SDF-1 synergistically enhances IL-3-induced activation of the Raf-1/MEK/Erk signaling pathway through activation of Rac and its effector Pak kinases to promote hematopoiesis and chemotaxis.

Stromal cell-derived factor 1 (SDF-1) cooperates with cytokines to promote hematopoiesis. Here we demonstrate that SDF-1 activates Erk synergistically with interleukin-3 (IL-3) in hematopoietic cells. Small GTPases Ras and Rac were prominently activated by IL-3 and SDF-1, respectively. In accordance with this, Raf-1 was significantly activated by IL-3 but not by SDF-1. SDF-1 strongly induced phosphorylation of Raf-1 on S338, the target site for the Rac effector Paks, and enhanced the IL-3-induced activation of Raf-1 and MEK. Furthermore, the synergistic activation of Erk was inhibited by expression of a dominant-negative mutant of Pak1 or that of Rac and was enhanced by an activated mutant of Pak1. SDF-1 and IL-3 also showed synergistic effects on expansion of hematopoietic cells and on induction of chemotaxis, which were both inhibited by the MEK inhibitor PD98059. These results suggest that SDF-1 synergistically enhances IL-3-induced Erk activation by up-regulating Raf-1 activity through the Rac effector Pak kinases to promote hematopoiesis.