Signalling of the Ret receptor tyrosine kinase through the c-Jun NH2-terminal protein kinases (JNKS): evidence for a divergence of the ERKs and JNKs pathways induced by Ret

Neuroprotective Potential of a Small Molecule RET Agonist in Cultured Dopamine Neurons and Hemiparkinsonian Rats

BACKGROUND

Parkinson's disease (PD) is a progressive neurological disorder where loss of dopamine neurons in the substantia nigra and dopamine depletion in the striatum cause characteristic motor symptoms. Currently, no treatment is able to halt the progression of PD. Glial cell line-derived neurotrophic factor (GDNF) rescues degenerating dopamine neurons both in vitro and in animal models of PD. When tested in PD patients, however, the outcomes from intracranial GDNF infusion paradigms have been inconclusive, mainly due to poor pharmacokinetic properties.

OBJECTIVE

We have developed drug-like small molecules, named BT compounds that activate signaling through GDNF's receptor, the transmembrane receptor tyrosine kinase RET, both in vitro and in vivo and are able to penetrate through the blood-brain barrier. Here we evaluated the properties of BT44, a second generation RET agonist, in immortalized cells, dopamine neurons and rat 6-hydroxydopamine model of PD.

METHODS

We used biochemical, immunohistochemical and behavioral methods to evaluate the effects of BT44 on dopamine system in vitro and in vivo.

RESULTS

BT44 selectively activated RET and intracellular pro-survival AKT and MAPK signaling pathways in immortalized cells. In primary midbrain dopamine neurons cultured in serum-deprived conditions, BT44 promoted the survival of the neurons derived from wild-type, but not from RET knockout mice. BT44 also protected cultured wild-type dopamine neurons from MPP+-induced toxicity. In a rat 6-hydroxydopamine model of PD, BT44 reduced motor imbalance and seemed to protect dopaminergic fibers in the striatum.

CONCLUSION

BT44 holds potential for further development into a novel, possibly disease-modifying, therapy for PD.

Glial-derived neurotrophic factor regulates enteric mast cells and ameliorates dextran sulfate sodium-induced experimental colitis

BACKGROUND & AIMS

Although interactions between enteric glial cells (EGCs) and enteric mast cells have been demonstrated to play an important role in the pathogenesis of inflammatory bowel disease (IBD), the exact mechanisms by which EGCs regulate enteric mast cells are still unknown. The aims of this study were to investigate whether glial-derived neurotrophic factor (GDNF), which has been confirmed to be produced mostly by EGCs, might regulate enteric mast cells and ameliorate dextran sulfate sodium (DSS)-induced experimental colitis.

METHODS

Recombinant adenoviral vectors encoding GDNF (Ad-GDNF) were administered intracolonically in experimental colitis induced by DSS. The disease activity index and histological score were measured. The expression of tumour necrosis factor-α (TNF-α), interleukin-6 and myeloperoxidase (MPO) activity were measured by ELISA assay. The expression of trypsin and β-hexosaminidase were evaluated. GDNF specific receptor (GFR-α1/RET) was detected. The calcium reflux was tested by microplate reader. The expression p-JNK was analyzed by western blot assay.

RESULTS

GDNF resulted in a significant inhibition of the activation of enteric mast cells by down-regulating JNK signal pathway, lessening intracellular calcium influx, and then reducing the degranulation as well as the expression of pro-inflammatory cytokines via combing with its receptor (GFR-α1/RET) in mast cells, and these inhibitory effects were abrogated by treatment with neutralizing antibody against GDNF. Moreover, the administration of GDNF led to an amelioration of experimental colitis.

CONCLUSIONS

GDNF are able to regulate enteric mast cells and ameliorate experimental colitis. GDNF might be an important mediator of the cross-talk between EGCs and enteric mast cells, and GDNF might be a useful therapeutic drug for IBD.

Ret Receptor Has Distinct Alterations and Functions in Breast Cancer

Ret receptor tyrosine kinase is a proto-oncogene that participates in development of various cancers. Several independent studies have recently identified Ret as a key player in breast cancer. Although Ret overexpression and function have been under investigation, mainly in estrogen receptor positive breast cancer, a more comprehensive analysis of the impact of recurring Ret alterations in breast cancer is needed. This review consolidates the current knowledge of Ret alterations and their potential effects in breast cancer. We discuss and integrate data on Ret changes in different breast cancer subtypes and potential function in progression, as well as the participation of distinct Ret network signaling partners in these processes. We propose that it will be essential to define a shared molecular feature of tumors with alteration in Ret receptor, be this at the genetic level or via overexpression in order to design effective therapies to target the Ret pathway. Here we review experimental evidence from basic research and pre-clinical studies concentrating on Ret alterations as potential biomarkers for recurrence, and we discuss the possibility that targeting the Ret pathway might in the future become a treatment for breast cancer.

RET isoform-specific interaction with scaffold protein Ezrin promotes cell migration and chemotaxis in lung adenocarcinoma

OBJECTIVES

Increased expression of REarranged during Transfection (RET) kinase is reported in 10-20 % of lung adenocarcinomas (LUAD) and is associated with metastasis and reduced survival. Ezrin is a scaffold protein that promotes protein interactions with the actin cytoskeleton to regulate cell migration and is also associated with invasion and metastasis in cancers. RET isoforms interact with unique combinations of scaffold proteins to promote distinct signaling pathways. We hypothesized that RET isoforms associate distinctly with Ezrin for cytoskeletal reorganization and LUAD cell migration processes.

METHODS

HCC1833 and A549 LUAD, SH-SY5Y neuroblastoma or HEK-293 cells expressing RET and Ezrin were stimulated with the RET ligand glial cell line-derived neurotrophic factor (GDNF) and treated with RET, Ezrin or Src inhibitors. Co-immunoprecipitation or pull-down assays coupled to immunoblotting were used to investigate protein activation and interactions. Immunofluorescence confocal microscopy assessed LUAD cytoskeletal reorganization and colocalization of RET and Ezrin. Live-cell fluorescence imaging was used to measure cell migration and chemotaxis.

RESULTS

GDNF promoted activation, interaction and colocalization of RET51 isoform and Ezrin. Inhibition of RET or Src impaired Ezrin interactions with RET and Src. GDNF stimulation enhanced the formation of actin-rich filopodia, in which both RET and Ezrin were enriched, and promoted chemotaxis in LUAD cells. However, inhibition of RET, Src or Ezrin suppressed filopodia formation, reduced colocalization of Ezrin with RET, and impaired cell migration and/ or chemotaxis. We further showed that GDNF-mediated activation of RET and Ezrin promoted RhoA-GTPase activity and signaling of ROCK1 and ROCK2 in LUAD cells.

CONCLUSIONS

Expression and activation of RET51 mediates unique protein interactions with Ezrin to promote LUAD cell chemotaxis for cancer cell dissemination, which may have implications in LUAD metastatic progression.

