Kuang Yu Chen

Department of Chemistry and The Cancer Institute of New Jersey, Rutgers - The State University of New Jersey, Piscataway, NJ. 08855-0939

Received 8/19/97 Accepted 8/25/97

5. PERSPECTIVE AND SUMMARY

Biochemical and molecular biological studies have shown that suppression of CBP/tk and E2F binding activities is responsible, respectively, for the attenuation of TK and DHFR gene expression in senescent cells (10, 11). A similar approach can be used to investigate the mechanism of regulation of other G1/S genes during cell senescence. The regulation of some G1/S genes during the progression of cell cycle has already been studied. For example, Schultze et al (25) have shown that a variant E2F site within cyclin A is responsible for cell cycle regulation of cyclin A gene. On the other hand, a CCAAT binding protein, termed CBP/cycA, has been suggested to be responsible for the G1/S transition in normal, adhesion-dependent, mesenchymal cells (26). Cyclin A contains two E2F sites (7/8 match) and a CCAAT element (Figure 7). It is certainly of interest to examine which promoter region in cyclin A, E2F or CCAAT, or both, is responsible for age-dependent regulation.

The age-dependent G1/S genes can be divided into two groups based on their promoter organizations. The Group 1 genes contain the tandem CCAAT elements (Figure 5) whereas the Group 2 genes contain E2F consensus sequence (Figure 7). The CBP/tk is likely to be a Y-box family protein. Since the Y-box family proteins may represent a large number of loosely related CCAAT binding proteins (27), it remains to be investigated whether CBP/tk is equivalent to NF-Y (12, 13). NF-Y has also been suggested to be involved in the cell cycle regulation of RNR R2 subunit (24). Recently, a CCAAT binding protein, termed CBP/cdc2, was shown to be involved in the cell cycle-dependent regulation of cdc2 gene (28). It remains to be seen whether CCAAT binding proteins CBP/tk (10), CBP/R2(24), CBP/cycA (26), and CBP/cdc2 (28) are identical or related to NF-Y or other Y-box binding proteins. Based on the similarity of the promoter organization among the Group 1 genes, it is tempting to speculate that these multiple CCAAT elements-containing regions may be responsible for the age-dependent down-regulation of these genes. This possibility is now under investigation. If indeed, the CCAAT binding proteins areinvolved, it will be of interest to compare the biochemistry of these CCAAT binding proteins with that of CBP/tk. At the same time, it is also important to carry out further study on the role of NF-Y in controlling target genes during cellular aging. All the genes in Group 2, with exception of H11 and cyclin A, contain perfect E2F consensus sequence within their promoter. The variant E2F site (7/8 match) in H1 and cyclin A has been shown to be involved in cell cycle regulation (26). Four of the Group 2 genes also contain CCAAT element in their promoters (Figure 7). Of particular note is the high sequence homology (15/19 match) between the Y-box containing region of cdc2 (-205/-187) and the 19-bp repeat (-52/-34) within the TK promoter. Most of the CCAAT element in Group 2 genes also shares Y-box feature. It is likely that E2F site alone (e.g. DHFR and TS) or together with some Y-box protein (e.g. cyclin A) will be sufficient to control the regulation of the Group 2 genes during the progression of cell cycle and during cell senescence.

Once the key transcription factors involved in the age-dependent down-regulation of G1/S genes have been characterized, attention will certainly be directed toward the regulation of these transcription factor genes. It is interesting to note that the characteristic promoter organization associated with the age-dependent G1/S genes is also present in NF-Y and E2F1 genes. Thus, NF-YA, NF-YB and E2F1 all contain multiple CCAAT repeats in tandem within their promoters. In addition, E2F1 contains four E2F sites. This raises a possibility that these transcription factors are part of autoregulatory circuit which may be an integral component of the genetic program of aging. If indeed this is the case, we will have to look for the earlier events in the cell cycle that lead to the suppression of transcription factors such as E2F1 and CBP/tk. Although the mid-G1 genes such as ODC and eIF-5A are fully expressed in senescent cells (Figure 1), we note that their gene products, namely ODC enzyme and eIF-5A protein, are greatly reduced in senescent cells (6, 19). Both proteins have been implicated in growth control and are essential for proliferation (6, 19). We wonder whether they may have a role in the down-regulation of transcription factors such as CBP/tk and E2F1, and the global attenuation of G1/S genes during senescence.