Role of c-Kit in Multiple Myeloma.

C-Kit is a receptor tyrosine kinase whose activation occurs by interaction with its ligand, the Stem Cell Factor (SCF). Two distinct isoforms of c-Kit have been identified, and are characterized by the presence or absence of the tetrapeptide sequence Gly-Asn-Asn-Lys (GNNK) in the juxtamembrane domain of the extracellular region. These isoforms signal with distinct properties. c-Kit has been reported to be expressed in the plasma cells of about 20% of patients with Multiple myeloma (MM), but not in normal plasma cells. Yet, its functional and biochemical signalling properties in myeloma cells have not been described. In this study we show that patients and cell lines of MM co-expressed both c-Kit isoforms (GNNK+ and GNNK−), being predominant the GNNK− isoform. c-Kit was activated by SCF in the cell line RPMI8226, and in cell lines derived from patients. To study the properties of the two c-Kit isoforms, we have expressed these isoforms in the cell line MM1S which do not express these receptors in a significantly high level. Both receptors types were expressed by retroviral infection. Addition of SCF provoked their tyrosine phosphorylation. Imatinib mesylate, that is a known inhibitor of c-Kit, inhibited the phosphorylation induced by SCF of both isoforms. Both receptors coupled to the PI3K/Akt pathway, but were unable to substantially activate the Erk1/2 MAPKs. Differences in the kinetics of phosphorylation between the isoforms GNNK+ and GNNK− were observed. The variant GNNK− was more rapidly and highly phosphorylated than the GNNK+ isoform, which was activated more slowly but this activation persisted a longer time. In MM1S-GNNK+ cells the principal substrate of Akt activated by SCF was the p70S6K, and this activation was dependent on the activity of the mammalian Target Of Rapamycin (mTOR). Finally, the expression of the c-Kit isoforms in MM1S partially prevented their death induced by dexamethasone. These data indicate that c-Kit expression in MM cells is functional and couples to survival pathways that may modulate cell death in response to therapeutic compounds used in the treatment of this disease.