Gllcocorticoid inhibition of amino acid transport in rat hepatoma cells

Dexamethasone rapidly and reversibly inhibits the initial rate of transport of α-aminoisobutyric acid in rat hepatoma cells in tissue culture. Colcemid and cytochalasin B neither inhibit transport nor interfere with its inhibition by dexamethasone, arguing that microtubules and microfilaments are not involved in this hormonal effect. Continuous protein synthesis is required both for the dexamethasone inhibition of transport and for its reversal, although cycloheximide alone inhibits transport in control cells by less than 25%. A model for the dexamethasone inhibition of amino acid transport is presented suggesting that glucocorticoids either block the synthesis or enhance the degradation of a rate-limiting protein in the transport system.     

Preferential uptake of cytotoxic porphyrins from hematoporphyrin derivative in murine L929 fibroblasts and Chinese hamster ovary K1 epithelial cells

Photodynamically induced loss of clonogenicity of murine L929 fibroblasts and Chinese hamster ovary K1 epithelial cells was determined with two different assays. It appeared that the loss of clonogenicity was much higher when 20 cells/cm2 were incubated with hematoporphyrin derivative (HPD) and illuminated, than when confluent cell layers were incubated with the same amount of HPD and illuminated prior to plating out. This dependency of cell killing on the experimental protocol was also observed when protoporphyrin (90-95% pure) was used as photosensitizer, but not when the cells were photodynamically treated with rose bengal or exposed to mitomycin C. Further, when cell layers were incubated with the residual solution that remained after the previous incubation of a confluent cell layer with HPD, illumination of these layers appeared to be almost non-toxic, although the overall porphyrin concentration in the residual solution was only slightly lower than in HPD. These results indicate that the porphyrins, responsible for loss of clonogenicity, are present in relatively small amounts in HPD and unpurified protoporphyrin and are preferentially taken up by the cells. Although 2-aminoisobutyric acid transport and DNA synthesis are among the most photosensitive targets with HPD, photodynamic treatment of L929 cells with the residual solution did not result in inhibition of the transport system and DNA synthesis. In contrast, the K+ content of the cells still decreased considerably, when utilizing the porphyrins, remaining in the residual solution as sensitizer. This indicates that under the present experimental conditions the disturbance of the membrane barrier function does not contribute to loss of clonogenicity of these cells and, moreover, that the photodynamically induced K+ leakage is caused by a component of HPD other than inhibition of 2-aminoisobutyric acid transport and DNA synthesis.     

Transport and metabolic effects of α-aminoisobutyric acid in saccharomyces cerevisiae

α-Aminoisobutyric acid is actively transported into yeast cells by the general amino acid transport system. The system exhibits a K_m for α-aminoisobutyric acid of 270 μM, a V_max of 24 nmol/min per mg cells (dry weight), and a pH optimum of 4.1–4.3. α-Aminoisobutyric acid is also transported by a minor system(s) with a V_max of 1.7 nmol/min per mg cells. Transport occurs against a concentration gradient with the concentration ratio reaching over 1000:1 (in/out). The α-aminoisobutyric acid is not significantly metabolized or incorporated into protein after an 18 h incubation. α-Aminoisobutyric acid inhibits cell growth when a poor nitrogen source such as proline is provided but not with good nitrogen sources such as NH₄⁺. During nitrogen starvation α-aminoisobutric acid strongly inhibits the synthesis of the nitrogen catabolite repression sensitive enzyme, asparaginase II. Studies with a mutant yeast strain (GDH-CR) suggest that α-aminoisobutyric acid inhibition of asparaginase II synthesis occurs because α-aminoisobutyric acid is an effective inhibitor of protein synthesis in nitrogen starved cells.     

