Insulin-like Growth factor-I Receptor Promoter Research Articles

BRCA1 is Expressed in Uterine Serous Carcinoma (USC) and Controls Insulin-Like Growth Factor I Receptor (IGF-IR) Gene Expression in USC Cell Lines

The insulin-like growth factor I receptor (IGF-IR) and BRCA1 affect cell growth and apoptosis. Little information is available about BRCA1 activity on the IGF signaling pathway. This study evaluated the effect of BRCA1 on IGF-IR expression. BRCA1 and IGF-IR immunohistochemistry on archival tissues (35 uterine serous carcinomas [USCs] and 17 metastases) were performed. USPC1 and USPC2 cell lines were transiently cotransfected with an IGF-IR promoter construct driving a luciferase reporter gene and a BRCA1 expression plasmid. Endogenous IGF-IR levels were evaluated by Western immunoblotting. We found high BRCA1 and IGF-IR protein expression in primary and metastatic USC tumors. All samples were immunostained for BRCA1-71% strongly stained; and 33/35 (94%) were stained positive for IGF-IR-2 (6%) strongly stained. No difference in BRCA1 and IGF-IR staining intensity was noted between BRCA1/2 mutation carriers and noncarriers. Metastatic tumors stained more intensely for BRCA1 than did the primary tumor site (P = 0.041) and with borderline significance for IGF-IR (P = 0.069). BRCA1 and IGF-IR staining did not correlate to survival. BRCA1 expression led to 35% and 54% reduction in IGF-IR promoter activity in the USPC1 and USCP2 cell lines, respectively. Western immunoblotting showed a decline in phosphorylated IGF-IR and phosphorylated AKT in both transiently and stably transfected cells. BRCA1 and IGF-IR are highly expressed in USC tumors. BRCA1 suppresses IGF-IR gene expression and activity. These findings suggest a possible biological link between the BRCA1 and the IGF-I signaling pathways in USC. The clinical implications of this association need to be explored.

Differential regulation of insulin-like growth factor-I receptor gene expression by wild type and mutant androgen receptor in prostate cancer cells

The progression of prostate cancer from an organ-confined, androgen-sensitive disease to a metastatic one is associated with dysregulation of androgen receptor (AR)-regulated target genes and with a decrease in insulin-like growth factor-I receptor (IGF-IR) expression. To investigate the differential effects of wild type (wt) and mutant AR on IGF-IR levels we employed a series of isogenic prostate-derived cell lines and human xenografts. We show that basal and phosphorylated IGF-IR levels progressively decreased as prostate cancer cells became more tumorigenic and metastatic. In addition, we show that wt, but not mutant, AR along with dihydrotestosterone treatment increased IGF-IR promoter activity and endogenous IGF-IR levels. ChIP analysis show enhanced AR binding to the IGF-IR promoter in AR-overexpressing cells. Finally, wt AR-overexpressing cells display an enhanced proliferation rate. In summary, we provide evidence that activated wt AR enhances IGF-IR transcription in prostate cancer cells via a mechanism that involves AR binding to the IGF-IR promoter. AR mutations alter the ability of the mutated protein to regulate IGF-IR expression. Our results suggest that prostate cancer progression is associated with a decrease in IGF-IR expression that could be the result of impaired ability of AR to stimulate IGF-IR gene expression.

Identification of Insulin-Like Growth Factor-I Receptor (IGF-IR) Gene Promoter-Binding Proteins in Estrogen Receptor (ER)-Positive and ER-Depleted Breast Cancer Cells

