αB-Crystallin, a member of the small heat shock protein (HSP) family, accumulates in reactive astrocytes in a variety of pathological conditions. We previously reported the upregulation of αB-crystallin in response to high extracellular potassium concentration. In the present study, we investigated the regulatory mechanism of aB-crystallin expression by KCL When human glioma U-251MG cells were exposed to continuous KCl treatment, induction of αB-crystallin mRNA was observed after 8h and persisted for a few days. Functional promoter analysis using deletion and mutation constructs revealed that the proximal heat shock element (HSE-P), which contributes to heat shock induction in HeLa cells, is essential for transcriptional activation of the eB-crystallin gene by KCl in U-251MG cells. Gel mobility shift and antibody supershift assays showed that KCl induces the HSE-binding activity of heat shock factor (HSF) 2, while heat shock induces that of HSF1. This is the first demonstration that HSF2 can be activated by KCl and is involved in the upregulation of αB-crystallin gene expression in glial cells.