Miz1 is required for early embryonic development during gastrulation

E3 ubiquitin ligase Mule ubiquitinates Miz1 and is required for TNFalpha-induced JNK activation

The zinc finger transcription factor Miz1 is a negative regulator of TNFalpha-induced JNK activation and cell death through inhibition of TRAF2 K63-polyubiquitination in a transcription-independent manner. Upon TNFalpha stimulation, Miz1 undergoes K48-linked polyubiquitination and proteasomal degradation, thereby relieving its inhibition. However, the underling regulatory mechanism is not known. Here, we report that HECT-domain-containing Mule is the E3 ligase that catalyzes TNFalpha-induced Miz1 polyubiquitination. Mule is a Miz1-associated protein and catalyzes its K48-linked polyubiquitination. TNFalpha-induced polyubiquitination and degradation of Miz1 were inhibited by silencing of Mule and were promoted by ectopic expression of Mule. The interaction between Mule and Miz1 was promoted by TNFalpha independently of the pox virus and zinc finger domain of Miz1. Silencing of Mule stabilized Miz1, thereby suppressing TNFalpha-induced JNK activation and cell death. Thus, our study reveals a molecular mechanism by which Mule regulates TNFalpha-induced JNK activation and apoptosis by catalyzing the polyubiquitination of Miz1.

The interaction between Myc and Miz1 is required to antagonize TGFbeta-dependent autocrine signaling during lymphoma formation and maintenance

The Myc protein suppresses the transcription of several cyclin-dependent kinase inhibitors (CKIs) via binding to Miz1; whether this interaction is important for Myc's ability to induce or maintain tumorigenesis is not known. Here we show that the oncogenic potential of a point mutant of Myc (MycV394D) that is selectively deficient in binding to Miz1 is greatly attenuated. Binding of Myc to Miz1 is continuously required to repress CKI expression and inhibit accumulation of trimethylated histone H3 at Lys 9 (H3K9triMe), a hallmark of cellular senescence, in T-cell lymphomas. Lymphomas that arise express high amounts of transforming growth factor beta-2 (TGFbeta-2) and TGFbeta-3. Upon Myc suppression, TGFbeta signaling is required to induce CKI expression and cellular senescence and suppress tumor recurrence. Binding of Myc to Miz1 is required to antagonize growth suppression and induction of senescence by TGFbeta. We demonstrate that, since lymphomas express high levels of TGFbeta, they are poised to elicit an autocrine program of senescence upon Myc inactivation, demonstrating that TGFbeta is a key factor that establishes oncogene addiction of T-cell lymphomas.

ARF antagonizes the ability of Miz-1 to inhibit p53-mediated transactivation

Although Myc-interacting zinc-finger protein-1 (Miz-1) is known to be a poxvirus and zinc-finger (POZ) transcription factor required for Myc transcriptional repression, additional regulatory function of Miz-1 is less well understood. Using a yeast two-hybrid screen, we identified human alternate reading frame (ARF) protein as a novel interaction partner of Miz-1. The zinc-finger domain of Miz-1 is involved in its binding to ARF. In addition, we found that Miz-1 was able to interact with p53 through its DNA-binding domain, thus to diminish the binding of p53 to its target promoter and inhibit p53-mediated gene transcription. Interestingly, the Miz-1-regulated p53 transcriptional suppression does not require the presence of ARF or Mdm2. Importantly, ARF and p53 were found to competitively bind to Miz-1 in regulating p53-mediated transcription, and this conclusion was verified by both in vitro binding assay and competitive chromatin immunoprecipitation assay using a bona fide p53 endogenous Bax and Puma promoters. Thus, our study reveals that Miz-1 acts as a p53 suppressor by interfering with p53 DNA-binding ability, and ARF is able to counteract the suppression of Miz-1 on p53 by direct binding to Miz-1, suggesting that Miz-1 is a novel mediator in the ARF-p53 pathway.

