Characterization of the INT6 mammary tumor gene product

Myc-associated zinc-finger protein promotes clear cell renal cell carcinoma progression through transcriptional activation of the MAP2K2-dependent ERK pathway

Background

The dysfunction of myc-related zinc finger protein (MAZ) has been proven to contribute to tumorigenesis and development of multiple cancer types. However, the biological roles and clinical significance of MAZ in clear cell renal carcinoma (ccRCC) remain unclear.

Methods

MAZ expression was examined in ccRCC and normal kidney tissue by quantitative real-time PCR and Western blot. Statistical analysis was used to evaluate the clinical correlation between MAZ expression and clinicopathological characteristics to determine the relationship between MAZ expression and the survival of ccRCC patients. The biological roles of MAZ in cells were investigated in vitro using MTT and colony assays. Luciferase reporter assays and chromatin immunoprecipitation (ChIP) were used to investigate the relationship between MAZ and its potential downstream signaling molecules.

Results

MAZ expression is elevated in ccRCC tissues, and higher levels of MAZ were correlated with poor survival of patients with ccRCC. MAZ upregulation elevates the proliferation ability of ccRCC cells in vitro, whereas silencing MAZ represses this ability. Our results further reveal that MAZ promotes cell growth, which is dependent on ERK signaling. Importantly, we found that MAZ positively regulates MAP2K2 expression in ccRCC cells. Mechanistically, MAZ binds to the MAP2K2 promoter and increases MAP2K2 transcription. Furthermore, MAP2K2 levels were shown to be increased in ccRCC tissues and to be associated with a poor prognosis of ccRCC patients. MAP2K2 upregulation activates the ERK signaling pathway and promotes ccRCC progression.

Conclusion

These results reveal that the MAZ/MAP2K2/ERK signaling axis plays a crucial role in promoting ccRCC progression, which suggests the potential therapeutic utility of MAZ in ccRCC.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12935-021-02020-9.

Expression of truncated eukaryotic initiation factor 3e (eIF3e) resulting from integration of mouse mammary tumor virus (MMTV) causes a shift from cap-dependent to cap-independent translation

Integration of mouse mammary tumor virus (MMTV) at the common integration site Int6 occurs in the gene encoding eIF3e, the p48 subunit of translation initiation factor eIF3. Integration is at any of several introns of the Eif3e gene and causes the expression of truncated Eif3e mRNAs. Ectopic expression of the truncated eIF3e protein resulting from integration at intron 5 (3e5) induces malignant transformation, but by an unknown mechanism. Because eIF3e makes up at least part of the binding site for eIF4G, we examined the effects of 3e5 expression on protein synthesis. We developed an NIH3T3 cell line that contains a single copy of the 3e5 sequence at a predetermined genomic site. Co-immunoprecipitation indicated diminished binding of eIF3 to eIF4G, signifying a reduction in recruitment of the mRNA-unwinding machinery to the 43 S preinitiation complex. Cell growth and overall protein synthesis were decreased. Translation driven by the eIF4G-independent hepatitis C virus internal ribosome entry sequence (HCV IRES) in a bicistronic mRNA was increased relative to cap-dependent translation. Endogenous mRNAs encoding XIAP, c-Myc, CYR61, and Pim-1, which are translated in a cap-independent manner, were shifted to heavier polysomes whereas mRNAs encoding GAPDH, actin, L32, and L34, which are translated in a cap-dependent manner, were shifted to lighter polysomes. We propose that expression of 3e5 diminishes eIF4G interaction with eIF3 and causes abnormal gene expression at the translational level. The correlation between up-regulation of cap-independent translation and MMTV-induced tumorigenesis contrasts with the well established model for malignant transformation involving up-regulation of highly cap-dependent translation.

Int6 regulates both proteasomal degradation and translation initiation and is critical for proper formation of acini by human mammary epithelium

INT6/EIF3E has been implicated in breast tumorigenesis, but its functional activities remain poorly defined. We found that repressing INT6 expression induced transformed properties in normal human mammary epithelium (MCF10A); in contrast, Int6 silencing induced apoptosis in HeLa cells. As in yeast, Int6 in human cells was required for assembly of active proteasomes. A reverse phase protein array screen identified SRC3/AIB1 as one oncoprotein whose level and stability increased when Int6 was silenced in MCF10A cells. Our data further show that Int6 binds SRC3 and its ubiquitin ligase Fbw7, thus perhaps mediating the interaction between SRC3-Fbw7 and proteasomes. Consistent with this, Int6 silencing did not increase SRC3 levels in HeLa cells, which have low Fbw7 levels.

