The insulin receptor substrate (IRS)-1 recruits phosphatidylinositol 3-kinase to Ret: evidence for a competition between Shc and IRS-1 for the binding to Ret

Personalized Medicine in Medullary Thyroid Carcinoma: A Broad Review of Emerging Treatments

Medullary thyroid carcinoma (MTC) arises from parafollicular cells in the thyroid gland, and although rare, it represents an aggressive type of thyroid cancer. MTC is recognized for its low mutational burden, with point mutations in RET or RAS genes being the most common oncogenic events. MTC can be resistant to cytotoxic chemotherapy, and multitarget kinase inhibitors (MKIs) have been considered a treatment option. They act by inhibiting the activities of specific tyrosine kinase receptors involved in tumor growth and angiogenesis. Several tyrosine kinase inhibitors are approved in the treatment of advanced MTC, including vandetanib and cabozantinib. However, due to the significant number of adverse events, debatable efficiency and resistance, there is a need for novel RET-specific TKIs. Newer RET-specific TKIs are expected to overcome previous limitations and improve patient outcomes. Herein, we aim to review MTC signaling pathways, the most recent options for treatment and the applications for personalized medicine.

Serum Metabolomic Signatures of Hirschsprung's Disease Based on GC-MS and LC-MS

Hirschsprung's disease (HSCR) is a congenital digestive tract malformation characterized by the absence of intramural ganglion cells in the myenteric and submucosal plexuses along variable lengths of the gastrointestinal tract. Although the improvement of surgical methods has allowed great progress in the treatment of HSCR, its incidence and postoperative prognosis are still not ideal. The pathogenesis of HSCR remains unclear to date. In this study, metabolomic profiling of HSCR serum samples was performed by an integrated analysis of gas chromatography-mass spectrometry (GC-MS) and liquid chromatography-high-resolution tandem mass spectrometry (LC-HRMS/MS) as well as multivariate statistical analyses. Based on the random forest algorithm and receiver operator characteristic analysis, 21 biomarkers related to HSCR were optimized. Several amino acid metabolism pathways were identified as important disordered pathways of HSCR, among which tryptophan metabolism was crucial. To our knowledge, this is the first serum metabolomics study focusing on HSCR, and it provides a new perspective for explaining the mechanism of HSCR.

RET aberrant cancers and RET inhibitor therapies: Current state-of-the-art and future perspectives

Precision oncology informed by genomic information has evolved in leaps and bounds over the last decade. Although non-small cell lung cancer (NSCLC) has moved to center-stage as the poster child of precision oncology, multiple targetable genomic alterations have been identified in various cancer types. RET alterations occur in roughly 2% of all human cancers. The role of RET as oncogenic driver was initially identified in 1985 after the discovery that transfection with human lymphoma DNA transforms NIH-3T3 fibroblasts. Germline RET mutations are causative of multiple endocrine neoplasia type 2 syndrome, and RET fusions are found in 10-20% of papillary thyroid cases and are detected in most patients with advanced sporadic medullary thyroid cancer. RET fusions are oncogenic drivers in 2% of Non-small cell lung cancer. Rapid translation and regulatory approval of selective RET inhibitors, selpercatinib and pralsetinib, have opened up the field of RET precision oncology. This review provides an update on RET precision oncology from bench to bedside and back. We explore the impact of selective RET inhibitor in patients with advanced NSCLC, thyroid cancer, and other cancers in a tissue-agnostic fashion, resistance mechanisms, and future directions.

TAZ enhances mammary cell proliferation in 3D culture through transcriptional regulation of IRS1

WW domain-containing transcriptional regulator 1 (TAZ) is a transcriptional co-activator and effector of the Hippo signaling pathway. In certain breast cancer subtypes, Hippo signaling is dysregulated leading to activation of TAZ and altered expression of TAZ transcriptional targets. Over the past decade, we and others have found that TAZ transcriptionally regulates genes that affect multiple aspects of breast cancer cell behaviour. However, while cancer cell-intrinsic oncogenic functions of TAZ have emerged, less is known about whether TAZ might also contribute to tumourigenesis by sensitizing tumour cells to factors present in the tumour microenvironment or in systemic circulation. Here, we show that TAZ directly regulates the expression of insulin receptor substrate 1 (IRS1) in breast cancer cells. TAZ or IRS1 overexpression induces a similar proliferative transformation phenotype in MCF10A mammary epithelial cells. TAZ enhances IRS1 mRNA, protein levels and downstream signaling in MCF10A. Mechanistically, TAZ interacts with the IRS1 promoter through the TEAD family of transcription factors and enhances its activity. Critically, TAZ-induced IRS1 upregulation contributes to the proliferation of TAZ-overexpressing MCF10A in 3-dimensional (3D) Matrigel culture. Therefore, we offer compelling evidence that TAZ regulates signaling through the insulin pathway in breast cancer cells. These findings highlight an additional mechanism by which TAZ may promote breast cancer tumourigenesis and progression by modulating cancer cell responses to exogenously produced factors.

The role of the ShcD and RET interaction in neuroblastoma survival and migration

Preliminary screening data showed that the ShcD adaptor protein associates with the proto-oncogene RET receptor tyrosine kinase. In the present study, we aimed to investigate the molecular interaction between ShcD and RET in human neuroblastoma cells and study the functional impact of this interaction. We were able to show that ShcD immunoprecipitated with RET from SK-N-AS neuroblastoma cell lysates upon GDNF treatment. This result was validated by ShcD-RET co-localization, which was visualized using a fluorescence microscope. ShcD-RET coexpression promoted ShcD and RET endosomal localization, resulting in unexpected inhibition of the downstream ERK and AKT pathways. Interestingly, ShcD-RET association reduced the viability and migration of SK-N-AS cells. Although ShcD was previously shown to trigger melanoma cell migration and tumorigenesis, our data showed an opposite role for ShcD in neuroblastoma SK-N-AS cells via its association with RET in GDNF-treated cells. In conclusion, ShcD acts as a switch molecule that promotes contrasting biological responses depending on the stimulus ad cell type.

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The melanoma associated Shc adaptor, ShcD, is found to interact with Ret oncogene receptor in SK-N-AS neuroblastoma cells.

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ShcD and Ret coexpression favoures their endosomal localization.

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ShcD-Ret association has suppressed ERK and AKT signalling.

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The functional consequence of ShcD and Ret interaction was shown to negatively affect cell survival and cellular migration in.

