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Functional and cellular constraints that shaped the PPARg binding landscape in human and mouse macrophages: human expression

ABSTRACT: Genome-wide comparisons of transcription factor binding sites in different species allow for a direct evaluation of the evolutionary constraints that shape transcription factor binding landscapes. To gain insights into the evolution of the PPARg-dependent transcriptional network we obtained binding data for PPARg, RXR and PU.1 in human macrophages and compared the profiles to matching data from mouse macrophages. We found that PPARg binding was highly divergent and only 5% of the PPARg bound regions were occupied in both species. Despite the low conservation of PPARg binding sites, conserved PPARg target genes contribute more than 30% to the functional target genes identified in human macrophages. In addition conserved target genes are strongly enriched for lipid metabolic functions. We detected the lineage-specification factor PU.1 at the majority of human PPARg binding sites. This confirmed the juxtaposed binding configuration found in mouse macrophages and demonstrated the preservation of tissue-specific adjacent PPARg-Pu.1 binding in the absence of individual binding site conservation. Finally, based on this of PPARg and PU.1 binding between human and mouse we suggest a mechanism by which PU.1 facilitates PPARg binding site turnover in macrophages.

ORGANISM(S): Homo sapiens

PROVIDER: GSE25137 | GEO | 2011/12/01

SECONDARY ACCESSION(S): PRJNA142767

REPOSITORIES: GEO

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Project description:The winged helix protein FOXA2 and the nuclear receptor PPARg are highly conserved, regionally-expressed transcription factors that regulate networks of genes controlling complex metabolic functions. Cistrome analysis for FOXA2 in mouse liver and PPARg in mouse adipocytes has previously produced consensus binding sites that are nearly identical to those used by the factors in human cells. Despite this conservation of the canonical binding motif, we report here that the great majority of specific binding regions for FOXA2 in human liver and for PPARg in human adipocytes are not in the orthologous locations to the mouse genome. Nevertheless, gene-centric analysis reveals strong shared transcription factor occupancy near genes in tissue-specific metabolic pathways that are functionally conserved across species. Genes with only species-specific binding sites fail to show enrichment for these pathways. Thus, the biological functions of transcription factors that control specific metabolic functions are highly shared across species. Two TFs, FOXA2 and PPARg, were studied for genome-wide conservation of binding between mouse and human in specific tissues/cell-types (liver for FOXA2, adipocytes for PPARg). The number of replicates for each TF was chosen to obtain a comparable number of reads between the TFs and species. Human FOXA2 ChIP-seq was performed on two biological replicates of human liver samples, in three technical replicates each. Input DNA was also collected and sequenced from both biological samples. Mouse FOXA2 ChIP-seq was performed on four biological replicates of mouse liver samples. The ChIP and sequencing were repeated on two of these biological replicates to create technical replicates for additional sequence reads. Input DNA was sequenced from three additional mouse livers. Human PPARg ChIP-seq was performed on a human adipocyte cell-line (SGBS) differentiated in two replicate cultures. Input DNA was also collected and sequenced from one culture. Mouse PPARg ChIP-seq was performed on 3T3-L1 cells differentiated into adipocytes in culture in a single replicate, and this sequence data was pooled with existing data previously generated by the same lab, already available in GEO (GSE21314). A standard pool of input DNA sample sequence from multiple mouse tissue was used for analyzing the Mouse PPARg ChIP-seq data.

Project description:In mammalian cells transcription factors (TFs) preferentially bind sites contained in regions of high nucleosomal occupancy, as determined by nucleotide-dependent computational analysis. This observation suggests that nucleosomes may act as gatekeepers of TF binding sites. We hypothesized that in mammalian genomes the information controlling nucleosome assembly may partially coincide with the information that enables TFs to recognize cognate sites in cis-regulatory elements while ignoring the myriad of non-functional, randomly occurring consensus binding sites. This way, nucleosome-mediated masking would be coupled to TF binding site functionality. The hematopoietic master regulator Pu.1 maintained nucleosome depletion at macrophage-specific enhancers that were otherwise occupied by nucleosomes in other cell types and in reconstituted chromatin. We identified a minimal set of DNA sequence and shape features that predicted Pu.1 binding with 78% accuracy. The same features predicted nucleosome occupancy in cells where Pu.1 was not expressed with higher accuracy than specifically designed models. Control of nucleosome deposition by DNA sequence and shape features that also specify TF consensus site functionality may allow maintaining the gatekeeper function of nucleosomes during evolution of cis-regulatory elements. Chromatin immuno-precipitations of the transcription factor Pu.1 followed by multiparallel sequencing performed in murine bone marrow-derived macrophages. Experiments carried out in cells infected either with a retroviral vector containing a short hairpin targeting Pu.1 or with the empty vector as control. The shPU.1 hairpin (sequence available upon request) was selected among five designed using a publicly available software (http://katahdin.mssm.edu/siRNA) and was cloned in a modified version of TtRMPVIR inducible retroviral vector (Genbank HQ456318) in which the puromycin resistance gene was inserted. The empty vector, containing an sh-Renilla sequence, was used as control. Chromatin immuno-precipitations of the transcription factor PU.1 binding to in vitro reconstituted chromatin followed by multiparallel sequencing Micrococcal nuclease digestion of chromatin extracted from bone marrow derived macrophages followed by multiparallel sequencing . Experiments were carried out in untreated cells (4 replicates) and cells infected either with a retroviral vector containing a short hairpin targeting Pu.1 or with the empty vector as control (2 replicates). The shPU.1 hairpin (sequence available upon request) was selected among five designed using a publicly available software (http://katahdin.mssm.edu/siRNA) and was cloned in a modified version of TtRMPVIR inducible retroviral vector (Genbank HQ456318) in which the puromycin resistance gene was inserted. The empty vector, containing an sh-Renilla sequence, was used as control. Micrococcal nuclease digestion of in vitro reconstituted chromatin followed by multiparallel sequencing

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Functional and cellular constraints that shaped the PPARg binding landscape in human and mouse macrophages: human expression

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