ARHGAP4 promotes leukemogenesis in acute myeloid leukemia by inhibiting DRAM1 signaling

Rho GTPase-activating protein 4 (ARHGAP4) is an important Rho family GTPase-activating protein that is strongly associated with the onset and progression of some tumors. We found that ARHGAP4 mRNA and protein are overexpressed in human acute myeloid leukemia (AML) patients and are associated with a poor prognosis. ARHGAP4 knockdown significantly impairs viability and colony formation capacity and induces apoptosis in AML cells. Further results demonstrate that ARHGAP4 deletion impairs AML progression in vivo. Interestingly, DRAM1 signaling is significantly activated in AML cells with ARHGAP4 knockdown. Our results also indicated that ARHGAP4 might function in AML cells by binding with p53 to inhibit DRAM1. Moreover, knockdown of DRAM1 rescues the defects of ARHGAP4 in AML cells. This newly described role of the ARHGAP4/DRAM1 axis in regulating AML progression may have important therapeutic implications.

RSV infection does not induce EMT

Respiratory syncytial virus (RSV) infection does not cause severe disease in most of us despite suffering from multiple RSV infections in our lives. However, infants, young children, older adults, and immunocompromised patients are unfortunately vulnerable to RSV-associated severe diseases. A recent study suggested that RSV infection causes cell expansion, resulting in bronchial wall thickening in vitro. Whether the virus-induced changes in the lung airway resemble epithelial-mesenchymal transition (EMT) is still unknown. Here, we report that RSV does not induce EMT in three different in vitro lung models: the epithelial A549 cell line, primary normal human bronchial epithelial cells, and pseudostratified airway epithelium. We found that RSV increases the cell surface area and perimeter in the infected airway epithelium, which is distinct from the effects of a potent EMT inducer, TGF-β1-driven cell elongation-indicative of cell motility. A genome-wide transcriptome analysis revealed that both RSV and TGF-β1 have distinct modulation patterns of the transcriptome, which suggests that RSV-induced changes are distinct from EMT.

Emergence of multiple set-points of cellular homeostatic tension

Stress fibers (SFs), a contractile actin bundle in nonmuscle mesenchymal cells, are known to intrinsically sustain a constant level of tension or tensional stress, a process called cellular tensional homeostasis. Malfunction in this homeostatic process has been implicated in many diseases such atherosclerosis, but its mechanisms remain incompletely understood. Interestingly, the homeostatic stress in individual SFs is altered upon recruitment of α-smooth muscle actin in particular cellular contexts to reinforce the preexisting SFs. While this transition of the set-point stress is somewhat a universal process observed across different cell types, no clear explanation has been provided as to why cells end up possessing different stable stresses. To address the underlying physics, here we describe that imposing a realistic assumption on the nature of SFs yields the presence of multiple set-points of the homeostatic stress, which transition among them depending on the magnitude of the cellular tension. We analytically derive non-dimensional parameters that characterize the extent of the transition and predict that SFs tend to acquire secondary stable stresses if they are subject to as large a change in stiffness as possible or to as immediate a transition as possible upon increasing the tension. This is a minimal and simple explanation, but given the frequent emergence of force-dependent transformation of various subcellular structures in addition to that of SFs, the theoretical concept presented here would offer an essential guide to addressing potential common mechanisms governing complicated cellular mechanobiological responses.

Prognosis, immune microenvironment, and personalized treatment prediction in Rho GTPase-activating protein 4-mutant cervical cancer: Computer strategies for precision oncology

AIMS

Cervical cancer with different mutations is associated with specific genomic differences. We developed a new mutation prediction model of the ARHGAP4 gene for cervical cancer.

MAIN METHODS

We conducted a panoramic analysis of CESC mutations based on The Cancer Genome Atlas-Cervical Squamous Cell Carcinoma and Endocervical Adenocarcinoma (TCGA-CESC) database. We made copy number variation analysis and correlation analysis of somatic mutations and tumor mutation load fraction. Then we established a prediction model of ARHGAP4 mutation, screened related genes based on the risk scores, calculated the correlation between the risk score and immune microenvironment, and analyzed drug sensitivity.

