Insulin-like growth factor binding proteins (IGFBPs) in breast cancer

Molecular regulation of phenolic compounds on IGF-1 signaling cascade in breast cancer

Breast cancer is a highly aggressive and heterogeneous disease with complex features that remains a major health problem and undermines the span and quality of life of women worldwide. Primary literature has shown the role of phenolic compounds in controlling the onset of breast cancer. The mechanism of action of phenolic compounds can be explained by their interaction with signal transduction pathways that regulate cell proliferation and induction of apoptosis. One of the targets of phenolic compounds is the insulin like growth factor 1 (IGF-1) signaling cascade, which plays a significant role in the growth and development of mammary tissues by leading proliferative and anti-apoptotic events. Increasing research evidence points to the function of the IGF-1 cascade system in the commencement, progression, and metastasis of breast tissue malignancy. In this review, we mainly discuss the function of the IGF-1 system, and the role of phenolic compounds in regulating the IGF-1 signaling cascade and curbing breast malignancies.

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.

Investigation of the Interplay between Circulating Lipids and IGF-I and Relevance to Breast Cancer Risk: An Observational and Mendelian Randomization Study

BACKGROUND

Circulating lipids and insulin-like growth factor 1 (IGF-I) have been reliably associated with breast cancer. Observational studies suggest an interplay between lipids and IGF-I, however, whether these relationships are causal and if pathways from these phenotypes to breast cancer overlap is unclear.

METHODS

Mendelian randomization (MR) was conducted to estimate the relationship between lipids or IGF-I and breast cancer risk using genetic summary statistics for lipids (low-density lipoprotein cholesterol, LDL-C; high-density lipoprotein cholesterol, HDL-C; triglycerides, TGs), IGF-I and breast cancer from GLGC/UKBB (N = 239,119), CHARGE/UKBB (N = 252,547), and Breast Cancer Association Consortium (N = 247,173), respectively. Cross-sectional observational and MR analyses were conducted to assess the bi-directional relationship between lipids and IGF-I in SHIP (N = 3,812) and UKBB (N = 422,389), and using genetic summary statistics from GLGC (N = 188,577) and CHARGE/UKBB (N = 469,872).

RESULTS

In multivariable MR (MVMR) analyses, the OR for breast cancer per 1-SD increase in HDL-C and TG was 1.08 [95% confidence interval (CI), 1.04-1.13] and 0.94 (95% CI, 0.89-0.98), respectively. The OR for breast cancer per 1-SD increase in IGF-I was 1.09 (95% CI, 1.04-1.15). MR analyses suggested a bi-directional TG-IGF-I relationship (TG-IGF-I β per 1-SD: -0.13; 95% CI, -0.23 to -0.04; and IGF-I-TG β per 1-SD: -0.11; 95% CI, -0.18 to -0.05). There was little evidence for a causal relationship between HDL-C and LDL-C with IGF-I. In MVMR analyses, associations of TG or IGF-I with breast cancer were robust to adjustment for IGF-I or TG, respectively.

CONCLUSIONS

Our findings suggest a causal role of HDL-C, TG, and IGF-I in breast cancer. Observational and MR analyses support an interplay between IGF-I and TG; however, MVMR estimates suggest that TG and IGF-I may act independently to influence breast cancer.

IMPACT

Our findings should be considered in the development of prevention strategies for breast cancer, where interventions are known to modify circulating lipids and IGF-I.

