Objective To explore the mechanism of cell apoptosis of immortalized human keratinocytes (HaCaT cells) and protein expression related to this process after long term exposure to sodium arsenite (NaAsO2,1.0 μmol/L).Methods Malignant transformation model was set up through long-term exposure of HaCaT cells to 1.0 μmol/L NaAsO2.Cell passage for 0,1,7,14,21,28 and 35 generations in the process of malignant transformation were collected for measurement of cell apoptosis rate by flow cytometry,and apoptosis related proteins by Western blotting,including activation of cysteine protease 3,8 (cleaved-caspase-3,8),C/EBP homologous protein (CHOP),B-cell leukemia/lymphoma 2 (Bcl-2),and Bcl-2 associated X protein (Bax).Results Along with the arsenite treatment,the apoptosis levels were significantly decreased (F =26.770,all P ＜ 0.05),the apoptosis levels (0.307 ± 0.049,0.213 ± 0.055,0.163 ± 0.057,0.147 ± 0.035,0.053 ± 0.012) of the 7th,14th,21st,28th and 35thgenerations of cells after arsenite treatment were lower than that of control group of the 0th generation (0.393 ±0.021,all P ＜ 0.05).Compared between generations,there were statistical differences of the protein expression levels of cleaved-caspase-3,Chop,Bax and Bcl-2 in arsenite group (cleaved-caspase-3:1.000 ± 0.000,1.030 ± 0.027,1.104 ± 0.069,1.016 ± 0.087,0.838 ± 0.075,0.753 ± 0.082,0.677 ± 0.073;Chop:1.000 ± 0.000,1.059 ± 0.018,0.934 ± 0.095,0.976 ± 0.216,0.793 ± 0.136,0.651 ± 0.042,0.564 ± 0.056;Bax:1.000 ± 0.000,1.069 ± 0.037,1.028 ± 0.042,0.954 ± 0.118,0.641 ± 0.135,0.531 ± 0.132,0.429 ± 0.085;Bcl-2:1.000 ± 0.000,1.072 ± 0.023,1.249 ± 0.134,1.334 ± 0.143,1.633 ± 0.221,1.507 ± 0.152,1.461 ± 0.145,F =7.730,7.355,27.802,12.438,all P ＜ 0.05),compared with control group of the 0th generation (1.000 ± 0.000) and the same generation control group (1.000 ± 0.000),after the 21st generation,the differences were statistically significant (all P ＜ 0.05),while there was no difference of the protein expression levels of cleaved-caspase-8 (F =0.832,P ＞ 0.05).Conclusion In the process of malignant transformation,the apoptosis levels of HaCaT cells are inhibited after long term sodium arsenite exposure through mitochondria and endoplasmic reticulum (ER) stress signaling pathways.