Differential effects of leukocyte common antigen-related protein on biochemical and biological activities of RET-MEN2A and RET-MEN2B mutant proteins

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

Neuroendocrine-derived peptides promote prostate cancer cell survival through activation of IGF-1R signaling

BACKGROUND

Neuroendocrine (NE) cells promote the progression of prostate cancer to a castration-resistant state through the production of paracrine growth factors. We have demonstrated this principle using in vitro and in vivo proliferative endpoints; however, the contributions of NE-derived pro-survival factors and anti-apoptosis to this phenomenon have not been thoroughly investigated.

METHODS

Here, we utilized conditioned-medium (CM) from LNCaP cells, engineered to undergo NE differentiation, and examined its effects on PC3 and LNCaP cell survival.

RESULTS

Statistically significant changes in clonogenic survival, Annexin V staining, PARP cleavage and trypan blue positivity of approximately twofold were observed in the presence of NE-derived CM relative to control-CM for both LNCaP and PC3 cells. These changes were partially abrogated by antagonists of the neuropeptides neurotensin, bombesin, and PTHrP. Selective inhibitors of IGF-1R, EGFR or Src caused significant and nearly complete blockade of prostate cancer cell survival due to NE secretions. Similar increases in cell survival were observed for LNCaP or PC3 cells treated with NE-derived medium in the presence of docetaxel. Increased phosphorylation of IGF-1R, following treatment with NE-derived medium, was accompanied by decreased protein tyrosine phosphatase, receptor type F (PTPRF) mRNA, and protein levels. Overexpression of PTPRF decreased cell survival, the amplitude and duration of IGF-1R phosphorylation, and enhanced PARP cleavage in the presence of NE-derived medium.

CONCLUSIONS

These data support the hypothesis that NE-derived factors act upon prostate cancer cells to stimulate pro-survival signaling and describe a novel mechanism of cross-talk between NE-derived factors and IGF-1R, mediated in part by PTPRF.

Receptor type protein tyrosine phosphatases (RPTPs) - roles in signal transduction and human disease

Protein tyrosine phosphorylation is a fundamental regulatory mechanism controlling cell proliferation, differentiation, communication, and adhesion. Disruption of this key regulatory mechanism contributes to a variety of human diseases including cancer, diabetes, and auto-immune diseases. Net protein tyrosine phosphorylation is determined by the dynamic balance of the activity of protein tyrosine kinases (PTKs) and protein tyrosine phosphatases (PTPs). Mammals express many distinct PTKs and PTPs. Both of these families can be sub-divided into non-receptor and receptor subtypes. Receptor protein tyrosine kinases (RPTKs) comprise a large family of cell surface proteins that initiate intracellular tyrosine phosphorylation-dependent signal transduction in response to binding of extracellular ligands, such as growth factors and cytokines. Receptor-type protein tyrosine phosphatases (RPTPs) are enzymatic and functional counterparts of RPTKs. RPTPs are a family of integral cell surface proteins that possess intracellular PTP activity, and extracellular domains that have sequence homology to cell adhesion molecules. In comparison to extensively studied RPTKs, much less is known about RPTPs, especially regarding their substrate specificities, regulatory mechanisms, biological functions, and their roles in human diseases. Based on the structure of their extracellular domains, the RPTP family can be grouped into eight sub-families. This article will review one representative member from each RPTP sub-family.

Maturation of ureter-bladder connection in mice is controlled by LAR family receptor protein tyrosine phosphatases

Congenital anomalies affecting the ureter-bladder junction are frequent in newborns and are often associated with other developmental defects. However, the molecular and morphological processes underlying these malformations are still poorly defined. In this study, we identified the leukocyte antigen-related (LAR) family protein tyrosine phosphatase, receptor type, S and F (Ptprs and Ptprf [also known as Lar], respectively), as crucially important for distal ureter maturation and craniofacial morphogenesis in the mouse. Embryos lacking both Ptprs and Ptprf displayed severe urogenital malformations, characterized by hydroureter and ureterocele, and craniofacial defects such as cleft palate, micrognathia, and exencephaly. The detailed analysis of distal ureter maturation, the process by which the ureter is displaced toward its final position in the bladder wall, leads us to propose a revised model of ureter maturation in normal embryos. This process was deficient in embryos lacking Ptprs and Ptprf as a result of a marked reduction in intrinsic programmed cell death, thereby causing urogenital system malformations. In cell culture, Ptprs bound and negatively regulated the phosphorylation and signaling of the Ret receptor tyrosine kinase, whereas Ptprs-induced apoptosis was inhibited by Ret expression. Together, these results suggest that ureter positioning is controlled by the opposing actions of Ret and LAR family phosphatases regulating apoptosis-mediated tissue morphogenesis.

