Genetic deletion of Gadd45b, a regulator of active DNA demethylation, enhances long-term memory and synaptic plasticity.

Dynamic epigenetic mechanisms including histone and DNA modifications regulate animal behavior and memory. While numerous enzymes regulating these mechanisms have been linked to memory formation, the regulation of active DNA demethylation (i.e., cytosine-5 demethylation) has only recently been investigated. New discoveries aim toward the Growth arrest and DNA damage-inducible 45 (Gadd45) family, particularly Gadd45b, in activity-dependent demethylation in the adult CNS. This study found memory-associated expression of gadd45b in the hippocampus and characterized the behavioral phenotype of gadd45b(-/-) mice. Results indicate normal baseline behaviors and initial learning but enhanced persisting memory in mutants in tasks of motor performance, aversive conditioning and spatial navigation. Furthermore, we showed facilitation of hippocampal long-term potentiation in mutants. These results implicate Gadd45b as a learning-induced gene and a regulator of memory formation and are consistent with its potential role in active DNA demethylation in memory.

Gadd45a levels in human breast cancer are hormone receptor dependent.

BACKGROUND: Gadd45a is a member of the Gadd45 family of genes that are known stress sensors. Gadd45a has been shown to serve as an effector in oncogenic stress in breast carcinogenesis in murine models. The present study was aimed at clarifying the expression of Gadd45a in human breast cancer and its correlation with clinicopathologic features. METHODS: The expression levels of Gadd45a in breast tissue samples of female breast surgery cases were examined by immunohistochemistry (IHC) using a Gadd45a antibody. Percent staining was determined and statistical analyses were applied to determine prognostic correlations. RESULTS: 56 female breast surgery cases were studied: Normal (11), Luminal A (9), Luminal B (11), HER2+ (10), Triple Negative (15). There was a highly significant difference in percent Gadd45a staining between groups [Mean]: Normal 16.3%; Luminal A 65.3%; Luminal B 80.7%; HER2+ 40.5%; TN 32%, P < 0.001, ANOVA. Gadd45a IHC levels for Normal cases found 82% negative/low. Luminal A breast cancer cases were found to be 67% high. Luminal B breast cancers were 100% high. Her2+ cases were 50% negative/low. Triple Negative cases were 67% negative/low. This difference in distribution of Gadd45a levels across breast cancer receptor subtypes was significant, P = 0.0009. CONCLUSIONS: Gadd45a levels are significantly associated with hormone receptor status in human breast cancer. Normal breast tissue displays low Gadd45a levels. High Gadd45a levels are associated with Luminal A and Luminal B subtypes. Absence of hormone receptors in Triple Negative subtype is associated with Negative/Low levels of Gadd45a. Further studies are indicated to elucidate the role of Gadd45a in breast cancer as a potential prognosticator or target for treatment.

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.

Gadd45a stress signaling regulates sFlt-1 expression in preeclampsia.

Preeclampsia, which affects approximately 5-8% of all pregnancies and is one of the leading causes of maternal and fetal morbidity and mortality, is a pregnancy induced complex of multiple pathological changes, including elevated blood pressure, proteinuria and edema manifested after 20 weeks gestation. There is growing evidence that placental stresses during pregnancy, notably hypoxia, and an increase in circulating soluble Flt-1 (sFlt-1) are important in the etiopathogenesis of preeclampsia. How placental stress results in elevated sFlt-1 expression is currently unknown. Here we provide novel data implicating the Gadd45a stress sensor protein as an upstream modulator of pathophysiological changes observed in preeclampsia. It is shown that Gadd45a expression and activation of its downstream effector p38 kinase are elevated in preeclamptic placentas compared to non-preeclamptic controls, and correlate with elevated sFlt-1. Furthermore, a regulatory loop is demonstrated where stress, including hypoxia, IL-6 or hypertonic stress, caused induction of Gadd45a, leading to p38 activation and ultimately increasing sFlt-1 secretion in endothelial cells. These data provide a compelling working frame to further test the role of Gadd45 stress sensors in the etiology of preeclampsia, and set the stage for considering novel therapeutic regimens, including p38 inhibitors, for treatment of preeclampsia.

Gadd45 stress sensors in malignancy and leukemia.

