High circulating concentrations of
fatty acids cause
triacylglycerol (TAG) accumulation in hepatocytes of dairy cows, a common metabolic disorder after calving. Low secretion of
apolipoprotein B (
APOB) and
very low density lipoprotein (VLDL) are thought to be the major factors for TAG accumulation in hepatocytes. Recent data in nonruminant models revealed that
sortilin 1 (SORT1) is a key regulator of VLDL secretion in part due to its ability to bind
APOB. Thus, SORT1 could play a role in the susceptibility of dairy cows to develop
fatty liver. To gain mechanistic insights in vivo and in vitro, we performed experiments using liver biopsies or isolated primary hepatocytes. For the in vivo study, blood and liver samples were collected from healthy multiparous dairy cows (n = 6; 9.0 ± 2.1 d in milk) and cows with
fatty liver (n = 6; 9.7 ± 2.2 d in milk). In vitro, hepatocytes isolated from 4 healthy female calves (1 d old, 42-51 kg) were challenged with (
fatty acids) or without (control) a 1.2 mM mixture of
fatty acids in an attempt to induce metabolic stress. Furthermore, hepatocytes were treated with empty adenovirus vectors (Ad-GFP) or SORT1 overexpressing adenovirus (Ad-SORT1) for 6 h, or SORT1 inhibitor for 2 h followed by a challenge with (Ad-GFP +
fatty acids, Ad-SORT1 +
fatty acids, or SORT1 inhibitor +
fatty acids) or without (Ad-GFP, Ad-SORT1, or SORT1 inhibitor) the 1.2 mM mixture of
fatty acids for 12 h. Data from liver biopsies were compared using a 2-tailed unpaired Student's t-test. Data from calf hepatocytes were analyzed by one-way ANOVA. Data revealed that both
fatty liver and in vitro challenge with
fatty acids were associated with greater concentrations of TAG and
mRNA and
protein abundance of SORT1, SREBF1, FASN, and ACACA. In contrast,
mRNA and
protein abundance of CPT1A and
APOB, and
mRNA abundance of
MTTP were markedly lower. Compared with
fatty acid challenge alone, SORT1 overexpression led to greater concentration of TAG and
mRNA abundance of SREBF1, FASN, ACACA, DGAT1, and DGAT2, and
protein abundance of SREBF1, FASN, and ACACA. In contrast, concentration of secreted VLDL-
APOB and
mRNA abundance of
APOB and
MTTP, and
protein abundance of CPT1A,
APOB, and
MTTP were lower. Compared with
fatty acid challenge alone, SORT1 inhibitor +
fatty acids led to lower concentrations of TAG and
mRNA abundance of SREBF1, FASN, and DGAT2, and
protein abundance of FASN, ACACA, and DGAT1. Concentrations of secreted VLDL-
APOB and
mRNA abundance of CPT1A and
protein abundance of CPT1A and
APOB were greater. Overall, in vitro data suggested that greater SORT1 abundance induced by exogenous
fatty acids caused a reduction in VLDL-
APOB secretion and increased hepatocyte TAG synthesis. Such mechanism was also apparent in tissue from cows with
fatty liver. Thus, targeted downregulation of hepatic SORT1 could represent a viable mechanism to unload
lipid during conditions where the influx of
fatty acids increases markedly.