Leucine and arginine regulate trophoblast motility through mTOR-dependent and independent pathways in the preimplantation mouse embryo.

Uterine implantation is a critical element of mammalian reproduction and is a tightly and highly coordinated event. An intricate and reciprocal uterine-embryo dialog exists to synchronize uterine receptivity with the concomitant activation of the blastocyst, maximizing implantation success. While a number of pathways involved in regulating uterine receptivity have been identified in the mouse, less is understood about blastocyst activation, the process by which the trophectoderm (TE) receives extrinsic cues that initiate new characteristics essential for implantation. Amino acids (AA) have been found to regulate blastocyst activation and TE motility in vitro. In particular, we find that arginine and leucine alone are necessary and sufficient to induce TE motility. Both arginine and leucine act individually and additively to propagate signals that are dependent on the activity of the mammalian target of rapamycin complex 1 (mTORC1). The activities of the well-established downstream targets of mTORC1, p70S6K and 4EBP, do not correlate with trophoblast motility, suggesting that an independent-rapamycin-sensitive pathway operates to induce trophoblast motility, or that other, parallel amino acid-dependent pathways are also involved. We find that endogenous uterine factors act to induce mTORC1 activation and trophoblast motility at a specific time during pregnancy, and that this uterine signal is later than the previously defined signal that induces the attachment reaction. In vivo matured blastocysts exhibit competence to respond to an 8-hour AA stimulus by activating mTOR and subsequently undergoing trophoblast outgrowth by the morning of day 4.5 of pregnancy, but not on day 3.5. By the late afternoon of day 4.5, the embryos no longer require any exposure to AA to undergo trophoblast outgrowth in vitro, demonstrating the existence and timing of an equivalent in vivo signal. These results suggest that there are two separate uterine signals regulating implantation, one that primes the embryo for the attachment reaction and another that activates mTOR and initiates invasive behavior.

Upregulation of the amino acid transporter ATB0,+ (SLC6A14) in colorectal cancer and metastasis in humans.

ATB(0,+) (SLC6A14) is a Na(+)/Cl(-)-coupled arginine transporter expressed at low levels in normal colon. Arginine is an essential amino acid for tumor cells. Arginine is also the substrate for nitric oxide synthases (NOSs). Since arginine and arginine-derived nitric oxide (NO) play a critical role in cancer, we examined the expression of ATB(0,+) in colorectal cancer. Paired normal and cancer tissues from colectomy specimens of 10 patients with colorectal cancer and from the liver tissue of one patient with hepatic metastasis from a colonic primary were used for the analysis of the levels of ATB(0,+) mRNA, inducible NOS (iNOS) mRNA and the corresponding proteins. Tissues samples from the colon, liver, and lymph nodes of an additional patient with metastatic colon cancer were analyzed for ATB(0,+) protein alone. We also examined the levels of nitrotyrosylated proteins. The ATB(0,+) mRNA increased 22.9+/-3.0-fold in colorectal cancer compared to normal tissue and the increase was evident in each of the 10 cases examined. iNOS mRNA increased 5.2+/-1.1-fold in cancer specimens. The changes in mRNA levels were associated with an increase in ATB(0,+), iNOS, and nitrotyrosylated proteins. The increased expression of ATB(0,+) and iNOS was also demonstrated in liver and lymph node specimens with metastases from colonic primaries. This study strongly suggests that the upregulation of ATB(0,+) may have a pathogenic role in colorectal cancer. Since ATB(0,+) is a versatile transporter not only for arginine but also for several drugs including NOS inhibitors, these findings have significant clinical and therapeutic relevance.

Possible role of amino acids, peptides, and sugar transporter in protein removal and innate lung defense.

This review presents the hypothesis that removal of polypeptides and glycoproteins from the alveolar space and airways is mediated in part by enzymatic degradation, followed by transporter mediated transepithelial transport of amino acids, peptides and sugar residues. Furthermore, the activity of these transporters ensures low availability of nutrients, and decrease bacterial growth. Thus, airway epithelial transporters for sugar, amino acids, peptides and other nutrients can contribute to the innate lung defense.

Up-regulation of the amino acid transporter ATB(0,+) (SLC6A14) in carcinoma of the cervix.

OBJECTIVE: ATB(0,+) is an energy-coupled transporter for arginine and amino acid-based prodrugs. The objective of the study was to examine the expression of this transporter in cervical cancer. METHODS: Specimens of normal ectocervical mucosa and cervical squamous cell carcinoma were used for determination of ATB(0,+) mRNA levels by RT-PCR. A commercial dot blot of paired normal cervix and cervical cancer cDNA was also used to quantify ATB(0,+) mRNA. ATB(0,+) mRNA and protein in tissue sections were analyzed by in situ hybridization and immunohistochemistry/immunofluorescence. RESULTS: ATB(0,+) mRNA increased 5.6-fold (P < 0.0004) in cervical cancer compared to normal cervix. This was associated with a parallel increase in ATB(0,+) protein. CONCLUSIONS: Expression of ATB(0,+) is minimal in normal cervix and is up-regulated in cervical cancer. The up-regulation of this highly concentrative transporter for arginine and prodrugs in cervical cancer has significant clinical and therapeutic relevance.

Expression of the amino acid transporter ATB 0+ in lung: possible role in luminal protein removal.

Normal lung function requires transepithelial clearance of luminal proteins; however, little is known about the molecular mechanisms of protein transport. Protein degradation followed by transport of peptides and amino acids may play an important role in this process. We previously cloned and functionally characterized the neutral and cationic amino acid transporter ATB(0+) and showed expression in the lung by mRNA analysis. In this study, the tissue distribution, subcellular localization, and function of the transporter in native tissue were investigated. Western blots showed expression of the ATB(0+) protein in mouse lung, stomach, colon, testis, blastocysts, and human lung. Immunohistochemistry revealed that ATB(0+) is predominantly expressed on the apical membrane of ciliated epithelial cells throughout mouse airways from trachea to bronchioles and in alveolar type I cells. Electrical measurements from mouse trachea preparations showed Na(+)- and Cl(-)-dependent, amino acid-induced short-circuit current consistent with the properties of ATB(0+). We hypothesize that, by removing amino acids from the airway lumen, the transporter contributes to protein clearance and, by maintaining a low nutrient environment, plays a role in lung defense.

The GABA transporter GAT1 and the MAGUK protein Pals1: interaction, uptake modulation, and coexpression in the brain.

GABAergic signaling in the CNS is terminated in part through uptake of GABA by GABA transporters. We used the yeast two-hybrid system to identify proteins that associate with the carboxy-terminus of the neuronal GABA transporter GAT1. We found an interaction between GAT1 and the MAGUK protein Pals1. When coexpressed in COS-7 cells, Pals1 co-immunoprecipitates with GAT1. We demonstrate cellular coexpression of GAT1 and Pals1 in the mouse hippocampus and striatum. Functionally, coexpression of GAT1 and Pals1 in COS-7 cells increases [3H]-GABA uptake by GAT1. The mechanism underlying increased uptake is increased levels of GAT1 protein. We hypothesize that Pals1 contributes to the stability of the GAT1, thus promoting the expression level of the transporter protein. In the CNS, Pals1 may stabilize GAT1 at appropriate levels in specific GABAergic neurons.