Amino acid transporters and nutrient-sensing mechanisms: new targets for treating insulin-linked disorders?

The IIS (insulin/IGF (insulin-like growth factor) signalling) cascade has an important role in regulating normal development and physiology, as evidenced by its effects in a host of major human diseases including cancer, Type 2 diabetes and neurodegeneration. Recently, it has become clear that multiple types of local nutrient-sensing mechanisms have an impact on cellular insulin-sensitivity through the downstream kinase TOR (target of rapamycin). In vivo analysis in flies has surprisingly highlighted PATs (proton-assisted amino acid transporters) as having a uniquely potent role in regulating IIS/TOR activity and growth, potentially via a novel signalling mechanism. Other molecules such as the heterodimeric amino acid transporter, CD98, which provides the principal route for cellular uptake of leucine, an amino acid implicated in regulating TOR, also appear to have important effects. As our understanding of how nutrient sensing has an impact on IIS/TOR increases, novel targets to modulate aberrant IIS in disease are likely to emerge, which could complement current strategies designed to block kinases in this pathway.

Tryptophan degradation by human placental indoleamine 2,3-dioxygenase regulates lymphocyte proliferation.

1. The physiological importance of human placental indoleamine 2,3-dioxygenase (EC 1.13.11.42), the first and rate-limiting enzyme in tryptophan metabolism, in regulating feto-maternal immunology has been studied. 2. Concentrations were measured in placental villous explant conditioned media of 14 amino acids that are known to be required for lymphocyte proliferation. In the absence of interferon-gamma only tryptophan and threonine were significantly lowered; in the presence of interferon-gamma (known to stimulate indoleamine 2,3-dioxygenase) tryptophan but not threonine depletion was much greater. 3. Peripheral blood mononuclear cell proliferation determined by measuring thymidine incorporation into DNA following culture in the medium previously conditioned by culture of villous explants was markedly reduced when placental indoleamine 2,3-dioxygenase was stimulated with interferon-gamma. Inhibition of placental indoleamine 2,3-dioxygenase by 1-methyl-tryptophan prevented inhibition of thymidine incorporation. Supplementation of the conditioned medium with tryptophan but no other amino acid completely reversed the inhibition of thymidine incorporation. 4. Flow cytometric analysis showed that CD4-positive T lymphocyte division was specifically suppressed by indoleamine 2,3-dioxygenase-mediated tryptophan depletion. This inhibition of T cell proliferation was due to arrest of cell cycle progression. 5. To study the mechanism of tryptophan sensing we examined the ability of 11 L-tryptophan analogues to support lymphocyte proliferation. Only L-tryptophan methyl and ethyl esters were able to stimulate proliferation in tryptophan-free media. Since both of these molecules are readily degraded to tryptophan by intracellular esterases this suggests that the tryptophan sensor is intracellular. 6. Our results show that mechanisms are present in the human placenta which are able to regulate cellular proliferation of the maternal immune system. This mechanism is dependent both on placental indoleamine 2,3-dioxygenase-mediated tryptophan degradation and on tryptophan sensing systems within lymphocytes.

Cationic amino acid transport through system y+L in erythrocytes of patients with lysinuric protein intolerance.

We test the hypothesis that lysinuric protein intolerance (LPI), a rare autosomal recessive defect of cationic amino acid transport, results from the absence of the recently described y+L amino acid transporter. We compare fluxes of lysine (1 microM) into erythrocytes of normal subjects with those of patients homozygous for the LPI mutation. No significant differences in fluxes through system y+L in normal or LPI cells were found, excluding the possibility that system y+L cannot be expressed in patients with LPI. Reasons for supposing that there may be tissue-specific processing of two recently described genes encoding the y+L transporter are discussed. Polymerase chain reaction measurement of expression of these two genes in an erythroleukemic cell line suggests that alternatively there may be an as-yet-unidentified additional member of this gene family.

External anions relieve DIDS inhibition of SO4 efflux from placental membrane vesicles.

Sulphate efflux from human placental microvillus membrane vesicles was inhibited by external DIDS (KI congruent to 10(-6) M). This inhibition was partially reversed on addition of the translocated substrates sulphate or selenate to the external medium: selenite which is not translocated does not protect against DIDS inhibition. These findings show that the mechanism responsible for sulphate efflux can be modified by substrate in the external medium.

Sulphate transport in human placental brush-border membrane vesicles: competitive inhibition by selenate.

The effect of selenate on sulphate uptake by human placental brush-border membrane vesicles has been investigated. Selenate added to the incubation medium inhibits 1 mM sulphate uptake in a dose-dependent fashion with a Ki of approx. 2.5 mM. The inhibition by selenate is competitive, suggesting that selenate and sulphate share a common transporter (an anion exchange system) which may be of particular importance for the transport of such essential trace elements to the fetus.