Thesis title: Dose-dependent effect of polyamines on cancer cell proliferation and viability
Spermidine (SPD) is a polyamine that is involved in various cellular processes, including proliferation, viability, and translation. Because cancer cells contain higher concentrations of polyamines than normal cells, their biosynthesis is considered an attractive target for cancer therapy. In contrast, toxic by-products of SPD, such as aldehydes including acrolein and H2O2, have been shown to induce cytotoxicity in vitro, and recent research has reported the potential use of SPD to limit intestinal tumorigenesis in vivo (Gobert et al., 2022). Therefore, the administration of exogenous polyamines appears to induce distinct cellular outcomes in a concentration-dependent manner, although this issue has not been addressed in detail. Here, I investigated the mechanisms underlying changes in cell proliferation and viability associated with the administration of exogenous SPD in colorectal cancer cell lines. Upon polyamine depletion by difluoromethylornithine (DFMO), SPD treatment resulted in a dose-dependent, bidirectional effect. At lower doses (< 20 µM), SPD induced cell proliferation via DHPS and EIF5A hypusination. Conversely, at higher doses, SPD (> 100 µM) induced cytotoxicity independently of DHPS. Fetal bovine serum (FBS) in the culture medium was essential for the cytotoxic effect of high SPD as it contains bovine serum amine oxidase (BSAO), and the addition of exogenous BSAO, which has amine oxidase activity equivalent to that of FBS, restored SPD toxicity. Of note, the DHPS (Deoxyhypusine Synthase) inhibitor GC7 prevented the cytotoxic effect of high SPD by inhibiting BSAO in a non-competitive inhibitory manner, as evaluated by cell-free biochemical assays. Together, these results reveal the concentration-dependent survival and cytotoxic effects of SPD in cancer cells and provide a novel target for the widely used DHPS inhibitor, GC7.