Erbin and the NF2 tumor suppressor Merlin cooperatively regulate cell-type-specific activation of PAK2 by TGF-beta

The molecular biology of NF2/Merlin on tumorigenesis and development

The neurofibromatosis type 2 (NF2) gene, known for encoding the tumor suppressor protein Merlin, is central to the study of tumorigenesis and associated cellular processes. This review comprehensively examines the multifaceted role of NF2/Merlin, detailing its structural characteristics, functional diversity, and involvement in various signaling pathways such as Wnt/β-catenin, Hippo, TGF-β, RTKs, mTOR, Notch, and Hedgehog. These pathways are crucial for cellular growth, proliferation, and differentiation. NF2 mutations are specifically linked to the development of schwannomas, meningiomas, and ependymomas, although the precise mechanisms of tumor formation in these specific cell types remain unclear. Additionally, the review explores Merlin's role in embryogenesis, highlighting the severe developmental defects and embryonic lethality caused by NF2 deficiency. The potential therapeutic strategies targeting these genetic aberrations are also discussed, emphasizing inhibitors of mTOR, HDAC, and VEGF as promising avenues for treatment. This synthesis of current knowledge underscores the necessity for ongoing research to elucidate the detailed mechanisms of NF2/Merlin and develop effective therapeutic strategies, ultimately aiming to improve the prognosis and quality of life for individuals with NF2 mutations.

Molecular mechanisms of pancreatic cancer liver metastasis: the role of PAK2

Background

Pancreatic cancer remains an extremely malignant digestive tract tumor, posing a significant global public health burden. Patients with pancreatic cancer, once metastasis occurs, lose all hope of cure, and prognosis is extremely poor. It is important to investigate liver metastasis of Pancreatic cancer in depth, not just because it is the most common form of metastasis in pancreatic cancer, but also because it is crucial for treatment planning and prognosis assessment. This study aims to delve into the mechanisms of pancreatic cancer liver metastasis, with the goal of providing crucial scientific groundwork for the development of future treatment methods and drugs.

Methods

We explored the mechanisms of pancreatic cancer liver metastasis using single-cell sequencing data (GSE155698 and GSE154778) and bulk data (GSE71729, GSE19279, TCGA-PAAD). Initially, Seurat package was employed for single-cell data processing to obtain expression matrices for primary pancreatic cancer lesions and liver metastatic lesions. Subsequently, high-dimensional weighted gene co-expression network analysis (hdWGCNA) was used to identify genes associated with liver metastasis. Machine learning algorithms and COX regression models were employed to further screen genes related to patient prognosis. Informed by both biological understanding and the outcomes of algorithms, we meticulously identified the ultimate set of liver metastasis-related gene (LRG). In the study of LRG genes, various databases were utilized to validate their association with pancreatic cancer liver metastasis. In order to analyze the effects of these agents on tumor microenvironment, we conducted an in-depth analysis, including changes in signaling pathways (GSVA), cell differentiation (pseudo-temporal analysis), cell communication networks (cell communication analysis), and downstream transcription factors (transcription factor activity prediction). Additionally, drug sensitivity analysis and metabolic analysis were performed to reveal the effects of LRG on gemcitabine resistance and metabolic pathways. Finally, functional experiments were conducted by silencing the expression of LRG in PANC-1 and Bx-PC-3 cells to validate its influence to proliferation and invasiveness on PANC-1 and Bx-PC-3 cells.

Results

Through a series of algorithmic filters, we identified PAK2 as a key gene promoting pancreatic cancer liver metastasis. GSVA analysis elucidated the activation of the TGF-beta signaling pathway by PAK2 to promote the occurrence of liver metastasis. Pseudo-temporal analysis revealed a significant correlation between PAK2 expression and the lower differentiation status of pancreatic cancer cells. Cell communication analysis revealed that overexpression of PAK2 promotes communication between cancer cells and the tumor microenvironment. Transcription factor activity prediction displayed the transcription factor network regulated by PAK2. Drug sensitivity analysis and metabolic analysis revealed the impact of PAK2 on gemcitabine resistance and metabolic pathways. CCK8 experiments showed that silencing PAK2 led to a decrease in the proliferative capacity of pancreatic cancer cells and scratch experiments demonstrated that low expression of PAK2 decreased invasion capability in pancreatic cancer cells. Flow cytometry reveals that PAK2 significantly inhibited apoptosis in pancreatic cancer cell lines. Molecules related to the TGF-beta pathway decreased with the inhibition of PAK2, and there were corresponding significant changes in molecules associated with EMT.

Conclusion

PAK2 facilitated the angiogenic potential of cancer cells and promotes the epithelial-mesenchymal transition process by activating the TGF-beta signaling pathway. Simultaneously, it decreased the differentiation level of cancer cells, consequently enhancing their malignancy. Additionally, PAK2 fostered communication between cancer cells and the tumor microenvironment, augments cancer cell chemoresistance, and modulates energy metabolism pathways. In summary, PAK2 emerged as a pivotal gene orchestrating pancreatic cancer liver metastasis. Intervening in the expression of PAK2 may offer a promising therapeutic strategy for preventing liver metastasis of pancreatic cancer and improving its prognosis.

