Up-regulation of system A activity in the regenerating rat liver

Long-term osmotic regulation of amino acid transport systems in mammalian cells

Mammalian cells accumulate organic osmolytes, either to adapt to permanent osmotic changes or to mediate cell volume increase in cell cycle progression. Amino acids may serve as osmolytes in a great variety of cells. System A, a transport system for neutral amino acids, is induced after hypertonic shock by a mechanism which requires protein synthesis and gene transcription. Indirect evidence supports the view that system A activity increases due to the interaction of pre-existing A carriers with putative activating proteins. The intracellular accumulation of most neutral amino acids after hypertonic shock depends, exclusively, on the increase in system A activity. Long-term activation of system A is dependent on the integrity of cytoskeletal structures, but in a different way depending on whether cells are polarized or not.

14-3-3 proteins within the hypothalamic-neurohypophyseal system of the osmotically stressed rat: transcriptomic and proteomic studies

The hypothalamic-neurohypophyseal system (HNS) mediates neuroendocrine responses to dehydration through the actions of the antidiuretic hormone vasopressin (VP) and the natriuetic peptide oxytocin (OT). VP and OT are synthesised as separate prepropeptide precursors in the cell bodies of magnocellular neurones in the hypothalamic supraoptic nucleus (SON) and paraventricular nucleus, the axons of which innervate the posterior pituitary gland (PP). Dehydration evokes a massive release of both peptides into the circulation, and this is accompanied by a function-related remodelling of the HNS. Microarray studies on mRNAs differentially expressed in the SON revealed that transcripts encoding the Ywhag and Ywhaz isoforms of the 14-3-3 family of regulatory proteins, are increased in the rat SON by 3 days of water deprivation; findings that we have confirmed by the real-time polymerase chain reaction. Because there is no necessary proportionality between transcript and protein abundance, we next examined Ywhag and Ywhaz translation products throughout the HNS in parallel with 14-3-3 post-translational modification, which is known to be an important determinant of functional activity. Both proteins are robustly expressed in the SON in VP- and OT-containing neurones, but the abundance of neither changes with dehydration. However, the total level of Ywhaz protein is increased in the neurointermediate lobe of the pituitary (NIL, which includes the PP), in parallel with a basic post-translationally modified isoform, suggesting transport from the cell bodies of the SON of newly-synthesised protein and changes in its activity. The level of an acidic, probably phosphorylated, Ywhag isoform is down-regulated in the SON by dehydration, although total levels are unchanged. Finally, based on the presence of a phosphorylated 14-3-3 binding motif, we have identified a 14-3-3 binding partner, proteasome subunit, beta type 7, in the NIL. Thus, we suggest that, through complex transcriptional, and post-translational processes, 14-3-3 proteins are involved in the regulation or mediation of HNS plasticity following dehydration.

Proteomic analysis of the 14-3-3 family inArabidopsis

In this study, various proteomics-based methods were utilized to examine the 14-3-3 protein family in Arabidopsis thaliana. A protein extract was prepared from an Arabidopsis hypocotyl suspension culture and analyzed by two-dimensional gel electrophoresis and immunoblotting with a 14-3-3 monoclonal antibody that recognizes multiple Arabidopsis isoforms. Protein spots that cross-reacted with the monoclonal antibody as well as the surrounding spots were analyzed by high performance liquid chromatography in conjunction with electrospray-tandem mass spectrometry. Nine separate spots contained 14-3-3s and each spot contained multiple 14-3-3 isoforms. Every isoform observed was verified by the identification of at least one isoform-specific peptide. Further analysis by mass spectrometry revealed that the isoforms Chi, Upsilon, Omega, Phi, and Lambda were acetylated on their N termini and no non-acetylated N termini were recovered. These data, together with the distribution of isoforms and the confirmation that 14-3-3s are not complexed during urea denaturing isoelectric focusing, supports the conclusion that Arabidopsis 14-3-3s are acetylated in vivo and are significantly affected by other post-translational modifications.

