IGFs and human cancer: implications regarding the risk of growth hormone therapy

Perturbations of the insulin-like growth factor (IGF) axis, including the autocrine production of IGFs, IGF binding proteins (IGFBPs) and IGFBP proteases such as prostate specific antigen (PSA), and cathepsin D have been identified in prostate, lung and breast cancer cells and tissues. Serum IGFBP-3 levels have been found to be negatively correlated to the risk of cancer. Interestingly, IGFBP-3 is a potent inhibitor of IGF action and also mediates apoptosis via an IGF-independent mechanism. Recent case-control studies have found an approximately 10% increase in the serum levels of IGF-I in patients with prostate, breast and lung cancers, which are among the most frequently diagnosed cancers. While the studies indicate an association between serum IGF-I levels and cancer risk, causality has not been established. Thus, serum IGF-I level may actually be a confounding variable, serving as a marker for autocrine tissue IGF-I production. Growth hormone (GH) therapy raises both IGF-I and IGFBP-3 levels in serum. However, the role of GH in controlling prostate, breast and lung growth and carcinogenesis remains unclear from animal studies. Increased GH levels as seen in acromegaly have been associated with benign prostatic hyperplasia but not with prostate, breast or lung cancers, although colon cancer mortality may be increased. Should serum IGF-I levels be proven to play a causal role in the pathogenesis of cancer, interpreting the risk associated with therapies such as GH replacement must take into account both the duration of exposure and the risk magnitude associated with the degree of serum IGF-I elevation. Since GH-deficient patients often have a subnormal IGF-I serum level, which normalizes on therapy, their cancer risk on GH therapy probably does not increase substantially above that of the normal population. Until further research in the area dictates otherwise, ongoing surveillance and routine monitoring of IGF-I levels in GH recipients should become standard of care.     

Analysis of insulin-like growth factor I gene expression in malignancy: evidence for a paracrine role in human breast cancer

Insulin-like growth factor I (IGF-I) activity has been reported to be produced by several human cancers. Identification of RNAs transcribed from the IGF-I gene has been complicated by the detection of multiple hybridizing bands on Northern analysis. To determine if any of these RNAs are transcribed from the IGF-I gene, we have used a sensitive and specific ribonuclease (RNAse) protection assay for IGF-I. We have also studied the breast cancer tissue expression of IGF-I using in situ hybridization histochemistry. We have found no IGF-I mRNA in breast (zero of 11) or colon cancer (zero of 9) cell lines; both of these tumors have been previously reported to express IGF-I mRNA. However, three of three neuroepithelioma and one of two Ewing's sarcoma cell lines express IGF-I mRNA; therefore, in these tumors IGF-I may be an autocrine growth factor. In contrast to breast cancer cell lines, RNA extracted from breast tissues has easily detectable IGF-I mRNA. In situ hybridizations show that IGF-I mRNA is expressed in the stromal cells, and not by normal or malignant epithelial cells. These findings suggest that although IGF-I is not produced by breast epithelial cells it may function as either a paracrine stimulator of epithelial cells or an autocrine stimulator of stromal cells.     

Characterization of a chimeric monoclonal antibody against the insulin-like growth factor-I receptor

The insulin-like growth factors (IGFs) signaling system has been shown to play important roles in neoplasia. The IGF receptor type 1 (IGF-IR) is overexpressed in many types of solid and hematopoietic malignancies, and there is substantial experimental and clinical evidence that targeting IGF-IR is a promising therapeutic strategy against cancer. It has been previously reported that a mouse monoclonal antibody (mAb), 4G11, blocked IGF-I binding to IGF-IR and downregulated the IGF-IR in MCF-7 cells. We cloned this antibody, constructed a human-mouse chimeric antibody, designated m590, and characterized it. The chimeric IgG1 m590 bound to cell-associated IGF-IR on NWT c43 stably transfected cells and MCF-7 breast cancer cells as efficiently as the parental murine antibody. Using purified IGF-IR extracellular domains, we found that both the chimeric m590 and the parental 4G11 antibodies bind to conformational epitopes on IGF-IR. Neither of these antibodies bound to the insulin receptor (IR) ectodomain. Furthermore, IgG1 m590 blocked the binding of IGF-I and IGF-II to IGF-IR, and inhibited both IGF-I and IGF-II induced phosphorylation of IGF-IR in MCF-7 cells. These results suggest that m590 could be an useful antibody in diagnosis and treatment of cancer, as well as a research tool.     

