Influence of the 5'-untranslated region of ornithine decarboxylase mRNA and spermidine on ornithine decarboxylase synthesis

The functional role of polyamines in eukaryotic cells

Polyamines, consisting of putrescine, spermidine and spermine are essential for normal cell growth and viability in eukaryotic cells. Since polyamines are cations, they interact with DNA, ATP, phospholipids, specific kinds of proteins, and especially with RNA. Consequently, the functions of these acidic compounds and some proteins are modified by polyamines. In this review, the functional modifications of these molecules by polyamines are presented. Structural change of specific mRNAs by polyamines causes the stimulation of the synthesis of several different proteins, which are important for cell growth and viability. eIF5 A, the only known protein containing a spermidine derivative, i.e. hypusine, also functions at the level of translation. Experimental results thus far obtained strongly suggest that the most important function of polyamines is at the level of translation.

Roles of polyamines in translation

Polyamines are organic polycations that bind to a variety of cellular molecules, including nucleic acids. Within cells, polyamines contribute to both the efficiency and fidelity of protein synthesis. In addition to directly acting on the translation apparatus to stimulate protein synthesis, the polyamine spermidine serves as a precursor for the essential post-translational modification of the eukaryotic translation factor 5A (eIF5A), which is required for synthesis of proteins containing problematic amino acid sequence motifs, including polyproline tracts, and for termination of translation. The impact of polyamines on translation is highlighted by autoregulation of the translation of mRNAs encoding key metabolic and regulatory proteins in the polyamine biosynthesis pathway, including S-adenosylmethionine decarboxylase (AdoMetDC), antizyme (OAZ), and antizyme inhibitor 1 (AZIN1). Here, we highlight the roles of polyamines in general translation and also in the translational regulation of polyamine biosynthesis.

Polyamine Metabolism and Gene Methylation in Conjunction with One-Carbon Metabolism

Recent investigations have revealed that changes in DNA methylation status play an important role in aging-associated pathologies and lifespan. The methylation of DNA is regulated by DNA methyltransferases (DNMT1, DNMT3a, and DNMT3b) in the presence of S-adenosylmethionine (SAM), which serves as a methyl group donor. Increased availability of SAM enhances DNMT activity, while its metabolites, S-adenosyl-l-homocysteine (SAH) and decarboxylated S-adenosylmethionine (dcSAM), act to inhibit DNMT activity. SAH, which is converted from SAM by adding a methyl group to cytosine residues in DNA, is an intermediate precursor of homocysteine. dcSAM, converted from SAM by the enzymatic activity of adenosylmethionine decarboxylase, provides an aminopropyl group to synthesize the polyamines spermine and spermidine. Increased homocysteine levels are a significant risk factor for the development of a wide range of conditions, including cardiovascular diseases. However, successful homocysteine-lowering treatment by vitamins (B6, B12, and folate) failed to improve these conditions. Long-term increased polyamine intake elevated blood spermine levels and inhibited aging-associated pathologies in mice and humans. Spermine reversed changes (increased dcSAM, decreased DNMT activity, aberrant DNA methylation, and proinflammatory status) induced by the inhibition of ornithine decarboxylase. The relation between polyamine metabolism, one-carbon metabolism, DNA methylation, and the biological mechanism of spermine-induced lifespan extension is discussed.

Modulation of protein synthesis by polyamines

Polyamines are ubiquitous small basic molecules that play important roles in cell growth and viability. Since polyamines mainly exist as a polyamine-RNA complex, we looked for proteins whose synthesis is preferentially stimulated by polyamines at the level of translation, and thus far identified 17 proteins in Escherichia coli and 6 proteins in eukaryotes. The mechanisms of polyamine stimulation of synthesis of these proteins were investigated. In addition, the role of eIF5A, containing hypusine formed from spermidine, on protein synthesis is described. These results clearly indicate that polyamines and eIF5A contribute to cell growth and viability through modulation of protein synthesis.

Ornithine decarboxylase mRNA is stabilized in an mTORC1-dependent manner in Ras-transformed cells

Upon Ras activation, ODC (ornithine decarboxylase) is markedly induced, and numerous studies suggest that ODC expression is controlled by Ras effector pathways. ODC is therefore a potential target in the treatment and prevention of Ras-driven tumours. In the present study we compared ODC mRNA translation profiles and stability in normal and Ras12V-transformed RIE-1 (rat intestinal epithelial) cells. While translation initiation of ODC increased modestly in Ras12V cells, ODC mRNA was stabilized 8-fold. Treatment with the specific mTORC1 [mTOR (mammalian target of rapamycin) complex 1] inhibitor rapamycin or siRNA (small interfering RNA) knockdown of mTOR destabilized the ODC mRNA, but rapamycin had only a minor effect on ODC translation initiation. Inhibition of mTORC1 also reduced the association of the mRNA-binding protein HuR with the ODC transcript. We have shown previously that HuR binding to the ODC 3'UTR (untranslated region) results in significant stabilization of the ODC mRNA, which contains several AU-rich regions within its 3'UTR that may act as regulatory sequences. Analysis of ODC 3'UTR deletion constructs suggests that cis-acting elements between base 1969 and base 2141 of the ODC mRNA act to stabilize the ODC transcript. These experiments thus define a novel mechanism of ODC synthesis control. Regulation of ODC mRNA decay could be an important means of limiting polyamine accumulation and subsequent tumour development.

Regulation of polyamine metabolism by translational control

Polyamines are low molecular weight, positively charged compounds that are ubiquitous in all living cells. They play a crucial role in many biochemical processes including regulation of transcription and translation, modulation of enzyme activities, regulation of ion channels and apoptosis. A strict balance between synthesis, catabolism and excretion tightly controls the cellular concentration of polyamines. The concentrations of rate-limiting enzymes in the polyamine synthesis and degradation pathways are regulated at different levels, including transcription, translation and degradation. Polyamines can modulate the translation of most of the enzymes required for their synthesis and catabolism through feedback mechanisms that are unique for each enzyme. Translational control is associated with cis-acting and trans-acting factors that can be influenced by the concentration of polyamines through mechanisms that are not completely understood. In this review, we present an overview of the translational control mechanisms of the proteins in the polyamine pathway, including ornithine decarboxylase (ODC), ODC antizyme, S-adenosylmethionine decarboxylase and spermidine/spermine N(1) acetyltransferase, highlighting the areas where more research is needed. A better understanding of the translational control of these enzymes would offer the possibility of a novel pharmacological intervention against cancer and other diseases.

