The pleiotypic response to amino acid deprivation is the result of interactions between components of the glycolysis and protein synthesis pathways

Amino acid transporter SLC38A5 is a tumor promoter and a novel therapeutic target for pancreatic cancer

Pancreatic ductal adenocarcinoma (PDAC) cells have a great demand for nutrients in the form of sugars, amino acids, and lipids. Particularly, amino acids are critical for cancer growth and, as intermediates, connect glucose, lipid and nucleotide metabolism. PDAC cells meet these requirements by upregulating selective amino acid transporters. Here we show that SLC38A5 (SN2/SNAT5), a neutral amino acid transporter is highly upregulated and functional in PDAC cells. Using CRISPR/Cas9-mediated knockout of SLC38A5, we show its tumor promoting role in an in vitro cell line model as well as in a subcutaneous xenograft mouse model. Using metabolomics and RNA sequencing, we show significant reduction in many amino acid substrates of SLC38A5 as well as OXPHOS inactivation in response to SLC38A5 deletion. Experimental validation demonstrates inhibition of mTORC1, glycolysis and mitochondrial respiration in KO cells, suggesting a serious metabolic crisis associated with SLC38A5 deletion. Since many SLC38A5 substrates are activators of mTORC1 as well as TCA cycle intermediates/precursors, we speculate amino acid insufficiency as a possible link between SLC38A5 deletion and inactivation of mTORC1, glycolysis and mitochondrial respiration, and the underlying mechanism for PDAC attenuation. Overall, we show that SLC38A5 promotes PDAC, thereby identifying a novel, hitherto unknown, therapeutic target for PDAC.

Apoptosis and its suppression in hepatocytes culture

In order to achieve the goal of developing extracorporeal liver support devices, it is necessary to optimise bioprocess environment such that viability and function are maximised. Optimising culture medium composition and controlling the constitution of the cellular microenvironment within the bioreactor have for many years been considered vital to achieving these aims. Coupled to this is the need to understand apoptosis, the prime suspect in the demise of animal cultures, including those of hepatocytes. Results presented here show that absent nutrients including glucose and amino acids play a substantial part in the induction of apoptosis. The use of chemical apoptosis inhibitors was utilised to investigate key components of hepatic apoptosis where caspases, predominantly caspase 8, were implicated in staurosporine (STS)-induced HepZ apoptosis. Caspase 9 and 3 activation although recorded was of less significance. Interestingly, these results were not consistent with those of mitochondrial membrane depolarisation where inhibition of caspase activation appeared to drive depolarisation. Inhibition of mitochondrial permeability transition and use of anti-oxidants was unsuccessful in reducing apoptosis, caspase activation and mitochondrial membrane depolarisation. In further studies, the anti-apoptotic gene bcl-2 was over-expressed in HepZ, resulting in a cell line that was more robust and resistant to death induced by glucose and cystine deprivation and treatment with STS. Bcl-2 did not however show significant cytoprotectivity where apoptosis was stimulated by deprivation of glutamine and serum. Overall, results indicated that although apoptosis can be curbed by use of chemical inhibitors and genetic manipulation, their success is dependent on apoptotic stimuli.

Enhanced stability of heterologous proteins by supramolecular self-assembly

Recently, we reported on the dual function of human ferritin heavy chain (hFTN-H) used for the fusion expression and solubility enhancement of various heterologous proteins: (1) high-affinity interaction with HSP70 chaperone DnaK and (2) formation of self-assembled supramolecules with limited and constant sizes. Especially the latter, the self-assembly function of hFTN-H is highly useful in avoiding the undesirable formation of insoluble macroaggregates of heterologous proteins in bacterial cytoplasm. In this study, using enhanced green fluorescent protein (eGFP) and several deletion mutants of Mycoplasma arginine deiminase (ADI(132-410)) as reporter proteins, we confirmed through TEM image analysis that the recombinant fusion proteins (hFTN-H::eGFP and hFTN-H::ADI(132-410)) formed intracellular spherical particles with nanoscale diameter ( approximately 10 nm), i.e., noncovalently cross-linked supramolecules. Surprisingly, the supramolecular eGFP and ADI showed much enhanced stability in bioactivity. That is, the activity level was much more stably maintained for the prolonged period of time even at high temperature, at high concentration of Gdn-HCl, and in wide range of pH. The stability enhancement by supramolecular self-assembly may make it possible to utilize the protein supramolecules as novel means for drug delivery, enzymatic material conversion (biotransformation), protein chip/sensor, etc. where the maintenance of protein/enzyme stability is strictly required.

