Abstract
Objective
Inflammation contributes to the development of type 2 diabetes mellitus (T2DM). While South Asians are more prone to develop T2DM than Europids, the inflammatory phenotype of the South Asian population remains relatively unknown. Therefore, we aimed to investigate potential differences in circulating levels of inflammation-related proteins in South Asians compared with Europids with T2DM.
Method
In this secondary analysis of three randomized controlled trials, relative plasma levels of 73 inflammation-related proteins were measured using an Olink Target Inflammation panel and the serum fibroblast growth factor 21 (FGF21) concentration using an ELISA kit in Dutch South Asians (n = 47) and Dutch Europids (n = 49) with T2DM.
Results
Of the 73 inflammation-related proteins, the relative plasma levels of six proteins were higher (stem cell factor, caspase-8, C–C motif chemokine ligand 28, interferon-gamma, sulfotransferase 1A1 and cystatin D; q-value <0.05), while relative levels of six proteins were lower (FGF21, human fibroblast collagenase, interferon-8, C–C motif chemokine ligand 4, C–X–C motif chemokine ligand 6 and monocyte chemoattractant protein-1; q-value <0.05) in South Asians compared with Europids. Of these, the effect size of FGF21 was the largest, particularly in females. We validated this finding by assessing the FGF21 concentration in serum. The FGF21 concentration was indeed lower in South Asians compared with Europids with T2DM in both males (−42.2%; P < 0.05) and females (−58.5%; P < 0.001).
Conclusion
Relative plasma levels of 12 inflammation-related proteins differed between South Asians and Europids with T2DM, with a significantly pronounced reduction in FGF21. In addition, the serum FGF21 concentration was significantly lower in South Asian males and females compared with Europids. Whether low FGF21 is an underlying cause or consequence of T2DM in South Asians remains to be determined.
Clinical trial registration
ClinicalTrials.gov (NCT01761318, registration date 20-12-2012; NCT02660047, registration date 21-03-2018; and NCT03012113, registration date 06-01-2017).
Introduction
One in ten adults is living with diabetes, and this number is expected to rise to one in nine by 2040 (1; https://www.diabetesaustralia.com.au/about-diabetes/diabetes-globally/). Of those affected, 90% have type 2 diabetes mellitus (T2DM), with ethnic groups showing different susceptibility to developing T2DM and associated diseases (2). For example, South Asians originating from the Indian subcontinent develop T2DM at a significantly younger age and a lower body mass index (BMI) than other ethnic counterparts, and given that they present nearly a quarter of the world’s population, understanding the underlying mechanism is of utmost importance (3, 4). The increased risk of the development of T2DM in South Asians is thought to be partially attributable to their metabolic phenotype, characterized by more central obesity, dyslipidemia and more insulin resistance relative to Europids (5, 6, 7). However, these factors alone do not entirely explain the increased risk (8).
In recent years, research has shown that inflammation plays a prominent role in developing T2DM and its associated complications (9, 10). Inflammation results from stress on the adipose tissue, leading to the attraction of immune cells by releasing cytokines and chemokines that promote inflammation (11, 12, 13). Interestingly, previous studies suggest the presence of a more proinflammatory phenotype in the South Asian population. Our prior research revealed higher C-reactive protein (CRP) levels already in cord blood from South Asian neonates compared with Europid neonates (14). Additionally, South Asians living with T2DM display a more activated interferon (IFN)-signaling pathway than Europids living with T2DM (15).
Considering the pivotal role of inflammation in the development of T2DM and its associated comorbidities, identifying the inflammation-related protein signature could improve our understanding of South Asians’ high T2DM risk. It may also provide valuable insight into the applicability of novel treatment modalities in this population. Therefore, in the current study, we aimed to investigate potential differences in circulating inflammation-related proteins in South Asians compared with Europids living with T2DM.
Methods
Participants and study design
Participants
This study is a secondary analysis of three previously performed randomized, double-blinded, placebo-controlled clinical trials.
