Drosophila Myc Interacts with Host Cell Factor (dHCF) to Activate Transcription and Control Growth

Abstract

The Myc proto-oncoproteins are transcription factors that recognize numerous target genes through hexameric DNA sequences called E-boxes. The mechanism by which they then activate the expression of these targets is still under debate. Here, we use an RNAi screen in Drosophila S2 cells to identify Drosophila host cell factor (dHCF) as a novel co-factor for Myc that is functionally required for the activation of a Myc-dependent reporter construct. dHCF is also essential for the full activation of endogenous Myc target genes in S2 cells, and for the ability of Myc to promote growth in vivo. Myc and dHCF physically interact, and they colocalize on common target genes. Furthermore, down-regulation of dHCF-associated histone acetyltransferase and histone methyltransferase complexes in vivo interferes with the Myc biological activities. We therefore propose that dHCF recruits such chromatin-modifying complexes and thereby contributes to the expression of Myc targets and hence to the execution of Myc biological activities.Myc genes were identified for their powerful transforming capabilities in vertebrate systems and later shown to be essential for normal development (reviewed in Refs. 1, 2). The molecular functions of Myc are evolutionarily conserved, and the single Myc homolog in Drosophila melanogaster (called Myc when referring to the protein, and diminutive or dm when referring to alleles) can substitute for vertebrate Myc (3) and vice versa (Ref. 4, for a recent review see Ref. 5). Drosophila Myc prominently controls cellular growth; null mutations prevent organismal growth and lead to death during early larval stages (6), whereas hypomorphic dm mutations prolong development and result in small adult flies, made up of smaller than wild-type cells (7). Conversely, overexpression of Myc results in bigger cells, but also stimulates apoptosis (7,8). These effects of Myc are mediated by the transcriptional regulation of a large number of target genes (9, 10), including genes transcribed by RNA polymerases I (11) and III (12).

Drosophila Mbm Is a Nucleolar Myc and Casein Kinase 2 Target Required for Ribosome Biogenesis and Cell Growth of Central Brain Neuroblasts

“…Amplification primers flanking E-box1 were 5=-CCACCAA CAATCGGATCTTA-3= and 5=-TGTCCTCCACTGTTGTGCAT-3=, and primers flanking E-box2 were 5=-TGTCCACATCCTGGTGTCC-3= and 5=-CGCTCGTTCCACAAGCTACT-3=. Primers used for the positive control, Nop5, and the negative control, Pka-C1, were described previously (31).…”

“…Luciferase assays were carried out as described previously (31), using a dual-luciferase reporter assay system (Promega) and a Glomax luminometer. The experiment was repeated three times, and within a single experiment, each transfection was performed in duplicate.…”

Evidence for Tissue-Specific JAK/STAT Target Genes inDrosophilaOptic Lobe Development

“…The role of Nop56 in animal development has not been well studied. Previous studies showed that Nop56 gene expression is activated by dMyc, implicating Nop56 in cellular growth in Drosophila (Orian et al 2003;Hulf et al 2005;Pierce et al 2008;Furrer et al 2010). Neumüller et al (2011), through RNA interference (RNAi) screens, identified Nop56 as a regulator of neuroblast proliferation in the central brain.…”

PAF1 complex component Leo1 helps recruit Drosophila Myc to promoters

“…To identify novel transcriptional cofactors for Myc, we carried out an RNAi screen in S2 cells (a cell line derived from Drosophila embryos). Out of 752 tested transcription-associated factors, we found 33 proteins that consistently affected the activity of the Myc-dependent reporter, including the scaffold protein dHCF (33) and the components of the PAF1 complex. Depletion of the PAF1 proteins does not strongly affect Myc protein levels (Fig.…”

“…To identify proteins that influence Myc's transcriptional output, we carried out an RNAi screen in Drosophila S2 cells (33) and identified the PAF1 complex as a functional interactor of Myc. The PAF1 complex is conserved from yeast to man.…”