Stimulation of in vivo nuclear transport dynamics of actin and its co-factors IQGAP1 and Rac1 in response to DNA replication stress

Nuclear actin filaments - a historical perspective

The view on nuclear filaments formed by non-skeletal β-actin has significantly changed over the decades. Initially, filamentous actin was observed in amphibian oocyte nuclei and only under specific cell stress conditions in mammalian cell nuclei. Improved labeling and imaging technologies have permitted insights into a transient but microscopically apparent filament network that is relevant for chromatin organization, biomechanics of the mammalian cell nucleus, gene expression, and DNA damage repair. Here, we will provide a historical perspective on the developing insight into nuclear actin filaments.

Chromatin meets the cytoskeleton: the importance of nuclear actin dynamics and associated motors for genome stability

The actin cytoskeleton is of fundamental importance for numerous cellular processes, including intracellular transport, cell plasticity, and cell migration. However, functions of filamentous actin (F-actin) in the nucleus remain understudied due to the comparatively low abundance of nuclear actin and the resulting experimental limitations to its visualization. Owing to recent technological advances such as super-resolution microscopy and the development of nuclear-specific actin probes, essential roles of the actin cytoskeleton in the context of genome maintenance are now emerging. In addition to the contributions of monomeric actin as a component of multiple important nuclear protein complexes, nuclear actin has been found to undergo polymerization in response to DNA damage and DNA replication stress. Consequently, nuclear F-actin plays important roles in the regulation of intra-nuclear mobility of repair and replication foci as well as the maintenance of nuclear shape, two important aspects of efficient stress tolerance. Beyond actin itself, there is accumulating evidence for the participation of multiple actin-binding proteins (ABPs) in the surveillance of genome integrity, including nucleation factors and motor proteins of the myosin family. Here we summarize recent findings highlighting the importance of actin cytoskeletal factors within the nucleus in key genome maintenance pathways.

IPO5 Mediates EMT and Promotes Esophageal Cancer Development through the RAS-ERK Pathway

Objective

In the development of many tumors, IPO5, as a member of the nuclear transporter family, exerts a significant function. Also, IPO5 is used as a therapeutic target for tumors based on some reports. By studying IPO5 expression in esophageal cancer tissues, the mechanism associated with IPO5 improving esophageal cancer development was explored in this study.

Methods

To gain differentially expressed genes, this study utilized mRNA microarray and TCGA database for comprehensive analysis of esophageal cancer tissues and normal esophageal cancer tissues, and then the differentially expressed gene IPO5 was screened by us. To assess esophageal cancer patients' prognosis, this study also applied the Kaplan-Meier analysis, and we also conducted the GSEA enrichment analysis to investigate IPO5-related signaling pathways. This study performed TISIDB and TIMER online analysis tools to study the correlation between IPO5 and immune regulation and infiltration. We took specimens of esophageal cancer from patients and detected the expression of IPO5 in tumor and normal tissues by immunohistochemistry. The IPO5 gene-silenced esophageal cancer cell model was constructed by lentivirus transfection. Through the Transwell invasion assay, CCK-8 assay, and cell scratch assay, this study investigated the effects of IPO5 on cell propagation, invasion, and transfer. What is more, we identified the influences of IPO5 on the cell cycle through flow cytometry and established a subcutaneous tumor-forming model in nude mice. Immunohistochemistry was used to verify the expression of KI-67, and this study detected the modifications of cell pathway-related proteins using Western blot and applied EMT-related proteins to explain the mechanism of esophageal cancer induced by IPO5.

Results

According to database survival analysis, IPO5 high-expression patients had shorter disease-free survival than IPO5 low-expression patients. Compared to normal tissues, the IPO5 expression in cancer tissues was significantly higher in clinical trials (P < 0.05). Through TISIDB and TIMER database studies, we found that IPO5 could affect immune regulation, and the age of IPO5 expression grows with the increase of immune infiltration level. The IPO5 expression in esophageal cancer cells was higher than normal, especially in ECA109 and OE33 cells (P < 0.01). After knocking out IPO5 gene expression, cell proliferation capacity and invasion capacity were reduced (P < 0.05) and decreased (P < 0.01) in the IPO5-interfered group rather than the negative control group. The growth cycle of esophageal carcinoma cells was arrested in the G2/M phase after IPO5 gene silencing (P < 0.01). Tumor-forming experiments in nude mice confirmed that after IPO5 deletion, the tumor shrank, the expression of KI67 decreased, the downstream protein expression level of the RAS pathway decreased after sh-IPO5 interference (P < 0.01), and the level of EMT marker delined (P < 0.05).

