Background: Micro RNAs (miRs) have emerged as key regulators during oncogenesis. They have been found to\nregulate cell proliferation, differentiation, and apoptosis. Mir-125b has been identified as an oncomir in various\nforms of tumours, but we have previously proposed that miR-125b is a suppressor of lymph node metastasis in\ncutaneous malignant melanoma. Our goal was therefore to further examine this theory.\nMethods: We used in-situ-hybridization to visualise miR-125b expression in primary tumours and in lymph node\nmetastasis. Then using a miRVector plasmid containing a miR-125b-1 insert we transfected melanoma cell line\nMel-Juso and then investigated the effect of the presence of a stable overexpression of miR-125b on growth\nby western blotting, flow cytometry and ?-galactosidase staining. The tumourogenicity of the transfected cells\nwas tested using a murine model and the tumours were further examined with in-situ-hybridization.\nResults: In primary human tumours and in lymph node metastases increased expression of miR-125b was found in\nsingle, large tumour cells with abundant cytoplasm. A stable overexpression of miR-125b in human melanoma cell\nline Mel-Juso resulted in a G0/G1 cell cycle block and emergence of large cells expressing senescence markers:\nsenescence-associated beta-galactosidase, p21, p27 and p53. Mel-Juso cells overexpressing miR-125b were\ntumourigenic in mice, but the tumours exhibited higher level of cell senescence and decreased expression of\nproliferation markers, cyclin D1 and Ki67 than the control tumours.\nConclusions: Our results confirm the theory that miR-125b functions as a tumour supressor in cutaneous malignant\nmelanoma by regulating cellular senescence, which is one of the central mechanisms protecting against the\ndevelopment and progression of malignant melanoma
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