* This work was supported by National Institutes of Health Grant R01 DK53307-01 (to M. H.), National Research Initiative/United States Department of Agriculture Grant 35200-10639 (to M. H.), and National Institutes of Health Graduate Student Fellowship 5T32 DK07319 (to J. F.).The costs of publication of this article were defrayed in part by the payment of page charges. The article must therefore be hereby marked “advertisement” in accordance with 18 U.S.C. Section 1734 solely to indicate this fact.
Adaptation to amino acid deficiency is critical for cell survival. In yeast, this adaptation involves phosphorylation of the translation eukaryotic initiation factor (eIF) 2α by the kinase GCN2. This leads to the increased translation of the transcription factor GCN4, which in turn increases transcription of amino acid biosynthetic genes, at a time when expression of most genes decreases. Here it is shown that translation of the arginine/lysine transporter cat-1 mRNA increases during amino acid starvation of mammalian cells. This increase requires both GCN2 phosphorylation of eIF2α and the translation of a 48-amino acid upstream open reading frame (uORF) present within the 5′-leader of the transporter mRNA. When this 5′-leader was placed in a bicistronic mRNA expression vector, it functioned as an internal ribosomal entry sequence and its regulated activity was dependent on uORF translation. Amino acid starvation also induced translation of monocistronic mRNAs containing the cat-1 5′-leader, in a manner dependent on eIF2α phosphorylation and translation of the 48-amino acid uORF. This is the first example of mammalian regulation of internal ribosomal entry sequence-mediated translation by eIF2α phosphorylation during amino acid starvation, suggesting that the mechanism of induced Cat-1 protein synthesis is part of the adaptive response of cells to amino acid limitation.
eukaryotic initiation factor
open reading frame
upstream open reading frame
Dulbecco's modified Eagle's medium
fetal bovine serum
internal ribosome entry site
IRES-specific trans-acting factor
Amino acid starvation of yeast and mammalian cells induces phosphorylation of the translation initiation factor eIF2α,1 altering the pattern of gene expression to remedy the stress and conserve resources for the starving cells (
). An adaptive response to amino acid starvation for any single amino acid has been extensively characterized in yeast; translation of the transcription factor GCN4 increases during total or single amino acid starvation, causing a transcriptional induction of the amino acid biosynthetic genes (
). Given that essential amino acids have to be transported into cells via membrane-spanning amino acid transporter proteins, part of the adaptive response involves the increased expression of amino acid biosynthetic and transporter genes (
). A significant part of this adaptive response is the increased expression of the cationic amino acid transporter 1 gene (cat-1), the main transporter of the essential amino acids arginine and lysine (
) vector, at the SalI/NcoI sites. The cat-1 5′-UTRf mutants were generated by PCR-directed mutagenesis. The mutf-stop was generated by mutating the uORF stop codon (Fig. 1A, TGA to TTA within thecat-1 5′-UTRf) thus placing the uORF/ATG in frame with the LUC start codon. The mutf-start was generated by mutating the uORF/ATG from ATG to TTG within thecat-1 5′-UTRf and cat-15′-UTRt DNAs. The cat-1 5′-UTRf andcat-1 5′-UTRt have an NcoI site at the 3′ end, which contains the start codon ATG for the LUC cistron. The context of the A+1TG LUC/start codon within the mutf-stop and mutf-start vectors is AGCGCCCA+1TG (compare this sequence with Fig.1A). The monocistronic expression vectors, cat-15′-UTRf/LUC and cat-15′-UTRmutf/LUC, were generated by cloning theSalI/XbaI cat-15′-UTRf/LUC and cat-1 5′-UTRmutf/LUC fragments from the bicistronic vectors into theEcoRI/XbaI site of the pUHD10–3 vector. In this vector, transcription is directed by the minimal cytomegalovirus promoter (
). Expression vectors for the PERK, PERK-mut, GCN2, eIF2α S-A, and eIF2α S-D were kindly provided by D. Ron (New York University School of Medicine, New York, NY). All vectors contained the corresponding inserts within theXbaI/HindIII site of pCDNA3. In this vector, transcription is directed by the cytomegalovirus promoter and the selectable marker gene was replaced with the CD2 cell surface marker gene (
). Treatments were performed by culturing cells (5 × 105 cells/35-mm dish) for 48 h in growth medium followed by culture under fed or starved conditions for the appropriate times. Thapsigargin was added to 400 nm as described (
). Stable C6-GCN2-mut cell lines were generated by transfecting the GCN2-mut-containing expression vector and aneo-expressing vector in a molar ratio of 10:1. Mass cultures were generated from cells selected in 0.1% G418.
