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.     

gadd45 is not required for activation of c-Jun N-terminal kinase or p38 during acute stress.

Cells respond to environmental stress with activation of c-Jun N-terminal kinase (JNK) and p38. Recent studies have implicated Gadd45 and two related proteins, MyD118/Gadd45beta and CR6/Gadd45gamma, as initiators of JNK/p38 signaling via their interaction with an upstream kinase MTK1. It was proposed that stress-induced expression of the Gadd45-related proteins leads to MTK1 activation and subsequent JNK/p38 activation. Using embryo fibroblasts from gadd45-null mice, we have addressed the requirement for Gadd45 in mediating JNK/p38 activation during acute stress. Comparison of JNK/p38 activities in response to methyl methanesulfonate, hydrogen peroxide, UVC irradiation, sorbitol, and anisomycin treatment of gadd45(+/+) and gadd45(-/-) fibroblasts revealed no deficiency in JNK/p38 activation in gadd45(-/-) fibroblasts. In addition, in wild type cells, JNK and p38 activation significantly preceded gadd45 induction with all stresses. Examination of myd118/gadd45beta and cr6/gadd45gamma expression in gadd45(+/+) and gadd45(-/-) fibroblasts revealed similar induction patterns in the two cell types, which, like gadd45 expression, was delayed relative to JNK/p38 activation. We conclude that gadd45 expression is not required for activation of JNK/p38 by environmental stresses, nor are stress-induced increases in myd118/gadd45beta and cr6/gadd45gamma expression necessary for kinase activation in response to such insults.     

Selective requirement of p38alpha MAPK in cytokine-dependent, but not antigen receptor-dependent, Th1 responses

The role of the p38 MAPK pathway in Th1 development has been controversial, because indirect manipulations of either upstream p38 activators or modifiers of p38 activity have had variable effects on IFN-gamma production in CD4+ T cells. Uncertainties regarding the specificity of pharmacologic inhibition or p38 dominant negative mutants diminish the strength of conclusions about the role of the p38alpha isoform in Th1 development. Also, the effects of some upstream p38 activators, such as MAPK kinase 3, on Th1 development are not as strong as the effects of other manipulations, such as the expression of a dominant negative p38 mutant. Finally, embryonic lethality has prevented a direct examination of p38alpha-deficient T cells. To test the requirement for p38alpha in Th1 differentiation, we generated Ag-specific p38alpha+/- and p38alpha-/- CD4+ T cells using RAG2-/- blastocyst complementation and retroviral expression of the DO11.10 TCR. IFN-gamma production in response to TCR signaling is normal in p38alpha-/- T cells cultured in Th1 conditions, implying normal Th1 development. However, p38alpha-/- Th1 cells have a much greater defect in IFN-gamma secretion stimulated by IL-12/IL-18 compared with TCR-induced IFN-gamma secretion. These results suggest that the activity of p38alpha in Th1 cells is relatively restricted to acting in one of two alternative pathways (i.e., cytokine induced) that can induce the production of IFN-gamma in differentiated Th1 cells, but that p38alpha is not required for the process of Th1 commitment and development itself.     

Induction of hyper Th1 cell-type immune responses by dendritic cells lacking the suppressor of cytokine signaling-1 gene

Suppressor of cytokine signaling (SOCS1/JAB) has been shown to play an important role in regulating dendritic cell (DC) function and suppressing inflammatory diseases and systemic autoimmunity. However, role of SOCS1 in DCs for the initiation of Th cell response has not been clarified. Here we demonstrate that SOCS1-deficient DCs induce stronger Th1-type responses both in vitro and in vivo. SOCS1-deficient DCs induced higher IFN-gamma production from naive T cells than wild-type (WT) DCs in vitro. Lymph node T cells also produced a higher amount of IFN-gamma when SOCS1-deficient bone marrow-derived DCs (BMDCs) were transferred in vivo. Moreover, SOCS1(-/-) BMDCs raised more effective anti-tumor immunity than WT BMDCs. Microarray analysis revealed that IFN-inducible genes were highly expressed in SOCS1-deficient DCs without IFN stimulation, suggesting hyper STAT1 activation in SOCS1(-/-) DCs. These phenotypes of SOCS1-deficient DCs were similar to those of CD8alpha(+) DCs, and in the WT spleen, SOCS1 is expressed at higher levels in the Th2-inducing CD4(+) DC subset, relative to the Th1-inducing CD8alpha(+) DC subset. We propose that reduction of the SOCS1 gene expression in DCs leads to CD8alpha(+) DC-like phenotype which promotes Th1-type hyperresponses.     

p38 alpha mitogen-activated protein kinase is activated by CD28-mediated signaling and is required for IL-4 production by human CD4+CD45RO+ T cells and Th2 effector cells.

