Stress-induced JNK activation is independent of Gadd45 induction

Single-cell transcriptomics identifies Gadd45b as a regulator of herpesvirus-reactivating neurons

Single-cell RNA sequencing (scRNA-seq) is a powerful technique for dissecting the complexity of normal and diseased tissues, enabling characterization of cell diversity and heterogeneous phenotypic states in unprecedented detail. However, this technology has been underutilized for exploring the interactions between the host cell and viral pathogens in latently infected cells. Herein, we use scRNA-seq and single-molecule sensitivity fluorescent in situ hybridization (smFISH) technologies to investigate host single-cell transcriptome changes upon the reactivation of a human neurotropic virus, herpes simplex virus-1 (HSV-1). We identify the stress sensor growth arrest and DNA damage-inducible 45 beta (Gadd45b) as a critical antiviral host factor that regulates HSV-1 reactivation events in a subpopulation of latently infected primary neurons. We show that distinct subcellular localization of Gadd45b correlates with the viral late gene expression program, as well as the expression of the viral transcription factor, ICP4. We propose that a hallmark of a "successful" or "aborted" HSV-1 reactivation state in primary neurons is determined by a unique subcellular localization signature of the stress sensor Gadd45b.

Benzo[a]pyrene induced p53-mediated cell cycle arrest, DNA repair, and apoptosis pathways in Chinese rare minnow (Gobiocypris rarus)

The p53 pathways play an important role in carcinogenesis. In mammals, p53 and p53 target genes have been extensively studied, but little is known about their functions and regulation in fish. In this study, the cDNA fragments of p53 network genes, including p53, p21, mdm2, gadd45α, gadd45β, igfbp-3, and bax, were cloned from Chinese rare minnow (Gobiocypris rarus). These genes displayed high amino acid sequence identities with their zebrafish orthologs. The mRNA levels of p53 network genes and pathological changes in the liver were determined after adult rare minnow were exposed to 0.4, 2, and 10 µg/L of benzo[a]pyrene (BaP) for 28 days. The results showed that p53, p21, mdm2, gadd45α, and bax mRNA expressions in the livers from males and females were significantly upregulated compared with those of the controls (p < 0.05), but gadd45β and igfbp-3 expression was not significantly changed. Microphotographs revealed enlargement of the cell nuclei and cellular degeneration in males, while atrophy and vacuolization of hepatocytes were observed in females (10 µg/L). These results suggested that BaP induced liver DNA repair and apoptosis pathways and caused adverse pathological changes in rare minnow. The strongly responsive p53 network genes in the livers suggest that rare minnow is suitable as an experimental fish to screen environmental carcinogens. In addition, the p53 network genes in rare minnow could feasibly be used to identify the mechanism of environmental carcinogenesis. © 2016 Wiley Periodicals, Inc. Environ Toxicol 32: 979-988, 2017.

CArG-driven GADD45α activated by resveratrol inhibits lung cancer cells

We report anticarcinogenic effects of suicide gene therapy that relies on the use of resveratrol-responsive CArG elements from the Egr-1 promoter to induce GADD45α. In A549 lung cancer cells, endogenous GADD45α was not induced upon resveratrol treatment. Therefore, induction of exogenous GADD45α resulted in growth inhibition. Resveratrol transiently induced Egr-1 through ERK/JNK-ElK-1. Hence, we cloned natural or synthetic Egr-1 promoter upstream of GADD45α cDNA to create a suicide gene therapy vector. Since natural promoter may have antagonized effects, we tested synthetic promoter that contains either five, six or nine repeats of CArG elements essential in the Egr-1 promoter to drive the expression of GADD45α upon resveratrol treatment. Further analysis confirmed that both synthetic promoter and natural Egr-1 promoter were able to “turn on” the expression of GADD45α when combined with resveratrol, and subsequently led to suppression of cell proliferation and apoptosis.

Possible Role of GADD45γ Methylation in Diffuse Large B-Cell Lymphoma: Does It Affect the Progression and Tissue Involvement?

Objective:

Diffuse large B-cell lymphoma (DLBCL) is the most common type of non-Hodgkin lymphoma among adults and is characterized by heterogeneous clinical, immunophenotypic, and genetic features. Different mechanisms deregulating cell cycle and apoptosis play a role in the pathogenesis of DLBCL. Growth arrest DNA damage-inducible 45 (GADD45γ) is an important gene family involved in these mechanisms. The aims of this study are to determine the frequency of GADD45γ methylation, to evaluate the correlation between GADD45γ methylation and protein expression, and to investigate the relation between methylation status and clinicopathologic parameters in DLBCL tissues and reactive lymphoid node tissues from patients with reactive lymphoid hyperplasia.

Materials and Methods:

Thirty-six tissue samples of DLBCL and 40 nonmalignant reactive lymphoid node tissues were analyzed in this study. Methylation-sensitive high-resolution melting analysis was used for the determination of GADD45γ methylation status. The GADD45γ protein expression was determined by immunohistochemistry.

Results:

GADD45γ methylation was frequent (50.0%) in DLBCL. It was also significantly higher in advanced-stage tumors compared with early-stage (p=0.041). In contrast, unmethylated GADD45γ was associated with nodal involvement as the primary anatomical site (p=0.040).

Conclusion:

The results of this study show that, in contrast to solid tumors, the frequency of GADD45γ methylation is higher and this epigenetic alteration of GADD45γ may be associated with progression in DLBCL. In addition, nodal involvement is more likely to be present in patients with unmethylated GADD45γ.

Down-regulation of heat shock protein HSP90ab1 in radiation-damaged lung cells other than mast cells

Ionizing radiation (IR) leads to fibrosing alveolitis (FA) after a lag period of several weeks to months. In a rat model, FA starts at 8 weeks after IR. Before that, at 5.5 weeks after IR, the transcription factors Sp1 (stimulating protein 1) and AP-1 (activator protein 1) are inactivated. To find genes/proteins that were down-regulated at that time, differentially expressed genes were identified in a subtractive cDNA library and verified by quantitative RT-PCR (reverse transcriptase polymerase chain reaction), western blotting and immunohistochemistry (IH). The mRNA of the molecular chaperone HSP90AB1 (heat shock protein 90 kDa alpha, class B member 1) was down-regulated 5.5 weeks after IR. Later, when FA manifested, HSP90ab1 protein was down-regulated by more than 90% in lung cells with the exception of mast cells. In most mast cells of the normal lung, both HSP90ab1 and HSP70, another major HSP, show a very low level of expression. HSP70 was massively up-regulated in all mast cells three months after irradiation whereas HSP90AB1 was up-regulated only in a portion of mast cells. The strong changes in the expression of central molecular chaperones may contribute to the well-known disturbance of cellular functions in radiation-damaged lung tissue.

