Stimulation of Steroid Receptor Coactivator-3 (SRC-3) Gene Overexpression by a Positive Regulatory Loop of E2F1 and SRC-3

SRC-3, a Steroid Receptor Coactivator: Implication in Cancer

Steroid receptor coactivator-3 (SRC-3), also known as amplified in breast cancer 1 (AIB1), is a member of the SRC family. SRC-3 regulates not only the transcriptional activity of nuclear receptors but also many other transcription factors. Besides the essential role of SRC-3 in physiological functions, it also acts as an oncogene to promote multiple aspects of cancer. This review updates the important progress of SRC-3 in carcinogenesis and summarizes its mode of action, which provides clues for cancer therapy.

Targeting the NCOA3-SP1-TERT axis for tumor growth in hepatocellular carcinoma

Hepatocellular carcinoma (HCC) has a high mortality rate and lacks an effective therapeutic target. Elevated expression of human telomerase reverse transcriptase (TERT) is an important hallmark in cancers, but the mechanism by which TERT is activated differentially in cancers is poorly understood. Here, we have identified nuclear receptor coactivator-3 (NCOA3) as a new modulator of TERT expression and tumor growth in HCC. NACO3 specifically binds to the TERT promoter at the -234 to -144 region and transcriptionally activates TERT expression. NCOA3 promotes HCC cell growth and tumor progression in vitro and in vivo through upregulating the TERT signaling. Knockdown of NACO3 suppresses HCC cell viability and colony formation, whereas TERT overexpression rescues this suppression. NCOA3 interacts with and recruits SP1 binding on the TERT promoter. Knockdown of NCOA3 also inhibits the expression of the Wnt signaling-related genes but has no effect on the Notch signaling-targeting genes. Moreover, NCOA3 is positively correlated with TERT expression in HCC tumor tissues, and high expression of both NCOA3 and TERT predicts a poor prognosis in HCC patients. Our findings indicate that targeting the NCOA3-SP1-TERT signaling axis may benefit HCC patients.

Steroid receptor coactivator-3 as a target for anaplastic thyroid cancer

Anaplastic thyroid carcinoma (ATC) is an aggressive malignancy without effective therapeutic options to improve survival. Steroid receptor coactivator-3 (SRC-3) is a transcriptional coactivator whose amplification and/or overexpression has been identified in many cancers. In this study, we explored the expression of SRC-3 in ATCs and the effects of a new class of SRC-3 inhibitor-2 (SI-2) in human ATC cells (THJ-11T and THJ-16T cells) and mouse xenograft models to assess therapeutic potential of SI-2 for the treatment of ATC. SRC-3 protein abundance was significantly higher in human ATC tissue samples and ATC cells than in differentiated thyroid carcinomas or normal controls. SI-2 treatment effectively reduced the SRC-3 expression in both ATC cells and ATC xenograft tumors induced by these cells. Cancer cell survival in ATC cells and tumor growth in xenograft tumors were significantly reduced by SI-2 treatment through induction of cancer cell apoptosis and cell cycle arrest. SI-2 also reduced cancer stem-like cells as shown by an inhibition of tumorsphere formation, ALDH activity, and expression of stem cell markers in ATC. These findings indicate that SRC-3 is a potential therapeutic target for treatment of ATC patients and that SI-2 is a potent and promising candidate for a new therapeutic agent.

Transcriptional Landscape of PARs in Epithelial Malignancies

G protein-coupled receptors (GPCRs), the largest family of cell receptors, act as important regulators of diverse signaling pathways. Our understanding of the impact of GPCRs in tumors is emerging, yet there is no therapeutic platform based on GPCR driver genes. As cancer progresses, it disrupts normal epithelial organization and maintains the cells outside their normal niche. The dynamic and flexible microenvironment of a tumor contains both soluble and matrix-immobilized proteases that contribute to the process of cancer advancement. An example is the activation of cell surface protease-activated receptors (PARs). Mammalian PARs are a subgroup of GPCRs that form a family of four members, PAR_1–4, which are uniquely activated by proteases found in the microenvironment. PAR₁ and PAR₂ play central roles in tumor biology, and PAR₃ acts as a coreceptor. The significance of PAR₄ in neoplasia is just beginning to emerge. PAR₁ has been shown to be overexpressed in malignant epithelia, in direct correlation with tumor aggressiveness, but there is no expression in normal epithelium. In this review, the involvement of key transcription factors such as Egr1, p53, Twist, AP2, and Sp1 that control PAR₁ expression levels specifically, as well as hormone transcriptional regulation by both estrogen receptors (ER) and androgen receptors (AR) are discussed. The cloning of the human protease-activated receptor 2; Par2 (hPar2) promoter region and transcriptional regulation of estrogen (E₂) via binding of the E₂–ER complex to estrogen response elements (ERE) are shown. In addition, evidence that TEA domain 4 (TEAD₄) motifs are present within the hPar2 promoter is presented since the YAP oncogene, which plays a central part in tumor etiology, acts via the TEAD₄ transcription factor. As of now, no information is available on regulation of the hPar3 promoter. With regard to hPar4, only data showing CpG methylation promoter regulation is available. Characterization of the PAR transcriptional landscape may identify powerful targets for cancer therapies.

SRC-3 is involved in maintaining hematopoietic stem cell quiescence by regulation of mitochondrial metabolism in mice

Quiescence maintenance is an important property of hematopoietic stem cells (HSCs), whereas the regulatory factors and underlying mechanisms involved in HSC quiescence maintenance are not fully uncovered. Here, we show that steroid receptor coactivator 3 (SRC-3) is highly expressed in HSCs, and SRC-3-deficient HSCs are less quiescent and more proliferative, resulting in increased sensitivity to chemotherapy and irradiation. Moreover, the long-term reconstituting ability of HSCs is markedly impaired in the absence of SRC-3, and SRC-3 knockout (SRC-3^-/-) mice exhibit a significant disruption of hematopoietic stem and progenitor cell homeostasis. Further investigations show that SRC-3 deficiency leads to enhanced mitochondrial metabolism, accompanied by overproduction of reactive oxygen species (ROS) in HSCs. Notably, the downstream target genes of peroxisome proliferator-activated receptor-coactivators 1α (PGC-1α) involved in the regulation of mitochondrial metabolism are significantly upregulated in SRC-3-deficient HSCs. Meanwhile, a significant decrease in the expression of histone acetyltransferase GCN5 accompanied by downregulation of PGC-1α acetylation is observed in SRC-3-null HSCs. Conversely, overexpression of GCN5 can inhibit SRC-3 deficiency-induced mitochondrial metabolism enhancement and ROS overproduction, thereby evidently rescuing the impairment of HSCs in SRC-3^-/- mice. Collectively, our findings demonstrate that SRC-3 plays an important role in HSC quiescence maintenance by regulating mitochondrial metabolism.

Histone acetyl transferase GCN5 promotes human hepatocellular carcinoma progression by enhancing AIB1 expression

BACKGROUND

General control non-depressible 5 (GCN5) is a crucial catalytic component of a transcriptional regulatory complex that plays important roles in cellular functions from cell cycle regulation to DNA damage repair. Although GCN5 has recently been implicated in certain oncogenic roles, its role in liver cancer progression remains vague.

