Cellular stress induces the tyrosine phosphorylation of caveolin-1 (Tyr(14)) via activation of p38 mitogen-activated protein kinase and c-Src kinase. Evidence for caveolae, the actin cytoskeleton, and focal adhesions as mechanical sensors of osmotic stress

Caveolae and caveolin-1 as targets of dietary polyphenols for protection against vascular endothelial dysfunction

Caveolae, consisting of caveolin-1 proteins, are ubiquitously present in endothelial cells and contribute to normal cardiovascular functions by acting as a platform for cellular signaling pathways as well as transcytosis and endocytosis. However, caveolin-1 is thought to have a proatherogenic role by inhibiting endothelial nitric oxide synthase activity and Nrf2 activation, or by promoting inflammation through NF-κB activation. Dietary polyphenols were suggested to exert anti-atherosclerotic effects by a mechanism involving the inhibition of endothelial dysfunction, by which they can regulate redox-sensitive signaling pathways in relation to NF-κB and Nrf2 activation. Some monomeric polyphenols and microbiota-derived catabolites from monomeric polyphenols or polymeric tannins might be responsible for the inhibition, because they can be transferred into the circulation from the digestive tract. Several polyphenols were reported to modulate caveolin-1 expression or its localization in caveolae. Therefore, we hypothesized that circulating polyphenols affect caveolae functions by altering its structure leading to the release of caveolin-1 from caveolae, and attenuating redox-sensitive signaling pathway-dependent caveolin-1 overexpression. Further studies using circulating polyphenols at a physiologically relevant level are necessary to clarify the mechanism of action of dietary polyphenols targeting caveolae and caveolin-1.

To squeeze or not: Regulation of cell size by mechanical forces in development and human diseases

Physical constraints, such as compression, shear stress, stretching and tension play major roles during development and tissue homeostasis. Mechanics directly impact physiology, and their alteration is also recognized as having an active role in driving human diseases. Recently, growing evidence has accumulated on how mechanical forces are translated into a wide panel of biological responses, including metabolism and changes in cell morphology. The aim of this review is to summarize and discuss our knowledge on the impact of mechanical forces on cell size regulation. Other biological consequences of mechanical forces will not be covered by this review. Moreover, wherever possible, we also discuss mechanosensors and molecular and cellular signaling pathways upstream of cell size regulation. We finally highlight the relevance of mechanical forces acting on cell size in physiology and human diseases.

Are There Lipid Membrane-Domain Subtypes in Neurons with Different Roles in Calcium Signaling?

Lipid membrane nanodomains or lipid rafts are 10-200 nm diameter size cholesterol- and sphingolipid-enriched domains of the plasma membrane, gathering many proteins with different roles. Isolation and characterization of plasma membrane proteins by differential centrifugation and proteomic studies have revealed a remarkable diversity of proteins in these domains. The limited size of the lipid membrane nanodomain challenges the simple possibility that all of them can coexist within the same lipid membrane domain. As caveolin-1, flotillin isoforms and gangliosides are currently used as neuronal lipid membrane nanodomain markers, we first analyzed the structural features of these components forming nanodomains at the plasma membrane since they are relevant for building supramolecular complexes constituted by these molecular signatures. Among the proteins associated with neuronal lipid membrane nanodomains, there are a large number of proteins that play major roles in calcium signaling, such as ionotropic and metabotropic receptors for neurotransmitters, calcium channels, and calcium pumps. This review highlights a large variation between the calcium signaling proteins that have been reported to be associated with isolated caveolin-1 and flotillin-lipid membrane nanodomains. Since these calcium signaling proteins are scattered in different locations of the neuronal plasma membrane, i.e., in presynapses, postsynapses, axonal or dendritic trees, or in the neuronal soma, our analysis suggests that different lipid membrane-domain subtypes should exist in neurons. Furthermore, we conclude that classification of lipid membrane domains by their content in calcium signaling proteins sheds light on the roles of these domains for neuronal activities that are dependent upon the intracellular calcium concentration. Some examples described in this review include the synaptic and metabolic activity, secretion of neurotransmitters and neuromodulators, neuronal excitability (long-term potentiation and long-term depression), axonal and dendritic growth but also neuronal cell survival and death.

Decrease in cholesterol in the cell membrane is essential for Nrf2 activation by quercetin

Quercetin is a flavonoid with various cytoprotective effects. We previously reported that quercetin exerts anti-allergic, anti-oxidative, and anti-fibrotic activities via the induction of heme oxygenase (HO)-1. However, the mechanisms by which quercetin induces HO-1 to exhibit cytoprotective effects are poorly understood. We focused on its action on the cell membrane, which is the first part of the cell to interact with the extracellular environment. The cell membrane contains lipid rafts and caveolae, which play important roles in cellular signaling. A recent study showed that nuclear factor E2-related factor 2 (Nrf2), a transcription factor regulating anti-oxidative enzymes including HO-1, interacts with caveolin-1 (Cav-1), a component of caveolae, to regulate cellular anti-oxidative capacity. In this study, we investigated the changes in the cell membrane that leads to the induction of HO-1 by quercetin. Quercetin decreased the amount of cholesterol in the raft fractions, which in turn promoted the induction of HO-1. It also changed the composition of the lipid rafts and decreased and increased the expression of Cav-1 in the raft and non-raft fractions, respectively. Nrf2, which was localized in the cell membrane under resting conditions, was translocated along with Cav-1 to the nucleus after exposure to quercetin. These findings indicate for the first time that the HO-1-dependent cytoprotective effects of quercetin are mediated by the structural changes in lipid rafts brought about by decreasing the amount of cholesterol in the cell membrane, which thereby results in the translocation of the Cav-1-Nrf2 complex to the nucleus and induces the expression of HO-1.

Pathophysiology and molecular mechanism of caveolin involved in myocardial protection strategies in ischemic conditioning

Multiple pathophysiological pathways are activated during the process of myocardial injury. Various cardioprotective strategies protect the myocardium from ischemia, infarction, and ischemia/reperfusion (I/R) injury through different targets, yet the clinical translation remains limited. Caveolae and its structure protein, caveolins, have been suggested as a bridge to transmit damage-preventing signals and mediate the protection of ultrastructure in cardiomyocytes under pathological conditions. In this review, we first briefly introduce caveolae and caveolins. Then we review the cardioprotective strategies mediated by caveolins through various pathophysiological pathways. Finally, some possible research directions are proposed to provide future experiments and clinical translation perspectives targeting caveolin based on the investigative evidence.

Mitochondrial Dysfunction and the Glycolytic Switch Induced by Caveolin-1 Phosphorylation Promote Cancer Cell Migration, Invasion, and Metastasis

Simple Summary

Caveolin-1 (CAV1) is a membrane protein that has been attributed a dual role in cancer, acting at early stages as a tumor suppressor and in later stages of the disease as a promoter of metastasis. In the latter case, enhanced expression of CAV1 favors the malignant phenotype and correlates with a poorer prognosis of the patients. Bearing in mind that the reprogramming of energy metabolism is required in cancer cells to meet both the bioenergetic and biosynthetic needs to sustain increased proliferation, migration, and invasion, we evaluated the metabolism of metastatic cells expressing or not CAV1. In this study, we show that the expression of CAV1 promotes in cancer cells a metabolic switch to an aerobic, glycolytic phenotype by blocking mitochondrial respiration.

Abstract

Cancer cells often display impaired mitochondrial function, reduced oxidative phosphorylation, and augmented aerobic glycolysis (Warburg effect) to fulfill their bioenergetic and biosynthetic needs. Caveolin-1 (CAV1) is a scaffolding protein that promotes cancer cell migration, invasion, and metastasis in a manner dependent on CAV1 phosphorylation on tyrosine-14 (pY14). Here, we show that CAV1 expression increased glycolysis rates, while mitochondrial respiration was reduced by inhibition of the mitochondrial complex IV. These effects correlated with increased reactive oxygen species (ROS) levels that favored CAV1-induced migration and invasion. Interestingly, pY14-CAV1 promoted the metabolic switch associated with increased migration/invasion and augmented ROS-inhibited PTP1B, a phosphatase that controls pY14 levels. Finally, the glycolysis inhibitor 2-deoxy-D-glucose reduced CAV1-enhanced migration in vitro and metastasis in vivo of murine melanoma cells. In conclusion, CAV1 promotes the Warburg effect and ROS production, which inhibits PTP1B to augment CAV1 phosphorylation on tyrosine-14, thereby increasing the metastatic potential of cancer cells.

