Modulation of transport systems for neutral and anionic amino acids in mesenchymal cells.

Introduction The concerted operation of several amino acidtransport systems ensures the optimal composition of the intracellular compartment as well as inter-organ amino acid flow [ l ] . The operation of these transport systems is linked through substrates accumulated by active transport systems (such as Systems A and X,;;) and then exchanged with other amino acids through exchange pathways (Systems L, ASC and &-) [2-41. Glutamine is an ideal substrate for the co-ordinated operation of transport systems for neutral amino acids. Indeed, this amino acid is a good substrate for System A but can also interact with Systems ASC and L [5,6] as well as with specific N-type transport systems, originally described in the liver [7] but also characterized in some mesenchymal models [8]. In the intracellular compartment, 1,-glutamine can be easily converted into 1.-glutamate. However, the intracellular pool of 1.-glutamate can also be fuelled through specific transport routes, such as System X,;; [9,10], an active transport mechanism which has the same operational features of the Na+,K+dependent glutamate carriers, cloned from nervous system and absorptive epithelia [ 1 1 131. Thus the high intracellular concentrations of glutamine and glutamate found in mesenchymal cells [ 141 are the result of the operation of both System A (through glutamine uptake) and System X,; (through glutamate uptake). In mesenchymal cells the kinetic features of these two mechanisms are very different; the V,,, of System A is much higher than that of System X,;; [2]; conversely, the affinity of System A for its substrates is much lower than that of System X,;;. Mammalian cells are usually cultured in media that contain high concentrations of glutamine and little glutamate, a situation resembling that observed in extracellular fluids in viva [ 15,161. Under these conditions, mesenchymal cells accumulate glutamine through System A