Insulin-like growth factor binding protein-5 (IGFBP-5) induces premature cell death in the mammary glands of transgenic mice

The role of insulin-like growth factor-1 in lactation

Generally, Insulin-like growth factor 1 (IGF-1) is believed to regulate lactation activity by promoting cell proliferation and differentiation. With the advancement of research, IGF-1 has been discovered to play an important role in different stages of lactation. In actual animal production, lactation ability directly affects milk yield and milk quality, which not only affects the survival and future growth of pups, but also is an important economic trait of some animals. In this paper, it is introduced that IGF-1 plays an important role in the whole lactation process, and what factors are involved in the regulation of IGF-1 in this process and how to improve lactation ability through IGF-1 in animal production, providing a theoretical basis for further exploration of IGF-1 in lactation, and also brings a theoretical foundation for the improvement of animal lactation ability.

IGFBP5 affects cardiomyocyte survival and functional recovery in mice following myocardial ischemia

Insulin-like growth factor-binding protein 5 (IGFBP5) has been shown to be useful for the diagnosis and treatment of multiple tumors and cerebrovascular diseases. However, it is unknown whether IGFBP5 is involved in myocardial repair following myocardial infarction (MI). Here we show high expression of IGFBP5 in multiple models of ischemic and hypoxic injury. IGFBP5 affected the proliferation of neonatal rat cardiomyocytes (NRCMs) and the cardiomyocyte apoptosis induced by oxygen-glucose deprivation (OGD). Subsequently, heart-specific IGFBP5 knockdown inhibited myocardial apoptosis and increased cardiomyocyte proliferation in mice with MI. During the chronic remodeling stage, heart-specific regulation of IGFBP5 ameliorated pathological cardiac remodeling and dysfunction. Mechanistically, IGFBP5 regulated cardiomyocyte survival through the insulin-like growth factor 1 (IGF1) receptor (IGF1R)/protein kinase B (PKB/AKT) pathway. In summary, our results provide mechanistic insights into the effect of IGFBP5 on cardiomyocyte during cardiac repair. IGFBP5 may represent a therapeutic target for myocardial ischemic injury.

The role of the IGF system in mammary physiology of ruminants

The IGF system plays a central role in all stages of mammary development, lactation and involution. IGFs exert their effects on the mammary gland through both endocrine and paracrine/autocrine mechanisms and the importance of circulating versus local IGF action remains an open question, especially in ruminants. At the whole organ level, a critical role for IGFs in ductal morphogenesis and lobuloalveolar development has been established, while at the cellular level the ability of IGFs to stimulate cell proliferation and control cell survival contributes to the number of milk-secreting cells in the gland. Much of this work has been conducted in rodents which provide an affordable research model and allow for genetic manipulation of specific components of the IGF system. Research into the role of the IGF system in dairy cows has generally supported information obtained with rodents though large gaps in our knowledge remain and species differences are not well defined. Examples include whether exogenous somatotropin exerts its effects on the mammary gland through local IGF-1 synthesis which is accepted dogma in rodents, what the role of IGF-1 versus IGF-2 is in the mammary gland, and how the IGFBPs regulate IGF bioactivity. This last area is particularly under-investigated in ruminants both at the whole animal and the cellular and molecular levels. Given that the IGF system may underlie many management practices that could contribute to enhancing productive efficiency of lactation, more research into the basic biology of this important system is warranted.

Mice with gene alterations in the GH and IGF family

Much of our understanding of GH's action stems from animal models and the generation and characterization of genetically altered or modified mice. Manipulation of genes in the GH/IGF1 family in animals started in 1982 when the first GH transgenic mice were produced. Since then, multiple laboratories have altered mouse DNA to globally disrupt Gh, Ghr, and other genes upstream or downstream of GH or its receptor. The ability to stay current with the various genetically manipulated mouse lines within the realm of GH/IGF1 research has been daunting. As such, this review attempts to consolidate and summarize the literature related to the initial characterization of many of the known gene-manipulated mice relating to the actions of GH, PRL and IGF1. We have organized the mouse lines by modifications made to constituents of the GH/IGF1 family either upstream or downstream of GHR or to the GHR itself. Available data on the effect of altered gene expression on growth, GH/IGF1 levels, body composition, reproduction, diabetes, metabolism, cancer, and aging are summarized. For the ease of finding this information, key words are highlighted in bold throughout the main text for each mouse line and this information is summarized in Tables 1, 2, 3 and 4. Most importantly, the collective data derived from and reported for these mice have enhanced our understanding of GH action.

The Multi-Faced Role of PAPP-A in Post-Partum Breast Cancer: IGF-Signaling is Only the Beginning

Insulin-like growth factor (IGF) signaling and control of local bioavailability of free IGF by the IGF binding proteins (IGFBP) are important regulators of both mammary development and breast cancer. A recent genome-wide association study (GWAS) identified small nucleotide polymorphisms that reduce the expression of IGFBP-5 as a risk factor of developing breast cancer. This observation suggests that genetic alterations leading to a decreased level of IGFBP-5 may also contribute to breast cancer. In the current review, we focus on Pregnancy-Associated Plasma Protein A (PAPP-A), a protease involved in the degradation of IGFBP-5. PAPP-A is overexpressed in the majority of breast cancers but its role in cancer has only begun to be explored. More specifically, this review aims at highlighting the role of post-partum involution in the oncogenic function of PAPP-A. Notably, we summarize recent studies indicating that PAPP-A plays a role not only in the degradation of IGFBP-5 but also in the deposition of collagen and activation of the collagen receptor discoidin 2 (DDR2) during post-partum involution. Finally, considering the immunosuppressive microenvironment of post-partum involution, we also discuss the unexpected finding made in Ewing Sarcoma that PAPP-A plays a role in immune evasion. While the immunosuppressive role of PAPP-A in breast cancer remains to be determined, collectively these studies highlight the multifaced role of PAPP-A in cancer that extends well beyond its effect on IGF-signaling.

MiR-21-3p Centric Regulatory Network in Dairy Cow Mammary Epithelial Cell Proliferation

MicroRNA-mediated gene regulation is important for the development of the mammary gland and the lactating process. A previous study has shown that the expression of microRNA-21 (miR-21) is different in the dry and early lactation period of the dairy cow mammary gland, but the molecular mechanisms underlying the lactation cycle are not fully understood. Here, the function of miR-21-3p on bovine mammary gland epithelial cells (BMECs) was detected by MTT assay and flow cytometry analysis, which showed that miR-21-3p significantly promoted the cell viability and proliferation. Then, the regulating mechanism of miR-21-3p on cell viability and proliferation was elucidated. Dual luciferase assay, RT-qPCR, and Western blot results revealed that IGFBP5 was a target gene of miR-21-3p. It was known that lncRNA could act as a competing endogenous RNA to sequester miRNAs and reduce the regulatory effect of miRNA-targeted genes. Based on our previous lncRNA-seq data and bioinformatics analysis, lncRNA NONBTAT017009.2 was potentially associated with miR-21-3p, and its expression was specifically inhibited with the transfection of miR-21-3p mimic into BMECs. Inversely, the overexpression of NONBTAT017009.2 significantly decreased the expression level of miR-21-3p in BMECs, while the expression of IGFBP5, the target gene of miR-21-3p, was significantly upregulated. In addition, the promoter region of miR-21 contained two STAT3 binding sites, and the dual luciferase reporter assays revealed that the overexpression of STAT3 significantly reduced the promoter activity of miR-21, implying that the transcription factor STAT3 may act as an upstream regulator affecting the regulation process of miR-21-3p. The overexpression of STAT3 significantly inhibited the expression of miR-21-3p, while the mRNA expression of IGFBP5 was significantly increased compared with the control group. Besides, there are no STAT3 binding sites in the promoter region of IGFBP5 as we predicted by gene-regulation and JASPAR software. Therefore, it could infer that STAT3 might regulate the expression of IGFBP5 by miR-21-3p. Taken together, these results established a regulatory network of miR-21-3p to illustrate the regulating mechanism on promoting cow mammary epithelial cell proliferation.