Polyamines, androgens, and skeletal muscle hypertrophy

The naturally occurring polyamines, spermidine, spermine, and their precursor putrescine, play indispensible roles in both prokaryotic and eukaryotic cells, from basic DNA synthesis to regulation of cell proliferation and differentiation. The rate-limiting polyamine biosynthetic enzymes, ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase, are essential for mammalian development, with knockout of the genes encoding these enzymes, Odc1 and Amd1, causing early embryonic lethality in mice. In muscle, the involvement of polyamines in muscle hypertrophy is suggested by the concomitant increase in cardiac and skeletal muscle mass and polyamine levels in response to anabolic agents including β-agonists. In addition to β-agonists, androgens, which increase skeletal mass and strength, have also been shown to stimulate polyamine accumulation in a number of tissues. In muscle, androgens act via the androgen receptor to regulate expression of polyamine biosynthetic enzyme genes, including Odc1 and Amd1, which may be one mechanism via which androgens promote muscle growth. This review outlines the role of polyamines in proliferation and hypertrophy, and explores their possible actions in mediating the anabolic actions of androgens in muscle.

Identification of proteins whose synthesis is preferentially enhanced by polyamines at the level of translation in mammalian cells

In Escherichia coli, several proteins whose synthesis is enhanced by polyamines at the level of translation have been identified. We looked for proteins that are similarly regulated in eukaryotes using a mouse mammary carcinoma FM3A cell culture system. Polyamine deficiency was induced by adding an inhibitor of ornithine decarboxylase, alpha-difluoromethylornithine, to the medium. Proteins enhanced by polyamines were determined by comparison of protein levels in control and polyamine-deficient cells using two-dimensional gel electrophoresis, and were identified by Edman degradation and/or LC/MALDI-TOF/TOF tandem mass spectrometry. Polyamine stimulation of the synthesis of these proteins at the level of translation was confirmed by measuring levels of the corresponding mRNAs and proteins, and levels of the [(35)S]methionine pulse-labeled proteins. The proteins identified in this way were T-complex protein 1, beta subunit (Cct2); heterogeneous nuclear ribonucleoprotein L (Hnrpl); and phosphoglycerate mutase 1 (Pgam1). Since Cct2 was most strongly enhanced by polyamines among three proteins, the mechanism of polyamine stimulation of Cct2 synthesis was studied using NIH3T3 cells transiently transfected with genes encoding Cct2-EGFP fusion mRNA with normal or mutated 5'-untranslated region (5'-UTR) of Cct2 mRNA. Polyamines most likely enhanced ribosome shunting on the 5'-UTR of Cct2 mRNA.

Minor contribution of an internal ribosome entry site in the 5'-UTR of ornithine decarboxylase mRNA on its translation

The mechanism of synthesis of ornithine decarboxylase (ODC) at the level of translation was studied using cell culture and cell-free systems. Synthesis of firefly luciferase (Fluc) from the second open reading frame (ORF) in a bicistronic construct transfected into FM3A and HeLa cells was enhanced by the presence of the 5'-untranslated region (5'-UTR) of ODC mRNA between the two ORFs. However, cotransfection of the gene encoding 2A protease inhibited the synthesis of Fluc. Synthesis of Fluc from the second cistron in the bicistronic mRNA in a cell-free system was not affected significantly by the 5'-UTR of ODC mRNA. Synthesis of ODC from ODC mRNA in a cell-free system was inhibited by 2A protease and cap analogue (m7GpppG). Rapamycin inhibited ODC synthesis by 40-50% at both the G1/S boundary and the G2/M phase. These results indicate that an IRES in the 5'-UTR of ODC mRNA does not function effectively.

Enhancement of +1 Frameshift by Polyamines during Translation of Polypeptide Release Factor 2 in Escherichia coli

Polypeptide release factor 2 (RF2) in Escherichia coli is known to be synthesized by a +1 frameshift at the 26th UGA codon of RF2 mRNA. Polyamines were found to stimulate the +1 frameshift of RF2 synthesis, an effect that was reduced by excess RF2. Polyamine stimulation of +1 frameshift of RF2 synthesis was observed at the early logarithmic phase, which is the important phase in determination of the overall rate of cell growth. A Shine-Dalgarno-like sequence was necessary for an efficient +1 frameshift of RF2 synthesis, but not for polyamine stimulation. Spectinomycin, tetracycline, streptomycin, and neomycin reduced polyamine stimulation of the +1 frameshift of RF2 synthesis. The results suggest that a structural change of the A site on 30 S ribosomal subunits is important for polyamine stimulation of the +1 frameshift. The level of mRNAs of ribosomal proteins and elongation factors having UAA as termination codon was enhanced by polyamines, and OppA synthesis from OppA mRNA having UAA as termination codon was more enhanced by polyamines than that from OppA mRNA having a UGA termination codon. Furthermore, synthesis of ribosomal protein L20 and elongation factor G from the mRNAs having a UAA termination codon was enhanced by polyamines at the level of translation and transcription. The results suggest that some protein synthesis from mRNAs having a UAA termination codon is enhanced at the level of translation through polyamine stimulation of +1 frameshift of RF2 synthesis. It is concluded that prfB encoding RF2 is a new member of the polyamine modulon.

Stable siRNA-mediated silencing of antizyme inhibitor: regulation of ornithine decarboxylase activity

Ornithine decarboxylase (ODC) is the rate-limiting enzyme involved in the biosynthesis of polyamines essential for cell growth and differentiation. Aberrant upregulation of ODC, however, is widely believed to be a contributing factor in tumorigenesis. Antizyme is a major regulator of ODC, inhibiting ODC activity through the formation of complexes and facilitating degradation of ODC by the 26S proteasome. Moreover, the antizyme inhibitor (AZI) serves as another factor in regulating ODC, by binding to antizyme and releasing ODC from ODC-antizyme complexes. In our previous report, we observed elevated AZI expression in tumor specimens. Therefore, to evaluate the role of AZI in regulating ODC activity in tumors, we successfully down-regulated AZI expression using RNA interference technology in A549 lung cancer cells expressing high levels of AZI. Two AZI siRNAs, which were capable to generate a hairpin dsRNA loop targeting AZI, could successively decrease the expression of AZI. Using biological assays, antizyme activity increased in AZI-siRNA-transfected cells, and ODC levels and activity were reduced as well. Moreover, silencing AZI expression decreased intracellular polyamine levels, reduced cell proliferation, and prolonged population doubling time. Our results directly demonstrate that downregulation of AZI regulates ODC activity, intracellular polyamine levels, and cell growth through regulating antizyme activity. This study also suggests that highly expressed AZI may be partly responsible for increased ODC activity and cellular transformation.

A unifying model for the role of polyamines in bacterial cell growth, the polyamine modulon

We reported previously that the synthesis of specific proteins such as OppA, Cya, and RpoS (sigma(38)), which are important for cell growth and viability, is stimulated by polyamines at the level of translation. In this study we found that the synthesis of FecI and Fis was also stimulated by polyamines at the level of translation. The FecI and Fis proteins enhance the expression of mRNAs that are involved in iron uptake and energy metabolism and the expression of rRNA and some tRNAs. The Shine-Dalgarno (SD) sequence of their mRNAs was not obvious or was not located at the usual position. When the SD sequences were created at the normal position on these mRNAs, protein synthesis was no longer influenced by polyamines. Thus, the common characteristic of these mRNAs was to have a weak or ineffective SD sequence. We propose that a group of genes whose expression is enhanced by polyamines at the level of translation be referred to as a "polyamine modulon." By DNA microarray, we found that 309 of 2,742 mRNA species were upregulated by polyamines. Among the 309 up-regulated genes, transcriptional enhancement of at least 58 genes might be attributable to increased levels of the transcription factors Cya, RpoS, FecI, and Fis, which are all organized in the polyamine modulon. This unifying molecular mechanism is proposed to underlie the physiological role of polyamines in controlling the growth of Escherichia coli.

