Phorbol ester inhibition of estrogen-induced uterine deoxyribonucleic acid synthesis

Protein kinase C (PKC) is a key regulatory enzyme in the control of growth and differentiated function in many cell types. Recently it has become clear that cross talk occurs between PKC and steroid hormone- activated signaling pathways. In this work we have thus used the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) to investigate the relationship between PKC activation and estrogen-induced proliferation in an in vivo model of hormone action, the immature rat uterus. A single injection of estradiol (E2) to immature female animals increases DNA synthesis in all major uterine cell types. Administration of TPA alone, simultaneous administration of TPA and E2, or administration of TPA 12 h after E2 did not alter uterine DNA synthesis. However, administration of TPA 6 h after E2 markedly decreased [3H]thymidine incorporation and the labeling indices in uterine epithelial, stromal, and myometrial cells. This inhibition represents a decrease in DNA synthesis per se rather than a change in the time course of the tissue response to the hormone. The inhibitory effect of TPA was reversible within 72 h and did not appear to be due to a decrease in the level or degree of occupancy of uterine estrogen receptors. These results suggest that a discrete regulatory event(s) in the pathway of estradiolinduced proliferation is inhibited by PKC activation approximately 6 h after hormonal stimulation.