Invited review: Accelerating mammary gland involution after drying-off in dairy cattle

Bovine mammary gland involution, as a part of the reproductive cycle in dairy cows, is a very important remodeling transformation of the mammary gland for the subsequent lactation. There is considerable incentive to accelerate mammary gland involution to improve udder health, shorten the dry period, and simplify the management process by reducing dietary changes. The complex process of mammary involution is characterized by morphological changes in the epithelial cells and mammary tissue, changes in the composition of mammary secretions, and changes in the integrity of tight junctions. Involution is facilitated by elements of the immune system and several types of proteases and is coordinated by various types of hormones. This review first describes the involution process and then argues for the need to accelerate it. Last, this review focuses on various intervention methods for accelerating involution. Our aim is to provide a comprehensive overview of bovine mammary gland involution as well as potential techniques and new opinions for dry cow management.

IGF-binding proteins

Insulin-like growth factor-binding proteins (IGFBPs) 1-6 bind IGFs but not insulin with high affinity. They were initially identified as serum carriers and passive inhibitors of IGF actions. However, subsequent studies showed that, although IGFBPs inhibit IGF actions in many circumstances, they may also potentiate these actions. IGFBPs are widely expressed in most tissues, and they are flexible endocrine and autocrine/paracrine regulators of IGF activity, which is essential for this important physiological system. More recently, individual IGFBPs have been shown to have IGF-independent actions. Mechanisms underlying these actions include (i) interaction with non-IGF proteins in compartments including the extracellular space and matrix, the cell surface and intracellular space, (ii) interaction with and modulation of other growth factor pathways including EGF, TGF-β and VEGF, and (iii) direct or indirect transcriptional effects following nuclear entry of IGFBPs. Through these IGF-dependent and IGF-independent actions, IGFBPs modulate essential cellular processes including proliferation, survival, migration, senescence, autophagy and angiogenesis. They have been implicated in a range of disorders including malignant, metabolic, neurological and immune diseases. A more complete understanding of their cellular roles may lead to the development of novel IGFBP-based therapeutic opportunities.

Analysis of the IGF-system in milk from farm animals - Occurrence, regulation, and biomarker potential

IGFs and IGF-binding proteins (IGFBPs) are abundantly present in milk and in dairy products. Compared to the IGFs, the IGFBP have received less attention in milk, although truncated IGFBPs and IGFBP-glycosylation have been described in milk. Thereby, complex control of local IGF-effects can be assumed on the levels of IGFBPs, proteases, and protease inhibitors. The present review collects the current knowledge both on presence and regulation of IGFs and IGFBPs in milk particularly from dairy animal species. As a rule higher levels of IGF-I, IGF-II, and IGFBPs are measured around parturition if compared to later time-points of lactation. In all farm animal species included in this review, it is found that the relative abundancies of IGFBPs in milk and serum are similar, with IGFBP-3 and -2 characterized by higher concentrations if compared to IGFBP-4 or -5. The concentrations of IGFs and IGFBPs in milk or dairy products can be altered by hormones, dairy processing, or fermentation. Because milk can be used for non-invasive biomarker research, quality management, and health monitoring, we discuss novel directions of IGF-analysis and potential on-site biomarker research in milk.

The innate and adaptive infiltrating immune systems as targets for breast cancer immunotherapy

A cancer cell-centric view has long dominated the field of cancer biology. Research efforts have focussed on aberrant cancer cell signalling pathways and on changes to cancer cell DNA. Mounting evidence demonstrates that many cancer-associated cell types within the tumour stroma co-evolve and support tumour growth and development, greatly modifying cancer cell behaviour, facilitating invasion and metastasis and controlling dormancy and sensitivity to drug therapy. Thus, these stromal cells represent potential targets for cancer therapy. Among these cell types, immune cells have emerged as a promising target for therapy. The adaptive and the innate immune system play an important role in normal mammary development and breast cancer. The number of infiltrating adaptive immune system cells with tumour-rejecting capacity, primarily, T lymphocytes, is lower in breast cancer compared with other cancer types, but infiltration occurs in a large proportion of cases. There is strong evidence demonstrating the importance of the immunosuppressive role of the innate immune system during breast cancer progression. A consideration of components of both the innate and the adaptive immune system is essential for the design and development of immunotherapies in breast cancer. In this review, we focus on the importance of immunosuppressive myeloid-derived suppressor cells (MDSCs) as potential targets for breast cancer therapy.

LPA receptor activity is basal specific and coincident with early pregnancy and involution during mammary gland postnatal development

During pregnancy, luminal and basal epithelial cells of the adult mammary gland proliferate and differentiate resulting in remodeling of the adult gland. While pathways that control this process have been characterized in the gland as a whole, the contribution of specific cell subtypes, in particular the basal compartment, remains largely unknown. Basal cells provide structural and contractile support, however they also orchestrate the communication between the stroma and the luminal compartment at all developmental stages. Using RNA-seq, we show that basal cells are extraordinarily transcriptionally dynamic throughout pregnancy when compared to luminal cells. We identified gene expression changes that define specific basal functions acquired during development that led to the identification of novel markers. Enrichment analysis of gene sets from 24 mouse models for breast cancer pinpoint to a potential new function for insulin-like growth factor 1 (Igf1r) in the basal epithelium during lactogenesis. We establish that β-catenin signaling is activated in basal cells during early pregnancy, and demonstrate that this activity is mediated by lysophosphatidic acid receptor 3 (Lpar3). These findings identify novel pathways active during functional maturation of the adult mammary gland.

An intergenic risk locus containing an enhancer deletion in 2q35 modulates breast cancer risk by deregulating IGFBP5 expression

Breast cancer is the most diagnosed malignancy and the second leading cause of cancer mortality in females. Previous association studies have identified variants on 2q35 associated with the risk of breast cancer. To identify functional susceptibility loci for breast cancer, we interrogated the 2q35 gene desert for chromatin architecture and functional variation correlated with gene expression. We report a novel intergenic breast cancer risk locus containing an enhancer copy number variation (enCNV; deletion) located approximately 400Kb upstream to IGFBP5, which overlaps an intergenic ERα-bound enhancer that loops to the IGFBP5 promoter. The enCNV is correlated with modified ERα binding and monoallelic-repression of IGFBP5 following oestrogen treatment. We investigated the association of enCNV genotype with breast cancer in 1,182 cases and 1,362 controls, and replicate our findings in an independent set of 62,533 cases and 60,966 controls from 41 case control studies and 11 GWAS. We report a dose-dependent inverse association of 2q35 enCNV genotype (percopy OR = 0.68 95%CI 0.55-0.83, P = 0.0002; replication OR = 0.77 95% CI 0.73-0.82, P = 2.1 × 10-19) and identify 13 additional linked variants (r2 > 0.8) in the 20Kb linkage block containing the enCNV (P = 3.2 × 10-15 - 5.6 × 10-17). These associations were independent of previously reported 2q35 variants, rs13387042/rs4442975 and rs16857609, and were stronger for ER-positive than ER-negative disease. Together, these results suggest that 2q35 breast cancer risk loci may be mediating their effect through IGFBP5.

