Defining a novel cis-element in the 3'-untranslated region of mammalian ribonucleotide reductase component R2 mRNA. cis-trans-interactions and message stability

Effects of cross-linked high-molecular-weight hyaluronic acid on epidural fibrosis: experimental study

OBJECT

Epidural fibrosis is nonphysiological scar formation, usually at the site of neurosurgical access into the spinal canal, in the intimate vicinity of and around the origin of the radicular sheath. The formation of dense fibrous tissue causes lumbar and radicular pain. In addition to radicular symptoms, the formation of scar tissue may cause problems during reoperation. The authors aimed to investigate the effects of cross-linked high-molecular-weight hyaluronic acid (HA), an HA derivative known as HA gel, on the prevention of epidural fibrosis by using histopathological and biochemical parameters.

METHODS

Fifty-six adult female Sprague-Dawley rats were evaluated. The rats were divided into 4 groups. Rats in the sham group (n = 14) underwent laminectomy and discectomy and received no treatment; rats in the control group (n = 14) underwent laminectomy and discectomy and received 0.9% NaCl treatment in the surgical area; rats in the HA group (n = 14) received HA treatment at the surgical area after laminectomy and discectomy; and rats in the HA gel group (n = 14) underwent laminectomy and discectomy in addition to receiving treatment with cross-linked high-molecular-weight HA in the surgical area. All rats were decapitated after 4 weeks, and the specimens were evaluated histopathologically and biochemically. The results were statistically compared using the Mann-Whitney U-test.

RESULTS

Compared with the sham and control groups, the HA and HA gel groups showed significantly lower fibroblast cell density and tissue hydroxyproline concentrations (p < 0.05). There was statistically significant lower dural adhesion and foreign-body reaction between the control and HA gel groups (p < 0.05). Granulation tissue and epidural fibrosis were significantly lower in the HA and HA gel groups compared with the sham group (p < 0.05). There were no significant differences in any histopathological parameters or biochemical values between Groups 3 and 4 (p > 0.05).

CONCLUSIONS

Cross-linked high-molecular-weight HA had positive effects on the prevention of epidural fibrosis and the reduction of fibrotic tissue density. The efficacy of this agent should also be verified in further experimental and clinical studies.

The role of transforming growth factor beta signaling in messenger RNA stability

Transforming growth factor beta (TGF-beta) is a biologically multipotent regulatory protein implicated in functions that include the regulation of cellular growth, differentiation, extracellular matrix formation, and wound healing. It also plays a role in the pathologies of Alzheimer's disease, cancer and autoimmune disorders. TGF-beta modulates gene expression by affecting transcriptional activation and mRNA turnover rate. Steady-state mRNA levels depend on both the transcriptional activity and mRNA half-life. The stability of mRNA can be modified by the binding of trans-acting factors to cis-elements on the message. These can protect the mRNA from cleavage by RNAses, or they may promote mRNA cleavage. Changes in mRNA stability can lead to changes in the proteome and subsequently in cellular metabolism. The SMAD family of proteins has been implicated in the transduction of the TGF-beta signal, where they regulate transcriptional activity. This review attempts to provide new insights into the role played by TGF-beta in the regulation of mRNA turnover.

Differential RNA expression and polyribosome loading of alternative transcripts of the Akap4 gene in murine spermatids

