Hiroaki Ikushima

TGFβ signalling: a complex web in cancer progression

The distortion of growth factor signalling is the most important prerequisite in tumour progression. Transforming growth factor-β (TGFβ) signalling regulates tumour progression by a tumour cell-autonomous mechanism or through tumour–stroma interaction, and has either a tumour-suppressing or tumour-promoting function depending on cellular context. Such inherent complexity of TGFβ signalling results in arduous, but promising, assignments for developing therapeutic strategies against malignant tumours. As numerous cellular context-dependent factors tightly maintain the balance of TGFβ signalling and contribute to the regulation of TGFβ-induced cell responses, in this Review we discuss how they maintain the balance of TGFβ signalling and how their collapse leads to tumour progression.

Autocrine TGF-β Signaling Maintains Tumorigenicity of Glioma-Initiating Cells through Sry-Related HMG-Box Factors

Despite aggressive surgery, radiotherapy, and chemotherapy, treatment of malignant glioma remains formidable. Although the concept of cancer stem cells reveals a new framework of cancer therapeutic strategies against malignant glioma, it remains unclear how glioma stem cells could be eradicated. Here, we demonstrate that autocrine TGF-beta signaling plays an essential role in retention of stemness of glioma-initiating cells (GICs) and describe the underlying mechanism for it. TGF-beta induced [corrected] expression of Sox2, a stemness gene, and this induction was mediated by Sox4, a direct TGF-beta target gene. Inhibitors of TGF-beta signaling drastically deprived tumorigenicity of GICs by promoting their differentiation, and these effects were attenuated in GICs transduced with Sox2 or Sox4. Furthermore, GICs pretreated with TGF-beta signaling inhibitor exhibited less lethal potency in intracranial transplantation assay. These results identify an essential pathway for GICs, the TGF-beta-Sox4-Sox2 pathway, whose disruption would be a therapeutic strategy against gliomas.

The IRF Family Transcription Factors at the Interface of Innate and Adaptive Immune Responses

The interferon-regulatory factor (IRF) family, originally identified as transcriptional regulators of the type I interferon system, consists of nine members in mammals. A large number of studies have revealed the versatile and critical functions performed by this transcription factor family in immunity and other biological processes. Most notably, the advances in the study of signal transducing innate immune receptors have placed many IRF members as central mediators in the regulation of innate immune responses. In parallel, mechanistic studies have made it clearer that many IRFs exert their function either in cooperation or competition with other factors. In this article, we discuss current advances on the multipurpose and critical functions of IRFs in the regulation of innate immunity, particularly as they instruct adaptive immunity.

Glioma-initiating Cells Retain Their Tumorigenicity through Integration of the Sox Axis and Oct4 Protein

Background: Glioma-initiating cells are underlying causes of development and progression of glioblastoma. Results: Depletion of Oct4 expression suppresses tumorigenic activity of glioma-initiating cells through down-regulation of Sox2. Conclusion: Oct4 maintains tumorigenicity of glioma-initiating cells in cooperation with the Sox axis. Significance: This study uncovers the transcriptional network of stemness genes in cancer-initiating cells. Although the concept of cancer stem cells or cancer-initiating cells had created a new paradigm for the treatment of malignant tumors, it remains unclear how cancer-initiating cells can be eradicated. We have previously reported that the transforming growth factor-β (TGF-β)-Sox4-Sox2 pathway is essential for glioma-initiating cells to retain their stemness, and inhibition of TGF-β signaling may lead to differentiation of glioma-initiating cells (Ikushima, H., Todo, T., Ino, Y., Takahashi, M., Miyazawa, K., and Miyazono, K. (2009) Cell Stem Cell 5, 504–514). Here we demonstrate that Oct4 plays essential roles in retention of the stemness properties of glioma-initiating cells through positive regulation of Sox2 expression. We also show that, in glioma-initiating cells, Oct4 is associated with Sox4 and that Oct4-Sox4 complexes cooperatively activate the enhancer activity of the SOX2 gene. In contrast, in fetal neural progenitor cells, Sox2 expression is enhanced by transcriptional complex containing Sox2 protein itself, and this self-reinforcing loop of Sox2 appears to be disrupted in glioma-initiating cells, suggesting that Sox2 expression in glioma-initiating cells is differently regulated from that in neural progenitor cells. Our findings reveal differences between glioma-initiating cells and fetal neural progenitor cells and may open the way to depriving glioma-initiating cells of tumorigenic activity without affecting normal tissues.

