Ribosomal S6 kinase-1 modulates interleukin-1β-induced persistent activation of NF-κB through phosphorylation of IκBβ
Activation of NF-κB requires the phosphorylation and degradation of its associated inhibitory proteins, IκB. Previously, we reported that the extracellular signal-regulated kinase (ERK) is required for IL-1β to induce persistent activation of NF-κB in cultured rat vascular smooth muscle cells (VSMCs). The present study examined the mechanism by which the ERK signaling cascade modulates the duration of NF-κB activation. In cultured rat VSMCs, IL-1β activated ERK and induced degradation of both IκBα and IκBβ, which was associated with nuclear translocation of both ribosomal S6 kinase (RSK)1 and NF-κB p65. RSK1, a downstream kinase of ERK, was associated with an IκBβ/NF-κB complex, which was independent of the phosphorylation status of RSK1. Treatment of VSMCs with IL-1β decreased IκBβ in the RSK1/IκBβ/NF-κB complex, an effect that was attenuated by inhibition of ERK activation. Knockdown of RSK1 by small interference RNA attenuated the IL-1β-induced IκBβ decrease without influencing ether ERK phosphorylation or the earlier IκBα degradation. By using recombinant wild-type and mutant IκBβ proteins, both active ERK2 and RSK1 were found to directly phosphorylate IκBβ, but only active RSK1 phosphorylated IκBβ on Ser19 and Ser23, two sites known to mediate the subsequent ubiquitination and degradation. In conclusion, in the ERK signaling cascade, RSK1 is a key component that directly phosphorylates IκBβ and contributes to the persistent activation of NF-κB by IL-1β.