An X chromosome-linked gene, Akap4, is expressed only during spermiogenesis and encodes the major fibrous sheath protein of the mouse sperm flagellum. All sperm contain the AKAP4 protein even though only X chromosome-bearing spermatids express the gene, indicating that the Akap4 mRNA and/or protein must be shared among the conjoined spermatids via the intercellular bridges. There are two mouse Akap4 cDNA clones, Akap82 and Fsc1, which represent mRNAs that arise by alternative processing of a single gene. Although Akap82 and Fsc1 encode identical mature proteins, they differ in their 5' UTRs. We hypothesized that the expression pattern of these two mRNAs might be relevant to the issue of mRNA and/or protein transport into adjacent spermatids. Expression of both transcripts began in round spermatids, but the amount of the Akap82 transcript in condensing spermatids increased twofold relative to Fsc1. Significantly, only the Akap82 transcript was found on polyribosomes and translated in spermatids. These results indicate that the Akap82 transcript and/or its protein must be shared among the conjoined X and Y chromosome-bearing spermatids. Although Fsc1 was not polysomal, both the Akap82 and Fsc1 transcripts were deadenylated during spermiogenesis, suggesting that deadenylation is not always correlated with loading of mRNAs onto polyribosomes in germ cells. The distinct 5' UTR sequences in Akap82 and Fsc1 did not differ in their ability to regulate translation of reporter constructs either in vivo or in vitro. Antisense RNA transcripts complementary to both the Akap82 and Fsc1 mRNAs were present, suggesting that translatability may be regulated by these RNAs.

Regulation of ribonucleotide reductase M2 expression by the upstream AUGs

Ribonucleotide reductase catalyzes a rate-limiting reaction in DNA synthesis by converting ribonucleotides to deoxyribonucleotides. It consists of two subunits and the small one, M2 (or R2), plays an essential role in regulating the enzyme activity and its expression is finely controlled. Changes in the M2 level influence the dNTP pool and, thus, DNA synthesis and cell proliferation. M2 gene has two promoters which produce two major mRNAs with 5′-untranslated regions (5′-UTRs) of different lengths. In this study, we found that the M2 mRNAs with the short (63 nt) 5′-UTR can be translated with high efficiency whereas the mRNAs with the long (222 nt) one cannot. Examination of the long 5′-UTR revealed four upstream AUGs, which are in the same reading frame as the unique physiological translation initiation codon. Further analysis demonstrated that these upstream AUGs act as negative cis elements for initiation at the downstream translation initiation codon and their inhibitory effect on M2 translation is eIF4G dependent. Based on the findings of this study, we conclude that the expression of M2 is likely regulated by fine tuning the translation from the mRNA with a long 5′-UTR during viral infection and during the DNA replication phase of cell proliferation.

Two Plasmodium falciparum ribonucleotide reductase small subunits, PfR2 and PfR4, interact with each other and are components of the in vivo enzyme complex

Ribonucleotide reductase (RNR) is a tetrameric enzyme, composed of two large (R1) and two small (R2) subunits, which regulates the nucleotide balance in cells by controlling the rate-limiting step for deoxyribonucleotide synthesis. We have identified a second copy of the small subunit gene, termed PfR4, encoding a 324 amino acid residue polypeptide that shares only 25% identity with the previously identified PfR2 small subunit of Plasmodium falciparum. PfR4 expression is cell-cycle-regulated, and the profile of transcript and protein expression corresponds to that of PfR2. A 1.3 kb PfR4 5'-flanking fragment contained a functional promoter activity. We have detected interaction between PfR2 and PfR4 by co-immunoprecipitation experiments. Indirect immunofluorescence analysis showed distinct localization of two small RNR subunits with some colocalization. The association of PfR1 large subunit with PfR4 was detected by GST pull-down assay. This interaction is reduced significantly when using a PfR4 truncated at the COOH terminus, suggesting the involvement of COOH-terminal residues in PfR4-PfR1 interaction. All three RNR subunits co-eluted on a Superose 12 size-exclusion column corresponding to fractions with a molecular mass of around 250 kDa. This suggests the existence of all three RNR subunits in Plasmodium in a native complex of alpha2betabeta' configuration.

RRM1-induced metastasis suppression through PTEN-regulated pathways

Lung cancer is the leading cause of cancer-related mortality in the United States. Only 15% of patients with this disease survive 5 years or longer. Early metastatic spread is the single most important reason for this poor outcome. The survival of patients with pathological stage I disease, that is, no evidence for metastatic spread, and molecular aberrations on chromosome 11p15.5 is equal to that of patients with stage II disease, that is, metastatic spread to hilar lymph nodes. RRM1 is a gene in this region, and it is haploinsufficient in at least 34% stage I patients. Here, we show that overexpression of RRM1 in human and mouse lung cancer cell lines induced PTEN expression, reduced phosphorylation of focal adhesion kinase (FAK), suppressed migration, invasion, and metastasis formation, and increased survival in an animal model. Increased PTEN expression was required for the RRM1-induced suppression of cell motility and FAK phosphorylation. We conclude that RRM1 functions as a metastasis suppressor gene through induction of PTEN expression.

