Signaling events mediated by α3β1 integrin are essential for mammary tumorigenesis

The constitutive activation of beta-catenin signaling in the mammary basal epithelial cell layer in transgenic K5 Delta N beta cat mice leads to basal-type tumor development. Integrins of the beta 1 family and integrin-mediated signaling events have an important role in breast tumor growth and progression. We show here that the deletion of alpha 3 beta 1 integrin, a major laminin receptor, from the basal layer of the mammary epithelium of K5 Delta N beta cat mice completely prevented the tumorigenesis induced by beta-catenin signaling. Moreover, the depletion of alpha 3 beta 1 integrin from a spontaneously transformed mouse mammary basal epithelial cell line (MEC) prevented the cells from forming colonies in soft agar and greatly reduced tumor development in orthotopic grafts. Inhibition of the integrin signaling intermediates Rac1 or PAK1 (P21-activated Kinase 1) in MEC affected tumor cell growth in soft agar, whereas the expression of activated forms of these effectors in alpha 3-depleted cells rescued the capacity of these cells to grow in non-adherent conditions. Similarly, the tumorigenic potential of alpha 3-depleted cells was restored by the expression of activated PAK1, as assessed by orthotopic transplantation assay. In three-dimensional Matrigel culture, MEC survival and proliferation were affected by the depletion of alpha 3 beta 1 integrin, which also significantly decreased the activation of focal adhesion kinase (FAK), mitogen-activated protein kinase (MAPK) and c-Jun NH2-terminal kinase (JNK). Our data suggest that the activation of signaling cascades downstream from alpha 3 beta 1 and involving the Rac1/PAK1 pathway, MAPK and JNK, promotes prosurvival and proproliferative signals required for the malignant growth of basal mammary epithelial cells, providing further insight into the molecular mechanisms underlying breast cancer initiation and progression.