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

Abstract

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 ϳ80% in pre-toxic FCloaded macrophages. Cholesterol efflux to HDL 2 , which is mediated by a non-ABCA1 pathway, was inhibited by only ϳ20% 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.

Genetic and Pharmacologic Inhibition of the Neutrophil Elastase Inhibits Experimental Atherosclerosis

“…More importantly, our data further demonstrate that ABCA1 protein degradation, but not its transcription and translation, was altered by NE gene inactivation or exogenous HNE treatment. Currently, 2 major signal pathways have been implicated in ABCA1 protein degradation: PEST sequence‐mediated calpain proteolysis55 and the proteasome‐lysosome system 56, 57. We found no evidence to suggest that the calpain pathway is the underlying mechanism of NE‐promoted proteolysis of ABCA1, since NE‐mediated ABCA1 protein degradation was not affected by calpain‐specific inhibition.…”

Macrophages, inflammation, and atherosclerosis

“…Although deletion or inhibition of ACAT should increase cholesterol efflux via increased accumulation of intracellular free cholesterol, we have shown that elimination of ACAT1 from macrophages of LDLR À/À mice increased arterial lesions and reduced the number of arterial macrophages, likely because of accelerated apoptosis. 32 Feng and Tabas 33 have reached similar conclusions by showing that cholesterol loading in macrophages induces accumulation of free cholesterol and provokes 'toxic' effects including accelerated degradation of ABCA1 protein and reduced efflux of cholesterol from the cell.…”

7‐Ketocholesterol favors lipid accumulation and colocalizes with Nile Red positive cytoplasmic structures formed during 7‐ketocholesterol–induced apoptosis: Analysis by flow cytometry, FRET biphoton spectral imaging microscopy, and subcellular fractionation

“…Therefore, in accordance with these previous investigations, the present work underlines that 7KC favors cholesterol accumulation. Interestingly, in vivo high plasmatic levels of 7KC-3-sulfate (a potent antagonist of liver X receptors that transactivates ATP-binding cassette transporters such as ABCA1, which increases the overall rate of reverse cholesterol transport) are suspected of contributing to cholesterol accumulation (50 -52), which in turn can decrease cholesterol efflux, endogenous cholesterol biosynthesis, and low-density lipoprotein receptor expression (53,54).…”