The insulin-like growth factor I receptor (IGF-IR) has been implicated in the etiology of breast cancer. Overexpression of the IGF-IR gene is a typical feature of most primary breast cancers, whereas low IGF-IR levels are seen at advanced stages. Hence, evaluation of IGF-IR levels might be important for assessing prognosis. In the present study, we employed a proteomic approach based on DNA affinity chromatography followed either by mass spectroscopy (MS) or Western blot analysis to identify transcription factors that may associate with the IGF-IR promoter in estrogen receptor (ER)-positive and ER-depleted breast cancer cells. A biotinylated IGF-IR promoter fragment was bound to streptavidin magnetic beads and incubated with nuclear extracts of breast cancer cells. IGF-IR promoter-binding proteins were eluted with high salt and analyzed by MS and Western blots. Among the proteins that were found to bind to the IGF-IR promoter we identified zinc finger transcription factors Sp1 and KLF6, ER-α, p53, c-jun, and poly (ADP-ribosylation) polymerase. Furthermore, chromatin immune-precipitation (ChIP) analysis confirmed the direct in vivo binding of some of these transcription factors to IGF-IR promoter DNA. The functional relevance of binding data was assessed by cotransfection experiments with specific expression vectors along with an IGF-IR promoter reporter. In summary, we identified nuclear proteins that are potentially responsible for the differential expression of the IGF-IR gene in ER-positive and ER-depleted breast cancer cells.

Transcription factor E2F1 is a potent transactivator of the insulin-like growth factor-I receptor (IGF-IR) gene

Objectives The insulin-like growth factor-I receptor (IGF-IR) plays an important role in cancer development. The E2F1 transcription factor activates S-phase promoting genes and mediates apoptosis. Microarray analyses of E2F1-induced genes revealed that genes associated with proliferation as well as apoptosis are upregulated by E2F1. Among other candidate genes, DNA microarrays identified the IGF-IR gene as a putative E2F1 target. The aim of this study was to investigate the involvement of E2F1 in regulation of IGF-IR gene transcription. Methods To examine the potential regulation of IGF-IR gene expression by E2F1, an E2F1 expression vector was transfected into P69 and M12 prostate cancer cell lines, after which IGF-IR levels were measured by Western blots. Transient transfections were used to evaluate IGF-IR promoter activity and chromatin immunoprecipitation (ChIP) assays were employed to assess E2F1-binding to the IGF-IR promoter. Results Results obtained showed that E2F1 expression induced a significant increment in endogenous IGF-IR levels. ChIP assays showed enhanced E2F1-binding to the IGF-IR promoter in E2F1-expressing cells. Transient coexpression of an E2F1 vector along with an IGF-IR promoter-luciferase reporter resulted in a ∼140-fold increase in IGF-IR promoter activity. Furthermore, deletion and bioinformatic analyses indicate that the ability of E2F1 to stimulate IGF-IR promoter activity was correlated with the number of E2F1 sites in the promoter region. Conclusions In summary, we provide evidence that E2F1 regulates IGF-IR gene transcription in prostate cancer cells via a mechanism that involves direct binding to specific elements in the proximal IGF-IR promoter.

Folic acid and its metabolites modulate IGF-I receptor gene expression in colon cancer cells in a p53-dependent manner

The insulin-like growth factor-I receptor (IGF-IR) has an important role in colorectal cancer development and progression. IGF-IR displays a potent anti-apoptotic activity and is overexpressed in primary tumors and colon cancer-derived cell lines. Folic acid, a member of the vitamin B family, is a chemopreventive agent whose deficiency has been linked to an enhanced colon cancer risk. The present study was aimed at testing the hypothesis that part of the modulatory effect of folic acid on malignant transformation may be attributed to its ability to regulate IGF-IR gene expression. Regulation of IGF-IR gene expression by folic acid was assessed using western blots, RT-PCR, transient transfections and chromatin immunoprecipitation assays. Activation of the IGF-IR signaling pathway was evaluated by measuring phosphorylation of ERK, and apoptosis was assayed using poly (ADP-ribose) polymerase cleavage and annexin V-FITC staining. Results obtained showed that folic acid induced a dose-dependent decrease in IGF-IR protein and mRNA levels in the HCT116 +/+ colon cancer cell line. This effect was associated with a significant reduction in IGF-IR promoter activity. Similar effects were elicited by the folic acid metabolites dihydrofolic acid and tetrahydrofolic acid. In addition, folic acid abrogated the IGF-I-stimulated phosphorylation of the downstream signaling molecule ERK1/2 and exhibited a pro-apoptotic activity. Moreover, folic acid induced a significant decrease in Sp1 binding to the IGF-IR promoter region. Finally, folic acid had no effect in wild-type p53-depleted HCT116 -/- and Caco-2 cells. In conclusion, the mechanism of action of folic acid involves regulation of IGF-IR gene expression. The ability of folic acid to downregulate the IGF-I signal transduction pathway may allow the micronutrient to function as a chemopreventive agent. Folic acid deficiency, on the other hand, may lead to increased IGF-IR gene expression, with ensuing pathological activation by endocrine and/or autocrine/paracrine IGF-I.

