In vitro enhancement of response to radiation of human lung cancer cells following insulin-like growth factor 1 receptor inactivation

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Abstract Target disruption of the insulin-like growth factor 1 receptor (IGF-1R) restricts proliferation of tumor cells and enhances their in vitro radiosensitivity. However, there is little information regarding the expression of IGF-1R in lung cancer and the effect of IGF-1R expression on response to radiotherapy in this tumour disease. In this study we have set forth to explore the putative link between IGF-1R expression and radioresistance of lung carcinoma cells. We evaluated the cell surface expression of IGF-1R in 5 small cell lung cancer (SCLC) cell lines and 6 non-SCLC (NSCLC) cell lines with different degrees of radiosensitivity, using 125I-IGF-1 binding experiments and subsequent Scatchard analysis. The radioresistant NSCLC cell lines expressed significantly greater IGF-1R at the cell membrane compared with the radiosensitive SCLC cell lines. We also investigated, using MTT assay, the antitumoral effect of IGF-1R blockade alone and in combination with γ-irradiation on NSCLC cell lines. The interactions between IGF-1R blockade (using a blocking antibody-IGF-IRab) and irradiation were then classified by the Multiplicative Method. In U1810 cell line, which displayed the highest IGF-1R level, synergy was found when combining IGF-1Rab with irradiation, additive interactions were found for four cell lines (A549, H157, H23 and H125) whereas only subadditive effect was observed in U1752 cell line over a 7-day observation period. Using two different methods (DNA synthesis and MTT assay), we assessed the possible protective effect of IGF-1R activation on radiation-induced cell death in the U1810 cells. Exogenous IGF-1 was able to obstruct in vitro serum starvation/radiation-induced cell death in U1810 cell line.Combined treatment with IGF-1Rab and irradiation induced an accumulation of cells in cell cycle phases G1 and G2 with concurrent reduction in the proportion of cells in the S phase in U1810 cells.Furthermore, activation of IGF-1R by its ligand partially abrogated serum starvation/radiation-induced G1, G2 arrest, as assessed by flow cytometry. Our results indicate that IGF-1R expression in lung cancer is one of the factors related to radioresistance and that inhibition of the receptor in combination with irradiation may prove to be a viable therapeutic strategy for the treatment of those types of lung cancer that display high expression of IGF-1R.