Inner retinal injury in experimental glaucoma is prevented upon AAV mediated Shp2 silencing in a caveolin dependent manner

SH2 domain containing tyrosine phosphatase 2 (Shp2; PTPN11) regulates several intracellular pathways downstream of multiple growth factor receptors. Our studies implicate that Shp2 interacts with Caveolin-1 (Cav-1) protein in retinal ganglion cells (RGCs) and negatively regulates BDNF/TrkB signaling. This study aimed to investigate the mechanisms underlying the protective effects of shp2 silencing in the RGCs in glaucomatous conditions.

Methods: Shp2 was silenced in the Cav-1 deficient mice and the age matched wildtype littermates using adeno-associated viral (AAV) constructs. Shp2 expression modulation was performed in an acute and a chronic mouse model of experimental glaucoma. AAV2 expressing Shp2 eGFP-shRNA under a strong synthetic CAG promoter was administered intravitreally in the animals' eyes. The contralateral eye received AAV-eGFP-scramble-shRNA as control. Animals with Shp2 downregulation were subjected to either microbead injections or acute ocular hypertension experimental paradigm. Changes in inner retinal function were evaluated by measuring positive scotopic threshold response (pSTR) while structural and biochemical alterations were evaluated through H&E staining, western blotting and immunohistochemical analysis of the retinal tissues.

Results: A greater loss of pSTR amplitudes was observed in the WT mice compared to Cav-1^-/- retinas in both the models. Silencing of Shp2 phosphatase imparted protection against inner retinal function loss in chronic glaucoma model in WT mice. The functional rescue also translated to structural preservation of ganglion cell layer in the chronic glaucoma condition in WT mice which was not evident in Cav-1^-/- mice retinas.

Conclusions: This study indicates that protective effects of Shp2 ablation under chronic experimental glaucoma conditions are dependent on Cav-1 in the retina, suggesting in vivo interactions between the two proteins.

Differences in the expression of caveolin-1 isoforms in cancer-associated and normal fibroblasts of patients with oral squamous cell carcinoma

OBJECTIVES

For many years, tumor development has been viewed as a cell-autonomous process; however, today we know that the tumor microenvironment (TME) and especially cancer-associated fibroblasts (CAFs) significantly contribute to tumor progression. Caveolin-1 (Cav-1) is a scaffolding protein which is involved in several cancer-associated processes as important component of the caveolae. Our goal was to shed light on the expression of the two different isoforms of Cav-1 in normal fibroblasts (NFs) and CAFs of patients with oral squamous cell carcinoma (OSCC).

MATERIALS AND METHODS

Fibroblasts from normal mucosa and CAFs were isolated and propagated in vitro. Gene expression of the different Cav-1 isoforms was assessed via quantitative real-time PCR (qPCR) and supplemented by protein expression analysis.

RESULTS

We could show that the Cav-1β isoform is more highly expressed in NFs and CAFs compared to Cav-1α. Furthermore, the different Cav-1 isoforms tended to be differently expressed in different tumor stages. However, this trend could not be seen consistently, which is in line with the ambiguous role of Cav-1 in tumor progression described in literature. Western blotting furthermore revealed that NFs and CAFs might differ in the oligomerization profile of the Cav-1 protein.

CONCLUSION

These differences in expression of Cav-1 between NFs and CAFs of patients with OSCC confirm that the protein might play a role in tumor progression and is of interest for further analyses.

CLINICAL RELEVANCE

Our findings support a possible role of the two isoforms of Cav-1 in the malignant transformation of OSCC.

Caveolin-1 promotes radioresistance in rhabdomyosarcoma through increased oxidative stress protection and DNA repair

The aim of this work was to investigate whether Caveolin-1 (Cav-1), a membrane scaffolding protein widely implicated in cancer, may play a role in radiation response in rhabdomyosarcoma (RMS), a pediatric soft tissue tumor. For this purpose, we employed human RD cells in which Cav-1 expression was stably increased via gene transfection. After radiation treatment, we observed that Cav-1 limited cell cycle arrest in the G2/M phase and enhanced resistance to cell senescence and apoptosis via reduction of p21^Cip1/Waf1, p16^INK4a and Caspase-3 cleavage. After radiotherapy, Cav-1-mediated cell radioresistance was characterized by low accumulation of H2AX foci, as confirmed by Comet assay, marked neutralization of reactive oxygen species (ROS) and enhanced DNA repair via activation of ATM, Ku70/80 complex and DNA-PK. We found that Cav-1-overexpressing RD cells, already under basal conditions, had higher glutathione (GSH) content and greater catalase expression, which conferred protection against acute treatment with hydrogen peroxide. Furthermore, pre-treatment of Cav-1-overexpressing cells with PP2 or LY294002 compounds restored the sensitivity to radiation treatment, indicating a role for Src-kinases and Akt pathways in Cav-1-mediated radioresistance. These findings were confirmed using radioresistant RD and RH30 lines generated by hypofractionated radiotherapy protocol, which showed marked increase of Cav-1, catalase and Akt, and sensitivity to PP2 and LY294002 treatment. In conclusion, these data suggest that concerted activity of Cav-1 and catalase, in cooperation with activation of Src-kinase and Akt pathways, may represent a network of vital mechanisms that allow irradiated RMS cells to evade cell death induced by oxidative stress and DNA damage.

Cross-talk of Signaling Pathways in the Pathogenesis of Allergic Asthma and Cataract

Allergic asthma is a chronic inflammatory disease, which involves many cellular and cellular components. Cataract is a condition that affects the transparency of the lens, which the opacity of the lens caused by any innate or acquired factor degrades its transparency or changes in color. Both of them belong to diseases induced by immune disorders or inflammation. We want to confirm the signaling pathways involved in the regulation of asthma and cataract simultaneously, and provide reference for the later related experiments. So we conducted a scoping review of many databases and searched for studies (Academic research published in Wiley, Springer and Bentham from 2000 to 2019) about the possible relationship between asthma and cataract. It was found that during the onset of asthma and cataract, Rho/Rock signaling pathway, Notch signaling pathway, Wnt/β-catenin signaling pathway, PI3K/AKT signaling pathway, JAK/STAT signaling pathway, MAPK signaling pathway, TGF-β1/Smad signaling pathway and NF-κB signaling pathway are all active, so they may have a certain correlation in pathogenesis. Asthma may be associated with cataract through the eight signaling pathways, causing inflammation or immune imbalance based on allergy that can lead to cataract. According to these studies, we speculated that the three most likely signaling pathways are PI3K/AKT, MAPK and NF-κB signaling pathway.

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.

Adhesion-dependent Caveolin-1 Tyrosine-14 phosphorylation is regulated by FAK in response to changing matrix stiffness

Integrin-mediated adhesion regulates cellular responses to changes in the mechanical and biochemical properties of the extracellular matrix. Cell-matrix adhesion regulates caveolar endocytosis, dependent on caveolin 1 (Cav1) Tyr14 phosphorylation (pY14Cav1), to control anchorage-dependent signaling. We find that cell-matrix adhesion regulates pY14Cav1 levels in mouse fibroblasts. Biochemical fractionation reveals endogenous pY14Cav1 to be present in caveolae and focal adhesions (FA). Adhesion does not affect caveolar pY14Cav1, supporting its regulation at FA, in which PF-228-mediated inhibition of focal adhesion kinase (FAK) disrupts. Cell adhesion on 2D polyacrylamide matrices of increasing stiffness stimulates Cav1 phosphorylation, which is comparable to the phosphorylation of FAK. Inhibition of FAK across varying stiffnesses shows it regulates pY14Cav1 more prominently at higher stiffness. Taken together, these studies reveal the presence of FAK-pY14Cav1 crosstalk at FA, which is regulated by cell-matrix adhesion.

