The translation in vitro of rat ornithine decarboxylase mRNA is blocked by its 5' untranslated region in a polyamine-independent way

The human DEAD-box helicase DDX3X as a regulator of mRNA translation

The human DEAD-box protein DDX3X is an RNA remodelling enzyme that has been implicated in various aspects of RNA metabolism. In addition, like many DEAD-box proteins, it has non-conventional functions that are independent of its enzymatic activity, e.g., DDX3X acts as an adaptor molecule in innate immune signalling pathways. DDX3X has been linked to several human diseases. For example, somatic mutations in DDX3X were identified in various human cancers, and de novo germline mutations cause a neurodevelopmental condition now termed 'DDX3X syndrome'. DDX3X is also an important host factor in many different viral infections, where it can have pro-or anti-viral effects depending on the specific virus. The regulation of translation initiation for specific mRNA transcripts is likely a central cellular function of DDX3X, yet many questions regarding its exact targets and mechanisms of action remain unanswered. In this review, we explore the current knowledge about DDX3X's physiological RNA targets and summarise its interactions with the translation machinery. A role for DDX3X in translational reprogramming during cellular stress is emerging, where it may be involved in the regulation of stress granule formation and in mediating non-canonical translation initiation. Finally, we also discuss the role of DDX3X-mediated translation regulation during viral infections. Dysregulation of DDX3X's function in mRNA translation likely contributes to its involvement in disease pathophysiology. Thus, a better understanding of its exact mechanisms for regulating translation of specific mRNA targets is important, so that we can potentially develop therapeutic strategies for overcoming the negative effects of its dysregulation.

DDX3 interacts with USP9X and participates in deubiquitination of the anti-apoptotic protein MCL1

Here, we describe a novel interaction between the RNA helicase DDX3 and the deubiquitinase ubiquitin-specific peptidase 9 X-linked (USP9X) in human cells. Domain mapping studies reveal that the C-terminal region of DDX3 interacted with the N terminus of USP9X. USP9X was predominantly localized in the cytoplasm where the interaction between DDX3 and USP9X occurred. USP9X was not visibly enriched in cytoplasmic stress granules (SGs) under oxidative stress conditions, whereas overexpression of GFP-DDX3 induced SG formation and recruited USP9X to SGs in HeLa cells. Luciferase reporter assays showed that depletion of USP9X had no significant effect on DDX3-mediated translation. Given that DDX3 is not ubiquitinated upon ubiquitin overexpression, it is unlikely that DDX3 serves as a substrate of USP9X. Importantly, we found that ubiquitinated MCL1 was accumulated upon depletion of USP9X and/or DDX3 in MG132-treated cells, suggesting that USP9X and DDX3 play a role in regulating MCL1 protein stability and anti-apoptotic function. This study indicates that DDX3 exerts anti-apoptotic effects probably by coordinating with USP9X in promoting MCL1 deubiquitination.

DDX3 depletion represses translation of mRNAs with complex 5' UTRs

DDX3 is an RNA chaperone of the DEAD-box family that regulates translation. Ded1, the yeast ortholog of DDX3, is a global regulator of translation, whereas DDX3 is thought to preferentially affect a subset of mRNAs. However, the set of mRNAs that are regulated by DDX3 are unknown, along with the relationship between DDX3 binding and activity. Here, we use ribosome profiling, RNA-seq, and PAR-CLIP to define the set of mRNAs that are regulated by DDX3 in human cells. We find that while DDX3 binds highly expressed mRNAs, depletion of DDX3 particularly affects the translation of a small subset of the transcriptome. We further find that DDX3 binds a site on helix 16 of the human ribosomal rRNA, placing it immediately adjacent to the mRNA entry channel. Translation changes caused by depleting DDX3 levels or expressing an inactive point mutation are different, consistent with different association of these genetic variant types with disease. Taken together, this work defines the subset of the transcriptome that is responsive to DDX3 inhibition, with relevance for basic biology and disease states where DDX3 is altered.

