Copy number, epigenetic state and expression of the rRNA genes in young and senescent rat embryo fibroblasts.

The recent cloning of the gene that causes the premature aging in Werner syndrome patients has evoked speculations that deficits in expression of the ribosomal RNA genes could be related to cellular aging in general. Here we compare the state of the rRNA genes and the rRNA metabolism in young and senescent (aged) rat embryo fibroblasts (REF). Southern blot analysis revealed that the copy number and the methylation state of the genes did not change significantly with increasing cumulative population doublings (CPD) of the culture. Hence, young (low numbers of CPD) and senescent REF (high numbers of CPD), respectively, have the same repertoire of rDNA units that can be transcribed. The rRNA synthesis in these cells was analyzed by incorporation of labeled uridine at conditions allowing the measurement of absolute rather than relative rRNA synthesis rates. We revealed that the cell density dependence of the rRNA synthesis diminishes in senescent cells. Exponentially growing young REF exhibited an rRNA synthesis of 16 amol uridine incorporation per minute and cell. The rRNA synthesis decreased 10-fold in quiescent cells at saturating cell densities. Exponentially growing REF near the end of their replicative lifespan exhibited a 2-fold lower rRNA synthesis rate compared to young cells. However, in senescent REF the rRNA synthesis rate decreased only 2-fold with increasing cell densities resulting in a 3-fold higher rRNA synthesis rate compared to young cells at saturating cell densities. These data could be confirmed by calculating the rRNA synthesis rates from the rRNA content, the rRNA half-life, and the proliferation rate of the cells. Hence, senescent REF exhibited a higher rRNA synthesis rate when compared to young cells at similar growth rates resulting in the generally observed higher rRNA content (and cell size) of senescent cells. We conclude that cellular senescence of REF is not accompanied by rRNA expression deficiencies.