Highly Osmotic Oxidized Sucrose-Crosslinked Polyethylenimine for Gene Delivery Systems

In this work, highly osmotic oxidized sucrose-crosslinked polyethylenimine (SP2K) polymers were developed for gene delivery systems, and the transfection mechanism is examined. First, periodate-oxidized sucrose and polyethylenimine 2K (PEI2K) were crosslinked with various feed ratios via reductive amination. The synthesis was confirmed by ¹H NMR and FTIR. The synthesized SP2K polymers could form positively charged (~40 mV zeta-potential) and nano-sized (150–200 nm) spherical polyplexes with plasmid DNA (pDNA). They showed lower cytotoxicity than PEI25K but concentration-dependent cytotoxicity. Among them, SP2K7 and SP2K10 showed higher transfection efficiency than PEI25K in both serum and serum-free conditions, revealing the good serum stability. It was found that SP2K polymers possessed high osmolality and endosome buffering capacity. The transfection experiments with cellular uptake inhibitors suggest that the transfection of SP2K polymers would progress by multiple pathways, including caveolae-mediated endocytosis. It was also thought that caveolae-mediated endocytosis of SP2K polyplexes would be facilitated through cyclooxygenase-2 (COX-2) expression induced by high osmotic pressure of SP2K polymers. Confocal microscopy results also supported that SP2K polyplexes would be internalized into cells via multiple pathways and escape endosomes efficiently via high osmolality and endosome buffering capacity. These results demonstrate the potential of SP2K polymers for gene delivery systems.

Caveolae and lipid sorting: Shaping the cellular response to stress

Caveolae are an abundant and characteristic surface feature of many vertebrate cells. The uniform shape of caveolae is characterized by a bulb with consistent curvature connected to the plasma membrane (PM) by a neck region with opposing curvature. Caveolae act in mechanoprotection by flattening in response to increased membrane tension, and their disassembly influences the lipid organization of the PM. Here, we review evidence for caveolae as a specialized lipid domain and speculate on mechanisms that link changes in caveolar shape and/or protein composition to alterations in specific lipid species. We propose that high membrane curvature in specific regions of caveolae can enrich specific lipid species, with consequent changes in their localization upon caveolar flattening. In addition, we suggest how changes in the association of lipid-binding caveolar proteins upon flattening of caveolae could allow release of specific lipids into the bulk PM. We speculate that the caveolae-lipid system has evolved to function as a general stress-sensing and stress-protective membrane domain.

Caveolin1 Tyrosine-14 Phosphorylation: Role in Cellular Responsiveness to Mechanical Cues

The plasma membrane is a dynamic lipid bilayer that engages with the extracellular microenvironment and intracellular cytoskeleton. Caveolae are distinct plasma membrane invaginations lined by integral membrane proteins Caveolin1, 2, and 3. Caveolae formation and stability is further supported by additional proteins including Cavin1, EHD2, Pacsin2 and ROR1. The lipid composition of caveolar membranes, rich in cholesterol and phosphatidylserine, actively contributes to caveolae formation and function. Post-translational modifications of Cav1, including its phosphorylation of the tyrosine-14 residue (pY14Cav1) are vital to its function in and out of caveolae. Cells that experience significant mechanical stress are seen to have abundant caveolae. They play a vital role in regulating cellular signaling and endocytosis, which could further affect the abundance and distribution of caveolae at the PM, contributing to sensing and/or buffering mechanical stress. Changes in membrane tension in cells responding to multiple mechanical stimuli affects the organization and function of caveolae. These mechanical cues regulate pY14Cav1 levels and function in caveolae and focal adhesions. This review, along with looking at the mechanosensitive nature of caveolae, focuses on the role of pY14Cav1 in regulating cellular mechanotransduction.

Volumetric Compression Induces Intracellular Crowding to Control Intestinal Organoid Growth via Wnt/β-Catenin Signaling

Enormous amounts of essential intracellular events are crowdedly packed inside picoliter-sized cellular space. However, the significance of the physical properties of cells remains underappreciated because of a lack of evidence of how they affect cellular functionalities. Here, we show that volumetric compression regulates the growth of intestinal organoids by modifying intracellular crowding and elevating Wnt/β-catenin signaling. Intracellular crowding varies upon stimulation by different types of extracellular physical/mechanical cues and leads to significant enhancement of Wnt/β-catenin signaling by stabilizing the LRP6 signalosome. By enhancing intracellular crowding using osmotic and mechanical compression, we show that expansion of intestinal organoids was facilitated through elevated Wnt/β-catenin signaling and greater intestinal stem cell (ISC) self-renewal. Our results provide an entry point for understanding how intracellular crowdedness functions as a physical regulator linking extracellular physical cues with intracellular signaling and potentially facilitate the design of engineering approaches for expansion of stem cells and organoids.

Polyethylenimine-based nanovector grafted with mannitol moieties to achieve effective gene delivery and transfection

Polyethylenimine (PEI), a kind of cationic non-viral gene delivery vector, is capable of stable and efficient transgene expression for gene delivery. However, low transfection efficiency in vivo along with high toxicity limited the further application of gene therapy in the clinic. To enhance gene transfection performance and reduce cytotoxicity of polyethylenimine, branched polyethylenimine-derived cationic polymers BPEI25 k-man-S/L/M/H with different grafting degree with mannitol moieties were prepared and the transfection efficiency was evaluated. Among them, BPEI25 k-man-L showed the best transfection efficiency, lower toxicity, and significantly enhanced long-term systemic transgene expression for 96 h in vivo even at a single-dose administration. The results of cellular uptake mechanism and western-blot experiments revealed that the mannitol modification of BPEI25 k induced and up-regulated the phosphorylation of caveolin-1 and thus enhanced the caveolae-mediated cellular uptake. This class of gene delivery system highlights a paradigmatic approach for the development of novel and safe non-viral vectors for gene therapy.

Caveolin-1 Ablation Imparts Partial Protection Against Inner Retinal Injury in Experimental Glaucoma and Reduces Apoptotic Activation

Retinal ganglion cell degeneration is a characteristic feature of glaucoma, and accordingly, protection of these cells constitutes a major therapeutic objective in the disease. Here, we demonstrate the key influence of caveolin (Cav) in regulating the inner retinal homeostasis in two models of experimentally elevated intraocular pressure (IOP). Two groups of Cav-1^-/- and wild-type mice were used in the study. Animals were subjected to experimentally induced chronic and acutely elevated IOP and any changes in their retinal function were assessed by positive scotopic threshold response recordings. TUNEL and cleaved caspase-3 assays were performed to evaluate apoptotic changes in the retina while Brn3a immunostaining was used as a marker to assess and quantify ganglion cell layer (GCL) changes. H&E staining was carried out on retinal sections to evaluate histological differences in retinal laminar structure. Cav-1 ablation partially protected the inner retinal function in both chronic and acute models of elevated IOP. The protective effects of Cav-1 loss were also evident histologically by reduced loss of GCL density in both models. The phenotypic protection in Cav-1^-/- glaucoma mice paralleled with increased TrkB phosphorylation and reduced endoplasmic reticulum stress markers and apoptotic activation in the inner retinas. This study corroborated previous findings of enhanced Shp2 phosphorylation in a chronic glaucoma model and established a novel role of Cav-1 in mediating activation of this phosphatase in the inner retina in vivo. Collectively, these findings highlight the critical involvement of Cav-1 regulatory mechanisms in ganglion cells in response to increased IOP, implicating Cav-1 as a potential therapeutic target in glaucoma.

