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According to the growth rate hypothesis (GRH), tumour cells have high inorganic phosphate (Pi) demands due to accelerated proliferation. Compared to healthy individuals, cancer patients present with a nearly 2.5-fold higher Pi serum concentration. In this work, we show that an increasing concentration of Pi had the opposite effect on Pi-transporters only in MDA-MB-231 when compared to other breast cell lines: MCF-7 or MCF10-A (non-tumoural breast cell line). Here, we show for the first time that high extracellular Pi concentration mediates ROS production in TNBC (MDA-MB-231). After a short-time exposure (1 h), Pi hyperpolarizes the mitochondrial membrane, increases mitochondrial ROS generation, impairs oxygen (O2) consumption and increases PKC activity. However, after 24 h Pi-exposure, the source of H2O2 seems to shift from mitochondria to an NADPH oxidase enzyme (NOX), through activation of PKC by H2O2. Exogenous-added H2O2 modulated Pi-transporters the same way as extracellular high Pi, which could be reversed by the addition of the antioxidant N-acetylcysteine (NAC). NAC was also able to abolish Pi-induced Epithelial-mesenchymal transition (EMT), migration and adhesion of MDA-MB-231. We believe that Pi transporters support part of the energy required for the metastatic processes stimulated by Pi and trigger Pi-induced H2O2 production as a signalling response to promote cell migration and adhesion.