The transcription factor onecut-2 controls the microphthalmia-associated transcription factor gene

Onecut Regulates Core Components of the Molecular Machinery for Neurotransmission in Photoreceptor Differentiation

Photoreceptor cells (PRC) are neurons highly specialized for sensing light stimuli and have considerably diversified during evolution. The genetic mechanisms that underlie photoreceptor differentiation and accompanied the progressive increase in complexity and diversification of this sensory cell type are a matter of great interest in the field. A role of the homeodomain transcription factor Onecut (Oc) in photoreceptor cell formation is proposed throughout multicellular organisms. However, knowledge of the identity of the Oc downstream-acting factors that mediate specific tasks in the differentiation of the PRC remains limited. Here, we used transgenic perturbation of the Ciona robusta Oc protein to show its requirement for ciliary PRC differentiation. Then, transcriptome profiling between the trans-activation and trans-repression Oc phenotypes identified differentially expressed genes that are enriched in exocytosis, calcium homeostasis, and neurotransmission. Finally, comparison of RNA-Seq datasets in Ciona and mouse identifies a set of Oc downstream genes conserved between tunicates and vertebrates. The transcription factor Oc emerges as a key regulator of neurotransmission in retinal cell types.

Natural skin-whitening compounds for the treatment of melanogenesis (Review)

Melanogenesis is the process for the production of melanin, which is the primary cause of human skin pigmentation. Skin-whitening agents are commercially available for those who wish to have a lighter skin complexions. To date, although numerous natural compounds have been proposed to alleviate hyperpigmentation, insufficient attention has been focused on potential natural skin-whitening agents and their mechanism of action from the perspective of compound classification. In the present article, the synthetic process of melanogenesis and associated core signaling pathways are summarized. An overview of the list of natural skin-lightening agents, along with their compound classifications, is also presented, where their efficacy based on their respective mechanisms of action on melanogenesis is discussed.

MITF-the first 25 years

In this review, Goding and Arnheiter present the current understanding of MITF's role and regulation in development and disease and highlight key areas where our knowledge of MITF regulation and function is limited.

All transcription factors are equal, but some are more equal than others. In the 25 yr since the gene encoding the microphthalmia-associated transcription factor (MITF) was first isolated, MITF has emerged as a key coordinator of many aspects of melanocyte and melanoma biology. Like all transcription factors, MITF binds to specific DNA sequences and up-regulates or down-regulates its target genes. What marks MITF as being remarkable among its peers is the sheer range of biological processes that it appears to coordinate. These include cell survival, differentiation, proliferation, invasion, senescence, metabolism, and DNA damage repair. In this article we present our current understanding of MITF's role and regulation in development and disease, as well as those of the MITF-related factors TFEB and TFE3, and highlight key areas where our knowledge of MITF regulation and function is limited.

MITF and UV responses in skin: From pigmentation to addiction

Ultraviolet radiation (UVR) has numerous effects on skin, including DNA damage, tanning, vitamin D synthesis, carcinogenesis, and immunomodulation. Keratinocytes containing damaged DNA secrete both α-melanocyte-stimulating hormone (α-MSH), which stimulates pigment production by melanocytes, and the opioid β-endorphin, which can trigger addiction-like responses to UVR. The pigmentation (tanning) response is an adaptation that provides some delayed protection against further DNA damage and carcinogenesis, while the opioid response may be an evolutionary adaptation for promoting sun-seeking behavior to prevent vitamin D deficiency. Here, we review the pigmentation response to UVR, driven by melanocytic microphthalmia-associated transcription factor (MITF), and evidence for UVR-induced melanomagenesis and addiction. We also discuss potential applications of a novel approach to generate protective pigmentation in the absence of UVR (sunless tanning) using a topical small-molecule inhibitor of the salt-inducible kinase (SIK) family.

