doi:10.1369/jhc.5A6677.2005
Volume 54 (2): 151-159, 2006 Copyright ©The Histochemical Society, Inc. The Immunoquantification of Caveolin-1 and eNOS in Human and Rabbit Diseased Blood Vessels
Departments of Cardiology (AZ,ZM,AC,DLH) and Cardiac Surgery (BFB) and Department of Medicine (AZ), University of Melbourne, Austin Health, Heidelberg, Australia, and Department of Anatomy and Cell Biology, Monash University, Clayton, Australia (MJB) Correspondence to: Dr. Anthony Zulli, Division of Cardiovascular Research, Department of Cardiology, Austin Health, Heidelberg 3084, Australia. E-mail: azulli{at}unimelb.edu.au
In this study, caveolin-1 (cav-1), an inhibitor of endothelial nitric oxide synthase (eNOS), was semi-quantified in diseased human and rabbit blood vessels. New Zealand White rabbits were fed, for 12 weeks, a high methionine diet (to induce intimal hyperplasia), 0.5% cholesterol diet, a normal diet, or the combination of both experimental diets. Excess segments of human internal mammary arteries (IMA) and radial arteries (RA) were obtained from patients undergoing coronary artery bypass surgery. eNOS and cav-1 were localized throughout both human and rabbit vessels. In rabbit arteries, eNOS was significantly increased in the endothelium overlying intimal thickening and atherosclerotic plaques compared with the adjacent endothelium overlying normal media. Interestingly, the endothelial cav-1:eNOS ratio increased 5-fold only in endothelium overlying plaques but decreased in endothelium overlying vessels with neo-intimal thickening. In human tissue, there was no difference between RA and IMA eNOS immunoreactivity in endothelium, intima, or media; however, RA endothelial, intimal, and medial cav-1 immunoreactivity increased 4-fold (p<0.02), 8-fold (p<0.001), and 4-fold (p<0.004), respectively, compared with IMA. Furthermore, the cav-1:eNOS immunostaining ratio in the media correlated with intimal thickening (r2 = 0.5). Our results suggest a close relationship between increased cav-1 and diseased blood vessels. (J Histochem Cytochem 54:151159, 2006)
Key Words: atherosclerosis radial artery caveolin-1 endothelial nitric oxide synthase immunohistochemistry
CAVEOLIN-1 (cav-1) is a structural protein (Glenney and Soppet 1992
As cav-1 is involved in endothelial function, evidence is accumulating to suggest a regulatory role for cav-1 in both intimal hyperplasia (Hassan et al. 2004
Frank and Lisanti (2004) To further identify a role for cav-1 in atherogenesis, we semi-quantified both eNOS and cav-1 immunohistochemistry in the endothelial cell layer, intima, and media of human internal mammary arteries (IMA) and radial arteries (RA). Furthermore, as we have previously reported in a rabbit model of dietary-induced intimal hyperplasia and atherosclerosis, we also semi-quantified both eNOS and cav-1 immunohistochemistry in the endothelium overlying the normal vessel wall, intimal thickening, and atherosclerotic areas of rabbit thoracic aorta.
Human Tissue Excess segments of IMA (n=10) and RA (n=4) were obtained from consenting patients undergoing coronary artery bypass surgery. Vessels were placed in cold oxygenated Krebs buffer and immediately transported to the laboratory. Vessels were cleaned of connective tissue and fat and fixed in 10% formalin overnight. All vessels were concomitantly processed for paraffin, and all vessels were mounted on a single paraffin block. Coronal sections (5 µm) were cut and mounted on gelatin-coated microscope slides.
Rabbit Tissue
Immunohistochemistry
Semi-quantification of eNOS and Cav-1 Proteins In each group, the color intensity value obtained with the MCID software (Imaging Research) was multiplied by the proportional staining area value, and each group was divided by the mean value obtained from the control group normal endothelial layer [described above as (a)]. Please note: this value also acts as the "internal control." Thus, the endothelial layer overlying normal media taken from the control group is reported as 1 ± SEM, and all other values are reported as "fold increase over control."
Cav-1:eNOS Ratio To determine the cav-1:eNOS ratio in human tissue, all RA endothelial, medial, and intimal cav-1 and eNOS quantitative results were "normalized" by dividing it by the mean results obtained from the IMA. In this way, the results obtained from the IMA acted as the "internal control" and its represented value is 1 ± SEM. For example, the RA cav-1 media data were divided by the mean data obtained from the IMA media. To determine the ratio, the normalized cav-1 data were then divided by the normalized eNOS data. For both the human and rabbit study, immunoquantification of eNOS and cav-1 was performed on three separate serial sections, three separate times on three separate days, and all results showed similar findings. In the Results section, we report only the third assessment.
Statistical Analysis Comparisons between endothelium overlying normal media and endothelium overlying neo-intimal thickening [(a) and (b), see above] for all groups were also analyzed by one-way ANOVA to determine whether there were significant differences among the four experimental groups, and Tukey's posthoc test was applied to determine where between the groups the statistical difference lay. In all cases, statistical significance was accepted at a level of p<0.05.
Human Tissue In the RA, eNOS and cav-1 were localized to the endothelium and media and to cells within the neointima (Figures 1A 1D). There was no significant difference in endothelial or intimal eNOS protein between the IMA and RA; however, medial eNOS was increased 1.5-fold in the RA compared with the IMA, but this failed to reach significance (Figure 2A ). However, endothelial and medial cav-1 immunoreactivity increased 4-fold (p<0.02 and p<0.004, respectively) compared with the IMA, whereas intimal cav-1 increased 8-fold compared with the IMA (p<0.004, Figure 2B). By plotting the intima:media ratio of all vessels studied vs the cav-1:eNOS ratio in the media, we found a strong correlation (r2 = 0.5; Figure 2C) between these variables.
