IGF-2 stimulated growth mediated by the somatomedin type 2 receptor

Single-transmembrane domain IGF-II/M6P receptor: potential interaction with G protein and its association with cholesterol-rich membrane domains

The IGF-II/mannose 6-phosphate (M6P) receptor is a single-transmembrane domain glycoprotein that plays an important role in the intracellular trafficking of lysosomal enzymes and endocytosis-mediated degradation of IGF-II. The receptor may also mediate certain biological effects in response to IGF-II binding by interacting with G proteins. However, the nature of the IGF-II/M6P receptor's interaction with the G protein or with G protein-coupled receptor (GPCR) interacting proteins such as β-arrestin remains unclear. Here we report that [(125)I]IGF-II receptor binding in the rat hippocampal formation is sensitive to guanosine-5'-[γ-thio]triphosphate, mastoparan, and Mas-7, which are known to interfere with the coupling of the classical GPCR with G protein. Monovalent and divalent cations also influenced [(125)I]IGF-II receptor binding. The IGF-II/M6P receptor, as observed for several GPCRs, was found to be associated with β-arrestin 2, which exhibits sustained ubiquitination after stimulation with Leu(27)IGF-II, an IGF-II analog that binds rather selectively to the IGF-II/M6P receptor. Activation of the receptor by Leu(27)IGF-II induced stimulation of extracellular signal-related kinase 1/2 via a pertussis toxin-dependent pathway. Additionally, we have shown that IGF-II/M6P receptors under normal conditions are associated mostly with detergent-resistant membrane domains, but after stimulation with Leu(27)IGF-II, are translocated to the detergent-soluble fraction along with a portion of β-arrestin 2. Collectively these results suggest that the IGF-II/M6P receptor may interact either directly or indirectly with G protein as well as β-arrestin 2, and activation of the receptor by an agonist can lead to alteration in its subcellular distribution along with stimulation of an intracellular signaling cascade.

Phospholipase C and protein kinase C-β 2 mediate insulin-like growth factor II-dependent sphingosine kinase 1 activation

We recently reported that IGF-II binding to the IGF-II/mannose-6-phosphate (M6P) receptor activates the ERK1/2 cascade by triggering sphingosine kinase 1 (SK1)-dependent transactivation of G protein-coupled sphingosine 1-phosphate (S1P) receptors. Here, we investigated the mechanism of IGF-II/M6P receptor-dependent sphingosine kinase 1 (SK1) activation in human embryonic kidney 293 cells. Pretreating cells with protein kinase C (PKC) inhibitor, bisindolylmaleimide-I, abolished IGF-II-stimulated translocation of green fluorescent protein (GFP)-tagged SK1 to the plasma membrane and activation of endogenous SK1, implicating PKC as an upstream regulator of SK1. Using confocal microscopy to examine membrane translocation of GFP-tagged PKCα, β1, β2, δ, and ζ, we found that IGF-II induced rapid, transient, and isoform-specific translocation of GFP-PKCβ2 to the plasma membrane. Immunoblotting of endogenous PKC phosphorylation confirmed PKCβ2 activation in response to IGF-II. Similarly, IGF-II stimulation caused persistent membrane translocation of the kinase-deficient GFP-PKCβ2 (K371R) mutant, which does not dissociate from the membrane after translocation. IGF-II stimulation increased diacylglycerol (DAG) levels, the established activator of classical PKC. Interestingly, the polyunsaturated fraction of DAG was increased, indicating involvement of phosphatidyl inositol/phospholipase C (PLC). Pretreating cells with the PLC inhibitor, U73122, attenuated IGF-II-dependent DAG production and PKCβ2 phosphorylation, blocked membrane translocation of the kinase-deficient GFP-PKCβ2 (K371R) mutant, and reduced sphingosine 1-phosphate production, suggesting that PLC/PKCβ2 are upstream regulators of SK1 in the pathway. Taken together, these data provide evidence that activation of PLC and PKCβ2 by the IGF-II/M6P receptor are required for the activation of SK1.

RNA-binding protein insulin-like growth factor mRNA-binding protein 3 (IMP-3) promotes cell survival via insulin-like growth factor II signaling after ionizing radiation

Ionizing radiation (IR) induces proapoptotic gene expression programs that inhibit cell survival. These programs often involve RNA-binding proteins that associate with their mRNA targets to elicit changes in mRNA stability and/or translation. The RNA-binding protein IMP-3 is an oncofetal protein overexpressed in many human malignancies. IMP-3 abundance correlates with tumor aggressiveness and poor prognosis. As such, IMP-3 is proving to be a highly significant biomarker in surgical pathology. Among its many mRNA targets, IMP-3 binds to and promotes translation of insulin-like growth factor II (IGFII) mRNA. Our earlier studies showed that reducing IMP-3 abundance with siRNAs reduced proliferation of human K562 chronic myeloid leukemia cells because of reduced IGF-II biosynthesis. However, the role of IMP-3 in apoptosis is unknown. Here, we have used IR-induced apoptosis of K562 cells as a model to explore a role for IMP-3 in cell survival. Knockdown of IMP-3 with siRNA increased susceptibility of cells to IR-induced apoptosis and led to reduced IGF-II production. Gene reporter assays revealed that IMP-3 acts through the 5' UTR of IGFII mRNA during apoptosis to promote translation. Finally, culture of IR-treated cells with recombinant IGF-II partially reversed the effects of IMP-3 knockdown on IR-induced apoptosis. Together, these results indicate that IMP-3 acts in part through the IGF-II pathway to promote cell survival in response to IR. Thus, IMP-3 might serve as a new drug target to increase sensitivity of CML cells or other cancers to IR therapy.

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.