Mammalian MAPK signal transduction pathways activated by stress and inflammation: a 10-year update

The mammalian stress-activated families of mitogen-activated protein kinases (MAPKs) were first elucidated in 1994, and by 2001, substantial progress had been made in identifying the architecture of the pathways upstream of these kinases as well as in cataloguing candidate substrates. This information remains largely sound. Nevertheless, an informed understanding of the physiological and pathophysiological roles of these kinases remained to be accomplished. In the past decade, there has been an explosion of new work using RNAi in cells, as well as transgenic, knockout and conditional knockout technology in mice that has provided valuable insight into the functions of stress-activated MAPK pathways. These findings have important implications in our understanding of organ development, innate and acquired immunity, and diseases such as atherosclerosis, tumorigenesis, and type 2 diabetes. These new developments bring us within striking distance of the development and validation of novel treatment strategies. Herein we first summarize the molecular components of the mammalian stress-regulated MAPK pathways and their regulation as described thus far. We then review some of the in vivo functions of these pathways.

GADD45 proteins: central players in tumorigenesis

The Growth Arrest and DNA Damage-inducible 45 (GADD45) proteins have been implicated in regulation of many cellular functions including DNA repair, cell cycle control, senescence and genotoxic stress. However, the pro-apoptotic activities have also positioned GADD45 as an essential player in oncogenesis. Emerging functional evidence implies that GADD45 proteins serve as tumor suppressors in response to diverse stimuli, connecting multiple cell signaling modules. Defects in the GADD45 pathway can be related to the initiation and progression of malignancies. Moreover, induction of GADD45 expression is an essential step for mediating anti-cancer activity of multiple chemotherapeutic drugs and the absence of GADD45 might abrogate their effects in cancer cells. In this review, we present a comprehensive discussion of the functions of GADD45 proteins, linking their regulation to effectors of cell cycle arrest, DNA repair and apoptosis. The ramifications regarding their roles as essential and central players in tumor growth suppression are also examined. We also extensively review recent literature to clarify how different chemotherapeutic drugs induce GADD45 gene expression and how its up-regulation and interaction with different molecular partners may benefit cancer chemotherapy and facilitate novel drug discovery.

Low-dose carcinogenicity of 2-amino-3-methylimidazo[4,5-f ]quinoline in rats: Evidence for the existence of no-effect levels and a mechanism involving p21(Cip / WAF1)

The carcinogenicity of the low amounts of genotoxic carcinogens present in food is of pressing concern. The purpose of the present study was to determine the carcinogenicity of low doses of the dietary genotoxic carcinogen 2-amino-3-methylimidazo[4,5-f]quinoline (IQ) and to investigate mechanisms by which IQ exerts its carcinogenic effects. A total of 1595 male F344 rats were divided into seven groups and administered with IQ at doses of 0, 0.001, 0.01, 0.1, 1, 10 and 100 p.p.m. in the diet for 16 weeks. We found that IQ doses of 1 p.p.m. and below did not induce preneoplastic lesions in either the liver or the colon, while IQ doses of 10 and 100 p.p.m. induced preneoplastic lesions in both of these organs. These results demonstrate the presence of no-effect levels of IQ for both liver and colon carcinogenicity in rats. The finding that p21(Cip/WAF1) was significantly induced in the liver at doses well below those required for IQ mediated carcinogenic effects suggests that induction of p21(Cip/WAF1) is one of the mechanisms responsible for the observed no-effect of low doses of IQ. Furthermore, IQ administration caused significant induction of CYP1A2 at doses of 0.01-10 p.p.m., but administration of 100 p.p.m. IQ induced CYP1A1 rather than CYP1A2. This result indicates the importance of dosage when interpreting data on the carcinogenicity and metabolic activation of IQ. Overall, our results suggest the existence of no-effect levels for the carcinogenicity of this genotoxic compound.

Hypermethylation of growth arrest DNA-damage-inducible gene 45 in non-small cell lung cancer and its relationship with clinicopathologic features

The growth arrest DNA-damage-inducible protein 45 (GADD45) can serve as a key coordinator of the stress response by regulating cell cycle progression, genomic stability, DNA repair, and other stress-related responses. Although deregulation of GADD45 expression has been reported in several types of human tumors, its role in lung cancer is still unknown. DNA hypermethylation of promoter CpG islands is known to be a major mechanism for epigenetic inactivation of tumor suppressor genes. We investigated the methylation status of GADD45 family genes (GADD45A, B, and G) in 139 patients with non-small cell lung cancer (NSCLC) using methylation-specific PCR (MSP) and correlated the results with clinicopathologic features of the patients. Methylation frequencies in tumors were 1.4% for GADD45A, 7.2% for GADD45B, and 31.6% for GADD45G. RT-PCR and MSP analysis showed that promoter methylation of the GADD45G gene resulted in downregulation of its mRNA expression. GADD45G methylation was significantly more frequent in female patients than male patients (P = 0.035). This finding suggests that methylation-associated down-regulation of the GADD45G gene may be involved in lung tumorigenesis.

Regulation of JNK and p38 MAPK in the immune system: signal integration, propagation and termination

Stress-activated MAP kinases (MAPKs), comprised of JNK and p38, play prominent roles in the innate and adaptive immune systems. Activation of MAPKs is mediated by a three-tiered kinase module comprised of MAPK kinase kinases (MAP3Ks), MAPK kinases (MAP2Ks) and MAPKs through sequential protein phosphorylation. Activated MAPKs, in turn, phosphorylate transcription factors and other targets to regulate gene transcription and immune responses. Recent studies have provided new insight into the upstream and downstream components of the MAPK pathway that facilitate the activation and propagation of MAPK signaling in immune responses. Moreover, MAPK activity is negatively regulated by MAPK phosphatases (MKPs), a group of dual-specificity phosphatases that dephosphorylate and inactivate the MAPKs. Here we discuss the recent advances in our understanding of these regulatory processes in MAPK signaling with a focus on their impacts on immune function.

Gadd45 in stress signaling

Gadd45 genes have been implicated in stress signaling in response to physiological or environmental stressors, which results in cell cycle arrest, DNA repair, cell survival and senescence, or apoptosis. Evidence accumulated implies that Gadd45 proteins function as stress sensors is mediated by a complex interplay of physical interactions with other cellular proteins that are implicated in cell cycle regulation and the response of cells to stress. These include PCNA, p21, cdc2/cyclinB1, and the p38 and JNK stress response kinases. What deterministic factors dictate whether Gadd45 and partner proteins function in either cell survival or apoptosis remains to be determined. An attractive working model to consider is that the extent of cellular/DNA damage, in a given cell type, dictates the association of different Gadd45 proteins with particular partner proteins, which determines the outcome.

