Caveolin-1 deficiency (-/-) conveys premalignant alterations in mammary epithelia, with abnormal lumen formation, growth factor independence, and cell invasiveness

Lipid rafts in health and disease

Lipid rafts are sphingolipid- and cholesterol-rich domains of the plasma membrane which contain a variety of signalling and transport proteins. Different subtypes of lipid rafts can be distinguished according to their protein and lipid composition. Caveolae are types of rafts that are rich in proteins of the caveolin family (caveolin-1, -2 and -3) which present a distinct signalling platform. The importance of lipid raft signalling in the pathogenesis of a variety of conditions, such as Alzheimer's, Parkinson's, cardiovascular and prion diseases, systemic lupus erythematosus and HIV, has been elucidated over recent years and makes these specific membrane domains an interesting target for pharmacological approaches in the cure and prevention of these diseases. This Review analyses the importance of lipid raft proteins and lipids in health and disease, with a focus on the current state of knowledge.

Caveolin 1 is overexpressed and amplified in a subset of basal-like and metaplastic breast carcinomas: a morphologic, ultrastructural, immunohistochemical, and in situ hybridization analysis

PURPOSE

The distribution and significance of caveolin 1 (CAV1) expression in different breast cell types and role in breast carcinogenesis remain poorly understood. Both tumor-suppressive and oncogenic roles have been proposed for this protein. The aims of this study were to characterize the distribution of CAV1 in normal breast, benign breast lesions, breast cancer precursors, and metaplastic breast carcinomas; to assess the prognostic significance of CAV1 expression in invasive breast carcinomas; and to define whether CAV1 gene amplification is the underlying genetic mechanism driving CAV1 overexpression in breast carcinomas.

EXPERIMENTAL DESIGN

CAV1 distribution in frozen and paraffin-embedded whole tissue sections of normal breast was evaluated using immunohistochemistry, immunofluorescence, and immunoelectron microscopy. CAV1 expression was immunohistochemically analyzed in benign lesions, breast cancer precursors, and metaplastic breast carcinomas and in a cohort of 245 invasive breast carcinomas from patients treated with surgery followed by anthracycline-based chemotherapy. In 25 cases, CAV1 gene amplification was assessed by chromogenic in situ hybridization.

RESULTS

In normal breast, CAV1 was expressed in myoepithelial cells, endothelial cells, and a subset of fibroblasts. Luminal epithelial cells showed negligible staining. CAV1 was expressed in 90% of 39 metaplastic breast carcinomas and in 9.4% of 245 invasive breast cancers. In the later cohort, CAV1 expression was significantly associated with 'basal-like' immunophenotype and with shorter disease-free and overall survival on univariate analysis. CAV1 gene amplification was found in 13% of cases with strong CAV1 expression.

CONCLUSIONS

The concurrent CAV1 amplification and overexpression call into question its tumor-suppressive effects in basal-like breast carcinomas.

Transit of hormonal and EGF receptor-dependent signals through cholesterol-rich membranes

The functional consequences of changes in membrane lipid composition that coincide with malignant growth are poorly understood. Sufficient data have been acquired from studies of lipid binding proteins, post-translational modifications of signaling proteins, and biochemical inhibition of lipidogenic pathways to indicate that growth and survival pathways might be substantially re-directed by alterations in the lipid content of membranes. Cholesterol and glycosphingolipids segregate into membrane patches that exhibit a liquid-ordered state in comparison to membrane domains containing relatively lower amounts of these classes of lipids. These "lipid raft" structures, which may vary in size and stability in different cell types, both accumulate and exclude signaling proteins and have been implicated in signal transduction through a number of cancer-relevant pathways. In prostate cancer cells, signaling from epidermal growth factor receptor (EGFR) to the serine-threonine kinase Akt1, as well as from IL-6 to STAT3, have been demonstrated to be influenced by experimental interventions that target cholesterol homeostasis. The recent finding that classical steroid hormone receptors also reside in these microdomains, and thus may function within these structures in a signaling capacity independent of their role as nuclear factors, suggests a novel means of cross-talk between receptor tyrosine kinase-derived and steroidogenic signals. Potential points of intersection between components of the EGFR family of receptor tyrosine kinases and androgen receptor signaling pathways, which may be sensitive to disruptions in cholesterol metabolism, are discussed. Understanding the manner in which these pathways converge within cholesterol-rich membranes may present new avenues for therapeutic intervention in hormone-dependent cancers.