The first two clinical trials were designed to investigate the effect of a 26-week liraglutide treatment on glycemic endpoints and ectopic fat deposition in participants with overweight, obesity and T2DM (16, 17). In total, 50 patients of Dutch Europid (hereafter ‘Europid’) origin (study 1) (16) and 47 of Dutch South Asian (hereafter ‘South Asian’) origin (study 2) (17) were included. South Asian ethnicity was defined as having four grandparents who originally descended from Suriname, Bangladesh, India, Nepal, Pakistan, Afghanistan, Bhutan or Sri Lanka. Inclusion criteria were males and females aged 18–69 years, BMI ≥ 25 kg/m2 and hemoglobin A1c (HbA1c) levels of 7.0–10.0% (53–86 mmol/mol) despite the use of metformin, sulfonylurea derivatives and insulin. The main exclusion criteria were the use of other glucose-lowering therapy and renal, hepatic or cardiovascular disease (i.e., presence of congestive heart failure – New York Heart Association (NYHA) classification III–IV, uncontrolled hypertension (systolic blood pressure > 180 mmHg and/or diastolic blood pressure > 110 mmHg) or an acute coronary or cerebrovascular accident within 30 days prior to study inclusion); gastric bypass surgery; chronic pancreatitis or previous acute pancreatitis; pregnancy or lactation and MRI contra-indications. Both trials were performed between 2013 and 2018.
The third clinical trial was a randomized, double-blinded, placebo-controlled crossover study designed to assess the effect of cold exposure and mirabegron on plasma lipids, energy expenditure and brown adipose tissue fat fraction in ten healthy lean South Asian males versus 10 age- and BMI-matched Europid males. Inclusion criteria were age 18–30 and a healthy BMI between 18–25 kg/m2. The study was performed between June 2017 and June 2018.
Study approval
All three trials were conducted according to the principles of the revised Declaration of Helsinki (18). Before inclusion, written informed consent was obtained from all participants. The local ethics committee approved all trials, which were conducted at the Leiden University Medical Center and registered at clinicaltrial.gov (NCT01761318, NCT02660047 and NCT03012113).
Study designs
The designs of all trials have been extensively described elsewhere (16, 17, 19).
At baseline, all participants from the three studies arrived at the outpatient clinic after at least a 6 hour fast for those with T2DM and a 10-hour overnight fast for those without T2DM. The initial assessment in all trials included body composition and bioelectrical impedance analysis (BIA; Bodystat 1500, Bodystat Ltd, UK), followed by the collection of venous blood samples. In the trials with patients with T2DM, after the initial blood sample collection, individuals underwent an MRI and proton magnetic resonance spectroscopy to measure the subcutaneous and visceral adipose tissue and hepatic triglyceride content (HTGC).
Blood collection
Following the collection of venous blood samples, serum (BD Vacutainer® SST II Advanced tubes) and plasma (BD Vacutainer® EDTA tubes) were obtained by centrifugation in all studies and stored at −80 °C until further analysis.
To measure the relative levels of circulating inflammation-related proteins in patients with T2DM, a commercially available protein biomarker panel ‘Target 96 Inflammation’ from Olink proteomics (Olink Bioscience, Sweden) was used. Olink Proteomics performed quality control; samples were excluded when their incubation and detection control deviated by more than ±0.3 normalized protein expression from the plate median (20). This resulted in the exclusion of one sample. A total of 73 of the 96 (i.e., 76%) proteins were detected in at least 75% of the plasma samples included in the analysis.
Plasma levels of total cholesterol, high-density lipoprotein-cholesterol, triglycerides, aspartate aminotransferase (AST) and alanine aminotransferase (ALT) were measured on a Modular P800 analyzer (Roche Diagnostic, Germany) for the patients with T2DM. Low-density lipoprotein-cholesterol (LDL-C) was calculated according to the Friedewald formula (21). HbA1c was initially measured with boronate affinity high-performance liquid chromatography (Primus Ultra; Siemens Healthcare Diagnostics, The Netherlands) because of logistical reasons and later with ion-exchange high-performance liquid chromatography (Tosoh G8, Sysmex Nederland B.V., The Netherlands). To ensure accurate and consistent results, HbA1c levels obtained from the boronate affinity method were corrected based on the correlation coefficient obtained from validation samples measured on both analyzers. Plasma CRP concentrations were measured on a Roche Modular analyzer (Roche Diagnostics). Serum fibroblast growth factor 21 (FGF21) concentrations in samples from all three trials were measured using a human FGF21 Quantikine ELISA kit (R&D Systems, USA).
