Effect of mutations at serines 1280-1283 on the mitogenic and transforming activities of the insulin-like growth factor I receptor

Heterozygous nonsense mutations near the C-terminal region of IGF1R in two patients with small-for-gestational-age-related short stature

OBJECTIVE

The type I insulin-like growth factor I receptor (IGF1R) plays an important role in growth. We aimed to evaluate the detailed mechanism underlying the effect of IGF1R on human growth.

PATIENTS AND METHODS

We have performed sequence analysis of IGF1R in 55 patients with SGA short stature in Japan, since 2004, and identified novel heterozygous nonsense mutations in 2 patients: an 8-year-old Japanese boy (case 1), with a birthweight of 2228 g (-3·3 SDS) and height of 46 cm (-2·1 SDS), and a 3-year-old Japanese girl (case 2), with a birthweight of 2110 g (-3·0 SDS) and height of 44·3 cm (-2·8 SDS). Both patients had a short stature (-3·2 SDS, -3·1 SDS). We determined the protein expression of mutated IGF1R, assessed the effect of the endoplasmic reticulum-associated degradation (ERAD) pathway on mutated IGF1R, assessed the dominant-negative effect of IGF1R and performed quantitative RT-PCR analysis of IGF1R mRNA expression in whole blood cells.

RESULTS

Two novel heterozygous nonsense mutations (case 1: p.Q1250X and case 2: p.W1249X) were identified. Although these mutations did not affect blood IGF1R mRNA levels, they significantly decreased the expression of IGF1R protein in transiently transfected cells. Treatment with the proteasome inhibitor MG132 showed significantly increased IGF1R protein.

CONCLUSIONS

Heterozygous nonsense mutations affecting the C-terminal region (p.Q1250X, p.W1249X) of IGF1R decreased the expression of IGF1R through the ERAD pathway. Our study revealed the importance of the C-terminal region and the dosage of this receptor for growth.

Something old, something new and something borrowed: emerging paradigm of insulin-like growth factor type 1 receptor (IGF-1R) signaling regulation

The insulin-like growth factor type 1 receptor (IGF-1R) plays a key role in the development and progression of cancer; however, therapeutics targeting it have had disappointing results in the clinic. As a receptor tyrosine kinase (RTK), IGF-1R is traditionally described as an ON/OFF system, with ligand stabilizing the ON state and exclusive kinase-dependent signaling activation. Newly added to the traditional model, ubiquitin-mediated receptor downregulation and degradation was originally described as a response to ligand/receptor interaction and thus inseparable from kinase signaling activation. Yet, the classical model has proven over-simplified and insufficient to explain experimental evidence accumulated over the last decade, including kinase-independent signaling, unbalanced signaling, or dissociation between signaling and receptor downregulation. Based on the recent findings that IGF-1R “borrows” components of G-protein coupled receptor (GPCR) signaling, including β-arrestins and G-protein-related kinases, we discuss the emerging paradigm for the IGF-1R as a functional RTK/GPCR hybrid, which integrates the kinase signaling with the IGF-1R canonical GPCR characteristics. The contradictions to the classical IGF-1R signaling concept as well as the design of anti-IGF-1R therapeutics treatment are considered in the light of this paradigm shift and we advocate recognition of IGF-1R as a valid target for cancer treatment.

Novel missense mutation in the IGF-I receptor L2 domain results in intrauterine and postnatal growth retardation

BACKGROUND

IGFs play key roles in intrauterine and postnatal growth through the IGF-I receptor (IGF-IR). We identified a family bearing a new heterozygous missense mutation at the L2 domain of IGF-IR (R431L).

METHOD

We analysed the nucleotide sequences of the IGF1R gene of the family. We prepared R(-) cells (fibroblasts with targeted disruption of the IGF-IR gene) expressing wild-type or R431L IGF-IR and performed functional analyses by evaluating IGF-I binding, IGF-I-stimulated DNA synthesis, tyrosine phosphorylation of IGF-IR and its substrates, and internalization by measuring [(125) I]IGF-I internalization. We also performed confocal microscopy analysis.

RESULTS

We identified a family bearing a new heterozygous missense mutation at the L2 domain of IGF-IR (R431L) through an 8-year-old girl and her mother, both born with intrauterine growth retardation. In experiments conducted using cells homozygously transfected with the IGF-IR R431L mutation; (i) IGF-I binding was not affected; (ii) DNA synthesis induced by IGF-I was decreased; (iii) IGF-IR internalization stimulated by IGF-I was decreased and (iv) IGF-I-stimulated tyrosine phosphorylation was reduced IGF-IR by low concentrations of IGF-I and on insulin receptor substrate (IRS)-1 and IRS-2.

CONCLUSION

A missense mutation (R431L) leads to the inhibition of cell proliferation, attenuation of IGF signalling and decrease in internalization of IGF-IR. The results of this study suggest a novel link between a mutation at the IGF-IR L2 domain and intrauterine and postnatal growth retardation.

Serine phosphorylation of the insulin-like growth factor I (IGF-1) receptor C-terminal tail restrains kinase activity and cell growth

Insulin-like growth factor I receptor (IGF-1R) signaling is essential for cell, organ, and animal growth. The C-terminal tail of the IGF-1R exhibits regulatory function, but the mechanism is unknown. Here, we show that mutation of Ser-1248 (S1248A) enhances IGF-1R in vitro kinase activity, autophosphorylation, Akt/mammalian target of rapamycin activity, and cell growth. Ser-1248 phosphorylation is mediated by GSK-3β in a mechanism that involves a priming phosphorylation on Ser-1252. GSK-3β knock-out cells exhibit reduced IGF-1R cell surface expression, enhanced IGF-1R kinase activity, and signaling. Examination of crystallographic structures of the IGF-1R kinase domain revealed that the (1248)SFYYS(1252) motif adopts a conformation tightly packed against the kinase C-lobe when Ser-1248 is in the unphosphorylated state that favors kinase activity. S1248A mutation is predicted to lock the motif in this position. In contrast, phosphorylation of Ser-1248 will drive profound structural transition of the sequence, critically affecting connection of the C terminus as well as exposing potential protein docking sites. Decreased kinase activity of a phosphomimetic S1248E mutant and enhanced kinase activity in mutants of its predicted target residue Lys-1081 support this auto-inhibitory model. Thus, the SFYYS motif controls the organization of the IGF-1R C terminus relative to the kinase domain. Its phosphorylation by GSK-3β restrains kinase activity and regulates receptor trafficking and signaling.

