Impact of interactions between normal and transformed epithelial cells and the relevance to cancer

The majority of human cancers are initiated when a single cell in an epithelial sheet becomes transformed. Cell transformation arises from the activation of oncoproteins and/or inactivation of tumor suppressor proteins. Recent studies have independently revealed that interaction and communication between transformed cells and their normal neighbors have a significant impact on the fate of the transformed cell. Several reports have shown that various phenomena occur at the interface between normal and transformed epithelial cells following the initial transformation event. In epithelia of Drosophila melanogaster, transformed and normal cells compete for survival in a process termed cell competition. This review will summarize current research and discuss the impact of these studies on our understanding of how primary tumors emerge and develop within a normal epithelium.

Partners in crime: the TGFβ and MAPK pathways in cancer progression

The TGFβ and Ras-MAPK pathways play critical roles in cell development and cell cycle regulation, as well as in tumor formation and metastasis. In the absence of cellular transformation, these pathways operate in opposition to one another, where TGFβ maintains an undifferentiated cell state and suppresses proliferation, while Ras-MAPK pathways promote proliferation, survival and differentiation. However, in colorectal and pancreatic cancers, the opposing pathways' mechanisms are simultaneously activated in order to promote cancer progression and metastasis. Here, we highlight the roles of the TGFβ and Ras-MAPK pathways in normal and malignant states, and provide an explanation for how the concomitant activation of these pathways drives tumor biology. Finally, we survey potential therapeutic targets in these pathways.

Protein-intrinsic and signaling network-based sources of resistance to EGFR- and ErbB family-targeted therapies in head and neck cancer

Agents targeting EGFR and related ErbB family proteins are valuable therapies for the treatment of many cancers. For some tumor types, including squamous cell carcinomas of the head and neck (SCCHN), antibodies targeting EGFR were the first protein-directed agents to show clinical benefit, and remain a standard component of clinical strategies for management of the disease. Nevertheless, many patients display either intrinsic or acquired resistance to these drugs; hence, major research goals are to better understand the underlying causes of resistance, and to develop new therapeutic strategies that boost the impact of EGFR/ErbB inhibitors. In this review, we first summarize current standard use of EGFR inhibitors in the context of SCCHN, and described new agents targeting EGFR currently moving through pre-clinical and clinical development. We then discuss how changes in other transmembrane receptors, including IGF1R, c-Met, and TGF-β, can confer resistance to EGFR-targeted inhibitors, and discuss new agents targeting these proteins. Moving downstream, we discuss critical EGFR-dependent effectors, including PLC-γ; PI3K and PTEN; SHC, GRB2, and RAS and the STAT proteins, as factors in resistance to EGFR-directed inhibitors and as alternative targets of therapeutic inhibition. We summarize alternative sources of resistance among cellular changes that target EGFR itself, through regulation of ligand availability, post-translational modification of EGFR, availability of EGFR partners for hetero-dimerization and control of EGFR intracellular trafficking for recycling versus degradation. Finally, we discuss new strategies to identify effective therapeutic combinations involving EGFR-targeted inhibitors, in the context of new system level data becoming available for analysis of individual tumors.

Protein kinase Cδ inactivation inhibits cellular proliferation and decreases survival in human neuroendocrine tumors

The concept of targeting cancer therapeutics toward specific mutations or abnormalities in tumor cells, which are not found in normal tissues, has the potential advantages of high selectivity for the tumor and correspondingly low secondary toxicities. Many human malignancies display activating mutations in the Ras family of signal-transducing genes or over-activity of p21(Ras)-signaling pathways. Carcinoid and other neuroendocrine tumors have been similarly demonstrated to have activation of Ras signaling directly by mutations in Ras, indirectly by loss of Ras-regulatory proteins, or via constitutive activation of upstream or downstream effector pathways of Ras, such as growth factor receptors or PI(3)-kinase and Raf/mitogen-activated protein kinases. We previously reported that aberrant activation of Ras signaling sensitizes cells to apoptosis when the activity of the PKCδ isozyme is suppressed and that PKCδ suppression is not toxic to cells with normal levels of p21(Ras) signaling. We demonstrate here that inhibition of PKCδ by a number of independent means, including genetic mechanisms (shRNA) or small-molecule inhibitors, is able to efficiently and selectively repress the growth of human neuroendocrine cell lines derived from bronchopulmonary, foregut, or hindgut tumors. PKCδ inhibition in these tumors also efficiently induced apoptosis. Exposure to small-molecule inhibitors of PKCδ over a period of 24  h is sufficient to significantly suppress cell growth and clonogenic capacity of these tumor cell lines. Neuroendocrine tumors are typically refractory to conventional therapeutic approaches. This Ras-targeted therapeutic approach, mediated through PKCδ suppression, which selectively takes advantage of the very oncogenic mutations that contribute to the malignancy of the tumor, may hold potential as a novel therapeutic modality.