Pak2-mediated phosphorylation promotes RORγt ubiquitination and inhibits colonic inflammation

Dysregulated interleukin-17 (IL-17) expression and its downstream signaling is strongly linked to inflammatory bowel diseases (IBDs). However, the molecular mechanisms by which the function of RORγt, the transcription factor of IL-17, is regulated remains elusive. By a mass spectrometry-based approach, we identify that Pak2, a serine (S)/threonine (T) kinase, directly associates with RORγt. Pak2 recognizes a conserved KRLS motif within RORγt and phosphorylates the S-316 within this motif. Genetic deletion of Pak2 in Th17 cells reduces RORγt phosphorylation, increases IL-17 expression, and induces severe colitis upon adoptive transfer to Rag1^−/− mice. Similarly, reconstitution of RORγt-S316A mutant in Rorc^−/− Th17 cells enhances IL-17 expression and colitis severity. Mechanistically, we demonstrate that Pak2-mediated phosphorylation causes a conformational change resulting in exposure of the ubiquitin ligase Itch interacting PPLY motif and degradation of RORγt. Thus, we have uncovered a mechanism by which the activity of RORγt is regulated that can be exploited therapeutically.

Kathania et al. show that Pak2, a Ser/Thr kinase, associates with RORγt and phosphorylates Ser-316 of RORγt. Deletion of Pak2 in Th17 cells enhances IL-17 expression and colitis severity. Pak2-mediated phosphorylation causes a conformational change resulting in increased ubiquitination of RORγt by the E3 ubiquitin ligase Itch.

Accessory proteins of the RAS-MAPK pathway: moving from the side line to the front line

Health and disease are directly related to the RTK-RAS-MAPK signalling cascade. After more than three decades of intensive research, understanding its spatiotemporal features is afflicted with major conceptual shortcomings. Here we consider how the compilation of a vast array of accessory proteins may resolve some parts of the puzzles in this field, as they safeguard the strength, efficiency and specificity of signal transduction. Targeting such modulators, rather than the constituent components of the RTK-RAS-MAPK signalling cascade may attenuate rather than inhibit disease-relevant signalling pathways.

Human Umbilical Cord Mesenchymal Stem Cells Ameliorate Hepatic Stellate Cell Activation and Liver Fibrosis by Upregulating MicroRNA-455-3p through Suppression of p21-Activated Kinase-2

Mesenchymal stem cells (MSCs) were shown to have potential therapeutic effects for treatment of liver fibrosis, and dysregulated expression of microRNAs (miRNAs) played a pivotal role in the pathogenesis of liver fibrosis by regulating their downstream target genes. However, the mechanism by which MSCs affect the progression of liver fibrosis by regulating miRNA expression remains unclear. Here, we investigated whether human umbilical cord MSCs (HUC-MSCs) attenuated hepatic fibrosis by regulating miR-455-3p and its target gene. Significantly upregulated miRNA (miR-455-3p) was screened out by GEO datasets analysis and coculture HUC-MSCs with hepatic stellate cell (HSC) LX-2 cells. p21-activated kinase-2 (PAK2) was forecasted to be the target gene of miR-455-3p by bioinformatics analyses and confirmed by luciferase reporter assay. HUC-MSCs were transplanted into mice with carbon tetrachloride- (CCl₄-) induced liver fibrosis, the result showed that HUC-MSC transplantation significantly ameliorated the severity of CCl₄-induced liver fibrosis, attenuated collagen deposition, improved liver function by reducing the expression of alanine aminotransferase (ALT) and aspartate aminotransferase (AST) in serum, upregulated miR-455-3p, and suppressed PAK2 expression of liver tissue in mice. Taken together, our study suggests that HUC-MSCs inhibit the activation of HSCs and mouse CCl₄-induced liver fibrosis by upregulation of miR-455-3p through targeting PAK2.

The leucine-rich repeat signaling scaffolds Shoc2 and Erbin: cellular mechanism and role in disease

Leucine-rich repeat-containing proteins (LRR proteins) are involved in supporting a large number of cellular functions. In this review, we summarize recent advancements in understanding functions of the LRR proteins as signaling scaffolds. In particular, we explore what we have learned about the mechanisms of action of the LRR scaffolds Shoc2 and Erbin and their roles in normal development and disease. We discuss Shoc2 and Erbin in the context of their multiple known interacting partners in various cellular processes and summarize often unexpected functions of these proteins through analysis of their roles in human pathologies. We also review these LRR scaffold proteins as promising therapeutic targets and biomarkers with potential application across various pathologies.

Extracellular Vesicles From Liver Progenitor Cells Downregulates Fibroblast Metabolic Activity and Increase the Expression of Immune-Response Related Molecules

Extracellular vesicles (EVs) mediate cell-to-cell crosstalk whose content can induce changes in acceptor cells and their microenvironment. MLP29 cells are mouse liver progenitor cells that release EVs loaded with signaling cues that could affect cell fate. In the current work, we incubated 3T3-L1 mouse fibroblasts with MLP29-derived EVs, and then analyzed changes by proteomics and transcriptomics. Results showed a general downregulation of protein and transcript expression related to proliferative and metabolic routes dependent on TGF-beta. We also observed an increase in the ERBB2 interacting protein (ERBIN) and Cxcl2, together with an induction of ribosome biogenesis and interferon-related response molecules, suggesting the activation of immune system signaling.

