Mannose-6-phosphate/insulin-like growth factor-II receptor is a receptor for retinoic acid

Cell-specific secretory granule sorting mechanisms: the role of MAGEL2 and retromer in hypothalamic regulated secretion

Intracellular protein trafficking and sorting are extremely arduous in endocrine and neuroendocrine cells, which synthesize and secrete on-demand substantial quantities of proteins. To ensure that neuroendocrine secretion operates correctly, each step in the secretion pathways is tightly regulated and coordinated both spatially and temporally. At the trans-Golgi network (TGN), intrinsic structural features of proteins and several sorting mechanisms and distinct signals direct newly synthesized proteins into proper membrane vesicles that enter either constitutive or regulated secretion pathways. Furthermore, this anterograde transport is counterbalanced by retrograde transport, which not only maintains membrane homeostasis but also recycles various proteins that function in the sorting of secretory cargo, formation of transport intermediates, or retrieval of resident proteins of secretory organelles. The retromer complex recycles proteins from the endocytic pathway back to the plasma membrane or TGN and was recently identified as a critical player in regulated secretion in the hypothalamus. Furthermore, melanoma antigen protein L2 (MAGEL2) was discovered to act as a tissue-specific regulator of the retromer-dependent endosomal protein recycling pathway and, by doing so, ensures proper secretory granule formation and maturation. MAGEL2 is a mammalian-specific and maternally imprinted gene implicated in Prader-Willi and Schaaf-Yang neurodevelopmental syndromes. In this review, we will briefly discuss the current understanding of the regulated secretion pathway, encompassing anterograde and retrograde traffic. Although our understanding of the retrograde trafficking and sorting in regulated secretion is not yet complete, we will review recent insights into the molecular role of MAGEL2 in hypothalamic neuroendocrine secretion and how its dysregulation contributes to the symptoms of Prader-Willi and Schaaf-Yang patients. Given that the activation of many secreted proteins occurs after they enter secretory granules, modulation of the sorting efficiency in a tissue-specific manner may represent an evolutionary adaptation to environmental cues.

Towards understanding the cell surface phenotype, metabolic properties and immune functions of resident macrophages of the peritoneal cavity and splenic red pulp using high resolution quantitative proteomics

Background:Resident macrophages (Mϕs) are distributed throughout the body and are important for maintaining tissue homeostasis and for defence against infections. Tissue Mϕs are highly adapted to their microenvironment and thought to mediate tissue-specific functions involving metabolism and immune defence that are not fully elucidated. Methods:We have used high resolution quantitative proteomics to gain insights into the functions of two types of resident tissue Mϕs: peritoneal cavity Mϕs and splenic red pulp Mϕs.  The cellular expression levels of many proteins were validated by flow cytometry and were consistently in agreement with the proteomics data.Results:Peritoneal and splenic red pulp macrophages displayed major differences in cell surface phenotype reflecting their adaptation to different tissue microenvironments and tissue-specific functions. Peritoneal Mϕs were shown to be enriched in a number of key enzymes and metabolic pathways normally associated with the liver, such as metabolism of fructose, detoxification, nitrogen homeostasis and the urea cycle. Supporting these observations, we show that peritoneal Mϕs are able to utilise glutamine and glutamate which are rich in peritoneum for urea generation.  In comparison, splenic red pulp Mϕs were enriched in proteins important for adaptive immunity such as antigen presenting MHC molecules, in addition to proteins required for erythrocyte homeostasis and iron turnover. We also show that these tissue Mϕs may utilise carbon and nitrogen substrates for different metabolic fates to support distinct tissue-specific roles.Conclusions:This study provides new insights into the functions of tissue Mϕs in immunity and homeostasis.  The comprehensive proteomics data sets are a valuable resource for biologists and immunologists.

Unravelling novel functions of the endosomal transporter mannose 6-phosphate/insulin-like growth factor receptor (CD222) in health and disease: An emerging regulator of the immune system

Properly balanced cellular responses require both the mutual interactions of soluble factors with cell surface receptors and the crosstalk of intracellular molecules. In particular, immune cells exposed unceasingly to an array of positive and negative stimuli must distinguish between what has to be tolerated and attacked. Protein trafficking is one of crucial pathways involved in this labour. The approximately >270-kDa protein transporter called mannose 6- phosphate/insulin-like growth factor 2 receptor (M6P/IGF2R, CD222) is a type I transmembrane glycoprotein present largely intracellularly in the Golgi apparatus and endosomal compartments, but also at the cell surface. It is expressed ubiquitously in a vast majority of higher eukaryotic cell types. Through binding and trafficking multiple unrelated extracellular and intracellular ligands, CD222 is involved in the regulation of a plethora of functions, and thus implicated in many physiological but also pathophysiological conditions. This review describes, first, general features of CD222, such as its evolution, genomic structure and regulation, protein structure and ligands; and second, its specific functions with a special focus on the immune system.

Iodide transport and breast cancer

Breast cancer is the second most common cancer worldwide and the leading cause of cancer death in women, with incidence rates that continue to rise. The heterogeneity of the disease makes breast cancer exceptionally difficult to treat, particularly for those patients with triple-negative disease. To address the therapeutic complexity of these tumours, new strategies for diagnosis and treatment are urgently required. The ability of lactating and malignant breast cells to uptake and transport iodide has led to the hypothesis that radioiodide therapy could be a potentially viable treatment for many breast cancer patients. Understanding how iodide is transported, and the factors regulating the expression and function of the proteins responsible for iodide transport, is critical for translating this hypothesis into reality. This review covers the three known iodide transporters - the sodium iodide symporter, pendrin and the sodium-coupled monocarboxylate transporter - and their role in iodide transport in breast cells, along with efforts to manipulate them to increase the potential for radioiodide therapy as a treatment for breast cancer.

Changes in lysosomal enzymes and mannose-6-phosphate receptors related to sexual maturation in bull epididymis

One of the most striking features of the mammalian epididymis is the secretion of lysosomal enzymes (LE). These LE may play a role in sperm maturation. In the present study we investigated the activity and distribution of four LE (?-galactosidase (?-Gal), N-acetyl-?-D-glucosaminidase (?-NAG), ?-mannosidase (?-Man) and ?-glucuronidase (?-Glu)) in bull epididymis at two different ages (6 months and 4 years) to determine whether these enzymes vary with sexual maturity. In young, sexually immature (SI) bulls we found high LE activity in the epididymal tissue that accounts for a developed and active lysosomal apparatus. In contrast, low LE activity was measured in sexually mature (SM) bulls, and ?-NAG and ?-Gal were mostly secreted into the lumen. We also attempted to correlate LE distribution with the expression and functionality of mannose-6-phosphate receptors (MPRs), which are thought to be involved in proper delivery of LE to lysosomes. The cation-dependent MPR was highly expressed in SI bulls, with expression decreasing during adulthood, whereas the expression of the cation-independent MPR was higher in SM than SI bulls. In addition, the four enzymes recovered from the epididymal lumen interact with both MPRs at each age. We conclude that the activity and distribution of LE in bull epididymis varies with sexual maturity and that the distribution is regulated differently by the two types of MPR. These findings could provide some molecular basis for male infertility.