RESULTS

In this study, we report that GCN5 was overexpressed in 17 (54.8 %) of 31 human hepatocellular carcinoma (HCC) specimens. Down-regulation of GCN5 inhibited HCC cell proliferation and xenograft tumor formation. GCN5 knockdown decreased the protein levels of the proliferation marker proliferating cell nuclear antigen (PCNA) and amplified in breast cancer 1 (AIB1), but increased the protein levels of cell cycle inhibitor p21(Cip1/Waf1) in HepG2 cells. GCN5 regulated AIB1 expression, at least in part, by cooperating with E2F1 to enhance AIB1 transcription. Consistently, GCN5 expression was positively correlated with AIB1 expression in human HCC specimens in two GEO profile datasets.

CONCLUSION

Since AIB1 plays a promoting role in HCC progression, our results propose that GCN5 promotes HCC progression at least partially by regulating AIB1 expression. This study implicates that GCN5 might be a potential molecular target for HCC diagnosis and treatment.

Inhibition of SRC-3 enhances sensitivity of human cancer cells to histone deacetylase inhibitors

SRC-3 is widely expressed in multiple tumor types and involved in cancer cell proliferation and apoptosis. Histone deacetylase (HDAC) inhibitors are promising antitumor drugs. However, the poor efficacy of HDAC inhibitors in solid tumors has restricted its further clinical application. Here, we reported the novel finding that depletion of SRC-3 enhanced sensitivity of breast and lung cancer cells to HDAC inhibitors (SAHA and romidepsin). In contrast, overexpression of SRC-3 decreased SAHA-induced cancer cell apoptosis. Furthermore, we found that SRC-3 inhibitor bufalin increased cancer cell apoptosis induced by HDAC inhibitors. The combination of bufalin and SAHA was particular efficient in attenuating AKT activation and reducing Bcl-2 levels. Taken together, these accumulating data might guide development of new breast and lung cancer therapies.

Circadian Clock Regulates Bone Resorption in Mice

The circadian clock controls many behavioral and physiological processes beyond daily rhythms. Circadian dysfunction increases the risk of cancer, obesity, and cardiovascular and metabolic diseases. Although clinical studies have shown that bone resorption is controlled by circadian rhythm, as indicated by diurnal variations in bone resorption, the molecular mechanism of circadian clock-dependent bone resorption remains unknown. To clarify the role of circadian rhythm in bone resorption, aryl hydrocarbon receptor nuclear translocator-like (Bmal1), a prototype circadian gene, was knocked out specifically in osteoclasts. Osteoclast-specific Bmal1-knockout mice showed a high bone mass phenotype due to reduced osteoclast differentiation. A cell-based assay revealed that BMAL1 upregulated nuclear factor of activated T cells, cytoplasmic, calcineurin-dependent 1 (Nfatc1) transcription through its binding to an E-box element located on the Nfatc1 promoter in cooperation with circadian locomotor output cycles kaput (CLOCK), a heterodimer partner of BMAL1. Moreover, steroid receptor coactivator (SRC) family members were shown to interact with and upregulate BMAL1:CLOCK transcriptional activity. Collectively, these data suggest that bone resorption is controlled by osteoclastic BMAL1 through interactions with the SRC family and binding to the Nfatc1 promoter. © 2016 American Society for Bone and Mineral Research.

HER2 Signaling Drives DNA Anabolism and Proliferation through SRC-3 Phosphorylation and E2F1-Regulated Genes

Approximately 20% of early-stage breast cancers display amplification or overexpression of the ErbB2/HER2 oncogene, conferring poor prognosis and resistance to endocrine therapy. Targeting HER2(+) tumors with trastuzumab or the receptor tyrosine kinase (RTK) inhibitor lapatinib significantly improves survival, yet tumor resistance and progression of metastatic disease still develop over time. Although the mechanisms of cytosolic HER2 signaling are well studied, nuclear signaling components and gene regulatory networks that bestow therapeutic resistance and limitless proliferative potential are incompletely understood. Here, we use biochemical and bioinformatic approaches to identify effectors and targets of HER2 transcriptional signaling in human breast cancer. Phosphorylation and activity of the Steroid Receptor Coactivator-3 (SRC-3) is reduced upon HER2 inhibition, and recruitment of SRC-3 to regulatory elements of endogenous genes is impaired. Transcripts regulated by HER2 signaling are highly enriched with E2F1 binding sites and define a gene signature associated with proliferative breast tumor subtypes, cell-cycle progression, and DNA replication. We show that HER2 signaling promotes breast cancer cell proliferation through regulation of E2F1-driven DNA metabolism and replication genes together with phosphorylation and activity of the transcriptional coactivator SRC-3. Furthermore, our analyses identified a cyclin-dependent kinase (CDK) signaling node that, when targeted using the CDK4/6 inhibitor palbociclib, defines overlap and divergence of adjuvant pharmacologic targeting. Importantly, lapatinib and palbociclib strictly block de novo synthesis of DNA, mostly through disruption of E2F1 and its target genes. These results have implications for rational discovery of pharmacologic combinations in preclinical models of adjuvant treatment and therapeutic resistance.

Biological relevance of Hsp90-binding immunophilins in cancer development and treatment

Immunophilins are a family of intracellular receptors for immunosuppressive drugs. Those immunophilins that are related to immunosuppression are the smallest proteins of the family, i.e., FKBP12 and CyPA, whereas the other members of the family have higher molecular weight because the show additional domains to the drug-binding site. Among these extra domains, the TPR-domain is perhaps the most relevant because it permits the interaction of high molecular weight immunophilins with the 90-kDa heat-shock protein, Hsp90. This essential molecular chaperone regulates the biological function of several protein-kinases, oncogenes, protein phosphatases, transcription factors and cofactors . Hsp90-binding immunophilins where first characterized due to their association with steroid receptors. They regulate the cytoplasmic transport and the subcellular localization of these and other Hsp90 client proteins, as well as transcriptional activity, cell proliferation, cell differentiation and apoptosis. Hsp90-binding immunophilins are frequently overexpressed in several types of cancers and play a key role in cell survival. In this article we analyze the most important biological actions of the best characterized Hsp90-binding immunophilins in both steroid receptor function and cancer development and discuss the potential use of these immunophilins for therapeutic purposes as potential targets of specific small molecules.

HER2 and uPAR cooperativity contribute to metastatic phenotype of HER2-positive breast cancer

Human epidermal growth factor receptor type 2 (HER2)-positive breast carcinoma is highly aggressive and mostly metastatic in nature though curable/manageable in part by molecular targeted therapy. Recent evidence suggests a subtype of cells within HER2-positive breast tumors that concomitantly expresses the urokinase plasminogen activator receptor (uPAR) with inherent stem cell/mesenchymal-like properties promoting tumor cell motility and a metastatic phenotype. This HER-positive/uPAR-positive subtype may be partially responsible for the failure of HER2-targeted treatment strategies. Herein we discuss and substantiate the cumulative preclinical and clinical evidence on HER2-uPAR cooperativity in terms of gene co-amplification and/or mRNA/protein co-overexpression. We then propose a regulatory signaling model that we hypothesize to maintain upregulation and cooperativity between HER2 and uPAR in aggressive breast cancer. An improved understanding of the HER2/uPAR interaction in breast cancer will provide critical biomolecular information that may help better predict disease course and response to therapy.

