Molecular and functional characteristics of ovarian surface epithelial cells transformed by KrasG12D and loss of Pten in a mouse model in vivo

Selenium-binding protein 1: its physiological function, dependence on aryl hydrocarbon receptors, and role in wasting syndrome by 2,3,7,8-tetrachlorodibenzo-p-dioxin

BACKGROUND

Selenium-binding protein 1 (Selenbp1) is suggested to play a role in tumor suppression, and may be involved in the toxicity produced by dioxin, an activator of aryl hydrocarbon receptors (AhR). However, the mechanism or likelihood is largely unknown because of the limited information available about the physiological role of Selenbp1.

METHODS

To address this issue, we generated Selenbp1-null [Selenbp1 (-/-)] mice, and examined the toxic effect of 2,3,7,8-tetrachlorodibenzo-p-dioxin (TCDD) in this mouse model.

RESULTS

Selenbp1 (-/-) mice exhibited only a few differences from wild-type mice in their apparent phenotypes. However, a DNA microarray experiment showed that many genes including Notch1 and Cdk1, which are known to be enhanced in ovarian carcinoma, are also increased in the ovaries of Selenbp1 (-/-) mice. Based on the different responses to TCDD between C57BL/6J and DBA/2J strains of mice, the expression of Selenbp1 is suggested to be under the control of AhR. However, wasting syndrome by TCDD occurred equally in Selenbp1 (-/-) and (+/+) mice.

CONCLUSIONS

The above pieces of evidence suggest that 1) Selenbp1 suppresses the expression of tumor-promoting genes although a reduction in Selenbp1 alone is not very serious as far as the animals are concerned; and 2) Selenbp1 induction by TCDD is neither a pre-requisite for toxicity nor a protective response for combating TCDD toxicity.

GENERAL SIGNIFICANCE

Selenbp1 (-/-) mice exhibit little difference in their apparent phenotype and responsiveness to dioxin compared with the wild-type. This may be due to the compensation of Selenbp1 function by a closely-related protein, Selenbp2.

Early transformative changes in normal ovarian surface epithelium induced by oxidative stress require Akt upregulation, DNA damage and epithelial-stromal interaction

Ovarian cancer is the deadliest gynecological malignancy due to detection of cancer at a late stage when the disease has metastasized. One likely progenitor cell type of ovarian cancer is the ovarian surface epithelium (OSE), which proliferates rapidly in the presence of inflammatory cytokines and oxidative stress following ovulation. To determine whether oxidative stress induces DNA damage leading to spontaneous transformative changes in normal OSE, an immortalized mouse OSE cell line (MOSE cells) or normal mouse ovarian organoids were treated with hydrogen peroxide (H2O2) and loss of contact inhibition was assessed by soft agar assay. In response to H2O2, OSE cells grown in 3D exhibited growth in soft agar but MOSE cells grown on 2D plastic did not, indicating a critical role for epithelial-stromal interactions in neoplastic initiation. Loss of contact inhibition in response to H2O2 correlated with an increase in proliferation, DNA damage and upregulation of the oncogene Akt1. Use of a reactive oxygen species scavenger or Akt inhibitor blocked H2O2-induced proliferation and growth in soft agar. Although parental MOSE cells did not undergo transformation by H2O2, MOSE cells stably overexpressing constitutively active myristoylated Akt or knockdown of phosphatase and tensin homolog (PTEN) exhibited loss of contact inhibition and increased proliferation. This study indicates that normal OSE undergo transformative changes induced by oxidative stress and that this process requires Akt upregulation and activation. A 3D model that retains tissue architecture is critical for studying this process and may lead to development of new intervention strategies directed at early stages of ovarian cancer.

Tumor repressor protein 53 and steroid hormones provide a new paradigm for ovarian cancer metastases

The functional status of the tumor repressor protein (TP53 or TRP53) is a defining feature of ovarian cancer. Mutant or null alleles of TP53 are expressed in greater than 90% of all high-grade serous adenocarcinomas. Wild-type TP53 is elevated in low-grade serous adenocarcinomas in women and in our Pten;Kras;Amhr2-Cre mutant mouse model. Disruption of the Trp53 gene in this mouse model did not lead to high-grade ovarian cancer but did increase expression of estrogen receptor α (ESR1) and markedly enhanced the responsiveness of these cells to estrogen. Specifically, when Trp53-positive and Trp53 null mutant mice were treated with estradiol or vehicle, only the Trp53 null and Esr1-positive tumors respond vigorously to estradiol in vivo and exhibit features characteristic of high-grade type ovarian cancer: invasive growth into the ovarian stroma, rampant metastases to the peritoneal cavity, and nuclear atypia. Estrogen promoted and progesterone suppressed the growth of Trp53 null ovarian tumors and tumor cells injected ip, sc, or when grown in matrigel. Exposure of the Trp53 depleted cells to estrogen also has a profound impact on the tumor microenvironment and immune-related events. These results led to the new paradigm that TRP53 status is related to the susceptibility of transformed ovarian surface epithelial cells to estradiol-induced metastases and nuclear atypia via increased levels of estradiol receptor α.

