A concerted mechanism involving ACAT and SREBPs by which oxysterols deplete accessible cholesterol to restrict microbial infection

Abstract

Most of the cholesterol in the plasma membranes (PMs) of animal cells is sequestered through interactions with phospholipids and transmembrane domains of proteins. However, as cholesterol concentration rises above the PM's sequestration capacity, a new pool of cholesterol, called accessible cholesterol, emerges. The transport of accessible cholesterol between the PM and the endoplasmic reticulum (ER) is critical to maintain cholesterol homeostasis. This pathway has also been implicated in the suppression of both bacterial and viral pathogens by immunomodulatory oxysterols. Here, we describe a mechanism of depletion of accessible cholesterol from PMs by the oxysterol 25-hydroxycholesterol (25HC). We show that 25HC-mediated activation of acyl coenzyme A: cholesterol acyltransferase (ACAT) in the ER creates an imbalance in the equilibrium distribution of accessible cholesterol between the ER and PM. This imbalance triggers the rapid internalization of accessible cholesterol from the PM, which is sustained for long periods of time through 25HC-mediated suppression of SREBPs and continued activation of ACAT. In support of a physiological role for this mechanism, 25HC failed to suppress Zika virus and human coronavirus infection in ACAT-deficient cells, and Listeria monocytogenes infection in ACAT-deficient cells and mice. We propose that selective depletion of accessible PM cholesterol triggered by ACAT activation and sustained through SREBP suppression underpins the immunological activities of 25HC and a functionally related class of oxysterols.

A cholesterol-binding bacterial toxin provides a strategy for identifying a specific Scap inhibitor that blocks lipid synthesis in animal cells

“…S7 A , Top ). In contrast, the addition of LDL, whose cholesterol content is liberated and delivered to the ER ( 51 ), or 25HC, which triggers the rapid internalization of cholesterol from the PM to the ER ( 54 ), resulted in a dramatic increase in cholesteryl [ 14 C]oleate formation ( SI Appendix , Fig. S7 A , Top ).…”

The role of cholesterol and its oxidation products in tuberculosis pathogenesis

“…Recent studies have shown a crucial role for 25-HC in reducing susceptibility to cholesterol-dependent cytolysins and dampening the intraepithelial spread of bacterial pathogens such as L. monocytogenes and S. flexneri [ 27 , 46 ] . The proposed mechanism suggests that 25-HC triggers the rapid mobilization of “accessible” plasma membrane cholesterol, which is not bound to sphingomyelin, and sequesters it intracellularly in lipid droplets as cholesterol esters [ 27 , 46 , 47 ] . Interestingly, while 25-HC appears to be protective against L. monocytogenes infection when secreted in a paracrine fashion, macrophages treated with 25-HC exhibited an increased bacterial burden [ 27 ] .…”

25-Hydroxycholesterol in health and diseases

“…4 ). First, 25-HC modifies PM cholesterol composition ( 87 ). By activating the LXR signaling pathway, and then ABCA1-mediated cholesterol efflux, 25-HC activates the RCT, thus resulting in a decrease of the cholesterol level at the PM, impeding the viral membrane fusion with cells.…”