Shc and Enigma are both required for mitogenic signaling by Ret/ptc2

Regulatory and Interacting Partners of PDLIM7 in Thyroid Cancer

Enigma protein, encoded by the PDLIM7 gene, is overexpressed in thyroid cancer in a stage-dependent manner, suggesting a potential involvement in the initiation and progression of thyroid cancer. The Enigma interacts with several cellular pathways, including PI3K/AKT, MDM2, and BMP-1. The Enigma is regulated by microRNAs. Specifically, we showed that the Enigma protein upregulation corresponds to the downregulation of Let-7 family genes. There is limited research on the interactions and regulation of the Enigma with other proteins/genes in thyroid cancer tissues, indicating a gap in current knowledge. Our aim is to establish the Enigma as a biomarker. We also aim to study the interacting partners of the Enigma signaling pathways and their probable miRNA regulation in thyroid cancer progression. Using Western blotting, densitometric analysis, immunoprecipitation (IP), and reverse IP, we detected the protein expression and protein-protein interactions in the corresponding papillary thyroid carcinomas (PTCs). Utilizing real-time qPCR assay and Pearson's correlation test, we highlighted the correlation between PDLIM7 and Let-7g gene expression in the same tissues. The results showed the differential upregulations of the Enigma protein in different stages of PTCs compared to benign tissues along with AKT, VDR, BMP-1, and MDM2 proteins. Loss of DBP was observed in a subset of PTCs. Strong interactions of the Enigma with PI3K/AKT and MDM2 were noted, along with a weaker BMP-1 interaction. Pearson's correlation coefficient analysis between PDLIM7 and let-7g gene expression was significant (p < 0.05); however, there was a weak inverse correlation (r = -0.27). The study suggests the potential utility of the PDLIM7-qPCR assay as a biomarker for thyroid cancer. The Enigma's interactions with key signaling pathways may provide valuable insights into the development of thyroid cancer. The study contributes to understanding the molecular mechanisms involving the Enigma protein in thyroid cancer and highlights its potential as a biomarker.

The unexpected versatility of ALP/Enigma family proteins

One of the most intriguing features of multicellular animals is their ability to move. On a cellular level, this is accomplished by the rearrangement and reorganization of the cytoskeleton, a dynamic network of filamentous proteins which provides stability and structure in a stationary context, but also facilitates directed movement by contracting. The ALP/Enigma family proteins are a diverse group of docking proteins found in numerous cellular milieus and facilitate these processes among others. In vertebrates, they are characterized by having a PDZ domain in combination with one or three LIM domains. The family is comprised of CLP-36 (PDLIM1), Mystique (PDLIM2), ALP (PDLIM3), RIL (PDLIM4), ENH (PDLIM5), ZASP (PDLIM6), and Enigma (PDLIM7). In this review, we will outline the evolution and function of their protein domains which confers their versatility. Additionally, we highlight their role in different cellular environments, focusing specifically on recent advances in muscle research using Drosophila as a model organism. Finally, we show the relevance of this protein family to human myopathies and the development of muscle-related diseases.

Ret signaling in ureteric bud epithelial cells controls cell movements, cell clustering and bud formation

Ret signaling promotes branching morphogenesis during kidney development, but the underlying cellular mechanisms remain unclear. While Ret-expressing progenitor cells proliferate at the ureteric bud tips, some of these cells exit the tips to generate the elongating collecting ducts, and in the process turn off Ret. Genetic ablation of Ret in tip cells promotes their exit, suggesting that Ret is required for cell rearrangements that maintain the tip compartments. Here, we examine the behaviors of ureteric bud cells that are genetically forced to maintain Ret expression. These cells move to the nascent tips, and remain there during many cycles of branching; this tip-seeking behavior may require positional signals from the mesenchyme, as it occurs in whole kidneys but not in epithelial ureteric bud organoids. In organoids, cells forced to express Ret display a striking self-organizing behavior, attracting each other to form dense clusters within the epithelium, which then evaginate to form new buds. The ability of forced Ret expression to promote these events suggests that similar Ret-dependent cell behaviors play an important role in normal branching morphogenesis.

Pathologic significance of a novel oncoprotein in thyroid cancer progression

BACKGROUND

The incidence of thyroid cancer is increasing worldwide, and there is an emerging need to develop accurate tools for diagnosis. Fine needle aspiration biopsy has greatly improved evaluation of thyroid nodules, but challenges with indeterminate lesions remain in up to 25% of biopsies. Novel tissue biomarkers may assist in improved nodule characterization. Microcalcifications occurring in thyroid cancers suggest proteins involved in bone formation may play a role in thyroid carcinogenesis. We evaluated the expression of the known osteogenic protein, Enigma, in thyroid cancer as a candidate oncoprotein and role in carcinogenesis based on association with other known oncoproteins such as bone morphogenetic protein-1 (BMP-1).

METHODS

The expression of both Enigma and BMP-1 were evaluated by immunohistochemistry (IHC) in an equal number of benign (n = 120) and different histological subtypes of malignant (n = 120) human archival thyroid nodules with and without calcification. The colocalization of Enigma with BMP-1 was evaluated by confocal microscopy using the BZ analyzer.

RESULTS

Enigma was strongly expressed in thyroid cancer tissue with a higher immunoreactive score in advanced thyroid cancer compared to less advanced and benign nodules. Enigma was localized either in cytoplasm or nucleus depending on the histological subtypes. Higher expression of Enigma was associated with the tumor size and lymph node involvement. There was clear and strong colocalization signal of Enigma and that of BMP-1. Expression of Enigma occurred without regard to calcification in cancer tissue.

CONCLUSION

Enigma may serve as an oncoprotein marker, identifying benign from malignant thyroid tissue on FNA. Enigma may have a role in carcinogenesis of thyroid cancer independent of tissue calcification, possibly in relation to interaction with BMP-1.

Enigma prevents Cbl-c-mediated ubiquitination and degradation of RETMEN2A

The Cbl proteins (Cbl, Cbl-b, and Cbl-c) are a highly conserved family of RING finger ubiquitin ligases (E3s) that function as negative regulators of tyrosine kinases in a wide variety of signal transduction pathways. In this study, we identify a new Cbl-c interacting protein, Enigma (PDLIM7). This interaction is specific to Cbl-c as Enigma fails to bind either of its closely related homologues, Cbl and Cbl-b. The binding between Enigma and Cbl-c is mediated through the LIM domains of Enigma as removal of all three LIM domains abrogates this interaction, while only LIM1 is sufficient for binding. Here we show that Cbl-c binds wild-type and MEN2A isoforms of the receptor tyrosine kinase, RET, and that Cbl-c enhances ubiquitination and degradation of activated RET. Enigma blocks Cbl-c-mediated RETMEN2A ubiquitination and degradation. Cbl-c decreased downstream ERK activation by RETMEN2A and co-expression of Enigma blocked the Cbl-c-mediated decrease in ERK activation. Enigma showed no detectable effect on Cbl-c-mediated ubiquitination of activated EGFR suggesting that this effect is specific to RET. Through mapping studies, we show that Cbl-c and Enigma bind RETMEN2A at different residues. However, binding of Enigma to RETMENA prevents Cbl-c recruitment to RETMEN2A. Consistent with these biochemical data, exploratory analyses of breast cancer patients with high expression of RET suggest that high expression of Cbl-c correlates with a good outcome, and high expression of Enigma correlates with a poor outcome. Together, these data demonstrate that Cbl-c can ubiquitinate and downregulate RETMEN2A and implicate Enigma as a positive regulator of RETMEN2A through blocking of Cbl-mediated ubiquitination and degradation.

