Inorganic Polyphosphate and Fasting: Interactions in the Context of Neuronal Senescence and Cancer Metabolism

Introduction: Inorganic polyphosphate (polyP) plays a key role in mitochondrial function, especially in aging and age-related diseases. While reduced polyP levels are mainly linked to neurodegeneration in the literature, its role in cancer is less clear, though it may promote tumor growth. Dietary restriction (DR), known to influence mitochondrial function, shows promise in mitigating age-related diseases, including neurodegenerative and oncological ones. This doctoral thesis explores the interaction between polyP and DR, focusing on its role in neuronal senescence and tumor proliferation and metabolism. Materials and methods: For the neuroscientific study, we used SH-SY5Y cells wild-type (Wt) and MitoPPX, which are enzymatically depleted of mitochondrial polyP thanks to the ectopic expression of the exopolyphosphatase (PPX). Both cell types were differentiated into neurons using retinoic acid. We assessed the effects of Short-Term Starvation (STS) on total and mitochondrial polyP levels, markers of senescence, cell viability, bioenergetics, and key mitochondrial functions. Moreover, in vivo experiments were conducted using C57BL/6J mice fed ad libitum or undergoing three cycles of intermittent fasting (IF). In the oncological study, we evaluated STS effects on total polyP levels and cell viability across different human tumor cell lines compared to control medium. In particular, the studied cancer cell lines were the following: HCT116, SH-SY5Y, OVCAR-5, MDA-MB-231, and MCF-7. We also examined the impact of pre-treatment with exogenous polyP on cell viability under both control and STS conditions in these cell lines. Results: Mitochondrial polyP depletion induces a neuronal senescent phenotype, characterized by increased β-galactosidase activity, morphological alterations, and proteomic changes. MitoPPX cells also show a reduced adenosine triphosphate (ATP)/adenosine diphosphate (ADP) ratio and increased mitochondrial levels of BAX without an increase in cell death. STS restores mitochondrial polyP levels, increases AMPK activity, and modifies the levels of some proteins involved in oxidative phosphorylation in MitoPPX cells compared to control medium. However, STS does not fully reverse the effects induced by the depletion of mitochondrial polyP on several neuronal senescence markers, including the levels of β-galactosidase activity and altered morphology. In C57BL/6J mice, there is an increase in ketone bodies and a reduction in blood glucose during IF cycles in both sexes, but brain polyP levels increase compared with the corresponding controls only in males. Moreover, in males, there is also an improvement in glucose tolerance right before the last cycle of fasting. In the oncological studies, 72 h of STS induce a reduction in cell viability, compared with control conditions, in all tumor cell lines studied without a concomitant modification in total polyP levels. Furthermore, pre-treatment with exogenous polyP has no effect on cell viability in any tumor cell lines, either under control or STS conditions. Finally, STS combined with an AKT inhibitor in MCF-7 cells, while having a synergistic effect on cell viability, is not associated with a modification in polyP levels. Conclusions: Our findings highlight polyP's crucial role in mitochondrial function, with its depletion contributing to neuronal senescence. DR may mitigate this effect by regulating polyP metabolism in the brain, possibly with sex-specific effects. In vitro studies on human tumor cell lines showed no changes in polyP levels with short-term nutrient deprivation and no impact of exogenous polyP pre-treatment on tumor proliferation.