IGF-I receptor signalling in transformation and differentiation

Growth factors in inflammatory bowel disease: the actions and interactions of growth hormone and insulin-like growth factor-I

Multiple growth hormones (GHs) and factors are relevant to inflammatory bowel disease (IBD) due to their trophic effects on epithelial cells to promote mucosal integrity, renewal, and repair, on mesenchymal cells to promote wound healing, and on intestinal immune cells to modulate inflammation. The anabolic effects of GHs and factors outside the intestine are relevant to minimizing nutritional insufficiency, catabolic state, and the inability to maintain body weight due to inflammation-induced malabsorption. GHs and factors can, however, have a dual role, whereby trophic effects can be beneficial but, if excessive, can promote complications including the increased risk of intestinal tumors/adenocarcinoma and fibrosis. This review focuses on GH and insulin-like growth factor (IGF-I), for which evidence suggests such a dual role may exist. The actions of GH and IGF-I on the healthy intestine are compared with effects during intestinal inflammation or associated complications. Interactions between these growth factors and others relevant to IBD are considered. The role of the newly discovered suppressors of cytokine signaling proteins, which may dictate the balance between beneficial and excessive actions of GH and IGF-I, is also addressed.

Time course of molecular responses of human skeletal muscle to acute bouts of resistance exercise

Resistance exercise (RE) training, designed to induce hypertrophy, strives for optimal activation of anabolic and myogenic mechanisms to increase myofiber size. Clearly, activation of these mechanisms must precede skeletal muscle growth. Most mechanistic studies of RE have involved analysis of outcome variables after many training sessions. This study measured molecular level responses to RE on a scale of hours to establish a time course for the activation of myogenic mechanisms. Muscle biopsy samples were collected from nine subjects before and after acute bouts of RE. The response to a single bout was assessed at 12 and 24 h postexercise. Further samples were obtained 24 and 72 h after a second exercise bout. RE was induced by neuromuscular electrical stimulation to generate maximal isometric contractions in the muscle of interest. A single RE bout resulted in increased levels of mRNA for IGF binding protein-4 (84%), MyoD (83%), myogenin (approximately 3-fold), cyclin D1 (50%), and p21-Waf1 (16-fold), and a transient decrease in IGF-I mRNA (46%). A temporally conserved, significant correlation between myogenin and p21 mRNA was observed (r = 0.70, P < or = 0.02). The mRNAs for mechano-growth factor, IGF binding protein-5, and the IGF-I receptor were unchanged by RE. Total skeletal muscle RNA was increased 72 h after the second serial bout of RE. These results indicate that molecular adaptations of skeletal muscle to loading respond in a very short time. This approach should provide insights on the mechanisms that modulate adaptation to RE and may be useful in evaluating RE training protocol variables with high temporal resolution.

EWS/FLI-1 silencing and gene profiling of Ewing cells reveal downstream oncogenic pathways and a crucial role for repression of insulin-like growth factor binding protein 3

Ewing tumors are characterized by abnormal transcription factors resulting from the oncogenic fusion of EWS with members of the ETS family, most commonly FLI-1. RNA interference targeted to the junction between EWS and FLI-1 sequences was used to inactivate the EWS/FLI-1 fusion gene in Ewing cells and to explore the resulting phenotype and alteration of the gene expression profile. Loss of expression of EWS/FLI-1 resulted in the complete arrest of growth and was associated with a dramatic increase in the number of apoptotic cells. Gene profiling of Ewing cells in which the EWS/FLI-1 fusion gene had been inactivated identified downstream targets which could be grouped in two major functional clusters related to extracellular matrix structure or remodeling and regulation of signal transduction pathways. Among these targets, the insulin-like growth factor binding protein 3 gene (IGFBP-3), a major regulator of insulin-like growth factor 1 (IGF-1) proliferation and survival signaling, was strongly induced upon treating Ewing cells with EWS/FLI-1-specific small interfering RNAs. We show that EWS/FLI-1 can bind the IGFBP-3 promoter in vitro and in vivo and can repress its activity. Moreover, IGFBP-3 silencing can partially rescue the apoptotic phenotype caused by EWS/FLI-1 inactivation. Finally, IGFBP-3-induced Ewing cell apoptosis relies on both IGF-1-dependent and -independent pathways. These findings therefore identify the repression of IGFBP-3 as a key event in the development of Ewing's sarcoma.

Selective modulation of type 1 insulin-like growth factor receptor signaling and functions by beta1 integrins

We show here that β₁ integrins selectively modulate insulin-like growth factor type I receptor (IGF-IR) signaling in response to IGF stimulation. The β_1A integrin forms a complex with the IGF-IR and insulin receptor substrate-1 (IRS-1); this complex does not promote IGF-I mediated cell adhesion to laminin (LN), although it does support IGF-mediated cell proliferation. In contrast, β_1C, an integrin cytoplasmic variant, increases cell adhesion to LN in response to IGF-I and its down-regulation by a ribozyme prevents IGF-mediated adhesion to LN. Moreover, β_1C completely prevents IGF-mediated cell proliferation and tumor growth by inhibiting IGF-IR auto-phosphorylation in response to IGF-I stimulation. Evidence is provided that the β₁ cytodomain plays an important role in mediating β₁ integrin association with either IRS-1 or Grb2-associated binder1 (Gab1)/SH2-containing protein-tyrosine phosphate 2 (Shp2), downstream effectors of IGF-IR: specifically, β_1A associates with IRS-1 and β_1C with Gab1/Shp2. This study unravels a novel mechanism mediated by the integrin cytoplasmic domain that differentially regulates cell adhesion to LN and cell proliferation in response to IGF.

The expression of insulin-like growth factor-I receptor correlates with Fuhrman grading of renal cell carcinomas

Recent reports have shown significant correlation between Fuhrman nuclear grade of renal cell carcinoma (RCC) and patient survival. However, no one specific gene alteration has yet been described to account for this correlation. This study investigated the expression of the insulin-like growth factor-I receptor (IGF-IR) in RCC and correlated the results to the tumor Fuhrman nuclear grade. Formalin-fixed, paraffin-embedded sections from 68 cases of RCC were stained using the immunohistochemical avidin-biotin-peroxidase method. An anti-human IGF-IR rabbit polyclonal antibody was used. The stains were semiquantitatively evaluated using the Allred score system, assessing intensity of stain and percentage of positive tumor cells. Statistical analysis was performed using the Kruskal-Wallis test. Strong and diffuse cytoplasmic IGF-IR stain (Allred score 7 to 8) was identified in 25 of 25 (100%) of grade 3 and 4 RCCs. Grade 2 RCCs had a median IGF-IR Allred score of 4. Ten of 10 (100%) grade 1 RCCs were negative. Even in the positive high-nuclear-grade tumors, areas of low nuclear grade, when present, were IGF-IR negative. Statistical analysis using the Kruskal-Wallis test demonstrated significant correlation between increasing Fuhrman nuclear grade and increasing IGF-IR Allred score (P <0.0001). Thus we report the novel finding of significant statistical correlation between IGF-IR protein expression and Fuhrman nuclear grade of RCC, and consequentially with patient survival.

