tances SCAP dissociates from COPII, the SCAP/SREBP complex remains in the ER, and proteolytic activation is blocked. In another feedback loop SREBP-1a and SREBP-1c are suppressed by polyunsaturated fatty acids . SREBP-1c transcription in the liver is controlled by liver X receptors, whose activation in turn is blocked by PUFA. In spite of the current research efforts in this field, our knowledge of intracellular cholesterol trafficking and homeostasis is far from complete. To gain a better handle on these events, we performed a genome-wide cDNA over-expression screen to identify modulators of SREBP activity. We used a cell-based luciferase assay that measures expression from an SREBP-specific promoter. We also performed secondary biological assays to further validate these hits. Additionally, employing a novel modification of Gene Set Enrichment Analysis we performed a pathway analysis on the high throughput screening data, as GSEA was originally developed for analyzing microarray experiments. GSEA applies a priori biological knowledge to genome-scale data sets to implicate pathways in the biological system of interest. In addition to known pathways regulating lipid metabolism, such as the SREBP and nuclear hormone receptor pathway, our analysis has led to the identification of a number of pathways previously not associated with the regulation of cellular cholesterol homeostasis. The data suggests that pathways involved in intracellular signal transduction such as tyrosine kinase signaling, G-protein / small GTPase pathways and ephrin signaling positively affect intracellular cholesterol homeostasis, while 
pathways acting at the extracellular level, such as matrix Eleutheroside E site proteins, cell-matrix and cell-adhesion proteins, and pathways involved in cell structure and organization, negatively regulate cellular cholesterol homeostasis. We have validated 12504917 the results of the primary screen through a series of secondary biological assays and find considerable overlap between the genes identified by secondary screening and the pathways identified via GSEA, indicating that pathway-centric analyses of biological screening data is a valid approach that may assist in target identification. Our results implicate multiple novel genes and pathways in intracellular cholesterol homeostasis and open up novel venues for the interrogation of lipid biology and lipid-linked disease. and renilla luciferase levels. Thus, a high luciferase ratio indicates SREBP pathway activation and vice-versa. For our experiments, this SREBP signaling assay was optimized by a series of steps. First, in order to use an optimal reporter construct the 36SREs cassette was sub-cloned and tested in a number of luciferase vectors including, pGL3-Basic and pTransLucent. In our hands, the pTransLucent vector displayed higher luciferase ratios and higher signal-to-noise ratio in a 23127512 384-well format and was chosen for further experiments. Second, two mammalian cell lines HEK-293 and HeLa were tested for cell line of choice. HEK-293 cells displayed higher assay reproducibility, luciferase signals and fold change under different experimental conditions and were thus chosen for this study. Third, a mutant SRE promoter driving a luciferase gene was generated and used as a specificity control for our experiments. This mutant SRE-luciferase construct was inactive under all experimental conditions. Fourth, to optimize the repression of SREBP signaling by cholesterol, a concentration response curve for 25-
