Evidence of a progesterone receptor in the liver of the green frog Rana esculenta and its down-regulation by 17 beta estradiol and progesterone

Progesterone is a versatile hormone showing an ample variety of effects. On e of the numerous functions attributed to progesterone is the modulation of vitellogenesis in oviparous vertebrates. As a prerequisite for the possibl e involvement of progesterone in vitellogenesis modulation, we investigated the presence of a progesterone receptor (PR) in the liver of the female gr een frog Rana esculenta. H-3-Progesterone (H-3-P) binding activity was foun d in both cytosol and nuclear extract of the liver of Rana esculenta. The p rogesterone-binding moiety showed the typical characteristics of a true rec eptor, such as high affinity, law capacity, and specificity for progesteron e. It also bound to DNA-cellulose and was eluted with a linear salt gradien t at a concentration of 0.05 M of NaCl. The progesterone-binding moiety was down regulated by steroid hormones, in that ovariectomy resulted in a sign ificant increase, in both cytosol and nuclear extract, of H-3-P binding act ivity with respect to intact females. On the contrary, H-3-P binding activi ty was almost undetectable after estradiol and/or progesterone treatment. T he progesterone binding moiety of Rana esculenta was analyzed by Western bl otting with the aid of a monoclonal antibody raised against the subunits A and B of the chicken PR. An immunoreactive band of about 67 kDa was observe d in the liver of both intact and treated females. The 67 kDa band showed a n increased intensity in ovariectomized animals, while it was faint followi ng treatment with estradiol and/or progesterone. This is the first report on the presence of a progesterone receptor (PR) in the liver of an amphibian. PR of Rana esculenta is down regulated by estra diol and/or progesterone and shows peculiar immunological and biochemical c haracteristics, which make it rather different from the PR of other vertebr ates. (C) 1999 Wiley-Liss, Inc.