Oncogenesis of RON receptor tyrosine kinase: a molecular target for malignant epithelial cancers

Targeting Receptor Tyrosine Kinases as a Novel Strategy for the Treatment of Triple-Negative Breast Cancer

Triple-negative breast cancer (TNBC) comprises a group of aggressive and heterogeneous breast carcinoma. Chemotherapy is the mainstay for the treatment of triple-negative tumors. Nevertheless, the success of chemotherapeutic treatments is limited by their toxicity and development of acquired resistance leading to therapeutic failure and tumor relapse. Hence, there is an urgent need to explore novel targeted therapies for TNBC. Receptor tyrosine kinases (RTKs) are a family of transmembrane receptors that are key regulators of intracellular signaling pathways controlling cell proliferation, differentiation, survival, and motility. Aberrant activity and/or expression of several types of RTKs have been strongly connected to tumorigenesis. RTKs are frequently overexpressed and/or deregulated in triple-negative breast tumors and are further associated with tumor progression and reduced survival in patients. Therefore, targeting RTKs could be an appealing therapeutic strategy for the treatment of TNBC. This review summarizes the current evidence regarding the antitumor activity of RTK inhibitors in preclinical models of TNBC. The review also provides insights into the clinical trials evaluating the use of RTK inhibitors for the treatment of patients with TNBC.

Molecular landscape and subtype-specific therapeutic response of nasopharyngeal carcinoma revealed by integrative pharmacogenomics

Nasopharyngeal carcinoma (NPC) is a malignant head and neck cancer type with high morbidity in Southeast Asia, however the pathogenic mechanism of this disease is poorly understood. Using integrative pharmacogenomics, we find that NPC subtypes maintain distinct molecular features, drug responsiveness, and graded radiation sensitivity. The epithelial carcinoma (EC) subtype is characterized by activations of microtubule polymerization and defective mitotic spindle checkpoint related genes, whereas sarcomatoid carcinoma (SC) and mixed sarcomatoid-epithelial carcinoma (MSEC) subtypes exhibit enriched epithelial-mesenchymal transition (EMT) and invasion promoting genes, which are well correlated with their morphological features. Furthermore, patient-derived organoid (PDO)-based drug test identifies potential subtype-specific treatment regimens, in that SC and MSEC subtypes are sensitive to microtubule inhibitors, whereas EC subtype is more responsive to EGFR inhibitors, which is synergistically enhanced by combining with radiotherapy. Through combinational chemoradiotherapy (CRT) screening, effective CRT regimens are also suggested for patients showing less sensitivity to radiation. Altogether, our study provides an example of applying integrative pharmacogenomics to establish a personalized precision oncology for NPC subtype-guided therapies.

Splicing factors control triple-negative breast cancer cell mitosis through SUN2 interaction and sororin intron retention

BACKGROUND

Triple negative breast cancer (TNBC) is an aggressive subtype of breast cancer with limited therapeutic opportunities. Recently, splicing factors have gained attention as potential targets for cancer treatment. Here we systematically evaluated the role of RNA splicing factors in TNBC cell proliferation.

METHODS

In this study, we performed an RNAi screen targeting 244 individual splicing factors to systematically evaluate their role in TNBC cell proliferation. For top candidates, mechanistic insight was gained using amongst others western blot, PCR, FACS, molecular imaging and cloning. Pulldown followed by mass spectrometry were used to determine protein-protein interactions and patient-derived RNA sequencing data was used relate splicing factor expression levels to proliferation markers.

RESULTS

We identified nine splicing factors, including SNRPD2, SNRPD3 and NHP2L1, of which depletion inhibited proliferation in two TNBC cell lines by deregulation of sister chromatid cohesion (SCC) via increased sororin intron 1 retention and down-regulation of SMC1, MAU2 and ESPL1. Protein-protein interaction analysis of SNRPD2, SNRPD3 and NHP2L1 identified that seven out of the nine identified splicing factors belong to the same spliceosome complex including novel component SUN2 that was also critical for efficient sororin splicing. Finally, sororin transcript levels are highly correlated to various proliferation markers in BC patients.

CONCLUSION

We systematically determined splicing factors that control proliferation of breast cancer cells through a mechanism that involves effective sororin splicing and thereby appropriate sister chromatid cohesion. Moreover, we identified SUN2 as an important new spliceosome complex interacting protein that is critical in this process. We anticipate that deregulating sororin levels through targeting of the relevant splicing factors might be a potential strategy to treat TNBC.

