Expression of a functional inducible nitric oxide synthase in hairy cell leukaemia and ESKOL cell line

The expression of nitric oxide synthase (NOS) isoforms was investigated in the established ESKOL hairy cell line and in leukemic cells of patients wit h hairy cell leukemia (HCL). By reverse transcription-polymerase chain reac tion (RT-PCR), these cells were found to spontaneously express inducible NO S (iNOS)-specific mRNA, but not endothelial constitutive NOS (ecNOS) mRNA. The iNOS protein was detected by immunofluorescence in the cytoplasm of per meabilized leukemic cells and ESKOL cells, using different anti-iNOS monocl onal antibodies. A protein of 135 kDa was identified by Western blotting in ESKOL and HCL lysates, confirming the presence of an iNOS in these cells. Cytosolic homogenates displayed NOS catalytic activity, as measured by the conversion of C-14-labelled L-arginine into C-14 L-citrulline and by detect ion in situ using the DAF-PDA (diaminofluorescein diacetate) NO-sensitive f luorescent probe. Ligation of CD23 (low affinity IgE receptor) was found to increase iNOS expression in ESKOL and conversely to decrease the percentag e of cells undergoing apoptosis, as measured by the percentage of cells exp ressing annexin V. These results indicate that, as in chronic B cell lympho cytic leukemia cells (B-CLL) a functional iNOS is expressed constitutively in hairy cells that contributes to protecting these tumoral cells from apop tosis.