Ethyl linoleate inhibits α-MSH-induced melanogenesis through Akt/GSK3β/β-catenin signal pathway

Ethyl linoleate is an unsaturated fatty acid used in many cosmetics for its various attributes, such as antibacterial and anti-inflammatory properties and clinically proven to be an effective anti-acne agent. In this study, we investigated the effect of ethyl linoleate on the melanogenesis and the mechanism underlying its action on melanogenesis in B16F10 murine melanoma cells. Our results revealed that ethyl linoleate significantly inhibited melanin content and intracellular tyrosinase activity in α-MSH-induced B16F10 cells, but it did not directly inhibit activity of mushroom tyrosinase. Ethyl linoleate inhibited the expression of microphthalmia-associated transcription factor (MITF), tyrosinase, and tyrosinase related protein 1 (TRP1) in governing melanin pigment synthesis. We observed that ethyl linoleate inhibited phosphorylation of Akt and glycogen synthase kinase 3β (GSK3β) and reduced the level of β-catenin, suggesting that ethyl linoleate inhibits melanogenesis through Akt/GSK3β/β-catenin signal pathway. Therefore, we propose that ethyl linoleate may be useful as a safe whitening agent in cosmetic and a potential therapeutic agent for reducing skin hyperpigmentation in clinics.

The master role of microphthalmia-associated transcription factor in melanocyte and melanoma biology

Certain transcription factors have vital roles in lineage development, including specification of cell types and control of differentiation. Microphthalmia-associated transcription factor (MITF) is a key transcription factor for melanocyte development and differentiation. MITF regulates expression of numerous pigmentation genes to promote melanocyte differentiation, as well as fundamental genes for maintaining cell homeostasis, including genes encoding proteins involved in apoptosis (eg, BCL2) and the cell cycle (eg, CDK2). Loss-of-function mutations of MITF cause Waardenburg syndrome type IIA, whose phenotypes include depigmentation due to melanocyte loss, whereas amplification or specific mutation of MITF can be an oncogenic event that is seen in a subset of familial or sporadic melanomas. In this article, we review basic features of MITF biological function and highlight key unresolved questions regarding this remarkable transcription factor.

SOX5 is involved in balanced MITF regulation in human melanoma cells

Background

Melanoma is a cancer with rising incidence and new therapeutics are needed. For this, it is necessary to understand the molecular mechanisms of melanoma development and progression. Melanoma differs from other cancers by its ability to produce the pigment melanin via melanogenesis; this biosynthesis is essentially regulated by microphthalmia-associated transcription factor (MITF). MITF regulates various processes such as cell cycling and differentiation. MITF shows an ambivalent role, since high levels inhibit cell proliferation and low levels promote invasion. Hence, well-balanced MITF homeostasis is important for the progression and spread of melanoma. Therefore, it is difficult to use MITF itself for targeted therapy, but elucidating its complex regulation may lead to a promising melanoma-cell specific therapy.

Method

We systematically analyzed the regulation of MITF with a novel established transcription factor based gene regulatory network model. Starting from comparative transcriptomics analysis using data from cells originating from nine different tumors and a melanoma cell dataset, we predicted the transcriptional regulators of MITF employing ChIP binding information from a comprehensive set of databases. The most striking regulators were experimentally validated by functional assays and an MITF-promoter reporter assay. Finally, we analyzed the impact of the expression of the identified regulators on clinically relevant parameters of melanoma, i.e. the thickness of primary tumors and patient overall survival.

Results

Our model predictions identified SOX10 and SOX5 as regulators of MITF. We experimentally confirmed the role of the already well-known regulator SOX10. Additionally, we found that SOX5 knockdown led to MITF up-regulation in melanoma cells, while double knockdown with SOX10 showed a rescue effect; both effects were validated by reporter assays. Regarding clinical samples, SOX5 expression was distinctively up-regulated in metastatic compared to primary melanoma. In contrast, survival analysis of melanoma patients with predominantly metastatic disease revealed that low SOX5 levels were associated with a poor prognosis.

Conclusion

MITF regulation by SOX5 has been shown only in murine cells, but not yet in human melanoma cells. SOX5 has a strong inhibitory effect on MITF expression and seems to have a decisive clinical impact on melanoma during tumor progression.

Electronic supplementary material

The online version of this article (doi:10.1186/s12920-016-0170-0) contains supplementary material, which is available to authorized users.