The role of the IGF system in cancer growth and metastasis: overview and recent insights

IGF-I receptor (IGF-IR) signaling and functions are mediated through the activities of a complex molecular network of positive (e.g., type I IGF) and negative (e.g., the type II IGF receptor, IGF-IIR) effectors. Under normal physiological conditions, the balance between the expression and activities of these molecules is tightly controlled. Changes in this delicate balance (e.g., overexpression of one effector) may trigger a cascade of molecular events that can ultimately lead to malignancy. In recent years, evidence has been mounting that the IGF axis may be involved in human cancer progression and can be targeted for therapeutic intervention. Here we review old and more recent evidence on the role the IGF system in malignancy and highlight experimental and clinical studies that provide novel insights into the complex mechanisms that contribute to its oncogenic potential. Controversies arising from conflicting evidence on the relevance of IGF-IR and its ligands to human cancer are discussed. Our review highlights the importance of viewing the IGF axis as a complex multifactorial system and shows that changes in the expression levels of any one component of the axis, in a given malignancy, should be interpreted with caution and viewed in a wider context that takes into account the expression levels, state of activation, accessibility, and functionality of other interacting components. Because IGF targeting for anticancer therapy is rapidly becoming a clinical reality, an understanding of this complexity is timely because it is likely to have an impact on the design, mode of action, and clinical outcomes of newly developed drugs.

Effect of kainic acid treatment on insulin-like growth factor-2 receptors in the IGF2-deficient adult mouse brain

Insulin-like growth factor-2 (IGF2) is a member of the insulin gene family with known neurotrophic properties. The actions of IGF2 are mediated via the IGF type 1 and type 2 receptors as well as through the insulin receptors, all of which are widely expressed throughout the brain. Since IGF2 is up-regulated in the brain after injury, we wanted to determine whether the absence of IGF2 can lead to any alteration on brain morphology and/or in the response of its receptor binding sites following a neurotoxic insult. No morphological differences were observed between the brains of IGF2 knockout (IGF2(-/-)) and wild-type control (IGF2(+/+)) mice. However, our in vitro receptor autoradiography results indicate that IGF2(-/-) mice had lower endogenous levels of [(125)I]IGF1 and [(125)I]insulin receptor binding sites in the hippocampus and cerebellum as compared to IGF2(+/+) mice, while endogenous [(125)I]IGF2 receptor binding showed a decrease only in the cerebellum. Seven days after kainic acid administration, the [(125)I]insulin receptor binding sites were significantly decreased in all brain regions of the IGF2(+/+) mice, while the levels of [(125)I]IGF1 and [(125)I]IGF2 binding sites were decreased only in select brain areas. The IGF2(-/-) mice, on the other hand, showed increased [(125)I]IGF1 and [(125)I]IGF2 and [(125)I]insulin receptor binding sites in selected regions such as the hippocampus and cerebellum. These results, taken together, suggest that deletion of IGF2 gene does not affect gross morphology of the brain but does selectively alter endogenous [(125)I]IGF1, [(125)I]IGF2 and [(125)I]insulin receptor binding sites and their response to neurotoxicity.

The RNA-binding protein IMP-3 is a translational activator of insulin-like growth factor II leader-3 mRNA during proliferation of human K562 leukemia cells

IMP-3, a member of the insulin-like growth factor-II (IGF-II) mRNA-binding protein (IMP) family, is expressed mainly during embryonic development and in some tumors. Thus, IMP-3 is considered to be an oncofetal protein. The functional significance of IMP-3 is not clear. To identify the functions of IMP-3 in target gene expression and cell proliferation, RNA interference was employed to knock down IMP-3 expression. Using human K562 leukemia cells as a model, we show that IMP-3 protein associates with IGF-II leader-3 and leader-4 mRNAs and H19 RNA but not c-myc and beta-actin mRNAs in vivo by messenger ribonucleoprotein immunoprecipitation analyses. IMP-3 knock down significantly decreased levels of intracellular and secreted IGF-II without affecting IGF-II leader-3, leader-4, c-myc, or beta-actin mRNA levels and H19 RNA levels compared with the negative control siRNA treatment. Moreover, IMP-3 knock down specifically suppressed translation of chimeric IGF-II leader-3/luciferase mRNA without altering reporter mRNA levels. Together, these results suggest that IMP-3 knock down reduced IGF-II expression by inhibiting translation of IGF-II mRNA. IMP-3 knock down also markedly inhibited cell proliferation. The addition of recombinant human IGF-II peptide to these cells restored cell proliferation rates to normal. IMP-3 and IMP-1, two members of the IMP family with significant structural similarity, appear to have some distinct RNA targets and functions in K562 cells. Thus, we have identified IMP-3 as a translational activator of IGF-II leader-3 mRNA. IMP-3 plays a critical role in regulation of cell proliferation via an IGF-II-dependent pathway in K562 leukemia cells.

Targeted Knockdown of the RNA-binding Protein CRD-BP Promotes Cell Proliferation via an Insulin-like Growth Factor II-dependent Pathway in Human K562 Leukemia Cells

The c-myc mRNA coding region determinant-binding protein (CRD-BP) was first identified as a masking protein that stabilizes c-myc mRNA in a cell-free mRNA degradation system. Thus, CRD-BP is thought to promote cell proliferation by maintaining c-Myc at critical levels. CRD-BP also appears to be an oncofetal protein, based upon its expression during mammalian development and in some tumors. By using K562 leukemia cells as a model, we show that CRD-BP gene silencing by RNA interference significantly promoted proliferation, indicating an inhibitory effect of CRD-BP on proliferation. Unexpectedly, CRD-BP knockdown had no discernible effect on c-myc mRNA levels. CRD-BP is also known as insulin-like growth factor II (IGF-II) mRNA-binding protein-1. It has been reported to repress translation of a luciferase reporter mRNA containing an IGF-II 5'-untranslated region known as leader 3 but not one containing IGF-II leader 4. CRD-BP knockdown markedly increased IGF-II mRNA and protein levels but did not alter translation of luciferase reporter mRNAs containing 5'-untranslated regions consisting of either IGF-II leader 3 or leader 4. Addition of antibody against IGF-II to cell cultures inhibited the proliferative effect of CRD-BP knockdown, suggesting that regulation of IGF-II gene expression, rather than c-myc mRNA levels, mediates the proliferative effect of CRD-BP knockdown. Thus, we have identified a dominant function for CRD-BP in cell proliferation of human K562 cells, involving a possible IGF-II-dependent mechanism that appears independent of its ability to serve as a c-myc mRNA masking protein.

