The stimulation of Na,K,Cl cotransport and of system A for neutral amino acid transport is a mechanism for cell volume increase during the cell cycle

NPPB prevents postoperative peritoneal adhesion formation by blocking volume-activated Cl- current

5-Nitro-2-(3-phenylpropylamino)-benzoic acid (NPPB) is a non-specific chloride channel blocker. Peritoneal adhesion is an inevitable complication of abdominal surgery and remains an important clinical problem, leading to chronic pain, intestinal obstruction, and female infertility. The aim of this study is to observe the effects of NPPB on peritoneal adhesions and uncover the underlying mechanism. The formation of postoperative peritoneal adhesions was induced by mechanical injury to the peritoneum of rats. MTT assay and wound-healing assay were used to evaluate proliferation and migration of primary cultured adhesion fibroblasts (AFB) respectively. Whole-cell chloride currents were measured using a fully automated patch-clamp workstation. Cell volume changes were monitored by light microscopy and video imaging. Our results demonstrated that NPPB could significantly prevent the formation of peritoneal adhesion in rats and inhibit the proliferation of AFB in a concentration-dependent manner. NPPB also reduced the migration of AFB cells with an IC₅₀ of 53.09 μM. A 47% hypotonic solution successfully activated the I_Cl,vol in AFB cells. The current could be blocked by extracellular treatment with NPPB. Moreover, 100 μM NPPB almost completely eliminated the capacity of regulatory volume decrease (RVD) in these cells. These data indicate that NPPB could prevent the formation of postoperative peritoneal adhesions. The possible mechanism may be through the inhibition of the proliferation and migration of AFB cells by modulating I_Cl,vol and cell volume. These results suggest a potential clinical use of NPPB for preventing the formation of peritoneal adhesions.

Carbon-11 and Fluorine-18 Labeled Amino Acid Tracers for Positron Emission Tomography Imaging of Tumors

Tumor cells have an increased nutritional demand for amino acids (AAs) to satisfy their rapid proliferation. Positron-emitting nuclide labeled AAs are interesting probes and are of great importance for imaging tumors using positron emission tomography (PET). Carbon-11 and fluorine-18 labeled AAs include the [1-¹¹C] AAs, labeling alpha-C- AAs, the branched-chain of AAs and N-substituted carbon-11 labeled AAs. These tracers target protein synthesis or amino acid (AA) transport, and their uptake mechanism mainly involves AA transport. AA PET tracers have been widely used in clinical settings to image brain tumors, neuroendocrine tumors, prostate cancer, breast cancer, non-small cell lung cancer (NSCLC) and hepatocellular carcinoma. This review focuses on the fundamental concepts and the uptake mechanism of AAs, AA PET tracers and their clinical applications.

Ion transporters in brain tumors

Ion transporters are important in regulation of ionic homeostasis, cell volume, and cellular signal transduction under physiological conditions. They have recently emerged as important players in cancer progression. In this review, we discussed two important ion transporter proteins, sodium-potassium-chloride cotransporter isoform 1 (NKCC-1) and sodium-hydrogen exchanger isoform 1 (NHE-1) in Glioblastoma multiforme (GBM) and other malignant tumors. NKCC-1 is a Na(+)- dependent Cl(-) transporter that mediates the movement of Na(+), K(+), and Cl(-) ions across the plasma membrane and maintains cell volume and intracellular K(+) and Cl(-) homeostasis. NHE-1 is a ubiquitously expressed cell membrane protein which regulates intracellular pH (pH(i)) and extracellular pH (pH(e)) homeostasis and cell volume. Here, we summarized recent pre-clinical experimental studies on NKCC-1 and NHE-1 in GBM and other malignant tumors, such as breast cancer, hepatocellular carcinoma, and lung cancer cells. These studies illustrated that pharmacological inhibition or down-regulation of these ion transporter proteins reduces proliferation, increases apoptosis, and suppresses migration and invasion of cancer cells. These new findings reveal the potentials of these ion transporters as new targets for cancer diagnosis and/or treatment.

System a amino acid transport-targeted brain and systemic tumor PET imaging agents 2-amino-3-[(18)F]fluoro-2-methylpropanoic acid and 3-[(18)F]fluoro-2-methyl-2-(methylamino)propanoic acid

INTRODUCTION

Amino acid based radiotracers target tumor cells through increased uptake by membrane-associated amino acid transport (AAT) systems. In the present study, four structurally related non-natural (18)F-labeled amino acids, (R)- and (S)-[(18)F]FAMP 1 and (R)- and (S)-[(18)F]MeFAMP 2 have been prepared and evaluated in vitro and in vivo for their potential utility in brain and systemic tumor imaging based upon primarily system A transport with positron emission tomography (PET).

METHODS

The transport of enantiomers of [(18)F]FAMP 1 and [(18)F]MeFAMP 2 was measured through in vitro uptake assays in human derived cancer cells including A549 (lung), DU145 (prostate), SKOV3 (ovary), MDA MB468 (breast) and U87 (brain) in the presence and absence of amino acid transporter inhibitors. The in vivo biodistribution of these tracers was evaluated using tumor mice xenografts at 15, 30, 60 and 120 min post injection.

RESULTS

All four tracers showed moderate to high levels of uptake (1-9%ID/5×10(5) cells) by the cancer cell lines tested in vitro. AAT cell inhibition assays demonstrated that (R)-[(18)F]1 and (S)-[(18)F]1 entered these tumor cells via mixed AATs, likely but not limited to system A and system L. In contrast, (R)-[(18)F]2 and (S)-[(18)F]2 showed high selectivity for system A AAT. Similar to the results of in vitro cell studies, the tumor uptake of all four tracers was good to high and persisted over the 2 hours time course of in vivo studies. The accumulation of these tracers was higher in tumor than most normal tissues including blood, brain, muscle, bone, heart, and lung, and the tracers with the highest in vitro selectivity for system A AAT generally demonstrated the best tumor imaging properties. Higher uptake of these tracers was observed in the pancreas, kidney and spleen compared to tumors.

CONCLUSIONS

These preclinical studies demonstrate good imaging properties in a wide range of tumors for all four amino acids evaluated with (R)-[(18)F]2 having the highest selectivity for system A AAT.

Cystine/cysteine metabolism in cultured Sf9 cells: influence of cell physiology on biosynthesis, amino acid uptake and growth

Spodoptera frugiperda (Sf9) insect cells proliferate in a cystine-free medium, with the same growth rate, reaching the same final cell density, as in a cystine-containing medium, provided that the inoculum is taken from a pre-culture sufficiently early, at 47-53 h. With an inoculum from a 103 h culture an extended lag phase accompanied by cell death was observed during the first 50 h of cystine-free culture, even though the culture had been adapted to cystine-free conditions for 10 passages. Cystine-free cultures seeded with a 103 h inoculum had lower growth rates and reached lower final cell densities than corresponding cystine-supplied cultures. Cysteine biosynthesis occurs from methionine via the β-cystathionine pathway. More methionine was consumed by the cells in cystine-free media, and cystathionine was secreted when methionine and cystine were supplied in excess. The data suggest that cysteine biosynthesis is up-regulated in proliferating cells but down-regulated when the cells enter the stationary phase.In cultures supplied with cystine (10-100 mg 1(-1)), the specific uptake rate and total consumption of cystine, as well as the uptake of glutamate, glutamine and glucose increased with increasing cystine concentrations. These results are interpreted in view of system x (c) (-) , a concentration dependent amino acid transporter. Similarly, the consumption of amino acids transported by system L (ile, leu, val, tyr) was enhanced in cystine-containing cultures, as compared to cystine-free cultures. Uptake of cystine, methionine and system L amino acids ceases abruptly in all cultures, even before growth ceased. The specific growth rate starts to decline early during the growth phase, but this growth behaviour could not be correlated to the depletion of nutrients. We therefore propose that the observed growth pattern is a result of (auto)regulatory events that control both proliferation and metabolism.

