Studies on the Regulation and Localization of 5‐Lipoxygenase in Human B‐Lymphocytes

Stimulated B‐lymphocytes, isolated from patients with chronic lymphocytic leukemia of B‐cell type (B‐CLL cells) or from human tonsils, produced similar amounts of leukotriene (LT) B4 and 5–hydroxy‐eicosatetraenoic acid (5‐HETE) as polymorphonuclear granulocytes. Unlike intact granulocytes or monocytes, human B‐lymphocytes require calcium ionophore, exogenous arachidonic acid and an oxidative environment in order to produce 5‐lipoxygenase products. Several thiol‐reactive compounds such as N‐ethylmaleimide, methyl methanethiosulfonate, azodicarboxylic acid bisfdimethylamide] (diamide) as well as hydrogen peroxide were all found to stimulate cellular leukotriene biosynthesis. Reverse transcriptase (RT)‐PCR analysis demonstrated the expression of 5‐lipoxygenase, 5‐lipoxygenase‐activating protein (FLAP) and LTA4 hydrolase mRNA in B‐CLL cells. Western blot analysis demonstrated a band corresponding to the molecular size of FLAP in the B‐CLL cell membrane. Furthermore, MK886, the FLAP‐binding cellular leukotriene biosynthesis inhibitor, reduced both LTB4 and 5‐HETE formation. Immunocy‐tochemistry showed that 5‐lipoxygenase was mainly localized in the nuclei of non‐activated B‐CLL cells, tonsillar B‐lymphocytes and monoclonal B‐cells. In contrast, neither human peripheral T‐lymphocytes nor Jurkat cells were stained. These results suggest that 5‐lipoxygenase and its products function in the nucleus of B‐lymphocytes.