The nuclear receptor coactivator AIB1 mediates insulin-like growth factor I-induced phenotypic changes in human breast cancer cells

Modeling migration and metastasis in Drosophila

Cell motility makes essential contributions to normal embryonic development and homeostasis. It is also thought to contribute in important ways to tumor metastasis. Because of this dual importance, cell migration has been extensively studied. The fruit fly Drosophila melanogaster has served as an important model organism for genetic analysis of many aspects of developmental biology, including cell migration. Here we describe the various types of cell movements that have been studied in detail, which represent models for epithelial-to-mesenchymal transition, transepithelial migration, inflammation, wound healing and invasion. We summarize what has been learned about the molecular control of cell migration from genetic studies in the fly. In addition, we describe recent efforts to model tumor metastasis directly in Drosophila by expressing oncogenes and/or mutating tumor suppressor genes. Together these studies suggest that Drosophila has much to offer as a model for varied aspects of tumor metastasis.

Deletion of Tip30 leads to rapid immortalization of murine mammary epithelial cells and ductal hyperplasia in the mammary gland

Transformation of mammary epithelial cells (MECs) from the normal to the neoplastic stage requires the dysregulation of tumor suppressor genes and proto-oncogenes. Tip30 is a tumor suppressor that can inhibit estrogen receptor-mediated transcription in MECs, but its role in MEC proliferation remains unknown. Here, we show that deleting the Tip30 gene leads to ductal hyperplasia in mouse mammary glands early in life and extensive mammary hyperplasia with age. Tip30(-/-) mammary glands transplanted into wild-type mammary fat pads also display mammary trees with extensive ductal hyperplasia. Strikingly, Tip30 deletion promotes proliferation of primary MECs and results in rapid immortalization of MECs in vitro relative to wild-type cells. Gene array analysis identified significant increases in the expression of mammary epithelial growth factors Wisp2 and Igf-1 in Tip30(-/-) cells. Knockdown of either Wisp2 or Igf-1 using short interfering RNA dramatically inhibited proliferation of Tip30(-/-) cells. Together, these results suggest that Tip30 is an intrinsic and negative regulator of MEC proliferation partly through the inhibition of Wisp2 and Igf-1 expression, and its absence in the mammary gland may predispose MECs to neoplastic transformation.

Regulation of SRC family coactivators by post-translational modifications

Initially identified as a group of auxiliary protein factors involved in transcriptional regulation by steroid hormone receptors as well as by other members of the nuclear receptor superfamily, the steroid receptor coactivators (SRCs) have since then been implicated in the transcriptional regulation of other transcription factors which are important components of very different signaling pathways. Members of the SRC family have been shown to interact with myogenin, MEF-2, transcriptional enhancer factor (TEF), NF-kappaB, AP-1, STAT, p53, and E2F1, suggesting that SRC coactivators participate in diverse cellular processes. Recent evidence indicates that various post-translational modifications play critical roles in determining the final transcriptional output and specificity of SRC coactivators. In this review, we summarized the current knowledge concerning post-translational modifications, dynamic interplay between different modifications, and patho-physiological relevance of the modifications of SRC proteins.

SRC-3/AIB1 protein and gene amplification levels in human esophageal squamous cell carcinomas

It has been suggested that the steroid receptor coactivatior-3 (SRC-3) gene, also known as AIB1, ACTR, RAC3, p/CIP and TRAM-1, located at 20q12, plays an oncogenic role in several types of human cancers. In this study, we examined the encoded protein expression of SRC-3 and its copy number in 221 human esophageal squamous cell carcinomas (ESCCs). In this ESCC series, the overexpression and increased copy number of SRC-3 gene was detected in 46 and 13% of ESCCs, respectively. In addition, overexpression of SRC-3 was observed more frequently in primary ESCCs in late T stages (T3/T4) than that in earlier T1/T2 stages (P<0.05), but no significant association of expression of SRC-3 and status of lymph node metastases was observed (P>0.05). These results suggest that overexpression of SRC-3, caused by gene amplification/gain or other molecular mechanisms, might provide a selective advantage for the development and local invasion of certain subsets of ESCC. In addition, a significant correlation (P<0.05) of overexpression of SRC-3 with increased cell proliferation (through detection of Ki-67 expression) was observed in these ESCCs. These findings suggest a potential role of SRC-3 in the control of ESCC cell proliferation; such may be responsible, at least in part, for tumorigenesis and/or progression of ESCC.

