Changes in insulin-like growth factor-I receptor expression and binding protein secretion associated with tamoxifen resistance and estrogen independence in human breast cancer cells in vitro

Decoding Insulin-Like Growth Factor Signaling Pathway From a Non-coding RNAs Perspective: A Step Towards Precision Oncology in Breast Cancer

Breast cancer (BC) is a highly complex and heterogenous disease. Several oncogenic signaling pathways drive BC oncogenic activity, thus hindering scientists to unravel the exact molecular pathogenesis of such multifaceted disease. This highlights the urgent need to find a key regulator that tunes up such intertwined oncogenic drivers to trim the malignant transformation process within the breast tissue. The Insulin-like growth factor (IGF) signaling pathway is a tenacious axis that is heavily intertwined with BC where it modulates the amplitude and activity of vital downstream oncogenic signaling pathways. Yet, the complexity of the pathway and the interactions driven by its different members seem to aggravate its oncogenicity and hinder its target-ability. In this review, the authors shed the light on the stubbornness of the IGF signaling pathway and its potential regulation by non-coding RNAs in different BC subtypes. Nonetheless, this review also spots light on the possible transport systems available for efficient delivery of non-coding RNAs to their respective targets to reach a personalized treatment code for BC patients.

IGFBP-2 and -5: important regulators of normal and neoplastic mammary gland physiology

The insulin-like growth factor (IGF) axis plays an important role in mammary gland physiology. In addition, dysregulation of this molecular axis may have a causal role in the aetiology and development of breast cancer (BC). This report discusses the IGF axis in normal and neoplastic mammary gland with special reference to IGF binding proteins (IGFBPs) -2 and -5. We describe how these high affinity binders of IGF-1 and IGF-2 may regulate local actions of growth factors in an autocrine and/or paracrine manner and how they also have IGF-independent effects in mammary gland. We discuss clinical studies which investigate both the prognostic value of IGFBP-2 and -5 expression in BC and possible involvement of these genes in the development of resistance to adjuvant endocrine therapies.

Insulin-like growth factor - oestradiol crosstalk and mammary gland tumourigenesis

Development and differentiation of the mammary gland are dependent on the appropriate temporal expression of both systemically acting hormones and locally produced growth factors. A large body of evidence suggests that molecular crosstalk between these hormonal and growth factor axes is crucial for appropriate cell and tissue function. Two of the most important trophic factors involved in this process are the oestrogen (E) and insulin-like growth factor (IGF) molecular axes. The reciprocal crosstalk that exists between these pathways occurs at transcriptional/post-transcriptional and translational/post-translational levels regulate the expression and activity of genes involved in this process. In a clinical context an important consequence of such crosstalk in the mammary gland is the role which it may play in the aetiology, maintenance and development of breast tumours. Although oestradiol (E2) acting through oestrogen receptors α and β (ERα/β) is important for normal mammary gland function it can also provide a mitogenic drive to ER+ breast tumours. Therefore over several years anti-oestrogen therapeutic regimens in the form of selective oestrogen receptor modulators (SERMs - e.g. tamoxifen), aromatase inhibitors (AI e.g. anastrozole) or selective oestrogen receptor down regulators (SERDs - e.g. fulvestrant) have been used in an adjuvant setting to control tumour growth. Although initial response is usually encouraging, large cohorts of patients eventually develop resistance to these treatments leading to tumour recurrence and poor prognosis. There are potentially many routes by which breast cancer (BC) cells could escape anti-oestrogen based therapeutic strategies and one of the most studied is the possible growth factor mediated activation of ER(s). Because of this, growth factor modulation of ER activity has been an intensively studied route of molecular crosstalk in the mammary gland. The insulin-like growth factors (IGF-1 and -2) are amongst the most potent mitogens for mammary epithelial cells and there is accumulating evidence that they interact with the E2 axis to regulate mitogenesis, apoptosis, adhesion, migration and differentiation of mammary epithelial cells. Such interactions are bi-directional and E2 has been shown to regulate the expression and activity of IGF axis genes with the general effect of sensitising breast epithelial cells to the actions of IGFs and insulin. In this short review we discuss the evidence for the involvement of crosstalk between the insulin-like growth factor (IGF) and oestrogen axes in the mammary gland and comment on the relevance of such studies in the aetiology and treatment of BC.

