Hepatic Oleate Regulates Insulin-like Growth Factor-Binding Protein 1 Partially through the mTORC1-FGF21 Axis during High-Carbohydrate Feeding

Abstract

Stearoyl-CoA desaturase-1 (SCD1) catalyzes the rate-liming step of monounsaturated fatty acid biosynthesis and is a key regulator of systemic glucose metabolism. Mice harboring either a global (GKO) or liver-specific deletion (LKO) of Scd1 display enhanced insulin signaling and whole-body glucose uptake. Additionally, GKO and LKO mice are protected from high-carbohydrate diet-induced obesity. Given that high-carbohydrate diets can lead to chronic metabolic diseases such as obesity, diabetes, and hepatic steatosis, it is critical to understand how Scd1 deficiency confers metabolically beneficial phenotypes. Here we show that insulin-like growth factor-binding protein 1 (IGFBP1), a hepatokine that has been reported to enhance insulin signaling, is significantly elevated in the liver and plasma of GKO and LKO mice fed a low-fat high-carbohydrate diet. We also observed that the expression of hepatic Igfbp1 is regulated by oleic acid (18:1n9), a product of SCD1, through the mTORC1-FGF21 axis both in vivo and in vitro.

Keywords: fibroblast growth factor 21; insulin-like growth factor-binding protein 1; mechanistic target of rapamycin; oleic acid; stearic acid; stearoyl-CoA desaturase-1.

Figure 1

Hepatic SCD1 deficiency up-regulates IGFBP1. (A) Relative hepatic Igfbp1 mRNA levels in 8–10 weeks old male WT (n = 4) and GKO (n = 6) mice fed HCD for 10 days. (B) Relative hepatic Igfbp1 mRNA levels in 8–13 weeks old male LOX (n = 13) and LKO (n = 12) mice fed HCD for 7–10 days. (C) Plasma IGFBP1 protein concentration in 8–10 weeks old male LOX (n = 7) and LKO (n = 7) mice fed HCD for 7–10 days. (D) Relative IGFBP1 mRNA levels in HepG2 cells treated for 24 h with BSA (n = 10) or BSA plus 4 μM (n = 12) of SCD inhibitor. Values are mean ± SEM; * p < 0.05 vs. WT, LOX, or BSA, *** p < 0.005 vs. WT, LOX or BSA by Student’s two-tailed t-test.

Figure 2

Oleic acid blunts IGFBP1 up-regulation in vivo and in vitro. (A) Relative hepatic Igfbp1 mRNA levels in 12–14 weeks old male LOX and LKO mice fed either HCD supplemented with 15% by weight tristearin (n = 4 LOX and n = 4 LKO) or 15% by weight triolein (n = 5 LOX; 7 LKO) for 10 days. (B) Relative hepatic Igfbp1 mRNA levels in 8–10 weeks old male WT (n = 4), GKO (n = 6), and SCD5tg (n = 5) mice fed HCD for 10 days. (C) Relative Igfbp1 mRNA level in HepG2 cells treated for 24 h with either BSA (n = 10), BSA plus 4 μM SCD inhibitor (n = 12), BSA plus 4μM SCD inhibitor, and 500 μM oleic acid (n = 12), or BSA plus SCD inhibitor and 500 μM steric acid (n = 12). (D) Secreted IGFBP1 protein concentration in HepG2 cells treated for 24 h with either BSA (n = 7), BSA plus 4 μM SCD inhibitor (n = 8), BSA plus 4 μM SCD inhibitor and 500 μM oleic acid (n = 7), or BSA plus SCD inhibitor and 500 μM steric acid (n = 8). Values are mean ± SEM; * p < 0.05 vs. WT, LOX, or BSA; ** p < 0.01 vs. WT, LOX, or BSA; *** p < 0.005 vs. WT, LOX triolein or BSA by Student′s two-tailed t-test.

Figure 3

Inhibition of mTORC1 by rapamycin suppresses IGFBP1 induction. (A) Relative hepatic Igfbp1 mRNA levels in 10–12 weeks old male LOX and LKO mice fed HCD diet for 10 days treated daily with either vehicle (n = 3 LOX; 3 LKO) or rapamycin (n = 5 LOX + R; 5 LKO + R). (B) Relative Igfbp1 mRNA levels of HepG2 cells treated with either BSA (n = 5), BSA plus 1μM SCD inhibitor (n = 5), or BSA plus 4 μM SCD inhibitor and rapamycin (n = 5). Values are mean ± SEM; * p < 0.05 vs. LOX or BSA; ** p < 0.01 vs. LOX+V or BSA; *** p < 0.005 vs. LOX or BSA by Student′s two-tailed t-test.

Figure 4

IGFBP1 is regulated by SCD1 through the mTORC1-FGF21 axis. (A) Relative hepatic Scd1 and Fgf21 mRNA levels in LOX/LOX (n = 6, 3 male; 3 female) and DLKO (n = 6, 3 male; 3 female) mice fed HCD for 10 days. (B) Relative hepatic Igfbp1 mRNA level in LOX/LOX (n = 6, 3 male; 3 female) and DLKO mice fed HCD (n = 6, 3 male; 3 female) for 10 days. (C) Relative Fgf21 mRNA level in HepG2 cells treated with BSA (n = 6), BSA plus 1 μM SCD inhibitor (n = 6), BSA plus 1 μM SCD inhibitor and 100 μM oleic acid (n = 5), BSA plus 1 μM SCD inhibitor and 100 μM steric acid (n = 5), BSA plus 1 μM SCD inhibitor and rapamycin (n = 5). (D) Relative FGF21 mRNA level in HepG2 cells treated with control lentiviral particles (n = 6), control lentiviral particles plus SCD inhibitor (n = 6), or shFGF21 lentiviral particles plus SCD inhibitor (n = 6). (E) Relative IGFBP1 mRNA level in HepG2 cells treated with control lentiviral particles (n = 6), control lentiviral particles plus SCD inhibitor (n = 5), or shFGF21 lentiviral particles plus SCD inhibitor (n = 5). Values are mean ± SEM; ns = not significant; * p < 0.05 vs. LOX/LOX, BSA, or control; ** p < 0.01 vs. LOX/LOX, BSA, or control; *** p < 0.005 vs. LOX/LOX, BSA, or control by Student′s two-tailed t-test.

Figure 5

Proposed model of IGFBP1 induction through the mTORC1-FGF21 axis.