In the present study, keratinocytes were coincubated with human neutrophils to determine whether or not an increase in leukotriene B4 formation can occur. Human keratinocytes used were cultured in serum-free, low-calcium medium, whereas neutrophils were purified from heparinized venous blood. After coincubations, formation of leukotriene B4 was determined by reversed-phase high-performance liquid chromatography, coupled with its characteristic UV scan. Confirmation and quantification was by radioimmunoassay. Our data revealed that incubations of keratinocytes (1.5 x 10(6)) alone stimulated with calcium ionophore resulted in no detectable amounts of leukotriene B4. In contrast, incubations of neutrophils (5 x 10(6)) alone resulted in the generation of 62.2 +/- 8.5 ng of LTB4. Coincubations of the neutrophils with keratinocytes (ratio 3:1) resulted in a 56-163% increase in leukotriene B4 formation. To delineate the source of the newly formed leukotriene B4, incubations of keratinocytes with leukotriene A4 revealed that keratinocytes can transform leukotriene A4 into leukotriene B4. These latter findings indicate that although keratinocytes cannot directly metabolize arachidonic acid into leukotriene B4 via the 5-lipoxygenase enzyme, they can transform neutrophil-derived leukotriene A4 into leukotriene B4, thus indicating the possible existence of a putative keratinocyte-leukotriene A4 hydrolase. It is therefore reasonable to speculate that the keratinocytes possess the capacity to generate leukotriene B4 in the epidermis when provided leukotriene A4 and thereby can amplify the inflammatory processes occurring during neutrophil exocytosis. These findings indicate that transcellular metabolism of arachidonic acid metabolites in the epidermis by keratinocytes and neutrophils may contribute to the high levels of leukotriene B4 in lesional skin of inflammatory skin diseases.
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