Conditional ablation of HMGB1 in mice reveals its protective function against endotoxemia and bacterial infection

Significance The high-mobility group box 1 (HMGB1) protein is abundantly expressed in the nucleus where it regulates chromatin function. More recently, it was found to also function in the cytoplasm and extracellular milieu for the regulation of immunity and inflammation. However, the in vivo study of HMGB1 has been hampered by the fact that HMGB1-deficient mice die soon after birth. In this study, we successfully generated Hmgb1-floxed mice to achieve conditional inactivation of the gene in a cell- and tissue-specific manner. We demonstrate that cytosolic HMGB1 in myeloid cells is critical for the protection of the host from endotoxemia and bacterial infection by inducing autophagy, a cellular response critical for maintaining cellular viability in the setting of various stresses including infection. High-mobility group box 1 (HMGB1) is a DNA-binding protein abundantly expressed in the nucleus that has gained much attention for its regulation of immunity and inflammation. Despite this, whether and how HMGB1 contributes to protective and/or pathological responses in vivo is unclear. In this study, we constructed Hmgb1-floxed (Hmgb1f/f) mice to achieve the conditional inactivation of the gene in a cell- and tissue-specific manner by crossing these mice with an appropriate Cre recombinase transgenic strain. Interestingly, although mice with HMGB1 ablation in myeloid cells apparently develop normally, they are more sensitive to endotoxin shock compared with control mice, which is accompanied by massive macrophage cell death. Furthermore, these mice also show an increased sensitivity to Listeria monocytogenes infection. We also provide evidence that the loss of HMGB1 in macrophages results in the suppression of autophagy, which is commonly induced by lipopolysaccharide stimulation or L. monocytogenes infection. Thus, intracellular HMGB1 contributes to the protection of mice from endotoxemia and bacterial infection by mediating autophagy in macrophages. These newly generated HMGB1 conditional knockout mice will serve a useful tool with which to study further the in vivo role of this protein in various pathological conditions.

TGF-β signal transduction spreading to a wider field: a broad variety of mechanisms for context-dependent effects of TGF-β.

Transforming growth factor (TGF)-β signaling is involved in almost all major cell behaviors under physiological and pathological conditions, and its regulatory system has therefore been vigorously investigated. The fundamental elements in TGF-β signaling are TGF-β ligands, their receptors, and intracellular Smad effectors. The TGF-β ligand induces the receptors directly to phosphorylate and activate Smad proteins, which then form transcriptional complexes to control target genes. One of the classical questions in the field of research on TGF-β signaling is how this cytokine induces multiple cell responses depending on cell type and cellular context. Possible answers to this question include cross-interaction with other signaling pathways, different repertoires of Smad-binding transcription factors, and genetic alterations, especially in cancer cells. In addition to these genetic paradigms, recent work has extended TGF-β research into new fields, including epigenetic regulation and non-coding RNAs. In this review, we first describe the basic machinery of TGF-β signaling and discuss several factors that comprise TGF-β signaling networks. We then address mechanisms by which TGF-β induces several responses in a cell-context-dependent fashion. In addition to classical frames, the interaction of TGF-β signaling with epigenetics and microRNA is discussed.

Recognition of tumor cells by Dectin-1 orchestrates innate immune cells for anti-tumor responses

The eradication of tumor cells requires communication to and signaling by cells of the immune system. Natural killer (NK) cells are essential tumor-killing effector cells of the innate immune system; however, little is known about whether or how other immune cells recognize tumor cells to assist NK cells. Here, we show that the innate immune receptor Dectin-1 expressed on dendritic cells and macrophages is critical to NK-mediated killing of tumor cells that express N-glycan structures at high levels. Receptor recognition of these tumor cells causes the activation of the IRF5 transcription factor and downstream gene induction for the full-blown tumoricidal activity of NK cells. Consistent with this, we show exacerbated in vivo tumor growth in mice genetically deficient in either Dectin-1 or IRF5. The critical contribution of Dectin-1 in the recognition of and signaling by tumor cells may offer new insight into the anti-tumor immune system with therapeutic implications. DOI: http://dx.doi.org/10.7554/eLife.04177.001

