Swelling-activated taurine and K⁺ transport in human cervical cancer cells: Association with cell cycle progression

The aim of this study was to investigate swelling-activated taurine and K⁺ transport in human cervical cancer cells under various culture conditions, testing the hypothesis that the progression of cell cycle was accompanied by differential activities of swelling-activated transport pathways. Aphidicolin, an inhibitor of deoxyribonucleic acid (DNA) synthesis, was used to synchronize the cell cycle. The distribution of cell cycle stage was determined by fluorescence-activated cell sorting (FACS). Hypotonicity activated taurine efflux, which was sensitive to tamoxifen and 5-nitro-2-(3-phenylpro-pylamino) benzoic acid (NPPB). Cell swelling also induced both Cl^--dependent and -independent K⁺ (⁸⁶Rb⁺) efflux, presumably mediated by KCl cotransport (KCC) and Ca²⁺-activated K⁺ channels, respectively. Cell cycle arrest in G0/G1 was accompanied by a remarkable decrease in the rate constant for swelling-activated taurine efflux, from 0.20±0.007 to 0.026±0.002 min^-1 (n=6). The activity of swelling-activated taurine efflux recovered progressively on re-entry into the cell cycle. After removal of aphidicolin and culture with 10% fetal calf serum for 10 h, the rate constant increased significantly from 0.026±0.002 to 0.093±0.002 min^-1 (n=6). After 24 h release from aphidicolin, the efflux rate constant had increased further to 0.195±0.006 min^-1 (n=6), a value not significantly different from that in normally proliferating cells. The differential activities of swelling-activated taurine transport matched well with our previous study showing a volume-sensitive anion channel associated with cell cycle progression. In contrast to the differential activities of swelling-activated taurine transport, swelling-activated K⁺ (⁸⁶Rb⁺) transport was independent of the progression of cell cycle. Most importantly, pharmacological blockade of swelling-activated taurine efflux by tamoxifen or NPPB caused proliferating cervical cancer cells to arrest in G0/G1, suggesting that the activity of this efflux was associated with G1/S checkpoint progression. This study provides new and important information on the functional significance of swelling-activated transport system in the regulation of cell cycle clock of human cervical cancer cells.