To the Editor:

Recent advances on the action of estrogens and progestogens in normal and pathological human endometrium

Hormonal control in the development of the normal endometrium is of the utmost importance. It is well established that the two main hormones involved in this process are estradiol and progesterone, which are also implicated in the pathological conditions concerning endometriosis and endometrial carcinoma. There are two types of endometrial carcinoma: type I which represents 80%–90% is hormone-dependent, whereas the remainder is type II and is hormone-independent. The endometrial tissue contains all the enzymatic systems in the formation and transformation of the various hormones, including aromatases, sulfatases, sulfotransferases, hydroxysteroid dehydrogenases, hydroxylases, and glucuronidases. It is interesting to note that increased sulfatase activity is correlated with severity of endometriosis. An increased sulfatase/sulfotransferase ratio represents a poor prognosis in patients with endometrial carcinoma. Treatment with hormone replacement therapy (estrogens+progestogens), as well as with tibolone, is most effective in protecting this tissue by climacteric alterations, owing to the significant decrease of ovarian hormones. In conclusion, enzymatic control can open appealing perspectives to protect this organ from possible pathological alterations.

Can 19-nortestosterone derivatives be aromatized in the liver of adult humans? Are there clinical implications?

CONTEXT

Previous studies in postmenopausal women have demonstrated that, after oral administration of norethisterone, a small proportion of the compound is rapidly converted into ethinylestradiol. The shape of the concentration - time curve suggested that this occurred in the liver. The results were confirmed by in vitro investigations with adult human liver tissue. In 2002, it was shown that, after oral treatment of women with tibolone, aromatization of the compound occurred, resulting in the formation of a potent estrogen, 7 alpha-methyl-ethinylestradiol. The result has been called into question, because the adult human liver does not express cytochrome P450 aromatase, which is encoded by the CYP 19 gene. Moreover, it has been claimed that the serum level of 7 alpha-methyl-ethinylestradiol measured by gas chromatography/mass spectrometry was an artifact.

REPLY

Aromatization of steroids is a complex process of consecutive oxidation reactions which are catalyzed by cytochrome P450 enzymes. The conversion of the natural C19 steroids, testosterone and androstenedione, into estradiol-17beta and estrone is dependent on the oxidative elimination of the angular C19-methyl group. This complex key reaction is catalyzed by the cytochrome P450 aromatase, which is expressed in many tissues of the adult human (e.g. ovary, fat tissue), but not in the liver. However, 19-nortestosterone derivatives are characterized by the lack of the C19-methyl group. Therefore, for the aromatization of these synthetic steroids, the action of the cytochrome P450 aromatase is not necessary and the oxidative introduction of double bonds into the A-ring can be catalyzed by other hepatic cytochrome P450 enzymes. The final key process in the formation of a phenolic A-ring, both in natural androgens and 19-nortestosterone derivatives, is the enolization of a 3-keto group to the C2-C3-enol or the C3-C4-enol moiety, which occurs without the action of enzymes.

CONCLUSION

19-nortestosterone derivatives (norethisterone, norethynodrel, tibolone) can readily be aromatized in the adult human liver. This leads to the formation of the potent estrogens ethinylestradiol from norethisterone or norethynodrel and 7 alpha-methyl-ethinylestradiol from tibolone. This may have clinical consequences, e.g. the elevated risk of venous thromboembolic disease in premenopausal women treated with high doses of norethisterone for bleeding disorders, or the elevated risk of stroke or endometrial disease in postmenopausal women treated with tibolone.