Involvement of Ral GTPase in v-Src-induced phospholipase D activation

FOXD1-dependent RalA-ANXA2-Src complex promotes CTC formation in breast cancer

BACKGROUND

Early metastasis is a key factor contributing to poor breast cancer (BC) prognosis. Circulating tumor cells (CTCs) are regarded as the precursor cells of metastasis, which are ultimately responsible for the main cause of death in BC. However, to date molecular mechanisms underlying CTC formation in BC have been insufficiently defined.

METHODS

RNA-seq was carried out in primary tissues from early-stage BC patients (with CTCs≥5 and CTCs = 0, respectively) and the validation study was conducted in untreated 80 BC patients. Multiple in vitro and in vivo models were used in functional studies. Luciferase reporter, ChIP-seq, CUT&Tag-seq, and GST-pulldown, etc. were utilized in mechanistic studies. CTCs were counted by the CanPatrol™ CTC classification system or LiquidBiospy™ microfluidic chips. ERK1/2 inhibitor SCH772984 was applied to in vivo treatment.

RESULTS

Highly expressed FOXD1 of primary BC tissues was observed to be significantly associated with increased CTCs in BC patients, particularly in early BC patients. Overexpressing FOXD1 enhanced the migration capability of BC cells, CTC formation and BC metastasis, via facilitating epithelial-mesenchymal transition of tumor cells. Mechanistically, FOXD1 was discovered to induce RalA expression by directly bound to RalA promotor. Then, RalA formed a complex with ANXA2 and Src, promoting the interaction between ANXA2 and Src, thus increasing the phosphorylation (Tyr23) of ANXA2. Inhibiting RalA-GTP form attenuated the interaction between ANXA2 and Src. This cascade culminated in the activation of ERK1/2 signal that enhanced metastatic ability of BC cells. In addition, in vivo treatment with SCH772984, a specific inhibitor of ERK1/2, was used to dramatically inhibit the CTC formation and BC metastasis.

CONCLUSION

Here, we report a FOXD1-dependent RalA-ANXA2-Src complex that promotes CTC formation via activating ERK1/2 signal in BC. FOXD1 may serve as a prognostic factor in evaluation of BC metastasis risks. This signaling cascade is druggable and effective for overcoming CTC formation from the early stages of BC.

The RAL Enigma: Distinct Roles of RALA and RALB in Cancer

RALA and RALB are highly homologous small G proteins belonging to the RAS superfamily. Like other small GTPases, the RALs are molecular switches that can be toggled between inactive GDP-bound and active GTP-bound states to regulate diverse and critical cellular functions such as vesicle trafficking, filopodia formation, mitochondrial fission, and cytokinesis. The RAL paralogs are activated and inactivated by a shared set of guanine nucleotide exchange factors (GEFs) and GTPase-activating proteins (GAPs) and utilize similar sets of downstream effectors. In addition to their important roles in normal cell biology, the RALs are known to be critical mediators of cancer cell survival, invasion, migration, and metastasis. However, despite their substantial similarities, the RALs often display striking functional disparities in cancer. RALA and RALB can have redundant, unique, or even antagonistic functions depending on cancer type. The molecular basis for these discrepancies remains an important unanswered question in the field of cancer biology. In this review we examine the functions of the RAL paralogs in normal cellular physiology and cancer biology with special consideration provided to situations where the roles of RALA and RALB are non-redundant.

A signalling cascade for Ral

Ras is the most mutated oncoprotein in cancer. Among the three oncogenic effectors of Ras - Raf, PI3 Kinase and RalGEF>Ral - signalling through RalGEF>Ral (Ras-like) is by far the least well understood. A variety of signals and binding partners have been defined for Ral, yet we know little of how Ral functions in vivo. This review focuses on previous research in Drosophila that defined a function for Ral in apoptosis and established indirect relationships among Ral, the CNH-domain MAP4 Kinase misshapen, and the JNK MAP kinase basket. Most of the described signalling components are not essential in C. elegans, facilitating subsequent analysis using developmental patterning of the C. elegans vulval precursor cells (VPCs). The functions of two paralogous CNH-domain MAP4 Kinases were defined relative to Ras>Raf, Notch and Ras>RalGEF>Ral signalling in VPCs. MIG-15, the nematode ortholog of misshapen, antagonizes both the Ral-dependent and Ras>Raf-dependent developmental outcomes. In contrast, paralogous GCK-2, the C. elegans ortholog of Drosophila happyhour, propagates the 2°-promoting signal of Ral. Manipulations via CRISPR of Ral signalling through GCK-2 coupled with genetic epistasis delineated a Ras>RalGEF>Ral>Exo84>GCK-2>MAP3K^MLK-1> p38^PMK-1 cascade. Thus, genetic analysis using invertebrate experimental organisms defined a cascade from Ras to p38 MAP kinase.

K15 promoter-driven enforced expression of NKIRAS exhibits tumor suppressive activity against the development of DMBA/TPA-induced skin tumors

NKIRAS1 and NKIRAS2 (also called as κB-Ras) were identified as members of the atypical RAS family that suppress the transcription factor NF-κB. However, their function in carcinogenesis is still controversial. To clarify how NKIRAS acts on cellular transformation, we generated transgenic mice in which NKIRAS2 was forcibly expressed using a cytokeratin 15 (K15) promoter, which is mainly activated in follicle bulge cells. The ectopic expression of NKIRAS2 was mainly detected in follicle bulges of transgenic mice with NKIRAS2 but not in wild type mice. K15 promoter-driven expression of NKIRAS2 failed to affect the development of epidermis, which was evaluated using the expression of K10, K14, K15 and filaggrin. However, K15 promoter-driven expression of NKIRAS2 effectively suppressed the development of skin tumors induced by treatment with 7,12-dimethylbenz(a)anthracene (DMBA)/12-O-tetradecanoylphorbol 13-acetate (TPA). This observation suggested that NKIRAS seemed to function as a tumor suppressor in follicle bulges. However, in the case of oncogenic HRAS-driven cellular transformation of murine fibroblasts, knockdown of NKIRAS2 expression drastically suppressed HRAS-mutant-provoked cellular transformation, suggesting that NKIRAS2 was required for the cellular transformation of murine fibroblasts. Furthermore, moderate enforced expression of NKIRAS2 augmented oncogenic HRAS-provoked cellular transformation, whereas an excess NKIRAS2 expression converted its functional role into a tumor suppressive phenotype, suggesting that NKIRAS seemed to exhibit a biphasic bell-shaped enhancing effect on HRAS-mutant-provoked oncogenic activity. Taken together, the functional role of NKIRAS in carcinogenesis is most likely determined by not only cellular context but also its expression level.

