UCSF

Cholesterol is a key structural component of mammalian cell membranes. Unlike other lipids such as fatty acids, cholesterol cannot be catabolized for energy production, necessitating feedback mechanisms to prevent pathological cholesterol accumulation. Feedback inhibition of cholesterol metabolism involves regulation of SREBP2, the master transcriptional regulator of cholesterol synthesis, and its chaperone protein SCAP. In states of high cholesterol content, cholesterol causes SCAP/SREBP2 to be retained in the ER. Interestingly, biochemical studies have shown that the oxysterol 25-hydroxycholesterol (25-HC) is also capable of blocking SREBP2 activation by trapping it in the ER. This has raised the question of whether endogenous oxysterols have a physiologic function in controlling cholesterol metabolism.

Ch25h is an ER transmembrane enzyme that catalyzes the conversion of cholesterol to 25-hydroxycholesterol. The knockout mouse for this enzyme was reported to have no baseline defects in cholesterol homeostasis, calling into question whether 25-HC is required for inhibition of SREBP2 in vivo. In this thesis, I find that Ch25h upregulation is required in macrophages to shut down their endogenous cholesterol biosynthesis in response to bacterial pathogen sensing. Deletion of Ch25h in activated macrophages results in increased expression of cholesterol biosynthesis genes, as well as a hyper-inflammatory phenotype characterized by overproduction of the cytokine IL-1. These findings identify a new role for regulation of the SREBP2 pathway in controlling set-points for inflammation.

IL-1 normally exists in the cytosol in a pro-form and must be cleaved by an oligomeric protein complex known as the ‘inflammasome’ to become active. Here, I find that Ch25h provides a check on IL-1 overproduction by preventing cholesterol-dependent activation of the AIM2 inflammasome, which is triggered by the presence of cytosolic DNA. I find that Ch25h-deficient activated macrophages have impaired mitochondrial function, signs of mitochondrial membrane damage, and cytosolic accumulation of mitochondrial DNA. Thus, Ch25h is induced in macrophages by type I IFN to protect mitochondria from cholesterol-dependent damage upon bacterial sensing. Since AIM2 is a sequence non-specific DNA binding protein, I suggest that this circuit exists to prevent macrophages from spuriously engaging inflammation in response to host-derived nucleic acids.