Localization and smooth muscle cell composition of atherosclerotic lesions in Watanabe heritable hyperlipidemic rabbits

Sympathectomy Causes Aggravated Lesions and Dedifferentiation in Large Rabbit Atherosclerotic Arteries without Involving Nitric Oxide

Previously [Histochem J 1997;29:279-286], we found that sympathectomy induced neointima formation in ear but not cerebral arteries of genetically hyperlipidemic rabbits. To clarify the influence of sympathetic nerves in atherosclerosis, and whether their influence involves vascular NO activity, we studied groups of normocholesterolemic intact (NI) and sympathectomized (NS), and hypercholesterolemic intact (HI) and sympathectomized (HS) rabbits (diet/6-hydroxydopamine for 79 days). Segments of basilar (BA) and femoral (FA) arteries were studied histochemically, to evaluate differentiation (anti-desmin, anti-vimentin, anti-h-caldesmon, and nuclear dye), by confocal microscopy, and by in vitro myography. In BAs, staining of NI and NS groups was similar. In hypercholesterolemic groups, a small neointima developed, more frequently in HS segments where smooth muscle cells (SMCs) positive for all antibodies appeared to be migrating into the neointima. In FAs, SMCs stained for the three antibodies in the NI group, but we observed desmin- and h-caldesmon-negative, vimentin-positive cells in some external medial layers of the NS, HI and HS groups, identical to adventitial fibroblasts. Large neointimas of the HS group contained vimentin-positive and largely desmin- and h-caldesmon-negative cells. Relaxation of BA or FA segments to acetylcholine was not decreased by sympathectomy. Sympathectomy increased the contraction of resting FAs to nitro-L-arginine (p = 0.0379). Thus, sympathectomy aggravates the tendency for FA SMCs to migrate and dedifferentiate, increasing atherosclerotic lesions, without decreasing NO activity, but has only minor effects on BAs.

Remodeling of the vascular tunica media is essential for development of collateral vessels in the canine heart

Previous studies have shown that neointima formation and adventitial remodeling play an important role in the enlargement of collateral vessels (CVs) during coronary arteriogenesis in the dog heart. In this study, we investigated the importance of remodeling of the tunica media in the same model. Basal membrane (BM), contractile and cytoskeletal components of smooth muscle cells (SMCs) were studied in growth of coronary CVs induced by chronic occlusion of the left circumflex (LCX) coronary artery by routine histology, electron microscopy (EM), and immunoconfocal microscopy using antibodies against alpha-smooth actin (alpha-SM actin), calponin, desmin, and laminin. In addition, matrix metalloproteinase-2 (MMP-2) and tissue inhibitor-1 of matrix metalloproteinase (TIMP-1) were investigated. The data showed that (1) in normal small arteries (NVs) laminin formed a network in which SMCs were encaged; alpha-SM actin, calponin and desmin were evenly expressed in SMCs; (2) in early (2 weeks) growing CVs the laminin network was disrupted, desmin was significantly reduced in SMCs, but alpha-SM actin and calponin still highly expressed; (3) in actively (6 weeks) growing CVs laminin was still weak in the tunica media (TM), but without network-like structure. Desmin was further reduced in SMCs of TM, whereas alpha-SM actin and calponin showed little changes, although they were significantly decreased in intimal SMCs; (4) in mature CVs, the network-like structure was re-formed, and alpha-SM actin, calponin, and desmin were all similar to that in normal vessels; (5) histology for BM confirmed laminin staining; (6) EM revealed that in NVs the SMCs contained abundant contractile filaments and were surrounded by a layer of BM whereas in growing CVs, BM structure was not observed, but the SMCs in the media still contained many myofilaments; (7) MMP-2 was highly expressed in the media of early growing vessels, but decreased in TM of actively growing vessels where TIMP-1 expression was high. In conclusion, our data revealed features of TM of growing CVs. Disruption and degradation of BM facilitate SMC proliferation, and together with reduction of desmin and fragmentation of the internal elastic lamina enable the vascular wall to expand and enlarge when blood pressure and shear stress increase. MMP2 may be an important player in regulating SMC phenotype, proliferation, migration and maintaining integrity of the vascular wall through governing proteolysis during arteriogenesis.

