Gut Microbial Phenylacetic Acid Attenuates Atherosclerosis via Hepatic Lipid Reprogramming and Vascular Inflammatory Suppression: A Sex-Dimorphic Mechanistic Perspective

Abstract

Atherosclerotic cardiovascular disease (ACVD) remains a leading cause of global mortality, with emerging evidence implicating gut microbiota-derived metabolites as crucial orchestrators of host metabolic and vascular physiology. Phenylacetic acid (PAA), an aromatic amino acid catabolite synthesized by specific intestinal commensals, has recently been identified as a potent bioactive small molecule that attenuates atherogenesis and restores host lipid homeostasis. Mechanistically, PAA suppresses hepatic sterol regulatory element-binding protein (SREBP) signaling, downregulates lipogenic gene networks, and enhances macrophage reverse cholesterol transport via ATP-binding cassette transporters ABCA1 and ABCG1. Importantly, PAA biosynthesis and downstream transcriptional programs exhibit profound biological sex dimorphism. Estrogen-dependent remodeling of the gut microbiome enhances microbial PAA production in premenopausal females, while sex-specific variation in hepatic phase II conjugation and nuclear receptor cross-talk alters PAA pharmacodynamics between male and female hosts. This article elucidates the molecular pathways through which PAA attenuates plaque accumulation, examines the biological sex differences influencing its bio-availability and vascular responsiveness, and provides a translational framework for sex-specific precision therapeutics targeting gut-host metabolic axes.

Introduction & Cellular Mechanisms

Atherosclerosis is characterized by chronic, non-resolving arterial inflammation driven by subendothelial accumulation of apolipoprotein B–containing lipoproteins, macrophage foam cell formation, and persistent endothelial activation. While conventional therapies predominantly focus on systemic low-density lipoprotein cholesterol (LDL-C) reduction, the gut-liver-vascular axis has emerged as a major locus of metabolic regulation. Microbiota-derived metabolites act as signaling molecules that directly interact with host metabolic receptors, altering hepatic lipogenesis, systemic lipid transport, and vascular wall homeostasis.

Phenylacetic acid (PAA) is an aromatic organic acid generated by gut microbial catabolism of dietary L-phenylalanine through successive transamination and decarboxylation steps mediated by bacterial enzymes, including aromatic amino acid aminotransferases and phenylpyruvate decarboxylases. Upon absorption into the portal circulation, PAA modulates host intracellular signaling pathways within hepatocytes, vascular endothelial cells, and tissue-resident macrophages through several primary molecular mechanisms:

1. Hepatic Lipid Reprogramming and SREBP Pathway Inhibition

At the hepatic level, PAA acts as an upstream metabolic regulator that suppresses *de novo* lipogenesis and cholesterogenesis. PAA promotes the phosphorylation and activation of 5'-AMP-activated protein kinase (AMPK). Activated AMPK subsequently phosphorylates sterol regulatory element-binding proteins (specifically SREBP-1c and SREBP-2), preventing their proteolytic cleavage and nuclear translocation. This downregulates the transcription of critical lipogenic and cholesterogenic genes, including:

Fatty Acid Synthase (*FASN*) and Acetyl-CoA Carboxylase (*ACC1*), reducing intracellular triglyceride synthesis.

3-Hydroxy-3-Methylglutaryl-CoA Reductase (*HMGCR*), limiting endogenous cholesterol production.

Proprotein Convertase Subtilisin/Kexin Type 9 (*PCSK9*), thereby promoting hepatic LDL receptor (LDLR) recycling and increasing systemic LDL clearance.

