Insulin-like growth factor receptor 1b is required for zebrafish primordial germ cell migration and survival

Insulin-like growth factor (IGF) signaling is a critical regulator of somatic growth during fetal and adult development, primarily through its stimulatory effects on cell proliferation and survival. IGF signaling is also required for development of the reproductive system, although its precise role in this regard remains unclear. We have hypothesized that IGF signaling is required for embryonic germline development, which requires the specification and proliferation of primordial germ cells (PGCs) in an extragonadal location, followed by directed migration to the genital ridges. We tested this hypothesis using loss-of-function studies in the zebrafish embryo, which possesses two functional copies of the Type-1 IGF receptor gene (igf1ra, igf1rb). Knockdown of IGF1Rb by morpholino oligonucleotides (MO) results in mismigration and elimination of primordial germ cells (PGCs), resulting in fewer PGCs colonizing the genital ridges. In contrast, knockdown of IGF1Ra has no effect on PGC migration or number despite inducing widespread somatic cell apoptosis. Ablation of both receptors, using combined MO injections or overexpression of a dominant-negative IGF1R, yields embryos with a PGC-deficient phenotype similar to IGF1Rb knockdown. TUNEL analyses revealed that mismigrated PGCs in IGF1Rb-deficient embryos are eliminated by apoptosis; overexpression of an antiapoptotic gene (Bcl2l) rescues ectopic PGCs from apoptosis but fails to rescue migration defects. Lastly, we show that suppression of IGF signaling leads to quantitative changes in the expression of genes encoding CXCL-family chemokine ligands and receptors involved in PGC migration. Collectively, these data suggest a novel role for IGF signaling in early germline development, potentially via cross-talk with chemokine signaling pathways.     

Crosstalk between the p190-B RhoGAP and IGF signaling pathways is required for embryonic mammary bud development

P190-B RhoGAP (p190-B, also known as ARHGAP5) has been shown to play an essential role in invasion of the terminal end buds (TEBs) into the surrounding fat pad during mammary gland ductal morphogenesis. Here we report that embryos with a homozygous p190-B gene deletion exhibit major defects in embryonic mammary bud development. Overall, p190-B-deficient buds were smaller in size, contained fewer cells, and displayed characteristics of impaired mesenchymal proliferation and differentiation. Consistent with the reported effects of p190-B deletion on IGF-1R signaling, IGF-1R-deficient embryos also displayed a similar small mammary bud phenotype. However, unlike the p190-B-deficient embryos, the IGF-1R-deficient embryos exhibited decreased epithelial proliferation and did not display mesenchymal defects. Because both IGF and p190-B signaling affect IRS-1/2, we examined IRS-1/2 double knockout embryonic mammary buds. These embryos displayed major defects similar to the p190-B-deficient embryos including smaller bud size. Importantly, like the p190-B-deficient buds, proliferation of the IRS-1/2-deficient mesenchyme was impaired. These results indicate that IGF signaling through p190-B and IRS proteins is critical for mammary bud formation and ensuing epithelial-mesenchymal interactions necessary to sustain mammary bud morphogenesis.     

Mannose 6-phosphate/insulin-like growth factor II receptor mediates the growth-inhibitory effects of retinoids.