Characterization of a chimeric monoclonal antibody against the insulin-like growth factor-I receptor

The insulin-like growth factors (IGFs) signaling system has been shown to play important roles in neoplasia. The IGF receptor type 1 (IGF-IR) is overexpressed in many types of solid and hematopoietic malignancies, and there is substantial experimental and clinical evidence that targeting IGF-IR is a promising therapeutic strategy against cancer. It has been previously reported that a mouse monoclonal antibody (mAb), 4G11, blocked IGF-I binding to IGF-IR and downregulated the IGF-IR in MCF-7 cells. We cloned this antibody, constructed a human-mouse chimeric antibody, designated m590, and characterized it. The chimeric IgG1 m590 bound to cell-associated IGF-IR on NWT c43 stably transfected cells and MCF-7 breast cancer cells as efficiently as the parental murine antibody. Using purified IGF-IR extracellular domains, we found that both the chimeric m590 and the parental 4G11 antibodies bind to conformational epitopes on IGF-IR. Neither of these antibodies bound to the insulin receptor (IR) ectodomain. Furthermore, IgG1 m590 blocked the binding of IGF-I and IGF-II to IGF-IR, and inhibited both IGF-I and IGF-II induced phosphorylation of IGF-IR in MCF-7 cells. These results suggest that m590 could be an useful antibody in diagnosis and treatment of cancer, as well as a research tool.     

Progestins induce down-regulation of insulin-like growth factor-I (IGF-I) receptors in human breast cancer cells: potential autocrine role of IGF-II.

Insulin-like growth factor-I (IGF-I) receptors are present in breast cancer cells and may play a role in breast cancer cell growth. We have studied the effect of progestins on IGF-I receptors in T47D human breast cancer cells. T47D cells constitutively express high levels of progesterone receptors and are a model for studying the regulation of cellular functions by progestins. Treatment of T47D cells with either progesterone or the synthetic progestin promegestone (R5020) decreased IGF-I receptor content by approximately 50%, as measured by Scatchard analysis and receptor biosynthesis studies. In contrast to progestins, estradiol, dexamethasone, and dihydrotestosterone did not influence IGF-I receptor content. No effect of R5020 was seen after 12 h of incubation, a near-maximal effect was seen after 24 h, and greatest effects were seen after 72 h. R5020 decreased IGF-I receptor mRNA abundance, indicating that progestins acted at the level of gene expression. However, progestins also increased the secretion of IGF-II, a ligand for the IGF-I receptor. In contrast to IGF-II, T47D cells did not express IGF-I. The addition of exogenous IGF-II to T47D cells down-regulated both IGF-I receptor binding and IGF-I receptor mRNA abundance. This study indicates, therefore, that progestins regulate IGF-I receptors in breast cancer cells and suggests that this regulation occurs via an autocrine pathway involving enhanced IGF-II secretion.     

The insulin-like growth factor system: towards clinical applications

Insulin-like growth factors (IGF-I and II) are important endocrine, paracrine and autocrine mediators of physiological growth. They promote cellular proliferation, survival and differentiation. Their effects are mediated mainly through the IGF-I receptor, but IGFs also bind to the IGF-II/mannose 6-phosphate and insulin receptors. IGF activity is modulated by a family of six high-affinity IGF binding proteins (IGFBPs); in most situations, IGFBPs inhibit IGF actions but they may also enhance them. Assays are now available for IGF-I, IGF-II and individual IGFBPs. IGF-I and IGFBP-3 assays have some utility in the diagnosis and management of acromegaly and growth hormone deficiency. There is a large body of in vitro and in vivo evidence supporting a pathogenic role for alterations in the IGF system in many diseases, including diabetes, cancer, cardiovascular disease and neuromuscular disease. More recently, epidemiological studies have linked high IGF-I levels with some cancers and low IGF-I levels with ischaemic heart disease. Preliminary studies of recombinant IGF-I as a treatment for diabetes, osteoporosis and neuromuscular disease have been performed in humans. In contrast, there is considerable interest in developing IGF inhibitors for the treatment of cancer. This apparent paradox highlights the need to develop therapeutics beyond the natural ligands and inhibitors, with characteristics such as ligand and tissue specificity. This will only become possible as we increase our understanding of this complex system. Additionally, as IGF and IGFBP assays are becoming more readily available, their role in the diagnosis and monitoring of diseases should be more clearly defined in the near future.     