The dichotomous role of TGF-β in controlling liver cancer cell survival and proliferation

Hepatocellular carcinoma (HCC) is the major form of primary liver cancer and one of the most prevalent and life-threatening malignancies globally. One of the hallmarks in HCC is the sustained cell survival and proliferative signals, which are determined by the balance between oncogenes and tumor suppressors. Transforming growth factor beta (TGF-β) is an effective growth inhibitor of epithelial cells including hepatocytes, through induction of cell cycle arrest, apoptosis, cellular senescence, or autophagy. The antitumorigenic effects of TGF-β are bypassed during liver tumorigenesis via multiple mechanisms. Furthermore, along with malignant progression, TGF-β switches to promote cancer cell survival and proliferation. This dichotomous nature of TGF-β is one of the barriers to therapeutic targeting in liver cancer. Thereafter, understanding the underlying molecular mechanisms is a prerequisite for discovering novel antitumor drugs that may specifically disable the growth-promoting branch of TGF-β signaling or restore its tumor-suppressive arm. This review summarizes how TGF-β inhibits or promotes liver cancer cell survival and proliferation, highlighting the functional switch mechanisms during the process.

Merlin regulates signaling events at the nexus of development and cancer

Background

In this review, we describe how the cytoskeletal protein Merlin, encoded by the Neurofibromin 2 (NF2) gene, orchestrates developmental signaling to ensure normal ontogeny, and we discuss how Merlin deficiency leads to aberrant activation of developmental pathways that enable tumor development and malignant progression.

Main body

Parallels between embryonic development and cancer have underscored the activation of developmental signaling pathways. Hippo, WNT/β-catenin, TGF-β, receptor tyrosine kinase (RTK), Notch, and Hedgehog pathways are key players in normal developmental biology. Unrestrained activity or loss of activity of these pathways causes adverse effects in developing tissues manifesting as developmental syndromes. Interestingly, these detrimental events also impact differentiated and functional tissues. By promoting cell proliferation, migration, and stem-cell like phenotypes, deregulated activity of these pathways promotes carcinogenesis and cancer progression. The NF2 gene product, Merlin, is a tumor suppressor classically known for its ability to induce contact-dependent growth inhibition. Merlin plays a role in different stages of an organism development, ranging from embryonic to mature states. While homozygous deletion of Nf2 in murine embryos causes embryonic lethality, Merlin loss in adult tissue is implicated in Neurofibromatosis type 2 disorder and cancer. These manifestations, cumulatively, are reminiscent of dysregulated developmental signaling.

Conclusion

Understanding the molecular and cellular repercussions of Merlin loss provides fundamental insights into the etiology of developmental disorders and cancer and has the potential, in the long term, to identify new therapeutic strategies.

The Scribble family in cancer: twentieth anniversary

Among the more than 160 PDZ containing proteins described in humans, the cytoplasmic scaffold Scribble stands out because of its essential role in many steps of cancer development and dissemination. Its fame has somehow blurred the importance of homologous proteins, Erbin and Lano, all belonging to the LRR and PDZ (LAP) protein family first described twenty years ago. In this review, we will retrace the history of LAP family protein research and draw attention to their contribution in cancer by detailing the features of its members at the structural and functional levels, and highlighting their shared-but also different-implication in the tumoral process.

Proximity biotinylation identifies a set of conformation-specific interactions between Merlin and cell junction proteins

Neurofibromatosis type 2 is an inherited, neoplastic disease associated with schwannomas, meningiomas, and ependymomas and that is caused by inactivation of the tumor suppressor gene NF2 The NF2 gene product, Merlin, has no intrinsic catalytic activity; its tumor suppressor function is mediated through the proteins with which it interacts. We used proximity biotinylation followed by mass spectrometry and direct binding assays to identify proteins that associated with wild-type and various mutant forms of Merlin in immortalized Schwann cells. We defined a set of 52 proteins in close proximity to wild-type Merlin. Most of the Merlin-proximal proteins were components of cell junctional signaling complexes, suggesting that additional potential interaction partners may exist in adherens junctions, tight junctions, and focal adhesions. With mutant forms of Merlin that cannot bind to phosphatidylinositol 4,5-bisphosphate (PIP₂) or that constitutively adopt a closed conformation, we confirmed a critical role for PIP₂ binding in Merlin function and identified a large cohort of proteins that specifically interacted with Merlin in the closed conformation. Among these proteins, we identified a previously unreported Merlin-binding protein, apoptosis-stimulated p53 protein 2 (ASPP2, also called Tp53bp2), that bound to closed-conformation Merlin predominately through the FERM domain. Our results demonstrate that Merlin is a component of cell junctional mechanosensing complexes and defines a specific set of proteins through which it acts.

Therapeutic effects of Erbin inhibitor on spinal cord contusion in mice

Erbin has been shown to maintain the integrity of cell structure, regulate the proliferation and differentiation of cell and transconduct signals in the pathways. This study was conducted to assess the therapeutic effects of an Erbin inhibitor on spinal cord contusion in mice. Spinal contusion models of mouse were constructed and treated with an Erbin inhibitor. The experimental animals were divided into control (normal animal without any treatment), models with spinal cord injury (SIM), and models receiving Erbin inhibitor (Inhibitor). The contents of 5-hydroxytryptamine (5-HT) and reactive oxygen species (ROS) in the brain and spinal cord tissues were measured using ELISA. The expression of ERK1/2, MAPK, NF-kB and NRG1 was quantified using qRT-PCR, Western blot analysis and immunohistochemistry. Flow cytometry was used to determine the formation of macrophages. Erbin interference vector was constructed and its interference effect on the expression of these genes was characterized in cultured bone marrow cells. Spinal contusion models were successfully constructed. Administering Erbin inhibitor inhibited the expression of ERK1/2, MAPK and NF-kB and up-regulated the expression of NRG1. Flow cytometry showed that Erbin inhibitor induced the formation of a large number of macrophages, which are beneficial to the recovery of spinal cord injury. Experiments with Erbin interference vector showed similar impacts on the expression of genes at cellular level as the inhibitor did. Our work has demonstrated that the Erbin inhibitor is very effective to treat spinal cord contusion in mice. The possible mechanism of therapeutic effect is that the inhibitor suppresses the ERK1/2/MAPK and/or NF-kB/MAPK signal pathways and enhances the NRG1-ErbB signaling pathway by reducing the expression of Erbin, leading to the inhibition of apoptosis, promotion of proliferation and differentiation, and subsequent repair of the damaged spinal cord.

