Academic literature on the topic 'G3BP2 expression'

ABSTRACTStress granules (SGs) are protein-mRNA aggregates that are formed in response to environmental stresses, resulting in translational inhibition. SGs are generally believed to play an antiviral role and are manipulated by many viruses, including various alphaviruses. GTPase-activating protein (SH3 domain)-binding protein 1 (G3BP1) is a key component and commonly used marker of SGs. Its homolog G3BP2 is a less extensively studied SG component. Here, we demonstrate that Chikungunya virus (CHIKV) infection induces cytoplasmic G3BP1- and G3BP2-containing granules that differ from bona fide SGs in terms of morphology, composition, and behavior. For several Old World alphaviruses it has been shown that nonstructural protein 3 (nsP3) interacts with G3BPs, presumably to inhibit SG formation, and we have confirmed this interaction in CHIKV-infected cells. Surprisingly, CHIKV also relied on G3BPs for efficient replication, as simultaneous depletion of G3BP1 and G3BP2 reduced viral RNA levels, CHIKV protein expression, and viral progeny titers. The G3BPs colocalized with CHIKV nsP2 and nsP3 in cytoplasmic foci, but no colocalization with nsP1, nsP4, or dsRNA was observed. Furthermore, G3BPs could not be detected in a cellular fraction enriched for CHIKV replication/transcription complexes, suggesting that they are not directly involved in CHIKV RNA synthesis. Depletion of G3BPs did not affect viral entry, translation of incoming genomes, or nonstructural polyprotein processing but resulted in severely reduced levels of negative-stranded (and consequently also positive-stranded) RNA. This suggests a role for the G3BPs in the switch from translation to genome amplification, although the exact mechanism by which they act remains to be explored.IMPORTANCEChikungunya virus (CHIKV) causes a severe polyarthritis that has affected millions of people since its reemergence in 2004. The lack of approved vaccines or therapeutic options and the ongoing explosive outbreak in the Caribbean underline the importance of better understanding CHIKV replication. Stress granules (SGs) are cytoplasmic protein-mRNA aggregates formed in response to various stresses, including viral infection. The RNA-binding proteins G3BP1 and G3BP2 are essential SG components. SG formation and the resulting translational inhibition are generally considered an antiviral response, and many viruses manipulate or block this process. Late in infection, we and others have observed CHIKV nonstructural protein 3 in cytoplasmic G3BP1- and G3BP2-containing granules. These virally induced foci differed from true SGs and did not appear to represent replication complexes. Surprisingly, we found that G3BP1 and G3BP2 were also needed for efficient CHIKV replication, likely by facilitating the switch from translation to genome amplification early in infection.

AbstractMetastasis is the leading cause of death of patients with esophageal squamous cell carcinoma (ESCC). Although an increasing number of studies have demonstrated the involvement of G3BP2 in several human cancers, how G3BP2 interacts with long noncoding RNAs and regulates mRNA transcripts in mediating ESCC metastasis remains unclear. In this study, we uncovered that G3BP2 was upregulated in ESCC. Further analysis revealed that upregulation of G3BP2 was significantly correlated with lymph node metastasis, depth of tumor invasion and unfavorable outcomes in ESCC patients. Both in vitro and in vivo functional assays demonstrated that G3BP2 dramatically enhanced ESCC cell migration and invasion. Mechanistically, LINC01554 maintained the high G3BP2 expression in ESCC by protecting G3BP2 from degradation through ubiquitination and the interaction domains within LINC01554 and G3BP2 were identified. In addition, RNA-seq revealed that HDGF was regulated by G3BP2. G3BP2 bound to HDGF mRNA transcript to stabilize its expression. Ectopic expression of HDGF effectively abolished the G3BP2 depletion-mediated inhibitory effect on tumor cell migration. Intriguingly, introduction of compound C108 which can inhibit G3BP2 remarkedly suppressed ESCC cell metastasis in vitro and in vivo. Collectively, this study describes a newly discovered regulatory axis, LINC01554/G3BP2/HDGF, that facilitates ESCC metastasis and will provide novel therapeutic strategies for ESCC.

Breast tumors contain tumorigenic cancer cells, termed “tumor-initiating cells” (TICs), which are capable of both replenishing themselves and giving rise to populations of nontumorigenic breast cancer cells (non-TICs). However, the molecular mechanisms responsible for breast tumor initiation remain poorly understood. Here we describe a chemical screening strategy to identify small molecules that enhance the effect of chemotherapeutic agents on TIC-enriched breast cancer cells. We identified proteins that interact with the lead compound C108, including the stress granule-associated protein, GTPase-activating protein (SH3 domain)-binding protein 2, G3BP2. G3BP2 regulates breast tumor initiation through the stabilization of Squamous cell carcinoma antigen recognized by T cells 3 (SART3) mRNA, which leads to increased expression of the pluripotency transcription factors Octamer-binding protein 4 (Oct-4) and Nanog Homeobox (Nanog). Our findings suggest that G3BP2 is important for the process of breast cancer initiation. Furthermore, these data suggest a possible connection between stress granule formation and tumor initiation in breast cancer cells.

