A radioligand binding assay for the insulin-like growth factor 2 receptor

Insulin-like growth factors 2 and 1 (IGF2 and IGF1) and insulin are closely related hormones that are responsible for the regulation of metabolic homeostasis, development and growth of the organism. Physiological functions of insulin and IGF1 are relatively well-studied, but information about the role of IGF2 in the body is still sparse. Recent discoveries called attention to emerging functions of IGF2 in the brain, where it could be involved in processes of learning and memory consolidation. It was also proposed that these functions could be mediated by the receptor for IGF2 (IGF2R). Nevertheless, little is known about the mechanism of signal transduction through this receptor. Here we produced His-tagged domain 11 (D11), an IGF2-binding element of IGF2R; we immobilized it on the solid support through a well-defined sandwich, consisting of neutravidin, biotin and synthetic anti-His-tag antibodies. Next, we prepared specifically radiolabeled [125I]-monoiodotyrosyl-Tyr2-IGF2 and optimized a sensitive and robust competitive radioligand binding assay for determination of the nanomolar binding affinities of hormones for D11 of IGF2. The assay will be helpful for the characterization of new IGF2 mutants to study the functions of IGF2R and the development of new compounds for the treatment of neurological disorders.

Cation-dependent mannose-6-phosphate receptor functions as a pattern recognition receptor in anti-bacterial immunity of Marsupenaeus japonicus

The cation-dependent mannose-6-phosphate receptor (CD-MPR) is a member of the P-type lectin family. As a type I transmembrane glycoprotein, it functions in the delivery of newly synthesized acid hydrolases from the trans-Golgi network to endosomes for their subsequent transfer to the lysosome by binding the mannose-6-phosphate receptor-recognition moieties in the hydrolases. However, the functions of CD-MPR in immune responses are seldom reported. In the present study, we identified a CD-MPR-like molecule in Marsupenaeus japonicus and designed it as MjCD-MPR. It was significantly upregulated after challenge with Vibrio anguillarum at the mRNA and protein levels. Knockdown of MjCD-MPR resulted in a significant increase in the amount of V. anguillarum in the hemolymph of shrimp, which suggested that MjCD-MPR plays a role in shrimp antibacterial defense. The recombinant extracytoplasmic region of MjCD-MPR could bind gram-positive and gram-negative bacteria by interaction with peptidoglycan, lipopolysaccharide, and lipoteichoic acid. MjCD-MPR showed no direct bacteriostatic or bacteriocidal activity. Knockdown of MjCD-MPR decreased the expression levels of several antimicrobial peptides (Alf-C1, Alf-E1, Crustin I-2, and Crustin I-3), suggesting that MjCD-MPR promotes the expression of antimicrobial peptides in shrimp. In summary, working as a pattern recognition receptor, MjCD-MPR recognizes invading bacteria and triggers the expression of AMPs against bacterial infection in shrimp.

Development of IGF signaling antibody arrays for the identification of hepatocellular carcinoma biomarkers

Purpose

Our objective was to develop a system to simultaneously and quantitatively measure the expression levels of the insulin-like growth factor (IGF) family proteins in numerous samples and to apply this approach to profile the IGF family proteins levels in cancer and adjacent tissues from patients with hepatocellular carcinoma (HCC).

Experimental Design

Antibodies against ten IGF family proteins (IGF-1, IGF-1R, IGF-2, IGF-2R, IGFBP-1, IGFBP-2, IGFBP-3, IGFBP-4, IGFBP-6, and Insulin) were immobilized on the surface of a glass slide in an array format to create an IGF signaling antibody array. Tissue lysates prepared from patient's liver cancer tissues and adjacent tissues were then applied to the arrays. The proteins captured by antibodies on the arrays were then incubated with a cocktail of biotinylated detection antibodies and visualized with a fluorescence detection system. By comparison with standard protein amount, the exact protein concentrations in the samples can be determined. The expression levels of the ten IGF family proteins in 25 pairs of HCC and adjacent tissues were quantitatively measured using this novel antibody array technology. The differential expression levels between cancer tissues and adjacent tissues were statistically analyzed.

Results

A novel IGF signaling antibody array was developed which allows the researcher to simultaneously detect ten proteins involved in IGF signal pathway with high sensitivity and specificity. Using this approach, we found that the levels of IGF-2R and IGFBP-2 in HCC tissues were higher than those in adjacent tissues.

