Regulation of mitochondrial fates and cellular metabolism via parkin-mediated mitophagy and interaction between apoptosis and autophagy pathways in cancer

Apoptosis is a cell death pathway that regulates tissue homeostasis, and is often altered in oncogenesis. Autophagy is a lysosome degradation pathway that mediates cellular adaptation in response to stresses. Altered autophagy pathways are proposed to be associated with pathogenesis of neurodegenerative diseases and oncogenesis. The goal of this work is to study the complex link between apoptosis and autophagy pathways, and their possible roles in the development of cancer. Using transgenic mice models, we found that impaired apoptosis by overexpression of a dominant negative form of Caspase-9 (Casp9DN) failed to accelerate T-cell lymphoma either by itself or in tumor-prone Bax overexpressing transgenic mice. Additionally, heterozygous disruption of Beclin 1, a central upstream autophagy regulator, failed to promote T-cell lymphoma in either Casp9DN or tumor-prone Bax overexpressing transgenic mice. However, caspase inhibition enhanced a unique form of selective mitochondrial autophagy, referred to as mitochondrial outer membrane permeabilization (MOMP)-induced mitophagy. Parkin, a protein mutated in early-onset Parkinson's disease, mediates mitophagy following protonophore (CCCP) treatment, suggesting that Parkin may also play a role in MOMP-induced mitophagy. Thus, two different types of mitochondrial stresses, MOMP and CCCP, cause mitochondrial depolarization and induce mitophagy. We therefore examined if there is a mechanistic link between two mitophagy pathways. Focusing on the roles of autophagy and apoptosis regulators using isogenic hematopoietic cell lines, our studies demonstrate that MOMP-induced mitophagy is dependent upon Bcl-2 family members, but independent of Parkin or ULK1 (an autophagy regulator). In contrast, CCCP-induced mitophagy is dependent upon Parkin and ULK1, but independent of Bcl-2 family members. However, we found that both pathways ultimately result in the following properties: reduced mitochondrial respiration rate, altered cellular metabolism, and high sensitivity to 2-DG (an inhibitor of glycolysis). Interestingly, 2-DG induced cell death in cells following Parkin-dependent mitophagy is independent of Bcl-2 and Bax/Bak. Overall, the work in this dissertation demonstrates that the two different mitochondrial stresses, MOMP and protonophore (CCCP) treatment, lead to two mechanistically distinct mitophagy pathways, but both alter mitochondrial respiration and cellular metabolism.