Central role of the exchange factor GEF-H1 in TNF-α-induced sequential activation of Rac, ADAM17/TACE, and RhoA in tubular epithelial cells

Regulation of the RhoA exchange factor GEF-H1 by profibrotic stimuli through a positive feedback loop involving RhoA, MRTF, and Sp1

RhoA and its effectors, the transcriptional coactivators myocardin-related transcription factor (MRTF) and serum response factor (SRF), control epithelial phenotype and are indispensable for profibrotic epithelial reprogramming during fibrogenesis. Context-dependent control of RhoA and fibrosis-associated changes in its regulators, however, remain incompletely characterized. We previously identified the guanine nucleotide exchange factor GEF-H1 as a central mediator of RhoA activation in renal tubular cells exposed to inflammatory or fibrotic stimuli. Here we found that GEF-H1 expression and phosphorylation were strongly elevated in two animal models of fibrosis. In the Unilateral Ureteral Obstruction mouse kidney fibrosis model, GEF-H1 was upregulated predominantly in the tubular compartment. GEF-H1 was also elevated and phosphorylated in a rat pulmonary artery banding (PAB) model of right ventricular fibrosis. Prolonged stimulation of LLC-PK1 tubular cells with tumor necrosis factor (TNF)-α or transforming growth factor (TGF)-β1 increased GEF-H1 expression and activated a luciferase-coupled GEF-H1 promoter. Knockdown and overexpression studies revealed that these effects were mediated by RhoA, cytoskeleton remodeling, and MRTF, indicative of a positive feedback cycle. Indeed, silencing endogenous GEF-H1 attenuated activation of the GEF-H1 promoter. Of importance, inhibition of MRTF using CCG-1423 prevented GEF-H1 upregulation in both animal models. MRTF-dependent increase in GEF-H1 was prevented by inhibition of the transcription factor Sp1, and mutating putative Sp1 binding sites in the GEF-H1 promoter eliminated its MRTF-dependent activation. As the GEF-H1/RhoA axis is key for fibrogenesis, this novel MRTF/Sp1-dependent regulation of GEF-H1 abundance represents a potential target for reducing renal and cardiac fibrosis.NEW & NOTEWORTHY We show that expression of the RhoA regulator GEF-H1 is upregulated in tubular cells exposed to fibrogenic cytokines and in animal models of kidney and heart fibrosis. We identify a pathway wherein GEF-H1/RhoA-dependent MRTF activation through its noncanonical partner Sp1 upregulates GEF-H1. Our data reveal the existence of a positive feedback cycle that enhances Rho signaling through control of both GEF-H1 activation and expression. This feedback loop may play an important role in organ fibrosis.

Linear motif specificity in signaling through p38α and ERK2 mitogen-activated protein kinases

Mitogen-activated protein kinase (MAPK) cascades are essential for eukaryotic cells to integrate and respond to diverse stimuli. Maintaining specificity in signaling through MAPK networks is key to coupling distinct inputs to appropriate cellular responses. Docking sites-short linear motifs found in MAPK substrates, regulators, and scaffolds-can promote signaling specificity through selective interactions, but how they do so remains unresolved. Here, we screened a proteomic library for sequences interacting with the MAPKs extracellular signal-regulated kinase 2 (ERK2) and p38α, identifying selective and promiscuous docking motifs. Sequences specific for p38α had high net charge and lysine content, and selective binding depended on a pair of acidic residues unique to the p38α docking interface. Finally, we validated a set of full-length proteins harboring docking sites selected in our screens to be authentic MAPK interactors and substrates. This study identifies features that help define MAPK signaling networks and explains how specific docking motifs promote signaling integrity.

Endothelial Glycocalyx

The glycocalyx is a polysaccharide structure that protrudes from the body of a cell. It is primarily conformed of glycoproteins and proteoglycans, which provide communication, electrostatic charge, ionic buffering, permeability, and mechanosensation-mechanotransduction capabilities to cells. In blood vessels, the endothelial glycocalyx that projects into the vascular lumen separates the vascular wall from the circulating blood. Such a physical location allows a number of its components, including sialic acid, glypican-1, heparan sulfate, and hyaluronan, to participate in the mechanosensation-mechanotransduction of blood flow-dependent shear stress, which results in the synthesis of nitric oxide and flow-mediated vasodilation. The endothelial glycocalyx also participates in the regulation of vascular permeability and the modulation of inflammatory responses, including the processes of leukocyte rolling and extravasation. Its structural architecture and negative charge work to prevent macromolecules greater than approximately 70 kDa and cationic molecules from binding and flowing out of the vasculature. This also prevents the extravasation of pathogens such as bacteria and virus, as well as that of tumor cells. Due to its constant exposure to shear and circulating enzymes such as neuraminidase, heparanase, hyaluronidase, and matrix metalloproteinases, the endothelial glycocalyx is in a continuous process of degradation and renovation. A balance favoring degradation is associated with a variety of pathologies including atherosclerosis, hypertension, vascular aging, metastatic cancer, and diabetic vasculopathies. Consequently, ongoing research efforts are focused on deciphering the mechanisms that promote glycocalyx degradation or limit its syntheses, as well as on therapeutic approaches to improve glycocalyx integrity with the goal of reducing vascular disease. © 2022 American Physiological Society. Compr Physiol 12: 1-31, 2022.

Downregulation of ADAM17 in pediatric immune thrombocytopenia impairs proplatelet formation

BACKGROUND

Immune thrombocytopenia (ITP) is the most common etiology of acquired thrombocytopenia diseases in children. ITP is characterized by the immune-mediated decreased formation and excessive destruction of platelets. The pathogenesis and management of pediatric ITP are distinct from adult ITP. A disintegrin and metalloproteinase 17 (ADAM17) mediates the shedding of platelet receptor glycoprotein Ib α (GPIb α) in extracellular domain, functioning in the platelet activation and clearance. Our study aims to probe the roles and mechanisms of ADAM17 in pediatric ITP.