MRI
The patients with T2DM underwent an MRI in the supine position at baseline, using a 3.0 Tesla MRI scanner (Ingenia, Philips Healthcare, The Netherlands) to assess visceral, abdominal adipose tissue volumes and HTGC, as extensively described previously (17).
Statistical analysis
Data are expressed as mean ± standard deviation. The normality of data was confirmed using the Shapiro–Wilk test, visual histograms and Q–Q plots. Baseline characteristics for the patients with T2DM were compared between ethnicities and sexes using a Chi-square test for binary values (i.e., the use of diabetic medication) and an independent t-test for normally distributed data. Not normally distributed data were log 10-transformed (i.e., subcutaneous adipose tissue, visceral adipose tissue, visceral/subcutaneous adipose tissue ratio, total cholesterol, triglycerides and CRP). Nonparametric tests were performed on data not normally distributed after log 10 transformation (i.e., age, diabetes duration, body fat percentage, HTGC, HbA1c, LDL-C, AST, ALT and metformin dose).
Olink data were analyzed using the Mann–Whitney U test to determine the difference in relative plasma levels of inflammation-related proteins between ethnicities. All proteomic analyses were corrected for multiple testing using Benjamini–Hochberg’s false discovery rate (FDR). The FDR-corrected P value (i.e., q-value) was set at <0.05. Volcano plots were constructed by calculating the fold change (FC) between South Asians and Europids on a log 2 scale. From here on, all data were split for both sexes.
To study the difference between serum FGF21 concentrations between ethnicities, an independent t-test was performed with log 10-transformed data. To determine whether the observed differences in FGF21 levels between ethnic groups were attributable to baseline phenotypical characteristics of the two cohorts, we conducted sensitivity analyses (Supplemental Table 2 (see the section on Supplementary materials given at the end of the article)). In these analyses, we examined FGF21 level differences between ethnicities while adjusting separately for age, BMI, waist circumference, visceral adipose tissue (VAT), HTGC, TC, LDL-C, AST, ALT, metformin dose and T2DM duration. The adjustments did not alter the results, indicating that the observed differences in FGF21 levels were not explained by these variables. Furthermore, nonparametric Spearman-rank correlations (rho) were applied to examine the association between relative plasma FGF21 levels from the Olink database and relative plasma IFN-gamma levels, HTCG and serum triglycerides and between serum FGF21 concentrations with serum CRP, HTGC and serum triglycerides, as the data were not normally distributed.
The statistical analysis of the baseline characteristics of the cohort without T2DM has been extensively described elsewhere (22). To compare serum FGF21 concentrations between ethnicities, an independent t-test was performed with log 10-transformed data of baseline values to attain a normal distribution. All statistical analyses of the Olink database were performed using RStudio (version 4.3.2, 2023), and other statistical analyses were performed using Statistical Package for the Social Sciences (version 29.0.1.0., IBM Corp., USA). All graphs were created with GraphPad Prism software version 9.3.1 for Windows (GraphPad Software, USA). The significance for the analysis of the difference in relative plasma levels of inflammation-related proteins between ethnicities was set at q < 0.05 and for all other analyses at P < 0.05.
Results
Baseline characteristics
At baseline, significant differences were seen in the characteristics between the South Asians and Europids with T2DM, as described previously (16, 17) (Supplemental Table 1). In short, South Asians exhibited a lower age (P = 0.012), weight (P < 0.001), length (P < 0.001), BMI (P = 0.002), waist circumference (P < 0.001), waist-to-hip ratio (P < 0.001), visceral adipose tissue volume (P = 0.006), HTGC (P < 0.001), total cholesterol (P = 0.003), LDL-C (P = 0.003), AST (P < 0.001) and dose of metformin (P = 0.037) compared with Europids. On the other hand, South Asians had a longer duration of diabetes (P < 0.001) and higher levels of ALT (P < 0.001) than their Europid counterparts. When splitting the groups based on sex, the differences in body composition and clinical parameters between South Asians and Europids persisted (Supplemental Table 1).
