Myc is an essential negative regulator of platelet-derived growth factor beta receptor expression

Macrophage-mediated PDGF Activation Correlates with Regenerative Outcomes Following Musculoskeletal Trauma

OBJECTIVE

Our objective was to identify macrophage subpopulations and gene signatures associated with regenerative or fibrotic healing across different musculoskeletal injury types.

BACKGROUND

Subpopulations of macrophages are hypothesized to fine tune the immune response after damage, promoting either normal regenerative, or aberrant fibrotic healing.

METHODS

Mouse single-cell RNA sequencing data before and after injury were assembled from models of musculoskeletal injury, including regenerative and fibrotic mouse volumetric muscle loss (VML), regenerative digit tip amputation (DTA), and fibrotic heterotopic ossification (HO). R packages Harmony , MacSpectrum and Seurat were used for data integration, analysis and visualizations.

RESULTS

There was substantial overlap between macrophages from the regenerative VML (2 mm injury) and regenerative bone (DTA) models, as well as a separate overlap between the fibrotic VML (3 mm injury) and fibrotic bone (HO) models. We identified 2 fibrotic-like (FL 1 and FL 2) along with 3 regenerative-like (RL 1, RL 2, and RL 3) subpopulations of macrophages, each of which was transcriptionally distinct. We found that regenerative and fibrotic conditions had similar compositions of pro-inflammatory and anti-inflammatory macrophages, suggesting that macrophage polarization state did not correlate with healing outcomes. Receptor/ligand analysis of macrophage-to-mesenchymal progenitor cell (MPC) crosstalk showed enhanced transforming growth factor beta (TGF-β) in fibrotic conditions and enhanced platelet derived growth factor (PDGF) signaling in regenerative conditions.

CONCLUSION

Characterization of macrophage subtypes could be used to predict fibrotic responses following injury and provide a therapeutic target to tune the healing microenvironment towards more regenerative conditions.

TRIM28 and TRIM27 are required for expressions of PDGFRβ and contractile phenotypic genes by vascular smooth muscle cells

Vascular smooth muscle cells (VSMCs) in the normal arterial media continually express contractile phenotypic markers which are reduced dramatically in response to injury. Tripartite motif-containing proteins are a family of scaffold proteins shown to regulate gene silencing, cell growth, and differentiation. We here investigated the biological role of tripartite motif-containing 28 (TRIM28) and tripartite motif-containing 27 (TRIM27) in VSMCs. We observed that siRNA-mediated knockdown of TRIM28 and TRIM27 inhibited platelet-derived growth factor (PDGF)-induced migration in human VSMCs. Both TRIM28 and TRIM27 can regulate serum response element activity and were required for maintaining the contractile gene expression in human VSMCs. At the same time, TRIM28 and TRIM27 knockdown reduced the expression of PDGF receptor-β (PDGFRβ) and the phosphorylation of its downstream signaling components. Immunoprecipitation showed that TRIM28 formed complexes with TRIM27 through its N-terminal RING-B boxes-Coiled-Coil domain. Furthermore, TRIM28 and TRIM27 were shown to be upregulated and mediate the VSMC contractile marker gene and PDGFRβ expression in differentiating human bone marrow mesenchymal stem cells. In conclusion, we identified that TRIM28 and TRIM27 cooperatively maintain the endogenous expression of PDGFRβ and contractile phenotype of human VSMCs.

C-Myc Deregulation is Involved in Melphalan Resistance of Multiple Myeloma: Role of PDGF-BB

Oncogenes are important regulators of cancer growth and progression and their action may be modulated by proteins of the growth factor family, such as angiogenic cytokines, known to be strongly involved in neoplastic evolution. Reciprocal interactions between oncogenes and angiogenic modulators may represent, in haematological neoplasms, including multiple myeloma (MM), a possible mechanism of drug resistance. The aim of this work is to investigate in vitro and in vivo whether or not c-myc deregulation is involved in the melphalan resistance elicited by myeloma patients and consequently to clarify the role of the angiogenic factor PDGF-BB in modulating c-myc protein expression. Fifty-one MM patients on chemotherapy with melphalan were analyzed for structural alterations of the c-myc gene, c-Myc protein expression, as well as for serum PDGF-BB release. For the in vitro study, two M14-derived established cell clones, differing for the c-Myc protein expression (c-Myc low -expressing or constitutively expressing clones) were used. Our results show that PDGF-BB is able to up-regulate Myc expression and reduce melphalan sensitivity of tumor cell clones, constitutively expressing c-myc gene product. In addition, down-regulation of c-Myc protein induces the expression of PDGF-β receptor molecules and reduces PDGF-BB release. In agreement with these results, in vivo data show that melphalan-resistant MM patients present overexpressed c-Myc protein and higher serum PDGF-β receptor levels compared to minor responding patients.

A circular RNA promotes tumorigenesis by inducing c-myc nuclear translocation

Circular RNAs (circRNAs) are a subclass of noncoding RNAs widely expressed in mammalian cells. We report here the tumorigenic capacity of a circRNA derived from angiomotin-like1 (circ-Amotl1). Circ-Amotl1 is highly expressed in patient tumor samples and cancer cell lines. Single-cell inoculations using circ-Amotl1-transfected tumor cells showed a 30-fold increase in proliferative capacity relative to control. Agarose colony-formation assays similarly revealed a 142-fold increase. Tumor-take rate in nude mouse xenografts using 6-day (219 cells) and 3-day (9 cells) colonies were 100%, suggesting tumor-forming potential of every cell. Subcutaneous single-cell injections led to the formation of palpable tumors in 41% of mice, with tumor sizes >1 cm³ in 1 month. We further found that this potent tumorigenicity was triggered through interactions between circ-Amotl1 and c-myc. A putative binding site was identified in silico and tested experimentally. Ectopic expression of circ-Amotl1 increased retention of nuclear c-myc, appearing to promote c-myc stability and upregulate c-myc targets. Expression of circ-Amotl1 also increased the affinity of c-myc binding to a number of promoters. Our study therefore reveals a novel function of circRNAs in tumorigenesis, and this subclass of noncoding RNAs may represent a potential target in cancer therapy.

ELL targets c-Myc for proteasomal degradation and suppresses tumour growth

Increasing evidence supports that ELL (eleven-nineteen lysine-rich leukaemia) is a key regulator of transcriptional elongation, but the physiological function of Ell in mammals remains elusive. Here we show that ELL functions as an E3 ubiquitin ligase and targets c-Myc for proteasomal degradation. In addition, we identify that UbcH8 serves as a ubiquitin-conjugating enzyme in this pathway. Cysteine 595 of ELL is an active site of the enzyme; its mutation to alanine (C595A) renders the protein unable to promote the ubiquitination and degradation of c-Myc. ELL-mediated c-Myc degradation inhibits c-Myc-dependent transcriptional activity and cell proliferation, and also suppresses c-Myc-dependent xenograft tumour growth. In contrast, the ELL(C595A) mutant not only loses the ability to inhibit cell proliferation and xenograft tumour growth, but also promotes tumour metastasis. Thus, our work reveals a previously unrecognized function for ELL as an E3 ubiquitin ligase for c-Myc and a potential tumour suppressor.