Unique structural and nucleotide exchange features of the Rho1 GTPase of Entamoeba histolytica

The single-celled human parasite Entamoeba histolytica possesses a dynamic actin cytoskeleton vital for its intestinal and systemic pathogenicity. The E. histolytica genome encodes several Rho family GTPases known to regulate cytoskeletal dynamics. EhRho1, the first family member identified, was reported to be insensitive to the Rho GTPase-specific Clostridium botulinum C3 exoenzyme, raising the possibility that it may be a misclassified Ras family member. Here, we report the crystal structures of EhRho1 in both active and inactive states. EhRho1 is activated by a conserved switch mechanism, but diverges from mammalian Rho GTPases in lacking a signature Rho insert helix. EhRho1 engages a homolog of mDia, EhFormin1, suggesting a role in mediating serum-stimulated actin reorganization and microtubule formation during mitosis. EhRho1, but not a constitutively active mutant, interacts with a newly identified EhRhoGDI in a prenylation-dependent manner. Furthermore, constitutively active EhRho1 induces actin stress fiber formation in mammalian fibroblasts, thereby identifying it as a functional Rho family GTPase. EhRho1 exhibits a fast rate of nucleotide exchange relative to mammalian Rho GTPases due to a distinctive switch one isoleucine residue reminiscent of the constitutively active F28L mutation in human Cdc42, which for the latter protein, is sufficient for cellular transformation. Nonconserved, nucleotide-interacting residues within EhRho1, revealed by the crystal structure models, were observed to contribute a moderating influence on fast spontaneous nucleotide exchange. Collectively, these observations indicate that EhRho1 is a bona fide member of the Rho GTPase family, albeit with unique structural and functional aspects compared with mammalian Rho GTPases.

Three Observations That Have Changed Our Understanding of Cyclin D1 and p27 in Cell Cycle Control

Our understanding of cell cycle control has been based largely upon studies of synchronized cultures, often focused upon the early stages of the cell cycle following stimulation of quiescent cultures. These studies showed that cyclin D1 and p27(Kip1) (p27) each respond to the growth environment of the cell and together control entry into the cell cycle. In contrast, all cell cycle phases were considered in these studies of actively growing cultures, including events leading to withdrawal from the cell cycle. This approach relies upon the techniques of microinjection, quantitative image analysis, and time-lapse microscopy. The results provide critical new detail to our understanding of the roles of cyclin D1 and p27 in cell cycle regulation. Three critical observations resulting from this work will be described here to demonstrate that 1) cyclin D1 levels oscillate through the normal cell cycle, 2) checkpoint kinases are able to suppress cyclin D1 during S phase, and 3) the level of p27 is determined by a dynamic interaction between cyclin D1 and p27 so as to determine the rate of cell cycle progression. Based upon these observations, a model of cell cycle control is presented in which ras activity stimulates cyclin D1 during G2 phase, resulting in commitment of the cell to continued cell cycle progression. During G1 phase, ras activity suppresses the level of p27 protein, most of which is bound to cyclin D1, resulting in regulation of the rate of proliferation. This model predicted the involvement of checkpoint kinases in regulating cyclin D1 and the role of checkpoint kinases in the protection of neural cells against reactive oxygen. The substantiation of these 2 predictions serves as general validation of the model.

Quantitative models of signal transduction networks: How detailed should they be?