METHODS

The differently expressed ADAM17 in megakaryocytes was obtained from children with ITP through the next-generation RNA-Sequence. Hematoxylin-eosin and Giemsa staining were performed for cell morphology identification. Flow cytometry was applied to assess autoantibodies against platelets, subtypes of lymphocytes, the surface expression level of ADAM17 and polyploidization of megakaryocytes, as well as the full-length GP Ib α.

RESULTS

ADAM17 was significantly downregulated in megakaryocytes and platelets in children with ITP. Higher values of PDW and positive autoantibodies presence were observed in children with ITP. Loss of ADAM17 in mice led to defects in proplatelet formation and significantly elevated expression of phosphorylated myosin light chain (p-MLC) in megakaryocytes.

CONCLUSIONS

Our study indicated that the downregulation of ADAM17 might be an innate cause of inefficient platelet production in pediatric ITP.

ADAMs family in kidney physiology and pathology

A disintegrin and metalloproteinases (ADAMs) family are proteolytic transmembrane proteases that modulate diverse cell functions and coordinate intercellular communication. ADAMs are responsible for regulating cell proliferation, differentiation, migration, and organ morphogenesis in kidney development. Abnormally activated ADAMs drive inflammation and fibrosis in response to kidney diseases such as acute kidney injury, diabetic kidney disease, polycystic kidney disease, and chronic allograft nephropathy. ADAM10 and ADAM17, known as the most characterized members of ADAMs, are extensively investigated in kidney diseases. Notably, ADAM proteases have the potential to be targets for developing novel treatment approaches in kidney diseases.

Kidney Adaptations Prevent Loss of Trace Elements in Wistar Rats with Early Metabolic Syndrome

Metabolic syndrome (MetS) represents a cluster of related metabolic abnormalities, including central obesity, hypertension, dyslipidemia, hyperglycemia, and insulin resistance. These metabolic derangements present significant risk factors for chronic kidney disease that carries to loss of essential micronutrients, which accelerates comorbidity apparition. The work aimed was to evaluate the trace element homeostasis regarding morphological adaptations and renal function in MetS early-onset. Fifty male Wistar rats were divided into two groups: (a) control group and (b) hypercaloric diet group that developed MetS early-onset after 3 months. Classical zoometric parameters do not show changes; however, biochemical modifications were observed such as hyperglycemia, protein glycation, insulin resistance, dyslipidemia, hyperinsulinemia, and hypoadiponectinemia. MetS early-onset group observed renal structural modifications, but no functional changes. The structural modifications observed were minimal glomerular injury, glomerular basement membrane thickening, as well as mesangial and tubular cells that showed growth and proliferation. In serum and kidney (cortex and medulla), the concentrations of Zn, Fe, Cr, Mg, Mn, Cu, Co, and Ni were no differences between the experimental groups, but excretory fractions of these were lower in the hypercaloric diet group. In conclusion, MetS early-onset coexist renal structural modification and a hyperreabsorptive activity of essential trace elements that avoid its loss; thus, the excretory fraction of oligo-elements could be used a biomarker of early renal injury caused by metabolic diseases in the clinical practice.

Natriuretic peptide receptor-C releases and activates guanine nucleotide-exchange factor H1 in a ligand-dependent manner

Although natriuretic peptide receptor-C (NPR-C) is involved in the clearance of natriuretic peptides from plasma, it also possesses other physiological functions, such as inhibition of adenylyl cyclase activity through Gαi. However, the physiological roles and intracellular signaling pathways of NPR-C have yet been not fully elucidated. In this study, we identified a RhoA-specific guanine nucleotide-exchange factor, GEF-H1, as a novel binding protein of NPR-C. We demonstrated that endogenous NPR-C interacted with GEF-H1 in HeLa cells, and that the interaction between NPR-C and GEF-H1 was dependent on a 37-amino acid cytoplasmic region of NPR-C. In contrast, another natriuretic peptide receptor, NPR-A, which includes the kinase homology and guanylyl cyclase domains in the intracellular region, did not interact with GEF-H1. We also revealed that the ligands of NPR-C (i.e., ANP, CNP, and osteocrin) caused dissociation of GEF-H1 from NPR-C. Furthermore, osteocrin treatment induced phosphorylation of GEF-H1 at Ser-886, enhanced the interaction of GEF-H1 with 14-3-3, and increased the amount of activated GEF-H1. These findings strongly supported that NPR-C may be involved in diverse physiological roles by regulating GEF-H1 signaling.

S-1-propenylcysteine improves TNF-α-induced vascular endothelial barrier dysfunction by suppressing the GEF-H1/RhoA/Rac pathway

Background

Vascular endothelial barrier function is maintained by cell-to-cell junctional proteins and contributes to vascular homeostasis. Various risk factors such as inflammation disrupt barrier function through down-regulation of these proteins and promote vascular diseases such as atherosclerosis. Previous studies have demonstrated that aged garlic extract (AGE) and its sulfur-containing constituents exert the protective effects against several vascular diseases such as atherosclerosis. In this study, we examined whether AGE and its sulfur-containing constituents improve the endothelial barrier dysfunction elicited by a pro-inflammatory cytokine, Tumor-necrosis factor-α (TNF-α), and explored their mode of action on TNF-α signaling pathway.

Methods

Human umbilical vein endothelial cells (HUVECs) were treated with test substances in the presence of TNF-α for various time periods. The endothelial permeability was measured by using a transwell permeability assay. The localization of cell-to-cell junctional proteins and actin cytoskeletons were visualized by immunostaining. RhoA and Rac activities were assessed by using GTP-binding protein pulldown assay. Gene and protein expression levels of signaling molecules were analyzed by real-time PCR and western blotting, respectively.

Results

We found that AGE and its major sulfur-containing constituent, S-1-propenylcysteine (S1PC), reduced hyperpermeability elicited by TNF-α in HUVECs. In addition, S1PC inhibited TNF-α-induced production of myosin light chain (MLC) kinase and inactivation of MLC phosphatase through the suppression of the Rac and RhoA signaling pathways, respectively, which resulted in the dephosphorylation of MLC2, a key factor of actin remodeling. Moreover, S1PC inhibited the phosphorylation and activation of guanine nucleotide exchange factor-H1 (GEF-H1), a common upstream key molecule and activator of Rac and RhoA. These effects of S1PC were accompanied by its ability to prevent the disruption of junctional proteins on the cell–cell contact regions and the increase of actin stress fibers induced by TNF-α.

