Rac1 controls the subcellular localization of the Rho guanine nucleotide exchange factor Net1A to regulate focal adhesion formation and cell spreading

Alteration of actin cytoskeletal organisation in fetal akinesia deformation sequence

Fetal akinesia deformation sequence (FADS) represents the severest form of congenital myasthenic syndrome (CMS), a diverse group of inherited disorders characterised by impaired neuromuscular transmission. Most CMS originate from defects in the muscle nicotinic acetylcholine receptor, but the underlying molecular pathogenesis is only poorly understood. Here we show that RNAi-mediated silencing of FADS-related proteins rapsyn and NUP88 in foetal fibroblasts alters organisation of the actin cytoskeleton. We show that fibroblasts from two independent FADS individuals have enhanced and shorter actin stress fibre bundles, alongside with an increased number and size of focal adhesions, with an otherwise normal overall connectivity and integrity of the actin-myosin cytoskeleton network. By proximity ligation assays and bimolecular fluorescence complementation, we show that rapsyn and NUP88 localise nearby adhesion plaques and that they interact with the focal adhesion protein paxillin. Based on these findings we propose that a respective deficiency in rapsyn and NUP88 in FADS alters the regulation of actin dynamics at focal adhesions, and thereby may also plausibly dictate myofibril contraction in skeletal muscle of FADS individuals.

Alternative Transcripts Diversify Genome Function for Phenome Relevance to Health and Diseases

Manipulation using alternative exon splicing (AES), alternative transcription start (ATS), and alternative polyadenylation (APA) sites are key to transcript diversity underlying health and disease. All three are pervasive in organisms, present in at least 50% of human protein-coding genes. In fact, ATS and APA site use has the highest impact on protein identity, with their ability to alter which first and last exons are utilized as well as impacting stability and translation efficiency. These RNA variants have been shown to be highly specific, both in tissue type and stage, with demonstrated importance to cell proliferation, differentiation and the transition from fetal to adult cells. While alternative exon splicing has a limited effect on protein identity, its ubiquity highlights the importance of these minor alterations, which can alter other features such as localization. The three processes are also highly interwoven, with overlapping, complementary, and competing factors, RNA polymerase II and its CTD (C-terminal domain) chief among them. Their role in development means dysregulation leads to a wide variety of disorders and cancers, with some forms of disease disproportionately affected by specific mechanisms (AES, ATS, or APA). Challenges associated with the genome-wide profiling of RNA variants and their potential solutions are also discussed in this review.

mRNA location and translation rate determine protein targeting to dual destinations

Numerous proteins are targeted to two or multiple subcellular destinations where they exert distinct functional consequences. The balance between such differential targeting is thought to be determined post-translationally, relying on protein sorting mechanisms. Here, we show that mRNA location and translation rate can also determine protein targeting by modulating protein binding to specific interacting partners. Peripheral localization of the NET1 mRNA and fast translation lead to higher cytosolic retention of the NET1 protein by promoting its binding to the membrane-associated scaffold protein CASK. By contrast, perinuclear mRNA location and/or slower translation rate favor nuclear targeting by promoting binding to importins. This mRNA location-dependent mechanism is modulated by physiological stimuli and profoundly impacts NET1 function in cell motility. These results reveal that the location of protein synthesis and the rate of translation elongation act in coordination as a "partner-selection" mechanism that robustly influences protein distribution and function.

Src stimulates Abl-dependent phosphorylation of the guanine exchange factor Net1A to promote its cytosolic localization and cell motility

The neuroepithelial cell transforming gene 1 (Net1) is a guanine nucleotide exchange factor for the small GTPase RhoA that promotes cancer cell motility and metastasis. Two isoforms of Net1 exist, Net1 and Net1A, both of which are sequestered in the nucleus in quiescent cells to prevent aberrant RhoA activation. Many cell motility stimuli drive cytosolic relocalization of Net1A, but mechanisms controlling this event are not fully understood. Here, we demonstrate that epithelial growth factor stimulates protein kinase Src- and Abl1-dependent phosphorylation of Net1A to promote its cytosolic localization. We show that Abl1 efficiently phosphorylates Net1A on Y373, and that phenylalanine substitution of Y373 prevents Net1A cytosolic localization. Furthermore, we found that Abl1-driven cytosolic localization of Net1A does not require S52, which is a phosphorylation site of a different kinase, c-Jun N-terminal kinase, that inhibits nuclear import of Net1A. However, we did find that MKK7-stimulated cytosolic localization of Net1A does require Y373. We also demonstrate that aspartate substitution at Y373 is sufficient to promote Net1A cytosolic accumulation, and expression of Net1A Y373D potentiates epithelial growth factor-stimulated RhoA activation, downstream myosin light chain 2 phosphorylation, and F-actin accumulation. Moreover, we show that expression of Net1A Y373D in breast cancer cells also significantly increases cell motility and Matrigel invasion. Finally, we show that Net1A is required for Abl1-stimulated cell motility, which is rescued by expression of Net1A Y373D, but not Net1A Y373F. Taken together, this work demonstrates a novel mechanism controlling Net1A subcellular localization to regulate RhoA-dependent cell motility and invasion.

