Recepteur d'origine nantais tyrosine kinase is a direct target of hypoxia-inducible factor-1alpha-mediated invasion of breast carcinoma cells

Small-molecule agents for cancer immunotherapy

Cancer immunotherapy, exemplified by the remarkable clinical benefits of the immune checkpoint blockade and chimeric antigen receptor T-cell therapy, is revolutionizing cancer therapy. They induce long-term tumor regression and overall survival benefit in many types of cancer. With the advances in our knowledge about the tumor immune microenvironment, remarkable progress has been made in the development of small-molecule drugs for immunotherapy. Small molecules targeting PRR-associated pathways, immune checkpoints, oncogenic signaling, metabolic pathways, cytokine/chemokine signaling, and immune-related kinases have been extensively investigated. Monotherapy of small-molecule immunotherapeutic drugs and their combinations with other antitumor modalities are under active clinical investigations to overcome immune tolerance and circumvent immune checkpoint inhibitor resistance. Here, we review the latest development of small-molecule agents for cancer immunotherapy by targeting defined pathways and highlighting their progress in recent clinical investigations.

A potential signaling axis between RON kinase receptor and hypoxia-inducible factor-1 alpha in pancreatic cancer

The Cancer Genome Atlas (TCGA) of a pancreatic cancer cohort identified high MST1R (RON tyrosine kinase receptor) expression correlated with poor prognosis in human pancreatic cancer. RON expression is null/minimal in normal pancreas but elevates from pan-in lesions through invasive carcinomas. We report using multiple approaches RON directly regulates HIF-1α, a critical driver of genes involved in cancer cell invasion and metastasis. RON and HIF-1α are highly co-expressed in the 101 human PDAC tumors analyzed and RON expression correlated with HIF-1α expression in a subset of PDAC cell lines. knockdown of RON expression in RON positive cells blocked HIF-1α expression, whereas ectopic RON expression in RON null cells induced HIF-1α expression suggesting the direct regulation of HIF-1α by RON kinase receptor. RON regulates HIF-1α through an unreported transcriptional mechanism involving PI3 kinase-mediated AKT phosphorylation and Sp1-dependent HIF-1α promoter activity leading to increased HIF-1α mRNA expression. RON/HIF-1α modulation altered the invasive behavior of PDAC cells. A small-molecule RON kinase inhibitor decreased RON ligand, MSP-induced HIF-1α expression, and invasion of PDAC cells. Immunohistochemical analysis on RON knockdown orthotopic PDAC tumor xenograft confirmed that RON inhibition significantly blocked HIF-1α expression. RON/HIF-1α co-expression also exists in triple-negative breast cancer cells, a tumor type that also lacks molecular therapeutic targets. This is the first report describing RON/HIF-1α axis in any tumor type and is a potential novel therapeutic target.

Evidence for 2-Methoxyestradiol-Mediated Inhibition of Receptor Tyrosine Kinase RON in the Management of Prostate Cancer

2-Methoxyestradiol (2-ME₂) possesses anti-tumorigenic activities in multiple tumor models with acceptable tolerability profile in humans. Incomplete understanding of the mechanism has hindered its development as an anti-tumorigenic compound. We have identified for the first-time macrophage stimulatory protein 1 receptor (MST1R) as a potential target of 2-ME₂ in prostate cancer cells. Human tissue validation studies show that MST1R (a.k.a RON) protein levels are significantly elevated in prostate cancer tissues compared to adjacent normal/benign glands. Serum levels of macrophage stimulatory protein (MSP), a ligand for RON, is not only associated with the risk of disease recurrence, but also significantly elevated in samples from African American patients. 2-ME₂ treatment inhibited mechanical properties such as adhesion and elasticity that are associated with epithelial mesenchymal transition by downregulating mRNA expression and protein levels of MST1R in prostate cancer cell lines. Intervention with 2-ME₂ significantly reduced tumor burden in mice. Notably, global metabolomic profiling studies identified significantly higher circulating levels of bile acids in castrated animals that were decreased with 2-ME₂ intervention. In summary, findings presented in this manuscript identified MSP as a potential marker for predicting biochemical recurrence and suggest repurposing 2-ME₂ to target RON signaling may be a potential therapeutic modality for prostate cancer.