Conclusions

The present study suggested that AGE and its major constituent, S1PC, improve endothelial barrier disruption through the protection of junctional proteins on plasma membrane.

Tumor necrosis factor-α induces claudin-3 upregulation in kidney tubular epithelial cells through NF-κB and CREB1

Claudins are essential for tight junction formation and paracellular transport, and they affect key cellular events including proliferation and migration. The properties of tight junctions are dynamically modulated by a variety of inputs. We previously showed that the inflammatory cytokine tumor necrosis factor-α (TNFα), a major pathogenic factor in kidney disease, alters epithelial permeability by affecting the expression of claudin-1, -2, and -4 in kidney tubular cells. Here, we explored the effect of TNFα on claudin-3 (Cldn-3), a ubiquitous barrier-forming protein. We found that TNFα elevated Cldn-3 protein expression in tubular epithelial cells (LLC-PK1 and IMCD3) as early as 3 h post treatment. Bafilomycin A and bortezomib, inhibitors of lysosomal and proteasomes, respectively, reduced Cldn-3 degradation. However, TNFα caused a strong upregulation of Cldn-3 in the presence of bafilomycin, suggesting an effect independent from lysosomes. Blocking protein synthesis using cycloheximide prevented Cldn-3 upregulation by TNFα, verifying the contribution of de novo Cldn-3 synthesis. Indeed, TNFα elevated Cldn-3 mRNA levels at early time points. Using pharmacological inhibitors and siRNA-mediated silencing, we determined that the effect of TNFα on Cldn-3 was mediated by extracellular signal regulated kinase (ERK)-dependent activation of NF-κB and PKA-induced activation of CREB1. These two pathways were turned on by TNFα in parallel and both were required for the upregulation of Cldn-3. Because Cldn-3 was suggested to modulate cell migration and epithelial-mesenchymal transition (EMT), and TNFα was shown to affect these processes, Cldn-3 upregulation may modulate regeneration of the tubules following injury.

Regulation and functions of the RhoA regulatory guanine nucleotide exchange factor GEF-H1

Since the discovery by Madaule and Axel in 1985 of the first Ras homologue (Rho) protein in Aplysia and its human orthologue RhoB, membership in the Rho GTPase family has grown to 20 proteins, with representatives in all eukaryotic species. These GTPases are molecular switches that cycle between active (GTP bound) and inactivate (GDP bound) states. The exchange of GDP for GTP on Rho GTPases is facilitated by guanine exchange factors (GEFs). Approximately 80 Rho GEFs have been identified to date, and only a few GEFs associate with microtubules. The guanine nucleotide exchange factor H1, GEF-H1, is a unique GEF that associates with microtubules and is regulated by the polymerization state of microtubule networks. This review summarizes the regulation and functions of GEF-H1 and discusses the roles of GEF-H1 in human diseases.

YAP and TAZ protect against white adipocyte cell death during obesity

The expansion of the white adipose tissue (WAT) in obesity goes along with increased mechanical, metabolic and inflammatory stress. How adipocytes resist this stress is still poorly understood. Both in human and mouse adipocytes, the transcriptional co-activators YAP/TAZ and YAP/TAZ target genes become activated during obesity. When fed a high-fat diet (HFD), mice lacking YAP/TAZ in white adipocytes develop severe lipodystrophy with adipocyte cell death. The pro-apoptotic factor BIM, which is downregulated in adipocytes of obese mice and humans, is strongly upregulated in YAP/TAZ-deficient adipocytes under HFD, and suppression of BIM expression reduces adipocyte apoptosis. In differentiated adipocytes, TNFα and IL-1β promote YAP/TAZ nuclear translocation via activation of RhoA-mediated actomyosin contractility and increase YAP/TAZ-mediated transcriptional regulation by activation of c-Jun N-terminal kinase (JNK) and AP-1. Our data indicate that the YAP/TAZ signaling pathway may be a target to control adipocyte cell death and compensatory adipogenesis during obesity.

FAK regulates actin polymerization during sperm capacitation via the ERK2/GEF-H1/RhoA signaling pathway

Actin polymerization is a crucial process during sperm capacitation. We have recently described the participation of FAK during actin polymerization in guinea pig spermatozoa. However, the mechanism by which FAK mediates these processes is unknown. Our previous data have shown that MAPK1 (hereafter referred to as ERK2) is activated during the first minutes of capacitation, and inhibition of ERK2 blocked actin polymerization and the acrosome reaction. In this current study, we found that FAK is involved in ERK2 activation - as FAK was phosphorylated at tyrosine residue 925 and bound to Grb2 - and that inhibition of FAK results in a significant decrease of ERK2 activation. We also confirmed the presence of Rho guanine nucleotide exchange factor 2 (ARHGEF2, hereafter referred to as GEF-H1), which is able to associate with RhoA during capacitation. RhoA activation and its participation in actin polymerization were also analyzed. Inhibition of FAK or ERK1/2 impeded GEF-H1 phosphorylation, RhoA activation, and the association between GEF-H1 and RhoA. Finally, we observed the presence of fibronectin on the sperm surface, its role in sperm-sperm interaction as well as participation of β-integrin in the activation of ERK2. Our results show that the signaling pathway downstream of fibronectin, via integrin, FAK, Grb2, MEK1/2, ERK2, GEF-H1 and RhoA regulates the actin polymerization associated with spermatozoa capacitation.

