Caveolae and the caveolins in human disease

Advanced Drug Delivery Reviews 49 (2001) 325–335www.elsevier.com/ locate /drugdeliv

Caveolae and the caveolins in human diseasea , a b*Lee Campbell , Mark Gumbleton , Kenneth Ritchie

aPharmaceutical Cell Biology, Welsh School of Pharmacy, Cardiff University, Cardiff CF10 3XF, UKbDepartment of Molecular and Experimental Medicine, The Scripps Research Institute, La Jolla, CA 92037, USA

Received 15 January 2001; accepted 3 April 2001

Abstract

There has been an exponential growth in caveolae research since the early 1990s. The caveolae membrane systemcomprises unique lipid and protein domains, and fulfills a role in a wide range of processes. At the plasma membranecaveolae serve to compartmentalise and integrate a wide range of signal transduction processes. A key structural andfunctional protein for caveolae is caveolin. Caveolin proteins possess a ‘scaffolding’ domain which for caveolins-1 and -3appear central to many of the reported signal regulation functions for caveolae. Caveolae or caveolin protein are increasinglyimplicated in the molecular pathology of a number of diseases. Opportunities exist for basic and applied investigatorsworking within the pharmaceutical sciences to exploit the caveolae membrane system to identify novel pharmacologicaltargets and therapeutic strategies, including the delivery of pharmacologically active caveolin based peptides. 2001Elsevier Science B.V. All rights reserved.

Keywords: Caveolin; Caveolae; Disease; Cancer; Atherosclerosis; Alzheimer’s disease; Diabetes; Muscular dystrophy; Inflammation

Contents

1. Introduction ............................................................................................................................................................................ 3252. Muscular dystrophy ................................................................................................................................................................. 3263. Cancer.................................................................................................................................................................................... 3274. Diabetes ................................................................................................................................................................................. 3295. Atherosclerosis ....................................................................................................................................................................... 3306. Inflammation .......................................................................................................................................................................... 3317. Alzheimer’s disease................................................................................................................................................................. 3318. Conclusion ............................................................................................................................................................................. 332References .................................................................................................................................................................................. 332

1. Introduction

First identified as flask shaped structures conspicu-ously positioned at the plasma membrane of endo-*Corresponding author. Tel.: 1 44-029-2087-5449; fax: 1 44-thelial cells [1], caveolae have traditionally been029-2087-5449.

E-mail address: [email protected] (L. Campbell). viewed as specialised transport organelles facilitating

0169-409X/01/$ – see front matter 2001 Elsevier Science B.V. All rights reserved.PI I : S0169-409X( 01 )00145-4

326 L. Campbell et al. / Advanced Drug Delivery Reviews 49 (2001) 325 –335

the cellular uptake of molecules from the external in the number, but a decrease in the size, of caveolaemilieu in a variety of cell types. Whilst this is compared to control samples, suggesting a relation-certainly the case in some instances (see article by ship between the disease state and the biophysicalJan Schnitzer in this series) the subsequent identifica- properties of caveolae [5].tion of caveolin-1 has led to more profound discov- Caveolin-3 represents the muscle specific membereries in the field of cell biology and biochemistry. of the caveolin super-gene family [6] and has beenFor example, caveolae are shown to compartmen- demonstrated to co-immunoprecipitate withtalise a diverse range of transduction signalling dystrophin, suggesting a biochemical associationmolecules, and in the particular case of caveolin-1 between the two [7]. Recently, however, it has beenserve as a general inhibitor of plasma membrane revealed that caveolin-3 competes with dystrophinsignalling cascades (see article of Jacques Couet et for a binding site within the C-terminus of theal. in this series). Despite the major progress toward b-dystroglycan molecule [8], the latter an additionalthe understanding of the fundamental functions of important functional subunit of the DGC. By impli-the caveolin protein family, their possible role in the cation caveolin-3 may serve to regulate the inter-aetiology of human disease is only now starting to action of dystrophin and b-dystroglycan within thisemerge. Several groups working independently on complex. Analysis using quantitative immuno-quite different aspects of human pathology have flouresence and Western blot procedures has con-provided breakthroughs in the understanding of how firmed the up-regulation of caveolin-3 at the level ofcaveolins are regulated and function in a broad the sarcolemma in biopsy tissue resected from DMDspectrum of disease states [2,3]. patients [9]. Closely following this, skeletal muscle

