Non-lactogenic effects of growth hormone on growth and insulin-like growth factor-I messenger ribonucleic acid of rat mammary gland

Pubertal mammary gland development is a key determinant of adult mammographic density

Mammographic density refers to the radiological appearance of fibroglandular and adipose tissue on a mammogram of the breast. Women with relatively high mammographic density for their age and body mass index are at significantly higher risk for breast cancer. The association between mammographic density and breast cancer risk is well-established, however the molecular and cellular events that lead to the development of high mammographic density are yet to be elucidated. Puberty is a critical time for breast development, where endocrine and paracrine signalling drive development of the mammary gland epithelium, stroma, and adipose tissue. As the relative abundance of these cell types determines the radiological appearance of the adult breast, puberty should be considered as a key developmental stage in the establishment of mammographic density. Epidemiological studies have pointed to the significance of pubertal adipose tissue deposition, as well as timing of menarche and thelarche, on adult mammographic density and breast cancer risk. Activation of hypothalamic-pituitary axes during puberty combined with genetic and epigenetic molecular determinants, together with stromal fibroblasts, extracellular matrix, and immune signalling factors in the mammary gland, act in concert to drive breast development and the relative abundance of different cell types in the adult breast. Here, we discuss the key cellular and molecular mechanisms through which pubertal mammary gland development may affect adult mammographic density and cancer risk.

Role of Growth Hormone in Breast Cancer

Breast cancer is one of the most common cancers diagnosed in women. Approximately two-thirds of all breast cancers diagnosed are classified as hormone dependent, which indicates that hormones are the key factors that drive the growth of these breast cancers. Ovarian and pituitary hormones play a major role in the growth and development of normal mammary glands and breast cancer. In particular, the effect of the ovarian hormone estrogen has received much attention in regard to breast cancer. Pituitary hormones prolactin and growth hormone have also been associated with breast cancer. Although the role of these pituitary hormones in breast cancers has been studied, it has not been investigated extensively. In this review, we attempt to compile basic information from most of the currently available literature to understand and demonstrate the significance of growth hormone in breast cancer. Based on the available literature, it is clear that growth hormone plays a significant role in the development, progression, and metastasis of breast cancer by influencing tumor angiogenesis, stemness, and chemoresistance.

The Rat Mammary Gland: Morphologic Changes as an Indicator of Systemic Hormonal Perturbations Induced by Xenobiotics

The development and morphology of the rat mammary gland are dependent upon several hormones including estrogens, androgens, progesterone, growth hormone and prolactin. In toxicology studies, treatment with xenobiotics may alter these hormones resulting in changes in the morphology of reproductive tissues such as the mammary gland. In the rat, male and female mammary glands exhibit striking morphologic differences that can be altered secondary to hormonal perturbations. Recognizing these morphologic changes can help the pathologist predict potential xenobiotic-induced perturbations in the systemic hormonal milieu. This review examines the development of the rat mammary gland and the influence of sex hormones on the morphology of the adult male and female rat mammary gland. Specific case examples from the literature and data from our laboratory highlight the dynamic nature of the rat mammary gland in response to hormonal changes.

Role of IGF1R in Breast Cancer Subtypes, Stemness, and Lineage Differentiation

Insulin-like growth factor (IGF) signaling is fundamental for growth and survival. A large body of evidence (laboratory, epidemiological, and clinical) implicates the exploitation of this pathway in cancer. Up to 50% of breast tumors express the activated form of the type 1 insulin-like growth factor receptor (IGF1R). Breast cancers are categorized into subtypes based upon hormone and ERRB2 receptor expression and/or gene expression profiling. Even though IGF1R influences tumorigenic phenotypes and drug resistance across all breast cancer subtypes, it has specific expression and function in each. In some subtypes, IGF1R levels correlate with a favorable prognosis, while in others it is associated with recurrence and poor prognosis, suggesting different actions based upon cellular and molecular contexts. In this review, we examine IGF1R expression and function as it relates to breast cancer subtype and therapy-acquired resistance. Additionally, we discuss the role of IGF1R in stem cell maintenance and lineage differentiation and how these cell fate influences may alter the differentiation potential and cellular composition of breast tumors.

Hormonal control of stem cell systems

Many organs respond to physiological challenges by changing tissue size or composition. Such changes may originate from tissue-specific stem cells and their supportive environment (niche). The endocrine system is a major effector and conveyor of physiological changes and as such could alter stem cell behavior in various ways. In this review, we examine how hormones affect stem cell biology in four different organs: the ovary, intestine, hematopoietic system, and mammary gland. Hormones control every stage of stem cell life, including establishment, expansion, maintenance, and differentiation. The effects can be cell autonomous or non-cell autonomous through the niche. Moreover, a single hormone can affect different stem cells in different ways or affect the same stem cell differently at various developmental times. The vast complexity and diversity of stem cell responses to hormonal cues allow hormones to coordinate the body's reaction to physiological challenges.

The pivotal role of insulin-like growth factor I in normal mammary development

Mammary development begins in puberty in response to an estrogen (E(2)) surge. E(2) does not act alone. It relies on pituitary growth hormone (GH) to induce insulin-like growth factor I (IGF-I) production in the mammary stromal compartment. In turn, IGF-I permits E(2) (and progesterone) action. During puberty, E(2) and IGF-I synergize for ductal morphogenesis. During pregnancy, progesterone joins IGF-I and E(2) to stimulate secretory differentiation necessary to produce milk. Prolactin stimulates milk production, while transforming growth factor-β inhibits proliferation. The orchestrated action of hormones, growth factors, and receptors necessary for mammary development and function are also critical in breast cancer.

Polymorphic CA repeat length in insulin-like growth factor 1 and risk of breast cancer in Iranian women

To evaluate the association between breast cancer and cytosine-adenine (CA) dinucleotide repeat length polymorphisms in the promoter region of the Insulin-like Growth Factor 1 (IGF-1) gene, a case-control study of 215 breast cancer patients and 224 controls was conducted in Iranian women. The most common allele and genotype in both controls and patients were an allele length of 19 and a homozygous genotype of (CA)(19)/(CA)(19). Women with two alleles longer than 19 were found to be at a higher risk of breast cancer with an odds ratio of 4.1 (P = 0.0002). In contrast, women with two alleles shorter than 20 were at lower risk of breast cancer. These results suggest a novel association between CA repeat length in IGF-1 and risk of breast cancer.