mRNA Location and Translation Rate Determine Protein Targeting to Dual Destinations

Numerous proteins are targeted to two or multiple subcellular destinations where they exert distinct functional consequences. The balance between such differential targeting is thought to be determined post-translationally, relying on protein sorting mechanisms. Here, we show that protein targeting can additionally be determined by mRNA location and translation rate, through modulating protein binding to specific interacting partners. Peripheral localization of the NET1 mRNA and fast translation lead to higher cytosolic retention of the NET1 protein, through promoting its binding to the membrane-associated scaffold protein CASK. By contrast, perinuclear mRNA location and/or slower translation rate favor nuclear targeting, through promoting binding to importins. This mRNA location-dependent mechanism is modulated by physiological stimuli and profoundly impacts NET1 function in cell motility. These results reveal that the location of protein synthesis and the rate of translation elongation act in coordination as a 'partner-selection' mechanism that robustly influences protein distribution and function.

Role of Small GTPase RhoA in DNA Damage Response

Accumulating evidence has suggested a role of the small GTPase Ras homolog gene family member A (RhoA) in DNA damage response (DDR) in addition to its traditional function of regulating cell morphology. In DDR, 2 key components of DNA repair, ataxia telangiectasia-mutated (ATM) and flap structure-specific endonuclease 1 (FEN1), along with intracellular reactive oxygen species (ROS) have been shown to regulate RhoA activation. In addition, Rho-specific guanine exchange factors (GEFs), neuroepithelial transforming gene 1 (Net1) and epithelial cell transforming sequence 2 (Ect2), have specific functions in DDR, and they also participate in Ras-related C3 botulinum toxin substrate 1 (Rac1)/RhoA interaction, a process which is largely unappreciated yet possibly of significance in DDR. Downstream of RhoA, current evidence has highlighted its role in mediating cell cycle arrest, which is an important step in DNA repair. Unraveling the mechanism by which RhoA modulates DDR may provide more insight into DDR itself and may aid in the future development of cancer therapies.

Cdk1 phosphorylation negatively regulates the activity of Net1 towards RhoA during mitosis

The Neuroepithelial transforming gene 1 (Net1) is a RhoA subfamily guanine nucleotide exchange factor that is overexpressed in a number of cancers and contributes to cancer cell motility and proliferation. Net1 also plays a Rho GTPase independent role in mitotic progression, where it promotes centrosomal activation of Aurora A and Pak2, and aids in chromosome alignment during prometaphase. To understand regulatory mechanisms controlling the mitotic function of Net1, we examined whether it was phosphorylated by the mitotic kinase Cdk1. We observed that Cdk1 phosphorylated Net1 on multiple sites in its N-terminal regulatory domain and C-terminus in vitro. By raising phospho-specific antibodies to two of these sites, we also demonstrated that both endogenous and transfected Net1 were phosphorylated by Cdk1 in cells. Substitution of the major Cdk1 phosphorylation sites with aliphatic or acidic residues inhibited the interaction of Net1 with RhoA, and treatment of metaphase cells with a Cdk1 inhibitor increased Net1 activity. Cdk1 inhibition also increased Net1 localization to the plasma membrane and stimulated cortical F-actin accumulation. Moreover, Net1 overexpression caused spindle polarity defects that were reduced in frequency by acidic substitution of the major Cdk1 phosphorylation sites. These data indicate that Cdk1 phosphorylates Net1 during mitosis and suggest that this negatively regulates its ability to signal to RhoA and alter actin cytoskeletal organization.

