Originally published as JHC exPRESS on February 16, 2009. doi:10.1369/jhc.2009.953273
Volume 57 (6): 559-566, 2009 Copyright ©The Histochemical Society, Inc. Use of Human Vascular Tissue Microarrays for Measurement of Advanced Glycation Endproducts
Department of Pathology, Johns Hopkins University School of Medicine, Baltimore, Maryland (MKH,JL,AMM,TCC), and Departments of Epidemiology and Medicine, Johns Hopkins Bloomberg School of Public Health and the Johns Hopkins University School of Medicine, Baltimore, Maryland (ES) Correspondence to: Marc K. Halushka, MD, PhD, Ross Building, RM 632L, 720 Rutland Avenue, Baltimore, MD 21205. E-mail: mhalush1{at}jhmi.edu
Advanced glycation endproducts (AGEs) are present in the vasculature and are associated with vascular disease. We determined levels of AGEs in eight distinct adult vascular tissues using tissue microarray (TMA) technology and associated these levels with clinical characteristics. Medium-to-large caliber blood vessels were harvested from 100 adult autopsies to create 17 TMAs. AGE levels were evaluated by IHC using a polyclonal anti-AGE antibody on over 700 unique blood vessels. Slides were digitally scanned, and quantitative analysis was performed using a color deconvolution image analysis technique. Medial AGE staining was strongly correlated between all eight blood vessels. In the media, AGE staining levels were significantly higher at older ages (p=0.009), in white subjects (p<0.001) and with longer postmortem interval (PMI; p<0.0001). These associations remained significant after simultaneous adjustment for age, race/ethnicity, PMI, and diabetes status. Diabetes was associated with elevated AGE levels but only after adjustment for confounding by clinical variables including race/ethnicity, hypertension, and kidney function. This extensive vascular study shows that AGE accumulation in the macrovasculature is a global process affecting atherosclerosis-prone and -resistant vessels. It also suggests ethnicity has a previously undescribed role in vascular tissue AGE levels. This manuscript contains online supplemental material at http://www.jhc.org. Please visit this article online to view these materials. (J Histochem Cytochem 57:559–566, 2009)
Key Words: advanced glycation endproducts blood vessels immunohistochemistry diabetes
ADVANCED GLYCATION ENDPRODUCTS (AGEs) play a major role in the development of diabetic vascular disease. AGEs are a collection of moieties formed by the Maillard reaction and are elevated with aging, diabetes, severe renal disease, dietary intake, and smoking (Vlassara and Palace 2002
Small studies of AGEs in vascular tissues have established a link between AGEs and vascular disease (Nakamura et al. 1993
Large-scale studies that compared robust phenotypic data and tissue AGE measures have used skin biopsies. Elevations in skin AGEs are associated with development of both micro- and macrovascular disease in persons with diabetes (Monnier et al. 1999
Large-scale studies that combine robust phenotypic data and AGE levels in human vascular tissues do not exist. Procurement of human tissues and measurement of AGE levels in vascular tissue have proven difficult. High-performance liquid chromatography (HPLC), liquid chromatography–mass spectrometry (LC-MS), and nuclear magnetic resonance (NMR), all methods to accurately measure AGEs, are expensive, rigorous, and labor intensive, making large-scale studies cost prohibitive (Thornalley et al. 2003
We overcame these problems by using tissue microarray (TMA) technology to measure AGEs in a high-throughput format (Kononen et al. 1998 This study was designed to determine whether AGE IHC staining intensity was correlated between atherosclerosis-prone and atherosclerosis-resistant vessels. We also sought to assess the association of AGEs with sociodemographic and clinical characteristics including age, race/ethnicity, sex, diabetes, and hypertension status. We hypothesized that AGE staining intensity would be higher at older ages, in smokers, and in persons with diabetes or renal disease. This is the first large-scale study to determine the distribution of AGEs in human vascular tissues using high-throughput technology.
Study Population and Tissue Harvesting One hundred adult autopsies were harvested at The Johns Hopkins Hospital or Bayview Medical Center. Tissues were taken from a variety of atherosclerosis-prone and -resistant large-to-medium caliber vessels. Tissues were fixed in 10% neutral buffered formalin (Cardinal Health; Dublin, OH) for a minimum of 24 hr, processed, and embedded in paraffin. Demographic and clinical information was collected from a review of patient medical records as described previously (Halushka et al. in press
TMA Creation
IHC
The antigen for this polyclonal-AGE antibody was a proprietary mixture of AGE-human serum albumin and AGE-BSA (Abcam, unpublished data), which was not expected to cross-react with CML epitopes. A blocking experiment was performed in our laboratory with AGE-BSA derived from both glucose and glycolaldehyde protocols (Valencia et al. 2004
Digital Slide Scanning and Staining Quantification Separate AGE staining values were generated for eight blood vessels in up to 100 subjects (Table 1 ). An overall average AGE staining value was generated separately for the media and intima layers of each subject, which was designed to be a comprehensive measure of overall AGE accumulation in relative terms. This value was calculated by performing a Z-transformation of the log base 10 of each separate AGE staining score for a given vessel and averaging this across all eight vessels from a given subject. This final score was multiplied by 100 for regression analysis. Additional tissues present on the TMAs (skin, lung parenchyma, renal parenchyma, renal arcuate/interlobar, and retinal vessels) were not analyzed for this study.
