EGFR Signaling Promotes Cartilage Matrix Breakdown and Stimulates  Chondrocyte Proliferation

Objectives:   The epidermal growth factor receptor (EGFR) is expressed by articular cartilage and displays increased expression levels in human osteoarthritis in vitro.  Global genetic loss of Mig6, an intracellular suppressor of EGFR signaling in mice results in loss of proteoglycan staining and precocious osteoarthritis in load bearing joints such as the knee and temporomandibular joints (TMJ).  These results suggest that the EGFR provides catabolic signals which may over-activate protease activity in articular cartilage, promoting osteoarthritis onset and progression.  In this project, we used a murine model of genetically-induced osteoarthritis through genetic gain of EGFR signaling to define the relationship among protease activity, EGFR signaling, and articular cartilage matrix degradation during knee osteoarthritis in vivo.

Methods: Knees were dissected from adult mice and immediately fixed and processed for paraffin embedding. Histological analysis was performed on 7-μm sections. Safranin O staining of glycosaminoglycans was performed by staining sections with Weigert's Iron Hematoxylin and 0.02% aqueous Fast Green, followed by rinsing with 1% acetic acid and staining with 0.1% aqueous Safranin O.  Tissue segments were viewed with a Nikon E800 microscope, and SPOT-RT digital camera and software. For quantitative statistical analysis of cartilage thickness, tissue areas were measured and the values were compared by independent groups t-test between means using statistical software.

Results: We have found that activation of EGFR signaling promotes protease-mediated osteoarthritic degeneration of articular cartilage and stimulates chondrocyte proliferation within the load bearing joint of the knee. Together, these observations suggest multiple roles for EGFR signaling in regulation of structural macromolecules in the articular cartilage matrix.

Conclusions: The results of this study provide insight into the mechanisms of articular cartilage degeneration and may contribute to future testing of the potential utility of agents which block the EGFR as a therapeutic treatment for OA of the knee and TMJ.