Exon Skipping in the RET Gene Encodes Novel Isoforms That Differentially Regulate RET Protein Signal Transduction

Rearranged during transfection (RET), a receptor tyrosine kinase that is activated by the glial cell line-derived neurotrophic factor family ligands (GFLs), plays a crucial role in the development and function of the nervous system and additionally is required for kidney development and spermatogenesis. RET encodes a transmembrane receptor that is 20 exons long and produces two known protein isoforms differing in C-terminal amino acid composition, referred to as RET9 and RET51. Studies of human pheochromocytomas identified two additional novel transcripts involving the skipping of exon 3 or exons 3, 4, and 5 and are referred to as RET(Δ) (E3) and RET(Δ) (E345), respectively. Here we report the presence of Ret(Δ) (E3) and Ret(Δ) (E345) in zebrafish, mice, and rats and show that these transcripts are dynamically expressed throughout development of the CNS, peripheral nervous system, and kidneys. We further explore the biochemical properties of these isoforms, demonstrating that, like full-length RET, RET(ΔE3) and RET(ΔE345) are trafficked to the cell surface, interact with all four GFRα co-receptors, and have the ability to heterodimerize with full-length RET. Signaling experiments indicate that RET(ΔE3) is phosphorylated in a similar manner to full-length RET. RET(ΔE345), in contrast, displays higher baseline autophosphorylation, specifically on the catalytic tyrosine, Tyr(905), and also on one of the most important signaling residues, Tyr(1062) These data provide the first evidence for a physiologic role of these isoforms in RET pathway function.

PI3K/Akt/mTOR signaling in medullary thyroid cancer: a promising molecular target for cancer therapy

The phosphatidylinositol 3-kinase (PI3K)/Akt/mammalian target of rapamycin (mTOR) pathway is a central hub for the regulation of cell proliferation, apoptosis, cell cycle, metabolism, and angiogenesis. Several studies have recently suggested that the PI3K/Akt/mTOR signaling pathway is implicated in the pathogenesis and progression of neuroendocrine tumors. Medullary thyroid cancer (MTC) is a neuroendocrine tumor developing from the C cells of the thyroid. Mutations in the RET proto-oncogene are involved in the pathogenesis of several forms of MTC. The deregulation of the PI3K/Akt/mTOR pathway seems to contribute to the tumorigenic activity of RET proto-oncogene mutations. Targeting this pathway through specific inhibitors at simple or multiple sites may represent an attractive potential therapeutic approach for patients with advanced MTCs. The aim of this review is to examine the role of the PI3K/Akt/mTOR pathway in the development and progression of MTC and the new therapeutic options that target this signaling pathway.

The Novel Functions of High-Molecular-Mass Complexes Containing Insulin Receptor Substrates in Mediation and Modulation of Insulin-Like Activities: Emerging Concept of Diverse Functions by IRS-Associated Proteins

Insulin-like peptides, such as insulin-like growth factors (IGFs) and insulin, induce a variety of bioactivities, such as growth, differentiation, survival, increased anabolism, and decreased catabolism in many cell types and in vivo. In general, IGFs or insulin bind to IGF-I receptor (IGF-IR) or insulin receptor (IR), activating the receptor tyrosine kinase. Insulin receptor substrates (IRSs) are known to be major substrates of receptor kinases, mediating IGF/insulin signals to direct bioactivities. Recently, we discovered that IRSs form high-molecular-mass complexes (referred to here as IRSomes) even without IGF/insulin stimulation. These complexes contain proteins (referred to here as IRSAPs; IRS-associated proteins), which modulate tyrosine phosphorylation of IRSs by receptor kinases, control IRS stability, and determine intracellular localization of IRSs. In addition, in these complexes, we found not only proteins that are involved in RNA metabolism but also RNAs themselves. Thus, IRSAPs possibly contribute to modulation of IGF/insulin bioactivities. Since it is established that disorder of modulation of insulin-like activities causes various age-related diseases including cancer, we could propose that the IRSome is an important target for treatment of these diseases.

Central role of RET in thyroid cancer

RET (rearranged during transfection) is a receptor tyrosine kinase involved in the development of neural crest derived cell lineages, kidney, and male germ cells. Different human cancers, including papillary and medullary thyroid carcinomas, lung adenocarcinomas, and myeloproliferative disorders display gain-of-function mutations in RET. Accordingly, RET protein has become a promising molecular target for cancer treatment.

Thyroid Cancer: Role of RET and Beyond

Specific thyroid cancer histotypes, such as papillary and medullary thyroid carcinoma, display genetic rearrangements or point mutations of the RET gene, resulting in its oncogenic conversion. The molecular mechanisms mediating RET rearrangement with other genes and the role of partner genes in tumorigenesis have been described. In addition, the RET protein has become a molecular target for medullary thyroid carcinoma treatment.

XB130, a tissue-specific adaptor protein that couples the RET/PTC oncogenic kinase to PI 3-kinase pathway

XB130 is a recently cloned 130 kDa-adaptor protein and Src kinase substrate, structurally similar to actin-filament-associated protein. Here we show that XB130 is predominantly expressed in the thyroid. Given that XB130 is a thyroid-specific tyrosine kinase substrate, we asked whether it is targeted by RET/PTC, a genetically rearranged, constitutively active, thyroid-specific tyrosine kinase that plays a pathogenic role in papillary thyroid cancer. RET/PTC induced robust tyrosine phosphorylation of XB130, which promoted its subsequent association with the p85alpha subunit of phosphatidylinositol 3-kinase (PI 3-kinase). We identified tyrosine 54 of XB130 as the major target of RET/PTC-mediated phosphorylation and a critical binding site for the SH2 domains of p85alpha. Importantly, downregulation of XB130 in TPC1 papillary thyroid cancer cells, harboring the RET/PTC1 kinase, strongly reduced Akt activity without altering ERK1/2 phosphorylation, and concomitantly inhibited cell-cycle progression and survival in suspension. In conclusion, XB130 is a novel substrate of the RET/PTC kinase that links RET/PTC signaling to PI 3-kinase activation, and thereby plays an important role in sustaining proliferation and survival of thyroid tumor cells.

