Inhibition of PAI-1 Blocks PD-L1 Endocytosis and Improves the Response of Melanoma Cells to Immune Checkpoint Blockade.

Immune checkpoint molecules, especially PD-1 and its ligand PD-L1, act as a major mechanism of cancer immune evasion. Although anti-PD-1/PD-L1 monotherapy increases therapeutic efficacy in melanoma treatment, only a subset of patients exhibits long-term tumor remission, and the underlying mechanism of resistance to PD-1/PD-L1 inhibitors remains unclear. In this study, we demonstrated that cell surface retention of PD-L1 is inversely correlated with PAI-1 expression in vitro, in vivo, and in clinical specimens. Moreover, extracellular PAI-1 induced the internalization of surface-expressed PD-L1 by triggering clathrin-mediated endocytosis. The endocytosed PD-L1 was transported to lysosomes for degradation by endolysosomal systems, resulting in the reduction of surface PD-L1. Notably, inhibition of PAI-1 by pharmacological inhibitor with tiplaxtinin led to elevated PD-L1 expression on the plasma membrane, both in vitro and in vivo. Strikingly, targeting PAI-1 by tiplaxtinin treatment synergizes with anti-PD-L1 immune checkpoint blockade therapy in a syngeneic murine model of melanoma. Our findings demonstrate a role for PAI-1 activity in immune checkpoint modulation by promoting surface PD-L1 for lysosomal degradation and provides an insight into the combination of PAI-1 inhibition and anti-PD-L1 immunotherapy as a promising therapeutic regimen for melanoma treatment.

Caveolins; An Assailant or An Ally of Various Cellular Disorders.

Caveolae have impressive morphological highlights of the cytomembrane of mammalian cells which involve in wide diversity of cellular functions involving signaling pathways and cholesterol hastening. Caveolin proteins possess a 'scaffolding' domain which for caveolin-1 and caveolin-3 appear to act a dominant role in signal regulation through caveolae. Caveolin-1 is treated to be protein in the cytomembrane entrapped with caveolae in endothelial cells and vascular smooth muscle cells which diminish nitric oxide (NO) by fill up the calcium/calmodulin (Ca2+/CaM) confining point of endothelial nitric oxide synthase (eNOS), decrease NO generation produce endothelial dysfunction and atherosclerotic injury development. It is a cholesterol-binding layer protein associated with cell cholesterol transport and also shows cardioprotective action through ischemic preconditioning (IPC) in diabetic and postmenopausal rat heart. Additionally it is ensnared in the procedures of tumorigenesis, prostate disease, and inflammation. The present study in the paper is to explore the structural functionalities of caveolins and their contributory role in CVS disorders and various other diseases.

[Interaction between functional nano-hydroxyapatite and cells and the underlying mechanisms].

OBJECTIVE: To explore the interaction between arginine functionalized hydroxyapatite (HAP/Arg) nanoparticles and endothelial cells, and to investigate mechanisms for endocytosis kinetics and endocytosis.
 METHODS: Human umbilical vein endothelial cells (HUVECs) were selected as the research model.Cellular uptake of HAP/Arg nanoparticles were observed by laser scanning confocal microscopy.Average fluorescence intensity of cells after ingestion with different concentrations of HAP/Arg nanoparticles were determined by flow cytometer and atomic force microscopy.
 RESULTS: The HAP/Arg nanoparticles with doped terbium existed in cytoplasm, and most of them distributed around the nucleus area after cellular uptake by HUVECs. Cellular uptake process of HAP/Arg nanoparticles in HUVECs was in a time and concentration dependent manner. 4 h and 50 mg/L was the best condition for uptake. HAP/Arg nanoparticles were easier to be up-taken into the cells than HAP nanoparticles without arginine functionalized.
 CONCLUSION: HAP/Arg nanoparticles are internalized by HUVECs cells through an active transport and energy-dependent endocytosis process, and it is up-taken by cells mainly through caveolin-mediated endocytosis, but the clathrin-dependent endocytic pathway is also involved..

Non-canonical roles for caveolin in regulation of membrane repair and mitochondria: implications for stress adaptation with age.

