The possible role of the arterial microcirculation in the pathogenesis of atherosclerosis

Comparison of Cell-based and Nanoparticle-based Therapeutics in Treating Atherosclerosis

Today, cardiovascular diseases are among the biggest public health threats worldwide. Atherosclerosis, a chronic inflammatory disease with complex aetiology and pathogenesis, predispose many of these conditions, including the high mortality rate-causing ischaemic heart disease and stroke. Nevertheless, despite the alarming prevalence and absolute death rate, established treatments for atherosclerosis are unsatisfactory in terms of efficacy, safety, and patient acceptance. The rapid advancement of technologies in healthcare research has paved new treatment approaches, namely cell-based and nanoparticle-based therapies, to overcome the limitations of conventional therapeutics. This paper examines the different facets of each approach, discusses their principles, strengths, and weaknesses, analyses the main targeted pathways and their contradictions, provides insights on current trends as well as highlights any unique mechanisms taken in recent years to combat the progression of atherosclerosis.

Metabolic syndrome update

The metabolic syndrome is a multiplex risk factor for atherosclerotic cardiovascular disease and type 2 diabetes. It is composed of atherogenic dyslipidemia, elevated blood pressure, insulin resistance and elevated glucose, a pro-thrombotic state, and a pro-inflammatory state. Excess energy intake and concomitant obesity are the major drivers of the syndrome. Lifestyle intervention can reverse metabolic risk factors, but at times, drug therapies or bariatric surgery may be required to control more overt risk factors.

Comparative study of the effect of rivaroxaban and fondaparinux on monocyte's coagulant activity and cytokine release

OBJECTIVES

Tissue factor (TF) exposed on activated monocytes and macrophages is involved in thrombosis through activation of factor X and cytokine release, responsible for inflammation and thrombosis. We investigated the effect of two anti-factor Xa drugs: rivaroxaban, a direct anti-Xa inhibitor, and fondaparinux, an antithrombin dependent anti-Xa inhibitor, on monocyte/macrophage procoagulant activity and cytokine release.

METHODS

Rivaroxaban and fondaparinux were tested at pharmacological concentrations on LPS-activated monocytes and on THP-1 cells, a human monocytic cell line, to assess 1) TF expression by flow cytometry 2) prothrombinase activity by its coagulant activity and 3) cytokine release in cell supernatants by antibody based cytokine array and ELISA for IL-8 and TNFα.

RESULTS AND CONCLUSION

Rivaroxaban and fondaparinux did not modify TF expression level on activated cells. In contrast procoagulant activity associated to monocytes and macrophages was dose dependently inhibited by rivaroxaban, but not significantly by fondaparinux. These results could explain why patients undergoing major orthopedic surgery with rivaroxaban prophylaxis were able to achieve significant reductions in venous thromboembolism, compared with drugs commonly used, i.e. fondaparinux and low molecular weight heparin. In addition, rivaroxaban and fondaparinux suppressed some chemokine secretion produced by activated macrophages. This may also contribute to their antithrombotic effect in clinic.

Statins in low doses reduce VEGF and bFGF serum levels in patients with type 2 diabetes mellitus

BACKGROUND/AIMS

Recent experimental research revealed that statins at low doses induce angiogenesis, which in turn may be related to the course of atherosclerosis. There are no clinical studies evaluating the effect of 'low-dose' statins on serum levels of angiogenesis regulators in diabetic subjects. We aimed to explain how low doses of statins modify the serum concentrations of two potent proangiogenic factors, vascular endothelial growth factor (VEGF) and basic fibroblast growth factor (bFGF), in patients with type 2 diabetes.

METHODS

Measurements of fasting glucose level, HbA1c, 1,5-anhydro-D-glucitol and lipid profile were taken from 47 patients with type 2 diabetes treated with low doses of atorvastatin (10 mg daily) or simvastatin (10-20 mg daily), from 45 statin-free patients with type 2 diabetes and from 23 nondiabetic subjects. Measurements of VEGF and bFGF in serum were taken using the BD™ Cytometric Bead Array.

RESULTS AND CONCLUSION

Statins used in low doses in patients with type 2 diabetes reduce the serum concentration of VEGF and bFGF which suggests antiangiogenic potential of these doses. Nevertheless, this effect could be neutralized by postprandial hyperglycemia.

