Mannose 6-phosphate/insulin-like growth factor II receptor fails to interact with G-proteins. Analysis of mutant cytoplasmic receptor domains

HDL3-signalling in HepG2 cells involves glycosyl-phosphatidylinositol-anchored proteins

In [3H]phosphatidylcholine (PC) prelabelled HepG2 cells, HDL3 stimulates a biphasic increase in 1.2-diacylglycerol (DAG). The early phase is mediated in part by a phospholipase C which is inhibited by 10 microM D 609, RHC-80267 or U-73122 and less by 100 microM propranolol. A phospholipase D is more likely involved in the late phase, as the DAG peak lags behind phosphatidic acid rise and is blocked by 100 microM propranolol. Cellular preincubation with 200 microg/ml antibodies against the inositolphosphoglycan (IPG) moiety of the GPI-anchor (Ab(IPG)), or depletion in GPI-anchored proteins by cellular pretreatment with 0.5 U/ml PI-PLC, 1 mM insulin and 2 HU/ml streptolysin-O, or depletion in membrane cholesterol content by filipin (5 microg/ml), digitonin (5 microg/ml) and cholesterol oxidase (0.5 U/ml) decreases the HDL3-signal, suggesting the involvement of a lipolytic cleavage of GPI-anchored proteins. Inhibition of proteases by 1 mM leupeptin/PMSF improves the response time to HDL3, with a DAG peak at 2-3 min. In the presence of protease-inhibitors, HDL3 releases in the culture medium several proteins with a residual IPG that binds Ab(IPG) after SDS-PAGE analysis and immunoblotting. HDL3-signalling pathways comprise tyrosine kinases, as preincubation with 100 microg/ml genistein or tyrphostin inhibits the HDL3-signal. HDL3 activates PC hydrolysis through a multistep pathway involving the cleavage of GPI-anchored proteins.

Overexpression of insulin-like growth factor-II induces accelerated myoblast differentiation

Previous studies have shown that exogenous insulin-like growth factors (IGFs) can stimulate the terminal differentiation of skeletal myoblasts in culture and have established a correlation between the rate and the extent of IGF-II secretion by muscle cell lines and the rate of biochemical and morphological differentiation. To investigate the hypothesis that autocrine secretion of IGF-II plays a critical role in stimulating spontaneous myogenic differentiation in vitro, we have established C2 muscle cell lines that stably express a mouse IGF-II cDNA under control of the strong, constitutively active Moloney sarcoma virus promoter, enabling us to study directly the effects of IGF-II overproduction. Similar to observations with other muscle cell lines, IGF-II overexpressing myoblasts proliferated normally in growth medium containing 20% fetal serum, but they underwent enhanced differentiation compared with controls when incubated in low-serum differentiation medium. Accelerated differentiation of IGF-II overexpressing C2 cells was preceded by the rapid induction of myogenin mRNA and protein expression (within 1 h, compared with 24-48 h in controls) and was accompanied by an enhanced proportion of the retinoblastoma protein in an underphosphrylated and potentially active form, by a marked increase in activity of the muscle-specific enzyme, creatine phosphokinase, by extensive myotube formation by 48 h, and by elevated secretion of IGF binding protein-5 when compared with controls. These results confirm a role for IGF-II as an autocrine/paracrine differentiation factor for skeletal myoblasts, and they define a model cell system that will be useful in determining the biochemical mechanisms of IGF action in cellular differentiation.

Insulin-like growth factor-II/mannose-6-phosphate receptor expression during early heart development

Expression of the insulin-like growth factor-II/mannose-6-phosphate (ICF-II/ M6P) receptor was examined during the major stages of heart morphogenesis in the chicken embryo. By using an affinity-purified antibody, Western blot analysis of total embryonic proteins from stages 5-24 revealed little if any IGF-II/M6P receptor protein until stage 7, approximately 8 hours prior to the appearance of the rudimentary myocardial tubes. Thereafter, receptor accumulation increased until stage 14, after which receptor protein levels remained constant, up to 7 days in ovo. Immunohistochemical localization revealed that, among all embryonic tissues at stages 10-24, the predominant site of receptor expression was the developing myocardium. Receptor expression was also immunohistochemically evaluated in a defined in vitro model of cardiogenesis in which explanted precardiac mesoderm is induced to undergo differentiation by co-explanted endoderm. In this system, as in vivo, IGF-II/M6P receptors were only detected after precardiac mesoderm had differentiated into a synchronously contractile multilayer which expressed cardiac alpha-actin. These findings indicate that the IGF-II/M6P receptor has an important role during early heart development.

