The INT6 cancer gene and MEK signaling pathways converge during zebrafish development

Assessment of the FRET-based Teen sensor to monitor ERK activation changes preceding morphological defects in a RASopathy zebrafish model and phenotypic rescue by MEK inhibitor

BACKGROUND

RASopathies are genetic syndromes affecting development and having variable cancer predisposition. These disorders are clinically related and are caused by germline mutations affecting key players and regulators of the RAS-MAPK signaling pathway generally leading to an upregulated ERK activity. Gain-of-function (GOF) mutations in PTPN11, encoding SHP2, a cytosolic protein tyrosine phosphatase positively controlling RAS function, underlie approximately 50% of Noonan syndromes (NS), the most common RASopathy. A different class of these activating mutations occurs as somatic events in childhood leukemias.

METHOD

Here, we evaluated the application of a FRET-based zebrafish ERK reporter, Teen, and used quantitative FRET protocols to monitor non-physiological RASopathy-associated changes in ERK activation. In a multi-level experimental workflow, we tested the suitability of the Teen reporter to detect pan-embryo ERK activity correlates of morphometric alterations driven by the NS-causing Shp2D61G allele.

RESULTS

Spectral unmixing- and acceptor photobleaching (AB)-FRET analyses captured pathological ERK activity preceding the manifestation of quantifiable body axes defects, a morphological pillar used to test the strength of SHP2 GoF mutations. Last, the work shows that by multi-modal FRET analysis, we can quantitatively trace back the modulation of ERK phosphorylation obtained by low-dose MEK inhibitor treatment to early development, before the onset of morphological defects.

CONCLUSION

This work proves the usefulness of FRET imaging protocols on both live and fixed Teen ERK reporter fish to readily monitor and quantify pharmacologically- and genetically-induced ERK activity modulations in early embryos, representing a useful tool in pre-clinical applications targeting RAS-MAPK signaling.

Eukaryotic initiation factor 3e subunit is positively associated with tumorigenesis and development of hepatocellular carcinoma

BACKGROUND

Eukaryotic initiation factor 3 (eIF3) is one of the most complex translation initiation factors in mammalian cells. Eukaryotic initiation factor 3e subunit (eIF3e) is a member of the 13 subunits of eIF3, and its dysregulated expression is closely related to the occurrence and development of cancer. However, the role of eIF3e in different types of malignant tumor is contradictory. So far, the role of eIF3e in primary hepatocellular carcinoma (HCC) has been unknown.

AIM

To study the role of eIF3e during tumorigenesis and development of HCC.

METHODS

Freshly isolated liver cancer tissues and adjacent tissues from five randomly selected patients with HCC were collected during operation of liver transplantation. The levels of mRNA and protein expression of eIF3e in liver cancer tissues and adjacent tissues were detected by qRT-PCR and Western blot, respectively. An eIF3e-containing plasmid, pcDNA3.1-eIF3e, or an eIF3e-silencing short hairpin RNA plasmid, pSuper-sheIF3e, was transiently transfected into HepG2 cells, and the levels of mRNA and protein expression of eIF3e were analyzed by qRT-PCR and Western Blot, respectively, to confirm that eIF3e expression was up-regulated by pcDNA3.1-eIF3e or down-regulated by pSuper-sheIF3e. To study the effect of altered eIF3e expression on cellular behavior, Cell Counting Kit-8 approach was used to evaluate cell proliferation, scratch test was used to observe cell migration, and flow cytometric analysis was performed to assess cell apoptosis. Finally, the effect of eIF3e on tumor growth was evaluated by tumor-forming experiment in nude mice.

RESULTS

Compared with the adjacent tissues, eIF3e was up-regulated in HCC tissues. Cellular experiments showed that the transcription levels of eIF3e mRNA in HepG2 and Huh7 cells were higher than those in HL7702 cells. After HepG2 cells were transiently transfected by pcDNA3.1-eIF3e or pSuper-sheIF3e, it was found that up-regulated eIF3e increased proliferation and migration of HepG2 cells, but had no effect on cellular apoptosis, while down-regulated eIF3e decreased proliferation and migration, and accelerated apoptosis. The results in nude mice showed that eIF3e promoted tumor growth in vivo.

CONCLUSION

Overexpression of eIF3e is positively associated with tumorigenesis and development of HCC. Therefore, eIF3e has the potential to become a novel target for clinical treatment of HCC.

