Expression of insulin-like growth factor binding proteins in human breast cancer correlates with estrogen receptor status

IGF-independent regulation of breast cancer growth by IGF binding proteins

The human IGFBP family consists of at least seven proteins, designated as IGFBP-1, -2, -3, -4, -5, -6, and-7. IGFBPs 1-6 bind IGF-I and IGF-II with high affinity whereas IGFBP-7, a newly identified IGFBP, binds IGFs with lower affinity and constitutes a low-affinity member of the IGFBP family. IGFBPs serve to transport the IGFs, prolong their half-lives, and modulate their biological action. At the cellular level, IGFBPs can either potentiate or inhibit the mitogenic effects of IGFs, depending upon cell types and IGFBP species (IGF-dependent action of IGFBPs). However, recent studies have indicated that IGFBPs, especially IGFBP-3, potently inhibit breast cancer cell growth in an IGF-independent manner. The IGF-independent action of IGFBP-3 requires interaction with cell-surface association proteins, presumably putative IGFBP-3 specific receptors, and is responsible for growth inhibitory action of the known growth suppressing factors such as TGF-beta, retinoic acid, and antiestrogens in breast cancer cells. Thus, IGFBP-3 appears to be a major factor in a negative control system involved in regulating human breast cancer cell growth in vitro. IGFBP-7, representing a low affinity IGFBP, appears to function as an IGF-independent cell growth regulator in breast cancer cells. Overall structural similarity between IGFBP-7 and classical high affinity IGFBPs 1-6 suggests that the mechanisms of action and signaling pathways used by IGFBP-7 may provide insight into the IGF-independent actions of the high affinity IGFBPs. A fuller understanding of the IGF-independent action of IGFBPs will allow us to understand how the growth of neoplastic cells can be modulated by the IGF/IGFBP system, and how other growth factors or pharmacological agents can interface with this system.

Characterization and regulation of insulin-like growth factor binding proteins in human hepatic stellate cells

Cultured hepatic stellate cells (HSCs), the cell type primarily involved in the progression of liver fibrosis, secrete insulin-like growth factor-I (IGF-I) and IGF binding protein (IGFBP) activity. IGF-I exerts a mitogenic effect on HSCs, thus potentially contributing to the fibrogenic process in an autocrine fashion. However, IGF-I action is modulated by the presence of specific IGFBPs that may inhibit and/or enhance its biologic effects. Therefore, we examined IGFBP-1 through IGFBP-6 mRNA and protein expression in HSCs isolated from human liver and activated in culture. Regulation of IGFBPs in response to IGF-I and other polypeptide growth factors involved in the hepatic fibrogenic process was also assessed. RNase protection assays and ligand blot analysis demonstrated that HSCs express IGFBP-2 through IGFBP-6 mRNAs and release detectable levels of IGFBP-2 through IGFBP-5. Because IGF-I, platelet-derived growth factor-BB (PDGF-BB), and transforming growth factor-beta (TGF-beta) stimulate HSC proliferation and/or matrix production, we tested their effect on IGFBPs released by HSCs. IGF-I induced IGFBP-3 and IGFBP-5 proteins in a time-dependent manner without an increase in the corresponding mRNAs. IGFBP-4 protein levels decreased in response to IGF-I. TGF-beta stimulated IGFBP-3 mRNA and protein but decreased IGFBP-5 mRNA and protein. In contrast, PDGF-BB failed to regulate IGFBPs compared with controls. Recombinant human IGFBP-3 (rhIGFBP-3) was then tested for its effect on IGF-I-induced mitogenesis in HSCs. rhIGFBP-3 inhibited IGF-I-stimulated DNA synthesis in a dose-dependent manner, with a peak effect observed at 25 nM IGFBP-3. Because TGF-beta is highly expressed in cirrhotic liver tissue, we determined whether IGFBP-3 mRNA expression is increased in liver biopsies obtained from patients with an active fibroproliferative response due to viral-induced chronic active hepatitis. In the majority of these samples, IGFBP-3 mRNA was increased compared with normal controls. These findings indicate that human HSCs, in their activated phenotype, constitutively produce IGFBPs. IGF-I and TGF-beta differentially regulate IGFBP-3, IGFBP-4, and IGFBP-5 expression, which, in turn, may modulate the in vitro and in vivo action of IGF-I.

