Role of ERK map kinase and CRM1 in IL‐1β‐stimulated release of HMGB1 from cortical astrocytes

Abstract

Reactive astrocytes are traditionally thought to impede brain plasticity after stroke. However, we previously showed that reactive astrocytes may also contribute to stroke recovery, partly via the release of a nuclear protein called high‐mobility group box 1 (HMGB1). Here, we investigate the mechanisms that allow stimulated astrocytes to release HMGB1. Exposure of rat primary astrocytes to IL‐1β for 24 h elicited a dose‐dependent HMGB1 response. Immunostaining and western blots of cell lysates showed increased intracellular levels of HMGB1. Western blots confirmed that IL‐1β induced a release of HMGB1 into astrocyte conditioned media. MAP kinase signaling was involved. Levels of phospho‐ERK were increased by IL‐1β, and the MEK/ERK inhibitor U0126 decreased HMGB1 upregulation in the stimulated astrocytes. Since HMGB1 is a nuclear protein, the role of the nuclear protein exporter, chromosome region maintenance 1 (CRM1), was assessed as a candidate mechanism for linking MAP kinase signaling to HMGB1 release. IL‐1β increased CRM1 expression in concert with a translocation of HMGB1 from nucleus into cytoplasm. Blockade of IL‐1β‐stimulated HMGB1 release with the ERK inhibitor U0126 was accompanied by a downregulation of CRM1. Our findings reveal that IL‐1β stimulates the release of HMGB1 from activated astrocytes via ERK MAP kinase and CRM1 signaling. These data suggest a novel pathway by which inflammatory cytokines may enhance the ability of reactive astrocytes to release prorecovery mediators after stroke.