PGC-1α in exercise and fasting-induced regulation of hepatic UPR in mice

Abstract

The aim of the present study was to test the hypothesis that PGC-1α is involved in the regulation of hepatic UPR and autophagy in response to both exercise and fasting in mice. Liver-specific PGC-1α knockout (LKO) mice and their floxed littermates (lox/lox) were used in two experimental parts. Liver and plasma were obtained from (1) fed and 18 h fasted mice and (2) immediately after, 2, 6, and 10 h after 1-h treadmill running as well as from resting mice, where one resting group was euthanized at time points corresponding to 0 and 2 h and another corresponding to 6 and 10 h of recovery. Hepatic eIF2α phosphorylation and sXBP1 mRNA content increased immediately after exercise and IRE1α phosphorylation as well as cleaved ATF6 protein content was higher 2 h into recovery than at rest in both genotypes. Fasting reduced hepatic IRE1α phosphorylation and protein content as well as PERK protein and sXBP1 mRNA content similarly in lox/lox and LKO mice. In addition, the hepatic LC3II/LC3I protein ratio increased immediately after exercise and with fasting in both genotypes, while fasting decreased p62 protein content in lox/lox mice. Liver-specific PGC-1α knockout did not affect these responses, but the LC3II/LC3I protein ratio was higher in LKO than lox/lox mice in both rest groups. In conclusion, the present study provides evidence for pathway-specific exercise-induced activation and fasting-induced downregulation of the UPR as well as exercise and fasting-induced regulation of autophagy in mouse liver. In addition, overall PGC-1α does not seem to be required for the fasting and exercise-induced regulation of UPR and autophagy, but may be involved in regulating basal hepatic autophagy.

Keywords: Exercise; Fasting; Liver; PGC-1α; UPR.

Fig. 1

Representative western blots from the exercise (a) and fasting (b) study. PEPCK, phosphoenolpyruvate carboxykinase; G6Pase, glucose-6-phosphatase; FAS, fatty acid synthase; BiP, binding immunoglobulin protein; ATF6, activating transcription factor 6; IRE1α, inositol-requiring enzyme 1 alpha; PERK, protein kinase-like ER-kinase; eIF2α, eukaryotic translation initiation factor 2 alpha; LC3, microtubule-associated protein 1 light chain 3. In addition, representative western blots of OXPHOS in resting mice (c)

Fig. 2

Hepatic glycogen (a), triglyceride (b), and PEPCK protein (c) content in liver-specific PGC-1α knockout (LKO) and littermate floxed (lox/lox) mice at rest and 0, 2, 6, and 10 h into recovery from 1-h treadmill running. Resting mice were either euthanized at time points corresponding to 0 and 2 h (rest a.m.) or 6 and 10 h (rest p.m.). Protein is given in arbitrary units (AU). Values are presented as means ± standard error (SE); n = 8–10. A single asterisk is significantly different from rest within given genotype, p < 0.05. A single dagger is significantly different from rest a.m. within given genotype, p < 0.05. Horizontal line indicates a main effect, p < 0.05

Fig. 3

Hepatic BiP protein (a), cleaved ATF6 protein (b), IRE1α^Ser924 phosphorylation (c), IRE1α protein (d), PERK^Thr980 phosphorylation (e), PERK protein (f), eIF2α^Ser51 phosphorylation (g), and eIF2α protein (h) content in liver-specific PGC-1α knockout (LKO) and littermate floxed (lox/lox) mice at rest and 0, 2, 6, and 10 h into recovery from 1-h treadmill running. Resting mice were either euthanized at time points corresponding to 0 and 2 h (rest a.m.) or 6 and 10 h (rest p.m.). Protein content and phosphorylation are given in arbitrary units (AU). Values are presented as means ± standard error (SE); n = 8–10. A single asterisk is significantly different from rest within given genotype, p < 0.05. A single dagger is significantly different from rest a.m. within given genotype, p < 0.05. Horizontal line indicates a main effect, p < 0.05

Fig. 4

Hepatic sXBP1 (a) and HSP72 (b) mRNA content in liver-specific PGC-1α knockout (LKO) and littermate floxed (lox/lox) mice and PGC-1α mRNA in lox/lox mice (c) at rest and 0, 2, 6, and 10 h into recovery from 1-h treadmill running. Resting mice were either euthanized at time points corresponding to 0 and 2 h (rest a.m.) or 6 and 10 h (rest p.m.). The target mRNA is normalized to single stranded (ss) DNA. Values are presented as means ± standard error (SE); n = 6–9. A single asterisk is significantly different from rest within given genotype, p < 0.05. A single dagger is significantly different from rest a.m. within given genotype, p < 0.05

Fig. 5

Hepatic LC3I (a), LC3II (b), LC3II/LC3I protein ratio (c) and p62 protein (d) in liver-specific PGC-1α knockout (LKO) and littermate floxed (lox/lox) mice at rest and 0, 2, 6, and 10 h into recovery from 1-h treadmill running. Resting mice were either euthanized at time points corresponding to 0 and 2 h (rest a.m.) or 6 and 10 h (rest p.m.). Protein content is given in arbitrary units (AU). Values are presented as means ± standard error (SE); n = 8–10. A single asterisk is significantly different from rest within given genotype, p < 0.05. A single dagger is significantly different from rest a.m. within given genotype, p < 0.05. A single number sign is significantly different from lox/lox within given group, p < 0.05. Horizontal line indicates a main effect, p < 0.05

Fig. 6

Hepatic glycogen (a), triglyceride (b), and PEPCK protein (c) content in fed (Fed) and 18 h fasted (Fast) liver-specific PGC-1α knockout (LKO), and littermate floxed (lox/lox) mice. Protein is given in arbitrary units (AU). Values are presented as means ± standard error (SE); n = 7–8. A single asterisk is significantly different from Fed within given genotype, p < 0.05

Fig. 7

Hepatic BiP protein (a), cleaved ATF6 protein (b), IRE1α^Ser724 phosphorylation (c), IRE1α protein (d), PERK^Thr980 phosphorylation (e), PERK protein (f), eIF2α^Ser51 phosphorylation (g), and eIF2α protein (h) content in fed (Fed) and 18 h fasted (Fast) liver-specific PGC-1α knockout (LKO) and littermate floxed (lox/lox) mice. Protein is given in arbitrary units (AU). Values are presented as means ± standard error (SE); n = 7–8. A single asterisk is significantly different from Fed within given genotype, p < 0.05. A single number sign is significantly different from lox/lox within given group, p < 0.05

Fig. 8

Hepatic sXBP1 (a) and HSP72 (b) mRNA content in fed (Fed) and 18 h fasted (Fast) liver-specific PGC-1α knockout (LKO) and littermate floxed (lox/lox) mice and PGC-1α mRNA in lox/lox mice (c). The target mRNA is normalized to single stranded (ss) DNA. Values are presented as means ± standard error (SE); n = 6–8. A single asterisk is significantly different from Fed within given genotype, p < 0.05

Fig. 9

Hepatic LC3I protein (a), LC3II protein (b), LC3II/LC3I (c) and p62 protein (d) protein ratio in fed (Fed) and 18 h fasted (Fast) liver-specific PGC-1α knockout (LKO) and littermate floxed (lox/lox) mice. Protein content is given in arbitrary units (AU). Values are presented as means ± standard error (SE); n = 7–8. A single asterisk is significantly different from Fed within given genotype, p < 0.05. A single number sign significantly different from lox/lox within given group, p < 0.05