A phosphorylation-dependent gating mechanism controls the SH2 domain interactions of the Shc adaptor protein

The Shc (Src homology collagen-like) adaptor protein plays a crucial role in linking stimulated receptors to mitogen-activated protein kinase activation through the formation of dynamic signalling complexes. Shc comprises an N-terminal phosphotyrosine binding (PTB) domain, a C-terminal Src homology 2 (SH2) domain and a central proline-rich collagen homology 1 domain. The latter domain contains three tyrosine residues that are known to become phosphorylated. We have expressed and purified the human p52Shc isoform and characterised its binding to different ligands. CD spectra revealed that some parts of the Shc protein are not fully folded, remaining largely unaffected by the binding of ligands. The PTB domain binds peptide and Ins-1,4,5-P(3) (but not Ins-1,3,5-P(3)) independently, suggesting two distinct sites of interaction. In the unphosphorylated Shc, the SH2 domain is non-functional. Ligand binding to the PTB domain does not affect this. However, phosphorylation of the three tyrosine residues promotes binding to the SH2 domain. Thus, Shc has an intrinsic phosphorylation-dependent gating mechanism where the SH2 domain adopts an open conformation only when tyrosine phosphorylation has occurred.

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.

The Shc-binding site of the betac subunit of the GM-CSF/IL-3/IL-5 receptors is a negative regulator of hematopoiesis

Tyrosine and serine phosphorylation of the common beta chain (beta(c)) of the granulocyte-macrophage colony-stimulating factor (GM-CSF), interleukin-3 (IL-3), and IL-5 receptors is widely viewed as a general mechanism that provides positive inputs by coupling the receptor to signaling pathways that stimulate several cellular functions. We show here that despite the known action of Tyr577 in beta(c) to recruit Shc-PI-3 kinase (PI3K) pathway members, Tyr577 plays, surprisingly, a negative regulatory role in cell function, and that this is mediated, at least in part, through the uncoupling of SH2-containing inositol 5'-phosphatase (SHIP) from beta(c). Fetal liver cells from beta(c)/beta(IL-3)(-/-) mice expressing human GM-CSF receptor alpha chain and beta(c) Tyr577Phe mutant showed enhanced colony formation and expansion of progenitor cells in response to GM-CSF. Dissection of these activities revealed that basal survival was increased, as well as cytokine-stimulated proliferation. As expected, the recruitment and activation of Shc was abolished, but interestingly, Gab-2 and Akt phosphorylation increased. Significantly, the activation of PI3K was enhanced and prolonged, accompanied by loss of SHIP activity. These results reveal a previously unrecognized negative signaling role for Tyr577 in beta(c) and demonstrate that uncoupling Shc from cytokine receptors enhances PI3K signaling as well as survival and proliferation.

Mammary tumorigenesis and metastasis caused by overexpression of insulin receptor substrate 1 (IRS-1) or IRS-2

Insulin receptor substrates (IRSs) are signaling adaptors that play a major role in the metabolic and mitogenic actions of insulin and insulin-like growth factors. Reports have recently noted increased levels, or activity, of IRSs in many human cancers, and some have linked this to poor patient prognosis. We found that overexpressed IRS-1 was constitutively phosphorylated in vitro and in vivo and that transgenic mice overexpressing IRS-1 or IRS-2 in the mammary gland showed progressive mammary hyperplasia, tumorigenesis, and metastasis. Tumors showed extensive squamous differentiation, a phenotype commonly seen with activation of the canonical beta-catenin signaling pathway. Consistent with this, IRSs were found to bind beta-catenin in vitro and in vivo. IRS-induced tumorigenesis is unique, given that the IRSs are signaling adaptors with no intrinsic kinase activity, and this supports a growing literature indicating a role for IRSs in cancer. This study defines IRSs as oncogene proteins in vivo and provides new models to develop inhibitors against IRSs for anticancer therapy.

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

The RET receptor tyrosine kinase is important for several different biological functions during development. The recruitment at the phosphorylated Tyr(1062) site in RET of a number of different phosphotyrosine binding (PTB) domain-containing adaptor proteins, including Shc and Frs2, plays a dominant role for the multiple different biological functions of the RET receptor during development, including stimulation of cell survival. Here, we demonstrate that a competitive recruitment of Shc as opposed to Frs2 mediates the survival signaling arising from RET activation. Based on results from a peptide array, we have genetically engineered the PTB domain binding site of RET to rewire its recruitment of the PTB proteins Shc and Frs2. An engineered RET that has a competitive interaction with Shc at the expense of Frs2, but not a RET receptor that only recruits Frs2, activates cell survival signaling pathways and is protective from cell death in neuronal SK-N-MC cells. Thus, cell type-specific functions involve a competitive recruitment of different PTB adaptor molecules by RET that activate selective 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.

RET as a diagnostic and therapeutic target in sporadic and hereditary endocrine tumors

The RET gene encodes a receptor tyrosine kinase that is expressed in neural crest-derived cell lineages. The RET receptor plays a crucial role in regulating cell proliferation, migration, differentiation, and survival through embryogenesis. Activating mutations in RET lead to the development of several inherited and noninherited diseases. Germline point mutations are found in the cancer syndromes multiple endocrine neoplasia (MEN) type 2, including MEN 2A and 2B, and familial medullary thyroid carcinoma. These syndromes are autosomal dominantly inherited. The identification of mutations associated with these syndromes has led to genetic testing to identify patients at risk for MEN 2 and familial medullary thyroid carcinoma and subsequent implementation of prophylactic thyroidectomy in mutation carriers. In addition, more than 10 somatic rearrangements of RET have been identified from papillary thyroid carcinomas. These mutations, as those found in MEN 2, induce oncogenic activation of the RET tyrosine kinase domain via different mechanisms, making RET an excellent candidate for the design of molecular targeted therapy. Recently, various kinds of therapeutic approaches, such as tyrosine kinase inhibition, gene therapy with dominant negative RET mutants, monoclonal antibodies against oncogene products, and nuclease-resistant aptamers that recognize and inhibit RET have been developed. The use of these strategies in preclinical models has provided evidence that RET is indeed a potential target for selective cancer therapy. However, a clinically useful therapeutic option for treating patients with RET-associated cancer is still not available.