Regulation of Energy Metabolism by Receptor Tyrosine Kinase Ligands

Metabolic diseases, such as diabetes, obesity, and fatty liver disease, have now reached epidemic proportions. Receptor tyrosine kinases (RTKs) are a family of cell surface receptors responding to growth factors, hormones, and cytokines to mediate a diverse set of fundamental cellular and metabolic signaling pathways. These ligands signal by endocrine, paracrine, or autocrine means in peripheral organs and in the central nervous system to control cellular and tissue-specific metabolic processes. Interestingly, the expression of many RTKs and their ligands are controlled by changes in metabolic demand, for example, during starvation, feeding, or obesity. In addition, studies of RTKs and their ligands in regulating energy homeostasis have revealed unexpected diversity in the mechanisms of action and their specific metabolic functions. Our current understanding of the molecular, biochemical and genetic control of energy homeostasis by the endocrine RTK ligands insulin, FGF21 and FGF19 are now relatively well understood. In addition to these classical endocrine signals, non-endocrine ligands can govern local energy regulation, and the intriguing crosstalk between the RTK family and the TGFβ receptor family demonstrates a signaling network that diversifies metabolic process between tissues. Thus, there is a need to increase our molecular and mechanistic understanding of signal diversification of RTK actions in metabolic disease. Here we review the known and emerging molecular mechanisms of RTK signaling that regulate systemic glucose and lipid metabolism, as well as highlighting unexpected roles of non-classical RTK ligands that crosstalk with other receptor pathways.

mTORC1 is a key mediator of RON-dependent breast cancer metastasis with therapeutic potential

Metastasis is the biggest challenge in treating breast cancer, and it kills >40,000 breast cancer patients annually in the US. Aberrant expression of the RON receptor tyrosine kinase in breast tumors correlates with poor prognosis and has been shown to promote metastasis. However, the molecular mechanisms that govern how RON promotes metastasis, and how to block it, are still largely unknown. We sought to determine critical effectors of RON using a combination of mutational and pharmacologic strategies. High-throughput proteomic analysis of breast cancer cells upon activation of RON showed robust phosphorylation of ribosomal protein S6. Further analysis revealed that RON strongly signals through mTORC1/p70S6K, which is mediated predominantly by the PI3K pathway. A targeted mutation approach to modulate RON signaling validated the importance of PI3K/mTORC1 pathway for spontaneous metastasis in vivo. Finally, inhibition of mTORC1 with an FDA-approved drug, everolimus, resulted in transient shrinkage of established RON-dependent metastases, and combined blockade of mTORC1 and RON delayed progression. These studies have identified a key downstream mediator of RON-dependent metastasis in breast cancer cells and revealed that inhibition of mTORC1, or combined inhibition of mTORC1 and RON, may be effective for treatment of metastatic breast cancers with elevated expression of RON.

Inhibition of RON kinase potentiates anti-CTLA-4 immunotherapy to shrink breast tumors and prevent metastatic outgrowth

The advent of immune checkpoint blockade as a new strategy for immunotherapy has changed the outlook for many aggressive cancers. Although complete tumor eradication is attainable in some cases, durable clinical responses are observed only in a small fraction of patients, underlining urgent need for improvement. We previously showed that RON, a receptor tyrosine kinase expressed in macrophages, suppresses antitumor immune responses, and facilitates progression and metastasis of breast cancer. Here, we investigated the molecular changes that occur downstream of RON activation in macrophages, and whether inhibition of RON can cooperate with checkpoint immunotherapy to eradicate tumors. Activation of RON by its ligand, MSP, altered the gene expression profile of macrophages drastically and upregulated surface levels of CD80 and PD-L1, ligands for T-cell checkpoint receptors CTLA-4 and PD-1. Genetic deletion or pharmacological inhibition of RON in combination with anti-CTLA-4, but not with anti-PD-1, resulted in improved clinical responses against orthotopically transplanted tumors compared to single-agent treatment groups, resulting in complete tumor eradication in 46% of the animals. Positive responses to therapy were associated with higher levels of T-cell activation markers and tumor-infiltrating lymphocytes. Importantly, co-inhibition of RON and anti-CTLA-4 was also effective in clearing metastatic breast cancer cells in lungs, resulting in clinical responses in nearly 60% of the mice. These findings suggest that RON inhibition can be a novel approach to potentiate responses to checkpoint immunotherapy in breast cancer.

Expression of RON in colorectal cancer and its relationships with tumor cell behavior and prognosis

Aims and background

The aims of the current study were to evaluate whether recepteur d'origine nantais (RON) affects tumor cell behavior and oncogenic signaling pathways in colorectal cancer, and to examine the relationship of its expression with various clinicopathological parameters and patient survival.

Methods

Immunohistochemistry, Western blot and RT-PCR were used to detect the expression of the RON gene in human colorectal cancer tissue. To study the biological role of RON in tumor cell behavior and cellular signaling pathways, we used small interfering RNA (siRNA) to knock down RON gene expression in human colorectal cancer cell lines.

Results

Knockdown of RON inhibited the induction of the invasive growth phenotype and the activation of oncogenic signaling pathways including Akt, MAPK and β-catenin. RON overexpression was associated with tumor size, lymphovascular invasion, depth of invasion, lymph node metastasis, distant metastasis, tumor stage and poor survival.

Conclusions

These results suggest that RON overexpression may help in predicting poor clinical outcomes in colorectal cancer.