Control of amino acid transport in the mammary gland of the pregnant mouse

The regulation of the uptake of the amino acid analog α-aminoisobutyric acid was studied in diced mammary glands from pregnant mice. Stimulation of uptake by insulin was not prevented by inhibitors of protein synthesis; protein synthesis inhibitors decreased uptake by 20%; this response occurred more promptly in insulintreated tissues. Elimination of extracellular amino acids led to a substantial increase in transport which was not abolished by inhibitors of protein synthesis. These results indicate that insulin does not increase amino acid transport in this system by altering synthesis and degradation of transport protein. They are consistent with a model in which the activity of the existing amino acid transport protein is subject to negative feedback regulation from the intracellular amino acid pool.     

A reduction in energy-dependent amino acid transport by microtubular inhibitors in ehrlich ascites tumor cells

Vincristine, other periwinkle alkaloids, and colchicine partially inhibit the energy dependent transport of α-aminoisobutyric acid in Ehrlich ascites tumor cells. The properties of this phenomenon were characterized in detail for vincristine. Maximum depression of the steady-state intracellular α-aminoisobutyric acid level was achieved with a vincristine concentration of > 0.5 m̈M. The inhibitory effect of vincristine increases as the extracellular α-aminoisobutyric acid concentration is increased reaching a maximum, however, of only ∼25% at a level of 5 mM, leaving a large gradient for α-aminoisobutyric acid across the cell membrane. Vincristine produced an asymmetrical effect on the bidirectional fluxes decreasing the initial uptake rate, while increasing the efflux of α-aminoisobutyric acid. Inhibition of net α-aminoisobutyric acid transport by vincristine was partially reversible (∼40%). Colchicine (50 m̈M) reduced the steady-state α-aminoisobutyric acid level by 30%, an effect that was not reversible. Inhibition by vinleurosine and vinrosidine was comparable to that of vincristine. Addition of glucose to the medium resulted in a small, but significant, decrease in the inhibitory effects of both vincristine and colchicine. The data indicate that these agents inhibit a small component of the uphill transport of α-aminoisobutyric acid in Ehrlich ascites tumor cells. The inhibitory effect of vincristine cannot be attributed to an increase in the passive permeability of the cell membrane to this agent. Rather, the data along with other studies from this laboratory suggest that vincristine reduces the energy-dependent transport of α-aminoisobutyric acid by either inhibiting cellular energy metabolism or by inhibiting the coupling of energy-metabolism to the transport of this amino acid and raises the possibility that cellular microtubules play a role in these processes.     

Further studies of the transport of amino acids in rat liver slices

Further studies of amino acid transport by the rat liver slice have shown that the transport of α-aminoisobutyric acid is inhibited by glycine as well as dinitrophenol, Na⁺-free medium, and iodoacetate. Glycine itself is actively transported by the rat liver slice, although some metabolism also takes place. Cystine is transported by a single transport system, although reduction to cysteine occurs intracellularly and to some extent in the medium also. Cysteine is transported faster than cystine and to greater concentration gradients. Kinetic studies showed that cystine was transported by a single system that was inhibited by glycine but not by α-amino-isobutyric acid. Two transport systems were involved in cysteine transport, each inhibited to a certain extent by α-aminoisobutyric acid and glycine. Lysine and valine both exist at a higher concentration intracellularly than in the plasma in vivo but no intracellular gradients were obtained after in vitro incubations. It is suggested that the intracellular gradients for these amino acids are maintained by protein catabolism.     

Amino acid transport by aggregates of cultured chicken heart cells. Effect of insulin.

Single heart cells dissociated from 14-day-old chicken embryos were reagregated into spheroidal clusters on a gyratory shaker and centrifuged to form cohesive discs of approximately 400 aggregates. These cultured cells accumulated 2-amino[1-14C]isobutyric acid against a gradient. When incubated for 3 hours in a protein-free, buffered, balanced salt solution, concentrative transport decreased to a stable basal level. Incubation in the presence of sodium bovine insulin prevented this fall in transport activity and resulted in increased 2-aminoisobutyric acid uptake to concentrations 40 time sthat in the medium during a subsequent 3-hour transport assay. Intracellular accumulation of 2-aminoisobutyric acid was linear during the initial 15 min of transport in the presence and absence of added insulin. Basal transport of 2-aminoisobutyric acid was temperature-dependent, requied extracellular sodium greater than 125 meg/liter, and demonstrated saturation with an apparent Vmax of 3.4 mmol/liter/10 min and an apparent Km of 2.6 mM. Basal transport activity was not reduced by cycloheximide or puromycin even after 3 hours of exposure...     

