Formation of 15alpha-hydroxyestriol and 15alpha-hydroxyestradiol from C19 15alpha-hydroxylated precursors in human pregnancy

Biosynthesis of estetrol in human pregnancy: Potential pathways

Estetrol (E4) has emerged as a novel and highly promising estrogen for therapeutic use. E4 is a weak natural estrogen produced only in pregnancy. Because of its novelty, there is considerable interest by clinicians in how it is produced in pregnancy. Although the fetal liver plays a key role in its production, the placenta is also involved. A current view is that estradiol (E2) formed in the placenta enters the fetal compartment and is then rapidly sulfated. E2 sulfate then undergoes 15α-/16α-hydroxylation in the fetal liver thereby forming E4 sulfate (phenolic pathway). However, another pathway involving 15α,16α-dihydroxy-DHEAS formed in the fetal liver and converted to E4 in the placenta also plays a significant role (neutral pathway). It is not known which pathway predominates, but both pathways appear to be important in E4 biosynthesis. In this commentary, we summarize the well-established pathways in the formation of estrogens in the nonpregnant and pregnant female. We then review what is known about the biosynthesis of E4 and describe the 2 proposed pathways involving the fetus and placenta.

Estetrol: a unique steroid in human pregnancy

Estetrol (E(4)) is an estrogenic steroid molecule synthesized exclusively by the fetal liver during human pregnancy and reaching the maternal circulation through the placenta. Its function is presently unknown. After its discovery in the mid-1960s, E(4) research revealed rather unique properties of this steroid and spawned a large body of state-of-the art publications. Nevertheless, 20 years later experimental work was virtually abandoned. In recent years based on new data, E(4) has experienced a vita nova, a revival of preclinical and clinical research activities with the goal to elucidate its physiological function and explore its potential for therapeutic use in humans. This review is intended to offer an historical account of the discovery of E(4) and the preclinical studies conducted during the heyday of E(4) research that ended in the mid-1980s.

15- and 16-hydroxylations of androgens and estrogens in the human fetal liver: a critical step in estetrol biosynthesis

To elucidate the main metabolic pathways which lead to the foeto-placental biosynthesis of estetrol (I), we investigated the 15 alpha- and 16 alpha-hydroxylations of potential precursors of this estrogen in the human fetal liver. We determined the 15 alpha- and 16 alpha-hydroxylation capacity of the fetal liver for each precursor by GC-MS. The results suggest that estetrol is derived only from estradiol sulfate (II) and DHEA sulfate (III). 15 alpha-Hydroxy-androstenedione (IV) can no longer be regarded as a good precursor of estetrol. The phenolic pathway appears to be a more likely route than the neutral pathway, even when derived from DHEA sulfate.

Aromatization of 15 alpha and 16 alpha hydroxylated androgens in the human placental using [1,2-3H]-substrates

This in vitro study reports data on the aromatization of [1,2-3H]-C19 steroids in the human term placenta [androstenedione (III), testosterone (IV), 15 alpha-hydroxy-androstenedione (V), 15 alpha-hydroxy-testosterone (VI), 16 alpha-hydroxy-androstenedione (VII)]. The hydroxylated androgens were microbiologically synthesized from commercially radiolabelled [1,2-3H]-androstenedione and testosterone. Androstenedione and testosterone were good substrates for the human placental aromatase (low Km values, high Vmax); they strongly inhibited the 15 and 16 hydroxylated androgens aromatizations. On the other hand, these hydroxylated compounds acted as poor substrates and were only non-competitive inhibitors of the androstenedione and testosterone aromatizations. However, 15 alpha-hydroxy-androstenedione could not be disregarded as a potential precursor of 15 alpha-hydroxylated estrogens in the human placenta.

Isolation fo 15alpha-hydroxypregnenolone and 15alpha-hydroxydehydroisoandrosterone from human pregnancy urine

15alpha-Hydroxydehydroisoandrosterone and 15alpha-hydroxypregnenolone were isolated from hydrolyzed extracts of human late pregnancy urine and identified by means of the isotope dilution technique. In two separate determinations the excretion rate of 15alpha-hydroxydehydroisoandrosterone was found to be 1.7 and 3.2 microgram per day while that of 15alpha-hydroxypregnenolone was 1.7 and 2.9 microgram per day. It is postulated that 15alpha-hydroxydehydroisoandrosterone might serve as a precursor of 15alpha-hydroxylated estrogens already isolated from late pregnancy urine. Similarly, 15alpha-hydroxypregnenolone might be an endogenous precursor of 15alpha-hydroxyprogesterone.