The use of intracellular receptors distinguishes classical steroid hormone signaling from signaling by hydrophilic signaling factors including peptide hormones and growth factors that use membrane-bound receptors. Intracellular receptors for steroids are located in the cytoplasm or in the nucleus of target cells. Steroid hormone receptors act as pisundra ligand-activated transcription factors, since their ligand (hormone) binds to them and activates them so that they can bind to DNA and regulate transcription (production of mRNA) for a specific gene, which is then translated into a protein.This classical form of steroid signaling is known as nuclear-initiated steroid signaling (NISS) (Figure 19.1). In this way, the steroid has an effect on the cell’s genome. It can take minutes, hours, or days for the effects of classical steroid hormone signaling to induce a biological response in the target cell in the form of production of a new protein.
FIGURE 19.1.Mechanisms of steroid signaling.
Within seconds or minutes after addition of some steroid hormones, some other signaling effects can be observed in target cells. These include changes in intracellular calcium concentration, activation of G proteins, and stimulation of protein kinase activity, which are not mediated by classical intracellular steroid receptors but through steroid receptors in the plasma membrane. Membrane-initiated steroid signaling (MISS) is now described in addition to classical steroid signaling. At present, less detailed information is known about MISS than about NISS, although MISS is an active area of research investigation. Both types of signaling are believed to be important for normal function of steroid hormones.Molecules that signal target cells using NISS and MISS include sex steroid hormones, glucocorticoids, and mineralocorticoids as well as vitamins A and D, retinoids, and thyroid hormones. Classical intracellular receptors have been grouped into two categories, type 1 receptors and type 2 receptors based on the details of their signaling mechanisms (Figure 19.2). Sex hormone, glucocorticoid, and mineralocorticoid receptors are type 1 receptors, while vitamin A, vitamin D, retinoid, and thyroid hormone receptors are type 2. In order to bind to and activate their intracellular receptors, the steroid hormones and vitamins must first move from the blood circulation across cell membranes. Steroid hormones are synthesized from a common precursor and have structures that enable them to enter into their target cells.
FIGURE 19.2.Categories of steroid receptors.
Cholesterol is the precursor of all classes of steroid hormones: glucocorticoids (e.g., cortisol), mineralocorticoids (e.g., aldosterone), and sex hormones—androgens, estrogens, and progestins (Figure 19.3). (Note: Glucocorticoids and mineralocorticoids are collectively called corticosteroids.) Cholesterol is first converted to pregnenolone and then to progesterone, which is a common precursor to all steroid hormones. Corticosteroids such as cortisol and aldosterone are produced from progesterone. While testosterone (an androgen) is also produced from progesterone, estradiol (an estrogen) is produced from testosterone (see LIR Biochemistry, Chapter 18 for more information regarding cholesterol).
FIGURE 19.3.Key steroid hormones produced from cholesterol.
Synthesis and secretion of steroid hormones occur in the adrenal cortex (cortisol, aldosterone, and androgens), ovaries and placenta (estrogens and progestins), and testes (testosterone) (Figure 19.4). Hormones exert their effects at the cellular level, as evidenced by aldosterone stimulation of renal reabsorption of sodium and excretion of potassium. Other biological effects of steroid hormones include cortisol’s stimulation of gluconeogenesis, estrogen’s regulation of the menstrual cycle, and testosterone’s promotion of anabolism. In order to exert such biological consequences, steroid hormones are transported by the blood from their sites of synthesis to their target organs.Because of their lipid nature and hydrophobicity, they must be complexed with a plasma protein in the aqueous environment of blood plasma. Plasma albumin can act as a nonspecific protein carrier and does carry aldosterone. However, specific steroid-carrier plasma proteins bind the steroid hormones more tightly than does albumin, for example, corticosteroid-binding globulin (transcortin) is responsible for transporting cortisol and sex hormone–binding protein transports sex steroids.
FIGURE 19.4.Actions of steroid hormones.
Inhibitors of steroid hormone synthesis as cancer therapy
Estrogen is derived from testosterone by the action of the enzyme aromatase. Aromatase inhibitors are used in the treatment of estrogen-responsive breast cancer in postmenopausal women. After menopause, the main source of estrogen is from aromatization of adrenal-produced androgens. Inhibitors of aromatase can reduce estrogen levels significantly and remove the main source of growth stimulation from estrogen-responsive tumors. Arrest of tumor growth and/or initiation of apoptosis (cell death) of estrogen-responsive breast tumors occur as a result of therapy with aromatase inhibitors.
