Dissociation between resistance to apoptosis and the transformed phenotype in IGF-I receptor signaling

High expression of SMYD3 indicates poor survival outcome and promotes tumour progression through an IGF-1R/AKT/E2F-1 positive feedback loop in bladder cancer

The AKT/mTOR pathway is critical for bladder cancer (BC) pathogenesis and is hyper-activated during BC progression. In the present study, we identified a novel positive feedback loop involving oncogenic factors histone methyltransferase SMYD3, insulin-like growth factor-1 receptor (IGF-1R), AKT, and E2F-1. SMYD3 expression was significantly up-regulated in BC tumors and positively associated with histological grade, lymph node metastasis, and shorter patient survival. Depletion of SMYD3 inhibited BC cell proliferation, colony formation, migration, invasion, and xenograft tumor growth. Mechanistically, SMYD3 inhibition led to the diminished AKT/mTOR signaling activity, thereby triggering deleterious effects on BC cells. Furthermore, SMYD3 directly activates the expression of IGF-1R, a critical activator of AKT in BC, by inducing hyper-methylation of histone H3-K4 and subsequent chromatin remodeling in the IGF-1R promoter region. On the other hand, E2F-1, a downstream factor of the AKT pathway, binds to the E2F-1 binding motifs at the SMYD3 promoter and consequently induces SMYD3 transcription and expression. Thus, SMYD3/IGF-1R/AKT/E2F-1 forms a positive feedback loop leading to the hyper-activated AKT signaling. Our findings provide not only profound insights into SMYD3-mediated oncogenic activity but also present a unique avenue for treating BC by directly disrupting this signaling circuit.

Expression of insulin-like growth factor-1 receptor in metastatic uveal melanoma and implications for potential autocrine and paracrine tumor cell growth

We investigated the importance of the insulin-like growth factor-1 receptor (IGF-1R) in hepatic metastases of uveal melanoma. The expression pattern of IGF-1R in archival tissue samples of hepatic metastasis from 24 patients was analyzed by immunohistochemistry. All the samples of hepatic metastases stained positive for IGF-1R. To investigate the biological role of IGF-1R on the growth of metastatic uveal melanoma, a long-term cell line obtained from a hepatic metastasis (TJU-UM001) was evaluated. TJU-UM001 expressed cell surface IGF-1R (>90%) and proliferated in response to exogenous and endogenous insulin-like growth factor-1 (IGF-1). Correlatively, anti-IGF-1R antibody completely blocked IGF-1-induced growth of TJU-UM001 cells. IGF-1 preferentially induced phosphorylation of Akt (S473) in quiescent TJU-UM001 cells, and this was blocked by anti-IGF-1R antibody. This study suggests that autocrine and paracrine mechanisms underlie IGF-1-induced growth of metastatic uveal melanoma and underscore the potential benefit of IGF-1 or IGF-1R antagonism in treatment for metastatic uveal melanoma.

Deregulations in the cyclin-dependent kinase-9-related pathway in cancer: implications for drug discovery and development

The CDK9-related pathway is an important regulator of mammalian cell biology and is also involved in the replication cycle of several viruses, including the human immunodeficiency virus type 1. CDK9 is present in two isoforms termed CDK9-42 and CDK9-55 that bind noncovalently type T cyclins and cyclin K. This association forms a heterodimer, where CDK9 carries the enzymatic site and the cyclin partner functions as a regulatory subunit. This heterodimer is the main component of the positive transcription elongation factor b, which stabilizes RNA elongation via phosphorylation of the RNA pol II carboxyl terminal domain. Abnormal activities in the CDK9-related pathway were observed in human malignancies and cardiac hypertrophies. Thus, the elucidation of the CDK9 pathway deregulations may provide useful insights into the pathogenesis and progression of human malignancies, cardiac hypertrophy, AIDS and other viral-related maladies. These studies may lead to the improvement of kinase inhibitors for the treatment of the previously mentioned pathological conditions. This review describes the CDK9-related pathway deregulations in malignancies and the development of kinase inhibitors in cancer therapy, which can be classified into three categories: antagonists that block the ATP binding site of the catalytic domain, allosteric inhibitors, and small molecules that disrupt protein-protein interactions.

Flavopiridol induces phosphorylation of AKT in a human glioblastoma cell line, in contrast to siRNA-mediated silencing of Cdk9: Implications for drug design and development

Cdk9 and Cdk7 are cdc2-like serine/threonine kinases that stabilize RNA transcript elongation through RNA polII carboxyl terminal domain (CTD) phosphorylation and are considered suitable targets for cancer therapy. The effects of flavopiridol and of siRNA-mediated inhibition of Cdk9 and/or Cdk7 were analyzed in human glioblastoma and human prostate cancer cell lines. One finding revealed that Cdk9 and Cdk7 could substitute each other in RNA polII CTD phosphorylation in contrast to the in vitro system. Thus, a simultaneous inhibition of Cdk9 and Cdk7 might be required both for targeting malignant cells and developing a platform for microarray analysis. However, these two pathways are not redundant, as indicated by differential effects observed in cell cycle regulation following siRNA-mediated inhibition of Cdk9 and/or Cdk7 in human PC3 prostate cancer cell line. Specifically, siRNA-mediated inhibition of Cdk9 caused a shift from G 0/G 1 to G 2/M phase in human PC3 prostate cancer cell line. Another finding showed that flavopiridol treatment induced a substantial AKT-Ser473 phosphorylation in human glioblastoma T98G cell line in contrast to siRNA-mediated inhibition of Cdk9 and Cdk9 combined with Cdk7, whereas siRNA-mediated silencing of Cdk7 caused a minor increase in AKT-Ser473 phosphorylation. AKT-Ser473 is a hallmark of AKT pathway activation and may protect cells from apoptosis. This finding also shows that Cdk9 and Cdk7 pathways are not redundant and may have important implications in drug development and for studying the mechanism of chemoresistance in malignant cells.

Targeting the insulin-like growth factor 1 receptor (IGF1R) signaling pathway for cancer therapy

INTRODUCTION

The IGF system controls growth, differentiation, and development at the cellular, organ and organismal levels. IGF1 receptor (IGF1R) signaling is dysregulated in many cancers. Numerous clinical trials are currently assessing therapies that inhibit either growth factor binding or IGF1R itself. Therapeutic benefit, often in the form of stable disease, has been reported for many different cancer types.

