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Papers In Press, published online ahead of print July 1, 2006
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Journal of Lipid Research, Vol. 47, 1583-1587, July 2006
Copyright © 2006 by American Society for Biochemistry and Molecular Biology
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United States Department of Agriculture, Agricultural Research Service, Western Human Nutrition Research Center, and Department of Nutrition, University of California-Davis, Davis, CA 95616
The online version of this article (available at http://www.jlr.org) contains additional tables and references in 9 sections. ![]()
Published, JLR Papers in Press, April 3, 2006.
1 To whom correspondence should be addressed. e-mail: dhwang{at}whnrc.usda.gov
| ABSTRACT |
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Supplementary key words peroxisome proliferator-activated receptor target genes conserved elements PACM
Abbreviations: DR1, direct repeat with a 1 bp spacer; GO, gene ontology; GWM, generalized weight matrix; PACM, PPAR-associated conserved motif; PPAR, peroxisome proliferator-activated receptor; PPRE, peroxisome proliferator response element; PWM, position weight matrix; ROC, receiver operating characteristic; UCSC, University of California at Santa Cruz
| INTRODUCTION |
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The three PPAR isotypes,
,
, and
, are differentially expressed across tissue types and developmental stages. However, all three bind peroxisome proliferator response elements (PPREs) in regulatory regions of their target genes. In this study, all known PPREs are collected from the literature and basic assumptions about PPREs are investigated. The most selective detection technique, position weight matrix (PWM)-based search of PPREs within upstream conserved elements, is applied to the entire human genome to develop a library of PPAR target genes. This technique is further assessed by microarray and gene ontology (GO) analysis, yielding new insights in PPAR biology.
| METHODS |
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Collection of reported functional PPREs
Reported PPREs from 78 publications and collection methods are detailed in the Supplemental Section 1.
Detection of PPREs in DNA sequences
PWMs were generated using the CONSENSUS algorithm (5) on lists of reported PPREs. DNA sequences were scored against PWMs using the PATSER program (5). A DNA sequence whose matrix score surpassed the cutoff value was a "detected" binding site. To evaluate PPREs for within-site correlations, the GMMPS program (6), which implements a generalized weight matrix (GWM) model, was used.
Evaluation of PPRE detection
Detection methods were evaluated using receiver operating characteristic (ROC) curves. Optimal discrimination techniques minimize the area under the curve. Each data point on the curve corresponds to a cutoff value: the false-negative coordinate is the fraction of reported PPREs that fall below the cutoff, and the false-positive coordinate is the fraction of random human promoter regions (5,000 bp) that contain a sequence that exceeds the cutoff. Although random promoter sequences may contain true binding sites, detection in such sequences is a direct measure of selectivity and a very useful benchmark when comparing detection methods.
Identification of overrepresented motifs
To determine the significance (P
0.05) of motif occurrence between reported and random sets, the data were modeled using a binomial distribution.
Microarray data analyses
The National Center for Biotechnology Information's PubMed and GEO databases were searched for PPAR microarray studies that published accession numbers of all regulated genes. Six were located (712). The two rat microarrays were excluded from tests involving conserved elements because such data were not available for the rat. See Supplemental Section 2 for further details.
GO analysis
The MAPPFinder tool (13) within GenMAPP version 2.0 was used to identify enriched GO terms with the Hs-Std_20041021.gdb and Mm-Std_20040824.gdb databases.
| RESULTS |
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alone or PPAR
alone were not better discriminators of PPREs overall (see Supplemental Figure 4B). Matrices constructed from PPREs reported to bind one of the isotypes were not better discriminators of PPREs of the same isotype than were matrices constructed from PPREs reported to bind the other isotype, even when flanking nucleotides were included (see Supplemental Figure 4CD). Higher order probability models were also evaluated. Use of a background model to reflect true GC content did not improve discrimination ability (data not shown). Furthermore, none of the nucleotide positions were cocorrelated under the GWM model, even when flanking nucleotides were included or PPREs were subgrouped by isotype.
Identification of PPREs and a novel motif in conserved elements
To improve selectivity, the search space was restricted to conserved elements within 5,000 bp upstream of reported human PPAR target genes. These elements, provided via the human Most Conserved track at UCSC, are conserved in space (neighboring nucleotides) and time (human, chimp, mouse, rat, dog, chicken, fugu, and zebrafish genomes), as identified by the phylogenetic hidden Markov model of Siepel et al. (16). Using our PWM (see Supplemental Section 3) to search these elements, PPREs were overrepresented among reported human genes compared with random genes (P < 0.00001). Furthermore, these PPREs occur with greater frequency than response elements of any PPAR or non-PPAR transcription factor in the TRANSFAC database using the MATCH program (17).
The upstream conserved elements were also searched for novel motifs using MEME (18). A motif of width 15 bp with the consensus TTCATTTGGACATTG was discovered. This motif, here named PACM, for PPAR-associated conserved motif (PWM in Supplemental Section 3), is more common than PPREs among these elements.
ROC curves for PPRE and PACM detection using our PWMs in upstream conserved elements are illustrated in Fig. 2A . With an average of only four conserved elements per gene, less than half of the reported human target genes have PPREs among their upstream conserved elements. Thus, this method cannot predict all direct PPAR targets. However, because the ROC curve is fairly sharp, a subset of targets can be predicted with high confidence. Of the reported genes that have any upstream conserved elements, 70% have a PPRE or PACM at matrix scores for which detection in a random promoter is <10%.
