Hyperhomocysteinemia enhances vascular inflammation and accelerates atherosclerosis in a murine model

Hyperhomocysteinemia leads to adverse cardiac remodeling in hypertensive rats

Hyperhomocysteinemia (Hhe), linked to cardiovascular disease by epidemiological studies, may be an important factor in adverse cardiac remodeling in hypertension. Specifically, convergence of myocardial and vascular alterations promoted by Hhe and hypertension may exacerbate cardiac remodeling and myocardial dysfunction. We studied male spontaneously hypertensive rats fed one of three diets: control, intermediate Hhe inducing, or severe Hhe inducing. After 10 wk of dietary intervention, cardiac function was assessed in vitro, and cardiac and coronary arteriolar remodeling were monitored by histomorphometric, immunohistochemical, and biochemical techniques. Results showed that Hhe induced diastolic dysfunction, as characterized by the diastolic pressure-volume curve, without significant changes in baseline systolic function. Perivascular collagen levels were increased by Hhe, and there was an increase in left ventricular hydroxyproline levels. Myocyte size was not affected. Coronary arteriolar wall thickness increased with Hhe due to smooth muscle hyperplasia. Mast cells increased in parallel with Hhe and collagen accumulation. In summary, 10 wk of Hhe caused coronary arteriolar remodeling, myocardial collagen deposition, and diastolic dysfunction in hypertensive rats.

Hyperhomocyst(e)inemia and Thrombophilia

OBJECTIVE

To review the role of an elevated total plasma homocysteine level (hyperhomocyst[e]inemia) in patients with venous or arterial thrombosis, as reflected by the medical literature and the consensus opinion of recognized experts in the field.

DATA SOURCES

Review of the medical literature, primarily from the last 10 years.

DATA EXTRACTION AND SYNTHESIS

The literature was reviewed to identify key points defining the condition, and the clinical study design of each article was examined. A draft manuscript was prepared and circulated prior to the conference to every participant in the College of American Pathologists Conference XXXVI: Diagnostic Issues in Thrombophilia. Each of the key points and associated recommendations was then presented for discussion at the conference. Recommendations were accepted if a consensus of the 70% of the experts attending the conference was reached. The results of the discussion were used to revise the manuscript into its final form.

CONCLUSIONS

Consensus was reached on 9 recommendations concerning the criteria for diagnosis, the method of testing, and the approach for clinical management. A major point of consensus was that no causal role of hyperhomocyst(e)inemia in venous or arterial thrombosis is yet established. Testing methods used to measure homocysteine directly are sensitive and reliable, and provide more information than does genotyping for markers linked to abnormal plasma homocysteine.

Increased monocyte adhesion to aortic endothelium in rats with hyperhomocysteinemia: role of chemokine and adhesion molecules

OBJECTIVE

The stimulatory effect of homocysteine (Hcy) on monocyte chemoattractant protein (MCP)-1 expression in vitro has been suggested to play an important role in Hcy-mediated atherosclerosis. We investigated whether such a stimulatory effect occurs in vivo, leading to monocyte adhesion to the endothelium.

METHODS AND RESULTS

Sprague-Dawley rats were divided into 4 groups. Hyperhomocysteinemia was induced in 1 group of rats after 4 weeks of a high-methionine diet (serum Hcy levels were 4- to 5-fold higher than levels in control rats). The number of ED-1-positive cells present on the surface of aortic endothelium was significantly elevated in hyperhomocysteinemic rats. There was a significant increase in the expression of MCP-1, vascular cell adhesion molecule-1 (VCAM-1), and E-selectin in the endothelium. Antibodies recognizing MCP-1, VCAM-1, or E-selectin could abolish the enhanced monocyte binding to the aortic endothelium of hyperhomocysteinemic rats. Endothelium-dependent aortic relaxation was impaired in hyperhomocysteinemic rats.

CONCLUSIONS

These results suggest that in the absence of other known risk factors, hyperhomocysteinemia stimulates the expression of MCP-1, VCAM-1, and E-selectin in vivo, leading to increased monocyte adhesion to the aortic endothelium. Such an effect may contribute significantly to the development of atherosclerosis by facilitating monocyte/macrophage infiltration into the arterial wall.

Increased serum homocysteine and sudden death resulting from coronary atherosclerosis with fibrous plaques

INTRODUCTION

Modest elevations of total homocysteine have been associated with increased risk for coronary atherosclerosis but correlation between elevated homocysteine and plaque morphology has not been described in humans.

METHODS

We determined serum homocysteine at postmortem from 87 men with coronary thrombus (62 of whom were diagnosed as acute), from 35 men with severe coronary disease without thrombus, and from 46 controls. In coronary deaths, atherosclerotic plaques at the sites of maximal luminal narrowing of the four epicardial coronary arteries were classified as fibrous plaques, fibrous cap atheromas, thin-cap atheromas, and healed ruptures, and macrophage infiltration was assessed semiquantitatively.

