Production of free glutamate in milk requires the leucine transporter LAT1

Abstract

The concentration of free glutamate (Glu) in rat's milk is ∼10 times higher than that in plasma. Previous work has shown that mammary tissue actively transports circulatory leucine (Leu), which is transaminated to synthesize other amino acids such as Glu and aspartate (Asp). To investigate the molecular basis of Leu transport and its conversion into Glu in the mammary gland, we characterized the expression of Leu transporters and [3H]Leu uptake in rat mammary cells. Gene expression analysis indicated that mammary cells express two Leu transporters, LAT1 and LAT2, with LAT1 being more abundant than LAT2. This transport system is sodium independent and transports large neutral amino acids. The Leu transport system in isolated rat mammary cells could be specifically blocked by the LAT1 inhibitors 2-aminobicyclo-[2.2.1]-heptane-2-carboxylic acid (BCH) and triiodothyronine (T3). In organ cultures, Glu secretion was markedly inhibited by these LAT1 inhibitors. Furthermore, the profiles of Leu uptake inhibition by amino acids in mammary cells were similar to those reported for LAT1. In vivo, concentrations of free Glu and Asp increased in milk by oral gavage with Leu at 6, 12, and 18 days of lactation. These results indicate that the main Leu transporter in mammary tissue is LAT1 and the transport of Leu is a limiting factor for the synthesis and release of Glu and Asp into milk. Our studies provide the bases for the molecular mechanism of Leu transport in mammary tissue by LAT1 and its active role on free Glu secretion in milk, which confer umami taste in suckling pups.

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]. Concurrently, the activity of BCAT and BCKD (two vital enzymes in BCAA catabolism) are increased in mammary tissue during lactation [68], which might be caused by reductions in insulin and growth hormone or increases in cortisol and glucagon [69].…”

Infant intakes of human milk branched chain amino acids are negatively associated with infant growth and influenced by maternal body mass index

“…46 The mammary gland expresses BCAA aminotransferase and the branched-chain α-ketoacid dehydrogenase complex that are responsible for the two-step breakdown of BCAAs. 48 Although only demonstrated in species other than humans, the lactating mammary gland is known to take up copious amounts of BCAAs and extensively degrades them for the synthesis of other non-essential amino acids, such as Glu. 48,49 Throughout the body, the catabolism of BCAAs may be an important means of dealing with a surplus in supply.…”

“…48 Although only demonstrated in species other than humans, the lactating mammary gland is known to take up copious amounts of BCAAs and extensively degrades them for the synthesis of other non-essential amino acids, such as Glu. 48,49 Throughout the body, the catabolism of BCAAs may be an important means of dealing with a surplus in supply. 38 Accordingly, in non-lactating individuals with metabolic disease, BCAA plasma concentrations increase as a function of impaired BCAA catabolism.…”

Amino acid transportation, sensing and signal transduction in the mammary gland: key molecular signalling pathways in the regulation of milk synthesis

“…These transporters are heterodimeric proteins, which comprise of a catalytic subunit (L-type amino acid transporter 1 (LAT1), encoded by SLC7A5, or L-type amino acid transporter 2 (LAT2), encoded by SLC7A6) and a glycoprotein 4F2 heavy chain (4F2hc). Both LAT1 and LAT2 are highly expressed in the mammary tissues (15) , but which subtype is predominately expressed in the mammary gland seems to be different among species (19)(20)(21) . In rats, the gene expression of LAT1 in the mammary gland is greater than that of LAT2 during the lactation period (19) .…”

“…Both LAT1 and LAT2 are highly expressed in the mammary tissues (15) , but which subtype is predominately expressed in the mammary gland seems to be different among species (19)(20)(21) . In rats, the gene expression of LAT1 in the mammary gland is greater than that of LAT2 during the lactation period (19) . The gene expression of LAT2, but not LAT1, is significantly increased with the progression of lactation in sows (21) .…”