Insulin-Like Growth Factor-II/Cation-Independent Mannose 6-Phosphate Receptor in Neurodegenerative Diseases.

The insulin-like growth factor II/mannose 6-phosphate (IGF-II/M6P) receptor is a multifunctional single transmembrane glycoprotein. Recent studies have advanced our understanding of the structure, ligand-binding properties, and trafficking of the IGF-II/M6P receptor. This receptor has been implicated in a variety of important cellular processes including growth and development, clearance of IGF-II, proteolytic activation of enzymes, and growth factor precursors, in addition to its well-known role in the delivery of lysosomal enzymes. The IGF-II/M6P receptor, distributed widely in the central nervous system, has additional roles in mediating neurotransmitter release and memory enhancement/consolidation, possibly through activating IGF-II-related intracellular signaling pathways. Recent studies suggest that overexpression of the IGF-II/M6P receptor may have an important role in regulating the levels of transcripts and proteins involved in the development of Alzheimer's disease (AD)-the prevalent cause of dementia affecting the elderly population in our society. It is reported that IGF-II/M6P receptor overexpression can increase the levels/processing of amyloid precursor protein leading to the generation of ß-amyloid peptide, which is associated with degeneration of neurons and subsequent development of AD pathology. Given the significance of the receptor in mediating the transport and functioning of the lysosomal enzymes, it is being considered for therapeutic delivery of enzymes to the lysosomes to treat lysosomal storage disorders. Notwithstanding these results, additional studies are required to validate and fully characterize the function of the IGF-II/M6P receptor in the normal brain and its involvement in various neurodegenerative disorders including AD. It is also critical to understand the interaction between the IGF-II/M6P receptor and lysosomal enzymes in neurodegenerative processes, which may shed some light on developing approaches to detect and prevent neurodegeneration through the dysfunction of the receptor and the endosomal-lysosomal system.

Overexpression of the Insulin-Like Growth Factor II Receptor Increases ß-Amyloid Production and Affects Cell Viability.

Amyloid ß (Aß) peptides originating from amyloid precursor protein (APP) in the endosomal-lysosomal compartments play a critical role in the development of Alzheimer's disease (AD), the most common type of senile dementia affecting the elderly. Since insulin-like growth factor II (IGF-II) receptors facilitate the delivery of nascent lysosomal enzymes from the trans-Golgi network to endosomes, we evaluated their role in APP metabolism and cell viability using mouse fibroblast MS cells deficient in the murine IGF-II receptor and corresponding MS9II cells overexpressing the human IGF-II receptors. Our results show that IGF-II receptor overexpression increases the protein levels of APP. This is accompanied by an increase of ß-site APP-cleaving enzyme 1 levels and an increase of ß- and γ-secretase enzyme activities, leading to enhanced Aß production. At the cellular level, IGF-II receptor overexpression causes localization of APP in perinuclear tubular structures, an increase of lipid raft components, and increased lipid raft partitioning of APP. Finally, MS9II cells are more susceptible to staurosporine-induced cytotoxicity, which can be attenuated by ß-secretase inhibitor. Together, these results highlight the potential contribution of IGF-II receptor to AD pathology not only by regulating expression/processing of APP but also by its role in cellular vulnerability.

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.

Synthetic low-calcaemic vitamin D(3) analogues inhibit secretion of insulin-like growth factor II and stimulate production of insulin-like growth factor-binding protein-6 in conjunction with growth suppression of HT-29 colon cancer cells.