Conclusion

Our IGF signaling antibody array which can detect the expression of ten IGF family members with high sensitivity and specificity will undoubtedly prove a powerful tool for drug and biomarker discovery.

Molecular inflammation and adipose tissue matrix remodeling precede physiological adaptations to pregnancy

Changes in adipose tissue metabolism are central to adaptation of whole body energy homeostasis to pregnancy. To gain insight into the molecular mechanisms supporting tissue remodeling, we have characterized the longitudinal changes of the adipose transcriptome in human pregnancy. Healthy nonobese women recruited pregravid were followed in early (8-12 wk) and in late (36-38 wk) pregnancy. Adipose tissue biopsies were obtained in the fasting state from the gluteal depot. The adipose transcriptome was examined via whole genome DNA microarray. Expression of immune-related genes and extracellular matrix components was measured using real-time RT-PCR. Adipose mass, adipocyte size, and cell number increased in late pregnancy compared with pregravid measurements (P < 0.001) but remained unchanged in early pregnancy. The adipose transcriptome evolved during pregnancy with 10-15% of genes being differently expressed compared with pregravid. Functional gene cluster analysis revealed that the early molecular changes affected immune responses, angiogenesis, matrix remodeling, and lipid biosynthesis. Increased expression of macrophage markers (CD68, CD14, and the mannose-6 phosphate receptor) emphasized the recruitment of the immune network in both early and late pregnancy. The TLR4/NF-κB signaling pathway was enhanced specifically in relation to inflammatory adipokines and chemokines genes. We conclude that early recruitment of metabolic and immune molecular networks precedes the appearance of pregnancy-related physiological changes in adipose tissue. This biphasic pattern suggests that physiological inflammation is an early step preceding the development of insulin resistance, which peaks in late pregnancy.

Mannose-6-phosphate/insulin-like growth factor-II receptor in human melanoma cells: effect of ligands and antibodies on the receptor expression

BACKGROUND

The M6P/IGF-II receptor belongs to the IGF system which plays a crucial role in tumorigenicity. While the role of the IGF-I receptor in signal transduction is well documented, previous experiments failed to uncover a clear signalling function for the M6P/IGF-II receptor. However, more recent studies have shown the capability of M6P/IGF-II receptor to initiate transmembrane signalling.

MATERIALS AND METHODS

Human melanoma cells were used to detect the cell surface expression of the M6P/IGF-II receptor and its modulation by different effectors and monoclonal anti-receptor antibodies.

RESULTS

M6P (5 mM) caused an increase of the luminescent receptor signal of about 50% . Pre-incubation of cells with Act-D (5 microg/mL) or CHI (10 microg/mL) following M6P stimulation in the presence of the inhibitors caused a reduction of receptor cell surface expression of 27% or 31%, respectively. The monoclonal antibody (mAb) 2G11 was able to mimic the M6P effect on the receptor up-regulation but the mAb MEM-238 did not. The synergistic effect detected with the combination of M6P and the mAb 2G11 and the failure of 2G11 to compete with the M6P action suggests that both effectors have different binding sites on the receptor. Unlike 2G11 the mAb MEM-238 prevented the M6P effect on receptor up-regulation confirming partially overlapping binding epitopes of both effectors. Brefeldin A was shown to have an inhibiting effect on the vesicular transport of the receptor protein to the plasma membrane and forskolin had an activating effect on the receptor exocytosis with the following enhanced integration into the plasma membrane.

CONCLUSION

Up-regulation of the tumour suppressor M6P/IGF-II receptor might represent an approach for anticancer therapy. In addition, results support recent data on the receptor's capability of signal transduction.

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.

The discovery and mechanism of action of novel tumor-selective and apoptosis-inducing 3,5-diaryl-1,2,4-oxadiazole series using a chemical genetics approach

