Characterization of a cytoplasmic androgen receptor in the male secondary sexual character of green frog (Rana esculenta)

Celebrating 50+ years of research on the reproductive biology and endocrinology of the green frog: An overview

This issue is dedicated to the late Professor Giovanni Chieffi, and this article is an overview of the research on Comparative Endocrinology of reproduction using Rana esculenta (alias Pelophylax esculentus) as a model system. Starting from the early 1970s till today, a large quantity of work have been conducted both in the fields of experimental endocrinology and in the definition of the diffuse neuroendocrine system, with a major focus on the increasing role of regulatory peptides. The various aspects investigated concerned the histological descriptions of principal endocrine glands of the hypothalamic-pituitary-gonadal (HPG) axis, the localization and distribution in the HPG of several different substances (i.e. neurosteroids, hypothalamic peptide hormones, pituitary gonadotropins, gonadal sex steroids, and other molecules), the determination of sex hormone concentrations in both serum and tissues, the hormone manipulations, as well as the gene and protein expression of steroidogenic enzymes and their respective receptors. All together these researches, often conducted considering different periods of the annual reproductive cycle of the green frog, allowed to understand the mechanism of cascade control/regulation of the HPG axis of R. esculenta, characterizing the role of different hormones in the two sexes, and testing the hypotheses about the function of single hormones in different target organs. It becomes evident from the review that, in their simplest form, several features of this species are specular as compared to those of other vertebrate species and that reproduction in this frog species is either under endogenous multi-hormonal control or by a wide array of different factors. Our excursus of this research, spanning almost five decades, shows that R. esculenta has been intensively and successfully used as an animal model in reproductive endocrinology as well as several field studies such as those involving environmental concerns that focus on the effects of endocrine disruptors and other environmental contaminants.

Aromatase and testosterone receptor in the liver of the female green frog, Rana esculenta

In the green frog, Rana esculenta, a peculiar feature of female reproductive endocrinology is an high level of circulating testosterone. Although several hypotheses have been set out to explain this phenomenon, the testosterone specific roles in female anuran have not been yet fully explored. This study results propose a testosterone implication in liver vitellogenin synthesis control, since in ovariectomized frogs the hormone induces an increase of circulating vitellogenin. The testosterone action could depend on its local conversion to 17beta-estradiol by aromatase which is present in frog liver tissue. Liver aromatase activity ranges from 7.5 to 26 fmoles E2 formed/mg protein/h and results higher as long as liver is engaged in vitellogenin synthesis. Aromatase activity seems depend on testosterone since it decreases after ovariectomy and is restored by testosterone injection in ovariectomized frogs. In green frog liver, testosterone binding molecules are present both in cytosol and nuclei. These molecule binding properties (Kd and Bmax in nM range; t 1/2 = 85 min; specificity) are in line with those of testosterone receptor of other lower vertebrate target tissue. In liver nuclei, testosterone receptor level undergoes modification throughout the sexual cycle which almost coincides with that of plasma testosterone level and liver aromatase activity. This could indicate that the testosterone induction of liver aromatase in frogs is via the testosterone receptor, as reported for aromatase of mammalian brain tissues.

The androgen receptor mRNA is up-regulated by testosterone in both the Harderian gland and thumb pad of the frog, Rana esculenta

Alpha 32P-labelled cDNA probe from plasmid containing rat androgen receptor (rAR) has been tested in hybridization experiments using RNAs from the Harderian gland and thumb pad of the edible frog, Rana esculenta. Northern blot analysis has shown a high degree of homology between the rAR cDNA and the frog androgen receptor mRNA (fAR mRNA); this has been supported by both the hybridization conditions (high stringency) and the molecular size of fAR mRNA which is quite similar to those described in mammals (9.4 kb). The role of androgens has been further investigated with respect to the kinetics of expression of fAR mRNA in in vivo experiments. In both the Harderian gland and thumb pad, testosterone has increased the levels of fAR mRNA as compared with the untreated groups. The use of cyproterone acetate (CPA) in combination with testosterone has resulted in a loss of the increase in fAR mRNA as compared to testosterone-treated groups, while CPA alone has resembled the control group. In primary cultures of frog Harderian gland and thumb pad cells, the steady-state levels of fAR mRNA have been increased in the cells exposed to testosterone as compared to those not exposed. These findings confirm that, in these androgen target tissues, testosterone exerts an up-regulation on its own receptors, increasing the accumulation of fAR mRNA in the same way as oestrogens up-regulate the expression of their own receptors in Xenopus liver and oviduct cells.