Many different theories of ageing have been proposed but none has served the unifying purpose of defining a molecular target that can limit the structural and functional decline associated with age that ultimately leads to the demise of the organism. We propose that the search for a molecule with these unique properties must account for regulation of the signalling efficiency of multiple cellular functions that degrade with age due to a loss of a particular protein. We suggest caveolin as one such molecule that serves as a regulator of key processes in signal transduction. We define a particular distinction between cellular senescence and ageing and propose that caveolin plays a distinct role in each of these processes. Caveolin is traditionally thought of as a membrane-localized protein regulating signal transduction via membrane enrichment of specific signalling molecules. Ultimately we focus on two non-canonical roles for caveolin - membrane repair and regulation of mitochondrial function - which may be novel features of stress adaptation, especially in the setting of ageing. The end result of preserving membrane structure and mitochondrial function is maintenance of homeostatic signalling, preserving barrier function, and regulating energy production for cell survival and resilient ageing.

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.

Role of aquaglyceroporins and caveolins in energy and metabolic homeostasis.

Aquaglyceroporins and caveolins are submicroscopic integral membrane proteins that are particularly abundant in many mammalian cells. Aquaglyceroporins (AQP3, AQP7, AQP9 and AQP10) encompass a subfamily of aquaporins that allow the movement of water, but also of small solutes, such as glycerol, across cell membranes. Glycerol constitutes an important metabolite as a substrate for de novo synthesis of triacylglycerols and glucose as well as an energy substrate to produce ATP via the mitochondrial oxidative phosphorylation. In this sense, the control of glycerol influx/efflux in metabolic organs by aquaglyceroporins plays a crucial role with the dysregulation of these glycerol channels being associated with metabolic diseases, such as obesity, insulin resistance, non-alcoholic fatty liver disease and cardiac hypertrophy. On the other hand, caveolae have emerged as relevant plasma membrane sensors implicated in a wide range of cellular functions, including endocytosis, apoptosis, cholesterol homeostasis, proliferation and signal transduction. Caveolae-coating proteins, namely caveolins and cavins, can act as scaffolding proteins within caveolae by concentrating signaling molecules involved in free fatty acid and cholesterol uptake, proliferation, insulin signaling or vasorelaxation, among others. The importance of caveolae in whole-body homeostasis is highlighted by the link between homozygous mutations in genes encoding caveolins and cavins with metabolic diseases, such as lipodystrophy, dyslipidemia, muscular dystrophy and insulin resistance in rodents and humans. The present review focuses on the role of aquaglyceroporins and caveolins on lipid and glucose metabolism, insulin secretion and signaling, energy production and cardiovascular homeostasis, outlining their potential relevance in the development and treatment of metabolic diseases.

Atypical G(αi) signal transduction.

G protein signaling is an extremely complex event that is involved in almost every cellular process. As such, G protein-coupled receptors are the most commonly found type of transmembrane receptors used by cells to initiate intracellular signaling events. However, the widely accepted model of cyclical GDP-GTP exchange in response to ligand binding to 7TMRs, followed by dissociation of the G protein subunits and activation of intracellular signaling cascades, has repeatedly been challenged in recent years. Some of the exceptions that have been brought forth include signaling by a non-dissociated, rearranged heterotrimer and the existence of "reverse-mode", active G proteins that interact with active receptors. Here, we focus on G(αi/o), one of the common G(α) classes, and outline a major exception to the classical model, that of G protein coupling to RTKs. We then describe a novel concept in G(αi/o) signaling, namely that the pathways induced by agonist binding circumvent the typical signaling pathways responsive to decreases in the second messenger cAMP, via adenylyl cyclase inhibition.

The role of caveolae in the pathophysiology of lung diseases.

Caveolae are flask-shaped plasma membrane invaginations formed by constitutive caveolin proteins and regulatory cavin proteins. Caveolae harbor a range of signaling components such as receptors, ion channels and regulatory molecules. There is now increasing evidence that caveolins and cavins play an important role in a variety of diseases. However, the mechanisms by which these caveolar proteins affect lung health and disease are still under investigation, with emerging data suggesting complex roles in disease pathophysiology. This review summarizes the current state of understanding of how caveolar proteins contribute to lung structure and function and how their altered expression and/or function can influence lung diseases.

