Amino acid transport in thymic- and spleen-derived lymphocytes

System A amino acid transporters regulate glutamine uptake and attenuate antibody-mediated arthritis

Proliferation of rapidly dividing bone marrow-derived cells is strongly dependent on the availability of free glutamine, whose uptake is mediated through different amino acid transporters. The sodium-coupled neutral amino acid transporter (SNAT) family was previously reported to be associated with the development of collagen-induced arthritis in mice. Here, we tested the hypothesis whether impairment of SNAT proteins influences immune cell function and in turn alters arthritis development. The 2-(methylamino)isobutyric acid (MeAIB), a SNAT-specific substrate, was used to modulate the function of SNAT proteins. We demonstrate that glutamine uptake by murine naive lymphocytes, and consequent cell proliferation, is strongly associated with system A transporters. Physiological impairment of SNAT proteins reduced the antibody-initiated effector phase of arthritis, mainly by affecting the levels of circulating monocytes and neutrophils. MeAIB was also shown to affect the proliferation of immortalized cells, through trans-inhibition of SNAT proteins. Based on our observations, we conclude that SNAT proteins regulate the initial stages of lymphocyte activation by regulating glutamine uptake, and that the effector phase of arthritis can be affected by non-metabolized SNAT substrates. Most probably, metabolically active cells within both the adaptive and the innate immune systems are regulated by SNAT proteins and play a role in modifying arthritis development.

Changes in neutral amino acid transport activity in myeloid leukemia cells differentiated by lipopolysaccharide

M1 cells derived from mouse myeloid leukemia have been reported to differentiate to macrophage-like cells upon treatment with substances such as lipopolysaccharide. Previously we found that in mouse peritoneal macrophages most of the neutral amino acids were taken up through a unique Na+-independent system. In this paper we have investigated the neutral amino acid transport in M1 cells and in those treated with lipopolysaccharide. In M1 cells serine, alanine and proline were taken up mainly by Na+-dependent transport systems, and leucine was largely transported by a Na+-independent system. By treating the cells with lipopolysaccharide, the activities of the Na+-dependent systems markedly decreased, whereas the activity of the Na+-independent system was little affected. The amino acid concentrations in the cells and the culture medium were measured. As a whole, the intracellular to extracellular distribution ratios for neutral amino acids that are preferred substrates for Na+-dependent systems were decreased on lipopolysaccharide treatment, whereas those for amino acids that are mainly transported by a Na+-independent system were slightly increased. From these results we conclude that M1 cells treated with lipopolysaccharide tend to differentiate to macrophage-like cells with respect to the neutral amino acid transport.

The transport of glutamine into rat mesenteric lymphocytes

The transport of glutamine into isolated rat mesenteric lymphocytes was studied. This transport appears to be dependent upon the Na+ gradient. The Km for glutamine transport was about 1.0 mM. A large number of amino acids were shown to inhibit the rate of transport of both serine and glutamine into lymphocytes. The transport of glutamine was competitively inhibited by serine and that for serine was similarly inhibited by glutamine. In contrast, histidine and 2-(methylamino)isobutyrate inhibited the transport of both serine and glutamine noncompetitively. It is concluded that glutamine is transported into rat mesenteric lymphocytes by a process similar to System ASC described for other cells.

Amino acid transport in human tonsillar lymphocytes with regard to patient's age and tonsillar diseases

Amino acid transport in human tonsillar lymphocytes was investigated in 32 patients of various ages and with different tonsillar diseases. Tonsillar lymphocytes appeared to possess at least four different transport systems for neutral amino acids including the activated Na+-dependent A system transport. The transport activity of neutral amino acid was significantly higher in child cases than in adult cases. In adult cases with focal tonsillitis, tonsillar lymphocytes were taken from patients with rheumatoid arthritis (RA) and skin diseases as secondary lesions who showed good improvement of the skin lesions after tonsillectomy. These lymphocytes were revealed to have more activated Na+-dependent transport than those from patients with skin diseases who showed poor improvement of the skin lesions after the tonsillectomy, and those from patients with recurrent tonsillitis. The characteristics of amino acid transport in human tonsillar lymphocytes and changes influenced by age and tonsillar diseases are discussed.