The aims of the present study were to compare the ability of various synthetic analogues of 1 alpha,25-dihydroxyvitamin D(3) [1 alpha,25-(OH)(2)D(3)] to inhibit proliferation of HT-29 cells, a human colon adenocarcinoma cell line. HT-29 cells were incubated for 144 h with various concentrations (0-100 nM) of 1 alpha,25-(OH)(2)D(3), or the analogues EB1089, CB1093 or 1 beta,25-(OH)(2)D(3). All these analogues except 1 beta,25-(OH)(2)D(3) inhibited cell proliferation, but relative potencies and efficacies of EB1089 and CB1093 were much greater than that of the native vitamin. Cells grew in serum-free medium, reaching a plateau density at day 10 of culture, and addition of 10 nM 1 alpha,25-(OH)(2)D(3) or 1 beta,25-(OH)(2)D(3) did not alter the long-term growth characteristics of HT-29 cells. However, cells treated with 10 nM EB1089 or CB1093 grew at a rate slower than control and reached final densities that were 53+/-1 and 36+/-2% lower than control, respectively. Immunoblot analysis of serum-free conditioned medium using a monoclonal anti-insulin-like growth factor-(IGF)-II antibody showed that both 10 nM EB1089 and CB1093 markedly inhibited secretion of both mature 7500 M(r) and higher M(r) forms of IGF-II. Ligand blot and immunoblot analyses of conditioned media revealed the presence of IGFBPs of M(r) 24,000 (IGFBP-4), 30,000 (glycosylated IGFBP-4), 35,000 (IGFBP-2) and 32,000-34,000 (IGFBP-6). The level of IGFBP-2 was decreased by 42+/-8 and 49+/-7% by 10 nM EB 1089 and CB1093, respectively, compared to controls. IGFBP-6 was increased approximately twofold by EB1089 and CB1093, and exogenously added IGFBP-6 inhibited HT-29 cell proliferation. These results suggest that inhibition of HT-29 cell proliferation by EB1089 and CB1093 may be attributed, at least in part, to the decreased secretion of IGF-II. The increase in IGFBP-6 concentration coupled with its high affinity for IGF-II may also contribute to decreased cellular proliferation by an indirect mechanism involving sequestration of endogenously produced IGF-II.

M6P/IGF2R imprinting evolution in mammals.

Imprinted gene identification in animals has been limited to eutherian mammals, suggesting a significant role for intrauterine fetal development in the evolution of imprinting. We report herein that M6P/IGF2R is not imprinted in monotremes and does not encode for a receptor that binds IGF2. In contrast, M6P/IGF2R is imprinted in a didelphid marsupial, the opossum, but it strikingly lacks the differentially methylated CpG island in intron 2 postulated to be involved in imprint control. Thus, invasive placentation and gestational fetal growth are not required for imprinted genes to evolve. Unless there was convergent evolution of M6P/ IGF2R imprinting and receptor IGF2 binding in marsupials and eutherians, our results also demonstrate that these two functions evolved in a mammalian clade exclusive of monotremes.

Mechanisms for high affinity mannose 6-phosphate ligand binding to the insulin-like growth factor II/mannose 6-phosphate receptor.

The two mannose 6-phosphate (Man-6-P) binding domains of the insulin-like growth factor II/mannose 6-phosphate receptor (Man-6-P/IGF2R), located in extracytoplasmic repeats 1-3 and 7-9, are capable of binding Man-6-P with low affinity and glycoproteins that contain more than one Man-6-P residue with high affinity. High affinity multivalent ligand binding sites could be formed through two possible mechanisms: the interaction of two Man-6-P binding domains within one Man-6-P/IGF2R molecule or by receptor oligomerization. To discriminate between these mechanisms, truncated FLAG epitope-tagged Man-6-P/IGF2R constructs, containing one or both of the Man-6-P binding domains, were expressed in 293T cells, and characterized for binding of pentamannose phosphate-bovine serum albumin (PMP-BSA), a pseudoglycoprotein bearing multiple Man-6-P residues. A construct containing all 15 repeats of the Man-6-P/IGF2R extracytoplasmic domain bound PMP-BSA with the same affinity as the full-length receptor (K(d) = 0.54 nm) with a curvilinear Scatchard plot. The presence of excess unlabeled PMP-BSA increased the dissociation rate of pre-formed (125)I-PMP-BSA/receptor complexes, suggesting negative cooperativity in multivalent ligand binding and affirming the role of multiple Man-6-P/IGF2R binding domains in forming high affinity binding sites. Truncated receptors containing only one Man-6-P binding domain and mutant receptor constructs, containing an Arg(1325) --> Ala mutation that eliminates binding to the repeats 7-9 binding domain, formed high affinity PMP-BSA binding, but with reduced stoichiometries. Collectively, these observations suggest that alignment of Man-6-P binding domains of separate Man-6-P/IGF2R molecules is responsible for the formation of high affinity Man-6-P binding sites and provide functional evidence for Man-6-P/IGF2R oligomerization.