ERK3 promotes endothelial cell functions by upregulating SRC-3/SP1-mediated VEGFR2 expression

Despite a regain of interest recently in ERK3 kinase signaling, the molecular regulations of both ERK3 gene expression and protein kinase activity are still largely unknown. While it is shown that disruption of ERK3 gene causes neonatal lethality, cell type-specific functions of ERK3 signaling remain to be explored. In this study, we report that ERK3 gene expression is upregulated by cytokines through c-Jun in endothelial cells; c-Jun binds to the ERK3 gene and regulates its transcription. We further reveal a new role for ERK3 in regulating endothelial cell migration, proliferation and tube formation by upregulating SRC-3/SP-1-mediated VEGFR2 expression. The underlying molecular mechanism involves ERK3-stimulated formation of a transcriptional complex involving coactivator SRC-3, transcription factor SP-1 and the secondary coactivator CBP. Taken together, our study identified a molecular regulatory mechanism of ERK3 gene expression and revealed a previously unknown role of ERK3 in regulating endothelial cell functions.

Identification of verrucarin a as a potent and selective steroid receptor coactivator-3 small molecule inhibitor

Members of the steroid receptor coactivator (SRC) family are overexpressed in numerous types of cancers. In particular, steroid receptor coactivator 3 (SRC-3) has been recognized as a critical coactivator associated with tumor initiation, progression, recurrence, metastasis, and chemoresistance where it interacts with multiple nuclear receptors and other transcription factors to enhance their transcriptional activities and facilitate cross-talk between pathways that stimulate cancer progression. Because of its central role as an integrator of growth signaling pathways, development of small molecule inhibitors (SMIs) against SRCs have the potential to simultaneously disrupt multiple signal transduction networks and transcription factors involved in tumor progression. Here, high-throughput screening was performed to identify compounds able to inhibit the intrinsic transcriptional activities of the three members of the SRC family. Verrucarin A was identified as a SMI that can selectively promote the degradation of the SRC-3 protein, while affecting SRC-1 and SRC-2 to a lesser extent and having no impact on CARM-1 and p300 protein levels. Verrucarin A was cytotoxic toward multiple types of cancer cells at low nanomolar concentrations, but not toward normal liver cells. Moreover, verrucarin A was able to inhibit expression of the SRC-3 target genes MMP2 and MMP13 and attenuated cancer cell migration. We found that verrucarin A effectively sensitized cancer cells to treatment with other anti-cancer drugs. Binding studies revealed that verrucarin A does not bind directly to SRC-3, suggesting that it inhibits SRC-3 through its interaction with an upstream effector. In conclusion, unlike other SRC SMIs characterized by our laboratory that directly bind to SRCs, verrucarin A is a potent and selective SMI that blocks SRC-3 function through an indirect mechanism.

Impaired hematopoiesis and delayed thrombopoietic recovery following sublethal irradiation in SRC‑3 knockout mice

The objective of the present study was to investigate the role of the steroid receptor coactivator-3 (SRC-3) in hematopoiesis of mouse bone marrow (BM) following total body irradiation (TBI). SRC-3^−/− mice and wild-type (WT) mice were exposed to 4.5 Gy γ rays. Immunoblotting analysis revealed that the SRC-3 protein (p160) levels in normal BM-nucleated cells in WT were higher than in SRC-3^−/− mice. Furthermore, peripheral blood cell counts, BM cellularity and colony-forming unit (CFU) assays were performed following irradiation. The results showed that peripheral blood cells were significantly lower in number and recovered less rapidly in irradiated SRC-3^−/− mice as compared with control animals. BM-nucleated cell and CFU counts were significantly decreased in SRC-3^−/− mice on the 7th and 14th day. Of note, the recovery of platelet (PLT) and megakaryocytic lineage were more depressed than the granulocytic and erythroid lineage in SRC-3^−/− mice. In conclusion, the present study demonstrated that the hematopoietic ability in SRC-3 knockout mice is severely impaired following a sublethal dose of irradiation.

Steroid receptor coactivator-3 regulates glucose metabolism in bladder cancer cells through coactivation of hypoxia inducible factor 1α

Cancer cell proliferation is a metabolically demanding process, requiring high glycolysis, which is known as "Warburg effect," to support anabolic growth. Steroid receptor coactivator-3 (SRC-3), a steroid receptor coactivator, is overexpressed and/or amplified in multiple cancer types, including non-steroid targeted cancers, such as urinary bladder cancer (UBC). However, whether SRC-3 regulates the metabolic reprogramming for cancer cell growth is unknown. Here, we reported that overexpression of SRC-3 accelerated UBC cell growth, accompanied by the increased expression of genes involved in glycolysis. Knockdown of SRC-3 reduced the UBC cell glycolytic rate under hypoxia, decreased tumor growth in nude mice, with reduction of proliferating cell nuclear antigen and lactate dehydrogenase expression levels. We further revealed that SRC-3 could interact with hypoxia inducible factor 1α (HIF1α), which is a key transcription factor required for glycolysis, and coactivate its transcriptional activity. SRC-3 was recruited to the promoters of HIF1α-target genes, such as glut1 and pgk1. The positive correlation of expression levels between SRC-3 and Glut1 proteins was demonstrated in human UBC patient samples. Inhibition of glycolysis through targeting HK2 or LDHA decelerated SRC-3 overexpression-induced cell growth. In summary, overexpression of SRC-3 promoted glycolysis in bladder cancer cells through HIF1α to facilitate tumorigenesis, which may be an intriguing drug target for bladder cancer therapy.

Bufalin is a potent small-molecule inhibitor of the steroid receptor coactivators SRC-3 and SRC-1

Virtually all transcription factors partner with coactivators that recruit chromatin remodeling factors and interact with the basal transcription machinery. Coactivators have been implicated in cancer cell proliferation, invasion, and metastasis, including the p160 steroid receptor coactivator (SRC) family composed of SRC-1 (NCOA1), SRC-2 (TIF2/GRIP1/NCOA2), and SRC-3 (AIB1/ACTR/NCOA3). Given their broad involvement in many cancers, they represent candidate molecular targets for new chemotherapeutics. Here, we report on the results of a high-throughput screening effort that identified the cardiac glycoside bufalin as a potent small-molecule inhibitor for SRC-3 and SRC-1. Bufalin strongly promoted SRC-3 protein degradation and was able to block cancer cell growth at nanomolar concentrations. When incorporated into a nanoparticle delivery system, bufalin was able to reduce tumor growth in a mouse xenograft model of breast cancer. Our work identifies bufalin as a potentially broad-spectrum small-molecule inhibitor for cancer.

