Does platelet-derived growth factor-A chain stimulate proliferation of arterial mesenchymal cells in human atherosclerotic plaques?

Chemical analysis of atherosclerotic plaque cholesterol combined with histology of the same tissue

Sensitive method for chemical analysis of free cholesterol (FC) and cholesterol esters (CE) was developed. Mouse arteries were dissected and placed in chloroform-methanol without tissue grinding. Extracts underwent hydrolysis of cholesteryl esters and derivatization of cholesterol followed by liquid chromatography/mass spectrometry (LC/MS/MS) analysis. We demonstrated that FC and CE could be quantitatively extracted without tissue grinding and that lipid extraction simultaneously worked for tissue fixation. Delipidated tissues can be embedded in paraffin, sectioned, and stained. Microscopic images obtained from delipidated arteries have not revealed any structural alterations. Delipidation was associated with excellent antigen preservation compatible with traditional immunohistochemical procedures. In ApoE(-/-) mice, LC/MS/MS revealed early antiatherosclerotic effects of dual PPARalpha,gamma agonist LY465606 in brachiocephalic arteries of mice treated for 4 weeks and in ligated carotid arteries of animals treated for 2 weeks. Reduction in CE and FC accumulation in atherosclerotic lesions was associated with the reduction of lesion size. Thus, a combination of LC/MS/MS measurements of CE and FC followed by histology and immunohistochemistry of the same tissue provides novel methodology for sensitive and comprehensive analysis of experimental atherosclerotic lesions.

Persistent cytomegalovirus infection is associated with increased expression of TGF- 1, PDGF-AA and ICAM-1 and arterial intimal thickening in kidney allografts

BACKGROUND

Cytomegalovirus (CMV) is a suggested risk factor for the development of chronic allograft nephropathy. Transforming growth factor-beta (TGF-beta) and platelet-derived growth factor (PDGF) are important molecules in this process. We analysed the impact of persistent CMV infection in kidney allografts on the expression of growth factors, adhesion molecules and inflammation markers.

METHODS

In a population of 172 renal transplant recipients, CMV was diagnosed in 82 patients by pp65 antigenaemia test and viral cultures. Biopsies taken after CMV infection were available from 48 of the 82 patients for the demonstration of CMV antigens by immunohistochemistry and in situ DNA hybridization. Biopsy material for further analyses was available from 16 CMV patients. Five patients with no previous CMV infection were used as controls. Biopsy histology was scored according to Banff 97 classification.

RESULTS

In 11 out of 16 patients, persistent CMV antigens and/or DNA were demonstrated in the biopsy >2 months after the last positive finding in blood or urine. Increased expression of TGF-beta1 was recorded in tubuli and in arterial endothelium in biopsies with a positive CMV finding compared with controls. Also, the expression of PDGF-AA was increased in tubuli and somewhat in arterial endothelium in CMV-positive biopsies. The expression of intercellular adhesion molecule-1 (ICAM-1) was increased significantly in peritubular capillary endothelium. Vascular intimal thickening was increased in the biopsies with persistent CMV infection.

CONCLUSIONS

Persistent CMV infection in kidney allografts was associated with increased vascular changes and increased expression of TGF-beta1, PDGF-AA and ICAM-1.

Differential binding of platelet-derived growth factor isoforms to glycosaminoglycans

The platelet-derived growth factor (PDGF) family comprises disulfide-bonded dimeric isoforms and plays a key role in the proliferation and migration of mesenchymal cells. Traditionally, it consists of homo- and heterodimers of A and B polypeptide chains that occur as long (A(L) and B(L)) or short (A(S) and B(S)) isoforms. Short isoforms lack the basic C-terminal extension that mediates binding to heparin. In the present study, we show that certain PDGF isoforms bind in a specific manner to glycosaminoglycans (GAGs). Experiments performed with wild-type and mutant Chinese hamster ovary cells deficient in the synthesis of GAGs revealed that PDGF long isoforms bind to heparan sulfate and chondroitin sulfate, while PDGF short isoforms only bind to heparan sulfate. This was confirmed by digestion of cell surface GAGs with heparitinase and chondroitinase ABC and by incubation with sodium chloride to prevent GAG sulfation. Furthermore, exogenous GAGs inhibited the binding of long isoforms to the cell membrane more efficiently than that of short isoforms. Additionally, we performed surface plasmon resonance experiments to study the inhibition of PDGF isoforms binding to low molecular weight heparin by GAGs. These experiments showed that PDGF-AA(L) and PDGF-BB(S) isoforms bound to GAGs with the highest affinity. In conclusion, PDGF activity at the cell surface may depend on the expression of various cellular GAG species.

