Plasma membrane studies on drug sensitive and resistant cell lines. IV. Rubidium transport and ouabain binding

Cell and membrane lipid analysis by proton magnetic resonance spectroscopy in five breast cancer cell lines

The lipid composition of five human breast cancer cell lines (MCF-7, T47D, ZR-75-1, SKBR3 and MDA-MB231) was assessed by proton magnetic resonance spectroscopy (MRS) in whole cells and membrane-enriched fractions. The proportions of the three main lipid resonances in 1D spectra were different for each cell line. These resonances included mobile methyl and methylene functions from fatty acids of triglycerides and phospholipids and N-trimethyl from choline of phospholipids. T47D and ZR-75-1 cells presented a high methylene/methyl ratio (6.02 +/- 0.35 and 6.28 +/- 0.90). This ratio was significantly lower for SKBR3, MCF-7 and MDA-MB231 cells (2.76 +/- 0.22, 2.27 +/- 0.57 and 1.39 +/- 0.39). The N-trimethyl/methyl ratio was high for MDA-MB231 and SKBR3 cells (1.38 +/- 0.54 and 0.86 +/- 0.32), but lower for MCF-7, T47D and ZR-75-1 cells (0.49 +/- 0.11, 0.16 +/- 0.07 and 0.07 +/- 0.03). 2D COSY spectra confirmed these different proportions in mobile lipids. From 1D spectra obtained on membrane preparations, T47D and ZR-75-1 were the only cell lines to retain a signal from mobile methylene functions. These differences might be related to the heterogeneity found for several parameters of these cells (tumorigenicity, growth rate, hormone receptors); an extended number of cases from fresh samples might enable clinical correlations.

Retinoic-acid-induced differentiation of HL-60 cells is associated with biphasic activation of the Na+-K+ pump

The human promyelocytic leukemia cell line, HL-60, can be induced to differentiate into granulocyte-like cells when cultured in the presence of 10(-6) M retinoic acid (RA) for several days. Following the addition of RA two kinds of changes occur. First, there are early changes that comprise an increase in the intracellular concentration of sodium ions [Na]i, which reaches its maximum after 6 h, and an increase in the activity of the Na+-pump, which is reflected by an ouabain-sensitive K+ influx that peaks at 8 h (170% of the control value) and that occurs without any change in the number of pump molecules, as measured by the binding of 3H-ouabain. Second, beginning after 12 h of culture with RA, a decrease in the number of ouabain-binding sites occurs, this being accompanied by an increase in the number of K+ ions actively transported by each site. The effect of modulation of Na+-pump activity on the RA-induced differentiation of HL-60 cells was studied using low, noncytotoxic concentrations of ouabain which, although alone having no differentiating effect, accelerated and potentiated the effect of RA on differentiation. When added in combination, these drugs induced rapid stimulation of the Na+-pump, which reached its peak after 2 h. These results indicate that a concomitant increase in the level of [Na+]i and in the activity of the Na+-pump constitute primary events in the interaction between RA and HL-60 cells, and that cation fluxes may play a role in the initiation of the process of differentiation.

Specific involvement of calmodulin and non-specific effect of tropomyosin in the sensitivity to ouabain of Na+,K+-ATPase in murine plasmocytoma cells

The Kd for ouabain for inhibition of Na+,K+-ATPase isolated from murine plasmocytoma MOPC 173 cells is 120 microM, but when isolated in the presence of EDTA, it is 100-fold lower (1.2 microM). Simultaneous addition of muscle tropomyosin and calcium to sensitive membranes restored the original insensitivity (tropomyosin bound to the membranes in an irreversible and saturable manner). For comparison 86Rb influx into intact cells, mediated by the Na+,K+-pump, is half-maximally inhibited at 50 microM ouabain. Calcium converts the enzyme to an insensitive form. This appeared to involve calmodulin because after extraction of calmodulin with EDTA and EGTA from sensitive membranes, they could not be made insensitive by the addition of tropomyosin and Ca2+. Addition of exogenous calmodulin to these calmodulin-depleted membranes was required, in addition to tropomyosin and Ca2+, to decrease the ouabain sensitivity. The involvement of calmodulin was further assessed by measuring the range of Ca2+ concentrations required to convert to the insensitive form. At saturating concentrations of tropomyosin, increasing free [Ca2+] up to 3 microM led to an heterogeneous population of Na+,K+-ATPase forms. The calcium dependency was a saturable process. The shift to the insensitive form was half maximal at 0.65 + 0.11 microM free Ca2+ and was abolished by the addition of troponin I or trifluoroperazine (0.1 mM). These results suggest that, in murine plasmocytoma cells, the intrinsic sensitivity of Na+,K+-ATPase to ouabain might be regulated by a calmodulin-dependent process within a submembrane contractile-like environment.