In vitro influence of nutrient deprivation and papaverine exposure in MCF-7 and MDA-MB-231 breast cancer cell lines

Abstract

Breast cancer treatment in South Africa is constrained by limited access to affordable targeted therapies, highlighting the urgent need for cost-effective solutions within the public health sector. Fasting, a free and accessible intervention, shows promise in reducing tumorigenic metabolism and chemotherapy side effects; however, the efficacy is frequently reduced by metabolic plasticity, where metabolic activity shifts from glycolysis to oxidative phosphorylation (OXPHOS) in order to resist nutrient deprivation (ND). Various studies have demonstrated that nutrient-deprived breast tumorigenic cells exhibit increased dependence on OXPHOS for optimal proliferation through augmented mitochondrial complex I expression, highlighting a notable metabolic vulnerability that could potentially be targeted by combining fasting with a mitochondrial complex I inhibitor. Papaverine (PPV), a repurposed mitochondrial complex I inhibitor, was selected for this study due to its low cost, established safety, and accessibility in South Africa. However, the combination of physiological ND and PPV remains to be elucidated. Thus, the aim of this study was to investigate the combined effects of fasting-mimetic glucose and glutamine deprivation and PPV exposure, on proliferation, morphology, cell cycle progression, oxidative stress, superoxide dismutase (SOD) activity, mitochondrial membrane potential (ΔΨM), and the activity of 5′ adenosine monophosphate–activated protein kinase (AMPK) and mammalian target of rapamycin (mTOR) in metabolically distinct breast tumorigenic cells. This study used two breast cancer cell lines: OXPHOS-dependent MCF-7 (luminal, hormone receptor–positive) and glycolysis-dependent MDA-MB-231 (basal, triple-negative). The results obtained demonstrate that ND significantly enhanced the antiproliferative effects of PPV in both MCF-7 and MDA-MB-231 cells. Neither PPV nor ND alone significantly inhibited cell proliferation, indicating that ND sensitizes tumorigenic cells to PPV-mediated mitochondrial complex I inhibition. Morphologically, exposure to ND and PPV significantly increased the number of cells exhibiting enlarged morphology and cell protrusions. Furthermore, cell cycle analysis revealed that exposure to ND and PPV did not affect cell cycle progression but did alter reactive oxygen species (ROS) generation, specifically reducing superoxide (O₂⁻) generation and increasing hydrogen peroxide (H₂O₂) production, potentially independent of SOD activity. Assessment of ΔΨM revealed cell line–specific responses to ND and PPV exposure, with significant depolarization observed in MCF-7 cells and hyperpolarization in MDA-MB-231 cells. Moreover, in MDA-MB-231 cells, ND and PPV exposure led to the activation of AMPK and the inhibition of mTORC1. These findings suggest that combining ND and PPV targets critical biochemical pathways and molecular markers in breast tumorigenic cells, offering a potentially cost-effective and accessible therapeutic approach for resource-limited settings while aligning with Sustainable Development Goal 3 to promote global health and well-being. Although the combination of fasting and PPV shows promise, further in vivo preclinical research is required.