Research resource: loss of the steroid receptor coactivators confers neurobehavioral consequences

Steroid receptor coactivators (SRCs) are important transcriptional modulators that regulate nuclear receptor and transcription factor activity to adjust transcriptional output to cellular demands. Highlighting their pleiotropic effects, dysfunction of the SRCs has been found in numerous pathologies including cancer, inflammation, and metabolic disorders. The SRC family is expressed strongly in the brain including the hippocampus, cortex, and hypothalamus. Studies focusing on the effect of SRC loss using congenic SRC knockout mice (SRC(-/-)) are limited in number, yet strongly indicate that the SRCs play important roles in regulating reproductive behavior, development, and motor coordination. To better understand the unique functions of the SRCs, we performed a neurobehavioral test battery focusing on anxiety and exploratory behaviors, motor coordination, sensorimotor gating, and nociception in both male and female null mice and compared them with their wild-type (WT) littermates. Results from the test battery reveal a role for SRC1 in motor coordination. Additionally, we found that SRC1 regulates anxiety responses in SRC1(-/-) male and female mice, and nociception sensitivity in SRC1(-/-) male but not female mice. By comparison, SRC2 regulates anxiety response with SRC2(-/-) females showing decreased anxiety in novel environments, as well as increased exploratory behavior in the open field compared with WT littermates. Additionally, SRC2(-/-) males were shown to have deficits in sensorimotor gating. Loss of SRC3 also shows sex differences in anxiety and exploratory behaviors. In particular, SRC3(-/-) female mice have increased anxiety and reduced exploratory activity and impairments in prepulse inhibition, whereas SRC3(-/-) male mice show no significant behavioral differences. In both genders, ablation of SRC3 decreases nocifensive behaviors. Collectively, these resource data suggest that loss of the SRCs results in behavioral phenotypes, underscoring the importance of understanding both the general and gender-based activity of SRCs in the brain.

AIB1 cooperates with ERα to promote epithelial mesenchymal transition in breast cancer through SNAI1 activation

Epithelial Mesenchymal Transition (EMT) plays a major role in cancer metastasis. Several genes have been shown to play a role in EMT, and one of these is Amplified-in-breast cancer 1 (AIB1), which has oncogenic function and is known to be amplified in breast cancer. However, the role of AIB1 in EMT remains largely undefined at the molecular level. In this study, the effect of AIB1 overexpression on the EMT of the breast cancer cell line T47D was investigated. Overexpression of AIB1 disrupted the epithelial morphology of the cells. At the same time, the cells displayed a strong metastasis and reduced level of the epithelial marker E-cadherin. In contrast, knockdown of AIB1 in T47D cells increased cell-cell adhesion and produced weak metastasis, as well as a higher level of E-cadherin expression. We proposed that the regulation of EMT by AIB1 occurred through the action of the transcription factor SNAI1, and demonstrated that such interaction required the participation of ERα and the presence of ERα-binding site on SNAI1 promoter. The expression level of E-cadherin and the extent of cell migration and invasion in SNAI1-knocked down T47D cells that overexpressed AIB1 were similar to those of T47D cells that did not overexpress AIB1 and had no SNAI1 knockdown. Taken together, these results suggested that AIB1 exerted its effect on EMT through its interaction with ERα, which could directly bind to the ERα-binding site on the SNAI1 promoter, allowing the AIB1-ERα complex to promote the transcription of SNAI1 and eventually led to repression of E-cadherin expression, consistent with the loss of E-cadherin being a hallmark of EMT.

Transcriptional repression of AIB1 by FoxG1 leads to apoptosis in breast cancer cells

The oncogene nuclear receptor coactivator amplified in breast cancer 1 (AIB1) is a transcriptional coactivator that is overexpressed in various types of human cancers. However, the molecular mechanisms controlling AIB1 expression in the majority of cancers remain unclear. In this study, we identified a novel interacting protein of AIB1, forkhead-box protein G1 (FoxG1), which is an evolutionarily conserved forkhead-box transcriptional corepressor. We show that FoxG1 expression is low in breast cancer cell lines and that low levels of FoxG1 are correlated with a worse prognosis in breast cancer. We also demonstrate that transient overexpression of FoxG1 can suppress endogenous levels of AIB1 mRNA and protein in MCF-7 breast cancer cells. Exogenously expressed FoxG1 in MCF-7 cells also leads to apoptosis that can be rescued in part by AIB1 overexpression. Using chromatin immunoprecipitation, we determined that FoxG1 is recruited to a region of the AIB1 gene promoter previously characterized to be responsible for AIB1-induced, positive autoregulation of transcription through the recruitment of an activating, multiprotein complex, involving AIB1, E2F transcription factor 1, and specificity protein 1. Increased FoxG1 expression significantly reduces the recruitment of AIB1, E2F transcription factor 1 and E1A-binding protein p300 to this region of the endogenous AIB1 gene promoter. Our data imply that FoxG1 can function as a pro-apoptotic factor in part through suppression of AIB1 coactivator transcription complex formation, thereby reducing the expression of the AIB1 oncogene.

The steroid receptor coactivator-3 is required for the development of castration-resistant prostate cancer

The transcriptional coactivator SRC-3 plays a key role in enhancing prostate cancer cell proliferation. Although SRC-3 is highly expressed in advanced prostate cancer, its role in castration-resistant prostate cancer (CRPC) driven by PTEN mutation is unknown. We documented elevated SRC-3 in human CRPC and in PTEN-negative human prostate cancer. Patients with high SRC-3 and undetectable PTEN exhibited decreased recurrence-free survival. To explore the causal relationship in these observations, we generated mice in which both Pten and SRC-3 were inactivated in prostate epithelial cells (Pten3CKO mice), comparing them with mice in which only Pten was inactivated in these cells (PtenCKO mice). SRC-3 deletion impaired cellular proliferation and reduced tumor size. Notably, while castration of PtenCKO control mice increased the aggressiveness of prostate tumors relative to noncastrated counterparts, deletion of SRC-3 in Pten3CKO mice reversed all these changes. In support of this finding, castrated Pten3CKO mice also exhibited decreased levels of phospho-Akt, S6 kinase (RPS6KB1), and phosphorylated S6 protein (RPS6), all of which mediate cell growth and proliferation. Moreover, these tumors appeared to be more differentiated as evidenced by higher levels of Fkbp5, an AR-responsive gene that inhibits Akt signaling. Lastly, these tumors also displayed lower levels of certain androgen-repressed genes such as cyclin E2 and MMP10. Together, our results show that SRC-3 drives CRPC formation and offer preclinical proof of concept for a transcriptional coactivator as a therapeutic target to abrogate CRPC progression.

Nuclear receptor coactivator-6 attenuates uterine estrogen sensitivity to permit embryo implantation

Uterine receptivity to embryo implantation is coordinately regulated by 17β-estradiol (E(2)) and progesterone (P(4)). Although increased E(2) sensitivity causes infertility, the mechanisms underlying the modulation of E(2) sensitivity are unknown. We show that nuclear receptor coactivator-6 (NCOA6), a reported coactivator for estrogen receptor α (ERα), actually attenuates E(2) sensitivity to determine uterine receptivity to embryo implantation under normal physiological conditions. Specifically, conditional knockout of Ncoa6 in uterine epithelial and stromal cells does not decrease, but rather markedly increases E(2) sensitivity, which disrupts embryo implantation and inhibits P(4)-regulated genes and decidual response. NCOA6 enhances ERα ubiquitination and accelerates its degradation, while loss of NCOA6 causes ERα accumulation in stromal cells during the preimplantation period. During the same period, NCOA6 deficiency also caused a failure in downregulation of steroid receptor coactivator-3 (SRC-3), a potent ERα coactivator. Therefore, NCOA6 controls E(2) sensitivity and uterine receptivity by regulating multiple E(2)-signaling components.

