Nucleotide sequence for a major messenger RNA for a 40 kilodalton polypeptide that is under translational control in mouse tumor cells

Molecular characterization, expression analysis and heterologous expression of two translationally controlled tumor protein genes from Cucumis sativus

The translationally controlled tumor protein (TCTP) is a family of abundant and ubiquitous proteins involved in several important primary functions. Cucumbers harbor two TCTP genes, CsTCTP1 and CsTCTP2; however, their functional roles remain unclear. In this study, we isolated CsTCTP1 and CsTCTP2 (XP-004134215 and XP-004135602, respectively) promoters, full-length cDNA and genomic sequences from Cucumis sativus. Bioinformatics analysis, containing cis-acting elements, structural domains, phylogenetic tree and conserved motifs, suggested the conservation and divergence of CsTCTP1 and CsTCTP2, thus providing knowledge regarding their functions. Expression analysis indicated that CsTCTP1 and CsTCTP2 exhibited tissue-specific expression and were regulated by biotic or abiotic stresses in C. sativus. Furthermore, CsTCTP1 and CsTCTP2 were regulated by ABA and may be associated with the TOR (target of rapamycin) signaling pathway. In a prokaryotic expression analysis, CsTCTP1 and CsTCTP2 showed positive responses to salt and heat stresses and a negative response to drought and HgCl2 stresses. TCTP may exert multiple functions in various cellular processes.

Looking into laminin receptor: critical discussion regarding the non-integrin 37/67-kDa laminin receptor/RPSA protein

The 37/67-kDa laminin receptor (LAMR/RPSA) was originally identified as a 67-kDa binding protein for laminin, an extracellular matrix glycoprotein that provides cellular adhesion to the basement membrane. LAMR has evolutionary origins, however, as a 37-kDa RPS2 family ribosomal component. Expressed in all domains of life, RPS2 proteins have been shown to have remarkably diverse physiological roles that vary across species. Contributing to laminin binding, ribosome biogenesis, cytoskeletal organization, and nuclear functions, this protein governs critical cellular processes including growth, survival, migration, protein synthesis, development, and differentiation. Unsurprisingly given its purview, LAMR has been associated with metastatic cancer, neurodegenerative disease and developmental abnormalities. Functioning in a receptor capacity, this protein also confers susceptibility to bacterial and viral infection. LAMR is clearly a molecule of consequence in human disease, directly mediating pathological events that make it a prime target for therapeutic interventions. Despite decades of research, there are still a large number of open questions regarding the cellular biology of LAMR, the nature of its ability to bind laminin, the function of its intrinsically disordered C-terminal region and its conversion from 37 to 67 kDa. This review attempts to convey an in-depth description of the complexity surrounding this multifaceted protein across functional, structural and pathological aspects.

Molecular basis of the acceleration of the GDP-GTP exchange of human ras homolog enriched in brain by human translationally controlled tumor protein

Ras homolog enriched in brain (Rheb), a small GTPase, positively regulates the mTORC1 pathway. The GDP-GTP exchange of Rheb has been suggested to be facilitated by translationally controlled tumor protein (TCTP). Here we demonstrate that human TCTP (hTCTP) interacts with human Rheb (hRheb) and accelerates its GDP release in vitro and that hTCTP activates the mTORC1 pathway in vivo. To investigate the underlying mechanism, we built structure models of GDP- and GTP-bound hRheb in complexes with hTCTP and performed molecular dynamics simulations of the models, which predict key residues involved in the interactions and region of hRheb undergoing conformational change during the GDP-GTP exchange. These results are verified with site-directed mutagenesis and in vitro biochemical and in vivo cell biological analyses. Furthermore, a crystal structure of the E12V mutant hTCTP, which lacks the guanine nucleotide exchange factor activity, shows that the deficiency appears to be caused by loss of a salt-bridging interaction with Lys-45 of hRheb. These data collectively provide insights into the molecular mechanisms of how hTCTP interacts with hRheb and activates the mTORC1 pathway.

Green tea (−)-epigallocatechin gallate inhibits insulin stimulation of 3T3-L1 preadipocyte mitogenesis via the 67-kDa laminin receptor pathway

Insulin and (−)-epigallocatechin gallate (EGCG) have been reported to regulate fat cell mitogenesis and adipogenesis, respectively. This study investigated the pathways involved in EGCG modulation of insulin-stimulated mitogenesis in 3T3-L1 preadipocytes. EGCG inhibited insulin stimulation of preadipocyte proliferation in a dose- and time-dependent manner. EGCG also suppressed insulin-stimulated phosphorylation of the insulin receptor-β, insulin receptor (IR) substrates 1 and 2 (IRS1 and IRS2), and mitogen-activated protein kinase pathway proteins, RAF1, MEK1/2, and ERK1/2, but not JNK. Furthermore, EGCG inhibited the association of IR with the IRS1 and IRS2 proteins, but not with the IRS4 protein. These data suggest that EGCG selectively affects particular types of IRS and MAPK family members. Generally, EGCG was more effective than epicatechin, epicatechin gallate, and epigallocatechin in modulating insulin-stimulated mitogenic signaling. We identified the EGCG receptor [also known as the 67-kDa laminin receptor (67LR)] in fat cells and found that its expression was sensitive to growth phase, tissue type, and differentiation state. Pretreatment of preadipocytes with 67LR antiserum prevented the effects of EGCG on insulin-stimulated phosphorylation of IRS2, RAF1, and ERK1/2 and insulin-stimulated preadipocyte proliferation (cell number and bromodeoxyuridine incorporation). Moreover, EGCG tended to increase insulin-stimulated associations between the 67LR and IR, IRS1, IRS2, and IRS4 proteins. These data suggest that EGCG mediates anti-insulin signaling in preadipocyte mitogenesis via the 67LR pathway.

