Distinctive regulation of the functional linkage between the human cation-independent mannose 6-phosphate receptor and GTP-binding proteins by insulin-like growth factor II and mannose 6-phosphate

GTP-binding protein-activator sequences in the insulin receptor

Some functions of the insulin receptor (insR) are assumed to be mediated by pertussis toxin-sensitive Gi/G(o) proteins. Here we have located G-protein-activator domains in the cytoplasmic region of the human insR. We searched the sequence of insR and found three candidate regions at residues 1039-1061, 1147-1168 and 1325-1345, referred to as ISRP1, ISRP2 and ISRP3, respectively. Among them, the Gi/G(o)-activating function was observed only in peptide ISRP3. ISRP1 specifically activated Gs, whereas ISRP2 had no effect on G proteins. ISRP2 and ISRP3 contained five of six autophosphorylated tyrosine residues in insR. After tyrosine phosphorylation, ISRP2 showed specific Gi-activating function, and ISRP3 potentiated its ability and became capable of activating G proteins generally. This is the first study that specifies G-protein-activator domains in insR and describes their modification by autophosphorylation.

Insulin-like growth factor (IGF) binding proteins and insulin-like growth factor secretion by cultured chondrocyte cells: identification, characterization and ontogeny during cell differentiation

Insulin-like growth factor binding proteins (IGF BP) and insulin-like growth factors (IGF) secretion by differentiating chondrocytes, derived from mouse embryonic limb bud and responsive to both IGF-I and -II [23], was investigated. The Western ligand blot analysis of the conditioned medium (CM) from days 1, 3, 5 and 7 of culture revealed the secretion of IGF BP of approx. 35-40, 28-30 and 24-26 kDa. The 35-40 kDa protein which comigrated with the 40 kDa protein in CM of trophoblast cells identified as IGF BP-3. The 28-30 kDa protein was identified as IGF BP-2 by Western immunoblotting with alpha-IGF BP-2 antisera. The 24-26 kDa protein was consistent with the nonglycosylated form of IGF BP-4. Secretion of three IGF BPs were increased with the age of the culture. This suggested that the major IGF BP secreted by differentiating chondrocytes in culture are IGF BP-2, -3 and -4. All three of these IGF BPs were stimulated by both IGF-I and -II. IGF-I was approx. 2-fold more potent than IGF-II. The investigation of the localized production of IGF revealed that chondrocytes, similar to IGF BP, secreted IGF-II in differentiation dependent manner. No IGF-I secretion was identified. Examination of the secretion of solubilized IGF-II receptor by the chondrocytes, in contrast to trophoblasts, failed to reveal the presence of IGF-II receptor in the CM. This suggested that, unlike many other cells, including trophoblasts, chondrocytes do not secrete solubilized IGF-II receptor. In summary, the present results suggested an interactive autocrine/paracrine action of IGF BP and IGF-II in the chondrocytes, while the IGF-I action is predominantly endocrine.

Insulinlike growth factors and binding proteins in colon cancer

Insulinlike growth factors (IGFs) express anabolic and mitogenic activity on wide variety of cells. Besides endocrine effects, IGFs have major autocrine and paracrine effects on many cellular functions. Two factors that significantly affect the extent of cellular response to IGFs include the membrane receptors for IGFs and the soluble binding proteins (BPs), which modulate the action of IGFs at the receptor level. IGFs, IGF receptors, and IGFs and their BPs (IGF-BPs) thus constitute three components of the IGF system. A role of IGFs in the transformation and proliferation of cancer cells has become increasingly evident in the past few years. Studies from several laboratories show that all three components of the IGF system may play an important role in the proliferation of colon cancers. It was recently shown that the relative expression of IGFs and IGF/BPs may critically control the metastatic potential of colon cancers. The purpose of this article is to summarize our current knowledge of the IGF system and to present support for a significant role of IGFs in the initiation and growth of colon cancers. The expression and structural aspects of IGFs, their receptors, and BPs are outlined first, followed by a discussion of the role of IGFs in gastrointestinal functions and in colon cancers.