Caveolae as plasma membrane sensors, protectors and organizers.

Caveolae are submicroscopic, plasma membrane pits that are abundant in many mammalian cell types. The past few years have seen a quantum leap in our understanding of the formation, dynamics and functions of these enigmatic structures. Caveolae have now emerged as vital plasma membrane sensors that can respond to plasma membrane stresses and remodel the extracellular environment. Caveolae at the plasma membrane can be removed by endocytosis to regulate their surface density or can be disassembled and their structural components degraded. Coat proteins, called cavins, work together with caveolins to regulate the formation of caveolae but also have the potential to dynamically transmit signals that originate in caveolae to various cellular destinations. The importance of caveolae as protective elements in the plasma membrane, and as membrane organizers and sensors, is highlighted by links between caveolae dysfunction and human diseases, including muscular dystrophies and cancer.

[Cavins: new sights of caveolae-associated protein].

Caveolae are specialized lipid rafts that form flask-shaped invaginations of the plasma membrane. Many researches show that caveolae are involved in cell signaling and transport. Caveolin-1 is the major coat protein essential for the formation of caveolae. Recently, several reports indicated that the other caveolae-associated proteins, Cavins, are required for caveola formation and organization. It's worth noting that Cavin-1 could cooperate with Caveolin-1 to accommodate the structural integrity and function of caveolae. Here, we reviewed that the relationship between Cavins and Caveolins and explore the role of them in regulating caveolae.

Endocytic regulation of voltage-dependent potassium channels in the heart.

Understanding the regulation of cardiac ion channels is critical for the prevention of arrhythmia caused by abnormal excitability. Ion channels can be regulated by a change in function (qualitative) and a change in number (quantitative). Functional changes have been extensively investigated for many ion channels including cardiac voltage-dependent potassium channels. By contrast, the regulation of ion channel numbers has not been widely examined, particularly with respect to acute modulation of ion channels. This article briefly summarizes stimulus-induced endocytic regulation of major voltage-dependent potassium channels in the heart. The stimuli known to cause their endocytosis include receptor activation, drugs, and low extracellular [K(+)], following which the potassium channels undergo either clathrin-mediated or caveolin-mediated endocytosis. Receptor-mediated endocytic regulation has been demonstrated for Kv1.2, Kv1.5, KCNQ1 (Kv7.1), and Kv4.3, while drug-induced endocytosis has been demonstrated for Kv1.5 and hERG. Low [K(+)](o)-induced endocytosis might be unique for hERG channels, whose electrophysiological characteristics are known to be under strong influence of [K(+)](o). Although the precise mechanisms have not been elucidated, it is obvious that major cardiac voltage-dependent potassium channels are modulated by endocytosis, which leads to changes in cardiac excitability.

Caveolae, caveolin, and cavins: potential targets for the treatment of cardiac disease.

Caveolae are omega-shaped membrane invaginations present in essentially all cell types of the cardiovascular system, including endothelial cells, smooth muscle cells, macrophages, cardiac myocytes, and fibroblasts. Numerous functions have been ascribed to this omega-shaped structure. Caveolae are enriched with different signaling molecules and ion channel regulatory proteins and function both in protein trafficking and signal transduction in these cell types. Caveolins are the structural proteins that are necessary for the formation of caveola membrane domains. Mechanistically, caveolins interact with a variety of downstream signaling molecules, as, for example, Src-family tyrosine kinase, p42/44 mitogen-activated protein (MAP) kinase, and endothelial nitric oxide synthase (eNOS) and hold the signal transducers in the inactive condition until activated with proper stimulus. Caveolae are gradually acquiring increasing attention as cellular organelles contributing to the pathogenesis of several structural and functional processes including cardiac hypertrophy, atherosclerosis, and heart failure. At present, very little is known about the role of caveolae in cardiac function and dysfunction, although recent studies with caveolin knock-out mouse have shown that caveolae and caveolins play a pivotal role in various human pathobiological conditions. This review will discuss the possible role and mechanism of action of caveolae and caveolins in different cardiac diseases.

Is caveolin involved in normal proximal tubule function? Presence in model PT systems but absence in situ.

