Abstract
Objective
Clusterin is closely correlated with insulin resistance and its associated comorbidities. This study aimed to investigate the correlation between serum clusterin levels and non-alcoholic fatty liver disease (NAFLD) and further explore the mediating role of insulin resistance in this relationship.
Methods
This study enrolled 195 inpatients (41 males and 154 females) aged 18–61 years. Twenty-four patients were followed up for 12 months after bariatric surgery. Serum clusterin levels were measured using a sandwich enzyme-linked immunosorbent assay. Fatty liver disease was diagnosed on the basis of liver ultrasonography. The fatty liver index (FLI) was calculated to quantify the degree of hepatic steatosis. The mediating role of homeostasis model assessment-insulin resistance (HOMA-IR) was assessed using mediation analysis.
Results
Participants with NAFLD had significantly higher serum clusterin levels than those without NAFLD (444.61 (325.76–611.52) mg/L vs 294.10 (255.20–373.55) mg/L, P < 0.01). With increasing tertiles of serum clusterin levels, the prevalence of NAFLD displayed an upward trend (P < 0.01). Multivariate linear regression analysis showed that serum clusterin levels were a positive determinant of FLI (standardized β = 0.271, P < 0.001) after adjusting for multiple metabolic risk factors. Serum clusterin levels significantly decreased after bariatric surgery (298.77 (262.56–358.10) mg/L vs 520.55 (354.94–750.21) mg/L, P < 0.01). In the mediation analysis, HOMA-IR played a mediating role in the correlation between serum clusterin levels and FLI; the estimated percentage of the total effect was 17.3%.
Conclusion
Serum clusterin levels were associated with NAFLD. In addition, insulin resistance partially mediated the relationship between serum clusterin levels and FLI.
Introduction
Non-alcoholic fatty liver disease (NAFLD) is a metabolic stress-induced liver injury that is pathologically manifested by excessive triglyceride deposition in hepatocytes. It is the most prevalent chronic liver disease worldwide, with a prevalence of approximately 24% (1). The estimated prevalence of NAFLD in Chinese people is 15% (2). Some patients with NAFLD develop non-alcoholic steatohepatitis, cirrhosis, and even liver cancer. Increasing evidence has suggested that NAFLD is a multisystem disease with significant impacts on extrahepatic organs and systemic glucose and lipid metabolism (3, 4). In addition, patients with NAFLD are susceptible to cardiovascular diseases, which often emerge before adverse liver outcomes (5), resulting in a severe reduction in quality of life and substantial economic and medical burdens. Therefore, early prevention and management are critical to improve the prognosis of patients with NAFLD.
Clusterin is a multifunctional, evolutionarily conserved glycoprotein that is widely expressed in human tissues and plays an essential role in a variety of pathophysiological events (6). As a chaperone protein, clusterin can bind to misfolded proteins and prevent their aggregation, thereby removing them from the extracellular environment and promoting their degradation (7). Previous studies have demonstrated that clusterin is valuable for regulating food intake, treating cancer, facilitating apoptotic cell clearance, and enhancing cognitive function in patients with Alzheimer's disease or acute brain injury (8, 9, 10, 11, 12). The connection between clusterin and metabolic diseases has garnered significant attention in recent years. Population-based studies have demonstrated strong associations between serum clusterin levels and obesity, type 2 diabetes, dyslipidemia, metabolic syndrome, and premature coronary artery disease (13, 14, 15, 16). One study has suggested that clusterin was a novel adipokine that linked cardiometabolic disease and obesity (17). In addition, liver-derived clusterin played an important role in regulating insulin-dependent muscle glucose uptake, hence regulating systemic metabolic homeostasis (18). However, the relationship between serum clusterin levels and NAFLD has not been fully elucidated. Animal research has confirmed that serum clusterin levels are elevated in mouse models of non-alcoholic steatohepatitis (19); however, this observation has not been explored in a population-based study.
