R.D.R. Camp

The identification of hydroxy fatty acids in psoriatic skin

Psoriasis is a common chronic inflammatory and proliferative skin disease characterised by epidermal neutrophil infiltration which may be induced by chemotactic substances in the involved epidermis. Superficial psoriatic scale was shown to contain biologically active amounts of leukotriene B4 and monohydroxy-eicosatetraenoic acid (HETE)-like material as determined by assay for chemokinetic activity in high performance liquid chromatography (HPLC) fractions of scale extracts. Extracts of scale and chamber fluid from abraded lesional and uninvolved psoriatic skin were purified by HPLC and appropriate fractions were analysed by gas chromatography - mass spectrometry (GC-MS). The following monohydroxy metabolites of arachidonic, linoleic and 11,14-eicosadienoic acids were identified: 15-HETE, 12-HETE, 11-HETE, 9-HETE, 8-HETE, 5-HETE, 13-hydroxy-octadecadienoic acid (13-HODD), 9-HODD and 15-hydroxy-eicosadienoic acid (15-HEDE). The results suggested that 12-HETE, 13-HODD and 9-HODD are the most abundant monohydroxy fatty acids in the psoriatic skin extracts described above. Assays of 13-HODD, 9-HODD and 15-HEDE for chemokinetic activity were negative with concentrations up to 10(-4)M. The biological significance of these three compounds in not known, but some of the hydroxylated metabolites of arachidonic acid may, by virtue of their chemotactic properties, be relevant to the pathogenesis of the psoriatic neutrophil infiltrate.

Leukotriene B4-like material in scale of psoriatic skin lesions

1 Acidic lipid extracts of scale from the lesions of the skin disease, psoriasis, were purified by straight phase high performance liquid chromatography (h.p.l.c). Assay of fractions by an agarose microdroplet chemokinesis method showed the presence of biologically active material that co‐eluted with standard leukotriene B4 (LTB4). 2 LTB4‐like chemokinetic activity was also detected in fractions collected on reversed phase h.p.l.c. of psoriatic scale extracts that were initially purified by straight phase h.p.l.c. 3 No LTB4‐like activity was detected after similar purification of scale obtained by abrasion of large areas of normal skin. 4 The LTB4‐like material found in extracts of psoriatic scale may play a role in the pathogenesis of the neutrophil infiltrate which characterizes psoriasis.

Contrasting in vitro lymphocyte chemotactic activity of the hydroxyl enantiomers of 12-hydroxy-5,8,10,14-eicosatetraenoic acid

1 The chemotactic activity of 12(R)‐hydroxy‐5,8,10,14‐eicosatetraenoic acid (12(R)‐HETE), 12(S)‐HETE and leukotriene B4 (LTB4) for human mixed peripheral blood lymphocytes has been assessed in a 48‐well microchemotaxis assay. Responses to the standard lymphocyte chemoattractants, zymosan‐activated plasma, casein and N‐formyl‐methionyl‐leucyl‐phenylalanine (fMLP) were also measured. 2 12(R)‐HETE was shown to be chemotactic for lymphocytes over the range 5 × 10−7 to 5 × 10−5 m. In contrast, negligible chemotactic responses to 12(S)‐HETE were obtained. 3 LTB4 was 200 times more potent than 12(R)‐HETE as a lymphocyte chemoattractant, although maximal responses to the two agonists were similar. 4 12(R)‐HETE and LTB4, which are present in extracts of samples from the skin lesions of psoriasis, may be, at least in part, responsible for the lymphocyte infiltrate which is a characteristic feature of this disease.

Leukotrienes C4 and D4 psoriatic skin lesions

Chemoattractant arachidonate lipoxygenase products have been recovered from the skin lesions of psoriasis, and may play a role in eliciting the intra-epidermal neutrophil infiltrate that characterises this disease. In view of evidence for lipoxygenase activity in psoriasis, the characteristic vasodilation in psoriatic lesions, and the vasodilator properties of leukotriene (LT) C4 and D4 in human skin, the presence of these LTs in psoriatic lesions has been investigated. Skin chamber fluid from abraded psoriatic lesions contained significantly greater amounts of immunoreactive material than that from clinically normal skin, as determined by a double antibody radioimmunoassay (RIA) that uses antiserum cross-reacting with both LTC4 and LTD4. Purification of lesional chamber fluid and scale extracts by high performance liquid chromatography (HPLC) and RIA of fractions showed immunoreactivity which co-eluted with standard LTC4 and LTD4. These findings suggest that LTC4 and LTD4 may play a role in mediating the vasodilation and increased blood flow that characterise psoriatic skin lesions.