Targeting RIPK3 oligomerization blocks necroptosis without inducing apoptosis

Receptor-interacting serine/threonine-protein kinase 3 (RIPK3) is a central protein in necroptosis with great potential as a target for treating necroptosis-associated diseases, such as Crohn's disease. However, blockade of RIPK3 kinase activity leads to unexpected RIPK3-initiated apoptosis. Herein, we found that PP2, a known SRC inhibitor, inhibits TNF-α-induced necroptosis without initiating apoptosis. Further investigation showed that PP2 acts as an inhibitor of not only SRC but also RIPK3. PP2 does not disturb the integrity of the RIPK1-RIPK3-mixed lineage kinase domain-like pseudokinase (MLKL) necroptosome or the autophosphorylation of RIPK3 at T231/S232 but disrupts RIPK3 oligomerization, thereby impairing the phosphorylation and oligomerization of MLKL. These results demonstrate the essential role of RIPK3 oligomerization in necroptosis and suggest a potential RIPK3 oligomerization-targeting strategy for therapeutic development.

The non-receptor tyrosine phosphatase type 14 blocks caveolin-1-enhanced cancer cell metastasis

Caveolin-1 (CAV1) enhanced migration, invasion, and metastasis of cancer cells is inhibited by co-expression of the glycoprotein E-cadherin. Although the two proteins form a multiprotein complex that includes β-catenin, it remained unclear how this would contribute to blocking the metastasis promoting function of CAV1. Here, we characterized by mass spectrometry the protein composition of CAV1 immunoprecipitates from B16F10 murine melanoma cells expressing or not E-cadherin. The novel protein tyrosine phosphatase PTPN14 was identified by mass spectrometry analysis exclusively in co-immunoprecipitates of CAV1 with E-cadherin. Interestingly, PTPN14 is implicated in controlling metastasis, but only few known PTPN14 substrates exist. We corroborated by western blotting experiments that PTPN14 and CAV1 co-inmunoprecipitated in the presence of E-cadherin in B16F10 melanoma and other cancer cells. Moreover, the CAV1(Y14F) mutant protein was shown to co-immunoprecipitate with PTPN14 even in the absence of E-cadherin, and overexpression of PTPN14 reduced CAV1 phosphorylation on tyrosine-14, as well as suppressed CAV1-enhanced cell migration, invasion and Rac-1 activation in B16F10, metastatic colon [HT29(US)] and breast cancer (MDA-MB-231) cell lines. Finally, PTPN14 overexpression in B16F10 cells reduced the ability of CAV1 to induce metastasis in vivo. In summary, we identify here CAV1 as a novel substrate for PTPN14 and show that overexpression of this phosphatase suffices to reduce CAV1-induced metastasis.

Caveolae: Formation, dynamics, and function

Caveolae are abundant surface pits formed by the assembly of cytoplasmic proteins on a platform generated by caveolin integral membrane proteins and membrane lipids. This membranous assembly can bud off into the cell or can be disassembled releasing the cavin proteins into the cytosol. Disassembly can be triggered by increased membrane tension, or by stress stimuli, such as UV. Here, we discuss recent mechanistic studies showing how caveolae are formed and how their unique properties allow them to function as multifunctional protective and signaling structures.

A role for caveola-forming proteins caveolin-1 and CAVIN1 in the pro-invasive response of glioblastoma to osmotic and hydrostatic pressure

In solid tumours, elevated interstitial fluid pressure (osmotic and hydrostatic pressure) is a barrier to drug delivery and correlates with poor prognosis. Glioblastoma (GBM) further experience compressive force when growing within a space limited by the skull. Caveolae are proposed to play mechanosensing roles, and caveola‐forming proteins are overexpressed in GBM. We asked whether caveolae mediate the GBM response to osmotic pressure. We evaluated in vitro the influence of spontaneous or experimental down‐regulation of caveola‐forming proteins (caveolin‐1, CAVIN1) on the proteolytic profile and invasiveness of GBM cells in response to osmotic pressure. In response to osmotic pressure, GBM cell lines expressing caveola‐forming proteins up‐regulated plasminogen activator (uPA) and/or matrix metalloproteinases (MMPs), some EMT markers and increased their in vitro invasion potential. Down‐regulation of caveola‐forming proteins impaired this response and prevented hyperosmolarity‐induced mRNA expression of the water channel aquaporin 1. CRISPR ablation of caveola‐forming proteins further lowered expression of matrix proteases and EMT markers in response to hydrostatic pressure, as a model of mechanical force. GBM respond to pressure by increasing matrix‐degrading enzyme production, mesenchymal phenotype and invasion. Caveola‐forming proteins mediate, at least in part, the pro‐invasive response of GBM to pressure. This may represent a novel target in GBM treatment.

Enhance transgene responses through improving cellular uptake and intracellular trafficking by bio-inspired non-viral vectors

Background

Gene therapy remains a significant challenge due to lots of barriers limiting the genetic manipulation technologies. As for non-viral delivery vectors, they often suffer insufficient performance due to inadequate cellular uptake and gene degradation in endosome or lysosome. The importance of overcoming these conserved intracellular barriers is increasing as the delivery of genetic cargo.

Results

A surface-functionalized non-viral vector involving the biomimetic mannitol moiety is initiated, which can control the cellular uptake and promote the caveolae-mediated pathway and intracellular trafficking, thus avoiding acidic and enzymatic lysosomal degradation of loaded gene internalized by clathrin-mediated pathway. Different degrees of mannitol moiety are anchored onto the surface of the nanoparticles to form bio-inspired non-viral vectors and CaP-MA-40 exhibits remarkably high stability, negligible toxicity, and significantly enhanced transgene expression both in vitro and in vivo.

Conclusions

This strategy highlights a paradigmatic approach to construct vectors that need precise intracellular delivery for innovative applications.

Alteration in memory cognition due to activation of caveolin-1 and oxidative damage in a model of dementia of Alzheimer's type

OBJECTIVE

The present study aims to investigate the role of caveolin-1 in dementia of Alzheimer's type using intracerebroventricular streptozotocin (ICV-STZ)-induced neurodegeneration model in rats.

MATERIALS AND METHODS

Male Wistar rats (220-260 g) were employed. STZ 3 mg/kg via ICV route was given once to cause neuronal injury. Daidzein - a caveolin inhibitor at 0.2, 0.4, and 0.6 mg/kg s.c. were given daily whereas minoxidil - a caveolin activator was given at 0.45 mg/kg, i.p. on alternate days for 28 days. STZ was also given at its submaximal dose 1.5 mg/kg to minoxidil group only.

RESULTS

ICV-STZ control animals exhibited cognitive and neurological deficits on the Morris water maze, elevated plus maze, and balance beam tests (P < 0.0001). Treatment with daidzein significantly restored memory impairments and decreased oxidative damage whereas minoxidil potentiates the effect of STZ causing significant impairment in memory. Significant oxidative stress such as lipid peroxidation and glutathione (P < 0.0001) were also observed due to ICV-STZ administration resulting in neuronal damage which was significantly prevented by treatment with daidzein in brain tissues.

CONCLUSION

The findings from the present investigation may conclude that the caveolin-1 from caveolae at the cell membrane induces memory deficits and oxidative stress phenotype that resemble the neurological phenotype of Alzheimer's disease. Further studies are warranted to gauge the effect of caveolin dyshomeostasis on the amyloidogenic cascade.

