IGF receptor function and regulation in autocrine human neuroblastoma cell growth

Cathepsin Inhibition Prevents Autophagic Protein Turnover and Downregulates Insulin Growth Factor-1 Receptor-Mediated Signaling in Neuroblastoma

Inhibition of the major lysosomal proteases, cathepsins B, D, and L, impairs growth of several cell types but leads to apoptosis in neuroblastoma. The goal of this study was to examine the mechanisms by which enzyme inhibition could cause cell death. Cathepsin inhibition caused cellular accumulation of fragments of the insulin growth factor 1 (IGF-1) receptor. The fragments were located in dense organelles that were characterized as autophagosomes. This novel discovery provides the first clear link between lysosomal function, autophagy, and IGF-1- mediated cell proliferation. A more in-depth analysis of the IGF1 signaling pathway revealed that the mitogen-activated protein kinase (MAPK) cell-proliferation pathway was impaired in inhibitor treated cells, whereas the Akt cell survival pathway remained functional. Shc, an adapter protein that transmits IGF-1 signaling through the MAPK pathway, was sequestered in autophagosomes; whereas IRS-2, an adapter protein that transmits IGF-1 signaling through the Akt pathway, was unaffected by cathepsin inhibition. Furthermore, Shc was sequestered in autophagosomes as its active form, indicating that autophagy is a key mechanism for downregulating IGF-1-induced cell proliferation. Cathepsin inhibition had a greater effect on autophagic sequestration of the neuronal specific adapter protein, Shc-C, than ubiquitously expressed Shc-A, providing mechanistic support for the enhanced sensitivity of neuronally derived tumor cells. We also observed impaired activation of MAPK by epidermal growth factor treatment in inhibitor-treated cells. The Shc adapter proteins are central to transducing proliferation signaling by a range of receptor tyrosine kinases; consequently, cathepsin inhibition may become an important therapeutic approach for treating neuroblastoma and other tumors of neuronal origin.

Inhibition of hepatocellular carcinoma cell growth by an anti-insulin-like growth factor-I receptor monoclonal antibody

Hepatocellular carcinoma (HCC) overexpresses insulin-like growth factor-I receptor (IGF-IR), as compared with normal hepatocytes. Since IGF-1R-mediated signaling promotes survival, oncogenic transformation and tumor growth and spread, it represents a potential target for treating HCC. Here, we have generated a murine anti-IGF-1R antibody, 4F2, that recognizes the IGF-IRα subunit and blocks in vitro IGF-I and IGF-II-induced cell proliferation of SMMC-7721 and Bel-7402 HCC cell lines. 4F2 can inhibit IGF-IR autophosphorylation, IRS-1 phosphorylation and the activation of the major downstream signaling molecules AKT and mitogen-activated protein kinase. Additionally, we observed a moderate increase in apoptosis as demonstrated by detection of changes in the expression of the pro-apoptotic and anti-apoptotic proteins Bax and Bcl-2 after 4F2 treatment. Combined treatment with 4F2 and doxorubicin was more effective in reducing cell proliferation and promoting apoptosis than either agent alone. These data support that therapeutic anti-IGF-IR antibodies are potential new agents for treating HCC.

Patterning N-type and S-type neuroblastoma cells with Pluronic F108 and ECM proteins

Influencing cell shape using micropatterned substrates affects cell behaviors, such as proliferation and apoptosis. Cell shape may also affect these behaviors in human neuroblastoma (NBL) cancer, but to date, no substrate design has effectively patterned multiple clinically important human NBL lines. In this study, we investigated whether Pluronic F108 was an effective antiadhesive coating for human NBL cells and whether it would localize three NBL lines to adhesive regions of tissue culture plastic or collagen I on substrate patterns. The adhesion and patterning of an S-type line, SH-EP, and two N-type lines, SH-SY5Y and IMR-32, were tested. In adhesion assays, F108 deterred NBL adhesion equally as well as two antiadhesive organofunctional silanes and far better than bovine serum albumin. Patterned stripes of F108 restricted all three human NBL lines to adhesive stripes of tissue culture plastic. We then investigated four schemes of applying collagen and F108 to different regions of a substrate. Contact with collagen obliterates the ability of F108 to deter NBL adhesion, limiting how both materials can be applied to substrates to produce high fidelity NBL patterning. This patterned substrate design should facilitate investigations of the role of cell shape in NBL cell behavior.

