Serum-dependent transcriptional networks identify distinct functional roles for H-Ras and N-Ras during initial stages of the cell cycle

Combined HRAS and NRAS ablation induces a RASopathy phenotype in mice

BACKGROUND

HRASKO/NRASKO double knockout mice exhibit exceedingly high rates of perinatal lethality due to respiratory failure caused by a significant lung maturation delay. The few animals that reach adulthood have a normal lifespan, but present areas of atelectasis mixed with patches of emphysema and normal tissue in the lung.

METHODS

Eight double knockout and eight control mice were analyzed using micro-X-ray computerized tomography and a Small Animal Physiological Monitoring system. Tissues and samples from these mice were analyzed using standard histological and Molecular Biology methods and the significance of the results analyzed using a Student´s T-test.

RESULTS

The very few double knockout mice surviving up to adulthood display clear craniofacial abnormalities reminiscent of those seen in RASopathy mouse models, as well as thrombocytopenia, bleeding anomalies, and reduced platelet activation induced by thrombin. These surviving mice also present heart and spleen hyperplasia, and elevated numbers of myeloid-derived suppressor cells in the spleen. Mechanistically, we observed that these phenotypic alterations are accompanied by increased KRAS-GTP levels in heart, platelets and primary mouse embryonic fibroblasts from these animals.

CONCLUSIONS

Our data uncovers a new, previously unidentified mechanism capable of triggering a RASopathy phenotype in mice as a result of the combined removal of HRAS and NRAS.

Concomitant deletion of HRAS and NRAS leads to pulmonary immaturity, respiratory failure and neonatal death in mice

We reported previously that adult (HRAS-/-; NRAS-/-) double knockout (DKO) mice showed no obvious external phenotype although lower-than-expected numbers of weaned DKO animals were consistently tallied after crossing NRAS-KO and HRAS-KO mice kept on mixed genetic backgrounds. Using mouse strains kept on pure C57Bl/6 background, here we performed an extensive analysis of the offspring from crosses between HRAS-KO and NRAS-KO mice and uncovered the occurrence of very high rates of perinatal mortality of the resulting DKO littermates due to respiratory failure during the first postnatal 24-48 h. The lungs of newborn DKO mice showed normal organ structure and branching but displayed marked defects of maturation including much-reduced alveolar space with thick separating septa and significant alterations of differentiation of alveolar (AT1, AT2 pneumocytes) and bronchiolar (ciliated, Clara cells) cell lineages. We also observed the retention of significantly increased numbers of undifferentiated progenitor precursor cells in distal lung epithelia and the presence of substantial accumulations of periodic acid-Schiff-positive (PAS+) material and ceramide in the lung airways of newborn DKO mice. Interestingly, antenatal dexamethasone treatment partially mitigated the defective lung maturation phenotypes and extended the lifespan of the DKO animals up to 6 days, but was not sufficient to abrogate lethality in these mice. RNA microarray hybridization analyses of the lungs of dexamethasone-treated and untreated mice uncovered transcriptional changes pointing to functional and metabolic alterations that may be mechanistically relevant for the defective lung phenotypes observed in DKO mice. Our data suggest that delayed alveolar differentiation, altered sphingolipid metabolism and ceramide accumulation are primary contributors to the respiratory stress and neonatal lethality shown by DKO mice and uncover specific, critical roles of HRAS and NRAS for correct lung differentiation that are essential for neonatal survival and cannot be substituted by the remaining KRAS function in this organ.