Omega-3 Fatty Acid-Derived Resolvin D2 Regulates Human Placental Vascular Smooth Muscle and Extravillous Trophoblast Activities

Omega-3 fatty acids are important to pregnancy and neonatal development and health. One mechanism by which omega-3 fatty acids exert their protective effects is through serving as substrates for the generation of specialized pro-resolving lipid mediators (SPM) that potently limit and resolve inflammatory processes. We recently identified that SPM levels are increased in maternal blood at delivery as compared to umbilical cord blood, suggesting the placenta as a potential site of action for maternal SPM. To explore this hypothesis, we obtained human placental samples and stained for the SPM resolvin D2 (RvD2) receptor GPR18 via immunohistochemistry. In so doing, we identified GPR18 expression in placental vascular smooth muscle and extravillous trophoblasts of the placental tissues. Using in vitro culturing, we confirmed expression of GPR18 in these cell types and further identified that stimulation with RvD2 led to significantly altered responsiveness (cytoskeletal changes and pro-inflammatory cytokine production) to lipopolysaccharide inflammatory stimulation in human umbilical artery smooth muscle cells and placental trophoblasts. Taken together, these findings establish a role for SPM actions in human placental tissue.

Regulation of RhoA activation and cell motility by c-Jun N-terminal kinases and Net1

Jnks are mitogen activated protein kinases that are best known for regulating transcription and apoptotic signaling. However, they also play important roles in controlling cell motility and invasion by phosphorylating many actin and microtubule regulatory proteins. These mechanisms have important implications for normal cell motility as well as cancer metastasis. Jnks are activated by growth factors and cytokines that stimulate cell motility, and this often requires upstream activation of Rho GTPases. Our recent work indicates that Jnks may also regulate Rho GTPase activation. Specifically, we found that Jnk-dependent phosphorylation of the RhoA guanine nucleotide exchange factor (RhoGEF) Net1A promotes its cytosolic accumulation to drive RhoA activation and actin cytoskeletal reorganization. Net1A is unusual among RhoGEFs in that it is sequestered in the nucleus to prevent aberrant RhoA activation. Importantly, Jnk-stimulated cytosolic localization of Net1A is sufficient to stimulate cell motility and extracellular matrix invasion in non-invasive breast cancer cells. Since Net1A expression is critical for cancer cell motility and invasion in vitro, and breast cancer metastasis in vivo, these data uncover a previously unappreciated regulatory mechanism that may contribute to metastasis in multiple types of cancer.

Whole-transcriptome splicing profiling of E7.5 mouse primary germ layers reveals frequent alternative promoter usage during mouse early embryogenesis

Alternative splicing (AS) and alternative promoter (AP) usage expand the repertories of mammalian transcriptome profiles and thus diversify gene functions. However, our knowledge about the extent and functions of AS and AP usage in mouse early embryogenesis remains elusive. Here, by performing whole-transcriptome splicing profiling with high-throughput next generation sequencing, we report that AS extensively occurs in embryonic day (E) 7.5 mouse primary germ layers, and may be involved in multiple developmental processes. In addition, numerous RNA splicing factors are differentially expressed and alternatively spliced across the three germ layers, implying the potential importance of AS machinery in shaping early embryogenesis. Notably, AP usage is remarkably frequent at this stage, accounting for more than one quarter (430/1,648) of the total significantly different AS events. Genes generating the 430 AP events participate in numerous biological processes, and include important regulators essential for mouse early embryogenesis, suggesting that AP usage is widely used and might be relevant to mouse germ layer specification. Our data underline the potential significance of AP usage in mouse gastrulation, providing a rich data source and opening another dimension for understanding the regulatory mechanisms of mammalian early development.

Summary: This study seeks to capture the alternative splicing landscape during mouse gastrulation, underlining the potential importance of alternative promoter usage in mammalian early embryogenesis.