Pasireotide, an IGF-I action inhibitor, prevents growth hormone and estradiol-induced mammary hyperplasia

Mammary hyperplasia increases breast cancer risk. Tamoxifen prevents breast cancer in women with atypical hyperplasia, but has serious side effects. As estradiol action requires IGF-I, direct inhibition of IGF-I action theoretically might be an efficacious alternative to tamoxifen. After hypophysectomy and oophorectomy, 21-day-old female rats were treated with GH and E₂. After 7 days all terminal end buds (TEBs) and 75% of ducts became hyperplastic. Co-treatment with pasireotide, a somatostatin analog that blocks GH secretion and IGF-I action in the mammary gland, prevented hormone-induced hyperplasia. The number and size of TEBs and moderately or floridly hyperplastic ducts was reduced by pasireotide (P < 0.01). In contrast, the same concentration of octreotide, which has a more selective somatostatin receptor subtype binding profile, was less effective than pasireotide. Tamoxifen inhibited hyperplasia when used alone with GH + E₂, but did not add to the inhibitory effect of pasireotide when the two treatments were combined. Both pasireotide and tamoxifen acted via the IGF-I receptor signaling pathway and both were found to inhibit mammary cell proliferation and stimulate apoptosis. The number of epithelial cells expressing phosphorylated insulin receptor substrate (IRS)-1 in response to GH and E₂ was reduced by pasireotide, as was staining intensity. These results support the concept that IGF-I inhibition, in this case by pasireotide, inhibits E₂ and GH-induced mammary hyperplasia. As tamoxifen did not further increase the inhibitory effect of pasireotide, the peptide appears to be at least as effective as tamoxifen in preventing GH + E₂-induced mammary hyperplasia.

Short-term administration of rhGH increases markers of cellular proliferation but not milk protein gene expression in normal lactating women

Growth hormone is one of few pharmacologic agents known to augment milk production in humans. We hypothesized that recombinant human GH (rhGH) increases the expression of cell proliferation and milk protein synthesis genes. Sequential milk and blood samples collected over four days were obtained from five normal lactating women. Following 24 h of baseline milk and blood sampling, rhGH (0.1 mg/kg/day) was administered subcutaneously once daily for 3 days. Gene expression changes were determined by microarray studies utilizing milk fat globule RNA isolated from each milk sample. Following rhGH administration, DNA synthesis and cell cycle genes were induced, while no significant changes were observed in the expression of milk synthesis genes. Expression of glycolysis and citric acid cycle genes were increased by day 4 compared with day 1, while lipid synthesis genes displayed a circadian-like pattern. Cell cycle gene upregulation occurred after a lag of ∼2 days, likely explaining the failure to increase milk production after only 3 days of rhGH treatment. We conclude that rhGH induces expression of cellular proliferation and metabolism genes but does not induce milk protein gene expression, as potential mechanisms for increasing milk production and could account for the known effect of rhGH to increase milk production following 7-10 days.

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.

IGF-I, GH, and sex steroid effects in normal mammary gland development

Although the pubertal surge of estrogen is the immediate stimulus to mammary development, the action of estrogen depends upon the presence of pituitary growth hormone and the ability of GH to stimulate production of IGF-I in the mammary gland. Growth hormone binds to its receptor in the mammary fat pad, after which production of IGF-I mRNA and IGF-I protein occurs. It is likely that IGF-I then works through paracrine means to stimulate formation of TEBs, which then form ducts by bifurcating or trifurcating and extending through the mammary fat pad. By the time pubertal development is complete a tree-like structure of branching ducts fills the rodent mammary fat pad. In addition to requiring IGF-I in order to act, estradiol also directly synergizes with IGF-I to enhance formation of TEBs and ductal morphogenesis. Together they increase IRS-1 phosphorylation and cell proliferation, and inhibit apoptosis. In fact, the entire process of ductal morphogenesis, in oophorectomized IGF-I(-/-) knockout female mice, can occur as a result of the combined actions of estradiol and IGF-I. IGF-I also permits progesterone action in the mammary gland. Together they have been shown to stimulate a form of ductal morphogenesis, which is anatomically different from the kind induced by IGF-I and estradiol. Although both progesterone and estradiol synergize with IGF-I by increasing IGF-I action parameters, there must be other, as yet unknown mechanisms that account for the anatomical differences in the different forms of ductal morphogenesis observed (hyperplasia in response to IGF-I plus estradiol and single layered ducts in response to IGF-I plus progesterone).

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.

Epithelial-specific and stage-specific functions of insulin-like growth factor-I during postnatal mammary development

Postnatal development of the mammary gland requires interactions between the epithelial and stromal compartments, which regulate actions of hormones and growth factors. IGF-I is expressed in both epithelial and stromal compartments during postnatal development of the mammary gland. However, little is known about how local expression of IGF-I in epithelium or stroma regulates mammary growth and differentiation during puberty and pregnancy-induced alveolar development. The goal of this study was to investigate the mechanisms of IGF-I actions in the postnatal mammary gland and test the hypothesis that IGF-I expressed in stromal and epithelial compartments has distinct functions. We established mouse lines with inactivation of the igf1 gene in mammary epithelium by crossing igf1/loxP mice with mouse lines expressing Cre recombinase under the control of either the mouse mammary tumor virus long-terminal repeat or the whey acidic protein gene promoter. Epithelial-specific loss of IGF-I during pubertal growth resulted in deficits in ductal branching. In contrast, heterozygous reduction of IGF-I throughout the gland decreased expression of cyclins A2 and B1 during pubertal growth and resulted in alterations in proliferation of the alveolar epithelium and milk protein levels during pregnancy-induced differentiation. Reduction in epithelial IGF-I at either of these stages had no effect on these indices. Taken together, our results support distinct roles for IGF-I expressed in epithelial and stromal compartments in mediating growth of the postnatal mammary gland.

Pubertal mammary gland development: insights from mouse models

During puberty the mammary gland develops from a rudimentary tree to a branched epithelial network of ducts which can support alveolar development and subsequent milk production during pregnancy and lactation. This process involves growth, proliferation, migration, branching, invasion, apoptosis and above all, tight regulation which allows these processes to take place simultaneously during the course of just a few weeks to create an adult gland. The process is under hormonal control and is thus coordinated with reproductive development. Mouse models, with overexpressed or knocked-out genes, have highlighted a number of pubertal mammary gland phenotypes and given significant insight into the regulatory mechanisms controlling this period of development. Here we review the published findings of the wide range of gene-manipulated mammary mouse models, documenting the common pubertal mammary gland phenotypes observed, and summarizing their contribution to our current understanding of how pubertal mammary gland development occurs.

SOM230 Inhibits Insulin-Like Growth Factor-I Action in Mammary Gland Development by Pituitary Independent Mechanism: Mediated through Somatostatin Subtype Receptor 3?

Somatostatin analogs (SAs) treat acromegaly by lowering pituitary GH secretion, which, in turn, lowers systemic IGF-I. The profound systemic effect is often greater than expected in the face of only partial GH suppression. Here we report that the SA SOM230 can also act by a nonpituitary-mediated inhibition of IGF-I action. SOM230 inhibited mammary development in intact and hypophysectomized female rats, a process requiring IGF-I. IGF-I overcame this inhibition. SOM230 also inhibited other actions of IGF-I (inhibition of apoptosis, phosphorylation of insulin receptor substrate-1, and cell division). SOM230 did not reduce IGF-I mRNA abundance in mammary gland but did stimulate IGF binding protein 5 (IGFBP5). IGFBP5 was 3.75 times higher in mammary epithelium of SOM230 than in placebo animals (P < 0.001). Administration of IGFBP-5 also inhibited GH-induced mammary development (P < 0.001). Measurement of sstr(1-5) (somatostatin subtype receptor) by real-time RT-PCR revealed that the mammary glands had an abundance of sstr(3) and lower amounts of sstr(4) and sstr(5) but no sstr(1) or sstr(2.) That mammary development was also inhibited to a lesser degree than SOM230 by octreotide, whose main action is through sstr(2), strongly suggests that sstr(3) is at least in part mediating the effects of the SAs. We conclude that 1) SAs inhibit IGF-I action in the mammary gland through a novel nonpituitary mechanism; 2) IGFBP-5, here shown to inhibit pubertal mammary development, might mediate the effect; and 3) Measurement of available sstr receptors in the mammary gland suggests that sstr(3) mediates the SA activity, but sstr(5) is also a possible mediator.