e15666 Background: Integration of Whole Exome Sequencing (WES) into clinical cancer therapeutics has revolutionized medicine in recent years. DNA sequencing alone may miss clinically actionable variants or identify aberrations that are not being transcribed. In this study we investigated the utility of integrating DNA and RNA sequencing in clinical samples. Methods: A cohort of 32 patient samples were analyzed by WES and RNA sequencing. Differential expression analysis was performed using a cohort of controls. Pathway analysis was performed using Ingenuity Pathway Analysis. WES and RNA analysis detected alterations including SNVs, indels, copy number events, fusions, alternate transcripts, TMB, MSI status and differential expression. Results: A total of 25 CRC (39-78yrs) and 7 pancreatic cancers (PCs) (51-91 yrs) were profiled by WES and RNA seq. A 66 yr old pt with neoplasm of rectosigmoid junction tumor was found to be KRAS wildtype and was treated with cetuximab plus FOLFIRI. Patient failed therapy after 2 yrs and sequencing revealed MET amplification which is a known mechanism of resistance to cetuximab treatment. Further, RNA expression analysis showed 44-fold increase in MET expression along with overexpression of AREG and EREG. Out of the 7 PCs, 3 cases that did not harbor KRAS mutation were found to harbor VTCN1/NRG1 fusion, FGFR1/G3BP2 fusion and BRAF V600E mutation respectively. A 50 year-old stage IV metastatic (met) PC pt was treated with standard of care regimens. Upon relapse the sample was found to harbor VTCN1/NRG1 fusion along with a TERT promoter mutation. RNA expression analysis revealed 54-fold increased expression of NRG1 which may lead to clinical trial enrollment. A 52 year-old male met Stage IV PC, was treated with rucaparib and irinotecan based on prior sequencing data. Upon relapse, the pt went on ATR inhibitor (BAY1895344) and progressed very quickly. Sequencing of the met lesion was found to harbor FGFR1/G3BP2 fusion which was also confirmed by RNA expression. A 55 year-old met pt with PC was treated with standard chemotherapy. Upon disease progression pt was sequenced and based on presence of BRAF V600E, pt was treated with trametinib and dabrafenib. Following disease progression on BRAF/MEKi, met sample was resequenced and RNA expression analysis found increased expression of MET, MACC1 and SMAD7 and 4-fold decrease in PTEN which could potentially cause resistance to BRAF/MEKi therapy. Conclusions: Our study highlights the utility of comprehensive testing integrating genomic and transcriptomic data, in identifying targeted therapy options for cancer patients.

Radiotherapy is commonly used to treat oral cancer patients in the current clinics; however, a subpopulation of patients shows poor radiosensitivity. Therefore, the aim of this study is to identify a biomarker or druggable target to enhance the effectiveness of radiotherapy on oral cancer patients. By performing an in silico analysis against public databases, we found that the upregulation of FOXD1, a gene encoding forkhead box d1 (Foxd1), is extensively detected in primary tumors compared to normal tissues and associated with a poor outcome in oral cancer patients receiving irradiation treatment. Moreover, our data showed that the level of FOXD1 transcript is causally relevant to the effective dosage of irradiation in a panel of oral cancer cell lines. The FOXD1 knockdown (FOXD1-KD) dramatically suppressed the colony-forming ability of oral cancer cells after irradiation treatment. Differentially expressed genes analysis showed that G3BP2, a negative regulator of p53, is predominantly repressed after FOXD1-KD and transcriptionally regulated by Foxd1, as judged by a luciferase-based promoter assay in oral cancer cells. Gene set enrichment analysis significantly predicted the inhibition of E2F-related signaling pathway but the activation of the interferons (IFNs) and p53-associated cellular functions, which were further validated by luciferase reporter assays in the FOXD1-KD oral cancer cells. Robustly, our data showed that FOXD1-KD fosters the expression of TXNIP, a downstream effector of IFN signaling and activator of p53, in oral cancer cells. These findings suggest that FOXD1 targeting might potentiate the anti-cancer effectiveness of radiotherapy and promote immune surveillance on oral cancer.

