High Dietary Selenium Intake Alters Lipid Metabolism and Protein Synthesis in Liver and Muscle of Pigs

Abstract

Background: Prolonged high intakes of dietary selenium have been shown to induce gestational diabetes in rats and hyperinsulinemia in pigs.

Objective: Two experiments were conducted to explore metabolic and molecular mechanisms for the diabetogenic potential of high dietary selenium intakes in pigs.

Methods: In Expt. 1, 16 Yorkshire-Landrace-Hampshire crossbred pigs (3 wk old, body weight = 7.5 ± 0.81 kg, 50% males and 50% females) were fed a corn-soybean meal basal diet supplemented with 0.3 or 1.0 mg Se/kg (as selenium-enriched yeast for 6 wk). In Expt. 2, 12 pigs of the same crossbreed (6 wk old, body weight = 16.0 ± 1.8 kg) were fed a similar basal diet supplemented with 0.3 or 3.0 mg Se/kg for 11 wk. Biochemical and gene and protein expression profiles of lipid and protein metabolism and selenoproteins in plasma, liver, muscle, and adipose tissues were analyzed.

Results: In Expt. 1, the 1-mg-Se/kg diet did not affect body weight or plasma concentrations of glucose and nonesterified fatty acids. In Expt. 2, the 3-mg-Se/kg diet, compared with the 0.3-mg-Se/kg diet, increased (P < 0.05) concentrations of plasma insulin (0.2 compared with 0.4 ng/mL), liver and adipose lipids (41% to 2.4-fold), and liver and muscle protein (10-14%). In liver, the 3-mg-Se/kg diet upregulated (P < 0.05) the expression, activity, or both of key factors related to gluconeogenesis [phosphoenolpyruvate carboxykinase (PEPCK); 13%], lipogenesis [sterol regulatory element binding protein 1 (SREBP1), acetyl-coenzyme A carboxylase (ACC), and fatty acid synthase (FASN); 46-90%], protein synthesis [insulin receptor (INSR), P70 ribosomal protein S6 kinase (P70), and phosphorylated ribosomal protein S6 (P-S6); 88-105%], energy metabolism [AMP-activated protein kinase (AMPK); up to 2.8-fold], and selenoprotein glutathione peroxidase 3 (GPX3; 1.4-fold) and suppressed (P < 0.05) mRNA levels of lipolysis gene cytochrome P450, family 7, subfamily A, polypeptide 1 (CYP7A1; 88%) and selenoprotein gene selenoprotein W1 (SEPW1; 46%). In muscle, the 3-mg-Se/kg diet exerted no effect on the lipid profiles but enhanced (P < 0.05) expression of P-S6 and mammalian target of rapamycin (mTOR; 42-176%; protein synthesis); selenoprotein P (SELP; 40-fold); and tumor suppressor protein 53 (P53) and peroxisome proliferator-activated receptor γ (PPARG; 52-58%; lipogenesis) and suppressed (P < 0.05) expression of INSR (59%; insulin signaling); selenoprotein S (SELS); deiodinases, iodothyronine, type I (DIO1); and thioredoxin reductase 1 (TXNRD1; 50%; selenoproteins); and ACC1 and FASN (35-51%; lipogenesis).

Conclusion: Our research showed novel roles, to our best knowledge, and mechanisms of high selenium intakes in regulating the metabolism of protein, along with that of lipid, in a tissue-specific fashion in pigs.

Keywords: AMPK; Se; lipid; protein; selenoprotein.

FIGURE 1

Adipose tissue concentrations of TC (A) and TGs (B) and liver concentrations of TC (C), TGs (D), and NEFAs (E) in pigs fed 0.3 or 3.0 mg Se/kg for 11 wk (Expt. 2). Values are means ± SEs, n = 6. *Different from control, P < 0.05. Control = 0.3 mg Se/kg; high Se = 3.0 mg Se/kg. NEFA, nonesterified fatty acid; TC, total cholesterol.