Claudin-2 suppresses GEF-H1, RHOA, and MRTF, thereby impacting proliferation and profibrotic phenotype of tubular cells

The tight junctional pore-forming protein claudin-2 (CLDN-2) mediates paracellular Na⁺ and water transport in leaky epithelia and alters cancer cell proliferation. Previously, we reported that tumor necrosis factor-α time-dependently alters CLDN-2 expression in tubular epithelial cells. Here, we found a similar expression pattern in a mouse kidney injury model (unilateral ureteral obstruction), consisting of an initial increase followed by a drop in CLDN-2 protein expression. CLDN-2 silencing in LLC-PK¹ tubular cells induced activation and phosphorylation of guanine nucleotide exchange factor H1 (GEF-H1), leading to Ras homolog family member A (RHOA) activation. Silencing of other claudins had no such effects, and re-expression of an siRNA-resistant CLDN-2 prevented RHOA activation, indicating specific effects of CLDN-2 on RHOA. Moreover, kidneys from CLDN-2 knockout mice had elevated levels of active RHOA. Of note, CLDN-2 silencing reduced LLC-PK1 cell proliferation and elevated expression of cyclin-dependent kinase inhibitor P27 (P27KIP1) in a GEF-H1/RHOA-dependent manner. P27KIP1 silencing abrogated the effects of CLDN-2 depletion on proliferation. CLDN-2 loss also activated myocardin-related transcription factor (MRTF), a fibrogenic RHOA effector, and elevated expression of connective tissue growth factor and smooth muscle actin. Finally, CLDN-2 down-regulation contributed to RHOA activation and smooth muscle actin expression induced by prolonged tumor necrosis factor-α treatment, because they were mitigated by re-expression of CLDN-2. Our results indicate that CLDN-2 suppresses GEF-H1/RHOA. CLDN-2 down-regulation, for example, by inflammation, can reduce proliferation and promote MRTF activation through RHOA. These findings suggest that the initial CLDN-2 elevation might aid epithelial regeneration, and CLDN-2 loss could contribute to fibrotic reprogramming.

NDR2 kinase contributes to cell invasion and cytokinesis defects induced by the inactivation of RASSF1A tumor-suppressor gene in lung cancer cells

Background

RASSF1A, a tumor suppressor gene, is frequently inactivated in lung cancer leading to a YAP-dependent epithelial-mesenchymal transition (EMT). Such effects are partly due to the inactivation of the anti-migratory RhoB GTPase via the inhibitory phosphorylation of GEF-H1, the GDP/GTP exchange factor for RhoB. However, the kinase responsible for RhoB/GEF-H1 inactivation in RASSF1A-depleted cells remained unknown.

Methods

NDR1/2 inactivation by siRNA or shRNA effects on epithelial-mesenchymal transition, invasion, xenograft formation and growth in SCID−/− Beige mice, apoptosis, proliferation, cytokinesis, YAP/TAZ activation were investigated upon RASSF1A loss in human bronchial epithelial cells (HBEC).

Results

We demonstrate here that depletion of the YAP-kinases NDR1/2 reverts migration and metastatic properties upon RASSF1A loss in HBEC. We show that NDR2 interacts directly with GEF-H1 (which contains the NDR phosphorylation consensus motif HXRXXS/T), leading to GEF-H1 phosphorylation. We further report that the RASSF1A/NDR2/GEF-H1/RhoB/YAP axis is involved in proper cytokinesis in human bronchial cells, since chromosome proper segregation are NDR-dependent upon RASSF1A or GEF-H1 loss in HBEC.

Conclusion

To summarize, our data support a model in which, upon RASSF1A silencing, NDR2 gets activated, phosphorylates and inactivates GEF-H1, leading to RhoB inactivation. This cascade induced by RASSF1A loss in bronchial cells is responsible for metastasis properties, YAP activation and cytokinesis defects.

Electronic supplementary material

The online version of this article (10.1186/s13046-019-1145-8) contains supplementary material, which is available to authorized users.

The Microtubule-Associated Innate Immune Sensor GEF-H1 Does Not Influence Mouse Norovirus Replication in Murine Macrophages

Norovirus is an acute infection of the gastrointestinal tract causing rapid induction of vomiting and diarrhoea. The infection is sensed and controlled by the innate immune system, particularly by the RNA helicase MDA-5 and type I and III interferons (IFNs). We have observed that intracellular replication of murine norovirus (MNV) occurs in membranous clusters proximal to the microtubule organising centre, a localisation dependent on intact microtubules. Recently, it was shown that the host protein guanine nucleotide exchange factor-H1 (GEF-H1) is a microtubule-associated innate immune sensor that activates interferon Regulatory Factor 3 to induce the production of type I IFNs. Thus, we interrogated the potential role of GEF-H1 in controlling MNV infections. We observed that GEF-H1 was recruited to the MNV replication complex; however RNAi-mediated suppression of GEF-H1 did not outwardly affect replication. We furthered our studies to investigate the impact of GEF-H1 on MNV innate detection and observed that GEF-H1 did not contribute to type I IFN induction during MNV infection or influenza virus infection but did result in a small reduction of interferon–β (IFNβ) during West Nile virus infection. Intriguingly, we discovered an interaction of GEF-H1 with the viral MNV non-structural protein 3 (NS3), an interaction that altered the location of GEF-H1 within the cell and prevented the formation of GEF-H1-induced microtubule fibres. Thus, our results indicate that GEF-H1 does not contribute significantly to the innate immune sensing of MNV, although its function may be modulated via interaction with the viral NS3 protein.

iRhom2 promotes lupus nephritis through TNF-α and EGFR signaling

Lupus nephritis (LN) often results in progressive renal dysfunction. The inactive rhomboid 2 (iRhom2) is a newly identified key regulator of A disintegrin and metalloprotease 17 (ADAM17), whose substrates, such as TNF-α and heparin-binding EGF (HB-EGF), have been implicated in the pathogenesis of chronic kidney diseases. Here, we demonstrate that deficiency of iRhom2 protects the lupus-prone Fcgr2b-/- mice from developing severe kidney damage without altering anti-double-stranded DNA (anti-dsDNA) Ab production by simultaneously blocking HB-EGF/EGFR and TNF-α signaling in the kidney tissues. Unbiased transcriptome profiling of kidneys and kidney macrophages revealed that TNF-α and HB-EGF/EGFR signaling pathways are highly upregulated in Fcgr2b-/- mice, alterations that were diminished in the absence of iRhom2. Pharmacological blockade of either TNF-α or EGFR signaling protected Fcgr2b-/- mice from severe renal damage. Finally, kidneys from LN patients showed increased iRhom2 and HB-EGF expression, with interstitial HB-EGF expression significantly associated with chronicity indices. Our data suggest that activation of iRhom2/ADAM17-dependent TNF-α and EGFR signaling plays a crucial role in mediating irreversible kidney damage in LN, thereby uncovering a target for selective and simultaneous dual inhibition of 2 major pathological pathways in the effector arm of the disease.

