Cloning and functional characterization of PTRF, a novel protein which induces dissociation of paused ternary transcription complexes

Caveolin and cavin family members: dual roles in cancer

Caveolae are specialized plasma membrane subdomains with distinct lipid and protein compositions, which play an essential role in cell physiology through regulation of trafficking and signaling functions. The structure and functions of caveolae have been shown to require the proteins caveolins. Recently, members of the cavin protein family were found to be required, in concert with caveolins, for the formation and function of caveolae. Caveolins have a paradoxical role in the development of cancer formation. They have been involved in both tumor suppression and oncogenesis, depending on tumor type and progress stage. High expression of caveolins and cavins leads to inhibition of cancer-related pathways, such as growth factor signaling pathways. However, certain cancer cells that express caveolins and cavins have been shown to be more aggressive and metastatic because of their increased potential for anchorage-independent growth. Here, we will survey the functional roles of caveolins and of different cavin family members in cancer regulation.

Emerging role of polymerase-1 and transcript release factor (PTRF/ Cavin-1) in health and disease

Polymerase-1 and release transcript factor (PTRF) was initially reported to be involved in the termination of the transcription process. More recently, it has been implicated in the formation of caveolae, cave-like structures in the plasma membrane. The effects of PTRF related to caveolae suggest that this protein may play important roles in health and disease. PTRF is highly expressed in various cells, including adipocytes, osteoblasts and muscle (cardiac, skeletal and smooth) cells. The role of PTRF in prostate cancer has been recently reviewed but there is growing evidence that PTRF is involved in other physiological processes such as cell repair and the regulation of glucose and lipid metabolism and, furthermore, altered expression of PTRF may be associated with disease. This review discusses the emerging role of PTRF in health and disease.

Binding of the termination factor Nsi1 to its cognate DNA site is sufficient to terminate RNA polymerase I transcription in vitro and to induce termination in vivo

Different models have been proposed explaining how eukaryotic gene transcription is terminated. Recently, Nsi1, a factor involved in silencing of ribosomal DNA (rDNA), was shown to be required for efficient termination of rDNA transcription by RNA polymerase I (Pol I) in the yeast Saccharomyces cerevisiae. Nsi1 contains Myb-like DNA binding domains and associates in vivo near the 3' end of rRNA genes to rDNA, but information about which and how DNA sequences might influence Nsi1-dependent termination is lacking. Here, we show that binding of Nsi1 to a stretch of 11 nucleotides in the correct orientation was sufficient to pause elongating Pol I shortly upstream of the Nsi1 binding site and to release the transcripts in vitro. The same minimal DNA element triggered Nsi1-dependent termination of pre-rRNA synthesis using an in vivo reporter assay. Termination efficiency in the in vivo system could be enhanced by inclusion of specific DNA sequences downstream of the Nsi1 binding site. These data and the finding that Nsi1 blocks efficiently only Pol I-dependent RNA synthesis in an in vitro transcription system improve our understanding of a unique mechanism of transcription termination.

Cavin-3 knockout mice show that cavin-3 is not essential for caveolae formation, for maintenance of body composition, or for glucose tolerance

The cavins are a family of proteins associated with caveolae, cavin-1, -2 and -3 being widely expressed while cavin-4 is restricted to striated muscle. Deletion of cavin-1 results in phenotypes including metabolic changes consistent with adipocyte dysfunction, and caveolae are completely absent. Deletion of cavin-2 causes tissue-specific loss of caveolae. The consequences of cavin-3 deletion are less clear, as there are divergent data on the abundance of caveolae in cavin-3 null mice. Here we examine the consequences of cavin-3 deficiency in vivo by making cavin-3 knockout mice. We find that loss of cavin-3 has minimal or no effects on the levels of other caveolar proteins, does not appear to play a major role in formation of protein complexes important for caveolar morphogenesis, and has no significant effect on caveolae abundance. Cavin-3 null mice have the same body weight and fat mass as wild type animals at ages 8 through 30 weeks on both normal chow and high fat diets. Likewise, the two mouse strains exhibit identical glucose tolerance tests on both diets. Microarray analysis from adipose tissue shows that the changes in mRNA expression between cavin-3 null and wild type mouse are minimal. We conclude that cavin-3 is not absolutely required for making caveolae, and suggest that the mechanistic link between cavin-3 and metabolic regulation remains uncertain.

Polymerase I and transcript release factor (PTRF) regulates adipocyte differentiation and determines adipose tissue expandability

Impaired adipogenesis renders an adipose tissue unable to expand, leading to lipotoxicity and conditions such as diabetes and cardiovascular disease. While factors important for adipogenesis have been studied extensively, those that set the limits of adipose tissue expansion remain undetermined. Feeding a Western-type diet to apolipoprotein E2 knock-in mice, a model of metabolic syndrome, produced 3 groups of equally obese mice: mice with normal glucose tolerance, hyperinsulinemic yet glucose-tolerant mice, and prediabetic mice with impaired glucose tolerance and reduced circulating insulin. Using proteomics, we compared subcutaneous adipose tissues from mice in these groups and found that the expression of PTRF (polymerase I and transcript release factor) associated selectively with their glucose tolerance status. Lentiviral and pharmacologically overexpressed PTRF, whose function is critical for caveola formation, compromised adipocyte differentiation of cultured 3T3-L1cells. In human adipose tissue, PTRF mRNA levels positively correlated with markers of lipolysis and cellular senescence. Furthermore, a negative relationship between telomere length and PTRF mRNA levels was observed in human subcutaneous fat. PTRF is associated with limited adipose tissue expansion underpinning the key role of caveolae in adipocyte regulation. Furthermore, PTRF may be a suitable adipocyte marker for predicting pathological obesity and inform clinical management.

Polymerase I and transcript release factor acts as an essential modulator of glioblastoma chemoresistance

OBJECTIVES

This study is to investigate if polymerase I and transcript release factor (PTRF) acts as a modulator in glioblastoma (GBM) chemoresistance.

