Role of post-translational modifications in regulating c-Myc proteolysis, transcriptional activity and biological function

Oncogenic activation of glypican-3 by c-Myc in human hepatocellular carcinoma

Glypican-3 (GPC3) is a heparan sulfate proteoglycan that has an important role in cell growth and differentiation, and its function in tumorigenesis is tissue-dependent. In hepatocellular carcinoma (HCC), the overexpression of GPC3 has been demonstrated to be a reliable diagnostic indicator. However, the mechanisms that regulate the expression and function of GPC3 remain unclear. The oncoprotein c-Myc is a transcription factor that plays a significant role in more than 50% of human tumors. We report here that GPC3 is a transcriptional target of c-Myc and that the expression of c-Myc is also regulated by GPC3, thus forming a positive feedback signaling loop. We found that the overexpression of c-Myc could induce GPC3 promoter-dependent luciferase activity in luciferase reporter experiments. Furthermore, mutational analysis identified c-Myc-binding sites within the GPC3 promoter. The exogenous overexpression of c-Myc increased the endogenous messenger RNA (mRNA) and protein levels of GPC3. Chromatin immunoprecipitation experiments revealed the binding of c-Myc to the endogenous GPC3 promoter, indicating that c-Myc can directly transcriptionally activate GPC3. Interestingly, GPC3 can also elevate c-Myc expression. Overexpression of GPC3 increased c-Myc protein levels, whereas the knockdown of GPC3 reduced c-Myc expression levels. Lastly, the elevated levels of c-Myc correlate with the overexpression of GPC3 in human HCC samples.

CONCLUSION

These data provide new mechanistic insight into the roles of GPC3 and of c-Myc in the development of HCC.

Systems-pharmacology dissection of a drug synergy in imatinib-resistant CML

Occurrence of the BCR-ABL(T315I) gatekeeper mutation is among the most pressing challenges in the therapy of chronic myeloid leukemia (CML). Several BCR-ABL inhibitors have multiple targets and pleiotropic effects that could be exploited for their synergistic potential. Testing combinations of such kinase inhibitors identified a strong synergy between danusertib and bosutinib that exclusively affected CML cells harboring BCR-ABL(T315I). To elucidate the underlying mechanisms, we applied a systems-level approach comprising phosphoproteomics, transcriptomics and chemical proteomics. Data integration revealed that both compounds targeted Mapk pathways downstream of BCR-ABL, resulting in impaired activity of c-Myc. Using pharmacological validation, we assessed that the relative contributions of danusertib and bosutinib could be mimicked individually by Mapk inhibitors and collectively by downregulation of c-Myc through Brd4 inhibition. Thus, integration of genome- and proteome-wide technologies enabled the elucidation of the mechanism by which a new drug synergy targets the dependency of BCR-ABL(T315I) CML cells on c-Myc through nonobvious off targets.

CD19 is a major B cell receptor-independent activator of MYC-driven B-lymphomagenesis

PAX5, a B cell-specific transcription factor, is overexpressed through chromosomal translocations in a subset of B cell lymphomas. Previously, we had shown that activation of immunoreceptor tyrosine-based activation motif (ITAM) proteins and B cell receptor (BCR) signaling by PAX5 contributes to B-lymphomagenesis. However, the effect of PAX5 on other oncogenic transcription factor-controlled pathways is unknown. Using a MYC-induced murine lymphoma model as well as MYC-transformed human B cell lines, we found that PAX5 controls c-MYC protein stability and steady-state levels. This promoter-independent, posttranslational mechanism of c-MYC regulation was independent of ITAM/BCR activity. Instead it was controlled by another PAX5 target, CD19, through the PI3K-AKT-GSK3β axis. Consequently, MYC levels in B cells from CD19-deficient mice were sharply reduced. Conversely, reexpression of CD19 in murine lymphomas with spontaneous silencing of PAX5 boosted MYC levels, expression of its key target genes, cell proliferation in vitro, and overall tumor growth in vivo. In human B-lymphomas, CD19 mRNA levels were found to correlate with those of MYC-activated genes. They also negatively correlated with the overall survival of patients with lymphoma in the same way that MYC levels do. Thus, CD19 is a major BCR-independent regulator of MYC-driven neoplastic growth in B cell neoplasms.

Transient structure and dynamics in the disordered c-Myc transactivation domain affect Bin1 binding

The crucial role of Myc as an oncoprotein and as a key regulator of cell growth makes it essential to understand the molecular basis of Myc function. The N-terminal region of c-Myc coordinates a wealth of protein interactions involved in transformation, differentiation and apoptosis. We have characterized in detail the intrinsically disordered properties of Myc-1–88, where hierarchical phosphorylation of S62 and T58 regulates activation and destruction of the Myc protein. By nuclear magnetic resonance (NMR) chemical shift analysis, relaxation measurements and NOE analysis, we show that although Myc occupies a very heterogeneous conformational space, we find transiently structured regions in residues 22–33 and in the Myc homology box I (MBI; residues 45–65); both these regions are conserved in other members of the Myc family. Binding of Bin1 to Myc-1–88 as assayed by NMR and surface plasmon resonance (SPR) revealed primary binding to the S62 region in a dynamically disordered and multivalent complex, accompanied by population shifts leading to altered intramolecular conformational dynamics. These findings expand the increasingly recognized concept of intrinsically disordered regions mediating transient interactions to Myc, a key transcriptional regulator of major medical importance, and have important implications for further understanding its multifaceted role in gene regulation.

