Fine-tuning protein recycling to control cholesterol homeostasis

Abstract

Intracellular levels of cholesterol are paramount to regulating cholesterol levels in blood. The Ubiquitin Proteasome System (UPS) can regulate the expression and activity of the master regulators of cholesterol levels in the cell: SREBP2 and LXR. The UPS, its E2 and E3 ligases and deubiquitylating enzymes (DUBs) can also fine-tune the expression, degradation and activity enzymes involved in the synthesis, uptake and efflux of cholesterol. Using molecular techniques, in this thesis we focus on the identification and description of new pathways that regulate intracellular cholesterol pools through the UPS. Two new E3 ligases involved in the regulation of intracellular cholesterol homeostasis were identified: MARCH6 and RNF145. Both E3 ligases respond to high cholesterol levels. MARCH6 degrades SM, an enzyme involved in cholesterol synthesis when intracellular cholesterol is high. RNF145's transcription is responsive to LXR, a transcription factor activated in the presence of certain cholesterol derivatives. We also look at a feedback mechanism in cholesterol homeostasis regulated by two E3 ligases: MARCH6 and IDOL. This feedback mechanism regulates which cholesterol source is preferentially used by the cell: either uptake or synthesis. The E2 ligases involved in the cholesterol-mediated degradation of HMGCR and SM were identified and serve as another mechanism to differentiate their breakdown. Furthermore, we identify a novel regulation of IDOL by DUBs, independent of its regulation by LXR