Thymocyte K+, Na+ and Water Balance During Dexamethasone- and Etoposide-Induced Apoptosis

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.

Monovalent ions and stress-induced senescence in human mesenchymal endometrial stem/stromal cells

Monovalent ions are involved in growth, proliferation, differentiation of cells as well as in their death. This work concerns the ion homeostasis during senescence induction in human mesenchymal endometrium stem/stromal cells (hMESCs): hMESCs subjected to oxidative stress (sublethal pulse of H₂O₂) enter the premature senescence accompanied by persistent DNA damage, irreversible cell cycle arrest, increased expression of the cell cycle inhibitors (p53, p21) cell hypertrophy, enhanced β-galactosidase activity. Using flame photometry to estimate K⁺, Na⁺ content and Rb⁺ (K⁺) fluxes we found that during the senescence development in stress-induced hMESCs, Na⁺/K⁺pump-mediated K⁺ fluxes are enhanced due to the increased Na⁺ content in senescent cells, while ouabain-resistant K⁺ fluxes remain unchanged. Senescence progression is accompanied by a peculiar decrease in the K⁺ content in cells from 800-900 to 500-600 µmol/g. Since cardiac glycosides are offered as selective agents for eliminating senescent cells, we investigated the effect of ouabain on ion homeostasis and viability of hMESCs and found that in both proliferating and senescent hMESCs, ouabain (1 nM-1 µM) inhibited pump-mediated K⁺ transport (ID₅₀ 5 × 10^-8 M), decreased cell K⁺/Na⁺ ratio to 0.1-0.2, however did not induce apoptosis. Comparison of the effect of ouabain on hMESCs with the literature data on the selective cytotoxic effect of cardiac glycosides on senescent or cancer cells suggests the ion pump blockade and intracellular K⁺ depletion should be synergized with target apoptotic signal to induce the cell death.

Ions, the Movement of Water and the Apoptotic Volume Decrease

The movement of water across the cell membrane is a natural biological process that occurs during growth, cell division, and cell death. Many cells are known to regulate changes in their cell volume through inherent compensatory regulatory mechanisms. Cells can sense an increase or decrease in their cell volume, and compensate through mechanisms known as a regulatory volume increase (RVI) or decrease (RVD) response, respectively. The transport of sodium, potassium along with other ions and osmolytes allows the movement of water in and out of the cell. These compensatory volume regulatory mechanisms maintain a cell at near constant volume. A hallmark of the physiological cell death process known as apoptosis is the loss of cell volume or cell shrinkage. This loss of cell volume is in stark contrast to what occurs during the accidental cell death process known as necrosis. During necrosis, cells swell or gain water, eventually resulting in cell lysis. Thus, whether a cell gains or loses water after injury is a defining feature of the specific mode of cell death. Cell shrinkage or the loss of cell volume during apoptosis has been termed apoptotic volume decrease or AVD. Over the years, this distinguishing feature of apoptosis has been largely ignored and thought to be a passive occurrence or simply a consequence of the cell death process. However, studies on AVD have defined an underlying movement of ions that result in not only the loss of cell volume, but also the activation and execution of the apoptotic process. This review explores the role ions play in controlling not only the movement of water, but the regulation of apoptosis. We will focus on what is known about specific ion channels and transporters identified to be involved in AVD, and how the movement of ions and water change the intracellular environment leading to stages of cell shrinkage and associated apoptotic characteristics. Finally, we will discuss these concepts as they apply to different cell types such as neurons, cardiomyocytes, and corneal epithelial cells.

Fibroblast Growth Factor 22 Inhibits ER Stress-Induced Apoptosis and Improves Recovery of Spinal Cord Injury

Currently, inhibiting or reducing neuronal cell death is the main strategy to improve recovery of spinal cord injury (SCI). Therapies using nerve growth factors to treat SCI mainly focused on reducing the area damaged by postinjury degeneration to promote functional recovery. In this report, we investigated the mechanism of ER (endoplasmic reticulum) stress-induced apoptosis and the protective action of fibroblast growth factor 22 (FGF22) in vivo. Our results demonstrated that ER stress-induced apoptosis plays a significant role in injury of SCI model rats. FGF22 administration promoted recovery and increased neuron survival in the spinal cord lesions of model mice. The protective effect of FGF22 is related to decreased expression of CHOP (C/EBP-homologous protein), GRP78 (glucose-regulated protein 78), caspase-12, X-box binding protein 1 (XBP1), eukaryotic initiation factor 2α (Eif-2α) and Bad which are ER stress-induced apoptosis response proteins. Moreover, FGF22 administration also increased the number of neurons and the expression of growth-associated protein 43 (GAP43) which was related to axon regeneration. We also demonstrated that the protective effect of FGF22 effectively reduces neuronal apoptosis and promotes axonal regeneration. Our study first illustrated that the function of FGF22 is related to the inhibition of ER stress-induced cell death in SCI recovery via activation of downstream signals. This study also suggested a new tendency of FGF22 therapy development in central neural system injuries, which involved chronic ER stress-induced apoptosis.

