Abstract. SLC5A8 and SLC5A12 are sodium-coupled monocarboxylate transporters (SMCTs), the former being a high-affinity type and the latter a low-affinity type. Both transport a variety of monocarboxylates in a Na + -coupled manner. They are expressed in the gastrointestinal tract, kidney, thyroid, brain, and retina. SLC5A8 is localized to the apical membrane of epithelial cells lining the intestinal tract and proximal tubule. In the brain and retina, its expression is restricted to neurons and the retinal pigment epithelium. The physiologic functions of SLC5A8 include absorption of short-chain fatty acids in the colon and small intestine, reabsorption of lactate and pyruvate in the kidney, and cellular uptake of lactate and ketone bodies in neurons. It also transports the B-complex vitamin nicotinate. SLC5A12 is also localized to the apical membrane of epithelial cells lining the intestinal tract and proximal tubule. In the brain and retina, its expression is restricted to astrocytes and Müller cells. SLC5A8 also functions as a tumor suppressor; its expression is silenced in tumors of colon, thyroid, stomach, kidney, and brain. The tumor-suppressive function is related to its ability to mediate concentrative uptake of butyrate, propionate, and pyruvate, all of which are inhibitors of histone deacetylases. SLC5A8 can also transport a variety of pharmacologically relevant monocarboxylates, including salicylates, benzoate, and g-hydroxybutyrate. Non-steroidal anti-inflammatory drugs such as ibuprofen, ketoprofen, and fenoprofen, also interact with SLC5A8. These drugs are not transportable substrates for SLC5A8, but instead function as blockers of the transporter. Relatively less is known on the role of SLC5A12 in drug transport.

ATB(0,+) [SLC6A14 (solute carrier family 6 member 14)] is an Na(+)/Cl(-)-coupled amino acid transporter whose expression is upregulated in cancer. 1-Methyltryptophan is an inducer of immune surveillance against tumour cells through its ability to inhibit indoleamine dioxygenase. In the present study, we investigated the role of ATB(0,+) in the uptake of 1-methyltryptophan as a potential mechanism for entry of this putative anticancer drug into tumour cells. These studies show that 1-methyltryptophan is a transportable substrate for ATB(0,+). The transport process is Na(+)/Cl(-)-dependent with an Na(+)/Cl(-)/1-methyltryptophan stoichiometry of 2:1:1. Evaluation of other derivatives of tryptophan has led to identification of alpha-methyltryptophan as a blocker, not a transportable substrate, for ATB(0,+). ATB(0,+) can transport 18 of the 20 proteinogenic amino acids. alpha-Methyltryptophan blocks the transport function of ATB(0,+) with an IC(50) value of approximately 250 muM under conditions simulating normal plasma concentrations of all these 18 amino acids. These results suggest that alpha-methyltryptophan may induce amino acid deprivation in cells which depend on the transporter for their amino acid nutrition. Screening of several mammary epithelial cell lines shows that ATB(0,+) is expressed robustly in some cancer cell lines, but not in all; in contrast, non-malignant cell lines do not express the transporter. Treatment of ATB(0,+)-positive tumour cells with alpha-methyltryptophan leads to suppression of their colony-forming ability, whereas ATB(0,+)-negative cell lines are not affected. The blockade of ATB(0,+) in these cells with alpha-methyltryptophan is associated with cell cycle arrest. These studies reveal the potential of ATB(0,+) as a drug target for cancer chemotherapy.

These studies show that SLC5A8 mediates the tumor-suppressive effects of the bacterial fermentation product butyrate in the colon.

SMCT1 is a sodium-coupled (Na + -coupled) transporter for L-lactate and short-chain fatty acids. Here, we show that the ketone bodies, b-D-hydroxybutyrate and acetoacetate, and the branched-chain ketoacid, a-ketoisocaproate, are also substrates for the transporter. The transport of these compounds via human SMCT1 is Na + -coupled and electrogenic. The Michaelis constant is 1.4 ± 0.1 mM for b-D-hydroxybutyrate, 0.21 ± 0.04 mM for acetoacetate and 0.21 ± 0.03 mM for a-ketoisocaproate. The Na + : substrate stoichiometry is 2 : 1.As L-lactate and ketone bodies constitute primary energy substrates for neurons, we investigated the expression pattern of this transporter in the brain.In situ hybridization studies demonstrate widespread expression of SMCT1 mRNA in mouse brain. Immunofluorescence analysis shows that SMCT1 protein is expressed exclusively in neurons. SMCT1protein co-localizes with MCT2, a neuron-specific Na + -independent monocarboxylate transporter. In contrast, there was no overlap of signals for SMCT1 and MCT1, the latter being expressed only in non-neuronal cells. We also demonstrate the neuron-specific expression of SMCT1 in mixed cultures of rat cortical neurons and astrocytes. This represents the first report of an Na + -coupled transport system for a major group of energy substrates in neurons. These findings suggest that SMCT1 may play a critical role in the entry of L-lactate and ketone bodies into neurons by a process driven by an electrochemical Na + gradient and hence, contribute to the maintenance of the energy status and function of neurons. Keywords: brain, ketone bodies, neuroenergetics, neuronspecific expression, sodium-coupled monocarboxylate transporter, sodium-coupled transport. Glucose is the major energy substrate used by the brain (Pellerin and Magistretti 2004). The metabolism of glucose in the brain is compartmentalized. Even though the metabolic utilization of glucose in the brain is increased during intense neuronal activity, uptake of glucose in astrocytes, rather than in neurons, accounts for most of this activity-associated glucose utilization (Pellerin and Magistretti 2004). Astrocytes convert glucose into L-lactate, a monocarboxylate, and release it into the extracellular medium to be taken up subsequently by neurons. Thus, the energy needs of the active neurons are not met directly by glucose oxidation but rather, by oxidation of L-lactate supplied by astrocytes. L-Lactate is therefore the primary metabolic fuel for neurons under normal physiological conditions. Normal levels of L-lactate in blood are quite significant (1-1.2 mM) and this L-lactate in the circulation is also available for neurons, in addition to the L-lactate generated by astrocytes.Under circumstances when glucose availability is limited, such as during the suckling period in mammals (Hawkins et al. 1971;Cremer 1982), prolonged starvation and uncontrolled diabetes (Gjedde and Crone 1975;Hawkins et al. 1986), neurons must rely on substrates other than L-lactate to fulfil their high energy demands a...

