Summary The PI3K/mTOR-pathway is the most commonly dysregulated pathway in epithelial cancers and represents an important target for cancer therapeutics. Here we show that dual inhibition of PI3K/mTOR in ovarian cancer-spheroids leads to death of inner matrix-deprived cells, whereas matrix-attached cells are resistant. This matrix-associated resistance is mediated by drug-induced upregulation of cellular survival programs that involve both FOXO-regulated transcription and cap-independent translation. Inhibition of any one of several upregulated proteins, including Bcl-2, EGFR, or IGF1R, abrogates resistance to PI3K/mTOR inhibition. These results demonstrate that acute adaptive responses to PI3K/mTOR inhibition in matrix-attached cells resemble well-conserved stress responses to nutrient and growth factor deprivation. Bypass of this resistance mechanism through rational design of drug combinations could significantly enhance PI3K-targeted drug efficacy.

Ezrin-radixin-moesin (ERM) proteins regulate the organization and function of specific cortical structures in polarized epithelial cells by connecting filamentous (F)-actin to plasma membrane proteins. The contribution of ERM proteins to these structures depends on a conformational change to an active state in which the C-terminal region interacts with F-actin and the N-terminal domain interacts with membrane ligands. The specific ligands necessary for stabilizing ERM proteins at the membrane are not known. By generating mice deficient for ERM-binding phosphoprotein 50͞ Na ؉ /H ؉ exchanger regulatory factor 1 (EBP50͞NHERF1), which binds the N-terminal domain of ERM proteins, we found that EBP50 is required for the maintenance of active ERM proteins at the cortical brush border membranes (BBM) of polarized epithelia. In EBP50(؊͞؊) mice, ERM proteins were significantly decreased specifically in BBM from kidney and small intestine epithelial cells, whereas they remained unchanged in the cytoplasm. In wild-type animals, EBP50 was localized to the BBM compartment where it was processed by cleavage of the ERM-binding motif. In BBM, active ERM proteins formed distinct complexes with full-length EBP50 and with F-actin, suggesting a switch mechanism in which proteolytically processed EBP50 would release ERM proteins to complex with F-actin. The structural defects found in the EBP50(؊͞؊) intestinal microvilli were reminiscent of those described in ezrin(؊͞؊) mice, suggesting a role for EBP50 in organizing apical epithelial membranes. mutant (knockout) mice ͉ brush border membranes ͉ intestine ͉ kidney

PTEN, a tumor suppressor frequently inactivated in many human cancers, directly antagonizes the activity of phosphatidylinositol-3-OH kinase (PI3K) by dephosphorylating phosphoinositides. We show here that PTEN interacts directly with the NHERF1 and NHERF2 (Na þ /H þ exchanger regulatory factor) homologous adaptor proteins through the PDZ motif of PTEN and the PDZ1 domain of NHERF1 or both PDZ domains of NHERF2. NHERFs were shown to interact directly with platelet-derived growth factor receptor (PDGFR), and we demonstrate the assembly of a ternary complex between PTEN, NHERFs and PDGFR. The activation of the PI3K pathway after PDGFR stimulation was prolonged in NHERF1(À/À) mouse embryonic fibroblasts as compared to wild-type cells, consistent with defective PTEN recruitment to PDGFR in the absence of NHERF1. Depletion of NHERF2 by small interfering RNA similarly increased PI3K signaling. Phenotypically, the loss of NHERF1 enhanced the PDGFinduced cytoskeletal rearrangements and chemotactic migration of the cells. These data indicate that, in normal cells, NHERF proteins recruit PTEN to PDGFR to restrict the activation of the PI3K.

Targeted therapeutics hold tremendous promise in inhibiting cancer cell proliferation. However, targeting proteins individually can be compensated for by bypass mechanisms and activation of regulatory loops. Designing optimal therapeutic combinations must therefore take into consideration the complex dynamic networks in the cell. In this study, we analyzed the insulin-like growth factor (IGF-1) signaling network in the MDA-MB231 breast cancer cell line. We used reverse-phase protein array to measure the transient changes in the phosphorylation of proteins after IGF-1 stimulation. We developed a computational procedure that integrated mass action modeling with particle swarm optimization to train the model against the experimental data and infer the unknown model parameters. The trained model was used to predict how targeting individual signaling proteins altered the rest of the network and identify drug combinations that minimally increased phosphorylation of other proteins elsewhere in the network. Experimental testing of the modeling predictions showed that optimal drug combinations inhibited cell signaling and proliferation, whereas nonoptimal combination of inhibitors increased phosphorylation of nontargeted proteins and rescued cells from cell death. The integrative approach described here is useful for generating experimental intervention strategies that could optimize drug combinations and discover novel pharmacologic targets for cancer therapy. Cancer Res; 70(17); 6704-14. ©2010 AACR. Major FindingsSimple and reliable strategies are needed to identify optimal combinations of molecular targeted drugs to treat individual cancer patients, to realize the fullest potential of a targeted therapeutic approach.

