Characterization of amino acid transport during erythroid cell differentiation

Functional characterization of two S-nitroso-L-cysteine transporters, which mediate movement of NO equivalents into vascular cells

System L amino acid transporters have been shown to be responsible for cellular uptake of S-nitroso-L-cysteine (l-CSNO). In this study, we examined the characteristics of L-CSNO uptake in Xenopus laevis oocytes expressing system L transporters and found that uptake increased only when both 4F2 heavy chain (4F2HC) and either L-type amino acid transporter 1 (LAT1) or LAT2 light chain were coexpressed. The K(m) for transport was 57 +/- 8 microM for 4F2HC-LAT1 and 520 +/- 52 microM for 4F2HC-LAT2. Vascular endothelial and smooth muscle cells were shown to express transcripts for 4F2HC and for both LAT1 and LAT2. Transport of L-CSNO into red blood cells, endothelial cells, and smooth muscle cells was inhibited by 2-aminobicyclo(2.2.1)heptane-2-carboxylic acid (BCH) and by large neutral amino acids demonstrating functional system L transporters in each cell type. Uptake of L-CSNO led to accumulation of cellular S-nitrosothiols and inhibition of both growth factor-induced ERK phosphorylation and TNF-alpha-mediated IkappaB degradation. Similar effects were seen when cells were incubated simultaneously with S-nitrosoalbumin and L-cysteine but not with d-cysteine or with S-nitrosoalbumin alone. In each case, nitrosylation of proteins and cellular responses were blocked by BCH. Together, these data suggest that transmembrane movement of nitric oxide (NO) equivalents from the plasma albumin NO reservoir is mediated by cysteine, which serves as a carrier. The mechanism requires transnitrosylation from S-nitrosoalbumin to free cysteine and activity of system L transporters, thereby providing a unique pathway for cellular responses to S-nitrosoalbumin.

Impact of forskolin and amino acid depletion upon System A activity and SNAT expression in BeWo cells

Amino acid transport System A (SysA) plays an important role in mediating the transplacental transfer of neutral amino acids from mother to fetus. Given that prior work has demonstrated that SysA activity is regulated, both over gestation and in response to dietary restriction during pregnancy, we examined the response of SysA activity and sodium-dependent neutral amino acid transporter (SNAT; responsible for SysA activity) expression to cAMP analogues and amino acid deprivation in BeWo cells, an accepted model of placental syncytia. SysA activity was unaffected by forskolin, a cAMP agonist, at 48 and 72 h. Amino acid depletion was associated with an up-regulation of SysA activity, largely mediated through an enhancement of SNAT2 (Slc38a2) expression at both the protein and mRNA level. SNAT1 (Slc38a1) expression did not change in response to amino acid depletion, while SNAT4 (Slc38a4) could not be detected. In summary, SysA activity in BeWo cells responds to amino acid depletion through the differential regulation of SNAT subtypes.

Induction of calcium-activated potassium channel activity by hemin in human erythroleukemia cells

The agent hemin has been demonstrated to be able to initiate a coordinated differentiation program in several cell types. In the present study, we examined the ability of hemin on inducing cell differentiation and Ca(2+)-activated K(+) channel activity in erythroleukemic K562 cells. Treating undifferentiated K562 cells with hemin (0.1 mM) for five days caused these cells to display differentiation-like characteristics including chromatin aggregation, nuclear degradation, pseudopod extension of the membrane and increased hemoglobin production. However, overall cell viability was not significantly changed by the presence of hemin. After hemin treatment for different periods, the Ca(2+)-activated K(+) channel was activated by the addition of ionomycin (1 microM), and was inhibited by either clotrimazole, charybdotoxin, or EGTA. Before hemin treatment there was no significant Ca(2+)-activated K(+) channel activity present in undifferentiated K562 cells. After hemin treatment for 5 days, a significant Ca(2+)-activated K(+) channel activity was detected. This increasing Ca(2+)-activated K(+) channel activity may be contributed from a subtype of Ca(2+)-activated K(+) channel, KCNN4. These results suggest that the ability of hemin to induce increasing Ca(2+)-activated K(+) channel activity may contribute to the mechanism of hemin-induced K562 cell differentiation.