Lipoprotein (a) binds and inactivates tissue factor pathway inhibitor: a novel link between lipoproteins and thrombosis

Lipoprotein (a) [Lp(a)] has been associated with both anti-fibrinolytic and atherogenic effects. However, no direct link currently exists between this atherogenic lipoprotein and intravascular coagulation. The current study examined the binding and functional effects of Lp(a), its lipoprotein constituents, apoliprotein (a) [apo(a)] and low-density lipoprotein (LDL), and lysine-plasminogen (L-PLG), which shares significant homology with apo(a), on tissue factor pathway inhibitor (TFPI), a major regulator of tissue factor-mediated coagulation. Results indicate that Lp(a), apo(a), and PLG but not LDL bound recombinant TFPI (rTFPI) in vitro and that apo(a) bound to a region spanning the last 37 amino acid residues of the c-terminus of TFPI. The apparent binding affinity for TFPI was much higher for Lp(a) (KD approximately 150 nM) compared to PLG (KD approximately 800 nM) and nanomolar concentrations of apo(a) (500 nM) inhibited PLG binding to TFPI. Lp(a) also inhibited in a concentration-dependent manner rTFPI activity and endothelial cell surface TFPI activity in vitro, whereas PLG had no such effect. Moreover physiologic concentrations of PLG (2 microM) had no effect on the concentration-dependent inhibition of TFPI activity induced by Lp(a). In human atherosclerotic plaque, apo(a) and TFPI immunostaining were shown to coexist in smooth muscle cell-rich areas of the intima. These data suggest a novel mechanism whereby Lp(a) through its apo(a) moiety may promote thrombosis by binding and inactivating TFPI.

Effects of TCV-116 on endothelin-1 and PDGF A-chain expression in angiotensin II-induced hypertensive rats

Angiotensin II (Ang II) has been shown to stimulate cardiac growth and collagen synthesis in cultured vascular smooth muscle cells and to increase fibroblast proliferation. Chronic infusion with Ang II increases blood pressure and activates growth mechanisms to produce hypertrophy of the heart. This study investigated the effects of an Ang II type 1 receptor antagonist, TCV-116, on preproendothelin-1 (preproET-1), ETA receptor and platelet-derived growth factor (PDGF) A-chain expression in the left ventricle of Wistar-Kyoto rats treated for 2 weeks with Ang II (200 ng x kg(-1) x min(-1)), and the relation of these effects to myocardial remodeling. Rats given Ang II alone (ANGII-V) were compared with rats also receiving TCV-116 (ANGII-TCV). In both groups, blood pressure was similar and significantly higher than in control rats. The preproET-1, ET(A) receptor and PDGF A-chain expressions in the left ventricle were significantly increased in ANGII-V compared with control rats, and were significantly suppressed in ANGII-TCV compared with ANGII-V rats. ANGII-V rats showed a significant increase of the type I collagen expression, wall-to-lumen ratio, perivascular fibrosis, and myocardial fibrosis, with all these parameters being significantly improved by TCV-116. Myocardial remodeling in Ang II-induced hypertensive rats was significantly ameliorated by a subdepressor dose of TCV-116, which may have been due to a decrease in ET-1 and PDGF A-chain expression in the left ventricle.

Induction of platelet-derived growth factor B-chain expression by transforming growth factor-beta involves transactivation by Smads

Transforming growth factor-beta (TGF-beta) regulates a diverse array of biological processes, such as proliferation, differentiation, extracellular matrix production, and apoptosis. In cultured vascular endothelial cells, TGF-beta induces the expression of platelet-derived growth factor (PDGF) B-chain, a mitogen and chemoattractant, at the level of transcription. The molecular mechanism(s) underlying this process are not presently understood. In this study, we performed serial 5' deletion and transient transfection analysis to define a region in the PDGF-B promoter mediating inducible responsiveness to TGF-beta. This region contains an atypical nucleotide recognition element for the Smad family of transcriptional regulators. Electrophoretic mobility shift analysis revealed that nuclear proteins bound to this site in a transient and specific manner. Supershift studies demonstrated the physical association of Smad4 with the promoter. Overexpression of Smad4 activated the PDGF-B promoter and superinduced PDGF-B promoter-dependent expression in cells exposed to TGF-beta. Moreover, simultaneous cotransfection of Smad3 and Smad4 activated the PDGF-B promoter. This effect was attenuated when Smad4 was substituted with its dominant negative counterpart. Mutation of the (-81)CAGA(-78) motif in the PDGF-B promoter abrogated Smad-inducible promoter-dependent expression. Overexpression of Smad2 and Smad3 transactivated the PDGF-B promoter in a synergistic manner. These findings demonstrate the existence of a novel, functional binding element in the proximal region of the PDGF-B promoter mediating responsiveness to TGF-beta.