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Detection of upstream PPREs using TRANSFAC matrices does not distinguish upregulated from nonregulated genes (Fig. 2B). Using our PPRE matrix, the ROC curves fall beneath the nondiscrimination line, but the difference is underwhelming (Fig. 2C). However, when the search space is restricted to highly conserved elements, we see a distinct selectivity for upregulated genes in the three PPAR
microarrays but not in the PPAR
microarray (Fig. 2D). None of the methods distinguish downregulated genes (data not shown).
Genome-wide prediction
We conducted a genome-wide search for PPREs among conserved elements in the 5,000 bp upstream of all human reference sequences. Of 24,033 genes, PPREs were detected upstream of 1,085 (cutoff score = 8). These genes and their PPREs are listed in Supplemental Section 5. The 1,207 genes with PACMs and the 172 genes with both PPREs and PACMs are also listed in Supplemental Sections 6 and 7.
GO analysis
Biological process GO terms that are statistically overrepresented (Z score
2) among the predicted PPAR target genes are given in Table 1
. Only GO terms locally associated (nonnested) with three or more predicted genes were retained. Predicted PPAR target genes are sorted by these GO terms in Supplemental Section 8. GO analysis was also conducted on the reported PPAR target genes and regulated genes from all six microarrays. For each of these gene sets (reported, microarray, and predicted), the enriched GO terms were grouped into general categories to elucidate functional clusters (Fig. 3
).
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| DISCUSSION |
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, it is not inherent to either the core DR1 or immediately flanking nucleotides.
Today, biologists commonly seek putative PPREs using the consensus or TRANSFAC matrix. Only 2 of the 73 reported DR1-like PPREs match an ideal DR1, and 8% have five or more mismatches from consensus. Unlike TRANSFAC matrix searches, our method detected PPREs with sufficient selectivity for a genome-wide search and preferentially selected upregulated genes in PPAR
microarray studies. Interestingly, the fact that downregulated genes were not favorably selected suggests that the primary mechanism by which PPARs suppress gene expression is not mediated through PPREs.
Although the PPAR targets predicted in our genome-wide search are not complete, they represent an important subset, namely those that are targets across many vertebrate species. A high false-negative rate (60%) was tolerated to minimize the false-positive rate (<10%). Nevertheless, the false-negative rate of our method is still an improvement over that of all microarrays analyzed (8497%). Genes necessarily excluded from the predicted library include those with PPREs outside of upstream conserved elements and those whose PPREs are not DR1-like. Experimental verification of individual genes in future studies is necessary for unequivocal validation and to demonstrate the particular biological contexts (anatomical site, developmental stage, time of observation, stimulus, coregulatory molecules, DNA topology, etc.) in which they are regulated. Lastly, as the search space was restricted to conserved elements, there exists the possibility that this library is relevant only to vertebrate development. However, the economy of biology is such that developmental genes are often functional in the mature animal as well, albeit with a different function.
GO analysis supports the validity of the prediction method. First, both the predicted and microarray data sets contain the functional groups represented by the set of reported PPAR target genes. Second, GO terms represented by the predicted genes match areas of known PPAR biological function, such as DNA damage response, mitogen-activated protein kinase signaling, Wnt receptor signaling, cell differentiation, and muscle development, even though direct PPAR targets in these areas were previously unknown. Our study provides the first evidence that PPARs directly target such genes.
The GO analysis also suggests new mechanistic insights. The overwhelming number of immune-related GO terms among microarray-regulated genes, but not the reported or predicted direct targets, strongly suggests that immune function is primarily regulated by PPARs through indirect means. The enrichment of chromatin modification GO terms among the predicted set implies an exciting new hypothesis: PPARs directly target chromatin-remodeling genes. The fact that these genes were not regulated in the microarray studies may be time-dependent; the earliest observation point was at 24 h. Chromatin-remodeling genes may be targeted very early after stimulus and only transiently. Because PPARs can launch broad physiological changes, one might expect that a temporary increase in the quantity of chromatin-remodeling proteins would be necessary to implement such changes.
Surprisingly, a novel motif, PACM, was more prevalent than PPREs among conserved elements upstream of reported human PPAR target genes. This element may be bound by an unidentified transcription factor that coordinates with PPARs to regulate a subset of PPAR targets, especially considering that PPAR
itself contains both PPRE and PACM among its upstream conserved elements. Only 172 genes across the entire human genome contain both PPREs and PACMs in their upstream conserved elements. GO analysis of these genes (see Supplemental Section 9) suggests that their protein products are involved in lipid metabolism, perhaps in developing neurological tissue.
In summary, the major contributions of this study include the resolution of basic research questions about PPREs, a PPRE detection technique with demonstrated discriminatory ability, the identification of a novel cis-acting element through which PPAR-associated regulation is likely mediated, and new insights with respect to PPAR regulatory function. Additionally, the methodology, PWM- or GWM-based search within conserved elements as identified by a phylogenetic hidden Markov model and the monitoring of error rates using signal detection theory, can be applied to other cis-acting elements. Finally, the library of genes that contain high-confidence predicted PPREs should be a valuable resource for PPAR biologists.
| ACKNOWLEDGMENTS |
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Manuscript received November 11, 2005 and in revised form January 12, 2006 and in re-revised form March 30, 2006.
| REFERENCES |
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