Geldanamycins trigger a novel Ron degradative pathway, hampering oncogenic signaling

STUB1/CHIP: New insights in cancer and immunity

The STUB1 gene (STIP1 homology and U-box-containing protein 1), located at 16q13.3, encodes the CHIP (carboxyl terminus of Hsc70-interacting protein), an essential E3 ligase involved in protein quality control. CHIP comprises three domains: an N-terminal tetratricopeptide repeat (TPR) domain, a middle coiled-coil domain, and a C-terminal U-box domain. It functions as a co-chaperone for heat shock protein (HSP) via the TPR domain and as an E3 ligase, ubiquitinating substrates through its U-box domain. Numerous studies suggest that STUB1 plays a crucial role in various physiological process, such as aging, autophagy, and bone remodeling. Moreover, emerging evidence has shown that STUB1 can degrade oncoproteins to exert tumor-suppressive functions, and it has recently emerged as a novel player in tumor immunity. This review provides a comprehensive overview of STUB1's role in cancer, including its clinical significance, impact on tumor progression, dual roles, tumor stem cell-like properties, angiogenesis, drug resistance, and DNA repair. In addition, we explore STUB1's functions in immune cell differentiation and maturation, inflammation, autoimmunity, antiviral immune response, and tumor immunity. Collectively, STUB1 represents a promising and valuable therapeutic target in cancer and immunology.

CHIP-mediated CIB1 ubiquitination regulated epithelial-mesenchymal transition and tumor metastasis in lung adenocarcinoma

CIB1 is a homolog of calmodulin that regulates cell adhesion, migration, and differentiation. It has been considered as an oncogene in many tumor cells; however, its role in lung adenocarcinoma (LAC) has not been studied. In this study, the expression levels of CIB1 in LAC tissues and adjacent normal tissues were examined by immunohistochemistry, and the relationship between CIB1 expression and patient clinicopathological characteristics was analyzed. The effects of CIB1 on epithelial–mesenchymal transition (EMT), migration, and metastasis of LAC cells were determined in vitro and vivo. Proteins interacting with CIB1 were identified using electrospray mass spectrometry (LS-MS), and CHIP was selected in the following assays. Carboxyl-terminus of Hsp70-interacting protein (CHIP) is a ubiquitin E3 ligase. We show that CHIP can degrade CIB1 via promoting polyubiquitination of CIB1 and its subsequent proteasomal degradation. Besides, lysine residue 10 and 65 of CIB1 is the ubiquitinated site of CIB1. Furthermore, CHIP-mediated CIB1 downregulation is critical for the suppression of metastasis and migration of LAC. These results indicated that CHIP-mediated CIB1 ubiquitination could regulate epithelial–mesenchymal and tumor metastasis in LAC.

Number and brightness analysis reveals that NCAM and FGF2 elicit different assembly and dynamics of FGFR1 in live cells

Both fibroblast growth factor-2 (FGF2) and neural cell adhesion molecule (NCAM) trigger FGF receptor 1 (FGFR1) signaling; however, they induce remarkably distinct receptor trafficking and cellular responses. The molecular basis of such a dichotomy and the role of distinct types of ligand-receptor interaction remain elusive. Number of molecules and brightness (N&B) analysis revealed that FGF2 and NCAM promote different FGFR1 assembly and dynamics at the plasma membrane. NCAM stimulation elicits long-lasting cycles of short-lived FGFR1 monomers and multimers, a behavior that might reflect a rapid FGFR1 internalization and recycling. FGF2, instead, induces stable dimerization at the dose that stimulates cell proliferation. Reducing the occupancy of FGFR1 in response to low FGF2 doses causes a switch towards cyclically exposed and unstable receptor dimers, consistently with previously reported biphasic response to FGF2 and with the divergent signaling elicited by different ligand concentrations. Similar instability was observed upon altering the endocytic pathway. Thus, FGF2 and NCAM induce differential FGFR1 clustering at the cell surface, which might account for the distinct intracellular fate of the receptor and, hence, for the different signaling cascades and cellular responses.

