TGFb1 Inhibition Increases the Radiosensitivity of Breast Cancer Cells In Vitro and Promotes Tumor Control by Radiation In Vivo

Purpose: To determine whether inhibition of TGFb signaling prior to irradiation sensitizes human and murine cancer cells in vitro and in vivo. Experimental Design: TGFb-mediated growth and Smad phosphorylation of MCF7, Hs578T, MDAMB-231, and T47D human breast cancer cell lines were examined and correlated with clonogenic survival following graded radiation doses with and without pretreatment with LY364947, a small molecule inhibitor of the TGFb type I receptor kinase. The DNA damage response was assessed in irradiated MDA-MB-231 cells pretreated with LY364947 in vitro and LY2109761, a pharmacokinetically stable inhibitor of TGFb signaling, in vivo. The in vitro response of a syngeneic murine tumor, 4T1, was tested using a TGFb neutralizing antibody, 1D11, with single or fractionated radiation doses in vivo. Results: Human breast cancer cell lines pretreated with TGFb small molecule inhibitor were radiosensitized, irrespective of sensitivity to TGFb growth inhibition. Consistent with increased clonogenic cell death, radiation-induced phosphorylation of H2AX and p53 was significantly reduced in MDA-MB-231 triple-negative breast cancer cells when pretreated in vitro or in vivo with a TGFb type I receptor kinase inhibitor. Moreover, TGFb neutralizing antibodies increased radiation sensitivity, blocked gH2AX foci formation, and significantly increased tumor growth delay in 4T1 murine mammary tumors in response to single and fractionated radiation exposures. Conclusion: These results show that TGFb inhibition prior to radiation attenuated DNA damage responses, increased clonogenic cell death, and promoted tumor growth delay, and thusmay be an effective adjunct in cancer radiotherapy. Clin Cancer Res; 17(21); 1–12. 2011 AACR.