Gut Bacteria-Derived Tryptamine Ameliorates Diet-Induced Obesity and Insulin Resistance in Mice

Abstract

Tryptophan is an essential amino acid that is metabolized in the intestine by gut bacteria into indole derivatives, including tryptamine. However, little is known about which bacterial tryptophan metabolites directly influence obesity. In this study, we identified tryptamine as a bacterial metabolite that significantly reduced fat mass following the intraperitoneal injection of five bacterial tryptophan end-products in a diet-induced obese mouse model. Interestingly, tryptamine, a serotonin analog, inhibited both lipogenesis and lipolysis in adipose tissue, which was further confirmed in a 3T3-L1 adipocyte cell culture study. Moreover, oral tryptamine supplementation markedly reduced fat mass and improved insulin sensitivity in a long-term, high-fat-diet, pair-feeding model. These studies demonstrate the therapeutic potential of tryptamine, a bacterial tryptophan metabolite, in ameliorating obesity and insulin resistance by directly regulating lipogenesis and lipolysis in white adipose tissue.

Keywords: adipose tissue; gut bacterial metabolite; high-fat diet; obesity; tryptamine.

Figure 1

Tryptamine, among other microbial tryptophan metabolites, reduced body weight and fat mass during short-term intraperitoneal injections under high-fat diet conditions. (a) Schematic representation of the tryptophan metabolic pathway in the host and gut microbiota (upper) and in vivo study design (bottom). (b–h) Mice were fed RC or an HFD for three weeks, with vehicle, tryptamine, indole, indole-3-acetate, indole-3-proprionic acid, or indole-3-carboxaldehyde administered daily via IP injection during the final 2 weeks. All groups fasted overnight prior to being sacrificed. (b) Body weight change curve for 3 weeks of RC or HFD feeding. (c) Final body weight gain, (d) fat mass, and (e) lean body mass. (f) Cumulative food intake for 3 weeks of RC or HFD. (g) Lipid parameters in plasma. (h) Tissue weight; n = 4–5 for each group. Data are presented as the mean ± SEM. # p < 0.05 according to the two-way ANOVA. * p < 0.05, ** p < 0.01, and *** p < 0.001 according to the one-way ANOVA. Statistical number was obtained by Student’s t-test. RC, regular chow; HFD, high-fat diet; Veh, vehicle; Trp, tryptophan; TA, tryptamine; I3A, indole-3-acetate; IPA, indole-3-propionic acid; ICA, indole-3-carboxaldehyde; IP, intraperitoneal; eWAT, epididymal white adipose tissues; SEM, standard error of the mean.

Figure 2

Tryptamine decreased respiratory exchange ratio during short-term intraperitoneal injection administration under high-fat diet conditions. (a) Oxygen consumption, (b) carbon dioxide production, (c) respiratory exchange ratio, (d) energy expenditure, (e) food intake, and (f) activity were measured in mice housed in individual metabolic cages for 48 h; n = 7–8 for each group. Data are presented as the mean ± SEM. # p < 0.05 and ### p < 0.001 according to the two-way ANOVA. Statistical number was obtained by Student’s t-test. Veh, vehicle; HFD, high-fat diet; TA, tryptamine; VO₂, oxygen consumption; VCO₂, carbon dioxide production; RER, respiratory exchange ratio; SEM, standard error of the mean.

Figure 3

Tryptamine decreased lipogenesis and lipolysis in white adipose tissue during short-term intraperitoneal injections under high-fat diet conditions. (a–e) Mice were fed RC or an HFD for three weeks, with Veh or TA administered daily via IP injection during the final 2 weeks. Analyses were performed in all groups that fasted overnight prior to being sacrificed. (a) Htr2a and Htr2b mRNA expression in eWAT. (b) Representative H&E staining images and cross-sectional area of eWAT. Scale bar = 100 μm. (c) Plasma free fatty acids. mRNA expression of genes involved in (d) lipogenesis and (e) lipolysis in eWAT; n = 4–5 for each group. (f,g) Adipocyte 3T3-L1 cells were incubated with vehicle or TA for 48 h. (f) Oil Red O staining. Scale bar = 100 μm. (g) Cellular triglyceride level. Data are presented as the mean ± SEM. * p < 0.05, ** p < 0.01, and *** p < 0.001 according to the one-way ANOVA. Statistical number was obtained by Student’s t-test. RC, regular chow; Veh, vehicle; HFD, high-fat diet; TA, tryptamine; IP, intraperitoneal; eWAT, epididymal white adipose tissues; Htr, 5-hydroxytryptamine; H&E, hematoxylin and eosin; CSA, cross-sectional area; Fasn, fatty acid synthase; Scd1, stearoyl-CoA desaturase 1; Dgat2, diglyceride acyltansferase 2; Cd36, cluster of differentiation 36; Hsl, hormone-sensitive lipase; Atgl, adipose triglyceride lipase; Mgll, monoglycerol lipase; TG, triglyceride; SEM, standard error of the mean.

Figure 4

Tryptamine supplementation attenuated HFD-induced obesity under long-term pair-feeding conditions. (a–g) Mice were fed an HFD or customized HFD containing 0.1% tryptamine for 16 weeks, with ad libitum feeding for the first 8 weeks and pair-feeding for the final 8 weeks. (a) Food intake. (b) Body weight change curve. (c) Body weight gain and (d) accumulative food intake over 16 weeks of HFD feeding. (e) Final fat mass and (f) lean body mass. (g) Representative H&E staining images and cross-sectional area of eWAT. Scale bar = 100 μm; n = 5–6 for each group. Data are presented as the mean ± SEM. # p < 0.05 and ## p < 0.01 according to the two-way ANOVA. Statistical number was achieved by a Student’s t-test. ns, non-significant; HFD, high-fat diet; TA, tryptamine; H&E, hematoxylin and eosin; eWAT, epididymal white adipose tissues; CSA, cross-sectional area; SEM, standard error of the mean.

Figure 5

Tryptamine supplementation improved whole-body insulin sensitivity under long-term pair-feeding HFD conditions. (a–d) Mice were fed an HFD or customized HFD containing 0.1% tryptamine for 16 weeks, with ad libitum feeding for the first 8 weeks and pair-feeding for the final 8 weeks. Analyses were performed in groups that fasted overnight prior to experiments. (a) Plasma glucose concentrations (left) and AUC (right) during glucose tolerance test. (b) Basal plasma glucose level, (c) plasma insulin concentrations (left) and AUC (right), and (d) HOMA-IR during the glucose tolerance test. (e,f) Insulin signaling analyses were performed in the HFD and TA groups after fasting for 6 h. Representative immunoblots of phospho Akt (p-Akt on Ser473) and total Akt in (e) eWAT and (f) liver of HFD and TA groups 30 min after insulin injection; n = 5 for each group. Data are presented as the mean ± SEM. # p < 0.05, ## p < 0.01, and ### p < 0.001 according to the two-way ANOVA. Statistical number was obtained by Student’s t-test. HFD, high-fat diet; TA, tryptamine; AUC, area under curve; HOMA-IR, homeostatic model assessment of insulin resistance; eWAT, epididymal white adipose tissue; SEM, standard error of the mean.