A mutation at tyrosine 1062 in MEN2A-Ret and MEN2B-Ret impairs their transforming activity and association with shc adaptor proteins

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.

Small Molecule Inhibitors as Therapeutic Agents Targeting Oncogenic Fusion Proteins: Current Status and Clinical

Oncogenic fusion proteins, arising from chromosomal rearrangements, have emerged as prominent drivers of tumorigenesis and crucial therapeutic targets in cancer research. In recent years, the potential of small molecular inhibitors in selectively targeting fusion proteins has exhibited significant prospects, offering a novel approach to combat malignancies harboring these aberrant molecular entities. This review provides a comprehensive overview of the current state of small molecular inhibitors as therapeutic agents for oncogenic fusion proteins. We discuss the rationale for targeting fusion proteins, elucidate the mechanism of action of inhibitors, assess the challenges associated with their utilization, and provide a summary of the clinical progress achieved thus far. The objective is to provide the medicinal community with current and pertinent information and to expedite the drug discovery programs in this area.

RET signaling pathway and RET inhibitors in human cancer

Rearranged during transfection (RET) receptor tyrosine kinase was first identified over thirty years ago as a novel transforming gene. Since its discovery and subsequent pathway characterization, RET alterations have been identified in numerous cancer types and are most prevalent in thyroid carcinomas and non-small cell lung cancer (NSCLC). In other tumor types such as breast cancer and salivary gland carcinomas, RET alterations can be found at lower frequencies. Aberrant RET activity is associated with poor prognosis of thyroid and lung carcinoma patients, and is strongly correlated with increased risk of distant metastases. RET aberrations encompass a variety of genomic or proteomic alterations, most of which confer constitutive activation of RET. Activating RET alterations, such as point mutations or gene fusions, enhance activity of signaling pathways downstream of RET, namely PI3K/AKT, RAS/RAF, MAPK, and PLCγ pathways, to promote cell proliferation, growth, and survival. Given the important role that mutant RET plays in metastatic cancers, significant efforts have been made in developing inhibitors against RET kinase activity. These efforts have led to FDA approval of Selpercatinib and Pralsetinib for NSCLC, as well as, additional selective RET inhibitors in preclinical and clinical testing. This review covers the current biological understanding of RET signaling, the impact of RET hyperactivity on tumor progression in multiple tumor types, and RET inhibitors with promising preclinical and clinical efficacy.

RET receptor signaling: Function in development, metabolic disease, and cancer

The RET proto-oncogene encodes a receptor tyrosine kinase whose alterations are responsible for various human cancers and developmental disorders, including thyroid cancer, non-small cell lung cancer, multiple endocrine neoplasia type 2, and Hirschsprung's disease. RET receptors are physiologically activated by glial cell line-derived neurotrophic factor (GDNF) family ligands that bind to the coreceptor GDNF family receptor α (GFRα). Signaling via the GDNF/GFRα1/RET ternary complex plays crucial roles in the development of the enteric nervous system, kidneys, and urinary tract, as well as in the self-renewal of spermatogonial stem cells. In addition, another ligand, growth differentiation factor-15 (GDF15), has been shown to bind to GFRα-like and activate RET, regulating body weight. GDF15 is a stress response cytokine, and its elevated serum levels affect metabolism and anorexia-cachexia syndrome. Moreover, recent development of RET-specific kinase inhibitors contributed significantly to progress in the treatment of patients with RET-altered cancer. This review focuses on the broad roles of RET in development, metabolic diseases, and cancer.

Novel Human-Derived RET Fusion NSCLC Cell Lines Have Heterogeneous Responses to RET Inhibitors and Differential Regulation of Downstream Signaling

Rearranged during transfection (RET) rearrangements occur in 1% to 2% of lung adenocarcinomas as well as other malignancies and are now established targets for tyrosine kinase inhibitors. We developed three novel RET fusion-positive (RET+) patient-derived cancer cell lines, CUTO22 [kinesin 5B (KIF5B)-RET fusion], CUTO32 (KIF5B-RET fusion), and CUTO42 (echinoderm microtubule-associated protein-like 4-RET fusion), to study RET signaling and response to therapy. We confirmed each of our cell lines expresses the RET fusion protein and assessed their sensitivity to RET inhibitors. We found that the CUTO22 and CUTO42 cell lines were sensitive to multiple RET inhibitors, whereas the CUTO32 cell line was >10-fold more resistant to three RET inhibitors. We discovered that our RET+ cell lines had differential regulation of the mitogen-activated protein kinase and phosphoinositide 3-kinase/protein kinase B (AKT) pathways. After inhibition of RET, the CUTO42 cells had robust inhibition of phosphorylated AKT (pAKT), whereas CUTO22 and CUTO32 cells had sustained AKT activation. Next, we performed a drug screen, which revealed that the CUTO32 cells were sensitive (<1 nM IC50) to inhibition of two cell cycle-regulating proteins, polo-like kinase 1 and Aurora kinase A. Finally, we show that two of these cell lines, CUTO32 and CUTO42, successfully establish xenografted tumors in nude mice. We demonstrated that the RET inhibitor BLU-667 was effective at inhibiting tumor growth in CUTO42 tumors but had a much less profound effect in CUTO32 tumors, consistent with our in vitro experiments. These data highlight the utility of new RET+ models to elucidate differences in response to tyrosine kinase inhibitors and downstream signaling regulation. Our RET+ cell lines effectively recapitulate the interpatient heterogeneity observed in response to RET inhibitors and reveal opportunities for alternative or combination therapies. SIGNIFICANCE STATEMENT: We have derived and characterized three novel rearranged during transfection (RET) fusion non-small cell lung cancer cell lines and demonstrated that they have differential responses to RET inhibition as well as regulation of downstream signaling, an area that has previously been limited by a lack of diverse cell line modes with endogenous RET fusions. These data offer important insight into regulation of response to RET tyrosine kinase inhibitors and other potential therapeutic targets.