The effects of milking frequency on insulin-like growth factor I signaling within the mammary gland of dairy cows

In dairy cows, short-term changes in milking frequency (MF) in early lactation have been shown to produce both an immediate and a long-term effect on milk yield. The effect of MF on milk yield is controlled locally within mammary glands and could be a function of changes in either number or activity of secretory mammary epithelial cells (MEC). Insulin-like growth factor I (IGF-I) signaling is one candidate factor that could mediate these effects, as it can be controlled locally within mammary glands. Both MEC number and activity can be affected by IGF-I signaling by activating the phosphoinositide 3-kinase (PI3K)/Akt and extracellular-signal-regulated kinase (ERK)1/2 pathways. To investigate the relationship between MF and IGF-I signaling, udder halves of 17 dairy cows were milked either 4 times a day (4×) or once a day (1×) for 14 d in early lactation. On d 14, between 3 and 5 h following milking, mammary biopsies were obtained from 10 cows from both udder halves, and changes in the expression of genes associated with IGF-I signaling and the activation of the PI3K/Akt and ERK1/2 pathways were measured. The mRNA abundance of IGF type I receptor, IGF binding protein (IGFBP)-3, and IGFBP-5 were lower following 4× milking relative to 1× milking. However, the mRNA abundance of IGF-I was not affected by MF. Both IGFBP3 and IGFBP5 are thought to inhibit IGF-I; therefore, decreases in their mRNA abundance may serve to stimulate the IGF-I signal in the 4×-milked mammary gland. The activation of PI3K/Akt pathway was lower in response to 4× milking relative to 1×, and the activation of the ERK1/2 was unaffected by MF, suggesting that they do not mediate the effects of MF.

Insulin-like growth factor binding proteins 4-6

Insulin-like growth factor binding proteins (IGFBPs) 4-6 have important roles as modulators of IGF actions. IGFBP-4 and IGFBP-6 predominantly inhibit IGF actions, whereas IGFBP-5 may enhance these actions under some circumstances. IGFBP-6 is unique among the IGFBPs for its marked IGF-II binding preference. IGFBPs 4-6 are found in the circulation as binary complexes with IGFs that can enter tissues. Additionally, about half of the circulating IGFBP-5 is found in ternary complexes with IGFs and an acid labile subunit; this high molecular complex cannot leave the circulation and acts as an IGF reservoir. IGFBPs 4-6 also have IGF-independent actions. These IGFBPs are regulated in a cell-specific manner and their dysregulation may play a role in a range of diseases including cancer. However, there is no clear clinical indication for measuring serum levels of these IGFBPs at present.

Pappa2 deletion in mice affects male but not female fertility

BACKGROUND

Recent studies have found associations between the gene encoding pregnancy associated plasma protein-A2 (PAPP-A2), a protease of insulin-like growth factor binding protein -5 (IGFBP-5), and measures of female reproductive performance in cattle. The purpose of the present study was to test the effects of Pappa2 deletion on reproduction in mice.

FINDINGS

We measured the fertility and offspring growth of Pappa2 deletion females, and also performed reciprocal matings (i.e., deletion males mated to control females) to control for the effects of offspring genotype. Ovarian and testicular IGFBP-5 levels were measured by Western blotting. As expected, deletion of Pappa2 increased ovarian IGFBP-5 levels. However, Pappa2 deletion in females had no effect on the interval between pairing and the birth of the first litter, the interval between the births of the first and second litters, or litter size. Offspring weight was lower in the offspring of Pappa2 deletion females, but effects of similar magnitude were observed in the offspring of Pappa2 deletion males, suggesting that the effects were due to heterozygosity for the deletion in the offspring. Pappa2 deletion in males had no effect on litter size or the interval between pairing and the birth of the first litter. However, the interval between the births of the first and second litters was significantly longer in deletion males.

CONCLUSIONS

Pappa2 deletion had no effect on female reproductive performance. In contrast, Pappa2 deletion had subtle effects on male fertility, although the underlying mechanism remains to be elucidated.

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.

Transcriptome analysis of the hormone-sensing cells in mammary epithelial reveals dynamic changes in early pregnancy

Background

Alveoli, the milk-producing units of the mammary gland, are generated during pregnancy by collaboration of different epithelial cell types. We present the first analysis of transcriptional changes within the hormone sensing population during pregnancy. Hormone-receptor positive (HR+) cells play a key role in the initiation of alveologenesis as they sense systemic hormonal changes and translate these into local instructions for neighboring HR- cells. We recently showed that IGF2 is produced specifically by HR+ cells in early pregnancy, but is undetectable in the virgin state. Here, we define the transcriptome of HR+ cells in early pregnancy with the aim to elucidate additional changes that are unique for this dynamic developmental time window.

Results

We harvested mammary glands from virgin, 3-day and 7-day pregnant mice and isolated a few hundred hormone-sensing cells per animal by FACS for microarray analysis. There was a high concordance between animals with a clear induction of cell cycle progression genes at day 3 of pregnancy and molecules involved in paracrine signalling at day 7.

Conclusions

These findings underscore the proliferative capacity of HR+ cells upon specific stimuli and elucidate developmentally-restricted changes in cellular communication. Since the majority of breast cancers are HR+, with a variable proportion of HR+ cells per tumor, we anticipate that this data set will aid further studies into the regulation of HR+ cell proliferation and the role of heterotypic signalling within tumors.

Electronic supplementary material

The online version of this article (doi:10.1186/s12861-015-0058-9) contains supplementary material, which is available to authorized users.

Evidence that breast cancer risk at the 2q35 locus is mediated through IGFBP5 regulation

GWAS have identified a breast cancer susceptibility locus on 2q35. Here we report the fine mapping of this locus using data from 101,943 subjects from 50 case-control studies. We genotype 276 SNPs using the 'iCOGS' genotyping array and impute genotypes for a further 1,284 using 1000 Genomes Project data. All but two, strongly correlated SNPs (rs4442975 G/T and rs6721996 G/A) are excluded as candidate causal variants at odds against >100:1. The best functional candidate, rs4442975, is associated with oestrogen receptor positive (ER+) disease with an odds ratio (OR) in Europeans of 0.85 (95% confidence interval=0.84-0.87; P=1.7 × 10(-43)) per t-allele. This SNP flanks a transcriptional enhancer that physically interacts with the promoter of IGFBP5 (encoding insulin-like growth factor-binding protein 5) and displays allele-specific gene expression, FOXA1 binding and chromatin looping. Evidence suggests that the g-allele confers increased breast cancer susceptibility through relative downregulation of IGFBP5, a gene with known roles in breast cell biology.