Proteomic Characterization of Messenger Ribonucleoprotein Complexes Bound to Nontranslated or Translated Poly(A) mRNAs in the Rat Cerebral Cortex

Receptor-triggered control of local postsynaptic protein synthesis plays a crucial role for enabling long lasting changes in synaptic functions, but signaling pathways that link receptor stimulation with translational control remain poorly known. Among the putative regulatory factors are mRNA-binding proteins (messenger ribonucleoprotein, mRNP), which control the fate of cytosolic localized mRNAs. Based on the assumption that a subset of mRNA is maintained in an inactive state, mRNP-mRNA complexes were separated into polysome-bound (translated) and polysome-free (nontranslated) fractions by sucrose density centrifugation. Poly(A) mRNA-mRNP complexes were purified from a postmitochondrial extract of rat cerebral cortex by oligo(dT)-cellulose affinity chromatography. The mRNA processing proteins were characterized, from solution, by a nanoflow reverse phase-high pressure liquid chromatography-mu-electrospray ionization mass spectrometry. The majority of detected mRNA-binding proteins was found in both fractions. However, a small number of proteins appeared to be fraction-specific. This subset of proteins is by far the most interesting because the proteins are potentially involved in controlling an activity-dependent onset of translation. They include transducer proteins, kinases, and anchor proteins. This study of the mRNP proteome is the first step in allowing future experimentation to characterize individual proteins responsible for mRNA processing and translation in dendrites.

Arginine deiminase enhances dexamethasone-induced cytotoxicity in human T-lymphoblastic leukemia CCRF-CEM cells

Since arginine deiminase (ADI; EC 3.5.3.6) inhibits cell proliferation by arresting cells in the G1 phase, we tested its synergistic effect on cell death induced by dexamethasone (DEX), which also induces apoptosis by G1 cell cycle arrest. ADI inhibited cell proliferation and induced apoptosis in human leukemic CEM cells in a dose-dependent manner. Simultaneous treatment with ADI and DEX showed synergistic effects on DNA fragmentation and LDH release. In addition, ADI exerted its anti-proliferative activity against DEX-resistant CEM cells. ADI suppressed expression of c-myc, a potential key regulator of cell proliferation and apoptosis, and increased expression of p27Kip1 cyclin-dependent kinase inhibitor. These results suggest that ADI efficiently increases the anti-cancer effect of DEX on human leukemic CEM cells through G1 cell cycle arrest involving downregulation of c-myc and upregulation of p27Kip1.

Amino acid transporters: roles in amino acid sensing and signalling in animal cells

Amino acid availability regulates cellular physiology by modulating gene expression and signal transduction pathways. However, although the signalling intermediates between nutrient availability and altered gene expression have become increasingly well documented, how eukaryotic cells sense the presence of either a nutritionally rich or deprived medium is still uncertain. From recent studies it appears that the intracellular amino acid pool size is particularly important in regulating translational effectors, thus, regulated transport of amino acids across the plasma membrane represents a means by which the cellular response to amino acids could be controlled. Furthermore, evidence from studies with transportable amino acid analogues has demonstrated that flux through amino acid transporters may act as an initiator of nutritional signalling. This evidence, coupled with the substrate selectivity and sensitivity to nutrient availability classically associated with amino acid transporters, plus the recent discovery of transporter-associated signalling proteins, demonstrates a potential role for nutrient transporters as initiators of cellular nutrient signalling. Here, we review the evidence supporting the idea that distinct amino acid "receptors" function to detect and transmit certain nutrient stimuli in higher eukaryotes. In particular, we focus on the role that amino acid transporters may play in the sensing of amino acid levels, both directly as initiators of nutrient signalling and indirectly as regulators of external amino acid access to intracellular receptor/signalling mechanisms.

