[3H]thymidine incorporation does not correlate with growth state in cultured alveolar type II cells

Quantitative measurement of [3H]thymidine [( 3H]TdR) incorporation into cultured cells is widely used as an indicator of cell proliferation. The observation that adult type II cells are able to incorporate large amounts of [3H]TdR despite the fact that they are not proliferating raised the question of the meaning of [3H]TdR incorporation in these cells. Comparing different systems of proliferating and nonproliferating type II cells and lung fibroblasts, we show that nonproliferating type II cells are able to synthesize some thymidine nucleotides used as immediate precursors for DNA synthesis and that most of the radioactivity incorporated into acid-insoluble material in these cells is actually in DNA. We found that hydroxyurea inhibited [3H]TdR incorporation into DNA, suggesting that nonreplicating type II cells use thymidine for scheduled, i.e., replicative, rather than unscheduled, or repair, DNA synthesis. However, newly synthesized DNA does not appear to be in a stable form, available for replication. These studies demonstrate that, in culture, adult type II cells initiate but are unable to complete scheduled DNA synthesis. They also establish that [3H]TdR incorporation cannot be used as an indicator of cell proliferation in cultured type II cells.