Critical role of insulin‑like growth factor binding protein‑5 in methamphetamine‑induced apoptosis in cardiomyocytes

Methamphetamine (MA) is a highly abused amphetamine-like psychostimulant. At present, the mechanisms underlying MA-induced cardiotoxicity are poorly understood. The cardiotoxic effects have yet not been clearly elucidated with respect to the apoptotic pathway. Insulin-like growth factor binding protein-5 (IGFBP5) is important for cell growth control and the induction of apoptosis. The aim of the present study was to analyze whether IGFBP5 is involved in MA-induced apoptosis as a novel target. MA-induced apoptosis was observed in neonatal rat ventricular myocytes (NRVMs) in a concentration-dependent manner using a terminal deoxyribonucleotide transferase-mediated dUTP nick end-labeling assay. Using reverse transcription polymerase chain reaction and western blotting, MA was demonstrated to induce concentration-dependent increases in the expression of IGFBP5. Silencing IGFBP5 with small interfering RNA significantly reduced apoptosis and suppressed the expression of caspase-3 in NRVMs following treatment with MA. To the best of our knowledge, the present study provided the first evidence suggesting that IGFBP5 is a potential therapeutic target in MA-induced apoptosis in vitro, providing a foundation for future in vivo studies.

IGF-I retards proper development of acinar structures formed by bovine mammary epithelial cells via sustained activation of Akt kinase

Insulin-like growth factor-I is involved in mammary gland development, promoting proliferation and inhibiting apoptosis of mammary epithelial cells (MECs). Mitogenic actions of IGF-I are mainly mediated by the phosphatidylinositol-3 kinase (PI3K)/Akt signaling pathway. We have found that in the presence of IGF-I bovine BME-UV1 MECs cultured on reconstituted basement membrane form large spheroids with disrupted polarity and no cavity in the center. These cells showed enhanced phosphorylation of Akt, decreased level of cleaved caspase-3, and sustained proliferative activity throughout the 16-d period of 3-dimensional culture. Inhibition of the PI3K/Akt pathway by a specific inhibitor of PI3K, LY294002, resulted in the restoration of the normal acinar phenotype. However, this effect was noted only when LY294002 was added in the second week of 3-dimensional culture, which corresponded with the time of cell cycle arrest and polarity formation under control conditions. Normal development of acini was also obtained when BME-UV1 cells were treated simultaneously with IGF-I and 17β-estradiol. The addition of 17β-estradiol regulated Akt activation, enabling the subsequent initiation of polarization processes. 17β-Estradiol also increased the level of IGFBP-3 protein in MECs cultured on Matrigel in the presence of IGF-I. The presented results indicate important interactions between signaling pathways activated by estrogen and IGF-I, which regulate alveologenesis in bovine mammary gland.

IGFBP5 mediates high glucose-induced cardiac fibroblast activation

This study examined whether IGF-binding protein 5 (IGFBP5) is involved in the high glucose-induced deteriorating effects in cardiac cells. Cardiac fibroblasts and cardiomyocytes were isolated from the hearts of 1- to 3-day-old Sprague Dawley rats. Treatment of fibroblasts with 25 mM glucose increased the number of cells and the mRNA levels of collagen III, matrix metalloproteinase 2 (MMP2), and MMP9. High glucose increased ERK1/2 activity, and the ERK1/2 inhibitor PD98059 suppressed high glucose-mediated fibroblast proliferation and increased collagen III mRNA levels. Whereas high glucose increased both mRNA and protein levels of IGFBP5 in fibroblasts, high glucose did not affect IGFBP5 protein levels in cardiomyocytes. The high glucose-induced increase in IGFBP5 protein levels was inhibited by PD98059 in fibroblasts. While recombinant IGFBP5 increased ERK phosphorylation, cell proliferation, and the mRNA levels of collagen III, MMP2, and MMP9 in fibroblasts, IGFBP5 increased c-Jun N-terminal kinase phosphorylation and induced apoptosis in cardiomyocytes. The knockdown of IGFBP5 inhibited high glucose-induced cell proliferation and collagen III mRNA levels in fibroblasts. Although high glucose increased IGF1 levels, IGF1 did not increase IGFBP5 levels in fibroblasts. The hearts of Otsuka Long-Evans Tokushima Fatty rats and the cardiac fibroblasts of streptozotocin-induced diabetic rats showed increased IGFBP5 expression. These results suggest that IGFBP5 mediates high glucose-induced profibrotic effects in cardiac fibroblasts.

Integrative eQTL-based analyses reveal the biology of breast cancer risk loci

Germline determinants of gene expression in tumors are infrequently studied due to the complexity of transcript regulation caused by somatically acquired alterations. We performed expression quantitative trait locus (eQTL)-based analyses using the multi-level information provided in The Cancer Genome Atlas (TCGA). Of the factors we measured, cis-acting eQTLs accounted for 1.2% of the total variation of tumor gene expression, while somatic copy-number alteration and CpG methylation accounted for 7.3% and 3.3%, respectively. eQTL analyses of 15 previously reported breast cancer risk loci resulted in the discovery of three variants that are significantly associated with transcript levels (false discovery rate [FDR] < 0.1). Our trans-based analysis identified an additional three risk loci to act through ESR1, MYC, and KLF4. These findings provide a more comprehensive picture of gene expression determinants in breast cancer as well as insights into the underlying biology of breast cancer risk loci.

Mammary gland development is delayed in mice deficient for aminopeptidase N

Development of the mammary gland requires the coordinated action of proteolytic enzymes during two phases of remodelling. Firstly, new ducts and side-branches thereof need to be established during pregnancy to generate an extensive ductal tree allowing the secretion and transport of milk. A second wave of remodelling occurs during mammary involution after weaning. We have analysed the role of the cell surface protease aminopeptidase N (Anpep, APN, CD13) during these processes using Anpep deficient and Anpep over-expressing mice. We find that APN deficiency significantly delays mammary gland morphogenesis during gestation. The defect is characterised by a reduction in alveolar buds and duct branching at mid-pregnancy. Conversely over-expression of Anpep leads to accelerated ductal development. This indicates that Anpep plays a critical role in the proteolytic remodelling of mammary tissue during adult mammary development.

IGFBP5 induces cell adhesion, increases cell survival and inhibits cell migration in MCF-7 human breast cancer cells

Maintenance of tissue boundaries is crucial for control of metastasis. We describe a new signalling pathway in which epithelial cell disruption can be minimised and thereby restricts epithelial-mesenchymal transgressions. This involves the release of insulin-like growth factor (IGF)-binding protein 5 (IGFBP5) from apoptotic cells, which increases the adhesion of epithelial cells on mesenchymal but not epithelial extracellular matrix (ECM), and involves the direct interaction of IGFBP5 and α2β1 integrins. IGFBP5 also induced cell adhesion to vitronectin in the absence of αVβ3 integrin, the vitronectin receptor, again through an α2β1-integrin-dependent action, suggesting that IGFBP5 can induce spreading on matrices, even in the absence of the integrins normally used in this process. Using IGFBP5 mutants we demonstrate that the effect is IGF-independent but requires the heparin-binding domain in the C-terminus of IGFBP5. A truncated mutant containing only the C-terminal of IGFBP5 also induced adhesion. Adhesion induced by IGFBP5 was dependent on Cdc42 and resulted in activation of integrin-linked kinase (ILK) and Akt. Consistent with these changes, IGFBP5 facilitated prolonged cell survival in nutrient-poor conditions and decreased phosphorylation of the stress-activated kinase p38 MAPK (MAPK14). Whereas IGFBP5 enhanced adhesion, it inhibited cell migration, although this was not evident using the truncated C-terminal mutant, suggesting that effects of IGFBP5 on adhesion and migration involve different mechanisms. We anticipate that these responses to IGFBP5 would reduce the metastatic potential of cells.

