Early prolonged prone position in noninvasively ventilated patients with SARS-CoV-2-related moderate-to-severe hypoxemic respiratory failure: clinical outcomes and mechanisms for treatment response in the PRO-NIV study.

BACKGROUND: Whether prone position (PP) improves clinical outcomes in COVID-19 pneumonia treated with noninvasive ventilation (NIV) is unknown. We evaluated the effect of early PP on 28-day NIV failure, intubation and death in noninvasively ventilated patients with moderate-to-severe acute hypoxemic respiratory failure due to COVID-19 pneumonia and explored physiological mechanisms underlying treatment response. METHODS: In this controlled non-randomized trial, 81 consecutive prospectively enrolled patients with COVID-19 pneumonia and moderate-to-severe (paO2/FiO2 ratio < 200) acute hypoxemic respiratory failure treated with early PP + NIV during Dec 2020-May 2021were compared with 162 consecutive patients with COVID-19 pneumonia matched for age, mortality risk, severity of illness and paO2/FiO2 ratio at admission, treated with conventional (supine) NIV during Apr 2020-Dec 2020 at HUMANITAS Gradenigo Subintensive Care Unit, after propensity score adjustment for multiple baseline and treatment-related variables to limit confounding. Lung ultrasonography (LUS) was performed at baseline and at day 5. Ventilatory parameters, physiological dead space indices (DSIs) and circulating inflammatory and procoagulative biomarkers were monitored during the initial 7 days. RESULTS: In the intention-to-treat analysis. NIV failure occurred in 14 (17%) of PP patients versus 70 (43%) of controls [HR = 0.32, 95% CI 0.21-0.50; p < 0.0001]; intubation in 8 (11%) of PP patients versus 44 (30%) of controls [HR = 0.31, 95% CI 0.18-0.55; p = 0.0012], death in 10 (12%) of PP patients versus 59 (36%) of controls [HR = 0.27, 95% CI 0.17-0.44; p < 0.0001]. The effect remained significant within different categories of severity of hypoxemia (paO2/FiO2 < 100 or paO2/FiO2 100-199 at admission). Adverse events were rare and evenly distributed. Compared with controls, PP therapy was associated with improved oxygenation and DSIs, reduced global LUS severity indices largely through enhanced reaeration of dorso-lateral lung regions, and an earlier decline in inflammatory markers and D-dimer. In multivariate analysis, day 1 CO2 response outperformed O2 response as a predictor of LUS changes, NIV failure, intubation and death. CONCLUSION: Early prolonged PP is safe and is associated with lower NIV failure, intubation and death rates in noninvasively ventilated patients with COVID-19-related moderate-to-severe hypoxemic respiratory failure. Early dead space reduction and reaeration of dorso-lateral lung regions predicted clinical outcomes in our study population. CLINICAL TRIAL REGISTRATION: ISRCTN23016116 . Retrospectively registered on May 1, 2021.

Diffusion-weighted quantitative MRI to diagnose benign conditions from malignancies of the anterior mediastinum: Improvement of diagnostic accuracy by comparing perfusion-free to perfusion-sensitive measurements of the apparent diffusion coefficient.

PURPOSE: To compare perfusion-free to perfusion-sensitive measurements of the apparent diffusion coefficient (ADC) to diagnose benign conditions from malignancies of the anterior mediastinum. MATERIALS AND METHODS: Seventy-six subjects were divided into a "benign conditions" group (A, n = 44) and a "malignancies" group (B, n = 32), based on histological findings. diffusion-weighted magnetic resonance imaging (DW-MRI) was performed at b of 0/150/800 sec/mm(2) . The ADCs were obtained on an ADC map by including (perfusion-sensitive = ADCb0-800 ) and excluding (perfusion-free = ADCb150-800 ) the b = 0 sec/mm(2) . The Mann-Whitney U-test was used to detect differences in ADCb0-800 compared with ADCb150-800 values between all cases, benign conditions, and malignancies. The same test was used to evaluate differences in ADCs between the two groups for each type of measurement (ADCb0-800 and ADCb150-800 ), and receiver-operating characteristic (ROC) curves were obtained to evaluate discrimination abilities with comparison of areas-under-ROC-curves (AUROC). Optimal cutpoints for discrimination between groups were determined by the Youden-Index with computation of accuracy. RESULTS: The median ADCb0-800 was significantly greater compared with ADCb150-800 for all cases (P = 0.0014), benign conditions (P = 0.0412), and malignancies (P = 0.0001). The median percentage of increase was 5.30% for group-A and 22.39% for group-B (P < 0.0001). AUROC of ADC in discriminating between groups was significantly greater for ADCb150-800 (0.932) compared with ADCb0-800 (0.831) (P = 0.001). The optimal cutpoint for distinction between groups was 1.52 × 10(-3) mm(2) /sec (sensitivity = 93.7%, specificity = 88.6%, accuracy = 90.8%) for ADCb150-800 and 1.75 × 10(-3) mm(2) /sec (sensitivity = 75.0%, specificity = 79.5%, accuracy = 77.6%) for ADCb0-800 . CONCLUSION: The use of perfusion-free ADC measurements significantly improves diagnostic accuracy of DW-MRI in differentiating benign conditions from malignancies of the anterior mediastinum. J. Magn. Reson. Imaging 2016;44:758-769.

