Hormone receptors act as the essential link between circulating hormones and molecular responses within target cells. Cell growth and function is regulated by a variety of extracellular signals, including not only hormones (in addition to the classical hormones, several so-called vitamins are now recognized to be hormones) but also growth factors and cytokines. Because there are very close similarities between the receptors for all three of these categories of effector, it seems sensible to include all three within this article. For example, the receptor for granulocyte macrophage colony stimulating factor (GM-CSF ), a class I cytokine receptor, is very similar in both structure and function to the receptors for growth hormone and prolactin (1). These are all now grouped as members of the class 1 cytokine receptor superfamily.
This classification of GM-CSF receptor along with growth hormone (GH) receptor immediately implies that there are a range of receptor families. Firstly, although the majority of hormones, growth factors, and cytokines have their receptors in the plasma membrane of target cells, molecules such as steroids, thyroid hormones, and vitamins A and D are all membrane-soluble, so they can cross the cell membrane down a concentration gradient. For this reason, their receptors are located within the cell. Thus, the first classification is by site: Receptors are either (a) within the plasma membrane or (b) within the soluble part of the cell.
The receptors found in the soluble part of the cell are all members of the same superfamily and are called the steroid receptor superfamily, which will be discussed in more detail later. The remaining receptors, perhaps best called transmembrane receptors, can be subclassified in a variety of ways. All have an extracellular domain (usually quite heavily glycosylated), a membrane-spanning domain, and an intracellular domain. The simplest is to classify them according to the nature of the membrane-spanning domain, but adding in some component of their cell signaling mechanism. This method of classification gives rise to four classes of transmembrane receptor. Firstly, there is the class of receptor in which there is a single membrane-spanning chain and the internal domain has no intrinsic tyrosine kinase activity. Examples of this are the growth hormone receptor, prolactin receptor, GM-CSF receptor, interleukin-3 receptor, and so on. Secondly, there is the single membrane-spanning chain that does have intrinsic tyrosine kinase activity, and this is characterized by the erbB family (eg, EGF receptor, erbB-2, etc.), the insulin receptor, platelet-derived growth factor (PDGF) receptor, and so on. Thirdly, there is the seven-helix membrane-spanning chain coupled to G-protein. This represents the majority of hormone receptors and neurotransmitters—for example, receptors for the gonadotrophs (leutinizing hormone, follicle stimulating hormone), glucagon, or adrenaline. For completion, there is the group of several (usually around 12) membrane-spanning subunits that permit the gated passage between extracellular and intracellular compartments for ion exchange.
It is not the purpose of this entry to consider cell signaling, and readers interested in the details of activation by receptors of G-proteins, adenylate cyclase, ras, raf, mitogen-associated protein (MAP) kinases, tyrosine kinases [including Janus kinases (Jaks)], signal transducers and activators of transcription (Stats) should consult the appropriate entry. However, hormone receptors would be of little use if the binding of ligand to the receptor did not result in activation of one or more of these pathways. In the case of the growth hormone, prolactin, and GM-CSF family of receptors, there is clear evidence of overlap between the Jak-Stat and Ras/Raf/MAP kinase pathways (2).
Hormone receptors must be biologically selective. This means that any ligand that activates a specific receptor must be associated with the physiological responses associated with that receptor. For example, estradiol-17b is an 18-carbon steroid that induces estrogenic responses through the estrogen receptor. Estrogen-17a has virtually no estrogenic activity. In studies on the binding of ligand to receptor, estradiol-17b binds to estrogen receptor with a dissociation constant of about 10 10 M, whereas the 17a isomer has a 100-fold lower affinity for the receptor, indicating that the hydroxyl group on carbon-17 plays an important role in the binding of ligand to receptor. However, diethylstilbestrol (DES), which is not a steroid and, on paper, bears little resemblance to the fused ring system of the steroids, has estrogenic activity at an equivalent dose to estradiol. Not surprisingly, in competition assays, estradiol and DES turn out to have very similar binding affinities for the estrogen receptor.
The class 1 cytokine receptor superfamily has already been mentioned, and it gives another interesting example of the importance of the nature of ligand receptor interaction. The three cytokines GM-CSF, IL-3, and IL-5 all have similar three-dimensional structures, in that they each contain a region with four a-helices. The receptors for all three ligands are all heterodimers made up of a- and b-subunits, but the b-subunit is common to all three. On examining ligand-receptor interaction (3), it was found that the a-helix nearest the ^-terminal end of the molecule interacts specifically with the common b-subunit of the receptor, whilst the a-helix nearest the C-terminal end gives the biological specificity because it will only interact with the a-subunit of the receptor that is specific for that ligand. The common b-subunit is the one that is involved in the cell signaling processes.
This use of a common subunit for cell signaling is not restricted to the class 1 cytokine receptor superfamily. For example, the receptors for luteinizing hormone (LH), follicle stimulating hormone, b-human chorionic gonadotrophin, and thyroid stimulating hormone are again all heterodimers with the b-subunit again being common—this time to act as the activator of the G-protein signaling system.
One of the interesting features of hormone receptors is that of selectivity in terms of target cell type. For example, a fat cell (adipocyte) will have receptors for LH, glucagon, and so on. If the cell is fully activated by LH, then that activity level of the target enzyme, hormone-sensitive lipase, will be maximal ie, addition of glucagon will give no further activity. However, if the dose of LH used is below saturating, then addition of an appropriate dose of glucagon would push lipase activity up to the maximum. If the fat cells are replaced by Sertoli cells, however, there are no glucagon receptors and so glucagon will have no effect on such cells. Sertoli cells do have LH receptors but do not contain hormone-sensitive lipase. Instead, they are rich in the enzymes required for metabolism of cholesterol, and binding of LH to its receptors on Sertoli cells will result in an increase in synthesis and secretion of testosterone. Thus a common receptor can, nevertheless, give rise to different physiological end-points in different target cells.