RON Signaling Is a Key Mediator of Tumor Progression in Many Human Cancers

With an increasing body of literature covering RON receptor tyrosine kinase function in different types of human cancers, it is becoming clear that RON has prominent roles in both cancer cells and in the tumor-associated microenvironment. RON not only activates several oncogenic signaling pathways in cancer cells, leading to more aggressive behavior, but also promotes an immunosuppressive, alternatively activated phenotype in macrophages and limits the antitumor immune response. These two unique functions of this oncogene, the strong correlation between RON expression and poor outcomes in cancer, and the high tolerability of a new RON inhibitor make it an exciting therapeutic target, the blocking of which offers an advantage toward improving the survival of cancer patients. Here, we discuss recent findings on the role of RON signaling in cancer progression and its potential in cancer therapy.

RON kinase isoforms demonstrate variable cell motility in normal cells

INTRODUCTION

Aberrant RON (Recepteur d'Origine Nantais) tyrosine kinase activation causes the epithelial cell to evade normal growth pathways, resulting in unregulated cell proliferation, increased cell motility and decreased apoptosis. Wildtype (wt) RON has been shown to play a role in metastasis of epithelial malignancies. It presents an important potential therapeutic target for colorectal, breast, gastric and pancreatic cancer. Little is known about functional differences amongst RON isoforms RON155, RON160 and RON165. The purpose of this study was to determine the effect of various RON kinase isoforms on cell motility.

METHODS

Cell lines with stable expression of wtRON were generated by inserting the coding region of RON in pTagRFP (tagged red fluorescence protein plasmid). The expression constructs of RON variants (RON155, RON160 and RON165) were generated by creating a mutagenesis-based wtRON-pTag RFP plasmid and stably transfected into HEK 293 cells. The wound closure scratch assay was used to investigate the effect on cell migratory capacity of wild type RON and its variants.

RESULTS

RON transfected cells demonstrated increased cell motility compared to HEK293 control cells. RON165 cell motility was significantly increased compared to RON160 (mean percentage of wound covered 37.37% vs. 32.40%; p = 0.03).

CONCLUSIONS

RON tyrosine kinase isoforms have variable cell motility. This may reflect a difference in the behavior of malignant epithelial cells and their capacity for metastasis.

Effect of Recepteur d'Origine Nantais expression on chemosensitivity and tumor cell behavior in colorectal cancer

Recepteur d'Origine Nantais (RON) expression is known to induce oncogenic properties including tumor cell growth, survival, motility, angiogenesis and chemoresistance. In the present study, we evaluated whether RON affects chemosensitivity and oncogenic behavior of colorectal cancer cells and investigated its prognostic value in colorectal cancer. To evaluate the impact of RON on chemosensitivity and tumor cell behavior, we treated colorectal cancer cells with small interfering RNAs specific to RON. This was followed by flow cytometric analyses and migration, Matrigel invasion and endothelial tube formation assays. The expression of RON was investigated by immunohistochemistry in colorectal cancer tissues. TUNEL assay and immunohistochemical staining for CD34 and D2-40 were deployed to determine apoptosis, angiogenesis and lymphangiogenesis. RON knockdown enhanced 5-fluorouracil (FU)-induced apoptosis by upregulating the activities of caspases and expression of proapoptotic genes. Moreover, it enhanced 5-FU-induced cell cycle arrest by decreasing the expression of cyclins and cyclin‑dependent kinases and inducing that of p21. Furthermore, RON knockdown augmented the 5-FU-induced inhibition of invasion and migration of colorectal cancer cells. The β-catenin signaling cascade was blocked by RON knockdown upon 5-FU treatment. RON knockdown also decreased endothelial tube formation and expression of VEGF-A and HIF-1α and increased angiostatin expression. Furthermore, it inhibited lymphatic endothelial cell tube formation and the expression of VEGF-C and COX-2. RON expression was observed to be associated with age, tumor size, lymphovascular and perineural invasion, tumor stage, lymph node and distant metastasis, and poor survival rate. The mean microvessel density value of RON-positive tumors was significantly higher than that of RON-negative ones. These results indicate that RON is associated with tumor progression by inhibiting chemosensitivity and enhancing angiogenesis in colorectal cancer.

SRSF2 promotes splicing and transcription of exon 11 included isoform in Ron proto-oncogene

The product of proto-oncogene Ron is a human receptor for the macrophage-stimulating protein (MSP). Upon activation, Ron is able to induce cell dissociation, migration and matrix invasion. Exon 11 skipping of Ron pre-mRNA produces Ron△165 protein that is constitutively active even in the absence of its ligand. Here we show that knockdown of SRSF2 promotes the decrease of exon 11 inclusion, whereas overexpression of SRSF2 promotes exon 11 inclusion. We demonstrate that SRSF2 promotes exon 11 inclusion through splicing and transcription procedure. We also present evidence that reduced expression of SRSF2 induces a decrease in the splicing of both introns 10 and 11; by contrast, overexpression of SRSF2 induces an increase in the splicing of introns 10 and 11. Through mutation analysis, we show that SRSF2 functionally targets and physically interacts with CGAG sequence on exon 11. In addition, we reveal that the weak strength of splice sites of exon 11 is not required for the function of SRSF2 on the splicing of Ron exon 11. Our results indicate that SRSF2 promotes exon 11 inclusion of Ron proto-oncogene through targeting exon 11. Our study provides a novel mechanism by which Ron is expressed.