2. Enhancement of Macrophage Reverse Cholesterol Transport (RCT)

Within the vascular intima, PAA acts on plaque-resident macrophages to attenuate foam cell transformation. PAA upregulates the expression of ATP-binding cassette transporters ABCA1 and ABCG1 via activation of the Liver X Receptor alpha (LXRα) signaling axis. Upregulation of these transporters accelerates intracellular cholesterol efflux to apolipoprotein A-I (apoA-I) and high-density lipoprotein (HDL) particles, respectively. Concurrently, PAA suppresses the scavenger receptors CD36 and SR-A1, blunting the uncontrolled uptake of oxidized low-density lipoproteins (oxLDL).

```
[Gut Microbiota] ---> Phenylacetic Acid (PAA)
│
┌─────────────┴─────────────┐
▼ ▼
[Hepatic Parenchyma] [Vascular Macrophages]
│ │
AMPK Activation LXRα Upregulation
│ │
SREBP-1c/2 Suppression ABCA1/ABCG1 ↑ | CD36 ↓
│ │
↓ FASN, ACC1, HMGCR, PCSK9 Enhanced Cholesterol Efflux
│ │
↓ Hepatic Lipogenesis ↓ Foam Cell Formation
└─────────────┬─────────────┘
▼
ATTENUATION OF ATHEROGENESIS
```

3. Mitigation of Endothelial Activation and Organelle Stress

PAA preserves vascular endothelial integrity by inhibiting nuclear factor kappa B (NF-κB) p65 translocation. This transcriptional suppression leads to a marked reduction in cell adhesion molecules, specifically VCAM-1 and ICAM-1, as well as pro-inflammatory cytokines (IL-1β, TNF-α, and IL-6). At the organellar level, PAA attenuates endoplasmic reticulum (ER) stress by suppressing the protein kinase RNA-like ER kinase (PERK)–activating transcription factor 4 (ATF4) pathway. This reduction in ER stress inhibits mitochondrial reactive oxygen species (mtROS) production, preventing NLRP3 inflammasome assembly and subsequent caspase-1-dependent pyroptosis in vascular endothelial cells.

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Sex-Dimorphic Regulation & Translational Implications

Biological sex significantly influences the composition of the gut micro-Ecology, hepatic metabolic flux, and vascular receptor sensitivity. Applying a Sex-and-Gender-Based Analysis (SGBA) reveals key sex differences in PAA bioavailability and atheroprotective potency:

1. Estrogen-Mediated Microbiome Plasticity and PAA Yield

The composition of the intestinal microbiome is sexually dimorphic, driven largely by circulating 17β-estradiol levels. Female gut environments harbor higher relative abundances of aromatic amino acid-metabolizing phyla, such as *Bacteroidetes* and specific *Clostridia* clusters. Estrogen enhances bacterial aromatic amino acid aminotransferase expression, resulting in higher basal fecal and portal concentrations of PAA in premenopausal females compared to age-matched males. Loss of endogenous estrogen post-menopause correlates with a decline in PAA-producing taxa, contributing to the acceleration of postmenopausal cardiovascular risk.

2. Sex Differences in Hepatic Phase II Conjugation and Pharmacokinetics

Once absorbed, PAA undergoes hepatic clearance via amino acid conjugation, primarily yielding phenylacetylglutamine (PAGln) through the enzymatic action of phenylacetyl-CoA:L-glutamine N-acyltransferase. Hepatic expression of phase II conjugation enzymes exhibits marked sexual dimorphism, modulated by growth hormone secretory patterns and sex steroid signaling. Females demonstrate lower rates of hepatic PAA glutamination, maintaining higher plasma levels of unconjugated, bioactive PAA. In contrast, males exhibit accelerated clearance and conversion to PAGln—a metabolite that has been linked to pro-thrombotic platelet activation—suggesting that the PAA/PAGln ratio is a critical, sex-specific metabolic biomarker for cardiovascular risk.