The insulin-like growth factor system and its pleiotropic functions in brain

In recent years, much interest has been devoted to defining the role of the IGF system in the nervous system. The ubiquitous IGFs, their cell membrane receptors, and their carrier binding proteins, the IGFBPs, are expressed early in the development of the nervous system and are therefore considered to play a key role in these processes. In vitro studies have demonstrated that the IGF system promotes differentiation and proliferation and sustains survival, preventing apoptosis of neuronal and brain derived cells. Furthermore, studies of transgenic mice overexpressing components of the IGF system or mice with disruptions of the same genes have clearly shown that the IGF system plays a key role in vivo.

Inhibition of insulin-like growth factor-I receptor (IGF-IR) signaling and tumor cell growth by a fully human neutralizing anti-IGF-IR antibody

Insulin-like growth factor-I receptor (IGF-IR) plays an important role in tumor cell growth and survival. On ligand stimulation, IGF-IR, a receptor tyrosine kinase, phosphorylates tyrosine residues on two major substrates, IRS-1 and Shc, which subsequently signal through the Ras/mitogen-activated protein kinase and phosphatidylinositol 3-kinase/AKT pathways. Here, we describe the characterization of a fully human anti-IGF-IR monoclonal antibody 19D12 that inhibits IGF binding and autophosphorylation of both IGF-IR/IGF-IR homodimers and IGF-IR/insulin receptor heterodimers. 19D12 does not recognize insulin receptor homodimers. In addition to inhibiting IGF-IR autophosphorylation, 19D12 also inhibits IRS-1 phosphorylation and activation of the major downstream signaling molecules AKT and extracellular signal-regulated kinase 1/2. Furthermore, the antibody down-regulates the total IGF-IR protein level and can exhibit antibody-dependent cellular cytotoxicity activity against a non-small cell adenocarcinoma cell line in vitro in the presence of isolated human natural killer cells. 19D12 binds tightly to the receptor, with an affinity of 3.8 pmol/L as measured by KinExA. In cell culture, 19D12 inhibits proliferation and soft agar growth of various tumor cell lines. In vivo, 19D12 inhibits the tumor growth of a very aggressive human ovarian tumor xenograft model A2780. These data support the development of this anti-IGF-IR monoclonal antibody as a promising anticancer agent.

Integrin expression regulates neuroblastoma attachment and migration

Neuroblastoma (NBL) is the most common malignant disease of infancy, and children with bone metastasis have a mortality rate greater than 90%. Two major classes of proteins, integrins and growth factors, regulate the metastatic process. We have previously shown that tumorigenic NBL cells express higher levels of the type I insulin-like growth factor receptor (IGF-IR) and that beta1 integrin expression is inversely proportional to tumorigenic potential in NBL. In the current study, we analyze the effect of beta1 integrin and IGF-IR on NBL cell attachment and migration. Nontumorigenic S-cells express high levels of beta1 integrin, whereas tumorigenic N-cells express little beta1 integrin. Alterations in beta1 integrin are due to regulation at the protein level, as translation is decreased in N-type cells. Moreover, inhibition of protein synthesis shows that beta1 integrin is degraded more slowly in S-type cells (SHEP) than in N-type cells (SH-SY5Y and IMR32). Inhibition of alpha5beta1 integrin prevents SHEP (but not SH-SY5Y or IMR32) cell attachment to fibronectin and increases SHEP cell migration. Increases in IGF-IR decrease beta1 integrin expression, and enhance SHEP cell migration, potentially through increased expression of alphavbeta3. These data suggest that specific classes of integrins in concert with IGF-IR regulate NBL attachment and migration.

Imbalance of the mitochondrial pro- and anti-apoptotic mediators in neuroblastoma tumours with unfavourable biology