Differential effects of glial cell line-derived neurotrophic factor and neurturin in RET/GFRalpha1-expressing cells

The c-ret protooncogene, RET, encodes a receptor tyrosine kinase. RET is activated by members of the glial cell line-derived neurotrophic factor (GDNF) family of ligands, which include GDNF, neurturin, artemin, and persephin. The ligands bind RET through GDNF family receptor alpha, termed GFRalpha1-4. Despite the importance of RET signaling in the development of the enteric nervous system and the kidney, the differential signaling mechanisms between RET ligands are poorly established. It has been suggested that signal specificity is achieved through binding of the ligand to its preferred GFRalpha. To compare the signaling profiles of GDNF and neurturin, we have identified a cell line, NG108-15, which endogenously expresses RET and GFRalpha1 but not GFRalpha2-4. Immunoblot data showed that GDNF caused a transient activation, whereas neurturin caused a sustained activation, of both p44/p42 MAP kinases and PLCgamma. Under serum starvation, NG108-15 cells differentiate and form neurites. Neurturin but not GDNF stimulated neurite outgrowth, which could be blocked by the selective PLC inhibitor U73122. On the other hand, GDNF but not neurturin promoted cell survival, and this could be blocked by the p44/p42 MAP kinase inhibitor PD98059. Our findings not only show the differential signaling of GDNF and neurturin but also suggest that this can be achieved through binding to the same GFRalpha subtype, leading to distinct biological responses.

Interaction of SH2-Bbeta with RET is involved in signaling of GDNF-induced neurite outgrowth

RET receptor signalling is essential for glial-cell-line-derived neurotrophic factor (GDNF)-induced survival and differentiation of various neurons such as mesencephalic neurons. To identify proteins that mediate RET-dependent signaling, yeast two-hybrid screening was performed with the intracellular domain of RET as bait. We identified a new interaction between RET and the adapter protein SH2-Bbeta. Upon GDNF stimulation of PC12-GFRalpha1-RET cells (that stably overexpress GDNF receptor alpha1 and RET), wild-type SH2-Bbeta co-immunoprecipitated with RET, whereas the dominant-negative SH2-Bbeta mutant R555E did not. RET interacted with endogenous SH2-Bbeta both in PC12-GFRalpha1-RET cells and in rat tissues. Mutagenesis analysis revealed that Tyr981 within the intracellular domain of RET was crucial for the interaction with SH2-Bbeta. Morphological evidence showed that SH2-Bbeta and RET colocalized in mesencephalic neurons. Furthermore, functional analysis indicated that overexpression of SH2-Bbeta facilitated GDNF-induced neurite outgrowth in both PC12-GFRalpha1-RET cells and cultured mesencephalic neurons, whereas the mutant R555E inhibited the effect. Moreover, inhibition of SH2-Bbeta expression by RNA interference caused a significant decrease of GDNF-induced neuronal differentiation in PC12-GFRalpha1-RET cells. Taken together, our results suggest that SH2-Bbeta is a new signaling molecule involved in GDNF-induced neurite outgrowth.

RET tyrosine kinase signaling in development and cancer

The variety of diseases caused by mutations in RET receptor tyrosine kinase provides a classic example of phenotypic heterogeneity. Gain-of-function mutations of RET are associated with human cancer. Gene rearrangements juxtaposing the tyrosine kinase domain to heterologous gene partners have been found in sporadic papillary carcinomas of the thyroid (PTC). These rearrangements generate chimeric RET/PTC oncogenes. In the germline, point mutations of RET are responsible for multiple endocrine neoplasia type 2 (MEN 2A and 2B) and familial medullary thyroid carcinoma (FMTC). Both MEN 2 mutations and PTC gene rearrangements potentiate the intrinsic tyrosine kinase activity of RET and, ultimately, activate the RET downstream targets. Loss-of-function mutations of RET cause Hirschsprung's disease (HSCR) or colonic aganglionosis. A deeper understanding of the molecular signaling of normal versus abnormal RET activity in cancer will enable the development of potential new treatments for patients with sporadic and inherited thyroid cancer or MEN 2 syndrome. We now review the role and mechanisms of RET signaling in development and carcinogenesis.

Biochemical and biological responses induced by coupling of Gab1 to phosphatidylinositol 3-kinase in RET-expressing cells

Grb2-associated binder-1 (Gab1) is a docking protein closely related to insulin receptor substrates. We previously reported that tyrosine 1062 in RET receptor tyrosine kinase activated by glial cell line-derived neurotrophic factor (GDNF) represents a binding site for the Shc-Grb2-Gab1 complex, and that the p85 subunit of phosphatidylinositol 3-kinase (PI3K) and SHP2 tyrosine phosphatase is associated with Gab1 in GDNF-treated cells. In the present study, we further analyzed the physiological roles of Gab1 downstream of RET, using Gab1 mutants that lack the binding sites for PI3K (Gab1 PI3K-m) or SHP-2 (Gab1 SHP2-m). Expression of Gab1 PI3K-m in SK-N-MC human primitive neuroectodermal tumor cells expressing wild-type RET markedly impaired Akt phosphorylation, Rac1 activation, and lamellipodia formation that were induced by GDNF whereas expression of Gab1 SHP2-m partially impaired Erk activation. Furthermore, expression of Gab1 PI3K-m, but not Gab1 SHP2-m, in TT human medullary thyroid carcinoma cells expressing RET with a multiple endocrine neoplasia 2A mutation enhanced cytochrome c release, and apoptosis induced by etoposide, suggesting that PI3K is involved in survival of TT cells via a mitochondrial pathway. These findings demonstrated that coupling of Gab1 to PI3K is important for biological responses in RET-expressing cells.

Differential requirement of Tyr1062 multidocking site by RET isoforms to promote neural cell scattering and epithelial cell branching

The receptor tyrosine kinase RET is alternatively spliced to yield two main isoforms, RET9 and RET51, which differ in their carboxyl terminal. Activated RET induces different biological responses such as morphological transformation, neurite outgrowth, proliferation, cell migration and branching. The two isoforms have been suggested to have separate intracellular signaling pathways and different roles in mouse development. Here we show that both isoforms are able to induce cell scattering of SK-N-MC neuroepithelioma cell line and branching tubule formation in MDCK cell line. However, the Y1062F mutation, which abrogates the transforming activity of both activated RET isoforms in NIH3T3 cells, does not abolish scattering and branching morphogenesis of RET51, whereas impairs these biological effects of RET9. The GDNF-induced biological effects of RET51 are inhibited by the simultaneous abrogation of both Tyr1062 and Tyr1096 docking sites. Thus, Tyr1096 may substitute the functions of Tyr1062. GRB2 is the only known adaptor protein binding to Tyr1096. Dominant-negative GRB2 expressed in MDCK cells together with RET9 or RET51 significantly reduces branching. Therefore, GRB2 is necessary for RET-mediated branching of MDCK cells.