The steroid receptor superfamily, which represents the receptors for those molecules which can diffuse across the plasma membrane, has been an important target for studies on regulation of gene expression. It has been recognized for many years that the effects induced by the majority of plasma membrane receptors are short-term (can be seen within seconds of ligand binding to receptor and are complete within a few hours, at most). Responses induced through the steroid receptor superfamily are generally long-term. The reason for this is that the hormones that act through transmembrane receptors are regulating the activity of preexisting enzymes, usually by increasing or decreasing the level of phosphorylation of specific serine, threonine or tyrosine residues. The steroid receptor superfamily acts through modulation of transcription of specific genes, so that it is the amount of total protein (enzyme) that is changed, rather than the activity of a preexisting protein.
Historically, long before receptor activation of gene transcription was proven, it was recognized that steroids induce synthesis of fresh enzyme. For example, glucocorticoid induction of tyrosine aminotransferase synthesis in hepatocytes has been known for many years. All members of the steroid hormone receptor superfamily contain similar domains (4). In particular, they all contain a DNA-binding domain that is remarkably well conserved among different members of the superfamily (receptors for estrogen, androgen, progesterone, glucocorticoid, mineralocorticoid, thyroid hormones, vitamin A derivatives, and vitamin D derivatives). They all contain a ligand-binding domain, and most (not glucocorticoid receptor) contain a nuclear localization sequence (see Nuclear Import, Export). This means that, as each receptor comes off its polysome, it is transported into the nucleus, and the empty receptor is located in the soluble part of the nucleus of target cells. Another common feature is that both the C- and N-termini of the molecule contain transactivation regions that are essential for the final activation of gene expression through interaction with various transcription factors (6).
Empty steroid receptor is found in association with various heat-shock proteins. In simplistic terms, these proteins are responsible for transporting the receptor to the nucleus and for preventing its binding to the DNA prior to the arrival of the hormone. For example, sites for binding a dimer of heat shock protein-90 (hsp90) are recognized in both the DNA-binding domain and the ligand-binding domain. Once ligand arrives and binds to the ligand-binding site, the receptor molecule undergoes some degree of allosteric change such that the hsp-90 molecule disengages and exposes not only the DNA-binding domain, but also a dimerization site. Thus two molecules of receptor, each with hormone attached, come together to form a dimer. This dimer now has high affinity for the specific sequence of nucleotides found in the appropriate hormone response element, which is normally (but not always) upstream of the structural gene(s) known to be activated by that hormone within the particular target cell. In order to get full activation of the gene, the dimer must have both transactivation sites functional, and all appropriate transcription factors must be in place.
Functional hormone receptors are essential to life. Many endocrine diseases are now recognized to occur because of changes in the activity of different hormone (growth factor or cytokine) receptors. For example, cases of diabetes can be due to loss of, or fall in, activity of the insulin receptor. Assays of receptor function are now becoming very important in various areas of medical diagnosis and treatment. A variety of diseases of the digestive tract may be ascribed to over- or underactivity of the epidermal growth factor receptor; and controlled use of selective tyrosine kinase inhibitors that block the function of the EGF receptor, but do not significantly effect other tyrosine kinase-mediated responses, may be very useful in treating such clinical problems.
Treatment for breast cancer is now closely dependent on the detection of estrogen receptor in the primary disease. Patients whose tumors do not contain functional estrogen receptor will not respond to endocrine therapies and thus can be put immediately onto another type of therapy. Previously, it was often the case that all patients received initial additive endocrine therapy and were only switched to alternative therapies once they had failed to show any response to the endocrine therapy. Now, they should be getting a more appropriate therapy at an earlier stage of the disease, at which time it has more chance of success.
Because of the increasing clinical importance of hormone receptor assays, there is much more interest in the reliability of such assays. A number of external quality control assays have been set up. Biochemical assay of the estrogen receptor is very well established, and the results from very large studies are available to give a good guide to expected results (7). As many labs begin to switch from biochemical to immunohistochemical (IHC) assays, it is important to establish the same external QA for these. One such QA scheme has been set up for the IHC of estrogen receptors in breast cancer biopsies and has shown that remarkably good agreement can be reached by quite large numbers of participating labs (8).
Hormone receptors mediate the physiological response of specific target cells to external signals. Depending on the natures of both the ligand and the receptor, the response can be short- or long-term. The chemical nature of the ligand determines whether the receptor will be membrane-bound or present within the internal soluble components of the target cell. A target cell for any specific external effector is defined by the presence of that receptor on or within the cell.
Plasma membrane receptors may be subclassified according to the structure of the transmembrane region and the nature of the subsequent cell signaling mechanism. These receptors act, principally, by altering the activity of preexisting proteins through increasing or decreasing the amount of phosphorylation on specific serine, threonine, or tyrosine residues. Soluble receptors are normally found within the nucleus and, when activated by ligand, become dimerized in such a way that they acquire high affinity for specific nucleotide sequences (hormone response elements) associated with the appropriate structural genes.
Much current medical research is directed toward modulating the activities of these different types of receptor through either directly blocking their action or interfering with the consequences of their activation (selective kinase inhibitors, phosphatases, etc.).