The ODC 3'-Untranslated Region and 5'-Untranslated Region Contain cis-Regulatory Elements: Implications for Carcinogenesis

It has been hypothesized that both the 3'-untranslated region (3'UTR) and the 5'-untranslated region (5'UTR) of the ornithine decarboxylase (ODC) mRNA influence the expression of the ODC protein. Here, we use luciferase expression constructs to examine the influence of both UTRs in keratinocyte derived cell lines. The ODC 5'UTR or 3'UTR was cloned into the pGL3 control vector upstream or downstream of the luciferase reporter gene, respectively, and luciferase activity was measured in both non-tumorigenic and tumorigenic mouse keratinocyte cell lines. Further analysis of the influence of the 3'UTR on luciferase activity was accomplished through site-directed mutagenesis and distal deletion analysis within this region. Insertion of either the 5'UTR or 3'UTR into a luciferase vector resulted in a decrease in luciferase activity when compared to the control vector. Deletion analysis of the 3'UTR revealed a region between bases 1969 and 2141 that was inhibitory, and mutating residues within that region increased luciferase activity. These data suggest that both the 5'UTR and 3'UTR of ODC contain cis-acting regulatory elements that control intracellular ODC protein levels.

Protocols for studying antizyme expression and function

Antizyme (AZ) is a key molecule in feedback regulation of cellular polyamines. It is induced by polyamines through stimulation of ribosomal frameshifting during its translation. In mammals, AZ is diverged into three paralogs, AZ1-3. Tissue and subcellular distribution are different among the paralogs, as determined by immunochemical methods or expression of fluorescent-tagged proteins. Only AZ2 is known to be phosphorylated. AZ regulates cellular polyamine levels through multiple mechanisms. It binds to ornithine decarboxylase (ODC) to form an inactive complex and to trigger degradation of ODC by 26S proteasomes. The AZ activity to promote ODC degradation can be measured both in vitro and in cells. AZ also inhibits cellular uptake of polyamines. This chapter comprises seven subchapters describing methods for studying expression and function of AZ.

A profusion of upstream open reading frame mechanisms in polyamine-responsive translational regulation

In many eukaryotic mRNAs one or more short 'upstream' open reading frames, uORFs, precede the initiator of the main coding sequence. Upstream ORFs are functionally diverse as illustrated by their variety of features in polyamine pathway biosynthetic mRNAs. Their propensity to act as sensors for regulatory circuits and to amplify the signals likely explains their occurrence in most polyamine pathway mRNAs. The uORF-mediated polyamine responsive autoregulatory circuits found in polyamine pathway mRNAs exemplify the translationally regulated dynamic interface between components of the proteome and metabolism.

The DEAD-box RNA helicase DDX3 associates with export messenger ribonucleoproteins as well as tip-associated protein and participates in translational control

Nuclear export of mRNA is tightly linked to transcription, nuclear mRNA processing, and subsequent maturation in the cytoplasm. Tip-associated protein (TAP) is the major nuclear mRNA export receptor, and it acts coordinately with various factors involved in mRNA expression. We screened for protein factors that associate with TAP and identified several candidates, including RNA helicase DDX3. We demonstrate that DDX3 directly interacts with TAP and that its association with TAP as well as mRNA ribonucleoprotein complexes may occur in the nucleus. Depletion of TAP resulted in nuclear accumulation of DDX3, suggesting that DDX3 is, at least in part, exported along with messenger ribonucleoproteins to the cytoplasm via the TAP-mediated pathway. Moreover, the observation that DDX3 localizes transiently in cytoplasmic stress granules under cell stress conditions suggests a role for DDX3 in translational control. Indeed, DDX3 associates with translation initiation complexes. However, DDX3 is probably not critical for general mRNA translation but may instead promote efficient translation of mRNAs containing a long or structured 5' untranslated region. Given that the DDX3 RNA helicase activity is essential for its involvement in translation, we suggest that DDX3 facilitates translation by resolving secondary structures of the 5'-untranslated region in mRNAs during ribosome scanning.