A novel series of 3,5-diaryl-oxadiazoles was identified as apoptosis-inducing agents through our cell and chemical genetics-based screening assay for compounds that induce apoptosis using a chemical genetics approach. Several analogues from this series including MX-74420 and MX-126374 were further characterized. MX-126374, a lead compound from this series, was shown to induce apoptosis and inhibit cell growth selectively in tumor cells. To elucidate the mechanism(s) by which this class of compounds alters the signal transduction pathway that ultimately leads to apoptosis, expression profiling using the Affymetrix Gene Chip array technology was done along with other molecular and biochemical analyses. Interestingly, we have identified several key genes (cyclin D1, transforming growth factor-beta1, p21, and insulin-like growth factor-BP3) that are altered in the presence of this compound, leading to characterization of the pathway for activation of apoptosis. MX-126374 also showed significant inhibition of tumor growth as a single agent and in combination with paclitaxel in murine tumor models. Using photoaffinity labeling, tail-interacting protein 47, an insulin-like growth factor-II receptor binding protein, was identified as the molecular target. Further studies indicated that down-regulation of tail-interacting protein 47 in cancer cells by small interfering RNA shows a similar pathway profile as compound treatment. These data suggest that 3,5-diaryl-oxadiazoles may be a new class of anticancer drugs that are tumor-selective and further support the discovery of novel drugs and drug targets using chemical genetic approaches.

HCG increases trophoblast migration in vitro via the insulin-like growth factor-II/mannose-6 phosphate receptor

We have previously shown that both HCG and insulin-like growth factor-II (IGF-II) stimulate trophoblastic invasion. Furthermore, the invasion-promoting function of IGF-II resulted from IGF-II mannose 6-phosphate receptor (IGF-II/M6PR) activation. Since HCG and IGF-II did not have an additive effect on cell migration of extravillous trophoblast (EVT) cell line, HTR-8 SVneo, we hypothesized that HCG actions are mediated via alterations in the expression and/or function of IGF-II axis. HCG treatment (50-50,000 mU/ml) of the HTR-8/SVneo cells did not alter the expression of either insulin-like growth factor-I or IGF-II mRNA or peptide synthesis, but caused (i) an increase in the (125)I-IGF-II binding to EVT cells, and (ii) an increase in the externalization rate of the IGF-II binding sites without affecting their internalization. This effect was due to the increase in the number of IGF-II binding sites in the plasma membrane without any change in the IGF-II binding affinity. Although HCG did not influence the abundance of IGF-II/M6PR mRNA or protein, anti-IGF-II/M6PR antibody decreased HCG-induced migration of EVT, supporting the hypothesis that HCG might stimulate EVT migration by increasing IGF-II binding to the plasma membrane and subsequently by increasing the IGF-II effect probably mediated via the IGF-II/M6PR.

An immuno-affinity method for the purification of mannose 6-phosphate receptor proteins

In a recent study, we have developed an ELISA method to quantify the mannose 6-phosphate receptor (MPR) proteins [J. Biochem. Biophys. Methods 52 (2002) 111]. In the present study, we have used the goat MPR 300 antibody and peptide specific antibodies to human MPR 46 to develop simple and efficient immuno-affinity matrices, which can be used to purify the MPR proteins from goat liver in a single step. The identity of the immuno-affinity purified receptors is confirmed by their molecular masses as well as by their immunoreactivity.

The insulin-like growth factor system in advanced breast cancer

Despite improvements in therapy, the prognosis for advanced breast cancer is poor and a search for new treatment targets and key regulators of tumour growth is warranted. Extensive data are available on the importance of the insulin-like growth factor (IGF) system in growth regulation of breast cancer cell lines in vitro, indicating that the IGF-I receptor (IGF-IR), IGF-I (and IGF-II) function as survival factors, while IGF binding protein (IGFBP)-3 may act as a growth inhibitor. There is a tight link between the growth regulatory pathways of IGFs and oestrogens in oestrogen-receptor(OR)-positive breast cancer cells. In vivo studies indicate a role of IGF-I and IGF-IR in breast cancer development. However, the importance of the IGF system in metastatic and highly aggressive breast tumours in vivo is not clear, and therapeutic strategies designed to interrupt IGF signalling have not yet proved to be an effective treatment modality in patients with metastatic breast cancer.

[Molecular therapy of breast carcinoma in the advanced phase]

Conventional chemotherapy regimens for the treatment of breast cancer have limited efficacy and are associated with significant toxicity, highlighting the need for novel targeted therapies. Increased expression and activation of receptor tyrosine kinases frequently occurs in human breast carcinomas and, therefore, several clinical trials are currently evaluating therapies targeting these receptors. Therapeutic strategies include blockade of individual receptors with monoclonal antibodies (e.g., trastuzumab) and inhibition of tyrosine kinase function (e.g., gefitinib). Trastuzumab is the first agent that has been approved for patients with human epidermal growth factor receptor 2 (HER2)-overexpressing breast cancer. Other growth-factor targeted drugs are in clinical development such as STI-571, farnesyl-transferase inhibitors and antibodies directed at the insulin-like growth factor.

