Metformin regulates lipid metabolism in a canine model of atrial fibrillation through AMPK/PPAR-α/VLCAD pathway

Abstract

Background: Atrial lipid metabolic remodeling is critical for the process of atrial fibrillation (AF). Abnormal Fatty acid (FA) metabolism in cardiomyocytes is involved in the pathogenesis of AF. MET (Metformin), an AMPK (AMP-activated protein kinase) activator, has been found to be associated with a decreased risk of AF in patients with type 2 diabetes. However, the specific mechanism remains unknown.

Methods: Fifteen mongrel dogs were divided into three groups: SR, ARP (pacing with 800 beats/min for 6 h), ARP plus MET (treated with MET (100 mg/kg/day) for two weeks before pacing). We assessed metabolic factors, speed limiting enzymes circulating biochemical metabolites (substrates and products), atrial electrophysiology and accumulation of lipid droplets.

Results: The expression of AMPK increased in the ARP group and significantly increased in the MET+ARP group comparing to the SR group. In the ARP group, the expressions of PPARα、PGC-1α and VLCAD were down-regulated, while the concentration of free fatty acid and triglyceride and the lipid deposition in LAA (left atrial appendage) increased. Moreover, AERP and AERPd have also been found abnormally in this process. Pretreatment with MET before receiving ARP reversed the alterations aforementioned.

Conclusions: The FA metabolism in LAA is altered in the ARP group, mainly characterized by the abnormal expression of the rate-limiting enzyme. Metformin reduces lipid accumulation and promotes β-oxidation of FA in AF models partially through AMPK/PPAR-α/VLCAD pathway. Our study indicates that MET may inhibit the FA lipid metabolic remodeling in AF.

Keywords: AMPK; Atrial fibrillation; Lipid metabolism; Metformin.

Fig. 1

Metformin decreases ARP-induced lipid accumulation in LAA. a, b Extract LAA lipids as shown in Materials and Methods, and detect the triglyceride and free fatty acid contents. Data are shown as mean ± SEM. c Quantitative analysis of oil red O positive area. d Representative images of oil red O staining for lipids (red color, × 40). SR, sinus rhythm; ARP, atrial rapid pacing; MET, metformin. *P <0.05 versus SR group; ^#P <0.05 versus ARP group; n = 5 per group. (The arrow points to the lipid droplets which stained with red color)

Fig. 2

Alterations in the protein expression of key metabolic factors in the left atrial appendage. a Representative images of the protein expression of AMPK, pAMPK, FAT/CD36, PPAR-α, PCG-1α, and VLCAD in the left atrial appendage of canines. b Quantitative analysis of the expression of AMPK, pAMPK, PPAR-α, PCG-1α, FAT/CD36 and VLCAD proteins in the left atrial appendage. Data are shown as mean ± SEM. SR, sinus rhythm; ARP, atrial rapid pacing; MET, metformin; AMPK, adenosine 5′-monophosphate (AMP)-activated protein kinase; pAMPK, phosphorylatied adenosine 5′-monophosphate (AMP)-activated protein kinase; FAT/CD36, fatty acid translocase; PGC-1α, peroxisome proliferator-activated receptor-gamma coactivator 1α; PPAR-α, peroxisome proliferator-activated receptor; VLCAD, Very long-chain specific acyl-CoA dehydrogenase.*P < 0.05 versus SR group; ^#P < 0.05 versus ARP group; n = 5 per group

Fig. 3

Alterations in the mRNA expression levels of key metabolic factors. Four lipid oxidation gene expression were examined by the RT-qPCR. a-c shown, PPAR-α, carnitine palmitoyl transferase (CPT)-1 and VLCAD were down-regulated in ARP group, but metformin attenuate those alterations. d shown, transcription level of ACC was decreased in ARP group and significantly decreased in MET+ARP group

Fig. 4

Alteration of electrophysiological parameters after ARP and the effect of MET. Alteration of AERP (A) and AERPd (B) after 6-h ARP or MET treatment. *P <0.05 vs. SR group; ^#P <0.05 vs. ARP group. ARP, atrial rapid pace; MET, metformin