Critical role of Myc activation in mouse hepatocarcinogenesis induced by the activation of AKT and RAS pathways

MYC activation at modest levels has been frequently found in hepatocellular carcinoma. However, its significance in hepatocarcinogenesis has remained obscure. Here we examined the role of Myc activation in mouse liver tumours induced by hepatocytic expression of myristoylated AKT (AKT) and/or mutant HRAS^V12 (HRAS) via transposon-mediated gene integration. AKT or HRAS alone required 5 months to induce liver tumours, whereas their combination generated hepatocellular carcinoma within 8 weeks. Co-introduction of AKT and HRAS induced lipid-laden preneoplastic cells that grew into nodules composed of tumour cells with or without intracytoplasmic lipid, with the latter being more proliferative and associated with spontaneous Myc expression. AKT/HRAS-induced tumorigenesis was almost completely abolished when MadMyc, a competitive Myc inhibitor, was expressed simultaneously. The Tet-On induction of MadMyc in preneoplastic cells significantly inhibited the progression of AKT/HRAS-induced tumours; its induction in transformed cells suppressed their proliferative activity with alterations in lipid metabolism and protein translation. Transposon-mediated Myc overexpression facilitated tumorigenesis by AKT or HRAS, and when it was co-introduced with AKT and HRAS, diffusely infiltrating tumours without lipid accumulation developed as early as 2 weeks. Examination of the dose-responses of Myc in the enhancement of AKT/HRAS-induced tumorigenesis revealed that a reduction to one-third retained enhancing effect but three-times greater introduction damped the process with increased apoptosis. Myc overexpression suppressed the mRNA expression of proteins involved in the synthesis of fatty acids, and when combined with HRAS introduction, it also suppressed the mRNA expression of proteins involved in their degradation. Finally, the MYC-positive human hepatocellular carcinoma was characterized by the cytoplasm devoid of lipid accumulation, prominent nucleoli and a higher proliferative activity. Our results demonstrate that in hepatocarcinogenesis induced by both activated AKT and HRAS, activation of endogenous Myc is an enhancing factor and adequate levels of Myc deregulation further facilitate the process with alterations in cellular metabolism.

Sos1 disruption impairs cellular proliferation and viability through an increase in mitochondrial oxidative stress in primary MEFs

Using a 4-hydroxytamoxifen (4OHT)-inducible, conditional Sos1-null mutation, we analyzed wild-type (WT), single Sos1-KO, Sos2-KO and double Sos1/2 KO primary mouse embryonic fibroblasts (MEF) with an aim at evaluating the functional specificity or redundancy of the Sos1 and Sos2 alleles at the cellular level. The 4OHT-induced Sos1-KO and Sos1/2-DKO MEFs exhibited distinct flat morphology, enlarged cell perimeter and altered cytoskeletal organization that were not observed in the WT and Sos2-KO counterparts. The Sos1-KO and Sos1/2-DKO MEFs also displayed significant accumulation, in comparison with WT and Sos2-KO MEFs, of cytoplasmic vesicular bodies identified as autophagosomes containing degraded mitochondria by means of electron microscopy and specific markers. Cellular proliferation and migration were impaired in Sos1-KO and Sos1/2-DKO MEFs in comparison with WT and Sos2-KO MEFs, whereas cell adhesion was only impaired upon depletion of both Sos isoforms. RasGTP formation was practically absent in Sos1/2-DKO MEFs as compared with the other genotypes and extracellular signal-regulated kinase phosphorylation showed only significant reduction after combined Sos1/2 depletion. Consistent with a mitophagic phenotype, in vivo labeling with specific fluorophores uncovered increased levels of oxidative stress (elevated intracellular reactive oxygen species and mitochondrial superoxide and loss of mitochondrial membrane potential) in the Sos1-KO and the Sos1/2-DKO cells as compared with Sos2-KO and WT MEFs. Interestingly, treatment of the MEF cultures with antioxidants corrected the altered phenotypes of Sos1-KO and Sos1/2-DKO MEFs by restoring their altered perimeter size and proliferative rate to levels similar to those of WT and Sos2-KO MEFs. Our data uncover a direct mechanistic link between Sos1 and control of intracellular oxidative stress, and demonstrate functional prevalence of Sos1 over Sos2 with regards to cellular proliferation and viability.