Molecular characterization of the loss of p75(NTR) expression in human prostate tumor cells

The low-affinity nerve growth factor receptor p75(NTR) is a 75-kDa glycoprotein that belongs to the tumor necrosis factor receptor superfamily and has been implicated in the induction of apoptosis in various tissues and cell lines. Immunohistochemistry on tissue sections from radical prostatectomies has shown that expression of p75(NTR) is limited to the epithelial cells. Western blot and immunohistochemical analyses have also shown a progressive loss of p75(NTR) expression in prostate epithelial cells during the malignant progression of organ-confined adenocarcinomas, with complete loss of expression in the naturally occurring prostate tumor cell lines DU-145, PC-3, LNCaP, and TSU-pr1, which were derived from metastases. Reintroduction of p75(NTR) expression into the TSU-pr1 tumor cell line was shown to reestablish the ability of these cells to undergo p75(NTR)-mediated apoptosis. It is not known whether this loss of expression is due to deletion of part or the entire p75(NTR) gene or to other factors. Through the use of southern blotting and polymerase chain reaction (PCR), we showed that loss of p75(NTR) protein expression was not due to deletion or loss of the gene. Furthermore, through reverse transcription-PCR, RNase protection, and the chromatin immunoprecipitation assay, we showed that transcription of the p75(NTR) gene occurred in these prostate tumor cell lines. Finally, through transient transfection using two constructs of p75(NTR), one containing the full 2-kb 3' untranslated region and one that contains only a few hundred bases of the 3' untranslated region (UTR), we showed that the 3' UTR may have a role in the loss of p75(NTR) expression in prostate cancer.

A conserved mechanism for controlling the translation of beta-F1-ATPase mRNA between the fetal liver and cancer cells

To characterize the mechanisms governing the biogenesis of mitochondria in cancer, we studied the mitochondrial phenotype and the mechanisms controlling the expression of the beta subunit of the mitochondrial H(+)-ATP synthase (beta-F1-ATPase) gene in the rat FAO and AS30D hepatomas. When compared with normal adult rat liver, the relative cellular content of the mitochondrial beta-F1-ATPase and glutamate dehydrogenase, as well as of mitochondrial DNA, was severely reduced in both cell lines. A paradoxical increase in the cellular abundance of beta-F1-ATPase mRNA was observed in cancer cells. Run-on transcription assays and the estimation of mRNA half-lives revealed that the increased abundance of beta-F1-ATPase mRNA results from the stabilization of the transcript in cancer. In vitro translation assays revealed a specific inhibition of the synthesis of the beta-precursor when translation reactions were carried out in the presence of extracts derived from cancer cells. The inhibitory effect was recapitulated using an RNA chimera that contained the 3'-untranslated region of beta-F1-ATPase mRNA. Hepatoma extracts also contained an increased activity of the developmentally regulated translation-inhibitory proteins that bind the 3'-untranslated region of beta-F1-ATPase mRNA. The results indicate that the expression of this gene in hepatoma cells is controlled by the same mechanisms that regulate its expression in the liver during fetal development.

The regulation of ribonucleoside diphosphate reductase by the tumor promoter orotic acid in normal rat liver in vivo