The effects of green tea (-)-epigallocatechin-3-gallate on reactive oxygen species in 3T3-L1 preadipocytes and adipocytes depend on the glutathione and 67 kDa laminin receptor pathways

Green tea (-)-epigallocatechin-3-gallate (EGCG) is known as to regulate obesity and fat cell activity. However, little information is known about the effects of EGCG on oxidative reactive oxygen species (ROS) of fat cells. Using 3T3-L1 preadipocytes and adipocytes, we found that EGCG increased ROS production in dose- and time-dependent manners. The concentration of EGCG that increased ROS levels by 180-500% was approximately 50 muM for a range of 8-16 h of treatment. In contrast, EGCG dose- and time-dependently decreased the amount of intracellular glutathione (GSH) levels. EGCG was more effective than (-)-epicatechin, (-)-epicatechin-3-gallate, and (-)-epigallocatechin in changing ROS and GSH levels. This suggests a catechin-specific effect. To further examine the relation of GSH to ROS as altered by EGCG, we observed that exposure of preadipocytes and adipocytes to N-acetyl-L-cysteine (a GSH precursor) blocked the EGCG-induced increases in ROS levels and decreases in GSH levels. These observations suggest a GSH-dependent effect of EGCG on ROS production. While EGCG was demonstrated to alter levels of ROS and GSH, its signaling was altered by an EGCG receptor (the so-called 67 kDa laminin receptor(67LR)) antiserum, but not by normal rabbit serum. These data suggest that EGCG mediates GSH and ROS levels via the 67LR pathway.

The 67 kDa laminin receptor: structure, function and role in disease

The 67LR (67 kDa laminin receptor) is a cell-surface receptor with high affinity for its primary ligand. Its role as a laminin receptor makes it an important molecule both in cell adhesion to the basement membrane and in signalling transduction following this binding event. The protein also plays critical roles in the metastasis of tumour cells. Isolation of the protein from either normal or cancerous cells results in a product with an approx. molecular mass of 67 kDa. This protein is believed to be derived from a smaller precursor, the 37LRP (37 kDa laminin receptor precursor). However, the precise mechanism by which cytoplasmic 37LRP becomes cell-membrane-embedded 67LR is unclear. The process may involve post-translational fatty acylation of the protein combined with either homo- or hetero-dimerization, possibly with a galectin-3-epitope-containing partner. Furthermore, it has become clear that acting as a receptor for laminin is not the only function of this protein. 67LR also acts as a receptor for viruses, such as Sindbis virus and dengue virus, and is involved with internalization of the prion protein. Interestingly, unmodified 37LRP is a ribosomal component and homologues of this protein are found in all five kingdoms. In addition, it appears to be strongly associated with histones in the eukaryotic cell nucleus, although the precise role of these interactions is not clear. Here we review the current understanding of the structure and function of this molecule, as well as highlighting areas requiring further research.

Novel Aspects of Prions, Their Receptor Molecules, and Innovative Approaches for TSE Therapy

1. Prion diseases are a group of rare, fatal neurodegenerative diseases, also known as transmissible spongiform encephalopathies (TSEs), that affect both animals and humans and include bovine spongiform encephalopathy (BSE) in cattle, scrapie in sheep, chronic wasting disease (CWD) in deer and elk, and Creutzfeldt-Jakob disease (CJD) in humans. TSEs are usually rapidly progressive and clinical symptoms comprise dementia and loss of movement coordination due to the accumulation of an abnormal isoform (PrP(Sc)) of the host-encoded prion protein (PrP(c)). 2. This article reviews the current knowledge on PrP(c) and PrP(Sc), prion replication mechanisms, interaction partners of prions, and their cell surface receptors. Several strategies, summarized in this article, have been investigated for an effective antiprion treatment including development of a vaccination therapy and screening for potent chemical compounds. Currently, no effective treatment for prion diseases is available. 3. The identification of the 37 kDa/67 kDa laminin receptor (LRP/LR) and heparan sulfate as cell surface receptors for prions, however, opens new avenues for the development of alternative TSE therapies.

Structure-function studies of the functional and binding epitope of the human 37 kDa laminin receptor precursor protein

Expression of the 37 kDa laminin receptor precursor protein (37LRP) correlates directly with increased invasiveness and the metastatic potential of tumors. The 37LRP matures to a 67 kDa protein which facilitates the binding of cancer cells to basement membranes. The palindrome peptide sequence LMWWML, corresponding to the 173-178-residue stretch of the human 37LRP sequence, has been identified as the laminin-1-binding site. Peptides from 37LRP of species that contain this palindrome-bind laminin-1 with high affinity. Nuclear magnetic resonance (NMR) conformational studies have been undertaken on a synthetic 15-residue peptide (KGAHSVGLMWWMLAR) containing the palindrome to establish the structural basis of this activity. To further correlate the structural data with laminin-1-binding function, analogous structural studies were conducted for a similar peptide (RGKHSIGLIWYLLAR) lacking the palindrome, originating from 37LRP sequence of Saccharomyces cerevisiae and exhibiting low laminin-1-binding affinity. Finally, in vitro cell invasion assays were performed to investigate the possibility that the laminin-1-binding affinity of the peptides influences their inhibitory activity.

The 67 kDa laminin receptor as a prognostic factor in human cancer

Different receptors for adhesion molecules, including the monomeric 67 kDa laminin receptor (67LR), are responsible for the interactions between tumor cells and components of the extracellular matrix that play an important role in tumor invasion and metastasis. Clinical data clearly demonstrate the importance of the 67LR in the progression of a wide variety of tumors, including breast, lung, ovary, and prostate carcinomas and lymphomas. Indeed, data on more than 4000 cases of different tumors from different organs studied by immunohistochemistry are all concordant with a role for the 67LR in invasiveness, metastasis, and even tumor growth. This receptor molecule appears to be unusual since the corresponding full-length gene encodes a 37 kDa precursor protein which, after acylation, dimerizes to generate the mature 67 kDa form. The primary function of the membrane receptor is to stabilize the binding of laminin to cell surface integrins, acting as an integrin-accessory molecule, although homology of the gene encoding the receptor precursor with other genes suggests additional functions. Studies conducted to define the structure, expression, and function of this laminin receptor represent a step toward developing therapeutic strategies that target this molecule. In particular, therapeutic approaches that downregulate expression of the receptor on tumor cells might lead to decreased tumor aggressiveness.