Growth hormone increases Stat5 and Stat1 expression in lactating goat mammary gland: a specific effect compared to milking frequency

In ruminants, both milking frequency and exogenous GH treatment affect milk production. In a previous report, we showed that the modulation of milk yield due to variations in milking frequency and GH treatment was associated with variations in mammary cell numbers. The aim of this study was to clarify the different mechanisms governing the effects of GH treatment and milking frequency on signal transducer and activator of transcription (Stat) expression and activation, and on the expression of genes involved in mammary cell differentiation. Six Saanen goats in late lactation were milked once daily from one half-udder and thrice daily from the other half-udder for 23 days. At the same time, the goats were divided into two groups: GH-treated versus control group. After slaughter of the goats, soluble mammary proteins and RNA were extracted from half-udder samples. Levels of Stat5, Stat3 and Stat1 proteins and the Stat activation by phosphorylation were analysed by Western blot. The amounts of Stat5 protein and mRNA were significantly elevated by GH treatment in all half-udders (milked once or thrice daily). Positive Stat5 immunoreactivity was principally localised in the nuclei of epithelial cells, with heterogeneous intensity between cells. No significant changes in Stat5 protein phosphorylation levels were observed. Furthermore, GH significantly increased Stat1 protein levels, without modifying the level of Stat1 tyrosine phosphorylation, and tended to reduce the abundance of Stat3 protein. In contrast, milking frequency failed to modify Stat gene expression, protein level and phosphorylation. Using Northern blot, we showed that levels of kappa casein and prolactin receptor mRNA were not affected by the treatments. These observations suggest that GH probably acts specifically on mammary cells by regulating the expression of Stat1, 3 and 5. In contrast, milking frequency does not act through this regulatory pathway.

Prolactin prevents acute stress-induced hypocalcemia and ulcerogenesis by acting in the brain of rat

Stress causes hypocalcemia and ulcerogenesis in rats. In rats under stressful conditions, a rapid and transient increase in circulating prolactin (PRL) is observed, and this enhanced PRL induces PRL receptors (PRLR) in the choroid plexus of rat brain. In this study we used restraint stress in water to elucidate the mechanism by which PRLR in the rat brain mediate the protective effect of PRL against stress-induced hypocalcemia and ulcerogenesis. We show that rat PRL acts through the long form of PRLR in the hypothalamus. This is followed by an increase in the long form of PRLR mRNA expression in the choroid plexus of the brain, which provides protection against restraint stress in water-induced hypocalcemia and gastric erosions. We also show that PRL induces the expression of PRLR protein and corticotropin-releasing factor mRNA in the paraventricular nucleus. These results suggest that the PRL levels increase in response to stress, and it moves from the circulation to the cerebrospinal fluid to act on the central nervous system and thereby plays an important role in helping to protect against acute stress-induced hypocalcemia and gastric erosions.

Developmental changes in insulin-like growth factor I receptor gene expression in the mouse mammary gland

The insulin-like growth factor I receptor (IGF-IR), which mediates the mitogenic action of IGF-I, has been shown to play an essential role in normal growth and development. However, the precise role of IGF-IR in the growth and differentiation of the mammary gland has not been elucidated. This study examines the profile of the IGF-IR gene and protein expression during normal postnatal mammary gland development in order to gain further insight into the role of the IGF-I/IGF-IR during mammary gland morphogenesis. Gene and protein expression were examined in developing mouse mammary glands (virgin, pregnant, lactating, involuting) by real time PCR analysis and Western blotting. Both IGF-IR gene and protein expression levels were high during early pregnancy. Interestingly, the level of gene expression was significantly down-regulated during late pregnancy (5.4 fold) and lactation (9-13 fold) and significantly up-regulated (3.9 fold) during late involution, to the level observed in the virgin mammary gland. By in situ hybridization, the IGF-IR transcripts were localized to the proliferating ductal epithelium of the mammary glands of virgin mice and to the differentiating ductal and alveolar epithelium of the mammary glands during pregnancy and lactation. In the involuting gland, the transcripts were localized to the regressing ductal epithelium. These data are direct evidence that IGF-IR expression is important for alveolar cell proliferation and suggest that the progression of involution may require the down-regulation of IGF-IR gene expression. Altogether, these results demonstrate that a developmental IGF-IR gene expression pattern exists in the mouse mammary gland and that increases in gene expression at specific phases of development may reflect an important role for IGF-I/IGF-IR at those phases of development.

Genetic Manipulation of Mammary Gland Development and Lactation

The mammalian genome is believed to contain some 30,000 to 40,000 different genes. Of these an estimated 42% have no known function. Genetically engineered mouse models (GEMM) have been a powerful tool available for determining gene function in vivo. In the mammary gland, a variety of genetic engineering approaches have been applied successfully to understanding the importance of specific gene products to mammary gland development and lactation. Our own laboratory has applied genetically engineered mice to facilitate understanding of the regulation of mammary gland development and lactation by insulin-like growth factors (IGF) and by the transcription factor, upstream stimulatory factor (USF-2). Our studies on transgenic mice that overexpress IGF-I have demonstrated the importance of IGF-dependent signaling pathways to maintenance of mammary epithelial cells during the declining phase of lactation. Our analysis of early developmental processes in mammary tissue from mice that carry a targeted mutation in the IGF-I receptor gene suggests that IGF-dependent stimulation of cell cycle progression is more important to early mammary gland development than potential antiapoptotic effects. Lastly, our studies on mice that carry a targeted mutation of the Usf2 gene have demonstrated that this gene is necessary for normal lactation and have highlighted the importance of this gene to the maintenance of protein synthesis. These studies, as well as studies of others, have highlighted both the strengths and limitations inherent in the use of GEMM. Limitations serve as the driving force behind development of new experimental strategies and genetic engineering schemes that will allow for a full understanding of gene function within the mammary gland.