AREAS COVERED

Canonical IGF signaling and non-canonical pathways involved in carcinogenesis. Three recent insights into IGF1R signaling, namely hybrid receptor formation with insulin receptor (INSR), insulin receptor substrate 1 nuclear translocation, and evidence for IGF1R/INSR as dependence receptors. Different approaches to targeting IGF1R and mechanisms of acquired resistance. Possible mechanisms by which IGF1R signaling supports carcinogenesis and specific examples in different human tumors.

EXPERT OPINION

Pre-clinical data justifies IGF1R as a target and early clinical trials have shown modest efficacy in selected tumor types. Future work will focus upon assessing the usefulness or disadvantages of simultaneously targeting the IGF1R and INSR, biomarker development to identify potentially responsive patients, and the use of IGF1R inhibitors in combination therapies or as an adjunct to conventional chemotherapy.

Elevated insulin-like growth factor 1 receptor signaling induces antiestrogen resistance through the MAPK/ERK and PI3K/Akt signaling routes

INTRODUCTION

Insulin-like growth factor 1 (IGF-1) receptor (IGF-1R) is phosphorylated in all breast cancer subtypes. Past findings have shown that IGF-1R mediates antiestrogen resistance through cross-talk with estrogen receptor (ER) signaling and via its action upstream of the epidermal growth factor receptor and human epidermal growth factor receptor 2. Yet, the direct role of IGF-1R signaling itself in antiestrogen resistance remains obscure. In the present study, we sought to elucidate whether antiestrogen resistance is induced directly by IGF-1R signaling in response to its ligand IGF-1 stimulation.

METHODS

A breast cancer cell line ectopically expressing human wild-type IGF-1R, MCF7/IGF-1R, was established by retroviral transduction and colony selection. Cellular antiestrogen sensitivity was evaluated under estrogen-depleted two-dimensional (2D) and 3D culture conditions. Functional activities of the key IGF-1R signaling components in antiestrogen resistance were assessed by specific kinase inhibitor compounds and small interfering RNA.

RESULTS

Ectopic expression of IGF-1R in ER-positive MCF7 human breast cancer cells enhanced IGF-1R tyrosine kinase signaling in response to IGF-1 ligand stimulation. The elevated IGF-1R signaling rendered MCF7/IGF-1R cells highly resistant to the antiestrogens tamoxifen and fulvestrant. This antiestrogen-resistant phenotype involved mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) and phosphatidylinositol 3-kinase/protein kinase B pathways downstream of the IGF-1R signaling hub and was independent of ER signaling. Intriguingly, a MAPK/ERK-dependent agonistic behavior of tamoxifen at low doses was triggered in the presence of IGF-1, showing a mild promitogenic effect and increasing ER transcriptional activity.

CONCLUSIONS

Our data provide evidence that the IGF-1/IGF-1R signaling axis may play a causal role in antiestrogen resistance of breast cancer cells, despite continuous suppression of ER transcriptional function by antiestrogens.

Cyclin T1 overexpression induces malignant transformation and tumor growth

Human PTE Fb is a protein kinase composed by CDK9 and Cyclin T that controls the elongation phase of RNA Pol II. This complex also affects the activation and differentiation program of lymphoid cells. In this study we found that several head and neck tumor cell lines overexpress PTE Fb. We also established that Cyclin T1 is able to induce transformation in vitro, as we determined by foci and colony formation assays. Nu/nu mice s.c. injected with stable transfected Cyclin T1 cells (NIH 3T3 Cyclin T1) developed tumors faster than animals injected with control cells (NIH 3T3 beta-gal). In vitro, NIH 3T3 Cyclin T1 cells show increased proliferation and CDK4-Rb phosphorylation. Even more, silencing E2F1 expression (shRNA E2F1) in NIH 3T3 cells resulted in a dramatic inhibition of Cyclin T1-induced foci. All these data demonstrate for the first time the Cyclin T1 oncogenic function and suggest a role for this protein in controlling cell cycle probably via Rb/E2F1 pathway.

The emerging role of insulin-like growth factor 1 receptor (IGF1r) in gastrointestinal stromal tumors (GISTs)

Recent years have seen a growing interest in insulin-like growth factor 1 receptor (IGF1R) in medical oncology. Interesting data have been reported also on IGF1r in gastrointestinal stromal tumors (GISTs) especially in children and in young adult patients whose disease does not harbour mutations on KIT and PDGFRA and are poorly responsive to conventional therapies. However, it is too early to reach conclusions on IGF1R as a novel therapeutic target in GIST because the receptor's biological role is still to be defined and the clinical significance in patients needs to be studied in larger studies. We update and comment the current literature on IGF1R in GISTs and discuss the future perspectives in this promising field.

Mechanism of growth inhibition by MicroRNA 145: the role of the IGF-I receptor signaling pathway

MicroRNA 145 (miR145) has been proposed as a tumor suppressor. It was previously shown that miR145 targets the 3' UTR of the insulin receptor substrate-1 (IRS-1) and dramatically inhibits the growth of colon cancer cells. miR145 also targets the type 1 insulin-like growth factor receptor (IGF-IR). We show here that an IRS-1 lacking its 3' UTR is no longer down-regulated by miR145 and rescues colon cancer cells from miR145-induced inhibition of growth. An IGF-IR resistant to miR145 (again by elimination of its 3' UTR) is not down-regulated by miR145 but fails to rescue colon cancer cells from growth inhibition. These and other results, taken together, indicate that down-regulation of IRS-1 plays a significant role in the tumor suppressor activity of miR145.

Growth inhibition of insulin-like growth factor I receptor monoclonal antibody to human colorectal cancer cells

AIM

Insulin-like growth factors (IGFs) and their receptors play a critical role in the growth and regulation of many type of malignant cells. The purpose of this study was to clarify the expression of Insulin-like growth factor I receptor (IGF-IR) in a human colorectal cancer cell line HT-29 and investigate the effects of 2 IGF-IR monoclonal antibodies (Mab) on the biological behavior of HT-29 cell line.

METHODS

Immunohistochemistry was used to test the expression of IGF-IR in HT-29 cell line, MTT assay was used to determine the antiproliferation effects of 2 kinds of IGF-IR monoclonal antibody on HT-29 cells. The effects of apoptosis rate and cell cycle of HT-29 cells were estimated by flow cytometry (FCM).