Estimating the activity of transcription factors by the effect on their target genes

Motivation: Understanding regulation of transcription is central for elucidating cellular regulation. Several statistical and mechanistic models have come up the last couple of years explaining gene transcription levels using information of potential transcriptional regulators as transcription factors (TFs) and information from epigenetic modifications. The activity of TFs is often inferred by their transcription levels, promoter binding and epigenetic effects. However, in principle, these methods do not take hard-to-measure influences such as post-transcriptional modifications into account.

Results: For TFs, we present a novel concept circumventing this problem. We estimate the regulatory activity of TFs using their cumulative effects on their target genes. We established our model using expression data of 59 cell lines from the National Cancer Institute. The trained model was applied to an independent expression dataset of melanoma cells yielding excellent expression predictions and elucidated regulation of melanogenesis.

Availability and implementation: Using mixed-integer linear programming, we implemented a switch-like optimization enabling a constrained but optimal selection of TFs and optimal model selection estimating their effects. The method is generic and can also be applied to further regulators of transcription.

Contact:rainer.koenig@uni-jena.de

Supplementary information:Supplementary data are available at Bioinformatics online.

Auto and cross regulatory elements control Onecut expression in the ascidian nervous system

The expression pattern of Onecut genes in the central and peripheral nervous systems is highly conserved in invertebrates and vertebrates but the regulatory networks in which they are involved are still largely unknown. The presence of three gene copies in vertebrates has revealed the functional roles of the Onecut genes in liver, pancreas and some populations of motor neurons. Urochordates have only one Onecut gene and are the closest living relatives of vertebrates and thus represent a good model system to understand its regulatory network and involvement in nervous system formation. In order to define the Onecut genetic cascade, we extensively characterized the Onecut upstream cis-regulatory DNA in the ascidian Ciona intestinalis. Electroporation experiments using a 2.5kb genomic fragment and of a series of deletion constructs identified a small region of 262bp able to reproduce most of the Onecut expression profile during embryonic development. Further analyses, both bioinformatic and in vivo using transient transgenes, permitted the identification of transcription factors responsible for Onecut endogenous expression. We provide evidence that Neurogenin is a direct activator of Onecut and that an autoregulatory loop is responsible for the maintenance of its expression. Furthermore, for the first time we propose the existence of a direct connection among Neurogenin, Onecut and Rx transcription factors in photoreceptor cell formation.

Silencing of GPNMB by siRNA inhibits the formation of melanosomes in melanocytes in a MITF-independent fashion

Background

Melanosomes are specialized membrane-surrounded organelles, which are involved in the synthesis, storage and transport of melanin. Glycoprotein (transmembrane) non-metastatic melanoma protein b (GPNMB), a melanosome-specific structural protein, shares significant amino acid sequence homology with Pmel-17. Proteomic analysis demonstrated that GPNMB is present in all stages (I-IV) of melanosomes. However, little is known about the role of GPNMB in melanosomes.

Methodology/Principal Findings

Using real-time quantitative PCR, Western blotting and immunofluorescence analysis, we demonstrated that the expression of GPNMB in PIG1 melanocytes was up-regulated by ultraviolet B (UVB) radiation. Transmission electron microscopy analysis showed that the total number of melanosomes in PIG1 melanocytes was sharply reduced by GPNMB-siRNA transfection. Simultaneously, the expression levels of tyrosinase (Tyr), tyrosinase related protein 1 (Trp1), Pmel17/gp100 and ocular albinism type 1 protein (OA1) were all significantly attenuated. But the expression of microphthalmia-associated transcription factor (MITF) was up-regulated. Intriguingly, in GPNMB silenced PIG1 melanocytes, UVB radiation sharply reduced MITF expression.

Conclusion

Our present work revealed that the GPNMB was critical for the formation of melanosomes. And GPNMB expression down-regulation attenuated melanosome formation in a MITF-independent fashion.

