Tumor-related Alternatively Spliced Rac1b Is Not Regulated by Rho-GDP Dissociation Inhibitors and Exhibits Selective Downstream Signaling

Alternative Splicing at the Crossroad of Inflammatory Bowel Diseases and Colitis-Associated Colon Cancer

The risk of developing colorectal cancer (CRC) is increased in ulcerative colitis patients compared to the general population. This increased risk results from the state of chronic inflammation, a well-known tumour-promoting condition. This review explores the pathologic and molecular characteristics of colitis-associated colon cancer (CAC), emphasizing the distinct features from sporadic CRC. We focus on the key signalling pathways involved in the transition to CAC, highlighting the emerging role of alternative splicing in these processes, namely on how inflammation-induced alternative splicing can significantly contribute to the increased CRC risk observed among UC patients. This review calls for more transcriptomic studies to elucidate the molecular mechanisms through which inflammation-induced alternative splicing drives CAC pathogenesis. A better understanding of these splicing events is crucial as they may reveal novel biomarkers for disease progression and have the potential to target changes in alternative splicing as a therapeutic strategy.

Assessing the Mechanism of Rac1b: An All-Atom Simulation Study of the Alternative Spliced Variant of Rac1 Small Rho GTPase

The Rho GTPase family plays a key role in cell migration, cytoskeletal dynamics, and intracellular signaling. Rac1 and its splice variant Rac1b, characterized by the insertion of an Extraloop, are frequently associated with cancer. These small GTPases switch between an active GTP-bound state and an inactive GDP-bound state, a process that is regulated by specific protein modulators. Among them, the Guanine nucleotide exchange factor (GEF) protein DOCK5 specifically targets Rho GTPases, promoting their activation by facilitating the exchange of GDP for GTP. In this study, we performed cumulative 10-μs-long all-atom molecular dynamics simulations of Rac1 and Rac1b, in isolation and in complex with DOCK5 and ELMO1, to investigate the impact of the Rac1b Extraloop. Our findings reveal that this Extraloop decreases the GDP residence time as compared to Rac1, mimicking the effect of accelerated GDP/GTP exchange induced by DOCK5. Furthermore, both Rac1b Extraloop and the ELMO1 protein stabilize the GTPase/DOCK5 complex, contributing to facilitate GDP dissociation. This shifts the balance between the GPT- and GDP-bound state of Rac1b toward the active GTP-bound state, sending a prooncogenic signal. Besides broadening our understanding of the biological functions of small Rho GTPases, this study provides key information to exploit a previously unexplored therapeutic niche to counter Rac1b-associated cancer.

The role of RAC1 in resistance to targeted therapies in cancer

RAC1 is a small 21 kDa RHO GTPase that plays a pivotal role in regulating actin cytoskeletal dynamics and cell growth. Alterations in the activity of RAC1 are implicated in a range of diseases, including cancer. Increased RAC1 activity, due to overexpression and/or activating mutations, drives transcriptional upregulation, reactive oxygen species production, mesenchymal-to-epithelial transition, membrane ruffling, and uncontrolled cell proliferation, which are hallmarks of an oncogenic phenotype. While RAC1-activating mutations alone do not appear sufficient to transform cells, their combination with other common mutations, such as BRAF, NRAS, or NF1, have been linked to drug resistance and significantly worsen patient prognosis and hinder treatment responses. The precise mechanisms underlying drug resistance, and the regulation of RAC1 splicing remain poorly understood. RAC1 is a challenging therapeutic target due to its ubiquitous presence and essential cellular functions. To date, there are no established standard treatments for cancers that harbour an additional RAC1 mutation or for RAC1-mediated drug resistance. Current experimental strategies aim to target RAC1 localization, its activators (e.g. guanine nucleotide exchange factors) and downstream effectors. Regulating RAC1 expression by targeting epigenetic regulators, and direct targeting of RAC1 itself, may also be possible in the near future.

The Roles of RAC1 and RAC1B in Colorectal Cancer and Their Potential Contribution to Cetuximab Resistance

Colorectal cancer (CRC) is one of the most diagnosed cancers and a leading contributor to cancer-related deaths in the United States. Clinically, standard treatment regimens include surgery, radiation, and chemotherapy; however, there has been increasing development and clinical use of targeted therapies for CRC. Unfortunately, many patients develop resistance to these treatments. Cetuximab, the first targeted therapy approved to treat advanced CRC, is a monoclonal antibody that targets the epidermal growth factor receptor and inhibits downstream pathway activation to restrict tumor cell growth and proliferation. CRC resistance to cetuximab has been well studied, and common resistance mechanisms include constitutive signal transduction through downstream protein mutations and promotion of the epithelial-to-mesenchymal transition. While the most common resistance mechanisms are known, a proportion of patients develop resistance through unknown mechanisms. One protein predicted to contribute to therapy resistance is RAC1, a small GTPase that is involved in cytoskeleton rearrangement, cell migration, motility, and proliferation. RAC1 has also been shown to be overexpressed in CRC. Despite evidence that RAC1 and its alternative splice isoform RAC1B play important roles in CRC and the pathways known to contribute to cetuximab resistance, there is a need to directly study the relationship between RAC1 and RAC1B and cetuximab resistance. This review highlights the recent studies investigating RAC1 and RAC1B in the context of CRC and suggests that these proteins could play a role in resistance to cetuximab.

Oxidative Stress in Breast Cancer: A Biochemical Map of Reactive Oxygen Species Production

This review systematizes information about the metabolic features of breast cancer directly related to oxidative stress. It has been shown those redox changes occur at all levels and affect many regulatory systems in the human body. The features of the biochemical processes occurring in breast cancer are described, ranging from nonspecific, at first glance, and strictly biochemical to hormone-induced reactions, genetic and epigenetic regulation, which allows for a broader and deeper understanding of the principles of oncogenesis, as well as maintaining the viability of cancer cells in the mammary gland. Specific pathways of the activation of oxidative stress have been studied as a response to the overproduction of stress hormones and estrogens, and specific ways to reduce its negative impact have been described. The diversity of participants that trigger redox reactions from different sides is considered more fully: glycolytic activity in breast cancer, and the nature of consumption of amino acids and metals. The role of metals in oxidative stress is discussed in detail. They can act as both co-factors and direct participants in oxidative stress, since they are either a trigger mechanism for lipid peroxidation or capable of activating signaling pathways that affect tumorigenesis. Special attention has been paid to the genetic and epigenetic regulation of breast tumors. A complex cascade of mechanisms of epigenetic regulation is explained, which made it possible to reconsider the existing opinion about the triggers and pathways for launching the oncological process, the survival of cancer cells and their ability to localize.

GTPase splice variants RAC1 and RAC1B display isoform-specific differences in localization, prenylation, and interaction with the chaperone protein SmgGDS

Identifying events that regulate the prenylation and localization of small GTPases will help define new strategies for therapeutic targeting of these proteins in disorders such as cancer, cardiovascular disease, and neurological deficits. Splice variants of the chaperone protein SmgGDS (encoded by RAP1GDS1) are known to regulate prenylation and trafficking of small GTPases. The SmgGDS-607 splice variant regulates prenylation by binding pre-prenylated small GTPases, but the effects of SmgGDS binding to the small GTPase RAC1 versus the splice variant RAC1B are not well defined. Here we report unexpected differences in the prenylation and localization of RAC1 and RAC1B, and their binding to SmgGDS. Compared to RAC1, RAC1B more stably associates with SmgGDS-607, is less prenylated, and accumulates more in the nucleus. We show that the small GTPase DIRAS1 inhibits binding of RAC1 and RAC1B to SmgGDS and reduces their prenylation. These results suggest that prenylation of RAC1 and RAC1B is facilitated by binding to SmgGDS-607, but the greater retention of RAC1B by SmgGDS-607 slows RAC1B prenylation. We show that inhibiting RAC1 prenylation by mutating the CAAX motif promotes RAC1 nuclear accumulation, suggesting that differences in prenylation contribute to the different nuclear localization of RAC1 versus RAC1B. Finally, we demonstrate RAC1 and RAC1B that cannot be prenylated bind GTP in cells, indicating that prenylation is not a prerequisite for activation. We report differential expression of RAC1 and RAC1B transcripts in tissues, consistent with these two splice variants having unique functions that might arise in part from their differences in prenylation and localization.