Differential effects of serum constituents on apoptosis induced by the cyclopentenone prostaglandin 15-deoxy-delta12,14-prostaglandin J2 in WISH epithelial cells

Cyclopentenone prostaglandins, delta12-PGJ2 and 15d-PGJ2, have potent anti-tumour and anti-inflammatory activities, and have been shown to induce apoptosis in amnion-derived WISH cells. In this study, we have investigated the protective effects of serum and its constituents (growth factors and albumin) on delta12-PGJ2 and 15d-PGJ2-induced apoptosis in WISH cells. Serum (0.5% w/v) was protective against both delta12-PGJ2 and 15d-PGJ2-induced apoptosis. This was not due to the presence of serum-derived growth factors (EGF, IGF-1 and IGF-2), since they had no significant effect on 15d-PGJ2-induced cell death. In contrast, IGF-1 partially inhibited etoposide-induced apoptosis, confirming the presence of a functional IGF-1 receptor signalling system. Albumin was identified as the key survival factor in serum, since albumin and delipidated albumin exhibited the same level of protection from 15d-PGJ2-induced apoptosis as serum itself. The potential for serum albumin to regulate the bioactivity of cyclopentenone PGs may be of considerable importance in pathological conditions where roles for cyclopentenone PGs have been identified.

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

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

Prorenin uptake in the heart: a prerequisite for local angiotensin generation?

Interference with locally generated angiotensin II most likely underlies the beneficial effects of renin-angiotensin system blockers in cardiac disorders. Since renin is not synthesized in the heart, this enzyme must be sequestered from the circulation in order to allow angiotensin generation at cardiac tissue sites. This review addresses the various ways through which circulating (i.e., kidney-derived) renin may reach cardiac tissue sites, considering in particular the possibility that prorenin, the inactive precursor of renin, is involved in cardiac angiotensin generation, as the plasma concentrations of prorenin are tenfold higher than those of renin. Renin and prorenin diffuse into the cardiac interstitial space and bind to cardiac (pro)renin receptors/renin-binding proteins. One of these receptors is the mannose 6-phosphate/insulin-like growth factor II receptor. This receptor not only binds mannose 6-phosphate-containing ligands like renin and prorenin, it also internalizes these enzymes, and activates prorenin intracellularly. This process possibly represents (pro)renin clearance, since intracellular angiotensin generation could not be demonstrated following (pro)renin uptake by cardiomyocytes. Angiotensin II-mediated myocyte proliferation did occur when incubating cardiomyocytes with prorenin plus angiotensionogen. The effects of prorenin plus angiotensinogen were comparable to those of 100nmol/l angiotensin II, although the angiotensin II levels in the medium during exposure of the cells to prorenin plus angiotensinogen were <1nmol/l. This suggests that cardiac angiotensin II generation by circulating renin occurs predominantly on the cell surface. The presence of ACE and/or renin on the cell membrane, in the microenvironment of angiotensin receptors, would allow maximal efficiency of local angiotensin II generation, i.e., immediate binding of angiotensin II to its receptors with minimal loss into the extracellular space.

Expression of insulin-like growth factors IGF-I and IGF-II, and their receptors during the growth and megakaryocytic differentiation of K562 cells

Insulin-like growth factors (IGFs) I and II are critical regulators of cell proliferation and differentiation and most of the growth promoting properties of both ligands are mediated by IGF-I receptor (IGF-IR). In the present study we have investigated the role of IGFs in K562 cell line during normal growth and 12-O-tetradecanoyl-phorbol-13-acetate (TPA)-induced megakaryocytic differentiation. Abundant expression of IGF-I, IGF-II and IGF-IR was demonstrated in resting cells and exogenous IGF-I and IGF-II increased 3H-thymidine incorporation in a dose dependent manner. In contrast, we found that basal growth of the cells was inhibited by using anti-IGF-IR mAb. Furthermore, also IGF-I and IGF-II induced DNA synthesis was significantly suppressed by anti-IGF-IR mAb. During megakaryocytic differentiation, expression of IGF-IR increased during first 12h, but after that the expression started to decrease together with IGF-I. Taken together, our data suggest that autocrine production of IGF-I and IGF-II may via IGF-IR play a significant role in the growth and megakaryocytic differentiation of K562 cells.

Mechanical strain stimulates ROS cell proliferation through IGF-II and estrogen through IGF-I

The mechanism by which mechanical strain stimulates bone cell proliferation was investigated and compared with that of estrogen in ROS 17/2.8 cells. Similarity of strain-related responses between ROS cells and osteoblasts was established by demonstrating that ROS cells respond to a short single period of strain in their substrate (1000-3500 microepsilon, 600 cycles, 1 Hz) by a similar strain magnitude-related increase in glucose 6-phosphate dehydrogenase activity as rat osteoblasts and osteocytes in explants in situ. ROS17/2.8 cells also showed similar proliferative responses to strain and 17beta-estradiol, as assessed by [3H]thymidine incorporation and cell counting, as primary cultures of long bone-derived osteoblast-like cells. Strain-related increase in proliferation in ROS cells was accompanied by a 4-fold increase in levels of insulin-like growth factor-II (IGF-II) in conditioned medium. Neither strain nor estrogen had an effect on the conditioned medium levels of IGF-I. Exogenous truncated IGFs tIGF-I and tIGF-II both increased proliferation in a dose-dependent manner. The neutralizing monoclonal antibody (nMAb) to IGF-I blocked proliferation stimulated by tIGF-I but not that due to tIGF-II and vice versa. IGF-I receptor blocking antibody (IGF-IRBAb) blocked the proliferative effect of tIGF-I but not that to tIGF-II. The proliferative effect of estrogen was abolished by IGF-I nMAb and IGF-IRBAb, but these antibodies had no effect on the proliferative response to strain. In contrast IGF-II nMAb abolished the proliferative effect of strain but had no effect on that of estrogen. These data show that ROS17/2.8 cells have similar responses to strain and estrogen qualitatively and quantitatively as rat osteoblasts in situ and rat long bone-derived osteoblast-like cells in primary culture. Estrogen-related proliferation in ROS17/2.8 cells appears to be mediated by IGF-I acting through the IGF-I receptor and does not involve IGF-II. In contrast, strain-related proliferation appears to be mediated by IGF-II and does not involve either IGF-I or the IGF-I receptor.

