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Abstract

Cholesterol is an essential component of membranes in higher eukaryotes, and a precursor for further important sterols. Abnormally high or low levels of cholesterol are associated with increased mortality, thus the amount of cholesterol available to the cells from diet or by de novo synthesis needs to be closely regulated. One of the most important coordinators of cholesterol homeostasis is the sterol regulatory element-binding protein (SREBP) family of transcription factors. Although fundamental mechanisms of the cholesterol metabolism as well as their regulation have been well described, it is believed that only a limited number of contributing factors have been discovered. The aim of this study was to identify novel targets of SREBP that contribute to the tight regulation of cellular cholesterol metabolism. For this, ten genes that had not previously been associated with cholesterol metabolism, but identified as putative SREBP targets through a combination of genome wide gene expression profiling and in silico promoter binding site predictions, were experimentally investigated. For nine of these candidate genes increased expression under sterol-depleted culture conditions was confirmed. Of these, four genes – C17ORF59, HES6, TMEM55B and SLC2A6 – were experimentally validated as regulated via SREBP transcription factors. One of the most promising candidate genes, C17ORF59, was characterized in further detail to elucidate its functional role in cholesterol metabolism. Knockdown of this previously uncharacterized gene lead to a redistribution of the low density lipoprotein (LDL) receptor from the plasma membrane towards the cell interior. Consequently, cellular uptake of LDL cholesterol is considerably reduced. Cellular localization studies revealed that the protein C17orf59 predominantly locates to the endo-lysosomal system. Furthermore, knockdown of C17orf59 lead to a scattering of the Golgi apparatus and is associated with a reduced vesicular stomatis virus glycoprotein (VSVG) transport to the cell surface. These results suggest that C17orf59 might play a role in the recycling of LDL receptor from sorting endosomes to the plasma membrane. By unrevealing C17orf59 as novel SREBP target and as relevant for LDL receptor localization and function, the findings of this study may contribute to a better understanding of cholesterol metabolism and introduce C17orf59 as putative therapeutic target for hypercholesterolemia and arteriosclerosis.