A preclinical algorithm of soluble surrogate biomarkers that correlate with therapeutic inhibition of the MET oncogene in gastric tumors.

The MET oncogene is amplified in a fraction of human gastric carcinoma cell lines, with consequent overexpression and constitutive activation of the corresponding protein product, the Met tyrosine kinase receptor. This genetically driven hyperactivation of Met is necessary for cancer cell growth and survival, so that Met pharmacological blockade results in cell-cycle arrest or apoptosis (oncogene addiction). MET gene amplification also occurs in vivo in a number of human gastric carcinomas, and clinical trials are now ongoing to assess the therapeutic efficacy of Met inhibitors in this type of malignancy. The aim of our study was to identify a preclinical algorithm of soluble surrogate biomarkers indicative of response to Met inhibition in gastric tumors, as a potential tool to integrate imaging criteria during patient follow-up. We started from a survey of candidate molecules based on antibody proteomics and gene expression profiling; after ELISA validation and analytical quantification, four biomarkers were identified that appeared to be strongly and consistently modulated by Met inhibition in a panel of Met-addicted gastric carcinoma cell lines, but not in Met-independent cell lines. Pharmacologic blockade of Met using specific small-molecule inhibitors led to reduced secretion of IL-8, GROα and the soluble form of uPAR and to increased production of IL-6 both in vitro (in culture supernatants) and in vivo (in the plasma of xenografted mice). If confirmed in patients, this information might prove useful to monitor clinical response to Met-targeted therapies in MET-amplified gastric carcinomas.

Unleashing the Guardian: The Targetable BCR-ABL/HAUSP/PML/PTEN Network in Chronic Myeloid Leukemia.

The complete eradication of Chronic Myeloid Leukemia is still challenging even in the era of highly selective and potent BCR-ABL tyrosine kinase inhibitors (TKIs). The 'Achilles heel' of TKI-based CML therapy is the inability of TKI to effectively target CML stem cells. Several pathways have been described to induce TKI insensitiveness in quiescent CML stem cells. In this review, we will describe the BCR-ABL/HAUSP/PML/PTEN network, whose signaling mediators converge to regulate the function of the tumor suppressor PTEN. We will also highlight the pharmacological strategies to modulate PTEN functions in order to sustain CML stem cell eradication.

Therapeutic inhibition of USP7-PTEN network in chronic lymphocytic leukemia: a strategy to overcome TP53 mutated/deleted clones.

Chronic Lymphocytic Leukemia (CLL) is a lymphoproliferative disorder with either indolent or aggressive clinical course. Current treatment regiments have significantly improved the overall outcomes even if higher risk subgroups - those harboring TP53 mutations or deletions of the short arm of chromosome 17 (del17p) - remain highly challenging. In the present work, we identified USP7, a known de-ubiquitinase with multiple roles in cellular homeostasis, as a potential therapeutic target in CLL. We demonstrated that in primary CLL samples and in CLL cell lines USP7 is: i) over-expressed through a mechanism involving miR-338-3p and miR-181b deregulation; ii) functionally activated by Casein Kinase 2 (CK2), an upstream interactor known to be deregulated in CLL; iii) effectively targeted by the USP7 inhibitor P5091. Treatment of primary CLL samples and cell lines with P5091 induces cell growth arrest and apoptosis, through the restoration of PTEN nuclear pool, both in TP53-wild type and -null environment. Importantly, PTEN acts as the main tumor suppressive mediator along the USP7-PTEN axis in a p53 dispensable manner. In conclusion, we propose USP7 as a new druggable target in CLL.

