SirT1 modulates the estrogen–insulin-like growth factor-1 signaling for postnatal development of mammary gland in mice
© Gu et al.; licensee BioMed Central Ltd. 2007
Received: 21 August 2006
Accepted: 3 January 2007
Published: 3 January 2007
Estrogen and insulin-like growth factor-1 (IGF-1) play important roles in mammary gland development and breast cancer. SirT1 is a highly conserved protein deacetylase that can regulate the insulin/IGF-1 signaling in lower organisms, as well as a growing number of transcription factors, including NF-κB, in mammalian cells. Whether SirT1 regulates the IGF-1 signaling for mammary gland development and function, however, is not clear. In the present study, this role of SirT1 was examined by studying SirT1-deficient mice.
SirT1-deficient (SirT1ko/ko) mice were generated by crossing a new strain of mice harboring a conditional targeted mutation in the SirT1 gene (SirT1co/co) with CMV-Cre transgenic mice. Whole mount and histology analyses, immunofluorescence staining, immunohistochemistry, and western blotting were used to characterize mammary gland development in virgin and pregnant mice. The effect of exogenous estrogen was also examined by subcutaneous implantation of a slow-releasing pellet in the subscapular region.
Both male and female SirT1ko/ko mice can be fertile despite the growth retardation phenotype. Virgin SirT1ko/ko mice displayed impeded ductal morphogenesis, whereas pregnant SirT1ko/ko mice manifested lactation failure due to an underdeveloped lobuloalveolar network. Estrogen implantation was sufficient to rescue ductal morphogenesis. Exogenous estrogen reversed the increased basal level of IGF-1 binding protein-1 expression in SirT1ko/ko mammary tissues, but not that of IκBα expression, suggesting that increased levels of estrogen enhanced the production of local IGF-1 and rescued ductal morphogenesis. Additionally, TNFα treatment enhanced the level of the newly synthesized IκBα in SirT1ko/ko cells. SirT1 deficiency therefore affects the cellular response to multiple extrinsic signals.
SirT1 modulates the IGF-1 signaling critical for both growth regulation and mammary gland development in mice. SirT1 deficiency deregulates the expression of IGF-1 binding protein-1 and attenuates the effect of IGF-1 signals, including estrogen-stimulated local IGF-1 signaling for the onset of ductal morphogenesis. These findings suggest that the enzymatic activity of SirT1 may influence both normal growth and malignant growth of mammary epithelial cells.
Mammalian SirT1 belongs to a family of nicotinamide adenine dinucleotide-dependent histone deacetylases [1, 2]. SirT1 is most closely related to yeast Sir2, the founding member of the evolutionarily conserved Sir2 family. Yeast Sir2 is required for silencing transcription at the telomeric region and mating type loci, and for suppression of ribosomal DNA recombination [3, 4]. The expression of an extra copy of Sir2 in either yeast mother cells or multicell organisms such as nematodes can significantly extend the lifespan [5, 6]. Inactivation of Sir2 enhances stress resistance and extends chronological lifespan of nondividing yeast cells, which is opposite to the requirement for Sir2 function in the reproductive lifespan . Whether SirT1 regulates the reproductive lifespan and/or the chronological lifespan in mammals remains unknown.
Sir2 is an integral part of an evolutionarily conserved insulin/insulin-like growth factor-1 (IGF-1) signaling (IIS) system in worms (Caenorhabditis elegans), fruit flies (Drosophila), mice, and humans [8, 9]. The IIS system includes membrane-bound receptors, cytoplasmic kinases, and nuclear transcription factors. To maintain the proper expression of the effector genes for the IIS system, these conserved components form a sophisticated regulatory system, which centers on a family of forkhead transcription factors (forkhead box 'other' proteins (FoxOs)), and operates on two levels. On one level, SirT1-mediated protein deacetylation attenuates the transcriptional activity of nuclear FoxO transcription factors [10–12]. On the second level, the FoxO transcription factors can be sequestered within the cytoplasm when phosphorylated by activated Akt kinases in response to insulin and IGF-1 signals . Conceivably, the IIS system senses the levels of insulin and IGF-1 and negatively regulates the expression of the effector genes. The IIS system is responsible for food storage, stress tolerance, and longevity in lower organisms, such as C. elegans [8, 9, 14]. In more advanced species, steroid hormones evolved to regulate the IIS system . In mice and humans, the IGF-1 signaling of the IIS system mediates local effects for growth and hormonal regulation for multiple tissues, including mammary glands [16, 17].