RalA and PLD1 promote lipid droplet growth in response to nutrient withdrawal

Lipid droplets (LDs) are dynamic organelles that undergo dynamic changes in response to changing cellular conditions. During nutrient depletion, LD numbers increase to protect cells against toxic fatty acids generated through autophagy and provide fuel for beta-oxidation. However, the precise mechanisms through which these changes are regulated have remained unclear. Here, we show that the small GTPase RalA acts downstream of autophagy to directly facilitate LD growth during nutrient depletion. Mechanistically, RalA performs this function through phospholipase D1 (PLD1), an enzyme that converts phosphatidylcholine (PC) to phosphatidic acid (PA) and that is recruited to lysosomes during nutrient stress in a RalA-dependent fashion. RalA inhibition prevents recruitment of the LD-associated protein perilipin 3, which is required for LD growth. Our data support a model in which RalA recruits PLD1 to lysosomes during nutrient deprivation to promote the localized production of PA and the recruitment of perilipin 3 to expanding LDs.

Structure and regulation of human phospholipase D

Mammalian phospholipase D (PLD) generates phosphatidic acid, a dynamic lipid secondary messenger involved with a broad spectrum of cellular functions including but not limited to metabolism, migration, and exocytosis. As a promising pharmaceutical target, the biochemical properties of PLD have been well characterized. This has led to the recent crystal structures of human PLD1 and PLD2, the development of PLD specific pharmacological inhibitors, and the identification of cellular regulators of PLD. In this review, we discuss the PLD1 and PLD2 structures, PLD inhibition by small molecules, and the regulation of PLD activity by effector proteins and lipids.

Ral GTPases promote breast cancer metastasis by controlling biogenesis and organ targeting of exosomes

Cancer extracellular vesicles (EVs) shuttle at distance and fertilize pre-metastatic niches facilitating subsequent seeding by tumor cells. However, the link between EV secretion mechanisms and their capacity to form pre-metastatic niches remains obscure. Using mouse models, we show that GTPases of the Ral family control, through the phospholipase D1, multi-vesicular bodies homeostasis and tune the biogenesis and secretion of pro-metastatic EVs. Importantly, EVs from RalA or RalB depleted cells have limited organotropic capacities in vivoand are less efficient in promoting metastasis. RalA and RalB reduce the EV levels of the adhesion molecule MCAM/CD146, which favors EV-mediated metastasis by allowing EVs targeting to the lungs. Finally, RalA, RalB, and MCAM/CD146, are factors of poor prognosis in breast cancer patients. Altogether, our study identifies RalGTPases as central molecules linking the mechanisms of EVs secretion and cargo loading to their capacity to disseminate and induce pre-metastatic niches in a CD146-dependent manner.

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.

NMR resonance assignments for the active and inactive conformations of the small G protein RalA

The Ral proteins (RalA and RalB) are small G proteins of the Ras family that have been implicated in exocytosis, endocytosis, transcriptional regulation and mitochondrial fission, as well as having a role in tumourigenesis. RalA and RalB are activated downstream of the master regulator, Ras, which causes the nucleotide exchange of GDP for GTP. Here we report the ¹H, ¹⁵ N and ¹³C resonance assignments of RalA in its active form bound to the GTP analogue GMPPNP. We also report the backbone assignments of RalA in its inactive, GDP-bound form. The assignments give insight into the switch regions, which change conformation upon nucleotide exchange. These switch regions are invisible in the spectra of the active, GMPPNP bound form but the residues proximal to the switches can be monitored. RalA is also an important drug target due to its over activation in some cancers and these assignments will be extremely useful for NMR-based screening approaches.

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.

Ral GTPases in Schwann cells promote radial axonal sorting in the peripheral nervous system

Small GTPases of the Rho and Ras families are important regulators of Schwann cell biology. The Ras-like GTPases RalA and RalB act downstream of Ras in malignant peripheral nerve sheath tumors. However, the physiological role of Ral proteins in Schwann cell development is unknown. Using transgenic mice with ablation of one or both Ral genes, we report that Ral GTPases are crucial for axonal radial sorting. While lack of only one Ral GTPase was dispensable for early peripheral nerve development, ablation of both RalA and RalB resulted in persistent radial sorting defects, associated with hallmarks of deficits in Schwann cell process formation and maintenance. In agreement, ex vivo-cultured Ral-deficient Schwann cells were impaired in process extension and the formation of lamellipodia. Our data indicate further that RalA contributes to Schwann cell process extensions through the exocyst complex, a known effector of Ral GTPases, consistent with an exocyst-mediated function of Ral GTPases in Schwann cells.

Glutamine as an Essential Amino Acid for KRas-Driven Cancer Cells

Cancer cells consume glutamine, a nonessential amino acid (NEAA), at exceedingly high rates to fulfill their energetic and biosynthetic requirements for proliferation. Glutamine plays distinct roles from essential amino acids in cell cycle progression and in the activation of mammalian target of rapamycin (mTOR). Furthermore, the need of cancer cells for glutamine can be exploited therapeutically - especially those driven by KRas. In this review we explore several distinct cellular roles for glutamine that contribute to glutamine addiction in KRas-driven cancer cells and discuss opportunities for therapeutic intervention created by glutamine addiction.

Multifactorial Regulation of G Protein-Coupled Receptor Endocytosis

Endocytosis is a process by which cells absorb extracellular materials via the inward budding of vesicles formed from the plasma membrane. Receptor-mediated endocytosis is a highly selective process where receptors with specific binding sites for extracellular molecules internalize via vesicles. G protein-coupled receptors (GPCRs) are the largest single family of plasma-membrane receptors with more than 1000 family members. But the molecular mechanisms involved in the regulation of GPCRs are believed to be highly conserved. For example, receptor phosphorylation in collaboration with β-arrestins plays major roles in desensitization and endocytosis of most GPCRs. Nevertheless, a number of subsequent studies showed that GPCR regulation, such as that by endocytosis, occurs through various pathways with a multitude of cellular components and processes. This review focused on i) functional interactions between homologous and heterologous pathways, ii) methodologies applied for determining receptor endocytosis, iii) experimental tools to determine specific endocytic routes, iv) roles of small guanosine triphosphate-binding proteins in GPCR endocytosis, and v) role of post-translational modification of the receptors in endocytosis.

Identification and expression of Ima, a novel Ral-interacting Drosophila protein

We report the identification of Ima, a novel Drosophila MAGUK-like protein, which contains two WW and four PDZ protein interaction domains and interacts with the small GTPase dRal in the yeast two-hybrid system and pull-down assays. The gene is expressed in distinct spatiotemporal patterns throughout embryonic development. Overexpression of Ima interferes with normal Drosophila development, indicating that the gene functions in a tissue specific manner.

