Potassium transport and content during G1 and S phase following serum stimulation of 3T3 cells

The Role of Intracellular Potassium in Cell Quiescence, Proliferation, and Death

This brief review explores the role of intracellular K+ during the transition of cells from quiescence to proliferation and the induction of apoptosis. We focus on the relationship between intracellular K+ and the growth and proliferation rates of different cells, including transformed cells in culture as well as human quiescent T cells and mesenchymal stem cells, and analyze the concomitant changes in K+ and water content in both proliferating and apoptotic cells. Evidence is discussed indicating that during the initiation of cell proliferation and apoptosis changes in the K+ content in cells occur in parallel with changes in water content and therefore do not lead to significant changes in the intracellular K+ concentration. We conclude that K+, as a dominant intracellular ion, is involved in the regulation of cell volume during the transit from quiescence, and the content of K+ and water in dividing cells is higher than in quiescent or differentiated cells, which can be considered to be a hallmark of cell proliferation and transformation.

Clinically relevant preservation conditions for mesenchymal stem/stromal cells derived from perinatal and adult tissue sources

The interplay between mesenchymal stem/stromal cells (MSCs) and preservation conditions is critical to maintain the viability and functionality of these cells before administration. We observed that Ringer lactate (RL) maintained high viability of bone marrow–derived MSCs for up to 72 h at room temperature (18°C–22°C), whereas adipose‐derived and umbilical cord‐derived MSCs showed the highest viability for 72 h at a cold temperature (4°C–8°C). These cells maintained their adherence ability with an improved recovery rate and metabolic profiles (glycolysis and mitochondrial respiration) similar to those of freshly harvested cells. Growth factor and cytokine analyses revealed that the preserved cells released substantial amounts of leukaemia inhibitory factors (LIFs), hepatocyte growth factor (HGF) and vascular endothelial growth factor‐A (VEGF‐A), as well as multiple cytokines (eg IL‐4, IL‐6, IL‐8, MPC‐1 and TNF‐α). Our data provide the simplest clinically relevant preservation conditions that maintain the viability, stemness and functionality of MSCs from perinatal and adult tissue sources.

Intracellular K+ and water content in human blood lymphocytes during transition from quiescence to proliferation

Many evidence shows that K⁺ ions are required for cell proliferation, however, changes in intracellular K⁺ concentration during transition of cells from quiescence to cycling are insufficiently studied. Here, we show using flame emission assay that a long-term increase in cell K⁺ content per g cell protein is a mandatory factor for transition of quiescent human peripheral blood lymphocytes (PBL) to proliferation induced by phytohemagglutinin, phorbol ester with ionomycin, and anti-CD3 antibodies with interleukin-2 (IL-2). The long-term increase in K⁺ content is associated with IL-2-dependent stage of PBL activation and accompanies the growth of small lymphocytes and their transformation into blasts. Inhibition of PBL proliferation with drugs specific for different steps of G0/G1/S transit prevented both blast-transformation and an increase in K⁺ content per cell protein. Determination of the water content in cells by measuring the density of cells in the Percoll gradient showed that, unlike the K⁺ content, the concentration of K⁺ in cell water remains unchanged, since water and K⁺ change in parallel. Correlation of proliferation with high cell K⁺ and water content has been confirmed by the data obtained in comparative study of PBL and permanently cycling Jurkat cells. Our data suggest that K⁺ is important for successful proliferation as the main intracellular ion that participates in regulation of cell water content during cell transition from quiescence to proliferation. We concluded that high K⁺ content in cells and the associated high water content is a characteristic feature of proliferating cells.

Proliferation-related changes in K+ content in human mesenchymal stem cells

Intracellular monovalent ions have been shown to be important for cell proliferation, however, mechanisms through which ions regulate cell proliferation is not well understood. Ion transporters may be implicated in the intracellular signaling: Na⁺ and Cl⁻ participate in regulation of intracellular pH, transmembrane potential, Ca²⁺ homeostasis. Recently, it is has been suggested that K⁺ may be involved in “the pluripotency signaling network”. Our study has been focused on the relations between K⁺ transport and stem cell proliferation. We compared monovalent cation transport in human mesenchymal stem cells (hMSCs) at different passages and at low and high densities of culture as well as during stress-induced cell cycle arrest and revealed a decline in K⁺ content per cell protein which was associated with accumulation of G1 cells in population and accompanied cell proliferation slowing. It is suggested that cell K⁺ may be important for successful cell proliferation as the main intracellular ion that participates in regulation of cell volume during cell cycle progression. It is proposed that cell K⁺ content as related to cell protein is a physiological marker of stem cell proliferation and may be used as an informative test for assessing the functional status of stem cells in vitro.

Central roles of Mg2+ and MgATP2- in the regulation of protein synthesis and cell proliferation: significance for neoplastic transformation

Growth factors are polypeptides that combine with specific membrane receptors on animal cells to stimulate proliferation, but they also stimulate glucose transport, uridine phosphorylation, intermediary metabolism, protein synthesis, and other processes of the coordinate response. There are a variety of nonspecific surface action treatments which stimulate the same set of reactions as the growth factors do, of which protein synthesis is most directly related to the onset of DNA synthesis. Mg(2+) is required for a very wide range of cellular reactions, including all phosphoryl transfers, and its deprivation inhibits all components of the coordinate response that have so far been tested. Growth factors raise the level of free Mg(2+) closer to the optimum for the initiation of protein synthesis. The resulting increase in protein synthesis accelerates progression through G1 to the onset of DNA synthesis and mitosis. None of the other 3 major cellular cations are similarly involved in growth regulation, although internal pH may play an auxiliary role. Almost 10(5) externally bound divalent cations are displaced from membranes for every attached insulin molecule, implying a conformational membrane change that releases enough Mg(2+) from the internal surface of the plasma membrane to account for the increase in free cytosolic Mg(2+). It is proposed that mTOR, the central control point for protein synthesis of the PI 3-K kinase cascade stimulated by insulin, is regulated by MgATP(2-) which varies directly with cytosolic Mg(2+). Other elements of the coordinate response to growth factors such as the increased transport of glucose and phosphorylation of uridine are also dependent upon an increase of Mg(2+). Deprivation of Mg(2+) in neoplastically transformed cultures normalizes their appearance and growth behavior and raises their abnormally low Ca(2+) concentration. Tight packing of the transformed cells at very high saturation density confers the same normalizing effects, which are retained for a few days after subculture at low density. The results suggest that the activity of Mg(2+) within the cell is a central regulator of normal cell growth, and the loss of its membrane-mediated control can account for the neoplastic phenotype.

Interleukin-2-dependent regulation of Na/K pump in human lymphocytes

The present study provides the first evidence that the abundance of catalytic alpha1-subunit of Na,K-ATPase increases in the course of T cell blast transformation. Immunodepressant cyclosporin A at anti-proliferative doses diminished the induction of alpha1 protein in activated lymphocytes. Furthermore, in competent T cells, IL-2 increases both the transport activity of Na/K pump and the content of Na,K-ATPase alpha1 protein in a time-dependent manner. A correlation was found between the long-term elevation in ouabain-sensitive Rb influxes and the increase in alpha1 protein content in late activated T cells. These results suggest that (1) the increased expression of Na,K-ATPase proteins underlie the cell cycle-dependent upregulation of ion pump during T cell transformation, and (2) IL-2 is involved in the regulated expression of Na,K-ATPase in human lymphocytes.

