title: BCR-ABL- and Ras-independent activation of Raf as a novel mechanism of Imatinib resistance in CML.
abstract: Although the BCR-ABL tyrosine kinase inhibitor Imatinib has undoubtedly revolutionized the therapy of chronic myeloid leukaemia (CML), acquired drug resistance remains a common problem in CML therapy. Resistance often arises from second-line mutations in BCR-ABL or overexpression of the BCR-ABL protein but in ~20% of CML cases resistance mechanisms do not involve altered BCR-ABL function. Imatinib-resistant CML cell lines have been widely used for comparative proteome/genome-wide expression screens in order to decipher resistance mechanisms but a clearcut molecular mechanism or molecular player in BCR-ABL-independent resistance to Imatinib has not yet evolved from those studies. Here, we report the identification of a novel mechanism for Imatinib resistance in CML cells with unaltered BCR-ABL function. Pharmacological analysis evidenced a constitutive, Imatinib-insensitive activation of the Erk-MAPK pathway in resistant cells. A systematic analysis of pathway constituents illustrated that Ras-GTP accumulation remained fully sensitive to Imatinib but c-Raf activity from serum-fed cultures was largely resistant to the drug's action. Sequencing excluded mutations in either B-Raf or c-Raf as the origin of resistance, indicating that a functional alteration in the regulation of c-Raf activity was responsible for this effect. Collectively, these findings highlight a novel mechanism of acquired Imatinib resistance based on the BCR-ABL and Ras-independent constitutive activation of the Erk-MAPK pathway through activated c-Raf, which could prove helpful for a better functional classification of the causes of Imatinib resistance in CML.
authors.affiliation: Institute of Human Genetics, Jena University Hospital, Kollegiengasse 10, D-07743 Jena, Germany. julia.hentschel@mti.uni-jena.de
authors.forename: Julia
authors.initials: J
authors.lastname: Hentschel
authors.forename: Ignacio
authors.initials: I
authors.lastname: Rubio
authors.forename: Melanie
authors.initials: M
authors.lastname: Eberhart
authors.forename: Christina
authors.initials: C
authors.lastname: Hipler
authors.forename: Jana
authors.initials: J
authors.lastname: Schiefner
authors.forename: Katrin
authors.initials: K
authors.lastname: Schubert
authors.forename: Ivan F
authors.initials: IF
authors.lastname: Loncarevic
authors.forename: Ute
authors.initials: U
authors.lastname: Wittig
authors.forename: Aria
authors.initials: A
authors.lastname: Baniahmad
authors.forename: Ferdinand
authors.initials: F
authors.lastname: von Eggeling
mesh_terms: D001549:Benzamides / Q000494:pharmacology; D048708:Cell Growth Processes / Q000502:physiology; D045744:Cell Line, Tumor; D002999:Clone Cells; D019008:Drug Resistance, Neoplasm; D004789:Enzyme Activation; D048049:Extracellular Signal-Regulated MAP Kinases / Q000378:metabolism; D016044:Fusion Proteins, bcr-abl / Q000378:metabolism*; D006801:Humans; D000068877:Imatinib Mesylate; D015464:Leukemia, Myelogenous, Chronic, BCR-ABL Positive / Q000188:drug therapy* / Q000201:enzymology*; D020930:MAP Kinase Kinase Kinases / Q000037:antagonists & inhibitors / Q000378:metabolism; D010879:Piperazines / Q000494:pharmacology*; D047428:Protein Kinase Inhibitors / Q000494:pharmacology*; D011743:Pyrimidines / Q000494:pharmacology*; D048490:raf Kinases / Q000378:metabolism*; D018631:ras Proteins / Q000378:metabolism*
chemical_list: C120227:2-(2-chloro-4-iodophenylamino)-N-cyclopropylmethoxy-3,4-difluorobenzamide; D001549:Benzamides; D010879:Piperazines; D047428:Protein Kinase Inhibitors; D011743:Pyrimidines; D000068877:Imatinib Mesylate; D016044:Fusion Proteins, bcr-abl; D048490:raf Kinases; D048049:Extracellular Signal-Regulated MAP Kinases; D020930:MAP Kinase Kinase Kinases; D018631:ras Proteins