Cell Shrinkage is Essential in Lysophosphatidic Acid Signaling in Ehrlich Ascites Tumor Cells

Abstract.

The present study aimed at elucidating the initial intracellular lysophosphatidic acid (LPA)-induced signaling events, in order to investigate the sequence in which LPA affects the intracellular concentration of free, cytosolic Ca²⁺, [Ca²⁺] _i , ion channels, the F-actin cytoskeleton, cell volume and the Na⁺/H⁺ exchanger. We found that stimulation of Ehrlich cells with LPA induced a transient, concentration-dependent increase in [Ca²⁺] _i , which is due to Ca²⁺ release from intracellular Ins(1,4,5)P₃-sensitive stores as well as an influx of Ca²⁺. The EC₅₀ values for LPA-induced Ca²⁺ mobilization were estimated at 0.03 nm and 0.4 nm LPA in the presence and absence of extracellular Ca²⁺, respectively. The LPA-induced increase in [Ca²⁺] _i resulted in (i) co-activation of Ca²⁺-activated, charybdotoxin (ChTX)-sensitive K⁺ and niflumic acid-sensitive Cl⁻ currents; (ii) a subsequent cell shrinkage and increased polymerization of F-actin, and (iii) activation of a Na⁺/H⁺ exchange, resulting in a concentration-dependent intracellular alkalinization. The EC₅₀ value for the LPA-induced rate of alkalinization was estimated at 0.37 nm LPA. When cell shrinkage was prevented, the LPA-induced activation of the Na⁺/H⁺ exchanger was impaired. In conclusion, the initial signaling events induced by LPA involves activation of volume regulatory mechanisms.