Our earlier studies have shown that in normal hepatocytes, orotic acid (OA) inhibits DNA synthesis induced by several growth factors in vitro and after two-thirds partial hepatectomy (PH) in vivo. As in the normal liver OA induces an imbalance in nucleotide pools (specifically, an increase in uridine nucleotides, including deoxyuridine nucleotides, and a decrease in adenosine nucleotides, including ATP) and creation of this imbalance is crucial for the mitoinhibitory effects of OA, we hypothesized that ribonucleoside diphosphate reductase (RNR), a key enzyme in DNA synthesis that is regulated by nucleotide/deoxynucleotide levels, might be one of the targets for the inhibition of DNA synthesis by OA. To test this hypothesis, we subjected male Fischer 344 rats (130-150 g) to two-thirds PH in the absence or in the presence of OA (a 300-mg tablet of OA methyl ester implanted intraperitoneally at the time of two-thirds PH). The rats were killed at different times later, and their livers were processed for analysis of levels of RNR enzyme activity, protein, and mRNA transcripts. The results obtained indicated that treatment with OA resulted in a near-100% inhibition of RNR induced by two-thirds PH in rat liver, as monitored by enzyme activity and protein level. Furthermore, this inhibition was paralleled by a decrease in the mRNA transcripts for both the M1 and M2 subunits of RNR. Nuclear run-off assays indicated that this decrease in the levels of mRNA transcripts could not be attributed to an effect on transcription. However, administration of OA 20 h after two-thirds PH, when RNR mRNA transcripts were maximally induced, resulted in increased degradation of the RNR M1 and M2 subunits. Taken together, these results indicate that OA treatment decreases RNR levels induced by two-thirds PH, at the levels of enzyme activity, protein, and mRNA transcripts, and the decreased levels of mRNA transcripts appeared to be due to increased degradation of the transcripts.

Ribonucleotide reductase R2 protein is phosphorylated at serine-20 by P34cdc2 kinase

Ribonucleotide reductase is a rate-limiting enzyme in DNA synthesis and is composed of two different proteins, R1 and R2. The R2 protein appears to be rate-limiting for enzyme activity in proliferating cells, and it is phosphorylated by p34cdc2 and CDK2, mediators of cell cycle transition events. A sequence in the R2 protein at serine-20 matches a consensus sequence for p34cdc2 and CDK2 kinases. We tested the hypothesis that the serine-20 residue was the major p34cdc2 kinase site of phosphorylation. Three peptides were synthesized (from Asp-13 to Ala-28) that contained either the wild type amino acid sequence (Asp-Gln-Gln-Gln-Leu-Gln-Leu-Ser-Pro-Leu-Lys-Arg-Leu-Thr-Leu-Ala, serine peptide) or a mutation, in which the serine residue was replaced with an alanine residue (alanine peptide) or a threonine residue (threonine peptide). Only the serine peptide and threonine peptide were phosphorylated by p34cdc2 kinase. In two-dimensional phosphopeptide mapping experiments of serine peptide and Asp-N endoproteinase digested R2 protein, peptide co-migration patterns suggested that the synthetic phosphopeptide containing serine-20 was identical to the major Asp-N digested R2 phosphopeptide. To further test the hypothesis that serine-20 is the primary phosphorylated residue on R2 protein, three recombinant R2 proteins (R2-Thr, R2-Asp and R2-Ala) were generated by site-directed mutagenesis, in which the serine-20 residue was replaced with threonine, aspartic acid or alanine residues. Wild type R2 and threonine-substituted R2 proteins (R2-Thr) were phosphorylated by p34cdc2 kinase, whereas under the same experimental conditions, R2-Asp and R2-Ala phosphorylation was not detected. Furthermore, the phosphorylated amino acid residue in the R2-Thr protein was determined to be phosphothreonine. Therefore, by replacing a serine-20 residue with a threonine, the phosphorylated amino acid in R2 protein was changed to a phosphothreonine. In total, these results firmly establish that a major p34cdc2 phosphorylation site on the ribonucleotide reductase R2 protein occurs near the N-terminal end at serine-20, which is found within the sequence Ser-Pro-Leu-Lys-Arg-Leu. Comparison of ribonucleotide reductase activities between wild type and mutated forms of the R2 proteins suggested that mutation at serine-20 did not significantly affect enzyme activity.