Inhibition of insulin-like growth factor I receptor autophosphorylation by novel 6-5 ring-fused compounds

The insulin-like growth factor 1 receptor (IGF1R) plays an important role in cell transformation, and it has emerged as a target for anti-cancer drug design. IGF1R is activated by autophosphorylation at three sites in the enzyme activation loop. We describe here a group of 6-5 ring-fused compounds that are the first reported inhibitors selective for the unphosphorylated (0P) form of IGF1R. These compounds do not significantly inhibit the fully activated, triply phosphorylated (3P) form. IGF1R was produced from baculovirus-infected Spodoptera frugiperda (Sf9) cells, and the 0P and 3P forms were purified to homogeneity. We used a continuous spectrophotometric assay to measure inhibition of the 0P and 3P forms. Analysis by native gel electrophoresis confirmed that the step inhibited in the autoactivation process was the transition between the 0P and IP forms of IGF1R. The compounds were also active against IGF1R autophosphorylation in intact Chinese hamster ovary (CHO) cells. Most of the compounds also inhibited the closely related insulin receptor to varying degrees, although some compounds showed selectivity for IGF1R or insulin receptor. This class of compounds could form the basis of design efforts to selectively block the autoinhibited conformation of IGF1R.

Additive regulation of hepatic gene expression by dwarfism and caloric restriction

Disrupted growth hormone/insulin-like growth factor-1 signaling (DF) and caloric restriction (CR) extend life span and delay the onset of age-related diseases in rodents. In combination, these interventions additively extend life span. To investigate the molecular basis for these effects, we performed genome-wide, microarray expression analysis of liver from homozygous and heterozygous Ames dwarf mice fed ad libitum or CR. CR and DF additively affected a group of 95 genes. Individually and together, DF and CR independently affected the expression of 212 and 77 genes, respectively. These results indicate that DF and CR affect overlapping sets of genes and additively affect a subset of genes. Together, the interventions produced changes in gene expression consistent with increased insulin, glucagon and catecholamine sensitivity, gluconeogenesis, protein turnover, lipid β-oxidation, apoptosis, and xenobiotic and oxidant metabolism; and decreased cell proliferation, lipid and cholesterol synthesis, and chaperone expression. These data suggest that the additive effects of DF and CR on life span develop from their additive effects on the level of expression of some genes and from their independent effects on other genes. These results provide a novel and focused group of genes closely associated with the regulation of life span in mammals.

Critical prosurvival roles for C/EBP beta and insulin-like growth factor I in macrophage tumor cells

One of the hallmarks of leukemic cells is their ability to proliferate and survive in the absence of exogenous growth factors (GFs). However, the molecular mechanisms used by myeloid tumor cells to escape apoptosis are not fully understood. Here we report that Myc/Raf-transformed macrophages require the transcription factor C/EBP beta to prevent cell death. In contrast to wild-type cells, C/EBP beta(-/-) macrophages were completely dependent on macrophage colony-stimulating factor or granulocyte-macrophage colony-stimulating factor for survival and displayed impaired tumorigenicity in vivo. Microarray analysis revealed that C/EBP beta-deficient cells expressed significantly reduced levels of the prosurvival factor insulin-like growth factor I (IGF-I). Overexpression of C/EBP beta stimulated transcription from the IGF-I promoter, indicating that IGF-I is a direct transcriptional target of C/EBP beta. Serological neutralization of IGF-I in C/EBP beta(+/+) tumor cell cultures induced apoptosis, showing that IGF-I functions as an autocrine survival factor in these cells. Macrophage tumor cells derived from IGF-I(-/-) mice were GF dependent, similar to C/EBP beta-deficient cells. Forced expression of either C/EBP beta or IGF-I in C/EBP beta(-/-) bone marrow cells restored Myc/Raf-induced transformation and permitted neoplastic growth without exogenous GFs. Thus, our findings demonstrate that C/EBP beta is essential for oncogenic transformation of macrophages and functions at least in part by regulating expression of the survival factor IGF-I.

The insulin-like growth factor-I (IGF-I) receptor kinase inhibitor NVP-ADW742, in combination with STI571, delineates a spectrum of dependence of small cell lung cancer on IGF-I and stem cell factor signaling

Stem cell factor (SCF)/Kit and insulin-like growth factor-I (IGF-I)/IGF-I receptor (IGF-IR) autocrine loops play a prominent role in the growth of small cell lung cancer (SCLC). Previous data suggested that IGF-I protects cells from apoptosis induced by STI571, an efficient inhibitor of Kit signal transduction, by activating the critical phosphatidylinositol 3-kinase-Akt pathway. To determine if inhibition of IGF-IR signaling would be therapeutically relevant in SCLC, the activity of a novel kinase inhibitor of IGF-IR, NVP-ADW742 (Novartis Pharma AG, Basel, Switzerland), was characterized. Pretreatment of the H526 cell line with NVP-ADW742 inhibited IGF-IR signaling and growth with IC50 values between 0.1 and 0.4 μm. SCF-mediated Kit phosphorylation and Akt activation were inhibited with IC50 values in the 1–5 μm range. However, NVP-ADW742 affected neither hepatocyte growth factor-mediated Akt activation nor activity of constitutively active Akt. The therapeutic potential of NVP-ADW742 was assessed by determining its effect on growth of several SCLC cell lines in serum. These studies clearly delineated two populations of cell lines as determined by differential sensitivity to NVP-ADW742. One population, which lacks active SCF/Kit autocrine loops, was inhibited with IC50 values between 0.1 and 0.5 μm. A second population, which has active SCF/Kit autocrine loops, was inhibited with IC50 values in the 4–7 μm range. When these cell lines were treated with a combination of STI571 and NVP-ADW742, no advantage was seen in the former group, whereas, in the latter group, a clearly synergistic response to the combination was seen when growth, apoptosis, or Akt activation was assessed. These data demonstrate that NVP-ADW742 is a potent and selective IGF-IR kinase inhibitor that can efficiently inhibit the growth of cells that are highly dependent on IGF-I signaling. However, for optimal growth inhibition of SCLC cells with an active SCF/Kit autocrine loop, a combination of a Kit inhibitor (STI571) and an IGF-IR inhibitor (NVP-ADW742) appears to be necessary. These observations suggest that, in tumors in which critical signal transduction pathways can be activated by alternative receptors, optimal therapy may require inhibition of multiple receptors.

Rapamycin sensitizes multiple myeloma cells to apoptosis induced by dexamethasone

Circumvention of chemoresistance in the B-cell neoplasm multiple myeloma (MM) might be achieved by targeting certain intracellular signaling pathways crucial for survival of the malignant clone. The use of the macrolide rapamycin, selectively inhibiting the phosphoprotein mammalian target of rapamycin (mTOR) downstream of, for example, insulin-like growth factor-I receptor (IGF-IR), possibly represents such a molecular mode of therapy. By using a panel of MM cell lines we showed that rapamycin induced G0/G1 arrest, an effect being associated with an increase of the cyclin-dependent kinase inhibitor p27 and a decrease of cyclins D2 and D3. Interestingly, in primary, mainly noncycling MM cells, rapamycin, at clinically achievable concentrations, induced apoptosis. More important, rapamycin sensitized both MM cell lines and primary MM cells to dexamethasone-induced apoptosis. This effect was associated with a decreased expression of cyclin D2 and survivin. The phosphorylation of the serine/threonine kinase p70S6K at Thr389 and Thr421/Ser424 was down-regulated by rapamycin and/or dexamethasone. Strikingly, the combinatorial treatment with rapamycin and dexamethasone suppressed the antiapoptotic effects of exogenously added IGF-I and interleukin 6 (IL-6) as well as their stimulation of p70S6K phosphorylation. The induction of apoptosis by rapamycin and dexamethasone despite the presence of survival factors was also demonstrated in primary MM cells, thus suggesting this drug combination to be active also in vivo. (Blood. 2004;103:3138-3147)