T cell proliferation and cytokine production usually require stimulation via both the TCR/CD3 complex and the CD28 costimulatory receptor. Using purified human CD4+ peripheral blood T cells, we show that CD28 stimulation alone activates p38 alpha mitogen-activated protein kinase (p38 alpha). Cell proliferation induced by CD28 stimulation alone, a response attributed to CD4+CD45RO+ memory T cells, was blocked by the highly specific p38 inhibitors SB 203580 (IC50 = 10-80 nM) and RWJ 67657 (IC50 = 0.5-4 nM). In contrast, proliferation induced by anti-CD3 plus anti-CD28 mAbs was not blocked. Inhibitors of p38 also blocked CD4+ T cell production of IL-4 (SB 203580 IC50 = 20-100 nM), but not IL-2, in response to CD3 and CD28 stimulation. IL-5, TNF-alpha, and IFN-gamma production were also inhibited, but to a lesser degree than IL-4. IL-4 production was attributed to CD4+CD45RO+ T cells, and its induction was suppressed by p38 inhibitors at the mRNA level. In polarized Th1 and Th2 cell lines, SB 203580 strongly inhibited IL-4 production by Th2 cells (IC50 = 10-80 nM), but only partially inhibited IFN-gamma and IL-2 production by Th1 cells (<50% inhibition at 1 microM). In both Th1 and Th2 cells, CD28 signaling activated p38 alpha and was required for cytokine production. These results show that p38 alpha plays an important role in some, but not all, CD28-dependent cellular responses. Its preferential involvement in IL-4 production by CD4+CD45RO+ T cells and Th2 effector cells suggests that p38 alpha may be important in the generation of Th2-type responses in humans.     

Gadd45α as an upstream signaling molecule of p38 MAPK triggers oxidative stress-induced sFlt-1 and sEng upregulation in preeclampsia

Preeclampsia (PE) is known to be associated with increased circulating levels of anti-angiogenic factors, such as soluble fms-related tyrosine kinase-1 (sFlt-1) and soluble endoglin (sEng). However, the way that placental oxidative stress results in the elevation of these two factors remains enigmatic. We have observed the overexpression of growth arrest and DNA damage-inducible 45 alpha (Gadd45α) and excessive activation of p38 mitogen-activated protein kinase (MAPK) in preeclamptic placentas compared with normotensive controls, together with increased levels of sFlt-1 and sEng in maternal sera in patients with PE. Moreover, Gadd45α knockdown or p38 inhibition provides protective effects in hypoxia/reoxygenation (H/R)-exposed human umbilical vein endothelial cells (HUVECs) by suppressing oxidative stress, inhibiting apoptosis, and promoting their potential for in vitro angiogenesis. A regulatory signaling pathway in which H/R intervention causes the induction of Gadd45α leading to p38 activation and ultimately an increase in sFlt-1 and sEng secretion in HUVECs has concurrently been established. Our study opens up a promising new avenue of investigation for increasing the understanding of the origin of sFlt-1 and sEng in PE and provides novel therapeutic targets for pregnancy complications arising from placental endothelial dysfunction.     