Thus, the purpose of this study was to investigate the association between serum clusterin levels and NAFLD in the Chinese population. In addition, due to the tight relationship between insulin resistance and clusterin (20), a mediation analysis would be conducted to determine the impact of the insulin resistance index (homeostasis model assessment-insulin resistance, HOMA-IR) in the association between clusterin levels and NAFLD.
Methods
Study participants
The participants of this study were inpatients who intended to undergo metabolic surgery at the Sixth People's Hospital Affiliated to Shanghai Jiao Tong University School of Medicine from July 2019 to August 2020. The original cohort consisted of 160 individuals aged between 18 and 65 years who were diagnosed with NAFLD. The exclusion criteria were as follows: viral hepatitis, autoimmune hepatitis, drug-induced hepatitis, excessive alcohol consumption (>210 g weekly for men and >140 g weekly for women), treatment with hepatoprotective drugs, severe renal dysfunction, mental disorders, and a history of malignant tumors. In total, 154 individuals who did not meet the exclusion criteria were included in this study. We then included 41 individuals without NAFLD who underwent cholecystectomy and did not meet the aforementioned exclusion criteria as controls. The 12-month follow-up data of the 24 individuals who underwent bariatric surgery were available for further analysis. All participants completed the questionnaire, which included information on previous and present diseases and medications, biochemical measurements, and physical examinations.
All the participants provided written informed consent. This study was approved by the Ethics Committee of Shanghai Sixth People’s Hospital Affiliated to Shanghai Jiao Tong University School of Medicine.
Assessment of serum clusterin levels
Fasting blood samples were collected from all participants after overnight fasting for 8–10 h before surgery. Serum clusterin levels were measured using a sandwich enzyme-linked immunosorbent assay (ELISA; DCLU00, R&D Systems). Serum samples were tested after a 7600-fold dilution. The intra- and inter-assay evaluation results were <3.45 and <7.33%, respectively.
Anthropometric and biochemical measurements
Height and weight were measured using a standard electronic scale. Body mass index (BMI) was calculated using the equation: BMI = weight (kg)/height (m)2. Waist circumference was measured using standardized methods as previously described (21). Systolic blood pressure (SBP) and diastolic blood pressure (DBP) were measured in the sitting position three times at a frequency of 3 min apart with an electronic sphygmomanometer, and the average value of the three measurements was considered the final value. Fasting blood samples were used to detect the following indicators. Serum alanine aminotransferase (ALT), aspartate aminotransferase (AST), gamma-glutamyl transpeptidase (GGT), total cholesterol (TC), triglyceride (TG), high-density lipoprotein cholesterol (HDL-c), and low-density lipoprotein cholesterol (LDL-c) levels were measured using enzymatic methods (HITACHI 7600-120, Hitachi, Japan). Fasting plasma glucose (FPG) levels were measured using the glucose oxidase method (HITACHI 7600-120, Hitachi). Serum fasting insulin (FINS) levels were measured with electrochemiluminescence immunoassay (Roche Cobas e601, Roche). Glycated hemoglobin A1c (HbA1c) levels were measured with high-performance liquid chromatography methods (VARIANT II, Bio-Rad Diagnostics). Serum C-reactive protein (CRP) levels were measured with immune-turbidimetric assays (BN-II, Siemens HealthCare Diagnostics). The HOMA-IR index was calculated using the following formula: HOMA-IR = FPG (mmol/L) × FINS (mU/L)/22.5 (22). The fatty liver index (FLI) was calculated using the following equation: FLI = (e (0.953 × loge (TG) + 0.139 × BMI + 0.718 × loge (GGT) + 0.053 × w-15.745)/(1 + e (0.953 × loge (TG) + 0.139 × BMI + 0.718 × loge (GGT) + 0.053 × w-15.745)) ×100 (23).
Definition of NAFLD
Liver ultrasonography was performed by an experienced sonographer among all participants using the Voluson 730 Expert B-mode ultrasonic diagnostic instrument (5.0 MHz transducer, GE Healthcare). NAFLD was diagnosed based on the 2018 edition of the Guidelines of Prevention and Treatment for Nonalcoholic Fatty Liver Disease (24).