RAL-1 controls multivesicular body biogenesis and exosome secretion

Exosomes are secreted vesicles arising from the fusion of multivesicular bodies (MVBs) with the plasma membrane. Despite their importance in various processes, the molecular mechanisms controlling their formation and release remain unclear. Using nematodes and mammary tumor cells, we show that Ral GTPases are involved in exosome biogenesis. In Caenorhabditis elegans, RAL-1 localizes at the surface of secretory MVBs. A quantitative electron microscopy analysis of RAL-1-deficient animals revealed that RAL-1 is involved in both MVB formation and their fusion with the plasma membrane. These functions do not involve the exocyst complex, a common Ral guanosine triphosphatase (GTPase) effector. Furthermore, we show that the target membrane SNARE protein SYX-5 colocalizes with a constitutively active form of RAL-1 at the plasma membrane, and MVBs accumulate under the plasma membrane when SYX-5 is absent. In mammals, RalA and RalB are both required for the secretion of exosome-like vesicles in cultured cells. Therefore, Ral GTPases represent new regulators of MVB formation and exosome release.

Role for RalA downstream of Rac1 in skeletal muscle insulin signalling

The small GTPase RalA is required for Rac1-mediated glucose uptake and activated by Rac1 in mouse skeletal muscle fibres. This might be the first demonstration of the involvement of RalA in Rac1-mediated insulin signalling in mature skeletal muscle.

The RAS-RAL axis in cancer: evidence for mutation-specific selectivity in non-small cell lung cancer

Activating RAS mutations are common in human tumors. These mutations are often markers for resistance to therapy and subsequent poor prognosis. So far, targeting the RAF-MEK-ERK and PI3K-AKT signaling pathways downstream of RAS is the only promising approach in the treatment of cancer patients harboring RAS mutations. RAL GTPase, another downstream effector of RAS, is also considered as a therapeutic option for the treatment of RAS-mutant cancers. The RAL GTPase family comprises RALA and RALB, which can have either divergent or similar functions in different tumor models. Recent studies on non-small cell lung cancer (NSCLC) have showed that different RAS mutations selectively activate specific effector pathways. This observation requires broader validation in other tumor tissue types, but if true, will provide a new approach to the treatment of RAS-mutant cancer patients by targeting specific downstream RAS effectors according to the type of RAS mutation. It also suggests that RAL GTPase inhibition will be an important treatment strategy for tumors harboring RAS glycine to cysteine (G12C) or glycien to valine (G12V) mutations, which are commonly found in NSCLC and pancreatic cancer.

Thermodynamic mapping of effector protein interfaces with RalA and RalB

RalA and RalB are members of the Ras family of small G proteins and are activated downstream of Ras via RalGEFs. The RalGEF-Ral axis represents one of the major effector pathways controlled by Ras and as such is an important pharmacological target. RalA and RalB are approximately 80% identical at the amino acid level; despite this, they have distinct roles both in normal cells and in the disease state. We have used our structure of RalB-RLIP76 to guide an analysis of Ral-effector interaction interfaces, creating panels of mutant proteins to probe the energetics of these interactions. The data provide a physical mechanism that underpins the effector selective mutations commonly employed to dissect Ral G protein function. Comparing the energetic landscape of the RalB-RLIP76 and RalB-Sec5 complexes reveals mutations in RalB that lead to differential binding of the two effector proteins. A panel of RLIP76 mutants was used to probe the interaction between RLIP76 and RalA and -B. Despite 100% sequence identity in the RalA and -B contact residues with RLIP76, differences still exist in the energetic profiles of the two complexes. Therefore, we have revealed properties that may account for some of the functional separation observed with RalA and RalB at the cellular level. Our mutations, in both the Ral isoforms and RLIP76, provide new tools that can be employed to parse the complex biology of Ral G protein signaling networks. The combination of these thermodynamic and structural data can also guide efforts to ablate RalA and -B activity with small molecules and peptides.

Phospholipase D signaling pathways and phosphatidic acid as therapeutic targets in cancer

Phospholipase D is a ubiquitous class of enzymes that generates phosphatidic acid as an intracellular signaling species. The phospholipase D superfamily plays a central role in a variety of functions in prokaryotes, viruses, yeast, fungi, plants, and eukaryotic species. In mammalian cells, the pathways modulating catalytic activity involve a variety of cellular signaling components, including G protein-coupled receptors, receptor tyrosine kinases, polyphosphatidylinositol lipids, Ras/Rho/ADP-ribosylation factor GTPases, and conventional isoforms of protein kinase C, among others. Recent findings have shown that phosphatidic acid generated by phospholipase D plays roles in numerous essential cellular functions, such as vesicular trafficking, exocytosis, autophagy, regulation of cellular metabolism, and tumorigenesis. Many of these cellular events are modulated by the actions of phosphatidic acid, and identification of two targets (mammalian target of rapamycin and Akt kinase) has especially highlighted a role for phospholipase D in the regulation of cellular metabolism. Phospholipase D is a regulator of intercellular signaling and metabolic pathways, particularly in cells that are under stress conditions. This review provides a comprehensive overview of the regulation of phospholipase D activity and its modulation of cellular signaling pathways and functions.

Ral GTPases in tumorigenesis: emerging from the shadows

Oncogenic Ras proteins rely on a series of key effector pathways to drive the physiological changes that lead to tumorigenic growth. Of these effector pathways, the RalGEF pathway, which activates the two Ras-related GTPases RalA and RalB, remains the most poorly understood. This review will focus on key developments in our understanding of Ral biology, and will speculate on how aberrant activation of the multiple diverse Ral effector proteins might collectively contribute to oncogenic transformation and other aspects of tumor progression.

Phospholipase D (PLD) drives cell invasion, tumor growth and metastasis in a human breast cancer xenograph model

Breast cancer is one of the most common malignancies in human females in the world. One protein that has elevated enzymatic lipase activity in breast cancers in vitro is phospholipase D (PLD), which is also involved in cell migration. We demonstrate that the PLD2 isoform, which was analyzed directly in the tumors, is crucial for cell invasion that contributes critically to the growth and development of breast tumors and lung metastases in vivo. We used three complementary strategies in a SCID mouse model and also addressed the underlying molecular mechanism. First, the PLD2 gene was silenced in highly metastatic, aggressive breast cancer cells (MDA-MB-231) with lentivirus-based shRNA, which were xenotransplanted in SCID mice. The resulting mouse primary mammary tumors were reduced in size (65%, p<0.05) and their onset delayed when compared to control tumors. Second, we stably overexpressed PLD2 in low-invasive breast cancer cells (MCF-7) with a biscistronic MIEG retroviral vector and observed that these cells were converted into a highly aggressive phenotype, as primary tumors that formed following xenotransplantation were larger, grew faster and developed lung metastases more readily. Third, we implanted osmotic pumps into SCID xenotransplanted mice that delivered two different small-molecule inhibitors of PLD activity (FIPI and NOPT). These inhibitors led to significant (>70%, p<0.05) inhibition of primary tumor growth, metastatic axillary tumors and lung metastases. In order to define the underlying mechanism, we determined that the machinery of PLD-induced cell invasion is mediated by phosphatidic acid (PA), WASp, Grb2 and Rac2 signaling events that ultimately affect actin polymerization and cell invasion. In summary, this study shows that PLD has a central role in the development, metastasis and level of aggressiveness of breast cancer, raising the possibility that PLD2 could be used as a new therapeutic target.

