AIM To clarify the mechanisms involved in the critical endoplasmic reticulum(ER) stress initiating unfolded protein response pathway modified by melatonin.METHODS Hepatoma cells, Hep G2, were cultured in vitro. Flow cytometry and TUNEL assay were used to measure Hep G2 cell apoptosis. Western blotting and quantitative reverse transcription-polymerase chain reaction methods were used to determine the protein and messenger RNA levels of ER stress and apoptosis related genes’ expression, respectively. Tissue microarray construction from patients was verified by immunohistochemical analysis.RESULTS In the present study, we first identified that melatoninselectively blocked activating transcription factor 6(ATF-6) and then inhibited cyclooxygenase-2 (COX-2) expression, leading to enhanced liver cancer cell apoptosis under ER stress condition. Dramatically increased CCAAT-enhancer-binding protein homologous protein level, suppressed COX-2 and decreased Bcl-2/Bax ratio by melatonin or ATF-6 si RNA contributed the enhanced Hep G2 cell apoptosis under tunicamycin (an ER stress inducer) stimulation. In clinical hepatocellular carcinoma patients, the close relationship between ATF-6 and COX-2 was further confirmed.CONCLUSION These findings indicate that melatonin as a novel selective ATF-6 inhibitor can sensitize human hepatoma cells to ER stress inducing apoptosis. AIM To clarify the mechanisms involved in the critical endoplasmic reticulum (ER) stress initiating unfolded protein response pathway modified by melatonin. METHODS Hepatoma cells, Hep G2, were cultured in vitro. Flow cytometry and TUNEL assay were used to measure Hep G2 cell apoptosis. Western blotting and quantitative reverse transcription-polymerase chain reaction methods were used to determine the protein and messenger RNA levels of ER stress and apoptosis related genes’ expression, respectively. Tissue microarray construction from patients was verified by immunohistochemical analysis .RESULTS In the present study, we first identified that melatonin selectively blocked activating transcription factor 6 (ATF-6) and then inhibited cyclooxygenase-2 (COX-2) expression, leading to enhanced liver cancer cell apoptosis under ER stress condition. Dramatically increased CCAAT-enhancer-binding protein homologous protein level, suppressed COX-2 and decreased Bcl-2 / Bax ratio by melatonin or ATF-6s i RNA contributed the enhanced Hep G2 cell apoptosis under tunicamycin (an ER stress inducer) stimulation. In clinical hepatocellular carcinoma patients, the close relationship between ATF-6 and COX-2 was confirmed. CONCLUSION These findings indicate that melatonin as a novel selective ATF-6 inhibitor can sensitize human hepatoma cells to ER stress inducing apoptosis.