RESULTS

IGF-IR was expressed in the membranes of HT-29 cell line; MTT results show that each IGF-IR (alpha-Subunit)Ab-3 Mab group has remarkable difference (p < 0.01) as compared with control group, each IGF-IRalpha(1H7)L Mab group has no remarkable difference (p > 0.05) as compared with control group, each IGF-IRalpha(1H7)L Mab group has light antiproliferation effect on the HT-29 cells. IGF-IR(alpha-Subunit)Ab-3 has stronger antiproliferation effect on the HT-29 cells and dose-depended growth inhibition effects were observed when concentration is less than 1.5 ug/ml;The 2 kinds of IGF-IR monoclonal antibody can arrest the cell cycle at any phase of HT-29 cells and induce apoptosis. Under the incubating conditions of containing 5 percent fetal bovine serum (FCS) and no FCS, the apoptosis percentage of IGF-IR(alpha-Subunit)Ab-3 group and IGF-IRalpha(1H7)L group have obviously increased (p < 0.01). After the number of cells was increased 30 times, the apoptosis percentage of IGF-IR(alpha-Subunit)Ab-3 group and IGF-IRalpha(1H7)L group have no remarkable differences as compared with control group (p > 0.05).

CONCLUSIONS

Blockade of the IGF-IR with IGF-IR Mab inhibiting proliferation, arresting cell cycle and inducing apoptosis of HT-29 cells may represent a valid approach to inhibit tumor growth.

Tumor suppressor pRb2/p130 gene and its derived product Spa310 spacer domain as perspective candidates for cancer therapy

Tumor suppressor pRb2/p130 gene belongs to the retinoblastoma (Rb) gene family, which also includes pRb/p105 and pRb/p107. The members of the Rb gene family have attracted a great deal of interest because of their essential role in regulating cell cycle and, consequently, cell proliferation. This mini review discusses the potential therapeutic applications both of pRb2/p130 and its derived product Spa310 spacer domain in cancer treatment.

Dual regulation of upstream binding factor 1 levels by IRS-1 and ERKs in IGF-1-receptor signaling

The Upstream Binding Factor 1 (UBF1) is a nucleolar protein that participates in the regulation of RNA polymerase I activity and ribosomal RNA (rRNA) synthesis. In 32D myeloid cells expressing the type 1 insulin-like growth factor receptor (IGF-IR), the UBF1 protein (but not its mRNA) is down regulated when the cells are shifted from Interleukin-3 (IL-3) to IGF-1. Ectopic expression of insulin receptor substrate-1 (IRS-1) in these cells inhibits the down-regulation of UBF1. We now show that the stability of UBF1 in 32D-derived cells requires also a signal from the extracellular regulated kinases (ERKs). When ERKs signaling is defective, as in cells over-expressing the insulin receptor (InR) or selected mutants of the IGF-1R, UBF1 is down-regulated, even in the presence of IRS-1. The down-regulation is corrected by the expression of an activated Ha-ras, which stimulates ERKs activity. Mutations at threonines 117 and 201 of UBF1, known to be phosphorylated by ERKs, cause its down-regulation. However, when IRS-2, instead of IRS-1, is ectopically expressed in 32D InR cells, ERKs phosphorylation is increased and UBF is stabilized. Taken together, these results indicate that in 32D-derived myeloid cells expressing either the IGF-IR or the InR, UBF1 levels are regulated by signaling from both IRS proteins and ERKs.

Regulation of survivin expression by IGF-1/mTOR signaling

Survivin is a dual regulator of cell proliferation and cell viability overexpressed in most human tumors. Although strategies to lower survivin levels have been pursued for rational cancer therapy, the molecular circuitries controlling survivin expression in tumors have not been completely elucidated. Here, we show that stimulation with insulin-like growth factor-1 (IGF-1) results in increased survivin expression in prostate cancer cells. This response is independent of de novo gene transcription, changes in mRNA expression or modifications of survivin protein stability. Instead, IGF-1 induced persistence and translation of a pool of survivin mRNA, in a reaction abolished by the mTOR (mammalian target of rapamycin) inhibitor, rapamycin. Forced expression of the mTOR target p70S6K1 reproduced the increase in survivin expression in prostate cancer cells, whereas acute ablation of endogenous p70S6K1 by small interfering RNA downregulated survivin levels. Rapamycin, alone or in combination with suboptimal concentrations of taxol reduced survivin protein levels, and decreased viability of prostate cancer cells. Therefore, IGF-1/mTOR signaling elevates survivin in prostate cancer cells via rapid changes in mRNA translation. Antagonists of this pathway may be beneficial to lower an antiapoptotic threshold maintained by survivin in prostate cancer.

The insulin-like growth factor I receptor is required for Akt activation and suppression of anoikis in cells transformed by the ETV6-NTRK3 chimeric tyrosine kinase

Signaling through the insulin-like growth factor I receptor (IGF-IR) axis is essential for transformation by many dominantly acting oncoproteins. However, the mechanism by which IGF-IR contributes to oncogenesis remains unknown. To examine this, we compared transformation properties of the oncogenic ETV6-NTRK3 (EN) chimeric tyrosine kinase in IGF-IR-null R- mouse embryo fibroblasts with R- cells engineered to reexpress IGF-IR (R+ cells). We previously showed that R- cells expressing EN (R- EN cells) are resistant to transformation but that transformation is restored in R+ cells. We now show that while R- EN cells have intact Ras-extracellular signal-regulated kinase signaling and cell cycle progression, they are defective in phosphatidylinositol-3-kinase (PI3K)-Akt activation and undergo detachment-induced apoptosis (anoikis) under anchorage-independent conditions. In contrast, R+ cells expressing EN (R+ EN cells) suppress anoikis and are fully transformed. The requirement for IGF-IR in R- EN cells is overcome by ectopic expression of either activated Akt or a membrane-targeted form of EN. Moreover, compared to R- EN cells, R+ EN cells show a dramatic increase in membrane localization of insulin receptor substrate 1 (IRS-1) in association with EN. Since EN is known to bind IRS-1 as an adaptor protein, our findings suggest that IGF-IR may function to localize EN/IRS-1 complexes to cell membranes, in turn facilitating PI3K-Akt activation and suppression of anoikis.

