Objective To investigate the roles of the cyclin D1/CDK4 and E2F-1/4 pathways and compare their work patterns in cell cycle changes induced by different doses of B[a]P. Methods Human embryo lung fibroblasts(HELFs)were treated with 2 μmol/L or 100 μmol/L B[a]P which were provided with some characteristics of transformed cells (T-HELFs).Cyclin D1,CDK4 and E2F-1/4 expressions were determined by Westem blotting.Flow cytometry was used to detect the distribution of cell cycle.Results After B[a]P treatment,the proportion of the first gap(G1)phase cells decreased.CDK4 and E2F-4 expression did not change significantly.In 2 μmol/L treated cells,a marked overexpression of cyclin D1 and E2F-1 was observed.However,in T-HELFs overexpression was limited to cyclin D1 only,and no overexpression of E2F-1 was observed.The decreases of G1 phase in response to B[a]P treatment were blocked in antisense cyclin D1 and antisense CDK4 transfected HELFs (A-D1 and A-K4)and T-HELFs(T-A-D1 and T-A-K4).Atier 2 μmol/L B[a]P treatment,overexpression of E2F-1 was attenuated in A-D1,and E2F-4 expression was decreased significantly in A-K4.In T-A-D1 and T-A-K4,E2F-4 expression was increased significantly,compared with T-HELFs.The E2F-1 expression remained unchanged in T-A-D1 and T-A-K4.Conclusions Cyclin D1/CDK4-E2F-1/4 pathways work in different patterns in response to low dose and high dose B[a]P treatment.In HELFs treated with 2 μmol/L B[a]P, cyclin D1 positively regulates the E2F-1 expression while CDK4 negatively regulates the E2F-4 expression;however,in HELFs treated with 100 μmol/L B[a]P,both cyclin D1 and CDK4 negatively regulate the E2F-4 expression.