The G-protein-coupled receptor kinase 5 inhibits NFκB transcriptional activity by inducing nuclear accumulation of IκBα

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Abstract
G-protein-coupled receptor (GPCR) kinases, GRKs, are known as serine/threonine kinases that regulate GPCR signaling, but recent findings propose functions for these kinases besides receptor desensitization. Indeed, GRK5 can translocate to the nucleus by means of a nuclear localization sequence, suggesting that this kinase regulates transcription events in the nucleus. To evaluate the effect of GRK5–IκBα interaction on NFκB signaling, we induced the overexpression and the knockdown of GRK5 in cell cultures. GRK5 overexpression causes nuclear accumulation of IκBα, leading to the inhibition of NFκB transcriptional activity. Opposite results are achieved by GRK5 knockdown through siRNA. A physical interaction between GRK5 and IκBα, rather than phosphorylative events, appears as the underlying mechanism. We identify the regulator of gene protein signaling homology domain of GRK5 (RH) and the N-terminal domain of IκBα as the regions involved in such interaction. To confirm the biological relevance of this mechanism of regulation for NFκB, we evaluated the effects of GRK5-RH on NFκB-dependent phenotypes. In particular, GRK5-RH overexpression impairs apoptosis protection and cytokine production in vitro and inflammation and tissue regeneration in vivo. Our results reveal an unexpected role for GRK5 in the regulation of NFκB transcription activity. Placing these findings in perspective, this mechanism may represent a therapeutic target for all those conditions involving excessive NFκB activity.