Implication of lysosomes on glucose-dependent E2F1-driven cell growth control and their novel role in mitotic progression

Lysosomes are the primary degradative organelles in mammalian cells. Lysosome enzymatic cocktail allows the degradation of cellular material including dysfunctional organelles and macromolecules through different converging pathways including macroautophagy. Lysosome function relies on both its acidification capacity, mediated by the vacuolar ATPase, and its cytosolic positioning, mediated by the motor proteins kinesins and dyneins. Emerging evidences point out a relevant role of lysosomes in cancer progression and denote an important function of these organelles in cellular homeostasis maintenance. Lysosomotropic drugs, such as Chloroquine-derivatives, are already being used in clinical trials for cancer treatment. This PhD thesis aims to study lysosome functions in cancer cells for the identification of novel vulnerabilities related to this organelle. Two different questions are addressed herein: 1) The implication of lysosomes on glucose-mediated E2F1-driven mTORC1 activation and 2) The role of lysosomes in cell division. E2F1 is overexpressed in numerous human cancers, including lung, breast and hepatocellular carcinomas. Traditionally, the major role reported for E2F1 in cancer is the activation of cell cycle. Previously, our group reported that E2F1 regulates cell growth, through the activation of mTORC1, a major regulator of protein synthesis and autophagy. Here we demonstrate that E2F1 induces the anterograde movement of lysosomes, which is associated with translocation of mTOR to lysosomes and v-ATPase activation. Moreover, we show that E2F1-dependent mTORC1 activation relies on glucose availability. E2F1 transcriptionally regulates several glycolytic enzymes, thus increasing glycolytic flux. We hypothesize that E2F1 is able to activate glycolysis and therefore affect v-ATPase activity and mTORC1 activation. Besides, E2F1 induces lysosome-dependent exocytosis which correlates with a metastatic phenotype. These novel functions of E2F1 in v-ATPase regulation and lysosomal trafficking provide insight into regulatory mechanisms by which E2F1 drives malignancy and highlight the potential role of lysosomes as a metabolic hub in mammalian cells. Studies on lysosome function are mainly focused in cells in interphase. However, the implication of these organelles during cell division remains unclear. Mitosis is a key event during cell cycle, in which the cell finally divides into two daughter cells. Mitotic progression comprises five active phases involving a dramatic rearrangement of cellular components in a short period of time. Until now, degradation of mitotic factors relied only on ubiquitination and proteasome-dependent degradation. In the present study, we show that impairment of lysosomal trafficking and function delays mitotic progression and increases mitotic errors, phenomena accompanied by an increase in toroidal-shaped nuclei, a product of mitosis impairment. Finally, we use a proteomic approach to discover novel lysosome protein targets involved in mitotic progression. Interestingly, we identified regulatory proteins of the cohesin complex necessary for correct chromosomal segregation. By characterizing a novel function of lysosomes specifically in mitosis, our work establishes a novel model of regulation of cell division independent of the proteasome. In summary, this work provides new insights into the function of lysosomes down-stream of E2F1 oncogenic signaling, which modulates G1/S transition and cell proliferation but also into lysosomal function in the completion of cell cycle by regulating mitotic progression. Our work highlights the importance of targeting lysosomes for novel cancer therapies.