LEUCINE ACTIVATES SYSTEM-A AMINO-ACID-TRANSPORT IN L6 RAT SKELETAL-MUSCLE CELLS

In this study, we present evidence showing that leucine is involved in the upregulation of system A amino acid transport activity in the L6 rat skeletal muscle cell line. At leucine concentrations of greater th an or equal to 0.05 mM, the uptake of N-methylamino-alpha-isobutyric a cid (MeAIB), a paradigm system A substrate, was stimulated by up to 50 %. Kinetic analysis revealed that this stimulation was a result of an increase in the maximal transport rate of MeAIB uptake, from 327 +/- 2 6 to 450 +/- 8 pmol . min(-1). mg . protein(-1) after incubation of ce lls with leucine. No significant change in the concentration at which MeAIB transport was half maximal was observed. System A activation was biphasic, reaching an initial plateau after 3 h, with a second phase of activation being observed after 5 h. The initial activation of syst em A transport occurred by a mechanism distinct from that activated by insulin-like growth factor-I (IGF-I) (3 nM), since the effects of leu cine and IGF-I were additive. This activation was not due to transstim ulation, since 2-amino-2-norbornane-carboxylic acid, a specific system L substrate, did not stimulate system A. Leucine's keto acid, ketoiso caproic acid, prevented the activation of system A transport, whereas aminooxyacetate, a transaminase inhibitor, augmented the increase in s ystem A activity by leucine. Both cycloheximide and actinomycin D inhi bited the leucine-induced increase in MeAIB uptake. The present result s indicate that leucine, or some cellular component regulated by it, i s capable of stimulating system A transport through control of DNA tra nscription, possibly of a gene encoding either a repressor or enhancer molecule of system A or perhaps of the gene encoding system A itself.