Both retinoids and the mannose 6-phosphate/insulin-like growth factor-II receptor (M6P/IGF2R) have been shown to play an important role in controlling cell growth during embryonic development and oncogenesis. Our recent work (Kang et al., Proc. Natl. Acad. Sci. USA, 94: 13671-13676, 1997; Kang et al., Proc. Natl. Acad. Sci. USA, 95: 13687-13691, 1998) revealed a direct biochemical interaction between retinoic acid (RA) and the M6P/IGF2R, thereby leading us to hypothesize that the M6P/IGF2R may mediate a growth-inhibiting effect of RA. To test this hypothesis, cell growth and apoptosis in response to RA and various receptor-selective retinoids were examined in cells that lack or overexpress the M6P/IGF2R. RA and those retinoids capable of binding to the M6P/IGF2R induced a remarkable morphological change with characteristics of round shape and reduced spreading, apoptosis, and growth inhibition in stably transfected mouse P388D1 cells overexpressing the M6P/IGF2R but not in the M6P/IGF2R-deficient P388D1 cells. These effects of RA were neither blocked by a potent RA nuclear receptor (RAR) antagonist (AGN193109), nor mimicked by a selective RAR agonist (TTNPB), suggesting that the observed effects of RA are independent of RARs. Similar effects of the retinoids were observed in cultured neonatal rat cardiac myocytes that have high levels of the M6P/IGF2R. Furthermore, overexpression of the M6P/IGF2R in a RA-resistant cancer cell line (HL-60R) that lacked functional RARs gave the cells a susceptibility to RA-induced apoptosis. These data suggest that the M6P/ IGF2R may play an important role in mediating retinoid-induced apoptosis/growth-inhibition and provide insight into the similar biological effects of RA and the M6P/IGF2R on fetal development and carcinogenesis.     

Role of insulin-like growth factors and their binding proteins in growth control and carcinogenesis

Interest in the role of the insulin-like growth factor (IGF) axis in growth control and carcinogenesis has recently been increased by the finding of elevated serum insulin-like growth factor I (IGF-I) levels in association with three of the most prevalent cancers in the United States: prostate cancer, colorectal cancer, and lung cancer. IGFs serve as endocrine, autocrine, and paracrine stimulators of mitogenesis, survival, and cellular transformation. These actions are mediated through the type 1 IGF-receptor (IGF-1R), a tyrosine kinase that resembles the insulin receptor. The availability of free IGF for interaction with the IGF-1R is modulated by the insulin-like growth factor-binding proteins (IGFBPs). IGFBPs, especially IGFBP-3, also have IGF-independent effects on cell growth. IGF-independent growth inhibition by IGFBP-3 is believed to occur through IGFBP-3-specific cell surface association proteins or receptors and involves nuclear translocation. IGFBP-3-mediated apoptosis is controlled by numerous cell cycle regulators in both normal and disease processes. IGFBP activity is also regulated by IGFBP proteases, which affect the relative affinities of IGFBPs, IGFs and IGF-1R. Perturbations in each level of the IGF axis have been implicated in cancer formation and progression in various cell types.     

Insulin and insulin-like growth factor I (IGF I) in early mouse embryogenesis

Growth factors have an important role in the regulation of cell growth, division and differentiation. They are also involved in the regulation of embryonic growth and differentiation. Insulin and insulin-like growth factor I (IGF I) play an important part in these events in the later stages of embryogenesis, when organogenesis is completed. In this study, we are presenting evidence that insulin and IGF I are also secreted by embryonic tissues during the prepancreatic stage of mouse development. We found measurable amounts of insulin and IGF I in 8- to 12-day-old mouse embryos. We also showed that embryonic cells derived from 8-, 9- and 10-day-old mouse embryos secrete insulin, IGF I and/or related molecules. Furthermore, the same growth factors, when added to the culture of 9-day-old mouse embryonic cells, stimulate their proliferation. These results lead to the conclusion that insulin can stimulate the growth of embryonic cells during the period when pancreas is not yet formed, which is indirect evidence for a paracrine (or autocrine) type of action.     