Impairment of Bone Healing by Insulin Receptor Substrate-1 Deficiency

Insulin receptor substrate-1 (IRS-1) is an essential molecule for intracellular signaling of insulin-like growth factor (IGF)-I and insulin, both of which are potent anabolic regulators of bone and cartilage metabolism. To investigate the role of IRS-1 in bone regeneration, fracture was introduced in the tibia, and its healing was compared between wild-type (WT) mice and mice lacking the IRS-1 gene (IRS-1(-/-) mice). Among 15 IRS-1(-/-) mice, 12 remained in a non-union state even at 10 weeks after the operation, whereas all 15 WT mice showed a rigid bone union at 3 weeks. This impairment was because of the suppression of callus formation with a decrease in chondrocyte proliferation and increases in hypertrophic differentiation and apoptosis. Reintroduction of IRS-1 to the IRS-1(-/-) fractured site using an adenovirus vector significantly restored the callus formation. In the culture of chondrocytes isolated from the mouse growth plate, IRS-1(-/-) chondrocytes showed less mitogenic ability and Akt phosphorylation than WT chondrocytes. An Akt inhibitor decreased the IGF-I-stimulated DNA synthesis of chondrocytes more potently in the WT culture than in the IRS-1(-/-) culture. We therefore conclude that IRS-1 deficiency impairs bone healing at least partly by inhibiting chondrocyte proliferation through the phosphatidylinositol 3-kinase/Akt pathway, and we propose that IRS-1 can be a target molecule for bone regenerative medicine.

Expression of insulin-like growth factor-I receptor in primary cutaneous carcinomas

BACKGROUND

Insulin-like growth factor-I (IGF-I) is the principal mediator of growth hormone, exerting its effects through binding of the insulin-like growth factor-I receptor (IGF-IR). Post-receptor activation leads to the production of transcription factors involved in cell proliferation, differentiation, transformation, and survival. Data indicate that IGF-IR is involved in tumorigenesis. To our knowledge, this receptor has not been previously studied in primary cutaneous carcinomas.

METHODS

Twenty-five cases of primary cutaneous carcinomas consisting of three keratoacanthoma-type squamous cell carcinomas (KAs), two squamous cell carcinomas in situ (SCCs in situ), eight squamous cell carcinomas (SCCs), three conventional basal cell carcinomas (BCCs), two morpheaform basal cell carcinomas (M-BCCs), and seven Merkel cell carcinomas (MCCs) were analyzed for IGF-IR immunohistochemical expression using IGF-IR mouse monoclonal antibody (dilution 1 : 50) using the avidin-biotin-peroxidase complex method.

RESULTS

Normal epidermis was negative for IGF-IR expression. Normal eccrine glands and outer root sheath strongly expressed IGF-IR. All KAs, SCCs in situ, SCCs, and BCCs were negative for IGF-IR expression. Six of seven (86%) of the MCCs stained with IGF-IR strongly, showing cell membrane accentuation and a perinuclear dot-like pattern.

CONCLUSION

The data suggest that IGF-IR immunopositivity in MCCs might constitute a diagnostic tool in discriminating between SCCs and BCCs. Although the possible pathogenic significance of the perinuclear dot-like staining pattern observed in these neoplasms is unknown, its pattern is similar to what has been previously described with cytokeratin-20 immunostaining.

The role of Shc and insulin-like growth factor 1 receptor in mediating the translocation of estrogen receptor alpha to the plasma membrane

Our previous studies demonstrated that 17beta-estradiol (E2) rapidly induces the interaction of estrogen receptor alpha (ERalpha) with the adapter protein Shc, the translocation of ERalpha to the cell membrane, and the formation of dynamic membrane structures in MCF-7 breast cancer cells. The present study examined how E2 causes ERalpha to translocate to the region of the plasma membrane and focused on mechanisms whereby Shc and the insulin-like growth factor-1 receptor (IGF-1R) mediate this process. Shc physically interacts with IGF-1R in the plasma membrane, and E2 activates IGF-1R. We reasoned that ERalpha, when bound to Shc, would be directed to the region of the plasma membrane by the same processes, causing membrane translocation of Shc. We confirmed that E2 rapidly induced IGF-1R phosphorylation and demonstrated that E2 induced formation of a ternary protein complex among Shc, ERalpha, and IGF-1R. Knock down of Shc with a specific small inhibitory RNA decreased the association of ERalpha with IGF-1R by 87%, suggesting that Shc is a crucial molecule in the formation of this ternary complex. Confocal microscopy studies provided further confirmation of the functional roles of Shc and the IGF-1R in the translocation of ERalpha to the region of the membrane. Down-regulation of Shc, ERalpha, or IGF-1R with specific small inhibitory RNAs all blocked E2-induced mitogen-activated protein kinase phosphorylation. Together, our results demonstrate that Shc and IGF-1R serve as key elements in the translocation of ERalpha to the cell membrane and in the facilitation of ERalpha-mediated rapid E2 action.

Multiple signaling pathways are involved in the regulation of IGF-I receptor inhibition of PTEN-enhanced apoptosis

PTEN is a dual protein and lipid phosphatase that dephosphorylates PIP3 at the 3' position, thereby antagonizing PI3-kinase activity. A reduction in PI3' kinase activity enhances the susceptibility of cells to apoptosis. By stably transfecting PC12 cells with an antisense PTEN construct, endogenous PTEN protein levels were reduced by approximately 50% and etoposide-induced apoptosis was markedly decreased. Furthermore, IGF-I receptor abrogation of this apoptotic effect was inhibited by both PI3' kinase and by specific inhibitors of p38 MAP kinase. Thus, we show for the first time that p38 MAP kinase is involved in this process.

Molecular mechanisms underlying IGF-I-induced attenuation of the growth-inhibitory activity of trastuzumab (Herceptin) on SKBR3 breast cancer cells

The clinical usefulness of trastuzumab (Herceptin; Genentech, San Francisco, CA) in breast cancer treatment is limited by the rapid development of resistance. We previously reported that IGF-I signaling confers resistance to the growth-inhibitory actions of trastuzumab in a model system, but the underlying molecular mechanism remains unknown. We used SKBR3/neo cells (expressing few IGF-I receptors) and SKBR3/IGF-IR cells (overexpressing IGF-I receptor) as our experimental model. IGF-I antagonized the trastuzumab-induced increase in the level of the Cdk inhibitor p27(Kip1). This resulted in decreased association of p27(Kip1) with Cdk2, restoration of Cdk2 activity and attenuation of cell-cycle arrest in G(1) phase, all of which had been induced by trastuzumab treatment in SKBR3/IGF-IR cells. We also found that the decrease in p27(Kip1) induced by IGF-I was accompanied by an increase in expression of Skp2, which is a ubiquitin ligase for p27(Kip1), and by increased Skp2 association with p27(Kip1). A specific proteasome inhibitor (LLnL) completely blocked the ability of IGF-I to reduce the p27(Kip1) protein level, while IGF-I increased p27(Kip1) ubiquitination. This suggests that the action of IGF-I in conferring resistance to trastuzumab involves targeting of p27(Kip1) to the ubiquitin/proteasome degradation machinery. Finally, specific inhibitors of MAPK and PI3K suggest that the IGF-I-mediated reduction in p27(Kip1) protein level by increased degradation predominantly involves the PI3K pathway. Our results provide an example of resistance to an antineoplastic therapy that targets one tyrosine kinase receptor by increased signal transduction through an alternative pathway in a complex regulatory network.