RESULTS

Median serum homocysteine postmortem as a result of acute thrombus was 10.4 micro mol/L (P=0.4 versus controls), 12.1 micro mol/L in men with organized thrombi (P=0.1 versus controls), 15.6 micro mol/L in men without thrombus (P=0.007 versus controls), and 9.8 micro mol/L in controls. The median homocysteine was 12.1 micro mol/L in 65 men with healed infarcts (P=0.03 versus controls). The number of fibrous plaques was associated with log-normalized homocysteine (P=0.004), independent of age, albumin, smoking, hypertension, and serum cholesterol. Homocysteine levels in the upper tertile (>15 micromol/L) were associated with sudden death without acute or organized thrombus (odds ratio 3.8, P=0.03) independent of age and other risk factors; the coexistence of diabetes increased the association (odds ratio 25.1, P=0.009, versus lowest tertile < or =8.5 micromol/L).

CONCLUSIONS

Increased serum homocysteine is associated with sudden death in the absence of acute coronary thrombosis, especially with concomitant diabetes, and with the presence of lipid-poor, fibrous plaques.

Dietary folate and the risk of nonfatal myocardial infarction

BACKGROUND

Elevated homocysteine levels have been associated with a higher risk of cardiovascular disease. Because folate intake can reduce homocysteine levels, we investigated the association between dietary folate intake and nonfatal myocardial infarction.

METHODS

We conducted a case-control study in three tertiary hospitals of Pamplona, Spain, between 1999 and 2001. Study physicians enrolled 171 patients less than 80 years of age with a first nonfatal myocardial infarction and 171 control patients matched by age, sex, hospital and calendar month. We excluded patients with any prior major cardiovascular disease. Participants were interviewed about medical factors and life-style and completed a food frequency questionnaire previously validated in Spain. We calculated energy-adjusted intakes of folate and estimated relative risks (RRs) of myocardial infarction and 95% confidence intervals (CIs) using conditional logistic regression. Relative risks were adjusted for conventional risk factors.

RESULTS

Only 6% of participants were taking vitamin supplements. The main sources of folate were green leafy vegetables, green beans, oranges, peppers and lettuces. The estimated matched RR of myocardial infarction for the top three quartiles of folate intake (, above 340 microg/day) was 0.57 (CI = 0.35-0.94), compared with the lowest quartile of intake. The multivariate adjusted RR was 0.51 (CI = 0.24-1.06). There was no apparent dose effect above this threshold.

CONCLUSIONS

Our results in a Mediterranean population with natural plant foods as the main source of folate provide further evidence to support the hypothesis that dietary folate intake may be an independent protective factor for myocardial infarction. The magnitude of the effect, its biological plausibility, and the consistency across studies offer support for a causal association.

Blockade of platelet-derived growth factor or its receptors transiently delays but does not prevent fibrous cap formation in ApoE null mice

Platelet-derived growth factor (PDGF) is a potent stimulant of smooth muscle cell migration and proliferation in culture. To test the role of PDGF in the accumulation of smooth muscle cells in vivo, we evaluated ApoE -/- mice that develop complex lesions of atherosclerosis. Fetal liver cells from PDGF-B-deficient embryos were used to replace the circulating cells of lethally irradiated ApoE -/- mice. One month after transplant, all monocytes in PDGF-B -/- chimeras are of donor origin (lack PDGF), and no PDGF-BB is detected in circulating platelets, primary sources of PDGF in lesions. Although lesion volumes are comparable in the PDGF-B +/+ and -/- chimeras at 35 weeks, lesions in PDGF-B -/- chimeras contain mostly macrophages, appear less mature, and have a reduced frequency of fibrous cap formation as compared with PDGF-B +/+ chimeras. However, after 45 weeks, smooth muscle cell accumulation in fibrous caps is indistinguishable in the two groups. Comparison of elicited peritoneal macrophages by RNase protection assay shows an altered cytokine and cytokine receptor profile in PDGF-B -/- chimeras. ApoE -/- mice were also treated for up to 50 weeks with a PDGF receptor antagonist that blocks all three PDGF receptor dimers. Blockade of the PDGF receptors similarly delays, but does not prevent, accumulation of smooth muscle and fibrous cap formation. Thus, elimination of PDGF-B from circulating cells or blockade of PDGF receptors does not appear sufficient to prevent smooth muscle accumulation in advanced lesions of atherosclerosis.

Renal tubulointerstitial injury in weanling rats with hyperhomocysteinemia

BACKGROUND

While hyperhomocysteinemia is associated with an increased risk of atherosclerosis and the related cardiovascular diseases, the effect of hyperhomocysteinemia on the kidney has not been clearly demonstrated. The purpose of this study was to investigate whether long-term hyperhomocysteinemia develops atherosclerotic lesions in the kidney.

METHODS

The effects of various dietary combinations, including folate deficiency, choline deficiency and methionine loading, on the plasma homocysteine concentration, renal function and renal histopathology were examined for 12 weeks in male weanling Fisher rats.

RESULTS

Folate deficiency, choline deficiency and methionine loading synergistically induced hyperhomocysteinemia up to 69.7 +/- 23.1 micromol/L (control, 11.6 +/- 3.9 micromol/L, P < 0.01) without any change in blood pressure. Creatinine clearance was negatively correlated with the plasma homocysteine concentration (r = -0.55, P < 0.01). Arterial and arteriolar wall thickening, and focal tubulointerstitial fibrosis were found in the kidneys of the hyperhomocysteinemic rat. The lesions of tubulointerstitial fibrosis appeared striped or wedge-shaped at the subcapsular cortex of the kidney. In addition, the expression of vascular endothelial growth factor, an indicator of hypoxia, was increased in the adjacent more intact area of the cortex. These findings suggest that the renal tubulointerstitial lesions were likely to be mediated by severe ischemia due to regional circulatory disturbance. Folate supplementation diminished these vascular and tubulointerstitial changes.