Swimming toward solutions: Using fish and frogs as models for understanding RASopathies

The RAS signaling pathway regulates cell growth, survival, and differentiation, and its inappropriate activation is associated with disease in humans. The RASopathies, a set of developmental syndromes, arise when the pathway is overactive during development. Patients share a core set of symptoms, including congenital heart disease, craniofacial anomalies, and neurocognitive delay. Due to the conserved nature of the pathway, animal models are highly informative for understanding disease etiology, and zebrafish and Xenopus are emerging as advantageous model systems. Here we discuss these aquatic models of RASopathies, which recapitulate many of the core symptoms observed in patients. Craniofacial structures become dysmorphic upon expression of disease-associated mutations, resulting in wider heads. Heart defects manifest as delays in cardiac development and changes in heart size, and behavioral deficits are beginning to be explored. Furthermore, early convergence and extension defects cause elongation of developing embryos: this phenotype can be quantitatively assayed as a readout of mutation strength, raising interesting questions regarding the relationship between pathway activation and disease. Additionally, the observation that RAS signaling may be simultaneously hyperactive and attenuated suggests that downregulation of signaling may also contribute to etiology. We propose that models should be characterized using a standardized approach to allow easier comparison between models, and a better understanding of the interplay between mutation and disease presentation.

Endothelin receptor Aa regulates proliferation and differentiation of Erb-dependent pigment progenitors in zebrafish

Skin pigment patterns are important, being under strong selection for multiple roles including camouflage and UV protection. Pigment cells underlying these patterns form from adult pigment stem cells (APSCs). In zebrafish, APSCs derive from embryonic neural crest cells, but sit dormant until activated to produce pigment cells during metamorphosis. The APSCs are set-aside in an ErbB signaling dependent manner, but the mechanism maintaining quiescence until metamorphosis remains unknown. Mutants for a pigment pattern gene, parade, exhibit ectopic pigment cells localised to the ventral trunk, but also supernumerary cells restricted to the Ventral Stripe. Contrary to expectations, these melanocytes and iridophores are discrete cells, but closely apposed. We show that parade encodes Endothelin receptor Aa, expressed in the blood vessels, most prominently in the medial blood vessels, consistent with the ventral trunk phenotype. We provide evidence that neuronal fates are not affected in parade mutants, arguing against transdifferentiation of sympathetic neurons to pigment cells. We show that inhibition of BMP signaling prevents specification of sympathetic neurons, indicating conservation of this molecular mechanism with chick and mouse. However, inhibition of sympathetic neuron differentiation does not enhance the parade phenotype. Instead, we pinpoint ventral trunk-restricted proliferation of neural crest cells as an early feature of the parade phenotype. Importantly, using a chemical genetic screen for rescue of the ectopic pigment cell phenotype of parade mutants (whilst leaving the embryonic pattern untouched), we identify ErbB inhibitors as a key hit. The time-window of sensitivity to these inhibitors mirrors precisely the window defined previously as crucial for the setting aside of APSCs in the embryo, strongly implicating adult pigment stem cells as the source of the ectopic pigment cells. We propose that a novel population of APSCs exists in association with medial blood vessels, and that their quiescence is dependent upon Endothelin-dependent factors expressed by the blood vessels.

Eukaryotic translation initiation factor 3 (eIF3) subunit e is essential for embryonic development and cell proliferation

Mammalian eukaryotic translation initiation factor 3 (eIF3) is the largest complex of the translation initiation factors. The eIF3 complex is comprised of thirteen subunits, which are named eIF3a to eIF3 m in most multicellular organisms. The eIF3e gene locus is one of the most frequent integration sites of mouse mammary tumor virus (MMTV), which induces mammary tumors in mice. MMTV-integration events result in the expression of C-terminal-truncated eIF3e proteins, leading to mammary tumor formation. We have shown that tumor formation can be partly caused by activation of hypoxia-inducible factor 2α. To investigate the function of eIF3e in mammals, we generated eIF3e-deficient mice. These eIF3e^-/- mice are embryonically lethal, while eIF3e^+/- mice are much smaller than wild-type mice. In addition, eIF3e^+/- mouse embryonic fibroblasts (MEFs) contained reduced levels of eIF3a and eIF3c subunits and exhibited reduced cellular proliferation. These results suggest that eIF3e is essential for embryonic development in mice and plays a role in maintaining eIF3 integrity.