Polyamines enhance synthesis of the RNA polymerase sigma 38 subunit by suppression of an amber termination codon in the open reading frame

The mechanisms by which polyamines stimulate synthesis of the RNA polymerase sigma(38) subunit in Escherichia coli were studied. Polyamine stimulation was observed only in strains in which the 33rd codon of RpoS mRNA is a UAG termination codon instead of a CAG codon for glutamine in wild-type E. coli. Readthrough of the termination codon by Gln-tRNA(supE) was stimulated by polyamines. This stimulation was found to be caused by an increase in both the level of suppressor tRNA(supE) and the binding affinity of Gln-tRNA(supE) for ribosomes. The stimulatory effect was observed with a UAG termination codon but not with UGA and UAA codons. Readthrough of the UAG termination codon at the 270th amino acid position of RpoS mRNA was also stimulated by polyamines, indicating that polyamines stimulate readthrough of a UAG codon regardless of its location within the RpoS mRNA. When cell viability of an E. coli strain having a termination codon in the 33rd position of RpoS mRNA was compared using cells cultured with or without putrescine, it was higher in cells cultured with putrescine than in cells cultured without putrescine. The level of sigma(38) subunit in the cells cultured with putrescine was higher than that in cells cultured without putrescine on days 2, 4, and 8, but the level of sigma(70) subunit was almost the same in cells cultured with or without putrescine. These results confirm that elevated expression of the rpoS gene is important for cell viability at late stationary phase.

Polyamine regulation of ornithine decarboxylase and its antizyme in intestinal epithelial cells

Ornithine decarboxylase (ODC) is feedback regulated by polyamines. ODC antizyme mediates this process by forming a complex with ODC and enhancing its degradation. It has been reported that polyamines induce ODC antizyme and inhibit ODC activity. Since exogenous polyamines can be converted to each other after they are taken up into cells, we used an inhibitor of S-adenosylmethionine decarboxylase, diethylglyoxal bis(guanylhydrazone) (DEGBG), to block the synthesis of spermidine and spermine from putrescine and investigated the specific roles of individual polyamines in the regulation of ODC in intestinal epithelial crypt (IEC-6) cells. We found that putrescine, spermidine, and spermine inhibited ODC activity stimulated by serum to 85, 46, and 0% of control, respectively, in the presence of DEGBG. ODC activity increased in DEGBG-treated cells, despite high intracellular putrescine levels. Although exogenous spermidine and spermine reduced ODC activity of DEGBG-treated cells close to control levels, spermine was more effective than spermidine. Exogenous putrescine was much less effective in inducing antizyme than spermidine or spermine. High putrescine levels in DEGBG-treated cells did not induce ODC antizyme when intracellular spermidine and spermine levels were low. The decay of ODC activity and reduction of ODC protein levels were not accompanied by induction of antizyme in the presence of DEGBG. Our results indicate that spermine is the most, and putrescine the least, effective polyamine in regulating ODC activity, and upregulation of antizyme is not required for the degradation of ODC protein.

Increase in cap- and IRES-dependent protein synthesis by overproduction of translation initiation factor eIF4G

The role of eIF4G during the initiation of protein synthesis was studied using mouse mammary carcinoma FM3A cells and FM4G cells that overproduce an N-terminally truncated form of eIF4G, which lacks the binding site of poly(A)-binding protein. An increase in eIF4G was correlated with an increase in protein synthesis and RNA helicase activity. Translation of mRNAshaving both short and long 5'-untranslated regions (5'-UTR) increased significantly in FM4G cells compared to that in FM3A cells. Both full-length and N-terminally truncated eIF4G transfectants of NIH3T3 cells formed colonies in soft agar and increased the saturation density of cell growth, indicating that both eIF4Gs function similarly. We also found that an internal ribosome entry site (IRES) exists in the 5'-UTR of ornithinedecarboxylase mRNA and that IRES-dependent protein synthesis increased in FM4G cells. Our results indicate that an increase in eIF4G contributes to the formation of active eIF4F similarly to that caused by an increase in eIF4E, as well as to a stimulation of IRES-dependent protein synthesis.

Polyamine regulation of ornithine decarboxylase synthesis in Neurospora crassa

Ornithine decarboxylase (ODC) of the fungus Neurospora crassa, encoded by the spe-1 gene, catalyzes an initial and rate-limiting step in polyamine biosynthesis and is highly regulated by polyamines. In N. crassa, polyamines repress the synthesis and increase the degradation of ODC protein. Changes in the rate of ODC synthesis correlate with similar changes in the abundance of spe-1 mRNA. We identify two sequence elements, one in each of the 5' and 3' regions of the spe-1 gene of N. crassa, required for this polyamine-mediated regulation. A 5' polyamine-responsive region (5' PRR) comprises DNA sequences both in the upstream untranscribed region and in the long 5' untranslated region (5'-UTR) of the gene. The 5' PRR is sufficient to confer polyamine regulation to a downstream, heterologous coding region. Use of the beta-tubulin promoter to drive the expression of various portions of the spe-1 transcribed region revealed a 3' polyamine-responsive region (3' PRR) downstream of the coding region. Neither changes in cellular polyamine status nor deletion of sequences in the 5'-UTR alters the half-life of spe-1 mRNA. Sequences in the spe-1 5'-UTR also impede the translation of a heterologous coding region, and polyamine starvation partially relieves this impediment. The results show that N. crassa uses a unique combination of polyamine-mediated transcriptional and translational control mechanisms to regulate ODC synthesis.

Gene structure and chromosomal localization of mouse S-adenosylmethionine decarboxylase

The structure of the mouse S-adenosylmethionine decarboxylase (AdoMetDC) gene has been determined. The mouse gene (AMD1) consisted of eight exons and seven introns, similar to the rat AdoMetDC gene, and was mapped to chromosome 10. The characteristics of AMD1 gene were as follows: (1) The region of the promoter necessary for maximal transcriptional activity was located about 400 nucleotides upstream of the transcriptional initiation point, and contained a TATA box and two GC boxes. The transcriptional activity of the promoter was nearly equal to that of the SV40 promoter. (2) Two polyadenylation signals for transcription were observed, and the larger AdoMetDC mRNA, which is the dominant form of mRNA, corresponded to mRNA that is generated using the second polyadenylation signal. (3) Using stable transfectants, we confirmed that the upstream open reading frame (uORF) in the 5'-untranslated region (5'-UTR) of AdoMetDC mRNA functioned as a negative regulatory element. Lower concentrations of polyamines affect both stimulation and inhibition of AdoMetDC synthesis, through the uORF in the mRNA, than affect general protein synthesis.