Intracellular RET signaling pathways activated by GDNF

Activation of REarranged during Transfection (RET) proto-oncogene is responsible for various human cancers such as papillary and medullary thyroid carcinomas and non-small cell lung carcinomas. RET activation in these tumors is caused by point mutations or gene rearrangements, resulting in constitutive activation of RET tyrosine kinase. Physiologically, RET is activated by glial cell line-derived neurotrophic factor (GDNF) ligands that bind to coreceptor GDNF family receptor alphas (GFRαs), leading to RET dimerization. GDNF-GFRα1-RET signaling plays crucial roles in the development of the enteric nervous system, kidney and lower urinary tract as well as in spermatogenesis. Intracellular tyrosine phosphorylation in RET and recruitment of adaptor proteins to phosphotyrosines are essential for various biological functions. Significance of intracellular RET signaling pathways activated by GDNF is discussed and summarized in this review.

PTPRA Phosphatase Regulates GDNF-Dependent RET Signaling and Inhibits the RET Mutant MEN2A Oncogenic Potential

The RET proto-oncogene encodes receptor tyrosine kinase, expressed primarily in tissues of neural crest origin. De-regulation of RET signaling is implicated in several human cancers. Recent phosphatome interactome analysis identified PTPRA interacting with the neurotrophic factor (GDNF)-dependent RET-Ras-MAPK signaling-axis. Here, by identifying comprehensive interactomes of PTPRA and RET, we reveal their close physical and functional association. The PTPRA directly interacts with RET, and using the phosphoproteomic approach, we identify RET as a direct dephosphorylation substrate of PTPRA both in vivo and in vitro. The protein phosphatase domain-1 is indispensable for the PTPRA inhibitory role on RET activity and downstream Ras-MAPK signaling, whereas domain-2 has only minor effect. Furthermore, PTPRA also regulates the RET oncogenic mutant variant MEN2A activity and invasion capacity, whereas the MEN2B is insensitive to PTPRA. In sum, we discern PTPRA as a novel regulator of RET signaling in both health and cancer.

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PTPRA inhibits ligand (GDNF-GFRα1)-mediated RET activity on Ras-MAPK signaling axis

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PTPRA dephosphorylate RET on key functional phosphotyrosine sites

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PTPRA catalytic (PTPase) domain 1 regulates RET-driven signaling

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PTPRA suppresses RET oncogenic mutant MEN2A in both Ras-MAPK and cell invasion models

GDF-15 as a Weight Watcher for Diabetic and Non-Diabetic People Treated With Metformin

Weight gain and obesity are global health concerns contributing to morbidity with increased risks of cardiovascular disease, diabetes, liver steatohepatitis and cancer. Pharmacological therapies or bariatric surgery are often required for those who fail to adhere to diet and lifestyle modifications. Metformin, a widely used antidiabetic agent, seems to have a health benefit beyond its anti-hyperglycemic properties, with few side effects. Emerging evidence shows weight loss to be associated with metformin in both diabetic and non-diabetic individuals. Recently, the growth differentiation factor 15 (GDF-15), a member of the transforming growth factor beta superfamily, has been identified as a key mediator of metformin-induced weight loss. Metformin increases the secretion of GDF-15, which binds exclusively to glial cell-derived neurotrophic factor family receptor alpha-like (GFRAL). This gut-brain cytokine works as a prominent player in reducing food intake and body weight in health and disease, like anorexia nervosa and cancer. Herein, we critically review advances in the understanding of the weight-reducing effects of metformin via the GDF-15 pathway.

Inventing Liraglutide, a Glucagon-Like Peptide-1 Analogue, for the Treatment of Diabetes and Obesity

Glucagon-like peptide-1 (GLP-1) has been in focus since the early 1980s as a long looked for incretin hormone, released from the gastrointestinal tract and with an important effect on glucose-dependent insulin secretion, providing efficient glucose lowering, with little risk for hypoglycemia. The enzyme dipeptidyl peptidase-4 (DPP-4) degrades GLP-1 very fast, and the remaining metabolite is cleared rapidly by the kidneys. Liraglutide is a fatty acid acylated analogue of GLP-1 that provides efficacy for 24 h/day. The mechanism of action for liraglutide is reviewed in detail with focus on pancreatic efficacy and safety, thyroid safety, and weight loss mechanism. Evolving science hypothesizes that GLP-1 has important effects on atherosclerosis, relevant for the cardiovascular benefit seen in the treatment of diabetes and obesity. Also, GLP-1 may be relevant in neurodegenerative diseases.

Non-homeostatic body weight regulation through a brainstem-restricted receptor for GDF15

Under homeostatic conditions, animals use well-defined hypothalamic neural circuits to help maintain stable body weight, by integrating metabolic and hormonal signals from the periphery to balance food consumption and energy expenditure. In stressed or disease conditions, however, animals use alternative neuronal pathways to adapt to the metabolic challenges of altered energy demand. Recent studies have identified brain areas outside the hypothalamus that are activated under these 'non-homeostatic' conditions, but the molecular nature of the peripheral signals and brain-localized receptors that activate these circuits remains elusive. Here we identify glial cell-derived neurotrophic factor (GDNF) receptor alpha-like (GFRAL) as a brainstem-restricted receptor for growth and differentiation factor 15 (GDF15). GDF15 regulates food intake, energy expenditure and body weight in response to metabolic and toxin-induced stresses; we show that Gfral knockout mice are hyperphagic under stressed conditions and are resistant to chemotherapy-induced anorexia and body weight loss. GDF15 activates GFRAL-expressing neurons localized exclusively in the area postrema and nucleus tractus solitarius of the mouse brainstem. It then triggers the activation of neurons localized within the parabrachial nucleus and central amygdala, which constitute part of the 'emergency circuit' that shapes feeding responses to stressful conditions. GDF15 levels increase in response to tissue stress and injury, and elevated levels are associated with body weight loss in numerous chronic human diseases. By isolating GFRAL as the receptor for GDF15-induced anorexia and weight loss, we identify a mechanistic basis for the non-homeostatic regulation of neural circuitry by a peripheral signal associated with tissue damage and stress. These findings provide opportunities to develop therapeutic agents for the treatment of disorders with altered energy demand.

