Comparison of two flow cytometric assays for cellular RNA--acridine orange and propidium iodide

Two flow cytometric assays for cellular RNA, two-step acridine orange (TSAO) and propidium iodide (PI), were compared with each other and with ultraviolet (uv) spectrophotometry of RNA to determine their ability to quantitate cellular RNA and to differentiate between proliferating and quiescent (Q) cells. The model system used for these comparisons was cells from unfed cultures of two mouse mammary tumor lines designated 66 and 67. The growth kinetics of unfed 67 and 66 cells were characterized by a decrease in cellular RNa (a factor of 2) with the decrease occurring throughout the entire population of cells as they entered plateau phase. The time course of the RNA decrease as monitored by the PI assay closely corresponded to that observed by uv spectrophotometric measurements. The TSAO assay, however, agreed with the uv spectroscopy and PI assays on the extent of the RNA decrease but showed no decrease in RNA until 24 hours after the other assays indicated that a significant decrease had occurred. The plateau cells from 67 and 66 unfed cultures had a greatly lowered RNA content and were predominately (greater than 97%) in the G1 phase of the cell cycle in terms of DNA content. However, when compared with exponentially growing cells, they were a distinct Q population. This distinction could be observed using either the TSAO or the PI assay. The TSAO assay provided better resolution of the Q population and has the added advantage of giving DNA distribution simultaneously. Therefore, both flow cytometric assays are particularly useful in this system; the TSAO for monitoring the Q population and the PI for determination of kinetic changes in total cellular RNA.