These results are in agreement with observation made by Lu et al. in cholangiocarcinoma [35], which confirm the PGE2-mediated miR-21 up-regulation in OTSCC and suggest a PGE2-miR-21-HPGD positive feed-forward loop that contributes to tumorigenesis (Fig. 4f). This evidence concerns the gene HPGD and cholangiocarcinoma.