Differential involvement of RalA and RalB in colorectal cancer

Identification of RAS-like oncoprotein B (RALB) as a potential prognostic and therapeutic target in head and neck squamous cell carcinoma

BACKGROUND/PURPOSE

The RAS superfamily oncogenes play significant roles in various types of malignant tumors. However, little is known about the role of RAS-like oncoprotein B (RALB) in head and neck squamous cell carcinoma (HNSCC). This study evaluated whether RALB can be a prognostic and therapeutic target for HNSCC.

MATERIALS AND METHODS

A total of 504 HNSCC samples from The Cancer Genome Atlas database were segregated into two groups: RALB-high and RALB-low. The clinical significance of RALB expression in HNSCC patients was investigated. Cell proliferation, migration, and invasion assays were performed in HN-1 and HN-5 cells by silencing RALB using siRNA. Gene enrichment and immune infiltration analyses were also performed.

RESULTS

RALB expression was elevated in HNSCC tissues compared with normal tissues and was an independent risk factor associated with poor prognosis. A nomogram including the RALB expression level was established to predict the prognosis of HNSCC patients and showed highest sensitivity and benefit in predicting the three-year survival. The inhibition of RALB expression effectively impeded the proliferation, invasion, and migration of HNSCC cells. Importantly, RALB levels were significantly correlated with T cell-mediated immune responses, especially in human papillomavirus-positive HNSCC samples.

CONCLUSION

This study identified RALB as a potential prognostic and therapeutic target for HNSCC, and provided insight into the relationship between RALB and revealed an innovative strategy for HNSCC immunotherapy.

Inflammasomes Are Influenced by Epigenetic and Autophagy Mechanisms in Colorectal Cancer Signaling

Inflammasomes contribute to colorectal cancer signaling by primarily inducing inflammation in the surrounding tumor microenvironment. Its role in inflammation is receiving increasing attention, as inflammation has a protumor effect in addition to inducing tissue damage. The inflammasome's function is complex and controlled by several layers of regulation. Epigenetic processes impact the functioning or manifestation of genes that are involved in the control of inflammasomes or the subsequent signaling cascades. Researchers have intensively studied the significance of epigenetic mechanisms in regulation, as they encompass several potential therapeutic targets. The regulatory interactions between the inflammasome and autophagy are intricate, exhibiting both advantageous and harmful consequences. The regulatory aspects between the two entities also encompass several therapeutic targets. The relationship between the activation of the inflammasome, autophagy, and epigenetic alterations in CRC is complex and involves several interrelated pathways. This article provides a brief summary of the newest studies on how epigenetics and autophagy control the inflammasome, with a special focus on their role in colorectal cancer. Based on the latest findings, we also provide an overview of the latest therapeutic ideas for this complex network.

Recent Advances in Therapeutic Strategies to Improve Colorectal Cancer Treatment

Colorectal cancer (CRC) is the second-leading cause of cancer-related mortality worldwide. CRC mortality results almost exclusively from metastatic disease (mCRC) for which systemic chemotherapy is often a preferred therapeutic option. Biomarker-based stratification of mCRC enables the use of precision therapy based on individual tumor mutational profiles. Activating mutations in the RAS/RAF/MAPK pathway downstream of EGFR signaling have, until recently, limited the use of EGFR-targeted therapies for mCRC; however, the development of anti-RAS and anti-RAF therapies together with improved strategies to limit compensatory signaling pathways is resulting in improved survival rates in several highly lethal mCRC sub-types (e.g., BRAF-mutant). The use of fluoropyrimidine (FP)-based chemotherapy regimens to treat mCRC continues to evolve contributing to improved long-term survival. Future advances in chemotherapy for mCRC will need to position development relative to the advances made in precision oncology.