It has been proposed that a lack of apoptosis plays an important role in neuroblastoma (NB) progression. We therefore screened cDNA array filters, including 198 apoptotic genes, in order to identify mRNA transcripts that are differentially expressed in tumours with unfavourable versus favourable biology. Twenty-one genes were analysed further using real-time reverse-transcriptase-polymerase chain reaction (RT-PCR). Significantly lower levels of DNCL1 (PIN; P(c)(corrected) = 0.0054) and NTRK1 (TrkA; P(c) = 0.039) were found in NB tumours with unfavourable biology. In addition, BID, BCL2, APAF1, CASP2, CASP3 and CASP9 were found to be preferentially expressed in tumours with favourable biology, whereas CDKN1A (p21), IL2RA, and MCL1, were found to be preferentially expressed in NB tumours with unfavourable biology. In conclusion, mRNA levels of transcripts encoding pro-apoptotic mediators of the mitochondrial apoptotic pathway were found to be expressed to a lower extent in tumours with unfavourable biology. Our data also suggest that the mitochondrial pathway is suppressed in advanced stages of NB tumours, due to an imbalance between anti-apoptotic and pro-apoptotic mediators which is a finding that may have therapeutic significance.

Amino acid transport in a human neuroblastoma cell line is regulated by the type I insulin-like growth factor receptor

Insulin-like growth factor I (IGF-I) and IGF-II stimulate cancer cell proliferation via interaction with the type I IGF receptor (IGF-IR). We put forward the hypothesis that IGF-IR mediates cancer cell growth by regulating amino acid transport, both when sufficient nutrients are present and when key nutrients such as glutamine are in limited supply. We examined the effects of alphaIR3, the monoclonal antibody recognizing IGF-IR, on cell growth and amino acid transport across the cell membrane in a human neuroblastoma cell line, SK-N-SH. In the presence of alphaIR3 (2 micro/ml), cell proliferation was significantly attenuated in both control (2 mM glutamine) and glutamine-deprived (0 mM glutamine) groups. Glutamine deprivation resulted in significantly increased glutamate (system X(AG)(-)), MeAIB (system A), and leucine (system L) transport, which was blocked by alphaIR3. Glutamine (system ASC) and MeAIB transport was significantly decreased by alphaIR3 in the control group. Addition of alphaIR3 significantly decreased DNA and protein biosynthesis in both groups. Glutamine deprivation increased the IGF-IR protein on the cell surface. Our results suggest that activation of IGF-IR promotes neuroblastoma cell proliferation by regulating trans-membrane amino acid transport.

Regulation of glutamine transport by the type I insulin-like growth factor receptor in a human neuroblastoma cell line

We examined the effects of the monoclonal antibody against the type I insulin-like growth factor receptor (IGF-IR), alphaIR3, on cell growth and membrane glutamine (Gln) transport in a human neuroblastoma cell line, SK-N-SH. In the presence of alphaIR3 (2 microg/ml), the cell proliferation rate was significantly attenuated. Gln transport was decreased in the alphaIR3-treated group with the mechanism of decreasing maximum transport velocity without affecting transport affinity. The addition of alphaIR3 significantly decreased DNA and protein biosynthesis. Our results support the concept that the activation of IGF-IR partially mediates neuroblastoma cell proliferation by regulating membrane Gln transport.

Bcl-2 and M-Myc coexpression increases IGF-IR and features of malignant growth in neuroblastoma cell lines

The bcl-2 and c-myc oncogenes cooperate to transform multiple cell types. In the pediatric malignancy NB(2), Bcl-2 is highly expressed. In tumors with a poor prognosis, N-Myc, a protein homologous to c-Myc, is overexpressed as a result of gene amplification. The present study was designed to determine whether Bcl-2 cooperates with N-Myc to bestow a tumorigenic phenotype to neuroblastoma (NB) cells. NB cell lines that at baseline express neither Bcl-2 nor N-Myc were stably transfected to express these gene products. In this model, we found Bcl-2 rescues N-Myc-expressing cells from apoptosis induced by serum withdrawal. Coexpression of Bcl-2 and N-Myc supports growth in low serum conditions and anchorage-independent growth in soft agar. Similarly, in vivo tumorigenic and angiogenic activity was dependent on coexpression. Our data further suggests that the mechanism underlying these changes involves the receptor for insulin growth factor type I (IGF-IR).