Single amino acid (arginine) deprivation: rapid and selective death of cultured transformed and malignant cells

The effects of arginine deprivation (-Arg) has been examined in 26 cell lines. Less than 10% of those with transformed or malignant phenotype survived for > 5 days, and many died more rapidly, notably leukaemic cells. Bivariate flow cytometry confirmed that vulnerable cell lines failed to move out of cell cycle into a quiescent state (G0), but reinitiated DNA synthesis. Many cells remained in S-phase, and/or had difficulty progressing through to G2 and M. Two tumour lines proved relatively 'resistant', A549 and MCF7. Although considerable cell loss occurred initially, both lines showed a 'cell cycle freeze', in which cells survived for > 10 days. These cells recovered their proliferative activity in +Arg medium, but behaved in the same manner to a second -Arg episode as they did to the first episode. In contrast, normal cells entered G0 and survived in -Arg medium for several weeks, with the majority of cells recovering with predictable kinetics in +Arg medium. In general, cells from a wide range of tumours and established lines die quickly in vitro following -Arg treatment, because of defective cell cycle checkpoint stringency, the efficacy of the treatment being most clearly demonstrated in co-cultures in which only the normal cells survived. The findings demonstrate a potentially simple, effective and non-genotoxic strategy for the treatment of a wide range of cancers.

Single amino acid (arginine) deprivation induces G1 arrest associated with inhibition of cdk4 expression in cultured human diploid fibroblasts

Withdrawal of a single amino acid (arginine) from freely cycling early passage primary human fibroblasts caused a halt to proliferation, characterized by an accumulation of cells in the G1 phase of the cell cycle. This arrest was accompanied by the suppression of cyclin D1- and cyclin E-associated kinase activities and the appearance of hypophosphorylated retinoblastoma protein. Arginine-deprived cells remained viable for in excess of 4 days and could be made to synchronously reenter the cell cycle by restoration of the amino acid, with kinetics characteristic of exit from a quiescent state. Stimulation of cells arrested by serum withdrawal did not result in S-phase entry when arginine was omitted from the culture medium. Although cyclin D1 accumulated on normal schedule, cdk4, which increased following restimulation in amino acid-replete medium, was not induced when arginine was absent. These results suggest that arginine deprivation-in common with other "suboptimal" conditions-inhibits the passage of normal human cells through the restriction point and implicate cdk4 as the key regulatory element in amino acid-sensitive cell cycle control.

In hybridoma cultures, deprivation of any single amino acid leads to apoptotic death, which is suppressed by the expression of the bcl-2 gene

The transfection of murine hybridomas with the apoptosis suppressor gene bcl-2 has been reported to result in the extension of batch culture duration, leading to significant improvements in culture productivity. In the present study, the effect of deprivation, individually, of each amino acid found in culture medium was examined to characterize the chemical environment of the culture in terms of its propensity to induce apoptosis. When cells were deprived of each amino acid, individually for 48 h, the majority of cell deaths in each case occurred by apoptosis, with essential amino acids being clearly most effective. For nearly all the amino acids, the viability of the bcl-2 cell line cultures was greater than 70% after 48 h, representing a substantial improvement in viability over control cell line cultures. Time course studies revealed that the induction of death could be divided into two phases. Initially, following the deprivation of a single essential amino acid, there was a period of time during which all the control cell line cultures retained high viability. The duration of this phase varied from 15 h in the case of lysine deprivation, through to 40 h in the case methionine deprivation. In the second phase of deprivation, the cultures exhibited an abrupt and rapid collapse in viability. The time taken for the viability to fall to 50% was similar for each amino acid. In every case, the duration of both phases of the bcl-2 cultures was considerably extended. Specific utilization rates were increased during the control cultures relative to the bcl-2 cultures for both the growth phase (ranging between 2% and 57% higher than the bcl-2 cultures) and the death phase (ranging between 172% to 1900% higher than the bcl-2 culture).

