Caveolin-1 is enriched in the peroxisomal membrane of rat hepatocytes

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

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

Caveolin-1 in oncogenic metabolic symbiosis

Metabolic phenotypes of cancer cells are heterogeneous and flexible as a tumor mass is a hurriedly evolving system capable of constant adaptation to oxygen and nutrient availability. The exact type of cancer metabolism arises from the combined effects of factors intrinsic to the cancer cells and factors proposed by the tumor microenvironment. As a result, a condition termed oncogenic metabolic symbiosis in which components of the tumor microenvironment (TME) promote tumor growth often occurs. Understanding how oncogenic metabolic symbiosis emerges and evolves is crucial for perceiving tumorigenesis. The process by which tumor cells reprogram their TME involves many mechanisms, including changes in intercellular communication, alterations in metabolic phenotypes of TME cells, and rearrangement of the extracellular matrix. It is possible that one molecule with a pleiotropic effect such as Caveolin-1 may affect many of these pathways. Here, we discuss the significance of Caveolin-1 in establishing metabolic symbiosis in TME.

Caveolin-1 protects against hepatic ischemia/reperfusion injury through ameliorating peroxynitrite-mediated cell death

Nitrative stress is considered as an important pathological process of hepatic ischemia and reperfusion injury but its regulating mechanisms are largely unknown. In this study, we tested the hypothesis that caveolin-1 (Cav-1), a plasma membrane scaffolding protein, could be an important cellular signaling against hepatic I/R injury through inhibiting peroxynitrite (ONOO(-))-induced cellular damage. Male wild-type mice and Cav-1 knockout (Cav-1(-/-)) were subjected to 1h hepatic ischemia following 1, 6 and 12h of reperfusion by clipping and releasing portal vessels respectively. Immortalized human hepatocyte cell line (L02) was subjected to 1h hypoxia and 6h reoxygenation and treated with Cav-1 scaffolding domain peptide. The major discoveries included: (1) the expression of Cav-1 in serum and liver tissues of wild-type mice was time-dependently elevated during hepatic ischemia-reperfusion injury. (2) Cav-1 scaffolding domain peptide treatment inhibited cleaved caspase-3 expression in the hypoxia-reoxygenated L02 cells; (3) Cav-1 knockout (Cav-1(-/-)) mice had significantly higher levels of serum transaminases (ALT&AST) and TNF-α, and higher rates of apoptotic cell death in liver tissues than wild-type mice after subjected to 1h hepatic ischemia and 6hour reperfusion; (4) Cav-1(-/-) mice revealed higher expression levels of iNOS, ONOO(-) and 3-nitrotyrosine (3-NT) in the liver than wild-type mice, and Fe-TMPyP, a representative peroxynitrite decomposition catalyst (PDC), remarkably reduced level of ONOO(-) and 3-NT and ameliorated the serum ALT, AST and TNF-α levels in both wild-type and Cav-1(-/-) mice. Taken together, we conclude that Cav-1 could play a critical role in preventing nitrative stress-induced liver damage during hepatic ischemia-reperfusion injury.

Caveolin-1 is upregulated in hepatic stellate cells but not sinusoidal endothelial cells after liver injury

Sinusoidal endothelial cells (SEC) and hepatic stellate cells (HSC) are closely associated specialized vascular cells residing in the hepatic sinusoid. These cells have been shown to play important roles in many different pathophysiologic processes, in particular in liver fibrosis/cirrhosis and portal hypertension. Caveolin-1 functions as a scaffolding protein, and has a variety of functions including in many disease states, such as liver cirrhosis. Although previous studies have shown that in the injured rat liver, caveolin-1 is upregulated, the precise cells in which remains unclear. Therefore, the purpose of this study was to clarify the cell type (or types) in which caveolin-1 is expressed in normal and injured rat liver. We have utilized both detailed immunohistochemical labeling with cell specific markers as well as cell isolation techniques (isolating sinusoidal endothelial cells, HSCs, and hepatocytes) in normal and injured (bile duct ligation) rat liver. We show here that in the normal liver caveolin-1 is expressed predominantly in HSCs and SECs but after liver injury there is upregulation of caveolin-1 in HSCs, but not in SECs. These data have functional implications for the cells in which caveolin-1 is regulated.