A Tool for Computation of Changes in Na+, K+, Cl- Channels and Transporters Due to Apoptosis by Data on Cell Ion and Water Content Alteration

Monovalent ions are involved in a vast array of cellular processes. Their movement across the cell membrane is regulated by numerous channels and transporters. Identification of the pathways responsible for redistribution of ions and cell water in living cells is hampered by their strong interdependence. This difficulty can be overcome by computational analysis of the whole cell flux balance. Our previous computational studies were concerned with monovalent ion fluxes in cells under the conditions of balanced ion distribution or during transition processes after stopping the Na+/K+ pump. Here we analyze a more complex case-redistribution of ions during cell apoptosis when the parameters keep changing during the process. New experimental data for staurosporine-induced apoptosis of human lymphoma cells U937 have been obtained: the time course of changes in cellular K+, Na+, Cl-, and water content, as well as Rb+ fluxes as a marker of the Na/K pump activity. Using a newly developed computational tool, we found that alteration of ion and water balance was associated with a 55% decrease in the Na+/K+-ATPase rate coefficient over a 4-h period, with a time-dependent increase in potassium channel permeability, and a decrease in sodium channel permeability. The early decrease in [Cl-]i and cell volume were associated with an ~5-fold increase in chloride channel permeability. The developed approach and the presented executable file can be used to identify the channels and transporters responsible for alterations of cell ion and water balance not only during apoptosis but in other physiological scenarios.

A comparative study of U937 cell size changes during apoptosis initiation by flow cytometry, light scattering, water assay and electronic sizing

A decrease in flow cytometric forward light scatter (FSC) is commonly interpreted as a sign of apoptotic cell volume decrease (AVD). However, the intensity of light scattering depends not only on the cell size but also on its other characteristics, such as hydration, which may affect the scattering in the opposite way. That makes estimation of AVD by FSC problematic. Here, we aimed to clarify the relationship between light scattering, cell hydration (assayed by buoyant density) and cell size by the Coulter technique. We used human lymphoid cells U937 exposed to staurosporine, etoposide or hypertonic stress as an apoptotic model. An initial increase in FSC was found to occur in apoptotic cells treated with staurosporine and hypertonic solutions; it is accompanied by cell dehydration and is absent in apoptosis caused by etoposide that is consistent with the lack of dehydration in this case. Thus, the effect of dehydration on the scattering signal outweighs the effect of reduction in cell size. The subsequent FSC decrease, which occurred in parallel to accumulation of annexin-positive cells, was similar in apoptosis caused by all three types of inducers. We conclude that an increase, but not a decrease in light scattering, indicates the initial cell volume decrease associated with apoptotic cell dehydration.

Detection and quantification of subtle changes in red blood cell density using a cell phone

Magnetic levitation has emerged as a technique that offers the ability to differentiate between cells with different densities. We have developed a magnetic levitation system for this purpose that distinguishes not only different cell types but also density differences in cells of the same type. This small-scale system suspends cells in a paramagnetic medium in a capillary placed between two rare earth magnets, and cells levitate to an equilibrium position determined solely by their density. Uniform reference beads of known density are used in conjunction with the cells as a means to quantify their levitation positions. In one implementation images of the levitating cells are acquired with a microscope, but here we also introduce a cell phone-based device that integrates the magnets, capillary, and a lens into a compact and portable unit that acquires images with the phone's camera. To demonstrate the effectiveness of magnetic levitation in cell density analysis we carried out levitation experiments using red blood cells with artificially altered densities, and also levitated those from donors. We observed that we can distinguish red blood cells of an anemic donor from those that are healthy. Since a plethora of disease states are characterized by changes in cell density magnetic cell levitation promises to be an effective tool in identifying and analyzing pathologic states. Furthermore, the low cost, portability, and ease of use of the cell phone-based system may potentially lead to its deployment in low-resource environments.

Stimulation of erythrocyte cell membrane scrambling by nystatin

The antifungal ionophore nystatin dissipates the Na(+) and K(+) gradients across the cell membrane, leading to cellular gain of Na(+) and cellular loss of K(+) . The increase of cellular Na(+) concentration may result in Ca(2+) accumulation in exchange for Na(+) . Increase of cytosolic Ca(2+) activity ([Ca(2+) ]i ) and loss of cellular K(+) foster apoptosis-like suicidal erythrocyte death or eryptosis, which is characterised by cell shrinkage and cell membrane scrambling leading to phosphatidylserine exposure at the erythrocyte surface. The present study explored whether nystatin stimulates eryptosis. Cell volume was estimated from forward scatter (FSC), phosphatidylserine exposure from annexin V binding and [Ca(2+) ]i from Fluo3-fluorescence in flow cytometry. A 48-hr exposure to nystatin (15 μg/ml) was followed by a significant increase of [Ca(2+) ]i , a significant increase of annexin V binding and a significant decrease of FSC. The annexin V binding after nystatin treatment was significantly blunted in the nominal absence of extracellular Ca(2+) . Partial replacement of extracellular Na(+) with extracellular K(+) blunted the nystatin-induced erythrocyte shrinkage but increased [Ca(2+) ]i and annexin V binding. Nystatin triggers cell membrane scrambling, an effect at least partially due to entry of extracellular Ca(2+) .

Magnesium loss in cyclosporine-treated patients is related to renal epidermal growth factor downregulation

BACKGROUND

Cyclosporine (CsA) treatment is associated with hypomagnesaemia due to a renal Mg(2+) leak. In animal studies a role for the Mg(2+) channel TRPM6 localized in the distal convoluted tubule and stimulated by epidermal growth factor (EGF) is suggested. We hypothesize that CsA-induced hypomagnesaemia is due to a renal magnesium leak, also in patients, resulting from a downregulation of the renal EGF production, thereby inhibiting the activation of TRPM6.

