Rit, a non-lipid-modified Ras-related protein, transforms NIH3T3 cells without activating the ERK, JNK, p38 MAPK or PI3K/Akt pathways

Multiomic characterization of oncogenic signaling mediated by wild-type and mutant RIT1

Aberrant activation of the RAS family of guanosine triphosphatases (GTPases) is prevalent in lung adenocarcinoma, with somatic mutation of KRAS occurring in ~30% of tumors. We previously identified somatic mutations and amplifications of the gene encoding RAS family GTPase RIT1 in lung adenocarcinomas. To explore the biological pathways regulated by RIT1 and how they relate to the oncogenic KRAS network, we performed quantitative proteomic, phosphoproteomic, and transcriptomic profiling of isogenic lung epithelial cells in which we ectopically expressed wild-type or cancer-associated variants of RIT1 and KRAS. We found that both mutant KRAS and mutant RIT1 promoted canonical RAS signaling and that overexpression of wild-type RIT1 partially phenocopied oncogenic RIT1 and KRAS, including induction of epithelial-to-mesenchymal transition. Our findings suggest that RIT1 protein abundance is a factor in its pathogenic function. Therefore, chromosomal amplification of wild-type RIT1 in lung and other cancers may be tumorigenic.

LZTR1: A promising adaptor of the CUL3 family

The study of the disorders of ubiquitin-mediated proteasomal degradation may unravel the molecular basis of human diseases, such as cancer (prostate cancer, lung cancer and liver cancer, etc.) and nervous system disease (Parkinson's disease, Alzheimer's disease and Huntington's disease, etc.) and help in the design of new therapeutic methods. Leucine zipper-like transcription regulator 1 (LZTR1) is an important substrate recognition subunit of cullin-RING E3 ligase that plays an important role in the regulation of cellular functions. Mutations in LZTR1 and dysregulation of associated downstream signaling pathways contribute to the pathogenesis of Noonan syndrome (NS), glioblastoma and chronic myeloid leukemia. Understanding the molecular mechanism of the normal function of LZTR1 is thus critical for its eventual therapeutic targeting. In the present review, the structure and function of LZTR1 are described. Moreover, recent advances in the current knowledge of the functions of LZTR1 in NS, glioblastoma (GBM), chronic myeloid leukemia (CML) and schwannomatosis and the influence of LZTR1 mutations are also discussed, providing insight into how LZTR1 may be targeted for therapeutic purposes.

The RIT1 C-terminus associates with lipid bilayers via charge complementarity

RIT1 is a member of the Ras superfamily of small GTPases involved in regulation of cellular signaling. Mutations to RIT1 are involved in cancer and developmental disorders. Like many Ras subfamily members, RIT1 is localized to the plasma membrane. However, RIT1 lacks the C-terminal prenylation that helps many other subfamily members adhere to cellular membranes. We used molecular dynamics simulations to examine the mechanisms by which the C-terminal peptide (CTP) of RIT1 associates with lipid bilayers. We show that the CTP is unstructured and that its membrane interactions depend on lipid composition. While a 12-residue region of the CTP binds strongly to anionic bilayers containing phosphatidylserine lipids, the CTP termini fray from the membrane allowing for accommodation of the RIT1 globular domain at the membrane-water interface.

The molecular functions of RIT1 and its contribution to human disease

RIT1 is a member of the Ras family of GTPases that direct broad cellular physiological responses through tightly controlled signaling networks. The canonical Ras GTPases are well-defined regulators of the RAF/MEK/ERK pathway and mutations in these are pathogenic in cancer and a class of developmental disorders termed RASopathies. Emerging clinical evidences have now demonstrated a role for RIT1 in RASopathies, namely Noonan syndrome, and various cancers including lung adenocarcinoma and myeloid malignancies. While RIT1 has been mostly described in the context of neuronal differentiation and survival, the mechanisms underlying aberrant RIT1-mediated signaling remain elusive. Here, we will review efforts undertaken to characterize the biochemical and functional properties of the RIT1 GTPase at the molecular, cellular, and organismal level, as well as provide a phenotypic overview of different human conditions caused by RIT1 mutations. Deeper understanding of RIT1 biological function and insight to its pathogenic mechanisms are imperative to developing effective therapeutic interventions for patients with RIT1-mutant Noonan syndrome and cancer.

FOXP1-induced lncRNA CLRN1-AS1 acts as a tumor suppressor in pituitary prolactinoma by repressing the autophagy via inactivating Wnt/β-catenin signaling pathway

As the commonest type of functional pituitary tumor, prolactinoma takes up around 40-60% of functional pituitary tumors. Despite dedications attributed to the treatment of prolactinoma, complete cure remains difficult. Hence, it is of significance to bring to light the underlying mechanism of prolactinoma. Long noncoding RNAs (lncRNAs) are a group of transcripts which can regulate various biological processes. In the present study, we explored an lncRNA that was differentially downregulated in prolactinoma samples. LncRNA clarin 1 antisense RNA 1 (CLRN1-AS1) was downregulated in 42 patient samples and inactivated the Wnt/β-catenin signaling pathway. Functionally, CLRN1-AS1 suppressed cell proliferation, promoted apoptosis, and inhibited autophagy. Subcellular fractionation assay revealed that CLRN1-AS1 was located in the cytoplasm of prolactinoma cells. Based on bioinformatics analysis and mechanism experiments, we determined that CLRN1-AS1 acted as a competing endogenous RNA (ceRNA) by sponging miR-217 to upregulate the dickkopf WNT signaling pathway inhibitor 1 (DKK1). Furthermore, Forkhead box P1 (FOXP1) was verified to be a transcription suppressor of CLRN1-AS1. In summary, this study revealed that FOXP1-induced CLRN1-AS1 regulated cellular functions in pituitary prolactinoma by sponging miR-217 to release the DKK1/Wnt/β-catenin signaling pathway.

HIF-1α-induced RIT1 promotes liver cancer growth and metastasis and its deficiency increases sensitivity to sorafenib

Ras-like-without-CAAX-1 (RIT1) belongs to the RAS superfamily of small GTPases, which plays critical roles in tumor progression. However, little is known about the roles of RIT1 in hepatocellular carcinoma (HCC). Here we found that RIT1 expression was positively associated with the presence of intrahepatic metastasis and the histological grade of HCC and higher RIT1 expression indicated shorter overall survival in HCC patients. In vitro and in vivo studies revealed that RIT1 functioned as an oncogene, as overexpression of RIT1 enhanced HCC cell proliferation and aggressive behavior, whereas silencing RIT1 expression repressed the malignant behaviors. Furthermore, RIT1 deficiency increased drug sensitivity to sorafenib treatment. We further demonstrated that hypoxia-inducible factor 1α (HIF-1α) directly transcriptionally upregulated RIT1, and its stableness was positively correlated with RIT1 expression in HCC tissues. Knockdown of RIT1 attenuated the invasion and migration induced by hypoxia. Collectively, our data highlight the significance of HIF-1α/RIT1 axis in driving HCC progression and sorafenib resistance.