Nordihydroguaiaretic acid (NDGA), an inhibitor of the HER2 and IGF-1 receptor tyrosine kinases, blocks the growth of HER2-overexpressing human breast cancer cells

We have reported that nordihydroguaiaretic acid (NDGA) inhibits the tyrosine kinase activities of the IGF-1 receptor (IGF-1R) and the HER2 receptor in breast cancer cells. Herein, we studied the effects of NDGA on the growth of estrogen receptor (ER) positive MCF-7 cells engineered to overexpress HER2 (MCF-7/HER2-18). These cells are an in vitro model of HER2-driven, ER positive, tamoxifen resistant breast cancer. NDGA was equally effective at inhibiting the growth of both parental MCF-7 and MCF-7/HER2-18 cells. Half maximal effects for both cell lines were in the 10-15 microM range. The growth inhibitory effects of NDGA were associated with an S phase arrest in the cell cycle and the induction of apoptosis. NDGA inhibited both IGF-1R and HER2 kinase activities in these breast cancer cells. In contrast, Gefitinib, an epidermal growth factor receptor inhibitor but not an IGF-1R inhibitor, was more effective in MCF-7/HER2-18 cells than in the parental MCF-7 cells and IGF binding protein-3 (IGFBP-3) was more effective against MCF-7 cells compared to MCF-7/HER2-18. MCF-7/HER2-18 cells are known to be resistant to the effects of the estrogen receptor inhibitor, tamoxifen. Interestingly, NDGA not only inhibited the growth of MCF-7/HER2-18 on its own, but it also demonstrated additive growth inhibitory effects when combined with tamoxifen. These studies suggest that NDGA may have therapeutic benefits in HER2-positive, tamoxifen resistant, breast cancers in humans.

Insulin-like growth factor-I receptor signaling in tamoxifen-resistant breast cancer: a supporting role to the epidermal growth factor receptor

There is considerable evidence that the epidermal growth factor receptor (EGFR) and IGF-I receptor (IGF-IR) cross-talk in breast cancer cells. In the present study, we have examined whether EGFR/IGF-IR cross-talk exists in EGFR-positive tamoxifen-resistant variants of MCF-7 (Tam-R) and T47D (T47D-R) breast cancer cell lines. Although Tam-R cells expressed reduced IGF-IR protein levels compared with their wild-type MCF-7 counterparts, phosphorylated IGF-IR protein levels were equivalent in the two cell lines under basal growth conditions, possibly as a consequence of increased IGF-II expression in Tam-R cells. IGF-II activated both IGF-IR and EGFR in Tam-R cells, whereas only activation of IGF-IR was observed in wild-type cells. In contrast, epidermal growth factor rapidly induced EGFR, but not IGF-IR, phosphorylation in Tam-R cells. IGF-II promoted direct association of c-SRC with IGF-IR, phosphorylated c-SRC, and increased EGFR phosphorylation at tyrosine 845, a c-SRC-dependent phosphorylation site. Pretreatment with either AG1024 (IGF-IR-specific inhibitor) or an IGF-II neutralizing antibody inhibited basal IGF-IR, c-SRC, and EGFR phosphorylation, and AG1024 significantly reduced Tam-R basal cell growth. The c-SRC inhibitor SU6656 also inhibited growth, reduced basal and IGF-II-induced c-SRC and EGFR phosphorylation, and blocked EGFR activation by TGFalpha. Similarly, in T47D-R cells, AG1024 and SU6656 inhibited basal and IGF-II-induced phosphorylation of c-SRC and EGFR, and SU6656 reduced TGFalpha-induced EGFR activity. These results suggest the existence of a unidirectional IGF-IR/EGFR cross-talk mechanism whereby IGF-II, acting through the IGF-IR, regulates basal and ligand-activated EGFR signaling and cell proliferation in a c-SRC-dependent manner in Tam-R cells. This cross-talk between IGF-IR and EGFR is not unique to Tam-R cells because this mechanism is also active in a tamoxifen-resistant T47D-R cell line.