Tumor necrosis factor alpha inhibits cyclin A expression and retinoblastoma hyperphosphorylation triggered by insulin-like growth factor-I induction of new E2F-1 synthesis

Cyclin A is required for cell cycle S phase entry, and its overexpression contributes to tumorigenesis. Release of pre-existing E2Fs from inactive complexes of E2F and hypophosphorylated retinoblastoma (RB) is the prevailing dogma for E2F transcriptional activation of target genes such as cyclin A. Here we explored the hypothesis that new synthesis of E2F-1 is required for insulin-like growth factor-I (IGF-I) to induce cyclin A accumulation and RB hyperphosphorylation, events that are targeted by tumor necrosis factor alpha (TNFalpha) to arrest cell cycle progression. We first established that IGF-I increases expression of cyclin A, causes hyperphosphorylation of RB, and augments the mass of E2F-1 in a time-dependent manner. As expected, E2F-1 small interfering RNA blocks the ability of IGF-I to increase synthesis of E2F-1. Most important, this E2F-1 small interfering RNA also blocks the ability of IGF-I to increase cyclin A accumulation and to hyperphosphorylate RB. We next established that TNFalpha dose-dependently inhibits IGF-I-induced phosphorylation of both RB and histone H1 by cyclin A-dependent cyclin-dependent kinases. Cyclin-dependent kinase 2 (Cdk2) mediates this suppression because co-immunoprecipitation experiments revealed that TNFalpha reduces the amount of IGF-I-induced cyclin A that binds Cdk2, leading to a reduction in Cdk2 enzymatic activity. TNFalpha antagonizes the ability of IGF-I to increase mass of both E2F-1 and cyclin A but not cyclin E or D1. The cytostatic property of TNFalpha is also shown by its ability to block IGF-I-stimulated luciferase activity of a cyclin A promoter reporter. Deletion of an E2F recognition site from this reporter eliminates the regulatory effects of both IGF-I and TNFalpha on cyclin A transcription, indicating the essential role of E2F-1 in mediating their cross-talk. Collectively, these results establish that TNFalpha targets IGF-I-induced E2F-1 synthesis, leading to inhibition of the subsequent accumulation in cyclin A, formation of cyclin A-Cdk2 complexes, hyperphosphorylation of RB, and cell cycle arrest.

Inducible expression of dominant negative insulin-like growth factor I receptor in MCF-7 breast cancer cells

The insulin-like growth factor I receptor (IGF-IR) is expressed in many cell types and is critical for normal growth and development. In the healthy mammary gland, the role of IGF-IR is not fully elucidated. However, IGF-IR, which is primarily expressed in the mammary epithelial cells, is known to play an obligatory role in cellular transformation, facilitating the progression to breast cancer. We have utilized the tetracycline regulatory (tet-on) system to generate an in vitro model system to allow us to further investigate IGF-I/IGF-IR function in mammary epithelial cells. A plasmid construct containing a mutant IGF-I receptor (IGF-IR-DN) fused to the tetracycline operator (tetOPh(CMV)-IGF-IR-DN) was stably transfected into MCF-7 human breast cancer cells. The conditional regulation of the IGF-IR-DN gene expression was studied in four independent clonal lines. The translated IGF-IR-DN protein was detected only in the stably transfected doxycycline- induced cells, and its expression was up-regulated (three- to sixfold) following induction. IGF-I stimulated cell proliferation diminished (twofold) in doxycycline- induced cells compared to uninduced cells, demonstrating that the transgene construct was functional and ruling out any pleiotropic effect that may be attributed to doxycycline. Interestingly, autophosphorylation of the IGF-IR and phosphorylation of the downstream substrate, insulin receptor substrate-1 (IRS-1), was not inhibited in doxycycline/IGF-I treated cells, suggesting the possibility that activation of downstream substrates other than the IRS-1 may be critical for optimal cell proliferation. This novel in vitro model should allow us to more directly examine the role of IGF-I/IGF-IR signaling and function in mammary epithelial cells.

Follicle-stimulating hormone promotes nuclear exclusion of the forkhead transcription factor FoxO1a via phosphatidylinositol 3-kinase in porcine granulosa cells

The forkhead family of transcription factors is conserved in evolution and known to play critical roles in the regulation of cellular differentiation and proliferation in many systems. The current studies demonstrate for the first time that forkhead homolog in rhabdomyosarcoma (FKHR) (FoxO1a) is expressed in porcine granulosa cells, and FSH stimulates FKHR phosphorylation and regulates its subcellular localization in this system. RT-PCR and Western blot studies demonstrated that FKHR is expressed and showed no change in FKHR message or protein levels in response to FSH (0-6 h). However, [32p]-orthophosphate labeling of cultured granulosa cells revealed robust phosphorylation after FSH treatment for 30 min. In addition, FSH caused nuclear exclusion of FKHR in these cells, apparently through the phosphatidylinositol 3-kinase signal transduction pathway. The cytosolic accumulation of FKHR protein that was observed in FSH-treated cells both by Western blot and immunohistochemistry was blocked when the cells were preincubated with the phosphatidylinositol 3-kinase inhibitor LY294002. Our data also demonstrate that Akt/protein kinase B, an established kinase for FKHR, is phosphorylated in response to FSH treatment. Interestingly, although FKHR was phosphorylated by 30 min after FSH treatment, the time course for Akt phosphorylation was relatively delayed and sustained. Although these studies do not preclude Akt involvement in FSH-stimulated FKHR phosphorylation, they do suggest that other kinases may contribute to rapid signaling to FKHR. Because FKHR has been shown to activate genes involved in apoptosis and growth inhibition, FSH may promote growth and survival by initiating the phosphorylation of FKHR, causing its nuclear exclusion, and reducing its effect as a cell cycle arrest or death-promoting transcription factor.

Changes in apoptosis-related pathways in acute myelocytic leukemia

Expression analysis of apoptotic genes was performed for 15 patients with acute myelocytic leukemia (AML) at the time of diagnosis to identify genes and signaling pathways involved in the regulation of cell survival and apoptosis during leukemogenesis. cDNA array analysis revealed 34 genes whose expression was significantly different compared to others. Tumor suppressor genes TP53 and CDKN2A were downregulated and protooncogenes JUN and GRB10 were upregulated. Furthermore, several cellular signaling pathways acting either in cell cycle regulation or in apoptosis were altered. Deregulation was found in pathways that contribute to genomic stability (by downregulation of either TP53 or CSE1L and by upregulation of GADD45A) and regulate cell cycle progression (by downregulation of CDKN2A and upregulation of RBBP4, CDC37, and NEDD5). Alterations at the transcriptional level were identified, namely, upregulation of JUN and E2F5. Abnormalities were observed in the regulation of the caspases through upregulation of CASP8 and by altered expression of BCL2-related pathway. Extrinsic apoptotic signals mediated by IGFs were deregulated and the glutathione detoxification pathway was downregulated. These findings provide insight into the regulation of balance between apoptosis and cell proliferation signals, and suggest that these genes and pathways may have an important role in the pathogenesis of AML.