METHODS

Renal transplant patients treated with CsA (n = 55) and 35 chronic kidney disease (CKD) patients were included. At three time points, with an interval of at least 1 month, blood and urine samples were taken to determine creatinine, Mg(2+), sodium and EGF.

RESULTS

Serum Mg(2+) was significantly lower in the CsA group versus the CKD group with significantly more CsA-treated patients developing hypomagnesaemia. Although the fractional excretion (FE) Mg(2+) did not differ significantly between the two groups, subanalysis of the patients with hypomagnesaemia showed a significantly higher FE Mg(2+) in CsA-treated patients compared with CKD patients (P = 0.05). The urinary EGF excretion was significantly decreased in the CsA group and was a predictor of the FE Mg(2+) in the two groups. Serum sodium was significantly decreased in the CsA group simultaneously with an increased FE Na(+).

CONCLUSIONS

In CsA-treated patients, the association of a low urinary EGF excretion and a decreased renal Mg(2+) reabsorption is in accordance with in vitro and animal studies. In the whole study population, log urinary EGF excretion is an independent predictor of the FE Mg(2+), supporting the role of EGF in magnesium reabsorption.

Role of ion transport in control of apoptotic cell death

Cell shrinkage is a hallmark and contributes to signaling of apoptosis. Apoptotic cell shrinkage requires ion transport across the cell membrane involving K(+) channels, Cl(-) or anion channels, Na(+)/H(+) exchange, Na(+),K(+),Cl(-) cotransport, and Na(+)/K(+)ATPase. Activation of K(+) channels fosters K(+) exit with decrease of cytosolic K(+) concentration, activation of anion channels triggers exit of Cl(-), organic osmolytes, and HCO3(-). Cellular loss of K(+) and organic osmolytes as well as cytosolic acidification favor apoptosis. Ca(2+) entry through Ca(2+)-permeable cation channels may result in apoptosis by affecting mitochondrial integrity, stimulating proteinases, inducing cell shrinkage due to activation of Ca(2+)-sensitive K(+) channels, and triggering cell-membrane scrambling. Signaling involved in the modification of cell-volume regulatory ion transport during apoptosis include mitogen-activated kinases p38, JNK, ERK1/2, MEKK1, MKK4, the small G proteins Cdc42, and/or Rac and the transcription factor p53. Osmosensing involves integrin receptors, focal adhesion kinases, and tyrosine kinase receptors. Hyperosmotic shock leads to vesicular acidification followed by activation of acid sphingomyelinase, ceramide formation, release of reactive oxygen species, activation of the tyrosine kinase Yes with subsequent stimulation of CD95 trafficking to the cell membrane. Apoptosis is counteracted by mechanisms involved in regulatory volume increase (RVI), by organic osmolytes, by focal adhesion kinase, and by heat-shock proteins. Clearly, our knowledge on the interplay between cell-volume regulatory mechanisms and suicidal cell death is still far from complete and substantial additional experimental effort is needed to elucidate the role of cell-volume regulatory mechanisms in suicidal cell death.

Balance of unidirectional monovalent ion fluxes in cells undergoing apoptosis: why does Na+/K+ pump suppression not cause cell swelling?

Cells dying according to the apoptotic program, unlike cells dying via an unprogrammed mode, are able to avoid swelling and osmotic bursting with membrane disruption.There are indications that apoptosis is accompanied by suppression of the Na+/K+ pump and changes in the K+ and Cl− channels. It remains unclear how ion fluxes through individual ion pathways are integrated so as to induce loss of intracellular ions and concomitant apoptotic volume decrease. A decrease in activity of the sodium pump during apoptosis should cause cell swelling rather than shrinkage. We have made the first systemic analysis of the monovalent ion flux balance in apoptotic cells. Experimental data were obtained for human U937 cells treated with staurosporine for 4–5 h, which is known to induce apoptosis. The data include cellular Cl− content and fluxes, K+, Na+, water content and ouabain-sensitive and -resistant Rb+ fluxes.Unidirectional monovalent ion fluxeswere calculated using these data and a cell model comprising the double Donnan system with the Na+/K+ pump, Cl−, K+, Na+ channels, the Na+–K+–2Cl−cotransporter (NKCC), the Na+–Cl− cotransporter (NC), and the equivalent Cl−/Cl− exchange.Apoptotic cell shrinkage was found to be caused, depending on conditions, either by an increase in the integral channel permeability of membrane for K+ or by suppression of the pump coupledwith a decrease in the integral channel permeability of membrane for Na+. The decrease in the channel permeability of membrane for Na+ plays a crucial role in cell dehydration in apoptosis accompanied by suppression of the pump. Supplemental Table S1 is given for easy calculating flux balance under specified conditions.

EGF increases TRPM6 activity and surface expression

Recent identification of a mutation in the EGF gene that causes isolated recessive hypomagnesemia led to the finding that EGF increases the activity of the epithelial magnesium (Mg2+) channel transient receptor potential M6 (TRPM6). To investigate the molecular mechanism mediating this effect, we performed whole-cell patch-clamp recordings of TRPM6 expressed in human embryonic kidney 293 (HEK293) cells. Stimulation of the EGF receptor increased current through TRPM6 but not TRPM7. The carboxy-terminal alpha-kinase domain of TRPM6 did not participate in the EGF receptor-mediated increase in channel activity. This activation relied on both the Src family of tyrosine kinases and the downstream effector Rac1. Activation of Rac1 increased the mobility of TRPM6, assessed by fluorescence recovery after photobleaching, and a constitutively active mutant of Rac1 mimicked the stimulatory effect of EGF on TRPM6 mobility and activity. Ultimately, TRPM6 activation resulted from increased cell surface abundance. In contrast, dominant negative Rac1 decreased TRPM6 mobility, abrogated current development, and prevented the EGF-mediated increase in channel activity. In summary, EGF-mediated stimulation of TRPM6 occurs via signaling through Src kinases and Rac1, thereby redistributing endomembrane TRPM6 to the plasma membrane. These results describe a regulatory mechanism for transepithelial Mg2+ transport and consequently whole-body Mg2+ homeostasis.

