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Cellular senescence, being the result of serial subculturing or of exogenous stresses, is considered to be a potent anticancer mechanism. However, it has been proposed that senescent cells may enhance the growth of adjacent malignant epithelial cells. On the other hand, exposure of tumors to repeated low doses of γ-irradiation is a common treatment regime. Nevertheless, γ-irradiation also affects the neighboring stromal cells and the interaction of the latter with cancer cells. Accordingly, in this study, we have exposed confluent cultures of human lung fibroblasts to repeated subcytotoxic doses of 4 Gy of γ-irradiation. We found that a single dose immediately activates a DNA damage response, leading to an intense, but reversible, cell cycle arrest. After a series of doses (total dose approximately 50 Gy) cellular senescence was accelerated, as shown by permanent growth arrest and the upregulation of specific biochemical and morphological senescence-associated markers. This process was found to be p53-dependent. Next, we studied the effect of these prematurely senescent cells on the growth of human malignant lung cell lines (A549 and H1299) and found that the presence of irradiation-mediated senescent cells strongly enhances the growth of these cancer cells in vitro and in immunocompromised (SCID) mice in vivo. This effect seems not to be related to an induction of epithelial-to-mesenchymal transdifferentiation but, to a significant extent, to the increased expression of matrix metalloproteases (MMPs), as a specific MMP inhibitor significantly restrains the growth of cancer in the presence of senescent fibroblasts. These findings indicate that lung fibroblasts that become senescent after ionizing radiation may contribute to lung cancer progression.