Receptor-mediated signal transduction networks, comprised of multiple biochemical pathways, control cell responses and are therefore central to normal and aberrant physiological processes. An appreciation for the inherent complexities of these networks has matured in recent years, to the point where it is now apparent that experimental measurements will need to be combined with computational modeling and analysis to best interpret and predict how individual mechanisms (proteinprotein interactions, enzymatic reactions, etc.,) are integrated at the network level. To progress along these lines, there is a major barrier to overcome: although a deep mechanistic understanding of signal transduction has been achieved, data sets of a suitably quantitative nature are still lacking. Based on our efforts to systematically acquire and analyze such measurements, we contend that the level of detail in models of signaling networks ought to be limited by the availability of quantitative data, rather than by the much greater availability of qualitative information about signaling interactions. Although this approach is sensible from a data-driven modeling perspective, it is controversial because it gives the false impression that molecular-level details are being ignored.

The heme oxygenase-1 protein is overexpressed in human renal cancer cells following activation of the Ras-Raf-ERK pathway and mediates anti-apoptotic signal

The stress-inducible cytoprotective enzyme heme oxygenase-1 (HO-1) may play a critical role in the growth and metastasis of tumors. We demonstrated that overexpressed HO-1 promotes the survival of renal cancer cells by inhibiting cellular apoptosis; we also showed that the proto-oncogene H-Ras becomes activated in these cells under stress following treatment with immunosuppressive agents. However, it is not known if there is an association between Ras activation and HO-1 overexpression. Here, we examined if the activation of H-Ras pathway could induce HO-1, and promote the survival of renal cancer cells (786-0 and Caki-1). In co-transfection assays, using HO-1 promoter-luciferase construct, we found that the activated H-Ras, H-Ras(12V), promoted HO-1 transcriptional activation. The inhibition of endogenous H-Ras by specific dominant-negative mutant/siRNA markedly ablated the HO-1 promoter activity. Active H-Ras increased HO-1 mRNA and protein expression. Moreover, transfection with effector domain mutant constructs of active H-Ras showed that H-Ras-induced HO-1 overexpression was primarily mediated through the Raf signaling pathway. Using pharmacological inhibitor, we observed that ERK is a critical intermediary molecule for Ras-Raf-induced HO-1 expression. Activation of H-Ras and ERK promoted nuclear translocation of the transcription factor Nrf2 for its binding to the specific sequence of HO-1 promoter. The knockdown of Nrf2 significantly inhibited H-Ras-induced HO-1 transcription. Finally, by FACS analysis using Annexin-V staining, we demonstrated that the H-Ras-ERK-induced and HO-1-mediated pathway could protect renal cancer cells from apoptosis. Thus, targeting the Ras-Raf-ERK pathway for HO-1 overexpression may serve as novel therapeutics for the treatment of renal cancer.

Notch and Ras promote sequential steps of excretory tube development in C. elegans

Receptor tyrosine kinases and Notch are crucial for tube formation and branching morphogenesis in many systems, but the specific cellular processes that require signaling are poorly understood. Here we describe sequential roles for Notch and Epidermal growth factor (EGF)-Ras-ERK signaling in the development of epithelial tube cells in the C. elegans excretory (renal-like) organ. This simple organ consists of three tandemly connected unicellular tubes: the excretory canal cell, duct and G1 pore. lin-12 and glp-1/Notch are required to generate the canal cell, which is a source of LIN-3/EGF ligand and physically attaches to the duct during de novo epithelialization and tubulogenesis. Canal cell asymmetry and let-60/Ras signaling influence which of two equivalent precursors will attach to the canal cell. Ras then specifies duct identity, inducing auto-fusion and a permanent epithelial character; the remaining precursor becomes the G1 pore, which eventually loses epithelial character and withdraws from the organ to become a neuroblast. Ras continues to promote subsequent aspects of duct morphogenesis and differentiation, and acts primarily through Raf-ERK and the transcriptional effectors LIN-1/Ets and EOR-1. These results reveal multiple genetically separable roles for Ras signaling in tube development, as well as similarities to Ras-mediated control of branching morphogenesis in more complex organs, including the mammalian kidney. The relative simplicity of the excretory system makes it an attractive model for addressing basic questions about how cells gain or lose epithelial character and organize into tubular networks.