Phosphatidic acid and lipid-sensing by mTOR

Mammalian target of rapamycin (mTOR) has been implicated as a sensor of nutrient sufficiency for dividing cells and is activated by essential amino acids and glucose. However, cells also require lipids for membrane biosynthesis. A central metabolite in the synthesis of membrane phospholipids is phosphatidic acid (PA), which is required for the stability and activity of mTOR complexes. Although PA is commonly generated by the phospholipase D-catalyzed hydrolysis of phosphatidylcholine, PA is also generated by diacylglycerol kinases and lysophosphatidic acid acyltransferases, which are at the center of phospholipid biosynthesis. It is proposed that the responsiveness of mTOR/TOR to PA evolved as a means for sensing lipid precursors for membrane biosynthesis prior to doubling the mass of a cell and dividing.

Inhibition of formyl peptide-stimulated phospholipase D activation by Fal-002-2 via blockade of the Arf6, RhoA and protein kinase C signaling pathways in rat neutrophils

Three recently developed selective phospholipase D (PLD) inhibitors N-(2-(4-(2-oxo-2,3-dihydro-1H-benzo[d]imidazol-1-yl)piperidin-1-yl)ethyl)-2-naphthamide (VU0155056), (S)-N-(1-(4-(5-chloro-2-oxo-2,3-dihydro-1H-benzo[d]imidazol-1-yl)piperidin-1-yl)propan-2-yl)-2-naphthamide (VU0155069), and N-(2-(4-oxo-1-phenyl-1,3,8-triazaspiro[4,5]decan-8-yl)ethyl)quinoline-3-carboxamide (VU0285655-1) inhibited O2 (•-) generation in formyl-Met-Leu-Phe (fMLP)-stimulated rat neutrophils. A novel 2-phenyl-4-quinolone compound 6-chloro-2-(2-chlorophenyl)-4-oxo-1,4-dihydroquinoline-3-carboxylate (Fal-002-2), which inhibited O2 (•-) generation, also reduced the fMLP- but not phorbol ester-stimulated PLD activity (IC50 16.0 ± 5.0 μM). Fal-002-2 attenuated the interaction of PLD1 with ADP-ribosylation factor (Arf) 6, Ras homology (Rho) A and protein kinase C (PKC) isoforms (α, βI, and βII), and also inhibited the membrane recruitment of Arf6 and RhoA in fMLP-stimulated neutrophils, but not in GTPγS-stimulated cell-free system. The cellular levels of GTP-bound Arf6 and GTP-bound RhoA were reduced by Fal-002-2. Fal-002-2 also attenuated the membrane recruitment of Rho-associated protein kinase 1, phosphorylation of myosin light chain 2 at Thr18/Ser19 and PLD1 at Thr147, and the interaction of Arf6 with both arfaptin 1 and phosphatidylinositol 4-phosphate 5-kinase 1A. The association between RhoA and Vav, the interaction of Vav with both Lyn and Lck, the membrane recruitment of Vav, and the phosphorylation of Vav at Tyr174, but not Src family at Tyr416, were all attenuated by Fal-002-2 in fMLP-stimulated neutrophils. These results indicate that Fal-002-2 is not a direct PLD inhibitor, but the inhibition of fMLP-stimulated PLD activity by Fal-002-2, which partly accounts for its suppression of O2 (•-) generation, is attributable to the blockade of both Arf6 and RhoA activation and attenuation of the interaction of Arf6, RhoA and PKC isoforms with PLD1 in rat neutrophils.

OX1 orexin/hypocretin receptor activation of phospholipase D

BACKGROUND AND PURPOSE

Orexin receptors potently signal to lipid messenger systems, and our previous studies have suggested that PLD would be one of these. We thus wanted to verify this by direct measurements and clarify the molecular mechanism of the coupling.

EXPERIMENTAL APPROACH

Orexin receptor-mediated PLD activation was investigated in CHO cells stably expressing human OX(1) orexin receptors using [(14) C]-oleic acid-prelabelling and the transphosphatidylation assay.

KEY RESULTS

Orexin stimulation strongly increased PLD activity - even more so than the phorbol ester TPA (12-O-tetradecanoyl-phorbol-13-acetate), a highly potent activator of PLD. Both orexin and TPA responses were mediated by PLD1. Orexin-A and -B showed approximately 10-fold difference in potency, and the concentration-response curves were biphasic. Using pharmacological inhibitors and activators, both orexin and TPA were shown to signal to PLD1 via the novel PKC isoform, PKCδ. In contrast, pharmacological or molecular biological inhibitors of Rho family proteins RhoA/B/C, cdc42 and Rac did not inhibit the orexin (or the TPA) response, nor did the molecular biological inhibitors of PKD. In addition, neither cAMP elevation, Gα(i/o) nor Gβγ seemed to play an important role in the orexin response.

CONCLUSIONS AND IMPLICATIONS

Stimulation of OX(1) receptors potently activates PLD (probably PLD1) in CHO cells and this is mediated by PKCδ but not other PKC isoforms, PKDs or Rho family G-proteins. At present, the physiological significance of orexin-induced PLD activation is unknown, but this is not the first time we have identified PKCδ in orexin signalling, and thus some specific signalling cascade may exist between orexin receptors and PKCδ.

Paxillin phosphorylation and complexing with Erk and FAK are regulated by PLD activity in MDA-MB-231 cells

MDA-MB-231 cells are highly aggressive human breast adenocarcinoma cells that depend on PLD activity for survival. In response to the stress of serum withdrawal, there is increased motility and invasiveness of these cells that is associated with a rapid increase in PLD activity. In addition, PLD activity is elevated in response to most mitogenic signals. Similar to PLD, paxillin, a focal adhesion adaptor protein, and Erk, mitogen-activated protein kinase, play vital roles in cell motility through regulation of focal adhesion dynamics. Here, we addressed whether there is a functional correlation between paxillin and PLD that may influence cancer cell motility. We investigated the role of PLD activity on paxillin regulation, Erk activation and formation of a paxillin-Erk and paxillin-FAK association. Inhibition of PLD activity led to an increase in paxillin tyrosine phosphorylation, a decrease in Erk activation, as measured by phosphorylation, and enhanced association of paxillin with Erk. In addition, we found that paxillin tyrosine phosphorylation depends upon Erk activity and may be a consequence of an increased association with FAK. Taken together, these results suggest that Erk activity is governed by PLD activity and regulates the tyrosine phosphorylation of paxillin, potentially explaining its role in cell motility. This study indicated that PLD, Erk, paxillin and FAK participate in the same signaling pathway in this breast cancer cell line.