Lrig1 is an endogenous inhibitor of Ret receptor tyrosine kinase activation, downstream signaling, and biological responses to GDNF

Glial cell line-derived neurotrophic factor (GDNF)/Ret signaling has potent trophic effects on ventral midbrain dopaminergic, motor, sensory, and sympathetic neurons. The molecular mechanisms that restrict Ret receptor tyrosine kinase activation are not well understood. Here, we show that Lrig1, a transmembrane protein containing leucine-rich repeats and Ig-like domains in its extracellular region, acts in a negative feedback loop to regulate the activity of Ret receptor tyrosine kinase. In particular, we demonstrate that Lrig1 is capable of physically interacting with Ret and that Lrig1/Ret association inhibits GDNF binding, recruitment of Ret to lipid rafts, receptor autophosphorylation, and mitogen-activated protein kinase (MAPK) activation in response to GDNF. In neuronal cells, Lrig1 overexpression also inhibits GDNF/Ret-induced neurite outgrowth in a cell-autonomous manner. Downregulation of Lrig1 using small interference RNA knock-down experiments potentiates both neuronal differentiation and MAPK activation in response to GDNF. Together, these results provide an insight into Lrig1 function and establish a new physiological mechanism to restrict signaling and biological responses induced by GDNF and Ret in neuronal cells.

Technology insight: gene therapy and its potential role in the treatment of medullary thyroid carcinoma

Metastatic medullary thyroid cancer (MTC) responds poorly to conventional treatments with chemotherapy and radiotherapy. Gene therapy--the transfer of genetic material for therapeutic purposes--might have therapeutic potential for patients with progressive metastatic MTC that is incurable by conventional treatments. To date, a number of gene-therapy strategies have been explored, primarily those that use replication-deficient adenovirus vectors to transfer therapeutic genes to tumor cells. Tissue-specific expression of the promoter for calcitonin and calcitonin-related polypeptide alpha has allowed therapeutic genes to be specifically expressed in calcitonin-secreting cells and in the MTC tumors derived from them; such tissue-specific expression contributes to improved safety of gene therapies and has the potential to increase their therapeutic index. In addition, the identification of an MTC-specific peptide ligand raises the possibility of developing an MTC-selective vector. In this article, we have described the exciting area of gene therapy in the management of MTC with a focus on preclinical in vitro and in vivo MTC models.

Medullary thyroid cancer: therapeutic targets and molecular markers

PURPOSE OF REVIEW

The present review will provide an update of important studies in medullary thyroid cancer (MTC) with an emphasis on targeted preclinical and translational research studies published over the past 2 years.

RECENT FINDINGS

Recent advances in the biology of MTC, particularly in RET proto-oncogene signaling, are now being translated into promising new therapies and biomarkers. Multifunction tyrosine kinase inhibitors that target RET, plus vascular endothelial growth factor receptors and additional kinases, are now being evaluated in Phase II clinical trials in MTC. Important unanswered questions include the optimal means for selecting high-risk patients, appropriate biomarkers for monitoring kinase inhibitor trials, and trial endpoints. Similar to ABL, epidermal growth factor receptors and other kinases, individual mutant RET forms have differential sensitivity to different inhibitors. In addition to RET, an old marker, calcitonin, has assumed increasing importance, but may not adequately reflect changes in tumor burden in RET inhibitor trials. A number of new therapeutic strategies are being developed that could be appropriate for the approximately 50% of patients who lack RET mutations in their tumors.

SUMMARY

Progress is being made toward effective targeted MTC therapy. Patients with advanced, progressive MTC should be considered for enrollment in clinical trials.

