# Inhaling Eugenol Inhibits NAFLD by Activating the Hepatic Ectopic Olfactory Receptor Olfr544 and Modulating the Gut Microbiota

## TL;DR

Inhaling eugenol, a compound from a Chinese medicinal plant, can help treat fatty liver disease by activating a liver receptor and improving gut bacteria.

## Contribution

Eugenol is shown to inhibit NAFLD via hepatic olfr544 activation and gut microbiota modulation, offering a novel inhalation therapy.

## Key findings

- Eugenol activates the hepatic olfactory receptor Olfr544, promoting fat oxidation and lipolysis.
- Eugenol enriches probiotic Lactobacillus strains and increases metabolites that inhibit lipid synthesis.
- Lower levels of key metabolites are observed in NAFLD patients, suggesting their therapeutic potential.

## Abstract

Non‐alcoholic fatty liver disease (NAFLD) is a major public health threat with currently limited therapeutic options. Inhalation therapy shows promise for treating metabolic disorders due to rapid absorption and high patient adherence, though relevant medications remain scarce. Eugenol (EUG), the primary component of Syzygium aromaticum volatile oil, emerges as a promising NAFLD inhibitor from lipid‐lowering aromatic Chinese medicine screening. EUG elicited significant anti‐steatotic effects in both cultured hepatocytes and comprehensive high‐fat diet‐induced NAFLD animal models. Mechanistically, EUG targeted the activation of the hepatic ectopic olfactory receptor Olfr544 and up‐regulated its downstream cyclic adenosine monophosphate (cAMP)/protein kinase A (PKA)/cAMP response element binding protein signaling pathway, which further promoted fat lipolysis and oxidation. This effect is prevented by Olfr544 knockdown models both in vitro and in vivo, with supporting bioinformatics analysis. Moreover, EUG reversed gut microbiota dysbiosis and enriched two probiotic strains L. ruteri XR23 and L. johnsonii XR25, and oral gavage potently mitigated NAFLD in mice, with the key metabolites 3‐indolepropionic acid (IPA) and 5‐hydroxyindole‐3‐acetic acid (5‐HIAA) inhibiting lipid synthesis. Lower levels of 5‐HIAA and IPA are observed in patients with NAFLD. These results highlight the considerable potential of EUG as an agonist of Olfr544 for treating NAFLD by inhalation.

A natural compound from the Chinese medicinal plant Syzygium aromaticum, eugenol, inhaling which can counteract NAFLD by activating the hepatic olfactory receptor olfr544 and enriching Lactobacillus sp. to promoting fat oxidation, catabolism and inhibit lipid synthesis, respectively.