Hypoxia Attenuates Trastuzumab Uptake and Trastuzumab-Emtansine (T-DM1) Cytotoxicity through Redistribution of Phosphorylated Caveolin-1

The antibody-drug conjugate trastuzumab-emtansine (T-DM1) offers an additional treatment option for patients with HER2-amplified tumors. However, primary and acquired resistance is a limiting factor in a significant subset of patients. Hypoxia, a hallmark of cancer, regulates the trafficking of several receptor proteins with potential implications for tumor targeting. Here, we have investigated how hypoxic conditions may regulate T-DM1 treatment efficacy in breast cancer. The therapeutic effect of T-DM1 and its metabolites was evaluated in conjunction with biochemical, flow cytometry, and high-resolution imaging studies to elucidate the functional and mechanistic aspects of hypoxic regulation. HER2 and caveolin-1 expression was investigated in a well-annotated breast cancer cohort. We find that hypoxia fosters relative resistance to T-DM1 in HER2⁺ cells (SKBR3 and BT474). This effect was not a result of deregulated HER2 expression or resistance to emtansine and its metabolites. Instead, we show that hypoxia-induced translocation of caveolin-1 from cytoplasmic vesicles to the plasma membrane contributes to deficient trastuzumab internalization and T-DM1 chemosensitivity. Caveolin-1 depletion mimicked the hypoxic situation, indicating that vesicular caveolin-1 is indispensable for trastuzumab uptake and T-DM1 cytotoxicity. In vitro studies suggested that HER2 and caveolin-1 are not coregulated, which was supported by IHC analysis in patient tumors. We find that phosphorylation-deficient caveolin-1 inhibits trastuzumab internalization and T-DM1 cytotoxicity, suggesting a specific role for caveolin-1 phosphorylation in HER2 trafficking. IMPLICATIONS: Together, our data for the first time identify hypoxic regulation of caveolin-1 as a resistance mechanism to T-DM1 with potential implications for individualized treatment of breast cancer.

Caveolin-1 Y14 phosphorylation suppresses tumor growth while promoting invasion

Caveolin-1 is a transmembrane protein with both tumor promoter and suppressor functions that remain poorly understood. Cav1 phosphorylation by Src kinase on tyrosine 14 is closely associated with focal adhesion dynamics and tumor cell migration, however the role of pCav1 in vivo in tumor progression remains poorly characterized. Herein, we expressed phosphomimetic Y14D, wild type, and non-phosphorylatable Y14F forms of Cav1 in MDA-MB-435 cancer cells. Expression of Cav1Y14D reduced cell proliferation and induced the TP53 tumor suppressor. Ectopic expression in MDA-MB-435 cells of Y14 phosphorylatable Cav1 was required for induction of TP53 in response to oxidative stress. Cav1Y14D promotes an apparent reversal of the Warburg effect and markedly inhibited tumor growth in vivo. However, Cav1 induced pseudopodial recruitment of glycolytic enzymes, and time-lapse intravital imaging showed increased invadopodia protrusion and extravasation into blood vessels for Cav1WT and Y14D but not for Y14F. Our results suggest that Cav1 Y14 phosphorylation levels play a role in the conflicting demands on metabolic resources associated with cancer cell proliferation versus motility.

Dynamic Regulation of Caveolin-1 Phosphorylation and Caveolae Formation by Mammalian Target of Rapamycin Complex 2 in Bladder Cancer Cells

The mammalian target of rapamycin (mTOR) and associated phosphatidylinositol 3-kinase/AKT/mTOR signaling pathway is commonly up-regulated in cancer, including bladder cancer. mTOR complex 2 (mTORC2) is a major regulator of bladder cancer cell migration and invasion, but the mechanisms by which mTORC2 regulates these processes are unclear. A discovery mass spectrometry and reverse-phase protein array-based proteomics dual approach was used to identify novel mTORC2 phosphoprotein targets in actively invading cancer cells. mTORC2 targets included focal adhesion kinase, proto-oncogene tyrosine-protein kinase Src, and caveolin-1 (Cav-1), among others. Functional testing shows that mTORC2 regulates Cav-1 localization and dynamic phosphorylation of Cav-1 on Y14. Regulation of Cav-1 activity by mTORC2 also alters the abundance of caveolae, which are specialized lipid raft invaginations of the plasma membrane associated with cell signaling and membrane compartmentalization. Our results demonstrate a unique role for mTORC2-mediated regulation of caveolae formation in actively migrating cancer cells.

Pharmacological inhibition of guanosine triphosphate cyclohydrolase1 elevates tyrosine phosphorylation of caveolin1 and cellular senescence

The role of 2,4-diamino-6-hydroxypyrimidine (DAHP), on cellular-senescence remains unclear as differential effects of DAHP have been reported in cardiovascular and cerebrovascular systems. We investigated the effect of pharmacologically-induced guanosine-triphosphate-cyclohydrolase1 (GTPCH1)-inhibition, through DAHP, on cellular-senescence in experimentally-induced diabetic and non-diabetic Wistar rats. Cellular-senescence was evaluated through senescence-associated events, namely, cell-cycle-arrest of peripheral blood mononuclear cells (PBMNCs); myocardial DNA fragmentation, total antioxidant capacity (TAC), telomerase-activity, nicotinamide adenine dinucleotide (NAD⁺)-content and tyrosine14-phosphorylation of caveolin1 (pY14) in similarly-aged, pubertal Wistar rats with streptozotocin (STZ) and/or DAHP. Oxidative stress (OS) indices such as myocardial biopterin concentrations (tetrahydrobiopterin-BH₄ and dihydrobiopterin-BH₂) and plasma total nitrite and nitrate (NOx) were determined. DAHP, per se, exhibited distinct senescence; in addition, in STZ+DAHP (the cardiomyopathy model), there was a marked accumulation of cells in G0G1 phase, as evidenced through flow-cytometry analysis, as-well-as fragmented DNA, than the respective controls suggesting the DAHP-mediated onset of senescence in circulating cells and the myocardium, with or without STZ. Concentrations of BH4 and BH2, and NOx were impaired in STZ and/or DAHP, indicating elevated OS in the treatment groups. In the independent treatment groups or the combination treatment, typical senescence indicators including myocardial telomerase-activity, NAD⁺-content and TAC were significantly reduced, while there was a marked elevation in the concentrations of pY14 as compared to the respective controls, reinforcing the occurrence of senescence in PBMNCs and the myocardium. We postulate that DAHP promotes early onset of cellular-senescence, potentially through OS-mediated cellular events in diabetic or non-diabetic models.

Bupivacaine-induced contraction is attenuated by endothelial nitric oxide release modulated by activation of both stimulatory and inhibitory phosphorylation (Ser1177 and Thr495) of endothelial nitric oxide synthase

This study examined the mechanism associated with the endothelium-dependent attenuation of vasoconstriction induced by bupivacaine (BPV), with a particular focus on the upstream cellular signaling pathway of endothelial nitric oxide synthase (eNOS) phosphorylation induced by BPV in human umbilical vein endothelial cells (HUVECs). BPV concentration-response curves were investigated in the isolated rat aorta. The effects of Nω-nitro-L-arginine methyl ester (L-NAME), 1H-[1,2,4]oxadiazolo[4,3-a]quinoxalin-1-one (ODQ), methylene blue, calmidazolium, the Src kinase inhibitor 4-amino-3-(4-chlorophenyl)-1-(t-butyl)-1H-pyrazolo[3,4-d]pyrimidine, 4-amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2) and the combination of L-arginine and L-NAME on BPV-induced contraction in endothelium-intact aorta preparations were examined. The effects of BPV alone and in combination with PP2 on the phosphorylation of eNOS (at Ser1177 or Thr495), caveolin-1 and Src kinase were examined in HUVECs. BPV-induced contraction was lower in endothelium-intact aortae than in endothelium-denuded aortae. L-NAME, ODQ, methylene blue and calmidazolium increased BPV-induced contraction in endothelium-intact aortae, whereas PP2 alone and combined treatment with L-arginine and L-NAME inhibited BPV-induced contraction. Low-concentration BPV (30 µM) induced both stimulatory (Ser1177) and inhibitory (Thr495) phosphorylation of eNOS in HUVECs. However, high-concentration BPV (150 µM) induced only stimulatory (Ser1177) eNOS phosphorylation. Additionally, phosphorylation of Src kinase, caveolin-1 and inhibitory eNOS (Thr495) induced by low-concentration BPV was inhibited by PP2. These results suggest that contraction induced by low-concentration BPV is attenuated by endothelial nitric oxide release, which is modulated both stimulatory (Ser1177) and inhibitory eNOS phosphorylation (Thr495). BPV-induced phosphorylation of eNOS (Thr495) is indirectly mediated by an upstream cellular signaling pathway involving Src kinase (Tyr416) and caveolin-1 (Tyr14).