Characterization of a human breast cancer cell line, MCF-7/RU58R-1, resistant to the pure antiestrogen RU 58,668

Breast cancer is the most common cancer disease in women in the western world. Tamoxifen has been the standard first line endocrine therapy for patients with estrogen receptor (ER) positive tumors. Unfortunately, almost all patients with advanced disease develop tamoxifen resistance. This has lead to a search for new potent antiestrogens. One of the new compounds under development is the pure antiestrogen RU 58,668. To study the mechanisms behind acquired resistance to RU 58,668, the RU 58,668-resistant cell line MCF-7/RU58(R)-1 (RU58(R)-1) was developed. The RU58(R)-1 cell line was responsive to tamoxifen, but cross-resistant to ICI 182,780 and the estrogen-sensitivity was reduced compared to the parental MCF-7 cell line. The protein levels of ERalpha, IGF-I Receptor (IGF-IR) and Bcl-2 were severely reduced, when RU58(R)-1 cells were cultured with RU 58,668 and the expression of progesterone receptor (PR) was lost. The ERalpha level increased upon withdrawal of RU 58,668 and the ERalpha protein was destabilized by RU 58,668 in both cell lines. Regulation of most of the investigated estrogen-sensitive mRNAs was found to be normal in the resistant cells. The protein levels of IGF-IR, Bcl-2 and the IGF Binding Protein 2 (IGFBP2) reverted towards MCF-7 levels upon RU 58,668 withdrawal, but the resistant phenotype was maintained. Thus, it appears as acquired resistance to RU 58,668 is not a result of loss of the ERalpha expression or function and we suggest that in the presence of RU 58,668, the RU58(R)-1 cell line probably uses other mitogenic pathways than the ERalpha pathway for growth and survival.

Tamoxifen-resistant glioma-cell sub-populations are characterized by increased migration and proliferation

Multifocal tumor recurrence of glioblastomas occurs in up to 14% of patients. In a parallel phase-II-study investigating post-operative treatment with tamoxifen (TAM), carboplatin and radiation therapy for glioblastomas, 16 of 49 patients (33%) showed multifocal recurrence, which developed after a mean of 46 weeks, raising the question of an association with therapy. We studied the interrelation of proliferation and migration in the presence of different protein-kinase-C(PKC) inhibitors (TAM, staurosporine, hypericin) in 2 glioma cell lines. In addition, 3 cell lines were selected for TAM resistance by repeated cycles of treatment with sub-lethal concentrations of TAM. The proliferative capacity and the invasive potential of selected sub-populations were assessed using growth-curve experiments, monolayer migration, and cell-adhesion assays. Treatment with all PKC inhibitors tested resulted in a dose-dependent decrease of proliferation, while motility was altered only at significantly higher doses. Resistance to TAM occurred in all 3 selected cell lines. The TAM-resistant sub-populations showed significantly increased proliferation, migration and adhesion as compared with the parental (non-selected) cell line. The higher incidence of multifocal disease after TAM treatment was paralleled by increased migratory potential of TAM-treated cells in vitro.

Crosstalk between the insulin-like growth factors and estrogens in breast cancer

Once it was recognized that breast tumor growth was stimulated by estrogens, successful therapeutic strategies based on depriving the tumor of this hormone were developed. Since the growth stimulatory properties of the estrogens are governed by the estrogen receptor (ER), understanding the mechanisms that activate ER are highly relevant. In addition to estrogens, peptide growth factors can also activate the ER. The insulin-like growth factors (IGFs) are potent mitogens for ER-positive breast cancer cell lines. This review will examine the evidence for interaction between these two pathways. The IGFs can activate the ER, while ER transcriptionally regulates genes required for IGF action. Moreover, blockade of ER function can inhibit IGF-mediated mitogenesis and interruption of IGF action can similarly inhibit estrogenic stimulation of breast cancer cells. Taken together, these observations suggest that the two growth regulatory pathways are tightly linked and that a further understanding of the mechanism of this crosstalk could lead to new therapeutic strategies in breast cancer.