The association of platelet-derived growth factor receptor expression, plaque morphology and histological features with symptoms in carotid atherosclerosis

Platelet-derived growth factor may influence smooth muscle cell migration and proliferation and, therefore, carotid plaque composition and stenosis. Platelet-derived growth factor receptor expression and histological features were compared in carotid plaques from symptomatic and asymptomatic patients. Immunocytochemistry and histology determined platelet-derived growth factor-alpha and -beta receptor expression, white blood cell infiltration, smooth muscle cell, elastin, cholesterol, collagen and intraplaque haemorrhage in carotid artery plaques removed at surgery or the post-mortem. Plaques with > 70% stenosis from asymptomatic (n = 10) and symptomatic patients (n = 27) had higher expression of platelet-derived growth factor and beta receptors and higher scores for macrophages and intraplaque haemorrhage than plaques with < 70% stenosis from asymptomatic patients (n = 33). Plaques with > 70% stenosis from symptomatic patients had significantly lower alpha receptor expression than plaques with > 70% stenosis from asymptomatic patients. The reduction of alpha receptor expression, which may inhibit smooth muscle cell migration, suggests that differential expression of platelet-derived growth factor receptor subunits in plaques may be related to symptoms.

Effects of somatostatin, somatostatin analogs, and endothelial cell somatostatin gene transfer on smooth muscle cell proliferation in vitro

OBJECTIVE

Somatostatin analogs inhibit neointimal hyperplasia and smooth muscle cell (SMC) proliferation in vivo. The gene transfer of somatostatin to endothelial cells (ECs) represents a potential means of local delivery of somatostatin to areas of arterial injury. This study tested the hypothesis that the retroviral gene transfer of somatostatin to ECs would inhibit SMC proliferation in vitro and evaluated the effects of somatostatin analogs on DNA synthesis and the growth of SMCs.

METHODS

Media transfer and coculture were used to determine the effects of somatostatin-producing ECs on SMC proliferation in vitro. The effects of a variety of somatostatin isoforms and analogs on the proliferation of SMCs, mitogenesis of serum-restimulated quiescent SMCs, and arterial explants were measured.

RESULTS

Despite the production of biologically relevant concentrations of somatostatin by ECs, no inhibition of SMC proliferation was noted. Somatostatin analogs inhibited DNA synthesis in arterial explants but did not inhibit either DNA synthesis or growth of cultured SMCs, which showed a likely effect of somatostatin on the initial transition in SMC phenotype.

CONCLUSION

Somatostatin exerts inhibitory effects on SMC proliferation only during the early transition to a proliferative phenotype. There are significant differences between this in vivo transition and the standard serum-restimulated model of cultured SMCs. These differences may account for the failure of somatostatin to inhibit SMC proliferation in the standard in vitro models.

Antiatherogenic effect of angiotensin converting enzyme inhibitor (benazepril) and angiotensin II receptor antagonist (valsartan) in the cholesterol-fed rabbits