MK-STYX [MAPK (mitogen-activated protein kinase) phospho-serine/threonine/tyrosine-binding protein] is a pseudophosphatase member of the dual-specificity phosphatase subfamily of the PTPs (protein tyrosine phosphatases). MK-STYX is catalytically inactive due to the absence of two amino acids from the signature motif that are essential for phosphatase activity. The nucleophilic cysteine residue and the adjacent histidine residue, which are conserved in all active dual-specificity phosphatases, are replaced by serine and phenylalanine residues respectively in MK-STYX. Mutations to introduce histidine and cysteine residues into the active site of MK-STYX generated an active phosphatase. Using MS, we identified G3BP1 [Ras-GAP (GTPase-activating protein) SH3 (Src homology 3) domain-binding protein-1], a regulator of Ras signalling, as a binding partner of MK-STYX. We observed that G3BP1 bound to native MK-STYX; however, binding to the mutant catalytically active form of MK-STYX was dramatically reduced. G3BP1 is also an RNA-binding protein with endoribonuclease activity that is recruited to ‘stress granules’ after stress stimuli. Stress granules are large subcellular structures that serve as sites of mRNA sorting, in which untranslated mRNAs accumulate. We have shown that expression of MK-STYX inhibited stress granule formation induced either by aresenite or expression of G3BP itself; however, the catalytically active mutant MK-STYX was impaired in its ability to inhibit G3BP-induced stress granule assembly. These results reveal a novel facet of the function of a member of the PTP family, illustrating a role for MK-STYX in regulating the ability of G3BP1 to integrate changes in growth-factor stimulation and environmental stress with the regulation of protein synthesis.

G3BP1 (Ras-GTPase-activating protein SH3 domain-binding protein) is responsible for normal RNA stress granule (SG) assembly and overexpressed in many cancer cells. Deletion of G3BP1 decreases the number and size of SGs. SGs are complex of RNA and proteins that stall translation of protein in response to stress. Given the function of G3BP1 in stress granule assembly and tumor suppression, it is believed that G3BP1 regulates cell growth and proliferation as well. Here, I constructed the recombinant protein expression vector and systemically optimized condition for the expression of human G3BP1 protein in E. coli. This research should be useful for investigating further functional analysis and atomic structure of G3BP1.

ABSTRACT Sindbis virus (SINV) is the prototype member of the Alphavirus genus, whose members cause severe human diseases for which there is no specific treatment. To ascertain host factors important in the replication of the SINV RNA genome, we generated a SINV expressing nsP4, the viral RNA-dependent RNA polymerase, with an in-frame 3×Flag epitope tag. Proteomic analysis of nsP4-containing complexes isolated from cells infected with the tagged virus revealed 29 associated host proteins. Of these, 10 proteins were associated only at a later time of infection (12 h), 14 were associated both early and late, and five were isolated only at the earlier time (6 h postinfection). These results demonstrate the dynamic nature of the virus-host interaction that occurs over the course of infection and suggest that different host proteins may be required for the multiple functions carried out by nsP4. Two related proteins found in association with nsP4 at both times of infection, GTPase-activating protein (SH3 domain) binding protein 1 (G3BP1) and G3BP2 were also previously identified as associated with SINV nsP2 and nsP3. We demonstrate a likely overlapping role for these host factors in limiting SINV replication events. The present study also identifies 10 host factors associated with nsP4 6 h after infection that were not found to be associated with nsP2 or nsP3. These factors are candidates for playing important roles in the RNA replication process. Identifying host factors essential for replication should lead to new strategies to interrupt alphavirus replication.

Abstract Introduction: Nilotinib is a selective bcr-abl tyrosine kinase inhibitor that is 30-fold more potent than Imatinib in vitro. To examine the molecular and functional effects of Nilotinib and Imatinib we performed gene expression and functional analyses in K562 cells following in vitro treatment with the two tyrosine kinase inhibitors. Particular emphasis was put on 1539 genes which we found to be differentially expressed in primary CD34+ cells from patients with CML in first chronic phase in comparison to CD34+ cells from normal bone marrow (Diaz-Blanco et al., Leukemia 2006). Methods: Affymetrix U133A 2.0 microarrays covering 21.722 probe sets were used to analyse the gene expression profile of 5x107 K562 cells after 24h in vitro treatment with Imatinib (0.5 μM) or Nilotinib (0.05 μM) (half maximal inhibitory concentration, IC 50). FISH analysis confirmed the K562 cell line to be BCR-ABL positive. Gene expression data of the treated cells were compared with the data of untreated cells. In addition, proliferation (Cell Titer 96 AQueous One Solution Cell Proliferation Assay, Promega), apoptosis (Cell Death Detection ELISAPLUS, Roche) and cell cycle (FITC BrdU Flow Kit, BD Pharmingen) assays were performed. A colony assay was performed to see differences in cell growth. Results: Looking at those 1539 differentially expressed genes in K562 cells which distinguish patients with CML from healthy donors, we found that Imatinib led to a significant downregulation of 187 and upregulation of 45 genes. In general, Nilotinib had a more pronounced effect than Imatinib regarding the number of genes affected and the degree of suppression. It caused downregulation of 418 and upregulation of 41 genes. Of note, genes affected by Nilotinib included all genes altered by Imatinib such as those related to bcr-abl signalling (Lyn, BCL2, Myc, PIK3CB, G3BP2). Downregulation of genes involved in cell cycle (CDK2, ORC5L, MCM3, POLE2, CCNG1) was only observed following Nilotinib exposure. The stronger effect of Nilotinib is in line with the results of cell cycle experiments showing that Nilotinib exposed cells had the lowest proportion of actively cycling cells. The proportion of apoptotic K562 cells was 5.5 fold greater following treatment with Nilotinib in comparison to Imatinib after 24 hours. Treatment with either Imatinib or Nilotinib produced a similar apoptotic rate and similar decrease in cell numbers after 96 hours. In the colony forming assay, the controls (K562 cells incubated with DMSO only) displayed strong leukemic growth which was inhibited by both Nilotinib and Imatinib, allowing only small clusters to appear. Conclusion: Nilotinib is apparently more potent than Imatinib with regard to the number of genes downregulated and the degree of their suppression. Many of the suppressed genes are associated with bcr-abl signalling and cell cycle.