Bile acid promotes liver regeneration via farnesoid X receptor signaling pathways in rats

Bile acids, which are synthesized from cholesterol in the hepatocytes of the liver, are amphipathic molecules with a steroid backbone. Studies have shown that bile acid exhibits important effects on liver regeneration. However, the mechanism underlying these effects remains unclear. The aim of the present study was to investigate the effect of bile acid and the farnesoid X receptor (FXR) on hepatic regeneration and lipid metabolism. Rats were fed with 0.2% bile acid or glucose for 7 days and then subjected to a 50 or 70% hepatectomy. Hepatic regeneration rate, serum and liver levels of bile acid, and expression of FXR and Caveolin‑1, were detected at 24, 48 or 72 h following hepatectomy. The expression of proliferating cell nuclear antigen (PCNA) in the liver was measured using immunohistochemistry at the end of the study. Hepatocytes isolated from rats were treated with bile acid, glucose, FXR agonist and FXR antagonist, separately or in combination. Lipid metabolism, the expression of members of the FXR signaling pathway and energy metabolism‑related factors were measured using ELISA kits or western blotting. Bile acid significantly increased the hepatic regeneration rate and the expression of FXR, Caveolin‑1 and PCNA. Levels of total cholesterol and high density lipoprotein were increased in bile acid‑ or FXR agonist‑treated hepatocytes in vitro. Levels of triglyceride, low density lipoprotein and free fatty acid were decreased. In addition, bile acid and FXR agonists increased the expression of bile salt export pump and small heterodimer partner, and downregulated the expression of apical sodium‑dependent bile acid transporter, Na+/taurocholate cotransporting polypeptide and cholesterol 7α‑hydroxylase. These results suggested that physiological concentrations of bile acid may promote liver regeneration via FXR signaling pathways, and may be associated with energy metabolism.

Caveolin-1 is essential for protecting against binge drinking-induced liver damage through inhibiting reactive nitrogen species

Caveolin-1 (Cav-1) is known to participate in many diseases, but its roles in alcoholic liver injury remain unknown. In the present study, we aimed to explore the roles of Cav-1 in protecting hepatocytes from ethanol-mediated nitrosative injury. We hypothesized that Cav-1 could attenuate ethanol-mediated nitrosative stress and liver damage through regulating epidermal growth factor receptor/signal transducer and activator of transcription 3/inducible nitric oxide synthase (EGFR/STAT3/iNOS)-signaling cascades. Ethanol-fed mice had time- and dose-dependent increases of Cav-1 in serum and liver with peak increase at 12 hours. Compared to wild-type mice, Cav-1 deficiency mice revealed higher expression of iNOS, higher levels of nitrate/nitrite and peroxynitrite, and had more serious liver damage, accompanied with higher levels of cleaved caspase-3 and apoptotic cell death in liver, and higher levels of alanine aminotransferase and aspartate aminotransferase in serum. Furthermore, the results revealed that the ethanol-mediated Cav-1 increase was in an extracellular signal-regulated kinase-dependent manner, and Cav-1 protected hepatocytes from ethanol-mediated apoptosis by inhibiting iNOS activity and regulating EGFR- and STAT3-signaling cascades. In agreement with these findings, clinical trials in human subjects revealed that serum Cav-1 level was time dependently elevated and peak concentration was observed 12 hours after binge drinking. Alcohol-induced liver lesions were negatively correlated with Cav-1 level, but positively correlated with nitrate/nitrite level, in serum of binge drinkers.

CONCLUSIONS

Cav-1 could be a cellular defense protein against alcoholic hepatic injury through inhibiting reactive nitrogen species and regulating EGFR/STAT3/iNOS-signaling cascades.

