The short form of RON is expressed in acute myeloid leukemia and sensitizes leukemic cells to cMET inhibitors

The MST1R/RON Tyrosine Kinase in Cancer: Oncogenic Functions and Therapeutic Strategies

Simple Summary

MST1R/RON receptor tyrosine kinase is a highly conserved transmembrane protein present on epithelial cells, macrophages, and recently identified in a T-cell subset. RON activation attenuates inflammation in healthy tissue. Interestingly, it is overexpressed in several epithelial neoplasms with increasing levels of expression associated with worse outcomes. Though the mechanisms involved are still under investigation, RON is involved in carcinogenesis via immune modulation of the immune tumor microenvironment, activation of numerous oncogenic pathways, and is protective under cellular stress. Alternatively, inhibition of RON abrogates tumor progression in both animal and human tissue models. Given this, RON is a targetable protein of great interest for cancer treatment. Here, we review RON’s function in tissue inflammation and cancer progression, and review cancer clinical trials to date that have used agents targeting RON signaling.

Abstract

The MST1R/RON receptor tyrosine kinase is a homologue of the more well-known MET receptor. Like MET, RON orchestrates cell signaling pathways that promote oncogenesis and enable cancer cell survival; however, it has a more unique role in the regulation of inflammation. RON was originally described as a transmembrane receptor expressed on tissue resident macrophages and various epithelial cells. RON is overexpressed in a variety of cancers and its activation modifies multiple signaling pathways with resultant changes in epithelial and immune cells which together modulate oncogenic phenotypes. While several RON isoforms have been identified with differences in structure, activation, and pathway regulation, increased RON expression and/or activation is consistently associated with worse outcomes. Tyrosine kinase inhibitors targeting RON have been developed, making RON an actionable therapeutic target.

RON in hepatobiliary and pancreatic cancers: Pathogenesis and potential therapeutic targets

The receptor protein tyrosine kinase RON belongs to the c-MET proto-oncogene family. Research has shown that RON has a role in cancer pathogenesis, which places RON on the frontline of the development of novel cancer therapeutic strategies. Hepatobiliary and pancreatic (HBP) cancers have a poor prognosis, being reported as having higher rates of cancer-related death. Therefore, to combat these malignant diseases, the mechanism underlying the aberrant expression and signaling of RON in HBP cancer pathogenesis, and the development of RON as a drug target for therapeutic intervention should be investigated. Abnormal RON expression and signaling have been identified in HBP cancers, and also act as tumorigenic determinants for HBP cancer malignant behaviors. In addition, RON is emerging as an important mediator of the clinical prognosis of HBP cancers. Thus, not only is RON significant in HBP cancers, but also RON-targeted therapeutics could be developed to treat these cancers, for example, therapeutic monoclonal antibodies and small-molecule inhibitors. Among them, antibody-drug conjugates have become increasingly popular in current research and their potential as novel anti-cancer biotherapeutics will be determined in future clinical trials.

The Receptor Tyrosine Kinase RON and Its Isoforms as Therapeutic Targets in Ewing Sarcoma

The receptor tyrosine kinase (RTK) RON is linked to an aggressive metastatic phenotype of carcinomas. While gaining interest as a therapeutic target, RON remains unstudied in sarcomas. In Ewing sarcoma, we identified RON among RTKs conferring resistance to insulin-like growth factor-1 receptor (IGF1R) targeting. Therefore, we explored RON in pediatric sarcoma cell lines and an embryonic Tg(kdrl:mCherry) zebrafish model, using an shRNA-based approach. To examine RON–IGF1R crosstalk, we employed the clinical-grade monoclonal antibody IMC-RON8, alone and together with the IGF1R-antibody IMC-A12. RON silencing demonstrated functions in vitro and in vivo, particularly within micrometastatic cellular capacities. Signaling studies revealed a unidirectional IGF1-mediated cross-activation of RON. Yet, IMC-A12 failed to sensitize cells to IMC-RON8, suggesting additional mechanisms of RON activation. Here, RT-PCR revealed that childhood sarcomas express short-form RON, an isoform resistant to antibody-mediated targeting. Interestingly, in contrast to carcinomas, treatment with DNA methyltransferase inhibitor did not diminish but increased short-form RON expression. Thus, this first report supports a role for RON in the metastatic progression of Ewing sarcoma. While principal molecular functions appear transferrable between carcinomas, Ewing sarcoma and possibly more common sarcoma subtypes, RON highlights that specific regulations of cellular networks and isoforms require better understanding to successfully transfer targeting strategies.

