Mass spectroscopic phosphoprotein mapping of Ral binding protein 1 (RalBP1/Rip1/RLIP76)

Crucial Role of RLIP76 in Promoting Glycolysis and Tumorigenesis by Stabilization of HIF-1α in Glioma Cells Under Hypoxia

Hypoxia is intimately associated with enhanced glycolysis in gliomas, and hypoxia-inducible factor 1α (HIF-1α) plays a critical role in this process. RLIP76 (Ral-interacting protein 76) functions as a multifunctional mediator and is aberrantly expressed in various malignant tumors, including glioma. However, the underlying mechanism of RLIP76 and HIF-1α in glioma glycolysis remains largely unclear. In the present study, we demonstrated that RLIP76 is a hypoxia-inducible molecule that contributes to facilitating glycolysis in glioma cells under hypoxic conditions. In addition, hypoxia-induced RLIP76 is a novel target of HIF-1α and enhances the two important HIF-1α-target glycolytic proteins glucose transporter type 1 (GLUT1) and lactate dehydrogenase A (LDHA) in hypoxia. Mechanistically, RLIP76 can directly bind to HIF-1α in the nucleus and regulate the stability of HIF-1α by alleviating HIF-1α ubiquitination and therefore activates GLUT1 and LDHA to accelerate glycolysis in hypoxia. Furthermore, the enhanced glycolysis is necessary for the role of RLIP76 to promote glioma development in vivo, confirming the ability of RLIP76 to regulate tumor cell glycolysis. Collectively, our results demonstrate a previously unappreciated function of RLIP76 in hypoxia-mediated glycolytic metabolism and implicate that RLIP76 might be a valuable therapeutic target for gliomas.

β-Arrestin2 Is Critically Involved in the Differential Regulation of Phosphosignaling Pathways by Thyrotropin-Releasing Hormone and Taltirelin

In recent years, thyrotropin-releasing hormone (TRH) and its analogs, including taltirelin (TAL), have demonstrated a range of effects on the central nervous system that represent potential therapeutic agents for the treatment of various neurological disorders, including neurodegenerative diseases. However, the molecular mechanisms of their actions remain poorly understood. In this study, we investigated phosphosignaling dynamics in pituitary GH1 cells affected by TRH and TAL and the putative role of β-arrestin2 in mediating these effects. Our results revealed widespread alterations in many phosphosignaling pathways involving signal transduction via small GTPases, MAP kinases, Ser/Thr- and Tyr-protein kinases, Wnt/β-catenin, and members of the Hippo pathway. The differential TRH- or TAL-induced phosphorylation of numerous proteins suggests that these ligands exhibit some degree of biased agonism at the TRH receptor. The different phosphorylation patterns induced by TRH or TAL in β-arrestin2-deficient cells suggest that the β-arrestin2 scaffold is a key factor determining phosphorylation events after TRH receptor activation. Our results suggest that compounds that modulate kinase and phosphatase activity can be considered as additional adjuvants to enhance the potential therapeutic value of TRH or TAL.

RLIP76: A Structural and Functional Triumvirate

RLIP76/RalBP1 is an ATP-dependent transporter of glutathione conjugates, which is overexpressed in various human cancers, but its diverse functions in normal cells, which include endocytosis, stress response and mitochondrial dynamics, are still not fully understood. The protein can be divided into three distinct regions, each with its own structural properties. At the centre of the protein are two well-defined domains, a GTPase activating protein domain targeting Rho family small G proteins and a small coiled-coil that binds to the Ras family small GTPases RalA and RalB. In engaging with Rho and Ral proteins, RLIP76 bridges these two distinct G protein families. The N-terminal region is predicted to be disordered and is rich in basic amino acids, which may mediate membrane association, consistent with its role in transport. RLIP76 is an ATP-dependent transporter with ATP-binding sites within the N-terminus and the Ral binding domain. Furthermore, RLIP76 is subject to extensive phosphorylation, particularly in the N-terminal region. In contrast, the C-terminal region is thought to form an extensive coiled-coil that could mediate dimerization. Here, we review the structural features of RLIP76, including experimental data and computational predictions, and discuss the implications of its various post-translational modifications.