alpha-ENaC is a functional element of the hypertonicity-induced cation channel in HepG2 cells and it mediates proliferation

The molecular correlate of hypertonicity-induced cation channels (HICCs) and their role in proliferation vs. apoptosis is a matter of debate. We report in this paper that, in whole-cell patch-clamp recordings, hypertonic stress (340→450 mosM) reversibly increased the Na⁺ conductance of HepG2 cells from 0.8 to 5.8 nS. The effect was dose-dependently inhibited by flufenamate and amiloride, known blockers of HICCs, with some 50% efficiency at 300 μM. In parallel, both drugs decreased HepG2 cell proliferation [in 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide (MTT) assays and with automatic cell counting]. Small interfering RNA (siRNA) silencing of the α-subunit of the epithelial Na⁺ channel (ENaC) reduced hypertonicity-induced Na⁺ currents to 60%, whereas the rate of HepG2 cell proliferation was approximately half of that of the control. Moreover, α-ENaC siRNA inhibited the regulatory volume increase of HepG2 cells (measured with scanning acoustic microscopy) by 60%. In florescence-activated cell sorting measurements, silencing of α-ENaC led to a significant decrease in the G1 and an increase in the G2/M phase of the cell cycle, whereas the S phase was not changing. Finally (determined by a caspase 3/7 assay), HICC inhibition by flufenamate and silencing of α-ENaC increased the rate of apoptosis in HepG2 cells. It is concluded that α-ENaC is one functional element of the HICC in HepG2 cells and that the channel is an important mediator of cell proliferation; likewise, HICC blockage shifts the system from a proliferative into a rather apoptotic one. This is the first report of a role of α-ENaC in cell proliferation.

New insights into function of the growth plate

The mammalian growth plate is a complex structure which is essential for the elongation of long bones. However, an understanding of how the growth plate functions at the cellular level is lacking. This review, summarises the factors involved in growth-plate regulation, its failure and the consequence of injury. We also describe some of the cellular mechanisms which underpin the increase in volume of the growth-plate chondrocyte which is the major determinant of the rate and extent of bone lengthening. We show how living in situ chondrocytes can be imaged using 2-photon laser scanning microscopy to provide a quantitative analysis of their volume. This approach should give better understanding of the cellular control of bone growth in both healthy and failed growth plates.

Nonnutritive effects of glutamine

Glutamine is the most abundant free amino acid of the human body. Besides its role as a constituent of proteins and its importance in amino acid transamination, glutamine has regulatory capacity in immune and cell modulation. Glutamine deprivation reduces proliferation of lymphocytes, influences expression of surface activation markers on lymphocytes and monocytes, affects the production of cytokines, and stimulates apoptosis. Moreover, glutamine administration seems to have a positive effect on glucose metabolism in the state of insulin resistance. Glutamine influences a variety of different molecular pathways. Glutamine stimulates the formation of heat shock protein 70 in monocytes by enhancing the stability of mRNA, influences the redox potential of the cell by enhancing the formation of glutathione, induces cellular anabolic effects by increasing the cell volume, activates mitogen-activated protein kinases, and interacts with particular aminoacyl-transfer RNA synthetases in specific glutamine-sensing metabolism. Glutamine is applied under clinical conditions as an oral, parenteral, or enteral supplement either as the single amino acid or in the form of glutamine-containing dipeptides for preventing mucositis/stomatitis and for preventing glutamine-deficiency in critically ill patients. Because of the high turnover rate of glutamine, even high amounts of glutamine up to a daily administration of 30 g can be given without any important side effects.

Ontogeny of the neutral amino acid transporter SNAT1 in the developing rat

System A is a highly regulated, Na+-dependent transporter that accepts neutral amino acids containing short, polar side chains. System A plays an important role during rat development as decreased pup weights are observed in dams infused during gestation with a non-metabolizable System A substrate. Given the potential importance of SNAT1 during development in the rat brain, we examined whether SNAT1 would be present at an earlier gestation during organogenesis in multiple organs by immunohistochemistry and immunoblotting. SNAT1 protein was observed in the developing lungs, intestines, kidneys, heart, pancreas, and skeletal muscle of rats at prenatal days 14, 17, 19, 21, and postnatal day 2 rats. SNAT1 protein expression decreased in the liver and intestine shortly after birth and as the rat matured. SNAT1 expression was constant throughout development in the lungs and kidney and increased in the heart from prenatal day 19 to postnatal day 2. Highest levels of expression in older animals were seen in organs undergoing rapid cell division.

Extracellular glutamine is a critical modulator for regulatory volume increase in human glioma cells

Mammalian cells regulate their volume to prevent unintentional changes in intracellular signaling, cell metabolism, and DNA integrity. Intentional cell volume changes occur as cells undergo proliferation, apoptosis, or cell migration. To regulate cell volume, cells use channels and transport systems to flux osmolytes across the plasma membrane followed by the obligatory movement of water. While essentially all cells are capable of regulatory volume decrease (RVD), regulatory volume increase (RVI) mechanisms have only been reported in some cell types. In this investigation, we used human glioma cells as a model system to determine conditions necessary for RVI. When exposed to hyperosmotic conditions through the addition of 30 mosM NaCl or sucrose, D54-MG and U251 glioma cell lines and glioma cells from acute patient biopsies shrunk transiently but were able to fully recover their original cell volume within 40-70 min. This ability was highly temperature sensitive and absolutely required the presence of low millimolar concentrations of l-glutamine in the extracellular solution. Other known substrates of glutamine transporters such as methyl-amino isobutyric acid (MeAIB), alanine, and threonine were unable to support RVI. The ability of cells to undergo RVI also required the presence of Na+, K+, and Cl- and was inhibited by the NKCC inhibitor, bumetanide, consistent with the involvement of a Na+/K+/2Cl- cotransporter (NKCC). Moreover, the expression of NKCC1 was demonstrated by Western blot. We concluded that regulatory volume increase in human glioma cells occurs through the uptake of Na+, K+, and Cl- by NKCC1 and is modulated by the presence of glutamine.