RIT1 suppresses esophageal squamous cell carcinoma growth and metastasis and predicts good prognosis

Ras-like without CAAX1 (RIT1) protein is a member of Ras family, which plays critical roles in signaling pathways and cellular process regulation. However, the role of RIT1 in esophageal squamous cell carcinoma (ESCC) is unclear. In this study, we found that the expression of RIT1 is downregulated in ESCC compared to corresponding non-tumor tissues. The low-level expression of RIT1 was correlated with poorer prognosis. Then we showed that RIT1 inhibited proliferation, invasion, and migration of ESCC cells, and silencing RIT1 by shRNA promoted tumorigenicity and metastasis in nude mice. We further demonstrated that RIT1 inhibited the malignant behaviors of ESCC through inhibiting the PI3K/AKT and MAPK pathway and epithelial–mesenchymal transition in ESCC cells. Our study also revealed that RIT1 increased drug sensitivity to cisplatin (CDDP), and this function could be carried out through downregulating stemness of ESCC. In conclusion, our study indicates for the first time that RIT1 displays tumor-suppressing functions in ESCC, and these functions were carried out by inhibiting MAPK and PI3K/AKT signaling pathway, inhibiting EMT, and downregulating cancer stemness of ESCC cells.

RIT1 controls actin dynamics via complex formation with RAC1/CDC42 and PAK1

RIT1 belongs to the RAS family of small GTPases. Germline and somatic RIT1 mutations have been identified in Noonan syndrome (NS) and cancer, respectively. By using heterologous expression systems and purified recombinant proteins, we identified the p21-activated kinase 1 (PAK1) as novel direct effector of RIT1. We found RIT1 also to directly interact with the RHO GTPases CDC42 and RAC1, both of which are crucial regulators of actin dynamics upstream of PAK1. These interactions are independent of the guanine nucleotide bound to RIT1. Disease-causing RIT1 mutations enhance protein-protein interaction between RIT1 and PAK1, CDC42 or RAC1 and uncouple complex formation from serum and growth factors. We show that the RIT1-PAK1 complex regulates cytoskeletal rearrangements as expression of wild-type RIT1 and its mutant forms resulted in dissolution of stress fibers and reduction of mature paxillin-containing focal adhesions in COS7 cells. This effect was prevented by co-expression of RIT1 with dominant-negative CDC42 or RAC1 and kinase-dead PAK1. By using a transwell migration assay, we show that RIT1 wildtype and the disease-associated variants enhance cell motility. Our work demonstrates a new function for RIT1 in controlling actin dynamics via acting in a signaling module containing PAK1 and RAC1/CDC42, and highlights defects in cell adhesion and migration as possible disease mechanism underlying NS.

Noonan syndrome (NS) belongs to the RASopathies, a group of developmental diseases caused by mutations in genes encoding RAS-MAPK pathway components. Germline mutations in RIT1 have been identified in NS. RIT1 belongs to the RAS superfamily, however, the cellular function of RIT1 remains elusive. We show that RIT1 binds p21-activated kinase 1 (PAK1), an effector of the RHO GTPases RAC1 and CDC42, which are important regulators of cytoskeletal dynamics. NS-associated RIT1 mutants enhance complex formation between RIT1, RAC1/CDC42 and PAK1. Expression of wild-type or mutant forms of RIT1 caused loss of stress fibers and mature focal adhesions and enhanced cell motility. Our data suggest that dysfunction in actin dynamics is a novel aspect in the pathophysiology of RASopathies.

RIT1 GTPase Regulates Sox2 Transcriptional Activity and Hippocampal Neurogenesis

Adult neurogenesis, the process of generating mature neurons from neuronal progenitor cells, makes critical contributions to neural circuitry and brain function in both healthy and disease states. Neurogenesis is a highly regulated process in which diverse environmental and physiological stimuli are relayed to resident neural stem cell populations to control the transcription of genes involved in self-renewal and differentiation. Understanding the molecular mechanisms governing neurogenesis is necessary for the development of translational strategies to harness this process for neuronal repair. Here we report that the Ras-related GTPase RIT1 serves to control the sequential proliferation and differentiation of adult hippocampal neural progenitor cells, with in vivo expression of active RIT1 driving robust adult neurogenesis. Gene expression profiling analysis demonstrates increased expression of a specific set of transcription factors known to govern adult neurogenesis in response to active RIT1 expression in the hippocampus, including sex-determining region Y-related HMG box 2 (Sox2), a well established regulator of stem cell self-renewal and neurogenesis. In adult hippocampal neuronal precursor cells, RIT1 controls an Akt-dependent signaling cascade, resulting in the stabilization and transcriptional activation of phosphorylated Sox2. This study supports a role for RIT1 in relaying niche-derived signals to neural/stem progenitor cells to control transcription of genes involved in self-renewal and differentiation.

Altered mRNA Splicing in SMN-Depleted Motor Neuron-Like Cells

Spinal muscular atrophy (SMA) is an intractable neurodegenerative disease afflicting 1 in 6–10,000 live births. One of the key functions of the SMN protein is regulation of spliceosome assembly. Reduced levels of the SMN protein that are observed in SMA have been shown to result in aberrant mRNA splicing. SMN-dependent mis-spliced transcripts in motor neurons may cause stresses that are particularly harmful and may serve as potential targets for the treatment of motor neuron disease or as biomarkers in the SMA patient population. We performed deep RNA sequencing using motor neuron-like NSC-34 cells to screen for SMN-dependent mRNA processing changes that occur following acute depletion of SMN. We identified SMN-dependent splicing changes, including an intron retention event that results in the production of a truncated Rit1 transcript. This intron-retained transcript is stable and is mis-spliced in spinal cord from symptomatic SMA mice. Constitutively active Rit1 ameliorated the neurite outgrowth defect in SMN depleted NSC-34 cells, while expression of the truncated protein product of the mis-spliced Rit1 transcript inhibited neurite extension. These results reveal new insights into the biological consequence of SMN-dependent splicing in motor neuron-like cells.