A 2-nt RNA enhancer on exon 11 promotes exon 11 inclusion of the Ron proto-oncogene

Ron is a human receptor for the macrophage-stimulating protein (MSP). Exon 11 skipping of Ron pre-mRNA produces the RonΔ165 protein that has a deletion of a 49 amino acid region in the β-chain extracellular domain. RonΔ165 is constitutively active even in the absence of its ligand. Through stepwise deletion analysis, we identified a 2-nt RNA enhancer, which is located 74 nt upstream from the 5′ splice site of exon 11, for exon 11 inclusion. Through double-base and single-base substitution analysis of the 2-nt RNA, we demonstrated that the GA, CC, UG and AC dinucleotides on exon 11, in addition to the wild-type AG sequence, function as enhancers for exon 11 inclusion of the Ron pre-mRNA.

Ron knockdown and Ron monoclonal antibody IMC-RON8 sensitize pancreatic cancer to histone deacetylase inhibitors (HDACi)

Recepteur d'origine nantais (Ron) is overexpressed in a panel of pancreatic cancer cells and tissue samples from pancreatic cancer patients. Ron can be activated by its ligand macrophage stimulating protein (MSP), thereby activating oncogenic signaling pathways. Crosstalk between Ron and EGFR, c-Met, or IGF-1R may provide a mechanism underlying drug resistance. Thus, targeting Ron may represent a novel therapeutic strategy. IMC-RON8 is the first Ron monoclonal antibody (mAb) entering clinical trial for targeting Ron overexpression. Our studies show IMC-RON8 downmodulated Ron expression in pancreatic cancer cells and significantly blocked MSP-stimulated Ron activation, downstream Akt and ERK phosphorylation, and survivin mRNA expression. IMC-RON8 hindered MSP-induced cell migration and reduced cell transformation. Histone deacetylase inhibitors (HDACi) are reported to target expression of various genes through modification of nucleosome histones and non-histone proteins. Our work shows HDACi TSA and Panobinostat (PS) decreased Ron mRNA and protein expression in pancreatic cancer cells. PS also reduced downstream signaling of pAkt, survivin, and XIAP, as well as enhanced cell apoptosis. Interestingly, PS reduced colony formation in Ron knockdown cells to a greater extent than Ron scramble control cells in colony formation and soft agarose assays. IMC-RON8 could also sensitize pancreatic cancer cells to PS, as reflected by reduced colony numbers and size in combination treatment with IMC-RON8 and PS compared to single treatment alone. The co-treatment further reduced Ron expression and pAkt, and increased PARP cleavage compared to either treatment alone. This study suggests the potential for a novel combination approach which may ultimately be of value in treatment of pancreatic cancer.

Macrophage-stimulating protein attenuates hydrogen peroxide-induced apoptosis in human renal HK-2 cells

Macrophage-stimulating protein (MSP) and its receptor, recepteur d'origine nantais (RON), play an important role in cell proliferation and migration. We have investigated the role of MSP in hydrogen peroxide (H2O2)-induced renal tubular apoptosis. Human renal proximal tubular (HK-2) cells were incubated with H2O2 for 24h in the presence of different concentrations of MSP, and cell viability was measured by MTT assay. The protein expression of Bax, Bcl-2, caspase-3, mitogen-activated protein kinases (MAPKs), phosphatidylinositol-3-kinase (PI3K)/Akt, and nuclear factor-kappa B (NF-κB) was determined by semiquantitative immunoblotting. Apoptosis was assessed by flow cytometry analysis after HK-2 cells were stained with fluorescein isothiocyanate-conjugated annexin V protein and propidium iodide. H2O2 treatment decreased cell viability in HK-2 cells; this was counteracted by MSP pretreatment. H2O2 treatment induced an increased ratio of Bax/Bcl-2, cleaved caspase-3, and the number of condensed nuclei, which was also counteracted by MSP. Flow cytometry analysis showed H2O2-induced apoptosis, and its prevention by MSP treatment. Increased protein expression of phospho-p38 MAPK was attenuated by MSP, while phospho-extracellular signal-regulated kinase and c-Jun-N-terminal kinase were not affected. H2O2 induced NF-κB activation and IκB-α degradation, but the increased nuclear NF-κB activation was counteracted by MSP or by a p38 MAPK inhibitor. H2O2 treatment decreased expression of phospho-PI3K and phospho-Akt, which was reversed by MSP pretreatment. These findings suggest that MSP attenuates H2O2-induced apoptosis in HK-2 cells by modulating the p38 and NF-κB, as well as PI3K/Akt, signaling pathways.