Absence of K-Ras Reduces Proliferation and Migration But Increases Extracellular Matrix Synthesis in Fibroblasts

The involvement of Ras-GTPases in the development of renal fibrosis has been addressed in the last decade. We have previously shown that H- and N-Ras isoforms participate in the regulation of fibrosis. Herein, we assessed the role of K-Ras in cellular processes involved in the development of fibrosis: proliferation, migration, and extracellular matrix (ECM) proteins synthesis. K-Ras knockout (KO) mouse embryonic fibroblasts (K-ras(-/-) ) stimulated with transforming growth factor-β1 (TGF-β1) exhibited reduced proliferation and impaired mobility than wild-type fibroblasts. Moreover, an increase on ECM production was observed in K-Ras KO fibroblasts in basal conditions. The absence of K-Ras was accompanied by reduced Ras activation and ERK phosphorylation, and increased AKT phosphorylation, but no differences were observed in TGF-β1-induced Smad signaling. The MEK inhibitor U0126 decreased cell proliferation independently of the presence of K-ras but reduced migration and ECM proteins expression only in wild-type fibroblasts, while the PI3K-AKT inhibitor LY294002 decreased cell proliferation, migration, and ECM synthesis in both types of fibroblasts. Thus, our data unveil that K-Ras and its downstream effector pathways distinctively regulate key biological processes in the development of fibrosis. Moreover, we show that K-Ras may be a crucial mediator in TGF-β1-mediated effects in this cell type. J. Cell. Physiol. 231: 2224-2235, 2016. © 2016 Wiley Periodicals, Inc.

Aspafilioside B induces G2/M cell cycle arrest and apoptosis by up-regulating H-Ras and N-Ras via ERK and p38 MAPK signaling pathways in human hepatoma HepG2 cells

We recently establish that aspafilioside B, a steroidal saponin extracted from Asparagus filicinus, is an active cytotoxic component. However, its antitumor activity is till unknown. In this study, the anticancer effect of aspafilioside B against HCC cells and the underlying mechanisms were investigated. Our results showed that aspafilioside B inhibited the growth and proliferation of HCC cell lines. Further study revealed that aspafilioside B could significantly induce G2 phase cell cycle arrest and apoptosis, accompanying the accumulation of reactive oxygen species (ROS), but blocking ROS generation with N-acetyl-l-cysteine (NAC) could not prevent G2/M arrest and apoptosis. Additionally, treatment with aspafilioside B induced phosphorylation of extracellular signal-regulated kinase (ERK) and p38 MAP kinase. Moreover, both ERK inhibitor PD98059 and p38 inhibitor SB203580 almost abolished the G2/M phase arrest and apoptosis induced by aspafilioside B, and reversed the expression of cell cycle- and apoptosis-related proteins. We also found that aspafilioside B treatment increased both Ras and Raf activation, and transfection of cells with H-Ras and N-Ras shRNA almost attenuated aspafilioside B-induced G2 phase arrest and apoptosis as well as the ERK and p38 activation. Finally, in vivo, aspafilioside B suppressed tumor growth in mouse xenograft models, and the mechanism was the same as in vitro study. Collectively, these findings indicated that aspafilioside B may up-regulate H-Ras and N-Ras, causing c-Raf phosphorylation, and lead to ERK and p38 activation, which consequently induced the G2 phase arrest and apoptosis. This study provides the evidence that aspafilioside B is a promising therapeutic agent against HCC.

Reversible, interrelated mRNA and miRNA expression patterns in the transcriptome of Rasless fibroblasts: functional and mechanistic implications

Background

4-Hydroxy-tamoxifen (4OHT) triggers Cre-mediated K-Ras removal in [H-Ras^-/-;N-Ras^-/-;K-Ras^lox/lox;RERT^ert/ert] fibroblasts, generating growth-arrested “Rasless” MEFs which are able to recover their proliferative ability after ectopic expression of Ras oncoproteins or constitutively active BRAF or MEK1.