Neurotrophic factor receptor RET: structure, cell biology, and inherited diseases

RET (REarranged during Transfection) is a transmembrane receptor tyrosine kinase that is activated by a complex consisting of a soluble glial cell line-derived neurotrophic factor (GDNF) family ligand (GFL) and a glycosyl phosphatidylinositol-anchored co-receptor, GDNF family receptors alpha (GFRalpha). RET signalling is crucial for the development of the enteric nervous system. RET also regulates the development of sympathetic, parasympathetic, motor, and sensory neurons, and is necessary for the postnatal maintenance of dopaminergic neurons. The effect of GFLs on sensory, motor, and dopaminergic neurons has raised clinical interest towards these ligands. Outside the nervous system, RET is crucial for development of the kidney and plays a key role in spermatogenesis. Inactivating mutations in RET cause the Hirschsprung's disease characterized by megacolon aganglionosis. In contrast, activating mutations give rise to different types of cancer, multiple endocrine neoplasia type 2A and type 2B, familial medullary thyroid carcinoma, and papillary thyroid carcinoma. The multiple disease phenotypes correlate with differences in the molecular and cell biological functions of different oncogenic RET proteins. In this review we summarize how the different domains of the RET protein contribute to its normal function and how mutations in these domains affect the function of the receptor.

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.

Microarray-based CGH of sporadic and syndrome-related pheochromocytomas using a 0.1-0.2 Mb bacterial artificial chromosome array spanning chromosome arm 1p

Pheochromocytomas (PCC) are relatively rare neuroendocrine tumors, mainly of the adrenal medulla. They arise sporadically or occur secondary to inherited cancer syndromes, such as multiple endocrine neoplasia type II (MEN2), von Hippel-Lindau disease (VHL), or neurofibromatosis type I (NF1). Loss of 1p is the most frequently encountered genetic alteration, especially in MEN2-related and sporadic PCC. Previous studies have revealed three regions of common somatic loss on chromosome arm 1p, using chromosome-based comparative genomic hybridization (CGH) and LOH analysis. To investigate these chromosomal aberrations with a higher resolution and sensitivity, we performed microarray-based CGH with 13 sporadic and 11 syndrome-related (10 MEN2A-related and 1 NF1-related) tumors. The array consisted of 642 overlapping bacterial artificial chromosome (BAC) clones mapped to 1p11.2-p36.33. Chromosomal deletions on 1p were detected in 18 of 24 cases (75%). Among 9 tumors with partial 1p loss, the deleted region was restricted to 1cen-1p32.3 in six cases (25%), indicating a region of genetic instability. The consensus regions of deletion in this study involved 1cen-1p21.1, 1p21.3-1p31.3, and 1p34.3-1p36.33. In conclusion, these data strongly suggest that chromosome arm 1p is the site for multiple tumor suppressor genes, although the potential candidate genes CDKN2C and PTPRF/LAR are not included in these regions.

Rosmarinic acid inhibits the formation of reactive oxygen and nitrogen species in RAW264.7 macrophages

Antioxidant action of Rosmarinic acid (Ros A), a natural phenolic ingredient in many Lamiaceae herbs such as Perilla frutescens, sage, basil and mint, was analyzed in relation to the Ikappa-B activation in RAW264.7 macrophages. Ros A inhibited nitric oxide (NO) production and inducible nitric oxide synthase (iNOS) protein synthesis induced by lipopolysaccharide (LPS), and also effectively suppressed phorbol 12-myristate 13-acetate (PMA)-induced superoxide production in RAW264.7 macrophages in a dose-dependent manner. Peroxynitrite-induced formation of 3-nitrotyrosine in bovine serum albumin and RAW264.7 macrophages were also inhibited by Ros A. Moreover, Western blot analysis demonstrated that LPS-induced phosphorylation of Ikappa-Balpha was abolished by Ros A. Ros A can act as an effective protector against peroxynitrite-mediated damage, and as a potent inhibitor of superoxide and NO synthesis; the inhibition of the formation of reactive oxygen and nitrogen species are partly based on its ability to inhibit the serine phosphorylation of Ikappa-Balpha.

