Graduation Date

Spring 5-9-2020

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Programs

Cancer Research

First Advisor

Michael A. Hollingsworth

Second Advisor

Kaustubh Datta

Third Advisor

Angie Rizzino

Fourth Advisor

Joyce Solheim

Abstract

Mitochondria are biosynthetic and bioenergetic hubs that confer cancer cells the metabolic flexibility to survive and grow in harsh tumor microenvironments. Accordingly, mitochondrial metabolism represents a promising target for pancreatic ductal adenocarcinoma (PDAC), which is frequently characterized as desmoplastic and nutrient poor. The findings presented in the first set of studies highlight the importance of mitochondria-dependent metabolic flexibility in PDAC cells upon exposure to acidic conditions. An acidic tumor microenvironment is a common feature of many solid tumors and exerts a profound influence on cancer biology. Similar to previous findings, we demonstrated that low extracellular pH induces epithelial-to-mesenchymal transition (EMT) and enhances cancer cell invasiveness. Although EMT is regularly associated with an increased glycolytic phenotype, we found that acidity-driven EMT was coupled to decreased glucose uptake and utilization. This corresponded with increased resistance to glucose starvation. Subsequently, we determined that cancer cells shifted their dependence from glycolysis to mitochondrial metabolism. This was confirmed using respiration-deficient cells, which did not exhibit enhanced invasion nor resistance to glucose starvation under acidic conditions. Furthermore, we found that lipid uptake and fatty acid oxidation (FAO) were increased, suggesting that fatty acids may have replaced glucose as the preferred choice of fuel. Importantly, we demonstrated that inhibition of FAO with etomoxir effectively suppressed low pH-induced invasion.

In the second set of studies, we investigated the modulation of cardiolipin metabolism as a novel approach for limiting mitochondrial activity and metabolic flexibility of PDAC cells. Cardiolipin is a mitochondrial-specific phospholipid found at increased levels following the activation of oncogenes as part of a lipogenic program. Furthermore, turnover of cardiolipin is remarkably slow in differentiated cells, providing a potential therapeutic window for cancer therapy. We found that PTPMT1, a mitochondrial phosphatase involved in the biosynthesis of cardiolipin, supported growth of pancreatic cancer cells both in vitro and in an orthotopic tumor model. Loss of PTPMT1 resulted in a time-dependent decrease in proliferation commensurate with reduced cardiolipin levels. Associated with cardiolipin depletion, we observed altered mitochondrial ultrastructure, impaired oxidative phosphorylation, and depleted TCA cycle intermediates. Remarkably, these changes did not induce cell death as determined by annexin V staining and PARP cleavage. However, both pharmacological and genetic inhibition of PTPMT1 activity resulted in increased sensitivity to glycolytic inhibition. Altogether, the results from these studies highlight the importance of mitochondrial metabolism for supporting malignant progression and offer novel insight regarding potential therapeutic avenues for PDAC treatment.

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