RLIP controls receptor-ligand signaling by regulating clathrin-dependent endocytosis

RLIP (Ral-interacting protein) is a multifunctional protein that couples ATP hydrolysis with the movement of substances. Its primary function appears to be in the plasma membrane, where it catalyzes the ATP-dependent efflux of glutathione-conjugates (GS-Es), as well as un-metabolized drugs and toxins. In the plasma membrane, its interaction with the clathrin adaptor protein AP2 localizes it to endocytic vesicle, where its GS-E-stimulated ATPase and transport activity are required for clathrin-dependent endocytosis (CDE). CDE is an essential mechanism for internalizing ligand-receptor complexes that signal proliferation (EGF, insulin, IGF1), apoptosis (TNFα, TRAIL, Fas-L), and differentiation and morphogenesis (TGFβ, WNT, Notch, SHH). Aberrant functioning of these pathways appears crucial for most cancer cells to evade apoptosis, invade surrounding tissues, and metastasize. Internalization of receptor-ligand complexes by CDE begins a sequence of events that can terminate, initiate, or modulate downstream signaling; the consequences of signaling through these downstream pathways may be inherently different in cancer and normal cells, a view supported by numerous basic and clinical observations. In this review, we will discuss the GS-E transport activity of RLIP, which determines the rate of ligand endocytosis, and how the inhibition and/or depletion of RLIP globally disrupts in ligand-receptor signaling.

RLIP76 Inhibition: A Promising Developmental Therapy for Neuroblastoma

Refractory and relapsed neuroblastoma (NB) present with significant challenges in clinical management. Though primary NBs largely with wild-type p53 respond well to interventions, dysfunctional signaling in the p53 pathways in a MYCN oncogene driven background is found in a number of children with NB. The p53-mutant NB is largely unresponsive to available therapies and p53-independent targeted therapeutics represents a vital need in pediatric oncology. We analyzed the findings on mercapturic acid pathway (MAP) transporter RLIP76, which has broad and critical effects on multiple pathways as essential for carcinogenesis, oxidative stress and drug-resistance, is over-expressed in NB. RLIP76 inhibition by antibodies or depletion by antisense causes apoptosis and sensitization to chemo-radiotherapy in many cancers. In addition, recent studies indicate that the interactions between p53, MYCN, and WNT regulate apoptosis resistance and protein ubiquitination. RLIP76 and p53 interact with each other and colocalize in NB cells. Targeted depletion/inhibition of RLIP76 causes apoptosis and tumor regression in NB irrespective of p53 status. In the present review, we discuss the mechanisms and the role of RLIP76 in oxidative stress, drug-resistance and clathrin-dependent endocytosis (CDE), and analyze the molecular basis for the role of RLIP76 targeted approaches in the context principal drivers of NB pathogenesis, progression and drug-resistance. The evidence from RLIP76 studies in other cancers, when taken in the context of our recent RLIP76 focused mechanistic studies in NB, provides strong basis for further characterization and development of RLIP76 targeted therapies for NB.

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.

RLIP76 regulates Arf6-dependent cell spreading and migration by linking ARNO with activated R-Ras at recycling endosomes

R-Ras small GTPase enhances cell spreading and motility via RalBP1/RLIP76, an R-Ras effector that links GTP-R-Ras to activation of Arf6 and Rac1 GTPases. Here, we report that RLIP76 performs these functions by binding cytohesin-2/ARNO, an Arf GTPase guanine exchange factor, and connecting it to R-Ras at recycling endosomes. RLIP76 formed a complex with R-Ras and ARNO by binding ARNO via its N-terminus (residues 1-180) and R-Ras via residues 180-192. This complex was present in Rab11-positive recycling endosomes and the presence of ARNO in recycling endosomes required RLIP76, and was not supported by RLIP76(Δ1-180) or RLIP76(Δ180-192). Spreading and migration required RLIP76(1-180), and RLIP76(Δ1-180) blocked ARNO recruitment to recycling endosomes, and spreading. Arf6 activation with an ArfGAP inhibitor overcame the spreading defects in RLIP76-depleted cells or cells expressing RLIP76(Δ1-180). Similarly, RLIP76(Δ1-180) or RLIP76(Δ180-192) suppressed Arf6 activation. Together these results demonstrate that RLIP76 acts as a scaffold at recycling endosomes by binding activated R-Ras, recruiting ARNO to activate Arf6, thereby contributing to cell spreading and migration.

