Zinc (Zn(2+)) is considered to be one of the factors aggravating brain damage after cerebral ischemia. Since Zn(2+) activates microglia, immune cells in the brain, this metal is proposed to modulate neuroinflammatory responses in the post-ischemic brain. Interleukin (IL)-23 is a heterodimeric cytokine composed of the p19 subunit unique to IL-23 and the p40 subunit common to IL-12. IL-23 has been shown to play a critical role in the progression of ischemic brain injury. However, whether Zn(2+) participates in the expression of IL-23 in microglia remains unknown. In this study, we examined the effect of Zn(2+) on IL-23 p19 mRNA expression using rat immortalized microglia HAPI cells. Exposure to Zn(2+) dose- and time-dependently induced the expression of IL-23 p19 mRNA in HAPI cells. Inhibitors of MAPK and NF-κB pathways failed to suppress this induction. Interestingly, we found that Zn(2+) stimulated the phosphorylation of eIF2α and promoted the nuclear accumulation of activating transcription factor 4 (ATF4). Treatment with salubrinal, an eIF2α dephosphorylation inhibitor, enhanced Zn(2+)-induced ATF4 accumulation and IL-23 p19 mRNA expression. In addition, reporter assay using the IL-23 p19 promoter region revealed that ATF4 directly transactivated IL-23 p19 promoter and that dominant-negative ATF4 suppressed Zn(2+)-induced activation of IL-23 p19 promoter. Taken together, these findings suggest that Zn(2+) up-regulates expression of the IL-23 p19 gene via the eIF2α/ATF4 axis in HAPI cells.
Keywords: Activating transcription factor 4; IL-23; Microglia; Phosphorylation; Zinc; eIF2α.