Dimerization of the insulin-like growth factor II/mannose 6-phosphate receptor.

The insulin-like growth factor II/mannose 6-phosphate receptor (IGF2R) interacts with lysosomal enzymes through two binding domains in its extracytoplasmic domain. We report in the accompanying article (Byrd, J. C., and MacDonald, R. G. (2000) J. Biol. Chem. 275, 18638-18646) that only one of the two extracytoplasmic mannose 6-phosphate (Man-6-P) binding domains is necessary for high affinity Man-6-P ligand binding, suggesting that, like the cation-dependent Man-6-P receptor, oligomerization of the IGF2R contributes to high affinity interaction with lysosomal enzymes. In the present study, we have directly characterized both naturally occurring and engineered forms of the IGF2R for their ability to form oligomeric structures. Whereas gel filtration chromatography suggested that purified bovine IGF2R species exist in a monomeric form, native gel electrophoresis allowed for the separation of dimeric and monomeric forms of the receptors with distinct phosphomannosyl ligand binding characteristics. The ability of the IGF2R to form oligomeric complexes was confirmed and localized to the extracytoplasmic domain through the use of epitope-tagged soluble IGF2R constructs bearing deletions of the transmembrane and cytoplasmic domains. Finally, chimeric receptors were engineered containing the extracytoplasmic and transmembrane domains of the IGF2R fused to the cytoplasmic domain of the epidermal growth factor receptor with which dimerization of the chimeras could be monitored by measuring autophosphorylation. Collectively, these results show that the IGF2R is capable of forming oligomeric complexes, most likely dimers, in the absence of Man-6-P ligands.

Training to treat cancer: future developments.

Both medical and clinical oncologists treat patients with cancer, but these specialties receive markedly different training. A questionnaire was sent to all UK oncology trainees regarding training. Rotation to other cancer-related specialties was successfully piloted. Such rotations might enhance interprofessional working.

Altered ligand binding by insulin-like growth factor II/mannose 6-phosphate receptors bearing missense mutations in human cancers.

The M6P/IGF2R gene, encoding the insulin-like growth factor II (IGF-II)/mannose 6-phosphate receptor (IGF2R), is frequently inactivated during carcinogenesis. M6P/IGF2R is postulated to be a tumor suppressor gene due to its ability to bind and degrade the mitogen IGF-II, promote activation of the growth inhibitor transforming growth factor beta, and regulate the targeting of lysosomal enzymes. In this study, we determined the effects of four M6P/IGF2R missense mutations associated with loss of heterozygosity in hepatocellular and breast cancers on the ligand binding properties of full-length membrane-bound receptors. Site-directed mutagenesis was used to prepare COOH-terminal, c-myc epitope-tagged human IGF2R cDNA expression constructs bearing point mutations that lead to the substitutions I1572T, G1464E, G1449V, and Q1445H, all of which are located in the receptor's extracytoplasmic domain. Ligand binding was measured in plasma membranes from 293T cells expressing full-length receptors. No binding of 125I-IGF-II to I1572T mutant receptors was observed. Binding to G1449V mutant receptors was decreased by 50% relative to wild-type (WT). However, IGF-II binding to the G1464E and Q1445H mutant receptors was equivalent to WT when plasma membranes were assayed immediately after preparation. The phosphomannosylated pseudoglycoprotein pentamannose 6-phosphate-BSA (PMP-BSA) was synthesized as a ligand for the M6P binding site. Binding of 125I-PMP-BSA was equivalent to WT for the I1572T, G1464E, and Q1445H mutations, but there was a 60% reduction in PMP-BSA binding to the G1449V mutant receptor. Thus, several missense mutations in M6P/IGF2R disrupt the ligand binding functions of the intact IGF2R, lending further support to the hypothesis that the M6P/IGF2R is a tumor suppressor gene.