Cell-free formation and interactome analysis of caveolae

Caveolae are linked to signaling protein regulation through interactions with caveolins. We describe a cell-free system for the biogenesis of caveolae and show phosphorylated-caveolins preferentially bind signaling proteins. Our validation in vivo shows that phosphorylated CAV1 recruits TRAF2 to the endosome to form a signaling platform.

Caveolae have been linked to the regulation of signaling pathways in eukaryotic cells through direct interactions with caveolins. Here, we describe a cell-free system based on Leishmania tarentolae (Lt) extracts for the biogenesis of caveolae and show its use for single-molecule interaction studies. Insertion of expressed caveolin-1 (CAV1) into Lt membranes was analogous to that of caveolin in native membranes. Electron tomography showed that caveolins generate domains of precise size and curvature. Cell-free caveolae were used in quantitative assays to test the interaction of membrane-inserted caveolin with signaling proteins and to determine the stoichiometry of interactions. Binding of membrane-inserted CAV1 to several proposed binding partners, including endothelial nitric-oxide synthase, was negligible, but a small number of proteins, including TRAF2, interacted with CAV1 in a phosphorylation-(CAV1^Y14)–stimulated manner. In cells subjected to oxidative stress, phosphorylated CAV1 recruited TRAF2 to the early endosome forming a novel signaling platform. These findings lead to a novel model for cellular stress signaling by CAV1.

Caveolin-1 deficiency protects pancreatic β cells against palmitate-induced dysfunction and apoptosis

Lipotoxicity leads to insulin secretion deficiency, which is among the important causes for the onset of type 2 diabetes mellitus. Thus, the restoration of β-cell mass and preservation of its endocrine function are long-sought goals in diabetes research. Previous studies have suggested that the membrane protein caveolin-1 (Cav-1) is implicated in β-cell apoptosis and insulin secretion, however, the underlying mechanisms still remains unclear. Our objective is to explore whether Cav-1 depletion protects pancreatic β cells from lipotoxicity and what are the underlying mechanisms. In this study, we found that Cav-1 silencing significantly promoted β-cell proliferation, inhibited palmitate (PA)-induced pancreatic β-cell apoptosis and enhanced insulin production and secretion. These effects were associated with enhanced activities of Akt and ERK1/2, which in turn downregulated the expression of cell cycle inhibitors (FOXO1, GSK3β, P21, P27 and P53) and upregulated the expression of Cyclin D2 and Cyclin D3. Subsequent inhibition of PI3K/Akt and ERK/MAPK pathways abolished Cav-1 depletion induced β-cell mass protection. Furthermore, under PA induced endoplasmic reticulum (ER) stress, Cav-1 silencing significantly reduced eIF2α phosphorylation and the expression of ER stress-responsive markers BiP and CHOP, which are among the known sensitizers of lipotoxicity. Our findings suggest Cav-1 as potential target molecule in T2DM treatment via the preservation of lipotoxicity-induced β-cell mass reduction and the attenuation of insulin secretion dysfunction.

The serum protein profile of early parity which induces protection against breast cancer

Early parity reduces the risk of breast cancer in women while nulliparity and late parity increase the risk of breast cancer. In order to translate this protection to women where early pregnancy is not feasible, much work has focused on understanding how parity confers protection against breast cancer, the molecular mechanisms by which this occurs is still not well understood. Healthy parous and nulliparous women were recruited for this study. We assessed serum protein profiles of early parous, late parous, and nulliparous women using the Phospho Explorer antibody array. Significantly altered proteins identified were validated by Western blot analysis. In silico analysis was performed with the data obtained. Our findings indicate increased phosphorylation levels of CDK1, AKT1 and Epo-R increased cell cycle and cell proliferation in late/nulliparous women. Increased levels of LIMK1, paxillin, caveolin-1, and tyrosine hydroxylase in late/nulliparous women demonstrate enhanced cell stress while decreased activity of p-p53 and pRAD51 in late/nulliparous women indicates decreased apoptosis and increased genomic instability. Further, increased levels of pFAK, pCD3zeta, pSTAT5B, MAP3K8 in early parous women favor enhanced innate/adaptive immunity. Overall, we have identified a unique protein signature that is responsible for the decreased risk of breast cancer and these proteins can also serve as biomarkers to predict the risk of breast cancer.

Glaucoma Pathogenesis and Neurotrophins: Focus on the Molecular and Genetic Basis for Therapeutic Prospects

BACKGROUND

Retinal ganglion cell (RGC) degeneration is a major feature of glaucoma pathology. Neuroprotective approaches that delay or halt the progression of RGC loss are needed to prevent vision loss which can occur even after conventional medical or surgical treatments to lower intraocular pressure.

OBJECTIVE

The aim of this review was to examine the progress in genetics and cellular mechanisms associated with endoplasmic reticulum (ER) stress, RGC dysfunction and cell death pathways in glaucoma.

MATERIALS AND METHODS

Here, we review the involvement of neurotrophins like brain derived neurotrophic factor (BDNF) and its high affinity receptor tropomyosin receptor kinase (TrkB) in glaucoma. The role of ER stress markers in human and animal retinas in health and disease conditions is also discussed. Further, we analysed the literature highlighting genetic linkage in the context of primary open angle glaucoma and suggested mechanistic insights into potential therapeutic options relevant to glaucoma management.

RESULTS

The literature review of the neurobiology underlying neurotrophin pathways, ER stress and gene associations provide critical insights into association of RGCs death in glaucoma. Alteration in signalling pathway is associated with increased risk of misfolded protein aggregation in ER promoting RGC apoptosis. Several genes that are linked with neurotrophin signalling pathways have been reported to be associated with glaucoma pathology.

CONCLUSION

Understanding genetic heterogeneity and involvement of neurotrophin biology in glaucoma could help to understand the complex pathophysiology of glaucoma. Identification of novel molecular targets will be critical for drug development and provide neuroprotection to the RGCs and optic nerve.

The Oxidative Stress-Induced Increase in the Membrane Expression of the Water-Permeable Channel Aquaporin-4 in Astrocytes Is Regulated by Caveolin-1 Phosphorylation

The reperfusion of ischemic brain tissue following a cerebral stroke causes oxidative stress, and leads to the generation of reactive oxygen species (ROS). Apart from inflicting oxidative damage, the latter may also trigger the upregulation of aquaporin 4 (AQP4), a water-permeable channel expressed by astroglial cells of the blood-brain barrier (BBB), and contribute to edema formation, the severity of which is known to be the primary determinant of mortality and morbidity. The mechanism through which this occurs remains unknown. In the present study, we have attempted to address this question using primary astrocyte cultures treated with hydrogen peroxide (H₂O₂) as a model system. First, we showed that H₂O₂ induces a significant increase in AQP4 protein levels and that this is inhibited by the antioxidant N-acetylcysteine (NAC). Second, we demonstrated using cell surface biotinylation that H₂O₂ increases AQP4 cell-surface expression independently of it's increased synthesis. In parallel, we found that caveolin-1 (Cav1) is phosphorylated in response to H₂O₂ and that this is reversed by the Src kinase inhibitor 4-Amino-5-(4-chlorophenyl)-7-(t-butyl)pyrazolo[3,4-d]pyrimidine (PP2). PP2 also abrogated the H₂O₂-induced increase in AQP4 surface levels, suggesting that  the phosphorylation of tyrosine-14 of Cav1 regulates  this  process. We  further  showed  that dominant-negative Y14F and phosphomimetic Y14D mutants caused a decrease and increase in AQP4 membrane expression respectively, and that the knockdown of Cav1 inhibits the increase in AQP4 cell-surface, expression following H₂O₂ treatment. Together, these findings suggest that oxidative stress-induced Cav1 phosphorylation modulates AQP4 subcellular distribution and therefore may indirectly regulate AQP4-mediated water transport.

