Abstract
Simple SummaryNegative energy balance can lead to the mobilization of large amounts of body fat. A large amount of non-esterified fatty acids (NEFA) produced by lipolysis not only can be used for energy generation by β-oxidation in the liver but can also act as a potential regulator of lipid metabolism. The present study provides evidence that NEFA can activate hepatocyte autophagy through the protein kinase R-like endoplasmic reticulum kinase (PERK) signaling pathway. Autophagy has been reported to contribute to lipid metabolism through promoting the breakdown of intracellular lipids. These findings enable a better understanding of the redistribution and utilization of energy during the perinatal period of dairy cows.During the perinatal period, the abnormally high plasma non-esterified fatty acids (NEFA) concentration caused by the negative energy balance (NEB) can impose a significant metabolic stress on the liver of dairy cows. Endoplasmic reticulum (ER) stress is an important adaptive response that can serve to maintain cell homeostasis in the event of stress. The protein kinase R-like endoplasmic reticulum kinase (PERK) pathway is the most rapidly activated cascade when ER stress occurs in cells and has an important impact on the regulation of hepatic lipid metabolism and autophagy modulation. However, it is unknown whether NEFA can affect autophagy through modulating the PERK pathway, under NEB conditions. In this study, we provide evidence that NEFA treatment markedly increased lipid accumulation, the phosphorylation level of PERK and eukaryotic initiation factor 2α (eIF2α), and the expression of glucose-regulated protein 78 (Grp78), activating transcription factor 4 (ATF4), and C/EBP homologous protein (CHOP). More importantly, NEFA treatment can cause a substantial increase in the protein levels of autophagy-related gene 7 (ATG7), Beclin-1 (BECN1), sequestosome-1 (p62), and microtubule-associated protein 1 light chain 3 (LC3)-II, and in the number of autophagosomes in primary bovine hepatocytes. The addition of GSK2656157 (PERK phosphorylation inhibitor) can significantly inhibit the effect of NEFA on autophagy and can further increase lipid accumulation. Overall, our results indicate that NEFA could promote autophagy via the PERK pathway in bovine hepatocytes. These findings provide novel evidence about the potential role of the PERK signaling pathway in maintaining bovine hepatocyte homeostasis.
Highlights
During the perinatal period, dairy cows have been found to display a negative energy balance (NEB) [1]
Nilered redstaining stainingofof primary bovine hepatocytes treated with indicated concentrations of esterified fatty acids (NEFA) or GSK2656157
We found that compared to the control group, there were relatively large numbers of autophagosomes in the hepatocytes treated with 1.2 mM non-esterified fatty acids (NEFA), and the endoplasmic reticulum (ER) was dilated in shape
Summary
Dairy cows have been found to display a negative energy balance (NEB) [1]. The high level of non-esterified fatty acids (NEFA) in the plasma induced by NEB can serve as an important trigger of fatty liver and ketosis in dairy cows [2]. As one of the three branches that can constitute the UPR, the protein kinase RNA-like ER kinase (PERK) pathway is the most rapidly activated cascade during this process [9]. This pathway can significantly inhibit protein translation through affecting the phosphorylated eukaryotic initiation factor 2α (eIF2α) but can stimulate the ER-associated degradation (ERAD) pathway by upregulating activating transcription factor 4 (ATF4) expression [10,11]. As a transcription factor, ATF4 could actively participate in the activation of the autophagy signaling pathway to maintain hepatocyte homeostasis [12]
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