Multi-kinase targeted therapy as a promising treatment strategy for ovarian tumors expressing sfRon receptor

The sfRon kinase is an important therapeutic target in ovarian cancer that contributes to prominent tumor growth and disease progression. We reasoned that a multi-kinase inhibition of sfRon pathway might be an effective strategy to achieve a sustained anti-tumor response, while simultaneously preventing treatment resistance. We performed a detailed dissection of sfRon signaling in vitro and demonstrated that S6K1 is a key component of a multi-kinase targeting strategy in sfRon expressing ovarian tumors. We selected AD80 compound that targets several kinases within sfRon pathway including AKT and S6K1, and compared its efficacy with inhibitors that selectively target either sfRon or PI3 kinase. Using human ovarian xenografts and clinically relevant patient-derived xenografts (PDXs), we demonstrated that in vivo treatment with single agent AD80 shows superior efficacy to a standard-care chemotherapy (cisplatin/paclitaxel), or to the direct inhibition of sfRon kinase by BMS777607. Our findings indicate that ovarian tumors expressing sfRon are most effectively treated with multi-kinase inhibitors simultaneously targeting AKT and S6K1, such as AD80, which results in long-term anti-tumor response and prevents metastasis development.

Hypoxia-induced alternative splicing: the 11th Hallmark of Cancer

Hypoxia-induced alternative splicing is a potent driving force in tumour pathogenesis and progression. In this review, we update currents concepts of hypoxia-induced alternative splicing and how it influences tumour biology. Following brief descriptions of tumour-associated hypoxia and the pre-mRNA splicing process, we review the many ways hypoxia regulates alternative splicing and how hypoxia-induced alternative splicing impacts each individual hallmark of cancer. Hypoxia-induced alternative splicing integrates chemical and cellular tumour microenvironments, underpins continuous adaptation of the tumour cellular microenvironment responsible for metastatic progression and plays clear roles in oncogene activation and autonomous tumour growth, tumor suppressor inactivation, tumour cell immortalization, angiogenesis, tumour cell evasion of programmed cell death and the anti-tumour immune response, a tumour-promoting inflammatory response, adaptive metabolic re-programming, epithelial to mesenchymal transition, invasion and genetic instability, all of which combine to promote metastatic disease. The impressive number of hypoxia-induced alternative spliced protein isoforms that characterize tumour progression, classifies hypoxia-induced alternative splicing as the 11th hallmark of cancer, and offers a fertile source of potential diagnostic/prognostic markers and therapeutic targets.

Discovery of Cell-Surface Vimentin (CSV) as a Sarcoma Target and Development of CSV-Targeted IL12 Immune Therapy

This chapter discusses a novel target of osteosarcoma (OS), cell-surface vimentin (CSV), and a novel generation of interleukin-12 (IL12), CSV-targeted IL12, for treating OS tumor metastasis. Vimentin is a known intracellular structural protein for mesenchymal cells but is also documented in tumor cells. Our recent study definitively revealed that vimentin can be translocated to the surface of very aggressive tumor cells, such as metastatic cells. This CSV property allows investigators to capture circulating tumor cells (CTCs) across any type of tumor, including OS. CTCs are known as the seeds of metastasis; therefore, targeting these cells using CSV is a logical approach for use in a metastatic OS setting. Interestingly, we found that the peptide VNTANST can bind to CSV when fused to the p40 subunit encoding the DNA of IL12. Systemic delivery of this CSV-targeted IL12 immune therapy inhibited OS metastasis and relapse in a mouse tumor model as detailed in this chapter. This CSV-targeted delivery of IL12 also reduced toxicity of IL12. In summary, this chapter details a novel approach for safe IL12 immune therapy via targeting CSV.

Antibody-drug conjugates targeting RON receptor tyrosine kinase as a novel strategy for treatment of triple-negative breast cancer

Treatment of triple-negative breast cancer (TNBC) is a challenge to oncologists. Currently, the lack of effective therapy has fostered a major effort to discover new targets and therapeutics to combat this disease. The recepteur d'origine nantais (RON) receptor has been implicated in the pathogenesis of TNBC. Clinical studies have revealed that aberrant RON expression is crucial in regulating TNBC malignant phenotypes. Increased RON expression also has prognostic value for breast cancer progress. These features provide the rationale to target RON for TNBC treatment. In this review, we discuss the importance of RON in TNBC tumorigenesis and the development of anti-RON antibody-drug conjugates (ADCs) for clinical application. The findings from preclinical studies lay the foundation for clinical trials of this novel biotherapeutic for TNBC therapy.