Expansion of the RASopathies

The Ras/mitogen activated protein kinase (MAPK) pathway is essential in the regulation of cell cycle, differentiation, growth, cell senescence and apoptosis, all of which are critical to normal development. A class of neurodevelopmental disorders, RASopathies, is caused by germline mutations in genes of the Ras/MAPK pathway. Through the use of whole exome sequencing and targeted sequencing of selected genes in cohorts of panel-negative RASopathy patients, several new genes have been identified. These include: RIT1, SOS2, RASA2, RRAS and SYNGAP1, that likely represent new, albeit rare, causative RASopathy genes. In addition, A2ML1, LZTR1, MYST4, SPRY1 and MAP3K8 may represent new rare genes for RASopathies, but, additional functional studies regarding the mutations are warranted. In addition, recent reports have demonstrated that chromosomal copy number variation in regions encompassing Ras/MAPK pathway genes may be a novel pathogenetic mechanism expanding the RASopathies.

Biochemical Classification of Disease-associated Mutants of RAS-like Protein Expressed in Many Tissues (RIT1)

RAS-like protein expressed in many tissues 1 (RIT1) is a disease-associated RAS subfamily small guanosine triphosphatase (GTPase). Recent studies revealed that germ-line and somatic RIT1 mutations can cause Noonan syndrome (NS), and drive proliferation of lung adenocarcinomas, respectively, akin to RAS mutations in these diseases. However, the locations of these RIT1 mutations differ significantly from those found in RAS, and do not affect the three mutational "hot spots" of RAS. Moreover, few studies have characterized the GTPase cycle of RIT1 and its disease-associated mutants. Here we developed a real-time NMR-based GTPase assay for RIT1 and investigated the effect of disease-associated mutations on GTPase cycle. RIT1 exhibits an intrinsic GTP hydrolysis rate similar to that of H-RAS, but its intrinsic nucleotide exchange rate is ∼4-fold faster, likely as a result of divergent residues near the nucleotide binding site. All of the disease-associated mutations investigated increased the GTP-loaded, activated state of RIT1 in vitro, but they could be classified into two groups with different intrinsic GTPase properties. The S35T, A57G, and Y89H mutants exhibited more rapid nucleotide exchange, whereas F82V and T83P impaired GTP hydrolysis. A RAS-binding domain pulldown assay indicated that RIT1 A57G and Y89H were highly activated in HEK293T cells, whereas T83P and F82V exhibited more modest activation. All five mutations are associated with NS, whereas two (A57G and F82V) have also been identified in urinary tract cancers and myeloid malignancies. Characterization of the effects on the GTPase cycle of RIT1 disease-associated mutations should enable better understanding of their role in disease processes.

Mutations in RIT1 cause Noonan syndrome with possible juvenile myelomonocytic leukemia but are not involved in acute lymphoblastic leukemia

Noonan syndrome is a heterogeneous autosomal dominant disorder caused by mutations in at least eight genes involved in the RAS/MAPK signaling pathway. Recently, RIT1 (Ras-like without CAAX 1) has been shown to be involved in the pathogenesis of some patients. We report a series of 44 patients from 30 pedigrees (including nine multiplex families) with mutations in RIT1. These patients display a typical Noonan gestalt and facial phenotype. Among the probands, 8.7% showed postnatal growth retardation, 90% had congenital heart defects, 36% had hypertrophic cardiomyopathy (a lower incidence compared with previous report), 50% displayed speech delay and 52% had learning difficulties, but only 22% required special education. None had major skin anomalies. One child died perinatally of juvenile myelomonocytic leukemia. Compared with the canonical Noonan phenotype linked to PTPN11 mutations, patients with RIT1 mutations appear to be less severely growth retarded and more frequently affected by cardiomyopathy. Based on our experience, we estimate that RIT1 could be the cause of 5% of Noonan syndrome patients. Because mutations found constitutionally in Noonan syndrome are also found in several tumors in adulthood, we evaluated the potential contribution of RIT1 to leukemogenesis in Noonan syndrome. We screened 192 pediatric cases of acute lymphoblastic leukemias (96 B-ALL and 96 T-ALL) and 110 cases of juvenile myelomonocytic leukemias (JMML), but detected no variation in these tumoral samples, suggesting that Noonan patients with germline RIT1 mutations are not at high risk to developing JMML or ALL, and that RIT1 has at most a marginal role in these sporadic malignancies.

Spectrum of mutations and genotype-phenotype analysis in Noonan syndrome patients with RIT1 mutations

RASopathies are autosomal dominant disorders caused by mutations in more than 10 known genes that regulate the RAS/MAPK pathway. Noonan syndrome (NS) is a RASopathy characterized by a distinctive facial appearance, musculoskeletal abnormalities, and congenital heart defects. We have recently identified mutations in RIT1 in patients with NS. To delineate the clinical manifestations in RIT1 mutation-positive patients, we further performed a RIT1 analysis in RASopathy patients and identified 7 RIT1 mutations, including two novel mutations, p.A77S and p.A77T, in 14 of 186 patients. Perinatal abnormalities, including nuchal translucency, fetal hydrops, pleural effusion, or chylothorax and congenital heart defects, are observed in all RIT1 mutation-positive patients. Luciferase assays in NIH 3T3 cells demonstrated that the newly identified RIT1 mutants, including p.A77S and p.A77T, and the previously identified p.F82V, p.T83P, p.Y89H, and p.M90I, enhanced Elk1 transactivation. Genotype-phenotype correlation analyses of previously reported NS patients harboring RIT1, PTPN11, SOS1, RAF1, and KRAS revealed that hypertrophic cardiomyopathy (56 %) was more frequent in patients harboring a RIT1 mutation than in patients harboring PTPN11 (9 %) and SOS1 mutations (10 %). The rates of hypertrophic cardiomyopathy were similar between patients harboring RIT1 mutations and patients harboring RAF1 mutations (75 %). Short stature (52 %) was less prevalent in patients harboring RIT1 mutations than in patients harboring PTPN11 (71 %) and RAF1 (83 %) mutations. These results delineate the clinical manifestations of RIT1 mutation-positive NS patients: high frequencies of hypertrophic cardiomyopathy, atrial septal defects, and pulmonary stenosis; and lower frequencies of ptosis and short stature.