LBP-p40 binds DNA tightly through associations with histones H2A, H2B, and H4

Laminin binding protein precursor p40 (LBP-p40) was long believed to be located exclusively in the cytoplasm. We recently reported localization of epitope-tagged LBP-p40 to the nucleus tightly associated with nuclear structure as well as on ribosomes. In this paper, we analyze the interaction of LBP-p40 with DNA and nuclear proteins in vitro. LBP-p40 was found to bind to a double-stranded DNA cellulose column at moderate salt. However, when mixed with a high salt nuclear extract, LBP-p40 was eluted from the DNA cellulose column only at higher salt. An LBP-p40 affinity column indicated that both histone H1 and in particular the core histones associate with LBP-p40. Using recombinant core histone molecules fused with glutathione S-transferase (GST), we demonstrate that histones H2A, H2B, and H4 are capable of interacting with LBP-p40, whereas H3 is not. These results suggest that association of LBP-p40 with histones H2A, H2B, and H4 confers tight binding of LBP-p40 to chromatin DNA in the nucleus.

TNF-alpha and IFN-gamma down-regulate the expression of the metastasis-associated bi-functional 37LRP/p40 gene and protein in transformed keratinocytes

The 37 LRP/p40 molecule is a bi-functional protein in which expression is increased in a large variety of cancers in association with their metastatic phenotype. Here we present the first data concerning the 37 LRP/p40 gene promoter activity and show that it is very active in a cervix carcinoma cell line. Interestingly, despite hallmarks of a housekeeping gene, we show that the 37 LRP/p40 gene promoter can be down-regulated by two potentially anticancerous cytokines, TNF-alpha and IFN-gamma. In addition, the dual fate of the protein, i.e., being intracellularly involved in the cell translation machinery and incorporated into a 67-kDa cell surface protein functioning as a laminin receptor (67LR), is differentially affected by the treatment. Our data suggest multiple regulation levels in the control of the 67LR/37LRP/p40 molecule expression and uncover new clues for the understanding of both the control of expression of this metastasis-associated molecule and the IFN-gamma and TNF-alpha anticancerous action.

Expression of a gene encoding a ribosomal p40 protein and identification of an active promoter site

The promoter of a gene encoding a ribosome-associated protein of 40 kDa from Arabidopsis thaliana (A-p40) was sequenced and the expression of the gene studied. A-p40 was expressed in the same organs and with the same variations as the eukaryotic elongation factor 1 alpha (eEF1A), another gene coding for a protein involved in translation Arabidopsis plants transformed with a beta-glucuronidase (GUS) gene driven by the A-p40 promoter confirm that A-p40 is expressed in actively dividing and growing cells. eEF1A promoter-GUS fusions have the same pattern of expression. Comparison of cis-acting elements from A-p40 and eEF1A revealed some common elements. A-p40 promoter deletions and transient gene expression in transfected Arabidopsis protopasts allowed the identification of trap40, a cis-acting element regulating gene expression. Gel retardation experiments indicate that eEF1A and A-p40 are regulated by different cis-acting elements. The role of such elements is discussed.

Identification of the active gene coding for the metastasis-associated 37LRP/p40 multifunctional protein

A 37LRP/p40 polypeptide is of major interest because it is consistently up-regulated in cancer cells in correlation with their invasive and metastatic phenotype. Furthermore, this polypeptide presents intriguing multifunctional properties because it has been characterized as the precursor of the metastasis-associated 67-kD laminin receptor (67LR) and as a cytoplasmic ribosomal-associated protein. The isolation of the 37LRP/p40 gene is a prerequisite for identifying the molecular mechanisms responsible for the constant up-regulation of the 67LR expression in cancer cells. To date, the active 37LRP/p40 gene has never been identified in any species due to the existence of multiple pseudogenes in most vertebrates genomes. In this study, we report for the first time the gene structure and potential regulatory sequences of the 37 LRP/p40 gene. The chicken genome was selected to undergo this characterization because it is the only known vertebrate that bears a single 37 LRP/p40 gene copy. The 37 LRP/p40 active gene is composed of 7 exons and 6 introns and bears features characteristic of a ribosomal protein gene. It does not bear a classical TATA box and it exhibits several transcription initiation sites as demonstrated by RNase protection assay and primer extension. Analysis of potential regulatory regions suggests that gene expression is driven not only by the 5' genomic region but also by the 5' untranslated and intron 1 sequences. On the basis of gene structure and extensive protein evolutionary study, we found that the carboxyterminal domain of the protein is a conserved lock-and-key structure/function domain that could be involved in the biosynthesis of the higher-molecular-weight 67-kD laminin receptor in vertebrates, whereas the central core of the protein would be responsible for the ribosome associated function. The first identification of the active 37LRP/p40 gene presented in this study is a critical step toward the isolation of the corresponding human gene and the understanding of the molecular mechanisms involved in the up-regulation of its expression during tumor invasion and metastasis.

Association of Plant p40 Protein with Ribosomes Is Enhanced When Polyribosomes Form during Periods of Active Tissue Growth

p40s are acidic proteins of eukaryotic cells occurring either free in the cytoplasm or in association with ribosomes, the latter occurring in both monosomes and polysomes. p40s may play a role in the regulation of protein synthesis, although the exact mechanism is not known. Leaves of all 10 plant species examined here, including both monocots and dicots, contained proteins detected on immunoblots with Arabidopsis thaliana p40 antiserum. The number and apparent size of the protein bands were variable even among closely related species. Abundance of p40 relative to ribosomal content during soybean (Glycine max L.) seed germination and during seed and leaf development was examined. p40 abundance correlated with periods of active tissue growth and high polysome content. The plant growth regulator indole acetic acid caused an increase in polysome formation in etiolated pea (Pisum sativum L.) plants and a concomitant recruitment of p40 into polysomes. Subcellular localization at the microscopy level indicated that the pattern of p40 staining is very similar to that for RNA, except that p40 is excluded from the nucleus. These data suggest that p40 is an accessory protein of the ribosome that might play a role in plant growth and development.