Mammalian SirT1 has evolved to modify the activity of a growing number of transcription factors, including p53, NF-κB, and PGC-1α, suggesting that SirT1 functions in a wide range of cellular responses to stress, inflammation, and nutrients [18–21]. SirT1-deficient mice display characteristic phenotypes of perinatal death and growth retardation as well as other diverse phenotypes, such as eye defects, with varying severity [22, 23]. The underlying causal mechanism for these phenotypes, however, remains unknown. We recently generated SirT1-deficient (SirT1ko/ko) mice and found that both male and female SirT1ko/ko mice can be fertile, which is in contrast to the sterile phenotypes observed in one strain of SirT1-deficient mice . This led to our study of the link between SirT1 and IGF-1 signaling using the mammary gland as a model organ.
The mammary gland is a unique organ because it develops after birth and undergoes dynamic changes throughout the reproductive lifespan of a female. At the onset of puberty, ovarian estrogen stimulates ductal morphogenesis during which mammary epithelial progenitor cells differentiate and proliferate while interacting with adipocytes and stromal cells within mammary fat pad [17, 24, 25]. Ovarian estrogen, in synergy with pituitary growth hormone (GH), stimulates stromal cells to produce local IGF-1. The local IGF-1, but not liver-produced systemic IGF-1, provides a paracrine signal for commencing ductal morphogenesis . Mice lacking GH, estrogen, IGF-1, GH receptor, or estrogen receptor alpha (ERα) fail to undergo postnatal ductal morphogenesis [17, 24–31], indicating that both steroid hormones and IGF-1 are on the common pathway for a critical developmental checkpoint. Once the arborated ductal network is established, cycles of differentiation, proliferation, and death of secretory alveolar epithelium repeat with each pregnancy [17, 24, 25].
In the present article we report the finding and characterization of impeded ductal morphogenesis in virgin SirT1ko/ko mice and lactation failure in SirT1ko/ko mothers. The characterization of these phenotypes has identified a SirT1-dependent regulatory mechanism by which SirT1 modulates the effectiveness of the estrogen–IGF-1 signaling for mammary gland development. The estrogen–IGF-1 signaling is defined as the ovarian estrogen-regulated, stromal cell-produced local IGF-1 signal for stimulating mammary epithelial cells.
Materials and methods
A previously described SirT1 targeting construct, KOII , was used to generate mice harboring a conditional targeted mutation in the SirT1 gene (SirT1co/co mice) (see Additional file 1). The breeding of SirT1co/co mice and CMV-Cre transgenic mice results in mice harboring a germline-transmitted deletion of exon 4 of the SirT1 gene (SirT1+/ko mice). Both SirT1co/co mice and SirT1+/co mice were used to establish breeding colonies for generating SirT1co/co and SirT1ko/ko mice, respectively. Both SirT1co/co mice and SirT1ko/ko mice were in a mixed 129SvJ/C57B6 background. Mice were housed in a special-pathogen-free facility and all procedures were approved by the University of Washington Animal Care and Use Committee. A PCR-based genotyping method was established to identify the wild-type, co, and ko loci of the SirT1 gene using three primers: 5' co primer, 5'-GGTTGACTTAGGTCTTGTCTG; 5' ko primer, 5'-AGGCGGATTTCTGAGTTCGA; 3' primer, 5'-CGTCCCTTGTAATGTTTCCC. Murine embryonic fibroblasts (MEFs) were isolated from the embryos between embryonic day 12.5 and embryonic day 14.5. The body weight was measured once a week.
