BLOCKADE OF MITOGEN-ACTIVATED PROTEIN-KINASE CASCADE SIGNALING IN INTERLEUKIN 6-INDEPENDENT MULTIPLE-MYELOMA CELLS

Interleukin 6 (IL-6) is a growth factor for multiple myeloma (MM) cell s, Set not all MM cell lines or patient cells require IL-6 for their g rowth, It is well known that IL-6 activates the signal transducers and activators of transcription (stat) 1-stat3 heterodimer, stat3 homodim er, and Ras-dependent mitogen-activated protein kinase (MAPK) cascades in multiple cell systems, We have shown previously that the MAPK path way is an important pathway for IL-6-mediated MM cell growth, In this study, we delineate the pattern of upstream MAPK cascade activation in IL-6-responsive B9 cells and in IL-6-nonresponsive U266, OCI-My5, and RPMI8226 MM cells to define sites of blockade of this pathway associa ted with loss of responsiveness to IL-6. In B9 cells, IL-6 triggered t he following in sequence: gp130 phosphorylation, gp130-to-protein tyro sine phosphatase 1D (PTP1D) binding, PTP1D phosphorylation, PTP1D comp lex formation with Grb2-Son of sevenless 1 (Sos1), and Sos1 phosphoryl ation, gp130 phosphorylation, gp130-to-PTP1D binding, PTP1D phosphoryl ation, and PTP1D-to-Grb2 binding are also induced by IL-6 in all IL-6- independent MM cell lines studied, However, Grb2 is not associated wit h Sos1, and neither Grb2-to-Sos1 binding nor Sos1 phosphorylation is t riggered by IL-6 in OCI-My5 MM cells, On the other hand, Grb2 and Sos1 are associated constitutively in U266 and RPMI8226 MR I cells, but ph osphorylation of Sos1 is not induced by IL-6, These data suggest that lack of Sos1 activation is associated with loss of IL-6 responsiveness in MM cell lines that grow independently of IL-6.