Regulation of Ras p21 and RalA GTPases activity by quinine in mammary epithelial cells

Quinine, a bitter compound, can act as an agonist to activate the family of bitter taste G protein-coupled receptor family of proteins. Previous work from our laboratory has demonstrated that quinine causes activation of RalA, a Ras p21-related small G protein. Ral proteins can be activated directly or indirectly through an alternative pathway that requires Ras p21 activation resulting in the recruitment of RalGDS, a guanine nucleotide exchange factor for Ral. Using normal mammary epithelial (MCF-10A) and non-invasive mammary epithelial (MCF-7) cell lines, we investigated the effect of quinine in regulating Ras p21 and RalA activity. Results showed that in the presence of quinine, Ras p21 is activated in both MCF-10A and MCF-7 cells; however, RalA was inhibited in MCF-10A cells, and no effect was observed in the case of MCF-7 cells. MAP kinase, a downstream effector for Ras p21, was activated in both MCF-10A and MCF-7 cells. Western blot analysis confirmed the expression of RalGDS in MCF-10A cells and MCF-7 cells. The expression of RalGDS was higher in MCF-10A cells in comparison to the MCF-7 cells. Although RalGDS was detected in MCF-10A and MCF-7 cells, it did not result in RalA activation upon Ras p21 activation with quinine suggesting that the Ras p21-RalGDS-RalA pathway is not active in the MCF-10A cells. The inhibition of RalA activity in MCF-10A cells due to quinine could be as a result of a direct effect of this bitter compound on RalA. Protein modeling and ligand docking analysis demonstrated that quinine can interact with RalA through the R79 amino acid, which is located in the switch II region loop of the RalA protein. It is possible that quinine causes a conformational change that results in the inhibition of RalA activation even though RalGDS is present in the cell. More studies are needed to elucidate the mechanism(s) that regulate Ral activity in mammary epithelial cells.

Low expression of RalGAPs associates with the poorer overall survival of head and neck squamous cell carcinoma

Background

The role of Ral and RalGAPs on the progression of head and neck squamous cell carcinoma (HNSC) remains unclear.

Methods

The predesigned siRNAs against RalGAPs were transfected into cells to evaluate the effect on RalA activation. The Data from TCGA and GTEx were combined to analyze the pan-cancer gene expression of RalA and RalGAPs in cancer and adjacent normal tissues. Kaplan-Meier analysis was used to assess the predictive value of RalA and RalGAPs expression on the overall survival of patients with HNSC. Methylation-specific PCR in vitro and next-generation bisulfite sequencing in vivo were used to evaluate the association between DNA methylation and the down-regulation of RalGAPs.

Results

RalGAPs negatively regulated RalA activation. HNSC patients with low level of RalGAPα2 had worse overall survival. The promoter of RalGAPα2 was widely methylated in comparison to RalGAPα1 and the DNA methylation level of RalGAPα2 promoter was increased in HNSC tissues and associated with the presence of neck lymph node metastasis.

Conclusions

RalA and RalGAPs could act as a specific predictor to assess the prognosis of HNSC. DNA methylation might be a potential mechanism that downregulated the RalGAPα2 expression.

Calmodulin extracts the Ras family protein RalA from lipid bilayers by engagement with two membrane-targeting motifs

RalA is a small GTPase and a member of the Ras family. This molecular switch is activated downstream of Ras and is widely implicated in tumor formation and growth. Previous work has shown that the ubiquitous Ca2+-sensor calmodulin (CaM) binds to small GTPases such as RalA and K-Ras4B, but a lack of structural information has obscured the functional consequences of these interactions. Here, we have investigated the binding of CaM to RalA and found that CaM interacts exclusively with the C terminus of RalA, which is lipidated with a prenyl group in vivo to aid membrane attachment. Biophysical and structural analyses show that the two RalA membrane-targeting motifs (the prenyl anchor and the polybasic motif) are engaged by distinct lobes of CaM and that CaM binding leads to removal of RalA from its membrane environment. The structure of this complex, along with a biophysical investigation into membrane removal, provides a framework with which to understand how CaM regulates the function of RalA and sheds light on the interaction of CaM with other small GTPases, including K-Ras4B.