Polyamine stimulation of the synthesis of oligopeptide-binding protein (OppA). Involvement of a structural change of the Shine-Dalgarno sequence and the initiation codon aug in oppa mRNA

We previously suggested that the degree of polyamine stimulation of oligopeptide-binding protein (OppA) synthesis is dependent on the secondary structure and position of the Shine-Dalgarno (SD) sequence of OppA mRNA. To study the structural change of OppA mRNA induced by polyamines and polyamine stimulation of initiation complex formation, four different 130-mer OppA mRNAs containing the initiation region were synthesized in vitro. The structural change of these mRNAs induced by polyamines was examined by measuring their sensitivity to RNase T(1), specific for single-stranded RNA, and RNase V(1), which recognizes double-stranded or stacked RNA. In parallel, the effect of spermidine on mRNA-dependent fMet-tRNA binding to ribosomes was examined. Our results indicate that the secondary structure of the SD sequence and initiation codon AUG is important for the efficiency of initiation complex formation and that spermidine relaxes the structure of the SD sequence and the initiation codon AUG. The existence of a GC-rich double-stranded region close to the SD sequence is important for spermidine stimulation of fMet-tRNA binding to ribosomes. Spermidine apparently binds to this GC-rich stem and causes a structural change of the SD sequence and the initiation codon, facilitating an interaction with 30 S ribosomal subunits.

Translational regulation of ornithine decarboxylase and other enzymes of the polyamine pathway

It has long been known that polyamines play an essential role in the proliferation of mammalian cells, and the polyamine biosynthetic pathway may provide an important target for the development of agents that inhibit carcinogenesis and tumor growth. The rate-limiting enzymes of the polyamine pathway, ornithine decarboxylase (ODC) and S-adenosylmethionine decarboxylase (AdoMetDC), are highly regulated in the cell, and much of this regulation occurs at the level of translation. Although the 5' leader sequences of ODC and AdoMetDC are both highly structured and contain small internal open reading frames (ORFs), the regulation of their translation appears to be quite different. The translational regulation of ODC is more dependent on secondary structure, and therefore responds to the intracellular availability of active eIF-4E, the cap-binding subunit of the eIF-4F complex, which mediates translation initiations. Cell-specific translation of AdoMetDC appears to be regulated exclusively through the internal ORF, which causes ribosome stalling that is independent of eIF-4E levels and decreases the efficiency with which the downstream ORF encoding AdoMetDC protein is translated. The translation of both ODC and AdoMetDC is negatively regulated by intracellular changes in the polyamines spermidine and spermine. Thus, when polyamine levels are low, the synthesis of both ODC and AdoMetDC is increased, and an increase in polyamine content causes a corresponding decrease in protein synthesis. However, an increase in active eIF-4E may allow for the synthesis of ODC even in the presence of polyamine levels that repress ODC translation in cells with lower levels of the initiation factor. In contrast, the amino acid sequence that is encoded by the upstream ORF is critical for polyamine regulation of AdoMetDC synthesis and polyamines may affect synthesis by interaction with the putative peptide, MAGDIS.

Enhancement of cell death due to decrease in Mg2+ uptake by OmpC (cation-selective porin) deficiency in ribosome modulation factor-deficient mutant

Ribosome modulation factor (RMF) is involved in stabilization of ribosomes during the transition from exponential growth to the stationary growth phase in Escherichia coli. A deficiency of RMF is known to reduce cell viability. Overaccumulation of spermidine also leads to a decrease in cell viability and to a decrease in the synthesis of RMF and of the cation-selective porin OmpC. Thus, a decrease in RMF levels may be involved in the decreased cell viability caused by excess spermidine. Because spermidine also influences the expression of OmpC, we examined whether OmpC deficiency enhances the cell death caused by RMF deficiency. The ompC mutant by itself did not affect protein synthesis or cell viability, but the double rmf ompC mutant produced a much larger decrease in protein synthesis and cell viability than did the single rmf mutant. There was also a decrease in the amount of ribosomes and in the Mg2+ content in the double rmf ompC mutant, and cell viability could be partially restored by the addition of Mg2+ to the growth medium. RMF deficiency was found to inhibit the synthesis of another cation-selective porin OmpF. Thus, the double rmf ompC mutant is deficient in both OmpC and OmpF, which probably accounts for the pronounced decrease in Mg2+ uptake in this mutant. The results indicate that both RMF and Mg2+, acting through stabilization of ribosomes, are important for cell viability at the stationary growth phase.

Bovine ornithine decarboxylase gene: cloning, structure and polymorphisms

Bovine ornithine decarboxylase (ODC) genomic clones were isolated from a bacteriophage lambda DASH genomic library. A total of 9452 bp sequence was determined which covers the entire sequence of the bovine ODC gene. Sequence analysis showed that the bovine ODC gene consisted of 12 exons which encode a protein identical to that inferred from a bovine ODC cDNA. Comparison of the structure and nucleotide sequence of the bovine, human and mouse ODC genes revealed that the gene was highly conserved. Primer extension analysis demonstrated that the transcription start point of bovine ODC mRNA was located 378 bp upstream from the A residue in the translation initiation codon. The 5'-untranslated region (UTR) of ODC mRNA was highly G + C rich, particularly in its 5'-most portion, and computer predictions suggested a very stable secondary structure for this region, with an overall free energy of formation of -134.4 kcal/mol. Conserved sequences and potential promoter elements including a TATA box, a possible CCAAT element, SP1 ranscription factor binding sites (GC boxes) and cAMP response elements (CRE) were identified in the 5'-flanking region of the gene. Two polymorphic restriction sites, a TaqI and a MspI, were mapped to the ODC gene and PCR-based methods for detection of the 2 polymorphisms were developed.

Molecular mechanism of polyamine stimulation of the synthesis of oligopeptide-binding protein

Polyamine stimulation of the synthesis of oligopeptide-binding protein (OppA) was shown to occur mainly at the level of translation by measuring OppA synthesis and its mRNA level. Several artificial oppA genes were constructed by site-directed mutagenesis. These synthesize different kinds of OppA mRNAs: mRNAs differing in the size of 5'-untranslated region; mRNAs having the Shine-Dalgarno (SD) sequence in a different position; mRNAs having different secondary structure in the region of the SD sequence; and fusion mRNAs consisting of the 5'-untranslated region of OppA mRNA and the open reading frame of beta-galactosidase. By measuring the synthesis of OppA or beta-galactosidase from these mRNAs, we found that the 171-nucleotide 5'-untranslated region and 145 nucleotides of the ORF of OppA mRNA are involved in the polyamine stimulation of OppA synthesis. When the secondary structure of the above region of OppA mRNA was analyzed by optimal computer folding, it was shown that the degree of polyamine stimulation of OppA protein synthesis was dependent on the structure of the SD sequence in addition to its position. Loose base pairing of the SD sequence with other regions of the mRNA caused strong polyamine stimulation, while intense base pairing of the SD sequence with other regions of the mRNA resulted in insignificant or weak polyamine stimulation.

