ApoE promotes hepatic selective uptake but not RCT due to increased ABCA1-mediated cholesterol efflux to plasma.

ApoE plays an important role in lipoprotein metabolism. This study investigated the effects of adenovirus-mediated human apoE overexpression (AdhApoE3) on sterol metabolism and in vivo reverse cholesterol transport (RCT). In wild-type mice, AdhApoE3 resulted in decreased HDL cholesterol levels and a shift toward larger HDL in plasma, whereas hepatic cholesterol content increased (P < 0.05). These effects were dependent on scavenger receptor class B type I (SR-BI) as confirmed using SR-BI-deficient mice. Kinetic studies demonstrated increased plasma HDL cholesteryl ester catabolic rates (P < 0.05) and higher hepatic selective uptake of HDL cholesteryl esters in AdhApoE3-injected wild-type mice (P < 0.01). However, biliary and fecal sterol output as well as in vivo macrophage-to-feces RCT studied with (3)H-cholesterol-loaded mouse macrophage foam cells remained unchanged upon human apoE overexpression. Similar results were obtained using hApoE3 overexpression in human CETP transgenic mice. However, blocking ABCA1-mediated cholesterol efflux from hepatocytes in AdhApoE3-injected mice using probucol increased biliary cholesterol secretion (P < 0.05), fecal neutral sterol excretion (P < 0.05), and in vivo RCT (P < 0.01), specifically within neutral sterols. These combined data demonstrate that systemic apoE overexpression increases i) SR-BI-mediated selective uptake into the liver and ii) ABCA1-mediated efflux of RCT-relevant cholesterol from hepatocytes back to the plasma compartment, thereby resulting in unchanged fecal mass sterol excretion and overall in vivo RCT.

Type I diabetes mellitus decreases in vivo macrophage-to-feces reverse cholesterol transport despite increased biliary sterol secretion in mice.

Type I diabetes mellitus (T1DM) increases atherosclerotic cardiovascular disease; however, the underlying pathophysiology is still incompletely understood. We investigated whether experimental T1DM impacts HDL-mediated reverse cholesterol transport (RCT). C57BL/6J mice with alloxan-induced T1DM had higher plasma cholesterol levels (P < 0.05), particularly within HDL, and increased hepatic cholesterol content (P < 0.001). T1DM resulted in increased bile flow (2.1-fold; P < 0.05) and biliary secretion of bile acids (BA, 10.5-fold; P < 0.001), phospholipids (4.5-fold; P < 0.001), and cholesterol (5.5-fold; P < 0.05). Hepatic cholesterol synthesis was unaltered, whereas BA synthesis was increased in T1DM (P < 0.001). Mass fecal BA output was significantly higher in T1DM mice (1.5-fold; P < 0.05), fecal neutral sterol excretion did not change due to increased intestinal cholesterol absorption (2.1-fold; P < 0.05). Overall in vivo macrophage-to-feces RCT, using [(3)H]cholesterol-loaded primary mouse macrophage foam cells, was 20% lower in T1DM (P < 0.05), mainly due to reduced tracer excretion within BA (P < 0.05). In vitro experiments revealed unchanged cholesterol efflux toward T1DM HDL, whereas scavenger receptor class BI-mediated selective uptake from T1DM HDL was lower in vitro and in vivo (HDL kinetic experiments) (P < 0.05), conceivably due to increased glycation of HDL-associated proteins (+65%, P < 0.01). In summary, despite higher mass biliary sterol secretion T1DM impairs macrophage-to-feces RCT, mainly by decreasing hepatic selective uptake, a mechanism conceivably contributing to increased cardiovascular disease in T1DM.

Impaired Bile Acid Metabolism and Gut Dysbiosis in Mice Lacking Lysosomal Acid Lipase.

Lysosomal acid lipase (LAL) is the sole enzyme known to be responsible for the hydrolysis of cholesteryl esters and triglycerides at an acidic pH in lysosomes, resulting in the release of unesterified cholesterol and free fatty acids. However, the role of LAL in diet-induced adaptations is largely unexplored. In this study, we demonstrate that feeding a Western-type diet to Lal-deficient (LAL-KO) mice triggers metabolic reprogramming that modulates gut-liver cholesterol homeostasis. Induction of ileal fibroblast growth factor 15 (three-fold), absence of hepatic cholesterol 7α-hydroxylase expression, and activation of the ERK phosphorylation cascade results in altered bile acid composition, substantial changes in the gut microbiome, reduced nutrient absorption by 40%, and two-fold increased fecal lipid excretion in LAL-KO mice. These metabolic adaptations lead to impaired bile acid synthesis, lipoprotein uptake, and cholesterol absorption and ultimately to the resistance of LAL-KO mice to diet-induced obesity. Our results indicate that LAL-derived lipolytic products might be important metabolic effectors in the maintenance of whole-body lipid homeostasis.