In this manuscript an overview of what is current- was found to be the only tissue type that presentedly known about caveolae and the caveolins with with any clear pathology in transgenic mice that hadrespect to non-infectious human disease is provided, been bred specifically to overexpress caveolin-3,and how this knowledge may predict likely targets despite the broad overexpression of this protein infor the alleviation of the clinical problem. other organ systems [10]. These mice exhibited

several distinctive features associated with the DMDphenotype. They displayed an abnormal gait accom-

2. Muscular dystrophy panied by marked muscle hypertrophy and elevatedserum creatinine, the latter a biochemical marker

Duchenne Muscular dystrophy (DMD) is the most used to clinically assess the degree of musclecommon and severest form of muscular dystrophy. degeneration. Additionally, an important observationMuscle wastage, which initially affects the pelvis, at the molecular level was made relating to theupper arms and upper legs, is rapid and eventually down-regulation of both dystrophin and b-involves all voluntary muscles with survival rarely dystroglycan in the skeletal muscles of these mice inbeyond the age of 30 years. DMD almost exclusively response to the elevated caveolin-3 levels [10].affects males reflecting its sex-linked recessive pat- Collectively, the above studies provide evidence thattern of inheritance. This genetic defect on the X- the up-regulation of caveolin-3 is an importantchromosome manifests itself at a molecular level by manifestation in DMD, however, further research isthe absence or mutant production of dystrophin, the required at the molecular level in order to understandprotein product of the Duchenne muscular dystrophy the exact mechanisms by which caveolin-3 contri-gene [4]. Dystrophin is a glycoprotein that forms a butes directly, if at all, to DMD progression.multi-subunit complex (termed the dystrophin– It has been postulated that the increased expres-glycoprotein complex or DGC) which spans the sion of caveolin-3 may reflect muscle regenerationmuscle plasma membrane (sarcolemma) forming a and differentiation in the DMD phenotype [11].link between cytoplasmic actin, the sarcolemma and Conversely, one could hypothesise that the aberrantthe extracellular matrix of muscle. Early freeze– expression of caveolin-3 in DMD may contribute tofracture studies of skeletal muscle plasma mem- muscle atrophy by potentiating apoptotic activitybranes from patients with DMD showed an increase within skeletal myocytes, a definite pathological

L. Campbell et al. / Advanced Drug Delivery Reviews 49 (2001) 325 –335 327

feature in this particular type of dystrophy [12]. type caveolin-3 in the presence of LGMD caveolin-3Support for this latter view may be derived from co-expression [18]. These findings may have impli-studies conducted on scalded mice [13]. For exam- cations in the design of future therapeutic regimensple, it appeared that caveolae in the muscle fibres far for the treatment of autosomal dominant LGMD.removed from the site of the actual burn, serve as In muscular dystrophy patients of unknown ge-centres for the initiation of apoptotic signals that netic etiology, but who phenotypically exhibit classi-ultimately lead to muscle degeneration (a significant cal symptoms of Duchenne/Becker or the limb–clinical complication in the management of burn girdle muscular dystrophy’s, mutations in the cyto-cases). It can be envisaged that similar apopototic plasmic domains of the caveolin-3 molecule haveactivity originating from caveolar domains in also been observed [15]. It may be of significancedystrophic muscle may contribute to the characteris- that these mutations lie within a sequence of thetic muscle degeneration observed in DMD. caveolin-3 protein which has previously been impli-