Up-regulation Of EIF3e Is Associated with The Progression of Esophageal Squamous Cell Carcinoma and Poor Prognosis in Patients

Introduction: Esophageal cancer is one of the most common malignant tumors in the world. Eukaryotic translation initiation factors 3e (eIF3e) makes a notable difference in the initiation of protein synthesis and tumor progression. However, the role of eIF3e in ESCC has not been revealed yet. This study aims to investigate the bio-functional and prognostic role of eIF3e in human ESCC tissues and cells. Methods: Immunohistochemical staining and Western blot were performed to detect the eIF3e expression in ESCC patients' tissues. The Kaplan-Meier product limit method and Cox regression were conducted to analyze the association between eIF3e expression, together with other related clinical/pathological features, and patients' prognosis. In the analysis of bio-functional role of eIF3e, CCK-8 and Transwell assay were performed to compare the proliferative and migrative ability after knockdown of eIF3e. Results: Up-regulation of eIF3e were demonstrated in ESCC tissues compared with the corresponding para-cancerous tissues. Overexpression of eIF3e was associated with deep tumor depth, lymph nodes metastasis, and advanced TNM stage. Importantly, the patients with high eIF3e expression suffered shorter overall and disease-free survival. Lymph node metastasis and histological grade served as independent prognostic predictors in patients' prognosis. Knockdown of eIF3e could inhibit cell proliferation and migration, in vitro. Conclusions: The eIF3e expression, or combined with other members of eIF3 complex, might predict poor prognosis of ESCC and serve as a potential breakthrough in the multimodality therapy of ESCC.

Suppression of RAC1-driven malignant melanoma by group A PAK inhibitors

Activating mutations in the RAC1 gene have recently been discovered as driver events in malignant melanoma. Expression of this gene is associated with melanocyte proliferation, and melanoma cells bearing this mutation are insensitive to BRAF inhibitors such as vemurafenib and dabrafenib, and also may evade immune surveillance due to enhanced expression of PD-L1. Activating mutations in RAC1 are of special interest, as small molecule inhibitors for the RAC effector p21-activated kinase (PAK) are in late-stage clinical development and might impede oncogenic signaling from mutant RAC1.

In this work, we explore the effects of PAK inhibition on RAC1^P29S signaling in zebrafish embryonic development, in the proliferation, survival, and motility of RAC1^P29S-mutant human melanoma cells, and on tumor formation and progression from such cells in mice. We report that RAC1^P29S evokes a Rasopathy-like phenotype on zebrafish development that can be blocked by inhibitors of PAK or MEK. We also found and that RAC1 mutant human melanoma cells are resistant to clinical inhibitors of BRAF but are uniquely sensitive to PAK inhibitors. These data suggest that suppressing the PAK pathway might be of therapeutic benefit in this type of melanoma.

A zebrafish melanoma model reveals emergence of neural crest identity during melanoma initiation

The "cancerized field" concept posits that cancer-prone cells in a given tissue share an oncogenic mutation, but only discreet clones within the field initiate tumors. Most benign nevi carry oncogenic BRAF(V600E) mutations but rarely become melanoma. The zebrafish crestin gene is expressed embryonically in neural crest progenitors (NCPs) and specifically reexpressed in melanoma. Live imaging of transgenic zebrafish crestin reporters shows that within a cancerized field (BRAF(V600E)-mutant; p53-deficient), a single melanocyte reactivates the NCP state, revealing a fate change at melanoma initiation in this model. NCP transcription factors, including sox10, regulate crestin expression. Forced sox10 overexpression in melanocytes accelerated melanoma formation, which is consistent with activation of NCP genes and super-enhancers leading to melanoma. Our work highlights NCP state reemergence as a key event in melanoma initiation.