Regulation of intracellular signaling and function by caveolin

Caveolae, flask-like invaginations of the plasma membrane, were discovered nearly 60 years ago. Originally regarded as fixation artifacts of electron microscopy, the functional role for these structures has taken decades to unravel. The discovery of the caveolin protein in 1992 (by the late Richard G.W. Anderson) accelerated progress in defining the contribution of caveolae to cellular physiology and pathophysiology. The three isoforms of caveolin (caveolin-1, -2, and -3) are caveolae-resident structural and scaffolding proteins that are critical for the formation of caveolae and their localization of signaling entities. A PubMed search for "caveolae" reveals ∼280 publications from their discovery in the 1950s to the early 1990s, whereas a search for "caveolae or caveolin" after 1990, identifies ∼7000 entries. Most work on the regulation of biological responses by caveolae and caveolin since 1990 has focused on caveolae as plasma membrane microdomains and the function of caveolin proteins at the plasma membrane. By contrast, our recent work and that of others has explored the localization of caveolins in multiple cellular membrane compartments and in the regulation of intracellular signaling. Cellular organelles that contain caveolin include mitochondria, nuclei and the endoplasmic reticulum. Such intracellular localization allows for a complexity of responses to extracellular stimuli by caveolin and the possibility of novel organelle-targeted therapeutics. This review focuses on the impact of intracellular localization of caveolin on signal transduction and cell regulation.

The liver connexin32 interactome is a novel plasma membrane-mitochondrial signaling nexus

Connexins are the structural subunits of gap junctions and act as protein platforms for signaling complexes. Little is known about tissue-specific connexin signaling nexuses, given significant challenges associated with affinity-purifying endogenous channel complexes to the level required for interaction analyses. Here, we used multiple subcellular fractionation techniques to isolate connexin32-enriched membrane microdomains from murine liver. We show, for the first time, that connexin32 localizes to both the plasma membrane and inner mitochondrial membrane of hepatocytes. Using a combination of immunoprecipitation-high throughput mass spectrometry, reciprocal co-IP, and subcellular fractionation methodologies, we report a novel interactome validated using null mutant controls. Eighteen connexin32 interacting proteins were identified. The majority represent resident mitochondrial proteins, a minority represent plasma membrane, endoplasmic reticulum, or cytoplasmic partners. In particular, connexin32 interacts with connexin26 and the mitochondrial protein, sideroflexin-1, at the plasma membrane. Connexin32 interaction enhances connexin26 stability. Converging bioinformatic, biochemical, and confocal analyses support a role for connexin32 in transiently tethering mitochondria to connexin32-enriched plasma membrane microdomains through interaction with proteins in the outer mitochondrial membrane, including sideroflexin-1. Complex formation increases the pool of sideroflexin-1 that is present at the plasma membrane. Together, these data identify a novel plasma membrane/mitochondrial signaling nexus in the connexin32 interactome.

II. Capsular vaso-mimicry formed by transgenic mammary tumor spheroids implanted ectopically into mouse dorsal skin fold: implications for cellular mechanisms of metastasis

Most cancer patients die of metastatic disease, not primary tumors, while biological mechanisms leading to metastases remain unclear and effective therapies are missing. Using a mouse dorsal skin chamber model we had observed that tumor growth and vasculature formation could be influenced by the way in vitro cultured (avascular) spheroids of N202 breast tumor cells were implanted; co-implantation of lactating breast tissue created stimulating microenvironment, whereas the absence of the graft resulted in temporary tumor dormancy. This report addressed the issue of cellular mechanisms of the vasculogenic switch that ended the dormancy. In situ ultrastructural analysis revealed that the tumors survived in ectopic microenvironment until some of host and tumor stem cells evolved independently into cells initiating the vasculogenic switch. The tumor cells that survived and proliferated under hypoxic conditions for three weeks were supported by erythrogenic autophagy of others. However, the host microenvironment first responded as it would to non-immunogenic foreign bodies, i.e., by encapsulating the tumor spheroids with collagen-producing fibroblasts. That led to a form of vaso-mimicry consisting of tumor cells amid tumor-derived erythrosomes (synonym of erythrocytes), megakaryocytes and platelets, and encapsulating them all, the host fibroblasts. Such capsular vaso-mimicry could potentially facilitate metastasis by fusing with morphologically similar lymphatic vessels or veins. Once incorporated into the host circulatory system, tumor cells could be carried away passively by blood flow, regardless of their genetic heterogeneity. The fake vascular segment would have permeability properties different from genuine vascular endothelium. The capsular vaso-mimicry was different from vasculogenic mimicry earlier observed in metastases-associated malignant tumors where channels formed by tumor cells were said to contain circulating blood. Structures similar to the vasculogenic mimicry were seen here as well but contained non-circulating erythrosomes formed between tumor nodules. The host's response to the implantation included coordinated formation of new vessels and peripheral nerves.