Decrease in cell viability due to the accumulation of spermidine in spermidine acetyltransferase-deficient mutant of Escherichia coli

Physiological functions of spermidine acetyltransferase in Escherichia coli have been studied using the spermidine acetyltransferase (speG) gene-deficient mutant CAG2242 and the cloned speG gene. The growth of E. coli CAG2242 in the defined M9 medium was normal in the presence and absence of 0.5mM spermidine. However, cell viability of E. coli CAG2242 at 48 h after the onset of growth decreased greatly by the addition of 0.5 mM spermidine. The amount of spermidine accumulated in the cells was approximately 3-fold that in the cells grown in the absence of spermidine. Transformation of the cloned speG gene to E. coli CAG2242 recovered the cell viability. Decreased in cell viability of E. coli CAG2242 was observed even when 0.5mM spermidine was added at 24 h after the onset of growth. The results indicate that accumulated spermidine functions at the late stationary phase of growth. The accumulation of spermidine caused a decrease in protein synthesis but not in DNA and RNA synthesis at 28 h after the onset of growth. The synthesis of several kinds of proteins was particularly inhibited. They included ribosome modulation factor and OmpC protein. Since the ribosome modulation factor is essential for cell viability at the stationary phase of growth (Yamagishi, M., Matsushima, H., Wada, A., Sakagami, M., Fujita, N., and Ishihama, A. (1993) EMBO J. 12, 625-630), the decrease in the protein was thought to be one of the reasons for the decrease in cell viability. The decrease in the ribosome modulation factor mainly occurred at the translational level.

Chlorpheniramine inhibits the synthesis of ornithine decarboxylase and the proliferation of human breast cancer cell lines

Proliferation of both mouse and human breast cancer cells was inhibited by chlorpheniramine (CPA) in a dose-response manner. At the beginning of the exponential phase of growth (two days after seeding), 250 microM CPA was able to reduce cell proliferation by 75% (in Ehrlich cell cultures) and 30% (in MCF-7 cultures). The antiproliferative effect of CPA was also tested on a poorly-differentiated and hormone-insensitive human breast cancer cell line (MDA-MB231) and on a highly proliferative human colon cancer cell line (clone 3). CPA was cytotoxic for MDA-MB231 cells at concentrations higher than 50 microM, and it was also cytotoxic for the colon cancer cell clone 3 at 250 microM CPA. Nevertheless, colon cancer cells were slightly stimulated at CPA concentrations less than 100 microM. CPA reduced (by 50-70%) the ornithine decarboxylase induction occurring early after culture seeding of experimental mammary tumors (Ehrlich carcinoma cells) and human breast cancer cells (MCF-7). The presented data suggest that in addition to ODC inhibition, CPA presents other still unknown cytotoxic effects.

Regulation of the human spermidine synthase mRNA translation by its 5'-untranslated region

An increased mRNA content of spermidine synthase was found in phytohemagglutinin stimulated human peripheral lymphocytes and in cultured human myeloma (Sultan) cells stimulated to grow by change of the culture medium. The many-fold increase in the amount of the message was accompanied by stimulation of the enzyme activity in activated lymphocytes, but not in stimulated myeloma cells. In the present study the effect of the 5'-untranslated region of spermidine synthase mRNA on the post-transcriptional control of its expression was studied both in vitro in rabbit reticulocyte system and in cultured mammalian cells. The results show that the GC-rich 5'-untranslated region of spermidine synthase mRNA has an inhibitory effect on its translation.

Lack of 12-O-tetradecanoylphorbol-13-acetate responsiveness of ornithine decarboxylase introns which have AP-1 consensus sequences

The AP-1 consensus sequences (TGAGTCA) are the major 12-O-tetradecanoylphorbol113-acetate (TPA) responsive elements shared by several TPA inducible genes, such as c-sis, c-fos, c-myc, collagenase, stromelysin, hMTIIA and SV40. However, the role of AP-1 binding sites, which are present in the introns 3, 5, and 11 of ODC gene, in the regulation of TPA-induced ornithine decarboxylase (ODC) gene transcription are unknown. We determined the TPA responsiveness of the AP-1 sequences in the introns of ODC gene in CV-1 cells which induce ODC activity and mRNA in response to TPA treatment. ODC introns containing AP-1 sequences were inserted into the chloramphenicol acetyltransferase (CAT) reporter gene. Transient transfection of CV-1 cells with the intron-CAT constructs followed by TPA treatment did not induce CAT activity. However, when flanking regions of the AP-1 site in intron 3 were narrowed down to 74 bp, TPA induced CAT activity by 5- to 7-fold. The TPA-inducibility could be eliminated by mutation of the AP-1 site (TGAGTCA-->TGATGCCA or TGATGA) in 74 bp of intron 3. These results indicate that the AP-1 sequences in the intact ODC introns may not be responsive to TPA. The flanking sequences of the AP-1 site may be crucial to determine whether the AP-1 site is accessible to the TPA-induced transcriptional factor(s).

Polyamine involvement in the cell cycle, apoptosis, and autoimmunity

The polyamines: putrescine, spermidine and spermine, are ubiquitous polycations which have numerous, unique interactions in eukaryotic cells. Polyamines are essential for cell growth, with the bulk of polyamine expression co-ordinated with the cell cycle. The length, charge, and charge distribution of polyamines permit them to interact with large anionic molecules such as DNA, RNA, and phospholipids. Here, a mechanism is proposed whereby cell cycle expression of polyamines at the start of S phase leads to disruption of transcription and splicing, giving priority to DNA and histone synthesis. Inappropriate initiation of this process in non-viable cells leads to apoptosis and may be an underlying cause of autoimmunity.

Polyamine profiles and growth properties of ornithine decarboxylase overexpressing MCF-7 breast cancer cells in culture

To determine the direct influence of the polyamine (PA) pathway on breast cancer phenotype, we employed a transfection approach to induce overexpression of the PA biosynthetic enzyme ornithine decarboxylase (ODC) in the hormone-responsive MCF-7 breast cancer cell line. Using a modified calcium phosphate method and an ODC cDNA coding for a truncated and more stable enzyme, we were able to achieve a moderate to marked degree of ODC overexpression (up to 150-fold) in a transient transfection system. ODC-overexpressing MCF-7 cells exhibited a selective increase in cellular putrescine content, while the levels of spermidine and spermine remained unaffected. Under defined culture conditions, overexpression of ODC resulted in a consistent but modest increase in [3H]thymidine incorporation into DNA which was similar in the presence and absence of 17-beta-estradiol, TGF-alpha, and IGF-I. In the presence of serum, the effect of ODC overexpression on basal [3H]-thymidine incorporation into DNA was inconsistent, possibly as a result of subtle differences in culture conditions. Overall, our results support the hypothesis that activation of the PA biosynthetic pathway may confer a growth advantage to breast cancer cells.