Ribonucleotide reductases

Ribonucleotide reductases provide the building blocks for DNA replication in all living cells. Three different classes of enzymes use protein free radicals to activate the substrate. Aerobic class I enzymes generate a tyrosyl radical with an iron-oxygen center and dioxygen, class II enzymes employ adenosylcobalamin, and the anaerobic class III enzymes generate a glycyl radical from S-adenosylmethionine and an iron-sulfur cluster. The X-ray structure of the class I Escherichia coli enzyme, including forms that bind substrate and allosteric effectors, confirms previous models of catalytic and allosteric mechanisms. This structure suggests considerable mobility of the protein during catalysis and, together with experiments involving site-directed mutants, suggests a mechanism for radical transfer from one subunit to the other. Despite large differences between the classes, common catalytic and allosteric mechanisms, as well as retention of critical residues in the protein sequence, suggest a similar tertiary structure and a common origin during evolution. One puzzling aspect is that some organisms contain the genes for several different reductases.

Dual regulation of stromelysin-3 by fibroblast growth factor-2 in murine osteoblasts

Osteoblasts express stromelysin-3, a matrix metalloproteinase associated with normal remodeling processes and with stromal fibroblasts surrounding many invasive carcinomas. Fibroblast growth factors (FGFs) play an important role in skeletal development, fracture repair, and osteoblast function. The osteoblastic cell line MC3T3 was used to study the regulation of stromelysin-3 by FGF-2. Acutely, FGF-2 decreased stromelysin-3 mRNA levels, whereas prolonged treatment caused an induction of stromelysin-3 mRNA. RNA stability studies and nuclear run-off assays indicated that acute treatment with FGF-2 decreased stromelysin-3 mRNA stability but did not alter gene transcription. However, the induction of stromelysin-3 after prolonged treatment with FGF-2 resulted from increased gene transcription, with no effect on RNA stability. The stimulatory effect was protein synthesis-dependent, whereas the inhibitory effect was not. This study demonstrates dual regulation of stromelysin-3 by FGF-2: acute destabilization of stromelysin-3 mRNA, followed by induction of gene transcription. This complex regulation may be important in the function of stromelysin-3 in bone and in remodeling processes, such as wound and fracture repair.

Purified recombinant Fmrp exhibits selective RNA binding as an intrinsic property of the fragile X mental retardation protein

Fragile X syndrome is caused by the transcriptional silencing of the FMR1 gene due to a trinucleotide repeat expansion. The encoded protein, Fmrp, has been found to be a nucleocytoplasmic RNA-binding protein containing both KH domains and RGG boxes that associates with polyribosomes as a ribonucleoprotein particle. RNA binding has previously been demonstrated with in vitro-translated Fmrp; however, it remained uncertain whether the selective RNA binding observed was an intrinsic property of Fmrp or required an associated protein(s). Here, baculovirus-expressed and affinity-purified FLAG-tagged murine Fmrp was shown to bind directly to both ribonucleotide homopolymers and human brain mRNA. FLAG-Fmrp exhibited selectivity for binding poly(G) > poly(U) >> poly(C) or poly(A). Moreover, purified FLAG-Fmrp bound to only a subset of brain mRNA, including the 3' untranslated regions of myelin basic protein message and its own message. Recombinant isoform 4, lacking the RGG boxes but maintaining both KH domains, was also purified and was found to only weakly interact with RNA. FLAG-purified I304N Fmrp, harboring the mutation of severe fragile X syndrome, demonstrated RNA binding, in contrast to previous suggestions. These data demonstrate the intrinsic property of Fmrp to selectively bind RNA and show FLAG-Fmrp as a suitable reagent for structural characterization and identification of cognate RNA ligands.

