The ratio of retinoblastoma (RB) to fos and RB to myc expression during the cell cycle.

The putative transregulatory activity of the RB (retinoblastoma tumor suppressor) gene product on the expression of the c-myc and c-fos proteins during the cell cycle was assessed in HL-60 promyelocytic leukemia cells. Multiparameter flow cytometry was used to simultaneously measure nuclear DNA content, RB protein, and MYC or FOS protein per cell. The amount of RB protein per cell increased with progression through the cell cycle. As the amount of RB protein increased, the ratio of RB to MYC or to FOS protein could be determined per cell as a function of cell cycle phase. Although the amount of RB protein per cell increased with progression through successive cell cycle phases, during S phase the relative rate of increase was not as rapid as that of nuclear DNA. The amount of MYC and FOS per cell also increased throughout the cell cycle, but also more slowly than DNA during S. The ratio of the amount of RB protein to MYC protein remained constant throughout the cell cycle, consistent with putative co-regulation suggested by previous studies of promoter structure. In contrast, the ratio of RB protein to FOS protein increased with progression through the phases of the cell cycle, consistent with a putative negative effect of RB on FOS which was found in previous studies with transgenes and reporters. There was no significant change in these ratios with myelo-monocytic differentiation. Although MYC and FOS have both been implicated as growth-promoting oncogenes putatively transregulated by RB, their behavior during the cell cycle relative to RB is thus distinguishable. Interestingly, in the case of all three of these putative cell cycle regulatory proteins, their cell cycle phase-specific expression levels are consistent with a minimum amount per cell that is necessary but not sufficient for progression to the next cell cycle phase.