Alzheimer’s disease (AD) is a progressive neurodegenerative disorder and the most common type of dementia, imposing a large burden on our increasingly aging population (1). The hallmark of AD pathogenesis is the accumulation and deposition of amyloid-β (Aβ) plaques in the brain (2). Amnestic mild cognitive impairment (MCI) is identified as a prodromal phase of AD, with approximately 10–20% of MCI patients progressing to dementia annually (3). Nevertheless, the prevalence of dementia continues to rise while therapeutic options remain limited, urging more researchers to target modifiable risk factors and develop effective strategies for prevention and treatment.

The ε4 allele of the apolipoprotein E gene (APOE4) is considered the strongest genetic risk factor for AD, increasing the risk of AD progression and decreasing the age of onset in a gene dose-dependent manner (4, 5). APOE4 not only leads to earlier and higher Aβ concentration but also affects non-Aβ pathways such as vascular function, immune responses, and tau-mediated neurodegeneration (6). APOE4 can also accelerate the progression of age-related cognitive impairment in individuals without dementia (7). However, specific associations between APOE4 and AD are still poorly understood because of the complexity of the underlying pathophysiological mechanisms (8).

APOE is primarily generated by astrocytes in the central nervous system (CNS) and more than 90% of peripheral APOE in the plasma is produced by the liver, and it plays a major role in numerous biological processes in both areas (9, 10). In the CNS, multiple mechanistic pathways regulated by APOE are collectively associated with cognitive function (11). As a crucial lipid carrier, APOE participates in supporting membrane homeostasis, synaptic integrity, and injury repair in the brain. In peripheral circulation, APOE modulates lipid-related metabolism through its release into the blood (6). A recent review summarized APOE4’s key role in cerebrovascular dysfunction through various mechanisms, including cerebral blood flow (CBF) disruption, cerebral amyloid angiopathy with slower Aβ clearance, and blood–brain barrier leakage, and these detrimental changes may promote cognitive decline (11). Many studies have indicated that APOE4 carriers exhibit abnormal CBF in certain brain regions when compared to noncarriers (11). However, the associations of APOE4 with CBF in cognitively impaired patients have been inconsistent and inconclusive in previous studies.

Interestingly, peripheral APOE might also affect cerebrovascular and cognition despite physical separation (12). Liu et al. demonstrated that accumulation of peripheral APOE4 was sufficient to damage brain function and exacerbate amyloid pathogenesis in mouse models (6). Since peripheral APOE4 primarily originates from the liver, a growing body of evidence suggests that the liver may also play an important role in the pathogenesis of dementia. Taken together, we propose that a complex interaction exists between APOE4 and liver function markers.

Therefore, we aimed to explore whether APOE4 associates with brain perfusion and to assess the potential association between APOE4 and liver function marker levels in patients with cognitive impairment.

By exploring the associations of APOE4 with CBF and serum liver function markers in 30 cognitively impaired patients, two main findings were observed in the present study. First, when compared with noncarriers, the APOE4 carriers demonstrated significantly higher absolute CBF in the left thalamus; higher standardized CBF in the bilateral occipital cortex, bilateral thalamus, and left precuneus; and hypoperfusion in the right lateral temporal cortex. This indicates that APOE4 exerted a strong effect on the perfusion of certain brain regions. Second, APOE4 carriers manifested significantly lower levels of bilirubin (including TB, DB, and IB) than noncarriers. It is possible that an underlying mechanism of bilirubin metabolism is influenced by APOE4, which may hold promise as a target for the diagnosis and treatment of AD.

Previous work has established that APOE4 is associated with cerebral hemodynamic dysregulation (11, 16); however, recent studies have shown mixed observations, with evidence of hypoperfusion and hyperperfusion in APOE4 carriers showing cognitive decline (17). Michels et al. found that, between cognitively healthy older adults and patients with MCI, APOE4 carriers demonstrated lower CBF than noncarriers in frontal, parietal, and temporal areas (18). Intriguingly, another study reported that APOE4 carriers showed increased CBF in the right parahippocampal gyrus, bilateral cingulate gyrus, and right posterior cingulate in the MCI group, as well as the right superior frontal gyrus in the AD group (19). This result indicated that APOE4 carriers were also able to exhibit hyperperfusion in certain brain regions. Consistent with this, a recent study observed that middle-aged APOE4 carriers manifested increased CBF in the anterior and middle cerebral arteries (20). These studies demonstrate the biphasic nature of CBF in APOE4 carriers, and we hypothesize that different perfusion patterns correspond to different stages of cognitive decline.

