Expression of cell cycle-dependent genes in young and senescent WI-38 fibroblasts.

We studied the expression of 11 cell cycle-dependent genes in senescent WI-38 fibroblasts and compared the results to those obtained in WI-38 cells from early passages (young cells). Every gene we examined is expressed in the senescent cells at levels similar to those in the young cells, including two genes maximally expressed at the G1/S phase boundary--genes for thymidine kinase and histone H3. The results clearly show that senescent, noncycling WI-38 cells are not similar to quiescent cells. Rather, such senescent WI-38 cells may be blocked just prior to the onset of DNA synthesis.