L-type amino acid transporter 1 (LAT1), which preferentially transports most of the essential amino acids, is highly upregulated in various cancer types and contributes to rapid growth and proliferation of cancer cells. However, molecular mechanisms responsible for the upregulation of LAT1 in cancer cell remain largely unknown. MicroRNAs (miRNAs) are small non-coding RNAs involved in gene silencing. They inhibit initiation of translation and induce mRNA decay, via binding to the target sequence in 3‘ untranslated region (3‘UTR) of specific mRNAs. By using multiple open-online tools for target prediction, we previously reported 5 candidate miRNAs that are commonly predicted to regulate LAT1 expression. One of them was miR-126, a well-established miRNA targeting LAT1. One of the other candidate miRNAs indeed targeted 3‘UTR of LAT1 mRNA in a luciferase reporter assay. Furthermore, transfection of the miRNA mimic significantly decreased the amount of LAT1 mRNA and protein in cultured cancer cell lines, confirming that miRNA could regulate LAT1 expression.
In this study, by using RNA-seq data from Cancer Genome Atlas, we revealed that 3 of the 5 miRNAs are significantly downregulated in cholangiocarcinoma compared to normal tissue. Correlation analysis using 284 miRNAs showed that the expression of the 3 miRNAs negatively correlate with that of LAT1 mRNA, classifying them in a group with top 5% negative correlation coefficients. Among those 3 miRNAs, we focused on the 2 miRNAs other than miR-126. Transfection of miRNA mimics significantly suppressed LAT1 expression at both mRNA and protein level in 5 cholangiocarcinoma cell lines. Moreover, proliferation of these cell lines were significantly inhibited by the miRNA mimics. Our results suggest that the decrease of these miRNAs could contribute to the upregulation of LAT1 in cholangiocarcinoma. These miRNAs could be potential tools for the suppression of cholangiocarcinoma through the attenuation of LAT1 expression.