Insulin-like growth factor-I stimulates amino acid transport in a glutamine-deprived human neuroblastoma cell line

It is still unknown how insulin-like growth factor-I (IGF-I) regulates cancer cell growth in the condition of the limited availability of key nutrients, such as glutamine. We investigated the effects of IGF-I on cell growth and amino acid transport in a glutamine-deprived human neuroblastoma cell line, SK-N-SH. Cell growth was measured, and 3H-labeled amino acid transport was assayed after treatment with or without IGF-I (50 ng/ml) in 2 mM (control) and 100 microM glutamine concentrations. Cell growth rates were dependent on glutamine concentrations. IGF-I stimulated cell growth in both 2 mM and 100 microM glutamine. IGF-I stimulated glutamine transport in 100 microM glutamine with the mechanism of increasing carrier Vmax, but had no effect in 2 mM glutamine. IGF-I also stimulated leucine, glutamate and 2-(methylamino)isobutyric acid transport in 100 microM glutamine. There were significant increases in [3H]thymidine and [3H]leucine incorporation in IGF-I-treated cells in both 2 mM and 100 microM glutamine. These data suggest that IGF-I stimulates cell growth by increasing amino acid transport in the condition of low glutamine levels in a human neuroblastoma cell line. This mechanism may allow to maintain cell growth even in nutrient-deprived tumor tissues.

Multi-hormonal regulation of IGFBP-6 expression in human neuroblastoma cells

Previous work has shown that neuroblastoma cells secrete IGFBP-2, -4 and -6 and that expression of these proteins is regulated by retinoic acid (at-RA) which promotes differentiation in these cells. Other agents also induce differentiation of neuroblastoma cells: these include the 9- cis and 13- cis isomers of at-RA, 1,25 dihydroxy- vitamin D3 (VD3), triidothyronine (T3) and 12-O-tetradecanoyl phorbol 13-acetate (TPA). Nine- cis and 13- cis isomers of at-RA increased IGFBP-6 expression, but decreased IGFBP-2 and IGFBP-4. VD3 stimulated IGFBP-6 and IGFBP-2 expression, whereas T3 inhibited IGFBP-6 expression without affecting IGFBP-2. TPA markedly enhanced expression of all three IGFBPs produced by SK-N-SH cells. Since IGFBP-6 secretion is associated with the arrest of proliferation in neuroblastoma cells and is regulated by the combined actions of differentiation factors, we subcloned the proximal promoter of human IGFBP-6 (nt -766/+1) into a pCAT expression vector so as to examine modulation of its transcriptional activity. VD3 and TPA were capable of stimulating promoter activity, T3 depressed it and at-RA and its 9- cis and 13- cis isomers had no effect. These results confirm the high sensitivity of IGFBP-6 expression to these differentiation agents, essentially at transcriptional level.

Insulin-like growth factor I is the key growth factor in serum that protects neuroblastoma cells from hyperosmotic-induced apoptosis

Neuroblastoma is a childhood tumor of the peripheral nervous system that remains largely uncurable by conventional methods. Mannitol induces apoptosis in neuroblastoma cell types and insulin-like growth factor I (IGF-I) protects these cells from hyperosmotic-induced apoptosis by affecting apoptosis-regulatory proteins. In the current study, we investigate factors that enable SH-SY5Y neuroblastoma cells to survive in the presence of an apoptotic stimulus. When SH-SY5Y cells are exposed to high mannitol concentrations, more than 60% of the cells are apoptotic within 48 h. Normal CS prevents hyperosmotic-induced apoptosis in a dose-dependent manner, with 0.6% CS protecting 50% of the cells, and 3% CS rescuing more than 70% of the cells from apoptosis. Serum also delays the commitment point for SH-SY5Y cells from 9 h to 35 h. A survey of several growth factors, including epidermal growth factor (EGF), platelet-derived growth factor (PDGF), nerve growth factor (NGF), fibroblast growth factor (FGF), and IGF-I reveals that IGF-I is a component of serum necessary for protection of neuroblastoma cells from death. Mitochondrial membrane depolarization occurs in greater than 40% of the cells after mannitol exposure and caspase-3 activation is increased in high mannitol conditions after 9 h. IGF-I blocks both the mitochondrial membrane depolarization and caspase-3 activation normally induced by hyperosmotic treatment in neuroblastoma cells. Our results suggest that (1) IGF-I is a key factor in serum necessary for protection from death and (2) IGF-I acts upstream from the mitochondria and the caspases to prevent apoptosis in human neuroblastoma.