CONCLUSION

These results indicate that diet-induced chronic hyperhomocysteinemia could induce vascular remodeling and tubulointerstitial injury in the kidney, and that these changes were ameliorated by folate supplementation.

Effect of low-dose aspirin on vascular inflammation, plaque stability, and atherogenesis in low-density lipoprotein receptor-deficient mice

BACKGROUND

Atherosclerosis is a complex vascular inflammatory disease. Low-dose aspirin is a mainstay in the prevention of vascular complications of atherosclerosis. We wished to determine the effect of low-dose aspirin on vascular inflammation, plaque composition, and atherogenesis in LDL receptor-deficient mice fed a high fat diet.

METHODS AND RESULTS

In LDL receptor-deficient mice fed a high fat diet compared with control mice, low-dose aspirin induced a significant decrease in circulating levels and vascular formation of soluble intercellular molecule-1, monocyte chemoattractant protein-1, tumor necrosis factor-alpha, interleukin-12p 40, without affecting lipid levels. This was associated with significant reduction of the nuclear factor kappaB activity in the aorta. Low-dose aspirin also significantly reduced the extent of atherosclerosis. Finally, aortic vascular lesions of the aspirin-treated animals showed 57% reduction (P<0.05) in the amount of macrophage cells, 77% increase in smooth muscle cells (P<0.05), and 23% increase in collagen (P<0.05).

CONCLUSIONS

Our results suggest that in murine atherosclerosis, low-dose aspirin suppresses vascular inflammation and increases the stability of atherosclerotic plaques, both of which, together with its antiplatelet activity, contribute to its antiatherogenic effect. We conclude that low-dose aspirin might be rationally evaluated in the progression and evolution of human atherosclerotic plaque.

Fenofibrate induces a selective increase of protein-bound homocysteine in rodents: a PPARalpha-mediated effect

Elevated levels of plasma homocysteine (Hcy) are associated with increased risk of cardiovascular disease though it is uncertain whether increases in Hcy represent a cause or a consequence of the disease process. Plasma Hcy exists in reduced, free oxidized, and protein-bound forms, that together comprise total Hcy (tHcy). Free reduced Hcy is thought to be the atherogenic, though minor, sub-fraction of tHcy. Recent reports have indicated that fenofibrate and other fibrates are capable of moderately increasing plasma tHcy. As many of the effects of fibrates are known to be mediated by the nuclear receptor PPARalpha, we determined the effect of fenofibrate on tHcy in PPARalpha-deficient mice. We further examined the effect of fenofibrate and fenofibrate plus folate supplementation on total as well as protein-bound Hcy in rats. Fenofibrate significantly increased serum tHcy in wild-type mice but not in PPARalpha deficient mice. In rats, fenofibrate increased serum tHcy by 69%, while the co-administration of folate with fenofibrate increased tHcy by only 7%. In spite of the above increase in tHcy in rats, only the protein-bound fraction of Hcy was increased. In a further study, fenofibrate also induced a significant increase in tHcy, while in spite of this, ex vivo peroxidation of VLDL+LDL was beneficially lowered and the lag time prolonged. In summary, fenofibrate increases serum tHcy in rodents in a PPARalpha-dependent manner. The increase in rats is solely due to protein-bound Hcy as atherogenic, reduced Hcy was unchanged. While awaiting corroboration in human, our results suggest that the extent and mechanism of the increase in total Hcy in patients treated with fenofibrate should not a priori be associated with relevant risk.

Endothelial dysfunction and atherothrombosis in mild hyperhomocysteinemia

Mildly elevated plasma homocysteine levels are an independent risk factor for atherothrombotic vascular disease in the coronary, cerebrovascular, and peripheral arterial circulation. Endothelial dysfunction as manifested by impaired endothelium-dependent regulation of vascular tone and blood flow, by increased recruitment and adhesion of circulating inflammatory cells to the endothelium, and by a loss of endothelial cell antithrombotic function contributes to the vascular disorders linked to hyperhomocysteinemia. Increased vascular oxidant stress through imbalanced thiol redox status and inhibition of important antioxidant enzymes by homocysteine results in decreased bioavailability of the endothelium-derived signaling molecule nitric oxide via oxidative inactivation. This plays a central role in the molecular mechanisms underlying the effects of homocysteine on endothelial function. Supplementation of folic acid and vitamin B12 has been demonstrated to be efficient in lowering mildly elevated plasma homocysteine levels and in reversing homocysteine-induced impairment of endothelium-dependent vasoreactivity. Results from ongoing intervention trials will determine whether homocysteine-lowering therapies contribute to the prevention and reduction of atherothrombotic vascular disease and may thereby provide support for the causal relationship between hyperhomocysteinemia and atherothrombosis.

Homocysteine: a risk factor for cardiovascular disease in subclinical hypothyroidism?