The IGF-II receptor system: a G protein-linked mechanism

Based on the finding that stimulation of the IGF-II receptor (IGF-IIR) is capable of activating Gi2 and calcium channels in BALB/c 3T3 fibroblasts, it was found that purified IGF-IIR can couple directly to purified Gi2 in phospholipid vesicles. IGF-IIR-Gi2 coupling can be characterized as follows. IGF-IIR directly couples to Gi2 in response to IGF-II in a stoichiometrical manner, suggesting that IGF-IIR works as a transmembrane signaling molecule and that the seven-transmembrane structure is not essential for receptor-G protein coupling. The mode of IGF-IIR-G12 interaction is similar to that of conventional receptor-G protein coupling, suggesting that a common G protein recognition mechanism is shared by IGF-IIR and conventional G-coupled receptors. The action of IGF-IIR is specific on Gi2 among various G proteins. Finally, the activity of IGF-IIR on Gi2 is similarly potent across the species of the proteins. These characteristics led to the discovery of a 14-amino-acid region in IGF-IIR that can directly interact with and activate Gi2, and is located at residues 2410-2423 of the human receptor. Subsequent work has indicated that this region is responsible for Gi-coupling function of intact IGF-IIR. The most important extensions of this discovery are the following: (1) The structure-function relationship for the Gi-activating function of this 14-amino-acid sequence, (2) the prediction of G protein-coupled functions of receptors based on the results obtained from 1), and (3) clarification of the detailed mechanism whereby ligand-receptor complex recognizes G proteins. This paper reviews what we have learned from IGF-IIR in terms of receptor-G protein interfaces and discusses future prospects.

Amino acids 356-372 constitute a Gi-activator sequence of the alpha 2-adrenergic receptor and have a Phe substitute in the G protein-activator sequence motif

The human alpha 2-adrenergic receptor contains the sequence KASRWRGRQNREKRFTF (amino acids 356-372) at the C-terminal end of its third intracellular loop. This sequence satisfies the structural criteria for G protein-activating sequences [(1992) J. Biol. Chem. 267, 8342-8346] except that the C-terminal sequence is B-B-X-X-Phe instead of B-B-X-B or B-B-X-X-B (B: basic residue, X: non-basic residue). Nevertheless, the synthetic peptide corresponding to this sequence (peptide alpha 2-F) was found to activate Gi and Go strongly with a saturated effect at 1-3 microM. Furthermore, the substitution of the C-terminal Phe of peptide alpha 2-F with Arg, Trp, and Tyr (but not Ala or Asp) did not appreciably affect the Gi-activating potency. It is suggested that the C-terminal basic residue of the B-B-X-X-B motif in Gi-activating sequences can be replaced by an aromatic residue.

Genetic analysis of genomic imprinting: an Imprintor-1 gene controls inactivation of the paternal copy of the mouse Tme locus

The Thp deletion on mouse chromosome 17 is lethal when inherited from the mother, because it deletes the T-associated maternal effect (Tme) locus, the paternal copy of which is inactivated by genomic imprinting. We have found a paternally nonimprinted Tme variant in crosses of Thp females with Mus m. musculus males. The data are consistent with the existence of a single Tme-unlinked gene, Imprintor-1 (Imp-1), with two alleles, one of which only causes imprinting at the Tme locus. Imp-1 is unlinked to the gene for cation-dependent Man-6-P receptor and acts prezygotically. Although Tme and Igf2r were thought to be identical, they show different patterns of imprinting in interspecies hybrids. The apparent nonequivalence of the Igf2r gene and Tme results in occurrence of viable mice lacking an active Igf2r gene. These mice are bigger at birth than their normal littermates, in accord with the proposed function of the IGF-II/Man-6-P receptor.

Metabolic actions of insulin-like growth factor II in cultured adult rat hepatocytes are not mediated through the insulin-like growth factor II receptor

Short- and long-term regulation of hepatic carbohydrate metabolism by insulin-like growth factor II was studied in primary cultures of adult rat hepatocytes and compared to the metabolic potency of insulin. Insulin-like growth factor II stimulated glycogen synthesis from [14C]glucose, uptake of [3H]aminoisobutyric acid and [14C]lactate formation from [14C]glucose up to three-fold. Basal glycogenolysis was inhibited to about 10%, and glucagon-activated glycogenolysis was blocked completely. The enzymatic activity of glucokinase and pyruvate kinase was induced two-fold, the glucagon-dependent induction of phosphoenolpyruvate carboxykinase was antagonized. Compared to insulin, half-maximal responses required up to 50 times higher insulin-like growth factor II concentrations ranging from 10-20 nmol/l. A similar difference was observed for binding affinity of insulin-like growth factor II to the insulin receptor. The interaction with the insulin-like growth factor II/mannose 6-phosphate (IGF-II/Man-6-P) receptor was examined by studying 125I-insulin-like growth factor II binding and uptake of lysosomal enzymes. The affinity of insulin-like growth factor II to the IGF-II/Man-6-P receptor was considerably higher than for the insulin receptor. Antibodies against the IGF-II/Man-6-P receptor did not affect metabolic responses to insulin-like growth factor II, while binding to its receptor and the receptor-mediated endocytosis of arylsulphatase A were strongly inhibited. Thus, in adult rat liver insulin-like growth factor II appeared to exert metabolic actions not via interaction with its own receptor but through low affinity binding to hepatic insulin receptors.

