Prostaglandin

A prostaglandin is any member of a group of lipid compounds that are derived enzymatically from fatty acids and have important functions in the animal body. Every prostaglandin contains 20 carbon atoms, including a 5-carbon ring. They are mediators and have a variety of strong physiological effects; although they are technically hormones, they are rarely classified as such.
The prostaglandins together with the thromboxanes and prostacyclins form the prostanoid class of fatty acid derivatives; the prostanoid class is a subclass of eicosanoids.
Contents
1 History and name
2 Biochemistry
2.1 Biosynthesis
2.2 Release of prostaglandins from the cell
2.2.1 Cyclooxygenases
2.2.2 Prostaglandin E synthase
2.2.3 Other terminal prostaglandin synthases
3 Function
4 Types
5 Role in pharmacology
5.1 Inhibition
5.2 Clinical uses
History and name
The name prostaglandin derives from the prostate gland. When prostaglandin was first isolated from seminal fluid in 1935 by the Swedish physiologist Ulf von Euler, and independently by M.W. Goldblatt, it was believed to be part of the prostatic secretions (in actuality prostaglandins are produced by the seminal vesicles); it was later shown that many other tissues secrete prostaglandins for various functions.
In 1971, it was determined that aspirin-like drugs could inhibit the synthesis of prostaglandins. The biochemists Sune K. Bergström, Bengt I. Samuelsson and John R. Vane jointly received the 1982 Nobel Prize in Physiology or Medicine for their research on prostaglandins.
Biochemistry
Biosynthesis
Biosynthesis of eicosanoids. (series-2)
Prostaglandins are found in virtually all tissues and organs. They are produced by all nucleated cells except lymphocytes. They are autocrine and paracrine lipid mediators that act upon platelets, endothelium, uterine and mast cells, among others. They are synthesized in the cell from the essential fatty acids (EFAs).
An intermediate is created by phospholipase-A2, then passed into one of either the cyclooxygenase pathway or the lipoxygenase pathway to form either prostaglandin and thromboxane or leukotriene. The cyclooxygenase pathway produces thromboxane, prostacyclin and prostaglandin D, E and F. The lipoxygenase pathway is active in leukocytes and in macrophages and synthesizes leukotrienes.
Name
EFA Type
Series
Gamma-linolenic acid (GLA) via DGLA
ω-6
series-1
Arachidonic acid (AA)
ω-6
series-2
Eicosapentaenoic acid (EPA)
ω-3
series-3
Release of prostaglandins from the cell
Prostaglandins were originally believed to leave the cells via passive diffusion because of their high lipophilicity. The discovery of the prostaglandin transporter (PGT, SLCO2A1), which mediates the cellular uptake of prostaglandin, demonstrated that diffusion cannot explain the penetration of prostaglandin through the cellular membrane. The release of prostaglandin has now also been shown to be mediated by a specific transporter, namely the multidrug resistance protein 4 (MRP4, ABCC4), a member of the ATP-binding cassette transporter superfamily. Whether MRP4 is the only transporter releasing prostaglandins from the cells is still unclear.
Cyclooxygenases
Prostaglandins are produced following the sequential oxidation of AA, DGLA or EPA by cyclooxygenases (COX-1 and COX-2) and terminal prostaglandin synthases. The classic dogma is as follows:
COX-1 is responsible for the baseline levels of prostaglandins.
COX-2 produces prostaglandins through stimulation.
However, while COX-1 and COX-2 are both located in the blood vessels, stomach and the kidneys, prostaglandin levels are increased by COX-2 in scenarios of inflammation. A third form of COX, termed COX-3 is thought to exist in the brain and may be associated with relief of Headaches when on NSAID therapy.
Prostaglandin E synthase
Prostaglandin E2 (PGE2) is generated from the action of prostaglandin E synthases on prostaglandin H2 (PGH2). Several prostaglandin E synthases have been identified. To date, microsomal prostaglandin E synthase-1 emerges as a key enzyme in the formation of PGE2.
Other terminal prostaglandin synthases
Terminal prostaglandin synthases have been identified that are responsible for the formation of other prostaglandins. For example, hematopoietic and lipocalin prostaglandin D synthases (hPGDS and lPGDS) are responsible for the formation of PGD2 from PGH2. Similarly, prostacyclin (PGI2) synthase (PGIS) converts PGH2 into PGI2. A thromboxane synthase (TxAS) has also been identified. Prostaglandin F synthase (PGFS) catalyzes the formation of 9α,11β-PGF2α,β from PGD2 and PGF2α from PGH2 in the presence of NADPH. This enzyme has recently been crystallyzed in complex with PGD2 and bimatoprost(a synthetic analogue of PGF2α).
Function
There are currently nine known prostaglandin receptors on various cell types. Prostaglandins ligate a subfamily of cell surface seven-transmembrane receptors, G-protein-coupled receptors. These receptors are termed DP1-2, EP1-4, FP, IP, and TP, corresponding to the receptor that ligates the corresponding prostaglandin (e.g., DP1-2 receptors bind to PGD2).
These varied receptors mean that Prostaglandins thus act on a variety of cells, and have a wide variety of actions:
cause constriction or dilation in vascular smooth muscle cells
cause aggregation or disaggregation of platelets
sensitize spinal neurons to pain
decrease intraocular pressure
regulate inflammatory mediation
regulate calcium movement
control hormone regulation
control cell growth
Prostaglandins are potent but have a short half-life before being inactivated and excreted. Therefore, they exert only a paracrine (locally active) or autocrine (acting on the same cell from which it is synthesized) function.
Types
Following is a comparison of the prostaglandin types Prostaglandin I2 (PGI2), Prostaglandin E2 (PGE2) and Prostaglandin F2α (PGF2α).
Type
Receptor
Function
PGI2
IP
vasodilation
inhibit platelet aggregation
bronchodilatation
PGE2
EP1
bronchoconstriction
GI tract smooth muscle contraction
EP2
bronchodilatation
GI tract smooth muscle relaxation
vasodilatation
EP3
↓ gastric acid secretion
↑ gastric mucus secretion
uterus contraction (when pregnant)
GI tract smooth muscle contraction
lipolysis inhibition
↑ autonomic neurotransmitters Unspecified
hyperalgesia
pyrogenic
PGF2α
FP
uterus contraction
bronchoconstriction
Role in pharmacology
Inhibition
See also: Prostaglandin antagonist and Mechanism of action of aspirin
Examples of prostaglandin antagonists are:
NSAIDs (inhibit cyclooxygenase)
Corticosteroids (inhibit phospholipase A2 production)
COX-2 selective inhibitors or coxibs
However, both NSAIDs and Coxibs can raise the risk of myocardial infarction.
Clinical uses
Synthetic prostaglandins are used:
To induce childbirth (parturition) or abortion (PGE2 or PGF2, with or without mifepristone, a progesterone antagonist);
To prevent closure of patent ductus arteriosus in newborns with particular cyanotic heart defects (PGE1)
To prevent and treat peptic ulcers (PGE)
As a vasodilator in severe Raynaud's phenomenon or ischemia of a limb
In pulmonary hypertension
In treatment of glaucoma (as in bimatoprost ophthalmic solution, a synthetic prostamide analog with ocular hypotensive activity)
To treat erectile dysfunction or in penile rehabilitation following surgery (PGE1 as alprostadil To treat egg binding in small birds