Mammalian intracellular protein homeostasis is maintained by a process called macroautophagy, or autophagy, and is a major protein degradation mechanism induced by cellular stress such as nutrient starvation. The discovery of key autophagy genes as well as signaling pathways involved in the regulation of autophagy has elucidated the essential protein machinery involved in autophagosome formation, fusion, and degradation. However, the specific mechanism as to how these genes and kinase pathways regulate autophagy is largely unknown. In addition to normal cellular physiology, autophagy has been implicated in the pathophysiology of cancer. This is evidenced by an increase in the frequency of spontaneous malignancies by heterozygous disruption of the autophagy gene Beclin-1; suggesting that Beclin-1 functions as a tumor suppressor gene. We show that Beclin-1 co-immunoprecipitates with the Class III PI3K hVps34 and its myristoylated protein adapter p150, and that this interaction requires amino acids 80-107 of Beclin-1. We also demonstrate that upon autophagic stimulation, the Beclin-1-hVps34-p150 complex is recruited to the membrane fraction in MCF-7 cells.
Rottlerin is a cytotoxic compound reported to increase the presence of autophagic vacuoles in the cytoplasm of pancreatic cells, membrane association of GFP-tagged LC3 to autophagosomes, and a marked induction of LC3-II protein, all important hallmarks of autophagy. Additionally, inhibition of Beclin-1 expression by RNAi was shown to inhibit Rottlerin-induced autophagy. Rottlerin was initially reported to specifically inhibit Protein Kinase C delta (PKCδ), and has also been shown to uncouple mitochondrial oxidative phosphorylation. The second portion of the dissertation aimed to explore the hypothesis that the cytotoxic effects of Rottlerin are related to its regulation of autophagy. Our results demonstrate that Rottlerin causes an accumulation of the autophagosomal marker LC3-II in MCF-7 and ZR75 breast cancer cells. This increase in accumulation of LC3-II was caused mainly by an impairment of the autophagy pathway at the lysosomal degradation step. However, a modest increase in autophagosome biogenesis contributes to the accumulation of LC3-II. Our studies concluded that exposure of breast cancer cells to Rottlerin induces caspase-independent cell death by reducing mitochondrial membrane potential (MMP), stimulating ER stress, and inhibiting autophagosome fusion with lysosomes, thereby rendering the autophagy survival pathway ineffective.