Molecular mechanisms by which Roux-en-Y gastric bypass (RYGB) improves glycemic control and metabolism in type 2 diabetes (T2D) remain incompletely understood. In the SLIMM-T2D trial, participants with T2D were randomized to RYGB or nonsurgical management and their fasting plasma proteome and metabolome were analyzed for up to 3 years. To identify analytes that mediate improvement in outcomes, we developed a high-throughput mediation analysis method (Hitman), which is significantly more powerful than existing methods. Top-ranking analyte mediators of glycemia improvement were growth hormone receptor and prolylhydroxyproline, which were more significant than any clinical mediator, including BMI. Betaalanine and Histidine Metabolism (both including CNDP1) were top differentially regulated pathways, and Valine, Leucine and Isoleucine Degradation was also a top differentiallyregulated pathway and a top mediator of improvement in insulin resistance. The identified analytes may serve as novel targets for T2D therapy. More broadly, Hitman can identify analyte mediators of outcomes in randomized trials for which high-throughput data are available.
Results
ClinicalProteomic and metabolomic data at baseline, the 3 month time point, and 1 year were available from 35 of the 38 SLIMM-T2D participants. Metabolic characteristics of these 35 participants did not differ between groups at baseline ( Table S1). The HbA1c and BMI of those that had proteomics or metabolomics per time point also did not differ from the SLIMM-T2D participants within their group at any time point (Figure S1). After 3 years, no DWM participants achieved study-defined glycemic goals (HbA1c<6.5% and fasting plasma glucose<126 mg/dL), whereas eight RYGB participants did, and seven of these eight participants were not receiving any antidiabetes medications (Simonson et al., 2018). The number of participants taking each diabetes medication class per arm at each time point is tabulated in Table S1.
ProteomeFasting proteomics were profiled at baseline, the 3 month time point, and years 1, 2 and 3 in RYGB and DWM. At baseline (pre-randomization), there were no statistically significant proteins as assessed by false discovery rate (FDR<0.15). For later, post-randomization time points, reduction in BMI was greater in RYGB, so groups were compared both with and without adjustment for each person's BMI change. Differences in the baseline-corrected proteome in RYGB vs. DWM (i.e. differences between groups in changes over time) emerged at the 3 month time point, with 14 significant proteins in the unadjusted analysis and 8 in the BMI-adjusted analysis. Proteins common to both analyses and downregulated in RYGB were: carnosine dipeptidase 1 (CNDP1, also known as Beta-Ala-His dipeptidase 1) and Fetuin-B (FETUB). Proteins in common upregulated in RYGB were: integrin-binding sialoprotein (IBSP), insulin-like growth factor binding protein (IGFBP2), endothelial cell-specific molecule 1 (ESM1), macrophage metalloelastase (MMP12), alpha-1-antichymotrypsin complex (SERPINA3), ...