MicroRNA-9 suppresses the growth, migration, and invasion of malignant melanoma cells via targeting NRP1

MicroRNAs (miRs) are a class of small noncoding RNAs that negatively regulate the gene expression by directly binding to the 3' untranslated region of their target mRNA, thus resulting in mRNA degradation or translational repression. miR-9 has recently been demonstrated to play a role in the development and progression of malignant melanoma (MM), but the regulatory mechanism of miR-9 in the malignant phenotypes of MM still remains largely unknown. In this study, a total of 73 pairs of MM tissues and adjacent normal tissues were collected. Real-time reverse transcription polymerase chain reaction and Western blot were used to detect the mRNA and protein expression of miR-9. MTT assay, wound healing assay, and transwell assay were conducted to determine the cell proliferation, migration, and invasion. Luciferase reporter assay was used to determine the targeting relationship between miR-9 and NRP1. Our data demonstrated that miR-9 expression was significantly downregulated in MM tissues compared with that in adjacent normal tissues. The decreased miR-9 level was significantly associated with the tumor stage and metastasis of MM. We also found that the expression level of miR-9 was decreased in MM cell lines (G361, B16, A375, and HME1) compared with normal skin HACAT cells. Ectopic expression of miR-9 led to a significant decrease in the ability of proliferation, migration, and invasion in A375 cells. NRP1 was further identified as a direct target gene of miR-9, and the protein expression of NRP1 was negatively regulated by miR-9 in A375 cells. Furthermore, overexpression of NRP1 reversed the suppressive effects of miR-9 on the malignant phenotypes of A375 cells. In vivo study revealed that miR-9 overexpression decreased the tumor growth, while overexpression of NRP1 increased MM growth. In summary, our findings suggest that the miR-9/NRP1 axis may serve as a potential target for the treatment of MM.