Recently, molecular evidence from patients suffer- cated in inhibiting neuronal nitric oxide synthaseing from two distinct forms of muscular dystrophy activity in skeletal muscle [19].has given further credence to a role for caveolin-3 in The current focus for drug delivery scientists hasthe muscle dystrophinopathies [14,15]. Patients suf- been to devise methods for the effective delivery offering from an autosomal dominant form of limb– recombinant DNA constructs of the dystrophin genegirdle muscular dystrophy (LGMD) have been found or truncated forms thereof, in order that the sustainedto have reduced expression of caveolin-3 [14]. expression of recombinant dystrophin may overcomeGenetic analysis of these families revealed two some of the clinical symptoms of this inheritedseparate mutations within the caveolin-3 gene, which disorder. Given that caveolae in skeletal muscle cellsresides at the 3p25 locus on chromosome 3 [16]. A demonstrate a propensity for the uptake of adminis-pinpoint mutation in which leucine is replaced by a tered naked DNA [20], it is tempting to speculateproline residue, was identified within the membrane that if the increased number of caveolae present inspanning region of the caveolin-3 molecule, and was DMD were functional in respect to internalisationsufficient to reduce caveolin-3 protein level by as then it may be they could be exploited in genemuch as 90% in the skeletal muscle of affected therapy delivery strategies.individuals [14]. Similarly, a microdeletion of ninebases within the caveolin scaffolding domain wasfound in these families, which had led to decreased 3. Cancercaveolin-3 protein levels and an identical clinicaloutcome [14]. Both of these mutated forms of Stringent control of proliferation and differentia-LGMD caveolin-3 have been shown to cause the tion are two fundamental cellular processes, and areretention of wild type caveolin-3 within the perinu- central to both normal tissue development andclear compartment of cells in vitro, i.e. in recombi- homeostasis. Identification of a whole plethora ofnant cell lines wild type caveolin-3 does not migrate key cell cycle regulatory elements, namely the cyclinto the sarcolemma when LGMD caveolin-3 is co- dependent kinases and their respective inhibitors, hasexpressed [17]. This latter finding represents a greatly enhanced our understanding of the growthpossible explanation to the autosomal dominant regulatory pathways that exist in the cell [21]. It ismode of inheritance of this particular muscular understood that perturbations to such pathways is adystrophy, i.e. both genes, mutated and normal, for pre-requisite for tumourgenesis and malignant dis-caveolin-3 can be expressed but functionally the ease. Recently, attention has focused on caveolin-1normal product is still compromised. An observation and its putative role as a modulator of cellularthat is yet to be fully explained relates to an transformation. Since caveolin-1 and caveolae areapproximate 6-fold decrease in the half life of the up-regulated in cells that have undergone terminalmutated forms of caveolin-3 (compared to the wild differentiation or contact inhibition [22], it has beentype) and the effect of proteosomal inhibitors which postulated that both play a critical role in theappear to restore a sarcolemma localisation of wild mediation of such events. Early studies reported the


loss of caveolin and caveolae in fibroblasts that had NIH 3T3 fibroblasts harbouring the caveolin-1 anti-been transfected with v-alb, bcr–alb and H-ras sense vector. Additionally, a reciprocal relationshiponcogenes [23]. The oncogenically transformed cells, between the activation state of the p42/44 MAPunlike their wild type counterparts, demonstrate an kinase cascade and caveolin-1 levels was observedability to form colonies when grown in soft agar. [34], indicating that the loss of caveolin can selec-Additionally, the size of the colonies formed showed tively effect signalling pathways. This latter result isa strong inverse correlation with the reduction in consistent with parallel studies showing the inhibi-cellular caveolin-1 levels. Although the study did not tion of signalling molecules along the p42/44 MAPprovide a direct association between caveolin and kinase cascade mediated by binding of MAP kinaseoncogenic transformation, the results obtained clear- components to the caveolin-1 scaffolding domainly advocate a role for caveolae and caveolin in the [35].modulation of cell proliferation and contact inhibi- A similar negative regulation by caveolin-1 hastion. Caveolin-1 and -2 forms of the gene family been reported for the Neu tyrosine kinase pathwayhave been co-localised to the 6-A2/7q31 and 7q31.1 [35]. Neu is a proto-oncogene product whose activa-sub-chromosomal loci in the mouse [24] and in man tion is implicated in oncogenic transformation of[25–28], respectively. The former chromosomal cultured fibroblasts and breast tumor-genesis, andlocation is a known tumour suppresser locus in mice, whose activation is associated with a poor prognosiswhile the latter is a known fragile site that is in breast cancer patients. Engelman and co-workersfrequently deleted in a wide spectrum of human [35] reported that in-vitro mutational activation ofcancers, including squamous cell carcinomas, Neu tyrosine kinase pathway results in a concomitantovarian adenocarcinomas, prostate and breast can- down-regulation of caveolin-1, but not caveolin-2.cers. Indeed caveolin-1 has been identified as one of With expression levels of caveolin-1 dramaticallyover 20 candidate ‘tumor suppressor’ genes that are reduced in mammary tumors derived from c-Neudown-regulated in human breast cancer [29]. expressing transgenic mice. Conversely, recombinant