Overexpression of eIF3e is correlated with colon tumor development and poor prognosis

EIF3e is a component of the eukaryotic translation initiation factor 3 (eIF-3) complexes, which is an essential factor for initiation of protein synthesis in mammalian cells. Translational control plays key roles in the complex mechanism of cancer development and progression. However, the clinical significance of eIF3e in colon cancer remains to be elucidated. We analyzed the eIF3e expression in a tissue microarray (TMA), which contained 173 colon cancer tissues paired with adjacent normal mucosa and lymph node metastasis. The expression of eIF3e was significantly elevated in colon cancer tissues in comparison with those in adjacent normal mucosa (P < 0.001) and lymph node metastasis (P < 0.001). The high expression of eIF3e in colon cancer was significantly correlated with tumor size (P < 0.001), lymph node involvement (P < 0.001), distant metastasis (P < 0.001), clinical stage (P < 0.001), histopathologic classification (P < 0.001), and vessel invasion (P = 0.036). Univariate and multivariate analysis revealed that eIF3e is an independent prognosis factor for overall survival and disease-free survival in colon cancer. Down-regulation of eIF3e in vitro inhibited colon cancer cell proliferation, clonality and promoted cell apoptosis. Taken together, high eIF3e expression may contribute to tumor progression and predict poor prognosis in colon cancer.

Int6/eIF3e is essential for proliferation and survival of human glioblastoma cells

Glioblastomas (GBM) are very aggressive and malignant brain tumors, with frequent relapses despite an appropriate treatment combining surgery, chemotherapy and radiotherapy. In GBM, hypoxia is a characteristic feature and activation of Hypoxia Inducible Factors (HIF-1α and HIF-2α) has been associated with resistance to anti-cancer therapeutics. Int6, also named eIF3e, is the “e” subunit of the translation initiation factor eIF3, and was identified as novel regulator of HIF-2α. Eukaryotic initiation factors (eIFs) are key factors regulating total protein synthesis, which controls cell growth, size and proliferation. The functional significance of Int6 and the effect of Int6/EIF3E gene silencing on human brain GBM has not yet been described and its role on the HIFs is unknown in glioma cells. In the present study, we show that Int6/eIF3e suppression affects cell proliferation, cell cycle and apoptosis of various GBM cells. We highlight that Int6 inhibition induces a diminution of proliferation through cell cycle arrest and increased apoptosis. Surprisingly, these phenotypes are independent of global cell translation inhibition and are accompanied by decreased HIF expression when Int6 is silenced. In conclusion, we demonstrate here that Int6/eIF3e is essential for proliferation and survival of GBM cells, presumably through modulation of the HIFs.

Continual low-level MEK inhibition ameliorates cardio-facio-cutaneous phenotypes in zebrafish

Cardio-facio-cutaneous (CFC) syndrome is caused by germline mutations in KRAS, BRAF and MEK1/2. The highly selective and potent MEK inhibitors that have been developed as anti-cancer agents hold potential as therapeutics for CFC syndrome. We have previously shown that the effects of CFC mutations on zebrafish gastrulation can be prevented by a 1-hour treatment with MEK inhibitors within a specific developmental time-window. However, MEK activity is essential for normal development and PD0325901 treatment outside this treatment window leads to additional developmental defects in MEK-dependent tissues. We now test ten different doses of PD0325901 at six developmental time points and assess the effects on body axis length, heart development and craniofacial structures in zebrafish embryos. Notably, we find that a continuous low-level dose of PD0325901 that has only minor inhibition of MEK activity can prevent the action of both the common CFC BRAF^Q257R kinase-active allele and the BRAF^G596V kinase-impaired mutant allele through the first 5 days of development. These results provide a detailed study of the effects of PD0325901 in development and show that, unlike in cancer, which requires robust inhibition of MAPK signalling, a partial reduction in phospho-ERK1/2 activity is sufficient to moderate the developmental effects of BRAF^CFC mutations.

Non-core subunit eIF3h of translation initiation factor eIF3 regulates zebrafish embryonic development

Eukaryotic translation initiation factor eIF3, which plays a central role in translation initiation, consists of five core subunits that are present in both the budding yeast and higher eukaryotes. However, higher eukaryotic eIF3 contains additional (non-core) subunits that are absent in the budding yeast. We investigated the role of one such non-core eIF3 subunit eIF3h, encoded by two distinct genes-eif3ha and eif3hb, as a regulator of embryonic development in zebrafish. Both eif3h genes are expressed during early embryogenesis, and display overlapping yet distinct and highly dynamic spatial expression patterns. Loss of function analysis using specific morpholino oligomers indicates that each isoform has specific as well as redundant functions during early development. The morphant phenotypes correlate with their spatial expression patterns, indicating that eif3h regulates development of the brain, heart, vasculature, and lateral line. These results indicate that the non-core subunits of eIF3 regulate specific developmental programs during vertebrate embryogenesis.

