HuR-dependent loading of miRNA RISC to the mRNA encoding the Ras-related small GTPase RhoB controls its translation during UV-induced apoptosis.

Of critical importance in the stress response is the post-transcriptional control of the expression of important genes involved in the control of cell survival and apoptosis. Here we report that miR-19, an oncogenic component of the miR-17-92/Oncomir-1 microRNA polycistron, regulates the expression of Ras homolog B (RhoB) in keratinocytes upon exposure to ultraviolet (UV) radiation. Strikingly, we could not find any evidence for deregulated expression of miR-19 during UV treatment. However, we show that miR-19-mediated regulation of antiapoptotic RhoB expression requires the binding of human antigen R (HuR), an AU-rich element binding protein, to the 3'-untranslated region of the rhoB mRNA. We propose that the loss of the interdependent binding between HuR and miR-19 to the rhoB mRNA upon UV exposure relieves this mRNA from miR-19-dependent inhibition of translation and contributes to the apoptotic response.

Transcription of the human uncoupling protein 3 gene is governed by a complex interplay between the promoter and intronic sequences.

AIMS/HYPOTHESIS: Uncoupling protein (UCP) 3 is an inner mitochondrial membrane transporter mainly produced in skeletal muscle in humans. UCP3 plays a role in fatty acid metabolism and energy homeostasis and modulates insulin sensitivity. In humans, UCP3 content is higher in fast-twitch glycolytic muscle than in slow-twitch oxidative muscle and is dysregulated in type 2 diabetes. Here, we studied the molecular mechanisms determining human UCP3 levels in skeletal muscle and their regulation by fasting in transgenic mice. METHODS: We produced a series of transgenic lines with constructs bearing different putative regulatory regions of the human UCP3 gene, including promoter and intron sequences. UCP3 mRNA and reporter gene expression and activity were measured in different skeletal muscles and tissues. RESULTS: The profile of expression and the response to fasting and thyroid hormone of human UCP3 mRNA in transgenic mice with 16 kb of the human UCP3 gene were similar to that of the endogenous human gene. Various parts of the UCP3 promoter did not confer expression in transgenic lines. Inclusion of intron 1 resulted in an expression profile in skeletal muscle that was identical to that of human UCP3 mRNA. Further dissection of intron 1 revealed that distinct regions were involved in skeletal muscle expression, distribution among fibre types and response to fasting. CONCLUSIONS/INTERPRETATION: The control of human UCP3 transcription in skeletal muscle is not solely conferred by the promoter, but depends on several cis-acting elements in intron 1, suggesting a complex interplay between the promoter and intronic sequences.

A new inducible protein expression system in fission yeast based on the glucose-repressed inv1 promoter.

Studies of the fission yeast Schizosaccharomyces pombe have made major contributions towards understanding cell-cycle control and many other important aspects of cell biology. A series of pREP expression vectors that utilize the thiamine-repressible nmt1 promoter are used routinely to manipulate the expression of genes in fission yeast. A shortcoming of the nmt1 promoter is that it is very slowly induced following removal of thiamine from the growth medium, requiring approx. 16h for full induction. Invertase, an enzyme responsible for sucrose metabolism, is regulated transcriptionally by glucose derepression in S. pombe. Using the inv1 promoter, we have developed the pINV1 set of inducible protein expression vectors. A shift from glucose to sucrose-based culture medium leads to a very rapid induction of the inv1 promoter. Genes that are regulated by the inv1 promoter are fully induced within 1h of the shift to sucrose-based medium. The pINV1 vectors utilize a simple induction protocol and enable studies in fission yeast requiring tight and rapid regulation of protein synthesis.

The DF-1 chicken fibroblast cell line: transformation induced by diverse oncogenes and cell death resulting from infection by avian leukosis viruses.

DF-1 is a continuous cell line of chicken embryo fibroblasts. The cells are free of endogenous sequences related to avian sarcoma and leukosis viruses and have normal fibroblastic morphology. DF-1 cells support the replication of avian retroviruses; diverse oncogenes induce foci of oncogenic transformation on monolayers of DF-1, and avian leukosis viruses of envelope subgroups B, D, and C induce cell death and form plaques. The new cell line will greatly facilitate studies on oncogenic transformation and cell killing by avian viruses.

Glutaredoxin is a direct target of oncogenic jun.

We have analysed differential gene expression in v-jun-transformed chicken embryo fibroblasts (CEF) compared to normal CEF by using the directional tag PCR subtraction method. From a first generation of putative Jun targets four clones were selected for study; they are upregulated in jun-transformed cells. Three of these clones showed homology to known genes: glutaredoxin, growth associated protein (GAP)-43/neuromodulin, and phenobarbital-induced cytochrome P450. The expression of these genes was analysed in fibroblasts transformed by various oncogenes. Expression of the glutaredoxin mRNA could be induced by a Jun-estrogen receptor chimaera in the absence of de novo protein biosynthesis. Based on this observation we conclude that glutaredoxin is a direct target of v-Jun.