RON Receptor Tyrosine Kinase as a Therapeutic Target for Eradication of Triple-Negative Breast Cancer: Efficacy of Anti-RON ADC Zt/g4-MMAE

Triple-negative breast cancer (TNBC) is a highly diverse group of malignant neoplasia with poor outcome. Currently, the lack of effective therapy has fostered a major effort to discover new targets to treat this malignant cancer. Here we identified the RON receptor tyrosine kinase as a therapeutic target for potential TNBC treatment. We analyzed RON expression in 168 primary TNBC samples via tissue microarray using anti-RON IHC staining and demonstrated that RON was widely expressed in 76.8% TNBC samples with overexpression in 76 cases (45.2%). These results provide the molecular basis to target RON for TNBC therapy. To this end, anti-RON monoclonal antibody Zt/g4-drug monomethyl auristatin E conjugate (Zt/g4-MMAE) was developed with a drug to antibody ratio of 3.29 and tested in a panel of TNBC cell lines with different phenotypes. In vitro, Zt/g4-MMAE rapidly induced RON internalization, resulted in cell-cycle arrest followed by massive cell death. The calculated IC₅₀ values ranged from 0.06 to 3.46 μg/mL dependent on individual TNBC cell lines tested. Zt/g4-MMAE also effectively killed TNBC stem-like cells with RON⁺/CD44⁺/CD24^- phenotypes and RON-negative TNBC cells through the bystander effect. In vivo, Zt/g4-MMAE at 10 mg/kg in a Q12 × 2 regimen completely eradicated TNBC xenografts without the regrowth of xenograft tumors. In conclusion, increased RON expression is a pathogenic feature in primary TNBC samples. Zt/g4-MMAE is highly effective in eradicating TNBC xenografts in preclinical models. These findings lay the foundation for using anti-RON Zt/g4-MMAE in clinical trials as a novel strategy for TNBC treatment.

Patent Highlights February-March 2018

A snapshot of noteworthy recent developments in the patent literature of relevance to pharmaceutical and medical research development.

Crosstalk between RON and androgen receptor signaling in the development of castration resistant prostate cancer

Castrate-resistant prostate cancer (CRPC) is the fatal form of prostate cancer. Although reactivation of androgen receptor (AR) occurs following androgen deprivation, the precise mechanism involved is unclear. Here we show that the receptor tyrosine kinase, RON alters mechanical properties of cells to influence epithelial to mesenchymal transition and functions as a transcription factor to differentially regulate AR signaling. RON inhibits AR activation and subset of AR-regulated transcripts in androgen responsive LNCaP cells. However in C4-2B, a castrate-resistant sub-line of LNCaP and AR-negative androgen independent DU145 cells, RON activates subset of AR-regulated transcripts. Expression of AR in PC-3 cells leads to activation of RON under androgen deprivation but not under androgen proficient conditions implicating a role for RON in androgen independence. Consistently, RON expression is significantly elevated in castrate resistant prostate tumors. Taken together our results suggest that RON activation could aid in promoting androgen independence and that inhibition of RON in combination with AR antagonist(s) merits serious consideration as a therapeutic option during hormone deprivation therapy.

Effects of PTCs on nonsense-mediated mRNA decay are dependent on PTC location

The récepteur d'origine nantais (RON) gene is a proto-oncogene that is responsible for encoding the human macrophage-stimulating protein (MSP) 1 receptor. MSP activation induces RON-mediated cell dissociation, migration and matrix invasion. Isoforms of RON that exclude exons 5 and 6 encode the RONΔ160 protein, which promotes cell transformation in vitro and tumor metastasis in vivo. Premature termination codons (PTCs) in exons activate the nonsense-mediated mRNA decay (NMD) signaling pathway. The present study demonstrated that PTCs at various locations in the alternative exons 5 and 6 could induce NMD of the majority of the spliced, or partially spliced, isoforms. However, the isoforms that excluded exon 6 or exons 5 and 6 were markedly increased when produced from mutated minigenes with inserted PTCs. Furthermore, the unspliced isoform of intron 5 was not observed to be decreased by the presence of PTCs. Notably, these effects may be dependent on the location of the PTCs. The current study demonstrated a novel mechanism underlying the regulation of NMD in alternative splicing.