The role of ROS in tumour development and progression

Eukaryotic cells have developed complex systems to regulate the production and response to reactive oxygen species (ROS). Different ROS control diverse aspects of cell behaviour from signalling to death, and deregulation of ROS production and ROS limitation pathways are common features of cancer cells. ROS also function to modulate the tumour environment, affecting the various stromal cells that provide metabolic support, a blood supply and immune responses to the tumour. Although it is clear that ROS play important roles during tumorigenesis, it has been difficult to reliably predict the effect of ROS modulating therapies. We now understand that the responses to ROS are highly complex and dependent on multiple factors, including the types, levels, localization and persistence of ROS, as well as the origin, environment and stage of the tumours themselves. This increasing understanding of the complexity of ROS in malignancies will be key to unlocking the potential of ROS-targeting therapies for cancer treatment.

The Role of Fast-Cycling Atypical RHO GTPases in Cancer

Simple Summary

For many years, cancer-associated mutations in RHO GTPases were not identified and observations suggesting roles for RHO GTPases in cancer were sparse. Instead, RHO GTPases were considered primarily to regulate cell morphology and cell migration, processes that rely on the dynamic behavior of the cytoskeleton. This notion is in contrast to the RAS proteins, which are famous oncogenes and found to be mutated at high incidence in human cancers. Recent advancements in the tools for large-scale genome analysis have resulted in a paradigm shift and RHO GTPases are today found altered in many cancer types. This review article deals with the recent views on the roles of RHO GTPases in cancer, with a focus on the so-called fast-cycling RHO GTPases.

Abstract

The RHO GTPases comprise a subfamily within the RAS superfamily of small GTP-hydrolyzing enzymes and have primarily been ascribed roles in regulation of cytoskeletal dynamics in eukaryotic cells. An oncogenic role for the RHO GTPases has been disregarded, as no activating point mutations were found for genes encoding RHO GTPases. Instead, dysregulated expression of RHO GTPases and their regulators have been identified in cancer, often in the context of increased tumor cell migration and invasion. In the new landscape of cancer genomics, activating point mutations in members of the RHO GTPases have been identified, in particular in RAC1, RHOA, and CDC42, which has suggested that RHO GTPases can indeed serve as oncogenes in certain cancer types. This review describes the current knowledge of these cancer-associated mutant RHO GTPases, with a focus on how their altered kinetics can contribute to cancer progression.

Pro-Inflammatory Cytokines Trigger the Overexpression of Tumour-Related Splice Variant RAC1B in Polarized Colorectal Cells

An inflammatory microenvironment is a tumour-promoting condition that provides survival signals to which cancer cells respond with gene expression changes. One example is the alternative splicing variant Rat Sarcoma Viral Oncogene Homolog (Ras)-Related C3 Botulinum Toxin Substrate 1 (RAC1)B, which we previously identified in a subset of V-Raf Murine Sarcoma Viral Oncogene Homolog B (BRAF)-mutated colorectal tumours. RAC1B was also increased in samples from inflammatory bowel disease patients or in an acute colitis mouse model. Here, we used an epithelial-like layer of polarized Caco-2 or T84 colorectal cancer (CRC) cells in co-culture with fibroblasts, monocytes or macrophages and analysed the effect on RAC1B expression in the CRC cells by RT-PCR, Western blot and confocal fluorescence microscopy. We found that the presence of cancer-associated fibroblasts and M1 macrophages induced the most significant increase in RAC1B levels in the polarized CRC cells, accompanied by a progressive loss of epithelial organization. Under these conditions, we identified interleukin (IL)-6 as the main trigger for the increase in RAC1B levels, associated with Signal Transducer and Activator of Transcription (STAT)3 activation. IL-6 neutralization by a specific antibody abrogated both RAC1B overexpression and STAT3 phosphorylation in polarized CRC cells. Our data identify that pro-inflammatory extracellular signals from stromal cells can trigger the overexpression of tumour-related RAC1B in polarized CRC cells. The results will help to understand the tumour-promoting effect of inflammation and identify novel therapeutic strategies.

MAPK Inhibition Requires Active RAC1 Signaling to Effectively Improve Iodide Uptake by Thyroid Follicular Cells

Simple Summary

The Sodium/Iodide Simulator (NIS) is responsible for the uptake of iodide in the thyroid follicular cells. NIS is present in most differentiated thyroid carcinomas (DTC), allowing radioactive iodine (RAI) to be used to destroy malignant cells. However, a significant proportion of DTCs stop picking up iodide and become resistant to RAI therapy. This is mainly due to the symporter no longer being produced or not being placed correctly at the cell’s membrane. This has been associated with mechanisms linked to malignant transformation, namely the overactivation of the so-called MAPK pathway. Thus, several drugs have been developed to inhibit this pathway, attempting to increase NIS levels and iodide uptake. However, MAPK inhibitors have had only partial success in restoring NIS expression. We found that the activity of another protein, the small GTPase RAC1, has an important role in this process, determining the outcome of MAPK inhibitors. Thus, our findings open new opportunities to find effective therapeutic alternatives for DTC resistant to RAI.

Abstract

The Sodium/Iodide Symporter (NIS) is responsible for the active transport of iodide into thyroid follicular cells. Differentiated thyroid carcinomas (DTCs) usually preserve the functional expression of NIS, allowing the use of radioactive iodine (RAI) as the treatment of choice for metastatic disease. However, a significant proportion of patients with advanced forms of TC become refractory to RAI therapy and no effective therapeutic alternatives are available. Impaired iodide uptake is mainly caused by the defective functional expression of NIS, and this has been associated with several pathways linked to malignant transformation. MAPK signaling has emerged as one of the main pathways implicated in thyroid tumorigenesis, and its overactivation has been associated with the downregulation of NIS expression. Thus, several strategies have been developed to target the MAPK pathway attempting to increase iodide uptake in refractory DTC. However, MAPK inhibitors have had only partial success in restoring NIS expression and, in most cases, it remained insufficient to allow effective treatment with RAI. In a previous work, we have shown that the activity of the small GTPase RAC1 has a positive impact on TSH-induced NIS expression and iodide uptake in thyroid cells. RAC1 is a downstream effector of NRAS, but not of BRAF. Therefore, we hypothesized that the positive regulation induced by RAC1 on NIS could be a relevant signaling cue in the mechanism underlying the differential response to MEK inhibitors, observed between NRAS- and BRAF-mutant tumors. In the present study, we found that the recovery of NIS expression induced through MAPK pathway inhibition can be enhanced by potentiating RAC1 activity in thyroid cell systems. The negative impact on NIS expression induced by the MAPK-activating alterations, NRAS Q61R and BRAF V600E, was partially reversed by the presence of the MEK 1/2 inhibitors AZD6244 and CH5126766. Notably, the inhibition of RAC1 signaling partially blocked the positive impact of MEK inhibition on NIS expression in NRAS Q61R cells. Conversely, the presence of active RAC1 considerably improved the rescue of NIS expression in BRAF V600E thyroid cells treated with MEK inhibitors. Overall, our data support an important role for RAC1 signaling in enhancing MAPK inhibition in the context of RAI therapy in DTC, opening new opportunities for therapeutic intervention.