Serum insulin-like growth factor-1
A commercial radioimmunoassay (ALPCO Inc., Windham, NH, USA) was used. Samples (300 μl) were prepared via an alcohol extraction followed by a 2-day disequilibrium incubation at 4°C. Samples were measured in duplicate and data were calculated as the samples were counted. Six standards ranging from 10 to 500 ng/ml were used to generate the standard curve. The lower limit of detection was 10 ng.
To determine the sensitivity of embryonic stem cells after ionizing radiation or hydrogen peroxide treatment, 300 embryonic stem cells were seeded onto a 10-cm plate for 24 hours. The plates were either exposed to a 137Cs source at indicated doses or were treated with embryonic stem media containing hydrogen peroxide at indicated concentrations for 30 minutes. Seven days after irradiation or hydrogen peroxide treatment, the embryonic stem colonies stained with crystal violet were counted.
Estrogen pellets in the form of 17β-estradiol, at 0.1 mg, and the 21-day releasing time were obtained from Innovative Research (Sarasota, FL, USA). One pelletwas subcutaneously implanted in the subscapular region ofindividual female mice using a sterilizedmetal trochar (Innovative Research).
Whole mount analysis
The inguinoabdominal mammary fat pads were spread on microscope slides, fixed in Carnoy's fixative overnight, hydrated and stained with carmine alum stain (Sigma, St. Louis, MO, USA) overnight, and were then dehydrated, treated with xylene to remove fat, and mounted with Secure Mount (Fisher Scientific, Pittsburgh, PA, USA) and cover slips.
Histological, immunohistochemical, and immunofluorescence analyses
The mice were given a single dose of bromodeoxyurdine (BrdU) at 100 g/kg body weight via intraperitoneal injection 2 hours before euthanasia. The mammary fat pads, as well as other tissues, were collected and fixed in 10% formalin solution (Fisher Scientific). Paraffin-embedded sections were prepared at 4 μm thickness followed by standard H & E staining for histological analysis and by immunohistochemical staining for BrdU-labeled cells (Sigma) and apoptotic cells (Promega, Madison, WI, USA). To detect the presence of mouse milk proteins, a rabbit anti-mouse milk specific protein antibody (Nordic Immunology, Tillburg, The Netherlands) was used for immunofluorescent staining, followed by Texas Red anti-rabbit secondary antibody (Molecular Probes) and DAPI staining for nuclei (Molecular Probe, Eugene, OR, USA). Microscopic analyses of all histological findings were carried out on an AxioVert 200M microscope with AxioVision 4.5 software (Carl Zeiss, München-Hallbergmoos, Germany).
Protein extracts were prepared from mammary tissues as well as from MEFs. The following primary antibodies were used: anti-SirT1 (Upstate Biotechology, Lake Placid, NY, USA), anti-IGF-1 binding protein-1 (IGFBP-1) (R&D Systems, Minneapolis, MN, USA), and anti-actin and anti-IκBα (Santa Cruz Biotechnology, Santa Cruz, CA, USA). The corresponding secondary antibodies were horseradish peroxidase-conjugated anti-rabbit (Pierce, Rockford, IL, USA), anti-rat, and anti-goat (Santa Cruz Biotechnology).
Growth retardation in SirT1ko/komice
Perinatal lethality, fertility, and lactation defect in SirT1ko/ko mice
ko/ko (n = 3)
ko/ko (n = 5)
Both male and female SirT1ko/ko mice can be fertile, which is in contrast with the reported sterile phenotype in one strain of SirT1-deficient mice . When male SirT1ko/ko mice impregnated female SirT1+/ko mice, the number of surviving SirT1ko/ko offspring was reduced due to the partial perinatal lethal phenotype (Table 1). In a reciprocal approach, male SirT1+/ko mice can also impregnate female SirT1ko/ko mice (Table 1). After parturition, SirT1ko/ko mothers exhibited normal nursing behavior. The pups did not survive for more than 3 days after birth, however, unless they were immediately removed and put under the care of a foster mother. All pups died of dehydration or starvation as a result of lack of milk in their stomachs.
