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Overexpression of a novel sorting nexin, SNX15, affects endosome morphology and protein trafficking.

Sorting nexin (SNX) 15 is a novel member of the SNX family of proteins. Although the functions of most SNXs have not yet been determined, several family members (e.g., SNX1, SNX2, SNX3, and SNX8) are orthologs of yeast proteins involved in protein trafficking. Overexpression of myc-tagged SNX15 in COS-7 cells altered the morphology of several endosomal compartments. In transient transfection experiments, myc-SNX15 was first seen in small punctate spots and small ring structures. Later, myc-SNX15 was found in larger rings. Finally, myc-SNX15 was observed in large, amorphous membrane-limited structures. These structures contained proteins from lysosomes, late endosomes, early endosomes, and the trans-Golgi network. However, the morphology of the endoplasmic reticulum and Golgi was not affected by overexpression of myc-SNX15. In myc-SNX15-overexpressing cells, the endocytosis of transferrin was severely inhibited and endocytosis of tac-trans-Golgi network (TGN) 38 and tac-furin was slowed. In addition, the recycling of internalized tac-TGN38 and tac-furin was also inhibited. Both the morphological and biochemical data indicate that SNX15 plays a crucial role in trafficking through the endocytic pathway. This is the first demonstration that a mammalian SNX protein is involved in protein trafficking.

Animals↗

Identification and characterization of specific DNA-binding complexes containing members of the Myc/Max/Mad network of transcriptional regulators.

In the past, eukaryotic cell-derived complexes of the Myc/Max/Mad network of transcriptional regulators have largely been refractory to DNA binding studies. We have developed electrophoretic mobility shift assay conditions to measure specific DNA binding of Myc/Max/Mad network complexes using a COS7 cell-based overexpression system. With the established protocol, we have measured on- and off-rates of c-Myc/Max, Max/Max, and Mad1/Max complexes and determined relative affinities. All three complexes appeared to bind with comparable affinity to a Myc E-box sequence. Furthermore, our data derived from competition experiments suggested that the Mad3/Max and Mad4/Max complexes also possess comparable DNA binding affinities. The conditions established for COS7 cell-overexpressed proteins were then used to identify c-Myc/Max, Max/Max, and Mnt/Max complexes in HL-60 cells. However, no Mad1/Max could be detected, despite the induction of Mad1 expression during differentiation. Whereas the DNA binding activity of c-Myc/Max complexes was down-regulated, Max/Max binding increased, and Mnt/Max binding remained unchanged. In addition, we have also tested for upstream stimulatory factor (USF) binding and observed that, in agreement with published data, USF comprises a major Myc E-box-binding factor that is more abundant than any of the Myc/Max/Mad network complexes. Similar to the Mnt/Max complex, the binding activity of USF remained constant during HL-60 differentiation. Our findings establish conditions for the analysis of DNA binding of Myc/Max/Mad complexes and indicate posttranslational regulation of the Max/Max complex.

Animals↗

Expression of mad, mxi1, max and c-myc during induced differentiation of hematopoietic cells: opposite regulation of mad and c-myc.

The Myc proto-oncoprotein family is considered to play an important role in the control of cell growth and differentiation. It appears that the interaction of Myc with its heterodimeric partner Max is essential for Myc function. Recently two other partners of Max, called Mad and Mxi1, have been identified. In an effort to gain insight into the network of these four proteins we have started to analyse the expression of the c-myc, max, mad and mxi1 genes at the mRNA level during hematopoietic cell growth and differentiation. In the human myeloid cell lines U-937, HL-60 and ML-1 c-myc expression was down-regulated as shown previously after induction of differentiation, whereas the expression of max was only slightly affected. In contrast to these two genes the expression of mad was induced upon differentiation in the three cell lines by TPA, retinoic acid, vitamin D3, dimethyl sulfoxide, and interferon-gamma and remained elevated for at least 3 days. A kinetic analysis showed that the induction of mad in U-937 in response to TPA was rapid (15 min) and at least in part transcriptional, reminiscent of immediate early genes. The expression of mxi1 was induced in U-937 by some inducers but not in HL-60 or ML-1. Its induction occurred slowly, peaking around 48 h. These analysis thus suggest that the expression of mad and c-myc is inversely regulated during induced hematopoietic differentiation.