Ral GTPases regulate cell-mediated cytotoxicity in NK cells

NK cells are key components of the immune response to virally infected and tumor cells. Recognition of target cells initiates a series of events in NK cells that culminates in target destruction via directed secretion of lytic granules. Ral proteins are members of the Ras superfamily of small GTPases; they regulate vesicular trafficking and polarized granule secretion in several cell types. In this study, we address the role of Ral GTPases in cell-mediated cytotoxicity. Using a human NK cell line and human primary NK cells, we show that both Ral isoforms, RalA and RalB, are activated rapidly after target cell recognition. Furthermore, silencing of RalA and RalB impaired NK cell cytotoxicity. RalA regulated granule polarization toward the immunological synapse and the subsequent process of degranulation, whereas RalB regulated degranulation but not polarization of lytic granules. Analysis of the molecular mechanism indicated that Ral activation in NK cells leads to assembly of the exocyst, a protein complex involved in polarized secretion. This assembly is required for degranulation, as interference with expression of the exocyst component Sec5 led to reduced degranulation and impaired cytotoxicity in NK cells. Our results thus identify a role for Ral in cell-mediated cytotoxicity, implicating these GTPases in lymphocyte function.

Different metastasis promotive potency of small G-proteins RalA and RalB in in vivo hamster tumor model

Background

Previously we have shown that oncogenic Ha-Ras stimulated in vivo metastasis through RalGEF-Ral signaling. RalA and RalB are highly homologous small G proteins belonging to Ras superfamily. They can be activated by Ras-RalGEF signaling pathway and influence cellular growth and survival, motility, vesicular transport and tumor progression in humans and in animal models. Here we first time compared the influence of RalA and RalB on tumorigenic, invasive and metastatic properties of RSV transformed hamster fibroblasts.

Methods

Retroviral vectors encoding activated forms or effector mutants of RalA or RalB proteins were introduced into the low metastatic HET-SR cell line. Tumor growth and spontaneous metastatic activity (SMA) were evaluated on immunocompetent hamsters after subcutaneous injection of cells. The biological properties of cells, including proliferation, clonogenicity, migration and invasion were determined using MTT, wound healing, colony formation and Boyden chamber assays respectively. Protein expression and phosphorylation was detected by Westen blot analysis. Extracellular proteinases activity was assessed by substrate-specific zymography.

Results

We have showed that although both Ral proteins stimulated SMA, RalB was more effective in metastasis stimulation in vivo as well as in potentiating of directed movement and invasion in vitro. Simultaneous expression of active RalA and RalB didn't give synergetic effect on metastasis formation. RalB activity decreased expression of Caveolin-1, while active RalA stimulated MMP-1 and uPA proteolytic activity, as well as CD24 expression. Both Ral proteins were capable of Cyclin D1 upregulation, JNK1 kinase activation, and stimulation of colony growth and motility. Among three main RalB effectors (RalBP1, exocyst complex and PLD1), PLD1 was essential for RalB-dependent metastasis stimulation.

Conclusions

Presented results are the first data on direct comparison of RalA and RalB impact as well as of RalA/RalB simultaneous expression influence on in vivo cell metastatic activity. We showed that RalB activation significantly more than RalA stimulates SMA. This property correlates with the ability of RalB to stimulate in vitro invasion and serum directed cell movement. We also found that RalB-PLD1 interaction is necessary for the acquisition of RalB-dependent high metastatic cell phenotype. These findings contribute to the identification of molecular mechanisms of metastasis and tumor progression.

Phospholipase D mediates nutrient input to mammalian target of rapamycin complex 1 (mTORC1)

The mammalian target of rapamycin (mTOR) is a critical sensor of nutritional sufficiency. Although much is known about the regulation of mTOR in response to growth factors, much less is known about the regulation of mTOR in response to nutrients. Amino acids have no impact on the signals that regulate Rheb, a GTPase required for the activation of mTOR complex 1 (mTORC1). Phospholipase D (PLD) generates a metabolite, phosphatidic acid, that facilitates association between mTOR and the mTORC1 co-factor Raptor. We report here that elevated PLD activity in human cancer cells is dependent on both amino acids and glucose and that amino acid- and glucose-induced increases in mTORC1 activity are dependent on PLD. Amino acid- and glucose-induced PLD and mTORC1 activity were also dependent on the GTPases RalA and ARF6 and the type III phosphatidylinositol-3-kinase hVps34. Thus, a key stimulatory event for mTORC1 activation in response to nutrients is the generation of phosphatidic acid by PLD.

Autoregulation of phospholipase D activity is coupled to selective induction of phospholipase D1 expression to promote invasion of breast cancer cells

Phospholipase D (PLD) is an important signaling enzyme implicated in the control of many biological processes, including cell proliferation and survival. Despite the importance of the duration and amplitude of PLD signaling in carcinogenesis, mechanisms that regulate PLD expression remain poorly understood. In our study, we define the regulatory components of the machinery that specifies selective PLD1 induction via signals propagated through PLD activity. We demonstrate for the first time that establishment of a positive feedback loop that is dependent on enzymatic activity originating from both PLD1 and PLD2 isozymes enhances selective expression of PLD1, but not PLD2. Phosphatidic acid, the product of PLD activity, leads to an increase in the Ras-ERK/PI3K-NFκB signaling cascade and enhances binding of NFκB to the PLD1 promoter, consequently inducing selective PLD1 expression in SK-BR3 breast cancer cells. Moreover, selective PLD inhibitor suppressed epidermal growth factor-induced matrix metalloproteinase upregulation and invasion by inhibiting PLD1 expression. In conclusion, we propose that autoregulation of PLD activity might be coupled to induction of PLD1 expression, and thereby play a role in carcinogenesis.