The role of homocysteine as a causal risk factor for cardiovascular disease remains controversial. Moderately elevated total plasma homocysteine levels have been reported in patients with overt hypothyroidism, a condition that is associated with an increased risk for cardiovascular disease. Recently, subclinical hypothyroidism has been identified as an independent risk factor for atherosclerosis and myocardial infarction in elderly women. Therefore, we measured prospectively total fasting plasma homocysteine levels in 37 consecutive subjects (6 males, 31 females, mean age 50 +/- 18 standard deviation [SD] years) with newly diagnosed subclinical hypothyroidism at baseline and after 3-4 months of levothyroxine supplementation. During levothyroxine treatment concentrations of thyrotropin (TSH) decreased from 10.1 +/- 5.8 (SD) to 1.5 +/- 1.8 mU/L. Fasting total plasma homocysteine levels were not elevated at baseline (9.9 +/- 2.9 micromol/L) and remained unchanged (9.6 +/- 3.5 micromol/L) after levothyroxine treatment. Serum folate or vitamin B(12) levels also remained unchanged. We conclude that subclinical hypothyroidism is not associated with hyperhomocysteinemia. Levothyroxine supplementation has no influence on total plasma homocysteine levels in subclinical hypothyroidism. Hence, total plasma homocysteine does not appear to contribute to the increased risk for atherosclerotic disease and myocardial infarction in patients with subclinical hypothyroidism.

Homocysteine and the stage of atherosclerotic disease: a study in patients suffering from clinically silent and clinically manifest atherosclerotic disease

BACKGROUND

Elevated serum levels of homocysteine are considered a risk factor for cardiovascular disease. It has been suggested that homocysteine is a late stage predictor of adverse cardiovascular events, which might explain reported controversies in literature. The objective of the present study was to study the relationship between homocysteine levels and the stage of atherosclerotic disease.

METHODS

In a cross-sectional study we compared the prevalence of hyperhomocysteinemia in high risk patients without silent or clinically overt atherosclerosis (group I, n = 196) with two patient groups: patients with clinically manifest atherosclerosis in the past or asymptomatic atherosclerosis (group II, n = 364) and patients who currently suffer from a clinically relevant manifestation of ischemic cardiovascular disease (group III, n = 967). In addition, we related homocysteine levels with a cumulative index of atherosclerotic disease (SMART-score).

RESULTS

Homocysteine levels (micromol L(-1)) for the different groups were 13.5 +/- 8.9 (group I), 13.7 +/- 8.2 (group II) and 14.7 +/- 7.7 (group III). After adjustment for age, body mass index, creatinine levels and current use of vitamins, no significant differences in the prevalence of hyperhomocysteinemia were observed, compared with the reference category. Linear regression analysis revealed a significant relationship between Smart score and homocysteine levels that remained after adjustment for potential confounders (Beta = 0.36 (0.14-0.59), P = 0.001).

CONCLUSION

If homocysteine levels are associated with the presence and indicators of atherosclerotic plaque burden, then this relationship is apparently not affected by the stage of atherosclerotic disease.

Hyperhomocysteinemia evoked by folate depletion: effects on coronary and carotid arterial function

High circulating concentrations of homocysteine (ie, hyperhomocysteinemia [Hhcy]) impair the vascular function of peripheral conduit arteries and arterioles perfusing splanchnic and skeletal muscle regions. The effects of HHcy on coronary resistance vessel function and other indexes of vascular function, ie, arterial permeability and stiffening, are unclear. We tested the hypotheses that HHcy impairs coronary resistance vessel reactivity; increases carotid arterial permeability; and initiates arterial stiffening. Male rats that consumed folate-replete (CON, n=44) or folate-deplete (HHcy, n=48) chow for 4 to 5 weeks had total plasma homocysteine concentrations of 7+/-2 or 58+/-4 micromol/L, respectively. Maximal acetylcholine-evoked relaxation (approximately 40% vs approximately 60%) and tension development from baseline in response to nitric oxide synthase inhibition (approximately 20% vs approximately 40%) were lower (both P<0.05) in coronary resistance vessels (approximately 120 microm, internal diameter) isolated from HHcy versus CON animals, respectively, whereas sodium nitroprusside-evoked relaxation and contractile responses to serotonin and potassium chloride were similar between groups. Permeability to 4400 MW and 65 000 MW fluorescently labeled (TRITC) dextran reference macromolecules (quantitative fluorescence microscopy) was approximately 44% and approximately 24% greater (P<0.05), respectively, in carotid arteries from HHcy versus CON rats. Maximal strain, evaluated by using a vessel elastigraph, was less ( approximately 32% vs 42%, P<0.05) in carotid arterial segments from HHcy versus CON animals, respectively. Finally, estimates of oxidative (copper-zinc+manganese superoxide dismutase activity) and glycoxidative (pentosidine) stress were elevated (P<0.05) in arterial tissue from HHcy versus CON rats. These findings suggest that moderately severe HHcy evoked by folate-depletion impairs endothelium-dependent relaxation of coronary resistance vessels, increases carotid arterial permeability, and initiates arterial stiffening. HHcy may produce these effects by a mechanism associated with increased oxidative and glycoxidative stress.

Understanding atherosclerosis through mouse genetics

During the past year studies with mouse models have significantly clarified our understanding of atherosclerosis. Noteworthy achievements include: the discovery of a number of novel genes and pathways; new evidence emphasizing the role of lymphocytes in atherogenesis; the development of mouse models exhibiting advanced lesions with evidence of thrombosis; and new results indicating an anti-atherogenic effect of testosterone.