DYRK2 priming phosphorylation of c-Jun and c-Myc modulates cell cycle progression in human cancer cells

Dysregulation of the G(1)/S transition in the cell cycle contributes to tumor development. The oncogenic transcription factors c-Jun and c-Myc are indispensable regulators at this transition, and their aberrant expression is associated with many malignancies. Degradation of c-Jun/c-Myc is a critical process for the G(1)/S transition, which is initiated upon phosphorylation by glycogen synthase kinase 3 β (GSK3β). However, a specific kinase or kinases responsible for priming phosphorylation events that precede this GSK3β modification has not been definitively identified. Here, we found that the dual-specificity tyrosine phosphorylation-regulated kinase DYRK2 functions as a priming kinase of c-Jun and c-Myc. Knockdown of DYRK2 in human cancer cells shortened the G(1) phase and accelerated cell proliferation due to escape of c-Jun and c-Myc from ubiquitination-mediated degradation. In concert with these results, silencing DYRK2 increased cell proliferation in human cancer cells, and this promotion was completely impeded by codeprivation of c-Jun or c-Myc in vivo. We also found marked attenuation of DYRK2 expression in multiple human tumor samples. Downregulation of DYRK2 correlated with high levels of unphosphorylated c-Jun and c-Myc and, importantly, with invasiveness of human breast cancers. These results reveal that DYRK2 regulates tumor progression through modulation of c-Jun and c-Myc.

CDK8 maintains tumor dedifferentiation and embryonic stem cell pluripotency

CDK8 is a cyclin-dependent kinase that mediates transcriptional control of pathways linked to both cancer and stem cells. In this study, we show that CDK8 is required for both tumor growth and maintenance of tumor dedifferentiation in vivo and uncover a common role for CDK8 in controlling cancer and stem cell function. Acute CDK8 loss in vivo strongly inhibited tumor growth and promoted differentiation. Transcriptional profiling identified a set of embryonic stem cell-related genes that are activated by CDK8 in cancer. Consistent with this, we found that CDK8 expression correlated to the embryonic stem cell pluripotency state and loss of CDK8 caused embryonic stem cells to differentiate. This effect was, at least partially, mediated by the ability of CDK8 to regulate MYC protein and downstream MYC target gene expression. Similar regulation of MYC target genes by CDK8 was observed in colon tumor cells, and increased expression of a CDK8-regulated, embryonic stem cell MYC target gene signature was associated with loss of differentiation and poor outcome in primary human colon cancers. Together, these observations reveal that CDK8 acts, at least in part, through MYC to maintain both tumors and embryonic stem cells in an undifferentiated state. This raises the intriguing possibility that targeting CDK8 therapeutically may specifically inhibit the stem-like properties of cancer cells.

Myc: the beauty and the beast

The iconic history of the Myc oncoprotein encompasses 3 decades of intense scientific discovery. There is no question that Myc has been a pioneer, advancing insight into the molecular basis of cancer as well as functioning as a critical control center for several diverse biological processes and regulatory mechanisms. This narrative chronicles the journey and milestones that have defined the understanding of Myc, and it provides an opportunity to consider future directions in this challenging yet rewarding field.

Differential regulation of N-Myc and c-Myc synthesis, degradation, and transcriptional activity by the Ras/mitogen-activated protein kinase pathway

Myc transcription factors are important regulators of proliferation and can promote oncogenesis when deregulated. Deregulated Myc expression in cancers can result from MYC gene amplification and translocation but also from alterations in mitogenic signaling pathways that affect Myc levels through both transcriptional and post-transcription mechanisms. For example, mutations in Ras family GTPase proteins that cause their constitutive activation can increase cellular levels of c-Myc by interfering with its rapid proteasomal degradation. Although enhanced protein stability is generally thought to be applicable to other Myc family members, here we show that c-Myc and its paralog N-Myc respond to oncogenic H-Ras (H-Ras(G12V)) in very different ways. H-Ras(G12V) promotes accumulation of both c-Myc and N-Myc, but although c-Myc accumulation is achieved by enhanced protein stability, N-Myc accumulation is associated with an accelerated rate of translation that overcomes a surprising H-Ras(G12V)-mediated destabilization of N-Myc. We show that H-Ras(G12V)-mediated degradation of N-Myc functions independently of key phosphorylation sites in the highly conserved Myc homology box I region that controls c-Myc protein stability by oncogenic Ras. Finally, we found that N-Myc and c-Myc transcriptional activity is associated with their proteasomal degradation but that N-Myc may be uniquely dependent on Ras-stimulated proteolysis for target gene expression. Taken together, these studies provide mechanistic insight into how oncogenic Ras augments N-Myc levels in cells and suggest that enhanced N-Myc translation and degradation-coupled transactivation may contribute to oncogenesis.

