TGFβ1 Inhibition Increases the Radiosensitivity of Breast Cancer Cells In Vitro and Promotes Tumor Control by Radiation In Vivo

Purpose: To determine whether inhibition of TGF beta signaling prior to irradiation sensitizes human and murine cancer cells in vitro and in vivo. Experimental Design: TGF beta-mediated growth and Smad phosphorylation of MCF7, Hs578T, MDA-MB-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 TGF beta 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 TGF beta signaling, in vivo. The in vitro response of a syngeneic murine tumor, 4T1, was tested using a TGF beta neutralizing antibody, 1D11, with single or fractionated radiation doses in vivo. Results: Human breast cancer cell lines pretreated with TGF beta small molecule inhibitor were radio-sensitized, irrespective of sensitivity to TGF beta 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 TGF beta type I receptor kinase inhibitor. Moreover, TGF beta neutralizing antibodies increased radiation sensitivity, blocked gamma H2AX 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 TGF beta inhibition prior to radiation attenuated DNA damage responses, increased clonogenic cell death, and promoted tumor growth delay, and thus may be an effective adjunct in cancer radiotherapy. Clin Cancer Res; 17(21); 6754-65. (C)2011 AACR.