The purpose of this study was to determine whether an angiotensin converting enzyme (ACE) inhibitor, benazepril, and an angiotensin receptor antagonist, valsartan, would decrease atherosclerotic severity in cholesterol-fed rabbits. Male rabbits were fed either: (a) normal rabbit chow; (b) 2% cholesterol diet; (c) 2% cholesterol diet supplemented by benazepril (3 mg/kg per day, subcutaneous injection); or (d) 2% cholesterol diet supplemented by valsartan (1 mg/kg per day, subcutaneous injection). After 12 weeks, the arteries were harvested for histomorphometry and immunohistochemistry. We observed that decreases in serum triglyceride (TG) and total cholesterol (TC) and ACE activity with benazepril-treatment were more than 60, 30, and 84%, respectively, in comparison with the cholesterol group; with valsartan-treatment, TG levels were 53% lower than in the cholesterol group, however, there was no significant difference in TC and ACE activity. The percentage of aortic surface atherosclerotic area, intimal thickness and the ratio of aortic intimal area to medial area were about 40% lower in the benazepril-treated group in comparison with those of the cholesterol group; the difference was more than 60% in the thoracic aorta. The valsartan-treated group had 23% less atherosclerotic area, less effective than benazepril treatment. The percent of PCNA-positive cells and the number of intimal proliferative cells/mm2 were significantly less in the benazepril-treated group compared with the cholesterol group (by 55 and 63%); these parameters were 35 and 17% lower, respectively, with valsartan. The ratio of proliferating macrophages to smooth muscle cells (SMCs) was 3:1 in the cholesterol group, 1:1 in the benazepril and 2:1 in the valsartan-treated group. These results indicate that benazepril could reduce atherosclerotic progression by decreasing macrophage proliferation and accumulation in the arterial wall. The mechanisms for reducing atherosclerotic progression by benazepril and valsartan may be related to reduction of TG and blockade of the angiotensin II action.

Cell proliferation in normal and atherosclerotic human aorta: proliferative splash in lipid-rich lesions

Local accumulation of cells (hypercellularity) in the intima of the arterial wall as a result of cell proliferation is recognized as one of the major manifestations of human atherosclerosis. In the present study we have used a monoclonal antibody against PCNA to identify the proliferative activity, in uninvolved intima of human aorta classified as diffuse intimal thickening, and in different types of atherosclerotic lesions: specifically, initial lesion, fatty streak, fibroatheroma and fibrous plaque. As compared with a diffuse intimal thickening, the cell number in the initial lesions, fatty streaks and in a fibrolipid plaque (fibroatheroma) was 1.5-3-fold higher, while the cellularity in a fibrous plaque (fibrotic lesion) was lower than in a fibroatheroma and comparable with the cell number in the initial lesions. Using monoclonal antibodies, inflammatory cells (T- and B-lymphocytes as well as monocytes-macrophages) have been revealed in the intima. However, most (84-93%) of the intimal cells were noninflammatory cells classified as resident cells possessing the antigens of smooth muscle cells and pericytes as well as a small number of cells unidentifiable with the antibodies used. The highest number of proliferating cells was found in a fibroatheroma (11-fold higher as compared with a diffuse intimal thickening). A significant, but lesser increase of PCNA-positive cells was revealed in other types of lesions, too. The proliferative 'splash' in lipid-rich lesions suggests a relationship between the lipid accumulation in atherosclerotic intima and the stimulation of proliferation. The highest proliferative index of resident cells (i.e. percentage of the PCNA-positive cells among the total number of resident cells) was revealed in fibrotic lesions. It was approximately eight-fold higher than in a diffuse intimal thickening. The proliferative index of inflammatory cells considerably exceeded that of resident cells. However, in all types of atherosclerotic lesions and in a diffuse intimal thickening it showed no significant differences and was similar to the proliferative index of inflammatory cells isolated from peripheral blood. This suggests that an increased number of resident cells in atherosclerotic lesions can be explained by stimulation of their proliferative activity, whereas an altered inflammatory cell number is rather a result of their penetration from the blood into the subendothelial intima with a constant proliferative index.

Expression of platelet-derived growth factor-A mRNA in human placenta: effect of magnesium infusion in pre-eclampsia

The expression of platelet-derived growth factor-A (PDGF-A) mRNA was examined in the cotyledons of normal human placentae and those from patients with pre-eclampsia. These patients exhibited pre-delivery blood pressure of 154+/-4/99+/-4 mmHg (mean+/-SEM) and met the criteria established for pre-eclampsia. During labour they received MgSO4 infusion for various time intervals (4-25 h). The PDGF-A message was quantitated to beta-actin by the solution hybridization nuclease protection assay. Since the two groups differed in two parameters (pre-eclampsia and MgSO4 treatment), the direct comparison was not feasible. An analysis of covariance revealed a significant difference in the message between the pre-eclamptic and control groups (P<0.01); the gestational age was not a significant covariate for either group but the time on MgSO4 in pre-eclampsia group was significant (P<0.002). A linear regression analysis of PDGF-A mRNA values for the pre-eclamptic group showed a time-dependent downregulation of the message by MgSO4 (P<0.01, r=- 0.796). These results show a uniform expression of PDGF-A mRNA in cotyledons of normal human placenta between 35 and 40 weeks of gestation. Furthermore, MgSO4 has an inhibitory effect on the expression of this message which may have aside from its anticonvulsive action beneficial effect on the function of pre-eclamptic placenta.