Expression of Intratumoral IGF-II Is Regulated by the Gene Imprinting Status in Triple Negative Breast Cancer from Vietnamese Patients

African American women suffer higher incidence and mortality of triple negative breast cancer (TNBC) than Caucasian women. TNBC is very aggressive, causing the worst clinical outcome. We previously demonstrated that tumors from these patients express high IGF-II and exhibit high activation of the IGF signaling pathways. IGF-II gene expression is imprinted (monoallelic), promotes tumor progression, and metastasis and regulates Survivin, a TNBC prognostic marker. Since BC mortality has increased among young Vietnamese women, we analyzed 48 (paired) TNBC samples from Vietnamese patients to assess IGF-II expression. We analyzed all samples by qrtPCR for identification of IGF-II heterozygosity and to determine allelic expression of the IGF-II gene. We also analyzed the tissues for proIGF-II and Survivin by RT-PCR and Western blotting. A total of 28 samples displayed IGF-II heterozygosity of which 78% were biallelic. Tumors with biallelic IGF-II gene expression exhibited the highest levels of proIGF-II and Survivin. Although 100% of these tissues corresponding normal samples were biallelic, they expressed significantly lower levels of or no proIGF-II and Survivin. Thus, IGF-II biallelic gene expression is differentially regulated in normal versus tumor tissues. We propose that intratumoral proIGF-II is dependent on the IGF-II gene imprinting status and it will promote a more aggressive TNBC.

Expression of insulin-like growth factor 2 receptor in corneal keratocytes during differentiation and in response to wound healing

PURPOSE

Insulin-like growth factor 2 receptor (IGF2R) associates with ligands that influence wound healing outcomes. However, the expression pattern of IGF2R and its role in the cornea is unknown.

METHODS

Human keratocytes were isolated from donor corneas. Fibroblasts (fibroblast growth factor 2 [FGF2]-treated) or myofibroblasts (TGF-β1-treated) were analyzed for IGF2R and α-smooth muscle actin (α-SMA) expression by Western blotting and immunolocalization. Mouse corneas were wounded in vivo and porcine corneas ex vivo. The IGF2R and α-SMA protein expression were visualized and quantified by immunohistochemistry. The IGF2R gene expression in human corneal fibroblasts was knocked-down with targeted lentiviral shRNA.

RESULTS

The IGF2R is expressed in epithelial and stromal cells of normal human, mouse, and porcine corneas. The IGF2R increases (11.2 ± 0.4-fold) in the epithelial and (11.7 ± 0.9-fold) stromal layers of in vivo wounded mouse corneas. Double-staining with α-SMA- and IGF2R-specific antibodies reveals that IGF2R protein expression is increased in stromal myofibroblasts in the wounded cornea relative to keratocytes in the normal cornea (11.2 ± 0.8-fold). Human primary stromal keratocytes incubated with FGF2 or TGF-β1 in vitro demonstrate increased expression (2.0 ± 0.4-fold) of IGF2R in myofibroblasts relative to fibroblasts. Conversion of IGF2R shRNA-lentiviral particle transduced corneal fibroblasts to myofibroblasts reveals a dependence on IGF2R expression, as only 40% ± 10% of cells transduced converted to myofibroblasts compared to 86% ± 3% in control cells.

CONCLUSIONS

The IGF2R protein expression is increased during corneal wound healing and IGF2R regulates human corneal fibroblast to myofibroblast differentiation.

Liposomes containing mannose-6-phosphate-cholesteryl conjugates for lysosome-specific delivery

We present a novel liposomal nanocarrier containing mannose 6-phosphate-cholesteryl conjugates and show its ability to reach the lysosomes by means of confocal and fluorescence microscopy measurements.

Retinoic acid induces autophagosome maturation through redistribution of the cation-independent mannose-6-phosphate receptor

Retinoic acids (RAs) have diverse biologic effects and regulate several cellular functions. Here, we investigated the role of RA on autophagy by studying its effects on autophagosome (AUT) maturation, as well as on upstream regulators of autophagosome biogenesis. Our studies, based on the use of pH-sensitive fluorescent reporter markers, suggested that RA promotes AUT acidification and maturation. By using competitive inhibitors and specific agonists, we demonstrated that this effect is not mediated by the classic RAR and RXR receptors. RA did not affect the levels of upstream regulators of autophagy, such as Beclin-1, phospho-mTOR, and phospho-Akt1, but induced redistribution of both endogenous cation-independent mannose-6-phosphate receptor CIMPR and transiently transfected GFP and RFP full-length CIMPR fusion proteins from the trans-Golgi region to acidified AUT structures. Those structures were found to be amphisomes (acidified AUTs) and not autophagolysosomes. The critical role of CIMPR in AUT maturation was further demonstrated by siRNA-mediated silencing of endogenous CIMPR. Transient CIMPR knockdown resulted in remarkable accumulation of nonacidified AUTs, a process that could not be reversed with RA. Our results suggest that RA induces AUT acidification and maturation, a process critical in the cellular autophagic mechanism.

Soluble M6P/IGF2R Released by TACE Controls Angiogenesis via Blocking Plasminogen Activation

Rationale:

The urokinase plasminogen activator (uPA) system is among the most crucial pericellular proteolytic systems associated with the processes of angiogenesis. We previously identified an important regulator of the uPA system in the mannose 6-phosphate/insulin-like growth factor 2 receptor (M6P/IGF2R).

Objective:

Here, we wanted to clarify whether and how did the soluble form of M6P/IGF2R (sM6P/IGF2R) contribute to modulation of the uPA system.

Methods and Results:

By using specific inhibitors and RNA interference, we show that the tumor necrosis factor α convertase (TACE, ADAM-17) mediates the release of the ectodomain of M6P/IGF2R from human endothelial cells. We demonstrate further that sM6P/IGF2R binds plasminogen (Plg) and thereby prevents Plg from binding to the cell surface and uPA, ultimately inhibiting in this manner Plg activation. Furthermore, peptide 18-36 derived from the Plg-binding site of M6P/IGF2R mimics sM6P/IGF2R in the inhibition of Plg activation and blocks cancer cell invasion in vitro, endothelial cell invasion in vivo, and tumor growth in vivo.

Conclusions:

The interaction of sM6P/IGF2R with Plg may be an important regulatory mechanism to inhibit migration of cells using the uPA/uPAR system.

Transcriptional regulation of the IGF signaling pathway by amino acids and insulin-like growth factors during myogenesis in Atlantic salmon

The insulin-like growth factor signalling pathway is an important regulator of skeletal muscle growth. We examined the mRNA expression of components of the insulin-like growth factor (IGF) signalling pathway as well as Fibroblast Growth Factor 2 (FGF2) during maturation of myotubes in primary cell cultures isolated from fast myotomal muscle of Atlantic salmon (Salmo salar). The transcriptional regulation of IGFs and IGFBP expression by amino acids and insulin-like growth factors was also investigated. Proliferation of cells was 15% d⁻¹ at days 2 and 3 of the culture, increasing to 66% d⁻¹ at day 6. Three clusters of elevated gene expression were observed during the maturation of the culture associated with mono-nucleic cells (IGFBP5.1 and 5.2, IGFBP-6, IGFBP-rP1, IGFBP-2.2 and IGF-II), the initial proliferation phase (IGF-I, IGFBP-4, FGF2 and IGF-IRb) and terminal differentiation and myotube production (IGF2R, IGF-IRa). In cells starved of amino acids and serum for 72 h, IGF-I mRNA decreased 10-fold which was reversed by amino acid replacement. Addition of IGF-I and amino acids to starved cells resulted in an 18-fold increase in IGF-I mRNA indicating synergistic effects and the activation of additional pathway(s) leading to IGF-I production via a positive feedback mechanism. IGF-II, IGFBP-5.1 and IGFBP-5.2 expression was unchanged in starved cells, but increased with amino acid replacement. Synergistic increases in expression of IGFBP5.2 and IGFBP-4, but not IGFBP5.1 were observed with addition of IGF-I, IGF-II or insulin and amino acids to the medium. IGF-I and IGF-II directly stimulated IGFBP-6 expression, but not when amino acids were present. These findings indicate that amino acids alone are sufficient to stimulate myogenesis in myoblasts and that IGF-I production is controlled by both endocrine and paracrine pathways. A model depicting the transcriptional regulation of the IGF pathway in Atlantic salmon muscle following feeding is proposed.