Yeast proteins related to the p40/laminin receptor precursor are essential components of the 40 S ribosomal subunit

We report here the isolation of two genes from the yeast, Saccharomyces cerevisiae, that encode proteins closely related to mammalian p40/laminin receptor precursors (LRPs). The yeast genes, designated YST1 and YST2, encode proteins with over 95% amino acid sequence identity with one another and over 60% identity with the human p40/laminin receptor precursor. The Yst/p40/37-LRP proteins are also more distantly related to the S2 family of ribosomal proteins. Analysis of the distribution of Yst1 tagged with the c-myc epitope revealed that the Yst proteins are components of the 40 S ribosomal subunit. Disruption of either YST1 or YST2 causes a significant reduction in growth rate, while disruption of both genes is lethal. Compared to wild type, polysome profiles in strains lacking either YST1 or YST2 show a pronounced shift from larger to smaller polysomes. This shift is accompanied by a substantial increase in free 60 S subunits and reduced levels of 40 S subunits. We conclude that the Yst proteins are required for translation and contribute to the assembly and/or stability of the 40 S ribosomal subunit.

Seventeen copies of the human 37 kDa laminin receptor precursor/p40 ribosome-associated protein gene are processed pseudogenes arisen from retropositional events

A cDNA coding for a 37 kDa polypeptide has been identified in several species as both the potential precursor of the 67 kDa laminin receptor (37LRP) and a putative ribosome-associated protein (p40). Interestingly, increased expression of this polypeptide (37LRP/p40) is consistently observed in invasive and metastatic cancer cells and is associated with poor prognosis. Southern-blot analysis of human genomic DNA predicted multiple copies of the 37LRP/p40 gene. In this study, we report that the number of copies of this sequence in the human genome is 26 +/- 2. We have sequenced and analyzed 19 genomic clones corresponding to the 37LRP/p40 gene and found that they were all processed pseudogenes. They all lack intronic sequences and show multiple genetic alterations leading in some cases to the appearance of stop codons. Moreover, they all bear characteristic features of retroposons as the presence of a poly(A)-tail at their 3' end and short direct repeated flanking DNA sequences. None of the pseudogenes analyzed present cis-elements in their 5' flanking region such as TATA or GC boxes. Our date reveal that over 50% of the 37LRP/p40 gene copies are pseudogenes most probably generated by retropositional events. The finding of multiple pseudogenes for the 37LRP/p40 suggests that the accumulation of several copies of this gene might have given a survival advantage to the cell in the course of evolution.

Control pathways of the 67 kDa laminin binding protein: surface expression and activity of a new ligand binding domain

A number of papers have been published on the clinical correlation of the expression of the 67 kDa laminin binding protein (LBP) with the metastatic potential of solid tumors. Both mRNA and protein expression levels have been reported, but both the relationship between them and the molecular nature of the 67 kDa surface product remain unclear. We have utilized a homotypic overexpression system to investigate the cell surface presentation of the 67 kDa LBP and the contribution of this protein to the invasive phenotype of cultured cell lines. We report here that the cellular mRNA levels do not directly reflect the levels of the 67 kDa LBP observed on the cell surface in this overexpression system. Methotrexate amplification of transfected plasmids expressing the 67 kDa LBP leads to an initial elevation of both the LBP mRNA and surface protein levels. This is accompanied by an altered, more flattened, cell morphology. Later, apparent adaptation of the cells to methotrexate is accompanied by a down-regulation of the surface expression of the protein. mRNA levels, however, remain elevated. A nine amino acid sequence, CDPGYIGSR (peptide 11), within the beta chain of laminin 1 has been identified as a probable binding domain for the 67 kDa LBP. Previous studies have identified a region of the 67 kDa LBP which may be involved in laminin interaction, although not necessarily via the peptide 11 domain. We have identified a second site within the amino acid coding sequence of the 67 kDa LBP which also shows biological activity both in vitro and in vivo. A peptide with this sequence, LBP residues 205-229, binds laminin-1 in a peptide 11 inhibitable manner. The receptor-derived peptide modulates invasion of basement membrane matrix in vitro and inhibits experimental lung colony formation when injected along with B16BL6 mouse melanoma cells. However, pretreatment of the melanoma cells with the peptide enhances lung colony formation. Thus, the interaction of the 67 kDa LBP with basement membrane matrix appears to involve a complex series of events including multiple adhesive sites and tight regulation of cell surface expression.

Overexpression of initiation factor eIF-4E does not relieve the translational repression of ribosomal protein mRNAs in quiescent cells

Translation of ribosomal protein (rp) mRNA is selectively repressed in mouse erythroleukemia (MEL) cells, which cease to proliferate upon differentiation, and in NIH 3T3 cells, for which growth is arrested by either serum starvation, contact inhibition, or treatment with the DNA polymerase inhibitor, aphidicolin. The efficiency of translation of rp mRNAs correlates with the expression of the gene encoding the cap binding protein, eIF-4E, as indicated by the fact that the abundance of the corresponding mRNA and protein also fluctuates in a growth-dependent manner. To examine the hypothesis that eIF-4E plays a role in regulation of the translation efficiency of rp mRNAs, we utilized an NIH 3T3-derived eIF-4E-overexpressing cell line. These cells overproduce eIF-4E to the extent that even under conditions of growth arrest, the abundance of the respective protein in its active (phosphorylated) form is higher than that found in exponentially growing NIH 3T3 cells. Nevertheless, this surplus amount of eIF-4E does not prevent the translational repression of rp mRNAs when the growth of these cells is arrested by blocking DNA synthesis with aphidicolin or hydroxyurea. In complementary experiments we used an in vitro translation system to compare the competitive potential of mRNAs, containing the translational cis-regulatory element (5' terminal oligopyrimidne tract) and mRNAs lacking such a motif, for the cap binding protein. Our results demonstrate that both types of mRNAs, regardless of their translational response to growth arrest, exhibit similar sensitivity to the cap analogue m7G(5')ppp(5')G. It appears, therefore, that the presence of the regulatory sequence at the 5' terminus of rp mRNAs does not lessen its competitive potential for the cap binding protein and that the growth-dependent decrease in the activity of eIF-4E does not play a key role in the repression of translation of rp mRNAs.