Insulin-like-growth-factor-I (IGF-I) antagonizes apoptosis induced by serum deficiency and doxorubicin in neuronal cell culture

We evaluated the effect of insulin-like growth factor (IGF)-I on neuronal cell viability and apoptosis induced by exposure to serum-free (SF) medium and to doxorubicin. In primary neuronal culture, IGF-I (0.5-2.0 microg/ml) slightly increased basal cell viability; SF medium tended to decrease viability (20-27%), and addition of IGF-I significantly antagonized this decrease (P< 0.05). In neuroblastoma (NB) SK-N-SH cell culture, IGF-I significantly increased viability (0.05-1.25 microg/ml) (P< 0.005); SF medium decreased it by 75%, and this decrease was prevented by IGF-I (0.5-1. 0 microg/ml) (P< 0.005). Flow cytometry studies showed an increased apoptosis on exposure to SF medium (88.8 vs 10.2%), which was suppressed to 38.3% by addition of IGF-I. Growth hormone (1-10 microU/ml) did not modify basal cell viability in either culture, and SF-induced cell death in NB cells. Doxorubicin (1-100 microM) caused neurotoxicity in primary and NB cultures (66-39% and 39-10% of controls, respectively), and increased apoptosis in NB cells (73. 8 vs 20.1%). IGF-I antagonized these neurotoxic/apoptotic effects (P< 0.05). This study suggests that IGF-I possesses a potent neuroprotective activity which may be involved in the resistance to doxorubicin.

Elevated serum levels of IGFBP-2 found in children suffering from acute leukaemia is accompanied by the occurrence of IGFBP-2 mRNA in the tumour clone

Insulin-like growth factor-binding proteins (IGFBPs) are important modulators of IGF action. In 50 children suffering from acute lymphoblastic leukaemia (ALL), we studied the serum levels of IGFBP-1,-2 and-3. The mean standard deviation score (SDS) values were estimated to be 0.7, 3.1 and -1.7 for the IGFBP-1,-2 and-3, respectively, compared with the normal range defined by a SDS from -2 to +2. IGFBP-1 and-3 were normal, but for IGFBP-2 we found a significantly elevated serum level compared with control groups (P < 0.05). However, during chemotherapy this increased serum IGFBP-2 normalized. In addition, we found a correlation between higher serum levels and the detection rate of the IGFBP-2 transcript in corresponding cells. In patients with ALL, the detection rates of IGFBP-2 mRNA were estimated to be 72% and 35% at the time of diagnosis and at day 33 of chemotherapy respectively; in the control groups (healthy children and children at their initial presentation of diabetes mellitus), the values were 28% and 33% respectively. Based on the correlation between IGFBP-2 serum levels and the corresponding gene expression as well as the normalization of IGFBP-2 levels during chemotherapy, we concluded that the increased serum level mainly originated from the tumour clone itself. Furthermore, possible functional consequences of elevated IGFBP-2 were outlined.

Bidirectional regulation of p38 kinase and c-Jun N-terminal protein kinase by insulin-like growth factor-I

We have previously shown that insulin-like growth factor I (IGF-I) activation of the IGF-I receptor rescues SH-SY5Y human neuroblastoma cells from high glucose-mediated programmed cell death (PCD). In the current study, we further explored the potential points in the cell death cascade where IGF-I receptor activation may afford neuroprotection. As an initial step, we examined the effects of the PCD stimulus, high glucose, on stress-activated protein kinases, specifically the two mitogen-activated protein kinases p38 kinase and c-Jun N-terminal kinase (JNK). High glucose treatment activated the tyrosine phosphorylation of both p38 kinase and JNK in a dose- and time-dependent fashion. We next examined the effects of IGF-I on JNK and p38 kinase under normoglycemic and hyperglycemic conditions. IGF-I activated p38 kinase alone and had additive effects on glucose-induced p38 kinase phosphorylation. In contrast, IGF-I inhibited glucose activation of JNK phosphorylation and JNK activity. IGF-I also inhibited the glucose-induced nuclear translocation of JNK, but did not effect glucose-induced translocation of p38 kinase. Finally, IGF-I inhibition of JNK phosphorylation was blocked by the mitogen-activated protein kinase/extracellular signal-regulated kinase inhibitor, PD98059. Collectively, these data imply cross-talk between the mitogen-activated protein kinase pathway and JNK and suggest that IGF-I activation of mitogen-activated protein kinases interferes with JNK activation and protects cells from PCD.

