Role of Fluorescence Line-Narrowing Spectroscopy and Related Luminescence-Based Techniques in the Elucidation of Mechanisms of Tumor Initiation by Polycyclic Aromatic Hydrocarbons and Estrogens^†

Formation of DNA adducts by various carcinogens represents the first critical event in the mechanism of tumor initiation. The carcinogenic polycyclic aromatic hydrocarbons (PAHs) are biologically activated by two major mechanisms:  one-electron oxidation to produce radical cations and monooxygenation to form bay-region diol epoxides. The PAH−DNA adducts formed by these mechanisms are stable adducts that remain in DNA unless removed by repair and depurinating adducts that are lost from DNA by cleavage of the glycosyl bond. Identification of PAH−DNA adducts has relied heavily on low-temperature, laser-based fluorescence spectroscopy under non-line-narrowing (NLN) and line-narrowing (FLN) conditions. These spectroscopies can be used for chemical identification, conformational analysis, and/or probing the microenvironment of DNA (or protein) adducts. Small and co-workers have pioneered the use of FLN spectroscopy in this research. For example, the structures of the depurinating adducts formed by the PAHs benzo[a]pyrene, 7,12-dimethylbenz[a]anthracene, and dibenzo[a,l]pyrene have been elucidated. Understanding of the mechanism of tumor initiation by PAHs has relied on identifying and quantifying the DNA adducts formed. The insights gained from the study of PAH−DNA adducts enabled us to discover the estrogen metabolites that form depurinating DNA adducts and can be potential endogenous initiators of human cancer. Small and co-workers have also studied the estrogen−DNA adducts and estrogen−thioether conjugates by using FLNS and related luminescence-based techniques and have demonstrated that the level of the 4-hydroxyestrone-1-N3-adenine depurinating adduct in breast tissue from a woman with breast carcinoma was significantly higher than that in breast tissue from women without breast cancer. The fluorescence- and phosphorescence-based techniques they are developing will be applied to analyzing estrogen adducts and conjugates as biomarkers of susceptibility to breast and other types of human cancer.