Regulation of uterine 5 alpha-reductase type 1 in mice.

A null mutation in the murine gene encoding steroid 5 alpha-reductase type 1 (5 alpha R1) leads to failure of normal parturition at term. This observation, together with the finding that mRNA levels of uterine 5 alpha R1 increase significantly at term in normal pregnant animals, indicates that 5 alpha R1 plays an important role in murine parturition. The current studies were conducted to elucidate the regulation of 5 alpha R1 in uterine tissues of nonpregnant and pregnant animals. Nonpregnant, ovariectomized ICR mice were treated with vehicle (control), 17 beta-estradiol (E(2)), progesterone (P(4) ), or E(2)+P(4) for 3 days. Thereafter, uterine tissues were obtained for histology, quantification of 5 alpha R1 specific activity, and Northern blot analysis of 5 alpha R1 mRNA expression. The 5 alpha R1 enzyme activity was significantly increased in animals treated with E(2)+P(4). However, activity was much less in uterine tissues from E(2)+P(4)-treated animals than in uterine tissues from pregnant animals near term. To evaluate further the regulation of 5 alpha R1 during gestation, mice underwent unilateral tubal ligation before timed matings. The 5 alpha R1 activity increased eightfold in uterine tissues from the fetal horn from Gestational Days 12 to 18. This temporal pattern in 5 alpha R1 activity paralleled marked increases in uterine diameter. Taken together, these studies indicate that expression of 5 alpha R1 is regulated by E(2)+P(4) in uterine tissues. Whereas E(2) alone is insufficient to induce enzyme activity, E(2) may be required to increase P(4) receptors and, thereby, mediate the effects of P(4) on 5 alpha R1 gene expression. Further increases in enzyme activity during late gestation are mediated by fetal occupancy, possibly through stretch-induced increases in endometrial growth. Thus, like other genes involved in parturition, expression of 5 alpha R1 is regulated by both hormonal and fetal-derived signaling pathways.