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.

Cerebrospinal fluid brain-derived proteins in the diagnosis of Alzheimer's disease and Creutzfeldt-Jakob disease

The differential diagnosis of dementia can be difficult in the early stages of disease, and with the emergence of new therapeutic agents for Alzheimer's disease (AD) there is an increasing need for reliable and accurate diagnostic tests. The concept of brain-specific proteins was first proposed in the 1960s and, since that time, methods have developed to measure these proteins in the cerebrospinal fluid (CSF). The concentration of individual brain-specific proteins can be altered in disease, and these changes are thought to reflect the underlying pathology. CSF tau protein and amyloid peptide A beta 42 concentrations are altered in AD and have been proposed as early diagnostic tests for this disease. The data from a number of studies suggest that these proteins may be of value, but are less specific than previously thought and further studies with neuropathological confirmation are required before these tests can be introduced into clinical practice. The detection of 14-3-3 in the CSF is an accurate test for sporadic Creutzfeldt-Jakob disease (CJD) and this accuracy has lead the World Health Organization to revise the clinical criteria for probable sporadic CJD to include a positive CSF 14-3-3. However, CSF 14-3-3 is less useful in the diagnosis of variant CJD, where studies are underway investigating the value of other CSF proteins.

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.

Developmental regulation of the concentrative nucleoside transporters CNT1 and CNT2 in rat liver

BACKGROUND/AIMS

The pattern of nucleoside transporter expression in hepatocytes was studied in the developing rat liver.

METHODS

Hepatocytes isolated from fetuses, neonates and adult rats were used for uridine uptake measurements and concentrative nucleoside transporter (CNT) expression.

RESULTS

Adult hepatocytes showed the highest Na-dependent uridine uptake, but fetal hepatocytes exhibited a significant NBTI-sensitive component of equilibrative Na+-independent transport, which was either negligible or absent in neonatal and adult rat hepatocytes. Low Na+-dependent uridine uptake was associated with low amounts of CNT1 and CNT2 transporter proteins, both with apparent Km values in the low micromolar range. Hepatocyte primary cultures from 20-day-old fetuses showed very low amounts of CNT2 mRNA, and expressed both carrier proteins. Incubation of fetal hepatocytes with dexamethasone and T3 resulted in a significant increase in Na+-dependent uridine uptake and an accumulation of the CNT2 protein and mRNA.

CONCLUSIONS

The expression of concentrative nucleoside carriers in hepatocytes from developing rat liver is developmentally regulated. Addition of endocrine factors known to induce differentiation of fetal hepatocytes results in selective up-regulation of CNT2 expression.

Posttranscriptional regulation of ATA2 transport during liver regeneration

The recent cloning of ATA2, a cDNA displaying characteristics identical to the System A transporter, has provided the first molecular tool for study of System A-mediated amino acid transport in liver. Despite the 233 +/- 9 and 472 +/- 11% increase in System A transport activity following partial hepatectomy at 6 and 12 h, respectively, the steady-state level of ATA2 mRNA did not show a corresponding marked increase. Examination of the kinetic properties of System A following partial hepatectomy revealed a K(m) of 0.26 +/- 0.04 mM which is consistent with the reported K(m) for ATA2. These results indicate that a System A transporter present in regenerating liver and ATA2 are identical, but that the increase in System A activity following partial hepatectomy does not result from an increase in steady-state levels of ATA2 mRNA. These observations suggest that ATA2-mediated transport of amino acids is regulated at the posttranscriptional level.

