MicroRNAs are important regulators of gene expression in normal development and disease. miR-9 is overexpressed in several cancer forms, including brain tumours, hepatocellular carcinomas, breast cancer and Hodgkin lymphoma (HL). Here we demonstrated a relevance for miR-9 in HL pathogenesis and identified two new targets Dicer1 and HuR. HL is characterized by a massive infiltration of immune cells and fibroblasts in the tumour, whereas malignant cells represent only 1% of the tumour mass. These infiltrates provide important survival and growth signals to the tumour cells, and several lines of evidence indicate that they are essential for the persistence of HL. We show that inhibition of miR-9 leads to derepression of DICER and HuR, which in turn results in a decrease in cytokine production by HL cells followed by an impaired ability to attract normal inflammatory cells. Finally, inhibition of miR-9 by a systemically delivered antimiR-9 in a xenograft model of HL increases the protein levels of HuR and DICER1 and results in decreased tumour outgrowth, confirming that miR-9 actively participates in HL pathogenesis and points to miR-9 as a potential therapeutic target.Oncogene ( Keywords: miR-9; cytokines; Hodgkin lymphoma; HuR; DICER1
INTRODUCTIONChronic inflammation is a well-recognized cause of cancer and up to 25% of all cancers are thought to be induced by either chronic infections or autoimmune diseases. 1 Inflammation not only works as a tumour-promoting agent but also influences other steps of tumorigenesis by inducing DNA damage, angiogenesis, invasion and metastasis. 2 Hodgkin lymphoma (HL) is one of the most frequent lymphomas in the western world 3 and it can be divided in several subtypes based on the morphology, composition of the infiltrates and the phenotype of the immune cells. The nodular sclerosis subtype is the most common subtype of classical HL (cHL) and accounts for about 60--70% of the cHL cases, followed by mixed cellularity cHL accounting for 20--25%.cHL is an unusual B-cell malignancy in which the tumour cells represent only 1% of the tumour bulk, whereas the vast majority of the cells are a mixture of infiltrating immune cells and fibroblasts actively attracted via chemokine secretion by the malignant cells, the so-called Hodgkin and Reed --Sternberg cells. 3 Several lines of evidence indicate that the infiltrating cells are necessary for the survival of the HL cells by providing growth and survival signals, and protection from NK cells. 3,4 MicroRNAs (miRNAs) are small non-coding RNAs that regulate gene expression at the posttranscriptional level by base-pairing to target mRNAs and promoting transcript instability and/or translational repression. 5 Mature miRNAs derive from long hairpin precursors that are sequentially processed by the RNase-III-type enzymes DROSHA and DICER1, respectively. 5 miRNAs have been implicated in several physiological and pathological events and can act as both tumour suppressors and oncogenes. 6