Modulation of c‐fos and c‐myc mRNA levels in normal human lymphocytes by calcium ionophore A23187 and phorbol ester

A transient expression of the proto‐oncogenes c‐fos and c‐myc is rapidly induced by growth factors or mitogens in different cell types including lectin‐stimulated lymphocytes. To elucidate the early events of lymphocyte activation, we determined (by Northern blot analysis) the steady‐state levels of c‐fos and c‐myc mRNA in normal human peripheral blood lymphocytes (PBL) stimulated with the Ca2+ ionophore A23187 and/or 12‐O‐tetradecanoylphorbol‐13‐acetate (TPA), whose biological activities are well defined. We report that ionophore A23187 (0.5 mUM) or, to a significantly lesser extent, TPA (0.5 ng/ml), neither of which is mitogenic alone at these concentrations, nevertheless can induce a transient accumulation of the protooncogene transcripts. These results indicate that a significant accumulation of c‐fos and c‐myc mRNA can occur without subsequent lymphocyte proliferation. The combination of these two mitogens increases the induced levels of both types of c‐onc mRNAs. The level of mRNA accumulation correlates with the degree of proliferation induced by mitogenic combinations of ionophore A23187 and TPA, as measured by [3H]thymidine incorporation. Thus, these compounds act synergistically both with respect to c‐fos and c‐myc mRNA accumulation and to mitogenicity in human PBL. We also studied the level of c‐fos transcripts following stimulation of the T lymphocyte T3/Ti receptor complex, using monoclonal anti‐T3 antibodies. We observed that mitogenic doses of anti‐T3 also induce an accumulation of c‐fos mRNA, whose induction also is synergized by TPA. These results suggest that transient accumulation of c‐fos‐, as well as c‐myc, mRNA is a normal event of the immune response. They confirm that Ca2+ ionophore combined with TPA can mimic some molecular features of the early steps of normal human PBL activation.