High-affinity insulin binding to an atypical insulin-like growth factor-I receptor in human breast cancer cells

Cell cycle control by the insulin-like growth factor signal: at the crossroad between cell growth and mitotic regulation

In proliferating cells and tissues a number of checkpoints (G1/S and G2/M) preceding cell division (M-phase) require the signal provided by growth factors present in serum. IGFs (I and II) have been demonstrated to constitute key intrinsic components of the peptidic active fraction of mammalian serum. In vivo genetic ablation studies have shown that the cellular signal triggered by the IGFs through their cellular receptors represents a non-replaceable requirement for cell growth and cell cycle progression. Retroactive and current evaluation of published literature sheds light on the intracellular circuitry activated by these factors providing us with a better picture of the pleiotropic mechanistic actions by which IGFs regulate both cell size and mitogenesis under developmental growth as well as in malignant proliferation. The present work aims to summarize the cumulative knowledge learned from the IGF ligands/receptors and their intracellular signaling transducers towards control of cell size and cell-cycle with particular focus to their actionable circuits in human cancer. Furthermore, we bring novel perspectives on key functional discriminants of the IGF growth-mitogenic pathway allowing re-evaluation on some of its signal components based upon established evidences.

The IGF-II-Insulin Receptor Isoform-A Autocrine Signal in Cancer: Actionable Perspectives

Insulin receptor overexpression is a common event in human cancer. Its overexpression is associated with a relative increase in the expression of its isoform A (IR^A), a shorter variant lacking 11 aa in the extracellular domain, conferring high affinity for the binding of IGF-II along with added intracellular signaling specificity for this ligand. Since IGF-II is secreted by the vast majority of malignant solid cancers, where it establishes autocrine stimuli, the co-expression of IGF-II and IR^A in cancer provides specific advantages such as apoptosis escape, growth, and proliferation to those cancers bearing such a co-expression pattern. However, little is known about the exact role of this autocrine ligand–receptor system in sustaining cancer malignant features such as angiogenesis, invasion, and metastasis. The recent finding that the overexpression of angiogenic receptor kinase EphB4 along with VEGF-A is tightly dependent on the IGF-II/IR^A autocrine system independently of IGFIR provided new perspectives for all malignant IGF2omas (those aggressive solid cancers secreting IGF-II). The present review provides an updated view of the IGF system in cancer, focusing on the biology of the autocrine IGF-II/IR^A ligand–receptor axis and supporting its underscored role as a malignant-switch checkpoint target.

IGFBP-2 - taking the lead in growth, metabolism and cancer

The activity of the Insulin-like Growth Factors (IGFs) ligands elicited via their receptors and transduced by various intracellular signal pathways is modulated by the IGF Binding Proteins (IGFBPs). Among all the IGFBPs, IGFBP-2 has been implicated in the regulation of IGF activity in most tissue and organs. Besides binding to IGFs in the circulation these IGF-regulatory activities of IGFBP-2 involve interactions with components of the extracellular matrix, cell surface proteoglycans and integrin receptors. In addition to these local peri-cellular activities, IGFBP-2 exerts other key functions within the nucleus, where IGFBP-2 directly or indirectly promotes transcriptional activation of specific genes. All of these IGFBP-2 activities, intrinsic or dependent on IGFs, contribute to its functional roles in growth/development, metabolism and malignancy as evidenced by studies in IGFBP-2 animal models and also by many in vitro studies. Finally, preclinical studies have demonstrated that IGFBP-2 administration can be beneficial in improving metabolic responses (inhibition of adipogenesis and enhanced insulin sensitivity), while blockade of IGFBP-2 appears to be an effective approach to inhibiting tumour growth and metastasis.

Two-layer modular analysis of gene and protein networks in breast cancer

Background

Genomic, proteomic and high-throughput gene expression data, when integrated, can be used to map the interaction networks between genes and proteins. Different approaches have been used to analyze these networks, especially in cancer, where mutations in biologically related genes that encode mutually interacting proteins are believed to be involved. This system of integrated networks as a whole exhibits emergent biological properties that are not obvious at the individual network level. We analyze the system in terms of modules, namely a set of densely interconnected nodes that can be further divided into submodules that are expected to participate in multiple biological activities in coordinated manner.