METHODS

Multidrug resistant (MDR) GBM cell line U251AR was established by exposing the U251 cell line to imatinib. The 2D-DIGE and MALDI-TOF/TOF-MS were performed on U251 and U251AR cell lines to screen MDR-related proteins. The expression of PTRF was determined by Western blot and quantitative RT-PCR analyses.

RESULTS

When compared with the parental U251 cells, expression of 21 proteins was significantly altered in U251AR cells. Among the 21 differentially expressed proteins, the expression of PTRF was up-regulated by 2.14 folds in U251AR cells when compared with that in the parental U251 cells. Knockdown of PTRF in GBM cell lines significantly increased chemosensitivity of cells to various chemical drugs and decreased the expression levels of caveolin1, a major structural component of caveolae. Expression levels of PTRF and caveolin1 were significantly up-regulated in the relapsed GBM patients. The mRNA level of PTRF and caveolin1 showed a positive correlation in the same GBM specimens.

CONCLUSIONS

Our results indicate that PTRF acts as a modulator in GBM chemoresistance.

Arterial dysfunction but maintained systemic blood pressure in cavin-1-deficient mice

Caveolae are omega-shaped plasma membrane micro-domains that are abundant in cells of the vascular system. Formation of caveolae depends on caveolin-1 and cavin-1 and lack of either protein leads to loss of caveolae. Mice with caveolin-1 deficiency have dysfunctional blood vessels, but whether absence of cavin-1 similarly leads to vascular dysfunction is not known. Here we addressed this hypothesis using small mesenteric arteries from cavin-1-deficient mice. Cavin-1-reporter staining was intense in mesenteric arteries, brain arterioles and elsewhere in the vascular system, with positive staining of both endothelial and smooth muscle cells. Arterial expression of cavin-1, -2 and -3 was reduced in knockout (KO) arteries as was expression of caveolin-1, -2 and -3. Caveolae were absent in the endothelial and smooth muscle layers of small mesenteric arteries as determined by electron microscopy. Arginase, a negative regulator of nitric oxide production, was elevated in cavin-1 deficient arteries as was contraction in response to the α1-adrenergic agonist cirazoline. Detailed assessment of vascular dimensions revealed increased media thickness and reduced distensibility, arguing that enhanced contraction was due to increased muscle mass. Contrasting with increased α1-adrenergic contraction, myogenic tone was essentially absent and this appeared to be due in part to increased nitric oxide production. Vasomotion was less frequent in the knock-out vessels. In keeping with the opposing influences on arterial resistance of increased agonist-induced contractility and reduced myogenic tone, arterial blood pressure was unchanged in vivo. We conclude that deficiency of cavin-1 affects the function of small arteries, but that opposing influences on arterial resistance balance each other such that systemic blood pressure in unstressed mice is well maintained.

Pleiotropic effects of cavin-1 deficiency on lipid metabolism

Mice and humans lacking caveolae due to gene knock-out or inactivating mutations of cavin-1/PTRF have numerous pathologies including markedly aberrant fuel metabolism, lipodystrophy, and muscular dystrophy. We characterized the physiologic/metabolic profile of cavin-1 knock-out mice and determined that they were lean because of reduced white adipose depots. The knock-out mice were resistant to diet-induced obesity and had abnormal lipid metabolism in the major metabolic organs of white and brown fat and liver. Epididymal white fat cells from cavin-1-null mice were small and insensitive to insulin and β-adrenergic agonists resulting in reduced adipocyte lipid storage and impaired lipid tolerance. At the molecular level, the lipolytic defects in white fat were caused by impaired perilipin phosphorylation, and the reduced triglyceride accumulation was caused by decreased fatty acid uptake and incorporation as well as the virtual absence of insulin-stimulated glucose transport. The livers of cavin-1-null mice were mildly steatotic and did not accumulate more lipid after high-fat feeding. The brown adipose tissues of cavin-1-null mice exhibited decreased mitochondria protein expression, which was restored upon high fat feeding. Taken together, these data suggest that dysfunction in fat, muscle, and liver metabolism in cavin-1-null mice causes a pleiotropic phenotype, one apparently identical to that of humans lacking caveolae in all tissues.

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.

Caveola-forming proteins caveolin-1 and PTRF in prostate cancer

The expression of caveola-forming proteins is dysregulated in prostate cancer. Caveolae are flask-shaped invaginations of the plasma membrane that have roles in membrane trafficking and cell signalling. Members of two families of proteins--caveolins and cavins--are known to be required for the formation and functions of caveolae. Caveolin-1, the major structural protein of caveolae, is overexpresssed in prostate cancer and has been demonstrated to be involved in prostate cancer angiogenesis, growth and metastasis. Polymerase I and transcript release factor (PTRF) is the only cavin family member necessary for caveola formation. When exogenously expressed in prostate cancer cells, PTRF reduces aggressive potential, probably via both caveola-mediated and caveola-independent mechanisms. In addition, stromal PTRF expression decreases with progression of the disease. Evaluation of caveolin-1 antibodies in the clinical setting is underway and it is hoped that future studies will reveal the mechanisms of PTRF action, allowing its targeting for therapeutic purposes.

Suppression of PTRF alleviates the polymicrobial sepsis induced by cecal ligation and puncture in mice

BACKGROUND

Sepsis and sepsis-associated organ failure are devastating conditions. Understanding the detailed cellular/molecular mechanisms involved in sepsis should lead to the identification of novel therapeutic targets.

METHODS

Cecal ligation and puncture (CLP) was used as a polymicrobial sepsis model in vivo to determine mortality and end-organ damage. Macrophages were adopted as the cellular model in vitro for mechanistic studies.