The mitogenic action of insulin-like growth factor I in normal human mammary epithelial cells requires the epidermal growth factor receptor tyrosine kinase

The signals used by insulin-like growth factor I (IGF-I) to stimulate proliferation in human mammary epithelial cells have been investigated. IGF-I caused the activation of both ERKs and Akt. Activation of ERKs was slower and more transient than that of Akt. ZD1839, a specific epidermal growth factor receptor (EGFR) tyrosine kinase inhibitor, prevented activation of ERKs but not Akt by IGF-I. Inhibition of the EGFR with function-blocking monoclonal antibodies also specifically blocked IGF-I-induced ERK activation. These effects occurred in primary mammary epithelial cells and in two cell lines derived from normal mammary epithelium but not in mammary fibroblasts or IGF-I-responsive breast carcinoma cell lines. Although IGF-I stimulated the proliferation of both normal and carcinoma cell lines, ZD1839 blocked this only in the normal line. ZD1839 had no effect on IGF-I receptor (IGF-IR) autophosphorylation in intact cells. IGF-I-induced ERK activation was insensitive to a broad spectrum matrix-metalloproteinase inhibitor and to CRM-197, an inhibitor of the EGFR ligand heparin-bound epidermal growth factor. EGFR was detectable within IGF-IR immunoprecipitates from normal mammary epithelial cells. Treatment of cells with IGF-I led to an increase in the amount of tyrosine-phosphorylated EGFR within these complexes. ZD1839 had no effect on complex formation but completely abolished their associated EGFR tyrosine phosphorylation. These findings indicate that IGF-I utilizes a novel EGFR-dependent signaling pathway involving the formation of a complex between the IGF-IR and the EGFR to activate the ERK pathway and to stimulate proliferation in normal human mammary epithelial cells. This form of regulation may be lost during malignant progression.

Beta-arrestin1 mediates insulin-like growth factor 1 (IGF-1) activation of phosphatidylinositol 3-kinase (PI3K) and anti-apoptosis

beta-arrestins (1 and 2) are widely expressed cytosolic proteins that play central roles in G protein-coupled receptor signaling. beta-arrestin1 is also recruited to the insulin-like growth factor 1 (IGF-1) receptor, a receptor tyrosine kinase, upon agonist binding. Here we report that, in response to IGF-1 stimulation, beta-arrestin1 mediates activation of phosphatidylinositol 3-kinase in a pathway that leads to the subsequent activation of Akt and anti-apoptosis. This process is independent of both Gi and ERK activity. The pathway fails in mouse embryo fibroblasts lacking both beta-arrestins and is restored by stable transfection of beta-arrestin1. Remarkably, this pathway is insensitive to chemical inhibition of IGF-1 receptor tyrosine kinase activity. These results suggest that, in addition to their roles in G protein-coupled receptor signaling, beta-arrestins couple the IGF-1 receptor tyrosine kinase to the phosphatidylinositol 3-kinase system and suggest that this mechanism is operative independently of the tyrosine kinase activity of the receptor.

Signaling mechanisms leading to the regulation of differentiation and apoptosis of articular chondrocytes by insulin-like growth factor-1

Cartilage development is initiated by the differentiation of mesenchymal cells into chondrocytes. Differentiated chondrocytes in articular cartilage undergo dedifferentiation and apoptosis during arthritis, in which NO production plays a critical role. Here, we investigated the roles and mechanisms of action of insulin-like growth factor-1 (IGF-1) in the chondrogenesis of mesenchymal cells and the maintenance and survival of differentiated articular chondrocytes. IGF-1 induced chondrogenesis of limb bud mesenchymal cells during micromass culture through the activation of phosphatidylinositol 3-kinase (PI3K) and Akt. PI3K activation is required for the activation of protein kinase C (PKC)-alpha and p38 kinase and inhibition of ERK1/2. These events are necessary for chondrogenesis. The growth factor additionally blocked NO-induced dedifferentiation and apoptosis of primary culture articular chondrocytes. NO production in chondrocytes induced down-regulation of PI3K and Akt activities, which was blocked by IGF-1 treatment. Stimulation of PI3K by IGF-1 resulted in blockage of NO-induced activation of p38 kinase and ERK1/2 and inhibition of PKCalpha and PKCzeta, which in turn suppressed dedifferentiation and apoptosis. Our results collectively indicate that IGF-1 regulates differentiation, maintenance of the differentiated phenotype, and apoptosis of articular chondrocytes via a PI3K pathway that modulates ERK, p38 kinase, and PKC signaling.

Insulin-like growth factor (IGF)-I and IGF binding proteins-2, -3, -4, -5 in porcine corpora lutea during the estrous cycle; evidence for inhibitory actions of IGFBP-3

In this study we measured protein concentrations of insulin-like growth factor (IGF)-I and IGF binding proteins (IGFBPs) 2-5 in porcine corpora lutea (CLs) throughout the estrous cycle (Experiment 1), and examined the effects of IGFBP-3 and IGFBP-3 antibody (AB) on luteal progesterone (P4) secretion in vitro (Experiment 2). For Experiment 1, (CLs) and serum were collected on days (D) 4, 7, 10, 13, 15 and 16 of the estrous cycle (n = 5 animals per day). IGF-I was extracted from CLs and sera, and measured by radioimmunoassay (RIA). IGFBPs were measured in CLs by ligand blots. For Experiment 2, CLs (from Experiment 1) were enzyme dissociated and luteal cells cultured (24 h) in Medium 199 (M199) containing (0-500 ng/ml) IGFBP-3 (+/-IGF-I; 100 ng/ml), or (0-10 microg/ml) IGFBP-3 AB. P4 in media was measured by RIA. In Experiment 1, luteal IGF-I concentrations (ng/g tissue) were maximal on day 4 and gradually decreased thereafter. Serum IGF-I concentrations (ng/ml) were highest on days 4 and 7, compared with days 10-15. Peak levels of luteal IGFBP-3 were also seen on days 4 and 7 of the cycle. Luteal IGFBP-2 concentrations showed a tendency to increase on day 16 (P < 0.05 versus day 10), but no significant changes in IGFBP-4 or -5 were seen. In Experiment 2, IGFBP-3 (w IGF) inhibited the steroidogenic actions of IGF-I, but had no significant actions alone (IGFBP-3 w/o IGF). Finally, IGFBP-3 AB stimulated P4 secretion on days 4 and 7, but not on days 10-16. We conclude that IGFBP-3 inhibits IGF-I actions in the porcine CL.