Upregulation and function of GADD45gamma in unilateral ureteral obstruction

We performed differential display analysis to determine transcriptional activity in the rat kidney, following unilateral ureteral obstruction and found a 12-fold increase in the expression of Growth Arrest and DNA Damage-45gamma (GADD45gamma), a stress-responsive molecule that interacts with cell-cycle proteins. GADD45gamma was strongly expressed in as little as 6 h following ureteric obstruction in the renal tubules, and was also found in kidney tissue of patients with chronic glomerulonephritis. Adenovirus-mediated expression of GADD45gamma in cultured renal tubular cells activated p38 along with a significant upregulation of C-C and C-X3-C chemokine ligands and fibrosis-related factors such as several matrix metalloproteinases, transforming growth factor-beta1, decorin, and bone morphogenetic protein 2. Silencing of GADD45gamma expression significantly blunted the upregulation of these inflammatory and fibrogenic mediators and monocyte infiltration in the ureteral obstructed rat kidney. Our study shows that GADD45gamma is quickly upregulated in the kidney with an obstructed ureter, enhancing the production of factors regulating the pathogenesis of kidney disease.

Mitogen-activated protein kinases regulate susceptibility to ventilator-induced lung injury

Background

Mechanical ventilation causes ventilator-induced lung injury in animals and humans. Mitogen-activated protein kinases have been implicated in ventilator-induced lung injury though their functional significance remains incomplete. We characterize the role of p38 mitogen-activated protein kinase/mitogen activated protein kinase kinase-3 and c-Jun-NH₂-terminal kinase-1 in ventilator-induced lung injury and investigate novel independent mechanisms contributing to lung injury during mechanical ventilation.

Methodology and Principle Findings

C57/BL6 wild-type mice and mice genetically deleted for mitogen-activated protein kinase kinase-3 (mkk-3^−/−) or c-Jun-NH₂-terminal kinase-1 (jnk1^−/−) were ventilated, and lung injury parameters were assessed. We demonstrate that mkk3^−/− or jnk1^−/− mice displayed significantly reduced inflammatory lung injury and apoptosis relative to wild-type mice. Since jnk1^−/− mice were highly resistant to ventilator-induced lung injury, we performed comprehensive gene expression profiling of ventilated wild-type or jnk1^−/− mice to identify novel candidate genes which may play critical roles in the pathogenesis of ventilator-induced lung injury. Microarray analysis revealed many novel genes differentially expressed by ventilation including matrix metalloproteinase-8 (MMP8) and GADD45α. Functional characterization of MMP8 revealed that mmp8^−/− mice were sensitized to ventilator-induced lung injury with increased lung vascular permeability.

Conclusions

We demonstrate that mitogen-activated protein kinase pathways mediate inflammatory lung injury during ventilator-induced lung injury. C-Jun-NH₂-terminal kinase was also involved in alveolo-capillary leakage and edema formation, whereas MMP8 inhibited alveolo-capillary protein leakage.

Gadd45 in the response of hematopoietic cells to genotoxic stress

Gadd45 genes have been implicated in stress signaling in response to physiological or environmental stressors, which results in either cell cycle arrest, DNA repair, cell survival and senescence, or apoptosis. Evidence accumulated implies that Gadd45 proteins function as stress sensors is mediated by a complex interplay of physical interactions with other cellular proteins that are implicated in cell cycle regulation and the response of cells to stress. These include PCNA, p21, cdc2/cyclinB1, and the p38 and JNK stress response kinases. Recently we have taken advantage of gadd45a and gadd45b deficient mice to determine the role gadd45a and gadd45b play in the response of bone marrow (BM) cells to genotoxic stress. Myeloid enriched BM cells from gadd45a and gadd45b deficient mice were observed to be more sensitive to ultraviolet radiation (UVC), VP-16, and daunorubicin (DNR)-induced apoptosis compared to wild-type (wt) cells. The increased apoptosis in gadd45a and gadd45b deficient cells was evident also by enhanced activation of caspase-3 and PARP cleavage and decreased expression of cIAP-1, Bcl-2, and Bcl-xL compared to wt cells. Reintroduction of gadd45 into gadd45 deficient BM cells restored the wt apoptotic phenotype. Both gadd45a and gadd45b deficient BM cells also displayed defective G2/M arrest following exposure to UVC and VP-16, but not to DNR, indicating the existence of different G2/M checkpoints that are either dependent or independent of gadd45. Additional work conducted in this laboratory has shown that in hematopoietic cells exposed to UV radiation gaddd45a and gadd45b cooperate to promote cell survival via two distinct signaling pathways involving activation of the Gadd45a-p38-NF-kB-mediated survival pathway and Gadd45b-mediated inhibition of the stress response MKK4-JNK pathway [O. Kovalsky, F.D. Lung, P.P. Roller, A.J. Fornace, Jr. Oligomerization of human Gadd45a protein. J Biol Chem. 276 (42) (2001) 39330-39339]. These data reveal novel mechanisms that mediate the pro-survival functions of gadd45a and gadd45b in hematopoietic cells following UV irradiation. Taken together, these findings identify gadd45a and gadd45b as anti-apoptotic genes that increase the survival of hematopoietic cells following exposure to UV radiation and certain anticancer drugs. This knowledge should contribute to a greater understanding of the genetic events involved in the pathogenesis of different leukemias and response of normal and malignant hematopoietic cells to chemo and radiation therapy. These observations set the stage to evaluate, in clinically relevant settings, the impact that the status of gadd45a and gadd45b might have on the efficacy of DNR or VP-16 in killing leukemic cells.

Gadd45a expression induces Bim dissociation from the cytoskeleton and translocation to mitochondria

Gadd45a, a p53- and BRCA1-regulated stress protein, has been implicated in the maintenance of genomic fidelity, probably through its roles in the control of cell cycle checkpoint and apoptosis. However, the mechanism(s) by which Gadd45a is involved in the induction of apoptosis remains unclear. We show here that inducible expression of Gadd45a protein causes dissociation of Bim, a Bcl2 family member, from microtubule-associated components and translocation to mitochondria. The Bim accumulation in mitochondria enhances interaction of Bim with Bcl-2, relieves Bax from Bcl-2-bound complexes, and subsequently results in release of cytochrome c into the cytoplasm. Suppression of endogenous Bim greatly inhibits Gadd45a induction of apoptosis. Interestingly, Gadd45a interacts with elongation factor 1alpha (EF-1alpha), a microtubule-severing protein that plays an important role in maintaining cytoskeletal stability, and inhibits EF-1alpha-mediated microtubule bundling, indicating that the interaction of Gadd45a with EF-1alpha disrupts cytoskeletal stability. A mutant form of Gadd45a harboring a deletion of EF-1alpha-binding domain fails to inhibit microtubule stability and to induce Bim translocation to mitochondria. Furthermore, coexpression of EF-1alpha antagonizes Gadd45a's property of suppressing cell growth and inducing apoptosis. These findings identify a novel link that connects stress protein Gadd45a to the apoptotic machinery and address the importance of cytoskeletal stability in apoptotic response to DNA damage.

