Knockout of SOD1 alters murine hepatic glycolysis, gluconeogenesis, and lipogenesis

Abstract

We previously observed a stronger effect of knockout of Cu,Zn-superoxide dismutase (SOD1) than that of Se-dependent glutathione peroxidase 1 (GPX1) on murine body weight and glucose homeostasis. Two experiments were conducted to determine how hepatic lipid profiles and key metabolic regulators were correlated with this difference. SOD1(-/-) and GPX1(-/-) mice and their respective wild-type (WT) littermates (n=6 or 7/group, male) were fed a Se-adequate Torula yeast-sucrose diet and killed at 6 months of age to collect liver samples. In Experiment 1, fasted SOD1(-/-) mice displayed pyruvate intolerance and a 61% decrease (P<0.05) in liver glycogen compared with their WT littermates. The former had lower (P<0.05) activities of phosphoenolpyruvate carboxykinase, total protein phosphatase, and protein phosphatase 2A, but a higher (P<0.05) activity of glucokinase in the liver than the latter. In contrast, hepatic concentrations of total cholesterol, triglycerides, and nonesterified fatty acids were increased by 11 to 100% (P<0.05) in the SOD1(-/-) mice. Meanwhile, these mice had elevated (P<0.05) hepatic protein levels of sterol-regulatory element binding proteins 1 and 2, p53 MAPK, total and phosphorylated AMP-activated protein kinase α1 protein, protein tyrosine phosphatase 1B, and protein phosphatase 2B. In Experiment 2, GPX1(-/-) mice and their WT littermates were compared, but showed no difference in any of the measures. In conclusion, knockout of SOD1, but not GPX1, led to a decreased liver glycogen storage synchronized with pyruvate intolerance and elevated hepatic lipid profiles in adult mice. This striking comparison was possibly due to unique impacts of these two knockouts on intracellular tone of H(2)O(2) and key regulators of liver gluconeogenesis, glycolysis, and lipogenesis.

Fig.1

Effects of SOD1 and GPX1 knockouts on mouse (6 months of age) pyruvate tolerance (A and B, 2 g/kg of body weight, fasted for 18 h) and intrahepatic glycogen content (C and D, fasted for 8 h). Values are means ± SE (n = 6–7). The asterisks* indicate genotype differences (P ≤ 0.05).

Fig.2

Effects of SOD1 and GPX1 knockouts on mouse (6 months of age, fasted for 8 h) hepatic PEPCK (A and B) and GK (C and D) activities. Values are means ± SE (n = 6–7). The asterisks* indicate genotype differences (P ≤ 0.05).

Fig.3

Effects of SOD1 and GPX1 knockouts on mouse (6 months of age, fasted for 8 h) hepatic total cholesterol (TC, A and B), total triglycerides (TG, C and D), and non-esterified fatty acid (NEFA, E and F). Values are means ± SE (n = 6–7). The asterisks* indicate genotype differences (P ≤ 0.05).

Fig.4

Effects of SOD1 and GPX1 knockouts on mouse (6 months of age, fasted for 8 h) hepatic total proteins of SREBP1, SREBP2, and p53. For a given protein, the upper band shows the representative images of three independent analyses (n = 3), and the lower value shows the relative density of protein bands (mean ± SE, n = 9). The superscript letters a and b indicate genotype differences (P ≤ 0.05).

Fig.5

Effects of SOD1 and GPX1 knockouts on mouse (6 months of age, fasted for 8 h) hepatic AMPK proteins. For a given protein, the upper band shows the representative images of three independent analyses (n = 3), and the lower value shows the relative density of protein bands (mean ± SE, n = 9). The superscript letters a and b indicate genotype differences (P ≤ 0.05).

Fig.6

Effects of SOD1 and GPX1 knockouts on mouse (6 months of age, fasted for 8 h) hepatic total activities of protein phosphatase, PP2A and GPX1 (A and B) and hepatic protein amount of PTP1B, PP2B and JNK2 (C). In panels A and B, values are means ± SE (n = 6–7). The asterisks* indicate genotype differences (P ≤ 0.05). In panel C for a given protein, the upper band shows the representative images of three independent analyses (n = 3), and the lower value shows the relative density of protein bands (mean ± SE, n = 9). The superscript letters a and b indicate genotype differences (P ≤ 0.05).

Fig.7

Schematic illustration of potential mechanisms of SOD1 knockout leading to the metabolic phenotype. →, Activation or increase; ⊥, Inhibition or decrease;⤍, Possibly affect;, Direct targets of H₂O₂; AMPKα1, AMP-activated protein kinase α1; c-fos, a component of transcription factor activator protein 1; GK, glucokinase; H₂O₂, hydrogen peroxide; JNK2, c-jun NH2 -terminal kinase; NEFA, non-esterified fatty acid; P, phosphorylation; p53, p53 mitogenactivated protein kinases; PEPCK, phosphoenolpyruvate carbooxykinase; PP2A, protein phosphatase 2A; PP2B, protein phosphatase 2B; PTP-1B, protein tyrosine phosphatase-1B; PTT, pyruvate tolerance test; SOD1, Cu, Zn-superoxide dismutase; −/−, knockout; SREBP, sterolregulatory binding protein; TC, total cholesterol; TG, total triglycerides.