Summary
The dimeric transcription factor nuclear factor κB (NF-κB) functions broadly in coordinating cellular responses during inflammation and immune reactions, and its importance in the pathogenesis of cancer is increasingly recognized. Many of the signal transduction pathways that trigger activation of cytoplasmic NF-κB in response to a broad array of immune and inflammatory stimuli have been elaborated in great detail. NF-κB can also be activated by DNA damage, though relatively less is known about the signal transduction mechanisms that link DNA damage in the nucleus with activation of NF-κB in the cytoplasm. Here, we focus on the conserved signaling pathway that has emerged that promotes NF-κB activation following DNA damage. Post-translational modification of NF-κB essential modulator (NEMO) plays a central role in linking the cellular DNA damage response to NF-κB via the ataxia telangiectasia mutated (ATM) kinase. Accumulating evidence suggests that DNA damage-dependent NF-κB activation may play significant biological roles, particularly during lymphocyte differentiation and progression of human malignancies.
We have examined the role of histone acetylation in the very earliest steps of differentiation of mouse embryonic stem cells in response to withdrawal of leukemia inhibitory factor (LIF) as a differentiation signal. The cells undergo dramatic changes in morphology and an ordered program of gene expression changes representing differentiation to all three germ layers over the first 3-5 days of LIF withdrawal. We observed a global increase in acetylation on histone H4 and to a lesser extent on histone H3 over this time period. Treatment of the cells with trichostatin A (TSA), a histone deacetylase inhibitor, induced changes in morphology, gene expression, and histone acetylation that mimicked differentiation induced by withdrawal of LIF. We examined localized histone acetylation in the regulatory regions of genes that were transcriptionally either active in undifferentiated cells, induced during differentiation, or inactive under all treatments. There was striking concordance in the histone acetylation patterns of specific genes induced by both TSA and LIF withdrawal. Increased histone acetylation in local regions correlated best with induction of gene expression. Finally, TSA treatment did not support the maintenance or progression of differentiation. Upon removal of TSA, the cells reverted to the undifferentiated phenotype. We concluded that increased histone acetylation at specific genes played a role in their expression, but additional events are required for maintenance of differentiated gene expression and loss of the pluripotent state.
Embryonic stem (ES) cells are pluripotent cells capable of unlimited self-renewal and differentiation into the three embryonic germ layers under appropriate conditions. Mechanisms for control of the early period of differentiation, involving exit from the pluripotent state and lineage commitment, are not well understood. An emerging concept is that epigenetic histone modifications may play a role during this early period. We have found that upon differentiation of mouse ES cells by removal of the cytokine leukemia inhibitory factor, there is a global increase in coupled histone H3 phosphorylation (Ser-10)-acetylation (Lys-14) (H3 phosphoacetylation). We show that this occurs through activation of both the extracellular signal-regulated kinase (ERK) and p38 MAPK signaling pathways. Early ES cell differentiation is delayed using pharmacological inhibitors of the ERK and p38 pathways. One common point of convergence of these pathways is the activation of the mitogen-and stress-activated protein kinase 1 (MSK1). We show here that MSK1 is the critical mediator of differentiation-induced H3 phosphoacetylation using both the chemical inhibitor H89 and RNA interference. Interestingly, inhibition of H3 phosphoacetylation also alters gene expression during early differentiation. These results point to an important role for both epigenetic histone modifications and kinase pathways in modulating early ES differentiation.
Embryonic stem (ES)2 cells are pluripotent cells with the capacity for unlimited self-renewal or differentiation into the three germ layers: endoderm, ectoderm, and mesoderm. This ability to form different cell types under appropriate conditions makes them a powerful tool in the study of biological mechanisms and treatment of disease. Mouse ES cells are derived from the inner cell mass of the developing blastocyst (1). In vivo, the inner cell mass becomes organized into a pluripotent epithelial layer, the epiblast, from which embryonic tissues are derived (2). In vitro, mouse ES cells can be maintained in an undifferentiated state with the addition of the cytokine leukemia inhibitory factor (LIF) to culture media. LIF primarily acts via the JAK-STAT signaling pathway to maintain pluripotency (3). Self-renewal also is enhanced by inhibition of mitogen-activated protein kinase (MAPK) pathways (4). Withdrawal of LIF results in spontaneous differentiation of the ES cells into all three lineages, which is marked by changes in gene expression and cell morphology (see Fig. 1A).Although much focus has been placed on factors involved in self-renewal, the mechanisms regulating exit from the pluripotent state followed by commitment to specific lineages remain largely unknown. An emerging concept is that alterations in epigenetic histone modifications may be important during this timeframe (5). The nucleosome, with DNA wrapped around an octamer of core histones (H2A, H2B, H3, and H4), forms the basic building blocks of chromatin. Histone tails are subject to numerous covalent modifications, including acetylation, phosp...
Background: Nuclear import of NEMO is critical for DNA damage-dependent NF-B signaling, but the mechanism remains unknown. Results: IPO3 binds NEMO, promotes its nuclear import, and is critical for DNA damage-dependent NF-B activation. Conclusion: IPO3 is a nonclassical nuclear import receptor for NEMO. Significance: IPO3 is a new player in DNA damage-dependent NF-B signaling with implications in cancer therapy.
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