Results

Comparison of the transcriptional profiles of Rasless fibroblasts with those of MEFs lacking only H-Ras and N-Ras identified a series of differentially expressed mRNAs and microRNAs specifically linked to the disappearance of K-Ras from these cells. The rescue of cell cycle progression in Rasless cells by activated BRAF or MEK1 resulted in the reversal of most such transcriptional mRNA and microRNA alterations.

Functional analysis of the differentially expressed mRNAs uncovered a significant enrichment in the components of pathways regulating cell division, DNA/RNA processing and response to DNA damage. Consistent with G1/S blockade, Rasless cells displayed repression of a series of cell cycle-related genes, including Cyclins, Cyclin-dependent kinases, Myc and E2F transcription targets, and upregulation of Cyclin-dependent kinase inhibitors. The profile of differentially expressed microRNAs included a specific set of oncomiR families and clusters (repressed miR-17 ~ 92, miR-106a ~ 363, miR-106b ~ 25, miR-212 ~ 132, miR-183 ~ 182, and upregulated miR-335) known for their ability to target a specific set of cellular regulators and checkpoint sensors (including Rb, E2F and Cdkns) able to modulate the interplay between the pro- and anti-proliferative or stress-response pathways that are reversibly altered in Rasless cells.

Conclusions

Our data suggest that the reversible proliferation phenotype of Rasless cells is the pleiotropic result of interplay among distinct pro- and anti-proliferative, and stress-response pathways modulated by a regulatory circuitry constituted by a specific set of differentially expressed mRNAs and microRNAs and preferentially targeting two cross-talking signalling axes: Myc-Rb-E2F-dependent and Cdkns-p53-dependent pathways.

Functional redundancy of Sos1 and Sos2 for lymphopoiesis and organismal homeostasis and survival

Sos1 and Sos2 are ubiquitously expressed, universal Ras guanine nucleotide exchange factors (Ras-GEFs) acting in multiple signal transduction pathways activated by upstream cellular kinases. The embryonic lethality of Sos1 null mutants has hampered ascertaining the specific in vivo contributions of Sos1 and Sos2 to processes controlling adult organism survival or development of hematopoietic and nonhematopoietic organs, tissues, and cell lineages. Here, we generated a tamoxifen-inducible Sos1-null mouse strain allowing analysis of the combined disruption of Sos1 and Sos2 (Sos1/2) during adulthood. Sos1/2 double-knockout (DKO) animals died precipitously, whereas individual Sos1 and Sos2 knockout (KO) mice were perfectly viable. A reduced percentage of total bone marrow precursors occurred in single-KO animals, but a dramatic depletion of B-cell progenitors was specifically detected in Sos1/2 DKO mice. We also confirmed a dominant role of Sos1 over Sos2 in early thymocyte maturation, with almost complete thymus disappearance and dramatically higher reduction of absolute thymocyte counts in Sos1/2 DKO animals. Absolute counts of mature B and T cells in spleen and peripheral blood were unchanged in single-KO mutants, while significantly reduced in Sos1/2 DKO mice. Our data demonstrate functional redundancy between Sos1 and Sos2 for homeostasis and survival of the full organism and for development and maturation of T and B lymphocytes.

Signal transduction pathways in chronic inflammatory autoimmune disease: small GTPases

Ras superfamily small GTPases represent a wide and diverse class of intracellular signaling proteins that are highly conserved during evolution. These enzymes serve as key checkpoints in coupling antigen receptor, growth factor, cytokine and chemokine stimulation to cellular responses. Once activated, via their ability to regulate multiple downstream signaling pathways, small GTPases amplify and diversify signaling cascades which regulate cellular proliferation, survival, cytokine expression, trafficking and retention. Small GTPases, particularly members of the Ras, Rap, and Rho family, critically coordinate the function and interplay of immune and stromal cells during inflammatory respones, and increasing evidence indicates that alterations in small GTPase signaling contribute to the pathological behavior of these cell populations in human chronic inflammatory diseases such as rheumatoid arthritis (RA) and systemic lupus erythematosus (SLE). Here, we review how Ras, Rap, and Rho family GTPases contribute to the biology of cell populations relevant to human chronic inflammatory disease, highlight recent advances in understanding how alterations in these pathways contribute to pathology in RA and SLE, and discuss new therapeutic strategies that may allow specific targeting of small GTPases in the clinic.