A targeting mutation of tyrosine 1062 in Ret causes a marked decrease of enteric neurons and renal hypoplasia

The Ret receptor tyrosine kinase plays a crucial role in the development of the enteric nervous system and the kidney. Tyrosine 1062 in Ret represents a binding site for the phosphotyrosine-binding domains of several adaptor and effector proteins that are important for the activation of intracellular signaling pathways, such as the RAS/ERK, phosphatidylinositol 3-kinase/AKT, and Jun-associated N-terminal kinase pathways. To investigate the importance of tyrosine 1062 for organogenesis in vivo, knock-in mice in which tyrosine 1062 in Ret was replaced with phenylalanine were generated. Although homozygous knock-in mice were born normally, they died by day 27 after birth and showed growth retardation. The development of the enteric nervous system was severely impaired in homozygous mutant mice, about 40% of which lacked enteric neurons in the whole intestinal tract, as observed in Ret-deficient mice. The rest of the mutant mice developed enteric neurons in the intestine to various extents, although the size and number of ganglion cells were significantly reduced. Unlike Ret-deficient mice, a small kidney developed in all knock-in mice, accompanying a slight histological change. The reduction of kidney size was due to a decrease of ureteric bud branching during embryogenesis. Thus, these findings demonstrated that the signal via tyrosine 1062 plays an important role in histogenesis of the enteric nervous system and nephrogenesis.

Dok-6, a Novel p62 Dok family member, promotes Ret-mediated neurite outgrowth

Activation of Ret, the receptor-tyrosine kinase for the glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs), results in the recruitment and assembly of adaptor protein complexes that function to transduce signals downstream of the receptor. Here we identify Dok-6, a novel member of the Dok-4/5 subclass of the p62 Dok family of intracellular adaptor molecules, and characterize its interaction with Ret. Expression analysis reveals that Dok-6 is highly expressed in the developing central nervous system and is co-expressed with Ret in several locations, including sympathetic, sensory, and parasympathetic ganglia, as well as in the ureteric buds of the developing kidneys. Pull-down assays using the Dok-6 phosphotyrosine binding (PTB) domain and GDNF-activated Ret indicate that Dok-6 binds to the phosphorylated Ret Tyr(1062) residue. Moreover, ligand activation of Ret resulted in phosphorylation of tyrosine residue(s) located within the unique C terminus of Dok-6 predominantly through a Src-dependent mechanism, indicating that Dok-6 is a substrate of the Ret-Src signaling pathway. Interestingly, expression of Dok-6 potentiated GDNF-induced neurite outgrowth in GDNF family receptor alpha1 (GFRalpha1)-expressing Neuro2A cells that was dependent upon the C-terminal residues of Dok-6. Taken together, these data identify Dok-6 as a novel Dok-4/5-related adaptor molecule that may function in vivo to transduce signals that regulate Ret-mediated processes such as axonal projection.

The RET Receptor Is Linked to Stress Response Pathways

RET is a transmembrane receptor required for the development of neuroendocrine and urogenital cell types. Activation of RET has roles in cell growth, migration, or differentiation, yet little is known about the gene expression patterns through which these processes are mediated. We have generated cell lines stably expressing either the RET9 or RET51 protein isoforms and have used these to investigate RET-mediated gene expression patterns by cDNA microarray analyses. As seen for many oncogenes, we identified altered expression of genes associated generally with cell-cell or cell-substrate interactions and up-regulation of tumor-specific transcripts. We also saw increased expression of transcripts normally associated with neural crest or other RET-expressing cell types, suggesting these genes may lie downstream of RET activation in development. The most striking pattern of expression was up-regulation of stress response genes. We showed that RET expression significantly up-regulated the genes for heat shock protein (HSP) 70 family members, HSPA1A, HSPA1B, and HSPA1L. Other members of several HSP families and HSP70-interacting molecules that were associated with stress response protein complexes involved in protein maturation were also specifically up-regulated by RET, whereas those associated with the roles of HSP70 in protein degradation were down-regulated or unaffected. The major mechanism of stress response induction is activation of the heat shock transcription factor HSF1. We showed that RET expression leads to increased HSF1 activation, which correlates with increased expression of stress response genes. Together, our data suggest that RET may be directly responsible for expression of stress response proteins and the initiation of stress response.

Tyrosine 981, a novel ret autophosphorylation site, binds c-Src to mediate neuronal survival

The glial cell line-derived neurotrophic factor (GDNF) family ligands (GFLs) are neurotrophic factors that influence several aspects of the developing and injured nervous system. GFLs signal through a common receptor tyrosine kinase (Ret) and one of the four ligand-binding co-receptors (GFRalpha1 to 4). Ligand-induced translocation of Ret to lipid rafts, where it interacts with the nonreceptor tyrosine kinase Src, is a prerequisite for full biological activity of these neurotrophic factors. This interaction and subsequent activation of Src are required for GFL-mediated neuronal survival, neurite outgrowth, or cell proliferation. Here we show by multiple approaches that Ret tyrosine 981 constitutes the major binding site of the Src homology 2 domain of Src and therefore the primary residue responsible for Src activation upon Ret engagement. Other tyrosines such as 1015 and 1029 may contribute to the overall interaction between Ret and Src, as judged by overexpression experiments. By generating a phosphospecific antibody, we demonstrate that tyrosine 981 is a novel autophosphorylation site in Ret. Importantly, we also show that this tyrosine becomes phosphorylated in dissociated sympathetic neurons after ligand stimulation. Mutation of tyrosine 981 to phenylalanine reduces GDNF-mediated survival in a transfected cerebellar granule neuron paradigm.

Structural Basis for the Specific Recognition of RET by the Dok1 Phosphotyrosine Binding Domain

Dok1 is a common substrate of activated protein-tyrosine kinases. It is rapidly tyrosine-phosphorylated in response to receptor tyrosine activation and interacts with ras GTPase-activating protein and Nck, leading to inhibition of ras signaling pathway activation and the c-Jun N-terminal kinase (JNK) and c-Jun activation, respectively. In chronic myelogenous leukemia cells, it has shown constitutive phosphorylation. The N-terminal phosphotyrosine binding (PTB) domain of Dok1 can recognize and bind specifically to phosphotyrosine-containing motifs of receptors. Here we report the crystal structure of the Dok1 PTB domain alone and in complex with a phosphopeptide derived from RET receptor tyrosine kinase. The structure consists of a beta-sandwich composed of two nearly orthogonal, 7-stranded, antiparallel beta-sheets, and it is capped at one side by a C-terminal alpha-helix. The RET phosphopeptide binds to Dok1 via a surface groove formed between strand beta5 and the C-terminal alpha-helix of the PTB domain. The structures reveal the molecular basis for the specific recognition of RET by the Dok1 PTB domain. We also show that Dok1 does not recognize peptide sequences from TrkA and IL-4, which are recognized by Shc and IRS1, respectively.