Na+/K+/Cl− cotransporter activates MAP-kinase cascade downstream to protein kinase C, and upstream to MEK

In this study, we demonstrated that the specific inhibitors of the Na+/K+/Cl- cotransporter (NKCC1), bumetanide and furosemide, inhibited extracellular regulated kinase (ERK) phosphorylation in Balb/c 3T3 fibroblasts, stimulated with a variety of mitogens. In addition to fibroblast growth factor (FGF) shown before, the various mitogens tested in the present study (endothelial growth factor (EGF), platelet-derived growth factor (PDGF), insulin, thrombin, and the phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate (TPA)). Enter, the Ras/Raf/MEK/ERK cascade via different growth factors receptors and through one of the two main routes. The results of the present study provide evidence that have led us to conclude that the target protein which is controlled by the Na+/K+/Cl- cotransporter, is downstream of tyrosine kinase receptors, as well as of the G-protein-coupled receptor (GPCR). Several additional lines of evidence supported the above conclusion: (i) furosemide inhibits phosphorylation of MAPK kinase (MEK) induced by receptor tyrosine kinase (RTK) ligands, such as PDGF, FGF, and EGF. (ii) Furosemide also inhibited ERK phosphorylation, induced by thrombin, a GPCR. (iii) Furosemide inhibited MEK and ERK phosphorylation even when ERK phosphorylation was induced by direct activation of protein kinase C (PKC) by TPA, which bypasses early steps of the mitogenic cascade. In addition, we found that furosemide did not affect PKC phosphorylation induced directly by TPA. Taken together, the results of the present study indicate that the signal transduction protein, controlled by the Na+/K+/Cl- cotransporter, must be downstream of the PKC, and at/or upstream to MEK in the Ras/Raf/MEK/ERK cascade.

Na(+)/K(+)/Cl(-) cotransporter activates mitogen-activated protein kinase in fibroblasts and lymphocytes

In a previous work, we have shown that overexpression of the Na(+)/K(+)/Cl(-) cotransporter (NKCC1) induces cell proliferation and transformation. We investigate in the present study the role of the NKCC1 in the mitogenic signal transduction. We show that overexpression of the cotransporter gene (NKCC1) in stablely transfected cells (Balb/c-NKCC1), resulted in enhanced phosphorylation of the extracellular regulated kinase (ERK) to produce double phosphorylated ERK (DP-ERK). Furthermore, the level of DP-ERK was reduced by 50-80% following the addition of bumetanide, a specific inhibitor of the Na(+)/K(+)/Cl(-) cotransporter, in quiescent as well as in proliferating cultures of the Balb/c-NKCC1 clone. In order to explore further the role of the Na(+)/K(+)/Cl(-) cotransporter in mitogenic signal transduction, we measured the effect of the two specific inhibitors of the cotransporter; bumetanide and furosemide, on DP-ERK level in immortalized non-transformed cells. In Balb/c 3T3 fibroblasts stimulated with FGF, bumetanide, and furosemide inhibited 50-60% of the ERK 1/2 phosphorylation. The inhibitor concentration needed for maximal inhibition of ERK 1/2 phosphorylation was similar to the concentration needed to block the K(+) influx mediated by the Na(+)/K(+)/Cl(-) cotransporter in these cells. To analyze whether the Na(+)/K(+)/Cl(-) cotransporter has a role in the mitogenic signal of normal cells, we measured the effect of bumetanide on ERK phosphorylation in human peripheral blood lymphocytes. The phosphorylation of ERK 1/2 in resting human lymphocytes, as well as in lymphocytes stimulated with phytohemagglutinin (PHA) was inhibited by bumetanide. The effect of bumetanide on ERK 2 phosphorylation was much lower than that of ERK 1 phosphorylation. The finding that the Na(+)/K(+)/Cl(-) cotransporter controls the ERK/MAPK (mitogen-activated protein kinase) signal transduction pathway, support our hypothesis that Na(+) and K(+) influxes mediated by this transporter plays a central role in the control of normal cell proliferation. Exploring the cellular ionic currents and levels, mediated by the Na(+)/K(+)/Cl(-) cotransporter, should lead to a better comprehension of cell proliferation and transformation machinery.

Overexpression of the Na(+)/K(+)/Cl(-) cotransporter gene induces cell proliferation and phenotypic transformation in mouse fibroblasts

Na(+)/K(+)/Cl(-) cotransporter activity is stimulated in early G(1) phase of the cell cycle and this stimulation was shown to be an essential event in fibroblast cell proliferation. In order to elucidate further the role of the Na(+)/K(+)/Cl(-) cotransporter in cell proliferation, we overexpressed the gene encoding the Na(+)/K(+)/Cl(-) cotransporter in mouse fibroblasts, and analyzed cellular phenotypic changes. Mouse Balb/c 3T3 cells were stably transfected with the cDNA of the shark rectal gland Na(+)/K(+)/Cl(-) cotransporter gene (NKCC1), and expressed in a mammalian vector under the cytomegalovirus promoter (Balb/c-NKCC1 cells). The transfected cells exhibited up to 10-fold greater bumetanide-sensitive Rb(+) influx compared to the control cells. The Balb/c-NKCC1 cells have acquired a typical transformation phenotype indicated by: (1) Loss of contact inhibition exhibited by growth to a higher cell density in confluent cultures, and formation of cell foci; (2) proliferation in low serum concentrations; and (3) formation of cell colonies in soft agar. The control cells transfected with the NKCC1 gene inserted in the opposite orientation in the vector retained their normal phenotype. Furthermore, the two specific inhibitors of the Na(+)/K(+)/Cl(-) cotransporter activity; bumetanide and furosemide inhibited the clonogenic efficiency in the NKCC1 transfected cells. These control experiments indicate that the apparent transformation phenotype acquired by the Balb/c-NKCC1 cells was not merely associated with the process of transfection and selecting for the neomycin-resistant clones, but rather with the overexpression of the Na(+)/K(+)/Cl(-) cotransporter gene. In order to ascertain that the regulated and normal expression of the Na(+)/K(+)/Cl(-) cotransporter control cell proliferation, the effect of bumetanide a specific inhibitor of the cotransporter, was tested on Balb/c 3T3 cell proliferation, induced by fibroblasts growth factor (FGF) and fetal calf serum (FCS). Bumetanide inhibited synchronized Balb/c 3T3 cell exit from the G(0)/G(1) arrest and entering S-phase. The inhibition was reversible, as removal of bumetanide completely released cell proliferation. Taken together, these results propose that the NKCC1 gene is involved in the control of normal cell proliferation, while its overexpression results in apparent cell transformation, in a manner similar to some protooncogenes.

Na, K-ATPase pump in activated human lymphocytes: on the mechanisms of rapid and long-term increase in K influxes during the initiation of phytohemagglutinin-induced proliferation

Functional expression of Na, K-ATPase pump as determined by ouabain-sensitive Rb influxes has been investigated in human peripheral blood lymphocytes, activated by phytohemagglutinin (PHA) from resting state to proliferation. It is found that a rapid twofold elevation of ouabain-sensitive Rb influx in response to PHA is followed by a long-term increase in pump activity, which precedes the DNA synthesis and is temporally related to the growth phase of mitogenic response. Unlike the early pump activation, the late enhanced pump activity is not the result of elevated cell Na content, it is inhibited by cycloheximide and requires new protein synthesis. Actinomycin D and alpha-amanitin, in doses, which suppress the PHA-induced increase in the RNA synthesis, do not abolish the elevated Rb influx until 20-24h of mitogenic activation and inhibit the late, growth-associated increase in Rb influx. It is concluded that (1) in mitogen-activated cells both short- and long-term control is involved in the enhanced pump activity, and (2) translational and transcriptional mechanisms may contribute to the long-term up-regulation of Na, K-ATPase pump during blast transformation of human lymphocytes.