Gadd45alpha does not modulate the carboplatin or 5-fluorouracil-induced apoptosis in human papillomavirus-positive cells

Gadd45alpha is shown to be induced by a wide spectrum of DNA-damaging agents and implicated in negative regulation of cell growth by causing G2-M arrest or induction of apoptosis. In the present study, we explored the involvement of p53 in the promoter activation of Gadd45alpha as well as the role of Gadd45alpha in carboplatin (Carb) or 5-fluorouracil (5-FU)-induced apoptosis in human papillomavirus virus (HPV)-positive HEp-2 and HeLa cells. We report that Carb or 5-FU upregulate Gadd45alpha and p53 in both these cells. Transient transfection of chloramphenicol acetyl transferase (CAT)-reporter construct driven by Gadd45alpha promoter clearly indicated that Gadd45alpha upregulation was mediated through activation of its promoter. Inhibition of p53 function by dominant-negative-p53 expression partially suppressed the activation of Gadd45alpha promoter. Further, the induction of apoptosis was assessed by detection of poly (ADP-ribose) polymerase (PARP) cleavage by Western blot analysis. Inhibition of upregulated Gadd45alpha expression by antisense expression vector did not modulate the Carb or 5-FU-induced apoptosis. Overall, we conclude that Gadd45alpha promoter activation partially depends on p53 function in HPV-positive cells. Moreover, Gadd45alpha protein does not modulate Carb or 5-FU-induced apoptosis in these cells.     

Gadd45a and Gadd45b protect hematopoietic cells from UV-induced apoptosis via distinct signaling pathways, including p38 activation and JNK inhibition

Gadd45a, Gadd45b, and Gadd45g (Gadd45/MyD118/CR6) are genes that are rapidly induced by genotoxic stress and have been implicated in genotoxic stress-induced responses, notably in apoptosis. Recently, using myeloid-enriched bone marrow (BM) cells obtained from wild-type (WT), Gadd45a-deficient, and Gadd45b-deficient mice, we have shown that in hematopoietic cells Gadd45a and Gadd45b play a survival function to protect hematopoietic cells from DNA-damaging agents, including ultra violet (UV)-induced apoptosis. The present study was undertaken to decipher the molecular paths that mediate the survival functions of Gadd45a and Gadd45b against genotoxic stress induced by UV radiation. It is 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-kappaB-mediated survival pathway and GADD45b-mediated inhibition of the stress response MKK4-JNK pathway.     

Transforming growth factor-beta-induced apoptosis is mediated by Smad-dependent expression of GADD45b through p38 activation

Transforming growth factor-beta (TGF-beta)-dependent apoptosis is important in the elimination of damaged or abnormal cells from normal tissues in vivo. In this report, we identify GADD45b as an effector of TGF-beta-induced apoptosis. GADD45b has been shown to be a positive mediator of apoptosis induced by certain cytokines and oncogenes. We show that Gadd45b is an immediateearly response gene for TGF-beta and that the proximal Gadd45b promoter is activated by TGF-beta through the action of Smad2, Smad3, and Smad4. We show that ectopic expression of GADD45b in AML12 murine hepatocytes is sufficient to activate p38 and to trigger apoptotic cell death, whereas antisense inhibition of Gadd45b expression blocks TGF-beta-dependent p38 activation and apoptosis. Furthermore, we also show that TGF-beta can activate p38 and induce apoptosis in mouse primary hepatocytes from wild-type mice, but not from Gadd45b-/- mice. All of these findings suggest that GADD45b participates in TGF-beta-induced apoptosis by acting upstream of p38 activation.     

Limited ability of antigen-specific Th1 responses to inhibit Th2 cell development in vivo

Th1 and Th2 cells mutually antagonize each other's differentiation. Consequently, allergen-specific Th1 cells are believed to be able to suppress the development of Th2 cells and to prevent the development of atopic disorders. To determine whether a pre-existing Ag-specific Th1 response can affect the development of Th2 cells in vivo, we used an immunization model of Ag-pulsed murine dendritic cell (DC) transfer to induce distinct Th responses. When transferred into naive mice, Ag-pulsed CD8alpha(+) DCs induced a Th1 response and the production of IgG2a, whereas CD8alpha(-) DCs primed a Th2 response and the production of IgE. In the presence of a pre-existing Ag-specific Th2 environment due to Ag-pulsed CD8alpha(-) DC transfer, CD8alpha(+) DCs failed to prime Th1 cells. In contrast, CD8alpha(-) DCs could prime a Th2 response in the presence of a pre-existing Ag-specific Th1 environment. Moreover, exogenous IL-4 abolished the Th1-inducing potential of CD8alpha(+) DCs in vitro, but the addition of IFN-gamma did not effectively inhibit the potential of CD8alpha(-) DCs to prime IL-4-producing cells. Thus, Th1 and Th2 cells differ in their potential to inhibit the development of the other. This suggests that the early induction of allergen-specific Th1 cells before allergy sensitization will not prevent the development of atopic disorders.     