The insulin-like growth factor family of ligands, receptors, and binding proteins

The insulin-like growth factors (IGFs) have important roles in normal cellular growth and development. The IGFs have also been implicated in regulation of tumor cell growth. Two ligands, IGF-I and IGF-II, have been identified that are expressed in both fetal and adult tissues. They interact with at least two specific cell surface receptors. The type I IGF receptor is homologous to the insulin receptor in structure and has tyrosine kinase activity. The type II receptor is identical to the mannose-6-phosphate receptor known to be important in the trafficking of lysosomal enzymes; its role in IGF signal transduction is not clear. Furthermore, a hybrid receptor composed of subunits from the insulin receptor and the type I IGF receptor have been identified. In addition to these receptors, six different IGF binding proteins have been identified, which modulate the activity of the IGFs in various ways. Thus, there is great potential for complex interactions between the family members that could ultimately regulate normal and neoplastic cell growth.

The molecular and cellular biology of insulin-like growth factor II

Insulin-like growth factor II (IGF-II) is a 67 amino acid polypeptide that belongs to the family of insulin-like peptides. The IGF-II gene is coupled to the insulin gene and paternally imprinted. Multiple IGF-II mRNAs with identical coding regions and 3' untranslated regions (UTRs) but different 5' UTRs are generated from 3 promoters. The transcripts are translationally discriminated and inactivated by a specific endonucleolytic cleavage in their 3' UTR. These features may be important in the control of IGF-II production. IGF-II functions in an auto- and paracrine manner and binds to two types of receptors. The IGF-I receptor that is a tyrosine kinase and closely related with the insulin receptor and the IGF-II/mannose 6-phosphate (IGF-II/Man 6-P) receptor that is identical with the cation-independent mannose 6-phosphate receptor. The mitogenic and metabolic actions of IGF-II are propagated by the IGF-I receptor. In contrast, the IGF-II/Man 6-P receptor, that target lysosomal enzymes from the Golgi apparatus or the plasma membrane to the lysosomes, mediates the rapid internalization and degradation of IGF-II. IGF-II is expressed at high levels during foetal life and it is a major growth factor for the foetus in rodents. The developmental profiles and tissue distribution of the IGF-I and the maternally imprinted IGF-II/Man 6-P receptors both parallel that of IGF-II. In this scenario IGF-II promotes the growth of the embryo through the IGF-I receptor, whereas the IGF-II/Man 6-P receptor balance the activity by controlling the extracellular level of IGF-II.

Identification of a Gs activator region of the beta 2-adrenergic receptor that is autoregulated via protein kinase A-dependent phosphorylation

We have localized a G protein activator region of the human beta 2-adrenergic receptor to region beta III-2 (from Arg259 to Lys273). The synthetic beta III-2, corresponding to the C-terminal end of the third cytoplasmic loop, activates Gs at nanomolar concentrations and weakly activates Gi. beta III-2 activates adenylyl cyclase at nanomolar concentrations in wild-type S49 lymphoma membranes, but not in membranes of unc mutant S49 cells, in which Gs is uncoupled from beta-adrenergic stimulation. Phosphorylation of beta III-2 by cAMP-dependent protein kinase A, which is involved in the desensitization of the beta-adrenergic receptor from Gs, drastically reduces the effect of beta III-2 on Gs while potentiating its action on Gi, resulting in a total loss of adenylyl cyclase-stimulating activity. These findings indicate that this receptor sequence is a multipotential G protein activator whose G protein specificity is regulated by protein kinase A.