The BCR-ABL/NF-κB signal transduction network: a long lasting relationship in Philadelphia positive Leukemias.

The Nuclear Factor-kappa B (NF-κB) family of transcription factors plays a key role in cancer pathogenesis due to the ability to promote cellular proliferation and survival, to induce resistance to chemotherapy and to mediate invasion and metastasis. NF-κB is recruited through different mechanisms involving either canonical (RelA/p50) or non-canonical pathways (RelB/p50 or RelB/p52), which transduce the signals originated from growth-factors, cytokines, oncogenic stress and DNA damage, bacterial and viral products or other stimuli. The pharmacological inhibition of the NF-κB pathway has clearly been associated with significant clinical activity in different cancers. Almost 20 years ago, NF-κB was described as an essential modulator of BCR-ABL signaling in Chronic Myeloid Leukemia and Philadelphia-positive Acute Lymphoblastic Leukemia. This review summarizes the role of NF-κB in BCR-ABL-mediated leukemogenesis and provides new insights on the long lasting BCR-ABL/NF-κB connection.

The Role of PTEN in Myeloid Malignancies.

PTEN deletion in the mouse and in the zebrafish highlights the essential role of this tumor suppressor in the development of myeloid malignancies, in particular acute myeloid leukemia and myeloproliferative disorders. In humans, extensive genetic sequences of myeloid malignancies did not reveal recurrent PTEN mutations and deletions. However, PTEN was shown to be functionally inactivated in several acute myeloid leukemia and chronic myeloid leukemia samples, through both post-trasductional modifications, changes in protein levels and cellular compartmentalization. Notably, non genomic inactivation of PTEN in myeloid malignancies could represent a challenging therapeutic opportunity for these diseases. Targeting those mechanisms that affect PTEN function could indeed promote PTEN reactivation with consequent cancer selective apoptosis induction. In this review we will describe the role of PTEN in the development of myeloid malignancies.

New alternative splicing BCR/ABL-OOF shows an oncogenic role by lack of inhibition of BCR GTPase activity and an increased of persistence of Rac activation in chronic myeloid leukemia.

In Chronic Myeloid Leukemia 80% of patients present alternative splice variants involving BCR exons 1, 13 or 14 and ABL exon 4, with a consequent impairment in the reading frame of the ABL gene. Therefore BCR/ABL fusion proteins (BCR/ABL-OOF) are characterized by an in-frame BCR portion followed by an amino acids sequence arising from the out of frame (OOF) reading of the ABL gene. The product of this new transcript contains the characteristic BCR domains while lacking the COOH-terminal Rho GTPase GAP domain. The present work aims to characterize the protein functionality in terms of cytoskeleton (re-)modelling, adhesion and activation of canonical oncogenic signalling pathways. Here, we show that BCR/ABL-OOF has a peculiar endosomal localization which affects EGF receptor activation and turnover. Moreover, we demonstrate that BCR/ABL-OOF expression leads to aberrant cellular adhesion due to the activation of Rac GTPase, increase in cellular proliferation, migration and survival. When overexpressed in a BCR/ABL positive cell line, BCR/ABL-OOF induces hyperactivation of Rac signaling axis offering a therapeutic window for Rac-targeted therapy. Our data support a critical role of BCR/ABL-OOF in leukemogenesis and identify a subset of patients that may benefit from Rac-targeted therapies.

Non genomic loss of function of tumor suppressors in CML: BCR-ABL promotes IκBα mediated p53 nuclear exclusion.

Tumor suppressor function can be modulated by subtle variation of expression levels, proper cellular compartmentalization and post-translational modifications, such as phosphorylation, acetylation and sumoylation. The non-genomic loss of function of tumor suppressors offers a challenging therapeutic opportunity. The reactivation of a tumor suppressor could indeed promote selective apoptosis of cancer cells without affecting normal cells. The identification of mechanisms that affect tumor suppressor functions is therefore essential. In this work, we show that BCR-ABL promotes the accumulation of the NFKBIA gene product, IκBα, in the cytosol through physical interaction and stabilization of the protein. Furthermore, BCR-ABL/IκBα complex acts as a scaffold protein favoring p53 nuclear exclusion. We therefore identify a novel BCR-ABL/IκBα/p53 network, whereby BCR-ABL functionally inactivates a key tumor suppressor.