Targeting Protein Kinases to Enhance the Response to anti-PD-1/PD-L1 Immunotherapy

The interaction between programmed cell death protein (PD-1) and its ligand (PD-L1) is one of the main pathways used by some tumors to escape the immune response. In recent years, immunotherapies based on the use of antibodies against PD-1/PD-L1 have been postulated as a great promise for cancer treatment, increasing total survival compared to standard therapy in different tumors. Despite the hopefulness of these results, a significant percentage of patients do not respond to such therapy or will end up evolving toward a progressive disease. Besides their role in PD-L1 expression, altered protein kinases in tumor cells can limit the effectiveness of PD-1/PD-L1 blocking therapies at different levels. In this review, we describe the role of kinases that appear most frequently altered in tumor cells and that can be an impediment for the success of immunotherapies as well as the potential utility of protein kinase inhibitors to enhance the response to such treatments.

A loss-of-function genetic screening identifies novel mediators of thyroid cancer cell viability

RET, BRAF and other protein kinases have been identified as major molecular players in thyroid cancer. To identify novel kinases required for the viability of thyroid carcinoma cells, we performed a RNA interference screening in the RET/PTC1(CCDC6-RET)-positive papillary thyroid cancer cell line TPC1 using a library of synthetic small interfering RNAs (siRNAs) targeting the human kinome and related proteins. We identified 14 hits whose silencing was able to significantly reduce the viability and the proliferation of TPC1 cells; most of them were active also in BRAF-mutant BCPAP (papillary thyroid cancer) and 8505C (anaplastic thyroid cancer) and in RAS-mutant CAL62 (anaplastic thyroid cancer) cells. These included members of EPH receptor tyrosine kinase family as well as SRC and MAPK (mitogen activated protein kinases) families. Importantly, silencing of the identified hits did not affect significantly the viability of Nthy-ori 3-1 (hereafter referred to as NTHY) cells derived from normal thyroid tissue, suggesting cancer cell specificity. The identified proteins are worth exploring as potential novel druggable thyroid cancer targets.

Suppressive action of miRNAs to ARP2/3 complex reduces cell migration and proliferation via RAC isoforms in Hirschsprung disease

Hirschsprung disease (HSCR) is a congenital disorder caused by the defective function of the embryonic enteric neural crest. The impaired migration of embryonic enteric neural crest plays an important role in the pathogenesis of this disease. Recent studies showed that the ARP2/3 complex and RAC isoforms had effects on actin cytoskeleton remodelling, which contributes to migration. Moreover, some regulatory relationships were identified between ARP2/3 complex and RAC isoforms. Although microRNAs (miRNAs) have been known to modulate target gene expression on the post-transcriptional level, little is known about the regulation among miRNAs, ARP2/3 complex and RAC isoforms. Here, we report that down-regulation of ARP2 and ARP3, two main subunits of ARP2/3 complex, suppressed migration and proliferation in 293T and SH-SY5Y cell lines via the inhibition of RAC1 and RAC2. Meanwhile, as the target genes, ARP2 and ARP3 are reduced by increased miR-24-1* and let-7a*, respectively, in 70 HSCR samples as compared with 74 normal controls. Co-immunoprecipitation showed that aberrant reduction in ARP2 and ARP3 could weaken the function of ARP2/3 complex. Our study demonstrates that the miR-24-1*/let-7a*-ARP2/3 complex-RAC isoforms pathway may represent a novel pathogenic mechanism for HSCR.

Altered expression of mir-222 and mir-25 influences diverse gene expression changes in transformed normal and anaplastic thyroid cells, and impacts on MEK and TRAIL protein expression

Thyroid cancer is the most common endocrine malignancy and accounts for the majority of endocrine cancer-related deaths each year. Our group and others have previously demonstrated dysfunctional microRNA (miRNA or miR) expression in the context of thyroid cancer. The objective of the present study was to investigate the impact of synthetic manipulation of expression of miR-25 and miR-222 in benign and malignant thyroid cells. miR-25 and miR-222 expression was upregulated in 8505C (an anaplastic thyroid cell line) and Nthy-ori (a SV40-immortalised thyroid cell line) cells, respectively. A transcriptomics-based approach was utilised to identify targets of the two miRNAs and real-time PCR and western blotting were used to validate a subset of the targets. Almost 100 mRNAs of diverse functions were found to be either directly or indirectly targeted by both miR-222 and miR-25 [fold change ≥2, false discovery rate (FDR) ≤0.05]. Gene ontology analysis showed the miR-25 gene target list to be significantly enriched for genes involved in cell adhesion. Fluidigm real-time PCR technologies were used to validate the downregulation of 23 and 22 genes in response to miR-25 and miR-222 overexpression, respectively. The reduction of the expression of two miR-25 protein targets, TNF-related apoptosis-inducing ligand (TRAIL) and mitogen-activated protein kinase kinase 4 (MEK4), was also validated. Manipulating the expression of both miR-222 and miR-25 influenced diverse gene expression changes in thyroid cells. Increased expression of miR-25 reduced MEK4 and TRAIL protein expression, and cell adhesion and apoptosis are important aspects of miR-25 functioning in thyroid cells.

A 6-Base Pair in Frame Germline Deletion in Exon 7 Of RET Leads to Increased RET Phosphorylation, ERK Activation, and MEN2A

CONTEXT

Multiple endocrine neoplasia type 2 (MEN2) is usually caused by missense mutations in the proto-oncogene, RET.

OBJECTIVE

This study aimed to determine the mutation underlying MEN2A in a female patient diagnosed with bilateral pheochromocytoma at age 31 years and with medullary thyroid carcinoma (MTC) 6 years later.

METHODS

Leukocyte DNA was used for exome and Sanger sequencing. Wild-type (WT) RET and mutants were expressed in HEK293 cells. Activation of MAPK/ERK and PI3K/AKT was analyzed by Western blotting and luciferase assay. The effect of RET mutants on cell proliferation was tested in a colony forming assay.

RESULTS

Exome sequencing revealed a 6-nucleotide/2-amino acid in-frame deletion in exon 7 of RET (c.1512_1517delGGAGGG, p.505_506del). In vitro expression showed that phosphorylation of the crucial tyrosine 905 was much stronger in the p.505_506del RET mutant compared with WT RET, indicating ligand-independent autophosphorylation. Furthermore, the p.505_506del RET mutant induced a strong activation of the MAPK/ERK pathway and the PI3K/AKT pathway. Consequently, the p.505_506del RET mutant cells increased HEK293 colony formation 4-fold compared with WT RET.

CONCLUSION

The finding of bilateral pheochromocytoma and MTC in our patient was highly suspicious of a RET mutation. Exome sequencing revealed a 6-base-pair deletion in exon 7 of RET, an exon not yet associated with MEN2. Increased ligand-independent phosphorylation of the p.505_506del RET mutant, increased activation of downstream pathways, and stimulation of cell proliferation demonstrated the pathogenic nature of the mutation. We therefore recommend screening the whole sequence of RET in MTC and pheochromocytoma patients with red flags for a genetic cause.

