Bengt Samuelsson

Arachidonic acid plays a central role in a biological control system where such oxygenated derivatives as prostaglandins, thromboxanes, and leukotrienes are mediators. The leukotrienes are formed by transformation of arachidonic acid into an unstable epoxide intermediate, leukotriene A4, which can be converted enzymatically by hydration to leukotriene B4, and by addition of glutathione to leukotriene C4. This last compound is metabolized to leukotrienes D4 and E4 by successive elimination of a gamma-glutamyl residue and glycine. Slow-reacting substance of anaphylaxis consists of leukotrienes C4, D4, and E4. The cysteinyl-containing leukotrienes are potent bronchoconstrictors, increase vascular permeability in postcapillary venules, and stimulate mucus secretion. Leukotriene B4 causes adhesion and chemotactic movement of leukocytes and stimulates aggregation, enzyme release, and generation of superoxide in neutrophils. Leukotrienes C4, D4, and E4, which are released from the lung tissue of asthmatic subjects exposed to specific allergens, seem to play a pathophysiological role in immediate hypersensitivity reactions. These leukotrienes, as well as leukotriene B4, have pro-inflammatory effects.

Arachidonic acid is released from membrane phospholipids upon cell stimulation (for example, by immune complexes and calcium ionophores) and converted to leukotrienes by a 5-lipoxygenase that also has leukotriene A4 synthetase activity. Leukotriene A4, an unstable epoxide, is hydrolyzed to leukotriene B4 or conjugated with glutathione to yield leukotriene C4 and its metabolites, leukotriene D4 and leukotriene E4. The leukotrienes participate in host defense reactions and pathophysiological conditions such as immediate hypersensitivity and inflammation. Recent studies also suggest a neuroendocrine role for leukotriene C4 in luteinizing hormone secretion. Lipoxins are formed by the action of 5- and 15-lipoxygenases on arachidonic acid. Lipoxin A causes contraction of guinea pig lung strips and dilation of the microvasculature. Both lipoxin A and B inhibit natural killer cell cytotoxicity. Thus, the multiple interaction of lipoxygenases generates compounds that can regulate specific cellular responses of importance in inflammation and immunity.

Arachidonic acid incubated with human platelets was converted into three compounds, 12L-hydroxy-5,8,10,14-eicosatetraenoic acid, 12L-hydroxy-5,8,10-heptadecatrienoic acid, and the hemiacetal derivative of 8-(1-hydroxy-3-oxopropyl)-9,12L-dihydroxy-5,10-heptadecadienoic acid. The formation of the two latter compounds from arachidonic acid proceeded by pathways involving the enzyme, fatty acid cyclo-oxygenase, in the initial step and with the prostaglandin endoperoxide, PGG(2), as an intermediate. The first mentioned compound was formed from 12L-hydroperoxy-5,8,10,14-eicosatetraenoic acid, which in turn was formed from arachidonic acid by the action of a novel lipoxygenase. Aspirin and indomethacin inhibited the fatty acid cyclo-oxygenase but not the lipoxygenase, whereas 5,8,11,14-eicosatetraynoic acid inhibited both enzymes. The almost exclusive transformation of the endoperoxide structure into non-prostaglandin derivatives supports the hypothesis that the endoperoxides can participate directly and not by way of the classical prostaglandins in regulation of cell functions. The observed transformations of arachidonic acid in platelets also explain the aggregating effect of this acid.

The Hippo pathway was initially discovered in Drosophila melanogaster as a key regulator of tissue growth. It is an evolutionarily conserved signaling cascade regulating numerous biological processes, including cell growth and fate decision, organ size ...Read More

Leukotrienes B4, C4, and D4, members of a recently discovered family of substances biosynthesized from arachidonic acid, were found to have potent microvascular actions in the hamster cheek pouch. When applied topically to the vascular network, leukotrienes C4 and D4 caused an intense constriction of arterioles, being similar to angiotensin in potency in this respect. The vasoconstriction induced by leukotrienes C4 and D4 was short-lived, and it was consistently followed by a marked and dose-dependent extravasation of macromolecules from postcapillary venules. Histamine did not constrict arterioles, but it elicited leakage of plasma, although on a molar basis it was no more than 1/1000th as potent as the leukotrienes. When used in the same concentration range as leukotrienes C4 and D4, leukotriene B4 did not evoke vasoconstriction or promote plasma leakage. On the other hand, leukotriene B4 caused a conspicuous and reversible adhesion of leukocytes to the endothelium in postcapillary venules. Our findings that leukotrienes induce microcirculatory alterations in vivo, closely resembling the early events in the acute inflammatory response, imply that leukotrienes, formed in several blood-borne and tissue-bound cells, may mediate important microcirculatory adjustments to noxious stimuli.