The RAL signaling network: Cancer and beyond

The RAL proteins RALA and RALB belong to the superfamily of small RAS-like GTPases (guanosine triphosphatases). RAL GTPases function as molecular switches in cells by cycling through GDP- and GTP-bound states, a process which is regulated by several guanine exchange factors (GEFs) and two heterodimeric GTPase activating proteins (GAPs). Since their discovery in the 1980s, RALA and RALB have been established to exert isoform-specific functions in central cellular processes such as exocytosis, endocytosis, actin organization and gene expression. Consequently, it is not surprising that an increasing number of physiological functions are discovered to be controlled by RAL, including neuronal plasticity, immune response, and glucose and lipid homeostasis. The critical importance of RAL GTPases for oncogenic RAS-driven cellular transformation and tumorigenesis still attracts most research interest. Here, RAL proteins are key drivers of cell migration, metastasis, anchorage-independent proliferation, and survival. This chapter provides an overview of normal and pathological functions of RAL GTPases and summarizes the current knowledge on the involvement of RAL in human disease as well as current therapeutic targeting strategies. In particular, molecular mechanisms that specifically control RAL activity and RAL effector usage in different scenarios are outlined, putting a spotlight on the complexity of the RAL GTPase signaling network and the emerging theme of RAS-independent regulation and relevance of RAL.

Design and Efficient Synthesis of RalA Inhibitors Containing the Dihydro-α-carboline Scaffold

Ras-related protein RalA is a member of the Ras small GTPases superfamily. Its activation plays an important role in regulating tumor initiation, invasion, migration, and metastasis. In this study, we designed a new type of RalA inhibitor containing a dihydro-α-carboline scaffold. The structurally new dihydro-α-carboline derivatives could be efficiently synthesized in good yields through a newly developed three-component [3+2+1] cyclization reaction. Evaluation of the biological activity showed that some of the dihydro-α-carboline derivatives can inhibit RalA/B and proliferative activities of NSCLC cell lines. The 4-(pyridin-3-yl)-dihydro-α-carboline compound (3 o) was found to be the most potent derivative, with IC₅₀ values of 0.43±0.03, 0.64±0.07, 0.93±0.10, and 1.54±0.15 μM against A549, H1299, H460, and H1975 cells, respectively. Mechanism investigation suggested that 3 o inhibits the RalA/B activation of A549, down-regulates Bcl-2, stimulates cytochrome c and PARP cleavage, and induces cell apoptosis. A molecular docking study revealed that 3 o can form stable hydrogen bonds with residues of RalA. Moreover, amide-π and alkyl-π interactions also contributed to the affinity between 3 o and RalA.

RAL GTPases mediate multiple myeloma cell survival and are activated independently of oncogenic RAS

Oncogenic RAS provides crucial survival signaling for up to half of multiple myeloma cases, but has so far remained a clinically undruggable target. RAL is a member of the RAS superfamily of small GTPases and is considered to be a potential mediator of oncogenic RAS signaling. In primary multiple myeloma, we found RAL to be overexpressed in the vast majority of samples when compared with pre-malignant monoclonal gammopathy of undetermined significance or normal plasma cells. We analyzed the functional effects of RAL abrogation in myeloma cell lines and found that RAL is a critical mediator of survival. RNAi-mediated knockdown of RAL resulted in rapid induction of tumor cell death, an effect which was independent from signaling via mitogen-activated protein kinase, but appears to be partially dependent on Akt activity. Notably, RAL activation was not correlated with the presence of activating RAS mutations and remained unaffected by knockdown of oncogenic RAS. Furthermore, transcriptome analysis yielded distinct RNA expression signatures after knockdown of either RAS or RAL. Combining RAL depletion with clinically relevant anti-myeloma agents led to enhanced rates of cell death. Our data demonstrate that RAL promotes multiple myeloma cell survival independently of oncogenic RAS and, thus, this pathway represents a potential therapeutic target in its own right.