Steroid Receptor Coactivator-3 and Activator Protein-1 Coordinately Regulate the Transcription of Components of the Insulin-Like Growth Factor/AKT Signaling Pathway

Steroid receptor coactivator (SRC)-3, also called amplified in breast cancer 1, is a member of the p160 nuclear receptor coactivator family involved in transcriptional regulation of target genes. SRC-3 is frequently amplified and/or overexpressed in hormone-sensitive and hormone-insensitive tumors. We reported previously that SRC-3 stimulated prostate cell growth in a hormone-independent manner through activation of AKT signaling pathway. However, the underlying mechanism remains undefined. Here, we exploited the mifepristone-induced SRC-3 LNCaP prostate cancer cell line generated in our laboratory to identify SRC-3-regulated genes by oligonucleotide microarray analysis. We found that SRC-3 up-regulates the expression of multiple genes in the insulin-like growth factor (IGF)/AKT signaling pathway that are involved in cell proliferation and survival. In contrast, knockdown of SRC-3 in PC3 (androgen receptor negative) prostate cancer cells and MCF-7 breast cancer cells reduces their expression. Similarly, in prostate glands of SRC-3 null mice, expressions of these components in the IGF/AKT signal pathway are also reduced. Chromatin immunoprecipitation assay revealed that SRC-3 was directly recruited to the promoters of these genes, indicating that they are direct targets of SRC-3. Interestingly, we showed that recruitment of SRC-3 to two target promoters, IRS-2 and IGF-I, requires transcription factor activator protein-1 (AP-1). Taken together, our results clearly show that SRC-3 and AP-1 can coordinately regulate the transcription of multiple components in the IGF/AKT pathway to ensure ligand-independent cell proliferation and survival of cancer cells.

Moderate overexpression of AIB1 triggers pre-neoplastic changes in mammary epithelium

Here we report a new model of pre-clinical breast cancer which has been generated by overexpressing the steroid receptor coactivator AIB1 at moderate levels in breast epithelium. Transgenic female mice display mammary hyperplasia at the onset of puberty, consistent with enhanced proliferation of primary mammary epithelial cultures and augmented levels of cyclin D1 and E-cadherin. Studies of BrdU incorporation revealed that AIB1 localizes to the nucleus during or after S phase, implicating a new role for AIB1 in cell-cycle progression subsequent to G1. Our findings suggest that moderate overexpression of AIB1 may represent one of the pre-neoplastic changes in breast tissue.

E6AP Mediates Regulated Proteasomal Degradation of the Nuclear Receptor Coactivator Amplified in Breast Cancer 1 in Immortalized Cells

The steroid receptor coactivator oncogene, amplified in breast cancer 1 (AIB1; also known as ACTR/RAC-3/TRAM-1/SRC-3/p/CIP), is amplified and overexpressed in a variety of epithelial tumors. AIB1 has been reported to have roles in both steroid-dependent and steroid-independent transcription during tumor progression. In this report, we describe that the cellular levels of AIB1 are controlled through regulated proteasomal degradation. We found that serum withdrawal or growth in high cell density caused rapid degradation of AIB1 protein, but not mRNA, in immortalized cell lines. Proteasome inhibitors prevented this process, and high molecular weight ubiquitylated species of AIB1 were detected. Nuclear export was required for proteasomal degradation of AIB1 and involved the ubiquitin ligase, E6AP. AIB1/E6AP complexes were detected in cellular extracts, and reduction of cellular E6AP levels with E6AP short interfering RNA prevented proteasomal degradation of AIB1. Conversely, overexpression of E6AP promoted AIB1 degradation. The COOH terminus of AIB1 interacted with E6AP in vitro and deletion of this region in AIB1 rendered it resistant to degradation in cells. From our results, we propose a model whereby signals promoted by changes in the cellular milieu initiate E6AP-mediated proteasomal degradation of AIB1 and thus contribute to the control of steady-state levels of this protein.

Mir-17-5p regulates breast cancer cell proliferation by inhibiting translation of AIB1 mRNA

MicroRNAs are an extensive family of approximately 22-nucleotide-long noncoding RNAs expressed in a wide range of eukaryotes, including humans, and they are important in development and disease. We found that microRNA Mir-17-5p has extensive complementarity to the mRNA of AIB1 (named for "amplified in breast cancer 1"). Cell culture experiments showed that AIB1 expression was downregulated by Mir-17-5p, primarily through translational inhibition. Expression of Mir-17-5p was low in breast cancer cell lines. We also found that downregulation of AIB1 by Mir-17-5p resulted in decreased estrogen receptor-mediated, as well as estrogen receptor-independent, gene expression and decreased proliferation of breast cancer cells. Mir-17-5p also completely abrogated the insulin-like growth factor 1-mediated, anchorage-independent growth of breast cancer cells. Our results reveal that Mir-17-5p has a role as a tumor suppressor in breast cancer cells.

