Inhibition of transforming growth factor-β signaling in normal lung epithelial cells confers resistance to ionizing radiation

Purpose: To address the functional role of radiation-induced transforming growth factor-beta (TGF-beta) signaling in a normal epithelial background, we selected a spontaneously immortalized lung epithelial cell line derived from the normal lung tissue of a dominant-negative mutant of the TGF-beta RII (Delta RII) transgenic mouse that conditionally expressed Delta RII under the control of the metallothionein promoter (MT-1), and assessed this cell line's response to radiation. Methods and Materials: A spontaneously immortalized lung epithelial cell culture (SILECC) was established and all analyses were performed within 50 passages. Colony-forming and terminal transferase dUPT nick end labeling (TUNEL) assays were used to assess clonogenic inhibition and apoptosis, respectively. Western-blot analysis was performed to assess the kinetics of p21, bax, and RII proteins. Transforming growth factor-beta-responsive promoter activity was measured using dual-luciferase reporter assay. Results: Exposure to ZnSO4 inhibited TGF-beta signaling induced either by recombinant TGF-beta 1 or ionizing radiation. The SILECC, treated with either ZnSO4, or neutralizing antibody against TGF-beta, showed a significant increase in radio-resistance compared to untreated cells. Furthermore, the expression of Delta RII inhibited the radiation-induced up-regulation of the TGF-beta effector gene p21(waf1/cip1) . Conclusions: Our findings imply that inhibition of radiation-induced TGF-beta signaling via abrogation of the RII function enhances the radio-resistance of normal lung epithelial cells, and this can be directly attributed to the loss of TGF-beta signaling function. (c) 2007 Elsevier Inc.