Endocytosing the death sentence

A series of recent studies have suggested that endocytosis of the mannose-6-phosphate receptor (MPR)* might play a critical role in delivering the death signal to cells targeted for destruction by the immune system (for review see Barry and Bleackley, 2002). These studies have raised a number of controversial issues regarding the trafficking of proteins from the plasma membrane of the target cell to their substrates in the cytosol. In this issue, Trapani and colleagues examine the death of cells in which endocytosis of the MPR is blocked and show that the death signal is delivered effectively in the absence of MPR endocytosis (Trapani et al., 2002, this issue). How then is the death sentence delivered?

Stimulation of human extravillous trophoblast migration by IGF-II is mediated by IGF type 2 receptor involving inhibitory G protein(s) and phosphorylation of MAPK

We have earlier shown that migration and invasiveness of first trimester human extravillous trophoblast cells are stimulated by IGF-II, independently of IGF type 1 receptor and that migration stimulation is the primary reason for increased extravillous trophoblast cell invasiveness induced by IGF-II. In the present study we examined the functional role of IGF type II receptor in IGF-II stimulation of extravillous trophoblast cell migration and the underlying signal transduction pathways including the participation of inhibitory G protein(s) and MAPK. The migratory ability of a well characterized in vitro propagated human first trimester extravillous trophoblast cell line expressing the phenotype of extravillous trophoblast cells in situ was quantitated with a Transwell migration assay under different experimental conditions. We found that the extravillous trophoblast cells expressed an abundance of IGF type 2 receptor as detected by immunostaining and Western blots, and recombinant human IGF-II promoted their migration in a dose- and time-dependent manner. Both polyclonal and monoclonal IGF type 2 receptor-blocking antibodies blocked migration-stimulating effects of IGF-II. Two synthetic IGF-II analogs ([Leu27]IGF-II, which can bind to IGF type 2 receptor and IGF-binding proteins, but not IGF type 1 receptor, and [QAYL-Leu27]IGF-II, which can bind to IGFR-II, but neither IGFR-I nor IGF-binding proteins) both stimulated extravillous trophoblast cell migration to levels higher than those induced by wild-type IGF-II. These results reveal that IGF-II action was mediated by IGF type 2 receptor, independently of IGF type 1 receptor and IGF-binding proteins. Treatment of extravillous trophoblast cell membrane preparations with IGF-II decreased adenylyl cyclase activity in a concentration-dependant manner, indicating the participation of inhibitory G proteins in IGF-II action. This was substantiated further with the findings that increasing intracellular cAMP using forskolin or (Bu)2cAMP inhibited basal extravillous trophoblast cell migration and blocked IGF-II stimulation of migration. IGF-II treatment rapidly stimulated phosphorylation of MAPK (ERK-1 and -2), which was blocked by pretreatment of extravillous trophoblast cells with the MAPK kinase (MEK) inhibitor PD98059. Treatment with this inhibitor also blocked extravillous trophoblast cell migration in the presence or absence of IGF-II. These results, taken together, reveal that IGF-II stimulates extravillous trophoblast cell migration by signaling through IGF type 2 receptor, involving inhibitory G proteins and activating the MAPK pathway.

Identification of cation-independent mannose 6-phosphate receptor/insulin-like growth factor type-2 receptor as a novel target of autoantibodies