PTEN loss and HOXA10 expression are associated with ovarian endometrioid adenocarcinoma differentiation and progression

Epithelial ovarian cancer is a heterogeneous disease that is subdivided into five major histotypes but the mechanisms driving their differentiation are not clear. Mutations in adenomatous polyposis coli (APC) and β-catenin are commonly observed in the human ovarian endometrioid adenocarcinoma (OEA) patients. However, the mechanisms subsequent to APC deletion in ovarian tumorigenesis have not been well characterized. We have conditionally deleted APC in the murine ovarian surface epithelium (OSE) and showed that its loss leads to development of epithelial inclusion cysts. High-grade OEAs with tightly packed villoglandular histology were observed in older APC-deleted mice. Phosphatase and tensin homolog (PTEN) expression was elevated in the early lesions but lost after progression to the more advanced tumors. Knockdown of APC or expression of a gain-of-function β-catenin similarly induced human OSE cells to develop tumors with endometrioid histology in xenografts. Expression of HOXA10 was induced in both the advanced APC-deleted murine tumors and in the tumor xenografts of human OSE cells with knocked-down APC. These results show that reduced APC activity is sufficient to induce formation of epithelial inclusion cysts and support OEA development and suggest that induced HOXA10 expression and loss of PTEN are key mechanisms driving endometrioid histotype differentiation and progression.

Immunobiology of human mucin 1 in a preclinical ovarian tumor model

Epithelial ovarian cancer is an aggressive malignancy, with a low 5-year median survival. Continued improvement on the development of more effective therapies depends in part on the availability of adequate preclinical models for in vivo testing of treatment efficacy. Mucin 1 (MUC1) glycoprotein is a tumor-associated antigen overexpressed in ovarian cancer cells, making it a potential target for immune therapy. To create a preclinical mouse model for MUC1-positive ovarian tumors, we generated triple transgenic (Tg) mice that heterozygously express human MUC1(+/-) as a transgene, and carry the conditional K-rasG12D oncoallele (loxP-Stop-loxP-K-ras(G12D/+)) and the floxed Pten gene (Pten/(loxP/loxP)). Injection of Cre recombinase-encoding adenovirus (AdCre) in the ovarian bursa of triple (MUC1KrasPten) Tg mice triggers ovarian tumors that, in analogy to human ovarian cancer, express strongly elevated MUC1 levels. The tumors metastasize loco-regionally and are accompanied by high serum MUC1, closely mimicking the human disease. Compared with the KrasPten mice with tumors, the MUC1KrasPten mice show increased loco-regional metastasis and augmented accumulation of CD4+Foxp3+ immune-suppressive regulatory T cells. Vaccination of MUC1KrasPten mice with type 1 polarized dendritic cells (DC1) loaded with a MUC1 peptide (DC1-MUC1) can circumvent tumor-mediated immune suppression in the host, activate multiple immune effector genes and effectively prolong survival. Our studies report the first human MUC1-expressing, orthotopic ovarian tumor model, reveal novel MUC1 functions in ovarian cancer biology and demonstrate its suitability as a target for immune-based therapies.

Proteomic profiling of a mouse model for ovarian granulosa cell tumor identifies VCP as a highly sensitive serum tumor marker in several human cancers