Recent advances in RASopathies

RASopathies or RAS/mitogen-activated protein kinase (MAPK) syndromes are a group of phenotypically overlapping syndromes caused by germline mutations that encode components of the RAS/MAPK signaling pathway. These disorders include neurofibromatosis type I, Legius syndrome, Noonan syndrome, Noonan syndrome with multiple lentigines (formerly called LEOPARD syndrome), Costello syndrome, cardiofaciocutaneous (CFC) syndrome, Noonan-like syndrome, hereditary gingival fibromatosis and capillary malformation-arteriovenous malformation. Recently, novel gene variants, including RIT1, RRAS, RASA2, A2ML1, SOS2 and LZTR1, have been shown to be associated with RASopathies, further expanding the disease entity. Although further analysis will be needed, these findings will help to better elucidate an understanding of the pathogenesis of these disorders and will aid in the development of potential therapeutic approaches. In this review, we summarize the novel genes that have been reported to be associated with RASopathies and highlight the cardiovascular abnormalities that may arise in affected individuals.

Synergistic anti-tumor effect of KLF4 and curcumin in human gastric carcinoma cell line

Kruppel-like factor 4 is a transcription factor which plays an important role in development and progression of various carcinomas. Curcumin characterized by excellent anti-cancer properties is regarded as a serviceable natural compound used in carcinoma therapy. This study aimed at exploring the impact of KLF4 overexpression in cooperation with curcumin on the proliferation, apoptosis and invasion of human gastric carcinoma BGC- 823 cells. Flow cytometry analysis, CCK-8 assays, transwell assays and Western blot results showed that KLF4 overexpression combined with curcumin had significant anti-proliferation, pro-apoptosis and anti-invasion effects on BGC-823 cells. We also found that KLF4 had synergistic effects with curcumin, better promoting apoptosis and inhibiting proliferation and invasion of gastric carcinona cells. These results indicate that KLF4 could be used as a potential therapeutic target; curcumin could act as an auxiliary and provide a promising therapeutic strategy in stomach cancer.

mTORC2 is required for rit-mediated oxidative stress resistance

Rit, a member of the Ras family of GTPases, has been shown to promote cell survival in response to oxidative stress, in part by directing an evolutionarily conserved p38 MAPK-Akt survival cascade. Aberrant Rit signaling has recently been implicated as a driver mutation in human cancer, adding importance to the characterization of critical Rit effector pathways. However, the mechanism by which Rit-p38 signaling regulated Akt activity was unknown. Here, we identify mTORC2 as a critical downstream mediator of Rit-dependent survival signaling in response to reactive oxygen species (ROS) stress. Rit interacts with Sin1 (MAPKAP1), and Rit loss compromises ROS-dependent mTORC2 complex activation, blunting mTORC2-mediated phosphorylation of Akt kinase. Taken together, our findings demonstrate that the p38/mTORC2/Akt signaling cascade mediates Rit-dependent oxidative stress survival. Inhibition of this previously unrecognized cascade should be explored as a potential therapy of Rit-dependent malignancies.

Separate cyclic AMP sensors for neuritogenesis, growth arrest, and survival of neuroendocrine cells

Dividing neuroendocrine cells differentiate into a neuronal-like phenotype in response to ligands activating G protein-coupled receptors, leading to the elevation of the second messenger cAMP. Growth factors that act at receptor tyrosine kinases, such as nerve growth factor, also cause differentiation. We report here that two aspects of cAMP-induced differentiation, neurite extension and growth arrest, are dissociable at the level of the sensors conveying the cAMP signal in PC12 and NS-1 cells. Following cAMP elevation, neuritogenic cyclic AMP sensor/Rapgef2 is activated for signaling to ERK to mediate neuritogenesis, whereas Epac2 is activated for signaling to the MAP kinase p38 to mediate growth arrest. Neither action of cAMP requires transactivation of TrkA, the receptor for NGF. In fact, the differentiating effects of NGF do not require activation of any of the cAMP sensors protein kinase A, Epac, or neuritogenic cyclic AMP sensor/Rapgef2 but, rather, depend on ERK and p38 activation via completely independent signaling pathways. Hence, cAMP- and NGF-dependent signaling for differentiation are also completely insulated from each other. Cyclic AMP and NGF also protect NS-1 cells from serum withdrawal-induced cell death, again by two wholly separate signaling mechanisms, PKA-dependent for cAMP and PKA-independent for NGF.

Oncogenic RIT1 mutations in lung adenocarcinoma

Lung adenocarcinoma is comprised of distinct mutational subtypes characterized by mutually exclusive oncogenic mutations in RTK/RAS pathway members KRAS, EGFR, BRAF and ERBB2, and translocations involving ALK, RET and ROS1. Identification of these oncogenic events has transformed the treatment of lung adenocarcinoma via application of therapies targeted toward specific genetic lesions in stratified patient populations. However, such mutations have been reported in only ∼55% of lung adenocarcinoma cases in the United States, suggesting other mechanisms of malignancy are involved in the remaining cases. Here we report somatic mutations in the small GTPase gene RIT1 in ∼2% of lung adenocarcinoma cases that cluster in a hotspot near the switch II domain of the protein. RIT1 switch II domain mutations are mutually exclusive with all other known lung adenocarcinoma driver mutations. Ectopic expression of mutated RIT1 induces cellular transformation in vitro and in vivo, which can be reversed by combined PI3K and MEK inhibition. These data identify RIT1 as a driver oncogene in a specific subset of lung adenocarcinomas and suggest PI3K and MEK inhibition as a potential therapeutic strategy in RIT1-mutated tumors.

Gain-of-function mutations in RIT1 cause Noonan syndrome, a RAS/MAPK pathway syndrome

RAS GTPases mediate a wide variety of cellular functions, including cell proliferation, survival, and differentiation. Recent studies have revealed that germline mutations and mosaicism for classical RAS mutations, including those in HRAS, KRAS, and NRAS, cause a wide spectrum of genetic disorders. These include Noonan syndrome and related disorders (RAS/mitogen-activated protein kinase [RAS/MAPK] pathway syndromes, or RASopathies), nevus sebaceous, and Schimmelpenning syndrome. In the present study, we identified a total of nine missense, nonsynonymous mutations in RIT1, encoding a member of the RAS subfamily, in 17 of 180 individuals (9%) with Noonan syndrome or a related condition but with no detectable mutations in known Noonan-related genes. Clinical manifestations in the RIT1-mutation-positive individuals are consistent with those of Noonan syndrome, which is characterized by distinctive facial features, short stature, and congenital heart defects. Seventy percent of mutation-positive individuals presented with hypertrophic cardiomyopathy; this frequency is high relative to the overall 20% incidence in individuals with Noonan syndrome. Luciferase assays in NIH 3T3 cells showed that five RIT1 alterations identified in children with Noonan syndrome enhanced ELK1 transactivation. The introduction of mRNAs of mutant RIT1 into 1-cell-stage zebrafish embryos was found to result in a significant increase of embryos with craniofacial abnormalities, incomplete looping, a hypoplastic chamber in the heart, and an elongated yolk sac. These results demonstrate that gain-of-function mutations in RIT1 cause Noonan syndrome and show a similar biological effect to mutations in other RASopathy-related genes.

