Localization of epidermal growth factor-stimulated Ras/Raf-1 interaction to caveolae membrane

Caveolin-1 affects early mycobacterial infection and apoptosis in macrophages and mice

Tuberculosis, caused by Mycobacterium tuberculosis, remains one of the deadliest infections in humans. Because Mycobacterium bovis Bacillus Calmette-Guérin (BCG) share genetic similarities with Mycobacterium tuberculosis, it is often used as a model to elucidate the molecular mechanisms of more severe tuberculosis infection. Caveolin-1 has been implied in many physiological processes and diseases, but it's role in mycobacterial infections has barely been studied. We isolated macrophages from Wildtype or Caveolin-1 deficient mice and analyzed hallmarks of infection, such as internalization, induction of autophagy and apoptosis. For in vivo assays we intravenously injected mice with BCG and investigated tissues for bacterial load with colony-forming unit assays, bioactive lipids with mass spectrometry and changes of protein expressions by Western blotting. Our results revealed that Caveolin-1 was important for early killing of BCG infection in vivo and in vitro, controlled acid sphingomyelinase (Asm)-dependent ceramide formation, apoptosis and inflammatory cytokines upon infection with BCG. In accordance, Caveolin-1 deficient mice and macrophages showed higher bacterial burdens in the livers. The findings indicate that Caveolin-1 plays a role in infection of mice and murine macrophages with BCG, by controlling cellular apoptosis and inflammatory host response. These clues might be useful in the fight against tuberculosis.

Unravelling molecular dynamics in living cells: Fluorescent protein biosensors for cell biology

Genetically encoded, fluorescent protein (FP)-based Förster resonance energy transfer (FRET) biosensors are microscopy imaging tools tailored for the precise monitoring and detection of molecular dynamics within subcellular microenvironments. They are characterised by their ability to provide an outstanding combination of spatial and temporal resolutions in live-cell microscopy. In this review, we begin by tracing back on the historical development of genetically encoded FP labelling for detection in live cells, which lead us to the development of early biosensors and finally to the engineering of single-chain FRET-based biosensors that have become the state-of-the-art today. Ultimately, this review delves into the fundamental principles of FRET and the design strategies underpinning FRET-based biosensors, discusses their diverse applications and addresses the distinct challenges associated with their implementation. We place particular emphasis on single-chain FRET biosensors for the Rho family of guanosine triphosphate hydrolases (GTPases), pointing to their historical role in driving our understanding of the molecular dynamics of this important class of signalling proteins and revealing the intricate relationships and regulatory mechanisms that comprise Rho GTPase biology in living cells.

Caveolin-1: A Review of Intracellular Functions, Tissue-Specific Roles, and Epithelial Tight Junction Regulation

Caveolin-1 (Cav1) is a vital protein for many cellular processes and is involved in both the positive and negative regulation of these processes. Cav1 exists in multiple cellular compartments depending on its role. Of particular interest is its contribution to the formation of plasma membrane invaginations called caveolae and its involvement in cytoskeletal interactions, endocytosis, and cholesterol trafficking. Cav1 participates in stem cell differentiation as well as proliferation and cell death pathways, which is implicated in tumor growth and metastasis. Additionally, Cav1 has tissue-specific functions that are adapted to the requirements of the cells within those tissues. Its role has been described in adipose, lung, pancreatic, and vascular tissue and in epithelial barrier maintenance. In both the intestinal and the blood brain barriers, Cav1 has significant interactions with junctional complexes that manage barrier integrity. Tight junctions have a close relationship with Cav1 and this relationship affects both their level of expression and their location within the cell. The ubiquitous nature of Cav1 both within the cell and within specific tissues is what makes the protein important for ongoing research as it can assist in further understanding pathophysiologic processes and can potentially be a target for therapies.

How cells sense and integrate information from different sources

Cell signaling is a fundamental cellular process that enables cells to sense and respond to information in their surroundings. At the molecular level, signaling is primarily carried out by transmembrane protein receptors that can initiate complex downstream signal transduction cascades to alter cellular behavior. In the human body, different cells can be exposed to a wide variety of environmental conditions, and cells express diverse classes of receptors capable of sensing and integrating different signals. Furthermore, different receptors and signaling pathways can crosstalk with each other to calibrate the cellular response. Crosstalk occurs through multiple mechanisms at different levels of signaling pathways. In this review, we discuss how cells sense and integrate different chemical, mechanical, and spatial signals as well as the mechanisms of crosstalk between pathways. To illustrate these concepts, we use a few well-studied signaling pathways, including receptor tyrosine kinases and integrin receptors. Finally, we discuss the implications of dysregulated cellular sensing on driving diseases such as cancer. This article is categorized under: Cancer > Molecular and Cellular Physiology Metabolic Diseases > Molecular and Cellular Physiology.

Anoctamins and Calcium Signalling: An Obstacle to EGFR Targeted Therapy in Glioblastoma?

Glioblastoma is the most common form of high-grade glioma in adults and has a poor survival rate with very limited treatment options. There have been no significant advancements in glioblastoma treatment in over 30 years. Epidermal growth factor receptor is upregulated in most glioblastoma tumours and, therefore, has been a drug target in recent targeted therapy clinical trials. However, while many inhibitors and antibodies for epidermal growth factor receptor have demonstrated promising anti-tumour effects in preclinical models, they have failed to improve outcomes for glioblastoma patients in clinical trials. This is likely due to the highly plastic nature of glioblastoma tumours, which results in therapeutic resistance. Ion channels are instrumental in the development of many cancers and may regulate cellular plasticity in glioblastoma. This review will explore the potential involvement of a class of calcium-activated chloride channels called anoctamins in brain cancer. We will also discuss the integrated role of calcium channels and anoctamins in regulating calcium-mediated signalling pathways, such as epidermal growth factor signalling, to promote brain cancer cell growth and migration.

Inhibition of EGFR Overcomes Acquired Lenvatinib Resistance Driven by STAT3-ABCB1 Signaling in Hepatocellular Carcinoma

Lenvatinib is an inhibitor of multiple receptor tyrosine kinases that was recently authorized for first-line treatment of hepatocellular carcinoma (HCC). However, the clinical benefits derived from lenvatinib are limited, highlighting the urgent need to understand mechanisms of resistance. We report here that HCC cells develop resistance to lenvatinib by activating EGFR and stimulating the EGFR-STAT3-ABCB1 axis. Lenvatinib resistance was accompanied by aberrant cholesterol metabolism and lipid raft activation. ABCB1 was activated by EGFR in a lipid raft-dependent manner, which significantly enhanced the exocytosis of lenvatinib to mediate resistance. Furthermore, clinical specimens of HCC showed a correlation between the activation of the EGFR-STAT3-ABCB1 pathway and lenvatinib response. Erlotinib, an EGFR inhibitor that has also been shown to inhibit ABCB1, suppressed lenvatinib exocytosis, and combined treatment with lenvatinib and erlotinib demonstrated a significant synergistic effect on HCC both in vitro and in vivo. Taken together, these findings characterize a mechanism of resistance to a first-line treatment for HCC and offer a practical means to circumvent resistance and treat the disease.

SIGNIFICANCE

HCC cells acquire resistance to lenvatinib by activating the EGFR-STAT3-ABCB1 pathway, identifying combined treatment with erlotinib as a strategy to overcome acquired resistance and improve the clinical benefit of lenvatinib.