KEY FINDINGS

The prediction model of ARHGAP4 mutation based on mRNA expression is closely related to the survival rate of cervical cancer patients and to the effect of immunotherapy. The prediction model is also related to the infiltration of immune cells and human leukocyte antigen family expression in the immune microenvironment. After computational analysis, three drugs (cytarabine, docetaxel, imatinib) were identified as potential agents for the ARHGAP4 mutation high-risk group, and two drugs (erlotinib, methotrexate) were shown to have therapeutic significance for patients in the low-risk group. The expression of ARHGAP4 was higher in cervical cancer tissues. The proliferation ability of HeLa and SiHa cells decreased after ARHGAP4 knockdown.

SIGNIFICANCE

This study provides not only a new approach for the prediction of the response of the cervical cancer patients to targeted drug therapy but also a new strategy for combining risk stratification with precision treatment.

Establishment of a system evaluating the contractile force of electrically stimulated myotubes from wrinkles formed on elastic substrate

Muscle weakness is detrimental not only to quality of life but also life expectancy. However, effective drugs have still not been developed to improve and prevent muscle weakness associated with aging or diseases. One reason for the delay in drug discovery is that no suitable in vitro screening system has been established to test whether drugs improve muscle strength. Here, we used a specific deformable silicone gel substrate to effectively and sensitively evaluate the contractile force generated by myotubes from wrinkles formed on the substrate. Using this system, it was found that the contractile force generated by an atrophic phenotype of myotubes induced by dexamethasone or cancer cell-conditioned medium treatment significantly decreased while that generated by hypertrophic myotubes induced by insulin-like growth factor-1 significantly increased. Notably, it was found that changes in the index related to contractile force can detect atrophic or hypertrophic phenotypes more sensitively than changes in myotube diameter or myosin heavy chain expression, both commonly used to evaluate myotube function. These results suggest that our proposed system will be an effective tool for assessing the contractile force-related state of myotubes, which are available for the development of drugs to prevent and/or treat muscle weakness.

Analysis of ARHGAP4 Expression With Colorectal Cancer Clinical Characteristics and Prognosis

Background

This study aims to analyze the correlation between ARHGAP4 in the expression and clinical characteristics of colorectal cancer (CRC), and the influence of ARHGAP4 expression on the prognosis of CRC, and to evaluate whether ARHGAP4 is a potential prognostic oncotarget for CRC.

Methods

ARHGAP4 was identified using the Gene Expression Omnibus database through weighted gene coexpression network analysis. Using the Gene Expression Profiling Interactive Analysis to perform and analyze the expression and prognosis of ARHGAP4 in CRC. The expression of AGRGAP4 and immune cells was analyzed by the Tumor IMmune Estimation Resource online database. Finally, immunohistochemistry was used to analyze the expression difference and prognosis of ARHGAP4 in CRC and adjacent normal tissues, as well as the relationship between AGRGAP4 expression and clinical features of CRC.

Results

We identified ARHGAP4 that is related to the recurrence of CRC from GSE97781 data. ARHGAP4 has not been reported in CRC. The high expression of ARHGAP4 in select colon adenocarcinoma indicates a poor prognosis by database analysis. In our clinical data results, ARHGAP4 is highly expressed in CRC and lowly expressed in normal tissues adjacent to cancer. Compared with the low-expression group, the high-expression group has a significantly poorer prognosis. In colon cancer, the B-cell, macrophage, neutrophil, and dendritic-cell levels are downregulated after ARHGAP4 gene knockout; the levels of CD8⁺ and CD4⁺ T cells, neutrophils, and dendritic cells are upregulated after the amplification of the ARHGAP4 gene. In addition, ARHGAP4 expression is related to N,M staging and clinical staging.