Inhibition of motor neuron death in vitro and in vivo by a p75 neurotrophin receptor intracellular domain fragment

The p75 neurotrophin receptor (p75(NTR); also known as NGFR) can mediate neuronal apoptosis in disease or following trauma, and facilitate survival through interactions with Trk receptors. Here we tested the ability of a p75(NTR)-derived trophic cell-permeable peptide, c29, to inhibit p75(NTR)-mediated motor neuron death. Acute c29 application to axotomized motor neuron axons decreased cell death, and systemic c29 treatment of SOD1(G93A) mice, a common model of amyotrophic lateral sclerosis, resulted in increased spinal motor neuron survival mid-disease as well as delayed disease onset. Coincident with this, c29 treatment of these mice reduced the production of p75(NTR) cleavage products. Although c29 treatment inhibited mature- and pro-nerve-growth-factor-induced death of cultured motor neurons, and these ligands induced the cleavage of p75(NTR) in motor-neuron-like NSC-34 cells, there was no direct effect of c29 on p75(NTR) cleavage. Rather, c29 promoted motor neuron survival in vitro by enhancing the activation of TrkB-dependent signaling pathways, provided that low levels of brain-derived neurotrophic factor (BDNF) were present, an effect that was replicated in vivo in SOD1(G93A) mice. We conclude that the c29 peptide facilitates BDNF-dependent survival of motor neurons in vitro and in vivo.

Glial cell line-derived neurotrophic factor induces cell proliferation in the mouse urogenital sinus

Glial cell line-derived neurotrophic factor (GDNF) is a TGFβ family member, and GDNF signals through a glycosyl-phosphatidylinositol-linked cell surface receptor (GFRα1) and RET receptor tyrosine kinase. GDNF signaling plays crucial roles in urogenital processes, ranging from cell fate decisions in germline progenitors to ureteric bud outgrowth and renal branching morphogenesis. Gene ablation studies in mice have revealed essential roles for GDNF signaling in urogenital development, although its role in prostate development is unclear. We investigated the functional role of GDNF signaling in the urogenital sinus (UGS) and the developing prostate of mice. GDNF, GFRα1, and RET show time-specific and cell-specific expression during prostate development in vivo. In the UGS, GDNF and GFRα1 are expressed in the urethral mesenchyme (UrM) and epithelium (UrE), whereas RET is restricted to the UrM. In each lobe of the developing prostate, GDNF and GFRα1 expression declines in the epithelium and becomes restricted to the stroma. Using a well-established organ culture system, we determined that exogenous GDNF increases proliferation of UrM and UrE cells, altering UGS morphology. With regard to mechanism, GDNF signaling in the UrM increased RET expression and phosphorylation of ERK1/2. Furthermore, inhibition of RET kinase activity or ERK kinases suppressed GDNF-induced proliferation of UrM cells but not UrE cells. We therefore propose that GDNF signaling in the UGS increases proliferation of UrM and UrE cells by different mechanisms, which are distinguished by the role of RET receptor tyrosine kinase and ERK kinase signaling, thus implicating GDNF signaling in prostate development and growth.

RET fusion genes are associated with chronic myelomonocytic leukemia and enhance monocytic differentiation

Myeloproliferative neoplasms are frequently associated with aberrant constitutive tyrosine kinase (TK) activity resulting from chimaeric fusion genes or point mutations such as BCR-ABL1 or JAK2 V617F. We report here the cloning and functional characterization of two novel fusion genes BCR-RET and FGFR1OP-RET in chronic myelomonocytic leukemia (CMML) cases generated by two balanced translocations t(10;22)(q11;q11) and t(6;10)(q27;q11), respectively. The two RET fusion genes leading to the aberrant activation of RET, are able to transform hematopoietic cells and skew the hematopoietic differentiation program towards the monocytic/macrophage lineage. The RET fusion genes seem to constitutively mimic the same signaling pathway as RAS mutations frequently involved in CMML. One patient was treated with Sorafenib, a specific inhibitor of the RET TK function, and demonstrated cytological and clinical remissions.

Low dose irradiation of thyroid cells reveals a unique transcriptomic and epigenetic signature in RET/PTC-positive cells

The high doses of radiation received in the wake of the Chernobyl incident and the atomic bombing of Hiroshima and Nagasaki have been linked to the increased appearance of thyroid cancer in the children living in the vicinity of the site. However, the data gathered on the effect of low doses of radiation on the thyroid remain limited. We have examined the genome wide transcriptional response of a culture of TPC-1 human cell line of papillary thyroid carcinoma origin with a RET/PTC1 translocation to various doses (0.0625, 0.5, and 4Gy) of X-rays and compared it to response of thyroids with a RET/PTC3 translocation and against wild-type mouse thyroids irradiated with the same doses using Affymetrix microarrays. We have found considerable overlap at a high dose of 4Gy in both RET/PTC-positive systems but no common genes at 62.5mGy. In addition, the response of RET/PTC-positive system at all doses was distinct from the response of wild-type thyroids with both systems signaling down different pathways. Analysis of the response of microRNAs in TPC-1 cells revealed a radiation-responsive signature of microRNAs in addition to dose-responsive microRNAs. Our results point to the fact that a low dose of X-rays seems to have a significant proliferative effect on normal thyroids. This observation should be studied further as opposed to its effect on RET/PTC-positive thyroids which was subtle, anti-proliferative and system-dependent.

How to Treat a Signal? Current Basis for RET-Genotype-Oriented Choice of Kinase Inhibitors for the Treatment of Medullary Thyroid Cancer

The significance of RET in thyroid cancer comes from solid evidence that, when inherited, an RET activating mutation primes C-cells to transform into medullary carcinomas. Moreover, environmental exposure to radiation also induces rearranged transforming RET “isoforms” that are found in papillary thyroid cancer. The RET gene codes for a tyrosine kinase receptor that targets a diverse set of intracellular signaling pathways. The nature of RET point mutations predicts differences in the mechanisms by which the receptor becomes activated and correlates with different forms of clinical presentation, age of onset, and biological aggressiveness. A number of RET-targeting Tyrosine Kinase Inhibitors (TKIs) are currently undergoing clinical trials to evaluate their effectiveness in the treatment of thyroid cancer, and it is conceivable that the RET genotype may also influence response to these compounds. The question that now emerges is whether, in the future, the rational for treatment of refractory thyroid cancer will be based on the management of an abnormal RET signal. In this paper we address the RET-targeting TKIs and review studies about the signaling properties of distinct RET mutants as a means to predict response and design combinatorial therapies for the soon to be available TKIs.

The role of galectin-1 in trophoblast differentiation and signal transduction

Galectins are proteins with the ability to bind β-galactosides through a conserved carbohydrate recognition domain. Galectin-1 exerts its biological effects by binding glycan ligands on proteins involved in cell adhesion and growth regulation. Galectin-1 inhibits trophoblast cell proliferation and induces syncytium formation. Its down-regulation in the syncytiotrophoblast has been associated with early pregnancy loss. In the choriocarcinoma-derived BeWo cells the galectin-1 induced growth inhibition is apoptosis-independent, but rather appears to be mediated by binding to cell surface receptors, such as the receptor tyrosine kinases REarranged during Transfection (RET) and Janus Kinase (JAK) 2 as well as vascular endothelial growth factor receptor 3. On the syncytiotrophoblast and extravillous trophoblast galectin-1 binds the Thomsen-Friedenreich disaccharide on mucin-1. The cell differentiation processes induced by binding to these receptors ultimately lead to the inhibition of proliferation and syncytium formation.

Ret-dependent and Ret-independent mechanisms of Gfl-induced sensitization

BACKGROUND

The GDNF family ligands (GFLs) are regulators of neurogenic inflammation and pain. We have previously shown that GFLs increase the release of the sensory neuron neuropeptide, calcitonin gene-related peptide (CGRP) from isolated mouse DRG.