Molecular determinants of magnesium homeostasis: insights from human disease

The past decade has witnessed multiple advances in our understanding of magnesium (Mg(2+)) homeostasis. The discovery that mutations in claudin-16/paracellin-1 or claudin-19 are responsible for familial hypomagnesemia with hypercalciuria and nephrocalcinosis provided insight into the molecular mechanisms governing paracellular transport of Mg(2+). Our understanding of the transcellular movement of Mg(2+) was similarly enhanced by the realization that defects in transient receptor potential melastatin 6 (TRPM6) cause hypomagnesemia with secondary hypocalcemia. This channel regulates the apical entry of Mg(2+) into epithelia. In so doing, TRPM6 alters whole-body Mg(2+) homeostasis by controlling urinary excretion. Consequently, investigation into the regulation of TRPM6 has increased. Acid-base status, 17beta estradiol, and the immunosuppressive agents FK506 and cyclosporine affect plasma Mg(2+) levels by altering TRPM6 expression. A mutation in epithelial growth factor is responsible for isolated autosomal recessive hypomagnesemia, and epithelial growth factor activates TRPM6. A defect in the gamma-subunit of the Na,K-ATPase causes isolated dominant hypomagnesemia by altering TRPM6 activity through a decrease in the driving force for apical Mg(2+) influx. We anticipate that the next decade will provide further detail into the control of the gatekeeper TRPM6 and, therefore, overall whole-body Mg(2+) balance.

Mechanisms and significance of cell volume regulation

Survival of human and animal cells requires avoidance of excessive alterations of cell volume. The osmolarity amassed by cellular accumulation of organic substances must be compensated by lowering cytosolic ion concentrations. The Na+/K+ ATPase extrudes Na+ in exchange for K+, which can permeate the cell membrane through K+ channels. K+ exit generates a cell-negative potential difference across the cell membrane, driving the exit of anions such as Cl-. The low cytosolic Cl- concentrations counterbalance the excess cellular osmolarity by organic substances. Cell volume regulation following cell swelling involves releasing ions through activation of K+ channels and/or anion channels, KCl-cotransport, or parallel activation of K+/H+ exchange and Cl-/HCO3- exchange. Cell volume regulation following cell shrinkage involves accumulation of ions through activation of Na+,K+,2Cl- cotransport, Na+/H+ exchange in parallel to Cl-/HCO3- exchange, or Na+ channels. The Na+ taken up is extruded by the Na+/K+ ATPase in exchange for K+. Shrunken cells further accumulate organic osmolytes such as sorbitol and glycerophosphorylcholine, and monomeric amino acids by altered metabolism and myoinositol (inositol), betaine, taurine, and amino acids by Na+ coupled transport. They release osmolytes during cell swelling. Challenges of cell volume homeostasis include transport, hormones, transmitters, and drugs. Moreover, alterations of cell volume participate in the machinery regulating cell proliferation and apoptotic cell death. Deranged cell volume regulation significantly contributes to the pathophysiology of several disorders such as liver insufficiency, diabetic ketoacidosis, hypercatabolism, fibrosing disease, sickle cell anemia, and infection.

Reduced Ca2+ entry and suicidal death of erythrocytes in PDK1 hypomorphic mice

The phosphoinositide-dependent kinase PDK1 is a key element in the phosphoinositol-3-kinase signalling pathway, which is involved in the regulation of ion channels, transporters, cell volume and cell survival. Eryptosis, the suicidal death of erythrocytes, is characterized by decrease in cell volume, cell membrane blebbing and phospholipids scrambling with phosphatidylserine exposure at the cell surface. Oxidative stress, osmotic shock or Cl- removal trigger eryptosis by activation of Ca2+-permeable cation channels and subsequent increase in cytosolic Ca2+ activity. To explore the impact of PDK1 for erythrocyte survival, eryptosis was analysed in hypomorphic mice (pdk1hm) expressing only some 25% of PDK1 and in their wild-type littermates (pdk1wt). Cell volume was estimated from forward scatter and phosphatidylserine exposure from annexin-V binding in fluorescence activated cell sorter analysis. Forward scatter was smaller in pdk1hm than in pdk1wt erythrocytes. Oxidative stress (100 microM tert-butylhydroperoxide), osmotic shock (+300 mM sucrose) and Cl- removal (replacement of Cl- with gluconate) all decreased forward scatter and increased the percentage of annexin-V-binding erythrocytes from both pdk1hm and pdk1wt mice. After treatment, the forward scatter was similar in both genotypes, but the percentage of annexin-V binding was significantly smaller in pdk1hm than in pdk1wt erythrocytes. According to Fluo-3 fluorescence, cytosolic Ca2+ activity was significantly smaller in pdk1hm than in pdk1wt erythrocytes. Treatment with Ca2+-ionophore ionomycin (1 microM) was followed by an increase in annexin-V binding to similar levels in pdk1hm and pdk1wt erythrocytes. The experiments reveal that PDK1 deficiency is associated with decreased Ca2+ entry into erythrocytes and thus with blunted eryptotic effects of oxidative stress, osmotic shock and Cl- removal.