RNA Analysis, Enzymatic Assays, and Evaluation of eIF2αs
RNA was isolated from transfected cells with the bicistronic mRNA expression vectors and analyzed by Northern blotting using a LUC-specific hybridization probe. The correct size RNAs for the full-length bicistronic transcripts were observed for all vectors tested in this study. Cell extracts were prepared and analyzed for LUC (Tropix luciferase assay kit) and CAT activities as described previously (
). Briefly, cells at 36 h after transfection were incubated with [35S]methionine (100 μCi/ml) and cell extracts were prepared in RIPA buffer. Equal amounts of protein extracts were analyzed on an SDS-acrylamide gel and prepared for autoradiography. Details of the phosphorylation status of the eIF2αs and regulation of translation have been described (
). To further study this regulation, a full-length 5′-UTR of the cat-1 mRNA was obtained by isolating a rat genomic DNA clone that contained the promoter region, the first exon, and the first intron of thecat-1 gene.
J. Fernandez, R. Mishra, A. Aulak, D. Robinson, and M. Hatzoglou, manuscript in preparation.
The transcription start site was determined in C6 cells (data not shown). This full-length UTR (Fig. 1A,cat-1 5′-UTRf) consists of exon 1 (154 nucleotides, in blue), exon 2 (98 nucleotides, in red), and the 5′ end of exon 3 (18 nucleotides, in red). An interesting feature of thecat-1 5′-UTR is the presence of a uORF. Bothcat-1 5′-UTRt and cat-15′-UTRf contained a 48-amino acid uORF, starting 46 bases 3′ to the mRNA cap site (Fig. 1, A andB).
The ability of the cat-1 5′-UTRf sequence to mediate internal translation initiation was tested in a bicistronic expression vector (
). The second cistron, encoding the firefly LUC enzyme, is efficiently translated only if it is preceded by an IRES in the intercistronic spacer region (Fig. 2A). Translation of the second cistron should be independent of translation of the first cistron. A vector that contained a stable RNA hairpin (hp) upstream of the CAT cistron (Fig. 2A) was used to demonstrate that when translation of the CAT cistron is inhibited, translation of the LUC cistron is unaffected. Two bicistronic expression vectors were generated by introducing the cat-1 5′-UTRf into the intercistronic space of the empty expression vector: CAT/cat-1 5′-UTRf/LUC and hpCAT/cat-15′-UTRf/LUC (Fig. 2A). The LUC/CAT ratio was 20-fold higher for the hpCAT/cat-1 5′ UTRf/LUC when compared with the CAT/cat-1 5′-UTRf/LUC. C6 cells were transiently transfected with these vectors, and LUC and CAT activities were analyzed. This increase was because of decreased CAT and sustained LUC activities in the hairpin-containing vector (data not shown). These data demonstrated that the full-length cat-15′-UTRf leader has IRES activity. Transfection of these vectors into C6 cells demonstrated that translational control mediated by the full-length cat-1 5′-UTRf was indistinguishable (data not shown) to the previously tested truncatedcat-1 5′-UTRt (
). Interestingly, the kinetics of induction of IRES-mediated translation during amino acid starvation were different from the kinetics of induction of eIF2α phosphorylation. Phosphorylation of eIF2α transiently increased during the first hour of amino acid starvation (
In yeast, uncharged tRNAs, which accumulate in cells depleted by any single amino acid, bind to and activate the yeast GCN2 cellular kinase, leading to phosphorylation of eIF2α and translational control (
). To gain an insight on the involvement of eIF2α phosphorylation on the increased cat-1 mRNA translation during amino acid starvation, the regulation of cat-1 IRES-mediated translation by individual amino acids was studied in C6 cells. Depletion of any single essential amino acid increased cat-1IRES activity with the same kinetics and to the same degree as described for total amino acid starvation (Fig. 2B and TableI). This is in agreement with translational control of the GCN4 mRNA by amino acid depletion (
We therefore tested whether eIF2α phosphorylation and mammalian GCN2 kinase activation are involved in cat-1 IRES activation during amino acid starvation. To test this hypothesis, C6 cells were transfected with the bicistronic expression vector containing thecat-1 5′-UTRt along with expression vectors expressing either the wild type mammalian GCN2 kinase (
), which allowed us to monitor expression of the mutant proteins by Western blot analysis (data not shown). Following 36 h of transfection, cells were incubated in either amino acid-containing (F) or amino acid-depleted (S) medium for 9 h as described previously (