The guanine nucleotide-exchange factor, eIF-2B

Eukaryotic initiation factor eIF-2B catalyses the exchange of guanine nucleotides on another translation initiation factor, eIF-2, which itself mediates the binding of the initiator Met-tRNA to the 40S ribosomal subunit during translation initiation. eIF-2B promotes the release of GDP from inactive [eIF-2.GDP] complexes, thus allowing formation of the active [eIF-2.GTP] species which subsequently binds the Met-tRNA. This guanine nucleotide-exchange step, and thus eIF-2B activity, are known to be an important control point for translation initiation. The activity of eIF-2B can be modulated in several ways. The best characterised of these involves the phosphorylation of the alpha-subunit of eIF-2 by specific protein kinases regulated by particular ligands. Phosphorylation of eIF-2 alpha leads to inhibition of eIF-2B. This mechanism is involved in the control of translation under a variety of conditions, including amino acid deprivation in yeast (Saccharomyces cerevisiae) where it causes translational upregulation of the transcription factor GCN4, and in virus-infected animal cells, where it involves a protein kinase activated by double-stranded RNA. There is now also growing evidence for direct regulation of eIF-2B. This appears likely to involve the phosphorylation of its largest subunit. Under certain circumstances eIF-2B may also be regulated by allosteric mechanisms. eIF-2B is a heteropentamer (subunits termed alpha, beta, gamma, delta and epsilon) and is thus more complex than most other guanine nucleotide-exchange factors. The genes encoding all five subunits have been cloned in yeast (exploiting the GCN4 regulatory system): all but the alpha appear to be essential for eIF-2B activity. However, this subunit may confer sensitivity to eIF-2 alpha phosphorylation. cDNAs encoding the alpha, beta, delta and epsilon subunits have been cloned from mammalian sources. There is substantial homology between the yeast and mammalian sequences. Attention is now directed towards understanding the roles of individual subunits in the function and regulation of eIF-2B.

Extensive accumulation of an extracellular L-amino-acid oxidase during gametogenesis of Chlamydomonas reinhardtii

In a previous study [Bulté, L. & Wollman, F.-A. (1992) Eur. J. Biochem. 204, 327-336], we identified a novel gamete-specific polypeptide of Chlamydomonas reinhardtii, M alpha. This 66-kDa polypeptide reacts with antibodies to cytochrome f and accumulates in gametes only in conditions that promote destabilisation of the cytochrome b6/f complex. Here, we show that M alpha is not a modification product of cytochrome f, but is part of protein M, a high-molecular-mass L-amino-acid oxidase located in the periplasm. It catalyzes oxidation of all L-amino acids tested, except cysteine. Using phenylalanine as a substrate, saturation of the enzymatic rate is reached at 2 microM. These characteristics suggest that protein M may operate in vivo as an efficient scavanger of ammonium from extracellular amino acids. The enzyme contains non-covalently bound FAD. It exists in two forms with essentially similar enzymatic properties, of 1.2-1.3 MDa and 0.9-1.0 MDa, respectively. The lighter form is an oligomer of M alpha, while the heavier form contains, in addition to M alpha, a second polypeptide of 135 kDa, M beta, in a molar ratio of 3-4 M alpha/M beta. Both polypeptides are glycosylated.

In vitro effect of extracellular AMP on MCF-7 breast cancer cells: inhibition of glycolysis and cell proliferation

MCF-7 human breast cancer cells propagated in vitro were treated with adenosine derivatives added to the culture medium. The effects on cell proliferation, glycolysis, and glutaminolysis were investigated. Of all adenosine derivatives tested, AMP was the most efficient inhibitor of cell proliferation. In AMP-treated cells, DNA synthesis decreased, whereas RNA and protein syntheses rose normally with time. In terms of carbohydrate metabolism, lactate production from glucose was drastically reduced; therefore, most of lactate produced must have been derived from glutamine. Increases in the enzyme activities involved in glutamate degradation and in the malate-aspartate shuttle were observed. In contrast, actual glycolytic flux rates declined, whereas key glycolytic enzyme activities increased. Metabolites such as fructose 1,6-bisphosphate and pyruvate accumulated in AMP-arrested cells. Based on the lowered NAD level in the AMP-treated cells, lactate dehydrogenase, but not malate dehydrogenase, was impaired; thereby the whole of glycolysis was inhibited. In compensation, glutamine catabolism was increased. NAD concentrations fell drastically because of the known inhibition of P-ribose-PP synthesis through heightened intracellular AMP levels. A hypothetical metabolic scheme to explain these results and to show how extracellular AMP may influence carbohydrate metabolism and cell proliferation is presented.