Regulation of amino acid transport in isolated rat hepatocytes during development

Short-term effects of thyroid hormones during development: Focus on signal transduction

Extranuclear or nongenomic effects of thyroid hormones are mediated by receptors located at the plasma membrane or inside cells, and are independent of protein synthesis. Recently the alphaVbeta3 integrin was identified as a cell membrane receptor for thyroid hormones, and a wide variety of nongenomic effects have now been shown to be induced through binding of thyroid hormones to this receptor. However, also other thyroid hormone receptors can produce nongenomic effects, including the cytoplasmic TRalpha and TRbeta receptors and probably also a G protein-coupled membrane receptor, and increasing importance is now given to thyroid hormone metabolites like 3,5-diiodothyronine and reverse T(3) that can mimick some nongenomic effects of T(3) and T(4). Signal transduction from the alphaVbeta3 integrin may proceed through at least three independent pathways (protein kinase C, Src or mitogen-activated kinases) but the details are still unknown. Thyroid hormones induce nongenomic effects on at least three important Na(+)-dependent transport systems, the Na(+)/K(+)-ATPase, the Na(+)/H(+) exchanger, and amino acid transport System A, leading to a mitogenic response in embryo cells; but modulation of the same transport systems may have different roles in other cells and at different developmental stages. It seems that thyroid hormones in many cases can modulate nongenomically the same targets affected by the nuclear receptors through long-term mechanisms. Recent results on nongenomic effects confirm the old theory that the primary role of thyroid hormones is to keep the steady-state level of functioning of the cell, but more and more mechanisms are discovered by which this goal can be achieved.

Short-term effects of thyroid hormones and 3,5-diiodothyronine on membrane transport systems in chick embryo hepatocytes

Rapid nongenomic effects of thyroid hormones L-T(3) and L-T(4) on two plasma membrane transport systems were investigated in 14-d-old and 19-d-old chick embryo hepatocytes. The Na(+)/H(+) exchanger activity was measured using the intracellular pH-sensitive fluorescent probe 2',7'-bis-(2-carboxyethyl)-5-(and-6)-carboxyfluorescein acetoxymethyl ester, whereas the amino acid transport was estimated by [1-(14)C]-2-aminoisobutyric acid uptake. System A amino acid transport activation was linear to hormone concentration, whereas the Na/H exchanger gave a bell-shaped dose-response curve, with a maximum at the physiological hormone concentration of 1 nM. The specificity of the effect was verified by the use of inhibitors and analogues. The thyroid hormone analog 3,5-diiodo-L-thyronine was able to mimic some of the hormone effects, but with a lower efficiency. The effect on the Na(+)/H(+) exchanger was identified for 14-d-old and 19-d-old cells, whereas the amino acid transport could only be activated at the late stage of embryo development. Both transport systems were activated through a signal transduction pathway involving PKC, MAPK pathway, and PI3K, even though the differences in response behavior indicate a differential modulation of the two transport systems by L-T(3) and L-T(4). These results clearly demonstrate the existence of rapid nongenomic action of thyroid hormones also in avian cells, and show that activation of System A amino acid transport is not directly correlated to changes in intracellular pH. For the first time, evidence is presented which suggests that short-term effects of thyroid hormones may play a role during fetal development and cell differentiation.

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.

Changes in the levels of glycerophosphoinositols during differentiation of hepatic and neuronal cells

Glycerophosphoinositols are metabolites formed by a phosholipase A2 and a lysolipase specifically acting on membrane phosphoinositol lipids. High levels of these compounds characterize epithelial cells and fibroblasts transformed by ras and other cellular oncogenes. Here we have analyzed the glycerophosphoinositol levels in cells that are considered models of cell differentiation. Using rat hepatocytes at different stages of liver development we have shown that the glycerophosphoinositol basal levels of fetal cells were up to fourfold higher than in adult hepatocytes. No changes in glycerophosphoinositol were observed in regenerating rat liver, a model of differentiated cells proliferating in a synchronous manner, where only glycerophosphoinositol 4-phosphate increased by 80%. Similarly to fetal hepatocytes, a modest but significant increase (30%) in the levels of glycerophosphoinositols was observed in undifferentiated NG-108-15 cells as compared to the same cells induced to differentiate by cAMP. In a different neuronal cell line, PC12 cells, increased glycerophosphoinositol levels characterized the differentiated cells. Based on these observations we suggest that high glycerophosphoinositol levels characterize cellular phenomena associated with the activation of ras/mitogen-activated protein kinase pathways.