Presence of Androgen Receptor Variant in Neuronal Lipid Rafts

Fast, nongenomic androgen actions have been described in various cell types, including neurons. However, the receptor mediating this cell membrane–initiated rapid signaling remains unknown. This study found a putative androgen receptor splice variant in a dopaminergic N27 cell line and in several brain regions (substantia nigra pars compacta, entorhinal cortex, and hippocampus) from gonadally intact and gonadectomized (young and middle-aged) male rats. This putative splice variant protein has a molecular weight of 45 kDa and lacks an N-terminal domain, indicating it is homologous to the human AR45 splice variant. Interestingly, AR45 was highly expressed in all brain regions examined. In dopaminergic neurons, AR45 is localized to plasma membrane lipid rafts, a microdomain involved in cellular signaling. Further, AR45 protein interacts with membrane-associated G proteins Gαq and Gαo. Neither age nor hormone levels altered AR45 expression in dopaminergic neurons. These results provide the first evidence of AR45 protein expression in the brain, specifically plasma membrane lipid rafts. AR45 presence in lipid rafts indicates that it may function as a membrane androgen receptor to mediate fast, nongenomic androgen actions.

Phosphorylated CAV1 activates autophagy through an interaction with BECN1 under oxidative stress

CAV1/Caveolin1, an integral membrane protein, is involved in caveolae function and cellular signaling pathways. Here, we report that CAV1 is a positive regulator of autophagy under oxidative stress and cerebral ischemic injury. Treatment with hydrogen peroxide enhanced autophagy flux and caused the localization of BECN1 to the mitochondria, whereas these changes were impaired in the absence of CAV1. Among many autophagy signals, only LC3 foci formation in response to hydrogen peroxide was abolished by CAV1 deficiency. Under oxidative stress, CAV1 interacted with a complex of BECN1/VPS34 through its scaffolding domain, and this interaction facilitated autophagosome formation. Interestingly, the phosphorylation of CAV1 at tyrosine-14 was essential for the interaction with BECN1 and their localization to the mitochondria, and the activation of autophagy in response to hydrogen peroxide. In addition, the expression of a phosphatase PTPN1 reduced the phosphorylation of CAV1 and inhibited autophagy. Further, compared to that in wild-type mice, autophagy was impaired and cerebral infarct damage was aggravated in the brain of Cav1 knockout mice. These results suggest that the phosphorylated CAV1 functions to activate autophagy through binding to the BECN1/VPS34 complex under oxidative stress and to protect against ischemic damage.

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.

Caveolins and cavins in the trafficking, maturation, and degradation of caveolae: implications for cell physiology

Caveolins (Cavs) are ~20 kDa scaffolding proteins that assemble as oligomeric complexes in lipid raft domains to form caveolae, flask-shaped plasma membrane (PM) invaginations. Caveolae ("little caves") require lipid-lipid, protein-lipid, and protein-protein interactions that can modulate the localization, conformational stability, ligand affinity, effector specificity, and other functions of proteins that are partners of Cavs. Cavs are assembled into small oligomers in the endoplasmic reticulum (ER), transported to the Golgi for assembly with cholesterol and other oligomers, and then exported to the PM as an intact coat complex. At the PM, cavins, ~50 kDa adapter proteins, oligomerize into an outer coat complex that remodels the membrane into caveolae. The structure of caveolae protects their contents (i.e., lipids and proteins) from degradation. Cellular changes, including signal transduction effects, can destabilize caveolae and produce cavin dissociation, restructuring of Cav oligomers, ubiquitination, internalization, and degradation. In this review, we provide a perspective of the life cycle (biogenesis, degradation), composition, and physiologic roles of Cavs and caveolae and identify unanswered questions regarding the roles of Cavs and cavins in caveolae and in regulating cell physiology.¹.

Caveolin1/protein arginine methyltransferase1/sirtuin1 axis as a potential target against endothelial dysfunction

Endothelial dysfunction (ED), an established response to cardiovascular risk factors, is characterized by increased levels of soluble molecules secreted by endothelial cells (EC). Evidence suggest that ED is an independent predictor of cardiac events and that it is associated with a deficiency in production or bioavailability of nitric oxide (NO) and/or an imbalance in the relative contribution of endothelium-derived relaxing and contracting factors. ED can be reversed by treating cardiovascular risk factors, hence, beyond ambiguity, ED contributes to initiation and progression of atherosclerotic disease. Majority of cardiovascular risk factors act by a common pathway, oxidative stress (OS), characterized by an imbalance in bioavailability of NO and reactive oxygen species (ROS). Enhanced ROS, through several mechanisms, alters competence of EC that leads to ED, reducing its potential to maintain homeostasis and resulting in development of cardiovascular disease (CVD). Influential mechanisms that have been implicated in the development of ED include (i) presence of elevated levels of NOS inhibitor, asymmetric dimethylarginine (ADMA) due to augmented enzyme activity of protein arginine methyl transferase-1 (PRMT1); (ii) decrease in NO generation by endothelial nitric oxide synthase (eNOS) uncoupling, or by reaction of NO with free radicals and (iii) impaired post translational modification of protein (PTM) such as eNOS, caveolin-1 (cav1) and sirtuin-1 (SIRT1). However, the inter-related mechanisms that concur to developing ED is yet to be understood. The events that possibly overlay include OS-induced sequestration of SIRT1 to caveolae facilitating cav1-SIRT1 association; potential increase in lysine acetylation of enzymes such as eNOS and PRMT1 leading to enhanced ADMA formation; imbalance in acetylation-methylation ratio (AMR); diminished NO generation and ED. Here we review current literature from research showing interdependent association between cav1-PRMT1-SIRT1 to the outcomes of experimental and clinical research aiming to preserve endothelial function with gene- or pharmaco-therapy.

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.

PrP(c) deficiency and dasatinib protect mouse intestines against radiation injury by inhibiting of c-Src

BACKGROUND & AIM

Despite extensive study of the contribution of cell death and apoptosis to radiation-induced acute intestinal injury, our knowledge of the signaling mechanisms involved in epithelial barrier dysfunction remains inadequate. Because PrP(c) plays a key role in intestinal homeostasis by renewing epithelia, we sought to study its role in epithelial barrier function after irradiation.

DESIGN

Histology, morphometry and plasma FD-4 levels were used to examine ileal architecture, wound healing, and intestinal leakage in PrP(c)-deficient (KO) and wild-type (WT) mice after total-body irradiation. Impairment of the PrP(c) Src pathway after irradiation was explored by immunofluorescence and confocal microscopy, with Caco-2/Tc7 cells. Lastly, dasatinib treatment was used to switch off the Src pathway in vitro and in vivo.

RESULTS

The decrease in radiation-induced lethality, improved intestinal wound healing, and reduced intestinal leakage promoted by PrP(c) deficiency demonstrate its involvement in acute intestinal damage. Irradiation of Cacao2/Tc7 cells induced PrP(c) to target the nuclei associated with Src activation. Finally, the protective effect triggered by dasatinib confirmed Src involvement in radiation-induced acute intestinal toxicity.

CONCLUSION

Our data are the first to show a role for the PrP(c)-Src pathway in acute intestinal response to radiation injury and offer a novel therapeutic opportunity.

Caveolae protect endothelial cells from membrane rupture during increased cardiac output

This study provides direct in vivo evidence that endothelial cell caveolae disassemble and hence flatten out under increased mechanical stress and that the presence of caveolae protects endothelial cell plasma membranes from damage.

Caveolae are strikingly abundant in endothelial cells, yet the physiological functions of caveolae in endothelium and other tissues remain incompletely understood. Previous studies suggest a mechanoprotective role, but whether this is relevant under the mechanical forces experienced by endothelial cells in vivo is unclear. In this study we have sought to determine whether endothelial caveolae disassemble under increased hemodynamic forces, and whether caveolae help prevent acute rupture of the plasma membrane under these conditions. Experiments in cultured cells established biochemical assays for disassembly of caveolar protein complexes, and assays for acute loss of plasma membrane integrity. In vivo, we demonstrate that caveolae in endothelial cells of the lung and cardiac muscle disassemble in response to acute increases in cardiac output. Electron microscopy and two-photon imaging reveal that the plasma membrane of microvascular endothelial cells in caveolin 1^−/− mice is much more susceptible to acute rupture when cardiac output is increased. These data imply that mechanoprotection through disassembly of caveolae is important for endothelial function in vivo.

