Genotoxic stress is an important and ubiquitous type of stress that cells are inevitably exposed to over the life span of an organism. Many potentially damaging agents both from the environment and from endogenous processes involving activated oxygen species and other reactive agents can damage the DNA in cells (Ref.1). A major factor in the cellular response to DNA damage and other types of stress, including nucleotide pool depletion and hypoxia, is the p53 tumor suppresser gene, which is activated by ATM/ATR kinases. Upon detection of DNA damage the p53 protein, which functions as a transcription factor, is stabilized, and its elevated level results in induction of p53 target genes. GADD45 (Growth Arrest and DNA Damage-inducible) is one of several known p53 target genes. However, while GADD45
induction by Gamma radiation is p53-dependent, other factors such as short wavelength ultraviolet light, MMS (Methyl Methanesulfonate), Ptg (Prostaglandin), and Camptothecin can induce GADD45
in the absence of active p53 (Ref.2).
In mammalian cells, three members of GADD45 genes, which include GADD45Alpha
, GADD45Beta (MyD118) and GADD45Gamma (CR6/OIG37) have been identified, which encode highly acidic nuclear proteins with similar and unusual charge characteristics (Ref.1). GADD45 family members are characterized by specific inducible patterns of expression. GADD45Alpha is induced in a p53-dependent manner by Gamma irradiation. GADD45Beta is upregulated by TGF-Beta (Transforming Growth Factor-Beta), which induces growth arrest and apoptosis in the myeloid cell line. Moreover, both GADD45Alpha and GADD45Beta are induced by IL-6 (Interleukin-6), a differentiation-inducing cytokine, and by DNA-damaging agents such as MMS. In contrast to the inducible expression pattern of GADD45Alpha and GADD45Beta associated with growth arrest, differentiation, and apoptosis, GADD45Gamma is induced in the proliferative response of T-Cells to IL-2. GADD45Gamma supports growth promotion when expressed at physiological levels (Ref.3). All GADD45 family proteins are involved in a variety of growth regulatory mechanisms, including DNA replication and repair, G2/M checkpoint control, and apoptosis. GADD45 binds to several proteins involved in these processes, including PCNA (Proliferating Cell Nuclear Antigen), the cell cycle kinase inhibitor p21(CIP1) (WAF1/CIP1), and CDC2 (Cell Division Cycle-2). GADD45 physically interacts with CDC2 and directly inhibits CDC2-Cyclin-B1 kinase through its disruption of the CDC2-Cyclin-B1 mitosis-promoting complex (Ref.4). GADD45Beta and GADD45Gamma are also involved myeloid differentiation. In response to environmental stress GADD45 and two related proteins, MyD118/GADD45Beta and CR6/GADD45Gamma, act as initiators of JNK (Jun N-terminal Kinase)/p38 signaling via their interaction with an upstream kinase MAP3K1 (MAP Kinase Kinase Kinase-1)/MEKK4 (MAPK /ERK Kinase Kinase-4) (Ref.5). This activation leads to phosphorylation and activation of transcription factors, such as c-Jun/Activator Protein-1 and ATF2 (Activating Transcription Factor-2), which mediate immediate early gene induction. (Ref.2). BRCA1 (Breast Cancer Susceptibility Protein-1), also induces GADD45 and immunodepletion of Oct1 and NF-YA (Nuclear transcription Factor-YAlpha) proteins or mutations in the Oct1 and CAAT motifs disrupt BRCA1 binding to the GADD45 promoter.
GADD45 can recognize damaged chromatin and modify chromatin accessibility. It facilitates topoisomerase relaxing and cleavage activity in the presence of core histones. This effect is mediated through destabilization of histone-DNA interactions, since GADD45
interacts directly with the core histone and prevents the formation of histone-DNA oligomer complexes.