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Amino acid starvation by asparaginase (ASNase) enhances phosphorylation of eukaryotic initiation factor 2 (eIF2) by general control nonderepressible 2 (GCN2) kinase, leading to reduced global mRNA translation rates. This conserves energy and allows cells time to reprogram stress-related gene expression to alleviate cell injury. This study addressed the importance of GCN2 for the immune system to adapt to amino acid starvation by ASNase. GCN2(+/+) and GCN2(-/-) mice were injected once daily with ASNase or saline for up to 7 d. In both thymus and spleen, activation of amino acid stress response genes to ASNase, such as asparagine synthetase and CAAT enhancer binding protein homologous protein, required GCN2. ASNase reduced food intake and body weight in both genotypes, but spleen and thymus wet weights and total cell numbers in thymus, spleen, bone marrow, and mesenteric lymph nodes were less in GCN2(-/-) mice treated with ASNase (genotype x ASNase, P < 0.05). In the thymus, GCN2(-/-) mice treated with ASNase demonstrated enhanced apoptosis and fewer cells in all subpopulations examined (CD3+, CD4-8-, CD4+8+, CD4+8-, CD4-8+) compared with GCN2(+/+) mice treated with ASNase (genotype x ASNase, P < 0.05). In the spleen, GCN2 deletion magnified ASNase-induced reductions in CD4+ T cells, CD8+ T cells, CD19+ B cells, and CD11b+ leukocytes (genotype x ASNase, P < 0.05). These results indicate that loss of GCN2 enhances immunosuppression by ASNase and that this eIF2 kinase is broadly required for amino acid stress management in the immune system.

Transcription initiation sites of the asparagine synthetase gene were investigated in human hepatoma cells after amino acid limitation by incubation in amino acid-complete minimal essential medium or medium lacking histidine. Cells incubated in complete minimal essential medium had mRNA transcripts with starting positions spanning across the 69 nucleotides immediately upstream of a previously designated transcription start site (+1), whereas the majority of mRNA transcripts started at nucleotide (+1) in cells incubated in histidine-free medium. Similar results were obtained regardless of whether the analysis was by 5' rapid amplification of cDNA ends or a ribonuclease protection assay. Low ASNS mRNA expression in amino acid-complete medium was associated with the wide range of initiation sites, whereas preferred alignment of the general transcription machinery at nucleotide (+1), observed in the amino acid deprived condition, was associated with a concurrent increase in transcription activity. To our knowledge, these results are the first example in a mammalian cell of transcription start selection by nutrient availability.

After amino acid deprivation, the mRNA content for both asparagine synthetase (AS) and the system A transporter ATA2 is increased. The purpose of the reported experiments was to characterize the molecular mechanism for the ATA2 gene and to contrast the ATA2 regulatory characteristics with those of AS. Amino acid limitation was initiated by incubation of HepG2 human hepatoma cells in either amino acid-free Krebs-Ringer bicarbonate buffer or culture medium lacking the single amino acid histidine. For ATA2, like AS, the elevated mRNA content was due to increased transcription. However, there were fundamental differences between the mechanisms for nutrient regulation of the AS and ATA2 genes. When cells were deprived of amino acids, there was a lag period of approximately 4 h before an increase in AS mRNA occurred, whereas the elevation of ATA2 mRNA was readily detectable at 2-4 h. Consistent with these observations, de novo protein synthesis was absolutely required for the activation of the AS gene, but the increase in ATA2 mRNA was largely independent of protein synthesis. Furthermore, in contrast to AS, transcription from the ATA2 gene was not increased by glucose deprivation. Given this lack of ATA2 transcriptional activation by glucose starvation and that the induction of the AS gene by glucose or amino acid starvation is mediated by common genomic elements, it is likely that the ATA2 gene does not contain the same genomic amino acid-responsive cis-elements as the AS gene.

The human genes for C/EBP homology protein (chop) and asparagine synthetase (AS) are model systems to investigate transcription induced by nutrient limitation and endoplasmic reticulum (ER) stress. The genomic cis-elements in the promoters of these two genes that mediate these responses have been identified and partially characterized. Multiple cis-elements are functional in each gene, but differences exist in the molecular mechanisms by which these genes respond to amino acid or glucose deprivation. Whereas chop expression is associated with cell stress and apoptosis, activation of the AS gene by ER stress indicates that asparagine may also be critical for cellular processes other than protein synthesis.

In mammals, plasma concentrations of amino acids (AA) are affected by nutritional or pathologic conditions. Alterations in AA profiles have been reported as a result of a deficiency of any one of the essential AA, a dietary imbalance of AA or an insufficient intake of protein. In recent years, evidence has accumulated that AA availability regulates the expression of several genes involved in the regulation of a number of cellular functions or AA metabolism. Nevertheless, the molecular mechanisms involved in the AA regulation of mammalian gene expression are limited, particularly the signaling pathways mediating the AA response. This work provides a better understanding of the signaling pathways involved in the AA control of gene expression. We studied the expression of C/EBP homologous protein (CHOP) and asparagine synthetase (AS) in response to deprivation of a single AA and investigated the possible link between protein synthesis inhibition due to amino acid limitation and gene expression. We have shown the following: 1) several mechanisms are involved in the AA control of gene expression. When omitted from the culture medium, each AA can activate one (or several) specific signaling pathways leading to the regulation of one specific pattern of genes. 2) AA limitation by itself can induce gene expression independently of a cellular stress due to protein synthesis inhibition. Together, these results suggest that AA control of gene expression involves several specific mechanisms by which one AA (or one group of AA) can activate one signaling pathway and thus alter one specific pattern of gene expression.