An early transient increase of intracellular Na+ may be one of the first components of the mitogenic signal. Direct detection by 23Na-NMR spectroscopy in quiescent 3T3 mouse fibroblasts stimulated by growth factors

23Na-NMR spectroscopy was designed to allow for continuous recording of intracellular Na+ in 3T3 fibroblasts stimulated by serum growth-factors in the presence of ion transport inhibitors. The metabolic state of cells at rest and following stimulation was monitored by 31P-NMR spectra of ATP and related high-energy phosphates. The study demonstrates that early activation of ion transporters by addition of serum is marked by the appearance of transient increase of the intracellular Na+, beginning 3 min after addition of serum to quiescent culture and lasting approx. 20 min. The initial rise in cellular Na+ results from an increased activity of the bumetanide-sensitive Na+/K+/Cl- cotransport and of the amiloride-sensitive Na+/H+ antiport. It is suppressed by any one of these inhibitors. Subsequent activation of the ouabain-sensitive Na+/K(+)-ATPase results in an increased Na+ efflux, leading to a return of intracellular Na+ to its initial baseline. Previous work had shown that the early activation of bumetanide-sensitive and amiloride sensitive ion-transporters by growth-factors was essential for induction of cell division, at least in some cell types. Preventing ion activation by adding ion-transport inhibitors lead to the inhibition of DNA synthesis 18 h later. This process was reversible upon elimination of these inhibitors. Even though alternative non-specific effects of these inhibitors cannot be ruled out, the observed transient peak in intracellular Na+ may be one of the earliest components of the mitogenic signal. On the basis of previous works, its effect seems to be related to the activation of Ca(2+)-dependent and cyclic AMP second messenger pathways. The different mechanisms whereby the activated Na+/K+/Cl- cotransport and the Na+/H+ antiport contribute to this signal need to be further investigated.

Bumetanide and furosemide inhibited vascular endothelial cell proliferation

In this study, we examined the role of the bumetanide-sensitive Na+/K+/Cl-cotransport in the mitogenic signal of vascular endothelial cell proliferation. The activity of the Na+/K+/Cl- cotransport is dramatically decreased in quiescent subconfluent cells, as compared to subconfluent cells growing in the presence of FGF. The Na+/K+/Cl- cotransport activity of quiescent subconfluent cultures deprived of FGF decreased to 6%, whereas that of quiescent cells grown to confluency was reduced to only 33% of the activity of subconfluent cells growing in the presence of FGF. The basal low activity of Na+/K+/Cl- cotransport in the quiescent subconfluent vascular endothelial cells was dramatically stimulated by FGF. In order to explore the role of the Na+/K+/Cl- cotransport in the mitogenic signal of the endothelial cells, the effect of two specific inhibitors of the cotransport -furosemide and -bumetanide was tested on cell proliferation induced by FGF. Bumetanide and furosemide inhibited synchronized cell proliferation measured by direct counting of cells and by DNA synthesis. Inhibition by furosemide and bumetanide was reversible; removal of these compounds completely released the cells to proliferate. These results indicate that the effect of these drugs is specific and is not due to an indirect toxic effect. This study clearly demonstrates that the FGF-induced activation of the Na+/K+/Cl- cotransport plays a role in the mitogenic signal pathway of vascular endothelial cells.

Change in K+ current of HeLa cells with progression of the cell cycle studied by patch-clamp technique

The K+ channel of HeLa S3 cells in metaphase was analyzed by inside-out and whole cell patch-clamp techniques. The channel had the characteristics of strong inward rectification, small conductance (22 pS at -100 mV), and dependence on intracellular Ca2+. We investigated the cell cycle dependency of the channel, using cells synchronized by harvesting them at the mitotic stage. The cell capacitance increased gradually with increases in the cell volume toward the S phase. The inward K+ currents through the channel at fixed membrane potentials were highest in early G1 and then decreased with time to a minimum in the S phase, increasing again in the M phase. The permeabilities at fixed membrane potentials were also highest in early G1, decreased to minima in the S phase, and increased again toward the next mitosis. In contrast, mean amplitude and the open probability of the single channel at a fixed membrane potential (-60 mV) did not change significantly during the cell cycle. Therefore the capacitance increases with progression of the cell cycle, whereas the permeability decreases from early G1 to an apparent minimum in the S phase. These changes may be caused by cell cycle-dependent changes in the number of channels.

K+ fluxes mediated by Na(+)-K(+)-Cl- cotransport and Na(+)-K(+)-ATPase pumps in renal tubule cell lines transformed by wild-type and temperature-sensitive strains of Simian virus 40

The relative contributions of Na(+)-K(+)-ATPase pumps and Na(+)-K(+)-Cl- cotransport to total rubidium (Rb+) influx into primary cultures of renal tubule cells (PC.RC) and cells transformed either with the wild-type or a temperature-sensitive mutant of the simian virus 40 (SV40), were measured under various growth conditions. The Na(+)-K(+)-ATPase-mediated component represented 74% and 44-48% of total Rb+ influx into PC.RC and SV40-transformed cells, respectively. Proliferating transformed cells showed substantial ouabain-resistant bumetanide-sensitive (Or-Bs) Rb+ influx (41-45% of total) which indicated the presence of a Na(+)-K(+)-Cl- cotransport. The Or-Bs component of Rb+ influx was greatly reduced when temperature-sensitive transformed renal cells (RC.SVtsA58) grown in Petri dishes or on permeable filters were shifted from the permissive (33 degrees C) to the restrictive temperature (39.5 degrees C) to arrest cell growth. The ouabain-sensitive Rb+ influx mediated by the Na(+)-K(+)-ATPase, the total and amiloride-sensitive Na+ uptakes were not modified following inhibition of cell proliferation. A similar fall in the Or-Bs influx was obtained when renal tubule cells transformed by the wild-type SV40 (RC.SV) were incubated with the K+ channel blocker, tetraethylammonium (TEA) ion, which we had previously shown to arrest cell growth without affecting cell viability (Teulon et al.: J. Cell. Physiol., 151:113-125, 1992). Reinitiation of cell growth by removal of TEA or return to 33 degrees C of the temperature-sensitive cells restored the Or-Bs component of Rb influx. Taken together, these results indicate that the Na(+)-K(+)-Cl- cotransport activity is critically dependent on cell growth conditions.

Effect of Ha-ras on mitogen-induced Ca(2+)- and K(+)-fluxes

Transforming Ha-ras enhances the mitogen-induced activation of both the Ca(2+)-influx and the furosemide-sensitive Na+/K+/2Cl-cotransporter. Both systems represent essential early steps of mitogenic signal transduction in NIH 3T3 fibroblasts.

Na+/K+/Cl- cotransport is stimulated by a Ca(++)-calmodulin-mediated pathway in BALB/c 3T3 fibroblasts

In the present study, we investigated the role of intracellular Ca++ in the stimulation of the Na+/K+/Cl- cotransport in synchronized BALB/c 3T3 cells. The Na+/K+/Cl- cotransport was stimulated by the growth factors EGF, TGF-alpha, IGF-1, and IGF-2, which do not activate protein kinase C, but do induce a transient increase in free cytoplasmic Ca++. In addition, direct activation of protein kinase C by the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA) did not affect the Na+/K+/Cl- cotransport activity of quiescent cells. The Na+/K+/Cl- cotransport was also stimulated by the above mitogens in cells pretreated with the phorbol ester TPA. This treatment led to a progressive decline in the activity of cellular protein kinase C. This result implies that cells deficient in protein kinase C may still support stimulation of the Na+/K+/Cl- cotransport. Taken as a whole, these findings suggest that the Na+/K+/Cl- cotransport is stimulated predominantly by a protein kinase C-independent mechanism in BALB/c 3T3 fibroblasts. Both the intracellular Ca++ antagonist 8-(N,N-diethylamino)octyl-3,4,5-trimethoxybenzoate (TMB-8) and two potent calmodulin antagonists, trifluoperazine (TFP) and chloropromazine (CP), blocked serum- and mitogen-stimulated Na+/K+/Cl- cotransport. These results suggest that the Na+/K+/Cl- cotransport is stimulated by an increase of intracellular Ca++ and subsequently by a Ca(++)-calmodulin-mediated pathway in the synchronized BALB/c 3T3 fibroblasts.