Conclusion

In esophageal cancer, IPO5 is highly expressed and correlates with survival rate. Esophageal cancer cell growth and migration were significantly affected by the inhibition of IPO5 in vitro and in vivo. IPO5 mediates EMT using the RAS-ERK signaling pathway activation and promotes esophageal cancer cell development in vivo and in vitro.

Identification of IQGAP1 as a SLC26A4 (Pendrin)-Binding Protein in the Kidney

Background: Several members of the SLC26A family of transporters, including SLC26A3 (DRA), SLC26A5 (prestin), SLC26A6 (PAT-1; CFEX) and SLC26A9, form multi-protein complexes with a number of molecules (e.g., cytoskeletal proteins, anchoring or adaptor proteins, cystic fibrosis transmembrane conductance regulator, and protein kinases). These interactions provide regulatory signals for these molecules. However, the identity of proteins that interact with the Cl^-/HCO₃ ^- exchanger, SLC26A4 (pendrin), have yet to be determined. The purpose of this study is to identify the protein(s) that interact with pendrin. Methods: A yeast two hybrid (Y2H) system was employed to screen a mouse kidney cDNA library using the C-terminal fragment of SLC26A4 as bait. Immunofluorescence microscopic examination of kidney sections, as well as co-immunoprecipitation assays, were performed using affinity purified antibodies and kidney protein extracts to confirm the co-localization and interaction of pendrin and the identified binding partners. Co-expression studies were carried out in cultured cells to examine the effect of binding partners on pendrin trafficking and activity. Results: The Y2H studies identified IQ motif-containing GTPase-activating protein 1 (IQGAP1) as a protein that binds to SLC26A4's C-terminus. Co-immunoprecipitation experiments using affinity purified anti-IQGAP1 antibodies followed by western blot analysis of kidney protein eluates using pendrin-specific antibodies confirmed the interaction of pendrin and IQGAP1. Immunofluorescence microscopy studies demonstrated that IQGAP1 co-localizes with pendrin on the apical membrane of B-intercalated cells, whereas it shows basolateral expression in A-intercalated cells in the cortical collecting duct (CCD). Functional and confocal studies in HEK-293 cells, as well as confocal studies in MDCK cells, demonstrated that the co-transfection of pendrin and IQGAP1 shows strong co-localization of the two molecules on the plasma membrane along with enhanced Cl^-/HCO₃ ^- exchanger activity. Conclusion: IQGAP1 was identified as a protein that binds to the C-terminus of pendrin in B-intercalated cells. IQGAP1 co-localized with pendrin on the apical membrane of B-intercalated cells. Co-expression of IQGAP1 with pendrin resulted in strong co-localization of the two molecules and increased the activity of pendrin in the plasma membrane in cultured cells. We propose that pendrin's interaction with IQGAP1 may play a critical role in the regulation of CCD function and physiology, and that disruption of this interaction could contribute to altered pendrin trafficking and/or activity in pathophysiologic states.

Optimizing metastatic-cascade-dependent Rac1 targeting in breast cancer: Guidance using optical window intravital FRET imaging

Assessing drug response within live native tissue provides increased fidelity with regards to optimizing efficacy while minimizing off-target effects. Here, using longitudinal intravital imaging of a Rac1-Förster resonance energy transfer (FRET) biosensor mouse coupled with in vivo photoswitching to track intratumoral movement, we help guide treatment scheduling in a live breast cancer setting to impair metastatic progression. We uncover altered Rac1 activity at the center versus invasive border of tumors and demonstrate enhanced Rac1 activity of cells in close proximity to live tumor vasculature using optical window imaging. We further reveal that Rac1 inhibition can enhance tumor cell vulnerability to fluid-flow-induced shear stress and therefore improves overall anti-metastatic response to therapy during transit to secondary sites such as the lung. Collectively, this study demonstrates the utility of single-cell intravital imaging in vivo to demonstrate that Rac1 inhibition can reduce tumor progression and metastases in an autochthonous setting to improve overall survival.