RESULTS

PTRF+/- mice survived longer and suffered less organ damage after CLP. Reductions in nitric oxide (NO) and iNOS biosynthesis were observed in plasma, macrophages, and vital organs in the PTRF+/- mice. Using an acute sepsis model after CLP, we found that iNOS-/- mice had a comparable level of survival as the PTRF+/- mice. Similarly, polymerase I transcript release factor (PTRF) deficiency resulted in decreased iNOS and NO/ROS production in macrophages in vitro. Mechanistically, lipopolysaccharide (LPS) enhanced the co-localization and interaction between PTRF and TLR4 in lipid rafts. Deletion of PTRF blocked formation of the TLR4/Myd88 complex after LPS. Consistent with this, lack of PTRF impaired the TLR4 signaling, as shown by the decreased p-JNK, p-ERK, and p-p38, which are upstream factors involved in iNOS transcription.

CONCLUSION

PTRF is a crucial regulator of TLR4 signaling in the development of sepsis.

Cavin-1 is essential for the tumor-promoting effect of caveolin-1 and enhances its prognostic potency in pancreatic cancer

Caveolin-1 exhibits a stage-dependent, functional fluctuation during pancreatic cancer development, but the underlying mechanisms remain unclear. Here, we report that cavin-1, a structural protein of caveolae, modulates the oncogenic function of caveolin-1 and cooperates with caveolin-1 to enhance pancreatic cancer aggressiveness. Cavin-1 expression is associated with caveolin-1 in pancreatic cancer tissue samples and cell lines, and predicts the metastatic potential of pancreatic cancer. Interactome analyses further revealed the physical interaction of cavin-1 and caveolin-1 and their colocalization in pancreatic cancer cells. Cavin-1 stabilizes caveolin-1 expression or activity by inhibiting its internalization and subsequent lysosomal degradation. More in-depth functional experiments showed that caveolin-1-enhanced aggressiveness of pancreatic cancer cells is dependent on the presence of cavin-1. In contrast, cavin-1 depletion inhibited the invasion and metastasis of pancreatic cancer cells, which could not be restored by caveolin-1-rescue construct. Tissue microarray analyses in two independent clinic cohorts also supported the augment of cavin-1 on the prognostic potency of caveolin-1, and showed that combination of cavin-1 with caveolin-1 predicted worse survival in pancreatic cancer patients. Of note, the phenotypes because of cavin-1 could not be achieved by other cavins such as cavin-2, and the tumor-promoting role of cavin-1 in pancreatic cancer was found to be largely dependent on caveolin-1 expression, which highlights the critical role of cavin-1/caveoin-1 in pancreatic cancer progression, and suggests that the interruption of cavin-1/caveolin-1 interaction is a promising therapeutic strategy for pancreatic cancer.

Cavin1; a regulator of lung function and macrophage phenotype

Caveolae are cell membrane invaginations that are highly abundant in adipose tissue, endothelial cells and the lung. The formation of caveolae is dependent on the expression of various structural proteins that serve as scaffolding for these membrane invaginations. Cavin1 is a newly identified structural protein whose deficiency in mice leads to loss of caveolae formation and to development of a lipodystrophic phenotype. In this study, we sought to investigate the functional role of Cavin1 in the lung. Cavin1 deficient mice possessed dramatically altered distal lung morphology and exhibited significant physiological alterations, notably, increased lung elastance. The changes in distal lung architecture were associated with hypercellularity and the accumulation of lung macrophages. The increases in lung macrophages occurred without changes to circulating numbers of mononuclear cells and without evidence for increased proliferation. However, the increases in lung macrophages were associated with higher levels of macrophage chemotactic factors CXCL2 and CCL2 in BAL fluid from Cavin1-/- mice suggesting a possible mechanism by which these cells accumulate. In addition, lung macrophages from Cavin1-/- mice were larger and displayed measurable differences in gene expression when compared to macrophages from wild-type mice. Interestingly, macrophages were also increased in adipose tissue but not in liver, kidney or skeletal muscle from Cavin1-/- mice, and similar tissue specificity for macrophage accumulation was observed in lungs and adipose tissue from Caveolin1-/- mice. In conclusion, this study demonstrates an important role for Cavin1 in lung homeostasis and suggests that caveolae structural proteins are necessary for regulating macrophage number and phenotype in the lung.

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.

Caveolin-1 and polymerase I and transcript release factor: new players in insulin-like growth factor-I receptor signaling

Caveolae are plasma membrane regions enriched in Caveolin proteins which regulate vesicular transport, endocytosis, and cell signaling. IGF-I receptor (IGF-IR) localizes in caveolae and tyrosine phosphorylates Caveolin-1 (Cav-1), the most represented caveolar protein. Cav-1 participates to IGF-IR internalization and signaling directly interacting with IGF-IR and its substrates. Recently, polymerase I and transcript release factor (PTRF) or Cavin-1, has been identified in the caveolar backbone. PTRF does not play a Cav-1 ancillary role and emerging data support a direct role of PTRF in IGF-IR signaling. PTRF and Cav-1 can bind IGF-IR and regulate IGF-IR internalization and plasma membrane replacement, mechanisms frequently deregulated in cancer cells. Although the exact roles of Cav-1 and IGF-IR in human cancer continue to be a matter of some debate, there is a strong evidence for an association between Cav-1 and IGF-IR in cancer development. With the discovery of IGF-IR interaction with PTRF in caveolae, new insight emerged to understand the growing functions of these domains in IGF-I action.

Single-molecule analysis reveals self assembly and nanoscale segregation of two distinct cavin subcomplexes on caveolae

In mammalian cells three closely related cavin proteins cooperate with the scaffolding protein caveolin to form membrane invaginations known as caveolae. Here we have developed a novel single-molecule fluorescence approach to directly observe interactions and stoichiometries in protein complexes from cell extracts and from in vitro synthesized components. We show that up to 50 cavins associate on a caveola. However, rather than forming a single coat complex containing the three cavin family members, single-molecule analysis reveals an exquisite specificity of interactions between cavin1, cavin2 and cavin3. Changes in membrane tension can flatten the caveolae, causing the release of the cavin coat and its disassembly into separate cavin1-cavin2 and cavin1-cavin3 subcomplexes. Each of these subcomplexes contain 9 ± 2 cavin molecules and appear to be the building blocks of the caveolar coat. High resolution immunoelectron microscopy suggests a remarkable nanoscale organization of these separate subcomplexes, forming individual striations on the surface of caveolae. DOI: http://dx.doi.org/10.7554/eLife.01434.001.

