Mutant p53 proteins alter signaling pathways involved in autophagy and redox regulation in cancer cells

Mutations in the TP53 gene occur in over 50% of the human cancers and most of them are missense mutations that result in the expression of mutant forms of p53. In addition, p53 mutated proteins acquire new biological properties referred as gain-of-function (GOF) that contribute to the induction and maintenance of cancer. Autophagy is an intracellular degradative process by which damaged macromolecules and organelles are targeted to lysosomes via autophagic vesicles and it is crucial to maintain primary biological activities during cellular stresses, such as nutrient starvation. Reactive oxygen species (ROS) are highly reactive byproducts of mitochondrial oxidative phosphorylation and are implicated in a plethora of biological events addressed to sustain each aspect of human cancer being able to act as second messengers in cellular signaling. The aim of this thesis was to dissect the molecular mechanisms by which oncogenic mutant p53 proteins promote cancer cell proliferation and chemoresistance in cancer cells by altering crucial signaling pathways involved in autophagy and redox homeostasis. We unveiled that GOF mutant p53 proteins, contrarily to its wild-type p53 counterpart, inhibit the autophagic pathway and enhance mitochondrial ROS in cancer cells, leading i) antiapoptotic effects, ii) proliferation and iii) chemoresistance of pancreas and breast cancer cells. We found that mutant p53 significantly counteracts the formation of autophagic vesicles and their fusion with lysosomes throughout the repression of some key autophagy-related proteins and enzymes with the concomitant stimulation of mTOR signaling. Consequently to the deregulation of AMPK signaling, the expression of its effector PGC-1α was also affected, driving a reduction of the antioxidant UCP2 protein expression and an increase of mitochondrial superoxide that acts as a critical mediator of oncogenic proprieties of mutant p53. As a paradigm of this mechanism, we showed that atg12 gene repression was mediated by the recruitment of the p50 NF-kB/mutant p53 protein complex onto two regions of the atg12 promoter suggesting the involvement of the p50-p50 homodimer as a transcriptional repressor of mutant p53 target genes. We have further correlated the low expression levels of the autophagic genes (atg12, becn1, sesn1, and dram1) with reduced relapse free survival (RFS) and distant metastasis free survival (DMFS) of breast cancer patients carrying TP53 gene mutations conferring a prognostic value to this mutant p53- and autophagy-related signature. Intriguingly, we demonstrated that mutant p53-driven mTOR stimulation, beyond its role on autophagy repression, sensitize cancer cells carrying mutant TP53 gene to the treatment with the mTOR inhibitor everolimus. The data reported in this thesis reveal novel mechanisms by which mutant p53 sustains tumor progression and lightened on the importance that play the redox cellular status and autophagy regulation in the human tumors carrying oncogenic mutant p53 proteins.