AIM:TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) has been reported to specifically induceapoptosis of cancer cells although only a small percentageof cell lines were sensitive to it.Cell lines not responding toTRAIL in vitro were said to be more prone to apoptosiswhen TRAIL was combined with another anticancer agent.Generally,factors affecting drug-sensitivity involve manyapoptosis-related proteins,including p53.The expressionof wild-type p53 gene was proposed as an important premisefor tumor cells responding to chemotherapy.The presentstudy was to investigate the cell killing action of TRAIL oncolon cancer cell line SW480,its synergistic effect withdoxorubicin,and the possible mechanisms.METHODS:SW480 cells were cultured in the regularcondition and incubated with different levels of agents.Morphologic changes in these cells after treatment wereobserved under phase-contrast microscope and cytotoxicityby TRAIL alone and in combination with doxorubicin wasquantified by a 1-day microculture tetrazolium dye (MTT)assay.In addition,flow cytometry assay (FCM) andtransmission electron microscopy were used to detectapoptosis among these cells.Variation of p53 protein levelamong different groups according to concentrations of agentswas measured by Western blot assay.RESULTS:(1) SW480 cells were not sensitive to TRAIL,with IC_(50)>1 mg·L~(-1) and dose-independent cytotoxicity.(2)SW480 cells were sensitive to doxorubicin at a certain degree,with dose-dependent cytotoxicity and IC_(50)=65.25±3.48μmol·L~(-1).(3) TRAIL could synergize with doxorubicin to killSW480 cells effectively,which was represented by theboosted killing effect of doxorubicin on theses cells.IC_(50) ofdoxorubicin against SW480 cells sharply reduced when itwas combined with TRAIL.(4) Subtoxic TRAIL (100 μg·L~(-1)),combined with subtoxic doxorubicin (0.86 μmol·L~(-1)),couldkill SW480 cells sufficiently.Cytotoxicity by MTT assayarrived at 80.12±2.67 %,which was significantly higherthan that by TRAIL or doxorubicin alone,with P=0.006 and0.003 respectively.This killing effect was partly due toapoptosis.It was proved by large amounts of apoptoticcells under phase-contrast microscopy,cell apoptosis rateof 76.82±1.93 % by FCM assay and typical apoptotic morphology observed through transmission electronmicroscopy.Increase of apoptosis after combined treatmenthad no relation with protein level of p53 (P>0.05).CONCLUSION:SW480 cells are not sensitive to TRAIL,butTRAIL can synergize with lower concentration of doxorubicinto induce apoptosis effectively.The status of p53 protein isnot involved in the mechanism of synergistic apoptosis.Itsuggests the potential therapeutic applicability of thecombination of TRAIL with doxorubicin against colon cancers. AIM: TRAIL (tumor necrosis factor-related apoptosis-inducing ligand) has been reported to specifically induceapoptosis of cancer cells although only a small percentage of cell lines were sensitive to it. Cell lines not responding to TRAIL in vitro were said to be more prone to apoptosiswhen TRAIL was combined with another anticancer agent. Generous, factors affecting drug-sensitivity involve manyapoptosis-related proteins, including p53. The expression of wild-type p53 gene was proposed as an important premise for tumor cells responding to chemotherapy. Present present was to investigate the cell killing action of TRAIL oncolon cancer cell line SW480, its synergistic effect with doroxorubicin, and the possible mechanisms. METHODS: SW480 cells were cultured in the regular condition and incubated with different levels of agents. Morphologic changes in these cells after treatment were observed under phase-contrast microscope and cytotoxicity by TRAIL alone and in combination with doxorubicin wasquantified by flow cytometry assay (FCM) and transmission electron microscopy were used to detectapoptosis among these cells. Variation of p53 protein levelamong different groups according to concentrations of agents was measured by Western blot assay. RESULTS: SW480 cells were not sensitive to TRAIL, with IC50> 1 mg · L -1 and dose-independent cytotoxicity. (2) SW480 cells were sensitive to doxorubicin at a certain degree with dose -dependent cytotoxicity and IC 50 (50) = 65.25 ± 3.48 μmol·L -1. (3) TRAIL could synergize with doxorubicin to killSW480 cells effectively, which was represented by the boosted killing effect of doxorubicin on the cells. ) ofdoxorubicin against SW480 cells sharply reduced when it was combined with TRAIL. (4) Subtoxic TRAIL (100 μg · L -1) combined with subtoxic doxorubicin (0.86 μmol·L -1) . Cytotoxicity by MTT assayarrived at 80.12 ± 2.67%, which was significantly higherthan that byTRAIL or doxorubicin alone, with P = 0.006 and 0.003 respectively. This killing effect was partly due toapoptosis. It was proved by large amounts of apoptotic cells under phase-contrast microscopy, cell apoptosis rate of 76.82 ± 1.93% by FCM assay and typical apoptotic morphology observed through transmission electron microscopy. Crease of apoptosis after combination treatment with no relation with protein level of p53 (P> 0.05). CONCLUSION: SW480 cells are not sensitive to TRAIL, but TRAIL can synergize with lower concentration of doxorubicinto induce apoptosis effectively. protein isnot involved in the mechanism of synergistic apoptosis. Itsuggests the potential therapeutic applicability of the combination of TRAIL with doxorubicin against colon cancers.