Gene therapy for thyroid cancer

Thyroid carcinomas are suitable targets for gene therapy because they can be highly lethal on one hand, while being susceptible to specific tumour targeting on the other hand. Several gene therapy modalities have been evaluated so far in experimental models of thyroid cancer, including tumour suppressor gene replacement, oncogene inhibition, suicide gene therapy, immunotherapy, antiangiogenesis, and viral oncolysis. All of these strategies have shown promising results, but clinical studies are lacking. Based on the clinical experience achieved in a pilot study in patients with advanced thyroid cancer and on clinical results in other types of solid cancer, it is suggested that combined gene therapy approaches, as well as multimodality therapeutic regimens, including gene therapy and conventional treatments, should be pursued to achieve clinically significant results.

Involvement of peroxynitrite in capsaicin-induced apoptosis of C6 glioma cells

Capsaicin induces apoptosis in some types of cells, but its mechanism remains obscure. In this study, peroxynitrite, a powerful oxidant generated from the reaction of superoxide and nitric oxide (NO) in biological system, was demonstrated to be responsible for capsaicin-mediated apoptosis in C6 glioma cells. Capsaicin-induced apoptosis was detected by terminal deoxynucleotidyl transferase-mediated dUTP nick-end labeling (TUNEL) assay, and also identified by Annexin V staining and comet assay. Capsazepine and ruthenium red, the vanilloid receptor 1 (VR1/TPRV1) antagonists, did not inhibit capsaicin-induced apoptosis. Exposure to capsaicin not only promoted the generation of superoxide and iNOS, but also markedly suppressed the expression of SODs. Nitrite and nitrate, the NO metabolites accumulated in the medium, and the nitrotyrosine was also increased in proteins of C6 glioma cells exposed to capsaicin. Pretreatment of cells with 4 microM ebselen (a peroxynitrite scavenger) showed effective inhibitory effect on the capsaicin-induced apoptosis. These results suggest that peroxynitrite can act as a potential mediator in the capsaicin-induced apoptosis in C6 glioma cells.

Functional significance of the LAR receptor protein tyrosine phosphatase family in development and diseases

The protein tyrosine phosphatases (PTPs) have emerged as critical players in diverse cellular functions. The focus of this review is the leukocyte common antigen-related (LAR) subfamily of receptor PTPs (RPTPs). This subfamily is composed of three vertebrate homologs, LAR, RPTP-sigma, and RPTP-delta, as well as few invertebrates orthologs such as Dlar. LAR-RPTPs have a predominant function in nervous system development that is conserved throughout evolution. Proteolytic cleavage of LAR-RPTP proproteins results in the noncovalent association of an extracellular domain resembling cell adhesion molecules and intracellular tandem PTPs domains, which is likely regulated via dimerization. Their receptor-like structures allow them to sense the extracellular environment and transduce signals intracellularly via their cytosolic PTP domains. Although many interacting partners of the LAR-RPTPs have been identified and suggest a role for the LAR-RPTPs in actin remodeling, very little is known about the mechanisms of action of RPTPs. LAR-RPTPs recently raised a lot of interest when they were shown to regulate neurite growth and nerve regeneration in transgenic animal models. In addition, LAR-RPTPs have also been implicated in metabolic regulation and cancer. This RPTP subfamily is likely to become important as drug targets in these various human pathologies, but further understanding of their complex signal transduction cascades will be required.Key words: protein tyrosine phosphatase, LAR, signal transduction, nervous system development.

The oncogenic activity of RET point mutants for follicular thyroid cells may account for the occurrence of papillary thyroid carcinoma in patients affected by familial medullary thyroid carcinoma

Activating germ-line point mutations in the RET receptor are responsible for multiple endocrine neoplasia type 2-associated medullary thyroid carcinoma (MTC), whereas somatic RET rearrangements are prevalent in papillary thyroid carcinomas (PTCs). Some rare kindreds, carrying point mutations in RET, are affected by both cancer types, suggesting that, under specific circumstances, point mutations in RET can drive the generation of PTC. Here we describe a family whose siblings, affected by both PTC and MTC, carried a germ-line point mutation in the RET extracellular domain, converting cysteine 634 into serine. We tested on thyroid follicular cells the transforming activity of RET(C634S), RET(K603Q), another mutant identified in a kindred with both PTC and MTC, RET(C634R) a commonly isolated allele in MEN2A, RET(M918T) responsible for MEN2B and also identified in kindreds with both PTC and MTC, and RET/PTC1 the rearranged oncogene that characterizes bona fide PTC in patients without MTC. We show that the various RET point mutants, but not wild-type RET, scored constitutive kinase activity and exerted mitogenic effects for thyroid PC Cl 3 cells, albeit at significantly lower levels compared to RET/PTC1. The low mitogenic activity of RET point mutants paralleled their reduced kinase activity compared to RET/PTC. Furthermore, RET point mutants maintained a protein domain, the intracellular juxtamembrane domain, that exerted negative effects on the mitogenic activity. In conclusion, RET point mutants can behave as dominant oncogenes for thyroid follicular cells. Their transforming activity, however, is rather modest, providing a possible explanation for the rare association of MTC with PTC.