Homocysteine upregulates vascular cell adhesion molecule-1 expression in cultured human aortic endothelial cells and enhances monocyte adhesion

Elevated plasma homocysteine is an independent risk factor for atherosclerosis. We hypothesized that homocysteine enhances monocyte/human aortic endothelial cell (HAEC) interactions, a pivotal early event in atherogenesis, by upregulating endothelial adhesion molecules. After incubation of cultured HAECs with reduced DL-homocysteine for up to 24 hours, adhesion of human monocytes to homocysteine-stimulated HAECs was significantly upregulated in a time- and dose-dependent fashion. Pretreatment of HAECs with 100 micromol/L homocysteine caused a 4.5-fold increase in the adhesion of normal human monocytes (P<0.001). Similarly, adhesion of monocytic U937 cells was maximally elevated by 3.5-fold at 100 micromol/L homocysteine (P<0.001). In support of our hypothesis, vascular cell adhesion molecule (VCAM)-1 mRNA expression increased 5-fold in HAECs after 3 hours of treatment with 100 micromol/L homocysteine, as assessed by quantitative reverse transcription- polymerase chain reaction. Neutralizing antibody studies confirmed the involvement of VCAM-1 in mediating monocyte adhesion to homocysteine-stimulated HAECs. Coincubation of HAECs with homocysteine and tumor necrosis factor-alpha synergistically elevated monocyte adhesion as well as VCAM-1 protein expression, with the latter evaluated by flow cytometry. Preincubation of HAECs with cyclooxygenase inhibitors completely abrogated homocysteine-induced monocyte adhesion, whereas scavenging reactive oxygen species and the elevation of NO caused partial inhibition only. These data support the notion that the proinflammatory effects of homocysteine may have important implications in atherogenesis.

Folic acid treatment reduces chemokine release from peripheral blood mononuclear cells in hyperhomocysteinemic subjects

Elevated plasma homocysteine concentration is an independent risk factor for cardiovascular disease. However, the mechanisms by which hyperhomocysteinemia induces vascular disease are uncertain. An early step in atherogenesis involves leukocyte migration into the arterial wall, a process regulated in part by chemokines. We hypothesized that homocysteine may exert its atherogenic effect in part through chemokine-mediated mechanisms, and in the present study, we examined the effects of folic acid supplementation for 6 weeks on chemokine levels in hyperhomocysteinemic individuals. Data showed the following: (1) Compared with control subjects, hyperhomocysteinemic subjects had elevated plasma levels of the CXC chemokines, epithelial neutrophil-activating peptide (ENA)-78 (P<0.05), and growth-regulated oncogene (GRO)alpha (P=0.088), and homocysteine was significantly correlated with ENA-78 and GROalpha. (2) During folic acid treatment, normalization of homocysteine levels was accompanied by a marked reduction in oxidized low density lipoprotein-stimulated release of CXC chemokines (ie, GROalpha, ENA-78, and interleukin-8) and CC chemokines (ie, monocyte chemoattractant peptide-1 and RANTES) in peripheral blood mononuclear cells from these individuals. (3) The oxidized low density lipoprotein-induced release of ENA-78 from peripheral blood mononuclear cells from control subjects was significantly reduced when cells were incubated in the presence of folic acid. These data may suggest that homocysteine exerts atherogenic effects in part by enhancing chemokine responses in cells involved in atherogenesis and that folic acid supplementation may downregulate these inflammatory responses.

Atherosclerosis and inflammation

The pathophysiology of coronary atherosclerosis is complex and multifactorial. The probability of the development of symptomatic coronary heart disease may be predicted by standard risk factor stratification involving hypertension, dyslipidemia, age, positive family history, and diabetes. However, risk factor stratification has been demonstrated to have significant limitations in the individual patient, which has generated a search for more specific and sensitive markers. Evidence is increasing that atherosclerosis is a disease characterized by inflammation, beginning with the earliest identifiable lesion (fatty streak) to the advanced vulnerable plaque. Clinical markers of inflammation, including C-reactive protein, modified low-density lipoprotein, homocysteine, tumor necrosis factor, and thermogenicity, have been identified as emerging risk factors that may add prognostic information in patient management. This review centers on inflammation as a potential pathogenetic factor in atherosclerosis and the role that clinical markers may play in the identification of patients at risk.

Matrix Metalloproteinases in Vascular Remodeling and Atherogenesis

Vascular remodeling, defined as any enduring change in the size and/or composition of an adult blood vessel, allows adaptation and repair. On the other hand, inappropriate remodeling, including its absence, underlies the pathogenesis of major cardiovascular diseases, such as atherosclerosis and restenosis. Since degradation of the extracellular matrix scaffold enables reshaping of tissue, participation of specialized enzymes called matrix metalloproteinases (MMPs) has become the object of intense recent interest in relation to physiological (“good”) and pathological (“bad”) vascular remodeling. Experimental evidence acquired in vitro and in vivo suggests that the major drivers of vascular remodeling, hemodynamics, injury, inflammation, and oxidative stress, regulate MMP expression and activity. Alternatively, nonspecific MMP inhibition seems to oppose remodeling, as suggested by the inhibition of intimal thickening and outward arterial remodeling. An emerging concept is that MMP-related genetic variations may contribute to heterogeneity in the presentation and natural history of atherosclerosis. The hypothesis that MMPs contribute to weakening of atherosclerotic plaques is especially attractive for the potential development of therapeutic interventions aimed at preventing plaque disruption (“the ugly”), a major cause of acute cardiovascular events. However, the current lack of appropriate experimental tools, including availability of specific MMP inhibitors and pertinent animal models, still limits our understanding of the many actions and relative contributions of specific MMPs. Our future potential ability to control vascular remodeling via regulation of MMPs will also depend on reaching a consensus of what is indeed “good” or “bad” vascular remodeling, concepts that have continued to evolve and change.