Effect of protein malnutrition on neutral amino acid transport by rat hepatocytes during development

Hepatocytes from suckling rats whose mothers were fed a low-protein diet (9% protein) showed a lower capacity for Na(+)-dependent L-alanine uptake [due to a decrease in maximal uptake rate (Vmax) of a low-affinity component of transport] and were not able to respond to insulin or glucagon, whereas those from suckling pups whose mothers were fed the control diet (17% protein) had already developed the ability to upregulate L-alanine transport after hormone treatment. When animals from low-protein-fed mothers were weaned onto a hypoprotein diet, the overall capacity for Na(+)-dependent L-alanine uptake (apparent Vmax) and its responsiveness to pancreatic hormones were restored. Hepatocytes from these animals showed a lower response to glucocorticoid treatment. Amino acid availability was dramatically decreased in suckling and weanling rats fed a low-protein diet. These results support the hypothesis that nutrient supply is an important factor in the proper development of hepatic transport functions during the suckling-weaning transition.

Regulatory and molecular aspects of mammalian amino acid transport

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L-alanine uptake by rat liver parenchymal and haematopoietic cells during the perinatal period

Alanine disposal by liver parenchymal and haematopoietic cells from 21-day fetuses, newborns and adult rats was studied. Preparations selectively enriched in either haematopoietic cells or hepatocytes were obtained by direct perfusion of fetal- and neonatal-rat livers. L-Alanine transport into liver parenchymal cells was best fitted to two Na(+)-dependent saturable systems. The high-affinity system showed a much higher activity (Vmax.) in hepatocytes from fetuses and newborns than in those from adult rats (2.4, 4.3 and 0.3 nmol/8 min per 10(6) cells for fetuses, newborns and adults respectively). Vmax. for the low-affinity component was slightly lower during the perinatal period than in the adult (about 30 nmol/8 min per 10(6) cells for hepatocytes from fetuses and newborns, versus 48 nmol/8 min per 10(6) cells for adult rat parenchymal cells). Haematopoietic cells from fetal-rat livers showed significant Na(+)-dependent L-alanine uptake which was completely abolished after birth. These results show that the transport systems involved in L-alanine uptake by liver parenchymal cells are fully developed before birth. This probably contributes to fulfilling the high requirement for neutral amino acids for protein synthesis during development. Haematopoietic cells may play an important role in liver amino acid metabolism during fetal life.

Different signal transduction by epidermal growth factor may be responsible for the difference in modulation of amino acid transport between fetal and adult hepatocytes

[1-14C]-2-aminoisobutyric acid (AIB) uptake and signal transduction pattern after epidermal growth factor (EGF) stimulation were examined in freshly isolated hepatocytes from 20-day-old fetuses and 3-month-old rats. EGF induced a transient increase of AIB transport after 10 min only in adult animals; the observed unresponsiveness of fetal liver is not dependent on a lack of EGF receptors which are present though to a lesser extent on the plasma membrane in this period. As far as the production of the second messengers, inositol trisphosphate (IP3) and calcium, is concerned, substantial differences were found: EGF increased IP3 production in adult hepatocytes, whereas it had no effect in fetal ones. Moreover, the addition of EGF induced a calcium transient in hepatocytes from adult animals, while there was no increase in fetal cells. The lack of EGF effect on amino acid transport in fetal cells could be due to its inability to produce both IP3 and calcium transients, suggesting that this transduction pathway is not activated during fetal life.

