Markku Pelto‐Huikko

We have cloned a novel member of the nuclear receptor superfamily. The cDNA of clone 29 was isolated from a rat prostate cDNA library and it encodes a protein of 485 amino acid residues with a calculated molecular weight of 54.2 kDa. Clone 29 protein is unique in that it is highly homologous to the rat estrogen receptor (ER) protein, particularly in the DNA-binding domain (95%) and in the C-terminal ligand-binding domain (55%). Expression of clone 29 in rat tissues was investigated by in situ hybridization and prominent expression was found in prostate and ovary. In the prostate clone 29 is expressed in the epithelial cells of the secretory alveoli, whereas in the ovary the granuloma cells in primary, secondary, and mature follicles showed expression of clone 29. Saturation ligand-binding analysis of in vitro synthesized clone 29 protein revealed a single binding component for 17beta-estradiol (E2) with high affinity (Kd= 0.6 nM). In ligand-competition experiments the binding affinity decreased in the order E2 > diethylstilbestrol > estriol > estrone > 5alpha-androstane-3beta,17beta-diol >> testosterone = progesterone = corticosterone = 5alpha-androstane-3alpha,17beta-diol. In cotransfection experiments of Chinese hamster ovary cells with a clone 29 expression vector and an estrogen-regulated reporter gene, maximal stimulation (about 3-fold) of reporter gene activity was found during incubation with 10 nM of E2. Neither progesterone, testosterone, dexamethasone, thyroid hormone, all-trans-retinoic acid, nor 5alpha-androstane-3alpha,I7beta-diol could stimulate reporter gene activity, whereas estrone and 5alpha-androstane-3beta,17beta-diol did. We conclude that clone 29 cDNA encodes a novel rat ER, which we suggest be named rat ERbeta to distinguish it from the previously cloned ER (ERalpha) from rat uterus.

The estrogen receptor (ER) is a ligand-activated transcription factor that mediates the effects of the steroid hormone 17 beta-estradiol, in both males and females. Since the isolation and cloning of ER, the consensus has been that only one such receptor exists. The finding of a second subtype of ER (ER beta) has caused considerable excitement amongst endocrinologists. In this article, we present data regarding the genomic structure and chromosomal localization of the human ER beta gene, demonstrating that two independent ER genes do exist in the human. Furthermore, we present data regarding the tissue distribution of human ER beta, showing that this receptor is expressed in multiple tissues. For instance, ER beta is found in developing spermatids of the testis, a finding of potential relevance for the ongoing debate on the effects of environmental estrogens on sperm counts. In addition, we find ER beta in ovarian granulosa cells, indicating that estrogens also participate in the regulation of follicular growth in the human.

In the mouse mutant dysgenetic lens (dyl) the lens vesicle fails to separate from the ectoderm, causing a fusion between the lens and the cornea. Lack of a proliferating anterior lens epithelium leads to absence of secondary lens fibers and a dysplastic, cataractic lens. We report the cloning of a gene, FoxE3, encoding a forkhead/winged helix transcription factor, which is expressed in the developing lens from the start of lens placode induction and becomes restricted to the anterior proliferating cells when lens fiber differentiation begins. We show that FoxE3 is colocalized with dyl in the mouse genome, that dyl mice have mutations in the part of FoxE3 encoding the DNA-binding domain, and that these mutations cosegregate with the dyl phenotype. During embryonic development, the primordial lens epithelium is formed in an apparently normal way in dyl mutants. However, instead of the proliferation characteristic of a normal lens epithelium, the posterior of these cells fail to divide and show signs of premature differentiation, whereas the most anterior cells are eliminated by apoptosis. This implies that FoxE3 is essential for closure of the lens vesicle and is a factor that promotes survival and proliferation, while preventing differentiation, in the lens epithelium.