Abstract: Objective: To investigate endoplasmic reticulum(ER) stress- induced the apoptosis of estrogen receptor negative breast cancer cells; moreover, the effect of calpain2 on ER stress-induced the apoptosis is explored as well. Methods Experimental group(DMEM),control group(DMEM+DMSO), and experimental group, Tunicamycin (TM) of various concentrations acted on cells for different durations， MTT method and flow cytometry were used for detecting the cell proliferation inhibition rate and apoptosis rate; Western blot was used for the expression of the marker protein glucose regulated protein of ERS was determined, and based on the highest point of GRP78 expression, the drug concentration and timing of TM established by ERS were determined, determining the expressions of apoptosis proteins caspase4 and CHOP, as well as the expression of calpain and its endogenous inhibition enzyme calpastatin; ERS inhibitor (4-PBA) and calpain inhibitor (calpeptin) were respectively subject to pretreated cells，and the influences of the above effects observed. Results ERS featured inhibiting efficacies on the cells’ proliferations (9, 12, 18 μmol/L) were respectively (33.88±1.32)%, (51.51±8.85)% and (55.77±2.61)%，and in comparison with the controls (P<0.01); 9 and 12 μmol/L TM’s cell apoptosis rates induced by ERS were respectively (16.70±0.46)％ and (28.10±1.09)％, in comparison to the controls (P<0.05); 9 μmol/L TM’s action on cells 24 h GRP78 yielded the highest expression and featured statistical significance in comparison to the controls (P<0.05), and those of 12 μmol/L were obviously down-regulated; ERS obviously up-regulated the expressions of caspase4, CHOP and calpain，and the expression of calpastatin down-regulated ( P<0.01~0.05). The effects can be reduced or blocked by 4-PBA and calpeptin (P<0.05). And the expressions of CHOP all down-regulated among them 4-PBA and the treated group( P<0.05). Conclusion: The strong ER stress can induce the apoptosis of breast cancer MDA-MB-231 cells; expect that, calpain2 may be engaged in its mechanism.

Key words: [Key word]: ER stress, GRP78, Calpain, Cell apoptosis, MDA-MB-231 cell