Epigenetic Silencing of 15-Hydroxyprostaglandin Dehydrogenase by Histone Methyltransferase EHMT2/G9a in Cholangiocarcinoma.

Cholangiocarcinoma (CCA) is a lethal malignancy with few therapeutic options. NAD+-dependent 15-hydroxyprostaglandin dehydrogenase (15-PGDH) has been shown to inhibit CCA cell growth in vitro and in xenograft models. However, the role of 15-PGDH in CCA development has not been investigated and the mechanism for 15-PGDH gene regulation remains unclear. Here, we evaluated the role of 15-PGDH in CCA development by using a mouse model with hydrodynamic tail vein injection of transposase-based plasmids expressing Notch1 intracellular domain and myr-Akt, with or without co-injection of 15-PGDH expression plasmids. Our results reveal that 15-PGDH overexpression effectively prevents CCA development. Through patient data mining and experimental approaches, we provide novel evidences that 15-PGDH is epigenetically silenced by histone methyltransferase G9a. We observe that 15-PGDH and G9a expressions are inversely correlated in both human and mouse CCAs. By using CCA cells and mouse models, we show that G9a inhibition restores 15-PGDH expression and inhibited CCA in vitro and in vivo. Mechanistically, our data indicate that G9a is recruited to 15-PGDH gene promoter via protein-protein interaction with the E-box binding Myc/Max heterodimer. The recruited G9a then silences 15-PGDH gene through enhanced methylation of H3K9. Our further experiments have led to the identification of STAT4 as a key transcription factor involved in the regulation of 15-PGDH by G9a. Collectively, our findings disclose a novel G9a-15PGDH signaling axis which is importantly implicated in CCA development and progression. IMPLICATIONS: The current study describes a novel G9a-15PGDH signaling axis which is importantly implicated in CCA development and progression.