P2Y2 RECEPTORS AND GRK2 ARE INVOLVED IN OSCILLATORY FLUID FLOW INDUCED ERK1/2 RESPONSES IN CHONDROCYTES

Mechanical loading is an important factor regulating cartilage metabolism maintained by chondrocytes. However, some of its underlying mechanisms remain poorly understood. In this study, we employed a chondrogenic cell line ATDC5 to investigate roles of P2Y2 and GRK2 in chondrocyte mechanotransduction. We first confirmed the expression of chondrocyte markers in differentiated ATDC5 cells. We then exposed both differentiated and undifferentiated ATDC5 cells to oscillatory fluid flow, and found that differentiated ATDC5 cells responded to oscillatory fluid flow by increasing COX-2 and aggrecan expressions. More importantly, fluid flow induced ERK1/2 response in differentiated cells was increased more than 10 times compared to that in undifferentiated cells. Furthermore, we found that P2Y2 mRNA and protein levels in differentiated ATDC5 cells were significant higher than those in undifferentiated cells. In contrast, GRK2 protein levels in differentiated cells were significantly lower than those in undifferentiated cells. Finally, overexpressions of P2Y2 and GRK2 in differentiated ATDC5 cells result in a 34% increase and a 21% decrease of the ERK1/2 phosphorylation respectively in response to oscillatory fluid flow, suggesting important roles of P2Y2 and GRK2 in chondrocyte mechanotransduction.