Abnormal insulin binding and membrane physical properties of a Friend erythroleukemia clone resistant to dimethylsulfoxide-induced differentiation

Changes in Friend murine erythroleukaemia cell membranes during induced differentiation determined by electrorotation

We used electrorotation measurements to investigate alterations in the plasma membranes of DS19 murine erythroleukaemia cells that accompanied erythropoietic differentiation induced by hexamethylene bisacetamide (HMBA). Following 3 days of HMBA treatment, the mean cell membrane specific capacitance determined from electrorotation spectra of individual, viable cells at physiological tonicity (300 mosmol/kg) fell from 1.74 to 1.53 microF/cm2, in agreement with trends observed earlier by dielectrophoretic measurements on bulk cell populations. Scanning and transmission electron microscopy revealed that the relatively high values found for cell membrane capacitance (> 1 microF/cm2) reflected the large area of plasma membrane associated with complex surface morphology including numerous microvilli. Furthermore, it demonstrated that the fall in membrane capacitance during HMBA treatment correlated with a reduction in the density of these complex surface features. Differences in the mechanical characteristics of the cell membranes of untreated and treated cells were then examined by exposing cells to osmotic stress. The intricacy of membrane morphology intensified with increasing osmolality of the suspending medium and this was reflected in higher specific capacitance values. When the osmolality was increased from 210 to 450 mosmol/kg, the mean membrane capacitance of untreated DS19 cells changed from 1.58 to 2.05 microF/cm2 while that for HMBA-treated cells changed from 1.47 to 1.72 microF/cm2, a significantly smaller response. This demonstrated that cells exposed to 72 h of differentiation treatment had an enhanced mechanical resilience as compared with their untreated counterparts, evidencing the early stages of the development of the membrane skeleton which becomes fully developed in mature erythrocytes. Our findings demonstrate the value of electrorotation measurements as a method for the non-invasive characterisation of viable leukaemic cells and their responses to stimuli and show that the membrane capacitance values so derived reflect membrane morphology.

Modifications of LDL-receptor-mediated endocytosis rates in CEM lymphoblastic cells grown in lipoprotein-depleted fetal calf serum

The efficiency of supplying cholesterol by the LDL endocytic pathway of lymphoblastic T CEM cells was compared when incubated in the presence of either fetal calf serum (FCS) or lipoprotein-depleted fetal calf serum (LDFCS). In the presence of FCS, there were 8600 +/- 2000 LDL receptors/cell with a Kd of (2.2 +/- 0.8).10(-8) M and a receptor cycling time of about 7 min; about 90% of the internalized LDL was degraded. LDL degradation produced 98% of total cellular cholesterol and only 2% came from endogenous synthesis. The absence of LDL in the culture medium of lymphoblastic CEM cells deeply modified certain metabolic and structural characteristics of the cells. Their cholesterol content decreased; the total number of LDL receptors increased 6-fold, whereas their affinity for the ligand decreased by the same factor (Kd = (1.2 +/- 0.2).10(-7) M); the receptor cycling time increased 3-fold. Finally, LDL degraded by cholesterol-depleted CEM cells amounted to about 40% of that degraded by untreated CEM cells.

Lipid characteristics of Friend erythroleukaemia cells differentiated by dimethyl sulphoxide or hexamethylenebisacetamide, and of non-inducible clones treated with the inducers

We studied the lipid changes in Friend erythroleukaemia (FEL) cells differentiated by dimethyl sulphoxide (DMSO) or hexamethylenebisacetamide (HMBA). We also examined clones of FEL cells non-inducible to differentiation by DMSO or HMBA for lipid changes not related to the differentiation process. A decrease in triacylglycerols was found in both DMSO- or HMBA-differentiated FEL cells, whereas a decrease in phosphatidylethanolamine was observed only in DMSO-differentiated FEL cells. The characteristics of the phospholipid fatty acid profiles of FEL cells, which proved to be similar to those of other transformed cells, were not significantly affected by erythroid differentiation.

Modification of membrane physical properties, biological response and insulin binding in Friend cells by low serum concentration

The effect of low serum concentration on plasma membrane fluidity and lipid composition, differentiation and insulin binding was investigated in three Friend erythroleukemia clones. Both FLC (clones No. 745) and F(+) (Ostertag F4N) Friend erythroleukemia cells can be induced to differentiate and to produce hemoglobin when exposed to DMSO. Clone R(3) (Ostertag F4-D5-1) is a DMSO-resistant clone when grown under normal conditions (15% serum) but could undergo differentiation with accumulation of protoporphyrin IX upon induction with DMSO when grown in low serum concentration (2.5% serum). Electron spin resonance measurements of the order parameters (S) and S(T parallel) demonstrate that R(3) has a more fluid plasma membrane than the FLC and F(+). The order parameters of the outer hyperfine splittings S(T parallel) at 37 degrees C are 0.60 +/- 0.009, 0.62 +/- 0.008 and 0.64 +/- 0.009 for R(3), F(+) and FLC, respectively. We have used the insulin receptors as a model for a polypeptide hormone receptor associated with the plasma membrane of the Friend clones. Insulin binding studies demonstrated that the receptor of R(3) had a decreased affinity for insulin manifest as a 10-fold increase in the amount of insulin required to compete for half of the tracer binding (18 nM for R(3) vs. 2 nM for FLC and F(+)). Computer-fit Scatchard plot analysis by the negative cooperativity model reveal that R(3) possessed a similar number of sites/cell (about 70,000) as the FLC or F(+) cells, with similar high and low affinities. Growing the DMSO-resistant clone R(3) in low serum concentration caused a decrease in receptor number by 35%, and an increase in receptor affinity to that seen with the differentiable clones. Thus, the abnormal properties of the plasma membrane and insulin receptor of the DMSO-resistant clone in our earlier report (Simon et al. (1984) Biochim. Biophys. Acta 803, 39-47) were partially reversed by growing the cells in a low serum concentration, restoring the cellular response to the differentiation agent.