Pregnancy-induced mammary gland development
Normal alveolar morphogenesis takes place under the control of additional hormones and growth factors during pregnancy and lactation [17, 24, 25]. These coordinated efforts are necessary to support a densely saturated lobuloalveolar system for secreting milk after parturition (see wild-type mice on lactation day 1 in Figures 2b and 3a). In some SirT1ko/ko mice, the alveolar morphogenesis was initiated, as indicated by the presence of a few secretory alveolar epithelial cells within scattered alveolar structures sprouted from a well-established ductal branching network (Figures 2c and 3a, lower panel). The extent of alveolar morphogenesis in SirT1ko/ko mice was inadequate, however, as shown by the severe deficit in milk production when compared with that in wild-type mice (Figure 2c).
Exogenous estrogen-induced ductal morphogenesis
Concurrent mammary epithelial cell proliferation and differentiation
Apoptosis is a part of the self-renewal and remodeling processes in normal mammary tissues [17, 24, 25]. Apoptotic epithelial cells can be readily identified in both ductal and alveolar epithelium of SirT1ko/ko mice on lactation day 1 (Figure 5a, lower panel). The levels of apoptotic cells were significantly higher than those in wild-type mice on lactation day 1, but seemed to be closer to that in wild-type mice in mid-pregnancy (e.g. day 13 of pregnancy) (Figure 5c). The rush of proliferation and differentiation during pregnancy-rescued ductal development in SirT1ko/ko mice manifested multiple features on lactation day 1 that could reassemble some characteristic features of wild-type mice in mid-pregnancy. Successive pregnancies, however, failed to improve the survival rate of either SirT1+/ko pups or SirT1ko/ko pups (three or four pregnancies in 3 months for each of three SirT1ko/ko females tested), indicating that the lactation defect persisted. A SirT1 deficiency therefore affected ductal morphogenesis in virgin mice, and lobuloalveolar proliferation in parous mice.
Deregulated negative feedback signals
The characterization of lactation failure has revealed two developmental defects in SirT1ko/ko female mice. The impeded ductal morphogenesis in virgin SirT1ko/ko mice bore some resemblance to the defective ductal morphogenesis seen in mice lacking GH, estrogen, or IGF-1, as well as in mice harboring mutations in the corresponding receptors, such as the GH receptor and ERα [17, 24–31]. These previous studies have established the notion that GH and estrogen synergistically stimulate ductal morphogenesis, which is mediated by stromal cell-produced local IGF-1 in mammary tissues. Impeded ductal morphogenesis in virgin SirT1ko/ko mice is therefore probably due to a deficit in IGF-1 signaling.
In addition to this early defect, pregnancy-induced mammary gland development in SirT1ko/ko mice appeared to be arrested at the beginning of lobuloalveolar development, which is reminiscent of the phenotype in mice lacking IκB kinase alpha (IKKα) activity for NF-κB activation . IKKα-deficient mice exhibit normal ductal morphogenesis at the onset of puberty and exhibit normal ductal side branching during early pregnancy. Owing to the absence of NF-κB-dependent cyclin D1 expression, however, IKKα-deficient mice display defective alveolar epithelial cell proliferation and fail to lactate . While SirT1 negatively regulates the transcription activity of FoxOs and NF-κB in mammalian cells [10, 11, 20], SirT1ko/ko mice displayed the loss-of-function phenotypes. We hypothesized that SirT1 deficiency deregulates the expression of negative feedback signals and thereby desensitizes the cells to various types of stimulation. Indeed, among the many FoxOs and NF-κB-regulated genes, IGFBP-1 and IκBα encode negative feedback signals for IGF-1 signaling and NF-κB activation, respectively. We found that the basal levels of both IGFBP-1 and IκBα were increased in adipose tissues from virgin SirT1ko/ko mice and in ductal epithelial cells from SirT1ko/ko mice on lactation day 1 (Figure 3b). Meanwhile, the expression pattern of IGFBP-1 and IκBα in wild-type mammary tissues, shown in Figure 3b, was in agreement with the findings of others [16, 38, 39]. Namely, the expression of IGFBP-1 was downregulated during pregnancy, whereas NF-κB activity was increased at a late stage of pregnancy. The loss of SirT1 therefore increased the basal level of both IGFBP-1 and IκBα in multilineages of cells, and potentially increased thresholds for activating SirT1-modulated signaling pathways.
