Since 1990, the National Cancer Institute (NCI) has screened more than 60,000 compounds against a panel of 60 human cancer cell lines. The 50-percent growth-inhibitory concentration (GI ₅₀ ) for any single cell line is simply an index of cytotoxicity or cytostasis, but the patterns of 60 such GI ₅₀ values encode unexpectedly rich, detailed information on mechanisms of drug action and drug resistance. Each compound's pattern is like a fingerprint, essentially unique among the many billions of distinguishable possibilities. These activity patterns are being used in conjunction with molecular structural features of the tested agents to explore the NCI's database of more than 460,000 compounds, and they are providing insight into potential target molecules and modulators of activity in the 60 cell lines. For example, the information is being used to search for candidate anticancer drugs that are not dependent on intact p53 suppressor gene function for their activity. It remains to be seen how effective this information-intensive strategy will be at generating new clinically active agents.

GADD45 is a ubiquitously expressed mammalian gene that is induced by DNA damage and certain other stresses. Like another p53-regulated gene, p21 ^WAF1/CIP1 , whose product binds to cyclin-dependent kinases (Cdk's) and proliferating cell nuclear antigen (PCNA), GADD45 has been associated with growth suppression. Gadd45 was found to bind to PCNA, a normal component of Cdk complexes and a protein involved in DNA replication and repair. Gadd45 stimulated DNA excision repair in vitro and inhibited entry of cells into S phase. These results establish GADD45 as a link between the p53-dependent cell cycle checkpoint and DNA repair.

The signaling pathway from phosphoinositide 3-kinase to the protein kinase Akt controls organismal life-span in invertebrates and cell survival and proliferation in mammals by inhibiting the activity of members of the FOXO family of transcription factors. We show that mammalian FOXO3a also functions at the G ₂ to M checkpoint in the cell cycle and triggers the repair of damaged DNA. By gene array analysis, FOXO3a was found to modulate the expression of several genes that regulate the cellular response to stress at the G ₂ -M checkpoint. The growth arrest and DNA damage response gene Gadd45a appeared to be a direct target of FOXO3a that mediates part of FOXO3a's effects on DNA repair. These findings indicate that in mammals FOXO3a regulates the resistance of cells to stress by inducing DNA repair and thereby may also affect organismal life-span.

Components of the ras signaling pathway contribute to activation of cellular p53. In MCF‐7 cells, p38 kinase activated p53 more effectively than other members of the ras pathway. p53 and p38 kinase exist in the same physical complex, and co‐expression of p38 stabilized p53 protein. In vitro, p38 kinase phosphorylated p53 at Ser33 and Ser46, a newly identified site. Mutation of these sites decreased p53‐mediated and UV‐induced apoptosis, and the reduction correlated with total abrogation of UV‐induced phosphorylation on Ser37 and a significant decrease in Ser15 phosphorylation in mutant p53 containing alanine at Ser33 and Ser46. Inhibition of p38 activation after UV irradiation decreased phosphorylation of Ser33, Ser37 and Ser15, and also markedly reduced UV‐induced apoptosis in a p53‐dependent manner. These results suggest that p38 kinase plays a prominent role in an integrated regulation of N‐terminal phosphorylation that regulates p53‐mediated apoptosis after UV radiation.

G ₁ /S and G ₂ /M cell cycle checkpoints maintain genomic stability in eukaryotes in response to genotoxic stress. We report here both genetic and functional evidence of a Gadd45-mediated G ₂ /M checkpoint in human and murine cells. Increased expression of Gadd45 via microinjection of an expression vector into primary human fibroblasts arrests the cells at the G ₂ /M boundary with a phenotype of MPM2 immunopositivity, 4 n DNA content and, in 15% of the cells, centrosome separation. The Gadd45-mediated G ₂ /M arrest depends on wild-type p53, because no arrest was observed either in p53-null Li–Fraumeni fibroblasts or in normal fibroblasts coexpressed with p53 mutants. Increased expression of cyclin B1 and Cdc25C inhibited the Gadd45-mediated G ₂ /M arrest in human fibroblasts, indicating that the mechanism of Gadd45-mediated G ₂ /M checkpoint is at least in part through modulation of the activity of the G ₂ -specific kinase, cyclin B1/p34 ^cdc2 . Genetic and physiological evidence of a Gadd45-mediated G ₂ /M checkpoint was obtained by using GADD45 -deficient human or murine cells. Human cells with endogenous Gadd45 expression reduced by antisense GADD45 expression have an impaired G ₂ /M checkpoint after exposure to either ultraviolet radiation or methyl methanesulfonate but are still able to undergo G ₂ arrest after ionizing radiation. Lymphocytes from gadd45 -knockout mice ( gadd45 −/−) also retained a G ₂ /M checkpoint initiated by ionizing radiation and failed to arrest at G ₂ /M after exposure to ultraviolet radiation. Therefore, the mammalian genome is protected by a multiplicity of G ₂ /M checkpoints in response to specific types of DNA damage.