Prostaglandin


The prostaglandins are a group of physiologically active lipid compounds called eicosanoids having diverse hormone-like effects in animals. Prostaglandins have been found in almost every tissue in humans and other animals. They are derived enzymatically from the fatty acid arachidonic acid. Every prostaglandin contains 20 carbon atoms, including a 5-carbon ring. They are a subclass of eicosanoids and of the prostanoid class of fatty acid derivatives.
The structural differences between prostaglandins account for their different biological activities. A given prostaglandin may have different and even opposite effects in different tissues in some cases. The ability of the same prostaglandin to stimulate a reaction in one tissue and inhibit the same reaction in another tissue is determined by the type of receptor to which the prostaglandin binds. They act as autocrine or paracrine factors with their target cells present in the immediate vicinity of the site of their secretion. Prostaglandins differ from endocrine hormones in that they are not produced at a specific site but in many places throughout the human body.
Prostaglandins are powerful, locally-acting vasodilators and inhibit the aggregation of blood platelets. Through their role in vasodilation, prostaglandins are also involved in inflammation. They are synthesized in the walls of blood vessels and serve the physiological function of preventing needless clot formation, as well as regulating the contraction of smooth muscle tissue. Conversely, thromboxanes are vasoconstrictors and facilitate platelet aggregation. Their name comes from their role in clot formation.
Specific prostaglandins are named with a letter followed by a number. For example, prostaglandin E1 is abbreviated PGE1 or PGE1, and prostaglandin I2 is abbreviated PGI2 or PGI2. The number is traditionally subscripted when the context allows; but, as with many similar subscript-containing nomenclatures, the subscript is simply forgone in many database fields that can store only plain text, and readers are used to seeing and writing it without subscript.

History and name

The name prostaglandin derives from the prostate gland, chosen when prostaglandin was first isolated from seminal fluid in 1935 by the Swedish physiologist Ulf von Euler, and independently by the Irish-English physiologist Maurice Walter Goldblatt. Prostaglandins were believed to be part of the prostatic secretions, and eventually were discovered to be produced by the seminal vesicles. Later, it was shown that many other tissues secrete prostaglandins and that they perform a variety of functions. The first total syntheses of prostaglandin F2α and prostaglandin E2 were reported by E. J. Corey in 1969, an achievement for which he was awarded the Japan Prize in 1989.
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

Prostaglandins are found in most tissues and organs. They are produced by almost all nucleated cells. They are autocrine and paracrine lipid mediators that act upon platelets, endothelium, uterine and mast cells. They are synthesized in the cell from the fatty acid arachidonic acid.
Arachidonic acid is created from diacylglycerol via phospholipase-A2, then brought to either the cyclooxygenase pathway or the lipoxygenase pathway. The cyclooxygenase pathway produces thromboxane, prostacyclin and prostaglandin D, E and F. Alternatively, the lipoxygenase enzyme pathway is active in leukocytes and in macrophages and synthesizes leukotrienes.

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, which mediates the cellular uptake of prostaglandin, demonstrated that diffusion alone 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, 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 oxygenation of arachidonic acid, DGLA or EPA by cyclooxygenases and terminal prostaglandin synthases. The classic dogma is as follows:
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 and growth.

Prostaglandin E synthase

- the most abundant prostaglandin - is generated from the action of prostaglandin E synthases on prostaglandin H2. 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 are responsible for the formation of PGD2 from PGH2. Similarly, prostacyclin synthase converts PGH2 into PGI2. A thromboxane synthase has also been identified.
Prostaglandin-F synthase catalyzes the formation of 9α,11β-PGF2α,β from PGD2 and PGF2α from PGH2 in the presence of NADPH. This enzyme has recently been crystallized in complex with PGD2 and bimatoprost.

Functions

There are currently ten known prostaglandin receptors on various cell types. Prostaglandins ligate a sub-family of cell surface seven-transmembrane receptors, G-protein-coupled receptors. These receptors are termed DP1-2, EP1-4, FP, IP1-2, and TP, corresponding to the receptor that ligates the corresponding prostaglandin.
The diversity of receptors means that prostaglandins act on an array of cells and have a wide variety of effects such as:
The following is a comparison of different types of prostaglandin, including prostaglandin I2, prostaglandin D2, prostaglandin E2, and prostaglandin F2α.
TypeReceptorReceptor typeFunction
PGI2IPGs
PGD2PTGDR and CRTH2 GPCR
  • produced by mast cells; recruits Th2 cells, eosinophils, and basophils
  • In mammalian organs, large amounts of PGD2 are found only in the brain and in mast cells
  • Critical to development of allergic diseases such as asthma
    • PGE2EP1Gq
  • bronchoconstriction
  • GI tract smooth muscle contraction
    • PGE2EP2Gs
  • bronchodilation
  • GI tract smooth muscle relaxation
  • vasodilation
    • PGE2EP3Gi
  • ↓ gastric acid secretion
  • ↑ gastric mucus secretion
  • uterus contraction
  • GI tract smooth muscle contraction
  • lipolysis inhibition
  • ↑ autonomic neurotransmitters
  • ↑ platelet response to their agonists and ↑ atherothrombosis in vivo
    • PGE2Unspecified
  • hyperalgesia
  • pyrogenic
    • PGF2αFPGq
  • uterus contraction
  • bronchoconstriction
  • urinary bladder contractions

    • Role in pharmacology

    Inhibition

    Examples of prostaglandin antagonists are:
    Synthetic prostaglandins are used:
    Cold exposure and IUDs may increase prostaglandin production.