Schwann cells promote EMT and the Schwann-like differentiation of salivary adenoid cystic carcinoma cells via the BDNF/TrkB axis

Perineural invasion (PNI) is a striking biological behavior observed in salivary adenoid cystic carcinoma (SACC). The present study was designed to establish a co-culture model of SACC cells with Schwann cells (SCs), and then study epithelial-mesenchymal transition (EMT) and the Schwann-like differentiation of SACC cells to investigate the likely molecular mechanism of PNI. The co-culture models of SCs with tumor cells (SACC-83, SACC-LM and MEC-1) were established using a Transwell system. An elevated concentration of brain-derived neurotrophic factor (BDNF) was detected by ELISA assay in the co-cultured medium of the SACC-83 group and SACC-LM group rather than the MEC-1 group. The EMT process and Schwann-like differentiation in SACC-83 cells were analyzed by RT-PCR, western blotting, immunofluorescence, photography, and migration and perineural invasion assays. The SACC-83 cells under the co-culture condition with SCs changed to a mesenchymal morphology and had higher migration and invasion capabilities compared with the solely cultured SACC-83 cells, accompanied by the downregulation of E-cadherin and upregulation of N-cadherin and vimentin. The co-cultured SACC-83 cells also developed Schwann-like differentiation with increased expression of SC markers, S100A4 and GFAP. However, inhibition of tropomyosin-related kinase B (TrkB) by K252a markedly blocked these effects. Additionally, the expression and correlation of TrkB, E-cadherin and S100A4 were analyzed by immunohistochemistry in 187 primary SACC cases. The levels of TrkB and 5100A4 expression were both positively associated with PNI in the SACC cases, while E-cadherin expression was negatively associated with PNI. Elevated expression of TrkB was significantly correlated with the downregulated expression of E-cadherin and the upregulated expression of S100A4 in the SACC cases. Our results suggest that SCs play a pivotal role in the PNI process by inducing the EMT process and the Schwann-like differentiation of SACC cells via the BDNF/TrkB axis. Interruption of the interreaction between SACC cells and SCs by targeting the BDNF/TrkB axis may be a potential strategy for anti-PNI therapy in SACC.