Advanced glycation end product associated skin autofluorescence: A mirror of vascular function?

https://doi.org/10.1016/j.exger.2012.04.011Get rights and content

Abstract

Advanced glycation end products (AGEs) seem to be involved in aging as well as in the development of cardiovascular diseases. During aging, AGEs accumulate in extracellular matrix proteins like collagen and contribute to vessel stiffness. Whether non-invasive measurement of AGE accumulation in the skin may reflect vessel function and vessel protein modification is unknown. Herein we set out to analyze the AGE-modifications in the collagens extracted from residual bypass graft material, the skin autofluorescence reflecting the accumulation of AGEs in the body as well as the pulse wave velocity reflecting vessel stiffness.

Collagen types I and III (pepsin digestible collagen fraction) were isolated from the veins of 52 patients by proteolysis. The residual collagen fraction was further extracted by collagenase digestion. Collagen was quantified by hydroxyproline assay and AGEs by the AGE intrinsic fluorescence. Skin autofluorescence was measured with an autofluorescence reader; pulse wave velocity with the VICORDER®.

The collagen AGE autofluorescence in patient vein graft material increased with patient age. The pepsin digestible collagen fraction was significantly less modified in comparison to the collagenase digestible fraction. Decreasing amounts of extracted collagenase digestible collagen correspond with increasing AGE autofluorescence. Skin autofluorescence and vessel stiffness were significantly linked to the AGE autofluorescence of the collagenase digestible collagen fraction from graft material. In conclusion we have found that skin autofluorescence and pulse wave velocity as non-invasive parameters significantly correlate with the AGE contained in graft material and therefore are strong predictors of vessel AGE modifications in patients with coronary heart disease. Whether the analysis of the skin autofluorescence leads to an improvement of the risk stratification in patients suffering from cardiovascular disease has to be further tested.

Highlights

► Strong correlation of the AGE autofluorescence from vascular collagen with age. ► Correlation of glycated HbA1c and blood glucose levels with the amount of PDCF. ► SAF and aPWV are strong predictors of vascular modifications in CHD. ► SAF could be an interesting surrogate parameter for clinical outcome in CHD.

Introduction

Cardiovascular diseases are, according to the World health organization, the leading cause of morbidity and mortality of the aging population in industrial countries. Major age-associated tissue modifications contribute to the progression of such diseases. Since the initial description of the non-enzymatic Maillard reaction finally resulting in the production of advanced glycation end products (AGEs), there is increasing evidence that these bioactive molecules accumulate in tissue and vessels with age and are involved in cardiovascular disease development (Araszkiewicz et al., 2011, Aronson, 2002, Baumann et al., 2009, Semba et al., 2009, Yamagishi et al., 2007). It is known that aging of the vascular system is characterized by decreasing elasticity, endothelial dysfunction, intimal thickening and accumulation of interstitial collagen (Greenwald, 2007, Najjar et al., 2005). Especially collagen of the fibrillary types I and III is important for the elasticity of the blood vessel system (Maurel et al., 1990). Physiological cross-links of the collagen fibers are important for the functionality of the matrix. However in aging, long-lived proteins like collagens are, due to slow turnover rates, especially susceptible to non-enzymatic glycation resulting in accumulation of AGEs (Reiser, 1998). These AGEs seem to establish additional cross-links resulting in loss of vessel elasticity. This mechanism so far was shown in a diabetic rat model (Reddy, 2004). Although a number of studies confirmed the correlation between AGE accumulation and increasing vascular stiffness the mechanism that may explain this association remains unclear (Campbell et al., 2011, Goldin et al., 2006, Semba et al., 2009).