The potential for novel anti-inflammatory therapies for coronary artery disease

Although drugs that lead to cholesterol and lipid lowering have proved to have significant effects in lowering cardiovascular morbidity and mortality, coronary artery disease remains a principal cause of death worldwide. There is a clear need to discover further therapeutic approaches to control this disease adequately. This review focuses on the mechanisms that have been implicated in the recruitment, activation and differentiation of inflammatory monocytes/macrophages in nascent vascular lesions into lipid-laden foam cells. These mechanisms might provide attractive targets for novel therapies for coronary artery disease.

Plasma aminothiol oxidation in chronic hemodialysis patients

BACKGROUND

Plasma aminothiols, including homocysteine, cysteine, and glutathione, function as an important extracellular redox system. We examined the plasma aminothiol concentration and redox status in ten chronic hemodialysis patients compared to ten age-matched healthy subjects.

METHODS

Plasma levels of reduced, free oxidized, and protein-bound homocysteine, cysteine, cysteinylglycine, and glutathione were determined using high-pressure liquid chromatography (HPLC).

RESULTS

Total plasma homocysteine, cysteine, and cysteinylglycine levels were significantly elevated in hemodialysis patients before dialysis compared to healthy subjects. Total plasma concentration of cysteine and homocysteine significantly decreased after hemodialysis. The ratio of free oxidized to free reduced homocysteine, cysteine, cysteinylglycine, and glutathione were each significantly elevated before dialysis compared to healthy subjects, and decreased significantly by the end of dialysis. The free oxidized to reduced ratio of cysteine and homocysteine were also significantly correlated with total plasma concentrations.

CONCLUSIONS

Plasma aminothiols are excessively oxidized in uremia, while the hemodialysis procedure is restorative of redox status. Oxidized aminothiols are candidate uremic toxins.

Cyclin A transcriptional suppression is the major mechanism mediating homocysteine-induced endothelial cell growth inhibition

Previously, it was reported that homocysteine (Hcy) specifically inhibits the growth of endothelial cells (ECs), suppresses Ras/mitogen-activated protein (MAP) signaling, and arrests cell growth at the G1/S transition of the cell cycle. The present study investigated the molecular mechanisms underlying this cell-cycle effect. Results showed that clinically relevant concentrations (50 μM) of Hcy significantly inhibited the expression of cyclin A messenger RNA (mRNA) in ECs in a dose- and time-dependent manner. G1/S-associated molecules that might account for this block were not changed, because Hcy did not affect mRNA and protein expression of cyclin D1 and cyclin E. Cyclin D1- and E-associated kinase activities were unchanged. In contrast, cyclin A–associated kinase activity and CDK2 kinase activity were markedly suppressed. Nuclear run-on assay demonstrated that Hcy decreased the transcription rate of the cyclin A gene but had no effect on the half-life of cyclin A mRNA. In transient transfection experiments, Hcy significantly inhibited cyclin A promoter activity in endothelial cells, but not in vascular smooth muscle cells. Finally, adenovirus-transduced cyclin A expression restored EC growth inhibition and overcame the S phase block imposed by Hcy. Taken together, these findings indicate that cyclin A is a critical functional target of Hcy-mediated EC growth inhibition.

Cyclin A transcriptional suppression is the major mechanism mediating homocysteine-induced endothelial cell growth inhibition

Previously, it was reported that homocysteine (Hcy) specifically inhibits the growth of endothelial cells (ECs), suppresses Ras/mitogen-activated protein (MAP) signaling, and arrests cell growth at the G1/S transition of the cell cycle. The present study investigated the molecular mechanisms underlying this cell-cycle effect. Results showed that clinically relevant concentrations (50 μM) of Hcy significantly inhibited the expression of cyclin A messenger RNA (mRNA) in ECs in a dose- and time-dependent manner. G1/S-associated molecules that might account for this block were not changed, because Hcy did not affect mRNA and protein expression of cyclin D1 and cyclin E. Cyclin D1- and E-associated kinase activities were unchanged. In contrast, cyclin A–associated kinase activity and CDK2 kinase activity were markedly suppressed. Nuclear run-on assay demonstrated that Hcy decreased the transcription rate of the cyclin A gene but had no effect on the half-life of cyclin A mRNA. In transient transfection experiments, Hcy significantly inhibited cyclin A promoter activity in endothelial cells, but not in vascular smooth muscle cells. Finally, adenovirus-transduced cyclin A expression restored EC growth inhibition and overcame the S phase block imposed by Hcy. Taken together, these findings indicate that cyclin A is a critical functional target of Hcy-mediated EC growth inhibition.