Relative plasma levels of inflammation-related proteins differ between South Asians and Europids with T2DM
Relative plasma levels of six inflammation-related proteins were higher in South Asians compared with Europids with T2DM (i.e., stem cell factor (SCF), caspase-8 (CASP-8), C–C motif chemokine ligand 28 (CCL28), IFN-gamma, sulfotransferase 1A1 (ST1A1) and cystatin D (CST5); FC > 0.27, q < 0.05; Fig. 1A). In addition, relative plasma levels of six inflammation-related proteins were lower in South Asians compared with Europids (i.e., FGF21, human fibroblast collagenase (MMP-1), interferon-8 (IL-8), C–C motif chemokine ligand 4, C–X–C motif chemokine ligand 6 (CXCL6) and monocyte chemoattractant protein-1 (MCP-1); FC < −0.24, q < 0.05; Fig. 1A). When splitting the data based on sex, the effects appeared to be specific for females because no significant differences in relative plasma levels were found in South Asian versus Europid males (Fig. 1B). In contrast, in South Asian versus Europid females, relative plasma levels of two inflammation-related proteins were higher (i.e., ST1A1 and IFN-gamma; FC > 0.74, q < 0.04) while relative plasma levels of six proteins were lower (i.e., FGF21, MCP-1, MMP-1, IL-8, vascular endothelial growth factor A and CXCL6; FC < −0.28, q < 0.04; Fig. 1C).
Comparison of plasma levels of inflammation-related proteins in South Asian (SA) and Europid (EU) males and females with type 2 diabetes mellitus. A volcano plot showing the comparison of relative plasma levels of 73 inflammation-related proteins in all SAs (n = 47) compared with all EUs (n = 48) with type 2 diabetes mellitus (A). Additionally, comparisons are shown for SA (n = 19) versus EU (n = 27) males with type 2 diabetes mellitus (B) and for SA (n = 28) versus EU (n = 20) females with type 2 diabetes mellitus (C). The x-axes show the log 2 fold change (log 2 FC) between SAs and EUs; the y-axes show the q-value on a log scale. P values were obtained from a Mann–Whitney U test and then corrected using Benjamini–Hochberg’s false discovery rate to yield q-values. Red circles represent significantly higher relative protein levels in SAs and blue circles represent significantly lower relative protein levels in SAs, compared with EUs (q < 0.05). The value of one EU male was excluded because of failure of the quality control of Olink, one EU male was excluded as insufficient plasma was available to perform protein analysis, and data of 23 proteins were omitted from the 96-inflammation panel because their levels were below the detection limit in more than 75% of the samples.
Citation: Endocrine Connections 14, 2; 10.1530/EC-24-0362
Circulating FGF21 concentrations are significantly lower in South Asians versus Europids with T2DM
Of note, of the 73 inflammation-related proteins measured, the relative plasma level of FGF21 differed the most between South Asians and Europids with T2DM. The relative plasma level of FGF21 was lower in South Asians compared with Europids with T2DM, both in males and females combined (FC −1.08; q = 0.002, Fig. 1A) and in females (FC −1.60; q = 0.003; Fig. 1C, Supplementary Fig. 1B). No significant difference in the relative plasma level of FGF21 was observed between South Asian males and Europid males with T2DM (FC −0.82; q = 0.120; Fig. 1B, Supplementary Fig. 1A).
To validate this finding, we next quantified FGF21 concentrations in the serum of the same cohort of South Asians and Europids with T2DM by ELISA. A strong correlation was found between relative plasma levels of FGF21 and serum FGF21 concentration (rho >0.860; P < 0.001) (data not shown). Similarly to the relative plasma levels, serum FGF21 concentrations were lower in South Asians compared with Europids with T2DM (215 ± 136 pg/mL vs 420 ± 337 pg/mL; −48.8%; P < 0.001; Fig. 2A). In addition, we now also observed lower serum FGF21 concentrations in South Asians compared with Europid males (182 ± 100 pg/mL vs 315 ± 249 pg/mL; −42.2%; P = 0.020; Fig. 2B) and in females (238 ± 154 pg/mL vs 574 ± 393 pg/mL; −58.5%; P < 0.001; Fig. 2C). Because of significant differences in baseline characteristics, we repeated the analysis with adjustments for potential confounders. However, this did not affect the results (Supplemental Table 2).
Comparison of the serum fibroblast growth factor 21 concentration in South Asian and Europid males and females with type 2 diabetes mellitus. Box plots showing the serum concentration of fibroblast growth factor 21 (FGF21) in all South Asians (n = 47, orange circles) compared with that of all Europids (n = 49, green circles) with type 2 diabetes mellitus (A). In addition, FGF21 concentrations are shown for South Asian (n = 19, orange circles) and Europid (n = 29, green circles) males (B) and South Asian (n = 28, orange circles) and Europid (n = 20, green circles) females (C). Circles represent individual values, boxes represent means and deviations represent standard deviations.