Antigen-specific stimulation of amino acid transport in bovine lymphocytes

Treatment of bovine lymphocytes isolated from animals which were either infected with Mycobacterium bovis or sensitized to a purified protein derivative (PPD-B) from this organism induced an increase in the transport of alpha-aminoisobutyric acid (AIB) and alpha-methylaminoisobutyric acid (MeAIB). PPD-B did not stimulate these transport activities in lymphocytes from nonsensitized animals. The transport stimulation was first measurable after about 7 hours of treatment, reached about a two-fold enhancement after 20 hours, and continued to increase to 30- to 40-fold after 6 days. The stimulation of AIB transport was inhibited by both ouabain and cycloheximide. Experiments to determine transport system specificities in nonstimulated lymphocytes showed that MeAIB transport was primarily by the Na+-dependent, A-system, and leucine transport was mostly by Na+-independent system(s). In contrast, AIB transport was about 25% by the A-system, 25% by at least one Na+-dependent, non-A-system, and 50% by one or more Na+-independent system(s). Analysis of the three components of AIB transport after treatment with PPD-B showed that: 1) transport by both the A-system and the Na+-independent system(s) was stimulated; 2) A-system transport was stimulated to a larger extent than Na+-independent transport; and 3) Na+-dependent, non-A-system transport was not stimulated significantly.

Amino acid transport in pig lymphocytes. Enhanced activity of transport system asc following mitogenic stimulation

Changes in neutral amino acid transport activity caused by addition of phytohaemagglutinin-P to quiescent peripheral pig lymphocytes have been evaluated by measurements of 14C-labelled neutral and analogue amino acids under conditions approaching initial entry rates. Utilizing methylaminoisobutyric acid, the best model substrate of System A, we confirmed our previous report (Borghetti, A.F., Kay, J.E. and Wheeler, K.P. (1979) Biochem. J. 182, 27-32) on the absence of this transport system in quiescent cells and its emergence following stimulation. Furthermore, we demonstrated the presence in quiescent cells of an Na+-dependent transport system for neutral amino acids that has been characterized as System ASC by several criteria including intolerance to methylaminoisobutyric acid, strict Na+-dependence, the property of transtimulation and specificity for pertinent substrates such as alanine, serine, cysteine and threonine. Analysis of the relationship between influx and substrate concentration revealed that two independent saturable components contribute to entry of alanine in quiescent cells: a low affinity (Km = approximately 4 mM) and a high affinity (Km = approximately 0.2 mM) component. The high affinity component could be inhibited in a competitive way by serine, cysteine and threonine, but methylaminoisobutyric acid did not change appreciably its constants. The enhanced activity of alanine transport through the ASC system observed in activated cells resulted from a large increase in the capacity (V) of the high affinity component without any substantial change in the apparent affinity constant (Km).

Role of insulin in the intermediary metabolism of the activated thymic-derived lymphocyte

The hypothesis that a role for insulin in the metabolism of T cells would be evident after cell activation when receptors appear was tested to validate the T cell model and to analyze the mechanism by which insulin may function in immunoregulation. Measuring the flux rates of 3-O-[methyl-3H]-D-glucose and aminoisobutyric acid, alpha-[1-14C], lactate production and oxidation, and glucose oxidation from carbon 1- and carbon 6-labeled substrates, it was determined that (a) mitogens such as phytohemagglutinin enhance basal T lymphocyte intermediary metabolism, (b) physiologic concentrations of insulin have no impact on the metabolism of unstimulated, cultured, receptor-negative lymphocytes, and (c) insulin provided to receptor bearing lymphocytes augments intermediary metabolism above mitogen stimulated levels. The importance of the pentose phosphate shunt pathway for energy metabolism in the stimulated lymphocyte was confirmed. These studies demonstrate that insulin has a classical physiologic role to play in the activated lymphocyte further validating the use of this cell to examine potential receptor defects in disorders of carbohydrate metabolism. By enhancing energy metabolism of stimulated lymphocytes, insulin serves biologic economy and thus may perform its immunoregulatory role.