Dietary lysine imbalance affects muscle proteome in zebrafish (Danio rerio): a comparative 2D-DIGE study

Lysine (Lys) is an indispensable amino acid (AA) and generally the first limiting AA in vegetable protein sources in fish feeds. Inadequate dietary Lys availability may limit protein synthesis, accretion and growth of fish. This experiment aimed to further elucidate the role of Lys imbalance on growth by examining the myotomal muscle proteome of juvenile zebrafish (Danio rerio). Quadruplicate groups of 8 fish were fed either a low-Lys [Lys(-), 1.34 g kg(-1)], medium/control (Lys, 2.47 g kg(-1)) or high-Lys [Lys(+), 4.63 g kg(-1)] diet. Fish growth was monitored from 33 to 49 days post-fertilization (dpf) and trunk myotomal muscle proteome of Lys(-) and Lys(+) treatments were screened by 2D-DIGE and MALDI ToF tandem mass spectrometry. Growth rate was negatively affected by diet Lys(-). Out of 527 ± 11 (mean ± S.E.M.) protein spots detected (∼10-150 kDa and 4-7 pI value), 30 were over-expressed and 22 under-expressed in Lys(-) fish (|fold-change| >1.2, p value <0.05). Higher myosin light chains abundance and other myofibrillar proteins in Lys(-) fish pointed to increased sarcomeric degradation, indicating a higher protein turnover for supplying basal energy-saving metabolism rather than growth and muscle protein accretion. The Lys deficiency also possibly induced a higher feeding activity, reflected in the over-expression of beta enolase and mitochondrial ATP synthase. Contrarily, in the faster growing fish [Lys(+)], over-expression of apolipoprotein A-I, F-actin capping protein and Pdlim7 point to increased energy storage as fat and enhanced muscle growth, particularly by mosaic hyperplasia. Thus using an exploratory approach, this study pinpoints interesting candidates for further elucidating the role of dietary Lys on growth of juvenile fish.

Enigma negatively regulates p53 through MDM2 and promotes tumor cell survival in mice

The human E3 ubiquitin ligase murine double minute 2 (MDM2) targets the tumor suppressor p53 for ubiquitination and degradation but also promotes its own ubiquitination and subsequent degradation. As the balance between MDM2 and p53 levels plays a crucial role in regulating cell proliferation and apoptosis, we sought to identify factors selectively inhibiting MDM2 self-ubiquitination. Here we have shown that the LIM domain protein Enigma directly interacts with MDM2 to form a ternary complex with p53 in vitro and in human hepatoma and colon carcinoma cell lines and mouse embryonic fibroblasts. We found that Enigma elicited p53 degradation by inhibiting MDM2 self-ubiquitination and increasing its ubiquitin ligase activity toward p53 in cells. Moreover, mitogenic stimuli such as serum, FGF, and HGF increased Enigma transcription via induction of serum response factor (SRF), leading to MDM2 stabilization and subsequent p53 degradation. We observed similar results in the livers of mice treated with HGF. In humans, we found SRF and Enigma coexpressed with MDM2 but not p53 in several liver and stomach tumors. Finally, we showed that Enigma promoted cell survival and chemoresistance by suppressing p53-mediated apoptosis in both cell lines and a mouse xenograft model. Our findings suggest a role for Enigma in tumorigenesis and uncover a mechanism whereby mitogens attenuate p53 antiproliferative activity through an SRF/Enigma/MDM2 pathway.

LMP1 regulates periodontal ligament progenitor cell proliferation and differentiation

LMP1 is an intracellular scaffold protein that contains a PDZ domain and three LIM domains. LMP1 has multiple functions including regulating mesenchymal stem cell (MSC) osteogenesis. Gene delivery of LMP1 induces bone formation in vivo in heterotopic and orthotopic sites. However, little is known about the physiological function and gene regulatory mechanisms of LMP1 in MSCs at the molecular level. Periodontal ligament (PDL) cells are a unique progenitor cell population that can differentiate into multiple cell types, including osteoblasts, adipocytes, or chondrocytes. This study sought to determine the physiological function and gene regulatory mechanisms of LMP1 in PDL cells at the molecular level. We show that LMP1 is upregulated in early stage of PDL cell osteogenic differentiation. Stable gene knockdown of LMP1 by shRNA inhibits DNA synthesis and corresponding cell proliferation in PDL cells, and further leads to decreased mineralization in vitro. Overexpression of LMP1 increases cell proliferation, and PDZ and ww-interacting domains are not sufficient to mediate this effect. Further, we found that in PDL cells, LMP1 is a downstream target gene of TGF-beta1 that is an early signal critical in preosteoblast proliferation and differentiation. TGF-beta1 stimulates PDL cell proliferation, however, this effect is compromised when LMP1 is knocked down. We further identified that the activation of TAK1-JNK/p38 kinase cascade is involved in the LMP1 gene regulation by TGF-beta1. We conclude that LMP1 is a downstream gene of TGF-beta1, involved in PDL cell proliferation. Our findings advance the understanding of the physiological function of LMP1 and define a regulatory mechanism of LMP1 in PDL progenitor cells and other MSCs.

Nucleocytoplasmic functions of the PDZ-LIM protein family: new insights into organ development

Recent work on the PDZ-LIM protein family has revealed that it has important activities at the cellular level, mediating signals between the nucleus and the cytoskeleton, with significant impact on organ development. We review and integrate current knowledge about the PDZ-LIM protein family and propose a new functional role, sequestering nuclear factors in the cytoplasm. Characterized by their PDZ and LIM domains, the PDZ-LIM family is comprised of evolutionarily conserved proteins found throughout the animal kingdom, from worms to humans. Combining two functional domains in one protein, PDZ-LIM proteins have wide-ranging and multi-compartmental cell functions during development and homeostasis. In contrast, misregulation can lead to cancer formation and progression. New emerging roles include interactions with integrins, T-box transcription factors, and receptor tyrosine kinases. Facilitating the assembly of protein complexes, PDZ-LIM proteins can act as signal modulators, influence actin dynamics, regulate cell architecture, and control gene transcription.

ALP/Enigma PDZ-LIM domain proteins in the heart

Actinin-associated LIM protein (ALP) and Enigma are two subfamilies of Postsynaptic density 95, discs large and zonula occludens-1 (PDZ)-Lin-11, Isl1 and Mec-3 (LIM) domain containing proteins. ALP family members have one PDZ and one LIM domain, whereas Enigma proteins contain one PDZ and three LIM domains. Four ALP and three Enigma proteins have been identified in mammals, each having multiple splice variants and unique expression patterns. Functionally, these proteins bind through their PDZ domains to alpha-actinin and bind through their LIM domains or other internal protein interaction domains to other proteins, including signaling molecules. ALP and Enigma proteins have been implicated in cardiac and skeletal muscle structure, function and disease, neuronal function, bipolar disorder, tumor growth, platelet and epithelial cell motility and bone formation. This review will focus on recent advances in the biological roles of ALP/Enigma PDZ-LIM domain proteins in cardiac muscle and provide insights into mechanisms by which mutations in these proteins are related to human cardiac disease.

RET/PTC-induced cell growth is mediated in part by epidermal growth factor receptor (EGFR) activation: evidence for molecular and functional interactions between RET and EGFR

RET/PTC rearrangements are one of the genetic hallmarks of papillary thyroid carcinomas. RET/PTC oncoproteins lack extracellular or transmembrane domains, and activation takes place through constitutive dimerization mediated through coiled-coil motifs in the NH(2) terminus of the chimeric protein. Based on the observation that the epidermal growth factor receptor (EGFR) kinase inhibitor PKI166 decreased RET/PTC kinase autophosphorylation and activation of downstream effectors in thyroid cells, despite lacking activity on the purified RET kinase, we proceeded to examine possible functional interactions between RET/PTC and EGFR. Conditional activation of RET/PTC oncoproteins in thyroid PCCL3 cells markedly induced expression and phosphorylation of EGFR, which was mediated in part through mitogen-activated protein kinase signaling. RET and EGFR were found to coimmunoprecipitate. The ability of RET to form a complex with EGFR was not dependent on recruitment of Shc or on their respective kinase activities. Ligand-induced activation of EGFR resulted in phosphorylation of a kinase-dead RET, an effect that was entirely blocked by PKI166. These effects were biologically relevant, as the EGFR kinase inhibitors PKI166, gefitinib, and AEE788 inhibited cell growth induced by various constitutively active mutants of RET in thyroid cancer cells as well as NIH3T3 cells. These data indicate that EGFR contributes to RET kinase activation, signaling, and growth stimulation and may therefore be an attractive therapeutic target in RET-induced neoplasms.