Under cell stress, global protein synthesis is inhibited to preserve energy. One mechanism is to sequester and silence mRNAs in ribonucleoprotein complexes known as stress granules (SGs), which contain translationally silent mRNAs, preinitiation factors, and RNA-binding proteins. Y-box binding protein 1 (YB-1) localizes to SGs, but its role in SG biology is unknown. We now report that YB-1 directly binds to and translationally activates the 5′ untranslated region (UTR) of G3BP1 mRNAs, thereby controlling the availability of the G3BP1 SG nucleator for SG assembly. YB-1 inactivation in human sarcoma cells dramatically reduces G3BP1 and SG formation in vitro. YB-1 and G3BP1 expression are highly correlated in human sarcomas, and elevated G3BP1 expression correlates with poor survival. Finally, G3BP1 down-regulation in sarcoma xenografts prevents in vivo SG formation and tumor invasion, and completely blocks lung metastasis in mouse models. Together, these findings demonstrate a critical role for YB-1 in SG formation through translational activation of G3BP1, and highlight novel functions for SGs in tumor progression.

RNA binding proteins (RBPs) play key roles in the post-transcriptional regulation of RNAs, which along with transcriptional regulation, is a major pathway that controls patterns of gene expression for development and proper cell signaling. Post-transcriptional control can occur at many different steps in RNA metabolism including; splicing, polyadenylation, mRNA stability, mRNA localization and translation. The over-expression of various RBPs in several different cancers leads to the notion that disrupted RNA metabolism has a role in carcinogenesis. Nevertheless, it is exceptionally challenging to discover the mechanisms behind RBP functions due to the difficulty in identifying the RNA targets of RBPs. This problem is compounded by the finding that RBPs frequently have multiple RNA targets which could be bound and regulated under different cellular contexts. Ras-GTPase-activating protein SH3-domain-binding proteins (G3BPs) are members of a highly conserved family of multi-functional RNA binding proteins, which appear to co-ordinate signal transduction and post-transcriptional gene regulation. Both G3BP1 and G3BP2 proteins are over-expressed in cancer, and G3BP1 promotes cell proliferation and survival. Aberrant expression of G3BP proteins is common in cancer, and their over-expression influence tumorigenesis. Furthermore, there is growing evidence that G3BPs are implicated in the aetiology of cancer metastasis. G3BP1 is involved in breast cancer epithelial to mesenchymal (EMT) metastasis via the Smad signalling pathway, whereas G3BP2 suppresses EMT metastasis by interacting with TWIST1 and localising it in the cytoplasm. The TWIST1-G3BP2 mechanotransduction pathway responds to biomechanical signals from the tumour environment and promotes EMT metastasis through the release of TWIST1 from G3BP2. G3BP2 has also been implicated in breast tumour initiation by stabilizing Squamous cell carcinoma antigen recognised by T cells 3 (SART3) transcripts which is responsible for the expression of pluripotent transcription factors Octamer-binding protein 4 (Oct-4) and Nanog Homeobox (Nanog). In addition, G3BPs possess antiviral activities and are targeted by various viruses, such as Polio virus, Chikungunya virus and Semliki Forest virus, to promote infection. Moreover, G3BPs, along with Caprin 1, have been reported to be responsible for the accumulation of interferon stimulated genes (ISGs) by facilitating their translation. Therefore, a detailed examination of G3BPs’ RNA transcripts may provide insights into the post-transcriptional mechanisms underlying tumorigenesis and viral infections. G3BPs are likely to be involved in the regulation of multiple transcript targets and the identification of more, or all, RNA targets of G3BPs will be an important step in a comprehensive understanding of molecular and cellular significances of G3BP’s activity by analyzing gene transcription, mRNA stability and translation, in different cellular contexts. Identification of different transcript targets of G3BPs will aid in the understanding of how G3BPs exert coordinated control of different cellular functions in a concerted fashion through their RNA targets. This research project was conceived from previous studies suggesting that G3BPs support translation of ISGs. The involvement of G3BPs in the translation of ISGs implies that G3BPs are involved in the regulation of the interferon system in response to viral infections and/or cellular stress, regulating the cellular immune response. Therefore, their antiviral property, or involvement in cancer metastasis could, in part, be due to the regulation of various ISGs which inhibit viral infections and promote cancer metastasis. The recent literature shows that interferon induced transmembrane (IFITM) proteins (IFITM1, IFITM2 and IFITM3) are antiviral proteins, involved in the restriction of various viruses and are also emerging to have a role in cancer progression and metastasis. Therefore, this gene family was selected as potential transcript targets of G3BPs. The main aim of this research study was to identify the individual roles of G3BP1 and G3BP2 in the regulation of