Background: Perineural invasion (PNI) is a negative prognostic indicator in head and neck squamous cell carcinoma (SCC). The molecular mechanisms of PNI are poorly understood and there are no targeted therapies for PNI. Contrary to previous belief that cancer cells invade nerves, recent studies suggest that Schwann cells (SCs) may play an important role. In response to tumor cells, Schwann cells dedifferentiate, become motile and travel towards tumor cells, and promote cancer cell dispersion by bringing cancer back to the nerve. Communication between cells through brain-derived neurotrophic factor (BDNF) activation of TrkB-receptor may contribute to these cellular events.
Objectives: Determine the effect of TrkB inhibition and BDNF on the degree of Schwann cell and cancer migration in vitro.
Study Design: Basic science research
Methods: A 2D-invasion assay was created by co-culturing human tongue SCC and differentiated human Schwann cells in a 2-well insert culture dish with a predefined cell-free gap. Dedifferentiated Schwann cells were converted into a quiescent, differentiated state by the addition of cAMP to the culture media for 72 hours. After cells adhered to the dish, Qtracker was used to fluorescently label the cells, the insert was removed, and the invasion assay was performed in three conditioned medias: DMSO (vehicle/control), BDNF, and TrkB Inhibitor. Time-lapse images of the co-cultures were performed over 72 hours and the degree of cell invasion was determined daily by measuring the number of cells that migrated towards the cell-free gap. Western blot was performed to measure BDNF and TrkB receptor expression in both cell lines. Immunohistochemistry was performed using O1 antibody to measure degree of Schwann cell differentiation to a quiescent state.
Results: Human tongue SCC and Schwann cells both expressed BDNF and TrkB on Western Blot. Changes in Schwann cell differentiation were seen on IHC. Schwann cell migration towards SCC exceeded SCC invasion towards the Schwann cells in all three conditions. However, Schwann cell migration was markedly lower with the addition of a TrkB inhibitor, when compared to control and BDNF conditions. The degree of SCC migration was similar across the three treatment groups.
Conclusion: Schwann cells have a unique property of dedifferentiation. In the presence of oral SCC, Schwann cells become more motile. These migratory properties can be suppressed through inhibition of TrkB receptor. These findings suggest that Schwann cell and TrkB signaling may have critical roles in the pathophysiology of PNI. Understanding the role of Schwann cells and neurotrophic factors in promoting PNI has important implications for therapeutic intervention; further studies are warranted.