EICOSANOIDS

EICOSANOIDS, unlike histamine, are not found preformed in the tissues; they are generated de novo from phospholipids

They are implicated in the control of many physiological processes and are among the most important mediators and modulators of the inflammatory reaction.

Interest in eicosanoids arose in the 1930’s after reports that semen contained a substance that contracted uterine smooth muscle.

The content of the semen was later identified to be prostaglandins.

Later it became clear that prostaglandins was not just one substance but a whole family of compounds, that they were generated in many if not most tissues and derived from arachidonic acid

STRUCTURE AND BIOSYNTHESISThe main source of the eicosanoids is arachidonic acid (5,8,11,14-eicosatetraenoic acid), a 20 – carbon unsaturated fatty acid containing four double bonds.

Arachidonic acid is found esterified in the phospholipids.

The principal eicosanoids are the prostaglandins, the thromboxanes and the leukotrienes, though other derivatives of arachidonate, for example the lipoxins, are also produced. The term prostanoid will be used here to encompass both prostaglandins and thromboxanes.

The initial and rate limiting step in eicosanoid synthesis is the liberation of arachidonate, either in a one step process or a two step process.

The form of phospholipipase A2 (PLA2) involved in the generation of arachidonic acid is mainly the intracellular form.

Its action can give rise not only to arachidonic acid, and thus eicosanoids, but also to lysoglyceryl-phosphorylcholine (lyso -PAF), which is the precursor of another mediator of inflammation – PAF.

Many stimuli can liberate arachidonic acid, and they vary with the cell type, for example thrombin on platelets, C5a on neurophils, bradykinin on fibroblasts and antigen – antibody reactions on mast cells. General cell damage also starts the process.

The free arachidonic acid is metabolised by several pathways :

by fatty acid cyclooxygenase of which there are two forms, COX – 1 and COX- 2. These enzymes initiate the biosynthesis of the prostaglandins and thromboxanes.

by various lipoxygenases which initiate the synthesis of leukotrienes, the lipoxins and other compounds.

The anti- inflammatory action of the glucocorticords largely the result of inhibition of induction of cyclooxygenase. These drugs may also stimulate production of the phospholipase A2 inhibitor lipocortin

The anti – inflammatory action of the non-steroidal anti inflammatory drugs (NSAIDs) results mainly from the fact that they inhibit the action of the fatty acid COXs.

Compounds that act selectively on COX-2, the form induced in inflammatory cells, are now available; and compounds that act at the specific sites of the eicosanoid synthesis ( e.g. inhibitors of 5-lipoxygenase and thromboxanes synthetase) are being or have been developed, as have specific antagonists of the prostaglandins and leukotrienes.

PROSTANOIDS : PRODUCTS OF THE PATHWAY

COX exist in two forms : COX -1 and COX-2.

COX -1 is found in most cells as a constitutive enzyme ( i.e. it is always present) and it is thought that the prostanoids it produces are involved in normal homeostasis ( e.g. regulating vascular responses).

COX -2 is induced in inflammatory cells by an inflammatory stimulus. This action is relevant to the mechanism of action of present and future NSAIDs.

Subsequent steps in arachidonate metabolism differ in different cells. In platelets, the pathway leads to thromboxane A2 synthesis, in vascular endothelium it leads to PGI2 synthesis and in macrophages it leads mainly to synthesis of PGI2. Mast cells synthesize PGI2

PROSTANOIDS RECEPTORS

There are five main classes of prostanoids receptors, all of which are G – protein coupled receptors. They are termed DP-, FP-, IP-, TP-, and EP-receptors respectively, based on the five classes of natural prostanoids PGD2, PGF2x, PGI2,PGE2 and thromboxane A2; the EP receptors are further divided into three subgroups.

ACTIONS OF THE PROSTANOIDS

The prostanoids affect most tissues, having a bewildering variety of effects:

PGD2 causes vasodilation, inhibition of platelet aggregation, relaxation of gastrointestinal muscle, uterine, relaxation, modification of release of hypothalamic/pituitary hormones ( its bronchoconstrictor effect is the result of an action on TP- receptors.)

PGD2a causes myometrial contraction in humans, luteolysis in some species e.g. cattle, and bronchocontriction in other species (cats and dogs)

PGI2 causes vasodilation, inhibition of platelet aggregation, renin release and natriuresis via effects on tubular reabsorption of Na+

Thromboxane A2 causes vasoconstriction, platelet aggregation and bronchoconstriction (much shown in guinea pigs than in human)

PGE2 has the following:

on EP1 – receptor it causes contraction of bronchial and gastrointestinal smooth muscle

on EP2 – receptor it causes bronchodilation, vasodilation,stimulation of intestinal fluid secretion and relaxation of gastrointestinal smooth muscle.

on EP3– receptor it causes constriction of intestinal smooth muscle, inhibition of gastric acid secretion, increase gastric mucus secretion, inhibition of lipolysis, inhibition of autonomic neurotransmitter release and stimulation of contraction of the pregnant human uterus.

THE ROLE OF THE PROSTANOIDS IN INFLAMMATION

PGE2, PGI2 and PGD2 are powerful vasodilators in their own right and synergise with other inflammatory vasodilator such as histamine and bradykinin.

It is the combined dilator action on the precapillary arterioles that contributes to the redness and increased blood flow in areas of acute inflammation.

These prostanoids do not directly increase the permeability of the postcapillary venules, but they potentiate this effect of histamine and bradykinin.

Similarly, they do not themselves produced pain but they potentiate the effect of bradykinin by sensitising afferent C fibres. The anti-inflammatory effects of the NSAIDs result largely from prevention of these actions of the prostaglandins.