Statistical analyses
All data were analyzed using SPSS version 24.0. All continuous variables were checked for normal distribution using the one-sample Kolmogorov–Smirnov test. Means ± s.d. or medians with interquartile ranges were reported for normally distributed and skewed distribution variables, respectively. For variables with normal distributions, the independent sample t-test was used to analyze the differences between the NAFLD and non-NAFLD groups. The Wilcoxon rank-sum test was used for variables with skewed distributions. The paired sample t-test was used to analyze the differences before and after bariatric surgery. Multivariate linear regression analyses were performed to investigate the relationship between serum clusterin levels and FLI. Three models were used, with FLI as the dependent variable and serum clusterin levels as the independent variable. Model 1 was a crude model. Age and gender were adjusted in Model 2. Age, gender, ALT, AST, TC, FPG, HbA1c, CRP, anti-hypertension therapy, lipid-lowering therapy, and anti-diabetes therapy were adjusted in Model 3. Statistical tests were two-sided, and statistical significance was set at P < 0.05.
A mediation analysis was conducted to evaluate the role of HOMA-IR in the relationship between serum clusterin levels and FLI. The causal pathway model, clusterin→HOMA-IR→FLI, was constructed. In the mediation model, serum clusterin level was the independent variable X, FLI index was the dependent variable Y, HOMA-IR was the mediator variable M, and age and gender were controlled. The ‘total effect’ was composed of a ‘direct effect’ of serum clusterin levels on FLI and an ‘indirect effect’ mediated by HOMA-IR. Mediation analysis was performed using process v3.4.1 by Andrew F. Hayes. The bootstrap method was used to calculate 95% CIs. When the significance of the total, direct, and indirect effects was <0.05 and the 95% CI did not contain 0, HOMA-IR was considered to partially mediate the relationship between serum clusterin levels and FLI. The mediating effect was quantified based on the percentage mediated (indirect effect divided by the total effect).
Results
Clinical characteristics of the study participants
This study included 195 participants (41 men and 154 women) with an age range of 18–61 years and an average age of 34.7 ± 10.0 years. The clinical characteristics of the study participants are presented in Table 1. Among the study population, 37 (19.0%) individuals had hypertension, 46 (23.6%) had type 2 diabetes, and 149 (76.4%) had dyslipidemia. Participants with NAFLD had significantly higher BMI, WC, SBP, DBP, ALT, AST, GGT, FPG, HbA1c, FINS, HOMA-IR, TG, and CRP levels (all P < 0.05) and significantly lower serum HDL-c levels (P < 0.01).
Clinical characteristics of the study participants.
| Variables | Total | NAFLD | |
|---|---|---|---|
| n = 195 | No (n = 41) | Yes (n = 154) | |
| Men/women | 41/154 | 6/35 | 35/119 |
| Age (years) | 34.7 ± 10.0 | 41.7 ± 10.0 | 32.8 ± 9.2b |
| BMI (kg/m2) | 34.6 (26.3–41.1) | 22.6 (21.1–24.5) | 38.3 (32.3–42.1)b |
| WC (cm) | 113.0 (95.0–124.0) | 81.0 (76.8–85.3) | 118.0 (107.0–128.0)b |
| Men | 118.0 (95.0–135.0)c | 85.5 (82.5–90.0) | 125.0 (110.3–136.0)b,c |