Unraveling the Anticancer Effect of Curcumin and Resveratrol

Resveratrol and curcumin are natural products with important therapeutic properties useful to treat several human diseases, including cancer. In the last years, the number of studies describing the effect of both polyphenols against cancer has increased; however, the mechanism of action in all of those cases is not completely comprehended. The unspecific effect and the ability to interfere in assays by both polyphenols make this challenge even more difficult. Herein, we analyzed the anticancer activity of resveratrol and curcumin reported in the literature in the last 11 years, in order to unravel the molecular mechanism of action of both compounds. Molecular targets and cellular pathways will be described. Furthermore, we also discussed the ability of these natural products act as chemopreventive and its use in association with other anticancer drugs.

Prognostic significance of MST1R dysregulation in renal cell tumors

Macrophage stimulating 1 receptor (MST1R) is a C-MET proto-oncogene family receptor tyrosine kinase. Promoter methylation patterns determine transcription of MST1R variants as hypermethylation of a region upstream of transcription start site (TSS) is associated with lack of MST1R long transcript (MST1R long) and expression of a short transcript with oncogenic potential. Thus, we aimed to investigate MST1R variant transcript regulation in renal cell tumors (RCT) and assess their prognostic potential. We found, in a series of 120 RCT comprising the four main subtypes (clear cell, papillary and chromophobe renal cell carcinoma, and oncocytoma), that higher methylation levels close to TSS were associated with total MST1R expression levels (MST1R total) in primary tumors (p=0.049) and renal cancer cell lines. After demethylating treatment, MST1R long/MST1R total ratio increased, as expected, in two renal cell carcinoma cell lines tested. However, in primary tumors with hypermethylation upstream of TSS, a decrease in MST1R long/MST1R total ratio was not detected, although higher expression ratio of nuclear factor-κB was apparent. Furthermore, survival analysis demonstrated that MST1R long/MST1R total ratio was independently associated with shorter disease-specific and disease-free survival, whereas MST1R total expression associated with shorter disease-specific survival. In conclusion, although promoter methylation patterns seem to determine MST1R global transcription regulation in renal cell carcinoma, other mechanisms might contribute to deregulate MST1R variant expression in RCT. Nevertheless, MST1R total expression and MST1R long/MST1R total ratio modulate the biological and clinical aggressiveness of renal cell carcinoma, as depicted by its prognostic significance, a finding that requires validation in a larger independent series.

Identification of Regulatory-RNAs for Alternative Splicing of Ron Proto-Oncogene

RON receptor tyrosine kinase is a proto-oncogene that induces cell migration and matrix invasion. RONΔ160 protein, which is produced by exclusion of exon 5 and 6, promotes cell migration, matrix invasion and protection from apoptosis. Alternative splicing regulation of exon 5 and 6 is not well understood. In this manuscript, we identified several new RNA regulatory elements for alternative splicing of Ron proto-oncogene. Firstly, we demonstrated that RNA sequences from EcoRI cleavage sites regulate alternative splicing of Ron exon 5 and 6. Secondly, we showed that the ~30 nt RNA at upstream end of exon 4 and the ~33 nt RNA at downstream end of exon 7 also modulate splicing of exon 5 and 6. Thirdly, our results indicate that the RNA sequences of the ends in exon 4 and 7 are required for the regulatory functions of the RNA from restriction enzyme cleavage sites. Our results provide a new insight for regulation of alternative splicing of Ron proto-oncogene.

Progress toward overcoming hypoxia-induced resistance to solid tumor therapy

Hypoxic tumors are associated with poor clinical outcome for multiple types of human cancer. This may be due, in part, to hypoxic cancer cells being resistant to anticancer therapy, including radiation therapy, chemotherapy, and targeted therapy. Hypoxia inducible factor 1, a major regulator of cellular response to hypoxia, regulates the expression of genes that are involved in multiple aspects of cancer biology, including cell survival, proliferation, metabolism, invasion, and angiogenesis. Here, we review multiple pathways regulated by hypoxia/hypoxia inducible factor 1 in cancer cells and discuss the latest advancements in overcoming hypoxia-mediated tumor resistance.