Tomato and soy polyphenols reduce insulin-like growth factor-I-stimulated rat prostate cancer cell proliferation and apoptotic resistance in vitro via inhibition of intracellular signaling pathways involving tyrosine kinase

We examined the ability of polyphenols from tomatoes and soy (genistein, quercetin, kaempferol, biochanin A, daidzein and rutin) to modulate insulin-like growth factor-I (IGF-I)-induced in vitro proliferation and apoptotic resistance in the AT6.3 rat prostate cancer cell line. IGF-I at 50 micro g/L in serum-free medium produced maximum proliferation and minimized apoptosis. Polyphenols exhibited different abilities to modulate IGF-I-induced proliferation, cell cycle progression (flow cytometry) and apoptosis (Annexin V/propidium iodide and terminal deoxynucleotidyltransferase-mediated deoxyuridine 5'-triphosphate nick end labeling). Genistein, quercetin, kaempferol and biochanin A exhibited dose-dependent inhibition of growth with a 50% inhibitory concentration (IC(50)) between 25 and 40 micro mol/L, whereas rutin and daidzein were less potent with an IC(50) of >60 micro mol/L. Genistein and kaempferol potently induced G(2)/M cell cycle arrest. Genistein, quercetin, kaempferol and biochanin A, but not daidzein and rutin, counteracted the antiapoptotic effects of IGF-I. Human prostate epithelial cells grown in growth factor-supplemented medium were also sensitive to growth inhibition by polyphenols. Genistein, biochanin A, quercetin and kaempferol reduced the insulin receptor substrate-1 (IRS-1) content of AT6.3 cells and prevented the down-regulation of IGF-I receptor beta in response to IGF-I binding. IGF-I-stimulated proliferation was dependent on activation of mitogen-activated protein kinase/extracellular signal-regulated kinase (ERK) and phosphatidylinositide 3-kinase pathways. Western blotting demonstrated that ERK1/2 was constitutively phosphorylated in AT6.3 cells with no change in response to IGF-I, whereas IRS-1 and AKT were rapidly and sensitively phosphorylated after IGF-I stimulation. Several polyphenols suppressed phosphorylation of AKT and ERK1/2, and more potently inhibited IRS-1 tyrosyl phosphorylation after IGF-I exposure. In summary, polyphenols from soy and tomato products may counteract the ability of IGF-I to stimulate proliferation and prevent apoptosis via inhibition of multiple intracellular signaling pathways involving tyrosine kinase activity.

Signaling differences from the A and B isoforms of the insulin receptor (IR) in 32D cells in the presence or absence of IR substrate-1

The A isoform of the insulin receptor (IR) is frequently overexpressed in cancer cells and is activated by IGF-II as well as by insulin, whereas the B isoform is predominant in differentiated tissues and responds poorly to IGF-II. The IR substrate-1 (IRS-1), a docking protein for the IR, is known to send a mitogenic signal and to be a powerful inhibitor of cell differentiation. We have investigated the biological effects of the two IR isoforms in parental 32D hemopoietic cells, which do not express IRS-1, and in 32D-derived cells in which IRS-1 is ectopically expressed. The effects of the two isoforms on cell survival, differentiation markers and nuclear translocation of IRS-1 were compared. The results confirm that the A isoform responds to IGF-II and preferentially sends mitogenic, antiapoptotic signals, whereas the B form, poorly responsive to IGF-II, tends to send differentiation signals.

Acute molecular responses of skeletal muscle to resistance exercise in able-bodied and spinal cord-injured subjects

Spinal cord injury (SCI) results in muscle atrophy, which contributes to a number of health problems, such as cardiovascular deconditioning, metabolic derangement, and osteoporosis. Electromyostimulation (EMS) holds the promise of ameliorating SCI-related muscle atrophy and, therefore, improving general health. To date, EMS training of long-term SCI subjects has resulted in some muscle hypertrophy but has fallen short of normalizing muscle mass. The aim of this study was to compare the molecular responses of vastus lateralis muscles from able-bodied (AB) and SCI subjects after acute bouts of EMS-induced resistance exercise to determine whether SCI muscles displayed some impairment in response. Analysis included mRNA markers known to be responsive to increased loading in rodent muscles. Muscles of AB and SCI subjects were subjected to EMS-stimulated exercise in two 30-min bouts, separated by a 48-h rest. Needle biopsy samples were obtained 24 h after the second exercise bout. In both the AB and SCI muscles, significant changes were seen in insulin-like growth factor binding proteins 4 and 5, cyclin-dependent kinase inhibitor p21, and myogenin mRNA levels. In AB subjects, the mRNA for mechano-growth factor was also increased. Before exercise, the total RNA concentration of the SCI muscles was less than that of the AB subjects but not different postexercise. The results of this study indicate that acute bouts of resistance exercise stimulate molecular responses in the skeletal muscles of both AB and SCI subjects. The responses seen in the SCI muscles indicate that the systems that regulate these molecular responses are intact, even after extended periods of muscle unloading.

Insulin-like growth factor I-stimulated melanoma cell migration requires phosphoinositide 3-kinase but not extracellular-regulated kinase activation

Dysregulated signaling contributes to altered cellular growth, motility, and survival during cancer progression. We have evaluated the ability of several factors to stimulate migration in WM1341D, a cell line derived from an invasive human vertical growth phase melanoma. Basic fibroblast growth factor, hepatocyte growth factor, interleukin-8, and CCL27 each slightly increased migration. Insulin-like growth factor I (IGF-I), however, stimulated a 15-fold increase in migration. This response required the IGF-I receptor, which activates phosphoinositide 3-kinase (PI3K) and mitogen-activated protein kinase/extracellular signal-regulated kinase (MAPK/ERK) pathways. Both pathways have been implicated in migration in a variety of cell types, but the signaling required for IGF-I-induced melanoma cell migration is not well defined. IGF-I-stimulated activation of MAPK/ERK signaling in WM1341D cells was inhibited by U0126, but a 33-fold higher dose of U0126 was needed to inhibit IGF-I-stimulated cellular migration. In contrast, similar concentrations of either wortmannin or LY294002 were required to inhibit both IGF-I-induced PI3K activation and migration. These results indicate that IGF-I-stimulated migration of WM1341D cells requires PI3K activation but is independent of MAPK/ERK signaling. Determining the contributions of IGF-I signaling pathways to migration will help us to understand melanoma progression and may lead to new therapeutic targets of this highly metastatic cancer.

Detection of protein tyrosine-kinase (PTK) gene expression pattern in normal and malignant T lymphocytes by combined PTK-specific polymerase chain reaction and parallel denaturing gradient gel electrophoresis

Protein tyrosine kinases (PTKs) control key functions of normal and malignant cells. Comparison of PTK gene expression among various cell populations may be achieved by amplification of the PTK cDNAs using degenerate primers which recognize two relatively invariable regions within their catalytic domain. This approach produces a mixture of PTK cDNA fragments with identical or very similar lengths which are difficult to separate by standard gel electrophoresis. These mixed products are then analyzed in a random fashion which leads to redundant cloning of some and potential omission of other PTKs. By using parallel denaturing gradient gel electrophoresis (DGGE) we have been able to separate the amplified PTK cDNAs derived from the same T-lymphocyte population and compare their expression between various types of normal and malignant T lymphocytes. One such PTK is the type I receptor for insulin-like growth factor, which we found to be preferentially expressed by neoplastic T cells on the both mRNA and protein levels. The combination of PCR which uses PTK-specific primers and parallel DGGE of the amplified PTK cDNAs may prove useful in studying mechanisms of cell activation and malignant transformation and in identifying targets for therapies based on selective inhibition of oncogenic PTKs.