Targeting Tyrosine Kinases in Acute Myeloid Leukemia: Why, Who and How?

Acute myeloid leukemia (AML) is a myeloid malignancy carrying a heterogeneous molecular panel of mutations participating in the blockade of differentiation and the increased proliferation of myeloid hematopoietic stem and progenitor cells. The historical "3 + 7" treatment (cytarabine and daunorubicin) is currently challenged by new therapeutic strategies, including drugs depending on the molecular landscape of AML. This panel of mutations makes it possible to combine some of these new treatments with conventional chemotherapy. For example, the FLT3 receptor is overexpressed or mutated in 80% or 30% of AML, respectively. Such anomalies have led to the development of targeted therapies using tyrosine kinase inhibitors (TKIs). In this review, we document the history of TKI targeting, FLT3 and several other tyrosine kinases involved in dysregulated signaling 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.

Characterization of RON protein isoforms in pancreatic cancer: implications for biology and therapeutics

The RON tyrosine kinase receptor is under investigation as a novel target in pancreatic cancer. While RON mutations are uncommon, RON isoforms are produced in cancer cells via a variety of mechanisms. In this study we sought to: 1) characterize RON isoform expression in pancreatic cancer, 2) investigate mechanisms that regulate isoform expression, and 3) determine how various isoforms effect gene expression, oncogenic phenotypes and responses to RON directed therapies. We quantified RON transcripts in human pancreatic cancer and found expression levels 2500 fold that of normal pancreas with RON isoform expression comprising nearly 50% of total transcript. RNA seq studies revealed that the short form (sfRON) and P5P6 isoforms which have ligand independent activity, induce markedly different patterns of gene expression than wild type RON. We found that transcription of RON isoforms is regulated by promoter hypermethylation as the DNA demethylating agent 5-aza-2′-deoxycytidine decreased all RON transcripts in a subset of pancreatic cancer cell lines. The viability of sfRON-expressing HPDE cells was reduced by a RON specific small molecule inhibitor, while a therapeutic monoclonal antibody had no demonstrable effects. In summary, RON isoforms may comprise half of total RON transcript in human pancreatic cancer and their expression is regulated at least in part by promoter hypermethylation. RON isoforms activate distinct patterns of gene expression, have transforming activity and differential responses to RON directed therapies. These findings further our understanding of RON biology in pancreatic cancer and have implications for therapeutic strategies to target RON activity.

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.

Acute myeloid leukaemia

Acute myeloid leukaemia (AML) is a disorder characterized by a clonal proliferation derived from primitive haematopoietic stem cells or progenitor cells. Abnormal differentiation of myeloid cells results in a high level of immature malignant cells and fewer differentiated red blood cells, platelets and white blood cells. The disease occurs at all ages, but predominantly occurs in older people (>60 years of age). AML typically presents with a rapid onset of symptoms that are attributable to bone marrow failure and may be fatal within weeks or months when left untreated. The genomic landscape of AML has been determined and genetic instability is infrequent with a relatively small number of driver mutations. Mutations in genes involved in epigenetic regulation are common and are early events in leukaemogenesis. The subclassification of AML has been dependent on the morphology and cytogenetics of blood and bone marrow cells, but specific mutational analysis is now being incorporated. Improvements in treatment in younger patients over the past 35 years has largely been due to dose escalation and better supportive care. Allogeneic haematopoietic stem cell transplantation may be used to consolidate remission in those patients who are deemed to be at high risk of relapse. A plethora of new agents - including those targeted at specific biochemical pathways and immunotherapeutic approaches - are now in trial based on improved understanding of disease pathophysiology. These advances provide good grounds for optimism, although mortality remains high especially in older patients.

