L-type amino acid transporter 1 (LAT1) expression in canine mammary gland tumors.

L-type amino acid transporter 1 (LAT1), an isoform of amino acid transport system L, transports branched or aromatic amino acids essential for fundamental cellular activities such as cellular growth, proliferation and maintenance. This amino acid transporter recently has received attention because of its preferential and up-regulated expression in a variety of human tumors, in contrast to its limited distribution and low-level expression in normal tissues. In this study, to explore the feasibility of using LAT1 expression as a molecular marker in mammary gland tumors (MGT), we performed a comparative study of LAT1 expression at the mRNA and protein levels in normal mammary gland cells and tumor cells. Conventional RT-PCR and Western blotting were performed on MGT tissues from 16 dogs and normal organs from nine healthy dogs. LAT1 expression was detected in ten of the 16 MGT patients. As is the case in human tissues, LAT1 showed limited expressional distribution in normal canine organs. For quantitative expressional comparison, extensive real-time RT-PCR was performed on mRNA samples from 53 MGT patients. The comparison demonstrated that LAT1 mRNA levels from MGT tissues were 20 times higher than those in normal mammary gland tissues. Additionally, histologically invasive MGT showed a higher expression of LAT1 than non-invasive tumors. These findings suggest that LAT1 could be a clinical marker and therapeutic target for invasive malignant MGT.