Enhancement of Tyrosine Hydroxylase Phosphorylation and Activity by Glial Cell Line-derived Neurotrophic Factor

Although glial cell-line derived neurotrophic factor (GDNF) acts as a potent survival factor for dopaminergic neurons, it is not known whether GDNF can directly alter dopamine synthesis. Tyrosine hydroxylase (TH) is the rate-limiting enzyme for dopamine biosynthesis, and its activity is regulated by phosphorylation on three seryl residues: Ser-19, Ser-31, and Ser-40. Using a TH-expressing human neuroblastoma cell line and rat primary mesencephalic neuron cultures, the present study examined whether GDNF alters the phosphorylation of TH and whether these changes are accompanied by increased enzymatic activity. Exposure to GDNF did not alter the TH protein level in either neuroblastoma cells or in primary neurons. However, significant increases in the phosphorylation of Ser-31 and Ser-40 were detected within minutes of GDNF application in both cell types. Enhanced Ser-31 and Ser-40 phosphorylation was associated with increased TH activity but not dopamine synthesis in neuroblastoma cells, possibly because of the absence of l-aromatic amino acid decarboxylase activity in these cells. In contrast, increased phosphorylation of Ser-31 and Ser-40 was found to enhance dopamine synthesis in primary neurons. Pharmacological experiments show that Erk and protein kinase A phosphorylate Ser-31 and Ser-40, respectively, and that their inhibition blocked both TH phosphorylation and activity. Our results indicate that, in addition to its role as a survival factor for dopaminergic neurons, GDNF can directly increase dopamine synthesis.

Cell signalling and gene expression mediated by RET tyrosine kinase

Germline mutations of the RET proto-oncogene cause multiple endocrine neoplasia (MEN) 2A or 2B by different mechanisms. As is the case for other receptor tyrosine kinases, mutant RET recruits a variety of signalling molecules via phosphorylated tyrosine residues present in the kinase domain and carboxy-terminal tail. As we previously reported, the signaling via phosphorylated tyrosine 1062 plays a crucial role in the transforming activities of both RET-MEN2A and RET-MEN2B mutant protein. Interestingly, this single tyrosine residue represents a binding site for several signalling molecules including SHC, Enigma, SNT/FRS2, DOK and IRS1 and is responsible for activation of the RAS/ERK, PI3-K/AKT, JNK, p38MAPK and ERK5 signalling pathways. Amongst these, the PI3-K/AKT and JNK pathways appeared to be more strongly activated in the cells expressing RET-MEN2B than in the cells expressing RET-MEN2A, suggesting the possibility that these pathways may be involved in the disease phenotype. In addition, RET is alternatively spliced to produce three isoforms and the splicing site is present just downstream of tyrosine 1062. These isoforms play different roles for the tumour development associated with MEN 2 or the development of the kidney and the enteric nervous system. Moreover, using differential display analysis, we identified several genes whose expression is highly induced by RET-MEN2B mutant proteins. The differential gene expression by RET-MEN2A and RET-MEN2B may also be important for the development of their phenotypes.

Protein kinase Calpha activation by RET: evidence for a negative feedback mechanism controlling RET tyrosine kinase

We have studied the role of protein kinase C (PKC) in signaling of the RET tyrosine kinase receptor. By using a chimeric receptor (E/R) in which RET kinase can be tightly controlled by the addition of epidermal growth factor (EGF), we have found that RET triggering induces a strong increase of PKCalpha, PKCdelta and PKCzeta activity and that PKCalpha, not PKCdelta and PKCzeta, forms a ligand-dependent protein complex with E/R. We have identified tyrosine 1062 in the RET carboxyl-terminal tail as the docking site for PKCalpha. Block of PKC activity by bisindolylmaleimide or chronic phorbol esters treatment decreased EGF-induced serine/threonine phosphorylation of E/R, while it caused a similarly sized increase of EGF-induced E/R tyrosine kinase activity and mitogenic signaling. Conversely, acute phorbol esters treatment, which promotes PKC activity, increased the levels of E/R serine/threonine phosphorylation and significantly decreased its phosphotyrosine content. A threefold reduction of tyrosine phosphorylation levels of the constitutively active RET/MEN2A oncoprotein was observed upon coexpression with PKCalpha. We conclude that RET binds to and activates PKCalpha. PKCalpha, in turn, causes RET phosphorylation and downregulates RET tyrosine kinase and downstream signaling, thus functioning as a negative feedback loop to modulate RET activity.

RET and NTRK1 proto-oncogenes in human diseases

RET and NTRK1 are receptor tyrosine kinase (RTK) proteins which play a role in the development and maturation of specific component of the nervous system. Their alterations have been associated to several human diseases, including some forms of cancer and developmental abnormalities. These features have contributed to the concept that one gene can be responsible for more than one disease. Moreover, both genes encoding for the two RTKs show genetic alterations that belong to either "gain of function" or "loss of function" class of mutations. In fact, receptor rearrangements or point mutations convert RET and NTRK1 in dominantly acting transforming genes leading to thyroid tumors, whereas inactivating mutations, associated with Hirschsprung's disease (HSCR) and congenital insensitivity to pain with anhidrosis (CIPA), impair RET and NTRK1 functions, respectively. In this review we have summarized the main features of the two receptors, their physiological and pathological roles. In addition, we attempted to identify the correlations between the different genetic alterations and the related pathogenetic mechanisms.

