TPH1 and cholangiocarcinoma: This phenomenon can be explained by the fact that cholangiocarcinoma cell lines (Mz-Cha-1, HuH-28, HuCC-T1, CCLP-1, SG231, and TFK1) show higher expression of the serotonin biosynthetic enzyme (TPH1), and lower expression of the serotonin degrading enzyme (MAO-A) 96.