The p53-family members p63 and p73 inhibit insulin-like growth factor-I receptor gene expression in colon cancer cells

The insulin-like growth factor-I receptor (IGF-IR) has a critical role in malignant transformation. Consistent with its antiapoptotic role, the IGF-IR gene is overexpressed in most types of cancer, including colorectal tumors. The recently identified p53 homologues, p63 and p73, exhibit some of the biological properties of p53, including the ability to transactivate p53-responsive genes and to induce apoptosis. In the present study, we examined the hypothesis that p63/p73 proteins may contribute to colon cancer cell proliferation via mechanism/s that involve regulation of IGF-IR gene expression. Using transient co-expression assays in colon cancer-derived HCT116 cells, we showed that both proteins inhibit IGF-IR promoter activity and endogenous IGF-IR levels in a dose-dependent manner, whereas mutant proteins are significantly impaired in their ability to suppress IGF-IR gene expression. These results are compatible with the notion that disruption of p63/p73-mediated signal transduction pathways in colon cancer may lead to increased IGF-IR gene transcription. In summary, we have identified the IGF-IR gene as a novel downstream target for p63/p73 action.

High expression level of insulin-like growth factor I receptor is associated with regional lymph node metastasis

4115 Background: Over-expression of the insulin-like growth factor I receptor (IGF-IR) has been found in many types of human cancers. In vivo, inhibition of the function of IGF-IR results in apoptosis and tumor growth attenuation, indicating that altered expression of IGF-IR may play an important role in human cancer. However, the role of IGF-IR in gastric cancer has not been explored. Methods: In this study, we determined the expression status of the IGF-IR in primary gastric cancer using immunohistochemistry. Results: In the 86 cases of resected gastric cancer examined, IGF-IR was over-expressed in 61.6% gastric tumor samples as compared to the adjacent cancer-free gastric mucosa (p=0.018). In patients with lymph node metastasis, high expression level of IGF-IR was detected in both the primary tumor site and metastatic tumor cells with lymph nodes (p=0.0102), implicating IGF-IR in the development of lymph node metastasis. It has been reported that the transcription factor Sp1 transactivates the IGF-IR promoter and regulates IGF-IR expression. We have previously showed that over-expression of Sp1 in gastric cancer associated with advanced stage and predicted poor survival. In the current study, the expression level of IGF-IR correlated with that of Sp1 (p=0.018). Further, the median survival for patients with low and high IGF-IR expression was 1242 days and 792 days, respectively. However, this survival difference was not statistically significant (p=0.16), indicating that a larger scale study is warranted to further determine the value of IGF-IR as a prognostic/survival marker. Conclusions: Taken together, our data suggest that alteration of IGF-IR expression is likely to play a significant role in the development and metastasis of gastric cancers. Although it is not an independent survival predictor, it is one of the essential down steam components of Sp1 that might affect patient outcome. No significant financial relationships to disclose.

Functional and physical interactions between BRCA1 and p53 in transcriptional regulation of the IGF-IR gene.

The insulin-like growth factor-I receptor (IGF-IR) mediates the biological actions of the IGFs, and is critical for normal mammary gland development as well as for malignant transformation. Transcription of the IGF-IR gene is under inhibitory control by a number of transcription factors with tumor suppressor activity, including BRCA1 and p53. To assess the potential functional interactions between BRCA1 and p53 in transcriptional control of the IGF-IR gene, co-transfections were performed on MCF-7 breast cancer cells using an IGF-IR promoter luciferase reporter construct together with expression vectors encoding BRCA1 and wild-type and mutant p53. Similar experiments were performed in the colorectal cancer cell line HCT116 (+/+), which expresses a wild-type p53 gene, and its HCT116 (-/-) derivative, which lacks p53. BRCA1 was able to suppress IGF-IR promoter activity both in the absence and presence of p53. However, BRCA1 had no effect in mutant p53-expressing cells. Co-immunoprecipitation experiments showed that BRCA1 and p53 physically interact. In summary, our data suggest that the transcriptional activity of BRCA1 depends on the cellular status of p53. Inability of mutant tumor suppressors to repress IGF-IR gene expression may result in increased IGF-IR levels and IGF binding, leading to a reduction in apoptosis and enhanced survival capacity of malignant cells.