Assessing adaptation of the cancer kinome in response to targeted therapies

Cancer cells are dependent on protein kinase signalling networks to drive proliferation and to promote survival, and, accordingly, kinases continue to represent a major target class for development of anti-cancer therapeutics. Kinase inhibitors nevertheless have yielded only limited success with many different malignancies due to the inability of single agents to sustain a durable clinical response. Cancer cell kinomes are highly resilient and able to bypass targeted kinase inhibition, leading to tumour resistance. A novel platform has been developed to analyse the activity of the expressed kinome using MIBs (multiplexed inhibitor beads), which consist of Sepharose beads with covalently immobilized inhibitors that preferentially bind activated kinases. Coupling MIB capture with MS (MIB-MS) allows simultaneous determination of the activity of over 75% of the expressed kinome, facilitating high-throughput assessment of adaptive kinase responses resulting from deregulated feedback and feedforward regulatory mechanisms. The adaptive response frequently involves transcriptional up-regulation of specific kinases that allow bypass of the targeted kinase. Understanding how the kinome reprogrammes to targeted kinase inhibition will allow novel therapeutic strategies to be developed for durable clinical responses.

p19Arf represses platelet-derived growth factor receptor β by transcriptional and posttranscriptional mechanisms

In addition to cancer surveillance, p19(Arf) plays an essential role in blocking signals stemming from platelet-derived growth factor receptor β (Pdgfrβ) during eye development, but the underlying mechanisms have not been clear. We now show that without Arf, pericyte hyperplasia in the eye results from enhanced Pdgfrβ-dependent proliferation from embryonic day 13.5 (E13.5) of mouse development. Loss of Arf in the eye increases Pdgfrβ expression. In cultured fibroblasts and pericyte-like cells, ectopic p19(Arf) represses and Arf knockdown enhances the expression of Pdgfrβ mRNA and protein. Ectopic Arf also represses primary Pdgfrβ transcripts and a plasmid driven by a minimal promoter, including one missing the CCAAT element required for high-level expression. p19(Arf) uses both p53-dependent and -independent mechanisms to control Pdgfrβ. In vivo, without p53, Pdgfrβ mRNA is elevated and eye development abnormalities resemble the Arf (-/-) phenotype. However, effects of p53 on Pdgfrβ mRNA do not appear to be due to direct p53 or RNA polymerase II recruitment to the promoter. Although p19(Arf) controls Pdgfrβ mRNA in a p53-dependent manner, it also blunts Pdgfrβ protein expression by blocking new protein synthesis in the absence of p53. Thus, our findings demonstrate a novel capacity for p19(Arf) to control Pdgfrβ expression by p53-dependent and -independent mechanisms involving RNA transcription and protein synthesis, respectively, to promote the vascular remodeling needed for normal vision.

Dynamic reprogramming of the kinome in response to targeted MEK inhibition in triple-negative breast cancer

Kinase inhibitors have limited success in cancer treatment because tumors circumvent their action. Using a quantitative proteomics approach, we assessed kinome activity in response to MEK inhibition in triple-negative breast cancer (TNBC) cells and genetically engineered mice (GEMMs). MEK inhibition caused acute ERK activity loss, resulting in rapid c-Myc degradation that induced expression and activation of several receptor tyrosine kinases (RTKs). RNAi knockdown of ERK or c-Myc mimicked RTK induction by MEK inhibitors, and prevention of proteasomal c-Myc degradation blocked kinome reprogramming. MEK inhibitor-induced RTK stimulation overcame MEK2 inhibition, but not MEK1 inhibition, reactivating ERK and producing drug resistance. The C3Tag GEMM for TNBC similarly induced RTKs in response to MEK inhibition. The inhibitor-induced RTK profile suggested a kinase inhibitor combination therapy that produced GEMM tumor apoptosis and regression where single agents were ineffective. This approach defines mechanisms of drug resistance, allowing rational design of combination therapies for cancer.

MicroRNA-9 is an activation-induced regulator of PDGFR-beta expression in cardiomyocytes

The platelet derived growth factor receptor (PDGFR) is an important target for novel anti-cancer therapeutics, but agents targeting PDGFR have been associated with cardiotoxicity. Cardiomyocyte PDGFR-β signaling in pressure-overloaded hearts induces compensatory angiogenesis via a paracrine-signaling cascade. Tight regulation of receptor tyrosine kinases in response to ligand stimulation is a critical part of any such cascade. The objective of the present study was to characterize the early and late regulation of PDGFR-β following ligand stimulation and define a potential role for microRNAs (miRNAs) predicted to interact with the 3'UTR of PDGFR-β in feedback regulation. Using two in-vitro model systems (U87 glioblastoma cells and neonatal cardiomyocytes), we observed that in response to stimulation with PDGF-BB, levels of PDGFR-β declined beginning at one hour, persisting for 48 h. PDGFR-β mRNA levels declined beginning at 6h after receptor activation. Early, but not late activation-induced receptor downregulation was proteasome dependent. Levels of miRNA-9 (miR-9) were significantly increased in U87 cells and cardiomyocytes beginning 6h after addition of ligand. In response to pressure overload, miR-9 levels were significantly reduced in the hearts of cardiac-specific PDGFR-β knockout mice. Luciferase reporter assays demonstrate that miR-9 directly interacts with its predicted seed in the 3'UTR of PDGFR-β. Increasing miR-9 levels reduces levels of PDGFR-β, resulting in a reduction in the paracrine angiogenic capacity of cardiomyocytes, consistent with the established function of cardiomyocyte PDGFR-β. Importantly, increase of anti-miR-9 in cardiomyocytes attenuates ligand-induced PDGFR-β downregulation. In conclusion, we have identified miR-9 as an activation-induced regulator of PDGFR-β expression in cardiomyocytes that is part of a negative feedback loop which serves to modulate PDGFR-β expression upon ligand-stimulation through direct interaction with the 3'UTR of PDFGR-β. This article is part of a Special Issue entitled 'Possible Editorial'.

Application of autologous derived-platelet rich plasma gel in the treatment of chronic wound ulcer: diabetic foot ulcer

The treatment of chronic wounds remains problematic, despite new insight into the cellular and molecular basis of wound healing. Although the aetio-pathogenesis of chronic wounds is said to be multi-factorial, it is evident from literature that effective and adequate wound debridement has produced the most consistent effect in chronic wound treatment. There is a growing body of evidence that suggests that wound healing in chronic diabetic foot ulcers is growth factor dependent and that the therapeutic delivery of these growth factors to wounds topically, has the potential ability to accelerate wound healing in conjunction with conventional wound care. Autologous derived platelet concentrate is activated to release growth factors that are stored in the platelet granules. These secretory proteins include cytokines and growth factors such as transforming growth factor-beta, vascular endothelia growth factor, platelet derived growth factor, and so on. The enhancement of soft tissue healing by the application of autologous derived platelet rich plasma gel (APG) is supported by basic science and some clinical studies. This review article will attempt to provide a concise report of current concepts on the use of APG in treating chronic ulcers.