II. Capsular vaso-mimicry formed by transgenic mammary tumor spheroids implanted ectopically into mouse dorsal skin fold: cellular mechanisms of metastasis

Most cancer patients die of metastatic disease, not primary tumors, while biological mechanisms leading to metastases remain unclear and effective therapies are missing. Using a mouse dorsal skin chamber model we had observed that tumor growth and vasculature formation could be influenced by the way in vitro cultured (avascular) spheroids of N202 breast tumor cells were implanted; co-implantation of lactating breast tissue created stimulating microenvironment, whereas the absence of the graft resulted in temporary tumor dormancy. This report addressed the issue of cellular mechanisms of the vasculogenic switch that ended the dormancy. In situ ultrastructural analysis revealed that the tumors survived in ectopic microenvironment until some of host and tumor stem cells evolved independently into cells initiating the vasculogenic switch. The tumor cells that survived and proliferated under hypoxic conditions for three weeks were supported by erythrogenic autophagy of others. However, the host microenvironment first responded as it would to non-immunogenic foreign bodies, i.e., by encapsulating the tumor spheroids with collagen-producing fibroblasts. That led to a form of vaso-mimicry consisting of tumor cells amid tumor-derived erythrosomes (synonym of erythrocytes), megakaryocytes and platelets, and encapsulating them all, the host fibroblasts. Such capsular vaso-mimicry could potentially facilitate metastasis by fusing with morphologically similar lymphatic vessels or veins. Once incorporated into the host circulatory system, tumor cells could be carried away passively by blood flow, regardless of their genetic heterogeneity. The fake vascular segment would have permeability properties different from genuine vascular endothelium. The capsular vaso-mimicry was different from vasculogenic mimicry earlier observed in metastases-associated malignant tumors where channels formed by tumor cells were said to contain circulating blood. Structures similar to the vasculogenic mimicry were seen here as well but contained non-circulating erythrosomes formed between tumor nodules. The host's response to the implantation included coordinated formation of new vessels and peripheral nerves.

Cross talk between ceramide and redox signaling: implications for endothelial dysfunction and renal disease

Recent studies have demonstrated that cross talk between ceramide and redox signaling modulates various cell activities and functions and contributes to the development of cardiovascular diseases and renal dysfunctions. Ceramide triggers the generation of reactive oxygen species (ROS) and increases oxidative stress in many mammalian cells and animal models. On the other hand, inhibition of ROS-generating enzymes or treatment of antioxidants impairs sphingomyelinase activation and ceramide production. As a mechanism, ceramide-enriched signaling platforms, special cell membrane rafts (MR) (formerly lipid rafts), provide an important microenvironment to mediate the cross talk of ceramide and redox signaling to exert a corresponding regulatory role on cell and organ functions. In this regard, activation of acid sphingomyelinase and generation of ceramide mediate the formation of ceramide-enriched membrane platforms, where transmembrane signals are transmitted or amplified through recruitment, clustering, assembling, or integration of various signaling molecules. A typical such signaling platform is MR redox signaling platform that is centered on ceramide production and aggregation leading to recruitment and assembling of NADPH oxidase to form an active complex in the cell plasma membrane. This redox signaling platform not only conducts redox signaling or regulation but also facilitates a feedforward amplification of both ceramide and redox signaling. In addition to this membrane MR redox signaling platform, the cross talk between ceramide and redox signaling may occur in other cell compartments. This book chapter focuses on the molecular mechanisms, spatial-temporal regulations, and implications of this cross talk between ceramide and redox signaling, which may provide novel insights into the understanding of both ceramide and redox signaling pathways.

