Yongjun Fan

To be, or not to be: NF-κB is the answer: role of Rel/NF-κB in the regulation of apoptosis

During their lifetime, cells encounter many life or death situations that challenge their very own existence. Their survival depends on the interplay within a complex yet precisely orchestrated network of proteins. The Rel/NF-κB signaling pathway and the transcription factors that it activates have emerged as critical regulators of the apoptotic response. These proteins are best known for the key roles that they play in normal immune and inflammatory responses, but they are also implicated in the control of cell proliferation, differentiation, apoptosis and oncogenesis. In recent years, there has been remarkable progress in understanding the pathways that activate the Rel/NF-κB factors and their role in the cells decision to either fight or surrender to apoptotic challenge. Whereas NF-κB is most commonly involved in suppressing apoptosis by transactivating the expression of antiapoptotic genes, it can promote programmed cell death in response to certain death-inducing signals and in certain cell types. This review surveys our current understanding of the role of NF-κB in the apoptotic response and focuses on many developments since this topic was last reviewed in Oncogene 4 years ago. These recent findings shed new light on the activity of NF-κB as a critical regulator of apoptosis in the immune, hepatic, epidermal and nervous systems, on the mechanisms through which it operates and on its role in tissue development, homoeostasis and cancer.

To be, or not to be: NF-kappaB is the answer--role of Rel/NF-kappaB in the regulation of apoptosis.

During their lifetime, cells encounter many life or death situations that challenge their very own existence. Their survival depends on the interplay within a complex yet precisely orchestrated network of proteins. The Rel/NF-κB signaling pathway and the transcription factors that it activates have emerged as critical regulators of the apoptotic response. These proteins are best known for the key roles that they play in normal immune and inflammatory responses, but they are also implicated in the control of cell proliferation, differentiation, apoptosis and oncogenesis. In recent years, there has been remarkable progress in understanding the pathways that activate the Rel/NF-κB factors and their role in the cells decision to either fight or surrender to apoptotic challenge. Whereas NF-κB is most commonly involved in suppressing apoptosis by transactivating the expression of antiapoptotic genes, it can promote programmed cell death in response to certain death-inducing signals and in certain cell types. This review surveys our current understanding of the role of NF-κB in the apoptotic response and focuses on many developments since this topic was last reviewed in Oncogene 4 years ago. These recent findings shed new light on the activity of NF-κB as a critical regulator of apoptosis in the immune, hepatic, epidermal and nervous systems, on the mechanisms through which it operates and on its role in tissue development, homoeostasis and cancer.

Current insights into the regulation of programmed cell death by NF-κB

The nuclear factor-kappaB (NF-κB) transcription factors have emerged as major regulators of programmed cell death (PCD) whether via apoptosis or necrosis. In this context, NF-κBs activity has important ramifications for normal tissue development, homoeostasis and the physiological functions of various cell systems including the immune, hepatic, epidermal and nervous systems. However, improper regulation of PCD by NF-κB can have severe pathologic consequences, ranging from neurodegeneration to cancer, where its activity often precludes effective therapy. Although NF-κB generally protects cells by inducing the expression genes encoding antiapoptotic and antioxidizing proteins, its role in apoptosis and necrosis can vary markedly in different cell contexts, and NF-κB can sensitize cells to death-inducing stimuli in some instances. This article describes our current knowledge of the role of NF-κB in apoptosis and necrosis, and focuses on the many advances since we last reviewed this rapidly evolving topic in Oncogene 3 years ago. There has been substantial progress in understanding NF-κBs mode of action in apoptosis and necrosis and the mechanisms that regulate its anti- vs proapoptotic activities. These recent developments shed new light on the role of NF-κB in many disease conditions including tumor development, tumor progression and anticancer treatment.

A molecular insight of Hes5‐dependent inhibition of myelin gene expression: old partners and new players

This study identifies novel mechanisms of Hes5 function in developmental myelination. We report here upregulation of myelin gene expression in Hes5−/− mice compared to wild‐type siblings and downregulation in overexpressing progenitors. This effect was only partially explained by the ability to regulate the levels of Mash1 and bind to N boxes in myelin promoters, as deletion of the DNA‐binding domain of Hes5 did not suppress its inhibitory role on myelin gene expression. Novel mechanisms of Hes5 function in the oligodendrocyte lineage include the regulation of feedback loops with the cell‐specific transcriptional activator Sox10. In progenitors with low levels of Sox10, Hes5 further decreases the bioavailability of this protein by transcriptional inhibition and direct sequestration of this activator. Increasing levels of Sox10 in progenitors, in turn, bind to Hes5 and titrate out its inhibitory effect by sequestration and displacement of the repressive complexes from myelin promoters. Thus, Hes5‐dependent modulation of myelin gene expression involves old players (i.e. Mash1) and novel mechanisms of transcriptional regulation that include cell‐specific regulatory loops with transcriptional activators (i.e. Sox10).