Miz1 is a signal- and pathway-specific modulator or regulator (SMOR) that suppresses TNF-alpha-induced JNK1 activation

The proinflammatory cytokine TNF-alpha exerts its pleiotropic functions through activation of multiple downstream effectors, including JNK1. Yet, the underlying regulatory mechanism is incompletely understood. Here, we report that the transcription factor Myc-interacting zinc-finger protein 1 (Miz1) selectively suppresses TNF-alpha-induced JNK1 activation and cell death independently of its transcription activity. Proteomics analysis and yeast two-hybrid screening reveal that Miz1 is a JNK-associated protein. The TNF-alpha-induced activation of JNK1 is augmented in Miz1-deficient mouse embryonic fibroblasts (Miz1(-/-) MEFs), but the augmentation is abrogated by reintroduction of Miz1 or its transcription-deficient mutant. The regulation by Miz1 is highly specific, because it regulates TNF-alpha-induced TRAF2 K63-linked polyubiquitination. Neither JNK1 activation by IL-1beta or UV nor TNF-alpha-induced activation of p38, ERK, or IkappaB kinase complex is affected by the loss of Miz1. The TNF-alpha-induced cell death also is accelerated in Miz1(-/-) MEFs. Upon TNF-alpha stimulation, Miz1 is degraded rapidly by the proteasome, relieving its suppression on JNK1 activation. Thus, our results show that in addition to being a transcription factor Miz1 acts as a signal- and pathway-specific modulator or regulator that specifically regulates TNF-alpha-induced JNK1 activation and cell death.

Myc increases self-renewal in neural progenitor cells through Miz-1

The mechanisms underlying the decision of a stem or progenitor cell to either self-renew or differentiate are incompletely understood. To address the role of Myc in this process, we expressed different forms of the proto-oncogene Myc in multipotent neural progenitor cells (NPCs) using retroviral transduction. Expression of Myc in neurospheres increased the proportion of self-renewing cells fivefold, and 1% of the Myc-overexpressing cells, but none of the control cells, retained self-renewal capacity even under differentiation-inducing conditions. A Myc mutant (MycV394D) deficient in binding to Miz-1, did not increase the percentage of self-renewing cells but was able to stimulate proliferation of NPCs as efficiently as wild-type Myc, indicating that these two cellular phenomena are regulated by at least partially different pathways. Our results suggest that Myc, through Miz-1, enhances self-renewal of NPCs and influences the way progenitor cells react to the environmental cues that normally dictate the cellular identity of tissues containing self-renewing cells.

Miz1 is required for hair follicle structure and hair morphogenesis

Previous work has implicated the Myc-binding transcription factor Miz1 in the control of keratinocyte proliferation and in the cellular response to TGFbeta. Miz1 is expressed in basal keratinocytes of the interfollicular epidermis and in hair follicles. Here we have conditionally knocked out the POZ/BTB transactivation domain of Miz1 in keratinocytes using a keratin 14 (K14)-Cre mouse deleter strain. K14Cre(+)/Miz1(lox/lox) mice have rough fur as a result of altered hair follicle orientation, irregular hair pigmentation and disturbed hair fiber structure. A regional thickening of the epidermis at the hair funnel orifice was accompanied by suprabasal proliferation, indicating a delayed exit of keratinocytes from the cell cycle. In addition, the catagen of the hair cycle was delayed in K14Cre(+)/Miz1(lox/lox) mice and intrafollicular keratinocyte proliferation was increased. In aged K14Cre(+)/Miz1(lox/lox) animals, the number of hair follicles remained unchanged but the number of visible hairs, especially of zigzag hairs, was reduced and a pigmentary incontinence into the dermis developed. Our data show that Miz1 is involved in controlling proliferation and differentiation in hair follicles and in hair fiber morphogenesis.

POZ for effect--POZ-ZF transcription factors in cancer and development

The BTB/POZ-ZF [Broad complex, Tramtrack, Bric à brac (BTB) or poxvirus and zinc finger (POZ)-zinc finger] protein family comprises a diverse group of transcription factors. POZ-ZF proteins have been implicated in many biological processes, including B cell fate determination, DNA damage responses, cell cycle progression and a multitude of developmental events, including gastrulation, limb formation and hematopoietic stem cell fate determination. Consequently, dysfunction of vertebrate POZ-ZF proteins, such as promyelocytic leukemia zinc finger (PLZF), B cell lymphoma 6 (Bcl-6), hypermethylated in cancer 1 (HIC-1), Kaiso, ZBTB7 and Fanconi anemia zinc finger (FAZF), has been linked directly or indirectly to tumorigenesis and developmental disorders. Here, we discuss recent advances in the POZ-ZF field and the implications for the design of future studies to elucidate the biological roles of these unique transcription factors.

