Thymidine triphosphate synthesis in senescent WI38 cells. Relationship to loss of replicative capacity.

The effect of in vitro age on thymidine triphosphate (TTP) synthesis was assessed in WI38 cultures according to the following measurements: (1) thymidine kinase activity of broken cell preparations; (2) in situ incorporation of [3H]thymidine into acid-soluble material; and (3) total intracellular TTP content as determined by an enzymatic assay. All three parameters were maximal in exponentially proliferating populations and minimal in quiescent monolayers; no significant differences between young and old cultures were observed despite the reduced replicative capacity of the latter. The addition of serum to density-arrested cultures induced both TTP synthesis and DNA replication after a lag of approx. 12 h; although a greater percentage of young cells initiated replication as compared with old, pool sizes expanded to a similar extent in both populations. Pool expansion did not require entry into S phase; the pool sizes of control and cytosyl arabinoside-treated cultures were comparable. These findings suggest that senescent cells retain the ability to synthesize TTP, even though they are incapable of replicating DNA. Because TTP synthesis is a cell cycle-dependent event that normally begins in late G1, senescent cells might be blocked in the latter portion of the prereplicative phase and not in G0 as are quiescent cells.

Trypsinization frequency and loss of proliferative capacity in WI-38 cells.

The effect of the trypsinization procedure employed in routine serial subcultivation on the proliferative life span of WI-38 cells was studied by varying the frequency of trypsinization and by using a nonenzymatic subcultivation technique. The results indicate that recurrent trypsinization does not contribute to loss of proliferative capacity and may, in fact, delay this loss to some degree.

Hydrocortisone effects on cell proliferation: specificity of response among various cell types.

The effect of 14 micrometer hydrocortisone (HC) on the proliferative activity of various vertebrate cell lines has been measured. Such activity in human deploid cell line WI-38 was enhanced by HC addition whereas in a number of other vertebrate cell lines under identical conditions HC was inhibitory to growth. We examined the effects of HC on normal human diploid cells other than WI-38; some were stimulated, some inhibited and some were unresponsive. The results suggest that cells derived from different human tissues retain in vitro some aspects of their cell-specific hormonal responses.