Enhanced susceptibility to suicidal death of erythrocytes from transgenic mice overexpressing erythropoietin

Eryptosis, a suicidal death of mature erythrocytes, is characterized by decrease of cell volume, cell membrane blebbing, and breakdown of cell membrane asymmetry with phosphatidylserine exposure at the cell surface. Triggers of eryptosis include increased cytosolic Ca(2+) activity, which could result from activation of Ca(2+)-permeable cation channels. Ca(2+) triggers phosphatidylserine exposure and activates Ca(2+)-sensitive K(+) channels, leading to cellular K(+) loss and cell shrinkage. The cation channels and thus eryptosis are stimulated by Cl(-) removal and inhibited by erythropoietin. The present experiments explored eryptosis in transgenic mice overexpressing erythropoietin (tg6). Erythrocytes were drawn from tg6 mice and their wild-type littermates (WT). Phosphatidylserine exposure was estimated from annexin binding and cell volume from forward scatter in fluorescence-activated cell sorting (FACS) analysis. The percentage of annexin binding was significantly larger and forward scatter significantly smaller in tg6 than in WT erythrocytes. Transgenic erythrocytes were significantly more resistant to osmotic lysis than WT erythrocytes. Cl(-) removal and exposure to the Ca(2+) ionophore ionomycin (1 microM) increased annexin binding and decreased forward scatter, effects larger in tg6 than in WT erythrocytes. The K(+) ionophore valinomycin (10 nM) triggered eryptosis in both tg6 and WT erythrocytes and abrogated differences between genotypes. An increase of extracellular K(+) concentration to 125 mM blunted the difference between tg6 and WT erythrocytes. Fluo-3 fluorescence reflecting cytosolic Ca(2+) activity was larger in tg6 than in WT erythrocytes. In conclusion, circulating erythrocytes from tg6 mice are sensitized to triggers of eryptosis but more resistant to osmotic lysis, properties at least partially due to enhanced Ca(2+) entry and increased K(+) channel activity.

Loss of mitochondrial membrane potential is inhibited by bombesin in etoposide-induced apoptosis in PC-3 prostate carcinoma cells

Neuroendocrine secretory products and their interactions with epithelial prostate cells are currently under investigation in order to understand their significance in the pathogenesis, prognosis, and therapy of prostate carcinoma. These neuropeptides have the potential to disrupt the balance between cell death and cell growth in the tumor. Our research was based on the role of bombesin in modulating the mitochondrial membrane potential (Delta psi(m)) in cell death induced by etoposide on PC-3 cells. Cells were cultured and stained with 5,5',6,6'-tetrachloro-1,1',3,3'-tetraethylbenzimidazolylcarbocyanine iodide (JC-1). At low membrane potentials, JC-1 produces a green fluorescence, and at high membrane potentials, it forms "J aggregates" with red fluorescence. Cells were examined in a confocal microscope. For quantitative analyses, regions of interest were selected. The size, number of pixels, and ratios between fluorescence intensity in the red and green channels in each region of interest were calculated. The loss of Delta psi(m) in etoposide-treated PC-3 cells was prevented by bombesin. The quantitative analysis of JC-1-stained cells revealed a significant decrease in the red (high Delta psi(m)) to green (low Delta psi(m)) ratio in etoposide-treated cells when compared with control cells, which was restored in the presence of bombesin (P < 0.00001). The interaction between treatments and area (P = 0.0002) was highly significant, and confirms that PC-3 cells keep their apoptosis machinery, showing an apoptotic volume decrease in response to etoposide. The protection by bombesin occurs by inhibition of apoptosis and maintenance of mitochondrial integrity. New therapeutic protocols and trials need to be developed to test drugs acting through the neutralization of antiapoptotic intracellular pathways mediated by neuroendocrine hormones.

Cell shrinkage and monovalent cation fluxes: role in apoptosis

The loss of cell volume or cell shrinkage has been a morphological hallmark of the programmed cell death process known as apoptosis. This isotonic loss of cell volume has recently been term apoptotic volume decrease or AVD to distinguish it from inherent volume regulatory responses that occurs in cells under anisotonic conditions. Recent studies examining the intracellular signaling pathways that result in this unique cellular characteristic have determined that a fundamental movement of ions, particularly monovalent ions, underlie the AVD process and plays an important role on controlling the cell death process. An efflux of intracellular potassium was shown to be a critical aspect of the AVD process, as preventing this ion loss could protect cells from apoptosis. However, potassium plays a complex role as a loss of intracellular potassium has also been shown to be beneficial to the health of the cell. Additionally, the mechanisms that a cell employs to achieve this loss of intracellular potassium vary depending on the cell type and stimulus used to induce apoptosis, suggesting multiple ways exist to accomplish the same goal of AVD. Additionally, sodium and chloride have been shown to play a vital role during cell death in both the signaling and control of AVD in various apoptotic model systems. This review examines the relationship between this morphological change and intracellular monovalent ions during apoptosis.