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

We have previously demonstrated that IGFBP-5 production by mammary epithelial cells increases dramatically during involution of the mammary gland. To demonstrate a causal relationship between IGFBP-5 and cell death we created transgenic mice expressing IGFBP-5 in the mammary gland using a mammary-specific promoter, β-lactoglobulin. DNA content in the mammary glands of transgenic mice was decreased as early as day 10 of pregnancy. Histological analysis indicated reduced numbers of alveolar end buds, with decreased ductal branching. Transgenic dams produced IGFBP-5 in their milk at concentrations similar to those achieved at the end of normal lactation. Mammary cell number and milk synthesis were both decreased by approximately 50% during the first 10 days of lactation. BrdU labelling was decreased, whereas DNA ladders were increased in transgenic animals on day 1 of lactation. On day 2 postpartum, the epithelial invasion of the mammary fat pad was clearly impaired in transgenic animals. The concentrations of the pro-apoptotic molecule caspase-3 and of plasmin were both increased in transgenic animals whilst the concentrations of 2 prosurvival molecules Bcl-2 and Bcl-xLwere both decreased. In order to examine whether IGFBP-5 acts by inhibiting the survival effect of IGF-I we examined IGF receptor phosphorylation and Akt phosphorylation and showed that both were inhibited. We attempted to “rescue” the transgenic phenotype by using growth hormone to increase endogenous IGF-I concentrations or by implanting minipumps delivering an IGF-1 analogue, R3-IGF-1, which binds weakly to IGFBP-5. Growth hormone treatment failed to affect mammary development suggesting that increased concentrations of endogenous IGF-1 are insufficient to overcome the high concentrations of IGFBP-5 produced by these transgenic animals. In contrast mammary development (gland weight and DNA content) was normalised by R3-IGF-I although milk production was only partially restored. This is the first demonstration that over-expression of IGFBP-5 can lead to; impaired mammary development, increased expression of the pro-apoptotic molecule caspase-3, increased plasmin generation and decreased expression of pro-survival molecules of the Bcl-2 family. It clearly demonstrates that IGF-I is an important developmental/survival factor for the mammary gland and, furthermore, this cell death programme may be utilised in a wide variety of tissues.

Regulation of cell apoptosis by insulin-like growth factor I

Correct temporal and spatial regulation of apoptosis is critical for normal mammary gland development and lactation. Previous work with a strain of transgenic mice that overexpress des(1-3)hIGF-I during pregnancy and lactation suggested that this growth factor inhibits apoptosis. The hypothesis tested within these studies is that overexpression of des(1-3)hIGF-I within the mammary gland inhibits apoptosis and the expression of apoptosis-associated genes that are known to be activated by the transcription factor AP-1. This inhibition of apoptosis was further posited to predispose the tissue to carcinogenesis. TUNEL analysis of mammary tissue from transgenic mice that overexpress des(1-3)hIGF-I under control of the rat whey acidic protein promoter showed only 25% (P < 0.05) of the number of apoptotic cells found in nontransgenic mice at the same stage of lactation. Northern analysis of RNA from these animals showed a 75% (P = 0.08) reduction in c-Jun mRNA abundance. Histological analysis of mammary tissue from nonlactating multiparous WAP-DES mice ranging in age from 13 to 25 months showed a variety of hyperplastic lesions. These lesions aberrantly expressed the transgene. At 23 months of age 50% of the transgenic mice within this study developed adenocarcinomas. These results support the conclusion that inhibition of apoptosis within the mammary gland by IGF-I involves decreased activity of AP-1 and predisposes the tissue to tumors.

Genetic manipulation of the IGF-I axis to regulate mammary gland development and function

Insulin-like growth factor I (IGF-I) is known to regulate mammary gland development. This regulation occurs through effects on both cell cycle progression and apoptosis. Our laboratory has studied the IGF-I-dependent regulation of these processes by using transgenic and knockout mouse models that exhibit alterations in the IGF-I axis. Our studies of transgenic mice that overexpress IGF-I during pregnancy and lactation have demonstrated that this growth factor slows the apoptotic loss of mammary epithelial cells during the declining phase of lactation but has minimal effects during early lactation on milk composition or lactational capacity. In contrast, our analysis of early developmental processes in mammary tissue from mice carrying a targeted mutation in the IGF-I receptor gene suggests that IGF-dependent stimulation of cell cycle progression is more important to early mammary gland development than potential anti-apoptotic effects. With both models, the effects of perturbing the IGF-I axis are dependent on the physiological state of the animal. The diminished ductal development that occurs in response to loss of the IGF-I receptor is dramatically restored during pregnancy, whereas the ability of overexpressed IGF-I to protect mammary cells from apoptosis does not occur if the mammary gland is induced to undergo forced involution. Data from our laboratory on the expression of IGF-signaling molecules in the mammary gland suggest that this effect of physiological context may be related to the expression of members of the insulin receptor substrate family.

The role of prolactin and growth hormone in breast cancer

This review will focus on the role for prolactin (PRL) and growth hormone (GH) in mammary tumor formation. Much attention has previously been focused on circulating levels of GH/PRL in relation to mammary tumor formation. We will review data demonstrating that these ligands also could be produced locally in different organs, including the mammary gland and mammary tumors, and suggest that this local production may be of importance for pathological conditions. We will also discuss mechanisms for crosstalk between steroids and GH/PRL. A crosstalk between GH- and PRL response is possible at multiple levels. In the human, GH can activate both the prolactin receptor (PRLR) and the growth hormone receptor (GHR). We have demonstrated that activation of the PRLR, but not the GHR, is inducing mammary tumors in transgenic mice. Furthermore, the elevated levels of insulin-like growth factor 1 (IGF-I) seen in the GHR activating transgenic mice is not sufficient for tumor induction. The induced tumors express functionally active prolactin that could be of importance for the tumor formation. Paracrine/aurocrine stimulation by PRL may be more important than PRL transported via the circulation. In women, the role for stimulation of the PRLR and/or the GHR in mammary tumor formation has not been proven, although experiments from primates suggest that the PRLR could be of importance.

Insulin-like growth factor I is essential for terminal end bud formation and ductal morphogenesis during mammary development

Previous studies from this laboratory have emphasized the essential role of GH in pubertal mammary development and shown that insulin-like growth factor I (IGF-I) was capable of substituting for GH in this process in rats and mice. The present study shows that, even when GH is present, no mammary development is possible unless IGF-I is present. IGF-I(-/-) null female animals were found to have significantly less mammary development than age matched wild-type controls (P <0.006) using several endpoints including the number of terminal end buds or TEBs (1.3 vs. 7.3), percent of the fat pad occupied by glandular elements (6.5 vs. 100), and number of ducts (15 vs. too numerous to count). That the deficiency in mammary gland development was related to the absence of IGF-I was underscored by the observation that des (1-3) IGF-I administration to IGF-I(-/-) null animals for 5 days caused significant mammary gland development as measured by TEB formation and branching of ducts. The number of TEBs rose from a mean of 1.3 in controls to 20.5 without added E2 (P < 0.009), and from 1.7 to 21 when des (1-3) IGF-I was given together with E2 (P < 0.006). The number of ducts increased significantly from a mean of 12 to 27 in response to IGF-I and E2, and from 15 to 24.5 with IGF-I alone. In contrast, administration of human GH with E2 had no stimulatory effect on mammary development in these animals, indicating that the full effect of GH in this process is mediated by IGF-I. To determine whether IGF-I was also responsible for further ductal morphogenesis, we administered des (1-3) IGF-I + E2 to the knockout animals for 14 days and compared the effects of this combination of hormones on mammary development with those observed after 5 days. We found that there was a significant increase from 5 to 14 days in the number of TEBs (mean: 21 vs. 41) and the area of the mammary fat pad occupied by glands (mean: 10 vs. 20%). There was elongation and thickening of the ducts which accounted for the increased area that was occupied by ductal structures. There was no significant increase in the number of ducts. However, there was the appearance of a large number of buds along the length of the ductal structures, suggesting the beginning of side branching. These results suggest that IGF-I, when given along with E2, is responsible for ductal morphogenesis.

