Epithelial cell cycling predicts p53 responsiveness to γ-irradiation during post-natal mammary gland development

An African-Specific Variant of TP53 Reveals PADI4 as a Regulator of p53-Mediated Tumor Suppression

TP53 is the most frequently mutated gene in cancer, yet key target genes for p53-mediated tumor suppression remain unidentified. Here, we characterize a rare, African-specific germline variant of TP53 in the DNA-binding domain Tyr107His (Y107H). Nuclear magnetic resonance and crystal structures reveal that Y107H is structurally similar to wild-type p53. Consistent with this, we find that Y107H can suppress tumor colony formation and is impaired for the transactivation of only a small subset of p53 target genes; this includes the epigenetic modifier PADI4, which deiminates arginine to the nonnatural amino acid citrulline. Surprisingly, we show that Y107H mice develop spontaneous cancers and metastases and that Y107H shows impaired tumor suppression in two other models. We show that PADI4 is itself tumor suppressive and that it requires an intact immune system for tumor suppression. We identify a p53-PADI4 gene signature that is predictive of survival and the efficacy of immune-checkpoint inhibitors.

SIGNIFICANCE

We analyze the African-centric Y107H hypomorphic variant and show that it confers increased cancer risk; we use Y107H in order to identify PADI4 as a key tumor-suppressive p53 target gene that contributes to an immune modulation signature and that is predictive of cancer survival and the success of immunotherapy. See related commentary by Bhatta and Cooks, p. 1518. This article is highlighted in the In This Issue feature, p. 1501.

Molecular and cellular basis of the dose-rate-dependent adverse effects of radiation exposure in animal models. Part I: Mammary gland and digestive tract

While epidemiological data are available for the dose and dose-rate effectiveness factor (DDREF) for human populations, animal models have contributed significantly to providing quantitative data with mechanistic insights. The aim of the current review is to compile both the in vitro experiments with reference to the dose-rate effects of DNA damage and repair, and the animal studies, specific to rodents, with reference to the dose-rate effects of cancer development. In particular, the review focuses especially on the results pertaining to underlying biological mechanisms and discusses their possible involvement in the process of radiation-induced carcinogenesis. Because the concept of adverse outcome pathway (AOP) together with the key events has been considered as a clue to estimate radiation risks at low doses and low dose-rates, the review scrutinized the dose-rate dependency of the key events related to carcinogenesis, which enables us to unify the underlying critical mechanisms to establish a connection between animal experimental studies with human epidemiological studies.

Regulation and coordination of the different DNA damage responses in Drosophila

Cells have evolved mechanisms that allow them to respond to DNA damage to preserve genomic integrity and maintain tissue homeostasis. These responses include the activation of the cell cycle checkpoints and the repair mechanisms or the induction of apoptosis that eventually will eliminate damaged cells. These “life” vs. “death” decisions differ depending on the cell type, stages of development, and the proliferation status of the cell. The apoptotic response after DNA damage is of special interest as defects in its induction could contribute to tumorigenesis or the resistance of cancer cells to therapeutic agents such as radiotherapy. Multiples studies have elucidated the molecular mechanisms that mediate the activation of the DNA damage response pathway (DDR) and specifically the role of p53. However, much less is known about how the different cellular responses such as cell proliferation control and apoptosis are coordinated to maintain tissue homeostasis. Another interesting question is how the differential apoptotic response to DNA damage is regulated in distinct cell types. The use of Drosophila melanogaster as a model organism has been fundamental to understand the molecular and cellular mechanisms triggered by genotoxic stress. Here, we review the current knowledge regarding the cellular responses to ionizing radiation as the cause of DNA damage with special attention to apoptosis in Drosophila: how these responses are regulated and coordinated in different cellular contexts and in different tissues. The existence of intrinsic mechanisms that might attenuate the apoptotic pathway in response to this sort of DNA damage may well be informative for the differences in the clinical responsiveness of tumor cells after radiation therapy.

Coordination between cell proliferation and apoptosis after DNA damage in Drosophila

Exposure to genotoxic stress promotes cell cycle arrest and DNA repair or apoptosis. These "life" or "death" cell fate decisions often rely on the activity of the tumor suppressor gene p53. Therefore, the precise regulation of p53 is essential to maintain tissue homeostasis and to prevent cancer development. However, how cell cycle progression has an impact on p53 cell fate decision-making is mostly unknown. In this work, we demonstrate that Drosophila p53 proapoptotic activity can be impacted by the G2/M kinase Cdk1. We find that cell cycle arrested or endocycle-induced cells are refractory to ionizing radiation-induced apoptosis. We show that p53 binding to the regulatory elements of the proapoptotic genes and its ability to activate their expression is compromised in experimentally arrested cells. Our results indicate that p53 genetically and physically interacts with Cdk1 and that p53 proapoptotic role is regulated by the cell cycle status of the cell. We propose a model in which cell cycle progression and p53 proapoptotic activity are molecularly connected to coordinate the appropriate response after DNA damage.

