Summary Sterol regulatory element binding proteins (SREBPs) are key transcriptional regulators of lipid metabolism. To define functional differences between the three mammalian SREBPs we are using genome-wide ChIP-seq with isoform-specific antibodies and chromatin from select tissues of mice challenged with different dietary conditions that enrich for specific SREBPs. We show hepatic SREBP-2 binds preferentially to two different gene-proximal motifs. A Gene ontology analysis suggests SREBP-2 targets lipid metabolic processes as expected but apoptosis and autophagy gene categories were also enriched. We show SREBP-2 directly activates autophagy genes during cell sterol depletion, conditions known to induce both autophagy and nuclear SREBP-2 levels. Additionally, SREBP-2 knockdown during nutrient depletion decreased autophagosome formation and lipid droplet association of the autophagosome targeting protein LC3. Thus, the lipid droplet could be viewed as a third source of cellular cholesterol, which along with sterol synthesis and uptake, is also regulated by SREBP-2.

The carbohydrate response element binding protein (ChREBP), a basic helix-loop-helix/leucine zipper transcription factor, plays a critical role in the control of lipogenesis in the liver. To identify the direct targets of ChREBP on a genome-wide scale and provide more insight into the mechanism by which ChREBP regulates glucose-responsive gene expression, we performed chromatin immunoprecipitation-sequencing and gene expression analysis. We identified 1153 ChREBP binding sites and 783 target genes using the chromatin from HepG2, a human hepatocellular carcinoma cell line. A motif search revealed a refined consensus sequence (CABGTG-nnCnG-nGnSTG) to better represent critical elements of a functional ChREBP binding sequence. Gene ontology analysis shows that ChREBP target genes are particularly associated with lipid, fatty acid and steroid metabolism. In addition, other functional gene clusters related to transport, development and cell motility are significantly enriched. Gene set enrichment analysis reveals that ChREBP target genes are highly correlated with genes regulated by high glucose, providing a functional relevance to the genome-wide binding study. Furthermore, we have demonstrated that ChREBP may function as a transcriptional repressor as well as an activator.

Lipid homeostasis in vertebrates is regulated by 3 sterol regulatory element binding protein (SREBP) isoforms. Here, we identify targets of SREBP-1 in mammalian liver using chromatin immunoprecipitation-high-throughput DNA sequencing. Antisera to SREBP-1 were used with liver chromatin from mice fed a high-carbohydrate diet after a fast, which leads to superinduction of hepatic SREBP-1c expression. SREBP-1-DNA complexes were subjected to massive parallel DNA sequencing using the Illumina Genome Analyzer II, resulting in 5.7 million sequence reads. Mapping these reads to the mouse reference genome identified 426 peaks of SREBP-1 binding vs. a control antibody. These binding peaks show a striking enrichment in proximal promoter regions, with 52% located within 1 kb upstream of a transcription start site. A previously undescribed sequence motif (5-ACTACANNTCCC-3) was present in 76% of the total peaks, and we show that it is a functional SREBP-1 response element. Our analysis also reveals that an Sp1 consensus site is present as a ''coregulatory'' motif in 50% of the SREBP-1 binding peaks, consistent with previous functional studies. SREBP-1 bound not only to many well-characterized SREBP-1 target genes but to several other previously unknown targets in lipid and carbohydrate metabolism as well as many putative target genes in other diverse biological pathways.ChIP-seq ͉ fasting/refeeding ͉ Sp1 ͉ Kolmogorov-Smirnov test A ll living organisms have evolved strategic regulatory systems to coordinate metabolic flux into potentially competing biochemical pathways to manage metabolite and end-product pools for optimal cellular fitness. This is particularly important in mammalian lipid homeostasis because cholesterol and fatty acids, the 2 major classes of mammalian lipids, are key to the structure and function of all cells, and unbalanced flux into these pathways can be both energy-inefficient and toxic. In vertebrates, lipid homeostasis is maintained in part by sterol regulatory element binding protein (SREBP) transcription factors, which are synthesized as Ϸ125-kDa precursors containing 2 membrane-spanning domains that localize the immature proteins to the endoplasmic reticulum membrane (1).Overlapping transcripts from 1 gene encode SREBP-1a and SREBP-1c isoforms, and the singular SREBP-2 is encoded from a distinct unlinked gene (2). There is an intricate system of nutrient sensing that relates cellular levels of cholesterol and other key lipid components to the expression and multistep membrane trafficking/protease pathway that converts the membrane-bound SREBPs into the much smaller nuclear targeted transcription factors.SREBPs activate genes encoding most of the key enzymes in cholesterol and fatty acid metabolism (3); thus, coupling SREBP processing to pathway flux provides a direct link between the activity of the regulatory factors and metabolic demand. Overexpression and targeted elimination studies in mice have produced crucial information on the roles of SREBPs in lipid regulation (3).The available evidence indi...

We used mouse hepatic chromatin enriched with an FXR antibody and chromatin immunoprecipitation-sequencing (ChIP-seq) to evaluate FXR binding on a genome-wide scale. This identified 1656 FXR-binding sites and 10% were located within 2 kb of a transcription start site which is much higher than predicted by random occurrence. A motif search uncovered a canonical nuclear receptor IR-1 site, consistent with in vitro DNA-binding studies reported previously. A separate nuclear receptor half-site for monomeric receptors such as LRH-1 was co-enriched and FXR activation of four newly identified promoters was significantly augmented by an LRH-1 expression vector in a co-transfection assay. There were 1038 genes located within 20 kb of a peak and a gene set enrichment analysis showed that genes identified by our ChIP-seq analysis are highly correlated with genes activated by an FXR-VP16 adenovirus in primary mouse hepatocytes providing functional relevance to the genome-wide binding study. Gene Ontology analysis showed FXR-binding sites close to many genes in lipid, fatty acid and steroid metabolism. Other broad gene clusters related to metabolism, transport, signaling and glycolysis were also significantly enriched. Thus, FXR may have a much wider role in cellular metabolism than previously appreciated.

Breast cancer is the most commonly diagnosed cancer in women, with 10% of disease attributed to hereditary factors. Although BRCA1 and BRCA2 account for a high percentage of hereditary cases, there are more than 25 susceptibility genes that differentially impact the risk for breast cancer. Traditionally, germline testing for breast cancer was performed by Sanger dideoxy terminator sequencing in a reflexive manner, beginning with BRCA1 and BRCA2. The introduction of next-generation sequencing (NGS) has enabled the simultaneous testing of all genes implicated in breast cancer resulting in diagnostic labs offering large, comprehensive gene panels. However, some physicians prefer to only test for those genes in which established surveillance and treatment protocol exists. The NGS based BRCAplus test utilizes a custom tiled PCR based target enrichment design and bioinformatics pipeline coupled with array comparative genomic hybridization (aCGH) to identify mutations in the six high-risk genes: BRCA1, BRCA2, PTEN, TP53, CDH1, and STK11. Validation of the assay with 250 previously characterized samples resulted in 100% detection of 3,025 known variants and analytical specificity of 99.99%. Analysis of the clinical performance of the first 3,000 BRCAplus samples referred for testing revealed an average coverage greater than 9,000X per target base pair resulting in excellent specificity and the sensitivity to detect low level mosaicism and allele-drop out. The unique design of the assay enabled the detection of pathogenic mutations missed by previous testing. With the abundance of NGS diagnostic tests being released, it is essential that clinicians understand the advantages and limitations of different test designs.