Eicosanoids in psoriasis

Pathophysiology of generalized pustular psoriasis

Generalized pustular psoriasis (GPP) is a serious dermatological disease characterized by fever, chills, rigors, and generalized pustule formation on the skin. Previous analyses in Japan have led to the proposal to divide GPP into two groups, one with a history of ordinary psoriasis (pso(+) GPP) and the other without a history of psoriasis (pso(-) GPP). Clinically the onset of the pustular outbreak is earlier in pso(-) GPP, which occurs more frequently after infections, whereas pso(+) GPP occurs more frequently following corticosteroid therapy. Substantial differences are also noted in HLA analyses. Activation of neutrophils is a basic mechanism in both types of GPP. Although the epidermal structural changes in GPP are usually not so prominent as those in psoriasis vulgaris, pso(+) GPP shows a more psoriasiform architecture than pso(-) GPP. Analysis of epidermal cell proliferation in GPP indicates that it is not less than that seen in psoriasis vulgaris. The occasional psoriasiform epidermal architecture especially seen in pso(+) GPP may be considered to be a steady-state condition achieved after epidermal cell proliferation has continued for a sustained period. Various inflammatory cytokines appear to be involved in the neutrophilic infiltrate seen in GPP.

Psoriasis and the arachidonic acid cascade

Arachidonic acid (5.8,11,14-eicosatetraenoic acid C20:4, n-6) is released from the cell membrane by the action of phospholipases on membrane phospholipids. Metabolites of arachidonic acid, which are generically termed eicosanoids, including prostaglandins, thromboxane, leukotrienes and hydroxyeicosatetraenoic acids, have been implicated as mediators or modulators of a number of physiological functions and pathological conditions in both normal and diseased human skin. Particularly, eicosanoids have been suspected to play an important role in the pathogenesis of psoriasis, because a number of phenomena observed in psoriasis can be explained, at least in part, by the action of eicosanoids. This review will focus on recent progress regarding the significance of eicosanoids in the pathogenesis of psoriasis. Recent developments in the molecular biology in the eicosanoids have renewed interest in the role of eicosanoids in psoriasis. New understanding of the etiology of psoriasis and advances in its treatment due to recent progress in eicosanoid biology will also be presented.

Effects of reactive oxygen species on the biosynthesis of 12 (S)-hydroxyeicosatetraenoic acid in mouse epidermal homogenate

Arachidonic acid is converted to 12-hydroxyeicosatetraenoic acid (12-HETE) in a homogenate of mouse epidermal cells. When the epidermal homogenate was preincubated with scavengers of reactive oxygen species (ROS), catalase or superoxide dismutase, significantly larger amounts of 12-HETE were produced as compared to untreated controls, suggesting that 12-lipoxygenase is quite prone to inactivation by ROS and peroxides. Mouse epidermal homogenate was then exposed to nine different ROS-generating systems to study the effects of superoxide, hydrogen peroxide, singlet oxygen, hypochlorite, peroxyl radicals, and alkyl hydroperoxides on the enzyme activity. Analysis by chiral phase high performance liquid chromatography demonstrated that the 12-HETE biosynthesized from arachidonic acid by mouse epidermal homogenate was the 12 (S)-enantiomer and excludes oxidation of arachidonic acid by ROS in a nonspecific free radical mechanism which leads to racemic 12-HETE. ROS generated by the interaction of xanthine with xanthine oxidase strongly inhibited epidermal 12 (S)-HETE biosynthesis. A flux of 0.7 nmol of superoxide/min/ml of reaction medium resulted in more than 50% inhibition of epidermal 12-lipoxygenase activity. The decrease in 12 (S)-HETE biosynthesis appeared to involve both superoxide and hydrogen peroxide. The efficacy of the latter species was also documented by exposure of mouse epidermal 12-lipoxygenase to glucose and glucose oxidase, which resulted in similar inhibitory effects on 12 (S)-HETE biosynthesis. The presence of the iron chelator diethylenetriaminepentaacetic acid during incubation of epidermal 12-lipoxygenase with both the xanthine/xanthine oxidase or the glucose/glucose oxidase systems partially protected the enzyme against inhibition, indicating that hydroxyl radical contributes to the overall inhibitory effect. Also, organic hydroperoxides inhibited epidermal 12-lipoxygenase, whereas singlet oxygen, hypochlorite, and peroxyl radicals were not effective. The results of this study lead to the proposal that 12-lipoxygenase activity may be regulated by ROS such as hydrogen peroxides, superoxide, and hydroxyl radicals.