Signaling mechanisms of inhibition of phospholipase D activation by CHS-111 in formyl peptide-stimulated neutrophils

A selective phospholipase D (PLD) inhibitor 5-fluoro-2-indolyl des-chlorohalopemide (FIPI) inhibited the O(2)(-) generation and cell migration but not degranulation in formyl-Met-Leu-Phe (fMLP)-stimulated rat neutrophils. A novel benzyl indazole compound 2-benzyl-3-(4-hydroxymethylphenyl)indazole (CHS-111), which inhibited O(2)(-) generation and cell migration, also reduced the fMLP- but not phorbol ester-stimulated PLD activity (IC(50) 3.9±1.2μM). CHS-111 inhibited the interaction of PLD1 with ADP-ribosylation factor (Arf) 6 and Ras homology (Rho) A, and reduced the membrane recruitment of RhoA in fMLP-stimulated cells but not in GTPγS-stimulated cell-free system. CHS-111 reduced the cellular levels of GTP-bound RhoA, membrane recruitment of Rho-associated protein kinase 1 and the downstream myosin light chain 2 phosphorylation, and attenuated the interaction between phosphatidylinositol 4-phosphate 5-kinase (PIP5K) and Arf6, whereas it only slightly inhibited the guanine nucleotide exchange activity of human Dbs (DH/PH) protein and did not affect the arfaptin binding to Arf6. CHS-111 inhibited the interaction of RhoA with Vav, the membrane association and the phosphorylation of Vav. CHS-111 had no effect on the phosphorylation of Src family kinases (SFK) but attenuated the interaction of Vav with Lck, Hck, Fgr and Lyn. CHS-111 also inhibited the interaction of PLD1 with protein kinase C (PKC) α, βI and βII isoenzymes, and the phosphorylation of PLD1. These results indicate that inhibition of fMLP-stimulated PLD activity by CHS-111 is attributable to the blockade of RhoA activation via the interference with SFK-mediated Vav activation, attenuation of the interaction of Arf6 with PLD1 and PIP5K, and the activation of Ca(2+)-dependent PKC in rat neutrophils.

Ras-related small GTPases RalA and RalB regulate cellular survival after ionizing radiation

PURPOSE

Oncogenic activation of Ras renders cancer cells resistant to ionizing radiation (IR), but the mechanisms have not been fully characterized. The Ras-like small GTPases RalA and RalB are downstream effectors of Ras function and are critical for both tumor growth and survival. The Ral effector RalBP1/RLIP76 mediates survival of mice after whole-body irradiation, but the role of the Ral GTPases themselves in response to IR is unknown. We have investigated the role of RalA and RalB in cellular responses to IR.

METHODS AND MATERIALS

RalA, RalB, and their major effectors RalBP1 and Sec5 were knocked down by stable expression of short hairpin RNAs in the K-Ras-dependent pancreatic cancer-derived cell line MIA PaCa-2. Radiation responses were measured by standard clonogenic survival assays for reproductive survival, gammaH2AX expression for double-strand DNA breaks (DSBs), and poly(ADP-ribose)polymerase (PARP) cleavage for apoptosis.

RESULTS

Knockdown of K-Ras, RalA, or RalB reduced colony-forming ability post-IR, and knockdown of either Ral isoform decreased the rate of DSB repair post-IR. However, knockdown of RalB, but not RalA, increased cell death. Surprisingly, neither RalBP1 nor Sec5 suppression affected colony formation post-IR.

CONCLUSIONS

Both RalA and RalB contribute to K-Ras-dependent IR resistance of MIA PaCa-2 cells. Sensitization due to suppressed Ral expression is likely due in part to decreased efficiency of DNA repair (RalA and RalB) and increased susceptibility to apoptosis (RalB). Ral-mediated radioresistance does not depend on either the RalBP1 or the exocyst complex, the two best-characterized Ral effectors, and instead may utilize an atypical or novel effector.

The RalB-RLIP76 complex reveals a novel mode of ral-effector interaction

RLIP76 (RalBP1) is a multidomain protein that interacts with multiple small G protein families: Ral via a specific binding domain, and Rho and R-Ras via a GTPase activating domain. RLIP76 interacts with endocytosis proteins and has also been shown to behave as a membrane ATPase that transports chemotherapeutic agents from the cell. We have determined the structure of the Ral-binding domain of RLIP76 and show that it comprises a coiled-coil motif. The structure of the RLIP76-RalB complex reveals a novel mode of binding compared to the structures of RalA complexed with the exocyst components Sec5 and Exo84. RLIP76 interacts with both nucleotide-sensitive regions of RalB, and key residues in the interface have been identified using affinity measurements of RalB mutants. Sec5, Exo84, and RLIP76 bind Ral proteins competitively and with similar affinities in vitro.

Phosphatidic acid signaling to mTOR: signals for the survival of human cancer cells

During the past decade elevated phospholipase D (PLD) activity has been reported in virtually all cancers where it has been examined. PLD catalyzes the hydrolysis of phosphatidylcholine to generate the lipid second messenger phosphatidic acid (PA). While many targets of PA signaling have been identified, the most critical target of PA in cancer cells is likely to be mTOR - the mammalian target of rapamycin. mTOR has been widely implicated in signals that suppress apoptotic programs in cancer cells - frequently referred to as survival signals. mTOR exists as two multi-component complexes known as mTORC1 and mTORC2. Recent data has revealed that PA is required for the stability of both mTORC1 and mTORC2 complexes - and therefore also required for the kinase activity of both mTORC1 and mTORC2. PA interacts with mTOR in a manner that is competitive with rapamycin, and as a consequence, elevated PLD activity confers rapamycin resistance - a point that has been largely overlooked in clinical trials involving rapamycin-based strategies. The earliest genetic changes occurring in an emerging tumor are generally ones that suppress default apoptotic programs that likely represent the first line of defense of cancer. Targeting survival signals in human cancers represents a rational anti-cancer therapeutic strategy. Therefore, understanding the signals that regulate PA levels and how PA impacts upon mTOR could be important for developing strategies to de-repress the survival signals that suppress apoptosis. This review summarizes the role of PA in regulating the mTOR-mediated signals that promote cancer cell survival.

Phosphatidic acid regulation of phosphatidylinositol 4-phosphate 5-kinases

Phosphatidic acid (PA) production by receptor-stimulated phospholipase D is believed to play an important role in the regulation of cell function. The second messenger function of PA remains to be elucidated. PA can bind and affect the activities of different enzymes and here we summarise the current status of activation of Type I phosphatidylinositol 4-phosphate 5-kinase by PA. Type 1 phosphatidylinositol 4-phosphate 5-kinase is also regulated by ARF proteins as is phospholipase D and we discuss the contributions of ARF and PA towards phosphatidylinositol(4,5)bisphosphate synthesis at the plasma membrane.

