Image: “Regulation of gene expression by steroid hormone receptor.” by Ali Zifan 03:07, 10 July 2016 (UTC) – Own work; Used information from: Campbell Biology (10th Edition) by: Jane B. Reece & Steven A. Wasserman. License: CC BY-SA 4.0
Synthesis of Steroid Hormones
Steroid hormones are produced in the adrenal cortex, testis, ovary, and some peripheral tissues (adipose tissue). They are synthesized from cholesterol, and they cannot be stored because of their lipophilic nature. This is the reason for their immediate release from the cell as soon as they are synthesized.
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Difference Between the Peptide Hormone and the Steroid Receptor
The steroid hormone can easily enter the target cell by crossing the cell membrane (unlike peptide hormones, which basically act at the level of the plasma membrane) to exert its effect. Once entered, they directly interact with cytoplasmic or nuclear receptors; then, the hormone-receptor complex activates DNA. DNA activation can have numerous effects, from DNA transcription activation, leading to the synthesis of a particular protein, to the repression of some DNA region.
Compared to peptide hormones, steroid hormones are slower acting and have relatively long half-lives. Examples of steroid hormones include cortisol, estrogen, and testosterone.Steroids play important roles in carbohydrate metabolism (glucocorticoids), mineral balance (mineralocorticoids), reproductive functions (gonadal steroids). They also play important roles in inflammatory responses, stress responses, bone metabolism, behavioral, emotional, and cognitive processes.
Mechanism of Actions of Steroid Hormone Receptors
Steroid hormone receptors are intracellular and before hormone binding. They might be located in the cytoplasm or in the nucleus. In the cytoplasm, after attachment of the steroid hormone, the complex moves to the nucleus, where it carries on its action. The receptor may be located in the nucleus before binding. In either case, after binding, the action occurs in the nucleus by DNA modification.
|Predominantly cytoplasmic||Predominantly nuclear|
|Glucocorticoid receptorMineralocorticoid receptorAndrogen receptor||Estrogen receptorThyroid hormone receptorVitamin D receptorRetinoic acid receptor|
The entry of the hormone-receptor complex into the nucleus is brought about by a specific part of the receptor known as the nuclear localization signal (NLS).
The unbound receptor’s entry into the nucleus without the hormone is avoided by the coverage of the localization signal areas by heat shock proteins. Upon binding of the hormone, the heat shock protein is liberated, exposing the nuclear localization signal, which in turn helps move the hormone-receptor complex into the nucleus.
In addition to this, the hormone’s binding to the hormone receptor brings about a series of conformational changes in the receptor.
The resultant hormone-receptor complex binds specifically to DNA promoter and enhancer elements, thereby affecting the expression of specific target genes.The binding of the hormone-receptor complex to the specific part of the DNA occurs via a specialized domain of the receptor called zinc finger. Zinc finger binding modulates the DNA. The further outcome is determined by the specific co-activator and the co-repressor, which binds to the complex.
Molecular Structure of the Steroid Hormone Receptor
Image: “Structural Organization of Nuclear Receptors.” by Boghog2 – Created by steustatiushistory.org using public domain software PyMol. License: Public Domain
Broadly, the intracellular hormone receptor consists of four domains, namely the variable domain, the DNA binding domain, hormone-binding domain, and the hinge region. These four units form a common structure of all the intracellular steroid hormone receptors.
The names of the subunits mirror their function. The function of the DNA binding region is to bind with the DNA. This centrally located, highly conserved, DNA binding domain (DBD) consists of two non-repetitive globular motifs, where zinc is coordinated with four cysteine residues. Their secondary and tertiary structure is distinct from classic zinc fingers. This region controls which gene will be activated. It interacts with the hormone-responsive elements in the DNA (HRE).
The hinge region plays a prominent role in moving the hormone-receptor complex into the nucleus.
The variable region, which begins at the N-terminal, is where the sequence of amino acids is variable among different receptors.
The hormone-binding domain consists of the ligand-binding domain, the localization signals, and sequence for binding with the heat shock proteins. These heat shock proteins (namely heat shock proteins hsp90 and hsp56) are required to maintain their inactive (but receptive) cytoplasmic conformation and are called chaperones.
Examples of the receptor
Hormones that bind to steroid hormone receptors include steroids (such as estrogen, progesterone, glucocorticoids), some amine hormones (such as thyroxines), and retinoids.
The binding can be at the cytoplasmic or the nuclear level. Steroid hormone receptors that are predominantly cytoplasmic include receptors for mineralocorticoids, glucocorticoids, and androgen hormones. Steroid hormone receptors that are predominantly nuclear include the estrogen receptor, thyroid hormone receptor, vitamin D receptor, and retinoic acid receptor.
