Plasma membrane accessible cholesterol is regulated by ACC1 and lipid droplets.

Proper maintenance of plasma membrane (PM) cholesterol is essential for diverse processes ranging from animal development to pathogen evasion. Despite decades of study, the mechanisms governing cellular cholesterol regulation are incomplete. Using genome-wide screens we find that ACC1, the rate-limiting enzyme in fatty acid biosynthesis, regulates PM cholesterol transport. ACC1 loss causes a ~10-fold increase in PM accessible cholesterol in cells and mice. Mechanistically, we find that ACC1 regulates lipid droplet (LD) catabolism, and LDs are intimately tied to PM accessible cholesterol levels since reductions or elevations in their numbers block or promote cholesterol trafficking, respectively. Furthermore, LDs are required for cholesterol trafficking induced by 25-hydroxycholesterol, a modulator of inflammation and an interferon-stimulated second messenger that protects cells from pathogen invasion. This work identifies an unrecognized role for ACC1 and LDs in cholesterol regulation, which has implications for diseases where LD numbers are altered, from metabolic syndromes to neurodegeneration.