The role of the neutral amino acid transporter SNAT2 in cell volume regulation

Sodium-dependent neutral amino acid transporter-2 (SNAT2), the ubiquitous member of SLC38 family, accounts for the activity of transport system A for neutral amino acids in most mammalian tissues. As the transport process performed by SNAT2 is highly energized, system A substrates, such as glutamine, glycine, proline and alanine, reach high transmembrane gradients and constitute major components of the intracellular amino acid pool. Moreover, through a complex array of exchange fluxes, involving other amino acid transporters, and of metabolic reactions, such as the synthesis of glutamate from glutamine, SNAT2 activity influences the cell content of most amino acids, thus determining the overall size and the composition of the intracellular amino acid pool. As amino acids represent a large fraction of cell organic osmolytes, changes of SNAT2 activity are followed by modifications in both cell amino acids and cell volume. This mechanism is utilized by many cell types to perform an effective regulatory volume increase (RVI) upon hypertonic exposure. Under these conditions, the expression of SNAT2 gene is induced and newly synthesized SNAT2 proteins are preferentially targeted to the cell membrane, leading to a significant increase of system A transport Vmax. In cultured human fibroblasts incubated under hypertonic conditions, the specific silencing of SNAT2 expression, obtained with anti-SNAT2 siRNAs, prevents the increase in system A transport activity, hinders the expansion of intracellular amino acid pool, and significantly delays cell volume recovery. These results demonstrate the pivotal role played by SNAT2 induction in the short-term hypertonic RVI and suggest that neutral amino acids behave as compatible osmolytes in hypertonically stressed cells.

Ca2+ signaling, intracellular pH and cell volume in cell proliferation

Mitogens control progression through the cell cycle in non-transformed cells by complex cascades of intracellular messengers, such as Ca2+ and protons, and by cell volume changes. Intracellular Ca2+ and proton concentrations are critical for linking external stimuli to proliferation, motility, apoptosis and differentiation. This review summarizes the role in cell proliferation of calcium release from intracellular stores and the Ca2+ entry through plasma membrane Ca2+ channels. In addition, the impact of intracellular pH and cell volume on cell proliferation is discussed.

Cell cycle-dependent activity of the volume- and Ca2+-activated anion currents in Ehrlich lettre ascites cells

Recent evidence implicates the volume-regulated anion current (VRAC) and other anion currents in control or modulation of cell cycle progression; however, the precise involvement of anion channels in this process is unclear. Here, Cl- currents in Ehrlich Lettre Ascites (ELA) cells were monitored during cell cycle progression, under three conditions: (i) after osmotic swelling (i.e., VRAC), (ii) after an increase in the free intracellular Ca2+ concentration (i.e., the Ca2+-activated Cl- current, CaCC), and (iii) under steady-state isotonic conditions. The maximal swelling-activated VRAC current decreased in G1 and increased in early S phase, compared to that in G0. The isotonic steady-state current, which seems to be predominantly VRAC, also decreased in G1, and increased again in early S phase, to a level similar to that in G0. In contrast, the maximal CaCC current (500 nM free Ca2+ in the pipette), was unaltered from G0 to G1, but decreased in early S phase. A novel high-affinity anion channel inhibitor, the acidic di-aryl-urea NS3728, which inhibited both VRAC and CaCC, attenuated ELA cell growth, suggesting a possible mechanistic link between cell cycle progression and cell cycle-dependent changes in the capacity for conductive Cl- transport. It is suggested that in ELA cells, entrance into the S phase requires an increase in VRAC activity and/or an increased potential for regulatory volume decrease (RVD), and at the same time a decrease in CaCC magnitude.

Oligonucleotide microarray analysis of differential transporter regulation in the regenerating rat liver

AIMS

The aim of this study was to investigate the regulation of hepatic transport systems during liver regeneration.

METHODS

A DNA oligonucleotide microarray was developed with probes for 400 transcripts. Data were confirmed using real-time PCR and on a functional level in the perfused rat liver. Liver homogenates were taken 3-48 h following 2/3-hepatectomy in rats and compared with sham-operated and non-operated controls.

RESULTS

A more than two-fold increase or decrease of expression was obtained in 183 genes following partial hepatectomy and in 16 genes in sham-operated rats. A strong induction during liver regeneration was detected for the amino acid transporters LAT4, SN2 and sodium-dependent neutral amino acid transporter (ASCT)2, whereas amino acid transport system (ATA)2 and ATA3 expressions remained unchanged. The upregulation of ASCT2 may be responsible for the increase in sodium-dependent neutral amino acid influx important for liver cell proliferation. Expression of the osmolyte transporters Smit, TauT and Bgt1 was almost unchanged indicating that osmolytes are not involved in the cell volume increase during liver regeneration. The basolateral bile salt transporter Ntcp messenger RNA (mRNA) was significantly downregulated, whereas bile salt export pump (Bsep) and multidrug resistance protein (Mrp)2 expressions remained almost unchanged. An increased mRNA expression following partial hepatectomy was detected for organic anion transporting polypeptide (Oatp)5, Octn1, Octn2 and SGLT2. In contrast, Mrp6, Oatp 2, Oatp 3, Oatp 4 and Oatp 7 were downregulated. A five-fold upregulation at the protein level was shown for the Na(+)-K(+)-2Cl- cotransporter sodium-potassium-2-chloride cotransporter (NKCC1).

CONCLUSIONS

The data show a differential regulation of hepatic transport systems during liver regeneration.

Cell size-proliferation relationship in rat glioma cells

The homeostasis of the central nervous system is highly controlled by glial cells and is dramatically altered in the case of glioma. In this respect, the complex connection between cell size and division is of particular importance and needs clarifying. In order to investigate this connection, cell number and volume were measured in C6 rat glioma cells under different experimental conditions, including continuous cell culture, Cl- channel blockade, and anisotonicity, and in the presence of an inhibitory conditioned medium collected from cell cultures or in a medium containing a low level of fetal calf serum. The rate of cell proliferation changed with cell volume in a bell-shaped manner, so that it is optimal within a cell volume window and appears to be controlled by low and high cell size checkpoints. The cell size-proliferation relationship can be defined by Boltzmann-like equations, which may reflect the effects of macromolecular crowding on proteins controlling the cell cycle progression. Altogether, these observations indicate that glioma cell proliferation is controlled predominantly but not exclusively by cell size-dependent mechanisms.

Perinuclear localization of Na-K-Cl-cotransporter protein after human cytomegalovirus infection

We (41) previously reported that Na-K-Cl-cotransporter (NKCC) function and microsomal protein expression are both dramatically reduced late in human cytomegalovirus (HCMV) infection of a human fibroblast cell line (MRC-5). We now report DNA microarray data showing that no significant HCMV-dependent NKCC gene repression can be detected 30 h postexposure (PE) to the virus. Consequently, we used plasma membrane biotinylation and subsequent subcellular fractionation in combination with semiquantitative immunoblotting and confocal microscopy to investigate the possibility that intracellular redistribution of the NKCC protein after HCMV infection could be a cause of the HCMV-induced loss of NKCC ion transport function. Our results show that the lifetime of plasmalemmal NKCC protein in quiescent, uninfected MRC-5 cells is approximately 48 h, and <20% of the total expressed NKCC protein are in the plasma membrane. The remainder (approximately 80%) was detected as diffusely distributed, small punctate structures in the cytoplasm. Following HCMV infection: 1) NKCC protein expression in the plasmalemma was sharply reduced (approximately 75%) within 24 h PE and thereafter continued to slowly decrease; 2) total cellular NKCC protein content remained unchanged or slightly increased during the course of the viral infection; and 3) HCMV infection caused NKCC protein to accumulate in the perinuclear region late in the HCMV infection (72 h PE). Thus our results imply that, in the process of productive HCMV infection, NKCC protein continues to be synthesized, but, instead of being delivered to the plasma membrane, it is clustered in a large, detergent-soluble perinuclear structure.