Role of TNF receptor-associated factor 2 and p38 mitogen-activated protein kinase activation during 4-1BB-dependent immune response

4-1BB is a costimulatory member of the TNFR family, expressed on activated CD4(+) and CD8(+) T cells. Previous results showed that 4-1BB-mediated T cell costimulation is CD28-independent and involves recruitment of TNFR-associated factor 2 (TRAF2) and activation of the stress-activated protein kinase cascade. Here we describe a role for the p38 mitogen-activated protein kinase (MAPK) pathway in 4-1BB signaling. Aggregation of 4-1BB alone induces p38 activation in a T cell hybridoma, whereas, in normal T cells, p38 MAPK is activated synergistically by immobilized anti-CD3 plus immobilized 4-1BB ligand. 4-1BB-induced p38 MAPK activation is inhibited by the p38-specific inhibitor SB203580 in both a T cell hybridoma and in murine T cells. T cells from TRAF2 dominant-negative mice are impaired in 4-1BB-mediated p38 MAPK activation. A link between TRAF2 and the p38 cascade is provided by the MAPK kinase kinase, apoptosis-signal-regulating kinase 1. A T cell hybrid transfected with a kinase-dead apoptosis-signal-regulating kinase 1 fails to activate p38 MAPK in response to 4-1BB signaling. To assess the role of p38 activation in an immune response, T cells were stimulated in an MLR in the presence of SB203580. In a primary MLR, SB203580 blocked IL-2, IFN-gamma, and IL-4 secretion whether the costimulatory signal was delivered via 4-1BB or CD28. In contrast, following differentiation into Th1 or Th2 cells, p38 inhibition blocked IL-2 and IFN-gamma without affecting IL-4 secretion. Nevertheless, IL-4 secretion by Th2 cells remained costimulation-dependent. Thus, critical T cell signaling events diverge following Th1 vs Th2 differentiation.     

Gliotoxin enhances radiotherapy via inhibition of radiation-induced GADD45a, p38, and NFkappaB activation

The purpose of the study was to elucidate the mechanism underlying the enhancement of radiosensitivity to 60Co gamma-irradiation in human hepatoma cell line HepG2 pretreated with gliotoxin. Enhancement of radiotherapy by gliotoxin was investigated in vitro with human hepatoma HepG2 cell line. Apoptosis related proteins were evaluated by Western blotting. Annexin V/PI and reactive oxygen species (ROS) were quantified by Flow Cytometric (FACS) analysis. Gliotoxin (200 ng/ml) combined with radiation (4 Gy) treated cells induced apoptosis. Cells treated with gliotoxin (200 ng/ml) prior to irradiation at 4 Gy induced the expression of bax and nitric oxide (NO). The gliotoxin-irradiated cells also increased caspase-3 activation and ROS. Gadd45a, p38, and nuclear factor kappa B (NFkappaB) activated in irradiated cells was inhibited by Gliotoxin. Specific inhibitors of p38 kinase, SB203580, significantly inhibited NFkappaB activation and increased the cytotoxicity effect in cells exposed to gliotoxin combined with irradiation. However, SB203580 did not suppress the activation of Gadd45a in irradiated cells. Gliotoxin inhibited anti-apoptotic signal pathway involving the activation of Gadd45a-p38-NFkappaB mediated survival pathway that prevent radiation-induced cell death. Therefore, gliotoxin, blocking inflammation pathway and enhancing irradiation-induced apoptosis, is a promising agent to increase the radiotherapy of tumor cells.     

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.     

