Levels of arachidonic acid and its metabolites in the skin in human allergic and irritant contact dermatitis

Correlation between mouse skin inflammation induced by arachidonic acid and eicosanoid synthesis

We and others have shown that arachidonic acid (AA), when applied topically to ear surfaces, causes an intense acute inflammatory reaction within minutes (as measured by ear thickness). In this study, we have investigated the cellular and biochemical changes associated with this phenomenon and have attempted to correlate these changes with the induction of inflammation. Measurement of vascular permeability by the accumulation of [125I]albumin showed that significant plasma exudation was observed at 15 min in AA-treated ears. Furthermore, the increase in [125I]albumin was time related and was nearly 10-fold greater than control at 1 h. No time-related change in plasma exudation was observed with control ears. Measurement of LTC4 by radioimmunoassay showed that there was a significant increase in LTC4 synthesis at 15 min after AA treatment. Maximal LTC4 synthesis occurred at 15 min and subsequently decreased to 30% of peak level at 30 min. Histological examination and myeloperoxidase measurement indicated that few neutrophils were present at these early time points and suggested that cells other than neutrophils are contributing to LTC4 synthesis. Ear thickness, [125I]albumin accumulation and leukotriene C4 (LTC4) synthesis in AA-treated ears were reduced significantly by topically administered mixed lipoxygenase (LO) and cyclooxygenase inhibitors such as BW755C and phenidone. Therefore, we suggest that AA-induced ear inflammation is a suitable screen for detecting LO inhibitors in vivo.

Determination of anaphylatoxin concentrations in suction blisters in patients with psoriasis

Concentrations of C3a and C4a anaphylatoxins in suction blister fluids were determined by radioimmunoassay in patients with psoriasis and normal controls. Comparison of anaphylatoxin levels between serum samples and blister contents in the same subjects revealed that the levels of both C3a and C4a anaphylatoxins were significantly higher in the former than the latter even in those raised on normal skin, suggesting that the classic complement pathway is activated during suction procedure. Therefore we cannot regard suction blister fluid to be simply representative of undisturbed interstitial tissue fluid as far as the complement system is concerned. There was no difference in anaphylatoxin levels between those from uninvolved skin of psoriatic patients and those from normal controls. However, significantly high anaphylatoxin levels were noted in fluids of suction blisters raised on lesional skin as compared with those produced on uninvolved skin in psoriatic patients.

Arachidonic acid metabolism by peripheral blood cells in psoriasis

The metabolism of arachidonic acid by mixed suspensions of leukocytes and platelets prepared from peripheral blood has been studied in 20 patients with psoriasis and 21 healthy controls. A lipoxygenase-derived product, identified as 12,20-dihydroxy-5,8,10,14-eicosatetraenoic acid was formed in increased amounts by the cell suspension from the psoriatic patients. This product results from the metabolism of platelet-derived 12-hydroxy-5,8,10,14-eicosatetraenoic acid by the polymorphonuclear leukocyte 20-hydroxylase enzyme. By contrast, synthesis of the cyclo-oxygenase products 12-hydroxy-5,8,10-heptadecatrienoic acid and thromboxane B2 was diminished. Benoxaprofen, which is known to be beneficial in psoriasis, diminished the levels of 12,20-dihydroxy-5,8,10,14-eicosatetraenoic acid formed in vitro.

