Control of DNA synthesis in growing BALB/c 3T3 mouse cells by a fibroblast growth regulatory factor

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A cell surface component from quiescent BALB/c 3T3 mouse cells that inhibits DNA synthesis and cell division when added to a culture of growing 3T3 cells has been detected. The inhibition of DNA synthesis by this factor was dependent on concentration and time of incubation; a transient exposure of cells to the factor followed by incubation in its absence for 20 hr was sufficient to elicit its inhibitory effect. The active component appears to be protein in nature, as judged by heat inactivation and trypsin sensitivity. Extracts obtained in an identical manner from quiescent 3T3 cells that had been preincubated in situ with uridine diphosphate N-acetyl-D-glucosamine (UDP-GlcNAc) did not inhibit DNA synthesis. The effect was specific for UDP-GlcNAc: incubation with three other nucleotide sugars yielded active component. Incubation of the inactive component from UDP-GlcNAc-treated cells with purified N-acetyl-β-D-glucosaminidase in vitro restored its inhibitory property. Extracts from growing cells failed to inhibit DNA synthesis. These results suggest that reversible glycosylation with N-acetyl-D-glucosamine residues may serve as a regulatory signal for the conversion of the active factor to its inactive form. We propose that the onset of quiescence of 3T3 cells is due to a casual relationship between depletion of growth factors in the culture medium and the presence of the active regulatory factor on the cell surface that inhibits DNA synthesis; conversion of the regulatory factor to its inactive form under favorable nutritional status may be viewed as a switch that allows DNA synthesis to resume.

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