Surprisingly, however, polyubiquitylated proteins do not accumulate or may even decrease in Int6-silenced cells that contain defective proteasomes. Considering that decreased ubiquitin might explain this observation and that Int6 might control ubiquitin levels in its role as a subunit of eIF3 (eukaryote translation initiation factor), we found that silencing Int6 reduced monoubiquitin protein levels, which correlated with a shift of ubiquitin mRNAs from larger polysomes to non-translating ribosomes. In contrast, levels of many housekeeping proteins did not change. This apparent reduction in the translation of ubiquitin genes correlated with a modest reduction in protein synthesis rate and formation of large polysomes. To further determine whether Int6 can selectively control translation, we analyzed translation of different 5′-UTR reporters and found that indeed, loss of Int6 had differential effects on these reporters.

Together the data suggest that Int6 depletion blocks ubiquitin-dependent proteolysis by decreasing both ubiquitin levels and the assembly of functional proteasome machinery, leading to accumulation of oncoproteins such as SRC3 that can transform mammary epithelium. Our data also raise the possibility that Int6 can further fine-tune protein levels by selectively controlling translation of specific mRNAs.

Common integration sites for MMTV in viral induced mouse mammary tumors

The paradigm of mammary cancer induction by the mouse mammary tumor virus (MMTV) is used to illustrate the body of evidence that supports the hypothesis that mammary epithelial stem/progenitor cells represent targets for oncogenic transformation. It is argued that this is not a special case applicable only to MMTV-induced mammary cancer, because MMTV acts as an environmental mutagen producing random interruptions in the somatic DNA of infected cells by insertion of proviral DNA copies. In addition to disrupting the host genome, the proviral DNA also influences gene expression through its associated enhancer sequences over significant inter-genomic distances. Genes commonly affected by MMTV insertion in multiple individual tumors include, the Wnt, FGF, RSpo gene families as well as eIF3e and Notch4. All of these gene families are known to play essential roles in stem cell maintenance and behavior in a variety of organs. The MMTV-induced mutations accumulate in cells that are long-lived and possess the properties of stem cells, namely, self-renewal and the capacity to produce divergent epithelial progeny through asymmetric division. The evidence shows that epithelial cells with these properties are present in normal mammary glands, may be infected with MMTV, become transformed to produce epithelial hyperplasia through MMTV-induced mutagenesis and progress to frank mammary malignancy. Retroviral marking via MMTV proviral insertion demonstrates that this process progresses from a single mammary epithelial cell that possesses all of the features ascribed to tissue-specific stem cells.

Cell cycle-related variation in subcellular localization of eIF3e/INT6 in human fibroblasts

The Int-6 gene is a site of mouse mammary tumour virus (MMTV) integration in murine tumours and INT6 protein has been identified independently as a subunit (eIF3e) of the eukaryotic translation initiation factor eIF3. In addition, the protein can interact with two other multi-subunit complexes: the COP9 signalosome (CSN) and the proteasome. The role of INT6 in tumourigenesis is nonetheless currently unclear. Here, using immunofluorescence microscopy, we show that eIF3e/INT6 is localized in part to the nucleus, while other eIF3 components are cytoplasmic. Primary human fibroblasts, but not their transformed counterparts, showed reduced nuclear INT6 staining in some cells, and this reduction was maximal in early S phase. This variation in eIF3e/INT6 may indicate regulated shuttling between cellular compartments and would be consistent with the presence of a nuclear export signal as well as a nuclear localization signal in the protein sequence. Loss of regulation of eIF3e/INT6 redistribution may therefore be a significant feature of malignancy in human cells.

Malignant transformation by the eukaryotic translation initiation factor 3 subunit p48 (eIF3e)

Several components of translation, e.g. eIF4E and PKR, are implicated in cancer. The e-subunit (p48) of mammalian initiation factor 3 is encoded by the Int6 gene, a common site for integration of the mouse mammary tumor virus genome, leading to the production of a truncated eukaryotic initiation factor-3e (eIF3e). Stable expression of a truncated eIF3e in NIH 3T3 cells causes malignant transformation by four criteria: foci formation; anchorage independent growth; accelerated growth; and lack of contact inhibition. Stable expression of full-length eIF3e does not cause transformation. The truncated eIF3e also inhibits the onset of apoptosis caused by serum starvation.