Mammary tumor regression elicited by Wnt signaling inhibitor requires IGFBP5

Wnt ligand-driven tumor growth is inhibited by the soluble Wnt inhibitor Fzd8CRD, but the mechanism through which this effect is mediated is unknown. In the MMTV-Wnt1 mouse model, regression of mammary tumors by Fzd8CRD treatment coincides with an acute and strong induction of insulin-like growth factor (IGF)-binding protein IGFBP5, an antagonist of IGF signaling that mediates involution of mammary gland in females after offspring are weaned. In this study, we show that repression of this IGF inhibitory pathway is crucial for Wnt-driven growth of mammary tumors. We found that IGFBP5 regulation was mediated by the β-catenin-dependent Wnt pathway. Wnt, in addition to IGF ligands, facilitated tumor growth by paracrine communication among tumor cells. In addition, Fzd8CRD caused precocious induction of IGFBP5 in normal mammary glands undergoing involution, implying an acceleration of the involution process by inhibition of Wnt signaling. The molecular and phenotypic parallel between tumor regression and mammary gland involution suggests that Wnt-driven mammary tumors use the same growth mechanism as proliferating normal mammary glands.

Forced involution of the functionally differentiated mammary gland by overexpression of the pro-apoptotic protein bax

The mammary gland is a developmentally dynamic, hormone-responsive organ that undergoes proliferation and differentiation within the secretory epithelial compartment during pregnancy. The epithelia are maintained by pro-survival signals (e.g., Stat5, Akt1) during lactation, but undergo apoptosis during involution through inactivation of cell survival pathways and upregulation of pro-apoptotic proteins. To assess if the survival signals in the functionally differentiated mammary epithelial cells can override a pro-apoptotic signal, we generated transgenic mice that express Bax under the whey acidic protein (WAP) promoter. WAP-Bax females exhibited a lactation defect and were unable to nourish their offspring. Mammary glands demonstrated: (1) a reduction in epithelial content, (2) hallmark signs of mitochondria-mediated cell death, (3) an increase in apoptotic cells by TUNEL assay, and (4) precocious Stat3 activation. This suggests that upregulation of a single pro-apoptotic factor of the Bcl-2 family is sufficient to initiate apoptosis of functionally differentiated mammary epithelial cells in vivo.

Fetal alcohol exposure increases mammary tumor susceptibility and alters tumor phenotype in rats

BACKGROUND

Altered fetal programming because of a suboptimal in utero environment has been shown to increase susceptibility to many diseases later in life. This study examined the effect of alcohol exposure in utero on N-nitroso-N-methylurea (NMU)-induced mammary cancer risk during adulthood.

METHODS

Study 1: Pregnant Sprague Dawley rats were fed a liquid diet containing 6.7% ethanol (alcohol-fed), an isocaloric liquid diet (pair-fed), or rat chow ad libitum (ad lib-fed) from day 11 to 21 of gestation. At birth, female pups were cross-fostered to ad lib-fed control dams. Adult offspring were given an I.P. injection of NMU at a dose of 50 mg/kg body weight. Mammary glands were palpated for tumors twice a week, and rats were euthanized at 23 weeks postinjection. Study 2: To investigate the role of estradiol (E2), animals were exposed to the same in utero treatments but were not given NMU. Serum was collected during the preovulatory phase of the estrous cycle.

RESULTS

At 16 weeks postinjection, overall tumor multiplicity was greater in the offspring from the alcohol-fed group compared to the control groups, indicating a decrease in tumor latency. At study termination, 70% of all animals possessed tumors. Alcohol-exposed animals developed more malignant tumors and more estrogen receptor-α-negative tumors relative to the control groups. In addition, IGF-binding protein-5 (IGFBP-5) mRNA and protein were decreased in tumors of alcohol-exposed animals. Study 2 showed that alcohol-fed animals had significantly increased circulating E2 when compared to either control group.

CONCLUSIONS

These data indicate that alcohol exposure in utero increases susceptibility to mammary tumorigenesis in adulthood and suggest that alterations in the IGF and E2 systems may play a role in the underlying mechanism.

t10,c12-CLA decreases adiposity in peripubertal mice without dose-related detrimental effects on mammary development, inflammation status, and metabolism

The trans 10, cis 12-conjugated linoleic acid (10,12-CLA) isomer reduces adiposity in several animal models. In the mouse, however, this effect is associated with adipose tissue inflammation, hyperinsulinemia and hepatic lipid accumulation. Moreover, 10,12-CLA was recently shown to promote mammary ductal hyperplasia and ErbB2/Her2-driven mammary cancer in the mouse. Reasons for detrimental effects of 10,12-CLA on the mouse mammary gland could relate to its effect on the mammary fat pad (MFP), which is essential for normal development. Accordingly, we hypothesized that mammary effects of 10,12-CLA were mediated through the MFP in a dose-dependent manner. Female FVB mice were fed 10,12-CLA at doses of 0%, 0.1%, 0.2%, or 0.5% of the diet from day 24 of age, and effects on mammary development and metabolism were measured on day 49. The 0.5% dose reduced ductal elongation and caused premature alveolar budding. These effects were associated with increased expression of inflammatory markers and genes shown to alter epithelial growth (IGF binding protein-5) and alveolar budding (TNF-α and receptor of activated NF-κB ligand). The 0.5% dose also caused hyperinsulinemia and hepatic lipid accumulation. In contrast, the 0.1% 10,12-CLA dose had no adverse effects on mammary development, metabolic events, and inflammatory responses, but remained effective in decreasing adipose weights and lipogenic gene expression. These results show that a low dose of 10,12-CLA reduces adiposity in the mouse without negative effects on mammary development, inflammation, and metabolism, and suggest that previously reported detrimental effects relate to the use of excessive doses.

Effects of reduced frequency of milk removal on gene expression in the bovine mammary gland

Regulation of milk synthesis and secretion is controlled mostly through local (intramammary) mechanisms. To gain insight into the molecular pathways comprising this response, an analysis of mammary gene expression was conducted in 12 lactating cows shifted from twice daily to once daily milking. Tissues were sampled by biopsy from adjacent mammary quarters of these animals during the two milking frequencies, allowing changes in gene expression to be assessed within each animal. Using bovine-specific, oligonucleotide arrays representing 21,495 unique transcripts, a range of differentially expressed genes were found as a result of less frequent milk removal, constituting transcripts and pathways related to apoptotic signaling (NF-kappaB, JUN, ATF3, IGFBP5, TNFSF12A) mechanical stress and epithelial tight junction synthesis (CYR61, CTGF, THBS1, CLDN4, CLDN8), and downregulated milk synthesis (LALBA, B4GALT1, UGP2, CSN2, GPAM, LPL). Quantitative real-time PCR was used to assess the expression of 13 genes in the study, and all 13 of these were correlated (P < 0.05) with values derived from array analysis. It can be concluded that the physiological changes that occur in the bovine mammary gland as a result of reduced milk removal frequency likely comprise the earliest stages of the involution response and that mechano-signal transduction cascades associated with udder distension may play a role in triggering these events.

