Cigarette smoking extract induced ER stress and apoptosis in ARPE-19 cells. ARPE-19 cells were exposed to CSE (0.004 μg/ml-320 μg/ml) for up to 24 h. A, protein levels of ER markers or pro-apoptosis markers were determined by Western blotting in cells treated with CSE for 6 h (p-eIF2α, eIF2α and GRP78) or 12 h (ATF4 and CHOP). B, expression of p-eIF2α (normalized with eIF2α), GRP78, ATF4 and CHOP were quantified by densitometry of Western blots. C, protein levels of ER stress markers, Nrf2, p-p38, and cleaved caspase-3 were determined by Western blotting in cells treated with 320 μg/ml of CSE for 6–24 h. D, expression of p-eIF2α (normalized with eIF2α), GRP78, ATF4, CHOP, ATF6, Nrf2, p-p38, and cleaved caspase-3 were quantified by densitometry. E, apoptosis of ARPE-19 cells was examined by TUNEL assay after CSE treatment for 24 h. Scale Bar: 50 μm. F, quantification of TUNEL-positive cells. G, cell viability of ARPE-19 cells with or without 320 μg/ml of CSE treatment for 24 h. All data were expressed as mean ± S.D., from three independent experiments. *, p < 0.05; **, p < 0.01 versus control.