Presence of thyroxine deiodinases in mammary gland: possible modulation of the enzyme-deiodinating activity by somatotropin

Thyroid hormones (TH) and somatotropin (ST) play critical role in lactation. One explanation of their multiple physiological actions is based on the functional interrelationships among ST, TH, and thyroxin deiodinase (5'D). This enzyme is present in the mammary tissue, milk cellular components, and whole milk and is responsible for intramammary production of triiodothyronine (T3). In rats in which the 5'D isozymes in the mammary gland and in the liver are similarly of type I (5'D-I), an enhancement of mammary 5'D-I causes a reduction of hepatic 5'D-I activities. This opposite rearrangement in the mammary and hepatic deiodinating activities is thought to be a factor of a homeorhetic response characterized by an increased and compartmentalized energy expenditure of the mammary gland. In the cow, the mammary 5'D is the type II (5'D-II) deiodinase. The 5'D-II, owing to its high catalytic efficiency, secures T3 production, making tissues relatively independent from the circulatory levels of TH and from variations in the hepatic 5'D-I activity. No significant alterations of 5'D-II isozymes were found during a low T3 syndrome. Location of tissue deiodinases in the cow, the 5'D-II in the mammary gland, and the 5'D-I in the liver make it so that T3 production in these two tissues can be dissociated in time to secure better local requirement for T3 supporting lactation. To date, attempts to evidence that the alterations in iodothyronines blood levels and in tissues' 5'Ds activity during lactation are due to ST action have not received clear experimental support in either cows or rats.

Regulation of milk protein gene expression

Studies using both transgenic mice and transfected mammary epithelial cells have established that composite response elements containing multiple binding sites for several transcription factors mediate the hormonal and developmental regulation of milk protein gene expression. Activation of signal transduction pathways by lactogenic hormones and cell-substratum interactions activate transcription factors and change chromatin structure and milk protein gene expression. The casein promoters have binding sites for signal transducers and activators of transcription 5, Yin Yang 1, CCAAT/enhancer binding protein, and the glucocorticoid receptor. The whey protein gene promoters have binding sites for nuclear factor I, as well as the glucocorticoid receptor and the signal transducers and activators of transcription 5. The functional importance of some of these factors in mammary gland development and milk protein gene expression has been elucidated by studying mice in which some of these factors have been deleted.

Developmental regulation of insulin-like growth factor-I and growth hormone receptor gene expression

During development, the insulin-like growth factor I (IGF-I) gene is expressed in a tissue specific manner; however, the molecular mechanisms governing its developmental regulation remain poorly defined. To examine the hypothesis that expression of the growth hormone (GH) receptor accounts, in part, for the tissue specific expression of the IGF-I gene during development, the developmental regulation of IGF-I and GH receptor gene expression in rat tissues was examined. The level of IGF-I and GH receptor mRNA was quantified in RNA prepared from rats between day 17 of gestation (E17) and 17 months of age (17M) using an RNase protection assay. Developmental regulation of IGF-I gene expression was tissue specific with four different patterns of expression seen. In liver, IGF-I mRNA levels increased markedly between E17 and postnatal day 45 (P45) and declined thereafter. In contrast, in brain, skeletal muscle and testis, IGF-I mRNA levels decreased between P5 and 4M but were relatively unchanged thereafter. In heart and kidney, a small increase in IGF-I mRNA levels was observed between the early postnatal period and 4 months, whereas in lung, minimal changes were observed during development. The changes in GH receptor mRNA levels were, in general, coordinate with the changes in IGF-I mRNA levels, except in skeletal muscle. Interestingly, quantification of GH receptor levels by Western blot analysis in skeletal muscle demonstrated changes coordinate with IGF-I mRNA levels. The levels of the proteins which mediate GH receptor signaling (STAT1, -3, and -5, and JAK2) were quantified by Western blot analysis. These proteins also are expressed in a tissue specific manner during development. In some cases, the pattern of expression was coordinate with IGF-I gene expression, whereas in others it was discordant. To further define molecular mechanisms for the developmental regulation of IGF-I gene expression, protein binding to IGFI-FP1, a protein binding site that is in the major promoter of the rat IGF-I gene and is important for basal promoter activity in vitro, was examined. Gel shift analyses using a 34-base pair oligonucleotide that contained IGFI-FP1 did not demonstrate changes in protein binding that paralleled those in IGF-I gene expression, suggesting that protein binding to IGFI-FP1 does not contribute to the developmental regulation of IGF-I gene expression, at least in brain and liver. In summary, the present studies demonstrate coordinate expression of the IGF-I gene and GH receptor during development and suggest that GH receptor expression contributes to the tissue specific expression of the IGF-I gene during development.

The biology of breast cancer

This article focuses on the major hormones and growth factors for which a critical role in normal mammary growth has been clearly defined. Certainly other hormonal systems and growth factors could also affect breast cancer initiation and progression, but their exact contribution to normal and/or malignant breast cell growth is poorly delineated. Examples of such factors include somatostatin, mammostatin, mammary-derived growth inhibitor (MDGI), mammary-derived growth factor-1 (MDGF-1), inhibins, activins, androgens, glucocorticoids, vitamin D, thyroid hormones, ecosinoids, and oxytocin. Clearly, the hormonal regulation of breast cancer cell growth and survival is multifaceted and very complex. In particular, the effects of estrogens and anti-estrogens on breast cells may depend on their interaction with a wide variety of other pathways. In addition, these interactions may vary among individual breast tumors depending on other genetic changes in the tumor cells that have not been discussed here, such as oncogene activation and loss of tumor suppressors. A more detailed understanding of how cells circumvent a dependency on these pathways is greatly needed in order to identify new biological targets and to design novel therapies for breast cancers that are resistant to anti-estrogen therapy. Such agents could be used alone or in combination with anti-estrogens to improve response to a second course of hormonal therapy.

IGF-I physiology and breast cancer

Recent studies imply that IGF-I levels vary greatly between normal women, and that premenopausal breast cancer risk is increased among women with higher IGF-I levels. It is known that tamoxifen lowers IGF-I levels, but further research is needed to determine whether antiestrogens will be of particular value in risk reduction for women with high IGF-I levels, and also to determine if IGF-I levels can indeed be used as an intermediate endpoint in risk reduction interventions. With respect to adjuvant therapy, we currently have convincing data that antiestrogens have moderate IGF-I lowering actions, but it remains unclear to what extent these contribute to the therapeutic effect of these compounds. Ongoing trials are addressing this question, as well as the hypothesis that interventions that increase IGF-I suppression will be associated with reduced relapse rates.