Distinct biochemical properties of the native members of the G12 G-protein subfamily. Characterization of G alpha 12 purified from rat brain

G12 and G13 are insufficiently characterized pertussis toxin-insensitive G-proteins. Here, we describe the isolation of G alpha 12 from rat brain membranes. G alpha 12 was purified to apparent homogeneity by three steps of conventional chromatography, followed by two cycles of subunit-exchange chromatography on immobilized G subunits. Purified G alpha 12 bound guanosine 5'-[gamma-thio]triphosphate slowly and substoichiometrically. For isolation of functionally active G alpha 12, it was mandatory to use sucrose monolaurate as a detergent. Comparative studies of both rat-brain-derived members of the G12 subfamily revealed differences in the affinity of G alpha 12 and G alpha 13 for G beta gamma. G alpha 12 required a higher Mg2+ concentration for AlF4- -induced dissociation from immobilized G beta gamma than did G alpha 13. In addition, the G12 subfamily members differed in their sedimentation velocities, as determined by sucrose-density-gradient centrifugation. Analysis of sedimentation coefficients revealed a higher tendency of G12 to form supramolecular structures in comparison to G13 and other G-proteins. These G13 structures were stabilized by sucrose monolaurate, which in turn may explain the necessity for this detergent for purification of functionally active G alpha 12. Despite these distinct biochemical characteristics of G12 and G13, both purified G-proteins coupled to a recombinant thromboxane A2 (TXA2) receptor reconstituted into phospholipid vesicles. These data indicate, (1) significant differences in the biochemical properties of native members of the G12 subfamily, and (2) their specific coupling to TXA2 receptors.

Species- and tissue-dependent diversity of G-protein beta subunit phosphorylation: evidence for a cofactor

We previously reported that, in the membranes of HL-60 cells during activation of G-proteins, a phosphate transfer reaction occurs which involves transient G-protein beta subunit (G beta) phosphorylation [Wieland, Nürnberg, Ulibarri, Kaldenberg-Stasch, Schultz and Jakobs (1993) J. Biol. Chem. 268, 18111-18118]. Here, the generality of this phenomenon is evaluated by studying membranes of various tissues obtained from different mammalian species. All membranes tested expressed at least G beta 1 and G beta 2 subunits. Cell membranes from bovine and porcine brain and liver, rat brain and human blood cells exhibited predominantly G beta 1 or both subtypes at roughly equal concentrations. In contrast, significantly more G beta 2 immunoreactivity was detected in membranes from human placenta. Bovine and porcine liver membranes exhibited weak, G beta-specific immunoreactive signals. Conversely, these membranes showed the highest levels of G beta phosphorylation after incubation with [gamma-32P]GTP or 35S-labelled guanosine 5'-[gamma-thio]triphosphate. Interestingly, G beta-specific phosphorylation of membranes from human erythrocytes and platelets was very weak. G beta phosphorylation was confirmed by immunoprecipitation with G beta-specific antibodies, and the target amino acid was identified as histidine. On SDS/PAGE, phosphorylated or thiophosphorylated G beta-proteins differed in their apparent molecular size from unmodified G beta-proteins. Moreover, phosphorylated G beta-proteins differed in a species-dependent fashion in their electrophoretic mobility. Solubilization of membrane proteins with detergent did not abolish G beta phosphorylation. In contrast, reconstituted purified Gi/Go proteins showed no G beta phosphorylation. From these experiments we conclude that: (i) G beta phosphorylation represents a general phenomenon occurring in the cells of various species to different degrees, (ii) phosphorylated G beta-proteins exhibit species-dependent diverse electrophoretic mobilities, and (iii) G beta phosphorylation requires a membrane-associated cofactor(s) which is lost during routine G-protein purification.