Characterization of ribonucleoprotein complexes and their binding sites on the neurofilament light subunit mRNA

Levels of neurofilament (NF) gene expression are important determinants of basic neuronal properties, but overexpression can lead to motoneuron degeneration in transgenic mice. In a companion study (Cañete-Soler, R., Schwartz, M. L., Hua, Y., and Schlaepfer, W. W. (1998) J. Biol. Chem. 273, 12650-12654), we show that levels of NF expression are regulated by altering mRNA stability and that stability determinants are present in the 3'-coding region (3'-CR) and 3'-untranslated region (3'-UTR) of the NF light subunit (NF-L) transcript. This study characterizes the ribonucleoprotein complexes that bind to the NF-L mRNA when cytoplasmic brain extracts are incubated with radioactive probes. Gel retardation assays reveal ribonucleoprotein complexes that are selectively competed with poly(C) or poly(U))/poly(A) homoribopolymers and are referred to as C-binding and U/A-binding complexes, respectively. The C-binding complex forms on the proximal 45 nucleotides of 3'-UTR, but its assembly is markedly enhanced by 23 nucleotides of flanking 3'-CR sequence. U/A-binding complexes form at multiple binding sites in the 3'-CR and 3'-UTR. A pattern of reciprocal binding suggests that the C-binding and U/A-binding complexes interact and may compete for common components or binding sites. Cross-linking studies reveal unique polypeptides in the C-binding and U/A-binding complexes. The findings provide the basis for probing mechanisms regulating NF-L mRNA stability and the relationship between NF overexpression and motoneuron degeneration in transgenic mice.

The R1 component of mammalian ribonucleotide reductase has malignancy-suppressing activity as demonstrated by gene transfer experiments

Our recent studies have shown that deregulated expression of R2, the rate-limiting component of ribonucleotide reductase, enhances transformation and malignant potential by cooperating with activated oncogenes. We now demonstrate that the R1 component of ribonucleotide reductase has tumor-suppressing activity. Stable expression of a biologically active ectopic R1 in ras-transformed mouse fibroblast 10T(1/2) cell lines, with or without R2 overexpression, led to significantly reduced colony-forming efficiency in soft agar. The decreased anchorage independence was accompanied by markedly suppressed malignant potential in vivo. In three ras-transformed cell lines, R1 overexpression resulted in abrogation or marked suppression of tumorigenicity. In addition, the ability to form lung metastases by cells overexpressing R1 was reduced by >85%. Metastasis suppressing activity also was observed in the highly malignant mouse 10T(1/2) derived RMP-6 cell line, which was transformed by a combination of oncogenic ras, myc, and mutant p53. Furthermore, in support of the above observations with the R1 overexpressing cells, NIH 3T3 cells cotransfected with an R1 antisense sequence and oncogenic ras showed significantly increased anchorage independence as compared with control ras-transfected cells. Finally, characteristics of reduced malignant potential also were demonstrated with R1 overexpressing human colon carcinoma cells. Taken together, these results indicate that the two components of ribonucleotide reductase both are unique malignancy determinants playing opposing roles in its regulation, that there is a novel control point important in mechanisms of malignancy, which involves a balance in the levels of R1 and R2 expression, and that alterations in this balance can significantly modify transformation, tumorigenicity, and metastatic potential.

Organ-specific activity of the 5' regulatory region of the glutamine synthetase gene in developing mice

Glutamine synthetase (GS) converts ammonia and glutamate into glutamine. We assessed the activity of the 5' regulatory region of the GS gene in developing transgenic mice carrying the chloramphenicol acetyltransferase (CAT) gene under the control of 3150 bp of the upstream sequence of the rat GS gene to obtain insight into the spatiotemporal regulation of its pattern of expression. To determine the organ-specific activity of the 5' regulatory region CAT and GS mRNA expression were compared by ribonuclease-protection and semi-quantitative in situ hybridization analyses. Three patterns were observed: the 5' region is active and involved in the regulation of GS expression throughout development (pericentral hepatocytes, intestines and epididymis); the 5' region shows no activity at any of the ages investigated (periportal hepatocytes and white adipose tissue); and the activity of the 5' region becomes repressed during development (stomach, muscle, brown adipose tissue, kidney, lung and testis). In the second group, an additional element must be responsible for the activation of GS expression. The last group included organs in which the 5' regulatory region is active, but not in the cells that express GS. In these organs, the activity of the 5' regulatory region must be repressed by other regulatory regions of the GS gene that are missing from the transgenic construct. These findings indicate that in addition to the 5' regulatory region, at least two unidentified elements are involved in the spatiotemporal pattern of expression of GS.