Inhibition of hormonal induction of tyrosine aminotransferase by polyamines in freshly isolated rat hepatocytes

Polyamine transport systems of Leishmania donovani promastigotes

The following observations are conjointly indicative of the presence of distinct energy-dependent, saturable and multiple polyamine transport systems in Leishmania donovani promastigotes, the causative agent for visceral leishmaniasis. Spermidine was influxed with as much as seven times higher rate than putrescine, while both spermidine and putrescine transporters exhibited equally high affinity for the respective polyamine. N-Ethylmaleimide arrested the complete functionality of both the transporters which could be restored by reduced glutathione. Putrescine transporter did not recognize spermine but spermidine was recognized to some extent, while spermidine transporter significantly recognized spermine but putrescine was absolutely spared. A few aromatic diamines viz., diaminobiphenyl and the analogs as well as aliphatic diamines viz., cadaverine and agmatine were selectively recognized by the putrescine transporter only. L. donovani promastigotes grown in presence of alpha-difluoromethylornithine, an irreversible inhibitor of ornithine decarboxylase, registered marked upregulation of putrescine transport while spermidine transport was only marginally induced. PA transport systems provide the alternative pool of polyamines in L. donovani promastigotes in the absence of an adequate intracellular PA repertoire.

Characterization of putrescine transport across the intestinal epithelium: study using isolated brush border and basolateral membrane vesicles of the enterocyte

Putrescine transport was investigated in isolated brush border and basolateral membrane vesicles prepared from the rabbit enterocyte. Brush border vesicles were oriented right-side-out and basolateral vesicles inside-out, forming a model representing uptake and extrusion across the intestinal epithelium. Putrescine transport across both membranes was initially rapid, and 66% of the equilibrium uptake was achieved within the first minute. According to osmoplots and measurements at 4 degrees C, 20% of total incorporation presented binding to the membrane. In order to estimate actual uptake into the vesicles, Km was calculated from the differences in putrescine incorporation at 37 degrees C and 4 degrees C, and was 12.7 mumol L-1 for brush border uptake and 38.2 mumol L-1 for basolateral extrusion. Putrescine uptake into brush border and basolateral membrane vesicles was not enhanced in the presence of an Na+ gradient. When Na+ was substituted with an uncharged solute, mannitol, putrescine incorporation was increased, indicating that putrescine uptake is not Na(+)-dependent and that cations might interfere with the carrier. Paraquat and methylglyoxalbis(guanylhydrazone), known to share the polyamine transport system, inhibited putrescine incorporation in both membrane vesicle preparations. Basolateral carrier showed significantly higher sensitivity to cations. We conclude that putrescine uptake across the apical membrane and extrusion across the basolateral membrane of the enterocyte are mediated by two different and independent carriers which differ in their electrical properties.

Recovery from acute glucagon challenge in isolated rat hepatocytes: is protein dephosphorylation synchronous or asynchronous?

One-dimensional SDS-PAGE of cytosolic phosphopeptides confirms that glucagon promotes the phosphorylation of 11 phosphopeptides in isolated rat hepatocytes pre-equilibrated with 32PO4(3-). Nine of these phosphopeptides are tentatively identified, whereas two phosphopeptides (48 kDa and 46 kDa) remain unidentified. Transfer of the glucagon-challenged hepatocytes to medium free of 32PO4(3-) and glucagon led to the rapid net dephosphorylation of the phosphopeptides and to a rapid decline in the specific radioactivity of the [32P]ATP pool. There were profound differences between the post-glucagon rates of net dephosphorylation of the different hepatic phosphopeptides, consistent with net dephosphorylation being asynchronous during the recovery phase from acute glucagon challenge. On the basis of descending rates of dephosphorylation, four major groups of phosphopeptides were delineated. Okadaic acid, a potent inhibitor of protein phosphatase 2A and to a lesser extent protein phosphatase 1, inhibited the dephosphorylation of all of the phosphopeptides. A role for protein phosphatase 2A in protein dephosphorylation may be indicated by the observation that spermine, a specific activator of protein phosphatase 2A, stimulates the dephosphorylation of some, but not all, of the glucagon-stimulated phosphopeptides. Although phosphorylation during the recovery phase from glucagon challenge may be a complicating factor, the results suggest that post-glucagon dephosphorylation is a complex asynchronous process. The physiological consequences of this asynchrony may be that the suppression of glycogenolysis and gluconeogenesis and the activation of glycolysis are early events in the recovery process.

