ABSTRACT
Background:Clinical and epidemiological studies revealed that estrogen replacement therapy was associated with a significant reduction in risk of colorectal cancer in postmenopausal women.In our previous studies,estrogen increased the expression of mismatch repair (MMR)gene MLH1 in colonic cancer cells,and re-expression of MLH1 in MLH-deficient colonic cancer cells significantly increased the estrogen-induced apoptosis.Aims:To investigate the signaling pathway implicated in the MLH1-mediated apoptosis in colonic cancer cells induced by estrogen and the roles of p53 and its related genes in this apoptotic pathway.Methods:Plasmid containing wild type human MLH1 (hMLH1)full length cDNA was transfected into MLH1-deficient human colonic cancer cell line HCT116.By using HCT116 cells transfected with empty plasmid as controls,the apoptotic DNA ladder was determined by electrophoresis and the expressions of p53 and other apoptosis-related proteins were assessed by Western blotting under the condition with or without estrogen stimulation. Results:17β-estradiol (E2 )at the concentration of 10 -8 mol/L induced significant apoptosis in HCT116 cells transfected with hMLH1.In HCT116 cells transfected with hMLH1 and stimulated with E2 (group D),the protein expressions of caspase-3,caspase-9,p53,Bax and cytoplasmic cytochrome C increased significantly when compared with HCT116 cells stimulated with E2 only (group B);expressions of the abovementioned proteins were also higher in group D than in group C (transfected with hMLH1 only).Conclusions:MMR gene MLH1 is involved in estrogen-induced apoptosis of human colonic cancer cell line HCT116 by activating p53 signaling and mitochondrial apoptotic pathway.