| Women | 110.0 (93.5–123.0) | 79.8 (75.8–84.3) | 116.0 (106.0–124.8)b |
| SBP (mmHg) | 128 (118–137) | 120 (109–128) | 130 (121–140)b |
| DBP (mmHg) | 82 (75–90) | 76 (71–82) | 84 (76–91)b |
| ALT (U/L) | 33 (21–56) | 22 (17–31) | 38 (24–61)b |
| AST (U/L) | 24 (18–36) | 21 (19–25) | 27 (18–42)b |
| GGT (U/L) | 33 (21–52) | 17 (14–35) | 37 (25–57)b |
| FPG (mmol/L) | 5.40 (4.88–6.37) | 4.90 (4.50–5.25) | 5.65 (5.13–6.54)b |
| HbA1c (%) | 5.8 (5.4–6.4) | 5.4 (5.3–5.6) | 5.9 (5.5–6.7)b |
| FINS (mU/L) | 21.99 (10.78–37.63) | 7.09 (4.83–10.09) | 27.91 (18.70–40.59)b |
| HOMA-IR | 5.97 (2.93–10.70) | 1.51 (1.00–2.24) | 7.46 (4.34–11.67)b |
| TC (mmol/L) | 4.88 ± 1.01 | 4.78 ± 0.91 | 4.91 ± 1.03 |
| TG (mmol/L) | 1.59 (1.19–2.17) | 1.27 (0.67–1.46) | 1.73 (1.29–2.30)b |
| HDL-c (mmol/L) | 1.09 (0.90–1.35) | 1.30 (1.00–1.61) | 1.05 (0.87–1.30)b |
| Men | 0.96 (0.77–1.14)d | 1.21 (1.00–1.31) | 0.88 (0.74–1.06)b,d |
| Women | 1.14 (0.97–1.44) | 1.33 (0.98–1.71) | 1.11 (0.94–1.35)a |
| LDL-c (mmol/L) | 2.96 ± 0.89 | 2.77 ± 0.88 | 3.00 ± 0.89 |
| FLI | 93.08 (50.77–98.49) | 15.67 (4.93–29.48) | 95.80 (86.03–98.98)b |
| Men | 97.80 (61.05–99.59)d | 25.58 (14.77–32.01) | 98.85 (92.35–99.72)b,d |
| Women | 91.46 (50.16–97.78) | 13.61 (4.65–29.51) | 94.70 (85.51–98.56)b |
| CRP (mg/L) | 3.10 (1.24–6.94) | 0.57 (0.13–1.30) | 4.44 (1.74–7.81)b |
| Hypertension, n (%) | 37 (19.0) | 1 (2.4) | 36 (23.4)b |
| Diabetes, n (%) | 46 (23.6) | 1 (2.4) | 45 (29.2)b |
| Dyslipidemia, n (%) | 149 (76.4) | 24 (58.5) | 125 (81.1)b |
| Antihypertensive therapy, n (%) | 27 (13.8) | 1 (2.4) | 26 (16.9)a |
| Antidiabetic therapy , n (%) | 21 (10.8) | 1 (2.4) | 20 (13.0) |
| Lipid-lowering agents, n (%) | 3 (1.5) | 0 | 3 (1.9) |
Continuous variables were expressed as means ± s.d. or medians with interquartile range. Categorical variables were expressed as numbers with percentages.aCompared with individuals without NAFLD, P < 0.01; bCompared with individuals without NAFLD, P < 0.05; cCompared with women, P < 0.01; dCompared with women, P < 0.05.
ALT, alanine aminotransferase; AST, aspartate aminotransferase; BMI, body mass index; CRP, C-reactive protein; FLI, fatty liver index; DBP, diastolic blood pressure; FINS, fasting insulin; FPG, fasting plasma glucose; GGT, gamma-glutamyl transferase; HbA1c, glycated hemoglobin A1c; HDL-c, high-density lipoprotein cholesterol; HOMA-IR, homeostasis model assessment-insulin resistance index; LDL-c, low-density lipoprotein cholesterol; SBP, systolic blood pressure; TC, total cholesterol; TG, triglyceride; WC, waist circumference.
In the entire population, serum clusterin level was 393.91 (292.54–577.40) mg/L. As depicted in Fig. 1A, participants with NAFLD had significantly higher serum clusterin levels than those without NAFLD (444.61 (325.76–611.52) mg/L vs 294.10 (255.20–373.55) mg/L, P < 0.01). The overall population was separated into three groups based on tertiles of serum clusterin levels: T1: <325.73 mg/L, T2: 325.73–528.94 mg/L, T3: >528.94 mg/L. With the increase in serum clusterin levels, the proportions of NAFLD at T1, T2, and T3 were 59.1, 81.3, and 96.9%, respectively, showing a significant upward trend (P < 0.01) (Fig. 1B).