Results

In the present work we construct two layers of the breast cancer network: the gene layer, where the correlation network of breast cancer genes is analyzed to identify gene modules, and the protein layer, where each gene module is extended to map out the network of expressed proteins and their interactions in order to identify submodules. Each module and its associated submodules are analyzed to test the robustness of their topological distribution. The constituent biological phenomena are explored through the use of the Gene Ontology. We thus construct a “network of networks”, and demonstrate that both the gene and protein interaction networks are modular in nature. By focusing on the ontological classification, we are able to determine the entire GO profiles that are distributed at different levels of hierarchy. Within each submodule most of the proteins are biologically correlated, and participate in groups of distinct biological activities.

Conclusions

The present approach is an effective method for discovering coherent gene modules and protein submodules. We show that this also provides a means of determining biological pathways (both novel and as well those that have been reported previously) that are related, in the present instance, to breast cancer. Similar strategies are likely to be useful in the analysis of other diseases as well.

The Interactions between Insulin and Androgens in Progression to Castrate-Resistant Prostate Cancer

An association between the metabolic syndrome and reduced testosterone levels has been identified, and a specific inverse relationship between insulin and testosterone levels suggests that an important metabolic crosstalk exists between these two hormonal axes; however, the mechanisms by which insulin and androgens may be reciprocally regulated are not well described. Androgen-dependant gene pathways regulate the growth and maintenance of both normal and malignant prostate tissue, and androgen-deprivation therapy (ADT) in patients exploits this dependence when used to treat recurrent and metastatic prostate cancer resulting in tumour regression. A major systemic side effect of ADT includes induction of key features of the metabolic syndrome and the consistent feature of hyperinsulinaemia. Recent studies have specifically identified a correlation between elevated insulin and high-grade PCa and more rapid progression to castrate resistant disease. This paper examines the relationship between insulin and androgens in the context of prostate cancer progression. Prostate cancer patients present a promising cohort for the exploration of insulin stabilising agents as adjunct treatments for hormone deprivation or enhancers of chemosensitivity for treatment of advanced prostate cancer.

Insulin-mediated acceleration of breast cancer development and progression in a nonobese model of type 2 diabetes

Epidemiologic studies suggest that type 2 diabetes (T2D) increases breast cancer risk and mortality, but there is limited experimental evidence supporting this association. Moreover, there has not been any definition of a pathophysiological pathway that diabetes may use to promote tumorigenesis. In the present study, we used the MKR mouse model of T2D to investigate molecular mechanisms that link T2D to breast cancer development and progression. MKR mice harbor a transgene encoding a dominant-negative, kinase-dead human insulin-like growth factor-I receptor (IGF-IR) that is expressed exclusively in skeletal muscle, where it acts to inactivate endogenous insulin receptor (IR) and IGF-IR. Although lean female MKR mice are insulin resistant and glucose intolerant, displaying accelerated mammary gland development and enhanced phosphorylation of IR/IGF-IR and Akt in mammary tissue, in the context of three different mouse models of breast cancer, these metabolic abnormalities were found to accelerate the development of hyperplastic precancerous lesions. Normal or malignant mammary tissue isolated from these mice exhibited increased phosphorylation of IR/IGF-IR and Akt, whereas extracellular signal-regulated kinase 1/2 phosphorylation was largely unaffected. Tumor-promoting effects of T2D in the models were reversed by pharmacological blockade of IR/IGF-IR signaling by the small-molecule tyrosine kinase inhibitor BMS-536924. Our findings offer compelling experimental evidence that T2D accelerates mammary gland development and carcinogenesis,and that the IR and/or the IGF-IR are major mediators of these effects.