14-3-3ζ as a prognostic marker and therapeutic target for cancer

IMPORTANCE OF THE FIELD

The ubiquitously expressed 14-3-3ζ protein is involved in numerous important cellular pathways involved in cancer. Recent research suggests 14-3-3ζ may play a central role regulating multiple pathways responsible for cancer initiation and progression. This review will provide an overview of 14-3-3 proteins and address the role of 14-3-3ζ overexpression in cancer.

AREAS COVERED IN THIS REVIEW

The review covers the basic role of 14-3-3 in regulation of multiple pathways with a focus on 14-3-3ζ as a clinically relevant biomarker for cancer recurrence.

WHAT THE READER WILL GAIN

14-3-3ζ overexpression has been found in multiple cancers; however, the clinical implications were unclear. Recently, 14-3-3ζ has been identified as a biomarker for poor prognosis and chemoresistance in multiple tumor types, indicating a potential clinical application for using 14-3-3ζ in selecting treatment options and predicting cancer patients' outcome.

TAKE HOME MESSAGE

14-3-3ζ is a potential prognostic marker of cancer recurrence and predictive marker for therapeutic resistance. The overexpression of 14-3-3ζ in multiple cancers suggests that it may be a common target to intervene tumor progression; therefore, more efforts are needed for the development of 14-3-3 inhibitors.

A candidate targeting molecule of insulin-like growth factor-I receptor for gastrointestinal cancers

Advances in molecular research in cancer have brought new therapeutic strategies into clinical usage. One new group of targets is tyrosine kinase receptors, which can be treated by several strategies, including small molecule tyrosine kinase inhibitors (TKIs) and monoclonal antibodies (mAbs). Aberrant activation of growth factors/receptors and their signal pathways are required for malignant transformation and progression in gastrointestinal (GI) carcinomas. The concept of targeting specific carcinogenic receptors has been validated by successful clinical application of many new drugs. Type I insulin-like growth factor (IGF) receptor (IGF-IR) signaling potently stimulates tumor progression and cellular differentiation, and is a promising new molecular target in human malignancies. In this review, we focus on this promising therapeutic target, IGF-IR. The IGF/IGF-IR axis is an important modifier of tumor cell proliferation, survival, growth, and treatment sensitivity in many malignant diseases, including human GI cancers. Preclinical studies demonstrated that downregulation of IGF-IR signals reversed the neoplastic phenotype and sensitized cells to anticancer treatments. These results were mainly obtained through our strategy of adenoviruses expressing dominant negative IGF-IR (IGF-IR/dn) against gastrointestinal cancers, including esophagus, stomach, colon, and pancreas. We also summarize a variety of strategies to interrupt the IGFs/IGF-IR axis and their preclinical experiences. Several mAbs and TKIs targeting IGF-IR have entered clinical trials, and early results have suggested that these agents have generally acceptable safety profiles as single agents. We summarize the advantages and disadvantages of each strategy and discuss the merits/demerits of dual targeting of IGF-IR and other growth factor receptors, including Her2 and the insulin receptor, as well as other alternatives and possible drug combinations. Thus, IGF-IR might be a candidate for a molecular therapeutic target in human GI carcinomas.

IGF-I activates caspases 3/7, 8 and 9 but does not induce cell death in colorectal cancer cells

BACKGROUND

Colorectal cancer is the third most common cancer in the western world. Chemotherapy is often ineffective to treat the advanced colorectal cancers due to the chemo-resistance. A major contributor to chemo-resistance is tumour-derived inhibition or avoidance of apoptosis. Insulin-like growth factor I (IGF-I) has been known to play a prominent role in colorectal cancer development and progression. The role of IGF-I in cancer cell apoptosis is not completely understood.

METHODS

Using three colorectal cancer cell lines and one muscle cell line, associations between IGF-I and activities of caspase 3/7, 8 and 9 have been examined; the role of insulin-like growth factor I receptor (IGF-IR) in the caspase activation has been investigated.

RESULTS

The results show that exogenous IGF-I significantly increases activity of caspases 3/7, 8 and 9 in all cell lines used; blocking IGF-I receptor reduce IGF-I-induced caspase activation. Further studies demonstrate that IGF-I induced caspase activation does not result in cell death. This is the first report to show that while IGF-I activates caspases 3/7, 8 and 9 it does not cause colorectal cancer cell death.

CONCLUSION

The study suggests that caspase activation is not synonymous with apoptosis and that activation of caspases may not necessarily induce cell death.

Modulation of caveolin-1 expression can affect signalling through the phosphatidylinositol 3-kinase/Akt pathway and cellular proliferation in response to insulin-like growth factor I

The IGFs mediate their effects on cell function through the type I IGF receptor and numerous intracellular signalling molecules, including the phosphatidylinositol 3-kinase (PI-3K)/Akt pathway. The type I IGF receptor also binds to the caveolae protein caveolin-1, but the impact of caveolae on IGF/PI-3K/Akt signalling remains controversial. We have examined the effect of complete (knockout) and partial (knockdown) caveolin-1 deficiency on cellular IGF effects mediated via the PI-3K/Akt pathway. Under basal conditions, caveolin-1-deficient mouse embryonic fibroblast cells [MF(-/-)] incorporated significantly more [3H]thymidine than wild-type mouse embryonic fibroblast cells [MF(+/+)]; however, small hairpin RNA-mediated knockdown of caveolin-1 (80% reduction) in 3T3L1 fibroblasts had no effect on basal proliferation. Interestingly, IGF-I induced proliferation was similar in MF(-/-) and MF(+/+) cells, whereas caveolin-1 knockdown promoted a hyperproliferative response to IGF-I [pkDCav3T3L1(80) 12.4+/-0.4-fold; pkDShuffle3T3L1 4.3+/-0.2-fold induction; P<0.01]. Immunoblot analysis showed that caveolin-1 knockdown had no affect on Akt expression or activation. However, in MF(-/-) cells, IGF-I-stimulated phosphorylation of Akt was reduced despite up-regulated Akt levels. Further investigation demonstrated that caveolin knockout up-regulated Akt-2 and Akt-3 isoform expression, but Akt-1 expression was down-regulated; interestingly, coimmunoprecipitation studies revealed Akt-1 as the predominant isoform to be phosphorylated in response to IGF-I. In summary, caveolin-1 deficiency promotes a hyperproliferative response to IGF-I that is unrelated to Akt expression/activation. However, cells that lack caveolin are able to respond appropriately to IGF-I through compensatory changes in Akt isoform expression. These data posit caveolin-1 as a component of the IGF/PI-3K/Akt signalling modulus regulating cellular proliferation with implications for diseases, including cancers, which have altered caveolin expression.