New mechanism for amino acid influx into human epidermal Langerhans cells: L-dopa/proton counter-transport system

We have characterized a stereospecific transport mechanism for L-dopa into human epidermal Langerhans cells (LCs). It is different from any other amino acid transport system. It is highly concentrative, largely pH-independent, and independent of exogenous Na+, glucose and oxygen, and fuelled by a renewable intracellular energy source inhibited by iodoacetate but not by arsenate. We propose that the mechanism is a unidirectional L-dopa/proton counter-transport system. We have recently demonstrated anaerobic glycolysis in human epidermis, which substantiates the need of proton pumps for resident LCs. The findings prompt a re-evaluation of the profound changes LCs undergo when exposed to oxygen in aerobic culture. L-dopa is not metabolized by LCs but can rapidly be dislocated to the intercellular space by certain extracellular amino acids, i.e. LCs can profit by L-dopa in a dualistic way, altogether a remarkable biological phenomenon.

Rat erythrocytes express the anionic amino acid transport protein EAAC1

EAAC1 is a widely expressed protein which transports anionic amino acids in a Na(+)-dependent fashion. Rat erythrocytes have generally been thought to be impermeant to anionic amino acids. Utilizing immunoelectron microscopy, we have demonstrated the presence of EAAC1-immunoreactive protein within rat erythrocytes. Immunoblotting revealed the presence of an approximately 60-kDa protein, consistent with EAAC1, in erythrocyte membranes. Specificity was confirmed by peptide competition. In conclusion, EAAC1 is expressed in rat erythrocytes.

L-alanine uptake by frog (Rana esculenta) red blood cells

L-Alanine uptake has been studied in frog red blood cells. The present study shows the presence of different carriers for this amino acid in these cells. In the physiological concentration range, most L-alanine is taken up through the Na(+)-dependent system ASC, although the sodium-independent systems asc and L are also active. The competitive inhibition data obtained makes difficult to differentiate the two Na(+)-independent activities in a clear contrast with data from fish or mammalian erythrocytes, indicating that despite its widespread occurrence in vertebrates, these carriers show characteristics that are species specific.

Increased red cell glutamine availability in sickle cell anemia: demonstration of increased active transport, affinity, and increased glutamate level in intact red cells

Sickle red blood cells (RBCs) have been shown to have an increase in total nicotinamide adenine dinucleotide (NAD) content by an as-yet-unknown mechanism. Because glutamine is an essential precursor in NAD biosynthesis, we have examined the rates of active RBC glutamine transport and glutamine transport kinetics with Michaelis-Menten constant (K[m]) and maximum velocity (V[max]) in RBCs from patients with sickle cell disease, patients with high reticulocyte counts, and normal volunteers. In addition, plasma and RBC levels of glutamine and glutamate in the three groups were analyzed. The rate of active glutamate transport in sickle RBCs increased threefold over that in high-reticulocyte RBCs and increased 15-fold over that in normal RBCs. Glutamine transport K(m) in sickle RBCs was decreased fivefold in comparison with that in the high-reticulocyte group and that in normal control subjects. Glutamine transport V(max) for sickle RBCs was twofold and eightfold higher in comparison with those in the high-reticulocyte RBCs and normal control RBCs, respectively. Finally, the level of RBC glutamate (a byproduct of glutamine in NAD synthesis) in the sickle group was significantly increased in comparison with that in the high-reticulocyte group, whereas the RBC glutamine level was not. The higher glutamate level in sickle cells may suggest a higher glutamine turnover in these cells. These data suggest that sickle RBCs have an increased glutamine availability and affinity that may facilitate the increase in total NAD in sickle RBCs.

