THE TRANSCRIPTIONAL EFFECTS AND DNA-BINDING SPECIFICITIES OF 17-BETA-ESTRADIOL AFTER DIMETHYLDIOXIRANE ACTIVATION

It was found recently that 17 beta-estradiol (E2) could be activated b y the epoxide-forming oxidant dimethyldioxirane (DMDO) resulting in th e inhibition of rat liver nuclear and nucleolar RNA synthesis in vitro (Carcinogenesis, 17, 1957-1961, 1996), To further study the mechanism of this inhibition, several synthetic DNAs with different base conten t and sequence were used to study the transcriptional effects and bind ing specificities of E2 after DMDO activation in vitro, The results sh ow: (1) E2 after activation had a strong inhibitory effect on the temp late function of both A-T and G-C containing double-stranded DNAs, e,g , poly[d(A-T)], polydG.polydC and poly[d(I-C)], and only a weak inhibi tion on the single-stranded DNA template, polydC. The inhibition was d ose-dependent, and only after DMDO activation. (2) H-3-IabeIed E2, aft er DMDO activation, was able to bind DNAs containing both A-T and GC b ases. The order of the binding preference was: calf thymus DNA > poly[ d(A-T)] > poly[d(G-C)], (3) The covalent binding nature of E2 to DNA a fter activation was further confirmed by P-32-post-labeling analysis u sing calf thymus DNA, (4) The absorption spectrum of E2 changed, after DMDO treatment, from a peak around 280-290 nm to 260-270 nm with a sh oulder appearing around 300-320 nm, These studies have not only confir med our earlier observation that E2, after DMDO activation, can inhibi t DNA-dependent RNA synthesis, but also provided new insights into the DNA-binding properties after activation. Additionally since epoxidati on is often required for the activation of chemical carcinogens to bin d DNA, these studies lend further support to our proposed hypothesis t hat E2 epoxidation may play an initiation role in estrogen carcinogene sis.