Control of cell size through phosphorylation of upstream binding factor 1 by nuclear phosphatidylinositol 3-kinase

The insulin-like growth factor I/insulin receptor substrate 1 axis controls, in a nonredundant way, approximately 50% of cell and body size in animals from Drosophila to mice and in cells in culture. Although other factors may also intervene, cell size is strongly dependent on ribosome biogenesis, which is under the control of RNA polymerase I activity. We have previously shown that insulin receptor substrate 1 (IRS-1) translocates to the nuclei and nucleoli, where it binds to the upstream binding factor (UBF) 1, a regulator of RNA polymerase I activity. Activation of UBF1 requires its phosphorylation. However, IRS-1 is not a kinase, and we searched for an intermediate kinase that can phosphorylate UBF1. We demonstrate here that IRS-1 binds also to the phosphatidylinositol 3-kinase (PI3-K) subunits in nuclear extracts, and that the p110 subunit of PI3-K directly phosphorylates and activates UBF1, an exclusively nucleolar protein. The interaction of IRS-1, PI3-K, and UBF1 in the nucleoli provides one of the mechanisms for the effects of IRS-1 on cell and body size.

Contribution of MEK/MAPK and PI3-K signaling pathway to the malignant behavior of Ewing's sarcoma cells: therapeutic prospects

Insulin-like growth factor receptor I (IGF-I)-mediated circuit is a major autocrine loop for Ewing's sarcoma (ES) cells and appears to be particularly important in the pathogenesis of this tumor. In this study, we analyzed the contribution of the 2 major pathways of the intracellular IGF-IR signaling cascade to the overall effects elicited by IGF-I in ES. Both the mitogen-activated protein kinase (MAPK) and phosphatidylinositol-3-kinase (PI3-K) signaling pathways appeared to be constitutively activated in ES, likely due to the presence of the IGF-IR-mediated autocrine loop. We demonstrated that both MEK/MAPK (PD98059 or U0126) and PI3-K inhibitors (LY294002) profoundly impaired ES cell growth in monolayer and soft agar basal conditions. Both PD98059 and LY294002 inhibited ES cell cycle progression by inducing G1 blockage, whereas only LY294002 significantly affected the survival of ES cells. Exogenous IGF-I completely reverted LY294002-induced growth inhibition by abrogating antiproliferative and proapoptotic effects of the PI3-K inhibitor. By contrast, IGF-I could not rescue cells from growth inhibition induced by PD98059. MEK/MAPK blockade also significantly reduced the migratory ability of ES cells, both in basal and IGF-I-induced conditions, and increased chemosensitivity to doxorubicin, a leader drug in the treatment of ES patients. Our findings therefore identify MAPK pathway as a promising target for pharmacologic intervention in ES.

Growth factor receptors as therapeutic targets: strategies to inhibit the insulin-like growth factor I receptor

Neoplastic transformation is often related to abnormal activation of growth factor receptors and their signaling pathways. The concept of targeting specific tumorigenic receptors and/or signaling molecules has been validated by the development and successful clinical application of drugs acting against the epidermal growth factor receptor 2 (HER2/neu, Erb2), the epidermal growth factor receptor 1 (EGFR, HER1), the Brc-Abl kinase, the platelet-derived growth factor receptor, and c-kit. This review will focus on the next promising therapeutic target, the insulin-like growth factor I receptor (IGF-IR). IGF-IR has been implicated in a number of neoplastic diseases, including several common carcinomas. From a pharmaceutical standpoint, of particular importance is that IGF-IR appears to be required for many transforming agents (genetic, viral, chemical) to act, but is not obligatory for the function of normal adult cells. The tumorigenic potential of IGF-IR is mediated through its antiapoptotic and transforming signaling, and in some cases through induction of prometastatic pathways. Preclinical studies demonstrated that downregulation of IGF-IR reversed the neoplastic phenotype and sensitized cells to antitumor treatments. The strategies to block IGF-IR function employed anti-IGF-IR antibodies, small-molecule inhibitors of the IGF-IR tyrosine kinase, antisense oligodeoxynucleotides and antisense RNA, small inhibitory RNA, triple helix, dominant-negative mutants, and various compounds reducing ligand availability. The experience with these strategies combined with the knowledge gained with current anti-growth factor receptor drugs should streamline the development of anti-IGF-IR therapeutics.

Autocrine stimulation by insulin-like growth factor I is involved in the growth, tumorigenicity and chemoresistance of human esophageal carcinoma cells

Insulin-like growth factor I (IGF-I) receptor (IGF-IR)-mediated signals are known to be involved in cell growth and transformation and prevention of apoptosis. In this study, we demonstrated the coexpression of IGF-I and IGF-IR in human esophageal carcinoma tissues. We also demonstrated the IGF-I autocrine system in esophageal carcinoma cell lines. Both the CE48T/VGH and CE81T/VGH cell lines showed proliferative responses to IGF-I stimulation. Autokinase activity of IGF-IR in these cells can be triggered by the exogenous addition of IGF-I. In addition, an IGF-I peptide antagonist, JB1, specifically inhibited ligand-induced receptor autophosphorylation in a dose-dependent manner. Under serum-free conditions, JB1 also reduced the degree of IGF-IR phosphorylation and cell numbers. Furthermore, the addition of JB1 decreased the number of CE81T/VGH colonies formed in methyl cellulose agar and the size and the incidence of tumors which grew in mice with severe combined immunodeficiency. These results imply that an IGF-I autocrine system in human esophageal carcinoma cells could stimulate tumor growth. Finally, we found that IGF-I prevented the apoptosis of CE81T/VGH cells induced by chemotherapeutic drugs, such as cisplatin, 5-fluorouracil and camptothecin. Thus, interruption of IGF-IR function may provide a way to retard tumor growth and increase the sensitivity of esophageal carcinoma to chemotherapy.

Effect of glutamine on glutathione, IGF-I, and TGF-beta 1

BACKGROUND

Our previous results have showed that oral glutamine (GLN) supplementation decreased carcinogenesis in 7,12-dimethylbenz[a]antracene (DMBA) breast cancer model. We also have found that GLN raises blood glutathione (GSH) levels in an implantable breast cancer model. The process of tumor growth was accompanied by depressed GSH production and increased levels of insulin-like growth factor-I (IGF-I) and transforming growth factor beta1 (TGF-beta 1). GSH is counter-regulatory to IGF-I. We therefore hypothesized that in DMBA model of breast cancer, the increased GSH levels seen with oral GLN would be associated with lowered levels of IGF-I &TGF-beta(1).