Citation: Endocrine Connections 14, 2; 10.1530/EC-24-0362
When measuring FGF21 concentrations in the serum of males without T2DM, serum FGF21 levels were generally lower compared with the males with T2DM and without differences between South Asians and Europids (83 ± 58 pg/mL vs 96 ± 76 pg/mL; −13.1%; P = 0.675; Supplementary Fig. 2).
Circulating FGF21 does not correlate with inflammation markers in both sexes
Because we previously showed that South Asians have a more activated IFN-signaling pathway compared with Europids with T2DM (15) and FGF21 is known for its anti-inflammatory properties in the context of T2DM (23), we next assessed whether relative FGF21 levels and the circulating FGF21 concentration were related to inflammation markers. First, we performed correlations with IFN-gamma levels. However, we did not find a significant correlation between relative plasma levels of FGF21 and IFN-gamma in South Asian males (rho = 0.088, P = 0.721) and Europid males (rho = −0.318, P = 0.106) (Supplementary Fig. 3A) or in South Asian females (rho = −0.171, P = 0.385) and Europid females (rho = 0.268, P = 0.254) (Supplementary Fig. 3C). In addition, we did not find a significant correlation between serum FGF21 concentrations and plasma CRP levels in South Asian males (rho = 0.125, P = 0.632) and Europid males (rho = 0.141, P = 0.466) (Supplementary Fig. 3A) or in South Asian females (rho = 0.034, P = 0.866) and Europid females (rho = −0.056, P = 0.819) (Supplementary Fig. 3B).
FGF21 relative levels and concentration positively correlate with serum triglycerides and HTGC in South Asians with T2DM
FGF21 is mainly synthesized by the liver and is known to play a role in lipid metabolism (24). Therefore, we next assessed whether relative plasma FGF21 levels and serum FGF21 concentrations were related to HTGC and serum triglyceride levels. We found that relative plasma FGF21 levels tended to be positively associated with HTGC in South Asian males (rho = 0.486, P = 0.035; Supplementary Fig. 4A) and were significantly positively related to HTGC in South Asian females (rho = 0.460, P = 0.014; Supplementary Fig. 4C). However, relative plasma FGF21 levels did not correlate with HTCG in both Europid males (rho = 0.155, P = 0.458; Supplementary Fig. 4A) and females (rho = 0.368, P = 0.110; Supplementary Fig 4B). Of note, relative plasma FGF21 levels were positively correlated with serum triglycerides in South Asian males (rho = 0.639, P = 0.003; Supplementary Fig. 4B) and females (rho = 0.528, P = 0.004; Supplementary Fig. 4D). Relative plasma FGF21 levels did tend to positively relate to serum triglycerides in Europid males (rho = 0.358, P = 0.067, Supplementary Fig. 4B). However, no such correlation was found in Europid females (rho = 0.224, P = 0.342, Supplementary Fig. 4D). Comparable results were found for serum FGF21 concentrations with both serum triglycerides and HTCG levels (Supplementary Fig. 5).
Discussion
In this study, we compared the relative plasma levels of 73 inflammation-related proteins between South Asians and Europids with T2DM. Relative plasma levels of six inflammation-related proteins were higher, and relative plasma levels of six proteins were lower in South Asians compared with Europids with T2DM. FGF21 was the most distinctive of all inflammation-related proteins measured and was lower in South Asians compared with Europid females with T2DM. We could validate this finding by measuring circulating serum FGF21 concentrations and observed lower concentrations in both male and female South Asians compared with Europids with T2DM. However, serum FGF21 concentrations did not differ in healthy South Asian versus Europid males, suggesting that the difference in FGF21 levels between ethnicities develops later in life. Furthermore, we found a tendency toward a positive correlation between relative plasma FGF21 levels, serum FGF21 concentration and both hepatic triglycerides and circulating triglycerides, especially in South Asians with T2DM.