Expression of the receptor tyrosine kinase recepteur d'origine nantais and its association with tumor progression in hypopharyngeal cancer

BACKGROUND

The purpose of this study was to investigate expression of recepteur d'origine nantais (RON) in human hypopharyngeal squamous cell carcinoma (SCC) and to determine whether RON affects tumor cell behavior in hypopharyngeal SCC cell line and if this would serve as a target for molecular therapy in a preclinical model.

METHODS

Reverse transcriptase-polymerase chain reaction, immunohistochemistry, Western blotting, cell invasion, migration, proliferation, and apoptosis assays were used to assess alteration of RON expression and its impact to cancer progression in human hypopharyngeal SCC.

RESULTS

Immunoreactivity of RON was observed in hypopharyngeal SCC tissues relative to adjacent normal mucosa in all cases. RON protein expression was significantly increased in metastatic lymph nodes than nonmetastatic lymph nodes by Western blotting. Knockdown of RON resulted in significantly reduced cell invasion, migration, and proliferation in human hypopharyngeal SCC cells. Knockdown of RON enhanced cell apoptosis through activation of caspase 3, caspase 7, and poly ADP-ribose polymerase (PARP).

CONCLUSION

These results indicate that knockdown of RON expression may be associated with the reversal of invasive phenotype in hypopharyngeal SCC.

Knockdown of RON inhibits AP-1 activity and induces apoptosis and cell cycle arrest through the modulation of Akt/FoxO signaling in human colorectal cancer cells

BACKGROUND/AIMS

Altered Recepteur d'Origine nantais (RON) expression transduces signals inducting invasive growth phenotype that includes cell proliferation, migration, matrix invasion, and protection of apoptosis in human cancer cells. The aims of the current study were to evaluate whether RON affects tumor cell behavior and cellular signaling pathways including activator protein-1 (AP-1) and Akt/forkhead box O (FoxO) in human colorectal cancer cells.

METHODS

To study the biological role of RON on tumor cell behavior and cellular signaling pathways in human colorectal cancer, we used small interfering RNA (siRNA) to knockdown RON gene expression in human colorectal cancer cell line, DKO-1.

RESULTS

Knockdown of RON diminished migration, invasion, and proliferation of human colorectal cancer cells. Knockdown of RON decreased AP-1 transcriptional activity and expression of AP-1 target genes. Knockdown of RON activated cleaved caspase-3, -7, -9, and PARP, and down-regulated the expression of Mcl-1, survivin and XIAP, leading to induction of apoptosis. Knockdown of RON induced cell cycle arrest in the G2/M phase of cancer cells by an increase of p27 and a decrease of cyclin D3. Knockdown of RON inhibited the phosphorylation of Akt/FoxO signaling proteins such as Ser473 and Thr308 of Akt and FoxO1/3a.

CONCLUSIONS

These results indicate that knockdown of RON inhibits AP-1 activity and induces apoptosis and cell cycle arrest through the modulation of Akt/FoxO signaling in human colorectal cancer cells.

RON is associated with tumor progression via the inhibition of apoptosis and cell cycle arrest in human gastric cancer

The recepteur d'origine nantais (RON) receptor tyrosine kinase is overexpressed in epithelial cancers, including gastric cancer. The aims of the present study were to evaluate whether RON affects tumor cell behaviors and oncogenic signaling pathways, and to document the relationship of its expression with various clinicopathological parameters in gastric cancer. The biological role of RON in tumor cell behaviors and oncogenic signaling pathways was investigated by using small interfering RNA in gastric cancer cell lines including AGS and MKN28. The expression of RON in gastric cancer tissues was investigated by using reverse transcription polymerase chain reaction and immunohistochemistry. Knockdown of RON suppressed tumor cell migration and invasion in AGS and MKN28, induced apoptosis through modulation of anti-apoptotic and pre-apoptotic genes and induced cell cycle arrest by decreasing cyclin D1, cyclin D3 and CDK4, and by inducing p21 and p27 expression. Signaling cascades, including Akt and mitogen-activated protein kinase (MAPK), were significantly blocked by knockdown of RON. Expression of RON was significantly associated with tumor size, depth of invasion, lymph node metastasis, tumor stage and poor survival. These results indicate that RON is associated with tumor progression via the inhibition of apoptosis and cell cycle arrest in human gastric cancer.

