Evidence for Actin Cytoskeleton-dependent and -independent Pathways for RelA/p65 Nuclear Translocation in Endothelial Cells^*

Abstract

Activation of the transcription factor NF-κB involves its release from the inhibitory protein IκBα in the cytoplasm and subsequently, its translocation to the nucleus. Whereas the events responsible for its release have been elucidated, mechanisms regulating the nuclear transport of NF-κB remain elusive. We now provide evidence for actin cytoskeleton-dependent and -independent mechanisms of RelA/p65 nuclear transport using the proinflammatory mediators, thrombin and tumor necrosis factor α, respectively. We demonstrate that thrombin alters the actin cytoskeleton in endothelial cells and interfering with these alterations, whether by stabilizing or destabilizing the actin filaments, prevents thrombin-induced NF-κB activation and consequently, expression of its target gene, ICAM-1. The blockade of NF-κB activation occurs downstream of IκBα degradation and is associated with impaired RelA/p65 nuclear translocation. Importantly, thrombin induces association of RelA/p65 with actin and this interaction is sensitive to stabilization/destabilization of the actin filaments. In parallel studies, stabilizing or destabilizing the actin filaments fails to inhibit RelA/p65 nuclear accumulation and ICAM-1 expression by tumor necrosis factor α, consistent with its inability to induce actin filament formation comparable with thrombin. Thus, these studies reveal the existence of actin cytoskeleton-dependent and -independent pathways that may be engaged in a stimulus-specific manner to facilitate RelA/p65 nuclear import and thereby ICAM-1 expression in endothelial cells.