Regulation of Programmed Cell Death by NF-κB and its Role in Tumorigenesis and Therapy

The Rel/NF-kappaB transcription factors are key regulators of programmed cell death (PCD). Their activity has significant physiological relevance for normal development and homeostasis in various tissues and important pathological consequences are associated with aberrant NF-kappaB activity, including hepatocyte apoptosis, neurodegeneration, and cancer. While NF-kappaB is best characterized for its protective activity in response to proapoptotic stimuli, its role in suppressing programmed necrosis has come to light more recently. NF-kappaB most commonly antagonizes PCD by activating the expression of antiapoptotic proteins and antioxidant molecules, but it can also promote PCD under certain conditions and in certain cell types. It is therefore important to understand the pathways that control NF-kappaB activation in different settings and the mechanisms that regulate its anti- vs pro-death activities. Here, we review the role of NF-kappaB in apoptotic and necrotic PCD, the mechanisms involved, and how its activity in the cell death response impacts cancer development, progression, and therapy. Given the role that NF-kappaB plays both in tumor cells and in the tumor microenvironment, recent findings underscore the NF-kappaB signaling pathway as a promising target for cancer prevention and treatment.

Divergent C-terminal transactivation domains of Rel|[sol]|NF-|[kappa]|B proteins are critical determinants of their oncogenic potential in lymphocytes

rel/nf-κB genes are amplified, overexpressed, or constitutively activated in many human hematopoietic tumors; however, the molecular mechanisms by which they contribute to tumorigenesis remain to be determined. Here, we explored the oncogenic potential of cellular Rel/NF-κB proteins in vitro and in vivo. We show that overexpression of wild-type mouse and human c-rel genes suffices to malignantly transform primary spleen cells in stringent soft agar assays and produce fatal tumors in vivo. In contrast relA and a constitutively active form of IKKβ did not. Importantly, a hybrid RelA protein with its C-terminal transactivation domain substituted by that of v-Rel was potently oncogenic in vitro and in vivo. The transactivation domain of v-Rel selectively conferred an oncogenic phenotype upon the Rel homology domain (RHD) of RelA, but not to the more divergent RHDs of p50/NF-κB1, p52/NF-κB2, or RelB. Collectively, our results highlight important differences in the intrinsic oncogenic activity of mammalian c-Rel and RelA proteins, and indicate that critical determinants of their differential oncogenicity reside in their divergent transactivation domains. These findings provide experimental evidence for a role of mammalian Rel/NF-κB factors in leukemia/lymphomagenesis in an in vivo animal model, and are consistent with the implication of c-rel in many human lymphomas.

Mutations in the v-Rel Transactivation Domain Indicate Altered Phosphorylation and Identify a Subset of NF-κB-Regulated Cell Death Inhibitors Important for v-Rel Transforming Activity

ABSTRACT Consistent with the constitutive activation of Rel/NF-κB in human hematopoietic tumors, the v-Rel oncoprotein induces aggressive leukemia/lymphomas in animal models. v-Rel is thus a valuable tool to characterize the role of Rel/NF-κB in cancer and the mechanisms involved. Prior studies by our group identified a serine-rich domain in v-Rel that was required for biological activity. Here, we investigated the molecular basis for the transformation defect of specific serine mutants. We show that the transforming efficiency of these mutants in primary lymphoid cells is correlated with their ability to mediate κB site-dependent transactivation and with specific changes in phosphorylation profiles. Interestingly, coexpression of the death antagonists Bcl-xL and Bcl-2 significantly increased their oncogenicity, whereas other NF-κB-regulated death inhibitors showed little or no effect. The fact that a subset of apoptosis inhibitors could rescue v-Rel transactivation mutants suggests that their reduced transcriptional activity may critically affect expression of defined death antagonists essential for oncogenesis. Consistent with this hypothesis, we observed selection for high endogenous expression of Bcl-2-related death antagonists in cells transformed by weakly transforming v-Rel mutants. These results emphasize the need for Rel/NF-κB to efficiently activate expression of a subset of antiapoptotic genes from the Bcl-2 family to manifest its oncogenic phenotype.