Disruption of ligand binding to the insulin-like growth factor II/mannose 6-phosphate receptor by cancer-associated missense mutations.

The insulin-like growth factor II/mannose 6-phosphate receptor (IGF2R) carries out multiple regulatory and transport functions, and disruption of IGF2R function has been implicated as a mechanism to increase cell proliferation. Several missense IGF2R mutations have been identified in human cancers, including the following amino acid substitutions occurring in the extracytoplasmic domain of the receptor: Cys-1262 --> Ser, Gln-1445 --> His, Gly-1449 --> Val, Gly-1464 --> Glu, and Ile-1572 --> Thr. To determine what effects these mutations have on IGF2R function, mutant and wild-type FLAG epitope-tagged IGF2R constructs lacking the transmembrane and cytoplasmic domains were characterized for binding of insulin-like growth factor (IGF)-II and a mannose 6-phosphate-bearing pseudoglycoprotein termed PMP-BSA (where PMP is pentamannose phosphate and BSA is bovine serum albumin). The Ile-1572 --> Thr mutation eliminated IGF-II binding while not affecting PMP-BSA binding. Gly-1449 --> Val and Cys-1262 --> Ser each showed 30-60% decreases in the number of sites available to bind both (125)I-IGF-II and (125)I-PMP-BSA. In addition, the Gln-1445 --> His mutant underwent a time-dependent loss of IGF-II binding, but not PMP-BSA binding, that was not observed for wild type. In all, four of the five cancer-associated mutants analyzed demonstrated altered ligand binding, providing further evidence that loss of IGF2R function is characteristic of certain cancers.

Growth inhibition and differentiation of the human colon carcinoma cell line, Caco-2, by constitutive expression of insulin-like growth factor binding protein-3.

The human colon carcinoma cell line, Caco-2, produces insulin-like growth factor binding protein-3 (IGFBP-3), the secretion of which correlates with markers of enterocyte differentiation. To investigate whether IGFBP-3 inhibits proliferation or induces differentiation, Caco-2 cells were stably transfected with an IGFBP-3 cDNA expression construct or pcDNA3 vector as a control. Accumulation of IGFBP-3 mRNA and secretion of the protein into conditioned medium 9 days after plating were readily detected in the transfected cells, whereas these parameters were undetectable in pcDNA3-transfected cells. Insulin-like growth factor binding protein-3-expressing cells grew at a rate similar to the controls for 6 days after plating, but achieved a much lower final density between days 10 and 12. By day 9 of culture, accumulation of sucrase-isomaltase mRNA, a marker of enterocytic differentiation of Caco-2 cells, was evident in the IGFBP-3-expressing cells, but was undetectable in the controls. These results indicate that IGFBP-3 may inhibit proliferation and induce early differentiation of Caco-2 cells.

Regulation of the insulin-like growth factor axis by increasing cell number in intestinal epithelial (IEC-6) cells.

Insulin-like growth factor binding protein-2 (IGFBP-2) production as a function of cell number by intestinal epithelial cells (IEC-6) was regulated such that the IGFBP-2 concentration in 24-h conditioned medium reached a maximum, which was maintained despite increasing cell number. Northern blot analysis revealed that this effect could largely be attributed to decreasing IGFBP-2 mRNA. In contrast to IGFBP-2, secretion of IGF-II and accumulation of IGF-II mRNA by IEC-6 cells correlated positively with cell number. The highest level of IGF-II protein detected by immunoblotting of conditioned medium occurred in post-confluent cells. IGF-I stimulated the cells to grow to a high cellular density and inhibited IGFBP-2 secretion in a concentration-dependent fashion. We conclude that expression of IGF-II and IGFBP-2 are regulated in IEC-6 cells by cellular density, and IGF-II may act as a survival factor at high cell density.