Stimulation of bumetanide-sensitive Na+/K+/Cl- cotransport by different mitogens in synchronized human skin fibroblasts is essential for cell proliferation

In this study, we examined the role of the bumetanide-sensitive Na+/K+/Cl- cotransport in the mitogenic signal of human skin fibroblast proliferation. The Na+/K+/Cl- cotransport was dramatically stimulated by either fetal calf serum, or by recombinant growth factors, added to quiescent G0/G1 human skin fibroblasts. The following mitogens, FGF, PDGF, alpha-thrombin, insulin-like growth factor-1, transforming growth factor-alpha, and the phorbol ester, 12-O-tetradecanoyl-phorbol-13-acetate, all stimulated the Na+/K+/Cl- cotransport. In addition, all the above mitogens induced DNA synthesis in the synchronized human fibroblasts. In order to explore the role of the Na+/K+/Cl- cotransport in the mitogenic signal, the effect of two specific inhibitors of the cotransport, furosemide and bumetanide, was tested on cell proliferation induced by the above recombinant growth factors. Bumetanide and furosemide inhibited synchronized cell proliferation as was measured by (a) cell exit from the G0/G1 phase measured by the use of flow cytometry, (b) cell entering the S-phase, determined by DNA synthesis, and (c) cell growth, measured by counting the cells. The inhibition by furosemide and bumetanide was reversible, removal of these compounds, completely released the cells from the block of DNA synthesis. In addition, the two drugs inhibited DNA synthesis only when added within the first 2-6 h of cell release. These results indicate that the effect of these drugs is specific, and is not due to an indirect toxic effect. This study clearly demonstrates that the growth factor-induced activation of the Na+/K+/Cl- cotransport plays a major role in the mitogenic signaling pathway of the human fibroblasts.

Phorbol ester TPA inhibits the stimulation of bumetanide-sensitive Na+/K+/Cl- transporter by different mitogens in quiescent BALB/c 3T3 mouse fibroblasts

In this study we examined the effect of the phorbol ester 12-O-tetradecanoyl-phorbol-13-acetate (TPA) on the bumetanide-sensitive Na+/K+/Cl- transporter in quiescent BALB/c 3T3 cells. We have shown that exposure of quiescent BALB/c 3T3 cultures to phorbol ester did not inhibit the basal bumetanide-sensitive Rb+ influx or efflux. In fact, at high concentration (100 ng/ml), TPA slightly stimulated the bumetanide-sensitive Rb+ influx and efflux. However, when the quiescent cultures were stimulated by serum or by defined growth factors, the stimulated fraction of the bumetanide-sensitive Rb+ influx was drastically inhibited by exposure of the cells to the phorbol ester TPA. Based on the above findings, we propose that activation of protein kinase C by the phorbol ester TPA does not inhibit the Na+/K+/Cl- cotransport activity; however it does suppress only the growth-factors-stimulated fraction of the cotransport in quiescent BALB/c 3T3 cells. These data propose that activation of kinase C has a regulatory feedback effect on the stimulation of the Na+/K+/Cl- cotransport activity by growth factors.

Bumetanide-sensitive Na+/K+/Cl- transporter is stimulated by phorbol ester and different mitogens in quiescent human skin fibroblasts

In this study we investigated the correlation between the mitogenic effect and stimulation of Rb+ (K+) fluxes in human skin fibroblasts treated by purified growth factors. Both K+ transporters, bumetanide-sensitive and ouabain-sensitive, are stimulated 2-3-fold after addition of either fetal calf serum or purified recombinant growth factors to quiescent G0/G1 human skin fibroblasts. Three groups of mitogens were compared: i) the phorbol ester 2-O-tetradecanoyl-phorbol-13-acetate (TPA); ii) growth factors that stimulate inositol phosphate hydrolysis and subsequently activate protein kinase C--fibroblast growth factor (FGF), platelet derived growth factor (PDGF), and alpha-thrombin; and iii) growth factors that do not activate kinase C--insulin-like growth factor-1 (IGF-1), and transforming like growth-factor-alpha (TGF-alpha). The three groups of mitogens stimulated human skin fibroblasts proliferation and Rb+ influxes in a similar dose-dependent fashion. The results indicate that both the bumetanide-sensitive and the ouabain-sensitive Rb+ fluxes are stimulated by protein kinase C-dependent and by the protein kinase C-independent pathways of the mitogenic signal.

Gonadotropin stimulates oocyte translation by increasing magnesium activity through intracellular potassium-magnesium exchange

We previously showed that gonadotropin increases the K+ activity in Xenopus oocytes and that this is a signal for increased translation. However, K+ need not act to control synthesis directly but may act through an unidentified downstream effector. Using microinjection to vary the salt content of oocytes and concomitantly measuring [3H]leucine incorporation, we found that small changes in Mg2+ greatly affect translation rates. (Ca2+ had little influence.) By measuring intracellular ion activities, we found that oocyte cations existed in a buffer-like (ion-exchange) equilibrium in which K+ and Mg2+ are the preponderant monovalent and divalent cations. Hence, increasing cellular K+ activity might increase translation by causing Mg2+ activity to rise. If so, the increased translation rates produced by hormone treatment or K+ injection would be prevented by EDTA, a Mg2+ chelating agent. This prediction was tested and confirmed. We conclude that, when gonadotropin increases K+ activity, the cell's internal ion-exchange equilibrium is altered thereby increasing Mg2+ activity and this up-regulates translation.

Effect of Na + flux inhibitors on induction of c-fos, c-myc, and ODC genes during cell cycle

The role of Na + transport systems in the mitogenic signal induced by growth factors was studied, and it was shown that two Na + transport systems contribute to the early increase in cytoplasmic Na + in response to serum growth factors, namely the amiloride-sensitive Na+/H+ antiport and the bumetanide-sensitive Na+/K+/Cl- cotransport. Bumetanide or amiloride, when added separately, inhibited part of the increase in cytoplasmic Na +, as a response to the addition of serum to quiescent BALB/c mouse 3T3 fibroblasts. Each drug also suppressed part of the stimulation of the ouabain-sensitive Rb + influx, which was controlled by intracellular Na +. However, when both drugs were added together with serum growth factors, a complete inhibition of the early increase in [Na +], and subsequently a complete blockage of Na+/K+ pump stimulation was obtained. Amiloride or bumetanide, when added separately, only partially inhibited DNA synthesis induced by serum, 24% and 8% respectively. However, when both drugs were added together, at the time of serum addition to the quiescent cells, cell entry into S-phase was completely inhibited. To investigate the mode of cell-cycle inhibition, analysis was done of the possible role of early Na + fluxes in the mitogenic signal transduced from cell membrane receptors to the nucleus. The effects of the two drugs amiloride and bumetanide on induction of three genes--c-fos, c-myc, and ornithin decarboxylase (ODC)--was measured during cell transition through the G1-phase. Amiloride and bumetanide, when added separately or in combination, did not inhibit the induction of c-fos, c-myc, and ODC mRNAs. These results suggest that stimulation of Na + fluxes by serum growth factors is essential for cell transition into the S-phase of cell cycle, but it plays no apparent role in the growth factor signal transduced from the cell surface to the interior of the cell, as manifested by c-fos, c-myc, and ODC genes induction.