Phenotypic diversity of smooth muscle cells isolated from human intracranial basilar artery

The present work examined heterogeneity of vascular smooth muscle cells cultured from human cerebral arteries that has not been previously reported. Primary smooth muscle cell cultures were isolated from human intracranial basilar arteries. Using a ring isolation method, multiple clones were generated from the cell cultures. These clones had two distinctly different morphologies: (1) fusiform; and (2) stellate. At confluence the fusiform-shaped clones grew in compact clusters with overlapping cells while the stellate-shaped clones were contact-inhibited growing in a monolayered pattern. The smooth muscle differentiation markers, alpha-smooth muscle-actin, calponin and smooth muscle-myosin heavy chains were expressed in all these clones. In response to serum stimulation, the stellate-shaped clones had a higher growth rate than the fusiform clones. This study reports that smooth muscle cells derived from human basilar arteries are heterogeneous.

The role of inflammation in vascular injury and repair

Inflammation plays a critical role in the vascular response to injury. In particular, mechanical injury using techniques such as balloon angioplasty and stenting results in complex inflammatory reactions which influence proliferation of vessel wall constituents such as endothelial cells, smooth muscle cells, and extracellular matrix proteins. Inflammatory cells are recruited to the injured vessel wall initially as a reparative mechanism; however, these same inflammatory processes are also pivotal in the development of restenotic lesions. Leukocytes serve as the primary inflammatory cells but we now know that platelets produce a number of important inflammatory mediators. This review describes the mechanisms that regulate endothelial cell migration, smooth muscle cell activation, and extracellular matrix protein production, all of which are key components in the inflammatory response to vascular injury.

Prolonged inhibition of nitric oxide synthesis in Yoshida hyperlipidemic rat: aorta functional and structural properties

To test whether inhibition of nitric oxide synthesis, associated with high levels of plasmatic lipids, can induce atherosclerotic lesions and phenotypic changes in smooth muscle cell composition in the aortic wall of an atherosclerotic-resistant species such as the rat, an inbred strain of hyperlipidemic Pittsburgh Yoshida rat was subjected to prolonged treatment (2 months) with the nitric oxide-synthase inhibitor L omega-nitro-arginine-methyl ester or with L-arginine. The two types of in vivo treatments were not able to modify in vitro aortic endothelium-mediated relaxation induced by acetylcholine or calcium-ionophore A-23187, the endothelium-independent sodium nitrite relaxation and the contractile response to serotonin. Histology and lipid infiltration of vascular specimens showed that L omega-nitro-arginine-methyl ester in vivo treatment did not induce any significant change in the aortic wall. Monoclonal antibodies to myosin isoforms and immunofluorescence procedures revealed the presence of an immature smooth muscle cell subpopulation in aortic specimens from saline-treated Pittsburgh Yoshida rats, whose expansion has been related in other species to atherogenesis. This peculiar cell phenotype disappeared in our animal model after prolonged L omega-nitro-arginine-methyl ester treatment. These data indicate that, despite interference with endothelium-mediated nitric oxide synthesis, atherosclerosis does not develop in this animal model and furnish for the first time a biological justification for atherogenesis resistance of rat, i.e., the lack of activation of an immature aortic smooth muscle cell population which in atherosclerosis-prone species is involved in lesion formation.

