Identification of small molecule inhibitors of Zcchc11 TUTase activity.

The RNA-binding protein Lin28 regulates the expression of the let-7 family of microRNAs (miRNAs) during early embryonic development. Lin28 recruits the 3' terminal uridylyl transferase (TUTase) Zcchc11 (TUT4) and/or Zcchc6 (TUT7) to precursor let-7 RNA (pre-let-7) to selectively block let-7 biogenesis. Uridylated pre-let-7 is targeted for decay by the downstream exonuclease Dis3l2 thereby preventing processing to mature let-7. Activation of this oncogenic pathway via up-regulation of Lin28 expression promotes cellular transformation, drives tumorigenesis in mouse models, and is frequently observed in a wide variety of cancer. Recent proof-of-principle experiments showed that Zcchc11 knockdown inhibits the tumorigenicity of Lin28-expressing human cancer cells and established this enzyme as a possible new therapeutic target for human malignancies. However, there are currently no known pharmacological agents capable of targeting this novel enzyme. In this study we developed and applied a sensitive biochemical assay that monitors Zcchc11 activity. Using this assay we performed an automated high-throughput screen of ∼ 15,000 chemicals to identify putative TUTase inhibitors. Several of these small molecules were validated as specific inhibitors of Zcchc11 activity. Our results demonstrate the feasibility of screening for TUTase inhibitors and present a relatively simple platform that can be exploited for future drug discovery efforts aimed at restoring let-7 expression in cancer.