Potassium salt microinjection into Xenopus oocytes mimics gonadotropin treatment

Gonadotropin stimulates protein synthesis and growth in ovarian oocytes. The hormone is also known to modify transfollicular K+ fluxes and is now shown to cause increased intraoocytic K+ activity (aK). The hormone's effect on aK was duplicated by microinjecting K+ salts into oocytes which were incubated in paraffin oil. This treatment mimicked the influence of gonadotropin on both the rate of protein synthesis and the synthesis of specific polypeptides. These findings suggest that gonadotropin-stimulated oocyte growth is attributable largely to the hormone's influence on transfollicular K+ fluxes. They support the hypothesis that the K+ flux and aK changes observed during cell activation are critical in causing subsequent increases in protein synthesis and growth.

Irreversible reduction in potassium fluxes accompanies terminal differentiation of human myoblasts to myotubes

Potassium and sodium fluxes believed to be important in the cellular response to serum and growth factors have not been widely investigated in cells which have undergone terminal differentiation. In this study we have analyzed two main K+ transport systems--the ouabain-sensitive Na+/K+ pump and the bumetanide-sensitive transporter--in human muscle in vitro at two developmental stages: proliferating myoblasts and differentiated myotubes. Myoblast differentiation to myotubes was accompanied by a marked decrease in both the ouabain-sensitive and the bumetanide-sensitive K+ (Rb+) influxes. The addition of serum to the terminally differentiated myotubes had no effect on these K+ transporters. However, serum addition to serum-deprived, undifferentiated myoblasts produced a marked stimulation of these K+ fluxes. The bumetanide-sensitive K+ transporter in human myoblasts and myotubes has the following properties: (1) It carries 30% and 40% of the total K+ influx in myoblasts and myotubes, respectively. (2) It performs net efflux of K+ in the undifferentiated myoblasts and zero net flux (self-exchange) in terminally differentiated myotubes. (3) It is dependent on extracellular Na+ and Cl- in addition to K+. (4) In myoblasts, the Km value for K+ is 1.36 mM, similar to the Km for K+ of the Na+/K+ pump. (5) It is resistant to ouabain (up to 2 mM) and sensitive to furosemide (K0.5 = 5 X 10(-6) M) and bumetanide (K0.5 = 10(-7) M). These data indicate that following terminal differentiation of proliferating myoblasts to mitotically inactive myotubes there is an irreversible reduction of K+ fluxes with a change in the net flux of K+ carried by the bumetanide-sensitive transporter.

Concentrations of elements in rat thymocytes measured by X-ray microanalysis

Elemental concentrations of rat thymocytes in vivo were studied by X-ray microanalysis of freeze-dried sections. Cells from different regions, the subcapsular zone, the cortex and the medulla were studied in thymic tissue from a number of animals. Generally thymocytes situated in the medulla had higher concentrations of K compared to those in the subcapsular zone. The concentration of Na in the nucleus was constant in the medulla in all animals but some variation in this element was seen between animals in the subcapsular zone. The distribution of K/Na ratio in individual thymocytes was different in each region of the thymus. Cells with low K/Na ratio (less than 5) were predominant in the subcapsular zone, whereas cells with higher values for K/Na ratio were found in the cortex and medulla. The subcapsular zone is the region where mitotic cells are mostly situated. The finding of thymocytes with higher concentrations of Na and low K/Na ratios in this region is in accord with in vitro studies on thymocyte stimulation.

Na+/K+/Cl- cotransport in cultured vascular smooth muscle cells: stimulation by angiotensin II and calcium ionophores, inhibition by cyclic AMP and calmodulin antagonists

The specific activity of the Na+/K+/Cl cotransporter was assayed by measuring the initial rates of furosemide-inhibitable 86Rb+ influx and efflux. The presence of all three ions in the external medium was essential for cotransport activity. In cultured smooth muscle cells furosemide and bumetanide inhibited influx by 50% at 5 and 0.2 microM, respectively. The dependence of furosemide-inhibitable 86Rb+ influx on external Na+ and K+ was hyperbolic with apparent Km values of 46 and 4 mM, respectively. The dependence on Cl was sigmoidal. Assuming a stoichiometry of 1:1:2 for Na+/K+/Cl-, a Km of 78 mM was obtained for Cl. In quiescent smooth muscle cells cotransport activity was approximately equal to Na+ pump activity with each pathway accounting for 30% of total 86Rb+ influx. Growing muscle cells had approximately 3 times higher cotransport activity than quiescent ones. Na+ pump activity was not significantly different in the growing and quiescent cultures. Angiotensin II (ANG) stimulated cotransport activity as did two calcium-transporting ionophores. A23187 and ionomycin. The removal of external Ca2+ prevented A23187, but not ANG, from stimulating the cotransporter. Calmodulin antagonists selectively inhibited 86Rb+ influx via the cotransporter. Beta-adrenoreceptor stimulation with isoproterenol, like other treatments which increase cAMP, inhibited cotransport activity. Cultured porcine endothelial cells had 3 times higher cotransport activity than growing muscle cells. Calmodulin antagonists inhibited cotransport activity, but agents which increase cAMP or calcium had no effect on cotransport activity in the endothelial cells.

Amiloride added together with bumetanide completely blocks mouse 3T3-cell exit from G0/G1-phase and entry into S-phase

In this study we tested the hypothesis that stimulation of univalent-cation fluxes which follow the addition of growth factors are required for cell transition through the G1-phase of the cell cycle. The effect of two drugs, amiloride and bumetanide, were tested on exit of BALB/c 3T3 cells from G0/G1-phase and entry into S-phase (DNA synthesis). Amiloride, an inhibitor of the Na+/H+ antiport, only partially inhibited DNA synthesis induced by serum. Bumetanide, an inhibitor of the Na+/K+ co-transport, only slightly suppressed DNA synthesis by itself, but when added together with amiloride completely blocked cell transition through G1 and entry into S-phase. Similar inhibitory effects of the two drugs were found on the induction of ornithine decarboxylase (ODC) (a marker of mid-G1-phase) in synchronized cells stimulated by either partially purified fibroblast growth factor (FGF) or serum. To test this hypothesis further, cells arrested in G0/G1 were stimulated by serum, insulin or FGF. All induced similar elevations of cellular K+ content during the early G1-phase of the cell cycle. However, serum and FGF, but not insulin, released the cells from the G0/G1 arrest, as measured by ODC enzyme induction. This result implies that the increase in cellular K+ content may be necessary but not sufficient for induction of early events during the G1-phase. The synergistic inhibitory effects of amiloride and bumetanide on the two activities stimulated by serum growth factors, namely ODC induction (mid-G1) and thymidine incorporation into DNA (S-phase), suggested that the amiloride-sensitive Na+/H+ antiport system together with the bumetanide-sensitive Na+/K+ transporter play a role in the mitogenic signal.