FIGURE 2

Relative mRNA levels of selenoproteins in the liver (A) and muscle (B) and molecules of insulin signaling, lipid synthesis, and lipid hydrolysis pathways in the liver (C) and muscle (D) of pigs fed 0.3 or 3.0 mg Se/kg for 11 wk (Expt. 2). Values are means ± SEs, n = 6. Means without a common letter differ, P < 0.05. Control = 0.3 mg Se/kg; high Se = 3.0 mg Se/kg. ACC1, acetyl-CoA carboxylase 1; AKT2, v-akt murine thymoma viral oncogene homolog 2; CYP7A1, cytochrome P450, family 7, subfamily A, polypeptide 1; DIO1, deiodinases, iodothyronine, type I; DIO3, deiodinases, iodothyronine, type III; FASN, fatty acid synthase; GK, glucokinase; GPX, glutathione peroxidase; INSR, insulin receptor; IRS1, insulin receptor substrate 1; P53, tumor suppressor protein 53; PPARG, peroxisome proliferator–activated receptor γ; SELH, selenoprotein H; SELS, selenoprotein S; SELT, selenoprotein T; SEPN1, selenoprotein N; SEPP1, selenoprotein P; SEPW1, selenoprotein W; SREBP, sterol regulatory element binding protein; TXNRD1, thioredoxin reductase 1.

FIGURE 3

Relative protein concentrations of liver GPX3, INSR, P-S6, P70, P-AMPKα, AMPKα, P-AMPKβ, P-ACC, and ACC (A) and muscle SELP, INSR, P-S6, P-ACC, and mTOR (B) in pigs fed 0.3 or 3.0 mg Se/kg for 11 wk (Expt. 2). Values below the protein band were relative densities and are expressed as means ± SEs, n = 4–6. Means without a common letter differ, P < 0.05. Control = 0.3 mg Se/kg; high Se = 3.0 mg Se/kg. ACC, acetyl-CoA carboxylase; AMPKα, AMP-activated protein kinase α; AMPKβ, AMP-activated protein kinase β; GPX3, glutathione peroxidase 3; INSR, insulin receptor; mTOR, mammalian target of rapamycin; P70, P70 ribosomal protein S6 kinase; P-ACC, phosphorylated acetyl-CoA carboxylase; P-AMPKα, phosphorylated AMP-activated protein kinase α; P-AMPKβ, phosphorylated AMP-activated protein kinase β; P-S6, phosphorylated ribosomal protein S6; SELP, selenoprotein P; S6, ribosomal protein S6.

FIGURE 4

GPX activities in muscle (A) and liver (B), FASN activities in muscle (C) and liver (D), and PEPCK activities in liver (E) of pigs fed 0.3 or 3.0 mg Se/kg for 11 wk (Expt. 2). The GPX activity unit is defined as 1 μmol NAD(P)H used/min. The FASN activity unit is defined as 1 mmol NAD(P)H used/min. Values are means ± SEs, n = 6. *Different from control, P < 0.05. Control = 0.3 mg Se/kg; high Se = 3.0 mg Se/kg. FASN, fatty acid synthase; GPX, glutathione peroxidase; PEPCK, phosphoenolpyruvate carboxykinase.

FIGURE 5

Scheme of postulated regulatory pathways and mechanisms for the effects of feeding a high-selenium diet on lipid and protein metabolism in tissues of pigs. The solid lines represent data-supported pathways, whereas the dashed lines indicate potential, unapproved pathways. CYP7A1, cytochrome P450, family 7, subfamily A, polypeptide 1; DIO1, deiodinases, iodothyronine, type I; FASN, fatty acid synthase; GPX, glutathione peroxidase; INSR, insulin receptor; mTOR, mammalian target of rapamycin; NEFA, nonesterified fatty acid; P53, tumor suppressor protein 53; P70, P70 ribosomal protein S6 kinase; P-ACC, phosphorylated acetyl-CoA carboxylase; P-AMPK, phosphorylated AMP-activated protein kinase; P-S6, phosphorylated ribosomal protein S6; PEPCK_ase, phosphoenolpyruvate carboxykinase activity; PPARG, peroxisome proliferator–activated receptor γ; SELP, selenoprotein P; SELS, selenoprotein S; SEPW1, selenoprotein W; SREBP1, sterol regulatory element binding protein 1; TC, total cholesterol; TXNRD1, thioredoxin reductase 1.