Residues essential for plasminogen binding by the cation-independent mannose 6-phosphate receptor

The 300 kDa cation-independent mannose 6-phosphate receptor (CI-MPR) is a multifunctional protein that binds diverse intracellular and extracellular ligands with high affinity. The CI-MPR is a receptor for plasminogen, and this interaction can be inhibited by lysine analogues. To characterize the molecular basis for this interaction, surface plasmon resonance (SPR) analyses were performed using truncated forms of the CI-MPR and plasminogen. The results show that the N-terminal region of the CI-MPR containing domains 1 and 2, but not domain 1 alone, of the receptor's 15-domain extracytoplasmic region binds plasminogen (K(d) = 5 +/- 1 nM) with an affinity similar to that of the full-length receptor (K(d) = 20 +/- 6 nM). In addition to its C-terminal serine protease domain, plasminogen contains lysine binding sites (LBS), which are located within each of its five kringle domains, except kringle 3. We show that kringles 1-4, but not kringles 1-3, bind the CI-MPR, indicating an essential role for the LBS in kringle 4 of plasminogen. To identify the lysine residue(s) of the CI-MPR that serve(s) as an essential determinant for recognition by the LBS of plasminogen, site-directed mutagenesis studies were carried out using a construct encoding the N-terminal three domains of the CI-MPR (Dom1-3His) which contains both a mannose 6-phosphate (Man-6-P) and plasminogen binding site. The results demonstrate two lysine residues (Lys53 located in domain 1 and Lys125 located in the loop connecting domains 1 and 2) of the CI-MPR are key determinants for plasminogen binding but are not required for Man-6-P binding.

Carbohydrate recognition by the mannose-6-phosphate receptors

The two P-type lectins, the 46kDa cation-dependent mannose-6-phosphate (Man-6-P) receptor (CD-MPR), and the 300kDa cation-independent Man-6-P receptor (CI-MPR), are the founding members of the growing family of mannose-6-phosphate receptor homology (MRH) proteins. A major cellular function of the MPRs is to transport Man-6-P-containing acid hydrolases from the Golgi to endosomal/lysosomal compartments. Recent advances in the structural analyses of both CD-MPR and CI-MPR have revealed the structural basis for phosphomannosyl recognition by these receptors and provided insights into how the receptors load and unload their cargo. A surprising finding is that the CD-MPR is dynamic, with at least two stable quaternary states, the open (ligand-bound) and closed (ligand-free) conformations, similar to those of hemoglobin. Ligand binding stabilizes the open conformation; changes in the pH of the environment at the cell surface and in endosomal compartments weaken the ligand-receptor interaction and/or weaken the electrostatic interactions at the subunit interface, resulting in the closed conformation.

Mannose-6-phosphate/insulin-like growth factor 2 receptor (M6P/IGF2R) in carcinogenesis

The cation-independent mannose-6-phosphate/insulin-like growth factor 2 receptor (M6P/IGF2R) is a multifunctional receptor. It is involved in a variety of cellular processes which become dysregulated in cancer. Its tumor suppressor role was recognized a long time ago. However, due to its multifunctionality, it is not easy to understand the extent of its relevance to normal cellular physiology. Accordingly, it is even more difficult understanding its role in carcinogenesis. This review presents critical and focused highlights of data relating to M6P/IGF2R, obtained during more than 25 years of cancer research.

High-affinity ligand binding by wild-type/mutant heteromeric complexes of the mannose 6-phosphate/insulin-like growth factor II receptor

The mannose 6-phosphate/insulin-like growth factor II receptor has diverse ligand-binding properties contributing to its roles in lysosome biogenesis and growth suppression. Optimal receptor binding and internalization of mannose 6-phosphate (Man-6-P)-bearing ligands requires a dimeric structure leading to bivalent high-affinity binding, presumably mediated by cooperation between sites on both subunits. Insulin-like growth factor II (IGF-II) binds to a single site on each monomer. It is hypothesized that IGF-II binding to cognate sites on each monomer occurs independently, but bivalent Man-6-P ligand binding requires cooperative contributions from sites on both monomers. To test this hypothesis, we co-immunoprecipitated differentially epitope-tagged soluble mini-receptors and assessed ligand binding. Pairing of wild-type and point-mutated IGF-II binding sites between two dimerized mini-receptors had no effect on the function of the contralateral binding site, indicating IGF-II binding to each side of the dimer is independent and manifests no intersubunit effects. As expected, heterodimeric receptors composed of a wild-type monomer and a mutant bearing two Man-6-P-binding knockout mutations form functional IGF-II binding sites. By contrast to prediction, such heterodimeric receptors also bind Man-6-P-based ligands with high affinity, and the amount of binding can be attributed entirely to the immunoprecipitated wild-type receptors. Anchoring of both C-terminal ends of the heterodimer produces optimal binding of both IGF-II and Man-6-P ligands. Thus, IGF-II binds independently to both subunits of the dimeric mannose 6-phosphate/insulin-like growth factor II receptor. Although wild-type/mutant hetero-oligomers form readily when mixed, it appears that multivalent Man-6-P ligands bind preferentially to wild-type sites, possibly by cross-bridging receptors within clusters of immobilized receptors.

Interactions of IGF-II with the IGF2R/cation-independent mannose-6-phosphate receptor mechanism and biological outcomes

The cation-independent mannose-6-phosphate/insulin-like growth factor-II receptor (IGF2R) is a membrane-bound glycoprotein consisting of 15 homologous extracellular repeat domains. The major function of this receptor is trafficking of lysosomal enzymes from the trans-Golgi network to the endosomes and their subsequent transfer to lysosomes. The IGF2R also plays a major role in binding and regulating the circulating and tissue levels of IGF-II. As this ligand is important for cell growth, survival, and migration, the maintenance of correct IGF-II levels influences its actions in normal growth and development. Deregulation of IGF2R expression has therefore been associated with growth related disease and cancer. This review highlights recent advances in understanding the IGF2R structure and mechanism of interaction with its ligands, in particular IGF-II. Recent mutagenesis studies combined with the crystal structure of domains 11-14 in complex with IGF-II have mapped the sites of interaction and explain how the IGF2R specificity for IGF-II is achieved. The role of domain 13 in high-affinity IGF-II binding is also revealed. Characterization of ligand:IGF2R interactions is vital for the understanding of the mechanism of IGF2R actions and will allow the development of specific cancer therapies in the future.

Mannose 6-phosphate/insulin-like growth factor 2 receptor limits cell invasion by controlling alphaVbeta3 integrin expression and proteolytic processing of urokinase-type plasminogen activator receptor

The multifunctional mannose 6-phosphate/insulin-like growth factor 2 receptor (M6P/IGF2R) is considered a tumor suppressor. We report here that RNA interference with M6P/IGF2R expression in urokinase-type plasminogen activator (uPA)/urokinase-type plasminogen activator receptor (uPAR) expressing human cancer and endothelial cells resulted in increased pericellular plasminogen activation, cell adhesion, and higher invasive potential through matrigel. M6P/IGF2R silencing led also to the cell surface accumulation of urokinase and plasminogen and enhanced expression of alphaV integrins. Genetic rescue experiments and inhibitor studies revealed that the enhanced plasminogen activation was due to a direct effect of M6P/IGF2R on uPAR, whereas increased cell adhesion to vitronectin was dependent on alphaV integrin expression and not uPAR. Increased cell invasion of M6P/IGF2R knockdown cells was rescued by cosilencing both uPAR and alphaV integrin. Furthermore, we found that M6P/IGF2R expression accelerates the cleavage of uPAR. M6P/IGF2R silencing resulted in an increased ratio of full-length uPAR to the truncated D2D3 fragment, incapable of binding most uPAR ligands. We conclude that M6P/IGF2R controls cell invasion by regulating alphaV integrin expression and by accelerating uPAR cleavage, leading to the loss of the urokinase/vitronectin/integrin-binding site on uPAR.