Stress-activated MAPKs and CRM1 regulate the subcellular localization of Net1A to control cell motility and invasion

The neuroepithelial cell transforming gene 1A (Net1A, an isoform of Net1) is a RhoA subfamily guanine nucleotide exchange factor (GEF) that localizes to the nucleus in the absence of stimulation, preventing it from activating RhoA. Once relocalized in the cytosol, Net1A stimulates cell motility and extracellular matrix invasion. In the present work, we investigated mechanisms responsible for the cytosolic relocalization of Net1A. We demonstrate that inhibition of MAPK pathways blocks Net1A relocalization, with cells being most sensitive to JNK pathway inhibition. Moreover, activation of the JNK or p38 MAPK family pathway is sufficient to elicit Net1A cytosolic localization. Net1A relocalization stimulated by EGF or JNK activation requires nuclear export mediated by CRM1. JNK1 (also known as MAPK8) phosphorylates Net1A on serine 52, and alanine substitution at this site prevents Net1A relocalization caused by EGF or JNK activation. Glutamic acid substitution at this site is sufficient for Net1A relocalization and results in elevated RhoA signaling to stimulate myosin light chain 2 (MLC2, also known as MYL2) phosphorylation and F-actin accumulation. Net1A S52E expression stimulates cell motility, enables Matrigel invasion and promotes invadopodia formation. These data highlight a novel mechanism for controlling the subcellular localization of Net1A to regulate RhoA activation, cell motility, and invasion.

mDia2 and CXCL12/CXCR4 chemokine signaling intersect to drive tumor cell amoeboid morphological transitions

Morphological plasticity in response to environmental cues in migrating cancer cells requires F-actin cytoskeletal rearrangements. Conserved formin family proteins play critical roles in cell shape, tumor cell motility, invasion and metastasis, in part, through assembly of non-branched actin filaments. Diaphanous-related formin-2 (mDia2/Diaph3/Drf3/Dia) regulates mesenchymal-to-amoeboid morphological conversions and non-apoptotic blebbing in tumor cells by interacting with its inhibitor diaphanous-interacting protein (DIP), and disrupting cortical F-actin assembly and bundling. F-actin disruption is initiated by a CXCL12-dependent mechanism. Downstream CXCL12 signaling partners inducing mDia2-dependent amoeboid conversions remain enigmatic. We found in MDA-MB-231 tumor cells CXCL12 induces DIP and mDia2 interaction in blebs, and engages its receptor CXCR4 to induce RhoA-dependent blebbing. mDia2 and CXCR4 associate in blebs upon CXCL12 stimulation. Both CXCR4 and RhoA are required for CXCL12-induced blebbing. Neither CXCR7 nor other Rho GTPases that activate mDia2 are required for CXCL12-induced blebbing. The Rho Guanine Nucleotide Exchange Factor (GEF) Net1 is required for CXCL12-driven RhoA activation and subsequent blebbing. These results reveal CXCL12 signaling, through CXCR4, directs a Net1/RhoA/mDia-dependent signaling hub to drive cytoskeleton rearrangements to regulate morphological plasticity in tumor cells. These signaling hubs may be conserved during normal and cancer cells responding to chemotactic cues.

Regulation of RhoA activation and cytoskeletal organization by acetylation

Rho GTPases regulate cell motility in a large part through control of actin cytoskeletal organization. The activation state of Rho proteins is regulated by a wide variety of guanine nucleotide exchange factors (GEFs) and GTPase activating proteins that are differentially expressed among cell types and disease states. The RhoA specific GEF neuroepithelial transforming gene 1 (Net1) is highly expressed in many cancer cells and stimulates cell motility, invasion and cell spreading in response to a variety of ligands. A key feature of Net1 proteins is that they are sequestered in the nucleus in the absence of a motility stimulus. We have recently found that accumulation of the Net1A isoform outside the nucleus, which is the primary Net1 isoform controlling cell motility, is regulated by its acetylation status. Here, we describe acetylation as a novel mechanism of RhoGEF regulation in cell motility that can be targeted in cancer and metastasis.

Rho GTPases: Novel Players in the Regulation of the DNA Damage Response?

The Ras-related C3 botulinum toxin substrate 1 (Rac1) belongs to the family of Ras-homologous small GTPases. It is well characterized as a membrane-bound signal transducing molecule that is involved in the regulation of cell motility and adhesion as well as cell cycle progression, mitosis, cell death and gene expression. Rac1 also adjusts cellular responses to genotoxic stress by regulating the activity of stress kinases, including c-Jun-N-terminal kinase/stress-activated protein kinase (JNK/SAPK) and p38 kinases as well as related transcription factors. Apart from being found on the inner side of the outer cell membrane and in the cytosol, Rac1 has also been detected inside the nucleus. Different lines of evidence indicate that genotoxin-induced DNA damage is able to activate nuclear Rac1. The exact mechanisms involved and the biological consequences, however, are unclear. The data available so far indicate that Rac1 might integrate DNA damage independent and DNA damage dependent cellular stress responses following genotoxin treatment, thereby coordinating mechanisms of the DNA damage response (DDR) that are related to DNA repair, survival and cell death.