The increase in intimal cav-1 was due to an increase in the number of cav-1-positive cells. These cells were also positive for -smooth muscle cell actin (Figure 3A
) but not macrophages (Figure 3B, Ham 56). In fact, the population of intimal cells within the RA intima was almost all -smooth muscle cell actin positive, with only scarce macrophages present (Figure 3B, inset).
Rabbit Tissue Cav-1 was identified in the endothelial layer overlying normal media and intimal thickening of the thoracic aorta of the control, methionine, cholesterol, and the combination group (Figures 4A , 4C, and 4Eonly combination group shown). In addition, cav-1 was also identified within the atherosclerotic plaques of the thoracic aorta from the high dietary cholesterol and high dietary cholesterol plus methionine group (Figure 4Eonly combination group shown).
Immunolocalization of eNOS in these experimental animals has been previously published (Zulli et al. 2003 -smooth muscle actin-positive cells (Figure 3C, possibly smooth muscle cells or myofibroblasts) and macrophages (Figure 3D, RAM-11 positive). In the endothelial layer overlying thickened intima of the control group, eNOS immunodensity significantly increased 2.8-fold compared with the endothelial layer overlying disease-free areas (p<0.05). Similarly, in the endothelial layer overlying thickened intima of the Meth group, eNOS immunodensity significantly increased 4.4-fold compared with the endothelial layer overlying disease-free areas (p<0.05). In the cholesterol group there was an 11-fold and 8-fold increase (p<0.05) in eNOS immunodensity in the endothelial layer overlying the thickened intima and atherosclerotic plaques, respectively, compared with the endothelial layer overlying disease-free areas. Likewise, in the MethChol group, we observed a 4-fold and 7-fold increase (p<0.05) in eNOS immunodensity in the endothelial layer overlying the thickened intima and atherosclerotic plaques, respectively, compared with the endothelial layer overlying disease-free areas. There was no significant difference in eNOS immunodensity in the endothelial layer overlying the thickened intima and atherosclerotic plaques between the Chol and MethChol groups. Also, there was no significant difference in eNOS immunodensity in the endothelial layers overlying disease-free areas between the control group, Chol group (2.5-fold increase, p=ns), Meth group (1.3-fold increase, p=ns), or the MethChol group (1.8-fold increase, p=ns; Figure 5A ).
Interestingly, cav-1 immunodensity was markedly increased only in the endothelium overlying plaques in both the Chol group (75-fold increase, p<0.05) and MethChol groups (77-fold increase, p<0.05) compared with the endothelial layer overlying disease-free areas in each respective group (Figure 5B). Such marked increase in cav-1 immunodensity led to an increase in the cav-1:eNOS ratio in the endothelial layer overlying plaques in both the Chol and MethChol groups (p<0.05, Figure 6 ). Conversely, the cav-1: eNOS ratio was virtually unchanged in all other endothelial layers overlying normal media in all groups and decreased consistently throughout all other endothelial layers overlying intimal thickening in all groups (Figure 6). We found no correlation between the endothelial layer cav-1 content overlying plaques in the Chol and MethChol groups and plaque size (Figure 7 , r2 = 0.02, p=0.4).
The major finding in this investigation is that cav-1 immunoreactivity is increased in both rabbit and human diseased blood vessels. We have also associated an increase in cav-1 compared with eNOS immunoreactivity with an increase in the intima:media ratio of human vessels and in the endothelial layer overlying atherosclerotic plaques in rabbit vessels.
An increase in endothelial cav-1 has been suggested to induce endothelial dysfunction. For example, recent studies have demonstrated that cav-1 regulates eNOS function in the endothelial cell. Evidence from cav-1 knockout mice showed that low concentrations of bradykinin (Drab et al. 2001
We have previously shown that endothelium-dependent relaxation was impaired in the abdominal aorta of these rabbits fed high dietary cholesterol and methionine and virtually abolished by the combination on these two diets, but eNOS protein was not compromised in any group (Zulli et al. 2003
Caveolae also play an important role in maintaining vascular tone via the detection of fluid shear stress generated by blood flowing over the endothelium. Thus, any change in local rheology of blood due to the presence of atherosclerosis would also affect the local shear stress present on the endothelial cells overlying intimal thickening and atherosclerotic plaques. In this case, an increase in shear stress would be expected in areas of intimal thickening and plaque formation, which in itself can increase cav-1 (Boyd et al. 2003
Here we report that the RA and IMA express similar endothelial, medial, and intimal eNOS protein immunoreactivity. These results are in contrast with Gaudino et al. (2003)
However, in this study we identify a possible role for cav-1 in human diseased vessels. We observed a significant increase in endothelial, intimal, and medial cav-1 immunoreactivity in the diseased radial arteries compared with diseased areas of the internal thoracic artery. Here we propose that, in contrast to endothelial cav-1, medial cav-1 might be inhibiting eNOS activity. For example, although it has been shown that the genetic transfer of eNOS mRNA to the RA improved vascular function, even though eNOS was previously present in these vessels (Cable et al. 1999
The diseased human radial arteries showed a marked increase in medial cav-1 protein, a phenomenon not observed in our rabbit model. We suggest that this difference could be due to different blood vessel types, as the IMA also showed minimal cav-1 immunoreactivity, which is similar to the results observed in our animal model. Although it has been shown that a lack of cav-1 gene exacerbates neo-intimal thickening (Hassan et al. 2004 In conclusion, we report that cav-1 immunoreactivity is increased within the atherosclerotic plaques of rabbits and within the media and intima of diseased human arteries, suggesting a close relationship between cav-1 and blood vessel disease.
Received for publication March 2, 2005; accepted June 15, 2005
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