Abnormal ornithine decarboxylase activity in transgenic mice increases tumor formation and infertility

A transgenic mouse line carrying ornithine decarboxylase cDNA as the transgene under the control of a mouse mammary tumor virus long terminal repeat (MMTV LTR) promoter was generated in order to study whether ornithine decarboxylase transgene expression will have any physiological or pathological effect during the entire life of a transgenic mouse. The high frequency of infertile animals and the loss of pups made the breeding of homozygous mice unsuccessful. However, a colony of heterozygous transgenic mice was followed for 2 years. In adult heterozygous transgenic mice, ornithine decarboxylase activity was significantly increased in the testis, seminal vesicle and preputial gland when compared to non-transgenic controls. In contrast, ornithine decarboxylase activity was decreased in the kidney and prostate of transgenic mice. No significant changes in ornithine decarboxylase activity were found in the ovary and mammary gland and only moderate changes in ornithine decarboxylase activity were detected in the heart, brain, pancreas and lung. The most common abnormalities found in adult animals (12 males and 20 females) of the transgenic line were inflammatory processes, including pancreatitis, hepatitis, sialoadenitis and pyelonephritis. Spontaneous tumors were observed in eight animals, including two benign tumors (one dermatofibroma, one liver hemangioma) and six malignant tumors (one lymphoma, one intestinal and three mammary adenocarcinomas and one adenocarcinoma in the lung). No significant pathological changes were found in 17 nontransgenic controls.

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.

Two zebrafish (Danio rerio) antizymes with different expression and activities

Cellular polyamines are regulated by a unique feedback mechanism involving ornithine decarboxylase (ODC) antizyme. The synthesis of mammalian antizyme requires a programmed translational frameshift event induced by polyamines. Antizyme represses ODC, a key enzyme for polyamine synthesis, through accelerating enzyme degradation by the 26 S proteasome. Antizyme also inhibits the cellular uptake of polyamines. In the present study we isolated two distinct zebrafish (Danio rerio) antizyme cDNA clones (AZS and AZL) from an embryonic library. Their sequences revealed that both clones required translational frameshifting for expression. Taking account of +1 frameshifting, AZS and AZL products were 214 and 218 residues long respectively and shared 51.8% amino acid identity. In rabbit reticulocyte lysates, both mRNA species were translated through spermidine-induced frameshifting. The presence of the two antizyme mRNA species in embryos, adult fish and a cultured cell line was confirmed by Northern blot analysis. The ratio of AZS mRNA to AZL mRNA in the adult fish was 1.8-fold higher than in the embryos. Whole-mount hybridization in situ demonstrated that both mRNA species are expressed in every tissue in embryo, but predominantly in the central nervous system and the eyes. Bacterial expression products of both cDNA species inhibited ODC activity, but only the AZS product accelerated ODC degradation in vitro. These results show that both zebrafish antizymes are induced by polyamines but their mRNA species are expressed differently during development. The difference in activities on ODC degradation suggests their functional divergence.

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.

Structure/function relationship studies on the T/S residues 173-177 of rat ODC

A well-conserved T/S cluster was detected among vertebrate ornithine decarboxylase by computer analysis (E. Viguera, O. Trelles, J.L. Urdiales, J.M. Matés, F. Sánchez-Jiménez, Trends Biochem. Sci. 19 (1994) 318-319). In the present report we studied the role of these residues (173, 176 and 177 in rat ornithine decarboxylase (ODC)) in enzymic activity and stability by in vitro expression, kinetic characterization and in vitro degradation of site-directed mutants. These T/S residues are substituted by a D/E-enriched fragment in other lower eukaryotic ODCs. The substitution of the T/S-enriched fragment (TLKTS) of rat ODC by the negative charged fragment of T. brucei ODC (KVEDC) did not affect protein stability, but increased Km values of the mutant enzyme. The substitution of the T/S residues by alanine also has a similar effect on rat ODC kinetic values. However, results indicate that polarity of the fragment must be an important factor for protein conformation, since the latter mutant, having no T/S or D/E residue in the fragment (ALKAA), showed reduced stability in vitro.