Small GTPase-a Key Role in Host Cell for Coronavirus Infection and a Potential Target for Coronavirus Vaccine Adjuvant Discovery

Small GTPases are signaling molecules in regulating key cellular processes (e.g., cell differentiation, proliferation, and motility) as well as subcellular events (e.g., vesicle trafficking), making them key participants, especially in a great array of coronavirus infection processes. In this review, we discuss the role of small GTPases in the coronavirus life cycle, especially pre-entry, endocytosis, intracellular traffic, replication, and egress from the host cell. Furthermore, we also suggest the molecules that have potent adjuvant activity by targeting small GTPases. These studies provide deep insights and references to understand the pathogenesis of coronavirus as well as to propose the potential of small GTPases as targets for adjuvant development.

Colloidal Self-Assembled Patterns Maintain the Pluripotency and Promote the Hemopoietic Potential of Human Embryonic Stem Cells

The generation of blood cells in a significant amount for clinical uses is still challenging. Human pluripotent stem cells-derived hemopoietic cells (hPSC-HCs) are a promising cell source to generate blood cells. Previously, it has been shown that the attached substrates are crucial in the maintenance or differentiation of hPSCs. In this study, a new family of artificial extracellular matrix (ECM) called colloidal self-assembled patterns (cSAPs: #1-#5) was used for the expansion of mouse and human PSCs. The optimized cSAP (i.e., #4 and #5) was selected for subsequent hemopoietic differentiation of human embryonic stem cells (hESCs). Results showed that the hematopoietic potential of hESCs was enhanced approx 3-4 folds on cSAP #5 compared to the flat control. The cell population of hematopoietic progenitors (i.e., CD34+CD43+ cells) and erythroid progenitors (i.e., CD71+GPA+ cells) were enhanced 4 folds at day 8 and 3 folds at day 14. RNA sequencing analysis of cSAP-derived hESCs showed that there were 300 genes up-regulated and 627 genes down-regulated compared to the flat control. The enriched signaling pathways, including up-regulation (i.e., Toll-like receptor, HIF-1a, and Notch) or down-regulation (i.e., FAs, MAPK, JAK/STAT, and TGF-β) were classic in the maintenance of hESC phenotype Real time PCR confirmed that the expression of focal adhesion (PTK2, VCL, and CXCL14) and MAPK signaling (CAV1) related genes was down-regulated 2-3 folds compared to the flat control. Altogether, cSAP enhances the pluripotency and the hematopoietic potential of hESCs that subsequently generates more blood-like cells. This study reveals the potential of cSAPs on the expansion and early-stage blood cell lineage differentiation of hPSCs.

α1-nAchR-Mediated Signaling Through Lipid Raft Is Required for Nicotine-Induced NLRP3 Inflammasome Activation and Nicotine-Accelerated Atherosclerosis

Background

Nicotine exerts direct effects on multiple cell types in the cardiovascular system by associating with its high-affinity nicotinic acetylcholine receptors (nAchRs). Lipid raft is a membrane microdomain that recruits various receptors and signaling molecules for coordinating cellular immune response and many others signaling processes. Here, we aim to identify the essential role of lipid raft in mediating nicotine-triggered inflammatory and nicotine-accelerated atherosclerosis, and to figure out the specific receptor of nicotine-induced Nod-like receptor protein 3 (NLRP3) inflammasome activation in macrophage.

Methods and Results

ApoE^–/– mice were fed with a high-fat diet to build atherosclerosis model. Methyl-β-cyclodextrin was used to interrupt intact lipid raft. We confirmed that nicotine triggered NLRP3 inflammasome activation and induced macrophage migration into atherosclerotic plaque, thus accelerated atherosclerosis in apoE^–/– mice fed with a high-fat diet. Mechanically, nicotine increased the expression of α1-nAChR and stimulated the accumulation of α1-nAChR in lipid raft, leading to NLRP3 inflammasome activation in macrophage. Conversely, silencing of α1-nAChR in macrophage sufficiently blocked the pro-inflammasome activation effect of nicotine, indicating that α1-nAChR was the specific receptor for nicotine in triggering NLRP3 inflammasome in macrophage. Furthermore, both the destruction of lipid raft by methyl-β-cyclodextrin and the interference of lipid raft clustering by silencing acid sphingomyelinase reversed nicotine-induced NLRP3 inflammasome activation by reducing the accumulation of α1-nAChR in lipid raft in macrophage, suggesting lipid raft–mediated accumulation of α1-nAChR was the key event in regulating the pro-inflammatory effects of nicotine in macrophage. Importantly, nicotine-induced NLRP3 inflammasome activation and macrophage migration into atherosclerotic plaque were reversed by methyl-β-cyclodextrin, making a significant improvement for atherosclerosis in apoE^–/– mice fed with a high-fat diet.

Conclusion

α1-nAChR-mediated signaling through lipid raft is required for NLRP3 inflammasome activation and pro-atherosclerotic property of nicotine.

LY6K promotes glioblastoma tumorigenicity via CAV-1-mediated ERK1/2 signaling enhancement

BACKGROUND

Lymphocyte antigen 6 complex, locus K (LY6K) is a putative oncogene in various cancers. Elevated expression of LY6K is correlated with poor patient prognosis in glioblastoma (GBM). The aim of this study is to advance our understanding of the mechanism by which LY6K contributes to GBM tumor biology.

METHODS

Bioinformatic data mining was used to investigate LY6K expression in relation to GBM clinical outcome. To understand the role of LY6K in GBM, we utilized patient-derived glioma stemlike cells (GSCs) and U87 cells and employed immunoblotting, immunofluorescent staining, radiation treatment, and orthotopic GBM xenograft models.

RESULTS

Our results show that increased expression of LY6K inversely correlates with GBM patient survival. LY6K promotes tumorigenicity in GBM cells both in vitro and in vivo. The mechanism underlying this tumorigenic behavior is enhancement of extracellular signal-regulated kinase 1 and 2 (ERK1/2) signaling. Interestingly, we observed that tumor-promoting LY6K-ERK1/2 signaling is mediated by the interaction of LY6K with caveolin-1, rather than through oncogenic receptor tyrosine kinase-mediated signaling. Moreover, association of LY6K with the cell membrane is crucial for its tumorigenic functions. Finally, DNA methylation maintains LY6K silencing, and hypomethylation of the LY6K promoter increases its expression. In GSCs, ionizing radiation leads to demethylation of the LY6K promoter, thereby increasing LY6K expression and GSC resistance to radiation.

CONCLUSIONS

Our study highlights the importance of the contribution of LY6K to GBM tumor biology and suggests LY6K as a potential membrane target for treating GBM.

Caveolin-1, a master regulator of cellular senescence

Cellular senescence is a feature of most somatic cells. It is characterized by an irreversible cell cycle arrest and by the ability to secrete a plethora of mediators of inflammation and growth factors, which can alter the senescent cell's microenvironment. Senescent cells accumulate in tissues over time and contribute to both aging and the development of age-associated diseases. Senescent cells have antagonistic pleiotropic roles in cancer. Given the inability of senescent cells to proliferate, cellular senescence is a powerful tumor suppressor mechanism in young individuals. However, accumulation of senescent stromal cells during aging can fuel cancer cell growth in virtue of their capacity to release factors that stimulate cell proliferation. Caveolin-1 is a structural protein component of caveolae, invaginations of the plasma membrane involved in a variety of cellular processes, including signal transduction. Mounting evidence over the last 10-15 years has demonstrated a central role of caveolin-1 in the development of a senescent phenotype and the regulation of both the anti-tumorigenic and pro-tumorigenic properties of cellular senescence. In this review, we discuss the cellular mechanisms and functions of caveolin-1 in the context of cellular senescence and their relevance to the biology of cancer.