Role of Recepteur D'origine Nantais on Gastric Cancer Development and Progression

Recepteur d'origine nantais (RON) is a receptor tyrosine kinase belonging to the subfamily of which c-MET is the prototype. Large epidemiologic studies have confirmed the strong association between RON and gastric cancer development. Constitutive activation of RON signaling directly correlates with tumorigenic phenotypes of gastric cancer and a poor survival rate in advanced gastric cancer patients. In this review, we focus on recent evidence of the aberrant expression and activation of RON in gastric cancer tumors and provide insights into the mechanism of RON signaling associated with gastric cancer progression and metastasis. Current therapeutics against RON in gastric cancer are summarized.

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.

A Decade of Boon or Burden: What Has the CHIP Ever Done for Cellular Protein Quality Control Mechanism Implicated in Neurodegeneration and Aging?

Cells regularly synthesize new proteins to replace old and abnormal proteins for normal cellular functions. Two significant protein quality control pathways inside the cellular milieu are ubiquitin proteasome system (UPS) and autophagy. Autophagy is known for bulk clearance of cytoplasmic aggregated proteins, whereas the specificity of protein degradation by UPS comes from E3 ubiquitin ligases. Few E3 ubiquitin ligases, like C-terminus of Hsc70-interacting protein (CHIP) not only take part in protein quality control pathways, but also plays a key regulatory role in other cellular processes like signaling, development, DNA damage repair, immunity and aging. CHIP targets misfolded proteins for their degradation through proteasome, as well as autophagy; simultaneously, with the help of chaperones, it also regulates folding attempts for misfolded proteins. The broad range of CHIP substrates and their associations with multiple pathologies make it a key molecule to work upon and focus for future therapeutic interventions. E3 ubiquitin ligase CHIP interacts and degrades many protein inclusions formed in neurodegenerative diseases. The presence of CHIP at various nodes of cellular protein-protein interaction network presents this molecule as a potential candidate for further research. In this review, we have explored a wide range of functionality of CHIP inside cells by a detailed presentation of its co-chaperone, E3 and E4 enzyme like functions, with central focus on its protein quality control roles in neurodegenerative diseases. We have also raised many unexplored but expected fundamental questions regarding CHIP functions, which generate hopes for its future applications in research, as well as drug discovery.

The RON receptor tyrosine kinase in pancreatic cancer pathogenesis and its potential implications for future targeted therapies

Pancreatic cancer remains a devastating disease with a mortality rate that has not changed substantially in decades. Novel therapies are therefore desperately needed. The RON receptor tyrosine kinase has been identified as an important mediator of KRAS oncogene addiction and is overexpressed in the majority of pancreatic cancers. Preclinical studies show that inhibition of RON function decreases pancreatic cancer cell migration, invasion, and survival and can sensitize pancreatic cancer cells to chemotherapy. This article reviews the current state of knowledge regarding RON biology and pancreatic cancer and discusses its potential as a therapeutic target.

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.

Ubiquitination-dependent regulation of signaling receptors in cancer

Ubiquitination of signaling cell surface receptors is a key mechanism regulating the availability of these receptors to interact with extracellular ligands. Accordingly, this regulation determines the sensitivity of cells to the humoral and locally secreted regulators of cell function, proliferation, and viability. Alterations in receptor ubiquitination and degradation are often encountered in cancers. Malignant cells utilize modified ubiquitination of signaling receptors to augment or attenuate signaling pathways on the basis of whether the outcome of this signaling is conducive or not for tumor growth and survival. These mechanisms as well as their significance for the treatment of human cancers are discussed.

Fibroblast growth factor receptor 3 (FGFR3) is a strong heat shock protein 90 (Hsp90) client: implications for therapeutic manipulation

Fibroblast growth factor receptor 3 (FGFR3) is a key regulator of growth and differentiation, whose aberrant activation causes a number of genetic diseases including achondroplasia and cancer. Hsp90 is a specialized molecular chaperone involved in stabilizing a select set of proteins termed clients. Here, we delineate the relationship of Hsp90 and co-chaperone Cdc37 with FGFR3 and the FGFR family. FGFR3 strongly associates with these chaperone complexes and depends on them for stability and function. Inhibition of Hsp90 function using the geldanamycin analog 17-AAG induces the ubiquitination and degradation of FGFR3 and reduces the signaling capacity of FGFR3. Other FGFRs weakly interact with these chaperones and are differentially influenced by Hsp90 inhibition. The Hsp90-related ubiquitin ligase CHIP is able to interact and destabilize FGFR3. Our results establish FGFR3 as a strong Hsp90 client and suggest that modulating Hsp90 chaperone complexes may beneficially influence the stability and function of FGFR3 in disease.