The RhoGEF Net1 is required for normal mammary gland development

Neuroepithelial transforming gene 1 (Net1) is a RhoA subfamily-specific guanine nucleotide exchange factor that is overexpressed in human breast cancer and is required for breast cancer cell migration and invasion. However, the role of Net1 in normal mammary gland development or function has never been assessed. To understand the role of Net1 in the mammary gland, we have created a conditional Net1 knockout mouse model. Whole-body deletion of Net1 results in delayed mammary gland development during puberty characterized by slowed of ductal extension and reduced ductal branching. Epithelial cells within the developing ducts show reduced proliferation that is accompanied by diminished estrogen receptor-α expression and activity. Net1-deficient mammary glands also exhibit reduced phosphorylation of regulatory subunits of myosin light chain and myosin light-chain phosphatase, indicating that RhoA-dependent actomyosin contraction is compromised. Net1 deficiency also leads to disorganization of myoepithelial and ductal epithelial cells and increased periductal collagen deposition. Mammary epithelial cell transplantation experiments indicate that reduced ductal branching and disorganization are cell autonomous. These data identify for the first time a role for NET1 in vivo and indicate that NET1 expression is essential for the proliferation and differentiation of mammary epithelial cells in the developing mammary gland.

Rho GTPase independent regulation of ATM activation and cell survival by the RhoGEF Net1A

ATM activation following DNA damage is a critical event which is required for efficient DNA repair and cell survival, yet signaling mechanisms controlling its activation are incompletely understood. The RhoGEF Net1 has previously been reported to control Rho GTPase activation and downstream cell survival outcomes following double strand DNA damage. However the role of Net1 isoforms in controlling ATM-dependent cell signaling has not been assessed. In the present work we show that expression of the Net1A isoform is specifically required for efficient activation of ATM but not the related kinase DNA-PK after ionizing radiation. Surprisingly Net1A overexpression also potently suppresses ATM activation and phosphorylation of its substrate H2AX. This effect does not require catalytic activity towards RhoA or RhoB, and neither Rho GTPase affects ATM activation, on its own. Consistent with a role in controlling ATM activation, Net1A knockdown also impairs DNA repair and cell survival. Taken together these data indicate that Net1A plays a plays a previously unrecognized, Rho GTPase-independent role in controlling ATM activity and downstream signaling after DNA damage to impact cell survival.

Vimentin regulates lung cancer cell adhesion through a VAV2-Rac1 pathway to control focal adhesion kinase activity

Vimentin is an intermediate filament protein whose expression correlates with increased metastatic disease, reduced patient survival and poor prognosis across multiple tumor types. Despite these well-characterized correlations, the molecular role of vimentin in cancer cell motility remains undefined. To approach this, we used an unbiased phosphoproteomics screen in lung cancer cell lines to discover cell motility proteins that show significant changes in phosphorylation upon vimentin depletion. We identified the guanine nucleotide exchange factor (GEF), VAV2, as having the greatest loss of phosphorylation owing to vimentin depletion. Since VAV2 serves as a GEF for the small Rho GTPase Rac1, a key player in cell motility and adhesion, we explored the vimentin-VAV2 pathway as a potential novel regulator of lung cancer cell motility. We show that VAV2 localizes to vimentin-positive focal adhesions (FAs) in lung cancer cells and complexes with vimentin and FA kinase (FAK). Vimentin loss impairs both pY142-VAV2 and downstream pY397-FAK activity showing that vimentin is critical for maintaining VAV2 and FAK activity. Importantly, vimentin depletion reduces the activity of the VAV2 target, Rac1, and a constitutively active Rac1 rescues defects in FAK and cell adhesion when vimentin or VAV2 is compromised. Based upon this data, we propose a model whereby vimentin promotes FAK stabilization through VAV2-mediated Rac1 activation. This model may explain why vimentin expressing metastatic lung cancer cells are more motile and invasive.

