Purpose: High levels of the chitinase 3-like protein HC-gp39 (human cartilage glycoprotein 39) have been found in the synovial fluid and sera of patients with arthritis. Although the function of this protein is unclear, in chondrocytes at least it appears to limit catabolic responses to cytokines such as Il-1b. Here we have investigated secretion of this protein by intervertebral disc cells and determined if its production is influenced by extracellular osmolarity.

Methods: Cells were isolated from bovine caudal discs by enzyme digestion and cultured in DMEM in alginate beads for 6 days. Medium osmolarity was increased in the physiological range by sodium/potassium addition. Supernatants were collected every 2 days and replaced with fresh media. At the end of experiment the supernatants were used for lactate determination and for detection of GP-39 by western blotting. Beads were assayed for glycosaminoglycans, cell viability and cell density.

Results: GP-39 was a major protein secreted by disc cells. It was evident on day 2 at low osmolarities. By day 4 concentrations in the medium had increased significantly and the protein was present mainly in fragmented form, particularly at high osmolarities. Osmolarity had no effect on cell density or viability. Rates of lactate production and GAG accumulation were greatest at high osmolarities.

Discussion: Changes in osmolarity, equivalent to those experienced by disc cells during the diurnal loss and regain of fluid content, had significant effects on cell metabolism and influenced production of GP-39. Osmotic changes might thus influence responses of disc cells to inflammatory signals.