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 microM) 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 proto-oncogene 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.