MicroRNA-mediated loss of ADAR1 in metastatic melanoma promotes tumor growth

Abstract

Some solid tumors have reduced posttranscriptional RNA editing by adenosine deaminase acting on RNA (ADAR) enzymes, but the functional significance of this alteration has been unclear. Here, we found the primary RNA-editing enzyme ADAR1 is frequently reduced in metastatic melanomas. In situ analysis of melanoma samples using progression tissue microarrays indicated a substantial downregulation of ADAR1 during the metastatic transition. Further, ADAR1 knockdown altered cell morphology, promoted in vitro proliferation, and markedly enhanced the tumorigenicity in vivo. A comparative whole genome expression microarray analysis revealed that ADAR1 controls the expression of more than 100 microRNAs (miRNAs) that regulate many genes associated with the observed phenotypes. Importantly, we discovered that ADAR1 fundamentally regulates miRNA processing in an RNA binding-dependent, yet RNA editing-independent manner by regulating Dicer expression at the translational level via let-7. In addition, ADAR1 formed a complex with DGCR8 that was mutually exclusive with the DGCR8-Drosha complex that processes pri-miRNAs in the nucleus. We found that cancer cells silence ADAR1 by overexpressing miR-17 and miR-432, which both directly target the ADAR1 transcript. We further demonstrated that the genes encoding miR-17 and miR-432 are frequently amplified in melanoma and that aberrant hypomethylation of the imprinted DLK1-DIO3 region in chromosome 14 can also drive miR-432 overexpression.

ADAR1 is required for differentiation and neural induction by regulating microRNA processing in a catalytically independent manner

“…Recent studies reported that non-enzymatic ADAR1 participates in small RNA biogenesis in different organisms [19,20]. ADAR1 was suggested to interact with DICER to promote miRNA processing [22] or to form a complex with DGCR8 to inhibit pri-miRNA processing [21]. We found that ADAR1 binds directly to pri-miR302s, competes with DGCR8 and thus inhibits miRNA processing ( Figure 6).…”

“…It was recently reported that ADAR1 could either bind with DICER [22] or DGCR8 [21] to regulate miR-NA processing. Interestingly, we found that ADAR1 in hESCs was predominately localized to the nucleus (data not shown) as previously reported [49].…”

“…However, the detailed mechanism(s) by which the non-catalytic activity of ADAR1 acts in regulating miRNA processing has remained elusive. Recently, two research groups showed that ADAR1 interacted with different components of the miRNA biogenesis pathway, and exerted different effects on miRNA production [21,22]. While Nemlich et al [21] showed that ADAR1 could interact with nuclear DGCR8, which would be mutually exclusive with the DROSHA-DGCR8 interaction and subsequently inhibit miRNA processing, Ota et al [22] showed that ADAR1 could interact with Dicer in the cytoplasm to promote miRNA processing in differentiated cells.…”

“…Recently, two research groups showed that ADAR1 interacted with different components of the miRNA biogenesis pathway, and exerted different effects on miRNA production [21,22]. While Nemlich et al [21] showed that ADAR1 could interact with nuclear DGCR8, which would be mutually exclusive with the DROSHA-DGCR8 interaction and subsequently inhibit miRNA processing, Ota et al [22] showed that ADAR1 could interact with Dicer in the cytoplasm to promote miRNA processing in differentiated cells. Although these studies clearly suggest that the non-catalytic activity of ADAR1 and the miRNA biogenesis pathway are connected at multiple levels, the different results also strongly argue that the underlying mechanisms still require further investigation.…”

miRNA biogenesis: Biological impact in the development of cancer

“…In addition to its previously mentioned editing function, ADAR1, can form a complex along with DGCR8, preventing the association between DGCR8 and Drosha during pri-miRNA processing. Moreover, it seems that ADAR1 can control the expression of more than 100 miRNAs which are positive regulators of metastatic programs in melanoma 91 ( Fig. 2A).…”