Regulation of MITF stability by the USP13 deubiquitinase

The microphthalmia-associated transcription factor (MITF) is essential for melanocyte development. Mutation-induced MAPK pathway activation is common in melanoma and induces MITF phosphorylation, ubiquitination, and proteolysis. Little is known about the enzymes involved in MITF ubiquitination/deubiquitination. Here we report the identification of a deubiquitinating enzyme, named ubiquitin-specific protease 13 (USP13) that appears to be responsible for MITF deubiquitination, utilizing a short hairpin RNA library against known deubiquitinating enzymes. Through deubiquitination, USP13 stabilizes and upregulates MITF protein levels. Conversely, suppression of USP13 (through knockdown) leads to dramatic loss of MITF protein, but not messenger RNA. Through its effects on MITF deubiquitination, USP13 was observed to modulate expression of MITF downstream target genes and, thereby, to be essential for melanoma growth in soft agar and in nude mice. These observations suggest that as a potentially drugable protease, USP13 might be a viable therapeutic target for melanoma.

A role for ATF2 in regulating MITF and melanoma development

The transcription factor ATF2 has been shown to attenuate melanoma susceptibility to apoptosis and to promote its ability to form tumors in xenograft models. To directly assess ATF2's role in melanoma development, we crossed a mouse melanoma model (Nras(Q61K)::Ink4a⁻/⁻) with mice expressing a transcriptionally inactive form of ATF2 in melanocytes. In contrast to 7/21 of the Nras(Q61K)::Ink4a⁻/⁻ mice, only 1/21 mice expressing mutant ATF2 in melanocytes developed melanoma. Gene expression profiling identified higher MITF expression in primary melanocytes expressing transcriptionally inactive ATF2. MITF downregulation by ATF2 was confirmed in the skin of Atf2⁻/⁻ mice, in primary human melanocytes, and in 50% of human melanoma cell lines. Inhibition of MITF transcription by MITF was shown to be mediated by ATF2-JunB-dependent suppression of SOX10 transcription. Remarkably, oncogenic BRAF (V600E)-dependent focus formation of melanocytes on soft agar was inhibited by ATF2 knockdown and partially rescued upon shMITF co-expression. On melanoma tissue microarrays, a high nuclear ATF2 to MITF ratio in primary specimens was associated with metastatic disease and poor prognosis. Our findings establish the importance of transcriptionally active ATF2 in melanoma development through fine-tuning of MITF expression.

MiR-495 and miR-218 regulate the expression of the Onecut transcription factors HNF-6 and OC-2

MicroRNAs are small, non-coding RNAs that posttranscriptionally regulate gene expression mainly by binding to the 3'UTR of their target mRNAs. Recent data revealed that microRNAs have an important role in pancreas and liver development and physiology. Using cloning and microarray profiling approaches, we show that a unique repertoire of microRNAs is expressed at the onset of liver and pancreas organogenesis, and in pancreas and liver at key stages of cell fate determination. Among the microRNAs that are expressed at these stages, miR-495 and miR-218 were predicted to, respectively, target the Onecut (OC) transcription factors Hepatocyte Nuclear Factor-6 (HNF-6/OC-1) and OC-2, two important regulators of liver and pancreas development. MiR-495 and miR-218 are dynamically expressed in developing liver and pancreas, and by transient transfection, we show that they target HNF-6 and OC-2 3'UTRs. Moreover, when overexpressed in cultured cells, miR-495 and miR-218 decrease the endogenous levels of HNF-6 and OC-2 mRNA. These results indicate that the expression of regulators of liver and pancreas development is modulated by microRNAs. They also suggest a developmental role for miR-495 and miR-218.

Transcriptional regulation in melanoma

Transcriptional regulation in melanoma is a complex process that tends to hijack the normal melanocyte signaling pathways involved in melanocyte development, pigmentation, and survival. At the center of these often overlapping networks of transcriptional activation and repression is microphthalmia-associated transcription factor (MITF), a melanocyte lineage marker that increases pigment production and exhibits diverse effects on cell survival, proliferation, and cell cycle arrest. The particular conditions that allow MITF to produce these potentially contradictory roles have not yet been fully elucidated, but analysis of the pathways involved provides opportunities to learn about new therapeutic strategies.

The mathematics of tanning

BACKGROUND

The pigment melanin is produced by specialized cells, called melanocytes. In healthy skin, melanocytes are sparsely spread among the other cell types in the basal layer of the epidermis. Sun tanning results from an UV-induced increase in the release of melanin to neighbouring keratinocytes, the major cell type component of the epidermis as well as redistribution of melanin among these cells. Here we provide a mathematical conceptualization of our current knowledge of the tanning response, in terms of a dynamic model. The resolution level of the model is tuned to available data, and its primary focus is to describe the tanning response following UV exposure.