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.

Co-operation of the 5' and 3' untranslated regions of ornithine decarboxylase mRNA and inhibitory role of its 3' untranslated region in regulating the translational efficiency of hybrid RNA species via cellular factor

The 5' untranslated region (UTR) has an inhibitory role in the translatability of ornithine decarboxylase (ODC) mRNA and of hybrid mRNA species, whereas the ODC 3' UTR causes a partial release of this inhibition. We designed experiments to explore whether the co-operation between ODC 5' UTR and 3' UTR in the translational regulation is due to a direct interaction of those sequences or whether it is mediated by their interaction with cellular factor(s). We stably transfected Chinese hamster ovary (CHO)-K1 cells and transiently transfected COS-1 cells with expression vectors carrying different chimaeric DNAs having the luciferase (LUC) coding sequence as reporter gene, the ODC 5' UTR or the ODC 3' UTR, or both, in the appropriate positions. We compared the results obtained by assaying the LUC activities of both transfected cell lines with each chimaeric DNA with those observed by translating the hybrid RNAs in a translation system in vitro. When the ODC 3' UTR was present, we observed a partial release of the translation inhibition owing to the ODC 5' UTR only in vivo. The releasing effect was restored in vitro by the addition of cytoplasmic extracts from wild-type CHO-K1 or COS-1 cells, prepared 2 and 8 h after their release from serum starvation. We also observed a partial inhibition of the translatability of the hybrid RNA owing to the presence of the ODC 3' UTR itself; the translational efficiency could be rescued by cell extract from 8 h serum-stimulated cells. The co-operation between the ODC-UTRs might be mediated by factors expressed by cells during particular phases of the cell cycle. Excess copies of the ODC-UTRs, expressed in trans, could compete in binding limited amounts of such regulatory factors and remove them from interaction with the endogenous ODC mRNA. This phenomenon should be reflected by modifications of the kinetics of ODC and/or LUC activities during serum stimulation. The overexpression of the ODC 3' UTR determined an increase in both endogenous ODC activity and LUC activity. Moreover, in the transfectants expressing the hybrid RNA species bearing the ODC 3' UTR the basal ODC activity is higher than that observed in control cells. We suggest that excess copies of the ODC 3' UTR mis-regulate the endogenous ODC translatability, probably by tying up regulatory molecules expressed by cells in limited amounts and sequestering them from the ODC mRNA species they should interact with.

Brain-derived peptides increase the expression of a blood-brain barrier GLUT1 glucose transporter reporter gene

The brain-derived peptide preparation Cerebrolysin (C1; EBEWE, Austria) increases the stability of blood-brain barrier (BBB)-GLUT1 transcript. To determine if the increase in BBB-GLUT1 mRNA stability is associated with an augmentation of gene expression, the present investigation studied the effect of C1 on the expression of a BBB-GLUT1-luciferase reporter gene in brain endothelial cultured (ECL) cells. Dose response studies showed that C1 markedly increased the expression of luciferase when the BBB-GLUT1-reporter gene was used. On the contrary, C1 produced no changes in the expression pattern of the control reporter gene, which lacks the GLUT1 regulatory sequence. Desensitization of the protein kinase C (PKC) receptor with the phorbol ester TPA, or inhibition with either 1-(5-isoquinolinylsulfonyl)-2-methylpiperazine (H7) or staurosporine, had no effect on the increased levels of luciferase induced by C1. Transfection efficiency was determined by measuring intracellular levels of the expression vector using a quantitative polymerase chain reaction (PCR) assay. The data presented here demonstrate that C1 increases BBB-GLUT1 gene expression in ECL cells through a mechanism that appears to be independent of activation of PKC.