Cell Differentiation↗

The basic region/helix-loop-helix/leucine zipper domain of Myc proto-oncoproteins: function and regulation.

A large body of evidence has been accumulated that demonstrates dominant effects of Myc proto-oncoproteins on different aspects of cellular growth. Myc is one of the few proteins that is sufficient to drive resting cells into the cell cycle and promote DNA synthesis. In line with this finding is that the constitutive expression of Myc in cells blocks their differentiation. These growth stimulating properties are most likely responsible for Myc's ability to initiate and promote tumor formation. Interestingly Myc can also sensitize cells to apoptosis, suggesting that this protein is part of a life-and-death switch. Molecularly Myc functions as a transcriptional regulator that needs to heterodimerize with Max to exert the biological activities described above and to regulate gene transcription. Myc and Max are just two members of a growing family of proteins referred to as the Myc/Max/Mad network. A hallmark of these proteins is that they possess a C-terminal basic region/helix-loop-helix/leucine zipper domain (bHLHZip). The bHLHZip domain specifies dimerization within the network and determines sequence specific DNA binding. Importantly this domain together with the N-terminal transactivation domain is essential for Myc biology. Here we have summarized the structural, functional, and regulatory aspects of the bHLHZip domain of Myc proteins.

Amino Acid Sequence↗

Mlx, a novel Max-like BHLHZip protein that interacts with the Max network of transcription factors.

Mad:Max heterodimers oppose the growth-promoting action of Myc:Max heterodimers by recruiting the mSin3-histone deacetylase (mSin3. HDAC) complex to DNA and functioning as potent transcriptional repressors. There are four known members of the Mad family that are indistinguishable in their abilities to interact with Max, bind DNA, repress transcription, and block Myc + Ras co-transformation. To investigate functional differences between Mad family proteins, we have identified additional proteins that interact with this family. Here we present the identification and characterization of the novel basic-helix-loop-helix zipper protein Mlx (Max-like protein x), which is structurally and functionally related to Max. The similarities between Mlx and Max include 1) broad expression in many tissues, 2) long protein half-life, and 3) formation of heterodimers with Mad family proteins that are capable of specific CACGTG binding. We show that transcriptional repression by Mad1:Mlx heterodimers is dependent on dimerization, DNA binding, and recruitment of the mSin3A.HDAC corepressor complex. In contrast with Max, Mlx interacts only with Mad1 and Mad4. Together, these findings suggest that Mlx may act to diversify Mad family function by its restricted association with a subset of the Mad family of transcriptional repressors.

3T3 Cells↗

YY1 as a regulator of replication-dependent hamster histone H3.2 promoter and an interactive partner of AP-2.

In analyzing cis-regulatory elements important for cell cycle control of the replication-dependent hamster histone H3.2 gene, we discovered a binding site for the transcription factor YY1 embedded within GC-rich sequences between the two tandem CCAAT repeats proximal to the TATA element. Base mutations that specifically eliminated YY1 binding resulted in suppression of the S phase induction of the H3.2 promoter. In addition, we discovered that YY1 is an interactive partner of AP-2, which also binds the H3.2 promoter and regulates its cell cycle-dependent expression. The critical domains for YY1 and AP-2A interaction are mapped, revealing that the N-terminal portion of YY1 (amino acids 1-300) and the DNA-binding/dimerization region of AP-2A are required. Our results suggest that YY1, acting as a transcription factor binding to its site on the promoter, or through protein-protein interaction with AP-2, may be part of a regulatory network including key cell cycle regulators such as c-Myc and Rb in controlling growth- and differentiation-regulated gene expression.

Animals↗

In silico identification of transcriptional regulators associated with c-Myc.

