PML/RARα -Regulated MicroRNA 181b Targets the Tumor Suppressor RASSF1A in Acute Promyelocytic Leukemia

Abstract 1361

In the acute promyelocytic leukemia (APL) bearing the translocation t(15;17), all-trans-retinoic acid (ATRA) treatment induces granulocytic maturation and complete remission of leukemia. Several factors are involved in the formation of the leukemic phenotype. Latest studies identified microRNAs as critical players in this network. In a micro array based microRNA screen we could identify miR-181b as downregulated in the APL cell line NB4 by treatment with pharmacological doses of ATRA. Additionally, we showed the transcriptional induction of miR-181b by the APL-associated PML-RARα oncogene which is released after treatment with ATRA. The overexpression of miR-181b by microRNA mimics leads to an inhibition of ATRA induced granulocytic differentiation. Furthermore, the downregulation of miR-181b by locked nucleic acids (LNAs) causes the induction of granulocytic transcription factor C/EBPβ. In a PML/RARα knock in mouse model we could show the enhanced expression of miR-181b. Also, APL patient samples show a significant enhanced expression of miR-181b in comparison to AML patient samples with normal karyotype. In addition, the miR-181b expression was strongly reduced in APL patient samples after ATRA treatment. In a microRNA target search we identified the novel ATRA regulated tumor suppressor RASSF1A as a putative target of miR-181b. In functional studies we showed that enforced expression of miR-181b reduces the protein level of RASSF1A by binding to the 3'UTR of RASSF1A mRNA. Accordingly, RASSF1A protein was enriched after knock down of miR-181b. The role of RASSF1A in ATRA induced differentiation was verified by knock down of RASSF1A protein by specific siRNA: Here we could show the reduction of ATRA induced CD11b expression. In conclusion, we identified miR-181b as a new player in the PML/RARα associated APL. Moreover, we firstly described the miR-181b target RASSF1A as a crucial factor in the ATRA activated granulocytic differentiation program in APL. Our data reveal the importance of deregulated miRNA biogenesis in cancer and may provide novel biomarkers and therapeutic targets in myeloid leukemia.