Rit subfamily small GTPases: regulators in neuronal differentiation and survival

Ras family small GTPases serve as binary molecular switches to regulate a broad array of cellular signaling cascades, playing essential roles in a vast range of normal physiological processes, with dysregulation of numerous Ras-superfamily G-protein-dependent regulatory cascades underlying the development of human disease. However, the physiological function for many "orphan" Ras-related GTPases remain poorly characterized, including members of the Rit subfamily GTPases. Rit is the founding member of a novel branch of the Ras subfamily, sharing close homology with the neuronally expressed Rin and Drosophila Ric GTPases. Here, we highlight recent studies using transgenic and knockout animal models which have begun to elucidate the physiological roles for the Rit subfamily, including emerging roles in the regulation of neuronal morphology and cellular survival signaling, and discuss new genetic data implicating Rit and Rin signaling in disorders such as cancer, Parkinson's disease, autism, and schizophrenia.

A high-throughput platform for lentiviral overexpression screening of the human ORFeome

In response to the growing need for functional analysis of the human genome, we have developed a platform for high-throughput functional screening of genes overexpressed from lentiviral vectors. Protein-coding human open reading frames (ORFs) from the Mammalian Gene Collection were transferred into lentiviral expression vector using the highly efficient Gateway recombination cloning. Target ORFs were inserted into the vector downstream of a constitutive promoter and upstream of an IRES controlled GFP reporter, so that their transfection, transduction and expression could be monitored by fluorescence. The expression plasmids and viral packaging plasmids were combined and transfected into 293T cells to produce virus, which was then used to transduce the screening cell line. We have optimised the transfection and transduction procedures so that they can be performed using robotic liquid handling systems in arrayed 96-well microplate, one-gene-per-well format, without the need to concentrate the viral supernatant. Since lentiviruses can infect both dividing and non-dividing cells, this system can be used to overexpress human ORFs in a broad spectrum of experimental contexts. We tested the platform in a 1990 gene pilot screen for genes that can increase proliferation of the non-tumorigenic mammary epithelial cell line MCF-10A after removal of growth factors. Transduced cells were labelled with the nucleoside analogue 5-ethynyl-2′-deoxyuridine (EdU) to detect cells progressing through S phase. Hits were identified using high-content imaging and statistical analysis and confirmed with vectors using two different promoters (CMV and EF1α). The screen demonstrates the reliability, versatility and utility of our screening platform, and identifies novel cell cycle/proliferative activities for a number of genes.

Analysis of Rit signaling and biological activity

Rit (Ras-like expressed in many tissues) is the founding member of a novel subgroup within the larger Ras superfamily of small GTP-binding proteins. Although Rit shares more than 50% amino acid identity with Ras, it contains a unique effector domain in common with the closely related Rin and Drosophila Ric proteins and lacks the C-terminal lipidation motifs critical for the membrane association and biological activity of many Ras proteins. Interestingly, whereas Rit has only modest transforming ability when assayed in NIH 3T3 cells, Rit exhibits neuronal differentiation activities comparable to those of oncogenic mutants of Ras when assayed in PC12 and other neuronal cell lines. This cell-type specificity is explained in part by the ability of Rit to selectively activate the neuronal Raf isoform, B-Raf. Importantly, Rit seems to play a critical role in neurotrophin-mediated MAP kinase signaling, because Rit gene silencing significantly alters NGF-dependent MAP kinase signaling and neuronal differentiation. In this chapter, we discuss the reagents and methods used to characterize Rit-mediated signaling to MAP kinase-signaling pathways to determine the extracellular stimuli that regulate Rit activation and to characterize Rit-induced neuronal differentiation.

Rit mutants confirm role of MEK/ERK signaling in neuronal differentiation and reveal novel Par6 interaction

Rit is a novel member of the Ras superfamily of small GTP-binding proteins that regulates signaling pathways controlling cellular fate determination. Constitutively activated mutants of Rit induce terminal differentiation of pheochromocytoma (PC6) cells resulting in a sympathetic neuron-like phenotype characterized by the development of highly-branched neurites. Rit signaling has been found to activate several downstream pathways including MEK/ERK, p38 MAPK, Ral-specific guanine nucleotide exchange factors (GEFs), and Rit associates with the Par6 cell polarity machinery. In this study, a series of Rit effector loop mutants was generated to test the importance of these cellular targets to Rit-mediated neuronal differentiation. We find that Rit-mediated neuritogenesis is dependent upon MEK/ERK MAP kinase signaling but independent of RalGEF activation. In addition, in vivo binding studies identified a novel mechanism of Par6 interaction, suggesting that the cell polarity machinery may serve to spatially restrict Rit signaling.

A novel cyclic AMP-dependent Epac-Rit signaling pathway contributes to PACAP38-mediated neuronal differentiation

Pituitary adenylate cyclase-activating polypeptide (PACAP38) stimulation results in the activation of G(s)alpha protein-coupled receptors to regulate neuronal differentiation in a cyclic AMP (cAMP)-dependent manner. These pathways involve protein kinase A (PKA)-dependent processes, but a growing body of evidence indicates that cAMP also regulates cellular functions through PKA-independent signaling cascades. Here we show that the Rit small GTPase is regulated by PACAP38 in a cAMP-dependent but PKA-independent fashion. Rit activation results from stimulation of the cAMP-activated guanine nucleotide exchange factor Epac but does not appear to rely upon the activation of Rap GTPases, the accepted cellular Epac substrates. Although RNA interference studies demonstrated that Epac is required for PACAP38-mediated Rit activation, neither Epac1 nor Epac2 activates Rit directly, indicating that Epac signals to Rit through a novel mechanism in which Rap signaling is not essential. Loss-of-function analysis demonstrated that Rit makes an important contribution to PACAP38-mediated neuronal differentiation. Surprisingly, although Rit is required for sustained extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein kinase signaling following nerve growth factor stimulation of pheochromocytoma 6 (PC6) cells, Rit silencing selectively suppressed PACAP38-elicited activation of p38, without obvious effects on ERK signaling in the same cells. Moreover, the ability of PACAP38 to stimulate CREB-dependent transcription and to promote neurite outgrowth was inhibited by Rit knockdown. Together, these studies identify an unsuspected connection between cAMP and Rit signaling pathways and imply that Rit can function downstream of G(s)alpha/cAMP/Epac in a novel signal transduction pathway necessary for PACAP38-mediated neuronal differentiation and CREB signaling.

