UC Berkeley

Cancer cells rely on profoundly altered metabolic pathways for their ability to outgrow normal cells and to survive nutritional, energetic and immunological challenges.

Pancreatic ductal adenocarcinoma (PDA) is an especially aggressive malignancy that displays a significantly altered metabolism and for which there is a lack of effective therapies. The profoundly altered metabolism of PDA, which allows it to scavenge nutrients that fuel its aberrant growth, could also represent a liability that can be exploited in therapeutic settings. PDA cells obtain metabolic building blocks via two scavenging processes, autophagy and macropinocytosis. Autophagy allows specific or bulk trafficking of intracellular organelles and proteins to the lysosome for degradation and recycling into its constituent monomers. Macropinocytosis allows for uptake of extracellular fluid, lipids, and proteins, which get trafficked to the lysosome for degradation. Both of these nutrient acquisition routes culminate at the lysosome,

a catabolic organelle that serves as a hub for recycling and storage of many important nutrient types. Recent studies have shown that lysosomes are upregulated in PDA cells via both transcriptional and post-transcriptional mechanisms, making lysosomal function paramount for PDA function and growth. While PDA cells are highly dependent on lysosomal function, little is known about the unique alterations in the PDA lysosomal proteome that allow for the increased metabolic flux from macropinocytosis and autophagy and their potential as novel therapeutic targets.

Here, I identified phospholipase B domain containing protein 1, PLBD1, as one of the most upregulated proteins in the lysosomes of PDA cells compared to non-PDA cells. PLBD1 expression is driven by the MiT/TFE family of proteins that are decoupled in PDA allowing constitutive activity. Loss of PLBD1 leads to morphologically enlarged lysosomes that are incapable of executing trafficking functions of autophagic machinery. Consistent with the key role of the lysosome in PDA growth, depleting PLBD1 leads to severe defects in proliferation of PDA cells but not their non-cancerous counterparts.

Another key requirement for PDA growth is the transfer of lysosome-derived nutrients to mitochondria for their utilization, but how this process occurs is largely unknown. In recent years, physical contact sites between organelles have emerged as a major mechanism allowing the transport of nutrients that enable metabolic reactions and signaling. To dissect the functions of this key but poorly understood process, I developed a proximity-based ligation assay as a measure for mitochondria-lysosome contact formation, as well as a chemically controlled tethering system to force contact between lysosomes and mitochondria. Overall, my graduate work identified PLBD1 as a major component of lysosomes in PDA cells and established valuable tools to study the dynamics of lysosomal nutrient transfer in PDA cells.