Membrane regulation of the Na+,K+-ATPase during the neuroblastoma cell cycle: correlation with protein lateral mobility

Cell cycle progression is regulated by intertwined redox oscillators

The different phases of the eukaryotic cell cycle are exceptionally well-preserved phenomena. DNA decompaction, RNA and protein synthesis (in late G1 phase) followed by DNA replication (in S phase) and lipid synthesis (in G2 phase) occur after resting cells (in G0) are committed to proliferate. The G1 phase of the cell cycle is characterized by an increase in the glycolytic metabolism, sustained by high NAD+/NADH ratio. A transient cytosolic acidification occurs, probably due to lactic acid synthesis or ATP hydrolysis, followed by cytosolic alkalinization. A hyperpolarized transmembrane potential is also observed, as result of sodium/potassium pump (NaK-ATPase) activity. During progression of the cell cycle, the Pentose Phosphate Pathway (PPP) is activated by increased NADP+/NADPH ratio, converting glucose 6-phosphate to nucleotide precursors. Then, nucleic acid synthesis and DNA replication occur in S phase. Along with S phase, unpublished results show a cytosolic acidification, probably the result of glutaminolysis occurring during this phase. In G2 phase there is a decrease in NADPH concentration (used for membrane lipid synthesis) and a cytoplasmic alkalinization occurs. Mitochondria hyperfusion matches the cytosolic acidification at late G1/S transition and then triggers ATP synthesis by oxidative phosphorylation. We hypothesize here that the cytosolic pH may coordinate mitochondrial activity and thus the different redox cycles, which in turn control the cell metabolism.

Bioelectrical regulation of cell cycle and the planarian model system

Cell cycle regulation through the manipulation of endogenous membrane potentials offers tremendous opportunities to control cellular processes during tissue repair and cancer formation. However, the molecular mechanisms by which biophysical signals modulate the cell cycle remain underappreciated and poorly understood. Cells in complex organisms generate and maintain a constant voltage gradient across the plasma membrane known as the transmembrane potential. This potential, generated through the combined efforts of various ion transporters, pumps and channels, is known to drive a wide range of cellular processes such as cellular proliferation, migration and tissue regeneration while its deregulation can lead to tumorigenesis. These cellular regulatory events, coordinated by ionic flow, correspond to a new and exciting field termed molecular bioelectricity. We aim to present a brief discussion on the biophysical machinery involving membrane potential and the mechanisms mediating cell cycle progression and cancer transformation. Furthermore, we present the planarian Schmidtea mediterranea as a tractable model system for understanding principles behind molecular bioelectricity at both the cellular and organismal level. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

A unifying model of the cell proliferation emphasizing plasma membrane fluxes

The regulation of cellular growth and proliferation is perhaps the most investigated and elusive problem in cell biology and seems to be possible to solve from almost any angle of study chosen. Among the non-systemic factors that have been discussed are genetic damage, genomic control, regulation by stimulatory and inhibitory peptide factors such as EGF, chalones, and fibronectin, protein kinase activation with tyrosine phosphorylation, adenylylcyclase and cAMP, cGMP, membrane perturbations and specifically in tumours the failure of the Pasteur effect in control of glycolysis, excessive membrane ATPase activity, and excessive hydrolytic and proteolytic activities at the cell surface. This article focuses on the central role of fluxes within the plasma membrane and re-examines the possibility that changes of flux of metabolites, ions, and reducing equivalents may be the common denominator regulating cellular proliferation.

Changes of electron spin resonance membrane fluidity in hexadecane-induced hyperproliferative epidermis

To study some of the biochemical and physical states of membranes associated with hyperproliferation, the effect of topical hexadecane on membrane fluidity in guinea pig epidermis was investigated by electron spin resonance using a 5-doxylstearic acid spin labeling agent. Guinea pig epidermal cells were separated into three regions of keratinocytes by Percoll density gradient centrifugation. Membrane fluidity and Na+, K+-ATPase activity were higher in hyperproliferating epidermal cells than in control. The free cholesterol content and the molar ratio of free cholesterol to phospholipid were found to decrease significantly. Also elevated levels of palmitic acid, stearic acid and omega-3 unsaturated fatty acid derived from phospholipid were observed. Normal differentiation of epidermis was found to be accompanied by a decrease in membrane fluidity, whereas a relatively high membrane fluidity was maintained in the hexadecane-induced hyperproliferation.