Macrophages transmit potent proangiogenic effects of oxLDL in vitro and in vivo involving HIF-1α activation: a novel aspect of angiogenesis in atherosclerosis

Neovascularization has been linked to the progression and vulnerability of atherosclerotic lesions. Angiogenesis is increased in lipid-rich plaque. Hypoxia-inducible factor alpha (HIF-1α) is a key transcriptional regulator responding to hypoxia and activating genes, which promote angiogenesis, among them vascular endothelial growth factor (VEGF). Oxidized low-density lipoprotein (oxLDL) is generated in lipid-rich plaque by oxidative stress. It triggers an inflammatory response and was traditionally thought to inhibit endothelial cells. New data, however, suggest that oxLDL can activate HIF-1α in monocytes in a hypoxia-independent fashion. We hypothesized that HIF-1α activation in monocyte-macrophages could transmit proangiogenic effects of oxLDL linking hyperlipidemia, inflammation, and angiogenesis in atherosclerosis. First, we examined the effect of oxLDL on HIF-1α and VEGF expression in monocyte-macrophages and on their proangiogenic effect on endothelial cells in vitro in a monocyte-macrophage/endothelial co-culture model. OxLDL strongly induced HIF-1α and VEGF in monocyte-macrophages and significantly increased tube formation in co-cultured endothelial cells. HIF-1α inhibition reversed this effect. Second, we demonstrated a direct proangiogenic effect of oxLDL in an in vivo angiogenesis assay. Again, HIF-1α inhibition abrogated the proangiogenic effect of oxLDL. Third, in a rabbit atherosclerosis model, we studied the effect of dietary lipid lowering on arterial HIF-1α and VEGF expression. The administration of low-lipid diet significantly reduced the expression of both HIF-1α and VEGF, resulting in decreased plaque neovascularization. Our data point to oxLDL as a proangiogenic agent linking hyperlipidemia, inflammation, and angiogenesis in atherosclerosis. This effect is dependent on macrophages and, at least in part, on the induction of the HIF-1α pathway.

Molecular imaging of inflammation and intraplaque vasa vasorum: a step forward to identification of vulnerable plaques?

Current developments in cardiovascular biology and imaging enable the noninvasive molecular evaluation of atherosclerotic vascular disease. Intraplaque neovascularization sprouting from the adventitial vasa vasorum has been identified as an independent predictor of intraplaque hemorrhage and plaque rupture. These intraplaque vasa vasorum result from angiogenesis, most likely under influence of hypoxic and inflammatory stimuli. Several molecular imaging techniques are currently available. Most experience has been obtained with molecular imaging using positron emission tomography and single photon emission computed tomography. Recently, the development of targeted contrast agents has allowed molecular imaging with magnetic resonance imaging, ultrasound and computed tomography. The present review discusses the use of these molecular imaging techniques to identify inflammation and intraplaque vasa vasorum to identify vulnerable atherosclerotic plaques at risk of rupture and thrombosis. The available literature on molecular imaging techniques and molecular targets associated with inflammation and angiogenesis is discussed, and the clinical applications of molecular cardiovascular imaging and the use of molecular techniques for local drug delivery are addressed.

Increased neovascularization in advanced lipid-rich atherosclerotic lesions detected by gadofluorine-M-enhanced MRI: implications for plaque vulnerability

BACKGROUND

Inflammation and neovascularization may play a significant role in atherosclerotic plaque progression and rupture. We evaluated gadofluorine-M-enhanced MRI for detection of plaque inflammation and neovascularization in an animal model of atherosclerosis.