Analysis of NIS Plasma Membrane Interactors Discloses Key Regulation by a SRC/RAC1/PAK1/PIP5K/EZRIN Pathway with Potential Implications for Radioiodine Re-Sensitization Therapy in Thyroid Cancer

The functional expression of the sodium-iodide symporter (NIS) at the membrane of differentiated thyroid cancer (DTC) cells is the cornerstone for the use of radioiodine (RAI) therapy in these malignancies. However, NIS gene expression is frequently downregulated in malignant thyroid tissue, and 30% to 50% of metastatic DTCs become refractory to RAI treatment, which dramatically decreases patient survival. Several strategies have been attempted to increase the NIS mRNA levels in refractory DTC cells, so as to re-sensitize refractory tumors to RAI. However, there are many RAI-refractory DTCs in which the NIS mRNA and protein levels are relatively abundant but only reduced levels of iodide uptake are detected, suggesting a posttranslational failure in the delivery of NIS to the plasma membrane (PM), or an impaired residency at the PM. Because little is known about the molecules and pathways regulating NIS delivery to, and residency at, the PM of thyroid cells, we here employed an intact-cell labeling/immunoprecipitation methodology to selectively purify NIS-containing macromolecular complexes from the PM. Using mass spectrometry, we characterized and compared the composition of NIS PM complexes to that of NIS complexes isolated from whole cell (WC) lysates. Applying gene ontology analysis to the obtained MS data, we found that while both the PM-NIS and WC-NIS datasets had in common a considerable number of proteins involved in vesicle transport and protein trafficking, the NIS PM complexes were particularly enriched in proteins associated with the regulation of the actin cytoskeleton. Through a systematic validation of the detected interactions by co-immunoprecipitation and Western blot, followed by the biochemical and functional characterization of the contribution of each interactor to NIS PM residency and iodide uptake, we were able to identify a pathway by which the PM localization and function of NIS depends on its binding to SRC kinase, which leads to the recruitment and activation of the small GTPase RAC1. RAC1 signals through PAK1 and PIP5K to promote ARP2/3-mediated actin polymerization, and the recruitment and binding of the actin anchoring protein EZRIN to NIS, promoting its residency and function at the PM of normal and TC cells. Besides providing novel insights into the regulation of NIS localization and function at the PM of TC cells, our results open new venues for therapeutic intervention in TC, namely the possibility of modulating abnormal SRC signaling in refractory TC from a proliferative/invasive effect to the re-sensitization of these tumors to RAI therapy by inducing NIS retention at the PM.

Different signaling and functionality of Rac1 and Rac1b in the progression of lung adenocarcinoma

Rac1 is a ubiquitously expressed Rho GTPase and an important regulator of the actin cytoskeleton. Its splice variant Rac1b exhibits a 19-amino acid (aa) in-frame insertion and is predominantly active. Both proteins were described in tumorigenesis or metastasis. We investigated the contribution of Rac1 and Rac1b to tumor progression of human non-small-cell lung adenocarcinoma (NSCLA). Rac1 protein was present in 8/8 NSCLA cell lines analyzed, whereas Rac1b was expressed in only 6/8. In wound-healing assays, enhanced green fluorescence protein (EGFP)-Rac1 slightly decreased cell migration, whereas proliferation was increased in both, Rac1- and Rac1b-expressing cells. In the in vivo chorioallantoic invasion model, EGFP-Rac1-expressing cells formed more invasive tumors compared to EGFP-Rac1b. This increased invasiveness correlated with enhanced phosphorylation of p38α, AKT and glycogen synthase kinase 3β (GSK3β), and activation of serum response- and Smad-dependent gene promoters by Rac1. In contrast, Rac1b solely activated the mitogen-activated protein kinase (MAPK) JNK2, together with TCF/LEF1- and nuclear factor kappa B (NFκB)-responsive gene reporters. Rac1b, as Rac1, phosphorylated p38α, AKT and GSK3β. Knockdown of the splicing factor epithelial splicing regulatory protein 1 (ESRP1), which mediates out-splicing of exon 3b from Rac1 pre-messenger RNA, resulted in increased Rac1b messenger RNA (mRNA) and suppression of the epithelial-mesenchymal transition (EMT)-associated transcription factor ZEB1. Our data demonstrate different signaling and functional activities of Rac1 and Rac1b and an important role for Rac1 in lung cancer metastasis.

RAC1B modulates intestinal tumourigenesis via modulation of WNT and EGFR signalling pathways

Current therapeutic options for treating colorectal cancer have little clinical efficacy and acquired resistance during treatment is common, even following patient stratification. Understanding the mechanisms that promote therapy resistance may lead to the development of novel therapeutic options that complement existing treatments and improve patient outcome. Here, we identify RAC1B as an important mediator of colorectal tumourigenesis and a potential target for enhancing the efficacy of EGFR inhibitor treatment. We find that high RAC1B expression in human colorectal cancer is associated with aggressive disease and poor prognosis and deletion of Rac1b in a mouse colorectal cancer model reduces tumourigenesis. We demonstrate that RAC1B interacts with, and is required for efficient activation of the EGFR signalling pathway. Moreover, RAC1B inhibition sensitises cetuximab resistant human tumour organoids to the effects of EGFR inhibition, outlining a potential therapeutic target for improving the clinical efficacy of EGFR inhibitors in colorectal cancer.

Aberrant Rac pathway signalling in glioblastoma

Glioblastoma is an aggressive and incurable form of brain cancer. Both mutation analysis in human glioblastoma and mouse modelling studies have shown that aberrant activation of the PI 3-kinase pathway is a central driver of glioblastoma malignancy. The small GTPase Rac is activated downstream of this pathway, mediating a subset of the effects of aberrant PI 3-kinase pathway activation. Here I discuss the current state of our knowledge on Rac activation mechanisms in glioblastoma. Current knowledge on roles for specific PI 3-kinase pathway responsive Rac guanine nucleotide exchange factors in glioblastoma is reviewed. Rac is best known for its role in promoting cell motility and invasion, but there is also evidence for roles in multiple other cellular processes with cancer relevance, including proliferation, differentiation, apoptosis, DNA damage responses, metabolism, angiogenesis and immunosuppression. I review what is known about the role of Rac in these processes in glioblastoma. Finally, I assess possible strategies to inhibit this pathway in glioblastoma through either direct inhibition of Rac or inhibition of upstream activators or downstream mediators of Rac signalling.

The Ratio of RAC1B to RAC1 Expression in Breast Cancer Cell Lines as a Determinant of Epithelial/Mesenchymal Differentiation and Migratory Potential

Breast cancer (BC) is a heterogenous disease encompassing tumors with different histomorphological phenotypes and transcriptionally defined subtypes. However, the non-mutational/epigenetic alterations that are associated with or causally involved in phenotype diversity or conversion remain to be elucidated. Data from the pancreatic cancer model have shown that the small GTPase RAC1 and its alternatively spliced isoform, RAC1B, antagonistically control epithelial–mesenchymal transition and cell motility induced by transforming growth factor β. Using a battery of established BC cell lines with either a well-differentiated epithelial or poorly differentiated mesenchymal phenotype, we observed subtype-specific protein expression of RAC1B and RAC1. While epithelial BC lines were RAC1B^high and RAC1^low, mesenchymal lines exhibited the reverse expression pattern. High RAC1B and/or low RAC1 abundance also correlated closely with a poor invasion potential, and vice versa, as revealed by measuring random cell migration (chemokinesis), the preferred mode of cellular movement in cells that have undergone mesenchymal transdifferentiation. We propose that a high RAC1B:RAC1 ratio in BC cells is predictive of an epithelial phenotype, while low RAC1B along with high RAC1 is a distinguishing feature of the mesenchymal state. The combined quantitative assessment of RAC1B and RAC1 in tumor biopsies of BC patients may represent a novel diagnostic tool for probing molecular subtype and eventually predict malignant potential of breast tumors.