Recepteur d'origine nantais (RON), more than a kinase: Role in castrate-resistant prostate cancer

Prostate cancer (PCA) is the second leading cause of cancer-related deaths in men in the United States. It is natural for a hormone-driven malignancy such as prostate cancer that androgen deprivation therapy (ADT) would be the preferred treatment for clinical disease management. However, after initial treatment response a vast majority of patients develop metastatic castrate-resistant prostate cancer (CRPC), which is fatal. While great headway has been made to understand the possible mechanisms that drive castrate-resistant disease, a bonafide cure remains elusive. Reactivation of androgen receptor (AR) signaling partly contributes to the emergence of CRPC. Here we briefly examine some of the known mechanisms of AR reactivation including intratumoral synthesis of androgens, modulation of AR coregulators, and AR variants with constitutive activity as well as activation of receptor tyrosine kinases. We primarily focus on the emerging dual function of the receptor tyrosine kinase (recepteur d'origine nantais; RON) as a traditional tyrosine kinase and transcription factor. We further discuss activation of RON as an alternate mechanism in the development of CRPC and available therapeutic approaches for clinical management of CRPC by combined inhibition of RON and AR.

Chrysin inhibits cell invasion by inhibition of Recepteur d'origine Nantais via suppressing early growth response-1 and NF-κB transcription factor activities in gastric cancer cells

Cell invasion is one of crucial reasons for cancer metastasis and malignancy. Recepteur d'origine Nantais (RON) has been reported to play an important role in the cancer cell invasion process. High accumulation and activation of RON has been implicated in gastric adenocarcinoma AGS cells. Chrysin is a naturally occurring phytochemical, a type of flavonoid, which has been reported to suppress tumor metastasis. However, the effects of chrysin on RON expression in gastric cancer are not well studied. In the present study, we examined whether chrysin affects RON expression in gastric cancer, and if so, its underlying mechanism. We examined the effect of chrysin on RON expression and activity, via RT-PCR, promoter study, and western blotting in human gastric cancer AGS cells. Chrysin significantly inhibited endogenous and inducible RON expression in a dose-dependent manner. After demonstrating that Egr-1 and NF-κB are the critically required transcription factors for RON expression, we discovered that chrysin suppressed Egr-1 and NF-κB transcription factor activities. Additionally, the phorbol-12-myristate-13-acetate- (PMA) induced cell invasion was partially abrogated by chrysin and an RON antibody. Our results suggest that chrysin has anticancer effects at least by suppressing RON expression through blocking Egr-1 and NF-κB in gastric cancer AGS cells.

Hypoxia promotes nuclear translocation and transcriptional function in the oncogenic tyrosine kinase RON

Tumor hypoxia drives metastatic progression, drug resistance, and posttreatment relapses, but how cancer cells adapt and evolve in response to hypoxic stress is not well understood. In this study, we address this question with the discovery that the receptor tyrosine kinase RON translocates into the nucleus of hypoxic cancer cells. In response to hypoxia, nuclear RON interacts with the hypoxia-inducible factor HIF-1α in a manner that relies on RON tyrosine kinase activity, binding to the c-JUN promoter and activating it. Mechanistic investigations revealed unexpectedly that nuclear RON played a more important role in activation of the c-JUN promoter than HIF-1α, leading to increased cell proliferation, survival adaptation, in vitro migration, and tumorigenicity under hypoxic conditions. Taken together, our results pointed to a novel function for RON as a transcriptional regulator that promotes the survival of cancer cells subjected to hypoxia. These results suggest novel implications for the use of small-molecule inhibitors or monoclonal antibodies targeting the RON kinase in the prevention or treatment of advanced cancer.

Pathogenesis of RON receptor tyrosine kinase in cancer cells: activation mechanism, functional crosstalk, and signaling addiction

The RON receptor tyrosine kinase, a member of the MET proto-oncogene family, is a pathogenic factor implicated in tumor malignancy. Specifically, aberrations in RON signaling result in increased cancer cell growth, survival, invasion, angiogenesis, and drug resistance. Biochemical events such as ligand binding, receptor overexpression, generation of structure-defected variants, and point mutations in the kinase domain contribute to RON signaling activation. Recently, functional crosstalk between RON and signaling proteins such as MET and EFGR has emerged as an additional mechanism for RON activation, which is critical for tumorigenic development. The RON signaling crosstalk acts either as a regulatory feedback loop that strengthens or enhances tumorigenic phenotype of cancer cells or serves as a signaling compensatory pathway providing a growth/survival advantage for cancer cells to escape targeted therapy. Moreover, viral oncoproteins derived from Friend leukemia or Epstein-Barr viruses interact with RON to drive viral oncogenesis. In cancer cells, RON signaling is integrated into cellular signaling network essential for cancer cell growth and survival. These activities provide the molecular basis of targeting RON for cancer treatment. In this review, we will discuss recent data that uncover the mechanisms of RON activation in cancer cells, review evidence of RON signaling crosstalk relevant to cancer malignancy, and emphasize the significance of the RON signaling addiction by cancer cells for tumor therapy. Understanding aberrant RON signaling will not only provide insight into the mechanisms of tumor pathogenesis, but also lead to the development of novel strategies for molecularly targeted cancer treatment.

