Soft Substrate Up-regulates the Interaction of STIM1 with Store-operated Ca2+Channels That Lead to Normal Epithelial Cell Apoptosis

Abstract

We have demonstrated that soft substrate induced apoptosis in polarized cells, but not in transformed cells by disturbance of Ca2+homeostasis. This study aims to further investigate the regulatory mechanisms underlying the disruption of Ca2+-signaling integrity in soft substrate–induced epithelial apoptosis. Soft substrate up-regulated the store-operated Ca2+(SOC) entry across the plasma membrane of normal cervical epithelial cells, which resulted in increased cytosolic Ca2+levels. Concomitantly, soft substrate induced the aggregation and translocation of stromal interacting molecule 1 (STIM1) toward the cell periphery to colocalize with Orai1, an essential pore subunit of SOC channel, detected by fluorescence resonance energy transfer approach and confocal image analyses. The disturbed Ca2+homeostasis resulted in the activation of μ-calpain, which cleaved α-spectrin, induced actin disorganization, and caused apoptosis. In contrast, soft substrate did not disturb Ca2+homeostasis or induce apoptosis in cervical cancer cells. Chelating extracellular Ca2+by EGTA and down-regulated SOC entry by small interfering RNA targeting STIM1 or inhibitors targeting Ca2+-binding site of calpain significantly inhibited soft substrate–induced activation of μ-calpain and epithelial cell apoptosis. Thus, soft substrate up-regulates the interaction of STIM1 with SOC channels, which results in the activation of μ-calpain and subsequently induces normal epithelial cell apoptosis.