Solution structure and dynamics of the small GTPase RalB in its active conformation: significance for effector protein binding

The small G proteins RalA/B have a crucial function in the regulatory network that couples extracellular signals with appropriate cellular responses. RalA/B are an important component of the Ras signaling pathway and, in addition to their role in membrane trafficking, are implicated in the initiation and maintenance of tumorigenic transformation of human cells. RalA and RalB share 85% sequence identity and collaborate in supporting cancer cell proliferation but have markedly different effects. RalA is important in mediating proliferation, while depletion of RalB results in transformed cells undergoing apoptosis. Crystal structures of RalA in the free form and in complex with its effectors, Sec5 and Exo84, have been solved. Here we have determined the solution structure of free RalB bound to the GTP analogue GMPPNP to an RMSD of 0.6 A. We show that, while the overall architecture of RalB is very similar to the crystal structure of RalA, differences exist in the switch regions, which are sensitive to the bound nucleotide. Backbone 15N dynamics suggest that there are four regions of disorder in RalB: the P-loop, switch I, switch II, and the loop comprising residues 116-121, which has a single residue insertion compared to RalA. 31P NMR data and the structure of RalB.GMPPNP show that the switch regions predominantly adopt state 1 (Ras nomenclature) in the unbound form, which in Ras is not competent to bind effectors. In contrast, 31P NMR analysis of RalB.GTP reveals that conformations corresponding to states 1 and 2 are both sampled in solution and that addition of an effector protein only partially stabilizes state 2.

Distinct roles of RalA and RalB in the progression of cytokinesis are supported by distinct RalGEFs

The Ras family G-proteins RalA and RalB make critical non-overlapping contributions to the generation of a tumorigenic regulatory network, supporting bypass of the normal restraints on both cell proliferation and survival. The Sec6/8 complex, or exocyst, has emerged as a principal direct effector complex for Ral GTPases. Here, we show that RalA and RalB support mitotic progression through mobilization of the exocyst for two spatially and kinetically distinct steps of cytokinesis. RalA is required to tether the exocyst to the cytokinetic furrow in early cytokinesis. RalB is then required for recruitment of the exocyst to the midbody of this bridge to drive abscission and completion of cytokinesis. The collaborative action of RalA and RalB is specified by discrete subcellular compartmentalization and unique pairs of RalGEF proteins that provide inputs from both Ras-family protein-dependent and protein-independent regulatory cues. This suggests that Ral GTPases integrate diverse upstream signals to choreograph multiple roles for the exocyst in mitotic progression.

SV40 small T antigen and PP2A phosphatase in cell transformation

The SV40 early region protein, SV40 small t antigen, promotes cell transformation through negative regulation of the protein phosphatase 2A (PP2A) family of serine-threonine phosphatases. More recently, reduced levels of PP2A activity have been found in different types of human cancer. This occurs either through inactivating mutations of PP2A structural subunits, or by upregulation of the cellular PP2A inhibitors, CIP2A and SET. Several distinct PP2A complexes have been identified that contribute directly to tumor suppression by regulating specific phosphorylation events. These studies provide us with new insights into the role of protein phosphatases in cancer initiation and maintenance.

Honokiol suppresses survival signals mediated by Ras-dependent phospholipase D activity in human cancer cells

PURPOSE

Elevated phospholipase D (PLD) activity provides a survival signal in several human cancer cell lines and suppresses apoptosis when cells are subjected to the stress of serum withdrawal. Thus, targeting PLD survival signals has potential to suppress survival in cancer cells that depend on PLD for survival. Honokiol is a compound that suppresses tumor growth in mouse models. The purpose of this study was to investigate the effect of honokiol on PLD survival signals and the Ras dependence of these signals.

EXPERIMENTAL DESIGN

The effect of honokiol upon PLD activity was examined in human cancer cell lines where PLD activity provides a survival signal. The dependence of PLD survival signals on Ras was investigated, as was the effect of honokiol on Ras activation.

RESULTS

We report here that honokiol suppresses PLD activity in human cancer cells where PLD has been shown to suppress apoptosis. PLD activity is commonly elevated in response to the stress of serum withdrawal, and, importantly, the stress-induced increase in PLD activity is selectively suppressed by honokiol. The stress-induced increase in PLD activity was accompanied by increased Ras activation, and the stress-induced increase in PLD activity in MDA-MB-231 breast cancer cells was dependent on a Ras. The PLD activity was also dependent on the GTPases RalA and ADP ribosylation factor. Importantly, honokiol suppressed Ras activation.

CONCLUSION

The data provided here indicate that honokiol may be a valuable therapeutic reagent for targeting a large number of human cancers that depend on Ras and PLD for their survival.

Multiple pathways regulated by the tumor suppressor PP2A in transformation

Reversible protein phosphorylation plays a central role in regulating intracellular signaling. Dysregulation of the mechanisms that regulate phosphorylation plays a direct role in cancer initiation and maintenance. Although abundant evidence supports the role of kinase oncogenes in cancer development, recent work has illuminated the role of specific protein phosphatases in malignant transformation. Protein phosphatase 2A (PP2A) is the major serine-threonine phosphatase in mammalian cells. Inactivation of PP2A by viral oncoproteins, mutation of specific subunits or overexpression of endogenous inhibitors contributes to cell transformation by regulating specific phosphorylation events. Here, we review recent progress in our understanding of how PP2A regulates mitogenic signaling pathways in cancer pathogenesis and how PP2A activity is modulated in human cancers.

Two conformational states of Ras GTPase exhibit differential GTP-binding kinetics

Previous (31)P NMR studies revealed that small GTPases H-Ras and K-Ras in complex with GTP assume two interconverting conformational states, state 1 and state 2. While state 2 corresponds to an active conformation, little is known about the function of state 1, an inactive conformation incapable of effector binding. To address the biochemical properties of state 1, we measured the (31)P NMR spectra of five Ras family small GTPases; H-Ras, M-Ras, Rap1A, Rap2A and RalA, and find that they exhibit distinctive state 2/state 1 populations with the ratios ranging from 0.072 for M-Ras to 16 for Rap2A. Further, we show that GTPases with higher populations of state 1 exhibit higher dissociation and association rate constants for GTP. These results imply that GTP loading to the nucleotide-free small GTPases preferentially yields state 1, which is subsequently converted to state 2, rendering the GTP-bound form functional.