Pre- and Postoperative Circulating IGF-I, IGFBP-3, and IGFBP-7 Levels in Relation to Endocrine Treatment and Breast Cancer Recurrence: A Nested Case-Control Study

Insulin-like growth factor-I (IGF-I) and its binding proteins (BPs) have been associated with breast cancer risk, especially high IGF-I concentrations and the biologically active fraction estimated as the IGF-I/IGFBP-3 molar ratio. The relation of circulating IGF-I and IGFBP-3 concentrations with risk of breast cancer recurrence has been less documented. In addition a new member to a sub-group of the IGFBP-superfamily was recently identified, the low affinity IGFBP-7. To date, the role of systemic IGFBP-7 in breast cancer progression has not been investigated. Our purpose was to establish whether circulating IGF-I, IGFBP-3, and IGFBP-7 levels are related to recurrence-risk in breast cancer. A case-control study was nested within the population-based BCBlood cohort of 853 breast cancer patients diagnosed 2002-2010 in Sweden and followed through 2012. In total, 95 patients with recurrence and 170 controls were matched on age and tumor characteristics. Plasma IGF analytes and tumor membrane IGF-I receptor (IGF-IR^m) positivity were analyzed and recurrence-risk was evaluated with conditional logistic regression. Preoperative tertiles of IGF-I and IGFBP-3 were both positively associated with recurrence-risk, but not IGFBP-7. The trend was of borderline significance for IGF-I, T1:REF, T2 OR:1.6, T3 OR: 2.2 adjusted P _trend=0.057 and significant for IGFBP-3 T1:REF, T2 OR:1.2, T3 OR: 2.1 adjusted P _trend=0.042. The models were adjusted for age, anthropometric factors, smoking, and treatments. There was a significant interaction between IGFBP-7 and IGF-IR^m positivity on recurrence, where the highest IGFBP-7 highest IGFBP-7 tertile conferred increased recurrence-risk in patients with IGF-IR^m positive tumors but not in those with IGF-IR^m negative tumors (P _interaction=0.024). By the 1-year visit, age-adjusted IGF-I levels were reduced by 17% while IGFBP-3 and IGFBP-7 were stable. IGF-I levels were significantly reduced by radiotherapy in all patients and by tamoxifen in patients with ER⁺ tumors. Postoperative changes >10% (n=208) in IGF-I, IGFBP-3, IGFBP-7, or the IGF-I/IGFBP-3 ratio did not predict recurrence after adjustment for preoperative levels, age, anthropometric factors, smoking, and treatments. In conclusion, this study suggests that preoperative IGF-I and IGFBP-3 levels, but not postoperative changes, might provide independent prognostic information and influence breast cancer recurrence. The role of IGFBP-7 in breast cancer merits further study.

Relationship of circulating insulin-like growth factor-I and binding proteins 1-7 with mammographic density among women undergoing image-guided diagnostic breast biopsy

Background

Mammographic density (MD) is a strong breast cancer risk factor that reflects fibroglandular and adipose tissue composition, but its biologic underpinnings are poorly understood. Insulin-like growth factor binding proteins (IGFBPs) are markers that may be associated with MD given their hypothesized role in breast carcinogenesis. IGFBPs sequester IGF-I, limiting its bioavailability. Prior studies have found positive associations between circulating IGF-I and the IGF-I:IGFBP-3 ratio and breast cancer risk. We evaluated the associations of IGF-I, IGFBP-3, and six other IGFBPs with MD.

Methods

Serum IGF measures were quantified in 296 women, ages 40–65, undergoing diagnostic image-guided breast biopsy. Volumetric density measures (MD-V) were assessed in pre-biopsy digital mammograms using single X-ray absorptiometry. Area density measures (MD-A) were estimated by computer-assisted thresholding software. Age, body mass index (BMI), and BMI²-adjusted linear regression models were used to examine associations of serum IGF measures with MD. Effect modification by BMI was also assessed.

Results

IGF-I and IGFBP-3 were not strongly associated with MD after BMI adjustment. In multivariable analyses among premenopausal women, IGFBP-2 was positively associated with both percent MD-V (β = 1.49, p value = 0.02) and MD-A (β = 1.55, p value = 0.05). Among postmenopausal women, positive relationships between IGFBP-2 and percent MD-V (β = 2.04, p = 0.003) were observed; the positive associations between IGFBP-2 and percent MD-V were stronger among lean women (BMI < 25 kg/m²) (β = 5.32, p = 0.0002; p interaction = 0.0003).