The tyrosine phosphatase Shp-2 mediates intracellular signaling initiated by Ret mutants

The Src homology 2-containing tyrosine phosphatase, Shp-2, is a crucial enzyme that mediates intracellular signaling and is implicated in cell proliferation and differentiation. Here we investigated the involvement of the Shp-2 tyrosine phosphatase in determining the downstream signaling pathways initiated by the Ret oncogene, carrying either the cysteine 634 to tyrosine or the methionine 918 to threonine substitutions. These mutations convert the receptor tyrosine kinase, Ret, into a dominant transforming protein and induce constitutive activation of its intrinsic tyrosine kinase activity leading to congenital and sporadic cancers in neuroendocrine organs. Using the PC12, rat pheochromocytoma cell line, as model system, we show that Shp-2 mediates immediate-early gene expression if induced by either of the mutant alleles. Furthermore, we show that Shp-2 activity is required for RetM918T-induced Akt activation. The results indicate that Shp-2 is a downstream mediator of the mutated receptors RetC634Y and RetM918T, thus suggesting that it may act as a limiting factor in Ret-associated endocrine tumors, in the neoplastic syndromes multiple endocrine neoplasia types 2A and 2B.

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.

Overexpression of leucocyte common antigen (LAR) P-subunit in thyroid carcinomas

Protein tyrosine phosphatase (PTPase) dephosphorylation and protein tyrosine kinase (PTKs) phosphorylation of key signal transduction proteins may be regulated by extracellular signals, making PTPases important in the regulation of cell proliferation. Leucocyte common antigen (LAR), a receptor-like PTPase, consists of E-subunit, containing the cell adhesion molecule-like receptor region, and P-subunit specific for a short segment of the extracellular region, the transmembrane peptide, and two cytoplasmic PTPase domains. We produced a monoclonal antibody against the LAR P-subunit for immunohistochemical screening of LAR expression in normal and tumourous tissues. Gliomas and gastric, colorectal, lung, breast and prostate cancers showed weak and relatively infrequent expression. Intense and diffuse expression, however, was detected in 95% (227 out of 239) of thyroid carcinomas, but only 12% (22 out of 128) of adenomas and no cases of benign thyroid disease were immunopositive. In contrast to broad staining in carcinomas, LAR expression in thyroid adenomas was often found in small focal or locally invasive areas. Western blot analysis similarly detected LAR P-subunit protein in thyroid carcinomas, but not in normal tissues. We believe this to be the first demonstration of LAR overexpression in thyroid carcinoma and may help to elucidate the role of PTPases in the development of malignancy.

Reduced tumorigenicity of murine leukemia cells expressing protein-tyrosine phosphatase, PTPepsilon C

Recently, we reported that a cytosolic isoform of protein-tyrosine phosphatase epsilon (PTP epsilon C), when overexpressed, inhibits terminal differentiation and apoptosis of murine M1 myeloblastic leukemia cells induced by interleukin-6. To determine whether these observed effects in vitro correspond to a tumorigenicity of PTP epsilon C-expresser (M1- epsilon C) cells in vivo, parent M1 and M1- epsilon C cells were intravenously inoculated into scid or nude mice, and survival of mice receiving these cell lines was monitored. Unexpectedly, both scid and nude mice inoculated with M1- epsilon C cells showed significantly prolonged survival time than those receiving parent M1 cells. While parent M1 cells inoculated by intravenous injection formed metastatic tumors in the spleen, expression of PTP epsilon C suppressed tumor development in the spleen. The results suggest a suppressive role of PTP epsilon C in tumorigenesis.

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

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

The GDNF/RET signaling pathway and human diseases

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