RESULTS

Inhibitors of the mitogen-activated protein kinase (MAPK) pathway abolished the enhancement of CGRP release by GDNF. Neurturin-induced enhancement in the stimulated release of CGRP, used as an indication of sensory neuronal sensitization, was abolished by inhibition of the phosphatidylinositol-3 kinase (PI-3K) pathway. Reduction in Ret expression abolished the GDNF-induced sensitization, but did not fully inhibit the increase in stimulus-evoked release of CGRP caused by neurturin or artemin, indicating the presence of Ret-independent GFL-induced signaling in sensory neurons. Integrin β-1 and NCAM are involved in a component of Ret-independent GFL signaling in sensory neurons.

CONCLUSIONS

These data demonstrate the distinct and variable Ret-dependent and Ret-independent signaling mechanisms by which GFLs induce sensitization of sensory neurons. Additionally, there is a clear disconnect between intracellular signaling pathway activation and changes in sensory neuronal function.

L-cysteine as a regulator for arsenic-mediated cancer-promoting and anti-cancer effects

Previous studies have shown that activities of tyrosine kinases and secretion of the active form of matrix metalloproteinase-2 (MMP-2) are correlated with promotion of tumor growth, while apoptotic cell death in cancer cells is correlated with anti-cancer effects. Although arsenic has been reported to have both cancer-promoting and anti-cancer effects, the mechanisms of the arsenic-mediated bidirectional effects remain unknown. We examined the effects of arsenic on both proto-oncogene c-RET-transfected NIH3T3 cells with benign characters and oncogenic RET-MEN2A-transfected NIH3T3 cells with malignant characters. Arsenic promoted not only c-RET tyrosine kinase activity but also genetically activated RET-MEN2A kinase activity with promotion of dimer formation of RET proteins. Arsenic also increased secretion of the active form of MMP-2 in both RET-MEN2A-transfectants and c-RET-transfectants. On the other hand, arsenic promoted poly-(ADP-ribose) polymerase (PARP) degradation and cell death in both malignant and non-malignant cells. Interestingly, l-cysteine inhibited the arsenic-mediated tumor-promoting effects (activation of kinases and MMP-2 secretion) but not arsenic-mediated anti-cancer effects (PARP degradation and cell death). Our results suggest redox-linked regulation of arsenic-mediated activities of kinases and MMP-2 secretion but not arsenic-mediated cell death. Our results also suggest that l-cysteine is an ideal supplement that inhibits arsenic-mediated tumor-promoting effects without affecting arsenic-mediated anti-cancer effects.

A redox-linked novel pathway for arsenic-mediated RET tyrosine kinase activation

We examined the biochemical effects of arsenic on the activities of RET proto-oncogene (c-RET protein tyrosine kinases) and RET oncogene (RET-MEN2A and RET-PTC1 protein tyrosine kinases) products. Arsenic activated c-RET kinase with promotion of disulfide bond-mediated dimerization of c-RET protein. Arsenic further activated RET-MEN2A kinase, which was already 3- to 10-fold augmented by genetic mutation compared with c-RET kinase activity, with promotion of disulfide bond-mediated dimerization of RET-MEN2A protein (superactivation). Arsenic also increased extracellular domain-deleted RET-PTC1 kinase activity with promotion of disulfide bond-mediated dimerization of RET-PTC1 protein. Arsenic increased RET-PTC1 kinase activity with cysteine 365 (C365) replaced by alanine with promotion of dimer formation but not with cysteine 376 (C376) replaced by alanine. Our results suggest that arsenic-mediated regulation of RET kinase activity is dependent on conformational change of RET protein through modulation of a special cysteine sited at the intracellular domain in RET protein (relevant cysteine of C376 in RET-PTC1 protein). Moreover, arsenic enhanced the activity of immunoprecipitated RET protein with increase in thiol-dependent dimer formation. As arsenic (14.2 microM) was detected in the cells cultured with arsenic (100 microM), direct association between arsenic and RET in the cells might modulate dimer formation. Thus, we demonstrated a novel redox-linked mechanism of activation of arsenic-mediated RET proto-oncogene and oncogene products.

Phosphorylation networks regulating JNK activity in diverse genetic backgrounds

Cellular signaling networks have evolved to enable swift and accurate responses, even in the face of genetic or environmental perturbation. Thus, genetic screens may not identify all the genes that regulate different biological processes. Moreover, although classical screening approaches have succeeded in providing parts lists of the essential components of signaling networks, they typically do not provide much insight into the hierarchical and functional relations that exist among these components. We describe a high-throughput screen in which we used RNA interference to systematically inhibit two genes simultaneously in 17,724 combinations to identify regulators of Drosophila JUN NH(2)-terminal kinase (JNK). Using both genetic and phosphoproteomics data, we then implemented an integrative network algorithm to construct a JNK phosphorylation network, which provides structural and mechanistic insights into the systems architecture of JNK signaling.

RET Gly691Ser mutation is associated with primary vesicoureteral reflux in the French-Canadian population from Quebec

Primary vesicoureteral reflux (pVUR) is a common, genetically heterogeneous congenital urinary tract abnormality in children. It causes urine to flow backward from the bladder to the ureter due to a developmental defect at the vesicoureteral junction, whose formation requires rearrangement during transformation (Ret)-mediated signaling pathways. To study the genetic causes of pVUR in Quebec patients, we used a sequencing-based candidate gene approach to screen the RET gene and found that 83 out of 118 pVUR patients are carriers of the rare A allele of single nucleotide polymorphism (SNP) rs1799939:G>A that results in a Gly691Ser mutation, a statistically significant increase in allelic frequency, that is absent at six flanking RET SNPs tested. Ser691 is a predicted phosphorylation site and our analysis of transfected cells showed that the Gly691Ser Ret mutant can efficiently interact and associate with a 75-80-kD tyrosine phosphorylated cellular protein, an event not seen with wild-type Ret. This interaction and/or the steric or electric hindrance created by phospho-Ser691 may interfere with the known regulatory functions of the normally phosphorylated phospho-Tyr687 and phospho-Ser696 on the cytoskeleton actin reorganization that are responsible for cell motility and morphology, which consequently may lead to the deficiency in ureteral development observed in pVUR. Our study demonstrates that the Ret Gly691Ser mutation is associated with pVUR and may be one of the genetic causes of this condition in the French-Canadian population in Quebec.