Plasma membrane ion channels in suicidal cell death

The machinery leading to apoptosis includes altered activity of ion channels. The channels contribute to apoptotic cell shrinkage and modify intracellular ion composition. Cl(-) channels allow the exit of Cl(-), osmolytes and HCO(3)(-) leading to cell shrinkage and cytosolic acidification. K(+) exit through K(+) channels contributes to cell shrinkage and decreases intracellular K(+) concentration, which in turn favours apoptotic cell death. K(+) channel activity further determines the cell membrane potential, a driving force for Ca(2+) entry through Ca(2+) channels. Ca(2+) may enter through unselective cation channels. An increase of cytosolic Ca(2+) may stimulate several enzymes executing apoptosis. Specific ion channel blockers may either promote or counteract suicidal cell death. The present brief review addresses the role of ion channels in the regulation of suicidal cell death with special emphasis on the role of channels in CD95 induced apoptosis of lymphocytes and suicidal death of erythrocytes or eryptosis.

Cell volume regulatory ion channels in cell proliferation and cell death

Alterations of cell volume are key events during both cell proliferation and apoptotic cell death. Cell proliferation eventually requires an increase of cell volume, and apoptosis is typically paralleled by cell shrinkage. Alterations of cell volume require the participation of ion transport across the cell membrane, including appropriate activity of Cl(-) and K(+) channels. Cl(-) channels modify cytosolic Cl(-) activity and mediate osmolyte flux, and thus influence cell volume. Most Cl(-) channels allow exit of HCO(3)(-), leading to cytosolic acidification, which in turn inhibits cell proliferation and favors apoptosis. K(+) exit through K(+) channels decreases cytosolic K(+) concentration, which may sensitize the cell for apoptotic cell death. K(+) channel activity further maintains the cell membrane potential, a critical determinant of Ca(2+) entry through Ca(2+) channels. Ca(2+) may, in addition, enter through Ca(2+)-permeable cation channels, which, in some cells, are activated by hyperosmotic shock. Increases of cytosolic Ca(2+) activity may trigger both mechanisms required for cell proliferation and mechanisms, leading to apoptosis. Thereby cell proliferation and apoptosis depend on magnitude and temporal organization of Ca(2+) entry, as well as activity of other signaling pathways. Accordingly, the same ion channels may participate in the stimulation of both cell proliferation and apoptosis. Specific ion channel blockers may thus abrogate both cellular mechanisms, depending on cell type and condition.

Stimulation of eryptosis by aluminium ions

Aluminium salts are utilized to impede intestinal phosphate absorption in chronic renal failure. Toxic side effects include anemia, which could result from impaired formation or accelerated clearance of circulating erythrocytes. Erythrocytes may be cleared secondary to suicidal erythrocyte death or eryptosis, which is characterized by cell shrinkage and exposure of phosphatidylserine (PS) at the erythrocyte surface. As macrophages are equipped with PS receptors, they bind, engulf and degrade PS-exposing cells. The present experiments have been performed to explore whether Al(3+) ions trigger eryptosis. The PS exposure was estimated from annexin binding and cell volume from forward scatter in FACS analysis. Exposure to Al(3+) ions (> or =10 microM Al(3+) for 24 h) indeed significantly increased annexin binding, an effect paralleled by decrease of forward scatter at higher concentrations (> or =30 microM Al(3+)). According to Fluo3 fluorescence Al(3+) ions (> or =30 microM for 3 h) increased cytosolic Ca(2+) activity. Al(3+) ions (> or =10 microM for 24 h) further decreased cytosolic ATP concentrations. Energy depletion by removal of glucose similarly triggered annexin binding, an effect not further enhanced by Al(3+) ions. The eryptosis was paralleled by release of hemoglobin, pointing to loss of cell membrane integrity. In conclusion, Al(3+) ions decrease cytosolic ATP leading to activation of Ca(2+)-permeable cation channels, Ca(2+) entry, stimulation of cell membrane scrambling and cell shrinkage. Moreover, Al(3+) ions lead to loss of cellular hemoglobin, a feature of hemolysis. Both effects are expected to decrease the life span of circulating erythrocytes and presumably contribute to the development of anemia during Al(3+) intoxication.

Mechanisms and significance of eryptosis

Suicidal death of erythrocytes (eryptosis) is characterized by cell shrinkage, membrane blebbing, activation of proteases, and phosphatidylserine exposure at the outer membrane leaflet. Exposed phosphatidylserine is recognized by macrophages that engulf and degrade the affected cells. Eryptosis is triggered by erythrocyte injury after several stressors, including oxidative stress. Besides caspase activation after oxidative stress, two signaling pathways converge to trigger eryptosis: (a) formation of prostaglandin E(2) leads to activation of Ca(2+)-permeable cation channels, and (b) the phospholipase A(2)-mediated release of platelet-activating factor activates a sphingomyelinase, leading to formation of ceramide. Increased cytosolic Ca(2+) activity and enhanced ceramide levels lead to membrane scrambling with subsequent phosphatidylserine exposure. Moreover, Ca(2+) activates Ca(2+)-sensitive K(2+) channels, leading to cellular KCl loss and cell shrinkage. In addition, Ca(2+) stimulates the protease calpain, resulting in degradation of the cytoskeleton. Eryptosis is inhibited by erythropoietin, which thus extends the life span of circulating erythrocytes. Eryptosis may be a mechanism of defective erythrocytes to escape hemolysis. Conversely, excessive eryptosis favors the development of anemia. Conditions with excessive eryptosis include iron deficiency, lead or mercury intoxication, sickle cell anemia, thalassemia, glucose 6- phosphate dehydrogenase deficiency, malaria, and infection with hemolysin-forming pathogens.

