Molecular networks of insulin signaling and amino acid metabolism in subcutaneous adipose tissue are altered by body condition in periparturient Holstein cows

Abstract

Peripartal cows mobilize not only body fat but also body protein to satisfy their energy requirements. The objective of this study was to determine the effect of prepartum BCS on blood biomarkers related to energy and nitrogen metabolism, and mRNA and protein abundance associated with AA metabolism and insulin signaling in subcutaneous adipose tissue (SAT) in peripartal cows. Twenty-two multiparous Holstein cows were retrospectively classified into a high BCS (HBCS; n = 11, BCS ≥ 3.5) or normal BCS (NBCS; n = 11, BCS ≤ 3.17) group at d 28 before expected parturition. Cows were fed the same diet as a total mixed ration before parturition and were fed the same lactation diet postpartum. Blood samples collected at -10, 7, 15, and 30 d relative to parturition were used for analyses of biomarkers associated with energy and nitrogen metabolism. Biopsies of SAT harvested at -15, 7, and 30 d relative to parturition were used for mRNA (real time-PCR) and protein abundance (Western blotting) assays. Data were subjected to ANOVA using the MIXED procedure of SAS (v. 9.4; SAS Institute Inc., Cary, NC), with P ≤ 0.05 being the threshold for significance. Cows in HBCS had greater overall plasma nonesterified fatty acid concentrations, due to marked increases at 7 and 15 d postpartum. This response was similar (BCS × Day effect) to protein abundance of phosphorylated (p) protein kinase B (p-AKT), the insulin-induced glucose transporter (SLC2A4), and the sodium-coupled neutral AA transporter (SLC38A1). Abundance of these proteins was lower at -15 d compared with NBCS cows, and either increased (SLC2A4, SLC38A1) or did not change (p-AKT) at 7 d postpartum in HBCS. Unlike protein abundance, however, overall mRNA abundances of the high-affinity cationic (SLC7A1), proton-coupled (SLC36A1), and sodium-coupled amino acid transporters (SLC38A2) were greater in HBCS than NBCS cows, due to upregulation in the postpartum phase. Those responses were similar to protein abundance of p-mTOR, which increased (BCS × Day effect) at 7 d in HBCS compared with NBCS cows. mRNA abundance of argininosuccinate lyase (ASL) and arginase 1 (ARG1) also was greater overall in HBCS cows. Together, these responses suggested impaired insulin signaling, coupled with greater postpartum AA transport rate and urea cycle activity in SAT of HBCS cows. An in vitro study using adipocyte and macrophage cocultures stimulated with various concentrations of fatty acids could provide some insights into the role of immune cells in modulating adipose tissue immunometabolic status, including insulin resistance and AA metabolism.