R-Ras and Rac GTPase cross-talk regulates hematopoietic progenitor cell migration, homing, and mobilization

Adult hematopoietic progenitor cells (HPCs) are maintained by highly coordinated signals in the bone marrow. The molecular mechanisms linking intracellular signaling network of HPCs with their microenvironment remain poorly defined. The Rho family GTPase Rac1/Rac2 has previously been implicated in cell functions involved in HPC maintenance, including adhesion, migration, homing, and mobilization. In the present studies we have identified R-Ras, a member of the Ras family, as a key signal mediator required for Rac1/Rac2 activation. We found that whereas Rac1 activity is up-regulated upon stem cell factor, integrin, or CXCL12 stimulation, R-Ras activity is inversely up-regulated. Expression of a constitutively active R-Ras mutant resulted in down-regulation of Rac1-activity whereas deletion of R-Ras led to an increase in Rac1/Rac2 activity and signaling. R-Ras(-/-) HPCs displayed a constitutively assembled cortical actin structure and showed increased directional migration. Rac1/Rac2 inhibition reversed the migration phenotype of R-Ras(-/-) HPCs, similar to that by expressing an R-Ras active mutant. Furthermore, R-Ras(-/-) mice showed enhanced responsiveness to G-CSF for HPC mobilization and exhibited decreased bone marrow homing. Transplantation experiments indicate that the R-Ras deficiency-induced HPC mobilization is a HPC intrinsic property. These results indicate that R-Ras is a critical regulator of Rac signaling required for HPC migration, homing, and mobilization.

Silencing of BNIP3 results from promoter methylation by DNA methyltransferase 1 induced by the mitogen-activated protein kinase pathway

We have previously shown that Ras mediates NO-induced BNIP3 expression via the MEK-E RK-HIF-1 pathway i n mouse macrophages, and that NO-induced death results at least in part from the induction of BNIP3. In the present study, we describe another aspect of Ras regulation of BNIP3 expression in pancreatic cancer cells. Human BNIP3 promoter-driven luciferase activity was efficiently induced by activated Ras in AsPC-1, Miapaca-2, PK-1 and PANC-1 cells. However, expression of endogenous BNIP3 was not induced, and BNIP3 up-regulation by hypoxia was also inhibited. Treatment of the cells with the DNMT inhibitor, 5-aza-2-deoxycytidine, restored BNIP3 induction, indicating that DNA methylation of the BNIP3 promoter was responsible for the inhibition of BNIP3 induction. Furthermore, inhibition of the MEK pathway with U0126 reduced DNMT1 expression, but not that of DNMT3a and 3b, and restored the hypoxia-inducibility of BNIP3, suggesting that the DNA methylation of the BNIP3 promoter was mediated by DNMT1 via the MEK pathway.

Multiple increased osteoclast functions in individuals with neurofibromatosis type 1

Skeletal abnormalities including scoliosis, tibial dysplasia, sphenoid wing dysplasia, and decreased bone mineral density (BMD) are associated with neurofibromatosis type 1 (NF1). We report the cellular phenotype of NF1 human-derived osteoclasts and compare the in vitro findings with the clinical phenotype. Functional characteristics (e.g., osteoclast formation, migration, adhesion, resorptive capacity) and cellular mechanistic alterations (e.g., F-actin polymerization, MAPK phosphorylation, RhoGTPase activity) from osteoclasts cultured from peripheral blood of individuals with NF1 (N = 75) were assessed. Osteoclast formation was compared to phenotypic, radiologic, and biochemical data. NF1 osteoprogenitor cells demonstrated increased osteoclast forming capacity. Human NF1-derived osteoclasts demonstrated increased migration, adhesion, and in vitro bone resorption. These activities coincided with increased actin belt formation and hyperactivity in MAPK and RhoGTPase pathways. Although osteoclast formation was increased, no direct correlation of osteoclast formation with BMD, markers of bone resorption, or the clinical skeletal phenotype was observed suggesting that osteoclast formation in vitro cannot directly predict NF1 skeletal phenotypes. While NF1 haploinsufficiency produces a generalized osteoclast gain-in-function and may contribute to increased bone resorption, reduced BMD, and focal skeletal defects associated with NF1, additional and perhaps local modifiers are likely required for the development of skeletal abnormalities in NF1.