Conclusions

In this comprehensive study of IGFBPs and MD, we observed a novel positive association between IGFBP-2 and MD, particularly among women with lower BMI. In concert with in vitro studies suggesting a dual role of IGFBP-2 on breast tissue, promoting cell proliferation as well as inhibiting tumorigenesis, our findings suggest that further studies assessing the role of IGFBP-2 in breast tissue composition, in addition to IGF-1 and IGFBP-3, are warranted.

Electronic supplementary material

The online version of this article (10.1186/s13058-019-1162-8) contains supplementary material, which is available to authorized users.

Insulin-like growth factor and epidermal growth factor signaling in breast cancer cell growth: focus on endocrine resistant disease

Breast cancer is the most common type of cancer for women worldwide with a lifetime risk amounting to a staggering total of 10%. It is well established that the endogenous synthesis of insulin-like growth factor (IGF) and epidermal growth factor (EGF) polypeptide growth factors are closely correlated to malignant transformation and all the steps of the breast cancer metastatic cascade. Numerous studies have demonstrated that both estrogens and growth factors stimulate the proliferation of steroid-dependent tumor cells, and that the interaction between these signaling pathways occurs at several levels. Importantly, the majority of breast cancer cases are estrogen receptor- (ER-) positive which have a more favorable prognosis and pattern of recurrence with endocrine therapy being the backbone of treatment. Unfortunately, the majority of patients progress to endocrine therapy resistant disease (acquired resistance) whereas a proportion of patients may fail to respond to initial therapy (de novo resistance). The IGF-I and EGF downstream signaling pathways are closely involved in the process of progression to therapy resistant disease. Modifications in the bioavailability of these growth factors contribute critically to disease progression. In the present review therefore, we will discuss in depth how IGF and EGF signaling participate in breast cancer pathogenesis and progression to endocrine resistant disease.

Insulin-like growth factor binding protein-4 and -5 modulate ligand-dependent estrogen receptor-α activation in breast cancer cells in an IGF-independent manner

Insulin-like growth factor binding proteins (IGFBPs) are modulators of numerous cellular processes including cell proliferation. Although IGFBPs classically act by sequestration of extracellular insulin-like growth factors (IGFs), thereby contributing to the fine-tuning of growth factor signals, IGF-independent actions of IGFBPs have also been described. In the breast, growth factor signaling in association with estradiol (E2)-stimulated estrogen receptor function is organized in a complex cross-talk. The importance of phosphatidylinositol 3-kinase/protein kinase B (Akt/PKB) pathway components for the E2-induced activation of estrogen receptor-alpha (ERα) is well accepted. Here we show that in the absence of IGFs, IGFBP-4 or IGFBP-5, either overexpressed in MCF-7 breast cancer cells or added exogenously, decreased the capability of E2 to induce ERα transcriptional activity. In addition, overexpression or addition of recombinant IGFBP-4 or IGFBP-5 resulted in reduction of E2-induced phosphorylation of Akt/PKB, GSK-3α/β and ERα in MCF-7 cells. The activation of the Akt/PKB-pathway describes a non-genomic effect of E2, which did not involve activation/phosphorylation of the IGF-I receptor (IGF-IR). Furthermore, knockdown of the IGF-IR did not affect the inhibition of E2-induced ERα phosphorylation by IGFBP-4 and 5. Moreover, IGFBP-4 and IGFBP-5 strongly decreased E2-triggered growth of MCF-7 cells. Our data suggest that IGFBPs interfere with the E2-induced activation of the Akt/PKB-pathway and prevent full hormone-dependent activation of ERα and breast cancer cell growth in an IGF- and IGF-IR-independent manner.

