Role of the Oncogenic Raf-1 in Orchestration of Discrete Nuclear Factor-κB-Activating Pathways

Raf-1, a key kinase in the Ras signaling pathway, plays critical roles in cell differentiation, proliferation, and tumorigenesis. However, knowledge of the Raf-1 in inflammation is limited. Using an inducible oncogenic Raf-1, we show that the Raf-1 orchestrates the discrete NF-κB activating pathways. While the Raf-1 activation induces a modest IκB degradation by enhancing the basal IκB kinase activity, it contradictorily suppresses the proinflammatory cytokine inducible IκB kinase complex, leading to an inhibition of TNF-α- and IL-1β-induced NF-κB activation. Despite considerable degrees of overlap, LPS signaling is not affected by Raf-1. By either conditionally reducing Raf-1 activity or completely disrupting the Raf-1 signaling by PD98059, a specific inhibitor of MEK1, the otherwise inhibited cytokine responses can be restored. Moreover, when the activity of Raf-1 is up-regulated during the cell cycle progression from the G₀ phase to the late G₁ phase, the enhanced Raf-1 activity suffices to shift the TNF-α response from the sensitive to the insensitive state. Together, these studies elucidate a mechanism by which signaling outputs are shaped by the intracellular Raf-1, thus explaining the “cellular context”-dependent cytokine response.