Cellular Distribution and Gene Regulation of Estrogen Receptors a and b in the Rat Pituitary Gland *

The pituitary gland is a heterogeneous tissue comprised of several hormone secreting and supporting cells, most of which are targeted by estrogens. Estrogen-induced changes in the pituitary are presumably mediated via the classical estrogen receptor, ERa. However, a novel receptor, ERb, and pituitary-specific truncated estrogen receptor products (TERPs) were recently identified. The objectives of this study were to examine the distribution of these receptors in the rat pituitary and compare their regulation by estradiol in SpragueDawley and the estrogen-sensitive Fischer 344 rats. Pituitary cryosections were subjected to immunocytochemistry for specific cell types, followed by in situ hybridization for ERa or ERb. ERa was expressed by approximately 45% of the lactotrophs and melanotrophs, 35% of the corticotrophs and folliculo-stellate cells, and 25% of the gonadotrophs. The expression of ERb showed a similar pattern but was generally lower than ERa. In two cell types, melanotrophs and gonadotrophs, ERb expression was significantly lower than ERa. In the second experiment, pituitary sections were immunostained for ERa, followed by in situ hybridization for ERb. Only a minute population (6–10%) of either anterior or intermediate lobe cells coexpressed ERa and ERb. In the next experiment, Fischer 344 and Sprague-Dawley rats were injected with oil or estradiol for 24 h. Total RNA from dissected anterior and posterior (neurointermediate) pituitaries was subjected to RT-PCR for ERa, ERb, or TERPs. Interestingly, ERa and ERb were unchanged by estradiol in either lobe of the pituitary. In contrast, estradiol increased pituitary TERP messenger RNA levels 4to 7-fold. A 20-kDa TERP protein was detected by Western blots in the pituitary but not the uterus. There were no differences in the estradiol-induced expression of any of the receptors between the two strains of rats. We conclude that: 1) ERb is expressed in all anterior and intermediate lobe cell types examined, albeit at a lower level than ERa; 2) no more than 10% of pituitary cells coexpress ERa and ERb; and 3) estradiol markedly increases TERP messenger RNA levels but does not alter the expression of ERa or ERb. We propose that estrogen receptor heterogeneity contributes to the diversity of pituitary cell responsiveness to estrogens. (Endocrinology 139: 3976–3983, 1998) T PITUITARY gland is a heterogeneous tissue consisting of the anterior, intermediate, and neural lobes. The anterior lobe is comprised of five hormone-producing cells and the supporting folliculo-stellate cells. The intermediate lobe contains primarily melanotrophs, whereas pituicytes (astroglial cells) and nerve endings make up the neural lobe. Several pituitary cell types are known targets for estrogens, including lactotrophs (1, 2), gonadotrophs (3), folliculostellate (4, 5), and intermediate lobe cells (6, 7). The classical estrogen receptor (ERa), which has been localized to most pituitary cells (8), is thought to mediate the direct effects of estrogen in the pituitary. As depicted in Fig. 1, ERa is comprised of five functional domains: a DNA binding domain (DBD), a hinge region, a hormone binding domain (HBD), and two transactivation domains (AF-1 and AF-2). Upon ligand activation, the receptor dimerizes, binds to a consensus DNA sequence named the estrogen response element (ERE), and alters gene transcription (reviewed in Refs. 9 and 10). Several ERa isoforms have been identified, especially in cancer cells (11, 12), but their function remains unclear. The rat pituitary expresses truncated estrogen receptor products (TERPs) that have not been detected in other tissues (13–16). These receptors are truncated at exon 5 and have two isoforms, TERP1 and TERP2. The more abundant TERP1 has a unique 31-bp sequence, whereas TERP2 contains the same sequence as TERP1 with an additional 66 bp (Fig. 1). Neither the specific pituitary cells expressing TERP nor its function are