THE ROLE OF BCL2 PROTEINS IN CHONDROCYTE BIOLOGY

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Degenerative cartilage disease (osteoarthritis, OA) is a major contributor to morbidity in the elderly population. Several lines of evidence support the concept that decreased chondrocyte viability and diminished or altered matrix synthesis (especially) in response to growth factors contribute to the increased incidence of OA with aging. The Bcl-2 family contains proteins that are both pro-apoptotic and anti-apoptotic. Recent studies have demonstrated a role for Bcl-2 proteins in regulating chondrocyte apoptosis. However it has not been shown directly that Bcl-2 plays a role in regulating articular chondrocyte apoptosis in vivo, nor has the expression of the major members of the Bcl-2 family been systematically examined in articular cartilage. In addition to the traditional role of Bcl-2 in regulating apoptosis, it is now clear that Bcl-2 is involved in the regulation of non-apoptotic pathways that control events such as axonal regeneration and epithelial cell differentiation. Evidence now exists that this type of pathway is operating in the chondrocyte and may be linked to the expression of aggrecan. The mechanisms that regulate aggrecan through a Bcl-2-dependent pathway have not been determined nor has it been established to what extent this pathway may regulate other cartilage matrix genes. Therefore, the general hypothesis that Bcl-2 proteins play a dual role in regulating both chondrocyte apoptosis and matrix gene expression will be explored through the following specific aims: 1) Determine if the inhibition of aggrecan expression in response to serum withdrawal or anti-sense Bcl-2 expression is part of a coordinate down-regulation of chondrocyte-specific gene expression by measuring mRNA levels coding for collagen II, IX, XI, link protein and others; 2) Establish the mechanism for decreased aggrecan (and other matrix gene) expression and probe the role of a specific transcription factor (NFkappaB) in the chondrocyte response to serum withdrawal or anti-sense Bcl-2; 3) Determine the relevance of the major members of the Bcl-2 in vivo by correlating the expression pattern of these proteins in the articular cartilage of rats and humans with aging, apoptosis and cartilage degeneration; also the effect of reduced Bcl-2 expression in a transgenic mouse model on chondrocyte apoptosis and matrix gene expression will be determined. This multifaceted experimental approach is intended to probe the basic role of Bcl-2 proteins in chondrocyte biology and to identify mechanisms that are important for cartilage maintenance and the pathogenesis of OA.