The scaffold protein IQGAP1 links heat-induced stress signals to alternative splicing regulation in gastric cancer cells

In response to oncogenic signals, Alternative Splicing (AS) regulators such as SR and hnRNP proteins show altered expression levels, subnuclear distribution and/or post-translational modification status, but the link between signals and these changes remains unknown. Here, we report that a cytosolic scaffold protein, IQGAP1, performs this task in response to heat-induced signals. We show that in gastric cancer cells, a nuclear pool of IQGAP1 acts as a tethering module for a group of spliceosome components, including hnRNPM, a splicing factor critical for the response of the spliceosome to heat-shock. IQGAP1 controls hnRNPM's sumoylation, subnuclear localisation and the relevant response of the AS machinery to heat-induced stress. Genome-wide analyses reveal that IQGAP1 and hnRNPM co-regulate the AS of a cell cycle-related RNA regulon in gastric cancer cells, thus favouring the accelerated proliferation phenotype of gastric cancer cells. Overall, we reveal a missing link between stress signals and AS regulation.

Chromatin mobility and relocation in DNA repair

The nucleus is a dynamic environment containing chromatin, membraneless organelles, and specialized molecular structures at the nuclear membrane. Within the spectrum of DNA repair activities are observations of increased mobility of damaged chromatin and the displacement of DNA lesions to specific nuclear environments. Here, we focus on the role that nuclear-specific filamentous actin plays in mobilizing damaged chromatin in response to DNA double-strand breaks and replication stress. We also examine nuclear pore complexes and promyelocytic leukemia-nuclear bodies as specialized platforms for homology-directed repair. The literature suggests an emerging model where specific types of DNA lesions are subjected to nuclear-derived forces that mobilize damaged chromatin and promote interaction with repair hubs to facilitate specialized repair reactions.

The decreased exclusion of nuclear eccDNA: From molecular and subcellular levels to human aging and age-related diseases

Extrachromosomal circular DNA (eccDNA) accumulates within the nucleus of eukaryotic cells during physiological aging and in age-related diseases (ARDs) and the accumulation could be caused by the declined exclusion of nuclear eccDNA in these states. This review focuses on the formation of eccDNA and the roles of some main factors, such as nuclear pore complexes (NPCs), nucleoplasmic reticulum (NR), and nuclear actin, in eccDNA exclusion. eccDNAs are mostly formed from non-coding DNA during DNA damage repair. They move to NPCs along nuclear actin and are excluded out of the nucleus through functional NPCs in young and healthy cells. However, it has been demonstrated that defective NPCs, abnormal NPC components and nuclear actin rods are increased in aged cells, various cancers and certain other ARDs such as cardiovascular diseases, premature aging, neurodegenerative diseases and myopathies. Therefore, mainly resulting from the increase of dysfunctional NPCs, the exclusion of nuclear eccDNAs may be reduced and eccDNAs thus accumulate within the nucleus in aging and the aforementioned ARDs. In addition, the protective function of non-coding DNA in tumorigenesis is further discussed.

GTPases, genome, actin: A hidden story in DNA damage response and repair mechanisms

The classical small Rho GTPase (Rho, Rac, and Cdc42) protein family is mainly responsible for regulating cell motility and polarity, membrane trafficking, cell cycle control, and gene transcription. Cumulative recent evidence supports important roles for these proteins in the maintenance of genomic stability. Indeed, DNA damage response (DDR) and repair mechanisms are some of the prime biological processes that underlie several disease phenotypes, including genetic disorders, cancer, senescence, and premature aging. Many reports guided by different experimental approaches and molecular hypotheses have demonstrated that, to some extent, direct modulation of Rho GTPase activity, their downstream effectors, or actin cytoskeleton regulation contribute to these cellular events. Although much attention has been paid to this family in the context of canonical actin cytoskeleton remodeling, here we provide a contextualized review of the interplay between Rho GTPase signaling pathways and the DDR and DNA repair signaling components. Interesting questions yet to be addressed relate to the spatiotemporal dynamics of this collective response and whether it correlates with different subcellular pools of Rho GTPases. We highlight the direct and indirect targets, some of which still lack experimental validation data, likely associated with Rho GTPase activation that provides compelling evidence for further investigation in DNA damage-associated events and with potential therapeutic applications in translational medicine.