Preimplantation exposure of mouse embryos to palmitic acid results in fetal growth restriction followed by catch-up growth in the offspring

Free fatty acids (FFAs) are energy substrates for many cell types, but in excess, some FFAs can accumulate in nonadipose cells, inducing apoptosis. Also known as lipotoxicity, this phenomenon may play a role in the development of obesity-related disease. Obesity is common among reproductive age women and is associated with adverse pregnancy and fetal outcomes; however, little is known about the effects of excess FFAs on embryos and subsequent fetal development. To address this knowledge gap, murine blastocysts were cultured in excess palmitic acid (PA), the most abundant saturated FFA in human serum, and ovarian follicular fluid. Targets susceptible to aberrations in maternal physiology, including embryonic IGF1 receptor (IGF1R) expression, glutamic pyruvate transaminase (GPT2) activity, and nuclei count, were measured. PA-exposed blastocysts demonstrated altered IGF1R expression, increased GPT2 activity, and decreased nuclei count. Trophoblast stem cells derived from preimplantation embryos were also cultured in PA. Cells exposed to increasing doses of PA demonstrated increased apoptosis and decreased proliferation. To demonstrate long-term effects of brief PA exposure, blastocysts cultured for 30 h in PA were transferred into foster mice, and pregnancies followed through Embryonic Day (ED)14.5 or delivery. Fetuses resulting from PA-exposed blastocysts were smaller than controls at ED14.5. Delivered pups were also smaller but demonstrated catch-up growth and ultimately surpassed control pups in weight. Altogether, our data suggest brief PA exposure results in altered embryonic metabolism and growth, with lasting adverse effects on offspring, providing further insight into the pathophysiology of maternal obesity.     

Hypoxia impairs primordial germ cell migration in zebrafish (Danio rerio) embryos

Background

As a global environmental concern, hypoxia is known to be associated with many biological and physiological impairments in aquatic ecosystems. Previous studies have mainly focused on the effect of hypoxia in adult animals. However, the effect of hypoxia and the underlying mechanism of how hypoxia affects embryonic development of aquatic animals remain unclear.

Methodology/Principal Findings

In the current study, the effect of hypoxia on primordial germ cell (PGC) migration in zebrafish embryos was investigated. Hypoxic embryos showed PGC migration defect as indicated by the presence of mis-migrated ectopic PGCs. Insulin-like growth factor (IGF) signaling is required for embryonic germ line development. Using real-time PCR, we found that the mRNA expression levels of insulin-like growth factor binding protein (IGFBP-1), an inhibitor of IGF bioactivity, were significantly increased in hypoxic embryos. Morpholino knockdown of IGFBP-1 rescued the PGC migration defect phenotype in hypoxic embryos, suggesting the role of IGFBP-1 in inducing PGC mis-migration.

Conclusions/Significance

This study provides novel evidence that hypoxia disrupts PGC migration during embryonic development in fish. IGF signaling is shown to be one of the possible mechanisms for the causal link between hypoxia and PGC migration. We propose that hypoxia causes PGC migration defect by inhibiting IGF signaling through the induction of IGFBP-1.     

Induction of insulin-like growth factor 2 expression in a mesenchymal cell line co-cultured with an ameloblast cell line

Various growth factors have been implicated in the regulation of cell proliferation and differentiation during tooth development. It has been unclear if insulin-like growth factors (IGFs) participate in the epithelium–mesenchyme interactions of tooth development. We previously produced three-dimensional sandwich co-culture systems (SW) containing a collagen membrane that induce the differentiation of epithelial cells. In the present study, we used the SW system to analyze the expression of IGFs and IGFRs. We demonstrate that IGF2 expression in mesenchymal cells was increased by SW. IGF1R transduces a signal; however, IGF2R does not transduce a signal. Recombinant IGF2 induces IGF1R and IGF2R expression in epithelial cells. IGF1R expression is increased by SW; however, IGF2R expression did not increase by SW. Thus, IGF2 signaling works effectively in SW. These results suggest that IGF signaling acts through the collagen membrane on the interaction between the epithelium and mesenchyme. In SW, other cytokines may be suppressed to induce IGF2R induction. Our results suggest that IGF2 may play a role in tooth differentiation.     