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.

INSULIN‐LIKE GROWTH FACTOR‐I RECEPTOR AS A CANDIDATE FOR A NOVEL MOLECULAR TARGET IN GASTROINTESTINAL CANCERS

Abnormal activation of growth factor receptors and their signal pathways are required for neoplastic transformation and tumor progression. The concept of targeting specific tumorigenic receptors has been validated by successful clinical application of multiple new drugs, such as those acting against HER2/neu, epidermal growth factor receptor 1, and c‐Kit. In this review, we focus on the next promising therapeutic molecular target of insulin‐like growth factor (IGF)‐I receptor (IGF‐Ir). The IGF/IGF‐Ir system is an important modifier of cancer cell proliferation, survival, growth, and treatment sensitivity in a number of neoplastic diseases, including human gastrointestinal carcinomas. Preclinical studies demonstrated that downregulation of IGF‐Ir signals reversed the neoplastic phenotype and sensitized cells to antitumor treatments. We summarize a variety of ways to disrupt IGF‐Ir function. Then, we introduce our strategy of adenoviruses expressing dominant negative of IGF‐Ir (IGF‐Ir/dn) against gastrointestinal cancers, including stomach, colon, and pancreas. IGF‐Ir/dn suppresses tumorigenicity both in vitro and in vivo and increases stressor‐induced apoptosis. IGF‐Ir/dn expression upregulates chemotherapy‐induced apoptosis and these combination therapies with chemotherapy are very effective against tumors in mice. Some drugs blocking IGF‐Ir function are now entering clinical trial, thus IGF‐Ir might be a candidate for a therapeutic target in several gastrointestinal malignancies.

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.

Cellular stress-related protein expression in Helicobacter pylori-infected gastric epithelial AGS cells

Helicobacter pylori infection leads to gastroduodenal inflammation, peptic ulceration, and gastric carcinoma. Moreover, H. pylori may induce disease-specific protein expression in gastric epithelial cells. The present study was aimed at determining differentially expressed proteins in H. pylori-infected gastric epithelial AGS cells. AGS cells were treated with H. pylori at a bacterium/cell ratio of 300:1 for 12 h. Altered protein patterns as separated by two-dimensional electrophoresis using pH gradients of 4-7 were conclusively identified by matrix-assisted laser desorption/ionization-time of flight mass spectrometry (MALDI-TOF MS) analysis of the peptide digests. Four differentially expressed proteins, whose expression levels were increased by more than two-fold in H. pylori-infected cells, were analyzed. These proteins (14-3-3 protein alpha/beta, cullin homolog 3, alpha-enolase, ezrin) are known to be related to cell proliferation, cell adhesion, and carcinogenesis, and may be mediated by cellular stress, such as reactive oxygen species. In conclusion, the identification of these differentially expressed proteins provide valuable information for the understanding of the pathophysiologic mechanisms of H. pylori-induced gastric diseases, and may be useful as prognostic indices of H. pylori-related gastric disorders.

24p3 in differentiation of myeloid cells

24p3 is a secreted lipocalin that has been variously related to apoptosis, proliferation, and the neutrophil lineage of blood cells. We have investigated the expression of 24p3 mRNA and protein in myeloid cell lines induced to differentiate by insulin-like growth factor 1 (IGF-1) and the granulocytic-colony simulating factor (G-CSF). Both these growth factors, which cause myeloid cells to differentiate into granulocytes, induced a marked increase in the expression of both 24p3 protein and mRNA. The mRNA especially appeared early after the cells were induced with either IGF-1 or G-CSF, at a time when the cells were still proliferating and are morphologically undifferentiated. 24p3 can be considered an early marker of granulocytic differentiation.

Paraptosis: mediation by MAP kinases and inhibition by AIP-1/Alix

Programmed cell death (pcd) may take the form of apoptotic or nonapoptotic pcd. Whereas cysteine aspartyl-specific proteases (caspases) mediate apoptosis, the mediators of nonapoptotic cell death programs are much less well characterized. Here, we report that paraptosis, an alternative, nonapoptotic cell death program that may be induced by the insulin-like growth factor I receptor (among other inducers), is mediated by mitogen-activated protein kinases (MAPKs) and inhibited by AIP-1/Alix. The inhibition by AIP-1/Alix is specific for paraptosis since apoptosis was not inhibited. Caspases were not activated in this paradigm, nor were caspase inhibitors effective in blocking cell death. However, insulin-like growth factor I receptor (IGFIR)-induced paraptosis was inhibited by MEK-2-specific inhibitors and by antisense oligonucleotides directed against c-jun N-terminal kinase-1 (JNK-1). These results suggest that IGFIR-induced paraptosis is mediated by MAPKs, and inhibited by AIP-1/Alix.

Interaction of 14-3-3 proteins with the insulin-like growth factor I receptor (IGFIR): evidence for a role of 14-3-3 proteins in IGFIR signaling

We have extended our previous yeast two-hybrid findings to show that 14-3-3beta also interacts with the insulin-like growth factor I receptor (IGFIR) in mammalian cells overexpressing both proteins and that the interaction involves serine 1283 and is dependent on receptor activation. Treatment of cells with the phorbol ester PMA stimulates the interaction of 14-3-3beta with the IGFIR in the absence of receptor tyrosine phosphorylation, suggesting that receptor activation leads to activation of an endogenous protein kinase that catalyzes the phosphorylation of serine 1283. To investigate the role of 14-3-3 proteins in IGF signal transduction, IGFIR structure-function studies were performed. Mutation of serine 1283 alone (S1283A) (a mutation that decreases but does not abolish the interaction of the IGFIR with 14-3-3) did not affect anchorage-independent growth of NIH 3T3 fibroblasts overexpressing the mutant receptor. However, the simultaneous mutation of this residue and the truncation of the C-terminal 27 residues of the receptor (Delta1310/S1283A) abolished the interaction of the receptor with 14-3-3 and reversed the enhanced colony formation observed with the IGFIR truncation mutation alone (Delta1310). The difference between the Delta1310 and Delta1310/S1283A transfectants in the soft agar assay was confirmed by tumorigenesis experiments. These findings suggest that 14-3-3 proteins interact with the IGFIR in vivo and that this interaction may play a role in a transformation pathway signaled by the IGFIR.