Experimental evidence for structure-activity features in common between mammalian histidine decarboxylase and ornithine decarboxylase

Common protein motifs between histidine decarboxylase (HDC) and ornithine decarboxylase (ODC) were detected by computational analysis. Mutants were generated and expressed in vitro. In both enzymes, terminal PEST-region-containing fragments are not essential for decarboxylation (PEST regions are sequence fragments enriched in proline, glutamic acid, serine and threonine residues in a hydrophilic fragment flanked by cationic amino acids). The substitution of a very well conserved histidine residue by alanine causes a severalfold increase of the apparent K(m) values for the respective substrates.

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 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.

Forced expression of antizyme abolishes ornithine decarboxylase activity, suppresses cellular levels of polyamines and inhibits cell growth

Ornithine decarboxylase (ODC) is a key enzyme in polyamine biosynthesis. It is a short-lived protein and negatively regulated by its products, polyamines. Its degradation is accelerated by the binding of antizyme, an ODC-inhibitory protein induced by polyamines. To evaluate the physiological importance of antizyme we examined the effect of forced expression of antizyme on cellular ODC and polyamine levels and cell growth. Antizyme almost completely abolished the induction of ODC by growth stimuli. This may have been caused by antizyme-induced rapid degradation of newly synthesized ODC, since the half-life of ODC complexes with antizyme was less than 5 min. Forced expression of antizyme caused reductions of cellular putrescine and spermidine levels, and inhibited cell growth, which was partially restored by the addition of putrescine. These observations suggested a critically important role of antizyme in polyamine metabolism.

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.

Storage of messenger RNA in eukaryotes: envelopment with protein, translational barrier at 5' side, or conformational masking by 3' side?

Messenger RNA can be stored in the cytoplasm of higher Eukaryotes in the form of masked messenger ribonucleoprotein particles (masked mRNPs, or informosomes). The typical example is the storage of mRNPs in germ cells (oocytes and spermatocytes). The masked mRNPs are inactive in translation, stable, i.e., protected against degradation, and unavailable for poly(A) tail processing, such as cytoplasmic polyadenylation and deadenylation. The major nonspecific mRNA-binding protein forming mRNPs and belonging to a special p50 family of basic, glycine-rich, phosphorylatable proteins seems to be necessary, but not sufficient for the masking. In some cases, mRNA-specific repressor proteins bound to the 5'-untranslated regions (5'-UTR) of mRNAs may be involved. Interactions of the 3'-untranslated regions (3'-UTR) with sequence-specific proteins seem to be of decisive importance for the masking of mRNPs. The hypothesis is proposed that the masking is achieved through a 3'-UTR-induced conformational rearrangement of mRNP; closing into a circle and condensation of mRNP are considered plausible.

