Investigating the Cellular Responses of Cancer Cells to Physiological and Hypoxic Oxygen Conditions
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AbstractMost incubators used in cell culture do not regulate O2 levels, making the headspace O2 concentration ~18%. In contrast, most human tissues are exposed to 2–9% O2 in vivo (physioxia). The main goal of this thesis project was to gain a better understanding of how supraphysiological O2 levels affect cell behavior in vitro, with a focus on cancer cell biology. Using RNA-seq, I studied how culture in either 5% O2 or 18% O2 affects gene expression in four human cancer cells. I found that O2 level in culture affected hundreds of genes, however, in a largely cell-type specific manner. Further, gene targets of the hypoxia-inducible factors (HIFs) were upregulated at 5% O2 compared to 18% O2 in all cell lines. This led me to investigate how culturing cancer cells at a baseline level of 5% O2 or 18% O2 affects their response to hypoxia (here, 1.1% O2). My results indicate that baseline O2 level substantially affects the transcriptional response of prostate cancer cells (PC-3) to hypoxia. Notably, cells grown in 18% O2 and then exposed to hypoxia showed an enhanced induction of HIF-regulated genes, particularly genes involved in glucose metabolism. This in turn resulted in an enhanced glucose uptake rate of cells taken from 18% O2 to hypoxia, compared to cells preadapted to 5% O2 and then exposed to hypoxia. Finally, while acute hypoxia did not affect proliferation or migration of PC-3 cells regardless of their baseline O2 level, cells preadapted to 5% O2 did show higher proliferation and migration rates compared to cells in 18% O2. We conclude that O2 levels in culture affect gene expression, glucose consumption, and growth in cancer cells, which, in turn, might result in differential sensitivity towards anticancer drugs, highlighting the importance of maintaining physiological O2 conditions in cell culture.
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