Laminin receptor expression in rat intestine and liver during development and differentiation

BACKGROUND/AIMS

Studies have identified a 67-kilodalton high-affinity laminin receptor (LR) whose expression has also been related to development, differentiation, and neoplastic transformation. The relationship of the 67-kilodalton LR to hepatic and enterocyte development and to enterocyte differentiation was investigated.

METHODS

LR messenger RNA (mRNA) was identified using a complementary DNA isolated from a rat crypt cell library. LR and integrin (alpha 6, beta 1, and beta 4) expression by rat intestinal crypt cells was compared with that of the more differentiated villus cells using Northern blotting. Developmental differences in LR expression were studied in fetal and neonatal rats. The pattern of LR expression in fetal and adult rat intestines was examined further by in situ hybridization.

RESULTS

LR mRNA levels were highest in fetal liver and intestine and adult rat crypt cells. LR mRNA levels were 9-10 times greater in crypt than in villus cells. Integrin subunit expression differed little between crypt and villus cells. Nascent transcription studies showed that the proportion of newly transcribed LR mRNA per total RNA synthesized was similar for crypt and villus cells, suggesting posttranscriptional control of LR mRNA levels in villus cells.

CONCLUSIONS

Increased LR mRNA expression is a feature of the fetal intestine and of the undifferentiated, mitotically active crypt cells.

Structural determination and characterization of a 40 kDa protein isolated from rat 40 S ribosomal subunit

We have purified a 40 kDa protein from the rat 40 S ribosomal subunit and determined its primary structure by amino acid and cDNA sequencing. The amino acid sequence of the 40 kDa protein shared 29-37% homology with prokaryotic ribosomal protein S2 of eubacteria and chloroplasts, indicating that the protein is a eukaryotic counterpart to prokaryotic S2. Moreover, the amino acid sequence shared 99% identity with those deduced from cDNAs for 68 kDa laminin binding proteins of human, murine and bovine origins. The cDNAs are capable of encoding polypeptides with predicted molecular mass of 33,000 which lacked typical signal sequences, N-linked glycosylation sites and putative transmembrane domains. These results indicate that the cDNAs for 68 kDa laminin binding proteins actually code for the 40 kDa ribosomal protein.

Host-cell receptors for Sindbis virus

Sindbis virus has a very wide host range, infecting many species of mosquitoes and other hematophagous insects and infecting many species of higher vertebrates. We have used two approaches to study host cell receptors used by Sindbis virus to enter cells. Anti-idiotype antibodies to neutralizing antibodies directed against glycoprotein E2 of the virus identified a 63-kDa protein as a putative receptor in chicken cells. In a second approach, monoclonal antibodies identified a 67 kDa protein, believed to be a high affinity laminin receptor, as a putative receptor in mammalian cells and in mosquito cells. We conclude that the virus attains its very wide host range by two mechanisms. In one mechanism, the virus is able to use more than one protein as a receptor. In a second mechanism, the virus utilizes proteins as receptors that are highly conserved across the animal kingdom.

The Drosophila stubarista phenotype is associated with a dosage effect of the putative ribosome-associated protein D-p40 on spineless

We describe the molecular characterization of the Drosophila melanogaster gene stubarista (sta) that encodes the highly conserved putative ribosome-associated protein D-p40. sta maps to cytological position 2A3-B2 on the X chromosome and encodes a protein (D-p40) of 270 amino acids. D-p40 shares 63% identity with the human p40 ribosomal protein. P element-mediated transformation of a 4.4-kb genomic fragment encompassing the 1-kb transcript corresponding to D-p40 was used to rescue both a lethal (sta2) and a viable (sta1) mutation at the stubarista (sta) locus. Developmental analysis of the sta2 mutation implicates a requirement for D-p40 during oogenesis and imaginal development, which is consistent with the expression of sta throughout development. In addition, we have analyzed the basis of the sta1 visible phenotype which consists of shortened antennae and bristles. sta1 is a translocation of the 1E1-2 to 2B3-4 region of the X chromosome onto the third chromosome at 89B21-C4. We provide genetic evidence that Dp(1;3)sta1 is mutant at the spineless (ss) locus and that it is associated with partial D-p40 activity. We demonstrate that sta1 acts as a recessive enhancer of ss; reduction in the amount of D-p40 provided by the transposed X chromosomal region of sta1 reveals a haplo-insufficient phenotype of the otherwise recessive ss mutations. This phenomenon is reminiscent of the enhancing effect observed with Minute mutations, one of which, rp49, has previously been shown to encode a ribosomal protein.

Detection of laminin receptor mRNA in human cancer cell lines and colorectal tissues by in situ hybridization

The 67-kd high-affinity laminin receptor (67 LR) is a gene product whose expression appears to be associated with the invasive and metastatic phenotype of a variety of human cancer cells. Northern blot hybridization has been routinely used to quantify the level of 67 LR mRNA from total cellular RNA extracts of homogenized tissue specimens or in vitro grown cell populations. This technique is useful to assess the average expression of the 67 LR mRNA of a particular sample but does not provide information about expression in specific cell types nor about heterogeneity of expression from cell to cell. In this study, we analyzed the expression of 67 LR mRNA in four human cancer cell lines with varying degrees of expression of 67 LR protein (renal cancer A-704, breast carcinoma MCF-7/4 and MCF-7/7, and pancreatic cancer Panc-1) using in situ hybridization performed with 67 LR riboprobes. Total cellular RNA was simultaneously extracted from the cell lines and hybridized on Northern blots with a 67 LR cDNA probe to assess the validity of the mRNA detection by in situ hybridization. Sixty-seven LR mRNA expression was higher in Panc-1 and MCF-7/4 cells than in MCF-7/7 and renal carcinoma A-704. There was a direct correlation (R2 = 0.88) between the in situ hybridization analysis and the mRNA levels detected by Northern blot analysis. The in situ hybridization method showed a heterogeneous expression of the 67 LR mRNA in the four cell lines with different subpopulations of cells showing a range from negative to high levels of the message. Sixteen freshly frozen human colorectal tissues (seven adenocarcinomas, five matched normal mucosae, and four adenomas) were also analyzed by in situ hybridization. The 67 LR mRNA was localized in normal and neoplastic epithelial cells. Adenocarcinoma cells showed a 1.6- to 5-fold higher expression (P < 0.02 according to the Wilcoxon-Mann-Whitney test) than did epithelial colonic cells from normal mucosae or adenomas. The signal tended to be stronger in poorly differentiated carcinomas and carcinomas with metastases than in moderately differentiated and nonmetastatic tumors. We conclude that the high expression of 67 LR mRNA in colorectal tumors is due to an increased production by tumor cells. Furthermore, in situ hybridization is an effective method to detect the expression of LR mRNA in cultured cell lines as well as in frozen tissue sections.