Tyrosine phosphorylation of tumor cell caveolin-1: impact on cancer progression

Caveolin-1 (CAV1) has long been implicated in cancer progression, and while widely accepted as an oncogenic protein, CAV1 also has tumor suppressor activity. CAV1 was first identified in an early study as the primary substrate of Src kinase, a potent oncoprotein, where its phosphorylation correlated with cellular transformation. Indeed, CAV1 phosphorylation on tyrosine-14 (Y14; pCAV1) has been associated with several cancer-associated processes such as focal adhesion dynamics, tumor cell migration and invasion, growth suppression, cancer cell metabolism, and mechanical and oxidative stress. Despite this, a clear understanding of the role of Y14-phosphorylated pCAV1 in cancer progression has not been thoroughly established. Here, we provide an overview of the role of Src-dependent phosphorylation of tumor cell CAV1 in cancer progression, focusing on pCAV1 in tumor cell migration, focal adhesion signaling and metabolism, and in the cancer cell response to stress pathways characteristic of the tumor microenvironment. We also discuss a model for Y14 phosphorylation regulation of CAV1 effector protein interactions via the caveolin scaffolding domain.

Caveolin-1 function at the plasma membrane and in intracellular compartments in cancer

Caveolin-1 (CAV1) is commonly considered to function as a cell surface protein, for instance in the genesis of caveolae. Nonetheless, it is also present in many intracellular organelles and compartments. The contributions of these intracellular pools to CAV1 function are generally less well understood, and this is also the case in the context of cancer. This review will summarize literature available on the role of CAV1 in cancer, highlighting particularly our understanding of the canonical (CAV1 in the plasma membrane) and non-canonical pathways (CAV1 in organelles and exosomes) linked to the dual role of the protein as a tumor suppressor and promoter of metastasis. With this in mind, we will focus on recently emerging concepts linking CAV1 function to the regulation of intracellular organelle communication within the same cell where CAV1 is expressed. However, we now know that CAV1 can be released from cells in exosomes and generate systemic effects. Thus, we will also elaborate on how CAV1 participates in intracellular communication between organelles as well as signaling between cells (non-canonical pathways) in cancer.

A Detergent-Free Method for Preparation of Lipid Rafts for the Shotgun Lipidomics Study

Lipid rafts are microdomains on plasma membrane that contain high levels of cholesterol and sphingolipids. Because of the detergent-resistant property of lipid rafts, lipid rafts isolated by methods that use detergents frequently yield different results. Artifacts can also be introduced through the use of detergents. These limitations could be overcome with a detergent-free method which eliminates possible artificial influences. Importantly, lipid rafts prepared with a detergent-free method is more compatible to mass spectrometric analysis since the ion suppression effect is largely reduced.This chapter describes a detergent-free two-step method for preparation of lipid rafts. Firstly, a purified plasma membrane fraction is prepared from cells by sedimentation of the postnuclear supernatant (PNS) in a Percoll gradient. Secondly, the as-prepared plasma membranes are sonicated to release lipid rafts which are further isolated by flotation in a continuous gradient of Optiprep solution. Then, we introduce a typical shotgun lipidomics workflow that can be used as a cost-effective and relatively high throughput method to determine the lipidomes of lipid rafts.The method also makes an easy start for lipidomics studies.

Lipid metabolism alteration contributes to and maintains the properties of cancer stem cells

Lipids, the basic components of the cell membrane, execute fundamental roles in almost all the cell activities including cell-cell recognition, signalling transduction and energy supplies. Lipid metabolism is elementary for life sustentation that balances activity between synthesis and degradation. An accumulating amount of data has indicated abnormal lipid metabolism in cancer stem cells (CSCs), and that the alteration of lipid metabolism exerts a great impact on CSCs' properties such as the capability of self-renewal, differentiation, invasion, metastasis, and drug sensitivity and resistance. CSCs' formation and maintenance cannot do without the regulation of fatty acids and cholesterol. In normal cells and embryonic development, fatty acids and cholesterol metabolism are regulated by some important signalling pathways (such as Hedgehog, Notch, Wnt signalling pathways); these signalling pathways also play crucial roles in initiating and/or maintaining CSCs' properties, and such signalling is shown to be commonly modulated by the abnormal lipid metabolism in CSCs; on the other hand, the altered lipid metabolism in turn modifies the cell signalling and generates additional impacts on CSCs. Metabolic rewiring is considered as an ideal hallmark of CSCs, and metabolic alterations would be promising therapeutic targets of CSCs for aggressive tumors. In this review, we summarize the most updated findings of lipid metabolic abnormalities in CSCs and prospect the potential applications of targeting lipid metabolism for anticancer treatment.

The Implications for Cells of the Lipid Switches Driven by Protein-Membrane Interactions and the Development of Membrane Lipid Therapy

The cell membrane contains a variety of receptors that interact with signaling molecules. However, agonist-receptor interactions not always activate a signaling cascade. Amphitropic membrane proteins are required for signal propagation upon ligand-induced receptor activation. These proteins localize to the plasma membrane or internal compartments; however, they are only activated by ligand-receptor complexes when both come into physical contact in membranes. These interactions enable signal propagation. Thus, signals may not propagate into the cell if peripheral proteins do not co-localize with receptors even in the presence of messengers. As the translocation of an amphitropic protein greatly depends on the membrane's lipid composition, regulation of the lipid bilayer emerges as a novel therapeutic strategy. Some of the signals controlled by proteins non-permanently bound to membranes produce dramatic changes in the cell's physiology. Indeed, changes in membrane lipids induce translocation of dozens of peripheral signaling proteins from or to the plasma membrane, which controls how cells behave. We called these changes "lipid switches", as they alter the cell's status (e.g., proliferation, differentiation, death, etc.) in response to the modulation of membrane lipids. Indeed, this discovery enables therapeutic interventions that modify the bilayer's lipids, an approach known as membrane-lipid therapy (MLT) or melitherapy.

The importance of caveolins and caveolae to dermatology: Lessons from the caves and beyond

Caveolae are flask-shaped invaginations of the cell membrane rich in cholesterol and sphingomyelin, with caveolin proteins acting as their primary structural components that allow compartmentalization and orchestration of various signalling molecules. In this review, we discuss how pleiotropic functions of caveolin-1 (Cav1) and its intricate roles in numerous cellular functions including lipid trafficking, signalling, cell migration and proliferation, as well as cellular senescence, infection and inflammation, are integral for normal development and functioning of skin and its appendages. We then examine how disruption of the homeostatic levels of Cav1 can lead to development of various cutaneous pathophysiologies including skin cancers, cutaneous fibroses, psoriasis, alopecia, age-related changes in skin and aberrant wound healing and propose how levels of Cav1 may have theragnostic value in skin physiology/pathophysiology.

Current advances in idiopathic pulmonary fibrosis: the pathogenesis, therapeutic strategies and candidate molecules

Idiopathic pulmonary fibrosis (IPF) is a type of chronic, progressive lung disease with unknown cause, which is characterized by increasing dyspnea and destruction of lung function with a high mortality rate. Evolving evidence demonstrated that the pathogenesis of IPF involved multiple signaling pathways such as inflammation, oxidative stress and fibrosis. However, drug discovery to prevent or revert IPF has been insufficient to cope with the development. Drug discovery targeting multiple links should be considered. In this review, we will brief the pathogenesis of IPF and discuss several small chemical entities toward the pathogenesis for IPF studied in animal models and clinical trials. The field of novel anti-IPF agents and the future directions for the prevention and treatment of IPF are detailed thoroughly discussed.