NUCLEAR-INITIATED STEROID SIGNALING
In classical steroid signaling, NISS, steroid hormones must leave the circulation and cross the plasma membrane of a target cell. Once inside the cell, they will encounter a specific receptor in the cytosol or in the nucleus. Hormone binding modifies the receptor, enabling it to regulate the transcription of specific genes.
Intracellular receptor structure
Intracellular receptors for steroid hormones are a highly conserved group of proteins that contain three major functional domains (Figure 19.5). The hormone (or ligand)-binding domain is in the COOH-terminal region of the receptor protein while the NH2-terminal region contains the gene regulatory domain. The DNA-binding domain of the protein forms an additional functional region. This region is highly conserved and contains zinc finger motifs containing cysteine amino acid residues that bind zinc and dictate DNA sequences to which the receptor will bind. Since these receptor proteins must enter the nucleus in order to bind to DNA and regulate transcription, they contain nuclear localization signals (NLS) to permit their trafficking into the nucleus (see also Chapter 11, Figure 11.9 for more information on trafficking of proteins into the nucleus).
FIGURE 19.5.Structure of steroid hormone receptors.
Mechanism of nuclear initiated steroid signaling
In the absence of hormone, estrogen and progesterone receptors are principally located in the nucleus of the target cell and glucocorticoid and androgen receptors are located in the cytoplasm. Receptors for vitamins A and D, retinoids, and thyroid hormone (type 2 steroid receptors) are found in the nucleus (see also Figure 19.2). Regardless of the receptor’s intracellular location, binding of a steroid hormone to its intracellular receptor causes activation of the receptor and enables it to translocate to the nucleus (Figure 19.6). The steroid hormone–receptor complex binds to the hormone response element (HRE) of the enhancer region and activates the gene promoter, causing transcription.
FIGURE 19.6. The nuclear-initiated steroid signaling (NISS) mechanism involves the activation of transcription by interaction of steroid hormone–receptor complex with hormone response element (HRE).
1. Sex steroid receptors, glucocorticoid receptors, and mineralocorticoid receptors: The activated receptor-ligand complex associates with coregulator or coactivator proteins that promote transcription. The receptor-ligand-coregulator complex binds to regulatory DNA sequences called HREs through zinc finger motifs. Ligand-bound type 1 receptor complexes bind to DNA as homodimers (two identical ligand-receptor complexes binding together). Binding of the activated hormone-receptor complexes to an HRE positions the activated receptor so that its gene regulatory domain interacts with proteins of the transcriptional complex bound to a promoter.2. Vitamins A and D, retinoid, and thyroid hormone receptors: For these type 2 steroid receptors, unoccupied receptors are complexed in the nucleus with corepressor proteins, inhibiting them from inducing transcription. Ligand binding to the receptor causes release of the corepressor proteins and allows for binding to coactivator proteins. Other type 2 receptors form heterodimers with the retinoid X receptor when binding to DNA to regulate the transcription of vitamin- or hormone-responsive genes.
Hormone specificity of gene transcription
An HRE is found in the promoter (or an enhancer element) for genes that respond to a specific steroid hormone, thus ensuring coordinated regulation of these genes. For example, a glucocorticoid-response element or GRE allows for a transcriptional response to a glucocorticoid such as cortisol. Each of the cortisol-responsive genes is under the control of its own GRE. Binding of the receptor-hormone complex to the glucocorticoid receptor (GR) causes a conformational change in the receptor that uncovers its zinc finger DNA-binding domain (Figure 19.7).The steroid-receptor complex then interacts with specific regulatory DNA sequences and the hormone-receptor complex in association with coactivator proteins controls the transcription of targeted genes. Overall, this process allows for the coordinate expression of a group of target genes, even when these genes are located on different chromosomes. The GRE can be located upstream or downstream of the genes it regulates and is able to function at great distances from those genes. The GRE, then, can function as a true enhancer.
FIGURE 19.7. Transcriptional regulation by intracellular steroid hormone receptors.
GRE, glucocorticoid response element (an example of an HRE); GR, glucocorticoid receptor.