Protoporphyrin-induced photodynamic effects on band 3 protein of human erythrocyte membranes

In previous studies it has been shown that protoporphyrin-induced photodynamic effects on red blood cells are caused by photooxidation of amino acid residues in membrane proteins and by the subsequent covalent cross-linking of these proteins. Band 3, the anion transport protein of the red blood cell membrane, has a relatively low sensitivity to photodynamic cross-linking. This cannot be attributed to sterical factors inherent in the specific localization of band 3 in the membrane structure. Solubilized band 3, for instance, showed a similar low sensitivity to cross-linking. By extracellular chymotrypsin cleavage of band 3 into fragments of 60 000 and 35 000 daltons it could be shown that both fragments were about equally sensitive to photodynamic cross-linking. The 17 000 dalton transmembrane segment, on the other hand, was completely insensitive. Inhibition of band 3-mediated sulfate transport proceeded much faster than band 3 interpeptide cross-linking, presumably indicating that the inhibition of transport is caused by photooxidation of essential amino acid residues or intrapeptide cross-linking. A close parallel was observed between photodynamic inhibition of anion transport and decreased binding of 4,4′-diisothiocyanodihydrostilbene-2,2′-disulfonate (H₂DIDS), suggesting that a photooxidation in the immediate vicinity of the H₂DIDS binding site may be responsible for transport inhibition.     

Relationship between glucocorticoid-mediated early decrease of protein synthesis and the steady state decreases of glucocorticoid receptor and TGF-beta activator protein.

The present studies were undertaken to better elucidate the mechanism(s) by which glucocorticoids inhibit the process of tissue repair. The aim was to determine the importance of the effect of glucocorticoids on decreasing the nuclear TGF-beta activator protein. The relationship amongst inhibition of noncollagen protein synthesis and the steady state levels of glucocorticoid receptor and the TGF-beta activator protein was examined. Both collagen and noncollagen synthesis were determined in skin fibroblast cell culture and in dermis. Fetal rat skin fibroblasts were treated for 24 h with dexamethsone. Noncollagen protein synthesis was decreased to approximately one-half that of collagen synthesis. Similar results were obtained in dermis in vivo. At 48 h, dexamethasone treatment resulted in practically no nuclear glucocorticoid receptor being noted and a 40-45% steady state decrease of the TGF-beta activator protein. We have recently reported that the TGF-beta activator protein exists as a protein complex with SP1 and NFKB (p 49). The present data indicate that although the marked decrease of the nuclear glucocorticoid receptor DNA binding following dexamethasone treatment is not comparable to the early 24 h decrease of noncollagen protein synthesis, the decrease of the TGF-beta activator protein complex binding to DNA is. The present studies indicate the importance of the effect of dexamethasone on the steady state level of the TGF-beta activator protein complex in the glucocorticoid-mediated process inhibition of tissue repair and the relationship of this decrease to the earlier inhibition of protein synthesis.     

Sulfhydyl group involvement in the modulation of neutral amino acid transport in thymocyte membrane vesicles

Membrane vesicles from rat thymocytes accumulate 2-aminoisobutyric acid in the presence of 0.1 M NaCl. Uptake is half maximal between 15 and 30 seconds after addition of the amino acid and reaches a plateau value after about 2 minutes. The uptake of 2-aminoisobutyric acid can be modulated by various sulfhydryl reagents. Reduced glutathione leads to an inhibition of uptake whereas oxidized glutathione increases uptake. Agents such as insulin and diamide which can induce disulfide formation lead to an activation of transport. These data indicate that uptake of the Na⁺-dependent amino acid, 2-aminoisobutyric acid, in thymocytes is modulated by a putative plasma membrane, sulfhydryl-containing protein.     

