The population of Western society is aging rapidly. Consequently, age-related diseases will increase greatly over the coming decades and will have a great impact on the quality of life of the elderly. Osteoarthritis (OA) is one of the most prevalent and disabling chronic conditions affecting the elderly and poses a significant public health problem (1). The most prominent feature of OA is the progressive destruction of articular cartilage, resulting in impaired joint motion, severe pain, and, ultimately, disability (2). As yet, the etiology of OA remains largely unknown. The incidence of OA increases with age: >50% of the population over 60 years of age is affected (3, 4). Although age is identified as the main risk factor for the development of OA, the mechanism by which aging is involved remains unclear. Age-related changes in the articular cartilage are expected to play an important role in the susceptibility of cartilage to OA.
Articular cartilage derives its mechanical properties from its extracellular matrix. This matrix is composed of type II collagen, which forms a 3-dimensional network that provides the cartilage with resistance to tensile forces (5). Highly negatively charged proteoglycans are embedded within this collagen network, and they generate a large swelling force that facilitates load support (the resilience of cartilage) (6, 7). One of the first characteristics of OA is damage to the collagen network, as reflected in increased swelling of the tissue and loss of proteoglycans (8–10). Both the collagen damage and the loss of proteoglycans adversely affect the mechanical properties of the cartilage. The chondrocytes within the cartilage are essential to maintaining the integrity of the tissue. They respond to tissue damage by increasing proteoglycan and collagen synthesis in an attempt to repair the damage (11, 12). If repair fails, damage will progress, leading to degeneration of the articular cartilage.
One of the major age-related changes in articular cartilage is the accumulation of advanced glycation end products (AGEs), which result from the spontaneous reaction of reducing sugars with proteins, or nonenzymatic glycation (NEG) (13, 14). The initial step in this reaction is the condensation of a sugar aldehyde with an ϵ-amino group of hydroxylysine or arginine residues in proteins. Subsequently, the initially formed Schiff base is stabilized by Amadori rearrangement. The Amadori product is further stabilized by oxidation and molecular rearrangements, ultimately generating a range of fluorophores and chromophores, which are collectively known as AGEs (14, 15). Most of these AGEs have not yet been isolated or characterized. Therefore, a few well-characterized AGEs are routinely used as markers for the process of NEG. Pentosidine, a fluorescent AGE formed by lysine and arginine residues, is often used for this purpose (16). AGEs are formed in all proteins, and since they can only be removed from the body when the protein is removed, AGEs accumulate in long-lived proteins such as collagens (17, 18). In human articular cartilage, a tissue with extremely slow turnover (half-life of type II collagen >100 years), pentosidine levels increase 50-fold from age 20 years to age 80 years (18–22).
AGEs are known to affect the physical and chemical properties of proteins. In particular, tissue strength is dependent upon the number of crosslinks present (15). Accumulation of AGEs is correlated with increased tissue stiffness in arteries, lenses, skin, tendons (23–27), and articular cartilage (20, 28). Moreover, an increase in AGEs renders tissue increasingly brittle, and thus more prone to mechanical damage. This effect has been shown in human lens capsules, cortical bone (25, 29), and articular cartilage (30). In addition to affecting the mechanical properties of tissue, AGEs interfere with cellular processes, such as adhesion of cells to the extracellular matrix, proliferation, and gene expression (31–33). Articular cartilage chondrocytes show decreased proteoglycan and collagen synthesis at increased AGE levels (21, 34). Degradation of AGE-modified collagen by matrix metalloproteinases is impaired as compared with that of unmodified collagen (34, 35).
The age-related accumulation of AGEs in articular cartilage increases tissue stiffness and decreases the capacity of the chondrocytes to remodel their extracellular matrix. Together, these effects render tissue more prone to damage and provide the molecular mechanism by which age-related accumulation of AGEs may eventually lead to the development of OA. In the present study, the hypothesis that the accumulation of AGEs predisposes to the development of OA was tested in the established canine anterior cruciate ligament transection (ACLT) model of OA (36–37).
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- MATERIALS AND METHODS
Age is by far the most important risk factor for the development of OA (1). By which mechanism aging is involved in the development of this debilitating disease remains largely unknown. Fatigue failure of the cartilage collagen network due to repetitive loading has long been recognized as one of the mechanisms involved in the development of OA (53, 54). With increasing age, the strength of the collagen matrix to withstand loading diminishes. Therefore, age-related changes in articular cartilage that influence the composition and strength of the cartilage matrix are very likely involved in the development of OA (55). One such change, the age-related accumulation of AGEs, has previously been shown to increase tissue stiffness, decrease extracellular matrix turnover (synthesis and degradation), and affect many cellular processes (13, 14, 20, 21, 28, 34, 47, 56). In the present study, we demonstrated in an in vivo model that this process of NEG is indeed causally involved in the age-related increase in susceptibility to OA.
Our study was designed such that, by selectively increasing the AGE levels in the cartilage matrix, only the effect of NEG on the susceptibility to OA could be studied, without the effects of other age-related changes. This approach clearly demonstrated that increased AGE levels predispose to the development of OA. As such, the spontaneous process of NEG is the first molecular mechanism described to date that is capable of explaining, at least in part, the strong age dependency of OA incidence. Furthermore, the present data suggest that the rate of a generally occurring aging process (i.e., AGE formation occurs in all tissue but is especially important in tissue with slow turnover, such as articular cartilage ) may predispose to the development of an age-related pathology such as OA. These data are consistent with the observations of Sell et al (57, 58), who showed an inverse relationship between the rate of glycation and the longevity of a species, which further supports the idea that AGE accumulation is an important process in aging and age-related diseases.
The involvement of AGE accumulation in OA emphasizes the dual nature of sugars: they are essential for life as building blocks and as a cellular energy source, and they initiate the formation of potentially detrimental AGEs. The recognition of NEG as a molecular mechanism that contributes to the development of OA provides new opportunities for therapies directed at the prevention of OA by inhibiting or reversing AGE formation. Inhibition of AGE formation by prophylactic treatment with compounds such as aminoguanidine, pyridoxamine, tenilsetam, or simple amino acids (e.g., lysine or arginine) has been shown to prevent AGE-related pathologies such as vascular stiffening, heart collagen accumulation, and protein crosslinking (59–63). Alternatively, AGE-directed therapy can consist of so-called AGE-breakers (64). Thiazolium compounds such as N-phenacylthiazolium bromide and phenyl-4,5-dimethylthiazolium chloride, which have been reported to break dicarbonyl-containing AGEs, showed efficacy in reversing AGE-related tendon crosslinking and cardiac stiffness (65, 66). Despite the fact that these therapies are relatively new, they provide proof that inhibition or reversal of AGE formation can have beneficial effects in AGE-mediated pathologies.
The involvement of AGEs in the etiology of OA, as demonstrated by our data, in combination with the emerging possibilities for AGE-directed therapies, provides possible tools for preventing or postponing the development of OA. This is an important development, since adequate therapy for OA is lacking, while the number of people with this debilitating disease is increasing because of the aging population.