Divergence of epidermal growth factor - transforming growth factor beta signaling in embryonic orofacial tissue

The epidermal growth factor (EGF) and transforming growth factor beta (TGFbeta) families of signaling molecules play a major role in growth and development of embryos. Abrogation of either signaling pathway results in defects in embryogenesis, including cleft palate. In the developing palate, both EGF and TGFbeta regulate cellular proliferation, extracellular matrix synthesis, and cellular differentiation but often in an opposing manner. Evidence from various adult cell types suggests the existence of cross talk between the EGF and TGFbeta signaling pathways, although it is unclear whether such cross talk exists in murine embryonic maxillary mesenchymal cells, from which the developing palate is derived. In this study, embryonic maxillary mesenchymal cells in culture were treated with EGF and TGFbeta, either singly or in combination, and the cells were subsequently examined for signaling interactions between these two pathways. Immunoblot analyses of nuclear extracts of embryonic maxillary mesenchymal cells revealed that TGFbeta-induced nuclear translocation of Smad 2 and Smad 3 proteins was not affected by EGF. Conversely, immunoblot analyses of whole-cell extracts of these cells indicated that EGF-induced phosphorylation of extracellular signal-regulated kinase proteins, ERK1 and ERK2, was not affected by TGFbeta. Expression of a transfected luciferase reporter gene driven by a promoter with Smad binding elements was induced by TGFbeta in these cells but was not affected by EGF. Last, TGFbeta was found to induce expression of the endogenous gelatinase B gene in embryonic maxillary mesenchymal cells; however, this effect was independent of any interaction of EGF. Collectively, data from this study suggest that the EGF and TGFbeta signal transduction pathways do not converge in murine embryonic maxillary mesenchymal cells.     

IGF2 actions on trophoblast in human placenta are regulated by the insulin-like growth factor 2 receptor, which can function as both a signaling and clearance receptor

Insulin-like growth factor 2 (IGF2) enhances proliferation and survival of human first-trimester cytotrophoblasts (CTB) by signaling through the insulin-like growth factor 1 receptor (IGF1R). However, the role of the IGF2 receptor (IGF2R) in regulating trophoblast kinetics is unclear: It could act as a clearance receptor for trafficking excess ligand to lysosomes for degradation and/or directly mediate IGF2 signaling. We used an IGF2R knockdown strategy in BeWo cells and placental villous explants to investigate trophoblast proliferation and survival in response to stimulation by IGF. Both IGF1 and IGF2 significantly (P < 0.001) increased mitosis and reduced apoptosis in serum-starved BeWo cells. Small interfering RNA (siRNA)-mediated knockdown of IGF2R further enhanced IGF2-stimulated mitosis (P < 0.01), and IGF2-mediated rescue of apoptosis (P < 0.001) in these cells. Leu(27)IGF2, an IGF2 analogue that binds to IGF2R but not IGF1R, also protected IGF2R-expressing BeWo cells from apoptosis but did not increase mitosis. IGF treatment of term placental villous explants with reduced syncytial expression of IGF2R increased CTB proliferation (P < 0.001) and decreased apoptosis (P < 0.01) compared to untreated controls. Moreover, IGF2-mediated rescue of CTB apoptosis was significantly greater than that in tissue with normal IGF2R expression. Leu(27)IGF2 promoted mitogenesis and survival only in explants with intact IGF2R expression. Given that altered CTB turnover is observed in pregnancies complicated by fetal growth restriction, the development of strategies to manipulate the IGF2R signaling axis in the syncytiotrophoblast may provide a therapeutic avenue for treating this condition.     

Role of IGF signaling in catch-up growth and accelerated temporal development in zebrafish embryos in response to oxygen availability