Insulin-like growth factors regulate neuronal differentiation and survival

Insulin-like growth factor I (IGF-I) and IGF-II are potent trophic factors for motor and sensory neurons and glial cells. The actions of IGF-I and IGF-II are mediated via the IGF-I receptor (IGF-IR). IGF:IGF-IR binding activates distinct signaling cascades, which in turn mediate the trophic effects of the IGFs. We discuss three main IGF coupled events: growth cone motility, long-term neurite outgrowth, and neuroprotection. Our data suggest that IGF-I enhances growth cone motility by promoting reorganization of actin and activation of focal adhesion proteins via the phosphatidylinositol-3 kinase (Pl-3K) pathway. Long-term treatment with IGF-I activates the mitogen-activated protein (MAP) kinase cascade and promotes neurite outgrowth. A separable, but likely linked, action of the IGFs via Pl-3K is protection of neurons from apoptosis. These pleotrophic effects of IGFs suggest that this family of growth factors may have potential clinical utility in the treatment of neurological disorders.

Human neuroblastoma cells use either insulin-like growth factor-I or insulin-like growth factor-II in an autocrine pathway via the IGF-I receptor: variability of IGF, IGF binding protein (IGFBP) and IGF receptor gene expression and IGF and IGFBP secretion in human neuroblastoma cells in relation to cellular proliferation

Neuroblastoma cells are thought to depend upon autocrine stimulation by IGF-II but not by IGF-I. We have studied the expression of IGF, IGFBP and IGF receptor mRNA in two human neuroblastoma cell lines, SK-N-MC and CHP, and asked whether or not the expression of the IGF system in these malignant cells determines their growth pattern. SK-N-MC cells grow with a cell doubling time of 36 hours in medium supplemented with 10% fetal calf serum whereas CHP cells only grow with a doubling time of 72 h. In addition, the SK-N-MC cell line has a plating efficiency ten times greater than the CHP cell line. RNase protection assays were performed using (32)P-labelled riboprobes and RNA that had been purified from SK-N-MC and CHP cells respectively. A 520 bases human IGF-I, a 556 bases human IGF-II, a 480 bases human IGF-I receptor and a 250 human IGF-II/mannose-6-phosphate (M6P) receptor probe were radiolabelled as were human IGFBP-1, -2, -3, -4, -5 and -6 probes. While both SKNMC and CHP neuroblastoma cells expressed mRNAs for IGFBP-2, -4, and -6 no signal was detected for IGFBP-1, and -3 and only SK-N-MC cells expressed IGFBP-5 mRNA. In addition, a 400 bases protected band was seen with the IGF-I receptor probe and a 260 bases protected band with the IGF-IIM6P receptor probe in either cell line. Interestingly, a 300 bases protected species was detected with the IGF-II probe in CHP cell RNA whereas SK-N-MC cells did not express IGF-II transcripts. Conversely, SK-N-MC cells expressed a 520 bases IGF-I transcript while CHP cells did not show IGF-I mRNA expression. As determined by specific radioimmunoassays SK-N-MC cells secreted 0.75+/-0.02 ng/ml IGF-I, 1.2+/-0.04 ng/ml IGF-II and 149+/-2.1 ng/ml IGFBP-2 within 24 h, whereas CHP cells secreted 0.1+/-0.01 ng/ml IGF-I, but 6.2+/-0.1ng/ml IGF-II and 254.8+/-5.5 ng/ml IGFBP-2 (N=5). IGFBP-2 secretion correlated positively with IGF-II secretion in CHP cells (r=0.85, P=0.05) and negatively with IGF-I (r= -0.9, P<0.01) in SK-N-MC cells. In conclusion, SK-N-MC cells which grow rapidly and have a high plating efficiency, express IGF-I, while CHP cells that grow more slowly express IGF-II. We hypothesize that neuroblastoma cells depend upon autocrine stimulation by either IGF-I or IGF-II. Variable sensitivity to growth inhibitors or apoptotic processes may be related to the differential expression of the IGF system.