Two human monoclonal autoantibodies, B-33 and B-24, were generated from the B cells of a patient with scleroderma. Both monoclonal antibodies (mAbs) were composed of mu and lambda chains, and recognized cytoplasmic vesicular structures by indirect immunofluorescence on Hep-2 cell line slides, although mAb B-24 showed an additional diffuse cytoplasmic staining pattern. By Western blot, mAb B-24 exhibited a polyreactive-like binding pattern, whereas mAb B-33 failed to recognize any electroblotted Hep-2 antigen. The polyreactive versus monospecific behaviour of mAbs B-24 and B-33 was further confirmed by enzyme-linked immunosorbent assay (ELISA) with a variety of foreign and autoantigens. The N-terminal sequence of a protein band isolated by affinity chromatography with mAb B-33 was identical to that of cation-independent mannose 6-phosphate receptor (CI-MPR), also known as the insulin-like growth factor type-2 receptor (IGF-2R). Immunofluorescence experiments on Hep-2 cell line slides demonstrated a striking co-localization between the staining pattern exhibited by these mAbs and the pattern obtained using a goat anti-CI-MPR serum, indicating the recognition by B-24 and B-33 of a structure located predominantly in late endosomes. Sequence analysis of the V-region gene segments of B-33 and B-24 showed both to be identical, except for the existence of a point mutation in B-33 located in the H-complementarity-determining region 3 (H-CDR3) (position 100D), which produces a non-conservative replacement of Gly by Ser. This single replacement appears to be responsible for the dramatic change in reactivity of human mAb B-33. The data shown here provide new evidence of the critical role played by the H-CDR3 region in distinguishing a polyspecific from a monospecific antibody. A population study demonstrated the existence of immunoglobulin G (IgG) reactivity against CI-MPR/IGF-2R in serum specimens from five individuals with different pathological conditions, thus indicating that this molecule is a potential target for the human autoimmune response.

High-affinity antibodies from hen's-egg yolks against human mannose-6-phosphate/insulin-like growth-factor-II receptor (M6P/IGFII-R): characterization and potential use in clinical cancer studies

The mannose-6-phosphate/insulin-like growth-factor-II receptor (M6P/IGFII-R) involved in trafficking of newly synthesized lysosomal enzymes, degradation of IGFII and activation of TGFbetaI, was suggested as being coded by a tumor-suppressor gene. No specific antibodies are currently available for clinical studies. Since M6P/IGFII-R is a highly conserved protein in mammals, we immunized chicken with human M6P/IGFII-R. Up to 200 mg of specific IgY from weekly pooled egg yolk was extracted by the polyethylene glycol procedure. Chicken IgY antibodies specifically recognized the human and bovine 270-kDa M6P/IGFII-R but not the 46-kDa M6P-R, as documented by immunoprecipitation and immunobloting. Using biosensor analysis, IgY antibodies were shown to bind M6P/IGFII-R with high affinity (K(D) = 7.5 x 10(-9) M). A solid-phase competitive ELISA using bovine M6P/IGFII-R coated on 96-well microplates, allowed us to titrate the M6P/IGFII-R in human sera at a sensitivity of 300 ng/ml. The M6P/IGFII-R was stained by immunoperoxidase in breast- and ovarian-cancer cell lines (T47D, MDA-MB231, MCF7 and BG1) and in frozen breast-cancer tissues, showing predominant localization in the trans-Golgi network. Staining specificity was shown with irrelevant IgY and by extinction with antigen excess. Quantitative immunohistochemical analysis of frozen sections from 40 invasive breast carcinomas indicated varying levels (from 5 to 400 units) of the M6P/IGFII-R protein which were not correlated with tumor size, histological grade and estrogen receptor or progesterone receptor. There was a trend (p = 0.08) between lymph-node invasiveness and low receptor level. Moreover, the M6P/IGFII-R level was significantly lower in cancer cells than in normal cells in 10 out of the 21 tumors in which the peritumoral normal glands could be quantified in parallel. These 2 last results agree with the hypothesis of a tumor-suppressor gene for this receptor and suggest more basic and clinical studies to prove it.

Alteration in pancreatic immunoreactivity of insulin-like growth factor (IGF)-binding protein (IGFBP)-6 and in intracellular degradation of IGFBP-3 in fibroblasts of IGF-II receptor/IGF-II-deficient mice

The Type-2 insulin-like growth factor receptor (IGF2R) mediates the transport of lysosomal hydrolases to lysosomes and the clearance of insulin-like growth factor II (IGF-II). Mutant mice lacking IGF2R usually die perinatally, but are completely rescued from lethality in the absence of IGF-II. IGF2R/IGF-II-deficient mice have elevated levels of circulating IGF binding protein (IGFBP)-3 and show a strong IGFBP-6 immunoreactivity in all pancreatic islet cells and in secretory granules of different size in acinar cells and interlobular connective tissue of exocrine pancreas. Fibroblasts derived from double mutant mice missort the lysosomal protease cathepsin D, and are able to degrade endocytosed (125I)IGFBP-3 intracellularly, however, with lower efficiency than in control cells. These results show that the deficiency of IGF2R and IGF-II affects the expression and metabolism of IGFBPs in a tissue- and cell type-specific manner.