Role of Lipid Rafts in Hematopoietic Stem Cells Homing, Mobilization, Hibernation, and Differentiation

Hematopoietic stem cells (HSCs) are multipotent, self-renewing cells that can differentiate into myeloid or lymphoid cells. The mobilization and differentiation processes are affected by the external environment, such as extracellular matrix and soluble molecules in the niche, where the lipid rafts (LRs) of the HSCs act as the receptors and control platforms for these effectors. LRs are membrane microdomains that are enriched in cholesterol, sphingolipid, and proteins. They are involved in diverse cellular processes including morphogenesis, cytokinesis, signaling, endocytic events, and response to the environment. They are also involved in different types of diseases, such as cancer, Alzheimer's, and prion disease. LR clustering and disruption contribute directly to the differentiation, homing, hibernation, or mobilization of HSCs. Thus, characterization of LR integrity may provide a promising approach to controlling the fate of stem cells for clinical applications. In this review, we show the critical role of LR modification (clustering, disruption, protein incorporation, and signal responding) in deciding the fate of HSCs, under the effect of soluble cytokines such as stem cell factor (SCF), transforming growth factor- β (TGF-β), hematopoietic-specific phospholipase Cβ2 (PLC-β2), and granulocyte colony-stimulating factor (G-CSF).

CD317 Activates EGFR by Regulating Its Association with Lipid Rafts

EGFR regulates various fundamental cellular processes, and its constitutive activation is a common driver for cancer. Anti-EGFR therapies have shown benefit in cancer patients, yet drug resistance almost inevitably develops, emphasizing the need for a better understanding of the mechanisms that govern EGFR activation. Here we report that CD317, a surface molecule with a unique topology, activated EGFR in hepatocellular carcinoma (HCC) cells by regulating its localization on the plasma membrane. CD317 was upregulated in HCC cells, promoting cell-cycle progression and enhancing tumorigenic potential in a manner dependent on EGFR. Mechanistically, CD317 associated with lipid rafts and released EGFR from these ordered membrane domains, facilitating the activation of EGFR and the initiation of downstream signaling pathways, including the Ras-Raf-MEK-ERK and JAK-STAT pathways. Moreover, in HCC mouse models and patient samples, upregulation of CD317 correlated with EGFR activation. These results reveal a previously unrecognized mode of regulation for EGFR and suggest CD317 as an alternative target for treating EGFR-driven malignancies. SIGNIFICANCE: Activation of EGFR by CD317 in hepatocellular carcinoma cells suggests CD317 as an alternative target for treating EGFR-dependent tumors.

Inhibition of EGFR Activation by Bivalent Ligands Based on a Cyclic Peptide Mimicking the Dimerization Arm Structure of EGFR

The epidermal growth factor receptor (EGFR) is a receptor in the ErbB family, and is overexpressed in some cancer cells. Recent research has shown that, since clustering of the EGFR increases the possibility of its dimerization and activation, the dimerization state of the EGFR on the cell surface is important for the recognition of the EGFR. In case a bivalent inhibitor has an optimized linker length, the clusters of the EGFR could be recognized with high affinity and kinase activation, which depends on EGF, could be suppressed. Peptide 1, which is derived from the dimerization arm of the EGFR, has been found previously to inhibit autophosphorylation of the EGFR. In this study, bivalent ligands based on peptide 1 with linkers of poly(L-proline) or poly-[(glycine)₄(L-serine)] have been designed and synthesized. Bivalent ligands with polyproline linkers could maintain the distance between the ligand moieties. The inhibitory activity of these bivalent ligands against EGFR autophosphorylation was measured and was found to increase as the linker enlarges up to a 15-mer proline linker. The inhibitory activity of a bivalent ligand 7b is significantly higher compared to the corresponding monomeric peptide 2a. This suggests that bivalent EGFR ligands with optimal and rigid linkers could recognize the clusters of the EGFR with higher affinity and suppress kinase activation involving EGF.

Regulation of Cell Survival, Apoptosis, and Epithelial-to-Mesenchymal Transition by Nitric Oxide-Dependent Post-Translational Modifications

SIGNIFICANCE

Nitric oxide (NO) is a physiopathological messenger generating different reactive nitrogen species (RNS) according to hypoxic, acidic and redox conditions. Recent Advances: RNS and reactive oxygen species (ROS) promote relevant post-translational modifications, such as nitrosation, nitration, and oxidation, in critical components of cell proliferation and death, epithelial-to-mesenchymal transition, and metastasis.

CRITICAL ISSUES

The pro- or antitumoral properties of NO are dependent on local concentration, redox state, cellular status, duration of exposure, and compartmentalization of NO generation. The increased expression of NO synthase has been associated with cancer progression. However, the experimental strategies leading to high intratumoral NO generation have been shown to exert antitumoral properties. The effect of NO and ROS on cell signaling is critically altered by factors modulating tumor progression such as oxygen content, metabolism, and inflammatory response. The review describes the alteration of key components involved in cell survival and death, metabolism, and metastasis induced by RNS- and ROS-related post-translational modifications.

FUTURE DIRECTIONS

The identification of the molecular targets affected by nitrosation, nitration, and oxidation, as well as their interactions with other post-translational modifications, will improve the understanding on the complex signaling and cell fate decision in cancer. The therapeutic NO-based strategies have to address the complex crosstalk among NO and ROS with regard to critical components affecting tumor cell survival, metabolism, and metastasis in the progression of cancer, as well as close interaction with ionizing radiation and chemotherapy.

High cholesterol in lipid rafts reduces the sensitivity to EGFR-TKI therapy in non-small cell lung cancer

Overcoming EGFR-TKI resistant which has the initial enthusiasm over substantial clinical responses is a formidable challenge on nowadays. In this study, we showed that cholesterol level in lipid rafts in gefitinib resistant non-small cell lung cancer (NSCLC) cell lines was remarkably higher than gefitinib sensitive cell line, and depletion of cholesterol increased gefitinib sensitivity. Furthermore, cholesterol-depleted enhanced gefitinib inhibit phosphorylation of EGFR, Akt-1, MEK1/2, and ERK1/2 and these were reversed in cholesterol add-back experiments. Gefitinib resistant cell lines showed high affinity of gefitinib and EGFR when cholesterol was depleted. Therefore, targeting cholesterol combined with EGFR-TKI is potentially a novel therapeutic strategy for gefitinib resistant treatment.

Caveolin-1 Inhibits Proliferation, Migration, and Invasion of Human Colorectal Cancer Cells by Suppressing Phosphorylation of Epidermal Growth Factor Receptor

Background

Although downregulation of caveolin-1 (Cav-1), which is a key constituent of membrane caveolae and a regulator of cellular processes, is associated with colorectal cancer (CRC), its involvement in the disease progression is largely unknown. This study aimed to explore the role of Cav-1 in CRC and the associated mechanisms.

Material/Methods

Fresh tissues from patients with CRC and human CRC SW480 cells were used to evaluate Cav-1 and Ki-67 expression using immunohistochemistry and Western blotting. The MTS and Transwell assays were performed to determine the effects of Cav-1 overexpression via pcDNA3.1/Cav-1 plasmid on cell proliferation and metastasis. The effect of Cav-1 on the epidermal growth factor receptor (EGFR) activation was evaluated by Western blotting. The correlation of Cav-1 expression with clinicopathological factors was statistically analyzed.