An association between a common variant (G972R) in the IRS-1 gene and sex hormone levels in post-menopausal breast cancer survivors

Insulin receptor substrate-1 (IRS-1) is a key downstream signaling molecule common to both the insulin and IGF signaling pathways that can interact with the estrogen pathway to regulate breast cell growth. We investigated whether a putative functional variant for IRS-1 (G972R) influences circulating levels of sex hormones, sex hormone binding globulin (SHBG), C-peptide, and insulin-like growth factor 1 (IGF-1) levels among post-menopausal African-American and non-Hispanic white breast cancer patients enrolled in the Health, Eating, Activity, and Lifestyle (HEAL) Study. Circulating levels of sex hormones and growth factors can influence breast cancer recurrence and survival. Serum estrone, estradiol, testosterone, SHBG, IGF-1 and C-peptide were measured in 468 patients at 30+ months post diagnosis. Non-protein bound hormone levels (free estradiol, free testosterone) were calculated. In African-American patients, the IRS-1 variant was associated with increased serum levels of estrone (p = 0.02), free estradiol (p = 0.04), total testosterone (p = 0.04), free testosterone (p = 0.006) and decreased levels of sex hormone-binding globulin (p = 0.02). No association was present for white patients. Our findings provide suggestive evidence that IRS-1 G972R variant may be associated with circulating levels of sex hormones and SHBG in African American breast cancer survivors.

Circulating levels of insulin-like growth factors, their binding proteins, and breast cancer risk

BACKGROUND

Earlier data support the hypothesis that the relation between circulating insulin-like growth factor-I (IGF-I) levels and breast cancer risk differs by menopausal status. The strong association of IGF-I with height in childhood and weak or no association between adult levels and adult height also suggest that IGF levels in young women may better reflect an exposure time period of importance to breast cancer. Few studies have assessed IGF binding protein-1 (IGFBP-1) or free IGF and breast cancer risk.

MATERIALS AND METHODS

We conducted a large case-control study nested within the prospective Nurses' Health Study. Plasma concentrations of IGF-I, free IGF, IGFBP-3, and IGFBP-1 were measured in blood samples collected in 1989 to 1990. Eight hundred women were identified who had a diagnosis of invasive or in situ breast cancer after blood collection, up to 1998, 27% of whom were premenopausal at blood collection. To those 800 women, one to two controls were age-matched for a total of 1,129 controls. We used logistic regression models to estimate the relative risk (RR) of breast cancer associated with IGF levels.

FINDINGS

Among postmenopausal women, neither IGF-I, IGFBP-3, IGFBP-1, nor free IGF was associated with breast cancer risk [RRs, top versus bottom quintile: IGF-I, 1.0; 95% confidence interval (95% CI), 0.7-1.4; IGFBP-3, 0.8; 95% CI, 0.6-1.1; IGFBP-1, 0.9; 95% CI, 0.6-1.5; and free IGF, 1.0; 95% CI, 0.6-1.4]. Among premenopausal women, IGFBP-3, IGFBP-1, and free IGF similarly were not associated with breast cancer risk (RRs, top versus bottom quintile: IGFBP-3, 1.2; 95% CI, 0.8-2.3; IGFBP-1, 1.5; 95% CI, 0.8-3.0; and free IGF, 1.1; 95% CI, 0.7-2.1). Higher IGF-I plasma levels, however, were associated with a modestly elevated breast cancer risk (RR, 1.6; 95% CI, 1.0-2.6) among the premenopausal women, with a stronger association among premenopausal women ages < or =50 (RR, 2.5; 95% CI, 1.4-4.3); further adjustment for IGFBP-3 did not greatly change these estimates.

INTERPRETATION

Circulating IGF-I levels seem to be modestly associated with breast cancer risk among premenopausal women, but not among postmenopausal women. IGFBP-3, IGFBP-1, and free IGF are not associated with breast cancer risk in either premenopausal or postmenopausal women in this cohort.