Studies on final differentiation of rat renal proximal tubular cells in culture

The ontogeny of effective Na and K permeability has been studied in renal epithelial cells isolated from the outermost superficial cortex from adult and young (10-15 days) rats. The cells were cultured for 2-4 days and exhibited phloridzin-inhibitable alpha-methylglucoside uptake, characteristic of renal proximal tubular cells (RPTC). Intracellular concentrations of K, Na, Cl, and P and kinetics of changes in intracellular ionic content after inhibition of Na-K-ATPase with 1 mM ouabain (or by incubation in low-K medium) were measured in individual cells using electron probe analysis. Intracellular concentrations of K, Na, Cl, and P were equivalent in young and adult rat RPTC. Adult rat and young rat cells preincubated in K-free medium rapidly recovered normal intracellular K and Na contents when returned to 5.5 mM K medium. The recovery was almost immediately blocked by ouabain. Effective permeabilities measured as half time of K efflux and Na influx after ouabain inhibition of Na-K-ATPase were higher in adult than in young RPTC cultured for less than 4 days. Effective K and Na permeabilities decreased significantly with increasing time in culture in adult but not in young rat RPTC. Among young rat RPTC, half times of Na and K fluxes were significantly correlated to age. Effective K and Na permeabilities were lower in both young and adult rat RPTC that had been serum deprived for 24 h than in cells that had been continuously cultured in serum. In cells cultured for 3 days and serum deprived for 1 day, the addition of serum significantly increased K and Na permeability both in young and adult RPTC, but the effect was more pronounced in young RPTC where permeability reached the same high values as in adult RPTC continuously cultured in serum. In conclusion, effective Na and K permeabilities and serum activation of "permeability units" change during ontogeny. These ontogenic changes might be blunted after a few days in culture due to dedifferentiation of adult rat RPTC.

Perturbation of Na+ and K+ gradients in human fibroblasts incubated in unsupplemented saline solutions

Changes in the intracellular concentrations of Na+ and K+ of fetal human fibroblasts have been followed after replacement of serum-containing growth media with unsupplemented and serum-supplemented saline solution (Earle's balanced salt solution). Incubation in unsupplemented salt solution was followed by a progressive increase of the internal Na+ counterbalanced by a decrease of internal K+, without major alterations of the internal osmolarity. After 3 h incubation the intracellular Na+ and K+ concentrations were 120 mM and 50 mM, respectively. These intracellular ion derangements were not associated with a failure of the (Na+ + K+)-ATPase pump, whose activity actually increased with enhanced intracellular Na+ concentration. Ion changes did not take place when serum (in excess of 0.5%, final concentration) was present in the saline solution and a complete restoration to normal of the Na+ and K+ gradients occurred upon addition of serum to cells previously incubated in plain saline solution. The effects of serum were mimicked by furosemide, thus suggesting that channels sensitive to this diuretic are involved in the movement of Na+ and K+ following fibroblast incubation in unsupplemented saline solution.

Fibroblast growth factor induces a transient net K+ influx carried by the bumetanide-sensitive transporter in quiescent BALB/c 3T3 fibroblasts

The bumetanide-sensitive transport system performed a net efflux of K+ in serum deprived quiescent cells. The addition of partially purified fibroblast growth factor (FGF) to G0/G1 phase 3T3 fibroblasts induced a transient net influx of K+, carried out by the bumetanide-sensitive transport system for 2-6 minutes. The stimulation of the bumetanide-sensitive K+ influx by FGF was followed by stimulation of the ouabain-sensitive K+ influx. In addition, both the bumetanide-sensitive and the ouabain-sensitive K+ influxes were found to be similarly stimulated when the G0/G1 3T3 cells were treated with insulin. These results suggest that growth factors such as FGF and insulin induce a change in the action of the bumetanide-sensitive transporter from performing net K+ efflux along its concentration gradient to an uphill transport pumping of K+ into the cell. We propose, therefore, that the bumetanide-sensitive transporter contributes to the increase in the intracellular K+ (and probable Na+) stimulated by growth factors such as FGF and insulin in early G1 phase of the cell cycle.

Serum stimulation of sodium transport in human fibroblasts containing low and high levels of intracellular sodium

The relationships between intracellular sodium content, sodium transport and serum effects were investigated in human fibroblasts. In the cells with low intracellular sodium (Na+iL; 0.04 mumol sodium/mg protein), serum stimulated the sodium-potassium pump as measured by ouabain-sensitive sodium efflux and rubidium influx and also exerted a transstimulation of ouabain-insensitive sodium transport resulting in net influx. In cells with high intracellular sodium (Na+iH; 0.42 mumol sodium/mg protein) all aspects of sodium transport were increased compared to Na+iL cells. In these cells serum caused no change in sodium-potassium pump activity but significantly increased the ouabain-insensitive sodium fluxes resulting in net efflux. In Na+iL cells, serum promoted net sodium influx through an amiloride-sensitive pathway that was undetectable in the basal state. In Na+iH cells the serum-stimulated net efflux was amiloride sensitive but this pathway also contributed to a major portion of sodium transport in the basal state. This study demonstrated that sodium-potassium pump activity is directed by the supply of internal sodium and that serum can increase this supply by promoting net influx, and that serum-induced sodium transport can be modified by intracellular sodium content.

31P NMR analysis of intracellular pH of Swiss Mouse 3T3 cells: effects of extracellular Na+ and K+ and mitogenic stimulation

Swiss mouse 3T3 cells grown on microcarrier beads were superfused with electrolyte solution during continuous NMR analysis. Conventional 31P and 19F probes of intracellular pH (pHc) were found to be impracticable. Cells were therefore superfused with 1 to 4 mM 2-deoxyglucose, producing a large intracellular, pH-sensitive signal of 2-deoxyglucose phosphate (2DGP). The intracellular incorporation of 2DGP inhibited the Embden-Meyerhof pathway. However, intracellular ATP was at least in part retained and the cellular responsivity to changes in extracellular ionic composition and to the application of growth factors proved intact. Transient replacement of external Na+ with choline or K+ reversibly acidified the intracellular fluids. Quiescent cells and mitogenically stimulated cells displayed the same dependence of shifts in pHc on external Na+ concentration (CoNa). PHc also depended on intracellular Na+ concentration (CcNa). Increasing ccNa by withdrawing external K+ (thereby inhibiting the Na,K-pump) caused reversible intracellular acidification; subsequently reducing CoNa produced a larger acid shift in pHc than with external K+ present. Comparison of separate preparations indicated that pHc was higher in stimulated than in quiescent cells. Transient administration of mitogens also reversibly alkalinized quiescent cells studied continuously. This study documents the feasibility of monitoring pHc of Swiss mouse 3T3 cells using 31P NMR analysis of 2DGP. The results support the concept of a Na/H antiport operative in these cells, both in quiescence and after mitogenic stimulation. The data document by an independent technique that cytoplasmic alkalinization is an early event in mitogenesis, and that full activity of the Embden-Meyerhof pathway is not required for the expression of this event.

Growth factor regulation of membrane transport in human fibroblasts and its relationship to stimulation of DNA synthesis

Serum stimulation of serum-deprived or density-inhibited normal cells enhances the level of various nutrient and ionic transport systems. Certain of these systems have been implicated in the regulation of cell proliferation. However, the use of serum stimulation to activate quiescent cells leads to enhancement of numerous transport systems with little understanding of which component or components of serum are related to activation of which transport systems. In this study we attempt to identify the specific effect of three known growth promoting factors (insulin, dexamethasone and epidermal growth factor [EGF]) on the activation of four membrane transport systems (A-amino acids, L-amino acids, glucose and K+) in normal and SV40-transformed WI38 human fibroblasts. We have also evaluated the effect of these growth factors on the stimulation of DNA synthesis in growth factor deprived cells. Thus, we can correlate the effect on a given transport system with the relative mitogenic stimulation produced by the growth factor. We conclude a) that a growth factor can effect a transport system differently in a normal versus transformed cell, b) that a specific growth factor can effect multiple transport systems and, c) with the exception of K+ transport, enhanced transport induced by a given growth factor does not necessarily correlate with the mitogenic potency of the growth factor. This latter point is of particular significance since the activation of K+ transport reflects, based on other studies, activation of the Na+-H+ exchanger which has been implicated in cell-cycle activation.