Aberrant activation of ROS1 represents a new molecular defect in chronic myelomonocytic leukemia.

Chronic myelomonocytic leukemia (CMML) is a clonal disorder sharing features of myelodysplastic syndromes and chronic myeloproliferative neoplasms. Although rare chromosomal aberrations and point mutations are reported in CMML, the molecular defects underlying CMML are largely unknown. ROS1 encodes a tyrosine kinase that is abnormally expressed and translocated in brain and lung cancers. In this study we show that ROS1 is abnormally activated in the CD34+ compartment of approximately 70% of CMML patients resulting in the activation of the Erk/Akt pathways through the Grb2/SOS complex thus revealing a central oncogenic role for ROS1 in CMML which might represent a molecular target.

Inhibition of MEK and PI3K/mTOR suppresses tumor growth but does not cause tumor regression in patient-derived xenografts of RAS-mutant colorectal carcinomas.

PURPOSE: Gene mutations along the Ras pathway (KRAS, NRAS, BRAF, PIK3CA) occur in approximately 50% of colorectal cancers (CRC) and correlate with poor response to anti-EGF receptor (EGFR) therapies. We assessed the effects of mitogen-activated protein (MAP)/extracellular signal-regulated kinase (ERK) kinase (MEK) and phosphoinositide 3-kinase (PI3K)/mTOR inhibitors, which neutralize the major Ras effectors, in patient-derived xenografts from RAS/RAF/PIK3CA-mutant metastatic CRCs (mCRC). EXPERIMENTAL DESIGN: Forty mCRC specimens harboring KRAS, NRAS, BRAF, and/or PIK3CA mutations were implanted in nonobese diabetic/severe combined immunodeficient (NOD/SCID) mice. Each xenograft was expanded into four treatment arms: placebo, the MEK inhibitor AZD6244, the PI3K/mTOR inhibitor, BEZ235, or AZD6244 + BEZ235. Cases initially treated with placebo crossed over to AZD6244, BEZ235, and the anti-EGFR monoclonal antibody cetuximab. RESULTS: At the 3-week evaluation time point, cotreatment of established tumors with AZD6244 + BEZ235 induced disease stabilization in the majority of cases (70%) but did not lead to overt tumor regression. Monotherapy was less effective, with BEZ235 displaying higher activity than AZD6244 (disease control rates, DCRs: AZD6244, 27.5%; BEZ235, 42.5%). Triple therapy with cetuximab provided further advantage (DCR, 88%). The extent of disease control declined at the 6-week evaluation time point (DCRs: AZD6244, 13.9%; BEZ235, 16.2%; AZD6244 + BEZ235, 34%). Cross-analysis of mice harboring xenografts from the same original tumor and treated with each of the different modalities revealed subgroups with preferential sensitivity to AZD6244 (12.5%), BEZ235 (35%), or AZD6244 + BEZ235 (42.5%); another subgroup (10%) showed equivalent response to any treatment. CONCLUSIONS: The prevalent growth-suppressive effects produced by MEK and PI3K/mTOR inhibition suggest that this strategy may retard disease progression in patients. However, data offer cautionary evidence against the occurrence of durable responses.

Oncogene addiction as a foundational rationale for targeted anti-cancer therapy: promises and perils.

A decade has elapsed since the concept of oncogene addiction was first proposed. It postulates that - despite the diverse array of genetic lesions typical of cancer - some tumours rely on one single dominant oncogene for growth and survival, so that inhibition of this specific oncogene is sufficient to halt the neoplastic phenotype. A large amount of evidence has proven the pervasive power of this notion, both in basic research and in therapeutic applications. However, in the face of such a considerable body of knowledge, the intimate molecular mechanisms mediating this phenomenon remain elusive. At the clinical level, successful translation of the oncogene addiction model into the rational and effective design of targeted therapeutics against individual oncoproteins still faces major obstacles, mainly due to the emergence of escape mechanisms and drug resistance. Here, we offer an overview of the relevant literature, encompassing both biological aspects and recent clinical insights. We discuss the key advantages and pitfalls of this concept and reconsider it as an illustrative principle to guide post-genomic cancer research and drug development.

Genetic and expression analysis of MET, MACC1, and HGF in metastatic colorectal cancer: response to met inhibition in patient xenografts and pathologic correlations.