RESULTS

The model appears capable of accounting for available experimental data on the tanning response in different skin and photo types. It predicts that the thickness of the epidermal layer and how far the melanocyte dendrites grow out in the epidermal layers after UV exposure influence the tanning response substantially.

CONCLUSION

Despite the paucity of experimental validation data the model is constrained enough to serve as a foundation for the establishment of a theoretical-experimental research programme aimed at elucidating the more fine-grained regulatory anatomy underlying the tanning response.

Efficient mapping of mendelian traits in dogs through genome-wide association

With several hundred genetic diseases and an advantageous genome structure, dogs are ideal for mapping genes that cause disease. Here we report the development of a genotyping array with approximately 27,000 SNPs and show that genome-wide association mapping of mendelian traits in dog breeds can be achieved with only approximately 20 dogs. Specifically, we map two traits with mendelian inheritance: the major white spotting (S) locus and the hair ridge in Rhodesian ridgebacks. For both traits, we map the loci to discrete regions of <1 Mb. Fine-mapping of the S locus in two breeds refines the localization to a region of approximately 100 kb contained within the pigmentation-related gene MITF. Complete sequencing of the white and solid haplotypes identifies candidate regulatory mutations in the melanocyte-specific promoter of MITF. Our results show that genome-wide association mapping within dog breeds, followed by fine-mapping across multiple breeds, will be highly efficient and generally applicable to trait mapping, providing insights into canine and human health.

Evolutionary sequence comparison of the Mitf gene reveals novel conserved domains

The microphthalmia-associated transcription factor (MITF) is a member of the MYC family of basic helix-loop-helix leucine zipper transcription factors. The corresponding gene was initially discovered in the mouse based on mutations which affect the development of several different cell types, including melanocytes and retinal pigment epithelium cells. Subsequently, it was shown to be associated with deafness and hypo-pigmentation disorders in humans. More recently, the gene has been shown to be critical in melanoma formation and to play a role in melanocyte stem cell maintenance. Thus, the mouse Mitf gene represents an important model system for the study of human disease as well as an interesting model for the study of transcription factor function in the organism. Here we use the evolutionary relationship of Mitf genes from numerous distantly related species, including vertebrates and invertebrates, to identify novel conserved domains in the Mitf protein and regions of possible functional importance in the 3' untranslated region. We also characterize the nine different 5' exons of the Mitf gene and identify a new 5' exon in the Drosophila Mitf gene. Our analysis sheds new light on the conservation of the Mitf gene and protein and opens the door for further functional analysis.

MITF: master regulator of melanocyte development and melanoma oncogene

Microphthalmia-associated transcription factor (MITF) acts as a master regulator of melanocyte development, function and survival by modulating various differentiation and cell-cycle progression genes. It has been demonstrated that MITF is an amplified oncogene in a fraction of human melanomas and that it also has an oncogenic role in human clear cell sarcoma. However, MITF also modulates the state of melanocyte differentiation. Several closely related transcription factors also function as translocated oncogenes in various human malignancies. These data place MITF between instructing melanocytes towards terminal differentiation and/or pigmentation and, alternatively, promoting malignant behavior. In this review, we survey the roles of MITF as a master lineage regulator in melanocyte development and its emerging activities in malignancy. Understanding the molecular function of MITF and its associated pathways will hopefully shed light on strategies for improving therapeutic approaches for these diseases.