An insulin-like growth factor II (IGF-II) affinity-enhancing domain localized within extracytoplasmic repeat 13 of the IGF-II/mannose 6-phosphate receptor.

Insulin-like growth factor II (IGF-II) and phosphomannosylated glycoproteins bind to distinct sites on the same receptor, the IGF-II/mannose 6-phosphate receptor (IGF2R). Analysis of truncated receptors (minireceptors) has been used to map the IGF-II binding site within the receptor's extracytoplasmic domain, which consists of 15 homologous repeats. A minireceptor consisting of repeat 11 contained the minimal elements for binding IGF-II, but with 5- to 10-fold lower relative binding affinity than the full-length receptor. We hypothesized that the complete, high-affinity IGF-II binding site is formed by interaction between the primary site in repeat 11 and a putative affinity-enhancing domain. To determine the minimum portion of the IGF2R's extracytoplasmic domain needed for expression of high-affinity IGF-II binding, a nested set of FLAG epitope-tagged minireceptors encompassing repeats 11 through 15 was prepared and transiently expressed in 293T cells. Minireceptors containing repeats 11-13 or 11-15 exhibited high affinity, comparable to the full-length receptor (IC50 = 1-2 nM), whereas constructs containing repeat 11 only or repeats 11-12 did not (IC50 = 10-20 nM). These data suggested that the affinity-enhancing domain is located within repeat 13, which contains a unique 43-residue insert that has approximately 50% sequence identity to the type II repeat of fibronectin. Although a repeat 13 minireceptor did not bind IGF-II on its own, an 11-13 minireceptor containing a deletion of the 43-residue insert exhibited low IGF-II binding affinity (IC50 = 10-20 nM). Expression of mutant receptors from a full-length IGF2R construct bearing a deletion of the 43-residue insert was very low relative to wild type. Depletion assays using IGF-II-Sepharose showed that the mutant receptor had lower affinity for IGF-II than the wild-type receptor. This study reveals that two independent receptor domains are involved in the formation of a high-affinity binding site for IGF-II, and that a complete repeat 13 is required for high-affinity IGF-II binding.

Production of insulin-like growth factor I (IGF-I) and IGF-binding proteins by rat intestinal stromal cells in vitro.

To determine if intestinal stromal cells secrete diffusible factors such as insulin-like growth factors (IGFs) capable of regulating epithelial cell growth in vitro, stromal cells were isolated by enzymatic digestion of rat intestine. Incorporation of [3H]thymidine into DNA and [14C]leucine into protein of IEC-6 cells, a model intestinal epithelial cell line, was significantly increased (two- to threefold) when the IEC-6 cells were co-cultured with stromal cells, relative to IEC-6 cells grown alone. Medium conditioned by stromal cells stimulated DNA synthesis of IEC-6 cells in a dose-dependent manner. Analysis of the conditioned medium revealed that intestinal stromal cells secreted IGF-I, but little IGF-II, in addition to an M(r) 32,000 IGF-binding protein (IGFBP-2) and an IGFBP having M(r) approximately 24,000. We conclude that rat intestinal stromal cells secrete one or more diffusible factors, which may include IGF-I and IGFBPs, capable of stimulating proliferation of IEC-6 cells in vitro.

Transplanted small bowel and expression of the insulin-like growth factor binding proteins.