Rin GTPase couples nerve growth factor signaling to p38 and b-Raf/ERK pathways to promote neuronal differentiation

In neuronal precursor cells, the magnitude and longevity of mitogen-activated protein (MAP) kinase cascade activation contribute to the nature of the cellular response, differentiation, or proliferation. However, the mechanisms by which neurotrophins promote prolonged MAP kinase signaling are not well understood. Here we defined the Rin GTPase as a novel component of the regulatory machinery contributing to the selective integration of MAP kinase signaling and neuronal development. Rin is expressed exclusively in neurons and is activated by neurotrophin signaling, and loss-of-function analysis demonstrates that Rin makes an essential contribution to nerve growth factor (NGF)-mediated neuronal differentiation. Most surprisingly, although Rin was unable to stimulate MAP kinase activity in NIH 3T3 cells, it potently activated isoform-specific p38alpha MAP kinase signaling and weakly stimulated ERK signaling in pheochromocytoma (PC6) cells. This cell-type specificity is explained in part by the finding that Rin binds and stimulates b-Raf but does not activate c-Raf. Accordingly, selective down-regulation of Rin in PC6 cells suppressed neurotrophin-elicited activation of b-Raf and p38, without obvious effects on NGF-induced ERK activation. Moreover, the ability of NGF to promote neurite outgrowth was inhibited by Rin knockdown. Together, these observations establish Rin as a neuronal specific regulator of neurotrophin signaling, required to couple NGF stimulation to sustain activation of p38 MAP kinase and b-Raf signaling cascades required for neuronal development.

Activated RIC, a small GTPase, genetically interacts with the Ras pathway and calmodulin during Drosophila development

The mammalian Rit and Rin proteins, along with the Drosophila homologue RIC, comprise a distinct and evolutionarily conserved subfamily of Ras-related small GTP-binding proteins. Unlike other Ras superfamily members, these proteins lack a signal for prenylation, contain a conserved but distinct effector domain, and, in the case of Rin and RIC, contain calmodulin-binding domains. To address the physiological role of this Ras subfamily in vivo, activated forms of the Drosophila Ric gene were introduced into flies. Expression of activated RIC proteins altered the development of well-characterized adult structures, including wing veins and photoreceptors of the compound eye. The effects of activated RIC could be mitigated by a reduction in dosage of several genes in the Drosophila Ras cascade, including Son of sevenless (Sos), Dsor (MEK), rolled (MAPK), and Ras itself. On the other hand, reduction of calmodulin exacerbated the defects caused by activated RIC, thus providing the first functional evidence for interaction of these molecules. We conclude that the activation of the Ras cascade may be an important in vivo requisite to the transduction of signals through RIC and that the binding of calmodulin to RIC may negatively regulate this small GTPase.

Rit contributes to nerve growth factor-induced neuronal differentiation via activation of B-Raf-extracellular signal-regulated kinase and p38 mitogen-activated protein kinase cascades

Rit is one of the original members of a novel Ras GTPase subfamily that uses distinct effector pathways to transform NIH 3T3 cells and induce pheochromocytoma cell (PC6) differentiation. In this study, we find that stimulation of PC6 cells by growth factors, including nerve growth factor (NGF), results in rapid and prolonged Rit activation. Ectopic expression of active Rit promotes PC6 neurite outgrowth that is morphologically distinct from that promoted by oncogenic Ras (evidenced by increased neurite branching) and stimulates activation of both the extracellular signal-regulated kinase (ERK) and p38 mitogen-activated protein (MAP) kinase signaling pathways. Furthermore, Rit-induced differentiation is dependent upon both MAP kinase cascades, since MEK inhibition blocked Rit-induced neurite outgrowth, while p38 blockade inhibited neurite elongation and branching but not neurite initiation. Surprisingly, while Rit was unable to stimulate ERK activity in NIH 3T3 cells, it potently activated ERK in PC6 cells. This cell type specificity is explained by the finding that Rit was unable to activate C-Raf, while it bound and stimulated the neuronal Raf isoform, B-Raf. Importantly, selective down-regulation of Rit gene expression in PC6 cells significantly altered NGF-dependent MAP kinase cascade responses, inhibiting both p38 and ERK kinase activation. Moreover, the ability of NGF to promote neuronal differentiation was attenuated by Rit knockdown. Thus, Rit is implicated in a novel pathway of neuronal development and regeneration by coupling specific trophic factor signals to sustained activation of the B-Raf/ERK and p38 MAP kinase cascades.

Autocrine activation of an osteopontin-CD44-Rac pathway enhances invasion and transformation by H-RasV12

Activated forms of Ras family members are prevalent in many cancers where Ras mutants transduce signals essential for transformation, angiogenesis, invasion and metastasis. As a cancer progression model, we used NIH3T3 cells to explore the mechanism of Ras-induced tumorigenesis. Ras family mutants H-RasV12 and Rit79L strongly induced foci formation, while Rho family mutants RhoA-QL, Rac1-QL and Cdc42-QL were less effective. A comparison of downstream transcriptional targets of Ras and Rho family members using a 26 383 element cDNA microarray revealed that the osteopontin (OPN) gene exhibited the best correlation between magnitude of gene expression change and level of foci formation (r=0.96, P<0.001). In association with H-RasV12- and Rit79L-mediated transformation, foci secreted OPN protein and upregulated the OPN receptor CD44, suggesting the novel initiation of an aberrant OPN-CD44-Rac autocrine pathway. In support of this were the following observations. First, RGD-deficient OPN protein-binding activity was present in H-RasV12-transformed cells but not in control cells, and binding activity was inhibited by the CD44 blocking antibody. Second, foci formation, cell invasion and Rac activity were induced by H-RasV12 and inhibited by the CD44 blocking antibody. Third, foci formation by H-RasV12 was substantially reduced by a short interfering RNA (siRNA) specifically targeting OPN expression for knockdown. Fourth, H-RasV12-mediated transformation was not blocked by the GRGDS peptide, suggesting that OPN effects were not mediated by the integrins. Lastly, OPN knockdown affected the downstream expression of 160 '2nd tier' genes, and at least a subset of these genes appears to be involved in transformation. Indeed, four genes were selected for knockdown, each resulting in a disruption of foci formation and/or invasion. These results underscore the role of aberrant autocrine signaling and transcriptional networking during tumorigenesis.