Vascular Tissue Measurements Intima and media thicknesses were measured using the calibrated (0.4982 µm/pixel) measuring tool in ImageJ (Rasband 1997–2004
Statistical Analysis
Study Population Characteristics One hundred subjects, 76 white, 20 African-American, 2 Asian, and 2 Hispanic individuals, were included in this study. Patients ranged in age from 20 to 101 years of age (mean, 64 ± 16 years). The average age of diabetic subjects was 63 ± 14 years. The average postmortem interval (PMI) was 18 ± 6 hr (range, 4–28 hr). Thirty subjects had a previous diagnosis of diabetes (26 type 2 diabetes mellitus subjects, 3 steroid-induced diabetes subjects, and 1 type 1 diabetes mellitus subject), and 62 subjects were hypertensive. The average duration of diabetes was 9.3 ± 8.0 years. Non-white subjects were significantly more likely to have a history of diabetes compared with whites (50% vs 24%, p=0.01).
AGE IHC
Measures of Completeness We were able to generate AGE staining values for 92% (735/800) of media segments and 76% (605/800) of intima segments. Absent values resulted from missing tissue specimens or tissue that could not be evaluated. Frequently, in the absence of vascular disease, the intima was not of sufficient thickness to accurately select as a ROI. Thus, the vessels that are the least prone to vascular disease, the internal mammary artery and the renal artery (beyond the aortic bifurcation), had the fewest intimal ROIs (57 and 49, respectively). The media and intima thicknesses were determined for 92% and 99% of vessels, respectively.
Relationship Between Intima and Media
Demographic and Clinical Characteristics and Vascular AGE Staining in Univariate and Multivariate Analyses Patient age (in years) was positively and significantly associated with AGE staining for four of the eight vessels. The correlations were strongest for the dorsalis pedis and mesenteric arteries (Figure 2 ). The overall correlation (average across all eight vessels) between patient age and AGE staining was also significant (p=0.009). Across vessels, white subjects tended to have greater AGE staining compared with non-white subjects. Significant differences were observed in the carotid, dorsalis pedis, internal mammary, and renal arteries (Figure 3 ) and overall (p=0.002). There was no significant relationship between ethnicity/race and AGE staining in the intima (data not shown). We also did not observe differences by sex in AGE staining of either the intima or the media (data not shown). Diabetes, renal disease, and smoking are thought to increase AGE levels (Vlassara and Palace 2002
Postmortem Interval and Vascular AGE Staining There was a positive association between PMI and AGE staining across vessels in both the media (r = 0.40, p<0.0001, n=100) and intima (r = 0.38, p=0.0001, n=100) by pairwise correlation. PMI was positively associated with AGE staining of media in eight of nine vessels (all except the dorsalis pedis) and with AGE staining in the intima in five of nine blood vessels (carotid, dorsalis pedis, iliac, internal mammary, and mesenteric arteries). Length of PMI was not associated with patient age, race/ethnicity, diabetes, or hypertensive status (all p>0.05). Two other potentially confounding variables, length of formalin fixation before processing and slide to slide staining variation, did not affect AGE staining (data not shown).
Multivariable Analysis to Assess Independence of Associations of Clinical Variables With Media AGE Staining
Artery Thickness and AGE Staining Generally, reduced medial thickness significantly correlated with AGE staining (six of eight vessels, p<0.05 by pairwise correlation). Increasing intimal thickness was positively correlated with medial AGE staining for four of eight vessels (p<0.05 by pairwise correlation).
To the best of our knowledge, this study represents the first large-scale survey of AGE staining in human vascular tissues using TMAs. This method of analysis enabled us to make several new observations and confirm and extend some known associations. By studying eight blood vessels per subject, we showed that AGE accumulation is a pervasive process in human vasculature, irrespective of a given vessel's propensity for atherosclerosis. Thus, any clinical variable shown to increase vascular AGE levels does so in a global fashion. By evaluating the intima and media separately, we showed that the two layers are significantly correlated across all vessels despite differences in absolute AGE accumulation (Table 3). Often a clear demarcation between AGE staining of the intima and media was observed as if the IEL was a barrier to AGE accumulation (Figure 1A).