The neuron-specific Rai (ShcC) adaptor protein inhibits apoptosis by coupling Ret to the phosphatidylinositol 3-kinase/Akt signaling pathway

Rai is a recently identified member of the family of Shc-like proteins, which are cytoplasmic signal transducers characterized by the unique PTB-CH1-SH2 modular organization. Rai expression is restricted to neuronal cells and regulates in vivo the number of postmitotic sympathetic neurons. We report here that Rai is not a common substrate of receptor tyrosine kinases under physiological conditions and that among the analyzed receptors (Ret, epidermal growth factor receptor, and TrkA) it is activated specifically by Ret. Overexpression of Rai in neuronal cell lines promoted survival by reducing apoptosis both under conditions of limited availability of the Ret ligand glial cell line-derived neurotrophic factor (GDNF) and in the absence of Ret activation. Overexpressed Rai resulted in the potentiation of the Ret-dependent activation of phosphatidylinositol 3-kinase (PI3K) and Akt. Notably, increased Akt phosphorylation and PI3K activity were also found under basal conditions, e.g., in serum-starved neuronal cells. Phosphorylated and hypophosphorylated Rai proteins form a constitutive complex with the p85 subunit of PI3K: upon Ret triggering, the Rai-PI3K complex is recruited to the tyrosine-phosphorylated Ret receptor through the binding of the Rai PTB domain to tyrosine 1062 of Ret. In neurons treated with low concentrations of GDNF, the prosurvival effect of Rai depends on Rai phosphorylation and Ret activation. In the absence of Ret activation, the prosurvival effect of Rai is, instead, phosphorylation independent. Finally, we showed that overexpression of Rai, at variance with Shc, had no effects on the early peak of mitogen-activated protein kinase (MAPK) activation, whereas it increased its activation at later time points. Phosphorylated Rai, however, was not found in complexes with Grb2. We propose that Rai potentiates the MAPK and PI3K signaling pathways and regulates Ret-dependent and -independent survival signals.

The long and short isoforms of Ret function as independent signaling complexes

Ret, the receptor tyrosine kinase for the glial cell line-derived neurotrophic factor family ligands (GFLs), is alternatively spliced to yield at least two isoforms, Ret9 and Ret51, which differ only in their C termini. To identify tyrosines in Ret that are autophosphorylation sites in neurons, we generated antibodies specific to phosphorylated Y905Ret, Y1015Ret, Y1062Ret, and Y1096Ret, all of which are autophosphorylated in cell lines. All four of these tyrosines in Ret became phosphorylated rapidly upon activation by GFLs in sympathetic neurons. These tyrosines remained phosphorylated in sympathetic neurons in the continued presence of GFLs, albeit at a lower level than immediately after GFL treatment. Comparison of GFL activation of Ret9 and Ret51 revealed that phosphorylation of Tyr(905) and Tyr(1062) was greater and more sustained in Ret9 as compared with Ret51. In contrast, Tyr(1015) was more highly phosphorylated over time in Ret51 than in Ret9. Surprisingly, Ret9 and Ret51 did not associate with each other in sympathetic neurons after glial cell line-derived neurotrophic factor stimulation, even though they share identical extracellular domains. Furthermore, the signaling complex associated with Ret9 was markedly different from the Ret51-associated signaling complex. Taken together, these data provide a biochemical basis for the dramatic functional differences between Ret9 and Ret 51 in vivo.

Role of Dok1 in cell signaling mediated by RET tyrosine kinase

Using a yeast two-hybrid screen, we identified Dok1 as a docking protein for RET tyrosine kinase. Dok1 bound more strongly to RET with a multiple endocrine neoplasia (MEN) 2B mutation than RET with a MEN2A mutation and was highly phosphorylated in the cells expressing the former mutant protein. Analysis by site-directed mutagenesis revealed that tyrosine 361 in mouse Dok1 represents a binding site for the Nck adaptor protein and tyrosines 295, 314, 361, 376, 397, and 408 for the Ras-GTPase-activating protein. We replaced tyrosine 361 or these six tyrosines with phenylalanine (designated Y361F or 6F) in Dok1 and introduced the mutant Dok1 genes into the cells expressing the wild-type RET or RET-MEN2B protein. Overexpression of Dok1 or Dok1-Y361F, but not Dok1-6F, suppressed the Ras/Erk activation induced by glial cell line-derived neurotrophic factor or RET-MEN2B, implying that this inhibitory effect requires the Ras-GTPase-activating protein binding to Dok1. In contrast, overexpression of Dok1, but not Dok1-Y361F or Dok1-6F, enhanced the c-Jun amino-terminal kinase (JNK) and c-Jun activation. This suggested that the association of Nck to tyrosine 361 in Dok1 is necessary for the JNK and c-Jun activation by glial cell line-derived neurotrophic factor or RET-MEN2B. Because a high level of the JNK phosphorylation was observed in the cells expressing RET-MEN2B, its strong activation via Nck binding to Dok1 may be responsible for aggressive properties of medullary thyroid carcinoma developed in MEN 2B.

Differential interaction of Enigma protein with the two RET isoforms

The receptor tyrosine kinase RET, with a known role in embryonic development and in human pathologies, is alternatively spliced to yield at least two functional isoforms, which differ only in their carboxyl terminal. Enigma protein is a member of the PDZ-LIM family and is known to interact with the short isoform of RET/PTC2, a chimeric oncoprotein isolated from papillary thyroid carcinoma. Here, we show that Enigma also interacts in intact cells with the short isoform of RET-wt and of its pathologic mutants associated to MEN2 syndromes, RET-C634R and RET-M918T. In contrast, Enigma binds all the corresponding RET long isoforms very poorly and colocalizes with short but not long RET/PTC2 isoforms. The RET docking tyrosine for Enigma is the last but one before the divergence between the two isoforms and we demonstrated that short-isoform-specific amino acid residues +2 to +4 to this tyrosine are required for the interaction of RET/PTC2 with Enigma.

Activation of RET tyrosine kinase regulates interleukin-8 production by multiple signaling pathways

Interleukin-8 (IL-8) is known to contribute to human cancer progression through its potential function as a mitogenic, angiogenic, or motogenic factor. We found a high level of IL-8 production in SK-N-MC human primitive neuroectodermal tumor cells transfected with the human RET gene (SK-N-MC (RET) cells) in response to glial cell line-derived neurotrophic factor (GDNF) stimulation. IL-8 was also produced at high levels in TT human medullary thyroid carcinoma and TPC-1 human papillary thyroid carcinoma cell lines both of which express activated RET tyrosine kinase. To investigate which signaling pathways are responsible for IL-8 expression, we treated SK-N-MC (RET) cells with several kinase inhibitors before GDNF stimulation. The results showed that a MEK1 inhibitor, PD98059, a p38MAPK inhibitor, SB202190, and a protein kinase C (PKC) inhibitor, Calphostin C, markedly decreased the IL-8 secretion from SK-N-MC (RET) cells at 24 h after GDNF stimulation. In contrast, a phosphatidylinositol 3-kinase (PI3-K) inhibitor, LY294002, increased its secretion. These results thus suggested that IL-8 production by RET tyrosine kinase is regulated by multiple signaling pathways.

