Soft substrate up-regulates the interaction of STIM1 with store-operated Ca²⁺ channels that lead to normal epithelial cell apoptosis

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.