High-affinity laminin receptor is a receptor for Sindbis virus in mammalian cells

Sindbis virus is an alphavirus with a very wide host range, being able to infect many birds and mammals as well as mosquitoes. We have isolated a monoclonal antibody that largely blocks virus binding to mammalian cells. This antibody was found to be directed against the C-terminal domain of the high-affinity laminin receptor, a 67-kDa protein present on the cell surface that binds with high affinity to basement membrane laminin and that is known to be important in development and in tumor invasion. This receptor is believed to be formed from a 295-amino-acid polypeptide that is modified in some unknown way after translation. The primary sequence of this 295-amino-acid protein is highly conserved among mammals. We found the hamster amino acid sequence to be identical to a mouse sequence and to differ at only two amino acids from a human sequence and at two amino acids from a bovine sequence. To verify the importance of the laminin receptor for infection by Sindbis virus, hamster cells were stably transfected with the gene encoding the 295-amino-acid protein under the control of a high-efficiency promoter. Such transfected hamster cells overexpressed the laminin receptor at the cell surface, bound severalfold more Sindbis virions than did the parental cells, and became infected by Sindbis virus with a higher efficiency. In contrast, cells transfected with the antisense gene expressed less laminin receptor on the surface and were less susceptible to the virus. Binding of the virus varied linearly with the amount of laminin receptor on the cell surface, whereas infectivity measured with a plaque assay varied with the 1.4 power of the receptor concentration, suggesting that interaction with more than one receptor aids virus penetration. By these criteria, the laminin receptor functions as the major receptor for Sindbis virus entry into mammalian cells. We also found that the anti-laminin receptor antibody partially blocked Sindbis virus binding to mosquito cells, suggesting that the laminin receptor is conserved in mosquitoes and functions as a Sindbis virus receptor in this host. The wide distribution of this highly conserved receptor may be in part responsible for the broad host range exhibited by the virus, which infects a wide range of mammals and birds as well as its mosquito vector and can infect many different tissues within these hosts.(ABSTRACT TRUNCATED AT 400 WORDS)

Nonintegrin laminin receptors in the nervous system: evidence for lack of a relationship to P40

Laminins are extracellular matrix proteins that mediate their effects on cells through integrin and nonintegrin receptors. Two receptors of 67 and 110 kD that bind laminin with a high affinity (Kd approximately nM) have been reported in neural cells. Here, we discuss these and other nonintegrin laminin receptors that have been implicated in neural function. In addition, we report studies characterizing a 43 kD protein, (P40), immunologically related to the 67 kD laminin receptor, which may be involved in retinal development. In our studies, polyclonal antisera (anti-P-20-A) to a synthetic peptide derived from the sequence of a cDNA for a putative high-affinity laminin receptor (67 kD) detected a protein of 43 kD in immunoblots of adult rat retinas. Immunohistochemistry with this antiserum showed that the retinal immunoreactivity was predominantly localized in the ganglion cell layer of both adult chicken and rat retinas where it appeared to be intracellular. Retinal ganglion cells were shown to be immunoreactive by retrogradely labeling them from the superior colliculus with a lipophilic dye and subsequently with anti-P-20-A antisera. Consistent with the preferential localization of the P-20-A immunoreactivity in ganglion cells, there was a substantial decrease in the amounts of P40 on Western blots following optic nerve section and resulting retinal ganglion cell death. Screening of a rat (PC12 cell) cDNA library with the anti-P-20-A antiserum further confirmed the specificity of the antiserum for the rat homologue of P40. Rat P40 is 97% identical to the mouse and 87% identical to human P40 at the nucleic acid level and 98% at the protein level. Restriction mapping of the rather abundant positive clones in the library that cross-hybridized with a human cDNA probe for P40 indicated that the full-length cDNA of 1.2 kb was the major and perhaps the only cDNA in the library. In Northern blots of adult rat retina, these clones hybridized to a single 1.2-kb transcript. Electroblots of retinal homogenates probed with radioiodinated laminin demonstrated binding to a broad band at 110 kD, but none at 43 kD. Taken together these findings suggest that P40 may not be a laminin receptor and are in keeping with the hydrophilic composition of the protein, its intracellular localization, as well as other features predicted by its nucleic acid sequence.(ABSTRACT TRUNCATED AT 400 WORDS)

A 33-kDa polypeptide with homology to the laminin receptor: component of translation machinery

A 33-kDa polypeptide (termed p40), which shares an antigenic determinant with a laminin receptor and is under translational control, is believed to serve as a precursor to the receptor and to be related to the neoplastic state. The present study of subcellular localization of this protein shows it to be a cytoplasmic component not associated with the plasma membrane. Most of the cellular p40 was found to be associated with polyribosomes as well as with 40S to 60S cytoplasmic particles. Conditions that lead to polysome disruption also caused release of the polysomal form of p40 as smaller particles, and polysome reconstitution was accompanied by uptake of p40 into these structures. Because of the large abundance of this protein in the cells (six to eight copies per ribosome), it is unlikely that it represents a factor that associates with the 40S preinitiation complex. The p40-containing particles appear to represent a newly discovered structure involved in the process of polysome formation.