Tyrosine phosphorylation of paxillin and focal adhesion kinase during insulin-like growth factor-I-stimulated lamellipodial advance

In the current studies, we examined whether focal adhesion kinase (FAK) and paxillin play a role in insulin-like growth factor-I (IGF-I)-stimulated morphological changes in neuronal cells. In SH-SY5Y human neuroblastoma cells, 10 nM IGF-I enhanced the extension of lamellipodia within 30 min. Scanning electron microscopy and staining with rhodamine-phalloidin showed that these lamellipodia displayed ruffles, filopodia, and a distinct meshwork of actin filaments. Immunofluorescent staining identified focal concentrations of FAK, paxillin, and phosphotyrosine within the lamellipodia. Immunoprecipitation experiments revealed that FAK and paxillin are tyrosine-phosphorylated during IGF-I-stimulated lamellipodial extension. Maximal phosphorylation of FAK and paxillin was observed 15-30 min after the addition of 10 nM IGF-I, whereas maximal IGF-I receptor phosphorylation occurred within 5 min. FAK, paxillin, and IGF-I receptor tyrosine phosphorylation had similar concentration-response curves and were inhibited by the receptor blocking antibody alphaIR-3. These results indicate that FAK and paxillin are tyrosine-phosphorylated during IGF-I-stimulated lamellipodial advance and suggest that the tyrosine phosphorylation of these two proteins helps mediate IGF-I-stimulated cell and growth cone motility. These responses contrast directly with recent reports showing insulin-stimulated dephosphorylation of FAK and paxillin.

Suramin disrupts insulin-like growth factor-II (IGF-II) mediated autocrine growth in human SH-SY5Y neuroblastoma cells

Suramin, traditionally used in the treatment of trypanosomiasis, is under investigation in the treatment of cancer. One side effect that limits its use is the onset of a sensorimotor polyneuropathy. In order to investigate the mechanism by which suramin induces polyneuropathy, we examined its effects on SH-SY5Y human neuroblastoma cells, an in vitro model of neuronal growth and differentiation. Addition of 50-400 micrograms/ml suramin to SH-SY5Y cells grown in 0.6% CS inhibited [3H]thymidine ([3H]TdR) incorporation and cell growth. Upon removal of suramin, [3H]TdR incorporation increased, demonstrating that levels of suramin used were cytostatic and not cytotoxic. Analysis of suramin-treated SH-SY5Y cells by flow cytometry revealed growth arrest in the G1/G0 phase of the cell cycle. IGF-II-induced SH-SY5Y growth is mediated by the type I IGF receptor (IGF-IR). Therefore, we examined its effect on IGF-IR tyrosine phosphorylation. Suramin prevented IGF-II-stimulated IGF-IR tyrosine phosphorylation. These results indicate that in SH-SY5Y cells, suramin acts as a cytostatic agent and can block IGF-II-dependent cell growth by preventing IGF-IR activation. Thus, suramin toxicity in the peripheral nervous system may be due, in part, to preventing IGF and other growth factors from activating their receptors.

Type I insulin-like growth factor receptor activation regulates apoptotic proteins

Activation of the type I insulin-like growth factor receptor (IGF-IR) blocks osmotic mediated programmed cell death (PCD) in neurons. We speculated that IGF-IR activation could afford neuroprotection either by effecting the negative regulators of the death pathway, Bcl-2 and Bcl-xL, or by altering activity of the ced-3/ICE-like proteases. Here we report that osmotic stress decreases total neuronal Bcl-2 by 4-fold and that hyperosmotic PCD correlates with proteolytic processing of neuronal ced-3/ICE-like proteases. IGF-IR activation maintains normal Bcl-2 levels, and signaling via the IGF-IR:phosphatidylinositol 3-kinase pathway prevents ICE/LAP-3 and Yama/CPP32 processing. Finally, increased neuronal IGF-IR expression enhances the negative death regulator Bcl-xL. We suggest that IGF-IR signaling exerts its short-term inhibitory effects upon PCD "upstream" of both Bcl proteins and ced-3/ICE-like proteases, while chronic increased IGF-IR expression may modulate susceptibility to death signals by mediating the negative death regulator, Bcl-xL.

A somatostatin analogue inhibits MAP kinase activation and cell proliferation in human neuroblastoma and in human small cell lung carcinoma cell lines

Somatostatin possesses antisecretory and antiproliferative activity on some human tumors. We herein report that, in a human neuroblastoma cell line, the somatostatin analogue BIM 23014 inhibited mitogen-activated protein (MAP) kinase activity stimulated by either insulin-like growth factor-1, whose receptor bears a tyrosine kinase, or carbachol, which acts at a G-protein coupled receptor. In a human small cell lung carcinoma line BIM inhibited serum-stimulated MAP kinase activation. These inhibitory actions occur in a dose range quite similar to that observed for suppression of proliferation induced by the analogue in the same cell lines. The decrease in cAMP elicited by the analogue in the two cell lines is not responsible for its inhibitory action on MAP kinase and cell growth. Moreover, the analogue did not modify intracellular [Ca2+] and pH. An involvement of a phosphatase activity is suggested.