Prostaglandins of the E series are also implicated in the production of fever. High concentrations are found in cerebrospinal fluid in infections, and there is evidence that the increase in temperature generated by endogenous fever – inducing cytokinin is mediated by PGE2. The antipyretic action of NSAIDs is the result partly of inhibition of the synthesis of PGE2 in the hypothalamus

LEUKOTRIENES

Leukotrienes are the products of the lipoxygenase pathways. The lipoxygenases,soluble enzymes located in the cytosol, are found in lungs, platelets, mast cells and white blood cells.

ACTIONS AND RECEPTORS OF THE LEUKOTRIENES

LTB4 act on specific LTB4 – receptors, defined by selective agonists and antagonists, the transduction mechanism being generation of inositol triphosphate and the increase of cytosolic Ca2+.

LTB4 is a powerful chemotactic agent for both neutrophils and macrophages. On neurophils,it also causes upregulation of the membrane adhesion molecules and increases the production of toxic oxygen products and the release of granule enzyme.

On the macrophages and lymphocytes, it stimulates proliferation and cytokinin release.

Cysteinyl – leukotrienes specific receptors for LTD4 have been defined on the basis of numerous selective antagonists, cysteinyl – leukotrienes have actions on the respiratory and cardiovascular system.

The respiratory : They are potent spasmogens, causing dose- related contraction of human bronchiolar muscle in vitro. LTE4 is less potent than LTC4 and LTD4, but its effect is much longer lasting.

All causes an increase in mucus secretion. Given by aerosol in vivo to human volunteers, they reduce specific airways conductance and maximum expiratory flow rate, the effect being more protracted than that produced by histamine.

The cardiovascular system: Small amount of LTC4 or LTD4, given intravenously cause a rapid, short-lived fall in blood pressure, and significant constriction of small coronary resistance vessels.

Given subcutaneously they are equipotent with histamine in causing wheal and flare.

Given topically in nose, LTD4 increased nasal blood flow and increases local vascular permeability

The role of leukotrienes in inflammation.

LTD4 can be found in inflammatory exudates and is present in the tissue in many inflammatory condition, including rheumatoid arthritis, psoriasis ( a chronic skin disease) and ulcerative colitis.

The cysteinyl – leukotrienes are present in the sputum of chronic bronchitis in amounts that are biologically active.

On antigen challenge, they are released from samples of human asthmatic lung in vitro and into nasal lavage fluid in in vivo in subjects with allergic rhinitis.

There is evidence that they contribute to the underlying bronchial hyper – reactivity in asthmatics and it is thought that they are among the main mediators of both the early and late phases of asthma.

The CysLT - receptor antagonists Zarfirlukast and montelukast are now in use for the treatment of asthma- the adjunct to other anti asthma drugs, the later in prevention of the acute attack. Iralukast is in the preclinical pipeline.

It is also possible that cystenyl - leukotrienes have a role in the cardiovascular changes of acute anaphylaxis

5 – lipoxygenase inhibitors - Zileuton

LIPOXINS act on specific receptors on polymorphs to oppose the LTB4, giving what might be called ‘stop signals’ to some aspects of inflammation.

PLATELET - ACTIVING FACTOR

PAF, which is also variously termed PAF – acether and AGEPC (acetyl – glyceryl – ether – phosphorylcholine), is biologically active lipid that can produce effects at exceedingly low concentrations

PAF has actions on variety of different target cells and is believed to be an important mediator in both acute and persisting allergic and inflammatory phenomena.

PAF is derived from its precursor, acyl -PAF, by phospholipase A2 activity, resulting in lyso -PAF, which is then acetylated to give PAF. This in turn can be deactylated to lyso- PAF

Source of PAF, they are generated and release from most inflammatory cells when these are stimulated and from platelets on stimulation.

Actions and role in inflammation.

Injected locally – local vasodilation and thus erythema, increased vascular permeability and wheal formation and hyperalgesia

Potent chemotaxin for neutrophils and monocytes

Injected locally – local vasodilation and thus erythema

activate phospholipase A2 to generate eicosanoids.

platelets it causes shape change and granule release.

PAF is also a spasmogen on both bronchial and ileal smooth muscle.

The anti – inflammatory actions of the glucocorticords may be caused, at least in part, by inhibition of PAF synthesis.

Competitive antagonist of PAF and /or specific inhibitors of lyso-PAF acetyltransferase could well be useful anti – inflammatory drugs and/or anti – asthmatic agents.

BRADYKININ

Bradykinin and the closely related peptides kallidin are vasoactive peptides formed by the action of enzymes on protein substrates termed kininogens.

The two peptides are virtually identical kallidin possessing one additional amino acid residue

Source and formation of bradykinin

An outline of the formation of bradykinin from high – molecular – weight kininogen in plasma by kallikrein

Inactive of bradykinin

The main enzyme that inactivate bradykinin related kinin are called kininases II, is the same as angiotensin – converting enzyme.

Actions and Role of bradykinin in inflammation.

Bradykinin (BK) causes vasodilation and increased vascular permeability. Its vasodilator action is partly as a result of generation of PGI2 and the release of NO.

It is a potent pain – producing agent, an effect that is potentiated by the prostaglandins.

Bradykinin is spasmogenic for several types of smooth muscle including that of the intestine and uterus; bronchial muscle is also contracted in some species.

The contraction is slow and sustained in comparison with that produced by histamine.

Bradykinin receptors

There are two types of bradykinin receptors, B1 and B2, which mediate very similar effects. Both are G- protein coupled receptors.