Progestational Activity of Natural and Synthetic Androgens

Uteroglobin, a progesterone‐regulated secretory protein in rabbit uterus, was used as a marker protein for studies on progestational activity of various natural and synthetic androgens. All the androgens investigated were able to induce uteroglobin synthesis in rabbit uterus; some of the synthetic androgens were even better inducers than progesterone itself. Our results suggest that androgens elicit their regulatory action on uteroglobin synthesis by way of progestin receptor mechanism, since: (i) There was an intimate correlation between the in vitro binding affinity to progestin receptor and the in vivo potency of the androgens to induce uteroglobin synthesis; (ii) Androgens were able to translocate cytosol progestin receptors to uterine nuclei; (iii) Dose‐response curves for uteroglobin induction were parallel for androgens and progestins, and (iv) Flutamide, a non‐steroidal antiandrogen, did not abolish androgen‐induced synthesis of uteroglobin or androgen‐promoted nuclear translocation of cytosol progestin receptors. Both progesterone and androgens seem to control uteroglobin synthesis through mechanisms involving formation of new mRNA species, since in each case there was an increase in the uterine preuteroglobin‐mRNA activity, as evaluated by a cell‐free in vitro translation assay, which correlated with the amount of uteroglobin secreted into the uterine fluid. Some of the androgens studied (7α, 17α‐dimethyl‐19‐nortestosterone, 17α‐ethyl‐19‐nortestosterone and 11‐methylene‐17α‐methyl‐19‐nortestosterone) enhanced uteroglobin synthesis to the same or greater extent than progesterone. Interestingly, these steroids are also known to be very potent androgens. The progestational actions of androgens may be applicable to human tissues, too, since all the androgens investigated were bound by the human uterine progestin receptor in a fashion identical with the rabbit receptor.