Critical roles of interactions among switch I-preceding residues and between switch II and its neighboring alpha-helix in conformational dynamics of the GTP-bound Ras family small GTPases

GTP-bound forms of Ras family small GTPases exhibit dynamic equilibrium between two interconverting conformations, "inactive" state 1 and "active" state 2. A great variation exists in their state distribution; H-Ras mainly adopts state 2, whereas M-Ras predominantly adopts state 1. Our previous studies based on comparison of crystal structures representing state 1 and state 2 revealed the importance of the hydrogen-bonding interactions of two flexible effector-interacting regions, switch I and switch II, with the γ-phosphate of GTP in establishing state 2 conformation. However, failure to obtain both state structures from a single protein hampered further analysis of state transition mechanisms. Here, we succeed in solving two crystal structures corresponding to state 1 and state 2 from a single Ras polypeptide, M-RasD41E, carrying an H-Ras-type substitution in residue 41, immediately preceding switch I, in complex with guanosine 5'-(β,γ-imido)triphosphate. Comparison among the two structures and other state 1 and state 2 structures of H-Ras/M-Ras reveal two new structural features playing critical roles in state dynamics; interaction of residues 31/41 (H-Ras/M-Ras) with residues 29/39 and 30/40, which induces a conformational change of switch I favoring its interaction with the γ-phosphate, and the hydrogen-bonding interaction of switch II with its neighboring α-helix, α3-helix, which induces a conformational change of switch II favoring its interaction with the γ-phosphate. The importance of the latter interaction is proved by mutational analyses of the residues involved in hydrogen bonding. These results define the two novel functional regions playing critical roles during state transition.

SmgGDS is a guanine nucleotide exchange factor that specifically activates RhoA and RhoC

SmgGDS is an atypical guanine nucleotide exchange factor (GEF) that promotes both cell proliferation and migration and is up-regulated in several types of cancer. SmgGDS has been previously shown to activate a wide variety of small GTPases, including the Ras family members Rap1a, Rap1b, and K-Ras, as well as the Rho family members Cdc42, Rac1, Rac2, RhoA, and RhoB. In contrast, here we show that SmgGDS exclusively activates RhoA and RhoC among a large panel of purified GTPases. Consistent with the well known properties of GEFs, this activation is catalytic, and SmgGDS preferentially binds to nucleotide-depleted RhoA relative to either GDP- or GTPγS-bound forms. However, mutational analyses indicate that SmgGDS utilizes a distinct exchange mechanism compared with canonical GEFs and in contrast to known GEFs requires RhoA to retain a polybasic region for activation. A homology model of SmgGDS highlights an electronegative surface patch and a highly conserved binding groove. Mutation of either area ablates the ability of SmgGDS to activate RhoA. Finally, the in vitro specificity of SmgGDS for RhoA and RhoC is retained in cells. Together, these results indicate that SmgGDS is a bona fide GEF that specifically activates RhoA and RhoC through a unique mechanism not used by other Rho family exchange factors.

RasGrf1 deficiency delays aging in mice

RasGRF1 is a Ras-guanine nucleotide exchange factor implicated in a variety of physiological processes including learning and memory and glucose homeostasis. To determine the role of RASGRF1 in aging, lifespan and metabolic parameters were analyzed in aged RasGrf1(-/-) mice. We observed that mice deficient for RasGrf1(-/-) display an increase in average and most importantly, in maximal lifespan (20% higher than controls). This was not due to the role of Ras in cancer because tumor-free survival was also enhanced in these animals. Aged RasGrf1(-/-) displayed better motor coordination than control mice. Protection against oxidative stress was similarly preserved in old RasGrf1(-/-). IGF-I levels were lower in RasGrf1(-/-) than in controls. Furthermore, SIRT1 expression was increased in RasGrf1(-/-) animals. Consistent with this, the blood metabolomic profiles of RasGrf1-deficient mice resembled those observed in calorie-restricted animals. In addition, cardiac glucose consumption as determined PET was not altered by aging in the mutant model, indicating that RasGrf1-deficient mice display delayed aging. Our observations link Ras signaling to lifespan and suggest that RasGrf1 is an evolutionary conserved gene which could be targeted for the development of therapies to delay age-related processes.

