LXR Driven Induction of HDL‐Cholesterol is Independent of Intestinal Cholesterol Absorption and ABCA1 Protein Expression

Abstract

We investigated whether: (1) liver X receptor (LXR)-driven induction of high-density lipoprotein cholesterol (HDL-C) and other LXR-mediated effects on cholesterol metabolism depend on intestinal cholesterol absorption; and (2) combined treatment with the LXR agonist GW3965 and the cholesterol absorption inhibitor ezetimibe results in synergistic effects on cholesterol metabolism that could be beneficial for treatment of atherosclerosis. Mice were fed 0.2 % cholesterol and treated with GW3965+ezetimibe, GW3965 or ezetimibe. GW3965+ezetimibe treatment elevated serum HDL-C and Apolipoprotein (Apo) AI, effectively reduced the intestinal cholesterol absorption and increased the excretion of faecal neutral sterols. No changes in intestinal ATP-binding cassette (ABC) A1 or ABCG5 protein expression were observed, despite increased mRNA expression, while hepatic ABCA1 was slightly reduced. The combined treatment caused a pronounced down-regulation of intestinal Niemann-Pick C1-like 1 (NPC1L1) and reduced hepatic and intestinal cholesterol levels. GW3965 did not affect the intestinal cholesterol absorption, but increased serum HDL-C and ApoAI levels. GW3965 also increased Apoa1 mRNA levels in primary mouse hepatocytes and HEPA1-6 cells. Ezetimibe reduced the intestinal cholesterol absorption, ABCA1 and ABCG5, but did not affect the serum HDL-C or ApoAI levels. Thus, the LXR-driven induction of HDL-C and ApoAI was independent of the intestinal cholesterol absorption and increased expression of intestinal or hepatic ABCA1 was not required. Inhibited influx of cholesterol via NPC1L1 and/or low levels of intracellular cholesterol prevented post-transcriptional expression of intestinal ABCA1 and ABCG5, despite increased mRNA levels. Combined LXR activation and blocked intestinal cholesterol absorption induced effective faecal elimination of cholesterol.

Liver X receptor α mediates hepatic triglyceride accumulation through upregulation of G0/G1 Switch Gene 2 expression

“…The LXR agonism leads to activation of the reverse pathway for cholesterol transport and both biliary and fecal cholesterol excretion. Despite their antiatherogenic benefits, the LXR agonists such as T09 also cause undesirable effects, including hypertriglyceridemia and hepatic steatosis (33,(50)(51)(52)(53)(54)(55)(56). In the present study, we demonstrate that, in the absence of G0S2, T09 is able to retain its positive effect on cholesterol/reverse cholesterol trafficking, while rendered incapable of inducing the negative side effects of increasing plasma TG and causing hepatic steatosis.…”

“…The further finding that loss of G0S2 affected TG content but not lipogenic gene expression in response to T09 suggests that, without G0S2, increased de novo FA synthesis may be insufficient to drive TG accumulation. More importantly, synthetic LXR agonists are well documented as putative pharmacological agents for hypercholesterolemia and atherosclerosis (50)(51)(52)(53)(54)(55)(56). The LXR agonism leads to activation of the reverse pathway for cholesterol transport and both biliary and fecal cholesterol excretion.…”

Chronic Activation of Liver X Receptor Sensitizes Mice to High Cholesterol Diet–Induced Gut Toxicity

“…Although there are reports suggesting that LXR activation inhibited intestinal cholesterol absorption by suppressing Npc1l1 (Altmann et al, 2004;Duval et al, 2006), it has been controversial whether the LXR responsive suppression of Npc1l1 is sufficient to cause a suppression of cholesterol uptake. For example, a report showed that GW3965 can reduce the intestinal expression of Npc1l1, but it was not sufficient to affect the intestinal cholesterol absorption under aN HCD (Kannisto et al, 2014). Another study reported that treatment with GW3965 can increase the intestinal cholesterol absorption in mice despite a reduced intestinal expression of Npc1l1 (Hu et al, 2012).…”

“…Npc1l1 ablation in mice resulted in markedly reduced intestinal cholesterol absorption (Altmann et al, 2004;Davis et al, 2004); however, the notion that LXR inhibits cholesterol absorption by suppressing Npc1l1 is not without controversy. Some studies have suggested that inhibition of Npc1l1 by LXR ligands did not affect cholesterol absorption (Kannisto et al, 2014). Others claimed that LXRb activity in the small intestine can promote Npc1l1-independent cholesterol absorption (Hu et al, 2012).…”

Liver X receptors as regulators of metabolism

“…The intestine is dedicated to a tight control of whole-body cholesterol homeostasis not only as an absorptive organ but also by contributing to the removal of excess cholesterol from the periphery. Both reverse cholesterol transport (RCT) pathway and trans-intestinal cholesterol excretion (TICE) are involved in this process (57), and LXR has been identified as a key player in both pathways in the intestine (26,28,(58)(59)(60)(61). The non-biliary RCT pathway targets plasma cholesterol to the proximal part of the small intestine and, thereby, induces the cellular cholesterol secretion into the intestine lumen.…”