PTEN modulates GDNF/RET mediated chemotaxis and branching morphogenesis in the developing kidney

The RET receptor tyrosine kinase is activated by GDNF and controls outgrowth and invasion of the ureteric bud epithelia in the developing kidney. In renal epithelial cells and in enteric neuronal precursor cells, activation of RET results in chemotaxis as Ret expressing cells invade the surrounding GDNF expressing tissue. One potential downstream signaling pathway governing RET mediated chemotaxis may require phosphatidylinositol 3-kinase (PI3K), which generates PI(3,4,5) triphosphate. The PTEN tumor suppressor gene encodes a protein and lipid phosphatase that regulates cell growth, apoptosis and many other cellular processes. PTEN helps regulate cellular chemotaxis by antagonizing the PI3K signaling pathway through dephosphorylation of phosphotidylinositol triphosphates. In this report, we show that PTEN suppresses RET mediated cell migration and chemotaxis in cell culture assays, that RET activation results in asymmetric localization of inositol triphosphates and that loss of PTEN affects the pattern of branching morphogenesis in developing mouse kidneys. These data suggest a critical role for the PI3K/PTEN axis in shaping the pattern of epithelial branches in response to RET activation.

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.

Enigma interacts with adaptor protein with PH and SH2 domains to control insulin-induced actin cytoskeleton remodeling and glucose transporter 4 translocation

APS (adaptor protein with PH and SH2 domains) initiates a phosphatidylinositol 3-kinase-independent pathway involved in insulin-stimulated glucose transport. We recently identified Enigma, a PDZ and LIM domain-containing protein, as a partner of APS and showed that APS-Enigma complex plays a critical role in actin cytoskeleton organization in fibroblastic cells. Because actin rearrangement is important for insulin-induced glucose transporter 4 (Glut 4) translocation, we studied the potential involvement of Enigma in insulin-induced glucose transport in 3T3-L1 adipocytes. Enigma mRNA was expressed in differentiated adipocytes and APS and Enigma were colocalized with cortical actin. Expression of an APS mutant unable to bind Enigma increased the insulin-induced Glut 4 translocation to the plasma membrane. By contrast, overexpression of Enigma inhibited insulin-stimulated glucose transport and Glut 4 translocation without alterations in proximal insulin signaling. This inhibitory effect was prevented with the deletion of the LIM domains of Enigma. Using time-lapse fluorescent microscopy of green fluorescent protein-actin, we demonstrated that the overexpression of Enigma altered insulin-induced actin rearrangements, whereas the expression of Enigma without its LIM domains was without effect. A physiological link between increased expression of Enigma and an alteration in insulin-induced glucose uptake was suggested by the increase in Enigma mRNA expression in adipose tissue of diabetic obese patients. Taken together, these data strongly suggest that the interaction between APS and Enigma is involved in insulin-induced Glut 4 translocation by regulating cortical actin remodeling and raise the possibility that modification of APS/Enigma ratio could participate in the alteration of insulin-induced glucose uptake in adipose tissue.

The Receptor-Type Protein Tyrosine Phosphatase J Antagonizes the Biochemical and Biological Effects of RET-Derived Oncoproteins

Thyroid cancer is frequently associated with the oncogenic conversion of the RET receptor tyrosine kinase. RET gene rearrangements, which lead to the generation of chimeric RET/papillary thyroid carcinoma (PTC) oncogenes, occur in PTC, whereas RET point mutations occur in familial multiple endocrine neoplasia type 2 (MEN2) and sporadic medullary thyroid carcinomas (MTC). We showed previously that the expression of the receptor-type protein tyrosine phosphatase J (PTPRJ) is suppressed in neoplastically transformed follicular thyroid cells. We now report that PTPRJ coimmunoprecipitates with wild-type RET and with the MEN2A-associated RET(C634R) oncoprotein but not with the RET/PTC1 and RET-MEN2B isoforms. Using mutated forms of PTPRJ and RET-MEN2A, we show that the integrity of the respective catalytic domains is required for the PTPRJ/RET-MEN2A interaction. PTPRJ expression induces dephosphorylation of the RET(C634R) and, probably via an indirect mechanism, RET/PTC1 oncoproteins on two key RET autophosphorylation sites (Tyr1062 and Tyr905). This results in a significant decrease of RET-induced Shc and extracellular signal-regulated kinase 1/2 phosphorylation levels. In line with this finding, adoptive PTPRJ expression reduced the oncogenic activity of RET(C634R) in an in vitro focus formation assay of NIH3T3 cells. As expected from the coimmunoprecipitation results, the RET(M918T) oncoprotein, which is associated to MEN2B and sporadic MTC, was resistant to the dephosphorylating activity of PTPRJ. Taken together, these findings identify RET as a novel substrate of PTPRJ and suggest that PTPRJ expression levels may affect tumor phenotype associated with RET/PTC1 and RET(C634R) mutants. On the other hand, resistance to PTPRJ may be part of the mechanism of RET oncogenic conversion secondary to the M918T mutation.

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.

Phosphotyrosine 1062 is critical for the in vivo activity of the Ret9 receptor tyrosine kinase isoform

The receptor tyrosine kinase Ret plays a critical role in the development of the mammalian excretory and enteric nervous systems. Differential splicing of the primary Ret transcript results in the generation of two main isoforms, Ret9 and Ret51, whose C-terminal amino acid tails diverge after tyrosine (Y) 1062. Monoisoformic mice expressing only Ret9 develop normally and are healthy and fertile. In contrast, animals expressing only Ret51 have aganglionosis of the distal gut and hypoplastic kidneys. By generating monoisoformic mice in which Y1062 of Ret9 has been mutated to phenylalanine, we demonstrate that this amino acid has a critical role in Ret9 signaling that is necessary for the development of the kidneys and the enteric nervous system. These findings argue that the distinct activities of Ret9 and Ret51 result from the differential regulation of Y1062 by C-terminal flanking sequences. However, a mutation which places Y1062 of Ret51 in a Ret9 context improves only marginally the ability of Ret51 to support renal and enteric nervous system development. Finally, monoisoformic mice expressing a variant of Ret9 in which a C-terminal PDZ-binding motif was mutated develop normally and are healthy. Our studies identify Y1062 as a critical regulator of Ret9 signaling and suggest that Ret51-specific motifs are likely to inhibit the activity of this isoform.

The interaction between the adaptor protein APS and Enigma is involved in actin organisation

APS (adaptor protein with PH and SH2 domains) is an adaptor protein phosphorylated by several tyrosine kinase receptors including the insulin receptor. To identify novel binding partners of APS, we performed yeast two-hybrid screening. We identified Enigma, a PDZ and LIM domain-containing protein that was previously shown to be associated with the actin cytoskeleton. In HEK 293 cells, Enigma interacted specifically with APS, but not with the APS-related protein SH2-B. This interaction required the NPTY motif of APS and the LIM domains of Enigma. In NIH-3T3 cells that express the insulin receptor, Enigma and APS were partially co-localised with F-actin in small ruffling structures. Insulin increased the complex formation between APS and Enigma and their co-localisation in large F-actin containing ruffles. While in NIH-3T3 and HeLa cells the co-expression of both Enigma and APS did not modify the actin cytoskeleton organisation, expression of Enigma alone led to the formation of F-actin clusters. Similar alteration in actin cytoskeleton organisation was observed in cells expressing both Enigma and APS with a mutation in the NPTY motif. These results identify Enigma as a novel APS-binding protein and suggest that the APS/Enigma complex plays a critical role in actin cytoskeleton organisation.

Protein expression patterns in zebrafish skeletal muscle: initial characterization and the effects of hypoxic exposure

Patterns of protein expression were examined in white skeletal muscle from adult zebrafish (Danio rerio). High resolution two-dimensional gel electrophoresis resolved between 300 and 400 spots with molecular masses between 20 and 120 kDa and isoelectric points between about 5 and 8. Forty spots, representing a range of protein size, charge, and abundance were excised, digested with trypsin, and subjected to matrix-assisted laser-desorption/ionisation-time of flight mass spectrometry for protein identification. Twenty-nine spots were identified, including enzymes of energy metabolism, contractile proteins, an iron transport protein, and a heat shock protein. In addition, several spots matched theoretical proteins predicted from genome sequencing. These theoretical proteins were tentatively identified by similarity to known proteins. Patterns of muscle protein expression were then measured after zebrafish were exposed to low oxygen (16 torr) for 48 h, an exposure previously shown to increase the survival of zebrafish at more severe reductions in oxygen. Exposure to low oxygen (hypoxia) did not change the general pattern of protein expression but did affect the amounts of six low abundance proteins. The relatively subtle effects of hypoxia on patterns of muscle protein expression contrasts the widespread changes previously documented in mRNA levels in this and other species of fish during hypoxic stress. The difference between protein and mRNA expression illustrates the need to integrate both measures for a more complete understanding of gene expression in fish during hypoxic exposure.