IGF and insulin receptor signaling in breast cancer

Major molecular abnormalities in breast cancer include the deregulation of several components of the IGF system. It is well recognized that the epithelial breast cancer cells commonly overexpress the IGF-I receptor while IGF-II is expressed by the tumor stroma. In view to the fact that the IGF-IR has mitogenic, pro-invasive and anti-apoptotic effects and mediates resistance to a variety of anti-cancer therapies, breast cancer is expected to be a candidate to therapeutic approaches aimed to inhibit the IGF-IR. However, there is increasing awareness that IGF system in cancer undergoes signal diversification by various mechanisms. One of these mechanisms is the aberrant expression of insulin receptor (IR) isoform A (IR-A), which is a high affinity receptor for both insulin and IGF-II, in breast cancer cells. Moreover, overexpression of both IGF-IR and IR-A in breast cancer cells, leads to overexpression of hybrid IR/IGF-IR receptors (HRs) as well. Upon binding to IGF-II, both IR-A and HRs may activate unique signaling patterns, which predominantly mediate proliferative effects. A better understanding of IGF system signal diversification in breast cancer has important implications for cancer prevention measures, which should include control of insulin resistance and associated hyperinsulinemia. Moreover, in addition to the IGF-IR, both IR-A and HRs should be also considered as molecular targets for anti-cancer therapies.

Insulin receptor is an independent predictor of a favorable outcome in early stage breast cancer

Fasting insulin is related to outcome in early breast cancer. We evaluated the expression of insulin receptor (IR) and its prognostic significance in patients with early stage breast cancer. Tumors from 191 patients with T1-3, N0-1, M0 breast cancer who were enrolled at a single center of a multicenter cohort study were used to construct microarrays with subsequent immunohistochemical evaluation of IR, IGF-IR, ER, PgR and HER2/neu. Correlation of biomarker expression with traditional prognostic factors, serum biochemistry (notably insulin) and clinical outcome was assessed. IR was strongly positive (Allred score = 8) in 54% of tumors. High IR expression significantly correlated with favorable prognostic markers (low tumor grade, lymph node negativity and progesterone receptor positivity) but not with fasting levels of circulating insulin. At a median follow-up of 9.1 years, high vs. low IR expression (an Allred score of 8 vs. 0-7) was associated with statistically significant improved distant disease-free survival (multivariate hazard ratio (HR) = 0.4; P = 0.027) and overall survival (multivariate HR = 0.26; P = 0.005). IR is highly expressed in the majority of early stage breast cancers but this expression is not clearly down-regulated in the presence of high insulin levels. Furthermore, high expression of IR is independently and significantly associated with more favorable clinical outcomes. Follow-up intervention research is recommended.

The insulin-like growth factor system and its pleiotropic functions in brain

In recent years, much interest has been devoted to defining the role of the IGF system in the nervous system. The ubiquitous IGFs, their cell membrane receptors, and their carrier binding proteins, the IGFBPs, are expressed early in the development of the nervous system and are therefore considered to play a key role in these processes. In vitro studies have demonstrated that the IGF system promotes differentiation and proliferation and sustains survival, preventing apoptosis of neuronal and brain derived cells. Furthermore, studies of transgenic mice overexpressing components of the IGF system or mice with disruptions of the same genes have clearly shown that the IGF system plays a key role in vivo.

Functional insulin receptors on human epithelial ovarian carcinoma cells: implications for IGF-II mitogenic signaling

The insulin receptor mediates a proliferative response in certain transformed cells, but little is known about its function in ovarian cancer. We used human epithelial ovarian carcinoma cell lines and lifespan-extended normal ovarian surface epithelial (OSE) cells to examine (125)I-insulin binding and mitogenic responses to insulin. All cancer cell and OSE cultures specifically bound (125)I-insulin. Except for OV202, the carcinoma lines had elevated insulin binding compared with OSE cells. All carcinoma lines except OV202 expressed insulin receptor as detected by flow cytometry and increased (3)H-thymidine incorporation or cell number in response to 0.1-10 nM insulin. Interestingly, similar concentrations of IGF-II also induced proliferation of the insulin-responsive cancer cell lines and displaced (125)I-insulin binding. Direct binding of (125)I-IGF-II to the insulin receptor was visualized by cross-linking and immunoprecipitation. Binding of IGF-II to the insulin receptor and a proliferative effect of insulin suggest the presence of insulin receptor isoform A. Real-time PCR analyses confirm that insulin receptor isoform A expression predominates over isoform B expression in the ovarian carcinoma cell lines. This report suggests that the insulin receptor may play a role in the regulation of ovarian cancer cell growth.