Mannose 6-Phosphate/Insulin-like growth factor II receptor mediates internalization and degradation of leukemia inhibitory factor but not signal transduction

Leukemia inhibitory factor (LIF) is a multifunctional cytokine belonging to the interleukin-6 subfamily of helical cytokines, all of which use the glycoprotein (gp) 130 subunit for signal transduction. The specific receptor for LIF, gp190, binds this cytokine with low affinity and is also required for signal transduction. We have recently reported that glycosylated LIF produced by transfected Chinese hamster ovary cells also binds to a lectin-like receptor, mannose 6-phosphate/insulin-like growth factor II receptor (Man-6-P/IGFII-R) (Blanchard, F., Raher, S., Duplomb, L., Vusio, P., Pitard, V., Taupin, J. L., Moreau, J. F., Hoflack, B., Minvielle, S., Jacques, Y., and Godard, A. (1998) J. Biol. Chem. 273, 20886-20893). The present study shows that (i) mannose 6-phosphate-containing LIF is naturally produced by a number of normal and tumor cell lines; (ii) other cytokines in the interleukin-6 family do not bind to Man-6-P/IGFII-R; and (iii) another unrelated cytokine, macrophage-colony-stimulating factor, is also able to bind to Man-6-P/IGFII-R in a mannose 6-phosphate-sensitive manner. No functional effects or signal transductions mediated by this lectin-like receptor were observed in various biological assays after LIF binding, and mannose 6-phosphate-containing LIF was as active as non-glycosylated LIF. However, mannose 6-phosphate-sensitive LIF binding resulted in rapid internalization and degradation of the cytokine on numerous cell lines, which suggests that Man-6-P/IGFII-R plays an important role in regulating the amounts of LIF available in vivo.

The mannose 6-phosphate/insulin-like growth factor II receptor is a nanomolar affinity receptor for glycosylated human leukemia inhibitory factor

Comparison of the binding properties of non-glycosylated, glycosylated human leukemia inhibitory factor (LIF) and monoclonal antibodies (mAbs) directed at gp190/LIF-receptor beta subunit showed that most of the low affinity (nanomolar) receptors expressed by a variety of cell lines are not due to gp190. These receptors bind glycosylated LIF produced in Chinese hamster ovary cells (CHO LIF) (Kd = 6.9 nM) but not Escherichia coli-derived LIF or CHO LIF treated with endoglycosidase F. CHO LIF binding to these receptors is neither affected by anti-gp190 mAbs nor by anti-gp130 mAbs and is specifically inhibited by low concentrations of mannose 6-phosphate (Man-6-P) (IC50 = 40 microM), suggesting that they could be related to Man-6-P receptors. The identity of this LIF binding component with the Man-6-P/insulin-like growth factor-II receptor (Man-6-P/IGFII-R) was supported by several findings. (i) It has a molecular mass very similar to that of the Man-6-P/IGFII-R (270 kDa); (ii) the complex of LIF cross-linked to this receptor is immunoprecipitated by a polyclonal anti-Man-6-P/IGFII-R antibody; (iii) this antibody inhibits LIF and IGFII binding to the receptor with comparable efficiencies; (iv) soluble Man-6-P/IGFII-R purified from serum binds glycosylated LIF (Kd = 4.3 nM) but not E. coli LIF. The potential role of Man-6-P/IGFII-R in LIF processing and biological activity is discussed.

Peptide growth factors and the adrenal cortex

Increasing evidence suggests that the actions of classical stimulants of adrenocortical growth and function, such as ACTH or dietary sodium restriction, may partially be mediated via locally produced regulators. Several peptide growth factors, such as basic fibroblast growth factor, insulin-like growth factors, and transforming growth factor-beta 1, have emerged in recent years as multifunctional molecules that typically play such regulatory roles. Adrenocortical cells are highly responsive to these growth factors, in particular in the regulation of cell growth and differentiated functions, such as steroidogenesis. In addition, growth factor expression in the adrenal cortex has been shown to be regulated by physiological stimulants. The spatial expression, release, and activation of these growth factors may, therefore, locally mediate or amplify the actions of the hypothalamo-pituitary axis and the renin-angiotensin system on adrenocortical proliferation, differentiation, and steroidogenesis.

Comparative effects of insulin-like growth factor II (IGF-II) and IGF-II mutants specific for IGF-II/CIM6-P or IGF-I receptors on in vitro hematopoiesis

This report presents the results of studies investigating the effect of insulin-like growth factor II (IGF-II) on the proliferation and differentiation of CD34+ bone marrow cells in serum-substituted liquid cultures. Bone marrow cells were enriched for CD34+ cells and then placed in liquid cultures supplemented with either interleukin 3 (IL-3) or IL-3 and c-kit ligand with and without the addition of IGF-II. When CD34+ cells were incubated with IL-3, cellularity increased throughout four weeks of culture. Cellularity was twofold greater when cultures also contained IGF-II. IGF-II also promoted an increase in cellularity in cultures with IL-3 and c-kit ligand. In combination with IL-3 or IL-3 and c-kit ligand, IGF-II promoted an earlier differentiation of granulocytes, as well as an increase in the number of megakaryocyte lineage cells. There were approximately two-fold more colony-forming units for granulocytes and macrophages (CFU-GM) and burst-forming units for erythroid cells (BFU-E) in cultures containing both IL-3 and IGF-II than in cultures with IL-3 alone. These results demonstrate that in cytokine-supplemented media, physiological concentrations of IGF-II augmented both the proliferation and differentiation of CD34+ bone marrow cells while maintaining a greater number of progenitor cells. To identify the receptors through which IGF-II enhances in vitro hematopoiesis, IGF-II was substituted with one of the mutant forms of IGF-II that selectively interacts with either IGF-II/CIM6-P receptors or with IGF-I and insulin receptors. The results with the mutant forms of IGF-II demonstrate that IGF-II augments in vitro hematopoiesis primarily through its interaction with IGF-I and possibly insulin receptors, rather than IGF-II/CIM6-P receptors.