Amino acid uptake regulation by cell growth in cultured hepatocytes isolated from fetal and adult rats

Amino acid uptake mediated by system A was studied in cultured fetal and adult hepatocytes, subjected to growth stimulation by EGF and insulin, or to growth inhibition by high cell density. The mitogenic stimulation induced a strong transport increase only in fetal cells, while the cell density-dependent growth inhibition, probably mediated by molecules present on adult hepatocyte membranes, provoked the decrease of amino acid uptake only in the adult cells. The results indicate that the different modulation of amino acid transport by cell growth is dependent on the age and the differentiation stage of hepatocytes.

Effect of simultaneous ingestion of L-amino acids and whole protein on plasma amino acid and urea nitrogen concentrations in humans

The effect of whole protein and L-amino acid ingestion on plasma amino acid (PAA) and urea nitrogen (UN) concentrations was investigated. Ten males ingested equivalent amounts of nitrogen as (trial 1) cottage cheese, (trial 2) an L-amino acid mixture, (trial 3) cottage cheese and L-amino acids. Mean changes in total PAA between trials 1 (342 mumol/liter) and 2 (719 mumol/liter) and trials 1 (342 mumol/liter) and 3 (981 mumol/liter) at 30 min and trials 1 (547 mumol/liter) and 3 (143 mumol/liter) at 150 min differed significantly. Mean changes in essential PAA between trials 1 (180 mumol/liter) and 2 (420 mumol/liter) and trials 1 (180 mumol/liter) and 3 (500 mumol/liter) at 30 min differed significantly. Mean changes in essential PAA between trials 1 (247 mumol/liter) and 3 (334 mumol/liter) at 60 min and between trials 1 (252 mumol/liter) and 3 (80 mumol/liter) at 150 min differed significantly. Mean increments in total and essential PAA were higher and peaked faster but decreased more quickly after trials 2 and 3 than after trial 1. Mean changes in plasma UN did not differ between trials. Ingestion of either L-amino acids, whole protein or the mixture of L-amino acids and whole protein was equally effective in increasing total PAA over 4 hr.

Epinephrine regulation of amino acid transport in rat hepatocytes isolated during development

The effect of epinephrine on the amino acid transport mediated by system A was investigated by determining the uptake of 2-amino [1-14C]isobutyric acid (AIB) in rat hepatocytes, freshly isolated at different stages of pre- and postnatal development. The data obtained show that the hormone increased AIB uptake, enhancing the Vmax, while Km was unchanged. This effect was evident in cells from adult, 18- to 20-day-old fetus, and neonate rat. Actinomycin D or cycloheximide abolished the hormone dependent increase. Experiments carried out with alpha- and beta-antagonists showed that the effect of epinephrine was beta-mediated in fetal life and alpha-mediated in adult life. Membrane binding experiments showed a higher value for epinephrine and beta-agonist dihydroalprenolol in the fetus versus the adult. The calcium depletion obtained after cell incubation with EGTA or calcium ionophore A23187 reduced the hormonal stimulation in the adult, and was ineffective in the prenatal period. An involvement of cAMP was present in the epinephrine modulation of AIB transport, both in adult and in fetal life.

Regulation of amino acid transport in hepatocytes isolated from adult and old rats

The 2-aminoisobutyric acid transport in hepatocytes isolated from 3- and 24-month-old rats was studied and some age-related differences were observed. The basal uptake appeared to be almost constant in cells from old animals during the incubation time, while, in the cells from adults, it showed a progressive increase, interpreted as being due to a derepression mechanism. Epinephrine and glucagon increased the transport in hepatocytes from animals of both ages, even if with a slightly different pattern; the hormones increased the Vmax, while the Km was unchanged at each age tested. However, the glucagon-induced increase in Vmax was lower in the older animals. The mechanism of hormonal action appeared to be similar in adult and old rats. In fact the uptake stimulation by glucagon and epinephrine showed a dependence on protein synthesis. The epinephrine effect was mediated by alpha-adrenergic receptors. No effect was exerted by extracellular amino acids on hepatocytes from 24-month-old animals, suggesting a loss of adaptative regulation mechanism with aging. This behaviour was reflected in the kinetic parameters; in fact the Vmax was not modified by extracellular amino acids at 24 months of age, while it appeared to be strongly decreased in the adult.