Ibuprofen disrupts a WNK1/GSK3β/SRPK1 protein complex required for expression of tumor-related splicing variant RAC1B in colorectal cells

A major risk factor promoting tumor development is chronic inflammation and the use of nonsteroidal anti-inflammatory drugs (NSAID), including ibuprofen, can decrease the risk of developing various types of cancer, including colorectal cancer (CRC). Although the molecular mechanism behind the antitumor properties of NSAIDs has been largely attributed to inhibition of cyclooxygenases (COXs), several studies have shown that the chemopreventive properties of ibuprofen also involve multiple COX-independent effects. One example is its ability to inhibit the alternative splicing event generating RAC1B, which is overexpressed in a specific subset of BRAF-mutated colorectal tumors and sustains cell survival. Here we describe the mechanism by which ibuprofen prevents RAC1B alternative splicing in a BRAF mutant CRC cell line: it leads to decreased translocation of SRPK1 and SRSF1 to the nucleus and is regulated by a WNK1/GSK3β/SRPK1 protein kinase complex. Surprisingly, we demonstrate that ibuprofen does not inhibit the activity of any of the involved kinases but rather promotes disassembly of this regulatory complex, exposing GSK3β serine 9 to inhibitory phosphorylation, namely by AKT, which results in nuclear exclusion of SRPK1 and SRSF1 hypophosphorylation. The data shed new light on the biochemical mechanisms behind ibuprofen’s action on alternative spliced RAC1B and may support its use in personalized approaches to CRC therapy or chemoprevention regimens.

The Small GTPase RAC1B: A Potent Negative Regulator of-and Useful Tool to Study-TGFβ Signaling

RAC1 and its alternatively spliced isoform, RAC1B, are members of the Rho family of GTPases. Both isoforms are involved in the regulation of actin cytoskeleton remodeling, cell motility, cell proliferation, and epithelial-mesenchymal transition (EMT). Compared to RAC1, RAC1B exhibits a number of distinctive features with respect to tissue distribution, downstream signaling and a role in disease conditions like inflammation and cancer. The subcellular locations and interaction partners of RAC1 and RAC1B vary depending on their activation state, which makes RAC1 and RAC1B ideal candidates to establish cross-talk with cancer-associated signaling pathways-for instance, interactions with signaling by transforming growth factor β (TGFβ), a known tumor promoter. Although RAC1 has been found to promote TGFβ-driven tumor progression, recent observations in pancreatic carcinoma cells surprisingly revealed that RAC1B confers anti-oncogenic properties, i.e., through inhibiting TGFβ-induced EMT. Since then, an unexpected array of mechanisms through which RAC1B cross-talks with TGFβ signaling has been demonstrated. However, rather than being uniformly inhibitory, RAC1B interacts with TGFβ signaling in a way that results in the selective blockade of tumor-promoting pathways, while concomitantly allowing tumor-suppressive pathways to proceed. In this review article, we are going to discuss the specific interactions between RAC1B and TGFβ signaling, which occur at multiple levels and include various components such as ligands, receptors, cytosolic mediators, transcription factors, and extracellular inhibitors of TGFβ ligands.

Rac1 Signaling: From Intestinal Homeostasis to Colorectal Cancer Metastasis

The small GTPase Rac1 has been implicated in a variety of dynamic cell biological processes, including cell proliferation, cell survival, cell-cell contacts, epithelial mesenchymal transition (EMT), cell motility, and invasiveness. These processes are orchestrated through the fine tuning of Rac1 activity by upstream cell surface receptors and effectors that regulate the cycling Rac1-GDP (off state)/Rac1-GTP (on state), but also through the tuning of Rac1 accumulation, activity, and subcellular localization by post translational modifications or recruitment into molecular scaffolds. Another level of regulation involves Rac1 transcripts stability and splicing. Downstream, Rac1 initiates a series of signaling networks, including regulatory complex of actin cytoskeleton remodeling, activation of protein kinases (PAKs, MAPKs) and transcription factors (NFkB, Wnt/β-catenin/TCF, STAT3, Snail), production of reactive oxygen species (NADPH oxidase holoenzymes, mitochondrial ROS). Thus, this GTPase, its regulators, and effector systems might be involved at different steps of the neoplastic progression from dysplasia to the metastatic cascade. After briefly placing Rac1 and its effector systems in the more general context of intestinal homeostasis and in wound healing after intestinal injury, the present review mainly focuses on the several levels of Rac1 signaling pathway dysregulation in colorectal carcinogenesis, their biological significance, and their clinical impact.

TNFα-mediated activation of NF-κB downregulates sodium-iodide symporter expression in thyroid cells

The sodium-iodide symporter (NIS) mediates transport of iodide across the basolateral membrane of thyroid cells. NIS expression in thyroid cancer (TC) cells allows the use of radioactive iodine (RAI) as a diagnostic and therapeutic tool, being RAI therapy the systemic treatment of choice for metastatic disease. Still, a significant proportion of patients with advanced TC lose the ability to respond to RAI therapy and no effective alternative therapies are available. Defective NIS expression is the main reason for impaired iodide uptake in TC and NIS downregulation has been associated with several pathways linked to malignant transformation. NF-κB signaling is one of the pathways associated with TC. Interestingly, NIS expression can be negatively regulated by TNF-α, a bona fide activator of NF-κB with a central role in thyroid autoimmunity. This prompted us to clarify NF-kB’s role in this process. We confirmed that TNF-α leads to downregulation of TSH-induced NIS expression in non-neoplastic thyroid follicular cell-derived models. Notably, a similar effect was observed when NF-κB activation was triggered independently of ligand-receptor specificity, using phorbol-myristate-acetate (PMA). TNF-α and PMA downregulation of NIS expression was reverted when NF-κB-dependent transcription was blocked, demonstrating the requirement for NF-kB activity. Additionally, TNF-α and PMA were shown to have a negative impact on TSH-induced iodide uptake, consistent with the observed transcriptional downregulation of NIS. Our data support the involvement of NF-κB-directed transcription in the modulation of NIS expression, where up- or down-regulation of NIS depends on the combined output to NF-κB of several converging pathways. A better understanding of the mechanisms underlying NIS expression in the context of normal thyroid physiology may guide the development of pharmacological strategies to increase the efficiency of iodide uptake. Such strategies would be extremely useful in improving the response to RAI therapy in refractory-TC.

Splicing Busts a Move: Isoform Switching Regulates Migration

Cell migration is essential for normal development, neural patterning, pathogen eradication, and cancer metastasis. Pre-mRNA processing events such as alternative splicing and alternative polyadenylation result in greater transcript and protein diversity as well as function and activity. A critical role for alternative pre-mRNA processing in cell migration has emerged in axon outgrowth during neuronal development, immune cell migration, and cancer metastasis. These findings suggest that migratory signals result in expression changes of post-translational modifications of splicing or polyadenylation factors, leading to splicing events that generate promigratory isoforms. We summarize this recent progress and suggest emerging technologies that may facilitate a deeper understanding of the role of alternative splicing and polyadenylation in cell migration.

