Cell membrane fluidity in K562 cells and its relation to receptor expression.

Cell membrane fluidity (CMF) and transferrin receptor (Tf-R) expression were investigated in K562 cells, a human chronic myelocytic leukemia cell line, treated by gamma-interferon (IFN-gamma). CMF was increased using spin-labeled electron spin resonance techniques, and Tf-R expression was measured by flow cytometric analysis with an EPICS-750 flow cytometer/cell sorter. Treatment of K562 cells in suspension culture with IFN-gamma for as long a time as 6 hr caused an increase in CMF, and then returned to the level of control cells at 12 hr. Conversely, by 24 hr after the beginning of treatment, the rigidity of CMF was increased. Thus, the changes of IFN-gamma-induced CMF was biphasic. While the early change of CMF is related to signal generation and transmission, the later change may reflect changes in lipid compositions and/or cytoskeletal complexes of the plasma cell membrane. A significant increase of Tf-R after 6 hr and 24 hr in number was obtained by treatment of K562 cells with IFN-gamma, but at 12 hr the number of Tf-R did not differ from the control. These results suggested that the early phase of upregulation of Tf-R induced by IFN-gamma was caused by increased CMF, and the late phase of upregulation of Tf-R was due to increased rigidity of CMF. In conclusion, the state of CMF associated with a certain receptor expression in cells is not rigid and can be modulated to some extent by exogenous influences. This may open possibilities of some adjuvant therapeutic measures in malignant diseases by increasing the antigenicity of tumor cells.