PP2A-B56α controls oncogene-induced senescence in normal and tumor human melanocytic cells

Oncoprotein C-MYC is overexpressed in human metastatic melanomas and melanoma-derived cells where it is required for suppression of oncogene-induced senescence (OIS). The genetic events that maintain high levels of C-MYC in melanoma cells and their role in OIS are unknown. Here, we report that C-MYC in cells from several randomly chosen melanoma lines was up-regulated at the protein level, and largely due to the increased protein stability. Of all known regulators of C-MYC stability, levels of B56α subunit of the PP2A tumor suppressor complex were substantially suppressed in all human melanoma cells compared to normal melanocytes. Accordingly, immuno-histochemical analysis revealed that the lowest and the highest amounts of PP2A-B56α were predominantly detected in metastatic melanoma tissues and in primary melanomas from patients with good clinical outcome, respectively. Importantly, PP2A-B56α overexpression suppressed C-MYC in melanoma cells and induced OIS, whereas depletion of PP2A-B56α in normal human melanocytes up-regulated C-MYC protein levels and suppressed BRAF^V600E- and, less efficiently, NRAS^Q61R-induced senescence. Our data reveal a mechanism of C-MYC overexpression in melanoma cells and identify a functional role for PP2A-B56α in OIS of melanocytic cells.

Troglitazone suppresses c-Myc levels in human prostate cancer cells via a PPARγ-independent mechanism

Troglitazone is a ligand for the peroxisome proliferator activated receptor gamma (PPARγ) that decreases growth of human prostate cancer cells in vitro and in vivo. However, the mechanism by which troglitazone reduces prostate cancer cell growth is not fully understood. To understand the signaling pathways involved in troglitazone-induced decreases in prostate cancer growth, we examined the effect of troglitazone on androgen-independent C4-2 human prostate cancer cells. Initial experiments revealed troglitazone inhibited C4-2 cell proliferation by arresting cells in the G(0)/G(1) phase of the cell cycle and inducing apoptosis. Since the proto-oncogene product c-Myc regulates both apoptosis and cell cycle progression, we next examined whether troglitazone altered expression of c-Myc. Troglitazone decreased c-Myc protein levels as well as expression of downstream targets of c-Myc in a dose-dependent manner. In C4-2 cells, troglitazone-induced decreases in c-Myc protein involve proteasome-mediated degradation of c-Myc protein as well as reductions in c-Myc mRNA levels. It appears that troglitazone stimulates degradation of c-Myc by increasing c-Myc phosphorylation, for the level of phosphorylated c-Myc was elevated in prostate cancer cells exposed to troglitazone. While troglitazone dramatically decreased the amount of c-Myc within C4-2 cells, the PPARγ ligands ciglitazone, rosiglitazone and pioglitazone did not reduce c-Myc protein levels. Furthermore the down-regulation of c-Myc by troglitazone was not blocked by the PPARγ antagonist GW9662 and siRNA-mediated decreases in PPARγ protein. Thus, our data suggest that troglitazone reduces c-Myc protein independently of PPARγ.

Romo1 is a negative-feedback regulator of Myc

Degradation of Myc protein is mediated by E3 ubiquitin ligases, including SCF(Fbw7) and SCF(Skp2), but much remains unknown about the mechanism of S-phase kinase-associated protein (Skp2)-mediated Myc degradation. In the present study, we show that upregulated Myc protein, which triggers the G1-S phase progression in response to growth-stimulatory signals, induces reactive oxygen species modulator 1 (Romo1) expression. Romo1 subsequently triggers Skp2-mediated ubiquitylation and degradation of Myc by a mechanism not previously reported in normal lung fibroblasts. We also show that reactive oxygen species (ROS) derived from steady-state Romo1 expression are necessary for cell cycle entry of quiescent cells. From this study, we suggest that the generation of ROS mediated by pre-existing Romo1 protein is required for Myc induction. Meanwhile, Romo1 expression induced by Myc during G1 phase stimulates Skp2-mediated Myc degradation in a negative-feedback mechanism.

