The central region of Gadd45 is required for its interaction with p21/WAF1

Cell cycle arrest represents an important response to genotoxic stress and the tumor suppressor p53 has been described to act as a critical effector in this biological event. Upon stress, p53 becomes transcriptionally active and up-regulates the transcription of downstream effector genes, which contain p53 recognition sites in their regulatory regions. Among the genes activated are p21 and GADD45, each of which independently exhibits growth-suppressive activity. The Gadd45 protein has been described to form a complex with p21, and thus, work was undertaken to map the regions of Gadd45 involved in this interaction and to examine the roles of those two proteins in growth suppression. In this report, a Gadd45 overlapping peptide library and a series of Gadd45 deletion mutants were used to define the domains of Gadd45 involved in the association with p21. Results using both in vitro and in vivo methods have shown that the interaction of Gadd45 with p21 involves a central region of Gadd45. Interestingly, the p21-binding domain of Gadd45 also encodes the Cdc2-binding activity, indicating that the central region of Gadd45 may serve as an important "core," through which Gadd45 protein is able to present cross-talk with other cell cycle regulators. In addition, GADD45 inhibition of Cdc2 kinase activity was compared with Myd118 and CR6, two other members of the GADD45 family. GADD45 was shown to generate the strongest inhibitory effect on Cdc2 activity. Finally, results from short-term survival assays further demonstrated that p21 and GADD45 act upon different cellular pathways to exert their growth-suppressive function.

The GADD45 inhibition of Cdc2 kinase correlates with GADD45-mediated growth suppression

Cell cycle growth arrest is an important cellular response to genotoxic stress. Gadd45, a p53-regulated stress protein, plays an important role in the cell cycle G(2)-M checkpoint following exposure to certain types of DNA-damaging agents such as UV radiation and methylmethane sulfonate. Recent findings indicate that Gadd45 interacts with Cdc2 protein and inhibits Cdc2 kinase activity. In the present study, a series of Myc-tagged Gadd45 deletion mutants and a Gadd45 overlapping peptide library were used to define the Gadd45 domains that are involved in the interaction of Gadd45 with Cdc2. Both in vitro and in vivo studies indicate that the interaction of Gadd45 with Cdc2 involves a central region of the Gadd45 protein (amino acids 65-84). The Cdc2-binding domain of Gadd45 is also required for Gadd45 inhibition of Cdc2 kinase activity. Sequence analysis of the central Gadd45 region reveals no homology to inhibitory motifs of known cyclin-dependent kinase inhibitors, indicating that the Cdc2-binding and -inhibitory domains on Gadd45 are a novel motif. The peptide containing the Cdc2-binding domain (amino acids 65-84) disrupted the Cdc2-cyclin B1 protein complex, suggesting that dissociation of this complex results from a direct interaction between the Gadd45 and Cdc2 proteins. GADD45-induced cell cycle G(2)-M arrest was abolished when its Cdc2 binding motif was disrupted. Importantly, a short term survival assay demonstrated that GADD45-induced cell cycle G(2)-M arrest correlates with GADD45-mediated growth suppression. These findings indicate that the cell cycle G(2)-M growth arrest mediated by GADD45 is one of the major mechanisms by which GADD45 suppresses cell growth.

Association with Cdc2 and inhibition of Cdc2/Cyclin B1 kinase activity by the p53-regulated protein Gadd45

Recently Gadd45, a p53-regulated stress protein, has been implicated in the activation of a G2/M checkpoint after damage by UV radiation and alkylating agents. While inhibitory phosphorylation of Cdc2 and suppression of cyclin B1 levels are known to be involved in G2 delays after genotoxic stress, Gadd45 has now been found to directly inhibit the activity of Cdc2/Cyclin B1 complex, while it had no appreciable effect on Cdk2/ Cyclin E activity even at very high levels of Gadd45. In contrast, p21CiP1/Waf1 is an universal cdk/cyclin inhibitor and inhibited both of the cyclin complexes tested here. Gadd45 was also able to physically interact with Cdc2, but not Cyclin B1. Addition of Gadd45 to immunoprecipitated Cdc2/Cyclin B1 in vitro led to a dissociation of this complex, and thus may represent a new checkpoint mechanism whereby Cdc2/Cyclin B1 can be inhibited. With the use of an antisense approach, reduced Gadd45 expression attenuated the suppression of Cdc2/Cyclin B1 activity in UV-irradiated human cells. Taken together, these results implicate Gadd45 in the control of G2/M cell cycle progression after certain stresses.

Gadd45, a p53-responsive stress protein, modifies DNA accessibility on damaged chromatin

This report demonstrates that Gadd45, a p53-responsive stress protein, can facilitate topoisomerase relaxing and cleavage activity in the presence of core histones. A correlation between reduced expression of Gadd45 and increased resistance to topoisomerase I and topoisomerase II inhibitors in a variety of human cell lines was also found. Gadd45 could potentially mediate this effect by destabilizing histone-DNA interactions since it was found to interact directly with the four core histones. To evaluate this possibility, we investigated the effect of Gadd45 on preassembled mononucleosomes. Our data indicate that Gadd45 directly associates with mononucleosomes that have been altered by histone acetylation or UV radiation. This interaction resulted in increased DNase I accessibility on hyperacetylated mononucleosomes and substantial reduction of T4 endonuclease V accessibility to cyclobutane pyrimidine dimers on UV-irradiated mononucleosomes but not on naked DNA. Both histone acetylation and UV radiation are thought to destabilize the nucleosomal structure. Hence, these results imply that Gadd45 can recognize an altered chromatin state and modulate DNA accessibility to cellular proteins.