HSP90 inhibition blocks ERBB3 and RET phosphorylation in myxoid/round cell liposarcoma and causes massive cell death in vitro and in vivo

Myxoid sarcoma (MLS) is one of the most common types of malignant soft tissue tumors. MLS is characterized by the FUS-DDIT3 or EWSR1-DDIT3 fusion oncogenes that encode abnormal transcription factors. The receptor tyrosine kinase (RTK) encoding RET was previously identified as a putative downstream target gene to FUS-DDIT3 and here we show that cultured MLS cells expressed phosphorylated RET together with its ligand Persephin. Treatment with RET specific kinase inhibitor Vandetanib failed to reduce RET phosphorylation and inhibit cell growth, suggesting that other RTKs may phosphorylate RET. A screening pointed out EGFR and ERBB3 as the strongest expressed phosphorylated RTKs in MLS cells. We show that ERBB3 formed nuclear and cytoplasmic complexes with RET and both RTKs were previously reported to form complexes with EGFR. The formation of RTK hetero complexes could explain the observed Vandetanib resistence in MLS. EGFR and ERBB3 are clients of HSP90 that help complex formation and RTK activation. Treatment of cultured MLS cells with HSP90 inhibitor 17-DMAG, caused loss of RET and ERBB3 phosphorylation and lead to rapid cell death. Treatment of MLS xenograft carrying Nude mice resulted in massive necrosis, rupture of capillaries and hemorrhages in tumor tissues. We conclude that complex formation between RET and other RTKs may cause RTK inhibitor resistance. HSP90 inhibitors can overcome this resistance and are thus promising drugs for treatment of MLS/RCLS.

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.

Destabilizing RET in targeted treatment of thyroid cancers

Metastatic differentiated thyroid cancers (DTC) are resistant to traditional chemotherapy. Kinase inhibitors have shown promise in patients with progressive DTC, but dose-limiting toxicity is commonplace. HSP90 regulates protein degradation of several growth-mediating kinases such as RET, and we hypothesized that HSP90 inhibitor (AUY922) could inhibit RET-mediated medullary thyroid cancer (MTC) as well as papillary thyroid cancer (PTC) cell growth and also radioactive iodine uptake by PTC cells. Studies utilized MTC cell lines TT (C634W) and MZ-CRC-1 (M918T) and the PTC cell line TPC-1 (RET/PTC1). Cell viability was assessed with MTS assays and apoptosis by flow cytometry. Signaling target expression was determined by western blot and radioiodine uptake measured with a gamma counter. Prolonged treatment of both MTC cell lines with AUY922 simultaneously inhibited both MAPK and mTOR pathways and significantly induced apoptosis (58.7 and 78.7% reduction in MZ-CRC-1 and TT live cells respectively, following 1 μM AUY922; P<0.02). Similarly in the PTC cell line, growth and signaling targets were inhibited, and also a 2.84-fold increase in radioiodine uptake was observed following AUY922 administration (P=0.015). AUY922 demonstrates in vitro activity against MTC and PTC cell lines. We observed a potent dose-dependent increase in apoptosis in MTC cell lines following drug administration confirming its anti-tumorigenic effects. Western blots confirm inhibition of pro-survival proteins including AKT suggesting this as the mechanism of cell death. In a functional study, we observed an increase in radioiodine uptake in the PTC cell line following AUY922 treatment. We believe HSP90 inhibition could be a viable alternative for treatment of RET-driven chemo-resistant thyroid cancers.

RET and EDNRB mutation screening in patients with Hirschsprung disease: Functional studies and its implications for genetic counseling

Hirschsprung disease (HSCR) is a major cause of chronic constipation in children. HSCR can be caused by germline mutations in RET and EDNRB. Defining causality of the mutations identified is difficult and almost exclusively based on in silico predictions. Therefore, the reported frequency of pathogenic mutations might be overestimated. We combined mutation analysis with functional assays to determine the frequencies of proven pathogenic RET and EDNRB mutations in HSCR. We sequenced RET and EDNRB in 57 HSCR patients. The identified RET-coding variants were introduced into RET constructs and these were transfected into HEK293 cells to determine RET phosphorylation and activation via ERK. An exon trap experiment was performed to check a possible splice-site mutation. We identified eight rare RET-coding variants, one possible splice-site variant, but no rare EDNRB variants. Western blotting showed that three coding variants p.(Pr270Leu), p.(Ala756Val) and p.(Tyr1062Cys) resulted in lower activation of RET. Moreover, only two RET variants (p.(Ala756Val) and p.(Tyr1062Cys)) resulted in reduced ERK activation. Splice-site assays on c.1880-11A>G could not confirm its pathogenicity. Our data suggest that indeed almost half of the identified rare variants are proven pathogenic and that, hence, functional studies are essential for proper genetic counseling.

Thyroid C-Cell Biology and Oncogenic Transformation

The thyroid parafollicular cell, or commonly named "C-cell," functions in serum calcium homeostasis. Elevations in serum calcium trigger release of calcitonin from the C-cell, which in turn functions to inhibit absorption of calcium by the intestine, resorption of bone by the osteoclast, and reabsorption of calcium by renal tubular cells. Oncogenic transformation of the thyroid C-cell is thought to progress through a hyperplastic process prior to malignancy with increasing levels of serum calcitonin serving as a biomarker for tumor burden. The discovery that multiple endocrine neoplasia type 2 is caused by activating mutations of the RET gene serves to highlight the RET-RAS-MAPK signaling pathway in both initiation and progression of medullary thyroid carcinoma (MTC). Thyroid C-cells are known to express RET at high levels relative to most cell types; therefore, aberrant activation of this receptor is targeted primarily to the C-cell, providing one possible cause of tissue-specific oncogenesis. The role of RET signaling in normal C-cell function is unknown though calcitonin gene transcription appears to be sensitive to RET activation. Beyond RET, the modeling of oncogenesis in animals and screening of human tumors for candidate gene mutations have uncovered mutation of RAS family members and inactivation of Rb1 regulatory pathway as potential mediators of C-cell transformation. A growing understanding of how RET interacts with these pathways, both in normal C-cell function and during oncogenic transformation, will help in the development of novel molecular-targeted therapies.

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.

Ponatinib (AP24534) is a novel potent inhibitor of oncogenic RET mutants associated with thyroid cancer

CONTEXT

The RET tyrosine kinase encoding gene acts as a dominantly transforming oncogene in thyroid carcinoma and other malignancies. Ponatinib (AP24534) is an oral ATP-competitive tyrosine kinase inhibitor that is in advanced clinical experimentation in leukemia.