Kidney proximal tubule (PT) cells are specialized for the uptake and transport of ions, solutes, peptides, and proteins. These functions are often regulated by hormones that signal at the cell surface and are internalized by clathrin-mediated endocytosis. However, the caveolin/caveolae pathway has also been implicated in normal PT function, often based on data from isolated PTs or PT cells in culture. Although we reported previously that caveolae and caveolin 1 are not detectable in PTs in vivo, reports of caveolin expression and function in PT cells appear periodically in the literature. Therefore, we reexamined caveolin expression in PTs in vivo, in isolated "purified" PTs following collagenase digestion, and in cultured PT cells. Caveolin 1 and 2 protein, mRNA, or immunofluorescence was undetectable in PTs in vivo, but PT cell cultures expressed caveolin 1 and/or 2. Furthermore, caveolin 1 and 2 mRNAs were detected in isolated PTs along with the endothelial markers CD31 and ICAM1. In contrast, no caveolin or endothelial marker mRNAs were detectable in samples isolated from snap-frozen kidneys by laser cut microdissection, which eliminates contamination by other cell types. We conclude 1) caveolin 1 and 2 are not normally expressed by PT cells in situ, 2) caveolin expression is "activated" in cultured PT cells, 3) contamination with non-PT, caveolin-expressing cells is a potential source of caveolin 1 and 2 that must be taken into account when isolated PTs are used in studies to correlate expression of these proteins with PT function.

New insights into caveolae, caveolins and endothelial function.

A growing body of evidence suggests that the cholesterol-rich invaginations of the plasma membrane, known as caveolae, are essential to cardiovascular homeostasis. The considerable attention devoted to these major plasmalemma macrostructures is attributable to their involvement in compartmentalization and clustering of signalling molecules that contribute to their function in the pathophysiology of the cardiovascular system. In light of recent developments in caveolae research, a better comprehension of the role of caveolae in the vasculature and how they mediate their activity is needed.

The regulation of endothelial nitric oxide synthase by caveolin: a paradigm validated in vivo and shared by the 'endothelium-derived hyperpolarizing factor'.

The endothelium plays a crucial role in the regulation of cardiovascular homeostasis through the release of vasoactive autacoids such as nitric oxide, prostacyclin, and a third factor or pathway termed 'endothelium-derived hyperpolarizing factor' (EDHF). Although the functional influence of NO and EDHF is sometimes reciprocal (i.e., their effects decrease or increase with the reduction in vessel diameter, respectively), recent insights led to the identification of caveolae and caveolin as common regulators of their production. In this review, we will first focus on the current understanding of the caveolin/eNOS paradigm and will then detail the most recent findings on the role of caveolae in driving EDHF-signaling pathways.

Down syndrome critical region protein 5 regulates membrane localization of Wnt receptors, Dishevelled stability and convergent extension in vertebrate embryos.

The Glypican family of heparan sulfate proteoglycans regulates Wnt signaling and convergent extension (CE) in vertebrate embryos. They are predicted to be glycosylphosphatidylinositol (GPI)-tethered membrane-bound proteins, but there is no functional evidence of their regulation by the GPI synthesis complex. Down syndrome critical region protein 5 (Dscr5, also known as Pigp) is a component of the GPI-N-acetylglucosaminyltransferase (GPI-GnT) complex, and is associated with specific features of Down syndrome. Here we report that Dscr5 regulates CE movements through the non-canonical Wnt pathway. Both dscr5 overexpression and knockdown impaired convergence and extension movements. Dscr5 functionally interacted with Knypek/Glypican 4 and was required for its localization at the cell surface. Knockdown of dscr5 disrupted Knypek membrane localization and caused an enhanced Frizzled 7 receptor endocytosis in a Caveolin-dependent manner. Furthermore, dscr5 knockdown promoted specific Dishevelled degradation by the ubiquitin-proteosome pathway. These results reveal a functional link between Knypek/Glypican 4 and the GPI synthesis complex in the non-canonical Wnt pathway, and provide the new mechanistic insight that Dscr5 regulates CE in vertebrate embryos by anchoring different Wnt receptors at the cell surface and maintaining Dishevelled stability.

Caveolins in the development and diseases of musculoskeletal system.

