Aim:BAG-1 is a multifunctional anti-apoptotic gene with four isoforms,and different BAG-1 isoforms have different anti-apoptotic functions.In this study,we transfected BAG-1 isoforms into the human breast cancer cell lines Hs578T (ER nega-tive) and MCF-7 (ER positive) to study their effect on apoptosis with or without estrogens.Methods: The constructed recombinant expression vectors carrying individual BAG-1 isoforms was used to transfect human breast cancer cell lines Hs578T (ER negative) and MCF-7 (ER positive).After stable cell lines were made,a variety of apoptosis-inducing agents,including doxorubicin,docetaxel,and 5-FU,was used to treat these cell lines with or without estrogen to test the role of BAG-1.The mechanism by which BAG-1 affected the function of Bcl-2 was exploredby using the cycloheximide chase assay.Results: The BAG-1 p50 and p46 isoforms significantly enhanced the resistance to apoptosis in both cell lines according to flow cytometry analysis.BAG-1 p33 and p29 failed to protect the transfected cells from apoptosis.The cell viability assay showed that only BAG-1 p50,but not p46,p33,or p29,increased estrogen-dependent function in ER-positive cell line MCF-7.Only BAG-1 p50 dramatically increased its anti-apoptotic ability in the presence of estrogen,while estrogen has very little effect on the anti-apoptotic ability of other BAG-1 isoforms.In the detection of the expression of K-ras,Hsp70,cytochrome c,Raf-1,ER-α,and Bcl-2 in MCF-7 cells by Weste blot,only Bcl-2 protein expression was significantly increased in MCF-7 cells transfected with BAG-1 p50 and p46,respectively.Furthermore,the cycloheximide chase assay indicated that the degradation of Bcl-2 protein was extended in the BAG-1 p50 and p46 transfected MCF-7 cells.Conclusion: Distinct isofors of BAG-1 have different anti-apoptotic functions in breast cancer cells,and that the BAG-1 p50 isoform can potentiate the role of estrogen in ER-positive breast cancer.