Genetic linkage analysis in recombinant inbred mice of P40, a putative clone for the high-affinity laminin receptor

Recombinant inbred (RI) mice were used to genetically map sequences of a 43-kDa protein, P40, that was originally identified as a high-affinity laminin receptor. More recent data have implicated this protein in development of the retina (Rabacchi et al. Development 109: 521-531, 1990), possibly via its proposed function in protein translation (G. Brawerman, personal communication). We have identified, in Southern blots, a set of P40-related sequences in BXD recombinant inbred mouse DNA. Ten restriction fragment length polymorphisms (RFLPs) segregate among the RI strains and display strain distribution patterns (SDPs) which are linked in varying degrees to previously typed loci on Chromosomes (Chrs) 1, 3, 6, 9, 11, 14, and 19. An intronic DNA probe from an incompletely processed P40 mRNA transcript overlaps with two of these loci mapping near the cholecystokinin gene locus on the distal arm of Chr 9 and to another site on the distal arm of Chr 6, suggesting that functional genes probably reside at least at these two sites. These P40 loci comprise part of a multimember gene family that is well dispersed in the mouse genome.

Genomic analysis of the 67-kDa laminin receptor in normal and pathological tissues: circumstantial evidence for retroposon features

We have cloned two cDNAs for the human 67-kDa laminin receptor (LR). In the present report we show that these clones hybridize to many restriction fragments in Southern experiments in human. This particular pattern is accounted for by the presence of up to 16 and 21 copies of the laminin receptor gene per haploid genome in human and mouse, respectively. In contrast, a single gene copy is found in chicken. Chromosomal localization reveals four main loci: LAMRP1, laminin receptor pseudogene 1 (Chr 3); LAMRP2, laminin receptor pseudogene 2 (Chr 12); LAMRP3, laminin receptor pseudogene 3 (Chr 14); LAMRP4, laminin receptor pseudogene 4 (Chr X). Comparison of our experimental results to the known features of processed retropseudogenes enabled us to conclude that the LR gene belongs to a retroposon family in mammals.

Oligopyrimidine tract at the 5' end of mammalian ribosomal protein mRNAs is required for their translational control

Mammalian ribosomal protein (rp) mRNAs are subject to translational control, as illustrated by their selective release from polyribosomes in growth-arrested cells and their underrepresentation in polysomes in normally growing cells. In the present experiments, we have examined whether the translational control of rp mRNAs is attributable to the distinctive features of their 5' untranslated region, in particular to the oligopyrimidine tract adjacent to the cap structure. Murine lymphosarcoma cells were transfected with chimeric genes consisting of selected regions of rp mRNA fused to non-rp mRNA segments, and the translational efficiency of the resulting chimeric mRNAs was assessed in cells that either were growing normally or were growth-arrested by glucocorticoid treatment. We observed that translational control of rpL32 mRNA was abolished when its 5' untranslated region was replaced by that of beta-actin. At the same time, human growth hormone (hGH) mRNA acquired the typical behavior of rp mRNAs when it was preceded by the first 61 nucleotides of rpL30 mRNA or the first 29 nucleotides of rpS16 mRNA. Moreover, the translational control of rpS16-hGH mRNA was abolished by the substitution of purines into the pyrimidine tract or by shortening it from eight to six residues with a concomitant cytidine----uridine change at the 5' terminus. These results indicate that the 5'-terminal pyrimidine tract plays a critical role in the translational control mechanism. Possible factors that might interact with this translational cis regulatory element are discussed.

Characterization of a putative clone for the 67-kilodalton elastin/laminin receptor suggests that it encodes a cytoplasmic protein rather than a cell surface receptor

The 67-kDa elastin binding protein shares many immunological and structural properties with the high-affinity 67-kDa tumor cell laminin receptor. Taking advantage of these similarities, we have screened a bovine cDNA library with a partial cDNA probe for the laminin receptor and have isolated and characterized a cDNA clone of 1038 bp that hybridizes to a single-size mRNA of 1.3 kb. The clone encodes a protein with a predicted molecular weight of 33K that lacks an N-terminal leader sequence, shows no posttranslational processing when translated in vitro in the presence of microsomes, and does not bind to elastin affinity columns. Although the bovine clone is nearly identical with clones encoding human and mouse proteins proported to be 67-kDa laminin receptor, physical and functional characteristics of the encoded protein suggest that it is a cytoplasmic protein that does not bind elastin. This finding calls into question the earlier conclusion that the clone encodes the 67-kDa receptor.

The high affinity murine laminin receptor is a member of a multicopy gene family

The high affinity laminin receptor is differentially expressed in metastasis. We now report that there are multiple copies (6 +/- 1) of the laminin receptor gene in the murine genome of normal diploid cells as well as in cell lines derived from cancer cells. We have analyzed three distinct cDNA clones isolated from an Okayama-Berg cDNA library of transformed mouse fibroblasts that may represent transcripts of three different laminin receptor genes. Polymorphic changes include insertion of bases at the 5' terminus, a base substitution within the coding region resulting in an amino acid change from phenylalanine to leucine, a base substitution obliterating a polyadenylation signal, as well as changes in the length of the 3' untranslated domains. The discovery of multiple transcripts of laminin receptor genes suggests that there is a strong selective pressure to maintain laminin receptor expression in murine cells.

A dorso-ventral asymmetry in the embryonic retina defined by protein conformation

In a search for determinants of retinotopic specification we previously identified an antigen in the dorsal embryonic retina as a protein called the 68-kDa laminin receptor. A dorso-ventral asymmetry in a laminin receptor seemed consistent with the known responsiveness of embryonic optic axons to laminin, but there were three peculiar points. (i) The molecular mass of this presumed laminin receptor in immunoblots is not 68 kDa but 43 kDa, and the molecular mass of the protein deduced from the mRNA is only 33 kDa. (ii) The antigen does not have the localization expected of a receptor for the extracellular matrix: the antibodies label mainly a granular cytoplasmic antigen in dorsal retina; an additional sparse cell-surface antigen present on a few cells does not show a dorso-ventral asymmetry. (iii) Despite the pronounced dorso-ventral difference seen immunohistochemically, in immunoblots the 43-kDa protein (p40) is evenly distributed throughout the retina. Here we show that (i) native p40 and in vitro-translated gene product are indistinguishable and their anomalous migration in denaturing gels probably is due to low pI; (ii) p40 is bound in a Mg2(+)-dependent manner to large cytoplasmic complexes that appear to include ribosomes; and (iii) there is a labile conformational difference in p40 between dorsal and ventral retina: dorsally it is more accessible to proteolysis, suggesting a more open conformation. In conjunction with the recent hypothesis that p40 constitutes a translation initiation factor (D. Auth and G. Brawerman, personal communication), these observations point to a dorso-ventral asymmetry in some aspect of protein translation, which in turn may set up differences in recognition factors on retinal growth cones.

