AbstractsBiology & Animal Science

In eukaryotic cells, the endoplasmic reticulum (ER) is a major storage organelle for calcium and a site of synthesis and folding of secretory proteins, cell membrane proteins. ER is sensitive to changes in the internal and external environment of the cell. Both physiological and pathological conditions may perturb the function of the ER resulting in ER stress. Given the fact that the chondrocyte, the only resident cell found in cartilage, is responsible for synthesis and turnover of the abundant extracellular matrix, Chondrocytes might be sensitive to ER stress. Here we report that glucose withdrawal, tunicamycin and thapsigargin induce ER stress in both primary chondrocytes and a chondrocyte cell line, detected by upregulation of GADD153 and caspase-12. Other agents such as IL-1beta or TNF alpha induced a minimal or no induction of GADD153, respectively. ER stress resulted in decreased accumulation of an alcian blue positive matrix by chondrocytes and decreased expression of type II collagen at the protein level. Further, quantitative real time PCR was used to demonstrate a downregulation of steady state mRNA levels coding for aggrecan, collagen II, and link protein in chondrocytes exposed to ER stress-inducing conditions. The Sox9-dependent collagen type II promoter activity was downregulated under an ER stress condition. Overexpression of GADD153 in chodrocytes resulted in decrease in Sox9-dependent collagen type II promoter activity, which was recovered by overexpression of Sox9. In addition, the ER stress resulted in decreased chondrocyte growth. Ultimately, ER stress resulted in chondrocyte apoptosis, as evidenced by DNA fragmentation and Annexin V staining. These findings have potentially important implications regarding consequences of ER stress in cartilage biology. BAG-1 (Bcl-2 associate athanogene-1) is a multifunctional protein linking cell proliferation, cell death, protein folding and cell stress. In vivo, BAG-1 is expressed in growth plate and articular cartilage, and the expression of BAG-1 decreases with age. Here we demonstrate for the first time that the major isoform of BAG-1 in chondrocytes is the 33KDa isoform, which is mainly located in ER. BAG-1 is involved in ER stress-induced apoptosis in chondrocytes. Induction of ER stress through multiple mechanisms all result in down-regulation of BAG-1 expression. In addition, direct suppression BAG-1 expression by siRNA resulted in chondrocyte growth arrest and apoptosis, whereas stable overexpression of BAG-1 delayed the onset of ER stress-mediated apoptosis. In addition to regulating apoptosis, we also observed decreased expression of collagen type II in BAG-1 deficient chondrocytes. In contrast, overexpression of BAG-1 resulted in increased expression of collagen II. Moreover, under ER stress condition, the reduced expression of collagen type II was delayed in chondrocytes overexpressing BAG-1. These results suggest a novel role for BAG-1 in supporting viability and matrix expression of chondrocytes.