Acetylation of the RhoA GEF Net1A controls its subcellular localization and activity

Net1 isoform A (Net1A) is a RhoA GEF that is required for cell motility and invasion in multiple cancers. Nuclear localization of Net1A negatively regulates its activity, and we have recently shown that Rac1 stimulates Net1A relocalization to the plasma membrane to promote RhoA activation and cytoskeletal reorganization. However, mechanisms controlling the subcellular localization of Net1A are not well understood. Here, we show that Net1A contains two nuclear localization signal (NLS) sequences within its N-terminus and that residues surrounding the second NLS sequence are acetylated. Treatment of cells with deacetylase inhibitors or expression of active Rac1 promotes Net1A acetylation. Deacetylase inhibition is sufficient for Net1A relocalization outside the nucleus, and replacement of the N-terminal acetylation sites with arginine residues prevents cytoplasmic accumulation of Net1A caused by deacetylase inhibition or EGF stimulation. By contrast, replacement of these sites with glutamine residues is sufficient for Net1A relocalization, RhoA activation and downstream signaling. Moreover, the N-terminal acetylation sites are required for rescue of F-actin accumulation and focal adhesion maturation in Net1 knockout MEFs. These data indicate that Net1A acetylation regulates its subcellular localization to impact on RhoA activity and actin cytoskeletal organization.

I'm coming to GEF you: Regulation of RhoGEFs during cell migration

Cell migration is a highly regulated multistep process that requires the coordinated regulation of cell adhesion, protrusion, and contraction. These processes require numerous protein–protein interactions and the activation of specific signaling pathways. The Rho family of GTPases plays a key role in virtually every aspect of the cell migration cycle. The activation of Rho GTPases is mediated by a large and diverse family of proteins; the guanine nucleotide exchange factors (RhoGEFs). GEFs work immediately upstream of Rho proteins to provide a direct link between Rho activation and cell–surface receptors for various cytokines, growth factors, adhesion molecules, and G protein-coupled receptors. The regulated targeting and activation of RhoGEFs is essential to coordinate the migratory process. In this review, we summarize the recent advances in our understanding of the role of RhoGEFs in the regulation of cell migration.

Controlling the switches: Rho GTPase regulation during animal cell mitosis

Animal cell division is a fundamental process that requires complex changes in cytoskeletal organization and function. Aberrant cell division often has disastrous consequences for the cell and can lead to cell senescence, neoplastic transformation or death. As important regulators of the actin cytoskeleton, Rho GTPases play major roles in regulating many aspects of mitosis and cytokinesis. These include centrosome duplication and separation, generation of cortical rigidity, microtubule-kinetochore stabilization, cleavage furrow formation, contractile ring formation and constriction, and abscission. The ability of Rho proteins to function as regulators of cell division depends on their ability to cycle between their active, GTP-bound and inactive, GDP-bound states. However, Rho proteins are inherently inefficient at fulfilling this cycle and require the actions of regulatory proteins that enhance GTP binding (RhoGEFs), stimulate GTPase activity (RhoGAPs), and sequester inactive Rho proteins in the cytosol (RhoGDIs). The roles of these regulatory proteins in controlling cell division are an area of active investigation. In this review we will delineate the current state of knowledge of how specific RhoGEFs, RhoGAPs and RhoGDIs control mitosis and cytokinesis, and highlight the mechanisms by which their functions are controlled.

Methylosome protein 50 promotes androgen- and estrogen-independent tumorigenesis

Methylosome protein 50 (MEP50) is a component of methylosome where MEP50 binds protein substrates and activates the oncogenic protein arginine methyl transferase 5 (PRMT5). MEP50 is also a coactivator for androgen receptor (AR) and estrogen receptor (ER), and transforms cells in the presence of androgen or estrogen. To extend the understanding of how MEP50 transforms cells, we investigated whether MEP50 could transform cells independent of AR and ER, and clarified whether PRMT5 could contribute to the MEP50-caused tumor formation. Microarray and Western blot analyses revealed the association of MEP50 with many human cancers including lung cancer. Knockdown of MEP50 retarded cell growth and migration in selected lung cancer cell lines, which expressed very low level of AR and ER and were insensitive to inhibitors of AR and ER. Moreover, overexpression of Myc-MEP50 enhanced cell transforming activities of 293T cells which are known lack of expression of AR and ER. Mechanistic analyses showed that MEP50 controlled G2 progression, upregulated cyclin-dependent kinase 1(CDK1)/cyclin B1, and activated the survival cascade Phosphoinositide 3-kinase (PI3K)/AKT. MEP50 promoted cell migration, and activated the cell migration pathways such as Ras-related C3 botulinum toxin substrate 1 (Rac1)/vasodilator-stimulated phosphoprotein (VASP), and forkhead box protein A2 (FOXA2)/slug/cadherin cascades. Further analyses revealed that MEP50 activated the survival factor PI3K through PRMT5-catalyzed dimethylation of PI3K. Collectively, it is concluded that MEP50 can transform cells independent of AR and ER, and PRMT5 has partial contribution to that process.