Inflammatory Microenvironment Modulation of Alternative Splicing in Cancer: A Way to Adapt

The relationship between inflammation and cancer has been long recognized by the medical and scientific community. In the last decades, it has returned to the forefront of clinical oncology since a wealth of knowledge has been gathered about the cells, cytokines and physiological processes that are central to both inflammation and cancer. It is now robustly established that chronic inflammation can induce certain cancers but also that solid tumors, in turn, can initiate and perpetuate local inflammatory processes that foster tumor growth and dissemination. Inflammation is the hallmark of the innate immune response to tissue damage or infection, but also mediates the activation, expansion and recruitment to the tissues of cells and antibodies of the adaptive immune system. The functional integration of both components of the immune response is crucial to identify and subdue tumor development, progression and dissemination. When this tight control goes awry, altered cells can avoid the immune surveillance and even subvert the innate immunity to promote their full oncogenic transformation. In this chapter, we make a general overview of the most recent data linking the inflammatory process to cancer. We start with the overall inflammatory cues and processes that influence the relationship between tumor and the microenvironment that surrounds it and follow the ever-increasing complexity of processes that end up producing subtle changes in the splicing of certain genes to ascertain survival advantage to cancer cells.

Splicing regulatory factors in breast cancer hallmarks and disease progression

By regulating transcript isoform expression levels, alternative splicing provides an additional layer of protein control. Recent studies show evidence that cancer cells use different splicing events to fulfill their requirements in order to develop, progress and metastasize. However, there has been less attention for the role of the complex catalyzing the complicated multistep splicing reaction: the spliceosome. The spliceosome consists of multiple sub-complexes in total comprising 244 proteins or splice factors and 5 associated RNA molecules. Here we discuss the role of splice factors in the oncogenic processes tumors cells need to fulfill their oncogenic properties (the so-called the hallmarks of cancer). Despite the fact that splice factors have been investigated only recently, they seem to play a prominent role in already five hallmarks of cancer: angiogenesis, resisting cell death, sustaining proliferation, deregulating cellular energetics and invasion and metastasis formation by affecting major signaling pathways such as epithelial-to-mesenchymal transition, the Warburg effect, DNA damage response and hormone receptor dependent proliferation. Moreover, we could relate expression of representative genes of four other hallmarks (enabling replicative mortality, genomic instability, avoiding immune destruction and evading growth suppression) to splice factor levels in human breast cancer tumors, suggesting that also these hallmarks could be regulated by splice factors. Since many splice factors are involved in multiple hallmarks of cancer, inhibiting splice factors might provide a new layer of oncogenic control and a powerful method to combat breast cancer progression.

The Intrinsic GDP/GTP Exchange Activities of Cdc42 and Rac1 Are Critical Determinants for Their Specific Effects on Mobilization of the Actin Filament System

The Rho GTPases comprise a subfamily of the Ras superfamily of small GTPases. Their importance in regulation of cell morphology and cell migration is well characterized. According to the prevailing paradigm, Cdc42 regulates the formation of filopodia, Rac1 regulates the formation of lamellipodia, and RhoA triggers the assembly of focal adhesions. However, this scheme is clearly an oversimplification, as the Rho subfamily encompasses 20 members with diverse effects on a number of vital cellular processes, including cytoskeletal dynamics and cell proliferation, migration, and invasion. This article highlights the importance of the catalytic activities of the classical Rho GTPases Cdc42 and Rac1, in terms of their specific effects on the dynamic reorganization of the actin filament system. GTPase-deficient mutants of Cdc42 and Rac1 trigger the formation of broad lamellipodia and stress fibers, and fast-cycling mutations trigger filopodia formation and stress fiber dissolution. The filopodia response requires the involvement of the formin family of actin nucleation promotors. In contrast, the formation of broad lamellipodia induced by GTPase-deficient Cdc42 and Rac1 is mediated through Arp2/3-dependent actin nucleation.

A New Twist to Ibuprofen: Alternative Action in Alternative Splicing

Ibuprofen is a nonsteroidal anti-inflammatory drug (NSAID) and is a widely used medication. One indication of NSAID use is long-term chemoprevention to decrease the risk of developing various types of cancer, in particular colorectal cancer. The molecular mechanism behind the antitumour properties of NSAID has been largely attributed to inhibition of the enzyme cyclooxygenase. In this review article, the authors highlight that additional mechanisms of NSAID, especially ibuprofen, action exist that are related to cell signalling and the modulation of gene expression, including alternative splicing. For example, the authors describe how ibuprofen inhibits expression of the tumour-related splicing variant RAC1b, which is overexpressed in a specific subset of colorectal tumours. The mechanism involves changes in the phosphorylation of splicing factors that regulate this alternative splicing event. According to recent studies, ibuprofen interferes with signal transmission via protein kinases, a process which is frequently altered in cancer cells.

RAC1b Overexpression Confers Resistance to Chemotherapy Treatment in Colorectal Cancer

Resistance to chemotherapy represents a major limitation in the treatment of colorectal cancer. Novel strategies to circumvent resistance are critical to prolonging patient survival. Rac1b, a constitutively activated isoform of the small GTPase Rac1, is upregulated with disease progression and promotes cell proliferation and inhibits apoptosis by activation of NF-κB signaling. Here, we show that Rac1b overexpression correlates with cancer stage and confirmed Rac1b expression is associated with increased growth through enhancing NF-κB activity. Rac1b knockdown reduced cellular proliferation and reduced NF-κB activity. Surprisingly, Rac1b expression and NF-κB activity were upregulated in cells treated with chemotherapeutics, suggesting that Rac1b facilitates chemo-resistance through activation of NF-κB signaling. Knockdown of Rac1b or Rac inhibition increases the sensitivity of the cells to oxaliplatin. When used in combination, inhibition of Rac prevents the increase in NF-κB activity associated with chemotherapy treatment and increases the sensitivity of the cells to oxaliplatin. Although Rac inhibition or oxaliplatin treatment alone reduces the growth of colorectal cancer in vivo, combination therapy results in improved outcomes compared with single agents alone. We provide the first evidence that Rac1b expression confers resistance to chemotherapy in colorectal cancer. Additionally, we show that the use of a Rac inhibitor prevents chemoresistance by blocking activation of chemotherapy induced NF-κB signaling, providing a novel strategy to overcome resistance to chemotherapy in colorectal cancer.

RAC1B: A Rho GTPase with Versatile Functions in Malignant Transformation and Tumor Progression

RAC1B is an alternatively spliced isoform of the monomeric GTPase RAC1. It differs from RAC1 by a 19 amino acid in frame insertion, termed exon 3b, resulting in an accelerated GDP/GTP-exchange and an impaired GTP-hydrolysis. Although RAC1B has been ascribed several protumorigenic functions such as cell cycle progression and apoptosis resistance, its role in malignant transformation, and other functions driving tumor progression like epithelial-mesenchymal transition, migration/invasion and metastasis are less clear. Insertion of exon 3b endows RAC1B with specific biochemical properties that, when compared to RAC1, encompass both loss-of-functions and gain-of-functions with respect to the type of upstream activators, downstream targets, and binding partners. In its extreme, this may result in RAC1B and RAC1 acting in an antagonistic fashion in regulating a specific cellular response with RAC1B behaving as an endogenous inhibitor of RAC1. In this review, we strive to provide the reader with a comprehensive overview, rather than critical discussions, on various aspects of RAC1B biology in eukaryotic cells.

Networks of mRNA Processing and Alternative Splicing Regulation in Health and Disease

mRNA processing events introduce an intricate layer of complexity into gene expression processes, supporting a tremendous level of diversification of the genome's coding and regulatory potential, particularly in vertebrate species. The recent development of massive parallel sequencing methods and their adaptation to the identification and quantification of different RNA species and the dynamics of mRNA metabolism and processing has generated an unprecedented view over the regulatory networks that are established at this level, which contribute to sustain developmental, tissue specific or disease specific gene expression programs. In this chapter, we provide an overview of the recent evolution of transcriptome profiling methods and the surprising insights that have emerged in recent years regarding distinct mRNA processing events - from the 5' end to the 3' end of the molecule.