Dysregulation of platelet-derived growth factor beta-receptor expression by DeltaNp73 in neuroblastoma

We have previously characterized how p53 family proteins control the transcriptional regulation of the platelet-derived growth factor beta-receptor (PDGFRB) and found that DeltaNp73alpha, acting dominant-negatively to p53 and p73, can upregulate PDGFRB promoter activity. Here, we report that PDGFRB regulation differs between two neuroblastoma cell lines, correlating with the actions of DeltaNp73. We found that PDGFRB was highly expressed in IMR-32 cells, and serum stimulation of IMR-32 cells did not downregulate PDGFRB expression, as seen in SH-SY5Y cells. In IMR-32, DeltaNp73 was found constitutively bound to the PDGFRB promoter, and silencing of DeltaNp73 resulted in repression of PDGFRB promoter activity as well as decreased PDGFRB protein expression. However, the anticancer drug cisplatin, known to stabilize and activate p53 and p73, downregulated PDGFRB expression not only in SH-SY5Y but also in IMR-32. Chromatin immunoprecipitation showed that cisplatin removed DeltaNp73 from the PDGFRB promoter and recruited p53 and p73, leading to binding of histone deacetylase 4. These results suggest a direct role of DeltaNp73 in the constantly enhanced PDGFRB expression seen in tumors.

Bcl-2 and c-Myc co-operate in the Epstein – Barr virus-immortalized human B-cell line GM607 but do not confer tumorigenicity

Eighty-five percent of follicular lymphomas possess a characteristic t(14;18) translocation that results in the deregulated expression of the proto-oncogene BCL-2. BCL-2 overexpression alone is insufficient for full cellular transformation and at least 1 other genetic event is believed to be necessary for follicular lymphoma development. Deregulated c-Myc expression has previously been shown to cooperate with Bcl-2 to transform murine fibroblast cell lines and lead to tumor development in mice. We have developed a human model system to study early transformation in lymphoid cells using immortalized lymphoblastoid cells. We sequentially introduced BCL-2 and c-MYC, 2 proto-oncogenes known to be involved in the transformation of B cells into Epstein-Barr virus (EBV)-immortalized human B cells. We show that the c-Myc and Bcl-2 overexpression, together with EBV immortalization were insufficient to cause full cellular transformation as measured by cell proliferation rates, soft agar and tumorigenicity assays. These results show that more than 3 genetic hits (EBV infection, Bcl-2 and c-Myc overexpression) were required for the full cellular transformation of human lymphoblastoid cells. However, subtle changes in cellular proliferation and sensitivity to apoptosis were documented, at non-limiting dilutions. These changes may confer a susceptibility to the modified cells such that they are more susceptible to the acquisition of additional genetic changes and evolve towards a fully transformed state. In addition, the model system developed may be suitable for the identification of further known and novel oncogenic events involved in the full transformation of B cells.

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.

The mych gene is required for neural crest survival during zebrafish development

Background

Among Myc family genes, c-Myc is known to have a role in neural crest specification in Xenopus and in craniofacial development in the mouse. There is no information on the function of other Myc genes in neural crest development, or about any developmental role of zebrafish Myc genes.

Principal Findings

We isolated the zebrafish mych (myc homologue) gene. Knockdown of mych leads to severe defects in craniofacial development and in certain other tissues including the eye. These phenotypes appear to be caused by cell death in the neural crest and in the eye field in the anterior brain.

Significance

Mych is a novel factor required for neural crest cell survival in zebrafish.

Integration of genome and chromatin structure with gene expression profiles to predict c-MYC recognition site binding and function

The MYC genes encode nuclear sequence specific-binding DNA-binding proteins that are pleiotropic regulators of cellular function, and the c-MYC proto-oncogene is deregulated and/or mutated in most human cancers. Experimental studies of MYC binding to the genome are not fully consistent. While many c-MYC recognition sites can be identified in c-MYC responsive genes, other motif matches-even experimentally confirmed sites-are associated with genes showing no c-MYC response. We have developed a computational model that integrates multiple sources of evidence to predict which genes will bind and be regulated by MYC in vivo. First, a Bayesian network classifier is used to predict those c-MYC recognition sites that are most likely to exhibit high-occupancy binding in chromatin immunoprecipitation studies. This classifier incorporates genomic sequence, experimentally determined genomic chromatin acetylation islands, and predicted methylation status from a computational model estimating the likelihood of genomic DNA methylation. We find that the predictions from this classifier are also applicable to other transcription factors, such as cAMP-response element-binding protein, whose binding sites are sensitive to DNA methylation. Second, the MYC binding probability is combined with the gene expression profile data from nine independent microarray datasets in multiple tissues. Finally, we may consider gene function annotations in Gene Ontology to predict the c-MYC targets. We assess the performance of our prediction results by comparing them with the c-myc targets identified in the biomedical literature. In total, we predict 460 likely c-MYC target genes in the human genome, of which 67 have been reported to be both bound and regulated by MYC, 68 are bound by MYC, and another 80 are MYC-regulated. The approach thus successfully identifies many known c-MYC targets and suggests many novel sites. Our findings suggest that to identify c-MYC genomic targets, integration of different data sources helps to improve the accuracy.

PI3K is involved in PDGF-beta receptor upregulation post-PDGF-BB treatment in mouse HSC

Increased expression of PDGF-beta receptors is a landmark of hepatic stellate cell activation and transdifferentiation into myofibroblasts. However, the molecular mechanisms that regulate the fate of the receptor are lacking. Recent studies suggested that N-acetylcysteine enhances the extracellular degradation of PDGF-beta receptor by cathepsin B, thus suggesting that the absence of PDGF-beta receptors in quiescent cells is due to an active process of elimination and not to a lack of expression. In this communication we investigated further molecular mechanisms involved in PDGF-beta receptor elimination and reappearance after incubation with PDGF-BB. We showed that in culture-activated hepatic stellate cells there is no internal protein pool of receptor, that the protein is maximally phosphorylated by 5 min and completely degraded after 1 h by a lysosomal-dependent mechanism. Inhibition of receptor autophosphorylation by tyrphostin 1296 prevented its degradation, but several proteasomal inhibitors had no effect. We also showed that receptor reappearance is time and dose dependent, being more delayed in cells treated with 50 ng/ml (48 h) compared with 10 ng/ml (24 h).

Biological responses to PDGF-BB versus PDGF-DD in human mesangial cells

Platelet-derived growth factor (PDGF)-BB and PDGF-DD mediate mesangial cell proliferation in vitro and in vivo. While PDGF-BB is a ligand for the PDGF alpha- and beta-receptor chains, PDGF-DD binds more selectively to the beta-chain, suggesting potential differences in the biological activities. Signal transduction and regulation of gene expression induced by PDGF-BB and -DD were compared in primary human mesangial cells (HMCs), which expressed PDGF alpha- and beta-receptor subunits. The growth factor concentrations used were chosen based on their equipotency in inducing HMCs proliferation and binding to the betabeta-receptor. Both growth factors, albeit at different concentrations induced phosphorylation and activation of extracellular signal-regulated kinase 1 (ERK1) and ERK2. In addition, PDGFs led to the phosphorylation and activation of signal transducers and activators of transcription 1 (STAT1) and STAT3. HMCs proliferation induced by either PDGF-BB or -DD could be blocked by signal transduction inhibitors of the mitogen-activated protein kinase-, Janus kinase (JAK)/STAT-, or phosphatidyl-inositol 3-kinase pathways. Using a gene chip array and subsequent verification by real-time reverse transcriptase (RT)-polymerase chain reaction, we found that in HMC genes for matrix metalloproteinase 13 (MMP-13) and MMP-14 and, to a low extent, cytochrome B5 and cathepsin L were exclusively regulated by PDGF-BB, whereas no exclusive gene regulation was detected by PDGF-DD. However, at the protein level, both MMP-13 and -14 were equally induced by PDGF-BB and -DD. PDGF-BB and -DD effect similar biological responses in HMCs albeit at different potencies. Rare apparently differential gene regulation did not result in different protein expression, suggesting that in HMCs both PDGFs exert their biological activity almost exclusively via the PDGF beta-receptor.

