General description of included patients and controls
A total of 78 subjects (40 females, 38 males, average age 58.83 ± 0.94 y) were enrolled and divided into four study populations: healthy control, T2D, T2D+obesity and T2D+hyperlipidemia groups. There were no significant among four groups including the indexes of age, SBP, DBP, ALT, AST, UA levels and the duration of T2D and antidiabetic agents’ intake. Interestingly, no patient with hyperlipidemia accepted the anti-hyperlipidemics. Furthermore, as expected, the close association of weight, BMI, FBG, insulin, HOMA-IR, TG, TC levels was found between patients and T2D vs. controls. The Cr levels in T2D groups were significantly higher than in the control group. Patients with T2D and hyperlipidemia had the highest levels of serum HDL and ApoB. The main clinical and biochemical characteristics are summarized in Table 1.
Changes of serum biomarkers among T2D, T2D+obesity, T2D+hyperlipidemia groups and controls
The patients with T2D and obesity had the highest levels of serum GDF15, which is statistically significant when comparing with the control group and T2D group. However, no significant difference was observed in four groups including serum apelin, CTRP13, GLP1, IGF1, IGFBP, IL-18, LCN, Nesfatin-1, REG1A, and Resistin 1 levels (Table 2).
Alteration of serum metabolic profiles among T2D, T2D+obesity, T2D+hyperlipidemia groups and controls
Using untargeted GC-MS metabolomics approach, the fingerprinting of human serum was obtained. The PCA and OPLS-DA plots shown in Figure 1 were used to determine the different metabolic profiles among T2D, T2D+obesity, T2D+hyperlipidemia groups and controls. The total PCA plots showed clear differentiations in the serum metabolomics between the T2D groups and Healthy controls (Figure 1.A). As expected, further PCA plots between two groups (T2D vs. Healthy control, T2D + obesity vs. Healthy control, and T2D + hyperlipidemia vs. Healthy control) were separate shown in Figure 1.B, Figure 1.C, and Figure 1.D. However, no clear differentiation was observed between T2D + obesity, T2D + hyperlipidemia and T2D groups with PCA plots (Figure E, F). Therefore, the adjusted analysis, OPLS-DA was applied and showed the clear differentiations (Figure E.1, F.1). The stable cumulative R2 and Q2 were observed for comparing the T2D + obesity group and the T2D + hyperlipidemia group to the T2D group (data not shown).
Multivariate analysis was used to screen the significantly different metabolites between all T2D groups and the Healthy control group, and T2D combined with obesity and hyperlipidemia and T2D groups. P value < 0.05 and Variable importance in the Projection (VIP) of OPLS-DA model > 1 were considered as statistically significance. The metabolites between groups were visualized using the volcano plots. Each point in the volcano plot stands as one different metabolite between groups. The X axis stands for the change folds and the Y axis stands for the P value of Student’s T test between two groups. The size of the point stands for the VIP value, the bigger size and the higher VIP value. The red color of each point stands for the up-regulation of the metabolite and the blue color of each point stands for the down-regulation of the metabolite significantly. The gray color of each point stands for non-significant change of metabolites between two groups. In our present study, we listed the comparison of the T2D vs. Healthy control groups (Figure 2.A), the T2D + obesity vs. the Healthy control groups (Figure 2.B), the T2D + hyperlipidemia vs. the Healthy control groups (Figure 2.C), the T2D + obesity vs. T2D groups (Figure 2.D) and the T2D + hyperlipidemia vs. the T2D groups (Figure 2.E). Finally, 20 metabolites (9 increased, 11 decreased) between the T2D and Healthy control groups, 20 metabolites (11 increased, 9 decreased) between the T2D + obesity and Healthy control groups, 32 metabolites (14 increased, 18 decreased) between the T2D + hyperlipidemia and Healthy control groups, 11 metabolites (7 increased, 4 decreased) between the T2D + obesity and the T2D groups, and 13 metabolites (4 increased, 9 decreased) between the T2D + hyperlipidemia and the T2D groups were found significantly distinct shown in Table 3. The common 2 metabolites (lactose 1, myristic acid) increased and 5 metabolites (xylose 2, 3-hydroxypropionic acid 1, allose 1, N-ethylglycine 1, and farnesal 2) decreased when comparing all T2D groups with the Healthy control group. The common 1 metabolite (bis (2-hydroxypropyl) amine 2) increased and 2 metabolites (urea and 3-hydroxypropionic acid 1) decreased when comparing T2D combined with obesity or hyperlipidemia group with the T2D group. Differential metabolites among all groups were displayed with the heat-map to visualize the change of metabolites, which showed the similar trends for comparisons of two groups (shown as the supplementary data, Figure S1).
Metabolic pathway analysis among T2D, T2D+obesity, T2D+hyperlipidemia groups and controls
With the analysis of online database KEGG (Kyoto Encyclopedia of Genes and Genomes, http://www.kegg.jp/kegg/pathway.html) pathway [11], the related pathways were found for differential metabolites between two groups. The recognized KEGG metabolic pathways as the backend database was furtherly analyzed to screen and score all related pathways and they were ranked as the bubble plots shown in Figure 3. In the plot, each bubble represents for one pathway. The bigger value of X-axis and the bigger size of the bubble showed the higher impact factor for the pathway. The P value was represented by the location of Y-axis and the color of the bubble. The deeper color of the bubble was; the more significant enrichment would be. In the present study, D-glutamine and D-glutamate, taurine and hypotaurine, and beta-alanine metabolism were analyzed that with higher impact factor between the T2D and Healthy control groups (Figure 3.A). Alanine, aspartate and glutamate, arginine and proline, glyoxylate and dicarboxylate and glycine, serine and threonine metabolism were recognized as the most related pathways between the T2D + obesity and Healthy control groups (Figure 3.B). Glycine, serine and threonine and taurine and hypotaurine metabolism were identified as the potential pathways between the T2D + hyperlipidemia and Healthy control groups (Figure 3.C). Additionally, the potential pathways were analyzed between T2D patients combined with obesity or hyperlipidemia and T2D groups, which exhibited that beta-alanine, glycine, serine and threonine and arginine and proline metabolism were the potential pathways between patients with T2D combined with obesity and patients with T2D (Figure 3.D), and purine and glycine, serine and threonine metabolism were potential pathways between patients with T2D combined with hyperlipidemia and patients with T2D (Figure 3.E).