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 estradiol-induced proliferation is inhibited by PKC activation approximately 6 h after hormonal stimulation.