Retinoic acid prevents Chlamydia pneumoniae-induced foam cell development in a mouse model of atherosclerosis

Chlamydia pneumoniae, a common respiratory pathogen, has been associated with cardiovascular disease. C. pneumoniae infection accelerates atherosclerotic lesion development in hyperlipidemic animals. Retinoic acid, an anti-oxidant, inhibits infection of endothelial cells by C. pneumoniae. The present study demonstrated that retinoic acid suppresses the acceleration of foam cell lesion development induced by C. pneumoniae in hyperlipidemic C57BL/6J mice. Retinoic acid treatment had no effect on foam cell lesion development in uninfected animals. Lung infection and duration was decreased in treated mice, suggesting one mechanism by which retinoic acid reduces C. pneumoniae-accelerated foam cell lesion formation in hyperlipidemic mice.

Strategies for carbohydrate recognition by the mannose 6-phosphate receptors

The two members of the P-type lectin family, the 46 kDa cation-dependent mannose 6-phosphate receptor (CD-MPR) and the 300 kDa cation-independent mannose 6-phosphate receptor (CI-MPR), are ubiquitously expressed throughout the animal kingdom and are distinguished from all other lectins by their ability to recognize phosphorylated mannose residues. The best-characterized function of the MPRs is their ability to direct the delivery of approximately 60 different newly synthesized soluble lysosomal enzymes bearing mannose 6-phosphate (Man-6-P) on their N-linked oligosaccharides to the lysosome. In addition to its intracellular role in lysosome biogenesis, the CI-MPR, but not the CD-MPR, participates in a number of other biological processes by interacting with various molecules at the cell surface. The list of extracellular ligands recognized by this multifunctional receptor has grown to include a diverse spectrum of Man-6-P-containing proteins as well as several non-Man-6-P-containing ligands. Recent structural studies have given us a clearer view of how these two receptors use related, but yet distinct, approaches in the recognition of phosphomannosyl residues.

Mannose-6-phosphate receptors (MPR 300 and 46) from the highly evolved invertebrate Asterias rubens (Echinodermate): biochemical and functional characterization of MPR 46 protein

Mammalian mannose 6-phosphate receptors (MPR 300 and 46) mediate transport of lysosomal enzymes to lysosomes. Recent studies established that the receptors are conserved throughout vertebrates. Although we purified the mollusc receptors and identified only a lysosomal enzyme receptor protein (LERP) in the Drosophila melanogaster, little is known about their structure and functional roles in the invertebrates. In the present study, we purified the putative receptors from the highly evolved invertebrate, starfish, cloned the cDNA for the MPR 46, and expressed it in mpr((-/-)) mouse embryonic fibroblast cells. Structural comparison of starfish receptor sequences with other vertebrate receptors gave valuable information on its extensive structural homology with the vertebrate MPR 46 proteins. The expressed protein efficiently sorts lysosomal enzymes within the cells establishing a functional role for this protein. This first report on the invertebrate MPR 46 further confirms the structural and functional conservation of the receptor not only in the vertebrates but also in the invertebrates.

Retinoic acid inhibits the infectivity and growth of Chlamydia pneumoniae in epithelial and endothelial cells through different receptors

Chlamydia pneumoniae is a human respiratory pathogen that has also been associated with cardiovascular disease. C. pneumoniae infection accelerates atherosclerotic plaque development in hyperlipidemic animals and promotes oxidation of low density lipoprotein in vitro. All-trans-retinoic acid (ATRA), an antioxidant, has been shown to inhibit C. pneumoniae infectivity for endothelial cells by preventing binding of the organism to the M6P/IGF2 receptor on the cell surface. This current study investigates whether ATRA similarly affects C. pneumoniae infectivity of epithelial cells, which are the primary site of infection in the respiratory tract, and the effects on intracellular growth in both endothelial and epithelial cells. Because ATRA binds to both the nuclear retinoid acid receptor (RAR) and the M6P/IGF2 receptor, 4-[(E)-2-(5,6,7,8-tetrahydro-5,5,8,8-tetramethyl-2-naphthalenyl)-1-propenyl]benzoic acid (TTNPB), an ATRA analog, which binds to the RAR but not the M6P/IGF2 receptor was used to differentiate the receptor mediating the effects of ATRA. The results of this study showed two separate effects of ATRA. The first effect is through interaction with the M6P/IGF2 receptor on the cell surface preventing attachment of the organism (inhibition by ATRA but not TTNPB) in endothelial cells and the second is through the nuclear receptor (inhibition by both ATRA and TTNPB) which inhibits growth in both epithelial and endothelial cells.

Retinoic acid stimulation of the sodium/iodide symporter in MCF-7 breast cancer cells is mediated by the insulin growth factor-I/phosphatidylinositol 3-kinase and p38 mitogen-activated protein kinase signaling pathways

CONTEXT

All-trans retinoic acid (tRA) induces differentiation in MCF-7 breast cancer cells, stimulates sodium/iodide symporter (NIS) gene expression, and inhibits cell proliferation. Radioiodine administration after systemic tRA treatment has been proposed as an approach to image and treat some differentiated breast cancer.

OBJECTIVE

The objective of this work was to study the relative role of genomic and nongenomic pathways in tRA stimulation of NIS expression in MCF-7 cells.

DESIGN

We inspected the human NIS gene locus for retinoic acid-responsive elements and tested them for function. The effects of signal transduction pathway inhibitors were also tested in tRA-treated MCF-7 cells and TSH-stimulated FRTL-5 rat thyroid cells, followed by iodide uptake assay, quantitative RT-PCR of NIS, and cell cycle phase analysis.

RESULTS

Multiple retinoic acid response elements around the NIS locus were identified by sequence inspection, but none of them was a functional tRA-induced element in MCF-7 cells. Inhibitors of the IGF-I receptor, Janus kinase, and phosphatidylinositol 3-kinase (PI3K), significantly reduced NIS mRNA expression and iodide uptake in tRA-stimulated MCF-7 cells but not FRTL-5 cells. An inhibitor of p38 MAPK significantly reduced iodide uptake in both tRA-stimulated MCF-7 cells and TSH-stimulated FRTL-5 cells. IGF-I and PI3K inhibitors did not significantly reduce the basal NIS mRNA expression in MCF-7 cells. Despite the chronic inhibitory effects on cell proliferation, tRA did not reduce the S-phase distribution of MCF-7 cells during the period of NIS induction.

CONCLUSION

The IGF-I receptor/PI3K pathway mediates tRA-stimulated NIS expression in MCF-7 but not FRTL-5 thyroid cells.

Mannose 6-phosphate receptor targeting and its applications in human diseases

The cation-independent mannose 6-phosphate receptor is a multifunctional protein which binds at the cell surface to two distinct classes of ligands, the mannose 6-phosphate (M6P) bearing proteins and IGF-II. Its major function is to bind and transport M6P-enzymes to lysosomes, but it can also modulate the activity of a variety of extracellular M6P-glycoproteins (i.e., latent TGFbeta precursor, urokinase-type plasminogen activator receptor, Granzyme B, growth factors, Herpes virus). The purpose of this review is to highlight the synthesis and potential use of high affinity M6P analogues able to target this receptor. Several M6P analogues with phosphonate, carboxylate or malonate groups display a higher affinity and a stronger stability in human serum than M6P itself. These derivatives could be used to favour the delivery of specific therapeutic compounds to lysosomes, notably in enzyme replacement therapies of lysosomal diseases or in neoplastic drug targeting. In addition, their potential applications in preventing clinical disorders, which are associated with the activities of other M6P-proteins involved in wound healing, cell growth or viral infection, will be discussed.