Abnormal accumulation and toxicity of polyamines in a difluoromethylornithine-resistant HTC cell variant

Mammalian cells possess an inducible, active polyamine transport system that is stringently regulated by feedback controls. This study provides evidence that DH23b cells, which were initially selected from the rat hepatoma HTC line for overproduction of ornithine decarboxylase, demonstrate an abnormality in the regulation of polyamine transport. Exposure of these cells to micromolar levels of spermidine or spermine resulted in inhibition of protein synthesis and eventual cell lysis. These effects were not due to by-products of polyamine oxidation by serum oxidases as neither inhibition of protein synthesis nor cell lysis was mitigated by aminoguanidine, reduced glutathione, dithiothreitol, or catalase. Although the polyamine transport system in the DH23b cells has the same Km and Vmax as that in the parental HTC line, the variant cells accumulated abnormally high levels of both spermidine (8-times normal) and spermine (4-times normal). In the HTC line, however, transport of both polyamines as well as putrescine was feedback inhibited within approx. 3 h, while in the variant cells uptake was not diminished by 12 h and terminated only with cell lysis. The DH23b cells appear to lack the normal mechanism responsible for feedback control of active polyamine incorporation. This defect provided the opportunity to manipulate intracellular levels of spermidine from 30 to approx. 800% of normal, allowing the demonstration that cellular protein synthesis is as sensitive to spermidine levels as previous in-vitro studies had suggested.

Characterisation of putrescine uptake by cultured adult mouse hepatocytes

Uptake of polyamines by cultured cells has been shown to be influenced by growth rate and/or differentiation. In this study, we have investigated whether the fully differentiated, non-proliferating adult mouse hepatocyte is capable of accumulating extracellular putrescine. When hepatocytes were cultured from 4 to 48 h, uptake of putrescine was found to increase substantially with time spent in culture. The Vmax for putrescine uptake increased 22-fold during this period with no change in apparent Km. Treatment of the cells with cycloheximide or actinomycin D at concentrations that did not affect cell viability inhibited the induction of putrescine uptake. Endogenous putrescine levels increased from 19.7 nmol/mg DNA after 4 h in culture to over 500 nmol/mg DNA after 48 h in culture. This increase was accompanied by a loss of over 90% of ornithine decarboxylase activity. Spermidine levels did not change over this time period, whereas spermine levels decreased by 35%. Difluoromethylornithine prevented the observed increase in intracellular putrescine but did not affect putrescine uptake. The increase in putrescine transport was not inhibited by culturing the hepatocytes in a high concentration of putrescine, spermidine or spermine. Moreover, the induction process was not stimulated by foetal calf serum but was selectively inhibited by the differentiating agents dimethylsulfoxide and retinoic acid. The results from those studies show that cultured mouse hepatocytes express a putrescine transport system that is poorly regulated by extracellular polyamines. The expression of the transporter requires the synthesis of mRNA and protein, and appears to be related to a time-dependent change in hepatocyte phenotype.

Transport and metabolism of polyamines in human lymphocytes

1. Polyamines are taken up by human peripheral lymphocytes in a concentration, time and pH dependent manner, with an energy-dependent transport system. 2. Each polyamine inhibits the uptake of the others, with the exception of putrescine. Spermine appears to have the highest affinity for the transporter/s. 3. Inhibition by ouabain, amiloride and vanadate suggests that the transport is dependent on Na+. 4. Polyamine content inside the cells increases by ca 6 and 3 times respectively during incubation with spermidine or spermine. 5. The incorporated polyamines are partially transformed into each other.

