Effects of deleting a tripeptide sequence observed in muscular dystrophy patients on the conformation of synthetic peptides corresponding to the scaffolding domain of caveolin-3

Caveolin-1, a master regulator of cellular senescence

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

Caveolin-1 promotes the tumor suppressor properties of oncogene-induced cellular senescence

Oncogene-induced senescence (OIS) is considered a powerful tumor suppressor mechanism. Caveolin-1 acts as a scaffolding protein to functionally regulate signaling molecules. We demonstrate that a lack of caveolin-1 expression inhibits oncogenic K-Ras (K-Ras^G12V)-induced premature senescence in mouse embryonic fibroblasts and normal human bronchial epithelial cells. Oncogenic K-Ras induces senescence by limiting the detoxification function of MTH1. We found that K-Ras^G12V promotes the interaction of caveolin-1 with MTH1, which results in inhibition of MTH1 activity. Lung cancer cells expressing oncogenic K-Ras have bypassed the senescence barrier. Interestingly, overexpression of caveolin-1 restores cellular senescence in both A549 and H460 lung cancer cells and inhibits their transformed phenotype. In support of these findings, our in vivo data demonstrate that overexpression of oncogenic K-Ras (K-Ras^G12D) induces cellular senescence in the lung of wildtype but not caveolin-1-null mice. A lack of K-Ras^G12D-induced premature senescence in caveolin-1-null mice results in the formation of more abundant lung tumors. Consistent with these data, caveolin-1-null mice overexpressing K-Ras^G12D display accelerated mortality. Finally, our animal data were supported by human sample analysis in which we show that caveolin-1 expression is dramatically down-regulated in lung adenocarcinomas from lung cancer patients, both at the mRNA and protein levels, and that low caveolin-1 expression is associated with poor survival. Together, our data suggest that lung cancer cells escape oncogene-induced premature senescence through down-regulation of caveolin-1 expression to progress from premalignant lesions to cancer.

Caveolin-1 controls mitochondrial function through regulation of m-AAA mitochondrial protease

Mitochondrial proteases ensure mitochondrial integrity and function after oxidative stress by providing mitochondrial protein quality control. However, the molecular mechanisms that regulate this basic biological function in eukaryotic cells remain largely unknown. Caveolin-1 is a scaffolding protein involved in signal transduction. We find that AFG3L2, a m-AAA type of mitochondrial protease, is a novel caveolin-1-interacting protein in vitro. We show that oxidative stress promotes the translocation of both caveolin-1 and AFG3L2 to mitochondria, enhances the interaction of caveolin-1 with AFG3L2 in mitochondria and stimulates mitochondrial protease activity in wild-type fibroblasts. Localization of AFG3L2 to mitochondria after oxidative stress is inhibited in fibroblasts lacking caveolin-1, which results in impaired mitochondrial protein quality control, an oxidative phosphorylation to aerobic glycolysis switch and reduced ATP production. Mechanistically, we demonstrate that a lack of caveolin-1 does not alter either mitochondrial number or morphology but leads to the cytoplasmic and proteasome-dependent degradation of complexes I, III, IV and V upon oxidant stimulation. Restoration of mitochondrial respiratory chain complexes in caveolin-1 null fibroblasts reverts the enhanced glycolysis observed in these cells. Expression of a mutant form of AFG3L2, which has reduced affinity for caveolin-1, fails to localize to mitochondria and promotes degradation of complex IV after oxidative stress. Thus, caveolin-1 maintains mitochondrial integrity and function when cells are challenged with free radicals by promoting the mitochondrial localization of m-AAA protease and its quality control functions.

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.

Inhibition of nuclear factor-erythroid 2-related factor (Nrf2) by caveolin-1 promotes stress-induced premature senescence

Reactive oxygen species can induce premature senescence. Caveolin-1 promotes oxidative stress–induced activation of the p53/p21Waf1/Cip1 pathway and development of premature senescence by acting as an endogenous inhibitor of the transcription factor Nrf2.