The development of powerful experimental strategies for functional genomics and accompanying computational tools has brought major advances in the delineation of transcriptional networks in organisms ranging from yeast to human. Regulation of transcription of eukaryotic genes is to a large extent combinatorial. Here, we used an in silico approach to identify transcription factors (TFs) that form recurring regulatory modules with c-Myc, a protein encoded by an oncogene that is frequently disregulated in human malignancies. A recent study identified, on a genomic scale, human genes whose promoters are bound by c-Myc and its heterodimer partner Max in Burkitt's lymphoma cells. Using computational methods, we identified nine TFs whose binding-site signatures are highly overrepresented in this promoter set of c-Myc targets, pointing to possible functional links between these TFs and c-Myc. Binding sites of most of these TFs are also enriched on the set of mouse homolog promoters, suggesting functional conservation. Among the enriched TFs, there are several regulators known to control cell cycle progression. Another TF in this set, EGR-1, is rapidly activated by numerous stress challenges and plays a central role in angiogenesis. Experimental investigation confirmed that c-Myc and EGR-1 bind together on several target promoters. The approach applied here is general and demonstrates how computational analysis of functional genomics experiments can identify novel modules in complex networks of transcriptional regulation.

Animals↗

Expression of the hTERT gene is regulated at the level of transcriptional initiation and repressed by Mad1.

Telomerase, an enzymatic activity responsible for the replication of chromosome end structures, is strongly upregulated in most human cancers. In contrast, most differentiated tissues are telomerase negative. The rate-limiting step for telomerase activity seems to be the expression of the catalytic subunit of the enzyme, encoded by the human telomerase reverse transcriptase (hTERT) gene. The precise mechanism of how hTERT is regulated has not been elucidated yet. We show here that the down-regulation of hTERT mRNA during 12-O-tetradecanoylphorbol-13-acetate-induced differentiation of human U937 cells is a consequence of a fast decrease in the rate of transcription rather than changes in its half-life. The only transcription factor that has so far been implicated in the regulation of hTERT expression is the c-Myc oncoprotein. Our analysis shows that another member of the myc/marx/mad network, mad1, encoding a transcriptional repressor that is significantly increased by 12-O-tetra-decanoylphorbol-13-acetate treatment, represses hTERT promoter-driven reporter gene activity in transient transfection assays. This effect is dependent on the NH2 terminal domain of Madl, which mediates the association with the transcriptional corepressor mSin3. Our findings suggest the involvement of an additional transcription factor in the regulation of hTERT expression and may provide a model for how hTERT activity is controlled during the differentiation process in human somatic tissues.

Basic Helix-Loop-Helix Leucine Zipper Transcriptio↗

A novel method for generation of signature networks as biomarkers from complex high throughput data.

Traditionally, gene signatures are statistically deduced from large gene expression and proteomics datasets and have been applied as an experimental molecular diagnostic technique that is sensitive to experimental design and statistical treatment. We have developed and applied the approach of "signature networks" which overcomes some of the drawbacks of clustering methods. We have demonstrated signature network assembly, functional analysis and logical operations on the networks that can be generated. In addition, we have used this technique in a proof of concept study to compare the effect of differential drug treatment using 4-hydroxytamoxifen and estrogen on the MCF-7 breast cancer cell line from a previously published study. We have shown that the two compounds can be differentiated by the networks of interacting genes. Both networks consist of a core module of genes including c-Fos as part of c-Fos/c-Jun heterodimer and c-Myc which is clearly visible. Using algorithms in our MetaCore software we are able to subtract the 4-hydroxytamoxifen and estrogen networks to further understand differences between these two treatments and show that the estrogen network is assembled around the core with other modules essential for all phases of the cell cycle. For example, Cyclin D1 is present in networks for the estrogen treated cells from two separate studies. These signature networks represent an approach to identify biomarkers and a general approach for discovering new relationships in complex high throughput toxicology data.

Algorithms↗

Expression of heterologous proteins in cultured rat hippocampal neurons using the Semliki Forest virus vector.

The Semliki Forest virus expression vector (Liljeström and Garoff: Bio/Technology 9:1356-1361, 1991) was tested in cultured rat hippocampal neurons using two Madin-Darby canine kidney (MDCK) cell membrane-associated proteins as reporters: rab8, a small GTPase involved in post-Golgi vesicle transport, and VIP21, an integral membrane protein of caveolae, trans-Golgi network, and post-Golgi vesicles. Expression of the c-myc epitope-tagged proteins was visualized by immunofluorescence microscopy. The proteins were first detected in neurons after 3-4 hr infection by the recombinant viruses. The infection efficiency on neurons was high: after 6 hr infection at a multiplicity of one, 50-60% of the cells expressed the reporter proteins. The neurons tolerated the infection well up to 8 hr. Their polarized organization was not disturbed, as judged from morphology and from distribution of the dendritic MAP2 and axonal synaptophysin marker proteins. The Semliki Forest virus vector thus seems suitable for short-term expression of proteins in cultured neurons.