Results

Overexpression of Cav-1 significantly reduced proliferation, migration, and invasion of SW480 cancer cells in vitro. The EGF-induced phosphorylation of EGFR and activations of the RAF-MEK-ERK and PI3K-AKT pathways were adversely regulated by Cav-1 overexpression in vitro. In 76 cases of CRC patients with EGFR expression, a negative correlation was observed between the level of Cav-1 and tumor-node-metastasis stage, lymph node metastasis, and distant metastasis (All p<0.05). Finally, the expression level of Cav-1 was negatively correlated with that of Ki-67.

Conclusions

This report is the first to show that overexpression of Cav-1significantly inhibits the proliferation, migration, and invasion potential of SW480 cells, possibly through reducing EGF-induced EGFR activation. High Cav-1 expression level may be a predictor of positive outcomes in patients with colorectal cancer.

The epidermal growth factor receptor regulates cofilin activity and promotes transmissible gastroenteritis virus entry into intestinal epithelial cells

Transmissible gastroenteritis virus (TGEV), a coronavirus, causes severe diarrhea and high mortality in newborn piglets. The porcine intestinal epithelium is the target of TGEV infection, but the mechanisms that TGEV disrupts the actin cytoskeleton and invades the host epithelium remain largely unknown. We not only found that TGEV infection stimulates F-actin to gather at the cell membrane but the disruption of F-actin inhibits TGEV entry as well. Cofilin is involved in F-actin reorganization and TGEV entry. The TGEV spike protein is capable of binding with EGFR, activating the downstream phosphoinositide-3 kinase (PI3K), then causing the phosphorylation of cofilin and F-actin polymerization via Rac1/Cdc42 GTPases. Inhibition of EGFR and PI3K decreases the entry of TGEV. EGFR is also the upstream activator of mitogen-activated protein kinase (MAPK) signaling pathways that is involved in F-actin reorganization. Additionally, lipid rafts act as signal platforms for the EGFR-associated signaling cascade and correlate with the adhesion of TGEV. In conlusion, these results provide valuable data of the mechanisms which are responsible for the TGEV pathogenesis and may lead to the development of new methods about controlling TGEV.

The Epidermal Growth Factor Receptor Forms Location-Dependent Complexes in Resting Cells

The epidermal growth factor receptor (EGFR) is a prototypical receptor tyrosine kinase involved in cell growth and proliferation and associated with various cancers. It is commonly assumed that after activation by binding of epidermal growth factor to the extracellular domain it dimerizes, followed by autophosphorylation of tyrosine residues at the intracellular domain. However, its oligomerization state before activation is controversial. In the absence of ligands, EGFR has been found in various, inconsistent amounts of monomeric, inactive dimeric, and oligomeric forms. In addition, evidence suggests that the active conformation is not a simple dimer but contains higher oligomers. As experiments in the past have been conducted at different conditions, we investigate here the influence of cell lines (HEK293, COS-7, and CHO-K1), temperature (room temperature and 37°C), and membrane localization on the quantitation of preformed dimers using SW-FCCS, DC-FCCS, quasi PIE-FCCS, and imaging FCCS. While measurement modality, temperature, and localization on upper or lower membranes have only a limited influence on the dimerization amount observed, the cell line and location to periphery versus center of the cell can change dimerization results significantly. The observed dimerization amount is strongly dependent on the expression level of endogenous EGFR in a cell line and shows a strong cell-to-cell variability even within the same cell line. In addition, using imaging FCCS, we find that dimers have a tendency to be found at the periphery of cells compared to central positions.

Manipulation of Host Cholesterol by Obligate Intracellular Bacteria

Cholesterol is a multifunctional lipid that plays important metabolic and structural roles in the eukaryotic cell. Despite having diverse lifestyles, the obligate intracellular bacterial pathogens Chlamydia, Coxiella, Anaplasma, Ehrlichia, and Rickettsia all target cholesterol during host cell colonization as a potential source of membrane, as well as a means to manipulate host cell signaling and trafficking. To promote host cell entry, these pathogens utilize cholesterol-rich microdomains known as lipid rafts, which serve as organizational and functional platforms for host signaling pathways involved in phagocytosis. Once a pathogen gains entrance to the intracellular space, it can manipulate host cholesterol trafficking pathways to access nutrient-rich vesicles or acquire membrane components for the bacteria or bacteria-containing vacuole. To acquire cholesterol, these pathogens specifically target host cholesterol metabolism, uptake, efflux, and storage. In this review, we examine the strategies obligate intracellular bacterial pathogens employ to manipulate cholesterol during host cell colonization. Understanding how obligate intracellular pathogens target and use host cholesterol provides critical insight into the host-pathogen relationship.

Caveolin-1: An Oxidative Stress-Related Target for Cancer Prevention

Aberrant oxidative metabolism is one of the hallmarks of cancer. Reactive species overproduction could promote carcinogenesis via inducing genetic mutations and activating oncogenic pathways, and thus, antioxidant therapy was considered as an important strategy for cancer prevention and treatment. Caveolin-1 (Cav-1), a constituent protein of caveolae, has been shown to mediate tumorigenesis and progression through oxidative stress modulation recently. Reactive species could modulate the expression, degradation, posttranslational modifications, and membrane trafficking of Cav-1, while Cav-1-targeted treatments could scavenge the reactive species. More importantly, emerging evidences have indicated that multiple antioxidants could exert antitumor activities in cancer cells and protective activities in normal cells by modulating the Cav-1 pathway. Altogether, these findings indicate that Cav-1 may be a promising oxidative stress-related target for cancer antioxidant prevention. Elucidating the underlying interaction mechanisms between oxidative stress and Cav-1 is helpful for enhancing the preventive effects of antioxidants on cancer, for improving clinical outcomes of antioxidant-related therapeutics in cancer patients, and for developing Cav-1 targeted drugs. Herein, we summarize the available evidence of the roles of Cav-1 and oxidative stress in tumorigenesis and development and shed novel light on designing strategies for cancer prevention or treatment by utilizing the interaction mode between Cav-1 and oxidative stress.

Caveolin-1, a stress-related oncotarget, in drug resistance

Caveolin-1 (Cav-1) is both a tumor suppressor and an oncoprotein. Cav-1 overexpression was frequently confirmed in advanced cancer stages and positively associated with ABC transporters, cancer stem cell populations, aerobic glycolysis activity and autophagy. Cav-1 was tied to various stresses including radiotherapy, fluid shear and oxidative stresses and ultraviolet exposure, and interacted with stress signals such as AMP-activated protein kinase. Finally, a Cav-1 fluctuation model during cancer development is provided and Cav-1 is suggested to be a stress signal and cytoprotective. Loss of Cav-1 may increase susceptibility to oncogenic events. However, research to explore the underlying molecular network between Cav-1 and stress signals is warranted.

Membrane-associated Ras dimers are isoform-specific: K-Ras dimers differ from H-Ras dimers

Are the dimer structures of active Ras isoforms similar? This question is significant since Ras can activate its effectors as a monomer; however, as a dimer, it promotes Raf's activation and MAPK (mitogen-activated protein kinase) cell signalling. In the present study, we model possible catalytic domain dimer interfaces of membrane-anchored GTP-bound K-Ras4B and H-Ras, and compare their conformations. The active helical dimers formed by the allosteric lobe are isoform-specific: K-Ras4B-GTP favours the α3 and α4 interface; H-Ras-GTP favours α4 and α5. Both isoforms also populate a stable β-sheet dimer interface formed by the effector lobe; a less stable β-sandwich interface is sustained by salt bridges of the β-sheet side chains. Raf's high-affinity β-sheet interaction is promoted by the active helical interface. Collectively, Ras isoforms' dimer conformations are not uniform; instead, the isoform-specific dimers reflect the favoured interactions of the HVRs (hypervariable regions) with cell membrane microdomains, biasing the effector-binding site orientations, thus isoform binding selectivity.