METHODS

Time-dated pubertal Sprague-Dawley rats were gavaged at time 0 with 1 g/kg/day glutamine (GLN) (n = 18), isonitrogenous Freamine (FA) (n = 18), or water (H(2)O) (n = 18). Rats were further randomized on day 7 to 100 mg/kg DMBA or oil. After 14 days, the animals were sacrificed and blood GSH, IGF-1, TGF-beta 1, breast tissue, and gut mucosa GSH levels were measured.

RESULTS

Oral GLN increased significantly blood, breast tissue, and gut mucosa levels of GSH in both DMBA and control groups in comparison with the control groups not treated with GLN. At the same time, the levels of blood IGF-I and TGF-beta 1 decreased significantly in both DMBA-treated and control groups. DMBA did not significantly affect any of these levels. CONCLUSIONS ;Oral GLN increased GSH levels and lowered IGF-I and TGF-beta 1 in a range that is considered clinically significant. However, the effect of GLN in maintaining normal gut GSH production in the presence of DMBA was much more significant. Inconsistent with our hypothesis, reduction in IGF and TGF-beta 1 levels did not correlate with DMBA's effect on gut GSH production.

Insulin-like growth factor I receptor signaling and nuclear translocation of insulin receptor substrates 1 and 2

The insulin receptor substrate 1 (IRS-1) can translocate to the nuclei and nucleoli of several types of cells. Nuclear translocation can be induced by an activated insulin-like growth factor 1 receptor (IGF-IR), and by certain oncogenes, such as the Simian virus 40 T antigen and v-src. We have asked whether IRS-2 could also translocate to the nuclei. In addition, we have studied the effects of functional mutations in the IGF-IR on nuclear translocation of IRS proteins. IRS-2 translocates to the nuclei of mouse embryo fibroblasts expressing the IGF-IR, but, at variance with IRS-1, does not translocate in cells expressing the Simian virus 40 T antigen. Mutations in the tyrosine kinase domain of the IGF-IR abrogate translocation of the IRS proteins. Other mutations in the IGF-IR, which do not interfere with its mitogenicity but inhibit its transforming capacity, result in a decrease in translocation, especially to the nucleoli. Nuclear IRS-1 and IRS-2 interact with the upstream binding factor, which is a key regulator of RNA polymerase I activity and, therefore, rRNA synthesis. In 32D cells, wild-type, but not mutant, IRS-1 causes a significant activation of the ribosomal DNA promoter. The interaction of nuclear IRS proteins with upstream binding factor 1 constitutes the first direct link of these proteins with the ribosomal DNA transcription machinery.

Neuroprotective effects of IGF-I against TNFalpha-induced neuronal damage in HIV-associated dementia

Human immunodeficiency virus type 1 (HIV-1) infection often results in disorders of the central nervous system, including HIV-associated dementia (HAD). It is suspected that tumor necrosis factor-alpha (TNFalpha) released by activated and/or infected macrophages/microglia plays a role in the process of neuronal damage seen in AIDS patients. In light of earlier studies showing that the activation of the insulin-like growth factor I receptor (IGF-IR) exerts a strong neuroprotective effect, we investigated the ability of IGF-I to protect neuronal cells from HIV-infected macrophages. Our results demonstrate that the conditioned medium from HIV-1-infected macrophages, HIV/CM, causes loss of neuronal processes in differentiated PC12 and P19 neurons and that these neurodegenerative effects are associated with the presence of TNFalpha. Furthermore, we demonstrate that IGF-I rescues differentiated neurons from both HIV/CM and TNFalpha-induced damage and that IGF-I-mediated neuroprotection is strongly enhanced by overexpression of the wt IGF-IR cDNA and attenuated by the antisense IGF-IR cDNA. Finally, IGF-I-mediated antiapoptotic pathways are continuously functional in differentiated neurons exposed to HIV/CM and are likely supported by TNFalpha-mediated phosphorylation of I(kappa)B. All together these results suggest that the balance between TNFalpha and IGF-IR signaling pathways may control the extent of neuronal injury in this HIV-related experimental setting.

Nuclear translocation of insulin receptor substrate-1 by oncogenes and Igf-I. Effect on ribosomal RNA synthesis

The insulin receptor substrate-1 (IRS-1) is one of the major substrates of both the insulin and IGF-I receptors and is generally localized in the cytosol/membrane fraction of the cell. We show here that a substantial fraction of IRS-1 is translocated to the nucleus in mouse embryo fibroblasts (MEF) expressing the simian virus 40 T antigen. Nuclear translocation of IRS-1 occurs also in MEF stimulated with IGF-I or in MEF expressing the oncogene v-src. Nuclear translocation of IRS-1 can be demonstrated by confocal microscopy, immunohistochemistry, or subcellular fractionation. An antibody to IRS-1 immunoprecipitates from nuclear fractions (but not from cytosolic fractions) the upstream binding factor, which is a key regulator of RNA polymerase I activity and ribosomal RNA (rRNA) synthesis. In agreement with this finding, in 32D murine hemopoietic cells, nuclear translocation of IRS-1 correlates with a markedly increased rRNA synthesis. Our experiments suggest that nuclear IRS-1 may play a specialized role in rRNA synthesis and/or processing.

Nuclear translocation of insulin receptor substrate-1 by the simian virus 40 T antigen and the activated type 1 insulin-like growth factor receptor

32D cells are a murine hemopoietic cell line that undergoes apoptosis upon withdrawal of interleukin-3 (IL-3) from the medium. 32D cells have low levels of the type 1 insulin-like growth factor (IGF-I) receptor and do not express insulin receptor substrate-1 (IRS-1) or IRS-2. Ectopic expression of IRS-1 delays apoptosis but cannot rescue 32D cells from IL-3 dependence. In 32D/IRS-1 cells, IRS-1 is detectable, as expected, in the cytosol/membrane compartment. The SV40 large T antigen is a nuclear protein that, by itself, also fails to protect 32D cells from apoptosis. Co-expression of IRS-1 with the SV40 T antigen in 32D cells results in nuclear translocation of IRS-1 and survival after IL-3 withdrawal. Expression of a human IGF-I receptor in 32D/IRS-1 cells also results in nuclear translocation of IRS-1 and IL-3 independence. The phosphotyrosine-binding domain, but not the pleckstrin domain, is necessary for IRS-1 nuclear translocation. Nuclear translocation of IRS-1 was confirmed in mouse embryo fibroblasts. These results suggest possible new roles for nuclear IRS-1 in IGF-I-mediated growth and anti-apoptotic signaling.