Skin levels of arachidonic acid-derived inflammatory mediators and histamine in atopic dermatitis and psoriasis

Since the biochemical events leading to cutaneous inflammation in atopic dermatitis and psoriasis are unknown, we studied the levels of arachidonic acid-derived mediators of inflammation as well as histamine in the suction blister fluid obtained from lesional and nonlesional skin of patients with these dermatoses. Mediator levels were determined radioimmunologically. Skin from healthy controls and uninvolved skin from patients contained very low or unmeasurable levels of the 5-lipoxygenase metabolite of arachidonic acid, leukotriene (LT) B4. In contrast, higher levels of LTB4-like immunoreactivity were detected in suction blister fluid from lesional atopic dermatitis skin, and even higher concentrations occurred in psoriasis lesions. LTB4-like immunoreactivity from atopic dermatitis suction blister fluid cochromatographed on reverse-phase high-pressure liquid chromatography with authentic LTB4, thus excluding cross-reaction of the LTB4-antibody with arachidonic acid or monohydroxyeicosatetraenoic acids. In contrast, suction blister concentrations of the cyclooxygenase metabolite of arachidonic acid prostaglandin (PG) E2 showed no significant differences between lesional and nonlesional patient skin and healthy control skin. PGD2 determined as a stable metabolite could not be detected in these samples. Histamine concentrations in lesional skin were within normal range. The elevated levels of the potent proinflammatory and immunomodulating mediator LTB4 could be involved in the pathogenesis of cutaneous inflammation in atopic dermatitis and psoriasis. In addition, they might explain the therapeutic efficiency of glucocorticosteroids, which among other actions inhibit the release of arachidonic acid from phospholipid stores by blocking the enzyme phospholipase A2. However, the specificity of disease expression in atopic dermatitis and psoriasis must be due to factors other than cutaneous LTB4 elevation.

Lipoxygenase products of arachidonic acid in human inflamed skin

Monohydroxy acids (HETEs) and leukotriene B4 (LTB4) metabolites of arachidonic acid were measured in skin of healthy volunteers after ultraviolet B irradiation, and in the uninvolved skin of psoriatics after topical dithranol application. Exudate was collected from suction bullae on control and inflamed abdominal skin, and analysed for 12-HETE and PGE2 by GC-MS and LTB4 by bioassay. 12-HETE and PGE2 were raised at 24 h but not at 72 h after u.v.B irradiation: control and 24 h values were 13.7 and 41.5 ng ml-1 (P less than 0.05, n = 6) for 12-HETE respectively, and 4.5 and 30.2 ng ml-1 (P less than 0.01, n = 6) for PGE2. Dithranol application raised PGE2 levels from 23.1 ng ml-1 in control exudate to 62 ng ml-1 (P less than 0.01, n = 6) at 24 h before declining to base levels at 72 h. However, 12-HETE was raised at 72 h (200 ng ml-1, P less than 0.01, n = 5) but not at 24 h (104 ng ml-1) compared to control levels (50 ng ml-1, n = 5). The levels of the LTB4 were low (less than 100 pg ml-1), and no significant increases were observed. Arachidonic acid in inflamed skin can be metabolised by the cyclo-oxygenase and lipoxygenase pathway. It is probable that the lipoxygenase product 12-HETE is involved in these inflammatory reactions.

The analysis of arachidonic acid metabolites in normal, uninvolved and lesional psoriatic skin

Collection of exudate from suction bullae is a commonly used method for sampling human skin for mediator analysis. It is satisfactory on skin of normal structure but is unreliable on lesional psoriatic skin in which there are major structural changes and excessive scaling. Collection of exudates from abraded sites was found to be a suitable alternative method for psoriatic skin. Arachidonic acid and 12-HETE, but not PGE2, were significantly higher in exudate from abraded lesional psoriatic skin (494 +/- 88, 45.9 +/- 4.2 and 9.6 +/- 1.8 ng/ml respectively, mean +/- sem, n = 5) compared to uninvolved skin (154 + 38, 18.5 + 5.1 and 7.7 + 1.9 ng/ml) or skin of normal volunteers (119 +/- 37, 14.5 +/- 6.7 and 4.5 +/- 1.6 ng/ml, n = 7) which were similar. The coefficient of variation for exudate collection and mediator analysis was usually less than 55%. The analysis of lipoxygenase and cyclooxygenase products was simplified by the use of chlorobutane to extract preferentially arachidonic acid and HETEs from neutral aqueous solutions.