Fendiline Enhances the Cytotoxic Effects of Therapeutic Agents on PDAC Cells by Inhibiting Tumor-Promoting Signaling Events: A Potential Strategy to Combat PDAC

The L-type calcium channel blocker fendiline has been shown to interfere with Ras-dependent signaling in K-Ras mutant cancer cells. Earlier studies from our lab had shown that treatment of pancreatic cancer cells with fendiline causes significant cytotoxicity and interferes with proliferation, survival, migration, invasion and anchorage independent growth. Currently there are no effective therapies to manage PDACs. As fendiline has been approved for treatment of patients with angina, we hypothesized that, if proven effective, combinatorial therapies using this agent would be easily translatable to clinic for testing in PDAC patients. Here we tested combinations of fendiline with gemcitabine, visudyne (a YAP1 inhibitor) or tivantinib (ARQ197, a c-Met inhibitor) for their effectiveness in overcoming growth and oncogenic characteristics of PDAC cells. The Hippo pathway component YAP1 has been shown to bypass K-Ras addiction, and allow tumor growth, in a Ras-null mouse model. Similarly, c-Met expression has been associated with poor prognosis and metastasis in PDAC patients. Our results presented here show that combinations of fendiline with these inhibitors show enhanced anti-tumor activity in Panc1, MiaPaCa2 and CD18/HPAF PDAC cells, as evident from the reduced viability, migration, anchorage-independent growth and self-renewal. Biochemical analysis shows that these agents interfere with various signaling cascades such as the activation of Akt and ERK, as well as the expression of c-Myc and CD44 that are altered in PDACs. These results imply that inclusion of fendiline may improve the efficacy of various chemotherapeutic agents that could potentially benefit PDAC patients.

RCC2, a regulator of the RalA signaling pathway, is identified as a novel therapeutic target in cisplatin-resistant ovarian cancer

Currently, cisplatin (DDP) is the first-line chemotherapeutic agent used for treatment of ovarian cancer, but gradually acquired drug resistance minimizes its therapeutic outcomes. We aimed to identify crucial genes associated with DDP resistance in ovarian cancer and uncover potential mechanisms. Two sets of gene expression data were downloaded from Gene Expression Omnibus, and bioinformatics analysis was conducted. In our study, the differentially expressed genes between DDP-sensitive and DDP-resistant ovarian cancer were screened in GSE15709 and GSE51373 database, and chromosome condensation 2 regulator (RCC2) and nucleoporin 160 were identified as 2 genes that significantly up-regulated in DDP-resistant ovarian cancer cell lines compared with DDP-sensitive cell lines. Moreover, RCC2, Ral small GTPase (RalA), and Ral binding protein-1 (RalBP1) expression was found to be significantly higher in DDP-resistant ovarian cancer tissues than in DDP-sensitive tissues. RCC2 plays a positive role in cell proliferation, apoptosis, and migration in DDP-resistant ovarian cancer cell lines in vitro and in vivo. Furthermore, RCC2 could interact with RalA, thus promoting its downstream effector RalBP1. RalA knockdown could reverse the effects of RCC2 overexpression on DDP-resistant ovarian cancer cell proliferation, apoptosis, and migration. Similarly, RalA overexpression could alleviate the effects of RCC2 knockdown in DDP-resistant ovarian cancer cells. Taken together, RCC2 may function as an oncogene, regulating the RalA signaling pathway, and intervention of RCC2 expression might be a promising therapeutic strategy for DDP-resistant ovarian cancer.-Gong, S., Chen, Y., Meng, F., Zhang, Y., Wu, H., Li, C., Zhang, G. RCC2, a regulator of the RalA signaling pathway, is identified as a novel therapeutic target in cisplatin-resistant ovarian cancer.

Investigation of the molecular mechanisms of hepatic injury upon naphthalene exposure in zebrafish (Danio rerio)

Naphthalene has been used worldwide as a commercial insecticide for decades, which when detected in the environment can have various negative effects on non-target organism, such as hepatotoxicity. However, the molecular mechanisms of how naphthalene acts to affect the liver in zebrafish (Danio rerio) remains unknown. In this study, we evaluated the potential toxic effects of naphthalene on livers in female adult zebrafish over a 21-day subacute exposure. Global hepatic gene expression was examined by microarrays and the results indicated the regulated genes were associated significantly with vital hepatic injury pathways and GO categories upon naphthalene exposure, such as disruptions in lipid metabolism, inflammatory response, and the carcinogenic processes. According to our observations of liver histology, nuclear enlargement as a potential indicator of cancers and hepatic lipometabolic disorder precisely were supported. The 96 h acute naphthalene tests on Tg(lysC:DsRed) and LiPan lines larvae revealed recruitment of neutrophils by the liver, as well as decreased liver size, which further confirmed hepatic inflammation response to naphthalene exposure. Thus, these findings advance the field of ecotoxicology by unveiling a new role of naphthalene as a leading cause of liver damage and provide potential biomarker-genes for environmental naphthalene monitoring.

