A delicate balance Post-transcriptional regulation of intracellular cholesterol metabolism

The maintenance of intracellular cholesterol homeostasis relies on a fascinating interplay between multiple complex molecular processes. The studies described in this thesis highlight the contribution of post-transcriptional regulation to the control of intracellular lipid metabolism.
The membranes of our cells are comprised of a complex heterogeneous mixture of different lipids and proteins. Cholesterol is a lipid that is essential for the integrity and function of our membranes. Cholesterol also serves as a critical precursor for steroid hormones, as cortisol and oestrogen, bile acids and vitamins, for instance vitamin D. Inversely, a dysregulated systemic cholesterol metabolism can be detrimental to human health, and is associated with many pathologies, including cardiovascular disease, neurological disorders, and cancer.
Chapter 1 comprises a general introduction, explaining the mechanisms that cells employ to tightly regulate their cholesterol levels. In chapter 2, we describe an unbiased screening approach, by which we identify the SREBP-regulating gene (SPRING) as a new moiety in the (post-)transcriptional regulation of cholesterol metabolism. In chapter 3, we focus on the post-translational regulation of 3-hydroxy-3-methyl-glutaryl-coenzyme A reductase (HMGCR), which is the first rate-limiting enzyme of cholesterol biosynthesis and the target of statins. We identify UBXD8 a necessary element for extraction and subsequent proteasomal degradation of HMGCR in response to oxysterols. In chapter 4, we move to the second rate-limiting enzyme of cholesterol biosynthesis: squalene monooxygenase (SQLE), which is subject to ubiquitylation and subsequent degradation in response to cholesterol. We identify UBE2J2 as the E2 ubiquitin conjugating enzyme necessary for this process, and show that the degradation of SQLE and HMGCR relies on the use of different E2 enzymes. In chapter 5, we investigate the function of the E3 ligase of SQLE, MARCH6, in endothelial cells. We find that MARCH6-regulated SQLE levels are an important determinant of endothelial junctional integrity and angiogenesis. Chapter 6 summarises the studies presented in this thesis and places the findings in a broader context.