Call volume and insulin signaling

Perturbations of cell hydration as provoked by changes in ambient osmolarity or under isoosmotic conditions by hormones, second messengers, intracellular substrate accumulation, or reactive oxygen intermediates critically contribute to the physiological regulation of cell function. In general an increase in cell hydration stimulates anabolic metabolism and proliferation and provides cytoprotection, whereas cellular dehydration leads to a catabolic situation and sensitizes cells to apoptotic stimuli. Insulin produces cell swelling by inducing a net K+ and Na+ accumulation inside the cell, which results from a concerted activation of Na+/H+ exchange, Na+/K+/2Cl- symport, and the Na+/K(+)-ATPase. In the liver, insulin-induced cell swelling is critical for stimulation of glycogen and protein synthesis as well as inhibition of autophagic proteolysis. These insulin effects can largely be mimicked by hypoosmotic cell swelling, pointing to a role of cell swelling as a trigger of signal transduction. This article discusses insulin-induced signal transduction upstream of swelling and introduces the hypothesis that cell swelling as a signal amplifyer represents an essential component in insulin signaling, which contributes to the full response to insulin at the level of signal transduction and function. Cellular dehydration impairs insulin signaling and may be a major cause of insulin resistance, which develops in systemic hyperosmolarity, nutrient deprivation, uremia, oxidative challenges, and unbalanced production of insulin-counteracting hormones. Hydration changes affect cell functions at multiple levels (such as transcriptom, proteom, phosphoproteom, and the metabolom) and a system biological approach may allow us to develop a more holistic view on the hydration dependence of insulin signaling in the future.

Amino acid transporters: roles in amino acid sensing and signalling in animal cells

Amino acid availability regulates cellular physiology by modulating gene expression and signal transduction pathways. However, although the signalling intermediates between nutrient availability and altered gene expression have become increasingly well documented, how eukaryotic cells sense the presence of either a nutritionally rich or deprived medium is still uncertain. From recent studies it appears that the intracellular amino acid pool size is particularly important in regulating translational effectors, thus, regulated transport of amino acids across the plasma membrane represents a means by which the cellular response to amino acids could be controlled. Furthermore, evidence from studies with transportable amino acid analogues has demonstrated that flux through amino acid transporters may act as an initiator of nutritional signalling. This evidence, coupled with the substrate selectivity and sensitivity to nutrient availability classically associated with amino acid transporters, plus the recent discovery of transporter-associated signalling proteins, demonstrates a potential role for nutrient transporters as initiators of cellular nutrient signalling. Here, we review the evidence supporting the idea that distinct amino acid "receptors" function to detect and transmit certain nutrient stimuli in higher eukaryotes. In particular, we focus on the role that amino acid transporters may play in the sensing of amino acid levels, both directly as initiators of nutrient signalling and indirectly as regulators of external amino acid access to intracellular receptor/signalling mechanisms.

The osmoregulatory and the amino acid-regulated responses of system A are mediated by different signal transduction pathways

The osmotic response of system A for neutral amino acid transport has been related to the adaptive response of this transport system to amino acid starvation. In a previous study (Ruiz-Montasell, B., M. Gómez-Angelats, F.J. Casado, A. Felipe, J.D. McGivan, and M. Pastor-Anglada. 1994. Proc. Natl. Acad. Sci. USA. 91:9569-9573), a model was proposed in which both responses were mediated by different mechanisms. The recent cloning of several isoforms of system A as well as the elucidation of a variety of signal transduction pathways involved in stress responses allow to test this model. SAT2 mRNA levels increased after amino acid deprivation but not after hyperosmotic shock. Inhibition of p38 activity or transfection with a dominant negative p38 did not alter the response to amino acid starvation but partially blocked the hypertonicity response. Inhibition of the ERK pathway resulted in full inhibition of the adaptive response of system A and no increase in SAT2 mRNA levels, without modifying the response to hyperosmolarity. Similar results were obtained after transfection with a dominant negative JNK1. The CDK2 inhibitor peptide-II decreased the osmotic response in a dose-dependent manner but did not have any effect on the adaptive response of system A. In summary, the previously proposed model of up-regulation of system A after hypertonic shock or after amino acid starvation by separate mechanisms is now confirmed and the two signal transduction pathways have been identified. The involvement of a CDK-cyclin complex in the osmotic response of system A links the activity of this transporter to the increase in cell volume previous to the entry in a new cell division cycle.

Aldosterone regulates the Na-K-2Cl cotransporter in vascular smooth muscle

Aldosterone increases cation transport and contractility of vascular smooth muscle, but the specific transporter involved and how it is linked to smooth muscle tone is unknown. Because the Na-K-2Cl cotransporter (NKCC1) contributes to vascular smooth muscle contraction and is regulated by vasoactive compounds, we sought to determine whether this transporter is a target of aldosterone in rat aorta. Treatment of adrenalectomized rats with aldosterone for 7 days resulted in a 63% increase in NKCC1 activity as measured by bumetanide-sensitive efflux of 86Rb+. Treatment of normal aortas in culture with aldosterone for 3 and 7 days resulted in 29% and 47% increases in NKCC1 activity, respectively. Aldosterone had no acute effect on 86Rb+ efflux. Stimulation of NKCC1 was blocked by spironolactone, a mineralocorticoid receptor antagonist, but not by RU38486, a glucocorticoid receptor antagonist. Aldosterone did not augment the stimulation of NKCC1 by phenylephrine and did not increase NKCC1 mRNA as determined by real-time polymerase chain reaction. We conclude that aldosterone regulates the Na-K-2Cl cotransporter in vascular smooth muscle through classic mineralocorticoid receptors but not through changes in the abundance of NKCC1 mRNA. This could account for the increase in Na+, K+, and Cl- fluxes previously observed in vascular smooth muscle from mineralocorticoid-treated animals and may contribute to increased vascular tone.

Regulation of amino acid and glucose transporters in endothelial and smooth muscle cells

While transport processes for amino acids and glucose have long been known to be expressed in the luminal and abluminal membranes of the endothelium comprising the blood-brain and blood-retinal barriers, it is only within the last decades that endothelial and smooth muscle cells derived from peripheral vascular beds have been recognized to rapidly transport and metabolize these nutrients. This review focuses principally on the mechanisms regulating amino acid and glucose transporters in vascular endothelial cells, although we also summarize recent advances in the understanding of the mechanisms controlling membrane transport activity and expression in vascular smooth muscle cells. We compare the specificity, ionic dependence, and kinetic properties of amino acid and glucose transport systems identified in endothelial cells derived from cerebral, retinal, and peripheral vascular beds and review the regulation of transport by vasoactive agonists, nitric oxide (NO), substrate deprivation, hypoxia, hyperglycemia, diabetes, insulin, steroid hormones, and development. In view of the importance of NO as a modulator of vascular tone under basal conditions and in disease and chronic inflammation, we critically review the evidence that transport of L-arginine and glucose in endothelial and smooth muscle cells is modulated by bacterial endotoxin, proinflammatory cytokines, and atherogenic lipids. The recent colocalization of the cationic amino acid transporter CAT-1 (system y(+)), nitric oxide synthase (eNOS), and caveolin-1 in endothelial plasmalemmal caveolae provides a novel mechanism for the regulation of NO production by L-arginine delivery and circulating hormones such insulin and 17beta-estradiol.