The Hepatocyte Growth Factor (HGF)/Met Axis: A Neglected Target in the Treatment of Chronic Myeloproliferative Neoplasms?

Met is the receptor of hepatocyte growth factor (HGF), a cytoprotective cytokine. Disturbing the equilibrium between Met and its ligand may lead to inappropriate cell survival, accumulation of genetic abnormalities and eventually, malignancy. Abnormal activation of the HGF/Met axis is established in solid tumours and in chronic haematological malignancies, including myeloma, acute myeloid leukaemia, chronic myelogenous leukaemia (CML), and myeloproliferative neoplasms (MPNs). The molecular mechanisms potentially responsible for the abnormal activation of HGF/Met pathways are described and discussed. Importantly, inCML and in MPNs, the production of HGF is independent of Bcr-Abl and JAK2V617F, the main molecular markers of these diseases. In vitro studies showed that blocking HGF/Met function with neutralizing antibodies or Met inhibitors significantly impairs the growth of JAK2V617F-mutated cells. With personalised medicine and curative treatment in view, blocking activation of HGF/Met could be a useful addition in the treatment of CML and MPNs for those patients with high HGF/MET expression not controlled by current treatments (Bcr-Abl inhibitors in CML; phlebotomy, hydroxurea, JAK inhibitors in MPNs).

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.

Targeting the PI3K/Akt/mTOR signaling pathway in B-precursor acute lymphoblastic leukemia and its therapeutic potential

B-precursor acute lymphoblastic leukemia (B-pre ALL) is a malignant disorder characterized by the abnormal proliferation of B-cell progenitors. The prognosis of B-pre ALL has improved in pediatric patients, but the outcome is much less successful in adults. Constitutive activation of the phosphatidylinositol 3-kinase (PI3K), Akt and the mammalian target of rapamycin (mTOR) (PI3K/Akt/mTOR) network is a feature of B-pre ALL, where it strongly influences cell growth and survival. RAD001, a selective mTORC1 inhibitor, has been shown to be cytotoxic against many types of cancer including hematological malignancies. To investigate whether mTORC1 could represent a target in the therapy of B-pre ALL, we treated cell lines and adult patient primary cells with RAD001. We documented that RAD001 decreased cell viability, induced cell cycle arrest in G0/G1 phase and caused apoptosis in B-pre ALL cell lines. Autophagy was also induced, which was important for the RAD001 cytotoxic effect, as downregulation of Beclin-1 reduced drug cytotoxicity. RAD001 strongly synergized with the novel allosteric Akt inhibitor MK-2206 in both cell lines and patient samples. Similar results were obtained with the combination CCI-779 plus GSK 690693. These findings point out that mTORC1 inhibitors, either as a single agent or in combination with Akt inhibitors, could represent a potential therapeutic innovative strategy in B-pre ALL.

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.

Class I phosphoinositide 3-kinases in normal and pathologic hematopoietic cells

Class I phosphoinositide 3-kinases which produce the D3-phosphoinositide second messenger phosphatidylinositol 3,4,5-trisphosphate in response to membrane receptors activation play a critical role in cell proliferation, survival, metabolism, and motility. These lipid kinases and the phosphatases regulating the level of D3-phosphoinositides have been an intense area of research these last two decades. The class I phosphoinositide 3-kinases signaling is found aberrantly activated in numerous human cancers, including in malignant hemopathies, and are important therapeutic targets for cancer therapy. Haematopoiesis is an ongoing process which generates the distinct blood cell types from a common hematopoietic stem cell through the action of a variety of cytokines. In the human adult hematopoiesis occurs primarily in the bone marrow, and defects in hematopoiesis result in diseases, such as anemia, thrombocytopenia, myeloproliferative syndromes, or leukemia. Here we give a brief overview of the role of class I phosphoinositide 3-kinases in hematopoietic stem cells, in hematopoietic lineage development and in leukemia, particularly in acute myeloid leukemia and summarize the potential therapeutic implications.