IFITM1, IFITM2 and IFITM3 (IFITM1-3) proteins in breast cancer cell lines. G3BPs were hypothesised to interact with the 3´-UTRs of the IFITM1-3 transcripts and regulate their translation. IFITM1-3 proteins are type I ISGs and are not expressed in all cell lines, therefore, the interferon (IFN) sensitive breast cancer line, MCF7, was selected to induce the expression of these proteins and to analyse the role of G3BPs in their regulation. Although IFITM1-3 are ISGs and their expression can be induced by type I IFN, the study design required a cell line which constitutively express these proteins as this would be beneficial to characterize the pathways regulating their translation. The interferon system is dysregulated in drug resistant cell lines in response to DNA damage, therefore, the breast cancer multidrug resistant (MDR) cell line, MDR.MCF7 (developed in the host laboratory) was chosen as a cell line which may constitutively express the IFITM1-3 proteins due to the dysregulation of the IFN system. Chapter 3 describes the induction and optimisation of IFITM1-3 proteins expression at both transcriptional and translational levels in MCF7 cells. The expression of IFITM1-3 proteins were also assessed in MDR.MCF7 cells. Chapter 4 and 5 were designed to study the individual role of G3BP1 and G3BP2, by performing siRNA-mediated knockdown of G3BPs in these cell lines and analysing the effects of their downregulation on the regulation of IFITM1-3 endogenous transcripts and proteins. The initial knockdown studies confirmed that both G3BP1 and G3BP2 are essential for the accumulation of IFITM1-3 proteins, without affecting their transcript levels. The research was extended to study the role of G3BPs in the regulation of IFITM1-3 through an interaction with their 3´-UTRs by performing luciferase reporter assays and RNase assisted RNA chromatography (RARC) assay. These assays confirmed that both G3BP1 and G3BP2 interact with the 3´-UTRs of IFITM1-3 and regulate their translation, supporting the hypothesis made at the start of this study. G3BPs have been reported to have a role in regulating the phosphorylation of the MEK/ERK pathway which is subsequently implicated in the translational regulation of ISGs. Based on these findings and other reports which show that one of the downstream effectors of the MEK pathway, eIF4E, is involved in the export of a certain subset of mRNAs, the role of this pathway was analysed in the regulation of IFITM1-3 proteins. Results have shown that knockdown of G3BPs in MDR.MCF7 cells led to a decrease in the phosphorylation status of MEK, ERK and eIF4E, supporting the idea that G3BPs could have role in the regulation of IFITM1-3 through this pathway as well. Preliminary studies were performed to further analyse this notion, by inhibiting the phosphorylation of MEK by using U0126, a well-known inhibitor of MEK, which acts by downstream inhibition of the phosphorylation of ERK and eIF4E. The mRNA and protein expression levels of IFITM1-3 were then analysed in the U0126 treated MDR.MCF7 cells by qRT-PCR and immunoblotting. The data analysis supports a role of G3BPs in the regulation of IFITM1-3 proteins via MEK/ERK pathway, although further experimental studies are required to confirm this role. Overall, this research study shows that both G3BP1 and G3BP2 are essential for the accumulation of IFITM1-3 proteins by interacting with the 3´-UTRs of their transcripts and also suggests an involvement of the MEK/ERK pathway in the translational regulation of IFITM1-3 via G3BPs. The data suggests that G3BPs intersect twice in the regulation of IFITM1-3 expression, firstly through MEK/ERK pathway and then through an interaction with the 3´-UTRs of IFITM1-3. However, the experiments performed here cannot resolve whether the two apparent functions are part of a single control mechanism or the two functions are mutually exclusive. Considering the relevance of these findings in the aetiology of cancer, further research is required to determine if these pathways can be targeted for future anti-cancer therapies.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Environment and Sc
Science, Environment, Engineering and Technology
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G3BP1 and G3BP2 are members of a highly conserved family of multi-functional RNA binding proteins, which appear to co-ordinate signal transduction and post-transcriptional gene regulation. Both proteins are over-expressed in cancer, and G3BP1 promotes cell proliferation and survival. Aberrant expression of various RNA binding proteins is common in cancer, and several of these proteins influence tumorigenesis. Therefore, detailed examination of RNA binding proteins, such as G3BPs, may provide insights into the post-transcriptional mechanisms underlying tumorigenesis. Tumours arise as a consequence of genetic mutation or alteration, which often result from stress-induced DNA damage. Cancer progression is facilitated by various epigenetic stress adaptation mechanisms. Stressful stimuli induce transitory translational shut-off, mediated by phosphorylation of eukaryotic initiation factor alpha;(eIF2alpha;). This phosphorylation event leads to formation of discrete cytoplasmic foci known as stress granules (SGs), which are translationally-silent