The insulin-like growth factor binding proteins (IGFBPs) act to modulate the growth and differentiation of the gastrointestinal mucosa by regulating cellular responses to the important mitogens, the insulin-like growth factors (IGFs). The transplanted small bowel must maintain the normal growth factor interrelationships and signaling pathways despite the potential of host rejection. Ostomy effluent of patients after small bowel or combined liver/small bowel transplantation was assayed for IGFBPs to investigate the effect of rejection on the IGF-IGFBP axis. Seventeen patients were studied over an 18-month period. The transplanted small bowel produced no measurable IGFBPs in the ostomy effluent under normal circumstances. However, when rejection was taking place the ostomy effluent was found to have measurable IGFBP levels in 6 of 12 episodes, the 6 episodes occurring in 6 different patients. Mostly, the IGFBPs present did not exhibit a serum-like pattern indicating the secretion of IGFBPs into the effluent was not the result of loss of mucosal barrier integrity. There were no statistically significant differences in protein content of the ostomy fluids in the presence or absence of rejection. Our results suggest that the gastrointestinal IGF axis is altered during some instances of transplanted small bowel rejection, with increased secretion of IGFBPs.

Stable interaction between G-actin and neurofilament light subunit in dopaminergic neurons.

Excessive accumulation of neurofilaments in the cell bodies and proximal axons of motor neurons is a major pathological hallmark of motor neuron diseases. In this communication we provide evidence that the neurofilament light subunit (68 kDa) and G-actin are capable of forming a stable interaction. Cytochalasin B, a cytoskeleton disrupting agent that interrupts actin-based microfilaments, caused aggregation of neurofilaments in cultured mesencephalic dopaminergic neurons, suggesting a possible interaction between neurofilaments and actin; which was tested further by using crosslinking reaction and affinity chromatography techniques. In the cross-linking experiment, G-actin interacted with individual neurofilament subunits and covalently cross-linked with disuccinimidyl suberate, a homobifunctional cross-linking reagent. Furthermore, G-actin was extensively cross-linked to the light neurofilament subunit with this reagent. The other two neurofilament subunits showed no cross-linking to G-actin. Moreover, neurofilament subunits were retained on a G-actin coupled affinity column and were eluted from this column by increasing salt concentration. All three neurofilament subunits became bound to the G-actin affinity column. However, a portion of the 160 and 200 kDa neurofilament subunits did not bind to the column, and the remainder of these two subunits eluted prior to the 68 kDa subunit, suggesting that the light subunit exhibited the highest affinity for G-actin. Moreover, neurofilaments demonstrated little or no binding to F-actin coupled affinity columns. The phosphorylation of neurofilament proteins with protein kinase C reduced its cross-linking to G-actin. The results of these studies are interpreted to suggest that the interaction between neurofilaments and actin, regulated by neurofilament phosphorylation, may play a role in maintaining the structure and hence the function of dopaminergic neurons in culture.

Flow cytometric analysis of vesicular pH in rat hepatocytes after ethanol administration.

We examined the effect of ethanol administration on intravesicular pH in intact hepatocytes by applying a flow cytometric technique to detect fluorescein-isothiocyanate-dextran (FITC-dextran) in acidic vesicles. Rats were pair-fed liquid diets containing either ethanol or isocaloric carbohydrate for 1 to 5 weeks. Our study showed that ethanol administration increased the in situ pH of hepatic lysosomes by 0.15 to 0.2 pH units. This pH increase was sufficient to cause a significant reduction in lysosomal protein degradation. Long-term ethanol administration also caused a significant alkalinization of hepatic endosomes, and this increased pH was sustained over the course of vesicular acidification in hepatocytes incubated in vitro. Direct exposure of hepatocytes from rats fed control diet to either 25 mmol/L ethanol or 50 micromol/L colchicine also brought about a rapid alkalinization of acidic vesicles in a manner that resembled that seen in hepatocytes from ethanol-fed rats. These same treatments augmented the vesicular alkalinization already present in cells from ethanol-fed animals. Although ethanol administration had no effect on the content of the hepatic mannose-6-phosphate/IGFII receptor, the results indicate that sustained alkalinization of endosomes could have important functional consequences by impairing M-6-P/IGFII receptor recycling, thereby disrupting the delivery of newly synthesized hydrolases to lysosomes. This decreased complement of hydrolases within lysosomes together with alkalinization of the intralysosomal compartment would result in an overall decrease in lysosomal proteolysis.