CSF-1 stimulates Na+K+-ATPase mediated 86Rb+ uptake in mouse bone marrow-derived macrophages

86Rb+ was used as an isotopic tracer for the measurement of K+-uptake into quiescent murine bone marrow-derived macrophages. 86Rb+ uptake was inhibited by ouabain indicating a Na+K+-ATPase is being measured. In support of this finding, increased sensitivity to ouabain inhibition was seen when the K+ content of the medium was reduced. A purified colony stimulating factor (CSF-1) was shown to stimulate the ouabain-sensitive 86Rb+ uptake in a dose-dependent manner. Such colony stimulating factor stimulation of 86Rb+ (K+) influx was rapid, with a maximal effect seen 10 minutes after growth factor addition followed by a gradual decrease. Thus increased Na+K+-ATPase activity was an early response of macrophages to the colony stimulating factor.

Characterization of a BALB/c 3T3 preadipose cell mutant with altered Na+K+Cl- cotransport activity

A BALB/c 3T3 cell mutant (3T3-E12) was isolated by its ability to survive at a low extracellular K+ concentration (0.14 mM). The growth rate of mutant cells was less dependent on external K+ than parental cells. Analysis of potassium transport revealed that 3T3-E12 cells have a decreased activity of the furosemide-sensitive Na+K+Cl- cotransport system, both in the efflux and influx modes. This is shown to be a result of a decrease in the apparent affinity of the transport system for K+ and Na+, but not Cl-. Upon exposure to the phorbol ester 12-0-tetradecanoyl-phorbol-13-acetate (TPA), BALB/c 3T3 cells exhibited a maximal volume decrease of 20%, while mutant cells shrunk by only 7%, suggesting that regulation of cell volume, at least four exposure to a tumor promoter, is impaired in mutant cells compared to parental 3T3 cells.

Serum-induced net K+ influx performed by the diuretic-sensitive transport system in quiescent NIH 3T3 mouse fibroblasts

In serum deprived NIH 3T3 mouse cells the diuretic-sensitive transport system performs K+ self-exchange. The addition of serum which stimulates cell proliferation induces a net influx of K+, carried out by the diuretic-sensitive transport system. Thus, serum growth factors appear to induce a change in the mechanism of action of the diuretic-sensitive transporter from K+ self-exchange to an uphill transport pumping K+ into the cell. I propose here that this uphill uptake of K+ contributes to the increase of intracellular K+ content, found in the early G1 phase of the cell cycle.

Effects of K+-deficiency and serum supplementation on protein turnover and nucleic acid synthesis in HeLa cells

When most of the K+ in a chemically defined medium was replaced with Rb+, cell growth of HeLa cells was strongly inhibited. The growth was partially but significantly restored by an addition of 5% dialyzed calf serum to the medium. The inhibition of cell growth in Rb+-substituted medium was partly due to suppression of protein synthesis by K+ deficiency, but the key mechanism of inhibition is still unknown. Rb+ substitution did not influence protein degradation or nucleic acid synthesis. The restoration of cell growth on addition of serum took place chiefly through stimulation of DNA synthesis. Protein and RNA syntheses were not affected by addition of serum, and serum-induced prevention of protein degradation was less in Rb+-substituted medium than in normal K+ medium.

Comparative measurements of potassium and chloride with ion-sensitive microelectrodes and x-ray microanalysis in cultured skeletal muscle fibers

Data of the intracellular electrolyte concentration of potassium and chloride in cultured muscle cells measured by x-ray analysis were compared by using the different activity coefficients with intracellular potassium and chloride activities measured with double-barrelled microelectrodes. By using an activity coefficient of 0.6, 95% of the potassium microelectrode measurements are in accordance with the x-ray analysis values, in spite of a scattering of the values. Membrane potential and intracellular potassium values are linearly related. x-ray analysis and ion-sensitive microelectrodes measured the cytoplasmic chloride in the same range. Taking into account known activity coefficients, an error of 25% must be assumed with the intracellular chloride measurements. However, x-ray analysis and ion-sensitive microelectrode investigations are reliable tools to study intracellular potassium and chloride changes, which play an important role in membrane characteristics.

Ionic changes associated with lead stimulation of DNA synthesis in Balb/c3T3 cells

Lead at slightly subtoxic concentrations markedly stimulated the rate of DNA synthesis in cultured animal cells. This stimulation was closely correlated with formation of a precipitate that was adsorbed and taken up by the cells under certain medium conditions. Data suggest that a precipitate-induced perturbation of the surface membrane leads to intracellular changes responsible for stimulation of DNA synthesis. Maximum stimulation of(3)H-thymidine incorporation by optimum concentrations of lead is delayed about 8 h compared to that in serum stimulation. In cells stimulated significantly by lead, but not in unstimu-lated cells, a reproducible rise of about 13% in intracellular magnesium occurred over a 24 h period, with an 8 h lag in the increase compared to that observed in serum stimulation. In view of the increases in intracellular magnesium consistently associated with and preceding stimulation of DNA synthesis by several different mitogens including serum and insulin, the present time-coordinated positive correlation between magnesium and DNA synthesis provides evidence for the primary involvement of this divalent cation in growth stimulation produced by lead.

Stimulation of DNA synthesis in kidney epithelial cells in culture by potassium

The hypothesis that the K+ concentration of extracellular fluid is a determinant of renal DNA synthesis was examined in quiescent, high-density cultures of monkey kidney epithelial cells of the BSC-1 line. The addition of KCl to the medium increased the number of cells engaged in DNA synthesis in a concentration-dependent manner. The capacity of K+ to stimulate DNA synthesis in a greater number of cells was additive with exogenous NaCl and calf serum and was associated with an increment in the steady-state cell K+ content. Studies with other monovalent cations indicated that the stimulatory effect of K+ on DNA synthesis was not mediated by increments in the chloride concentration or osmotic pressure of the medium. The addition of K+ to confluent cultures was associated with a concentration-dependent increase in cell multiplication. The commitment of cells to increased multiplication required exposure of the culture to added KCl for longer than 3 but not more than 6 h. Addition of KCl to cultures of mouse fibroblasts did not alter DNA synthesis, multiplication, or cell K+ content. These observations indicate that increased availability of K+ in the extracellular fluid can stimulate DNA synthesis in kidney epithelial cells in culture.

The element distribution in ultrathin cryosections of cultivated fibroblast cells

A preparation method for measurements of the intracellular distribution of elements in tissue culture cells is described which is based on cryofixation, cryoultramicrotomy, cryotransfer and X-ray microanalysis in a scanning transmission electron microscope. Dry weight concentrations of phosphorus, sulfur, chlorine and potassium in the nucleus, the cytoplasm and the mitochondria of L929 fibroblast cells of the mouse are reported. The preparation and quantitation procedures are discussed with respect to present limitations and possible improvements of the method.

