👤 Karen A Méndez-Lara

The HDL (high-density lipoprotein)-mediated stimulation of cellular cholesterol efflux initiates macrophage-specific reverse cholesterol transport (m-RCT), which ends in the fecal excretion of macrophage-derived unesterified cholesterol (UC). Early studies established that LDL (low-density lipoprotein) particles could act as efficient intermediate acceptors of cellular-derived UC, thereby preventing the saturation of HDL particles and facilitating their cholesterol efflux capacity. However, the capacity of LDL to act as a plasma cholesterol reservoir and its potential impact in supporting the m-RCT pathway in vivo both remain unknown. We investigated LDL contributions to the m-RCT pathway in hypercholesterolemic mice. Macrophage cholesterol efflux induced in vitro by LDL added to the culture media either alone or together with HDL or ex vivo by plasma derived from subjects with familial hypercholesterolemia was assessed. In vivo, m-RCT was evaluated in mouse models of hypercholesterolemia that were naturally deficient in CETP (cholesteryl ester transfer protein) and fed a Western-type diet. LDL induced the efflux of radiolabeled UC from cultured macrophages, and, in the simultaneous presence of HDL, a rapid transfer of the radiolabeled UC from HDL to LDL occurred. However, LDL did not exert a synergistic effect on HDL cholesterol efflux capacity in the familial hypercholesterolemia plasma. The m-RCT rates of the LDLr (LDL receptor)-KO (knockout), LDLr-KO/APOB100, and PCSK9 (proprotein convertase subtilisin/kexin type 9)-overexpressing mice were all significantly reduced relative to the wild-type mice. In contrast, m-RCT remained unchanged in HAPOB100 Tg (human APOB100 transgenic) mice with fully functional LDLr, despite increased levels of plasma APO (apolipoprotein)-B-containing lipoproteins. Hepatic LDLr plays a critical role in the flow of macrophage-derived UC to feces, while the plasma increase of APOB-containing lipoproteins is unable to stimulate m-RCT. The results indicate that, besides the major HDL-dependent m-RCT pathway via SR-BI (scavenger receptor class B type 1) to the liver, a CETP-independent m-RCT path exists, in which LDL mediates the transfer of cholesterol from macrophages to feces. Graphical Abstract: A graphical abstract is available for this article. Show less

Diabesity and fatty liver have been associated with low levels of high-density lipoprotein cholesterol, and thus could impair macrophage-specific reverse cholesterol transport (m-RCT). Liver X receptor (LXR) plays a critical role in m-RCT. Abcg5/g8 sterol transporters, which are involved in cholesterol trafficking into bile, as well as other LXR targets, could be compromised in the livers of obese individuals. We aimed to determine m-RCT dynamics in a mouse model of diabesity, the db/db mice. These obese mice displayed a significant retention of macrophage-derived cholesterol in the liver and reduced fecal cholesterol elimination compared with nonobese mice. This was associated with a significant downregulation of the hepatic LXR targets, including Abcg5/g8. Pharmacologic induction of LXR promoted the delivery of total tracer output into feces in db/db mice, partly due to increased liver and small intestine Abcg5/Abcg8 gene expression. Notably, a favorable upregulation of the hepatic levels of ABCG5/G8 and NR1H3 was also observed postoperatively in morbidly obese patients, suggesting a similar LXR impairment in these patients. In conclusion, our data show that downregulation of the LXR axis impairs cholesterol transfer from macrophages to feces in db/db mice, whereas the induction of the LXR axis partly restores impaired m-RCT by elevating the liver and small intestine expressions of Abcg5/g8. Show less