The interplay between IQGAP1 and small GTPases in cancer metastasis

The metastatic spread of tumor cells to distant anatomical locations is a critical cause for disease progression and leads to more than 90 % of cancer-related deaths. IQ motif-containing GTPase-activating protein 1 (IQGAP1), a prominent regulator in the cancer metastasis process, is a scaffold protein that interacts with components of the cytoskeleton. As a critical node within the small GTPase network, IQGAP1 acts as a binding partner of several small GTPases, which in turn function as molecular switches to control most cellular processes, including cell migration and invasion. Given the significant interaction between IQGAP1 and small GTPases in cancer metastasis, we briefly elucidate the role of IQGAP1 in regulating cancer metastasis and the varied interactions existing between IQGAP1 and small GTPases. In addition, the potential regulators for IQGAP1 activity and its interaction with small GTPases are also incorporated in this review. Overall, we comprehensively summarize the role of IQGAP1 in cancer tumorigenicity and metastasis, which may be a potential anti-tumor target to restrain cancer progression.

Nuclear F-actin counteracts nuclear deformation and promotes fork repair during replication stress

Filamentous actin (F-actin) provides cells with mechanical support and promotes the mobility of intracellular structures. Although F-actin is traditionally considered to be cytoplasmic, here we reveal that nuclear F-actin participates in the replication stress response. Using live and super-resolution imaging, we find that nuclear F-actin is polymerized in response to replication stress through a pathway regulated by ATR-dependent activation of mTORC1, and nucleation through IQGAP1, WASP and ARP2/3. During replication stress, nuclear F-actin increases the nuclear volume and sphericity to counteract nuclear deformation. Furthermore, F-actin and myosin II promote the mobility of stressed-replication foci to the nuclear periphery through increasingly diffusive motion and directed movements along the nuclear actin filaments. These actin functions promote replication stress repair and suppress chromosome and mitotic abnormalities. Moreover, we find that nuclear F-actin is polymerized in vivo in xenograft tumours after treatment with replication-stress-inducing chemotherapeutic agents, indicating that this pathway has a role in human disease.

Nuclear actin in cancer biology

The presence of actin in the nucleus has been a matter of debate for many years. In recent years many important roles of actin in the nucleus (transcriptional regulation, chromatin remodeling, DNA repair, cell division, maintenance of nuclear architecture) have been identified, and the precise control of nuclear actin levels has been demonstrated. The vital importance of the actin driven processes in the cell make it highly likely that dysregulation of nuclear actin dynamics and structure can be linked to tumor induction and -progression. In this chapter I summarize our current knowledge about nuclear actin in the cancer context.

Roles of the actin cytoskeleton in aging and age-associated diseases

The integrity of the cytoskeleton is essential to diverse cellular processes such as phagocytosis and intracellular trafficking. Disruption of the organization and dynamics of the actin cytoskeleton leads to age-associated symptoms and diseases, ranging from cancer to neurodegeneration. In addition, changes in the integrity of the actin cytoskeleton disrupt the functioning of not only somatic and stem cells but also gametes, resulting in aberrant embryonic development. Strategies to preserve the integrity and dynamics of the cytoskeleton are, therefore, potentially therapeutic to age-related disorders. The objective of this article is to revisit the current understanding of the roles played by the actin cytoskeleton in aging, and to review the opportunities and challenges for the transition of basic research into intervention development. It is hoped that, with the snapshot of evidence regarding changes in actin dynamics with advanced age, insights into future research directions can be attained.

Nuclear actin filaments in DNA repair dynamics

Recent development of innovative tools for live imaging of actin filaments (F-actin) enabled the detection of surprising nuclear structures responding to various stimuli, challenging previous models that actin is substantially monomeric in the nucleus. We review these discoveries, focusing on double-strand break (DSB) repair responses. These studies revealed a remarkable network of nuclear filaments and regulatory mechanisms coordinating chromatin dynamics with repair progression and led to a paradigm shift by uncovering the directed movement of repair sites.

The mTOR pathway: Implications for DNA replication

DNA replication plays a central role in genome health. Deleterious alteration of replication dynamics, or "replication stress", is a key driver of genome instability and oncogenesis. The replication stress response is regulated by the ATR kinase, which functions to mitigate replication abnormalities through coordinated efforts that arrest the cell cycle and repair damaged replication forks. mTOR kinase regulates signaling networks that control cell growth and metabolism in response to environmental cues and cell stress. In this review, we discuss interconnectivity between the ATR and mTOR pathways, and provide putative mechanisms for mTOR engagement in DNA replication and the replication stress response. Finally, we describe how connectivity between mTOR and replication stress may be exploited for cancer therapy.

