469 Background: Neurotrophin tyrosine kinase receptor 3 (NTRK3) is a receptor tyrosine kinase that has been shown to be an oncogene in breast cancer and possibly in hepatocellular carcinoma. NTRK3 is a trophic dependence receptor, which is a recently described class of receptors that initiate signaling in both the ligand bound and unbound states. Through a genome-wide screen for aberrantly methylated genes, we identified aberrantly methylated NTRK3 as a frequently methylated gene in colon cancer. The aim for the present study is to determine if NTRK3 is an epigenetically silenced tumor suppressor gene in colorectal cancer. Methods: NTRK3 promoter methylation was analyzed in human colon cancer cell lines, normal colon epithelium tissue, colorectal adenomas and colorectal cancers using quantitative methylation-specific PCR and bisulfite sequencing. NTRK3 mRNA and protein expression were studied using quantitative real-time PCR, immunohistochemistry and western blotting respectively. The tumor suppressor function of NTRK3 was examined by assessing the effect of NTRK3 on cell apoptosis, cell migration and in vitro colony formation assays in colon cancer cell lines stably transfected with an NTRK3 expression construct in the presence or absence of NT-3. Results: NTRK3 is methylated in 60% of colon adenomas and in 57% of colorectal cancers. The aberrant methylation of NTRK3 suppresses NTRK3 expression and releases colon cancer cells from NTRK3 mediated apoptosis induced by the expression of NTRK3 in the absence of the ligand NT-3 via the activation of MAPK/ERK pathway. Methylation of NTRK3 also releases colon cancer cells from NTRK3 mediated suppression of motility and anchorage independent growth. The addition of NT3 to colon cancer cells transfected with NTRK3 inhibits the tumor suppressor effects of NTRK3. Conclusions: The aberrant methylation of NTRK3 is likely functionally relevant for colorectal cancer formation as NTRK3 appears to be a conditional tumor suppressor gene in the colon depending on the expression status of its ligand NT-3. NTRK3 is a novel aberrantly methylated conditional tumor suppressor gene that is frequently methylated in colon adenomas and cancers and whose discovery reveals possible novel treatment approaches to colon cancer.
The tumor-suppressor gene Nkx2.8 suppresses bladder cancer proliferation through upregulation of FOXO3a and inhibition of the MEK/ERK signaling pathway
