Loss of CALU disrupts intracellular Ca2+ homeostasis and triggers apoptosis in glioma cells.

Glioma is an aggressive primary tumor of the central nervous system. Calcium (Ca2+) signaling between the ER and mitochondria is essential for cell survival and death regulation. Calumenin (CALU), an ER-resident Ca2+-binding protein, has been implicated in several cancers, but its role in glioma remains unclear. Public datasets (TCGA, CGGA) were analyzed to assess CALU expression and prognosis. We manipulated CALU through transfection technology and tested its role in Ca2+ responses, intracellular Ca2+ store-associated fluorescence, and mitochondrial Ca2+-associated fluorescence signals by means of Fluo-4, Mag-Fluo-4, and Rhod-2 probes, respectively. Mitochondrial Ca2+uptake was further assessed using an isolated mitochondrial Calcium Green-5 N assay. Mitochondrial function was assessed through the detection of MMP and mPTP utilizing JC-1 staining and calcein-AM/cobalt assay. Mitochondrial ROS and total ROS were estimated via mitoSox red staining and DCFH-DA assay. The cell apoptosis was appraised utilizing flow cytometry and TUNEL staining. A xenograft model using U87 glioma cells was established to further explore the role of CALU in vivo. CALU expression was highly expressed in gliomas and correlated with poor survival and mitochondrial Ca2+ transport genes. CALU depletion reduced intracellular Ca2+ store-associated fluorescence, enhanced store-operated Ca2+ entry (SOCE), and induced sustained cytosolic Ca2+elevation accompanied by enhanced mitochondrial Ca2+-associated fluorescence signals and enhanced mitochondrial Ca2+ uptake. These changes were associated with decreased MMP, increased mitochondrial calcein fluorescence, elevated ROS generation, and facilitated apoptosis. In vivo, CALU knockdown reduced tumor growth, increased 4-HNE and cleaved caspase-3 expression and reduced Ki-67 expression, which were partially reversed by NAC treatment. These findings suggest that CALU may contribute to the maintenance of intracellular Ca2+ homeostasis in glioma cells. Loss of CALU induces SOCE-associated Ca2+dysregulation, mitochondrial dysfunction, and ROS-mediated apoptosis, highlighting CALU as a potential therapeutic target for glioma.