Response of human mammary epithelial cells to DNA damage induced by BPDE: Involvement of novel regulatory pathways

The responses of a line of normal human mammary epithelial cells, HME87, to treatment with the ultimate carcinogen benzo[a]pyrene diol epoxide (BPDE) were analyzed using a directed gene expression analysis technique, RAGE. Under conditions where cell number was decreased by 50% 24 or 48 h post-treatment, flow cytometry demonstrated no establishment of a G₁/S arrest nor induction of apoptosis; cells continued to enter S phase from G₁ for at least 24 h but were blocked at G₂/M. Using the RAGE technique, changes in gene expression were assayed for over 1000 genes, and multiple time-point data were collected for 90 genes. In accord with the cell cycle studies, expression of the p21-WAF1 gene, the major mediator of p53-dependent G₁/S arrest, did not increase until 24 h posttreatment. The expression of other target genes for transactivation by p53 was increased at early time points, including GADD45, an effector of the G₂/M checkpoint, and WIP1. Analyses of proteins in treated cells indicated that p53 was phosphorylated at Ser15 but not at Ser20 within 30 min of treatment, and this correlated with an increase in the total amount of p53 protein. Significant expression changes were noted in a number of transcription factor genes, including ATF3 and E2A, genes that have not been previously connected to a response to DNA damage involving bulky chemical adducts. In addition, expression of the XPC gene was induced by BPDE treatment; the XPC product is thought to be involved in recognition of DNA damage by the nucleotide excision repair system.