Caveolins, essential scaffold proteins of caveolae, have emerged as versatile molecules that can influence numerous signaling pathways and disease processes. This brief review is focused on the role of caveolins in the physiology and pathology of musculoskeletal system and will discuss the key features of caveolins as related to differentiation and diseases of bone and muscle cells.

Clinical and translational implications of the caveolin gene family: lessons from mouse models and human genetic disorders.

Here we review the clinical and translational implications of the caveolin gene family for understanding the pathogenesis of human diseases, including breast and prostate cancers, pulmonary hypertension, cardiomyopathy, diabetes, and muscular dystrophy. Detailed phenotypic analysis of caveolin knockout mice has served to highlight the crucial role of a caveolin deficiency in the pathogenesis of many human disease processes. Mutations in the human caveolin genes are associated with a number of established genetic disorders (such as breast cancer, lipodystrophy, muscular dystrophy, and cardiomyopathy), making the caveolins important and novel targets for drug development. The implementation of new strategies for caveolin replacement therapy-including caveolin mimetic peptides-is ongoing.

Possible new druggable targets for the treatment of nephrosis. Perhaps we should find them in caveolea?

Nephrosis refers to a condition resulting from proteinuric kidney disease, leading to irreversible renal parenchymal damage and end-stage renal disease when left untreated. Furthermore, nephrosis appears to be a communicable disease carrying risks and complications to other organs such as the heart. Key pathophysiolgical processes involved in initiating and progressing renal damage in nephrosis and its complications may include altered glomerular hemodynamics after initial renal damage and loss of nephrons, nephrotoxicity of increased renal protein traffic enforcing intrinsic 'common pathway' mechanisms of renal scarring, and generalized endothelial dysfunction proceeding CV disease. The reader is first provided a basic overview on key mechanisms, targets and therapies in nephrosis while referred to some excellent updates hereon for more detailed information. The broader purpose of this short review, however, is to highlight caveolae/caveolins and caveolar function as central modulators in all the above key processes of nephrosis. Caveolae - little caves in the plasma membrane that are particularly abundant in endothelial cells, amongst others - are now known to be involved not only in endothelial transcytosis (e.g. of albumin) but also in cholesterol homeostasis (LDL-transport) and, importantly, in signal transduction such as insulin signalling and nitric oxide signalling in endothelial function and regulation of vasomotor tone, as well as signalling by growth factor receptors - such as TGF-beta - which may participate in renal scarring. It is suggested that caveolae may represent crucial sites where possible new druggable targets in nephrosis may be found.

Invadopodia biogenesis is regulated by caveolin-mediated modulation of membrane cholesterol levels.

Invadopodia are proteolytically active protrusions formed by invasive tumoural cells when grown on an extracellular matrix (ECM) substratum. Clearly, invadopodia are specialized membrane domains acting as sites of signal transduction and polarized delivery of components required for focalized ECM degradation. For these reasons, invadopodia are a model to study focal ECM degradation by tumour cells. We investigated the features of invadopodia membrane domains and how altering their composition would affect invadopodia biogenesis and function. This was achieved through multiple approaches including manipulation of the levels of cholesterol and other lipids at the plasma membrane, alteration of cholesterol trafficking by acting on caveolin 1 expression and phosphorylation. We show that cholesterol depletion impairs invadopodia formation and persistence, and that invadopodia themselves are cholesterol-rich membranes. Furthermore, the inhibition of invadopodia formation and ECM degradation after caveolin 1 knock-down was efficiently reverted by simple provision of cholesterol. In addition, the inhibitory effect of caveolin 3(DGV) expression, a mutant known to block cholesterol transport to the plasma membrane, was similarly reverted by provision of cholesterol. We suggest that invadopodia biogenesis, function and structural integrity rely on appropriate levels of plasma membrane cholesterol, and that invadopodia display the properties of cholesterol-rich membranes. Also, caveolin 1 exerts its function in invadopodia formation by regulating cholesterol balance at the plasma membrane. These findings support the connection between cholesterol, cancer and caveolin 1, provide further understanding of the role of cholesterol in cancer progression and suggest a mechanistic framework for the proposed anti-cancer activity of statins, tightly related to their blood cholesterol-lowering properties.