Nuclease activity associated with mammalian mRNA in its native state: possible basis for selectivity in mRNA decay

Polysome and messenger ribonucleoprotein (mRNP) preparations from various mammalian cells contain tightly bound nuclease activity that causes degradation of the mRNA in the preparations. This activity was found to cosediment with all polysome size classes as well as with free mRNPs and to remain associated with the mRNPs released from polysomes by treatment with EDTA. No association with ribosomal subunits was evident. The rates of mRNA degradation were not affected by serial dilution, an indication that enzyme and substrate are tightly associated. beta-Globin mRNA in purified reticulocyte polysomes was cleaved at AU sequences in the 3'-terminal region. Cleavages at the same sites occurred when deproteinized reticulocyte RNA was incubated with mouse sarcoma 180 (S-180) polysomes. The S-180 preparations caused additional cleavages, primarily at UG sequences. A P40 mRNA in S-180 polysomes was cleaved primarily in the 3' noncoding region, but the cleavages in a P21 mRNA were seen in the 5' noncoding region only. Actin mRNA was cleaved in an internal region, yielding large relatively stable 3'- and 5'-terminal fragments. These data suggest the occurrence of highly specific interactions between one or more mRNA-bound nucleases and individual mRNA species.

Laminin and fibronectin increase the steady state level of the 67 kD high affinity metastasis-associated laminin receptor mRNA in human cancer cells

Among the various known laminin binding proteins, the 67 kD high affinity laminin receptor (LR) is intimately involved during tumor invasion and metastasis. In this study, we report that laminin and fibronectin, two attachment glycoproteins, significantly increased the total cellular level of 67 kD LR mRNA in two human cancer cell lines, T47D breast carcinoma cells and A2058 melanoma cells. Neither GRGDS nor YIGSR synthetic peptides induced such a stimulatory effect. Since the steady state level of LR mRNA has been shown to control the number of receptors expressed at the cell surface, these results suggest that contact of the cancer cells with laminin and fibronectin in the host matrix may be an important regulatory mechanism by which cancer cells maintain a high number of LR at their cell surface as they progress through the several steps of tumoral invasion and metastasis.

Nuclease activity associated with mammalian mRNA in its native state: possible basis for selectivity in mRNA decay

Polysome and messenger ribonucleoprotein (mRNP) preparations from various mammalian cells contain tightly bound nuclease activity that causes degradation of the mRNA in the preparations. This activity was found to cosediment with all polysome size classes as well as with free mRNPs and to remain associated with the mRNPs released from polysomes by treatment with EDTA. No association with ribosomal subunits was evident. The rates of mRNA degradation were not affected by serial dilution, an indication that enzyme and substrate are tightly associated. beta-Globin mRNA in purified reticulocyte polysomes was cleaved at AU sequences in the 3'-terminal region. Cleavages at the same sites occurred when deproteinized reticulocyte RNA was incubated with mouse sarcoma 180 (S-180) polysomes. The S-180 preparations caused additional cleavages, primarily at UG sequences. A P40 mRNA in S-180 polysomes was cleaved primarily in the 3' noncoding region, but the cleavages in a P21 mRNA were seen in the 5' noncoding region only. Actin mRNA was cleaved in an internal region, yielding large relatively stable 3'- and 5'-terminal fragments. These data suggest the occurrence of highly specific interactions between one or more mRNA-bound nucleases and individual mRNA species.

Endothelial cells use alpha 2 beta 1 integrin as a laminin receptor

Human umbilical vein endothelial cells attach and spread on laminin-coated substrates. Affinity chromatography was used to identify the attachment receptor. Fractionation of extracts from surface-iodinated endothelial cells on human laminin-Sepharose yielded a heterodimeric complex, the subunits of which migrated with molecular sizes corresponding to 160/120 kD and 160/140 kD under nonreducing and reducing conditions, respectively. The purified receptor bound to laminin and slightly less to fibronectin and type IV collagen in a radioreceptor assay. This endothelial cell laminin receptor was classified as an alpha 2 beta 1 integrin because monoclonal and polyclonal antibodies directed against the alpha 2 and bet 1 subunits immunoprecipitated the receptor. Cytofluorometric analysis and immunoprecipitation showed that the alpha 2 subunit is an abundant integrin alpha subunit in the endothelial cells and that the alpha subunits associated with laminin binding in other types of cells are expressed in these cells only at low levels. The alpha 2 beta 1 integrin appears to be a major receptor for laminin in the endothelial cells, because an anti-alpha 2 monoclonal antibody inhibited the attachment of the endothelial cells to human laminin. These results define a new role for the alpha 2 subunit in laminin binding and suggest that the ligand specificity of the alpha 2 beta 1 integrin, which is known as a collagen receptor in other types of cells, can be modulated by cell type-specific factors to include laminin binding.

Characteristics of a multicopy gene family predominantly consisting of processed pseudogenes

The monoclonal antibody MOC-32 detected a 40 kDa protein in Western blot analysis. Immunological screening of an expression library of human SCLC cells with MOC-32 led to the isolation of overlapping cDNA clones. One of these, cHD4, was 1.0 kbp long and of about the same size as its corresponding mRNA. Preceded by an in phase stop codon, an open reading frame of 885 bp was present in cHD4 and a translational product of only 33 kDa could be calculated. Biochemical and immunological analysis established the relationship between the 40 kDa antigen and the isolated coding sequences and resolved the apparent discrepancy between the calculated molecular weight and the observed electrophoretic mobility. Nucleotide sequence comparison of cHD4 to the EMBL database revealed that cHD4 was nearly identical to a sequence claimed to encode a laminin binding protein. Southern blot and nucleotide sequence analysis indicated the presence of multiple copies of the gene in the human genome. At least five of these appeared to represent processed pseudogenes.