Humoral regulation of intestinal adaptation

After the loss of small bowel through disease or surgery the residual bowel adapts by increasing its functional capacity. This process of adaptation involves dilatation, hypertrophy and mucosal hyperplasia, particularly distal to the area of bowel loss or disease. The response of the residual bowel is mediated by a complex interplay of factors including luminal nutrition, pancreaticobiliary secretions, luminal or local growth factors and also humoral or endocrine factors. The experimental model commonly used to characterize the adaptive response, massive small bowel resection (MSBR), involves 80% resection of the small bowel in the rat. Of the various putative humoral factors, most work has focused on the products of the ileal L cells: enteroglucagon and peptide YY. Plasma levels of both hormones are increased after MSBR and indeed their mRNA levels are also increased as a result of an increase in the amount of message per L cell. Whilst PYY probably serves as an 'ileal brake' to slow the movement of the luminal contents and hence increase their mucosal contact time, the role of the enteroglucagon is unresolved. The molecular cloning of the proglucagon gene has revealed, firstly, that there are a number of biologically active peptides which derive from the propeptide and, secondly, that tissue-specific differential processing occurs. Most studies do not clearly define which of these products of proglucagon is being measured and is termed as glucagon-like or enteroglucagon immunoreactivity. The insulin-like growth factors (IGF) have a potent mitogenic action on the bowel. Their role after MSBR is likely to be largely paracrine. Though IGF-I mRNA levels do not increase after MSBR, the precipitous and early fall in ileal IGF-binding protein-3 (IGFBP-3) mRNA levels suggests a fall in IGFBP-3 levels may increase local IGF-I bioactivity. Polyamine synthesis is a critical component of the adaptive response, although the stimulus to their dramatic increase in synthesis after MSBR remains to be elucidated. Other humoral factors such as cholecystokinin, neurotensin and bombesin probably have minor indirect roles in the adaptive response. Components of the epidermal growth factor/transforming growth factor alpha response pathway family of growth factors may be involved as paracrine regulators. There is thus strong evidence that humoral factors play an important role in intestinal adaptation; characterization of the nature of the humoral factors and their relationship with other influences such as luminal nutrition and pancreatic biliary secretions may facilitate the development of new therapeutic strategies for the short bowel syndromes.

Single amino-acid replacement is responsible for the stabilization of ornithine decarboxylase in HMOA cells

The half-life of ornithine decarboxylase (ODC) in HMOA cells, a variant cell line derived from hepatoma tissue culture (HTC) cells, is markedly increased compared with that in the parental cell line. In the present study, we examined which of the three relevant factors is responsible for the ODC stabilization in HMOA cells, namely ODC itself, a regulatory protein antizyme and an ODC-degrading activity. SDS/PAGE analysis of radiolabeled ODC revealed that ODC from HMOA cells migrated somewhat faster than that from HTC cells, suggesting that HMOA ODC was structurally altered. Direct sequencing of reverse-transcription/polymerase-chain-reaction (RT-PCR) products of ODC mRNA from HMOA cells revealed a T to G replacement, causing a Cys441-->Trp replacement near the C-terminus. No alteration was found in the whole coding region of antizyme mRNA. An authentic mutant ODC cDNA with the same replacement was transfected and expressed in C55.7 ODC-deficient Chinese hamster ovary cells. Upon cycloheximide treatment, the mutant ODC activity did not decrease appreciably for at least 3 h, whereas wild-type ODC activity decreased with a half-life of 1 h. In-vitro-synthesized mutant ODC with the Cys441-->Trp (or Ala) replacement was also stable in a reticulocyte-lysate ODC-degradation system. Metabolically labeled and purified mouse ODC was degraded in HMOA cell extracts in the presence of ATP and antizyme as rapidly as in HTC cell extracts, indicating that HMOA cells have a normal ODC degrading activity. These results indicated that the single amino acid replacement, Cys441-->Trp, is responsible for the stabilization of ODC in HMOA cells and that Cys441 is important for rapid ODC turnover.

The induction of ornithine decarboxylase by ornithine takes place at post-transcriptional level in perifused Ehrlich carcinoma cells

The increase in ODC activity during perifusion of Ehrlich carcinoma cells with 0.5 mM ornithine correlates with an increase in 'de novo' synthetized ODC protein. ODC synthesis was followed by immunoprecipitation of equal quantities of 35S-labelled proteins after 10, 20 and 30 min of labelling. In addition, the rate of 'de novo' protein synthesis is very much elevated in cells perifused with saline buffer supplemented with 0.5 mM ornithine than in cells perifused with the saline buffer only. In spite of the higher specific ODC activity observed in cells perifused with saline buffer plus 0.5 mM ornithine respect to cells perifused with only saline buffer for 3.5 h, no elevation in ODC mRNA was observed when the cells were perifused in the presence of 0.5 mM ornithine.

