The fraction of cells in G1 with bound retinoblastoma protein increases with the duration of the cell cycle.
The retinoblastoma gene product (pRB) is a nuclear phosphoprotein with growth-suppressing effects. During early G1 phase, pRB is underphosphorylated and bound in the nucleus. The association between the duration of the cell cycle/G1 phase and the fraction of cells in G1 with bound pRB was studied in the human pre-B cell line Reh. The cell-cycle duration was varied by growing cells at different concentrations (25, 10, 2, 0.5 and 0%) of fetal calf serum (FCS); pRB binding was studied by flow cytometry. The culture doubling time increased from 21 h in 25% FCS to 54 h in 0.5% FCS. Cell death occurred in the absence of FCS, and the culture doubling time therefore could not be defined. The fraction of cells in G1 did not change significantly with decreasing FCS concentration (0.47 in 25% FCS, 0.52 in 0% FCS). In contrast, the fraction of G1 cells with bound pRB increased from 0.12 in 25% FCS to 0.65 in 0% FCS. Continuous labelling with bromodeoxyuridine demonstrated that the growth fraction was close to unity at all FCS concentrations down to 0.5%, hence, the duration of the cell cycle was equal to the culture doubling time under these conditions. The duration of early G1 phase (where pRB is underphosphorylated and bound) increased 10-fold, while the duration of late G1 phase increased twofold, for Reh cells grown in 0.5% FCS compared with cells grown in 25% FCS. The increase in the duration of late G1, and the increased S and G2+M phase transit times, indicate that other factors, in addition to pRB kinase activity, regulate the duration of G1 and the cell cycle of serum-deprived Reh cells.
Jonassen
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TS
Department of Biophysics, Institute for Cancer Research, The Norwegian Radium Hospital, Montebello, Oslo.
Seglen
P O
PO
Stokke
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T
D000818:Animals; D001402:B-Lymphocytes / Q000166:cytology* / Q000378:metabolism*; D002417:Cattle / Q000097:blood / Q000196:embryology; D002453:Cell Cycle / Q000502:physiology*; D002460:Cell Line; D004305:Dose-Response Relationship, Drug; D005312:Fetal Blood / Q000502:physiology; D016193:G1 Phase / Q000502:physiology; D006412:Hematopoietic Stem Cells / Q000166:cytology* / Q000378:metabolism*; D006801:Humans; D016160:Retinoblastoma Protein / Q000378:metabolism*; D013997:Time Factors
D016160:Retinoblastoma Protein