Increased neuronal endocytosis and protease delivery to early endosomes in sporadic Alzheimer's disease: neuropathologic evidence for a mechanism of increased beta-amyloidogenesis

The early endosome is the first vacuolar compartment along the endocytic pathway. It is the site of internalization and initial processing of amyloid precursor protein (APP) and apolipoprotein E (ApoE), two proteins of etiological importance in Alzheimer's disease, and a putative site of beta-amyloid peptide (Abeta) formation. Here, we identify early endosomes in human pyramidal neurons, using specific compartmental markers and morphometry, and show that in Alzheimer's disease individual endosomes display up to 32-fold larger volumes than the normal average. Endosomal enlargement contributed to an average 2.5-fold larger total endosomal volume per neuron, implying a marked increase in endocytic activity. Endosomal alterations were evident in most pyramidal neurons in Alzheimer brain, detectable at early stages of the disease but absent in several other neurodegenerative disorders examined. In addition, mature and proenzyme forms of the proteases cathepsin B and cathepsin D, a candidate APP secretase, were identified in most early endosomes in Alzheimer brains but were detectable in only a minor proportion of endosomes in normal brain. Expression of the cation-dependent 46 kDa mannose 6-phosphate receptor was elevated in pyramidal neurons of Alzheimer brains, which could be a possible basis for the altered cathepsin trafficking pattern. Enhanced endocytic activity, coupled with increased trafficking to endosomes of proteases, which may have the ability under pathological conditions to generate Abeta, constitutes a potential mechanism by which beta-amyloidogenesis may become accelerated in sporadic AD and also be subject to influences by ApoE.

The insulin-like growth factor-II/mannose-6-phosphate receptor is present in fetal bovine tissues throughout gestation

The insulin-like growth factor-II (IGF-II) and the IGF-II/mannose-6-phosphate (M6P) receptor are thought to play an important role in fetal growth and development. We have studied the expression of the IGF-II/M6P receptor in fetal bovine tissues from 5 through 36 weeks' gestation. Tissues from bovine fetuses were extracted in buffer containing 2% Triton-X-100 and 2% sodium dodecyl sulfate (SDS). Aliquots of the protein extracts were analyzed by SDS polyacrylamide gel electrophoresis and the protein bands were transferred onto nitrocellulose. Immunoblotting was performed with anti-bovine IGF-II/M6P receptor antiserum. In a subset of experiments, ligand blotting was carried out with radiolabeled IGF-II and subsequent autoradiography. IGF-II/M6P receptors were expressed in all tissues examined, with the highest amount of receptor being present in fetal lung and liver. Low amounts of receptors were measured in fetal brain. The amount of receptor was developmentally regulated throughout fetal life. The developmental regulation of receptor expression varied among the different tissues. In conclusion, the IGF-II/M6P receptor is present in all fetal bovine tissues examined. The presence of the IGF-II/M6P receptor seems to be developmentally regulated during bovine fetal life. We hypothesize that this receptor exerts important biologic effects during fetal growth and tissue and organ development.

Soluble insulin-like growth factor II/mannose 6-phosphate receptor carries multiple high molecular weight forms of insulin-like growth factor II in fetal bovine serum

We have characterized a soluble form of the insulin-like growth factor II/mannose 6-phosphate receptor (sIGF-II/MPR) and bound ligands from bovine serum. Fetal serum contained 2-8 mg/liter sIGF-II/MPR. Affinity-purified receptor isolated by adsorption to phosphomannan-agarose and elution with mannose 6-phosphate contained nearly stoichiometric amounts of bound 7.5-kDa IGF-II. In addition, at least 12 distinct 12-20-kDa proteins immunologically related to IGF-II also copurified with receptor. Receptor was separated from its associated ligands by acidification and gel filtration chromatography. Sequence analysis revealed that the 12-20-kDa proteins have the same amino termini as mature 7.5-kDa IGF-II. Protease and glycosidase treatments revealed that the different high molecular weight IGF-II species contain an identical COOH-terminal extension that is differentially glycosylated with O-linked sugars. Radiolabeled tracer experiments demonstrated that the sIGF-II/MPR carries approximately 1/4 of the IGF-II in fetal bovine serum. These results support a significant role for sIGF-II/MPR in the transport of circulating IGF-II isoforms during development.