Mystique is a new insulin-like growth factor-I-regulated PDZ-LIM domain protein that promotes cell attachment and migration and suppresses Anchorage-independent growth

By comparing differential gene expression in the insulin-like growth factor (IGF)-IR null cell fibroblast cell line (R- cells) with cells overexpressing the IGF-IR (R+ cells), we identified the Mystique gene expressed as alternatively spliced variants. The human homologue of Mystique is located on chromosome 8p21.2 and encodes a PDZ LIM domain protein (PDLIM2). GFP-Mystique was colocalized at cytoskeleton focal contacts with alpha-actinin and beta1-integrin. Only one isoform of endogenous human Mystique protein, Mystique 2, was detected in cell lines. Mystique 2 was more abundant in nontransformed MCF10A breast epithelial cells than in MCF-7 breast carcinoma cells and was induced by IGF-I and cell adhesion. Overexpression of Mystique 2 in MCF-7 cells suppressed colony formation in soft agarose and enhanced cell adhesion to collagen and fibronectin. Point mutation of either the PDZ or LIM domain was sufficient to reverse suppression of colony formation, but mutation of the PDZ domain alone was sufficient to abolish enhanced adhesion. Knockdown of Mystique 2 with small interfering RNA abrogated both adhesion and migration in MCF10A and MCF-7 cells. The data indicate that Mystique is an IGF-IR-regulated adapter protein located at the actin cytoskeleton that is necessary for the migratory capacity of epithelial cells.

Chronic expression of RET/PTC 3 enhances basal and insulin-stimulated PI3 kinase/AKT signaling and increases IRS-2 expression in FRTL-5 thyroid cells

The RET/PTC3 oncogene is a genetically rearranged and constitutively activated tyrosine kinase receptor that is common in papillary thyroid cancer. Because RET/PTC3 is chronically overexpressed in these thyroid cancer cells, and RET/PTC3-expressing tumors are associated with overactivity of tyrosine kinase signaling pathways and a more aggressive clinical course, we questioned whether chronic RET/PTC3 expression enhances cellular responses to thyroid mitogens in vitro. We stably transfected FRTL-5 cells with the RET/PTC3 gene; transfected and control cell lines were cultured without insulin, TSH, or serum. Thymidine incorporation into DNA was enhanced in the RET/PTC3 cells, but transformation was not observed. RET/PTC3 cells demonstrated higher basal and insulin-stimulated levels of activated Akt, both of which were reduced by LY294002, a PI3 kinase inhibitor, but not PD98059, a MEK inhibitor. By contrast, mitogen activated protein kinase (MAP kinase) was only minimally activated in RET/PTC3 cells before and after stimulation. Consistent with preferential activation of PI3 kinase, increased levels of total and phosphorylated IRS2 protein, relative activation of PDK-1, and enhanced IRS2-p85 interactions were identified in RET/PTC3-expressing cells. RET/PTC3 cells were also sensitized to insulin-induced thymidine incorporation; this effect was blocked by PI3 kinase (LY294002) rather than MEK 1/2 (PD98059) inhibitors. In summary, we have demonstrated that RET/PTC3 expression enhances basal and insulin-stimulated DNA synthesis through PI3 kinase, cooperatively activates Akt with insulin via PI3 kinase, and preferentially activates the Akt rather than MAP kinase pathway in FRTL-5 cells.

Tbx5 and Tbx4 transcription factors interact with a new chicken PDZ-LIM protein in limb and heart development

The T-domain transcription factors, Tbx5 and Tbx4, play important roles in vertebrate limb and heart development. To identify interacting and potential Tbx-regulating proteins, we performed a yeast two-hybrid screen with the C-terminal domain of Tbx5 as bait. We identified a new PDZ-LIM protein composed of one N-terminal PDZ and three C-terminal LIM domains, which we named chicken LMP-4. Among the Tbx2, 3, 4, 5 subfamily, we observed exclusive interaction with Tbx5 and Tbx4 proteins. Tbx3 nor Tbx2 can substitute for LMP-4 binding. While chicken LMP-4 associates with Tbx5 or Tbx4, it uses distinct LIM domains to bind to the individual proteins. Subcellular co-localization of LMP-4 and Tbx proteins supports the protein interaction and reveals interference of LMP-4 with Tbx protein distribution, tethering the transcription factors to the cytoskeleton. The protein-protein interaction indicates regulation of Tbx function at the level of transcription factor nuclear localization. During chicken limb and heart development, Tbx5/LMP-4 and Tbx4/LMP-4 are tightly co-expressed in a temporal and spatial manner, suggesting that they operate in the same pathway. Surprisingly, chicken LMP-4 expression domains outside those of Tbx5 in the heart led to the discovery of Tbx4 expression in the outflow tract and the right ventricle of this organ. The Tbx4-expressing cells coincide with those of the recently discovered secondary anterior heart-forming field. The discrete posterior or anterior expression domains in the heart and the exclusive fore- or hindlimb expression of Tbx5 and Tbx4, respectively, suggest common pathways in the heart and limbs. The identification of a new Tbx5/4-specific binding factor further suggests a novel mechanism for Tbx transcription factor regulation in development and disease.

Caveolin-1, Id3a and two LIM protein genes are upregulated by estrogen in vascular smooth muscle cells

Estrogen has diverse effects on the vasculature, such as vasodilation, endothelial growth and inhibition of vascular smooth muscle cell (VSMC) proliferation and migration. However, little is known about the genes that are regulated by estrogen in the vascular wall. Wistar rats were ovariectomized or sham-operated (Sham group), and 2 weeks after the operation, were subjected to subcutaneous implantation of placebo pellets (OVX + V group) or estradiol pellets (OVX + E group). Endothelium-denuded aortic tissue was examined 2 weeks after implantation. By applying high-density oligonucleotide microarray analysis, the expression of approximately 7000 genes was analyzed. Among the genes with different expression levels between the OVX + E group and the OVX + V group, those that have been reported to be expressed in the vasculature or muscle tissue, were chosen. Finally, four genes, caveolin-1, two LIM proteins (enigma and SmLIM) and Id3a, were identified. Microarray as well as real-time polymerase chain reaction showed that the expression levels of these genes were significantly higher in the OVX + E group than in the OVX + V group. To clarify whether estrogen directly upregulates these genes in the vascular wall, Northern blot analysis was performed using cultured rat VSMC. Addition of 100 nmol/L estradiol for 24 hours increased the mRNA levels of all four genes. Although the precise mechanism remains unclear, regulation of these genes by estrogen might contribute to its effect on VSMC.