The accumulation of advanced glycation end products is a general feature of the aging tissue due to glycation and oxidation reactions (Dunn et al., 1991). As shown by Sakata et al. the modification of collagen by AGEs decreases the susceptibility of collagen to enzymatic digestion (Sakata et al., 1995). In order to extract collagen from tissues a subsequent treatment with pepsin under acidic conditions followed by the digestion of the remaining collagen with enzymes like collagenase and proteinase seems to be the biochemical gold standard (Monnier et al., 1986, Sakata et al., 1995, Turk et al., 1999). Pepsin digestion causes solubilization of a part of the collagen, presumably by degrading the nonhelical telopeptide regions of the collagen molecule, where functional crosslinking is known to occur (Schnider and Kohn, 1981). The result of this treatment is a pepsin digestible collagen fraction (PDCF). Further collagen degradation of the pepsin insoluble fraction is achieved by collagenase treatment resulting in the isolation of a collagenase digestible collagen fraction (CDCF). The PDCF represents the less and the CDCF the more AGE modified collagen fraction.

The non-invasive measurement of skin autofluorescence (SAF) to estimate the skin tissue AGE burden is widely established and was shown to be a strong and independent predictor of cardiovascular mortality in diabetic and hemodialysis patients (Lutgers et al., 2009, Maury et al., 2011, McIntyre et al., 2011, Meerwaldt et al., 2007, Mulder et al., 2009, Noordzij et al., 2011, Samborski et al., 2011, Ueno et al., 2008). For evaluation of central arterial changes the measurement of the carotid to femoral (aortic) pulse wave velocity (aPWV) is an accepted procedure. The aPWV is used as an indicator of central arterial stiffness and seems to be a good predictor of increased cardiovascular risk (Baulmann et al., 2010, Mortensen et al., 2010, Nuernberger et al., 2007). Normally the pressure wave reflected from the periphery reaches the heart during diastole. But with increasing arterial stiffness due to aging or diseases, the velocity of the wave increases and the reflected pressure wave returns to the heart earlier. The pressure wave then reaches the heart during systole, resulting in increased cardiac afterload, elevated systolic and decreased diastolic blood pressure. Especially the decrease in diastolic pressure compromises the coronary blood flow and increases the risk for myocardial ischemia (Sutton-Tyrrell et al., 2005).

However so far it is unknown, whether the measurement of the skin autofluorescence as a non-invasive predictor of AGE burden and the measurement of the aPWV as a non-invasive marker for arterial stiffness do reflect the vascular modifications in situ. We hypothesized that these non-invasive parameters are related to vascular AGE accumulation and therefore studied the association among skin AGE fluorescence, aPWV and vascular collagen modification in graft material from patients with coronary heart disease.

Section snippets

Patients

In the present study we assessed 52 male patients with diagnosed coronary heart disease. For all patients height, weight and waist circumference were measured. BMI was determined as kg/m2. Smoking status was classified as a non-smoker, former smoker or current smoker (smoking within the last 2 months). Diabetes was defined as previously diagnosed diabetes. At the preoperative visit, all the study parameters including skin autofluorescence, aPWV, blood for laboratory analysis and the use of

Patient diagnosis, BMI, smoking and diabetes state

In the present study we assessed 52 male patients (mean age: 68.7 ± 10.15 years) with diagnosed coronary heart disease. 23.1% of the participants had a BMI (≥ 30 kg/m2). Regarding the smoking history 42.3% were non-smoker, 28.8% were former smoker and 25% were smoking within the last 2 months. 36.5% of the patients had diagnosed type 2 diabetes. From the diabetic participants 15.8% were only on diet, 57.9% were treated with tablets and 23.3% were on insulin therapy. Further demographic and clinical

Discussion

Coronary heart disease and resulting ischemic heart failure are the leading causes of death among the elderly in western countries. Cardiac surgery has become an accepted therapy for these patients. However, an increasing age of the general population reinforces the urgency for new approaches and strategies to identify people at risk. These strategies should include parameters of basic aging processes to understand age-associated modifications of the cardiovascular system individually. This

Sources of funding

Financial support was given from the Roux Funding Program of the Martin-Luther-University Halle-Wittenberg.

Disclosures

None.

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