Abstract
Sitosterolemia is a disease characterized by very high levels of sitosterol and other plant sterols and premature atherothrombotic vascular disease. One theory holds that plant sterols can directly promote atherosclerosis, but the mechanism is not known. Unesterified, or "free," cholesterol (FC) is a potent inducer of macrophage death, which causes plaque necrosis, a precursor to atherothrombosis. FC-induced macrophage death, however, requires dysfunction of the sterol esterifying enzyme acyl-coenzyme A-cholesterol acyltransferase (ACAT), which likely occurs slowly during lesion progression. In contrast, plant sterols are relatively poorly esterified by ACAT, and so they may cause macrophage death and plaque necrosis in an accelerated manner. In support of this hypothesis, we show here that macrophages incubated with sitosterol-containing lipoproteins accumulate free sterols and undergo death in the absence of an ACAT inhibitor. As with FC loading, sitosterol-induced macrophage death requires sterol trafficking to the endoplasmic reticulum, and sitosterol-enriched endoplasmic reticulum membranes show evidence of membrane protein dysfunction. However, whereas FC induces caspase-dependent apoptosis through activation of the unfolded protein response and JNK, sitosterol-induced death is caspase-independent and involves neither the unfolded protein response nor JNK. Rather, cell death shows signs of necroptosis and autophagy and is suppressed by inhibitors of both processes. These data establish two new concepts. First, a relatively subtle change in sterol structure fundamentally alters the type of death program triggered in macrophages. Understanding the basis of this alteration should provide new insights into the molecular basis of death pathway signaling. Second, sitosterol-induced macrophage death does not require ACAT dysfunction and so may occur in an accelerated fashion. Pending future in vivo studies, this concept may provide at least one mechanism for accelerated plaque necrosis and atherothrombotic disease in patients with sitosterolemia.
Highlights
Incubation of Macrophages with Sitosteryl Ester-containing Lipoproteins Leads to Free Sitosterol Accumulation in the Absence of an acyl-CoA:cholesterol acyltransferase (ACAT) Inhibitor—When lipoprotein-cholesterol is delivered to macrophages, the cholesterol is efficiently esterified by the endoplasmic reticulum (ER) enzyme ACAT, preventing FC accumulation and FC-induced macrophage death [20, 56, 57]
Lipoprotein-cholesterol accumulation induces death only when ACAT-mediated esterification is inhibited in vitro or becomes dysfunctional in vivo, which we propose occurs in a gradual manner during the course of advanced atherosclerosis [20]
We compared free sterol accumulation in control and ACAT-inhibited macrophages incubated with these particles, as well as acetyl-LDL reconstituted with cholesteryl oleate
Summary
Our data show that such lipoproteins are robust inducers of cell death in ACAT-competent macrophages and that the mechanism is fundamentally different from that caused by free cholesterol. We compared free sterol accumulation in control and ACAT-inhibited macrophages incubated with these particles, as well as acetyl-LDL reconstituted with cholesteryl oleate (rAcL-CO).
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have