Engineering the Recruitment of Phosphotyrosine Binding Domain-containing Adaptor Proteins Reveals Distinct Roles for RET Receptor-mediated Cell Survival

Targeting the RET tyrosine kinase in neuroblastoma: A review and application of a novel selective drug design strategy

The RET (REarranged during Transfection) gene, which encodes for a transmembrane receptor tyrosine kinase, is an established oncogene associated with the etiology and progression of multiple types of cancer. Oncogenic RET mutations and rearrangements resulting in gene fusions have been identified in many adult cancers, including medullary and papillary thyroid cancers, lung adenocarcinomas, colon and breast cancers, and many others. While genetic RET aberrations are much less common in pediatric solid tumors, increased RET expression has been shown to be associated with poor prognosis in children with solid tumors such as neuroblastoma, prompting an interest in RET inhibition as a form of therapy for these children. A number of kinase inhibitors currently in use for patients with cancer have RET inhibitory activity, but these inhibitors also display activity against other kinases, resulting in unwanted side effects and limiting their safety and efficacy. Recent efforts have been focused on developing more specific RET inhibitors, but due to high levels of conservation between kinase binding pockets, specificity remains a drug design challenge. Here, we review the background of RET as a potential therapeutic target in neuroblastoma tumors and the results of recent preclinical studies and clinical trials evaluating the safety and efficacy of RET inhibition in adults and children. We also present a novel approach to drug discovery leveraging the chemical phenomenon of atropisomerism to develop specific RET inhibitors and present preliminary data demonstrating the efficacy of a novel RET inhibitor against neuroblastoma tumor cells.

Mitogen-Inducible Gene 6 Inhibits Angiogenesis by Binding to SHC1 and Suppressing Its Phosphorylation

The mitogen-inducible gene 6 (MIG6) is an adaptor protein widely expressed in vascular endothelial cells. However, it remains unknown thus far whether it plays a role in angiogenesis. Here, using comprehensive in vitro and in vivo model systems, we unveil a potent anti-angiogenic effect of MIG6 in retinal development and neovascularization and the underlying molecular and cellular mechanisms. Loss of function assays using genetic deletion of Mig6 or siRNA knockdown increased angiogenesis in vivo and in vitro, while MIG6 overexpression suppressed pathological angiogenesis. Moreover, we identified the cellular target of MIG6 by revealing its direct inhibitory effect on vascular endothelial cells (ECs). Mechanistically, we found that the anti-angiogenic effect of MIG6 is fulfilled by binding to SHC1 and inhibiting its phosphorylation. Indeed, SHC1 knockdown markedly diminished the effect of MIG6 on ECs. Thus, our findings show that MIG6 is a potent endogenous inhibitor of angiogenesis that may have therapeutic value in anti-angiogenic therapy.

XB130/Tks5 scaffold protein interaction regulates Src-mediated cell proliferation and survival

XB130 and Tks5 interact endogenously and form a complex with Src tyrosine kinase. Tks5, like XB130, plays a role in cell proliferation and cell survival, and the interaction between XB130 and Tks5 is critical for regulation of Src-mediated cell proliferation and survival.

The scaffold protein XB130 regulates cell growth, survival, and migration. Yeast two-hybrid screening suggests that XB130 interacts with another scaffold protein, Tks5. We hypothesized that XB130 and Tks5 form a macromolecular complex to mediate signal transduction cascades for the regulation of cell growth and survival. Coimmunoprecipitation demonstrated that XB130 and Tks5 interact endogenously and form a complex with Src tyrosine kinase. Structure–function studies showed that the fifth SH3 domain of Tks5 binds to the N-terminus of XB130, which contains polyproline-rich motifs. Cell growth and survival studies revealed that down-regulation of XB130 and/or Tks5 reduced cell proliferation, resulting in cell cycle inhibition at the G1 phase and increased caspase 3 activity and apoptosis. Moreover, cell proliferation and survival were increased by overexpression of XB130 or Tks5 but decreased when XB130/Tks5 binding was disrupted by overexpression of XB130 N-terminal deleted mutant and/or Tks5 fifth SH3 domain W1108A mutant. Furthermore, down-regulation of XB130 and/or Tks5 inhibited serum- and growth factor–induced Src activation and downstream phosphorylation of PI3K and Akt. Our results suggest that Tks5, similar to XB130, plays a role in cell proliferation and cell survival and that the interaction between XB130 and Tks5 appears to be critical for regulation of Src-mediated cellular homeostasis.