Role of insulin-like growth factors in autocrine growth of human retinoblastoma Y79 cells

In this study, we have demonstrated that human retinoblastoma Y79 cells produce insulin-like growth factors (IGFs) type I and type II and release them into the medium. We have also ascertained, by means of competitive studies and cross-linking procedure, that Y79 cells contain the type-I IGF receptor (IGF-IR). Furthermore, surface-bound IGF-I is internalised by the receptor, then degraded to amino acids. Insulin, IGF-I and IGF-II caused down-regulation of IGF-IR; the effect is concentration and time dependent. Scatchard analysis demonstrated that incubation with insulin markedly decreased the binding capacity measured for IGF-I while the apparent Kd value calculated for IGF-I binding was not significantly modified. IGF-I, IGF-II and insulin induced tyrosine phosphorylation of IGF-IR. Tyrosine phosphorylation of this receptor with, however, a less strong signal, was detectable even in cells cultured in serum-free medium without the addition of any exogenous growth factor. Similar results have been found concerning the tyrosine phosphorylation of insulin receptor substrate-1 (IRS 1). Tyrosine phosphorylation of both IGF-IR and IRS 1, either under basal conditions or after stimulation with growth factors, was strongly inhibited when alpha-IR3, a monoclonal antibody to IGF-IR, was added to the culture. IGF-I was capable of inducing Y79 cell proliferation and its effect was entirely inhibited by the addition of alpha-IR3. This antibody also markedly reduced the proliferation of Y79 cells cultured in serum-free medium not supplemented with stimulatory factors. Our results indicate that IGF-I and IGF-IR mediate an autocrine growth mechanism in Y79 cells.

Insulin-like growth factor I rescues SH-SY5Y human neuroblastoma cells from hyperosmotic induced programmed cell death

Insulin-like growth factor I (IGF-I) and the type I IGF receptor are widely distributed in developing and adult mammalian nervous systems. In vitro, IGF-I is a mitogen for primary neurons and also for cells from the SH-SY5Y human neuroblastoma cell line, a well-characterized model system of neuronal growth. In the current study, we examined the effects of osmotic stress on SH-SY5Y cell viability and the mechanism by which IGF-I serves as a neuronal osmoprotectant. Within 24 hr, exposure of SH-SY5Y cells to hyperosmotic serum-free media decreased (1) the number of viable cells, (2) the rate of 3H-thymidine incorporation, and (3) cell cycle progression. The inclusion of 10 nM IGF-I with hyperosmotic media prevented the loss of cell viability. The osmoprotective effects of IGF-I were inhibited by alpha-IR3, a blocking antibody of the type I IGF receptor. The observed loss of SH-SY5Y cell viability following hyperosmotic shock was due to an induction of programmed cell death as determined by flow cytometry and gel electrophoresis. Our results suggest that IGF-I can protect SH-SY5Y cells from hyperosmotic induced programmed cell death.

Effects of serum and insulin-like growth factors on human neuroblastoma cell growth

Insulin-like growth factors (IGF-I and IGF-II) are mitogenic polypeptides expressed in both developing and adult tissues. To examine the effects of IGFs on neuronal growth, we used SH-SY5Y neuroblastoma cells as an in vitro model of nervous system development. In the current study, we found that either IGF-I (0.1 to 10 nM), insulin (0.1 to 5 micrograms/ml) or calf serum (0.1 to 3%) increased SH-SY5Y proliferation over a 3 day period in a dose dependent manner. In each case, treatment with anti-IGF-I receptor antibodies blocked cell proliferation. IGF-II mRNA levels correlated with SH-SY5Y cell density; subconfluent cells expressed high levels of IGF-II mRNA while low levels of IGF-II mRNA were present in confluent cells. Similarly, serum deprivation increased IGF-I receptor mRNA by 4-fold. Collectively, these results support the concept that an IGF/IGF-I receptor system at least partially mediates SH-SY5Y cell proliferation and suggests the importance of IGFs in regulating neuronal growth.