Identification and subcellular localization of leukotriene A4-hydrolase activity in human epidermis

Eicosanoids and Keratinocytes in Wound Healing

Significance: Eicosanoids are biologically active lipid mediators derived from arachidonic acid that are important in injury and inflammatory responses. Cyclooxygenase-1 and cyclooxygenase-2 mediate the production of prostanoids, whereas 5-lipoxygenase mediates the production of leukotrienes and hydroxyeicosatetraenoic acids. These lipid mediators have traditionally been known to recruit cells of the immune system to a site of injury and inflammation. However, they also interact with various cells that are resident to the wound bed, including modulation of keratinocyte activity. Recent Advances: Recent work has identified multiple prostanoid and leukotriene receptors on keratinocytes, indicating that eicosanoids directly interact with them. Recent work also shows that keratinocytes are capable of producing prostanoids and leukotrienes. Critical Issues: Much of the critical work has been performed in cell culture and mouse in vivo models. This has greatly expanded our understanding of the eicosanoid interactions with keratinocytes and wound healing in general. However, few of these in vivo models have been able to critically evaluate keratinocyte migration and re-epithelialization. Future Directions: As research continues in this exciting field, the cellular pathways stimulated by the eicosanoids will become better defined. Future research with excisional wound models in mice and pigs and ex vivo human skin models will better isolate the contribution of eicosanoid-mediated effects on keratinocyte migration and re-epithelialization.

Transcellular biosynthesis contributes to the production of leukotrienes during inflammatory responses in vivo

Leukotrienes are lipid mediators that evoke primarily proinflammatory responses by activating receptors present on virtually all cells. The production of leukotrienes is tightly regulated, and expression of 5-lipoxygenase, the enzyme required for the first step in leukotriene synthesis, is generally restricted to leukocytes. Arachidonic acid released from the cell membrane of activated leukocytes is rapidly converted to LTA(4) by 5-lipoxygenase. LTA(4) is further metabolized to either LTC(4) or LTB(4) by the enzyme LTC(4) synthase or LTA(4) hydrolase, respectively. Unlike 5-lipoxygenase, these enzymes are expressed in most tissues. This observation previously has led to the suggestion that LTA(4) produced by leukocytes may, in some cases, be delivered to other cell types before being converted into LTC(4) or LTB(4). While in vitro studies indicate that this process, termed transcellular biosynthesis, can lead to the production of leukotrienes, it has not been possible to determine the significance of this pathway in vivo. Using a series of bone marrow chimeras generated from 5-lipoxygenase- and LTA(4) hydrolase-deficient mice, we show here that transcellular biosynthesis contributes to the production of leukotrienes in vivo and that leukotrienes produced by this pathway are sufficient to contribute significantly to the physiological changes that characterize an ongoing inflammatory response.

Involvement of leukotriene B(4) in substance P-induced itch-associated response in mice

Intradermal injection of substance P elicits an itch sensation in human subjects and an itch-associated response in mice. The substance P-induced itch-associated response in mice is not inhibited by antihistamine. Therefore, the mechanisms of substance P-induced itch-associated response are unclear. In this study, we demonstrated one of the mechanisms. Substance P induces an arachidonate cascade to produce prostaglandins and leukotriene. In this study we considered whether arachidonate metabolites are involved in the substance P-induced itch-associated response. A phospholipase A(2) inhibitor arachidoryltrifluoromethyl ketone inhibited the substance P-induced itch-associated response in mice. Pre treatment with the glucocorticoids betamethasone and dexamethasone also produced inhibition of the substance P-induced itch-associated response in mice as well as humans. The 5-lipoxygenase inhibitor zileuton, but not the cyclooxygenase inhibitors indomethacin and diclofenac, suppressed substance P-induced itch-associated response. The leukotriene B(4) receptor antagonist 5-[2-(2-carboxyethyl)-3-[6-(4-methoxyphenyl)-5E-hexenyl]oxyphenoxy]valeric acid produced inhibition, whereas pranlukast (leukotriene C(4)/D(4)/E(4) receptor antagonist) and 5(Z)-7-[1S,2S, 3S,5R-3-(trans-b-styren)sulfonamido-6,6-dimethylbi cyclo(3,1,1)hept-2-yl]-5-heptenoic acid (EP(1) receptor antagonist) were without effect. Furthermore, when the production of leukotriene B(4) and prostaglandin E(2) was measured in skin injected with substance P and in mouse keratinocytes applied with substance P, the level of both products increased. As leukotriene B(4), but not prostaglandin E(2), also induces the itch-associated response in mice, these results suggest that leukotriene B(4) and keratinocytes, cutaneous cells which produced leukotriene B(4), play an important role in substance P-induced itch-scratch response in mice. Leukotriene B(4) receptor antagonist and 5-lipoxygenase inhibitor may be novel antipruritic drugs.