Study on the role of endogenous polyamines in glucagon, isoproterenol or serum-mediated induction of tyrosine aminotransferase in cultured heart cells

In confluent and serum-starved embryonic heart cell cultures, the addition of serum (10%), glucagon (GLU, 0.1 microM) or isoproterenol (ISO, 10 microM), causes the onset of ornithine decarboxylase (ODC) activity, with a maximum after 5-6 hr. This is paralleled by polyamine accumulation and by the induction of TAT, which, in the case of GLU and ISO, exhibits maximal activity at 4-3 hr respectively, followed by a net decline. Cyclic AMP (cAMP) also accumulates after exposure to GLU or ISO. However, under different conditions of ODC inhibition, serum fails to induce TAT, thus supporting a relevant role of cellular polyamines in serum action. Conversely, cAMP and TAT responses to GLU or ISO are markedly improved under prevention of polyamine accumulation, which also leads to a longer lasting TAT inducibility. The suggestion is made that polyamines are not required in the cAMP-dependent mechanism of TAT induction, but rather in the restoration of the basal activity of the enzyme.

Spermine antagonises the effects of dexamethasone, glucagon and cyclic AMP in increasing the activity of phosphatidate phosphohydrolase in isolated rat hepatocytes

Rat hepatocytes were incubated in monolayer culture, under serum free conditions, for 8 h. Glucagon (10 nM), 8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate (100 microM) and dexamethasone (100 nM) increased the activity of phosphatidate phosphohydrolase by approx. 2-, 3.6- and 3.3-fold, respectively. Spermine alone had no significant effect. Spermine (2.5 mM) almost completely inhibited the glucagon induced increase in phosphohydrolase activity. It only partially inhibited the dexamethasone and cyclic AMP mediated inductions. Spermidine had no significant effect in this respect. The results are discussed in relation to the known effects of polyamines on glycerolipid synthesis, in particular, and on intermediary metabolism.

Effects of an antitumoural rhodium complex on thioacetamide-induced liver tumor in rats. Changes in the activities of ornithine decarboxylase, tyrosine aminotransferase and of enzymes involved in fatty acid and glycerolipid synthesis

Rats were injected daily for 8 weeks with 50 mg of thioacetamide per kg to produce liver tumours. Some of these rats were given three doses of 50 mg of an antitumoural Rh(III) complex/kg at 14, 9 and 5 days before the end of the thioacetamide treatment. Thioacetamide decreased the rate of weight gain of the rats and the Rh(III) complex partly restored it. The activities of ATP citrate lyase, acetyl-CoA carboxylase and fatty acid synthetase in the livers were decreased by thioacetamide treatment and the Rh(III) complex partly reversed this effect. By contrast the activity of malic enzyme was increased by both thioacetamide and the Rh(III) complex and this effect probably relates to NADPH production for detoxification rather than for lipogenesis. Treatment with thioacetamide increased the rate of synthesis of di- and triacylglycerols from glycerol phosphate by liver homogenates, the activity of phosphatidate phosphohydrolase and the incorporation of [3H]glycerol into liver triacylglycerol in vivo. The Rh(III) complex did not produce a significant reversal of these effects of thioacetamide on glycerolipid synthesis. The total uptake of intraportally injected [3H]glycerol by the livers of thioacetamide treated rats was decreased and this was associated with a lowered activity of glycerol kinase. Thioacetamide increased the activity of hepatic ornithine decarboxylase by about 40-fold, but the Rh(III) complex did not reverse this effect. However, the decrease in tyrosine aminotransferase activity that was produced by thioacetamide was partly reversed by the Rh(III) complex. These results are discussed in relation to the tumour-promoting effects of thioacetamide and the antitumoural action of the Rh(III) complex.

The polyamine-dependent modulation of phenylalanine hydroxylase phosphorylation state and enzymic activity in isolated liver cells

The role of polyamines in the control of phenylalanine hydroxylase phosphorylation state and enzymic activity was investigated. Pre-treatment of liver cells with spermine (1 mM) abolishes the glucagon (1 nM)-stimulated increase in hydroxylase phosphorylation. Concurrently there is a decrease in phenylalanine hydroxylation flux, reflecting decreased enzyme activity; 50% inhibition occurs at approx. 10 microM-spermine. These results are discussed in the context of reports concerning the properties of protein phosphatase 2A.

