Hepatic oleate regulates one-carbon metabolism during high carbohydrate feeding

Abstract

Obesity is a major risk factor for type 2 diabetes, coronary heart disease, and strok. These diseases are associated with profound alterations in gene expression in metabolic tissues. Epigenetic-mediated regulation of gene expression is one mechanism through which environmental factors, such as diet, modify gene expression and disease predisposition. However, epigenetic control of gene expression in obesity and insulin resistance is not fully characterized. We discovered that liver-specific stearoyl-CoA desaturase-1 (Scd1) knockout mice (LKO) fed a high-carbohydrate low-fat diet exhibit dramatic changes in hepatic gene expression and metabolites of the folate cycle and one-carbon metabolism respectively for the synthesis of S-adenosylmethionine (SAM). LKO mice show an increased ratio of S-adenosylmethionine to S-adenosylhomocysteine, a marker for increased cellular methylation capacity. Furthermore, expression of DNA and histone methyltransferase genes is up-regulated while the mRNA and protein levels of the non-DNA methyltransferases including phosphatidylethanolamine methyltransferase (PEMT), Betaine homocysteine methyltransferase (Bhmt), and the SAM-utilizing enzymes such as glycine-N-methyltransferase (Gnmt) and guanidinoacetate methyltransferase (Gamt) are generally down-regulated. Feeding LKO mice a high carbohydrate diet supplemented with triolein, but not tristearin, and increased endogenous hepatic synthesis of oleate but not palmitoleate in Scd1 global knockout mice normalized one carbon gene expression and metabolite levels. Additionally, changes in one carbon gene expression are independent of the PGC-1α-mediated ER stress response previously reported in the LKO mice. Together, these results highlight the important role of oleate in maintaining one-carbon cycle homeostasis and point to observed changes in one-carbon metabolism as a novel mediator of the Scd1 deficiency-induced liver phenotype.

Keywords: Metabolomics; Nuclear magnetic resonance (NMR) spectroscopy; Oleate; One carbon cycle; Stearoyl-CoA desaturase.

Figure 1:. Hepatic SCD1 deficiency leads to altered one-carbon metabolite levels.

Lox and LKO mice were fed HSVLF with or without 20% stearate supplementation for 10 days. WT, GKO, GLS5, and GLS3 female mice were fed HSVLF for 10 days (n = 6–10/group). Metabolites were measured in WT, LKO, GKO, GLS5, and GLS3. Values are mean ± SEM, *p <0.05 vs. Lox, #p <0.05 vs. LKO, one-way ANOVA analysis.

Figure 2:. Hepatic gene and protein expressions of one carbon cycle are normalized in oleate-supplemented LD-fed LKO mice and in GLS5 and SCD3Tg mice.

Lox and LKO mice were fed HSVLF with or without 20% triolein supplementation for 10 days. WT, GKO, GLS5, and GLS3 female mice were fed HSVLF for 10 days (n = 6–10/group). Relative mRNA (A) and protein expression levels (B-C) of the indicated 1-carbon and methyltransferase genes and proteins were measured in WT, GKO, GLS5, and GLS3. Values are mean ± SEM, *p <0.05 vs. Lox, #p <0.05 vs. LKO, one-way ANOVA analysis.

Figure 3.. Changes in one carbon gene expression are independent of PGC-1α expression

Lox, LKO, and DLKO mice were fed HSVLF for 10 days, and relative hepatic expression of the folate and the indicated one-carbon genes were assessed (n = 4–6/group). Values are mean ± SEM, *p <0.05 vs. Lox, #p <0.05 vs SCD1 LKO; #p <0.05 vs DLKO one-way ANOVA analysis.

Figure 4.

Proposed scheme for SCD1-mediated regulation of one-carbon metabolism