Varying patterns of expression of insulin-like growth factors I and II and their receptors in murine mammary adenocarcinomas of different metastasizing ability

We studied the expression of insulin-like growth factors I (IGF-I) and II (IGF-II) and their receptors (IGF-R) in 2 related murine mammary adenocarcinoma in vivo lines, M3 and MM3, with different metastasizing ability. We further investigated the effects of IGFs on the secretion of a key enzyme in the metastatic cascade, the urokinase-type plasminogen activator (uPA) in M3 and MM3 cells. M3 is a spontaneous mammary tumor originated in BALB/c mice, with a 40% incidence of lung metastases. MM3 variant, obtained by successive s.c. implants of M3 lung metastases into syngeneic mice, shows a 95% incidence of lung metastases. Similar levels of expression of IGF-I protein were found in M3 and MM3 tumors, whereas IGF-II expression was 4-fold higher im MM3. RNAse protection assays showed similar levels of IGF-I mRNA in M3 and MM3 tumors and revealed a 4-fold increase in IGF-II transcripts in MM3 tumors compared with M3. Authentic IGF-I and II messages were also found in primary cultures of M3 and MM3 cells. IGF-I mRNA levels were similar in both cultures and, as described for solid tumors a 5-fold increase in IGF-II message was detected in MM3 cells. The presence of type I and mannose-6-phosphate (Man-6P)/type II IGF-R was demonstrated in both M3 and MM3 tumors. A 2-fold increase of type I IGF-R was detected in MM3 tumors compared with M3. Man-6P/type II IGF-R levels were 2-fold lower in MM3 tumors than in M3. As observed in tumor membranes, type I IGF-R concentrations were higher and Man-6P/type II IGF-R lower in cultures of MM3 epithelial cells compared with MM3 cells. In addition, we found that IGF-I enhanced secreted uPA activity in both M3 and MM3 cells while IGF-II only stimulated uPA secretion in MM3 cells.

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.

Identification of functional insulin-like growth factor-II/mannose-6-phosphate receptors in isolated bone cells

The role of the IGF-II/cation independent mannose-6-phosphate (IGF-II/M6P) receptor in the transduction of cellular effects evoked by IGF-II has been extensively debated in the literature. Many reports suggest that IGF-II transduces its effects through the IGF-I receptor, while others show that IGF-II utilizes the type II receptor to affect cellular activity. This study 1) verifies the presence of the IGF-II/M6P receptor in rat calvarial osteoblasts, and 2) evaluates the ability of the receptor to initiate intracellular signals. Using conventional receptor binding assays, it was found that osteoblasts bind IGF-II with high affinity. Scatchard analyses indicated that there are 5.08 x 10(4) IGF-II/M6P receptors per osteoblast with a Kd near 2.0 nM). The receptor protein was further identified by cross-linking with 125I-IGF-II. Northern analysis was used to identify an mRNA transcript for the IGF-II/M6P receptor protein. To examine if the IGF-II/M6P receptor can initiate second messenger signals, the ability of IGF-II to evoke Ca2+ transients was evaluated. Osteoblasts pretreated with IGF-I did not lose their ability to respond to IGF-II. Further, a polyclonal antibody against the rat IGF-II/M6P receptor (R-II-PAB1) 1) was able to evoke its own Ca2+ response, and 2) was able to block the generation of Ca2+ transients caused by IGF-II. The data in this report show that the osteoblastic Ca2+ response to IGF-II appears to be caused by an intracellular release of Ca2+ which is mediated by the IGF-II/M6P receptor making it possible to envision how the receptor may be an important modulator of osteoblast mediated osteogenesis.

Expression of insulin-like growth factor II in hepatitis B, cirrhosis and hepatocellular carcinoma: its relationship with hepatitis B virus antigen expression

Expression of insulin-like growth factor II in two human hepatocellular carcinoma cell lines and in hepatitis B, cirrhosis and hepatocellular carcinoma in 419 cases were investigated, and its relationship with the expression of hepatitis B virus X gene was studied by means of immunohistochemical and electron microscopic techniques. The results demonstrated that hepatocellular carcinoma cells (SMMC 7721 and QGY 7703) in culture could express insulin-like growth factor II. Expression seemed to be regulated by cell density, which was suggested as the molecular basis of the contact inhibition of cell proliferation. In tissue sections, cells with high expression of insulin-like growth factor II were observed not only in hepatocellular carcinoma (93%) but also in 95% of the pericancerous liver tissues, 72% of cirrhotic livers, 64% of chronic active hepatitis and 37% of chronic persistent hepatitis. In most cases of hepatocellular carcinoma, insulin-like growth factor II was localized in the cytoplasm of the cancer cells. In the benign liver disorders, four types of cells that highly expressed insulin-like growth factor II were observed: (a) a kind of small liver cell we named the small polygonal liver cell; (b) multinuclear giant hepatocytes; (c) hepatocytes in most of hyperplastic and neoplastic nodules, small hepatocyte nodules and some of regenerative nodules; and (d) some proliferating ductular cells. Even more interestingly, insulin-like growth factor II expression was shown to be closely related to the expression of hepatitis B virus X gene product. We suggest that the activation of insulin-like growth factor II gene and its overexpression may be a crucial step in the processes of hepatitis B virus-associated hepatocarcinogenesis and that the X gene product may activate the insulin-like growth factor II gene through a transactivation mechanism. In addition, we studied the characteristics of small polygonal liver cells, and the roles they may play in the regeneration and carcinogenesis of hepatitis B virus-infected liver are discussed.

Human osteosarcoma (U-2 OS) cells express both insulin-like growth factor-I (IGF-I) receptors and insulin-like growth factor-II/mannose-6-phosphate (IGF-II/M6P) receptors and synthesize IGF-II: autocrine growth stimulation by IGF-II via the IGF-I receptor