The nuclear receptors subfamily 3 and the 3 keto-steroid receptors are the two most commonly studied steroid hormone receptor families. The receptors of the estrogen groups form the NR3A members.
It should also be remembered that steroid hormones can also act through other receptors such as G protein-coupled receptors in addition to steroid hormone receptors.
Classification of the Nuclear Receptors
The nuclear–receptors–subfamily3 is divided into group A and group C. The group A is divided into two, namely the estrogen receptor alpha and the estrogen receptor beta.Group C is also known as the 3keto-steroid receptor group, which in turn consists of subgroups: 1=glucocorticoid, 2=mineralocorticoid, 3=progesterone, and 4=androgen receptor.
Classification based on the mechanism of action
Depending on their mechanism of action and subcellular distribution, there are two classes of nuclear receptors. The basic difference between the two types is the presence of heat shock proteins. While type 1 contains a heat shock protein bound with the inactive receptor and resides in the cytosol, type 2 resides in the nucleus and has no heat shock protein,. Still, in the absence of its steroid hormone ligand, type 2 nuclear receptors are often complexed with corepressor proteins. Ligand binding to the nuclear receptor causes dissociation of the corepressor and recruitment of coactivator proteins. Type II nuclear receptors include principally subfamily 1, for example, the retinoic acid receptor, retinoid X receptor, and thyroid hormone receptor.
The intricacies of binding with the steroid hormones have already been described in the mechanism of action. The latest discoveries in this field have also identified other types of nuclear receptors, namely type 3 and type 4. Type 3 is basically variant of type 1 and type 4 basically is a monomer unit which binds to DNA (rather than the heterodimer).
Image: “Mechanism of class I nuclear receptor action. A class I nuclear receptor (NR), in the absence of ligand, is located in the cytosol. Hormone binding to the NR triggers dissociation of heat shock proteins (HSP), dimerization, and translocation to the nucleus, where the NR binds to a specific sequence of DNA known as a hormone response element (HRE). The nuclear receptor DNA complex, in turn, recruits other proteins that are responsible for transcription of downstream DNA into mRNA, which is eventually translated into protein, which results in a change in cell function.” by Boghog2 – Made by Boghog2. License: Public Domain
Image: “Mechanism of class II nuclear receptor action. A class II nuclear receptor (NR), regardless of ligand-binding status, is located in the nucleus bound to DNA. For the purpose of illustration, the nuclear receptor shown here is the thyroid hormone receptor (TR) heterodimerized to the RXR. In the absence of ligand, the TR is bound to corepressor protein. Ligand binding to TR causes dissociation of corepressor and recruitment of coactivator protein, which, in turn, recruits additional proteins such as RNA polymerase that are responsible for transcription of downstream DNA into RNA and eventually protein.” by Boghog2 – Own work. License: Public Domain
It should be noted that both type 1 and type 2 ultimately are related to the genomic DNA for the manifestation of their action, and there is another group, which manifests the non-genomic structures. Nuclear receptors in non-genomic structures are membrane-associated instead of being localized in the cytosol or nucleus. These membrane-associated receptors function through alternative signal transduction mechanisms, not involving gene regulation.
The aldosterone receptors are located in the tubule of the kidney, and their function is to modulate the action of the aldosterone, which binds with them. On binding of aldosterone, the activity of the Na + K+ ATPase on the basolateral membrane is increased.
In addition to this, the epithelial sodium channels, the ROMK potassium channels are also activated. Some of the hormone receptors are coupled with the signals process in the cytoplasm rather than the action of the gene level in the nucleus.
There are two types of co-regulatory proteins, namely the coactivator and the corepressor. The binding of the nuclear receptor to the response element in the DNA recruits a number of other proteins.
These proteins can have various functions ranging from chromatin remodeling to acting as bridging molecules. Generally, the coactivators have intrinsic histone acetyltransferase activity, whereas the corepressors recruit the histone deacetylase activity. The coactivators are generally recruited by the agonist ligand, and the corepressors are generally recruited by the antagonist ligand.
Special Forms of Steroid Hormone Receptors
The sex hormone-binding globulin which generally functions in the transport mechanism has shown to be linked with the cell membrane of the cells in some of the cases, leading to the manifestation of the action of the steroid hormone. The sex hormone-binding globulin thus acts as a form of a receptor with manifestation based on the characteristic of the hormone which binds with it.
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GPR30 is a form of G protein-coupled receptor that functions as a steroid hormone receptor and binds estrogen. It is well known that in the central nervous system, the neurosteroid that can bind to the GABA channels at the glutamate NMDA receptor. Progesterone acts at one of the most important cation channels in the sperm which modulate the motility of the sperm known as the CATSPAR channel.