Expression of ileal glucagon and peptide tyrosine-tyrosine genes. Response to inhibition of polyamine synthesis in the presence of massive small-bowel resection

Massive small-bowel resection results in a marked adaptive response in the residual terminal ileum. Increased polyamine synthesis is a necessary component of this response. The ileal L-cell-derived peptides enteroglucagon and peptide tyrosine tyrosine (PYY) have been implicated as humoral mediators of this response. We have previously reported a rapid and sustained increase in glucagon mRNA concentrations after massive small-bowel resection. In this study using an inhibitor of the rate-limiting enzyme in polyamine biosynthesis, ornithine decarboxylase, we have demonstrated that the response of the glucagon and PYY genes to massive small-bowel resection is dependent on polyamine biosynthesis. In addition, we have examined the response of both the ornithine decarboxylase and c-jun genes in this model of intestinal adaptation.

Dependence of the adenovirus tripartite leader on the p220 subunit of eukaryotic initiation factor 4F during in vitro translation. Effect of p220 cleavage by foot-and-mouth-disease-virus L-protease on in vitro translation

The adenovirus tripartite leader (TPT) 5' untranslated region (5'UTR) allows translation in poliovirus-infected cells, in which the p220 subunit of eukaryotic initiation factor 4F is degraded. This p220-independent translation was investigated by measuring in vitro translation in a reticulocyte lysate of a reporter gene, chloramphenicol acetyltransferase, coupled to the TPT 5'UTR. The p220 subunit was degraded by translation of a foot-and-mouth-disease L-protease construct. Surprisingly, the TPT 5'UTR was dependent on intact p220, as are other naturally capped mRNA species. Translation of encephalomyocarditis virus RNA was p220 independent, as expected from its ability to support internal, cap-independent initiation. In vitro protein-synthesis experiments with purified initiation factors confirmed the dependence of TPT mRNA translation on eukaryotic initiation factor 4F. The relationship between adenovirus TPT-5'UTR-directed translation and poliovirus-induced host cell shut-off is discussed.

Chlorpheniramine inhibits the ornithine decarboxylase induction of Ehrlich carcinoma growing in vivo

The antihistaminic (+/-)-chlorpheniramine significantly reduced the progression of Ehrlich carcinoma when it was administered at 0.5 mg/mouse/day from the third day on, after tumour inoculation. The ODC activity of tumour cells was diminished by 70% on day 7 after tumour transplantation, when maximum ODC activity is detected in non-treated tumour growing 'in vivo'. Northern blot analyses indicated that the inhibitory effect of this 1,4-diamine takes place at a post-transcriptional level. Results obtained from serum-free cultured cells indicated that chlorpheniramine inhibits the ODC synthesis rate.

Ornithine decarboxylase gene of Neurospora crassa: isolation, sequence, and polyamine-mediated regulation of its mRNA

Ornithine decarboxylase (ODC), which initiates the biosynthesis of the polyamines putrescine, spermidine, and spermine, is encoded by the spe-1 gene of the fungus Neurospora crassa. This gene and its cDNA have been cloned and sequenced. The gene has a single 70-nucleotide intron in the coding sequence. The cDNA, comprising the entire coding region, recognizes a single 2.4-kb mRNA in Northern (RNA) blots. The mRNA transcript, defined by S1 mapping, has an extremely long, 535-base leader without strong secondary-structure features or an upstream reading frame. The translational start of the protein is ambiguous: a Met-Val-Met sequence precedes the Pro known to be the N terminus of the ODC polypeptide. The polypeptide encoded by the N. crassa spe-1 gene (484 amino acids) has 46% amino acid identity with that of Saccharomyces cerevisiae (466 amino acids) and 42% with that of mouse (461 amino acids). Alignment of the longer N. crassa sequence with S. cerevisiae and mouse sequences creates gaps in different sites in the S. cerevisiae and mouse sequences, suggesting that N. crassa ODC is closer to an ancestral form of the enzyme than that of either yeast or mouse ODC. N. crassa ODC, which turns over rapidly in vivo in the presence of polyamines, has two PEST sequences, found in most ODCs and other proteins with rapid turnover. In striking contrast to other eucaryotic organisms, the variation in the rate of ODC synthesis in response to polyamines in N. crassa is largely correlated with proportional changes in the abundance of ODC mRNA. Spermidine is the main effector of repression, while putrescine has a weaker effect. However, putrescine accumulation appears to increase the amount of active ODC that is made from a given amount of ODC mRNA, possibly by improving its translatability. Conversely, prolonged starvation for both putrescine and spermidine leads to the differentially impaired translation of ODC mRNA.