Regulation of breast cancer growth by insulin-like growth factors

The IGFs may be important autocrine, paracrine or endocrine growth factors for human breast cancer. IGF-I and II stimulate growth of cultured human breast cancer cells. IGF-I is slightly more potent, paralleling its higher affinity for the IGF-I receptor. Antibody blockade of the IGF-I receptor inhibits growth stimulation induced by both IGFs, suggesting that this receptor mediates the growth effects of both peptides. However, IGF-I receptor blockade does not inhibit estrogen (E₂)-induced growth suggesting that secreted IGFs are not the major mediators of E₂ action. Several breast cancer cell lines express IGF-II mRNA by both Northern analysis and RNase protection assay. IGF-II activity is found in conditioned medium by radioimmuno and radioreceptor assay, after removal of somatomedin binding proteins (BP) which are secreted in abundance. IGF-I is undetectable. BPs of 25 and 40 K predominate in ER-negative cell lines while BPs of 36 K predominate in ER-positive cells. Blockade of the IGF-I receptor inhibits anchorage-independent and monolayer growth in serum of a panel of breast cancer cell lines. Growth of one line (MDA-231) was also inhibited in vivo by receptor antibody treatment of nude mice. The antibody had no effect on growth of MCF-7 tumors. These data suggest the IGFs are important regulators of breast cancer cell proliferation and that antagonism of this pathway may offer a new treatment strategy.     

Functional characterization of the insulin-like growth factor I receptor on Jurkat T cells

Insulin-like growth factor I (IGF-I) has been shown to be important in the maintenance, development, and proliferation of various types of leukocytes, particularly T cells. Radio-receptor binding assays demonstrate that Jurkat T cells bind ¹²⁵I-IGF-I with an affinity of 1.77 nM (K_d) and express approximately 230 receptors/cell. Specificity studies show insulin also binds the IGF-I receptor with an affinity 20-fold lower than that of IGF-I. Interaction of IGF-I with its receptor on Jurkat T cells induces the phosphorylation of tyrosine kinase which is detectable by Western blotting. The 95,000 MW protein detected is equivalent to the molecular weight of the β chain of the IGF-I receptor described in other types of cells. These studies characterize the binding of IGF-I to its receptor on Jurkat T cells, demonstrate that IGF-I binding induces tyrosine phosphorylation, and support the hypothesis that IGF-I is important in the induction of T cell activation.     

Heterogeneity of insulin-like growth factor-I affinity for the insulin-like growth factor-II receptor: comparison of natural, synthetic and recombinant DNA-derived insulin-like growth factor-I

Although insulin-like growth factors (IGF) I and II bind with high affinity to structurally discrete receptors, they bind with a lesser affinity to each other's receptor. We have evaluated the affinity of five different IGF-I preparations (three natural IGF-I preparations, one synthetic preparation, and one recombinant DNA-derived) for the IGF-II receptor in rat placental membranes, 18-54,SF cells and BRL-3A cells. In all tissues tested, the natural IGF-I preparations demonstrated an affinity for the IGF-II receptor which was 10-20% that of IGF-II. However, the recombinant and synthetic IGF-I preparations exhibited substantially lower affinities than natural IGF-I for this receptor, with only 10-25% reduction in (125-I)iodo IGF-II binding at peptide concentrations up to 400 ng/ml. Radioimmunoassay of the natural IGF-I preparations with an antibody directed against the unique C-peptide region of IGF-II demonstrated that contamination of IGF-I preparations with immunoreactive IGF-II could not exceed 5%. These results demonstrate that IGF-I purified from human plasma has a different affinity for the IGF-II receptor than does synthetic or recombinant IGF-I. Furthermore, these data are consistent with the hypothesis that IGF-I, itself, may be heterogeneous, and that subforms may vary in their affinities for the IGF receptors. Alternatively, IGF-I preparations which have been considered to be pure may be contaminated with small amounts of IGF-II, resulting in overestimation of the affinity of IGF-I for the type II IGF receptor.     

