Annexin A6 suppresses breast cancer cell proliferation by inhibiting excessive receptor‐activated increase in cytosolic calcium

Abstract

Annexin A6 (AnxA6) is an unusual member of the annexin family of Ca2+-dependent membrane binding proteins that upon cell membrane association inhibits store-operated Ca2+ influx. This suggests that AnxA6 plays a role in a feedback mechanism that regulates receptor-activated rise in cytosolic Ca2+. Here we show that in the AnxA6-expressing BT-549 breast cancer cells AnxA6 translocates from the cytosol to the plasma membrane at high extracellular Ca2+ (2 mM). Its depletion in these cells inhibited the EGF receptor induced Ca2+ influx without affecting store-independent Ca2+ entry. Decreased AnxA6 expression is therefore accompanied by a 2-fold increase in intracellular Ca2+ concentrations and enhanced anchorage-independent cell growth. Moreover, depletion of AnxA6 in BT-549 cells not only led to impaired cell-cell cohesion and cell spreading onto collagen type IV but also inhibited cell motility and invasiveness. Finally, whereas the MAP kinase (ERK1/2) pathway remained constitutively active, Ca2+-dependent activation of focal adhesion kinase and the phosphoinositide-3 (PI3) kinase/Akt pathway were strongly inhibited in the AnxA6-depleted BT-549 cells. These data suggest that loss of AnxA6 promotes breast cancer tumorigenesis by uncoupling the feedback mechanism that prevents abnormal or sustained increases in intracellular Ca2+. Supported by grants: 1 SC1 CA134018-01 and DOD W81XWH-07-1-0254