Ornithine decarboxylase gene of Neurospora crassa: isolation, sequence, and polyamine-mediated regulation of its mRNA

Ornithine decarboxylase (ODC), which initiates the biosynthesis of the polyamines putrescine, spermidine, and spermine, is encoded by the spe-1 gene of the fungus Neurospora crassa. This gene and its cDNA have been cloned and sequenced. The gene has a single 70-nucleotide intron in the coding sequence. The cDNA, comprising the entire coding region, recognizes a single 2.4-kb mRNA in Northern (RNA) blots. The mRNA transcript, defined by S1 mapping, has an extremely long, 535-base leader without strong secondary-structure features or an upstream reading frame. The translational start of the protein is ambiguous: a Met-Val-Met sequence precedes the Pro known to be the N terminus of the ODC polypeptide. The polypeptide encoded by the N. crassa spe-1 gene (484 amino acids) has 46% amino acid identity with that of Saccharomyces cerevisiae (466 amino acids) and 42% with that of mouse (461 amino acids). Alignment of the longer N. crassa sequence with S. cerevisiae and mouse sequences creates gaps in different sites in the S. cerevisiae and mouse sequences, suggesting that N. crassa ODC is closer to an ancestral form of the enzyme than that of either yeast or mouse ODC. N. crassa ODC, which turns over rapidly in vivo in the presence of polyamines, has two PEST sequences, found in most ODCs and other proteins with rapid turnover. In striking contrast to other eucaryotic organisms, the variation in the rate of ODC synthesis in response to polyamines in N. crassa is largely correlated with proportional changes in the abundance of ODC mRNA. Spermidine is the main effector of repression, while putrescine has a weaker effect. However, putrescine accumulation appears to increase the amount of active ODC that is made from a given amount of ODC mRNA, possibly by improving its translatability. Conversely, prolonged starvation for both putrescine and spermidine leads to the differentially impaired translation of ODC mRNA.

Cowpea mosaic virus middle component RNA contains a sequence that allows internal binding of ribosomes and that requires eukaryotic initiation factor 4F for optimal translation

Cowpea mosaic virus (CPMV) middle component RNA (M-RNA) encodes two proteins of 105 and 95 kDa, of which translation starts at nucleotide (nt) 161 and nt 512, respectively. In vitro translation of both proteins directed by T7 transcripts of M-RNA was stimulated fourfold by eukaryotic initiation factor 4F (eIF-4F), the cap-binding protein complex. The ratio of the synthesis of both proteins after translation was not influenced by eIF-4F or by any known eIF. Part of the CPMV 5' sequence was cloned downstream of the 5' untranslated region of ornithine decarboxylase (ODC); the latter untranslated sequence has a highly stable secondary structure, preventing efficient translation of ODC. Insertion of nt 161 to 512 of CPMV M-RNA upstream of the ODC initiation codon resulted in a marked increase in ODC translation, which indicates that the CPMV sequence contains an internal ribosome-binding site. The insertion conferred stimulation by eIF-4F on ODC translation, showing that eIF-4F is able to stimulate internal initiation.