IGFBP-5 induces epithelial and fibroblast responses consistent with the fibrotic response

Fibrosis involves activation of fibroblasts, increased production of collagen and fibronectin and transdifferentiation into contractile myofibroblasts. The process resembles aspects of wound-healing but remains unresolved and can be life-threatening when manifest in the kidneys, lungs and liver, in particular. The causes are largely unknown, but recent suggestions that repetitive micro-injury results in the eventual failure of epithelial cell repair due to replicative senescence are gaining favour. This is consistent with the onset of fibrotic diseases in middle age. Because epithelial injury often involves blood loss, inflammatory responses associated with the fibrotic response have been considered as therapeutic targets. However, this has proved largely unsuccessful and focus is now switching to earlier events in the process. These include EMT (epithelial-mesenchymal transition) and fibroblast activation in the absence of inflammation. TGFbeta1 (transforming growth factor-beta1) induces both EMT and fibroblast activation and is considered to be a major pro-fibrotic factor. Recently, IGFBP-5 [IGF (insulin-like growth factor)-binding protein-5] has also been shown to induce similar effects on TGFbeta1, and is strongly implicated in the process of senescence. It also stimulates migration of peripheral blood mononuclear cells, implicating it in the inflammatory response. In this paper, we examine the evidence for a role of IGFBP-5 in fibrosis and highlight its structural relationship with other matrix proteins and growth factors also implicated in tissue remodelling.

A mouse mammary gland involution mRNA signature identifies biological pathways potentially associated with breast cancer metastasis

Mouse mammary gland involution resembles a wound healing response with suppressed inflammation. Wound healing and inflammation are also associated with tumour development, and a 'wound-healing' gene expression signature can predict metastasis formation and survival. Recent studies have shown that an involuting mammary gland stroma can promote metastasis. It could therefore be hypothesised that gene expression signatures from an involuting mouse mammary gland may provide new insights into the physiological pathways that promote breast cancer progression. Indeed, using the HOPACH clustering method, the human orthologues of genes that were differentially regulated at day 3 of mammary gland involution and showed prolonged expression throughout the first 4 days of involution distinguished breast cancers in the NKI 295 breast cancer dataset with low and high metastatic activity. Most strikingly, genes associated with copper ion homeostasis and with HIF-1 promoter binding sites were the most over-represented, linking this signature to hypoxia. Further, six out of the ten mRNAs with strongest up-regulation in cancers with poor survival code for secreted factors, identifying potential candidates that may be involved in stromal/matrix-enhanced metastasis formation/breast cancer development. This method therefore identified biological processes that occur during mammary gland involution, which may be critical in promoting breast cancer metastasis that could form a basis for future investigation, and supports a role for copper in breast cancer development.

IGF-independent effects of insulin-like growth factor binding protein-5 (Igfbp5) in vivo

IGF activity is regulated tightly by a family of IGF binding proteins (IGFBPs). IGFBP-5 is the most conserved of these and is up-regulated significantly during differentiation of several key lineages and in some cancers. The function of IGFBP-5 in these physiological and pathological situations is unclear, however, several IGFBP-5 sequence motifs and studies in vitro suggest IGF-independent actions. Therefore, we aimed to compare the phenotypes of mice overexpressing wild-type Igfbp5 or an N-terminal mutant Igfbp5 with negligible IGF binding affinity. Both significantly inhibited growth, even at low expression levels. Even though wild-type IGFBP-5 severely disrupted the IGF axis, we found no evidence for interaction of mutant IGFBP-5 with the IGF system. Further, overexpression of wild-type IGFBP-5 rescued the lethal phenotype induced by "excess" IGF-II in type 2 receptor-null mice; mutant IGFBP-5 overexpression could not. Therefore, wild-type IGFBP-5 provides a very effective mechanism for the inhibition of IGF activity and a powerful in vivo mechanism to inhibit IGF activity in pathologies such as cancer. This study is also the first to suggest significant IGF-independent actions for IGFBP-5 during development.

Growth hormone and insulin-like growth factor-I in the transition from normal mammary development to preneoplastic mammary lesions

Adult female mammary development starts at puberty and is controlled by tightly regulated cross-talk between a group of hormones and growth factors. Although estrogen is the initial driving force and is joined by luteal phase progesterone, both of these hormones require GH-induced IGF-I in the mammary gland in order to act. The same group of hormones, when experimentally perturbed, can lead to development of hyperplastic lesions and increase the chances, or be precursors, of mammary carcinoma. For example, systemic administration of GH or IGF-I causes mammary hyperplasia, and overproduction of IGF-I in transgenic animals can cause the development of usual or atypical hyperplasias and sometimes carcinoma. Although studies have clearly demonstrated the transforming potential of both GH and IGF-I receptor in cell culture and in animals, debate remains as to whether their main role is actually instructive or permissive in progression to cancer in vivo. Genetic imprinting has been shown to occur in precursor lesions as early as atypical hyperplasia in women. Thus, the concept of progression from normal development to cancer through precursor lesions sensitive to hormones and growth factors discussed above is gaining support in humans as well as in animal models. Indeed, elevation of estrogen receptor, GH, IGF-I, and IGF-I receptor during progression suggests a role for these pathways in this process. New agents targeting the GH/IGF-I axis may provide a novel means to block formation and progression of precursor lesions to overt carcinoma. A novel somatostatin analog has recently been shown to prevent mammary development in rats via targeted IGF-I action inhibition at the mammary gland. Similarly, pegvisomant, a GH antagonist, and other IGF-I antagonists such as IGF binding proteins 1 and 5 also block mammary gland development. It is, therefore, possible that inhibition of IGF-I action, or perhaps GH, in the mammary gland may eventually play a role in breast cancer chemoprevention by preventing actions of both estrogen and progesterone, especially in women at extremely high risk for developing breast cancer such as BRCA gene 1 or 2 mutations.

Mammary gland differentiation inversely correlates with GDF-8 expression

GDF-8 is recognised as an inhibitor of muscle cell growth and differentiation. Although initially thought to be restricted to muscle cells it is now accepted that GDF-8 expression has a broader tissue distribution. We demonstrate GDF-8 expression in the mouse mammary gland, which is predominantly associated with epithelial cells and displays an inverse correlation to the differentiated state of the gland. Specifically, the highest GDF-8 mRNA levels correlate with periods of maximal ductal growth, diminish as pregnancy progressed and are down-regulated to minimal levels by the onset of lactation as the epithelium differentiates. A similar profile is observed for both GDF-8 protein processing and reflects Smad2/3 phosphorylation profile. However, in contrast to muscle cells, GDF-8 neither reduces proliferation nor induces p21 expression levels in mammary epithelial cells. These data implicate a role for GDF-8 in mammary epithelial cell differentiation and demonstrate that GDF-8 has cell-type specific activities.

Role of insulin-like growth factor binding proteins in mammary gland development

Insulin-like growth factors (IGFs) play an important role in mammary gland development and their effects are, in turn, influenced by a family of 6 IGF-binding proteins (IGFBPs). The IGFBPs are expressed in time- and tissue-specific fashion during the periods of rapid growth and involution of the mammary gland. The precise roles of these proteins in vivo have, however, been difficult to determine. This review examines the indirect evidence (evolution, chromosomal location and roles in lower life-forms) the evidence from in vitro studies and the attempts to examine their roles in vivo, using IGFBP-deficient and over-expression models. Evidence exists for a role of the IGFBPs in inhibition of the survival effects of IGFs as well as in IGF-enhancing effects from in vitro studies. The location of the IGFBPs, often associated with the extracellular matrix, suggests roles as a reservoir of IGFs or as a potential barrier, restricting access of IGFs to distinct cellular compartments. We also discuss the relative importance of IGF-dependent versus IGF-independent effects. IGF-independent effects include nuclear localization, activation of proteases and interaction with a variety of extracellular matrix and cell surface proteins. Finally, we examine the increasing evidence for the IGFBPs to be considered as part of a larger family of extracellular matrix proteins involved in morphogenesis and tissue re-modeling.