Insulin-like growth factor-1 (IGF-1) and IGF-1 receptor (IGF-1R) expression in human lung in RDS and BPD

We hypothesize that IGF-1 and IGF-1R proteins are upregulated in lung epithelia and fibroblasts in RDS compared to normal development, and are further upregulated in BPD. We used immunohistochemistry to evaluate IGF-1 and IGF-R expression in lungs from autopsies of human stillbirths and RDS and BPD patients. IGF-1 and IGF-R immunostaining were present in fetal, RDS, and BPD lungs. In RDS, IGF-1 was present in alveolar epithelium and prominent in columnar and cuboidal airway epithelia. In BPD lungs, immunostaining was intensely increased in both airway and alveolar epithelia and in mesenchyme. The immunostaining index in bronchial epithelial cells and peribronchial myofibroblasts was significantly higher in BPD compared to RDS. IGF-1R expression was minimal in fetal lung and found mainly in mesenchyme. IGF-1R was increased in mesenchyme in RDS. In BPD it was especially increased in peribronchial and perialveolar mesenchyme. Immunostaining index for IGF-1R in epithelial cells and peribronchial myofibroblasts was increased in BPD compared to RDS. IGF-1 and IGF-R expression is low during fetal development, but is acutely upregulated in RDS, and persists with further upregulation in BPD.

Increased expression of the mannose 6-phosphate/insulin-like growth factor-II receptor in breast cancer cells alters tumorigenic properties in vitro and in vivo

The mannose 6-phosphate/insulin-like growth factor-II receptor (M6P/IGF-IIR) is thought to act as a suppressor of tumor growth by binding the mitogenic peptide IGF-II and modulating its extracellular levels via degradation. This receptor has been found to be absent or nonfunctional in a high proportion of breast tumors as a result of LOH and mutation of the gene. In our study, we have examined the effect of increasing expression of M6P/IGF-IIR on breast cancer cell tumorigenicity. MDA-MB-231 breast cancer cells stably transfected with M6P/IGF-IIR cDNA exhibited not only a greatly reduced ability to form tumors but also a markedly reduced growth rate in nude mice. In vitro, increased M6P/IGF-IIR expression resulted in 2-fold reduced uptake of IGF-II and was associated with reduced cellular invasiness and motility. Cells with increased M6P/IGF-IIR expression exhibited reduced phosphorylation of IGF-I receptor and p44/42 MAPK compared to vector transfectants, or wild-type MDA-MB-231 cells. These results therefore suggest that M6P/IGF-IIR levels can modulate breast cancer cell tumorigenicity by a mechanism that may involve altered IGF-I receptor signaling.

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.

IGF-I elicits growth of human intestinal smooth muscle cells by activation of PI3K, PDK-1, and p70S6 kinase

Endogenous IGF-I regulates growth of human intestinal smooth muscle cells by jointly activating phosphatidylinositol 3-kinase (PI3K) and ERK1/2. The 70-kDa ribosomal S6 kinase (p70S6 kinase) is a key regulator of cell growth activated by several independently regulated kinases. The present study characterized the role of p70S6 kinase in IGF-I-induced growth of human intestinal smooth muscle cells and identified the mechanisms of p70S6 kinase activation. IGF-I-induced growth elicited via either the PI3K or ERK1/2 pathway required activation of p70S6 kinase. IGF-I elicited concentration-dependent activation of PI3K, 3-phosphoinositide-dependent kinase-1 (PDK-1), and p70S6 kinase that was sequential and followed similar time courses. IGF-I caused time-dependent and concentration-dependent phosphorylation of p70S6 kinase on Thr(421)/Ser(424), Thr(389), and Thr(229) that paralleled p70S6 kinase activation. p70S6 kinase(Thr(421)/Ser(424)) phosphorylation was PI3K dependent and PDK-1 independent, whereas p70S6 kinase(Thr(389)) and p70S6 kinase(Thr(229)) phosphorylation and p70S6 kinase activation were PI3K dependent and PDK-1 dependent. IGF-I elicited sequential Akt(Ser(308)), Akt(Ser(473)), and mammalian target of rapamycin(Ser(2448)) phosphorylation; however, transfection of muscle cells with kinase-inactive Akt1(K179M) showed that these events were not required for IGF-I to activate p70S6 kinase and stimulate proliferation of human intestinal muscle cells.

A novel view on the mechanisms of action of insulin and other insulin superfamily peptides: involvement of adenylyl cyclase signaling system

A new signaling mechanism common to mammalian insulin, insulin-like growth factor I, relaxin and mollusc insulin-like peptide, and involving receptor-tyrosine kinase==>G(i) protein (betagamma)==>phosphatidylinositol-3-kinase==>protein kinase Czeta==>adenylyl cyclase==>protein kinase A was discovered in the muscles and some other tissues of vertebrates and invertebrates. The authors' data were used to reconsider the problem of participation of the adenylyl cyclase-cAMP system in the regulatory effects of insulin superfamily peptides. A hypothesis has been put forward according to which the adenylyl cyclase signaling mechanism producing cAMP has a triple co-ordinating role in the regulatory action of insulin superfamily peptides on the main cell processes, inducing the mitogenic and antiapoptotic effects and inhibitory influence on some metabolic effects of the peptides. It is suggested that cAMP is a key regulator responsible for choosing the transduction pathway by concerted launching of one (proliferative) program and switching off (suppression) of two others, which lead to cell death and to the predomination of anabolic processes in a cell. The original data obtained give grounds to conclude that the adenylyl cyclase signaling system is a mechanism of signal transduction not only of hormones with serpentine receptors, but also of those with receptors of the tyrosine kinase type (insulin superfamily peptides and some growth factors).