Posttranscriptional deregulation of MYC via PTEN constitutes a major alternative pathway of MYC activation in T-cell acute lymphoblastic leukemia

Cumulative evidence indicates that MYC, one of the major downstream effectors of NOTCH1, is a critical component of T-cell acute lymphoblastic leukemia (T-ALL) oncogenesis and a potential candidate for targeted therapy. However, MYC is a complex oncogene, involving both fine protein dosage and cell-context dependency, and detailed understanding of MYC-mediated oncogenesis in T-ALL is still lacking. To better understand how MYC is interspersed in the complex T-ALL oncogenic networks, we performed a thorough molecular and biochemical analysis of MYC activation in a comprehensive collection of primary adult and pediatric patient samples. We find that MYC expression is highly variable, and that high MYC expression levels can be generated in a large number of cases in absence of NOTCH1/FBXW7 mutations, suggesting the occurrence of multiple activation pathways in addition to NOTCH1. Furthermore, we show that posttranscriptional deregulation of MYC constitutes a major alternative pathway of MYC activation in T-ALL, operating partly via the PI3K/AKT axis through down-regulation of PTEN, and that NOTCH1(m) might play a dual transcriptional and posttranscriptional role in this process. Altogether, our data lend further support to the significance of therapeutic targeting of MYC and/or the PTEN/AKT pathways, both in GSI-resistant and identified NOTCH1-independent/MYC-mediated T-ALL patients.

LYL1 degradation by the proteasome is directed by a N-terminal PEST rich site in a phosphorylation-independent manner

Background

The Lymphoblastic leukemia 1 (LYL1) gene is a proto-oncogenic transcription factor found upregulated in patients with T-cell acute lymphoblastic leukemia (T-cell ALL). Initially, the upregulation was described to be as a result of a translocation. However, further studies revealed that transcriptional upregulation of LYL1could also occur without translocations. In addition, post-translational mechanisms, such as protein degradation could influence LYL1 expression as well.

Methodology/Principal Findings

In this study, we considered possible post-translational regulation of Lyl1, and investigated fundamental mechanisms governing LYL1 degradation in cell-based culture assays. We identify a PEST sequence motif located in the N-terminus of LYL1, which determines the efficiency of LYL1 degradation by the proteasome. The absence of the PEST degradation site leads to accumulation or upregulation of LYL1. We also show that LYL1 is phosphorylated by MAPK at S₃₆, and determined that proteasomal degradation of LYL1 occurs in a phosphorylation-independent manner.

Conclusions/Significance

Understanding LYL1 degradation is a step forward not only towards deciphering the normal function and regulation of LYL1, but could suggest post-translational mechanisms for upregulation of LYL1 that may contribute to its oncogenic role.

Noninvasive molecular imaging of c-Myc activation in living mice

The cytoplasmic Myc protein (c-Myc) regulates various human genes and is dysregulated in many human cancers. Phosphorylation mediates the protein activation of c-Myc and is essential for the function of this transcription factor in normal cell behavior and tumor growth. To date, however, the targeting of Myc as a therapeutic approach for cancer treatment has been achieved primarily at the nonprotein level. We have developed a molecular imaging sensor for noninvasive imaging of c-Myc activity in living subjects using a split Firefly luciferase (FL) complementation strategy to detect and quantify the phosphorylation-mediated interaction between glycogen synthase kinase 3beta (GSK3beta) and c-Myc. This sensor system consists of two fusion proteins, GSK 35-433-CFL and NFL-c-Myc, in which specific fragments of GSK3beta and c-Myc are fused with C-terminal and N-terminal fragments of the split FL, respectively. The sensor detects phosphorylation-specific GSK3beta-c-Myc interaction, the imaging signal of which correlates with the steady-state and temporal regulation of c-Myc phosphorylation in cell culture. The sensor also detects inhibition of c-Myc activity via differential pathways, allowing noninvasive monitoring of c-Myc-targeted drug efficacy in intact cells and living mice. Notably, this drug inhibition is detected before changes in tumor size are apparent in mouse xenograft and liver tumor models. This reporter system not only provides an innovative way to investigate the role of functional c-Myc in normal and cancer-related biological processes, but also facilitates c-Myc-targeted drug development by providing a rapid quantitative approach to assessing cancer response to therapy in living subjects.

ERK activation drives intestinal tumorigenesis in Apc(min/+) mice

Toll-like receptor (TLR) signaling is essential for intestinal tumorigenesis in Apc(min/+) mice, but the mechanisms by which Apc enhances tumor growth are unknown. Here we show that microflora-MyD88-ERK signaling in intestinal epithelial cells (IECs) promotes tumorigenesis by increasing the stability of the c-Myc oncoprotein. Activation of ERK (extracellular signal-related kinase) phosphorylates c-Myc, preventing its ubiquitination and subsequent proteasomal degradation. Accordingly, Apc(min/+)/Myd88(-/-) mice have lower phospho-ERK (p-ERK) levels and fewer and smaller IEC tumors than Apc(min/+) mice. MyD88 (myeloid differentiation primary response gene 88)-independent activation of ERK by epidermal growth factor (EGF) increased p-ERK and c-Myc and restored the multiple intestinal neoplasia (Min) phenotype in Apc(min/+)/Myd88(-/-) mice. Administration of an ERK inhibitor suppressed intestinal tumorigenesis in EGF-treated Apc(min/+)/Myd88(-/-) and Apc(min/+) mice and increased their survival. Our data reveal a new facet of oncogene-environment interaction, in which microflora-induced TLR activation regulates oncogene expression and related IEC tumor growth in a susceptible host.