Tissue distribution of an endogenous ligand to the Na, K ATPase molecule

A variety of evidence indicates the presence of a circulating ligand to the Na, K ATPase molecule that is involved in the regulation of extracellular sodium metabolism. To examine the potential role of endogenous ligands to the Na, K ATPase molecule in the regulation of intracellular sodium metabolism, the tissue distribution of digitalis-like activity was quantitated in several brain regions and peripheral organs. The digitalis-like activity of desalted and delipidated extracts of tissue was widely distributed and produced a displacement of tritiated ouabain that was parallel to the displacement produced by cold ouabain. These results suggest that tissue contains an endogenous ligand to the Na, K ATPase molecule and that this ligand may regulate intracellular sodium metabolism in an autocoid-like manner.

Regulation of intracellular pH during the G1/S-phase transition of the neuroblastoma cell cycle

Changes in active K+ and Na+ influx during the cell cycle of neuroblastoma (clone Neuro-2A) have suggested activation of an Na+, H+ exchange system during the G1/S-phase transition. Here we report that pHi, measured by the digitonin null-point method, is constant during G1-phase and the G1/S-phase transition and decreases in early S-phase. In addition pHi is shown to be most sensitive to the diuretic amiloride in the G1/S-phase transition, in agreement with the ion influx data. It is concluded from these data, that pHi is tightly regulated during the early cell cycle phases by the Na+, H+ exchange system, in particular during the G1/S-phase transition.

Differentiation of human keratinocytes: changes in lipid synthesis, plasma membrane lipid composition, and 125I-EGF binding upon administration of 25-hydroxycholesterol and mevinolin

We have studied the relationship between differentiation capacity, plasma membrane composition, and epidermal growth factor (EGF) receptor expression of normal keratinocytes in vitro. The plasma membrane composition of the cells was modulated experimentally by cholesterol depletion, using specific inhibitors of cholesterol synthesis, such as 25-hydroxycholesterol and mevinolin. Exposure of the cells towards these inhibitors resulted in a drastic decrease of cholesterol biosynthesis, as determined from 14C-acetate incorporation into the various lipid fractions. This effect on cholesterol biosynthesis was reflected by changes in plasma membrane composition, as determined by lipid analysis of isolated plasma membrane fractions, these resulting in a decreased cholesterol-phospholipid ratio. The experimental modulation of plasma membrane composition by 25-hydroxycholesterol or mevinolin were accompanied by a decreased cornified envelope formation and by high expression of EGF binding sites. These phenomena were more pronounced in cells induced to differentiate by exposure of cells grown under low Ca2+ to normal Ca2+ concentrations, as compared to cells grown persistently under low Ca2+ concentrations. These results suggest a close correlation between plasma membrane composition, differentiation capacity, and EGF receptor expression.

Binding of polycation DEAE-dextran to Chinese hamster ovary cells induces reversible Ca2+ influx and inhibits capping of concanavalin A acceptor proteins

Binding of the polycation DEAE-dextran to the cell surface of HA-1 CHO cells caused a marked increase in 45Ca2+ exchange influx. The effect was fairly selective for Ca2+, undirectional (efflux was not increased) and was rapidly reversed by treatment with polyanion dextran sulfate. 45Ca2+ influx could not be stimulated by treatment with multivalent lectins or fibronectin. In addition to stimulating 45Ca2+ flux, DEAE-dextran inhibited the capping of concanavalin-A acceptor proteins. Inhibition of capping occurred over the same DEAE-dextran concentration range (20-200 micrograms/ml) which stimulated 45Ca2+ uptake, possibly implicating increased cellular [Ca2+] in the inhibition of concanavalin A acceptor protein capping in this cell type. The profound effect of DEAE-dextran on cellular Ca2+ uptake and the rapid reversal of the effect by dextran sulfate might make the polycation a useful agent for the induction of transient increases in cellular [Ca2+].