14-3-3 binding to the IGF-1 receptor is mediated by serine autophosphorylation

The phosphoserine-binding 14-3-3 proteins have been implicated in playing a role in mitogenic and apoptotic signaling pathways. Binding of 14-3-3 proteins to phosphoserine residues in the C-terminus of the insulin-like growth factor-1 receptor (IGF-1R) has been described to occur in a variety of cell systems, but the kinase responsible for this serine phosphorylation has not been identified yet. Here we present evidence that the isolated dimeric insulin-like growth factor-1 receptor kinase domain (IGFKD) contains a dual specific (i.e. tyrosine/serine) kinase activity that mediates autophosphorylation of C-terminal serine residues in the enzyme. From the total phosphate incorporation of approximately 4 mol per mol kinase subunit, 1 mol accounts for serine phosphate. However, tyrosine autophosphorylation proceeds more rapidly than autophosphorylation of serine residues (t(1/2) approximately 1 min vs. t(1/2) approximately 5 min). Moreover, dot-blot and far-Western analyses reveal that serine autophosphorylation of IGFKD is sufficient to promote binding of 14-3-3 proteins in vitro. The proof that dual kinase activity of IGFKD is necessary and sufficient for 14-3-3 binding was obtained with an inactive kinase mutant that was phosphorylated on serine residues in a stoichiometric reaction with the catalytically active enzyme. Thus, the IGF-1R itself might be responsible for the serine autophosphorylation which leads to recognition of 14-3-3 proteins in vivo.

Microarray analysis of insulin and insulin-like growth factor-1 (IGF-1) receptor signaling reveals the selective up-regulation of the mitogen heparin-binding EGF-like growth factor by IGF-1

Insulin and insulin-like growth factor-1 (IGF-1) act through highly homologous receptors that engage similar intracellular signaling pathways, yet these hormones serve largely distinct physiological roles in the control of metabolism and growth, respectively. In an attempt to uncover the molecular mechanisms underlying their divergent functions, we compared insulin receptor (IR) and IGF-1 receptor (IGF-1R) regulation of gene expression by microarray analysis, using 3T3-L1 cells expressing either TrkC/IR or TrkC/IGF-1R chimeric receptors to ensure the highly selective activation of each receptor tyrosine kinase. Following stimulation of the chimeric receptors for 4 h, we detected 11 genes to be differentially regulated, of which 10 were up-regulated to a greater extent by the IGF-1R. These included genes involved in adhesion, transcription, transport, and proliferation. The expression of mRNA encoding heparin-binding epidermal growth factor-like growth factor (HB-EGF), a potent mitogen, was markedly increased by IGF-1R but not IR activation. This effect was dependent on MAPK, but not phosphatidylinositol 3-kinase, and did not require an autocrine loop through the epidermal growth factor receptor. HB-EGF mitogenic activity was detectable in the medium of 3T3-L1 preadipocytes expressing activated IGF-1R but not IR, indicating that the transcriptional response is accompanied by a parallel increase in mature HB-EGF protein. The differential abilities of the IR and IGF-1R tyrosine kinases to stimulate the synthesis and release of a growth factor may provide, at least in part, an explanation for the greater role of the IGF-1R in the control of cellular proliferation.

Effects of genetic blockade of the insulin-like growth factor receptor in human colon cancer cell lines

BACKGROUND & AIMS

Insulin-like growth factor (IGF)-I receptor (IGF-Ir) signaling is required for maintenance of growth and tumorigenicity of several tumor types. We have previously shown successful therapy in a lung cancer xenograft model using an adenovirus expressing antisense IGF-Ir. In this study, we sought to better dissect the mechanism and develop potentially more effective IGF-Ir-targeted therapeutics by developing and testing tetracycline-regulated and recombinant adenoviruses expressing dominant negative receptors.

METHODS

Truncated IGF-I receptors (IGF-Ir/tf; 482 and 950 amino acids long, respectively [IGF-Ir/482st and IGF-Ir/950st]) were cloned into tetracycline-regulated vectors and recombinant adenoviruses and then studied in colorectal cancer cells. We assessed the effect of IGF-Ir/tf on signaling blockade, colony formation, stress response (serum starvation and heat), chemotherapy-induced apoptosis, and in vivo therapeutic efficacy in xenografts.

RESULTS

Activation of IGF-Ir/tf expression by withdrawal of tetracycline suppressed tumorigenicity both in vitro and in vivo and up-regulated stressor-induced apoptosis. It effectively blocked both IGF-I- and IGF-II-induced activation of Akt-1. IGF-Ir/tf expression increased chemotherapy-induced apoptosis, and this combination therapy was very effective against tumors in mice. These findings were confirmed in a therapy model against established tumors using adenoviruses expressing IGF-Ir/tf. Moreover, IGF-Ir/482st was more effective than IGF-Ir/950st because of its bystander effect.

CONCLUSIONS

Anti-tumor activity of IGF-Ir/tf is mediated through inhibition of Akt-1 and enhances the efficacy of chemotherapy. Adenovirus IGF-Ir/482st may be a useful anticancer therapeutic for colorectal carcinoma.

Zebrafish insulin-like growth factor-I receptor: molecular cloning and developmental expression

The biological actions of the insulin-like growth factors (IGFs) are mediated primarily by the IGF-I receptor (IGF-IR), and the IGF family has been highly conserved throughout vertebrate evolution. In this study we report the isolation of a 3 kb cDNA clone for the zebrafish IGF-IR that includes the complete 3' untranslated region and polyA tail and mapping of the receptor gene to zebrafish linkage group 7. The open reading frame deduced from the cDNA sequence encompasses the juxtamembrane and protein tyrosine kinase portions of the receptor, and is 70 and 67% identical to the corresponding regions of the IGF-IRs of the turbot and Xenopus, respectively. By RT-PCR, zebrafish IGF-IR expression was detected from early blastula to early larval stages of development. Using whole mount in situ hybridization, IGF-IR expression was detected after gastrulation. Expression was evident in most tissues but was particularly evident in the tail, in eye and ear primordia and in the brain.