Western blotting analysis as a tool to study receptor tyrosine kinases

Receptor tyrosine kinases (RTKs) are involved in critical aspects of cell physiology ranging from cell -survival, proliferation, growth, migration, and differentiation. A tight control of the extent and duration of signals elicited by activated RTKs is crucial for preventing over-stimulation, which can ultimately lead to unrestrained proliferative ability and neoplastic growth. Ligand-induced downregulation of RTKs has emerged as a key negative regulatory mechanism that can accomplish signaling attenuation, by removing activated receptors from the cell surface and committing them to degradation. The ability of RTKs to escape from ligand-induced downregulation has been reported as a recurrent mechanism of oncogenic deregulation in cancer.Western blotting procedures have been extensively proven as straightforward assays to evaluate -protein expression levels and have been widely applied to study RTKs downregulation.

The crystal structure of a constitutively active mutant RON kinase suggests an intramolecular autophosphorylation hypothesis

A complex of RON(M1254T) with AMP-PNP and Mg(2+) reveals a substratelike positioning of Tyr1238 as well as likely catalysis-competent placement of the AMP-PNP and Mg(2+) components and indicates a tendency for cis phosphorylation. The structure shows how the oncogenic mutation may cause the constitutive activation and suggests a mechanistic hypothesis for the autophosphorylation of receptor tyrosine kinases.

Heat shock factor 1 deficiency via its downstream target gene alphaB-crystallin (Hspb5) impairs p53 degradation

Heat shock factor Hsf1 regulates the stress-inducibility of heat shock proteins (Hsps) or molecular chaperones. One of the functions attributed to Hsps is their participation in folding and degradation of proteins. We recently showed that hsf1(-/-) cells accumulate ubiquitinated proteins. However, a direct role for Hsf1 in stability of specific proteins such as p53 has not been elucidated. We present evidence that cells deficient in hsf1 accumulate wild-type p53 protein. We further show that hsf1(-/-) cells express lower levels of alphaB-crystallin and cells deficient in alphaB-crystallin also accumulate p53 protein. Reports indicate that alphaB-crystallin binds to Fbx4 ubiquitin ligase, and they target cyclin D1 for degradation through a pathway involving the SCF (Skp1-Cul1-F-box) complex. Towards determining a mechanism for p53 degradation involving alphaB-crystallin and Hsf1, we have found that ectopic expression of Fbx4 in wild-type mouse embryo fibroblasts (MEFs) expressing mutant p53 (p53R175H) leads to increase in its degradation, while MEFs deficient in hsf1 or alphaBcry are defective in degradation of this p53 protein. In addition, immunoprecipitated p53R175H from wild-type MEFs is able to pull-down both alphaB-crystallin and Fbx4. Finally, immunoprecipitated wild-type p53 from doxorubicin treated U2OS cells can pull-down endogenous alphaB-crystallin and Fbx4. These results indicate that hsf1- and alphaBcry-deficient cells accumulate p53 due to reduced levels of alphaB-crystallin in these cells. Elevated levels of p53 in hsf1- and alphaBcry-deficient cells lead to their increased sensitivity to DNA damaging agents. These data reveal a novel mechanism for protein degradation through Hsf1 and alphaB-crystallin.

Ecology-based screen identifies new metabolites from a Cordyceps-colonizing fungus as cancer cell proliferation inhibitors and apoptosis inducers

OBJECTIVES

This study aims to identify new anti-cancer agents from Cordyceps-colonizing fungi, using an ecology-based approach. It also aims to explore their anti-cell proliferative mechanisms, and to evaluate their anti-tumour effects in vivo.

MATERIALS AND METHODS

Extracts from Cordyceps-colonizing fungi were tested on HeLa cells, and active extracts were separated to obtain anti-tumour metabolites; their structures were elucidated by mass and nuclear magnetic resonance spectroscopy. Cell cycle analysis was evaluated using flow cytometry. Tumour formation assays were performed using C57BL/6J mice.

