Reduction in Raf kinase inhibitor protein expression is associated with increased Ras-extracellular signal-regulated kinase signaling in melanoma cell lines.
Mutations in the Raf signaling pathway are known to play a pivotal role in the progression of malignant melanoma. In this study, we provide evidence that the Raf-1 kinase inhibitory protein (RKIP) and its effects on Raf-1-mediated activation of mitogen-activated protein/extracellular signal-regulated kinase kinase are important for the metastatic potential of malignant melanoma. Screening nine melanoma cell lines at mRNA and protein levels, we detected significant down-regulation of RKIP expression in comparison with normal melanocytes. Loss of RKIP expression in transformed cells in vivo was confirmed in immunohistochemical analyses demonstrating reduction of RKIP expression already in primary melanoma and even stronger down-regulation or complete loss in melanoma metastases. Stable transfection of the melanoma cell line Mel Im with an RKIP expression plasmid blocked the Raf kinase pathway, resulting in down-regulation of extracellular signal-regulated kinase 1/2 and activator protein 1 activity. In very good agreement with the in vivo finding that down-regulation of RKIP expression is most obvious in melanoma metastasis, overexpression of RKIP in the highly invasive Mel Im cell line leads to a significant inhibition of invasiveness in vitro. Taken together, our results suggest that loss of RKIP in malignant melanoma contributes to enhanced invasiveness of transformed cells and therefore to progression of the disease.
Institute of Pathology, Medical School of the University of Regensburg, Franz-Josef-Strauss-Allee 11, D-93053 Regensburg, Germany.
Marion M
MM
Schuierer
Frauke
F
Bataille
Suzanne
S
Hagan
Walter
W
Kolch
Anja-Katrin
AK
Bosserhoff
D000727:Androgen-Binding Protein / Q000037:antagonists & inhibitors*; D002455:Cell Division; D002465:Cell Movement; D015536:Down-Regulation; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D008544:Melanocytes / Q000378:metabolism / Q000473:pathology; D008545:Melanoma / Q000201:enzymology*; D020928:Mitogen-Activated Protein Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism*; D049488:Phosphatidylethanolamine Binding Protein; D019908:Proto-Oncogene Proteins c-raf / Q000378:metabolism; D015398:Signal Transduction; D012878:Skin Neoplasms / Q000201:enzymology*; D014162:Transfection; D014407:Tumor Cells, Cultured
D000727:Androgen-Binding Protein; D004791:Enzyme Inhibitors; C511482:PEBP1 protein, human; D049488:Phosphatidylethanolamine Binding Protein; D019908:Proto-Oncogene Proteins c-raf; D020928:Mitogen-Activated Protein Kinases