Bcl-2 regulates chondrocyte morphology and aggrecan gene expression independent of caspase activation and full apoptosis

Abstract

Bcl-2 is widely expressed in a variety of cell types and is known to block apoptosis through a conserved pathway. However, recent reports have demonstrated that Bcl-2 regulates cell behavior independent of its control of apoptosis. Chondrocytes express a unique set of matrix proteins, including the proteoglycan aggrecan, and have been widely used to study the relationship between trophic factors and apoptosis. In this article, we report that Bcl-2 affects the morphology and regulates the expression of aggrecan in a rat chondrocyte cell line (IRC). Endogenous Bcl-2 and aggrecan mRNA were both down-regulated in response to serum withdrawal in parental IRC cells, while constitutive expression of Bcl-2 maintained aggrecan levels under conditions of serum withdrawal. In addition, expression of anti-sense Bcl-2 resulted in decreased aggrecan mRNA and produced a fibroblastic morphology compared with parental cells. The caspase inhibitor ZVAD-fmk effectively blocked full apoptosis of IRC cells in response to serum withdrawal or anti-sense Bcl-2 but did not prevent the down-regulation of aggrecan expression from either signal. These results suggest a novel role for Bcl-2 in regulating the differentiated phenotype of chondrocytes and the expression of a differentiation-specific gene independent of its control of apoptosis.