Identification of cholesterol-regulating genes by targeted RNAi screening

Targeted disruption of steroidogenic acute regulatory protein D4 leads to modest weight reduction and minor alterations in lipid metabolism

Steroidogenic acute regulatory protein (StAR)D4 is a member of the StAR related lipid transfer family. Homology comes from the approximately 210 amino acid lipid binding domain implicated in intracellular transport, cell signaling, and lipid metabolism. StARD4 was identified as a gene downregulated 2-fold by dietary cholesterol (Soccio, R. E., R. M. Adams, K. N. Maxwell, and J. L. Breslow. 2005. Differential gene regulation of StarD4 and StarD5 cholesterol transfer proteins. Activation of StarD4 by sterol regulatory element-binding protein-2 and StarD5 by endoplasmic reticulum stress. J. Biol. Chem. 280: 19410-19418). A mouse knockout was created to investigate StARD4's functionality and role in lipid metabolism. Homozygous knockout mice exhibited normal Mendelian mating genetics, but weighed less than wild-type littermates, an effect not accounted for by energy metabolism or food intake. Body composition as analyzed by DEXA scan showed no significant difference. No significant alterations in plasma or liver lipid content were observed on a chow diet, but female knockout mice showed a decrease in gallbladder bile cholesterol and phospholipid concentration. When challenged with a 0.2% lova-statin diet, StARD4 homozygous mice exhibited no changes. However, when challenged with a 0.5% cholesterol diet, female StARD4 homozygous mice showed a moderate decrease in total cholesterol, LDL, and cholesterol ester concentrations. Microarray analysis of liver RNA found few changes. However, NPC1's expression, a gene not on the microarray, was decreased approximately 2.5-fold in knockouts. These observations suggest that StARD4's role can largely be compensated for by other intracellular cholesterol transporters.

Chemical screen to reduce sterol accumulation in Niemann-Pick C disease cells identifies novel lysosomal acid lipase inhibitors

Niemann-Pick C disease (NPC) is a lysosomal storage disorder causing abnormal accumulation of unesterified free cholesterol in lysosomal storage organelles. High content phenotypic microscopy chemical screens in both human and hamster NPC-deficient cells have identified several compounds that partially revert the NPC phenotype. Cell biological and biochemical studies show that several of these molecules inhibit lysosomal acid lipase, the enzyme that hydrolyzes LDL-derived triacylglycerol and cholesteryl esters. The effects of reduced lysosomal acid lipase activity in lowering cholesterol accumulation in NPC mutant cells were verified by RNAi-mediated knockdown of lysosomal acid lipase in NPC1-deficient human fibroblasts. This work demonstrates the utility of phenotypic cellular screens as a means to identify molecular targets for altering a complex process such as intracellular cholesterol trafficking and metabolism.