Aberrant protein palmitoylation promotes hepatic lipid accumulation and injury in dairy cows.

Abstract

Elevated circulating nonesterified fatty acids (NEFA) represent a key pathological feature in dairy cows with fatty liver. Palmitic acid (PA), a major component of NEFA, can be enzymatically attached to proteins via a reversible post-translational modification known as palmitoylation, which potently modulates protein activity and function. Studies have revealed that aberrant hepatic palmitoylation is a crucial mechanism promoting lipid accumulation in non-ruminants. Nevertheless, the extent and pathological relevance of hepatic protein palmitoylation in dairy cows with fatty liver have largely remained unexplored. Therefore, this study was conducted to determine the status of hepatic protein palmitoylation in dairy cows with fatty liver and to elucidate its functional role in the development of hepatic steatosis. Blood and liver samples were collected from 10 dairy cows with fatty liver (hepatic triglyceride [TG] content >5%) and 10 control cows (hepatic TG content <1%) that had a similar number of lactations (median: 3, range: 2 to 4) and days in milk (median: 9 d, range: 5 to 14 d). To determine the effects of NEFA on palmitoylation, hepatocytes isolated from calves were treated with 1.2 mM NEFA for 12 h. To investigate the effects of palmitoylation on lipid accumulation in bovine hepatocytes, the cells were treated with 1.2 mM NEFA for 12 h in the presence or absence of a palmitoylation inhibitor (2-bromohexadecanoic acid). The results revealed that dairy cows with fatty liver exhibited liver injury and elevated hepatic palmitoyl-CoA content. Moreover, fatty liver dairy cows showed higher hepatic mRNA abundance of ZDHHC4/5/14/20 and lower mRNA abundance of ZDHHC3/19/21/23/24. In contrast, the mRNA abundance of depalmitoylase-related genes, including lysophospholipase 1 (LYPLA1 and LYPLA2), palmitoyl-protein thioesterase 1 (PPT1 and PPT2) and abhydrolase domain containing 17 (ABHD17A, ABHD17B and ABHD17C), was lower in the liver of cows with fatty liver than in control cows. Consistently, a greater abundance of palmitoylated proteins was observed in the liver of dairy cows with fatty liver. In vitro, NEFA treatment induced lipid accumulation, cell injury, and aberrant protein palmitoylation in bovine hepatocytes. Additionally, the upregulation of palmitoyltransferases and downregulation of depalmitoylases observed in cows with fatty liver were recapitulated in NEFA-treated bovine hepatocytes. Importantly, pharmacological inhibition of palmitoylation significantly alleviated NEFA-induced lipid accumulation and cell damage in bovine hepatocytes. Overall, these findings establish protein palmitoylation as both a critical pathological mechanism and a promising therapeutic target for fatty liver in dairy cows.