Gadd45a, a p53- and BRCA1-regulated stress protein, in cellular response to DNA damage

Mammalian cells exhibit complex, but intricate cellular responses to genotoxic stress, including cell cycle checkpoints, DNA repair and apoptosis. Inactivation of these important biological events may result in genomic instability and cell transformation, as well as alterations of therapeutic sensitivity. Gadd45a, a p53- and BRCA1-regulated stress-inducible gene, has been characterized as one of the important players that participate in cellular response to a variety of DNA damage agents. Interestingly, the signaling machinery that regulates Gadd45a induction by genotoxic stress involves both p53-dependent and -independent pathways; the later may employ BRCA1-related or MAP kinase-mediated signals. Gadd45a protein has been reported to interact with multiple important cellular proteins, including Cdc2 protein kinase, proliferating cell nuclear antigen (PCNA), p21Waf1/Cip1 protein, core histone protein and MTK/MEKK4, an up-stream activator of the JNK/SAPK pathway, indicating that Gadd45a may play important roles in the control of cell cycle checkpoint, DNA repair process, and signaling transduction. The importance of Gadd45a in maintaining genomic integrity is well manifested by the demonstration that disruption of endogenous Gadd45a in mice results in genomic instability and increased carcinogenesis. Therefore, Gadd45a appears to be an important component in the cellular defense network that is required for maintenance of genomic stability.

Linking JNK signaling to NF-kappaB: a key to survival

In addition to marshalling immune and inflammatory responses, transcription factors of the NF-kappaB family control cell survival. This control is crucial to a wide range of biological processes, including B and T lymphopoiesis, adaptive immunity, oncogenesis and cancer chemoresistance. During an inflammatory response, NF-kappaB activation antagonizes apoptosis induced by tumor necrosis factor (TNF)-alpha, a protective activity that involves suppression of the Jun N-terminal kinase (JNK) cascade. This suppression can involve upregulation of the Gadd45-family member Gadd45beta/Myd118, which associates with the JNK kinase MKK7/JNKK2 and blocks its catalytic activity. Upregulation of XIAP, A20 and blockers of reactive oxygen species (ROS) appear to be important additional means by which NF-kappaB blunts JNK signaling. These recent findings might open up entirely new avenues for therapeutic intervention in chronic inflammatory diseases and certain cancers; indeed, the Gadd45beta-MKK7 interaction might be a key target for such intervention.

Ultraviolet Irradiation Represses PATCHED Gene Transcription in Human Epidermal Keratinocytes through an Activator Protein-1-Dependent Process

Basal cell carcinoma (BCC) is one of the major types of skin cancer arising from keratinocytes. The SONIC HEDGEHOG pathway is deregulated in 100% of sporadic BCCs, as indicated by the overexpression of PATCHED, whose product encodes the receptor of SONIC HEDGEHOG, in 100% of analyzed BCCs. Reverse transcription-PCR analysis revealed that exposure to UVB irradiation, which is a risk factor known to contribute to BCC development, induces a strong and sharp decrease of PATCHED mRNA level both in vitro and ex vivo. Transcription of a reporter gene driven by the 4.4-kb 5'-regulatory region of the human PATCHED gene was shown to be down-regulated after UVB irradiation. Furthermore, overexpression of c-JUN, a member of the activator protein (AP)-1 family, induced repression of the PATCHED promoter. The role of AP-1 in UVB-induced PATCHED repression was confirmed in mouse embryonic fibroblasts knocked out for c-JUN NH(2)-terminal protein kinase. This study thus provides the first evidence of UV-induced down-regulation at the transcriptional level of the BCC-associated tumor suppressor PATCHED relying on activation of the AP-1 oncogenic pathway.

GADD45beta/GADD45gamma and MEKK4 comprise a genetic pathway mediating STAT4-independent IFNgamma production in T cells

The stress-inducible molecules GADD45beta and GADD45gamma have been implicated in regulating IFNgamma production in CD4 T cells. However, how GADD45 proteins function has been controversial. MEKK4 is a MAP kinase kinase kinase that interacts with GADD45 in vitro. Here we generated MEKK4-deficient mice to define the function and regulation of this pathway. CD4 T cells from MEKK4-/- mice have reduced p38 activity and defective IFNgamma synthesis. Expression of GADD45beta or GADD45gamma promotes IFNgamma production in MEKK4+/+ T cells, but not in MEKK4-/- cells or in cells treated with a p38 inhibitor. Thus, MEKK4 mediates the action of GADD45beta and GADD45gamma on p38 activation and IFNgamma production. During Th1 differentiation, the GADD45beta/GADD45gamma/MEKK4 pathway appears to integrate upstream signals transduced by both T cell receptor and IL12/STAT4, leading to augmented IFNgamma production in a process independent of STAT4.

Gadd45beta is important for perpetuating cognate and inflammatory signals in T cells

Gadd45beta (growth arrest and DNA damage-inducible, beta) is involved in cell cycle arrest, apoptosis, signal transduction and cell survival. In T cells, Gadd45b was rapidly induced by T cell receptor (TCR) and inflammatory signals. Deficiency of Gadd45beta in CD4+ T cells impaired their responses to TCR stimulation or inflammatory cytokines. ERK, p38 and JNK activation were all substantially suppressed in Gadd45beta-deficient CD4+ T cells. Cytokine production by Gadd45beta-deficient CD4+ T cells was also impaired. Furthermore, Gadd45beta mediated inflammatory cytokine production by dendritic cells, and Gadd45beta-deficient mice showed an impaired T helper type 1 response during Listeria monocytogenes infection. Gadd45beta is therefore a critical feedback regulator that perpetuates both cognate and inflammatory signals.

