The role of suppression of hepatic SCD1 expression in the metabolic effects of dietary methionine restriction

Abstract

Dietary methionine restriction (MR) produces concurrent increases in energy intake and expenditure, but the proportionately larger increase in energy expenditure (EE) effectively limits weight gain and adipose tissue accretion over time. Increased hepatic fibroblast growth factor-21 (FGF21) is essential to MR-dependent increases in EE, but it is unknown whether the downregulation of hepatic stearoyl-coenzyme A desaturase-1 (SCD1) by MR could also be a contributing factor. Global deletion of SCD1 mimics cold exposure in mice housed at 23 °C by compromising the insular properties of the skin. The resulting cold stress increases EE, limits fat deposition, reduces hepatic lipids, and increases insulin sensitivity by activating thermoregulatory thermogenesis. To examine the efficacy of MR in the absence of SCD1 and without cold stress, the biological efficacy of MR in Scd1^-/- mice housed near thermoneutrality (28 °C) was evaluated. Compared with wild-type mice on the control diet, Scd1^-/- mice were leaner, had higher EE, lower hepatic and serum triglycerides, and lower serum leptin and insulin. Although dietary MR increased adipose tissue UCP1 expression, hepatic Fgf21 messenger RNA, 24 h EE, and reduced serum triglycerides in Scd1^-/- mice, it failed to reduce adiposity or produce any further reduction in hepatic triglycerides, serum insulin, or serum leptin. These findings indicate that even when thermal stress is minimized, global deletion of SCD1 mimics and effectively masks many of the metabolic responses to dietary MR. However, the retention of several key effects of dietary MR in this model indicates that SCD1 is not a mediator of the biological effects of the diet.

Keywords: FGF21; acides aminés essentiels; animal models; apport alimentaire; calorimétrie indirecte; dietary intake; essential amino acids; indirect calorimetry; modèles animaux.

Fig. 1

Assessment of chronic effects of dietary methionine restriction (MR) on energy balance in wild-type (WT) and stearoyl-coenzyme A desaturase-1 (Scd1^−/−) mice housed initially at 23 °C and then after transfer to 28 °C. Animals were housed at 23 °C and acclimated to the control (CON) diet for 1 week beginning at 5 weeks of age, after which half the mice of each genotype were randomized to receive the MR diet. Average body weight (A), adiposity (B), daily energy intake per mouse (C), and energy expenditure (D) were measured after 5 weeks on the respective diets. Thereafter the mice were housed at 28 °C for an additional 5 weeks and the responses presented in figure panels A–D were remeasured. In tissues harvested thereafter, brown adipose tissue (BAT) uncoupling protein-1 (UCP1) expression (E) was measured by Western blotting in cell lysates and standardized to pyruvate dehydrogenase complex E2 (PDC-E2), while IWAT Ucp1 messenger RNA (mRNA) (F) was measured by real-time polymerase chain reaction and expressed relative to cyclophilin. Means ± SE of body weight, adiposity, and energy intake were measured after 5 weeks at 23 °C and 5 weeks at 28 °C, and means annotated with different letters differ at P < 0.05. Energy expenditure was measured after 5 weeks at 23 °C and 5 weeks at 28 °C, and least-square means ± SE were calculated by ANCOVA as described in the Materials and methods section. BAT UCP1 protein expression and IWAT Ucp1 mRNA were expressed as means ± SE and means annotated with different letters differ at p < 0.05.

Fig. 2

Effects of dietary methionine restriction (MR) on hepatic stearoyl-coenzyme A desaturase-1 (SCD1) expression and liver and serum triglycerides in wild-type (WT) and Scd1^−/− mice. Hepatic Scd1 messenger RNA (mRNA) (A) was measured via real-time polymerase chain reaction and standardized to cyclophilin. Whole-cell extracts (WCE) from COS cells transfected with empty vector (EV) or Scd1 were analyzed by Western blot (B) to establish specificity of the antibody. Expression of SCD1 was determined in hepatic microsomes prepared from WT mice and Scd1^−/− mice. Lanes 1–4 in panel C are from WT mice, and are representative of 5 animals per diet, while lanes 5–6 from Scd1^−/− mice were included to establish absence of SCD1 expression in Scd1^−/− mice. SCD1 expression was normalized to β-actin and means annotated with different letters differ at P < 0.05 (C). Means ± SE of serum (D) and hepatic (E) triglyceride were determined in 5 mice from each genotype and diet, and means annotated with different letters differ at P < 0.05. CON, control; mM, mmol/L.

Fig. 3

Effects of dietary methionine restriction (MR) on hepatic fibroblast growth factor-21 (Fgf21) and serum hormones in wild-type (WT) and stearoyl-coenzyme A desaturase-1 (Scd1^−/−) mice. Hepatic Fgf21 expression (A) and serum adiponectin (B), insulin (C), and leptin (D) were measured in samples harvested at study’s end. Fgf21 messenger RNA (mRNA) expression was determined using real-time polymerase chain reaction and standardized to cyclophilin expression. Serum adiponectin, insulin, and leptin were determined using enzyme-linked immunosorbent assays (ELISA). Data are expressed as means ± SE from 5 mice of each genotype and diet combination, and means annotated with different letters differ at P < 0.05. CON, control.