miR-183-5p Inhibits Occurrence and Progression of Acute Myeloid Leukemia via Targeting Erbin

Erbin has been shown to have significant effects on the development of solid tumors. However, little is known about its function and regulatory mechanism in hematological malignancies. The biological function of Erbin on cell proliferation was measured in vitro and in vivo. The predicted target of Erbin was validated by dual-luciferase reporter assay and rescue experiment. We found that overexpression of Erbin could inhibit the cell proliferation and promote the cell differentiation of acute myeloid leukemia (AML) cells, whereas depletion of Erbin could enhance the cell proliferation and block the cell differentiation in AML cells in vitro and in vivo. Besides, miR-183-5p was identified as the upstream regulator that negatively regulated the Erbin expression. The results were confirmed by dual-luciferase reporter and RNA pull-down assay. Furthermore, we found that miR-183-5p negatively regulated Erbin, resulting in enhanced cell proliferation of AML cells via activation of RAS/RAF/MEK/ERK and PI3K/AKT/FoxO3a pathways. The activation of RAS/RAF/MEK/ERK and PI3K/AKT/FoxO3a pathways was mediated by Erbin interacting with Grb2. These results were also validated by rescue experiments in vitro and in vivo. All above-mentioned findings indicated that the miR-183-5p/Erbin signaling pathway might represent a novel prognostic biomarker or therapeutic target for treatment of AML.

Proliferation-independent role of NF2 (merlin) in limiting biliary morphogenesis

The architecture of individual cells and cell collectives enables functional specification, a prominent example being the formation of epithelial tubes that transport fluid or gas in many organs. The intrahepatic bile ducts (IHBDs) form a tubular network within the liver parenchyma that transports bile to the intestine. Aberrant biliary 'neoductulogenesis' is also a feature of several liver pathologies including tumorigenesis. However, the mechanism of biliary tube morphogenesis in development or disease is not known. Elimination of the neurofibromatosis type 2 protein (NF2; also known as merlin or neurofibromin 2) causes hepatomegaly due to massive biliary neoductulogenesis in the mouse liver. We show that this phenotype reflects unlimited biliary morphogenesis rather than proliferative expansion. Our studies suggest that NF2 normally limits biliary morphogenesis by coordinating lumen expansion and cell architecture. This work provides fundamental insight into how biliary fate and tubulogenesis are coordinated during development and will guide analyses of disease-associated and experimentally induced biliary pathologies.

Cell-penetrating peptides selectively targeting SMAD3 inhibit profibrotic TGF-β signaling

TGF-β is considered a master switch in the pathogenesis of organ fibrosis. The primary mediators of this activity are the SMAD proteins, particularly SMAD3. In the current study, we have developed a cell-penetrating peptide (CPP) conjugate of the HIV TAT protein that is fused to an aminoterminal sequence of sorting nexin 9 (SNX9), which was previously shown to bind phosphorylated SMAD3 (pSMAD3). We determined that specifically preventing the nuclear import of pSMAD3 using the TAT-SNX9 peptide inhibited profibrotic TGF-β activity in murine cells and human lung fibroblasts as well as in vivo with no demonstrable toxicity. TGF-β signaling mediated by pSMAD2, bone morphogenetic protein 4 (BMP4), EGF, or PDGF was unaffected by the TAT-SNX9 peptide. Furthermore, while the TAT-SNX9 peptide prevented TGF-β's profibrotic activity in vitro as well as in 2 murine treatment models of pulmonary fibrosis, a 3-amino acid point mutant that was unable to bind pSMAD3 proved ineffective. These findings indicate that specifically targeting pSMAD3 can ameliorate both the direct and indirect fibroproliferative actions of TGF-β.

Erbin interacts with c-Cbl and promotes tumourigenesis and tumour growth in colorectal cancer by preventing c-Cbl-mediated ubiquitination and down-regulation of EGFR

The epidermal growth factor receptor (EGFR) is implicated in many types of cancer, including colorectal cancer (CRC), and has become one of the most common candidates for targeted therapy. Here, we found that Erbin, a member of the leucine-rich repeat and PDZ domain (LAP) family, plays a key role in EGFR signalling. Erbin inhibited EGFR ubiquitination and stabilized the EGFR protein by interacting with c-Cbl. Moreover, the PDZ domain of Erbin was critical for the interaction between Erbin and c-Cbl and EGFR ubiquitination. Interestingly, Erbin expression was elevated in tumour samples from CRC patients, increased in advanced clinical stage disease and correlated with EGFR expression. In vivo studies using mouse xenograft models of CRC showed that Erbin promotes tumour growth, and that the effects of Erbin on tumour growth are mainly related to the regulatory effects of Erbin on EGFR. The azoxymethane (AOM)-induced colon carcinogenesis model in Erbin(ΔC) (/) (ΔC) mice, with the PDZ domain of Erbin deleted, demonstrated that the PDZ domain of Erbin and its regulation of EGFR signalling are necessary for the tumourigenesis and tumour growth of CRC. We found that Erbin promotes tumourigenesis and tumour growth in CRC by stabilizing EGFR. Our study sheds light on developing Erbin, especially its PDZ domain, as a potential target for CRC treatment.