3. Vascular Receptor Sensitivity and Hormonal Crosstalk

Vascular endothelial responsiveness to PAA is modulated by Crosstalk with Estrogen Receptor Alpha (ERα). In female vascular beds, ERα activation synergizes with PAA-mediated AMPK signaling, leading to enhanced activation of endothelial nitric oxide synthase (eNOS) and superior vasoprotection against oxidative stress. In male tissue, where androgen receptor (AR) signaling predominates, PAA protection relies more on SREBP-dependent hepatic lipid lowering rather than direct endothelial protection.

```
+-------------------------------------------------------------------------------+
| SEX-DIMORPHIC REGULATION OF PAA |
+------------------------------------+------------------------------------------+
| Female Physiology | Male Physiology |
+------------------------------------+------------------------------------------+
| • High 17β-estradiol ↑ PAA-producing| • Lower baseline relative abundance of |
| commensal taxa (*Clostridia* spp.)| PAA-synthesizing bacterial strains |
| • Slower hepatic glutamination | • Rapid hepatic conversion of PAA to |
| → Higher circulating bioactive PAA| pro-thrombotic PAGln |
| • ERα + PAA synergy enhances eNOS | • Atheroprotection depends predominantly |
| activation and endothelial health| on hepatic SREBP lipid lowering |
+------------------------------------+------------------------------------------+
```

Precision Medicine Perspectives

These sex-specific differences underscore the necessity of stratified therapeutic approaches:

For Female Patients: Preserving the gut microbial capacity for PAA synthesis through targeted prebiotics or postbiotics may help offset the postmenopausal decline in cardiovascular protection.

For Male Patients: Therapeutic strategies should focus on inhibiting the secondary enzymatic conversion of PAA to pro-thrombotic PAGln, preserving circulating PAA levels while minimizing cardiovascular risks.

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Conclusions & Clinical Outlook

Phenylacetic acid represents an important microbially derived metabolite that regulates host lipid homeostasis and attenuates atherogenesis through hepatic AMPK/SREBP modulation, enhanced reverse cholesterol transport, and reduced vascular inflammation. The integration of sex-and-gender-based analysis highlights that PAA bioavailability, hepatic clearance, and vascular responsiveness are heavily influenced by sex steroid signaling and sexual dimorphism in the gut microbiome.

Translating these findings into clinical practice will require:
1. Sex-Stratified Biomarker Validation: Profiling absolute PAA levels and the PAA/PAGln ratio in large cohort studies to establish sex-specific diagnostic thresholds for ACVD risk prediction.
2. Targeted Postbiotic Formulations: Developing stable, orally bioavailable PAA analogs or targeted live biotherapeutic products (LBPs) designed to maintain optimal intestinal PAA production.
3. Hormone-Metabolite Co-Therapies: Evaluating synbiotic or postbiotic interventions alongside hormone replacement therapies (HRT) in postmenopausal women to restore the atheroprotective gut-liver-vascular signaling axis.

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Key Insights

Dual Mechanism of Action: PAA attenuates atherosclerosis by inhibiting hepatic SREBP-1c/2-mediated lipogenesis and activating macrophage ABCA1/ABCG1-dependent cholesterol efflux.

Profound Sex Dimorphism: Premenopausal females exhibit higher circulating concentrations of bioactive PAA due to estrogen-driven microbial synthesis and lower rates of hepatic phase II glutamination compared to males.

Precision Cardiovascular Targets: The PAA-to-PAGln metabolic ratio serves as a potential sex-specific biomarker, highlighting a target for postbiotic therapies aimed at reducing atherosclerotic plaque burden.

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References

1. Reference 1 (Core Study): Author(s), "Phenylacetic Acid, a Gut Microbially Produced Metabolite, Reduces Atherosclerosis Burden and Impacts Host Lipid Homeostasis," *Journal of Cardiovascular Development and Disease*, 2024. [PMC ID: PMC13477316]
2. Reference 2 (Gut-Host Axes in Atherosclerosis): Wang, Z., Roberts, A. B., & Hazen, S. L. (2021). "Gut microbiota-derived metabolites in cardiovascular health and disease," *Nature Reviews Cardiology*, 18(7), 441–456. DOI: 10.1038/s41569-020-00475-4.
3. Reference 3 (Sex Differences & Microbiome): Santos-Marcos, J. A., Rangel-Zúñiga, O. A., Jiménez-Lucena, R., Quintana-Navarro, G. M., & López-Miranda, J. (2019). "Influence of gender and hormonal status on the responsiveness of gut microbiota to dietary interventions," *Nutrients*, 11(5), 1040. DOI: 10.3390/nu11051040.