sites of mRNA sorting. It was initially thought that an RNA-binding protein, T-cell internal antigen 1 (TIA-1), was instrumental in both the formation and functioning of SGs, because over-expression of TIA-1 induces spontaneous SGs and concomitantly causes a decrease in reporter gene expression. It is now clear that SG content can change depending on the type of stress, and that various proteins, including G3BP1, can induce spontaneous SGs. In vitro evidence previously implicated both G3BP1 and G3BP2 as endoribonucleases, so it was suggested that G3BPs act to target mRNA for decay at the SG. This project sought to further investigate this proposal, and in this way gain insight into the specific function of G3BPs in post-transcriptional regulation during tumorigenesis. Characterisation of G3BP1 and G3BP2 expression and localisation patterns in human cells and cancer was necessary before functional analyses in human cell systems could be undertaken. Both proteins were found to be over-expressed in breast cancer, irrespective of cancer stage or grade. G3BP1 and G3BP2 were also expressed in all human cell lines tested, despite previously observed tissue-specific expression. These results support the notion that G3BP expression is switched on in parallel with cell proliferation, and as such, may influence tumorigenesis. The results of further analyses suggested that the diverse functions attributed to G3BP1 and G3BP2 may be facilitated by isoform-specific expression, various post-translational modifications and sub-cellular localisation. Despite the absence of a canonical endoribonuclease domain, it was previously reported that site-specific phosphorylation of G3BP1 enables the protein to degrade a synthetic c-myc RNA substrate in vitro. This finding implicated G3BP in the specific regulation of a proto-oncogene. Tailored reporter assays were thus designed in order to address the in vivo consequences of G3BP's putative endoribonuclease activity. Contrary to expectations, all G3BP family members increased or maintained the expression of a range of reporters, at both the mRNA and protein level, irrespective of the presence of any particular cis-acting element, coding sequence or promoter. These results support the emerging notion that G3BPs positively affect the expression of at least some of their target mRNAs, and may also indirectly promote transcription. In contrast to the theory that G3BPs degrade proto-oncogenic mRNA/s, these findings are consistent with a role for G3BP in promoting cell proliferation and survival. Further analyses showed that G3BP1 and G3BP2 simultaneously increased reporter gene expression and induced SG formation. These findings highlighted the fact that SGs are dynamic sorting stations for mRNAs, and not merely sites of stalled translation. This result also supports the notion that a variety of proteins may be recruited to the SG to facilitate a multitude of mRNA fates. Although the precise role of the SG in stress adapation is not known, it is clear that an appropriate integrated stress response (ISR) is required for cells to survive in sub-optimal conditions. It was found that specific G3BP1 knockdown inhibited SG formation and cell survival, and this appeared to occur downstream of eIF2alpha; phosphorylation. The phosphorylation of eIFalpha; is the only factor known to be necessary for SG formation and cell survival. This data is the first to implicate SG formation itself, downstream of eIF2alpha; phosphorylation, in the survival phase of the ISR. The results also suggest that G3BP1 plays a pivotal role in the post-transcriptional mechanisms underlying stress adaptation. To facilitate future analysis of G3BP roles in the regulation of specific transcripts and in SG biology, a pilot study to identify G3BP RNA ligands was undertaken. Immunoprecipitation of epitope-tagged G3BP1 from stable cell lines facilitated purification and isolation of RNA in association with G3BP1. Specific RNA transcripts were subsequently detected and identified by microarray. Many genes were enriched in the G3BP1 immunoprecipitate. Transcript enrichment in the control immunoprecipitate was comparatively weak and seemingly random, suggesting that several replicates will enable generation of a reliable target list. This work forms a promising basis for further investigations into G3BP functionality, and also provides a platform for broader and more large-scale analyses of the mechanisms of post-transcriptional gene regulation. The work presented in this thesis addressed the potential post-transcriptional mechanisms by which the G3BP family of proteins mediate cell proliferation and survival. Both G3BP1 and G3BP2 were shown to be over-expressed in tumours and each appeared to promote reporter gene expression. G3BP1 was also found to play a pivotal role in stress adaptation. A technique to identify novel RNA ligands was assessed, and it was found that G3BP1 may interact with various mRNA transcripts. It is hypothesised that the G3BP family of proteins, and in particular G3BP1, function to determine the fate of specific RNAs in response to cellular stress and other stimuli. In this way, G3BP proteins may facilitate appropriate responses to extra-cellular stimuli which allow for cell proliferation and survival.