BRCA1–Sp1 interactions in transcriptional regulation of the IGF-IR gene

The insulin-like growth factor-I receptor (IGF-IR) plays a critical role in breast tumorigenesis and is overexpressed in most primary tumors. BRCA1 is a transcription factor involved in numerous cellular processes, including DNA damage repair, cell growth, and apoptosis. Consistent with its tumor suppressor role, we demonstrated that BRCA1 repressed the activity of co-transfected IGF-IR promoter reporter constructs in a number of breast cancer-derived cell lines. Results of electrophoretic mobility shift assay showed that BRCA1 did not exhibit any specific binding to the IGF-IR promoter, although it prevented binding of Sp1. Co-immunoprecipitation experiments demonstrated that BRCA1 action was associated with specific interaction with Sp1 protein. Furthermore, using a series of glutathione S-transferase-tagged BRCA1 fragments, we mapped the Sp1-binding domain to a segment located between aa 260 and 802. In summary, our data suggest that the IGF-IR gene is a novel downstream target for BRCA1 action.

WT1-p53 Interactions in Insulin-like Growth Factor-I Receptor Gene Regulation

The insulin-like growth factor-I receptor (IGF-IR) plays a critical role in transformation. The expression of the IGF-IR gene is negatively regulated by a number of transcription factors, including the WT1 and p53 tumor suppressors. Previous studies have suggested both physical and functional interactions between the WT1 and p53 proteins. The potential functional interactions between WT1 and p53 in control of IGF-IR promoter activity were addressed by transient coexpression of vectors encoding different isoforms of WT1, together with IGF-IR promoter-luciferase reporter constructs, in p53-null osteosarcoma-derived Saos-2 cells, wild-type p53-expressing kidney tumor-derived G401 cells, and mutant p53-expressing, rhabdomyosarcoma-derived RD cells. Similar studies were also performed to compare p53-expressing Balb/c-3T3 and clonally derived p53-null, (10)1 fibroblasts and the colorectal cancer cell line HCT116 +/+, which expresses a wild-type p53 gene, and its HCT116 -/- derivative, in which the p53 gene has been disrupted by homologous recombination. WT1 splice variants lacking a KTS insert between zinc fingers 3 and 4 suppressed IGF-IR promoter activity in the absence of p53 or in the presence of wild-type p53. WT1 variants that contain the KTS insert are impaired in their ability to bind to the IGF-IR promoter and are unable to suppress IGF-IR promoter. In the presence of mutant p53, WT1 cannot repress the IGF-IR promoter. Coimmunoprecipitation experiments showed that p53 and WT1 physically interact, whereas electrophoretic mobility shift assay studies revealed that p53 modulates the ability of WT1 to bind to the IGF-IR promoter. In summary, the transcriptional activity of WT1 proteins and their ability to function as tumor suppressors or oncogenes depends on the cellular status of p53.

ATM-dependent expression of the insulin-like growth factor-I receptor in a pathway regulating radiation response

