GLOBAL CHANGE OF GENE-EXPRESSION AT LATE G1 S BOUNDARY MAY OCCUR IN HUMAN IMR-90 DIPLOID FIBROBLASTS DURING SENESCENCE/

The hallmark of cellular aging is the failure of senescent diploid cel ls to enter or to complete the S phase of the cell cycle. The cause fo r such failure may hold the key for our understanding of the molecular basis of cellular aging. We have previously shown that aging of IMR-9 0 human diploid fibroblasts in culture is accompanied by a five to sev enfold decrease in both thymidine kinase activity and thymidine kinase mRNA level (Chang and Chen, 1988, J. Biol. Chem., 263:11431-11435). T o examine whether attenuation of gene expression at G1/S boundary is u nique for thymidine kinase or it may involve most, if not all, of othe r G1/S genes, we compared the expressions of two classes of G1/S genes in young and in old IMR-90 cells following serum stimulation. We foun d that the expression of all these genes, including thymidylate syntha se (TS), dihydrofolate reductase (DHFR), ribonucleotide reductase (PNR ), proliferating cell nuclear antigen (PCNA), histone H1, histone H2A + 2B, histone H3, and histone H4, was induced to high levels in young IMR-90 cells but not in old IMR-90 cells. The mRNA revels of all G1/S genes in young cells were more than tenfold higher than that in old ce lls 12 hr after serum stimulation. The enzymes encoded by TS and DHFR genes and dUTPase also exhibited similar age-dependent attenuation in activities. in contrast, expression of growth-related genes such as eI F-5A, c-Ha-ras, and beta-actin did not show significant differences be tween young and old cells after serum stimulation. Computer analysis o f the promoter region of these G1/S genes revealed an Sp-1 binding sit e as the most common cis-element. Taken together, our results suggest that the suppression of G1/S gene expressions during senescence may be a global phenomenon and that G1/S genes may be coordinately controlle d. (C) 1994 Wiley-Liss, Inc.