OBJECTIVE

We tested whether ponatinib inhibited RET kinase and oncogenic activity.

METHODS

Ponatinib activity was studied by an in vitro RET immunocomplex kinase assay and immunoblotting. The effects of ponatinib on proliferation of human TT, MZ-CRC-1, and TPC-1 thyroid carcinoma cells, which harbor endogenous oncogenic RET alleles, and of NIH3T3 fibroblasts transfected with oncogenic RET mutants were determined. Ponatinib activity on TT cell xenografted tumors in athymic mice was measured.

RESULTS

Ponatinib inhibited immunopurified RET kinase at the IC₅₀ of 25.8 nM (95% confidence interval [CI] = 23.15-28.77 nM). It also inhibited (IC₅₀ = 33.9 nM; 95% CI = 26.41-43.58 nM) kinase activity of RET/V804M, a RET mutant displaying resistance to other tyrosine kinase inhibitor. Ponatinib blunted phosphorylation of point-mutant and rearranged RET-derived oncoproteins and inhibited proliferation of RET-transformed fibroblasts and RET mutant thyroid carcinoma cells. Finally, after 3 weeks of treatment with ponatinib (30 mg/kg/d), the volume of TT cell (medullary thyroid carcinoma) xenografts was reduced from 133 mm³ to an unmeasurable size (difference = 133 mm³, 95% CI = -83 to 349 mm³) (P < .001). Ponatinib-treated TT cell tumors displayed a reduction in the mitotic index, RET phosphorylation, and signaling.

CONCLUSIONS

Ponatinib is a potent inhibitor of RET kinase and has promising preclinical activity in models of RET-driven medullary thyroid carcinoma.

Cellular signaling pathway alterations and potential targeted therapies for medullary thyroid carcinoma

Parafollicular C-cell-derived medullary thyroid cancer (MTC) comprises 3% to 4% of all thyroid cancers. While cytotoxic treatments have been shown to have limited efficacy, targeted molecular therapies that inhibit rearranged during transfection (RET) and other tyrosine kinase receptors that are mainly involved in angiogenesis have shown great promise in the treatment of metastatic or locally advanced MTC. Multi-tyrosine kinase inhibitors such as vandetanib, which is already approved for the treatment of progressive MTC, and cabozantinib have shown distinct advantages with regard to rates of disease response and control. However, these types of tyrosine kinase inhibitor compounds are able to concurrently block several types of targets, which limits the understanding of RET as a specific target. Moreover, important resistances to tyrosine kinase inhibitors can occur, which limit the long-term efficacy of these treatments. Deregulated cellular signaling pathways and genetic alterations in MTC, particularly the activation of the RAS/mammalian target of rapamycin (mTOR) cascades and RET crosstalk signaling, are now emerging as novel and potentially promising therapeutic treatments for aggressive MTC.

New insights into Dok-4 PTB domain structure and function

The seven members of the Dok adapter protein family share a highly conserved phosphotyrosine-binding (PTB) domain. In the case of Dok-1, 2 and 3, the PTB domain binds to the lipid phosphatase Ship1, a key component of their inhibitory signaling mechanisms in immune cells. In contrast to most other Dok family members, Dok-4 is expressed widely but is poorly understood, largely because of limited knowledge of its partner molecules. We previously showed that, in contrast to the Dok-1 PTB domain (defined as aa 107-260), the homologous sequence in Dok-4 (aa 100-233) bound very poorly to Ret, a known Dok-4 partner. In the current study, we show that binding of Dok-4 to Ret requires residues C-terminal to the previously defined PTB domain boundaries (up to aa 246). These residues are predicted to form an extension in a critical C-terminal α-helix. We show that the Dok-4 PTB domain also binds the phosphorylated NPXY motifs in Ship1 but not Ship2. Finally, we found that a rare human single nucleotide polymorphism causing a R186H substitution in the PTB domain abolishes tyrosine phosphorylation of Dok-4 by Ret. In addition to providing a clearer understanding of Dok-4 PTB domain structure and function, our findings point to a potential mechanism for Dok-4 inhibitory signaling in T-cells and to the possibility of a rare Dok-4-related phenotype in humans.

Alternative splicing results in RET isoforms with distinct trafficking properties

The RET gene encodes a receptor tyrosine kinase that is alternatively spliced to two protein isoforms that differ in their C-terminal peptide sequences (RET9, RET51). These unique C-terminal tails produce distinct subcellular localizations and intracellular trafficking properties, which affect downstream signaling.

RET encodes a receptor tyrosine kinase that is essential for spermatogenesis, development of the sensory, sympathetic, parasympathetic, and enteric nervous systems and the kidneys, as well as for maintenance of adult midbrain dopaminergic neurons. RET is alternatively spliced to encode multiple isoforms that differ in their C-terminal amino acids. The RET9 and RET51 isoforms display unique levels of autophosphorylation and have differential interactions with adaptor proteins. They induce distinct gene expression patterns, promote different levels of cell differentiation and transformation, and play unique roles in development. Here we present a comprehensive study of the subcellular localization and trafficking of RET isoforms. We show that immature RET9 accumulates intracellularly in the Golgi, whereas RET51 is efficiently matured and present in relatively higher amounts on the plasma membrane. RET51 is internalized faster after ligand binding and undergoes recycling back to the plasma membrane. This differential trafficking of RET isoforms produces a more rapid and longer duration of signaling through the extracellular-signal regulated kinase/mitogen-activated protein kinase pathway downstream of RET51 relative to RET9. Together these differences in trafficking properties contribute to some of the functional differences previously observed between RET9 and RET51 and establish the important role of intracellular trafficking in modulating and maintaining RET signaling.

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.