The insulin-like growth factors (IGF) and IGF type I receptor during postnatal growth of the murine mammary gland: sites of messenger ribonucleic acid expression and potential functions

The goals of this study were to determine the cellular sites of insulin-like growth factor (IGF) and IGF type-I receptor (IGF-IR) expression and to begin to elucidate functional roles for the IGFs during postnatal development of the murine mammary gland. Using in situ hybridization analyses, we determined that IGF-I, IGF-II, and IGF-IR messenger RNAs were expressed in the highly proliferative terminal end buds during pubertal ductal growth. Consistent with these data, IGF-I (in combination with mammogenic hormones) promoted ductal growth in pubertal stage mammary glands cultured in vitro. During postpubertal and pregnancy stages, IGF-II and IGF-IR continued to be expressed in ductal epithelium. Expression of IGF-II in ductal and alveolar epithelium correlated with the pattern of rapidly proliferating cells, as determined by incorporation of 5-Bromo-2'-deoxyuridine, suggesting a potential autocrine or paracrine role for IGF-II as a mitogen for ductal epithelial cells. IGF-I expression was reinitiated in mammary epithelium in the differentiated alveoli at the end of pregnancy, suggesting an additional role for this factor in maintenance of the alveoli during lactation. Taken together, these data support an in vivo role for locally-produced IGFs in promoting ductal growth during puberty and suggest that IGF-I and IGF-II may have distinct functions during pregnancy-induced alveolar development.

60K gelatinase involved in mammary gland involution is regulated by beta-oestradiol

Cell matrix interactions are critical in the expression and maintenance of differentiated functions in mammary gland. Matrix metalloproteinases (MMPs), by acting on different matrix components, contribute to the remodelling of extracellular matrix. Of the three major gelatinases found in rat mammary gland at different stages of ontogeny, 60K gelatinase, a Ca2+ dependent neutral MMP, seems to be involved in involution, as it appears at the late stage of involution. Further investigations on its regulation by hormones which influence the mammary gland function were carried out. Administration of beta-oestradiol caused the appearance of 60K gelatinase on the 2nd day of involution, while in untreated controls this activity was absent. On treatment of mammary epithelial cells of the 2nd day involuting tissue in culture with beta-oestradiol, the 60K gelatinase activity appeared, while the untreated controls did not show the activity. The effect of beta-oestradiol was studied further by metabolic labelling of the epithelial cells from the 2nd day involuting tissue. A concentration dependent increase in the amount of radiolabelled 60K gelatinase was found on treatment with beta-oestradiol. The upregulation of the 60K gelatinase activity in vivo was also found by immunocytochemical staining of the beta-oestradiol treated tissues. The effect of beta-oestradiol appears to be specific for 60K as the activity of other gelatinases (130K and 68K) in the mammary gland were not affected. Furthermore, a drastic regression of the mammary gland as evidenced by histochemical analysis and a marked decrease in the milk protein production in beta-oestradiol treated tissues indicated the onset of early involution. These results indicate that the 60K gelatinase which is upregulated during involution or on induction of early involution may play a key role in remodelling of extracellular matrix in mammary gland and further that this enzyme is subject to modulation by beta-oestradiol.

Western diet, early puberty, and breast cancer risk

The typical high fat, low fibre diet of the industrialised West, particularly when associated with inadequate exercise, is likely to advance the onset of puberty. This will manifest in girls as an earlier menarche, earlier onset of breast development, and an earlier growth spurt. Both earlier menarche and adult tallness are markers of increased risk to breast cancer. Earlier menarche in the West is usually associated with earlier onset of hyperinsulinaemia, and multiple case-control studies report that hyperinsulinaemia too is a marker of increased breast cancer risk. Although the Western diet is linked both to earlier menarche and also to earlier hyperinsulinaemia, the mechanism involved is not necessarily the same. While menarche is likely to be triggered by a threshold level of fatness, manifestation of insulin resistance is genetically-determined and strongly influenced by the fatty acid profile of the diet. The putative mechanisms by which they influence mammary carcinogenesis also differ. Early menarche is reported to be associated with a raised oestradiol level persisting into early adult life. On the other hand, hyperinsulinaemia is commonly associated with abnormal aromatase activity in the ovaries. In addition, the concomitant increase in bioactive levels of insulin-like growth factor-I may synergise with oestrogen in stimulating proliferative activity in mammary epithelium. Dietary modification and exercise regimens are proposed in families at high risk to breast cancer. The measures have been shown to reduce insulin levels in both children and adults, and serial monitoring of insulin and sex steroid levels could be used to detect a metabolic-endocrine effect.

Role of the insulin-like growth factors and their binding proteins in lactation

Insulin-like growth factor (IGF)-I is thought to mediate a portion of the effects of bST on lactation in dairy cows. Serum concentrations of IGF-I are increased in lactating cows that were treated with bST, and IGF-I receptors are present in bovine mammary tissue. In addition, close arterial infusion of IGF-I into the mammary gland of goats increases milk yield. Little evidence exists to support a direct galactopoietic effect of IGF-I in ruminants. However, IGF-I is a potent mitogen for mammary epithelial cells and may also influence the inhibition of apoptosis of this cell type. The IGF are found in association with a family of individual binding proteins. The high affinity of the IGF for these proteins relative to the IGF receptor allows them to modulate IGF-I bioactivity in the mammary gland at the cellular level. Mammary epithelial cells synthesize multiple forms of IGF binding proteins, and one of these, IGF binding protein-3, is specifically regulated by the IGF. Stimulation of DNA synthesis by IGF-I is enhanced in bovine mammary epithelial cells that overexpress the IGF binding protein-3. These data indicate that IGF-I can stimulate the synthesis of an IGF binding protein, which enhances its own mitogenic activity. However, whether this mechanism is operative in the lactating mammary gland in vivo is unknown. Given the complexity of the interactions between the IGF and their binding proteins, more information is needed before the role of these growth factors in regulating growth, differentiation, and apoptosis of mammary epithelial cells is delineated.

Endocrine effects of IGF-I on normal and transformed breast epithelial cells: potential relevance to strategies for breast cancer treatment and prevention

Insulin-like growth factors (IGFs) are mitogenic and anti-apoptotic peptides that influence the proliferative behavior of many cell types, including normal and transformed breast epithelial cells. IGF-I has properties of both a tissue growth factor and a systemic hormone: there is evidence that IGF bioactivity in tissues is influenced not only by local factors such as tissue expression of IGFs, IGF binding proteins (IGFBPs), and IGFBP proteases, but also by factors that regulate whole-body IGF physiology and circulating IGF-I levels. Experimental evidence that interventions that reduce circulating IGF-I levels reduce proliferation of breast neoplasms has raised interest in the possibility of developing novel endocrine therapies that target the growth hormone/IGF-I axis. Furthermore, influences of the growth hormone/IGF-I axis on normal breast epithelial cells may underlie recent epidemiological observations that suggest that premenopausal women with high circulating IGF-I level are at increased risk for breast cancer. These studies suggest that the growth hormone/IGF-I axis deserves investigation as a possible target for novel breast cancer prevention strategies.