Ras Conformational Ensembles, Allostery, and Signaling

Ras proteins are classical members of small GTPases that function as molecular switches by alternating between inactive GDP-bound and active GTP-bound states. Ras activation is regulated by guanine nucleotide exchange factors that catalyze the exchange of GDP by GTP, and inactivation is terminated by GTPase-activating proteins that accelerate the intrinsic GTP hydrolysis rate by orders of magnitude. In this review, we focus on data that have accumulated over the past few years pertaining to the conformational ensembles and the allosteric regulation of Ras proteins and their interpretation from our conformational landscape standpoint. The Ras ensemble embodies all states, including the ligand-bound conformations, the activated (or inactivated) allosteric modulated states, post-translationally modified states, mutational states, transition states, and nonfunctional states serving as a reservoir for emerging functions. The ensemble is shifted by distinct mutational events, cofactors, post-translational modifications, and different membrane compositions. A better understanding of Ras biology can contribute to therapeutic strategies.

Farnesylated and methylated KRAS4b: high yield production of protein suitable for biophysical studies of prenylated protein-lipid interactions

Prenylated proteins play key roles in several human diseases including cancer, atherosclerosis and Alzheimer's disease. KRAS4b, which is frequently mutated in pancreatic, colon and lung cancers, is processed by farnesylation, proteolytic cleavage and carboxymethylation at the C-terminus. Plasma membrane localization of KRAS4b requires this processing as does KRAS4b-dependent RAF kinase activation. Previous attempts to produce modified KRAS have relied on protein engineering approaches or in vitro farnesylation of bacterially expressed KRAS protein. The proteins produced by these methods do not accurately replicate the mature KRAS protein found in mammalian cells and the protein yield is typically low. We describe a protocol that yields 5-10 mg/L highly purified, farnesylated, and methylated KRAS4b from insect cells. Farnesylated and methylated KRAS4b is fully active in hydrolyzing GTP, binds RAF-RBD on lipid Nanodiscs and interacts with the known farnesyl-binding protein PDEδ.

Plasma membrane regulates Ras signaling networks

Ras GTPases activate more than 20 signaling pathways, regulating such essential cellular functions as proliferation, survival, and migration. How Ras proteins control their signaling diversity is still a mystery. Several pieces of evidence suggest that the plasma membrane plays a critical role. Among these are: (1) selective recruitment of Ras and its effectors to particular localities allowing access to Ras regulators and effectors; (2) specific membrane-induced conformational changes promoting Ras functional diversity; and (3) oligomerization of membrane-anchored Ras to recruit and activate Raf. Taken together, the membrane does not only attract and retain Ras but also is a key regulator of Ras signaling. This can already be gleaned from the large variability in the sequences of Ras membrane targeting domains, suggesting that localization, environment and orientation are important factors in optimizing the function of Ras isoforms.

Fluorofenidone attenuates TGF-β1-induced lung fibroblast activation via restoring the expression of caveolin-1

Caveolin-1 plays an important role in the pathogenesis of idiopathic pulmonary fibrosis. We previously showed that fluorofenidone (FD), a novel pyridine agent, can attenuate bleomycin-induced experimental pulmonary fibrosis and restore the production of caveolin-1. In this study, we explore mainly whether caveolin-1 plays a critical role in the anti-pulmonary fibrosis effects of FD in vitro. The normal human lung fibroblasts (NHLFs) were cultured with transforming growth factor-β1 (TGF-β1) and then were treated with FD. Subsequently, NHLFs transfected with cav-1-siRNA were treated with TGF-β1 and/or FD. The expressions of α-smooth muscle actin (α-SMA), fibronectin, collagen I, caveolin-1, phosphorylated extracellular signal-regulated kinase (p-ERK), phosphorylated c-Jun N-terminal kinase (p-JNK), and phosphorylated P38 were measured by Western blot and/or real-time polymerase chain reaction. Fluorofenidone attenuated TGF-β1-induced expressions of α-SMA, fibronectin, and collagen I; inhibited phosphorylation of ERK, JNK, and P38; and restored caveolin-1 protein expression but cannot increase caveolin-1 mRNA level in vitro. After caveolin-1 was silenced, FD could not downregulate TGF-β1-induced expressions of α-SMA, fibronectin, and collagen I or phosphorylation of ERK, JNK, and P38. These studies demonstrate that FD, a potential antifibrotic agent, may attenuate TGF-β1-induced activation of NHLFs by restoring the expression of caveolin-1.

Membrane lipid therapy: Modulation of the cell membrane composition and structure as a molecular base for drug discovery and new disease treatment

Nowadays we understand cell membranes not as a simple double lipid layer but as a collection of complex and dynamic protein-lipid structures and microdomains that serve as functional platforms for interacting signaling lipids and proteins. Membrane lipids and lipid structures participate directly as messengers or regulators of signal transduction. In addition, protein-lipid interactions participate in the localization of signaling protein partners to specific membrane microdomains. Thus, lipid alterations change cell signaling that are associated with a variety of diseases including cancer, obesity, neurodegenerative disorders, cardiovascular pathologies, etc. This article reviews the newly emerging field of membrane lipid therapy which involves the pharmacological regulation of membrane lipid composition and structure for the treatment of diseases. Membrane lipid therapy proposes the use of new molecules specifically designed to modify membrane lipid structures and microdomains as pharmaceutical disease-modifying agents by reversing the malfunction or altering the expression of disease-specific protein or lipid signal cascades. Here, we provide an in-depth analysis of this emerging field, especially its molecular bases and its relevance to the development of innovative therapeutic approaches.

Mechanisms of membrane binding of small GTPase K-Ras4B farnesylated hypervariable region

K-Ras4B belongs to a family of small GTPases that regulates cell growth, differentiation and survival. K-ras is frequently mutated in cancer. K-Ras4B association with the plasma membrane through its farnesylated and positively charged C-terminal hypervariable region (HVR) is critical to its oncogenic function. However, the structural mechanisms of membrane association are not fully understood. Here, using confocal microscopy, surface plasmon resonance, and molecular dynamics simulations, we observed that K-Ras4B can be distributed in rigid and loosely packed membrane domains. Its membrane binding domain interaction with phospholipids is driven by membrane fluidity. The farnesyl group spontaneously inserts into the disordered lipid microdomains, whereas the rigid microdomains restrict the farnesyl group penetration. We speculate that the resulting farnesyl protrusion toward the cell interior allows oligomerization of the K-Ras4B membrane binding domain in rigid microdomains. Unlike other Ras isoforms, K-Ras4B HVR contains a single farnesyl modification and positively charged polylysine sequence. The high positive charge not only modulates specific HVR binding to anionic phospholipids but farnesyl membrane orientation. Phosphorylation of Ser-181 prohibits spontaneous farnesyl membrane insertion. The mechanism illuminates the roles of HVR modifications in K-Ras4B targeting microdomains of the plasma membrane and suggests an additional function for HVR in regulation of Ras signaling.