). As shown, expression of the mutant kinase inhibitedcat-1 IRES activation by amino acid starvation (Fig.2C, compare the -fold change of the ratio LUC/CAT). Induction in LUC activity was 9-fold for control, 15-fold for the GCN2 wild type, and 2-fold for GCN2 mutant-expressing cells (Table I). The increases in LUC/CAT ratios were 13-, 21-, and 2.5-fold respectively (Fig. 2C and Table I). The CAT activity decreased by 30% (Table I). When the bicistronic cat-15′-UTRf-containing mRNA was tested, regulation of translation of the LUC cistron was indistinguishable from thecat-1 5′-UTRt (data not shown). It was concluded that cat-1 IRES translational activation by amino acid starvation depends on GCN2 activity. However, these experiments were transient transfections and did not allow the evaluation of the degree of eIF2α phosphorylation by amino acid starvation in cells expressing the mutant GCN2 kinase. This was demonstrated in a stable C6 cell line that expressed the mutant GCN2 kinase (Fig.3A). Amino acid starvation of this cell line caused a small increase in phospho-eIF2α levels (phospho-eIF2α levels increased 3-fold in control (data not shown) and 1.4-fold in GCN2-mut cells). However, cat-1IRES-mediated translation did not increase (Fig. 3B and Table II). The ability of eIF2α to be phosphorylated in the GCN2-mut cells was tested by treating them with the endoplasmic reticulum (ER) stress-causing agent thapsigargin. As expected (
), thapsigargin treatment of cells induced eIF2α phosphorylation by 3-fold (Fig. 3A). The expression of the GCN2 mutant kinase decreased during the time course of amino acid starvation, as was expected, because of the decreased cap-dependent translation of the GCN2-mut mRNA (Fig.3A). 4E-BP-1 is dephosphorylated during the time course of amino acid starvation, thus decreasing cap-dependent translation by sequestering the cap-binding protein eIF4E (Fig.3A). However, expression of the mutant kinase is critical the first hour of amino acid starvation when the endogenous GCN2 kinase is activated (
) and phosphorylates eIF2α. At 1 h of amino acid starvation, the mutant kinase was expressed at the same level as in amino acid-fed cells (Fig. 3A). These data further demonstrate that decreasing cap-dependent translation is not the cause for increased cat-1 IRES-activation. This is in agreement with our previous finding that treatment of cells with rapamycin did not alter cat-1 IRES-mediated translation (
). We therefore conclude that eIF2α phosphorylation by the GCN2 kinase is required for increased cat-1 IRES-mediated translation by amino acid starvation. This finding is in agreement with the regulation of translation of the yeast GCN4 mRNA by the yeast GCN2 kinase (
). To determine the specific requirement for GCN2, the ER stress-induced ER-resident transmembrane kinase PERK (16)was tested. We transfected C6 cells with the bicistronic mRNA expression vector that contained the cat-1 5′-UTRt along with expression vectors expressing either the wild type PERK (
). As with GCN2, overexpression of the mutant kinase forms dimers with the wild type PERK; however, this should not affect the activation of GCN2 by amino acid starvation. The expression of the mutant kinase was monitored by Western blot analysis of cell extracts prepared from transfected cells using an antibody against a Myc epitope (
) that was contained at the NH2 terminus of the kinase (data not shown). The levels of the mutant PERK kinase were sustained at the same level for the first 6 h of amino acid starvation and gradually declined thereafter (data not shown). Following 36 h of transfection, cells were incubated in either amino acid-fed or amino acid-starved medium for 9 h as described previously (
). Expression of the PERK kinases did not affect induction of cat-1 IRES activity by amino acid starvation (Fig. 2C, compare the -fold change of the ratio LUC/CAT in PERK and PERK-mut cells). Amino acid starvation increased the LUC activity by 8-fold and decreased the CAT activity by 35 and 25%, respectively, in PERK wild type- and PERK mutant-expressing cells (Table I). The LUC/CAT ratio increased 12-fold for the PERK and 11-fold for the PERK-mut cells (Fig. 2C). It should be noted that the expression of the mutant PERK kinase decreased basal cat-1 IRES activity. This was because of a decrease in basal eIF2α phosphorylation levels in these cells (data not shown).