Animals respond to adverse environments by slowing down or arresting growth and development. Upon returning to normal conditions, they often show compensatory acceleration in growth and developmental rate. This phenomenon, known as `catch-up' growth, is widely documented in the animal kingdom. The underlying molecular mechanisms, however, are poorly understood. Using the zebrafish embryo as an experimental model system, we tested the hypothesis that changes in IGF signaling activities play an important role in the accelerated growth and temporal development resulting from re-oxygenation following hypoxia. We show that chronic hypoxia reduced, and re-oxygenation accelerated, embryonic growth and developmental rate. Whereas hypoxia repressed the Igf1 receptor and its downstream Erk1/2 and Akt signaling activities, re-oxygenation restored their activities. Specific inhibition of Igf1 receptor signaling during re-oxygenation by genetic and pharmacological approaches attenuated catch-up growth. Further analysis showed that whereas PI3K-Akt is required in both normal and catch-up growth, Mek1/2-Erk1/2 activation induced by elevated IGF signaling during re-oxygenation is particularly crucial for catch-up growth. These results suggest that the evolutionarily conserved IGF signaling pathway coordinates growth and temporal development in zebrafish embryos in response to oxygen availability.     

Insulin-like growth factors promote vasculogenesis in embryonic stem cells

The ability of embryonic stem cells to differentiate into endothelium and form functional blood vessels has been well established and can potentially be harnessed for therapeutic angiogenesis. However, after almost two decades of investigation in this field, limited knowledge exists for directing endothelial differentiation. A better understanding of the cellular mechanisms regulating vasculogenesis is required for the development of embryonic stem cell-based models and therapies. In this study, we elucidated the mechanistic role of insulin-like growth factors (IGF1 and 2) and IGF receptors (IGFR1 and 2) in endothelial differentiation using an embryonic stem cell embryoid body model. Both IGF1 or IGF2 predisposed embryonic stem to differentiate towards a mesodermal lineage, the endothelial precursor germ layer, as well as increased the generation of significantly more endothelial cells at later stages. Inhibition of IGFR1 signaling using neutralizing antibody or a pharmacological inhibitor, picropodophyllin, significantly reduced IGF-induced mesoderm and endothelial precursor cell formation. We confirmed that IGF-IGFR1 signaling stabilizes HIF1α and leads to up-regulation of VEGF during vasculogenesis in embryoid bodies. Understanding the mechanisms that are critical for vasculogenesis in various models will bring us one step closer to enabling cell based therapies for neovascularization.     

Activation of a novel calcineurin-mediated insulin-like growth factor-1 receptor pathway, altered metabolism, and tumor cell invasion in cells subjected to mitochondrial respiratory stress

We have previously shown that disruption of mitochondrial membrane potential by depletion of mitochondrial DNA (mtDNA) or treatment with a mitochondrial ionophore, carbonyl cyanide m-chlorophenylhydrazone, initiates a stress signaling, which causes resistance to apoptosis, and induces invasive behavior in C2C12 myocytes and A549 cells. In the present study we show that calcineurin (Cn), activated as part of this stress signaling, plays an important role in increased glucose uptake and glycolysis. Here we report that, although both insulin and insulin-like growth factor-1 receptor levels (IR and IGF1R, respectively) are increased in response to mitochondrial stress, autophosphorylation of IGF1R was selectively increased suggesting a shift in receptor pathways. Using an approach with FK506, an inhibitor of Cn, and mRNA silencing by small interference RNA we show that mitochondrial stress-activated Cn is critical for increased GLUT 4 and IGF1R expression and activation. The importance of the IGF1R pathway in cell survival under mitochondrial stress is demonstrated by increased apoptosis either by IGF1R mRNA silencing or by treatment with IGF1R inhibitors (AG1024 and picropodophyllin). This study describes a novel mechanism of mitochondrial stress-induced metabolic shift involving Cn with implications in resistance to apoptosis and tumor proliferation.     

p53 coordinates cranial neural crest cell growth and epithelial-mesenchymal transition/delamination processes

