Static and dynamic compressive strains influence nitric oxide production and chondrocyte bioactivity when encapsulated in PEG hydrogels of different crosslinking densities

Objective: Mechanical loading is an important regulator of chondrocytes; however, many of the mechanisms involved in chondrocyte mechanotransduction still remain unclear. Here, poly(ethylene glycol) (PEG) hydrogels are proposed as a model system to elucidate chondrocyte response due to cell deformation, which is controlled by gel crosslinking (p(x)). Methods: Bovine articular chondrocytes (50 x 10(6) cells/mL) were encapsulated in gels with three p(x)s and subjected to static (15% strain) or dynamic (0.3 Hz or 1 Hz, 15% amplitude strain) loading for 48 h. Cell deformation was examined by confocal microscopy. Cell response was assessed by total nitric oxide (NO) production, proteoglycan (PG) synthesis ((SO42-)-S-35-incorporation) and cell proliferation (CP) ([H-3]-thymidine incorporation). Oxygen consumption was assessed using an oxygen biosensor. Results: An increase in p(x) led to lower water contents, higher compressive moduli, and higher cell deformations. Chondrocyte response was dependent on both loading regime and p(x). For example, under a static strain, NO was not affected, while CID and PG synthesis were inhibited in low px and stimulated in high p(x). Dynamic loading resulted in either no effect or an inhibitory effect on NO, CP, and PG synthesis. Overall, our results showed correlations between NO and CP and/or PG synthesis under static and dynamic (0.3 Hz) loading. This finding was attributed to the hypoxic environment that resulted from the high cell-seeding density. Conclusion: This study demonstrates gel p(x) and loading condition influence NO, CP, and PG synthesis. Under a hypoxic environment and certain loading conditions, NO appears to have a positive effect on chondrocyte bioactivity. (c) Published by Elsevier Ltd on behalf of Osteoarthritis Research Society International.