Advanced glycation endproducts and osteoarthritis

Aging bone and cartilage: cross-cutting issues

Aging is a major risk factor for osteoarthritis and osteoporosis. Yet, these are not necessary outcomes of aging, and the relationship between age-related changes in bone and cartilage and development of disease is not clear. There are some well-described cellular changes associated with aging in multiple tissues that appear to be fundamental to the decline in function of cartilage and bone. A better understanding of age-related changes in cells and tissues is necessary to mitigate or, hopefully, avoid loss of bone and cartilage with aging. In addition, a better understanding of the dynamics of tissue maintenance in vivo is critical to developing tissue replacement and repair therapies. The role of stem cells in this process, and why tissues are not well maintained with advancing age, are frontiers for future aging research.

Activation of receptor for advanced glycation end products in osteoarthritis leads to increased stimulation of chondrocytes and synoviocytes

OBJECTIVE

The major risk factor for osteoarthritis (OA) is aging, but the mechanisms underlying this risk are only partly understood. Age-related accumulation of advanced glycation end products (AGEs) could be one of these mechanisms. We undertook this study to investigate the role of the receptor for AGEs (RAGE) in mediating the cellular effects of AGEs on chondrocytes and fibroblast-like synoviocytes (FLS).

METHODS

AGE levels in human cartilage were determined by fluorescence, browning, and pentosidine levels. Chondrocyte activation by AGEs was assessed as the release of proteoglycans and the synthesis of matrix metalloproteinase 1 (MMP-1) and type II collagen messenger RNA (mRNA). The activation of FLS by AGEs was measured by MMP-1 production and invasion through matrix proteins.

RESULTS

Patients with focal degeneration of cartilage showed increased AGE levels in their healthy cartilage compared with the levels in healthy cartilage from donors without cartilage degeneration (P < 0.01 for both fluorescence and browning; P not significant for pentosidine content). Stimulation of bovine chondrocytes with glycated albumin increased the release of proteoglycans by 110% (P < 0.001) and the production of MMP-1 mRNA by 200% (P = 0.028). In addition, OA FLS produced 240% more MMP-1 when stimulated with glycated albumin (P < 0.001). Glycated matrix or albumin increased the catabolic activity of OA FLS, which was assessed as invasive behavior, by 150% and 140% (P = 0.001 and P = 0.010), respectively. Effects of stimulation with AGEs were blocked by a neutralizing antibody against RAGE, but not by an isotype control.

CONCLUSION

This study shows that AGEs trigger RAGE on chondrocytes and FLS, leading to increased catabolic activity and therefore to cartilage degradation. AGEs, via RAGE, could therefore contribute to the development and/or progression of OA.

Advanced glycation end products enhance expression of pro-apoptotic genes and stimulate fibroblast apoptosis through cytoplasmic and mitochondrial pathways

Both aging and diabetes are characterized by the formation of advanced glycation end products (AGEs). Both exhibit other similarities including deficits in wound healing that are associated with higher rates of fibroblast apoptosis. In order to investigate a potential mechanism for enhanced fibroblast apoptosis in diabetes and aged individuals, experiments were carried out to determine whether the predominant advanced glycation end product in skin, N-epsilon-(carboxymethyl) lysine (CML)-collagen, could induce fibroblast apoptosis. In vivo experiments established that CML-collagen but not unmodified collagen induced fibroblast apoptosis and that apoptosis was dependent upon caspase-3, -8, and -9 activity. In vitro experiments demonstrated that CML-collagen but not control collagen induced a time- and dose-dependent increase in fibroblast apoptosis. By use of blocking antibodies, apoptosis was shown to be mediated through receptor for AGE signaling. AGE-induced apoptosis was largely dependent on the effector caspase, caspase-3, which was activated through both cytoplasmic (caspase-8-dependent) and mitochondrial (caspase-9) pathways. CML-collagen had a global effect of enhancing mRNA levels of pro-apoptotic genes that included several classes of molecules including ligands, receptors, adaptor molecules, mitochondrial proteins, and others. However, the pattern of expression was not identical to the pattern of apoptotic genes induced by tumor necrosis factor alpha.

Osteoarthritis gene therapy

Osteoarthritis (OA) is the Western world's leading cause of disability. It is incurable, costly and responds poorly to treatment. This review discusses strategies for treating OA by gene therapy. As OA affects a limited number of weight-bearing joints and has no major extra-articular manifestations, it is well suited to local, intra-articular gene therapy. Possible intra-articular sites of gene transfer include the synovium and the cartilage. Most experimental progress has been made with gene transfer to synovium, a tissue amenable to genetic modification by a variety of vectors, using both in vivo and ex vivo protocols. The focus so far has been upon the transfer of genes whose products enhance synthesis of the cartilaginous matrix, or inhibit its breakdown, although there is certainly room for alternative targets. It is possible to build a convincing case implicating interleukin-1 (IL-1) as a key mediator of cartilage loss in OA, and the therapeutic effects of IL-1 receptor anatagonist (IL-1Ra) gene transfer have been confirmed in three different experimental models of OA. As transfer of IL-1Ra cDNA to human arthritic joints has already been accomplished safely, we argue that clinical studies of intra-articular IL-1Ra gene transfer in OA are indicated and should be funded. Of the available vector systems, recombinant adeno-associated virus may provide the best combination of safety with in vivo delivery using current technology.