IGFBPs and neoplastic models. New concepts for roles of IGFBPs in regulation of cancer cell growth

The insulin-like growth factor binding proteins (IGFBPs) are a family of seven structurally homologous proteins that bind insulin-like growth factor 1 (IGF-I) and IGF-II with high affinity, thereby modu-lating the actions of IGFs. Several lines of recent evidence from various cell systems have suggested that IGFBPs, especially IGFBP-3, may play more active, IGF-independent, roles in growth regulation of cancer cells. In support of this hypothesis, the author has recently shown that IGFBP-3 binds specifically and with high affinity to the surface of various cell types and directly inhibits monolayer growth of these cells in an IGF-independent manner, presumably by specific interaction with cell membrane proteins that function as an IGFBP-3 receptor. The author's current studies demonstrate that a new class of IGFBP, IGFBP-7, constitutes a low affinity member of the IGFBP family, but primarily functions as a modulator of cell growth in an IGF-independent manner, similar to the action observed with IGFBP-3 in breast cancer cells. The author's studies on the mechanisms of action of the low affinity IGFBPs will provide insight into the IGF-independent actions of the classical high affinity IGFBPs and their impact on cancer cell growth.

Associations between insulin-like growth factors and their binding proteins and other prognostic indicators in breast cancer

Recent studies have suggested that insulin-like growth factors (IGFs) and insulin-like growth factor binding proteins (IGFBPs) may be implicated in the development and progression of breast cancer. Prostate-specific antigen (PSA), a serine protease, may play a role in the regulation of IGFs' function through cleavage of IGFBP-3, resulting in release of active IGFs from IGFBP-3. As IGFs, IGFBPs and PSA are all present in breast cancer, possible associations among these proteins were speculated. In this study, we have measured PSA, IGF-I, IGF-II, IGFBP-1 and IGFBP-3 in tumour tissue cytosols from 200 women with primary breast cancer, and have examined relationships between IGFs or IGFBPs and PSA along with other markers, including p53 protein, steroid hormone receptors (oestrogen and progesterone), cathepsin-D, epidermal growth factor receptor, Her-2/neu protein, S-phase fraction and DNA ploidy. Correlations or associations between PSA and IGF-I, IGF-II, IGFBP-1 or IGFBP-3 were not observed. IGF-II was positively correlated with both IGFBP-3 and IGFBP-1. IGF-I was not associated with either of the two binding proteins, nor with IGF-II. Both IGF-II and IGFBP-3 were inversely associated with the oestrogen receptor, and IGFBP-3 was also positively associated with S-phase fraction. Our finding of IGF-II and IGFBP-3 in association with unfavourable prognostic indicators of breast cancer suggests that IGFs may be involved in the progression of breast cancer.

Inhibition of insulin- and insulin-like growth factor-I-stimulated growth of human breast cancer cells by 1,25-dihydroxyvitamin D3 and the vitamin D3 analogue EB1089

1,25 Dihydroxyvitamin D3 (1,25-(OH)2D3) and a number of synthetic vitamin D3 analogues with low calcaemic activity, have been shown to inhibit breast cancer cell growth in vitro as well as in vivo. The purpose of the present study was to investigate a possible interaction of 1,25-(OH)2D3 and the vitamin D3 analogue EB1089 with the insulin-IGF-I regulatory system. The oestrogen receptor-positive MCF-7 human breast cancer cells used in this study are able to grow autonomously and their growth is stimulated by insulin. In order to avoid interference of IGF-binding proteins (IGF-BPs), we used an analogue of IGF-I, long R3 IGF-I, which stimulated MCF-7 cell growth similar to insulin. The growth stimulation by insulin and by long R3 IGF-I was completely inhibited by 1,25-(OH)2D3 and EB1089. Autonomous growth was also inhibited by 1,25-(OH)2D3 and EB1089. The analogue EB1089 was active at 50 times lower concentrations than 1,25-(OH)2D3. It was shown that growth inhibition was not achieved through downregulation of insulin and IGF-I binding after 48 h. Paradoxically, after prolonged treatment (8 days), an upregulation of insulin and IGF-I binding was observed. Two possible intracellular mediators of the insulin-IGF mitogenic signal are C-FOS and mitogen-activated protein (MAP) kinase. Insulin-induced C-FOS mRNA was inhibited by 1,25-(OH)2D3, suggesting that it could be involved in the growth inhibition by 1,25-(OH)2D3. MAP kinase activation appeared not to be involved in growth stimulation by both insulin and IGF-I. Together, the present study demonstrates that vitamin D3 compounds can block the mitogenic activity of insulin and IGF-I, which may contribute to their tumour suppressive activity observed in vivo.