The involvement of the PKR eIF2α kinase was tested next. PKR is activated by binding double-stranded RNA (
). To address the role of the PKR kinase, mouse embryo fibroblasts that either expressed the endogenous PKR kinase (PKR+/+) or had the kinase inactivated (PKR0/0) by homologous recombination were used (
). cat-1 IRES-mediated translation was measured following 9 h of amino acid starvation. The basal activity for LUC and CAT were similar in PKR+/+ and PKR0/0 cells (Fig.2C and Table I). Amino acid starvation of cells induced the LUC activity by 4-fold and decreased the CAT by 40% in both cell lines (Fig. 2C and Table I). The LUC/CAT ratio increased 6-fold in the PKR+/+ and 5.5-fold in the PKR0/0 cells (Fig. 2C). The increase of cat-1 IRES activity was lower than C6 cells (4-fold as compared with 7-fold in C6 cells). The lower induction in PKR cells may be because of cell type differences. The latter is supported by the fact that the basal LUC activity in PKR cells was 4-fold higher than the equivalent in C6 cells (Table I). These data suggest that eIF2α phosphorylation andcat-1 IRES translational induction during amino acid starvation depend on GCN2 activation.
To further demonstrate that phosphorylation of eIF2α is required for cat-1 IRES activation, we overexpressed an eIF2α that can either not be phosphorylated (Ser-51 changed to Ala) or mimics the phosphorylated protein (Ser-51 changed to Asp), and we assessed the translational control of the cat-1 IRES. Stress-induced phosphorylation of eIF2α occurs at Ser-51. It has been shown previously that overexpression of the variant Ser-51 to Ala eIF2α (S-A), in COS cells in vivo affects translation by converting the endogenous eIF2 complex to a variant form (
). Furthermore, substitution of Ser-51 by Asp (S-D) mimics the phosphorylated eIF2α and if overexpressed in amino acid-fed cells, it would be expected to mimic amino acid starvation. C6 cells were transfected with the bicistronic expression vector that contained the cat-1 5′-UTRf, along with expression vectors expressing either the eIF2α S-A or S-D. Efficient transfection of the eIF2αs was monitored by detecting expression of the CD2 cell surface protein, which was included in the expression vector (data not shown). The expression of the eIF2α S-A and S-D proteins in C6 cells was determined as described under “Experimental Procedures.” In the presence of amino acids, expression of the eIF2α S-A had no effect on translational activity of the IRES (Fig.4A and TableIII). By comparison, expression of the eIF2α S-D increased IRES-mediated translation to even higher levels (Fig. 4A, S-D, LUC/CAT: 16-fold) than observed by amino acid starvation (Fig. 2C, con, LUC/CAT: 13-fold). Amino acid starvation of cells expressing the eIF2α S-D did not increase IRES activity further (data not shown). Expression of the eIF2α S-A was tested next, to see whether it would impair induction of translation mediated by the cat-1 5′-UTRfduring amino acid starvation. As shown, overexpression of the eIF2α S-A impaired translational activation by amino acid starvation (Fig.4B and Table III). We conclude that eIF2α phosphorylation regulates in an indirect manner cat-1 IRES-mediated translation.
