Mitochondrial DNA mutations in cancer
The contribution of mitochondrial DNA mutations (mtDNA) and functional alterations of oxidative phosphorylation associated with them in tumor progression is still debated, as these mutations may both stimulate and inhibit tumorigenic potential of tumor cells. Recent studies carried out by our research group have shown that some mutations in mtDNA and in particular those that cause disassembly of the I complex of the respiratory chain (CI) inhibit tumor growth both in vitro and in animal models. It has also been shown that in these tumors there is a close correlation between the lack of CI and the degradation of the factor induced by hypoxia (HIF1α), which regulates the metabolic and hypoxic adaptation processes necessary for the progression towards malignancy. HIF1α levels are modulated at post-transcriptional level by prolyl-hydroxylases (PHDs) whose activity depends on pO2 and levels of α-ketoglutarate (αKG) and succinate (SA), intermediate of the tricarboxylic acid cycle (TCA). In normoxia, PHDs hydroxylate HIF1α and direct it to the proteasome, causing its degradation. Instead, under hypoxia condition the PHDs are not active and allow the stabilization of HIF1α and its transcriptional activity. Recently, we have shown that disassembly of CI induces an accumulation of NADH and αKG allowing the chronic destabilization of HIF1α even under normoxia (pseudonormoxia) by inhibiting tumor growth both in vitro and in vivo. However, it has been reported that in a breast cancer model the increase in NADH consumption reduces tumorigenic potential and metatasis formation by inhibiting the Akt / mTORC1-regulated pathway and induction of autophagy. It is therefore evident that the role of NADH and TCA homeostasis in the regulation of tumor progression has yet to be defined. Furthermore, it is clear that CI can represent a potential therapeutic target in cancer, particularly for those tumors dependent on oxidative phosphorylation. The objective of this research project is to clarify the molecular mechanisms that correlate the induction of pseudonormoxia with the arrest of tumor growth in the absence of CI. In particular, we will analyze how the levels of NADH, differently regulated by mutations in the mitochondrial genome, regulate hypoxic and metabolic adaptation, autophagy and, ultimately, tumor progression. Furthermore, this project aims to demonstrate that induction of pseudonormoxia alters tumorigenic potential both in vitro and in vivo in different tumor models. For this purpose, compounds will be used that induce (i) complete disassembly of CI, (ii) an increase in NADH levels, (iii) activation of PHDs by αKG accumulation.
Metabolic reprogramming in tumor cells
To promote continued proliferation and adapt to adverse environmental conditions, cancer cells must reprogram their metabolism. In particular, the regulation of the anaplerotic reactions of the tricarboxylic acid cycle (TCA) supports the metabolic plasticity required by tumor cells. For example, tumor cells with respiratory chain defects prefer the reductive carboxylation of glutamine as the main source of carbon for biosynthesis. Furthermore, our research group has shown that the lack of respiratory complex I (CI) induces a severe bioenergetic defect that profoundly alters the metabolic state of tumor cells and that may contribute to the inability of these cells to adapt to the scarcity of nutrients and oxygen, thus inhibiting tumor progression. To understand the metabolic response of tumor cells to the lack of CI, we generated a series of osteosarcoma, colon carcinoma and ovarian cancer clones missing from the NDUFS3 CI subunit through gene editing technologies. The NDUFS3-KO cells have highlighted bioenergetic defect that prevents its growth under metabolic stress conditions and stimulates its glycolytic metabolism. In these models we will analyze the levels of TCA metabolites, glutamine carboxylation and lipid metabolism.
Role of Complex I of the respiratory chain in the regulation of the balance between biogenesis and degradation of mitochondria
The dynamics of the mitochondrial network is strictly dependent on the balance between the mechanisms of biogenesis, fission/fusion and degradation (mitophagy) of these organelles. These processes are strictly controlled from a molecular point of view involving various proteins capable of regulating a sort of "signaling" of the mitochondria to the nucleus. How and whether Complex I participates in the regulation of the biogenesis and degradation of these organelles and how these two processes can contribute to the regulation of tumorigenesis in vitro and in vivo is not yet clear. In this context, using complex tumor cell models of Complex I, the mitochondrial and biogenesis process will be analyzed.