Characterization of insulin-like growth factor binding proteins and regulation of IGFBP3 in human mesangial cells

IGF-I regulates renal growth and development. Insulin-like growth factor binding proteins (IGFBPs) are synthesized by the kidney and may modulate the local autocrine and/or paracrine actions of IGF-I. We have previously demonstrated that mesangial cells (MC) release IGF-I and IGF-binding activity; however, the specific IGFBPs produced by these cells and the factors involved in their regulation are unknown. We examined MC for expression of IGFBP-1 to -6 mRNAs and proteins. RNase protection assays using total RNA demonstrated that MC express all of the IGFBPs. [125I]IGF-I Western ligand blot of conditioned medium demonstrated that MC release IGFBPs of 24, 29, 32 kDa, and a doublet at 46 kDa, consistent with IGFBP-4, -5, -2 and -3, respectively. IGFBP species of 28 and 34 kDa were also detected. Since IGF-I and TGF-beta are implicated in glomerular hypertrophy and matrix expansion, we tested their effect on IGFBPs released by MC. IGF-I (100 ng/ml), TGF-beta (2 ng/ml) and forskolin (10(-5) M) differentially regulated the abundance of IGFBPs released in the conditioned medium in a time-dependent manner. IGF-I and TGF-beta were potent inducers of the release of IGFBP3 protein; however, TGF-beta, but not IGF-I, increased IGFBP3 mRNA levels. Recombinant IGFBP3 was tested for its effect on IGF-I-induced mitogenesis. IGFBP3 inhibited IGF-I-stimulated DNA synthesis in a dose-dependent manner with a peak effect observed at 50 nM IGFBP3. Although TGF-beta is a potent inhibitor of IGF-I-stimulated DNA synthesis, this effect is not mediated via IGFBPs. Expression of IGFBP-1 to -6 by MC suggests that these proteins may modulate IGF-I bioavailability in the glomerulus. IGF-I itself, TGF-beta and cAMP agonists may indirectly modulate the effects of IGF-I via the release of IGFBPs by MC.

Insulin-like growth factor receptors and binding proteins

The insulin-like growth factor receptors are integral membrane proteins and demonstrate separate, but important effects on the regulation of cellular processes. The IGF-I receptor signals multiple cascades via its inherent tyrosine kinase activity. The IGF-II/M-6-P receptor on the other hand is primarily involved in targeting of enzymes to various subcellular compartments. In contrast, the insulin-like binding proteins are secreted by the cells and accumulate in the extracellular matrix or on the external surface of the cell. They are also involved in regulating cellular processes more indirectly. They modulate the interactions of the IGFs with their receptors, and in addition, may have some IGF-independent effects probably by direct interaction with integrin and other cell membrane receptor proteins. The recent studies, as outlined in this review, strongly suggest an important, if not essential role for the IGF system in normal physiology and disease states. The challenge now is to define the mechanisms involved in these effects. More studies are required to fully understand the post-receptor mechanism involved in IGF-I receptor signal transduction and the mechanisms whereby the IGFBPs exert their interesting effects. Understanding these mechanisms will enable investigators to create new therapeutic modalities for diseases that are affected by the IGF system.