Cancer therapeutics: targeting the dark side of Myc

The potent Myc oncoprotein plays a pivotal role as a regulator of tumorigenesis in numerous human cancers of diverse origin. Experimental evidence shows that inhibiting Myc significantly halts tumour cell growth and proliferation. This review summarises recent progress in understanding the function of Myc as a transcription factor, with emphasis on key protein interactions and target gene regulation. In addition, major advances in drug development aimed at eliminating Myc are described, including antisense and triple helix forming oligonucleotides, porphyrins and siRNA. Future anti-Myc strategies are also discussed that inhibit Myc at the level of expression and/or function. Targeting the dark side of Myc with novel therapeutic agents promises to have a profound impact in combating cancer.

Molecular pathogenesis of chronic wounds: the role of beta-catenin and c-myc in the inhibition of epithelialization and wound healing

Lack of understanding of the molecular mechanisms and pathogenesis of impaired healing in chronic ulcers is a serious health issue that contributes to excessive limb amputations and mortality. Here we show that beta-catenin and its downstream targets in keratinocytes, c-myc, and keratins K6 and K16, play important roles in the development of chronic wounds. In contrast to normal epidermis, we observed a significant nuclear presence of beta-catenin and elevated c-myc expression at the nonhealing wound edge of chronic ulcers from 10 patients. In vitro studies indicated that stabilization of nuclear beta-catenin inhibited wound healing and keratinocyte migration by blocking epidermal growth factor response, inducing c-myc and repressing the K6/K16 keratins (cytoskeletal components important for migration). The molecular mechanism of K6/K16 repression involved beta-catenin and arginine methyltransferase (CARM-1) acting as co-repressors of glucocorticoid receptor monomers. We conclude that activation of the beta-catenin/c-myc pathway(s) contributes to impaired healing by inhibiting keratinocyte migration and altering their differentiation. The presence of activated beta-catenin and c-myc in the epidermis of chronic wounds may serve as a molecular marker of impaired healing and may provide future targets for therapeutic intervention.

Schlafen-1 causes a cell cycle arrest by inhibiting induction of cyclin D1

Schlafen-1 (Slfn-1), the prototypic member of the Schlafen family of proteins, was described as an inducer of growth arrest in T-lymphocytes and causes a cell cycle arrest in NIH3T3 fibroblasts prior to the G1/S transition. How Slfn-1 exerts its effects on the cell cycle is not currently known. We report that synchronized murine fibroblasts expressing Slfn-1 do not exit G1 when stimulated with fetal calf serum, platelet-derived growth factor BB (PDGF-BB) or epidermal growth factor (EGF). The induction of cyclin D1 by these stimuli was blocked in the presence of Slfn-1 as were all downstream cell cycle processes. Overexpression of cyclin D1 in growth-arrested, Slfn-1-expressing cells induced an increase in cell growth consistent with this protein being the biological target of Slfn-1. Activation of the mitogen-activated protein kinase pathway by EGF or phorbol 12-myristate 13-acetate was unaffected by Slfn-1 expression. PDGF signaling was, however, almost completely blocked. This was due to a lack of PDGF receptor expression in Slfn-1-expressing cells consistent with Slfn-1 blocking the cell cycle in G1 where PDGF receptor expression is normally down-regulated. Finally, overexpression of Slfn-1 inhibited the activation of the cyclin D1 promoter. Slfn-1 therefore causes a cell cycle arrest during G1 by inhibiting induction of cyclin D1 by mitogens.

Suppression of MEK/ERK signalling by Myc: role of Bin-1

We report for the first time that over-expression of Myc suppresses mitogen-activated ERK kinase (MEK)/extracellular regulated kinase (ERK) signalling in chick embryo fibroblasts (CEF). Myc does not interfere with individual components of the signalling cascade, since efficient signal propagation via MEK and ERK in Myc-infected CEF can be seen. However, using the Myc-binding domain (MBD) of Bin-1, which binds to and negatively regulates the activity of Myc, we selectively suppressed Myc-induced apoptosis, without affecting its transforming properties. This was accompanied by a restoration in MEK/ERK signalling, suggesting a critical role for this pathway in regulating apoptosis in these cells. This was also confirmed using a specific pharmacological inhibitor of MEK. Experiments with conditioned media suggest that over-expression of Myc may inhibit autocrine growth factor production, which can be restored by co-expression of MBD. Although the identity of the growth factor(s) is not known, we propose a feedback mechanism whereby Myc interferes with growth factor signalling.

Physiology and gene expression characteristics of carcinogen-initiated and tumor-transformed glial progenitor cells derived from the CNS of methylnitrosourea (MNU)-treated Sprague-Dawley rats

Glial progenitors from the brain of normal adult Sprague-Dawley rats were compared to their initiated and malignant counterparts that were isolated from apparently normal brains of animals exposed to methylnitrosourea (MNU). Fibroblast growth factor-2 (FGF-2) or platelet-derived growth factor (PDGF)-A or -B induced differentiation of normal progenitors to a pro-astrocytic or oligodendrocytic morphology, respectively, whereas the combination of these factors resulted in their terminal differentiation to oligodendrocytes and senescence. In contrast, initiated progenitors did not exit the cell cycle when stimulated with PDGF and/or FGF-2. cDNA oligoarray analysis and RT-PCR verification showed an early upregulation/ induction of growth factor/receptors, PDGF-A, PDGFR-beta, IGFR-1, IGF-1 and -2, IL-6, MEGF-5, FRAG-1, IRS-2, HSPG, and FGFR-1, followed by a late increase in the expression IGFBP-6, PDGF-alpha, FGFR-4A, c/ERB-A, and FGFR-4, 2, and 1 during the tumorigenic progression. Western blot analyses demonstrated that MNU exposure caused progressive reduction of p21 protein levels, an increase of Rb phosphorylation, activation of AKT and CDK2, and upregulation of FGF receptors. Double immunofluorescence labeling showed progressive increase in nuclear colocalization of FGFR1, 2, and 4, which peaked in malignant lines. It is postulated that transition of normal rat glial progenitors to an initiated state is driven by IGF-1 and 2, IL-6, and the upregulation of the receptors PDGFR-beta and FGFR-1, 2, and 4. Deregulation of the cell cycle in this state involves reduction of p21 protein, concomitant upregulation of CDC2, and an increase in Rb phosphorylation that favors expression and nuclear translocation of FGFR-4 and FRAG-1 and 2. These events are associated with progressive activation of AKT and RAS. Malignant transformation is enhanced by near elimination of p21 and PC3, induction of AP-1 (upregulation of JUN-B, c-JUN, FRA-1), activation of the NF-kB pro-survival pathway, and inhibition of the TGF-beta pro-apoptotic pathway possibly in response to changes in the expression of nerve growth factor (NGF) I-A and NGFI-B. These data demonstrate that the events leading to malignancy in the rat brain in response to MNU treatment are to a great extent similar to those described for secondary glial malignancies in humans.