Caveolin-1 in the anterior cingulate cortex modulates chronic neuropathic pain via regulation of NMDA receptor 2B subunit

Chronic pain is still a basic science and clinical challenge. Unraveling of the neurobiological mechanisms involved in chronic pain will offer novel targets for the development of therapeutic strategies. It is well known that central sensitization in the anterior cingulate cortex (ACC) plays a critical role in initiation, development, and maintenance of chronic pain. However, the underlying mechanisms still remain elusive. Here, we reported that caveolin-1 (Cav-1), a scaffolding protein in membrane rafts, was persistently upregulated and activated in the ACC neurons after chronic constriction injury (CCI) in mice. Knockdown or blocking of Cav-1 in the contralateral ACC to the injury side reversed CCI-induced pain behavioral and neuronal sensitization and overexpression of Cav-1 in the ipsilateral ACC-induced pain behavior in the unaffected hindpaw. Furthermore, we found that Cav-1 directly binding with NMDA receptor 2B subunit (NR2B) and promotion of NR2B surface levels in the ACC contributed to modulation of chronic neuropathic pain. Disrupting the interaction of Cav-1 and NR2B through microinjection of a short peptide derived from the C-terminal of NR2B into the ACC exhibited a significant anti-nociception effect associated with decrease of surface NR2B expression. Moreover, Cav-1 increased intracellular Ca(2+) concentration and activated the ERK/CREB signaling pathway in an NR2B-dependent manner in the ACC. Our findings implicate that Cav-1 in the ACC neurons modulates chronic neuropathic pain via regulation of NR2B and subsequent activation of ERK/CREB signaling, suggesting a possible caveolin-mediated process would participate in neuronal transmission pathways implicated in pain modulation.

H-Ras mediates the inhibitory effect of epidermal growth factor on the epithelial Na+ channel

The present study investigates the role of small G-proteins of the Ras family in the epidermal growth factor (EGF)-activated cellular signalling pathway that downregulates activity of the epithelial Na⁺ channel (ENaC). We found that H-Ras is a key component of this EGF-activated cellular signalling mechanism in M1 mouse collecting duct cells. Expression of a constitutively active H-Ras mutant inhibited the amiloride-sensitive current. The H-Ras-mediated signalling pathway that inhibits activity of ENaC involves c-Raf, and that the inhibitory effect of H-Ras on ENaC is abolished by the MEK1/2 inhibitor, PD98059. The inhibitory effect of H-Ras is not mediated by Nedd4-2, a ubiquitin protein ligase that regulates the abundance of ENaC at the cell surface membrane, or by a negative effect of H-Ras on proteolytic activation of the channel. The inhibitory effects of EGF and H-Ras on ENaC, however, were not observed in cells in which expression of caveolin-1 (Cav-1) had been knocked down by siRNA. These findings suggest that the inhibitory effect of EGF on ENaC-dependent Na⁺ absorption is mediated via the H-Ras/c-Raf, MEK/ERK signalling pathway, and that Cav-1 is an essential component of this EGF-activated signalling mechanism. Taken together with reports that mice expressing a constitutive mutant of H-Ras develop renal cysts, our findings suggest that H-Ras may play a key role in the regulation of renal ion transport and renal development.

Prospects for multitarget lipid-raft-coated silica beads: a remarkable online biomaterial for discovering multitarget antitumor lead compounds

Application of lipid raft biomaterial with multiple cancer-related receptors for screening novel multitarget antitumour lead compounds.

Lipid rafts, KCa/ClCa/Ca2+ channel complexes and EGFR signaling: Novel targets to reduce tumor development by lipids?

Membrane lipid rafts are distinct plasma membrane nanodomains that are enriched with cholesterol, sphingolipids and gangliosides, with occasional presence of saturated fatty acids and phospholipids containing saturated acyl chains. It is well known that they organize receptors (such as Epithelial Growth Factor Receptor), ion channels and their downstream acting molecules to regulate intracellular signaling pathways. Among them are Ca2+ signaling pathways, which are modified in tumor cells and inhibited upon membrane raft disruption. In addition to protein components, lipids from rafts also contribute to the organization and function of Ca2+ signaling microdomains. This article aims to focus on the lipid raft KCa/ClCa/Ca2+ channel complexes that regulate Ca2+ and EGFR signaling in cancer cells, and discusses the potential modification of these complexes by lipids as a novel therapeutic approach in tumor development. This article is part of a Special Issue entitled: Membrane channels and transporters in cancers.

Caveolin-1 is required for kinase suppressor of Ras 1 (KSR1)-mediated extracellular signal-regulated kinase 1/2 activation, H-RasV12-induced senescence, and transformation

The molecular scaffold kinase suppressor of Ras 1 (KSR1) regulates the activation of the Raf/MEK/extracellular signal-regulated kinase (ERK) signal transduction pathway. KSR1 disruption in mouse embryo fibroblasts (MEFs) abrogates growth factor-induced ERK activation, H-Ras(V12)-induced replicative senescence, and H-Ras(V12)-induced transformation. Caveolin-1 has been primarily described as a major component of the coating structure of caveolae, which can serve as a lipid binding adaptor protein and coordinates the assembly of Ras, Raf, MEK, and ERK. In this study, we show that KSR1 interacts with caveolin-1 and is responsible for MEK and ERK redistribution to caveolin-1-rich fractions. The interaction between KSR1 and caveolin-1 is essential for optimal activation of ERK as a KSR1 mutant unable to interact with caveolin-1 does not efficiently mediate growth factor-induced ERK activation at the early stages of pathway activation. Furthermore, abolishing the KSR1-caveolin-1 interaction increases growth factor demands to promote H-Ras(V12)-induced proliferation and has adverse effects on H-Ras(V12)-induced cellular senescence and transformation. These data show that caveolin-1 is necessary for optimal KSR1-dependent ERK activation by growth factors and oncogenic Ras.

Interactions of caveolin-1 scaffolding and intramembrane regions containing a CRAC motif with cholesterol in lipid bilayers

Caveolin-1 is a major structural protein of caveolae and specifically binds cholesterol (Chol). The caveolin scaffolding domain is thought to be involved in caveolin-Chol interaction through the sequence V94-T-K-Y-W-F-Y-R101, a motif that matches a cholesterol recognition amino-acid consensus (CRAC). In the present work, three CRAC-containing peptides, corresponding to caveolin-1 94-101, 82-101 and 93-126, were tested to study the role of the CRAC motif in the caveolin-Chol interaction in 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) bilayers using differential scanning calorimetry (DSC), fluorescence and circular dichroism (CD). The Y97I substituents of the three peptides and one peptide segment corresponding to caveolin-1 101-126 that excludes the CRAC motif were also tested for comparison. Our results showed the potency of these CRAC-containing peptides in sequestering Chol into domains and the enhanced role of the intramembrane domain and scaffolding domain for the potency. Of the three CRAC-containing peptides, the peptide 93-126 was particularly effective in promoting Chol segregation, while the peptide 82-101 was less potent in promoting the formation of domains than the peptide 93-126, but was more potent than the peptide 94-101. The domain partition of DPPC/Chol bilayers was not observed in the presence of the peptide 101-126, in contrast to the case in the presence of the peptide 93-126 at the same concentrations of peptide and Chol. The potency of the CRAC motif in Chol segregation was lowered by the Y97I mutation. The difference in structure may be a factor that contributes to different effects of these peptides on the distribution of Chol in the lipid membrane.