RNA polymerase I termination: Where is the end?

The synthesis of ribosomal RNA (rRNA) precursor molecules by RNA polymerase I (Pol I) terminates with the dissociation of the protein-DNA-RNA ternary complex. Based on in vitro results the mechanism of Pol I termination appeared initially to be rather conserved and simple until this process was more thoroughly re-investigated in vivo. A picture emerged that Pol I termination seems to be connected to co-transcriptional processing, re-initiation of transcription and, possibly, other processes downstream of Pol I transcription units. In this article, our current understanding of the mechanism of Pol I termination and how this process might be implicated in other biological processes in yeast and mammals is summarized and discussed. This article is part of a Special Issue entitled: Transcription by Odd Pols.

Inverse Correlation between Caveolin-1 Expression and Clinical Severity in Psoriasis Vulgaris

Objective:

This study investigated the clinical significance of expression of caveolin-1 - a plasma membrane protein involved in caveola formation, endocytosis, signal transduction and angiogenesis - inthe pathogenesis of psoriasis vulgaris.

Methods:

A total of 20 patients with psoriasis vulgaris and 20 healthy volunteers were recruited. The expressions of caveolin-1, Ki-67 (marker of cell proliferation) and CD34 (marker of angiogenesis) in skin biopsies were detected by immunohistochemistry, and the level of caveolin-1 protein was quantified by Western blotting. Clinical severity was assessed using the Psoriasis Area and Severity Index (PASI) score. Correlations between caveolin-1 expression and psoriasis severity, cell proliferation and angiogenesis were analysed using the Spearman rank correlation test.

Results:

Expression of caveolin-1 was significantly lower in psoriasis samples than in healthy skin samples. In psoriasis lesions, the level of caveolin-1 expression was inversely correlated with the severity of psoriasis, cell proliferation and angiogenesis.

Conclusions:

The level of caveolin-1 expression seems to be related to the clinical severity of psoriasis, and may play a role in the abnormal keratinocyte hyperplasia and angiogenesis seen in this condition.

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.

The Reb1-homologue Ydr026c/Nsi1 is required for efficient RNA polymerase I termination in yeast

Several DNA cis-elements and trans-acting factors were described to be involved in transcription termination and to release the elongating RNA polymerases from their templates. Different models for the molecular mechanism of transcription termination have been suggested for eukaryotic RNA polymerase I (Pol I) from results of in vitro and in vivo experiments. To analyse the molecular requirements for yeast RNA Pol I termination, an in vivo approach was used in which efficient termination resulted in growth inhibition. This led to the identification of a Myb-like protein, Ydr026c, as bona fide termination factor, now designated Nsi1 (NTS1 silencing protein 1), since it was very recently described as silencing factor of ribosomal DNA. Possible Nsi1 functions in regard to the mechanism of transcription termination are discussed.

Two preferentially expressed proteins protect vascular endothelial cells from an attack by peptide-specific CTL

Vascular endothelial cells (EC) are an exposed tissue with intimate contact with circulating Ag-specific CTL. Experimental in vitro and clinical data suggested that endothelial cells present a different repertoire of MHC class I-restricted peptides compared with syngeneic leukocytes or epithelial cells. This endothelial-specific peptide repertoire might protect EC from CTL-mediated cell death. The HLA-A*02-restricted peptide profile of human EC and syngeneic B lymphoblastoid cells was biochemically analyzed and compared. For EC selective peptides, source protein expression, peptide binding affinity, and peptide-HLA-A*02 turnover were measured. The significance of abundant peptide presentation for target cell recognition by immunodominant CTL was tested by small interfering RNA treatment of EC to knock down the source proteins. High amounts of two peptides, PTRF(56-64) and CD59(106-114), were consistently detected in EC. This predominance of two endothelial peptides was explained by cell type-specific source protein expression that compensated for poor HLA-A*02 binding affinity and short half-live of peptide/HLA-A*02 complexes. Knocking down the source proteins containing the abundant endothelial peptide motifs led to a nearly 100-fold increase of surface expression of SMCY(311-319), an immunodominant minor histocompatibility Ag, as detected by cytotoxicity assays using SMCY(311-319)-specific CTL. We conclude that EC express and present preferentially two distinct HLA-A*02-restricted peptides at extraordinary high levels. These abundant self-peptides may protect EC from CTL-mediated lysis by competing for HLA-A*02 binding sites with immunodominant scarcely expressed antigenic peptides.

Caveolin-1, caveolae, and glioblastoma

Glioblastoma multiforme (GBM) is the most common malignant brain tumor and is characterized by high invasiveness, poor prognosis, and limited therapeutic options. Biochemical and morphological experiments have shown the presence of caveolae in glioblastoma cells. Caveolae are flask-shaped plasma membrane subdomains that play trafficking, mechanosensing, and signaling roles. Caveolin-1 is a membrane protein that participates in the formation of caveolae and binds a multitude of signaling proteins, compartmentalizing them in caveolae and often directly regulating their activity via binding to its scaffolding domain. Caveolin-1 has been proposed to behave either as a tumor suppressor or as an ongogene depending on the tumor type and progress. This review discusses the existing information on the expression and function of caveolin-1 and caveolae in GBM and the role of this organelle and its defining protein on cellular signaling, growth, and invasiveness of GBM. We further analyze the available data suggesting caveolin-1 could be a target in GBM therapy.