Insulin and the insulin-like growth factors I and II are mitogenic to cultured rat sciatic nerve segments and stimulate [3H]thymidine incorporation through their respective receptors

The factors that control proliferation of Schwann cells during peripheral nerve regeneration are not yet known. In this study we investigated the effects of insulin, insulin-like growth factor I and II (IGF-I and IGF-II), IGF-I analogues, and factors that interfere with their respective receptors, on [3H]thymidine incorporation into cultured nerve segments from the rat sciatic nerve. Segments cultured in nM (0.1-1.7 nM) concentrations of insulin, truncated IGF-I (tIGF-I), long R3IGF-I, or IGF-II exhibited an increase in [3H]thymidine incorporation compared with control segments. IGF-II was most potent. JB1, an IGF-I antagonist, counteracted the effects of tIGF-I and insulin. The results suggest that non-neuronal cells in the nerve segment, probably Schwann cells, possess distinct receptors for insulin, IGF-I, and IGF-II and that these receptors may be involved in the control of Schwann cell proliferation during peripheral nerve regeneration.

Neither type of mannose 6-phosphate receptor is sufficient for targeting of lysosomal enzymes along intracellular routes

Mouse embryonic fibroblasts that are deficient in the two mannose 6-phosphate receptors (MPRs) MPR 46 and MPR 300 missort the majority (> or = 85%) of soluble lysosomal proteins into the medium. Human MPR 46 and MPR 300 were expressed in these cells to test whether overexpression of a single type of MPR can restore transport of lysosomal proteins to lysosomes. Only a partial correction of the missorting was observed after overexpression of MPR 46. Even at MPR 46 levels that are five times higher than the wild-type level, more than one third of the newly synthesized lysosomal proteins accumulates in the secretions. Two-fold overexpression of MPR 300 completely corrects the missorting of lysosomal enzymes. However, at least one fourth of the lysosomal enzymes are transported along a secretion-recapture pathway that is sensitive to mannose 6-phosphate in medium. In control fibroblasts that express both types of MPR, the secretion-recapture pathway is of minor importance. These results imply that neither overexpression of MPR 46 nor MPR 300 is sufficient for targeting of lysosomal proteins along intracellular routes.

Potential roles of heterotrimeric G proteins of the endomembrane system

Heterotrimeric G proteins are recognized as versatile switches linking cell surface receptors to cellular effectors. Beside their location at the plasma membrane G proteins are found on intracellular membranes. Studies with modulators of G protein activity suggest that G proteins associated with organelle membranes are involved in various steps of secretion and vesicular function. In contrast to hormonal responses involving G proteins little is currently known about possible receptors or activators and effectors interacting with intracellular G proteins. This short review focuses on recent developments elucidating the role of organelle-associated G proteins.

Inhibition of IGF II-induced redistribution of mannose 6-phosphate receptors by the phosphatidylinositol 3-kinase inhibitor, wortmannin

The effects of wortmannin, a selective inhibitor of phosphatidylinositol (PI)3-kinase on insulin-like growth factor II (IGF II)-induced redistribution of the 300 kDa mannose 6-phosphate/IGF II receptor (MPR 300) has been studied in human hepatoma HepG2 cells. IGF II increased the expression of MPR 300 at the cell surface threefold that was completely abolished by wortmannin at 100-300 nM. Higher concentrations of wortmannin also reduced the basal MPR 300-dependent uptake of ligands to 68% of controls. Neither the transport of two lysosomal enzymes nor the secretion of the IGF binding protein-1 were affected by wortmannin. These results show that activation of PI3-kinase plays a critical role in the IGF II-stimulated redistribution of MPR 300 initiated rather by IGF II binding to tyrosine kinase receptors than to the MPR 300.

Insulin-like growth factor receptors and binding proteins

The insulin-like growth factor receptors are integral membrane proteins and demonstrate separate, but important effects on the regulation of cellular processes. The IGF-I receptor signals multiple cascades via its inherent tyrosine kinase activity. The IGF-II/M-6-P receptor on the other hand is primarily involved in targeting of enzymes to various subcellular compartments. In contrast, the insulin-like binding proteins are secreted by the cells and accumulate in the extracellular matrix or on the external surface of the cell. They are also involved in regulating cellular processes more indirectly. They modulate the interactions of the IGFs with their receptors, and in addition, may have some IGF-independent effects probably by direct interaction with integrin and other cell membrane receptor proteins. The recent studies, as outlined in this review, strongly suggest an important, if not essential role for the IGF system in normal physiology and disease states. The challenge now is to define the mechanisms involved in these effects. More studies are required to fully understand the post-receptor mechanism involved in IGF-I receptor signal transduction and the mechanisms whereby the IGFBPs exert their interesting effects. Understanding these mechanisms will enable investigators to create new therapeutic modalities for diseases that are affected by the IGF system.