Differential contextual responses of normal human breast epithelium to ionizing radiation in a mouse xenograft model

Radiotherapy is a key treatment option for breast cancer, yet the molecular responses of normal human breast epithelial cells to ionizing radiation are unclear. A murine subcutaneous xenograft model was developed in which nonneoplastic human breast tissue was maintained with the preservation of normal tissue architecture, allowing us to study for the first time the radiation response of normal human breast tissue in situ. Ionizing radiation induced dose-dependent p53 stabilization and p53 phosphorylation, together with the induction of p21(CDKN1A) and apoptosis of normal breast epithelium. Although p53 was stabilized in both luminal and basal cells, induction of Ser392-phosphorylated p53 and p21 was higher in basal cells and varied along the length of the ductal system. Basal breast epithelial cells expressed ΔNp63, which was unchanged on irradiation. Although stromal responses themselves were minimal, the response of normal breast epithelium to ionizing radiation differed according to the stromal setting. We also demonstrated a dose-dependent induction of γ-H2AX foci in epithelial cells that was similarly dependent on the stromal environment and differed between basal and luminal epithelial cells. The intrinsic differences between human mammary cell types in response to in vivo irradiation are consistent with clinical observation that therapeutic ionizing radiation is associated with the development of basal-type breast carcinomas. Furthermore, there may be clinically important stromal-epithelial interactions that influence DNA damage responses in the normal breast. These findings demonstrate highly complex responses of normal human breast epithelium following ionizing radiation exposure and emphasize the importance of studying whole-tissue effects rather than single-cell systems.

Estrogens, regulation of p53 and breast cancer risk: a balancing act

The paradoxical effects of ovarian hormones in both the promotion and prevention of breast cancer have been debated for over 30 years. Genetic studies have demonstrated that ovarian hormones act through NF-kappaB to stimulate proliferation and ductal elongation, whereas the p53 tumor suppressor protein plays a central role in rendering the mammary epithelium resistant to tumorigenesis. Transcriptional profiles now suggest that ovarian hormones stimulate a constellation of genes that interact with NF-kappaB and p53 to arbitrate the competing demands for proliferation and surveillance. Genes that participate in chromatin remodeling are among the acute transcriptional responses to estrogens and progestins. These genes are proposed to initiate epigenetic programs that influence the balance between proliferation and surveillance, and render the breast epithelium resistant to tumors.

Transcriptional responses to estrogen and progesterone in mammary gland identify networks regulating p53 activity

Estrogen and progestins are essential for mammary growth and differentiation but also enhance the activity of the p53 tumor suppressor protein in the mammary epithelium. However, the pathways by which these hormones regulate p53 activity are unknown. Microarrays were used to profile the transcriptional changes within the mammary gland after administration of either vehicle, 17beta-estradiol (E), or progesterone (P) individually and combined (EP). Treatment with EP yielded 1182 unique genes that were differentially expressed compared to the vehicle-treated group. Although 30% of genes were responsive to either E or P individually, combined treatment with both EP had a synergistic effect accounting for 60% of the differentially regulated genes. Analysis of protein-protein interactions identified p53, RelA, Snw1, and Igfals as common targets of genes regulated by EP. RelA and p53 form hubs within a network connected by genes that are regulated by EP and that may coordinate the competing functions of RelA and p53 in proliferation and survival of cells. Induction of early growth response 1 (Egr1) and Stratifin (Sfn) (also known as 14-3-3sigma) by EP was confirmed by reverse transcription-quantitative PCR and shown to be p53 independent. In luciferase reporter assays, Egr1 was shown to enhance transcriptional activation by p53 and inhibit nuclear factor kappaB activity. These results identify a gene expression network that provides redundant activation of RelA to support proliferation as well as sensitize p53 to ensure proper surveillance and integration of their competing functions through factors such as Egr1, which both enhance p53 and inhibit RelA.