Hormone-regulatable neoplastic transformation induced by a Jun-estrogen receptor chimera.

The v-jun oncogene encodes a nuclear DNA binding protein that functions as a transcription factor and is part of the activator protein 1 complex. Oncogenic transformation by v-jun is thought to be mediated by the aberrant expression of specific target genes. To identify such Jun-regulated genes and to explore the mechanisms by which Jun affects their expression, we have fused the full-length v-Jun and an amino-terminally truncated form of v-Jun to the hormone-binding domain of the human estrogen receptor. The two chimeric proteins function as ligand-inducible transactivators. Expression of the fusion proteins in chicken embryo fibroblasts causes estrogen-dependent transformation.

Oncogenic transformation induced by the Qin protein is correlated with transcriptional repression.

The retroviral oncogene qin codes for a protein that belongs to the family of the winged helix transcription factors. The viral Qin protein, v-Qin, differs from its cellular counterpart, c-Qin, by functioning as a stronger transcriptional repressor and a more efficient inducer of tumors. This observation suggests that repression may be important in tumorigenesis. To test this possibility, chimeric proteins were constructed in which the Qin DNA-binding domain was fused to either a strong repressor domain (derived from the Drosophila Engrailed protein) or a strong activator domain (from the herpes simplex virus VP16 protein). The chimeric transcriptional repressor, Qin-Engrailed, transformed chicken embryo fibroblasts in culture and induced sarcomas in young chickens. The chimeric activator, Qin-VP16, failed to transform cells in vitro or in vivo and caused cellular resistance to oncogenic transformation by Qin. These data support the conclusion that the Qin protein induces oncogenic transformation by repressing the transcription of genes which function as negative growth regulators or tumor suppressors.

Avian winged helix proteins CWH-1, CWH-2 and CWH-3 repress transcription from Qin binding sites.

The chicken winged helix proteins, CWH-1, CWH-2 and CWH-3, were isolated and identified by homology cloning using the winged helix sequence of the retroviral oncogene qin as a probe. The CWH proteins act as growth stimulators in chicken embryo fibroblasts and in this activity resemble the Qin protein. Qin is a transcriptional regulator that functions as a repressor, and its oncogenic potential is correlated with the ability to repress transcription. In this communication we show that CWH proteins are localized in the cell nucleus, recognize the Qin DNA binding site and also function as transcriptional repressors. The repression activity of CWH-3 was mapped to the region of amino acids 211 to 311, a domain that is homologous to the major repression domain of Qin.

Aberrant cell growth induced by avian winged helix proteins.

Winged helix transcription factors act as important regulators of embryonal development and tissue differentiation in vertebrates and invertebrates. Identification of the retroviral oncogene v-qin as a member of the winged helix family showed that these developmental regulators also have oncogenic potential. We used low-stringency hybridization of a chicken embryonic cDNA library to isolate cDNA clones coding for the three chicken winged helix (CWH) proteins, CWH-1, CWH-2, and CWH-3. The CWH genes are transcribed in a tissue-restricted pattern in adult and embryonic chicken tissues. The CWH proteins bind to conserved DNA binding sites for winged helix proteins in a sequence-specific manner. Expression of the CWH proteins from replication-competent retroviral RCAS vectors induces changes in morphology and growth pattern of chicken embryo fibroblasts. CWH-1 and CWH-3 also induce anchorage-independent growth in agar. Chicken embryo fibroblasts expressing the RCAS constructs release replication-competent viruses that are able to elicit the same cellular changes as the parental plasmid DNA. Our results suggest that winged helix transcription factors not only function as regulators of development and differentiation but also have the potential to stimulate abnormal cell proliferation.

Nuclear factor I interferes with transformation induced by nuclear oncogenes.

The four nuclear factor I genes (NFI-A, NFI-B, NFI-C, and NFI-X) give rise to multiple isoforms by alternative splicing in many tissues. These NFI proteins cooperate with AP-1, Myc, and other transcription factors in regulating transcription of numerous cellular and viral genes. We have investigated the growth-regulatory potential of NFI by overexpressing cDNAs from chicken NFI genes -A, -B, -C, and -X in chicken embryo fibroblasts (CEF). None of the NFI cDNAs induced oncogenic transformation of CEF. However, overexpression of each of the NFI proteins caused similar morphological alteration of the cells, inducing them to become flattened and polygonal and to show increased adherence. The growth properties of these cells were similar to normal CEF. When these morphologically altered CEF were challenged by superinfection with oncogenic retroviruses, they were resistant to transformation by the nuclear oncogenes jun, fos, junD, myc, and qin but were readily transformed by cytoplasmic oncogenes src, mil/raf, ras, and fps. The NFI-A1 protein was able to alter transactivation by the cellular and viral Jun proteins in a promoter-dependent manner. The changes in cell morphology and reduced susceptibility to nuclear oncogenes were not seen with a carboxy-terminal truncation in the transactivation domain of NFI, suggesting that this region of the protein is essential for the observed effects. The dichotomy between the activities of nuclear and of cytoplasmic oncogenes in this system is discussed.