Nuclear receptor coactivator RAC3 inhibits autophagy

RAC3 is an oncogene naturally overexpressed in several tumors. Besides its role as coactivator, it can exert several protumoral cytoplasmic actions. Autophagy was found to act either as a tumor suppressor during the early stages of tumor development, or as a protector of the tumor cell in later stages under hypoxic conditions. We found that RAC3 overexpression inhibits autophagy when induced by starvation or rapamycin and involves RAC3 nuclear translocation-dependent and -independent mechanisms. Moreover, hypoxia inhibits the RAC3 gene expression leading to the autophagy process, allowing tumor cells to survive until angiogenesis occurs. The interplay between RAC3, hypoxia, and autophagy could be an important mechanism for tumor progression and a good target for a future anticancer therapy.

Steroid receptor coactivator-3 as a potential molecular target for cancer therapy

INTRODUCTION

Steroid receptor coactivator-3 (SRC-3), also called amplified-in-breast cancer-1 (AIB1), is an oncogenic coactivator in endocrine and non-endocrine cancers. Functional studies demonstrate SRC-3 promotes numerous aspects of cancer, through its capacity as a coactivator for nuclear hormone receptors and other transcription factors, and via its ability to control multiple growth pathways simultaneously. Targeting SRC-3 with specific inhibitors therefore holds future promise for clinical cancer therapy.

AREAS COVERED

We discuss critical advances in understanding SRC-3 as a cancer mediator and prospective drug target. We review SRC-3 structure and function and its role in distinct aspects of cancer. In addition, we discuss SRC-3 regulation and degradation. Finally, we comment on a recently discovered SRC-3 small molecular inhibitor.

EXPERT OPINION

Most targeted chemotherapeutic drugs block only a single cellular pathway. In response, cancers frequently acquire resistance by upregulating alternative pathways. SRC-3 coordinates multiple signaling networks, suggesting SRC-3 inhibition offers a promising therapeutic strategy. Development of an effective SRC-3 inhibitor faces critical challenges. Better understanding of SRC-3 function and interacting partners, in both the nucleus and cytosol, is required for optimized inhibitor development. Ultimately, blockade of SRC-3 oncogenic function may inhibit multiple cancer-related signaling pathways.

The RIP140 gene is a transcriptional target of E2F1

RIP140 is a transcriptional coregulator involved in energy homeostasis and ovulation which is controlled at the transcriptional level by several nuclear receptors. We demonstrate here that RIP140 is a novel target gene of the E2F1 transcription factor. Bioinformatics analysis, gel shift assay, and chromatin immunoprecipitation demonstrate that the RIP140 promoter contains bona fide E2F response elements. In transiently transfected MCF-7 breast cancer cells, the RIP140 promoter is transactivated by overexpression of E2F1/DP1. Interestingly, RIP140 mRNA is finely regulated during cell cycle progression (5-fold increase at the G1/S and G2/M transitions). The positive regulation by E2F1 requires sequences located in the proximal region of the promoter (-73/+167), involves Sp1 transcription factors, and undergoes a negative feedback control by RIP140. Finally, we show that E2F1 participates in the induction of RIP140 expression during adipocyte differentiation. Altogether, this work identifies the RIP140 gene as a new transcriptional target of E2F1 which may explain some of the effect of E2F1 in both cancer and metabolic diseases.

The dark side of E2F1: in transit beyond apoptosis

E2F1 plays a critical role in cell-cycle progression and the induction of apoptosis in response to DNA damage. The latest evidence has uncovered that this tumor suppressor is most relevant for cancer progression and chemoresistance. Increased abundance of E2F1 triggers invasion and metastasis by activating growth receptor signaling pathways, which in turn promote an antiapoptotic tumor environment. The data shed light on the molecular mechanisms underlying E2F1-induced prometastatic activity and predict its radical switch from a mediator of cell death toward an accelerator of tumor progression. This raises the perspective of new drug targets at late-stage cancer.

The HMGA1 protoncogene frequently deregulated in cancer is a transcriptional target of E2F1

Reactivation of the HMGA1 protoncogene is very frequent in human cancer, but still very little is known on the molecular mechanisms leading to this event. Prompted by the finding of putative E2F binding sites in the human HMGA1 promoter and by the frequent deregulation of the RB/E2F1 pathway in human carcinogenesis, we investigated whether E2F1 might contribute to the regulation of HMGA1 gene expression. Here we report that E2F1 induces HMGA1 by interacting with a 193 bp region of the HMGA1 promoter containing an E2F binding site surrounded by three putative Sp1 binding sites. Both gain and loss of function experiments indicate that Sp1 functionally interacts with E2F1 to promote HMGA1 expression. However, while Sp1 constitutively binds HMGA1 promoter, it is the balance between different E2F family members that tunes the levels of HMGA1 expression between quiescence and proliferation. Finally, we found increased HMGA1 expression in pituitary and thyroid tumors developed in Rb(+/-) mice, supporting the hypothesis that E2F1 is a novel important regulator of HMGA1 expression and that deregulation of the RB/E2F1 path might significantly contribute to HMGA1 deregulation in cancer.

AIB1 as an independent prognostic marker in hepatocellular carcinoma after hepatic resection

BACKGROUND

Amplified in breast cancer 1 (AIB1) has been shown to promote growth and invasion in several types of human cancers and to have a prognostic role in some of cancers. However, its prognostic significance in hepatocellular carcinoma (HCC) remains unknown. This study aimed to address the issue.

METHODS

Immunohistochemical staining of AIB1 was performed for HCC and paired paratumorous liver (PTL) tissues from 139 patients. Associations between AIB1 expression with clinicopathological variables and patient survival were evaluated.

RESULTS

The expression rate of AIB1 was significantly higher in HCC (71/139, 51.1%) than in PTL tissues (1/139, 0.72%, P < 0.001). AIB1 expression in HCC was significantly associated with serum α-fetoprotein levels (P = 0.001) and Edmondson-Steiner grade (P = 0.038). Higher AIB1 expression in HCC was associated with shorter cumulative overall survival of the patients. Multivariate Cox regression analysis revealed that AIB1 was of independent prognostic significance for HCC.

CONCLUSIONS

AIB1 is independently associated with poor prognosis of HCC.

SRC-3 has a role in cancer other than as a nuclear receptor coactivator

Steroid receptor coactivator-3 (SRC-3), also known as AIB1, is a member of the p160 steroid receptor coactivator family. Since SRC-3 was found to be amplified in breast cancer in 1997, the role of SRC-3 in cancer has been broadly investigated. SRC-3 initially was identified as a transcriptional coactivator for nuclear receptors such as the estrogen receptor (ER), involved in the proliferation of hormone-dependent cancers. However, increasing clinical evidence shows that dysregulation of SRC-3 expression in several human hormone-independent cancers is correlated with pathological factors and clinical prognosis. Recently, both in vivo and in vitro studies demonstrate that SRC-3 may influence a number of cancer cellular processes in several ways independent of nuclear receptor signaling. In addition, an SRC-3 transgenic mice model shows that SRC-3 induces tumors in several mouse tissues. These results indicate that the role of SRC-3 in cancer is not just as a nuclear receptor coactivator. The focus of this review is to examine possible SRC-3 roles in cancer, other than as a nuclear receptor coactivator.

