Decreased Membrane Fluidity During Differentiation of Human

The human promyelocytic leukemia cell line. HL-60. undergoes differentiation in the presence of dimethyl sulfoxide (DMSO). phorbol esters, and other agents. Studies were undertaken to determine whether agents that promote differentiation influence membrane fluidity and to assess whether changes in membrane fluidity occur during the process of differentiation. Cells were grown in the presence of DMSO, 1 .25%. or tetradecanolyl-phorbol 1 3-acetate (TPA). 1 .6 x 10 5M. Surface membranes were isolated from cells by sucrose density sedimentation following cell disruption with nitrogen cavitation. Membrane fluidity was assessed by the fluorescence polarization of the membrane probe 1.6diphenyl-1 .3,5-hexatriene (DPH). A progressive decrease in membrane fluidity was first observed on day 3 following exposure of HL-60 cells to DMSO. and this reached a maximum on day 5. coincident with morphological and cytochemical differentiation. Similar changes in membrane fluidity were observed in liposomes prepared from extracted membrane lipids. The ratio of membrane cholesterol to phospholipid was increased 37% on day 5, and there was a small increase in the ratio of saturated to unsaturated fatty acids, with no significant change in phospholipid composition. No changes in membrane fluidity were observed during 1 8 hr of exposure of HL-60 cells to TPA, after which these cells became glass adherent. These studies demonstrate that significant changes in membrane lipid composition and fluidity occur during the process of differentiation in vitro. However. agents that stimulate leukemic cells to differentiate do not themselves appear to perturb bulk membrane fluidity.