GLP-1 receptor agonists and the thyroid: C-cell effects in mice are mediated via the GLP-1 receptor and not associated with RET activation

Liraglutide and exenatide are glucagon-like peptide receptor (GLP-1R) agonists used in the treatment of type 2 diabetes. Both molecules have been associated with the development of thyroid C-cell tumors after lifetime exposure in rodents. Previously, it has been reported that these tumors are preceded by increased plasma calcitonin and C-cell hyperplasia. We can now document that the murine C-cell effects are mediated via GLP-1R. Thus, 13 wk of continuous exposure to GLP-1R agonists was associated with marked increases in plasma calcitonin and in the incidence of C-cell hyperplasia in wild-type mice. In contrast, similar effects were not seen in GLP-1R knockout mice. Human C-cell cancer is often caused by activating mutations in the rearranged-during-transfection (RET) protooncogene. We developed an immunohistochemical method to assess RET activation in tissues. Liraglutide dosing to mice was not found to activate RET. Further evaluation of the signaling pathways demonstrated that liraglutide increased ribosomal S6, but not MAPK kinase, phosphorylation. These observations are consistent with effects of GLP-1R agonists on rodent C cells being mediated via mammalian target of rapamycin activation in a RET- and MAPK-independent manner.

RET PLCγ phosphotyrosine binding domain regulates Ca2+ signaling and neocortical neuronal migration

The receptor tyrosine kinase RET plays an essential role during embryogenesis in regulating cell proliferation, differentiation, and migration. Upon glial cell line-derived neurotrophic factor (GDNF) stimulation, RET can trigger multiple intracellular signaling pathways that in concert activate various downstream effectors. Here we report that the RET receptor induces calcium (Ca²⁺) signaling and regulates neocortical neuronal progenitor migration through the Phospholipase-C gamma (PLCγ) binding domain Tyr1015. This signaling cascade releases Ca²⁺ from the endoplasmic reticulum through the inositol 1,4,5-trisphosphate receptor and stimulates phosphorylation of ERK1/2 and CaMKII. A point mutation at Tyr1015 on RET or small interfering RNA gene silencing of PLCγ block the GDNF-induced signaling cascade. Delivery of the RET mutation to neuronal progenitors in the embryonic ventricular zone using in utero electroporation reveal that Tyr1015 is necessary for GDNF-stimulated migration of neurons to the cortical plate. These findings demonstrate a novel RET mediated signaling pathway that elevates cytosolic Ca²⁺ and modulates neuronal migration in the developing neocortex through the PLCγ binding domain Tyr1015.

Anti-proliferative effect and action mechanism of dexamethasone in human medullary thyroid cancer cell line

INTRODUCTION

Dexamethasone is known to inhibit the cell proliferation of certain transformed cell lines. In this study, the effect and action mechanism of dexamethasone were examined in the human medullary thyroid cancer cell line, TT cells.

METHODS

TT cells were treated with or without dexamethasone. 5-Bromo-2'-deoxyuridine uptake assay was used to evaluate cell proliferation. Cell cycle and its regulatory proteins were assessed by flow cytometry and western blot analysis, respectively. Apoptosis was analyzed by Hoechst staining and Annexin V assay.

RESULTS

Dexamethasone significantly reduced TT cell proliferation by 60% (p < 0.01). A substantial portion of cells was arrested at the G1 phase. The expression levels of cyclin D1, cyclin-dependent kinase (CDK)4, and CDK2 were decreased. In addition, the phosphorylation of retinoblastoma protein, which is a critical checkpoint protein in the transition of G1 to S phase, was decreased. On the other hand, the expression level of p27(Kip1), which is a cyclin/CDK inhibitor, was enhanced. Hoechst staining showed many fragmented nuclei in the dexamethasone-treated cells. The proportion of early apoptotic cells was also increased in the Annexin V assay.

CONCLUSION

Dexamethasone inhibited the proliferation of TT cells through cell cycle arrest at the G1 phase and increased apoptosis.

Splicing variants impact in thyroid normal physiology and pathological conditions

RNA splicing is an essential, precisely regulated process that occurs after gene transcription and before mRNA translation, in which introns may be removed and exons, retained. Variability in splicing patterns is a major source of protein diversity from the genome and function to generate a tremendously diverse proteome from a relatively small number of genes. Changes in splice site choice can determine different effects on the encoded protein. Small changes in peptide sequence can alter ligand binding, enzymatic activity, allosteric regulation, or protein localization. Errors in splicing regulation have been implicated in a number of different disease states. This study reviewed the mechanisms of splicing and their repercussion in endocrinology, emphasizing its importance in some thyroid physiological and pathological conditions.

Analysis of DOK-6 function in downstream signaling of RET in human neuroblastoma cells

Point mutations and structural alterations of the RET tyrosine kinase gene cause multiple endocrine neoplasia type 2 (MEN 2) and papillary thyroid carcinoma, respectively. RET activation by glial cell line-derived neurotrophic factor (GDNF) is essential for the development of the enteric nervous system and the kidney. The signal through RET tyrosine kinase requires several adaptor proteins including the DOK (downstream of kinase) family of proteins. Of the seven members of the DOK protein family, DOK-1, -4, -5, and -6 have been reported to play roles in the GDNF-RET signaling pathway. Although DOK-6 has been shown to bind to RET and promote GDNF-induced neurite outgrowth in mouse Neuro2A cells, DOK-6 function in human cells remains unclear. In the present study, we investigated the role of DOK-6 in GDNF-RET signaling in human cells including neuroblastoma cells. DOK-6 was constitutively localized to the plasma membrane via its pleckstrin homology (PH) domain, and was phosphorylated following RET activation via a MEN2A mutation or GDNF stimulation. However, DOK-6 could not significantly affect downstream signaling and neurite outgrowth in human neuroblastoma cells. The binding affinity of the DOK-6 phosphotyrosine-binding (PTB) domain to RET was much lower than that of the DOK-1, DOK-4, and SHC PTB domains to RET. These findings indicate that DOK-6 is involved in RET signaling with less influence when compared with DOK-1, DOK-4, and SHC.

Targeting the RET pathway in thyroid cancer

The RET (rearranged during transfection) protooncogene encodes a single pass transmembrane receptor that is expressed in cells derived from the neural crest and the urogenital tract. As part of a cell-surface complex, RET binds glial derived neurotrophic factor (GDNF) ligands in conjunction with GDNF-family alpha co-receptors (GFRalpha). Ligand-induced activation induces dimerization and tyrosine phosphorylation of the RET receptor with downstream activation of several signal transduction pathways. Activating germline RET mutations play a central role in the development of the multiple endocrine neoplasia (MEN) syndromes MEN2A, MEN2B, and familial medullary thyroid carcinoma (FMTC) and also in the development of the congenital abnormality Hirschsprung's disease. Approximately 50% of patients with sporadic MTC have somatic RET mutations, and a significant portion of papillary thyroid carcinomas result from chromosomal inversions or translocations, which activate RET (RET/PTC oncogenes). The RET protooncogene has a significant place in cancer prevention and treatment. Timely thyroidectomy in kindred members who have inherited a mutated RET allele, characteristic of MEN2A, MEN2B, or FMTC, can prevent MTC, the most common cause of death in these syndromes. Also, recently developed molecular therapeutics that target the RET pathway have shown activity in clinical trials of patients with advanced MTC, a disease for which there has been no effective therapy.