Nuclear receptor coactivators: structural and functional biochemistry

Transcription of eukaryotic cell is a multistep process tightly controlled by concerted action of macromolecules. Nuclear receptors are ligand-activated sequence-specific transcription factors that bind DNA and activate (or repress) transcription of specific sets of nuclear target genes. Successful activation of transcription by nuclear receptors and most other transcription factors requires "coregulators" of transcription. Coregulators make up a diverse family of proteins that physically interact with and modulate the activity of transcription factors and other components of the gene expression machinery via multiple biochemical mechanisms. The coregulators include coactivators that accomplish reactions required for activation of transcription and corepressors that suppress transcription. This review summarizes our current knowledge of nuclear receptor coactivators with an emphasis on their biochemical mechanisms of action and means of regulation.

The retinoblastoma tumor suppressor controls androgen signaling and human prostate cancer progression

Retinoblastoma (RB; encoded by RB1) is a tumor suppressor that is frequently disrupted in tumorigenesis and acts in multiple cell types to suppress cell cycle progression. The role of RB in tumor progression, however, is poorly defined. Here, we have identified a critical role for RB in protecting against tumor progression through regulation of targets distinct from cell cycle control. In analyses of human prostate cancer samples, RB loss was infrequently observed in primary disease and was predominantly associated with transition to the incurable, castration-resistant state. Further analyses revealed that loss of the RB1 locus may be a major mechanism of RB disruption and that loss of RB function was associated with poor clinical outcome. Modeling of RB dysfunction in vitro and in vivo revealed that RB controlled nuclear receptor networks critical for tumor progression and that it did so via E2F transcription factor 1-mediated regulation of androgen receptor (AR) expression and output. Through this pathway, RB depletion induced unchecked AR activity that underpinned therapeutic bypass and tumor progression. In agreement with these findings, disruption of the RB/E2F/nuclear receptor axis was frequently observed in the transition to therapy resistance in human disease. Together, these data reveal what we believe to be a new paradigm for RB function in controlling prostate tumor progression and lethal tumor phenotypes.

The cooperative function of nuclear receptor coactivator 1 (NCOA1) and NCOA3 in placental development and embryo survival

Nuclear receptor coactivator 1 [NCOA1/steroid receptor coactivator (SRC)-1] and NCOA3 (SRC-3/AIB1/ACTR) constitute two thirds of the SRC (steroid receptor coactivator) family. Although in vitro experiments have suggested overlapping functions between NCOA1 and NCOA3, their in vivo functional relationship is poorly understood. In this study, NCOA1 and NCOA3 double knockout mice were generated to determine the compensatory roles of NCOA1 and NCOA3 in development. NCOA1(-/-) mice survived normally, whereas most NCOA3(-/-) embryos were viable at embryonic d 13.5 (E13.5). In contrast, the majority of double-knockout (DKO) embryos died by E13.5. NCOA1 and NCOA3 are expressed in the labyrinth, and labyrinths of NCOA1(+/-);NCOA3(-/-) and DKO placentas were small compared with wild-type and single-knockout labyrinths. DKO labyrinths exhibited low densities of maternal blood sinuses and fetal capillaries and displayed fetomaternal blood transfusion. At the interface between maternal and fetal circulations, layer I sinusoidal trophoblast giant cells showed a reduced density of microvilli. Layer III syncytiotrophoblasts appeared to accumulate large lipid droplets and have reduced density and deepened invaginations of the intrasyncytial bays. The endothelial layer in DKO labyrinth showed abnormal morphologies and had large lipid droplets. Furthermore, disruption of NCOA1 and NCOA3 increased labyrinth trophoblast proliferation and their progenitor gene expression but decreased their differentiation gene expression. NCOA1 and NCOA3 deficiencies also affected the expression of several genes for placental morphogenesis including TGFβ-, peroxisome proliferator-activated receptor-β-, and peroxisome proliferator-activated receptor-γ-regulated genes and for glucose transportation including GLUT1 and Cx26. These findings demonstrate that NCOA1 and NCOA3 cooperatively regulate placental morphogenesis and embryo survival.

Estrogen receptor regulates E2F1 expression to mediate tamoxifen resistance

Antiestrogen resistance often develops with prolonged exposure to hormone therapies, including tamoxifen, and is a major problem in the treatment of breast cancer. Understanding the mechanisms involved in the development of antiestrogen resistance is an important step in the development of new targeted therapies. Two forms of antiestrogen resistance exist: de novo resistance and acquired resistance. To mimic acquired resistance, we have established a tamoxifen-resistant breast cancer cell line (MCF-7TamR) by treating parental MCF-7 cells with tamoxifen over a period of 6 months to select for cells with the resistant phenotype. Characterization of the MCF-7TamR cells under normal, hormone-deprived, and tamoxifen-treated conditions suggests that these cells continue to grow in the presence of tamoxifen. Additionally, a greater percentage of resistant cells enter the S phase under tamoxifen conditions, compared with parental MCF-7 cells. Consistent with these growth results, molecular analysis indicates that tamoxifen-resistant cells express higher levels of cyclin E1, cdk2, ACTR, and E2F1. Faslodex or ICI 182, 780 (ICI)-mediated degradation of estrogen receptor (ER) reduced the proliferation of these cells, as well as the level of E2F1 expression in tamoxifen-resistant cells, suggesting that tamoxifen resistance and E2F1 expression are in part dependent on ER. We further showed that tamoxifen enhances the ERalpha/Sp-1 interaction and promotes the recruitment of ERalpha and Sp-1 to the proximal promoter of E2F1 in resistant cells. Collectively, our findings suggest that tamoxifen resistance is a result of increased ERalpha/Sp-1 interaction, which enhances the expression of E2F1 to promote tamoxifen resistance.

Deregulated E2F and the AAA+ coregulator ANCCA drive proto-oncogene ACTR/AIB1 overexpression in breast cancer

The proto-oncogene ACTR/AIB1, a coactivator for transcription factors such as the nuclear receptors and E2Fs, is frequently overexpressed in various cancers including breast cancers. However, the underlying mechanism is poorly understood. Here, we identified several functional, noncanonical E2F binding sites in the ACTR first exon and intron that are critical for ACTR gene activation. We also found that the newly identified AAA+ coregulator AAA+ nuclear coregulator cancer associated (ANCCA) is recruited to the ACTR promoter and directly controls ACTR expression in breast cancer cells. Importantly, immunohistochemistry analysis indicated that ACTR overexpression is highly correlated with the expression of E2F1 and ANCCA in a cohort of human primary and lymph node-metastasized breast cancer specimens. Along with previous findings from us and others that ACTR is involved in its own gene regulation, these results suggest that one major mechanism of ACTR overexpression in cancer is the concerted, aberrant function of the nuclear coregulators such as ANCCA and ACTR, and they point to therapeutic strategies that target the Rb-E2F axis and/or the coregulator ANCCA for ACTR-overexpressing cancers.