Insulin regulation of protein phosphorylation in hepatocytes. Studies using two effectors: amiloride and natural aliphatic polyamines

The effects of amiloride and of natural aliphatic polyamines on basal and hormone-stimulated protein phosphorylations in hepatocytes were studied. Cells isolated from adult rats were incubated in suspension with (32P)-orthophosphate, in the absence or presence of the effectors at varying concentrations and for different times; hepatocytes were then exposed to various hormones for 10 min. Phosphoproteins contained in total cell lysates were analyzed by one- and two-dimensional gel electrophoresis and autoradiography. Amiloride and spermine (the most effective amine) decreased the basal level of phosphorylation of proteins of 46, 34 and 22 kDal, and increased that of 18 kDal and 93 kDal proteins. These effects were maximal with external concentrations of 1 mM and 7.5-10 mM amiloride and spermine, respectively. They were detectable after a lag period of about 10 min and reached a plateau after 45 min. Pretreatment of cells with these effectors almost completely prevented stimulation of the phosphorylation of the 46 and 34 kDal proteins by insulin. In contrast, the effects of vasopressin on the same proteins were only partly inhibited, whereas those of glucagon appeared largely unaffected. The major effect observed in intact cells (i.e., decreased phosphorylation) could be reproduced in a cell-free system where no kinase activity persisted. Amiloride or spermine added directly to cell extracts strongly accelerated the dephosphorylation of 46 kDal protein and also of the 61 kDal protein identified as pyruvate kinase. Furthermore, restoration of the activity of this enzyme occurred concomitantly with dephosphorylation of the 61 kDal protein, an observation supporting the notion that amiloride and spermine could activate a phosphoprotein phosphatase.

Polyamine uptake by bovine adrenocortical cells

Bovine adrenocortical cells of fasciculo-reticulata origin in primary culture actively accumulate polyamines from the extracellular medium in an energy-dependent process. At low extracellular concentration (e.g., 1 microM putrescine), the transport system resulted in a several-hundred-fold concentration of polyamine in the cellular compartment within 1-2 h of incubation. Putrescine uptake appeared to be the sum of a sodium-dependent, saturable process, with an apparent Km of about 10 microM and of a non-saturable, sodium-independent component. By contrast, spermine was taken up by the cells mostly in a sodium-independent manner. Cross-competition experiments suggested that both polyamines were at least partly transported by the same system. Using specific corresponding probes, it was shown that the polyamine uptake was independent of the amino acid transport systems of the A, L and N types known in a number of cell systems. Adrenocortical cell polyamine content is known to be modulated by adrenocorticotropin through induction of ornithine decarboxylase activity. The existence of a specific uptake system in these cells opens the possibility of a more rapid pathway for the regulation of cellular polyamine levels. It remains to be examined whether this polyamine transport system is under hormonal control, and whether this can support the suggestion that polyamines may represent a form of intracellular messengers in the mechanism of hormone action.

Effects of cyclic AMP, glucocorticoids and insulin on the activities of phosphatidate phosphohydrolase, tyrosine aminotransferase and glycerol kinase in isolated rat hepatocytes in relation to the control of triacylglycerol synthesis and gluconeogenesis

Rat hepatocytes were incubated in monolayer culture in modified Leibovitz L-15 medium containing either 10% (v/v) newborn-calf serum or 0.2% (w/v) fatty-acid-poor bovine serum albumin. The addition of 100 nM-dexamethasone increased the activities of both phosphatidate phosphohydrolase and tyrosine aminotransferase by about 3.5-fold after 8h, and these activities continued to rise until at least 24h. Incubating the hepatocytes in the albumin-containing medium with 10 microM- or 100 microM-8-(4-chlorophenylthio)adenosine 3',5'-cyclic monophosphate increased the activities of the phosphohydrolase and aminotransferase by 2.6- and 3.4-fold respectively after 8h. These increases were blocked by actinomycin D. The increases in the activities that were produced by the cyclic AMP analogue and dexamethasone were independent and approximately additive. Insulin when added alone did not alter the phosphohydrolase activity, but it increased the aminotransferase activity by 34%. The dexamethasone-induced increase in the phosphohydrolase activity was completely blocked by 7-144 microM-insulin, whereas that of the aminotransferase was only partly suppressed. Insulin had no significant Effects on the increases in the activities of phosphatidate phosphohydrolase and tyrosine aminotransferase that were produced by the cyclic AMP analogue, but this may be because the analogue is fairly resistant to degradation by the phosphodiesterase. The activity of glycerol kinase was not significantly changed by incubating the hepatocytes with insulin, dexamethasone and the cyclic AMP analogue alone or in combinations. It is proposed that high concentrations of cyclic AMP and glucocorticoids increase the total activity of phosphatidate phosphohydrolase in the liver and provide it with an increased capacity for synthesizing triacylglycerols and very-low-density lipoproteins, which is expressed when the availability of fatty acids is high. There appears to be a co-ordinated hormonal control of triacyglycerol synthesis and gluconeogenesis in diabetes and in metabolic stress to enable the liver to supply other organs with energy.

