Enterically derived high-density lipoprotein restrains liver injury through the portal vein

INTRODUCTION

High-density lipoprotein (HDL) participates in cholesterol homeostasis and may also have anti-inflammatory or anti-microbial roles through its interaction with numerous plasma proteins. The liver synthesizes most HDL in the body, but the intestine also produces HDL. However, a role for intestinal HDL distinct from that produced by the liver has not been identified. While remodeling its cargo, HDL particles circulate through tissue spaces, but so far, HDL trafficking within tissues has been scarcely studied.

RATIONALE

We reasoned that understanding HDL-trafficking patterns might bring insight into its roles in health and disease, including whether HDL made by the intestine is functionally redundant with that produced by the liver. Using a knock-in mouse that we previously generated to phototag HDL in any tissue location, we aimed to trace the fate of HDL synthesized by the intestine.

RESULTS

Phototagged HDL derived from small bowel enterocytes was generated most abundantly by the ileum and did not travel into draining lymphatic vessels as enterocyte-derived chylomicrons do. Instead, intestinal HDL rapidly entered the portal vein, the major blood supply to the liver. This finding raised the issue of whether the liver might benefit from intestinal HDL and pointed us to an older concept that HDL might neutralize a key microbial signal that can escape a permeable gut: lipopolysaccharide (LPS) from Gram-negative bacteria. Past studies using multiple models have shown that LPS engagement of its receptor, Toll-like receptor 4 (TLR4), in the liver drives significant liver pathology, including inflammation that progresses to fibrosis. Using biochemical, proteomic, and functional approaches, we observed that the intestine produces a particular subspecies of HDL called HDL₃. Unlike another HDL subspecies (HDL₂), HDL₃ sequestered LPS so efficiently that it could not bind to TLR4⁺ liver macrophages. In this way, HDL₃ produced by the intestine protected the liver from the inflammation and fibrosis observed in a variety of mouse models of liver injury that parallel clinically relevant conditions in humans, including surgical resection of the small bowel, alcohol consumption, or high-fat diets. Administration of an oral drug targeting the transcription factor liver X receptor, the master regulator of genes associated with HDL biogenesis, raised enteric HDL levels and protected the mice from liver pathology. This protection was lost if mice did not express enterically derived HDL, indicating that intestinal HDL was a key target of the drug. Six samples of human portal venous blood with matched systemic venous blood confirmed the enrichment of HDL₃.

Mechanistically, LPS-binding protein (LBP) was enriched in HDL₃ particles and was required for HDL₃ to mask LPS from detection by TLR4. This finding was unexpected because LBP otherwise promotes TLR4 signaling by shuttling LPS to CD14, which then shuttles it to TLR4. Thus, HDL₃ interacts with a known component of the TLR4-signaling platform, LBP, to hide LPS from detection. Without binding to TLR4, the HDL₃­­-LBP­­-LPS complex was not retained in liver. Instead, it exited the liver while the LPS associated with it was inactivated. The enzyme acyloxyacyl hydrolase, which is produced in part by liver macrophages and which deacylates critical fatty acid residues in LPS for TLR4 activation, could still access and act upon HDL₃-associated LPS to detoxify it. Low-density lipoprotein bound LPS, but not LBP, and was thus unable to prevent LPS activation of liver macrophages. LBP is in the same family of lipid-binding proteins as phospholipid transfer protein and cholesterol ester transfer protein, which have well-established roles in remodeling the lipid configuration of HDL. Another microbial lipid, lipoteichoic acid from Gram-positive bacteria, is known to bind LBP. We found that it too complexed with HDL₃ and suppressed the activation of liver macrophages.

CONCLUSION

The production of HDL by small bowel enterocytes in a form that potently masks LPS comprises a disease tolerance strategy to protect the liver from injury of enteric origin. Enteric HDL may thus be a suitable pharmacologic target for protecting the liver against gut-derived LPS leakage in alcoholic and nonalcoholic settings.