Melanocytes and the microphthalmia transcription factor network

The first mouse microphthalmia transcription factor (Mitf ) mutation was discovered over 60 years ago, and since then over 24 spontaneous and induced mutations have been identified at the locus. Mitf encodes a member of the Myc supergene family of basic helix-loop-helix zipper (bHLH-Zip) transcription factors. Like Myc, Mitf regulates gene expression by binding to DNA as a homodimer or as a heterodimer with another related family member, in the case of Mitf the Tfe3, Tfeb, and Tfec proteins. The study of Mitf has provided many insights into the biology of melanocytes and helped to explain how melanocyte-specific gene expression and signaling is regulated. The human homologue of MITF is mutated in patients with the pigmentary and deafness disorder Waardenburg Syndrome Type 2A (WS2A). The mouse Mitf mutations therefore serve as a model for the study of this human disease. Mutations and/or aberrant expression of several MITF family member genes have also been reported in human cancer, including melanoma (MITF), papillary renal cell carcinoma (TFE3, TFEB), and alveolar soft part sarcoma (TFE3). Genes in the MITF/TFE pathway may therefore also represent valuable therapeutic targets for the treatment of human cancer. Here we review recent developments in the analysis of Mitf function in vivo and in vitro and show how traditional genetics, modern forward genetics and in vitro biochemical analyses have combined to produce an intriguing story on the role and actions of a gene family in a living organism.

The transcription network regulating melanocyte development and melanoma

The enormous variety of pigmentation phenotypes in nature reflects a series of remarkable events that begin in the neural crest and end with the manufacture and distribution of pigment by mature melanocytes located in the epidermis and hair follicles. While the origins of melanoblasts from multipotent precursors in the neural crest is striking in itself, yet more so is the fact that these pioneer melanoblasts manage to undertake and survive their long migration, and in doing so proliferate and maintain their identity before ultimately arriving at their destination and undergoing differentiation. With the application of the powerful combination of genetics and molecular and cell biology the mystery surrounding the genesis of the melanocyte lineage is slowly being unravelled. At its heart is the powerful alliance between signal transduction and transcription that coordinates the program of gene expression that confers on a cell its identity, provides its passport for migration, and instructs it in the arts of survival and timely reproduction. The realization that the proliferation and migration of melanoblasts during development resembles closely the proliferation and metastasis of melanoma, a highly dangerous and increasingly common cancer, serves to highlight the value of the melanocyte system as a model for addressing key issues of general significance in both development and cancer.

Mutation in intron 6 of the hamster Mitf gene leads to skipping of the subsequent exon and creates a novel animal model for the human Waardenburg syndrome type II

In the course of analysis of ENU-induced mutations in Syrian hamsters, a novel dominant anophthalmic white mutant (Wh(V203)) with hearing loss was recovered. Because of this phenotype and a close linkage to the Tpi gene, the Mitf gene was considered as a candidate gene. In the Mitf cDNA, a deletion of 76 bp covering the entire exon 7 was detected. Further molecular analysis revealed a T --> A exchange 16 bp upstream of the end of intron 6, leading to skipping of exon 7. These 16 bp at the end of intron 6 are identical in hamster, rat, mouse, and humans, indicating high conservation during evolution and a functional importance in splicing. Since the loss of exon 7 changes the open reading frame of the MITF transcript, translation will be stopped after 10 new amino acids. The truncated protein is predicted to contain only a part of the basic region and will miss the two helical domains and the leucine zipper. The Wh(V203) mutation in the Syrian hamster affects the same functional domains of the Mitf transcription factor as the human R124X mutation, causing human Waardenburg syndrome type II. Therefore, the Wh(V203) hamster mutant provides a novel model for this particular syndrome.

Transcriptional regulation of mitfa accounts for the sox10 requirement in zebrafish melanophore development

The transcription factor Sox10 is required for the specification, migration and survival of all nonectomesenchymal neural crest derivatives including melanophores. sox10(-/-) zebrafish lack expression of the transcription factor mitfa, which itself is required for melanophore development. We demonstrate that the zebrafish mitfa promoter has sox10 binding sites necessary for activity in vitro, consistent with studies using mammalian cell cultures that have shown that Sox10 directly regulates Mitf expression. In addition, we demonstrate that these sites are necessary for promoter activity in vivo. We show that reintroduction of mitfa expression in neural crest cells can rescue melanophore development in sox10(-/-) embryos. This rescue of melanophores in sox10(-/-) embryos is quantitatively indistinguishable from rescue in mitfa(-/-) embryos. These findings show that the essential function of sox10 in melanophore development is limited to transcriptional regulation of mitfa. We propose that the dominant melanophore phenotype in Waardenburg syndrome IV individuals with SOX10 mutations is likely to result from failure to activate MITF in the normal number of melanoblasts.

