Induction of c-myc expression in human B lymphocytes by B-cell growth factor and anti-immunoglobulin.

Purified human B lymphocytes were examined for transcriptional expression of c-myc in response to mitogenic stimulation by the method of in situ hybridization using 35S-labeled DNA probes. The level of c-myc expression increased 10- to 20-fold within 2 hr after the addition of anti-mu, formalinized Staphylococcus aureus Cowan strain I, or B-cell growth factor, as compared to resting B cells. After 72-96 hr of mitogenic stimulation, c-myc expression remained elevated 5-fold, but expression among individual cells had become more heterogeneous than at early time points. To determine whether c-myc expression in human B lymphocytes is phase specific within the cell cycle, mitogen-stimulated cells were sorted by DNA content into populations of cells in G0/G1, S, and G2/M phases of the cell cycle. Examination of c-myc expression in phase-specific cells revealed that c-myc expression was elevated in all phases of the cell cycle, but it appeared to be maximally expressed in S phase. These studies suggest that c-myc expression in normal human B lymphocytes is cell-cycle dependent and remains elevated in all phases of the cycling cell.