Immune cell regulators in mouse mammary development and involution

Dramatic changes in cell composition and function occur in the mammary gland during a pregnancy-lactation-involution cycle. We investigated the transcriptional changes associated with these biological events by using microarray analysis and identified the critical genes involved by using genetically modified mice. Two surprising findings arose from these studies. First, the microarray data showed that postlactational regression was associated with an acute phase inflammatory response, in addition to cell death. Conditional deletion of signal transducer and activator of transcription (Stat)3 or the nuclear factor-kappaB regulatory kinase inhibitor of kappa B kinase beta resulted in a failure of cell death induction during involution, an indication that these signaling pathways are essential mediators of the involution process. Both Stat3 and nuclear factor-kappaB have been shown to regulate acute phase gene expression in addition to apoptosis regulators. Four distinct transcriptional profiles are present in the first 4 d of involution, whereas there are 3 in lactation. At the peak of lactation (i.e., d 10 in mouse), more than 400 genes reach their maximum expression before declining dramatically in the first 12 h of involution. A reciprocal pattern was observed for more than 500 genes that were specifically upregulated within the first 12 h of forced involution. We are now investigating the role of a subset of these genes in involution. We also uncovered a role for genes normally associated with immune cell signaling in the differentiation of luminal mammary epithelial cells during pregnancy. Genetic deletion of the transcription factor Stat6 resulted in delayed development during pregnancy, and this phenotype was recapitulated in mammary tissue from IL-4 and IL-13 doubly deficient mice. Furthermore, we showed that mammary epithelial cells secrete T-cell regulatory cytokines. T-helper type 1 cytokines, such as interferon-gamma and IL-12a, are secreted by undifferentiated mammary epithelial cells, whereas T-helper type 2 cytokines, including IL-4 and IL-13, are secreted by differentiated cells. This unexpected finding demonstrates a role for immune cell signaling in mammary epithelial cell fate and function.

Multifunctional roles of insulin-like growth factor binding protein 5 in breast cancer

The insulin-like growth factor axis, which has been shown to protect cells from apoptosis, plays an essential role in normal cell physiology and in cancer development. The family of insulin-like growth factor binding proteins (IGFBPs) has been shown to have a diverse spectrum of functions in cell growth, death, motility, and tissue remodeling. Among the six IGFBP family members, IGFBP-5 has recently been shown to play an important role in the biology of breast cancer, especially in breast cancer metastasis; however, the exact mechanisms of action remain obscure and sometimes paradoxical. An in-depth understanding of IGFBP-5 would shed light on its potential role as a target for breast cancer therapeutics.

Lactation failure in Src knockout mice is due to impaired secretory activation

Background

Mammary gland development culminates in lactation and is orchestrated by numerous stimuli and signaling pathways. The Src family of nonreceptor tyrosine kinases plays a pivotal role in cell signaling. In order to determine if Src plays a role in mammary gland development we have examined mammary gland development and function during pregnancy and lactation in mice in which expression of Src has been eliminated.

Results

We have characterized a lactation defect in the Src-/- mice which results in the death of over 80% of the litters nursed by Src-/- dams. Mammary gland development during pregnancy appears normal in these mice; however secretory activation does not seem to occur. Serum prolactin levels are normal in Src-/- mice compared to wildtype controls. Expression of the prolactin receptor at both the RNA and protein level was decreased in Src-/- mice following the transition from pregnancy to lactation, as was phosphorylation of STAT5 and expression of milk protein genes. These results suggest that secretory activation, which occurs following parturition, does not occur completely in Src-/- mice. Failed secretory activation results in precocious involution in the mammary glands of Src-/- even when pups were suckling. Involution was accelerated following pup withdrawal perhaps as a result of incomplete secretory activation. In vitro differentiation of mammary epithelial cells from Src-/- mice resulted in diminished production of milk proteins compared to the amount of milk proteins produced by Src+/+ cells, indicating a direct role for Src in regulating the transcription/translation of milk protein genes in mammary epithelial cells.

Conclusion

Src is an essential signaling modulator in mammary gland development as Src-/- mice exhibit a block in secretory activation that results in lactation failure and precocious involution. Src appears to be required for increased expression of the prolactin receptor and successful downstream signaling, and alveolar cell organization.

Insulin-like growth factor-binding protein-5 enters vesicular structures but not the nucleus

In addition to its extracellular function as a secreted protein, IGF-binding protein (IGFBP)-5 has been postulated to act as a signaling molecule in the nucleus. This study aims to assess the significance of this postulated nuclear localization. By confocal immunofluorescence microscopy, we detected IGFBP-5 in the vesicular compartment of mammary epithelial cells in culture, while no nuclear staining was observed. Immunohistochemistry performed on paraffin sections of the involuting mammary gland revealed IGFBP-5 positive staining of epithelial cells only outside the nucleus. To evaluate the contribution of reuptake of extracellular IGFBP-5, T47D cells were incubated with Alexa Fluor 647-labeled IGFBP-5. The protein was taken up into intracellular vesicles and again was neither detectable in the cytoplasm outside of vesicular structures nor in the nucleus. Quantification of the time and concentration dependence of uptake by immunoblotting revealed that the process was saturable at IGFBP-5 concentrations between 1 and 2 mum and partially reversible with 30% remaining in the cell after a 1-h chase. The observation of nuclear uptake of IGFBP-5 was restricted to artificial conditions such as expression of non-secreted forms of IGFBP-5 or selective permeabilization of the plasma membrane by digitonin.

Delayed mammary gland involution in mice with mutation of the insulin-like growth factor binding protein 5 gene

IGFs (IGF-I and IGF-II) are essential for development, and their bioactivities are tightly regulated by six related IGF-binding proteins (IGFBPs). IGFBP-5 is the most highly conserved binding protein and is expressed in several key developmental lineages as well as in multiple adult tissues including the mammary gland. To explore IGFBP-5 actions in vivo, we produced IGFBP-5 knockout (KO) mice. Whole-body growth, selected organ weights, and body composition were essentially normal in IGFBP-5 KO mice, presumably because of substantial compensation by remaining IGFBP family members. The IGFBP-5 KO mice also exhibited normal mammary gland development and were capable of nursing their pups. We then directly evaluated the proposed role of IGFBP-5 in apoptosis and remodeling of mammary gland during involution. We found that the process of involution after forced weaning was delayed in IGFBP-5 KO mice, with both the appearance of apoptotic cells and the reappearance of adipocytes retarded in mutant mice, compared with controls. We also determined the effects of IGFBP-5 deletion on mammary gland development in pubertal females after ovariectomy and stimulation with estradiol/progesterone. In this paradigm, IGFBP-5 KO mammary glands exhibited enhanced alveolar bud formation consistent with enhanced IGF-I action. These results demonstrate that IGFBP-5, although not essential for normal growth, is required for normal mammary gland involution and can regulate mammary gland morphogenesis in response to hormone stimulation.