Beyond RET: potential therapeutic approaches for advanced and metastatic medullary thyroid carcinoma

Medullary thyroid carcinoma (MTC) is a rare calcitonin-producing neuroendocrine tumour that originates from the parafollicular C-cells of the thyroid gland. The RET proto-oncogene encodes the RET receptor tyrosine kinase, which has essential roles in cell survival, differentiation and proliferation. Activating mutations of RET are associated with the pathogenesis of MTC and have been demonstrated in nearly all hereditary and in 30-50% of sporadic MTC cases, making this receptor an excellent target for small-molecule inhibitors for this tumour. Clinical trials of small organic inhibitors of tyrosine kinase receptors (TKIs) targeting the RET receptor have shown efficacy for treatment of metastatic MTC with 30-50% of patients responding to these agents. Despite the importance of the RET receptor in MTC, it is clear that other signal transduction pathways, tyrosine kinase receptors, and tumour suppressor genes are involved in MTC tumourigenesis and progression. A better understanding of molecular cross-talk between these signal pathways and the RET receptor may lead to combinatorial therapy that will improve outcomes beyond what is currently possible with RET-directed TKIs. Finally, there is evidence that immunological-based therapy using dendritic cell vaccination strategies have been effective for reducing tumour mass in a small number of patients. The identification of additional MTC-specific tumour antigens and a better understanding of specific epitopes in these tumour antigens may lead to improvement of response rates.

Oncoprotein signaling mediates tumor-specific inflammation and enhances tumor progression

The RET/PTC3 (RP3) fusion protein is an oncogene expressed during the development of thyroid cancer and in thyroid epithelial cells of patients with Hashimoto's thyroiditis. RP3 has two immunological properties: 1) it encodes a chimeric protein including peptides that may be targets of antitumor immune responses and 2) it is a tyrosine kinase that can activate NF-kappaB transcriptional programs, induce secretion of proinflammatory mediators, and stimulate innate immunity. To distinguish the antigenic properties of the RP3 oncoprotein from its signaling function, a transplantable tumor system was developed. Tumors expressing the functional, but not mutant, form of RP3 show enhanced infiltration of CD8(+) lymphocytes, myeloid-derived CD11b(+)Gr1(+) cells, and enhanced growth in immunocompetent mice. In contrast, RP3 signaling mutant-expressing tumors maintained enhanced infiltration of CD8(+) lymphocytes did not enhance recruitment of CD11b(+)Gr1(+) cells and showed a decreased tumor incidence. These results implicate a role for RP3 function in enhancing a tumor-suppressive innate inflammatory response. These experiments support a mechanism whereby oncogenes can directly recruit and activate innate and adaptive immune cells, resulting in enhanced tumor progression.

The beta-catenin axis integrates multiple signals downstream from RET/papillary thyroid carcinoma leading to cell proliferation

RET/papillary thyroid carcinoma (RET/PTC) oncoproteins result from the in-frame fusion of the RET receptor tyrosine kinase domain with protein dimerization motifs encoded by heterologous genes. Here, we show that RET/PTC stimulates the beta-catenin pathway. By stimulating PI3K/AKT and Ras/extracellular signal-regulated kinase (ERK), RET/PTC promotes glycogen synthase kinase 3beta (GSK3beta) phosphorylation, thereby reducing GSK3beta-mediated NH(2)-terminal beta-catenin (Ser33/Ser37/Thr41) phosphorylation. In addition, RET/PTC physically interacts with beta-catenin and increases its phosphotyrosine content. The increased free pool of S/T(nonphospho)/Y(phospho)beta-catenin is stabilized as a result of the reduced binding affinity for the Axin/GSK3beta complex and activates the transcription factor T-cell factor/lymphoid enhancer factor. Moreover, through the ERK pathway, RET/PTC stimulates cyclic AMP-responsive element binding protein (CREB) phosphorylation and promotes the formation of a beta-catenin-CREB-CREB-binding protein/p300 transcriptional complex. Transcriptional complexes containing beta-catenin are recruited to the cyclin D1 promoter and a cyclin D1 gene promoter reporter is active in RET/PTC-expressing cells. Silencing of beta-catenin by small interfering RNA inhibits proliferation of RET/PTC-transformed PC Cl3 thyrocytes, whereas a constitutively active form of beta-catenin stimulates autonomous proliferation of thyroid cells. Thus, multiple signaling events downstream from RET/PTC converge on beta-catenin to stimulate cell proliferation.

Loss of Sprouty1 rescues renal agenesis caused by Ret mutation

Renal morphogenesis requires a balance between positive and negative signals, which are provided in part by the receptor tyrosine kinase Ret and the putative tumor suppressor Sprouty1, respectively. Tyrosine 1062 of Ret is a binding site for several adaptor and effector proteins, such as Grb2/Sos/Ras, which activate the ERK pathway. Mice lacking Ret tyrosine 1062 nearly mimic the phenotype of Ret-knockout mice, which includes renal agenesis. Sprouty1 regulates Ret activity by modulating the ERK pathway, but the mechanism by which this occurs is uncertain. Here, we show that loss of Sprouty1 rescues the renal agenesis and early postnatal lethality caused by lack of Ret tyrosine 1062. The kidneys and lower urinary tracts of double-mutant mice developed normally. This effect was specific to the urinary system, because loss of Sprouty1 did not rescue the defects in the enteric nervous system characteristic of animals lacking Ret tyrosine 1062. These results suggest that Sprouty1 can modulate ERK signaling downstream of Ret, independent of Grb2/Sos/Ras, during renal morphogenesis.