SirT1 deficiency did not affect the ability of mammary epithelial progenitor cells to differentiate into functional alveolar epithelial cells despite the lactation failure (Figure 2c). SirT1 deficiency may therefore alter the homeostasis of the signals for ductal morphogenesis and/or the cellular response to the signals. Increased expression of both IGFBP-1 and IκBα in adipose tissues and mammary epithelial cells could simply reflect a global deregulation of gene expression in SirT1ko/ko cells in which the activity of both FoxOs and NF-κB is increased. IGFBP-1 is a potent inhibitor of IGF-1 in vivo , suggesting that the elevated basal level of IGFBP-1 in adipose tissues in SirT1ko/ko mice could reduce the effect of IGF-1. Moreover, deregulated IGFBP-1 expression, but not deregulated IκBα expression, was reversed in response to increased levels of estrogen in the mammary fat pads after implantation (Figure 4b). The finding is relevant to the fact that the expression of IGFBP-1 is downregulated as the level of estrogen increases in pregnant wild-type mice (Figure 3b). Increased levels of estrogen therefore stimulate the production of stromal cell-derived local IGF-1, which overcomes the IGFBP-1 barrier in SirT1ko/ko mice and, ultimately, reverses the increased levels of IGFBP-1 in mammary tissues.
Our current system might not be suitable for studying the effects of SirT1 deficiency on NF-κB activation in vivo. NF-κB signaling exhibits distinct biological responses because the expression of IκBα depends on NF-κB activation while the other isoforms of IκB (β and ε) are independent of NF-κB feedback . For example, TNFα stimulation induces dampened oscillatory behavior after the first hour, whereas lipopolysacharide treatment leads to stable NF-κB activation [42, 43]. The NF-κB signaling in the developing mammary gland is under the temporal control of RANK signaling and IKKα activation . The finding of increased levels of newly synthesized IκBα within the first hour indicated that SirT1 deficiency could disrupt the temporal control of NF-κB signaling to all types of stimulation (Figure 6). It remains unknown, however, whether the reduced IGF-1 signaling would affect the efficacy of RANK signaling and IKKα activation in developing mammary glands. The fact that individual SirT1ko/ko mice manifested varying degrees of ductal morphogenesis (Figure 3a) makes it difficult to dissect the potential compound effect of SirT1 deficiency on both upstream IKKα activation and downstream NF-κB signaling. Other experimental systems, such as mammary epithelial cell-specific SirT1ko/ko female mice, may be used to address this issue.
Our study of SirT1ko/ko mice has unveiled a regulatory mechanism by which SirT1 modulates the efficacy of estrogen-stimulated local IGF-1 signaling for ductal morphogenesis. SirT1 deficiency alters the homeostasis of gene expression and attenuates the efficacy of IGF-1 signaling. As a result, SirT1ko/ko mice manifest partial perinatal lethality and lactation failure phenotypes, implicating the role of evolutionarily conserved SirT1 in both postnatal survival and offspring survival of mammals.
SirT1 balances the homeostasis of IGF-1 and IGFBP-1 in vivo
IGFBP-1 is one of six IGF-1 binding proteins, but is the only one that can be negatively regulated by insulin . IGF-1 binding proteins inhibit the action of local IGF-1 in a spatial and temporal manner [40, 49]. In mammalian cells, FoxO3 can directly bind to the promoter of the IGFBP-1 gene and activate the transcription [50–52]. SirT1 acts to negatively regulate the activity of nuclear FoxO3 and to decrease the expression of IGFBP-1 when the IGF-1 signal is low (Figure 7a). Increased levels of IGFBP-1 can diminish the action of IGF-1 signals in vivo. In that way, SirT1 balances the homeostasis of IGF-1 and IGFBP-1 for varying levels of IGF-1 signals.