RLIP76 Targeted Therapy for Kidney Cancer

Despite recent improvements in chemotherapeutic approaches to treating kidney cancer, this malignancy remains deadly if not found and removed at an early stage of the disease. Kidney cancer is highly drug-resistant, which may at least partially result from high expression of transporter proteins in the cell membranes of kidney cells. Although these transporter proteins can contribute to drug-resistance, targeting proteins from the ATP-binding cassette transporter family has not been effective in reversing drug-resistance in kidney cancer. Recent studies have identified RLIP76 as a key stress-defense protein that protects normal cells from damage caused by stress conditions, including heat, ultra-violet light, X-irradiation, and oxidant/electrophilic toxic chemicals, and is crucial for protecting cancer cells from apoptosis. RLIP76 is the predominant glutathione-electrophile-conjugate (GS-E) transporter in cells, and inhibiting it with antibodies or through siRNA or antisense causes apoptosis in many cancer cell types. To date, blocking of RLIP76, either alone or in combination with chemotherapeutic drugs, as a therapeutic strategy for kidney cancer has not yet been evaluated in human clinical trials, although there is considerable potential for RLIP76 to be developed as a therapeutic agent for kidney cancer. In the present review, we discuss the mechanisms underlying apoptosis caused by RLIP76 depletion, the role of RLIP76 in clathrin-dependent endocytosis deficiency, and the feasibility of RLIP76-targeted therapy for kidney cancer.

The RLIP76 N-terminus binds ARNO to regulate PI 3-kinase, Arf6 and Rac signaling, cell spreading and migration

RLIP76 is a multifunctional protein involved in tumor growth and angiogenesis, and a promising therapeutic target in many cancers. RLIP76 harbors docking sites for many proteins, and we have found that it interacts with ARNO, a guanine nucleotide exchange factor for Arf6, and that RLIP76 regulates activation of Rac1 via Arf6, and regulates cell spreading and migration in an ARNO and Arf6-dependent manner. Here we show that ARNO interacts with the RLIP76 N-terminal domain, and this domain was required for RLIP76-dependent cell spreading and migration. We identified two sites in the RLIP76 N-terminus with differential effects on ARNO binding and downstream signaling: Ser29/Ser30 and Ser62. Ser29/30 mutation to Alanine inhibited ARNO interaction and was sufficient to block RLIP76-dependent cell spreading and migration, as well as RLIP76-dependent Arf6 activation. In contrast, RLIP76(S62A) interacted with ARNO and supported Arf6 activation. However, both sets of mutations blocked Rac1 activation. RLIP76-mediated Rac and Arf6 activation required PI3K activity. S29/30A mutations inhibited RLIP76-dependent PI3K activation, but S62A mutation did not. Together these results show that ARNO interaction with the RLIP76 N-terminus regulates cell spreading and motility via PI3K and Arf6, independent of RLIP76 control of Rac.

Emerging treatments in lung cancer - targeting the RLIP76 molecular transporter

Multidrug resistance in lung cancer cells is a significant obstacle in the treatment of lung cancer. Resistance to chemotherapeutic agents is often the result of efflux of the drugs from cancer cells, mediated by adenosine triphosphate (ATP)-dependent drug transport across the plasma membrane. Thus, identifying molecular targets in the cancer cell transport machinery could be a key factor in successful combinatorial therapy, along with chemotherapeutic drugs. The transport protein Ral-interacting protein of 76 kDa (RLIP76), also known as Ral-binding protein 1 (RalBP1), is a highly promising target for lung cancer treatment. RLIP76 is an ATP-dependent non-ATP-binding cassette (ABC) transporter, responsible for the major transport function in many cells, including many cancer cell lines, causing efflux of glutathione-electrophile conjugates of both endogenous metabolites and environmental toxins. RLIP76 is expressed in most human tissues, and is overexpressed in non-small-cell lung cancer cell lines and in many tumor types. The blockade of RLIP76 by various approaches has been shown to increase the sensitivity to radiation and chemotherapeutic drugs, and leads to apoptosis in cells. In xenograft tumor models in mice, RLIP76 blockade or depletion results in complete and sustained regression across many cancer cell types, including lung cancer cells. In addition to its transport function, RLIP76 has many other cellular and physiological functions based on its domain structure, which includes a unique Ral-binding domain and a Rho GTPase activating protein (RhoGAP)-catalytic domain as well as docking sites for multiple signaling proteins. As a Ral effector, RhoGAP, and adapter protein, RLIP76 has been shown to play important roles in endocytosis, mitochondrial fission, cell spreading and migration, actin dynamics during gastrulation, and Ras-induced tumorigenesis. Additionally, RLIP76 is also important for stromal cell function in tumors, as it was recently shown to be required for efficient endothelial cell function and angiogenesis in solid tumors. However, RLIP76 knockout mice are viable, and blockade effects appear to be selective for implanted tumors in mice, suggesting the possibility that RLIP76-targeting drugs may be successful in clinical trials. In this review, we outline the many cellular and physiological functions of RLIP76 in normal and cancer cells, and discuss the potential for RLIP76-based therapeutics in lung cancer treatment.