Activation of Rac-1 and RhoA contributes to podocyte injury in chronic kidney disease

Rho-family GTPases like RhoA and Rac-1 are potent regulators of cellular signaling that control gene expression, migration and inflammation. Activation of Rho-GTPases has been linked to podocyte dysfunction, a feature of chronic kidney diseases (CKD). We investigated the effect of Rac-1 and Rho kinase (ROCK) inhibition on progressive renal failure in mice and studied the underlying mechanisms in podocytes. SV129 mice were subjected to 5/6-nephrectomy which resulted in arterial hypertension and albuminuria. Subgroups of animals were treated with the Rac-1 inhibitor EHT1846, the ROCK inhibitor SAR407899 and the ACE inhibitor Ramipril. Only Ramipril reduced hypertension. In contrast, all inhibitors markedly attenuated albumin excretion as well as glomerular and tubulo-interstitial damage. The combination of SAR407899 and Ramipril was more effective in preventing albuminuria than Ramipril alone. To study the involved mechanisms, podocytes were cultured from SV129 mice and exposed to static stretch in the Flexcell device. This activated RhoA and Rac-1 and led via TGFβ to apoptosis and a switch of the cells into a more mesenchymal phenotype, as evident from loss of WT-1 and nephrin and induction of α-SMA and fibronectin expression. Rac-1 and ROCK inhibition as well as blockade of TGFβ dramatically attenuated all these responses. This suggests that Rac-1 and RhoA are mediators of podocyte dysfunction in CKD. Inhibition of Rho-GTPases may be a novel approach for the treatment of CKD.

FRAX597, a small molecule inhibitor of the p21-activated kinases, inhibits tumorigenesis of neurofibromatosis type 2 (NF2)-associated Schwannomas

The p21-activated kinases (PAKs) are immediate downstream effectors of the Rac/Cdc42 small G-proteins and implicated in promoting tumorigenesis in various types of cancer including breast and lung carcinomas. Recent studies have established a requirement for the PAKs in the pathogenesis of Neurofibromatosis type 2 (NF2), a dominantly inherited cancer disorder caused by mutations at the NF2 gene locus. Merlin, the protein product of the NF2 gene, has been shown to negatively regulate signaling through the PAKs and the tumor suppressive functions of Merlin are mediated, at least in part, through inhibition of the PAKs. Knockdown of PAK1 and PAK2 expression, through RNAi-based approaches, impairs the proliferation of NF2-null schwannoma cells in culture and inhibits their ability to form tumors in vivo. These data implicate the PAKs as potential therapeutic targets. High-throughput screening of a library of small molecules combined with a structure-activity relationship approach resulted in the identification of FRAX597, a small-molecule pyridopyrimidinone, as a potent inhibitor of the group I PAKs. Crystallographic characterization of the FRAX597/PAK1 complex identifies a phenyl ring that traverses the gatekeeper residue and positions the thiazole in the back cavity of the ATP binding site, a site rarely targeted by kinase inhibitors. FRAX597 inhibits the proliferation of NF2-deficient schwannoma cells in culture and displayed potent anti-tumor activity in vivo, impairing schwannoma development in an orthotopic model of NF2. These studies identify a novel class of orally available ATP-competitive Group I PAK inhibitors with significant potential for the treatment of NF2 and other cancers.

P21-activated kinase-2 is a critical mediator of transforming growth factor-β-induced hepatoma cell migration

BACKGROUND AND AIM

Transforming growth factor-β (TGF-β) has been shown to play a central role in the promotion of cell motility, but its functional mechanism has remained unclear. With the aim of investigating the diagnostic and treatment modalities for patients with hepatocellular carcinoma (HCC), the signaling pathway that may contribute to TGF-β-mediated cell invasion in hepatoma cells was evaluated.

METHODS

Three hepatoma cell lines, HepG2, PLC/PRF/5, and HLF, were treated with TGF-β, and the involvement of the non-canonical TGF-β pathway was analyzed by cell migration assays. HepG2 cells were treated with a p21-activated kinase-2 (PAK2)-targeting small interfering RNA and analyzed for their cell motility. The relationships between the PAK2 status and the clinicopathological characteristics of 62 HCC patients were also analyzed.

RESULTS

The cell migration assays showed that Akt is a critical regulator of TGF-β-mediated cell migration. Western blotting analyses showed that TGF-β stimulated Akt and PAK2 in all three hepatoma cell lines, and phosphorylated PAK2 was blocked by Akt inhibitor. Suppression of PAK2 expression by small interfering RNA resulted in increased focal adhesions with significantly repressed cell migration in the presence of TGF-β. Clinicopathological analyses showed that the phosphorylation level of PAK2 was closely associated with tumor progression, metastasis, and early recurrence of HCC.

CONCLUSIONS

PAK2 may be a critical mediator of TGF-β-mediated hepatoma cell migration, and may represent a potential target for the treatment of HCC.