Molecular events in follicular thyroid tumors

Knowledge of the molecular events that govern human thyroid tumorigenesis has grown considerably in the past ten years. Key genetic alterations and new oncogenic pathways have been identified. Molecular genetic aberrations in thyroid carcinomas bear noteworthy resemblance to those in acute myelogenous leukemias. Thyroid carcinomas and myeloid leukemias both possess transcription factor gene rearrangements-PPARgamma-related translocations in thyroid carcinoma and RARalpha-related and CBF-related translocations (amongst others) in myeloid leukemia. PPARgamma and RARalpha are closely related members ofthe same nuclear receptor subfamily, and the PML-RARalpha and PAX8-PPARgamma fusion proteins both function as dominant negative inhibitors of their wild-type parent proteins. Thyroid carcinomas and myeloid leukemias also both harbor NRAS mutations (15-25% of both cancers) and receptor tyrosine kinase mutations--RET mutations in thyroid carcinomas and FLT3 mutations in myeloid leukemias. The NRAS and tyrosine receptor kinase mutations are not observed in the same thyroid carcinoma or leukemia patients, suggesting that multiple initiating pathways exist in both. Lastly, thyroid carcinomas and myeloid leukemias possess p53 mutations at relatively low frequency (10-15%) in patients who tend to be older and have more aggressive, therapy resistant disease. Such parallels are unlikely to occur by chance alone and argue that common mechanisms underlie these diverse epithelial and hematologic cancers. The comparison of thyroid carcinomas and myeloid leukemias may highlight areas of thyroid cancer investigation worthy of further focus. For example, few collaborating mutations have been defined in thyroid carcinomas even though they play a clear role in myeloid leukemias, as exemplified by RARalpha rearrangements and FLT3 mutations that together dictate the promyleocytic leukemia phenotype. Functional interactions between collaborating mutations are possible at multiple levels, and it is tempting to speculate that some thyroid carcinomas might develop through an unique combination or co-activation of RET and RAS and/or RET and PPARgamma (and/or other) signaling systems. In fact, the ELE1-RET (PTC3) fusion protein contains the ELE1 nuclear receptor co-activator domain and it appears to physically associate with and inhibit wild-type PPARgamma in some papillary carcinomas. The similarities of the fusion proteins in thyroid carcinoma and myeloid leukemia suggest that a more directed search for fusion genes in non-thyroid carcinomas is warranted. In fact, novel fusion genes have been identified recently in aggressive midline, secretory breast, and renal cell carcinomas, although the epithelial nature of the latter is not well-documented. Interestingly, these cancers all tend to present more frequently in adolescence and young adulthood in a manner similar to thyroid and myeloid malignancies that have fusion genes. The analyses of cancers that present earlier in life may enhance fusion gene recognition in other carcinoma types. Definition and biologic characterization of the precursor cells that give rise to thyroid carcinoma will also be important. Myeloid leukemias are thought to arise from stem/progenitor cells that acquire disturbed self-renewal and differentiation capacities but retain characteristics of the myeloid lineages. Although the presence of comparable stem/progenitor cells in the thyroid are not defined, distinct thyroid cancer lineages and patterns of differentiation exist and candidate stem/progenitor cells such as the p63-immunoreactive solid cell nests are apparent. A last important area is development of molecular-based therapies for thyroid carcinoma patients resistant to standard radio-iodine treatment. Treatments for such cancers are limited and pathways defined by thyroid cancer mutations are prime targets for pharmacologic interventions with molecular inhibitors. Tyrosine kinase inhibitors and nuclear receptor ligands have proven dramatically effective in some myeloid leukemia patients. Various molecular inhibitors are being investigated now in thyroid cancer models. Such developments predict that the thyroid cancer model will continue to provide biologic insights into human carcinoma biology and that improved pathologic diagnosis and treatment for thyroid cancer patients sit on the not too distant horizon.

Microsomal Prostaglandin E2 Synthase-1 Is Induced by Conditional Expression of RET/PTC in Thyroid PCCL3 Cells through the Activation of the MEK-ERK Pathway

RET/PTC rearrangements are believed to be tumor-initiating events in papillary thyroid carcinomas. We identified microsomal prostaglandin E2 synthase-1 (mPGES-1) as a RET/PTC-inducible gene through subtraction hybridization cloning and expression profiling with custom microarrays. The inducible prostaglandin E2 (PGE2) biosynthetic enzymes cyclooxygenase-2 (COX-2) and mPGES-1 are up-regulated in many cancers. COX-2 is overexpressed in thyroid malignancies compared with benign nodules and normal thyroid tissues. Eicosanoids may promote tumorigenesis through effects on tumor cell growth, immune surveillance, and angiogenesis. Conditional RET/PTC1 or RET/PTC3 expression in PCCL3 thyroid cells markedly induced mPGES-1 and COX-2. PGE2 was the principal prostanoid and up-regulated (by approximately 60-fold), whereas hydroxyeicosatetraenoic acid metabolites were decreased, consistent with shunting of prostanoid biosynthesis toward PGE2 by coactivation of the two enzymes. RET/PTC activated mPGES-1 gene transcription. Based on experiments with kinase inhibitors, with PCCL3 cell lines with doxycycline-inducible expression of RET/PTC mutants with substitutions of critical tyrosine residues in the kinase domain, and lines with inducible expression of activated mutants of H-RAS and MEK1, RET/PTC was found to regulate mPGES-1 through Shc-RAS-MEK-ERK. These data show a direct relationship between activation of a tyrosine kinase receptor oncogene and regulation of PGE2 biosynthesis. As enzymes involved in prostanoid biosynthesis can be targeted with pharmacological inhibitors, these findings may have therapeutic implications.

RET/PTC-induced dedifferentiation of thyroid cells is mediated through Y1062 signaling through SHC-RAS-MAP kinase

Constitutive activation of the RET proto-oncogene in papillary thyroid carcinomas results from rearrangements linking the promoter(s) and N-terminal domains of unrelated genes to the C-terminus of RET tyrosine kinase (RET/PTC). RET/PTC expression has been demonstrated to inhibit transcription of thyroid-specific genes. To study the signal transduction pathways responsible for this, we generated PCCL3 thyroid cells with doxycycline-inducible expression of RET/PTC3, RET/PTC3(Y541F), or PTC2/PDZ. Acute expression of RET/PTC(Y541F) appropriately interacted with Shc, an intermediate in the activation of the Ras pathway, but failed to activate PLCgamma. By contrast, PTC2/PDZ failed to bind Shc, but interacted normally with PLCgamma. Acute expression of RET/PTC3 or RET/PTC3(Y541F), but not PTC2/PDZ, inhibited TSH-induced Tg and NIS expression, suggesting that activation of Shc-Ras, but not PLCgamma, is required for RET/PTC-induced dedifferentiation. Accordingly, acute expression of H-Ras(V12) or of a constitutively active MEK1 also blocked TSH-induced expression of Tg and NIS. Moreover, MEK inhibitors restored Tg and NIS levels. In conclusion, activation of the Ras/Raf/MEK/MAPK pathway through Shc mediates RET/PTC-induced thyroid cell dedifferentiation. This suggests that inhibition of this pathway may promote redifferentiation in poorly differentiated thyroid carcinomas with constitutive activation of either Ras or RET/PTC.

RET/PTC (rearranged in transformation/papillary thyroid carcinomas) tyrosine kinase phosphorylates and activates phosphoinositide-dependent kinase 1 (PDK1): an alternative phosphatidylinositol 3-kinase-independent pathway to activate PDK1

Thyroid cancers are a leading cause of death due to endocrine malignancies. RET/PTC (rearranged in transformation/papillary thyroid carcinomas) gene rearrangements are the most frequent genetic alterations identified in papillary thyroid carcinoma. Although the oncogenic potential of RET/PTC is related to intrinsic tyrosine kinase activity, the substrates for this enzyme are yet to be identified. In this report, we show that phosphoinositide-dependent kinase 1 (PDK1), a pivotal serine/threonine kinase in growth factor-signaling pathways, is a target of RET/PTC. RET/PTC and PDK1 colocalize in the cytoplasm. RET/PTC phosphorylates a specific tyrosine (Y9) residue located in the N-terminal region of PDK1. Y9 phosphorylation of PDK1 by RET/PTC requires an intact catalytic kinase domain. The short (iso 9) and long forms (iso 51) of the RET/PTC kinases (RET/PTC1 and RET/PTC3) induce Y9 phosphorylation of PDK1. Moreover, Y9 phosphorylation of PDK1 by RET/PTC does not require phosphatidylinositol 3-kinase or Src activity. RET/PTC-induced phosphorylation of the Y9 residue results in increased PDK1 activity, decrease of cellular p53 levels, and repression of p53-dependent transactivation. In conclusion, RET/PTC-induced tyrosine phosphorylation of PDK1 may be one of the mechanisms by which it acts as an oncogenic tyrosine kinase in thyroid carcinogenesis.