Amino Acid Sequence↗

Changes in the transcriptome and synthetic lethal dependencies following KRAS mutant expression reveal profound tissue specificity.

Oncogenic KRAS mutations exhibit a striking tissue-restricted tropism, occurring with high frequency in pancreatic, colorectal, and lung adenocarcinomas while remaining rare in other lineages. The molecular basis for why these specific tissues are uniquely permissive to KRAS transformation, and how this context shapes therapeutic vulnerabilities, remains poorly defined. Here, we utilized CRISPR-mediated genome engineering to generate endogenous, conditional KRAS-mutant isogenic cell line models across three primary permissive lineages (lung, colon, and pancreas) and the nonpermissive breast lineage. Integrated genome-wide CRISPR fitness screens and comparative transcriptome analyses revealed that KRAS-driven synthetic lethal (SL) dependencies are profoundly shaped by their tissue of origin. Strikingly, we observed minimal overlap in SL hits across lineages, with only three genes shared among the permissive lines, suggesting that the KRAS oncogene operates through divergent, context-specific genetic networks. Mechanistically, we show that KRAS activation induces a universal MYC-driven metabolic signature, but the specific machinery required to sustain this state is lineage-restricted. We identified a dependency on the diphthamide synthesis pathway to maintain translational fidelity amid a KRAS-induced hypertranslational state. These findings demonstrate that even when driven by the same oncogene, tumors exhibit distinct regulatory landscapes and unique genetic vulnerabilities. Our results provide a framework for developing lineage-aware therapeutic strategies, moving beyond universal KRAS inhibition toward targeted interventions tailored to a tumor's specific tissue context.

Proto-Oncogene Proteins p21(ras)↗

Targeting of the transcription factor Max during apoptosis: phosphorylation-regulated cleavage by caspase-5 at an unusual glutamic acid residue in position P1.

Max is the central component of the Myc/Max/Mad network of transcription factors that regulate growth, differentiation and apoptosis. Whereas the Myc and Mad genes and proteins are highly regulated, Max expression is constitutive and no post-translational regulation is known. We have found that Max is targeted during Fas-induced apoptosis. Max is first dephosphorylated and subsequently cleaved by caspases. Two specific cleavage sites for caspases in Max were identified, one at IEVE(10) decreasing S and one at SAFD(135) decreasing G near the C-terminus, which are cleaved in vitro by caspase-5 and caspase-7 respectively. Mutational analysis indicates that both sites are also used in vivo. Thus Max represents the first caspase-5 substrate. The unusual cleavage after a glutamic acid residue is observed only with full-length, DNA-binding competent Max protein but not with corresponding peptides, suggesting that structural determinants might be important for this activity. Furthermore, cleavage by caspase-5 is inhibited by the protein kinase CK2-mediated phosphorylation of Max at Ser-11, a previously mapped phosphorylation site in vivo. These findings suggest that Fas-mediated dephosphorylation of Max is required for cleavage by caspase-5. The modifications that occur on Max in response to Fas signalling affect the DNA-binding activity of Max/Max homodimers. Taken together, our findings uncover three distinct processes, namely dephosphorylation and cleavage by caspase-5 and caspase-7, that target Max during Fas-mediated apoptosis, suggesting the regulation of the Myc/Max/Mad network through its central component.

Amino Acid Chloromethyl Ketones↗

Structural and thermodynamical characterization of the complete p21 gene product of Max.

