Correlation analysis between the expression of cathepsin D and the accumulation of advanced glycation end products in skin tissues

Abstract

Objective To determine the expression of cathepsin D and advanced glycation end products (AGEs) in skin tissues from patients of different ages or skin tissues with different degrees of sun exposure, to evaluate their correlation, and to preliminarily investigate the role of cathepsin D in the degradation and accumulation of AGEs in photoaged skin. Methods Skin tissues were collected from sun-exposed and sun-protected body sites in patients aged 15 - 20 years, 35 - 40 years, 55 - 60 years or 75 - 80 years. These skin tissues were divided into 8 groups according to age of patients and degrees of sun exposure, and there were 6 specimens in each group. Immunohistochemical and immunofluorescent methods were used to measure the expression of cathepsin D and AGEs in the skin tissues. Statistical analysis was carried out by factorial design analysis of variance, Wilcoxon rank sum test and Kruskal-Wallis rank sum test for analyzing associations of the expression of cathepsin D and AGEs with age and sun exposure, as well as by Pearson correlation analysis for assessing the correlation between cathepsin D expression and AGEs expression. Results Immunohistochemical study showed that the expression of cathepsin D markedly decreased along with the increase of age, but the accumulation of AGEs gradually increased along with the increase of age. In the same age group, the cathepsin D expression was lower in the sun-exposed skin tissues than in the sun-protected skin tissues, while the accumulation of AGEs was more in the sun-exposed skin tissues than in the sun-protected skin tissues. Factorial design analysis of variance showed that sun exposure could decrease the expression of cathepsin D (F = 58.70, P < 0.001) , but increase the accumulation of AGEs (F = 158.18, P < 0.001) . Moreover, the increase of age could lead to decreased expression of cathepsin D (F = 79.49, P < 0.001) , and increased expression of AGEs (F = 106.06, P < 0.001) . Compared with the sun-protected skin tissues, the sun-exposed skin tissues in all the age groups showed significantly lower absorbance value of cathepsin D (35 - 40 years: 0.020 ± 0.005 vs. 0.032 ± 0.005; 55 - 60 years: 0.012 ± 0.004 vs. 0.026 ± 0.002; 75 - 80 years: 0.002 ± 0.001 vs. 0.013 ± 0.004; all P < 0.001) , but higher absorbance value of AGEs (35 - 40 years: 0.030 ± 0.008 vs. 0.010 ± 0.003; 55 - 60 years: 0.066 ± 0.010 vs. 0.021 ± 0.004; 75 - 80 years: 0.085 ± 0.015 vs. 0.035 ± 0.009; all P < 0.001) except the age group of 15 - 20 years. No matter whether the skin tissues were sun-exposed or sun-protected, there were significant differences in the expression of cathepsin D and AGEs among different age groups (all P < 0.001) . The results of double immunofluorescence staining were similar to those of immunohistochemical study. Pearson correlation analysis showed that the expression of cathepsin D in the sun-exposed skin tissues was highly negatively correlated with the accumulation of AGEs (r = -0.915, P < 0.05) , while they were moderately negatively correlated in the sun-protected skin tissues (r = -0.730, P < 0.05) . Conclusions Along with the increase of age, the expression of cathepsin D in skin tissues decreased, but the expression of AGEs increased. In the sun-protected skin tissues, the expression of cathepsin D was moderately negatively correlated with the expression of AGEs, while they were highly negatively correlated in the sun-exposed skin tissues, suggesting that cathepsin D may play an important role in the degradation and accumulation of AGEs in photoaged skin. Key words: Skin aging; Cathepsin D; Glycosylation end products, advanced; Age factors; Sunlight