Identification of SRC3/AIB1 as a preferred coactivator for hormone-activated androgen receptor

Transcription activation by androgen receptor (AR), which depends on recruitment of coactivators, is required for the initiation and progression of prostate cancer, yet the mechanisms of how hormone-activated AR interacts with coactivators remain unclear. This is because AR, unlike any other nuclear receptor, prefers its own N-terminal FXXLF motif to the canonical LXXLL motifs of coactivators. Through biochemical and crystallographic studies, we identify that steroid receptor coactivator-3 (SRC3) (also named as amplified in breast cancer-1 or AIB1) interacts strongly with AR via synergistic binding of its first and third LXXLL motifs. Mutagenesis and functional studies confirm that SRC3 is a preferred coactivator for hormone-activated AR. Importantly, AR mutations found in prostate cancer patients correlate with their binding potency to SRC3, corroborating with the emerging role of SRC3 as a prostate cancer oncogene. These results provide a molecular mechanism for the selective utilization of SRC3 by hormone-activated AR, and they link the functional relationship between AR and SRC3 to the development and growth of prostate cancer.

The roles and action mechanisms of p160/SRC coactivators and the ANCCA coregulator in cancer

Chromosomal aberrations involving genes encoding members of the p160/SRC transcriptional coactivator family such as AIB1/ACTR and TIF2 implicated the coactivators in malignancy of human cells. Significant progress has been made in the last decade toward uncovering their roles in the development and progression of solid tissue tumors as well as leukemia and understanding of the underlying molecular mechanisms. Here, we review their genetic aberrations and dysregulation in expression in breast cancer, prostate cancer, and other nonhormone-responsive cancers. The experimental evidence gathered from studies using cell culture and animal models strongly supports a critical and, in some circumstances, their oncogenic function. We summarize results that the SRCs may contribute to tumorigenesis and disease progression through transcription factors such as E2F, PEA3, and AP-1 and through an intimate control of signaling pathways of growth factors-Akt and the receptor tyrosine kinases. The finding that a recently identified nuclear receptor coregulator ANCCA, like the SRCs, is frequently overexpressed in many types of cancers again underscores their broader roles in cancer.

Normal and cancer-related functions of the p160 steroid receptor co-activator (SRC) family

The three homologous members of the p160 SRC family (SRC1, SRC2 and SRC3) mediate the transcriptional functions of nuclear receptors and other transcription factors, and are the most studied of all the transcriptional co-activators. Recent work has indicated that the SRCgenes are subject to amplification and overexpression in various human cancers. Some of the molecular mechanisms responsible for SRC overexpression, along with the mechanisms by which SRCs promote breast and prostate cancer cell proliferation and survival, have been identified, as have the specific contributions of individual SRC family members to spontaneous breast and prostate carcinogenesis in genetically manipulated mouse models. These studies have identified new challenges for cancer research and therapy.

The role of steroid receptor coactivator-3 (SRC-3) in human malignant disease

BACKGROUND

The p160 steroid receptor coactivator (SRC) family is critical to the transcriptional activation function of nuclear hormone receptors. A key member of this family is SRC-3, initially found to be amplified and expressed in breast cancer it has subsequent been shown to be expressed in malignant disease arising from a wide range of other organs. An understanding of the potential role of SRC-3 in the pathogenesis and its possible prognostic role in a broad range of tumours will improve our general understanding of carcinogenesis as well as potentially leading to a new prognostic marker as well as new therapeutic targets.

METHODS

Relevant papers were identified by searching the PubMed and MEDLINE databases for article published until 28th February 2009. Only articles published in English were considered. The search terms included "SRC-3", "AIB1" in association with the following terms: "human", "cancer" and "malignant disease". The search focused on malignant disease arising outside of the mammary gland. Full articles were obtained and references were checked for additional material when appropriate.

RESULTS

SRC-3 is amplified and expressed in a wide spectrum of human malignant diseases and appears to be a potential prognostic marker in a number of different tumours.

CONCLUSION

SRC-3 appears to be implicated in the possible risk of developing prostate and ovarian cancer. Its presence appears to be a marker of aggressive disease. Further research is required to determine its predictive and prognostic utility given the relative paucity of studies for each specific malignant disease.

The role and regulation of the nuclear receptor co-activator AIB1 in breast cancer

AIB1 (amplified in breast cancer 1), also called SRC-3 and NCoA-3, is a member of the p160 nuclear receptor co-activator family and is considered an important oncogene in breast cancer. Increased AIB1 levels in human breast cancer have been correlated with poor clinical prognosis. Overexpression of AIB1 in conjunction with members of the epidermal growth factor receptor (EGF/HER) tyrosine kinase family, such as HER2, is associated with resistance to tamoxifen therapy and decreased disease-free survival. A number of functional studies in cell culture and in rodents indicate that AIB1 has a pleiotropic role in breast cancer. Initially AIB1 was shown to have a role in the estrogen-dependent proliferation of breast epithelial cells. However, AIB1 also affects the growth of hormone-independent breast cancer and AIB1 levels are limiting for IGF-1-, EGF- and heregulin-stimulated biological responses in breast cancer cells and consequently the PI3 K/Akt/mTOR and other EGFR/HER2 signaling pathways are controlled by changes in AIB1 protein levels. The cellular levels and activity of AIB1 are in turn regulated at the levels of transcription, mRNA stability, post-translational modification, and by a complex control of protein half life. In particular, AIB1 activity as well as its half-life is modulated through a number of post-translational modifications including serine, threonine and tyrosine phosphorylation via kinases that are components of multiple signal transduction pathways. This review summarizes the possible mechanisms of how dysregulation of AIB1 at multiple levels can lead to the initiation and progression of breast cancer as well as its role as a predictor of response to breast cancer therapy, and as a possible therapeutic target.

Disruption of the SRC-1 gene in mice suppresses breast cancer metastasis without affecting primary tumor formation

Steroid receptor coactivator-1 (SRC-1) is a coactivator for nuclear hormone receptors such as estrogen and progesterone receptors and certain other transcription factors such as Ets-2 and PEA3. SRC-1 expression in breast cancer is associated with HER2 and c-Myc expression and with reduced disease-free survival. In this study, SRC-1(-/-) mice were backcrossed with FVB mice and then cross-bred with MMTV-polyoma middle T antigen (PyMT) mice to investigate the role of SRC-1 in breast cancer. Although mammary tumor initiation and growth were similar in SRC-1(-/-)/PyMT and wild-type (WT)/PyMT mice, genetic ablation of SRC-1 antagonized PyMT-induced restriction of mammary ductal differentiation and elongation. SRC-1(-/-)/PyMT mammary tumors were also more differentiated than WT/PyMT mammary tumors. The intravasation of mammary tumor cells and the frequency and extent of lung metastasis were drastically reduced in SRC-1(-/-)/PyMT mice compared with WT/PyMT mice. Metastatic analysis of transplanted WT/PyMT and SRC-1(-/-)/PyMT tumors in SRC-1(-/-) and WT recipient mice revealed that SRC-1 played an intrinsic role in tumor cell metastasis. Furthermore, SRC-1 was up-regulated during mammary tumor progression. Disruption of SRC-1 inhibited Ets-2-mediated HER2 expression and PyMT-stimulated Akt activation in the mammary tumors. Disruption of SRC-1 also suppressed colony-stimulating factor-1 (CSF-1) expression and reduced macrophage recruitment to the tumor site. These results suggest that SRC-1 specifically promotes metastasis without affecting primary tumor growth. SRC-1 may promote metastasis through mediating Ets-2-mediated HER2 expression and activating CSF-1 expression for macrophage recruitment. Therefore, functional interventions for coactivators like SRC-1 may provide unique approaches to control breast cancer progression and metastasis.