Proliferation and the monoclonal origins of atherosclerotic lesions

Benditt's observation of the monoclonal origin of the atherosclerotic lesion has been controversial because it appeared to conflict with conventional wisdom. A new method based on a polymerase chain reaction amplification of the DNA of an X-inactivated gene from microdissected tissue confirms that Benditt was correct. However, this monoclonal expansion can also be found in nonatherosclerotic intima and media. These new data suggest that plaque clonality may represent expansion of preexisting patches of cells arising during development of the media. This developmental view does not conflict with other recent evidence that plaque expansion is associated with mutation or viral events. However, if plaques arise from patches, then early developmental mechanisms may be critical to the later evolution of the lesions.

Egr-1-induced endothelial gene expression: a common theme in vascular injury

A number of pathophysiologically relevant genes, including platelet-derived growth factor B-chain (PDGF-B), are induced in the vasculature after acute mechanical injury. In rat aorta, the activated expression of these genes was preceded by a marked increase in the amount of the early-growth-response gene product Egr-1 at the endothelial wound edge. Egr-1 interacts with a novel element in the proximal PDGF-B promoter, as well as with consensus elements in the promoters of other genes induced by endothelial injury. This interaction is crucial for injury-induced PDGF-B promoter-dependent expression. Sp1, whose binding site in the PDGF-B promoter overlaps that of Egr-1, occupies this element in unstimulated cells and is displaced by increasing amounts of Egr-1. These findings implicate Egr-1 in the up-regulated expression of PDGF-B and other potent mediators in mechanically injured arterial endothelial cells.

An essential role for platelet-derived growth factor in neointima formation in human saphenous vein in vitro

The role of platelet-derived growth factor (PDGF), a potent vascular smooth muscle cells (SMC) mitogen and chemoattractant, was investigated during neointima formation in human saphenous vein organ culture. PDGFA and B messenger ribonucleic acid (mRNA) expression was detected by RNase protection assay and in situ hybridisation and PDGF protein by immunocytochemistry. The expression of PDGFA and B mRNA was low in veins before culture while PDGF protein was detected in all cell types. A neointima consisting of densely packed SMC developed after 14 days of culture. The dense packing and high expression of PDGFA and B mRNA in neointimal SMC led to higher PDGF protein concentrations in the neointima, the role of which was examined by culturing with neutralising anti-(human PDGF) antibodies. The anti-PDGF antibodies significantly reduced neointimal thickness by approximately 66% and the number of neointimal cells by approximately 50%, without affecting neointimal or medial proliferation indices or cell viability. These results suggest that PDGF played an essential role in SMC migration into the neointima in human saphenous vein.

Platelet-derived growth factor A-chain expression in developing and mature human kidneys and in Wilms' tumor

Regulated expression of PDGF A-chain may be important in kidney development. We employed two polyclonal antisera to detect expression of PDGF A-chain in fetal and normal adult kidneys by immunohistochemistry. Specificity of the antisera was demonstrated by Western blots of fetal and adult kidneys, demonstrating monospecific bands at 10 to 15 kD, and by absorption studies with PDGF-A peptide. PDGF A-chain is uniformly expressed by visceral glomerular epithelial cells and the epithelial cells of the distal nephron, including collecting ducts and contiguous urothelium lining the renal pelvis, in both fetal and adult kidneys. Fetal kidneys also demonstrate expression of PDGF A-chain at the earliest stages of vesicle formation from the metanephric blastema; this expression is then only intermittently detectable in developing glomeruli until differentiation of visceral epithelial cells occurs. Fetal and mature arterial smooth muscle cells, and some express PDGF A-chain. In situ hybridization with a riboprobe made from PDGF A-chain cDNA showed close correlation of mRNA expression with protein immunohistochemistry. PDGF A-chain expression was also identified in epithelial elements of 5/6 Wilms' tumors studied. These are the first studies to localize PDGF A-chain expression in human kidney and suggest sites of activity for PDGF A-chain in development, neoplasia, and in the renal arterial sclerosis of aging.