Table IIIAbsolute values of LUC and CAT activities from experiments described in Fig. 4
Translation of the uORF and eIF2α Phosphorylation Are Required for cat-1 IRES Activation by Amino Acid Starvation
What is the mechanism by which eIF2α phosphorylation leads to induction ofcat-1 mRNA translation? Given the importance of upstream ORFs in the regulation of translation of the GCN4 expression in yeast (
), we hypothesized that the 48-amino acid uORF may play a role in cat-1 IRES activation by amino acid starvation. The presence of uORFs in mRNAs is believed to regulate translation of the downstream ORF (
). To determine the importance of the uORF incat-1 IRES-mediated translation, the regulation ofcat-1 IRES activity by amino acid starvation was tested in bicistronic mRNAs that either contained the uORF or a uORF having a mutated initiation codon (AUG was mutated to TTG, in both thecat-1 5′-UTRt and the cat-15′-UTRf bicistronic expression vectors). C6 cells were transfected independently with the vectors CAT/cat-1 5′ UTRt/LUC (data not shown), CAT/cat-15′-UTRmutt-start/LUC (data not shown), CAT/cat-15′-UTRf/LUC, and CAT/cat-15′-UTRmutf-start/LUC (Fig. 4C). The media were changed 36 h after transfection to either amino acid-containing (F) or amino acid-depleted (S) and analyzed at different times (Fig.4C, the 9-h time point is shown). Mutation of the uORF abolished induction of the LUC activity, in both the full-length (Fig.4C and Table III) or the cat-15′-UTRt (data not shown). To further support the data that the uORF is important in cat-1 IRES-mediated translation, the TGA stop codon was mutated to TTA within the CAT/cat-15′-UTRf/LUC vector (CAT/cat-15′-UTRmutf-stop/LUC). In this vector translation initiating at the AUG of the uORF was in frame with the LUC/AUG (see “Experimental Procedures”), generating a fusion protein with LUC (Fig. 4C, inset). The presence of both LUC and fusion LUC proteins in mutf-stop cells indicates that translation initiates at both the uORF/AUG and the LUC/AUG (see “Discussion”). This mutation abolished induction by amino acid starvation (Fig. 4C and Table III). Therefore, the presence and translation of the 48-amino acid uORF appears to be required for induction of translation mediated by the cat-1 IRES. We have shown earlier that expression of the eIF2α S-D protein into C6 cells increased cat-1 IRES-mediated translation independent of amino acid availability (Fig. 4A).
We therefore tested if the eIF2α S-D-dependent induction required translation of the uORF. The cat-15′-UTRmutf-start vector was cotransfected with the eIF2α S-D expression vector, and the LUC and CAT activities were measured. The LUC/CAT ratio did not increase with overexpression of the variant eIF2α S-D (Fig. 4A (right panel) and Table III). Similar to cat-1 5′-UTRf, thecat-1 5′-UTRmutf-mediated translation was not induced by overexpression of the eIF2α S-A (Fig. 4A,right panel). It is concluded that the eIF2α phosphorylation-dependent increase in IRES-mediated translation requires the translation of the uORF.
Amino Acid Starvation Induces Translation Mediated by the cat-1 5′-Leader in Monocistronic mRNAs via a Mechanism That Involves eIF2α Phosphorylation
The results described in Figs. Figure 1, Figure 2, Figure 3, Figure 4 demonstrate that the IRES activity of thecat-1 5′-leader increases in amino acid-depleted cells in a manner dependent on eIF2α phosphorylation and translation of the 48-amino acid uORF. Translation of the uORF within the bicistronic mRNA occurs via an IRES at the 5′ end of the leader (data not shown). Because the authentic cat-1 mRNA is monocistronic, we determined the eIF2α and uORF-dependent regulation of translation mediated by the full-length leader in monocistronic mRNAs. Previous studies using the truncatedcat-1 5′-leader in monocistronic mRNAs suggested that translation of the uORF plays a role in translational induction by amino acid starvation (
). Expression vectors for monocistronic mRNAs were constructed that contained either the cat-15′-UTRf/LUC (Fig. 2A) or the cat-15′-UTRmutf/LUC (Figs. 1 and 2A) at position +76 from the transcription start site of the CMV promoter (see “Experimental Procedures”). Therefore, the uORF of thecat-1 5′-leader in these monocistronic mRNAs was 122 nucleotides from the 5′ cap site of the mRNA. It should be noted that the translation of the uORF within the monocistronic mRNAs can be cap-dependent, IRES-mediated, or both. It is expected that the translation rates of the uORF in monocistronicversus bicistronic mRNAs will be different. Induction of the cat-1 5′-UTRf-mediated translation during amino acid starvation may be lower in monocistronic versusbicistronic mRNAs. C6 cells were transfected independently with the monocistronic mRNA expression vectors alone, or with expression vectors for eIF2α S-A. Amino acid starvation induced LUC activity by 3.5-fold in cat-1 5′-UTRf/LUC-expressing cells, after 9 h of amino acid starvation. This induction was abolished in cells expressing the eIF2α S-A (Fig.5). These data are in agreement with data obtained with bicistronic mRNAs (Fig. 2C). The effect of expression of eIF2α S-D in amino acid-fed C6 cells oncat-1 5′-UTRf-mediated translation in monocistronic mRNAs was determined next. Similar to data obtained with bicistronic mRNAs, translation mediated by thecat-1 5′-leader in monocistronic mRNAs increased in cells expressing eIF2α S-D (Fig. 5). In agreement with our findings in bicistronic mRNAs, translation of the cat-15′-UTRmutf-start/LUC mRNA did not increase by amino acid starvation or eIF2α phosphorylation (Fig. 5). In agreement with the bicistronic mRNA studies (Fig. 3), no induction of LUC activity was observed in C6-GCN2-mut cells transfected with the cat-15′-UTRf/LUC vector (data not shown). These data suggest that the IRES element within the 5′-leader of the cat-1mRNA is responsible for increased translation of thecat-1 mRNA during amino acid starvation and eIF2α phosphorylation.
