Autophagy inhibition by chloroquine sensitizes HT-29 colorectal cancer cells to concurrent chemoradiation

AIM:To investigate whether the inhibition of autophagy by chloroquine(CQ)sensitizes rectal tumors to radiation therapy(RT)or concurrent chemoradiation(chemoRT).METHODS:In vitro,HCT-116 and HT-29 colorectal cancer(CRC)cell lines were treated as following:(1)PBS;(2)CQ;(3)5-fluorouracil(5-FU);(4)RT;(5)CQ and RT;(6)5-FU and RT;(7)CQ and 5-FU;and(8)5-FU and CQ and RT.Each group was then exposed to various doses of radiation(0-8 Gy)depending on the experiment.Cell viability and proliferative capacity were measured by3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide(MTT)and clonogenic assays.Clonogenic survivalcurves were constructed and compared across treatment groups.Autophagy status was determined by assessing the LC3-Ⅱto LC3-Ⅰratio on western blot analysis,autophagosome formation on electron microscopy and identification of a perinuclear punctate pattern with GFPlabeled LC3 on fluorescence microscopy.Cell cycle arrest and cell death were evaluated by FACS and AnnexinⅤanalysis.All experiments were performed in triplicate and statistical analysis was performed by the student’s t test to compare means between treatment groups.RESULTS:RT(2-8 Gy)induced autophagy in HCT-116and HT-29 CRC cell lines at 4 and 6 h post-radiation,respectively,as measured by increasing LC3-Ⅱto LC3-Ⅰratio on western blot.Additionally,electron microscopy demonstrated autophagy induction in HT-29 cells24 h following irradiation at a dose of 8 Gy.Drug treatment with 5-FU(25μmol/L)induced autophagy and the combination of 5-FU and RT demonstrated synergism in autophagy induction.CQ(10μmol/L)alone and in combination with RT effectively inhibited autophagy and sensitized both HCT-116 and HT-29 cells to treatment with radiation(8 Gy;P<0.001 and 0.00001,respectively).Significant decrease in clonogenic survival was seen only in the HT-29 cell line,when CQ was combined with RT at doses of 2 and 8 Gy(P<0.5 and P=0.05,respectively).There were no differences in cell cycle progression or Annexin V staining upon CQ addition to RT.CONCLUSION:Autophagy inhibition by CQ increases CRC cell sensitivity to concurrent treatment with 5-FU and RT in vitro,suggesting that addition of CQ to chemoRT improves CRC treatment response.