Insulin/insulin-like growth factor-I and estrogen cooperate to stimulate cyclin E-Cdk2 activation and cell Cycle progression in MCF-7 breast cancer cells through differential regulation of cyclin E and p21(WAF1/Cip1)

Estrogens and insulin/insulin-like growth factor-I (IGF-I) are potent mitogens for breast epithelial cells and, when co-administered, induce synergistic stimulation of cell proliferation. To investigate the molecular basis of this effect, a MCF-7 breast cancer cell model was established where serum deprivation and concurrent treatment with the pure estrogen antagonist, ICI 182780, inhibited growth factor and estrogen action and arrested cells in G(0)/G(1) phase. Subsequent stimulation with insulin or IGF-I alone failed to induce significant S-phase entry. However, these treatments increased cyclin D1, cyclin E, and p21 gene expression and induced the formation of active Cdk4 complexes but resulted in only minor increases in cyclin E-Cdk2 activity, likely due to recruitment of the cyclin-dependent kinase (CDK) inhibitor p21(WAF1/Cip1) into these complexes. Treatment with estradiol alone resulted in a greater increase in cyclin D1 gene expression but markedly decreased p21 expression, with a concurrent increase in Cdk4 and Cdk2 activity and subsequent synchronous entry of cells into S phase. Co-administration of insulin/IGF-I and estrogen induced synergistic stimulation of S-phase entry coincident with synergistic activation of high molecular mass (approximately 350 kDa) cyclin E-Cdk2 complexes lacking p21. To determine if the ability of estrogen to deplete p21 was central to these effects, cells stimulated with insulin and estradiol were infected with an adenovirus expressing p21. Induction of p21 to levels equivalent to those following treatment with insulin alone markedly inhibited the synergism between estradiol and insulin on S-phase entry. Thus the ability of estradiol to antagonize the insulin-induced increase in p21 gene expression, with consequent activation of cyclin E-Cdk2, is a central component of the synergistic stimulation of breast epithelial cell proliferation induced by simultaneous activation of the estrogen and insulin/IGF-I signaling pathways.

Insulin receptor activation by IGF-II in breast cancers: evidence for a new autocrine/paracrine mechanism

IGF-II, produced by breast cancer epithelial and stromal cells, enhances tumor growth by activating the IGF-I receptor (IGF-I-R) via autocrine and paracrine mechanisms. Previously we found that the insulin receptor (IR), which is related to the IGF-I-R, is overexpressed in breast cancer cells. Herein, we find that, in breast cancer the IR is activated by IGF-II. In eight human breast cancer cell lines studied there was high affinity IGF-II binding to the IR, with subsequent IR activation. In these lines, IGF-II had a potency up to 63% that of insulin. In contrast, in non malignant human breast cells, IGF-II was less than 1% potent as insulin. Via activation of the IR tyrosine kinase IGF-II stimulated breast cancer cell growth. Moreover, IGF-II also activated the IR in breast cancer tissue specimens; IGF-II was 10-100% as potent as insulin. The IR occurs in two isoforms generated by alternative splicing of exon 11; these isoforms are IR-A (Ex11-) and IR-B (Ex11+). IR-A was predominantly expressed in breast cancer cells and specimens and the potency of IGF-II was correlated to the expression of this isoform (P<0.0001). These data indicate, therefore, that the IR-A, which binds IGF-II with high affinity, is predominantly expressed in breast cancer cells and represents a new autocrine/paracrine loop involved in tumor biology.

Insulin receptor what role in breast cancer?