Conclusion

ARHGAP4 is highly expressed in CRC, and the high expression of ARHGAP4 has a poor prognosis. The expression of ARHGAP4 in CRC is related to the immune cells such as B cells, CD8⁺ and CD4⁺ T cells, macrophages, neutrophils, and dendritic cells. ARHGAP4 is correlated with N,M staging and clinical staging in CRC. ARHGAP4 may be a potential biomarker for the prognosis of CRC.

Inner Nuclear Membrane Protein, SUN1, is Required for Cytoskeletal Force Generation and Focal Adhesion Maturation

The linker of nucleoskeleton and cytoskeleton (LINC) complex is composed of the inner nuclear membrane-spanning SUN proteins and the outer nuclear membrane-spanning nesprin proteins. The LINC complex physically connects the nucleus and plasma membrane via the actin cytoskeleton to perform diverse functions including mechanotransduction from the extracellular environment to the nucleus. Mammalian somatic cells express two principal SUN proteins, namely SUN1 and SUN2. We have previously reported that SUN1, but not SUN2, is essential for directional cell migration; however, the underlying mechanism remains elusive. Because the balance between adhesive force and traction force is critical for cell migration, in the present study, we focused on focal adhesions (FAs) and the actin cytoskeleton. We observed that siRNA-mediated SUN1 depletion did not affect the recruitment of integrin β1, one of the ubiquitously expressed focal adhesion molecules, to the plasma membrane. Consistently, SUN1-depleted cells normally adhered to extracellular matrix proteins, including collagen, fibronectin, laminin, and vitronectin. In contrast, SUN1 depletion reduced the activation of integrin β1. Strikingly, the depletion of SUN1 interfered with the incorporation of vinculin into the focal adhesions, whereas no significant differences in the expression of vinculin were observed between wild-type and SUN1-depleted cells. In addition, SUN1 depletion suppressed the recruitment of zyxin to nascent focal adhesions. These data indicate that SUN1 is involved in the maturation of focal adhesions. Moreover, disruption of the SUN1-containing LINC complex abrogates the actin cytoskeleton and generation of intracellular traction force, despite the presence of SUN2. Thus, a physical link between the nucleus and cytoskeleton through SUN1 is required for the proper organization of actin, thereby suppressing the incorporation of vinculin and zyxin into focal adhesions and the activation of integrin β1, both of which are dependent on traction force. This study provides insights into a previously unappreciated signaling pathway from the nucleus to the cytoskeleton, which is in the opposite direction to the well-known mechanotransduction pathways from the extracellular matrix to the nucleus.

Wrinkle force microscopy: a machine learning based approach to predict cell mechanics from images

Combining experiments with artificial intelligence algorithms, we propose a machine learning based approach called wrinkle force microscopy (WFM) to extract the cellular force distributions from the microscope images. The full process can be divided into three steps. First, we culture the cells on a special substrate allowing to measure both the cellular traction force on the substrate and the corresponding substrate wrinkles simultaneously. The cellular forces are obtained using the traction force microscopy (TFM), at the same time that cell-generated contractile forces wrinkle their underlying substrate. Second, the wrinkle positions are extracted from the microscope images. Third, we train the machine learning system with GAN (generative adversarial network) by using sets of corresponding two images, the traction field and the input images (raw microscope images or extracted wrinkle images), as the training data. The network understands the way to convert the input images of the substrate wrinkles to the traction distribution from the training. After sufficient training, the network is utilized to predict the cellular forces just from the input images. Our system provides a powerful tool to evaluate the cellular forces efficiently because the forces can be predicted just by observing the cells under the microscope, which is much simpler method compared to the TFM experiment. Additionally, the machine learning based approach presented here has the profound potential for being applied to diverse cellular assays for studying mechanobiology of cells.

A deep learning approach, termed wrinkle force microscopy, allows for conducting traction force microscopy by observing bright-field cell images but without using its conventional requirements such as fluorescent microbeads and confocal microscopy.