Interleukin-2 dependent feline T-lymphoblastoid cells designated as MYA-1 cells were established. The cells were free from exogenous retroviruses and sensitive for replication of feline immunodeficiency virus (FIV). FIV can grow more efficiently in MYA-1 cells than in feline primary peripheral blood mononuclear cells. This line of cells will be useful not only for isolation and propagation of FIV, but also for further investigation of properties of FIV.

D-serine, an endogenous co-agonist of NMDA receptors in vertebrate retina, may modulate glutamate sensitivity of retinal neurons. This study determined at the functional and molecular level the transport process responsible for D-serine in retinal Müller cells. RT-PCR and immunoblotting showed that serine racemase (SR), the synthesizing enzyme for D-serine, is expressed in the rMC-1 Müller cell line and primary cultures of mouse Müller cells (1°MCs). The relative contributions of different amino acid transport systems to D-serine uptake were determined based on differential substrate specificities and ion dependencies. D-serine uptake was obligatorily dependent on Na + , eliminating Na + -independent transporters (asc-1 and system L) for D-serine in Müller cells. The Na + :substrate stoichiometry for the transport process was 1:1. Dserine transport was inhibited by alanine, serine, cysteine, glutamine, and asparagine, but not anionic amino acids or cationic amino acids, suggesting that D-serine transport in Müller cells occurs via ASCT2 rather than ASCT1 or ATB 0,+ . The expression of mRNAs specific for ASCT1, ASCT2, and ATB 0,+ was analyzed by RT-PCR confirming the expression of ASCT2 (and ASCT1) mRNA, but not ATB 0,+ , in Müller cells. Immunoblotting detected ASCT2 in neural retina and in 1°MCs; immunohistochemistry confirmed these data in retinal sections and in cultures of 1°MCs. The efflux of D-serine via ASCT2 by ASCT2 substrates was demonstrable using the Xenopus laevis oocyte heterologous expression system. These data provide the first molecular evidence for SR and ASCT2 expression in a Müller cell line and in 1°MCs and suggest that D-serine, synthesized in Müller cells by SR, is effluxed via ASCT2 to regulate NMDA receptors in adjacent neurons.

Age-related changes in major lymphocyte subsets were analyzed in 195 cynomolgus monkeys (Macaca fascicularis) aged from one month to 31 years. The percentages of CD20+ B cells in peripheral blood lymphocytes (PBL) decreased with age to five years of age, but after that, no significant change was observed. The percentages of CD16+ NK cells gradually increased during the first five years and reached the peak at from four to ten years of age, whereas the percentages of CD3+ T cells in PBL were relatively constant throughout the life. Among the T cells, the CD4+ CD8- T cells decreased, but CD4- CD8+ T cells increased within the first decade of life. We further analyzed the expressions of CD28 and CD29 molecules on T cells to determine the relation between age-related activation and phenotypic changes. Almost all CD4+ CD8- T cells (> 90%) were CD28+ at all ages analyzed, but a clear age-related decrease in CD28 expression was demonstrated in CD4- CD8+ T cells during the first ten years. In the case of CD29 expression, age-related increases in CD29hi cells were apparent in both CD4+ CD8- and CD4- CD8+ T cells during the first ten years. The percentages of CD29hi cells, however, were higher in CD4- CD8+ T cells than in CD4+ CD8- T cells in all ages analyzed. These results indicated that the age-related changes in percentages of major lymphocyte subsets as well as in phenotypes of T cells might be related to the maturation of the immune system including an increase in memory cells in cynomolgus monkeys.

Two strains of feline immunodeficiency virus (FIV) were isolated directly from peripheral blood mononuclear cells (PBMCs) of Japanese domestic cats or indirectly from PBMCs of specific pathogen free (SPF) cats inoculated with whole blood from naturally infected cats with FIV by cocultivation with primary PBMCs from SPF cats. Two isolates, designated as FIV TM 1 and FIV TM 2, had a lentivirus-like morphology by electron microscopy, a tropism for interleukin-2 dependent T-lymphocytes and Mg2+-dependent reverse transcriptase activity. By immunoblotting the isolates gave bands at 130, 48, 44, 40, 28, 17, and 13 kDa, and these bands except 130 kDa were detected in FIV Petaluma strain when reacted with the plasma of cats infected naturally with FIV TM 1 strain.