Serum clusterin levels in participants without NAFLD and with NAFLD (A). The prevalence of NAFLD according to serum clusterin tertile groups (B).
Citation: Endocrine Connections 12, 7; 10.1530/EC-22-0545
Individuals with NAFLD exhibited considerably higher FLI levels than those without NAFLD (95.80 (86.03–98.98) vs 15.67 (4.93–29.48), P < 0.01). In addition, FLI levels were higher in men than that in women (97.80 (61.05–99.59) vs 91.46 (50.16–97.78), P < 0.01).
Relationship between serum clusterin levels and FLI
Spearman correlation analysis revealed positive associations between serum clusterin levels and FLI in men (r = 0.330, P = 0.035) and women (r = 0.403, P < 0.001), and the correlations remained significant both in men (r = 0.349, P = 0.027) and women (r = 0.357, P < 0.001) after adjusting for age.
Multivariate linear regression analysis was performed to examine the association between serum clusterin levels and FLI (Table 2). Serum clusterin levels were positively associated with FLI in the crude model (standardized β = 0.457, P < 0.001). In model 2, after controlling for age and gender, serum clusterin levels were also linked to FLI (standardized β = 0.364, P < 0.001), and this association remained significant in model 3 (standardized β = 0.271, P < 0.001), after adjusting for TC, ALT, AST, CRP, FPG, HbA1c, anti-hypertension therapy, lipid-lowering therapy, and anti-diabetes therapy. Similarly, controlling for covariates revealed that the third tertile of serum clusterin levels was associated with a higher FLI, compared with the lowest tertile (P < 0.001).
Associations of serum clusterin levels with FLI.
| Per 1 unit increase | Tertile 1 | Tertile 2 | Tertile 3 | ||||
|---|---|---|---|---|---|---|---|
| Standardized β (95% CI) | P | Standardized β (95% CI) | P | Standardized β (95% CI) | P | ||
| Participants (n) | 65 | 65 | 65 | ||||
| Model 1 | 0.457 (0.330–0.583) | <0.001 | Reference | 0.226 (0.078–0.374) | 0.003 | 0.488 (0.329–0.635) | <0.001 |
| Model 2 | 0.364 (0.245–0.484) | <0.001 | Reference | 0.145 (0.007–0.283) | 0.040 | 0.380 (0.239–0.520) | <0.001 |
| Model 3 | 0.271 (0.151–0.391) | <0.001 | Reference | 0.100 (−0.033–0.234) | 0.141 | 0.260 (0.118–0.401) | <0.001 |
Model 1: no adjustments; Model 2: adjusted for age, gender; Model 3: adjusted for age, gender, TC, ALT, AST, CRP, FPG, HbA1c, anti-hypertension therapy, anti-diabetes therapy, and lipid-lowing therapy. β for per 1-unit increase was estimated from 1-unit increase of log2-transformed clusterin levels. TC, total cholesterol; ALT, alanine aminotransferase; AST, aspartate aminotransferase; FPG, fasting plasma glucose; HbA1c, glycated hemoglobin A1c; CRP, C-reactive protein; FLI, fatty liver index.
Changes in serum clusterin levels were also investigated when metabolic disorders improved. Twenty-four participants with NAFLD were followed up for 12 months after bariatric surgery. The BMI (26.59 ± 4.17 vs 37.56 ± 5.04 kg/m2, P < 0.01) and WC (93.48 ± 12.83 vs 118.69 ± 11.12 cm, P < 0.01) levels decreased significantly. In addition, participants had considerably lower SBP, DBP, TC, TG, LDL-c, FPG, FINS, HOMA-IR, and CRP levels. As shown in Fig. 2, serum clusterin levels were significantly reduced (298.77 (262.56–358.10) vs 520.55 (354.94–750.21) mg/L, P < 0.01). Similarly, FLI exhibited a declining trend (4.77 (2.12–12.47) vs 61.15 (39.89–85.43), P < 0.01) (Fig. 2). Spearman correlation analysis showed that changes in FLI were positively associated with changes in BMI (r = 0.723, P < 0.001) and changes in WC (r = 0.559, P = 0.004). Partial correlation analysis showed that, after adjusting for changes in BMI, changes in serum clusterin levels were positively associated with changes in FLI (r = 0.466, P = 0.025).