Cell confluence regulates claudin-2 expression: possible role for ZO-1 and Rac

Claudin-2 (Cldn-2) is a channel-forming tight junction (TJ) protein in the proximal tubules that mediates paracellular Na+ transport and has also emerged as a regulator of proliferation and migration. Expression of Cldn-2 is altered by numerous stimuli, but the underlying mechanisms remain incompletely understood. Here we show that Cldn-2 protein and mRNA expression were low in subconfluent tubular cells and increased during junction maturation. Cldn-1 or occludin did not exhibit similar confluence-dependence. Conversely, disruption of TJs by Ca2+ removal or silencing of zonula occludens-1 (ZO-1) or ZO-2 induced a large drop in Cldn-2 abundance. Immunofluorescent staining revealed a more uneven Cldn-2 staining in nascent, Cldn-1-positive TJs. Subconfluence and ZO-1 silencing augmented Cldn-2 degradation and reduced Cldn-2 promoter activity, suggesting that insertion into the TJs slows Cldn-2 turnover. Indeed, blocking endocytosis or lysosomal degradation increased Cldn-2 abundance. Cell confluence increased expression of the junctional adapters ZO-1 and -2, and the small GTPase Rac, and elevated Rac activity and p21-activated kinase (Pak) phosphorylation, suggesting that they might mediate confluence-dependent Cldn-2 regulation. Indeed, Rac silencing or Pak inhibition strongly reduced Cldn-2 protein abundance, which was likely the combined effect on turnover, as these interventions reduced Cldn-2 promoter activity and augmented Cldn-2 degradation. Taken together, our data suggest that TJ integrity and maturity, ZO-1 expression/TJ localization, and Rac/Pak control Cldn-2 degradation and synthesis. A feedback mechanism connecting Cldn-2 expression with junction remodeling, e.g., during wound healing, epithelial-mesenchymal transition, or tumor metastasis formation, may have important downstream effects on permeability, proliferation, and migration.

EGFR transactivation is involved in TNF-α-induced expression of thymic stromal lymphopoietin in human keratinocyte cell line

BACKGROUND

Thymic stromal lymphopoietin (TSLP) is an epithelial cell-derived cytokine involved in the pathology of inflammatory skin diseases, such as atopic dermatitis and psoriasis. Tumor necrosis factor (TNF)-α, a key cytokine in inflammatory skin diseases, is a known TSLP inducer. TNF-α activates NF-κB and induces transactivation of epidermal growth factor receptor (EGFR) in epithelial cells. However, the detailed mechanism of TSLP induction by TNF-α has remained unclear.

OBJECTIVE

We investigated the involvement of TNF-α-induced EGFR transactivation in TSLP expression.

METHODS

HaCaT cells were stimulated with TNF-α or EGF in the presence or absence of an EGFR kinase inhibitor or other signaling inhibitors. The expression of TSLP mRNA was analyzed by RT-PCR and the phosphorylation level of signal proteins was analyzed by western blot. TSLP promoter and NF-κB transcription activities were analyzed by luciferase assay.

RESULTS

TNF-α-induced TSLP expression was inhibited by the EGFR kinase inhibitor AG1478. While TSLP expression was induced by EGF, it was inhibited by the MEK inhibitor, U0126. Inhibitors of p38 and ADAM proteases suppressed the TNF-α-induced TSLP expression and EGFR phosphorylation, but not the EGF-induced expression.

CONCLUSION

TNF-α-induced EGFR transactivation results in TSLP induction through ERK activation. The activation of p38 and ADAM proteases mediates TNF-α-induced EGFR phosphorylation. These findings suggested that the TNF-α-induced EGFR transactivation pathway could be a target for the treatment of inflammatory skin diseases.

Epithelial junctions and Rho family GTPases: the zonular signalosome

The establishment and maintenance of epithelial cell-cell junctions is crucially important to regulate adhesion, apico-basal polarity and motility of epithelial cells, and ultimately controls the architecture and physiology of epithelial organs. Junctions are supported, shaped and regulated by cytoskeletal filaments, whose dynamic organization and contractility are finely tuned by GTPases of the Rho family, primarily RhoA, Rac1 and Cdc42. Recent research has identified new molecular mechanisms underlying the cross-talk between these GTPases and epithelial junctions. Here we briefly summarize the current knowledge about the organization, molecular evolution and cytoskeletal anchoring of cell-cell junctions, and we comment on the most recent advances in the characterization of the interactions between Rho GTPases and junctional proteins, and their consequences with regards to junction assembly and regulation of cell behavior in vertebrate model systems. The concept of “zonular signalosome” is proposed, which highlights the close functional relationship between proteins of zonular junctions (zonulae occludentes and adhaerentes) and the control of cytoskeletal organization and signaling through Rho GTPases, transcription factors, and their effectors.

Dynasore enhances the formation of mitochondrial antiviral signalling aggregates and endocytosis-independent NF-κB activation

BACKGROUND AND PURPOSE

Dynasore has been used extensively as an inhibitor of clathrin-mediated endocytosis. While studying the role of endocytosis in LPS-induced signalling events, we discovered that dynasore itself induced activation of NF-κB, independently of its effects on endocytosis and without involving the Toll-like receptor 4 signalling pathways. The purpose of this study was to characterize this novel effect and to explore the underlying mechanism of action.