Protein kinase B activation by reactive oxygen species is independent of tyrosine kinase receptor phosphorylation and requires SRC activity

Reactive oxygen species (ROS) participate as second messengers in the mitogenic signal transduction. Most of the experimental data supporting the role of ROS as signaling molecules have been obtained by using H2O2. Exposure of cells to H2O2 rapidly increases tyrosine phosphorylation of tyrosine kinase receptors (TKRs) in the absence of growth factor binding, thus inducing the activation of downstream signaling cascades, like that of protein kinase B (AKT). Another molecule able to induce an increase of intracellular ROS levels is diethylmaleate (DEM), which acts by depleting the ROS scavenger reduced glutathione (GSH). A comparison of the effects exerted by H2O2 and DEM shows that the latter induces redox modifications milder than those generated by H2O2. We also demonstrated that DEM-induced redox modifications are not accompanied by platelet-derived growth factor-receptor (PDGF-R) and epidermal growth factor-receptor Tyr phosphorylation, although they are able to activate ERKs and AKT, with kinetics different from those observed following H2O2 treatment. The activation of these two pathways is not blocked by AG1296, a selective inhibitor of PDGF-R Tyr kinase, thus confirming that the effects of DEM are not mediated by the TKR phosphorylation. On the contrary, PP2 (4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazole[3,4-d]pyrimidine), an inhibitor of Src kinase, completely prevents DEM- and H2O2-induced AKT activation but has no effect on the pathway of ERKs. Finally, nitration of Tyr residues in PDGF-R is observed in DEM-treated cells, thus suggesting that ROS-induced modifications different from Tyr phosphorylation can occur at the growth factor-receptor level and can be involved in the regulation of signaling pathways.

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.

Early gene activation in chronic leukemic B lymphocytes induced toward a plasma cell phenotype

Chronic lymphocytic leukemia (CLL) is characterized by the accumulation of lymphocytes that are arrested at an intermediate stage of B lymphocyte development. CLL B lymphocytes transform (mature) to a plasmacytic phenotype with loss of CD19 and CD20 and the appearance of cytoplasmic immunoglobulin when treated in vitro with phorbol esters. We have used array hybridization technology to describe gene expression patterns for untreated and tetradecanoyl phorbol acetate (TPA)-treated CLL B cells at 5, 10, and 20 min following initial TPA exposure. Three genes, early growth response factor 1 (EGR-1), dual specificity phosphatase 2, and CD69 (early T-cell activation antigen), showed a 2.0-fold or greater increase in mRNA transcription at four or more of six time points in two studies. Upregulation of expression of these genes was confirmed by real-time polymerase chain reaction in the TPA-treated cells of four CLL patients. A progressive increase in gene expression was observed during the 20-min time course for all three genes. In addition, protein expression of EGR-1 and CD69 was increased as measured by immunofluorescence cell analysis. Several genes (PKC, n-myc, jun D, and BCL-2) previously reported as overexpressed in CLL lymphocytes were overexpressed in these studies also, but were not altered by TPA treatment. Genes for proteins whose upregulation requires hours of TPA exposure (the 4F2hc component of the L-system amino acid transporter, prohibition, and hsp60) were assessed, and their later expression contrasted with the early expression of EGR-1, dual specificity phosphatase 2, and CD69. EGR-1 encodes a zinc-finger transcription factor that is induced by pokeweed mitogen and TPA and promotes B lymphocyte maturation. The dual specificity phosphatase 2 encodes an enzyme that reverses mitogen activated protein kinase cell activation by dephosphorylation. The CD69 protein is induced by TPA in thymocytes and is a type II transmembrane signaling molecule in hematopoietic cells. These findings suggest that the products of these three genes may be central to early steps in the TPA-induced evolution of CLL B cells to a plasmacytic phenotype.

Internalization of glial cell-derived neurotrophic factor receptor GFR alpha 1 in the absence of the ret tyrosine kinase coreceptor

1. Glial cell-derived neurothrophic factor (GDNF) interacts with a cell surface receptor, GFRalpha1, that is linked via a glycosyl-phosphatidylinositol (GPI) lipid to the cell membrane. The neurotrophic activities of GDNF are mediated by binding to GFRalpha1 and further interaction of the GDNF-GFRalpha1 complex with a coreceptor tyrosine kinase encoded by the c-Ret protooncogene. There is also evidence for the existence of cell signaling by GDNF and GFRalpha1 in the absence of Ret. 2. To further delineate the Ret-dependent and -independent functions of GDNF, cellular internalization of GDNF and GFRalpha1 was examined in cells lines and primary neurons. 3. Relative to other GPI-anchored receptors, efficient endocytosis (approximately 30-40% of total surface-bound ligand internalized after 2 min) of GNDF and GFRalpha1 was observed in neuroblastoma and transfected-fibroblast cell lines that lack Ret. Primary hippocampal neurons from transgenic mice that express a wild-type GFRalpha1 together with a mutant, tyrosine kinase-inactive Ret also internalized GDNF efficiently (approximately 20% of total surface-bound ligand internalized after 2 min). We also observed a ligand dependence for GFRalpha1 internalization in the cell lines that lack Ret. Furthermore, a comparison in the presence and absence of Ret indicates that this coreceptor tyrosine kinase slows internalization at early time points. 4. The data suggest different mechanisms of internalization for GDNF-GFRalpha1 in the absence and presence of the Ret coreceptor.

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.

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.

Clik1: a novel kinase targeted to actin stress fibers by the CLP-36 PDZ-LIM protein

In this report we have characterized a novel, ubiquitously expressed kinase, Clik1, that is predominantly nuclear and undergoes autophosphorylation. Yeast two-hybrid analysis indicated a highly specific association between Clik1 and CLP-36, which was identified in 36 out of 37 Clik1-interacting clones. CLP-36 is a PDZ-LIM protein that localizes to actin stress fibers in nonmuscle cells and associates with α-actinin via its PDZ-domain. The association of CLP-36 with Clik1, in turn, is mediated by the C-terminal part of CLP-36 containing the LIM domain, and association was not noted with the closely related ALP PDZ-LIM protein. Interestingly, the association with CLP-36 led to relocalization of the otherwise nuclear Clik1 kinase to actin stress fibers, where it disrupted the periodic staining pattern of CLP-36. Taken together these results establish the CLP-36 PDZ-LIM protein as an adapter, recruiting the Clik1 kinase to actin stress fibers in nonmuscle cells, and suggest that Clik1 represents a novel regulator of actin stress fibers.

Overexpressed LIM mineralization proteins do not require LIM domains to induce bone

Rat LIM mineralization protein 1 (LMP-1, an LIM domain protein) mediates bone morphogenetic protein 6 (BMP-6) induction of bone nodule formation in fetal rat calvarial osteoblast (ROB) cultures. We have isolated the complementary DNA (cDNA) for the human homologue of LMP-1 from an adult human heart cDNA library and showed that when overexpressed it is osteoinductive in the same culture system. The recently revised cDNA sequence of Enigma, the protein product of which binds to the insulin receptor and the tyrosine kinase receptor ret, now matches the nucleotide sequence of human LMP-1 (hLMP-1). A truncated, 223 amino acid (AA) LMP-1(t) protein has identical effects as the full-length protein, despite the deletion of the LIM domains. Two splice variants of human LMP-1 have been detected. Human LMP-2 has a 119-base pair (bp) deletion between bp 325 and 444 and a 17-bp insertion at bp 444. The resulting derived protein contains 423 AA with the LIM domains intact and does not induce bone formation when overexpressed in ROB cultures. Human LMP-3 has the same 17 nucleotide insertion at bp 444, resulting in a shift in the reading frame that causes a stop codon to occur at bp 505-507. The resulting 153 AA protein does not have the LIM domains, but overexpression of hLMP-3 induces bone formation in osteoblast cultures. These findings suggest that the LIM domains are not required for LMPs to induce bone formation. In addition, a small region (36 AA) of the LMP-1 protein may be required for bone formation.

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.

The GDNF/RET signaling pathway and human diseases

Glial cell line-derived neurotrophic factor (GDNF) and related molecules, neurturin, artemin and persephin, signal through a unique multicomponent receptor system consisting of RET tyrosine kinase and glycosyl-phosphatidylinositol-anchored coreceptor (GFRalpha1-4). These neurotrophic factors promote the survival of various neurons including peripheral autonomic and sensory neurons as well as central motor and dopamine neurons, and have been expected as therapeutic agents for neurodegenerative diseases. In addition, it turned out that the GDNF/RET signaling plays a crucial role in renal development and regulation of spermatogonia differentiation. RET mutations cause several human diseases such as papillary thyroid carcinoma, multiple endocrine neoplasia types 2A and 2B, and Hirschsprung's disease. The mutations resulted in RET activation or inactivation by various mechanisms and the biological properties of mutant proteins appeared to be correlated with disease phenotypes. The signaling pathways activated by GDNF or mutant RET are being extensively investigated to understand the molecular mechanisms of disease development and the physiological roles of the GDNF family ligands.