MSP-RON signalling in cancer: pathogenesis and therapeutic potential

Since the discovery of MSP (macrophage-stimulating protein; also known as MST1 and hepatocyte growth factor-like (HGFL)) as the ligand for the receptor tyrosine kinase RON (also known as MST1R) in the early 1990s, the roles of this signalling axis in cancer pathogenesis has been extensively studied in various model systems. Both in vitro and in vivo evidence has revealed that MSP-RON signalling is important for the invasive growth of different types of cancers. Currently, small-molecule inhibitors and antibodies blocking RON signalling are under investigation. Substantial responses have been achieved in human tumour xenograft models, laying the foundation for clinical validation. In this Review, we discuss recent advances that demonstrate the importance of MSP-RON signalling in cancer and its potential as a therapeutic target.

Protein-bound polysaccharide decreases invasiveness and proliferation in pancreatic cancer by inhibition of hedgehog signaling and HIF-1α pathways under hypoxia

To develop an effective therapeutic approach to pancreatic ductal adenocarcinoma (PDAC), we focused on the antitumor mechanism of protein-bound polysaccharide (PSK) under hypoxia. PSK decreased proliferation in PDAC cells under hypoxia but not normoxia. PSK also showed anti-tumor effects in vivo, inhibited invasiveness of PDAC cells, and decreased the expression of HIF-1α and hedgehog (Hh) signaling-related molecules under hypoxia. Inhibition of HIF-1α and Hh signaling reduced proliferation and invasiveness in PDAC cells under hypoxia. In conclusion, we found new PSK-related pathways in invasiveness and proliferation in PDAC under hypoxia. PSK may be a promising therapeutic drug to treat refractory PDAC.

EGCG inhibits recepteur d'origine nantais expression by suppressing Egr-1 in gastric cancer cells

Abnormal accumulation and activation of the recepteur d'origine nantais (RON) has been implicated in epithelial tumor carcinogenesis. In the present study, we examined the effect of epigallocatechin-3-gallate (EGCG), the major green tea catechin, on the induction of RON and tumor growth in human gastric cancer. EGCG inhibited phorbol 12-myristate 13-acetate (PMA)-induced RON expression and reduced RON transcriptional activity. However, (-)-epigalloca-techin (EGC), (-)-epicatechin gallate (ECG) and (-)‑epicatechin (EC) did not affect RON expression. Experiments with deleted and site-directed mutagenesis of the RON promoter indicated that Egr-1 binding sites in the RON promoter may be the EGCG‑response element acting as a cis-element in gastric cancer cells. EGCG also inhibited PMA-induced Egr-1 expression and DNA binding in a dose-dependent manner. Furthermore, gastric cancer cells pretreated with PMA showed markedly enhanced invasiveness, which was partially abrogated by EGCG and siRNA-targeted RON and Egr-1. EGCG significantly reduced tumor growth in an in vivo tumor model, whereas RON expression was downregulated. These results suggest that EGCG may exert at least part of its anticancer effect by controlling RON expression through suppression of Egr-1 activation.

Hypoxia-regulated target genes implicated in tumor metastasis

Hypoxia is an important microenvironmental factor that induces cancer metastasis. Hypoxia/hypoxia-inducible factor-1α (HIF-1α) regulates many important steps of the metastatic processes, especially epithelial-mesenchymal transition (EMT) that is one of the crucial mechanisms to cause early stage of tumor metastasis. To have a better understanding of the mechanism of hypoxia-regulated metastasis, various hypoxia/HIF-1α-regulated target genes are categorized into different classes including transcription factors, histone modifiers, enzymes, receptors, kinases, small GTPases, transporters, adhesion molecules, surface molecules, membrane proteins, and microRNAs. Different roles of these target genes are described with regards to their relationship to hypoxia-induced metastasis. We hope that this review will provide a framework for further exploration of hypoxia/HIF-1α-regulated target genes and a comprehensive view of the metastatic picture induced by hypoxia.