Caveolin-1alpha and -1beta perform nonredundant roles in early vertebrate development

Caveolins are integral membrane proteins that localize predominantly to lipid rafts, where they oligomerize to form flask-shaped organelles termed caveolae and play important roles in membrane trafficking, signal transduction, and other cellular processes. To investigate potential roles for caveolin-1 (cav-1) in development, cav-1alpha and -1beta cDNAs were functionally characterized in the zebrafish. Cav-1alpha and -1beta mRNAs exhibited overlapping but distinct expression patterns throughout embryogenesis. Targeted depletion of either Cav-1 isoform, using antisense morpholino oligomers, resulted in a substantial loss of caveolae and dramatic neural, eye, and somite defects by 12 hours after fertilization, the time at which mRNA levels of both isoforms substantially increased in wild-type animals. Morphant phenotypes were rescued by injection of homotypic (cav-1alpha/cav-1alpha) but not heterotypic (cav-1alpha/cav-1beta) zebrafish and human cav-1 cRNAs, revealing nonredundant and evolutionarily conserved functions for the individual Cav-1 isoforms. Mutation of a known Cav-1 phosphorylation site unique to Cav-1alpha (Y14-->F) resulted in a failure to rescue the cav-1alpha morphant phenotype, verifying an essential role for Cav-1alpha specifically and implicating this residue in early developmental functions. Cav-1alpha and -1beta morphants also exhibited disruption in the actin cytoskeleton. These results support important and previously unanticipated roles for the Caveolin-1 isoforms in vertebrate organogenesis.

Caveolin-1, mammary stem cells, and estrogen-dependent breast cancers

Estrogen exposure is considered a significant risk factor for breast cancer development. Estrogen receptor (ER) alpha is expressed at low levels in normal epithelia, and its expression is dramatically up-regulated as transformation progresses during mammary hyperplasia and adenocarcinoma development. The mechanism(s) driving ERalpha up-regulation during mammary tumorigenesis remains unclear. Caveolin-1 (Cav-1) is the structural protein of plasmalemmal invaginations, termed caveolae, which functions as a tumor suppressor gene. Interestingly, Cav-1 dominant-negative mutations are exclusively found in ERalpha-positive breast cancer samples. In support of these clinical findings, ERalpha expression is increased in Cav-1 (-/-) null mammary epithelia, and estrogen stimulation further enhances the growth of Cav-1-deficient three-dimensional epithelial structures. These phenotypes correlate with augmented levels of cyclin D1. In addition, Cav-1 gene inactivation induces the accumulation of a cell population with the characteristics of adult mammary stem cells. Primary cultures of Cav-1 (-/-) mammary epithelial cells exhibit premalignant changes, such as abnormal lumen formation, epidermal growth factor-independent growth, defects in cell substrate attachment, and increased cell invasiveness. Thus, Cav-1 gene inactivation promotes premalignant alterations in mammary epithelia and induces increased ERalpha expression levels and the up-regulation of cyclin D1. As tumor formation is a multihit process, Cav-1 mutations that occur during the early stages of mammary transformation may be a critical upstream/initiating event leading to increased ERalpha levels.

Muscular atrophy of caveolin-3-deficient mice is rescued by myostatin inhibition

Caveolin-3, the muscle-specific isoform of caveolins, plays important roles in signal transduction. Dominant-negative mutations of the caveolin-3 gene cause autosomal dominant limb-girdle muscular dystrophy 1C (LGMD1C) with loss of caveolin-3. However, identification of the precise molecular mechanism leading to muscular atrophy in caveolin-3-deficient muscle has remained elusive. Myostatin, a member of the muscle-specific TGF-beta superfamily, negatively regulates skeletal muscle volume. Here we report that caveolin-3 inhibited myostatin signaling by suppressing activation of its type I receptor; this was followed by hypophosphorylation of an intracellular effector, Mad homolog 2 (Smad2), and decreased downstream transcriptional activity. Loss of caveolin-3 in P104L mutant caveolin-3 transgenic mice caused muscular atrophy with increase in phosphorylated Smad2 (p-Smad2) as well as p21 (also known as Cdkn1a), a myostatin target gene. Introduction of the myostatin prodomain, an inhibitor of myostatin, by genetic crossing or intraperitoneal administration of the soluble type II myostatin receptor, another inhibitor, ameliorated muscular atrophy of the mutant caveolin-3 transgenic mice with suppression of p-Smad2 and p21 levels. These findings suggest that caveolin-3 normally suppresses the myostatin-mediated signal, thereby preventing muscular atrophy, and that hyperactivation of myostatin signaling participates in the pathogenesis of muscular atrophy in a mouse model of LGMD1C. Myostatin inhibition may be a promising therapy for LGMD1C patients.