p73 competes with co-activators and recruits histone deacetylase to NF-Y in the repression of PDGF beta-receptor

We investigated mechanisms of the p73alpha-mediated repression of the platelet-derived growth factor beta-receptor (PDGFRB) promoter caused by its interaction with NF-Y. Treatment of cells with the histone deacetylase (HDAC) inhibitor, Trichostatin A, increases PDGFRB promoter activity through the CCAAT motif and counteracts the repression caused by p73alpha. Activation of the PDGFRB promoter by the co-activator p300 also occurs through the CCAAT motif. Expression of p73alpha counteracts both p300- and P/CAF-mediated activation of the PDGFRB promoter, and expression of p300 or P/CAF attenuates the p73alpha-mediated repression of the promoter activity. In concordance, p73alpha decreases the p300-mediated acetylation of NF-YC, p300 competes with p73alpha for binding NF-YB, and P/CAF competes with p73alpha for binding NF-YB and NF-YC. Furthermore, p73alpha, but not the oncogenic DeltaNp73alpha, binds directly to HDAC1. We performed chromatin immunoprecipitation with antibodies against p73, DeltaNp73, NFYB, p300 and HDAC1 at different periods after serum stimulation in serum-starved NIH3T3 cells. A marked decrease of DeltaNp73, NF-YB and p300 was detected 6 hours after serum stimulation when the expression of PDGFRB decreased. Conversely, HDAC1 was found bound at its maximum and the anti-p73 detecting both TAp73 and DeltaNp73 was found at all time points, indicating that p73, but not DeltaNp73, remains bound at this time. Double immunofluorescence staining of TAp73 and HDAC1 revealed that both of these molecules exist in the nucleus at this time point, supporting the presence of endogenous interaction. These results suggest that p73 and DeltaNp73 behave as physiological regulators for the transcription of the PDGFRB promoter.

p19ARF directly and differentially controls the functions of c-Myc independently of p53

Increased expression of the oncogenic transcription factor c-Myc causes unregulated cell cycle progression. c-Myc can also cause apoptosis, but it is not known whether the activation and/or repression of c-Myc target genes mediates these diverse functions of c-Myc. Because unchecked cell cycle progression leads to hyperproliferation and tumorigenesis, it is essential for tumour suppressors, such as p53 and p19ARF (ARF), to curb cell cycle progression in response to increased c-Myc (refs 2, 3). Increased c-Myc has previously been shown to induce ARF expression, which leads to cell cycle arrest or apoptosis through the activation of p53 (ref. 4). Here we show that ARF can inhibit c-Myc by a unique and direct mechanism that is independent of p53. When c-Myc increases, ARF binds with c-Myc and dramatically blocks c-Myc's ability to activate transcription and induce hyperproliferation and transformation. In contrast, c-Myc's ability to repress transcription is unaffected by ARF and c-Myc-mediated apoptosis is enhanced. These differential effects of ARF on c-Myc function suggest that separate molecular mechanisms mediate c-Myc-induced hyperproliferation and apoptosis. This direct feedback mechanism represents a p53-independent checkpoint to prevent c-Myc-mediated tumorigenesis.

pRb, Myc and p53 are critically involved in SV40 large T antigen repression of PDGF beta-receptor transcription

The expression of the PDGF beta-receptor is tightly regulated during a normal cell cycle. c-Myc and p73alpha repress transcription of the receptor through interaction with NF-Y. In ST15A cells which stably express the temperature-sensitive SV40 large T antigen (LT) the receptor expression and ligand binding decreased under the permissive condition. Transient expression of the LT, but not small t, decreased the endogenous receptor expression at mRNA and protein levels in NIH3T3 cells but not in the myc-null HO15.19 cells. The wild-type LT, but not the various pRb or p53 binding defective LT mutants, represses the PDGF beta-receptor promoter activity. Moreover, the inability of the LT-mediated repression in the myc-null cells, the Rb-null 3T3 cells, and the Saos-2 cells lacking pRb and p53, indicates that Myc, pRb and p53 are all necessary elements. PDGF beta-receptor promoter-luciferase assays revealed that the CCAAT motif is important for the repression. Furthermore, p53 was found to increase the promoter activity mainly via the upstream Sp1 binding sites together with the CCAAT motif in the NIH 3T3 cells. This was confirmed by Schneider's Drosophila line (SL2) cells deficient in both endogenous NF-Y and Sp1. Chromatin immunoprecipitation using ST15A cells revealed that both LT and p53 bound the PDGF beta-receptor promoter and the binding of p53 diminished when LT was expressed in the permissive condition. However, LT binds the promoter in the absence of pRb and p53 in Saos-2 cells stably expressing LT. These results suggest that LT binds the promoter and interferes with NF-Y and Sp1 to repress it in the presence of Myc, pRb and p53.

Promoter-binding and repression of PDGFRB by c-Myc are separable activities

The c-Myc transcription factor represses the mRNA expression of the platelet-derived growth factor receptor beta gene (PDGFRB). Using chromatin immunoprecipitation, we show that c-Myc binds to the proximal promoter of the PDGFRB gene in proliferating rat fibroblasts. Interestingly, mutant c-Myc proteins that are unable to repress PDGFRB gene expression, c-Myc(dBR) and c-Myc(d106-143), are still able to bind to the promoter in vivo. Hence, promoter-binding and repression of PDGFRB by c-Myc are separable activities. We also show that Myc repression of PDGFRB is not dependent on previously described or known transactivator-binding regions, suggesting Myc may be recruited to the promoter by multiple or yet unidentified transcription factors. In the presence of intact promoter-binding by Myc, trichostatin A (TSA) can block Myc repression of PDGFRB in vivo, again demonstrating that promoter-binding and repression are separable. Taken together, we hypothesize that Myc repression of PDGFRB expression occurs by a multi-step mechanism in which repression is initiated after Myc is recruited to the promoter.

