Inhibition of BALB/c‐3T3 cells in late G1: Commitment to DNA synthesis controlled by somatomedin C

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Abstract
Methylglyoxal bis‐(guanylhydrazone) (mGBG) blocked the stimulation of DNA synthesis in quiescent, density‐inhibited BALB/c‐3T3 cells treated with platelet‐derived growth factor (PDGF) and platelet‐poor plasma (PPP). Competence formation produced by a transient exposure to PDGF was not effected by mGBG. In contrast, mGBG effectively inhibited the PPP‐stimulated progression of competent cells through the G1 phase of the cell cycle, although maximal inhibition was observed when mGBG was present during both the exposure to PDGF‐ and PPP‐supplemented media. When quiescent cells were treated with PDGF and PPP‐supplemented media in the presence of mGBG for 12–18 hours and the mGBG was then removed, cells entered the S phase after a 4 hour lag. The rate of entry into the S phase, but not the time necessary for the cells to progress from the mGBG block into the S phase, was dependent on the concentration of PPP present after removal of the mGBG. Either somatomedin C or insulin, but not epidermal growth factor, fibroblast growth factor, or PDGF were able to substitute for PPP in allowing cells to enter the S phase after the cells were released from the mGBG block. A marked inhibition of (3H)‐leucine incorporation in serum‐stimulated cultures was produced at mGBG concentrations which caused no decrease in the amount of (3H)‐uridine incorporated during a short (15 minute) pulse. The ability of hormones to allow cells to progress to the late G1 phase and become committed to DNA synthesis after a mGBG inhibition was not related to their ability to restore the normal rate of protein synthesis as determined by (3H)‐leucine incorporation.