Sex hormone receptor repertoire in breast cancer

Abstract

Classification of breast cancer as endocrine sensitive, hormone dependent, or estrogen receptor (ER) positive refers singularly to ER α . One of the oldest recognized tumor targets, disruption of ER α -mediated signaling, is believed to be the mechanistic mode of action for all hormonal interventions used in treating this disease. Whereas ER α is widely accepted as the single most important predictive factor (for response to endocrine therapy), the presence of the receptor in tumor cells is also of prognostic value. Even though the clinical relevance of the two other sex hormone receptors, namely, ER β and the androgen receptor remains unclear, two discordant phenomena observed in hormone-dependent breast cancers could be causally related to ER β -mediated effects and androgenic actions. Nonetheless, our understanding of regulatory molecules and resistance mechanisms remains incomplete, further compromising our ability to develop novel therapeutic strategies that could improve disease outcomes. This review focuses on the receptor-mediated actions of the sex hormones in breast cancer.

Figure 1

(a) Characterization of the estrogen receptor gene and protein. ERα and ERβ are encoded by two distinct genes; the ERα gene is localized to chromosome 6q24–27; the ERβ gene is located on chromosome 14q21-22. The gene transcripts are composed of 9 exons. The two encoded proteins differ in number of amino acids with ERα being slightly longer, 595 versus 530. Both proteins have five distinct domains, three of which have relative degrees of homology. The domain with the greatest disparity resides in the A/B domain, which may account for many of the antagonistic actions observed between the two ERs. (b) Characterization of the androgen receptor gene and protein. The human androgen receptor gene is situated on the long arm of the X-chromosome, q11-12. The organization of the protein coding region is similar to that of the estrogen receptor but is divided over 8 exons. The sequence encoding the N-terminal domain (NTD) is found in exon 1. Similar to the ER, the functionality of the DNA-binding domain (DBD) is provided by a motif comprised of two zinc fingers (UU) encoded by exons 2 and 3. The first zinc finger mediates DNA recognition while the second mediates DNA-dependent dimerization. Message for the ligand-binding domain (LBD) is distributed over the remaining five exons.

Figure 2

Monomeric modeling of ERα LBD. The LBD (a) is a three-dimensional configuration composed of three folded strata. The central core is comprised of helices 5, 6, 9, and 10, which is positioned between two layers, one composed of helices 1–4; the other helices 7, 8, and 11. The size of the binding cavity is approximately two-fold greater than the molecular volume of estradiol. E2 binds diagonally (b) between helices 11, 6, and 3 and induces a conformational change in the LBD. Devoid of contact with the ligand, helix 12 (in red) is repositioned, thus providing a protective seal over the binding cavity. Reorientation of helix 12 in this manner intrinsically generates AF2, recruits coactivators, and promotes transcription. However, when the receptor binds selective estrogen receptor modulator or SERM (c), the length of the SERMs side chain exceeds the confines of the binding cavity. As a result, helix 12 is misaligned over the binding pocket. Diffraction studies indicate helix 12 is rotated 130° toward the N terminus of the LBD.

Figure 3

Schematic representation of the genomic effect of estradiol. Nuclear translocation of ligand-bound receptor. (A) Activation of targeted genes in osteoblasts necessitates recruitment of two chromatin remodeling complexes known as SWI/SNF-A (switching defective/sucrose nonfermenting), as well as coactivators of the p160 family including SRC-1, SRC-2, and SRC-3 (steroid receptor coactivators 1, 2, and 3). All of the SRCs possess three nuclear receptor (NR) boxes located in the receptor interacting domain, which enables direct interaction with ERα; and two activation domains, AD1 and AD2, which serve as binding sites for p300/CBP (cointegrator-associated protein), and CARM1 (coactivator-associated arginine methyltransferase 1). The importance of these coregulators, especially p300/CBP, relates to the latter's interaction with the AF-2 domain of the ERs, which prompts recruitment of histone acetylases to the receptor. Together with the SRC complexes, the epigenetic enzyme disrupts DNA stability, thus allowing transcription of the target genes. (B) Repression of transcription occurs in a manner opposite to that of activation. Upon SERM binding, the receptor undergoes conformational changes that enhance interactions with corepressors. Three of the most well-known repressors of ERα-mediated transcriptional activity are NCoR (nuclear corepressor), silencing mediator of retinoid and thyroid receptor (SMRT), and repressor of ERα activity (REA). This transcriptional comodulatory apparatus results in inhibition of transcription.

Figure 4

ERβ splice variants. The AF1 and AF2 domains exhibit the least homology between ERβ1 and ERα. ERβ2 is nearly identical to ERβ1 except that the last 61 amino acids (aa) are replaced by 26 novel aa (indicated by the green portion of the receptor). As such, the β2 splice variant is comprised of 495 aa. Similarly, the last 65 aa of ERβ5 has been replaced by 7 other aa resulting in a 472 aa protein. Of note, truncation of the LBD/AF1 domains of β2 and β5 results in loss of ligand-binding activity, while conservation of their DBD maintains the likelihood of ERE binding. Estrogen binding to the delta5 splice variant is compromised by deletion of nucleotides 812–950 (139 base pairs (bp)) in the LBD. The result of the 18 aa insertion into what is essentially ERβ2 markedly diminishes ligand binding.

Figure 5

Sex steroidogenesis via the intracrine pathway. Androgens and estrogens synthesized in peripheral tissue including the breast require the presence of DHEA, an inactive precursor derived from the adrenal gland, and all of the appropriate enzymes. In addition to their synthesis, sex steroids are also inactivated in the same tissue thus minimizing widespread systemic effects. DHEA = dehydroepiandrosterone; DHEA-S = DHEA sulfated; 4-dione = androstenedione; A-dione = 5α-androstenedione; 3α-, 3β-, and 17βHSD = hydroxysteroid dehydrogenase; 5α-red = reductase.