Spermine promotes the translocation of phosphatidate phosphohydrolase from the cytosol to the microsomal fraction of rat liver and it enhances the effects of oleate in this respect

Spermine (0.5-2 mM) promoted the translocation of phosphatidate phosphohydrolase from the soluble to the microsomal fraction in a cell-free system derived from rat liver. By contrast, spermidine (1 mM) and putrescine (1 mM) had no significant effect on the translocation when added alone. Spermine, and to a lesser extent, spermidine, enhanced the translocating action of oleate and increased its effectiveness in transferring the phosphohydrolase from the soluble to the microsomal fraction. It is proposed that the phosphohydrolase becomes metabolically active when it combines with membranes and that polyamines might help to regulate this interaction. This could facilitate the action of fatty acids and enable cells to increase their capacity for triacylglycerol synthesis to match an increased availability of fatty acids.

Pretranslational regulation of tyrosine aminotransferase and phosphoenolpyruvate carboxykinase (GTP) synthesis by glucagon and dexamethasone in adult rat hepatocytes

The regulation of synthesis of the gluconeogenic cytosolic enzyme phosphoenolpyruvate carboxykinase (PEPCK) and of tyrosine aminotransferase (TAT) by glucagon and glucocorticoid hormones was studied in hepatocytes maintained in suspension culture for 7 h. Specific antibodies were used to measure relative rates of enzyme synthesis after pulse-labelling of the cells with [3H]leucine or [35S]methionine. Concomitantly, amounts of mRNA were quantified after translation in vitro in a reticulocyte lysate and specific immunoprecipitation of the proteins. Glucagon stimulated the rate of synthesis of PEPCK by 4-6-fold and that of TAT by 6-8-fold in 2h. In contrast, dexamethasone had little effect on PEPCK synthesis, whereas it increased TAT synthesis by 5-9-fold. When used in combination, the two hormones displayed additive effects on TAT synthesis, whereas the glucocorticoid hormone strongly potentiated stimulation of PEPCK synthesis by glucagon. In every instance, changes in rates of synthesis of the two enzymes were totally accounted for by increases in amounts of the corresponding functional mRNA, suggesting a pretranslational site of action for both glucagon and dexamethasone.

Regulation of protein phosphorylation by polyamines in hepatocytes

The effects of natural aliphatic polyamines on basal and hormone-stimulated protein phosphorylations in hepatocytes were studied. Cells isolated from adult rats were incubated in suspension with [32P]orthophosphate, in the absence or presence of polyamines at varying concentrations and for different times; hepatocytes were then exposed to various hormones for 10 min. Phosphoproteins contained in total cell lysates were analyzed by one- and two-dimensional gel electrophoresis and autoradiography. Spermine, the most effective amine, decreased the basal level of phosphorylation of proteins with 46, 34 and 22 kDa, and increased that of a 18 kDa protein. These effects, maximal with an external concentration of 7.5-10 mM, were detectable after a lag period of about 10 min and reached a plateau after 45 min. Pretreatment of cells with the polyamine almost completely prevented stimulation of the phosphorylation of the 46 and 34 kDa proteins by insulin; in contrast, the effects of phenylephrine on the same proteins were only partly inhibited, whereas those of glucagon appeared largely unaffected. The major polyamine effect observed in intact cells (i.e., decreased phosphorylation) could be reproduced in a cell-free system where no kinase activity persisted. Indeed, spermine added directly to cell extracts strongly accelerated dephosphorylation of the 46 kDa protein and also of the 61 kDa protein identified as pyruvate kinase; furthermore, restoration of the activity of this enzyme occurred concomitantly with dephosphorylation of the 61 kDa protein in the presence of spermine.