Insulin-like growth factor-I inhibits growth regulatory responses engaged by estrogen and progesterone in the mouse mammary gland

The pathways and key players that regulate parity-induced protection against breast cancer, conferred by estrogen (E) and progesterone (P), have not fully been explained. Interestingly, in rodents, high levels of circulating insulin-like growth factor-I (IGF-I) appear to block this parity-induced protection. Using an in-vitro mouse mammary gland whole organ culture system, we investigated the mechanisms by which IGF-I affects the protective effects of E+P. Earlier studies have demonstrated that important changes induced by parity include both the enhanced sensitivity to p53 activation, and the cell death that occurs in response to DNA damage. Remarkably, when IGF-I was added to cultures containing E+P and irradiated, both cell death and expression of the tumor suppressor protein p53 were inhibited (P<0.03 and <0.006, respectively). In addition, quantitative real-time PCR analysis of numerous genes identified by microarray as being upregulated in response to prolonged exposure to pregnancy levels of E+P suggested that a subset was affected by the addition of IGF-I. IGF-I suppressed the hormone-induced expression of insulin-like growth factor-binding protein-3, transforming growth factor beta-1, and cellular retinol-binding protein-1 (P<0.01 for all genes); all of which are genes that are important in growth suppression and the regulation of survival of cells. Overall, the observed reductions in p53 sensitivity and hormone-induced gene expression in response to IGF-I in vitro suggest a mechanism for the blockade in parity-induced protection previously seen in response to IGF-I in vivo.

Estrogen and progesterone induce persistent increases in p53-dependent apoptosis and suppress mammary tumors in BALB/c-Trp53+/- mice

Introduction

Treatment with estrogen and progesterone (E+P) mimics the protective effect of parity on mammary tumors in rodents and depends upon the activity of p53. The following experiments tested whether exogenous E+P primes p53 to be more responsive to DNA damage and whether these pathways confer resistance to mammary tumors in a mouse model of Li-Fraumeni syndrome.

Methods

Mice that differ in p53 status (Trp53^+/+, Trp53^+/-, Trp53^-/-) were treated with E+P for 14 days and then were tested for p53-dependent responses to ionizing radiation. Responses were also examined in parous and age-matched virgins. The effects of hormonal exposures on tumor incidence were examined in BALB/c-Trp53^+/- mammary tissues.

Results

Nuclear accumulation of p53 and apoptotic responses were increased similarly in the mammary epithelium from E+P-treated and parous mice compared with placebo and age-matched virgins. This effect was sustained for at least 7 weeks after E+P treatment and did not depend on the continued presence of ovarian hormones. Hormone stimulation also enhanced apoptotic responses to ionizing radiation in BALB/c-Trp53^+/- mice but these responses were intermediate compared with Trp53^+/+ and Trp^-/- tissues, indicating haploinsufficiency. The appearance of spontaneous mammary tumors was delayed by parity in BALB/c-Trp53^+/- mice. The majority of tumors lacked estrogen receptor (ER), but ER⁺ tumors were observed in both nulliparous and parous mice. However, apoptotic responses to ionizing radiation and tumor incidence did not differ among outgrowths of epithelial transplants from E+P-treated donors and nulliparous donors.

Conclusion

Therefore, E+P and parity confer a sustained increase in p53-mediated apoptosis within the mammary epithelium and suppress mammary tumorigenesis, but this effect was not retained in epithelial outgrowths.

Altered mammary gland development in the p53+/m mouse, a model of accelerated aging

The tumor suppressor p53 is important for inhibiting the development of breast carcinomas. However, little is known about the effects of increased p53 activity on mammary gland development. Therefore, the effect of p53 dosage on mammary gland development was examined by utilizing the p53+/m mouse, a p53 mutant which exhibits increased wild-type p53 activity, increased tumor resistance, a shortened longevity, and a variety of accelerated aging phenotypes. Here we report that p53+/m virgin mice exhibit a defect in mammary gland ductal morphogenesis. Transplants of mammary epithelium into p53+/m recipient mice demonstrate decreased outgrowth of wild-type and p53+/m donor epithelium, suggesting systemic or stromal alterations in the p53+/m mouse. Supporting these data, p53+/m mice display decreased levels of serum IGF-1 and reduced IGF-1 signaling in virgin glands. The induction of pregnancy or treatment of p53+/m mice with estrogen, progesterone, estrogen and progesterone in combination, or IGF-1 stimulates ductal outgrowth, rescuing the p53+/m mammary phenotype. Serial mammary epithelium transplants demonstrate that p53+/m epithelium exhibits decreased transplant capabilities, suggesting early stem cell exhaustion. These data indicate that appropriate levels of p53 activity are important in regulating mammary gland ductal morphogenesis, in part through regulation of the IGF-1 pathway.