Changes in urinary taurine and hypotaurine excretion after two-thirds hepatectomy in the rat

This study followed the time course of urinary taurine and hypotaurine excretion after two-thirds hepatectomy in rats. The excretion of both taurine and hypotaurine was elevated during 18 h following the hepatectomy, with maximal excretion during the first 6 h. Twelve and 24 h after partial hepatectomy, the hepatic hypotaurine concentration was increased but liver taurine did not differ significantly from controls. No changes were observed in hypotaurine and taurine concentrations of heart, kidney, lung, muscle tissue and spleen. We postulate that partial hepatectomy induces a rapid increase of hepatic (hypo)taurine synthesis from precursor amino acids. The increased (hypo)taurine concentrations spill over into urine.

Differential expression and regulation of nucleoside transport systems in rat liver parenchymal and hepatoma cells

Primary cultures of rat-liver parenchymal cells show carrier-mediated nucleoside uptake by a mechanism that mainly involves concentrative, Na+-dependent transport activity. In contrast, the hepatoma cell line FAO shows high nucleoside transport activity, although it is mostly accounted for by Na+-independent transport processes. This is associated with a low amount of sodium purine nucleoside transporter (SPNT) mRNA. SPNT encodes a purine-preferring transporter expressed in liver parenchymal cells. To analyze whether SPNT expression is modulated during cell proliferation, SPNT mRNA levels were determined in the early phase of liver growth after partial hepatectomy and in synchronized FAO cells that had been induced to proliferate. SPNT mRNA amounts increased as early as 2 hours after partial hepatectomy. FAO cells induced to proliferate after serum refeeding show an increase in SPNT mRNA levels, which is followed by an increase in Na+-dependent nucleoside uptake and occurs before the peak of 3H-thymidine incorporation into DNA. FAO cells also express significant equilibrative nucleoside transport activity, which may be accounted for by the expression of the nitrobenzylthioinosine (NBTI)-sensitive and -insensitive isoforms, rat equilibrative nucleoside transporter 1 (rENT1) and rENT2, respectively. Interestingly, rENT2 mRNA levels follow a similar pattern to that described for SPNT when FAO cells are induced to proliferate, whereas rENT1 appears to be constitutively expressed. Liver parenchymal cells show low and negligible mRNA levels for rENT1 and rENT2 transporters, respectively, although most of the equilibrative transport activity found in hepatocytes is NBTI-resistant. It is concluded that: 1) SPNT expression is regulated both in vivo and in vitro in a way that appears to be dependent on cell cycle progression; 2) SPNT expression may be a feature of differentiated hepatocytes; and 3) equilibrative transporters are differentially regulated, rENT2 expression being cell cycle-dependent. This is consistent with its putative role as a growth factor-induced delayed early response gene.

Cytoskeletal-dependent activation of system A for neutral amino acid transport in osmotically stressed mammalian cells: a role for system A in the intracellular accumulation of osmolytes

System A activity for neutral amino acid transport is increased after hypertonic shock in NBL-1 (an epithelial cell line) and CHO-K1 cells (a nonepithelial cell line) by a mechanism which is consistent with the synthesis of a regulatory protein that activates preexisting system A carrier proteins (Ruiz-Montasell et al., 1994, Proc. Natl. Acad. Sci. USA, 91,9569-9573). In this study, we have further investigated this biological response by determining the role of cytoskeletal structures in system A regulation by hypertonic stress. Using inhibitors of the microfilament and microtubule networks, we show that the increase in system A activity after hypertonic treatment requires the integrity of both cytoskeletal structures in NBL-1 cells, although the increase in system A activity triggered by amino acid starvation is completely insensitive to any of these drugs. In contrast, the enhancement of system A activity in osmotically stressed CHO-K1 cells is not sensitive to inhibitors of the microtubule network. In both cell types, the results suggest that the inhibitors block the increase of system A activity. System A transport decreases when CHO-K1 cells return to isotonic conditions by a mechanism that is insensitive to inhibitors of protein and mRNA synthesis. The increase in system A transport activity is also followed by the accumulation of neutral amino acids (fourfold for alanine), which is totally blocked by the same agents (cycloheximide and actinomycin D) that prevent the increase in system A activity after hypertonic treatment, thus indicating that system A is crucial for maintaining a high concentration of organic osmolytes inside the cell.

