Decreased microviscosity of membrane lipids in leukemic cells: two possible mechanisms

Steady-state fluorescence polarization studies with the fluorescent lipid probe 1,6-diphenyl 1,3,5-hexatriene were done to determine the degree of microviscosity of cellular membrane lipids and serum lipoproteins in human normal donors and leukemic patients. The results show a marked decrease in microviscosity of cellular membrane lipids in both intact lymphocytes and isolated cellular plasma membranes obtained from leukemic patients in clinical relapse as compared to intact lymphocytes and isolated cellular plasma membranes obtained from normal donors and leukemic patients in complete clinical remission. Concomitant to these dynamic changes in cellular membrane lipids, the degree of microviscosity of lipids in the blood serum of leukemic patients in clinical relapse is markedly reduced as compared to serum obtained from normal donors and leukemic patients in complete clinical remission. Moreover, an in vitro incubation of leukemic lymphocytes with normal low density lipoproteins results in an increased microviscosity of cellular membrane lipids. In addition to the interrelation between cellular membrane lipids and serum lipoproteins, plasma membrane vesicles with a high degree of lipid microviscosity were isolated from the blood serum and pleural effusion of leukemic patients in clinical relapse. Such membrane vesicles could not be detected in normal serum. Therefore, we suggest that the two major mechanisms associated with the decreased microviscosity of membrane lipids in human leukemic cells are an abnormal exchange in lipids between the leukemic cell surface membrane and leukemic serum lipoproteins and an exfoliation of plasma membrane vesicles with a high degree of microviscosity from the cell surface of leukemic cells.