Mutant Brca2/p53 mice exhibit altered radiation responses in the developing mammary gland

Appropriate balance between proliferation and apoptosis is critical for mammary gland development and is often altered during tumorigenesis. Carcinogens like radiation induce DNA damage and activate protective responses such as cell cycle arrest and apoptosis. We used mice carrying Brca2(-/-) and/or p53(-/-) mutations to evaluate the individual and combined effects of these genes on cell proliferation and apoptosis in the developing mammary gland. Mice were exposed to 5Gy of radiation or chamber exposure (controls) followed by injection with BrdU. Mammary glands were collected 6 h post-radiation exposure and evaluated for proliferation (BrdU) and apoptosis (TUNEL) in terminal end buds (TEB) and ducts. Under control conditions, the Brca2 mutation reduced proliferation and apoptosis in TEB but not ducts, whereas the p53 mutation reduced apoptosis in TEB and ducts but did not influence proliferation. Despite these alterations in proliferation and/or apoptosis, neither mutation, either individually or combined, significantly altered the overall balance between the two as measured by the proliferation to apoptosis ratio (growth index). Following irradiation, the Brca2 mutation had no significant effect on proliferation or apoptosis, whereas the p53 mutation resulted in reduced apoptosis in TEB and ducts but did not significantly influence proliferation. Neither mutation by itself altered the growth index in the TEB after irradiation although combined Brca2/p53 mutation caused significantly increased proliferation, reduced apoptosis, and an elevated growth index in TEB and ducts. These results reveal both independent and collaborative growth regulatory roles for Brca2 and p53 under normal and adverse environmental conditions. Additionally, we demonstrate the importance of gene-environment interactions by showing that Brca2- and p53-deficient mice can compensate for their genetic deficiencies under control conditions but not after exposure to radiation. We also demonstrate distinct spatial differences in the cellular functions of Brca2 and p53 and show that combined mutation of both genes is more detrimental than loss of either gene alone.

Estrogen and progesterone regulate radiation-induced p53 activity in mammary epithelium through TGF-beta-dependent pathways

DNA damage normally induces p53 activity, but responses to ionizing radiation in the mammary epithelium vary among developmental stages. The following studies examined the hormones and growth factors that regulate radiation-responsiveness of p53 in mouse mammary epithelium. Immunoreactive p21/WAF1 and TUNEL staining were used as indicators of p53 activity following exposure to ionizing radiation. In ovariectomized mice, radiation-induced accumulation of p21/WAF1 was minimal in the mammary epithelial cells (<1%). Systemic injections of estrogen and progesterone (E+P) for 72 h were necessary to recover maximal expression of p21/WAF1 following ionizing radiation (55%). The effects of E+P on radiation-induced p21/WAF1 were p53-dependent as responses were absent in Trp53-/- mice. Though hormonal treatments stimulated increases in the proportion of cycling cells (PCNA-positive), this was not directly correlated with p53 activity. Whole organ cultures were used to determine whether E+P act directly upon the mammary gland. Treatment with E+P was sufficient to render p53 responsive to radiation, but TGF-beta-neutralizing antibodies blocked responsiveness. In the absence of E+P, TGF-beta1 alone did not alter p53 activity. These results demonstrate that estrogen and progesterone together with TGF-beta signaling are necessary for maintenance of p53 activity in the mammary epithelium.

Sensitivity to DNA damage is a common component of hormone-based strategies for protection of the mammary gland

An early full-term pregnancy significantly reduces the risk of getting breast cancer in women. In animals, this protection can be mimicked by a short-term exposure to physiologic doses of estrogen plus progesterone. Sensitization of p53 and up-regulation of transforming growth factor beta are believed to be important aspects of the mechanism by which protection is imparted. Little is known, however, about the use of this pathway in response to other chemopreventive agents. In this article, we investigated the ability of retinoids, such as 9-cis retinoic acid, all-trans retinoic acid, and N-4-hydroxyphenylretinamide (4-HPR), to sensitize the ductal epithelial cells of virgin mammary glands to DNA damage responses. Using a whole-organ culture system, we observed enhanced cell death in response to gamma-irradiation in the virgin tissues treated with retinoids for 72 hours. These retinoids were partially dependent on p53 and transforming growth factor beta to exert their radiosensitizing effects. However, 4-HPR seemed to sensitize other cells or activate these pathways in a different manner as costimulation with ovarian hormones and 4-HPR was additive, whereas coculture of ovarian hormones and the natural retinoids did not increase amount of death. Taken together, these data suggest that sensitization of the mammary epithelium to p53-dependent apoptosis is a common pathway, which is engaged by retinoids as well as ovarian hormones.

Pregnancy-induced changes in cell-fate in the mammary gland

The protective effect of an early full-term pregnancy is a well established phenomenon; in contrast, the molecular and cell-specific mechanisms that govern parity-specific changes in the mammary gland have not been well described. Recent studies signify a dramatic advance in our understanding of this phenomenon, and indicate a 'cell fate' model for parity-related changes that lead to protection against breast cancer.