Ras and Rap signaling in synaptic plasticity and mental disorders

The Ras family GTPases (Ras, Rap1, and Rap2) and their downstream mitogen-activated protein kinases (ERK, JNK, and p38MAPK) and PI3K signaling cascades control various physiological processes. In neuronal cells, recent studies have shown that these parallel cascades signal distinct forms of AMPA-sensitive glutamate receptor trafficking during experience-dependent synaptic plasticity and adaptive behavior. Interestingly, both hypo- and hyperactivation of Ras/ Rap signaling impair the capacity of synaptic plasticity, underscoring the importance of a "happy-medium" dynamic regulation of the signaling. Moreover, accumulating reports have linked various genetic defects that either up- or down-regulate Ras/Rap signaling with several mental disorders associated with learning disability (e.g., Alzheimer's disease, Angelman syndrome, autism, cardio-facio-cutaneous syndrome, Coffin-Lowry syndrome, Costello syndrome, Cowden and Bannayan-Riley-Ruvalcaba syndromes, fragile X syndrome, neurofibromatosis type 1, Noonan syndrome, schizophrenia, tuberous sclerosis, and X-linked mental retardation), highlighting the necessity of happy-medium dynamic regulation of Ras/Rap signaling in learning behavior. Thus, the recent advances in understanding of neuronal Ras/Rap signaling provide a useful guide for developing novel treatments for mental diseases.

Akt signaling is required for glioblastoma maintenance in vivo

Glioblastoma multiforme (GBM) can be induced in mice through the combined expression of activated forms of KRas and Akt in glial progenitor cells. We have previously demonstrated that KRas is required for the maintenance of these tumors in vivo as inhibition of KRas expression resulted in apoptotic tumor regression and significantly increased survival. To determine the reliance of these tumors on Akt signaling in vivo, we generated a viral vector that allows the expression of Akt to be controlled post-delivery. Survival rates were compared between those animals with continued Akt expression and animals in which expression of Akt was suppressed. Although a fifth of the tumors were refractory to treatment, inhibition of Akt significantly increased the survival of tumor-bearing mice and nearly a fourth of the mice remained in remission four months after the treatment period. These data suggest that Akt is required for glioblastoma maintenance in the context of activated Ras and that loss of Akt expression results in increased survival; therefore, the PI3K/AKT signaling pathway is a viable therapeutic target in this context.

Altered dipsogenic responses and expression of angiotensin receptors in the offspring exposed to prenatal high sucrose

The present study determined water and salt intake as well as expression of AT(1) and AT(2) receptors in the brain and kidney in the adult offspring rats prenatally exposed to high sucrose. Following the exposure during pregnancy, water intake and salt intake at baseline levels were not changed in the adult offspring. However, after 24h water deprivation, consumption of water and salt was significantly increased compared to that of the control. Plasma sodium and osmolality levels remained the same between the offspring in the control and the exposed groups, while hematocrit was higher in the offspring exposed to prenatal high sucrose immediately following water deprivation. Density of renal AT(1) receptor protein was the same between the control and the exposed group, while AT(2) receptor protein in the kidney was significantly increased in the offspring exposed to prenatal high sucrose in association of thicker basal membrane of glomerular. In the forebrain, both AT(1) and AT(2) receptor levels were significantly increased in the offspring with history of prenatal high sucrose. In addition, water deprivation induced more c-fos expression in the central dipsogenic areas, including the paraventricular and supraoptic nuclei in the offspring exposed to prenatal high sucrose. The results suggested that prenatal high intake of sucrose may affect development of pathways in regulation of dipsogenic behavior in face of dehydration, which was associated with altered expression of AT(1) or/and AT(2) receptors in the kidney and brain.