Blockade of ARHGAP11A reverses malignant progress via inactivating Rac1B in hepatocellular carcinoma

Background

The molecular signaling events involving in high malignancy and poor prognosis of hepatocellular carcinoma (HCC) are extremely complicated. Blockade of currently known targets has not yet led to successful clinical outcome. More understanding about the regulatory mechanisms in HCC is necessary for developing new effective therapeutic strategies for HCC patients.

Methods

The expression of Rho GTPase-activating protein 11A (ARHGAP11A) was examined in human normal liver and HCC tissues. The correlations between ARHGAP11A expression and clinicopathological stage or prognosis in HCC patients were analyzed. ARHGAP11A was downregulated to determine its role in the proliferation, invasion, migration, epithelial-to-mesenchymal transition (EMT) development, and regulatory signaling of HCC cells in vitro and in vivo.

Results

ARHGAP11A exhibited high expression in HCC, and was significantly correlated with clinicopathological stage and prognosis in HCC patients. Moreover, ARHGAP11A facilitated Hep3B and MHCC97-H cell proliferation, invasion, migration and EMT development in vitro. ARHGAP11A knockdown significantly inhibited the in vivo growth and metastasis of HCC cells. Furthermore, ARHGAP11A directly interacted with Rac1B independent of Rho GTPase- activating activity. Rac1B blockade effectively interrupted ARHGAP11A-elicited HCC malignant phenotype. Meanwhile, upregulation of Rac1B reversed ARHGAP11A knockdown mediated mesenchymal-to-epithelial transition (MET) development in HCC cells.

Conclusion

ARHGAP11A facilitates malignant progression in HCC patients via ARHGAP11A-Rac1B interaction. The ARHGAP11A/Rac1B signaling could be a potential therapeutic target in the clinical treatment of HCC.

Electronic supplementary material

The online version of this article (10.1186/s12964-018-0312-4) contains supplementary material, which is available to authorized users.

Activated Rho GTPases in Cancer-The Beginning of a New Paradigm

Involvement of Rho GTPases in cancer has been a matter of debate since the identification of the first members of this branch of the Ras superfamily of small GTPases. The Rho GTPases were ascribed important roles in the cell, although these were restricted to regulation of cytoskeletal dynamics, cell morphogenesis, and cell locomotion, with initially no clear indications of direct involvement in cancer progression. This paradigm has been challenged by numerous observations that Rho-regulated pathways are often dysregulated in cancers. More recently, identification of point mutants in the Rho GTPases Rac1, RhoA, and Cdc42 in human tumors has finally given rise to a new paradigm, and we can now state with confidence that Rho GTPases serve as oncogenes in several human cancers. This article provides an exposé of current knowledge of the roles of activated Rho GTPases in cancers.

The Rac1 splice form Rac1b favors mouse colonic mucosa regeneration and contributes to intestinal cancer progression

We previously have identified the ectopic expression of Rac1b, an activated and novel splice variant of Rac1, in a subset of human colorectal adenocarcinomas, as well as in inflammatory bowel diseases and in colitis mouse model. Rac1b overexpression has been further evidenced in breast, pancreatic, thyroid, ovarian, and lung cancers. In this context, the aim of our study was to investigate the physiopathological implications of Rac1b in intestinal inflammation and carcinogenesis in vivo. The ectopic expression of Rac1b was induced in mouse intestinal epithelial cells after crossing Rosa26-LSL-Rac1b and villin-Cre mice. These animals were let to age or were challenged with dextran sulfate sodium (DSS) to induce experimental colitis, or either received azoxymethane (AOM)/DSS treatment, or were bred with Apc^Min/+ or Il10^-/- mice to trigger intestinal tumors. Rac1b ectopic expression increased the intestinal epithelial cell proliferation and migration, enhanced the production of reactive oxygen species, and promoted the Paneth cell lineage. Although Rac1b overexpression alone was not sufficient to drive intestinal neoplasia, it enhanced Apc-dependent intestinal tumorigenesis. In the context of Il10 knockout, the Rac1b transgene strengthened colonic inflammation due to induced intestinal mucosa permeability and promoted cecum and proximal colon carcinogenesis. In contrast, Rac1b alleviated carcinogen/acute inflammation-associated colon carcinogenesis (AOM/DSS). This resulted at least partly from the early mucosal repair after resolution of inflammation. Our data highlight the critical role of Rac1b in driving wound-healing after resolution of intestinal inflammation, and in cooperating with Wnt pathway dysregulation and chronic inflammation to promote intestinal carcinogenesis.

Alternative-splicing defects in cancer: Splicing regulators and their downstream targets, guiding the way to novel cancer therapeutics

Defects in alternative splicing are frequently found in human tumors and result either from mutations in splicing-regulatory elements of specific cancer genes or from changes in the regulatory splicing machinery. RNA splicing regulators have emerged as a new class of oncoproteins and tumor suppressors, and contribute to disease progression by modulating RNA isoforms involved in the hallmark cancer pathways. Thus, dysregulation of alternative RNA splicing is fundamental to cancer and provides a potentially rich source of novel therapeutic targets. Here, we review the alterations in splicing regulatory factors detected in human tumors, as well as the resulting alternatively spliced isoforms that impact cancer hallmarks, and discuss how they contribute to disease pathogenesis. RNA splicing is a highly regulated process and, as such, the regulators are themselves tightly regulated. Differential transcriptional and posttranscriptional regulation of splicing factors modulates their levels and activities in tumor cells. Furthermore, the composition of the tumor microenvironment can also influence which isoforms are expressed in a given cell type and impact drug responses. Finally, we summarize current efforts in targeting alternative splicing, including global splicing inhibition using small molecules blocking the spliceosome or splicing-factor-modifying enzymes, as well as splice-switching RNA-based therapeutics to modulate cancer-specific splicing isoforms. This article is categorized under: RNA in Disease and Development > RNA in Disease RNA Processing > Splicing Regulation/Alternative Splicing.

Fast-cycling Rho GTPases

The Rho GTPases were discovered more than 30 years ago, and they were for a long time considered to follow simple cycling between GDP-bound and GTP-bound conformations, as for the Ras subfamily of small GTPases. The Rho GTPases consist of 20 members, but at least 10 of these do not follow this classical GTPase cycle. Thus, based on their kinetic properties, these Rho GTPases can instead be classified as atypical. Some of these atypical Rho GTPases do not hydrolyze GTP, and some have significantly increased intrinsic GDP/GTP exchange activity. This review focuses on this latter category of atypical Rho GTPases, the so-called 'fast-cycling' Rho GTPases. The different members of these fast-cycling atypical Rho GTPases are described in more detail here, along with their potential regulatory mechanisms. Finally, some insights are provided into the involvement of the atypical Rho GTPases in human pathologies.

Targeting Colon Cancers with Mutated BRAF and Microsatellite Instability

The subgroup of colon cancer (CRC) characterized by mutation in the BRAF gene and high mutation rate in the genomic DNA sequence, known as the microsatellite instability (MSI) phenotype, accounts for roughly 10% of the patients and derives from polyps with a serrated morphology. In this review, both features are discussed with regard to therapeutic opportunities. The most prevalent cancer-associated BRAF mutation is BRAF V600E that causes constitutive activation of the pro-proliferative MAPK pathway. Unfortunately, the available BRAF-specific inhibitors had little clinical benefit for metastatic CRC patients due to adaptive MAPK reactivation. Recent contributions for the development of new combination therapy approaches to pathway inhibition will be highlighted. In addition, we review the promising role of the recently developed immune checkpoint therapy for the treatment of this CRC subtype. The MSI phenotype of this subgroup results from an inactivated DNA mismatch repair system and leads to frameshift mutations with translation of new amino acid stretches and the generation of neo-antigens. This most likely explains the observed high degree of infiltration by tumour-associated lymphocytes. As cytotoxic lymphocytes are already part of the tumour environment, their activation by immune checkpoint therapy approaches is highly promising.