Identification of genes that modulate sensitivity of U373MG glioblastoma cells to cis-platinum

Scatter factor (hepatocyte growth factor) and its receptor c-Met are increasingly expressed during progression from low-grade to high-grade gliomas. Scatter factor/c-Met signaling induces glioma cell motility, invasion, angiogenesis and resistance to DNA-damaging agents. The latter is relevant to the understanding of the resistance of human gliomas to chemotherapy and radiotherapy. The goal of this study was to identify a set of genes that may contribute to scatter factor-mediated protection of U373MG cells against cis-platinum, a DNA cross-linking agent. We used DNA microarray assays, confirmatory semiquantitative reverse transcription-polymerase chain reaction analysis and functional assays to identify genes involved in the scatter factor-induced resistance of U373MG to cis-platinum. We identified a group of genes that are overexpressed in cells treated with scatter factor plus cis-platinum relative to cells treated with cis-platinum alone and confirmed some of these gene expression alterations by reverse transcription-polymerase chain reaction. Inhibiting the expression of three of these genes--polycystic kidney disease 1, amplified in breast cancer 1 and DEAD/H box helicase 21--using small interfering RNAs reduced survival of cis-platinum-treated cells and partially reversed the scatter factor protection against cis-platinum. Dominant-negative Akt and IkappaB super-repressor expression vectors inhibited the scatter factor protection, and abrogated the ability of scatter factor to alter the expression of DEAD/H box helicase 21 and polycystin (PKD1) within the context of cis-platinum exposure. The Akt and nuclear factor-kappaB inhibitors had no effect on amplified in breast cancer 1 expression. These studies implicate DEAD/H box helicase 21, polycystin (PKD1) and amplified in breast cancer 1 as novel transcription-dependent regulators of scatter factor-mediated glioma cell protection against cytotoxic death, and identify other potential regulators for future study.

Signaling regulation of genomic and nongenomic functions of estrogen receptors

Estrogen receptors (ERs) mediate the effects of 17beta-estradiol under physiologic and pathologic conditions. ERs trigger 17beta-estradiol-sensitive gene transcription by binding to specific estrogen response elements (i.e. genomic mechanism). The cellular effects of estrogen are also influenced by membrane- or cytoplasm-initiated responses (i.e. nongenomic mechanism). Both ER-evoked genomic and nongenomic effects originate from a unique signaling network. Furthermore, estrogen-initiated rapid pathways and ERalpha interactions with specific partners (e.g. AIB1, PELP1/MNAR; MTA1, MTA1s and p130Cas) influence both ER functions. Here, we summarize the recent findings related to multiple regulatory levels of the signaling networks responsible for ERs-mediated responses in breast cancer cells.

IGF-I mediated survival pathways in normal and malignant cells

The type-I and -II insulin-like growth factors (IGF-I, II) are now established as survival- or proliferation-factors in many in vitro systems. Of note IGFs provide trophic support for multiple cell types or organ cultures explanted from various species, and delay the onset of programmed cell death (apoptosis) through the mitochondrial (intrinsic pathway) or by antagonizing activation of cytotoxic cytokine signaling (extrinsic pathway). In some instances, IGFs protect against other forms of death such as necrosis or autophagy. The effect of IGFs on cell survival appears to be context specific, being determined both by the cell origin (tissue specific) and the cellular stress that induces loss of cellular viability. In many human cancers, there is a strong association with dysregulated IGF signaling, and this association has been extensively reviewed recently. IGF-regulation is also disrupted in childhood cancers as a consequence of chromosomal translocations. IGFs are implicated also in acute renal failure, traumatic injury to brain tissue, and cardiac disease. This article focuses on the role of IGFs and their cellular signaling pathways that provide survival signals in stressed cells.

SRC-3/AIB1: transcriptional coactivator in oncogenesis

Steroid receptor coactivator-3 (SRC-3, also known as NCoA3, AIB1, p/CIP, RAC3, ACTR, and TRAM1), localized on a frequently amplified region, 20q12, has been associated with multiple cancers, including breast, gastric and prostate cancers. Although SRC-3 has been implicated as an oncogene, compelling evidence has only recently emerged implicating it as a causal factor in the genesis of human cancers. Here, we summarize recent evidence that indicates aberrant SRC-3 expression is important in hormone-sensitive and -insensitive human cancers.