Desmoglein-1/Erbin interaction suppresses ERK activation to support epidermal differentiation

Genetic disorders of the Ras/MAPK pathway, termed RASopathies, produce numerous abnormalities, including cutaneous keratodermas. The desmosomal cadherin, desmoglein-1 (DSG1), promotes keratinocyte differentiation by attenuating MAPK/ERK signaling and is linked to striate palmoplantar keratoderma (SPPK). This raises the possibility that cutaneous defects associated with SPPK and RASopathies share certain molecular faults. To identify intermediates responsible for executing the inhibition of ERK by DSG1, we conducted a yeast 2-hybrid screen. The screen revealed that Erbin (also known as ERBB2IP), a known ERK regulator, binds DSG1. Erbin silencing disrupted keratinocyte differentiation in culture, mimicking aspects of DSG1 deficiency. Furthermore, ERK inhibition and the induction of differentiation markers by DSG1 required both Erbin and DSG1 domains that participate in binding Erbin. Erbin blocks ERK signaling by interacting with and disrupting Ras-Raf scaffolds mediated by SHOC2, a protein genetically linked to the RASopathy, Noonan-like syndrome with loose anagen hair (NS/LAH). DSG1 overexpression enhanced this inhibitory function, increasing Erbin-SHOC2 interactions and decreasing Ras-SHOC2 interactions. Conversely, analysis of epidermis from DSG1-deficient patients with SPPK demonstrated increased Ras-SHOC2 colocalization and decreased Erbin-SHOC2 colocalization, offering a possible explanation for the observed epidermal defects. These findings suggest a mechanism by which DSG1 and Erbin cooperate to repress MAPK signaling and promote keratinocyte differentiation.

Microarray analysis of gene expression in vestibular schwannomas reveals SPP1/MET signaling pathway and androgen receptor deregulation

Vestibular schwannomas are benign neoplasms that arise from the vestibular nerve. The hallmark of these tumors is the biallelic inactivation of neurofibromin 2 (NF2). Transcriptomic alterations, such as the neuregulin 1 (NRG1)/ErbB2 pathway, have been described in schwannomas. In this study, we performed a whole transcriptome analysis in 31 vestibular schwannomas and 9 control nerves in the Affymetrix Gene 1.0 ST platform, validated by quantitative real-time PCR (qRT-PCR) using TaqMan Low Density arrays. We performed a mutational analysis of NF2 by PCR/denaturing high-performance liquid chromatography (dHPLC) and multiplex ligation-dependent probe amplification (MLPA), as well as a microsatellite marker analysis of the loss of heterozygosity (LOH) of chromosome 22q. The microarray analysis demonstrated that 1,516 genes were deregulated and 48 of the genes were validated by qRT-PCR. At least 2 genetic hits (allelic loss and/or gene mutation) in NF2 were found in 16 tumors, seven cases showed 1 hit and 8 tumors showed no NF2 alteration. MET and associated genes, such as integrin, alpha 4 (ITGA4)/B6, PLEXNB3/SEMA5 and caveolin-1 (CAV1) showed a clear deregulation in vestibular schwannomas. In addition, androgen receptor (AR) downregulation may denote a hormonal effect or cause in this tumor. Furthermore, the osteopontin gene (SPP1), which is involved in merlin protein degradation, was upregulated, which suggests that this mechanism may also exert a pivotal role in schwannoma merlin depletion. Finally, no major differences were observed among tumors of different size, histological type or NF2 status, which suggests that, at the mRNA level, all schwannomas, regardless of their molecular and clinical characteristics, may share common features that can be used in their treatment.

c-Myb regulates cell cycle-dependent expression of Erbin: an implication for a novel function of Erbin

In the present study, we demonstrated the cell cycle periodicity of Erbin expression with the maximal expression of Erbin in G2/M phase. A significant increase in Erbin promoter activity was observed in G2/M phase-synchronized cells. Sequence analysis revealed a c-Myb site in the core promoter region of Erbin. Mutagenesis of c-Myb consensus sequences abrogated the increased Erbin promoter activity in G2/M phase. ChIP and oligonucleotide pull-down assays validated that the recruitment of c-Myb to the consensus sequences was specific. The interaction of c-Myb with c-Myb site in the Erbin promoter was significantly enhanced in G2/M phase. Ectopic overexpression of c-Myb led to the up-regulation of Erbin promoter activity and c-Myb silencing by small interfering RNA significantly decreased Erbin protein level. Transfection of c-Myb rescued Erbin expression that was impaired by c-Myb knockdown. It proves that c-Myb and the c-Myb response element mediate the cell cycle-dependent expression of Erbin. Inactivation of Erbin causes an acceleration of the G1/S transition, the formation of multipolar spindles and abnormal chromosome congression. These results unravel a critical role of c-Myb in promoting Erbin transcription in G2/M phase and also predict an unappreciated function of Erbin in cell cycle progression.

Tgf-beta induced Erk phosphorylation of smad linker region regulates smad signaling

The Transforming Growth Factor-Beta (TGF-β) family is involved in regulating a variety of cellular processes such as apoptosis, differentiation, and proliferation. TGF-β binding to a Serine/Threonine kinase receptor complex causes the recruitment and subsequent activation of transcription factors known as smad2 and smad3. These proteins subsequently translocate into the nucleus to negatively or positively regulate gene expression. In this study, we define a second signaling pathway leading to TGF-β receptor activation of Extracellular Signal Regulated Kinase (Erk) in a cell-type dependent manner. TGF-β induced Erk activation was found in phenotypically normal mesenchymal cells, but not normal epithelial cells. By activating phosphotidylinositol 3-kinase (PI3K), TGF-β stimulates p21-activated kinase2 (Pak2) to phosphorylate c-Raf, ultimately resulting in Erk activation. Activation of Erk was necessary for TGF-β induced fibroblast replication. In addition, Erk phosphorylated the linker region of nuclear localized smads, resulting in increased half-life of C-terminal phospho-smad 2 and 3 and increased duration of smad target gene transcription. Together, these data show that in mesenchymal cell types the TGF-β/PI3K/Pak2/Raf/MEK/Erk pathway regulates smad signaling, is critical for TGF-β-induced growth and is part of an integrated signaling web containing multiple interacting pathways rather than discrete smad/non-smad pathways.