Regulation of Id1 protein expression in mouse embryo fibroblasts by the type 1 insulin-like growth factor receptor

The activated type 1 insulin-like growth factor (IGF-IR) increases the expression of Id1 proteins in mouse embryo fibroblasts (MEF). Up-regulation depends on a functional receptor and on multiple pathways originating from different domains of the receptor. In MEF, Id1 protein expression is also up-regulated by serum and certain oncogenes. Signaling through Stat3 plays an important, but not exclusive, role in the up-regulation of Id1 protein levels. In all instances, the increase in Id1 protein expression is paralleled by a corresponding increase in Id1 promoter activity, as measured with a reporter gene.

Expression of the oncogenic NPM-ALK chimeric protein in human lymphoid T-cells inhibits drug-induced, but not Fas-induced apoptosis

Anaplastic large cell lymphomas (ALCLs) are frequently associated with the t(2;5)(p23;q35) translocation, leading to the expression of NPM-ALK, a fusion protein linking nucleophosmin and anaplastic lymphoma kinase, a receptor tyrosine kinase. In ALCLs, dimerization of NPM-ALK leads to constitutive autophosphorylation and activation of the kinase, necessary for NPM-ALK oncogenicity. To investigate whether NPM-ALK, like other oncogenic tyrosine kinases, can inhibit drug-induced apoptosis, we permanently transfected NPM-ALK into Jurkat T-cells. As in ALCLs, NPM-ALK was expressed as a constitutively kinase-active 80 kDa protein, and could be detected by immunocytochemistry in nucleoli, nuclei and cytoplasm. Doxorubicin-induced apoptosis (assessed by cell morphology and annexin V-FITC binding) was significantly inhibited in two independent NPM-ALK-expressing clones (5.2+/-1.8 and 7.5+/-0.8% apoptosis), compared to control vector-transduced cells (36+/-6.7%). Similar results were observed with etoposide. In contrast, Fas-induced apoptosis was not inhibited. Cytochrome c release into the cytosol was delayed in doxorubicin-, but not anti-Fas-treated transfectant cells, indicating that apoptosis inhibition occurred upstream of mitochondrial events. Using NPM-ALK mutants, we demonstrated that inhibition of drug-induced apoptosis: (1) requires functional kinase activity, (2) does not involve phospholipase C-gamma, essential for NPM-ALK-mediated mitogenicity and (3) appears to be phosphoinositide 3-kinase independent, despite a strong Akt/PKB activation observed in wild type NPM-ALK-expressing cells. These results suggest that the NPM-ALK antiapoptotic and mitogenic pathways are distinct.

The role of the insulin receptor substrate-1 in the differentiation of rat hippocampal neuronal cells

H19-7/IGF-IR cells are rat hippocampal cells expressing a human IGF-I receptor, which differentiate to a neuronal phenotype when stimulated by IGF-I at 39 degrees C. H19-7/IGF-IR cells have low levels of expression of insulin receptor substrate-l (IRS-1), a major substrate of the IGF-IR. IGF-I induces serine-phosphorylation and down-regulation of the endogenous IRS-1 upon differentiation of H19-7/IGF-IR cells. The profound influence of IRS-1 on differentiation of H19-7/IGF-IR cells was confirmed by transfecting these cells with a plasmid expressing mouse IRS-1. Over-expression of wild type IRS-1 in H19-7/IGF-IR cells abolishes IGF-I-induced differentiation at 39 degrees C. A mutant of IRS-1 lacking the PTB domain loses the ability to inhibit the differentiation program. H19-7/IGF-IR/IRS-1 cells at 39 degrees C show a stronger and prolonged activation of Akt, when compared to H19-7/IGF-IR cells. The role of Akt in the inhibition of the differentiation program was confirmed by using the inhibitor of Class I PI3 kinases LY29400, which restores IGF-I-induced differentiation of H19-7/IGF-IR/IRS-1 cells. H19-7/IGF-IR/IRS-1 cells show a strong reduction in MAP kinases signaling, which is related to the superactivation of Akt. This was confirmed by expressing in H19-7/IGF-IR cells a constitutively active Akt, which inhibited MAP kinases activation in these cells. These experiments confirm the importance of MAPK in the mechanism of IGF-I-mediated differentiation of H19-7/IGF-IR cells

IGF-I receptor signalling in transformation and differentiation

The type 1 insulin-like growth factor receptor (IGF-IR) sends several signals, some of which are contradictory. When the concentrations of insulin receptor substrate 1 (IRS-1), a major substrate of the IGF-IR, are high, the signal is mitogenic, anti-apoptotic, and can even cause malignant transformation. However, in the absence of IRS-1, the IGF-IR sends a differentiation signal, which leads to granulocytic differentiation in haemopoietic cells. The mitogenic signal of the IGF-IR/IRS-1 combination depends largely, but not exclusively, on the activation of the phosphatidylinositol-3 kinase (PI3K).

Anti-apoptotic signaling of the insulin-like growth factor-I receptor through mitochondrial translocation of c-Raf and Nedd4

The type 1 insulin-like growth factor receptor (IGF-IR) sends a strong anti-apoptotic signal by at least three different pathways. By using mutants of the IGF-IR, we showed that one of the pathways depends on residues of the IGF-IR (serines 1280--1283) that interact with 14.3.3 proteins. The result is the activation of Raf-1 and the mitochondrial translocation of both Raf-1 and Nedd4, a target of caspases. A mutant IGF-IR in which the serines at positions 1280--1283 have been mutated to alanine does not protect from apoptosis and fails to translocate Nedd4 or Raf-1 to the mitochondria. This failure is accompanied by a loss of cytochrome c from the mitochondria. The 14.3.3/Raf-1/Nedd4 pathway is operative in the presence or absence of the insulin receptor substrate-1.

Regulation of Id2 gene expression by the insulin-like growth factor I receptor requires signaling by phosphatidylinositol 3-kinase

The Id proteins play an important role in proliferation, differentiation, and tumor development. We report here that Id gene expression can be regulated by the insulin-like growth factor I receptor (IGF-IR), a receptor that also participates in the regulation of cellular proliferation and differentiation. Specifically, we found that the IGF-IR activated by its ligand was a strong inducer of Id2 gene expression in 32D murine hemopoietic cells. This activation was not simply the result of cellular proliferation, as Id2 gene expression was higher in 32D cells stimulated by IGF-I than in cells exponentially growing in interleukin-3. The up-regulation of Id2 gene expression was largely dependent on the presence of insulin receptor substrate-1, a major substrate of the IGF-IR and a potent activator of the phosphatidylinositol 3-kinase (PI3K) pathway. The role of PI3K activity in the up-regulation of Id2 gene expression by the IGF-IR was confirmed by different methods and in different cell types. In 32D cells, the up-regulation of Id2 gene expression by the PI3K pathway correlated with interleukin-3 independence and inhibition of differentiation.