Caveolae: One Function or Many?

Caveolae are small, bulb-shaped plasma membrane invaginations. Mutations that ablate caveolae lead to diverse phenotypes in mice and humans, making it challenging to uncover their molecular mechanisms. Caveolae have been described to function in endocytosis and transcytosis (a specialized form of endocytosis) and in maintaining membrane lipid composition, as well as acting as signaling platforms. New data also support a model in which the central function of caveolae could be related to the protection of cells from mechanical stress within the plasma membrane. We present evidence for these diverse roles and consider in vitro and in vivo experiments confirming a mechanoprotective role. We conclude by highlighting current gaps in our knowledge of how mechanical signals may be transduced by caveolae.

Caveolin-1 phosphorylation regulates vascular endothelial insulin uptake and is impaired by insulin resistance in rats

AIMS/HYPOTHESIS

As insulin entry into muscle interstitium is rate-limiting for its overall peripheral action, defining the route and regulation of its entry is critical. Caveolin-1 is required for caveola formation in vascular endothelial cells (ECs) and for EC insulin uptake. Whether this requirement reflects simply the need for caveola availability or involves a more active role for caveolae/caveolin-1 is not known. Here, we examined the role of insulin-stimulated tyrosine 14 (Tyr(14))-caveolin-1 phosphorylation in mediating EC insulin uptake and the role of cellular Src-kinase (cSrc), TNF-α/IL-6 and high fat diet (HFD) in regulating this process.

METHODS

Freshly isolated ECs from normal or HFD-fed rats and/or cultured ECs were treated with FITC-labelled or regular insulin with or without a Src or phosphotidylinositol-3-kinase inhibitor, TNF-α or IL-6, or transfecting FLAG-tagged wild-type (WT) or mutant (Y14F) caveolin-1. Tyr(14)-caveolin-1/Tyr(416) cSrc phosphorylation and FITC-insulin uptake were quantified by immunostaining and/or western blots.

RESULTS

Insulin stimulated Tyr(14)-caveolin-1 phosphorylation during EC insulin uptake. Inhibiting cSrc, but not phosphotidylinositol-3-kinase, reduced insulin-stimulated caveolin-1 phosphorylation. Furthermore, inhibiting cSrc reduced FITC-insulin uptake by ∼50%. Overexpression of caveolin-1Y14F inhibited, while overexpression of WT caveolin-1 increased, FITC-insulin uptake. Exposure of ECs to TNF-α or IL-6, or to 1-week HFD feeding eliminated insulin-stimulated caveolin-1 phosphorylation and inhibited FITC-insulin uptake to a similar extent.

CONCLUSIONS/INTERPRETATION

Insulin stimulation of its own uptake requires caveolin-1 phosphorylation and Src-kinase activity. HFD in vivo and proinflammatory cytokines in vitro both inhibit this process.

Herpes Simplex Virus 1 Suppresses the Function of Lung Dendritic Cells via Caveolin-1

Caveolin-1 (Cav-1), the principal structural protein of caveolae, has been implicated as a regulator of virus-host interactions. Several viruses exploit caveolae to facilitate viral infections. However, the roles of Cav-1 in herpes simplex virus 1 (HSV-1) infection have not fully been elucidated. Here, we report that Cav-1 downregulates the expression of inducible nitric oxide synthase (iNOS) and the production of nitric oxide (NO) in dendritic cells (DCs) during HSV-1 infection. As a result, Cav-1 deficiency led to an accelerated elimination of virus and less lung pathological change following HSV-1 infection. This protection was dependent on iNOS and NO production in DCs. Adoptive transfer of DCs with Cav-1 knockdown was sufficient to confer the protection to wild-type (WT) mice. In addition, Cav-1 knockout (KO) (Cav-1(-/-)) mice treated with an iNOS inhibitor exhibited significantly reduced survival compared to that of the nontreated controls. We found that Cav-1 colocalized with iNOS and HSV-1 in caveolae in HSV-1-infected DCs, suggesting their interaction. Taken together, our results identified Cav-1 as a novel regulator utilized by HSV-1 to evade the host antiviral response mediated by NO production. Therefore, Cav-1 might be a valuable target for therapeutic approaches against herpesvirus infections.

Phosphorylation of caveolin-1 on tyrosine-14 induced by ROS enhances palmitate-induced death of beta-pancreatic cells

A considerable body of evidence exists implicating high levels of free saturated fatty acids in beta pancreatic cell death, although the molecular mechanisms and the signaling pathways involved have not been clearly defined. The membrane protein caveolin-1 has long been implicated in cell death, either by sensitizing to or directly inducing apoptosis and it is normally expressed in beta cells. Here, we tested whether the presence of caveolin-1 modulates free fatty acid-induced beta cell death by reexpressing this protein in MIN6 murine beta cells lacking caveolin-1. Incubation of MIN6 with palmitate, but not oleate, induced apoptotic cell death that was enhanced by the presence of caveolin-1. Moreover, palmitate induced de novo ceramide synthesis, loss of mitochondrial transmembrane potential and reactive oxygen species (ROS) formation in MIN6 cells. ROS generation promoted caveolin-1 phosphorylation on tyrosine-14 that was abrogated by the anti-oxidant N-acetylcysteine or the incubation with the Src-family kinase inhibitor, PP2 (4-amino-5-(4-chlorophenyl)-7(dimethylethyl)pyrazolo[3,4-d]pyrimidine). The expression of a non-phosphorylatable caveolin-1 tyrosine-14 to phenylalanine mutant failed to enhance palmitate-induced apoptosis while for MIN6 cells expressing the phospho-mimetic tyrosine-14 to glutamic acid mutant caveolin-1 palmitate sensitivity was comparable to that observed for MIN6 cells expressing wild type caveolin-1. Thus, caveolin-1 expression promotes palmitate-induced ROS-dependent apoptosis in MIN6 cells in a manner requiring Src family kinase mediated tyrosine-14 phosphorylation.

Oxidative stress-induced inhibition of Sirt1 by caveolin-1 promotes p53-dependent premature senescence and stimulates the secretion of interleukin 6 (IL-6)

Oxidative stress can induce premature cellular senescence. Senescent cells secrete various growth factors and cytokines, such as IL-6, that can signal to the tumor microenvironment and promote cancer cell growth. Sirtuin 1 (Sirt1) is a class III histone deacetylase that regulates a variety of physiological processes, including senescence. We found that caveolin-1, a structural protein component of caveolar membranes, is a direct binding partner of Sirt1, as shown by the binding of the scaffolding domain of caveolin-1 (amino acids 82-101) to the caveolin-binding domain of Sirt1 (amino acids 310-317). Our data show that oxidative stress promotes the sequestration of Sirt1 into caveolar membranes and the interaction of Sirt1 with caveolin-1, which lead to inhibition of Sirt1 activity. Reactive oxygen species stimulation promotes acetylation of p53 and premature senescence in wild-type but not caveolin-1 null mouse embryonic fibroblasts (MEFs). Either down-regulation of Sirt1 expression or re-expression of caveolin-1 in caveolin-1 null MEFs restores reactive oxygen species-induced acetylation of p53 and premature senescence. In addition, overexpression of caveolin-1 induces stress induced premature senescence in p53 wild-type but not p53 knockout MEFs. Phosphorylation of caveolin-1 on tyrosine 14 promotes the sequestration of Sirt1 into caveolar membranes and activates p53/senescence signaling. We also identified IL-6 as a caveolin-1-specific cytokine that is secreted by senescent fibroblasts following the caveolin-1-mediated inhibition of Sirt1. The caveolin-1-mediated secretion of IL-6 by senescent fibroblasts stimulates the growth of cancer cells. Therefore, by inhibiting Sirt1, caveolin-1 links free radicals to the activation of the p53/senescence pathway and the protumorigenic properties of IL-6.