Background: The intermediate-conductance Ca²⁺-activated K⁺ channel (K_Ca3.1) is one of the key molecules that modulate Ca²⁺ signaling by controlling Ca²⁺ entry through Ca²⁺ release activated Ca²⁺ channels in T cell. Our previous study demonstrated that the up-regulation of K_Ca3.1 in CD4⁺ T cells was involved in the pathogenesis of inflammatory bowel disease (IBD), and a significant decrease in IBD disease severity was elicited by administration of the K_Ca3.1 inhibitor, TRAM-34. K_Ca3.1 is possible therapeutic target for IBD, however, the mechanism of K_Ca3.1 up-regulation in CD4⁺ T cells remains unclear. Recent studies showed that histone deacetylase inhibition in vivo decreases disease responses in IBD. We examined the involvement of HDACs in up-regulation of K_Ca3.1 in CD4⁺ T cells from IBD model mice.

Methods: The mouse model of IBD was prepared by exposing male C57BL/6 mice to 5% dextran sulfate sodium for 7 days. Isolated splenocytes from IBD model mice were cultivated in the presence of the lectin, concanavalin A (Con A, 5 μg/mL) for 48 hr. 24 hours after the cultivation, cell suspensions were treated with vehicle control or HDAC inhibitors. CD4⁺ T cells were then isolated using the Dynabeads technology. The expression levels of K_Ca3.1, HDACs, and K_Ca3.1-related genes were determined by real-time PCR and/or western blot assays. K_Ca3.1 inhibitor-induced depolarization responses were monitored by voltage-sensitive dye imaging. All HDAC inhibitors were supplied by Professor Suzuki (Kyoto Prefectural University of Medicine, Japan).

Results: Real-time PCR assay showed the significant level expression of class I HDAC members (HDAC1, 2 and 3) in CD4⁺ T cells of normal mice, and they were significantly up-regulated in IBD model. Several activator protein 1 (AP1) components that are possible regulators of K_Ca3.1 transcription also up-regulated in IBD model. The treatment with the pan-HDAC inhibitor vorinostat (1 μM) induced both down-regulation of K_Ca3.1 and decrease in K_Ca3.1 activity in CD4⁺ T cells. Moreover, the selective inhibition of HDAC2 and HDAC3 significantly down-regulated K_Ca3.1.

Conclusion: These findings suggest that HDAC2/3 may be involved in the post-transcriptional regulation of K_Ca3.1 in inflammatory CD4⁺ cells.