Changes in serum clusterin levels, FLI, BMI, and WC 12 months after bariatric surgery.
Citation: Endocrine Connections 12, 7; 10.1530/EC-22-0545
Effects of serum clusterin levels on FLI, with HOMA-IR as a mediator
Previous studies have validated the association between serum clusterin levels and HOMA-IR, a well-established influencing factor for NAFLD. To evaluate the role of HOMA-IR in the association between serum clusterin levels and FLI, the mediation analysis was performed. In the following analysis, age and gender were corrected. The results were shown in Fig. 3. In this mediation model, serum clusterin levels were directly correlated with FLI (standardized β = 0.364, P < 0.001) and correlated with HOMA-IR (standardized β = 0.258, P < 0.001). When HOMA-IR and serum clusterin levels were simultaneously included, both variables demonstrated moderate associations with FLI (standardized β = 0.246, P < 0.001; standardized β = 0.301, P < 0.001). The 95% CI for the direct effect of serum clusterin levels on FLI, as determined by the bootstrap test, was 0.181–0.421, and the 95% CI for the mediating effect of HOMA-IR was 0.023–0.117. These findings suggest that HOMA-IR partially mediates the association between serum clusterin levels and FLI. HOMA-IR mediation effect accounted for 17.3% of the total effect.
Mediation analysis of HOMA-IR in the relationship between serum clusterin levels and FLI.
Citation: Endocrine Connections 12, 7; 10.1530/EC-22-0545
Discussion
In this study, we investigated the relationship between serum clusterin levels and NAFLD. Serum clusterin levels were considerably elevated in patients with NAFLD and were independently linked to FLI after adjusting for confounding factors. Serum clusterin levels were reduced 12 months after weight loss treatment. Furthermore, HOMA-IR served as a mediator for the relationship between serum clusterin levels and FLI.
NAFLD is the hepatic component of metabolic syndrome and is characterized by excessive lipid deposition in the liver; it is intimately linked to abdominal fat deposits and insulin resistance (25). Previous research has demonstrated a correlation between serum clusterin levels and metabolic syndrome (13). However, population-based studies of the relationship between clusterin levels and NAFLD are limited. Sun et al. (26) recruited 41 inpatients with NAFLD and found no significant difference in serum clusterin levels between patients with steatosis grades of 1 and ≥ 2. In contrast, we found that serum clusterin levels were positively and independently associated with FLI. This could be due to the limited sample size in Sun et al.’s study. We found that those with NAFLD had higher BMI levels, worse glucose and lipid metabolic status, and higher CRP levels. Won et al. (13) have found that CRP and BMI were independently and positively associated with serum clusterin levels. Because BMI is a well-known indicator of whole-body adiposity, we speculated that in the obese state, adipose tissue could secrete more clusterin to the bloodstream. In addition, it is reasonable to speculate that inflammatory situations enhance the release of clusterin from adipose tissues or from the liver, as obesity is usually associated with systemic inflammation.
NAFLD is associated with a series of complex events. The ‘double-hit’ theory used to be a widely accepted explanation for the etiology of NAFLD (27). However, mounting evidence has suggested that the pathophysiology of NAFLD is exceedingly complex and goes beyond the ‘double-hit’ theory. Consequently, the ‘multiple-hit’ theory has been regarded as a more plausible explanation. Genetic susceptibility to NAFLD, defective metabolic and signaling pathways in hepatocytes, cellular contacts in the liver, and crosstalk between adipose tissue and the liver are all possible contributors to the progression of hepatic damage (28). Of note, insulin resistance is still a crucial factor in the pathogenesis of NAFLD and NASH. In this study, we found that HOMA-IR may partially moderate the link between serum clusterin levels and FLI, indicating that circulating clusterin can modulate systemic glucose metabolism and insulin sensitivity via target organs. The results of animal studies appeared to support our speculation. Seo et al. (18) discovered that clusterin was mostly released by the liver and functioned in a paracrine manner in skeletal muscle by binding to its receptor LRP2, consequently modulating systemic glucose homeostasis.