Merlin: a tumour suppressor with functions at the cell cortex and in the nucleus

Inhibition of proliferation by cell-to-cell contact is essential for tissue organization, and its disruption contributes to tumorigenesis. The FERM domain protein Merlin, encoded by the NF2 tumour suppressor gene, is an important mediator of contact inhibition. Merlin was thought to inhibit mitogenic signalling and activate the Hippo pathway by interacting with diverse target-effectors at or near the plasma membrane. However, recent studies highlight that Merlin pleiotropically affects signalling by migrating into the nucleus and inducing a growth-suppressive programme of gene expression through its direct inhibition of the CRL4DCAF1 E3 ubiquitin ligase. In addition, Merlin promotes the establishment of epithelial adhesion and polarity by recruiting Par3 and aPKC to E-cadherin-dependent junctions, and by ensuring the assembly of tight junctions. These recent advances suggest that Merlin acts at the cell cortex and in the nucleus in a similar, albeit antithetic, manner to the oncogene β-catenin.

Cell type specificity of signaling: view from membrane receptors distribution and their downstream transduction networks

Studies on cell signaling pay more attention to spatial dynamics and how such diverse organization can relate to high order of cellular capabilities. To overview the specificity of cell signaling, we integrated human receptome data with proteome spatial expression profiles to systematically investigate the specificity of receptors and receptor-triggered transduction networks across 62 normal cell types and 14 cancer types. Six percent receptors showed cell-type-specific expression, and 4% signaling networks presented enriched cell-specific proteins induced by the receptors. We introduced a concept of "response context" to annotate the cell-type dependent signaling networks. We found that most cells respond similarly to the same stimulus, as the "response contexts" presented high functional similarity. Despite this, the subtle spatial diversity can be observed from the difference in network architectures. The architecture of the signaling networks in nerve cells displayed less completeness than that in glandular cells, which indicated cellular-context dependent signaling patterns are elaborately spatially organized. Likewise, in cancer cells most signaling networks were generally dysfunctional and less complete than that in normal cells. However, glioma emerged hyper-activated transduction mechanism in malignant state. Receptor ATP6AP2 and TNFRSF21 induced rennin-angiotensin and apoptosis signaling were found likely to explain the glioma-specific mechanism. This work represents an effort to decipher context-specific signaling network from spatial dimension. Our results indicated that although a majority of cells engage general signaling response with subtle differences, the spatial dynamics of cell signaling can not only deepen our insights into different signaling mechanisms, but also help understand cell signaling in disease.

p21-Activated kinase 2 (PAK2) inhibits TGF-β signaling in Madin-Darby canine kidney (MDCK) epithelial cells by interfering with the receptor-Smad interaction

TGF-β (transforming growth factor β) plays a variety of cellular functions mainly through the Smad pathway. Phosphorylation of the carboxyl SXS motif in R-Smads (Smad2 and Smad3) by the type I receptor TβRI is a key step for their activation. It has been reported that the serine/threonine kinase PAK2 (p21-activated kinase 2) can mediate TGF-β signaling in mesenchymal cells. Here, we report that PAK2 restricts TGF-β-induced Smad2/3 activation and transcriptional responsiveness in MDCK epithelial cells. Mechanistically, PAK2 associates with Smad2 and Smad3 in a kinase activity-dependent manner and blocks their activation. PAK2 phosphorylates Smad2 at Ser-417, which is adjacent to the L3 loop that contributes to the TβRI-R-Smad association. Consistently, substitution of Ser-417 with glutamic acid attenuates the interaction of Smad2 with TβRI. Together, our results indicate that PAK2 negatively modulate TGF-β signaling by attenuating the receptor-Smad interaction and thus Smad activation.

Peptidoglycan: a critical activator of the mammalian immune system during infection and homeostasis

Peptidoglycan is a conserved structural component of the bacterial cell wall with molecular motifs unique to bacteria. The mammalian immune system takes advantage of these properties and has evolved to recognize this microbial associated molecular pattern. Mammals have four secreted peptidoglycan recognition proteins, PGLYRP-1-4, as well as two intracellular sensors of peptidoglycan, Nod1 and Nod2. Recognition of peptidoglycan is important in initiating and shaping the immune response under both homeostatic and infection conditions. During infection, peptidoglycan recognition drives both cell-autonomous and whole-organism defense responses. Here, we examine recent advances in the understanding of how peptidoglycan recognition shapes mammalian immune responses in these diverse contexts.

ERBIN is a new SARA-interacting protein: competition between SARA and SMAD2 and SMAD3 for binding to ERBIN

SARA, an early endosomal protein, plays a key role in TGFβ signalling, as it presents SMAD2 and SMAD3 for phosphorylation by the activated TGFβ receptors. Here, we show that ERBIN is a new SARA-interacting protein that can be recruited by SARA to early endosomes. ERBIN was recently shown to bind and segregate phosphorylated SMAD2 and SMAD3 (SMAD2/3) in the cytoplasm, thereby inhibiting SMAD2/3-dependent transcription. SARA binds to ERBIN using a new domain, which we have called the ERBID (ERBIN-binding domain), whereas ERBIN binds to SARA using a domain (amino acids 1208-1265) that also interacts with SMAD2 and SMAD3, which we have called the SSID (SARA- and SMAD-interacting domain). We additionally show that SARA competes with SMAD2/3 for binding to ERBIN. In agreement, overexpression of SARA or the ERBID peptide reverses the inhibitory effect of ERBIN on SMAD2/3-dependent transcription. Taken together, these data suggest that the response of cells to TGFβ and activin A can be influenced by the relative concentrations of SARA, ERBIN and SMAD2/3.