A therapeutic role for targeting c-Myc/Hif-1-dependent signaling pathways

Deregulated c-Myc occurs in approximately 30% of human cancers. Similarly, hypoxia-inducible factor (HIF) is commonly overexpressed in a variety of human malignancies. Under physiologic conditions, HIF inhibits c-Myc activity; however, when deregulated oncogenic c-Myc collaborates with HIF in inducing the expression of VEGF, PDK1 and hexokinase 2. Most of the knowledge of HIF derives from studies investigating a role of HIF under hypoxic conditions, however, HIF-1alpha stabilization is also found in normoxic conditions. Specifically, under hypoxic conditions HIF-1-mediated regulation of oncogenic c-Myc plays a pivotal role in conferring metabolic advantages to tumor cells as well as adaptation to the tumorigenic micromilieu. In addition, our own results show that under normoxic conditions oncogenic c-Myc is required for constitutive high HIF-1 protein levels and activity in Multiple Myeloma (MM) cells, thereby influencing VEGF secretion and angiogenic activity within the bone marrow microenvironment. Further studies are needed to delineate the functional relevance of HIF, MYC, and the HIF-MYC collaboration in MM and other malignancies, also integrating the tumor microenvironment and the cellular context. Importantly, early studies already demonstrate promising preclinical of novel agents, predominantly small molecules, which target c-Myc, HIF or both.

MYC in oncogenesis and as a target for cancer therapies

MYC proteins (c-MYC, MYCN, and MYCL) regulate processes involved in many if not all aspects of cell fate. Therefore, it is not surprising that the MYC genes are deregulated in several human neoplasias as a result from genetic and epigenetic alterations. The near "omnipotency" together with the many levels of regulation makes MYC an attractive target for tumor intervention therapy. Here, we summarize some of the current understanding of MYC function and provide an overview of different cancer forms with MYC deregulation. We also describe available treatments and highlight novel approaches in the pursuit for MYC-targeting therapies. These efforts, at different stages of development, constitute a promising platform for novel, more specific treatments with fewer side effects. If successful a MYC-targeting therapy has the potential for tailored treatment of a large number of different tumors.

Dihydroartemisinin accelerates c-MYC oncoprotein degradation and induces apoptosis in c-MYC-overexpressing tumor cells

Artemisinin and its derivatives (ARTs) are effective antimalarial drugs and also possess profound anticancer activity. However, the mechanism accounted for its distinctive activity in tumor cells remains unelucidated. We computed Pair wise Pearson correlation coefficients to identify genes that show significant correlation with ARTs activity in NCI-55 cell lines using data obtained from studies with HG-U133A Affymetrix chip. We found c-myc is one of the genes that showed the highest positive correlation coefficients among the probe sets analyzed (r=0.585, P<0.001). Dihydroartemisinin (DHA), the main active metabolite of ARTs, induced significant apoptosis in HL-60 and HCT116 cells that express high levels of c-MYC. Stable knockdown of c-myc abrogated DHA-induced apoptosis in HCT116 cells. Conversely, forced expression of c-myc in NIH3T3 cells sensitized these cells to DHA-induced apoptosis. Interestingly, DHA irreversibly down-regulated the protein level of c-MYC in DHA-sensitive HCT116 cells, which is consistent to persistent G1 phase arrest induced by DHA. Further studies demonstrated that DHA accelerated the degradation of c-MYC protein and this process was blocked by pretreatment with the proteasome inhibitor MG-132 or GSK 3beta inhibitor LiCl in HCT116 cells. Taken together, ARTs might be useful in the treatment of c-MYC-overexpressing tumors. We also suggest that c-MYC may potentially be a biomarker candidate for prediction of the antitumor efficacies of ARTs.

Glycoproteomics in neurodegenerative diseases

Protein glycosylation regulates protein function and cellular distribution. Additionally, aberrant protein glycosylations have been recognized to play major roles in human disorders, including neurodegenerative diseases. Glycoproteomics, a branch of proteomics that catalogs and quantifies glycoproteins, provides a powerful means to systematically profile the glycopeptides or glycoproteins of a complex mixture that are highly enriched in body fluids, and therefore, carry great potential to be diagnostic and/or prognostic markers. Application of this mass spectrometry-based technology to the study of neurodegenerative disorders (e.g., Alzheimer's disease and Parkinson's disease) is relatively new, and is expected to provide insight into the biochemical pathogenesis of neurodegeneration, as well as biomarker discovery. In this review, we have summarized the current understanding of glycoproteins in biology and neurodegenerative disease, and have discussed existing proteomic technologies that are utilized to characterize glycoproteins. Some of the ongoing studies, where glycoproteins isolated from cerebrospinal fluid and human brain are being characterized in Parkinson's disease at different stages versus controls, are presented, along with future applications of targeted validation of brain specific glycoproteins in body fluids.