In vivo coupling of insulin-like growth factor II/mannose 6-phosphate receptor to heteromeric G proteins. Distinct roles of cytoplasmic domains and signal sequestration by the receptor

We examined the signaling function of the IGF-II/mannose 6-phosphate receptor (IGF-IIR) by transfecting IGF-IIR cDNAs into COS cells, where adenylyl cyclase (AC) was inhibited by transfection of constitutively activated G alpha i cDNA (G alpha i2Q205L). In cells transfected with IGF-IIR cDNA, IGF-II decreased cAMP accumulation promoted by cholera toxin or forskolin. This effect of IGF-II was not observed in untransfected cells or in cells transfected with IGF-IIRs lacking Arg2410-Lys2423. Thus, IGF-IIR, through its cytoplasmic domain, mediates the Gi-linked action of IGF-II in living cells. We also found that IGF-IIR truncated with C-terminal 28 residues after Ser2424 caused G beta gamma-dominant response of AC in response to IGF-II by activating Gi. Comparison with the G alpha i-dominant response of AC by intact IGF-IIR suggests that the C-terminal 28-residue region inactivates G beta gamma. This study not only provides further evidence that IGF-IIR has IGF-II-dependent signaling function to interact with heteromeric G proteins with distinct roles by different cytoplasmic domains, it also suggests that IGF-IIR can separate and sequestrate the G alpha and G beta gamma signals following Gi activation.

The two mannose 6-phosphate receptors transport distinct complements of lysosomal proteins

Mammalian cells express two different mannose 6-phosphate receptors (MPR 46 and MPR 300), which both mediate targeting of Man-6-P-containing lysosomal proteins to lysosomes. To assess the contribution of either and both MPRs to the transport of lysosomal proteins, fibroblasts were established from mouse embryos that were homozygous for disrupted alleles of either MPR 46 or MPR 300 or both MPRs. Fibroblasts missing both MPRs secreted most of the newly synthesized lysosomal proteins and were unable to maintain the catabolic function of lysosomes. The intracellular levels of lysosomal proteins decreased to < 20%, and undigested material accumulated in the lysosomal compartment. Fibroblasts lacking either MPR exhibited only a partial missorting and maintained, in general, half-normal to normal levels of lysosomal proteins. The same species of lysosomal proteins were found in secretions of double MPR-deficient fibroblasts as in secretions of single MPR-deficient fibroblasts, but at different ratios. This clearly indicates that neither MPR has an exclusive affinity for one or several lysosomal proteins. Furthermore, neither MPR can substitute in vivo for the loss of the other. It is proposed that the heterogeneity of the Man-6-P recognition marker within a lysosomal protein and among different lysosomal proteins has necessitated the evolution of two MPRs with complementary binding properties to ensure an efficient targeting of lysosomal proteins.

Cloning and expression of the cDNA of chicken cation-independent mannose-6-phosphate receptor

We cloned and sequenced the 8767-bp full-length cDNA for the chicken cation-independent mannose-6-phosphate receptor (CI-MPR), of interest because, unlike its mammalian homologs, it does not bind insulin-like growth factor II (IGF-II). The cDNA encodes a protein of 2470 aa that includes a putative signal sequence, an extracytoplasmic domain consisting of 15 homologous repeat sequences, a 23-residue transmembrane sequence, and a 161-residue cytoplasmic sequence. Overall, it shows 60% sequence identity with human and bovine CI-MPR homologs, and all but two of 122 cysteine residues are conserved. However, it shows much less homology in the N-terminal signal sequence, in repeat 11, which is proposed to contain the IGF-II-binding site in mammalian CI-MPR homologs, and in the 14-aa residue segment in the cytoplasmic sequence that has been proposed to mediate G-protein-coupled signal transduction in response to IGF-II binding by the human CI-MPR. Transient expression in COS-7 cells produced a functional CI-MPR which exhibited mannose-6-phosphate-inhibitable binding and mediated endocytosis of recombinant human beta-glucuronidase. Expression of the functional chicken CI-MPR in mice lacking the mammalian CI-MPR should clarify the controversy over the physiological role of the IGF-II-binding site in mammalian CI-MPR homologs.