Repression of the miR-143/145 cluster by oncogenic Ras initiates a tumor-promoting feed-forward pathway

Although activating mutations in RAS oncogenes are known to result in aberrant signaling through multiple pathways, the role of microRNAs (miRNAs) in the Ras oncogenic program remains poorly characterized. Here we demonstrate that Ras activation leads to repression of the miR-143/145 cluster in cells of human, murine, and zebrafish origin. Loss of miR-143/145 expression is observed frequently in KRAS mutant pancreatic cancers, and restoration of these miRNAs abrogates tumorigenesis. miR-143/145 down-regulation requires the Ras-responsive element-binding protein (RREB1), which represses the miR-143/145 promoter. Additionally, KRAS and RREB1 are targets of miR-143/miR-145, revealing a feed-forward mechanism that potentiates Ras signaling.

Systematic quantification of negative feedback mechanisms in the extracellular signal-regulated kinase (ERK) signaling network

Cell responses are actuated by tightly controlled signal transduction pathways. Although the concept of an integrated signaling network replete with interpathway cross-talk and feedback regulation is broadly appreciated, kinetic data of the type needed to characterize such interactions in conjunction with mathematical models are lacking. In mammalian cells, the Ras/ERK pathway controls cell proliferation and other responses stimulated by growth factors, and several cross-talk and feedback mechanisms affecting its activation have been identified. In this work, we take a systematic approach to parse the magnitudes of multiple regulatory mechanisms that attenuate ERK activation through canonical (Ras-dependent) and non-canonical (PI3K-dependent) pathways. In addition to regulation of receptor and ligand levels, we consider three layers of ERK-dependent feedback: desensitization of Ras activation, negative regulation of MEK kinase (e.g. Raf) activities, and up-regulation of dual-specificity ERK phosphatases. Our results establish the second of these as the dominant mode of ERK self-regulation in mouse fibroblasts. We further demonstrate that kinetic models of signaling networks, trained on a sufficient diversity of quantitative data, can be reasonably comprehensive, accurate, and predictive in the dynamical sense.

Ras homolog enriched in brain (Rheb) enhances apoptotic signaling

Rheb is a homolog of Ras GTPase that regulates cell growth, proliferation, and regeneration via mammalian target of rapamycin (mTOR). Because of the well established potential of activated Ras to promote survival, we sought to investigate the ability of Rheb signaling to phenocopy Ras. We found that overexpression of lipid-anchored Rheb enhanced the apoptotic effects induced by UV light, TNFα, or tunicamycin in an mTOR complex 1 (mTORC1)-dependent manner. Knocking down endogenous Rheb or applying rapamycin led to partial protection, identifying Rheb as a mediator of cell death. Ras and c-Raf kinase opposed the apoptotic effects induced by UV light or TNFα but did not prevent Rheb-mediated apoptosis. To gain structural insight into the signaling mechanisms, we determined the structure of Rheb-GDP by NMR. The complex adopts the typical canonical fold of RasGTPases and displays the characteristic GDP-dependent picosecond to nanosecond backbone dynamics of the switch I and switch II regions. NMR revealed Ras effector-like binding of activated Rheb to the c-Raf-Ras-binding domain (RBD), but the affinity was 1000-fold lower than the Ras/RBD interaction, suggesting a lack of functional interaction. shRNA-mediated knockdown of apoptosis signal-regulating kinase 1 (ASK-1) strongly reduced UV or TNFα-induced apoptosis and suppressed enhancement by Rheb overexpression. In conclusion, Rheb-mTOR activation not only promotes normal cell growth but also enhances apoptosis in response to diverse toxic stimuli via an ASK-1-mediated mechanism. Pharmacological regulation of the Rheb/mTORC1 pathway using rapamycin should take the presence of cellular stress into consideration, as this may have clinical implications.

Bimodal expression of Sprouty2 during the cell cycle is mediated by phase-specific Ras/MAPK and c-Cbl activities

Sprouty2 is an important inhibitor of cell proliferation and signal transduction. In this study, we found a bimodal expression of Sprouty2 protein during cell cycle progression after exit from quiescence, whereas elevated Sprouty4 expression in the G1 phase stayed high throughout the rest of the cell cycle. Induction of the mitogen-activated protein kinase via activated Ras was crucial for increased Sprouty2 expression at the G0/G1 transition. Following the first peak, accelerated proteasomal protein degradation caused a transient attenuation of Sprouty2 abundance during late G1. Since the decline in its expression was abolished by dominant negative c-Cbl and the timely restricted interaction between Sprouty2 and c-Cbl disappeared at the second peak of Sprouty2 expression, we conclude that the second phase in the cell cycle-specific expression profile of Sprouty2 is solely dependent on ubiquitination by c-Cbl. Our results suggest that Sprouty2 abundance is the result of strictly coordinated activities of Ras and c-Cbl.