Negative regulation of TGF-β1-induced MKK6-p38 and MEK-ERK signalling and epithelial-mesenchymal transition by Rac1b

Prompted by earlier findings that the Rac1-related isoform Rac1b inhibits transforming growth factor (TGF)-β1-induced canonical Smad signalling, we studied here whether Rac1b also impacts TGF-β1-dependent non-Smad signalling such as the MKK6-p38 and MEK-ERK mitogen-activated protein kinase (MAPK) pathways and epithelial-mesenchymal transition (EMT). Transient depletion of Rac1b protein in pancreatic cancer cells by RNA interference increased the extent and duration of TGF-β1-induced phosphorylation of p38 MAPK in a Smad4-independent manner. Rac1b depletion also strongly increased basal ERK activation - independent of the kinase function of the TGF-β type I receptor ALK5 - and sensitised cells towards further upregulation of phospho-ERK levels by TGF-β1, while ectopic overexpression of Rac1b had the reverse effect. Rac1b depletion increased an EMT phenotype as evidenced by cell morphology, gene expression of EMT markers, cell migration and growth inhibition. Inhibition of MKK6-p38 or MEK-ERK signalling partially relieved the Rac1b depletion-dependent increase in TGF-β1-induced gene expression and cell migration. Rac1b depletion also enhanced TGF-β1 autoinduction of crucial TGF-β pathway components and decreased that of TGF-β pathway inhibitors. Our results show that Rac1b antagonises TGF-β1-dependent EMT by inhibiting MKK6-p38 and MEK-ERK signalling and by controlling gene expression in a way that favors attenuation of TGF-β signalling.

Rac1 in human diseases: The therapeutic potential of targeting Rac1 signaling regulatory mechanisms

Abnormal Rac1 signaling is linked to a number of debilitating human diseases, including cancer, cardiovascular diseases and neurodegenerative disorders. As such, Rac1 represents an attractive therapeutic target, yet the search for effective Rac1 inhibitors is still underway. Given the adverse effects associated with Rac1 signaling perturbation, cells have evolved several mechanisms to ensure the tight regulation of Rac1 signaling. Thus, characterizing these mechanisms can provide invaluable information regarding major cellular events that lead to aberrant Rac1 signaling. Importantly, this information can be utilized to further facilitate the development of effective pharmacological modulators that can restore normal Rac1 signaling. In this review, we focus on the pathological role of Rac1 signaling, highlighting the benefits and potential drawbacks of targeting Rac1 in a clinical setting. Additionally, we provide an overview of available compounds that target key Rac1 regulatory mechanisms and discuss future therapeutic avenues arising from our understanding of these mechanisms.

The role of TGF-β and its crosstalk with RAC1/RAC1b signaling in breast and pancreas carcinoma

This article focusses on the role of TGF-β and its signaling crosstalk with the RHO family GTPases RAC1 and RAC1b in the progression of breast and pancreatic carcinoma. The aggressive nature of these tumor types is mainly due to metastatic dissemination. Metastasis is facilitated by desmoplasia, a peculiar tumor microenvironment and the ability of the tumor cells to undergo epithelial-mesenchymal transition (EMT) and to adopt a motile and invasive phenotype. These processes are controlled entirely or in part by TGF-β and the small RHO GTPase RAC1 with both proteins acting as tumor promoters in late-stage cancers. Data from our and other studies point to signaling crosstalk between TGF-β and RAC1 and the related isoform, RAC1b, in pancreatic and mammary carcinoma cells. Based on the exciting observation that RAC1b functions as an endogenous inhibitor of RAC1, we propose a model on how the relative abundance or activity of RAC1 and RAC1b in the tumor cells may determine their responses to TGF-β and, ultimately, the metastatic capacity of the tumor.

RAC1b overexpression stimulates proliferation and NF-kB-mediated anti-apoptotic signaling in thyroid cancer cells

Overexpression of tumor-associated RAC1b has been recently highlighted as one of the most promising targets for therapeutic intervention in colon, breast, lung and pancreatic cancer. RAC1b is a hyperactive variant of the small GTPase RAC1 and has been recently shown to be overexpressed in a subset of papillary thyroid carcinomas associated with unfavorable outcome. Using the K1 PTC derived cell line as an in vitro model, we observed that both RAC1 and RAC1b were able to induce a significant increase on NF-kB and cyclin D1 reporter activity. A clear p65 nuclear localization was found in cells transfected with RAC1b-WT, confirming NF-kB canonical pathway activation. Consistently, we observed a RAC1b-mediated decrease in IκBα (NF-kB inhibitor) protein levels. Moreover, we show that RAC1b overexpression stimulates G1/S progression and protects thyroid cells against induced apoptosis, the latter through a process involving the NF-kB pathway. Present data support previous findings suggesting an important role for RAC1b in the development of follicular cell-derived thyroid malignancies and point out NF-kB activation as one of the molecular mechanisms associated with the pro-tumorigenic advantage of RAC1b overexpression in thyroid carcinomas.

Rac1b enhances cell survival through activation of the JNK2/c-JUN/Cyclin-D1 and AKT2/MCL1 pathways

Rac1b is a constitutively activated, alternatively spliced form of the small GTPase Rac1. Previous studies showed that Rac1b promotes cell proliferation and inhibits apoptosis. In the present study, we used microarray analysis to detect genes differentially expressed in HEK293T cells and SW480 human colon cancer cells stably overexpressing Rac1b. We found that the pro-proliferation genes JNK2, c-JUN and cyclin-D1 as well as anti-apoptotic AKT2 and MCL1 were all upregulated in both lines. Rac1b promoted cell proliferation and inhibited apoptosis by activating the JNK2/c-JUN/cyclin-D1 and AKT2/MCL1 pathways, respectively. Very low Rac1b levels were detected in the colonic epithelium of wild-type Sprague-Dawley rats. Knockout of the rat Rac1 gene exon-3b or knockdown of endogenous Rac1b in HT29 human colon cancer cells downregulated only the AKT2/MCL1 pathway. Our study revealed that very low levels of endogenous Rac1b inhibit apoptosis, while Rac1b upregulation both promotes cell proliferation and inhibits apoptosis. It is likely the AKT2/MCL1 pathway is more sensitive to Rac1b regulation.

SPSB1-mediated HnRNP A1 ubiquitylation regulates alternative splicing and cell migration in EGF signaling

Extracellular signals have been shown to impact on alternative pre-mRNA splicing; however, the molecular mechanisms and biological significance of signal-induced splicing regulation remain largely unknown. Here, we report that epidermal growth factor (EGF) induces splicing changes through ubiquitylation of a well-known splicing regulator, hnRNP A1. EGF signaling upregulates an E3 ubiquitin (Ub) ligase adaptor, SPRY domain-containing SOCS box protein 1 (SPSB1), which recruits Elongin B/C-Cullin complexes to conjugate lysine 29-linked polyUb chains onto hnRNP A1. Importantly, SPSB1 and ubiquitylation of hnRNP A1 have a critical role in EGF-driven cell migration. Mechanistically, EGF-induced ubiquitylation of hnRNP A1 together with the activation of SR protein kinases (SRPKs) results in the upregulation of a Rac1 splicing isoform, Rac1b, to promote cell motility. These findings unravel a novel crosstalk between protein ubiquitylation and alternative splicing in EGF/EGF receptor signaling, and identify a new EGF/SPSB1/hnRNP A1/Rac1 axis in modulating cell migration, which may have important implications for cancer treatment.

