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Crosstalk between tumor and stromal cells in the tumor microenvironment alter its properties in ways that facilitate the invasive behavior of tumor cells. Here, we demonstrate that cancer-associated fibroblasts (CAF) increase the stiffness of the extracellular matrix (ECM) and promote anisotropic fiber orientation, two mechanical signals generated through a Snail1/RhoA/αSMA–dependent mechanism that sustains oriented tumor cell migration and invasiveness. Snail1-depleted CAF failed to acquire myofibroblastic traits in response to TGFβ, including RhoA activation, αSMA-positive stress fibers, increased fibronectin fibrillogenesis, and production of a stiff ECM with oriented fibers. Snail1 expression in human tumor–derived CAF was associated with an ability to organize the ECM. In coculture, a relatively smaller number of Snail1-expressing CAF were capable of imposing an anisotropic ECM architecture, compared with nonactivated fibroblasts. Pathologically, human breast cancers with Snail1+ CAF tended to exhibit desmoplastic areas with anisotropic fibers, lymph node involvement, and poorer outcomes. Snail1 involvement in driving an ordered ECM was further confirmed in wound-healing experiments in mice, with Snail1 depletion preventing the anisotropic organization of granulation tissue and delaying wound healing. Overall, our results showed that inhibiting Snail1 function in CAF could prevent tumor-driven ECM reorganization and cancer invasion. Cancer Res; 75(2); 284–95. ©2014 AACR.

40 p.-6 fig., Snail1 transcriptional factor is essential for triggering epithelial-to-mesenchymal transition (EMT) and inducing tumor cell invasion. We report here an additional, EMTindependent action of Snail1 on tumor invasion: its expression in cancer-associated fibroblasts is necessary for enhancement by these cells on epithelial cells tumor invasion. Snail1 expression in fibroblast requires signals derived from tumor cells such as TGF-; reciprocally, in fibroblasts Snail1 organizes a complex program that favors collective invasion of epithelial cells at least in part by the secretion of diffusible signaling molecules, such as prostaglandin E2. The capability of human or murine tumor-derived cancer associated fibroblasts to promote tumor invasion is associated to Snail1 expression and obliterated by Snail1 depletion. In vivo experiments show that Snail1 depletion in mice prevents the invasion of breast tumors and epithelial tumor cells co-xenografted with Snail1-depleted fibroblasts originate tumors with lower invasion than those transplanted with control fibroblasts. Therefore, these results demonstrate that the role of Snail1 in tumor invasion is not limited to EMT but dependent on its expression in stromal fibroblasts where it orchestrates its activation and the crosstalk with epithelial tumor cells. Moreover, they point to the interference of Snail expression as a promising target for preventing the action of stromal fibroblasts on tumor progression., This work was funded by grants from Fundación Científica de la Asociación Española contra el Cáncer to AGH, JIC and FB, and Ministerio de Economía(SAF2013-48849-C2-1-R) and Fundació La Marató de TV3 (120130) to AGH. We also thank the support from Instituto Carlos III (RD12/0036/005 and RD12/0036/004,1 part of the Plan Nacional I+D+I and cofounded by the ISCIII-Subdirección General de Evaluación and Fondo Europeo de Desarrollo Regional-FEDER) and Generalitat de Catalunya (2014 SGR 32).