PURPOSE: We determined the gene copy numbers for MET, for its transcriptional activator MACC1 and for its ligand hepatocyte growth factor (HGF) in liver metastases from colorectal carcinoma (mCRC). We correlated copy numbers with mRNA levels and explored whether gain and/or overexpression of MET and MACC1 predict response to anti-Met therapies. Finally, we assessed whether their genomic or transcriptional deregulation correlates with pathologic and molecular parameters of aggressive disease. EXPERIMENTAL DESIGN: One hundred three mCRCs were analyzed. Copy numbers and mRNA were determined by quantitative PCR (qPCR). Thirty nine samples were implanted and expanded in NOD (nonobese diabetic)/SCID (severe combined immunodeficient) mice to generate cohorts that were treated with the Met inhibitor JNJ-38877605. In silico analysis of MACC1 targets relied on genome-wide mapping of promoter regions and on expression data from two CRC datasets. RESULTS: No focal, high-grade amplifications of MET, MACC1, or HGF were detected. Chromosome 7 polysomy and gain of the p-arm were observed in 21% and 8% of cases, respectively, and significantly correlated with higher expression of both Met and MACC1. Met inhibition in patient-derived xenografts did not modify tumor growth. Copy number gain and overexpression of MACC1 correlated with unfavorable pathologic features better than overexpression of Met. Bioinformatic analysis of putative MACC1 targets identified elements besides Met, whose overexpression cosegregated with aggressive forms of colorectal cancer. CONCLUSIONS: Experiments in patient-derived xenografts suggest that mCRCs do not rely on Met genomic gain and/or overexpression for growth. On the basis of pathologic correlations and bioinformatic analysis, MACC1 could contribute to CRC progression through mechanisms other than or additional to Met transcriptional upregulation.

A molecularly annotated platform of patient-derived xenografts ("xenopatients") identifies HER2 as an effective therapeutic target in cetuximab-resistant colorectal cancer.

UNLABELLED: Only a fraction of patients with metastatic colorectal cancer receive clinical benefit from therapy with anti-epidermal growth factor receptor (EGFR) antibodies, which calls for the identification of novel biomarkers for better personalized medicine. We produced large xenograft cohorts from 85 patient-derived, genetically characterized metastatic colorectal cancer samples ("xenopatients") to discover novel determinants of therapeutic response and new oncoprotein targets. Serially passaged tumors retained the morphologic and genomic features of their original counterparts. A validation trial confirmed the robustness of this approach: xenopatients responded to the anti-EGFR antibody cetuximab with rates and extents analogous to those observed in the clinic and could be prospectively stratified as responders or nonresponders on the basis of several predictive biomarkers. Genotype-response correlations indicated HER2 amplification specifically in a subset of cetuximab-resistant, KRAS/NRAS/BRAF/PIK3CA wild-type cases. Importantly, HER2 amplification was also enriched in clinically nonresponsive KRAS wild-type patients. A proof-of-concept, multiarm study in HER2-amplified xenopatients revealed that the combined inhibition of HER2 and EGFR induced overt, long-lasting tumor regression. Our results suggest promising therapeutic opportunities in cetuximab-resistant patients with metastatic colorectal cancer, whose medical treatment in the chemorefractory setting remains an unmet clinical need. SIGNIFICANCE: Direct transfer xenografts of tumor surgical specimens conserve the interindividual diversity and the genetic heterogeneity typical of the tumors of origin, combining the flexibility of preclinical analysis with the informative value of population-based studies. Our suite of patient-derived xenografts from metastatic colorectal carcinomas reliably mimicked disease response in humans, prospectively recapitulated biomarker-based case stratification, and identified HER2 as a predictor of resistance to anti-epidermal growth factor receptor antibodies and of response to combination therapies against HER2 and epidermal growth factor receptor in this tumor setting.

Inhibition of Src impairs the growth of met-addicted gastric tumors.

