The effects of chronic AMPK activation on hepatic triglyceride accumulation and glycerol 3-phosphate acyltransferase activity with high fat feeding - PubMed (original) (raw)

The effects of chronic AMPK activation on hepatic triglyceride accumulation and glycerol 3-phosphate acyltransferase activity with high fat feeding

Bradley S Henriksen et al. Diabetol Metab Syndr. 2013.

Abstract

Background: High fat feeding increases hepatic fat accumulation and is associated with hepatic insulin resistance. AMP Activated Protein Kinase (AMPK) is thought to inhibit lipid synthesis by the acute inhibition of glycerol-3-phosphate acyltransferase (GPAT) activity and transcriptional regulation via sterol regulatory element binding protein-1c (SREBP-1c).

Methods: The purpose of this study was to determine if chronic activation of AMPK prevented an increase in GPAT1 activity in rats fed a high fat diet. Rats were fed a control (C), or a high fat (HF) diet (60% fat) for 6 weeks and injected with saline or a daily aminoimidazole carboxamide ribnucleotide (AICAR) dose of 0.5 mg/g body weight.

Results: Chronic AMPK activation by AICAR injections resulted in a significant reduction in hepatic triglyceride accumulation in both the C and HF fed animals (C, 5.5±0.7; C+AICAR, 2.7 ±0.3; HF, 21.8±3.3; and HF+AICAR, 8.0±1.8 mg/g liver). HF feeding caused an increase in total GPAT and GPAT1 activity, which was not affected by chronic AMPK activation (GPAT1 activity vs. C, C+AICAR, 92±19%; HF, 186±43%; HF+AICAR, 234±62%). Markers of oxidative capacity, including citrate synthase activity and cytochrome c abundance, were not affected by chronic AICAR treatment. Interestingly, HF feeding caused a significant increase in long chain acyl-CoA dehydrogenase or LCAD (up 66% from C), a marker of fatty acid oxidation capacity.

Conclusions: These results suggest that chronic AMPK activation limits hepatic triglyceride accumulation independent of a reduction in total GPAT1 activity.

Keywords: AMPK; GPAT1; LCAD; SREBP-1c; mTOR.

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Figures

Figure 1

Figure 1

Phospho-AMP-activated protein kinase (pAMPK) content in the liver was increased with acute 5-aminoimidazole-4-carboxamide riboside (AICAR) treatment (n=5-7). Livers from AICAR-treated rats were removed 1 hr after injection. Asterisk (*) denotes a main effect of AICAR (p<0.05). Graph represents means ± SE.

Figure 2

Figure 2

Chronic activation of liver AMPK with daily injections of AICAR limited the normal increase in triglyceride accumulation that occurs with high fat feeding such that it was not significantly different from the control group (n = 7-12). Letters are used to represent significance; same letter means no significant difference (P < 0.05). Graph represents means ± SE.

Figure 3

Figure 3

Total mammalian target of rapamycin complex (mTOR) protein content of liver extracts revealed no significant differences with high fat feeding or chronic AMPK activation (n = 4-5). Bands for all 4 groups were taken side by side with no interruption. Graph represents means ± SE.

Figure 4

Figure 4

Phospho-raptor content was increased in livers treated with AICAR (n= 4-5). Asterisk (*) denotes a main effect of AICAR (p<0.05). Graph represents means ± SE.

Figure 5

Figure 5

Eukaryotic initiation factor 4E-binding protein (4EBP) following HF and AICAR treatments. a. Total Eukaryotic initiation factor 4E-binding protein (4EBP) results show an increase with the treatment of AICAR (n = 4-5). Bands for all 4 groups were taken side by side with no interruption Asterisk (*) denotes a main effect of AICAR treatment (P<0.05). Graph represents means ± SE. b Eukaryotic initiation factor 4E-binding protein (4EBP) phosphorylation (percentage of the total protein in the 2 hypophosphorylated bands compared to total) showed an increased phosphorylation with AICAR treatment (n = 4-5). Bands for all 4 groups were taken side by side with no interruption. Letters are used to represent significance; same letter means no significant difference (P < 0.05). Graph represents means ± SE.

Figure 6

Figure 6

Chronic activation of AMPK and sterol regulatory element binding protein-1c (SREBP-1c). a. Chronic activation of AMPK caused a reduction in the total abundance of uncleaved Sterol regulatory element binding protein-1c (SREBP-1c) in rats fed either chow or high fat diet (n = 3-5). Bands for all 4 groups were taken side by side with no interruption. Letters are used to represent significance. A significant main effect of AICAR was observed (p<0.05). Graph represents means ± SE. b. Chronic activation of AMPK caused a reduction in the total abundance of cleaved (65-68 kDa bands) SREBP-1c in the liver of rats fed either chow or high fat diet (n = 4-5). Bands for all 4 groups were taken side by side with no interruption. Letters are used to represent significance. A significant main effect of AICAR and high fat feeding was observed (p<0.05). Graph represents means ± SE.

Figure 7

Figure 7

Total acetyl coA carboxylase (ACC) content had a main effect of chronic AMPK activation (n = 7-10). High fat feeding blunted the decrease in total ACC content with the HF + AICAR group. Bands for all 4 groups were taken side by side with no interruption. Letters are used to represent significance. Asterisk (*) denotes a main effect of AICAR (P < 0.05). Graph represents means ± SE.

Figure 8

Figure 8

GPAT activity following high fat feeding and AICAR treatments. a. High fat feeding increased total glycerol-3-phosphate acyl-transferase (GPAT) activity (n = 5-8) in liver. A main AICAR effect on total GPAT activity was absent. Asterisk (*) denotes a main effect of high fat feeding (P < 0.05). Graph represents means ± SE. b. High fat feeding increased NEM-sensitive glycerol-3-phosphate acyl-transferase (GPAT1) activity in liver (n = 5-8). A main AICAR effect on total GPAT activity was absent. *Main treatment effect0020 (P < 0.05). Graph represents means ± SE.

Figure 9

Figure 9

Citrate synthase activity in the liver did not increase with either high fat feeding or chronic AMPK activation (n = 4-5). Graph represents means ± SE.

Figure 10

Figure 10

Long chain acyl CoA dehydrogenase (LCAD) following high fat feeding and AICAR treatments. a. There was a main high fat effect on total long chain acyl CoA dehydrogenase (LCAD) content in the liver (n = 8-10). Asterisk (*) indicates a main effect of high fat diet (P < 0.05). Graph represents means ± SE. b. Chronic activation of AMPK did not have an effect on total long chain acyl CoA dehydrogenase (LCAD) content in the liver (n = 4-5). Bands for all 4 groups were taken side by side with no interruption. Letters are used to represent significance; same letter means no significant difference (P < 0.05). Graph represents means ± SE.

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