Cholesterol modulation of membrane fluidity and ecto-nucleotide triphosphatase activity in human normal and CLL lymphocytes

Lymphocytes isolated from the peripheral blood of patients with chronic lymphocytic leukemia (CLL) and from healthy donors were studied for membrane fluidity and lipid phase separation. The degree of fluidity of the surface membrane lipids was estimated by fluorescence polarization analysis using the probe 1,6-diphenyl-1,3,5-hexatriene (DPH) embedded in the lipid core of the surface membrane of intact cells. CLL lymphocyte showed a more fluid lipid bilayer in their plasma membrane than normal lymphocytes. Treatment of both groups of lymphocytes with cholosteryl hemisuccinate (CHS) complexed with PVP and BSA resulted in rigidification of cell membranes. A lipid thermotropic transition temperatures was observed at 23.6 + 1.1° in normal lymphocytes and 16.3 + 1.0° in CLL lymphocytes, which rose to 32.3 + 1.3° in normal and was abolished in CLL lymphocytes after treatment with CHS. 'Compound lipid fluidizer' reduced the thermotropic transition temperature to 17.5 + 1.0° and 15.1 + 0.9° in normal and CLL lymphocytes, respectively. A statistically significant increase (p < 0.01) in the specific activity of the ecto-ATPase was observed in CLL lymphocytes as compared to normal cells. A dramatic increase of the ecto-ATPase activity was also observed when normal lymphocytes were treated with 0.05-0.5 m M CHS. The biological significance of these results is discussed in terms of modifications in the lipid-protein interactions in lymphocyte plasma membrane induced by CHS and the implications in the proliferation of leukemic cells.