A long non-coding RNA in the let-7 complex acting as a potent and specific death effector of cancer cells

The let-7 complex in Drosophila encodes three evolutionarily conserved microRNAs: miR-100, let-7, and miR-125. These act as heterochronic genes in regulating developmental timing in response to the steroid hormone ecdysone and play important roles in cell differentiation. Here we identify two additional long non-coding RNAs in the let-7 complex, we named let-A and let-B. Both are transcribed in the large first intron of the primary RNA encoding the microRNAs. We show these RNAs to be sequentially expressed in early pupal stages in response to ecdysone signaling, albeit exhibiting a different expression pattern compared to the microRNA let-7. Surprisingly, ectopic expression of let-A in Drosophila cancer cells induces rapid cell death. Dead cells further release RNA molecules in the medium that is becoming toxic to other cancer cells. In vivo grown tumors lose their tumorigenicity after being incubated in the let-A induced medium. Moreover, feeding flies carrying transplanted tumor cells with such induced medium leads to reduced growth of tumors in a subset of hosts. Our results uncover a new lncRNA which can act as a potent and specific cell death effector for Drosophila tumor cells.