Photodynamic effects of haematoporphyrin derivative on DNA repair in murine L929 fibroblasts.

Illumination with red light of murine L929 fibroblasts that had been sensitized with haematoporphyrin derivative caused DNA single-strand breaks after a lag time of about 20 min, as revealed by alkaline elution. The cells appeared not to be capable of recovering from this damage. The photodynamic effect of haematoporphyrin derivative on DNA repair was assessed by monitoring the repair kinetics of DNA damage induced by either X-rays, u.v. light (254 nm) or methyl methanesulphonate treatment subsequent to a non-DNA-damaging photodynamic treatment with haematoporphyrin derivative. On 'post-incubation', the normally rapid repair of X-ray-induced DNA strand breaks did not occur, whereas with u.v. light and methyl methanesulphonate treatment after photodynamic treatment prolonged post-incubation resulted in an increase in the number of strand breaks rather than the normally observed decrease. This clearly shows that, after a photodynamic treatment with haematoporphyrin derivative that itself did not cause strand breaks, excision repair in L929 cells is severely inhibited at a stage beyond the incision step.     

Acetylcholine synthesis in rat brain: dissimilar effects of clozapine and chlorpromazine.

Experiments have been conducted in which glucagon-induced stimulation of α-aminoisobutyric acid (AIB) transport in rat liver, presumably mediated by cyclic AMP, was markedly inhibited by actinomycin D, cycloheximide or puromycin. Inhibition of transport occurred despite the presence of high concentrations of cyclic AMP, suggesting that the nucleotide may act prior to, or at the same point as, the antibiotics. Kinetic studies showed that 1) the half-life of the glucagon-stimulated system(s) was less than 1 hour, and 2) glucagon treatment doubled V_max without having any effect on the apparent K_m for hepatic AIB transport. The results suggest that glucagon stimulates hepatic AIB transport by increasing the synthesis of some critical protein having a rapid turnover rate; this effect may be due to a prior increase in RNA synthesis.     

Inhibition of Amino Acid Transport and Protein Synthesis by HgCl2 and Methylmercury in Astrocytes: Selectivity and Reversibility

Abstract: The previously reported observation that submi-cromolar concentrations of HgCl₂ inhibit glutamate uptake reversibly in astrocytes, without effect on 2-deoxyglucose uptake, suggested that elemental mercury vapor, which is oxidized to mercuric mercury in the brain, might cause neurodegenerative change through the mediation of glutamate excitotoxicity. Here, selectivity is explored further by measuring the inhibition of other amino acid transporters and protein synthesis as a function of HgCl₂ concentration. The properties of MeHgCl were compared under identical conditions, and some morphological correlates of function were examined. Inhibition of amino acid transport by HgCl₂ was selective, whereas MeHgCl was nonselective. The 50% inhibitory concentrations of HgCl₂ for uptake of α-aminoiso-butyric acid by system A, uptake of α-aminoisobutyric acid or kynurenine by a system L variant, and uptake of γ-ami-nobutyric acid were all two- to fourfold greater than that for uptake of glutamate. The submicromolar concentrations of HgCl₂ that inhibited glutamate transport also inhibited protein synthesis, but in a rapidly reversible fashion, and elicited only discrete ultrastructural changes (heterochromatin. increased numbers of lysosomal bodies, and increased complexity of cell surface). In contrast, inhibition of protein synthesis by MeHgCl was acutely (1-h) irreversible and became marked only at concentrations higher than those that elicited gross morphologic change in the form of "bleb"-like swellings. The results lend support to the proposed excitotoxic mediation of mercury vapor neurotoxicity and reveal a sharp contrast between the effects of HgCl₂ and MeHgCl on astrocytes.     