Human RAS superfamily proteins and related GTPases

The tumor oncoproteins HRAS, KRAS, and NRAS are the founding members of a larger family of at least 35 related human proteins. Using a somewhat broader definition of sequence similarity reveals a more extended superfamily of more than 170 RAS-related proteins. The RAS superfamily of GTP (guanosine triphosphate) hydrolysis-coupled signal transduction relay proteins can be subclassified into RAS, RHO, RAB, and ARF families, as well as the closely related Galpha family. The members of each family can, in turn, be arranged into evolutionarily conserved branches. These groupings reflect structural, biochemical, and functional conservation. Recent findings have provided insights into the signaling characteristics of representative members of most RAS superfamily branches. The analysis presented here may serve as a guide for predicting the function of numerous uncharacterized superfamily members. Also described are guanosine triphosphatases (GTPases) distinct from members of the RAS superfamily. These related proteins employ GTP binding and GTPase domains in diverse structural contexts, expanding the scope of their function in humans.

Signaling specificity by Ras family GTPases is determined by the full spectrum of effectors they regulate

Ras family GTPases (RFGs) regulate signaling pathways that control multiple biological processes. How signaling specificity among the closely related family members is achieved is poorly understood. We have taken a proteomics approach to signaling by RFGs, and we have analyzed interactions of a panel of RFGs with a comprehensive group of known and potential effectors. We have found remarkable differences in the ability of RFGs to regulate the various isoforms of known effector families. We have also identified several proteins as novel effectors of RFGs with differential binding specificities to the various RFGs. We propose that specificity among RFGs is achieved by the differential regulation of combinations of effector families as well as by the selective regulation of different isoforms within an effector family. An understanding of this new level of complexity in the signaling pathways regulated by RFGs is necessary to understand how they carry out their many cellular functions. It will also likely have critical implications in the treatment of human diseases such as cancer.

Small GTPase Rin induces neurite outgrowth through Rac/Cdc42 and calmodulin in PC12 cells

The novel Ras-like small GTPase Rin is expressed prominently in adult neurons, and binds calmodulin (CaM) through its COOH-terminal–binding motif. It might be involved in calcium/CaM-mediated neuronal signaling, but Rin-mediated signal transduction pathways have not yet been elucidated. Here, we show that expression of Rin induces neurite outgrowth without nerve growth factor or mitogen-activated protein kinase activation in rat pheochromocytoma PC12 cells. Rin-induced neurite outgrowth was markedly inhibited by coexpression with dominant negative Rac/Cdc42 protein or CaM inhibitor treatment. We also found that expression of Rin elevated the endogenous Rac/Cdc42 activity. Rin mutant proteins, in which the mutation disrupted association with CaM, failed to induce neurite outgrowth irrespective of Rac/Cdc42 activation. Disruption of endogenous Rin function inhibited the neurite outgrowth stimulated by forskolin and extracellular calcium entry through voltage-dependent calcium channel evoked by KCl. These findings suggest that Rin-mediated neurite outgrowth signaling requires not only endogenous Rac/Cdc42 activation but also Rin–CaM association, and that endogenous Rin is involved in calcium/CaM-mediated neuronal signaling pathways.

The dark side of Ras: regulation of apoptosis

Mutational activation of Ras promotes oncogenesis by disrupting a multitude of normal cellular processes. Perhaps, best characterized and understood are the mechanisms by which oncogenic Ras promotes deregulated cell cycle progression and uncontrolled cellular proliferation. However, it is now clear that oncogenic Ras can also deregulate processes that control apoptosis. In light of the diversity of downstream effector targets known to facilitate Ras function, it is perhaps not surprising that Ras regulation of cell survival is complex, involving the balance and interplay of multiple signaling networks. While our understanding of these events is still far from complete, and is complicated by cell type and signaling context differences, several important mechanisms have begun to emerge. We review the role and mechanism of specific effectors in regulating the antiapoptotic (Raf, phosphatidylinositol 3-kinase and Tiam1) and apoptotic (Nore1 and RASSF1) actions of oncogenic Ras, and discuss the possibility that the effector actions of p120RasGAP make a significant contribution to Ras regulation of apoptotic events.

Functional interaction between the small GTP-binding protein Rin and the N-terminal of Brn-3a transcription factor

Brn-3a is a transcription factor belonging to the class IV of POU domain transcription factors. It is expressed throughout the peripheral nervous system but especially in postmitotic sensory neurons of dorsal root ganglia. Brn-3a is known to regulate different genes involved in neuronal differentiation and survival. It has been shown that some of these genes require the N-terminal domain of Brn-3a in order to be activated and this effect is observed only in neurons suggesting that it may require a neuronal-specific cofactor. In order to identify this putative factor(s) we screened a cDNA library via a variant of the original yeast two-hybrid system. By using the N-terminal of Brn-3a as the bait, we have repeatedly isolated a protein named Rin, an incompletely characterized small GTP-binding protein expressed only in neurons. In this work, we describe the evidence for a functional interaction between Brn-3a and Rin and demonstrate the role of Rin in modulating the activation of the Brn-3a regulated egr-1 promoter by the N-terminal domain of Brn-3a.

Rit promotes MEK-independent neurite branching in human neuroblastoma cells

Rit, by sequence homology, is a member of the Ras subfamily of small guanine triphosphatases (GTPases). In PC6 cells, Rit signals through pathways both common to and different from those activated by Ras to promote cell survival and neurite outgrowth. However, the specific morphological changes induced by Rit in human cells are not known. Here, we show in a human neuronal model that Rit increases neurite outgrowth and branching through MEK-dependent and MEK-independent signaling mechanisms, respectively. Adenoviral expression of wild-type or constitutively active Rit increased neurite initiation, elongation and branching on endogenous matrix or a purified laminin-1 substratum of SH-SY5Y cells as assessed using image analysis. This outgrowth was morphologically distinct from that promoted by constitutively active Ras or Raf (evidenced by increased branching and elongation). Constitutively active Rit increased phosphorylation of ERK 1/2, but not Akt, and the MEK inhibitor PD 098059 blocked constitutively active Rit-induced neurite initiation but not elongation or branching. These results suggest that Rit plays a key role in human neuronal development and regeneration through activating both known and as yet undefined signaling pathways.