Role of IGF-I in normal mammary development

Growth hormone (GH) is now believed to be the pituitary factor that is responsible for mammary ductal morphogenesis. Mammary development at puberty occurs because of synergy between GH and estrogen on formation of terminal end buds (TEBs). TEBs extend into the substance of the mammary gland fat pad, resulting in ductal morphogenesis. Ultimately, the whole mammary fat pad accommodates a complex network of ducts. IGF-I or des(1-3) IGF-I mimic the actions of GH on TEB formation in hypophysectomized, gonadectomized rats. Since GH stimulates IGF-I mRNA within the mammary gland synergistically, we hypothesize that IGF-I partially mediates actions of GH in mammary gland development. Studies in transgenic mice overexpressing IGF-I, des(1-3) IGF-I, or IGFBP-3 show that IGF-I causes ductal hypertrophy in the lactating mouse and prevention of post-lactational mammary gland involution. One of the mechanisms for this effect involves apoptosis. The potential role of GH or IGF-I in mammary carcinogenesis, and the applicability of animal studies to humans, are discussed.

Ontogeny and epithelial-stromal interactions regulate IGF expression in the ovine mammary gland

Although the insulin-like growth factors (IGF-I and -II) have been implicated in the stimulation of mammogenesis, little is known of their regulation in the mammary gland. In this study we removed epithelial tissue from one of the two mammary glands of 1-week-old ewe lambs and examined IGF-I and -II mRNA expression during postnatal development in both the intact mammary gland and in the gland cleared of epithelial tissue. Expression of IGF-I mRNA was highest at 6 and 10 weeks of age, coincident with the prepubertal phase of rapid mammary growth, then declined and remained low until expression increased during late pregnancy. IGF-I mRNA was more abundant in the mammary fat pad adjacent to parenchyma (MFP) than in the contralateral fat pad that had been surgically cleared of epithelium (CFP). The level of IGF-II mRNA in parenchyma was highest at 1-23 weeks of age due to an increase in the abundance of specific mRNAs. Expression was lower in the fat pads, with generally higher levels in the intact MFP than the CFP, and in these tissues IGF-II expression was shown to increase with age between 6 and 23 weeks. We also investigated the influence of the ovary and estrogen on the expression of IGFs. While IGF-I mRNA abundance was unaffected by ovariectomy, exogenous estrogen resulted in higher levels of expression in the MFP of ovariectomized ewes and tended to increase its level in the parenchyma of intact ewes. Ovariectomy increased IGF-II mRNA within mammary parenchyma whereas estrogen suppressed levels in both the parenchyma and MFP. These findings demonstrate that IGF-I and -II mRNAs are expressed locally within the developing ovine mammary gland and are regulated by stage of ontogeny, ovarian hormones, and epithelial stromal interaction.

Growth hormone treatment induces mammary gland hyperplasia in aging primates

The decline of growth hormone (GH) and insulin-like growth factor I (IGF-I) production during aging has been likened to the decrease in gonadal steroids in menopause. The repletion of GH/IGF-I levels in aging individuals is suggested to restore the lean tissue anabolism characteristic of youth. In addition to anabolic effects on musculo-skeletal tissues, GH also stimulates mammary glandular growth in some species, although its effects on primate mammary growth remain unclear. Some clinical observations implicate GH in human mammary growth, for example, gynecomastia occurs in some children treated with GH (ref. 6), and tall stature and acromegaly are associated with an increased incidence of breast cancer. To investigate the effects of GH/IGF-I augmentation on mammary tissue in a model relevant to aging humans, we treated aged female rhesus monkeys with GH, IGF-I, GH + IGF-I or saline diluent for 7 weeks. IGF-I treatment was associated with a twofold increase, GH with a three- to fourfold increase, and GH + IGF-I with a four'-to fivefold increase in mammary glandular size and epithelial proliferation index. These mitogenic effects were directly correlated with circulating GH and IGF-I levels, suggesting that either GH or its downstream effector IGF-I stimulates primate mammary epithelial proliferation.

Breast cancer: further metabolic-endocrine risk markers?

There is evidence that increased oestrogen receptor (ER) expression in normal mammary epithelium may be a risk marker for the development of breast cancer. Insulin-like growth factor 1 (IGF1) is a potent inducer of mitosis and has been shown to synergize with oestrogen in stimulating the growth of human breast cancer in vitro. In these cells oestradiol has been shown to upregulate IGF type 1 receptor (IGFR), and recently a similar effect has been reported in normal human breast tissue xenografts in vivo. It has been postulated that the combined effect of oestradiol and IGF1 may stimulate proliferation in normal mammary epithelium and increase breast cancer risk. The bioavailability of IGF1 to the tissues is modulated by IGF-binding proteins (IGFBPs), and higher circulating levels of IGF1 and lower levels of IGFBP3 have been reported in breast cancer patients. Breast cancer specimens show a positive correlation between ER status and IGF receptor status, and also a negative correlation between ER status and IGFBP3 expression. Finally, ectopic growth hormone expression has been shown in a majority of specimens of normal and malignant breast tissue, and this may contribute to breast cancer risk, possibly by increasing the local level of bioavailable IGF1. Expansion of such findings may provide clinically useful markers of increased risk to breast cancer in women.

Early mammary development: growth hormone and IGF-1

The first step in pubertal mammary development is the appearance of terminal end buds arising from pleuropotent stem cells present in the immature ductal tree of the prepubertal animal. Work from this laboratory indicates that growth hormone is the pituitary hormone responsible for terminal end bud development. Growth hormone likely acts through the production of IGF-1. This minireview focuses on the hormonal control of early mammary development with special emphasis on the roles of growth hormone and IGF-1.

Growth hormone-binding protein in the goat: characterization, evolution under exogenous growth hormone treatment, and correlation with liver growth hormone receptor levels

This report describes the identification and characterization of a specific, high-affinity growth hormone-binding protein (GHBP) in lactating goat serum. Serum samples were incubated with [125I]human GH as ligand and in the absence or in the presence of bovine GH as competitor. GH-GHBP complex formation was performed by high-performance liquid chromatography, and the radioactivity was recorded on-line with a Berthold LB detector connected to a computer. The results showed that a serum protein was able to bind specifically to human GH and bovine GH but not to ovine prolactin. Scatchard plots indicated an affinity constant of 4.5 x 10(8) M-1 and a maximum binding capacity of 4.8 x 10(-10) mol/l. In addition, we conducted a 4-wk study to determine the effects of recombinant bovine GH administration on milk production in lactating goats. The effects of recombinant bovine GH treatment on milk production and on the regulation of GHBP and hepatic GH receptor levels were studied. As expected, recombinant bovine GH injected daily increased yields of milk, fat, protein (40, 61, and 40%, respectively), and circulating insulin-like growth factor 1 concentrations compared with controls. During the pretreatment and treatment periods, the control goats exhibited a constant amount of GHBP in serum. No consistent effect of GH treatment on GHBP level was observed. The binding of [125I]bovine GH to hepatic microsomal membranes of GH-treated goats was significantly decreased compared with that of control goats. After MgCl2 desaturation of membranes, the results demonstrated that the down-regulation of GH hepatic receptors, observed for the treated goat group, was induced by receptor occupancy without modification of binding affinity. The GH receptor gene expression, analyzed by slot blot and hybridization with an [alpha-32P]GH receptor cDNA probe, was not modified by the GH treatment. In lactating goats, the galactopoietic effect of exogenous GH involved a hepatic receptor occupancy. The individual concentration of GHBP in serum cannot explain the individual variations of responses to GH treatment in goats.

