Mitogen-activated protein kinase signaling in bovine articular chondrocytes in response to fluid flow does not require calcium mobilization

In the present study, the role of mitogen-activated protein kinases (MAPKs) in chondrocyte mechanotransduction was investigated. We hypothesized that MAPKs participate in fluid flow-induced chondrocyte mechanotransduction. To test our hypothesis, we studied cultured chondrocytes subjected to a well-defined mechanical stimulus generated with a laminar flow chamber. The extracellular signal-regulated kinases 1 and 2 (ERK1/2) were activated 1.6-3-fold after 5-15 min of fluid flow exposure corresponding to a chamber wall shear stress of 1.6 Pa. Activation of ERK1/2 was observed in the presence of both 10% FBS and 0.1% BSA, suggesting that the flow effects do not require serum agonists. Treatment with thapsigargin or EGTA had no significant effect on the ERK1/2 activation response to flow, suggesting that Ca2+ mobilization is not required for this response. To assess downstream effects of the activated MAPKs on transcription, flow studies were performed using chondrocytes transfected with a chimeric luciferase construct containing 2.4 kb of the promoter region along with exon 1 of the human aggrecan gene. Two-hour exposure of transfected chondrocytes to fluid flow significantly decreased aggrecan promoter activity by 40%. This response was blocked by treatment of chondrocytes with the MEK-1 inhibitor PD98059. These findings demonstrate that, under the conditions of the present study, fluid flow-induced signals activate the MEK-1/ERK signaling pathway in articular chondrocytes, leading to down-regulation of expression of the aggrecan gene.