High effectiveness of triptolide, an active diterpenoid triepoxide, in suppressing Kir-channel currents from human glioma cells

Triptolide (Trip), a diterpene triepoxide isolated from medicinal vine Trypterygium wilfordii Hook. F. possessed multiple biological activities including antineoplastic actions. However, no report concerning its effects on ion currents has been published. In this study, we attempted to determine whether this compound has any effects on ion currents in malignant glioma cells. The mRNA expression of KCNJ10 (Kir4.1) was detected in U373 glioma cells. The inwardly rectifying K⁺ currents (I_K(IR)) in U373 cells were almost fully blocked by BaCl₂ (1 mM). Trip (30 nM-10 μM) effectively decreased the amplitude of I_K(IR) in a concentration-dependent manner with an IC₅₀ value of 0.72 μM. In chlorotoxin-treated U373 cells, Trip-mediated block of I_K(IR) remained effective. Addition of Trip (3 μM) slightly inhibited the amplitude of Ca²⁺-activated K⁺ current and sustained K⁺ outward current in U373 cells. In cell-attached configuration, when Trip was added to the bath, the activity of inwardly rectifying K⁺ (Kir) channels diminished with no change in single-channel conductance. Its suppression of Kir channels was accompanied by a reduction in the slow component of mean open time. Under current-clamp conditions, addition of Trip depolarized the membrane along with changes in frequency histogram of resting potential. Block by this component of Kir4.1 channels may be an important mechanism underlying its actions on the functional activity of glioma cells. Targeting at Kir4.1 channels may be clinically useful as an adjunctive regimen to anti-cancer drugs.