EXPERIMENTAL APPROACH

We utilized gel electrophoresis, microscopy, gene knockdown and luciferase-based promoter activity to evaluate the effect of dynasore on cell signalling pathways and to delineate the mechanisms involved in its effects,

KEY RESULTS

Dynasore activated the NF-κB and IFN-β pathways by activating mitochondrial antiviral signalling protein (MAVS). We showed that MAVS is activated by NOX/Rac and forms high molecular weight aggregates, similar to that observed in response to viral infection. We also demonstrated that dynasore-induced activation of JNK occurs downstream of MAVS and is required for activation of NF-κB and IFN-β.

CONCLUSION AND IMPLICATIONS

These findings demonstrate a novel effect of dynasore on cell signalling. We describe a novel Rac1-, ROS- and MAVS-mediated signalling cascade through which dynasore dramatically activates NF-κB, mimicking the viral induction of this key inflammatory signalling pathway. Our results call attention to the need for a broader interpretation of results when dynasore is used in its traditional fashion as an inhibitor of clathrin-mediated endocytosis. These results suggest the intriguing possibility that dynasore or one of its analogues might be of value as an antiviral therapeutic strategy or vaccine adjuvant.

Tumor necrosis factor-α induces a biphasic change in claudin-2 expression in tubular epithelial cells: role in barrier functions

The inflammatory cytokine tumor necrosis factor-α (TNF-α) is a pathogenic factor in acute and chronic kidney disease. TNF-α is known to alter expression of epithelial tight junction (TJ) proteins; however, the underlying mechanisms and the impact of this effect on epithelial functions remain poorly defined. Here we describe a novel biphasic effect of TNF-α on TJ protein expression. In LLC-PK1 tubular cells, short-term (1-6 h) TNF-α treatment selectively elevated the expression of the channel-forming TJ protein claudin-2. In contrast, prolonged (>8 h) TNF-α treatment caused a marked downregulation in claudin-2 and an increase in claudin-1, -4, and -7. The early increase and the late decrease in claudin-2 expression involved distinct mechanisms. TNF-α slowed claudin-2 degradation through ERK, causing the early increase. This increase was also mediated by the EGF receptor and RhoA and Rho kinase. In contrast, prolonged TNF-α treatment reduced claudin-2 mRNA levels and promoter activity independent from these signaling pathways. Electric Cell-substrate Impedance Sensing measurements revealed that TNF-α also exerted a biphasic effect on transepithelial resistance (TER) with an initial decrease and a late increase. Thus there was a good temporal correlation between TNF-α-induced claudin-2 protein and TER changes. Indeed, silencing experiments showed that the late TER increase was at least in part caused by reduced claudin-2 expression. Surprisingly, however, claudin-2 silencing did not prevent the early TER drop. Taken together, the TNF-α-induced changes in claudin-2 levels might contribute to TER changes and could also play a role in newly described functions of claudin-2 such as proliferation regulation.

Asef controls vascular endothelial permeability and barrier recovery in the lung

This is the first report of Asef involvement in the regulation of endothelial vascular permeability in vitro and in vivo. Asef activation in endothelial cells by hepatocyte growth factor suppressed the Rho-dependent pathway of agonist-induced endothelial permeability and promoted Rac1-dependent endothelial barrier recovery.

Increased levels of hepatocyte growth factor (HGF) in injured lungs may reflect a compensatory response to diminish acute lung injury (ALI). HGF-induced activation of Rac1 GTPase stimulates endothelial barrier protective mechanisms. This study tested the involvement of Rac-specific guanine nucleotide exchange factor Asef in HGF-induced endothelial cell (EC) cytoskeletal dynamics and barrier protection in vitro and in a two-hit model of ALI. HGF induced membrane translocation of Asef and stimulated Asef Rac1-specific nucleotide exchange activity. Expression of constitutively activated Asef mutant mimicked HGF-induced peripheral actin cytoskeleton enhancement. In contrast, siRNA-induced Asef knockdown or expression of dominant-negative Asef attenuated HGF-induced Rac1 activation evaluated by Rac-GTP pull down and FRET assay with Rac1 biosensor. Molecular inhibition of Asef attenuated HGF-induced peripheral accumulation of cortactin, formation of lamellipodia-like structures, and enhancement of VE-cadherin adherens junctions and compromised HGF-protective effect against thrombin-induced RhoA GTPase activation, Rho-dependent cytoskeleton remodeling, and EC permeability. Intravenous HGF injection attenuated lung inflammation and vascular leak in the two-hit model of ALI induced by excessive mechanical ventilation and thrombin signaling peptide TRAP6. This effect was lost in Asef^−/− mice. This study shows for the first time the role of Asef in HGF-mediated protection against endothelial hyperpermeability and lung injury.

Dual regulation of tumor necrosis factor-α on myosin light chain phosphorylation in vascular smooth muscle

We previously demonstrated that inhibitor κB kinase 2 (IKK2) is a myosin light chain kinase (MLCK). In the present study, we assess whether the prototypical activator of IKK2 tumor necrosis factor-α (TNF-α) regulates the MLCK activity of IKK2 and thus MLC phosphorylation in vascular smooth muscle cells (VSMCs). Kinase activity assay revealed that TNF-α downregulated the MLCK activity of IKK2 in human VSMCs (HVSMCs). However, Western blot analysis did not demonstrate a significant effect of TNF-α on MLC phosphorylation in HVSMCs, and myograph analysis did not reveal a significant effect of TNF-α on the contraction of the aorta from Sprague-Dawley rats and C57Bl/6j mice, suggesting a dual regulation of MLC phosphorylation by TNF-α. Confirming this notion, TNF-α significantly increased MLC phosphorylation in IKK2(-/-) but not wild-type cells. Furthermore, our results show that TNF-α increased GTP-bound RhoA and MLC phosphatase subunit MYPT1 phosphorylation and markedly reduced MLC phosphorylation in the presence of Rho-kinase inhibitor Y-27632, suggesting that downregulation of MLCK activity of IKK2 by TNF-α is antagonized by simultaneous RhoA/Rho-kinase activation. These results indicate that TNF-α dually regulates MLC phosphorylation through both IKK2 and RhoA/Rho-kinase pathways.