Heterotrimeric G proteins and the single-transmembrane domain IGF-II/M6P receptor: functional interaction and relevance to cell signaling

The G protein-coupled receptor (GPCR) family represents the largest and most versatile group of cell surface receptors. Classical GPCR signaling constitutes ligand binding to a seven-transmembrane domain receptor, receptor interaction with a heterotrimeric G protein, and the subsequent activation or inhibition of downstream intracellular effectors to mediate a cellular response. However, recent reports on direct, receptor-independent G protein activation, G protein-independent signaling by GPCRs, and signaling of nonheptahelical receptors via trimeric G proteins have highlighted the intrinsic complexities of G protein signaling mechanisms. The insulin-like growth factor-II/mannose-6 phosphate (IGF-II/M6P) receptor is a single-transmembrane glycoprotein whose principal function is the intracellular transport of lysosomal enzymes. In addition, the receptor also mediates some biological effects in response to IGF-II binding in both neuronal and nonneuronal systems. Multidisciplinary efforts to elucidate the intracellular signaling pathways that underlie these effects have generated data to suggest that the IGF-II/M6P receptor might mediate transmembrane signaling via a G protein-coupled mechanism. The purpose of this review is to outline the characteristics of traditional and nontraditional GPCRs, to relate the IGF-II/M6P receptor's structure with its role in G protein-coupled signaling and to summarize evidence gathered over the years regarding the putative signaling of the IGF-II/M6P receptor mediated by a G protein.

Insulin-Like Growth Factor (IGF) 2 Stimulates Steroidogenesis and Mitosis of Bovine Granulosa Cells Through the IGF1 Receptor: Role of Follicle-Stimulating Hormone and IGF2 Receptor1

Little is known regarding the role of insulin-like growth factor 2 (IGF2) and the regulation of the IGF2 receptor (IGF2R) during follicular development. Granulosa cells were collected from small (1-5 mm) and large (8-22 mm) bovine follicles and were treated with IGF2 for 1-2 days in serum-free medium, and steroid production, cell proliferation, specific (125)I-IGF2 binding, and gene expression were quantified. IGF2 increased both estradiol and progesterone production by granulosa cells, and cells from large follicles were more responsive to the effects of IGF2 than those from small follicles. Abundance of aromatase (CYP19A1) mRNA was stimulated by IGF2 and IGF1. The effective dose (ED(50)) of IGF2 stimulating 50% of the maximal estradiol production was 63 ng/ml for small follicles and 12 ng/ml for large follicles, and these values were not affected by FSH. The ED(50) of IGF2 for progesterone production was 20 ng/ml for both small and large follicles. IGF2 also increased proliferation of granulosa cells by 2- to 3-fold, as determined by increased cell numbers and (3)H-thymidine incorporation into DNA. Treatment with IGF1R antibodies reduced the stimulatory effect of IGF2 and IGF1 on estradiol production and cell proliferation. Specific receptors for (125)I-IGF2 existed in granulosa cells, and 2-day treatment with estradiol, FSH, or cortisol had no significant effect on specific (125)I-IGF2 binding. Also, FSH treatment of small- and large-follicle granulosa cells had no effect on IGF2R mRNA levels, whereas IGF1 decreased IGF2R mRNA and specific (125)I-IGF2 binding. Granulosa cell IGF2R mRNA abundance was 3-fold greater in small than in large follicles. These findings support the hypothesis that both IGF2 and its receptor may play a role in granulosa cell function during follicular development. In particular, increased free IGF1 in developing follicles may decrease synthesis of IGF2R, thereby allowing for more IGF2 to be bioavailable (free) for induction of steroidogenesis and mitogenesis via the IGF1R.

The luminal domain participates in the endosomal trafficking of the cation-independent mannose 6-phosphate receptor

Although the role of the cytoplasmic tail of the cation-independent mannose 6-phosphate receptor (CIMPR) has been well established in the receptor trafficking, that of the luminal domain is still controversial. We noticed that the peripheral distribution of GFP, fused to the transmembrane and cytoplasmic domains of CIMPR (G-CIMPR-tail), was distinct from that of endogenous CIMPR or of GFP fused to the full-length CIMPR (G-CIMPR-full). By live-cell imaging, trans-Golgi-network (TGN)-derived transport carriers containing G-CIMPR-full more frequently stopped and overlapped with transferrin-containing endosomes in the peripheral region than those containing G-CIMPR-tail. G-CIMPR-full was recycled back to the perinuclear TGN more slowly than that for G-CIMPR-tail, evidenced by fluorescence recovery after photobleaching analysis. Moreover, endogenous CIMPR and G-CIMPR-full, but not GFP-CIMPR-tail, drastically altered the characteristic distribution after treatment with chloroquine. A mutant receptor, G-CIMPR-full R/A, that cannot recognize the mannose 6-phosphate (M6P)-signal, behaved similarly to G-CIMPR-full, indicating that these differences are not attributable to the M6P-ligands binding situation. Interestingly, we also found that U18666A treatment was able to discriminate the M6P-ligand binding-dependent trafficking of CIMPR. Based on these findings, we propose that the CIMPR luminal domain is required for tight interaction with endocytic compartments, and retention by them, and that there are additional transport steps, in which the binding to M6P-ligands is involved.

Mannose-6-phosphate/Insulin-like Growth Factor II Receptor Expression and Tumor Development

The mannose-6-phosphate/insulin-like growth factor II receptor (M6P/IGF-IIR) is a multi-functional transmembrane glycoprotein whose major function is to bind and transport M6P-bearing glycoproteins from the trans-Golgi network or the cell surface to lysosomes. The cell surface M6P/IGF-IIR also bind and internalizes the insulin-like growth factor II. The receptor gene is considered a « candidate » tumor suppressor gene. The phenotypic consequences of loss of M6P/IGF-IIR through somatic mutation are potentially very complex since M6P/IGF-IIR has a number of roles in cellular physiology. Loss of function mutations in M6P/IGF-IIR gene could contribute to multi-step carcinogenesis. In the light of the multi-functional cellular potential roles of the M6P/IGF-IIR the purpose of this review is to highlight some recent data concerning its normal functions and the potential role of its loss in tumor pathophysiology with the aim to try to clarify the possible underlying mechanisms of its involvement in tumor development.

Synthesis and receptor binding affinity of carboxylate analogues of the mannose 6-phosphate recognition marker

The mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF2R) is involved in multiple physiological pathways including targeting of lysosomal enzymes, degradation of IGF2, and cicatrization through TGFbeta activation. To target potential therapeutics to this membrane receptor, four carboxylate analogues of mannose 6-phosphate (M6P) were synthesized. Three of them, two isosteric carboxylate analogues and a malonate derivative, showed a binding affinity for the M6P/IGF2R equivalent to or higher than that of M6P. Contrary to M6P, all these analogues were particularly stable in human serum. Moreover, these derivatives did not present any cytotoxic activity against two human cell lines. These analogues represent a new potential for the lysosomal targeting of enzyme replacement therapy in lysosomal diseases or to prevent the membrane-associated activities of the M6P/IGF2R.