Peptidyl-Prolyl Isomerase Pin1 Markedly Enhances the Oncogenic Activity of the Rel Proteins in the Nuclear Factor-κB Family

The peptidyl-prolyl isomerase Pin1 is frequently up-regulated in human cancers in which Rel/nuclear factor-kappaB (NF-kappaB) is constitutively activated, but its role in these cancers remains to be determined, and evidence is still lacking to show that Pin1 contributes to cell transformation by Rel/NF-kappaB. Rel/NF-kappaB transcriptional and oncogenic activities are modulated by several posttranslational modifications and coregulatory proteins, and previous studies showed that cytokine treatment induces binding of Pin1 to the RelA subunit of NF-kappaB, thereby enhancing RelA nuclear localization and stability. Here we show that Pin1 associates with the Rel subunits of NF-kappaB that are implicated in leukemia/lymphomagenesis and modulates their transcriptional and oncogenic activities. Pin1 markedly enhanced transformation of primary lymphocytes by the human c-Rel protein and also increased cell transformation by the potent viral Rel/NF-kappaB oncoprotein v-Rel, in contrast to a Pin1 mutant in the WW domain involved in interaction with NF-kappaB. Pin1 promoted nuclear accumulation of Rel proteins in the absence of activating stimuli. Importantly, inhibition of Pin1 function with the pharmacologic inhibitor juglone or with Pin1-specific shRNA led to cytoplasmic relocalization of endogenous c-Rel in human lymphoma-derived cell lines, markedly interfered with lymphoma cell proliferation, and suppressed endogenous Rel/NF-kappaB-dependent gene expression. Together, these results show that Pin1 is an important regulator of Rel/NF-kappaB transforming activity and suggest that Pin1 may be a potential therapeutic target in Rel/NF-kappaB-dependent leukemia/lymphomas.

An optimal range of transcription potency is necessary for efficient cell transformation by c-Rel to ensure optimal nuclear localization and gene-specific activation

c-Rel is overexpressed in several B-cell lymphomas and c-rel gene overexpression can transform primary chicken lymphoid cells and induce tumors in animals. Although c-Rel is generally a stronger transcriptional activator than its viral derivative v-Rel, its oncogenic activity is significantly weaker. Among the mutations acquired during c-Rels evolution into v-Rel are deletion of c-Rels transactivation domain 2 (cTAD2) and mutations in cTAD1. Given the critical role of the Rel TADs in cell transformation, we investigated how mutations in c-Rels cTAD1 and cTAD2 contribute to its oncogenicity and that of v-Rel. Mutations in cTAD2 noticeably increased c-Rels transforming activity by promoting its nuclear localization and gene-specific transactivation, despite an overall decrease in κB site-dependent transactivation potency. Conversely, substitution of vTAD by cTAD1 increased v-Rels transactivation and transforming efficiencies, whereas its substitution by the stronger cTAD2 compromised activation of mip-1β but not irf-4 and was detrimental to cell transformation. These results suggest that the Rel TADs differentially contribute to gene-specific activation and that an optimal range of transcription potency is necessary for efficient transformation. These findings may have important implications for understanding how Rel TAD mutations can lead to a more oncogenic phenotype.

Molecular Basis of Oncogenesis by NF-κB

The Rel/NF-κB transcription factors are renowned for their fundamental contribution to normal immune, inflammatory and acute phase responses. A growing body of evidence also underscores their important role in the control of cellular gene expression, cell proliferation and apoptosis. Thus, it comes as no surprise that sustained Rel/NF-κB activity has emerged as a hallmark of many human cancers. Experimental evidence indicates a strong correlation between the transcriptional activity of Rel/NF-κB and its role in malignant cell transformation. The important role of NF-κB in the control of the apoptotic response also supports its participation in the resistance of tumor cells to therapeutic treatment. This review focuses on the mechanisms that underlie the contribution of Rel/NF-κB to cancer and highlights how appreciation of its role in this context has evolved from a bird’s eye view to a true recognition of its RELevant function in oncogenesis.