LTA4 hydrolase in human skin: decreased activity, but normal concentration in lesional psoriatic skin. Evidence for different LTA4 hydrolase activity in human lymphocytes and human skin

Leukotriene A4 (LTA4) hydrolase which transforms LTA4 into the proinflammatory compound LTB4 has been identified in human epidermis. The purpose of this study was to investigate the potential role of this enzyme in psoriasis, in which LTB4 is present in biologically active concentrations. The concentration and activity of LTA4 hydrolase was determined in normal skin and in matched samples of involved and uninvolved psoriatic skin. The enzyme content was determined using an affinity-purified antibody. This antibody was also used for immunohistochemical staining of skin biopsies. Immunohistochemically LTA4 hydrolase was localized predominantly in the basal and spinous layers in normal skin and in involved and uninvolved psoriatic skin. The LTA4 hydrolase content varied between 2.8 and 3.1 micrograms enzyme/mg protein and was found to be similar in normal and psoriatic skin, involved as well as uninvolved. In contrast, the activity of the enzyme was decreased significantly in involved psoriatic skin (9.9 +/- 2.1 micrograms LTB4/mg enzyme per min) compared with matched uninvolved psoriatic skin (16.4 +/- 3.5 micrograms LTB4/mg enzyme per min), but was decreased only insignificantly compared with normal skin (12.4 +/- 1.8 micrograms LTB4/mg enzyme per min). It was found that the conversion of LTA4 to LTB4 results in inactivation of LTA4 hydrolase activity. This finding is compatible with the idea that the decreased LTA4 hydrolase activity in involved psoriatic skin reflects transcellular LTB4 formation in vivo. In peripheral lymphocytes the enzyme content was 1.3 +/- 0.3 microgram enzyme/mg protein in normal lymphocytes and 1.4 +/- 0.3 microgram enzyme/mg protein in psoriatic lymphocytes, which was significantly lower than in the skin. In contrast, the specific LTA4 hydrolase activities in normal and psoriatic lymphocytes (23.4 +/- 1.3 and 21.3 +/- 1.7 micrograms LTB4/mg enzyme per min) were significantly higher than in normal skin. These findings may indicate the existence of LTA4 hydrolase isoforms in human lymphocytes and human skin.

The significance of polyunsaturated fatty acids in cutaneous biology

The skin epidermis displays a highly active metabolism of polyunsaturated fatty acids (PUFA). Dietary deficiency of linoleic acid (LA) and 18-carbon (n-6) PUFA results in characteristic scaly skin disorder and excessive epidermal water loss. Arachidonic acid, a 20-carbon (n-6) PUFA is metabolized via the cyclooxygenase pathway into predominantly prostaglandin E2 (PGE2) PGF2 alpha, and PGD2 and via the lipoxygenase pathway into predominantly 15-hydroxyeicosatetraenoic acid (15-HETE). The prostaglandins modulate normal skin physiological processes at low concentrations and inflammatory reactions at high concentrations. Similarly, the very active epidermal 15-lipoxygenase transforms dihomogammalinolenic acid (DGLA) into 15-hydroxy eicosatrienoic acid (15-HETrE), eicosapentaenoic acid (EPA) into 15-hydroxyeicosapentaenoic acid (15-HEPE) and docosahexaenoic acid (DHA) into 17-hydroxydocosahexaenoic acid (17-HDoHE), respectively. These monohydroxy acids exhibit anti-inflammatory properties. In contrast, the 18-carbon (n-6) PUFA is transformed into 13-hydroxy-9,11-octadecadienoic acid (13-HODE), which exerts antiproliferative properties in the tissue. Thus, the supplementation of diets with appropriate purified vegetable oils and/or fish oil may generate local cutaneous anti-inflammatory metabolites which could serve as a less toxic in vivo monotherapy or as adjuncts to standard therapeutic regimens for the management of skin inflammatory disorders.