Molecular fingerprinting of BMP2- and BMP4-treated embryonic maxillary mesenchymal cells

OBJECTIVE

To determine the differences in gene expression between control-, bone morphogenetic protein (BMP)2- and BMP4-treated murine embryonic maxillary mesenchymal (MEMM) cells.

DESIGN

Transcript profiles of BMP2-, BMP4- and vehicle-treated MEMM cells were compared utilizing the murine high-density GeneChip arrays from Affymetrix. The raw chip data (probe intensities) were pre-processed using robust multichip averaging with GC-content background correction and further normalized with GeneSpring v7.2 software. Cluster analysis of the microarray data was performed with the GeneSpring software. Changes in the gene expression were verified by TaqMan quantitative real-time PCR.

RESULTS

Expression of approximately 50% of the 45 101 genes and expressed sequence tags examined in this study were detected in BMP2-, BMP4- and vehicle-treated MEMM cells and that of several hundred genes was significantly altered (up or downregulated) in these cells in response to BMP2 and BMP4. Expression profiles of each of the 26 mRNAs tested by TaqMan quantitative real-time PCR were found to be consistent with the microarray data. Genes whose expression was modulated following BMP2 or BMP4 treatment, could be broadly classified based on the functions of the encoded proteins such as the growth factors and signaling molecules, transcription factors, and proteins involved in epithelial-mesenchymal interactions, extracellular matrix synthesis, cell adhesion, proliferation, differentiation, and apoptosis.

CONCLUSION

Utilization of the Affymetrix GeneChip microarray technology has enabled us to delineate a detailed transcriptional map of BMP2 and BMP4 responsiveness in embryonic maxillary mesenchymal cells and offers revealing insights into crucial molecular regulatory mechanisms employed by these two growth factors in orchestrating embryonic orofacial cellular responses.

Myc regulates keratinocyte adhesion and differentiation via complex formation with Miz1

Myc plays a key role in homeostasis of the skin. We show that Miz1, which mediates Myc repression of gene expression, is expressed in the epidermal basal layer. A large percentage of genes regulated by the Myc–Miz1 complex in keratinocytes encode proteins involved in cell adhesion, and some, including the α6 and β1 integrins, are directly bound by Myc and Miz1 in vivo. Using a Myc mutant deficient in Miz1 binding (MycV394D), we show that Miz1 is required for the effects of Myc on keratinocyte responsiveness to TGF-β. Myc, but not MycV394D, decreases keratinocyte adhesion and spreading. In reconstituted epidermis, Myc induces differentiation and loss of cell polarization in a Miz1-dependent manner. In vivo, overexpression of β1 integrins restores basal layer polarity and prevents Myc-induced premature differentiation. Our data show that regulation of cell adhesion is a major function of the Myc–Miz1 complex and suggest that it may contribute to Myc-induced exit from the epidermal stem cell compartment.

N-Myc functions in transcription and development

N-Myc is a member of the Myc family of proteins, which are best known for their potent oncogenic activities and association with a large proportion of human cancers. Intense scrutiny of the oncogenic properties of Myc family proteins over the last several decades has revealed a great deal about their transcriptional and oncogenic activities. Myc proteins have broad effects on transcription and can stimulate a variety of cell behaviors that contribute to the malignant phenotype. N-Myc and c-Myc also play essential functions during embryonic development, and loss of these proteins has deleterious effects in most, if not all, tissues and organ systems. What remains to be fully unraveled is the relationship between the diverse activities associated with deregulated and overexpressed Myc and their normal roles during embryonic development and tissue homeostasis. In this review I summarize our understanding of the transcriptional activities of Myc family proteins and the roles of N-myc in morphogenesis, particularly as they relate to cellular proliferation and apoptosis.

