Human keratinocytes lack the components to produce leukotriene B4

The cellular origin of leukotriene B4 (LTB4), a potent pro-inflammatory molecule present in psoriatic lesions, has yet to be determined. In the present study, cultured human keratinocytes were evaluated for their ability to produce LTB4. Keratinocytes stimulated under a variety of conditions did not produce detectable amounts of LTB4, as measured by enzyme immunoassay and liquid chromatographic techniques. Prostaglandin E2 and 15-hydroxyeicosatetraenoic acid were the only eicosanoids detected. The capacity of keratinocytes to synthesize 5-lipoxygenase (5-LO) products, or lack thereof, was further evaluated by preparing subcellular fractions and examining them for the presence of 5-LO activity and the proteins responsible for LTB4 production. Using Western blot analysis, we detected no bands that migrated with the 78-kDa 5-LO enzyme. Subcellular fractions were also examined for the presence of the 5-LO-activating protein (FLAP). This protein, which is essential to 5-LO activity, could not be detected in any keratinocyte preparation examined. Consistent with the absence of proteins, the mRNAs for 5-LO and FLAP were undetectable by reverse transcriptase polymerase chain reactions analysis. These results demonstrate that human keratinocytes lack the crucial proteins necessary for LTB4 production.