Patterns of cation-chloride cotransporter expression during embryonic rodent CNS development

Intracellular Cl- plays a key role in cellular volume regulation, cell cycle control and shaping the polarity of inhibitory postsynaptic responses mediated by anion-permeable GABA and glycine receptors. In this study, we have investigated the expression patterns of members of the cation-chloride cotransporters (CCCs), including the K-Cl cotransporters KCC1-4 and the Na-K-2 Cl cotranporter NKCC1 during rodent embryonic brain development. At the time of neurogenesis (embryonic days; E12.5-14.5), KCC1 was only detectable in the developing choroid plexus. KCC2 mRNA was detectable as early as E12.5 in the ventral part of the (cervical) spinal cord, and by E14.5, the expression had spread to TUJ1-positive differentiating regions of the rhombencephalon, diencephalon and olfactory bulb, in parallel with neuronal maturation. KCC3 mRNA was scarce in the cortical plate at E14.5, and slightly up-regulated at birth. In contrast, KCC4 mRNA was abundantly expressed in the ventricular zone and was down-regulated perinatally. At E14.5, NKCC1 was highly expressed in the vimentin-positive radial glia of the proliferative zone of the subcortical region. At later embryonic stages, during gliogenesis (E17-P0), there was a shift in NKCC1 expression to the neuron specific Class III beta-tubulin (betaIII) positive region of the cortical plate. These unique spatiotemporal expression patterns of distinct CCCs during embryonic development suggests that Cl- regulatory mechanisms are critically involved in the control of neuronal development.

Regulatory volume decrease is actively modulated during the cell cycle

Nasopharyngeal carcinoma cells, CNE-2Z, when swollen by 47% hypotonic solution, exhibited a regulatory volume decrease (RVD). The RVD was inhibited by extracellular applications of the chloride channel blockers tamoxifen (30 microM; 61% inhibition), 5-nitro-2-(3-phenylpropylamino)benzoic acid (NPPB, 100 microM; 60% inhibition), and ATP (10 mM; 91% inhibition). The level and time constant of RVD varied greatly between cells. Most cells conducted an incomplete RVD, but a few had the ability to recover their volume completely. There was no obvious correlation between cell volume and RVD capacity. Flow cytometric analysis showed that highly synchronous cells were obtained by the mitotic shake-off technique and that the cells progressed through the cell cycle synchronously when incubated in culture medium. Combined application of DNA synthesis inhibitors, thymidine and hydroxyurea arrested cells at the G1/S boundary and 87% of the cells reached S phase 4 h after being released. RVD capacity changed significantly during the cell cycle progression in cells synchronized by shake-off technique. RVD capacity being at its highest in G1 phase and lowest in S phase. The RVD capacity in G1 (shake-off cells sampled after 4 h of incubation), S (obtained by chemical arrest), and M cells (selected under microscope) was 73, 33, and 58%, respectively, and the time constants were 435, 769, and 2,000 sec, respectively. We conclude that RVD capacity is actively modulated in the cell cycle and RVD may play an important role in cell cycle progress.

Radiolabeled amino acids for tumor imaging with PET: radiosynthesis and biological evaluation of 2-amino-3-[18F]fluoro-2-methylpropanoic acid and 3-[18F]fluoro-2-methyl-2-(methylamino)propanoic acid

Novel radiopharmaceuticals, including amino acids, that target neoplasms through their altered metabolic states have shown promising results in preclinical and clinical studies. Two fluorinated analogues of alpha-aminoisobutyric acid, 2-amino-3-fluoro-2-methylpropanoic acid (FAMP) and 3-fluoro-2-methyl-2-(methylamino)propanoic acid (N-MeFAMP), have been radiolabeled with fluorine-18, characterized in amino acid uptake assays, and evaluated in vivo in normal rats and a rodent tumor model. The key steps in the syntheses of both radiotracers involved the preparation of cyclic sulfamidate precursors. Radiosyntheses of both [18F]FAMP and [18F]N-MeFAMP via no-carrier-added nucleophilic substitution provided high yields (>78% decay-corrected) in high radiochemical purity (>99%). Amino acid transport assays using 9L gliosarcoma cells demonstrated that both compounds are substrates for the A type amino acid transport system, with [18F]N-MeFAMP showing higher specificity than [18F]FAMP for A type transport. Tissue distribution studies in normal Fischer rats and Fischer rats implanted intracranially with 9L gliosarcoma tumor cells were also performed. At 60 min postinjection, the tumor vs normal brain ratio of radioactivity was 36:1 in animals receiving [18F]FAMP and 104:1 in animals receiving [18F]N-MeFAMP. On the basis of these studies, both [18F]FAMP and [18F]N-MeFAMP are promising imaging agents for the detection of intracranial neoplasms via positron emission tomography.

ATA2-mediated amino acid uptake following partial hepatectomy is regulated by redistribution to the plasma membrane

System A, the Na(+)-dependent amino acid transport activity, is encoded by the ATA2 gene and up-regulated following partial hepatectomy (PH), and its competitive inhibition interferes with liver regeneration. Rabbit polyclonal antibody was raised against a portion of the ATA2 gene product followed by immunodetection of ATA2 in isolated liver plasma membrane and lysate. The level of ATA2 increased in the plasma membrane following PH, while the relatively high quantity of ATA2 found in liver lysate remained constant. We also have shown that Northern analysis of steady-state ATA2 mRNA revealed no significant change following PH. These data show that ATA2-mediated transport is not regulated by the steady-state level of ATA2 mRNA but is regulated by the amount of ATA2 and redistribution to the plasma membrane. We hypothesize that ATA2 activity is regulated by recruitment of ATA2 protein from an intracellular compartment. In addition, the pattern of expression of System A activity in oocytes, transport kinetics, and sensitivity to chemical modification indicate the presence of a second System A isoform in liver that differs substantially from ATA2.

Volume increase in growth plate chondrocytes during hypertrophy: the contribution of organic osmolytes

During the differentiation cascade of growth plate chondrocytes, cells undergo as much as a 10-15-fold increase in volume. This volume increase, which occurs to different extents in growth plates growing at different rates, has been demonstrated to be the single most significant variable in understanding the quantitative aspects of the cellular kinetics of long bone growth. Our hypothesis is that this volume increase, which occurs through cell swelling by water imbibition, requires intracellular accumulation of osmolytes through activation or upregulation of membrane transport mechanisms. Significant intracellular accumulation of inorganic osmolytes, such as Na+, K+, and Cl-, is potentially disruptive to normal cellular metabolism, whereas intracellular accumulation of organic osmolytes is considered to be more compatible with metabolic function. Thus, we concentrated on determining the contributions of organic osmolytes--betaine, amino acids, inositol, and sorbitol--to volume increase. Pooled cryostat sections of young bovine growth plates were extracted followed by automated analysis for their content of amino acids. Analysis for betaine and the sugar alcohols was done by extraction and derivatization, followed by high-performance liquid chromatography (HPLC). Parallel stereological analyses correlated osmolyte changes to stages of chondrocytic differentiation, specifically comparing intracellular concentration and amount in proliferative vs. hypertrophic chondrocytes. Calculations demonstrated that, maximally, these organic osmolytes, in total, account for 6%-7% of the intracellular osmolytes required to sustain the volume increase, and that the most significant contribution is from betaine. This suggests that intracellular accumulation of organic osmolytes is not a primary strategy used by growth plate chondrocytes during volume increase of their terminal differentiation. The data also suggest that there is a differential regulation of transporters of these osmolytes such that intracellular concentrations are constantly modified as cells proceed through the differentiation cascade.