Down-regulation of caveolin-1 appears to be a over-expression of caveolin-1 blocked Neu-mediatedcommon feature in epithelial cells that originate from signal transduction in-vivo, an inhibitory effect iden-human cancers. Reductions in caveolin-1 levels tified as mediated through interaction of the catalyticmediated at the level of transcriptional regulation region of Neu tyrosine kinase with the caveolin-1have been observed in carcinoma cell lines derived scaffolding domain. The results imply that certainfrom lung [30], cervical [31] and breast [32] tissues, synthetic peptide sequences derived from thewith a strong correlation between tumorigenicity and caveolin-1 scaffolding domain may have some thera-caveolin-1 levels. Such data provides further support peutic value in the treatment of breast cancer [35].to the hypothesis that down-regulation of caveolin-1 Hypermethylation of CpG nucleotide islands in theremoves the ‘molecular brake’ on cell signalling promoter region of known tumour suppressor genes,events that drive cell transformation and prolifer- e.g. p16, is one mechanism by which genes areation. inactivated and specific cancers induced without the

The possibility that caveolin may represent a novel requirement of point mutations or gene deletion [36].tumour suppresser protein was further evaluated by The existence of CpG islands within the first andGalbiati and colleagues [33]. These workers under- second exons of both caveolin-1 and caveolin-2took an elaborate series of experiments utilising a genes has recently been reported [37]. Indeed CpGcaveolin-1 antisense vector system in NIH 3T3 islands located in the 59promoter region of thefibroblasts to specifically target and down-regulate caveolin-1 gene in MCF-7 and T-47D breast car-caveolin-1 protein. Their antisense experiments dem- cinoma cell lines are recognised to be methylatedonstrated that the loss of caveolin-1 was sufficient to [37], with the transcription of caveolin-1 in theconfer anchorage-independent growth and drive cel- above cancer cell lines repressed. Thus the caveolin-lular transformation. Tumour formation in immuno- 1 gene may represent a new target for novel methyla-deficient nude mice became apparent within 1–2 tion inhibitors whose subsequent reactivation ofweeks following the subcutaneous implantation of caveolin-1 could serve to suppress tumour growth.


