Serum modulates mast cell responses to IgE antigen stimulation

Serum induces the expression of the fos and jun gene families, which encode the transcription factor AP‐1. Since we previously found that activation of mast cells by IgE‐antigen (Ag) induces the mRNA accumulation of c‐fos, c‐jun, junB and junD proto‐oncogenes, we were prompted to investigate whether serum could affect such accumulation in these cells. In addition, we investigated whether serum could modulate inhibition of DNA synthesis in immunologically stimulated mast cells. Mast cells, which were cultured in the presence of fetal calf serum (FCS), were characterized by a high proliferation rate and high accumulation of the mRNA of c‐fos, junB and junD proto‐oncogenes. After sustained FCS deprivation both DNA synthesis and the level of c‐fos mRNA were significantly decreased, as expected, whereas the level of c‐jun, junB and junD mRNA were not affected. As opposed to mast cells which were cultured in the presence of FCS, immunological stimulation of FCS‐deprived cells resulted in DNA synthesis inhibition and an increase in c‐fos expression. The results also show that the level of c‐fos mRNA was increased by either IgE‐Ag or FCS up to a similar level, while these two triggers could not act synergistically to enhance this expression further. Thus, changes in DNA synthesis, induced by FCS, block the ability of the immunological challenge to inhibit mast cell growth and to enhance c‐fos mRNA accumulation.