Nuclear Actin and Actin-Binding Proteins in DNA Repair

Nuclear actin has been implicated in a variety of DNA-related processes including chromatin remodeling, transcription, replication, and DNA repair. However, the mechanistic understanding of actin in these processes has been limited, largely due to a lack of research tools that address the roles of nuclear actin specifically, that is, distinct from its cytoplasmic functions. Recent findings support a model for homology-directed DNA double-strand break (DSB) repair in which a complex of ARP2 and ARP3 (actin-binding proteins 2 and 3) binds at the break and works with actin to promote DSB clustering and homology-directed repair. Further, it has been reported that relocalization of heterochromatic DSBs to the nuclear periphery in Drosophila is ARP2/3 dependent and actin-myosin driven. Here we provide an overview of the role of nuclear actin and actin-binding proteins in DNA repair, critically evaluating the experimental tools used and potential indirect effects.

Phosphatidylinositol-3-phosphate-mediated actin domain formation linked to DNA synthesis upon insulin treatment in rat hepatoma-derived H4IIEC3 cells

Phosphatidylinositol-3-phosphate (PI3P) is a lipid that accumulates in the early endosomal membrane, and acts as a scaffold to recruit proteins that contain a PI3P-binding domain, such as the FYVE domain. In this study, we examined the effect of PI3P depletion on the insulin response in rat hepatoma-derived H4IIEC3 cells. We found that insulin treatment induced the transient formation of an actin domain structure, a mesh-like tangled network of actin filaments where phosphorylated Akt, endosomal proteins, and PI3P accumulated. Actin domain formation was repressed by the depletion of PI3P by SAR405, an inhibitor of the class III PI3 kinase, Vps34, by the inhibition of PI3P function by the competitive binding of an excess amount of GST-fused 2xFYVE protein to intracellular PI3P, and by the use of diabetic model cells, in which PI3P was depleted. SAR405 did not affect the phosphorylation level of Akt, and the transcriptional regulation of gluconeogenic and cholesterol synthetic genes after insulin treatment. Interestingly, insulin-induced DNA synthesis was specifically inhibited by SAR405, cytochalasin B, and also in diabetic model cells. These results suggest that PI3P is required for the formation of actin domains, which affected a signaling pathway downstream of Akt associated with DNA synthesis in H4IIEC3 cells.

The Nature of Actin-Family Proteins in Chromatin-Modifying Complexes

Actin is not only one of the most abundant proteins in eukaryotic cells, but also one of the most versatile. In addition to its familiar involvement in enabling contraction and establishing cellular motility and scaffolding in the cytosol, actin has well-documented roles in a variety of processes within the confines of the nucleus, such as transcriptional regulation and DNA repair. Interestingly, monomeric actin as well as actin-related proteins (Arps) are found as stoichiometric subunits of a variety of chromatin remodeling complexes and histone acetyltransferases, raising the question of precisely what roles they serve in these contexts. Actin and Arps are present in unique combinations in chromatin modifiers, helping to establish structural integrity of the complex and enabling a wide range of functions, such as recruiting the complex to nucleosomes to facilitate chromatin remodeling and promoting ATPase activity of the catalytic subunit. Actin and Arps are also thought to help modulate chromatin dynamics and maintain higher-order chromatin structure. Moreover, the presence of actin and Arps in several chromatin modifiers is necessary for promoting genomic integrity and an effective DNA damage response. In this review, we discuss the involvement of actin and Arps in these nuclear complexes that control chromatin remodeling and histone modifications, while also considering avenues for future study to further shed light on their functional importance.

Ras GTPase-Activating-Like Protein IQGAP1 (IQGAP1) Promotes Breast Cancer Proliferation and Invasion and Correlates with Poor Clinical Outcomes

BACKGROUND Breast cancer is one of the most common female cancers in the world. As a key integrator of cell signaling pathways, IQGAP1 contributes to the development and progression of several cancers. However, the exact effects and molecular mechanisms of IQGAP1 in breast cancer progression remain poorly understood. MATERIAL AND METHODS In the present study, IQGAP1 expression was measured in 96 paired breast cancer samples and the corresponding adjacent non-cancerous tissues by immunohistochemistry and quantitative polymerase chain reaction. To further explore the biological function of IQGAP1 in breast cancer cells, we knocked down IQGAP1 expression in MCF-7 cells and overexpressed it in SK-BR-3 cells. RESULTS IQGAP1 was specifically upregulated in breast cancer tissues compared with the corresponding adjacent non-cancerous tissues. Moreover, IQGAP1 expression was positively correlated with breast cancer survival rate. IQGAP1 also promoted breast cancer cell proliferation and cell cycle progression and suppressed apoptosis. CONCLUSIONS In conclusion, our results suggest that IQGAP1 plays an important role in the cell proliferation and invasion of human breast cancer cells, thus indicating that IQGAP1 may be a potential therapeutic target for the treatment of human breast cancer.