Enhanced expression of an insulin growth factor-like binding protein (mac25) in senescent human mammary epithelial cells and induced expression with retinoic acid

mac25, the subject of this report, was selected by the differential display of mRNA method in a search for genes overexpressed in senescent human mammary epithelial cells. mac25 had previously been cloned as a discrete gene, preferentially expressed in normal, leptomeningial cells compared with meningioma tumors. mac25 is another member of the insulin growth factor-binding protein (IGFBP) family. Insulin-like growth factors are potent mitogens for mammary epithelial cells, and the IGFBPs have been shown to modulate this mitogenic activity. We report here that mac25, unlike most IGFBPs, is down-regulated at the transcription level in mammary carcinoma cell lines, suggesting a tumor-suppressor role. The gene was mapped to chromosome 4q12. We found that mac25 accumulates in senescent cells and is up-regulated in normal, growing mammary epithelial cells by all-trans-retinoic acid or the synthetic retinoid fenretinide. These findings suggest that mac25 may be a downstream effector of retinoid chemoprevention in breast epithelial cells and that its tumor-suppressive role may involve a senescence pathway.

Antiproliferative actions of insulin-like growth factor binding protein (IGFBP)-3 in human breast cancer cells

A number of lines of evidence suggest that IGFs are important mitogens in human breast cancer: (1) IGFs are the most potent growth factor in human breast cancer cells; (2) estrogen stimulates expression of IGF-II and the type 1 IGF receptor; and (3) stromal cells express IGFs, which may act in a paracrine manner. Numerous studies have demonstrated that IGFBPs modulate the mitogenic effects of IGFs in the local environment. In particular, we have recently demonstrated that IGFBP-3 inhibits the growth of Hs578T and MDA-MB-231 human breast cancer cells in an IGF-independent manner. Further studies revealed the existence of cell surface-associated IGFBP-3 receptors. Receptor binding and the subsequent antiproliferative action of IGFBP-3 was inhibited by IGFs, owing to the formation of an IGF-IGFBP-3 complex that prevents the binding of IGFBP-3 to its receptors. In addition, exogeneously added soluble heparin or heparan sulfate inhibited the binding of IGFBP-3 to the cell surface in a dose-dependent manner. However, when heparin and heparan sulfate linkages of glycosaminoglycans on the cell surface were enzymatically remove, IGFBP-3 binding was only minimally affected. These data suggest that soluble heparin or heparan sulfate forms a complex with IGFBP-3, thereby inhibiting receptor binding of IGFBP-3, rather than competing with cell-surface glycosaminoglycans for binding of IGFBP-3. Additionally, the role of IGFBP-3 in the antiproliferative effects of transforming growth factor (TGF)-beta and retinoic acid (RA) is supported by our observations that: (1) inhibition of IGFBP-3 gene expression using an IGFNBP-3 antisense oligodeoxynucleotide not only blocks TGF-beta and RA simulation of IGFBP-3 production by up to 90%m but also inhibits their antiproliferative effects by 40-60%; and (2) treatment with IGF-II and IGF-II analogs diminish TGF-beta effects by blocking TGF-beta induced binding of IGFBP-3 to the cell surface. Taken together, our results support the hypothesis that IGFBP-3 is an important antiproliferative factor in human breast cancer, acting in an IGF-independent manner in addition to its ability to modulate the binding of IGF peptides to IGF receptors.

Insulin-like growth factors and breast cancer

Several years of research have indicated that the insulin-like growth factor (IGF) family of ligands, receptors and binding proteins are expressed in human breast cancer. The ligands are potent mitogens for breast cancer cell lines, and blockade of IGF signaling inhibits tumor growth. The IGFs can be regulated in normal and neoplastic tissue, indicating their important role in proliferation. For example, estrogen, a hormone important in the growth and progression of breast cancer is able to alter expression of IGF ligands, receptors and binding proteins. In addition, recent data now indicate that IGF ligands can also activate estrogen receptor (ER) in a ligand-independent manner. The apparent cross-talk between IGF and ER signaling is especially important to consider since anti-estrogens, such as tamoxifen, are a major modality for the treatment of breast cancer. Recent data suggest that IGFs may also be involved in tamoxifen resistance, through upregulation of the IGF-I receptor. Thus blockade of IGF signaling in combination with tamoxifen may prove to be a beneficial treatment for breast cancer patients.