Hormone receptor antagonists as cancer therapies
Receptor antagonists bind to the hormone receptor and prevent binding of the natural hormone to its receptor. Selective estrogen receptor modulators (SERMs) are important therapies for treatment and prevention of breast cancer. Owing to their selectivity, SERMs have different effects in different tissues. One, tamoxifen, blocks estrogen receptors in breast, thereby inhibiting estrogen-dependent growth of tumors. Tamoxifen is used in premenopausal women with estrogen-receptor positive breast cancer. Tamoxifen has other effects in other tissues. For example, it can increase estrogen signaling in the endometrium, with the potential for endometrial malignancy.
MEMBRANE-INITIATED STEROID SIGNALING
Rapid effects of steroid hormones that occur within seconds to minutes after exposure of target cells to steroid hormones are now believed to result from actions of steroid receptors localized to the plasma membrane. MISS induces biological effects more quickly than classical NISS since it promotes modifications to existing proteins (e.g., phosphorylation) and does not require synthesis of new proteins. Details remain to be determined for many aspects of MISS; however, some details of this signaling process are known, particularly for membrane estrogen receptors. Membrane forms of androgen, glucocorticoid, progesterone, mineralocorticoid, and thyroid hormones have also been identified and have similar signaling processes to the membrane estrogen receptor.Evidence also exists for the presence of membrane-bound vitamin D receptors. Cross talk between intracellular and membrane-bound pools of steroid receptors is believed to occur and both NISS and MISS mechanisms can be used to evoke biological responses. Convergence of signals at the membrane, cytoplasm, and nucleus causes the overall biological effects of steroid hormones. For example, kinases activated by MISS may phosphorylate coactivators required for transcriptional activation via NISS. Additionally, signaling from membrane steroid receptors may contribute to gene transcription independently of nuclear steroid receptors.
Membrane steroid receptors are believed to have the same protein structure as intracellular steroid receptors but are localized to membrane caveolae, invaginated regions of the membrane that have a flask-like shape (Figure 19.8) (see also Chapter 3, Figure 3.13). In its membrane-bound form, the steroid hormone receptor may either associate with the outer surface of the plasma membrane in the flask of an individual caveola or may be tethered by a scaffolding protein to the plasma membrane. After the receptor is bound by its specific steroid hormone, it becomes activated and may form homodimers or heterodimers with other membrane steroid receptors.
FIGURE 19.8.Membrane-initiated steroid signaling (MISS). SH, steroid harmone.
Mechanism of membrane-initiated steroid signaling
Activated receptors bound by their specific steroid hormone then associate with a complex of signaling proteins that can include G proteins, growth factor receptors, the tyrosine kinase Src, and the GTP-binding protein Ras. The epidermal growth factor receptor (EGFR) is often implicated in MISS, and its activation can result in sustained MAP kinase signaling in a cell responding to a steroid via MISS. Second messengers can be induced and ion channels regulated. Protein kinases that often function in response to G protein activation, including serine/threonine kinases PKA and PKC, can be activated as well as PI3 kinase that functions in catalytic receptor signaling (see also Chapters 17 and 18). Phosphorylation of target proteins by activated kinases causes a rapid change in their activity and a rapid biological response by the cell.
- Classical steroid signaling involves the use of intracellular receptors that function as ligand-activated transcription factors and therefore regulate the synthesis of new cellular proteins. This form of steroid signaling is referred to as NISS.
- Steroid hormones are synthesized from cholesterol and exert actions on the adrenal cortex, the ovaries, and the testes.
- Intracellular steroid receptors are present in the cytosol or nucleus of the target cell. They contain ligand-binding, gene regulatory, and DNA-binding domains.
- Hormone binding to its intracellular receptor results in receptor dimerization and activation. If in the cytosol, hormone-receptor complexes will first traffic to the nucleus. Once in the nucleus, they bind to DNA and activate transcription of hormone-responsive genes.
- Membrane receptors for steroid hormones permit more rapid signaling events in response to hormone binding. These have the same protein structure as intracellular steroid receptors but are instead localized to membrane caveolae.
- MISS involves modifications to existing cellular proteins, often through phosphorylation.
- Convergence of steroid signaling pathways at the membrane, cytoplasm, and nucleus permits an overall biological response to the steroid hormone.