Actions of insulin on MCF-7 cells that are synchronized with hydroxyurea

The role of insulin in stimulating metabolic processes in MCF-7 cells was studied in cells synchronized at the G1:S interphase of the cell cycle using hydroxyurea. Cells released from the hydroxyurea block progressed through one S-phase of the cell cycle when insulin was absent from the medium. When free insulin was present the cells continued through more than one S-phase. Since cells accumulate at G0 in serum- and hormone-free conditions it is apparent that insulin has an essential action in the MCF-7 cells between the G0 and S-phase of the cell cycle. Insulin is also known to stimulate the incorporation of [3H]leucine into a pH 4.6 precipitable phosphoprotein fraction in the MCF-7 cells. Insulin was shown to express this action, even when the cells were maintained at the G1:S interphase with hydroxyurea. Insulin is thus able to effect a differentiative action, i.e. a stimulation of phosphoprotein synthesis, under conditions where insulin's effect on [3H]thymidine incorporation into DNA is prevented.