Signal transduction mediated by the Ras/Raf/MEK/ERK pathway from cytokine receptors to transcription factors: potential targeting for therapeutic intervention

The Ras/Raf/Mitogen-activated protein kinase/ERK kinase (MEK)/extracellular-signal-regulated kinase (ERK) cascade couples signals from cell surface receptors to transcription factors, which regulate gene expression. Depending upon the stimulus and cell type, this pathway can transmit signals, which result in the prevention or induction of apoptosis or cell cycle progression. Thus, it is an appropriate pathway to target for therapeutic intervention. This pathway becomes more complex daily, as there are multiple members of the kinase and transcription factor families, which can be activated or inactivated by protein phosphorylation. The diversity of signals transduced by this pathway is increased, as different family members heterodimerize to transmit different signals. Furthermore, additional signal transduction pathways interact with the Raf/MEK/ERK pathway to regulate positively or negatively its activity, or to alter the phosphorylation status of downstream targets. Abnormal activation of this pathway occurs in leukemia because of mutations at Ras as well as genes in other pathways (eg PI3K, PTEN, Akt), which serve to regulate its activity. Dysregulation of this pathway can result in autocrine transformation of hematopoietic cells since cytokine genes such as interleukin-3 and granulocyte/macrophage colony-stimulating factor contain the transacting binding sites for the transcription factors regulated by this pathway. Inhibitors of Ras, Raf, MEK and some downstream targets have been developed and many are currently in clinical trials. This review will summarize our current understanding of the Ras/Raf/MEK/ERK signal transduction pathway and the downstream transcription factors. The prospects of targeting this pathway for therapeutic intervention in leukemia and other cancers will be evaluated.

Evidence that Myc activation depletes the epidermal stem cell compartment by modulating adhesive interactions with the local microenvironment

Activation of Myc (c-Myc) causes epidermal cells to exit the stem cell compartment and differentiate into sebocytes and interfollicular epidermis at the expense of the hair lineages. To investigate how Myc exerts these effects we analysed the transcription of more than 10000 genes following Myc activation in the basal layer of mouse epidermis for 1 or 4 days. The major classes of induced genes were involved in synthesis and processing of RNA and proteins, in cell proliferation and in differentiation. More than 40% of the downregulated genes encoded cell adhesion and cytoskeleton proteins. Repression of these genes resulted in profound changes in the adhesive and motile behaviour of keratinocytes. Myc activation inhibited cell motility and wound healing, correlating with decreased expression of a large number of extracellular matrix proteins. Cell adhesion and spreading were also impaired, and this correlated with decreased expression of the alpha6beta4 integrin, decreased formation of hemidesmosomes and decreased assembly of the actomyosin cytoskeleton. We propose that Myc stimulates exit from the stem cell compartment by reducing adhesive interactions with the local microenvironment or niche, and that the failure of hair differentiation reflects an inability of keratinocytes to migrate along the outer root sheath to receive hair inductive stimuli.

Analysis of Myc bound loci identified by CpG island arrays shows that Max is essential for Myc-dependent repression

The c-myc proto-oncogene encodes a transcription factor, c-Myc, which is deregulated and/or overexpressed in many human cancers. Despite c-Myc's importance, the identity of Myc-regulated genes and the mechanism by which Myc regulates these genes remain unclear. By combining chromatin immunoprecipitation with CpG island arrays, we identified 177 human genomic loci that are bound by Myc in vivo. Analyzing a cohort of known and novel Myc target genes showed that Myc-associated protein X, Max, also bound to these regulatory regions. Indeed, Max is bound to these loci in the presence or absence of Myc. The Myc:Max interaction is essential for Myc-dependent transcriptional activation; however, we show that Max bound targets also include Myc-repressed genes. Moreover, we show that the interaction between Myc and Max is essential for gene repression to occur. Taken together, the identification and analysis of Myc bound target genes supports a model whereby Max plays an essential and universal role in the mechanism of Myc-dependent transcriptional regulation.

Functional analysis of the N-terminal domain of the Myc oncoprotein

Myc is a multifunctional nuclear phosphoprotein that can drive cell cycle progression, apoptosis and cellular transformation. Myc orchestrates these activities at the molecular level by functioning as a regulator of gene transcription to activate or repress specific target genes. Previous studies have shown that both the Myc N-terminal domain (NTD) and the C-terminal domain (CTD) are essential for Myc functions. The role of the CTD is relatively well understood as it encodes a basic helix-loop-helix leucine zipper motif important for DNA binding and protein-protein interactions. By contrast, the role of the NTD and the specific domains responsible for different Myc activities are not as well defined. To investigate the regions of the NTD necessary for Myc function and to determine whether these activities are overlapping or independent of one another, we have conducted a detailed structure-function analysis of the Myc NTD. We assessed the ability of a number of deletion and point mutants within the highly conserved regions of the Myc NTD to induce cell cycle progression, apoptosis and transformation as well as repress and activate expression of endogenous target genes. Our analyses highlight the complexity of the Myc NTD and extend previous studies. For example, we show most Myc mutants that were compromised as repressors of gene transcription retained the ability to activate gene transcription, reinforcing the concept that these activities can be uncoupled. Repression of two different target genes could be distinguished by specific mutants, further supporting the notion of at least two different Myc repression mechanisms. Mutants disabled at both inducing and repressing gene transcription could not maximally drive the biological activities of Myc, indicating these functions are tightly linked. Indeed, a close association of Myc repression and apoptosis was also observed.

Loss of a FYN-regulated differentiation and growth arrest pathway in advanced stage neuroblastoma

Tumor stage, age of patient, and amplification of MYCN predict disease outcome in neuroblastoma. To gain insight into the underlying molecular pathways, we have obtained expression profiles from 94 primary neuroblastoma specimens. Advanced tumor stages show a characteristic expression profile that includes downregulation of multiple genes involved in signal transduction through Fyn and the actin cytoskeleton. High expression of Fyn and high Fyn kinase activity are restricted to low-stage tumors. In culture, expression of active Fyn kinase induces differentiation and growth arrest of neuroblastoma cells. Expression of Fyn predicts long-term survival independently of MYCN amplification. Amplification of MYCN correlates with deregulation of a distinct set of genes, many of which are target genes of Myc. Our data demonstrate a causal role for Fyn kinase in the genesis of neuroblastoma.

p73 independent of c-Myc represses transcription of platelet-derived growth factor beta-receptor through interaction with NF-Y

We recently reported that c-Myc represses the transcription of platelet-derived growth factor (PDGF) beta-receptor (Izumi, H., Molander, C., Penn, L. Z., Ishisaki, A., Kohno, K., and Funa, K. (2001) J. Cell Sci. 114, 1533-1544). We demonstrate here that the p53 family protein p73alpha represses PDGF beta-receptor transcription essentially by the same mechanism. p73alpha but not p73beta or p53 represses the transcription in concordance with its ability to bind NF-YC and NF-YB. None of other p73 isoforms (i.e. p73beta, p73gamma, p73epsilon), C-terminal deletion mutants of p73alpha, and p53 is able to bind NF-Y with the exception of p63alpha. This finding suggests that the sterile alpha-motif domain present only in p73alpha and p63alpha is the interaction site. For the repression, the N-terminal transactivation domain of p73alpha is also indispensable, arguing for the importance of the activity of p73alpha in the mechanism. p73alpha binds the C-terminal HAP domain of NF-YC previously found to be the interaction site with c-Myc and TBP. Because c-Myc induces and activates p73alpha (Zaika, A., Irwin, M., Sansome, C., and Moll, U. M. (2001) J. Biol. Chem. 276, 11310-11316) and they bind each other (Uramoto, H., Izumi, H., Ise, T., Tada, M., Uchiumi, T., Kuwano, M., Yasumoto, K., Funa, K., and Kohno, K. (2002) J. Biol. Chem. 277, in press), we examined whether the repression by p73 is dependent on c-Myc. However, Myc-null rat fibroblasts are also susceptible to p73alpha-induced repression. Serum stimulation of NIH3T3 cells gradually decreased the amount of endogenous NF-Y binding to the PDGF beta-receptor promoter, whereas NF-YA expression in the nuclear extracts remains unchanged. Our results indicate that serum stimulation induces c-Myc and p73alpha, leading to the down-regulation of PDGF beta-receptor expression by repressing its transcription.