Rho GTPase-independent regulation of mitotic progression by the RhoGEF Net1

Neuroepithelial transforming gene 1 (Net1) is a RhoA-subfamily-specific guanine nucleotide exchange factor that is overexpressed in multiple human cancers and is required for proliferation. Molecular mechanisms underlying its role in cell proliferation are unknown. Here we show that overexpression or knockdown of Net1 causes mitotic defects. Net1 is required for chromosome congression during metaphase and generation of stable kinetochore microtubule attachments. Accordingly, inhibition of Net1 expression results in spindle assembly checkpoint activation. The ability of Net1 to control mitosis is independent of RhoA or RhoB activation, as knockdown of either GTPase does not phenocopy effects of Net1 knockdown on nuclear morphology, and effects of Net1 knockdown are effectively rescued by expression of catalytically inactive Net1. We also observe that Net1 expression is required for centrosomal activation of p21-activated kinase and its downstream kinase Aurora A, which are critical regulators of centrosome maturation and spindle assembly. These results identify Net1 as a novel regulator of mitosis and indicate that altered expression of Net1, as occurs in human cancers, may adversely affect genomic stability.

Timing is everything: Rac1 controls Net1A localization to regulate cell adhesion

Cell adhesion to the extracellular matrix elicits a temporal reorganization of the actin cytoskeleton that is regulated first by Rac1 and later by RhoA. The signaling mechanisms controlling late stage RhoA activation are incompletely understood. Net1A is a RhoA/RhoB-specific guanine nucleotide exchange factor that is required for cancer cell motility. The ability of Net1A to stimulate RhoA activation is negatively regulated by nuclear sequestration. However, mechanisms controlling the plasma membrane localization of Net1A had not previously been reported. Recently we have shown that Rac1 activation stimulates plasma membrane relocalization and activation of Net1A. Net1A relocalization is independent of its catalytic activity and does not require its C-terminal pleckstrin homology or PDZ interacting domains. Rac1 activation during cell adhesion stimulates a transient relocalization of Net1A that is terminated by proteasomal degradation of Net1A. Importantly, plasma membrane localization of Net1A is required for efficient myosin light chain phosphorylation, focal adhesion maturation, and cell spreading. These data show for the first time a physiological mechanism controlling Net1A relocalization from the nucleus. They also demonstrate a previously unrecognized role for Net1A in controlling actomyosin contractility and focal adhesion dynamics during cell adhesion.

Regulation of focal adhesion kinase activation, breast cancer cell motility, and amoeboid invasion by the RhoA guanine nucleotide exchange factor Net1

Net1 is a RhoA guanine nucleotide exchange factor (GEF) that is overexpressed in a subset of human cancers and contributes to cancer cell motility and invasion in vitro. However, the molecular mechanism accounting for its role in cell motility and invasion has not been described. In the present work, we show that expression of both Net1 isoforms in breast cancer cells is required for efficient cell motility. Although loss of Net1 isoform expression only partially blocks RhoA activation, it inhibits lysophosphatidic acid (LPA)-stimulated migration as efficiently as knockdown of RhoA itself. However, we demonstrate that the Net1A isoform predominantly controls myosin light-chain phosphorylation and is required for trailing edge retraction during migration. Net1A interacts with focal adhesion kinase (FAK), localizes to focal adhesions, and is necessary for FAK activation and focal adhesion maturation during cell spreading. Net1A expression is also required for efficient invasion through a Matrigel matrix. Analysis of invading cells demonstrates that Net1A is required for amoeboid invasion, and loss of Net1A expression causes cells to shift to a mesenchymal phenotype characterized by high β1-integrin activity and membrane type 1 matrix metalloproteinase (MT1-MMP) expression. These results demonstrate a previously unrecognized role for the Net1A isoform in controlling FAK activation during planar cell movement and amoeboid motility during extracellular matrix (ECM) invasion.