It has been shown here that the activity of an IRES element within the mRNA 5′-leader sequence of the arginine/lysine transportercat-1 increases during amino acid starvation. This increase depends both on eIF2α phosphorylation by GCN2 and translation of a 48-amino acid uORF within the leader.
The GCN2 kinase functions in the general amino acid control pathway of yeast Saccharomyces cerevisiae. Starvation for any single amino acid activates GCN2, which in turn phosphorylates eIF2α, causing a decrease in eIF2·GTP·Met-tRNAMet ternary complexes. Interestingly, translation of the GCN4 mRNA increases at the time ternary complexes decrease and global translation initiation is inhibited (
). Both of these examples of translational control by amino acid starvation involve direct induction of translation by eIF2α phosphorylation.
The translational regulation of the cat-1 mRNA by amino acid starvation has some features similar to the translational control of the GCN4 and ATF4 mRNAs. We have clearly shown that GCN2-dependent phosphorylation of eIF2α is required for increased translation of the cat-1 mRNA during amino acid starvation.
An interesting finding of the current study was that amino acid limitation in the absence of eIF2α phosphorylation is ineffective in inducing cat-1 IRES-mediated translation. Two pieces of data support this conclusion: (i) induction of cat-1 IRES activity was abolished by the expression of the eIF2α S-A during amino acid starvation and (ii) amino acid starvation did not inducecat-1 IRES-mediated translation in the C6-GCN2-mut cells. The importance of eIF2α phosphorylation on cat-1 IRES activation was further demonstrated by the finding that expression of the eIF2α S-D that mimics the phosphorylated eIF2α variant increases cat-1 IRES-mediated translation in the presence of amino acids. This increase was higher than the increase caused by amino acid starvation. We conclude that eIF2α phosphorylation is the key regulator for cat-1 IRES activation. Our data and our knowledge on amino acid control of mammalian cells (
) and, as shown here, signals the induction of translation of mRNAs that encode proteins essential for survival. The importance of translational control as a signaling pathway in metabolic processes is supported by the recent work of Kaufman and co-workers (
). Furthermore, in contrast to the ATF4 and GCN4mRNAs that require eIF2α phosphorylation at the time of induction, translational induction of the cat-1 IRES activity occurs indirectly. Phosphorylation of eIF2α occurs during the first hour of amino acid starvation, whereas IRES-mediated translation increases at 6–12 h.
How is the cat-1 ORF translated within the bicistronic mRNA? To study exclusively regulation of IRES-mediated translation in this study, we constructed an artificial bicistronic mRNA expression vector where the cat-1 ORF was replaced with the LUC ORF. This bicistronic mRNA allowed us to study IRES-mediated translation independently of cap-dependent translation. The eIF2α-dependent increase in LUC activity caused by amino acid starvation in bicistronic mRNAs can be explained by either increased reinitiation following translation of the uORF or increased IRES-mediated translation.
Is increased cat-1 IRES-mediated translation during starvation caused by translation of the uORF followed by increased reinitiation at the cat-1 ORF? This is possible, because translation of the cat-1 mRNA uORF is required for induction of IRES-mediated translation. Our data do not support this model. As discussed above and described earlier for the GCN4(
) mRNAs, increased reinitiation at thecat-1/ORF should occur in the first hour of amino acid starvation when eIF2·GTP·Met-tRNAMet ternary complexes are low. However, cat-1 IRES-mediated translation of the LUC did not increase during the first 3 h of amino acid starvation. Moreover, by the time that IRES activity increased, eIF2α phosphorylation had fallen to base-line levels.