Consistent with previous literature, we report the positive association of age with AGE staining and interpret this as validation of our quantitative staining method (Dyer et al. 1993
We identified a novel relationship between ethnicity and AGE staining of the media. The higher levels of AGE staining in white subjects persisted even after adjustment for age, sex, PMI, smoking status, and clinical variables (diabetes, hypertension, and kidney function). The non-white cohort (84% African American) had less AGE staining despite having a greater burden of diabetes. However, because the non-white cohort also had higher levels of detrimental risk factors (hypertension, poor kidney function), an association between AGE staining and diabetes status was observed only in our models that accounted for these race/ethnic differences. Our survey of the literature has found no previous reports of differences in tissue AGE levels by race/ethnicity, although this may be caused by the small sample size and race/ethnic homogeneity of previous studies. Possible causes include differences in genetics, diet, or environmental factors that we are unable to account for in this study (Krajcovicova-Kudlackova et al. 2002
There are some important limitations of this study. IHC has a relatively small dynamic range compared with other imaging methods such as immunofluorescence (IF) (Rimm 2006
It is not entirely clear why a stronger association between diabetes status and AGE staining was not observed in these data, but heterogeneity in disease variables likely had an effect on our findings. To obtain a sufficient number of samples in a reasonable period of time (3 years), essentially every autopsy that conformed to our inclusion and exclusion criteria was used (Halushka et al. in press
Autolysis that occurs with extended postmortem interval has been thought to decrease cellular viability and lower antibody staining intensity (Martinez-Diaz et al. 2004 Intimal analysis did not provide as much information as medial analysis. A problem with analyzing the intima was the lack of sufficient intimal thickness to select as an ROI in many non-atherosclerotic vessels. Therefore, what could have been the weakest staining intimas were not evaluated. This partly explains the difference in dynamic range between medial and intimal staining (7.9 vs 3.2). The limited utility of the intima may also be methodological because the antibody was titrated to achieve maximal dynamic range in the media. The stronger intimal staining likely fully saturated the intensity measurement, creating an artificial ceiling to AGE staining scores.
It is important to point out the trade-offs between this new TMA method with quantitative IHC and older analytic methods. We learned in this study that large differences in AGE content of intima and media exist. In studies that may grind up vascular tissues for use in analytic devices such as HPLC, keeping an exact ratio of intima to media may be an underappreciated step to prevent confounding of results, particularly with variably sized atherosclerotic plaques (Vogt et al. 1982
We specifically assessed the role of potential confounding variables such as days of formalin fixation before processing, slide to slide staining heterogeneity, and PMI. By identifying PMI as a confounding variable, we were able to account for it in our regression analysis unlike prior studies that used autopsy tissues but failed to perform this necessary analysis. The effect of these variables is unknown in prior IHC studies (Nakamura et al. 1993 The correlation across all vessels suggests that measurement of AGEs in the media of any medium-to-large caliber artery may be a marker of vascular tissue AGE levels throughout the body. What then is the ideal vessel to study? AGE staining in the dorsalis pedis artery was significantly and independently associated with aging and ethnicity, and it correlated with AGE levels in all other vessels studied. The dorsalis pedis is in an easily accessible location on the dorsal aspect of the foot, just beneath the skin. In the future, it may be a useful artery to study non-invasive imaging measures of vascular AGEs that would have global vascular applicability. This study breaks ground in several areas. It is the first large-scale survey of AGE staining in human vascular tissues, encompassing eight unique vessels in 100 adult subjects. We found that, within a given individual, the AGE levels in these vessels are correlated, regardless of whether they were atherosclerosis-prone or atherosclerosis-resistant vessels. The overall medial AGE staining significantly correlated with increasing subject age, white ethnicity, and PMI in one model and with diabetes after further adjustment for comorbidites (hypertension and kidney function) in a second model. This TMA-based study represents a new method of large-scale vascular tissue analysis. We believe that future TMAs created from a single type of blood vessel from hundreds of individuals and derived from surgical (non-autopsy) materials will be of great use in evaluating specific AGEs such as CML, carboxyethyllysine, pentosidine, and others in relation to clinical variables and outcomes.
We thank Kristen Lecksell for excellent digital slide scanning and Dr. Frederick Brancati for helpful conversations.
This study was supported by a junior faculty award from the American Diabetes Association (1-05-JF-20 to MKH) and the National Institutes of Health/National Institute of Diabetes and Digestive and Kidney Diseases (K01 DK076595 to ES) . Received for publication November 20, 2008; accepted January 27, 2009
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