited vascular SMC migration. Thus, p27 might control neointimal thickening through the regulation of both cell proliferation and migration.
Our studies with fat-fed rabbits showed that aortic arch tissue displays increased cell proliferation and atherogenicity as compared with femoral artery. We found that primary cultures of ASMCs and FSMCs maintained marked differences in their growth and migratory potential, which might be related, at least in part, to their distinct primary embryonic lineage (neural crest and mesoderm, respectively) (1,27,38). Indeed, ASMCs and CSMCs, which are thought to derive from neural crest ectoderm, behaved similarly in our proliferation and migration assays. We chose to examine ASMCs and FSMCs as an in vitro model to elucidate molecular mechanisms involved in the establishment of dissimilar atherogenicity in distinct vessel segments. Greater ASMC proliferation and migration corre- . Infected cells were selected with puromycin. Puromycin-resistant cells were maintained for 72 h in mitogen-free ITC medium, and then cultures were stimulated with 10 ng/ml PDGF-BB for the indicated time. A and B, cells were lysed in buffer A to perform immunoblot analysis. P-ERK1/2 and ERK1/2 indicate phosphorylated (active) and total ERK1/2, respectively. Densitometric analysis was performed to estimate the relative level of P-ERK1/2 and p27. Each P-ERK and p27 value was divided by its corresponding loading control (total ERK and tubulin, respectively; nd, not detected). For p27, results are shown relative to Rev-LacZ at 0 h (set as 1). C, lated with the lower expression of p27 when compared with FSMCs, and retrovirus-mediated overexpression of p27 attenuated the growth and migratory potential of ASMCs. Previous studies also support the notion that distinct regulation of p27 expression plays an important role in establishing differences in the phenotypic response of vascular SMCs toward a variety of stimuli. First, Yang et al. (29) reported reduced proliferation of human internal mammary artery compared with saphenous vein SMCs. Importantly, PDGF-BB markedly down-regulated p27 protein level in saphenous vein, but this response was much less pronounced in internal mammary artery. Thus, sustained p27 expression despite growth stimuli may contribute to the resistance to growth of SMCs from internal mammary artery and to the longer patency of arterial versus venous grafts. Second, p27 may regulate the proliferative response of vascular SMCs toward fibroblast growth factor 2 (FGF2 or basic FGF). Whereas FGF2 plays a critical role in the induction of medial SMC proliferation after balloon angioplasty (30,43,44), neutralizing antibodies to FGF2 failed to inhibit neointimal SMC proliferation in balloon-injured arteries (45). Moreover, only a small increase in growth was observed when arteries with existing neointimal lesions were exposed to FGF2 (30,43). Attenuated FGF2-dependent proliferation of neointimal SMCs occurred despite a robust induction of positive cell cycle regulators (30). Interestingly, neointimal SMCs expressed high levels of p27 compared with medial SMCs, and FGF2 infusion did not reduce the level of this inhibitor in arteries with established neointimal lesions.
Protein turnover is thought to play a major role in the regulation of p27 expression. Phosphorylation of p27 on Thr-187 triggers its ubiquitination and rapid turnover in the proteasome (18). Our Western blot assays demonstrate that the majority (90%) of p27 in ASMCs corresponds to a slow migrating form that undergoes phosphorylation on Thr-187 and ubiquitination. In marked contrast, ϳ96% p27 in FSMCs corresponded to a faster migrating p27 band that was not recognized by the phospho-specific antibody and did not contain ubiquitinated protein. Thus, the relative amount of p27 phosphorylated on Thr-187 and ubiquitinated appears higher in ASMCs compared with FSMCs, which might account for the lower level of p27 detected in ASMCs. Of note, ubiquitinated p27 in the faster migrating band that does not contain phosphorylated Thr-187 was also detected in ASMCs (cf. Fig. 4E). This finding is in agreement with recent studies demonstrating an additional pathway for p27 ubiquitination and proteolysis independent of phosphorylation of p27 on Thr-187 (46,47).
We investigated additional regulatory networks involved in the establishment of vascular SMC-phenotypic variance. A wealth of evidence implicates the rapid activation of the MAPK signal transduction pathway during the pathogenesis of cardiovascular disease (19,21). For example, it has been suggested that persistent activation and hyperexpression of ERK1/2 are critical elements to initiate and perpetuate cell proliferation during diet-induced atherogenesis in the rabbit (48). Moreover, ERK1/2 activation occurs rapidly after angioplasty of porcine and rat arteries, (20,22), and all three MAPKs are activated in human-failing hearts (49). Our results indicate that ERK1/2 contribute to establishing phenotypic differences between ASMCs and FSMCs. First, mitogen-dependent activation of ERK1/2 was more robust in ASMCs than in FSMCs. Second, reduced ERK1/2 activation by exposure of ASMCs to PD98059 impaired their growth and migratory capacity. By contrast, forced activation of ERK1/2 greatly increased FSMC proliferation and migration. We observed increased p27 expression upon ERK1/2 blockade in ASMCs and diminished p27 expression upon forced ERK1/2 activation in FSMCs. Thus, in agree-ment with previous studies in NIH 3T3 fibroblasts and cancer cells (50 -53), our findings suggest an important role for the MAPK pathway in the control of p27 expression in ASMCs and FSMCs. Solid ERK1/2 activation in mitogen-stimulated ASMC cultures might facilitate p27 degradation, thus favoring proliferation and migration of these cells. In contrast, weaker ERK1/2 activation might contribute to comparably higher expression of p27 in FSMCs, thus hindering their proliferative and migratory responses. In consideration of this model, it is noteworthy that PDGF-BB induced similar MAPK activation in cultures of saphenous vein and internal mammary artery despite distinct regulation of p27 in these cells (29), suggesting that MAPK-independent mechanisms of p27 regulation might operate in the SMCs of different vascular beds.
In conclusion, we propose that intrinsic differences in MAPKdependent signaling and p27 expression in rabbit ASMCs and FSMCs contribute to establishing variance in their proliferative and migratory potential. These dissimilarities might be attributable, at least in part, to their distinct primary embryonic origin. Further clarification of the molecular networks underlying vascular SMC-phenotypic variance should shed significant insight into the mechanisms leading to regional variability in the susceptibility to intimal lesion development.