G3BP1 and G3BP2 are members of a highly conserved family of multi-functional RNA binding proteins, which appear to co-ordinate signal transduction and post-transcriptional gene regulation. Both proteins are over-expressed in cancer, and G3BP1 promotes cell proliferation and survival. Aberrant expression of various RNA binding proteins is common in cancer, and several of these proteins influence tumorigenesis. Therefore, detailed examination of RNA binding proteins, such as G3BPs, may provide insights into the post-transcriptional mechanisms underlying tumorigenesis. Tumours arise as a consequence of genetic mutation or alteration, which often result from stress-induced DNA damage. Cancer progression is facilitated by various epigenetic stress adaptation mechanisms. Stressful stimuli induce transitory translational shut-off, mediated by phosphorylation of eukaryotic initiation factor alpha;(eIF2alpha;). This phosphorylation event leads to formation of discrete cytoplasmic foci known as stress granules (SGs), which are translationally-silent sites of mRNA sorting. It was initially thought that an RNA-binding protein, T-cell internal antigen 1 (TIA-1), was instrumental in both the formation and functioning of SGs, because over-expression of TIA-1 induces spontaneous SGs and concomitantly causes a decrease in reporter gene expression. It is now clear that SG content can change depending on the type of stress, and that various proteins, including G3BP1, can induce spontaneous SGs. In vitro evidence previously implicated both G3BP1 and G3BP2 as endoribonucleases, so it was suggested that G3BPs act to target mRNA for decay at the SG. This project sought to further investigate this proposal, and in this way gain insight into the specific function of G3BPs in post-transcriptional regulation during tumorigenesis. Characterisation of G3BP1 and G3BP2 expression and localisation patterns in human cells and cancer was necessary before functional analyses in human cell systems could be undertaken. Both proteins were found to be over-expressed in breast cancer, irrespective of cancer stage or grade. G3BP1 and G3BP2 were also expressed in all human cell lines tested, despite previously observed tissue-specific expression. These results support the notion that G3BP expression is switched on in parallel with cell proliferation, and as such, may influence tumorigenesis. The results of further analyses suggested that the diverse functions attributed to G3BP1 and G3BP2 may be facilitated by isoform-specific expression, various post-translational modifications and sub-cellular localisation. Despite the absence of a canonical endoribonuclease domain, it was previously reported that site-specific phosphorylation of G3BP1 enables the protein to degrade a synthetic c-myc RNA substrate in vitro. This finding implicated G3BP in the specific regulation of a proto-oncogene. Tailored reporter assays were thus designed in order to address the in vivo consequences of G3BP's putative endoribonuclease activity. Contrary to expectations, all G3BP family members increased or maintained the expression of a range of reporters, at both the mRNA and protein level, irrespective of the presence of any particular cis-acting element, coding sequence or promoter. These results support the emerging notion that G3BPs positively affect the expression of at least some of their target mRNAs, and may also indirectly promote transcription. In contrast to the theory that G3BPs degrade proto-oncogenic mRNA/s, these findings are consistent with a role for G3BP in promoting cell proliferation and survival. Further analyses showed that G3BP1 and G3BP2 simultaneously increased reporter gene expression and induced SG formation. These findings highlighted the fact that SGs are dynamic sorting stations for mRNAs, and not merely sites of stalled translation. This result also supports the notion that a variety of proteins may be recruited to the SG to facilitate a multitude of mRNA fates. Although the precise role of the SG in stress adapation is not known, it is clear that an appropriate integrated stress response (ISR) is required for cells to survive in sub-optimal conditions. It was found that specific G3BP1 knockdown inhibited SG formation and cell survival, and this appeared to occur downstream of eIF2alpha; phosphorylation. The phosphorylation of eIFalpha; is the only factor known to be necessary for SG formation and cell survival. This data is the first to implicate SG formation itself, downstream of eIF2alpha; phosphorylation, in the survival phase of the ISR. The results also suggest that G3BP1 plays a pivotal role in the post-transcriptional mechanisms underlying stress adaptation. To facilitate future analysis of G3BP roles in the regulation of specific transcripts and in SG biology, a pilot study to identify G3BP RNA ligands was undertaken. Immunoprecipitation of epitope-tagged G3BP1 from stable cell lines facilitated purification and isolation of RNA in association with G3BP1. Specific RNA transcripts were subsequently detected and identified by microarray. Many genes were enriched in the G3BP1 immunoprecipitate. Transcript enrichment in the control immunoprecipitate was comparatively weak and seemingly random, suggesting that several replicates will enable generation of a reliable target list. This work forms a promising basis for further investigations into G3BP functionality, and also provides a platform for broader and more large-scale analyses of the mechanisms of post-transcriptional gene regulation. The work presented in this thesis addressed the potential post-transcriptional mechanisms by which the G3BP family of proteins mediate cell proliferation and survival. Both G3BP1 and G3BP2 were shown to be over-expressed in tumours and each appeared to promote reporter gene expression. G3BP1 was also found to play a pivotal role in stress adaptation. A technique to identify novel RNA ligands was assessed, and it was found that G3BP1 may interact with various mRNA transcripts. It is hypothesised that the G3BP family of proteins, and in particular G3BP1, function to determine the fate of specific RNAs in response to cellular stress and other stimuli. In this way, G3BP proteins may facilitate appropriate responses to extra-cellular stimuli which allow for cell proliferation and survival.
