Effects of shear stress on nitric oxide and matrix protein gene expression in human osteoarthritic chondrocytes in vitro

Mechanical loading alters articular cartilage metabolism. However, mechanisms underlying intracellular signaling and communication between cells in response to mechanical stresses remain enigmatic. This study tested the hypothesis that shear stress-induced nitric oxide (NO) production participates in the regulation of matrix protein gene expression. The data presented here demonstrate that exposure of human osteoarthritic chondrocytes to a continuously applied shear stress (1.64 Pa) upregulated NO synthase gene expression and increased NO release by 1.8-, 2.4-, and 3.5-fold at 2, 6, and 24 h, respectively. Exposure of chondrocytes to a short duration of shear stress for 2 h resulted in the release of accumulation of NO in the culture medium. Exposure of chondrocytes to shear stress for 2, 6, and 24 h inhibited type II collagen mRNA signal levels by 27%, 18% and 20% after a constant post-shear incubation period of 24 h. Aggrecan mRNA signal levels were inhibited by 30%, 32% and 41% under identical conditions. Addition of an NO antagonist increased type II collagen mRNA signal levels by an average of 1.8-fold (137% of the un-sheared control) and reestablished the aggrecan mRNA signal levels by an average of 1.4-fold after shear stress (92% of the un-sheared control) (ANOVA p < 0.05). These data support the hypothesis that shear stress-induced NO release may influence the development of degenerative joint diseases by inhibiting matrix macromolecule synthesis.