Abstract
Osteoarthritis (OA) is characterized by progressive erosion of articular cartilage due to degradation of the cartilage matrix. The major enzymes involved are the matrix metalloproteases and aggrecanases, which are either derived from the synovium or synthesized by chondrocytes as OA progresses. This abnormal enzyme synthesis is part of a phenotypic change from normal to ‘degradative’ chondrocytes. If this change could be prevented, then disease progression might be slowed.
In early OA, degradative chondrocytes are only present in the superficial zone, but with increasing severity of OA, more chondrocytes become degradative cells so that, in high-grade OA, these cells are also located in the deep zone. We hypothesized the existence of a ‘factor X’, which diffuses from the superficial to the deep zone and induces cells to change phenotype and express the pro-teases. We further hypothesize that this factor is released by degradative chondrocytes. To test the hypothesis, we co-cultured explants of human superficial-zone OA cartilage (which contains degradative cells and thus factor X) with explants of deep-zone cartilage from fracture neck of femur patients (#NOF), which contains mostly normal chondrocytes that do not express the proteases. We investigated MMP expression by real time RT-PCR and protein synthesis by immunohistochemistry.
Before culture, MMP-2, -3, -9, or -13 were expressed in the superficial-zone OA cartilage, but not in deep-zone #NOF cartilage, as expected. After 4 weeks with separate culture of superficial zones and deep zones, no MMPs was expressed in deep zone chondrocytes, suggesting that culture per se did not induce expression of these enzymes. Neither did culture abolish expression in the superficial zone, as confirmed by RT-PCR and immunohistochemistry. However, when superficial-zone cartilage was co-cultured with deep-zone cartilage, MMP-3 expression were induced in deep- zone chon-drocytes, suggesting that a diffusible factor X, derived from degradative chondrocytes, had induced normal articular chondrocytes to express MMP-3. These experiments provide evidence for the existence of a factor that, when diffusing through the cartilage matrix, has the potential to induce normal non-enzyme expressing cells to become degradative chondrocytes.
Correspondence should be addressed to Mr Carlos Wigderowitz, Senior Lecturer, University Department of Orthopaedic and Trauma Surgery, Ninewells Hospital and Medical School, Dundee DD1 9SY.