The PI3K-Akt pathway is activated in cancer by genetic or epigenetic events and efforts are under way to develop targeted therapies. PTEN tumor suppressor is the major brake of the pathway and a common target for inactivation in glioblastoma, one of the most aggressive and therapy-resistant cancers. To achieve potent inhibition of the PI3K-Akt pathway in glioblastoma, we need to understand its mechanism of activation by investigating the interplay between its regulators. We show here that PTEN modulates the PI3K-Akt pathway in glioblastoma within a tumor suppressor network that includes NHERF1 and PHLPP1. The NHERF1 adaptor, previously characterized by our group as a PTEN ligand and regulator, shows also PTEN-independent Akt-modulating effects that led us to identify the PHLPP1/PHLPP2 Akt phosphatases as NHERF1 ligands. NHERF1 interacts via its PDZ domains with PHLPP1/PHLPP2 and scaffolds heterotrimeric complexes with PTEN. Functionally, PHLPP1 requires NHERF1 for membrane localization and growth suppressive effects. PHLPP1 loss boosts Akt phosphorylation only in PTEN-negative cells and cooperates with PTEN loss for tumor growth. In a panel of low-grade and high-grade glioma patient samples, we show for the first time a significant disruption of all three members of the PTEN-NHERF1-PHLPP1 tumor suppressor network in high-grade tumors, correlating with Akt activation and patients’ abysmal survival. We thus propose a PTEN-NHERF1-PHLPP PI3K-Akt pathway inhibitory network that relies on molecular interactions and can undergo parallel synergistic hits in glioblastoma.

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Anchorage-independent growth is a hallmark of tumor growth and results from enhanced proliferation and altered cell-cell and cell-matrix interactions. By using genedeficient mouse embryonic fibroblasts (MEFs), we showed for the first time that NHERF1/EBP50 (Na/H exchanger regulator factor 1/ezrin-radixin-moesin binding phosphoprotein 50), an adapter protein with membrane localization under physiological conditions, inhibits cell motility and is required to suppress anchorage-independent growth. Both NHERF1 PDZ domains are necessary for the tumor suppressor effect. NHERF1 associates directly through the PDZ2 domain with b-catenin and is required for b-catenin localization at the cell-cell junctions in MEFs. Mechanistically, the absence of NHERF1 selectively decreased the interaction of b-catenin with E-cadherin, but not with N-cadherin. The ensuing disorganization of E-cadherinmediated adherens junctions as well as the observed moderate increase in b-catenin transcriptional activity contributed most likely to the anchorage-independent growth of NHERF1-deficient MEFs. In vivo, NHERF1 is specifically localized at the apical brush-border membrane in intestinal epithelial cells and is required to maintain a fraction of the cortical b-catenin at this level. Thus, NHERF1 emerges as a cofactor essential for the integrity of epithelial tissues by maintaining the proper localization and complex assembly of b-catenin.

This review summarizes the emerging roles of NHERF1/EBP50 adaptor protein in tumorigenesis. NHERF1/EBP50 (Na(+)/H(+) exchanger regulating factor 1; ezrin-radixin-moesin (ERM) binding phosphoprotein of 50 kDa) is a PDZ domain-containing protein with physiological localization at the plasma membrane. We discuss in this review the functions of NHERF1/EBP50 as a linker between membrane proteins and the cytoskeleton network, as well as its involvement in different types of cancer, such as breast and liver cancers. Recent evidence obtained from our laboratory and from other groups shows that NHERF1/EBP50 is an important player in cancer progression. It appears that, depending on its subcellular distribution, NHERF1/EBP50 may behave either as a tumor suppressor, when it is localized at the plasma membrane, or as an oncogenic protein, when it is shifted to the cytoplasm. We provide here an overview of the mechanisms by which this adaptor protein controls cell transformation, and propose a model suggesting a dual role of NHERF1/EBP50 in cancer.