Recently, insulin-like growth factor-I and -II (IGF-I and -II) have been implicated in the growth promotion of tumors in vivo and tumor cells in vitro. We have studied the human osteosarcoma cell line U-2 OS in order 1) to gain more insight into the growth promoting actions of the IGFs and 2) to establish an in vitro tissue culture model of IGF action in human tumor cells. Specific binding of 125I-IGF-I and 125I-IGF-II to IGF-I receptors and IGF-II/mannose-6-phosphate (M6P) receptors on U-2 OS cells was demonstrated in competitive binding experiments and in affinity crosslinking experiments. Western blotting of cell extracts confirmed the expression of the IGF-II/M6P receptor. In addition, in Northern blotting experiments using total RNA from U-2 OS cells IGF-I receptor RNA of 11 kb and IGF-II/M6P receptor RNA of approximately 9 kb were detected. Solution hybridization experiments confirmed the presence of IGF-I receptor and IGF-II/M6P receptor RNA. In a subset of experiments DNA synthesis was measured as 3H-thymidine uptake into cellular DNA of U-2 OS cells. Normal rat serum stimulated DNA synthesis maximally. IGF-I-deficient serum from hypophysectomized rats as well as IGF-I or IGF-II without serum were approximately twofold and tenfold, respectively, less potent than serum in stimulating 3H-thymidine uptake. The concentrations of IGF-I and IGF-II needed for half maximal stimulation of DNA synthesis corresponded well with the respective concentrations required for half maximal inhibition of 125I-IGF-I binding to U-2 OS cells. The anti-IGF-I receptor antibody alpha IR3 blocked the IGF-I and IGF-II stimulated increase of 3H-thymidine uptake. In addition, basal DNA synthesis was partially inhibited by the anti-IGF-I receptor antibody. These data suggest that U-2 OS cells synthesize and secrete IGF-like peptides. Northern blotting experiments confirmed that U-2 OS cells express an IGF-II RNA species of 5.3 kb but no IGF-I transcripts. In a series of RNase protection assays, protected RNA fragments were detected with an IGF-II riboprobe. Also, cell-conditioned medium from U-2 OS cells contained 1-2 ng/ml IGF-II immunoreactivity as measured in an IGF-binding protein blocked IGF-II radioimmunoassay.

IN CONCLUSION

1) U-2 OS cells express IGF-I and IGF-II/M6P receptors. 2) U-2 OS tumor cells respond to the addition of exogenous IGF-I and IGF-II with an increase of DNA synthesis.(ABSTRACT TRUNCATED AT 400 WORDS)

Calcium-mediated signal transduction: biology, biochemistry, and therapy

The process of proliferation, invasion and metastasis is a complex one which involves both the autonomy of the malignant cells and their interaction with the cellular and extracellular environments. The way in which the tumor cells respond to cellular and extracellular stimuli is regulated through transduction of those signals and translation into cellular activity. Transmembrane signal transduction involves three major categories of events: ion channel activation, transmission through guanine nucleotide binding protein intermediates with production of second messengers, and phosphorylation events. A frequent common denominator of these different pathways is a cellular calcium homeostasis. Calcium may be both a result of and a regulator of many of these signal transduction pathways and has been shown to have a role in the regulation of proliferation, invasion, and metastatic potential. The understanding and application of the basic tenets of these pathways to tumor cell proliferation, invasion, and metastases opens a new target for therapeutic intervention. We have identified a novel agent, CAI, which through inhibition of stimulated calcium influx inhibits proliferation and migration in vitro, and growth and dissemination in human cancer xenografts in vivo. CAI offers a new approach to cancer therapy, signal transduction therapy.

Expression, purification and binding to the receptor of human insulin-like growth factor II

Human insulin-like growth factor II (IGF-II) was expressed as a fused protein with 14 additive amino acids in Escherichia coli with a high yield by an expression system using T7 RNA polymerase. Purification of the expressed protein was simply performed using only differential ultrafiltrations, giving a homogeneous preparation upon polyacrylamide gel electrophoresis and high-performance liquid chromatography. The expressed peptide was reacted with a monoclonal antibody raised against native IGF-II on a blotted membrane. Furthermore, the peptide was bound to IGF-II receptor in solubilized rat fetus membrane, though the affinity was slightly inferior to that of native IGF-II. In addition, fusion IGF-II immobilized on a gel matrix was useful for one-step purification of the IGF-II receptor with a high yield from solubilized rat fetus membranes.

Insulin-like growth factor binding proteins in the human adrenal gland

Insulin-like growth factors (IGFs) are thought to be important regulators of adrenocortical growth and steroidogenesis. IGFs are usually complexed with a family of specific IGF-binding proteins (IGFBPs) in serum, other body fluids, and in conditioned media of a variety of cell types. IGFBPs may either inhibit or potentiate the effects of IGFs. In the present study we have investigated the gene expression of the IGFBPs and IGF receptors in human fetal (HFA) and adult (HAA) adrenals. Northern blotting and/or reverse transcription polymerase chain reaction (RT-PCR) methods were used. IGFBP secretion into the cell culture medium was studied in primary cell cultures by Western ligand blotting and by radioimmunoassays. IGFBP-1 mRNA expression was low in adrenals: Northern blots were negative, but RT-PCR revealed IGFBP-1 mRNA in HFA. IGFBP-2 mRNA was equally expressed in both HFA and HAA with no differences in signal intensities by Northern blotting. IGFBP-3 mRNA was detected in HFA but not in HAA by Northern blotting. IGFBP-4 mRNA was expressed equally in both HFA and HAA. IGFBP-5 and -6 mRNA expression was more abundant in HAA than in HFA. IGF-I and type I and type II IGF receptor mRNAs were equally expressed in both HFA and HAA. 12-O-tetradecanoyl phorbol-13-acetate (TPA), a protein kinase regulator, upregulated IGFBP-1 in HFA cultures as determined by RIA, but ACTH was without effect. IGFBP-2 was not regulated by TPA or ACTH neither at protein nor at mRNA level. IGFBP-3 was downregulated by TPA both at protein and mRNA levels, but it was not affected by ACTH.(ABSTRACT TRUNCATED AT 250 WORDS)

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.

Comparative studies of the granulopoietic enhancing effects of biosynthetic human insulin-like growth factors I and II

The effect of biosynthetic human insulin-like growth factor I (IGF-I) and IGF-II on the in vitro growth of human marrow myeloid progenitors in the presence of recombinant human granulocyte colony stimulating factor (rhG-CSF), granulocyte-macrophage CSF (rhGM-CSF), or interleukin-3 (rhIL-3), was investigated. IGF-I and IGF-II similarly enhanced the growth of myeloid progenitors in cultures stimulated with any of the above hemopoietic regulators. Analysis of colony composition showed an increase in the numbers of granulocyte colonies, but no alteration in the numbers of macrophage or granulocyte/macrophage colonies. IGF-I induced an increase of 62 +/- 16%, 84 +/- 13%, and 107 +/- 18% in granulocyte colony numbers in the presence of G-CSF, GM-CSF, or IL-3, respectively. The values for IGF-II were 66 +/- 13%, 96 +/- 12%, and 91 +/- 12%. Similar enhancement of myeloid colony formation by both peptides was also detected in G-CSF and GM-CSF-stimulated cultures of marrow cells that had been depleted of accessory cells, while neither peptide exerted any effect in the presence of IL-3 in such cultures. The growth-promoting effects of IGF-I and IGF-II were completely abrogated by monoclonal antibodies directed against the IGF-I (Type I) membrane receptor. IGF-I and IGF-II thus appear to exert their effects on human marrow myeloid progenitors via a direct mechanism involving the Type I receptor.

