Ras effector pathways modulate scatter factor-stimulated NF-κB signaling and protection against DNA damage

Scatter factor (SF) (hepatocyte growth factor) is a pleiotrophic cytokine that accumulates within tumors in vivo and protects tumor cells against cytotoxicity and apoptosis due to DNA damaging agents in vitro. Previous studies have established that SF-mediated cell protection involves antiapoptotic signaling from its receptor (c-Met) to PI3 kinase → c-Akt → Pak1 (p21-activated kinase -1) → NF-κB (nuclear factor-kappa B). Here, we found that Ras proteins (H-Ras and R-Ras) enhance SF-mediated activation of NF-κB and protection of DU-145 and MDCK (Madin-Darby canine kidney) cells against the topoisomerase IIα inhibitor adriamycin. Studies of Ras effector loop mutants and their downstream effectors suggest that Ras/PI3 kinase and Ras/Raf1 pathways contribute to SF stimulation of NF-κB signaling and cell protection. Further studies revealed that Raf1 positively regulates the ability of SF to stimulate NF-κB activity and cell protection. The ability of Raf1 to stimulate NF-κB activity was not due to the classical Raf1 → MEK1/2 → ERK1/2 pathway. However, we found that a MEK3/6 → p38 pathway contributes to SF-mediated activation of NF-κB. In contrast, RalA, a target of the Ras/RalGDS pathway negatively regulated the ability of SF to stimulate NF-κB activity and cell protection. Ras, Raf1 and RalA modulate SF stimulation of NF-κB activity, in part, by regulating IκB kinase (IKK)-β kinase activity. These findings suggest that Ras/Raf1/RalA pathways may converge to modulate NF-κB activation and SF-mediated survival signaling at the IKK complex and/or a kinase upstream of this complex.