Evidence for the transforming activity of a truncated Int6 gene, in vitro

Int6/eIF3-p48 was first identified as a common integration site for MMTV in mouse mammary tumors. In all cases, the MMTV integration event resulted in an interruption of the normal Int6 transcript from one allele leaving the second allele intact and operative. We hypothesize that insertion of MMTV into Int6 results in a mutated allele that encodes a shortened Int6 mRNA and protein (Int6sh), which either modifies normal Int6 function or possesses a new independent function. To confirm the transforming potential of the mutation and its dominant function, we transfected two mammary epithelial cell lines, MCF10A (human), and HC11 (mouse), with Int6sh under the control of the elongation factor-1alpha (eEF1A) promoter. Expression of Int6sh in MCF10A and HC11 mammary epithelial cells leads to anchorage-independent growth in soft agar indicative of a transformed phenotype. Colonies selected from agar exhibited high levels of mutated Int6sh and wild type Int6 RNA transcripts by RT-PCR and Northern blot analysis. In addition, Int6sh transformed MCF10A and HC11 cells formed nodular growths, in vivo, in immune compromised hosts. NIH3T3 cells, mouse embryo fibroblasts, were also transformed to anchorage-independent growth in vitro by Int6sh expression. These observations provide direct evidence that the Int6 mutations observed in MMTV-induced tumors and hyperplasia contribute to the malignant transformation of the mammary epithelial cells.

PCI complexes: pretty complex interactions in diverse signaling pathways

Three protein complexes (the proteasome regulatory lid, the COP9 signalosome and eukaryotic translation initiation factor 3) contain protein subunits with a well defined protein domain, the PCI domain. At least two (the COP9 signalosome and the lid) appear to share a common evolutionary origin. Recent advances in our understanding of the structure and function of the three complexes point to intriguing and unanticipated connections between the cellular functions performed by these three protein assemblies, especially between translation initiation and proteolytic protein degradation.

Glycoconjugate distribution in early human notochord and axial mesenchyme

Glycosylation patterns of cells and tissues give insights into spatially and temporally regulated developmental processes and can be detected histochemically using plant lectins with specific affinities for sugar moieties. The early development of the vertebral column in man is a process which has never been investigated by lectin histochemistry. Therefore, we studied binding of several lectins (AIA, Con A, GSA II, LFA, LTA, PNA, RCA I, SBA, SNA, WGA) in formaldehyde-fixed sections of the axial mesenchyme of 5 human embryos in Carnegie stages 12-15. During these developmental stages, an unsegmented mesenchyme covers the notochord. Staining patterns did not show striking temporal variations except for SBA which stained the cranial axial mesenchyme only in the early stage 12 embryo and for PNA, of which the staining intensity in the mesenchyme decreased with age. The notochord appeared as a highly glycosylated tissue. Carbohydrates detected may correspond to adhesion molecules or to secreted substances like proteoglycans or proteins which could play an inductive role, for example, for the neural tube. The axial perinotochordal unsegmented mesenchyme showed strong PNA binding. Therefore, its function as a PNA-positive "barrier" tissue is discussed. The endoderm of the primitive gut showed a lectin-binding pattern that was similar to that of the notochord, which may correlate with interactions between these tissues during earlier developmental stages.

Yin6, a fission yeast Int6 homolog, complexes with Moe1 and plays a role in chromosome segregation

The INT6 gene has been implicated in human breast cancer formation, but its function is unknown. We isolated an Int6 homolog from fission yeast, Yin6, by its binding to a conserved protein in the Ras pathway, Moe1. Yin6 and Moe1 converge on the same protein complex to promote microtubule instability/disassembly. Yin6 and Moe1 interact cooperatively: when either protein is absent, the other becomes mislocalized with decreased protein levels. Furthermore, whereas full-length human Int6 rescues the phenotypes of the yin6-null (yin6Delta) mutant cells and binds human Moe1, truncated Int6 proteins found in tumors do not. Importantly, yin6Delta alone impairs chromosome segregation weakly, but yin6Delta together with ras1Delta causes severe chromosome missegregation. These data support a model in which INT6 mutations in humans either alone or together with additional mutations, such as a RAS mutation, may contribute to tumorigenesis by altering genome stability.