Antipsoriatic anthrones: aspects of oxygen radical formation, challenges and prospects

1. Antipsoriatic anthrones are among the most commonly used topical agents for the treatment of psoriasis. 2. These drugs generate reactive oxygen species during their auto-oxidation under physiological conditions. 3. Recent studies have indicated that activation of molecular oxygen by anthrones may play a critical role in their mechanism of action at the molecular level. 4. This article summarizes the evidence pointing toward the significance of oxygen activation and radical formation in the antipsoriatic action and induction of skin inflammation of anthrones. 5. Also, the role of oxygen radical formation in the development of novel anthrones devoid of inflammatory and staining properties is discussed.

Inactivation of mouse epidermal 12-lipoxygenase by anthralin--implications for the role of oxygen radicals

In activation of 12-lipoxygenase (12-LO) in mouse epidermal homogenate by the antipsoriatic drug anthralin has been studied in detail. In view of the chemical instability of anthralin in a physiological buffer, the biological effects ascribed to the molecule itself may be related to some of its breakdown products. However, the inhibitory activity could not be attributed to the known stable oxidation product of anthralin, danthron, which did not decrease (12-LO activity. Addition of the antioxidants 2,6-di-tert-butyl-4-methylphenol (BHT) or beta-carotene, or the hydroxyl radical scavenger sodium benzoate, protected against anthralin-mediated 12-LO inactivation, suggesting that pro-oxidant species derived from anthralin play a key role in the inhibitory action. Even though inhibitory effects of anthralin against catalase and superoxide dismutase (SOD) have been observed under the conditions applied in this study, these antioxidant enzymes also partially prevented the inhibition of 12-LO by anthralin when added to the incubation mixtures. Control experiments without anthralin revealed that the oxygen radical scavengers and antioxidant enzymes, themselves, did not appreciably influence epidermal 12-LO activity. A mechanism underlying the inactivation of epidermal 12-LO by anthralin is proposed, which involves active oxygen species formed during the auto-oxidation of the drug.

Dietary modulation of phospholipid fatty acid composition and lipoxygenase products in mouse lung homogenates

This study investigated the potential of dietary fats to modulate the arachidonic acid content of mouse lung phospholipids and the formation of lipoxygenase products from arachidonic and eicosapentaenoic acids. Prior to breeding, female mice were fed for five months diets with 10 wt% of either olive oil, safflower oil, fish oil, or linseed oil. The same diets were fed to the females during gestation and to the pups from day 18 to day 42 postpartum. On day 42, the phospholipids were extracted from fresh lung tissue and separated into classes [phosphatidylethanolamine (PE), phosphatidylserine (PS), phosphatidylcholine (PC), and phosphatidylinositol (PI)] by thin-layer chromatography. Methyl esters of phospholipid fatty acids and unesterified fatty acids were analyzed by gas chromatography. At comparable dietary n-3/n-6 ratios, arachidonic acid was reduced 85 and 75% in lungs from mice fed linseed oil and fish oil, respectively, compared to lungs of safflower oil-fed mice. Dietary fats affected the proportion of arachidonic acid in phospholipids in the order: PE > PC > PS > PI. Following incubation of homogenized lung tissue, the total amount of 12-lipoxygenase products was lowest in lungs from mice fed olive oil, and 12-hydroxyeicosatetraenoic acid was lowest in incubated lungs from mice fed linseed oil. Comparison of the amounts of lipoxygenase substrate fatty acids in the individual phospholipids with the lipoxygenase products suggested that the major substrate pool for the 12-lipoxygenase pathway in mouse lung homogenates was PC.