Involvement of PKC-α in regulatory volume decrease responses and activation of volume-sensitive chloride channels in human cervical cancer HT-3 cells

1. The present study was carried out to identify the specific protein kinase C (PKC) isoform involved in regulatory volume decrease (RVD) responses, and to investigate the signal transduction pathways underlying the activation of volume-sensitive chloride channels in human cervical cancer HT-3 cells. The role of Ca²⁺ in RVD and in the activation of chloride currents was also studied. 2. The time course of RVDs was prolonged by microinjection of PKC-α antibody but not by PKC-β or PKC-γ antibody, and also by exposure to Ca²⁺-free medium, in particular when combined with microinjection of EDTA. Immunofluorescence staining showed that hypotonic superfusion evoked the translocation of PKC-α to the cell membrane, whereas PKC-β or PRC-γ remained unaffected. The translocation of PKC-α was observed a few minutes after hypotonic stress, reaching peak intensity at 30 min, and returned to the cytoplasm 60 min after hypotonic exposure. Western blot analyses showed an increased PKC-α level in terms of intensity and phosphorylation in the cell membrane, while neither PKC-β nor PKC-γ was activated upon hyposmotic challenge. 3. Whole-cell patch-clamp studies demonstrated that neomycin and PKC blockers such as staurosporine and H7 inhibited volume-sensitive chloride currents. The inhibitory effect of neomycin on chloride currents can be reversed by the PKC activator phorbol 12-myristate, 13-acetate (PMA). Moreover, the PKC inhibitor and PKC-α antibody, but not PKC-β or PKC-γ antibody, significantly attenuated the chloride currents. The activation of volume-sensitive chloride currents were insensitive to the changes of intracellular Ca²⁺ but required the presence of extracellular Ca²⁺. 4. Our results suggest the involvement of PKC-α and extracellular Ca²⁺ in RVD responses and the activation of volume-sensitive chloride channels in HT-3 cells.