Combined zebrafish-yeast chemical-genetic screens reveal gene-copper-nutrition interactions that modulate melanocyte pigmentation

Hypopigmentation is a feature of copper deficiency in humans, as caused by mutation of the copper (Cu(2+)) transporter ATP7A in Menkes disease, or an inability to absorb copper after gastric surgery. However, many causes of copper deficiency are unknown, and genetic polymorphisms might underlie sensitivity to suboptimal environmental copper conditions. Here, we combined phenotypic screens in zebrafish for compounds that affect copper metabolism with yeast chemical-genetic profiles to identify pathways that are sensitive to copper depletion. Yeast chemical-genetic interactions revealed that defects in intracellular trafficking pathways cause sensitivity to low-copper conditions; partial knockdown of the analogous Ap3s1 and Ap1s1 trafficking components in zebrafish sensitized developing melanocytes to hypopigmentation in low-copper environmental conditions. Because trafficking pathways are essential for copper loading into cuproproteins, our results suggest that hypomorphic alleles of trafficking components might underlie sensitivity to reduced-copper nutrient conditions. In addition, we used zebrafish-yeast screening to identify a novel target pathway in copper metabolism for the small-molecule MEK kinase inhibitor U0126. The zebrafish-yeast screening method combines the power of zebrafish as a disease model with facile genome-scale identification of chemical-genetic interactions in yeast to enable the discovery and dissection of complex multigenic interactions in disease-gene networks.

An oncogenic role of eIF3e/INT6 in human breast cancer

Altered expression of the eukaryotic translation initiation factor 3 (eIF3) subunit eIF3e/INT6 has been described in various types of human cancer, but the nature of its involvement in tumorigenesis is not yet clear. Using immunohistochemical analysis of 81 primary breast cancers, we found that high tumor grade correlated significantly with elevated cytoplasmic eIF3e level in epithelial tumor cells. Analysis of protein synthesis after siRNA-mediated knockdown in breast cancer cell lines indicated that eIF3e is not required for bulk translation. Microarray analysis of total and polysomal RNAs nonetheless identified distinct sets of mRNAs regulated either positively or negatively by eIF3e; functional classification of these revealed a marked enrichment of genes involved in cell proliferation, invasion and apoptosis. Validated mRNA targets regulated positively at the translational level by eIF3e included urokinase-type plasminogen activator and apoptotic regulator BCL-XL, whereas synthesis of proteins including the mitotic checkpoint component MAD2L1 was negatively regulated. Finally, eIF3e-depleted breast carcinoma cells showed reduced in vitro invasion and proliferation. Taken together, our study data suggest that eIF3e has a positive role in breast cancer progression. It regulates the translation, and in some cases abundance, of mRNAs involved in key aspects of cancer cell biology.

Kinase-activating and kinase-impaired cardio-facio-cutaneous syndrome alleles have activity during zebrafish development and are sensitive to small molecule inhibitors

The Ras/MAPK pathway is critical for human development and plays a central role in the formation and progression of most cancers. Children born with germ-line mutations in BRAF, MEK1 or MEK2 develop cardio-facio-cutaneous (CFC) syndrome, an autosomal dominant syndrome characterized by a distinctive facial appearance, heart defects, skin and hair abnormalities and mental retardation. CFC syndrome mutations in BRAF promote both kinase-activating and kinase-impaired variants. CFC syndrome has a progressive phenotype, and the availability of clinically active inhibitors of the MAPK pathway prompts the important question as to whether such inhibitors might be therapeutically effective in the treatment of CFC syndrome. To study the developmental effects of CFC mutant alleles in vivo, we have expressed a panel of 28 BRAF and MEK alleles in zebrafish embryos to assess the function of human disease alleles and available chemical inhibitors of this pathway. We find that both kinase-activating and kinase-impaired CFC mutant alleles promote the equivalent developmental outcome when expressed during early development and that treatment of CFC-zebrafish embryos with inhibitors of the FGF-MAPK pathway can restore normal early development. Importantly, we find a developmental window in which treatment with a MEK inhibitor can restore the normal early development of the embryo, without the additional, unwanted developmental effects of the drug.