Anchorage-independent growth of fibroblasts that express a truncated IGF-I receptor

The purpose of this investigation was to study signaling by an insulin-like growth factor I receptor (IGF-I R) that lacks the extracellular portion of the receptor. We transfected IGF-I R-negative mouse embryo fibroblasts with a truncated IGF-I R consisting of only the transmembrane and cytoplasmic part of the beta subunit. Proliferation as assessed by counting cells was the same for vector only transfectants and the truncated receptor transfectants in defined medium containing EGF and PDGF. In contrast, anchorage-independent growth as measured by colony formation in soft agar was markedly increased for the truncated IGF-I R transfectants compared to the vector transfectants. MAP-kinase activity in the truncated IGF-I R transfectants was not higher than in the vector transfectants; however, PI 3-kinase activity was significantly higher in the IGF-I R transfectants. These results provide evidence that an IGF-I receptor consisting of only the transmembrane and cytoplasmic domain of the beta subunit can signal pathways leading to anchorage-independent growth.

Characterization of insulin-like growth factor I (IGF-I) receptor mutants for their effects on IGF-I- and interleukin 4-mediated DNA synthesis of 32D cells

Recently we demonstrated that overexpression of the wild type insulin-like growth factor I receptor (IGF-IRWT) in 32D myeloid progenitor cells led to cell proliferation in response to interleukin 4 (IL-4) as well as insulin-like growth factor I (IGF-I) in the absence of insulin receptor substrate expression (Soon, L., Flechner, L., Gutkind, J. S., Wang, L. H., Baserga, R., Pierce, J. H., and Li, W. (1999) Mol. Cell. Biol. 19, 3816-3828). To understand the structural importance of insulin-like growth factor I receptor (IGF-IR) in mediating IL-4- and IGF-I-induced DNA synthesis, we transfected various mutants of IGF-IR to 32D cells. Our results show that most mutants, including Y1250F, Y1251F, Y1250F/Y1251F, S1280A/S1281A/S1282A/S1283A, Y1316F, and 1245d, still retained mitogenic response toward IGF-I or IL-4. However, the Y950F, Y1131F, and Y1135F mutants were not able to respond to either ligand. The H1293F/K1294R and 1293d mutants reduced response toward IGF-I but not to IL-4. Phosphorylation of Shc was greatly reduced in those three mutants that lost mitogenic response. The MAPK activity was much lower in Y1131F and Y1135F mutants, indicating the importance of the Shc/MAPK pathway in IGF-I-induced mitogenesis. Importantly, the synergistic effect of these two factors on DNA synthesis was not affected in cells expressing most of the mutants, even in those three that had lower mitogenic response toward a single ligand. These results suggest that an unidentified pathway(s) may be induced upon co-addition of IGF-I and IL-4 that sustains the intact mitogenesis.

Molecular biology and cellular physiology of refractoriness to androgen ablation therapy in advanced prostate cancer

We review the extensive body of data on the molecular aetiology of hormone refractory disease in metastatic prostate cancer patients. Particular emphasis is placed on the crucial role of the bone micro-environment, especially the intercellular interactions of metastatic prostate cancer cells and osteoblasts in promoting the establishment of hormone refractory disease. Resistance of tumour cells to anticancer therapies is generally viewed as a phenomenon almost exclusively determined by chromosomal defects and/or gene mutations. However, it is now well-documented that the local milieu of the bone metastases can also protect tumour cells from anticancer therapy- induced apoptosis, either independently or synergistically with resistance-related genetic alterations. A key determinant of this protection is the urokinase/plasmin cascade which modulates the local concentration of survival factors, such as insulin-like growth factor (IGF-1). The molecular pathways whereby this major growth and survival factor for prostate cancer cells exerts its anti-apoptotic effect on prostate cancer cells are discussed.

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.

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.

Inhibition of the type I insulin-like growth factor receptor expression and signaling: novel strategies for antimetastatic therapy

The receptor for the type 1 insulin-like growth factor (IGF-1R) plays a critical role in the acquisition of the malignant phenotype. Using a highly metastatic murine lung carcinoma model, it was demonstrated that this receptor regulates several cellular functions that can impact on the metastatic potential of the cells, including cellular proliferation, anchorage-independent growth, cell migration, and invasion. The tumor model was used to develop several strategies for altering receptor expression and function as means of abrogating the metastatic potential of the cells. They include stable expression in the tumor cells of IGF-1R antisense RNA and dominant negative receptor mutants in which tyrosines in the kinase domain were substituted with phenylalanine. In addition, a novel strategy was used based on altering post ligand-binding receptor turnover. This led to inhibition of receptor re-expression and signaling and resulted in increased tumor cell apoptosis. When combined with the development of viral vectors designed to deliver genetic information with high efficiency, these strategies could form the basis for development of highly specific, antimetastatic therapy in tumors with known IGF-IR involvement.

14-3-3 proteins: structure, function, and regulation

The 14-3-3 proteins are a family of conserved regulatory molecules expressed in all eukaryotic cells. A striking feature of the 14-3-3 proteins is their ability to bind a multitude of functionally diverse signaling proteins, including kinases, phosphatases, and transmembrane receptors. This plethora of interacting proteins allows 14-3-3 to play important roles in a wide range of vital regulatory processes, such as mitogenic signal transduction, apoptotic cell death, and cell cycle control. In this review, we examine the structural basis for 14-3-3-ligand interactions, proposed functions of 14-3-3 in various signaling pathways, and emerging views of mechanisms that regulate 14-3-3 actions.

IGF-I receptor activation and BCL-2 overexpression prevent early apoptotic events in human neuroblastoma

The type I insulin-like growth factor receptor (IGF-IR) is important for mitogenesis, transformation, and survival of tumor cells. The current study examines the effect of IGF-IR expression and activation on apoptosis in SHEP human neuroblastoma cells. SHEP cells undergo apoptosis which is prevented by IGF-I addition or overexpression of the IGF-IR (SHEP/IGF-IR cells). High mannitol treatment activates caspase-3 by 1 h in SHEP cells while caspase-3 activation is delayed by 3 h in SHEP/IGF-IR cells. Transfection with Bcl-2 (SHEP/Bcl-2 cells) prevents serum withdrawal and mannitol induced apoptosis and caspase-3 activation. Mannitol induces mitochondrial membrane depolarization in both SHEP and SHEP/IGF-IR cells. IGF-IR activation or overexpression of Bcl-2 in SHEP cells prevents mitochondrial membrane depolarization. Collectively, these results suggest that IGF-IR or Bcl-2 overexpression in neuroblastoma cells promotes cell survival by preventing mitochondrial membrane depolarization and caspase-3 activation, ultimately leading to increased tumor growth.