Endocytosis and signaling: cell logistics shape the eukaryotic cell plan

Our understanding of endocytosis has evolved remarkably in little more than a decade. This is the result not only of advances in our knowledge of its molecular and biological workings, but also of a true paradigm shift in our understanding of what really constitutes endocytosis and of its role in homeostasis. Although endocytosis was initially discovered and studied as a relatively simple process to transport molecules across the plasma membrane, it was subsequently found to be inextricably linked with almost all aspects of cellular signaling. This led to the notion that endocytosis is actually the master organizer of cellular signaling, providing the cell with understandable messages that have been resolved in space and time. In essence, endocytosis provides the communications and supply routes (the logistics) of the cell. Although this may seem revolutionary, it is still likely to be only a small part of the entire story. A wealth of new evidence is uncovering the surprisingly pervasive nature of endocytosis in essentially all aspects of cellular regulation. In addition, many newly discovered functions of endocytic proteins are not immediately interpretable within the classical view of endocytosis. A possible framework, to rationalize all this new knowledge, requires us to "upgrade" our vision of endocytosis. By combining the analysis of biochemical, biological, and evolutionary evidence, we propose herein that endocytosis constitutes one of the major enabling conditions that in the history of life permitted the development of a higher level of organization, leading to the actuation of the eukaryotic cell plan.

Lipid rafts, caveolae and GPI-linked proteins

Lipid rafts and caveolae are specialized membrane microdomains enriched in sphingolipids and cholesterol. They function in a variety of cellular processes including but not limited to endocytosis, transcytosis, signal transduction and receptor recycling. Here, we outline the similarities and differences between lipid rafts and caveolae as well as discuss important components and functions of each.

Epigenetic alteration of PRKCDBP in colorectal cancers and its implication in tumor cell resistance to TNFα-induced apoptosis

PURPOSE

PRKCDBP is a putative tumor suppressor in which alteration has been observed in several human cancers. We investigated expression and function of PRKCDBP in colorectal cells and tissues to explore its candidacy as a suppressor in colorectal tumorigenesis.

EXPERIMENTAL DESIGN

Expression and methylation status of PRKCDBP and its effect on tumor growth were evaluated. Transcriptional regulation by NF-κB signaling was defined by luciferase reporter and chromatin immunoprecipitation assays.

RESULTS

PRKCDBP expression was hardly detectable in 29 of 80 (36%) primary tumors and 11 of 19 (58%) cell lines, and its alteration correlated with tumor stage and grade. Promoter hypermethylation was commonly found in cancers. PRKCDBP expression induced the G(1) cell-cycle arrest and increased cellular sensitivity to various apoptotic stresses. PRKCDBP was induced by TNFα, and its level correlated with tumor cell sensitivity to TNFα-induced apoptosis. PRKCDBP induction by TNFα was disrupted by blocking NF-κB signaling while it was enhanced by RelA transfection. The PRKCDBP promoter activity was increased in response to TNFα, and this response was abolished by disruption of a κB site in the promoter. PRKCDBP delayed the formation and growth of xenograft tumors and improved tumor response to TNFα-induced apoptosis.

CONCLUSIONS

PRKCDBP is a proapoptotic tumor suppressor which is commonly altered in colorectal cancer by promoter hypermethylation, and its gene transcription is directly activated by NF-κB in response to TNFα. This suggests that PRKCDBP inactivation may contribute to tumor progression by reducing cellular sensitivity to TNFα and other stresses, particularly under chronic inflammatory microenvironment.

Polymerase I and transcript release factor (PTRF)/cavin-1 is a novel regulator of stress-induced premature senescence

According to the "free radical theory" of aging, premature senescence induced by oxidative stress contributes to organismal aging. Polymerase I and transcript release factor (PTRF)/cavin-1 is a structural protein component of caveolae, invaginations of the plasma membrane involved in signal transduction. We show that oxidative stress up-regulates PTRF/cavin-1 protein expression and promotes the interaction between PTRF/cavin-1 and caveolin-1, another structural protein component of caveolae. Consistent with these data, the number of caveolae is dramatically increased in cells subjected to oxidative stress. We demonstrate that down-regulation of PTRF/cavin-1 by shRNA significantly inhibits oxidative stress-induced premature senescence. Mechanistically, we found that PTRF/cavin-1 expression is necessary for the oxidant-induced sequestration of Mdm2, a negative regulator of p53, into caveolar membranes, away from p53, and activation of the p53/p21(Waf1/Cip1) pathway. Expression of a mutant form of PTRF/cavin-1, which fails to localize to caveolar membranes after oxidative stress, inhibits oxidative stress-induced activation of p53 and induction of premature senescence. Thus, PTRF/cavin-1 is a novel regulator of oxidative stress-induced premature senescence by acting as a link between free radicals and activation of the p53/p21(Waf1/Cip1) pathway.

Regulation of cellular senescence by the essential caveolar component PTRF/Cavin-1

Polymerase I and transcript release factor (PTRF, also known as Cavin-1) is an essential component in the biogenesis and function of caveolae. Here, we show that PTRF expression is increased in senescent human fibroblasts. Importantly, overexpression of PTRF induced features characteristic of cellular senescence, whereas reduced PTRF expression extended the cellular replicative lifespan. Interestingly, we found that PTRF localized primarily to the nuclei of young and quiescent WI-38 human fibroblasts, but translocated to the cytosol and plasma membrane during cellular senescence. Furthermore, electron microscopic analysis demonstrated an increased number of caveolar structures in senescent and PTRF-transfected WI-38 cells. Our data suggest that the role of PTRF in cellular senescence is dependent on its targeting to caveolae and its interaction with caveolin-1, which appeared to be regulated by the phosphorylation of PTRF. Taken together, our findings identify PTRF as a novel regulator of cellular senescence that acts through the p53/p21 and caveolar pathways.

Polymerase transcriptase release factor (PTRF) anchors MG53 protein to cell injury site for initiation of membrane repair

Plasma membrane repair is an essential process for maintenance of homeostasis at the cellular and tissue levels, whereas compromised repair capacity contributes to degenerative human diseases. Our recent studies show that MG53 is essential for muscle membrane repair, and defects in MG53 function are linked to muscular dystrophy and cardiac dysfunction. Here we report that polymerase I and transcript release factor (PTRF), a gene known to regulate caveolae membrane structure, is an indispensable component of the membrane repair machinery. PTRF acts as a docking protein for MG53 during membrane repair potentially by binding exposed membrane cholesterol at the injury site. Cells lacking expression of endogenous PTRF show defective trafficking of MG53 to membrane injury sites. A mutation in PTRF associated with human disease results in aberrant nuclear localization of PTRF and disrupts MG53 function in membrane resealing. Although RNAi silencing of PTRF leads to defective muscle membrane repair, overexpression of PTRF can rescue membrane repair defects in dystrophic muscle. Our data suggest that membrane-delimited interaction between MG53 and PTRF contributes to initiation of cell membrane repair, which can be an attractive target for treatment or prevention of tissue injury in human diseases.