Differential expression of alpha-actin mRNA and immunoreactive protein in brain microvascular pericytes and smooth muscle cells

Hypertension has been linked to opening of the blood-brain barrier and may be related to the expression of the smooth muscle alpha-actin gene in contractile cells at the brain microvasculature. However, the cellular origin (i.e., endothelial cells, pericytes, smooth muscle cells) of the alpha-actin mRNA in the brain microvasculature is not clearly identified. Therefore, we investigated the abundance of actin mRNA by Northern blot analysis in isolated brain microvessels and in brain microvascular endothelial or pericytes in tissue culture. All samples showed the characteristic 2.1 kb transcript corresponding to cytoplasmic beta and gamma isoform mRNA. The 1.7 kb transcript corresponding to smooth muscle alpha-actin was detected in freshly isolated bovine brain microvessels, in primary cultures of brain microvascular pericytes, or endothelial cells; the latter cultures contain both endothelial cells and pericytes. The alpha-actin mRNA was absent in a cloned bovine brain endothelial cell line. The relative abundance of the alpha/(beta + gamma) actin transcript ratio was: cultured pericytes > freshly isolated microvessels > endothelial primary. The cellular distribution of the smooth muscle alpha-actin immunoreactive protein was studied by immunocytochemistry in cytospun/methanol-fixed isolated bovine brain microvessels with a monoclonal antibody directed to the amino-terminal decapeptide of the smooth muscle alpha-actin isoform. This antibody reacted strongly with precapillary arterioles of isolated microvessels, whereas no immunostaining was observed in either capillary endothelial cells or in pericytes. In conclusion, the alpha-actin mRNA is expressed in brain microvascular pericytes in tissue culture, but the immunoreactive alpha-actin protein is not expressed in brain microvascular pericytes in vivo. These data suggest that either 1) alpha-actin gene expression is induced in capillary pericytes in tissue culture or 2) alpha-actin mRNA in brain capillary pericytes in vivo is subject to translational repression resulting in no detectable alpha-actin protein under normal conditions.

Non-AP-1 tumor promoter 12-O-tetradecanoylphorbol-13-acetate-responsive sequences in the human ornithine decarboxylase gene

To define the mechanisms of 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced transcription of the ornithine decarboxylase (ODC) gene, we isolated a genomic clone (hODC41B) of ODC from a human leukocyte genomic DNA library. The restriction endonuclease map, in comparison with the previously published sequences of the human ODC gene, indicated that hODC41B contained a 15.7-kb sequence that extended from the sixth exon to about 10 kb upstream of the ODC gene. A 2.5-kb genomic fragment containing the 5' flanking region and the first exon was subcloned and sequenced. Sequence analysis revealed multiple putative promoter/enhancer elements (a TATA box, a CAAT box, 17 GC boxes, and a cAMP-responsive element) but no consensus AP-1 sequences (TGAGTCA) in the 2.5-kb 5' flanking region. However, three AP-1 sequences were located in introns 3, 5, and 11. We constructed a series of chimeric genes containing part of the first exon and increasingly longer 5' flanking sequences of the ODC gene fused to either bacterial chloramphenicol acetyltransferase (CAT) or luciferase reporter genes. TPA inducibility was determined by transient transfection and measurement of CAT or luciferase expression in HeLa cells. The induction of CAT activity by TPA decreased with decreasing lengths of the 5' flanking sequences up to nt -82. The TPA induction from the construct -72 ODC CAT was threefold to sevenfold, and the TPA inducibility of the same fragment was about ninefold to 30-fold with the luciferase reporter gene. Further deletion analysis revealed TPA-responsive sequences in ODC nt -42 to +54. Gel mobility shift assays using alpha-32P-end labeled ODC nt -42 to +60 revealed that nt -42 to +60 specifically bound HeLa cell nuclear proteins. HeLa cell nuclear protein binding to ODC nt -42 to +60 could not be completely competed by AP-1-, AP-2-, AP-3-, or SP1-responsive sequences.

Regulation of protein synthesis by mRNA structure

In addition to the m7G cap structure, the length of the 5' UTR and the position and context of the AUG initiator codon (which have been discussed elsewhere in this volume), higher order structures within mRNA represent a critical parameter for translation. The role of RNA structure in translation initiation will be considered primarily, although structural elements have also been found to affect translation elongation and termination. We will first describe the different effects of higher order RNA structures per se, and then consider specific examples of RNA structural elements which control translation initiation by providing binding sites for regulatory proteins.

Molecular cloning of the S-adenosylmethionine synthetase gene in Drosophila melanogaster

We have isolated and sequenced cDNA clones encoding the Drosophila melanogaster S-adenosylmethionine synthetase. The deduced amino acid sequence contains 405 amino acid residues and shows high homology to rat, yeast, Arabidopsis and Escherichia coli counterparts. The gene is transcribed throughout Drosophila development but its main activity is seen in adult males and females. The highest transcription activity is seen in female ovaries. The transcript has an unusually long 5'-untranslated region, which might be of importance for translational regulation.

Correlation between the inhibition of cell growth by bis(ethyl)polyamine analogues and the decrease in the function of mitochondria

The antiproliferating effect of nine kinds of bis(ethyl)polyamine analogues [three kinds each of bis(ethyl)triamine, bis(ethyl)tetraamine and bis(ethyl)pentaamine] was compared using FM3A cells. The inhibitory effect was in the order BE4444 > BE3443 > BE4334 > or = BE444 > BE343 > BE333 > BE44 > BE34 > BE33. Our results indicate that not only polyamine deficiency but also the accumulation of polyamine analogues is involved in the inhibition of cell growth. Accumulation of bis(ethyl)polyamine analogues caused the inhibition of protein synthesis and the decrease in the ATP content. The protein synthetic system in mitochondria was more strongly inhibited by bis(ethyl)polyamine analogues than that in the cytoplasm. Under conditions such that cytoplasmic protein synthesis was inhibited by 50% by bis(ethyl)polyamine analogues, mitochondrial protein synthesis was almost completely inhibited. Mitochondrial Ile-tRNA formation was inhibited by bis(ethyl)polyamine analogues at the concentrations that cytoplasmic Ile-tRNA formation was stimulated. This may be one of the reasons for the selective inhibition of mitochondrial protein synthesis. This inhibition was followed by the decrease in ATP content, swelling of mitochondria and depletion of mitochondrial DNA. These results suggest that the early event of metabolic change caused by bis(ethyl)polyamine analogues in cells is the inhibition of protein synthesis, especially of mitochondrial protein synthesis.