Insulin-like growth factor-I and binding protein-3 and risk of cancer

Insulin-like growth factor (IGF)-I is an important mitogen required by some cell types to progress from the G1 phase to the S phase of the cell cycle. IGF binding proteins (IGFBPs) can have opposing actions, in part by binding IGF-I, but also by direct inhibitory effects on target cells. As mitogens and anti-apoptotic agents, IGFs may be important in carcinogenesis, possibly by increasing the risk of cellular transformation by enhancing cell turnover. Indeed, many types of neoplastic cells express or overexpress IGF-I receptors, which stimulate mitogenesis when activated by IGF-I in vitro. In vivo, tissue IGF bioactivity is determined not only by circulating IGF-I and IGFBP levels, but also by local production of IGFs, IGFBPs, and possibly IGFBP proteases that enhance IGF-I availability by cleaving IGFBPs. Because determinants of tissue IGF bioactivity appear to be regulated in parallel with circulating IGF-I level, it is reasonable to hypothesize that the substantial intraindividual variability in circulating levels of IGF-I and IGFBP-3 may be important in determining risk of some cancers. In recent epidemiologic studies, relatively high plasma IGF-I and low IGFBP-3 levels have been independently associated with greater risk of prostate cancer in men, breast cancer among premenopausal women, and colorectal adenoma and cancer in men and women and possibly lung cancer. These include prospective data from the Physicians' Health Study and the Nurses' Health Study. In general, two- to fourfold elevated risks have been observed for prostate cancer in men in the top quartile of IGF-I relative to those in the bottom quartile, and low levels of IGFBP-3 were associated with an approximate doubling of risk. For breast cancer, an association with IGF-I for postmenopausal women was not apparent, but strong associations were observed for premenopausal cases in the Nurses' Health Study. Further study is needed to confirm this subgroup finding in women. Recent data also indicate that high IGF-I and low IGFBP-3 increase risk of colorectal cancer and large or villous adenomas. Of note, for colorectal neoplasia, fourfold elevated risks were observed in men and women with low IGFBP-3, whereas high IGF-I was associated with a doubling of risk. These emerging epidemiologic data indicate that high levels of IGF-I and low levels of IGFBP-3 are associated with an increased risk of at least several types of carcinoma that are common in economically developed countries. Further study is required to determine the clinical relevance of these findings.     

A novel unidirectional cross-talk from the insulin-like growth factor-I receptor to leptin receptor in human breast cancer cells

Obesity is a major risk factor for the development and progression of breast cancer. Increased circulating levels of the obesity-associated hormones leptin and insulin-like growth factor-I (IGF-I) and overexpression of the leptin receptor (Ob-R) and IGF-I receptor (IGF-IR) have been detected in a majority of breast cancer cases and during obesity. Due to correlations between increased leptin, Ob-R, IGF-I, and IGF-IR in breast cancer, we hypothesized that molecular interactions may exist between these two signaling pathways. Coimmunoprecipitation and immunoblotting showed that IGF-IR and Ob-R interact in the breast cancer cell lines MDA-MB-231, MCF7, BT474, and SKBR3. Stimulation of cells with IGF-I promoted Ob-R phosphorylation, which was blocked by IGF-IR kinase inhibition. In addition, IGF-I activated downstream signaling molecules in the leptin receptor and IGF-IR pathways. In contrast to IGF-I, leptin did not induce phosphorylation of IGF-IR, indicating that receptor cross-signaling is unidirectional, occurring from IGF-IR to Ob-R. Our results show, for the first time, a novel interaction and cross-talk between the IGF-I and leptin receptors in human breast cancer cells.     

Characterization of insulin-like growth factor I (IGF-I) receptors of human breast cancer cells

Studies of binding of IGF-I to a plasma-membrane-enriched subcellular fraction prepared from MCF-7 human breast cancer cells reveal the presence of 0.2 pmols specific binding sites for this mitogen per mg membrane protein, with an equilibrium affinity constant of 1.45 nM-1. Competition studies with insulin, IGF-II, and an anti-IGF-I receptor antibody are consistent with the presence of specific IGF-I receptors, and SDS-PAGE showed binding to a 130 kDa subunit identical to that of receptors from human placenta. In addition, we show that IGF-I is more potent than estradiol and comparable to EGF in stimulating in vitro proliferation of MCF-7 cells, and that IGF-I-stimulated proliferation of these cells is inhibited by a blocking monoclonal antibody against the IGF-I receptor. These results demonstrate that IGF-I is an important mitogen for MCF-7 cells and that the mitogenic effect is mediated by specific IGF-I receptors.     

