A high-density lipoprotein (HDL) subspecies produced by small intestine, which potently shields the endotoxicity of bacterial lipopolysaccharide (LPS), may protect against gut-derived liver injury, according to a study led by scientists at Washington University School of Medicine (WUSM) in St. Louis.

Enteric HDL may therefore represent a novel pharmacological target for protecting the liver against gut-derived LPS leakage in both alcoholic and nonalcoholic settings, including nonalcoholic steatohepatitis (NASH), the authors reported in the July 23, 2021, edition of Science.

Finding new such new targets is timely, as "there are currently no effective treatment approaches for NASH or alcoholic liver damage, despite the increasing prevalence of these conditions," study leader Gwendalyn Randolph told BioWorld Science.

"For small bowel resections, treatments include glucagon-like peptide- 2 (GLP-2) agonists, but these target improved gut absorption and there is no treatment for LPS leakage per se," said the professor in the Department of Pathology at WUSM.

The portal vein collects and transports intestinal host and microbiome nutrients and metabolites to the liver, where the latter may drive NASH and fibrosis.

Enteric LPS from Gram-negative bacteria also causes liver injury after intestinal insult, but the mechanisms limiting this via the gut-portal axis remain poorly defined.

Theoretically, HDL may protect the liver through its potential to neutralize LPS, while HDL-cholesterol (HDL-C) is important in cholesterol transport, with only the liver and small intestine producing apolipoprotein A1 (apoA1) HDL-C core protein.

When the gene encoding the cholesterol transporter ABCA1 essential for HDL biogenesis is deleted in intestinal epithelial cells, plasma HDL-C is reduced by around 25, but the approximate 75% HDL-C reduction after liver-specific ABCA1 loss suggests the intestine is a secondary HDL-C source.

However, how enteric-derived HDL-C is delivered to the liver is poorly understood, limiting our understanding of its role in the gut-liver axis.

This was investigated in the new Science study conceived by Randolph and her then postdoctoral researcher, study first author Yong-Hyun Han, now an assistant professor in the Laboratory of Pathology and Physiology at Kangwon National University in Korea.

They demonstrated that enteric-derived HDL-C entered portal blood, while intestinal epithelial cells produced small HDL3 particles with potent LPS-neutralizing properties.

"We studied mice genetically modified so that only the intestine could not make HDL particles and found that the portal veins in these mice contained little HDL compared to systemic blood," explained Randolph. "Then we used a previously developed mouse model wherein HDL glowed under fluorescent light to demonstrate that HDL was entering the portal blood."

"We characterized how hepatic immune cells responded to LPS in the presence of portal blood HDL and found that it potently prevented liver cell inflammation via neutralization of gut-derived LPS."

Intestine-derived HDL in its HDL3 subspecies form was then shown to cross the portal vein coupled with LPS-binding protein (LBP).

High-speed centrifugation of HDL was shown to remove the bound LBP, which negated HDL LPS-neutralizing effect, but that was reinstated when LBP was replaced, noted Randolph.

"We then used chromatography to purify the HDL while retaining the bound LBP, which did not lose its LPS-neutralizing effect, showing that LBP was necessary for HDL3 in portal blood to neutralize LPS."

She and her team further demonstrated that HDL3 prevented LPS binding to and inflammatory activation of liver macrophages and supported extracellular LPS inactivation.

This is clinically important, as "macrophage activation sets in motion an inflammatory reaction leading to hepatic fibrosis, liver failure or cancer," said Randolph. "That HDL can act at that early step to stop this chain of events by preventing macrophage activation, ultimately reduces liver injury."

In mouse models of intestinal damage, genetic manipulation leading to loss of intestine-derived HDL exacerbated liver injury.

"To test how the inability to make intestine-derived HDL affected liver injury, we surgically resected the mouse small intestine to generate intestinal insult, or fed the mice a high fat diet or gave them alcohol," said Randolph.

"In all three models, the extent of hepatic inflammation or collagenous tissue indicating fibrosis was approximately double in mice unable to make HDL versus wild-type controls."

Administration of GW-3965, a commercially available oral agonist of transcription factor liver X receptor (LXR), the master regulator of HDL biogenesis genes, was shown to upregulate enteric HDL production and protect mice from liver damage.

"In mice treated with GW-3965, liver damage was evident to only about half the extent of that seen in untreated controls and about one-quarter of the disease in mice that could not make enteric HDL," said Randolph.

These findings may have translational potential, as "we acquired blood from the portal vein and antecubital veins of 6 humans and showed enrichment of HDL3 in their portal versus systemic blood," said Randolph. "This reflects the mouse blood distribution findings and leads us to believe there will be many other similarities between the species."

Furthermore, a gene profiling and functional analysis revealed that the intestinal epithelium must express ABCA1 in order for a low-dose, oral LXR agonist to protect the liver.

The finding that the LXR agonist relies on enteric HDL production, which in turn requires intestinal ABCA1 expression, shows that the LXR agonist is acting through enteric HDL, noted Randolph.

"This suggests that LXR agonists might not be the only way to achieve liver protection, since the real clinical target is pharmacological elevation of enteric HDL and there may be other ways to achieve that clinically," she said.

"Newer versions of LXR agonists, which could be formulated so that they are confined to the intestine, are of particular interest, as are other methods to increase intestinal HDL."

In the near future, "we expect to see great progress in this area and are excited to have identified intestinal HDL as being a target for an unmet clinical need," said Randolph.

"A major unanswered question concerns how late in liver disease HDL can be targeted and still improve hepatic status, but given the liver's natural regenerative capacity, we hope that targeting HDL could still be effective after injury has progressed to clinical manifestation."