Identification of novel CBP interacting proteins in embryonic orofacial tissue

cAMP response element-binding protein (CREB)-binding protein (CBP) plays an important role as a general co-integrator of multiple signaling pathways and interacts with a large number of transcription factors and co-factors, through its numerous protein-binding domains. To identify nuclear factors associated with CBP in developing orofacial tissue, a yeast two-hybrid screen of a cDNA library derived from orofacial tissue from gestational day 11 to 13 mouse embryos was conducted. Using the carboxy terminus (amino acid residues 1676-2441) of CBP as bait, several novel proteins that bind CBP were identified, including an Msx-interacting-zinc finger protein, CDC42 interaction protein 4/thyroid hormone receptor interactor 10, SH3-domain GRB2-like 1, CCR4-NOT transcription complex subunit 3, adaptor protein complex AP-1 beta1 subunit, eukaryotic translation initiation factor 2B subunit 1 (alpha), and cyclin G-associated kinase. Results of the yeast two-hybrid screen were confirmed by glutathione S-transferase pull-down assays. The identification of these proteins as novel CBP-binding partners allows exploration of new mechanisms by which CBP regulates and integrates diverse cell signaling pathways.

ZENON, a novel POZ Kruppel-like DNA binding protein associated with differentiation and/or survival of late postmitotic neurons

The rat tyrosine hydroxylase gene promoter contains an E-box/dyad motif and an octameric and heptameric element that may be recognized by classes of transcription factors highly expressed during nervous system development. In a one-hybrid genetic screen, we used these sites as targets to isolate cDNAs encoding new transcription factors present in the brain. We identified ZENON, a novel rat POZ protein that contains two clusters of Kruppel-like zinc fingers and that presents several features of a transcription factor. ZENON is found in nuclei following transient transfection with the cDNA. The N-terminal zinc finger cluster contains a DNA binding domain that interacts with the E box. Cotranfection experiments revealed that ZENON induces tyrosine hydroxylase promoter activity. Unlike other POZ proteins, the ZENON POZ domain is not required for either activation of transcription or self-association. In the embryonic neural tube, ZENON expression is restricted to neurons that have already achieved mitosis and are engaged in late stages of neuronal differentiation (late postmitotic neurons). ZENON neuronal expression persists in the adult brain; therefore, ZENON can be considered a marker of mature neurons. We propose that ZENON is involved in the maintenance of panneuronal features and/or in the survival of mature neurons.

Role of the proto-oncogene Pokemon in cellular transformation and ARF repression

Aberrant transcriptional repression through chromatin remodelling and histone deacetylation has been postulated to represent a driving force underlying tumorigenesis because histone deacetylase inhibitors have been found to be effective in cancer treatment. However, the molecular mechanisms by which transcriptional derepression would be linked to tumour suppression are poorly understood. Here we identify the transcriptional repressor Pokemon (encoded by the Zbtb7 gene) as a critical factor in oncogenesis. Mouse embryonic fibroblasts lacking Zbtb7 are completely refractory to oncogene-mediated cellular transformation. Conversely, Pokemon overexpression leads to overt oncogenic transformation both in vitro and in vivo in transgenic mice. Pokemon can specifically repress the transcription of the tumour suppressor gene ARF through direct binding. We find that Pokemon is aberrantly overexpressed in human cancers and that its expression levels predict biological behaviour and clinical outcome. Pokemon's critical role in cellular transformation makes it an attractive target for therapeutic intervention.

Msx-1 and Msx-2 in mammary gland development

Homeobox genes do not generally function alone to determine cell fate and morphogenesis. Rather it is the distinct combination of various members of the homeobox family of genes and their spatiotemporal patterns of expression that determine cell identity and function. Functional redundancy often makes it difficult to clearly discern the role of any one given homeobox gene. The roles that Msx1 and Msx2 play in branching morphogenesis of the mammary gland are only now becoming more evident. Many signaling pathways and transcription factors are implicated in how these homeobox genes correctly determine the morphological development of the gland. Overexpression of Msx1 and Msx2 may also be involved in tumorigenesis. Additional studies are needed to elucidate the roles of these genes in both breast development and cancer.

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.