Epidermal growth factor receptor-PI3K signaling controls cofilin activity to facilitate herpes simplex virus 1 entry into neuronal cells

Herpes simplex virus type 1 (HSV-1) establishes latency in neurons and can cause severe disseminated infection with neurological impairment and high mortality. This neurodegeneration is thought to be tightly associated with virus-induced cytoskeleton disruption. Currently, the regulation pattern of the actin cytoskeleton and the involved molecular mechanisms during HSV-1 entry into neurons remain unclear. Here, we demonstrate that the entry of HSV-1 into neuronal cells induces biphasic remodeling of the actin cytoskeleton and an initial inactivation followed by the subsequent activation of cofilin, a member of the actin depolymerizing factor family that is critical for actin reorganization. The disruption of F-actin dynamics or the modulation of cofilin activity by mutation, knockdown, or overexpression affects HSV-1 entry efficacy and virus-mediated cell ruffle formation. Binding of the HSV-1 envelope initiates the epidermal growth factor receptor (EGFR)-phosphatidylinositide 3-kinase (PI3K) signaling pathway, which leads to virus-induced early cofilin phosphorylation and F-actin polymerization. Moreover, the extracellular signal-regulated kinase (ERK) kinase and Rho-associated, coiled-coil-containing protein kinase 1 (ROCK) are recruited as downstream mediators of the HSV-1-induced cofilin inactivation pathway. Inhibitors specific for those kinases significantly reduce the virus infectivity without affecting virus binding to the target cells. Additionally, lipid rafts are clustered to promote EGFR-associated signaling cascade transduction. We propose that HSV-1 hijacks cofilin to initiate infection. These results could promote a better understanding of the pathogenesis of HSV-1-induced neurological diseases.

The actin cytoskeleton is involved in many crucial cellular processes and acts as an obstacle to pathogen entry into host cells. Because HSV-1 establishes lifelong latency in neurons and because neuronal cytoskeletal disruption is thought to be the main cause of HSV-1-induced neurodegeneration, understanding the F-actin remodeling pattern by HSV-1 infection and the molecular interactions that facilitate HSV-1 entry into neurons is important. In this study, we showed that HSV-1 infection induces the rearrangement of the cytoskeleton as well as the initial inactivation and subsequent activation of cofilin. Then, we determined that activation of the EGFR-PI3K-Erk1/2 signaling pathway inactivates cofilin and promotes F-actin polymerization. We postulate that by regulating actin cytoskeleton dynamics, cofilin biphasic activation could represent the specific cellular machinery usurped by pathogen infection, and these results will greatly contribute to the understanding of HSV-1-induced early and complex changes in host cells that are closely linked to HSV-1 pathogenesis.

The caveolin-1 scaffolding domain peptide decreases phosphatidylglycerol levels and inhibits calcium-induced differentiation in mouse keratinocytes

Phospholipase D2 (PLD2) has been found localized in low-density caveolin-rich membrane microdomains. Our previous study suggested that PLD2 and aquaporin 3 (AQP3) interact in these domains to inhibit keratinocyte proliferation and promote differentiation by cooperating to produce phosphatidylglycerol. To examine the effect of membrane microdomain localization on the PLD2/AQP3 signaling module and keratinocyte proliferation and differentiation, we treated mouse keratinocytes with 3 µM cell-permeable caveolin-1 scaffolding domain peptide or a negative control peptide and stimulated cell differentiation using a moderately elevated extracellular calcium concentration (125 uM) to maximally promote differentiation and phosphatidylglycerol production. Cell proliferation, differentiation, total PLD activity, phosphatidylglycerol levels, and AQP3 activity were monitored. The caveolin-1 scaffolding domain peptide itself had no effect on phosphatidylglycerol levels or keratinocyte proliferation or differentiation but prevented the changes induced by a moderately elevated calcium concentration, whereas a negative control did not. The caveolin-1 scaffolding domain peptide had little effect on total PLD activity or glycerol uptake (AQP3 activity). We conclude that the caveolin-1 scaffolding domain peptide disrupts the functional association between AQP3 and PLD2 and prevents both the inhibited proliferation and the stimulated differentiation in response to elevated extracellular calcium levels. The interaction of caveolin-1 and PLD2 is indirect (i.e., lipid mediated); together with the proliferation-promoting effects of caveolin-1 knockout on epidermal keratinocytes, we propose that the caveolin-1 scaffolding domain pepetide exerts a dominant-negative effect on caveolin-1 to alter lipid rafts in these cells.

Cavin-3 dictates the balance between ERK and Akt signaling

Cavin-3 is a tumor suppressor protein of unknown function. Using both in vivo and in vitro approaches, we show that cavin-3 dictates the balance between ERK and Akt signaling. Loss of cavin-3 increases Akt signaling at the expense of ERK, while gain of cavin-3 increases ERK signaling at the expense Akt. Cavin-3 facilitates signal transduction to ERK by anchoring caveolae to the membrane skeleton of the plasma membrane via myosin-1c. Caveolae are lipid raft specializations that contain an ERK activation module and loss of the cavin-3 linkage reduces the abundance of caveolae, thereby separating this ERK activation module from signaling receptors. Loss of cavin-3 promotes Akt signaling through suppression of EGR1 and PTEN. The in vitro consequences of the loss of cavin-3 include induction of Warburg metabolism (aerobic glycolysis), accelerated cell proliferation, and resistance to apoptosis. The in vivo consequences of cavin-3 knockout are increased lactate production and cachexia. DOI:http://dx.doi.org/10.7554/eLife.00905.001.

Hierarchical scaffolding of an ERK1/2 activation pathway

Background

Scaffold proteins modulate cellular signaling by facilitating assembly of specific signaling pathways. However, there is at present little information if and how scaffold proteins functionally interact with each other.

Results

Here, we show that two scaffold proteins, caveolin-1 and IQGAP1, are required for phosphorylation of the actin associated pool of extracellular signal regulated kinase 1 and 2 (ERK1/2) in response to protein kinase C activation. We show by immunofluorescence and proximity ligation assays, that IQGAP1 tethers ERK1/2 to actin filaments. Moreover, siRNA experiments demonstrate that IQGAP1 is required for activation of actin-bound ERK1/2. Caveolin-1 is also necessary for phosphorylation of actin-bound ERK1/2 in response to protein kinase C, but is dispensible for ERK1/2 association with actin. Simultaneous knock down of caveolin-1 and IQGAP1 decreases total phorbol ester-induced ERK1/2 phosphorylation to the same degree as single knock down of either caveolin-1 or IQGAP1, indicating that caveolin-1 and IQGAP1 operate in the same ERK activation pathway. We further show that caveolin-1 knock down, but not IQGAP1 knock down, reduces C-Raf phosphorylation in response to phorbol ester stimulation.

Conclusions

Based on our data, we suggest that caveolin-1 and IQGAP1 assemble distinct signaling modules, which are then linked in a hierarchical arrangement to generate a functional ERK1/2 activation pathway.

Plant sterols as anticancer nutrients: evidence for their role in breast cancer

While many factors are involved in the etiology of cancer, it has been clearly established that diet significantly impacts one’s risk for this disease. More recently, specific food components have been identified which are uniquely beneficial in mitigating the risk of specific cancer subtypes. Plant sterols are well known for their effects on blood cholesterol levels, however research into their potential role in mitigating cancer risk remains in its infancy. As outlined in this review, the cholesterol modulating actions of plant sterols may overlap with their anti-cancer actions. Breast cancer is the most common malignancy affecting women and there remains a need for effective adjuvant therapies for this disease, for which plant sterols may play a distinctive role.