Human breast cancer MDA-MB-231 cells fail to express the neurofibromin protein, lack its type I mRNA isoform and show accumulation of P-MAPK and activated Ras

Neurofibromin is a tumor suppressor protein, which is similar in function t o the GTPase activating protein (GAP), p120GAP, in that it accelerates inac tivation of Ras. Mutations in the NF1 gene cause neurofibromatosis type 1, NF1, an autosomal dominant disease with a diverse spectrum of clinical mani festations, including neurofibromas. Ras activation (GTP binding) is induce d by the GTP exchange factor Sos and its inactivation is regulated through the GAPs (p120GAP and neurofibromin). Strikingly, neurofibromin was nearly absent in MB-231 human breast cancer cells and present in the remaining fou r cell lines studied, with higher levels in BT-474 and MB-453 than in MCF-7 and BT-20 cells, as tested with polyclonal antibodies to both the N-termin al as well as the C-terminal peptides. Coordinated with the near absence of neurofibromin, these cells also presented with much greater levels of P-MA PK and activated Ras. Further, RT-PCR analysis demonstrated the absence of expression of NFI mRNA type I isoform only in the MB-231 cell lines. This r esult documents for the first time an altered NFI expression at the protein and mRNA levels in MDA-MB-231 breast cancer cells. Published by Elsevier S cience Ireland Ltd.