Peroxisome Dynamics: Molecular Players, Mechanisms, and (Dys)functions

Peroxisomes are remarkably versatile cell organelles whose size, shape, number, and protein content can vary greatly depending on the organism, the developmental stage of the organism’s life cycle, and the environment in which the organism lives. The main functions usually associated with peroxisomes include the metabolism of lipids and reactive oxygen species. However, in recent years, it has become clear that these organelles may also act as intracellular signaling platforms that mediate developmental decisions by modulating extraperoxisomal concentrations of several second messengers. To fulfill their functions, peroxisomes physically and functionally interact with other cell organelles, including mitochondria and the endoplasmic reticulum. Defects in peroxisome dynamics can lead to organelle dysfunction and have been associated with various human disorders. The purpose of this paper is to thoroughly summarize and discuss the current concepts underlying peroxisome formation, multiplication, and degradation. In addition, this paper will briefly highlight what is known about the interplay between peroxisomes and other cell organelles and explore the physiological and pathological implications of this interorganellar crosstalk.

Mitochondrial cholesterol: a connection between caveolin, metabolism, and disease

Caveolin (CAV) is an essential component of caveolae, cholesterol-enriched invaginations of the plasma membrane of most mammalian cells. However, CAV is not restricted to plasma membrane caveolae, and pools of CAV are present in myriad intracellular membranes. CAV proteins tightly bind cholesterol and contribute to regulation of cholesterol fluxes and distributions within cells. In this context, we recently showed that CAV1 regulates the poorly understood process controlling mitochondrial cholesterol levels. Cholesterol accumulates in mitochondrial membranes in the absence of CAV1, promoting the organelle's dysfunction with important metabolic consequences for cells and animals. In this article, we suggest a working hypothesis that addresses the role of CAV1 within the homeostatic network that regulates the influx/efflux of mitochondrial cholesterol.

Lipid raft redox signaling: molecular mechanisms in health and disease

Lipid rafts, the sphingolipid and cholesterol-enriched membrane microdomains, are able to form different membrane macrodomains or platforms upon stimulations, including redox signaling platforms, which serve as a critical signaling mechanism to mediate or regulate cellular activities or functions. In particular, this raft platform formation provides an important driving force for the assembling of NADPH oxidase subunits and the recruitment of other related receptors, effectors, and regulatory components, resulting, in turn, in the activation of NADPH oxidase and downstream redox regulation of cell functions. This comprehensive review attempts to summarize all basic and advanced information about the formation, regulation, and functions of lipid raft redox signaling platforms as well as their physiological and pathophysiological relevance. Several molecular mechanisms involving the formation of lipid raft redox signaling platforms and the related therapeutic strategies targeting them are discussed. It is hoped that all information and thoughts included in this review could provide more comprehensive insights into the understanding of lipid raft redox signaling, in particular, of their molecular mechanisms, spatial-temporal regulations, and physiological, pathophysiological relevances to human health and diseases.

Altered distribution of caveolin-1 in early liver steatosis

BACKGROUND

Caveolin-1, the main structural protein of caveolae, is involved in cholesterol homoeostasis, transcytosis, endocytosis and signal transduction and thought to play an important role in lipidogenesis. Little is known about the pathophysiological role of caveolin-1 in nonalcoholic fatty liver disease (NAFLD), a condition frequently associated with the metabolic syndrome and characterized by abnormal accumulation of intrahepatic triglycerides with a potentially harmful risk of evolution to liver fibrosis, cirrhosis and hepatocellular carcinoma.

MATERIALS AND METHODS

Liver steatosis (micro/macrovesicular) was induced in adult rats fed a choline-deficient diet for 14days and compared with a control normal diet. The expression and subcellular distribution of caveolin-1 was assessed using light and electron microscopy by immunohistochemical and immunocytochemical techniques and by Western blotting.

RESULTS

Caveolin-1 was mainly associated with the hepatocyte basolateral plasma membrane. Fatty hepatocytes were characterized by a significant increase in the expression of caveolin-1 around and within the lipid droplets as well as in the inner membrane of mitochondria.

CONCLUSIONS

Our data suggest the involvement of caveolin-1 in the case of abnormal lipogenesis and mitochondrial function typical of steatotic hepatocytes in NAFLD. Addressing the role played by caveolin-1 in liver membranes in NAFLD may help future therapeutic choices in a frequent metabolic liver disease.