Effect of external K+ on protein and DNA synthesis during and after heat shock in rat hepatoma cells

The effects of extracellular K+ concentrations on protein and DNA synthesis after non-lethal heat shock were studied in the hepatoma cell lines Reuber H35 and HTC. Elevation of the extracellular K+ concentration by equimolar replacement of Na+ by K+ in growth media of Reuber H35 and HTC cells caused an increase of the intracellular K+ content in both cell lines. This property was subsequently used to study the effect of elevated intracellular K+ concentrations on protein and DNA synthesis after hyperthermic treatment at 42 degrees C for 30 min. In normal K+ medium, protein and DNA synthesis were inhibited rapidly after the start of the hyperthermic treatment in both Reuber H35 and HTC cells. Increasing the external K+ concentration of the medium did not influence the inhibition and subsequent recovery of protein synthesis after heat shock in both cell lines. In contrast, in media with elevated K+ concentrations, DNA synthesis after heat-shock was inhibited less in Reuber H35 cells than in cells incubated in normal K+ medium and, furthermore, showed no inhibition in HTC cells. The protective effect of external K+ on DNA synthesis after heat shock was maximal between 50 and 70 mM in the temperature range 42-44 degrees C.

Lateral mobility of plasma membrane lipids in Xenopus eggs: regional differences related to animal/vegetal polarity become extreme upon fertilization

Regional differences in the lateral mobility properties of plasma membrane lipids have been studied in unfertilized and fertilized Xenopus eggs by fluorescence photobleaching recovery (FPR) measurements. Out of a variety of commonly used lipid probes only the aminofluorescein-labeled fatty acids HEDAF (5-(N-hexadecanoyl)-aminofluorescein) and TEDAF (5-(N-tetradecanoyl)-aminofluorescein) appear to partition into the plasma membrane. Under all experimental conditions used these molecules show partial recovery upon photobleaching indicating the existence of lipidic microdomains. In the unfertilized egg the mobile fraction of plasma membrane lipids (approximately 50%) has a fivefold smaller lateral diffusion coefficient (D = 1.5 X 10(-8) cm2/sec) in the animal than in the vegetal plasma membrane (D = 7.6 X 10(-8) cm2/sec). This demonstrates the presence of an animal/vegetal polarity within the Xenopus egg plasma membrane. Upon fertilization this polarity is strongly (greater than 100X) enhanced leading to the formation of two distinct macrodomains within the plasma membrane. At the animal side of the egg lipids are completely immobilized on the time scale of FPR measurements (D less than 10(-10) cm2/sec), whereas at the vegetal side D is only slightly reduced (D = 4.4 X 10(-8) cm2/sec). The immobilization of animal plasma membrane lipids, which could play a role in the polyspermy block, probably arises by the fusion of cortical granules which are more numerous here. The transition between the animal and the vegetal domain is sharp and coincides with the boundary between the presumptive ecto- and endoderm. The role of regional differences in the plasma membrane is discussed in relation to cell diversification in early development.

Loss of EGF binding and cation transport response during differentiation of mouse neuroblastoma cells

Mouse neuroblastoma cells (clone N1E-115) differentiate in culture upon withdrawal of serum growth factors and acquire the characteristics of neurons. We have shown tht exponentially growing N1E-115 cells possess functional epidermal growth factor (EGF) receptors but that the capacity for binding EGF and for stimulation of DNA synthesis is lost as the cells differentiate. Furthermore, in exponentially growing cells, EGF induces a rapid increase in amiloride-sensitive Na+ influx, followed by stimulation of the (Na+-K+)ATPase, indicating that activation of the Na+/H+ exchange mechanism in N1E-115 cells [1] may be induced by EGF. The ionic response is also lost during differentiation, but we have shown that the stimulation of both Na+ and K+ influx is directly proportional to the number of occupied receptors in all cells whether exponentially growing or differentiating, thus only indirectly dependent on the external EGF concentration. The linearity of the relationships indicates that there is no rate-limiting step between EGF binding and the ionic response. Our data would suggest that as neuroblastoma cells differentiate and acquire neuronal properties, their ability to respond to mitogens, both biologically and in the activation of cation transport processes, progressively decreases owing to the loss of the appropriate receptors.