The ATM gene is mutated in the syndrome of ataxia telangiectasia (AT), associated with neurologic dysfunction, growth abnormalities, and extreme radiosensitivity. Insulin-like growth factor-I receptor (IGF-IR) is a cell surface receptor with tyrosine kinase activity that can mediate mitogenesis, cell transformation, and inhibition of apoptosis. We report here that AT cells express low levels of IGF-IR and show decreased IGF-IR promoter activity compared with wild-type cells. Complementation of AT cells with the ATM cDNA results in increased IGF-IR promoter activity and elevated IGF-IR levels, whereas expression in wild-type cells of a dominant negative fragment of ATM specifically reduces IGF-IR expression, results consistent with a role for ATM in regulating IGF-IR expression at the level of transcription. When expression of IGF-IR cDNA is forced in AT cells via a heterologous viral promoter, near normal radioresistance is conferred on the cells. Conversely, in ATM cells complemented with the ATM cDNA, specific inhibition of the IGF-IR pathway prevents correction of the radiosensitivity. Taken together, these results establish a fundamental link between ATM function and IGF-IR expression and suggest that reduced expression of IGF-IR contributes to the radiosensitivity of AT cells. In addition, because IGF-I plays a major role in human growth and metabolism and serves as a survival and differentiation factor for developing neuronal tissue, these results may provide a basis for understanding other aspects of the AT syndrome, including the growth abnormalities, insulin resistance, and neurodegeneration.

ATM-dependent expression of the insulin-like growth factor-I receptor in a pathway regulating radiation response.

The ATM gene is mutated in the syndrome of ataxia telangiectasia (AT), associated with neurologic dysfunction, growth abnormalities, and extreme radiosensitivity. Insulin-like growth factor-I receptor (IGF-IR) is a cell surface receptor with tyrosine kinase activity that can mediate mitogenesis, cell transformation, and inhibition of apoptosis. We report here that AT cells express low levels of IGF-IR and show decreased IGF-IR promoter activity compared with wild-type cells. Complementation of AT cells with the ATM cDNA results in increased IGF-IR promoter activity and elevated IGF-IR levels, whereas expression in wild-type cells of a dominant negative fragment of ATM specifically reduces IGF-IR expression, results consistent with a role for ATM in regulating IGF-IR expression at the level of transcription. When expression of IGF-IR cDNA is forced in AT cells via a heterologous viral promoter, near normal radioresistance is conferred on the cells. Conversely, in ATM cells complemented with the ATM cDNA, specific inhibition of the IGF-IR pathway prevents correction of the radiosensitivity. Taken together, these results establish a fundamental link between ATM function and IGF-IR expression and suggest that reduced expression of IGF-IR contributes to the radiosensitivity of AT cells. In addition, because IGF-I plays a major role in human growth and metabolism and serves as a survival and differentiation factor for developing neuronal tissue, these results may provide a basis for understanding other aspects of the AT syndrome, including the growth abnormalities, insulin resistance, and neurodegeneration.

BRCA1 Suppresses Insulin-like Growth Factor-I Receptor Promoter Activity: Potential Interaction between BRCA1 and Sp1

The insulin-like growth factor I receptor (IGF-I-R) has an important role in breast cancer etiology. The receptor is overexpressed by most breast cancers, where it functions as a potent antiapoptotic agent. BRCA1 is a tumor suppressor gene that is mutated in a large fraction of familial breast and ovarian cancers. Cotransfection of Saos-2, MCF7, and CHO cells with IGF-I-R promoter constructs driving luciferase reporter genes, and with a BRCA1 expression vector, suppressed promoter activity in a dose-dependent manner. Functional interactions between BRCA1 and Sp1 in the regulation of the IGF-I-R gene were studied in Schneider cells, a Drosophila cell line which lacks endogenous Sp1. In these cells BRCA1 suppressed 45% of the Sp1-induced trans-activation of the IGF-I-R promoter. These results suggest that BRCA1 is capable of suppressing the IGF-I-R promoter in a number of cell lines, thus resulting in low levels of receptor mRNA and protein. Mutant versions of BRCA1 lacking trans-activational activity can potentially derepress the IGF-I-R promoter. Activation of the overexpressed receptor by locally produced or circulating IGFs may be a crucial step in breast and ovarian cancer progression.

Regulation of insulin-like growth factor I receptor promoter activity by wild-type and mutant versions of the WT1 tumor suppressor.