Modulation of the inhibitor properties of dipeptidyl (acyloxy)methyl ketones toward the CaaX proteases

Dipeptidyl (acyloxy)methyl ketones (AOMKs) have been identified as mechanism-based inhibitors of certain cysteine proteases. These compounds are also inhibitors of the integral membrane proteins Rce1p and Ste24p, which are proteases that independently mediate a cleavage step associated with the maturation of certain isoprenylated proteins. The enzymatic mechanism of Rce1p is ill-defined, whereas Ste24p is a zinc metalloprotease. Rce1p is required for the proper processing of the oncoprotein Ras and is viewed as a potential target for cancer therapy. In this study, we synthesized a small library of dipeptidyl AOMKs to investigate the structural elements that contribute to the inhibitor properties of this class of molecules toward Rce1p and Ste24p. The compounds were evaluated using a fluorescence-based in vitro proteolysis assay. The most potent dipeptidyl AOMKs contained an arginine residue and the identity of the benzoate group strongly influenced potency. A 'warhead' free AOMK inhibited Rce1p and Ste24p. The data suggest that the dipeptidyl AOMKs are not mechanism-based inhibitors of Rce1p and Ste24p and corroborate the hypothesis that Rce1p is not a cysteine protease.

Tumor suppressors Sav/Scrib and oncogene Ras regulate stem-cell transformation in adult Drosophila malpighian tubules

An increasing body of evidence suggests that tumors might originate from a few transformed cells that share many properties with normal stem cells. However, it remains unclear how normal stem cells are transformed into cancer stem cells (CSCs). Here, we demonstrated that mutations causing the loss of tumor suppressor Salvador (Sav) or Scribble (Scrib) or activation of the oncogene Ras transform normal stem cells into CSCs through a multistep process in the adult Drosophila Malpighian Tubules (MTs). In wild-type MTs, each stem cell generates one self-renewing and one differentiating daughter cell. However, in flies with loss-of-function sav or scrib or gain-of-function Ras mutations, both daughter cells grew and behaved like stem cells, leading to the formation of tumors in MTs. Ras functioned downstream of Sav and Scrib in regulating the stem-cell transformation. The Ras-transformed stem cells exhibited many of the hallmarks of cancer, such as increased proliferation, reduced cell death, and failure to differentiate. We further demonstrated that several signal transduction pathways (including MEK/MAPK, RhoA, PKA, and TOR) mediate Ras' function in the stem-cell transformation. Therefore, we have identified a molecular mechanism that regulates stem-cell transformation, and this finding may lead to strategies for preventing tumor formation in certain organs.

Acetylation-dependent oncogenic activity of metastasis-associated protein 1 co-regulator

High expression of metastasis-associated protein 1 co-regulator (MTA1), a component of the nuclear remodelling and histone deacetylase complex, has been associated with human tumours. However, the precise role of MTA1 in tumorigenesis remains unknown. In this study, we show that induced levels of MTA1 are sufficient to transform Rat1 fibroblasts and that the transforming potential of MTA1 is dependent on its acetylation at Lys626. Underlying mechanisms of MTA1-mediated transformation include activation of the Ras-Raf pathway by MTA1 but not by acetylation-inactive MTA1; this was due to the repression of Galphai2 transcription, which negatively influences Ras activation. We observed that acetylated MTA1-histone deacetylase (HDAC) interaction was required for the recruitment of the MTA1-HDAC complex to the Galphai2 regulatory element and consequently for the repression of Galphai2 transcription and expression leading to activation of the Ras-Raf pathway. The findings presented in this study provide for the first time--to the best of our knowledge--evidence of acetylation-dependent oncogenic activity of a cancer-relevant gene product.