14-3-3 proteins and growth control

The 14-3-3 proteins constitute a family that is highly conserved in a wide range of organisms, including higher eukaryotes, invertebrates and plants. Variants of 14-3-3 proteins assembled in homo- and heterodimers were found to interact with diverse cellular proteins. Until recently, the biological role of 14-3-3 members was still poorly understood. However, the results of an increasing number of studies on their structure and function are converging to define 14-3-3 proteins as a novel type of adaptor that modulates interactions between components involved in signal transduction pathway and in cell cycle control.

The differential effects of pp120 (Ceacam 1) on the mitogenic action of insulin and insulin-like growth factor 1 are regulated by the nonconserved tyrosine 1316 in the insulin receptor

pp120 (Ceacam 1) undergoes ligand-stimulated phosphorylation by the insulin receptor, but not by the insulin-like growth factor 1 receptor (IGF-1R). This differential phosphorylation is regulated by the C terminus of the beta-subunit of the insulin receptor, the least conserved domain of the two receptors. In the present studies, deletion and site-directed mutagenesis in stably transfected hepatocytes derived from insulin receptor knockout mice (IR(-/-)) revealed that Tyr(1316), which is replaced by the nonphosphorylatable phenylalanine in IGF-1R, regulated the differential phosphorylation of pp120 by the insulin receptor. Similarly, the nonconserved Tyr(1316) residue also regulated the differential effect of pp120 on IGF-1 and insulin mitogenesis, with pp120 downregulating the growth-promoting action of insulin, but not that of IGF-1. Thus, it appears that pp120 phosphorylation by the insulin receptor is required and sufficient to mediate its downregulatory effect on the mitogenic action of insulin. Furthermore, the current studies revealed that the C terminus of the beta-subunit of the insulin receptor contains elements that suppress the mitogenic action of insulin. Because IR(-/-) hepatocytes are derived from liver, an insulin-targeted tissue, our observations have finally resolved the controversy about the role of the least-conserved domain of insulin and IGF-1Rs in mediating the difference in the mitogenic action of their ligands, with IGF-1 being more mitogenic than insulin.

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.

Insulin and insulin-like growth factor-1 receptors in an evoluted fish, the turbot: cDNA cloning and mRNA expression

The insulin receptor (IR) and the structurally related insulin-like growth factor type 1 receptor (IGF-1R) belong to the tyrosine kinase (TK) receptor family. In this study, we have carried out the molecular characterization of both receptors from turbot (Psetta maxima), an evoluted teleost flatfish. The cDNA encoding the precursors of IGF-1R and the nearly entire IR (only the first 16 amino acids of the alpha subunit are missing when compared to the published human sequence) were cloned from an embryonic cDNA library. The deduced polypeptides contain all the topological features characterizing the insulin/IGF-1 receptor family. They are highly conserved compared to their mammalian counterparts, particularly within domains involved in the catalytic activity and in the transduction pathways. Nevertheless, some differences in the primary sequences, especially in the carboxy-terminal domain of the precursors, may affect the functions fulfilled by these receptors. As in mammals, the long IGF-1R 5'-untranslated sequence contains open reading frames and potential Sp1 binding sites. Northern blot analyses have revealed a major IR transcript of 11 kb, which is approximately the size of IGF-1R transcript (Eliès, G., Groigno, L., Wolff, J., Boeuf, G., Boujard, D., 1996. Characterization of the insulin-like growth factor type 1 receptor messenger in two teleost species. Mol. Cell. Endocrinol. 124, 131-140.). If IR and IGF-1R transcripts are detected by RT-PCR at all developmental stages and in all tissues examined, in situ hybridization studies have shown that these mRNA can be visualized as ubiquitous signals only in young larvae, whereas IGF-1R and IR expression appears weaker during later development and in adult tissues.

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.

Characterization of an antibody that can detect an activated IGF-I receptor in human cancers

The type 1 insulin-like growth factor receptor (IGF-IR) plays an important role in malignant transformation and in apoptosis. Its role in human cancer has now been firmly established. IGF-IR signaling occurs only when the receptor is activated by its ligands, which induce autophosphorylation of the receptor at several tyrosine residues. Although the IGF-IR (phosphorylated or not) can be detected in human cancers with conventional antibodies, it would be desirable to obtain antibodies that can detect the IGF-IR only when activated by its ligands. We describe and characterize in this paper such an antibody and show that it can be used in sections of human cancers to detect an autophosphorylated IGF-IR. This antibody will be useful in detecting autocrine or paracrine influences on normal and tumor cells and could eventually be also useful in diagnostic and prognostic studies of human primary and metastatic cancer.

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

Programmed Cell Death (PCD) is known to play an important role in both the development and the growth rate of human tumors. It has in fact been suggested that suppression of the apoptotic pathway is a requirement for the establishment of the transformed phenotype. In order to elucidate the relationship between resistance to apoptosis and transformation, we have asked in this investigation whether or not the two processes can be directly correlated. For this purpose, we have used mouse embryo fibroblasts (MEF) expressing either the wild-type or several mutants of the type 1 insulin-like growth factor receptor (IGF-IR). The wild-type IGF-IR has both transforming and anti-apoptotic activities, and we have asked whether these two activities can be or not separated in mutant receptors. Using this well-defined system, our results show that certain mutants of the IGF-IR that have strong anti-apoptotic and mitogenic activities, are incapable of transforming MEF (colony formation in soft agar). We have, instead, a good correlation between mitogenic and anti-apoptotic activities, suggesting the possibility that the two processes may share similar signaling pathways from the IGF-IR. On the other hand, our results indicate that transformation requires an additional signal, above and beyond the mitogenic and survival signals. Our conclusion is that, at least in this system, the establishment of the malignant phenotype and resistance to apoptosis can be dissociated, implying the possibility of separate targeting.