METHODS AND RESULTS

Sixteen rabbits with aortic plaque and 6 normal control rabbits underwent gadofluorine-M-enhanced MRI. Eight rabbits had advanced atherosclerotic lesions, whereas the remaining 8 had early lesions. Magnetic resonance atherosclerotic plaque enhancement was meticulously compared with plaque inflammation and neovessel density as assessed by histopathology. Advanced plaques and early atheroma were enhanced after gadofluorine-M injection. Control animals displayed no enhancement. After accounting for the within-animal correlation of observations, mean contrast-to-noise ratio was significantly higher in advanced plaques than compared with early atheroma (4.29+/-0.21 versus 3.00+/-0.32; P=0.004). Macrophage density was higher in advanced plaques in comparison to early atheroma (geometric mean=0.50 [95% CI, 0.19 to 1.03] versus 0.25 [0.07 to 0.42]; P=0.05). Furthermore, higher neovessel density was observed in advanced plaques (1.83 [95% CI, 1.51 to 2.21] versus 1.29 [0.99 to 1.69]; P=0.05). The plaque accumulation of gadofluorine-M correlated with increased neovessel density as shown by linear regression analysis (r=0.67; P<0.001). Confocal and fluorescence microscopy revealed colocalization of gadofluorine-M with plaque areas containing a high density of neovessels.

CONCLUSIONS

Gadofluorine-M-enhanced MRI is effective for in vivo detection of atherosclerotic plaque inflammation and neovascularization in an animal model of atherosclerosis. These findings suggest that gadofluorine-M enhancement reflects the presence of high-risk plaque features believed to be associated with plaque rupture. Gadofluorine-M plaque enhancement may therefore provide functional assessment of atherosclerotic plaque in vivo.

Low vasa vasorum densities correlate with inflammation and subintimal thickening: potential role in location--determination of atherogenesis

OBJECTIVES

To assess the role of coronary vasa vasorum (VV) spatial distribution in determining the location of early atherosclerotic lesion development.

METHODS AND RESULTS

Six, 3-month-old, female, crossbred swine were fed 2% high-cholesterol (HC) diet for 3 months prior to euthanasia. Six other pigs were fed normal diet (N) for the entire 6 months. Right coronary arteries were harvested and scanned intact with micro-CT (20mum cubic-voxel-size). After scanning, randomly selected cross-sectional histological sections were stained for nuclear-factor kappaB (NF-kappaB), hypoxia-inducible factor-1alpha (HIF-1alpha), macrophages, von-Willebrand-factor, dihydroethidium (DHE), tumor necrosis factor-alpha (TNF-alpha) and interleukin-6 (IL-6). The number of positive stained cells, as well as intima-to-media ratio, were compared with VV density (#/mm(2)) obtained from micro-CT images (which closely matched the location of the histological sections) in each of four equal quadrants of the coronary vessel wall. In normal, as well as HC pigs, the number of NF-kappaB (r=0.73 and 0.70), HIF-1alpha (r=0.74 and 0.77), TNF-alpha (r=0.58 and 0.72) and IL-6 (r=0.70 and 0.72) positive cells as well as the expression of DHE (Kendall tau coefficient -0.64 and -0.63) inversely correlated with VV density. In HC the VV density also inversely correlated with intima/media ratios (r=0.65).

CONCLUSIONS

Our data suggest that low VV density territories within the coronary vessel wall are susceptible to hypoxia, oxidative stress and microinflammation and may therefore be starting points of early atherogenesis.

Pathological vascularization of the coronary intima

Secondary vascularization was found in the walls of diseased coronary arteries from 36 individuals who died after sudden cardiac arrest. In the series presented here, coronary arteries of hospital patients dying from all causes also showed a 100% incidence of pathological neovascularization. A process linking neovascularization and chronic inflammation to an ultimate infectious aetiology is suggested.

Angiogenesis in Atherogenesis

Atherogenesis is the pathobiological process, which underlies atherosclerotic cardiovascular disease and evolves in the 3 stages of initiation, progression, and complication to clinical significance. Of note, this process is associated with neovascularization, and it was not until recently that the implications of angiogenesis in atherogenesis were delineated. This article gives an updated overview on this topic and briefly reflects on the similarities with neovessel formation in carcinogenesis.