Vitamin C-treated murine bone marrow-derived dendritic cells preferentially drive naïve T cells into Th1 cells by increased IL-12 secretions

Vitamin C has been reported to shift immune responses toward Th1. In this study, we evaluated whether this effect was by way of dendritic cells. Murine dendritic cells (DCs) were prepared from bone marrow precursors. DCs treated with vitamin C secreted an increased amount of IL-12p70 after activation with LPS. These cells rendered naïve T cells to secrete more Th1 cytokine, IFN-γ, and less Th2-cytokine, IL-5 in the culture supernatants. Vitamin C-treatment also increased phosphorylation of p38 and ERK1/2 in DCs. p38 inhibitor in culture media suppressed the effect of vitamin C to elevate IL-12p70 secretion. In contrast, ERK inhibitor elevated IL-12p70 secretion. In summary, vitamin C taken up into DCs increased IL-12p70 secretion of these cells by modulating the activation of signal molecules, and thus shifted immune responses toward Th1. These data provide us a new insight on the role of vitamin C in modulating immune responses.     

Gadd45a and Gadd45b modulate innate immune functions of granulocytes and macrophages by differential regulation of p38 and JNK signaling

Gadd45 proteins function as stress sensors in response to various physiological and environmental stressors, interacting with other cellular proteins implicated in cellular stress responses, including p38 and JNK. This study shows that mice lacking either Gadd45a or Gadd45b are defective in the recruitment of granulocytes and macrophages to the intra-peritoneal cavity following intra-peritoneal administration of the bacterial cell wall pathogen-associated molecular pattern lipopolysaccharide (LPS). Bone marrow derived granulocytes and macrophages lacking either Gadd45a or Gadd45b are shown to be impaired in their chemotactic response to LPS, as well as other inflammatory stimuli such as N-formyl-methionine-leucine-phenylalanine and IL-8. Evidence was obtained also implicating Gadd45a and Gadd45b in other myeloid innate immune functions, including reactive oxygen species production, phagocytosis, and adhesion. Gadd45a and Gadd45b activation of p38 kinase was implicated in the response of granulocytes to LPS mediated chemotaxis, whereas Gadd45a and Gadd45b curtailment of JNK activation was linked to chemotaxis of macrophages in response to LPS. Collectively, these data highlight a novel role for both Gadd45a and Gadd45b in myeloid innate immune functions by differential modulation of p38 and JNK signaling in granulocytes compared to macrophages.     

Activation of discoidin domain receptor 1 facilitates the maturation of human monocyte-derived dendritic cells through the TNF receptor associated factor 6/TGF-beta-activated protein kinase 1 binding protein 1 beta/p38 alpha mitogen-activated protein kinase signaling cascade

Maturation of dendritic cells (DCs) is critical for their ability to stimulate resting naive T cells in primary immune responses. Previous studies demonstrated that collagen, such as type I collagen, could facilitate DC maturation; however, the basis of collagen-mediated DC maturation remains unclear. Discoidin domain receptor 1 (DDR1) is a nonintegrin collagen receptor constitutively expressed in a variety of epithelial cells, including tumor cells, and is inducible in leukocytes. In this study, we evaluated the role of DDR1 in DC maturation using human monocyte-derived DCs. Two DDR1 isoforms, DDR1a and DDR1b, were expressed in both immature and mature DCs. Activation of DDR1 on immature DCs resulted in their partial maturation; however, DDR1 activation markedly amplified TNF-alpha- and LPS-induced phenotypic and functional maturation of DCs through activation of p38 mitogen-activated protein kinase (MAPK), suggesting the involvement of DDR1b in this process. Activation of DDR1b on differentiated DDR1b-overexpressing THP-1 cells or DDR1 on mature DCs induced the formation of TNFR associated factor 6 (TRAF6)/TGF-beta-activated kinase 1 binding protein 1beta/p38alpha MAPK complex and p38alpha autophosphorylation. Transfection of differentiated DDR1b-overexpressing THP-1 cells with dominant negative TRAF6 completely abrogated DDR1b-mediated p38 MAPK phosphorylation, indicating a critical role of TRAF6 in DDR1b-mediated p38 MAPK activation. Taken together, our data suggest that DDR1b-collagen interaction augments the maturation of DCs in a tissue microenvironment through a unique TRAF6/TGF-beta-activated kinase 1 binding protein 1beta/p38alpha MAPK signaling cascade and contributes to the development of adaptive immune responses.     