Structure and physiology of the RET receptor tyrosine kinase

The identification of the ret oncogene by Masahide Takahashi and Geoffrey Cooper in 1985 was both serendipitous and paradigmatic ( Takahashi et al. 1985). By transfecting total DNA from a human lymphoma into mouse NIH3T3 cells, they obtained one clone, which in secondary transformants yielded more than 100-fold improvement in transformation efficiency. Subsequent investigations revealed that the ret oncogene was not present as such in the primary lymphoma, but was derived by DNA rearrangement during transfection from normal human sequences of the ret locus. At the time, activation by DNA rearrangement had not been previously described for a transforming gene with the NIH3T3 transfection assay. The discovery of ret opened a field of study that has had a profound impact in cancer research, developmental biology, and neuroscience, and that continues to yield surprises and important insights to this day.

FRS2 via fibroblast growth factor receptor 1 is required for platelet-derived growth factor receptor beta-mediated regulation of vascular smooth muscle marker gene expression

Vascular smooth muscle cells (VSMC) exhibit phenotypic plasticity and change from a quiescent contractile phenotype to a proliferative synthetic phenotype during physiological arteriogenesis and pathological conditions such as atherosclerosis and restenosis. Platelet-derived growth factor (PDGF)-BB is a potent inducer of the VSMC synthetic phenotype; however, much less is known about the role of fibroblast growth factor-2 (FGF2) in this process. Here, we show using signal transduction mutants of FGF receptor 1 (FGFR1) expressed in rat VSMC that the adaptor protein FRS2 is essential for FGFR1-mediated phenotypic modulation and down-regulation of VSMC smooth muscle alpha-actin (SMA) gene expression. In addition, we show that PDGF-BB and FGF2 act synergistically to induce cell proliferation and down-regulate SMA and SM22alpha in VSMC. Furthermore, we show that PDGF-BB induces tyrosine phosphorylation of FGFR1 and that this phosphorylation is mediated by PDGF receptor-beta (PDGFRbeta), but not c-Src. We demonstrate that FRS2 co-immunoprecipitates with PDGFRbeta in a complex that requires FGFR1 and that both the extracellular and the intracellular domains of FGFR1 are required for association with PDGFRbeta, whereas the cytoplasmic domain of FGFR1 is required for FRS2 association with the FGFR1-PDGFRbeta complex. Knockdown of FRS2 in VSMC by RNA interference inhibited PDGF-BB-mediated down-regulation of SMA and SM22alpha without affecting PDGF-BB mediated cell proliferation or ERK activation. Together, these data support the notion that PDGFRbeta down-regulates SMA and SM22alpha through formation of a complex that requires FGFR1 and FRS2 and prove novel insight into VSMC phenotypic plasticity.

Cell migration by a FRS2-adaptor dependent membrane relocation of ret receptors

During development neural progenitor cells migrate with extraordinary precision to inhabit tissues and organs far from their initial position. Little is known about the cellular basis for directional guidance by tyrosine kinase receptors (RTKs). RET is a RTK with important functions in guiding the migration of neuronal cells, and RET dysregulation leads to clinical disease such as agangliosis of the colon. We show here that RET migration in neuroepitheliomal and non-neuronal cells is elicited by the activation of specific signaling pathways initiated by the competitive recruitment of the FRS2 adaptor molecule to tyrosine 1062 (Y1062) in RET. FRS2 selectively recruited RET to focal complexes and led to activation of SRC family kinases and focal adhesion kinase (FAK). Activation of SRC depended on its direct interaction with RET at a different intracellular tyrosine (Y981) and activation of molecular signaling from these two separate sites in concert regulated migration. Our data suggest that an important function for FRS2 is to concentrate RET in membrane foci, leading to an engagement of specific signaling complexes localized in these membrane domains.

