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Yu Y, Zhang S, Wang X, Yang Z, Ou G. Overexpression of TrkB promotes the progression of colon cancer. APMIS 2010; 118: 188–95.
Studies have confirmed that TrkB plays important roles in facilitating metastasis in various types of malignant tumors. In the present study, 30 cases of colon cancer and matched non-tumors were examined for the expression of TrkB by Western blot. The expression of TrkB was also examined in 90 colon tumor sections by immunohistochemical methods, and D2-40 staining was used to evaluate the correlation between TrkB expression and lymphatic vessel density. To investigate the effects of TrkB on the progression of colon cancer, siRNA specific for TrkB was transfected into LoVo cells, and proliferation, apoptosis and invasion of trasfected cells were examined using MTT [3-(4,5-Dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide], flow cytometry and Transwell assays, respectively. Our results showed that TrkB was up-regulated in colon tumors compared with the non-tumorous counterparts, and the overexpression of TrkB was closely correlated with lymphatic vessel density (LVD) and metastasis. Inhibition of TrkB by siRNA increased the apoptotic rates of transfected cells, while the numbers of proliferative and invasive cells were decreased. In summary, our data suggest that overexpression of TrkB in colon cancer possibly plays roles in inhibiting apoptosis, promoting proliferation and invasion, facilitating tumor progression by lymphangiogenesis-associated metastasis.
Colon cancer is the third most common cancer worldwide, and the incidence of colon cancer is increasing. The prognosis of patients with colon cancer principally correlates with the proliferative, apoptotic and invasive potentials of tumor cells, which are regulated by some critical genes. Tropomysin-related kinase B (TrkB) is a member of the Trk family, and functions as a receptor tyrosine kinase, which is necessary for the normal development of the nervous system (1, 2). Recent studies have shown the suppression of anoikis and induction of metastasis by TrkB (3, 4). TrkB is up-regulated in various primary human tumors, including neuroblastoma (5) and hepatocellular carcinoma (6), especially in metastatic gastric (7) and pancreatic tumors (8). TrkB is also reported to have intimate correlations with the prognosis of cancer patients (9–11) or angiogenesis (12). Therefore, the overexpression of TrkB might play an important role in the progression of malignant tumors.
Studies have shown that enhanced TrkB signaling promotes cell survival (13), and when activated, TrkB leads to the activation of downstream signaling molecules like phosphoinositide-3 kinase/protein kinase B (PI3K/Akt) (14, 15), which results in the differential regulation of tumorous processes as apoptosis and especially metastasis. Although point mutation was observed in colorectal cancer (16), whether TrkB positively participates in primary colon cancer has not yet been determined.
This study was designed to investigate the expression and clinical significance of TrkB in surgically resected colon cancer with different clinicopathological features. We report here that high TrkB expression is common in colon tumors, and increased expression of TrkB is correlated with LVD and lymph node metastasis. We also report here that the interruption of TrkB expression by TrkB-siRNA promoted apoptosis of LoVo cells. However, suppression of TrkB inhibited the proliferation and invasion of LoVo cells. These results identify TrkB as a potential promoter in the progression of colon cancer.
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- Materials and methods
TrkB is a receptor tyrosine kinase and mediates various signaling pathways. TrkB is critical to tumorigenesis (25) and metastasis (26), by promoting cell proliferation and survival. TrkB expression was found up-regulated in a variety of human tumors including neuroblastoma, ovarian cancer, and pancreatic tumors. Targeting at interfering TrkB expression may be helpful in the progression of effective anticancer therapies.
This study investigated TrkB expression to determine the clinical significance of TrkB in the progression of colon cancer. We examined 30 colon tumors and paired non-tumors by Western blot, and found a statistically significant TrkB overexpression in colon cancer. TrkB expression was significantly higher in neoplastic tissues, which suggested that TrkB overexpression correlates with the proliferation and survival of colon tumor cells. Despite the need for further research, these data suggest a crucial role of TrkB in facilitating tumorigenesis of colon cancer.
The expression of TrkB was also examined in 90 sections of colon cancer by means of immunohistochemistry. Through immunostaining, TrkB was detected extensively in colon tumors, further demonstrating that the overexpression of TrkB was common in colon tumors, regardless of tumor size and differentiation. A previous study has shown that TrkB expression was correlated with angiogenesis of ovarian cancer (12), while we found in this study that patients with higher TrkB expression had a higher LVD and a significant metastatic phenotype. These results indicate that TrkB probably facilitated metastasis of colon cancer partially by associated lymphoangiogenesis.
A study has shown that trk tyrosine kinase inhibitor induced cell death (27), and here we investigated the effects of TrkB-siRNA on the proliferation, apoptosis, and invasion of LoVo cells. After TrkB-siRNA transfection, LoVo cells exhibited a much lower level of TrkB and suppressed proliferation, which suggested that TrkB possibly promoted the proliferation and was likely necessary for the growth of LoVo cells. Studies have shown that TrkB inhibited anoikis of tumors (28), and we noticed that the apoptotic rate of transfected cells was increased by TrkB-siRNA, confirming the negative role of TrkB in cell apoptosis, which might help survival of normal LoVo cells. It is well accepted that the invasive ability of tumor cells plays a critical role for a successful metastasis. We observed in this study that the invasion of LoVo cells was largely inhibited by the deficiency of TrkB, indicating the important role of TrkB in LoVo cell invasion and even metastasis of colon cancer.
In conclusion, our study demonstrated that the overexpression of TrkB was common in colon cancer, which suggested that TrkB might play a critical role in inducing tumorigenesis of colon cancer. We also found that the increased expression of TrkB was correlated with higher metastatic ability, possibly by lymphoangiogenesis. The augmented apoptosis, attenuated proliferation and invasion of LoVo cells were also observed by the interruption of TrkB expression using specific siRNA. Taken together, TrkB may provide a helpful target for inhibitory therapies of progression of colon cancer. The regulatory signaling downstream of TrkB in colon cancer deserves further investigation.