Uptake of alpha-aminoisobutyric acid in pig spleen lymphocytes stimulated by sodium periodate.

The effect of sodium periodate on the ability of pig spleen lymphocytes to transport the nonmetabolizable amino acid, α-aminoisobutyric acid, was studied. NaIO₄-treated cells exhibited a lowered rate of uptake of α-aminoisobutyric acid in contrast to phytohemagglutinin- and concanavalin A-treated cells. However, when periodate-treated cells were preincubated with untreated cells for 2 h, the mixed cells exhibited twofold stimulation in the uptake of α-aminoisobutyric acid as compared to untreated cells. The increased uptake of α-aminoisobutyric acid in mixed cells was due to a change in the V but not in the K_m. The observed increased uptake of α-aminoisobutyric acid in mixed cells was inhibited (24%) by ouabain, although the level of uptake in untreated and NaIO₄-treated cells was not affected. Na⁺,K⁺-ATPase activity in mixed cells, which was ouabain sensitive, was stimulated 56%. Studies also showed that there was a decrease in the fluorescence polarization (P value) of diphenyl hexatriene in mixed cells (P = 0.21) as compared to untreated cells (P = 0.24). These results demonstrate that NaIO₄ treatment induces a change in the lymphocyte cell membrane and transport of α-aminoisobutyric acid. Incubation of NaIO₄-treated cells with untreated cells is required for the stimulatory effect in the uptake of α-aminoisobutyric acid, and the stimulation appears to be due to changes in Na⁺,K⁺-ATPase activity and membrane fluidity.     

Hyperthermia, polyamine depletion, and inhibition of X-ray-induced DNA strand break repair

We have recently demonstrated that HeLa cells that had been depleted of polyamines by treatment with inhibitors of polyamine biosynthesis were deficient in their ability to repair X-ray-induced DNA strand breaks. Since it had previously been demonstrated that hyperthermic shock also inhibited strand break repair following X irradiation and that hyperthermia resulted in a leakage of polyamines from cells, it seemed of interest to examine whether the inhibition of repair by hyperthermia was related to this loss of cellular polyamines. In the present paper it is demonstrated that both polyamine depletion and hyperthermia inhibit strand closure, and that a combined treatment further reduces the rate of repair. In cells not depleted of polyamines, repair is restored to normal levels if hyperthermia treatment is followed by a 4-h incubation at 37 degrees C before X irradiation. In polyamine-depleted cells, this 37 degrees C incubation does not result in a return of repair ability. Polyamine supplementation was not effective in reversing hyperthermia-dependent repair inhibition, and, in fact, restoration of repair in control cells following hyperthermic shock corresponded to a time at which polyamines show a maximum decrease in those cells. These results suggest that the inhibition of repair and the increased radiosensitivity observed in hyperthermically treated cells is not related to polyamine depletion. However, data further suggest that polyamine-depleted cells may have other alterations, perhaps in chromatin, which render them more sensitive to thermal inhibition of repair.     

Axon Viability and Mitochondrial Function are Dependent on Local Protein Synthesis in Sympathetic Neurons

(1) Axons contain numerous mRNAs and a local protein synthetic system that can be regulated independently of the cell body. (2) In this study, cultured primary sympathetic neurons were employed, to assess the effect of local protein synthesis blockade on axon viability and mitochondrial function. (3) Inhibition of local protein synthesis reduced newly synthesized axonal proteins by 65% and resulted in axon retraction after 6 h. Acute inhibition of local protein synthesis also resulted in a significant decrease in the membrane potential of axonal mitochondria. Likewise, blockade of local protein transport into the mitochondria by transfection of the axons with Hsp90 C-terminal domain decreased the mitochondrial membrane potential by 65%. Moreover, inhibition of the local protein synthetic system also reduced the ability of mitochondria to restore axonal levels of ATP after KCl-induced depolarization. (4) Taken together, these results indicate that the local protein synthetic system plays an important role in mitochondrial function and the maintenance of the axon.     

