EPIDERMIS CONTAINS PLATELET-TYPE 12-LIPOXYGENASE THAT IS OVEREXPRESSED IN GERMINAL LAYER KERATINOCYTES IN PSORIASIS

Human epidermal cells exhibited none of the cytosolic lipoxygenase act ivity that is prominent in mucosal epithelial cells, but instead conta ined a microsomal activity that converted arachidonic acid to 12-hydro xyeicosatetraenoic acid (12-HETE). Identification of the extractable 1 2-HETE-forming activity as a 12-lipoxygenase (distinct from cytochrome P-450) included (S)-12-stereospecificity of product formation, trappi ng of 12-hydroperoxyeicosatetraneoic acid as an intermediate reaction product, and lack of NADPH dependence for activity. Epidermal cell pol y(A)(+) RNA contained high levels of a 2.3-kb mRNA that selectively hy bridized with human platelet 12-lipoxygenase cDNA, and partial cDNA se quence of this mRNA indicated identity to platelet la-lipoxygenase. Th e epidermal 12-lipoxygenase was not recognized by antibodies against t he leukocyte-type 12- and 15-lipoxygenases (found in leukocytes, retic ulocytes, and mucosal epithelial cells) but was detected by an antipla telet la-lipoxygenase antibody. The epidermal 12-lipoxygenase antigen was selectively expressed in germinal layer keratinocytes in healthy a nd psoriatic skin, and these layers exhibited hyperplasia and increase d immunostaining in inflamed psoriatic skin. Together with previous re sults, these observations indicate that 1) epidermis generates 12-HETE by either cytochrome P-450- or lipoxygenase-based mechanisms dependin g on reaction conditions, and 2) 12-lipoxygenases (originally describe d in hematopoietic cell types) may be expressed in at least two distin ct isoforms in epithelial barriers in humans, and in the case of the s kin, a microsomal (platelet-type) 12-lipoxygenase is selectively overe xpressed in germinal layer keratinocytes during psoriatic inflammation .