Nordihydroguaiaretic acid depletes ATP and inhibits a swelling-activated, ATP-sensitive taurine channel

Abstract

The mechanism by which nordihydroguaiaretic acid (NDGA), a lipoxygenase inhibitor, prevents swelling-activated organic osmolyte efflux was examined in the human hepatoma cell line Hep G2. When swollen in hypotonic medium, Hep G2 cell exhibited a regulatory volume decrease that was associated with the release of intracellular taurine, an amino acid found at a concentrations of 22.0 +/- 2.5 nmol/mg protein (approximately 5 mM) in these cells. Rate coefficients for swelling-activated [3H]taurine uptake and efflux were unaffected when extracellular taurine was increased from 0.1 to 25 mM, indicating that taurine is released via a channel. Taurine efflux was rapidly activated after cell swelling and immediately inactivated when cells were returned to normal size by restoration of isotonicity. Swelling-activated taurine efflux was not altered by replacement of extracellular Na+ with choline+ or K+ but was inhibited when cellular ATP levels were decreased with a variety of chemical agents, consistent with an ATP-regulated channel previously described in other cell types. NDGA inhibited swelling-activated [3H]taurine efflux in Hep G2 cells at concentrations of 50-150 microM; however, these same concentrations of NDGA also lowered cell ATP levels. Likewise, ketoconazole, an inhibitor of cytochrome P-450 monoxygenases, inhibited [3H]taurine efflux only at concentrations at which cell ATP levels were also lowered. In contrast, other inhibitors of cyclooxygenase (indomethacin, 100 microM) or of lipoxygenases (caffeic acid, 100 microM), as well as arachidonic acid itself (100 microM), had no effect on either taurine efflux or cell ATP. The present findings characterize a swelling-activated, ATP-sensitive osmolyte channel in Hep G2 cells and demonstrate that inactivation of the channel by NDGA is related to the ability of this drug to deplete cellular ATP.