Upregulation of the amino acid transporter ATB^0,+ (SLC6A14) in colorectal cancer and metastasis in humans

ATB^0,+ (SLC6A14) is a Na⁺/Cl^--coupled arginine transporter expressed at low levels in normal colon. Arginine is an essential amino acid for tumor cells. Arginine is also the substrate for nitric oxide synthases (NOSs). Since arginine and arginine-derived nitric oxide (NO) play a critical role in cancer, we examined the expression of ATB^0,+ in colorectal cancer. Paired normal and cancer tissues from colectomy specimens of 10 patients with colorectal cancer and from the liver tissue of one patient with hepatic metastasis from a colonic primary were used for the analysis of the levels of ATB^0,+ mRNA, inducible NOS (iNOS) mRNA and the corresponding proteins. Tissues samples from the colon, liver, and lymph nodes of an additional patient with metastatic colon cancer were analyzed for ATB ^0,+ protein alone. We also examined the levels of nitrotyrosylated proteins. The ATB^0,+ mRNA increased 22.9 ± 3.0-fold in colorectal cancer compared to normal tissue and the increase was evident in each of the 10 cases examined. iNOS mRNA increased 5.2 ± 1.1-fold in cancer specimens. The changes in mRNA levels were associated with an increase in ATB^0,+, iNOS, and nitrotyrosylated proteins. The increased expression of ATB^0,+ and iNOS was also demonstrated in liver and lymph node specimens with metastases from colonic primaries. This study strongly suggests that the upregulation of ATB^0,+ may have a pathogenic role in colorectal cancer. Since ATB^0,+ is a versatile transporter not only for arginine but also for several drugs including NOS inhibitors, these findings have significant clinical and therapeutic relevance.