The initial aim of this study was to identify novel serum diagnostic markers for the human ovarian granulosa cell tumor (GCT), a tumor that represents up to 5% of all ovarian cancers. To circumvent the paucity of human tissues available for analyses, we used the Ctnnb1^tm1Mmt/+;Pten^{tm1Hwu/tmiHwu};Amhr2^{tm3(cre)Bhr/+} transgenic mouse model, which features the constitutive activation of CTNNB1 signaling combined with the loss of Pten in granulosa cells and develops GCTs that mimic aggressive forms of the human disease. Proteomic profiling by mass spectrometry showed that vinculin, enolase 1, several heat shock proteins, and valosin containing protein (VCP) were more abundantly secreted by cultured mouse GCT cells compared to primary cultured GC. Among these proteins, only VCP was present in significantly increased levels in the preoperative serum of GCT cancer patients compared to normal subjects. To determine the specificity of VCP, serum levels were also measured in ovarian carcinoma, non-Hodgkin's lymphoma and breast, colon, pancreatic, lung, and prostate cancer patients. Increased serum VCP levels were observed in the majority of cancer cases, with the exception of patients with lung or prostate cancer. Moreover, serum VCP levels were increased in some GCT, ovarian carcinoma, breast cancer, and colon cancer patients who did not otherwise display increased levels of widely used serum tumor markers for their cancer type (e.g. inhibin A, inhibin B, CA125, CEA, or CA15.3). These results demonstrate the potential use of VCP as highly sensitive serum marker for GCT as well as several other human cancers.

Consequences of RAS and MAPK activation in the ovary: the good, the bad and the ugly

This review summarizes studies providing evidence (1) that endogenous RAS activation regulates important physiological events during ovulation and luteinization (2) that expression of the mutant, active KRAS(G12D) in granulosa cells in vivo causes abnormal follicle growth arrest leading to premature ovarian failure and (3) that KRAS(G12D) expression in ovarian surface epithelial (OSE) cells renders them susceptible to the pathological outcome of transformation and tumor formation. These diverse effects of RAS highlight how critical its activation is linked to cell- and stage-specific events in the ovary that control normal processes and that can also lead to altered granulosa cell and OSE cell fates.

Wild-type tumor repressor protein 53 (Trp53) promotes ovarian cancer cell survival

Loss of Pten in the Kras(G12D);Amhr2-Cre mutant mice leads to the transformation of ovarian surface epithelial (OSE) cells and rapid development of low-grade, invasive serous adenocarcinomas. Tumors occur with 100% penetrance and express elevated levels of wild-type tumor repressor protein 53 (TRP53). To test the functions of TRP53 in the Pten;Kras (Trp53+) mice, we disrupted the Trp53 gene yielding Pten;Kras(Trp53-) mice. By comparing morphology and gene expression profiles in the Trp53+ and Trp53- OSE cells from these mice, we document that wild-type TRP53 acts as a major promoter of OSE cell survival and differentiation: cells lacking Trp53 are transformed yet are less adherent, migratory, and invasive and exhibit a gene expression profile more like normal OSE cells. These results provide a new paradigm: wild-type TRP53 does not preferentially induce apoptotic or senescent related genes in the Pten;Kras(Trp53+) cancer cells but rather increases genes regulating DNA repair, cell cycle progression, and proliferation and decreases putative tumor suppressor genes. However, if TRP53 activity is forced higher by exposure to nutlin-3a (a mouse double minute-2 antagonist), TRP53 suppresses DNA repair genes and induces the expression of genes that control cell cycle arrest and apoptosis. Thus, in the Pten;Kras(Trp53+) mutant mouse OSE cells and likely in human TP53+ low-grade ovarian cancer cells, wild-type TRP53 controls global molecular changes that are dependent on its activation status. These results suggest that activation of TP53 may provide a promising new therapy for managing low-grade ovarian cancer and other cancers in humans in which wild-type TP53 is expressed.

Minireview: animal models and mechanisms of ovarian cancer development

Ovarian cancer in women is a complex and deadly disease, where the molecular events that initiate and control tumor formation remain poorly defined. Therefore, mouse models provide one approach for determining the mechanisms by which specific oncogenic factors cause ovarian surface epithelial cell and granulosa cell transformation. This minireview summarizes the phenotypes of current mouse models that have been generated and some of the underlying mechanisms they have provided.

The loss of Hoxa5 function causes estrous acyclicity and ovarian epithelial inclusion cysts

Hox genes encode transcription factors that play essential roles during embryo morphogenesis and organogenesis. Expression of several Hox members persists at the adult age, indicating a wide spectrum of action from embryonic to postnatal life. In the present study, we reported that in adult mice, the Hoxa5 gene shows a dynamic expression profile in the ovary that depends on the estrous cycle, the gestational status, and the age of the female, suggesting that Hoxa5 may have distinct physiological functions in the ovary. Consistent with a role for Hoxa5 in ovarian function, Hoxa5(-/-) nulliparous females exhibit precocious puberty and an early onset of estrous acyclicity. They show a prolonged estrous cycle with increased metestrus-diestrus length, a phenotype that worsens with age. Older mutant females also develop ovarian epithelial inclusion cysts reminiscent of human endosalpingiosis. Immunolabeling studies suggest that these cysts originate from the ovarian surface epithelium, a source of epithelial ovarian carcinomas. Staining of the Hoxa5(-/-) ovarian cysts by the ovarian cancer markers paired box gene 8 (PAX8) and Wilms' tumor 1 (WT1) further strengthens the notion that these cysts may constitute preneoplastic lesions. Moreover, the deregulation of the estrous cycle and the presence of ovarian epithelial cysts in Hoxa5(-/-) older females correlate with a reduced expression of specific epidermal growth factor receptor signaling components, namely Egfr, Areg, and Btc. Altogether, our data unveil that Hoxa5, a stroma-specific gene, plays a significant role in ovarian biology and may be involved in ovarian cancer predisposition.