Insulin-like growth factor II promotes in vitro cholinergic development of mouse septal neurons: comparison with the effects of insulin-like growth factor I

We investigated the effect of insulin-like growth factors II and I (IGFII and IGFI) on septal primary cultures from mouse embryonic day 15 brains. The addition of IGFII to septal cultures enhanced total choline acetyltransferase (ChAT) activity in a dose-dependent manner. Maximal stimulation of ChAT activity was observed at 10 ng/ml IGFII. The effect of IGFII on ChAT activity was completely blocked by anti-IGFII/M-6-P receptor antibodies, whereas the antisera alone had no effect on the enzyme activity. Double-labeled immunohistochemical studies revealed that most ChAT-positive neurons expressed IGFII/M-6-P receptor immunoreactivity. These results indicate that the trophic effect of IGFII results from the direct action of this molecule through the IGFII/M-6-P receptor in septal cholinergic neurons. IGFI also stimulated ChAT activity, but with less potency than IGFII. Antibodies against the IGFII/M-6-P receptor inhibited approximately 50% of the IGFI response, suggesting that the effect of IGFI is mediated in part by the IGFII/M-6-P receptor. Thus, it appears that IGFII and IGFI are potent trophic factors for central cholinergic neurons and could potentially play a significant role in the differentiation, maintenance and regeneration of these neurons.

Tail-specific antibodies that block return of 46,000 M(r) mannose 6-phosphate receptor to the trans-Golgi network

Recycling of 46,000 M(r) mannose 6-phosphate receptor (MPR 46) was investigated by microinjection of Fab fragments against small epitopes within the cytoplasmic domain of the receptor. Fab fragments against the peptide 43-47 (Ala-Tyr-Arg-Gly-Val) efficiently blocked return of MPR 46 to the TGN. Antibody-induced redistribution resulted in accumulation of MPR 46 within an endosomal compartment, from which it recycled to the plasma membrane. Rab5 and rab7, markers for early and late endosomes, respectively, were not detectable in the compartment of redistributed MPR 46, suggesting that it represents a specialized endosomal subcompartment. The bulk of redistributed MPR 46 did not colocalize with endocytosed fluid-phase marker, suggesting that it accumulates at a site where MPR 46 has been segregated from endocytosed material, which is destined for transport to lysosomes. Peptide 43-47 contains a tyrosine (residue 44) which has been shown earlier to be part of an internalization signal for MPR 46 (Johnson, K. F., W. Chan, and S. Kornfeld. 1990. Proc. Natl. Acad. Sci. USA. 87:10010-10014). The role of tyrosine residue 44 as part of a putative multifunctional sorting signal is discussed.

Effects of insulin-like growth factors (IGFs) and IGF receptor antibodies on the proliferation of human breast cancer cells

It has been shown previously that MCF-7 cells proliferate in response to nanomolar concentrations of IGF-I and IGF-II. It has also been reported that the actions of both peptides are mediated through the IGF-I receptor. To further characterize these observations, we used MCF-7 and Hs578T cell lines in the serum-free/phenol red-free system developed by Ogasawara and Sibarsku, 1988. Cell proliferation was studied in the presence of insulin, IGF-I and -II and a series of growth factor receptor antibodies. No effect was observed on Hs578T cell proliferation with any of the growth factors. However, MCF-7 cells were stimulated 4-5 fold with IGF-I and insulin, while IGF-II was only slightly less potent. alpha IR3, a monoclonal antibody directed against the IGF-I receptor, was stimulatory when added alone. However, alpha IR3 blocked approximately 50% of the IGF-I response, only 5% of the insulin response, and did not block the IGF-II effect on cell proliferation. These data suggest that alpha IR3 and IGF-I are acting as agonists through the IGF-I receptor, but that insulin and IGF-II are acting through other receptors. Two different IGF-II/M-6-P receptor antibodies and an insulin receptor antibody failed to significantly block IGF-II actions. All three antibodies were stimulatory when added alone. beta-gal inhibited 27% of the IGF-II response and had no effect when added alone. Since beta-gal decreases the binding affinity of the IGF-II/M-6-P receptor for IGF-II and does not bind to the IGF-I or insulin receptor, these data suggest the possibility that IGF-II mitogenic action is mediated through the IGF-II/M-6-P receptor. In summary, these data indicate that nanomolar concentration of insulin, IGF-I and IGF-II are potent mitogens in MCF-7 cells and can potentially stimulate cell proliferation through all three receptors.