Tirzepatide induces a thermogenic-like amino acid signature in brown adipose tissue

Abstract

Objectives: Tirzepatide, a dual GIP and GLP-1 receptor agonist, delivered superior glycemic control and weight loss compared to selective GLP-1 receptor (GLP-1R) agonism in patients with type 2 diabetes (T2D). These results have fueled mechanistic studies focused on understanding how tirzepatide achieves its therapeutic efficacy. Recently, we found that treatment with tirzepatide improves insulin sensitivity in humans with T2D and obese mice in concert with a reduction in circulating levels of branched-chain amino (BCAAs) and keto (BCKAs) acids, metabolites associated with development of systemic insulin resistance (IR) and T2D. Importantly, these systemic effects were found to be coupled to increased expression of BCAA catabolic genes in thermogenic brown adipose tissue (BAT) in mice. These findings led us to hypothesize that tirzepatide may lower circulating BCAAs/BCKAs by promoting their catabolism in BAT.

Methods: To address this question, we utilized a murine model of diet-induced obesity and employed stable-isotope tracer studies in combination with metabolomic analyses in BAT and other tissues.

Results: Treatment with tirzepatide stimulated catabolism of BCAAs/BCKAs in BAT, as demonstrated by increased labeling of BCKA-derived metabolites, and increases in levels of byproducts of BCAA breakdown, including glutamate, alanine, and 3-hydroxyisobutyric acid (3-HIB). Further, chronic administration of tirzepatide increased levels of multiple amino acids in BAT that have previously been shown to be elevated in response to cold exposure. Finally, chronic treatment with tirzepatide led to a substantial increase in several TCA cycle intermediates (α-ketoglutarate, fumarate, and malate) in BAT.

Conclusions: These findings suggest that tirzepatide induces a thermogenic-like amino acid profile in BAT, an effect that may account for reduced systemic levels of BCAAs in obese IR mice.

Keywords: BAT; BCAAs; BCKAs; GIPR; GLP-1R; Tirzepatide.

Graphical abstract

Figure 1

Tirzepatide augments the catabolism of branched-chain amino acids (BCAAs) in brown adipose tissue (BAT). Obese insulin resistant mice were dosed once daily for 14-days with vehicle or tirzepatide (TZP (10 nmol/kg), n = 6 per group). Daily body weight (A) and food intake (B). Schematic representation of uniformally-¹³C labelled alpha-ketoisovalerat (KIV, [U-13C]KIV) administration protocol (C). Plasma labeling (% and absolute amount) of [U-13^C]KIV (D and E). The percentage of labelled valine and 3-HIB in BAT (F). The absolute amounts of labelled valine and 3-hydroxyisobutyrate (3-HIB) in BAT (G). The absolute amounts of unlabeled valine, leucine, isoleucine, 3-HIB (H), glutamate, alanine, serine and glycine (I) citrate (Cit), alpha-keto glutarate (2-KG), succinate, fumarate (Fum) and malate (Mal) in BAT (J). Data are presented as mean ± SEM. Statistical analyses performed included student unpaired t-test (F-J), two-way ANOVA (A, B, D and E), followed by Dunnett's multiple comparisons test where appropriate, ∗p < 0.05.

Figure 2

Tirzepatide augments catabolism of branched-chain amino acids (BCAAs) in brown adipose tissue (BAT) independent of weight loss. Obese insulin resistant mice were dosed with vehicle, tirzepatide (TZP (10 nmol/kg)) or pair-fed to TZP daily for 14-days, n = 6 per group. Daily body weight (A) and food intake (B). The percentage of labelled valine and 3-HIB in liver, BAT and skeletal muscle (SKM, C and D). The absolute amounts of unlabeled valine, 3-HIB, leucine, isoleucine, alanine, glutamate, glycine, serine and urea cycle intermediates citrulline, ornithine and arginine in BAT (E-I). Data are presented as mean ± SEM. Statistical analyses performed included two-way ANOVA (A and B) and one-way ANAOVA (C-I) followed by Dunnett's multiple comparisons test where appropriate, ∗p < 0.05 vs vehicle, ^#p < 0.05 vs pair-fed.

Figure 3

Tirzepatide increases TCA cycle intermediate levels in brown adipose tissue independent of weight loss. Obese insulin resistant mice were dosed with vehicle, tirzepatide (TZP (10 nmol/kg)) or pair-fed to TZP daily for 14-days, n = 6 per group. The absolute amounts of citrate (A), alpha-ketoglutarate (2-KG, B), succinate (C), fumarate (D) and malate (E) content in BAT (G and H). Data are presented as mean ± SEM. Statistical analysis was performed using a one-way ANAOVA followed by Dunnett's multiple comparisons test where appropriate, ∗p < 0.05.

Figure 4

Chronic treatment with tirzepatide increases amino acid content in the liver in a weight-dependent manner. Obese insulin resistant mice were dosed with vehicle, tirzepatide (TZP (10 nmol/kg)) or pair-fed to TZP daily for 14-days, n = 6 per group. The absolute amino acid content in the liver. Data are presented as mean ± SEM. Statistical analysis was performed using a one-way ANAOVA followed by Dunnett's multiple comparisons test where appropriate, ∗p < 0.05.

Figure 5

Chronic treatment with tirzepatide augments amino acid content in skeletal muscle in a weight-dependent manner. Obese insulin resistant mice were dosed with vehicle, tirzepatide (TZP (10 nmol/kg)) or pair-fed to TZP daily for 14-days, n = 6 per group. The absolute amino acid content in skeletal muscle. Data are presented as mean ± SEM. Statistical analysis was performed using a one-way ANAOVA followed by Dunnett's multiple comparisons test where appropriate, ∗p < 0.05.

Figure 6

Chronic Treatment with Tirzepatide induces a thermogenic-like amino acid profile in brown adipose tissue (BAT). Obese insulin resistant mice were dosed with vehicle, tirzepatide (TZP (10 nmol/kg)) or pair-fed to TZP daily for 3 or 14-days, n = 6 per group. Effect of TZP on body weight (A) and tissue levels of BCAAs in BAT (B). Statistical analyses performed included one-way ANOVA and two-way ANOVA. ∗Unadjusted P < 0.05. ∗∗Unadjusted P < 0.01, ∗∗∗Unadjusted P < 0.001.