Truncated forms of the insulin-like growth factor II (IGF-II)/mannose 6-phosphate receptor encompassing the IGF-II binding site: characterization of a point mutation that abolishes IGF-II binding.

Complete understanding of the functional significance of insulin-like growth factor II (IGF-II) binding by the IGF-II/mannose-6-phosphate (Man-6-P) receptor requires mapping and ultimately mutational analysis of the receptor's IGF-II binding domain. Recent advances have localized the IGF-II binding site to extracytoplasmic repeats 10-11. To improve resolution of the binding site map, a nested set of epitope-tagged, truncated forms of the human IGF-II/Man-6-P receptor were transiently expressed in COS-7 cells. The IGF-II binding properties of truncated receptors immunoprecipitated from cell lysates and conditioned media were determined by affinity cross-linking. From the largest truncated receptor, encompassing extracytoplasmic repeats 8-11 (M(r) 68 K), through the smallest, comprised primarily of repeat 11 (M(r) 23 K), all were able to bind and cross-link to IGF-II. As a group, the truncated receptors had similar affinities for IGF-II, but with relative binding affinities 5-to 10-fold lower than those of full-length receptors. A point mutation substituting threonine for isoleucine at residue 1572, located in the NH2-terminal half of repeat 11, completely abolished IGF-II binding. We conclude that repeat 11 of the IGF-II/Man-6-P receptor's extracytoplasmic domain contains the minimal elements required for binding and cross-linking to IGF-II, and that lle1572 and other residues within the NH2-terminal half of repeat 11 are particularly important for IGF-II interaction.

Expression of insulin-like growth factor-II and insulin-like growth factor binding proteins during Caco-2 cell proliferation and differentiation.

The components of the insulin-like growth factor (IGF) axis and their roles in regulating proliferation and differentiation of the human colon adenocarcinoma cell line, Caco-2, have been investigated. Caco-2 cells proliferated in serum-free medium at 75% the rate observed in medium containing 10% fetal bovine serum. IGF-I (10 nM) increased Caco-2 cell growth in serum-free medium, but not to the rate seen with serum. Multiple IGF-II mRNA species were produced by Caco-2 cells, but IGF-I mRNA was undetectable. Secretion of radioimmunoassayable IGF-II corresponded with steady-state levels of IGF-II mRNA, neither of which was observed to change markedly over the course of 16 days of Caco-2 cell differentiation. Levels of sucrase-isomaltase mRNA, a marker for enterocytic differentiation, increased 12-fold between days 5 and 16 of culture. Northern blotting of total RNA and ligand blot and immunoblot analyses of serum-free conditioned medium revealed that Caco-2 cells produce several IGF binding proteins (IGFBPs), including IGFBP-2, -3, and -4, as well as a 31,000 M(r) species that was not identified. The pattern of IGFBP secretion changed dramatically during Caco-2 cell differentiation: IGFBP-3 and IGFBP-2 increased 8.5-fold and 5-fold, respectively, whereas IGFBP-4 and the 31,000 M(r) species decreased 43% and 90%. Caco-2 cell clones stably transfected with a human IGFBP-4 cDNA construct exhibited a 60% increase in steady-state level of IGFBP-4 mRNA, and secreted twice as much IGFBP-4 protein as controls. Moreover, IGFBP-4-overexpressing cells proliferated at only 25% the rate of control cells in serum-free medium, in conjunction with a 70% increase in expression of sucrase-isomaltase. In summary, these studies indicate that a complex IGF axis is involved in autocrine regulation of Caco-2 cell proliferation and differentiation.