Gene expression in flow sorted mouse teratocarcinoma X human fibroblast heterokaryons

Mouse teratocarcinoma cells and primary human fibroblasts were fluorescently labelled with fluorescein isothiocyanate (FITC)- and trimethylrhodamine isothiocyanate (TRITC)-stearylamine respectively. After fusion populations highly enriched for red-green heterokaryons (around 80%) were isolated from the fusion mixture using a FACS II cell sorter. To study gene expression in the early hybrids [35S] methionine-labelled proteins synthesized by the sorted cells at two and three days after fusion were analysed by two-dimensional gel electrophoresis. Three spots were denser in gels of the fused cells than in those of 1:1 mixtures of parental cells. For one of these proteins it could be demonstrated that this reflects the enhanced synthesis of a mouse-specific protein present only in small amounts in teratocarcinoma cells. All three proteins were synthesized in relatively large amounts by differentiated mouse cells. Collagen (type I) synthesis by the sorted hybrid cells was studied by analysing the [3H] proline-labelled material secreted into the medium. Analysis by sodium dodecyl sulphate (SDS)-gel electrophoresis and two-dimensional nonequilibrium pH gradient electrophoresis showed that the material secreted by the fused cells five days after fusion was the same as that secreted by the human fibroblasts. No evidence was obtained for synthesis of mouse alpha 2(I) collagen. The amount of collagen produced by the sorted cells five days after fusion was about half the amount produced by the human fibroblasts. Immunofluorescence studies also showed that collagen synthesis was not suppressed after fusion both in heterokaryons and synkaryons. In conclusion, we did not find evidence for activation of a previously completely silent mouse gene in the fused cells. The results show, however, that the fused cells do resemble the differentiated fibroblasts rather than the undifferentiated teratocarcinoma cells.