Regulation of deoxyribonucleic acid polymerase activity in uterine luminal epithelium after multiple doses of estrogen

Estradiol (E2) exerts both inhibitory and stimulatory effects on DNA synthesis in the rat uterine luminal epithelium (LE). This inhibitory effect is due to a shift in the time course of DNA synthesis, i.e. in animals receiving a single injection of E2, a peak of DNA synthesis occurs 24 h after treatment, but in animals receiving multiple injections of E2, DNA synthesis is suppressed until 10–12 h after hormone treatment ceases. In these previous studies LE DNA synthesis was assessed by measuring tritiated thymidine incorporation. In the present study, we sought to determine if the molecular basis for this decrease in DNA synthesis was due to a suppression of DNA polymerase activity in LE nuclei. Animals receiving a single injection of E2 exhibit a peak of nuclear DNA polymerase activity 20–24 h later. Animals receiving multiple injections of E2 (0, 12, 15, and 18 h) show more than a 50% decrease in DNA polymerase activity at 20–24 h, due to a shift in the maximum increase in enzyme activity to 32–36 h after the initial treatment. The observed differences between these groups are not due to different levels of DNase activity or different degrees of leakage of the nuclear enzyme. The observed enzyme activity is due to DNA polymerase-a, since it requires ATP as well as deoxyribonucleoside triphosphates, and is aphidicolin sensitive. These results indicate that the inhibitory effect of E2 on LE DNA synthesis is due at least in part to a suppression of nuclear DNA polymerasea activity.