Activation of a G protein complex by aggregation of beta-1,4-galactosyltransferase on the surface of sperm

Fertilization is initiated by the species-specific binding of sperm to the extracellular coat of the egg. One sperm receptor for the mouse egg is beta-1,4-galactosyltransferase (GalTase), which binds O-linked oligosaccharides on the egg coat glycoprotein ZP3. ZP3 binding induces acrosomal exocytosis through the activation of a pertussis toxin-sensitive heterotrimeric guanine nucleotide-binding protein (G protein). The cytoplasmic domain of sperm surface GalTase bound to and activated a heterotrimeric G protein complex that contained the Gi alpha subunit. Aggregation of GalTase by multivalent ligands elicited G protein activation. Sperm from transgenic mice that overexpressed GalTase had higher rates of G protein activation than did wild-type sperm, which rendered transgenic sperm hypersensitive to their ZP3 ligand. Thus, the cytoplasmic domain of cell surface GalTase appears to enable it to function as a signal-transducing receptor for extracellular oligosaccharide ligands.

Does the overexpression of pro-insulin-like growth factor-II in transfected human embryonic kidney fibroblasts increase the secretion of lysosomal enzymes?

Insulin-like growth factor-II (IGF-II) and lysosomal enzymes bearing the mannose 6-phosphate (Man6P) recognition marker, bind to two distinct binding sites of the IGF-II/M6P receptor. The two classes of ligands reciprocally modulate the binding of the other class of ligand to the receptor [Kiess, W., Thomas, C. L., Greenstein, L., Lee, L., Sklar, M. M., Rechler, M. M., Sahagian, G. G. & Nissley, S. P. (1989) J. Biol. Chem. 264, 4710-4714]. We asked whether or not overexpression of pro-IGF-II by cells in culture leads to missorting of lysosomal enzymes. Human embryonal kidney fibroblasts were transfected with the full-length human IGF-II cDNA or a control cDNA. Solution hybridization/RNase protection experiments using a human IGF-II riboprobe showed that two transfectants expressed large quantities of IGF-II mRNA, whereas the non-transfected cells did not. The analysis of conditioned media revealed that these cells secrete approximately 0.15 micrograms and 1.0 micrograms immunoreactive IGF-II/ml and 22 x 10(6) cells and 24 x 10(6) cells within 24 hours. Immunoreactive IGF-II was shown by Western blotting to represent 17-kDa pro-IGF-II. The amount of the lysosomal enzyme, beta-hexosaminidase, was approximately twofold increased in the conditioned media from pro-IGF-II overexpressing cells compared with control media, as shown by Western-blot analysis and immunoprecipitation of media extracts of metabolically labeled cells. The synthesis rate of beta-hexosaminidase was not affected by pro-IGF-II overexpression. In addition, the basal amount of another newly synthesized lysosomal enzyme, the cathepsin D precursor, was also twofold higher in pro-IGF-II overexpressing cells than in control cells. In contrast, the surface binding and cellular uptake rate of a Man6P-containing neoglycoprotein did not differ between the cell lines. The results indicate that the overexpression of pro-IGF-II doubles the secretion and/or reduces the re-uptake of beta-hexosaminidase and cathepsin D to approximately 20% of the total synthesized enzymes in human embryonal kidney fibroblasts compared to control cells. We hypothesize that, in cells synthesizing high amounts of pro-IGF-II, the growth factor may modulate the targeting of a portion of lysosomal enzymes, mainly by partially enhancing the secretion of newly synthesized enzymes and, in addition, possibly by affecting the re-uptake mechanism.

Insulin-like growth factors and breast cancer

Several years of research have indicated that the insulin-like growth factor (IGF) family of ligands, receptors and binding proteins are expressed in human breast cancer. The ligands are potent mitogens for breast cancer cell lines, and blockade of IGF signaling inhibits tumor growth. The IGFs can be regulated in normal and neoplastic tissue, indicating their important role in proliferation. For example, estrogen, a hormone important in the growth and progression of breast cancer is able to alter expression of IGF ligands, receptors and binding proteins. In addition, recent data now indicate that IGF ligands can also activate estrogen receptor (ER) in a ligand-independent manner. The apparent cross-talk between IGF and ER signaling is especially important to consider since anti-estrogens, such as tamoxifen, are a major modality for the treatment of breast cancer. Recent data suggest that IGFs may also be involved in tamoxifen resistance, through upregulation of the IGF-I receptor. Thus blockade of IGF signaling in combination with tamoxifen may prove to be a beneficial treatment for breast cancer patients.