The insulin-like growth factor I (IGF-I) receptor is a transmembrane tyrosine kinase that mediates the growth-promoting effects of IGF-I and IGF-II. Changes in IGF-I receptor messenger RNA levels are reflected in cell surface receptor number, and modulation of IGF-I receptor levels affects tumorigenicity in numerous cellular models; thus, control of IGF-I receptor gene expression appears to be an important level at which cellular proliferation and tumorigenic potential may be regulated. We have previously shown that the product of the WT1 Wilms' tumor suppressor gene represses IGF-I receptor gene expression both in vitro and in vivo, and that decreased WT1 levels are correlated with up-regulation of IGF-I receptor gene expression in Wilms' tumor, benign prostatic hyperplasia, and breast cancer. Gene regulation by WT1 is complex, in that the WT1 gene encodes a variety of products as a result of alternative splicing and RNA editing, and a number of missense point mutations have been characterized in Wilms' tumor-associated syndromes. Additionally, the WT1 protein has been demonstrated to self-associate through its N-terminal domain, although the role of this intermolecular interaction in transcriptional regulation by WT1 is unclear. In this report, we analyze the relative activity of wild-type and mutant versions of the WT1 protein with respect to IGF-I receptor promoter activity in transient transfection assays and assess the potential contribution of WT1 self-association to IGF-I receptor regulation using the yeast two-hybrid system. Of the naturally occurring variations in WT1 structure, only the presence of a three-amino acid KTS insert in the zinc finger domain introduced by alternative splicing of exon 9 had a significant effect on WT1 repression of IGF-I receptor promoter activity. The N- and C-terminal domains of WT1 also exhibited partial repression, as did the most common mutant version of the WT1 protein associated with Denys-Drash syndrome. Mutations in the WT1 N-terminus attenuated WT1 self-association in the yeast two-hybrid system, but did not impair transcriptional repression. Our results suggest that 1) the DNA-binding capacity of WT1 is critical for maximal repression of the IGF-I receptor promoter, but some effects may be mediated through protein-protein interactions involving the N-terminal domain; 2) WT1 self-association may not be required for repression of the IGF-I receptor promoter; and 3) the Denys-Drash syndrome version of the WT1 protein may exhibit residual or possible gain of function activity in some contexts rather than exerting dominant negative effects, as has been proposed previously.

P53 regulates insulin-like growth factor-I (IGF-I) receptor expression and IGF-I-induced tyrosine phosphorylation in an osteosarcoma cell line: interaction between p53 and Sp1.

The insulin-like growth factor-I receptor (IGF-IR) is involved in tumorigenesis. The aim of the present study was to investigate whether the IGF-IR is a physiological target for p53 in osteosarcoma cells. The p53-induced regulation of IGF-IR levels was studied in a tetracycline-regulated expression system. When expressed in Saos-2, osteosarcoma cells that lack p53, wild-type p53 decreased, whereas mutated p53 increased IGF-IR expression, and IGF-I-induced tyrosine phosphorylation of the IGF-IR. Similarly, wild-type p53 decreased IGF-I-induced tyrosine phosphorylation of IRS-1. A functional and physical interaction between p53 and Sp1, in the regulation of the IGF-R, was studied in osteosarcoma cells. Expression of p53 decreased IGF-IR promoter activity, whereas no effect on promoter activity was seen by Sp1 expressed alone. However, Sp1 counteracted the inhibitory effect of p53 on IGF-IR promoter activity in a dose-dependent manner. Furthermore, wild-type and mutated p53 were coimmunoprecipitated with Sp1, indicating a physical interaction between p53 and Sp1. In conclusion, p53 regulates IGF-IR expression, as reflected by a reduction in IGF-IR protein and a parallel reduction in IGF-I-induced tyrosine phosphorylation of the IGF-IR and IRS-1 in an osteosarcoma cell line. These data indicate that the IGF-I receptor is a physiological target for p53 in osteosarcoma cells. Furthermore, data supporting an interaction between p53 and Sp1 in the regulation of the promoter activity of IGF-IR are presented.