Biogenesis and function of extracellular vesicles in cancer

Extracellular vesicles (EVs) are heterogeneous multi-signal messengers that support cancer growth and dissemination by mediating the tumor-stroma crosstalk. Exosomes are a subtype of EVs that originate from the limiting membrane of late endosomes, and as such contain information linked to both the intrinsic cell "state" and the extracellular signals cells received from their environment. Resolving the signals affecting exosome biogenesis, cargo sorting and release will increase our understanding of tumorigenesis. In this review we highlight key cell biological processes that couple exosome biogenesis to cargo sorting in cancer cells. Moreover, we discuss how the bidirectional communication between tumor and non-malignant cells affect cancer growth and metastatic behavior.

Ral signaling pathway in health and cancer

The Ral (Ras-Like) signaling pathway plays an important role in the biology of cells. A plethora of effects is regulated by this signaling pathway and its prooncogenic effectors. Our team has demonstrated the overactivation of the RalA signaling pathway in a number of human malignancies including cancers of the liver, ovary, lung, brain, and malignant peripheral nerve sheath tumors. Additionally, we have shown that the activation of RalA in cancer stem cells is higher in comparison with differentiated cancer cells. In this article, we review the role of Ral signaling in health and disease with a focus on the role of this multifunctional protein in the generation of therapies for cancer. An improved understanding of this pathway can lead to development of a novel class of anticancer therapies that functions on the basis of intervention with RalA or its downstream effectors.

Tyrosine phosphorylation of RalGDS by c-Met receptor blocks its interaction with Ras

RalGDS is a guanine nucleotide exchange factor that promotes the active GTP-bound form of Ral GTPases, RalA and RalB. GTP-bound Ras has the capacity to activate Ral GTPases at least in part by binding to the C-terminal Ras-binding domain (RBD) of RalGDS and directing the protein to Ral GTPases in the plasma membrane. In many cases, activation of Ral proteins complements other Ras effector pathways to carry out a cell function, but in others it opposes them. Moreover, in many cases activation of Ral proteins contributes to the oncogenic potential of Ras. However, in some cell types Ral proteins suppresses tumor formation, suggesting oncogenic stimuli that function through Ras may need to suppress Ral activation in order to transform cells. In this paper, we demonstrate a potential biochemical mechanism for such phenomena by showing that c-Met receptors promote the tyrosine phosphorylation of RalGDS at Y752 in its RBD, which blocks the binding of Ras to RalGDS.

Synthesis of novel Ral inhibitors: An in vitro and in vivo study

Chemical synthesis was performed to produce a series of 6-amino-1,3-disubstituted-4-phenyl-1,4-dihydro pyrano[2,3-c]pyrazole-5-carbonitrile compounds (14-57) which were characterized by ¹H NMR, ¹³C NMR and LC/MS-MS. These compounds were assessed for their effect on the in vitro anchorage independent growth of human lung cancer cell line H2122 and IC₅₀ values calculated. Two of the more potent compounds, BQU057 40 and BQU082 57 also displayed a dose dependent effect on RalA and RalB activity in H2122 spheroids using the common RalBP1 pull-down assay. Mouse PK and tissue distribution studies on 40 and 57 were performed and demonstrated that parent drug was present in tumor 3.0h post ip (50mg/Kg) dose.

Inhibition of Ral GTPases Using a Stapled Peptide Approach

Aberrant Ras signaling drives numerous cancers, and drugs to inhibit this are urgently required. This compelling clinical need combined with recent innovations in drug discovery including the advent of biologic therapeutic agents, has propelled Ras back to the forefront of targeting efforts. Activated Ras has proved extremely difficult to target directly, and the focus has moved to the main downstream Ras-signaling pathways. In particular, the Ras-Raf and Ras-PI3K pathways have provided conspicuous enzyme therapeutic targets that were more accessible to conventional drug-discovery strategies. The Ras-RalGEF-Ral pathway is a more difficult challenge for traditional medicinal development, and there have, therefore, been few inhibitors reported that disrupt this axis. We have used our structure of a Ral-effector complex as a basis for the design and characterization of α-helical-stapled peptides that bind selectively to active, GTP-bound Ral proteins and that compete with downstream effector proteins. The peptides have been thoroughly characterized biophysically. Crucially, the lead peptide enters cells and is biologically active, inhibiting isoform-specific RalB-driven cellular processes. This, therefore, provides a starting point for therapeutic inhibition of the Ras-RalGEF-Ral pathway.

