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Glucocorticoids (GC) are a class of steroid hormones characterised by an ability to bind with the glucocorticoid receptor (GR) and trigger similar effects. Glucocorticoids are distinguished from mineralocorticoids and sex steroids by their specific receptors, target cells, and effects. In technical terms, corticosteroid refers to both glucocorticoids and mineralocorticoids, but is often used as a synonym for glucocorticoid.
Cortisol (or hydrocortisone) is the most important human glucocorticoid. It is essential for life, and regulates or supports a variety of important cardiovascular, metabolic, immunologic, and homeostatic functions. Glucocorticoid receptors are found in the cells of almost all vertebrate tissues.
The name glucocorticoid derives from early observations that these hormones were involved in glucose metabolism. In the fasted state, cortisol stimulates several processes that collectively serve to increase and maintain normal concentrations of glucose in blood. These effects include:
Glucocorticoids have potent anti-inflammatory and immunosuppressive properties. This is particularly evident when they are administered at pharmacological doses, but also is important in normal immune responses. As a consequence, glucocorticoids are widely used as drugs to treat inflammatory conditions such as arthritis and dermatitis, and as adjunction therapy for conditions such as autoimmune diseases.
Glucocorticoids have multiple effects on fetal development. An important example is their role in promoting maturation of the lung and production of the surfactant necessary for extrauterine lung function. Mice with homozygous disruptions in the corticotropin-releasing hormone gene (see below) die at birth due to pulmonary immaturity.
Excessive glucocorticoid levels resulting from administration as a drug or hyperadrenocorticism have effects on many systems. Some examples include inhibition of bone formation, suppression of calcium absorption (both of which can lead to osteoporosis), delayed wound healing, muscle weakness, and increased risk of infection. These observations suggest a multitude of less-dramatic physiologic roles for glucocorticoids.
Mode of action
Glucocorticoids bind to the cytosolic glucocorticoid receptor. This type of receptor is activated by ligand binding. After a hormone binds to the corresponding receptor, the newly-formed receptor-ligand complex translocates itself into the cell nucleus, where it binds to many glucocorticoid response elements (GRE) in the promoter region of the target genes. The opposite mechanism is called transrepression. The activated hormone receptor interacts with specific transcription factors and prevents the transcription of targeted genes. Glucocorticoids are able to prevent the transcription of any of immune genes, including the IL-2 gene.
The ordinary glucocorticoids do not distinguish among transactivation and transrepression and influence both the "wanted" immune and "unwanted" genes regulating the metabolic and cardiovascular functions. At the current time, intensive research is aimed at discovering selectively acting glucocorticoids that will be able to repress only the immune system.
A variety of synthetic glucocorticoids, some far more potent than cortisol, have been created for therapeutic use. They differ in the pharmacokinetics (absorption factor, half-life, volume of distribution, clearance) and in pharmacodynamics (for example the capacity of mineralocorticoid activity: retention of sodium (Na+) and water; see also: renal physiology). Because they permeate the intestines easily, they are primarily administered per os (by mouth), but also by other methods, such as topically on skin. More than 90 percent of them bind different plasma proteins, however with a different binding specificity. Endogenous glucocorticoids and some synthetic corticoids have high affinity to the protein transcortin (also called CBG, corticosteroid-binding protein), whereas all of them bind albumin. In the liver, they quickly metabolise by conjugation with a sulfate or glucuronic acid, and are secreted in the urine.
Glucocorticoid potency, duration of effect, and overlapping mineralocorticoid potency varies (Table).
Cortisol (hydrocortisone) is the standard of comparison for glucocorticoid potency. Hydrocortisone is the name used for pharmaceutical preparations of cortisol. Data refer to oral dosing, except when mentioned. Note that oral potency may be less than parenteral potency because significant amounts (up to 50% in some cases) may not be absorbed from the intestine. Note that fludrocortisone, DOCA, and aldosterone are not considered glucocorticoids, and are included in this table to provide perspective on mineralocorticoid potency.
Hydrocortisone cream or ointment is available nonprescription up to 1% strength. In general, stronger forms require prescription.