Amino acid transport and protein synthesis in energetically-stable calcium-tolerant isolated cardiac myocytes

Myocytes isolated by enzymic dispersion from adult rat ventricular tissue are shown to be energetically stable in the presence of 0.5 mM CaCl₂. ATP and ADP content and rates of lactate production are comparable with those of intact myocardial tissue and consistent with these cells being tightly coupled. Addition of 2,4-dinitrophenol precipitates rapid changes in adenine nucleotide concentrations and a 10-fold increase in lactate production. Cardiac myocytes selectively transport neutral amino acids of the A and L classes. Transport of the amino acid analogue α-aminoisobutyric acid is an active, temperature-dependent and insulin-sensitive process. The apparent K_m for α-aminoisobutyric acid transport is similar to that determined for embryonic cardiac cells. Mature myocytes incorporate labelled amino acids into cytoplasmic proteins with molecular weights ranging from 10 000 to 150 000. Newly synthesised protein is metabolically stable. The data establishes calcium-tolerant myocytes as an experimental system offering many advantages over whole hearts for short- and long-term studies of protein synthesis and catabolism.     

Inhibition of repair of X-ray-induced DNA damage by heat: the role of hyperthermic inhibition of DNA polymerase alpha activity

HeLa S3 cells growing in suspension have been used to investigate possible mechanisms underlying the inhibitory action of hyperthermia (44 degrees C) on the repair of DNA strand breaks as caused by a 6-Gy X-irradiation treatment. The role of hyperthermic inactivation of DNA polymerase alpha was investigated using the specific DNA polymerase alpha inhibitor, aphidicolin. It was found that both heat and aphidicolin (greater than or equal to 2 micrograms ml-1) could decrease DNA repair rates in a dose-dependent way. When the applications of heat and aphidicolin were combined, each at nonmaximal doses, no full additivity in effects was observed on DNA repair rates. When the heat and radiation treatment were separated in time by postheat incubation at 37 degrees C, restoration to normal repair kinetics was observed within 8 h after hyperthermia. When heat was combined with aphidicolin addition, restoration of the aphidicolin effect to control level was also observed about 8 h after hyperthermia. It is suggested that although DNA polymerase alpha seems to be involved in the repair of X-ray-induced DNA damage, and although this enzyme is partially inactivated by heat, other forms of heat damage have to be taken into account to explain the observed repair inhibition.     

Effects of cycloheximide on thermotolerance expression, heat shock protein synthesis, and heat shock protein mRNA accumulation in rat fibroblasts.

A single hyperthermic exposure can render cells transiently resistant to subsequent high temperature stresses. Treatment of rat embryonic fibroblasts with cycloheximide for 6 h after a 20-min interval at 45 degrees C inhibits protein synthesis, including heat shock protein (hsp) synthesis, and results in an accumulation of hsp 70 mRNA, but has no effect on subsequent survival responses to 45 degrees C hyperthermia. hsp 70 mRNA levels decreased within 1 h after removal of cycloheximide but then appeared to stabilize during the next 2 h (3 h after drug removal and 9 h after heat shock). hsp 70 mRNA accumulation could be further increased by a second heat shock at 45 degrees C for 20 min 6 h after the first hyperthermic exposure in cycloheximide-treated cells. Both normal protein and hsp synthesis appeared increased during the 6-h interval after hyperthermia in cultures which received two exposures to 45 degrees C for 20 min compared with those which received only one treatment. No increased hsp synthesis was observed in cultures treated with cycloheximide, even though hsp 70 mRNA levels appeared elevated. These data indicate that, although heat shock induces the accumulation of hsp 70 mRNA in both normal and thermotolerant cells, neither general protein synthesis nor hsp synthesis is required during the interval between two hyperthermic stresses for Rat-1 cells to express either thermotolerance (survival resistance) or resistance to heat shock-induced inhibition of protein synthesis.