FATTY-ACID METABOLISM IN PHORBOL ESTER-DIFFERENTIATING HUMAN-LEUKEMIA CELLS

Abstract

Human promyelocytic leukemia cells (HL-60) undergo differentiation when treated with the tumor promoter 12-O-tetradecanoylphorbol-13-acetate (TPA). As the primary target for TPA action is membranes, studies were undertaken to determine whether phorbol ester exposure would influence fatty acid metabolism in these cells. In cells incubated with labeled fatty acids for 1 h the percentage of distribution of lipid radioactivity is highest in the phospholipid fraction of control cultures, whereas in TPA-supplemented cells, substantially more label is associated with triacylglycerols. The specific activity of phospholipids and triacylglycerols was lower in treated than in control cells; the amount of cellular triacylglycerols increased 3.2-fold (lipid per mg protein). The increase in the amount of cellular phospholipids in TPA-treated cells is not as pronounced (approximately 50% > control), and this only occurs at higher concentrations of TPA. At early times after TPA exposure there is no stimulation of the cellular uptake of labeled fatty acids; differentiating cells (24-48 h of TPA) when incubated with label contained more radioactivity than did control cultures. Cells treated with TPA for 48 h show a marked decrease in the conversion of [1-14C]stearic acid to monoenoic product (22% of control); this decrease is dose dependent and occurs within 24-48 h of treatment. Although the phospholipid fatty acid composition of differentiating cells was similar to control cells, acyl groups of triacylglycerols isolated from treated cells showed a marked decrease in the percentage of unsaturates. TPA treatment of HL-60 cells evidently has a profound effect on fatty acid metabolism. The lack of an effect of TPA on fatty acid metabolism after short-term exposure to the promoter suggests that the modifications observed may be the result of cellular differentiation rather than a direct effect exerted by the presence of TPA in the culture media.