Regulation of leucine catabolism by metabolic fuels in mammary epithelial cells

Abstract

Lactation is associated with elevated catabolism of branched-chain amino acids (BCAA) in mammary glands to produce glutamate, glutamine, alanine, aspartate, and asparagine. This study determined effects of metabolic fuels on the catabolism of leucine (a representative BCAA) in bovine mammary epithelial cells. Cells were incubated at 37 °C for 2 h in Krebs buffer containing 0.5 mM L-leucine and either L-[1-(14)C]leucine or L-[U-(14)C]leucine. The medium also contained 0-5 mM D-glucose, 0-2 mM L-glutamine, 0-4 mM DL-β-hydroxybutyrate, or 0-2 mM oleic acid. Rates of leucine decarboxylation were 60 % lower, but rates of α-ketoisocaproate production were 34 % higher, in the presence of 2 mM glucose than in its absence. All variables of leucine catabolism did not differ between 2 and 5 mM glucose or between 0 and 4 mM DL-β-hydroxybutyrate. Compared with 0-0.25 mM glutamine, 0.5 and 2 mM L-glutamine reduced leucine transport, transamination, and decarboxylation. In contrast, increasing the concentration of oleic acid from 0 to 2 mM dose-dependently stimulated leucine transamination, decarboxylation, and oxidation of carbons 2-6. Oleic acid also enhanced the abundance of cytosolic BCAA transaminase, while reducing the phosphorylated level (inactive state) of the E1α subunit of the mitochondrial branched-chain α-ketoacid dehydrogenase complex. Thus, hypoglycemia or ketosis in early lactation does not likely affect BCAA metabolism in mammary epithelial cells. Increasing circulating levels of BCAA and oleic acid may have great potential to increase the syntheses of glutamate, glutamine, aspartate, alanine, and asparagine by lactating mammary glands, thereby leading to enhanced production of milk for suckling neonates.

Acute alcohol exposure, acidemia or glutamine administration impacts amino acid homeostasis in ovine maternal and fetal plasma

“…Results of this study support the notions that glutamine is a functional amino acid for improving animal and human health (Wu 2010). Our findings are also consistent with the proposition that, under conditions (e.g., FAS in this study) when rates of utilization of amino acids are greater than rates of their endogenous synthesis, there should be a dietary requirement for glutamine and other traditionally classified “nonessential amino acids” to maintain whole-body homeostasis (e.g., acid-base balance) and adequate rates of tissue protein synthesis (Lei et al 2012; Rezaei et al 2012; Wu et al 2012; Yao et al 2012). …”

Impaired Mitochondrial Metabolism and Mammary Carcinogenesis

“…For example, branched chain amino acid metabolism plays an important role in mammary epithelial cell physiology during lactation. The branched chain amino acids (Leu, Ile, Val) are metabolized to produce glutamate, glutamine, aspartate, alanine, and asparagine to be secreted in milk [12,13]. …”

Novel metabolic and physiological functions of branched chain amino acids: a review

“…BCAA catabolism could enhance the syntheses of glutamate, glutamine, aspartate, alanine, and asparagine in the mammary gland and increase the production of milk for suckling neonates [11, 12]. In addition, the major leucine transporter LAT1 is a limiting factor for the synthesis of glutamate and aspartate in mammary tissue [67].…”