Reverse engineering gene network identifies new dysferlin-interacting proteins

Dysferlin (DYSF) is a type II transmembrane protein implicated in surface membrane repair of muscle. Mutations in dysferlin lead to Limb Girdle Muscular Dystrophy 2B (LGMD2B), Miyoshi Myopathy (MM), and Distal Myopathy with Anterior Tibialis onset (DMAT). The DYSF protein complex is not well understood, and only a few protein-binding partners have been identified thus far. To increase the set of interacting protein partners for DYSF we recovered a list of predicted interacting protein through a systems biology approach. The predictions are part of a "reverse-engineered" genome-wide human gene regulatory network obtained from experimental data by computational analysis. The reverse-engineering algorithm behind the analysis relates genes to each other based on changes in their expression patterns. DYSF and AHNAK were used to query the system and extract lists of potential interacting proteins. Among the 32 predictions the two genes share, we validated the physical interaction between DYSF protein with moesin (MSN) and polymerase I and transcript release factor (PTRF) in mouse heart lysate, thus identifying two novel Dysferlin-interacting proteins. Our strategy could be useful to clarify Dysferlin function in intracellular vesicles and its implication in muscle membrane resealing.

Caveolae and lipid trafficking in adipocytes

The abundance of caveolae in adipocytes suggests a possible cell-specific role for these structures, and because these cells take up and release fatty acids as their quantitatively most robust activity, modulation of fatty acid movement is one such role that is supported by substantial in vitro and in vivo data. In addition, caveolae are particularly rich in cholesterol and sphingolipids, and indeed, fat cells harbor more cholesterol than any other tissue. In this article, we review the role of adipocyte caveolae with regard to these important lipid classes.

IGF-IR internalizes with Caveolin-1 and PTRF/Cavin in HaCat cells

BACKGROUND

Insulin-like growth factor-I receptor (IGF-IR) is a tyrosine kinase receptor (RTK) associated with caveolae, invaginations of the plasma membrane that regulate vesicular transport, endocytosis and intracellular signaling. IGF-IR internalization represents a key mechanism of down-modulation of receptors number on plasma membrane. IGF-IR interacts directly with Caveolin-1 (Cav-1), the most relevant protein of caveolae. Recently it has been demonstrated that the Polymerase I and Transcript Release Factor I (PTRF/Cavin) is required for caveolae biogenesis and function. The role of Cav-1 and PTRF/Cavin in IGF-IR internalization is still to be clarified.

METHODOLOGY/PRINCIPAL FINDINGS

We have investigated the interaction of IGF-IR with Cav-1 and PTRF/Cavin in the presence of IGF1in human Hacat cells. We show that IGF-IR internalization triggers Cav-1 and PTRF/Cavin translocation from plasma membrane to cytosol and increases IGF-IR interaction with these proteins. In fact, Cav-1 and PTRF/Cavin co-immunoprecipitate with IGF-IR during receptor internalization. We found a different time course of co-immunoprecipitation between IGF-IR and Cav-1 compared to IGF-IR and PTRF/Cavin. Cav-1 and PTRF/Cavin silencing by siRNA differently affect surface IGF-IR levels following IGF1 treatment: Cav-1 and PTRF/Cavin silencing significantly affect IGF-IR rate of internalization, while PTRF/Cavin silencing also decreases IGF-IR plasma membrane recovery. Since Cav-1 phosphorylation could have a role in IGF-IR internalization, the mutant Cav-1Y14F lacking Tyr14 was transfected. Cav-1Y14F transfected cells showed a reduced internalization of IGF-IR compared with cells expressing wild type Cav-1. Receptor internalization was not impaired by Clathrin silencing. These findings support a critical role of caveolae in IGF-IR intracellular traveling.

CONCLUSIONS/SIGNIFICANCE

These data indicate that Caveolae play a role in IGF-IR internalization. Based on these findings, Cav-1 and PTRF/Cavin could represent two relevant and distinct targets to modulate IGF-IR function.

Quantitative proteomics of caveolin-1-regulated proteins: characterization of polymerase i and transcript release factor/CAVIN-1 IN endothelial cells

Caveolae are organelles abundant in the plasma membrane of many specialized cells including endothelial cells (ECs), epithelial cells, and adipocytes, and in these cells, caveolin-1 (Cav-1) is the major coat protein essential for the formation of caveolae. To identify proteins that require Cav-1 for stable incorporation into membrane raft domains, a quantitative proteomics analysis using isobaric tagging for relative and absolute quantification was performed on rafts isolated from wild-type and Cav-1-deficient mice. In three independent experiments, 117 proteins were consistently identified in membrane rafts with the largest differences in the levels of Cav-2 and in the caveola regulatory proteins Cavin-1 and Cavin-2. Because the lung is highly enriched in ECs, we validated and characterized the role of the newly described protein Cavin-1 in several cardiovascular tissues and in ECs. Cavin-1 was highly expressed in ECs lining blood vessels and in cultured ECs. Knockdown of Cavin-1 reduced the levels of Cav-1 and -2 and weakly influenced the formation of high molecular weight oligomers containing Cav-1 and -2. Cavin-1 silencing enhanced basal nitric oxide release from ECs but blocked proangiogenic phenotypes such as EC proliferation, migration, and morphogenesis in vitro. Thus, these data support an important role of Cavin-1 as a regulator of caveola function in ECs.