Apart from discrepancies in sample characteristics, differences in perfusion imaging modalities, and post-processing approaches (20), cerebrovascular compensation also contributes to the conflicting observations. APOE4 carriers experience dysregulation of capillary blood flow with reduced oxygen transport to the brain tissue; consequently, increased CBF is needed to maintain sufficient oxygen supply and normal cognition (20, 21). Nevertheless, cerebrovascular compensation is limited with hyperperfusion in the early stage, resisting the deleterious impacts of APOE4 and subsequent hypoperfusion, indicating a relative breakdown of this compensatory mechanism (22, 23). Our findings help to indicate that the lateral temporal cortex may be one of the first brain regions to exhibit cerebrovascular compensation breakdown. In addition, we also provided evidence that APOE4 may be associated with CBF in the occipital cortex and thalamus, which has not been previously reported, and our findings also support that the thalamus is a vulnerable brain region in AD (24).

We also found that APOE4 carriers exhibited lower bilirubin levels than noncarriers. Bilirubin originates from the lysis of senescent erythrocytes and appears in the peripheral circulation in two forms: conjugated bilirubin (DB) and unconjugated bilirubin (IB). The primary form in the circulatory system is unconjugated bilirubin, which transforms into conjugated bilirubin by combining with albumin (ALB). Bilirubin participates in the decomposition and absorption of lipids (25) and its presence is used to diagnose conditions, such as hepatobiliary disorders and hemolytic anemia (26). Despite the lack of research on the relationship between APOE4 and bilirubin levels, some studies have provided relevant evidence. Zhong et al. reported that serum IB values and IB/ALB were significantly higher in patients with neurodegenerative dementia and Aβ deposition (27), indicating that bilirubin may be involved in the pathologic process of Aβ deposition. In contrast, higher serum bilirubin has been linked to increased regional homogeneity in certain brain regions within normal young adults (25), and AD patients have shown decreased concentrations of total bilirubin compared with normal subjects (28, 29), which supports our observations. Bilirubin performs powerful antioxidative and anti-inflammatory functions and can be considered a protective factor for scavenging superoxide during neurotransmission (25, 26). Although the specific mechanisms of APOE4 and bilirubin metabolism seem extremely complex, the above results imply that bilirubin could be a therapeutic target for cognitive impairment. Additionally, bile acid profiles produced by the liver were also linked to fluid and imaging biomarkers in patients with MCI and AD (30). Taken together, the liver may play a key role in the pathogenesis of cognitive impairment and its influence may be seriously underestimated.

The present study had several limitations. First, the sample size utilized in this study was relatively small, which may have compromised the reliability of the findings. Future studies should include larger cohorts of individuals diagnosed with AD and MCI, and the additional inclusion of normal controls would provide more substantial insights into the impact of APOE4 on brain perfusion and liver metabolic indicators. Second, as a cross-sectional design, the results were exploratory, and causal relationships could not be inferred from the analysis. Longitudinal research with interventions targeting cognitively impaired patients will help to identify a deeper understanding of the subject matter. Third, although ASL provides a noninvasive quantitative measurement of CBF and enhances the ability to detect cognitive decline and the progression of dementia (31–33), the ASL analysis results must still be interpreted carefully. The lack of harmonization of techniques complicates the analysis of data acquired from multiple scanners, and physiologic variations in human patients can cause strong variability in ASL acquired from the same patient at different time points, which may affect the confidence of the statistical analysis results in this study.

In conclusion, APOE4 inheritance exerts a strong impact on CBF dysfunction and represents a risk factor for AD. Different perfusion patterns may correspond to different stages of cognitive decline in APOE4 carriers. The lateral temporal cortex may be one of the first brain regions to show cerebrovascular compensation breakdown, and the thalamus may be a vulnerable brain region in AD. Further studies identifying APOE4−modulated CBF-specific mechanisms may provide options for the prevention and treatment of dementia via preclinical and clinical pathways. From a neurobiological perspective, APOE4 may be associated with bilirubin metabolism, as APOE4 carriers demonstrated lower bilirubin levels than noncarriers; this relationship could potentially provide specific neural targets for the diagnosis and treatment of AD.

The original contributions presented in the study are included in the article/Supplementary material, further inquiries can be directed to the corresponding author.

The studies involving humans were approved by Medical Research Ethics Committee of the Third Affiliated Hospital of Shenzhen University Medical College. The studies were conducted in accordance with the local legislation and institutional requirements. The participants provided their written informed consent to participate in this study.

HW: Formal analysis, Writing – original draft, Writing – review & editing, Data curation, Investigation. LS: Formal analysis, Writing – review & editing. SL: Data curation, Writing – review & editing. YL: Formal analysis, Writing – review & editing. CX: Data curation, Writing – review & editing. GQ: Data curation, Writing – review & editing. QG: Data curation, Writing – review & editing. CC: Data curation, Writing – review & editing. TL: Formal analysis, Writing – review & editing. KL: Formal analysis, Writing – review & editing. FZ: Conceptualization, Formal analysis, Funding acquisition, Investigation, Methodology, Project administration, Resources, Supervision, Validation, Writing – review & editing.