Regulative potential of glutamine--relation to glutathione metabolism

Glutamine (GLN) is the most abundant free amino acid (AA) in the human body. Under GLN-free conditions, which can be obtained when cells are cultivated in vitro, tissue cells cannot grow. Therefore, when classifying GLN as a "non-essential" AA, one must consider that in the human body GLN is synthesized from essential AAs and is continuously delivered from skeletal muscle to other organs. It is fascinating that a relatively simple AA like GLN can stimulate a large variety of cellular reactions. GLN stimulates not only the growth of cells but also the expression of surface antigens, the formation of cytokines, and the synthesis of heat shock proteins. Further, a GLN deficiency leads to a cell cycle arrest in G(0) to G(1) and reduces apoptosis. Interestingly, many of these biological activities also are associated with the cellular reduced oxygen potential, which depends mainly on the ratio of reduced to oxidized glutathione. Experimental animal studies have shown that the administration of GLN increases tissue concentrations of reduced glutathione. This review describes the relation of GLN to reduced glutathione metabolism and discusses the alteration of reduced glutathione metabolism under a variety of clinical conditions such as reperfusion injury, myocardial infarction, respiratory insufficiency, cancer, diabetes, liver disease, and clinical protein catabolism.

Growth factors stimulate the Na-K-2Cl cotransporter NKCC1 through a novel Cl(-)-dependent mechanism

The Na-K-2Cl cotransporter NKCC1 is an important volume-regulatory transporter that is regulated by cell volume and intracellular Cl(-). This regulation appears to be mediated by phosphorylation of NKCC1, although there is evidence for additional, cytoskeletal regulation via myosin light chain (MLC) kinase. NKCC1 is also activated by growth factors and may contribute to cell hypertrophy, but the mechanism is unknown. In aortic endothelial cells, NKCC1 (measured as bumetanide-sensitive (86)Rb(+) influx) was rapidly stimulated by serum, lysophosphatidic acid, and fibroblast growth factor, with the greatest stimulation by serum. Serum increased bumetanide-sensitive influx significantly more than bumetanide-sensitive efflux (131% vs. 44%), indicating asymmetric stimulation of NKCC1, and produced a 17% increase in cell volume and a 25% increase in Cl(-) content over 15 min. Stimulation by serum and hypertonic shrinkage were additive, and serum did not increase phosphorylation of NKCC1 or MLC, and did not decrease cellular Cl(-) content. When cellular Cl(-) was replaced with methanesulfonate, influx via NKCC1 increased and was no longer stimulated by serum, whereas stimulation by hypertonic shrinkage still occurred. Based on these results, we propose a novel mechanism whereby serum activates NKCC1 by reducing its sensitivity to inhibition by intracellular Cl(-). This resetting of the Cl(-) set point of the transporter enables the cotransporter to produce a hypertrophic volume increase.

Localization of a Na(+)-K(+)-2Cl(-) cotransporter in the rabbit lens

Earlier work from this laboratory demonstrated a bumetanide-inhibitable K(+) uptake activity in cultured bovine lens epithelial cells, but not at the anterior surfaces of intact bovine lenses isolated in an Ussing-type chamber. Presently the distribution of the bumetanide-sensitive Na(+)-K(+)-2Cl(-) cotransporter within the lens was re-examined. To complement previous results, (86)Rb(+) uptake experiments were done in a chamber design that limited exposure of the radiolabel to specific surfaces of rabbit lenses under short-circuit conditions. In addition, the cotransporter protein (NKCC1, but not NKCC2) was immune-detected in Western blots. For the latter, membrane preparations were obtained from capsule-plus-epithelial specimens, and from three cortical fractions, i.e. the anterior, equatorial, and posterior regions, as well as a fifth, nuclear fraction. K(+) influxes across the anterior-polar, equatorial, and posterior-polar surfaces were 0.375, 0.348 and 0.056 microEq (hr cm(2))(-1) respectively, rates that were not significantly reduced by the presence of 0.1 mM bumetanide (P > 0.15, as unpaired data). In contrast, bumetanide-sensitive K(+) influx rates were measured across the anterior and equatorial surfaces under hypertonic, but not under hypo-osmotic conditions. In culture, bumetanide and ouabain were equipotent in reducing by approximately half the K(+) uptake of quiescent, rabbit lens epithelial cells under control, iso-osmotic conditions, indicating a cell-culture induced up-regulation of the cotransport activity by an undetermined mechanism. The immunoblotting of lens membrane proteins elicited approximately 170-180 kDa bands accordant with the identity of the NKCC1 isoform in the epithelial and cortical equatorial fractions. Thus, NKCC1 was readily demonstrated using membrane specimens taken from within the lens. Its activity in the intact organ may be activated by conditions fostering cell shrinkage, and perhaps, agents stimulating epithelial cell elongation, given its distribution within the lens.

Insulin resistance induced by loop diuretics and hyperosmolarity in perfused rat liver

Insulin-induced cell swelling was recently suggested to reflect an independent signal for metabolic insulin effects such as inhibition of hepatic proteolysis, which is transmitted at the level of autophagosome formation via p38MAPK activation [Häussinger et al., Gastroenterology 116 (1999), 921-935]. Here, the role of insulin-induced cell swelling in the overall context of insulin signalling towards proteolysis inhibition was studied in perfused rat liver. Loop diuretics and hyperosmolarity, which impair insulin-stimulated cell swelling, strongly blunt Erk-2 and p38MAPK activation as well as proteolysis inhibition by insulin, but are without effect on insulin-induced tyrosine phosphorylation of IR-beta and IRS-1. Inhibitors of phosphatidylinositol-3-kinase (PI3-kinase) also block insulin-induced cell swelling, MAP kinase activation and proteolysis inhibition, but the antiproteolytic response to hypoosmolarity remains unaffected. We suggest that PI3-kinase-mediated cell swelling induced by insulin is required to amplify the insulin signal to MAP kinases and thus proteolysis regulation. The perturbation of insulin-induced cell swelling may be of pathophysiological relevance for the development of insulin resistance in clinical situations associated with hyperosmotic dehydration and loop diuretic treatment.

Methohexital affects neutrophil (PMN) dynamic free amino acid pool and immune functions in vitro

BACKGROUND AND OBJECTIVE

The objective of this study was to determine the dose as well as the duration of exposure-dependent effects of methohexital on neutrophil [polymorphonuclear leucocyte (PMN)] free amino acid profiles and, in a parallel study, on PMN immune functions.

METHODS

Whole blood samples were taken from 20 volunteers and incubated with methohexital [0 (control), 3.6, 26, 130 and 260 microg mL-1] for 10, 30, 60 or 120 min. PMN amino acid profiles were documented using advanced PMN separation and high-performance liquid chromatography procedures. Superoxide anion (O2-) and hydrogen peroxide production (H2O2), and activity of released myeloperoxidase (MPO), were determined photometrically.