RLIP76: a versatile transporter and an emerging target for cancer therapy

In the last few years, extensive research has been made to elucidate the functional significance of RLIP76. The resulting novel breakthroughs have helped us understand its transport and signaling functions. RLIP76 is a ubiquitously expressed, key stress-defensive, anti-apoptotic, multi-functional protein that transports glutathione-conjugates of electrophilic compounds, thus controlling the intracellular concentration of pro-apoptotic oxidized lipid byproducts and other xenobiotics such as chemotherapeutic agents. These properties place RLIP76 at a very important position in the hierarchy of the stress defense mechanism adopted by the cell. Selective over-expression of RLIP76 in malignant cells of diverse origin is one of the possible mechanisms by which these cells overcome chemotherapy and radiation induced oxidative damage. RLIP76 has also been shown to be an effective transporter of many conventional chemotherapeutic drugs. Such transport, if inhibited, can lead to increased cellular accumulation of drugs which in turn translates to enhanced drug sensitivity. Recent studies have shown that inhibition and/or depletion of RLIP76 by antibodies, siRNA, or antisense can lead to drastic and sustained regression of lung, kidney, melanoma, colon, and prostate cancer xenografts with no observed recurrence of tumors. All these findings converge on the fact that such inhibition/depletion of RLIP76 can be used clinically to terminate cancer growth and progression. In the present review, we will discuss the role of RLIP76 as a multi-drug transporter, its involvement in cancer, and the prospects of using RLIP76 inhibition as an emerging treatment for cancer.

Src phosphorylation of RhoGDI2 regulates its metastasis suppressor function

RhoGDI2 is a suppressor of metastasis in human bladder cancer. Although diminished RhoGDI2 expression in tumors is associated with decreased patient survival, normal expression in some metastatic tumors led us to wonder whether other mechanisms regulate RhoGDI2 function. Protein interaction analysis identified Src as a novel RhoGDI2 interaction partner. Gene expression profiling and immunohistochemistry of human tumors revealed that Src levels diminish as a function of bladder cancer stage. In addition, diminished Src levels and RhoGDI2 levels appear mutually exclusive in individual tumors, indicating that both genes are likely involved in the same signaling pathway leading to metastasis suppression. Studies confirmed that activated Src kinase binds and phosphorylates RhoGDI2 in vitro and vivo. Mutagenesis revealed that Tyr-153 and, to a lesser degree, Tyr-24 were the primary Src phosphorylation sites. Phosphorylation decreased the amount of Rac1 in RhoGDI2 complexes and increased RhoGDI2 association with cell membranes. Stable expression of phosphomimetic Tyr-153 RhoGDI2 in metastatic human bladder cancer cell lines had no effect on primary tumor growth but suppressed metastasis more potently than WT RhoGDI2. These data suggest that phosphorylation by Src enhances RhoGDI2 metastasis suppression and that loss of Src relieves metastasis suppression in tumor cells that maintain RhoGDI2 expression. Our findings also suggest caution in using Src inhibitors in the hope of delaying progression in patients with bladder cancer.

RLIP76: A novel glutathione-conjugate and multi-drug transporter

RLIP76, a stress-responsive, multi-functional protein with multi-specific transport activity towards glutathione-conjugates (GS-E) and chemotherapeutic agents, is frequently over-expressed in malignant cells. Our recent studies suggest that it plays a prominent anti-apoptotic role selectively in cancer cells. We have previously shown that RLIP76 accounts for up to 80% of the transport of GS-E and blocking the RLIP76-mediated transport of GS-E in cells results in the accumulation of pro-apoptotic endogenous electrophiles and on-set of apoptosis. Here we demonstrate that when RLIP76 mediate transport of GS-E is abrogated either by anti-RLIP76 IgG or accumulation of 4-hydroxynonenal (4-HNE) and its GSH-conjugate (GS-HNE) occurs and a massive apoptosis is observed in cells, indicate that the inhibition of RLIP76 transport activity at the cell surface is sufficient for observed anti-tumor activity. RLIP76 is linked with certain cellular functions including membrane plasticity and movement (as a primary 'effector' in the Ral pathway, perhaps functioning as a GTPase activating protein, or GAP), and as a component of clathrin-coated pit-mediated receptor-ligand endocytosis-a process that mediates movement of membrane vesicles.