Cyclin A transcriptional suppression is the major mechanism mediating homocysteine-induced endothelial cell growth inhibition

Previously, it was reported that homocysteine (Hcy) specifically inhibits the growth of endothelial cells (ECs), suppresses Ras/mitogen-activated protein (MAP) signaling, and arrests cell growth at the G(1)/S transition of the cell cycle. The present study investigated the molecular mechanisms underlying this cell-cycle effect. Results showed that clinically relevant concentrations (50 microM) of Hcy significantly inhibited the expression of cyclin A messenger RNA (mRNA) in ECs in a dose- and time-dependent manner. G(1)/S-associated molecules that might account for this block were not changed, because Hcy did not affect mRNA and protein expression of cyclin D1 and cyclin E. Cyclin D1- and E-associated kinase activities were unchanged. In contrast, cyclin A-associated kinase activity and CDK2 kinase activity were markedly suppressed. Nuclear run-on assay demonstrated that Hcy decreased the transcription rate of the cyclin A gene but had no effect on the half-life of cyclin A mRNA. In transient transfection experiments, Hcy significantly inhibited cyclin A promoter activity in endothelial cells, but not in vascular smooth muscle cells. Finally, adenovirus-transduced cyclin A expression restored EC growth inhibition and overcame the S phase block imposed by Hcy. Taken together, these findings indicate that cyclin A is a critical functional target of Hcy-mediated EC growth inhibition.

Relative roles of albumin and ceruloplasmin in the formation of homocystine, homocysteine-cysteine-mixed disulfide, and cystine in circulation

Disulfide forms of homocysteine account for >98% of total homocysteine in plasma from healthy individuals. We recently reported that homocysteine reacts with albumin-Cys(34)-S-S-cysteine to form homocysteine-cysteine mixed disulfide and albumin-Cys(34) thiolate anion. The latter then reacts with homocystine or homocysteine-cysteine mixed disulfide to form albumin-bound homocysteine (Sengupta, S., Chen, H., Togawa, T., DiBello, P. M., Majors, A. K., Büdy, B., Ketterer, M. E., and Jacobsen, D. W. (2001) J. Biol. Chem. 276, 30111-30117). We now extend these studies to show that human albumin, but not ceruloplasmin, mediates the conversion of homocysteine to its low molecular weight disulfide forms (homocystine and homocysteine-cysteine mixed disulfide) by thiol/disulfide exchange reactions. Only a small fraction of homocystine is formed by an oxidative process in which copper bound to albumin, but not ceruloplasmin, mediates the reaction. When copper is removed from albumin by chelation, the overall conversion of homocysteine to its disulfide forms is reduced by only 20%. Ceruloplasmin was an ineffective catalyst of homocysteine oxidation, and immunoprecipitation of ceruloplasmin from human plasma did not inhibit the capacity of plasma to mediate the conversion of homocysteine to its disulfide forms. In contrast, ceruloplasmin was a highly efficient catalyst for the oxidation of cysteine and cysteinylglycine to cystine and bis(-S-cysteinylglycine), respectively. However, when thiols (cysteine and homocysteine) that are disulfide-bonded to albumin-Cys(34) are removed by treatment with dithiothreitol to form albumin-Cys(34)-SH (mercaptalbumin), the conversion of homocysteine to its disulfide forms is completely blocked. In conclusion, albumin mediates the formation of disulfide forms of homocysteine by thiol/disulfide exchange, whereas ceruloplasmin converts cysteine to cystine by copper-dependent autooxidation.

Familial amyloid polyneuropathy: receptor for advanced glycation end products-dependent triggering of neuronal inflammatory and apoptotic pathways

Familial amyloid polyneuropathy (FAP) is a neurodegenerative disorder associated with extracellular deposition of mutant transthyretin (TTR) amyloid fibrils, particularly in the peripheral nervous system. We have hypothesized that binding of TTR fibrils to the receptor for advanced glycation end products (RAGE) on critical cellular targets is associated with a destructive stress response underlying peripheral nerve dysfunction. Analysis of nerve biopsy samples from patients with FAP (n = 16) at different stages of disease (0-3), compared with age-matched controls (n = 4), by semiquantitative immunohistology and in situ hybridization showed increased levels of RAGE, beginning at the earliest stages of the disease (FAP 0; p < 0.02) and especially localized in axons. Upregulation of proinflammatory cytokines (tumor necrosis factor-alpha and interleukin-1beta) (approximately threefold; p < 0.02) and the inducible form of nitric oxide synthase (iNOS) ( approximately 2.5-fold; p < 0.04) was also observed in a distribution overlapping RAGE expression. Tyrosine nitration and increased activated caspase-3 in axons from FAP patients (p < 0.03) were apparent. Although these data suggest the presence of ongoing neuronal stress, there was no upregulation of neurotrophins (nerve growth factor and neurotrophin-3) in FAP nerves. Studies on cultured neuronal-like, Schwann, and endothelial cells incubated with TTR fibrils displayed RAGE-dependent expression of cytokines and iNOS at early times (6 and 12 hr, respectively), followed by later (24 hr) activation of caspase-3 and DNA fragmentation. We propose that the interaction of TTR fibrils with RAGE may contribute to cellular stress and toxicity in FAP. Furthermore, there is an apparent lack of responsiveness of Schwann cells in FAP nerve to provide neurotrophic factors.