AP-1 mediated relief of repressive activity of the CD30 promoter microsatellite in Hodgkin and Reed-Sternberg cells

Overexpression of CD30 is the hallmark of Hodgkin and Reed-Sternberg (H-RS) cells and drives constitutive nuclear factor-kappaB activation that is the molecular basis for the pathophysiology of Hodgkin's lymphoma. Transcription of the CD30 gene is controlled by the core promoter that is driven by Sp-1 and the microsatellite sequences (MSs) that represses core promoter activity. To understand the mechanism(s) of CD30 overexpression in H-RS cells, we structurally and functionally characterized the CD30 MSs. Although the CD30 MS of H-RS cell lines was polymorphic, it was not truncated compared with that of control cells. A strong core promoter activity and constitutive Sp-1 binding were revealed in all cell lines examined irrespective of the levels of CD30 expression. In transient reporter gene assays, all MS clones derived from H-RS cell lines repressed the core promoter activity in unrelated cell lines, but not in the H-RS cell lines. An AP-1-binding site was found in the MS at nucleotide position of -377 to -371, the presence of which was found to relieve repression of the core promoter in H-RS cell lines but not in other tumor cell lines. H-RS cell lines showed constitutive and strong AP-1-binding activity, but other cell lines did not. The AP-1 complex contained JunB, whose overexpression activated reporter constructs driven by the CD30 promoter including the MSs, and was dependent on the AP-1 site. JunB expression was detected in H-RS cells in vitro and in vivo, but not in reactive cells or tumor cells of non-Hodgkin's lymphoma of diffuse large B-cell type. Transduction of JunB small interfering RNAs suppressed CD30 promoter activity in L428 cells but not in control cells. Taken together, overexpression and binding of JunB to the AP-1 site appear to relieve the repression of the core promoter by the CD30 MS in H-RS cells, which provide one basis for the constitutive overexpression of CD30 in Hodgkin's lymphoma.

Overexpression of RbAp46 facilitates stress-induced apoptosis and suppresses tumorigenicity of neoplastigenic breast epithelial cells

We have found previously that the retinoblastoma (Rb) suppressor associated protein 46 (RbAp46) is a gene upregulated by the Wilms' tumor suppressor, WT1, and functions as a potent growth inhibitor. To investigate the effect of RbAp46 overexpression on early development of breast cancer, we established stable cell lines from neoplastigenic breast epithelial cells, MCF10AT3B, a cell line derived from a model of human proliferative disease, to constitutively express exogenous RbAp46. We have found that expression of RbAp46 suppressed colony formation of MCF10AT3B cells in soft-agar, and inhibited tumor formation of these cells in nude mice. Expression of RbAp46 sensitized MCF10AT3B cells to apoptosis induced by serum deprivation and hydrocortisone withdrawal. Furthermore, we have found that the c-Jun NH2-terminal kinase (JNK) pathway and GADD45, a growth arrest- and DNA damage-inducible gene, are constitutively activated in RbAp46-expressing cells. Our data suggested that high levels of RbAp46 expression inhibit the tumorigenicity of neoplastigenic breast epithelial cells by facilitating JNK-dependent apoptotic cell death. Our data also suggested that dysregulation of RbAp46 gene may be involved in the early development of breast cancer.

Induction of ATF3 by ionizing radiation is mediated via a signaling pathway that includes ATM, Nibrin1, stress-induced MAPkinases and ATF-2

Exposure of human cells to genotoxic agents induces various signaling pathways involved in the execution of stress- and DNA-damage responses. Inappropriate functioning of the DNA-damage response to ionizing radiation (IR) is associated with the human diseases ataxia-telangiectasia (A-T) and Nijmegen Breakage syndrome (NBS). Here, we show that IR efficiently induces Jun/ATF transcription factor activity in normal human diploid fibroblasts, but not in fibroblasts derived from A-T and NBS patients. IR was found to enhance the expression of c-Jun and, in particular, ATF3, but, in contrast to various other stress stimuli, did not induce the expression of c-Fos. Using specific inhibitors, we found that the ATM- and Nibrin1-dependent activation of ATF3 does neither require p53 nor reactive oxygen species, but is dependent on the p38 and JNK MAPkinases. Via these kinases, IR activates ATF-2, one of the transcription factors acting on the atf3 promoter. The activation of ATF-2 by IR resembles ATF-2 activation by certain growth factors, since IR mainly induced the second step of ATF-2 phosphorylation via the stress-inducible MAPkinases, phosphorylation of Thr69. As IR does not enhance ATF-2 phosphorylation in ATM and Nibrin1-deficient cells, both ATF-2 and ATF3 seem to play an important role in the protective response of human cells to IR.

Comparative analysis of the genetic structure and chromosomal mapping of the murine Gadd45g/CR6 gene

Gadd45g/CR6, Gadd45b/MyD118, and Gadd45a/Gadd45 are members of a gene family that displays distinct patterns of gene expression in response to stimuli that induce differentiation, growth arrest, and/or apoptosis. All three of these highly conserved proteins interact with a number of critical cell cycle and cell survival regulatory proteins such as PCNA, p21(WAF1/CIP1), CDK1 (cdc2-p34), and MTK1/MEKK4, and have been reported to influence the activity of the p38 and JNK kinases. Species-blot analysis showed that Gadd45g is an evolutionarily conserved gene and sequence analysis showed that Gadd45g has a gene structure conserved with that of other members of its gene family. A comparison of the putative transcription factor binding sites found in the sequences of the gene family members suggests, that like Gadd45b, NF-kappaB and STATs may be responsible for the differences in regulation of expression observed between Gadd45g and Gadd45a. Analysis of the Gadd45b/MyD118 promoter shows that there are three different enhanceosome-like regions that may allow cell-type specific responses to TGF-beta1 by the Gadd45b/MyD118 promoter. Fluorescent in situ hybridization (FISH) confirmed the localization of the Gadd45g gene to mouse chromosome band 13A5-B, which has been reported to contain a quantitative trait locus that regulates body weight in mice. This suggests that alleles of the Gadd45g gene may function in the regulation of body weight, in addition to its currently recognized roles in differentiation and stress responses.

Loss of oncogenic H-ras-induced cell cycle arrest and p38 mitogen-activated protein kinase activation by disruption of Gadd45a

The activation of p53 is a guardian mechanism to protect primary cells from malignant transformation; however, the details of the activation of p53 by oncogenic stress are still incomplete. In this report we show that in Gadd45a(-/-) mouse embryo fibroblasts (MEF), overexpression of H-ras activates extracellular signal-regulated kinase (ERK) and c-Jun N-terminal kinase (JNK) but not p38 kinase, and this correlates with the loss of H-ras-induced cell cycle arrest (premature senescence). Inhibition of p38 mitogen-activated protein kinase (MAPK) activation correlated with the deregulation of p53 activation, and both a p38 MAPK chemical inhibitor and the expression of a dominant-negative p38alpha inhibited p53 activation in the presence of H-ras in wild-type MEF. p38, but not ERK or JNK, was found in a complex with Gadd45 proteins. The region of interaction was mapped to amino acids 71 to 96, and the central portion (amino acids 71 to 124) of Gadd45a was required for p38 MAPK activation in the presence of H-ras. Our results indicate that this Gadd45/p38 pathway plays an important role in preventing oncogene-induced growth at least in part by regulating the p53 tumor suppressor.