Reactive oxygen species (ROS) can induce premature cellular senescence, which is believed to contribute to aging and age-related diseases. The nuclear erythroid 2 p45–related factor-2 (Nrf2) is a transcription factor that mediates cytoprotective responses against stress. We demonstrate that caveolin-1 is a direct binding partner of Nrf2, as shown by the binding of the scaffolding domain of caveolin-1 (amino acids 82–101) to the caveolin-binding domain of Nrf2 (amino acids 281–289). Biochemical studies show that Nrf2 is concentrated into caveolar membranes in human and mouse fibroblasts, where it colocalizes with caveolin-1, under resting conditions. After oxidative stress, caveolin-1 limits the movement of Nrf2 from caveolar membranes to the nucleus. In contrast, Nrf2 is constitutively localized to the nucleus before and after oxidative stress in caveolin-1–null mouse embryonic fibroblasts (MEFs), which do not express caveolin-1. Functional studies demonstrate that caveolin-1 acts as an endogenous inhibitor of Nrf2, as shown by the enhanced up-regulation of NQO1, an Nrf2 target gene, in caveolin-1–null MEFs and the activation or inhibition of a luciferase construct carrying an antioxidant responsive element (ARE) after down-regulation of caveolin-1 by small interfering RNA or overexpression of caveolin-1, respectively. Expression of a mutant form of Nrf2 that cannot bind to caveolin-1 (Φ→A-Nrf2) hyperactivates ARE and inhibits oxidative stress–induced activation of the p53/p21^Waf1/Cip1 pathway and induction of premature senescence in fibroblasts. Finally, we show that overexpression of caveolin-1 in colon cancer cells inhibits oxidant-induced activation of Nrf2-dependent signaling, promotes premature senescence, and inhibits their transformed phenotype. Thus, by inhibiting Nrf2-mediated signaling, caveolin-1 links free radicals to the activation of the p53/senescence pathway.

Interaction of caveolin-1 with Ku70 inhibits Bax-mediated apoptosis

Caveolin-1, the structural protein component of caveolae, acts as a scaffolding protein that functionally regulates signaling molecules. We show that knockdown of caveolin-1 protein expression enhances chemotherapeutic drug-induced apoptosis and inhibits long-term survival of colon cancer cells. In vitro studies demonstrate that caveolin-1 is a novel Ku70-binding protein, as shown by the binding of the scaffolding domain of caveolin-1 (amino acids 82-101) to the caveolin-binding domain (CBD) of Ku70 (amino acids 471-478). Cell culture data show that caveolin-1 binds Ku70 after treatment with chemotherapeutic drugs. Mechanistically, we found that binding of caveolin-1 to Ku70 inhibits the chemotherapeutic drug-induced release of Bax from Ku70, activation of Bax, translocation of Bax to mitochondria and apoptosis. Potentiation of apoptosis by knockdown of caveolin-1 protein expression is greatly reduced in the absence of Bax expression. Finally, we found that overexpression of wild type Ku70, but not a mutant form of Ku70 that cannot bind to caveolin-1 (Ku70 Φ→A), limits the chemotherapeutic drug-induced Ku70/Bax dissociation and apoptosis. Thus, caveolin-1 acts as an anti-apoptotic protein in colon cancer cells by binding to Ku70 and inhibiting Bax-dependent cell death.

Caveolin-1, cellular senescence and age-related diseases

According to the "free radical theory" of aging, normal aging occurs as the result of tissue damages inflicted by reactive oxygen species (ROS) when ROS production exceeds the antioxidant capacity of the cell. ROS induce cellular dysfunctions such as stress-induced premature senescence (SIPS), which is believed to contribute to normal organismal aging and play a role in age-related diseases. Consistent with this hypothesis, increased oxidative damage of DNA, proteins, and lipids have been reported in aged animals and senescent cells accumulate in vivo with advancing age. Caveolin-1 acts as a scaffolding protein that concentrates and functionally regulates signaling molecules. Recently, great progress has been made toward understanding of the role of caveolin-1 in stress-induced premature senescence. Data show that caveolin-mediated signaling may contribute to explain, at the molecular level, how oxidative stress promotes the deleterious effects of cellular senescence such as aging and age-related diseases. In this review, we discuss the cellular mechanisms and functions of caveolin-1 in the context of SIPS and their relevance to the biology of aging.