Role of urokinase plasminogen activator receptor in gastric cancer: a potential therapeutic target

Recent advancements in cancer research have led to major breakthroughs; however, the impact on overall cancer-related death rate remains unacceptable. Thus, further insights into tumor markers and subsequent development of targeted therapies are urgently needed. For decades the urokinase plasminogen activator (uPA) system has been thought to drive tumor progression by mediating directed extracellular proteolysis on the surface of migrating or invading cells. Intervention with this proteolysis by targeting of uPA receptor (uPAR) has been proposed to represent a novel approach for inhibiting tumor progression. Recent data have provided new insights into the role of uPAR in gastric cancer progression. In addition to mediating proteolysis, this receptor also appears to mediate cell signaling, proliferation, and survival, and these observations have revealed novel ways to target uPAR. In this review, we discuss uPAR expression in gastric cancer, the relationship between uPAR and Helicobacter pylori, and recent insights into uPAR-signaling mechanisms. The role of uPAR as a cancer target in gastric cancer is also summarized.

Upregulation of recepteur d'origine nantais tyrosine kinase and cell invasiveness via early growth response-1 in gastric cancer cells

Abnormal accumulation and activation of the recepteur d'origine nantais (RON) has been implicated in carcinogenesis of epithelial tumors. RON expression was induced by the tumor promoter, phorbol 12-myristate 13-acetate (PMA), in gastric adenocarcinoma AGS cells. Studies with deleted and site-directed mutagenesis of Egr-1 promoter and with expression vectors encoding Egr-1 confirmed that Egr-1 is essential for RON expression. In addition, AGS cells pretreated with PMA showed remarkably enhanced invasiveness, which was partially abrogated by siRNA-targeted RON and Egr-1. These results suggest that tumor promoter induces RON expression via Egr-1, which, in turn, stimulates cell invasiveness in AGS cells.

Novel function of transcription factor Nrf2 as an inhibitor of RON tyrosine kinase receptor-mediated cancer cell invasion

Recepteur d' origine nantais (RON), a tyrosine kinase receptor, is aberrantly expressed in human tumors and promotes cancer cell invasion. RON receptor activation is also associated with resistance to tamoxifen treatment in breast cancer cells. Nrf2 is a positive regulator of cytoprotective genes. Using chromatin immunoprecipitation (ChIP) and site-directed mutagenesis studies of the RON promoter, we identified Nrf2 as a negative regulator of RON gene expression. High Nrf2 and low RON expression was observed in normal mammary tissue whereas high RON and low or undetectable expression of Nrf2 was observed in breast tumors. The Nrf2 inducer sulforaphane (SFN) as well as ectopic Nrf2 expression or knock-down of the Nrf2 negative regulator keap1, which stabilizes Nrf2, inhibited RON expression and invasion of carcinoma cells. Consequently, our studies identified a novel functional role for Nrf2 as a "repressor" and RON kinase as a molecular target of SFN, which mediates the anti-tumor effects of SFN. These results are not limited to breast cancer cells since the Nrf2 inducer SFN stabilized Nrf2 and inhibited RON expression in carcinoma cells from various tumor types.

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.

MSP-induced RON activation upregulates uPAR expression and cell invasiveness via MAPK, AP-1 and NF-κB signals in gastric cancer cells

Overexpression of recepteur d'Origine nantais (RON) and urokinase plasminogen activator receptor (uPAR) have been observed in human gastric cancers. However, the interaction between RON and uPAR in gastric cancer is unclear. The present study investigated the effect of macrophage-stimulating protein (MSP, the RON ligand) on uPAR expression and the underlying signal pathways in human gastric cancer AGS cells. uPAR messenger RNA expression was induced by MSP in a time- and concentration-dependent manner. MSP also induced uPAR promoter activity. The introduction of RON-specific small interfering RNA (siRNA) significantly affected the MSP-induced uPAR transcription. Deleted and site-directed mutagenesis studies demonstrated the involvement of the binding sites of transcription factor nuclear factor-kappaB (NF-κB) and activator protein (AP)-1 in the MSP-induced uPAR expression. Studies with expression vectors encoding mutated-type NF-κB signaling molecules and AP-1 decoy confirmed that NF-κB and AP-1 were essential for the MSP-induced uPAR expression. In addition, MSP induced the activation of extracellular signal-regulated kinase-1/2 (Erk-1/2), c-Jun amino terminal kinase (JNK) and p38 mitogen-activated protein kinase (MAPK). Dominant-negative mutants (K97M and TAM67) and specific inhibitors of Erk-1/2 and JNK were able to suppress the MSP-induced uPAR expression. AGS cells pretreated with MSP showed a remarkably enhanced invasiveness, which was partially abrogated by siRNA-targeted RON and uPAR-neutralizing antibodies. The above results suggest that MSP induces uPAR expression via MAPK, AP-1 and NF-κB signaling pathways and, in turn, stimulates cell invasiveness in human gastric cancer AGS cells.