Ionic responses rapidly elicited by porcine platelet-derived growth factor in Swiss 3T3 cells

Addition of porcine platelet-derived growth factor (PDGF) to quiescent cultures of Swiss 3T3 cells caused a marked, dose-dependent stimulation of Na+ influx and Na-K pump-mediated 86Rb+ uptake. Porcine PDGF (a single component in SDS polyacrylamide gels) stimulated ion fluxes to the same maximal extent as partially purified preparations, and exhibited half-maximal effect at 6 ng/ml (2 X 10(-10) M). Maximal effect was achieved at 30 ng/ml (10(-9) M). In the presence of insulin, PDGF elicited mitogenesis at comparable concentrations. PDGF stimulated ion uptake in a time-dependent fashion; maximal effect was obtained after 5 min of exposure to the growth factor. PDGF stimulates Na+ influx via an amiloride-sensitive pathway, suggesting that PDGF enhances the activity of a Na+/H+ antiport system. In accordance with this possibility, the mitogen caused an increase of intracellular pH by 0.15 pH units, as judged by the steady-state distribution of labelled 5,5-dimethyloxazolidine-2,4-dione (DMO). Porcine PDGF stimulated E-type prostaglandin synthesis and cAMP accumulation but these events could be dissociated from the stimulation of the ionic fluxes, which was detected within minutes and was not blocked by indomethacin. It is suggested that PDGF elicits multiple signals to stimulate cell proliferation in 3T3 cells.

Extracellular Na+ and initiation of DNA synthesis: role of intracellular pH and K+

Initiation of DNA synthesis in confluent quiescent 3T3 cell cultures stimulated by epidermal growth factor (EGF), vasopressin, and insulin was abolished by removing extracellular Na+. The inhibition was reversible, time- and Na+-concentration-dependent, and not due to an effect on binding or internalization of 125I-EGF. Stimulation by combinations of other growth factors with different mechanisms of action was also affected by decreasing extracellular Na+, but with different half-maximal Na+ concentrations. When choline was used as an osmotic substitute for Na+, the decrease in DNA synthesis was correlated with the decrease in intracellular K+. In contrast, when sucrose was used there was stimulation of the Na+-K+ pump and maintenance of intracellular K+ that resulted in a somewhat higher rate of DNA synthesis at lowered extracellular Na+ compared to choline. Mitogenesis induced by epidermal growth factor, vasopressin, and insulin led to cytoplasmic alkalinization as determined by an increase in uptake of the weak acid 5,5-dimethyloxazolidine-2,4-dione. Experimental decrease in extracellular Na+ blocked this cellular alkalinization. Therefore, under some conditions the supply of extracellular Na+ may limit cellular proliferation because of a reduction in the provision of Na+ to the Na+/H+ antiport and resultant failure of alkalinization. We conclude that Na+ flux and its effect on intracellular K and pH has a major role in the complex system that regulates proliferation.

Alterations in monovalent cation transport in Sindbis virus-infected chick cells

Influx experiments using the potassium tracer 86Rb+ indicated that the activity of the Na+K+ ATPase, or sodium pump, was reduced 40-50% as a consequence of Sindbis virus infection of avian fibroblasts. The inhibition of this ouabain-sensitive, active transport system temporally correlated with a decrease in the intracellular K+ concentration and the termination of cellular protein synthesis. By contrast, the rate of influx facilitated by the furosemide-sensitive (Na+K+Cl-) cotransport system was only slightly depressed. Efflux experiments indicated that no alterations in the relative rate of nonspecific permeability or "leakage" of K+ could be detected in chick cells infected by Sindbis virus. The amount of [3H]ouabain bound to Sindbis virus-infected cells paralleled the reduction in Na+K+ ATPase activity. These binding studies revealed no difference in the number of Na+ pump sites. The Km of ouabain binding, however, increased approximately 3.5-fold in the virus-infected cells. No change in the apparent affinity of the Na+ pump for K+ could be detected, yet the Vmax for ouabain-sensitive K+ transport was decreased. These experiments suggest that a reduction in Na+K+ ATPase turnover results in the altered intracellular monovalent cation levels found in Sindbis virus-infected chick cells.

Rb+ influxes differentiate between growth arrest of cells by different agents

The effect of cell cycle on Rb+ (K+) fluxes was studied in NIH 3T3 mouse fibroblasts. Serum starvation or isoleucine deprivation resulted in cell arrest at an early G1/G0 phase, accompanied by a marked decrease in both ouabain-sensitive and ouabain-resistant Rb+ influx. On the other hand, cells arrested at late G1/G0 phase by hydroxyurea treatment have high ouabain-sensitive and ouabain-resistant Rb+ influx. Butyric acid treatment resulted in cell arrest at an early G1/G0 phase, but in contrast to serum or isoleucine starvation did not decrease Rb+ influxes. It is thus shown that quiescent cells may have Rb+ influx rates as high as that of logarithmically growing cells. The results are consistent with the hypothesis that an increased ion permeability of the cell is initiated at a critical stage in G1/G0 phase, and that butyric acid may arrest the cell beyond that stage.

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

The pumping activity of the plasma membrane-bound Na+,K+-ATPase shows considerable variation during the cell cycle of mouse neuroblastoma Neuro-2A cells. Addition of external ATP at millimolar concentrations, which selectively enhances the plasma membrane permeability of Neuro-2A cells for sodium ions, stimulates the Na+,K+-ATPase pumping activity at all phases of the cell cycle from a factor of 1.05 in mitosis up to 2.2 in G1 phase. Determination of the number of Na+,K+-ATPase copies per cell by direct 3H-ouabain binding studies in the presence of external ATP shows a gradual increase in the number of pump sites on passing from mitosis to the late S/G2-phase by approximately a factor of 2. From these data the pumping activity per copy of Na+,K+-ATPase, optimally stimulated with respect to its various substrate ions, has been determined during the various phases of the cell cycle. This optimally stimulated pumping activity per enzyme copy, which is a reflection of the physicochemical state of the plasma membrane, is high in mitosis, almost twofold lower in early G1 phase, and increases gradually again during the other phases of the cell cycle. This shows that the observed regulation of Na+,K+-ATPase activity during the cell cycle is caused by a combination of three independent factors--namely variation in intracellular substrate availability (Na+), changes in number of enzyme copies per cell, and modulation of the plasma membrane environment of the protein molecules. The modulation of the optimal pumping activity per enzyme copy shows a good correlation (rho = 0.96) with the known modulation of protein lateral mobility during the cell cycle, such that a high protein lateral mobility correlates with a low enzyme activity. It is concluded that changes in plasma membrane properties take place during the Neuro-2A cell cycle that result in changes in the rate of protein lateral diffusion and Na+,K+-ATPase activity in directly correlated way.

Na+/H+ antiport in Swiss 3T3 cells: mitogenic stimulation leads to cytoplasmic alkalinization

Addition of Na+ to Na+-depleted Swiss 3T3 cells causes a rapid and dramatic increase in intracellular pH, as monitored by uptake of the weak acid 5,5-dimethyloxazolidine-2,4-dione. The effect of Na+ is concentration dependent (half-maximal effect at 38 mM); this cation can be replaced by Li+ but not by K+ or the choline ion. Amiloride prevents the Na+-induced increase in intracellular pH and also blocks the entry of Na+ into 3T3 cells; the half-maximal concentrations of amiloride for inhibiting the two processes are similar (40 microM). Increase in extracellular pH caused an increase in the initial rate of Na+ influx that was of sufficient magnitude to stimulate the activity of the Na+/K+ pump in quiescent 3T3 cells. Taken together, these findings suggest the presence of a functional Na+/H+ antiport in Swiss 3T3 cells. Addition of the potent mitogenic combination platelet-derived growth factor, vasopressin, and insulin to quiescent Swiss 3T3 cells increased the intracellular pH from 7.21 +/- 0.07 to 7.36 +/- 0.09 in 10 independent experiments (P less than 0.001). This combination of growth factors also stimulated Na+ entry and ouabain-sensitive Rb+ uptake. The data support the hypothesis that early changes in ion fluxes play a role in signaling mitogenesis in 3T3 cells.