Interaction of synthetic peptides corresponding to the scaffolding domain of Caveolin-3 with model membranes

Caveolin-1 and -3 are among the few proteins in which the functional domains are contiguous and modular. The interaction of synthetic peptides spanning the scaffolding domain of caveolin-3 with model membranes has been investigated. The peptides include the scaffolding domain, the aromatic and positively charged residues at the C-terminal end of this domain as well as deletion of three amino acids TFT, observed in certain patients with limb girdle muscular dystrophy. All of the peptides appear to be peripherally bound to the bilayer surface. However, no preferential binding to sphingomyelin and cholesterol-containing lipid vesicles was observed. Deletion of TFT appears to affect the association with lipid vesicles compared with the native sequence. Association with lipids decreases considerably when TFT as well as the aromatic-rich segment YWFYR, which occurs at the extreme C-terminus of the scaffolding domain, are deleted.

Role of Caveolae and Caveolins in Health and Disease

Although they were discovered more than 50 years ago, caveolae have remained enigmatic plasmalemmal organelles. With their characteristic “flasklike” shape and virtually ubiquitous tissue distribution, these interesting structures have been implicated in a wide range of cellular functions. Similar to clathrin-coated pits, caveolae function as macromolecular vesicular transporters, while their unique lipid composition classifies them as plasma membrane lipid rafts, structures enriched in a variety of signaling molecules. The caveolin proteins (caveolin-1, -2, and -3) serve as the structural components of caveolae, while also functioning as scaffolding proteins, capable of recruiting numerous signaling molecules to caveolae, as well as regulating their activity. That so many signaling molecules and signaling cascades are regulated by an interaction with the caveolins provides a paradigm by which numerous disease processes may be affected by ablation or mutation of these proteins. Indeed, studies in caveolin-deficient mice have implicated these structures in a host of human diseases, including diabetes, cancer, cardiovascular disease, atherosclerosis, pulmonary fibrosis, and a variety of degenerative muscular dystrophies. In this review, we provide an in depth summary regarding the mechanisms by which caveolae and caveolins participate in human disease processes.

A caveolin-3 mutant that causes limb girdle muscular dystrophy type 1C disrupts Src localization and activity and induces apoptosis in skeletal myotubes

Caveolins are membrane proteins that are the major coat proteins of caveolae, specialized lipid rafts in the plasma membrane that serve as scaffolding sites for many signaling complexes. Among the many signaling molecules associated with caveolins are the Src tyrosine kinases, whose activation regulates numerous cellular functions including the balance between cell survival and cell death. Several mutations in the muscle-specific caveolin, caveolin-3, lead to a form of autosomal dominant muscular dystrophy referred to as limb girdle muscular dystrophy type 1C (LGMD-1C). One of these mutations (here termed the `TFT mutation') results in a deletion of a tripeptide (ΔTFT(63-65)) that affects the scaffolding and oligomerization domains of caveolin-3. This mutation causes a 90-95% loss of caveolin-3 protein levels and reduced formation of caveolae in skeletal muscle fibers. However, the effects of this mutation on the specific biochemical processes and cellular functions associated with caveolae have not been elucidated. We demonstrate that the TFT caveolin-3 mutation in post-mitotic skeletal myotubes causes severely reduced localization of caveolin-3 to the plasma membrane and to lipid rafts, and significantly inhibits caveolar function. The TFT mutation reduced the binding of Src to caveolin-3, diminished targeting of Src to lipid rafts, and caused abnormal perinuclear accumulation of Src. Along with these alterations of Src localization and targeting, there was elevated Src activation in myotubes expressing the TFT mutation and an increased incidence of apoptosis in those cells compared with control myotubes. The results of this study demonstrate that caveolin-3 mutations associated with LGMD-1C disrupt normal cellular signal transduction pathways associated with caveolae and cause apoptosis in muscle cells, all of which may reflect pathogenetic pathways that lead to muscle degeneration in these disorders.

Role of caveolin and caveolae in insulin signaling and diabetes

Caveolae are specialized membrane microdomains present within the plasma membrane of the vast majority of cell types. They have a unique composition in that they are highly enriched in cholesterol, sphingolipids, and their coat proteins the caveolins (-1, -2, and -3). In recent years it has been recognized that caveolae act as signaling platforms, serving as a concentrating point for numerous signaling molecules, as well as regulating flux through many distinct signaling cascades. Although caveolae are found in a variety of cell types, they are most abundant in adipose tissue. This fact has led to the intense study of the function of these organelles in adipocytes. It has now become apparent that effective insulin signaling in the adipocyte may be strictly dependent on localization of at least two insulin-responsive elements to caveolae (insulin receptor and GLUT4), as well as on a direct functional interaction between caveolin-1 and the insulin receptor. We present a critical discussion of these recent findings.