Phenotypic overlap between monocytes and vascular endothelial cells

During embryonic development, endothelial cells (ECs) develop organ specific properties. ECs express specific markers, which are helpful in identifying these cells in vivo and in culture. Interestingly, most of the supposed specific endothelial markers are present on both ECs and hematopoietic precursors or mature blood cells, which correspond to the idea of a common embryonic precursor. Monocytes/makrophages and monocyte-derived dendritic cells, as more differentiated hematopoietic cell populations, show a wide phenotypic overlap with particularly hepatic sinusoidal, and microvascular endothelial cells within inflamed tissue, such as neovascularizised complicated atherosclerotic plaques. Furthermore, under local angiogenic growth conditions monocytes or monocyte precursors or immature dendritic cells may differentiate into endothelial like cells. First evidence suggests an endothelium-independent revascularization potential carried by monocyte-derived macrophages. These macrophages have been shown to form tunnel-like structures in ischemic regions. Future studies have to address the question, whether monocyte-/dendritic cell-derived endothelial like cells can develop a similar functional behaviour in vasoregulation, coagulation and fibrinolysis, as described for vascular endothelial cells, and thus may contribute to neoangiogenesis by a direct vessel-forming role.

Cerivastatin, an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme a reductase, inhibits endothelial cell proliferation induced by angiogenic factors in vitro and angiogenesis in in vivo models

Cerivastatin is an inhibitor of 3-hydroxy-3-methylglutaryl coenzyme A reductase. It inhibits the biosynthesis of cholesterol and its precursors: farnesyl pyrophosphate and geranylgeranyl pyrophosphate (GGPP), which are involved in Ras and RhoA cell signaling, respectively. Statins induce greater protection against vascular risk than that expected by cholesterol reduction. Therefore, cerivastatin could protect plaque against rupture, an important cause of ischemic events. In this study, the effect of cerivastatin was tested on angiogenesis because it participates in plaque progression and plaque destabilization. Cerivastatin inhibits in vitro the microvascular endothelial cell proliferation induced by growth factors, whereas it has no effect on unstimulated cells. This growth arrest occurs at the G(1)/S phase and is related to the increase of the cyclin-dependent kinase inhibitor p21(Waf1/Cip1). These effects are reversed by GGPP, suggesting that the inhibitory effect of cerivastatin is related to RhoA inactivation. This mechanism was confirmed by RhoA delocalization from cell membrane to cytoplasm and actin fiber depolymerization, which are also prevented by GGPP. It was also shown that RhoA-dependent inhibition of cell proliferation is mediated by the inhibition of focal adhesion kinase and Akt activations. Moreover, cerivastatin inhibits in vivo angiogenesis in matrigel and chick chorioallantoic membrane models. These results demonstrate the antiangiogenic activity of statins and suggest that it may contribute to their therapeutic benefits in the progression and acute manifestations of atherosclerosis.

Oxidized LDL differentially regulates MMP-1 and TIMP-1 expression in vascular endothelial cells

We have reported recently that oxidized low-density lipoprotein (oxLDL) stimulates matrix metalloproteinase-1 (MMP-1) expression in human vascular endothelial cells. The present study was conducted to examine the effect of oxLDL on expression of Tissue inhibitor of metalloproteinase-1 (TIMP-1), an endogenous inhibitor of MMPs, in human vascular endothelial cells. Our enzyme-linked immunosorbent assay and Northern blot analysis showed that oxLDL inhibited TIMP-1 secretion and expression by human umbilical vein endothelial cells. In contrast, PMA stimulated TIMP-1 expression and secretion. Both oxLDL and PMA increased MMP-1 expression and secretion significantly as previously reported. Inhibition by oxLDL of TIMP-1 expression was also observed in human aortic endothelial cells. Collagenase activity as detected by an enzymatic activity assay demonstrated, as expected, an increase in collagenase activity in the culture medium from oxLDL-treated cells as compared with that from untreated cells. The presented data indicates that oxLDL differentially regulates TIMP-1 and MMP-1 expression, whereas PMA coordinately regulates TIMP-1 and MMP-1 in vascular endothelial cells. The lack of coordination in the secretion of MMP-1 and TIMP-1 induced by oxLDL leads to an increased collagen-degrading activity that may contribute to destabilization of atherosclerotic plaques.

