Selective Expression of Estrogen Receptor α and β Isoforms in Human Pituitary Tumors¹

Abstract

The physiological effects of estrogen on the pituitary, including cellular proliferation and regulation of hormone synthesis, are mediated by the nuclear estrogen receptor (ER). The ER acts as a dimer to modulate gene transcription and contains specific functional domains encoded in different exons. Two separate, but related, forms of the receptor (ERalpha and ERbeta) exist, with distinct tissue and cell patterns of expression. Additional ER isoforms, generated by alternative messenger ribonucleic acid (mRNA) exon splicing, have been defined in several tissues and are postulated to play a role in tumorigenesis or in modulating the estrogen response. We examined 71 human pituitary adenomas of varying phenotypes and 6 normal pituitary specimens for ER mRNA forms by RT-PCR and hybridization blotting analysis. All prolactinomas (n = 14) contained ERalpha, and several contained ERbeta (5 of 14) mRNA. In comparison, 6 tumors that expressed PRL and GH expressed ERbeta (4 of 6) more frequently than ERalpha (3 of 6). ERbeta mRNA was also found more frequently in null cell (8 of 24 ERalpha and 14 of 24 ERbeta) and gonadotrope (13 of 21 ERalpha and 18 of 21 ERbeta) tumors. Additionally, ERbeta was found in 4 of 6 tumors that contained only GH, although ERalpha was not observed in this tumor type. Expression of the two ER forms within a tumor type was overlapping, but some tumors contained only 1 isoform. Expression of ERalpha mRNA splice variants also varied with cell type. All normal pituitaries contained ERalpha deletions of exon 4, 5, and 7, whereas only 2 of 6 samples contained the exon 2 deletion variant. Although the same ERalpha mRNA variants were observed among the various tumor types, the proportion of specific splice variants expressed varied. For example, most ER-positive prolactinomas expressed ERalpha variants with deletions of exon 2, 4, or 5, whereas gonadotropin tumors preferentially expressed the ERalpha exon 7 deletion variant. A novel ERbeta mRNA splice variant, missing exon 2, was observed in a majority of all ERbeta-positive tumors. Immunoblotting analysis of ERalpha and ERbeta proteins supported the mRNA results. Because ERalpha and ERbeta have different biological responses to selective ER modulators, and the ER deletion variants have biological effects distinct from those of the full-length ER, expression of these isoforms may influence the biological properties of these tumors and affect their ability to respond to estrogen and antiestrogen therapies.

How Valuable Is the RT-qPCR of Pituitary-Specific Transcription Factors for Identifying Pituitary Neuroendocrine Tumor Subtypes According to the New WHO 2017 Criteria?

“…ERα has been limited to LTs and GTs, where it could modulate gonadal steroid estrogen (E 2 )-mediated gene expression [23]. In a previous study of 71 human PitNETs, the analysis of ER mRNA by RT-PCR showed significant expression of ERα in all LTs and in 61% of GTs [24]. Simultaneously, another study evaluating the mRNA expression of ERα and ERβ in 38 PitNETs found co-expression of both receptors in 60% of LTs and 29% of GTs.…”

Oestrogen Receptors, Receptor Variants and Oestrogen Actions in the Hypothalamic‐Pituitary Axis

“…In females, disruption of the ERβ gene results in reduced litter size (40), but this subfertility is suggested to occur through abnormalities in other reproductive tissues; in males, there is little decrease in fertility (41, 42). These models suggest that only ERα plays an important role in the pituitary, even though low levels of ERβ mRNA expression have been described in nearly every adult pituitary cell type (34, 43–46). However, ERβ may play a role in oestrogen actions in specific cell types, or during specific developmental stages.…”

“…However, ERβ may play a role in oestrogen actions in specific cell types, or during specific developmental stages. For example, in human pituitary, only ERβ is expressed in corticotropes and somatotropes, and this may explain some direct effects of oestrogen on ACTH and GH that were puzzling in cells that do not express ERα (43, 44). Few studies have measured ERβ protein, and the protein expression may have a more restricted pattern than the mRNA (47).…”

“…These responses can either be stimulatory, as for human ERαδ3 in HeLa cervical cells on an AP1‐containing promoter (70), or inhibitory, as observed with human ERαδ3 and δ5 in breast cells with an ERE promoter (70), or rat ERαδ4 and ERαδ3 in GH4 pituitary cells with an ERE promoter (71). Several ERα mRNA splice variants have been identified in human pituitary tumours (43, 44), with some selectivity of expression among various cell types, but it is unclear if significant amounts of ER variant proteins are made in any human tissue.…”

Truncated Estrogen Receptor Product-1 Suppresses Estrogen Receptor Transactivation by Dimerization with Estrogen Receptors α and β

“…ER and other steroid receptors are thought to act primarily as homodimers, although ER␣-ER␤ dimers can be observed on DNA and in solution (5)(6)(7). Because the isoforms are expressed in a tissue-and cell-specific manner (8,9) and have differential affinities for synthetic and environmental ligands (10), dimer formation can have important biological consequences. For example, ER isoforms ␣ and ␤ have markedly different responses to estrogen antagonists, and the ratio of the two isoforms could determine cellular responses to these drugs (10 -12).…”