High IGFBP-3 levels in marrow plasma in early-stage MDS: effects on apoptosis and hemopoiesis

The pathophysiology of the myelodysplastic syndromes (MDS) is incompletely understood. Tumor necrosis factor (TNF)alpha levels are elevated, particularly in early-stage MDS, and apoptosis in marrow cells is upregulated. Observations in other models have shown a role for insulin-like growth factor binding protein 3 (IGFBP-3) in TNFalpha-mediated apoptosis. We observed increased levels of IGFBP-3 in the marrow plasma of patients with MDS (P = 0.005) and hypothesized that altered IGFBP-3 levels contribute to the dysregulation of hemopoiesis in MDS by affecting proliferation and apoptosis. Western analysis of marrow plasma from MDS patients revealed an increase in the ratio of intact vs fragmented IGFBP-3 in early-stage MDS (relative to controls) that decreased with MDS disease progression, suggesting increased proteolysis with more advanced disease. Thus, these results provide evidence for dysregulation of IGFBP-3 in patients with MDS. While the data are complex, they are consistent with a modulatory effect of IGFBP-3 on hemopoiesis in MDS. Conceivably, understanding these mechanisms may allow for the development of novel therapeutic strategies.

Hormonal regulation of the Grb14 signal modulator and its role in cell cycle progression of MCF-7 human breast cancer cells

Growth factor receptor bound (Grb)14 is a member of the Grb7 family of src homology (SH)2 domain-containing proteins. These proteins perform both adaptor and modulatory roles in receptor tyrosine kinase (RTK) signaling, although their regulation is poorly understood. In this study, a positive correlation between Grb14 protein expression and ER alpha status in breast cancer cell lines led us to investigate regulation of Grb14 by estradiol and insulin, which synergize in the regulation of breast cancer cell proliferation. In MCF-7 cells maintained in charcoal-stripped serum, Grb14 expression was downregulated by estradiol and increased by the pure anti-estrogen ICI 182780. Under serum-free conditions, insulin enhanced Grb14 expression but this effect was repressed by estradiol when both hormones were used in combination. Using a system in which c-Myc induction drives cell cycle progression independently of estradiol, we demonstrated that Grb14 regulation was specific to estradiol treatment. Finally, we demonstrated a novel functional role for Grb14 whereby its overexpression inhibited not only insulin- but also estrogen-induced cell cycle progression. This was associated with decreased extracellular signal-regulated kinase (Erk)1/2 activation in insulin-stimulated Grb14-overexpressing cells. These data represent the first demonstration of regulation of Grb14 expression levels in response to hormonal stimuli, and are consistent with its role as a repressor of insulin signaling where it is induced as a negative feedback mechanism. A role for Grb14 is also shown in estrogen/insulin crosstalk since estradiol blocks the insulin-induced induction of this protein.

The therapeutic potential of agents targeting the type I insulin-like growth factor receptor

The type I insulin-like growth factor receptor (IGF-1R) is a receptor tyrosine kinase that mediates insulin-like growth factor I (IGF-1) and IGF-2 signalling. Increased expression levels and/or enhanced activity of IGF-1R have been observed in many types of cancer. It is well documented that IGF-1R plays important roles in the proliferation, transformation, motility and metastasis of cancer cells. Therefore, IGF-1R has surfaced as an attractive target for cancer therapy. There are several aspects of this receptor that need to be considered when thinking about inhibitory strategies. In this review, several points relevant to targeting IGF-1R will be discussed, including the signalling pathways downstream of the receptor, the potential role for the insulin receptor in regulating IGF action and multiple cancer phenotypes regulated by this receptor. In addition, there are several strategies that could be used to inhibit IGF action. Inhibition of receptor function by lowering protein expression, decreasing kinase activity by small-molecule inhibitors, disrupting receptor function by monoclonal antibody blockade and neutralising circulating ligand all represent potential therapeutic strategies. As these strategies move forward to clinical trial, several important considerations need to be incorporated into the clinical trial design.