Absorption of amino acids from the human mouth

Certain amino acids were transported across buccal mucosa in vivo by a carrier-mediated process. Metabolic loss of L-amino acids from the mouth in a 5 min test period was negligible. The buccal mucosal transport process was stereospecific for most L-amino acids tested. The uptake of L-methionine and L-leucine showed a tendency to saturation with increasing substrate concentration. The absorption of L-leucine, L-isoleucine and L-methionine as single amino acids was inhibited in the presence of each other suggesting at least one common transport mechanism. Administration of equimolar amounts of amino acids revealed a specific pattern of absorption that could be classified into fast, intermediate, and slow groups. Absorption of some amino acids was at least partly dependent on the presence of sodium ions in the luminal solution. In conclusion, our studies demonstrate that the human buccal mucosa is permeable to L-amino acids in a selective manner, and may resemble absorption pattern similar to other locations of the gastrointestinal tract.

Changes in alanine and glutamine transport during rat red blood cell maturation

Alanine and glutamine transport have been studied during red blood cell maturation in the rat. Kinetic parameters of Na(+)-dependent L-alanine transport were: Km 0.43 and 1.88 mM and Vmax 158 and 45 nmoles/ml ICW/min for reticulocytes and erythrocytes, respectively. During red cell maturation in the rat there is a loss of capacity and affinity of the system ASC for L-alanine transport. The values for Na(+)-dependent L-glutamine transport in reticulocytes were Km 0.51 mM and Vmax 157 nmoles/ml ICW/min. On the other hand, a total loss of L-glutamine transport mediated by both N and ASC systems is demonstrated in mature red cells. This seems to indicate that during rat red cell maturation the system N disappears. Furthermore, the system ASC specificity in mature cells changes, and glutamine enters the red cell by non-mediated diffusion processes.

Transport of cationic amino acids by the mouse ecotropic retrovirus receptor

Susceptibility of rodent cells to infection by ecotropic murine leukaemia viruses (MuLV) is determined by binding of the virus envelope to a membrane receptor that has multiple membrane-spanning domains. Cells infected by ecotropic MuLV synthesize envelope protein, gp70, which binds to this receptor, thereby preventing additional infections. The consequences of envelope-MuLV receptor binding for the infected host cell have not been directly determined, partly because the cellular function of the MuLV receptor protein is unknown. Here we report a coincidence in the positions of the first eight putative membrane-spanning domains found in the virus receptor and in two related proteins, the arginine and histidine permeases of Saccharomyces cerevisiae (Fig. 1), but not in any other proteins identified by computer-based sequence comparison of the GenBank data base. Xenopus oocytes injected with receptor-encoding messenger RNA show increased uptake of L-arginine, L-lysine and L-ornithine. The transport properties and the expression pattern of the virus receptor behave in ways previously attributed to y+, the principal transporter of cationic L-amino acids in mammalian cells.

Differential expression of amino acid transport systems A and ASC during erythroleukemia cell differentiation

The human erythroleukemic cell K-562 serves as an in vitro model to study changes in cell surface antigens and mechanisms regulating globin gene expression associated with in vivo erythropoiesis. In this report we have examined the regulation of amino acid transport systems, in particular, systems A and ASC, during differentiation of erythroleukemic cells. For additional comparison we examined the uptake of leucine, 3-aminoendobicyclo-(3,2,1)-octane-3-carboxylic acid (BCO), arginine, and glutamate. Hexamethylene-bis-acetamide (HMBA), dimethyl sulfoxide, and butyrate induce cell differentiation with a block in G1-G0 phase of the cell cycle. These agents caused a significant downregulation of 2-(methylamino)isobutyric acid uptake by system A. In contrast, the Na(+)-dependent threonine uptake by system ASC remained unaltered. The uptake of leucine, BCO, arginine, and glutamate by as yet unidentified systems was, however, stimulated after HMBA treatment. Hemin, a potent inducer of hemoglobin synthesis in K-562 cells, does not block cell cycle events and, interestingly, had no significant effect on both systems A and ASC. These differences in inducer actions suggest that system A activity may be related to specific stages of cell differentiation and perhaps to other cellular signals.