RCC2 over-expression in tumor cells alters apoptosis and drug sensitivity by regulating Rac1 activation

Background

Small GTP binding protein Rac1 is a component of NADPH oxidases and is essential for superoxide-induced cell death. Rac1 is activated by guanine nucleotide exchange factors (GEFs), and this activation can be blocked by regulator of chromosome condensation 2 (RCC2), which binds the switch regions of Rac1 to prevent access from GEFs.

Methods

Three cancer cell lines with up- or down-regulation of RCC2 were used to evaluate cell proliferation, apoptosis, Rac1 signaling and sensitivity to a group of nine chemotherapeutic drugs. RCC2 expression in lung cancer and ovarian cancer were studied using immunochemistry stain of tumor tissue arrays.

Results

Forced RCC2 expression in tumor cells blocked spontaneous- or Staurosporine (STS)-induced apoptosis. In contrast, RCC2 knock down in these cells resulted in increased apoptosis to STS treatment. The protective activity of RCC2 on apoptosis was revoked by a constitutively activated Rac1, confirming a role of RCC2 in apoptosis by regulating Rac1. In an immunohistochemistry evaluation of tissue microarray, RCC2 was over-expressed in 88.3% of primary lung cancer and 65.2% of ovarian cancer as compared to non-neoplastic lung and ovarian tissues, respectively. Because chemotherapeutic drugs can kill tumor cells by activating Rac1/JNK pathway, we suspect that tumors with RCC2 overexpression would be more resistant to these drugs. Tumor cells with forced RCC2 expression indeed had significant difference in drug sensitivity compared to parental cells using a panel of common chemotherapeutic drugs.

Conclusions

RCC2 regulates apoptosis by blocking Rac1 signaling. RCC2 expression in tumor can be a useful marker for predicting chemotherapeutic response.

Initiation of DNA replication requires actin dynamics and formin activity

Nuclear actin regulates transcriptional programmes in a manner dependent on its levels and polymerisation state. This dynamics is determined by the balance of nucleocytoplasmic shuttling, formin- and redox-dependent filament polymerisation. Here, using Xenopus egg extracts and human somatic cells, we show that actin dynamics and formins are essential for DNA replication. In proliferating cells, formin inhibition abolishes nuclear transport and initiation of DNA replication, as well as general transcription. In replicating nuclei from transcriptionally silent Xenopus egg extracts, we identified numerous actin regulators, and disruption of actin dynamics abrogates nuclear transport, preventing NLS (nuclear localisation signal)-cargo release from RanGTP-importin complexes. Nuclear formin activity is further required to promote loading of cyclin-dependent kinase (CDK) and proliferating cell nuclear antigen (PCNA) onto chromatin, as well as initiation and elongation of DNA replication. Therefore, actin dynamics and formins control DNA replication by multiple direct and indirect mechanisms.

From Cytoskeleton to Gene Expression: Actin in the Nucleus

Although most people still associate actin mainly with the cytoskeleton, several lines of evidence, with the earliest studies dating back to decades ago, have emphasized the importance of actin also inside the cell nucleus. Actin has been linked to many gene expression processes from gene activation to chromatin remodeling, but also to maintenance of genomic integrity and intranuclear movement of chromosomes and chromosomal loci. Recent advances in visualizing different forms and dynamic properties of nuclear actin have clearly advanced our understanding of the basic concepts by which actin operates in the nucleus. In this chapter we address the different breakthroughs in nuclear actin studies, as well as discuss the regulation nuclear actin and the importance of nuclear actin dynamics in relation to its different nuclear functions. Our aim is to highlight the fact that actin should be considered as an essential component of the cell nucleus, and its nuclear actions should be taken into account also in experiments on cytoplasmic actin networks.

