Abstract: Ample evidence suggests that the stromal microenvironment in which tumor cells develop profoundly influences many steps of epithelial tumorigenesis. Fibroblasts, a major cell type of the stromal compartment, may function as a stress monitor to interact with epithelial cells and to modulate their response. The molecular details of the epithelium-stroma interaction, however, remain not well understood. We here demonstrate that fibroblasts engage in a senescence-like cellular program in response to cumulative sublethal stress insults. We further show that the accumulation of senescence-like fibroblasts in the stroma can compromise the mammary tissue homeostasis and deregulate epithelial-stromal interactions. Using protracted low-dose ionizing radiation (IR) as a model of low-level environmental stress, we demonstrate that primary human mammary stromal fibroblasts (HMFs) respond to low-dose IR by displaying a senescence-like phenotype in a dose-dependent and accumulative manner. Compared with un-irradiated HMFs, IR-induced senescence-like fibroblasts (SFs) profoundly stimulate the growth of malignant mammary epithelial cells (MECs) in a two-dimensional coculture assay. In agreement with the growth promoting activity, functional genomic analysis reveals that SFs transmit mitogenic signals to MECs by upregulating mitotic regulators. To further understand the heterotypic influences of SFs on MECs, we adopted a three-dimensional (3D) coculture system to study the epithelial-stromal interactions in a tissue-like context. We provide a direct experimental proof that SFs dysregulate the 3D growth of MECs due to the substantial cytoskeletal alterations in SFs and the upregulation of multiple secreted-type matrix metalloproteinases. The resultant alterations in cell-cell and cell-matrix interactions lead to aberrant proliferation, tissue morphogenesis, matrix invasion and stress responses of MECs depending on their genetic status. Untransformed MECs respond to SFs by growing into enlarged cysts with central cell-free lumens; MECs with inactivated apoptosis- and autophagy-dependent cell death pathways formed filled masses that resemble incipient tumors; malignant MECs that have undergone epithelial-mesenchymal transition fully express their invasive potential in the presence of SFs. Taken together, our results suggest that low-level environmental stress can foster an oncogenic environment that interacts with permissive genetic traits in epithelial cells to promote breast carcinogenesis.