Physiological roles and regulation of hepatic angiopoietin-like protein 3 in Japanese Black cattle (Bos taurus) during the fattening period.

Angiopoietin-like protein 3 (ANGPTL3) is expressed predominantly in the liver and plays a major role in regulating the circulating triglyceride and lipoprotein fraction concentrations by inhibiting lipoprotein lipase (LPL) activity. Given these physiological roles, ANGPTL3 may play an important role in metabolic changes related to fat accumulation during the fattening period in Japanese Black. This study aimed to reveal the physiological roles of hepatic ANGPTL3 in Japanese Black steers (Bos taurus) during the fattening period and investigate the regulatory effects of hepatic ANGPTL3. To investigate the gene expression and protein localization of ANGPTL3, 18 tissue samples were collected from tree male Holstein bull calves aged 7 weeks. Biopsied liver tissues and blood samples were collected from 21 Japanese Black steers during the early (T1; 13 months of age), middle (T2; 20 months), and late fattening phases (T3; 28 months). Relative mRNA expression, blood metabolite concentrations, hormone concentrations, growth, and carcass traits were analyzed. To identify the regulatory factors of hepatic ANGPTL3, primary bovine hepatocytes collected by 2 Holstein calves aged 7 weeks were incubated with insulin, palmitate, oleate, propionate, acetate, or beta-hydroxybutyric acid (BHBA). The ANGPTL3 gene was most highly expressed in the liver, with minor expression in the renal cortex, lungs, reticulum, and jejunum in Holstein bull calves. In Japanese Black steers, relative ANGPTL3 mRNA expressions were less as fattening progressed, and blood triglyceride, total cholesterol, and non-esterified fatty acid (NEFA) concentrations increased. Relative ANGPTL8 and Liver X receptor alpha (LXRα) mRNA expressions decreased in late and middle fattening phases, respectively. Furthermore, relative ANGTPL3 mRNA expression was positively correlated with ANGPTL8 (r = 0.650; P < 0.01) and ANGPTL4 (r = 0.540; P < 0.05) in T3 and T1, respectively, and LXRα showed no correlation with ANGPTL3. Relative ANGTPL3 mRNA expression was negatively correlated with total cholesterol (r = -0.434; P < 0.05) and triglyceride (r = -0.645; P < 0.01) concentrations in T3 and T1, respectively; There was no significant correlation between ANGTPL3 and carcass traits. Relative ANGTPL3 mRNA expression in cultured bovine hepatocytes was downregulated in oleate treatment. Together, these findings suggest that ANGPTL3 downregulation in late fattening phases is associated with the changes in lipid metabolism.