Expression and mutational status of RON in neoplastic lesions of the breast: analysis of MSP/RON signaling in ductal carcinoma in situ and invasive ductal carcinoma

Recepteur d'origine nantais (RON) is a receptor tyrosine kinase closely related to MET and involved in tumorigenesis. We investigated the roles of aberrations in RON and its ligand, macrophage-stimulating protein (MSP), in invasive ductal carcinoma (IDC, n = 81), ductal carcinoma in situ (DCIS, n = 26), and in benign lesions (n = 20) of mammary gland. Expression of RON and MSP was evaluated by immunohistochemistry and the mutational status of a region containing the proteolytic cleavage site in exon 1 and each exon of the kinase domain (exon 14-20) of RON was screened by polymerase chain reaction-single strand conformational polymorphism (PCR-SSCP) analysis. The proportion of cases positive for RON expression was significantly different between malignant [86% (92/107)] and benign [40% (8/20)] lesions. RON expression was positive in both IDC and DCIS [90% (73/81) and 73% (19/26), respectively], whereas MSP expression was present in 54% (44/81) of IDC and absent in DCIS. RON expression correlated significantly with the histological grade of DCIS. No mutations were detected in the examined regions of RON in breast cancer samples as confirmed by PCR-SSCP. The findings suggest the involvement of RON expression in the development of breast cancer, and that an autocrine/paracrine loop of RON seems to affect tumor invasiveness.

Tyrosine kinase receptor RON and its ligand MSP in Merkel cell carcinoma

The RON gene, encoding the tyrosine-kinase receptor for macrophage-stimulating protein (MSP), is involved in a range of neoplastic processes. However, no aberration in RON or MSP has been identified in Merkel cell carcinoma (MCC). We investigated the RON signaling pathway in MCC. Fourteen cases of MCC were tested for the expression of RON and its ligand, MSP, using reverse transcription PCR (RT-PCR) and immunohistochemistry. The mutation of RON was also examined. RT-PCR identified transcription of both RON and MSP in all nine cases that were available for the examination. Immunohistochemistry showed the expression of RON (9/14 cases) and MSP (9/14 cases). Six cases out of 14 were positive for both RON and MSP. Normal Merkel cells were negative for RON and MSP expression. Missense mutation in the tyrosine kinase region of RON was detected in one of the 14 cases, and the case showed expression of RON. Transcription of RON and MSP was observed in MCC, and a considerable number of MCC cases expressed both RON and MSP, while Merkel cells do not express these molecules. The results suggest that RON signaling seems to play at least some role in the pathogenesis of MCC.

Isolation of Fully Human Antagonistic RON Antibodies Showing Efficient Block of Downstream Signaling and Cell Migration

RON belongs to the c-MET family of receptor tyrosine kinases. As its well-known family member MET, RON and its ligand macrophage-stimulating protein have been implicated in the progression and metastasis of tumors and have been shown to be overexpressed in cancer. We generated and tested a large number of human monoclonal antibodies (mAbs) against human RON. Our screening yielded three high-affinity antibodies that efficiently block ligand-dependent intracellular AKT and MAPK signaling. This effect correlates with the strong reduction of ligand-activated migration of T47D breast cancer cell line. By cross-competition experiments, we showed that the antagonistic antibodies fall into three distinct epitope regions of the RON extracellular Sema domain. Notably, no inhibition of tumor growth was observed in different epithelial tumor xenografts in nude mice with any of the antibodies. These results suggest that distinct properties beside ligand antagonism are required for anti-RON mAbs to exert antitumor effects in vivo.

Identification of new possible targets for leukemia treatment by kinase activity profiling

To date, the biology of acute leukemia has been unclear, and defining new therapeutic targets without prior knowledge remains complicated. The use of high-throughput techniques would enable us to learn more about the biology of the disease, and make it possible to directly assess a broader range of therapeutic targets. In this study we have identified comprehensive tyrosine kinase activity profiles in leukemia samples using the PamChip® kinase activity profiling system. Strikingly, 31% (44/120) of the detected peptides were active in all three groups of leukemia samples. The recently reported activity of platelet-derived growth factor receptor (PDGFR) and neurotrophic tyrosine kinase receptors (NTRK1 and NTRK2) in leukemia could be appreciated in our array results. In addition, high levels of peptide phosphorylation were demonstrated for peptides related to macrophage stimulating 1 receptor (MST1R). A provisional signal transduction scheme of the common active peptides was constructed and used to specifically select an inhibitor for leukemic blast cell survival assays. As expected, a dose-dependent decrease in leukemic blast cell survival was achieved for all leukemia samples. Our data demonstrate that kinase activity profiling in leukemic samples is feasible and provides novel insights into the pathogenesis of leukemia. This approach can be used for the rapid discovery of potential drug targets.

Deletion or insertion in the first immunoglobulin-plexin-transcription (IPT) domain differentially regulates expression and tumorigenic activities of RON receptor Tyrosine Kinase

Background

Activation of the RON receptor tyrosine kinase, a member of the c-MET family, regulates tumorigenic phenotypes. The RON extracellular domains are critical in regulating these activities. The objective of this study was to determine the role of the first IPT domain in regulating RON-mediated tumorigenic activities and the underlying mechanisms.