Caveolin-1 mutations in human breast cancer: functional association with estrogen receptor alpha-positive status

A Japanese study reported that up to 16% of breast cancer samples harbor a sporadic mutation within the human Cav-1 gene, namely P132L. To date, however, no studies have examined the United States' population. Here, we developed a novel allele-specific real-time PCR assay to detect the Cav-1 P132L mutation in mammary tumor cells isolated by laser capture microdissection from formalin-fixed paraffin-embedded breast cancer samples. We report that the Cav-1 P132L mutation is present in approximately 19% of estrogen receptor alpha (ERalpha)-positive breast cancers but not in ERalpha-negative breast cancers. This is the first demonstration that the P132L mutation is exclusively associated with ERalpha-positive mammary tumors. We also identified six novel Cav-1 mutations associated with ERalpha-positive breast cancers (W128Stop, Y118H, S136R, I141T, Y148H, and Y148S). Thus, the overall incidence of Cav-1 mutations in ERalpha-positive breast cancers approaches 35% (greater than one-third). To mechanistically dissect the functional relationship between Cav-1 gene inactivation and ERalpha expression, we isolated primary mammary epithelial cells from wild-type and Cav-1-/- mice and cultured them in a three-dimensional system, allowing them to form mammary acinar-like structures. Under conditions of growth factor deprivation, Cav-1-deficient mammary acini displayed increased ERalpha levels and enhanced sensitivity toward estrogen-stimulated growth, with specific up-regulation of cyclin D1. Finally, we discuss the possibility that sporadic Cav-1 mutations may act as an initiating event in human breast cancer pathogenesis.

Effect of Caveolin-1 on growth of human gastric cancer cell Line MGC803

AIM: To investigate the effect of Caveolin-1 on the proliferation and differentiation of gastric carcinoma cell line MGC803, and explore the probability that Caveolin-1 can be used for gene therapy.

METHODS: Caveolin-1 gene and Pcl-neo control plasmid were transfected into human MGC803 cell line by lipofectin, respectively. The positive clones were selected by G418. We also stabled a positive control group which was treated with PD98059 for 48 hours. Then the expression of Caveolin-1 in each group was detected by Western blot. Cell morphology was observed under optical microscope. Cell population doubling time was determined by cell counting method and cell cycle was analyzed by flow cytometry.

RESULTS: The expression of Caveolin-1 was significantly higher in the cells treated with Caveolin-1 or PD98059 than that in the empty controls (P < 0.001, q = 23.067 or 13.3376). Furthermore, Caveolin-1 expression was also markedly higher in the cells transfected with Caveolin-1 than that in the positive controls (P < 0.001, q = 9.7294). Under light microscope, marked changes occurred in cell morphous after gene transfection. Before transfection, the cells had a significant heteromorphism, with the features of large cell body, little cytoplasm, obvious karyokinesis. While in Caveolin-1-transfected MGC803 cells, the cell malignancy declined as the cellular heteromorphism diminished, with the ratio of nuclear-to-cytoplasm decreased, and the karyokinesis disappeared. Caveolin-1-transfected cells had an extended doubling time (65.46 h vs 46.67 h, P < 0.05, q = 4.8695). At same time, the population of Caveolin-1-transfected cells in G₀/G₁ phase was obviously increased (P < 0.01, q = 9.1824) while that in S phase was decreased (P < 0.01, q = 7.827). There were also notable differences in cell cycle distribution between Caveolin-1-transfected cells and the positive controls (G₀/G₁: P < 0.01, q = 4.9323; S: P < 0.05, q = 3.3295).

CONCLUSION: Caveolin-1 not only induces the differentiation of MGC803 cells, but also blocks them at in G₀/G₁ phase. Caveolin-1 can inhibit the proliferation of MGC803 cells in vitro by prolonging the cell doubling time.