Ral-Arf6 crosstalk regulates Ral dependent exocyst trafficking and anchorage independent growth signalling

Integrin dependent regulation of growth factor signalling confers anchorage dependence that is deregulated in cancers. Downstream of integrins and oncogenic Ras the small GTPase Ral is a vital mediator of adhesion dependent trafficking and signalling. This study identifies a novel regulatory crosstalk between Ral and Arf6 that controls Ral function in cells. In re-adherent mouse fibroblasts (MEFs) integrin dependent activation of RalA drives Arf6 activation. Independent of adhesion constitutively active RalA and RalB could both however activate Arf6. This is further conserved in oncogenic H-Ras containing bladder cancer T24 cells, which express anchorage independent active Ral that supports Arf6 activation. Arf6 mediates active Ral-exocyst dependent delivery of raft microdomains to the plasma membrane that supports anchorage independent growth signalling. Accordingly in T24 cells the RalB-Arf6 crosstalk is seen to preferentially regulate anchorage independent Erk signalling. Active Ral we further find uses a Ral-RalBP1-ARNO-Arf6 pathway to mediate Arf6 activation. This study hence identifies Arf6, through this regulatory crosstalk, to be a key downstream mediator of Ral isoform function along adhesion dependent pathways in normal and cancer cells.

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Ral mediates Arf6 activation downstream of integrins and oncogenic Ras.

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Arf6 mediates Ral-exocyst dependent delivery of raft microdomains.

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Active Ral supports anchorage independent Arf6 activation in bladder cancer T24 cells.

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Ral-Arf6 crosstalk in T24 cells regulates anchorage independent Erk signalling.

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A Ral-RalBP1-ARNO-Arf6 pathway mediates the Ral-Arf6 crosstalk.

Effects of RAL signal transduction in KRAS- and BRAF-mutated cells and prognostic potential of the RAL signature in colorectal cancer

Our understanding of oncogenic signaling pathways has strongly fostered current concepts for targeted therapies in metastatic colorectal cancer. The RALA pathway is novel candidate due to its independent role in controlling expression of genes downstream of RAS.

We compared RALA GTPase activities in three colorectal cancer cell lines by GTPase pull-down assay and analyzed the transcriptional and phenotypic effects of transient RALA silencing. Knocking-down RALA expression strongly diminished the active GTP-bound form of the protein. Proliferation of KRAS mutated cell lines was significantly reduced, while BRAF mutated cells were mostly unaffected. By microarray analysis we identified common genes showing altered expression upon RALA silencing in all cell lines. None of these genes were affected when the RAF/MAPK or PI3K pathways were blocked.

To investigate the potential clinical relevance of the RALA pathway and its associated transcriptome, we performed a meta-analysis interrogating progression-free survival of colorectal cancer patients of five independent data sets using Cox regression. In each dataset, the RALA-responsive signature correlated with worse outcome.

In summary, we uncovered the impact of the RAL signal transduction on genetic program and growth control in KRAS- and BRAF-mutated colorectal cells and demonstrated prognostic potential of the pathway-responsive gene signature in cancer patients.