Hepatocyte growth factor-induced Asef-IQGAP1 complex controls cytoskeletal remodeling and endothelial barrier

Hepatocyte growth factor (HGF) attenuates agonist-induced endothelial cell (EC) permeability and increases pulmonary endothelial barrier function via Rac-dependent enhancement of the peripheral actin cytoskeleton. However, the precise mechanisms of HGF effects on the peripheral cytoskeleton are not well understood. This study evaluated a role for Rac/Cdc42-specific guanine nucleotide exchange factor Asef and the multifunctional Rac effector, IQGAP1, in the mechanism of HGF-induced EC barrier enhancement. HGF induced Asef and IQGAP1 co-localization at the cell cortical area and stimulated formation of an Asef-IQGAP1 functional protein complex. siRNA-induced knockdown of Asef or IQGAP1 attenuated HGF-induced EC barrier enhancement. Asef knockdown attenuated HGF-induced Rac activation and Rac association with IQGAP1, and it abolished both IQGAP1 accumulation at the cell cortical layer and IQGAP1 interaction with actin cytoskeletal regulators cortactin and Arp3. Asef activation state was essential for Asef interaction with IQGAP1 and protein complex accumulation at the cell periphery. In addition to the previously reported role of the IQGAP1 RasGAP-related domain in the Rac-dependent IQGAP1 activation and interaction with its targets, we show that the IQGAP1 C-terminal domain is essential for HGF-induced IQGAP1/Asef interaction and Asef-Rac-dependent activation leading to IQGAP1 interaction with Arp3 and cortactin as a positive feedback mechanism of IQGAP1 activation. These results demonstrate a novel feedback mechanism of HGF-induced endothelial barrier enhancement via Asef/IQGAP1 interactions, which regulate the level of HGF-induced Rac activation and promote cortical cytoskeletal remodeling via IQGAP1-Arp3/cortactin interactions.

Hepatocyte growth factor triggers distinct mechanisms of Asef and Tiam1 activation to induce endothelial barrier enhancement

Previous reports described an important role of hepatocyte growth factor (HGF) in mitigation of pulmonary endothelial barrier dysfunction and cell injury induced by pathologic agonists and mechanical forces. HGF protective effects have been associated with Rac-GTPase signaling pathway activated by Rac-specific guanine nucleotide exchange factor Tiam1 and leading to enhancement of intercellular adherens junctions. This study tested involvement of a novel Rac-specific activator, Asef, in endothelial barrier enhancement by HGF and investigated a mechanism of HGF-induced Asef activation. Si-RNA-based knockdown of Tiam1 and Asef had an additive effect on attenuation of HGF-induced Rac activation and endothelial cell (EC) barrier enhancement. Tiam1 and Asef activation was abolished by pharmacologic inhibitors of HGF receptor and PI3-kinase. In contrast to Tiam1, Asef interacted with APC and associated with microtubule fraction upon HGF stimulation. EC treatment by low dose nocodazole to inhibit peripheral microtubule dynamics partially attenuated HGF-induced Asef peripheral translocation, but had negligible effect on Tiam1 translocation. These effects were associated with attenuation of HGF-induced barrier enhancement in EC pretreated with low ND dose and activation of Rac and its cytoskeletal effectors PAK1 and cortactin. These data demonstrate, that in addition to microtubule-independent Tiam1 activation, HGF engages additional microtubule- and APC-dependent pathway of Asef activation. These mechanisms may complement each other to provide the fine tuning of Rac signaling and endothelial barrier enhancement in response to various agonists.

Paxillin mediates stretch-induced Rho signaling and endothelial permeability via assembly of paxillin-p42/44MAPK-GEF-H1 complex

Suboptimal ventilator support or regional ventilation heterogeneity in inflamed lungs causes excessive tissue distension, which triggers stretch-induced pathological signaling and may lead to vascular leak and lung dysfunction. Focal adhesions (FAs) are cell-substrate adhesive complexes participating in cellular mechanotransduction and regulation of the Rho GTPase pathway. Stretch-induced Rho regulation remains poorly understood. We used human lung endothelial cells (ECs) exposed to pathological cyclic stretch (CS) at 18% distension to test the hypothesis that FA protein paxillin participates in CS-induced Rho activation by recruiting the Rho-specific guanine nucleotide exchange factor GEF-H1. CS induced phosphorylation of paxillin and activated p42/44-MAP kinase, Rho GTPase, and paxillin/GEF-H1/p42/44-MAPK association. CS caused nearly 2-fold increase in EC permeability, which was attenuated by paxillin knockdown. Expression of the paxillin-Y31/118F phosphorylation mutant decreased the CS-induced paxillin/GEF-H1 association (16.3 ± 4.1%), GEF-H1 activation (28.9 ± 9.2%), and EC permeability (28.7 ± 8.1%) but not CS-induced p42/44-MAPK activation. Inhibition of p42/44-MAPK suppressed CS-induced paxillin/GEF-H1 interactions (15.9 ± 7.9%), GEF-H1 activation (11.7 ± 4.3%), and disruption of EC monolayer. Expression of GEF-H1T678A lacking p42/44-MAPK phosphorylation site attenuated Rho activation (31.2±11.6%). We conclude that MAPK-dependent targeting of GEF-H1 to paxillin is involved in the regulation of CS-induced Rho signaling and EC permeability. This study proposes a novel concept of paxillin-GEF-H1-p42/44-MAPK module as a regulator of pathological mechanotransduction.-Gawlak, G., Tian, Y., O'Donnell, J. J., III, Tian, X., Birukova, A. A., Birukov, K. G. Paxillin mediates stretch-induced Rho signaling and endothelial permeability via assembly of paxillin-p42/44MAPK-GEF-H1 complex.