Identifying genes regulated in a Myc-dependent manner

The c-myc proto-oncogene can direct a diverse array of biological activities, including cell cycle progression, apoptosis, and differentiation. It is believed that Myc can affect this wide variety of activities by functioning as a regulator of gene transcription, although few targets have been identified to date. To delineate the molecular program regulated downstream of Myc, we used a cDNA microarray approach and identified 52 putative targets out of >6000 cDNAs analyzed. To further distinguish the subset of genes whose regulation was dependent upon Myc per se from those regulated in response to activation of general mitogenic or apoptotic programs, the putative cDNA targets were then screened by a series of assays. By this approach 37 putative targets were ruled out and 15 Myc target genes were uncovered. Interestingly, comparing our results with other high throughput screens reveals that certain putative Myc targets previously reported are shown not to be regulated downstream of Myc (e.g. ribosomal proteins, HSP90beta), whereas others are further supported by our analyses (e.g. pdgfbetar, nucleolin). The identity of genes specifically regulated downstream of Myc provides the critical tools required to understand the role Myc holds in the transformation process and to delineate how Myc functions as a regulator of gene transcription.

Sp1 is a key regulator of the PDGF beta-receptor transcription

The mouse PDGF beta-receptor promoter is tightly controlled by NF-Y that binds to a CCAAT box located upstream of the initiation site [1, 2]. In this report, we show that Sp1 plays an essential role in the PDGF beta-receptor transcription. Within the upstream GC rich area there are two Sp1 binding sites located in close proximity to the CCAAT box. Deletion of the GC rich region resulted in a 50% decrease of the transcriptional activity of the promoter, and a complete loss of its responsiveness to over-expression of Sp1. There was an additive effect between NF-Y and Sp I in reporter activity when they were co-transfected together with the promoter-reporter construct. Furthermore, transfection of NF-Y failed to enhance transcriptional activity when the Sp1 binding sites were deleted from the promoter, suggesting an important role for Sp1 in this NF-Y controlled transcription. We have recently reported that c-Myc represses PDGF beta-receptor transcription through its interference with the transactivation activity of NF-Y [3]. In the case of p21(wafl/cip1) transcription, c-Myc was shown to repress its transcription by sequestering Sp1 [4]. However, we could not find any effect of Sp1 in the c-Myc-mediated repression on the PFDGF beta-receptor promoter, since the deletion of SpI binding sites could not attenuate the repression by c-Myc on the promoter activity.

The myc oncogene: MarvelouslY Complex

The activated product of the myc oncogene deregulates both cell growth and death check points and, in a permissive environment, rapidly accelerates the affected clone through the carcinogenic process. Advances in understanding the molecular mechanism of Myc action are highlighted in this review. With the revolutionary developments in molecular diagnostic technology, we have witnessed an unprecedented advance in detecting activated myc in its deregulated, oncogenic form in primary human cancers. These improvements provide new opportunities to appreciate the tumor subtypes harboring deregulated Myc expression, to identify the essential cooperating lesions, and to realize the therapeutic potential of targeting Myc. Knowledge of both the breadth and depth of the numerous biological activities controlled by Myc has also been an area of progress. Myc is a multifunctional protein that can regulate cell cycle, cell growth, differentiation, apoptosis, transformation, genomic instability, and angiogenesis. New insights into Myc's role in regulating these diverse activities are discussed. In addition, breakthroughs in understanding Myc as a regulator of gene transcription have revealed multiple mechanisms of Myc activation and repression of target genes. Moreover, the number of reported Myc regulated genes has expanded in the past few years, inspiring a need to focus on classifying and segregating bona fide targets. Finally, the identity of Myc-binding proteins has been difficult, yet has exploded in the past few years with a plethora of novel interactors. Their characterization and potential impact on Myc function are discussed. The rapidity and magnitude of recent progress in the Myc field strongly suggests that this marvelously complex molecule will soon be unmasked.

Platelet-derived growth factor-B and -C and active alpha-receptors in medulloblastoma cells

The malignant childhood brain tumor medulloblastoma belongs to the group of primitive neuroectodermal tumours (PNETs). Medulloblastomas are thought to arise from remnants of the transient external germinal layer in the cerebellum. Proliferation, differentiation, and motility of cells in the central nervous system are regulated by growth factors, e.g., platelet-derived growth factor (PDGF). Recently, it was shown that higher level of PDGF alpha-receptor expression is characteristic of metastatic medulloblastomas. We have investigated five medulloblastoma/PNET cell lines and found that the PDGF alpha-receptor is actively signalling in most of them, an activity most likely driven by endogenously produced PDGF-C. PDGF-C is normally present in cells of the developing external germinal layer and our results are consistent with the idea that medulloblastomas are derived from such cells undergoing early neuronal differentiation. Moreover, the expression of PDGF and its receptors was associated with neuronal characteristics, but not with high levels of c-myc expression in the medullablastoma cells.

Amplification of PPM1D in human tumors abrogates p53 tumor-suppressor activity

Expression of oncogenic Ras in primary human cells activates p53, thereby protecting cells from transformation. We show that in Ras-expressing IMR-90 cells, p53 is phosphorylated at Ser33 and Ser46 by the p38 mitogen-activated protein kinase (MAPK). Activity of p38 MAPK is regulated by the p53-inducible phosphatase PPM1D, creating a potential feedback loop. Expression of oncogenic Ras suppresses PPM1D mRNA induction, leaving p53 phosphorylated at Ser33 and Ser46 and in an active state. Retrovirus-mediated overexpression of PPM1D reduced p53 phosphorylation at these sites, abrogated Ras-induced apoptosis and partially rescued cells from cell-cycle arrest. Inactivation of p38 MAPK (the product of Mapk14) in vivo by gene targeting or by PPM1D overexpression expedited tumor formation after injection of mouse embryo fibroblasts (MEFs) expressing E1A+Ras into nude mice. The gene encoding PPM1D (PPM1D, at 17q22/q23) is amplified in human breast-tumor cell lines and in approximately 11% of primary breast tumors, most of which harbor wildtype p53. These findings suggest that inactivation of the p38 MAPK through PPM1D overexpression resulting from PPM1D amplification contributes to the development of human cancers by suppressing p53 activation.

