The effect of estrogen on aromatase and vascular endothelial growth factormessenger ribonucleic acid in the normal nonhuman primate mammary gland

In the present study, the baboon was used as a model to investigate the eff ects of steroid hormones on vascular endothelial growth factor (VEG/PF) and aromatase expression and on proliferation of the normal mammary gland. Imm unocytochemistry revealed that both aromatase and VEG/PF acre expressed in the epithelial cells of the terminal ductal lobular units. Mammary tissue b iopsies were obtained from female baboons during the follicular and luteal phases of the menstrual cycle, 4 weeks after ovariectomy (OVX), and after 2 weeks of treatment with estradiol benzoate (E2B; 500 mu g/day, im). Althou gh there was little apparent difference in aromatase messenger ribonucleic acid(mRNA) in tissue from follicular and luteal phases or after ovariectomy , aromatase mRNA was decreased in tissue from ovariectomized (OVX) animals treated for 2 Reeks with E2B. Furthermore, aromatase activity in tissue fro m these animals was markedly reduced compared to activity in tissue from th e OVX animals before treatment (P < 0.001). In one animal in which mammary aromatase activity was measured sequentially during the follicular and lute al phases, aromatase activity was increased significantly after OVX and was reduced to the level in the intact animal by subsequent treatment with E2B . This effect on both aromatase activity and mRNA occurred rapidly 2 and 4 h after injection with E2B, in contrast to its effect on aromatase, E2B tre atment of OVX animals stimulated VEG/PF mRNA 2 and 4 h after injection. In histoculture of mammary biopsies from these animals in the follicular and l uteal phases of the menstrual cycle or after OVX, [H-3]thymidine incorporat ion was increased significantly by incubation with testosterone (T) as well as estrogen (P < 0.01). The effect of T was blocked by aromatase inhibitor , 4-hydroxyandrostenedione, suggesting that the tissue is responsive to E p roduced by aromatization of T in the tissue. When mammary tissue from OVX a nimals was cultured with T, there was a significantly greater increase in [ H-3]thymidine incorporation than in histocultures of tissue from intact ani mals (P < 0.01). However, in histocultures of tissue from the OVX animals t reated with E2B (500 mu g) for 2 weeks, [H-3] thymidine incorporation was s imilar to the level in tissue of intact animals incubated with T. No signif icant changes occurred in [H-3]thymidine incorporation with the nonaromatiz able androgen dihydrotestosterone or progesterone alone. These findings sug gest that estrogens produced locally by aromatization of T have a functiona l role in mammary tissue. Aromatase expressed in the mammary gland could be important in maintaining local estrogen concentrations, particularly after menopause. Estrogen appears to regulate transcription of both aromatase an d VEG/PF in the mammary gland, suggesting a regulatory loop by which local estrogens could stimulate VEG/PF production. Thus, paracrine/autocrine mech anisms that can enhance the proliferation of malignant cells and their meta static spread already exist before transformation.