Proinflammatory properties of unsaturated fatty acids and their monohydroxy metabolites

The proinflammatory effects of unsaturated fatty acids and, where appropriate, their monohydroxy derivatives, have been investigated both by application to human skin and with respect to human polymorphonuclear leukocyte (PMN) migration. Of the fatty acids applied to the skin only eicosapentaenoic and arachidonic acids (EPA; AA) produced consistent, measurable erythema. The monohydroxy derivatives of the two fatty acids also caused erythema, the 12-hydroxy isomers being the most potent. Chemokinetic activity towards PMNs was observed in the presence of AA, EPA and alpha-linolenic acid using an agarose microdroplet chemokinesis assay. In contrast to their in vivo properties, the 5-hydroxy isomers of AA and EPA were the most potent, being approximately 10 times more chemokinetically active than the other isomers. Quantification of the hydroxyeicosatetraenoic and hydroxyeicosapentaenoic acids (HETEs; HEPEs) in the lesional skin of psoriatic patients demonstrated that, of the metabolites measured, 12-HETE was present in the greatest amounts. Twenty five times more 12-HETE than 12- or 15-HEPE was detected, these being the most abundant of the HEPEs formed. The monohydroxy derivatives of AA and EPA may contribute to the inflammatory changes observed in psoriasis. The HETEs appear to be of greater importance than the HEPEs in view of the relative amounts present.

Prostaglandins, hydroxy fatty acids, leukotrienes and inflammation of the skin

There is good evidence that prostaglandins E2, I2 and D2 are mediators of the vascular events of experimentally induced acute inflammation in human skin, but their role in inflammatory skin diseases such as atopic eczema remains to be determined.

The role of chemo-attractant lipoxygenase products in the pathogenesis of psoriasis.

The histopathology of chronic, stable plaque psoriasis is characterized by acanthosis with mitotic figures in the lower epidermis, abnormalities of differentiation, vascular dilatation and tortuosity, and leukocyte infiltrates. These infiltrates are composed of neutrophils and mononuclear cells, the former apparently migrating from the dermal papilla, through the Malpighian layer and into the parakeratotic stratum corneum. Mononuclear cells are evident in the superficial dermis and in the lower half of the epidermis (Ragaz & Ackerman, 1979; Soltani & Van Scott, 1972; Helwig, 1958; Pinkus & Mehregan, 1966). The nature of the earliest histopathological change in developing psoriatic lesions is the subject of controversy with, for example, one group (Chowaniec et al, 1981) reporting infiltration of polymorphonuclear leukocytes into the epidermis to be the earliest change, another (Ragaz & Ackerman, 1979) finding a superficial perivascular lymphocyte infiltrate, and yet another (Brody, 1984a, b) reporting mast cell degranulation as the primary event in acute guttate psoriasis. In addition, Braun-Falco and Schmoeckel (1977) found macrophages to be the predominant infiammatory cells in initial psoriatic lesions. In postulating a role for an endogenously released chemical mediator in the genesis of a pathological change, it is important (i) to recover elevated levels of the mediator from the involved tissue in vivo at an appropriate time, (2) to reproduce at least in part the pathological change by introduction ofthe mediator into the tissue in vivo, in appropriate concentrations, (3) to demonstrate mechanisms for the release and removal of the mediator in the involved tissue, (4) to modify the pathological process by administration of specific enzyme inhibitors or receptor antagonists in vivo, (5) to determine the relative amounts of other chemical mediators with similar modes of action in the involved tissue in vivo and (6) to determine whether production ofthe mediator occurs in other lesions, i.e. whether it is specific to the pathological process under study. It is noteworthy that most ofthe above, which are an extension ofthe criteria of Dale (1934), can only be investigated by in vivo studies. We have chosen to examine the role of acidic lipid neutrophil chemo-attractants in the pathogenesis ofthe infiammatory changes in chronic plaque psoriasis. The evidence obtained to date is presented here in six sections, according to the above criteria.

Psoriatic skin lesions contain a novel lipid neutrophil chemokinetic compound which is distinct from known chemoattractant eicosanoids

1 Lipid extracts of scale from the lesions of the skin disease psoriasis were purified by high performance liquid chromatography (h.p.l.c.). Assay of fractions by an agarose microdroplet method showed the presence of a novel neutrophil chemokinetic compound which possessed the chromatographic properties of a monohydroxy fatty acid, yet was distinct from the chemoattractant eicosanoid, 12‐hydroxyeicosatetraenoic acid, previously isolated in psoriasis. 2 The novel material, termed compound X, was also detected in fractions collected on h.p.l.c. of extracts of chamber fluid samples obtained from abraded psoriatic lesions, but was not detectable in samples from clinically normal skin. 3 Comparison of the straight and reversed phase h.p.l.c. retention times of compound X with those of a range of standard monohydroxy fatty acids, together with further analysis by gas chromatography — mass spectrometry and assay of selected standards for neutrophil chemokinetic activity, failed to reveal the structural identity of compound X. 4 The finding of a further compound in psoriatic lesions, which stimulates neutrophil movement, highlights the complexity of inflammatory mediator production in this disease.

Generalized pruritus and its management

Generalized pruritus is defined here as the syndrome of widespread itching occurring in the absence of primary skin lesions. The subject has been reviewed by Cunliffe (1979) and this contribution is restricted to a discussion of some aetiological factors and advances in therapy.

The clinical and pharmacological effect of lonapalene (RS‐43179), a 5‐lipoxygenase inhibitor, applied topically in psoriasis

33 increased in psoriatic cells (011410014) compared with normal cells (003710001) (P < o 001). There was greater conversion both of total PI and individual PI fractions in samples from psoriatics, implying increased PLC activity. In addition, there was a correlation between MNL PDE activity and MNL cytosol PLC activity (r = o-9). These studies suggest a link between cyclic nucleotide and phosphoinositide regulatory abnormalities which may lead to immune and proliferative aspects of cell dysfunction in psoriasis.