Targeting acute hypoxic cancer cells by doxorubicin-immunoliposomes directed by monoclonal antibodies specific to RON receptor tyrosine kinase

PURPOSE

Hypoxia contributes to acquired drug resistance in various cancer cells. The underlying mechanism is cellular insensitivity regulated by hypoxia-inducible factors (HIF), which impairs drug uptake, transport, and metabolism. The current study determines anti-RON antibody-directed cytotoxicity of doxorubicin (Dox)-immunoliposomes (IL) in hypoxic colon cancer cells.

METHODS

Cells were cultured under hypoxia (1% O(2), 5% CO(2), and 96% N(2)) for 24 h. Dox-loaded IL were formulated followed by post-insertion of monoclonal antibody Zt/g4 specific to RON. Western blotting was used to detect HIF-1α and RON expression. Cellular uptake of Zt/g4-conjugated IL was determined by confocal and internalization assays. Cell viability was assessed by the MTT assay.

RESULTS

RON and HIF-1α expression were observed in hypoxic colon HCT116 and SW620 cells. Resistance to Dox-induced cytotoxicity was acquired in hypoxic cells with increased IC(50) values. However, acquired resistance was attenuated by Zt/g4-directed Dox-IL, which displays increased cytotoxic activities. IL binding and uptake revealed that hypoxic RON expression is functional, which mediates high levels of Zt/g4-Dox-IL binding and cytoplasmic internalization. Zt/g4-Dox-IL is effective in killing hypoxic HCT116 and SW620 cells with reduced IC(50) values compared to Dox and pegylated-liposomal Dox. These effects were dependent on hypoxic RON expression. HCC1937 cells with diminished RON expression under hypoxia were insensitive to Zt/g4-Dox-IL-induced cytotoxic effect.

CONCLUSIONS

RON expressed by hypoxic colon cancer cells is thus a potential targeting molecule for delivery of chemotherapeutics. The ability of anti-RON mAb to direct Dox-IL cytotoxicity could be developed for attenuating hypoxia-acquired drug resistance in various cancer cells.

An unusual function of RON receptor tyrosine kinase as a transcriptional regulator in cooperation with EGFR in human cancer cells

Homodimerization of RON (MST1R), a receptor tyrosine kinase, usually occurs in cells stimulated by a ligand and leads to the downstream activation of signaling pathways. Here we report that bladder cancer cells, in response to physiological stress, use an alternative mechanism for signaling activation. Time-course studies indicated that RON migrated directly from the membrane to the nucleus of bladder cancer cells in response to serum starvation. Biochemical and genetic studies implied that this nuclear internalization was complexed with epidermal growth factor receptor (EGFR) and required the docking of importins. In vivo analysis confirmed that nuclear RON was present in 38.4% (28/73) of primary bladder tumors. Chromatin immunoprecipitation (ChIP) on microarray analysis further revealed that this internalized complex bound to at least 134 target genes known to participate in three stress-responsive networks: p53, stress-activated protein kinase/c-jun N-terminal kinase and phosphatidylinositol 3-kinase/Akt. These findings suggest that RON, in a complex with EGFR, acts as a transcriptional regulator in response to acute disturbances (e.g. serum starvation) imposed on cancer cells. In an attempt to re-establish homeostasis, these cells bypass regular mechanisms required by ligand stimulation and trigger the RON-directed transcriptional response, which confers a survival advantage.

Hypoxia and metastasis in breast cancer

In this review we summarize the evidence for a role for hypoxic response in the biology of metastasis, with a particular emphasis on the metastasis of breast cancer and the function of the hypoxia inducible factor (HIF).