Animal models for the study of milk secretion

Milk secretion is regulated by a complex interaction of galactopoietic hormones which is not yet fully understood. Recent studies have demonstrated that this systemic control is modulated within the mammary gland by local mechanisms responsive to the frequency and completeness of milk removal. New insights into the endocrine and local (paracrine and autocrine) regulation of milk secretion have come from the adaptation of traditional endocrinological techniques to take advantage of new molecular tools, and from technical advances in other fields. This paper reviews recently developed animal models for the study of milk secretion and describes their application to provide new information into the roles of two key galactopoietic hormones, growth hormone and prolactin, and the modulation of their actions by local, intramammary mechanisms.

Growth hormone induction of hepatic serine protease inhibitor 2.1 transcription is mediated by a Stat5-related factor binding synergistically to two gamma-activated sites

A growth hormone (GH)-inducible nuclear factor (GHINF) from rat liver has been purified to near homogeneity. On SDS-polyacrylamide gel electrophoresis and UV-cross-linking, a major band of mass approximately 93 kDa and a minor band of approximately 70 kDa are detected in the purified fraction. DNase I footprinting using purified GHINF yields a protected region of -149/-115 on the rat serine protease inhibitor 2.1 (Spi 2.1) promoter encompassed within the growth hormone response element (GHRE). Mutational analysis demonstrated that GHINF binds synergistically to two gamma-interferon-activated sites (GAS) within the GHRE, with the 3' element being the pivotal binding domain. Functional assays show that both GAS elements are necessary for full GH response. GHINF has no immunoreactivity with either a C-terminal Stat1 antibody or an N-terminal Stat3 antibody, while cross-reacting with a C-terminal Stat5 monoclonal antibody. GHINF will bind to two GAS elements from the Stat5 binding region of the beta-casein gene. These studies indicate that GHINF is a Stat5-related factor binding synergistically to two GAS elements to activate Spi 2.1 transcription.

Insulin-like growth factor-I and insulin-like growth factor binding protein-3 inhibit involution of the mammary gland following lactation: studies in transgenic mice

The role of the insulin-like growth factor system on mammary gland development and involution following pregnancy and lactation was studied. Transgenic mice expressing rat IGF-I and human IGFBP-3 specifically in the mammary gland tissue, were created using the whey acidic protein gene. Mammary gland development was normal in transgenic animals expressing either rIGF-I or hIGFBP-3. In contrast, involution of the gland was delayed in both groups of transgenic mice. Specifically, the number of apoptotic cells was less in the involuting glands of transgenic mice compared with control animals. These results confirm the hypothesis that the IGF system regulates mammary gland function.

The Met-HGF/SF autocrine signaling mechanism is involved in sarcomagenesis

Hepatocyte growth factor/scatter factor (HGF/SF) can elicit a wide variety of effects upon cells expressing its receptor, the tyrosine kinase proto-oncogene product Met, including mitogenicity, motility, and morphogenesis. Normally, met expression is restricted to epithelial cells and is activated in a paracrine fashion by HGF/SF secreted from cells of mesenchymal origin. In this chapter, we review data showing that: (i) met over-expression in HGF/SF-expressing NIH/3T3 fibroblasts leads to sarcomagenesis and metastasis via an autocrine mechanism; (ii) Met-HGF/SF autocrine signalling occurs to a low level in normal fibroblasts and to a much greater extent in human sarcomas and sarcoma cell lines; (iii) met expression is enhanced as p53-deficient fibroblasts are passaged in vitro and (iv) met and HGF/SF over-expression are selected for during tumorigenesis of p53-deficient late-passage fibroblasts. Thus, loss of p53 predisposes a mesenchymal cell to over-express met and high level Met-HGF/SF autocrine signaling in mesenchymal cells promotes both sarcomagenesis and metastasis through inappropriate induction of the pleiotropic responses to Met-HGF/SF stimulation.

Relationships between nutrition, puberty and mammary development in cattle

The available studies concerning the relationships between nutrition, puberty and mammary development demonstrate the importance of pubertal mammary growth for the future development and ultimate milk-producing capacity of the mammary gland. A relationship between reproductive development and mammary development is also evident and mammary development at puberty is clearly influenced by the feeding level at that time. The role of specific nutrients has not been thoroughly investigated, but results suggest that specific fatty acids may be involved in the regulation of mammary growth. Mammary growth during puberty is affected by oestrogen and GH, but their respective roles and mechanisms of action have not yet been clarified.

Transgenic strategies in reproductive endocrinology

The present discussion surveys some of the recently published studies utilizing transgenic strategies to address questions in reproductive endocrinology. Beginning with a brief introduction of the transgenic method itself, the following areas are covered: 1. Sexual development and Müllerian-inhibiting substance; 2. Hypogonadal mice and hypothalamic GnRH; 3. The GnRH neuron: generation of immortalized rare cell types; 4. Glycoprotein hormones: immortalized cells, development and evolution; 5. Growth hormone and reproduction; and, 6. Gestation and the insulin-like growth factors. In each section, the discussion attempts to be integrative with respect to the significance of the results to physiological, cellular and molecular biology. We believe this approach is appropriate, as transgenic science itself is necessarily an integration of all of these levels of investigation and participation from those working at all levels is needed.

Intact and amino-terminally shortened forms of insulin-like growth factor I induce mammary gland differentiation and development

Growth hormone (GH) plays a role in regulating growth and differentiation of immature glandular structures in the mammary gland, but the mechanisms by which the hormone exerts these effects are unknown. We have previously found that GH stimulates insulin-like growth factor I (IGF-I) I mRNA production within the mammary glands of hypophysectomized rats. In this study we set out to determine if IGF-I administration could mimic the action of GH in initiating mammary gland differentiation and development. Two forms of IGF-I, intact and amino-terminally shortened [des-(1-3)-IGF-I], were found to induce the development of terminal end buds and the formation of alveolar structures in the mammary glands of hypophysectomized, castrated, and estradiol-treated sexually immature male rats. The effect of both forms of IGF-I was similar to that obtained with human GH, but the truncated form was at least 5 times more potent than intact IGF-I. These findings suggest that the inductive effect of GH on glandular differentiation is mediated by the GH-induced production of IGF-I or a related molecule within the mammary gland itself.