ABCA1-mediated Cholesterol Efflux Is Defective in Free Cholesterol-loaded Macrophages

In advanced atherosclerosis, macrophage foam cells progressively accumulate large amounts of unesterified or “free” cholesterol (FC), a process that is thought to contribute to foam cell death and lesional necrosis. The cellular consequences of early FC accumulation, including those that lead to further FC accumulation, are poorly understood. In this context, we show that cholesterol and phospholipid efflux mediated by ABCA1, which is initially induced in the cholesterol-loaded macrophage, was inhibited by ∼80% in pre-toxic FC-loaded macrophages. Cholesterol efflux to HDL2, which is mediated by a non-ABCA1 pathway, was inhibited by only ∼20% in FC-loaded macrophages. FC loading led to decreased levels of ABCA1 protein via increased degradation of ABCA1, and not by decreased transcription or translation of AbcA1 mRNA. The decrease in ABCA1 protein occurred relatively early and was not prevented by caspase inhibitors, indicating that it was not a consequence of FC-induced apoptosis. However, inhibition of proteasomal function by lactacystin largely prevented the degradation of ABCA1. Importantly, the FC-induced decrease in ABCA1 function and protein was almost entirely prevented in macrophages that had partial deficiency of npc1 or were exposed to nanomolar concentrations of U18666A, both of which lead to defective cholesterol trafficking to the endoplasmic reticulum, but leave trafficking to the plasma membrane largely intact. Thus, a relatively early event during FC loading of macrophages is increased degradation of ABCA1, which appears to require trafficking of cholesterol to a peripheral cellular site, but not bulk trafficking of excess cholesterol to the plasma membrane. These findings provide new insight into the post-translational regulation of ABCA1 and the pathobiology of the FC-loaded macrophage.