Nuclear factor-kappaB activation is associated with somatic and germ line RET mutations in medullary thyroid carcinoma

The nuclear factor-kappaB (NF-kappaB) family of transcription factors regulates a wide variety of cellular processes including cell growth, differentiation, and apoptosis. NF-kappaB has been shown to be activated through several signaling pathways that involve growth factor receptors. The aim of the study was to assess the immunohistochemical expression of members of the NF-kappaB family and the putative targets of NF-kappaB in a series of medullary thyroid carcinomas (MTCs), in correlation with RET mutational status. A tissue microarray was constructed from paraffin-embedded blocks of 48 MTCs (13 familial, 35 sporadic) previously evaluated for germ line and somatic RET mutations. Immunohistochemical evaluation included members of the NF-kappaB (p50, p65, p52, c-Rel, RelB) family, as well as putative targets of NF-kappaB such as Flip, Bcl-xL, and cyclin D1. Nuclear immunostaining for members of NF-kappaB was frequent in MTCs (p50, 19%; p65, 68%; p52, 86.6%; c-Rel, 75%; RelB, 36%). MTCs with germ line or somatic RET mutations (29 cases) showed NF-kappaB nuclear translocation (particularly of p65, P = .035) more frequently than MTCs without RET mutations (19 cases). Immunostaining for putative targets of NF-kappaB showed a significant statistical association between p65 and Bcl-xL (P = .024). In addition, Bcl-xL expression was statistically higher in the tumors with exon 16 RET mutation in comparison with those with exon 10 and 11 RET mutations or wild-type RET (P = .002). Moreover, the significance of RETsignaling in NF-kappaB activation was evaluated in the RET-mutated TT cell line. TT cells were infected with lentiviruses carrying short hairpin RNA to knock down RET expression, and NF-kappaB activity was assessed by luciferase reporter assays. Silencing of RET in the TT cell line produced a significant decrease in NF-kappaB activation and reduction in ERK1/2. The results suggest that the NF-kappaB is frequently activated in MTCs. The results also support the hypothesis that RET activation by somatic or germ line mutations may be responsible for NF-kappaB activation in MTCs.

RET signaling-induced SPHK1 gene expression plays a role in both GDNF-induced differentiation and MEN2-type oncogenesis

RET, the receptor of glial cell line-derived neurotrophic factor (GDNF) family ligands, is important for the development of kidney and peripheral neurons. GDNF promotes survival and differentiation of neurons. Mutation of RET leads to the constitutive signal activation causing papillary thyroid carcinoma and multiple endocrine neoplasia type 2 (MEN2). In this study, we report that GDNF/RET signaling up-regulates sphingosine kinase (SPHK) enzyme activity, SPHK1 protein and SPHK1 message in TGW human neuroblastoma cells. Silencing of SPHK1 using siRNA inhibited GDNF-induced neurite formation, GAP43 expression, and cell growth, suggesting the important role of SPHK1 in GDNF signal transduction. Furthermore, NIH3T3 cells transfected with MEN2A type mutated RET but not c-RET demonstrated the up-regulation of SPHK activity, SPHK1 protein and SPHK1 message compared with NIH3T3 cells. The cell growth and anchorage-independent colony formation of MEN2A-NIH3T3 was inhibited with siRNA of SPHK1, while no effect of scramble siRNA was observed. These results suggest the oncogenic role of SPHK1 in MEN2A type tumor. Promoter analysis showed that activator protein 2 and specificity protein 1 binding motif of the 5' promoter region of SPHK1 gene is important for its induction by GDNF. Furthermore, we demonstrated that ERK1/2 and PI3 kinase are involved in GDNF-induced SPHK1 transcription by using specific inhibitors.

Rapid down-regulation of Ret following exposure of dopaminergic neurons to neurotoxins

The survival and functional maintenance of vertebrate neurons depends on the availability of specific neurotrophic factors. We studied the influence of neurotrophic support on responses of dopaminergic neurons to 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, a neurotoxin known to damage the nigrostriatal dopaminergic pathway in humans and other mammals. Treatment of mice with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine caused decreases in levels of Ret, a tyrosine kinase receptor for glial cell line-derived neurotrophic factor (GDNF) in the striatum, under the condition in which tyrosine hydroxylase was moderately decreased and the GDNF family receptor alpha1, another receptor of GDNF that is the ligand-binding subunit, were unaffected. Down-regulation of Ret was also observed in dopamine-producing PC12 cells undergoing apoptosis induced by rotenone, another toxic substance for dopaminergic neurons, while other cellular components were not affected. Ret was also extremely vulnerable to other apoptotic inducing conditions. Taken together, these results indicate that Ret, an important signal molecule in dopaminergic neurons, may be down-regulated in the early stages of neuronal degeneration caused by various neurotoxic substances, and may lead to reduced neurotrophic influences.

Prion protein resides in membrane microclusters of the immunological synapse during lymphocyte activation

Expression of prion protein (PrP) has been reported for a variety of cell types including neuronal cells, haematopoietic stem cells, antigen-presenting cells, as well as lymphocytes. However, besides this widespread occurrence little is known about the physiological roles exhibited by this enigmatic protein. In this study, the contribution of PrP to the classical T-lymphocyte activation process was characterized by clustering the T-cell receptor component CD3epsilon as well as PrP with soluble and surface-immobilized antibodies, respectively. We present evidence that PrP is a component of signaling structures recently described as plasma membrane microclusters established during T-lymphocyte activation. The formation of immunological synapses, however, did not depend on the presence of PrP as proven by siRNA knockdown experiments, indicating very subtle physiological roles of PrP in vivo within the immune system.

Effects of pharmacological inhibition of small GTPases on axon extension and migration of enteric neural crest-derived cells

In the developing enteric nervous system, there is a close association between migrating neural crest-derived cells and the axons of early differentiating neurons. We used pharmacological inhibitors of small GTPases to determine if crest cell migration and axon growth could be uncoupled in cultured intact explants of embryonic mouse gut and slices of embryonic gut grown on collagen gels containing GDNF. Inhibition of the Rho effectors, ROCKI/II, or Rac/Cdc42 inhibited both cell migration and neurite growth in intact explants of embryonic gut. The effects of both ROCKI/II and Rac/Cdc42 inhibitors were more severe on cell migration and axon extension in gut explants from Ret(+/-) mice than in explants from wildtype mice, indicating that Rho GTPases probably act downstream of the receptor tyrosine kinase, Ret. Inhibition of ROCKI/II had different effects on migration and axon extension in gut slices grown on collagen gels containing GDNF from that seen in intact explants of gut. We conclude that ROCKI/II and Rac/Cdc42 are required for both neural crest-derived cell migration and axon growth in the developing gut.

The dependence receptor Ret induces apoptosis in somatotrophs through a Pit-1/p53 pathway, preventing tumor growth

Somatotrophs are the only pituitary cells that express Ret, GFRalpha1 and GDNF. This study investigated the effects of Ret in a somatotroph cell line, in primary pituitary cultures and in Ret KO mice. Ret regulates somatotroph numbers by inducing Pit-1 overexpression, leading to increased p53 expression and apoptosis, both of which can be prevented with Ret or Pit-1 siRNA. The Pit-1 overexpression is mediated by sustained activation of PKCdelta, JNK, c/EBPalpha and CREB induced by a complex of Ret, caspase 3 and PKCdelta. In the presence of GDNF, Akt is activated, and the Pit-1 overexpression and resulting apoptosis are blocked. The adenopituitary of Ret KO mice is larger than normal, showing Pit-1 and somatotroph hyperplasia. In normal animals, activation of the Ret/Pit-1/p53 pathway by retroviral introduction of Ret blocked tumor growth in vivo. Thus, somatotrophs have an intrinsic mechanism for controlling Pit-1/GH production through an apoptotic/survival pathway. Ret might be of value for treatment of pituitary adenomas.