Regulation of growth factor signaling by FRS2 family docking/scaffold adaptor proteins

The FRS2 family of adaptor/scaffold proteins has two members, FRS2alpha and FRS2beta. Both proteins contain N-terminal myristylation sites for localization on the plasma membrane and a PTB domain for binding to limited species of receptor tyrosine kinases (RTKs), including the FGF receptor, the neurotophin receptor, RET, and ALK. Activation of these RTKs allows FRS2 proteins to become phosphorylated of tyrosine residues and then bind to Grb2 and Shp2, a SH2 domain-containing adaptor and a tyrosine phosphatase, respectively. Subsequently, Shp2 activates a Ras/ERK pathway and Grb2 activates a Ras/ERK, phosphatidyl inositol (PI)-3 kinase and ubiquitination/degradation pathways by binding to SOS, Gab1, and Cbl via the SH3 domains of Grb2. FRS2alpha acts as 'a conning center' in FGF signaling mainly because it induces sustained levels of activation of ERK via Shp2-binding sites and Grb2-binding sites, though the contribution of the former is greater. Indeed, FRS2alpha knockout mice and mice with mutated Shp2-binding sites exhibit a variety of phenotypes due to defects in FGF signaling in vivo. Although FRS2beta binds to the EGF receptor, it does not induce tyrosine phosphorylation on the receptor. Instead, it inhibits EGF signaling, resulting in inhibition of EGF-induced cell proliferation and cell transformation. Based on these findings, the involvement of FRS2 proteins in tumorigenesis should be studied extensively to be validated as candidate biomarkers for the effectiveness of treatments targeting RTKs such as the FGF receptor and EGF receptor.

Histone H2AX-dependent GABA(A) receptor regulation of stem cell proliferation

Stem cell self-renewal implies proliferation under continued maintenance of multipotency. Small changes in numbers of stem cells may lead to large differences in differentiated cell numbers, resulting in significant physiological consequences. Proliferation is typically regulated in the G1 phase, which is associated with differentiation and cell cycle arrest. However, embryonic stem (ES) cells may lack a G1 checkpoint. Regulation of proliferation in the 'DNA damage' S/G2 cell cycle checkpoint pathway is known for its role in the maintenance of chromatin structural integrity. Here we show that autocrine/paracrine gamma-aminobutyric acid (GABA) signalling by means of GABA(A) receptors negatively controls ES cell and peripheral neural crest stem (NCS) cell proliferation, preimplantation embryonic growth and proliferation in the boundary-cap stem cell niche, resulting in an attenuation of neuronal progenies from this stem cell niche. Activation of GABA(A) receptors leads to hyperpolarization, increased cell volume and accumulation of stem cells in S phase, thereby causing a rapid decrease in cell proliferation. GABA(A) receptors signal through S-phase checkpoint kinases of the phosphatidylinositol-3-OH kinase-related kinase family and the histone variant H2AX. This signalling pathway critically regulates proliferation independently of differentiation, apoptosis and overt damage to DNA. These results indicate the presence of a fundamentally different mechanism of proliferation control in these stem cells, in comparison with most somatic cells, involving proteins in the DNA damage checkpoint pathway.