Single transmembrane domain insulin-like growth factor-II/mannose-6-phosphate receptor regulates central cholinergic function by activating a G-protein-sensitive, protein kinase C-dependent pathway

The insulin-like growth factor-II/mannose-6-phosphate (IGF-II/M6P) receptor is a single-pass transmembrane glycoprotein that plays an important role in the intracellular trafficking of lysosomal enzymes and endocytosis-mediated degradation of IGF-II. However, its role in signal transduction after IGF-II binding remains unclear. In the present study, we report that IGF-II/M6P receptor in the rat brain is coupled to a G-protein and that its activation by Leu27IGF-II, an analog that binds rather selectively to the IGF-II/M6P receptor, potentiates endogenous acetylcholine release from the rat hippocampal formation. This effect is mediated by a pertussis toxin (PTX)-sensitive GTP-binding protein and is dependent on protein kinase Calpha (PKCalpha)-induced phosphorylation of downstream substrates, myristoylated alanine-rich C kinase substrate, and growth associated protein-43. Additionally, treatment with Leu27IGF-II causes a reduction in whole-cell currents and depolarization of cholinergic basal forebrain neurons. This effect, which is blocked by an antibody against the IGF-II/M6P receptor, is also sensitive to PTX and is mediated via activation of a PKC-dependent pathway. These results together revealed for the first time that the single transmembrane domain IGF-II/M6P receptor expressed in the brain is G-protein coupled and is involved in the regulation of central cholinergic function via the activation of specific intracellular signaling cascades.

Delayed Onset ofIgf2-Induced Mammary Tumors inIgf2rTransgenic Mice

The insulin-like growth factor-II (IGF-II) receptor (IGF2R) regulates the level or activity of numerous proteins, including factors that control growth and differentiation. Frequent loss or inactivation of this receptor in a diverse group of tumors indicates that it may act as a tumor suppressor, but it is not known which functions of this receptor are selected against in the tumors. Lysosomal targeting and degradation of the growth-promoting IGF-II has been proposed as a mechanism for the tumor suppressor effects of IGF2R. As a genetic test of this hypothesis in vivo, we have produced Igf2r transgenic mice that ubiquitously express the transgene and have crossed these mice with mice that develop mammary tumors as a consequence of Igf2 overexpression. Our findings indicate that the presence of the Igf2r transgene delays mammary tumor onset and decreases tumor multiplicity in Igf2 transgenic mice. These findings are relevant to human tumors and preneoplastic conditions accompanied by altered IGF2 expression.

Lysosomes and lysosomal proteins in cancer cell death (new players of an old struggle)

Death of cancer cells influences tumor development and progression, as well as the response to anticancer therapies. This can occur through different cell death programmes which have recently been shown to implicate components of the acidic organelles, lysosomes. The role of lysosomes and lysosomal enzymes, including cathepsins and some lipid hydrolases, in programmed cell death associated with apoptotic or autophagic phenotypes is presented, as evidenced from observations on cultured cells and living animals. The possible molecular mechanisms that underlie the action of lysosomes during cell death are also described. Finally, the contribution of lysosomal proteins and lysosomes to tumor initiation and progression is discussed. Elucidation of this role and the underlying mechanisms will shed a new light on these 'old' organelles and hopefully pave the way for the development of novel anticancer strategies.

Synthesis and biological activity of M6-P and M6-P analogs on fibroblast and keratinocyte proliferation

A new synthetic route to obtain the carboxylate analog of mannose 6-phosphate (M6-P) is presented. The effects of the M6-P, the carboxylate and two other analogs (the phosphonate and the alpha,beta ethylenic carboxylate) on the proliferation of human keratinocytes and dermal fibroblasts as well as on the proliferation of a murine fibroblast cell line, 3T3-J2 are tested. We observed that M6-P is a potent inhibitor of proliferation of both fibroblasts and keratinocytes. Among its analogs, the phosphonate showed a similar effect on human dermal fibroblasts but not on keratinocytes.

Chlamydia pneumoniae uses the mannose 6-phosphate/insulin-like growth factor 2 receptor for infection of endothelial cells

Several mechanisms for attachment and entry of Chlamydia have been proposed. We previously determined that the major outer membrane protein of Chlamydia trachomatis is glycosylated with a high-mannose oligosaccharide, and a similar structure inhibited the attachment and infectivity of C. trachomatis in epithelial cells. Because insulin-like growth factor 2 (IGF2) was shown to enhance the infectivity of Chlamydia pneumoniae but not C. trachomatis in endothelial cells, a hapten inhibition assay was used to analyze whether the mannose 6-phosphate (M6P)/IGF2 receptor that also binds M6P could be involved in infection of endothelial cells (HMEC-1) by Chlamydia. M6P and mannose 6-phosphate-poly[N-(2-hydroxyethyl)-acrylamide] (M6P-PAA) inhibited the infectivity of C. pneumoniae AR-39, but not C. trachomatis serovar UW5 or L2, while mannan inhibited the growth of C. trachomatis, but not C. pneumoniae. Using metabolically labeled organisms incubated with cells at 4 degrees C (organisms attach but do not enter) or at 37 degrees C (organisms attach and are internalized), M6P-PAA was shown to inhibit attachment and internalization of C. pneumoniae in endothelial cells but did not inhibit attachment or internalization of C. trachomatis serovar E or L2. These findings indicate that C. pneumoniae can utilize the M6P/IGF2 receptor and that the use of this receptor for attachment and entry differs between C. pneumoniae and C. trachomatis.

Developmental differences between cation-independent and cation-dependent mannose-6-phosphate receptors in rat brain at perinatal stages

Mannose-6-phosphate receptors (MPRs) play a role in the selective transport of macromolecules bearing mannose-6-phosphate residue to lysosomes. To date, two types of MPRs have been described in most of cells and tissues: the cation-dependent (CD-MPR) and cation-independent mannose-6-phosphate receptor (CI-MPR). In order to elucidate their possible role in the central nervous system, the expression and binding properties of both MPRs were studied in rat brain along perinatal development. It was observed that the expression of CI-MPR decreases progressively from fetuses to adults, while the CD-MPR increases around the 10th day of birth, and maintains these values up to adulthood. Binding assays showed differences in the Bmax and KD values between the ages studied, and they did not correlate with the expression levels of both MPRs. Variations in lysosomal enzyme activities and expression of phosphomannosylated ligands during development correlated more with CD-MPR than with CI-MPR expression. These results suggest that both receptors play a different role in rat brain during perinatal development, being CD-MPR mostly involved in lysosome maturation.

Domain interactions of the mannose 6-phosphate/insulin-like growth factor II receptor

The mannose 6-phosphate/insulin-like growth factor II receptor (M6P/IGF2R) forms oligomeric structures important for optimal function in binding and internalization of Man-6-P-bearing extracellular ligands as well as lysosomal biogenesis and growth regulation. However, neither the mechanism of inter-receptor interaction nor the dimerization domain has yet been identified. We hypothesized that areas near the ligand binding domains of the receptor would contribute preferentially to oligomerization. Two panels of minireceptors were constructed that involved truncations of either the N- or C-terminal regions of the M6P/IGF2R encompassing deletions of various ligand binding domains. alpha-FLAG or alpha-Myc-based immunoprecipitation assays showed that all of the minireceptors tested were able to associate with a full-length, Myc-tagged M6P/IGF2R (WT-M). In the alpha-FLAG but not alpha-Myc immunoprecipitation assays, the degree of association of a series of C-terminally truncated minireceptors with WT-M showed a positive trend with length of the minireceptor. In contrast, length did not seem to affect the association of the N-terminally truncated minireceptors with WT-M, except that the 12th extracytoplasmic repeat appeared exceptionally important in dimerization in the alpha-FLAG assays. The presence of mutations in the ligand-binding sites of the minireceptors had no effect on their ability to associate with WT-M. Thus, association within the heterodimers was not dependent on the presence of functional ligand binding domains. Heterodimers formed between WT-M and the minireceptors demonstrated high affinity IGF-II and Man-6-P-ligand binding, suggesting a functional association. We conclude that there is no finite M6P/IGF2R dimerization domain, but rather that interactions between dimer partners occur all along the extracytoplasmic region of the receptor.

