Abstract
Choline and methionine serve essential roles in the liver that may interact with glucose metabolism. Our objectives were to quantify glucose export, cellular glycogen, and expression of genes controlling oxidation and gluconeogenesis in primary bovine neonatal hepatocytes exposed to increasing concentrations of choline chloride (CC) and D,L-methionine (DLM) with or without fatty acids (FA). Primary hepatocytes isolated from 3 Holstein calves were maintained as monolayer cultures for 24 h before treatment with CC (61, 128, 2028, 4528 μmol/L) and DLM (16, 30, 100, 300 μmol/L) with or without a 1 mmol/L FA cocktail in a factorial design. After 24 h, media was harvested to quantify glucose, β-hydroxybutyrate (BHB), and cells harvested to quantify glycogen, DNA, and gene expression. No interactions between CC and DLM were detected. The potential two-way interaction between CC or DLM and FA was partitioned into three contrasts when P ≤ 0.20: linear without FA, linear with FA, difference of slope. Fatty acids did not affect glucose or cellular glycogen but increased pyruvate carboxylase (PC), cytosolic and mitochondrial phosphoenolpyruvate carboxykinase (PEPCKc, PEPCKm), and glucose-6-phosphatase (G6PC) expression. Increasing CC decreased glucose but increased cellular glycogen. Expression of PC and PEPCKc was increased by CC during FA treatment. Increasing DLM did not affect metabolites or PC expression, although PEPCKc was marginally decreased. Methionine did not affect G6PC, while CC had a marginal quadratic effect on G6PC. Oxidative and gluconeogenic enzymes appear to respond to FA in primary bovine neonatal hepatocytes. Increased PC and PEPCKc by CC during FA treatment suggest increased gluconeogenic capacity. Changes in G6PC may have shifted glucose-6-phosphate towards cellular glycogen; however, subsequent examination of G6PC protein is needed. Unaltered PC and marginally decreased PEPCKc suggest increased oxidative capacity with DLM, although BHB export was unaltered. The differential regulation supports unique effects of CC and DLM within bovine hepatocytes.
Highlights
Hepatic metabolism responds to increased glucose demand at the onset of lactation by doubling gluconeogenesis [1]
The primary objectives of that experiment were to examine the regulation of genes controlling methyl group transfer and lipid metabolism, and quantify very low-density lipoproteins (VLDL) and reactive oxygen species (ROS) secretion in response to increasing concentrations of choline chloride (CC) and DLM in the absence and presence of fatty acids (FA) [21]
Relevant to the current discussion, we reported that increasing CC increased the export of VLDL and decreased ROS secretion, while DLM did not affect VLDL or ROS [21]
Summary
Hepatic metabolism responds to increased glucose demand at the onset of lactation by doubling gluconeogenesis [1]. Proportional contributions of lactate and amino acids to glucose carbon increase [2] as propionate, the primary gluconeogenic precursor, is limited by a voluntary reduction in feed intake around the time of calving [3,4]. Cows experience negative energy balance peripartum and respond by mobilizing fatty acids (FA) from adipose tissue. Two competing pathways, are determined by carbon availability in the TCA cycle. Excessive hepatic FA uptake and imbalance of oxidative pathway capacity likely contributes to the development of hyperketonemia and fatty liver, two common metabolic disorders that challenge the performance and health of dairy cows [6]
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