Developmental expression of the IGF-II/mannose 6-phosphate receptor

The first indication that the insulin-like growth factor-II/mannose 6-phosphate receptor (IGF-II/M6PR) is developmentally regulated came from studies of the serum form of the receptor in the rat. By immunoblotting, the circulating form of the receptor, which was 10 kDa smaller than the tissue receptor, was high in 19 day fetal and 3, 10, and 20 day postnatal sera and then declined sharply. We next used quantitative immunoblotting to measure the total tissue IGF-II/M6PR in the rat. The receptor levels were high in fetal tissues and in most tissues declined dramatically in late gestation and/or in the early postnatal period. The rank order of receptor expression was heart > placenta > lung = intestine > muscle = kidney > liver > brain. In heart, the receptor was 1.7% of total protein in the extract. More recently, we have examined the expression of IGF-II/M6PR mRNA using Northern blotting and a solution hybridization/RNase protection assay. The rank order of receptor mRNA concentration among fetal tissues agreed with the rank order of receptor protein. The concentration of receptor mRNA was significantly lower in postnatal tissue than in fetal tissue. Thus IGF-II/M6PR mRNA concentration is an important determinant of receptor protein in most tissues. What is the function of the IGF-II/M6PR in embryonic and fetal tissues? The M6PR in birds and frogs does not bind IGF-II. It is intriguing that the rat IGF-II/M6PR is prominent during the embryonic and fetal periods, times at which the differences between mammals, on the one hand, and frogs and birds, on the other, are most striking.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of growth and function of human gingival fibroblasts by fibroblast-activating factor derived from Porphyromonas gingivalis W50

The effect of a 24-kDa fibroblast-activating factor (FAF) isolated from outer membrane vesicles of Porphyromonas gingivalis W50 on the modulation of [3H]thymidine uptake and cell proliferation was examined in selected fibroblast and transformed cell lines. FAF enhanced the proliferation of human gingival fibroblasts, human skin fibroblasts, and human umbilical vein endothelial cells in subconfluent and confluent cells, suggesting that FAF might be functioning as a competence factor. The transformed cell lines, U-937 and HEp-2, were not responsive. FAF and several human-derived growth factors showed a synergistic effect on proliferation. [3H]proline and [3H]leucine were rapidly incorporated into fibroblasts in the presence of FAF; however, there was no selective induction of collagen synthesis. FAF was not active in the induction of interleukin-1 and interleukin-6. It is hypothesized that FAF from P. gingivalis functions as a growth factor for human fibroblasts but is without activity for transformed cells.

Biosynthetic labeling of beta-hexosaminidase B: inhibition of the cellular uptake of lysosomal secretions containing [3H]hexosaminidase B by insulin-like growth factor-II in rat C6 glial cells

The insulin-like growth factor-II/mannose-6-phosphate receptor binds two classes of ligands, IGF-II and lysosomal enzymes containing the mannose-6-phosphate recognition marker. To study the interaction of the two classes of ligands at the receptor level, we have isolated 'high uptake' forms of lysosomal enzymes containing mannose-6-phosphate that had been radiolabeled biosynthetically using a tissue culture model: Tay-Sachs disease fibroblasts were incubated in medium containing [3H]mannose, ammonium chloride and mannose-6-phosphate. Under the conditions of these experiments, the Tay-Sachs disease fibroblasts synthesized and secreted radiolabeled hexosaminidase B, as confirmed by measuring enzymatic activity of cell-conditioned medium. The enzyme secreted was recognized by antibodies raised against purified hexosaminidase A and B but not by nonimmune control sera in Western blotting and immunoprecipitation experiments. The radiolabeled cell-conditioned medium was partially purified by ion-exchange chromatography on a DEAE-Sephadex column. When partially purified [3H]hexosaminidase B was incubated with rat C6 glial cells which express large numbers of IGF-II/mannose-6-phosphate receptors, the enzyme was taken up specifically via the IGF-II/mannose-6-phosphate receptor as evidenced by carbohydrate competition experiments. The specific uptake of the radiolabeled lysosomal enzyme was partially inhibited by IGF-II and an antibody against the IGF-II/mannose-6-phosphate receptor (No. 3637). We conclude that the cellular uptake of a biosynthetically labeled lysosomal enzyme, hexosaminidase B, is partially inhibited by IGF-II. We hypothesize that IGF-II might be capable of modulating lysosomal pathways in vivo.

Effects of insulin-like growth factors and growth hormone on the in vitro proliferation of T lymphocytes

The insulin-like growth factors I and II (IGF-I and IGF-II) promote proliferation and differentiation of many cell types. We report that recombinant IGF-I and IGF-II augment both the lectin- and anti-CD3-induced proliferation of human peripheral blood mononuclear cells (PBMC) at concentrations proportional to their binding affinities. IGF-I and IGF-II also augmented the lectin-induced proliferation of purified T lymphocytes. Effects of IGF-I were found in cultures of T cells vigorously depleted for monocytes and supplemented with saturating concentrations of interleukin-1. The latter results indicate that the effect of IGF-I on the proliferation of T lymphocytes can occur independent of monocytes or monocyte-derived factors.

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.

Interactions of pro-cathepsin D and IGF-II on the mannose-6-phosphate/IGF-II receptor

The mannose-6-phosphate (Man-6P)/IGF-II receptor is a multifunctional receptor which binds with a high affinity on distinct sites two strikingly different classes of ligands: IGF-II, and Man-6P bearing molecules such as lysosomal enzymes or other biologically relevant ligands (TGF beta precursor, EGF receptor, proliferin...). Binding of each ligand on its cognate site is severely decreased in the presence of the other type of ligand, thus revealing that the two distinct sites are strongly interacting (steric hindrance, conformational change). Any imbalance in ligands and receptor concentration in various pathological situations (transformation, tumor, altered hormonal levels...) is thus likely to perturb their associated biological functions in the targeting and routing of lysosomal enzymes or Man-6P ligands or in the autocrine/paracrine IGF-II--induced cellular responses.