Efficient in vitro and in vivo gene regulation of a retrovirally delivered pro-apoptotic factor under the control of the Drosophila HSP70 promoter

We have developed a self-inactivating retroviral vector system with an internal, inducible Drosophila HSP70 promoter. This vector system delivers the desired transgene into cells rapidly and efficiently. It generates mixed populations of transduced cells where the transgene is inducible, and does not require the isolation of specific clones. Since the transgene is not expressed (or poorly expressed) at the restrictive condition (34 degrees C), mixed populations can be selected in which tumor suppressors or other inhibitory genes can be strongly induced upon changing the conditions (39 degrees C or the plant amino acid L-canavanine). This retroviral vector should be very useful for the expression of sequences that are poorly tolerated by cells, and is also active in animals.

Limited redundancy of survival signals from the type 1 insulin-like growth factor receptor

The type 1 insulin-like growth factor receptor (IGF-IR) is effective in protecting cells from a variety of apoptotic injuries. In 32D murine hemopoietic cells, the IGF-IR sends three separate survival signals, through insulin receptor substrate-1, Shc, and mitochondrial Raf translocation. We report here that these three pathways for survival have a limited redundancy. If one of these pathways is blocked, the IGF-IR can still protect 32D cells from apoptosis induced by interleukin-3 withdrawal. However, when two of the three pathways are inactivated, the receptor is no longer capable to protect cells from apoptosis. The survival signal can use any two pathway combinations.

Mechanisms of regulation of cell adhesion and motility by insulin receptor substrate-1 in prostate cancer cells

LNCaP cells are human prostatic cancer cells that have a frame-shift mutation of the tumor suppressor gene PTEN and do not express the insulin receptor substrate-1 (IRS-1), a major substrate of the type 1 insulin-like growth factor receptor (IGF-IR). Ectopic expression of IRS-1 in LNCaP cells increases cell adhesion and decreases cell motility by an IGF-I-independent mechanism. We show now that these effects of IRS-1 are accompanied by serine phosphorylation of IRS-1 and are inhibited by inhibitors of phosphatidylinositol 3-kinase (PI3K). We have confirmed the requirement for PI3K activity and serine phosphorylation by the use of IRS-1 mutants, expressed in LNCaP cells. Serine phosphorylation inhibits IGF-I-induced tyrosyl phosphorylation of IRS-1, which is restored by the expression of wild-type PTEN or by inhibition of PI3K activity. Finally, IRS-1 in LNCaP cells co-immunoprecipitates with integrin alpha 5 beta 1, and the association is again IGF-I-independent. We conclude that in LNCaP cells, IRS-1 is serine phosphorylated by PI3K, generating effects that are different, and even opposite, from those generated by IGF-I.

The contradictions of the insulin-like growth factor 1 receptor

In recent years, the type 1 insulin-like growth factor receptor (IGF-IR) has emerged as a receptor that plays a very important role in the growth of cells, both in vivo and in vitro. The ability of the IGF-IR to induce mitogenesis and to promote survival of cells against a variety of apoptotic agents is well documented. Somewhat less known are other functions of the IGF-IR, like its ability to induce differentiation, to regulate cell size and to affect the organization of the cytoskeleton of cells. This review will focus on these lesser known functions of the IGF-IR. At the same time, we will emphasize how the IGF-IR can send contradictory signals, which depend on different domains of the receptor and the availability of downstream transducing molecules.

Suppressed oxidant-induced apoptosis in cadmium adapted alveolar epithelial cells and its potential involvement in cadmium carcinogenesis

Apoptosis involves a series of genetically programmed events associated with endonucleolytic cleavage of DNA. This process is triggered by a variety of agents, including oxidants such as hydrogen peroxide (H(2)O(2)) and it plays a key role in eliminating pre-neoplastic cells from the lung. Failure to do so could favor tumor promotion. The current study demonstrated that alveolar epithelial cells, adapted to cadmium (CdCl(2)) by repeated in vitro exposure, exhibit lower levels of H(2)O(2)-induced apoptosis than similarly challenged non-adapted cells. An immunologic assay, measuring cytoplasmic histone-associated DNA fragments, indicated maximal apoptosis 24 h after exposure to 400 microM H(2)O(2). Non-adapted cells showed a 13-fold increase in oxidant-induced apoptosis while Cd-adapted cells had only a 4-fold elevation. A terminal deoxyribonucleotidyl transferase mediated dUTP nick end labeling (TUNEL) method was used to assess the percentage of cells with DNA breaks consistent with apoptosis. Cd-adapted and non-adapted cells that were not exposed to H(2)O(2) did not differ in TUNEL positivity. However, after H(2)O(2) treatment, the percentage of TUNEL positive cells was 4-fold higher in non-adapted cultures than in adapted ones. Suppression of oxidant-induced apoptosis is due, in part, to up-regulation in the gene expression of several resistance factors including metallothioneins (MT-1 and MT-2), glutathione S-transferases (GST-alpha and GST-pi), and gamma-glutamylcysteine synthetase catalytic subunit (gamma-GCS). These steady-state mRNA changes, determined by Northern blotting, were accompanied by increased levels of MT and gamma-GCS protein, GST activity, and glutathione (GSH). Suppressed oxidant-induced apoptosis, resulting at least in part from these response modifications, could leave pre-neoplastic or neoplastic cells alive, favor clonal expansion, and ultimately lead to cancer development.

IGF-I receptor signaling in a prostatic cancer cell line with a PTEN mutation

LNCaP prostatic cancer cells are characterized by having a PTEN mutation, low levels of type 1 insulin-like growth factor receptor (IGF-IR) and no IRS-1, one of the major substrates of the IGF-IR. The absence of IRS-1, an activator of PI3-kinase, is compensated in these cells by the mutation in PTEN, an inhibitor of PI3-kinase. However, IGF-IR signaling in the absence of IRS-1 can cause cell differentiation and growth arrest. We hypothesized that these three characteristics may not be unrelated, specifically that, together, they may favor the metastatic spread of prostatic cancer cells without decreasing their growth potential. In support of this hypothesis, we report here that: (1) IRS-1 expression increases cell adhesion and decreases cell motility; (2) over-expression of the IGF-IR, in the absence of IRS-1, causes growth arrest and (3) a combination of IGF-IR and IRS-1 restores the transformed phenotype of LNCaP cells. These findings suggest a mechanism by which prostatic cancer cells can achieve metastatic potential without interfering with their growth potential. Oncogene (2000).

