Annexin A6 (AnxA6) is a Ca2+-dependent membrane-binding protein involved in vesicular traffic. The likely participation of AnxA6 in the response of lymphocytes to Ca2+ signals has not been investigated yet. The present study focuses on intracellular relocation of AnxA6 in human Jurkat T lymphoblasts upon stimulation followed by transient increase of intracellular [Ca2+] and exocytosis of interleukin-2 (IL-2). Stimulation of the cells under different experimental conditions (by lowering pH and/or by rising extracellular [Ca2+] in the presence of ionomycin) induced time-dependent transients of intracellular [Ca2+] and concomitant changes in AnxA6 intracellular localization and in IL-2 secretion, with only minor effects on cell viability and apoptosis. In resting conditions (in the presence of EGTA or with no ionophore) AnxA6 was localized uniformly in the cytosol, whereas it translocated to vesicular structures beneath the plasma membrane within 5 min following stimulation of Jurkat T cells and rise of intracellular [Ca2+] at pH 7.4. Lowering the extracellular pH value from 7.4 to 6.0 significantly enhanced this process. AnxA6 changed its location from the cytosol to the secretory granules and early endosomes which seem to represent membranous targets for annexin. In conclusion, AnxA6 is sensitive to variations in intracellular [Ca2+] upon stimulation of Jurkat T cells, as manifested by a switch in its intracellular localization from the cytosol to vesicular structures located in close proximity to the plasma membrane, suggestive of participation of AnxA6 in calcium- and proton-dependent secretion of cytokines by lymphocytes.
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