L-type amino acid transporter 1 expression is upregulated and associated with cellular proliferation in colorectal cancer

Previous studies have shown that the L-type amino acid transporter 1 (LAT1) is highly expressed in many types of cancer. Upregulated LAT1 expression is considered to be associated with cancer cell proliferation. In the present study, we investigated LAT1 expression in 210 patients with colorectal cancer (CRC) and 40 patients with colonic adenoma using an immunohistochemical method, and analyzed the associations between LAT1 expression and clinicopathological factors and prognosis. The biological significance of LAT1 was also examined under conditions with sub-normal amounts of essential amino acids using colon cancer cell lines. High expression of LAT1 was observed in 152 of 210 CRC patients (72.4%) and 12 of 40 patients with colonic adenoma (30%), and this difference in the frequency of LAT1 expression between CRC and adenoma was significant (P<0.001). High expression of LAT1 was associated with venous invasion (P=0.027). The restriction of amino acids suppressed cell proliferation in CRC cells with higher LAT1 expression, while cellular proliferation was not suppressed in the cells expressing lower levels of LAT1. Mammalian target of rapamycin (mTOR) expression was also downregulated under restricted availability of amino acids, suggesting that the restriction of amino acids induced an antitumor effect through inhibition of the LAT1/mTOR pathway. In summary, the present study demonstrated that LAT1 expression is frequently upregulated in CRC and is associated with cancer cell proliferation via the mTOR pathway.