Echographical assessment of the early stage of experimental atherosclerosis of the descending aorta in rabbits

To assess the early stage of atherosclerosis of the thoracic descending aorta, we evaluated morphological atheromatous lesions (atherosis) and the stiffness parameter of the artery (beta; sclerosis) in 24 male rabbits using echography. Male Japanese white rabbits weighing 2.5-3.0 kg were fed a diet containing 1% cholesterol for 7 (n = 8) or 14 weeks (n = 8). Rabbits fed a normal diet were used as controls (n = 8). Atheromatous lesions were evaluated with intravascular ultrasound (IVUS: Aloka, 20 MHz, 6F). We also calculated beta using M-mode echography (7.5 or 10 MHz) and direct aortic pressure measurement. Thickening of the intima-media complex was clearly observed with IVUS in the 14-week group but was not detected in the others. Histologically, only a thin layer of foamy cells on the intima (thickness < 20 microns) was observed in the 7-week group. The value of beta was significantly increased in both the 7-week (4.7 +/- 2.2) and 14-week groups (4.5 +/- 0.8) compared with controls (1.7 +/- 0.9, both p < 0.01). These results suggest that the development of atherosis might be preceded by vascular sclerosis during the early stage of atherosclerosis when the serum cholesterol level is high: at a time when the thin layer of foamy cells could not be detected by conventional IVUS.

Increased expression of a brain/embryo-type myosin heavy chain isoform (MIIB2) in mesangial proliferative glomerulonephritis

Proliferation of mesangial cells is frequently found in glomerulonephritis, such as IgA glomerulonephritis. Recent reports suggest that a brain/embryo-type myosin isoform (MIIB2) is involved in cell proliferation. We have studied the expression of MIIB2 in renal biopsy samples from patients with various renal diseases and in the renal tissues from the rat model of mesangial proliferative glomerulonephritis induced with anti-Thy 1.1 antibody. Immunohistochemical analysis of the biopsy samples using an anti-brain-type myosin heavy chain-specific monoclonal antibody (HBM1) indicated that 92% of the samples from patients with IgA glomerulonephritis contained a significant population of mesangial cells that reacted with the antibody. Most of the samples from patients with other types of proliferative glomerular diseases also contained HBM1-reactive mesangial cells, while none of the samples from patients with non-proliferative glomerular diseases contained a significant population of HBM1-reactive mesangial cells. The expression of a brain/embryo-type myosin heavy chain isoform (MIIB2) in the mesangial cells began at five days after injection of anti-Thy 1.1 antibody and peaked at the tenth day. On the other hand, the expression of the proliferating cell nuclear antigen in the mesangial cells was induced at two days after injection of anti-Thy 1.1 antibody and was maximal at the fourth day. These results indicate that the expression of the MIIB2 isoform by mesangial cells is accelerated in proliferative glomerulonephritis and suggest that the myosin isoform is involved in the phenotypic transformations of the glomerular tissues rather than in the cell proliferation.

Correlation between the presence of an immature smooth muscle cell population in tunica media and the development of atherosclerotic lesion. A study on different-sized rabbit arteries from cholesterol-fed and Watanabe heritable hyperlipemic rabbits

Mapping the distribution of an immature smooth muscle cell (SMC) subpopulation in large- and small-sized arterial vessels was carried out in normocholesterolemic rabbits and compared with the mapping atherosclerotic lesions in endogenously (Watanabe heritable hyperlipemic, WHHL) and exogenously derived (cholesterol-fed, CT) hypercholesterolemic rabbits. This cell subset is identified by a specific myosin isoform content and displays an intermediate degree of differentiation between fetal- and adult-type SMC. Monoclonal anti-myosin antibodies, immunofluorescence procedures, and different arterial segments of a rabbit vessel tree, i.e. from aorta to dental pulp (common carotid, external carotid, lingual, facial, maxillary, inferior alveolar arteries, and dental branches of alveolar arteries) were studied. WHHL of different ages (3 to 12 months), and two different concentrations of CT (2% and 0.2%) in the diet for 3 and 12 months, respectively, were used. The results of the present study indicate that: (1) using a diet with a higher percentage of CT (rabbits fed 2% CT-diet for 3 months) there is maximum expansion of atherosclerotic lesions from the aorta up to the maxillary artery; (2) localization of atherosclerotic lesions with a lower CT content in the diet is dependent on the duration of feeding and may involve the aorta up to the external carotid artery; (3) the development of the atherosclerotic lesion in hypercholesterolemic rabbit is strictly related to the appearance of an intermediate SMC subtype; (4) atherosclerotic lesions occur only in those arterial sites which, in corresponding normocholesterolemic rabbit, contain intermediate-type SMC; and (5) no differences can be found in the distribution of SMC subpopulations present in the lesions from WHHL, CT-fed animals, or at various arterial levels, whereas some discrepancies can be shown in aortic atherogenesis.

