Serum factors can modulate the developmental clock of gamma- to beta-globin gene switching in somatic cell hybrids

The fusion of human fetal erythroid (HFE) cells with mouse erythroleukemia (MEL) cells produces stable synkaryons (HFE x MEL) which can be monitored for extended periods of time in culture. Initially these hybrids express a human fetal globin program (gamma >> beta), but after weeks or months in culture, they switch to an adult pattern of globin expression (beta >> gamma). The rate at which hybrids switch to the adult phenotype is roughly dependent on the gestational age of the fetal erythroid cells used in the fusion, suggesting that the rate of switching in vitro may be determined by a developmental clock type of mechanism, possibly involving the cumulative number of divisions experienced by the human fetal cells. To investigate whether the number or rate of cell divisions postfusion can influence the rate of switching, we monitored the rate of switching in hybrids from independent fusions under growth-promoting (serum-replete) and growth-suppressing (serum-deprived) conditions. We found that hybrids grown under serum-deprived or serumless conditions switched more rapidly to adult globin expression than did their counterparts in serum-replete conditions. Neither the number of cumulative cell divisions nor time in culture per se predicted the rate of switching in vitro. Our data suggest that factors present in serum either retard switching of hybrids by their presence or promote switching by their absence, indicating that globin switching in vitro can be modulated by the environment; however, once switching in HFE x MEL hybrids is complete, serum factors cannot reverse this process.