Detection of insulin-like growth factor binding proteins (IGFBPs) by ligand blotting in breast cancer tissues

Eighty breast cancer specimens were examined for insulin-like growth factor binding protein (IGFBP) expression by ligand blotting. Five distinct IGFBP species were found: a doublet at 48 and 44 kDa was IGFBP-3, the 34-kDa band was IGFBP-2, and a band at 24 kDa was IGFBP-4. A 32-kDa band was compatible with the migration position reported for IGFBP-5. IGFBP-3 was inversely correlated with ER expression, while IGFBP-4 was positively correlated with both ER and PgR. IGFBP-4 was also inversely correlated with S-phase fraction. Thus, IGFBP expression correlates with other parameters of breast cancer biology and may play a role in regulating tumor growth.

The insulin-like growth factor system as a target in breast cancer

Evidence from several experimental systems has shown that the insulin-like growth factors (IGFs) can stimulate breast cancer proliferation. Since IGF action is mediated by interaction with specific cell surface receptors, interruption of these signalling pathways could result in inhibition of cellular growth. In all extracellular fluids, the IGFs are associated with high affinity binding proteins, the IGFBPs can bind the IGFs and prevent receptor activation, and thus might have a role in a targeted approach to breast cancer therapy. Here we present our studies using IGFBP-1 to inhibit growth of the breast cancer cell line MCF-7.

Recombinant insulin-like growth factor binding protein-1 inhibits IGF-I, serum, and estrogen-dependent growth of MCF-7 human breast cancer cells

The insulin-like growth factors (IGFs) are potent mitogens for breast cancer cells and their activity is modulated by high affinity binding proteins (IGFBPs). We have recently shown that IGFBP-1 purified from human amniotic fluid neutralizes IGF-I-dependent growth of MCF-7 cells. In this study we examined the effects of recombinant IGFBP-1 (rBP-1) on IGF-I, estradiol (E2), and serum-induced monolayer and anchorage independent growth (AIG) of MCF-7 cells. Under serum-free conditions, rBP-1 had no effect on MCF-7 basal monolayer growth. However, 40 nM rBP-1 completely blocked the mitogenic action of both IGF-I and 5% charcoal stripped serum (CSS). This concentration of rBP-1 partially inhibited E2-induced growth, while 80 nM rBP-1 completely abolished E2 mitogenicity. The addition of either excess IGF-I or 5 nM [Arg3]IGF-I, a species that does not bind IGFBPs, neutralized rBP-1 inhibitory effects. In AIG assays, 80 nM rBP-1 reduced colony number by at least 70% and decreased colony size in all treatment groups compared to control. We examined rBP-1 effects on both IGF-I binding to MCF-7 membranes and activation of type I IGF receptor (IGFR1) and found that 80 nM rBP-1 reduced IGF-I receptor binding to levels of nonspecific binding and completely abolished ligand-dependent IGFR1 phosphorylation. However, neither treatment with 5% CSS nor exposure to E2 resulted in IGFR1 phosphorylation suggesting that different mechanism(s) are responsible for rBP-1 inhibitory action under this condition. Our data suggest rBP-1 may serve as an antagonist of human breast cancer growth by interfering with growth factor-mediated cell proliferation.

Regulation of insulin-like growth factor binding proteins in ovarian cancer cells by oestrogen

Insulin-like growth factor-I (IGF-I), its receptor and its binding proteins are expressed by ovarian cancer cells. In this study, we examined oestradiol (E2) regulation of IGF-I and IGF binding proteins (IGFBP) in an oestrogen-responsive ovarian cancer cell line, PE04. In serum-free conditions, PE04 cell monolayer growth was increased 1.64-fold by 3 nmol/l E2 compared with controls, although IGF-I mRNA levels were not increased. In contrast to IGF-I mRNA, IGFBP mRNA was regulated by E2. E2 caused a marked decrease in IGFBP-3 RNA, but IGFBP-2, -4 and -6 levels were only minimally depressed. IGFBP-5 mRNA levels were increased by E2. Tamoxifen had less effect on IGFBP mRNA regulation. Ligand blotting showed that E2 reduced IGFBP levels in conditioned media. IGFBP RNA was also detected in human ovarian tissue samples. Thus, IGFBP expression can be regulated in oestrogen-responsive ovarian cancer by E2.