title: Expression of c-Kit isoforms in multiple myeloma: differences in signaling and drug sensitivity.
abstract: BACKGROUND
  c-Kit is expressed in the plasma cells from 30% of patients with multiple myeloma. Two different isoforms of c-Kit, characterized by the presence or absence of the tetrapeptide sequence GNNK in the extracellular domain, have been described. However, their expression and function in myeloma cells are unknown. We explored the function and expression of these c-Kit isoforms in myeloma cells.
  
  
  METHODS
  Expression of c-Kit isoforms was investigated by reverse transcriptase polymerase chain reaction in fresh plasma cells from patients and cell lines. The function of these c-Kit isoforms was analyzed upon expression in myeloma cells. Signaling was investigated by western blotting using antibodies specific for activated forms of several signaling proteins. The impact of c-Kit on the action of drugs commonly used in the treatment of multiple myeloma was investigated by MTT proliferation assays.
  
  
  RESULTS
  Fresh plasma cells from patients as well as myeloma cell lines expressed the two isoforms of c-Kit. Retroviral infection of myeloma cells with vectors that code for c-Kit-GNNK+ or c-Kit-GNNK- forms demonstrated differences in the kinetics of phosphorylation between these isoforms. Stem cell factor-induced activation of the GNNK- form was faster and more pronounced than that of the GNNK+ form, whose activation, however, lasted for longer. The c-Kit receptors weakly activated the Erk1/2 and Erk5 pathways. Both receptors, however, efficiently coupled to the PI3K/Akt pathway, and stimulated p70S6K activation. The latter was sensitive to the mTOR inhibitor, rapamycin. Studies of drug sensitivity indicated that cells expressing the GNNK- form were more resistant to the anti-myeloma action of bortezomib and melphalan.
  
  
  CONCLUSIONS
  Our data indicate that c-Kit expression in multiple myeloma cells is functional, and coupled to survival pathways that may modulate cell death in response to therapeutic compounds used in the treatment of this disease.
authors.affiliation: Centro de Investigación del Cáncer-IBMCC, CSIC-Universidad de Salamanca, and Department of Hematology, University Hospital of Salamanca, Salamanca, Spain.
authors.forename: Juan Carlos
authors.initials: JC
authors.lastname: Montero
authors.forename: Ricardo
authors.initials: R
authors.lastname: López-Pérez
authors.forename: Jesús F
authors.initials: JF
authors.lastname: San Miguel
authors.forename: Atanasio
authors.initials: A
authors.lastname: Pandiella
mesh_terms: D000970:Antineoplastic Agents / Q000494:pharmacology; D001897:Boronic Acids / Q000494:pharmacology; D000069286:Bortezomib; D016923:Cell Death; D002460:Cell Line; D045744:Cell Line, Tumor; D015972:Gene Expression Regulation, Neoplastic; D006801:Humans; D016130:Immunophenotyping; D008558:Melphalan / Q000494:pharmacology; D009101:Multiple Myeloma / Q000188:drug therapy / Q000378:metabolism*; D020033:Protein Isoforms; D011494:Protein Kinases / Q000378:metabolism; D019009:Proto-Oncogene Proteins c-kit / Q000096:biosynthesis* / Q000737:chemistry*; D011719:Pyrazines / Q000494:pharmacology; D015398:Signal Transduction; D020123:Sirolimus / Q000494:pharmacology; D058570:TOR Serine-Threonine Kinases
chemical_list: D000970:Antineoplastic Agents; D001897:Boronic Acids; D020033:Protein Isoforms; D011719:Pyrazines; D000069286:Bortezomib; D011494:Protein Kinases; C546842:MTOR protein, human; D019009:Proto-Oncogene Proteins c-kit; D058570:TOR Serine-Threonine Kinases; D008558:Melphalan; D020123:Sirolimus