Deciphering adaptor specificity in GFL-dependent RET-mediated proliferation and neurite outgrowth

Glial cell derived neurotrophic factor (GDNF)-dependent receptor tyrosine kinase RET activity is required for proper development of the nervous system and genitourinary tract. Loss-of-function mutations in RET are associated with enteric nervous system abnormalities (Hirschsprung disease) and renal deficits (Potter's syndrome), whereas activating mutations lead to hereditary cancer syndromes (multiple endocrine neoplasia type 2A and type 2B). RET activation is crucial for the proper regulation of a variety of cellular processes including cell migration, proliferation and neurite outgrowth. By analyzing a series of RET mutants we found that Y1062 was critical for stimulating GDNF-mediated proliferation as well as proliferation stimulated by GDNF-independent oncogenic forms of RET. Studies using small interfering RNA driven by lentivirus to knock-down expression of particular adaptor proteins that interact with RET phospho-Y1062, demonstrated that only Src-homology 2 and growth factor receptor binding protein 2 were necessary for RET-mediated proliferation by wild type and oncogenic forms of RET. Interestingly, we discovered that Y1062 was also required for GDNF-stimulated neurite outgrowth. However, small interfering RNAs to either Src-homology 2 or growth factor receptor binding protein 2 or a panel of other adaptor proteins known to interact with RET Y1062 were incapable of blocking GDNF-stimulated neurite formation, indicating that differential use of intracellular adaptors is responsible for regulating alternative RET-stimulated cellular events such as proliferation versus a differentiation response like neurite outgrowth.

Beta-arrestin is a necessary component of Wnt/beta-catenin signaling in vitro and in vivo

The Wnt/beta-catenin signaling pathway is crucial for proper embryonic development and tissue homeostasis. The phosphoprotein dishevelled (Dvl) is an integral part of Wnt signaling and has recently been shown to interact with the multifunctional scaffolding protein beta-arrestin. Using Dvl deletion constructs, we found that beta-arrestin binds a region N-terminal of the PDZ domain of Dvl, which contains casein kinase 1 (CK1) phosphorylation sites. Inhibition of Wnt signaling by CK1 inhibitors reduced the binding of beta-arrestin to Dvl. Moreover, mouse embryonic fibroblasts lacking beta-arrestins were able to phosphorylate LRP6 in response to Wnt-3a but decreased the activation of Dvl and blocked beta-catenin signaling. In addition, we found that beta-arrestin can bind axin and forms a trimeric complex with axin and Dvl. Furthermore, treatment of Xenopus laevis embryos with beta-arrestin morpholinos reduced the activation of endogenous beta-catenin, decreased the expression of the beta-catenin target gene, Xnr3, and blocked axis duplication induced by X-Wnt-8, CK1epsilon, or DshDeltaDEP, but not by beta-catenin. Thus, our results identify beta-arrestin as a necessary component for Wnt/beta-catenin signaling, linking Dvl and axin, and open a vast array of signaling avenues and possibilities for cross-talk with other beta-arrestin-dependent signaling pathways.

Screening for PTB domain binding partners and ligand specificity using proteome-derived NPXY peptide arrays

Modular interaction domains that recognize peptide motifs in target proteins can impart selectivity in signaling pathways. Phosphotyrosine binding (PTB) domains are components of cytoplasmic docking proteins that bind cell surface receptors through NPXY motifs. We have employed a library of human proteome-derived NXXY sequences to explore PTB domain specificity and function. SPOTS peptide arrays were used to create a comprehensive matrix of receptor motifs that were probed with a set of 10 diverse PTB domains. This approach confirmed that individual PTB domains have selective and distinct recognition properties and provided a means to explore over 2,500 potential PTB domain-NXXY interactions. The results correlated well with previously known associations between full-length proteins and predicted novel interactions, as well as consensus binding data for specific PTB domains. Using the Ret, MuSK, and ErbB2 receptor tyrosine kinases, we show that interactions of these receptors with PTB domains predicted to bind by the NXXY arrays do occur in cells. Proteome-based peptide arrays can therefore identify networks of receptor interactions with scaffold proteins that may be physiologically relevant.