Fluorescence enhancement of DNA-bound TO-PRO-3 by incorporation of bromodeoxyuridine to monitor cell cycle kinetics

Abstract

Background: The detection of DNA-incorporated bromodeoxyuridine (BrdUrd) in mammalian cells is a well-known and important technique to study cell cycle. The use of TO-PRO-3 for detection of BrdUrd substitution of DNA by dual-laser flow cytometry has been investigated. Methods: Fluorescence enhancement of TO-PRO-3 in BrdUrd-labeled cells is registered in combination with the fluorescence emission of the intercalating dye propidium iodide (PI) as a total DNA stain to give bivariate DNA/BrdUrd histograms. By the low concentration of only 0.3 μM TO-PRO-3, BrdUrd detection is optimized, and undisturbed total DNA content by PI can be detected as well. TO-PRO-3 is excited by a red HeNe laser and PI by an argon ion laser. Results: In order to understand the binding of TO-PRO-3, energy transfer from PI to TO-PRO-3 has been measured as well as the influence of an external DNA binding dye such as Hoechst 33258 with Adenine-Thymine (AT) binding specificity. Cell cycle studies of human SCL-2 keratinocytes and mouse 3T3 cells prove the method to be as generally applicable as the classical BrdUrd/Hoechst quenching technique, but without need for expensive ultraviolet laser excitation. No BrdUrd sensitivity could be found for the similar dyes TO-PRO- 1 and YO-PRO-3, whereas TO-PRO-5 and YOYO-3 showed only very little sensitivity to BrdUrd labeling as compared with TO-PRO-3. Conclusions: Cell cycle studies of mammalian cells can be done by dual-laser flow cytometry without the need for ultraviolet lasers by using the BrdUrd-dependent fluorescence enhancement of TO-PRO-3. Total DNA content can be measured simultaneously using PI.