Hyperplastic growth of aortic smooth muscle cells in renovascular hypertensive rabbits is characterized by the expansion of an immature cell phenotype

Smooth muscle cells (SMCs) of rabbit aorta undergo marked changes in myosin isoform content during development. Analysis of nonmuscle myosin composition at the protein level has permitted the identification of three phases in the SMC differentiation process: fetal, postnatal, and adult. Using monoclonal antibodies specific for smooth muscle and nonmuscle myosins and extra domain A of fibronectin as well as cDNA probes for platelet-derived growth factors (PDGF) and various procollagens, we have evaluated the differentiation pattern of aortic SMCs in two-kidney, one-clip hypertensive rabbits. Morphometric and bromo-deoxyuridine studies indicate that hypertrophy of aortic media along with intimal thickening occurring in hypertensive animals is due to SMC hyperplasia. Western blotting experiments performed on aortic specimens from hypertensive animals with antimyosin antibodies revealed the appearance of a myosin isoform pattern of the "immature" type. Immunofluorescence tests showed that these cells are localized in the thickened intima or distributed in the underlying media (sparsely or in groups). Similarly, the fibronectin variant showing the extra domain A, peculiar to "phenotypically modulated" SMCs, appeared in intimal thickening, and its expression followed the time course of nonmuscle myosin expression. Counting of postnatal-type SMCs in the aortic media revealed that this cell population increases markedly with hypertension (2- up to 15-fold at 4 months) and then declines to near control level in 8-month hypertensive rabbits. Diminution of postnatal-type SMCs at later stages of hypertension was temporally correlated with the slowing down of aortic wall hypertrophy. Average levels of mRNAs, as determined by densitometric analysis in aortas from 1- and 2.5-month hypertensive rabbits, showed an increased expression for PDGF beta receptor (up to twofold), procollagen type I (alpha 1, threefold), procollagen type III (alpha 1, twofold), and fibronectin (up to threefold) compared with controls. Conversely, the steady-state levels of mRNAs for PDGF (A and B chain), PDGF alpha receptor, TGF-beta 1, and procollagen type IV (alpha 1) did not increase significantly. These results provide evidence that in adult renovascular hypertensive rabbits, the hyperplastic growth of aortic SMCs is accompanied by the expansion of an "immature" cell phenotype characteristic of the early stages of development.

Glycosaminoglycan fractions from human arteries presenting diverse susceptibilities to atherosclerosis have different binding affinities to plasma LDL

The topographic distribution of atherosclerotic lesions is influenced by biochemical factors intrinsic to the arterial wall. In the present work we have investigated whether the composition/chemical structure of glycosaminoglycans constitutes one of these factors. Normal human arteries were obtained at necropsy, and in order of decreasing susceptibility to atherosclerosis, consisted of the abdominal and thoracic aortas and the iliac and pulmonary arteries. The results showed similar concentrations of total glycosaminoglycan and collagen. Of the glycosaminoglycans known to interact with low-density lipoprotein (LDL), dermatan sulfate was present in all arteries in comparable concentrations, but the aortas had a 30% higher content of chondroitin 4/6-sulfate, which in turn was slightly enriched in 6-sulfated disaccharide units. LDL-affinity chromatography with dermatan sulfate+chondroitin 4/6-sulfate fractions demonstrated that increasing affinity to LDL matched an increasing susceptibility to atherosclerosis. Analysis of glycosaminoglycans in the eluates indicated a positive correlation between affinity to LDL and increasing molecular weight and the existence of a fraction of glycosaminoglycans of high affinity to LDL in the aortas only. These results suggest that arterial glycosaminoglycans participate in the multifactorial mechanisms that modulate the differential localization of atherosclerotic lesions.