The small GTPase Ral mediates SDF-1-induced migration of B cells and multiple myeloma cells

Chemokine-controlled migration plays a critical role in B-cell development, differentiation, and function, as well as in the pathogenesis of B-cell malignancies, including the plasma cell neoplasm multiple myeloma (MM). Here, we demonstrate that stimulation of B cells and MM cells with the chemokine stromal cell-derived factor-1 (SDF-1) induces strong migration and activation of the Ras-like GTPase Ral. Inhibition of Ral, by expression of the dominant negative RalN28 mutant or of RalBPDeltaGAP, a Ral effector mutant that sequesters active Ral, results in impaired SDF-1-induced migration of B cells and MM cells. Of the 2 Ral isoforms, RalA and RalB, RalB was found to mediate SDF-1-induced migration. We have recently shown that Btk, PLCgamma2, and Lyn/Syk mediate SDF-1-controlled B-cell migration; however, SDF-1-induced Ral activation is not affected in B cells deficient in these proteins. In addition, treatment with pharmacological inhibitors against PI3K and PLC or expression of dominant-negative Ras did not impair SDF-1-induced Ral activation. Taken together, these results reveal a novel function for Ral, that is, regulation of SDF-1-induced migration of B cells and MM cells, thereby providing new insights into the control of B-cell homeostasis, trafficking, and function, as well as into the pathogenesis of MM.

Mutation of Y179 on phospholipase D2 (PLD2) upregulates DNA synthesis in a PI3K-and Akt-dependent manner

Phospholipase D2 (PLD2), one of the two mammalian members of the PLD family, has been implicated in cell proliferation, transformation, tumor progression and survival. However, as precise mechanistic details are still unknown, we investigated here if the PLD2 isoform would signal through the PI3K/AKT pathway. Transient expression of PLD2 in COS7 cells with either the WT or with a Y179F mutant, resulted in an increased basal phosphorylation of AKT in residues T308 and S473, in a PI3K-dependent manner. Transfection of PLD2-Y179F (but not the wild type) caused an increased (>2-fold) DNA synthesis even in the absence of extracellular stimuli. Other signaling mechanisms downstream such PLD/PI3K dependence (that might lead to DNA synthesis regulation) were further studied. PLD2-Y179F caused an increase in phosphorylation of p42/p44 ERK and in the expression of G0/G1 phase transition markers (p21 CIP, PCNA), and these effects, too, were dependent on PI3K. Interestingly, Akt, once activated induced the phosphorylation of PLD2 on residue T175, an effect that was inhibited by LY296004. Lastly, if PLD2-Y179F is further mutated in residue K758 (PLD2 Y179F-K758R), which renders inactive a catalytic site, DNA synthesis is then abrogated, indicating that the activity of the enzyme (i.e. synthesis of PA) is necessary for the observed effects. In conclusion, the unavailability of residue Y179 on PLD2 to become phosphorylated leads to an augmentation of DNA synthesis concomitantly with MEK and AKT phosphorylation, in a process that is dependent on PI3K and independent of any extracellular stimuli. This might be critical for the maintenance of the PLD2-regulated proliferative status.

Phospholipase D provides a survival signal in human cancer cells with activated H-Ras or K-Ras

Phospholipase D (PLD) is elevated in rodent fibroblasts expressing activated H-Ras mutants. We therefore examined the PLD activity in human cancer cells with activating Ras mutations. T24 bladder carcinoma cells express an activated H-Ras gene and Calu-1 lung carcinoma cells express an activated K-Ras gene. We report here that both of these cancer cell lines express highly elevated levels of PLD activity and that the PLD activity is dependent upon Ras. We also show that the PLD activity is dependent upon the Ras effector molecules RalA and phosphatidylinositol-3-kinase (PI3K). PLD activity has been shown to provide a survival signal in breast cancer cell lines that suppressed stress-induced apoptosis. Suppression of PLD activity in the T24 and Calu-1 cells resulted in apoptotic cell death in the absence of serum, indicating that the elevated PLD activity provided a survival signal in these cancer cell lines. Suppression of Ras, RalA, or PI3K also led to apoptosis in the absence of serum. These data indicate that a critical component of Ras signaling in human cancer cells is the activation of PLD and that targeting PLD survival signals in cancer cells could be an effective strategy to induce apoptosis in human cancers with activating Ras mutations.

Development and characterization of gemcitabine-resistant pancreatic tumor cells

BACKGROUND

Pancreatic cancer is an exceptionally lethal disease with an annual mortality nearly equivalent to its annual incidence. This dismal rate of survival is due to several factors including late presentation with locally advanced, unresectable tumors, early metastatic disease, and rapidly arising chemoresistance. To study the mechanisms of chemoresistance in pancreatic cancer we developed two gemcitabine-resistant pancreatic cancer cell lines.

METHODS

Resistant cells were obtained by culturing L3.6pl and AsPC-1 cells in serially increasing concentrations of gemcitabine. Stable cultures were obtained that were 40- to 50-fold increased in resistance relative to parental cells. Immunofluorescent staining was performed to examine changes in beta-catenin and E-cadherin localization. Protein expression was determined by immunoblotting. Migration and invasion were determined by modified Boyden chamber assays. Fluorescence-activated cell sorting (FACS) analyses were performed to examine stem cell markers.

RESULTS

Gemcitabine-resistant cells underwent distinct morphological changes, including spindle-shaped morphology, appearance of pseudopodia, and reduced adhesion characteristic of transformed fibroblasts. Gemcitabine-resistant cells were more invasive and migratory. Gemcitabine-resistant cells were increased in vimentin and decreased in E-cadherin expression. Immunofluorescence and immunoblotting revealed increased nuclear localization of total beta-catenin. These alterations are hallmarks of epithelial-to-mesenchymal transition (EMT). Resistant cells were activated in the receptor protein tyrosine kinase, c-Met and increased in expression of the stem cell markers CD (cluster of differentiation)24, CD44, and epithelial-specific antigen (ESA).

CONCLUSIONS

Gemcitabine-resistant pancreatic tumor cells are associated with EMT, a more-aggressive and invasive phenotype in numerous solid tumors. The increased phosphorylation of c-Met may also be related to chemoresistance and EMT and presents as an attractive adjunctive chemotherapeutic target in pancreatic cancer.

The tumor suppressor PP2A Abeta regulates the RalA GTPase

The serine-threonine protein phosphatase 2A (PP2A) is a heterotrimeric enzyme family that regulates numerous signaling pathways. Biallelic mutations of the structural PP2A Abeta subunit occur in several types of human tumors; however, the functional consequences of these cancer-associated PP2A Abeta mutations in cell transformation remain undefined. Here we show that suppression of PP2A Abeta expression permits immortalized human cells to achieve a tumorigenic state. Cancer-associated Abeta mutants fail to reverse tumorigenic phenotype induced by PP2A Abeta suppression, indicating that these mutants function as null alleles. Wild-type PP2A Abeta but not cancer-derived Abeta mutants form a complex with the small GTPase RalA. PP2A Abeta-containing complexes dephosphorylate RalA at Ser183 and Ser194, inactivating RalA and abolishing its transforming function. These observations identify PP2A Abeta as a tumor suppressor gene that transforms immortalized human cells by regulating the function of RalA.