Mechanisms Of Hormone Action: II Intracellular Receptors
Estrogen stimulates the proliferation of breast cancer tissue and exposure to estrogens may be important in the pathogenesis of this disease. During the treatment of women with breast cancer it is routine practice to establish the presence (ER +ve) or absence (ER -ve) of estrogen receptors in cancer cells. Women who have ER + ve tumours are more likely to respond to endocrine manipulation following surgery and/or chemotherapy (50–60% response rate in ER +ve cancers, 5–10% in ER−ve tumours). The most commonly used endocrine therapy is the drug tamoxifen which has estrogen-antagonist effects in the breast, probably mediated by the recruitment of corepressors for estrogen receptor action. It produces a significant fall in tumour recurrence and death rates for women with ER +ve disease, irrespective of age. The possible use of tamoxifen and the newer, selective estrogen receptor modulator drugs (SERMs, e.g. raloxifene, toremifine) for the prevention of breast cancer are under investigation. Trastuzumab, a humanized IgG1 against human epidermal growth factor receptor-2 (HER-2+), is now used to treat early breast cancer that overexpresses HER-2.
Lipophilic hormones, such as steroids and the thyroid hormones, pass easily through the plasma membrane into the cell, where they combine with specific receptor proteins (Fig. 4a). In the inactive state, for the subfamily of glucocorticoid, progesterone, estrogen and androgen receptors, the receptor is bound to a heat shock protein (HSP 90; Fig. 4b).
When the hormone binds to the receptor, the HSP dissociates from it, the receptors form homodimers and the hormone– receptor complex binds to DNA at specific sites, termed hormone response elements (HREs), which lie upstream from transcription initiation sites. Transcription and subsequent protein synthesis are altered. The thyroid hormone and retinoic acid receptors are not associated with HSPs in their inactive state, and are able to associate with their response elements on the DNA in the absence of the hormones, and act as transcription inhibitors (see also below). Activation of receptors expressing the actions of the hormones appears to be achieved through phosphorylation, although at present this process is poorly understood.
Nature of the steroid receptor
The steroid receptors form part of a larger ‘superfamily’ of nuclear DNA-binding receptors, including androgen, estrogen, glucocorticoid, thyroid and vitamin D receptors (Fig. 4c). They all have two main regions, a hydrophobic hormone-binding region and a DNA-binding region, which consists of two ‘zinc fingers’, rich in cysteine and basic amino acids. The structures of the receptors are known. Region 1 is the DNA-binding region, and is the most conserved among the members of the receptor family, in that it has a high sequence homology from receptor to receptor, as shown in Fig. 4c. It is thought that the first zinc finger determines the specificity of the binding of the receptor to DNA, while the second finger stabilizes the receptor to its response element of the DNA. Regions 2 and 3 of the receptors determine the hormone specificity of binding, and are not well conserved among the different receptors.
Two distinct, main receptor forms have been discovered, called ER-α and ER-β respectively. They have different affinities for estradiol and different anatomical distribution. For example only ER-α has been found in the liver, and ER-β is the pre- dominant form in prostate. These differences may account, in part, for the wide diversity of estrogen action in different tissues and under different physiological and pathological states. It has been found, for example, that in healthy ovarian tissue the β form predominates, but in ovarian cancer the a form predominates. It is possible that the β form somehow regulates the activity of the α form. The α and β forms have several nuclear coactivators and repressors, and their activity depends also on their rates of turnover.
Estrogen receptor antagonists have found a powerful use in the prevention and treatment of breast cancer (see Clinical scenario above). These compounds interfere with the processing of the normal intracellular hormone–receptor interaction. This can occur at one or more of several sites (Fig. 4d). The receptor itself may be blocked or post-receptor-binding events, for example receptor dimerization, receptor turnover or mRNA or protein synthesis, may be inhibited. Examples of estrogen receptor blockers are the SERMS (selective estrogen receptor modulators) such as tamoxifen, raloxifene and to remifene. These are interesting because they appear to act as agonists in some tissues such as bone and liver cells, and may therefore be important preventive measures for reducing the rate of development of osteoporosis and for lowering blood cholesterol. SERMS may act by activating as yet unidentified coactivators or corepressors and may modulate estrogen receptor turnover. Their action may also be dictated by whether they combine with ER-a or ER-b receptors.
Thyroid hormone receptors
Like other members of the nuclear receptor family, thyroid hormone receptors function as hormone-activated transcription factors. In contrast to steroid hormone receptors, however, thyroid hormone receptors bind to DNA in the absence of hormone, leading usually to transcriptional repression. When thyroid hormone binds to the receptor, however, it causes a conformational change in the receptor that changes it to function as a transcriptional activator. As with many other receptors, several isoforms have been discovered. Currently, four different isoforms are recognized, namely: α-1, α-2, β-1 and β-2. These different forms appear to be very important in development; different isoforms are expressed at different stages of development and in different organs and tissues. For example α-1, α-2 and βb-1 are expressed in virtually all tissues in which thyroid hormones act, but β-2 is synthesized mainly in the developing ear, and in the anterior pituitary gland and hypothalamus. Receptor α-1 is the first isoform detected in the conceptus, and the β form appears to be essential for normal brain development shortly after birth.