The SRC-3/AIB1 coactivator is degraded in a ubiquitin- and ATP-independent manner by the REGgamma proteasome

Steroid receptor coactivator-3 (SRC-3/AIB1) is an oncogene frequently amplified and overexpressed in breast cancers. Here we report that SRC-3 interacts with REGgamma, a proteasome activator known to stimulate the trypsin-like activity of the 20S proteasome. RNAi knockdown and gain-of-function experiments suggest that REGgamma promotes SRC-3 protein degradation. Cellular levels of REGgamma expression affect estrogen-receptor target-gene expression and cell growth as a result of its ability to promote degradation of the SRC-3 protein. In vitro proteasome proteolysis assays using purified REGgamma, SRC-3, and the 20S proteasome reinforce these conclusions and demonstrate that REGgamma promotes the degradation of SRC-3 in a ubiquitin- and ATP-independent manner. This work demonstrates the first example of a physiologically relevant endogenous cellular target for the REGgamma-proteasome complex. It also highlights the fact that an alternative mode of proteasome-mediated protein degradation, independent of the 19S proteasome regulatory cap, targets the SRC-3 protein for degradation.

Steroid receptor coactivator-3, a homolog of Taiman that controls cell migration in the Drosophila ovary, regulates migration of human ovarian cancer cells

Border cell migration is a process that occurs during Drosophila ovarian development in which cells derived from a simple epithelium migrate and invade neighboring tissue. This process resembles the behavior of cancerous cells that derive from the simple epithelium of the human ovary. One important regulator of border cell migration is Taiman, a homolog of steroid receptor coactivator-3 (SRC-3). Because increasing evidence indicates that similarities exist between the molecular control of migration of border cells and of cancer cells, we investigated whether SRC-3 controls ovarian cancer cell migration. Little or no SRC-3 expression was detected in normal ovarian surface epithelium, ovarian cysts and borderline ovarian tumors that lack stromal invasion. In contrast, SRC-3 was abundantly expressed in high-grade ovarian carcinomas. Inhibiting SRC-3 expression in ovarian cancer cells markedly reduced cell spreading and migration, and altered intracellular localization of focal adhesion kinase. This inhibitory effect on cell migration was independent of the estrogen receptor (ER) status of the cells. These studies reveal a novel role for SRC-3 in ovarian cancer progression by promoting cell migration, independently of its role in estrogen receptor signaling.

Ovarian cancer metastasis: integrating insights from disparate model organisms

Despite considerable efforts to improve early detection, and advances in chemotherapy, metastasis remains a major challenge in the clinical management of ovarian cancer. Studies of new murine models are providing novel insights into the pathophysiology of ovarian cancer, but these models are not readily amenable to genetic screens. Genetic analysis of border-cell migration in the Drosophila melanogaster ovary provides clues that will improve our understanding of ovarian cancer metastasis at the molecular level, and also might lead to potential therapeutic targets.

Identification of target genes in breast cancer cells directly regulated by the SRC-3/AIB1 coactivator

Steroid receptor coactivator-3 (SRC-3/AIB1) is a coactivator for nuclear receptors and other transcription factors and an oncogene that contributes to growth regulation and development of mammary and other tumor types. Because of its biological functions, it is important to identify genes regulated by SRC-3. However, because coactivators do not bind DNA directly, extensive work is required to determine whether genes identified by RNA profiling approaches are direct or indirect targets. Here, we report the use of chromatin immunoprecipitation (ChIP)-based assays that involve genomic mapping and computational analyses of immunoprecipitated DNA to identify SRC-3-binding target genes in estradiol (E2)-treated MCF-7 breast cancer cells. We identified 18 SRC-3 genomic binding sites and demonstrated estrogen receptor-alpha (ERalpha) binding to all of them. Both E2-dependent and -independent SRC-3/ERalpha-binding sites were identified. RNA polymerase II ChIP assays were used to determine the correlation between SRC-3 and ERalpha binding and recruitment of the transcriptional machinery. These assays, in conjunction with analyses of RNA obtained from E2-treated cells, lead to the identification of SRC-3/ERalpha-associated genes. The ability of SRC family coactivators to regulate the expression of one of these genes, PARD6B/Par6, was confirmed by using cells individually depleted of SRC-1, SRC-2, or SRC-3 by small interfering RNA. The method described herein can be used to identify genes regulated by non-DNA-binding factors, such as other coactivators or corepressors, as well as DNA-binding transcription factors, and provides information on their binding location that can accelerate further gene characterization.