Discovery and characterization of small molecules that target the GTPase Ral

The Ras-like GTPases RalA and RalB are important drivers of tumour growth and metastasis. Chemicals that block Ral function would be valuable as research tools and for cancer therapeutics. Here we used protein structure analysis and virtual screening to identify drug-like molecules that bind to a site on the GDP-bound form of Ral. The compounds RBC6, RBC8 and RBC10 inhibited the binding of Ral to its effector RALBP1, as well as inhibiting Ral-mediated cell spreading of murine embryonic fibroblasts and anchorage-independent growth of human cancer cell lines. The binding of the RBC8 derivative BQU57 to RalB was confirmed by isothermal titration calorimetry, surface plasmon resonance and (1)H-(15)N transverse relaxation-optimized spectroscopy (TROSY) NMR spectroscopy. RBC8 and BQU57 show selectivity for Ral relative to the GTPases Ras and RhoA and inhibit tumour xenograft growth to a similar extent to the depletion of Ral using RNA interference. Our results show the utility of structure-based discovery for the development of therapeutics for Ral-dependent cancers.

Ral GTPase down-regulation stabilizes and reactivates p53 to inhibit malignant transformation

Ral GTPases are critical effectors of Ras, yet the molecular mechanism by which they induce malignant transformation is not well understood. In this study, we found the expression of K-Ras, RalB, and sometimes RalA, but not AKT1/2 and c-Raf, to be required for maintaining low levels of p53 in human cancer cells that harbor mutant K-Ras and wild-type p53. Down-regulation of K-Ras, RalB, and sometimes RalA increases p53 protein levels and results in a p53-dependent up-regulation of the expression of p21(WAF). K-Ras, RalA, and RalB depletion increases p53 stability as demonstrated by ataxia telangiectasia-mutated kinase activation, increased Ser-15 phosphorylation, and a significant (up to 6-fold) increase in p53 half-life. Furthermore, depletion of K-Ras and RalB inhibits anchorage-independent growth and invasion and interferes with cell cycle progression in a p53-dependent manner. Depletion of RalA inhibits invasion in a p53-dependent manner. Thus, expression of K-Ras and RalB and possibly RalA proteins is critical for maintaining low levels of p53, and down-regulation of these GTPases reactivates p53 by significantly enhancing its stability, and this contributes to suppression of malignant transformation.

A role for Ral GTPase-activating protein subunit β in mitotic regulation

Ral proteins are small GTPases that play critical roles in normal physiology and in oncogenesis. There is little information on the GTPase-activating proteins (GAPs) that downregulate their activity. Here, we provide evidence that the noncatalytic β subunit of RalGAPα1/2 β complexes is involved in mitotic control. RalGAPβ localizes to the Golgi and nucleus during interphase, and relocalizes to the mitotic spindle and cytokinetic intercellular bridge during mitosis. Depletion of RalGAPβ causes chromosome misalignment and decreases the amount of mitotic cyclin B1, disturbing the metaphase-to-anaphase transition. Overexpression of RalGAPβ interferes with cell division, leading to binucleation and multinucleation, and cell death. We propose that RalGAPβ plays an essential role in the sequential progression of mitosis by controlling the spatial and temporal activation of Ral GTPases in the spindle assembly checkpoint (SAC) and cytokinesis. Deregulation of RalGAPβ might cause genomic instability, leading to human carcinogenesis.

Ral and Rheb GTPase activating proteins integrate mTOR and GTPase signaling in aging, autophagy, and tumor cell invasion

Diverse environmental cues converge on and are integrated by the mTOR signaling network to control cellular growth and homeostasis. The mammalian Tsc1-Tsc2 GTPase activating protein (GAP) heterodimer is a critical negative regulator of Rheb and mTOR activation. The RalGAPα-RalGAPβ heterodimer shares sequence and structural similarity with Tsc1-Tsc2. Unexpectedly, we observed that C. elegans expresses orthologs for the Rheb and RalA/B GTPases and for RalGAPα/β, but not Tsc1/2. This prompted our investigation to determine whether RalGAPs additionally modulate mTOR signaling. We determined that C. elegans RalGAP loss decreased lifespan, consistent with a Tsc-like function. Additionally, RalGAP suppression in mammalian cells caused RalB-selective activation and Sec5- and exocyst-dependent engagement of mTORC1 and suppression of autophagy. Unexpectedly, we also found that Tsc1-Tsc2 loss activated RalA/B independently of Rheb-mTOR signaling. Finally, RalGAP suppression caused mTORC1-dependent pancreatic tumor cell invasion. Our findings identify an unexpected crosstalk and integration of the Ral and mTOR signaling networks.