Gadd45 proteins, including Gadd45a, Gadd45b, and Gadd45g, have been implicated in stress signaling in response to physiological and environmental stress, including oncogenic stress, which can result in cell cycle arrest, DNA repair, cell survival, senescence, and apoptosis. The function of Gadd45 as a stress sensor is mediated via a complex interplay of physical interactions with other cellular proteins implicated in cell cycle regulation and the response of cells to stress, notably PCNA, p21, cdc2/cyclinB1, and the p38 and JNK stress response kinases. Altered expression of Gadd45 has been observed in multiple types of solid tumors as well as in hematopoietic malignancies. Using genetically engineered mouse models and bone-marrow transplantation, evidence has been obtained indicating that Gadd45 proteins can function to either promote or suppress tumor development and leukemia; this is dependent on the molecular nature of the activated oncogene and the cell type, via engagement of different signaling pathways.

Gadd45a functions as a promoter or suppressor of breast cancer dependent on the oncogenic stress.

Gadd45a plays a pivotal role as a stress sensor that modulates cellular responses to various stress stimuli including oncogenic stress. We reported that the stress sensor Gadd45a gene functions as a tumor suppressor in Ras-driven breast tumorigenesis via increasing JNK-mediated apoptosis and p38-mediated senescence. In contrast, here, we show that Gadd45a promotes Myc-driven breast cancer by negatively regulating MMP10 via GSK3 ß/ß-catenin signaling, resulting in increased tumor vascularization and growth. These novel findings indicate that Gadd45a functions as either tumor promoter or suppressor, is dependent on the oncogenic stress, and is mediated via distinct signaling pathways. Collectively, these novel findings highlight the significance of the type of oncogenic alteration on how stress response genes function during initiation and progression of tumorigenesis. Because Gadd45a is a target for BRCA1 and p53, these findings have implications regarding BRCA1/p53 tumor suppressor functions.

Stress sensor Gadd45 genes as therapeutic targets in cancer.

Gadd45 genes have been implicated in stress signaling responses to various physiological or environmental stressors, resulting in cell cycle arrest, DNA repair, cell survival and senescence, or apoptosis. Evidence accumulated up to date suggests that Gadd45 proteins function as stress sensors, mediating their activity through 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. Disregulated expression of Gadd45 has been observed in multiple types of solid tumors as well as in hematopoietic malignancies. Also, evidence has accumulated that Gadd45 proteins are intrinsically associated with the response of tumor cells to a variety of cancer therapeutic agents. Thus, Gadd45 proteins may represent a novel class of targets for therapeutic intervention in cancer. Additional research is needed to better understand which of the Gadd45 stress response functions may be targeted for chemotherapeutic drug design in cancer therapy.

Conserved DNA methylation in Gadd45a(-/-) mice.

Gadd45a (growth arrest and DNA-damage-inducible protein 45 alpha) plays a pivotal role in cellular stress responses and is implicated in DNA repair, cell cycle arrest and apoptosis.(1) Recently, it was proposed that GADD45A is a key regulator of active DNA demethylation by way of its role in DNA repair.(2) Barreto et al. reported that Gadd45a overexpression activated transcription from methylation-silenced reporter plasmids and promoted global DNA demethylation. siRNA-mediated knockdown of Gadd45a levels resulted in increased levels of DNA methylation at specific endogenous loci. Based on these exciting results, Gadd45a(-/-) mice might be predicted to have a hypermethylation phenotype. We report here that neither global nor locus-specific methylation is increased in Gadd45a(-/-) mice.

Gadd45a suppresses Ras-driven mammary tumorigenesis by activation of c-Jun NH2-terminal kinase and p38 stress signaling resulting in apoptosis and senescence.

The Gadd45 family of proteins is known to play a central role as cellular stress sensors that modulate the response of mammalian cells to stress inflicted by physiologic and environmental stressors. Gadd45a was shown to be a direct target to the p53 and BRCA1 tumor suppressor genes, whose loss of function is known to play a vital role in breast carcinogenesis; however, the role of Gadd45a in the suppression of breast cancer remains unclear. To address this issue, Gadd45a-deficient mice were crossed with breast cancer prone mouse mammary tumor virus-Ras mice to generate mice that express activated Ras and differ in their Gadd45a status. Using this mouse model, we show that the loss of Gadd45a accelerates Ras-driven mammary tumor formation, exhibiting increased growth rates and a more aggressive histologic phenotype. Moreover, it is shown that accelerated Ras-driven tumor formation in the absence of Gadd45a results in both a decrease in apoptosis, which is linked to a decrease in c-Jun NH(2)-terminal kinase (JNK) activation, and a decrease in Ras-induced senescence, which is correlated with a decrease in p38 kinase activation. Altogether, these results provide a novel model for the tumor-suppressive function of Gadd45a in the context of Ras-driven breast carcinogenesis, showing that Gadd45a elicits its function through activation of the stress-induced JNK and p38 kinases, which contribute to increase in apoptosis and Ras-induced senescence.