Alterations in serum clusterin levels after bariatric surgery are not fully understood. Ghanim et al. (29) discovered a 22% decrease in serum clusterin levels in patients with obesity 6 months after bariatric surgery. Nevertheless, another Spanish study found no difference in serum clusterin levels between the baseline and follow-up periods (30). Notably, recent studies have revealed that a decrease in clusterin levels is concurrent with an increase in insulin sensitivity. Jeon et al. (31) pointed out that exercise training significantly reduced serum clusterin levels in postmenopausal women with diabetes. In addition, treatment with the insulin sensitizer rosiglitazone has been found to lower serum clusterin levels in patients with diabetes (20). Our findings demonstrated that serum clusterin levels were dramatically reduced after bariatric surgery, accompanied by improved metabolic status. Partial correlation analysis showed that ∆Clusterin was positively associated with ∆FLI after adjusting for ∆BMI. If the relationship between the two can still be confirmed after expanding the sample size in the future, serum clusterin levels might be a biomarker for hepatic steatosis improvement. In addition, Finelli et al. (32) have pointed out that reducing visceral fat content is beneficial to alleviate the degree of hepatic steatosis. Consistent with this, we also found a positive association between ∆WC and ∆FLI. Whether changes in serum clusterin levels were associated with reduced visceral fat content is worthy of further investigation in the future.
Clusterin is a glycoprotein ubiquitously expressed in the human body. Previous studies have indicated that clusterin may exert various autocrine and paracrine bioactivities. The liver is one of the primary target organs of clusterin. In the present study, after correcting for several confounding factors, serum clusterin levels were independently associated with FLI. However, the link between hepatic clusterin and hepatic lipid and glucose metabolism remains debatable. Animal studies have shown that both endogenous and exogenous liver clusterin overexpression can prevent the development of diet-induced hepatic steatosis and fibrosis (33, 34). The mechanism might be due to the fact that clusterin could regulate the expression of Srebp1c, a crucial factor in liver lipid synthesis, hence lowering hepatic lipid deposition in mice fed a high-fat diet (34). However, another in vitro experiment revealed that clusterin negatively affected hepatocytes. David et al. (17) discovered that clusterin had the ability to upregulate the expression of Gck, which is a core regulator of the gluconeogenesis pathway, thereby blocking insulin signal transduction and insulin resistance. They argued that the downregulation of Srebp1c by clusterin might have greater deleterious effects by boosting gluconeogenesis, even though Srebp1c can control liver fat deposition (35). Currently, there is no plausible explanation for the above contradictory findings, and the influence of liver clusterin on the metabolism of hepatic glucose and lipids still requires additional research.
The strength of the study includes the novelty of the evidence from the population, the robust analysis of the mediated role of insulin resistance, and the before and after changes of clusterin associated with the changes in metabolic parameters. This study had some limitations that should be clarified. First, this study could not demonstrate a causal relationship between clusterin levels and NAFLD. The second limitation was that we did not use a biopsy as accurate measurement, simply using FLI to estimate liver fat content. Finally, the sample size of the follow-up cohort was limited, and the follow-up duration was only 1 year. It is vital to investigate the dynamic changes in serum clusterin levels.
In conclusion, our study suggests that serum clusterin levels are elevated in individuals with NAFLD and are independently associated with FLI. In addition, insulin resistance partially mediated the relationship between serum clusterin levels and FLI.
Declaration of interest
The authors declare that they have no conflict of interest.
Funding
This work was supported by Clinical Research Plan of SHDC (No. SHDC2020CR1017B), Shanghai Research Center for Endocrine and Metabolic Diseases (2022ZZ01002), and Shanghai Municipal Key Clinical Specialty.
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