Despite much research that lends support for This may be of significance since the re-occurrencecaveolin-1 serving as a tumour suppressor protein, of aggressive prostate cancer following radical pros-the reciprocal relationship between caveolin-1 ex- tatectomy, and the mortality rates, are higher inpression and cell transformation is not, however, by African–Americans than their white counterpartsany means unequivocal. Much of the current litera- [39]. However, differences in life style and otherture has been generated by the use of in-vitro socio-economic factors could not be discounted asoverexpression systems for caveolin-1 that are as- contributing factors to this increased virulence [39].sumed to mimic the function of endogenously ex- At the molecular level the targeted down-regula-pressed caveolin. In this respect Hurlstone and co- tion of caveolin-1 (anti-sense strategies) in severalworkers [28] have questioned the role for caveolin-1 androgen insensitive mouse prostate cell lines isas a tumour suppressor gene in breast tissue in vivo. shown to convert them to an androgen sensitiveIntriguingly, these studies utilised patient samples phenotype [40]. Thus, as expected the overexpres-and should be regarded as reflecting the ‘true’ in- sion of recombinant caveolin-1 in prostate cancervivo pathological situation. Employing standard im- cell lines renders them insensitive to c-myc inducedmunohistochemical techniques these investigators apoptosis upon androgen withdrawal [41]. Despitedetected caveolin-1 in myoepithelial and stromal the relationship existing between caveolin-1 expres-cells but not in the glandular epithelial cells of sion and prostate cancer progression, knowledge ofnormal breast tissue, the major cell type transformed the exact mechanisms by which caveolin-1 elicits thein many breast carcinomas. Given this information development of aggressive prostate cancer and an-the apparent overall loss of caveolin-1 in breast drogen insensitivity remains poor. Several unre-carcinomas may simply reflect the higher mass of solved issues remain, such as the impact of caveolinglandular epithelial cells, relative to the other on the AKT signalling pathway (an important inter-caveolin-1 positive cell types, as indeed one would mediate in prostate cancer progression) and thepredict with tumour progression. possible interaction of caveolin with the androgen

Another in vivo study utilising archival biopsy receptor.material has been documented that actually links theincreased expression of caveolin-1 with metastaticdisease, specifically the progression of prostatic 4. Diabetescancers [38]. For this reason the role of caveolin inprostate cancer progression merits special attention. Type II diabetes is generally characterised byImmunohistochemical staining in specimens derived impaired insulin secretion, however, other factorsfrom 189 prostate cancer patients having undergone such as insulin resistance and inappropriate hepaticradical prostatectomy indicate that caveolin-1 is glucose production are also seen. Additionally, it hasabsent in the glandular epithelium of normal prostate been recently proposed that type II diabetes may alsobut present in adjacent poorly differentiated prostate involve impaired glucose transport [42]. Beforecancer cells; caveolin-1 expression was found to be considerations upon how caveolae may be involvedcorrelated with a standard prostate cancer disease in various facets of type II diabetes, an overview ofmarker, the Gleason score [38]. Interestingly, the what is currently known about glucose transport withpattern of caveolin-1 staining in such cells assumes a respected to caveolae and caveolin is first needed.peculiar granular appearance within the cytoplasm as The insulin-stimulated glucose uptake which oc-opposed to its more traditional punctate appearance curs primarily in muscle, and to a lesser extentat the cell membrane seen in other cell types. The adipose tissue, is mediated by a glucose transporter,functional significance of this distribution in prostate GLUT4, which conveys glucose into the cell down acancer cells remains to be evaluated. Additionally, glucose concentration gradient [43]. This glucosecomparative studies have shown that caveolin-1 is transporter is predominantly localised to small vesi-the only biomarker significantly elevated (others cles and tubulo-vesicular structures within the cellscreened include p53 and bcl-2) in African–Ameri- cytoplasm, and translocates to the plasma membranecan but not in white American prostate cancers [39]. in response to insulin receptor signalling. Signifi-