Regulation of translation by specific protein/mRNA interactions

This review will focus on cases of specific translational control by protein/RNA interactions in the 5'- or 3'-UTR of eukaryote mRNA where either the cis-acting RNA determinant or the trans-acting protein (or preferably both) have been identified with fair certainty. Examples of messages that are regulated by 5' motifs, which are proposed to occlude ribosome binding when bound by their specific factors, include ferritin and ribosomal protein mRNAs and the autoregulated thymidylate synthase and poly(A)-binding mRNAs. However, it has become increasingly evident recently that 3' UTR determinants and their specific binding proteins also regulate translation efficiency either directly, or indirectly via an influence on the polyadenylation status of the mRNA. It is still unclear how events at the 3' end of mRNA influence ribosome binding. Most, if not all, of the mRNAs known to be regulated by 3' UTR motifs are subject to regulation during early development or during differentiation such as several spermatocyte and oocyte mRNAs and erythroid lipoxygenase mRNA. To date, in all cases where translation is controlled directly by specific protein/mRNA interactions, the protein seems to act as a negative regulator, a translational repressor, whose binding to the specific site on the mRNA results in inhibition of initiation. The only cases of translational activation known so far concern internal initiation of translation of picornaviral RNAs, but this topic is beyond the scope of this review.

Overproduction of S-adenosylmethionine decarboxylase in ethylglyoxal-bis(guanylhydrazone)-resistant mouse FM3A cells

A variant cell line, termed SAM-1, which overproduced S-adenosylmethionine decarboxylase (AdoMetDC), was isolated by treatment of mouse FM3A cells with N-methyl-N'-nitro-N-nitrosoguanidine and subsequent incubation with ethylglyoxal bis(guanylhydrazone), an inhibitor of the enzyme. The cells were resistant to ethylglyoxal bis(guanylhydrazone), and showed AdoMetDC activity approximately five-times higher than control cells. The rate of AdoMetDC synthesis and the amount of AdoMetDC existing in SAM-1 cells were about five-times those in control cells. The amount of AdoMetDC mRNA existing in SAM-1 cells was five-times more than that in control cells. The amount of 5'-([(Z)-4-amino-2-butenyl]methylamino)-5'-deoxyadenosine, an irreversible inhibitor of AdoMetDC, necessary to inhibit cell growth was also five-times more in SAM-1 cells than in control cells. However, the following were the same in both SAM-1 and control cells; the amount of genomic DNA for AdoMetDC, the size and nucleotide sequence of 5' untranslated region of AdoMetDC mRNA, the deduced amino acid sequence (334 residues) from the nucleotide sequence of AdoMetDC cDNA and the degradation rate (t1/2 = about 4 h) of AdoMetDC. In addition, AdoMetDC mRNA in control cells was slightly more stable than that in SAM-1 cells. The results indicate that the overproduction of AdoMetDC in SAM-1 cells was caused by the increase of AdoMetDC mRNA. The variant cell line is convenient for studying the regulation of AdoMetDC and the physiological function of polyamines.

Interaction of TPA and ultraviolet B radiation in regulation of ODC gene expression in rat keratinocytes

Ultraviolet B radiation (UVB) and phorbol esters are known to promote tumor formation in skin; however, the interaction between UVB and phorbol esters in the regulation of gene expression remains incompletely understood. To define the interaction of UVB and phorbol esters in the control of keratinocyte gene expression, we have studied the effects of the phorbol ester 12-O-tetradecanoylphorbol-13-acetate (TPA) and UVB on the regulation of ornithine decarboxylase (ODC) gene expression in a rat keratinocyte cell line. Both UVB and TPA alone increased ODC activity and induced the expression of the ODC gene. The combination of UVB and TPA produced a further increment in ODC gene expression at 12 h, but UVB markedly attenuated the TPA induction of ODC mRNA transcripts at 3 h. Protein synthesis inhibition with cycloheximide also induced ODC mRNA transcripts, but did not eliminate the further induction of ODC gene expression by UVB or TPA. No changes in actin gene expression following exposure to TPA/UVB were detected in the same experiments. UVB and TPA alone or in combination had no effect on the transcriptional activity of an ODC-chloramphenicol acetyltransferase fusion gene in transfected rat keratinocytes. The results of these studies suggest a complex posttranscriptional interaction of phorbol esters and UVB in the control of keratinocyte gene expression.

Control of c-myc mRNA half-life in vitro by a protein capable of binding to a coding region stability determinant

Polysome-associated c-myc mRNA is degraded relatively rapidly in cells and in an in vitro mRNA decay system containing extracts from cultured mammalian cells. Using this system, a competition/screening assay was devised to search for factors that bind to specific regions of polysome-associated c-myc mRNA and thereby alter its half-life. mRNA stability was first assayed in reactions containing exogenous competitor RNAs corresponding to portions of c-myc mRNA itself. The addition of a 182-nucleotide sense strand fragment from the carboxy-terminal portion of the c-myc-coding region destabilized c-myc mRNA by at least eightfold. This RNA fragment had no effect on the stability of other mRNAs tested. Moreover, c-myc mRNA was not destabilized in reactions containing unrelated competitor RNAs or sense strand RNA from the c-myc 5' region. Polysome-associated globin mRNA containing the c-myc-coding region segment in-frame was also destabilized in vitro by the 182-nucleotide RNA. As determined by UV-cross-linking experiments, the 182-nucleotide RNA fragment was recognized by and bound to an approximately 75-kD polysome-associated protein. On the basis of these data plus Northern blotting analyses of c-myc mRNA decay products, we suggest that the approximately 75-kD protein is normally bound to a c-myc-coding region determinant and protects that region of the mRNA from endonuclease attack. Possible links between the protective protein, translation, ribosome pausing, and c-myc mRNA turnover are discussed.

Regulation of ornithine decarboxylase gene expression, polyamine levels, and DNA synthetic rates by all-trans-retinoic acid in cultured human keratinocytes

Regulation of ornithine decarboxylase (ODC) gene expression and cell growth by all-trans-retinoic acid in the presence and absence of exogenous putrescine were examined in normal keratinocyte cultures maintained in serum-free medium containing 0.15 mM Ca++. Putrescine and the higher polyamines are negative feedback regulators of ODC synthesis and are essential for cell growth. Human keratinocytes were incubated with and without 1 microM putrescine and the effects of 5 x 10(-7) M retinoic acid on ODC mRNA levels, ODC activity, polyamine levels, and DNA synthetic rates were determined. Northern blot analysis of total RNA isolated from breast reduction keratinocytes treated with retinoic acid up to 24 h showed a time-dependent suppression of ODC mRNA levels that was unaffected by putrescine. ODC activity was suppressed more rapidly in keratinocytes grown in the absence of putrescine; however, at 24 h, ODC activity was suppressed to an equal extent under both culture conditions. The effect of retinoic acid on polyamine levels was determined in the absence of exogenous putrescine. Retinoic acid treatment markedly suppressed putrescine and N1-acetylspermidine levels, whereas spermidine and spermine levels were relatively unaffected. The effect of retinoic acid on DNA synthetic rates, as measured by 3H-thymidine incorporation, was variable. Retinoic acid either stimulated or had little effect on keratinocyte DNA synthetic rates in cells derived from breast reductions and cultured in the absence of putrescine; these effects were not opposed by the presence of exogenous putrescine. In contrast, DNA synthesis in keratinocytes derived from neonatal foreskins was consistently suppressed by retinoic acid, independent of the polyamine status. Our data, therefore, suggest that the effect of retinoic acid on cell growth, as indicated by DNA synthetic rates, does not necessarily parallel its effect on ODC activity and mRNA levels.