Targeted mutation of serine 697 in the Ret tyrosine kinase causes migration defect of enteric neural crest cells

The RET receptor tyrosine kinase plays a critical role in the development of the enteric nervous system (ENS) and the kidney. Upon glial-cell-line-derived neurotrophic factor (GDNF) stimulation, RET can activate a variety of intracellular signals, including the Ras/mitogen-activated protein kinase, phosphatidylinositol 3-kinase (PI3K)/AKT, and RAC1/JUN NH(2)-terminal kinase (JNK) pathways. We recently demonstrated that the RAC1/JNK pathway is regulated by serine phosphorylation at the juxtamembrane region of RET in a cAMP-dependent manner. To determine the importance of cAMP-dependent modification of the RET signal in vivo, we generated mutant mice in which serine residue 697, a putative protein kinase A (PKA) phosphorylation site, was replaced with alanine (designated S697A mice). Homozygous S697A mutant mice lacked the ENS in the distal colon, resulting from a migration defect of enteric neural crest cells (ENCCs). In vitro organ culture showed an impaired chemoattractant response of the mutant ENCCs to GDNF. JNK activation by GDNF but not ERK, AKT and SRC activation was markedly reduced in neurons derived from the mutant mice. The JNK inhibitor SP600125 and the PKA inhibitor KT5720 suppressed migration of the ENCCs in cultured guts from wild-type mice to comparable degrees. Thus, these findings indicated that cAMP-dependent modification of RET function regulates the JNK signaling responsible for proper migration of the ENCCs in the developing gut.

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.

RET oncoproteins induce tyrosine phosphorylation changes of proteins involved in RNA metabolism

We report the identification of proteins induced in response to RET/PTC2, an oncogene implicated in thyroid cancers. Anti-phosphotyrosine antibody affinity resin was used to purify Tyr(P)-containing and interacting proteins from 293T and NIH3T3 cells which were transfected with kinase active or inactive RET/PTC and RETMEN2 oncogenes. Proteins were separated by one-dimensional SDS-PAGE, extracted by in-gel digestion, and identified by MALDI-TOF peptide mass fingerprinting. The expression and tyrosine phosphorylation of Sam68, a protein implicated in mRNA nucleocytoplasmic translocation and splicing, were further examined in RET-transfected cells and thyroid tumors. Of relevance, cells transfected with RETMEN2B examined for anti-phosphotyrosine bound proteins, showed other proteins implicated in splicing: DEAD-box p68 RNA helicase, SYNCRIP, and hnRNP K. Western blotting analysis suggested that these proteins are singularly tyrosine phosphorylated in RETMEN2B-transfected cells, and that they constitutively bind with Sam68. The study concludes that regulation of splicing factors is likely to be important in RET-mediated thyroid carcinogenesis.

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.

Activation of the Erk8 mitogen-activated protein (MAP) kinase by RET/PTC3, a constitutively active form of the RET proto-oncogene

Mitogen-activated protein (MAP) kinases have a central role in several biological functions, including cell adhesion and spreading, chemotaxis, cell cycle progression, differentiation, and apoptosis. Extracellular signal-regulated kinase 8 (Erk8) is a large MAP kinase whose activity is controlled by serum and the c-Src non-receptor tyrosine kinase. Here, we show that RET/PTC3, an activated form of the RET proto-oncogene, was able to activate Erk8, and we demonstrate that such MAP kinase participated in RET/PTC3-dependent stimulation of the c-jun promoter. By using RET/PTC3 molecules mutated in specific tyrosine autophosphorylation sites, we characterized Tyr(981), a known binding site for c-Src, as a major determinant of RET/PTC3-induced Erk8 activation, although, surprisingly, the underlying mechanism did not strictly depend on the activity of Src. In contrast, we present evidence that RET/PTC3 acts on Erk8 through Tyr(981)-mediated activation of c-Abl. Furthermore, we localized the region responsible for the modulation of Erk8 activity by the RET/PTC3 and Abl oncogenes in the Erk8 C-terminal domain. Altogether, these results support a role for Erk8 as a novel effector of RET/PTC3 and, therefore, RET biological functions.

Direct phosphorylation of proliferative and survival pathway proteins by RET

BACKGROUND

Gain-of-function mutations in the RET tyrosine kinase receptor cause the multiple endocrine neoplasia syndromes type 2a and 2b, and medullary thyroid cancer. We have previously shown that RET signals through focal adhesion kinase (FAK) in medullary thyroid cancer cells and that extracellular signal-regulated kinase (ERK) activity can be blocked by pp2, an inhibitor of both Src and RET. We hypothesized that RET could directly phosphorylate FAK and ERK.

METHODS

RET and ERK kinase activity were measured with the use of an in vitro kinase assay. The relative contribution of RET in phosphorylation of ERK was tested by treating cells with PD98059, an inhibitor of MEK, and the RET inhibitor PP2, then measuring ERK activity.

RESULTS

Immunoprecipitated, mutant RET from cells or the recombinant RET kinase domain was able to directly phosphorylate tyrosine residues on FAK. Specifically Y576/577, Y861, and Y925, but not the autophosphorylation site Y397 of FAK, were phosphorylated by RET. Similarly ERK 2 could be phosphorylated at Y187 (Y204 in ERK1). Inhibition of both MEK (upstream of ERK) and RET was more potent than inhibition of either alone in decreasing ERK activity. Furthermore, tyrosine residues in DOK1, the p85 subunit of phosphatidylinositol 3' kinase, JNK 1 and 2, P-38, and phospholipase-gamma were directly phosphorylated by RET.

CONCLUSIONS

RET directly phosphorylates tyrosine residues on FAK, ERK 1/2, DOK1, the p85 subunit of of phosphatidylinositol 3' kinase, JNK 1 and 2, P-38, and phospholipase-gamma. These data indicate a direct interaction between RET and a broad range of effector molecules that may contribute to tumor pathogenesis.

RET tyrosine kinase signaling in development and cancer

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

A Drosophila model of multiple endocrine neoplasia type 2

Dominant mutations in the Ret receptor tyrosine kinase lead to the familial cancer syndrome multiple endocrine neoplasia type 2 (MEN2). Mammalian tissue culture studies suggest that RetMEN2 mutations significantly alter Ret-signaling properties, but the precise mechanisms by which RetMEN2 promotes tumorigenesis remain poorly understood. To determine the signal transduction pathways required for RetMEN2 activity, we analyzed analogous mutations in the Drosophila Ret ortholog dRet. Overexpressed dRetMEN2 isoforms targeted to the developing retina led to aberrant cell proliferation, inappropriate cell fate specification, and excessive Ras pathway activation. Genetic analysis indicated that dRetMEN2 acts through the Ras-ERK, Src, and Jun kinase pathways. A genetic screen for mutations that dominantly suppress or enhance dRetMEN2 phenotypes identified new genes that are required for the phenotypic outcomes of dRetMEN2 activity. Finally, we identified human orthologs for many of these genes and examined their status in human tumors. Two of these loci showed loss of heterozygosity (LOH) within both sporadic and MEN2-associated pheochromocytomas, suggesting that they may contribute to Ret-dependent oncogenesis.