The mechanism by which eIF2α phosphorylation increasescat-1 IRES-mediated translation is not known. Our working hypothesis is that eIF2α phosphorylation induces the synthesis of a protein that interacts with the IRES, inducing its activity. This is possible, because eIF2α phosphorylation directly induces translation of mRNAs, such as ATF4 (
It has clearly been shown here that translation of the 48-amino acid uORF is essential for the eIF2α-mediated increase in cat-1IRES activity during amino acid starvation in both monocistronic and bicistronic mRNAs. What is the mechanism of this requirement? Models that involve the translation of a small uORF have been described previously (
) have very elegantly demonstrated that peptide-specific attenuation within uORF 2 of the human cytomegalovirus gpUL4 mRNA (gp48) inhibits downstream translation by 10-fold. Interestingly, translation of these uORFs in the gp48 and cat-1 mRNAs is mediated by suboptimal AUGs (and Fig. 1A). Efficient initiation codons contain purines at positions −3 and +4 (with +1 being the A of A+1UG). This can explain that, when the uORF/AUG was placed in frame with the LUC/AUG (mutf-stop bicistronic mRNA), translation initiation occurred at the LUC/AUG as well as at the uORF/AUG (Fig. 4). However, in all known examples (
) of translation attenuation within a uORF, translation of the downstream cistron is inhibited. In contrast, it is shown here that translation of the uORF is required for increased IRES-mediated translation.
One possible mechanism of stimulation is that translation of thecat-1/uORF resolves a secondary structure at the 5′ end of the cat-1 mRNA leader, allowing a newly synthesized protein to interact with the RNA sequences and form an active IRES. It is shown in Fig. 1 that the 5′ end of the cat-1 leader is involved in a stable RNA structure (ΔG = −98.11). It is therefore possible that the 5′ end of the structure of the uORF RNA sequence has an inhibitory role on cat-1 IRES activation and translation of the uORF relieves this inhibition. Our model proposes the following translational control mechanism for the naturalcat-1 mRNA. Translation of the uORF within thecat-1 mRNA leader in amino acid fed cells is in part cap-dependent (the GC-rich region at the 5′ end of the leader may inhibit cap-dependent translation, Fig.1A) (
). Under these conditions the cat-1/ORF is translated via (i) leaky scanning of the uORF (the ribosomes do not recognize the uORF/AUG, initiating at the cat-1/ORF/AUG); (ii) reinitiation following translation of the uORF; or (iii) an IRES, acting independently of uORF translation. During amino acid starvation when cap-dependent protein synthesis decreases (
), our model predicts that IRES-mediated translation prevails in a manner dependent on translation of the uORF and eIF2α phosphorylation. This probably occurs via formation of a new IRES or activation of an existing one. Future studies will determine the validity of these translational control models.
The proposed model for regulation of translation mediated by thecat-1 IRES is paralleled by recent findings on regulation of IRES-mediated translation (
). However, induction of the cat-1IRES-mediated translation was not the result of induction of apoptosis (data not shown). Two IRES elements that specifically function during the G2/M phase of the cell cycle were reported for the mRNAs for ornithine decarboxylase (
). This tropism of the fibroblast growth factor 2 IRES further supports the regulation of IRES activity by cell-type specific ITAFs.
We propose that some mammalian mRNAs use IRES-mediated translation to provide cells with proteins essential for survival when the nutrient supply is limited. Among these proteins is the Cat-1 arginine/lysine transporter, which also provides cells with the substrate for NO synthesis (
). The translational regulation described in this study further extends the complex regulatory mechanisms that control the cell's supply of arginine and lysine. Our finding, that arginine and lysine transport increases under nutritional stress, supports further the importance of these amino acids in cell survival.
We thank Dr. A. Shatkin for critical reading of the manuscript. We express our appreciation to Drs. D. Ron and H. Harding for input during the early stages of this project and providing us with the reagents used in this study. We also thank Dr. P. Sarnow for providing us with the bicistronic mRNA expression vectors. We thank D. Robinson for assisting us with the cloning of the 5′-UTRs. We gratefully thank Irene Krukovets for excellent technical assistance.