Insulin-like Growth Factor-1 Regulates the Mechanosensitivity of Chondrocytes by Modulating TRPV4

Both mechanical and IGF-1 stimulation are required for normal articular cartilage development and maintenance of the extracellular matrix. While much effort has been made to define the signaling pathways associated with these anabolic stimuli, we focused on how these pathways interact to regulate chondrocyte function. The Transient Receptor Potential Vanilloid 4 (TRPV4) channel is central to chondrocyte mechanotransduction and regulation of cartilage homeostasis. However, the mechanism by which TRPV4 is mechanically gated or regulated is not clear. In this study we propose that insulin-like growth factor 1 (IGF-1), which is important in regulating matrix production during mechanical load, modulates TRPV4 channel activity. Whole-cell patch clamp studies demonstrated that IGF-1 blocks hypotonic swelling-induced TRPV4 currents. IGF-1 increases stress fiber formation and apparent cell stiffness, indicating a potential mechanism for regulating TRPV4 channel response to mechanical stimulation. TRPV4 channel response to mechanical stimulation is demonstrated through increase in intracellular calcium during hypotonic swelling. This response to hypotonic swelling is significantly reduced in the presence of IGF-1, however disruption of F-actin following IGF-1 treatment returns cellular response. A down-stream response to mechanical stimulation of chondrocytes is ATP release. Data here indicate that activation of TRPV4 through hypotonic swelling induces ATP release, but this release is greatly reduced with IGF-1 treatment. Taken together this study indicates that IGF-1 modulates TRPV4 channel response to mechanical stimulation by increasing cell stiffness. As chondrocyte response to mechanical stimulation is greatly altered during OA progression, IGF-1 presents as a promising candidate for prevention and treatment of articular cartilage damage.