RET proto-oncogene: a review and update of genotype-phenotype correlations in hereditary medullary thyroid cancer and associated endocrine tumors

Hereditary medullary thyroid carcinoma (MTC) is caused by autosomal dominant gain-of-function mutations in the RET proto-oncogene. Associations between specific RET mutations (genotype) and the aggressiveness of MTC and presence or absence of other endocrine neoplasms (phenotype) are well documented. Mutations in six exons (10, 11, 13, 14, 15, and 16) located in either cysteine-rich or tyrosine kinase domains cause one of three distinctive clinical subtypes: familial MTC, multiple endocrine neoplasia (MEN) type 2A (including variants with Hirschsprung's disease and cutaneous lichen amyloidosis), and MEN 2B. Hallmarks of MEN 2A include MTC, pheochromocytoma, and hyperparathyroidism. MEN 2B is associated with an earlier onset of MTC and pheochromocytoma, the absence of hyperparathyroidism, and the presence of striking physical stigmata (e.g., coarse facies, ganglioneuromatosis, and marfanoid habitus). Familial MTC is not associated with other endocrine neoplasms; however, the accurate distinction between familial MTC and MEN 2A may be difficult in kindreds with small size, incomplete histories, or a predominance of young individuals who may not have yet fully manifested the syndrome. Genetic testing detects greater than 95% of mutation carriers and is considered the standard of care for all first-degree relatives of patients with newly diagnosed MTC. Recommendations on the timing of prophylactic thyroidectomy and the extent of surgery are based upon a model that utilizes genotype- phenotype correlations to stratify mutations into three risk levels.

Phase I Study of the Farnesyltransferase Inhibitor BMS-214662 Given Weekly in Patients with Solid Tumors

Purpose: A phase I trial of BMS-214662, a selective farnesyltransferase inhibitor with significant preclinical antitumor activity in which drug was given as a weekly 1-hour infusion for four of six weeks, was conducted to evaluate the tolerability, pharmacokinetics, and pharmacodynamic effect on farnesyltransferase activity in peripheral blood mononuclear cells.

Experimental Design: BMS-214662 was given to 27 patients with solid tumors at 10 escalating dose levels (28-220 mg/m2) allowing intrapatient dose escalation; pharmacokinetics and pharmacodynamics were done at the first seven dose levels.

Results: Grade 4 neutropenia (four patients) was the most common dose-limiting toxicity followed by aminotransferase elevation (grade 3 alanine aminotransferase and grade 4 aspartate aminotransferase) and grade 3 dehydration. Most frequent toxicities were neutropenia in 11 (14%), anemia in 15 (19%), fatigue in 9 (12%), and nausea and diarrhea in 6 (8%) of courses, respectively. One minor response lasting 18 weeks in a patient with non–small cell lung cancer, serum calcitonin level reduction accompanied by disease stabilization in two of four patients with medullary thyroid carcinoma, and stable disease in 16 of 25 evaluable patients was seen. No correlation was observed between dose and Cmax, total body clearance (mean, 26.15 ± 10.88 L per hour per m2), volume of distribution at steady state (mean, 39.51 ± 17.91 L/m2), or half-life (mean, 2.63 ± 1.81 hours); a moderate correlation existed between dose given and systemic drug exposure (AUC). Substantial inhibition of peripheral blood mononuclear cell farnesyltransferase activity but near complete recovery by 24 hours was seen.

Conclusion: BMS-214667 was well tolerated as a weekly 1-hour i.v. infusion for four of six weeks with evidence of pharmacodynamic effect. The study was terminated before maximum tolerated dose was reached. Alternative schedules of drug administration might result in improved pharmacodynamic profile.

Activation of c-Jun amino-terminal kinase by GDNF induces G2/M cell cycle delay linked with actin reorganization

It is well known that the cell cycle is controlled by several cyclin/cyclin-dependent kinase (Cdk) complexes whose expression and phosphorylation states vary with orderly periodicity. During the cell cycle, activity of the cyclin/Cdk complexes can be regulated directly or indirectly by a number of molecules, including protein kinases and phosphatases, p53, and Cdk inhibitors. Here, we show that the addition of glial cell line-derived neurotrophic factor (GDNF) induced G2/M cell cycle delay in human SK-N-MC neuroectodermal tumor cells that express RET tyrosine kinase, accompanying actin reorganization. Cell cycle delay at G2/M was characterized by accelerated and prolonged Cdc2 phosphorylation and stabilization of cyclin B1 and Wee1 kinase expression. Interestingly, we found that phosphorylation and/or expression of Cdc2, cyclinB1, and Wee1 was controlled by the Rac1/c-Jun NH2-terminal kinase (JNK) pathway. Immunohistochemical analysis suggested that the G2/M cell cycle delay may be necessary to prevent the mitotic progression of SK-N-MC cells with perturbed actin cytoskeletons.

RET-familial medullary thyroid carcinoma mutants Y791F and S891A activate a Src/JAK/STAT3 pathway, independent of glial cell line-derived neurotrophic factor

The RET proto-oncogene encodes a receptor tyrosine kinase whose dysfunction plays a crucial role in the development of several neural crest disorders. Distinct activating RET mutations cause multiple endocrine neoplasia type 2A (MEN2A), type 2B (MEN2B), and familial medullary thyroid carcinoma (FMTC). Despite clear correlations between the mutations found in these cancer syndromes and their phenotypes, the molecular mechanisms connecting the mutated receptor to the different disease phenotypes are far from completely understood. Luciferase reporter assays in combination with immunoprecipitations, and Western and immunohistochemistry analyses were done in order to characterize the signaling properties of two FMTC-associated RET mutations, Y791F and S891A, respectively, both affecting the tyrosine kinase domain of the receptor. We show that these RET-FMTC mutants are monomeric receptors which are autophosphorylated and activated independently of glial cell line-derived neurotrophic factor. Moreover, we show that the dysfunctional signaling properties of these mutants, when compared with wild-type RET, involve constitutive activation of signal transducers and activators of transcription 3 (STAT3). Furthermore, we show that STAT3 activation is mediated by a signaling pathway involving Src, JAK1, and JAK2, differing from STAT3 activation promoted by RET(C634R) which was previously found to be independent of Src and JAKs. Three-dimensional modeling of the RET catalytic domain suggested that the structural changes promoted by the respective amino acids substitutions lead to a more accessible substrate and ATP-binding monomeric conformation. Finally, immunohistochemical analysis of FMTC tumor samples support the in vitro data, because nuclear localized, Y705-phosphorylated STAT3, as well as a high degree of RET expression at the plasma membrane was observed.

The RET proto-oncogene: a potential target for molecular cancer therapy

The inhibition of activated receptor tyrosine kinases has defined a new era of selective cancer therapy. The value of these approaches has been demonstrated for a growing number of tyrosine kinases. Gain-of-function alterations within the RET proto-oncogene are responsible for the development of medullary, as well as papillary, thyroid carcinoma and make it a candidate for the design of targeted therapies. Recently, various strategies have been used to block the activity of RET in pre-clinical models, providing evidence that RET is a potential target for a selective cancer-therapy approach, especially when considering that the inhibition of RET activity is sufficient to revert neoplastic characteristics. Although the ideal clinically useful therapeutic option has yet to be developed, successes with other selective tyrosine kinase inhibitors encourages further effort.

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.

