TRPM4 Channels Mediate Hypertonicity-induced, Ca²⁺-impermeable, Non-selective Cation Currents in a Cervical Cancer Cell Line, HeLa Cells

Abstract

When extracellular osmolarity exceeds intracellular osmolarity, cells initially shrink but then approach the original cell volume by so-called regulatory cell volume increase (RVI). RVI operates under physiological conditions so that impairment of RVI leads to immediate cell cycle arrest and apoptosis. In a cervical cancer cell line, HeLa cells, extracellular hypertonicity induced non-selective cation currents (I_HO currents) which transported mainly Na⁺ into cells to induce RVI as assessed with the patch-clamp method. Ion channels mediating these currents were Ca²⁺-impermeable and sensitive to flufenamic acid (FFA) and econazole but not to amiloride or ruthenium red. RT-PCR indicated that HeLa cells express transient receptor potential (TRP) C1, C6, M3, M4, M7, M8, V1 and V2 subunits which form non-selective cation channels. From these results, we speculated that TRPM4 may mediate I_HO currents. Indeed, transduction of a dominant-negative TRPM4 subunit significantly inhibited I_HO currents. TRPM4 channels became insensitive to hypertonic stimulus when intracellular Ca²⁺ was strongly buffered. Thus, extracellular hypertonicity activates TRPM4 channels through intracellular Ca²⁺ to induce RVI in HeLa cells. These results indicate that intra-uterus or -vaginal application of drugs blocking TRPM4 channels may cause antiproliferative/proapoptotic effects on cervical cancer.