Familial amyloid polyneuropathy: receptor for advanced glycation end products-dependent triggering of neuronal inflammatory and apoptotic pathways

Familial amyloid polyneuropathy (FAP) is a neurodegenerative disorder associated with extracellular deposition of mutant transthyretin (TTR) amyloid fibrils, particularly in the peripheral nervous system. We have hypothesized that binding of TTR fibrils to the receptor for advanced glycation end products (RAGE) on critical cellular targets is associated with a destructive stress response underlying peripheral nerve dysfunction. Analysis of nerve biopsy samples from patients with FAP (n = 16) at different stages of disease (0-3), compared with age-matched controls (n = 4), by semiquantitative immunohistology and in situ hybridization showed increased levels of RAGE, beginning at the earliest stages of the disease (FAP 0; p < 0.02) and especially localized in axons. Upregulation of proinflammatory cytokines (tumor necrosis factor-alpha and interleukin-1beta) (approximately threefold; p < 0.02) and the inducible form of nitric oxide synthase (iNOS) ( approximately 2.5-fold; p < 0.04) was also observed in a distribution overlapping RAGE expression. Tyrosine nitration and increased activated caspase-3 in axons from FAP patients (p < 0.03) were apparent. Although these data suggest the presence of ongoing neuronal stress, there was no upregulation of neurotrophins (nerve growth factor and neurotrophin-3) in FAP nerves. Studies on cultured neuronal-like, Schwann, and endothelial cells incubated with TTR fibrils displayed RAGE-dependent expression of cytokines and iNOS at early times (6 and 12 hr, respectively), followed by later (24 hr) activation of caspase-3 and DNA fragmentation. We propose that the interaction of TTR fibrils with RAGE may contribute to cellular stress and toxicity in FAP. Furthermore, there is an apparent lack of responsiveness of Schwann cells in FAP nerve to provide neurotrophic factors.

Advances in experimental dyslipidemia and atherosclerosis

Among the models of dyslipidemia and atherosclerosis, a number of wild-type, naturally defective, and genetically modified animals (rabbits, mice, pigeons, dogs, pigs, and monkeys) have been characterized. In particular, their similarities to and differences from humans in respect to relevant biochemical, physiologic, and pathologic conditions have been evaluated. Features of atherosclerotic lesions and their specific relationship to plasma lipoprotein particles have been critically reviewed and summarized. All animal models studied have limitations: the most significant advantages and disadvantages of using a specific animal species are outlined here. New insights in lipid metabolism and genetic background with regard to variations in pathogenesis of dyslipidemia-associated atherogenesis have also been reviewed. Evidence suggests that among wild-type species, strains of White Carneau pigeons and Watanabe Heritable Hyperlipidemic and St. Thomas's Hospital rabbits are preferable to the cholesterol-fed wild-type animal species in dyslipidemia and atherosclerosis research. Evidence for the usefulness of both wild-type and transgenic animals in studying the involvement of inflammatory pathways and Chlamydia pneumoniae infection in pathogenesis of atherosclerosis has also been summarized. Transgenic mice and rabbits are excellent tools for studying specific gene-related disorders. However, despite these significant achievements in animal experimentation, there are no suitable animal models for several rare types of fatal dyslipidemia-associated disorders such as phytosterolemia and cerebrotendinous xanthomatosis. An excellent model of diabetic atherosclerosis is unavailable. The question of reversibility of atherosclerosis still remains unanswered. Further work is needed to overcome these deficiencies.

Dietary supplementation with methionine and homocysteine promotes early atherosclerosis but not plaque rupture in ApoE-deficient mice

Hyperhomocysteinemia is an independent risk factor for atherothrombosis. However, causality is unproven, and it remains unknown whether hyperhomocysteinemia promotes atherosclerosis, plaque rupture, and/or thrombosis. We evaluated the short- and long-term effects of hyperhomocysteinemia on plaque size and structure in 99 atherosclerosis-prone apolipoprotein E-deficient mice. Hyperhomocysteinemia was induced by methionine (Met) or homocysteine (HcyH) supplementation: low Met (+11 g Met/kg food), high Met (+33 g Met/kg food), low HcyH (0.9 g HcyH/L drinking water), and high HcyH (1.8 g HcyH/L drinking water). Met and HcyH supplementation significantly raised plasma total homocysteine levels by 4- to 16-fold above those observed in mice fed a control diet (up to 146.1 micromol/L). Compared with controls, aortic root plaque size was significantly larger in supplemented groups after 3 months (56% and 173% larger in high-Met and high-HcyH, respectively) but not after 12 months. Hyperhomocysteinemia was associated with an increase in the amount of collagen in plaques after both 3 and 12 months. Mechanical testing of the tail tendons revealed no weakening of collagen after 12 months of hyperhomocysteinemia. Many plaques in both control and supplemented mice appeared rupture prone morphologically, but all aortic root plaques and all but 1 coronary plaque had an intact surface without rupture or thrombosis. Thus, diet-induced hyperhomocysteinemia promotes early atherosclerosis and plaque fibrosis but does not, even in the long term, weaken collagen or induce plaque rupture.