Osmotic shock-induced suicidal death of erythrocytes

Osmotic shock triggers eryptosis, a suicidal death of erythrocytes characterized by cell shrinkage, cell membrane blebbing and phosphatidylserine exposure at the cell surface. Phosphatidylserine-exposing erythrocytes are recognized by macrophages, engulfed, degraded and thus cleared from circulating blood. Eryptosis following osmotic shock is mediated by two distinct signalling pathways. On the one hand, osmotic shock stimulates a cyclooxygenase leading to formation of prostaglandin E2 and subsequent activation of Ca2+-permeable cation channels. On the other hand, osmotic shock activates a phospholipase A2 leading to release of platelet activating factor, which in turn activates a sphingomyelinase and thus stimulates the formation of ceramide. The increased cytosolic Ca2+ concentrations on the one hand and ceramide on the other trigger phospholipid scrambling of the cell membrane with the subsequent shift of phosphatidylserine from the inner to the outer cell membrane leaflet. Ca2+ further activates Ca2+-sensitive K+ channels leading to cellular KCl loss and further cell shrinkage. The cation channels are inhibited by Cl- anions, erythropoietin and dopamine. The sphingomyelinase is inhibited by high concentrations of urea. Thus, the high Cl- and urea concentrations in renal medulla presumably prevent the triggering of eryptosis despite hyperosmolarity. The mechanisms involved in eryptosis may not only affect the survival of erythrocytes but may be similarly operative in nucleated cells exposed to osmotic shock.

The epithelial Mg2+ channel transient receptor potential melastatin 6 is regulated by dietary Mg2+ content and estrogens

The kidney is the principal organ responsible for the regulation of the body Mg(2+) balance. Identification of the gene defect in hypomagnesemia with secondary hypocalcemia recently elucidated transient receptor potential melastatin 6 (TRPM6) as the gatekeeper in transepithelial Mg(2+) transport, whereas its homolog, TRPM7, is implicated in cellular Mg(2+) homeostasis. The aim of this study was to determine the tissue distribution in mouse and regulation of TRPM6 and TRPM7 by dietary Mg(2+) and hormones. This study demonstrates that TRPM6 is expressed predominantly in kidney, lung, cecum, and colon, whereas TRPM7 is distributed ubiquitously. Dietary Mg(2+) restriction in mice resulted in hypomagnesemia and renal Mg(2+) and Ca(2+) conservation, whereas a Mg(2+)-enriched diet led to increased urinary Mg(2+) and Ca(2+) excretion. Conversely, Mg(2+) restriction significantly upregulated renal TRPM6 mRNA levels, whereas a Mg(2+) enriched diet increased TRPM6 mRNA expression in colon. Dietary Mg(2+) did not alter TRPM7 mRNA expression in mouse kidney and colon. In addition, it was demonstrated that 17beta-estradiol but not 1,25-dihydroxyvitamin D(3) or parathyroid hormone regulates TRPM6 renal mRNA levels. Renal TRPM7 mRNA abundance remained unaltered under these conditions. The renal TRPM6 mRNA level in ovariectomized rats was significantly reduced, whereas 17beta-estradiol treatment normalized TRPM6 mRNA levels. In conclusion, kidney, lung, cecum, and colon likely constitute the main sites of active Mg(2+) (re)absorption in the mouse. In addition, Mg(2+) restriction and 17beta-estradiol upregulated renal TRPM6 mRNA levels, whereas a Mg(2+)-enriched diet stimulated TRPM6 mRNA expression in colon, supporting the gatekeeper function of TRPM6 in transepithelial Mg(2+) transport.

Ion channels in cell proliferation and apoptotic cell death

Cell proliferation and apoptosis are paralleled by altered regulation of ion channels that play an active part in the signaling of those fundamental cellular mechanisms. Cell proliferation must--at some time point--increase cell volume and apoptosis is typically paralleled by cell shrinkage. Cell volume changes require the participation of ion transport across the cell membrane, including appropriate activity of Cl- and K+ channels. Besides regulating cytosolic Cl- activity, osmolyte flux and, thus, cell volume, most Cl- channels allow HCO3- exit and cytosolic acidification, which inhibits cell proliferation and favors apoptosis. K+ exit through K+ channels may decrease intracellular K+ concentration, which in turn favors apoptotic cell death. K+ channel activity further maintains the cell membrane potential, a critical determinant of Ca2+ entry through Ca2+ channels. Cytosolic Ca2+ may trigger mechanisms required for cell proliferation and stimulate enzymes executing apoptosis. The switch between cell proliferation and apoptosis apparently depends on the magnitude and temporal organization of Ca2+ entry and on the functional state of the cell. Due to complex interaction with other signaling pathways, a given ion channel may play a dual role in both cell proliferation and apoptosis. Thus, specific ion channel blockers may abrogate both fundamental cellular mechanisms, depending on cell type, regulatory environment and condition of the cell. Clearly, considerable further experimental effort is required to fully understand the complex interplay between ion channels, cell proliferation and apoptosis.