DNA damage-induced inflammation and nuclear architecture

Nuclear architecture and the chromatin state affect most-if not all- DNA-dependent transactions, including the ability of cells to sense DNA lesions and restore damaged DNA back to its native form. Recent evidence points to functional links between DNA damage sensors, DNA repair mechanisms and the innate immune responses. The latter raises the question of how such seemingly disparate processes operate within the intrinsically complex nuclear landscape and the chromatin environment. Here, we discuss how DNA damage-induced immune responses operate within chromatin and the distinct sub-nuclear compartments highlighting their relevance to chronic inflammation.

Persistent nuclear actin filaments inhibit transcription by RNA polymerase II

Actin is abundant in the nucleus and it is clear that nuclear actin has important functions. However, mystery surrounds the absence of classical actin filaments in the nucleus. To address this question, we investigated how polymerizing nuclear actin into persistent nuclear actin filaments affected transcription by RNA polymerase II. Nuclear filaments impaired nuclear actin dynamics by polymerizing and sequestering nuclear actin. Polymerizing actin into stable nuclear filaments disrupted the interaction of actin with RNA polymerase II and correlated with impaired RNA polymerase II localization, dynamics, gene recruitment, and reduced global transcription and cell proliferation. Polymerizing and crosslinking nuclear actin in vitro similarly disrupted the actin-RNA-polymerase-II interaction and inhibited transcription. These data rationalize the general absence of stable actin filaments in mammalian somatic nuclei. They also suggest a dynamic pool of nuclear actin is required for the proper localization and activity of RNA polymerase II.

MicroRNA-1 properties in cancer regulatory networks and tumor biology

Short non-coding microRNAs have been identified to orchestrate crucial mechanisms in cancer progression and treatment resistance. MicroRNAs are involved in posttranscriptional modulation of gene expression and therefore represent promising targets for anticancer therapy. As mircoRNA-1 (miR-1) exerted to be predominantly downregulated in the majority of examined tumors, miR-1 is classified to be a tumor suppressor with high potential to diminish tumor development and therapy resistance. Here we review the complex functionality of miR-1 in tumor biology.

Rac1 augments Wnt signaling by stimulating β-catenin-lymphoid enhancer factor-1 complex assembly independent of β-catenin nuclear import

β-Catenin transduces the Wnt signaling pathway and its nuclear accumulation leads to gene transactivation and cancer. Rac1 GTPase is known to stimulate β-catenin-dependent transcription of Wnt target genes and we confirmed this activity. Here we tested the recent hypothesis that Rac1 augments Wnt signaling by enhancing β-catenin nuclear import; however, we found that silencing/inhibition or up-regulation of Rac1 had no influence on nuclear accumulation of β-catenin. To better define the role of Rac1, we employed proximity ligation assays (PLA) and discovered that a significant pool of Rac1-β-catenin protein complexes redistribute from the plasma membrane to the nucleus upon Wnt or Rac1 activation. More importantly, active Rac1 was shown to stimulate the formation of nuclear β-catenin-lymphoid enhancer factor 1 (LEF-1) complexes. This regulation required Rac1-dependent phosphorylation of β-catenin at specific serines, which when mutated (S191A and S605A) reduced β-catenin binding to LEF-1 by up to 50%, as revealed by PLA and immunoprecipitation experiments. We propose that Rac1-mediated phosphorylation of β-catenin stimulates Wnt-dependent gene transactivation by enhancing β-catenin-LEF-1 complex assembly, providing new insight into the mechanism of cross-talk between Rac1 and canonical Wnt/β-catenin signaling.

IQGAP1: insights into the function of a molecular puppeteer

The intracellular spatiotemporal organization of signaling events is critical for normal cellular function. In response to environmental stimuli, cells utilize highly organized signaling pathways that are subject to multiple layers of regulation. However, the molecular mechanisms that coordinate these complex processes remain an enigma. Scaffolding proteins (scaffolins) have emerged as critical regulators of signaling pathways, many of which have well-described functions in immune cells. IQGAP1, a highly conserved cytoplasmic scaffold protein, is able to curb, compartmentalize, and coordinate multiple signaling pathways in a variety of cell types. IQGAP1 plays a central role in cell-cell interaction, cell adherence, and movement via actin/tubulin-based cytoskeletal reorganization. Evidence also implicates IQGAP1 as an essential regulator of the MAPK and Wnt/β-catenin signaling pathways. Here, we summarize the recent advances on the cellular and molecular biology of IQGAP1. We also describe how this pleiotropic scaffolin acts as a true molecular puppeteer, and highlight the significance of future research regarding the role of IQGAP1 in immune cells.