Functional specificity of ras isoforms: so similar but so different

H-ras, N-ras, and K-ras are canonical ras gene family members frequently activated by point mutation in human cancers and coding for 4 different, highly related protein isoforms (H-Ras, N-Ras, K-Ras4A, and K-Ras4B). Their expression is nearly ubiquitous and broadly conserved across eukaryotic species, although there are quantitative and qualitative differences of expression depending on the tissue and/or developmental stage under consideration. Extensive functional studies have determined during the last quarter century that these Ras gene products are critical components of signaling pathways that control eukaryotic cell proliferation, survival, and differentiation. However, because of their homology and frequent coexpression in various cellular contexts, it remained unclear whether the different Ras proteins play specific or overlapping functional roles in physiological and pathological processes. Initially, their high degree of sequence homology and the observation that all Ras isoforms share common sets of downstream effectors and upstream activators suggested that they were mostly redundant functionally. In contrast, the notion of functional specificity for each of the different Ras isoforms is supported at present by an increasing body of experimental observations, including 1) the fact that different ras isoforms are preferentially mutated in specific types of tumors or developmental disorders; 2) the different transforming potential of transfected ras genes in different cell contexts; 3) the distinct sensitivities exhibited by the various Ras family members for modulation by different GAPs or GEFs; 4) the demonstration that different Ras isoforms follow distinct intracellular processing pathways and localize to different membrane microdomains or subcellular compartments; 5) the different phenotypes displayed by genetically modified animal strains for each of the 3 ras loci; and 6) the specific transcriptional networks controlled by each isoform in different cellular settings.

MicroRNA destabilization enables dynamic regulation of the miR-16 family in response to cell-cycle changes

The miR-16 family, which targets genes important for the G1-S transition, is a known modulator of the cell cycle, and members of this family are often deleted or downregulated in many types of cancers. Here, we report the reciprocal relationship-that of the cell cycle controlling the miR-16 family. Levels of this family increase rapidly as cells are arrested in G0. Conversely, as cells are released from G0 arrest, levels of the miR-16 family rapidly decrease. Such rapid changes are made possible by the unusual instabilities of several family members. The repression mediated by the miR-16 family is sensitive to these cell-cycle changes, which suggests that the rapid upregulation of the miR-16 family reinforces cell-cycle arrest in G0. Upon cell-cycle re-entry, the rapid decay of several members allows levels of the family to decrease, alleviating repression of target genes and allowing proper resumption of the cell cycle.

cAMP-dependent protein kinase regulates post-translational processing and expression of complex I subunits in mammalian cells

Work is presented on the role of cAMP-dependent protein phosphorylation in post-translational processing and biosynthesis of complex I subunits in mammalian cell cultures. PKA-mediated phosphorylation of the NDUFS4 subunit of complex I promotes in cell cultures in vivo import/maturation in mitochondria of the precursor of this protein. The import promotion appears to be associated with the observed cAMP-dependent stimulation of the catalytic activity of complex I. These effects of PKA are counteracted by activation of protein phosphatase(s). PKA and the transcription factor CREB play a critical role in the biosynthesis of complex I subunits. CREB phosphorylation, by PKA and/or CaMKs, activates at nuclear and mitochondrial level a transcriptional regulatory cascade which promotes the concerted expression of nuclear and mitochondrial encoded subunits of complex I and other respiratory chain proteins.