Caveolae, caveolins, and cavins: complex control of cellular signalling and inflammation

Caveolae are specialized lipid rafts that form flask-shaped invaginations of the plasma membrane. They are involved in cell signalling and transport and have been shown critically regulate vascular reactivity and blood pressure. The organization and functions of caveolae are mediated by coat proteins (caveolins) and support or adapter proteins (cavins). The caveolins, caveolin-1, -2, and -3, form the structural backbone of caveolae. These proteins are also highly integrated into caveolae function and have their own activity independent of caveolae. The cavins, cavins 1-4, are involved in regulation of caveolae and modulate the function of caveolins by promoting the membrane remodelling and trafficking of caveolin-derived structures. The relationships between these different proteins are complex and intersect with many aspects of cell function. Caveolae have also been implicated in chronic inflammatory conditions and other pathologies including atherosclerosis, inflammatory bowel disease, muscular dystrophy, and generalized dyslipidaemia. The pathogenic role of the caveolins is an emerging area, however, the roles of cavins in disease is just beginning to be explored. This review will examine the relationship between caveolins and cavins and explore the role of caveolae in inflammatory signalling mechanisms.

Cavin proteins: New players in the caveolae field

Caveolae are specialized lipid microdomains, forming small invaginations in the plasma membrane, known to be implicated in multiple functions including lipid storage, cell signaling and endocytosis. Formation of these wide flask-shaped invaginations is dependent on the expression of a caveolar coat protein, namely caveolin. Until now, the accepted paradigm was that caveolin was the sole and only structural protein of caveolae since its expression was necessary and sufficient to drive caveolae biogenesis. The recent characterizations of PTRF/cavin-1 and subsequently other cavin family members in caveolae formation have highlighted additional levels of complexity in the biogenesis of these plasma membrane invaginations. In this review, recent advances on the role of the different cavin family members in the regulation of caveolae structures as well as potential new functions will be discussed.

Exploring the caves: cavins, caveolins and caveolae

Caveolae are ampullate (flask-shaped) invaginations that are abundant in the plasma membrane of many mammalian cell types. Although caveolae are implicated in a wide range of processes including endothelial transcytosis, lipid homeostasis and cellular signalling, a detailed molecular picture of many aspects of their function has been elusive. Until recently, the only extensively characterised protein components of caveolae were the caveolins. Recently, data from several laboratories have demonstrated that a family of four related proteins, termed cavins 1-4, plays key roles in caveolar biogenesis and function. Salient properties of the cavin family include their propensity to form complexes with each other and their different but overlapping tissue distribution. This review summarises recent data on the cavins, and sets them in the context of open questions on the construction and function of caveolae. The discovery of cavins implies that caveolae might have unexpectedly diverse structural properties, in accord with the wide range of functions attributed to these 'little caves'.

MURC/Cavin-4 and cavin family members form tissue-specific caveolar complexes

Polymerase I and transcript release factor (PTRF)/Cavin is a cytoplasmic protein whose expression is obligatory for caveola formation. Using biochemistry and fluorescence resonance energy transfer–based approaches, we now show that a family of related proteins, PTRF/Cavin-1, serum deprivation response (SDR)/Cavin-2, SDR-related gene product that binds to C kinase (SRBC)/Cavin-3, and muscle-restricted coiled-coil protein (MURC)/Cavin-4, forms a multiprotein complex that associates with caveolae. This complex can constitutively assemble in the cytosol and associate with caveolin at plasma membrane caveolae. Cavin-1, but not other cavins, can induce caveola formation in a heterologous system and is required for the recruitment of the cavin complex to caveolae. The tissue-restricted expression of cavins suggests that caveolae may perform tissue-specific functions regulated by the composition of the cavin complex. Cavin-4 is expressed predominantly in muscle, and its distribution is perturbed in human muscle disease associated with Caveolin-3 dysfunction, identifying Cavin-4 as a novel muscle disease candidate caveolar protein.

Transcription termination by nuclear RNA polymerases

Gene transcription in the cell nucleus is a complex and highly regulated process. Transcription in eukaryotes requires three distinct RNA polymerases, each of which employs its own mechanisms for initiation, elongation, and termination. Termination mechanisms vary considerably, ranging from relatively simple to exceptionally complex. In this review, we describe the present state of knowledge on how each of the three RNA polymerases terminates and how mechanisms are conserved, or vary, from yeast to human.

SRBC/cavin-3 is a caveolin adapter protein that regulates caveolae function

Caveolae are a major membrane domain common to most cells. One of the defining features of this domain is the protein caveolin. The exact function of caveolin, however, is not clear. One possible function is to attract adapter molecules to caveolae in a manner similar to how clathrin attracts molecules to coated pits. Here, we characterize a candidate adapter molecule called SRBC. SRBC binds PKCdelta and is a member of the STICK (substrates that interact with C-kinase) superfamily of PKC-binding proteins. We also show it co-immunoprecipitates with caveolin-1. A leucine zipper in SRBC is essential for both co-precipitation with caveolin and localization to caveolae. SRBC remains associated with caveolin when caveolae bud to form vesicles (cavicles) that travel on microtubules to different regions of the cell. In the absence of SRBC, intracellular cavicle traffic is markedly impaired. We conclude that SRBC (sdr-related gene product that binds to c-kinase) and two other family members [PTRF (Pol I and transcription release factor) and SDPR] function as caveolin adapter molecules that regulate caveolae function.

Deletion of Cavin/PTRF causes global loss of caveolae, dyslipidemia, and glucose intolerance

Caveolae are specialized invaginations of the plasma membrane found in numerous cell types. They have been implicated as playing a role in a variety of physiological processes and are typically characterized by their association with the caveolin family of proteins. We show here by means of targeted gene disruption in mice that a distinct caveolae-associated protein, Cavin/PTRF, is an essential component of caveolae. Animals lacking Cavin have no morphologically detectable caveolae in any cell type examined and have markedly diminished protein expression of all three caveolin isoforms while retaining normal or above normal caveolin mRNA expression. Cavin-knockout mice are viable and of normal weight but have higher circulating triglyceride levels, significantly reduced adipose tissue mass, glucose intolerance, and hyperinsulinemia--characteristics that constitute a lipodystrophic phenotype. Our results underscore the multiorgan role of caveolae in metabolic regulation and the obligate presence of Cavin for caveolae formation.