Neural crest development involves epithelial-mesenchymal transition (EMT), during which epithelial cells are converted into individual migratory cells. Notably, the same signaling pathways regulate EMT function during both development and tumor metastasis. p53 plays multiple roles in the prevention of tumor development; however, its precise roles during embryogenesis are less clear. We have investigated the role of p53 in early cranial neural crest (CNC) development in chick and mouse embryos. In the mouse, p53 knockout embryos displayed broad craniofacial defects in skeletal, neuronal and muscle tissues. In the chick, p53 is expressed in CNC progenitors and its expression decreases with their delamination from the neural tube. Stabilization of p53 protein using a pharmacological inhibitor of its negative regulator, MDM2, resulted in reduced SNAIL2 (SLUG) and ETS1 expression, fewer migrating CNC cells and in craniofacial defects. By contrast, electroporation of a dominant-negative p53 construct increased PAX7(+) SOX9(+) CNC progenitors and EMT/delamination of CNC from the neural tube, although the migration of these cells to the periphery was impaired. Investigating the underlying molecular mechanisms revealed that p53 coordinates CNC cell growth and EMT/delamination processes by affecting cell cycle gene expression and proliferation at discrete developmental stages; disruption of these processes can lead to craniofacial defects.     

Directionality of heart looping: effects of Pitx2c misexpression on flectin asymmetry and midline structures

The insulin-like growth factors (IGFs) are well known mitogens, both in vivo and in vitro, while functions in cellular differentiation have also been indicated. Here, we demonstrate a new role for the IGF pathway in regulating head formation in Xenopus embryos. Both IGF-1 and IGF-2, along with their receptor IGF-1R, are expressed early during embryogenesis, and the IGF-1R is present particularly in anterior and dorsal structures. Overexpression of IGF-1 leads to anterior expansion of head neural tissue as well as formation of ectopic eyes and cement gland, while IGF-1 receptor depletion using antisense morpholino oligonucleotides drastically reduces head structures. Furthermore, we demonstrate that IGF signaling exerts this effect by antagonizing the activity of the Wnt signal transduction pathway in the early embryo, at the level of beta-catenin. Thus, the IGF pathway is required for head formation during embryogenesis.     

Upregulation of microRNA-7 contributes to inhibition of the growth and metastasis of osteosarcoma cells through the inhibition of IGF1R

We aim to uncover the methylation of microRNA-7 (miR-7) promoter in osteosarcoma (OS) and the inner mechanism of miR-7 on the progression of OS cells. Expression and methylation state of miR-7 in OS tissues and cells were detected. With the aim to unearth the ability of miR-7 in OS, the proliferation, cell cycle progression, apoptosis, invasion, migration of OS cells, and the tumor growth in nude mice were determined. Meanwhile, IGF1R expression was detected and the association between miR-7 and IGF1R was confirmed. The proliferating cell nuclear antigen (PCNA) expression was tested by immunohistochemical staining, and the lung metastasis was observed by H&E staining. miR-7 expression was decreased and methylation state of miR-7 was increased in OS tissues and cells. Upregulated miR-7 inhibited proliferation, cell cycle progression, invasion,and migration, while inducing apoptosis of OS cells and the tumor growth as well as PCNA expression in nude mice. Expression of IGF1R was downregulated in OS cells with overexpression of miR-7. Experiments verified the binding site between miR-7 and IGF1R. Our study demonstrates that abnormal methylation of miR-7 contributes to decreased miR-7 in OS. In addition, miR-7 represses the initiation and progression of OS cells through the inhibition of IGF1R.     

Insulin‐like growth factor‐1 regulates the SIRT1‐p53 pathway in cellular senescence

Cellular senescence, which is known to halt proliferation of aged and stressed cells, plays a key role against cancer development and is also closely associated with organismal aging. While increased insulin‐like growth factor (IGF) signaling induces cell proliferation, survival and cancer progression, disrupted IGF signaling is known to enhance longevity concomitantly with delay in aging processes. The molecular mechanisms involved in the regulation of aging by IGF signaling and whether IGF regulates cellular senescence are still poorly understood. In this study, we demonstrate that IGF‐1 exerts a dual function in promoting cell proliferation as well as cellular senescence. While acute IGF‐1 exposure promotes cell proliferation and is opposed by p53, prolonged IGF‐1 treatment induces premature cellular senescence in a p53‐dependent manner. We show that prolonged IGF‐1 treatment inhibits SIRT1 deacetylase activity, resulting in increased p53 acetylation as well as p53 stabilization and activation, thus leading to premature cellular senescence. In addition, either expression of SIRT1 or inhibition of p53 prevented IGF‐1‐induced premature cellular senescence. Together, these findings suggest that p53 acts as a molecular switch in monitoring IGF‐1‐induced proliferation and premature senescence, and suggest a possible molecular connection involving IGF‐1‐SIRT1‐p53 signaling in cellular senescence and aging.     