Ultraviolet-irradiation-induced apoptosis is mediated via ligand independent activation of tumor necrosis factor receptor 1

Ultraviolet (UV)-irradiation has been shown to induce jun N-terminal kinase activity via aggregation-mediated activation of tumor necrosis factor receptor 1 (TNFR1) but the role of TNFR1 in mediating UV-induced apoptosis has not been explored. Using p53-null cells, we demonstrate that UV-stimulated ligand independent activation of TNFR1 plays a major role in mediating the apoptotic effects of UV-irradiation. UV-irradiation and TNF alpha acted in a synergistic manner to induce apoptosis. UV-irradiation stimulated the aggregation-mediated activation of TNFR1 which was coupled with activation of caspase 8, the most proximal caspase in TNF alpha signaling pathway. CrmA and the dominant negative versions of FADD, caspase 8 and caspase 10, that block the apoptotic axis of TNFR1 at different levels, also independently inhibited the UV-induced apoptosis. The engagement of the membrane initiated events was specific for UV-irradiation since neither CrmA nor the dominant negative FADD, caspase 8 or caspase 10 blocked the ionizing radiation-induced apoptosis. Cisplatin and melphalan, the UV-mimetic agents known to elicit UV-type DNA damage, also induced apoptosis but differed from UV in that both of the former agents engaged the caspase cascade at a level distal to FADD. Consistent with these findings cisplatin also did not stimulate TNFR1 aggregation. Together these results indicate that DNA damage per se was not sufficient to activate the membrane TNFR1. Based on our results we propose that the plasma membrane initiated events play a predominant role in mediating UV-irradiation-induced apoptosis and that UV-irradiation appears to engage the apoptotic axis of TNFR1 and perhaps those of other membrane death receptors to transduce its apoptotic signals.

Tumor suppressor p53 can participate in transcriptional induction of the GADD45 promoter in the absence of direct DNA binding

The GADD45 gene is a growth arrest-associated gene that is induced by certain DNA-damaging agents and other stresses, such as starvation, in all mammalian cells. In addition to a strong p53-binding element in an intronic sequence, we have recently found that p53, while not required or sufficient alone, may contribute to the stress responsiveness of the promoter. Much of the responsiveness was localized to a GC-rich motif in the proximal promoter which contains multiple Egr1 sites and a larger WT1 site; this 20-bp WT1 motif is identical to the WT1-binding site in the PDGF-A gene. In extracts from a human breast carcinoma cell line expressing p53 and WT1, which is known to associate with p53 in vivo, evidence was obtained that these proteins are in a complex that binds this 20-bp element. A combination of p53 and WT1 expression vectors strongly induced a GADD45-reporter construct, while mutation of the WT1-Egr1 site in the promoter prevented this induction. Abrogation of p53 function by a dominant-negative vector or abrogation of WT1 function by an antisense vector markedly reduced the induction of this promoter. Since p53 does not bind directly to the promoter, these results indicate that p53 can contribute to the positive regulation of a promoter by protein-protein interactions.

Rb binds c-Jun and activates transcription

The retinoblastoma protein (Rb) acts as a critical cell-cycle regulator and loss of Rb function is associated with a variety of human cancer types. Here we report that Rb binds to members of the AP-1 family of transcription factors, including c-Jun, and stimulates c-Jun transcriptional activity from an AP-1 consensus sequence. The interaction involves the leucine zipper region of c-Jun and the B pocket of Rb as well as a C-terminal domain. We also present evidence that the complexes are found in terminally differentiating keratinocytes and cells entering the G1 phase of the cell cycle after release from serum starvation. The human papillomavirus type 16 E7 protein, which binds to both c-Jun and Rb, inhibits the ability of Rb to activate c-Jun. The results provide evidence of a role for Rb as a transcriptional activator in early G1 and as a potential modulator of c-Jun expression during keratinocyte differentiation.

The human papillomavirus type 16 E7 gene product interacts with and trans-activates the AP1 family of transcription factors

The E7 gene product of human papillomavirus type 16 (HPV16) binds to the retinoblastoma gene product (pRb) and dissociates pRb-E2F complexes. However, the observation that the ability of E7 to bind pRb is not required for the HPV16-induced immortalization of primary keratinocytes prompted a search for other cellular factors bound by E7. Using a glutathione-S-transferase (GST) fusion protein system, we show that E7 complexes with AP1 transcription factors including c-Jun, JunB, JunD and c-Fos. The ability of E7 to complex with c-Jun in vivo is demonstrated by co-immunoprecipitation and the yeast two-hybrid system. An analysis of E7 point mutants in the GST system indicates that the E7 zinc-finger motif, but not the pRb binding domain, is involved in these interactions. Using c-Jun deletion mutants, E7 binding maps between amino acids 224 and 286 of c-Jun. E7 trans-activates c-Jun-induced transcription from a Jun responsive promoter, and this activity correlates with the ability of E7 mutants to bind Jun proteins. Finally, a transcriptionally inactive c-Jun deletion, which can bind E7, interferes with the E7-induced transformation of rat embryo fibroblasts in cooperation with an activated ras, indicating that the Jun-E7 interaction is physiologically relevant and that Jun factors may be targeted in the E7 transformation pathway.