Quercetin inhibits matrix metalloproteinase-1 expression in human vascular endothelial cells through extracellular signal-regulated kinase

Studies have shown that intake of quercetin was inversely associated with mortality from coronary heart disease. Since recent studies documented that disruption of atherosclerotic plaques is the key event triggering acute myocardial infarction, and vascular endothelium-derived matrix metalloproteinase-1 (MMP-1) contributes to plaque destabilization, we examined the effect of quercetin on MMP-1 expression in human vascular endothelial cells. Our results showed that quercetin significantly inhibited basal and oxidized LDL (oxLDL)-stimulated MMP-1 expression. Our data also indicated that extracellular signal-regulated kinase (ERK) mediated the basal and oxLDL-stimulated expression of MMP-1, and quercetin is a potent inhibitor of ERK, suggesting that quercetin may inhibit MMP-1 expression by blocking the ERK pathway. Finally, we showed that quercetin stimulated tissue inhibitor of metalloproteinase-1 expression in oxLDL- and PMA-treated cells. In conclusion, the present study demonstrated for the first time that quercetin inhibited MMP-1 expression in vascular endothelial cells, suggesting that quercetin might contribute to plaque stabilization.

Modified LDL - trigger of atherosclerosis and inflammation in the arterial intima

Atherosclerosis is characterized by chronic inflammation of an injured intima. The pathological processes are initiated by accumulation of morphologically distinct, modified forms of LDL, and followed by cellular infiltration and foam cell formation. Activated intimal cells secrete enzymes and agents capable of modifying LDL, and the modified lipids of LDL, in turn, are able to activate intimal cells and to trigger various inflammatory signals. These processes can initiate and maintain a vicious circle in the intima and lead to lesion progression. In this review, we focus on the LDL modifications relevant to the initial lipid accumulation and discuss their pro-inflammatory effects.

Local neovascularization and cellular composition within vulnerable regions of atherosclerotic plaques of human carotid arteries

An improved immunohistochemical method has been used to assess neovascularization within the vulnerable 'shoulder' regions of atherosclerotic plaques from carotid arteries. A combination of monoclonal antibodies (CD31, CD34, +/- von Willebrand factor) was shown to be far more effective than conventional techniques in demonstrating extensive vascularizations within the 'shoulder' and cap regions of late-stage plaques. Such sites were shown to be microfocal, often appearing as a plexus of both large and small vessels which occupied a significant proportion of the 'shoulder' area. These regions of marked neovascularization were commonly associated with accumulations of macrophages, mast cells, and T-cells, indicative of local inflammatory reactions. The matrix components elastin and collagen type VI showed variable distributions which suggested extensive tissue remodelling, whereas collagen type IV was recognized as a basement membrane protein of most blood vessels, as well as being associated with 'stellate' smooth muscle cells. Evidence of local microvascular damage within the shoulder regions of some specimens was demonstrated by extravascular red blood cells, macrophages containing haemosiderin, and perivascular fibrin deposition. These local haemorrhages derived from microvessels beneath the lining of the arterial lumen are a further indication of how the microfocal vascularization of the plaque 'shoulder' might contribute to further complications of inflammation and plaque destabilization in late-stage disease.

Adventitial angiogenesis early after coronary angioplasty : correlation with arterial remodeling

The spatial correlation between arterial wall microvessels and the accumulation of atherosclerotic plaque is well documented. The role of these microvessels in the development of primary and restenotic lesions is not known. To investigate the effect of interventional procedures on arterial wall microvessels, we studied the adventitial microvascularity of porcine coronary arteries subjected to angioplasty. Twenty-two juvenile domestic swine were subjected to single or repeated (double) balloon angioplasty of the coronary arteries, with the interval between the first and second injury being 14 days. The number, density, and size of adventitial microvessels were measured 1 hour as well as 3, 7, 14, and 28 days after injury. One hour after single balloon injury, there were very few intact adventitial microvessels. Adventitial microvessel number, microvessel area density, and microvessel size were maximal 3 days after single (SI) and double (DI) injury but subsequently underwent progressive regression. Adventitial endothelial cell replication, as assessed by the incorporation of bromodeoxyuridine, was very low for the majority of arteries. Maximal endothelial cell replication indices were observed 3 days after SI and DI (eg, 12.0+/-3.3%). Early after SI the central arterial lumen area transiently increased, then renarrowed. The lumen area did not change after DI. Arterial remodeling occurred, as the accumulation of intimal and medial mass was correlated with expansion of the external elastic lamina. Adventitial microvessel area density was correlated with central arterial luminal area (R=0.34, P=0.04). The adventitial microvessel area density and the microvessel size index were greater late after DI compared with SI. These data indicate that adventitial angiogenesis occurs within 3 days after balloon injury and that regression of adventitial microvessels after SI corresponds with arterial narrowing. Changes in the adventitial microvasculature may be a component of arterial remodeling after balloon angioplasty.

