The PML/RARα-Regulated MiR-181a/b-Cluster Targets the Tumor Suppressor RASSF1A in Acute Promyelocytic Leukemia

In 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 the genomically clustered miR-181a and miR-181b as downregulated in the APL cell line NB4 by treatment with pharmacological doses of ATRA. In addition, the expression of the miR-181a/b-cluster was strongly reduced in bone marrow samples of APL patient while ATRA-based therapy. Furthermore, we showed the transcriptional induction of miR-181a and miR-181b by the APL-associated PML-RARα oncogene in vitro and in vivo. In PR9 cells, carrying a zinc-driven PML/RARα construct, and in PML/RARα-knock in mice the expression of the fusion gene leads to upregulation of the microRNA-cluster expression. Analysis of bone marrow samples of APL patients showed an enhanced expression of miR-181a and miR-181b in comparison to AML patient samples with normal karyotype, whereas other AML subgroups show no significant regulation. Based on siRNA experiments we could propose AP-1 and GATA-2 as potential co-activators for the PML/RARα-dependent regulation of the miR-181a/b-cluster. In functional studies in NB4 cells we observed after lentiviral knock down of miR-181a and miR-181b a significant reduction of colony size and number as well as proliferation rate. In contrast, transient overexpression of miR-181a and miR-181b led to an inhibition of ATRA-induced expression of the differentiation marker CD11b. In a microRNA target search we identified the novel ATRA regulated tumor suppressor RASSF1A as a putative target of miR-181a and miR-181b. In functional studies we showed that enforced expression of miR-181a and 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. Overexpression of RASSF1A in NB4 cells strongly induced apoptosis. Additional, we could show by western blot that the miR-181a/b-cluster and RASSF1A modulate cell cycle via regulation of cyclin D1. In conclusion, we identified the miR-181a/b-cluster as an important player in the PML/RARα associated APL. Moreover, we firstly described the miR-181a/b target RASSF1A as a crucial factor in the ATRA activated granulocytic differentiation program in APL. Our data reveal the importance of deregulated microRNA biogenesis in cancer and may provide novel biomarkers and therapeutic targets in myeloid leukemia.