Forchlorfenuron and Novel Analogs Cause Cytotoxic Effects in Untreated and Cisplatin-Resistant Malignant Mesothelioma-Derived Cells

Malignant mesothelioma (MM) is a currently incurable, aggressive cancer derived from mesothelial cells, most often resulting from asbestos exposure. The current first-line treatment in unresectable MM is cisplatin/pemetrexed, which shows very little long-term effectiveness, necessitating research for novel therapeutic interventions. The existing chemotherapies often act on the cytoskeleton, including actin filaments and microtubules, but recent advances indicate the ‘fourth’ form consisting of the family of septins, representing a novel target. The septin inhibitor forchlorfenuron (FCF) and FCF analogs inhibit MM cell growth in vitro, but at concentrations which are too high for clinical applications. Based on the reported requirement of the chloride group in the 2-position of the pyridine ring of FCF for MM cell growth inhibition and cytotoxicity, we systematically investigated the importance (cell growth-inhibiting capacity) of the halogen atoms fluorine, chlorine, bromine and iodine in the 2- or 3-position of the pyridine ring. The MM cell lines ZL55, MSTO-211H, and SPC212, and—as a control—immortalized Met-5A mesothelial cells were used. The potency of the various halogen substitutions in FCF was mostly correlated with the atom size (covalent radius); the small fluoride analogs showed the least effect, while the largest one (iodide) most strongly decreased the MTT signals, in particular in MM cells derived from epithelioid MM. In the latter, the strongest effects in vitro were exerted by the 2-iodo and, unexpectedly, the 2-trifluoromethyl (2-CF3) FCF analogs, which were further tested in vivo in mice. However, FCF-2-I and, more strongly, FCF-2-CF3 caused rapidly occurring strong symptoms of systemic toxicity at doses lower than those previously obtained with FCF. Thus, we investigated the effectiveness of FCF (and selected analogs) in vitro in MM cells which were first exposed to cisplatin. The slowly appearing population of cisplatin-resistant cells was still susceptible to the growth-inhibiting/cytotoxic effect of FCF and its analogs, indicating that cisplatin and FCF target non-converging pathways in MM cells. Thus, a combination therapy of cisplatin and FCF (analogs) might represent a new avenue for the treatment of repopulating chemo-resistant MM cells in this currently untreatable cancer.

The emerging roles of srGAPs in cancer

GTPase activating proteins (GAPs) were initially considered as the inhibitors of cell signaling pathways because of their nature to activate the intrinsic GTPase activity of the RhoGTPases. But recent studies of dysregulated GAPs in many cancers such as glioblastoma, colorectal cancer, breast cancer, and renal cancer have elucidated the important roles of GAPs in carcinogenesis and GAPs have been shown to perform multiple nonconventional functions in different contexts. We have discussed the recent developments in the roles played by different types of srGAPs (SLIT-ROBO Rho GTPase-activating proteins) in cancer.

ARHGAP4-SEPT2-SEPT9 complex enables both up- and down-modulation of integrin-mediated focal adhesions, cell migration, and invasion

The Rho family of GTPases are inactivated in a cell context–dependent manner by Rho-GTPase-activating proteins (Rho-GAPs), but their signaling mechanisms are poorly understood. Here we demonstrate that ARHGAP4, one of the Rho-GAPs, forms a complex with SEPT2 and SEPT9 via its Rho-GAP domain and SH3 domain to enable both up- and down-modulation of integrin-mediated focal adhesions (FAs). We show that silencing ARHGAP4 and overexpressing its two mutually independent upstream regulators, SEPT2 and SEPT9, all induce reorganization of FAs to newly express Integrin Beta 1 and also enhance both cell migration and invasion. Interestingly, even if these cell migration/invasion–associated phenotypic changes are induced upon perturbations to the complex, it does not necessarily cause enhanced clustering of FAs. Instead, its extent depends on whether the microenvironment contains ligands suitable for the up-regulated Integrin Beta 1. These results provide novel insights into cell migration, invasion, and microenvironment-dependent phenotypic changes regulated by the newly identified complex.