RESULTS

Based on ecological considerations, the selected extracts were subjected to initial anti-tumour screening. Bioassay-guided fractionation of the active extract afforded two new epipolythiodioxopiperazines, named gliocladicillins A (1) and B (2). (A) 1 and B (2) inhibited growth of HeLa, HepG2 and MCF-7 tumour cells. Further study demonstrated that both preparations arrested the cell cycle at G(2)/M phase in a dose-dependent manner, and induced apoptosis through up-regulation of expression of p53, p21, and cyclin B, and activation of caspases-8, -9 and -3. These data imply that gliocladicillins A (1) and B (2) induce tumour cell apoptosis through both extrinsic and intrinsic pathways. In addition, in vivo studies showed that they displayed significant inhibitory effects on cell population growth of melanoma B16 cells implanted into immunodeficient mice.

CONCLUSIONS

Gliocladicillins A (1) and B (2) are effective anti-tumour agents in vitro and in vivo and should be further evaluated for their potential in clinical use.

Degradation of activated protein kinases by ubiquitination

Protein kinases are important regulators of intracellular signal transduction pathways and play critical roles in diverse cellular functions. Once a protein kinase is activated, its activity is subsequently downregulated through a variety of mechanisms. Accumulating evidence indicates that the activation of protein kinases commonly initiates their downregulation via the ubiquitin/proteasome pathway. Failure to regulate protein kinase activity or expression levels can cause human diseases.

Met-related receptor tyrosine kinase Ron in tumor growth and metastasis

The Ron receptor is a member of the Met family of cell surface receptor tyrosine kinases and is primarily expressed on epithelial cells and macrophages. The biological response of Ron is mediated by binding of its ligand, hepatocyte growth factor-like protein/macrophage stimulating-protein (HGFL). HGFL is primarily synthesized and secreted from hepatocytes as an inactive precursor and is activated at the cell surface. Binding of HGFL to Ron activates Ron and leads to the induction of a variety of intracellular signaling cascades that leads to cellular growth, motility and invasion. Recent studies have documented Ron overexpression in a variety of human cancers including breast, colon, liver, pancreas, and bladder. Moreover, clinical studies have also shown that Ron overexpression is associated with both worse patient outcomes as well as metastasis. Forced overexpression of Ron in transgenic mice leads to tumorigenesis in both the lung and the mammary gland and is associated with metastatic dissemination. While Ron overexpression appears to be a hallmark of many human cancers, the mechanisms by which Ron induces tumorigenesis and metastasis are still unclear. Several strategies are currently being undertaken to inhibit Ron as a potential therapeutic target; current strategies include the use of Ron blocking proteins, small interfering RNA (siRNA), monoclonal antibodies, and small molecule inhibitors. In total, these data suggest that Ron is a critical factor in tumorigenesis and that inhibition of this protein, alone or in combination with current therapies, may prove beneficial in the treatment of cancer patients.

Smad4-dependent TGF-beta signaling suppresses RON receptor tyrosine kinase-dependent motility and invasion of pancreatic cancer cells

Transforming growth factorbeta (TGF-beta) signals through Smad-dependent and Smad-independent pathways. However, Smad signaling is altered by allelic deletion or intragenic mutation of the Smad4 gene in more than half of pancreatic ductal adenocarcinomas. We show here that loss of Smad4-dependent signaling leads to aberrant expression of RON, a phosphotyrosine kinase receptor, and that signaling by RON cooperates with Smad4-independent TGF-beta signaling to promote cell motility and invasion. Restoring Smad4 expression in a pancreatic ductal adenocarcinoma cell line that is deficient in Smad4 repressed RON expression. Conversely, small interference RNA knock down of Smad4 or blocking TGF-beta signaling with a TGF-beta type I receptor kinase inhibitor in Smad4-intact cell lines induced RON expression. TGF-beta-induced motility and invasion were inhibited in cells that express Smad4 and that have low levels of RON compared with isogenically matched cells that were deficient in Smad4. Furthermore, knocking down RON expression in Smad4-deficient cells suppressed TGF-beta-mediated motility and invasion. We further determined that Smad4-dependent signaling regulated RON expression at the transcriptional level by real-time reverse transcription PCR and RON promoter luciferase reporter assays. Functional inactivation by site-directed mutations of two Smad binding sites on the RON promoter inhibited TGF-beta-mediated repression of RON promoter activity. These studies indicate that loss of Smad4 contributes to aberrant RON expression and that cross-talk of Smad4-independent TGF-beta signaling and the RON pathway promotes an invasive phenotype.