The homeostasis of IGF-1 and IGFBP-1 clearly affects fertility, as shown by the infertility phenotype in both IGF-1-deficient mice and IGFBP-1 transgenic mice [28, 40, 49]. The serum level of estrogen in IGF-1-deficient female mice appears significantly reduced, whereas in ERα-deficient mice, which are also infertile, there is a threefold increase . In contrast, SirT1ko/ko mice can be fertile despite the fact that the fertility was reduced as compared with that of wild-type females (Table 1). Although our preliminary assessment indicated that the serum level of estrogen in SirT1ko/ko female mice was not overtly altered, it remains possible that a subtle irregularity or an individual variation of the estrogen level or ERα expression might be attributed to the reduced fertility. An alternative explanation for reduced fertility is that SirT1 deficiency deregulates the activity of FoxO3 and manifests an opposite effect of FoxO3 deficiency. FoxO3-deficient mice exhibit early depletion of ovarian follicles, which is reminiscent of premature ovarian failure in women [47, 54]. Premature ovarian failure manifests as early cessation of ovarian function and as premature menopause caused by genetic mutations, chemo/radiation therapy, or other unknown factors. It is possible that SirT1 deficiency counterbalances normal release of ovarian follicles and reduces the fertility in mice.
SirT1 modulates the efficacy of the IGF-1 signal for ductal morphogenesis
We hypothesize that, in virgin SirT1ko/ko mice, increased levels of IGFBP-1 in adipose tissues effectively diminish IGF-1 signals, including the estrogen–IGF-1 signaling, and halt the differentiation of mammary epithelial progenitor cells (Figure 7b). Several results of this study provided support for this hypothesis. First, the expression of IGFBP-1 is increased in mammary fat pads from virgin SirT1ko/ko mice, in which there is no evidence of any duct (Figures 2a and 3a). Second, both pregnancy and estrogen implantation can rescue impeded ductal morphogenesis in SirT1ko/ko mice (Figures 3a and 4a), demonstrating that increased levels of estrogen can enhance the production of local IGF-1. Moreover, the varying degrees of rescue among individual mice, as well as individual mammary fat pads of the same mouse, also indicated that local IGF-1 must be halted in SirT1ko/ko mice (Figure 3a, and data not shown). Finally, increased levels of estrogen specifically activate the IGF-1 signaling in mammary tissues, as shown by the reversal of IGFBP-1 expression in SirT1ko/ko mice after estrogen implantation (Figure 4b). These results indicate that a gradient of IGF-1 signals, including both circulating and local IGF-1, may regulate mammary gland development at embryonic, postnatal, and reproductive stages, and that increased expression of IGFBP-1 decreases the efficacy of IGF-1 signals.
Mammary development may undergo estrogen/IGF-1-independent phases and estrogen/IGF-1-dependent phases, which encompasses a late stage of embryonic development to the onset of puberty and involves maternal IGF-1, circulating IGF-1, and estrogen-stimulated local IGF-1. By embryonic day 16.5, mammary mesenchymes are connected to primitive fat pads and begin to proliferate, which results in rudimentary ducts [17, 25, 55, 56]. We propose that this process marks the transition from estrogen/IGF-1-independent mammary stem cells to IGF-1-dependent epithelial progenitor cells, and that maternal IGF-1 is sufficient to stimulate the differentiation of epithelial progenitor cells to estrogen-independent ductal epithelial cells but not estrogen-dependent epithelial cells (Figure 7b). This explains why the development of rudimentary ducts is normal in either IGF-1 or ERα knockout mice and that TEBs form and regress immediately before and after birth, respectively [17, 26, 27]. During the prepubertal period, the rudimentary ducts grow isometrically under the influence of circulating IGF-1. Like maternal IGF-1, circulating IGF-1 is not sufficient to induce the differentiation of estrogen-independent ductal epithelial cells into estrogen-dependent epithelial cells, which will express ERα and become estrogen dependent (Figure 7b). At the onset of puberty, ovarian estrogen, in synergy with pituitary GH, causes a surge in the production of stromal cell-derived local IGF-1 and stimulates the differentiation of estrogen-independent ductal epithelial cells to estrogen-dependent epithelial cells, which begins postnatal development of mammary gland (Figure 7b). Impeded ductal morphogenesis results in virgin SirT1ko/ko mice because local IGF-1 is apparently not sufficient to overcome increased IGFBP-1 in mammary adipose tissues. SirT1 therefore positively regulates the efficacy of the estrogen–IGF-1 signaling for ductal epithelial cell proliferation and differentiation.