Distinct anabolic response of osteoblast to low-intensity pulsed ultrasound

Low-intensity pulsed ultrasound, a form of mechanical energy transmitted as high-frequency acoustical pressure waves, provides noninvasive therapeutic treatment for accelerating fracture repair and distraction osteogenesis. Relatively young osteoblasts respond to ultrasound by transiently upregulating message levels of immediate-early genes as well as that of osteocalcin and insulin-like growth factor I (IGF-I). Osteocytes derived from newborn rat tibia and calvaria responded to a lesser extent only in c-fos and cyclooxygenase-2 (COX-2) messages. Compared with the stretched osteocytes, which use stretch-activated and parathyroid hormone (PTH)-potentiated Ca2+ influx as an entry route to the protein kinase A (PKA) signal transduction pathways, there was no evidence of Ca2+ internalization by any of the cells tested on exposure to the ultrasound. On the other hand, inhibitors of p38 mitogen-activated protein kinase (MAPK) and upstream phosphoinositide 3-kinase (PI3K) blocked COX-2 and osteocalcin upregulation by the ultrasound-exposed ST2, murine bone marrow-derived cells. This is distinct from the aforementioned osteocytic response to low-frequency stretching and implies the involvement of integrins. Our findings suggested that accelerated fracture repair and distraction osteogenesis by the low-intensity pulsed ultrasound depend, at least in part, on the stimulation of osteoblastic cells at relatively early stages of osteogenic lineage. Bone is under control of multiple regulatory mechanisms so that diverse physical forces can be reflected to the microenvironment of each cell, in turn, to the entire bone.

Insulin/IGF and target of rapamycin signaling: a TOR de force in growth control

'They come in all sizes.' Apart from its origin and use in the clothing industry, this saying reflects the fact that the size of organisms spans an enormous range. Whether destined to be large or small, species grow in an organized fashion to reach their final specified size. For growth to proceed, food must be metabolized to liberate energy in the form of adenosine triphosphate (ATP) and protein building blocks in the form of amino acids. One major orchestrator of this complex growth process in diverse metazoan species is the insulin/insulin-like growth factor (IGF) system. This review summarizes current studies primarily from Drosophila regarding the function of the insulin/IGF system in the control of growth.

Genes, growth factors and acanthosis nigricans

Acanthosis nigricans (AN) occurs most commonly in association with hyperinsulinaemia and more rarely as a paraneoplastic syndrome. It is also a feature of several genetic disorders. Indirect evidence suggests a role for tyrosine kinase growth factor receptor signalling in the pathogenesis of AN. Defects in the insulin receptor gene causing insulin resistance and AN are well recognized, but recent data in several other syndromes of this association, including lipodystrophic disorders, have identified causative defects in other pathways. The mechanism of AN due to insulin resistance is most probably direct or indirect activation of the insulin-like growth factor 1 receptor by high levels of circulating insulin. However, more direct evidence for abnormal tyrosine kinase receptor signalling in AN has been provided by studies of craniosynostosis and skeletal dysplasia syndromes with AN, which have identified activating mutations in fibroblast growth factor receptors.

Insulin-like growth factor 1 induces hypoxia-inducible factor 1-mediated vascular endothelial growth factor expression, which is dependent on MAP kinase and phosphatidylinositol 3-kinase signaling in colon cancer cells

Stimulation of human colon cancer cells with insulin-like growth factor 1 (IGF-1) induces expression of the VEGF gene, encoding vascular endothelial growth factor. In this article we demonstrate that exposure of HCT116 human colon carcinoma cells to IGF-1 induces the expression of HIF-1 alpha, the regulated subunit of hypoxia-inducible factor 1, a known transactivator of the VEGF gene. In contrast to hypoxia, which induces HIF-1 alpha expression by inhibiting its ubiquitination and degradation, IGF-1 did not inhibit these processes, indicating an effect on HIF-1 alpha protein synthesis. IGF-1 stimulation of HIF-1 alpha protein and VEGF mRNA expression was inhibited by treating cells with inhibitors of phosphatidylinositol 3-kinase and MAP kinase signaling pathways. These inhibitors also blocked the IGF-1-induced phosphorylation of the translational regulatory proteins 4E-BP1, p70 S6 kinase, and eIF-4E, thus providing a mechanism for the modulation of HIF-1 alpha protein synthesis. Forced expression of a constitutively active form of the MAP kinase kinase, MEK2, was sufficient to induce HIF-1 alpha protein and VEGF mRNA expression. Involvement of the MAP kinase pathway represents a novel mechanism for the induction of HIF-1 alpha protein expression in human cancer cells.

Insulin-like growth factor binding protein-6 inhibits neuroblastoma cell proliferation and tumour development

In neuroblastoma cells, survival and proliferation are dependent upon the insulin-like growth factor (IGF) system. IGFs actively participate in cell growth, whereas IGFBP-6, is associated with the arrest of growth. With a view to blocking IGF-II action, we produced recombinant human IGFBP-6 capable of binding IGFs with affinities between 1.23 and 6.36 x 10(9) M(-1). Ex vivo mitogenic activities were tested on two human neuroblastoma cell lines, in which 100 ng/ml IGFBP-6 completely abolished the effects of both endogenous and exogenous IGF-II. In vivo, nude mice previously injected with neuroblastoma cells were submitted to either 15 daily injections of 4-20 microg IGFBP-6 or implantation of mini-pumps diffusing 20-100 microg IGFBP-6 over 2 weeks. The result was an average 18% reduction in the incidence and development of tumours. Delivery of the IGFBP-6 via mini-pumps also delayed tumour appearance by 6-15 days. Our results therefore show the involvement of IGFBP-6 in neuroblastoma cell growth, both ex vivo in terms of proliferation and in vivo in terms of tumour development.

The Drosophila insulin/IGF receptor controls growth and size by modulating PtdInsP3 levels

Understanding the control of size is of fundamental biological and clinical importance. Insulin/IGF signaling during development controls growth and size, possibly by coordinating the activities of the Ras and PI 3-kinase signaling pathways. We show that in Drosophila mutating the consensus binding site for the Ras pathway adaptor Drk/Grb2 in Chico/IRS does not interfere with growth whereas mutating the binding sites of the PI 3-kinase adaptor p60 completely abrogates Chico function. Furthermore, we present biochemical and genetic evidence that loss of the homolog of the tumor suppressor gene, Pten, results in increased PtdInsP3 levels and that these increased levels are sufficient to compensate for the complete loss of the Insulin/insulin-like growth factor receptor function. This reduction of Pten activity is also sufficient to vastly increase organism size. These results suggest that PtdInsP3 is a second messenger for growth and that levels of PtdInsP3 during development regulate organismal size.

Invited Review: Autocrine/paracrine IGF-I and skeletal muscle adaptation

This brief review presents the basic premises suggesting that insulin-like growth factor I (IGF-I), functioning in an autocrine/paracrine mode, is an important mediator of skeletal muscle adaptation. Key intracellular signaling mechanisms associated with ligation of the primary IGF-I receptor are highlighted to illustrate the mechanisms by which IGF-I may promote muscle hypertrophy. In addition, a number of recent findings are presented that highlight the potential for interactions between IGF-I-related signaling pathways and intracellular signaling mechanisms activated by cytokines or hormonal systems.

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.

Signal transduction pathways: the molecular basis for targeted therapies

The elucidation of the signal transduction pathways that regulate cell growth and differentiation has led to a number of exciting opportunities for novel cancer therapies. It is now well known that growth factors and cell matrix molecules activate cognate growth factor receptors and integrins, respectively, to initiate a complex signaling cascade that ultimately targets the nucleus, cell surface, and mitochondria. Signaling to these target molecules results in the regulation of gene transcription, cell adhesion and motility, and cell survival, all of which are integral parts of cellular growth control mechanisms. As a result of increased understanding of cell growth regulation mechanisms, a number of novel therapeutic agents have been developed and tested in preclinical models and, to some extent, in clinical trials. Based on our current understanding of growth regulation in normal and cancer cells, one would predict that these new agents could influence proliferation and survival of cancer cells, as well as their response to traditional cytotoxic therapies. In this overview, the mechanistic basis for the use of signal transduction-targeted novel therapeutics is presented, along with predictions regarding how they may interact with ionizing radiation in different subgroups of patients.

