Perturbation of growth and differentiation of friend murine erythroleukemia cells by 5‐bromodeoxyuridine incorporation in early S phase

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Abstract
Cultured Friend murine erythroleukemia cells (Friend cells) are induced to undergo erythroid differentiation when grown in the presence of dimethylsulfoxide (DMSO) and other compounds. The effects of unifilar substitution of bromouracil (BU) for thymidine in the DNA (BU‐DNA) of Friend cells were examined. Cells were grown in the presence of 5‐bromodeoxyuridine (BrdU) for one generation, then centrifuged and resuspended in medium containing DMSO without BrdU. These cells exhibited a delay in the appearance of heme‐producing, benzidine‐reactive (B+) cells and a decreased rate of cell proliferation in comparison to the control not containing BU‐DNA. A transient inhibition of entry into S phase was observed when control cells or cells containing BU‐DNA were grown in the presence of DMSO for 10 to 20 hours. This transient inhibition was increased in the BrdU culture. Thus, BU‐substitution in Friend cells alters other cellular functions in addition to erythroid differentiation. The rate of increase in the percent of cells committed to differentiate (those forming B+ colonies in plasma clots) was similar in the BrdU and control cultures until 40 to 50 hours. After this time, a delay in the appearance of committed cells was observed in the BrdU culture. The effect of BrdU on the appearance of B+ cells was more pronounced and occurred earlier than its effect on the rate of commitment. Therefore, the delay in the appearance of B+ cells in the BrdU culture was due primarily to perturbation of post‐commitment events such as the accumulation of hemoglobin. We also examined the effect on growth and differentiation after BrdU was incorporated during different intervals of S phase in cells synchronized by centrifugal elutriation or by double thymidine block and hydroxyurea treatment. The delay in the appearance of B+ cells and inhibition of cell proliferation were only observed when BrdU was incorporated in the first half of S phase. BrdU (10 μM) had no effect on growth or differentiation when present during late S or G1 and G2. These results, using two very different methods to achieve cell synchrony, indicate that the effects of BrdU on growth and differentiation described above are due to its incorporation into DNA sequences replicating during early S.