It is commonly believed that the insulin receptor mainly mediates the metabolic effects of insulin, whereas the closely related IGF-I receptor is considered a major factor for the regulation of cell proliferation. Experimental and epidemiological evidence indicates, however, that insulin and insulin receptors may play an important role in breast cancer. This article reviews evidence indicating that (a) insulin receptors are overexpressed in human breast cancer, (b) insulin stimulates growth in breast cancer cells, (c) cells transfected with human insulin receptor may acquire a ligand-dependent transformed phenotype, and (d) breast cancer is associated with insulin resistance and hyperinsulinemia. These findings may open new possibilities in breast cancer prevention, prognosis assessment, and therapy. (Trends Endocrinol Metab 1997; 8:306-312). (c) 1997, Elsevier Science Inc.

ASPB10 insulin induction of increased mitogenic responses and phenotypic changes in human breast epithelial cells: evidence for enhanced interactions with the insulin-like growth factor-I receptor

The human insulin analogue ASPB10 has been reported to have increased affinity for the insulin receptor and to cause breast cancer in female rats. In the study reported here, we investigated whether ASPB10 has an increased mitogenic potency and induces a transformed phenotype in cultured human breast cells. In both MCF-10 cells (a non-malignant human breast line) and MCF-7 cells (a human breast cancer cell line), ASPB10 was approximately twofold more potent than insulin in competing for 125I-insulin binding but sevenfold to tenfold more potent than insulin in competing for 125I-insulin-like growth factor (IGF)-I binding. In addition, ASPB10 was twofold more potent than insulin in stimulating insulin receptor autophosphorylation but significantly more potent in stimulating IGF-I receptor autophosphorylation in both cell lines. Moreover, ASPB10 was approximately sevenfold more potent than insulin in stimulating the growth of MCF-10 and MCF-7 cells. This increased mitogenic effect of ASPB10 was significantly inhibited (but not abolished) when cells were cultured in the presence of alpha-IR3, a monoclonal antibody to the IGF-I receptor. ASPB10, but not insulin, caused phenotypic changes (focus formation) in MCF-10 cells. Neither agent caused colony formation in soft agar in MCF-10 cells, but ASPB10 was more potent than insulin in stimulating colony formation in MCF-7 cells. These observations indicate that in human breast cells, ASPB10 has enhanced mitogenic effects and induces phenotypic changes as a consequence of its activation of both insulin and IGF-I receptors.

Insulin receptors in syncytiotrophoblast and fetal endothelium of human placenta. Immunohistochemical evidence for developmental changes in distribution pattern

The localisation of insulin receptors (IR) was investigated on cryosections of human non-pathologic first trimester and full term placentae by indirect immunohistochemistry with three different monoclonal antibodies (MABS). In placentae from 6 to 10 weeks postmenstruation (p-m.), only syncytiotrophoblast was stained, predominantly that of mesenchymal villi and syncytial sprouts, which are areas of high proliferative activity. In placentae from 11 to 14 weeks p-m., endothelial cells commenced to react with the IR MABS and the syncytiotrophoblast was less intensely labelled than at weeks 6 to 10 p-m. In term placentae, the microvillous membrane of the syncytiotrophoblast showed only patches of weak immunoreactivity. In contrast, the endothelial cells in the placenta but not in the umbilical cord were strongly stained. The amniotic epithelium in the chorionic plate and fibroblasts in the stroma were conspicuously labelled. The data indicate: (1) the receptor density on villous syncytiotrophoblast decreases and that of fetal endothelium increases' throughout gestation; (2) syncytiotrophoblast of human term placentae expresses a low level per unit area of surface IR; and (3) the majority of IR in human term placentae is located in fetal endothelium. Apart from yet unknown functional effects of maternal and fetal insulin at the placental barrier, the results suggest a growth promoting effect on the trophoblast of maternal insulin in first trimester as well as developmental effects of fetal insulin on the feto-placental vessels at term.

