Uncoupling of c-myc mRNA expression from G1 events in human T lymphocytes

Normal human T-cells were grown in interleukin 2 (IL-2) until they became quiescent. Treatment with the phorbol-ester Phorbol-12, 13-dibutyrate (PBt2) made the cells competent to respond to IL-2 by expression of high affinity IL-2 receptors. These cells could be considered as G1 cells as they also displayed increased uridine incorporation, but they did not proceed into the S phase. c-myc mRNA expression was induced both by the phorbol ester and by IL-2, provided that receptors were present. A pulse with PBt2 increased c-myc mRNA transiently, while responsiveness to IL-2 and IL-2 receptors was maintained long after c-myc expression had subsided. IL-2 induced and maintained c-myc mRNA levels for more than 10h. Thus c-myc mRNA was expressed both when the cells were made competent to respond to the growth factor and when the cells progressed towards and through the S phase. In T lymphocytes c-myc mRNA expression was thus regulated in a complex way and was not necessary for maintaining G1 functions.