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.

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.

Human mononuclear phagocytes express the insulin-like growth factor-II/mannose-6-phosphate receptor

The insulin-like growth factor (IGF)-II/mannose-6-phosphate (M6P) receptor, which targets acid hydrolases to lysosomes, is a multifunctional protein with separate binding sites for IGF-II and M6P. The purpose of this study was to determine if alveolar macrophages (AM) and their precursor cells, blood monocytes, expressed this receptor. AM expressed IGF-II/M6P receptors as detected by [125]IGF-II surface binding that was not reduced by recombinant IGF-I or IGF-I receptor monoclonal antibody (alpha IR3). Surface binding was also detected on blood monocytes and could be upregulated approximately 4-fold by incubation with lipopolysaccharide. There were no differences in surface binding by AM lavaged from individuals with asbestos exposure or from normal volunteers. Using the polymerase chain reaction and reverse transcriptase to reverse-transcribe mRNA from mononuclear phagocytes, specific IGF-II/M6P receptor cDNA was amplified and detected by agarose gel electrophoresis from both AM and blood monocytes. The IGF-II/M6P receptor has an intracellular transport role in many cells cycling from the cell surface to the cytoplasm, or binding to phosphorylated acid hydrolases in the Golgi and transporting them to an acidic prelysosomal site where they dissociate and fuse to the lysosomes and IGF-II/M6P recycles to the trans-Golgi. These functions may be particularly important in asbestosis and other interstitial lung diseases where AM are activated, intracellular lysosomes are a prominent morphologic feature, and acid hydrolases are found in recovered lavage fluid.

Regulation of insulin-like growth factor (IGF) I receptor expression during muscle cell differentiation. Potential autocrine role of IGF-II

Muscle is an important target tissue for insulin-like growth factor (IGF) action. The presence of specific, high affinity IGF receptors, as well as the expression of IGF peptides and binding proteins by muscle suggest that a significant component of IGF action in this tissue is mediated through autocrine and/or paracrine mechanisms. To explore autocrine/paracrine action of IGFs in muscle, we studied the regulation of the IGF-I receptor and the expression of IGF peptides during differentiation of the mouse BC3H-1 muscle cell line. Differentiation from myoblasts to myocytes was associated with a 60% decrease in IGF-I receptor sites determined by Scatchard analysis. Analysis of mRNA abundance and protein labeling studies indicated that the decrease in IGF-I receptor sites was associated with similar reductions in IGF-I receptor gene expression and receptor biosynthesis. IGF-II peptide gene expression was detected in myoblasts and increased 15-fold with differentiation; the increase in IGF-II gene expression preceded the decrease in IGF-I receptor gene expression. In contrast, IGF-I peptide gene expression was low in myoblasts and decreased slightly with differentiation. To explore the potential role of endogenous IGF-II in the differentiation-associated decrease in IGF-I receptor expression, we investigated the effects of IGF-II treatment in myoblasts. The addition of IGF-II to undifferentiated myoblasts resulted in downregulation of the IGF-I receptor which was associated with decreased IGF-I receptor biosynthesis and decreased IGF-I receptor mRNA abundance. These studies suggest, therefore, that IGF-I receptor expression during muscle cell differentiation may be regulated, at least in part, through autocrine production of IGF-II.

Evidence against a role for insulin-like growth factor II in the autonomous growth of rat 18,54-SF cells

The 18,54-SF rat cell line multiplies in serum-free medium and has been reported to produce insulin-like growth factor II (IGF-II) and to possess IGF-II/mannose 6-phosphate (Man-6-P) receptors, raising the possibility of autocrine growth stimulation by IGF-II acting through this receptor. When serum-free medium was changed every 24 h the 18,54-SF cells multiplied at the same rate as when the medium was not changed. An antibody (No. 3637) which blocks the binding of IGF-II to the IGF-II/Man-6-P receptor did not decrease or increase the growth rate of the 18,54-SF cells when medium was changed every 24 h. When the medium was changed every 12 h the rate of accumulation of cells in the culture was decreased. Addition of IGF-I or IGF-II at 10, 50, and 100 ng/ml every 12 h did not correct this decrease in cell number. When the medium was replaced every 12 h, cells in the periphery of the cultures gradually became nonviable as assessed by trypan blue uptake. However, the percentage of cells synthesizing DNA in the center of the cultures (approximately 50% during a 3 h pulse with tritiated thymidine) was the same whether the medium was changed every 12 h or left unchanged. Addition of IGF-I or IGF-II to the fresh medium change every 12 h did not increase the percentage of cells synthesizing DNA.

Receptor binding, endocytosis, and mitogenesis of insulin-like growth factors I and II in fetal rat brain neurons

Cell surface binding, internalization, and biological effects of insulin-like growth factors (IGFs) I and II have been studied in primary neuronal cultures from developing rat brain (embryonic day 15). Two types of IGF binding sites are present on the cell surface. The IGF-I receptor alpha-subunit (Mr 125,000) binds IGF-I with a KD of 1 nM and IGF-II with 10 times lower affinity. The mannose-6-phosphate (Man-6-P)/IGF-II receptor (Mr 250,000) binds IGF-II with a KD of 0.5 nM and IGF-I with 100 times lower affinity. Surface-bound IGF-I and IGF-II are internalized by their respective receptors binding and internalization of IGF-II but not those of IGF-I. Neuronal synthesis of RNA and DNA is increased twofold by IGF-I with 10 times higher potency than IGF-II. Antibody 3637, which blocks receptor binding of IGF-II, has no effect on the DNA response to IGF-I or IGF-II. Double immunocytochemical staining with antibodies to bromodeoxyuridine and neurofilament shows that greater than 80% of the bromodeoxyuridine-positive cells become neurofilament positive. It is concluded that IGF-I and IGF-II bind to two receptors on the surface of neuronal precursor cells that mediate endocytosis and degradation of IGF-I and IGF-II. Proliferation of neuronal precursor cells is stimulated by IGF-I and IGF-II via activation of the IGF-I receptor.