Expression of insulin-like growth factor-binding protein 5 in neuroblastoma cells is regulated at the transcriptional level by c-Myb and B-Myb via direct and indirect mechanisms

Neuroblastoma (NB), a malignant childhood tumor deriving from the embryonic neural crest, is sensitive to the growth-stimulating effects of insulin-like growth factors (IGFs). Aggressive cases of this disease often acquire autocrine loops of IGF production, but the mechanisms through which the different components of the IGF axis are regulated in tumor cells remain unclear. Upon conditional expression of c-Myb in a NB cell line, we detected up-regulation of IGF1, IGF1 receptor, and insulin-like growth factor-binding protein 5 (IGFBP-5) expression. Analysis of the IGFBP-5 promoter revealed two potential Myb binding sites at position -59 to -54 (M1) and -429 to -424 (M2) from the transcription start site; both sites were bound by c-Myb and B-Myb in vitro and in vivo. Reporter assays carried out using the proximal region of the human IGFBP-5 promoter demonstrated that c-Myb and B-Myb enhanced transcription. However, site-directed mutagenesis and deletion of the Myb binding sites coupled with reporter assays revealed that M2 but not M1 was important for Myb-dependent transactivation of the IGFBP-5 promoter. The double mutant M1/M2 was still transactivated by c-Myb, suggesting the existence of Myb binding-independent mechanisms of IGFBP-5 promoter regulation. A constitutively active AKT transactivated the IGFBP-5 promoter, whereas the phosphatidylinositol 3-kinase inhibitor LY294002 suppressed it. Moreover, the kinase dead dominant negative K179M AKT mutant was able to inhibit transcription from the M2 and M1/M2 IGFBP-5 mutant promoters. Deletion analysis of the IGFBP-5 promoter revealed that the AKT-responsive region lies between nucleotides -334 and -83. Together, these data suggest that the Myb binding-independent transactivation of the IGFBP-5 promoter was due to the activation of the phosphatidylinositol 3-kinase/AKT pathway likely mediated by IGF1 receptor-dependent signals. Finally, IGFBP-5 was able to modulate proliferation of NB cells in a manner dependent on its concentration and on the presence of IGFs.

Exploring the interface between metallo-proteinase activity and growth factor and cytokine bioavailability

Metallo-proteinases are implicated in many processes involved in tissue remodeling, cell motility, morphogenesis, and cell and organ growth and differentiation. Recent data suggest that several members of the metzincin family including the matrix metallo-proteinases (MMPs), adamalysin-related proteinases, and the newly described pappalysins, are intimately involved in the activation and/or release of cytokines and growth factors. We review how metzincins, working through unique mechanisms, influence the extracellular milieu of several important cytokines and growth factors including transforming growth factor-beta (TGF-beta), TNF-alpha, IGFs and HB-epidermal growth factor (EGF). Because metzincins can modulate the bioavailability of these peptides, they may serve as unique target molecules to control cytokine and growth factor action in the extracellular environment.

Insulin-like growth factors and cancer

Interest in insulin-like growth factors (IGFs) and their effect on carcinogenesis has increased recently because high serum concentrations of IGF1 are associated with an increased risk of breast, prostate, colorectal, and lung cancers. Physiologically, IGF1 is the major mediator of the effects of the growth hormone; it thus has a strong influence on cell proliferation and differentiation and is a potent inhibitor of apoptosis. The action of IGF1 is predominantly mediated through the IGF1 receptor (IGF1R). IGF1R is involved in several oncogenic transformation processes. The availability of unbound IGF1 for interaction with IGF1R is modulated by IGF-binding proteins (IGFBP1-6). IGFBPs, especially IGFBP3, have independent effects on cell growth, for example, IGFBP3 has proapoptotic activities both dependent on and independent of p53.

PIP3 inhibition of RGS protein and its reversal by Ca2+/calmodulin mediate voltage-dependent control of the G protein cycle in a cardiac K+ channel

Regulators of G protein signaling (RGS) accelerate intrinsic GTP hydrolysis on alpha subunits of trimeric G proteins and play crucial roles in the physiological regulation of G protein-mediated cell signaling. The control mechanisms of the action of RGS proteins per se are poorly clarified, however. We recently showed a physiological mode of action of a RGS protein in cardiac myocytes. The voltage-dependent formation of Ca2+/calmodulin facilitated the GTPase activity of RGS by an unidentified mechanism, which underlay the "relaxation" behavior of G protein-gated K+ (K(G)) channels. Here we report the mechanism which is the reversal by Ca2+/calmodulin of phosphatidylinositol-3,4,5,-trisphosphate (PIP3)-mediated inhibition of RGS. Purified RGS4 protein alone inhibited GTP-induced K(G) channel activity in inside-out patches from atrial myocytes. The inhibitory effect of RGS4 was reduced by PIP3 and restored by addition of Ca2+/calmodulin. The intracellular application of anti-PIP3 antibody abolished the RGS-dependent relaxation behavior of K(G) current in atrial myocytes. This study, therefore, reveals a general physiological control mechanism of RGS proteins by lipid-protein interaction.

The Raf/MEK/ERK signal transduction cascade as a target for chemotherapeutic intervention in leukemia

The Raf/MEK/ERK (MAPK) signal transduction cascade is a vital mediator of a number of cellular fates including growth, proliferation and survival, among others. The focus of this review centers on the MAPK signal transduction pathway, its mechanisms of activation, downstream mediators of signaling, and the transcription factors that ultimately alter gene expression. Furthermore, negative regulators of this cascade, including phosphatases, are discussed with an emphasis placed upon chemotherapeutic intervention at various points along the pathway. In addition, mounting evidence suggests that the PI3K/Akt pathway may play a role in the effects elicited via MAPK signaling; as such, potential interactions and their possible cellular ramifications are discussed.

An unexpected biochemical and functional interaction between gp130 and the EGF receptor family in breast cancer cells

Oncostatin M (OSM), an interleukin-6 type cytokine, acts via the gp130 signaling receptor to inhibit proliferation and induce differentiation of breast cancer cells. EGF, a mitogen for breast cells, signals via EGFR/ErbB tyrosine kinase receptors which are implicated in breast cancer pathogenesis. Here we show paradoxically that EGF enhanced the OSM-induced inhibition of proliferation and induction of cellular differentiation in both estrogen receptor positive and negative breast cancer cells. This functional synergism was also seen with heregulin but not SCF, PDGF or IGF-1, indicating that it was specific to EGF-related growth factors. Immunoprecipitation experiments revealed that gp130 was constitutively associated with ErbB-2 and ErbB-3. There was a similar association between the OSMRbeta and ErbB-2. Furthermore, EGF unexpectedly induced tyrosine phosphorylation of gp130. We show that OSM induced phosphorylation of STAT3. Both OSM and EGF activated the p42/44 MAP kinases, but while the MEK inhibitor, PD98059, ablated the OSM-induced inhibition, it only partially ablated the inhibitory effects of OSM plus EGF. Thus, we have demonstrated that the receptors and signalling pathways of two apparently unrelated growth factors were intimately linked, resulting in an unexpected biological effect. This provides a new mechanism for generating signalling diversity and has potential clinical implications in breast cancer.