Functional and molecular characteristics of system L in human breast cells

The functional and molecular properties of system in human mammary cancer cells (MDA-MB-231 and MCF-7) have been examined. All transport experiments were conducted under Na+-free conditions. α-Aminoisobutyric acid (AIB) uptake by MDA-MB-231 and MCF-7 cells was almost abolished by BCH (2-amino-2-norbornane-carboxylic acid). AIB uptake by MDA-MB-231 cells was also inhibited by -alanine (83.6%), -lysine (75.6%) but not by -proline. Similarly, -lysine and -alanine, respectively, reduced AIB influx into MCF-7 cells by 45.3% and 63.7%. The Km of AIB uptake into MDA-MB-231 and MCF-7 cells was, respectively, 1.6 and 8.8 mM, whereas the Vmax was, respectively, 9.7 and 110.0 nmol/mg protein/10 min. AIB efflux from MDA-MB-231 and MCF-7 cells was trans-stimulated by BCH, -glutamine, -alanine, -leucine, -lysine and AIB (all at 2 mM). In contrast, -glutamate, -proline, -arginine and MeAIB had no effect. The interaction between -lysine and AIB efflux was one of low affinity. The fractional release of AIB from MDA-MB-231 cells was trans-accelerated by -leucine and -tryptophan but not by -alanine. MDA-MB-231 and MCF-7 cells expressed LAT1 and CD98 mRNA. MCF-7 cells also expressed LAT2 mRNA. The results suggest that AIB transport in mammary cancer cells under Na+-free conditions is predominantly via system which acts as an exchange mechanism. The differences in the kinetics of AIB transport between MDA-MB-231 and MCF-7 cells may be due to the differential expression of LAT2.