Na+,K(+)-ATPase expression during the early phase of liver growth after partial hepatectomy

Na+,K(+)-ATPase expression has been studied in the early phase of liver growth after partial hepatectomy to ascertain whether its increased activity is due to stable effects, involving de novo synthesis and insertion of pumps into the plasma membrane. Na+,K(+)-ATPase activity progressively increases after partial hepatectomy, reaching a three-fold induction above basal values 12 h after surgery. mRNA amounts of both alpha 1 and beta 1 subunits are rapidly increased up to two-fold for alpha 1 and nearly three-fold for beta 1, at 9 and 12 h post-hepatectomy, respectively. This correlates with increased abundance of both subunit proteins. The results prove that the increase of Na+,K(+)-ATPase activity correlates with higher expression of both subunit proteins and mRNAs, although the characteristics of the induction suggest that some translational and post-translational events may be equally involved in the increased activity of the pump.

Coordinate induction of Na(+)-dependent transport systems and Na+,K(+)-ATPase in the liver of obese Zucker rats

Solute uptake into liver plasma membrane vesicles from either lean or obese Zucker rats was monitored. D-Glucose and L-leucine uptakes at physiological concentrations of the substrate were not different in lean and obese Zucker rats. In agreement with a previous report (Ruiz et al. (1991) Biochem. J. 280, 367-372) L-alanine uptake was significantly enhanced in those preparations from obese animals. Na(+)-coupled uridine transport was markedly enhanced also in obese rats. The effect was due to an increase in Vmax (5.5 +/- 0.6 vs. 2.1 +/- 0.2 pmol/mg protein per 3 s, P < 0.01) without any significant change in Km (11.0 +/- 2.8 vs. 9.0 +/- 2.7 microM for obese and lean rats, respectively). Na+,K(+)-ATPase activity was also higher in liver plasma membrane vesicles from rat liver and it correlated with a higher amount of alpha 1-subunit protein in both, plasma membrane vesicles and homogenates from obese rat livers. In summary, in the hypertrophic liver of obese Zucker rats a coordinate induction of several Na(+)-dependent transport systems occurs and, in order to sustain the metabolic pressure associated with this adaptation, a significant induction of the Na+,K(+)-ATPase expression is also found. These data also provide new evidence for regulation of the recently characterized Na(+)-dependent nucleoside transporter.

Up-regulation of liver system A for neutral amino acid transport in euglycemic hyperinsulinemic rats

To determine the role of insulin on the in vivo modulation of liver system A activity, we used the euglycemic hyperinsulinemic clamp coupled to the measurement of solute uptakes into plasma membrane vesicles partially purified from livers of hyperinsulinemic rats and their saline-infused controls. The clamp was performed in chronically catheterized rats, either in the fasted state, 24 h after surgery (Group I), or after 3 days of recovery (Group II). System A activity, measured as the MeAIB-inhibitable L-alanine uptake, was selectively induced by hyperinsulinemia, although the effect was much greater in Group II than in Group I rats (137% vs. 24% over the basal values, respectively). This might be explained by the higher basal levels found in those liver plasma membrane vesicles from Group I fasted animals. Hyperinsulinemia also decreased blood amino acids but to a similar extent in both experimental groups. This suggests that amino acid depletion by itself may not cause up-regulation of system A. Other transport activities involved in neutral amino acid transport (Systems ASC, N and L) were not modified by the clamp. The induction of system A cannot be explained by changes in the dissipation rate of the Na+ transmembrane gradient, because the differences between insulin- and saline-infused rats remained even when the electrochemical Na+ gradient was disrupted in the presence of monensin. Thus, hyperinsulinemia might induce an increase in the number of transporters inserted into the plasma membrane.

Regulatory and molecular aspects of mammalian amino acid transport

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