Induction of neurite extension and survival in pheochromocytoma cells by the Rit GTPase

The Rit, Rin, and Ric proteins comprise a distinct and evolutionarily conserved subfamily of the Ras-like small G-proteins. Although these proteins share the majority of core effector domain residues with Ras, recent studies suggest that Rit uses novel effector pathways to regulate NIH3T3 cell proliferation and transformation, while the functions of Rin and Ric remain largely unknown. Since we demonstrate that Rit is expressed in neurons, we investigated the role of Rit signaling in promoting the differentiation and survival of pheochromocytoma cells. In this study, we show that expression of constitutively active Rit (RitL79) in PC6 cells results in neuronal differentiation, characterized by the elaboration of an extensive network of neurite-like processes that are morphologically distinct from those mediated by the expression of oncogenic Ras. Although activated Rit fails to stimulate mitogen-activated protein kinase/extracellular-signal-regulated kinase (MAPK/ERK) signaling pathways in COS cells, RitL79 induced the phosphorylation of ERK1/2 in PC6 cells. We also find that Rit-mediated effects on neurite outgrowth can be blocked by co-expression of dominant-negative mutants of C-Raf1 or mitogen-activated protein kinase kinase 1 (MEK1). Moreover, expression of dominant-negative Rit is sufficient to inhibit NGF-induced neurite outgrowth. Expression of active Rit inhibits growth factor-withdrawal mediated apoptosis of PC6 cells, but does not induce phosphorylation of Akt/protein kinase B, suggesting that survival does not utilize the phosphatidylinositol 3-kinase (PI3K)/Akt pathway. Instead, pharmacological inhibitors of MEK block Rit-stimulated cell survival. Taken together, these studies suggest that Rit represents a distinct regulatory protein, capable of mediating differentiation and cell survival in PC6 cells using a MEK-dependent signaling pathway to achieve its effects.

Nerve growth factor-dependent activation of the small GTPase Rin

The Rit and Rin proteins comprise a distinct and evolutionarily conserved subfamily of Ras-related small GTPases. Although we have defined a role for Rit-mediated signal transduction in the regulation of cell proliferation and transformation, the function of Rin remains largely unknown. Because we demonstrate that Rin is developmentally regulated and expressed in adult neurons, we examined its role in neuronal signaling. In this study, we show that stimulation of PC6 cells with either epidermal growth factor or nerve growth factor (NGF) results in rapid activation of Rin. This activation correlates with the onset of Ras activation, and dominant-negative Ras completely inhibits Rin activation induced by NGF. Further examination of Ras-mediated Rin activation suggests that this process is dependent upon neuronally expressed regulatory factors. Expression of mutationally activated H-Ras fails to activate Rin in non-neuronal cells, but results in potent stimulation of Rin-GTP levels in a variety of neuronal cell lines. Furthermore, although constitutively activated Rin does not induce neurite outgrowth on its own, both NGF-induced and oncogenic Ras-induced neurite outgrowth were inhibited by the expression of dominant-negative Rin. Together, these studies indicate that Rin activation is a direct downstream effect of growth factor-dependent signaling in neuronal cells and suggest that Rin may function to transduce signals within the mature nervous system.

Involvement of phosphatidylinositol 3-kinase, but not RalGDS, in TC21/R-Ras2-mediated transformation

Oncogenic Ras and activated forms of the Ras-related protein TC21/R-Ras2 share similar abilities to alter cell proliferation. However, in contrast to Ras, we found previously that TC21 fails to activate the Raf-1 serine/threonine kinase. Thus, TC21 must utilize non-Raf effectors to regulate cell function. In this study, we determined that TC21 interacts strongly with some (RalGDS, RGL, RGL2/Rlf, AF6, and the phosphatidylinositol 3-kinase (PI3K) catalytic subunit p110delta), and weakly with other Ras small middle dotGTP-binding proteins. In addition, library screening identified novel TC21-interacting proteins. We also determined that TC21, similar to Ras, mediates activation of phospholipase Cepsilon. We then examined if RalGDS, a RalA guanine nucleotide exchange factor, or PI3K are effectors for TC21-mediated signaling and cell proliferation in murine fibroblasts. We found that overexpression of full-length RalGDS reduced the focus forming activity of activated TC21. Furthermore, expression of activated Ras, but not TC21, enhanced GTP loading on RalA. In fact, TC21 attenuated insulin-stimulated RalA small middle dotGTP formation. In contrast, like Ras, expression of activated TC21 resulted in membrane translocation and an increase in the PI3K-dependent phosphorylation of Akt, and inhibition of PI3K activity interfered with TC21 focus formation. Finally, unlike Ras, TC21 did not activate the Rac small GTPase, indicating that Ras may not activate Rac by PI3K. Taken together, these results suggest that PI3K, but not RalGDS, is an important mediator of cell proliferation by TC21.

Potent transforming activity of the small GTP-binding protein Rit in NIH 3T3 cells: evidence for a role of a p38gamma-dependent signaling pathway

A novel branch of the Ras family, Rit, was recently identified. Rit exhibits a distinct C-terminus and effector domain, and does not activate mitogen-activated protein kinase (MAPK) but can cooperate with Raf to transform fibroblasts. Here, we found that when overexpressed, activated mutants of Rit transform NIH 3T3 cells efficiently, and stimulate p38gamma but not MAPK, p38alpha, p38gamma, p38delta, or ERK5. Furthermore, we provide evidence that p38gamma activation is required for the ability of Rit to stimulate gene expression and cellular transformation. These findings suggest that this unique GTPase stimulates proliferative pathways distinct from those regulated by other Ras family members.

RERG is a novel ras-related, estrogen-regulated and growth-inhibitory gene in breast cancer

Using microarray analysis, we identified a unique ras superfamily gene, termed RERG (ras-related and estrogen-regulated growth inhibitor), whose expression was decreased or lost in a significant percentage of primary human breast tumors that show a poor clinical prognosis. Importantly, high RERG expression correlated with expression of a set of genes that define a breast tumor subtype that is estrogen receptor-positive and associated with a slow rate of tumor cell proliferation and a favorable prognosis for these cancer patients. RERG mRNA expression was induced rapidly in MCF-7 cells stimulated by beta-estradiol and repressed by tamoxifen treatment. Like Ras, RERG protein exhibited intrinsic GDP/GTP binding and GTP hydrolysis activity. Unlike Ras proteins, RERG lacks a known recognition signal for COOH-terminal prenylation and was localized primarily in the cytoplasm. Expression of RERG protein in MCF-7 breast carcinoma cells resulted in a significant inhibition of both anchorage-dependent and anchorage-independent growth in vitro and inhibited tumor formation in nude mice. These features of RERG are strikingly different from most Ras superfamily GTP-binding pro-teins and suggest that the loss of RERG expression may contribute to breast tumorigenesis.