Growth hormone suppresses the expression of IGFBP-5, and promotes the IGF-I-induced phosphorylation of Akt in bovine mammary epithelial cells

Growth hormone (GH) plays a specific role to inhibit apoptosis in the bovine mammary gland through the insulin-like growth factor (IGF)-I system, however, the mechanism of GH action is poorly understood. In this study, we show that GH dramatically inhibits the expression of IGFBP-5, and GH along with IGF-I enhanced the phosphorylation of Akt through the reduction of IGF binding protein (IGFBP)-5. To determine how GH affects Akt through IGF-I in bovine mammary epithelial cells (BMECs), we examined the phosphorylation of Akt in GH treated BMECs and found that IGF-I induced phosphorylation of Akt was significantly enhanced by the treatment with GH. We demonstrated that GH reduces mRNA and protein expression of IGFBP-5 in BMECs, but it does not affect the expression of IGFBP-3. To determine that the enhanced effect of the Akt phosphorylation by the treatment of GH is due to the inhibition of the expression of IGFBP-5, we examined the effect of IGFBP-3 and -5 on the phosphorylation of Akt through IGF-I in the GH-treated BMECs. The phosphorylation of Akt was inhibited in a dose-dependent manner when IGFBP-5 was added at varying concentrations and was also inhibited in the presence of IGFBP-3. The results of this study suggest that GH plays an important role on mammary gland involution in bovine mammary epithelial cells.

Mammary gland involution as a multi-step process

Mammary gland involution is a highly complex multi-step process in which the lactating gland returns to a morphologically near pre-pregnant state. This developmental stage is characterized by a high degree of epithelial cell death, redevelopment of the mammary adipose tissue and tissue remodelling. Many factors involved have been described and these have been reviewed intensively in this journal (Furth, P. A., J. Mammary Gland Biol. Neoplasia, 4:123-127, 1999) and elsewhere. Microarray analysis technology has now not only allowed us to identify genes not previously associated with involution (Stein, T., Morris, J.S., Davis, C.R.,Weber-Hall, S.J., Duffy, M.A., Heath, V.J., et al., Breast Cancer Res., 6: R75-R91, 2004; Clarkson, R.W., Wayland, M.T., Lee, J., Freeman, T., Watson, C.J., Breast Cancer Res., 6: R92-R109, 2004; Clarkson, R.W., Watson, C.J., J. Mammary Gland Biol. Neoplasia, 8: 309-319, 2003), it has also enabled us to define multiple phases of the controlled regulatory response to forced weaning on the basis of their transcriptional profiles. This review provides a synthesis of published data, integrating the time course of transcriptional activity in the mouse mammary gland with a gene ontology approach to identify the pathways involved.

The molecular culprits underlying precocious mammary gland involution

Mammary gland involution, characterized by extensive apoptosis and structural remodelling of the gland, is the process by which the gland is returned to the pre-pregnant state. A key advantage of the mammary gland is the ability to synchronize involution through forced weaning, thus allowing the dissection of biochemical pathways involved in the involution process. Over the past few years, significant advances have been made in understanding the signaling pathways and downstream effectors that regulate epithelial cell apoptosis in the first phase of involution, and the importance of matrix metalloproteinases and their inhibitors in both phases of involution. The precise nature of the triggers for apoptosis, however, and the ultimate perpetrators of cell death are not yet clear. This review focuses on genes whose perturbation, either by targeted deletion or overexpression in transgenic mouse models, leads to precocious involution. The accumulating data point to a complex network of signal transduction pathways that synergize to regulate apoptosis in the involuting mammary gland.

Insulin-like growth factors and breast cancer therapy

Despite improvements in breast cancer therapy in recent years, additional therapies need to be developed. New therapies may have activity by themselves or may have utility in combination with other agents. Population, preclinical, and basic data suggest the insulin-like growth factor (IGF) system functions to maintain the malignant phenotype in breast cancer. Since the IGFs act via transmembrane tyrosine kinase receptors, targeting of the key receptors could provide a new pathway in breast cancer. In addition, IGF action enhances cell survival, so combination of anti-IGF therapy with conventional cytotoxic drugs could lead to synergistic effects. In this review, we will discuss the rationale for targeting the IGF system, potential methods to disrupt IGF signaling, and identify potential interactions between IGF inhibitors and other anti-tumor strategies. We will also identify important issues to consider when designing clinical trials.

Acute myeloid leukemia-associated Mkl1 (Mrtf-a) is a key regulator of mammary gland function

Transcription of immediate-early genes--as well as multiple genes affecting muscle function, cytoskeletal integrity, apoptosis control, and wound healing/angiogenesis--is regulated by serum response factor (Srf). Extracellular signals regulate Srf in part via a pathway involving megakaryoblastic leukemia 1 (Mkl1, also known as myocardin-related transcription factor A [Mrtf-a]), which coactivates Srf-responsive genes downstream of Rho GTPases. Here we investigate Mkl1 function using gene targeting and show the protein to be essential for the physiologic preparation of the mammary gland during pregnancy and the maintenance of lactation. Lack of Mkl1 causes premature involution and impairs expression of Srf-dependent genes in the mammary myoepithelial cells, which control milk ejection following oxytocin-induced contraction. Despite the importance of Srf in multiple transcriptional pathways and widespread Mkl1 expression, the spectrum of abnormalities associated with Mkl1 absence appears surprisingly restricted.

Insulin-like growth factor binding protein (IGFBP)-5 is upregulated during both differentiation and apoptosis in primary cultures of mouse mammary epithelial cells

We have previously demonstrated that insulin-like growth factor binding protein-5 (IGFBP-5) is upregulated following treatment of the mouse mammary epithelial cell line HC11 with lactogenic hormones (dexamethasone, insulin, and prolactin-DIP). In addition, we have also shown that IGFBP-5 is upregulated in mammary epithelial cells in vivo during involution of the rodent mammary gland. We have, therefore, postulated that there may be a dual regulation of IGFBP-5 expression during the temporally separated processes of differentiation and apoptosis of mammary epithelial cells. To test this hypothesis further, we have used a phenotypically differentiated model, which comprises primary cultures of mouse mammary epithelial cells grown on a layer of EHS (Engelbreth-Holm-Swarm) extracellular matrix. We show that lactogenic hormone treatment (hydrocortisone, insulin, and prolactin-HIP) of these cultures induces the upregulation of IGFBP-5 thus replicating the results obtained with the HC11 cell line. In addition, following the induction of apoptosis in primary cultures of mammary epithelial cells by treatment with TGFbeta-3, IGFBP-5 expression is also upregulated. In parallel with this upregulation of IGFBP-5, there is also an increase in the levels of cleaved caspase-3, a well-characterized marker of cellular apoptosis. These findings confirm previous in vivo work demonstrating an increase in IGFBP-5 expression during involution of the mouse mammary gland. When HC11 cells are cultured under serum-free conditions (a well-characterized apoptotic insult in cell culture), there is also an increase in cleaved caspase-3 levels. Unexpectedly, in the presence of TGFbeta-3, caspase-3 levels are attenuated. In the presence of DIP, caspase-3 levels are also decreased in HC11 cells. As described previously, TGFbeta-3 inhibits beta-casein synthesis in HC11 cells. In the HC11 cell line (in contrast to primary cultures of mammary epithelial cells), there is no evidence for TGFbeta-3 induction of IGFBP-5 under either serum-free or DIP-supplemented conditions. We believe our data with primary cultures of mammary epithelial cells support the hypothesis of dual regulation of IGFBP-5 expression during both differentiation and apoptosis in the mammary gland and emphasizes the importance of using appropriate cell culture models to investigate such phenomena in this tissue. We discuss the possible implications of our observations in relation to the physiological processes of pregnancy, lactation, and involution in the mammary gland and the associated changes in mammary epithelial cell function.