Baculoviruses and nucleosome management

Negatively-supercoiled-ds DNA molecules, including the genomes of baculoviruses, spontaneously wrap around cores of histones to form nucleosomes when present within eukaryotic nuclei. Hence, nucleosome management should be essential for baculovirus genome replication and temporal regulation of transcription, but this has not been documented. Nucleosome mobilization is the dominion of ATP-dependent chromatin-remodeling complexes. SWI/SNF and INO80, two of the best-studied complexes, as well as chromatin modifier TIP60, all contain actin as a subunit. Retrospective analysis of results of AcMNPV time course experiments wherein actin polymerization was blocked by cytochalasin D drug treatment implicate actin-containing chromatin modifying complexes in decatenating baculovirus genomes, shutting down host transcription, and regulating late and very late phases of viral transcription. Moreover, virus-mediated nuclear localization of actin early during infection may contribute to nucleosome management.

Crosstalk between DNA repair and cancer stem cell (CSC) associated intracellular pathways

DNA damaging agents (ionizing radiation and chemotherapeutics) are considered as most effective in cancer treatment. However, there is a subpopulation of carcinoma cells within the tumour demonstrating resistance to DNA damaging treatment approaches. It is suggested that limited tumour response to this kind of therapy can be associated with specific molecular properties of carcinoma stem cells (CSCs) representing the most refractory cell subpopulation. This review article presents novel data about molecular features of CSCs underlying DNA damage response and related intracellular signalling.

Endometrial stromal sarcomas: immunoprofile with emphasis on HMB45 reactivity

OBJECTIVES

We describe the morphologic and immunohistochemical features of 17 endometrial stromal neoplasms, 16 sarcomas, and one stromal nodule.

METHODS

We reviewed 35 cases interpreted as endometrial stromal neoplasms, but 17 high-grade endometrial stromal sarcomas (ESS) and one case of mixed endometrial sarcoma and leiomyosarcoma were excluded from the study. Data from the Surveillance Epidemiology and End Results program on low- and high-grade ESS for 1973 through 2003 were obtained.

RESULTS

One uterine primary ESS had collections of clear cells (20%), while a metastatic ESS contained predominantly clear cells (90%). CD10 (88.2%) and smooth muscle actin (70.5%) were the most common positive immunohistochemical markers. The latter marker was located in the cytoplasm in 47% of the ESS and in the nucleus in 23.5%, a previously unreported feature. HMB45 was detected in 23.5% of the ESS, which contrasts with the 2% reported by other authors.

CONCLUSIONS

The presence of clear cells and HMB45 reactivity does not justify the term perivascular epithelioid cell tumors for these neoplasms. Two of 17 patients with ESS died of metastatic disease. However, among 274 cases of ESS (all stages included) collected by the Surveillance Epidemiology and End Results Program of the National Cancer Institute during a 30-year period, the 10-year survival rate was 94%.

Rho, nuclear actin, and actin-binding proteins in the regulation of transcription and gene expression

Actin cytoskeleton is one of the main targets of Rho GTPases, which act as molecular switches on many signaling pathways. During the past decade, actin has emerged as an important regulator of gene expression. Nuclear actin plays a key role in transcription, chromatin remodeling, and pre-mRNA processing. In addition, the "status" of the actin cytoskeleton is used as a signaling intermediate by at least the MKL1-SRF and Hippo-pathways, which culminate in the transcriptional regulation of cytoskeletal and growth-promoting genes, respectively. Rho GTPases may therefore regulate gene expression by controlling either cytoplasmic or nuclear actin dynamics. Although the regulation of nuclear actin polymerization is still poorly understood, many actin-binding proteins, which are downstream effectors of Rho, are found in the nuclear compartment. In this review, we discuss the possible mechanisms and key proteins that may mediate the transcriptional regulation by Rho GTPases through actin.

To be or not to be assembled: progressing into nuclear actin filaments

The paradigm states that cytoplasmic actin operates as filaments and nuclear actin is mainly monomeric, acting as a scaffold in transcription complexes. However, why should a powerful function of actin, namely polymerization, not be used in the nucleus? Recent progress in the field forces us to rethink this issue, as many actin filament assembly proteins have been linked to nuclear functions and new experimental approaches have provided the first direct visualizations of polymerized nuclear actin.