siRNA-mediated type 1 insulin-like growth factor receptor silencing induces chemosensitization of a human liver cancer cell line with mutant P53

Overexpression of type 1 insulin-like growth factor receptor (IGF1R) contributes to the progression and metastasis of liver cancer, implying that IGF1R gene is a suitable target of RNA interference (RNAi) for liver cancer therapy. To investigate the possible regulation of IGF1R by P53, we examined the level of IGF1R expression in liver cancer cell lines in response to adriamycin. Levels of IGF1R mRNA and protein in cell lines with wild-type P53 decreased dramatically after P53 induction, but no such reduction of IGF1R was observed in cell lines with mutated P53. Inhibition of wild-type P53 in HEPG2 cells by small interfering RNA (siRNA) significantly upregulated the expression of IGF1R. IGF1R inhibition by siRNA in Huh7 cells with mutated P53 significantly depressed cell proliferation. To investigate the sensitivity of cancer cells to adriamycin after inhibition of IGF1R, we depressed IGF1R expression using siRNA, and then added adriamycin at an IC50 dose. After a further 48 h incubation with adriamycin, proliferation was significantly depressed in the cells treated with siRNA targeting IGF1R, in comparison with siRNA targeting scramble. Furthermore, both TUNEL and pro-caspase-3 expression assay showed a significant increase in apoptosis after combined treatment with adriamycin and siRNA targeting IGF1R. Our results demonstrate that IGF1R is downregulated by P53, and that siRNA targeting of IGF1R increases liver cancer cells sensitivity to adriamycin and promotes apoptosis. siRNA targeting of IGF1R could be potentially useful for increasing sensitivity to anti-cancer drugs, especially in drug-resistant cells with mutated P53.     

Signals transduced via insulin-like growth factor I receptor (IGF(R)) mediate resistance to retinoic acid-induced cell growth arrest in a human neuroblastoma cell line

Retinoic Acid (RA) has been shown to control growth and induce differentiation in a number of human neuroblastoma (NB) cell lines. However, a number of NB cell lines may be termed resistant to RA as they fail to growth arrest and differentiate. In studying the mechanism mediating RA-resistance, we noted that invariably RA-resistant NB cell lines constitutively express Insulin-like Growth Factor 2 (IGF2) (Gaetano, 1991b). The NB cell line LAN-1-15N (15N) represented an interesting model in which to study the development of RA-resistance as initially 15N cells are growth arrested by RA, however with prolonged culture (8-10 days) cells begin to proliferate. Coincidentally, RA induces IGF2 mRNA and protein secretion in 15N NB cells (Matsumoto, 1992). In this study we isolated RA-resistant 15N cell lines and analyzed their growth properties and changes in cell cycle related (cdc2, cdk2, cyclins A, B, D and E) and early response (fos and jun) gene expression to evaluate the role IGF2 may play in mediating RA resistance. We found that exogenous IGF2 stimulates growth in 15N and is capable of altering RA induced inhibition of NB cell growth. Finally we show that by blocking the Insulin-like Growth Factor Receptor (IGF1(R)) with a monoclonal antibody (alpha-IR3) in the presence of RA the growth of RAR cell lines could be completely blocked. These data are consistent with the concept that signals by IGF2 and transduced via the IGF1(R) can mediate resistance to the growth inhibiting properties of RA.