The role of insulin-like growth factor II and its receptor in mouse preimplantation development

Insulin-like growth factor II (IGF-II) and its receptor, the IGF-II/mannose-6-phosphate (IGF-II/M6P) receptor, are first expressed from the zygotic genome at the two-cell stage of mouse development. However, their role is not clearly defined. Insulin-like growth factor II is believed to mediate growth through the heterologous type 1 IGF and insulin receptors, whereas the IGF-II/M6P receptor is believed to act as a negative regulator of somatic growth by limiting the availability of excess levels of IGF-II. These studies demonstrate that IGF-II does have a role in growth regulation in the early embryo through the IGF-II/M6P receptor. Insulin-like growth factor II stimulated cleavage rate in two-cell embryos in vitro. Moreover, this receptor is required for the glycaemic response of two-cell embryos to IGF-II and for normal progression of early embryos to the blastocyst stage. Improved development of embryos in crowded culture supports the concept of an endogenous embryonic paracrine activity that enhances cell proliferation. These responses indicate that the IGF-II/M6P receptor is functional and likely to participate in such a regulatory circuit. The functional role of IGF-II and its receptor is discussed with reference to regulation of early development.

Demonstration of tumor suppression by mannose 6-phosphate/insulin-like growth factor 2 receptor

The mannose 6-phosphate/IGF-2 receptor has been proposed to be a tumor suppressor gene on the basis of loss of heterozygosity and mutations in tumors from cancer patients. To test this hypothesis, the receptor was expressed in 66cl4, a mouse mammary tumor cell line deficient in the receptor. Expression of the receptor corrected the abnormal lysosomal trafficking phenotype displayed by these cells. Receptor expression had no apparent effect on growth or invasiveness of the cells in vitro but effectively inhibited formation of mammary tumors in BALB/c mice. Analysis of cell proliferation and apoptosis in tumors indicated that the primary effect of the receptor was to inhibit cell proliferation. Proliferation indices for receptor-deficient and receptor-expressing tumors, as determined by BrdU incorporation, were 24.6 and 7.6%, respectively. No significant effect of receptor expression on apoptosis was observed. Receptor expression similarly inhibited tumor growth in BALB/c scid mice indicating that cytotoxic T cells and other components of the immune system missing in scid mice are not involved in the receptor's tumor suppressing effect. These findings establish a role for the receptor as a bona fide tumor suppressor gene and together with previous studies, suggest an important role for the receptor in human and rodent cancers.

Gene expression studies provide clues to the pathogenesis of uterine leiomyoma: new evidence and a systematic review

BACKGROUND

Uterine leiomyomas are extremely common and a major cause of pelvic pain, bleeding, infertility, and the leading indication for hysterectomy. Familial and epidemiological studies provide compelling evidence that genetic alterations play an important role in leiomyoma development.

METHODS

Using Affymetrix U133A GeneChip we analysed expression profiles of 22,283 genes in paired samples of leiomyoma and adjacent normal myometrium. We compared our results with previously published data on gene expression in uterine leiomyoma and identified the overlapping gene alterations.

RESULTS

We detected 80 genes with average differences of > or = 2-fold and false discovery rates of < 5% (14 overexpressed and 66 underexpressed). A comparative analysis including eight previous gene expression studies revealed eight prominent genes (ADH1, ATF3, CRABP2, CYR61, DPT, GRIA2, IGF2, MEST) identified by at least five different studies, eleven genes (ALDH1, CD24, CTGF, DCX, DUSP1, FOS, GAGEC1, IGFBP6, PTGDS, PTGER3, TYMS) reported by four studies, twelve genes (ABCA, ANXA1, APM2, CCL21, CDKN1A, CRMP1, EMP1, ESR1, FY, MAP3K5, TGFBR2, TIMP3) identified by three studies, and 40 genes reported by two different studies.

CONCLUSIONS

Review of gene expression data revealed concordant changes in genes regulating retinoid synthesis, IGF metabolism, TGF-beta signaling and extracellular matrix formation. Gene expression studies provide clues to the relevant pathways of leiomyoma development.

Insulin-like growth factor II induces apoptosis in osteoblasts

The ability of insulin-like growth factor II (IGF-II) to modulate apoptosis was studied in murine osteoblasts. At 72 h of culture, 0.01, 0.1, and 1.0 nM IGF-II produced a dose-dependent increase in apoptosis assayed by TdT-mediated dUTP-biotin nick end labeling (TUNEL) and confirmed with acridine orange-ethidium bromide staining. A maximal increase of 5.0-fold above control was found with 1 nM IGF-II. A time course of treatment with 0.1 nM IGF-II demonstrated a significant increase in apoptosis compared to vehicle-treated cells by 48 h. IGF-II-induced apoptosis could not be inhibited by a blocking antibody to the IGF-I receptor. Human osteoblast cultures demonstrated a similar dose-dependent increase in apoptosis with IGF-II. No significant effect of IGF-II was found on proliferation in murine osteoblast cultures. Western blot analysis demonstrated that IGF-II decreased Bcl-2 protein levels, but not Bax, resulting in a significant reduction in the Bcl-2/Bax ratio. To determine if overexpression of Bcl-2 could block IGF-II-induced apoptosis, osteoblasts were isolated from a transgenic mouse that overexpresses human Bcl-2 in bone through a construct utilizing the 2.3 kb promoter region of the Type I collagen gene linked to a 1.8 kb region of human Bcl-2 (Col2.3Bcl-2). At 72 h, IGF-II significantly increased apoptosis in a dose-dependent manner in osteoblast cultures from the control littermates. In osteoblasts from Col2.3Bcl-2 mice, no significant effect on apoptosis was found with 0.01, 0.1, or 1.0 nM IGF-II. Western blot analysis of Bcl-2 and Bax levels demonstrated a transient decrease in the Bcl-2/Bax ratio at 24 h with no decrease in the ratio at 48 or 72 h. Thus, IGF-II appears to promote osteoblast apoptosis, and overexpression of Bcl-2 is able to block IGF-II-induced apoptosis.

Identification of a low affinity mannose 6-phosphate-binding site in domain 5 of the cation-independent mannose 6-phosphate receptor

The 300-kDa cation-independent mannose 6-phosphate receptor (CI-MPR) and the 46-kDa cation-dependent MPR (CD-MPR) are type I integral membrane glycoproteins that play a critical role in the intracellular delivery of newly synthesized mannose 6-phosphate (Man-6-P)-containing acid hydrolases to the lysosome. The extracytoplasmic region of the CI-MPR contains 15 contiguous domains, and the two high affinity ( approximately 1 nm) Man-6-P-binding sites have been mapped to domains 1-3 and 9, with essential residues localized to domains 3 and 9. Domain 5 of the CI-MPR exhibits significant sequence homology to domains 3 and 9 as well as to the CD-MPR. A structure-based sequence alignment was performed that predicts that domain 5 contains the four conserved key residues (Gln, Arg, Glu, and Tyr) identified as essential for carbohydrate recognition by the CD-MPR and domains 3 and 9 of the CI-MPR, but lacks two cysteine residues predicted to form a disulfide bond within the binding pocket. To determine whether domain 5 harbors a carbohydrate-binding site, a construct that encodes domain 5 alone (Dom5His) was expressed in Pichia pastoris. Microarray analysis using 30 different oligosaccharides demonstrated that Dom5His bound specifically to a Man-6-P-containing oligosaccharide (pentamannosyl 6-phosphate). Frontal affinity chromatography showed that the affinity of Dom5His for Man-6-P was approximately 300-fold lower (K(i) = 5.3 mm) than that observed for domains 1-3 and 9. The interaction affinity for the lysosomal enzyme beta-glucuronidase was also much lower (K(d) = 54 microm) as determined by surface plasmon resonance analysis. Taken together, these results demonstrate that the CI-MPR contains a third Man-6-P recognition site that is located in domain 5 and that exhibits lower affinity than the carbohydrate-binding sites present in domains 1-3 and 9.

