High-mobility group box 1 up-regulates aquaporin 4 expression via microglia–astrocyte interaction

• HMGB1 initiates IL-1β production in microglia.
• IL-1β induces transfers of NF-κB into the nucleus in astrocytes.
• The nuclear import of NF-κB leads to AQP4 expression.

To clarify the mechanism of high-mobility group box (HMGB) 1-induced brain edema formation, this study focused on the effect of HMGB1 on aquaporin (AQP) 4, a water channel, in rat brain. Treatments for 6 h with 100–1000 ng/ml HMGB1, not showing self-toxicity, of primary-cultured rat astrocytes didnot increase AQP4 mRNA, unexpectedly. In contrast, intracerebroventricular (i.c.v.) injection of 300 ng of HMGB1 significantly increased AQP4 protein after 8 h and formed edema after 24 h in vivo. Thus, we investigated the roles of microglia as well as astrocytes. HMGB1 (1000 ng/ml) drastically increased interleukin (IL)-1β in the primary-cultured rat microglia after 2 h. The exposure of microglia to conditioned medium with HMGB1 and 3 mM adenosine 5′-triphosphate for 6 h significantly increased AQP4 mRNA in astrocytes after 6 h. Although 1000 ng/ml HMGB1 didnot induce transfer of nuclear factor (NF)-κB into the nucleus in astrocytes after 1 h, the conditioned medium containing IL-1β led to its nuclear import. As factors likely to be involved in the nuclear import of NF-κB besides IL-1β, nitric oxide and tumor necrosis factor-α didnot contribute under these conditions. Finally, i.c.v. injection of 30 nmol parthenolide, an NF-κB inhibitor, reversed 300 ng of HMGB1 injection-induced AQP4 protein increase after 8 h in vivo. The effect of parthenolide and the outcomes obtained so far suggest that HMGB1 indirectly up-regulates AQP4 expression through diffusible factor(s) such as IL-1β from microglia since HMGB1 by itself didnot affect NF-κB intracellular localization in astrocytes.