Increased autophagy mediates the adaptive mechanism of the mammary gland in dairy cows with hyperketonemia

Abstract

Hyperketonemia is a metabolic disease in dairy cows, associated with negative nutrition balance (NNB) induced by low dry matter intake (DMI) and increased nutrient requirements. Hyperketonemia could induce metabolic stress, which might indirectly affect mammary tissue. Autophagy is a highly conserved physiological process that results in the turnover of intracellular material, and is involved in maintaining cellular homeostasis under the challenge of metabolic stress induced by NNB. The aim of this study was to investigate the autophagy status and autophagy-related pathways AMP-activated kinase α (AMPKα) and mechanistic target of rapamycin (mTOR) in the mammary glands of dairy cows with hyperketonemia. Cows with hyperketonemia [CWH, n = 10, blood β-hydroxybutyrate (BHB) concentration 1.2 to 3.0 mmol/L] and cows without hyperketonemia (CWOH, n = 10, BHB < 1.2 mmol/L) from 3 to 12 DIM were randomly selected from the herd. The mammary tissue and blood samples were collected from these cows between 0630 and 0800 h, before feeding, at 3 to 12 d in milk. Serum concentrations of glucose, BHB, and fatty acids were determined using an autoanalyzer with commercial kits between 0630 and 0800 h, before feeding. Concentrations of fatty acids, BHB (median and interquartile range: CWH, 2.44 and 1.3, 2.82 mM; CWOH, 0.49 and 0.41, 0.57 mM), and milk fat were greater in CWH. The DMI, glucose concentration, milk production, and milk protein levels were lower in CWH. The mRNA abundance of autophagosome formation-related gene, beclin 1 (BECN1), phosphatidylinositol 3-kinase catalytic subunit type 3 (PIK3C3), autophagy-related gene (ATG) 5, ATG7, ATG12, microtubule-associated protein 1 light chain 3 (MAP1LC3, also called LC3) and sequestosome-1 (SQSTM1, also called p62) were greater in the mammary glands of CWH. The protein abundance of LC3-II and phosphorylation level of Unc-51-like kinase 1 (ULK1) were greater in CWH, but the total ubiquitinated proteins and protein abundance of p62 were lower. Transmission electron microscopy showed an increased number of autophagosomes in the mammary glands of CWH. Furthermore, the phosphorylation of AMPKα was greater, but the phosphorylation of mTOR was lower in the mammary glands of CWH. These results indicate that activity of mTOR pathways and autophagy activity, and upregulation of AMPKα, may be response mechanisms to mitigate metabolic stress induced by hyperketonemia in the mammary glands of dairy cows.