Novel mechanism of regulation of Rac activity and lamellipodia formation by RET tyrosine kinase

Rac activation in neuronal cells plays an important role in lamellipodia formation that is a critical event for neuritogenesis. It is well known that the Rac activity is regulated via activation of phosphatidylinositol 3-kinase (PI3K) by a variety of receptor tyrosine kinases. Here we show that increased serine phosphorylation on RET receptor tyrosine kinase following cAMP elevation promotes lamellipodia formation of neuronal cells induced by glial cell line-derived neurotrophic factor (GDNF). We identified serine 696 in RET as a putative phosphorylation site by protein kinase A and found that mutation of this serine almost completely inhibited lamellipodia formation by GDNF without affecting activation of the PI3K/AKT signaling pathway. Mutation of tyrosine 1062 in RET, whose phosphorylation is crucial for activation of PI3K, also inhibited lamellipodia formation by GDNF. Inhibition of lamellipodia formation by mutation of either serine 696 or tyrosine 1062 was associated with decrease of the Rac1-guanine nucleotide exchange factor (GEF) activity, suggesting that this activity is regulated by two different signaling pathways via serine 696 and tyrosine 1062 in RET. Moreover, in the presence of serine 696 mutation, lamellipodia formation was rescued by replacing tyrosine 687 with phenylalanine. These findings propose a novel mechanism that receptor tyrosine kinase modulates actin dynamics in neuronal cells via its cAMP-dependent phosphorylation.

Ureteric bud outgrowth in response to RET activation is mediated by phosphatidylinositol 3-kinase

The c-ret gene encodes a receptor tyrosine kinase (RET) essential for the development of the kidney and enteric nervous system. Activation of RET requires the secreted neurotrophin GDNF (glial cell line-derived neurotrophic factor) and its high affinity receptor, a glycosyl phosphatidylinositol-linked cell surface protein GFRalpha1. In the developing kidney, RET, GDNF, and GFRalpha1 are all required for directed outgrowth and branching morphogenesis of the ureteric bud epithelium. Using MDCK renal epithelial cells as a model system, activation of RET induces cell migration, scattering, and formation of filopodia and lamellipodia. RET-expressing MDCK cells are able to migrate toward a localized source of GDNF. In this report, the intracellular signaling mechanisms regulating RET-dependent migration and chemotaxis are examined. Activation of RET resulted in increased levels of phosphatidylinositol 3-kinase (PI3K) activity and Akt/PKB phosphorylation. This increase in PI3K activity is essential for regulating the GDNF response, since the specific inhibitor, LY294002, blocks migration and chemotaxis of MDCK cells. Using an in vitro organ culture assay, inhibition of PI3K completely blocks the GDNF-dependent outgrowth of ectopic ureter buds. PI3K is also essential for branching morphogenesis once the ureteric bud has invaded the kidney mesenchyme. The data suggest that activation of RET in the ureteric bud epithelium signals through PI3K to control outgrowth and branching morphogenesis.

Shc and CEACAM1 interact to regulate the mitogenic action of insulin

CEACAM1, a tumor suppressor (previously known as pp120), is a plasma membrane protein that undergoes phosphorylation on Tyr(488) in its cytoplasmic tail by the insulin receptor tyrosine kinase. Co-expression of CEACAM1 with insulin receptors decreased cell growth in response to insulin. Co-immunoprecipitation experiments in intact NIH 3T3 cells and glutathione S-transferase pull-down assays revealed that phosphorylated Tyr(488) in CEACAM1 binds to the SH2 domain of Shc, another substrate of the insulin receptor. Overexpressing Shc SH2 domain relieved endogenous Shc from binding to CEACAM1 and restored MAP kinase activity, growth of cells in response to insulin, and their colonization in soft agar. Thus, by binding to Shc, CEACAM1 sequesters this major coupler of Grb2 to the insulin receptor and down-regulates the Ras/MAP kinase mitogenesis pathway. Additionally, CEACAM1 binding to Shc enhances its ability to compete with IRS-1 for phosphorylation by the insulin receptor. This leads to a decrease in IRS-1 binding to phosphoinositide 3'-kinase and to the down-regulation of the phosphoinositide 3'-kinase/Akt pathway that mediates cell proliferation and survival. Thus, binding to Shc appears to constitute a major mechanism for the down-regulatory effect of CEACAM1 on cell proliferation.

RET/PTC1 oncogene signaling in PC Cl 3 thyroid cells requires the small GTP-binding protein Rho

Thyroid papillary carcinomas are characterized by RET/PTC rearrangements that cause the tyrosine kinase domain of the RET receptor to fuse with N-terminal sequences encoded by heterologous genes. This results in the aberrant expression of a ligand-independent and constitutively active RET kinase. We analysed actin reorganization induced by the RET/PTC1 oncogene in PC Cl 3 rat thyroid epithelial cells. Differently from oncogenes Src, Ras and Raf, RET/PTC1 caused actin filaments to form prominent stress fibers. Moreover, stress fibers were identified in human thyroid papillary carcinoma cell lines harboring RET/PTC1 rearrangements but not in thyroid carcinoma cells negative for RET/PTC rearrangements. RET/MEN 2A, a constitutively active but unrearranged membrane-bound RET oncoprotein, did not induce stress fibers in PC Cl 3 cells. Induction of stress fibers by RET/PTC1 was restricted to thyroid cells; it did not occur in NIH3T3 fibroblasts or MCF7 mammary cells. RET/PTC1-mediated stress fiber formation depended on Rho but not Rac small GTPase activity. In addition, inhibition of Rho, but not of Rac, caused apoptosis of RET/PTC1-expressing thyroid cells. We conclude that Rho is implicated in the actin reorganization and cell survival mediated by the chimeric RET/PTC1 oncogene in thyroid epithelial cells, both phenotypes being cell type- and oncogene type-specific.

The RET receptor: function in development and dysfunction in congenital malformation

Germline mutations in the RET proto-oncogene are responsible for two unrelated neural crest disorders: Hirschsprung disease, a congenital absence of the enteric nervous system in the hindgut, and multiple endocrine neoplasia type 2, a dominantly inherited cancer syndrome. Moreover, somatic rearrangements of RET are causally involved in the genesis of papillary thyroid carcinoma. The receptor tyrosine kinase encoded by the RET gene acts as the subunit of a multimolecular complex that binds four distinct ligands and activates a signalling network crucial for neural and kidney development. Over the past few years, a clearer picture of the mode of RET activation and of its multifaceted role during development has started to emerge. These findings, which provide new clues to the molecular mechanisms underlying RET signalling dysfunction in Hirschsprung disease, are summarized in this review.