The Onecut transcription factor HNF-6 (OC-1) is required for timely specification of the pancreas and acts upstream of Pdx-1 in the specification cascade

The pancreas derives from cells in the ventral and dorsal foregut endoderm that express the transcription factor Pdx-1. These specified cells give rise to the precursors of the endocrine, ductal, and exocrine pancreatic cells. The identification of transcription factors that regulate the onset of Pdx-1 expression is therefore essential to understand pancreas development. No such factor that acts both in the ventral and in the dorsal endoderm is known. We showed previously that the Onecut transcription factor HNF-6 promotes differentiation of the endocrine cell precursors in which it stimulates expression of the proendocrine gene Ngn-3. By analyzing the phenotype of HNF-6 null mice, we now demonstrate that HNF-6 also controls an earlier step in pancreas development. Indeed, the pancreas of Hnf6(-/-) mice was hypoplastic. This did not result from decreased proliferation or from increased apoptosis, but from retarded pancreatic specification of endodermal cells. The onset of Pdx-1 expression was delayed both in the ventral and in the dorsal endoderm, leading to a reduction in the number of endodermal cells expressing Pdx-1 at the time of pancreatic budding. In normal embryos, HNF-6 was detected in the endoderm prior to the expression of Pdx-1. Moreover, HNF-6 could directly stimulate the Pdx1 promoter. Our data therefore identify HNF-6 as the first factor known to control Pdx-1 expression both in the ventral and in the dorsal endoderm. We conclude that HNF-6 controls the timing of pancreas specification and that HNF-6 acts upstream of Pdx-1 in this developmental process. Together with the known role of HNF-6 in pancreatic endocrine cell differentiation, our data point to HNF-6 as a key regulator of pancreas development.

Pax3 down-regulation and shut-off of melanogenesis in melanoma B16/F10.9 by interleukin-6 receptor signaling

The microphthalmia-associated transcription factor (Mitf) is essential for melanocytic lineage development and for expression of melanogenic enzymes, such as tyrosinase. Interleukin-6 receptor/interleukin-6 chimera (IL6RIL6) induces in B16/F10.9 melanoma cells a loss of melanogenesis preceded by a sharp decrease in Mitf mRNA and gene promoter activity. In the Mitf promoter, the main cis-acting element mediating the IL6RIL6 effect is shown to be the binding site of Pax3, a paired homeodomain factor regulating among other things the development of melanocytes. Pax3 protein and mRNA levels decline steadily after IL6RIL6 treatment, and overexpression of an ectopic Pax3 cDNA suppresses the Mitf promoter inhibition. Loss of the synergism between Pax3 and Sox10, a high mobility group domain costimulatory factor, seems to be critical in the rapid decrease in Mitf gene expression. The Pax3 down-regulation in IL6RIL6-induced F10.9 cell is linked to growth arrest and transdifferentiation to a glial cell phenotype. IL6RIL6 stimulates the interleukin-6 family cytokine receptor gp130, leading to the rapid phosphorylation of Stat3 on tyrosine 705. This phosphorylation is required for Pax3 down-regulation and Mitf promoter silencing since these are inhibited in F10.9 cells overexpressing the Stat3 DN-mutant Y705F.

OC-3, a novel mammalian member of the ONECUT class of transcription factors

Transcription factors of the ONECUT class possess a single cut domain and a divergent homeodomain. They regulate gene networks by controlling the expression of other transcription factors and they play an important role in cell differentiation and metabolism. We identified earlier in mammals HNF-6 (ONECUT-1), the founding member of the class, and ONECUT-2 (OC-2). We have now characterized in the mouse a third ONECUT member, which we call OC-3. Its gene is located on chromosome 10. The sequence of OC-3 (490 residues) displays 51% amino acid identity with HNF-6 and 50% with OC-2. OC-3 has a DNA-binding specificity similar to that of HNF-6 and it is a stimulator of gene transcription. OC-3 mRNA is found in brain, stomach, and upper intestine in the adult and embryonic mouse. Our earlier work on HNF-6 and the expression patterns of the three mammalian ONECUT genes suggest that they all participate to the control of organ development from the foregut and midgut endoderm.