The Hsp90 molecular chaperone: an open and shut case for treatment

The molecular chaperone Hsp90 (90 kDa heat-shock protein) is a remarkably versatile protein involved in the stress response and in normal homoeostatic control mechanisms. It interacts with 'client proteins', including protein kinases, transcription factors and others, and either facilitates their stabilization and activation or directs them for proteasomal degradation. By this means, Hsp90 displays a multifaceted ability to influence signal transduction, chromatin remodelling and epigenetic regulation, development and morphological evolution. Hsp90 operates as a dimer in a conformational cycle driven by ATP binding and hydrolysis at the N-terminus. The cycle is also regulated by a group of co-chaperones and accessory proteins. Here we review the biology of the Hsp90 molecular chaperone, emphasizing recent progress in our understanding of structure-function relationships and the identification of new client proteins. In addition we describe the exciting progress that has been made in the development of Hsp90 inhibitors, which are now showing promise in the clinic for cancer treatment. We also identify the gaps in our current understanding and highlight important topics for future research.

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.

Multiple variants of the RON receptor tyrosine kinase: Biochemical properties, tumorigenic activities, and potential drug targets

Aberrant expression of the RON (Recepteur d'Origine Nantais) receptor tyrosine kinase, accompanied by generation of multiple splicing or truncated variants, contributes to pathogenesis of epithelial cancers. Currently, six variants including RONDelta170, Delta165, Delta160, Delta155, Delta110, and Delta55 with various deletions or truncations in the extracellular or intracellular regions have been identified. The extracellular sequences contain functional structures such as sema domain, PSI motif, and IPT units. The deletion or truncation results in constitutive phosphorylation and increased kinase activities. Oncogenic RONDelta160, generated by exclusion of the first IPT unit, is a typical example. In contrast, the deletion adjacent to the conserved MET(1254) in the kinase domain converts RON into a dominant negative agent. Among three mechanisms underlying isoform production, the switch from constitutive to alternative pre-mRNA splicing is the major event in producing RON variants in cancer cells. Most of the RON variants have the ability to activate multiple signaling cascades with a different substrate specificity and phosphorylation profile. They regulate cell migration, invasion, and proliferation, which contribute to the invasive phenotype and promote the malignant progression. Thus, determining the pathogenesis of RON variants is critical in understanding the mechanisms underlying cancer initiation and progression. Targeting oncogenic signals elicited by RON or its variants by special antibody or small interfering RNA could provide a novel strategy for the treatment of malignant epithelial cancers.

Degradation of HER2 by Cbl-Based Chimeric Ubiquitin Ligases

Targeting disease-causing proteins for ubiquitination and degradation by chimeric molecules represents a promising alternative therapeutic strategy in cancer. Here, several Cbl-based chimeric ubiquitin ligases were recombined to achieve effective down-regulation of HER2. These chimeric molecules consisted of the Cbl NH(2)-terminal tyrosine kinase binding domain, linker, and RING domain, with the Src homology 2 domain replaced with that from growth factor receptor binding protein 2 (Grb2), Grb7, p85, or Src. The chimeric proteins not only interacted with HER2 but also enhanced the down-regulation of endogenous overexpressed HER2. After the chimeric proteins were introduced into HER2-overexpressing breast cancer SK-BR-3 cells or ovarian cancer SK-OV-3 cells, they effectively promoted HER2 ubiquitination and degradation in a RING finger domain-dependent manner. Consequently, expression of these chimeric molecules led to an inhibition of colony formation, increased the proportion of cells in the G(1) cycle, and suppressed tumorigenicity. Collectively, our findings suggest that the Cbl-based chimeric ubiquitin ligases designed in the present study may represent a novel approach for the targeted therapy of HER2-overexpressing cancers.