High-grade serous ovarian cancer arises from fallopian tube in a mouse model

Although ovarian cancer is the most lethal gynecologic malignancy in women, little is known about how the cancer initiates and metastasizes. In the last decade, new evidence has challenged the dogma that the ovary is the main source of this cancer. The fallopian tube has been proposed instead as the primary origin of high-grade serous ovarian cancer, the subtype causing 70% of ovarian cancer deaths. By conditionally deleting Dicer, an essential gene for microRNA synthesis, and Pten, a key negative regulator of the PI3K pathway, we show that high-grade serous carcinomas arise from the fallopian tube in mice. In these Dicer-Pten double-knockout mice, primary fallopian tube tumors spread to engulf the ovary and then aggressively metastasize throughout the abdominal cavity, causing ascites and killing 100% of the mice by 13 mo. Besides the clinical resemblance to human serous cancers, these fallopian tube cancers highly express genes that are known to be up-regulated in human serous ovarian cancers, also demonstrating molecular similarities. Although ovariectomized mice continue to develop high-grade serous cancers, removal of the fallopian tube at an early age prevents cancer formation--confirming the fallopian tube origin of the cancer. Intriguingly, the primary carcinomas are first observed in the stroma of the fallopian tube, suggesting that these epithelial cancers have a mesenchymal origin. Thus, this mouse model demonstrates a paradigm for the origin and initiation of high-grade serous ovarian carcinomas, the most common and deadliest ovarian cancer.

Evaluating the progenitor cells of ovarian cancer: analysis of current animal models

Serous ovarian cancer is one of the most lethal gynecological malignancies. Progress on effective diagnostics and therapeutics for this disease are hampered by ambiguity as to the cellular origins of this histotype of ovarian cancer, as well as limited suitable animal models to analyze early stages of disease. In this report, we will review current animal models with respect to the two proposed progenitor cells for serous ovarian cancer, the ovarian surface epithelium and the fallopian tube epithelium.

The insulin-like growth factor 1 pathway is a potential therapeutic target for low-grade serous ovarian carcinoma

OBJECTIVE

To validate the overexpression of insulin-like growth factor 1 (IGF-1) and its receptor (IGF-1R) in low-grade serous ovarian carcinoma (SOC), and to investigate whether the IGF-1 pathway is a potential therapeutic target for low-grade SOC.

METHODS

Gene expression profiling was performed on serous borderline ovarian tumors (SBOTs) and low-grade SOC, and overexpression of IGF-1 in low-grade SOC was validated by RT-PCR and immunohistochemistry. The effect of exogenous IGF-1 on cell proliferation was determined in cell lines by cell proliferation assays, cell migration assays, and Western blot. Signaling pathways downstream of IGF-1 and the effects of the AKT inhibitor MK-2206 were investigated by Western blot analysis and by generating IGF-1R short hairpin RNA stable knockdown cell lines. Low- and high-grade cell lines were treated with the dual IGF-1R- and insulin receptor-directed tyrosine kinase inhibitor OSI-906, and cellular proliferation was measured.

RESULTS

mRNA analysis and immunostaining revealed significantly higher IGF-1 expression in low-grade SOCs than in SBOTs or high-grade SOCs. In response to exogenous treatment with IGF-1, low-grade cell lines exhibited more intense upregulation of phosphorylated AKT than did high-grade cell lines, an effect that was diminished with IGF-1R knockdown and MK-2206 treatment. Low-grade SOC cell lines were more sensitive to growth inhibition with OSI-906 than were high-grade cell lines.

CONCLUSIONS

IGF-1 is overexpressed in low-grade SOCs compared with SBOTs and high-grade SOCs. Additionally, low-grade SOC cell lines were more responsive to IGF-1 stimulation and IGF-1R inhibition than were high-grade lines. The IGF-1 pathway is therefore a potential therapeutic target in low-grade SOC.