Deregulation of Rho GTPases in cancer

In vitro and in vivo studies and evidence from human tumors have long implicated Rho GTPase signaling in the formation and dissemination of a range of cancers. Recently next generation sequencing has identified direct mutations of Rho GTPases in human cancers. Moreover, the effects of ablating genes encoding Rho GTPases and their regulators in mouse models, or through pharmacological inhibition, strongly suggests that targeting Rho GTPase signaling could constitute an effective treatment. In this review we will explore the various ways in which Rho signaling can be deregulated in human cancers.

Tumor cell expression of MMP3 as a prognostic factor for poor survival in pancreatic, pulmonary, and mammary carcinoma

Breast, lung, and pancreatic cancers collectively represent one third of all diagnosed tumors and are responsible for almost 40% of overall cancer mortality. Despite improvements in current treatments, efforts to develop more specific therapeutic options are warranted. Here we identify matrix metalloproteinase 3 (MMP3) as a potential target within all three of these tumor types. MMP3 has previously been shown to induce expression of Rac1b, a tumorigenic splice isoform of Rac1. In this study we find that MMP3 and Rac1b proteins are both strongly expressed by the tumor cells of all three tumor types and that expression of MMP3 protein is prognostic of poor survival in pancreatic cancer patients. We also find that MMP3 gene expression can serve as a prognostic marker for patient survival in breast and lung cancer. These results suggest an oncogenic MMP3-Rac1b signaling axis as a driver of tumor progression in three common poor prognosis tumor types, further suggesting that new therapies to target these pathways could have substantial therapeutic benefit.

Current perspectives of molecular pathways involved in chronic inflammation-mediated breast cancer

Inflammation has multifaceted role in cancer progression including initiation, promotion and invasion by affecting the immune surveillance and associated signaling pathways. Inflammation facilitates the over-expression of cytokines, chemokines and growth factors involved in progression of different cancers including breast cancer progression. Deregulation of biological processes such as oxidative stress, angiogenesis, and autophagy elicit favorable immune response towards chronic inflammation. Apart from the role in carcinogenesis, chronic inflammation also favors the emergence of drug resistance clones by inducing the growth of breast cancer stem-like cells. Immunomodulation mediated by cytokines, chemokines and several other growth factors present in the tumor microenvironment regulate chronic inflammatory response and alter crosstalk among various signaling pathways such as NF-κB, Nrf-2, JAK-STAT, Akt and MAPKs involved in the progression of breast cancer. In this review, we focused on cellular and molecular processes involved in chronic inflammation, crosstalk among different signaling pathways and their association in breast cancer pathogenesis.

Expression of tumor-related Rac1b antagonizes B-Raf-induced senescence in colorectal cells

Mutations in the BRAF oncogene have been identified as a tumor-initiating genetic event in mainly melanoma, thyroid and colon cancer, resulting in an initial proliferative stimulus that is followed by a growth arrest period known as oncogene-induced senescence (OIS). It remains unknown what triggers subsequent escape from OIS to allow further tumor progression. A previous analysis revealed that around 80% of colorectal tumors carrying a mutation in BRAF also overexpress splice variant Rac1b. We used normal NCM460 colonocytes as a model to express oncogenic B-Raf-V600E in the presence or absence of co-transfected Rac1b and then analyzed the effect on expression of senescence markers. When oncogenic B-Raf-V600E was expressed we observed the induction of the senescence-associated β-galactosidase and of the cell-cycle inhibitors p14, p15 and p21 whereas proliferation marker Ki67 was suppressed. Upon co-expression of splice variant Rac1b, but not of Rac1, the B-Raf-induced senescence phenotype was reverted and expression of the cell-cycle inhibitors downregulated in a reactive oxygen-species dependent manner. We thus provide evidence that co-expression of splice variant Rac1b counteracts B-Raf-induced senescence, indicating the selection for increased Rac1b expression as one potential mechanism by which colorectal tumor cells can escape from B-Raf-induced OIS.

Posttranscriptional Regulation of Splicing Factor SRSF1 and Its Role in Cancer Cell Biology

Over the past decade, alternative splicing has been progressively recognized as a major mechanism regulating gene expression patterns in different tissues and disease states through the generation of multiple mRNAs from the same gene transcript. This process requires the joining of selected exons or usage of different pairs of splice sites and is regulated by gene-specific combinations of RNA-binding proteins. One archetypical splicing regulator is SRSF1, for which we review the molecular mechanisms and posttranscriptional modifications involved in its life cycle. These include alternative splicing of SRSF1 itself, regulatory protein phosphorylation events, and the role of nuclear versus cytoplasmic SRSF1 localization. In addition, we resume current knowledge on deregulated SRSF1 expression in tumors and describe SRSF1-regulated alternative transcripts with functional consequences for cancer cell biology at different stages of tumor development.

ROS-induced epithelial-mesenchymal transition in mammary epithelial cells is mediated by NF-kB-dependent activation of Snail

Epithelial-mesenchymal transition (EMT) is characterized by loss of cell-cell junctions, polarity and epithelial markers, and in turn, acquisition of mesenchymal features and motility. Changes associated with this developmental process have been extensively implicated in breast cancer progression and metastasis. Matrix metalloproteinases (MMPs) have been identified as specific inducers of EMT in mammary epithelial cells. MMP-3 induces EMT associated with malignant transformation via a pathway dependent upon production of reactive oxygen species (ROS). While the process by which exposure to MMP-3 leads to induction of ROS has been extensively studied, exactly how the MMP-3-induced ROS stimulate EMT remains unknown. Here, we used profiling methods to identify MMP-3-induced transcriptional alterations in mouse mammary epithelial cells, finding common overlap with changes mediated by nuclear factor-κB (NF-κB) and found in advanced breast cancer. In cultured cells, we found that Snail, an ROS-dependent key mediator of MMP-3-induced changes, is regulated by NF-κB in response to MMP-3. More specifically, we found MMP-3 to cause binding of p65 and cRel NF-κB subunits to the Snail promoter, leading to its transcription. Our results identify a specific pathway by which MMPs induce EMT and malignant characteristics, and provide insight into potential therapeutic approaches to target MMP-associated breast cancers.

Sanguinarine inhibits Rac1b-rendered cell survival enhancement by promoting apoptosis and blocking proliferation

AIM

Small GTPase Rac1 is a member of the Ras superfamily, which plays important roles in regulation of cytoskeleton reorganization, cell growth, proliferation, migration, etc. The aim of this study was to determine how a constitutively active Rac1b regulated cell proliferation and to investigate the effects of the Rac1b inhibitor sanguinarine.

METHODS

Three HEK293T cell lines stably overexpressing GFP, Rac1-GFP or Rac1b-GFP were constructed by lentiviral infection. The cells were treated with sanguinarine (1 μmol/L) or its analogue berberine (1 μmol/L) for 4 d. Cell proliferation was evaluated by counting cell numbers and with a BrdU incorporation assay. The levels of cleaved PARP-89 (an apoptosis marker) and cyclin-D1 (a proliferative index) were measured using Western blotting.

RESULTS

In 10% serum-containing media, overexpressing either Rac1 or Rac1b did not significantly change the cell proliferation. In the serum-starved media, however, the survival rate of Rac1b cells was significantly increased, whereas that of Rac1 cells was moderately increased. The level of cleaved PARP-89 was significantly increased in serum-starved Rac1 cells, but markedly reduced in serum-starved Rac1b cells. The level of cyclin-D1 was significantly increased in both serum-starved Rac1 and Rac1b cells. Treatment with sanguinarine, but not berberine, inhibited the proliferation of Rac1b cells, which was accompanied by significantly increased the level of PARP-89, and decreased both the level of cyclin-D1 and the percentage of BrdU positive cells.

CONCLUSION

Rac1b enhances the cell proliferation under a growth-limiting condition via both anti-apoptotic and pro-proliferative mechanisms. Sanguinarine, as the specific inhibitor of Rac1b, is a potential therapeutic agent for malignant tumors with up-regulated Rac1b.