l-Leucine transport in human breast cancer cells (MCF-7 and MDA-MB-231): kinetics, regulation by estrogen and molecular identity of the transporter

The transport of l-leucine by two human breast cancer cell lines has been examined. l-Leucine uptake by MDA-MB-231 and MCF-7 cells was via a BCH-sensitive, Na⁺-independent pathway. l-Leucine uptake by both cell lines was inhibited by l-alanine, d-leucine and to a lesser extent by l-lysine but not by l-proline. Estrogen (17β-estradiol) stimulated l-leucine uptake by MCF-7 but not by MDA-MB-231 cells. l-Leucine efflux from MDA-MB-231 and MCF-7 cells was trans-stimulated by BCH in a dose-dependent fashion. The effect of external BCH on l-leucine efflux from both cell types was almost abolished by reducing the temperature from 37 to 4 °C. There was, however, a significant efflux of l-leucine under zero-trans conditions which was also temperature-sensitive. l-Glutamine, l-leucine, d-leucine, l-alanine, AIB and l-lysine all trans-stimulated l-leucine release from MDA-MB-231 and MCF-7 cells. In contrast, d-alanine and l-proline had little or no effect. The anti-cancer agent melphalan inhibited l-leucine uptake by MDA-MB-231 cells but had no effect on l-leucine efflux. Quantitative real-time PCR revealed that LAT1 mRNA was approximately 200 times more abundant than LAT2 mRNA in MCF-7 cells and confirmed that MDA-MB-231 cells express LAT1 but not LAT2 mRNA. LAT1 mRNA levels were higher in MCF-7 cells than in MDA-MB-231 cells. Furthermore, LAT1 mRNA was more abundant than CD98hc mRNA in both MDA-MB-231 and MCF-7 cells. The results suggest that system L is the major transporter for l-leucine in both MDA-MB-231 and MCF-7 cells. It is possible that LAT1 may be the major molecular correlate of system L in both cell types. However, not all of the properties of system L reflected those of LAT1/LAT2/CD98hc.