Prognostic relevance of c-MYC gene amplification and polysomy for chromosome 8 in suboptimally-resected, advanced stage epithelial ovarian cancers: a Gynecologic Oncology Group study

OBJECTIVE

The Gynecologic Oncology Group (GOG) examined the prognostic relevance of c-MYC amplification and polysomy 8 in epithelial ovarian cancer (EOC).

METHODS

Women with suboptimally-resected, advanced stage EOC who participated in GOG-111, a multicenter randomized phase III trial of cyclophosphamide+cisplatin vs. paclitaxel+cisplatin, and who provided a tumor block through GOG-9404 were eligible. Fluorescence in situ hybridization (FISH) with probes for c-MYC and the centromere of chromosome 8 (CEP8) was used to examine c-MYC amplification (> or =2 copies c-MYC/CEP8) and polysomy 8 (> or =4 CEP8 copies).

RESULTS

c-MYC amplification, defined as > or =2 copies c-MYC/CEP8, was observed in 29% (28/97) of EOCs and levels were ranged from 2.0-3.3 copies of c-MYC/CEP8. c-MYC amplification was not associated with patient age, race, GOG performance status, stage, cell type, grade, measurable disease status following surgery, tumor response or disease status following platinum-based combination chemotherapy. Women with vs. without c-MYC amplification did not have an increased risk of disease progression (hazard ratio [HR]=1.03; 95% confidence interval [CI]=0.65-1.64; p=0.884) or death (HR=1.08; 95% CI=0.68-1.72; p=0.745). c-MYC amplification was not an independent prognostic factor for progression-free survival (HR=1.03, 95% CI=0.57-1.85; p=0.922) or overall survival (HR=1.01, 95% CI=0.56-1.80; p=0.982). Similar insignificant results were obtained for c-MYC amplification categorized as > or =1.5 copies c-MYC/CEP8. Polysomy 8 was observed in 22 patients without c-MYC amplification and 3 with c-MYC amplification, and was associated with age and measurable disease status, but not other clinical covariates or outcomes.

CONCLUSIONS

c-MYC amplification and polysomy 8 have limited predictive or prognostic value in suboptimally-resected, advanced stage EOC treated with platinum-based combination chemotherapy.

Identification of domains responsible for ubiquitin-dependent degradation of dMyc by glycogen synthase kinase 3beta and casein kinase 1 kinases

In the present study, we report that ubiquitin-mediated degradation of dMyc, the Drosophila homologue of the human c-myc proto-oncogene, is regulated in vitro and in vivo by members of the casein kinase 1 (CK1) family and by glycogen synthase kinase 3beta (GSK3beta). Using Drosophila S2 cells, we demonstrate that CK1alpha promotes dMyc ubiquitination and degradation with a mechanism similar to the one mediated by GSK3beta in vertebrates. Mutation of ck1alpha or -epsilon or sgg/gsk3beta in Drosophila wing imaginal discs results in the accumulation of dMyc protein, suggesting a physiological role for these kinases in vivo. Analysis of the dMyc amino acid sequence reveals the presence of conserved domains containing potential phosphorylation sites for mitogen kinases, GSK3beta, and members of the CK1 family. We demonstrate that mutations of specific residues within these phosphorylation domains regulate dMyc protein stability and confer resistance to degradation by CK1alpha and GSK3beta kinases. Expression of the dMyc mutants in the compound eye of the adult fly results in a visible defect that is attributed to the effect of dMyc on growth, cell death, and inhibition of ommatidial differentiation.

Antifolate/folate-activated HGF/c-Met signalling pathways in mouse kidneys-the putative role of their downstream effectors in cross-talk with androgen receptor

This in vivo study of mouse kidneys was focused on the identification of protein mediators involved in the cross-talk between two signalling pathways. One pathway was triggered by testosterone via an androgen receptor, AR, and the other induced by CB 3717/folate via HGF, and its membrane receptor c-Met. Sequential activation of these pathways leads to a drastic decrease of testosterone-induced ornithine decarboxylase, ODC, expression. We proved that CB 3717/folate-induced ODC expression is Akt-dependent. CB 3717/folate activates Akt and ERK1/2 kinases, PTEN phosphatase and also up-regulates cyclin D2 and PCNA, but decreases GSK3beta and cyclin D1 protein levels. Testosterone activation of AR induces GSK3beta and PTEN. Results of the sequential activation of the studied signalling pathways suggest that Akt, GSK3beta and possibly ERK1/2 kinases may participate in the negative cross-talk and attenuation of AR transactivity, while the involvement of PTEN and cyclin D1 seems to be doubtful.