Non-Smad transforming growth factor-β signaling regulated by focal adhesion kinase binding the p85 subunit of phosphatidylinositol 3-kinase

TGF-β modulates numerous diverse cellular phenotypes including growth arrest in epithelial cells and proliferation in fibroblasts. Although the Smad pathway is fundamental for the majority of these responses, recent evidence indicates that non-Smad pathways may also have a critical role. Here we report a novel mechanism whereby the nonreceptor tyrosine focal adhesion kinase (FAK) functions as an adaptor necessary for cell type-specific responses to TGF-β. We show that in contrast to Smad actions, non-Smad pathways, including c-Abl, PAK2, and Akt, display an obligate requirement for FAK. Interestingly, this occurs in Src null SYF cells and is independent of FAK tyrosine phosphorylation, kinase activity, and/or proline-rich sequences in the C-terminal FAT domain. FAK binds the phosphatidylinositol 3-kinase (PI3K) p85 regulatory subunit following TGF-β treatment in a subset of fibroblasts but not epithelial cells and has an obligate role in TGF-β-stimulated anchorage-independent growth and migration. Together, these results uncover a new scaffolding role for FAK as the most upstream component regulating the profibrogenic action of TGF-β and suggest that inhibiting this interaction may be useful in treating a number of fibrotic diseases.

Identification of Erbin interlinking MuSK and ErbB2 and its impact on acetylcholine receptor aggregation at the neuromuscular junction

Erbin, a binding partner of ErbB2, was identified as the first member of the LAP family of proteins. Erbin was shown at postsynaptic membranes of the neuromuscular junction (NMJ) or in cultured C2C12 myotubes (1) to be concentrated, (2) to regulate the Ras-Raf-Mek pathway, and (3) to inhibit TGF-beta signaling. In the CNS, Erbin interacts with PSD-95. Furthermore, agrin-MuSK signaling initiates formation of AChR aggregates at the postsynaptic membrane. In search of proteins interacting with MuSK, we identified Erbin as a MuSK binding protein. We verified the interaction of MuSK with Erbin, or both concomitantly with ErbB2 by coimmunoprecipitation, and we mapped the interacting epitopes between Erbin and MuSK. We demonstrated elevated mRNA levels of Erbin at synaptic nuclei and colocalized Erbin and MuSK at postsynaptic membranes. We identified several Erbin isoforms at the NMJ, all of which contained the MuSK binding domain. By knocking down Erbin, we observed agrin-dependent AChR aggregates on murine primary skeletal myotubes and C2C12 cells, and in the absence of agrin, microclusters, both of significantly lower density. Complementary, AChR-epsilon-reporter expression was reduced in myotubes overexpressing Erbin. We show that myotubes also express other LAP protein family members, namely Scribble and Lano, and that both affect physical dimensions of agrin-dependent AChR aggregates and density of microclusters formed in the absence of agrin. Moreover, MuSK-Erbin-ErbB2 signaling influences TGF-beta signaling. Our data define the requirement of Erbin on the cross talk between agrin and neuregulin signaling pathways at the NMJ.

Development of small-molecule inhibitors of the group I p21-activated kinases, emerging therapeutic targets in cancer

The p21-activated kinases (PAKs), immediate downstream effectors of the small G-proteins of the Rac/cdc42 family, are critical mediators of signaling pathways regulating cellular behaviors and as such, have been implicated in pathological conditions including cancer. Recent studies have validated the requirement for PAKs in promoting tumorigenesis in breast carcinoma and neurofibromatosis. Thus, there has been considerable interest in the development of inhibitors to the PAKs, as biological markers and leads for the development of therapeutics. While initial approaches were based on screening for competitive organic inhibitors, more recent efforts have focused on the identification of allosteric inhibitors, organometallic ATP-competitive inhibitors and the use of PAK1/inhibitor crystal structures for inhibitor optimization. This has led to the identification of highly selective and potent inhibitors, which will serve as a basis for further development of inhibitors for therapeutic applications.

The tale of transforming growth factor-beta (TGFbeta) signaling: a soigné enigma

Transforming growth factor-beta (TGFbeta) is a secreted cytokine, which intricately controls a plethora of physiological and pathological processes during development and carcinogenesis. TGFbeta exerts antiproliferative effects and functions as a tumor suppressor during early stages of tumorigenesis, whereas at later stages it functions as a tumor promoter aiding in metastatic progression through an autocrine TGFbeta loop. Intricate knowledge of TGFbeta signaling and its regulation are still evolving. In this review, we make an attempt to showcase the associated enigma of TGFbeta signaling in its dual functional role as tumor suppressor and metastatic promoter during early and late stages of carcinogenesis, respectively.

Tgf-beta superfamily signaling in embryonic development and homeostasis

TGF-beta superfamily signaling pathways emerged with the evolution of multicellular animals, suggesting that these pathways contribute to the increased diversity and complexity required for the development and homeostasis of these organisms. In this review we begin by exploring some key developmental and disease processes requiring TGF-beta ligands to underscore the fundamental importance of these pathways before delving into the molecular mechanism of signal transduction, focusing on recent findings. Finally, we discuss how these ligands act as morphogens, how their activity and signaling range is regulated, and how they interact with other signaling pathways to achieve their specific and varied functional roles.