RESULTS

After methohexital, significant dose (> or = 26 microg mL-1) as well as duration of exposure-dependent (> or = 30 min) increases in histidine, isoleucine, leucine, valine, methionine, serine, glycine, threonine, and decreases in glutamine, glutamate, aspartate, asparagine, arginine, ornithine, citrulline, alanine and taurine were observed (P < or = 0.05). Concerning PMN immune functions, methohexital significantly decreased O2-, H2O2 formation and MPO (> or = 26 microg mL-1, > or = 30 min, P < or = 0.05).

CONCLUSIONS

Altogether, there is significant relevance to the pharmacological regimens which enhance the supply of methohexital in whole blood. In regards to our results, we suggest that considerable changes in PMN 'dynamic free amino acid pool', for example induced by methohexital, may be one of the determinants in cell nutrition adversely affecting PMN metabolism. It is partially through its effect on the PMN free amino acid pool that maleficent pharmacological stress may have an unintentional influence on PMN immune functions.

The role of system A for neutral amino acid transport in the regulation of cell volume

System A is a secondary active, sodium dependent transport system for neutral amino acids. Strictly coupled with Na,K-ATPase, its activity determines the size of the intracellular amino acid pool, through a complex network of metabolic reaction and exchange fluxes. Many hormones and drugs affect system A activity in specific cell models or tissues. In all the cell models tested thus far the activity of the system is stimulated by amino acid starvation, cell cycle progression, and the incubation under hypertonic conditions. These three conditions produce marked alterations of cell volume. The stimulation of system A activity plays an important role in cell volume restoration, through an expansion of the intracellular amino acid pool. Under normal conditions, system A substrates represent a major fraction of cell compatible osmolytes, organic compounds that exert a protein stabilizing effect. It is, therefore, likely that the activation of system A represents a portion of a more complex response triggered by exposure to stresses of various nature. Since system A transporters have been recently cloned, the molecular bases of these regulatory mechanisms will probably be elucidated in a short time.

Effects of diazepam on neutrophil (PMN) free amino acid profiles and immune functions in vitro. Metabolical and immunological consequences of L-alanyl-L-glutamine supplementation

The objective of this study was to determine the effects of diazepam, L-alanyl-L-glutamine (ala-gln) or diazepam combined with ala-gln on polymorphonuclear leukocyte (PMN) free amino acid profiles. In a parallel study the effects on PMN immune functions were also documented for the first time. The incubation of whole blood with diazepam led to significant changes in PMN free glutamine, aspartate, glutamate, ornithine, arginine, citrulline, taurine and methionine as well as branched chain and neutral amino acid concentrations. Ala-gln caused significant increases in PMN glutamine and alanine and asparagine, aspartate, glutamate, ornithine, arginine, serine and glycine profiles. Regarding PMN immune functions, diazepam significantly decreased superoxide anion (O(2)(-)) and hydrogen peroxide production (H(2)O(2)) and myeloperoxidase activity (MPO) while ala-gln significantly increased PMN immune functions. Ala-gln supplemented to diazepam largely reversed the changes in PMN amino acid profiles and PMN immune functions brought about by diazepam. Overall, diazepam or ala-gln lead to significant changes in PMN free amino acids. Important PMN immune functions also seem to be affected. In regards to the results, there is significant relevance to the pharmacological regimens which enhance the supply of diazepam or ala-gln in whole blood suggesting that considerable changes in PMN "labile free amino acid pool" occur. These regimens often follow beneficial nutritional therapy or maleficent pharmacological stress and may be one of the determinants in cell nutrition which influence PMN function. It is partially through its effect on PMN labile free amino acid pool that ala-gln supplemented to diazepam may maintain PMN immune functions in vitro.

Glutathione transport system in NIH3t3 fibroblasts

The current study characterizes a mediated transport for GSH uptake in murine fibroblasts NIH3T3. The presence of GSH mediated transport is indicated by the behaviour of GSH uptake time-course, as well as by kinetic saturation and the specific inhibition of the initial rate of GSH transport. Moreover, a concentrative GSH uptake has been measured, whose driving force may be due to a change of membrane potential and the direct involvement of ATP. Hyperbolic kinetic saturation shows a single mediated transport with high affinity for GSH (Km = 0.209 +/- 0.03 mM). High specificity of this GSH transporter for the entire structure of GSH is also demonstrated. To summarize, GSH uptake into NIH3T3 cells occurs by an active transport system and shows characteristics similar to ATP-dependent mechanisms previously identified in hepatocyte membranes. Moreover, a possible physiological role of this GSH transporter related to NIH3T3 cell proliferation has been hypothesized.

Amino acid transport in podocytes

It has recently been shown that formation of podocyte foot processes is dependent on a constant source of lipids and proteins (Simons M, Saffrich R, Reiser J, and Mundel P. J Am Soc Nephrol 10: 1633-1639, 1999). Here we characterize amino acid transport mechanisms in differentiated cultured podocytes and investigate whether it may be disturbed during podocyte injury. RT-PCR studies detected mRNA for transporters of neutral amino acids (ASCT1, ASCT2, and B(0/+)), cationic AA (CAT1 and CAT3), and anionic AA (EAAT2 and EAAT3). Alanine (Ala), asparagine, cysteine (Cys), glutamine (Gln), glycine (Gly), leucine (Leu), methionine (Met), phenylalanine (Phe), proline (Pro), serine (Ser), threonine (Thr), glutamic acid (Glu), arginine (Arg), and histidine (His) depolarized podocytes and increased their whole cell conductances. Depletion of extracellular Na(+) completely inhibited the depolarization induced by Ala, Gln, Glu, Gly, Leu, and Pro and decreased the depolarization induced by Arg and His, indicating the presence of Na(+)-dependent amino acid transport. Incubation of podocytes with 100 microg/ml puromycin aminonucleoside for 24 h significantly attenuated the effects induced by the various amino acids by approximately 70%. The data indicate the existence of different amino acid transporter systems in podocytes. Alteration of amino acid transport may participate in podocyte injury and disturbed foot process formation.

Sodium-potassium-chloride cotransport

Obligatory, coupled cotransport of Na(+), K(+), and Cl(-) by cell membranes has been reported in nearly every animal cell type. This review examines the current status of our knowledge about this ion transport mechanism. Two isoforms of the Na(+)-K(+)-Cl(-) cotransporter (NKCC) protein (approximately 120-130 kDa, unglycosylated) are currently known. One isoform (NKCC2) has at least three alternatively spliced variants and is found exclusively in the kidney. The other (NKCC1) is found in nearly all cell types. The NKCC maintains intracellular Cl(-) concentration ([Cl(-)](i)) at levels above the predicted electrochemical equilibrium. The high [Cl(-)](i) is used by epithelial tissues to promote net salt transport and by neural cells to set synaptic potentials; its function in other cells is unknown. There is substantial evidence in some cells that the NKCC functions to offset osmotically induced cell shrinkage by mediating the net influx of osmotically active ions. Whether it serves to maintain cell volume under euvolemic conditons is less clear. The NKCC may play an important role in the cell cycle. Evidence that each cotransport cycle of the NKCC is electrically silent is discussed along with evidence for the electrically neutral stoichiometries of 1 Na(+):1 K(+):2 Cl- (for most cells) and 2 Na(+):1 K(+):3 Cl(-) (in squid axon). Evidence that the absolute dependence on ATP of the NKCC is the result of regulatory phosphorylation/dephosphorylation mechanisms is decribed. Interestingly, the presumed protein kinase(s) responsible has not been identified. An unusual form of NKCC regulation is by [Cl(-)](i). [Cl(-)](i) in the physiological range and above strongly inhibits the NKCC. This effect may be mediated by a decrease of protein phosphorylation. Although the NKCC has been studied for approximately 20 years, we are only beginning to frame the broad outlines of the structure, function, and regulation of this ubiquitous ion transport mechanism.