The association between integrin-associated protein and SHPS-1 regulates insulin-like growth factor-I receptor signaling in vascular smooth muscle cells

Growth factor signaling is usually analyzed in isolation without considering the effect of ligand occupancy of transmembrane proteins other than the growth factor receptors themselves. In smooth muscle cells, the transmembrane protein Src homology 2 domain containing protein tyrosine phosphatase substrate-1 (SHPS-1) has been shown to be an important regulator of insulin-like growth factor-I (IGF-I) signaling. SHPS-1 is phosphorylated in response to IGF-I, leading to recruitment of Src homology 2 domain tyrosine phosphatase (SHP-2). Subsequently, SHP-2 is transferred to IGF-I receptor and regulates the duration of IGF-I receptor phosphorylation. Whether ligand occupancy of SHPS-1 influences SHPS-1 phosphorylation or SHP-2 recruitment, thereby altering growth factor signaling, is unknown. Previous studies have shown that integrin associated protein (IAP) associates with SHPS-1. We undertook these studies to determine whether this interaction controlled SHPS-1 phosphorylation and/or SHP-2 recruitment and thereby regulated IGF-I signaling. Disruption of IAP-SHPS-1 binding, by using an IAP monoclonal antibody or cells expressing mutant forms of IAP that did not bind to SHPS-1, inhibited IGF-I-stimulated SHPS-1 phosphorylation and SHP-2 recruitment. This was associated with a lack of SHP-2 transfer to IGF-I receptor and sustained receptor phosphorylation. This resulted in an inability of IGF-I to stimulate sustained mitogen-activated protein kinase activation, cell proliferation, and cell migration. The effect was specific for IGF-I because disruption of the IAP-SHPS-1 interaction had no effect on platelet-derived growth factor-stimulated SHPS-1 phosphorylation or cell migration. In summary, our results show that 1) ligand occupancy of SHPS-1 is a key determinant of its ability to be phosphorylated after IGF-I stimulation, and 2) the interaction between IAP and SHPS-1 is an important regulator of IGF-I signaling because disruption of the results in impaired SHP-2 recruitment and subsequent inhibition of IGF-I-stimulated cell proliferation and migration.     

Increases in free, unbound insulin-like growth factor I enhance insulin responsiveness in human hepatoma G2 cells in culture

Insulin-like growth factor-binding protein (IGFBP)-1 binds to insulin-like growth factor (IGF)-I and -II with high affinity and has been shown to modulate IGF-I actions in vivo and in vitro. The synthesis of IGFBP-1 is suppressed by insulin, and administration of IGFBP-1 to rats results in impaired glucose metabolism. A synthetic peptide (bp1-01) has been shown to have a high affinity and specificity for human IGFBP-1 and to inhibit IGF-I binding. The current studies were undertaken to determine if, after incubation of bp1-01 with IGF-I.IGFBP-1 complexes, anabolic and insulin-like effects of IGF-I could be detected in human hepatoma (HepG2) cell cultures and to determine the receptor subtype(s) through which these effects were mediated. Incubation of HepG2 cells with bp1-01 (200 nm) increased IGF-I-stimulated protein synthesis by 44% and glycogen synthesis by 170% compared with stimulation by IGF-I alone. Incubation with bp1-01 also enhanced IGF-I-stimulated tyrosine phosphorylation of the IGF-I/insulin hybrid receptor and insulin receptor substrate 1. Exposure of the cells to bp1-01 alone enhanced glycogen synthesis and phosphorylation of IGF-I/insulin hybrid receptors. This was not a direct effect of bp1-01 because it did not bind to the receptor and did not activate tyrosine kinase activity in the presence of an anti-IGF-I receptor antibody. The addition of bp1-01 (200 nm) plus insulin to HepG2 cell culture medium resulted in increased tyrosine phosphorylation of the hybrid receptor, insulin receptor substrate 1, and the glycogen synthesis response compared with the effects of insulin alone. This enhancement of hybrid receptor phosphorylation and glycogen synthesis by bp1-01 peptide was diminished by preincubation with an inhibitory antibody for the alpha subunit of IGF-I receptor (alphaIR3). bp1-01 stimulated the hybrid receptor phosphorylation response to IGF-I, and this effect was inhibited by prior incubation of the cells with alphaIR3. In conclusion, bp1-01 competes with IGF-I for binding to IGFBP-1, which leads to release of free IGF-I from IGF-I.IGFBP-1 complexes. This released IGF-I stimulates biologic actions that are mediated predominantly through the IGF-I/insulin hybrid receptor.     

