SERUM FACTORS REGULATE 5-LIPOXYGENASE ACTIVITY IN MATURATING HL-60 CELLS

5-Lipoxygenase activity in DMSO-differentiated HL60 cells is regulated by human serum. The serum effect depended on the differentiation stat e of the cells. For a stimulatory effect to occur, it was required tha t the cells had been treated with DMSO before addition of serum. After this regimen, the HL60 cells acquired the same high 5-LO activity as found for human neutrophils isolated from peripheral blood (about 9-ti mes higher than for HL60 cells treated only with DMSO). On the other h and, when serum was added together with DMSO and present during the en tire differentiation period (seven days), or withdrawn after the first four days, the 5-LO activity did not increase. 5-LO activity of HL60 cells covaried with the expression of the CD14 molecule, a marker for myeloid cell maturation which was recently identified as a receptor fo r the complex of LPS and LPS-binding protein. These serum effects on 5 -LO activity were only observed for intact cells. The prominent increa se in 5-LO activity induced by serum was not concomitant with similar changes in the expression of 5-LO or 5-LO-activating protein (FLAP), a s judged from analyses of immunoreactive protein and mRNA. Also, the h igh 5-LO activity induced by serum was rather insensitive to the drug MK886 under our standard assay conditions, which included addition of exogenous arachidonic acid (40 muM). The results indicate that additio nal cellular components of importance for 5-LO activity in HL60 cells become operative after serum treatment, and that mere expression of 5- LO and FLAP is insufficient for high 5-LO activity in intact cells.