TRAF2 facilitates vaccinia virus replication by promoting rapid virus entry

Tumor necrosis factor receptor (TNFR)-associated factor 2 (TRAF2) is a pivotal intracellular mediator of signaling pathways downstream of TNFR1 and -2 with known pro- and antiviral effects. We investigated its role in the replication of the prototype poxvirus vaccinia virus (VACV). Loss of TRAF2 expression, either through small interfering RNA treatment of HeLa cells or through genetic knockout in murine embryonic fibroblasts (MEFs), led to significant reductions in VACV growth following low-multiplicity infection. In single-cycle infections, there was delayed production of both early and late VACV proteins as well as accelerated virus-induced alterations to cell morphology, indicating that TRAF2 influences early stages of virus replication. Consistent with an early role, uncoating assays showed normal virus attachment but delayed virus entry in the absence of TRAF2. Although alterations to c-Jun N-terminal kinase (JNK) signaling were apparent in VACV-infected TRAF2^−/− MEFs, treatment of wild-type cells with a JNK inhibitor did not affect virus entry. Instead, treatment with an inhibitor of endosomal acidification greatly reduced virus entry into TRAF2^−/− MEFs, suggesting that VACV is reliant on the endosomal route of entry in the absence of TRAF2. Thus, TRAF2 is a proviral factor for VACV that plays a role in promoting efficient viral entry, most likely via the plasma membrane.

IMPORTANCE Tumor necrosis factor receptor-associated factors (TRAFs) are key facilitators of intracellular signaling with roles in innate and adaptive immunity and stress responses. We have discovered that TRAF2 is a proviral factor in vaccinia virus replication in both HeLa cells and mouse embryonic fibroblasts and that its influence is exercised through promotion of efficient virus entry.

Role of guanine nucleotide exchange factor-H1 in complement-mediated RhoA activation in glomerular epithelial cells

Visceral glomerular epithelial cells (GEC), also known as podocytes, are vital for the structural and functional integrity of the glomerulus. The actin cytoskeleton plays a central role in maintaining GEC morphology. In a rat model of experimental membranous nephropathy (passive Heymann nephritis (PHN)), complement C5b-9-induced proteinuria was associated with the activation of the actin regulator small GTPase, RhoA. The mechanisms of RhoA activation, however, remained unknown. In this study, we explored the role of the epithelial guanine nucleotide exchange factor, GEF-H1, in complement-induced RhoA activation. Using affinity precipitation to monitor GEF activity, we found that GEF-H1 was activated in glomeruli isolated from rats with PHN. Complement C5b-9 also induced parallel activation of GEF-H1 and RhoA in cultured GEC. In GEC in which GEF-H1 was knocked down, both basal and complement-induced RhoA activity was reduced. On the other hand, GEF-H1 knockdown augmented complement-mediated cytolysis, suggesting a role for GEF-H1 and RhoA in protecting GEC from cell death. The MEK1/2 inhibitor, U0126, and mutation of the ERK-dependent phosphorylation site (T678A) prevented complement-induced GEF-H1 activation, indicating a role for the ERK pathway. Further, complement induced GEF-H1 and microtubule accumulation in the perinuclear region. However, both the perinuclear accumulation and the activation of GEF-H1 were independent of microtubules and myosin-mediated contractility, as shown using drugs that interfere with microtubule dynamics and myosin II activity. In summary, we have identified complement-induced ERK-dependent GEF-H1 activation as the upstream mechanism of RhoA stimulation, and this pathway has a protective role against cell death.

Control of vascular permeability by atrial natriuretic peptide via a GEF-H1-dependent mechanism

Microtubule (MT) dynamics is involved in a variety of cell functions, including control of the endothelial cell (EC) barrier. Release of Rho-specific nucleotide exchange factor GEF-H1 from microtubules activates the Rho pathway of EC permeability. In turn, pathologic vascular leak can be prevented by treatment with atrial natriuretic peptide (ANP). This study investigated a novel mechanism of vascular barrier protection by ANP via modulation of GEF-H1 function. In pulmonary ECs, ANP suppressed thrombin-induced disassembly of peripheral MT and attenuated Rho signaling and cell retraction. ANP effects were mediated by the Rac1 GTPase effector PAK1. Activation of Rac1-PAK1 promoted PAK1 interaction with the Rho activator GEF-H1, inducing phosphorylation of total and MT-bound GEF-H1 and leading to attenuation of Rho-dependent actin remodeling. In vivo, ANP attenuated lung injury caused by excessive mechanical ventilation and TRAP peptide (TRAP/HTV), which was further exacerbated in ANP(-/-) mice. The protective effects of ANP against TRAP/HTV-induced lung injury were linked to the increased pool of stabilized MT and inactivation of Rho signaling via ANP-induced, PAK1-dependent inhibitory phosphorylation of GEF-H1. This study demonstrates a novel protective mechanism of ANP against pathologic hyperpermeability and suggests a novel pharmacological intervention for the prevention of increased vascular leak via PAK1-dependent modulation of GEF-H1 activity.

Colchicine attenuates renal injury in a model of hypertensive chronic kidney disease

Hypertension is a risk factor for chronic kidney disease, particularly when associated with impaired renal autoregulation and thereby increased intraglomerular pressure (Pgc). Elevated Pgc can be modeled in vitro by exposing glomerular mesangial cells to mechanical strain. We previously showed that RhoA mediates strain-induced matrix production. Here, we show that RhoA activation is dependent on an intact microtubule network. Upregulation of the profibrotic cytokine connective tissue growth factor (CTGF) by mechanical strain is dependent on RhoA activation and inhibited by microtubule disruption. We tested the effects of the microtubule depolymerizing agent colchicine in 5/6 nephrectomized rats, a model of chronic kidney disease driven by elevated Pgc. Colchicine inhibited glomerular RhoA activation and attenuated both glomerular sclerosis and interstitial fibrosis without affecting systemic blood pressure. Upregulation of the matrix proteins collagen I and fibronectin, as well as CTGF, was attenuated by colchicine. Activity of the profibrotic cytokine TGF-β, as assessed by Smad3 phosphorylation, was also inhibited by colchicine. Microtubule disruption significantly decreased renal infiltration of lymphocytes and macrophages. Our studies thus indicate that colchicine modifies hypertensive renal fibrosis. Its protective effects are likely mediated by inhibition of RhoA signaling and renal infiltration of inflammatory cells. Already well-established in clinical practice for other indications, prevention of hypertension-associated renal fibrosis may represent a new potential use for colchicine.