Mast cells accompany microvessels in human coronary atheromas: implications for intimal neovascularization and hemorrhage

Mast cells have been assigned a role in neovascularization. Therefore, we examined the deep regions of human coronary atheromas, the areas known to be prone to neovascularization, for the presence of mast cells. Specimens of atherosclerotic human coronary intima from 37 autopsy cases with ages of 24-84 years were stained with elastica-van Gieson to detect atheroma formation and with monoclonal antibody against von Willebrand factor to detect neovascularization. Mast cells were detected by staining the atheromas with monoclonal antibodies against the two major proteases of mast cells, tryptase and chymase. Of the 24 coronary atheromas found, 13 contained mast cells in the deep regions. All these 13 deep regions also displayed neovascularization, and the number of microvessels and the number of mast cells around the microvessels correlated strongly with the size of the atheroma. On the other hand, of the 11 deep regions lacking mast cells, only one displayed neovascularization. In the neovascularized areas of the coronary atheromas, the mast cells were in close proximity to the microvessels. All the mast cells contained tryptase, and some of them chymase, both known for their angiogenic and matrix-degrading potential. In light microscopic studies, degranulated mast cells were observed indicating activation of these cells, with release of tryptase and chymase. The selective localization of activated mast cells containing angiogenic factors around newly formed microvessels in human coronary atheromas suggests that mast cells play a role in the neovascularization of these lesions. Moreover, mast cells may also, by virtue of their neutral proteases, injure the microvessels, and thereby produce intraplaque hemorrhages and, ultimately, unstable lesions.

Computed tomography-derived intrarenal blood flow in renovascular and essential hypertension

The effect of renal artery stenosis on intrarenal perfusion and volume in renovascular hypertensive patients is unclear. Alterations in these attributes may ultimately be involved in deterioration of renal function. We measured whole kidney, cortical, and medullary perfusion and volume with electron beam computed tomography (EBCT) in 33 hypertensive patients, with well-preserved renal function, scheduled for renal angiography. EBCT-derived whole kidney perfusion was lower in patients with atherosclerotic renal artery stenosis (RAS; N = 20) than in fibromuscular dysplasia (FMD; N = 10) or essential hypertension (N = 28; P < 0.05), as was cortical perfusion (2.44 +/- 0.16 vs. 3.26 +/- 0.17 and 3.07 +/- 0.09 ml/min/cc tissue, respectively, P < 0.005), but medullary perfusion was similar. Whole kidney, cortical, and medullary perfusion correlated inversely with degree of stenosis in FMD, but not in atherosclerotic RAS. Renal volumes were similar. These results demonstrate that, in contrast to patients with FMD, in patients with atherosclerotic RAS the decrease in cortical perfusion is not directly related to the degree of stenosis in the main renal artery. Factors other than the stenosis itself may play a role in the pathophysiology of atherosclerotic RAS and associated renal failure.

The effect of balloon angioplasty on vasa vasorum blood flow in canine coronary arteries

To assess the effects of balloon dilatation on vasa vasorum flow, we performed percutaneous transluminal coronary angioplasty on the circumflex arteries of 12 dogs. Left anterior descending and circumflex coronary vasa vasorum flows were measured with radioactive microspheres at baseline, during, and 10 minutes after a 3-minute, 8 atm balloon inflation. With inflation, vasa vasorum flow at the balloon dilatation site profoundly decreased (from 0.25 +/- 0.08 to 0.03 +/- 0.01 ml/min/gm). The flow returned to normal within 10 minutes after deflation. This effect was not mediated by hemodynamic deterioration during coronary occlusion and did not occur in the contralateral coronary artery. Endomyocardial flow in the distribution of the dilated artery decreased markedly during balloon inflation (from 1.14 +/- 1.9 to 0.08 +/- 0.04 ml/min/gm), which confirmed coronary occlusion. We conclude that a prolonged decrease in vasa vasorum flow is not produced by experimental balloon angioplasty, which makes it unlikely that a sustained vasa vasorum flow reduction plays a role in the maintenance of patency or the induction of restenosis.