Cellular context‐dependent “colors” of transforming growth factor‐β signaling

Transforming growth factor (TGF)‐β signaling has interesting characteristics in the context of cancer. Although perturbations of TGF‐β signaling are strongly implicated in cancer progression, TGF‐β signaling has both tumor‐suppressive and tumor‐promoting effects. For example, TGF‐β inhibits cancer cell proliferation in some cellular contexts, but promotes it in others. Although several approaches to treating cancer have been considered using TGF‐β‐based therapeutic strategies, the contradictory behaviors of TGF‐β have made these approaches complex. To put them to practical use, either the tumor‐suppressive or tumor‐promoting arm needs to be specifically manipulated. However, there is virtually no method to specifically regulate a certain cell response induced by TGF‐β. In this review, we first consider the basic machinery of TGF‐β signaling, and describe several cell responses induced by TGF‐β stimulation in specific contexts. Mechanisms by which TGF‐β can induce several responses in a cellular context‐dependent fashion are discussed with established paradigms and models. We also address perspectives on the specific control of only a subset of numerous cell responses induced by TGF‐β stimulation. Such methods will aid specific regulation of either the tumor‐suppressive or tumor‐promoting arm of the TGF‐β pathway and in realization of TGF‐β‐based treatment of malignant tumors. (Cancer Sci 2010; 101: 306–312)

An Id‐like molecule, HHM, is a synexpression group‐restricted regulator of TGF‐β signalling

Transforming growth factor (TGF)‐β induces various cellular responses principally through Smad‐dependent transcriptional regulation. Activated Smad complexes cooperate with transcription factors in regulating a group of target genes. The target genes controlled by the same Smad‐cofactor complexes are denoted a synexpression group. We found that an Id‐like helix‐loop‐helix protein, human homologue of Maid (HHM), is a synexpression group‐restricted regulator of TGF‐β signalling. HHM suppressed TGF‐β‐induced growth inhibition and cell migration but not epithelial—mesenchymal transition. In addition, HHM inhibited TGF‐β‐induced expression of plasminogen activator inhibitor‐type 1 (PAI‐1), PDGF‐B, and p21WAF, but not Snail. We identified a basic‐helix‐loop‐helix protein, Olig1, as one of the Smad‐binding transcription factors affected by HHM. Olig1 interacted with Smad2/3 in response to TGF‐β stimulation, and was involved in transcriptional activation of PAI‐1 and PDGF‐B. HHM, but not Id proteins, inhibited TGF‐β signalling‐dependent association of Olig1 with Smad2/3 through physical interaction with Olig1. HHM thus appears to regulate a subset of TGF‐β target genes including the Olig1‐Smad synexpression group. HHM is the first example of a cellular response‐selective regulator of TGF‐β signalling with clearly determined mechanisms.

RB1CC1 Protein Positively Regulates Transforming Growth Factor-β Signaling through the Modulation of Arkadia E3 Ubiquitin Ligase Activity

Transforming growth factor-β (TGF-β) signaling is controlled by a variety of regulators, of which Smad7, c-Ski, and SnoN play a pivotal role in its negative regulation. Arkadia is a RING-type E3 ubiquitin ligase that targets these negative regulators for degradation to enhance TGF-β signaling. In the present study we identified a candidate human tumor suppressor gene product RB1CC1/FIP200 as a novel positive regulator of TGF-β signaling that functions as a substrate-selective cofactor of Arkadia. Overexpression of RB1CC1 enhanced TGF-β signaling, and knockdown of endogenous RB1CC1 attenuated TGF-β-induced expression of target genes as well as TGF-β-induced cytostasis. RB1CC1 down-regulated the protein levels of c-Ski but not SnoN by enhancing the activity of Arkadia E3 ligase toward c-Ski. Substrate selectivity is primarily attributable to the physical interaction of RB1CC1 with substrates, suggesting its role as a scaffold protein. RB1CC1 thus appears to play a unique role as a modulator of TGF-β signaling by restricting substrate specificity of Arkadia.