The b-HLH-LZ family of transcription factors contains numerous proteins including the Myc and Mad families of proteins. Max heterodimerizes with other members to bind the E-Box DNA sequence in target gene promoters. Max is the only protein in this network that recognizes and binds E-Box DNA sequences as a homodimer in vitro and represses transcription of Myc target genes in vivo. Key information such as the structure of p21 Max, the complete gene product, and its KD in the absence of DNA are still unknown. Here, we report the characterization of the secondary and quaternary structures, the dimerization and DNA binding of p21 Max and a thermodynamically stable mutant. The helical content of p21 Max indicates that its N-terminal and C-terminal regions are unstructured in the absence of DNA. NMR experiments further support the location of folded and unfolded domains. We also show that p21 Max has an apparent KD (37 degrees C) of 7 x 10(-6), a value 10-100 times smaller than the b-HLH-LZ itself. We demonstrate that electrostatic repulsions are responsible for the higher KD of the b-HLH-LZ. Finally, we show that a p21 Max double mutant forms a very stable dimer with a KD (37 degrees C) of 3 x 10(-10) and that the protein/DNA complex depicts a higher temperature of denaturation than p21 Max/DNA complex. Our results indicate that Max could homodimerize, bind DNA, and repress transcription in vivo and that its mutant could be more efficient at repressing the expression of c-Myc target genes.

Amino Acid Sequence↗

Design and properties of a Myc derivative that efficiently homodimerizes.

bHLH and bHLHZip are highly conserved structural domains mediating DNA binding and specific protein-protein interactions. They are present in a family of transcription factors, acting as dimers, and their selective dimerization is utilized to switch on and off cell proliferation, differentiation or apoptosis. Myc is a bHLHZip protein involved in growth control and cancer, which operates in a network with the structurally related proteins Max, Mad and Mnt. It does not form homodimers, working as a heterodimer with Max; Max, instead, forms homodimers and heterodimers with Mad and Mnt. Myc/Max dimers activate gene transcription, while Mad/Max and Mnt/Max complexes are Myc/Max antagonists and act as repressors. Modifying the molecular recognition of dimers may provide a tool for interfering with Myc function and, in general, for directing the molecular switches operated via bHLH(Zip) proteins. By molecular modelling and mutagenesis, we analysed the contribution of single amino acids to the molecular recognition of Myc, creating bHLHZip domains with altered dimerization specificity. We report that Myc recognition specificity is encoded in a short region within the leucine zipper; mutation of four amino acids generates a protein, Omomyc, that homodimerizes efficiently and can still heterodimerize with wild type Myc and Max. Omomyc sequestered Myc in complexes with low DNA binding efficiency, preventing binding to Max and inhibiting Myc transcriptional activator function. Consistently with these results, Omomyc produced a proliferation arrest in NIH3T3 cells. These data demonstrate the feasibility of interfering with fundamental biological processes, such as proliferation, by modifying the dimerization selectivity of a bHLHZip protein; this may facilitate the design of peptides of potential pharmacological interest.

3T3 Cells↗

The proto-oncogene c-myc in hematopoietic development and leukemogenesis.

The proto-oncogene c-myc has been shown to play a pivotal role in cell cycle regulation, metabolism, apoptosis, differentiation, cell adhesion, and tumorigenesis, and participates in regulating hematopoietic homeostasis. It is a transcription regulator that is part of an extensive network of interacting factors. Most probably, different biological responses are elicited by different overlapping subsets of c-Myc target genes, both induced and suppressed. Results obtained from studies employing mouse models are consistent with the need for at least one, and possibly two, mutations in addition to deregulated c-myc for malignant tumor formation. Repression of c-myc is required for terminal differentiation of many cell types, including hematopoietic cells. It has been shown that deregulated expression of c-myc in both M1 myeloid leukemic cells and normal myeloid cells derived from murine bone marrow, not only blocked terminal differentiation and its associated growth arrest, but also induced apoptosis, which is dependent on the Fas/CD95 pathway. There is evidence to suggest that the CD95/Fas death receptor pathway is an integral part of the apoptotic response associated with the end of the normal terminal myeloid differentiation program, and that deregulated c-myc expression can activate this signaling pathway prematurely. The ability of egr-1 to promote terminal myeloid differentiation when co-expressed with c-myc, and of c-fos to partially abrogate the block imparted by deregulated c-myc on myeloid differentiation, make these two genes candidate tumor suppressors. Several different transcription factors have been implicated in the down-regulation of c-myc expression during differentiation, including C/EBPalpha, CTCF, BLIMP-1, and RFX1. Alterations in the expression and/or function of these transcription factors, or of the c-Myc and Max interacting proteins, such as MM-1 and Mxi1, can influence the neoplastic process. Understanding how c-Myc controls cellular phenotypes, including the leukemic phenotype, should provide novel tools for designing drugs to promote differentiation and/or apoptosis of leukemic cells.