Domains of the insulin-like growth factor I receptor required for the activation of extracellular signal-regulated kinases

The type 1 insulin-like growth factor receptor (IGF-IR) activates the extracellular signal-regulated kinases (ERK1 and -2). The two major substrates of the IGF-IR, insulin receptor substrate-1 (IRS-1) and the Shc proteins, are known to contribute to this activation. We investigated the domains of the IGF-IR required for the activation of the ERK proteins. To facilitate this study, we used a cell line (32D cells) that lacks IRS-1. In the absence of IRS-1, ERK activation is inhibited if the IGF-IR is mutated at two domains: tyrosine Y950 and a serine quartet at 1280-1283. Expression of IRS-1 in 32D cells expressing the double mutant IGF-IR restores ERK activation. The importance of the C-terminus of the IGF-IR in ERK activation (in the absence of IRS-1) is confirmed by the failure of the insulin receptor to give a sustained activation of ERK. In this model system, there is a good, but not exact, correlation between ERK activation and cell survival after withdrawal of growth factors.

Differentiation and malignant transformation: two roads diverged in a wood

Growth factors and their receptors are known to send at times contradictory signals, such as proliferation or differentiation. Recent developments in our knowledge of growth factor receptors and their signaling pathways have clarified the basis for this puzzling behavior. Separate domains of a receptor and/or the availability of specific substrates determine the fate of a cell stimulated by the same growth factor. In some models, the difference between malignant transformation and differentiation (leading to cell death) may depend on the presence or absence of a single agonist or antagonist molecule. The type 1 insulin-like growth factor receptor will serve as the paradigm for this review. J. Cell. Biochem. Suppls. 32/33:68-75, 1999.

IGF and insulin action in the mammary gland: lessons from transgenic and knockout models

Transgenic and knockout mice have become valuable experimental systems with which to study specific molecular events within the mammary gland of an intact animal. These models have provided a wealth of information about the effects of a number of oncogenes and growth factors. This review focuses on results obtained from the application of transgenic and knockout models to determine the roles of insulin and insulin-like growth factors (IGF) in the regulation of mammary gland development, lactation and tumorigenesis. Transgenic models which overexpress IGF-I or -II display specific alterations in mammary gland development and an increased incidence of mammary tumors. Analysis of mammary gland development in knockout mice which are deficient in IGF-I or the IGF-I receptor supports the conclusion that the IGF system is important for normal mammary gland development. This review discusses these observations in detail and attempts to fit them into a larger picture of IGF and insulin action in the mammary gland.

Multiple signaling pathways of the insulin-like growth factor 1 receptor in protection from apoptosis

The type 1 insulin-like growth factor receptor (IGF-1R), activated by its ligands, protects several cell types from a variety of apoptotic injuries. The main signaling pathway for IGF-1R-mediated protection from apoptosis has been previously elucidated and rests on the activation of phosphatidylinositol 3-kinase, Akt/protein kinase B, and the phosphorylation and inactivation of BAD, a member of the Bcl-2 family of proteins. In 32D cells (a murine hemopoietic cell line devoid of insulin receptor substrate 1 [IRS-1]), the IGF-1R activates alternative pathways for protection from apoptosis induced by withdrawal of interleukin-3. One of these pathways leads to the activation of mitogen-activated protein kinase, while a third pathway results in the mitochondrial translocation of Raf and depends on the integrity of a group of serines in the C terminus of the receptor that are known to interact with 14.3.3 proteins. All three pathways, however, result in BAD phosphorylation. The presence of multiple antiapoptotic pathways may explain the remarkable efficacy of the IGF-1R in protecting cells from apoptosis.

Synthetic peptide sequence from the C-terminus of the insulin-like growth factor-I receptor that induces apoptosis and inhibition of tumor growth

Although the type 1 insulin-like growth factor receptor (IGF-IR) is a potent inhibitor of apoptosis, its C-terminus sequence sends contradictory signals, including a clearly proapoptotic signal. We have synthesized a peptide, peptide 2, having the sequence of the IGF-IR from residue 1282 to residue 1298 (C-terminus of the beta subunit). To favor its uptake into cells, we linked it to a stearic acid moiety at its NH-terminus. Peptide 2 is taken up by the cells, where it inhibits DNA synthesis and causes apoptosis, while a scrambled peptide (with stearic acid) and peptide 2 without stearic acid are completely ineffective. Peptide 2 is more effective when cells are in anchorage-independent conditions than when they grow in monolayer cultures. Accordingly, we find that peptide 2 can inhibit the growth of a human prostatic cell line in nude mice. The proapoptotic effect of peptide 2 is inhibited by the expression of Bcl-2 or by a dominant negative mutant of caspase 9. These and other data indicate that peptide 2 does not seem to be competing directly with the IGF-IR for common substrates, but that its proapoptotic effect is related to its ability to activate the caspase cascade.

Growth and differentiation signals by the insulin-like growth factor 1 receptor in hemopoietic cells are mediated through different pathways

The type 1 insulin-like growth factor receptor (IGF-IR) plays an important role in the growth of cells both in vivo and in vitro. The IGF-IR is also capable of inducing differentiation in a number of cell types, raising the question of how the same receptor can send two seemingly contradictory signals, one for growth and one for differentiation. Using 32D cells, which are murine hemopoietic cells, we show that the activated IGF-IR can induce differentiation along the granulocytic pathway in a manner similar to the granulocyte colony-stimulating factor. We find that one of the major substrates of the IGF-IR, the insulin receptor substrate-1 inhibits IGF-I-mediated differentiation of 32D cells. In the absence of insulin receptor substrate-1, functional impairment of another major substrate of the IGF-IR, the Shc proteins, is associated with a decrease in the extent of differentiation. Although the end points of the respective pathways remain to be defined, these results show for the first time that IGF-I-mediated growth or differentiation of hemopoietic cells may depend on a balance between two of its substrates.