Cationic and anionic amino acid transport studies in rat red blood cells

The transport of L-proline, L-lysine and L-glutamate in rat red blood cells has been studied. L-proline and L-lysine uptake were Na(+)-independent. When the concentration dependence was studied both showed a non-saturable uptake assimilable to a difussion-like process, with high Kd values (0.718 and 0.191 min-1 for L-proline and L-lysine respectively). Rat red blood cells showed high impermeability to L-glutamate. No sodium dependence was observed and the Kd value was low (0.067 min-1). Our results show firstly, that rat red blood cells do not have amino acid transport systems for anionic and cationic amino acids and secondly that erythrocytes show no sodium-dependent L-proline transport, and that these cells are very permeable to this amino acid.

Expression of Na(+)-H+ exchange and ATP-dependent proton extrusion in growing rat IMCD cells

As the last step of urinary acidification, the inner medullary collecting duct (IMCD) is thought to secrete protons into the tubular lumens by means of a H(+)-translocating adenosinetriphosphatase (H(+)-ATPase). However, recent studies have also shown the existence of Na(+)-H+ exchange activity in IMCD cells. Although the physiological function of the antiporter in IMCD cells is unknown, activation of Na(+)-H+ exchange in other cell-culture systems has been suggested to be closely associated with the process of cell growth. Thus presence of Na(+)-H+ exchange may relate to the growth phase of these cells. To examine intracellular pH (pHi) regulation in growing IMCD cells, we studied proton transport by Na(+)-dependent and Na(+)-independent mechanisms by microfluorimetry using the pHi-sensitive dye 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein acetoxymethyl ester (BCECF/AM). Actively growing cells, defined by [3H]thymidine incorporations, demonstrated an amiloride-sensitive Na(+)-dependent pHi recovery after an acid load. No pHi recovery was evident in the absence of Na+, indicating the importance of Na(+)-H+ exchange for pHi recovery. However, when evaluated in quiescent cells, Na(+)-dependent pHi recovery appeared to be diminished. Instead, a Na(+)-independent pHi recovery which was inhibitable by ATP depletion and by 1 mM N-ethylmaleimide was present, suggesting function of a H(+)-ATPase. These findings indicate that Na(+)-dependent proton extrusion activity (Na(+)-H+ exchange) but not Na(+)-independent proton extrusion activity is expressed during the rapid growth phase of IMCD cells, whereas the more quiescent cells express Na(+)-independent ATP-dependent proton extrusion activity and a possibly less active Na(+)-H+ exchanger.

Changes in neutral amino acid transport activity in myeloid leukemia cells differentiated by lipopolysaccharide

M1 cells derived from mouse myeloid leukemia have been reported to differentiate to macrophage-like cells upon treatment with substances such as lipopolysaccharide. Previously we found that in mouse peritoneal macrophages most of the neutral amino acids were taken up through a unique Na+-independent system. In this paper we have investigated the neutral amino acid transport in M1 cells and in those treated with lipopolysaccharide. In M1 cells serine, alanine and proline were taken up mainly by Na+-dependent transport systems, and leucine was largely transported by a Na+-independent system. By treating the cells with lipopolysaccharide, the activities of the Na+-dependent systems markedly decreased, whereas the activity of the Na+-independent system was little affected. The amino acid concentrations in the cells and the culture medium were measured. As a whole, the intracellular to extracellular distribution ratios for neutral amino acids that are preferred substrates for Na+-dependent systems were decreased on lipopolysaccharide treatment, whereas those for amino acids that are mainly transported by a Na+-independent system were slightly increased. From these results we conclude that M1 cells treated with lipopolysaccharide tend to differentiate to macrophage-like cells with respect to the neutral amino acid transport.