Results

Two RON variants, RON160 and RON^E5/6in with deletion and insertion in the first IPT domain, respectively, were molecularly cloned. RON160 was a splicing variant generated by deletion of 109 amino acids encoded by exons 5 and 6. In contrast, RON^E5/6in was derived from a transcript with an insertion of 20 amino acids between exons 5 and 6. Both RON160 and RON^E5/6in were proteolytically matured into two-chain receptor and expressed on the cell surface. RON160 was constitutively active with tyrosine phosphorylation. However, activation of RON^E5/6in required ligand stimulation. Deletion resulted in the resistance of RON160 to proteolytic digestion by cell associated trypsin-like enzymes. RON160 also resisted anti-RON antibody-induced receptor internalization. These features contributed to sustained intracellular signaling cascades. On the other hand, RON^E5/6in was highly susceptible to protease digestion, which led to formation of a truncated variant known as RONp110. RON^E5/6in also underwent rapid internalization upon anti-RON antibody treatment, which led to signaling attenuation. Although ligand-induced activation of RON^E5/6in partially caused epithelial to mesenchymal transition (EMT), it was RON160 that showed cell-transforming activities in cell focus formation and anchorage-independent growth. RON160-mediated EMT is also associated with increased motile/invasive activity.

Conclusions

Alterations in the first IPT domain in extracellular region differentially regulate RON mediated tumorigenic activities. Deletion of the first IPT results in formation of oncogenic variant RON160. Enhanced degradation and internalization with attenuated signaling cascades could be the mechanisms underlying non-tumorigenic features of RON^E5/6in.

Directing traffic in neural cells: determinants of receptor tyrosine kinase localization and cellular responses

The trafficking of receptor tyrosine kinases (RTKs) to distinct subcellular locations is essential for the specificity and fidelity of signal transduction and biological responses. This is particularly important in the PNS and CNS in which RTKs mediate key events in the development and maintenance of neurons and glia through a wide range of neural processes, including survival, proliferation, differentiation, neurite outgrowth, and synaptogenesis. The mechanisms that regulate the targeting of RTKs to their subcellular destinations for appropriate signal transduction, however, are still elusive. In this review, we discuss evidence for the spatial organization of signaling machinery into distinct subcellular compartments, as well as the role for ligand specificity, receptor sorting signals, and lipid raft microdomains in RTK targeting and the resultant cellular responses in neural cells.

RON receptor tyrosine kinase as a target for delivery of chemodrugs by antibody directed pathway for cancer cell cytotoxicity

Overexpression of the RON receptor tyrosine kinase exists in various cancers and contributes to malignant progression. To validate RON as a targeting moiety for delivery of chemoagents for enhanced tumor cytotoxicity, immunoliposomes (IL) loaded with doxorubicin (Dox) were formulated followed by postinsertion of monoclonal antibodies Zt/g4, Zt/c1, or their Fab fragments specific to the RON extracellular domains. Flow cytometry analysis showed that Zt/g4 or Zt/c1-IL binds to cancer cells and causes RON internalization as evident in confocal analysis of intracellular fluorescence intensity. The antibody-directed IL uptake by cancer cells is in both dose and time-dependent manners. Studies of cytotoxicity of individual IL in vitro against colon or breast cancer cell lines revealed that Zt/g4 directed Dox-IL displayed increased cytotoxic activities with a significant reduction of IC(50) values. An average of 8-fold increases in cytotoxic efficiency was achieved among four cell lines tested. Moreover, Zt/g4 directed Dox-IL also displayed the effective killing of cancer cells that are insensitive to pegylated liposomal doxorubicin. The effect of Zt/c1-Dox-IL was not as strong as Zt/g4-Dox-IL, and only moderate activities were observed. IL coupled with the Fab fragments of Zt/g4 or Zt/c1 show moderate activities against cancer cells. The ineffectiveness seemed to be related to the weak activities of the Fab fragments in the induction of RON internalization, which resulted in reduced drug uptakes. We conclude that anti-RON antibody-directed drug delivery is effective for increased uptake of cytotoxic drugs. Antibody-based RON targeting could be developed into a potential therapeutic for treatment of malignant cancers.

Identification of a novel series of potent RON receptor tyrosine kinase inhibitors

A novel series of N-(3-fluoro-4-(2-substituted-thieno[3,2-b]pyridin-7-yloxy)phenyl)-1-phenyl-5-(trifluoromethyl)-1H-pyrazole-4-carboxamides targeting RON receptor tyrosine kinase was designed and synthesized. SAR study of the series allowed us to identify compounds possessing either inhibitory activity of RON kinase enzyme in the low nanomolar range with low residual activity against the closely related c-Met or potent dual inhibitory activity against RON and c-Met, with no significant activity against VEGFR2 in both cases.