Physiologic replacement of glucocorticoid
Any glucocorticoid can be given in a dose that provides approximately the same glucocorticoid effects as normal cortisol production; this is referred to as physiologic, replacement, or maintenance dosing. This is approximately 6-12 mg/m²/day (m² refers to body surface area (BSA), and is a measure of body size; an average man is 1.7 m²).
Medical uses and effects of high-dose glucocorticoids
In much higher doses (termed pharmacologic doses), glucocorticoids are used to suppress various allergic, inflammatory, and autoimmune disorders. They are also administered as posttransplantory immunosuppressants to prevent the acute transplant rejection and the graft-versus-host disease. Nevertheless, they do not prevent an infection and also inhibit later reparative processes.
Some drugs used are cortisol (hydrocortisone), prednisone and dexamethasone.
Glucocorticoids suppress the cell-mediated immunity. They act by inhibiting genes that code for the cytokines IL-1, IL-2, IL-3, IL-4, IL-5, IL-6, IL-8 and IFN-γ, the most important of which is the IL-2. Smaller cytokine production reduces the T cell proliferation.
Glucocorticoids also suppress the humoral immunity, causing B cells to express smaller amounts of IL-2 and of IL-2 receptors. This diminishes both B cell clone expansion and antibody synthesis. The diminished amounts of IL-2 also causes fewer T lymphocyte cells to be activated.
Since glucocorticoid is a steroid, it regulates transcription factors; another factor it down-regulates is the expression of Fc receptors on macrophages, so there is a decreased phagocytosis of opsonised cells.
Glucocorticoids influence all types of inflammatory events, no matter what their cause. They induce the lipocortin-1 (annexin-1) synthesis, which then binds to cell membranes, preventing the phospholipase A2 from coming into contact with its substrate arachidonic acid. This leads to diminished eicosanoid production. The cyclooxygenase (both COX-1 and COX-2) expression is also suppressed, potentiating the effect. In other words, the two main products in inflammation, Prostaglandins and Leukotrienes, are inhibited by the action of Glucocorticoids.
Glucocorticoids also stimulate the lipocortin-1 escaping to the extracellular space, where it binds to the leukocyte membrane receptors and inhibits various inflammatory events: epithelial adhesion, emigration, chemotaxis, phagocytosis, respiratory burst, and the release of various inflammatory mediators (lysosomal enzymes, cytokines, tissue plasminogen activator, chemokines, etc.) from neutrophils, macrophages, and mastocytes.
Glucocorticoid drugs currently being used act nonselectively, so in the long run they may impair many healthy anabolic processes. To prevent this, much research has been focused recently on the elaboration of selectively-acting glucocorticoid drugs. These are the side-effects that could be prevented:
In high doses, hydrocortisone (cortisol) and those glucocorticoids with appreciable mineralocorticoid potency can exert a mineralocorticoid effect as well, although in physiologic doses this is prevented by rapid degradation of cortisol by 11β-hydroxysteroid dehydrogenase isoenzyme 2 (11β-HSD2) in mineralocorticoid target tissues. Mineralocorticoid effects can include salt and water retention, extracellular fluid volume expansion, hypertension, potassium depletion, and metabolic alkalosis.
The combination of clinical problems produced by prolonged, excess glucocorticoids, whether synthetic or endogenous, is termed Cushing's syndrome.
Adrenal suppression and withdrawal
In addition to the effects listed above, use of high-dose steroids for more than a week begins to produce suppression of the patient's adrenal glands because the exogenous glucocorticoids suppress hypothalamic corticotropin-releasing hormone (CRH) and pituitary adrenocorticotropic hormone (ACTH). With prolonged suppression, the adrenal glands atrophy (physically shrink), and can take months to recover full function after discontinuation of the exogenous glucocorticoid.
During this recovery time, the patient is vulnerable to adrenal insufficiency during times of stress, such as illness. While there is wide individual variation in suppressive dose and time for adrenal recovery, clinical guidelines have been devised to estimate potential adrenal suppression and recovery, to reduce risk to the patient. The following is one example, but many variations exist or may be appropriate in individual circumstances.
|This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Glucocorticoid". A list of authors is available in Wikipedia.|