Regulation of ureteric bud outgrowth by Pax2-dependent activation of the glial derived neurotrophic factor gene

The outgrowth of the ureteric bud from the posterior nephric duct epithelium and the subsequent invasion of the bud into the metanephric mesenchyme initiate the process of metanephric, or adult kidney, development. The receptor tyrosine kinase RET and glial cell-derived neurotrophic factor (GDNF) form a signaling complex that is essential for ureteric bud growth and branching morphogenesis of the ureteric bud epithelium. We demonstrate that Pax2 expression in the metanephric mesenchyme is independent of induction by the ureteric bud. Pax2 mutants are deficient in ureteric bud outgrowth and do not express GDNF in the uninduced metanephric mesenchyme. Furthermore, Pax2 mutant mesenchyme is unresponsive to induction by wild-type heterologous inducers. In normal embryos, GDNF is sufficient to induce ectopic ureter buds in the posterior nephric duct, a process inhibited by bone morphogenetic protein 4. However, GDNF replacement in organ culture is not sufficient to stimulate ureteric bud outgrowth from Pax2 mutant nephric ducts, indicating additional defects in the nephric duct epithelium of Pax2 mutants. Pax2 can activate expression of GDNF in cell lines derived from embryonic metanephroi. Furthermore, Pax2 protein can bind to upstream regulatory elements within the GDNF promoter region and can transactivate expression of reporter genes. Thus, activation of GDNF by Pax2 coordinates the position and outgrowth of the ureteric bud such that kidney development can begin.

RET oncogene activation in papillary thyroid carcinoma

The RET proto-oncogene encodes a cell membrane tyrosine-kinase receptor protein whose ligands belong to the glial cell line-derived neurotrophic factor. RET functions as a multicompetent receptor complex that includes alphaGFRs and RET. Somatic rearrangements of RET designated as RET/PTC (from papillary thyroid carcinoma) were identified in papillary thyroid carcinoma before RET was recognized as the susceptibility gene for MEN2. There are now at least at least 15 types of RET/PTC rearrangements involving RET and 10 different genes. RET/PTC1 and RET/PTC3 are by far the most common rearrangements. All of the rearrangements are due to DNA damage and result in the fusion of the RET tyrosine-kinase (RET-TK) domain to the 5'-terminal region of heterologous genes. RET/PTC rearrangements are very common in radiation-induced tumors but have been detected in variable proportions of sporadic (i.e., non-radiation associated) papillary carcinomas. It is estimated that up to approximately half the papillary thyroid carcinomas in the United States and Canada harbor RET/PTC rearrangements, most commonly RET/PTC-1, followed by RET/PTC-3 and occasionally RET/PTC-2. The cause of these rearrangements in sporadic papillary carcinomas is not known, but the close association between their presence and the papillary carcinoma phenotype indicates that they play a causative role in tumor development. The proposed mechanisms of RET/PTC-induced tumorigenesis and the clinical and pathologic implications of RET/PTC activation are discussed.

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.

Docking protein FRS2 links the protein tyrosine kinase RET and its oncogenic forms with the mitogen-activated protein kinase signaling cascade

The receptor tyrosine kinase RET functions as the signal transducing receptor for the GDNF (for "glial cell-derived neurotrophic factors") family of ligands. Mutations in the RET gene were implicated in Hirschsprung disease (HSCR), multiple endocrine neoplasia type 2 (MEN 2), and thyroid carcinomas. In this report we demonstrate that the docking protein FRS2 is tyrosine phosphorylated by ligand-stimulated and by constitutively activated oncogenic forms of RET. Complex formation between RET and FRS2 is mediated by binding of the phosphotyrosine-binding domain of FRS2 to pY1062, a residue in RET that also functions as a binding site for Shc. However, overexpression of FRS2 but not Shc potentiates mitogen-activated protein (MAP) kinase activation by RET oncoproteins. We demonstrate that oncogenic RET-PTC proteins are associated with FRS2 constitutively, leading to tyrosine phosphorylation of FRS2, MAP kinase stimulation, and cell proliferation. However, loss-of-function HSCR-associated RET mutants exhibit impaired FRS2 binding and reduced MAP kinase activation. These experiments demonstrate that FRS2 couples both ligand-regulated and oncogenic forms of RET, with the MAP kinase signaling cascade as part of the response of RET under normal biological conditions and pathological conditions, such as MEN 2 and papillary thyroid carcinomas.

Key role of Shc signaling in the transforming pathway triggered by Ret/ptc2 oncoprotein

The RET/PTC oncogenes, generated by chromosomal rearrangements in papillary thyroid carcinomas, are constitutively activated versions of protoRET, a gene encoding two protein isoforms of a transmembrane tyrosine kinase receptor. By using Ret/ptc2 short isoform (iso9), we have previously demonstrated that Tyr586 (Tyr1062 of protoRet) is the docking site for both the PTB and the SH2 domains of Shc. To determine the relevance of this interaction for the transforming activity of Ret/ptc oncogenes, we have generated and characterized novel Ret/ptc mutants unable to activate Shc: Ret/ptc2 long isoform (iso51)-Y586F and both isoforms of Ret/ptc2-N583A. These mutants neither activate Shc nor transform NIH3T3 cells. Since Tyr1062 shows features of a multifunctional docking site, we have used a Shc mutant (Shc Y317F) to directly assess Shc role. We have demonstrated that in our cell system Shc Y317F behaves like a dominant interfering mutant on the activation of the Grb2-Sos pathway by endogenous Shc triggered by Ret/ptc2. A strong reduction of the transforming activity of Ret/ptc2 in presence of this mutant was also demonstrated. Our data suggest that Shc activation play a key role in the transforming pathways triggered by Ret/ptc oncoproteins. Moreover, we have shown that coexpression of the Shc-Y317F mutant with Ret/ptc2 specifically causes apoptosis, and that the surviving cells lose the long-term expression of one of the two genes.

Identification of SNT/FRS2 docking site on RET receptor tyrosine kinase and its role for signal transduction

SNT/FRS2 is a lipid anchored docking protein that contains an amino-terminal myristylation signal, followed by a phosphotyrosine-binding (PTB) domain and a carboxy-terminal region with multiple tyrosine residues. Here we show that the SNT/FRS2 PTB domain binds to RET receptor tyrosine kinase activated by glial cell line-derived neurotrophic factor (GDNF) or multiple endocrine neoplasia (MEN) 2 mutations. Analyses by site directed-mutagenesis revealed that it binds to tyrosine 1062 in RET that is also known to be a binding site for the SHC adaptor protein. Whereas SHC bound to RET was associated with GRB2 and GAB1 proteins, SNT/FRS2 was associated with GRB2 only, suggesting that SNT/FRS2 is involved mainly in the activation of the RAS/mitogen activated protein kinase (MAPK) pathway but not the phosphatidylinositol 3-kinase (PI3-K)/AKT pathway. In addition, phosphorylated SNT/FRS2 appeared to directly complex with SHP-2 tyrosine phosphatase. These results suggest that tyrosine 1062 in RET provides a site for the interaction of multiple signaling molecules and that the balance of SHC and SNT/FRS2 binding may affect the nature of the intracellular signaling for cell proliferation, differentiation and survival induced by activated RET.