PGE2-induced Apoptotic Cell Death in K562 Human Leukaemia Cells

Prostaglandin-E2 (PGE2) is known to trigger suicidal death of nucleated cells (apoptosis) and enucleated erythrocytes (eryptosis). In erythrocytes PGE2 induced suicidal cell death involves activation of nonselective cation channels leading to Ca2+ entry followed by cell shrinkage and triggering of Ca2+ sensitive cell membrane scrambling with phosphatidylserine (PS) exposure at the cell surface. The present study was performed to explore whether PGE2 induces apoptosis of nucleated cells similarly through cation channel activation and to possibly disclose the molecular identity of the cation channels involved. To this end, Ca2+ activity was estimated from Fluo3 fluorescence, mitochondrial potential from DePsipher fluorescence, phosphatidylserine exposure from annexin binding, caspase activation from caspAce fluorescence, cell volume from FACS forward scatter, and DNA fragmentation utilizing a photometric enzyme immunoassay. Stimulation of K562 human leukaemia cells with PGE2 (50 microM) increased cytosolic Ca2+ activity, decreased forward scatter, depolarized the mitochondrial potential, increased annexin binding, led to caspase activation and resulted in DNA fragmentation. Gene silencing of the Ca2+-permeable transient receptor potential cation channel TRPC7 significantly blunted PGE2-induced triggering of PS exposure and DNA fragmentation. In conclusion, K562 cells express Ca2+-permeable TRPC7 channels, which are activated by PGE2 and participate in the triggering of apoptosis.

Potassium and Sodium Balance in U937 Cells During Apoptosis With and Without Cell Shrinkage

Staurosporine (STS) and etoposide (Eto) induced apoptosis of the human histiocytic lymphoma cells U937 were studied to determine the role of monovalent ions in apoptotic cell shrinkage. Cell shrinkage, defined as cell dehydration, was assayed by measurement of buoyant density of cells in continuous Percoll gradient. The K+ and Na+ content in cells of different density fractions was estimated by flame emission analysis. Apoptosis was evaluated by confocal microscopy and flow cytometry of acridine orange stained cells, by flow DNA cytometry and by effector caspase activity. Apoptosis of U937 cells induced by 1 muM STS for 4 h was found to be paralleled by an increase in buoyant density indicating cell shrinkage. An increase in density was accompanied by a decrease in K+ content (from 1.1 to 0.78 mmol/g protein), which exceeded the increase in Na+ content (from 0.30 to 0.34 mmol/g) and resulted in a significant decrease of the total K+ and Na+ content (from 1.4 to 1.1 mmol/g). In contrast to STS, 50 microM Eto for 4 h or 0.8-8 microM Eto for 18-24 h induced apoptosis without triggering cell shrinkage. During apoptosis of U937 cells induced by Eto the intracellular K(+)/Na+ ratio decreased like in the cells treated with STS, but the total K+ and Na+ content remained virtually the same due to a decrease in K+ content being nearly the same as an increase in Na+ content. Apoptotic cell dehydration correlated with the shift of the total cellular K+ and Na+ content. There was no statistically significant decrease in K+ concentration per cell water during apoptosis induced by either Eto (by 13.5%) or STS (by 8%), whereas increase in Na+ concentration per cell water was statistically significant (by 27% and 47%, respectively). The data show that apoptosis can occur without cell shrinkage-dehydration, that apoptosis with shrinkage is mostly due to a decrease in cellular K+ content, and that this decrease is not accompanied by a significant decrease of K+ concentration in cell water.

Hypervitaminosis D Mediates Compensatory Ca2+Hyperabsorption in TRPV5 Knockout Mice

Vitamin D plays an important role in Ca(2+) homeostasis by controlling Ca(2+) (re)absorption in intestine, kidney, and bone. The epithelial Ca(2+) channel TRPV5 mediates the Ca(2+) entry step in active Ca(2+) reabsorption. TRPV5 knockout (TRPV5(-/-)) mice show impaired Ca(2+) reabsorption, hypercalciuria, hypervitaminosis D, and intestinal hyperabsorption of Ca(2+). Moreover, these mice demonstrate upregulation of intestinal TRPV6 and calbindin-D(9K) expression compared with wild-type mice. For addressing the role of the observed hypervitaminosis D in the maintenance of Ca(2+) homeostasis and the regulation of expression levels of the Ca(2+) transport proteins in kidney and intestine, TRPV5/25-hydroxyvitamin-D(3)-1alpha-hydroxylase double knockout (TRPV5(-/-)/1alpha-OHase(-/-)) mice, which show undetectable serum 1,25(OH)(2)D(3) levels, were generated. TRPV5(-/-)/1alpha-OHase(-/-) mice displayed a significant hypocalcemia compared with wild-type mice (1.10 +/- 0.02 and 2.54 +/- 0.01 mM, respectively; P < 0.05). mRNA levels of renal calbindin-D(28K) (7 +/- 2%), calbindin-D(9K) (32 +/- 4%), Na(+)/Ca(2+) exchanger (12 +/- 2%), and intestinal TRPV6 (40 +/- 8%) and calbindin-D(9K) (26 +/- 4%) expression levels were decreased compared with wild-type mice. Hyperparathyroidism and rickets were present in TRPV5(-/-)/1alpha-OHase(-/-) mice, more pronounced than observed in single TRPV5 or 1alpha-OHase knockout mice. It is interesting that a renal Ca(2+) leak, as demonstrated in TRPV5(-/-) mice, persisted in TRPV5(-/-)/1alpha-OHase(-/-) mice, but a compensatory upregulation of intestinal Ca(2+) transporters was abolished. In conclusion, the elevation of serum 1,25(OH)(2)D(3) levels in TRPV5(-/-) mice is responsible for the upregulation of intestinal Ca(2+) transporters and Ca(2+) hyperabsorption. Hypervitaminosis D, therefore, is of crucial importance to maintain normocalcemia in impaired Ca(2+) reabsorption in TRPV5(-/-) mice.