Adaptive increase of amino acid transport system A requires ERK1/2 activation

Amino acid starvation markedly stimulates the activity of system A, a widely distributed transport route for neutral amino acids. The involvement of MAPK (mitogen-activated protein kinase) pathways in this adaptive increase of transport activity was studied in cultured human fibroblasts. In these cells, a 3-fold stimulation of system A transport activity required a 6-h amino acid-free incubation. However, a rapid tyrosine phosphorylation of ERK (extracellular regulated kinase) 1 and 2, and JNK (Jun N-terminal kinase) 1, but not of p38, was observed after the substitution of complete medium with amino acid-free saline solution. ERK1/2 activity was 4-fold enhanced after a 15-min amino acid-free incubation and maintained at stimulated values thereafter. A transient, less evident stimulation of JNK1 activity was also detected, while the activity of p38 was not affected by amino acid deprivation. PD98059, an inhibitor of ERK1/2 activation, completely suppressed the adaptive increase of system A transport activity that, conversely, was unaffected by inhibitors of other transduction pathways, such as rapamycin and wortmannin, as well as by chronic treatment with phorbol esters. In the presence of either L-proline or 2-(methylaminoisobutyric) acid, two substrates of system A, the transport increase was prevented and no sustained stimulation of ERK1/2 was observed. To identify the stimulus that maintains MAPK activation, cell volume was monitored during amino acid-free incubation. It was found that amino acid deprivation caused a progressive cell shrinkage (30% after a 6-h starvation). If proline was added to amino acid-starved, shrunken cells, normal values of cell volume were rapidly restored. However, proline-dependent volume rescue was hampered if cells were pretreated with PD98059. It is concluded that (a) the triggering of adaptive increase of system A activity requires a prolonged activation of ERK1 and 2 and that (b) cell volume changes, caused by the depletion of intracellular amino acid pool, may underlie the activation of MAPKs.

Inhibition of system A amino acid transport and hepatocyte proliferation following partial hepatectomy in the rat

System A, the sodium-dependent neutral amino acid transport activity, has a 3-fold increase in its initial uptake velocity into hepatocytes following partial hepatectomy (PH) in the rat. The purpose of this study was to examine the effect of inhibition of System A-mediated amino acid transport on hepatocyte proliferation and liver regeneration. We describe the in vivo competitive inhibition of System A activity following PH by the nonmetabolizable, System A-specific substrate, alpha-(methylamino)isobutyric acid (MeAIB). Administration of MeAIB 60 minutes before PH decreased the incorporation of [(3)H]thymidine into DNA by 45% +/- 5% and 76% +/- 17% at 24 and 36 hours, respectively. The readministration of MeAIB every 12 hours further decreased DNA synthesis by 92% +/- 18% and 82% +/- 11% at 24 and 36 hours. The recovery of liver mass of rats receiving MeAIB was decreased by 46.4% +/- 5.1% at 24 hours after PH. In vitro, 5 mmol/L MeAIB inhibited proliferation of primary hepatocytes by 56% +/- 4% and 61% +/- 12% 48 hours after incubation with 10% fetal calf serum or epidermal growth factor (5 ng/mL), respectively. Thus, MeAIB inhibition of System A transport activity decreased both in vivo and in vitro inducement of hepatocyte proliferation. Treatment with MeAIB did not significantly change the incorporation of [(3)H]leucine into total liver protein, but changes in serum amino acids and hepatocyte cell volume were observed, suggesting System A transport activity during hepatocyte proliferation functions primarily to provide amino acids to fuel liver-specific biochemical pathways and to increase cell volume.

Physiological significance of volume-regulatory transporters

Research over the past 25 years has identified specific ion transporters and channels that are activated by acute changes in cell volume and that serve to restore steady-state volume. The mechanism by which cells sense changes in cell volume and activate the appropriate transporters remains a mystery, but recent studies are providing important clues. A curious aspect of volume regulation in mammalian cells is that it is often absent or incomplete in anisosmotic media, whereas complete volume regulation is observed with isosmotic shrinkage and swelling. The basis for this may lie in an important role of intracellular Cl- in controlling volume-regulatory transporters. This is physiologically relevant, since the principal threat to cell volume in vivo is not changes in extracellular osmolarity but rather changes in the cellular content of osmotically active molecules. Volume-regulatory transporters are also closely linked to cell growth and metabolism, producing requisite changes in cell volume that may also signal subsequent growth and metabolic events. Thus, despite the relatively constant osmolarity in mammals, volume-regulatory transporters have important roles in mammalian physiology.

Amino acids are compatible osmolytes for volume recovery after hypertonic shrinkage in vascular endothelial cells

The response to chronic hypertonic stress has been studied in human endothelial cells derived from saphenous veins. In complete growth medium the full recovery of cell volume requires several hours and is neither associated with an increase in cell K+ nor hindered by bumetanide but depends on an increased intracellular pool of amino acids. The highest increase is exhibited by neutral amino acid substrates of transport system A, such as glutamine and proline, and by the anionic amino acid glutamate. Transport system A is markedly stimulated on hypertonic stress, with an increase in activity roughly proportional to the extent and the duration of the osmotic shrinkage. Cycloheximide prevents the increase in transport activity of system A and the recovery of cell volume. It is concluded that human endothelial cells counteract hypertonic stress through the stimulation of transport system A and the consequent expansion of the intracellular amino acid pool.

Na+-K+-2Cl- cotransport in Ehrlich cells: regulation by protein phosphatases and kinases

To identify protein kinases (PK) and phosphatases (PP) involved in regulation of the Na+-K+-2Cl- cotransporter in Ehrlich cells, the effect of various PK and PP inhibitors was examined. The PP-1, PP-2A, and PP-3 inhibitor calyculin A (Cal-A) was a potent activator of Na+-K+-2Cl- cotransport (EC50 = 35 nM). Activation by Cal-A was rapid (<1 min) but transient. Inactivation is probably due to a 10% cell swelling and/or the concurrent increase in intracellular Cl- concentration. Cell shrinkage also activates the Na+-K+-2Cl- cotransport system. Combining cell shrinkage with Cal-A treatment prolonged the cotransport activation compared with stimulation with Cal-A alone, suggesting PK stimulation by cell shrinkage. Shrinkage-induced cotransport activation was pH and Ca2+/calmodulin dependent. Inhibition of myosin light chain kinase by ML-7 and ML-9 or of PKA by H-89 and KT-5720 inhibited cotransport activity induced by Cal-A and by cell shrinkage, with IC50 values similar to reported inhibition constants of the respective kinases in vitro. Cell shrinkage increased the ML-7-sensitive cotransport activity, whereas the H-89-sensitive activity was unchanged, suggesting that myosin light chain kinase is a modulator of the Na+-K+-2Cl- cotransport activity during regulatory volume increase.