Cross-talk between IGF-I and TGF-beta signaling pathways

Insulin-like growth factor-I (IGF-I) has gained broad recognition as an important survival factor for epithelial cells in numerous tissues. The IGF-I receptor signaling pathway is deregulated in the majority of carcinomas, and such deregulation has also been reported to be tightly associated with enhanced tumor progression and metastasis. One of the key proteins that transduces IGF-I signals and is phospho-activated downstream of the IGF-I receptor, is the non-receptor serine/threonine kinase proto-oncogene protein kinase B (PKB, also known as Akt). This kinase serves as a major molecular node to control the function of many cell survival and death proteins through phosphorylation-mediated protein modification. The end result of the activation of Akt is enhanced cell survival and proliferation, pre-requisites for malignant transformation. Recent studies show that IGF-I signals cross-talk at multiple levels with various components of the TGF-beta signaling pathway, which depending on context may function either as tumor suppressor or as tumor promoter. Thus, a better understanding of how the IGF-I and TGF-beta signaling pathways are mutually interconnected is likely to unveil novel targets for the therapeutic intervention of many cancers.     

Fibroblast growth factor inhibits insulin-like growth factor-II (IGF-II) gene expression and increases IGF-I receptor abundance in BC3H-1 muscle cells.

Muscle is an important target tissue for insulin-like growth factor (IGF) action. We have previously reported that muscle cell differentiation is associated with down-regulation of the IGF-I receptor at the level of gene expression that is concomitant with an increase in the expression and secretion of IGF-II. Furthermore, treatment of myoblasts with IGF-II resulted in a similar decrease in IGF-I receptor mRNA abundance, suggesting an autocrine role of IGF-II in IGF-I receptor regulation. To explore further the role of IGF-II in IGF-I receptor regulation, BC3H-1 mouse muscle cells were exposed to differentiation medium in the presence of basic fibroblast growth factor (FGF), a known inhibitor of myogenic differentiation. FGF treatment of cells resulted in a 50% inhibition of IGF-II gene expression compared to that in control myoblasts and markedly inhibited IGF-II secretion. Concomitantly, FGF resulted in a 60-70% increase in IGF-I binding compared to that in control myoblasts. Scatchard analyses and studies of gene expression demonstrated that the increased IGF-I binding induced by FGF reflected parallel increases in IGF-I receptor content and mRNA abundance. These studies indicate that FGF may up-regulate IGF-I receptor expression in muscle cells through inhibition of IGF-II peptide expression and further support the concept of an autocrine role of IGF-II in IGF-I receptor regulation. In addition, these studies suggest that one mechanism by which FGF inhibits muscle cell differentiation is through inhibition of IGF-II expression.     

Insulin-like growth factors modulate the growth inhibitory effects of retinoic acid on MCF-7 breast cancer cells

Retinoids are currently being tested for the treatment and prevention of several human cancers, including breast cancer. However, the anti-cancer and growth inhibitory mechanisms of retinoids are not well understood. All-trans retinoic acid (RA) inhibits the growth of the estrogen receptor-positive (ER+) breast cancer cell line, MCF-7, in a reversible and dose-dependent manner. In contrast, insulin-like growth factors (IGF-I,IGF-II) and insulin are potent stimulators of the proliferation of MCF-7 and several other breast cancer cell lines. Pharmacologic doses of RA (≤10^?6M) completely inhibit IGF-I-stimulated MCF-7 cell growth. Published data suggest that the growth inhibitory action of RA on IGF-stimulated cell growth is linear and dose-dependent, similar to RA inhibition of unstimulated or estradiol-stimulated MCF-7 cell growth. Surprisingly, we have found that IGF-I or insulin-stimulated cell growth is increased to a maximum of 132% and 127%, respectively, by cotreatment with 10^?7 M RA, and that 10^?9–10^?7 M RA increase cell proliferation compared to IGF-I or insulin alone. MCF-7 cells that stably overexpress IGF-II are also resistant to the growth inhibitory effects of 10^?9–10^?7 M RA. Treatment with the IGF-I receptor blocking antibody, αIR-3, restores RA-induced growth inhibition of IGF-I-treated or IGF-II-overexpressing MCF-7 cells, indicating that the IGF-I receptor is mediating these effects. IGFs cannot reverse all RA effects since the altered cell culture morphology of RA-treated cells is similar in growth-inhibited cultures and in IGF-II expressing clones that are resistant to RA-induced growth inhibition. These results indicate that RA action on MCF-7 cells is biphasic in the presence of IGF-I or insulin with 10^?9–10^?7 M RA enhancing cell proliferation and ≥ 10^?6M RA causing growth inhibition. As IGF-I and IGF-II ligands are frequently detectable in breast tumor tissues, their potential for modulation of RA effects should be considered when evaluating retinoids for use in in vivo experimental studies and for clinical purposes. Additionally, the therapeutic use of inhibitors of IGF action in combination with RA is suggested by these studies. © 1995 Wiley-Liss Inc.