The Ron receptor tyrosine kinase is not required for adenoma formation in Apc(Min/+) mice

The Ron receptor tyrosine kinase is overexpressed in approximately half of all human colon cancers. Increased Ron expression positively correlates with tumor progression, and reduction of Ron levels in human colon adenocarcinoma cells reverses their tumorigenic properties. Nearly all colon tumors demonstrate loss of the adenomatous polyposis coli (APC) tumor suppressor, an early initiating event, subsequently leading to beta-catenin stabilization. To understand the role of Ron in early stage intestinal tumorigenesis, we generated Apc-mutant (Apc(Min/+)) mice with and without Ron signaling. Interestingly, we report here that significantly more Apc(Min/+) Ron-deficient mice developed higher tumor burden than Apc(Min/+) mice with wild-type Ron. Even though baseline levels of intestinal crypt proliferation were increased in the Apc(Min/+) Ron-deficient mice, loss of Ron did not influence tumor size or histological appearance of the Apc(Min/+) adenomas, nor was beta-catenin localization changed compared to Apc(Min/+) mice with Ron. Together, these data suggest that Ron may be important in normal intestinal tissue homeostasis, but that the expression of this receptor is not required for the formation and growth of adenomas in Apc(Min/+) mice.

Prognostic role of the recepteur d'origine nantais (RON) expression in ovarian cancer patients

OBJECTIVE

The aim of the study was to investigate the potential clinical relevance of immunohistochemically assessed RON expression in a large, single institution series of primary untreated advanced ovarian cancer patients.

METHODS

Immunohistochemical analysis was performed by using the polyclonal rabbit anti-RON-beta antibody (C-20, clone sc-322, Santa Cruz, California). Results were expressed as the total proportion of immunostained tumor cells (RON positivity), or the percentage of cells showing strong staining of RON expression (H-RON positivity).

RESULTS

In the overall series RON positive immunoreaction was observed in 103/141 cases, while H-Ron positivity was detected in 577141 (40.4%) cases. No association between RON and H-RON expression with response to first-line treatment was documented. During the follow up period, progression and death of disease were observed in 111 (78.7%) and 76 (53.9%) cases, respectively. Cases with strong H-RON expression has a shorter overall survival (median=35 months) than cases with low RON levels (median=59 months) (X(2)=-2.1, p value=0.032). In multivariate analysis, only platinum resistance, and extent of residual tumor retained an independent negative prognostic role for OS, with the percentages of H-RON positively immunostained cells showing a borderline statistical significance (p value=0.0643). The unfavourable role of elevated percentages of H-RON expression was maintained only in the subgroup of platinum resistant recurrent ovarian cancer patients (X(2)=3.89, p value=0.048) compared to the platinum sensitive ones (X(2)=1.98, p value=0.16).

CONCLUSIONS

The assessment of RON expression deserves further attention as a parameter helpful to identify poor prognosis ovarian cancer patients potentially candidates to investigational agents.

Ron-receptor tyrosine kinase in tumorigenesis and metastasis

The Ron-receptor tyrosine kinase has been increasingly recognized for its tumorigenic potential in the last decade. Ron-receptor activation leads to the activation of common receptor tyrosine kinase downstream-signaling pathways, and most prominently in tumor models, activation of MAPK, PI3K and beta-catenin. Numerous experimental models of mammalian tumorigenesis have demonstrated that increased Ron-receptor activity correlates with increased tumorigenesis in a variety of organs of epithelial origin. The evidence for Ron as an oncogene in human tumor biology is growing. The Ron receptor is overexpressed and over activated in a large number of human tumors, and overexpression of Ron correlates with a worse clinical outcome for patients in at least two human cancer states, namely breast and bladder cancer. Several experimental approaches have been demonstrated to successfully block Ron activity and function, and given these convincing data, approaches to block Ron-receptor activity in targeted human cancers should prove to be fruitful in the setting of future clinical research trials.

Regulation of RON tyrosine kinase-mediated invasion of breast cancer cells

RON (recepteur d'origine nantais), a tyrosine kinase receptor for macrophage-stimulating protein (MSP) was implicated in tumor progression. However, it was not investigated how this important oncogene is regulated. We show that MSP promotes invasion of MDA MB 231 and MDA MB 468 but not MCF-7 breast cancer cells. Reverse transcription-PCR and Western analysis indicated the expression of RON message and protein, respectively, in MDA MB 231 and MDA MB 468 cells but not in MCF-7 cells. RON expression correlated with Sp1 expression. Initial analysis of a 1.2-kb and 400-bp RON promoter in MDA MB 231 and MDA MB 468 cells suggested the presence of all the necessary regulatory elements within 400 bp from the transcription start site. Site-directed mutagenesis of the 400-bp RON promoter revealed that the overlapping Sp1 sites at-94 (Sp1-3/4) and Sp1 site at -113 (Sp1-5) are essential for RON gene transcription. Electrophoretic mobility shift assays and chromatin immunoprecipitation analysis indicated that Sp1 binding to these sites is required for RON promoter activity. Ectopic Sp1 expression in Sp1 null SL2 cells confirmed the involvement of these Sp1 sites in the regulation of oncogenic RON tyrosine kinase. Treatment of MDA MB 231 cells with mithramycin A, an inhibitor of Sp1 binding, or siRNA knock-down of Sp1 blocked RON gene expression and MSP-mediated invasion of MDA MB 231 cells. This is the first report demonstrating a clear link between Sp1-dependent RON tyrosine kinase expression and invasion of breast carcinoma cells.