In particular, mitochondrial Ca2+ overload, together with the accompanying ROS production, has been a critical factor for mitochondrial permeability transition pore (mPTP) opening. cell death response of cancer cells exposed to chemotherapeutics. In this review, we discuss the emerging SCH00013 role of ERCmitochondrial Ca2+ fluxes underlying these cancer-related features. the cytosolic process glycolysis. In aerobic conditions, pyruvate is transported into the mitochondria and metabolized to CO2 through the tricarboxylic acid (TCA) cycle. The TCA cycle is coupled to oxidative phosphorylation (OXPHOS), which is a pathway for the production of large amounts of ATP. In contrast, in anaerobic conditions, pyruvate is fermented to lactate, a process often referred to as anaerobic glycolysis, which is less energy effective. Nevertheless, proliferative cells exhibit enhanced glycolysis, producing high levels of lactate, even in the presence of O2 (aerobic glycolysis) (2). Cancer cells, which are characterized by uncontrolled proliferation and suppressed apoptosis, tend to switch to aerobic glycolysis despite the presence of sufficient O2 to support the OXPHOS pathway. As such, these cells display an elevated glucose consumption albeit without a proportional increase in its oxidation to CO2 together with an increased lactate production and lactate export, a phenomenon known as Warburg effect (3C5). Although glycolysis can produce ATP at a faster rate than OXPHOS (6) and may fuel biosynthesis with intermediates, cancer cells do not rely purely on glycolysis. The reprogrammed cellular metabolism in tumors also maintains sufficient levels of OXPHOS by using pyruvate generated by glycolysis. Indeed, the TCA cycle appears to complement glycolysis, supplying enough ATP, NADH, and biomass precursors for the biosynthesis of other macromolecules, like phospholipids and nucleotides (7). For instance, the TCA SCH00013 cycle intermediate oxaloacetate is used as a substrate for the biosynthesis of uridine monophosphate, a precursor of uridine-5-triphosphate and cytidine triphosphate involving a rate-limiting step executed by dihydroorotate dehydrogenase, which, in turn, catalyzes the synthesis of pyrimidines SCH00013 in the inner mitochondrial membrane (8). Its dehydrogenase activity depends on the electron transport chain (ETC), where it feeds the electrons of the dihydroorotate oxidation to the ETC by reducing respiratory ubiquinone. Thus, adequate ETC activity and proper pyrimidine biosynthesis are intimately linked (8). Mitochondrial Ca2+ Signals as Regulators of Cell Death and Survival Ca2+, a cofactor of several rate-limiting TCA enzymes [pyruvate-, isocitrate-, and -ketoglutarate dehydrogenases (PDH, IDH, and KGDH)], plays a pivotal role in the regulation of mitochondrial metabolism and bioenergetics (9). As such, Ca2+ present SCH00013 in the mitochondrial matrix is required for sufficient NADH and ATP production (10). Transfer of Ca2+ Signals from the Endoplasmic Reticulum (ER) to the Mitochondria The accumulation of Ca2+ into the mitochondria strictly depends on the ER, which serves as the main intracellular Ca2+-storage organelle. Ca2+ is stored in the ER by the action of ATP-driven sarco/endoplasmic reticulum Ca2+-ATPase (SERCA) with SERCA2b (11) as the housekeeping isoform and by ER luminal Ca2+-binding proteins like calreticulin and calnexin (12). Ca2+ can be released from the ER intracellular Ca2+-release channels, including inositol 1,4,5-trisphosphate receptors (IP3Rs) and ryanodine receptors (RyRs). IP3Rs, which are activated by the second messenger IP3, are IFITM1 ubiquitously expressed in virtually all human cell types (13, 14). IP3 is produced through the hydrolysis of phosphatidyl inositol 4,5-bisphosphate by phospholipase C (PLC)/, an enzyme activated in response to hormones, neurotransmitters, and antibodies. IP3R activity can be suppressed by compounds like xestospongin B (15), which directly inhibits IP3Rs, or U73122, which inhibits PLC activity (16). Although 2-APB (17) and xestospongin C (18) are also used as IP3R inhibitors, these compounds affect other Ca2+-transport systems. For instance, 2-APB is known to inhibit store-operated Ca2+ entry through Orai1 (19) and SERCA (20), and to activate SCH00013 Orai3 channels (19). In addition, similarly to its analogs like DPB162-AE, 2-APB can induce a Ca2+ leak from the ER, partially mediated by ER-localized Orai3 channels (20C23). Xestospongin C also inhibits SERCA with a potency that is equal to its inhibitory action on IP3Rs (24). RyRs are predominantly expressed in excitable cells, including several muscle types, neuronal cells, and pancreatic cells (25). In most cells, RyRs are mainly activated by cytosolic Ca2+ Ca2+-induced Ca2+ release, while in skeletal muscle they are activated through a direct coupling with the dihydropyridine receptor upon depolarization (26). RyR activity can be counteracted by dantrolene (27) and high concentrations of ryanodine (28). The efficient Ca2+ exchange between the ER and the mitochondria takes place in.