Animals↗

Functions of myc:max in the control of cell proliferation and tumorigenesis.

Deregulation and elevated expression of members of the Myc family of bHLHZip transcription factors are observed in a high percentage of tumors. This close association with human cancers has led to a tremendous effort to define their biological and biochemical activities. Although Myc family proteins have the capacity to elicit a wide range of cell behaviors, their principal function appears to be to drive cells into the cell cycle and to keep them there. However, forced expression of Myc profoundly sensitizes normal cells to apoptosis. Therefore, tumor formation caused by deregulated Myc expression requires cooperating events that disrupt pathways that mediate apoptosis. Myc-dependent tumor formation may also be impeded by a set of related bHLHZip proteins with the demonstrated potential to act as Myc antagonists in cell culture experiments. In this review, we examine the complex activities of Myc family proteins and how their actions might be regulated in the context of a network of bHLHZip proteins.

Animals↗

myc-immortalized microglial cells express a functional platelet-activating factor receptor.

The autacoid platelet-activating factor (PAF) takes part in a complex network of interactions regarding the cellular components of nervous tissues. Efforts aimed at characterizing the effects of PAF in the brain have been recently focalized on neurons because PAF exerts pleiotropic effects on these cells. Less attention has instead been paid to the glial component of the brain. We have used microglial cell lines immortalized from 13-day-old mouse embryo brains by a myc-transducing retrovirus. When exposed to physiological doses of PAF, immortalized microglial cells showed increases in intracellular free calcium concentrations due to release of calcium from internal stores, as well as to extracellular calcium influxes. These profiles of reactivity were independent from the immortalizing process, being observable in primary microglial cultures and in immortalized clones showing different proliferative rates. PAF was also able to induce transient expression of the c-fos protooncogene in serum-starved cultures and induced a strong chemotactic response in microglial cells. In contrast with control macrophage cultures, PAF did not promote prostaglandin or leukotriene synthesis in immortalized cells. This was most likely due to the low amount of total arachidonic acid found in immortal microglia, with respect to that observed in freshly isolated cells. Our data suggest that several of the effects observed after PAF stimulation might be independent from PAF-induced arachidonic acid metabolism. The availability of an in vitro microglial model might now help in studying the proinflammatory effects of PAF, both direct or microglia mediated, in the neural environment.

Animals↗

Fumonisin B(1)-induced alterations in cytokine expression and apoptosis signaling genes in mouse liver and kidney after an acute exposure.

Fumonisin B(1) (FB(1)), a carcinogenic mycotoxin produced primarily by fungus Fusarium verticillioides in corn, causes several fatal animal diseases. In mice, liver is the primary site of its toxicity. Our previous study showed that maximum induction of interferon gamma (IFNgamma) and tumor necrosis factor alpha (TNFalpha) was observed at 4 and 8 h, respectively, after an acute po FB(1) treatment. To further investigate the time-related induction of other cytokines and genes involved in apoptosis signaling, male BALB/c mice were administered orally with either saline or 25 mg/kg of FB(1) and sampled 4 or 8 h after treatment. Expression of various genes was analyzed by ribonuclease protection assay. FB(1) treatment caused increased expression of TNFalpha and interleukin (IL)-1beta in both liver and kidney, whereas IL-1alpha and IL-1 receptor antagonist (IL-1Ra) expression was induced only in the liver. Expression of TNFalpha signaling molecules, TNF receptor 55 and receptor interacting protein, was increased in liver and kidney after FB(1) treatment. Caspase 8 expression was increased only in liver with no changes in kidney with FB(1). FB(1) treatment induced expression of Fas in liver and kidney with no alterations in Fas signaling molecules, Fas ligand, Fas-associated death domain and Fas-associated protein factor. Treatment of mice with FB(1) increased the expression of B-Myc, c-Myc and Max, oncogenic transcription factors in the kidney. FB(1) toxicity caused induction of cytokine network in liver with involvement of TNFalpha signaling pathway. Increased expression of caspase 8 involved in the TNFalpha signaling pathway may contribute to the apoptosis, whereas IL-1Ra induction could contribute to the proliferating effects observed in FB(1) toxicity.

Animals↗