A novel method for monitoring tumor proliferation in vivo using fluorescent dye DiD.

Monitoring single cell proliferation in vivo is difficult, but optimizing this technique is essential in order to expand our knowledge of the regulation of tumor proliferation. In this study, we used a lipophilic fluorescent dye, DiD, that rapidly and stably integrates into the phospholipid cell membrane. We cultured DiD-stained prostate cancer cell lines for 10 days and isolated cells by flow cytometry based on expression levels of DiD. We found that a decrease in DiD intensity was correlated to the reduction of EdU, where the DiD-high population proliferated more slowly than the DiD-low population and the DiD-low population exhibited a higher mitotic index. We also found that DiD was detected after 3 weeks of implantation in an in vivo setting. Importantly, DiD dye did not have any effect on normal cell growth, whereas a gold standard fluorescent dye for measuring cell proliferation, CFSE, slowed cell proliferation. Although further study is indicated, DiD can be useful for identifying the molecular mechanisms underlying tumor proliferation in vivo.