An epithelial-mesenchymal transition (EMT) characterizes the progression of many carcinomas and it is linked to the acquisition of an invasive phenotype. Given that the tumor microenvironment is an active participant in tumor progression, an important issue is whether a reactive stroma can modulate this process. Using a novel EMT model of colon carcinoma spheroids, we demonstrate that their transforming-growth factor-␤1 (TGF-␤)-induced EMT is accelerated dramatically by the presence of activated macrophages, and we identify tumor necrosis factor-␣ (TNF-␣) as the critical factor produced by macrophages that accelerates the EMT. A synergy of TNF-␣ and TGF-␤ signaling promotes a rapid morphological conversion of the highly organized colonic epithelium to dispersed cells with a mesenchymal phenotype, and this process is dependent on enhanced p38 MAPK activity. Moreover, exposure to TNF-␣ stimulates a rapid burst of ERK activation that results in the autocrine production of this cytokine by the tumor cells themselves. These results establish a novel role for the stroma in influencing EMT in colon carcinoma, and they identify a selective advantage to the stromal presence of infiltrating leukocytes in regulating malignant tumor progression. INTRODUCTIONMany epithelial tumors undergo an epithelial-mesenchymal transition (EMT) that facilitates their invasion. The EMT is also an essential component of embryonic development, tissue remodeling, and wound repair (reviewed in Arias, 2001;Thiery, 2002). During this transition, the epithelial phenotype, characterized by strong cell-cell junctions and polarity, is replaced by a mesenchymal phenotype, with reduced cell-cell interactions, a fibroblastic morphology and increased motility. Given the importance of the EMT in carcinoma progression, there is considerable interest in understanding the mechanisms that contribute to this complex process. Although the exact mechanisms that underlie the EMT have not been elucidated, TGF-␤ has been implicated as a key inducer of the process (Oft et al., 1998;Portella et al., 1998;Lehmann et al., 2000;Bhowmick et al., 2001a;Ellenrieder et al., 2001;Fujimoto et al., 2001). TGF-␤ stimulates proliferation of many cell types, particularly those of mesenchymal origin, and it is also a potent inhibitor of epithelial cell proliferation. Contrary to an early ascribed role as a tumor suppressor (Markowitz and Roberts, 1996), TGF-␤ has been found to be abundantly expressed in many epithelial tumors and it acts in both an autocrine manner on the tumor cells themselves and as a paracrine modulator of the stroma (reviewed in Gold, 1999;de Caestecker et al., 2000;Yue and Mulder, 2001). In colon carcinoma, TGF-␤ also acts differently depending on the differentiation stage of the tumor, in general by switching from an early inhibitor of proliferation to a stimulator of growth and invasion during tumor progression (Hsu et al., 1994).TGF-␤ has been implicated as a major factor in the EMT, but this is likely to be a "multifactorial" process. Moreover, the strom...

During embryonic development, epithelial cells must escape the structural constraints imposed by tissue architecture and adopt a phenotype more amenable to cell movement, a process known as the epithelial-mesenchymal transition (EMT). The progression of carcinomas to invasive and metastatic disease may also involve localized occurrences of EMT. However, data that support the actual occurrence of EMT in specific carcinomas and the relevance of this process to the progression of these tumors had been scant. This review highlights recent studies that substantiate the importance of the EMT to colorectal carcinoma. Specifically, a novel model for studying the EMT of colorectal carcinoma has been used to gain insight into the nature of the EMT itself and to identify molecular events that contribute to disease progression. Although loss of E-cadherin function is a primal event for the EMT, the expression of specific integrins such as alpha(v)beta6 as a consequence of the EMT enables invasive cells to interact with interstitial matrices and to sustain activation of TGF-beta. Of note, alpha(v)beta6 expression in tumors is a marker of cells that have undergone an EMT and it is prognostic for tumors that will progress more rapidly to terminal disease. The EMT also induces autocrine signaling involving VEGF and Flt-1 that enable invasive cells to become 'self-sufficient' for survival. Thus, the EMT appears to be an integral component of colorectal cancer progression and its analysis can yield novel targets for prognosis and therapy.

We used a spheroid model of colon carcinoma to analyze integrin dynamics as a function of the epithelial-mesenchymal transition (EMT), a process that provides a paradigm for understanding how carcinoma cells acquire a more aggressive phenotype. This EMT involves transcriptional activation of the beta6 integrin subunit and a consequent induction of alphavbeta6 expression. This integrin enhances the tumorigenic properties of colon carcinoma, including activation of autocrine TGF-beta and migration on interstitial fibronectin. Importantly, this study validates the clinical relevance of the EMT. Kaplan-Meier analysis of beta6 expression in 488 colorectal carcinomas revealed a striking reduction in median survival time of patients with high beta6 expression. Elevated receptor expression did not simply reflect increasing tumor stage, since log-rank analysis showed a more significant impact on the survival of patients with early-stage, as opposed to late-stage, disease. Cox regression analysis confirmed that this integrin is an independent variable for these tumors. These findings define the alphavbeta6 integrin as an important risk factor for early-stage disease and a novel therapeutic candidate for colorectal cancer.

No abstract