Insulin-like growth factor mediated stromal-epithelial interactions in human breast cancer

The prominent 'desmoplastic' or stromal reaction seen in many invasive breast carcinomas lead to early speculation that stromal cells play a role in breast cancer pathogenesis. Experimental evidence now supports this hypothesis and interactions between stromal cells and epithelial cells appear to be important for both normal mammary development and neoplasia. The identification of genes that are selectively expressed in the stroma of malignant breast lesions has recently provided new insights into the molecular basis of stromal-epithelial interactions. Stromally expressed genes include growth factors, proteases and extracellular matrix proteins, all biological activities with potential roles in malignant progression. Investigations discussed here concern the nature of the paracrine signals provided by malignant epithelial cells that activate changes in stromal gene expression, the effect that the stromally derived factors have on the behavior of malignant epithelial cells and the identification of novel factors and receptors in either stroma or epithelia that contribute to their mutual interactions. These questions will be addressed in the context of this laboratory's studies on insulin-like growth factors, as these molecules show marked differences in stromal expression between benign and malignant breast tissue and thus provide a useful paradigm for investigations into the paracrine environment of an evolving breast tumor.

Insulinlike growth factors and binding proteins in colon cancer

Insulinlike growth factors (IGFs) express anabolic and mitogenic activity on wide variety of cells. Besides endocrine effects, IGFs have major autocrine and paracrine effects on many cellular functions. Two factors that significantly affect the extent of cellular response to IGFs include the membrane receptors for IGFs and the soluble binding proteins (BPs), which modulate the action of IGFs at the receptor level. IGFs, IGF receptors, and IGFs and their BPs (IGF-BPs) thus constitute three components of the IGF system. A role of IGFs in the transformation and proliferation of cancer cells has become increasingly evident in the past few years. Studies from several laboratories show that all three components of the IGF system may play an important role in the proliferation of colon cancers. It was recently shown that the relative expression of IGFs and IGF/BPs may critically control the metastatic potential of colon cancers. The purpose of this article is to summarize our current knowledge of the IGF system and to present support for a significant role of IGFs in the initiation and growth of colon cancers. The expression and structural aspects of IGFs, their receptors, and BPs are outlined first, followed by a discussion of the role of IGFs in gastrointestinal functions and in colon cancers.

Ultrastructural localization of insulin receptors in human placenta

PROBLEM

Investigations to date on localization of placental insulin receptors (IR) at the ultrastructural level focused on the maternal facing side of the placenta but did not allow localization of IRs to intracellular compartments.

METHOD

The ultrastructural localization of IR in placentae from early and term gestation was studied using the immunogold technique and a monoclonal antibody against the external domain of the IR (clone MA20).

RESULTS

At 10 wk gestation, there were high levels of IR in both cytotrophoblast and syncytiotrophoblast, with sparse microvillous labeling. Fetal vessels contained various amounts of IR while stromal cells were also labeled. By 13 wk, this pattern was unchanged; however, at term, fewer IR were present in the syncytiotrophoblast, with more label on microvilli than before. Fetal vessels were well labeled, while stromal cells appeared unchanged. Intense labeling of fetal erythrocytes provided an internal positive control, while omission of the primary antiserum produced loss of binding.

CONCLUSION

The data suggest a spatiotemporal alteration in placental IR distribution during pregnancy, possibly reflecting a shift in control of placental growth and metabolism from mother to fetus.

Purified hybrid insulin/insulin-like growth factor-I receptors bind insulin-like growth factor-I, but not insulin, with high affinity