Analysis of senescence-responsive stress fiber proteome reveals reorganization of stress fibers mediated by elongation factor eEF2 in HFF-1 cells

Stress fibers (SFs), which are actomyosin structures, reorganize in response to various cues to maintain cellular homeostasis. Currently, the protein components of SFs are only partially identified, limiting our understanding of their responses. Here we isolate SFs from human fibroblasts HFF-1 to determine with proteomic analysis the whole protein components and how they change with replicative senescence (RS), a state where cells decline in the ability to replicate after repeated divisions. We found that at least 135 proteins are associated with SFs, and 63 of them are up-regulated with RS, by which SFs become larger in size. Among them, we focused on eEF2 (eukaryotic translation elongation factor 2) as it exhibited on RS the most significant increase in abundance. We show that eEF2 is critical to the reorganization and stabilization of SFs in senescent fibroblasts. Our findings provide a novel molecular basis for SFs to be reinforced to resist cellular senescence.

Molecular subversion of Cdc42 signalling in cancer

Cdc42 is a member of the Rho family of small GTPases and a master regulator of the actin cytoskeleton, controlling cell motility, polarity and cell cycle progression. This small G protein and its regulators have been the subject of many years of fruitful investigation and the advent of functional genomics and proteomics has opened up new avenues of exploration including how it functions at specific locations in the cell. This has coincided with the introduction of new structural techniques with the ability to study small GTPases in the context of the membrane. The role of Cdc42 in cancer is well established but the molecular details of its action are still being uncovered. Here we review alterations found to Cdc42 itself and to key components of the signal transduction pathways it controls in cancer. Given the challenges encountered with targeting small G proteins directly therapeutically, it is arguably the regulators of Cdc42 and the effector signalling pathways downstream of the small G protein which will be the most tractable targets for therapeutic intervention. These will require interrogation in order to fully understand the global signalling contribution of Cdc42, unlock the potential for mapping new signalling axes and ultimately produce inhibitors of Cdc42 driven signalling.

Statistical profiling reveals correlations between the cell response to and the primary structure of Rho-GAPs

Rho-GTPase-activating proteins (Rho-GAPs) are essential upstream regulators of the Rho family of GTPases. Currently, it remains unclear if the phenotypic change caused by perturbations to a Rho-GAP is predictable from its amino acid sequence. Here we analyze the relationship between the morphological response of cells to the silencing of Rho-GAPs and their primary structure. For all possible pairs of 57 different Rho-GAPs expressed in MCF10A epithelial cells, the similarity in the Rho-GAP silencing-induced morphological change was quantified and compared to the similarity in the primary structure of the corresponding pairs. We found a distinct correlation between the morphological and sequence similarities in a specific group of RhoA-targeting Rho-GAPs. Thus, the family-wide analysis revealed a common feature shared by the specific Rho-GAPs.

High Throughput strategies Aimed at Closing the GAP in Our Knowledge of Rho GTPase Signaling

Since their discovery, Rho GTPases have emerged as key regulators of cytoskeletal dynamics. In humans, there are 20 Rho GTPases and more than 150 regulators that belong to the RhoGEF, RhoGAP, and RhoGDI families. Throughout development, Rho GTPases choregraph a plethora of cellular processes essential for cellular migration, cell–cell junctions, and cell polarity assembly. Rho GTPases are also significant mediators of cancer cell invasion. Nevertheless, to date only a few molecules from these intricate signaling networks have been studied in depth, which has prevented appreciation for the full scope of Rho GTPases’ biological functions. Given the large complexity involved, system level studies are required to fully grasp the extent of their biological roles and regulation. Recently, several groups have tackled this challenge by using proteomic approaches to map the full repertoire of Rho GTPases and Rho regulators protein interactions. These studies have provided in-depth understanding of Rho regulators specificity and have contributed to expand Rho GTPases’ effector portfolio. Additionally, new roles for understudied family members were unraveled using high throughput screening strategies using cell culture models and mouse embryos. In this review, we highlight theses latest large-scale efforts, and we discuss the emerging opportunities that may lead to the next wave of discoveries.