Interleukin-6 (LI-6) is a known growth and survival factor in multiple myeloma via activation of extracellular signal-regulated kinase and phosphatidylinositol 3-kinase signaling cascade. In this report we show that Grb2-associated binder (Gab) family adapter proteins Gab1 and Gab2 are expressed by multiple myeloma cells; and that interleukin-6 induces their tyrosine phosphorylation and association with downstream signaling molecules. We further demonstrate that these events are Src family tyrosine kinase-dependent and specifically identify the role of hematopoietic cell kinase (Hck) as a new Gab family adapter protein kinase. Conversely, inhibition of Src family tyrosine kinases by the pyrazolopyrimidine PP2, as in kinase-inactive Hck mutants, significantly reduces IL-6-triggered activation of extracellular signal-regulated kinase and AKT-1, leading to significant reduction of multiple myeloma cell proliferation and survival. Taken together, these results delineate a key role for Hck-mediated phosphorylation of Gab1 and Gab2 docking proteins in IL-6-induced proliferation and survival of multiple myeloma cells and identify tyrosine kinases and downstream adapter proteins as potential new therapeutic targets in multiple myeloma. 1 is a potent growth and survival factor in multiple myeloma (MM) initiating signaling pathways via binding to the IL-6 receptor. Early IL-6 signaling events that ultimately lead to the activation of extracellular signal-regulated kinase (ERK) and phosphatidylinositol 3-kinase (PI3K) are poorly understood. The IL-6 receptor is composed of two ␣-chains (IL-6 receptor ␣, gp80, 80 kDa), which are the binding molecules, and two -chains (gp130, 130 kDa), which are the signal transducers. IL-6 binding to its receptor triggers both the association of IL-6R␣ with gp130 and gp130 phosphorylation. The binding and activation of Src homology-containing tyrosine phosphatase (SHP)-2 and signal transducer and activator of transcription (STAT) 3 are dependent on the phosphorylation of cytoplasmic gp130 domain residues Tyr-759 and Tyr-767, Tyr-814, Tyr-905, and Tyr-915, respectively (1, 2). Hematopoietic cell kinase (Hck) is a member of the highly conserved Src family of protein-tyrosine kinases (SFKs), which mediate mitogenesis, differentiation, survival, migration, and adhesion (3). In contrast to the ubiquitous expression pattern of Src, Yes, and Fyn, the expression of Hck is restricted to the hematopoietic system; specifically, Hck is preferentially expressed in hematopoietic cells of the myeloid and B-lymphoid lineages (4, 5). In B-lymphoid lineages Hck is a common feature of pro-B-cells and its expression decreases with B-cell differentiation (6). Importantly, Hck binds to an "acidic" domain comprising amino acids 771-811 of gp130 and is independent of STAT3 and SHP2 association via tyrosine residues 759, 767, and 814. Functionally, this region is responsible for the activation of Hck and subsequently for the phosphorylation of ERK and the dephosphorylation of Pyk2, therefore mediating cell prolif...