Role of the altered transmural permeability in the pathomechanism of arteriosclerosis. History of arteriosclerosis theories. Role of the altered permeability in experimental arteriosclerosis models

After presenting an overview on classification and history of arteriosclerosis theories, the physiological factors involved in the transmural permeability of the arteries are discussed in detail. The development and characteristic features of the altered transmural permeability were studied in various experimental models such as in rat's hypercholesterolemia, local aortic hypoxia, lymphedema of the vascular wall and in hypertension. Results appear to show that alterations in permeability invariably developed in all of the pathological conditions examined, they were transient in nature and preceded the onset of intimal proliferation(s). The disturbance of transmural permeability might be the common pathologic clue which initiates uniform vascular responses to injuries produced by a variety of noxious stimuli. The possible role of the altered transmural permeability in the induction of smooth muscle cell proliferation is also discussed and evidence is provided that after withdrawal of stimulus for vascular injury intimal proliferation will not develop despite the manifest disorders in permeability.

Atherosclerosis: pathology, pathogenesis, and role of risk factors

Atherosclerosis is a complex disease that represents the end product of the interaction of many different causative agents. Those that originate external to the arterial wall usually are called primary risk factors. Many other influences, the secondary risk factors, modulate the primary factors. The penetrance of the secondary factors is variable. They can have a major effect in some people but not in others. The idea of risk factor is important because it provides the conceptual framework upon which to build an intervention program for prevention of atherosclerosis. The development of atherosclerosis can be viewed as a two-step process (Table 2). The first is injury to the arterial wall. The second is response to injury. The primary risk factors can be regarded as the injurious agents. Examples are factors causing endothelial damage, influx of plasma lipoproteins, toxic products of smoking, hemodynamic injury of hypertension, and perhaps microvascular injury from diabetes mellitus. The response to injury represents typical pathologic changes--proliferation of smooth muscle cells, mononuclear infiltration, phagocytosis of products of injury, secretion of connective tissue elements, neovascularization, and necrosis. Regulation of these latter processes is poorly understood and is a worthy subject for future research. Modulation of the primary injurious factors through alteration of secondary risk factors is currently the only significant approach to prevention of atherosclerosis. Future investigation may provide more direct ways to prevent or retard atherogenesis, either by more effective modification of primary factors or by reducing the magnitude of response to these factors.

A comparison of coronary and internal mammary arteries and implications of the results in the etiology of arteriosclerosis

Comparison was made between the intimal thickening of the anterior descending branch of the left coronary artery and the internal mammary artery in 352 necropsy examinations. The coronary arteries showed severe intimal thickening, progressing in severity throughout life, whereas the internal mammary showed no more than slight changes at any age. These observations, together with the variation in severity of the changes in different portions of the same vessel, and the freedom from this disease of the smaller arteries throughout the body, strongly suggest that a local or anatomic factor is the dominant influence in coronary artery disease.

Pathogenesis of rheumatoid arthritis: a vascular hypothesis

Past research into the pathogenesis of RA has generally concerned itself with established inflammation. The present review summarizes alterations in microvascular anatomy and function which occur during the hypoxic state, in various experimental and disease conditions. It further shows that tissue hypoxia is a common finding in RA and that the microvascular alterations of RA are similar to those produced by experimental hypoxia. The available data suggest that microcirculatory compromise, concomitant with an increase in metabolic needs of synovial tissue, may initiate tissue injury via anoxia and acidosis, resulting in hydrolytic enzyme release, increased vascular permeability and acceleration of inflammatory processes. It is further believed that the microcirculatory abnormality may be generalized, accounting for the systemic manifestations often seen in RA. Factors effecting arteriolar blood flow obstruction are reviewed to identify areas for future investigation in RA and other disorders involving microvasculopathy. The multitude of longknown and newly recognized factors predisposing to vasospasm and vasodilatation have been outlined as a guide to possible mechanisms which may be operative in RA. An attempt has been made to gather and synthesize the available data in the hope that it may stimulate other investigators to pursue more definitive research into specific areas which may show early microvascular abnormalities in the pathophysiology of RA. Identification of factors operative early in the pathogenesis of RA, before it becomes self-perpetuating, may well be a step in the direction of preventing the ravages of this disease, or providing insight to more effective control.