A new germline point mutation in Ret exon 8 (cys515ser) in a family with medullary thyroid carcinoma

BACKGROUND

A novel Cys-Ser Ret germline point mutation in a 58-year-old woman with bilateral medullary thyroid carcinoma (MTC) prompted us to perform genetic analysis of the family and evaluate the biological consequences of such a mutation.

METHODS

Ret analysis by direct sequencing was performed in five family members. The biological activity and biochemical properties of the Ret- Cys515Ser mutant were analyzed in NIH-3T3 cells.

RESULTS

The proband's son, age 35, had the Ret- Cys515Ser mutation and the L769 CTT/CTG exon 13 polymorphic variant, which was also found in his father. Clinical evaluation of the son also revealed bilateral multifocal microscopic MTC and papillary thyroid carcinoma (PTC). In vitro and in vivo analysis indicated ligand-independent activation of the Ret-Cys515Ser mutant due to aberrant disulfide homodimerization, increased mitogenic activity, and ability to induce anchorage-independent growth in NIH-3T3 cells in comparison to wild-type Ret, suggesting a possible role of Cys515Ser in tumor development.

CONCLUSIONS

The Cys515Ser mutation adds to cysteine substitution groups that have been described in association with MTC. Our data also highlight the importance of performing a complete genetic analysis in patients who present with MTC.

Ret isoform function and marker gene expression in the enteric nervous system is conserved across diverse vertebrate species

The enteric nervous system (ENS) derives from migratory neural crest cells that colonize the developing gut tube, giving rise to an integrated network of neurons and glial cells, which together regulate important aspects of gut function, including coordinating the smooth muscle contractions of the gut wall. The absence of enteric neurons in portions of the gut (aganglionosis) is the defining feature of Hirschsprung's disease (HSCR) and has been replicated in a number of mouse models. Mutations in the RET tyrosine kinase account for over half of familial cases of HSCR and mice mutant for Ret exhibit aganglionosis. RET exists in two main isoforms, RET9 and RET51 and studies in mouse have shown that RET9 is sufficient to allow normal development of the ENS. In the last several years, zebrafish has emerged as a model of vertebrate ENS development, having been supported by a number of demonstrations of conservation of gene function between zebrafish, mouse and human. In this study we further analyse the potential similarities and differences between ENS development in zebrafish, mouse and human. We demonstrate that zebrafish Ret is required in a dose-dependent manner to regulate colonization of the gut by neural crest derivatives, as in human. Additionally, we show that as in mouse and human, zebrafish ret is produced as two isoforms, ret9 and ret51. Moreover, we show that, as in mouse, the Ret9 isoform is sufficient to support colonization of the gut by enteric neurons. Finally, we identify zebrafish orthologues of genes previously identified to be expressed in the mouse ENS and demonstrate that these genes are expressed in the developing zebrafish ENS, thereby identifying useful ENS markers in this model organism. These studies reveal that the similarities between gene expression and gene function across vertebrate species is more extensive than previously appreciated, thus supporting the use of zebrafish as a general model for vertebrate ENS development and the use of zebrafish genetic screens as a way to identify candidate genes mutated in HSCR cases.

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.

RET(MEN 2B) is active in the endoplasmic reticulum before reaching the cell surface

MEN 2B (multiple endocrine neoplasia type 2B) is an autosomal dominant cancer syndrome caused by an oncogenic form of the receptor tyrosine kinase REarranged during transfection (RET). The MEN 2B syndrome is associated with an abnormal autophosphorylation of the mutated receptor even without ligand-stimulation. Here, we characterize the activation of a RET(MEN 2B) variant carrying the point mutation Met918Thr, and show that the 150 kDa precursor of RET(MEN 2B) becomes phosphorylated already during synthesis in the endoplasmic reticulum (ER). At least three different tyrosine residues (Tyr905, Tyr1062, Tyr1096) of the RET(MEN 2B) precursor are phosphorylated before the oncogenic receptor reaches the cell surface. We also demonstrate that the precursor of RET(MEN 2B) interacts with both growth factor receptor-bound protein and Src homology 2 domain-containing already in the ER, and that this interaction is dependent on the kinase activity of RET. With the aid of two RET mutants (RET(MEN 2B/S32L) and RET(MEN 2B/F393L)), which accumulate in the ER, we show that the oncogenic precursor of the receptor has the capacity to activate AKT, extracellular signal-regulated kinase and signal transducer and activator of transcription 3 from the ER. Taken together, our data demonstrate that the oncogenic precursor of RET(MEN 2B) is phosphorylated, interacts with adapter proteins and induces downstream signalling from the ER.

RET/Papillary Thyroid Carcinoma Oncogenic Signaling through the Rap1 Small GTPase

RET/papillary thyroid carcinoma (PTC) oncoproteins result from the in-frame fusion of the RET receptor tyrosine kinase with protein dimerization motifs encoded by heterologous genes. Here, we show that RET/PTC1 activates the Rap1 small GTPase. The activation of Rap1 was dependent on the phosphorylation of RET Tyr(1062). RET/PTC1 recruited a complex containing growth factor receptor binding protein 2-associated binding protein 1 (Gab1), CrkII (v-crk sarcoma virus CT10 oncogene homologue II), and C3G (Rap guanine nucleotide exchange factor 1). By using dominant-negative and small interfering duplex (small interfering RNA) oligonucleotides, we show that RET/PTC1-mediated Rap1 activation was dependent on CrkII, C3G, and Gab1. Activation of Rap1 was involved in the RET/PTC1-mediated stimulation of the BRAF kinase and the p42/p44 mitogen-activated protein kinases. Proliferation and stress fiber formation of RET/PTC1-expressing PC Cl 3 thyroid follicular cells were inhibited by the dominant-negative Rap1(N17) and by Rap1-specific GTPase-activating protein. Thus, Rap1 is a downstream effector of RET/PTC and may contribute to the transformed phenotype of RET/PTC-expressing thyrocytes.

A complex additive model of inheritance for Hirschsprung disease is supported by both RET mutations and predisposing RET haplotypes

PURPOSE

The RET proto-oncogene is considered to be the major susceptibility gene involved in Hirschsprung disease. Traditional RET germline mutations account for a small subset of Hirschsprung disease patients, but several studies have shown that there is a specific haplotype of RET associated with the sporadic forms of Hirschsprung disease. We have investigated for RET germline mutations and analyzed the RET haplotypic distribution in carriers versus noncarriers of RET germline mutations.