Prolactin receptors in the male Rana esculenta

The binding of 125I-labeled ovine prolactin (oPRL) to membrane preparations of several tissues from the male green frog, Rana esculenta, collected during the year is reported. PRL binding to kidney fractions was generally high (range 5-45%). A maximum was observed in the month of October, whereas the lowest value was found during the summer season. The binding to skin fractions was equally high (range 5-25%) and the annual profile parallels that of renal fractions. In the liver, a lower specific binding (range 3-4%) occurred consistently during the year, whereas no detectable binding was found in the muscle. The 125I-oPRL binding was inhibited by oPRL and oGH but not by oFSH or oLH. Scatchard analysis gave dissociation constants of 0.4-1 x 10(-10) M and binding capacity of about 20 fmol/mg of membrane proteins was observed in both the skin and kidney fractions. No receptor sites were detectable in 30-day hypophysectomized animals. The administration of oPRL or a crude homogenate of the frog hypophysis induced the appearance of specific PRL binding. Testosterone is able to restore prolactin binding in hypophysectomized animals, as PRL treatment does.

Androgen binding in the skin of mature male brown trout, Salmo trutta L

Specific, saturable binding of [3H]testosterone (Kd = 12.5 nM, Nmax = 1.1 pmol mg-1 protein) has been identified in skin cytosol of mature male brown trout. Binding with this affinity and capacity resembles more closely that observed in the plasma of mature male brown trout (Kd = 20.6 nM, Nmax = 6.6 pmol mg-1 protein) than that expected of a specific steroid receptor. However, the fraction of skin cytosol precipitating with 30% ammonium sulphate has a higher affinity for testosterone (Kd = 1.9 nM) and a lower capacity (Nmax = 96.3 fmol mg-1 protein) than does whole cytosol. Furthermore, the high-salt extract of crude skin nuclear pellet binds [3H]testosterone with affinity (Kd = 1.3 nM) and capacity (Nmax = 69.4 fmol mg-1 protein) similar to those of ammonium sulphate-precipitated cytosol. Specific binding in all three fractions of skin is abolished by treatment with proteolytic enzymes and a component of both cytosol and nuclear extract, which binds [3H]testosterone specifically, is retained on DNA-cellulose columns, eluting with high-salt buffer. Specifically bound [3H]testosterone is displaced most efficiently by testosterone. 5 alpha-dihydrotestosterone, estradiol-17 beta, and 11-ketotestosterone also compete but are 5- to 10-fold less potent. Consistent and reproducible binding of [3H]11-ketotestosterone to skin cytosol fractions or nuclear extract could not be demonstrated. It is concluded that testosterone binding with the above characteristics partially fulfills the criteria required of a putative steroid hormone receptor. The results are discussed with reference to the relative roles of testosterone and 11-ketotestosterone during sexual maturation.

The effects of thyroidectomy on androgen and prolactin receptors in the dorsal skin and caudal fin of Triturus cristatus carnifex

The interactions between thyroid hormones and receptors for steroid hormones and prolactin in dorsal skin and caudal fin of Triturus cristatus carnifex were studied during the annual cycle. Thyroidectomy induces an increase of prolactin binding in the dorsal skin and caudal fin in the animals captured in March. In these thyrodectomized animals the androgen receptors became undetectable. Results indicate that in Triturus cristatus carnifex the thyroid induces an increase of androgen receptors and a decrease, that is removed by thyroidectomy, of prolactin receptors.