Identification of proteins associating with poly(A)-binding-protein mRNA

Synthesis of poly(A)-binding protein is regulated at the translational level. We have investigated the binding of proteins to this mRNA on the premise that the protein(s) of the mRNP complex may be involved in regulating the expression of the mRNA. We found the first 243 nucleotides of the 5' untranslated region to contain sequences essential for RNP formation. A large, single-stranded bulge structure encompassing stretches rich in adenine nucleotides and a potential stem-loop domain appear to be the primary sites for protein binding. Removal of the 243-nucleotide segment results in a drastic reduction in protein binding and a concomitant increase in translational efficiency in vitro. We suggest that proteins binding to this region, including poly(A)-binding protein itself, may be essential for regulating translation of this mRNA.

Expression and post-transcriptional regulation of ornithine decarboxylase during early Xenopus development

In this paper we show that large changes in ornithine decarboxylase (ODC) activity occurred during early Xenopus development. Following fertilization, this enzyme activity rises with a quantitatively correlated accumulation of putrescine and spermidine. This increase in ODC activity was associated with an increased translation of the maternal ODC mRNA, which was stable in the embryo and whose polyadenylation increased slightly between fertilization and the mid-blastula transition (MBT). ODC activity was stable in cycloheximide-treated embryos, indicating that before the MBT this enzyme was not degraded. After the MBT, ODC activity fell, but no decrease in this mRNA was observed. In gastrulae, ODC mRNA was both increased in amount and polyadenylated. The reduced ODC activity at this stage of development was not associated with a fall in ribosome loading of the mRNA. Treatment of post-MBT embryos with cycloheximide lead to an accentuation of the normally observed decrease in ODC activity. Expression of Xenopus ODC in mutant ODC-deficient Chinese hamster ovary cells (C 55.7 cells) showed that the Xenopus enzyme was rapidly degraded and can be regulated post-translationally by polyamines, indicating that the post-MBT fall in ODC activity could be caused by a change in protein turnover or by polyamine-mediated regulation.

Translational control mechanism of ornithine decarboxylase by asparagine and putrescine in primary cultured hepatocytes

Asparagine stimulated the translation of ornithine decarboxylase (ODC) mRNA more than 10-fold in cultured hepatocytes which had been pretreated with glucagon in simple salt/glucose medium. Putrescine suppressed the increase in the rate of ODC synthesis caused by asparagine without significant change in the amount of ODC mRNA, suggesting that putrescine inhibited the effect of asparagine at least in part at the level of translation. Polysomal distribution of ODC mRNA was analyzed to examine the site of translational regulation by these effectors. In uninduced hepatocytes, most of the ODC mRNA was sedimented slightly after the 40 S ribosomal subunit. This ODC mRNA was sequestered from translational machinery since it was not shifted to the polysome fraction when peptide elongation was specifically inhibited by a low concentration of cycloheximide. In asparagine-treated cells, 40% of total ODC mRNA was in the polysomal fraction and formed heavier polysomes, indicating that asparagine stimulated both recruitment of ODC mRNA from the untranslatable pool and the initiation steps of translation. Putrescine did not change the distribution pattern of ODC mRNA on polysomes significantly. Thus, 30% of ODC mRNA remained on polysomes even when ODC synthesis was completely inhibited by putrescine. Paradoxically more than 70% of ODC mRNA was shifted into polysomes by putrescine in the presence of low concentrations of cycloheximide. These results, together with changes in the polysome profile, suggested that putrescine nonspecifically stimulated the recruitment of ODC mRNA from the untranslatable pool, whereas it specifically inhibited its translation at both the initiation and the elongation steps.

On the translational control of ornithine decarboxylase expression by polyamines

Ornithine decarboxylase (ODC, EC 4.1.1.17) expression is subject to negative feedback regulation by the polyamines. The results of previous studies favor either translational or post-translational regulation. To facilitate further analysis of the mechanism by which polyamines affect ODC expression we have used a cell line (L1210-DFMOr) that overproduces ODC. This cell line was isolated by selection for resistance to the antiproliferative effect of the ODC inhibitor alpha-difluoromethylornithine (DFMO). These cells respond similarly to polyamine depletion and repletion as do their wild-type counterparts. When L1210-DFMOr cells were grown in the presence of 20 mM DFMO (i.e., when their polyamine content was reduced to an extent that still permitted a normal growth rate) ODC represented 4-5% of the soluble protein synthesized. After transfer of the cells to a medium lacking DFMO (i.e., when their polyamine pools were repleted), the rate of incorporation of [35S]methionine into ODC was one order of magnitude lower. Since this difference in incorporation of radioactivity into ODC remained the same irrespective of the pulse-label time used (between 2 and 20 min) it is likely to represent a true difference in ODC synthesis rate. Consequently, the pulse-label experiments cannot be explained by rapid degradation of the enzyme during the labeling period. The difference in ODC synthesis rate was not accompanied by a corresponding difference in the steady-state level of ODC mRNA. Analyses of the distribution of ODC mRNA in polysome profiles did not demonstrate any major difference between cells grown in the absence or presence of DFMO, even though the ODC synthesis rate differed by as much as 10-fold. However, the distribution of the ODC mRNA in the polysome profiles indicated that the message was poorly translated. Thus, most of the ODC mRNA was present in fractions containing ribosomal subunits or monosomes. Inhibition of elongation by cycloheximide treatment resulted in a shift of the ODC mRNA from the region of the gradient containing ribosomal subunits to that containing mono- and polysomes, indicating that most of the ODC mRNA was accessible to translation. Taken together these data lend support to a translational control mechanism which involves both initiation and elongation.

Spermidine regulation of ornithine decarboxylase synthesis by a GC-rich sequence of the 5'-untranslated region

The nucleotides in the 70-170 region upstream from the initiator AUG have been shown to be important in the strong stimulation of ornithine decarboxylase (ODC) synthesis by low spermidine concentrations and in the inhibition of ODC synthesis at high spermidine concentrations [Ito, K., et al. (1990) J. Biol. Chem. 265, 13036-13041]. In this region, a GC-rich sequence as well as a small open reading frame (MGQASQATVL) existed. In order to clarify which of these was of greater importance for the spermidine regulation of ODC synthesis, the synthesis was performed with various ODC mRNAs, possessing different sizes and nucleotide sequences in the 5'-untranslated region. The results show that a GC-rich sequence, but not a small potential leader peptide, plays an important role in the spermidine regulation of ODC synthesis.