METHODS

We have screened the coding region of RET in 106 Spanish Hirschsprung disease patients using dHPLC technology. Statistical comparisons of the distribution of RET haplotypes between sporadic patients with and without a RET germline mutation were performed.

RESULTS

Nine novel germline mutations and one previously described were identified. A significant over-transmission of the "Hirschsprung disease haplotype" was detected when comparing transmitted versus nontransmitted alleles in the group of Hirschsprung disease triads without mutation. However, no distortion of the transmission of alleles was found in the group of mutated families.

CONCLUSIONS

These results would be concordant with a complex additive model of inheritance. The whole findings seem to suggest that low-penetrance mutations would be necessary but not sufficient and the additional presence of the "Hirschsprung disease haplotype" could contribute to the manifestation of the disease.

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.

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.

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.

Evaluation of potential mechanisms underlying genotype-phenotype correlations in multiple endocrine neoplasia type 2

Distinct dominant activating mutations in the RET proto-oncogene are responsible for the development of multiple endocrine neoplasia type 2 (MEN 2). Concise examination of the mutated codons led to the detection of a striking genotype-phenotype correlation between the mutated codon and the MEN 2 phenotype in terms of onset and aggressiveness of the disease, suggesting that manifestation and clinical progression is conditioned by the type of mutation. To gain insight into the molecular basis for this genotype-phenotype correlation, we analysed the impact of common and rare mutations identified in MEN 2A (C609Y, C634R), MEN 2B (A883F, M918T) and familial medullary thyroid carcinoma (Y791F) patients on several aspects of cell transformation, including proliferation, apoptosis, anchorage-independent growth and signaling. We found that tumor cells arising from distinct extracellular or intracellular MEN 2 mutations clearly differ in their proliferation properties owing to the activation of different molecular pathways, but importantly, also in resistance to apoptosis. Whereas MEN 2A mutants resulted in accelerated cell proliferation, MEN 2B-RET mutants significantly enhanced suppression of apoptosis, which may account, at least partially, for some of the clinical differences in MEN 2 patients.

Critical and distinct roles for key RET tyrosine docking sites in renal development

Molecular mechanisms that lead to congenital anomalies of kidneys and the lower urinary tract (CAKUT) are poorly understood. To elucidate the molecular basis for signaling specificity of GDNF-mediated RET signaling in kidney development, we characterized mice that exclusively express either the human RET9 or RET51 isoform, or express these isoforms with individual mutations in docking tyrosines for PTB and SH2-domain-containing adaptors Src (Y981), PLCgamma (Y1015), and Shc (Y1062). Our results provide evidence for differential and isoform-specific roles of these docking sites in murine kidney development. Homozygous Ret(RET9) and Ret(RET51) mice were viable and show normally developed kidneys, indicating redundant roles of human RET isoforms in murine kidney development. In the context of the RET51 isoform, only mutation of the docking Tyr 1015 (Y1015F) resulted in severe renal anomalies. These included bilateral megaureters and multicystic kidneys that were caused by supernumerary ureteric buds that fail to separate from the wolffian duct as well as decreased branching morphogenesis. Similar kidney and ureter defects were observed in RET9(Y1015F) mice that contain the Y1015F mutation in the RET9 isoform. Interestingly, loss of RET9(Y1062)-mediated AKT/MAPK activation resulted in renal agenesis or kidney rudiments, whereas mutation of this residue in RET51 had no obvious effect on AKT/MAPK activity and renal development. These results reveal novel roles of key RET-dependent signaling pathways in embryonic kidney development and provide murine models and new insights into the molecular basis for CAKUT.

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.

BAY 43-9006 inhibition of oncogenic RET mutants

BACKGROUND

Medullary and papillary thyroid carcinomas are often associated with oncogenic activation of the RET tyrosine kinase. We evaluated whether the biaryl urea BAY 43-9006, which is known to inhibit several other tyrosine kinases, blocks RET kinase function and oncogenic activity.

METHODS

We examined BAY 43-9006 activity against oncogenic RET in vitro and in cellular RET signaling in oncogenic RET-transfected NIH3T3 fibroblasts by using immunocomplex kinase assays and immunoblotting with phospho-specific antibodies. The effects of BAY 43-9006 on proliferation of human TPC1 and TT thyroid carcinoma cells, which harbor spontaneous oncogenic RET alleles, and on RAT1 fibroblasts transformed with oncogenic RET mutants, including mutants that are resistant to other chemotherapeutic agents, were determined using growth curves and flow cytometry. Growth of TT cell-derived xenograft tumors in athymic mice treated orally with BAY 43-9006 or with vehicle was measured. All statistical tests were two-sided.

RESULTS

BAY 43-9006 inhibited oncogenic RET kinase activity at half-maximal inhibitory concentrations (IC50s) of 50 nM or less in NIH3T3 cells. It also arrested the growth of NIH3T3 and RAT1 fibroblasts transformed by oncogenic RET and of thyroid carcinoma cells that harbor spontaneous oncogenic RET alleles. Moreover, BAY 43-9006 inhibited the growth of cells carrying RET V804L (IC50 = 110 nM, 95% confidence interval [CI] = 88 to 133 nM) or RET V804M (IC50 = 147 nM, 95% CI = 123 nM to 170 nM), both mutants that are resistant to anilinoquinazolines and pyrazolopyrimidines. After 3 weeks of oral treatment with BAY 43-9006 (60 mg/kg/day), the volume of TT cell xenografts (n = 7) was reduced from 72.5 to 44 mm3 (difference = 28.5 mm3, 95% CI = 7 mm3 to 50 mm3), whereas in vehicle-treated mice (n = 7), mean tumor volume increased to 408 mm3 (difference = 320 mm3, 95% CI = 180 mm3 to 460 mm3; untreated versus treated, P =.02). This inhibition paralleled a decrease in RET phosphorylation.

CONCLUSIONS

BAY 43-9006 is a powerful inhibitor of the RET kinase. Its potential as a therapeutic tool for RET-positive thyroid tumors, including those expressing V804 mutations merits study.

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.