PURPOSE: We examined whether inhibition of Src tyrosine kinase, a downstream effector of the MET oncogene, can hinder the malignant properties of gastric tumors dependent on Met for growth and survival. EXPERIMENTAL DESIGN: Sensitivity to Src inhibition was determined in vitro by measuring clonogenic survival (anchorage-independent growth) and in vivo by establishing xenograft models. Four "Met-addicted" gastric carcinoma cell lines (GTL16, MKN45, HS746T, and SNU5) and three Met-independent gastric carcinoma cell lines (KATO III, AGS, and NCI-N87) were treated with the Src inhibitor saracatinib (AZD0530). In GTL16 and KATO III, Src neutralization was also achieved by dasatinib and RNA interference. The biochemical and transcriptional consequences of Src inhibition were explored using anti-phosphoprotein antibodies and oligonucleotide microarrays. RESULTS: Inhibition of Src in Met-addicted gastric carcinoma cell lines (a) decreased the phosphorylation/activation levels of signaling intermediates involved in cell proliferation and protection from apoptosis and down-modulated the expression of several cell cycle regulators; (b) reduced anchorage-independent growth; (c) enhanced impairment of cell viability produced by Met inhibition; and (d) delayed tumorigenesis in xenotransplantation models. Immunohistochemical analysis of tumor xenograft tissues following systemic treatment with saracatinib showed a reduction of tumor cell proliferation index, increased apoptosis, and diminished phospho-focal adhesion kinase and phospho-paxillin staining. Tumor stroma parameters such as angiogenesis or inflammatory infiltration were unaffected. In clonogenic survival assays, gastric carcinoma cells without addiction to Met were less sensitive than Met-addicted cells to Src inhibition. CONCLUSIONS: Src is as a key downstream transducer of Met-driven tumor growth. Targeting Src might provide therapeutic benefit in Met-addicted tumors.

Only a subset of Met-activated pathways are required to sustain oncogene addiction.

Tumor onset and progression require the accumulation of many genetic and epigenetic lesions. In some cases, however, cancer cells rely on only one of these lesions to maintain their malignant properties, and this dependence results in tumor regression upon oncogene inactivation ("oncogene addiction"). Determining which nodes of the many networks operative in the transformed phenotype specifically mediate this response to oncogene neutralization is crucial to identifying the vulnerabilities of cancer. Using the Met receptor as the major model system, we combined multiplex phosphoproteomics, genome-wide expression profiling, and functional assays in various cancer cells addicted to oncogenic receptor tyrosine kinases. We found that Met blockade affected a limited subset of Met downstream signals: Little or no effect was observed for several pathways downstream of Met; instead, only a restricted and pathway-specific signature of transducers and transcriptional effectors downstream of Ras or phosphoinositide 3-kinase (PI3K) was inactivated. An analogous signature was also generated by inhibition of epidermal growth factor receptor in a different cellular context, suggesting a stereotyped response that likely is independent of receptor type or tissue origin. Biologically, Met inhibition led to cell-cycle arrest. Inhibition of Ras-dependent signals and PI3K-dependent signals also resulted in cell-cycle arrest, whereas cells in which Met was inhibited proliferated when Ras or PI3K signaling was active. These findings uncover "dominant" and "recessive" nodes among the numerous oncogenic networks regulated by receptor tyrosine kinases and active in cancer, with the Ras and PI3K pathways as determinants of therapeutic response.

Only a subset of Met-activated pathways are required to sustain oncogene addiction.

Tumor onset and progression require the accumulation of many genetic and epigenetic lesions. In some cases, however, cancer cells rely on only one of these lesions to maintain their malignant properties, and this dependence results in tumor regression upon oncogene inactivation ("oncogene addiction"). Determining which nodes of the many networks operative in the transformed phenotype specifically mediate this response to oncogene neutralization is crucial to identifying the vulnerabilities of cancer. Using the Met receptor as the major model system, we combined multiplex phosphoproteomics, genome-wide expression profiling, and functional assays in various cancer cells addicted to oncogenic receptor tyrosine kinases. We found that Met blockade affected a limited subset of Met downstream signals: Little or no effect was observed for several pathways downstream of Met; instead, only a restricted and pathway-specific signature of transducers and transcriptional effectors downstream of Ras or phosphoinositide 3-kinase (PI3K) was inactivated. An analogous signature was also generated by inhibition of epidermal growth factor receptor in a different cellular context, suggesting a stereotyped response that likely is independent of receptor type or tissue origin. Biologically, Met inhibition led to cell-cycle arrest. Inhibition of Ras-dependent signals and PI3K-dependent signals also resulted in cell-cycle arrest, whereas cells in which Met was inhibited proliferated when Ras or PI3K signaling was active. These findings uncover "dominant" and "recessive" nodes among the numerous oncogenic networks regulated by receptor tyrosine kinases and active in cancer, with the Ras and PI3K pathways as determinants of therapeutic response.