Characterization of the aminopeptidase activity of epidermal leukotriene A4 hydrolase against the opioid dynorphin fragment 1-7

Leukotriene A4 hydrolase is a bifunctional cytosolic enzyme, which both hydrolyses leukotriene A4 (LTA4) into leukotriene B4 (LTB4) and exerts aminopeptidase activity against opioid peptides. In the present study we have investigated whether the peptides angiotensin I and II, bradykinin, kallidine, histamine, dynorphin fragment 1-7 and substance P can act as substrates for epidermal and neutrophil LTA4 hydrolase. Among the tested substrates, dynorphin fragment 1-7 was found to be the best substrate for the enzyme. The aminopeptidase activity of epidermal and neutrophil LTA4 hydrolase against dynorphin fragment 1-7 was further characterized. The enzyme was purified from human epidermis and human neutrophils by anion exchange chromatography (Q-Sepharose) and affinity chromatography on a column with the LTA4 hydrolase inhibitor bestatin coupled to AH-Sepharose. The incubation of the dynorphin fragment 1-7 with LTA4 hydrolase resulted in the formation of tyrosine. The presence of the N-terminal amino acid tyrosine is essential for the interaction of opioids with their receptors, and this finding indicates that the LTA4 hydrolase can inactivate dynorphin fragment 1-7. After the two purification steps no other aminopeptidases acting at the N-terminal tyrosine of dynorphin fragment 1-7 was present in the preparation. This was demonstrated by the abolishment of the degradation at the N-terminal end of dynorphin fragment 1-7 when preincubating the enzyme preparation with LTA4 before the incubation with the dynorphin fragment 1-7. The abolishment of the aminopeptidase activity shows that activation of the hydrolase part of the enzyme, with conversion of LTA4 into the potent proinflammatory compound LTB4, results in an inhibition of the aminopeptidase activity of the enzyme. As a result, the catabolism of dynorphin fragment 1-7 and probably of other opioid peptides is inhibited, resulting in sustained biological effects of these opioids. This phenomenon may be important for the maintenance of inflammation in skin conditions, such as psoriasis and atopic dermatitis, in which LTB4 is formed.

Purification and characterization of leukotriene A4 hydrolase from human epidermis

The leukotriene A4 hydrolase is a central enzyme in leukotriene B4 formation. Unlike 5-lipoxygenase, leukotriene A4 hydrolase activity is present in normal human epidermis, where it is likely to be involved in transcellular leukotriene formation. In this study the leukotriene A4 hydrolase was purified from human epidermis and human cultured keratinocytes and compared with leukotriene A4 hydrolase from human neutrophils. To purify leukotriene A4 hydrolase from human epidermis a new non-specific affinity chromatography column, with the leukotriene A4 hydrolase inhibitor bestatin coupled to AH-Sepharose, was introduced. The epidermal leukotriene A4 hydrolase was purified to apparent homogeneity and the molecular weight was determined to be approximately 70,000 Da by SDS-PAGE. The pI was 5.1-5.4 for the epidermal as well as the keratinocyte and neutrophil leukotriene A4 hydrolase, as determined by chromatofocusing. Only minor differences in the amino acid composition were seen between the three enzyme sources. The optimal pH for the hydrolase activity was 7.5-8.5 for the epidermal and neutrophil leukotriene A4 hydrolases. Finally, it was also shown that the epidermal leukotriene A4 hydrolase undergoes suicide inactivation when transforming leukotriene A4 into leukotriene B4. It was concluded that there is a close resemblance between the epidermal leukotriene A4 hydrolase and the hydrolase found in other cell types. Therefore, the human epidermis may be a good model for the in vivo study of transcellular leukotriene formation.

Human epidermis transforms exogenous leukotriene A4 into peptide leukotrienes: possible role in transcellular metabolism

Leukotriene B4 formation can take place by cell interaction between keratinocytes and neutrophils. Thus, keratinocytes without proven 5-lipoxygenase activity can transform neutrophil-derived leukotriene A4 into leukotriene B4. The purpose of the present study was to investigate whether human epidermis is able to transform leukotriene A4 sequentially into the peptide leukotrienes (LTC4, LTD4 and LTE4). Epidermis isolated using the suction blister technique or keratomed skin specimens were incubated with either neutrophils or exogenously added leukotriene A4. Peptide leukotrienes were determined by integrated optical density after RP-HPLC separation, and the identity of leukotrine C4 was confirmed by (1) the retention time similarity with authentic leukotriene C4; (2) the UV spectrum determined with an on-line diode array detector; and (3) conversion by gamma-glutamyl transpeptidase of the peak coeluting with authentic leukotriene C4 into a new peak coeluting with authentic leukotriene D4. The results of this study showed that while human epidermis cannot form detectable amounts of peptide leukotrienes by itself, it can transform exogenous leukotriene A4 into peptide leukotrienes. Furthermore, coincubation of human epidermis and neutrophils resulted in a marked increase (90%) in peptide leukotriene formation when compared with neutrophils alone, indicating that human epidermis can transform neutrophil-derived leukotriene A4 into peptide leukotrienes. These results indicate that human skin contains leukotriene C4 synthase activity capable of producing significant amounts of leukotriene C4 from leukotriene A4, and that the keratinocytes may play a more active role in peptide leukotriene formation in the skin than previously thought.(ABSTRACT TRUNCATED AT 250 WORDS)