Enhancement of radiation response in follicular thyroid cancer cells by histone deacetylase inhibitors

Introduction: Radiotherapy represents the most applied non-surgical strategy for cancer treatment. Approximately, 50% of all cancer patients will receive radiation therapy during their course of treatment. Despite the recent technical improvements, many patients still suffer from locally recurrent disease. Combined treatment with chemical radiosensitizers could further increase the therapeutic ratio of radiotherapy. Objectives: To study the effect of the histone deacetylase inhibitors (HDACI), sodium butyrate (NaB) and valproic acid (VA), in the radiosensitivity of poorly differentiated thyroid cancer cells. Materials and Methods: Human follicular thyroid cancer cells (WRO) were incubated with NaB and VA and irradiated with a ¹³⁷Cs radiation source (7,7 Gy/min) at different radiation doses. The following parameters were studied at different post-irradiation times: cell survival was assessed by clonogenic assay, cell cycle arrest, cell death, DNA damage through the detection of phosphorylated gamma-H2AX by immunofluorescence and DNA damage repair protein expression by Western Blot. Results: Survival fraction at 2 Gy diminished from 68.4±1.4 (control) to 42.0±2.7 (p<0.0001) (NaB) and from 69.0±0.2 (control) to 56.3±1.6 (p<0,01) (VA). Cell death was significantly increased in irradiated cells with or without both HDACI. We observed that cells died predominantly by necrosis especially at longer time points after irradiation. There was a cell cycle arrest at G2/M phase in irradiated cells 6 and 24 h after irradiation. DNA damage analysis showed an increase in the number of foci/cell in those cells treated with both HDACI at 30 minutes, 4 and 24 h after irradiation (p<0.001). We also observed a significant increase at all the studied times in the foci diameter in all the irradiated cells. Interestingly, there was a higher percentage of big foci (>1μm) in the cells treated with both HDACI at 4 and 24 h after irradiation (p<0.01 y p<0.001, 4 and 24 h, respectively). At 4 h after irradiation, Ku80 expression was downregulated by VA (p<0.001) and Rad51 was dowregulated by NaB as well (p<0.001). Conclusion: In these experimental model, HDACI sodium butyrate and valproic acid increased poorly differentiated thyroid cancer cells radiosensitivity through the inhibition of DNA damage repair capacity and cell death increase. This combined treatment modality could provide a potential therapeutic strategy for those locally recurrent and unresponsive differentiated thyroid cancers. (author)