MMTV-induced mammary tumorigenesis: gene discovery, progression to malignancy and cellular pathways

The study of the mouse mammary tumor virus (MMTV) has provided important insights into the mechanisms of gene transcription regulation by steroid hormones, the mode of action of heritable super antigens and the progressive nature of neoplastic transformation in the mammary gland. Here we describe the current situation with respect to the latter aspect of MMTV biology and the prospects for further advance in our understanding of breast cancer in humans that may be expected from a continued study of MMTV-induced mammary neoplasia. MMTV is a heritable somatic mutagen whose target range is limited. Commonly, the tumorigenic capacity of MMTV is restricted to mammary gland, whereas infection is found in a variety of cell types. In order to replicate, proviral DNA must be inserted into the cell DNA and cell division is required to fix the mutation. Yet only in the mammary epithelium does this lead to neoplastic transformation. This suggests a unique relationship between MMTV and mammary epithelium. In evaluating this relationship, we and others have discovered genes and potential gene pathways that are pertinent in mammary differentiation and neoplasia. In addition, the clonal nature of these progressive events from normal to malignant phenotype has become increasingly clear. The weight of these observations compel us to conclude that mammary neoplasms arise from multipotent mammary epithelial cells through a process of acquired mutations that are reflected in the increasingly malignant nature of the population of progeny produced by these damaged stem cells.

Characterization of the interaction between the interferon-induced protein P56 and the Int6 protein encoded by a locus of insertion of the mouse mammary tumor virus

For determining cellular functions of the interferon-inducible human cytoplasmic protein P56, we undertook a Saccharomyces cerevisiae two-hybrid screen that identified Int6 as a P56-interacting protein. That the interaction also occurs in human cells was confirmed by coimmunoprecipitation and the observed cytoplasmic displacement of nuclear Int6 upon coexpression of P56. Because Int6 has been claimed to be both a cytoplasmic and a nuclear protein, we investigated the structural basis of this discrepancy. By mutational analyses, we showed that the Int6 protein contains a bipartite nuclear localization signal and a nuclear export signal at the far end of the amino terminus. The 20 amino-terminal residues of Int6, when they were attached to a different nuclear protein, were sufficient to translocate that protein to the cytoplasm. Within this region, replacement of any of the three leucine residues with alanine destroyed the function of the export signal. The specific domain of P56 that is required for its interaction with Int6 was mapped using the yeast two-hybrid assay and a mammalian coimmunoprecipitation assay. Both assays demonstrated that the C-terminal region of P56 containing three specific tetratricopeptide motifs is required for this interaction. In contrast, removal of an internal domain of P56 enhanced the interaction, as quantified by the two-hybrid assay.

Interaction between the Ret finger protein and the Int-6 gene product and co-localisation into nuclear bodies

The mouse int-6 gene was identified in mammary tumors as an integration site for the mouse mammary tumor virus. Its human counterpart encodes a product that interacts with the Tax viral oncoprotein of the human T cell leukaemia virus type 1. This interaction impedes the localisation of over-expressed Int-6 in nuclear bodies containing the promyelocytic leukaemia gene product (PML). In this study, Int-6 is characterised as a 52 kDa protein that is localised within nuclear bodies in primary lymphocytes. Screening of a human B cell cDNA library for proteins that interact with Int-6 led to isolation of four clones coding for the p110 subunit of eIF3, in accordance with previous detection of Int-6 in purified forms of this translation initiation factor. Another clone was interesting with respect to the subcellular localisation of Int-6. It encodes the Ret finger protein (Rfp) which interacts with PML and localises within a subset of PML nuclear bodies. The interaction of Rfp with Int-6 is mediated through a region in Rfp designated ‘Rfp domain’, distinct from that involved in the interaction with PML. Int-6 and Rfp are co-localised in certain PML nuclear bodies in lymphocytes and transfection studies in HeLa cells strongly suggest that Rfp triggers translocation of Int-6 to nuclear bodies.