Xiro homeoproteins coordinate cell cycle exit and primary neuron formation by upregulating neuronal-fate repressors and downregulating the cell-cycle inhibitor XGadd45-gamma

The iroquois (iro) homeobox genes participate in many developmental processes both in vertebrates and invertebrates, among them are neural plate formation and neural patterning. In this work, we study in detail Xenopus Iro (Xiro) function in primary neurogenesis. We show that misexpression of Xiro genes promotes the activation of the proneural gene Xngnr1 but suppresses neuronal differentiation. This is probably due to upregulation of at least two neuronal-fate repressors: XHairy2A and XZic2. Accordingly, primary neurons arise at the border of the Xiro expression domains. In addition, we identify XGadd45-gamma as a new gene repressed by Xiro. XGadd45-gamma encodes a cell-cycle inhibitor and is expressed in territories where cells will exit mitosis, such as those where primary neurons arise. Indeed, XGadd45-gamma misexpression causes cell cycle arrest. We conclude that, during Xenopus primary neuron formation, in Xiro expressing territories neuronal differentiation is impaired, while in adjacent cells, XGadd45-gamma may help cells stop dividing and differentiate as neurons.

Regulation of MTK1/MEKK4 kinase activity by its N-terminal autoinhibitory domain and GADD45 binding

A variety of cellular stresses activate the stress-responsive mitogen-activated protein (MAP) kinases p38 and JNK. In this study, we studied the activation mechanism of a human MAP kinase kinase kinase, MTK1 (also known as MEKK4), which mediates activation of both p38 and JNK. MTK1 has an extensive N-terminal noncatalytic domain composed of approximately 1,300 amino acids. Full-length or near full-length MTK1 is catalytically inactive when expressed in Saccharomyces cerevisiae cells, as it is in mammalian cells. Deletion of a segment including positions 253 to 553 activates kinase, indicating that this segment contains the autoinhibitory domain. In the autoinhibited conformation, the MTK1 kinase domain cannot interact with its substrate, MKK6. By a functional complementation screening with yeast cells, GADD45 proteins (GADD45alpha, beta, and gamma) were identified as MTK1 activators. GADD45 proteins bind a site in MTK1 near the inhibitory domain and relieve autoinhibition. Mutants of full-length MTK1 were isolated that can interact with MKK6 in the absence of the activator GADD45 proteins. These MTK1 mutants are constitutively active, in both yeast and mammalian cells. A model of MTK1 autoinhibition by the N-terminal inhibitory domain and activation by GADD45 binding is presented.

Activation of the c-Jun N-terminal kinase (JNK) signaling pathway is essential during PBOX-6-induced apoptosis in chronic myelogenous leukemia (CML) cells

The mitogen-activated protein (MAP) kinase family is activated in response to a wide variety of external stress signals such as UV irradiation, heat shock, and many chemotherapeutic drugs and leads to the induction of apoptosis. A novel series of pyrrolo-1,5-benzoxazepines have been shown to potently induce apoptosis in chronic myelogenous leukemia (CML) cells, which are resistant to many chemotherapeutic agents. In this study we have delineated part of the mechanism by which a representative compound known as PBOX-6 induces apoptosis. We have investigated whether PBOX-6 induces activation of MAP kinase signaling pathways in CML cells. Treatment of K562 cells with PBOX-6 resulted in the transient activation of two JNK isoforms, JNK1 and JNK2. In contrast, PBOX-6 did not activate the extracellular signal-regulated kinase (ERK) or p38. Apoptosis was found to occur independently of the small GTPases Ras, Rac, and Cdc42 but involved phosphorylation of the JNK substrates, c-Jun and ATF-2. Pretreatment of K562 cells with the JNK inhibitor, dicoumarol, abolished PBOX-6-induced phosphorylation of c-Jun and ATF-2 and inhibited the induced apoptosis, suggesting that JNK activation is an essential component of the apoptotic pathway induced by PBOX-6. Consistent with this finding, transfection of K562 cells with the JNK scaffold protein, JIP-1, inhibited JNK activity and apoptosis induced by PBOX-6. JIP-1 specifically scaffolds JNK, MKK7, and members of the mixed-lineage kinase (MLK) family, implicating these kinases upstream of JNK in the apoptotic pathway induced by PBOX-6 in K562 cells.

Myeloid differentiation (MyD) primary response genes in hematopoiesis

Myeloid Differentiation (MyD) primary response and Growth Arrest DNA-Damage (Gadd) genes comprise a set of overlapping genes, including known (IRF-1, EGR-1, Jun) and novel (MyD88, Gadd45alpha MyD118/Gadd45beta, GADD45gamma, MyD116/Gadd34) genes, that have been cloned by virtue of there being co-ordinately induced upon the onset of terminal myeloid differentiation. This review delineates the role MyD genes play in blood cell development, where they function as positive regulators of terminal differentiation, lineage specific blood cell development and control of blood cell homeostasis, including growth inhibition and apoptosis.

Fas, ceramide and serum withdrawal induce apoptosis via a common pathway in a type II Jurkat cell line

Ceramide is a key mediator of apoptosis, yet its role in Fas-mediated apoptosis is controversial. Some reports have indicated that ceramide is either a primary signaling molecule in Fas-induced cell death, or that it functions upstream of Fas by increasing FasL expression. Other studies have suggested that ceramide is not relevant to Fas-induced cell death. We have approached this problem by studying ceramide-induced apoptosis in unique Jurkat cell clones selected for resistance to membrane-bound FasL-induced death. Resistance of the mutant Jurkat cells was specific for FasL killing, since the mutant clones were sensitive to other apoptotic stimuli such as cycloheximide and staurosporine. We tested the effects of serum withdrawal, one of the strongest inducers of ceramide, and of exogenous ceramide on apoptosis of both wild-type and FasL-resistant clones. Wild-type Jurkat cells were remarkably sensitive to serum withdrawal and to exogenous ceramide. In contrast all FasL-resistant mutant clones were resistant to these apoptosis-inducing conditions. In contrast to previous work, we did not detect an increase in FasL in either wild-type or mutant clones. Moreover activation of stress-activated protein kinases (JNK/SAPKs) after serum withdrawal and exogenous ceramide treatment was detected only in the wild-type and not in the resistant clones. Because of the parallel resistance of the mutant clones to Fas and to ceramide-induced apoptosis, our data support the notion that ceramide is a second messenger for the Fas/FasL pathway and that serum withdrawal, through production of ceramide, shares a common step with the Fas-mediated apoptotic pathway. Finally, our data suggest that activation of JNK/SAPKs is a common mediator of the three pathways tested.