Hybrid insulin/insulin-like growth factor-I (IGF-I) receptors have previously been described in human placenta, but it has not been possible to study their properties in the presence of classical insulin receptors and type I IGF receptors. To facilitate the purification of hybrids, we produced an anti-peptide monoclonal antibody IGFR 1-2, directed against the C-terminal peptide of the type I IGF receptor beta-subunit. The antibody bound native human and rat type I IGF receptors, and reacted specifically with the beta-subunit on immunoblots. Solubilized placental microsomal membranes were depleted of classical type I IGF receptors by incubation with an immobilized monoclonal antibody IGFR 24-55, which reacts well with type I receptors but very poorly with hybrid receptors. Residual hybrid receptors were then isolated by incubation with immobilized antibody IGFR 1-2, and recovered by elution with excess of synthetic peptide antigen. Binding properties of hybrids were compared with those of immuno-affinity-purified insulin receptors and type I IGF receptors, by using the radioligands 125I-IGF-I and 125I-insulin. Hybrids bound approx. 20 times as much 125I-IGF-I as 125I-insulin at tracer concentrations (approx. 0.1 nM). The binding of 125I-insulin, but not 125I-IGF-I, to hybrids increased after treatment with dithiothreitol to reduce disulphide bonds between the alpha-subunits. Hybrids behaved very similarly to type I receptors with respect to the inhibition of 125I-IGF-I binding by unlabelled IGF-I and insulin. By contrast, the affinity of hybrids for insulin was approx. 10-fold lower than that of classical insulin receptors, as assessed by inhibition of 125I-insulin binding by unlabelled hormone. It is concluded that the properties of insulin receptors, but not IGF receptors, are markedly affected by assembly as hybrid compared with classical structures, and that hybrids are more likely to be responsive to IGF-I than insulin under physiological conditions.

Interference of the IGF system as a strategy to inhibit breast cancer growth

Experimental evidence suggests that human breast cancer cells can be regulated by the IGF-I and IGF-II present in the tumor stromal elements and/or by the endogenous tumor cell IGF-II in a paracrine or autocrine fashion. Thus, blockade of the receptor signalling pathway could lead to diminished tumor growth. Blockade of the type I IGF receptor by a monoclonal antibody (alpha IR3) has been used as a strategy to demonstrate the importance of the IGF pathway. Although alpha IR3 could not block serum-free growth of breast cancer cell lines, it could inhibit anchorage independent growth in most cell lines in the presence of serum. In vivo, alpha IR3 administered at the time of tumor cell inoculation could inhibit MDA-MB-231 tumor formation in athymic mice; however, inhibition of established tumors was not seen. Moreover, alpha IR3 could not inhibit tumor formation of the MCF-7 cell line in vivo. These results suggest that blockade of the type I IGF receptor can inhibit the growth of some breast cancer cells both in vitro and in vivo. Future anti-growth factor strategies include the combination of anti-IGF receptor antibodies with IGF neutralizing modalities, the dual blockade of growth factor receptors (epidermal growth factor receptor and type I IGF receptor), and combinations of steroid hormone antagonists and anti-growth factor treatments to maximize tumor inhibition.

The insulin receptor and type I IGF receptor: comparison of structure and function

The insulin receptor and type I IGF receptor are closely related in structure and function. The receptors are heterotetrameric glycoproteins, of structure alpha beta beta alpha, which are widely distributed in mammalian tissues. A third member of this receptor family has been described, the insulin receptor-related receptor for which a ligand has still to be identified. It has also been demonstrated that the insulin receptor and IGF receptor form alpha beta beta alpha hybrids in cells expressing both receptors. The key elements in the function of any receptor are recognition of ligand and transmission of an intracellular signal. In the insulin and IGF receptors, determinants of binding specificity are contained within amino-terminal and cysteine-rich domains of the extracellular alpha-subunit. Intracellular signalling is dependent on ligand activated tyrosine kinase activity in the transmembrane beta-subunit, which phosphorylates both the receptor itself and the specific substrate insulin receptor substrate-1 (IRS-1). Phosphorylated IRS-1 binds the enzyme phosphatidylinositol 3-kinase and may act as a multivalent docking site for SH2 domains of other proteins involved in signalling. The possibility that some signalling molecules interact directly with the receptors has not been ruled out. The specificity of action of insulin and IGFs in vivo depends on differences between the respective receptors in tissue distribution, ligand binding specificity and intrinsic signalling capacity. However, the detailed aspects of gene and receptor structure which underly these functional differences are still poorly understood. Moreover, the issue of specificity is complicated by the existence of hybrid and atypical receptors, which in principle could bind and respond to both insulin and IGF-I, although the physiological significance of these receptor subtypes is at present unclear.