Differential Notch and TGFβ Signaling in Primary Colorectal Tumors and Their Corresponding Metastases

Background: Loss of epithelial morphology and the acquisition of mesenchymal characteristics may contribute to metastasis formation during colorectal tumorigenesis. The Wnt, Notch and TGFβ signaling pathways control tissue homeostasis and tumor development in the gut. The relationship between the activity of these pathways and the expression of epithelial and mesenchymal markers was investigated in a series of primary colorectal tumors and their corresponding metastases. Methods: Tissue samples of primary colorectal tumors, normal colonic mucosa, and regional and systemic metastases were processed for immunohistochemistry in a tissue microarray format. The expression of mesenchymal (vimentin, fibronectin) and epithelial (E-cadherin) markers was related to markers of Wnt (β-catenin), Notch (HES1) and TGFβ (phospho-SMAD2) signalling. In addition, the KRAS mutation status was assessed. Results: When compared to normal mucosa, primary colorectal tumors showed a marked increase in the levels of cytoplasmic vimentin and nuclear β-catenin, phospho-SMAD2 and HES1. Increased vimentin expression correlated with the presence of oncogenic KRAS and with nuclear β-catenin. The corresponding liver, lymph node, brain and lung metastases did not express vimentin and displayed significantly lower levels of nuclear phospho-SMAD2 and HES1, while retaining nuclear β-catenin. Conclusions: Primary colorectal carcinomas display aberrant expression of vimentin, and have activated Notch and TGFβ signaling pathways. Surprisingly, many regional and distant metastases have lost nuclear HES1 and pSMAD2, suggesting that the activity of the Notch and TGFβ pathways is reduced in secondary colorectal tumors.