cantly, the tissues (fat and skeletal muscle) primarily pancreas has recently been demonstrated to mediateresponsible for ‘storage-destined’ glucose uptake are insulin-independent glucose uptake in muscle andalso tissue types rich in caveolae. Jarett and co- adipose cells [53]. This insulin mimicry is shown toworkers [44] reported that adipocyte insulin recep- involve the assembly of a number of signallingtors are localised to caveolae, an observation con- molecules at the target membrane, includingsistent with the role of caveolin in potentiating caveolin. Thus given what is known about caveolininsulin receptor signalling (see also article of Couet and the modulation of glucose uptake at the cellularet al. in this series). Insulin stimulation not only level, it is not inconceivable to suggest suchresults in the relocation of GLUT4 vesicles to the caveolin-1 ‘scaffold domain peptides’ or peptideplasma membrane from intracellular stores, but also mimetics may in the future provide for novel thera-the mobilisation of caveolin from a cytosolic pool to peutic strategies in the management of type IIthe plasma membrane, where GLUT4 and caveolin diabetes. In particular those difficult cases whichco-localise [45–47]. Observations made in freshly present with insulin resistance.isolated adipocytes also suggest that the movementof GLUT4 to caveolin-rich domains is necessary forglucose uptake. Gustavsson and colleagues [46] used 5. Atherosclerosisrat epididymal adipocytes to link the long-standingdiscrepancy between the rapid increase of GLUT4 in Hypercholesterolemia is the elevation of serumthe membrane of insulin treated cells and the slower cholesterol levels above the normal physiologicalincrease in glucose uptake. This lag phase in glucose range, and is thought to be a causal factor in theuptake maybe due to GLUT4 vesicles arriving at the development of atherosclerosis. Chronic elevation ofplasma membrane in an occluded state, only becom- cholesterol levels inhibits endothelium-dependenting viable glucose transporters following movement vasodilation. Despite the exact nature of this endo-to plasma membrane caveolin rich domains [46]. thelial dysfunction remaining elusive, a decreased

As previously mentioned, insulin resistance and availability of the vasodilator nitric oxide (NO) isimpaired glucose uptake are independent risk factors thought to be involved. Caveolin-1 appears to exertfor the development of type II diabetes, although the an inhibitory effect on endothelial-derived nitrictwo are not mutually exclusive since insulin resist- oxide synthase (eNOS) [54,55], raising the possi-ance predominately concerns pertubations to insulin- bility of caveolin-mediated inhibition of eNOS instimulated glucose uptake mechanisms at the cellular hypercholesterolemia. Indeed, recent research sup-level [48]. In this context it is interesting to note that ports this hypothesis that decreased NO productionmutations within the caveolin-1 binding motif of the maybe attributed to the reduced activity of eNOS ininsulin receptor have been identified in patients with response to elevated caveolin-1 levels during theinsulin resistant diabetes [49,50]. Such genetic altera- hypercholesterolaemic state.tions would be expected to attenuate insulin sig- Exposure of bovine aortic endothelial cells tonalling given that interactions with caveolin, me- serum obtained from hypercholesterolaemic (HC)diated through it’s scaffolding domain, are consid- individuals results in the increased expression ofered necessary for enhancement of insulin signalling caveolin-1 in the cultured cells [56]. Moreover, basalcascades. These genetic alterations in the insulin NO release is reduced (without any effect on eNOSreceptor therefore will ultimately lead to compro- protein levels), and a concomitant increase is ob-mised glucose uptake in peripheral tissue. Further, served in the formation of the inhibitory caveolin–recent studies have documented the down-regulation eNOS complex [56]. Further, using the same modelof caveolin-1 in adipocyte cultures that have been system these investigators also found that statins, aexposed to TNFa [51]. This may be of significance group of pro-drugs used in the treatment of hy-since TNFa is thought to be an important mediator percholesterolaemia, serve to de-stabilise the inhib-in the development of insulin resistant diabetes [52]. itory caveolin–eNOS complex [57]. In culturedInterestingly, ‘Amaryl’, a sulfonylurea drug used to bovine endothelial cells the pertubation of thestimulate insulin release from the b cells of the caveolin–eNOS complex can occur from a reduction


in caveolin-1 expression in response to decreased this vector allows the effective delivery of the AP–cholesterol synthesis within treated cells [57]. This Cav peptide constructs to the cellular cytoplasm viahas been found to be paralleled by an increase in NO unconventional modes of endocytosis and at theproduction. Such modulation of eNOS at the post- same time circumvents the intrinsic proteolytic ac-translational level may explain one mechanism by tivity of cells [62]. These constructs are shown to bewhich statins are proposed to have anti-athrogenic avidly taken up by the endothelium and adventitiaproperties, even in subjects that present with normal within isolated mouse aortic rings, where they in-cholesterol levels. hibited acetylcholine stimulated NO production.

