The invasion of tumor cells plays a critical role for a successful metastasis. In this study, we investigated a potential signaling that regulates the invasion of A549 cells. Our data suggested a novel signaling by which BDNF facilitates the invasion of A549 cells via TrkB/Pyk2/ERK pathway, which possibly contributes to the metastasis of those lung cancer cells. Our study indicated that TrkB plays a critical role in promoting the invasion of A549 cells, which is mediated by a mechanism closely associated with the activations of Pyk2 and ERK.
The expression of TrkB is up-regulated in a variety of human tumors, such as hepatoma, pancreatic ductal adenocarcinoma, Wilms' tumor, astrocytoma and glioblastoma [24–27]. This study evaluated TrkB expression to determine the clinical significance of TrkB for the advanced NSCLC. We examined 60 cases of NSCLC by means of immunohistochemistry and found a statistical evidence of TrkB higher expression in NSCLC, and patients with higher TrkB expression had a significant metastatic phenotype, supporting the potential role of TrkB in survival and metastasis of tumor cells [28, 29]. Therefore, the higher expression of TrkB probably plays an important role in the progression of NSCLC.
To investigate the potential function in BDNF-induced cell invasion, TrkB expression was compared between HBE and A549 cells. We found that the expression of TrkB in HBE cells was much lower, and A549cells with higher expression of TrkB seemed to be more invasive. Thus, TrkB was considered to be involved in the invasion of A549 cells. Compared with A549 cells, BDNF had no effects on Pyk2 and ERK activations or the invasion of HBE cells, which suggested that in TrkB-positive A549 cells, up-regulated TrkB was readily activated upon BDNF, and signaling pathways initiated by TrkB led to an immediate activation of Pyk2 and Pyk2-mediated functions.
Recent studies have been shown that inactivation of Trk by tyrosine kinase inhibitors was correlated with the inhibited invasion of tumor cells  and aiming at interfering TrkB expression or activation might be helpful in the progression of effective anticancer therapies. Our TrkB knockdown experiments in this study demonstrated a critical role of TrkB in BDNF-induced Pyk2 and ERK activations and the invasion of A549 cells. Further investigations should be carried out for the detailed activation and interaction between TrkB and Pyk2 in other lung cancer cell lines or in vivo.
The involvement of Pyk2 in the invasion of TrkB-positive A549 cells was clearly evident that the phosphorylation in Tyr402 was up-regulated by BDNF as well as cell invasion. Since BDNF-induced cell invasion was significantly reduced in Pyk2 knockdown cells, it was indicated that Pyk2 was required for regulating the invasion of A549 cells. Pyk2-mediated functions were performed by activating multiple downstream signaling molecules, including ERK, p38, c-Src and paxillin, which led to the differential regulation of cell invasion in various cell types [31–34]. The activation of ERK was observed after BDNF treatment, which was inhibited by Pyk2-siRNA and concomitant with a decreased cell invasion. Thus, we considered that ERK activated by Pyk2 was participated in the invasion of A549 cells stimulated by BDNF. Further experiments are necessary to clarify if other signaling molecules are involved in BDNF-induced cell invasion.
Taken together, our study confirmed that TrkB was overexpressed in NSCLCs. When activated by BDNF, TrkB induced Pyk2 phosphorylation in Tyr402, which led to ERK activation and promoted cell invasion. Our data thus revealed a TrkB/Pyk2/ERK signaling pathway that regulated the invasion of A549 cells and provided potential targets for the metastasis of NSCLC. Nevertheless, other signaling pathway(s) involved in the TrkB-associated invasion of lung cancer cells required further studies.