The RFG oligomerization domain mediates kinase activation and re-localization of the RET/PTC3 oncoprotein to the plasma membrane

The RET/PTC3 oncogene arises from the fusion between the N-terminal encoding domain of the RFG gene and the tyrosine kinase encoding domain of RET receptor. RET/PTC3 is very frequent in papillary thyroid carcinomas, especially in children exposed to the Chernobyl accident. We have studied the functional consequences of the RFG-RET fusion. Here we show that the N-terminal coiled-coil domain of RGF mediates oligomerization and activation of the kinase and of the transforming capability of RET/PTC3. In addition, the RFG coiled-coil domain mediates a physical association between RET/PTC3 and RGF proteins, rendering RFG a bona fide substrate of RET/PTC3 kinase. Finally, we show that the coiled-coil domain of RGF is essential for the distribution of the RET/PTC3 protein at the membrane/particulate cell compartment level, where also most of the RFG protein is localized. We propose that fusion to the RFG coiled-coil domain provides RET kinase with a scaffold that mediates oligomerization and re-localization of the RET/PTC3 protein, a process that may be crucial for the signalling of this specific RET/PTC variant.

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

Tyrosine 1062 of Ret, which represents an intracytoplasmic docking site for multiple signaling molecules, is essential for Ret-mediated activation of phosphatidylinositol 3-Kinase (PI3-K). PI3-K, in turn, has been implicated in inducing cell survival and neoplastic transformation mediated by Ret. We have examined the mechanisms by which Ret stimulates PI3-K. Here we show that the Insulin Receptor Substrate-1 (IRS-1) is tyrosine phosphorylated and associated with the p85 regulatory subunit of PI3-K in response to Ret activation. IRS-1 coimmunoprecipitates with Ret and co-expression of IRS-1 results in the potentiation of Ret-mediated activation of Akt(PKB), a bona fide effector of PI3-K. The association with the PTB domain of IRS-1 depends on the phosphorylation of tyrosine 1062 of Ret. The deletion of asparagine 1059 (delN1059) and the substitution of leucine 1061 (L1061P), two Ret mutations identified in families affected by congenital megacolon (Hirschsprung's disease), impair the binding of IRS-1 to Ret as well as Ret-mediated Akt(PKB) stimulation. Finally, we show that Shc, which was previously identified as another ligand of Y1062 of Ret, competes with IRS-1 for the binding to Ret pY1062. All together, these findings suggest that IRS-1 is a component of the signaling pathway which leads to Ret-mediated PI3-K activation, a pathway which can be targeted by Hirschsprung-associated Ret mutations. The alternative binding of Shc and IRS-1 to Ret pY1062 can be a system to modulate the activation of different intracellular signaling pathways and to elicit different biological responses following Ret activation.

Signaling complexes and protein-protein interactions involved in the activation of the Ras and phosphatidylinositol 3-kinase pathways by the c-Ret receptor tyrosine kinase

Proximal signaling events and protein-protein interactions initiated after activation of the c-Ret receptor tyrosine kinase by its ligand, glial cell line-derived neurotrophic factor (GDNF), were investigated in cells carrying native and mutated forms of this receptor. Mutation of Tyr-1062 (Y1062F) in the cytoplasmic tail of c-Ret abolished receptor binding and phosphorylation of the adaptor Shc and eliminated activation of Ras by GDNF. Phosphorylation of Erk kinases was also greatly attenuated but not eliminated by this mutation. This residual wave of Erk phosphorylation was independent of the kinase activity of c-Ret. Mutation of Tyr-1096 (Y1096F), a binding site for the adaptor Grb2, had no effect on Erk activation by GDNF. Activation of phosphatidylinositol-3 kinase (PI3K) and its downstream effector Akt was also reduced in the Y1062F mutant but not completely abolished unless Tyr-1096 was also mutated. Ligand stimulation of neuronal cells induced the assembly of a large protein complex containing c-Ret, Grb2, and tyrosine-phosphorylated forms of Shc, p85(PI3K), the adaptor Gab2, and the protein-tyrosine phosphatase SHP-2. In agreement with Ras-independent activation of PI3K by GDNF in neuronal cells, survival of sympathetic neurons induced by GDNF was dependent on PI3K but was not affected by microinjection of blocking anti-Ras antibodies, which did compromise neuronal survival by nerve growth factor, suggesting that Ras is not required for GDNF-induced survival of sympathetic neurons. These results indicate that upon ligand stimulation, at least two distinct protein complexes assemble on phosphorylated Tyr-1062 of c-Ret via Shc, one leading to activation of the Ras/Erk pathway through recruitment of Grb2/Sos and another to the PI3K/Akt pathway through recruitment of Grb2/Gab2 followed by p85(PI3K) and SHP-2. This latter complex can also assemble directly onto phosphorylated Tyr-1096, offering an alternative route to PI3K activation by GDNF.

The RET proto-oncogene in human cancers

The activation of the RET proto-oncogene contributes to the development of human cancers in two different ways. Somatic rearrangements of RET with a variety of activating genes, which contribute to unscheduled expression and constitutive dimerization of the chimeric RET/PTC oncoproteins in thyroid follicular cells, are frequently found in radiation-induced papillary thyroid carcinomas. Germ-line mutations, mainly point mutations, that lead to constitutive activation of RET tyrosine kinase activity are responsible for the development of the inherited cancer syndrome, multiple endocrine neoplasia type 2. There appears to be a correlation between specific types of RET mutation and clinical phenotypes of the cancers involved. The biological effects and the signaling pathways induced by different forms of RET activation have been investigated in a variety of cultured cells as well as in genetically engineered animal models. The identification of RET mutations in most MEN 2 families (95%) has translated into improved care for MEN 2 patients. However, further investigation of the signaling pathways contributing to tumorigenesis in relevant tissues will eventually help us to develop novel strategies to prevent or to treat human papillary thyroid carcinomas, MEN 2 disease, as well as the sporadic cancers relevant to MEN 2 disease.

Multiple endocrine neoplasia type 2 and RET: from neoplasia to neurogenesis

Multiple endocrine neoplasia type 2 (MEN 2) is an inherited cancer syndrome characterised by medullary thyroid carcinoma (MTC), with or without phaeochromocytoma and hyperparathyroidism. MEN 2 is unusual among cancer syndromes as it is caused by activation of a cellular oncogene, RET. Germline mutations in the gene encoding the RET receptor tyrosine kinase are found in the vast majority of MEN 2 patients and somatic RET mutations are found in a subset of sporadic MTC. Further, there are strong associations of RET mutation genotype and disease phenotype in MEN 2 which have led to predictions of tissue specific requirements and sensitivities to RET activity. Our ability to identify genetically, with high accuracy, subjects with MEN 2 has revolutionised our ability to diagnose, predict, and manage this disease. In the past few years, studies of RET and its normal ligand and downstream interactions and the signalling pathways it activates have clarified our understanding of the roles played by RET in normal cell survival, proliferation, and differentiation, as well as in disease. Here, we review the current knowledge of the normal functions of RET and the effects of mutations of this gene in tumorigenesis and in normal development.

Characterization of intracellular signals via tyrosine 1062 in RET activated by glial cell line-derived neurotrophic factor

Glial cell line derived neurotrophic factor (GDNF) signals through a multicomponent receptor complex consisting of RET receptor tyrosine kinase and a member of GDNF family receptor alpha (GFRalpha). Recently, it was shown that tyrosine 1062 in RET represents a binding site for SHC adaptor proteins and is crucial for both RAS/mitogen activated protein kinase (MAPK) and phosphatidylinositol 3-kinase (PI3-K)/AKT signaling pathways. In the present study, we characterized how these two pathways diverge from tyrosine 1062, using human neuroblastoma and primitive neuroectodermal tumor cell lines expressing RET at high levels. In response to GDNF stimulation, SHC bound to GAB1 and GRB2 adaptor proteins as well as RET, and SHC and GAB1 were highly phosphorylated on tyrosine. The complex formation consisting of SHC, GAB1 and GRB2 was almost abolished by replacement of tyrosine 1062 in RET with phenylalanine. Tyrosine-phosphorylated GAB1 was also associated with p85 subunit of PI3-K, resulting in PI3-K and AKT activation, whereas SHC-GRB2-SOS complex was responsible for the RAS/ERK signaling pathway. These results suggested that the RAS and PI3-K pathways activated by GDNF bifurcate mainly through SHC bound to tyrosine 1062 in RET. Furthermore, using luciferase reporter-gene assays, we found that the RAS/ERK and PI3-K signaling pathways are important for activation of CREB and NF-kappaB in GDNF-treated cells, respectively. Oncogene (2000) 19, 4469 - 4475.