Changes in the secretion of insulin-like growth factor binding proteins -2 and -4 associated with the development of tamoxifen resistance and estrogen independence in human breast cancer cell lines

We investigated the secretion of insulin-like growth factor binding proteins (IGFBPs) by estrogen-dependent ZR-75-1 and MCF-7 human breast cancer cells, and tamoxifen-resistant (ZR-75-9a1 and LY2) and estrogen-independent (ZR-PR-LT) variants which express altered levels of IGF-I receptor. IGFBP species (35 kDa and 44 kDa) were detectable in conditioned serum-free medium (SFM) by immunoblotting and positively identified as IGFBP-2 and -3, respectively. Secretion of IGFBP-2 into SFM by the tamoxifen-resistant and estrogen-independent cell lines was markedly reduced and secretion into SFM of the 24-kDa species, assigned the identity of IGFBP-4, was also reduced in the tamoxifen-resistant lines. There was no clear correlation between patterns of IGFBP secretion and IGF-I receptor expression.

Altered expression of the IGF-1 receptor in a tamoxifen-resistant human breast cancer cell line

The relationship between oestrogen (E2) and insulin-like growth factor-one (IGF-1) was examined in both tamoxifen-sensitive (MCF 7/5-21) and tamoxifen-resistant (MCF 7/5-23) subclones of the MCF 7 cell line. Both subclones were grown in defined, serum-free (SF) medium over a period of 7 days with the addition of E2 or IGF-1 or a combination of both agents. Growth of both MCF 7/5-21 and 7/5-23 cells was stimulated (245% and 350%, respectively) by E2. However, only the growth of MCF 7/5-23 cells was stimulated (266%) by IGF-1. A combination of E2 and IGF-1 significantly enhanced MCF 7/5-21 and 7/5-23 cell growth (581% and 695%, respectively). E2-induced IGF-1 receptor (IGF-1R) levels (as measured by 125I-IGF-1 binding and Northern analyses) in only MCF 7/5-23 cells. This effect was partially inhibited by tamoxifen. In medium containing serum, the growth of only the MCF 7/5-23 cells was significantly inhibited by the IGF-1R monoclonal antibody, alphaIR-3. The detection of E2-induced expression of IGF-2 using RT-PCR was demonstrated in the MCF 7/5-23 cells. These experiments indicate that E2 may sensitize tamoxifen-resistant MCF 7/5-23 cells to the growth stimulatory actions of IGF-2 via up-regulation of the IGF-1R and describes a cell-survival mechanism that may manifest itself as tamoxifen resistance.

Type I insulin-like growth factor receptor function in breast cancer

Experimental evidence suggests an important role of the type I IGF receptor (IGF-IR) in breast cancer development. Breast tumors and breast cancer cell lines express the IGF-IR. IGF-IR levels are higher in cancer cells than in normal breast tissue or in benign mammary tumors. The ligands of the IGF-IR are potent mitogens promoting monolayer and anchorage-independent growth of breast cancer cells. Interference with IGF-IR activation, expression, or signaling inhibits growth and induces apoptosis in breast cancer cells. In addition, recent studies established the involvement of the IGF-IR in the regulation of breast cancer cell motility and adhesion. We have demonstrated that in MCF-7 cells, overexpression of the IGF-IR promotes E-cadherin-dependent cell aggregation, which is associated with enhanced cell proliferation and prolonged survival in three-dimensional culture. The expression or function of the IGF-IR in breast cancer cells is modulated by different humoral factors, such as estrogen, progesterone, IGF-II, and interleukin-1. The IGF-IR and the estrogen receptor (ER) are usually co-expressed and the two signaling systems are engaged in a complex functional cross-talk controlling cell proliferation. Despite the convincing experimental evidence, the role of the IGF-IR in breast cancer etiology, especially in metastatic progression, is still not clear. The view emerging from cellular and animal studies is that abnormally high levels of IGF-IRs may contribute to the increase of tumor mass and/or aid tumor recurrence, by promoting proliferation, cell survival, and cell-cell interactions. However, in breast cancer, except for the well established correlation with ER status, the associations of the IGF-IR with other prognostic parameters are still insufficiently documented.