Nickel compounds induce apoptosis in human bronchial epithelial Beas-2B cells by activation of c-Myc through ERK pathway

Nickel compounds are carcinogenic to humans and have been shown to alter epigenetic homeostasis. The c-Myc protein controls 15% of human genes and it has been shown that fluctuations of c-Myc protein alter global epigenetic marks. Therefore, the regulation of c-Myc by nickel ions in immortalized but not tumorigenic human bronchial epithelial Beas-2B cells was examined in this study. It was found that c-Myc protein expression was increased by nickel ions in non-tumorigenic Beas-2B and human keratinocyte HaCaT cells. The results also indicated that nickel ions induced apoptosis in Beas-2B cells. Knockout of c-Myc and its restoration in a rat cell system confirmed the essential role of c-Myc in nickel ion-induced apoptosis. Further studies in Beas-2B cells showed that nickel ion increased the c-Myc mRNA level and c-Myc promoter activity, but did not increase c-Myc mRNA and protein stability. Moreover, nickel ion upregulated c-Myc in Beas-2B cells through the MEK/ERK pathway. Collectively, the results demonstrate that c-Myc induction by nickel ions occurs via an ERK-dependent pathway and plays a crucial role in nickel-induced apoptosis in Beas-2B cells.

Reflecting on 25 years with MYC

Just over 25 years ago, MYC, the human homologue of a retroviral oncogene, was identified. Since that time, MYC research has been intense and the advances impressive. On reflection, it is astonishing how each incremental insight into MYC regulation and function has also had an impact on numerous biological disciplines, including our understanding of molecular oncogenesis in general. Here we chronicle the major advances in our understanding of MYC biology, and peer into the future of MYC research.

Nucleophosmin and its AML-associated mutant regulate c-Myc turnover through Fbw7 gamma

Mutations leading to aberrant cytoplasmic localization of nucleophosmin (NPM) are the most frequent genetic alteration in acute myelogenous leukemia (AML). NPM binds the Arf tumor suppressor and protects it from degradation. The AML-associated NPM mutant (NPMmut) also binds p19Arf but is unable to protect it from degradation, which suggests that inactivation of p19Arf contributes to leukemogenesis in AMLs. We report here that NPM regulates turnover of the c-Myc oncoprotein by acting on the F-box protein Fbw7γ, a component of the E3 ligase complex involved in the ubiquitination and proteasome degradation of c-Myc. NPM was required for nucleolar localization and stabilization of Fbw7γ. As a consequence, c-Myc was stabilized in cells lacking NPM. Expression of NPMmut also led to c-Myc stabilization because of its ability to interact with Fbw7γ and delocalize it to the cytoplasm, where it is degraded. Because Fbw7 induces degradation of other growth-promoting proteins, the NPM–Fbw7 interaction emerges as a central tumor suppressor mechanism in human cancer.

c -MYC amplification and expression in astrocytic tumors

The aim of this study was to evaluate the nuclear and cytoplasmic expression of c-MYC protein in human astrocytic tumors of different histopathological grades and to determine whether its expression correlates with c-MYC gene amplification. An immunohistochemical study of c-MYC protein was performed in 140 paraffin-embedded astrocytic tumors of different grades. Among them, 30 specimens were analyzed for c-MYC gene amplification by FISH. Expression of nuclear and cytoplasmic c-MYC was observed, respectively, in 65.0 and 66.4% of the cases studied. The distribution of the positive cases according to the tumor grade increased in both nuclear and cytoplasmic staining with malignancy. The median nuclear LI also increased with tumor grade, with highest c-MYC nuclear expression in grade III. The median cytoplasmic labeling scores showed a significant difference between grade I tumors and diffuse tumors, which presented high and similar median scores. Cytoplasmic c-MYC localization was linked to high nuclear c-MYC expression. FISH results disclosed that the presence of two signals was inversely correlated with histopathological grade, while the presence of >/=5 signals increased according to degree of malignancy. Moreover, the presence of two signals was associated with low nuclear LI and the presence of four or more signals with high nuclear LI. These results indicate that c-MYC expression in astrocytic tumors is strongly associated with increased c-MYC gene copy number and suggest that c-MYC plays a role in the early tumorigenesis of astrocytomas.

Tumor cell-selective regulation of NOXA by c-MYC in response to proteasome inhibition

The proteasome controls a plethora of survival factors in all mammalian cells analyzed to date. Therefore, it is puzzling that proteasome inhibitors such as bortezomib can display a preferential toxicity toward malignant cells. In fact, proteasome inhibitors have the salient feature of promoting a dramatic induction of the proapoptotic protein NOXA in a tumor cell-restricted manner. However, the molecular determinants that control this specific regulation of NOXA are unknown. Here, we show that the induction of NOXA by bortezomib is directly dependent on the oncogene c-MYC. This requirement for c-MYC was found in a variety of tumor cell types, in marked contrast with dispensable roles of p53, HIF-1alpha, and E2F-1 (classical proteasomal targets that can regulate NOXA mRNA under stress). Conserved MYC-binding sites identified at the NOXA promoter were validated by ChIP and reporter assays. Down-regulation of the endogenous levels of c-MYC abrogated the induction of NOXA in proteasome-defective tumor cells. Conversely, forced expression of c-MYC enabled normal cells to accumulate NOXA and subsequently activate cell death programs in response to proteasome blockage. c-MYC is itself a proteasomal target whose levels or function are invariably up-regulated during tumor progression. Our data provide an unexpected function of c-MYC in the control of the apoptotic machinery, and reveal a long sought-after oncogenic event conferring sensitivity to proteasome inhibition.

