Relationships between Osteopontin, Osteoprotegerin, and Other Extracellular Matrix Proteins in Calcifying Arteries

Abstract

Osteopontin (OPN) and osteoprotegerin (OPG) are glycoproteins that participate in the regulation of tissue biomineralization. The aim of the project is to verify the hypothesis that the content of OPN and OPG in the aorta walls increases with the development of atherosclerosis and that these proteins are quantitatively related to the main proteins in the extracellular arteries matrix. Quantitative and qualitative analyses of the OPN and OPG content in 101 aorta sections have been conducted. Additionally, an enzyme-linked immunosorbent assay (ELISA) test has been performed to determine the collagen types I-IV and elastin content in the tissues. Correlations between the biochemical data and patients' age/sex, atherosclerosis stages, and calcification occurrences in the tissue have been established. We are the first to report correlations between OPN or OPG and various types of collagen and elastin content (OPG/type I collagen correlation: r = 0.37, p = 0.004; OPG/type II collagen: r = 0.34, p = 0.007; OPG/type III collagen: r = 0.39, p = 0.002, OPG/type IV collagen: r = 0.27, p = 0.03; OPG/elastin: r = 0.42, p = 0.001; OPN/collagen type I: r = 0.34, p = 0.007; OPN/collagen type II: r = 0.52, p = 0.000; OPN/elastin: r = 0.61, p = 0.001). OPN overexpression accompanies calcium deposit (CA) formation with the protein localized in the calcium deposit, whereas OPG is located outside the CA. Although OPN and OPG seem to play a similar function (inhibiting calcification), these glycoproteins have different tissue localizations and independent expression regulation. The independent expression regulation presumably depends on the factors responsible for stimulating the synthesis of collagens and elastin.

Keywords: atherosclerosis; calcification; extracellular matrix proteins; osteopontin; osteoprotegerin.

Figure 1

Dependence of OPG and OPN on the degree of atherosclerosis (A,B).

Figure 2

Scatterplot for OPN and other analytes for which a statistically significant correlation was obtained (A–C).

Figure 3

Scatterplot for OPG and other analytes for which a statistically significant correlation was obtained (A–E).

Figure 4

Comparison of the OPN and OPG contents in calcified and uncalcified samples (A,B).

Figure 5

Representative immunodetection images of OPN (panels A–C) and OPG (panels D–F) in the aortic wall of a person with very low-grade atherosclerosis, without signs of calcification. 40× magnification (panels A,D), 200× (panels B,E), 400× magnification (panels C,F).

Figure 6

Representative immunodetection images of OPN (panels A–C) and OPG (panels D–F) in the aortic wall of a person with advanced atherosclerosis, with a visible calcium deposit. Magnification 40× (panels A,D), 200× (panel B,E), magnification 400× (panels C,F).