HDAC inhibitors induce apoptosis but not cellular senescence in Gadd45α-deficient E1A+Ras cells

HDAC inhibitors (HDIs) induce irreversible cell cycle arrest and senescence in E1A+Ras expressing cells. Furthermore, HDIs activate Gadd45α/NF-κB signaling pathway to suppress apoptosis thereby promoting the cell survival. Here, to clarify the role of Gadd45α in realization of the antiapoptotic program, we compared wild-type E1A+Ras cells and the cells with knockout of gadd45α gene (Gadd45α−/− cells). As in Gadd45α-expressing E1A+Ras cells, HDIs induce irreversible cell cycle arrest in Gadd45α−/− cells, but the arrested cells do not senesce and eventually die due to activation of the apoptotic death program. These data suggest that the expression of Gadd45α is involved in maintaining the balance of pro- and anti-apoptotic stimuli, while lack or loss of Gadd45 directs the cells to apoptosis after HDIs treatment. Appropriately Gadd45α-deficient cells demonstrate a higher level of pro-apoptotic signals, whereas the anti-apoptotic program is suppressed. The elevated apoptotic background of Gadd45α−/− cells is accompanied by higher levels of Ser15-phosphorylated p53 and p21/Waf1 proteins that additionally commit the cells to HDIs-induced apoptosis. Additionally, loss of Gadd45α protein activates the DDR signaling pathway as demonstrated by nuclear pATM staining, accumulation of γH2AX foci and an increase of single-strand DNA breaks. Thus, in wild-type E1A+Ras cells the p53-dependent expression of Gadd45α is necessary not only for DNA repair and HDI-induced cellular senescence, but also to withstand to apoptosis after DNA damage and stress. Therefore the use of HDIs in combination with agents that block Gadd45α function may have promise for cancer therapy.     

Role of Gadd45a in Wip1-dependent regulation of intestinal tumorigenesis

Conversion of intestinal stem cells into tumor-initiating cells is an early step in Apc^Min-induced polyposis. Wild-type p53-induced phosphatase 1 (Wip1)-dependent activation of a DNA damage response and p53 has a permanent role in suppression of stem cell conversion, and deletion of Wip1 lowers the tumor burden in Apc^Min mice. Here we show that cyclin-dependent kinase inhibitor 2a, checkpoint kinase 2, and growth arrest and DNA damage gene 45a (Gadd45a) exert critical functions in the tumor-resistant phenotype of Wip1-deficient mice. We further identified Gadd45a as a haploinsufficient gene in the regulation of Wip1-dependent tumor resistance in mice. Gadd45a appears to function through its ability to activate the Jnk-dependent signaling pathway that in turn is a necessary mediator of the proapoptotic functions of p53 that respond to activation of the β-catenin signaling pathway. We propose that silencing of Gadd45a is sufficient to override p53 activation in the presence of active β-catenin under conditions of an enhanced DNA damage response.     

Gadd45 modulation of intrinsic and extrinsic stress responses in myeloid cells

Gadd45 proteins modulate signaling in response to physiological and environmental stressors. Expression of gadd45 genes is rapidly induced by different stressors, including differentiation-inducing cytokines and genotoxic stress. Induction of gadd45 genes at the onset of myeloid differentiation suggested that Gadd45 protein(s) play a role in hematopoiesis, yet no apparent abnormalities were observed in either the bone marrow (BM) or peripheral blood compartments of mice deficient for either gadd45a or gadd45b. However, under conditions of hematological stress, including acute stimulation with cytokines, myelo-ablation and inflammation, both gadd45a-deficient and gadd45b-deficient mice exhibited deficiencies. This is discussed within the context of what is known about Gadd45 proteins in stress signaling, hematopoietic development and the innate immune response. Furthermore, 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, displaying defective G2/M arrest following exposure to UVC and VP-16, but not to DNR. Novel mechanisms that mediate the pro-survival functions of Gadd45 in hematopoietic cells following UV irradiation were demonstrated, involving activation of the Gadd45a-p38-NF-kappaB survival pathway and Gadd45b mediated inhibition of the stress response MKK4-JNK apoptotic pathway. The ramifications regarding the pathogenesis of different leukemias and the response of normal and malignant hematopoietic cells to chemo- and radiation-therapy, as well as other challenges to the hematopoietic compartment, are discussed.