Thesis (PhD Doctorate)
Doctor of Philosophy (PhD)
School of Biomolecular and Biomedical Sciences
Full Text

Proteína 2 ligante de SH3 ativadora da GTPase de Ras (G3BP2) parece estar envolvido em vias importantes de sobrevivência celular como a via do NF-B. Já o fator tecidual (TF) e sua isoforma asHTF parecem estar envolvidos nos processos de angiogênese, proliferação e metástase. Assim, os genes G3BP2, TF e asHTF parecem possíveis fatores prognósticos em carcinoma epidermóide (CE) de cabeça e pescoço. Para avaliar a importância desses genes nesse tipo de câncer, foi realizada a quantificação da expressão do RNAm desses marcadores por PCR (reação em cadeia da polimerase) em tempo real em tumores primário e mucosas adjacente de 148 pacientes com CE de cavidade oral, língua e laringe e a proteína de TF e asHTF foi determinada por Western blotting em 17 pacientes com CE de cavidade oral. Os dados da expressão do RNAm foram correlacionados com variáveis clínico-patológicos e sobrevida dos pacientes. A expressão relativa do RNAm de G3BP2 foi menor nos tumores primários em relação à mucosa adjacente em todos os sítios analisados. TF e asHTF não apresentaram nenhuma correlação entre a expressão relativa no tumor primário e na mucosa adjacente para os três sítios analisados. Quando comparados aos tumores sem comprometimento linfonodal (pN0), tumores de cavidade oral e língua com comprometimento linfonodal (pN+) apresentaram maior expressão de TF (P=0,003; P=0,033, respectivamente) e de asHTF (P=0,004; P=0,018, respectivamente Teste não paramétrico de Mann-Whitney). Em seguida, construímos as curvas de sobrevida pelo método de Kaplan-Meier considerando como positivos os pacientes com expressão maior que a mediana para cada marcador. Para os CE de cavidade oral, pacientes com expressão positiva dos genes TF (P=0,034 Teste Log-rank) e asHTF (P=0,010) apresentaram pior sobrevida livre de doença na análise univariada. Quando realizado a análise multivariada tanto TF como asHTF mantiveram a significância estatística (P=0,002 para ambos os genes), sugerindo, assim, serem fatores prognósticos independent. A análise da proteína apontou 76,4% de concordância no padrão de expressão do RNAm de TF e 58,8% para a expressão do RNAm de asHTF. Não foi observada nenhuma correlação entre esses marcadores com outros dados clínico-patológicos e nem com a sobrevida dos pacientes para os tumores de língua e laringe. Assim, sugerimos que a expressão de TF e asHTF parecem ser marcadores prognósticos nos CE de cavidade oral principalmente em relação a sobrevida livre de doença. Apoio FAPESP 06/53755-5
GTPase activating protein (SH3 domain) binding protein 2 (G3BP2) seems to be involved in pathways important for cell survival as NF- B pathway. Tissue factor (TF) and its isoform asHTF appear to be involved in angiogenesis, proliferation and metastasis. Thus, the genes G3BP2, TF and asHTF seem possible prognostic factors in head and neck squamous cell carcinoma (SCC). To assess the importance of these genes in this type of cancer, we performed mRNA expression analysis of these markers by real time PCR (polymerase chain reaction) in primary tumors and adjacent mucosa of 148 patients with oral cavity, tongue and larynx SCC. TF and asHTF protein was determined by Western blotting in 17 patients with oral cavity SCC. mRNA expression levels were correlated with clinical and pathological variables and survival of patients. G3BP2 mRNA expression was lower in primary tumors as compared to adjacent mucosa at all sites analyzed. No difference was found between TF and asHTF expression in primary tumors and adjacent mucosa in the whole group. TF and asHTF levels mRNA expression were higher in pN+ tumors as compared to pN0 in oral cavity (P=0.003, P = 0.004, respectively) and tongue (P= 0.033, P= 0.018, respectively - Mann-Whitney test) tumors. For Kaplan Meier survival analysis patients were categorized positive (expression > tumor median relative expression) and negative (expression tumor median relative expression). For oral cavity SCC, patients with positive expression of TF (P =0.034 - log-rank test) and asHTF (P =0.010) genes presented shorter disease-free survival in univariate analysis. In multivariate analysis both TF as asHTF retained statistical significance (P=0.002 for both genes), suggesting, therefore, are independent prognostic factors. Protein analysis showed 76.4% agreement with mRNA expression for TF and 58.8% for asHTF. In patients with tongue and larynx tumors there correlation were not found. Thus, we suggest that the TF and asHTF expression seem to be prognostic markers in oral cavity SCC especially in relation to disease-free survival. Supported by FAPESP 06/53755-5