Regulated expression of the RB "tumor suppressor gene" in normal lymphocyte mitogenesis: elevated expression in transformed leukocytes and role as a "status quo" gene.

Expression of the RB retinoblastoma tumor suppressor gene product is regulated early during the stimulation of normal human peripheral blood lymphocytes, suggesting a regulatory role for the amount of this protein in mitogenesis of normal cells. When normal human peripheral blood lymphocytes were mitogenically stimulated with pokeweed mitogen, bivariate flow cytometric measurements of cellular DNA and RB protein content showed an early decrease in the amount of RB protein per cell, anteceding onset of S phase. A subsequent increase in the amount of RB protein per cell occurred with cell proliferation. Thus the amount of RB protein relative to the total cell mass underwent a biphasic response with mitogenesis. The resulting proliferating cells had a slightly elevated level of RB protein per cell compared to the unstimulated cells. Comparison of other proliferating leukocytes to normal lymphocytes showed that both EBV virally transformed lymphocytes and human promyelocytic leukemia cells (HL-60) had elevated levels of RB protein per cell compared to normal peripheral blood lymphocytes. Mitogenic stimulation or transformation by other means thus is associated with regulation of the amount of RB protein per cell, suggesting a regulatory role for the RB protein in normal cell growth control.