Structural study on a phosphorylated mannotetraose obtained from the phosphomannan of Candida albicans NIH B-792 strain by acetolysis

A mixture of phosphorylated manno-oligosaccharides was isolated from the acid-stable domain of phosphomannan of Candida albicans NIH B-792 strain (serotype B) by acetolysis and was fractionated on a column of Bio-Gel P-2 equilibrated with 50 mM pyridine-CH3COOH buffer, pH 5.0. A monophosphorylated mannotetraose was isolated as the major constituent. Structural analyses of this phosphate-containing tetraose and its reduction product with NaBH4 by 1H, 13C, and two-dimensional homonuclear Hartmann-Hahn NMR spectroscopies, subsequently, gave results consistent with the structure described below (where Manp represents the mannopyranose unit): [formula: see text] It was unexpected that the major phosphorylated branch in the acid-stable domain of the parent phosphomannan of this C. albicans strain is a relatively short mannotetraosyl residue containing solely alpha-1,2-linked mannopyranose units, and a phosphate group as a 6-O-ester on the intermediary unit adjacent to the nonreducing terminal group. These findings indicate that the size of the major phosphorylated branch of this phosphomannan is the same as that of Saccharomyces cerevisiae.

Distinct mode of G protein activation due to single residue substitution of active IGF-II receptor peptide Arg2410-Lys2423: evidence for stimulation acceptor region other than C-terminus of Gi alpha

Arg2410-Lys2423 (RVGLVRGEKARKGK, peptide 14) of the human insulin-like growth factor II receptor directly activates Gi and deletion of C-terminal 4 residues from peptide 14 nullifies this activity. A study was thus made of the effects of peptides modified in the C-terminal structure. RVGLVRGEKAAKGK and RVGLVRGEKARKGA scarcely activated Gi, whereas RVGLVRGEKARAGK (peptide A5) activated Gi as much as peptide 14 did. However, peptide A5 action did not depend on Mg2+ concentration and was little affected by pertussis toxin modification of Gi alpha. Peptide A5 may thus recognize the region on Gi alpha that is distinct from the extreme C-terminus. It is consequently considered that (i) the first and the last basic residues in the C-terminal motif of peptide 14 determine the capacity for recognition of Gi and (ii) there is a region different from the C-terminus of Gi alpha, through which the C-terminal second basic residue-altered peptide 14 activates Gi in a Mg(2+)-independent manner.

Insulin-like growth factor II overexpression does not affect sorting of lysosomal enzymes in NIH-3T3 cells

The sorting of newly synthesized mannose 6-phosphate (M6P)-containing proteins and of the major excreted protein (MEP), a lysosomal thiol proteinase, was studied in NIH-3T3 cells transfected with the cDNA of human insulin-like growth factor II (IGF II) or with the vector alone. Extracts from media and cells labelled with [35S] methionine were used for chromatography on a M6P/IGF II receptor affinity matrix or for immunoprecipitation to assess the distribution of newly synthesized M6P-containing proteins and MEP, respectively. The results indicate that the overexpression of IGF II did not affect the synthesis and the sorting of M6P-containing proteins and of MEP. The binding and uptake of the lysosomal enzyme arylsulfatase A were not affected in IGF II overexpressing cells.

Molecular recognition and targeting of lysosomal proteins

Recent studies have established that in mammalian cells insulin-like growth factor-II can couple the large mannose-6-phosphate receptor to a GTP-binding protein and that the insulin-like growth factor-II-induced activation of the GTP-binding protein is inhibited by mannose-6-phosphate and lysosomal enzymes. In mouse, the gene for the large mannose-6-phosphate receptor is maternally imprinted.

Insulin-like growth factor receptors in testicular vascular tissue from normal and diabetic rats

Testicular blood vessels contain IGF-I and IGF-II/M6P receptors. Binding to these receptors was altered following treatment with streptozotocin to induce diabetes. Intensity of labelling and size of receptors were examined using SDS-gel electrophoresis and autoradiography. The IGF-I and IGF-II/M6P receptor of the diabetic rat testicular microvessels appear to have a lower molecular weight as compared to controls. Macro- and microvascular tissues from diabetic rats apparently contain more IGF-I receptors than normal Sprague-Dawley rats. Using immunohistochemical techniques, the IGF-II/M6P receptor appears to dissociate easier from diabetic rat testicular arteries than from control animal blood vessels. M6P appears to increase both IGF-I and IGF-II binding to the rat IGF-II/M6P receptor, at least as visualized using affinity crosslinking analysis. Whether these differences in the IGF receptors are involved in the development of diabetic vascular disease is not yet known.