Sex steroids and growth factors in the regulation of mammary gland proliferation, differentiation, and involution

The mammary gland is subjected to major morphological and biochemical changes during the lactation cycle. It is therefore not surprising that this dynamic process is strictly controlled. The importance of the sex steroid hormones 17beta-estradiol and progesterone for normal development of the mammary gland was recognized several decades ago and has been unequivocally confirmed since. Furthermore, it is now also established that the influence of sex steroids is not restricted to mammogenesis, but that these hormones also control involution. Another important regulatory role is played by growth factors that have been shown to modulate survival (epidermal growth factor, amphiregulin, transforming growth factor alpha, insulin like growth factor, and tumor necrosis factor alpha) or apoptosis (tumor necrosis factor alpha, transforming growth factor beta) of mammary cells. However, the molecular mechanism underlying the influence of sex steroid hormones and/or growth factors on the development and function of the mammary gland remains largely unknown to date. Also scarce is information on the interaction between both groups of modulators. Nevertheless, based on the current indications compiled in this review, an important functional role for sex steroid hormones in the lactation cycle in co-operation with growth factors can be suggested.

Adipocyte-secreted factors synergistically promote mammary tumorigenesis through induction of anti-apoptotic transcriptional programs and proto-oncogene stabilization

Mammary epithelial cells are embedded in a unique extracellular environment to which adipocytes and other stromal cells contribute. Mammary epithelial cells are critically dependent on this milieu for survival. However, it remains unknown which adipocyte-secreted factors are required for the survival of the mammary epithelia and what role these adipokines play in the process of ductal carcinoma tumorigenesis. Here, we take a systematic molecular approach to investigate the multiple ways adipocytes and adipokines can uniquely influence the characteristics and phenotypic behavior of malignant breast ductal epithelial cells. Microarray analysis and luciferase reporter assays indicate that adipokines specifically induce several transcriptional programs involved in promoting tumorigenesis, including increased cell proliferation (IGF2, FOS, JUN, cyclin D1), invasive potential (MMP1, ATF3), survival (A20, NFkappaB), and angiogenesis. One of the key changes in the transformed ductal epithelial cells associated with the cell cycle involves the induction of NFkappaB (five-fold) and cyclin D1 (three-fold). We show that by regulating the transcription of these molecules, the synergistic activity of adipocyte-derived factors can potentiate MCF-7 cell proliferation. Furthermore, compared to other stromal cell-secreted factors, the full complement of adipokines shows an unparalleled ability to promote increased cell motility, migration, and the capacity for angiogenesis. Adipocyte-secreted factors can affect tumorigenesis by increasing the stabilization of pro-oncogenic factors such as beta-catenin and CDK6 as a result of a reduction in the gene expression of their inhibitors (i.e. p18). An in vivo coinjection system using 3T3-L1 adipocytes and SUM159PT cells effectively recapitulates the host-tumor interactions in primary tumors. Type VI collagen, a soluble extracellular matrix protein abundantly expressed in adipocytes, is further upregulated in adipocytes during tumorigenesis. It promotes GSK3beta phosphorylation, beta-catenin stabilization, and increased beta-catenin activity in breast cancer cells and may critically contribute towards tumorigenesis when not counterbalanced by other factors.

Hormone replacement therapy and breast carcinoma risk in Hispanic and non-Hispanic women

BACKGROUND

Hormone replacement therapy (HRT) alleviates menopausal symptoms and prevents osteoporosis, but there is concern that long-term use may have an adverse impact on breast carcinoma risk. Epidemiologic studies report inconsistent findings regarding the relationship between HRT and postmenopausal breast carcinoma risk and there is little information on the HRT-associated risk among minority women.

METHODS

To investigate the effects of HRT on breast carcinoma risk among Hispanic women, we examined data from the New Mexico Women's Health Study (NMWHS), a statewide case-control study comprising 366 postmenopausal women with breast carcinoma and 403 controls. Conditional logistic regression was used to estimate the odds ratios (OR) and 95% confidence intervals (95% CI).

