SummaryDespite the established role of mitochondria in tumorigenesis, the molecular mechanisms by which mitochondrial Ca2+ (mtCa2+) signaling regulates tumor growth and metastasis remain unknown. The crucial role of mtCa2+ in tumorigenesis is highlighted by the altered expression of proteins mediating mtCa2+ uptake and extrusion in cancer cells. Here, we demonstrate that expression of the mitochondrial Na+/Ca2+ exchanger NCLX (SLC8B1) is decreased in colorectal tumors and is associated with advanced-stage disease in patients. We reveal that downregulation of NCLX leads to mtCa2+ overload, mitochondrial depolarization, mitophagy, and reduced tumor size. Concomitantly, NCLX downregulation drives metastatic spread, chemoresistance, the expression of epithelial-to-mesenchymal transition (EMT), hypoxia, and stem cell pathways. Mechanistically, mtCa2+ overload leads to an increase in mitochondrial reactive oxygen species (mtROS) which activates HIF1α signaling supporting the metastatic behavior of tumor cells lacking NCLX. Our results reveal that loss of NCLX expression is a novel driver of metastatic progression, indicating that control of mtCa2+ levels is a novel therapeutic approach in metastatic colorectal cancer.HighlightsThe expression of NCLX is decreased in colorectal tumors and is associated with advanced-stage disease in patients.NCLX plays a dichotomous role in colorectal tumor growth and metastasis.NCLX downregulation causes mitophagy and reduced colorectal cancer tumor growth.NCLX downregulation induces stemness, chemoresistance and metastasis through mtCa2+/ROS/HIF1α signaling axis.Graphical AbstractSignificanceMitochondrial Ca2+ (mtCa2+) homeostasis is essential for cellular metabolism and growth and plays a critical role in cancer progression. mtCa2+ uptake is mediated by an inner membrane protein complex containing the mitochondrial Ca2+ uniporter (MCU). mtCa2+ uptake by the MCU is followed by a ∼100-fold slower mtCa2+ extrusion mediated by the inner mitochondrial membrane ion transporter, the mitochondrial Na+/Ca2+ exchanger NCLX. Because NCLX is a slower transporter than the MCU, it is a crucial rate-limiting factor of mtCa2+ homeostasis that cannot easily be compensated by another Ca2+ transport mechanism. This represents the first study investigating the role of NCLX in tumorigenesis and metastasis. We demonstrate for the first time that colorectal cancers exhibit loss of NCLX expression and that this is associated with advanced-stage disease. Intriguingly, decreased NCLX function has a dichotomous role in colorectal cancer. Thus, we reveal that NCLX loss leads to reduced primary tumor growth and overall tumor burden in vivo. Yet, the consequential increases in mtCa2+ elicit pro-survival, hypoxic and gene transcription pathways that enhance metastatic progression. This dichotomy is a well-established feature of chemoresistant and recurrent tumor cells including cancer stem cells. Moreover, the downstream changes elicited by NCLX loss are reminiscent of mesenchymal colorectal cancer subtypes that display poor patient survival. Our data indicate that the demonstrated changes to the mtCa2+/mtROS/HIF1α signaling axis elicited through the loss of NCLX are a key adaptation and driver of metastatic colorectal cancer.
Dichotomous role of the human mitochondrial Na+/Ca2+/Li+ exchanger NCLX in colorectal cancer growth and metastasis