Hypercholesterolaemia, however, is also associ- Additionally, the same study demonstrated that AP–ated with the production of reactive oxygen species Cav could also block inflammation and reduce(superoxide anions) [58] which have been recently oedema formation in mice following the intraperi-reported to dissociate eNOS from caveolin enriched toneal administration of carrageenan. The in-vivoplasma membrane domains, in addition to reducing reduction of inflammation by AP–Cav peptides wasthe number of caveolae present in BAEC [57]. This found to be comparable to that of the anti-inflamma-dissociation may indeed be beneficial in removing tory steroid dexamethasone (0.1 mg/kg). This ap-the ‘molecular brake’ on NO production, although proach represents the first to show in-vivo, thatironically NO is itself destroyed rapidly by reactive peptides based upon the caveolin scaffolding domainoxygen species [59]. Despite the intriguing nature of motif can be effectively used for attenuation ofthe link between caveolin, eNOS and nitric oxide disease processes.production, it is important to remember that endo-thelial dysfunction is also thought to involve altera-tions in signal transduction, reduced availability of 7. Alzheimer’s diseaseL-arginine, intimal thickening acting as a diffusionalbarrier to NO, reduced responsiveness of the smooth Alzheimer’s disease (AD) is the most commonmuscle to NO, and modification of the expression of form of dementia in adults aged 60 and above, andeNOS in addition to the previously mentioned de- significantly is the fourth leading cause of death instruction of NO by reactive oxygen species [60]. In American adults behind heart disease, cancer andconsideration of recent research, the role of caveolin stroke. Post-mortem examination of an AD patients’in the proatherogenic process induced by hyper- brain tissue reveals senile (neuritic plaques) andcholesterolaemia is still to be resolved, but this is neurofibrillary tangles present in the limbic andnevertheless an intriguing area for research. For associated cortices. There is increasing evidence thatmore definitive answers, investigations, particularly caveolae may have a role to play in the generation ofin-vivo, are required. such plaques and hence may have a pivotal role in

the initiation and progression of Alzheimer’s disease.The senile plaques found in Alzheimer’s disease

6. Inflammation contain Ab-amyloid protein as a major proteincomponent. This protein is generated by a series of

While the benefits of NO upon the functioning of proteolytic cleavages from amyloid precursor proteinthe circulatory system are well documented, the (APP) by b- and g-secretases. However, productioninappropriate production of NO in response to of Ab-amyloid protein maybe prevented if the APPlocalised injury may lead to a more disseminated is processed by a-secretase, which cleaves APPendothelial dysfunction. within the Ab sequence. Interestingly, APP has been

A pioneering paper recently published reports the reported to be present in caveolae-like membranein-vivo delivery of peptide constructs, based upon domains [63,64]. In agreement with this, Simonspeptides of the caveolin scaffolding domain (desig- [65] has shown inhibition of formation of b-amyloid,nated AP–Cav) aimed at attenuating inflammatory a product of the serial cleavage of APP, by depletingprocesses [61]. Synthesised as a fusion protein to the cellular cholesterol levels of hippocampal neu-ANTENNAPEDIA, a drosophila transcription factor, rons. Despite these results implicating the presence