RESULTS

After adjustment for sociodemographic, medical, and reproductive factors, we found an increased risk associated with long-term users of estrogen replacement therapy (ERT; OR for ERT use longer than 140 months was 2.57, 95%CI, 1.25-5.28, compared with ERT use shorter than 17 months). In addition, there was a positive trend with the duration of ERT use (P < 0.01). Hispanic postmenopausal women appeared to be at a significantly greater risk than non-Hispanic white women (OR for ERT use longer than 140 months in Hispanics was 5.53, 95%CI, 1.47-20.87; OR in non-Hispanics was 2.65, 95%CI, 0.95-7.34, compared with ERT use shorter than 17 months). In contrast, no significant association was observed for combined estrogen-progesterone use in either Hispanic or non-Hispanic women.

CONCLUSIONS

The results of this study indicated that postmenopausal women had significantly increased breast carcinoma risk for long-term ERT use. The risks among Hispanic women were substantially higher than among non-Hispanic white women although they were not statistically significant.

Hormone/growth factor interactions mediating epithelial/stromal communication in mammary gland development and carcinogenesis

Epithelial/mesenchymal interactions begin during embryonic development of the mammary gland and continue throughout mammary gland development into adult life. Stromal and epithelial growth factors that may mediate interactions between these compartments of the mammary gland are reviewed. Since mammogenic hormones are the primary regulators of mammary gland development, special consideration is given to hormonal regulation of growth factors in order to explore the integration of hormones and growth factors in the regulation of mammary gland growth and neoplasia. Examination of hormonal regulation of the fibroblast growth factor (FGF)-7/FGFR2-IIIb receptor system in the mammary gland reveals that mammogenic hormones differentially regulate the synthesis of stromal growth factors and their epithelial receptors. These effects serve to optimize the action of estrogen and progesterone on mammary gland development and illustrate that the ratio of these two hormones is critical in regulating this growth factor axis. The role of stromal/epithelial mitogenic microenvironments in modulating the genotype and phenotype of preneoplastic and neoplastic lesions by chemical carcinogens is discussed. Finally, changes in growth factor expression during mammary tumor progression are described to illustrate the relative roles that stromally-derived and epithelial-derived growth factors may play during progression to hormone independent tumor growth.

Insulin-like growth factor (IGF) binding protein-3 regulation of IGF-I is altered in an acidic extracellular environment

While extracellular acidification within solid tumors is well-documented, how reduced pH impacts regulation of insulin-like growth factor-I (IGF-I) has not been studied extensively. Because IGF-I receptor binding is affected by IGF binding proteins (IGFBPs), we examined how pH impacted IGFBP-3 regulation of IGF-I. IGF-I binding in the absence of IGFBP-3 was diminished at reduced pH. Addition of IGFBP-3 reduced IGF-I cell binding at pH 7.4 but increased surface association at pH 5.8. This increase in IGF-I binding at pH 5.8 corresponded with an increase in IGFBP-3 cell association. This, however, was not due to an increase in affinity of IGFBP-3 for heparin at reduced pH although both heparinase III treatment and heparin addition reduced IGFBP-3 enhancement of IGF-I binding. An increase in IGF-I binding to IGFBP-3, though, was seen at reduced pH using a cell-free assay. We hypothesize that the enhanced binding of IGF-I at pH 5.8 is facilitated by increased association of IGFBP-3 at this pH and that the resulting cell associated IGF-I is IGFBP-3 and not IGF-IR bound. Increased internalization and nuclear association of IGF-I at pH 5.8 in the presence of IGFBP-3 was evident, yet cell proliferation was reduced by IGFBP-3 at both pH 5.8 and 7.4 indicating that IGFBP-3-cell associated IGF-I does not signal the cell to proliferate and that the resulting transfer of bound IGF-I from IGF-IR to IGFBP-3 results in diminished proliferation. Solution binding of IGF-I by IGFBP-3 is one means by which IGF-I-induced proliferation is inhibited. Our work suggests that an alternative pathway exists by which IGF-I and IGFBP-3 both associate with the cell surface and that this association inhibits IGF-I-induced proliferation.