Protein kinase C-delta is an important signaling molecule in insulin-like growth factor I receptor-mediated cell transformation

To investigate the potential role of protein kinase C-delta (PKC-delta) in insulin-like growth factor I receptor (IGF-IR)-mediated cell transformation, an oncogenic gag-IGF-IR beta-fusion receptor lacking the entire extracellular domain, which was designated NM1, and a full-length IGF-IR were coexpressed with either wild-type PKC-delta (PKC-deltaWT) or an ATP-binding mutant of PKC-delta (PKC-deltaK376R) in NIH 3T3 fibroblasts. While overexpression of PKC-deltaWT did not affect NM1- and IGF-IR-induced focus and colony formation of NIH 3T3 cells, expression of PKC-deltaK376R severely impaired these events. In contrast, NM1-mediated cell growth in monolayer was not affected by coexpressing PKC-deltaK376R. PKC-deltaWT and PKC-deltaK376R were constitutively phosphorylated on a tyrosine residue(s) in the NM1- and IGF-IR-expressing cells and were associated with them in an IGF-I-independent manner. Activated IGF-IR was able to phosphorylate purified PKC-delta in vitro and stimulated its kinase activity. Furthermore, the level of endogenous PKC-delta protein was up-regulated through transcriptional activation in response to long-term IGF-IR activation. Taken together, our results demonstrate that PKC-delta plays an important role in IGF-IR-mediated cell transformation, probably via association of the receptor with PKC-delta and its activation through protein up-regulation and tyrosine phosphorylation. Competition with endogenous PKC-delta for NM1 and IGF-IR association by PKC-deltaK376R is probably an important mechanism underlying the PKC-deltaK376R-mediated inhibition of cell transformation by NM1 and IGF-IR.

IGF-I stimulates intestinal muscle cell growth by activating distinct PI 3-kinase and MAP kinase pathways

Insulin-like growth factor I (IGF-I), acting via its cognate receptor, plays an autocrine role in the regulation of growth of intestinal muscle cells. In the present study the signaling pathways mediating the growth effects of IGF-I were characterized in cultured human intestinal smooth muscle cells. Growth induced by a maximally effective concentration of IGF-I (100 nM), measured as [3H]thymidine incorporation, was only partially inhibited by LY-294002 [phosphatidylinositol 3-kinase (PI 3-kinase) inhibitor] or PD-98059 [mitogen-activated protein (MAP) kinase kinase (MEK) inhibitor] (86 +/- 7% and 35 +/- 6% inhibition, respectively) alone but was abolished by the two combined (114 +/- 18% inhibition), implying the participation of both pathways. IGF-I elicited time- and concentration-dependent increases in PI 3-kinase activity. This effect was inhibited only by LY-294002 (89 +/- 12%). IGF-I elicited time- and concentration-dependent phosphorylation of p44/p42 MAP kinase and increased MAP kinase activity. These effects were inhibited only by PD-98059 (78 +/- 9% and 98 +/- 7%, respectively). We conclude that in human intestinal muscle cells IGF-I activates distinct PI 3-kinase and MAP kinase signaling pathways, which act in conjunction to mediate growth.

Insulin receptor substrate-1 is the predominant signaling molecule activated by insulin-like growth factor-I, insulin, and interleukin-4 in estrogen receptor-positive human breast cancer cells

Because insulin-like growth factor-I (IGF-I), insulin, and interleukin-4 (IL-4) have known biological effects in breast cancer cells and signal through insulin-receptor substrate (IRS) adaptor proteins, we examined the expression and function of IRS-1 and IRS-2 in breast tumors and cell lines. IRS-1 and IRS-2 were expressed by cell lines and primary breast tumor specimens. IGF-I, insulin, and IL-4 treatment of MCF-7 and ZR-75, and IGF-I treatment of T47-D breast cancer cells, resulted in much greater tyrosine phosphorylation of IRS-1 compared with IRS-2. Furthermore, IGF-I stimulated greater tyrosine phosphorylation of IRS-1 than either insulin or IL-4. IGF-I treatment also enhanced association of the p85 regulatory subunit of phosphatidylinositol 3-kinase with IRS-1 and stimulated increased enzymatic activity compared with IL-4 and insulin in all three cell lines. Similarly, mitogen-activated protein kinase activity was greater in IGF-I-stimulated cells. To determine the functional significance of the activation of these pathways, we inhibited activation of phosphatidylinositol 3-kinase with wortmannin and mitogen-activated protein kinase with PD098059. Both compounds inhibited IGF-stimulated growth, suggesting that both pathways contributed to the mitogenic response to IGF-I. We conclude that IRS-1, and not IRS-2, is the predominant signaling molecule activated by IGF-I, insulin, and IL-4. Furthermore, enhanced tyrosine phosphorylation of IRS-1 by IGF-I, compared with either insulin or IL-4, is associated with greater activation of mitogenic downstream signaling pathways resulting in enhanced cell growth.

Structure and function of the insulin-like growth factor I receptor

Insulin-like growth factors I and II (IGF-I, IGF-II) were originally identified as potent mitogens and as the mediators of growth hormone action. Besides being mitogenic, however, these polypeptide growth factors play a crucial role in cell survival, and contribute to transformation and to maintenance of the malignant phenotype. Here we will discuss signaling by the IGFs, focusing specifically on the structure and function of the IGF-I receptor and the domains of this receptor responsible for distinct IGF functions: mitogenesis, transformation, and protection from apoptosis. We will also compare the structural domains of the related but functionally distinct receptor for insulin.

14-3-3 proteins interact with the insulin-like growth factor receptor but not the insulin receptor

We have used a yeast two-hybrid system to identify proteins which bind to the cytosolic portion of the type 1 insulin-like growth factor (IGF) receptor (IGFIR) but not the insulin receptor (IR). This analysis identified 14-3-3beta and zeta proteins. 14-3-3beta also binds to the IGFIR but not the IR in vitro and 14-3-3-IGFIR complexes are present in insect cells overexpressing the IGFIR cytoplasmic domain. 14-3-3 proteins are substrates of the IGFIR in the yeast system and in vitro. The interaction of 14-3-3 with the IGFIR requires receptor-kinase activity and maps to the C-terminus of the receptor, but does not depend on tyrosine residues in this or the juxtamembrane regions. Instead, the binding maps to serine residue 1283 and requires phosphorylation of this residue. 14-3-3 proteins are phosphoserine-binding proteins which have been shown to interact directly with components of the mitogenic and apoptotic signalling pathways, suggesting that they participate in growth regulation. Our findings suggest that 14-3-3 proteins may play a role in IGFIR signal transduction and may contribute to the differences in IGF and IR signalling capabilities.