Myeloid differentiation (MyD)/growth arrest DNA damage (GADD) genes in tumor suppression, immunity and inflammation

Myeloid differentiation (MyD) primary response and growth arrest DNA damage (Gadd) genes comprise a set of overlapping genes, including known (IRF-1, EGR-1, Jun) and novel (MyD88, Gadd45alpha, MyD118/Gadd45beta, GADD45gamma, MyD116/ Gadd34) genes, that have been cloned by virtue of being co-ordinately induced upon the onset of terminal myeloid differentiation and following exposure of cells to stress stimuli. In recent years it has become evident that MyD/Gadd play a role in blood cell development, where they function as positive regulators of terminal differentiation, lineage-specific blood cell development and control of blood cell homeostasis, including growth inhibition and apoptosis. MyD/Gadd are also involved in inflammatory responses to invading micro-organisms, and response to environmental stress and physiological stress, such as hypoxia, which results in ischemic tissue damage. An intricate network of interactions among MyD/GADD genes and gene products appears to control their diverse functions. Deregulated growth, increased cell survival, compromised differentiation and deficiencies in DNA repair are hallmarks of malignancy and its progression. Thus, the role MyD/Gadd play in negative growth control, including cell cycle arrest and apoptosis, and in DNA repair, make them attractive molecular targets for tumor suppression. The role MyD/Gadd play in innate immunity and host response to hypoxia also make these genes and gene products attractive molecular targets to treat immunity and inflammation disorders, such as septic shock and ischemic tissue damage.

Induction of gadd45beta by NF-kappaB downregulates pro-apoptotic JNK signalling

In addition to coordinating immune and inflammatory responses, NF-kappaB/Rel transcription factors control cell survival. Normally, NF-kappaB dimers are sequestered in the cytoplasm by binding to inhibitory IkappaB proteins, and can be activated rapidly by signals that induce the sequential phosphorylation and proteolysis of IkappaBs. Activation of NF-kappaB antagonizes apoptosis or programmed cell death by numerous triggers, including the ligand engagement of 'death receptors' such as tumour-necrosis factor (TNF) receptor. The anti-apoptotic activity of NF-kappaB is also crucial to oncogenesis and to chemo- and radio-resistance in cancer. Cytoprotection by NF-kappaB involves the activation of pro-survival genes; however, its basis remains poorly understood. Here we report that NF-kappaB complexes downregulate the c-Jun amino-terminal kinase (JNK) cascade, thus establishing a link between the NF-kappaB and the JNK pathways. This link involves the transcriptional upregulation of gadd45beta/myd118 (ref. 4), which downregulates JNK signalling induced by the TNF receptor (TNF-R). This NF-kappaB-dependent inhibition of the JNK pathway is central to the control of cell death. Our findings define a protective mechanism that is mediated by NF-kappaB complexes and establish a role for the persistent activation of JNK in the apoptotic response to TNF-alpha.

Induction of apoptosis by protein inhibitor of activated Stat1 through c-Jun NH2-terminal kinase activation

Members of the protein inhibitor of activated signal transducer and activator of transcription (STAT) family (PIAS family) of proteins act as negative regulators of STATs in cytokine signaling. We report here that PIAS proteins have proapoptotic activity. PIAS1 induced apoptosis in both human 293T cells and human osteosarcoma U2OS cells. PIAS1 is localized in the nucleus as distinct nuclear dots. Ectopic expression of PIAS1 in U2OS cells activated JNK1 (c-Jun NH(2)-terminal kinase). A dominant-negative JNK1, capable of inhibiting PIAS1-induced JNK1 activation, blocked PIAS1-mediated apoptosis. Furthermore, a mutant PIAS1, lacking the first 9 amino acid residues, failed to repress Stat1-mediated gene activation although it retained its ability to activate JNK and to trigger apoptosis. Our results identify a novel function of PIAS1 in the induction of JNK-dependent apoptosis, independent of the previously known inhibitory activity of PIAS1 in STAT-mediated gene activation.

Contrasting roles of NF-kappaB and JNK in arsenite-induced p53-independent expression of GADD45alpha

Growth arrest and DNA damage-inducible protein 45alpha (GADD45alpha) is an important cell cycle checkpoint protein that arrests cells at G2/M phase by inhibiting the activity of G2-specific kinase, cyclin B/p34cdc2. We report here that arsenite induces GADD45alpha expression in a p53-independent fashion and that this GADD45alpha induction by arsenite is regulated by NF-kappaB and c-Jun-N-terminal kinase (JNK) oppositely. In human bronchial epithelial cells overexpressing a kinase-mutated form of IkappaB kinase beta (IKKbeta-KM), the activation of NF-kappaB was inhibited. However, the G2/M cell cycle arrest and expression of GADD45alpha was substantially enhanced in response to arsenite in these cells. Expression of a dominant-negative mutant of SEK1 that blocks JNK activation decreased arsenite-induced GADD45alpha expression. Analysis of GADD45alpha expression in both wild-type and p53-/- fibroblasts indicated that the induction of GADD45alpha by arsenite was independent of the status of p53 protein.

GADD45gamma mediates the activation of the p38 and JNK MAP kinase pathways and cytokine production in effector TH1 cells

The p38 and JNK stress-activated MAPK signal transduction pathways are activated by T cell receptor (TCR) signaling and are required for IFN-gamma production by TH1 effector cells. Here, we show that the expression of GADD45gamma is induced during T cell activation and that the level of expression is higher in TH1 cells than in TH2 cells. TH1 cells from GADD45gamma(-/-) mice are severely compromised in their abilities to activate p38 and JNK in response to TCR signaling, produce much less IFN-gamma upon restimulation, and are deficient in activation-induced cell death (AICD). Additionally, GADD45gamma deficiencies caused reduced contact hypersensitivity in mice. Thus, GADD45gamma mediates activation of the p38 and JNK pathways and effector function of TH1 cells.