New perspectives in PDGF receptor downregulation: the main role of phosphotyrosine phosphatases

Uncontrolled activation of receptor tyrosine kinases (RTKs) is implicated in the proliferation of cancerous cells, and deficiencies in RTKs results in pathological conditions such as developmental abnormalities and immunodeficiencies. Tight regulation of RTK cascades is therefore critical for eliciting an appropriate type and level of response to external stimuli. The aim of this work is to compare different RTK downregulation mechanisms, such as ligandinduced internalisation, ubiquitin-mediated proteolysis and dephosphorylation by protein phosphotyrosine phosphatase (PTPs). We choose platelet-derived growth factor receptor (PDGF-r) in NIH3T3 cells as a model of RTK. Our data suggest that PDGF-r internalisation could be mainly considered as a positive signaling system, as it is involved in MAPK activation rather than a downregulation of the mitotic signal. Inhibition of receptor ubiquitination does not result in regulation of PDGF-r tyrosine phosphorylation and does not lead to variation of intracellular signalling pathways. The overall PDGF-r protein degradation upon PDGF stimulation does not exceed 30-40% of the total receptor; thus the receptor remains functionally active for further stimulation. On the contrary, PTP-dependent dephosphorylation of the activated receptors appears to play a crucial role. In fact, inhibition of PTP upon PDGF stimulation results in upregulation of receptor phosphorylation level, of PI3K recruitment and activation and of cell cycle rate. On the contrary, PTP-dependent dephosphorylation does not affect the endosomic pool of activated receptor. Furthermore, we demonstrate that PDGF-r downregulation by means of PTP dephosphorylation is important for both short term (2 hours) and long-lasting (up to 8 hours) PDGF-r activation. Herein we propose a revisited model of PDGF-r downregulation in which PTPs dephosphorylation retains a major role, conferring on receptor internalisation a signal transduction function.

Function and regulation of the transcription factors of the Myc/Max/Mad network

The members of the Myc/Max/Mad network function as transcriptional regulators. Substantial evidence has been accumulated over the last years that support the model that Myc/Max/Mad proteins affect different aspects of cell behavior, including proliferation, differentiation, and apoptosis, by modulating distinct target genes. The unbalanced expression of these genes, e.g. in response to deregulated Myc expression, is most likely an important aspect of Myc's ability to stimulate tumor formation. Myc and Mad proteins affect target gene expression by recruiting chromatin remodeling activities. In particular Myc interacts with a SWI/SNF-like complex that may contain ATPase activity. In addition Myc binds to TRRAP complexes that possess histone acetyl transferase activity. Mad proteins, that antagonize Myc function, recruit an mSin3 repressor complex with histone deacetylase activity. Thus the antagonism of Myc and Mad proteins is explained at the molecular level by the recruitment of opposing chromatin remodeling activities.

Translocations involving c-myc and c-myc function

c-MYC is the prototype for oncogene activation by chromosomal translocation. In contrast to the tightly regulated expression of c-myc in normal cells, c-myc is frequently deregulated in human cancers. Herein, aspects of c-myc gene activation and the function of the c-Myc protein are reviewed. The c-myc gene produces an oncogenic transcription factor that affects diverse cellular processes involved in cell growth, cell proliferation, apoptosis and cellular metabolism. Complete removal of c-myc results in slowed cell growth and proliferation, suggesting that while c-myc is not required for cell proliferation, it acts as an integrator and accelerator of cellular metabolism and proliferation.

Myc potentiates apoptosis by stimulating Bax activity at the mitochondria

The ability of the c-Myc oncoprotein to potentiate apoptosis has been well documented; however, the mechanism of action remains ill defined. We have previously identified spatially distinct apoptotic pathways within the same cell that are differentially inhibited by Bcl-2 targeted to either the mitochondria (Bcl-acta) or the endoplasmic reticulum (Bcl-cb5). We show here that in Rat1 cells expressing an exogenous c-myc allele, distinct apoptotic pathways can be inhibited by Bcl-2 or Bcl-acta yet be distinguished by their sensitivity to Bcl-cb5 as either susceptible (serum withdrawal, taxol, and ceramide) or refractory (etoposide and doxorubicin). Myc expression and apoptosis were universally associated with Bcl-acta and not Bcl-cb5, suggesting that Myc acts downstream at a point common to these distinct apoptotic signaling cascades. Analysis of Rat1 c-myc null cells shows these same death stimuli induce apoptosis with characteristic features of nuclear condensation, membrane blebbing, poly (ADP-ribose) polymerase cleavage, and DNA fragmentation; however, this Myc-independent apoptosis is not inhibited by Bcl-2. During apoptosis, Bax translocation to the mitochondria occurs in the presence or absence of Myc expression. Moreover, Bax mRNA and protein expression remain unchanged in the presence or absence of Myc. However, in the absence of Myc, Bax is not activated and cytochrome c is not released into the cytoplasm. Reintroduction of Myc into the c-myc null cells restores Bax activation, cytochrome c release, and inhibition of apoptosis by Bcl-2. These results demonstrate a role for Myc in the regulation of Bax activation during apoptosis. Moreover, apoptosis that can be triggered in the absence of Myc provides evidence that signaling pathways exist which circumvent Bax activation and cytochrome c release to trigger caspase activation. Thus, Myc increases the cellular competence to die by enhancing disparate apoptotic signals at a common mitochondrial amplification step involving Bax activation and cytochrome c release.

Mechanism for the transcriptional repression by c-Myc on PDGF (β)-receptor

c-Myc plays a key role in the cell cycle dependent control of the PDGF (β)-receptor mRNA. The mouse platelet-derived growth factor (PDGF) (β)-receptor promoter contains a CCAAT motif, and NF-Y plays an essential role in its transcription. Coexpression of c-Myc represses PDGF (β)-receptor luciferase reporter activity, and the CCAAT motif in the promoter is indispensable for this repression. Here we show that c-Myc binds NF-Y subunits, YB and YC, by immunoprecipitation from cotransfected COS-1 cells. The in vitro-translated c-Myc also binds the glutathione S-transferase (GST)-NF-YB fusion protein and GST-NF-YC, but not GST-NF-YA. The most C-terminal region of HAP domains of NF-YB and NF-YC, and the Myc homology boxes, but not the C-terminal bHLHZip domain, are indispensable for the coimmunoprecipitation, and also for the repression of PDGF (β)-receptor. c-Myc binds NF-Y complex without affecting the efficiency of NF-Y binding to DNA. However, the expression of Myc represses the transcriptional activation of NF-YC when fused to the GAL4 DNA binding domain. Furthermore, this repression was seen only when Myc homology boxes are present, and NF-YC contains the c-Myc binding region.

Function of the c-Myc oncoprotein in chromatin remodeling and transcription

Deregulated expression of the c-myc proto-oncogene contributes to malignant progression of a variety of tumors. The c-Myc protein (or Myc) is a transcription factor that positively or negatively regulates expression of distinct sets of target genes. Transcriptional activation by Myc is mediated through dimerization with Max and binding to the DNA consensus sequence CA(C/T)GTG (the E-box). Transcriptional inhibition is mediated through distinct DNA elements, and may be due to functional interference with factors that transactivate via these sequences. We review here our current knowledge on these transcriptional activities of Myc and their relationship to its biological function. The findings that Myc interacts with subunits of histone acetyl-transferase (HAT) complexes and of the ATP-dependent chromatin remodeling complex, SWI/SNF, suggest that localized changes in chromatin structure may mediate Myc function. We present a working hypothesis for the concerted action of HAT and SWI/SNF complexes in Myc-activated transcription and argue that this model should prompt re-thinking of the experimental strategies and criteria used to identify Myc target genes.