Budding yeast RNA polymerases I and II employ parallel mechanisms of transcriptional termination

Both RNA polymerase I and II (Pol I and Pol II) in budding yeast employ a functionally homologous "torpedo-like" mechanism to promote transcriptional termination. For two well-defined Pol II-transcribed genes, CYC1 and PMA1, we demonstrate that both Rat1p exonuclease and Sen1p helicase are required for efficient termination by promoting degradation of the nascent transcript associated with Pol II, following mRNA 3' end processing. Similarly, Pol I termination relies on prior Rnt1p cleavage at the 3' end of the pre-rRNA 35S transcript. This is followed by the combined actions of Rat1p and Sen1p to degrade the Pol I-associated nascent transcript that consequently promote termination in the downstream rDNA spacer sequence. Our data suggest that the previously defined in vitro Pol I termination mechanism involving the action of the Reb1p DNA-binding factor to "road-block" Pol I transcription close to the termination region may have overlooked more complex in vivo molecular processes.

PTRF triggers a cave in

Caveolae are small membrane invaginations important for cell signaling that are characterized by the presence of caveolin proteins. Hill et al. (2008) have now identified PTRF as a new constituent of the caveolar coat. In the absence of PTRF, caveolae flatten and caveolin-1 is released into the cell membrane, where it is rapidly internalized and degraded.

Caveolae structure and function

Studies on the structure and function of caveolae have revealed how this versatile subcellular organelle can influence numerous signalling pathways. This brief review will discuss a few of the key features of caveolae as it relates to signalling and disease processes.

A critical role of cavin (polymerase I and transcript release factor) in caveolae formation and organization

Cavin (PTRF) has been shown to be a highly abundant protein component of caveolae, but its functional role there is unknown. Here, we confirm that cavin co-localizes with caveolin-1 in adipocytes by confocal microscopy and co-distributes with caveolin-1 in lipid raft fractions by sucrose gradient flotation. However, cavin does not directly associate with caveolin-1 as solubilization of caveolae disrupts their interaction. Cholesterol depletion with beta-cyclodextrin causes a significant down-regulation of cavin from plasma membrane lipid raft fractions. Overexpression of cavin in HEK293-Cav-1 cells and knockdown of cavin in 3T3-L1 adipocytes enhances and diminishes caveolin-1 levels, respectively, indicating an important role for cavin in maintaining the level of caveolin-1. A truncated form of cavin, eGFP-cavin-1-322, which lacks 74 amino acids from the C-terminal, reveals a microtubular network localization by confocal microscopy. Disruption of cytoskeletal elements with latrunculin B or nocodazole diminishes cavin expression without affecting the caveolin-1 amount. We propose that the presence of cavin on the inside surface of caveolae stabilizes these structures, probably through interaction with the cytoskeleton, and cavin therefore plays an important role in caveolae formation and organization.

Cellular spelunking: exploring adipocyte caveolae

It has been known for decades that the adipocyte cell surface is particularly rich in small invaginations we now know to be caveolae. These structures are common to many cell types but are not ubiquitous. They have generated considerable curiosity, as manifested by the numerous publications on the topic that describe various, sometimes contradictory, caveolae functions. Here, we review the field from an "adipocentric" point of view and suggest that caveolae may have a function of particular use for the fat cell, namely the modulation of fatty acid flux across the plasma membrane. Other functions for adipocyte caveolae that have been postulated include participation in signal transduction and membrane trafficking pathways, and it will require further experimental scrutiny to resolve controversies surrounding these possible activities.

Cell-type-specific regulation of RNA polymerase I transcription: a new frontier

Ribosomal RNA transcription was one of the first model systems for molecular characterization of a transcription regulatory mechanism and certainly one of the best studied in the widest range of organisms. In multicellular organisms, however, the issue of cell-type-specific regulation of rRNA transcription has not been well addressed. Here I propose that a systematic study of cell-type-specific regulation of rRNA transcription may reveal new regulatory mechanisms that have not been previously realized. Specifically, issues concerning the cell-type-specific requirement for rRNA production, the universality of Pol I transcription complex and the division of rDNA into regulatory subdomains are discussed.

Association and insulin regulated translocation of hormone-sensitive lipase with PTRF

Polymerase I and transcript release factor (PTRF) is in human adipocytes mainly localized at the plasma membrane. This localization was under control of insulin, which translocated PTRF to the cytosol and nucleus, indicating a novel role for PTRF in insulin transcriptional control. In the plasma membrane PTRF was specifically bound to a triacylglycerol-metabolizing subclass of caveolae containing hormone-sensitive lipase (HSL). In response to insulin PTRF was translocated to the cytosol in parallel with HSL. PTRF and HSL were quantitatively immunoprecipitated from the cytosol by antibodies against either PTRF or HSL. The findings indicate also a novel extranuclear function for PTRF in the control of lipolysis.

A subcomplex of RNA polymerase III subunits involved in transcription termination and reinitiation

While initiation of transcription by RNA polymerase III (Pol III) has been thoroughly investigated, molecular mechanisms driving transcription termination remain poorly understood. Here we describe how the characterization of the in vitro transcriptional properties of a Pol III variant (Pol IIIdelta), lacking the C11, C37, and C53 subunits, revealed crucial information about the mechanisms of Pol III termination and reinitiation. The specific requirement for the C37-C53 complex in terminator recognition was determined. This complex was demonstrated to slow down elongation by the enzyme, adding to the evidence implicating the elongation rate as a critical determinant of correct terminator recognition. In addition, the presence of the C37-C53 complex required the simultaneous addition of C11 to Pol IIIdelta for the enzyme to reinitiate after the first round of transcription, thus uncovering a role for polymerase subunits in the facilitated recycling process. Interestingly, we demonstrated that the role of C11 in recycling was independent of its role in RNA cleavage. The data presented allowed us to propose a model of Pol III termination and its links to reinitiation.