Differential Regulation of Insulin-like Growth Factor-I (IGF-I) Receptor Gene Expression by IGF-I and Basic Fibroblastic Growth Factor

Insulin-like growth factor-I receptor (IGF-IR) gene expression is regulated by various stimuli, including hormones, growth factors, and nutritional status. We have investigated the molecular mechanism by which two growth factors, insulin-like growth factor-I (IGF-I) and basic fibroblast growth factor (bFGF) regulate IGF-IR gene expression. bFGF increases the endogenous IGF-IR mRNA levels and IGF-IR promoter activity. This effect is mediated by a region of the IGF-IR promoter located between nucleotides -476 and -188 in the 5'-flanking region. In contrast, IGF-I decreases the IGF-IR mRNA levels. IGF-I down-regulates IGF-IR transcriptional activity as deduced from experiments in which the levels of pre-mRNA and mRNA were measured. IGF-I reduced pre-mRNA and mRNA levels in parallel, while the mRNA stability was found to be unchanged by IGF-I treatment. While these results strongly suggest an effect of IGF-I on IGF-IR transcriptional activity, no specific IGF-I response element was demonstrated in the 5'-untranslated region or 5'-flanking region studied. Thus, bFGF and IGF-I have differential effects on IGF-IR gene transcription, with the IGF-I response region as yet unidentified.

The GC factor regulates the expression of the insulin-like growth factor-I receptor.

We have transfected a plasmid expressing the transcriptional regulator GC Factor (GCF) into cell lines and have found that the GCF: 1 causes a decrease in the levels of insulin-like growth factor I receptor (IGF-IR) mRNA; 2 causes a decrease in the number of IGF-IRs; and 3 represses the activity of the IGF-IR promoter. In addition, we show that the regulation of IGF-IR expression by GCF plays a physiological role in the control of cellular proliferation in vitro.

Regulation of insulin-like growth factor I receptor gene expression by Sp1: physical and functional interactions of Sp1 at GC boxes and at a CT element.

The insulin-like growth factor I (IGF-I) receptor mediates signal transduction by the IGFs and plays a critical role in growth and development. The proximal promoter region of the rat IGF-I receptor gene contains multiple Sp1 consensus-binding sites (GC boxes). Various promoter fragments fused to a luciferase reporter gene were transiently cotransfected together with an Sp1 expression vector into Drosophila Schneider cells, which lack endogenous Sp1. A proximal promoter fragment containing 476 nucleotides of 5'-flanking region and 640 nucleotides of 5'-untranslated region was strongly activated by Sp1 (an average of 116-fold), and progressive 5'-deletions of the promoter that sequentially removed GC boxes reduced Sp1 activation to 15-fold over basal promoter activity. DNase I footprinting studies with purified Sp1 protein revealed four GC boxes in the 5'-flanking region of the promoter and one homopurine/homopyrimidine motif (CT element) in the 5'-untranslated region that bound Sp1. Mutation of the CT element reduced Sp1 activation by 70%. Taken together, these results demonstrate that Sp1 can regulate expression of the IGF-I receptor promoter by acting both on GC boxes in the 5'-flanking region of the promoter and on a CT element in the 5'-untranslated region.

Cloning and characterization of the human insulin-like growth factor-I receptor gene 5'-flanking region.

The insulin-like growth factor-I receptor (IGFIR) is a membrane-bound glycoprotein that mediates the action of insulin-like growth factors. The cDNAs for the human IGFIR have been cloned and expressed, but the structures of the gene and its promoter have not been elucidated. In this study, we isolated an IGFIR promoter clone from a human chromosome 15 library. This clone contained the promoter, first exon, and a portion of the first intron. Sequence analysis of the 5' region that contained the promoter revealed that it lacked both TATA and CAAT boxes. The promoter contained binding sites for the transcription factors Sp1, AP-2, and the epidermal growth factor receptor transcription factor (ETF). Primer extension analysis of IGFIR mRNA indicated the presence of a single transcription start site 1,012 bp upstream from the ATG. When the putative promoter was ligated into a promoterless CAT vector and transfected mto HEPG2 cells, CAT activity was expressed, indicating that promoter activity was contained in this fragment. Other constructs containing the promoter and portions of the 5' untranslated region were used in transfection studies, and indicated that the 5' untranslated regions may play a role in promoter activity. Comparison of the human IGFIR promoter with that of the rat IGFIR promoter revealed significant sequence homology. Comparison of the IGFIR promoter with that of the human insulin receptor (IR) revealed structural similarities, although the arrangement of promoter elements differed.