Among the six inflammation-related proteins with higher relative levels in South Asians (i.e., SCF, CASP-8, CCL28, IFN-gamma, ST1A1 and CST5), all proteins are described in pro-inflammatory pathways (25, 26, 27, 28, 29, 30). Additionally, our findings align with our previous research showing higher mRNA levels of B-cell markers and IFN-signaling genes, indicating a more activated IFN-signaling pathway at the gene level in the blood of South Asians compared with Europids with T2DM (15). In this study, we replicate these findings at the protein level, showing a higher relative protein level of IFN-gamma and ST1A1, a central IFN-gamma intracellular mediator, in South Asians compared with Europids. Given the potential role of IFN-gamma in inducing insulin resistance in metabolic tissues, these findings may at least in part underly the increased insulin resistance among South Asians compared with Europids (29).
Significant differences in relative levels of inflammation-related proteins between South Asians and Europids were only found in females. However, serum FGF21 concentrations were significantly different in both males and females. Given that we assessed 73 inflammation-related proteins simultaneously, we adjusted our statistical analysis to account for multiple comparisons by controlling for the FDR. Consequently, it is possible that, if we had measured solely the FGF21 protein in plasma, we might also have a significant difference in FGF21 among males. Furthermore, the substantial difference in relative protein levels between South Asians and Europids in females only also suggests a potential influence of sex hormones on inflammation-related protein levels. The role of sex hormones on inflammation is a known factor (31, 32). Females exhibit higher inflammatory markers (CRP, tumor necrosis factor-alpha and interleukin 6) than males during their reproductive years and variations in inflammatory markers throughout the menstrual cycle (33, 34). Furthermore, pro-inflammatory markers are typically increased in postmenopausal females (35). In addition, sex hormones contribute to differences in body composition, particularly in fat distribution between males and females (36). Unfortunately, we have not measured sex hormones in this population nor do we have information on the menstrual cycles, use of hormonal anticonception or menopausal status in this population. However, in this study, both South Asian and Europid females with T2DM had significantly higher body fat percentages than the males. A higher fat percentage is positively associated with more inflammatory markers (37), which could contribute to the significant differences in relative protein levels between females but not males of both ethnicities.
Serum FGF21 concentrations were lower both in male and female South Asians compared with Europids with T2DM. FGF21 is an essential mediator of lipid metabolism by regulating lipolysis in white adipose tissue and increasing substrate utilization by increasing fatty acid oxidation in the liver (38, 39). It is mainly released from the liver and adipose tissue upon different metabolic stressors on the body, such as fasting, cold exposure and overfeeding (40). In addition, FGF21 has anti-inflammatory properties (41). People with obesity have a higher concentration of FGF21 than healthy people (42), likely in response to the increased metabolic stress on the body. Despite this, in Europids, 48 weeks of treatment with the FGF21 analog pegbelfermin improved signs of metabolic dysfunction-associated steatohepatitis (43). The increased FGF21 concentration in people living with obesity could indicate a compensatory mechanism for the possible reduction of FGF21 sensitivity in obesity. In our study, the serum FGF21 concentration was significantly lower in South Asians compared with Europids with T2DM. This could indicate that South Asians may not adequately compensate for this reduced sensitivity by increasing the FGF21 levels, potentially contributing to their increased risk of developing obesity-associated complications, including T2DM. Furthermore, exogenous FGF21 treatment could hold promise as a potential preventative and therapeutical option for obesity-associated complications in South Asians. Alternatively, since South Asians are known to exhibit higher inflammation compared with Europids, the lower FGF21 concentration observed in South Asians could be a compensatory response to the increased inflammation in this population (15, 44). However, we did not find a significant negative correlation between the circulating FGF21 concentrations and pro-inflammatory markers CRP and IFN-gamma. Given the complex mechanisms regulating FGF21, this does not rule out the possibility of a compensatory mechanism to increased inflammation. We did not observe significant differences in serum FGF21 levels between young South Asians and Europids without T2DM, suggesting that the differences observed in individuals with T2DM may contribute to the development of metabolic diseases. However, potential (epi-)genetic factors influencing the South Asian phenotype later in life cannot be ruled out. Therefore, further research is necessary to identify the potential underlying factors. Measuring the circulating FGF21 level in larger groups of lean individuals without metabolic disease or in individuals with obesity and pre-diabetes could provide more insight into its potential relationship with the development of T2DM.