Boronic acid–lectin affinity chromatography. 1. Simultaneous glycoprotein binding with selective or combined elution

We introduce a novel combination of boronic acid affinity chromatography with lectin affinity chromatography, dubbed as boronic acid-lectin affinity chromatography (BLAC). Concanavalin A and wheat germ agglutinin lectins were mixed with the pesudo-lectin boronic acid to form the BLAC affinity column and their performance was evaluated with standard glycoproteins. Optimization of the binding and elution buffers for the BLAC system is described. The BLAC columns were employed to isolate glycoproteins of interest using both selective and/or combined elution.

Concise Review: Role and Function of the Ubiquitin-Proteasome System in Mammalian Stem and Progenitor Cells

Highly ordered degradation of cell proteins by the ubiquitin-proteasome system, a sophisticated cellular proteolytic machinery, has been identified as a key regulatory mechanism in many eukaryotic cells. Accumulating evidence reveals that the ubiquitin-proteasome system is involved in the regulation of fundamental processes in mammalian stem and progenitor cells of embryonic, neural, hematopoietic, and mesenchymal origin. Such processes, including development, survival, differentiation, lineage commitment, migration, and homing, are directly controlled by the ubiquitin-proteasome system, either via proteolytic degradation of key regulatory proteins of signaling and gene expression pathways or via nonproteolytic mechanisms involving the proteasome itself or posttranslational modifications of target proteins by ubiquitin or other ubiquitin-like modifiers. Future characterization of the precise roles and functions of the ubiquitin-proteasome system in mammalian stem and early progenitor cells will improve our understanding of stem cell biology and may provide an experimental basis for the development of novel therapeutic strategies in regenerative medicine. Disclosure of potential conflicts of interest is found at the end of this article.

The Kaposi's sarcoma-associated herpesvirus LANA protein stabilizes and activates c-Myc

The Kaposi's sarcoma-associated herpesvirus (KSHV) latency-associated nuclear antigen (LANA) protein is functionally pleiotropic. LANA contributes to KSHV-associated pathogenesis, in part, by increasing entry of cells into S phase through a process that is driven by LANA interaction with the serine-threonine kinase glycogen synthase kinase 3 (GSK-3) and stabilization of beta-catenin. We now show that LANA affects the activity of another protein involved in cell cycle regulation, c-Myc. Sequencing of c-Myc coding sequences revealed that c-Myc in KSHV-positive primary effusion lymphoma (PEL) cell lines is wild type in the N-terminal region that regulates c-Myc protein stability. Despite this, c-Myc in PEL cells is stabilized. In LANA-expressing cells, inactivation of nuclear GSK-3 reduced phosphorylation of c-Myc at Thr58 and contributed to c-Myc stabilization by decreasing c-Myc ubiquitination. Phosphorylation of c-Myc on Ser62 also affects c-Myc stability and function. We now show that LANA increases the level of phosphorylated extracellular signal-regulated kinase 1 (ERK1) and increases ERK phosphorylation of c-Myc on Ser62. LANA also interacted with c-Myc, and c-Myc amino acids 147 to 220 were required for this interaction. LANA (L1006P) retained the ability to bind to c-Myc and activate ERK1, indicating that these events did not require LANA interaction with GSK-3. Thus, LANA stabilizes c-Myc; prevents the phosphorylation of c-Myc at Thr58, an event that promotes Myc-induced apoptosis; and independently stimulates phosphorylation of c-Myc at Ser62, an event that transcriptionally activates c-Myc. LANA-mediated manipulation of c-Myc function is likely to contribute to KSHV-associated tumorigenesis through the induction of c-Myc regulated cellular genes, as well as by the stimulation of cell cycle progression.

Transductal fluxes of water and monovalent ions in ferret salivary glands

The net transductal fluxes of water and monovalent ions were measured in the parotid and submandibular salivary glands of the ferret, Mustela putorius furo, during stimulation of secretion with pilocarpine. The duct systems of these glands were found to be impermeable to water using the split-oil droplet method for stationary microperfusion of lobular and main ducts. The net transductal fluxes of Na+, K+, Cl-, and HCO3- were characterized by analysis of ductal fluid samples obtained simultaneously from the intercalated, lobular, and main ducts of these glands.