The potential effect of modulating SirT1 activity
Breast cancer results from the accumulation of inherited and/or somatic mutations. Lifetime exposure to estrogen is a major risk factor for breast cancer, and a number of lifestyle factors, such as diet, body fat, alcohol consumption, exercise, parity, and hormone replacement therapy, may influence a woman's exposure to estrogen. The compound personal risk to a woman is difficult to assess because the molecular link between these factors and the effect of estrogen is poorly understood. The result of this study demonstrated that SirT1 positively modulates the efficacy of the estrogen–IGF-1 signal. This suggests that if SirT1 serves as a lifetime sensor to dietary restriction and acute withdrawal of nutrients [14, 57], its activity could ultimately influence the risk of breast cancer. Interestingly, an epidemiological study of Dutch famine in 1945 indicated that an exposure to famine at prepubertal age increased the risk of breast cancer, most clearly seen in women who never gave birth . Moreover, the serum levels of estrogen and IGF-1 were increased among those exposed to the famine, which suggests that an epigenetic adaptation phenomenon may take place. Assessing the potential correlation between the enzymatic activity of SirT1 and the risk of breast cancer is therefore of great interest in order to identify feasible targets for chemoprevention against breast cancer.
Several dietary polyphenols as well as small molecules have been identified as either agonists or antagonists of SirT1 [59, 60]. The pharmacological effects of these compounds in humans remain elusive given that SirT1 targets multiple transcription factors and exerts pleiotropic effects. Nonetheless, our study of SirT1ko/ko mice demonstrated the potential utility of SirT1 antagonists. Specifically, reducing SirT1 activity alters the homeostasis of cellular responses, including deregulation of the expression of IGFBP-1 and IκBα, which can attenuate the stimulation from the estrogen–IGF-1 signaling and potentially desensitize mammary epithelial cells to NF-κB activation. SirT1 is therefore a candidate target for chemoprevention against breast cancer.
The study of mammary gland development is an excellent model system for unraveling how SirT1 modulates the efficacy of the estrogen–IGF-1 signaling and regulates the timing of ductal morphogenesis. These new mechanistic insights may also aid in understanding the role of SirT1 in breast cancer as well as in other organs in which local IGF-1 plays an important role.
estrogen receptor alpha
forkhead box 'other' protein
insulin-like growth factor-1
insulin-like growth factor-1 binding protein-1
- H & E:
hemotoxylin and eosin
insulin/insulin-like growth factor-1 signaling
inhibitors of NF-κB alpha subunit
IκB kinase alpha
murine embryonic fibroblast
polymerase chain reaction
receptor activator of NF-κB
silencing information regulator
terminal end bud
tumor necrosis factor.
The study is supported by grants from the American Cancer Society (RSG-04-019-01-CNE) and the Nathan Shock Center of Excellence in the Biology of Aging at the University of Washington (to YG). The authors thank Jennifer Blasi, Thomas Gernon, Janet Leath, Jennifer Lee, Bryce Sopher, Loraine Warner, and Heather-Marie Wilson for their contributions in the early phases of the study, and Dr Fred Alt, Dr Mary Helen Barcellos-Hoff, Dr Mark Groudine, Dr Warren Ladiges, Dr George Martin, Dr Peter Rabinovitch, and Dr Jeffrey Schwartz for their support and advice.
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