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 prot eins during the cell cycle was assessed in HL-60 promyelocytic leukemi a cells, Multiparameter flow cytometry was used to simultaneously meas ure nuclear DNA content, RB protein, and MYC or FOS protein per cell, The amount of RB protein per cell increased with progression through t he 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 increase d with progression through successive cell cycle phases, during S phas e the relative rate of increase was not as rapid as that of nuclear DN A, The amount of MYC and FOS per cell also increased throughout the ce ll cycle, but also more slowly than DNA during S, The ratio of the amo unt of RB protein to MYC protein remained constant throughout the cell cycle, consistent with putative co-regulation suggested by previous s tudies 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 w as found in previous studies with transgenes and reporters, There was no significant change in these ratios with myelo-monocytic differentia tion, Although MYC and FOS have both been implicated as growth-promoti ng oncogenes putatively transregulated by RB, their behavior during th e cell cycle relative to RB is thus distinguishable. Interestingly, in the case of all three of these putative cell cycle regulatory protein s, their cell cycle phase-specific expression levels are consistent wi th a minimum amount per cell that is necessary but not sufficient for progression to the next cell cycle phase.