South Asians are known to develop T2DM at a lower BMI and younger age compared with Europids. This pattern is consistent with our study population as the BMI and waist circumference were lower in the South Asians compared with Europids, while their diabetes duration was more prolonged. A recent study analyzed a panel of inflammation-related proteins among people living with obesity, with and without metabolic syndrome. The study found a significant upregulation of FGF21 among those with both obesity and metabolic syndrome compared with people living with obesity without metabolic syndrome (45). Given that South Asians in our study had lower total cholesterol and HTGC than Europids, we cannot exclude that they were less metabolically compromised than Europids, potentially explaining their lower FGF21 concentration.
On the other hand, despite the lower HTGC in South Asians, the hepatic stress marker ALT was significantly elevated compared with Europids. This suggests that despite the lower HTGC, South Asians may still have elevated (metabolic) stress on the liver. Additionally, South Asians had a significantly longer duration of T2DM and consequently (beneficial) treatment, which might have affected HTGC. Therefore, while the lower HTGC observed in the South Asian population could explain the lower FGF21 concentration compared with Europids, elevated liver stress indicated by higher ALT levels could suggest that other factors may contribute to the lower FGF21 concentration observed.
One of the strengths of our study is the extensive analysis of inflammation-related proteins in a large sample of males and females of two ethnicities, allowing us to identify differences between ethnicities objectively. In addition, our population’s almost equal distribution of sexes will enable us to explore potential sex differences. However, our study is not without limitations. Because of differences in the metabolic phenotype in South Asians and Europids, matching both ethnic groups remains difficult. In addition, we have only blood samples to provide information about inflammation. Ideally, we would have measured inflammation proteins in metabolically active tissues, like the liver and white adipose tissue. This would allow us to localize the assessment of the sources of inflammation.
In conclusion, we showed that of the 73 measurable inflammation-related proteins, the relative plasma levels of 12 proteins were significantly different in South Asians compared with Europids with T2DM, with FGF21 being the most prominent concerning effect size. This observation was further supported by the finding of a lower serum FGF21 concentration in South Asians compared with Europids with T2DM. The difference in FGF21 between these ethnicities warrants further tissue-specific studies, given its potential as a metabolic target.
Supplementary materials
This is linked to the online version of the paper at https://doi.org/10.1530/EC-24-0362.
Declaration of interest
The authors declare that there is no conflict of interest that could be perceived as prejudicing the impartiality of this work.
Funding
This work was supported by the Dutch Research Council NWO (VENI Grant no. 09150161910073 to MRB), the Dutch Diabetes Research Foundation (Grant no. 2023.82.010 to MRB), the Fundación Alfonso Martin Escudero, the Maria Zambrano fellowship by the Ministerio de Universidades y la Unión Europea–NextGenerationEU (Grant no. RR_C_2021_04 to BMT) and The Netherlands Cardiovascular Research Initiative: an initiative with support of the Dutch Heart Foundation (Grant nos. CVON2014-02 ENERGISE and CVON2017 GENIUS-2 to PCNR). Novo Nordisk A/S (Bagsvaerd, Denmark) funded this investigator-initiated study and was not involved in the design of the study; the collection, analysis and interpretation of data; writing the report or the decision to submit the report for publication. We also thank Roba Metals B.V. IJsselstein (Utrecht, The Netherlands) for financial support.
Author contribution statement
CAH, BMT, MES, PCNR and MRB conceptualized the study; methodology was devised by CAH, BMT, MES and MRB; formal analyses were performed by CAH, BMT, MES and MRB; original investigation was performed by MBB, HJE, KJN and LGMJ; original draft was written by CAH; review and editing were carried out by CAH, BMT, MES, MBB, HJE, HJL, JWAS, IMJ, KJN, LGMJ, PCNR and MRB; visualization was performed by CAH; supervision was performed by BMT, MRB and PCNR; project administration was carried out by CAH; and funding was acquired by BMT, IMJ PCNR and MRB.
Data availability
The lead contact will share all data reported in this paper upon reasonable request.
Acknowledgment
We express our gratitude to all individuals who participated in the three clinical trials. We thank Trea Streefland (Division of Endocrinology, LUMC, Leiden, The Netherlands) for her excellent technical assistance. Furthermore, we thank Dr Julia I.P. van Heck (Department of Internal Medicine, Radboud University Medical Center, Nijmegen, The Netherlands) for her help with the analysis of the Olink database.
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