Opposite effect of NF-kappa B and c-Jun N-terminal kinase on p53-independent GADD45 induction by arsenite

Cell cycle checkpoint, a major genomic surveillance mechanism, is an important step in maintaining genomic stability and integrity in response to environmental stresses. Using cells derived from human bronchial epithelial cells, we demonstrate that NF-kappaB and c-Jun N-terminal kinase (JNK) reciprocally regulate arsenic trioxide (arsenite)-induced, p53-independent expression of GADD45 protein, a cell cycle checkpoint protein that arrests cells at the G(2)/M phase transition. Inhibition of NF-kappaB activation by stable expression of a kinase-mutated form of IkappaB kinase caused increased and prolonged induction of GADD45 by arsenite. In contrast, the induction of GADD45 by arsenite was transient and less potent in cells where the NF-kappaB activation pathway was normal. Analysis of the cell cycle profile by flow cytometry indicated that NF-kappaB inhibition potentiates arsenite-induced G(2)/M cell cycle arrest. Abrogation of JNK activation, on the other hand, decreased GADD45 expression induced by arsenite, suggesting a role for JNK activation in GADD45 induction. These results indicate a molecular mechanism by which NF-kappaB and JNK may differentially contribute to cell cycle regulation in response to arsenite.

Mammalian mitogen-activated protein kinase signal transduction pathways activated by stress and inflammation

The molecular details of mammalian stress-activated signal transduction pathways have only begun to be dissected. This, despite the fact that the impact of these pathways on the pathology of chronic inflammation, heart disease, stroke, the debilitating effects of diabetes mellitus, and the side effects of cancer therapy, not to mention embryonic development, innate and acquired immunity, is profound. Cardiovascular disease and diabetes alone represent the most significant health care problems in the developed world. Thus it is not surprising that understanding these pathways has attracted wide interest, and in the past 10 years, dramatic progress has been made. Accordingly, it is now becoming possible to envisage the transition of these findings to the development of novel treatment strategies. This review focuses on the biochemical components and regulation of mammalian stress-regulated mitogen-activated protein kinase (MAPK) pathways. The nuclear factor-kappa B pathway, a second stress signaling paradigm, has been the subject of several excellent recent reviews (258, 260).

JNK activation is associated with intracellular beta-amyloid accumulation

c-Jun has been implicated in the pathogenesis of Alzheimer's disease (AD), but the upstream cascade leading to c-Jun activation in AD is not known. Activation of c-Jun N-terminal kinase (JNK) is obviously a candidate for the upstream event. We tested this possibility focusing on PS1-linked AD. First, we observed that JNK is actually activated in cerebral neurons of PS1-linked AD patients, using immunohistochemistry and Western blot analyses with anti-activated JNK antibodies. We analyzed the relationship between beta-amyloid (beta A) and JNK activation by using aged transgenic mice overexpressing mutant (M146L) PS1 and human AD brains. The mice showed no neuronal loss but a very few diffuse beta A deposits, corresponding to the early stage of PS1-linked AD brain. Some neurons were reactive for anti-beta A antibodies in the cerebral cortex. Interestingly, JNK activation was observed in neurons showing intracellular beta A immunoreactivity in transgenic mice. Association between intracellular beta A and JNK activation was confirmed in cortical neurons of sporadic and PS1-linked AD patients. Furthermore, introduction of beta A peptides into the primary culture cortical neurons induced JNK activation and cell death. Collectively, these results suggested that intracellular beta A accumulation might trigger JNK activation leading to neuronal death.

The mammalian UV response: c-Jun induction is required for exit from p53-imposed growth arrest

The mammalian UV response results in rapid and dramatic induction of c-jun. Induction of a protooncogene, normally involved in mitogenic responses, by a genotoxic agent that causes growth arrest seems paradoxical. We now provide an explanation for the role of c-Jun in the UV response of mouse fibroblasts. c-Jun is necessary for cell-cycle reentry of UV-irradiated cells, but does not participate in the response to ionizing radiation. Cells lacking c-Jun undergo prolonged cell-cycle arrest, but resist apoptosis, whereas cells that express c-Jun constitutively do not arrest and undergo apoptosis. This function of c-Jun is exerted through negative regulation of p53 association with the p21 promoter. Cells lacking c-Jun exhibit prolonged p21 induction, whereas constitutive c-Jun inhibits UV-mediated p21 induction.

Regulation of Gadd45gamma expression by C/EBP

The Gadd45gamma (growth arrest and DNA damage-inducible) gene is activated transcriptionally by at least two kinds of agents: DNA damaging agent such as methyl methanesulfonate (MMS) and UV radiation, or cytokines such as interleukin (IL)-6, IL-2 and granulocyte colony-stimulating factor (G-CSF). To investigate the sequences and transcription factors involved in induction of Gadd45gamma after treatment with IL-6, the human gene was cloned and sequenced. We found C/EBP (CCAAT/enhancer-binding protein) family proteins, major transcription factors in the IL-6 signal transduction pathway, could regulate the transcriptional activity of the Gadd45gamma promoter. In addition, a noncanonical C/EBP-binding site within the Gadd45gamma promoter where C/EBPbeta and C/EBPdelta could bind, was identified by electrophoretic mobility shift assay (EMSA) and reporter gene analysis. Furthermore, we found a coordinated expression profile between Gadd45gamma mRNA and C/EBPs (beta and delta) protein during the differentiation of M1 cells: the amount of Gadd45gamma transcripts became maximal when both C/EBPbeta and C/EBPdelta levels were high, on day 1 of differentiation of M1 cells after treatment with IL-6. These findings suggest that mitotic growth arrest coupled to M1 cell differentiation is mediated by C/EBPs stimulation of growth arrest-associated genes such as Gadd45gamma.

Gadd45 family proteins are coactivators of nuclear hormone receptors

Gadd45 family genes encode nuclear acidic proteins composed of Gadd45, MyD118, and CR6. Sequence analysis showed that Gadd45 family proteins (Gadd45, MyD118, and CR6) contain LXXLL signature motifs considered necessary and sufficient for the binding of several coactivators to nuclear receptors. Interaction between Gadd45 or CR6 and RXR alpha was confirmed by a two-hybrid test in yeast. Results from a series of GST pulldown assays showed that these Gadd45 family proteins interact with several nuclear hormone receptors including RXR alpha, RAR alpha, ER alpha, PPAR alpha, PPAR beta, and PPAR gamma2 in vitro. Interaction between Gadd45 family proteins and nuclear hormone receptors resulted in modest activation of transactivating function of nuclear hormone receptors in reporter systems. When fused to DNA binding domain of GAL4, Gadd45 and CR6 activated the UAS-mediated transcription in mammalian cells. These results suggest that Gadd45 family proteins bind to nuclear hormone receptors and act as nuclear coactivators.