Interleukin 24 is induced by the RET/PTC3 oncoprotein and is an autocrine growth factor for epithelial cells

Thyroid cancers, like hematological malignancies, are commonly associated with chromosomal translocations leading to the formation of fusion proteins. Through altered signaling by fusion proteins, cell death and survival pathways are disrupted and the physiological balance of cell-cell communication may be lost. A consequence of this disruption is the release of factors by stressed cells that alert the host. One type of host response is leukocytic infiltration that may develop into chronic inflammation or autoimmune disease. Although inflammation can be associated with neoplastic tissue, the mechanism driving this process is largely unknown. Therefore, to address the mechanism of cancer inflammation we investigated the effects of an oncogene in a murine model system. A comprehensive genetic analysis revealed several soluble factors that were induced by RET/papillary thyroid carcinoma (PTC)3 gene expression including several proinflammatory cytokines, chemokines and immunologically relevant costimulatory molecules. Following a large genetic screen using RP3-expressing thyroid cells, we identified a highly abundant transcript and later identified it as interleukin 24 (Il24), a cytokine with diverse tumor suppressor and inflammatory activities. We show that RET/PTC3 induces Il24 expression in rat thyrocytes and that this expression is dependent on the signaling properties of its tyrosine kinase. Likewise, RET/PTC3 induces large amounts of Il24 following expression in murine thyrocytes, but its expression is dramatically reduced in poorly differentiated carcinomas, a finding that parallels the loss of RET/PTC3 expression. Consistent with its behavior as a tumor suppressor, the loss of Il24 coincided with the loss of RET/PTC3 in poorly differentiated mouse tumors. A functional role of Il24 in the autocrine growth/survival of RET/PTC3-expressing thyroid cells was identified helping to support its role in cellular transformation. These data suggest that the induction of Il24 by oncogenes may support tumor growth at the early stages of cancer.

The RET Receptor Is Linked to Stress Response Pathways

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

Mitogenic effect of glial cell line-derived neurotrophic factor is dependent on the activation of p70S6 kinase, but independent of the activation of ERK and up-regulation of Ret in SH-SY5Y cells

Glial cell line-derived neurotrophic factor (GDNF) activates c-Ret tyrosine kinase and several downstream intracellular pathways; the biological effects caused by the activation of each of these pathways, however, remain to be elucidated. Here we report the ability of GDNF to induce proliferation, rather than differentiation, of neuroblastoma cells (SH-SY5Y) by targeting the signaling pathway responsible for mediating this proliferative effect. GDNF induces the phosphorylation of Akt and p70S6 kinase (p70S6K) in SH-SY5Y cells in which Ret protein expression is relatively low. Interestingly, treating SH-SY5Y cells with retinoic acid greatly increases Ret protein levels and GDNF-induced Ret tyrosine phosphorylation, but does not affect the mitogenic action of GDNF and the activation of the Akt/p70S6K pathway. In contrast, the activation of the ERK pathway and the resulting induction of immediate-early genes parallel the increases in Ret protein levels. Rapamycin, a specific inhibitor of p70S6K activation by the mammalian target of rapamycin, completely prevents GDNF-induced proliferation and activation of p70S6K. These results suggest that GDNF promotes cell proliferation via the activation of p70S6K, independent of the ERK signaling pathway, and that GDNF activates the Akt/p70S6K pathway more efficiently than the ERK pathway in the cells in which Ret expression is low.

Tyrosine kinase oncoprotein, RET/PTC3, induces the secretion of myeloid growth and chemotactic factors

Differentiated thyroid carcinomas are the most frequent endocrine neoplasms, but account for few cancer-related deaths. Although the indolent growth of these cancers correlates well with longevity, the biological basis for this good prognosis is not known. In contrast, two of the most frequent autoimmune diseases involve the thyroid suggesting a high propensity for this organ to invoke destructive immunity. Unfortunately, the mechanism linking malignancy and autoimmunity is not clear, although the expression of the oncogenic fusion protein RET/PTC3 (RP3) in both of these disorders may provide a clue. Interestingly, the signaling caused by activated RET kinase involves overlapping pathways and some common to the inflammatory response. Accordingly, we analyzed the function of RP3 and a mutant RP3 molecule to induce proinflammatory pathways in thyroid epithelial cells. Indeed, we find that RP3 alone causes increases in nuclear NF-kappaB activity and secretion of MCP-1 and GM-CSF. Finally, transfer of RP3-expressing thyrocytes into mice in vivo attracted dense macrophage infiltrates, which lead to rapid thyroid cell death. Further, cytokine synthesis and inflammation was largely abrogated by mutation of RP3 Tyr588; an important protein-binding site for downstream signaling. Together, these studies implicate oncogene-induced cytokine-signaling pathways in a new mechanism linking inflammation with cancer.

Antitumor capacity of a dominant-negative RET proto-oncogene mutant in a medullary thyroid carcinoma model

Gain-of-function mutations in the RET proto-oncogene resulting in a constitutively active receptor tyrosine kinase have been identified as responsible for three subtypes of multiple endocrine neoplasia type 2 (MEN-2) and the development of sporadic medullary and papillary thyroid carcinoma. An important strategy in cancer gene therapy is the inhibition of oncogenic signal transduction by interfering with the molecular mechanisms of activation. In the present study, we tested the therapeutic capacity of an adenovirus expressing a dominant-negative (dn) RET mutant, RET(51).flag, under the control of a synthetic C cell-selective calcitonin promoter (TSE2.CP1) against human medullary thyroid cancer (MTC). Infection of human MTC-derived TT cells with Ad-TSE2.CP1-dn-RET(51).flag resulted in the accumulation of immature RET protein in the endoplasmic reticulum and a strong reduction of oncogenic RET receptor on the cell surface, indicating that RET(51).flag exhibits a dominant-negative effect over endogenous oncogenic protein. Analysis of potential downstream mechanisms associated with the inhibition of oncogenic RET signaling by overexpression of mutant RET(51).flag revealed a significant loss of cell viability in TT cells due to the induction of apoptosis. Finally, we examined the antitumor activity of the dominant-negative RET approach in vivo. Inoculation of Ad-TSE2.CP1- dn-RET(51).flag-expressing MTC cells into nude mice led to complete suppression of tumor growth. Moreover, a single intratumoral injection of Ad-TSE2.CP1-dn-RET(51).flag into established thyroid tumors resulted in prolonged survival of treated mice compared with the controls. Our data suggest that adenoviral delivery of dn-RET(51).flag may be a reliable strategy of effective molecular intervention for RET oncogene-related MTC.

Neurotrophic factors and their receptors in axonal regeneration and functional recovery after peripheral nerve injury

Over a half a century of research has confirmed that neurotrophic factors promote the survival and process outgrowth of isolated neurons in vitro. The mechanisms by which neurotrophic factors mediate these survival-promoting effects have also been well characterized. In vivo, peripheral neurons are critically dependent on limited amounts of neurotrophic factors during development. After peripheral nerve injury, the adult mammalian peripheral nervous system responds by making neurotrophic factors once again available, either by autocrine or paracrine sources. Three families of neurotrophic factors were compared, the neurotrophins, the GDNF family of neurotrophic factors, and the neuropoetic cytokines. Following a general overview of the mechanisms by which these neurotrophic factors mediate their effects, we reviewed the temporal pattern of expression of the neurotrophic factors and their receptors by axotomized motoneurons as well as in the distal nerve stump after peripheral nerve injury. We discussed recent experiments from our lab and others which have examined the role of neurotrophic factors in peripheral nerve injury. Although our understanding of the mechanisms by which neurotrophic factors mediate their effects in vivo are poorly understood, evidence is beginning to emerge that similar phenomena observed in vitro also apply to nerve regeneration in vivo.