of APP in caveolae, it is worth noting that domains ways will ultimately result in perturbations to normalwith caveolae-like properties also exist in cells which cell function and as a consequence may lead to thelack the scaffolding protein caveolin. These latter induction of pathological processes. With much offindings are, however, verified by further research the current evidence suggesting that the alteredwhich has shown a biochemical association exists expression of caveolins can contribute to the aetiolo-between APP and caveolin-1 in cell lines [66]. gy of such disease states as cancer, muscularAdditionally, functional data is also recognised dystrophy and Alzheimer’s disease, a good case forwhere over expression of caveolin-1 promotes a- specifically replacing or targeting the caveolin familysecretase-mediated cleavage of APP, whilst caveolin- of proteins is provided. Now the pharmacological1 based antisense oligonucleotides inhibit the pro- activity of the caveolin-1 based peptides has beenduction of the a-secretase cleavage product of APP successfully shown in an in-vivo animal model it is[66]. Clearly, in consideration of the important role probably not too optimistic to expect similar resultsplayed by a-secretase in the prevention of Ab- in humans within the not too distant future.amyloid protein production these results raise thepossibility that caveolae and caveolin proteins couldplay a key role in the prevention of Alzheimer’s Referencesdisease. However, other studies have implicatedcaveolin, specifically caveolin-3, in the generation of [1] G.E. Palade, Blood capillaries of the heart and other organs,

Circulation 24 (1961) 594–561.senile plaques. These studies [67] reported an up-[2] J.A. Engleman, X. Zhang, F. Galbiati, D. Volonte, F. Sotiga,regulation of caveolin-3 along with presenilin-1 and

R.G. Pestell, C. Minetti, P.E. Scherer, T. Okamoto, M.P.presenilin-2, in reactive astrocytes surrounding senile Lisanti, Molecular genetics of the caveolin gene family:im-plaques in the brains of Alzheimer’s patients. Over- plications for human cancers, diabetes, Alzheimer disease,expression of caveolin-3 has been found to stimulate and muscular dystrophy, Am. J. Hum. Genet. 63 (1998)

1578–1587.b-secretase-mediated processing of APP in transfect-[3] A. Shlegel, R.G. Pestell, M.P. Lisanti, Caveolins in choles-ed COS-7, raising the possibility that the over-ex-

terol trafficking and signal transduction: implications forpression of caveolin-3 in astrocytes may lead to the human disease, Front. Biosci. 5 (2000) D929–D937.increased production of harmful metabolites of APP [4] E.P. Hoffman, R.H. Brown, L.M. Kunkel, Dystrophin: the[67]. protein product of the Duchenne muscular dystrophy, Cell 51

(1987) 919–928.Additional studies are required in order that the[5] E. Bonilla, K. Fishbeck, D.L. Schotland, Freeze–fractureexact domain of the caveolin-3 molecule responsible

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cloning of caveolin-3 gene family expressed predominantlyfor the treatment of Alzheimer’s patients. However,in muscle, J. Biol. Chem. 271 (1996) 2255–2261.one limitation to the in vitro study of such mechanis-

[7] K.S. Song, P.E. Sherer, Z. Tang, T. Okamoto, S. Li, M.tic events within primary isolates of astrocytes is the Chafel, C. Chu, D.S. Kohtz, M.P. Lisanti, Expression ofdifferential expression of caveolin-1 and caveolin-3 caveolin-3 in skeletal muscle, cardiac, and smooth musclein culture. It is apparent that following the isolation cells. Caveolin-3 is a component of the sarcolemma and

co-fractionates with dystrophin and dystrophin-associatedprocedure the expression of caveolin-3 in astrocytesglycoproteins, J. Biol. Chem. 271 (1996) 15160–15165.is down-regulated while that of caveolin-1 is up-

[8] F. Sotgia, J.K. Lee, K. Das, M. Bedford, T.C. Petrucci, P.regulated [67]. Macioce, M. Sargiacomo, F.D. Bricarelli, C. Minetti, M.

Sudol, M.P. Lisanti, Caveolin-3 directly interacts with theC-terminal tail of beta-dystroglycan, J. Biol. Chem. 275(2000) 38048–38058.8. Conclusion

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Over the last 5 years it has become quite clear that Lisanti, C. Minetti, Increased number of caveolae andcaveolin proteins are involved in the modulation of caveolin-3 in Duchenne muscular dystrophy, Biochem. Bio-signal transduction events. Aberrations in such path- phys. Res. Commun. 261 (1999) 547–550.


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Caveolae and the caveolins in human disease

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