The level of DNA modification by (+)-syn-(11S,12R,13S,14R)- and (-)-anti-(11R,12S,13S,14R)-dihydrodiol epoxides of dibenzo[a,l]pyrene determined the effect on the proteins p53 and p21(WAF1) in the human mammary carcinoma cell line MCF-7

The polycyclic aromatic hydrocarbon (PAH) dibenzo[a,l]-pyrene (DB[a,l]P), t he most carcinogenic PAH tested in rodent bioassays, exerts its pathobiolog ical activity via metabolic formation of electrophilically reactive DNA-bin ding fjord region (+)-syn-(11S,12R,13S,14R)- or (-)-anti-(11R,12S,13S,14R)- DB[a,l]P-dihydrodiol epoxides (DB[a,l]PDEs), DB[a,l]P is metabolized to the se DB[a,l]PDEs which bind to DNA in human mammary carcinoma MCF-7 cells. Th e molecular response of MCF-7 cells to DNA damage caused by DB[a,l]PDEs was investigated by analyzing effects on the expression of the tumor suppresso r protein p53 and one of its target gene products, the cyclin-dependent kin ase inhibitor p21(WAF1). Treatment of MCF-7 cells with (+)-syn- and (-)-ant i-DB[a,l]PDE at a concentration range of 0.001-0.1 mu M resulted in DB[a,l] PDE-DNA adduct levels between 2 and 30, and 3 and 80 pmol/mg DNA, respectiv ely, 8 h after exposure. (-)-anti-DB[a,l]PDE exhibited a higher binding eff iciency that correlated with a significantly stronger p53 response at low c oncentrations of the dihydrodiol epoxides, The level of p53 increased by 6- 8 h after treatment. The p21WAF1 protein amount exceeded control levels by 12 h and remained elevated for 96 h, At a dose of 0.01 mu M (+)-syn-DB[a,l] PDE, an increase in p21WAF1 was observed in the absence of a detectable cha nge in p53 levels. The results indicate that the increase in p53 induced by DB[a,l]PDEs in MCF-7 cells requires an adduct level of similar to 15 pmol/ mg DNA and suggest that the level of adducts rather than the specific struc ture of the DB[a,l]PDE-DNA adduct formed triggers the p53 response. The PAH -DNA adduct level formed may determine whether p53 and p21(WAF1) pathways r espond, resulting in cell-cycle arrest, or fail to respond and increase the risk of mutation induction by these DNA lesions.