A novel transgenic technique that allows specific marking of the neural crest cell lineage in mice

Neural crest cells are embryonic, multipotent stem cells that give rise to various cell/tissue types and thus serve as a good model system for the study of cell specification and mechanisms of cell differentiation. For analysis of neural crest cell lineage, an efficient method has been devised for manipulating the mouse genome through the Cre-loxP system. We generated transgenic mice harboring a Cre gene driven by a promoter of protein 0 (P0). To detect the Cre-mediated DNA recombination, we crossed P0-Cre transgenic mice with CAG-CAT-Z indicator transgenic mice. The CAG-CAT-Z Tg line carries a lacZ gene downstream of a chicken beta-actin promoter and a "stuffer" fragment flanked by two loxP sequences, so that lacZ is expressed only when the stuffer is removed by the action of Cre recombinase. In three different P0-Cre lines crossed with CAG-CAT-Z Tg, embryos carrying both transgenes showed lacZ expression in tissues derived from neural crest cells, such as spinal dorsal root ganglia, sympathetic nervous system, enteric nervous system, and ventral craniofacial mesenchyme at stages later than 9.0 dpc. These findings give some insights into neural crest cell differentiation in mammals. We believe that P0-Cre transgenic mice will facilitate many interesting experiments, including lineage analysis, purification, and genetic manipulation of the mammalian neural crest cells.

Characterization of the Drosophila ortholog of mouse eIF-3p48/INT-6

The mouse mammary tumor virus (MMTV) has been shown to integrate frequently into INT-6 in MMTV-induced mouse mammary tumors. The INT6 gene has been highly conserved through evolution and has recently been shown to encode the p48 component of the eucaryotic translation initiation factor 3 (eIF-3) complex. We report here the isolation of the Drosophila eIF-3p48/INT-6. The gene comprises three exons within 1.8kb of genomic DNA located at cytogenetic position 73C2 in the Drosophila genome. The 1.5kb eIF-3p48/INT-6 RNA species encodes a protein composed of 364 amino-acid residues whose sequence is 71% similar to that of the mouse/human eIF-3/INT-6 amino-acid sequence. eIF-3p48/INT-6 RNA is expressed throughout development in Drosophila and the encoded protein is associated with the microsomal subcellular fraction.

The regional integration of retroviral sequences into the mosaic genomes of mammals

We have reviewed here three sets of data concerning the integration of retroviral sequences in the mammalian genome: (i) our experimental localization of a number of proviruses integrated in isochores characterized by different GC levels; (ii) results from other laboratories on the localization of retroviral sequences in open chromatin regions and/or next to CpG islands; and (iii) our compositional analysis of genes located in the neighborhood of integrated retroviral sequences. The three sets of data have provided a very consistent picture in that a compartmentalized, isopycnic integration of expressed proviruses appears to be the rule ('isopycnic' refers to the compositional match between viral and host sequences around the integration site). The results reviewed here suggest that: (i) integration of proviral sequences is targeted initially towards 'open chromatin regions'; while these exist in both GC-rich and GC-poor isochores, the 'open chromatin regions' of GC-rich isochores are the main targets for integration of retroviral sequences because of their much greater abundance; (ii) isopycnicity is associated with stability of integration; indeed, even non-expressed integrated retroviral sequences tend to show an isopycnic localization in the genome; (iii) transcription of integrated viral sequences (like transcription of host genes) appears to be associated, as a rule, with an isopycnic localization, as indicated by transcribed sequences that show an isopycnic integration and act in trans; (iv) selection plays a role in the choice of specific sites within an isopycnic region; in exceptional cases [such as mouse mammary tumor virus (MMTV) activating GC-rich oncogenes], selection may override isopycnicity.

The chromosome location of the human homolog of the mouse mammary tumor-associated gene INT6 and its status in human breast carcinomas

The INT6 gene is a common integration site for the mouse mammary tumor virus in mouse mammary tumors. We have determined that the human homolog of INT6 is located on chromosome region 8q22-q23. A processed INT6 pseudogene is located on chromosome 6q. INT6 is composed of 13 exons that span 45 kb of genomic DNA. The deduced amino acid sequence of the gene product is identical to the mouse protein and contains three potential translation start signals. We have examined 100 primary breast carcinoma DNAs for evidence of genetic alteration affecting INT6. Loss of heterozyosity (LOH) was detected in 11 of 39 (28%) of the tumor samples informative for a polymorphic sequence in intron 7 of INT6. Since single-strand conformation and hybrid mismatch analysis of the remaining allele in these tumor DNAs failed to detect any mutations, we conclude that the target gene for LOH must be closely linked to INT6.