Differential expression of heparin-binding EGF-like growth factor (HB-EGF) mRNA in normal human keratinocytes induced by a variety of natural and synthetic retinoids

It was recently revealed that epidermal growth following topical treatment with all-trans retinoic acid (atRA) was at least partly induced by heparin-binding epidermal growth factor-like growth factor (HB-EGF) released from suprabasal keratinocytes. Since proliferation of keratinocytes appears to be one of the critical roles of atRA in depigmentation treatment and promotion of wound healing, HB-EGF is considered suitable for assessing the therapeutic value of topical retinoids. In this study, HB-EGF mRNA expression in normal human keratinocytes after atRA treatment was examined, and the effects of a variety of natural and synthetic retinoids were compared. The results of reverse transcription polymerase chain reaction (RT-PCR) suggested that induction of differentiation increased HB-EGF mRNA expression in cultured keratinocytes. Real-time PCR analyses revealed that HB-EGF mRNA expression was elevated dose-dependently with atRA, peaking at 12 h. This elevation was more prominent in confluent keratinocytes than in subconfluent cells, suggesting that differentiated keratinocytes are more subject to stimulation of HB-EGF expression by atRA than proliferating keratinocytes. HB-EGF mRNA was upregulated in differentiation-induced keratinocytes by all retinoids used in this study at 1 micromol/l, and marked upregulation was seen when treated with three isotypes of retinoic acid (atRA, and 9-cis and 13-cis retinoic acid). RARalpha-selective agonists (Am80, Am580, ER-38925, and TAC-101) and a panagonist of RARs (Re80) caused relatively low elevation of HB-EGF transcripts, as did all-trans retinol (Rol) and all-trans retinal (Ral). Although another panagonist (Ch55) showed the highest elevation of HB-EGF mRNA, it was relatively cytotoxic at the concentration employed. Ral and Rol were found to upregulate HB-EGF when used at 100 micromol/l to 1 mmol/l, to a similar extent of atRA at 1-10 micromol/l. The capacity of retinoids to upregulate HB-EGF may be an important index for investigation and development of an ideal synthetic retinoid, which has maximum benefits and minimum side-effects

The insulin-like growth factor-II/mannose-6-phosphate receptor: structure, distribution and function in the central nervous system

The insulin-like growth factor-II/mannose-6-phosphate (IGF-II/M6P) receptor is a multifunctional single transmembrane glycoprotein which, along with the cation-dependent M6P (CD-M6P) receptor, mediates the trafficking of M6P-containing lysosomal enzymes from the trans-Golgi network (TGN) to lysosomes. Cell surface IGF-II/M6P receptors also function in the degradation of the non-glycosylated IGF-II polypeptide hormone, as well as in the capture and activation/degradation of extracellular M6P-bearing ligands. In recent years, the multifaceted role of the receptor has become apparent, as several lines of evidence have indicated that in addition to its role in lysosomal enzyme trafficking, clearance and/or activation of a variety of growth factors and endocytosis-mediated degradation of IGF-II, the IGF-II/M6P receptor may also mediate transmembrane signal transduction in response to IGF-II binding under certain conditions. However, very little is known about the physiological significance of the receptor in the function of the central nervous system (CNS). This review aims to delineate what is currently known about IGF-II/M6P receptor structure, its ligand binding properties and role in lysosomal enzyme transport. It also summarizes the recent data regarding the role of the receptor in the CNS, including its distribution, possible importance for normal and activity-dependent functioning as well as its implications in neurodegenerative disorders such as Alzheimer's disease (AD).

Structure of uPAR, plasminogen, and sugar-binding sites of the 300 kDa mannose 6-phosphate receptor

The 300 kDa cation-independent mannose 6-phosphate receptor (CI-MPR) mediates the intracellular transport of newly synthesized lysosomal enzymes containing mannose 6-phosphate on their N-linked oligosaccharides. In addition to its role in lysosome biogenesis, the CI-MPR interacts with a number of different extracellular ligands at the cell surface, including latent transforming growth factor-beta, insulin-like growth factor-II, plasminogen, and urokinase-type plasminogen activator receptor (uPAR), to regulate cell growth and motility. We have solved the crystal structure of the N-terminal 432 residues of the CI-MPR at 1.8 A resolution, which encompass three out of the 15 repetitive domains of its extracytoplasmic region. The three domains, which exhibit similar topology to each other and to the 46 kDa cation-dependent mannose 6-phosphate receptor, assemble into a compact structure with the uPAR/plasminogen and the carbohydrate-binding sites situated on opposite faces of the molecule. Knowledge of the arrangement of these three domains has allowed us to propose a model of the entire extracytoplasmic region of the CI-MPR that provides a context with which to envision the numerous binding interactions carried out by this multi-faceted receptor.

Differential modulation of rat hepatic stellate phenotype by natural and synthetic retinoids

Activation of hepatic stellate cells (HSC) is a central event in the pathogenesis of liver fibrosis during chronic liver injury. We examined the expression of retinoic acid (RAR) and retinoid X receptors (RXR) during HSC activation and evaluated the influence of natural and synthetic retinoic acids (RA) on the phenotype of culture-activated HSC. The expression of the major RAR/RXR subtypes and isoforms was analyzed by Northern hybridization. Presence of functional receptor proteins was established by gel shift analysis. Retinoic acids, RAR, and RXR selective agonists and an RAR antagonist were used to evaluate the effects of retinoid signalling on matrix synthesis by Northern blotting and immunoprecipitation, and on cell proliferation by BrdU incorporation. The 9-cisRA and synthetic RXR agonists reduced HSC proliferation and synthesis of collagen I and fibronectin. All-trans RA and RAR agonists both reduced the synthesis of collagen I, collagen III, and fibronectin, but showed a different effect on cell proliferation. Synthetic RAR agonists did not affect HSC proliferation, indicating that ATRA inhibits cell growth independent of its interaction with RARs. In contrast, RAR specific antagonists enhance HSC proliferation and demonstrate that RARs control proliferation in a negative way. In conclusion, natural RAs and synthetic RAR or RXR specific ligands exert differential effects on activated HSC. Our observations may explain prior divergent results obtained following retinoid administration to cultured stellate cells or to animals subjected to fibrogenic stimuli.

Increased expression of the mannose 6-phosphate/insulin-like growth factor-II receptor in breast cancer cells alters tumorigenic properties in vitro and in vivo

The mannose 6-phosphate/insulin-like growth factor-II receptor (M6P/IGF-IIR) is thought to act as a suppressor of tumor growth by binding the mitogenic peptide IGF-II and modulating its extracellular levels via degradation. This receptor has been found to be absent or nonfunctional in a high proportion of breast tumors as a result of LOH and mutation of the gene. In our study, we have examined the effect of increasing expression of M6P/IGF-IIR on breast cancer cell tumorigenicity. MDA-MB-231 breast cancer cells stably transfected with M6P/IGF-IIR cDNA exhibited not only a greatly reduced ability to form tumors but also a markedly reduced growth rate in nude mice. In vitro, increased M6P/IGF-IIR expression resulted in 2-fold reduced uptake of IGF-II and was associated with reduced cellular invasiness and motility. Cells with increased M6P/IGF-IIR expression exhibited reduced phosphorylation of IGF-I receptor and p44/42 MAPK compared to vector transfectants, or wild-type MDA-MB-231 cells. These results therefore suggest that M6P/IGF-IIR levels can modulate breast cancer cell tumorigenicity by a mechanism that may involve altered IGF-I receptor signaling.