Generation and validation of a mouse line with a floxed SRC-3/AIB1 allele for conditional knockout

The steroid receptor coactivator-3 (SRC-3), also known as AIB1, ACTR, p/CIP and NCOA3, is a transcriptional coactivator for nuclear receptors and certain other transcription factors. SRC-3 is widely expressed and plays important physiological functions and pathogenic roles in breast and prostate cancers. SRC-3 knockout (SRC-3(-/-)) mice display genetic background-dependent embryonic lethality and multiple local and systemic abnormalities. Since both the partial lethality and the systemic effects caused by global SRC-3 knockout interfere with downstream investigation of tissue-specific function of SRC-3, we have generated floxed SRC-3 (SRC-3(f/f)) mice with conditional alleles carrying loxP sites in introns 10 and 12 by a gene-targeting strategy. The two SRC-3(f/f) mouse lines (A and B) are indistinguishable from wild type mice. To test if deletion of the floxed exons 11 and 12 for SRC-3 nuclear receptor interaction domains and disruption of its downstream sequence for transcriptional activation domains would inactivate SRC-3 function, SRC-3(f/f) mice were crossbred with EIIa-Cre mice to generate SRC-3(d/d) mice with germ line deletion of the floxed SRC-3 gene. Both lines of SRC-3(d/d) mice exhibited growth retardation and low IGF-I levels, which was similar to that observed in SRC-3(-/-) mice. The line A SRC-3(d/d) mice showed normal viability, while line B SRC-3(d/d) mice showed partial lethality similar to SRC-3-/- mice, probably due to variable distributions of genetic background during breeding. These results demonstrate that the floxed SRC-3 mouse lines have been successfully established. These mice will be useful for investigating the cell type- and developmental stage-specific functions of SRC-3.

The AIB1 oncogene promotes breast cancer metastasis by activation of PEA3-mediated matrix metalloproteinase 2 (MMP2) and MMP9 expression

Amplified-in-breast cancer 1 (AIB1) is an overexpressed transcriptional coactivator in breast cancer. Although overproduced AIB1 is oncogenic, its role and underlying mechanisms in metastasis remain unclear. Here, mammary tumorigenesis and lung metastasis were investigated in wild-type (WT) and AIB1(-/-) mice harboring the mouse mammary tumor virus-polyomavirus middle T (PyMT) transgene. All WT/PyMT mice developed massive lung metastasis, but AIB1(-/-)/PyMT mice with comparable mammary tumors had significantly less lung metastasis. The recipient mice with transplanted AIB1(-/-)/PyMT tumors also had much less lung metastasis than the recipient mice with transplanted WT/PyMT tumors. WT/PyMT tumor cells expressed mesenchymal markers such as vimentin and N-cadherin, migrated and invaded rapidly, and formed disorganized cellular masses in three-dimensional cultures. In contrast, AIB1(-/-)/PyMT tumor cells maintained epithelial markers such as E-cadherin and ZO-1, migrated and invaded slowly, and still formed polarized acinar structures in three-dimensional cultures. Molecular analyses revealed that AIB1 served as a PEA3 coactivator and formed complexes with PEA3 on matrix metalloproteinase 2 (MMP2) and MMP9 promoters to enhance their expression in both mouse and human breast cancer cells. In 560 human breast tumors, AIB1 expression was found to be positively associated with PEA3, MMP2, and MMP9. These findings suggest a new alternative strategy for controlling the deleterious roles of these MMPs in breast cancer by inhibiting their upstream coregulator AIB1.

Genetic screening reveals an essential role of p27kip1 in restriction of breast cancer progression

The genetic changes and mechanisms underlying the progression of estrogen-dependent breast cancers to estrogen-independent, antiestrogen-resistant, and metastatic breast cancers are unclear despite being a major problem in endocrine therapy. To identify genes responsible for this progression, we carried out a genetic screening by an enhanced retroviral mutagen (ERM)-mediated random mutagenesis in the estrogen-dependent T47D breast cancer cells. We found that T47D cells contain only one p27kip1 (p27) allele coding for the p27 cyclin-dependent kinase (CDK) inhibitor. An ERM insertion into the p27 locus of T47D cells disrupted the p27 gene and created estrogen-independent and antiestrogen-resistant breast cancer cells that still maintained functional estrogen receptors. Disruption of p27 in T47D cells resulted in several changes, and most of these changes could be rescued by p27 restoration. First, CDK2 activity was increased in the absence of estrogen or in the presence of estrogen antagonists tamoxifen or ICI 182780; second, amplified in breast cancer 1 (AIB1), a cancer overexpressed transcriptional coactivator, was hyperphosphorylated, which made AIB1 a better coactivator for E2F1; and third, growth factor receptor binding protein 2-associated binder 2 (Gab2) and Akt activity were increased following E2F1 overactivation, leading to a significant enhancement of cell migration and invasion. Furthermore, the p27-deficient cells, but not T47D control cells, developed lung metastasis in an ovarian hormone-independent manner when they were i.v. injected into nude mice. In sum, loss of p27 activated AIB1, E2F1, Gab2, and Akt; increased cell migration and invasion; caused antiestrogen insensitivity; and promoted metastasis of breast cancer cells. These findings suggest that p27 plays an essential role in restriction of breast cancer progression.

Genetic ablation of the amplified-in-breast cancer 1 inhibits spontaneous prostate cancer progression in mice

Although the amplified-in-breast cancer 1 (AIB1; SRC-3, ACTR, or NCoA3) was defined as a coactivator for androgen receptor (AR) by in vitro studies, its role in AR-mediated prostate development and prostate cancer remained unexplored. We report here that AIB1 is expressed in the basal and stromal cells but not in the epithelial cells of the normal mouse prostates. AIB1 deficiency only slightly delayed prostate growth and had no effect on androgen-dependent prostate regeneration, suggesting an unessential role of AIB1 in AR function in the prostate. Surprisingly, when prostate tumorigenesis was induced by the SV40 transgene in transgenic adenocarcinoma of the mouse prostate (TRAMP) mice, AIB1 expression was observed in certain epithelial cells of the prostate intraepithelial neoplasia (PIN) and well-differentiated carcinoma and in almost all cells of the poorly differentiated carcinoma. After AIB1 was genetically inactivated in AIB1-/-/TRAMP mice, the progression of prostate tumorigenesis in most AIB1-/-/TRAMP mice was arrested at the well-differentiated carcinoma stage. Wild-type (WT)/TRAMP mice developed progressive, multifocal, and metastatic prostate tumors and died between 25 and 34 weeks. In contrast, AIB1-/-/TRAMP mice only exhibited PIN and early-stage well-differentiated carcinoma by 39 weeks. AIB1-/-/TRAMP prostates showed much lower cell proliferation than WT/TRAMP prostates. Most AIB1-/-/TRAMP mice could survive more than 35 weeks and died with other types of tumors or unknown reasons. Our results indicate that induction of AIB1 expression in partially transformed epithelial cells is essential for progression of prostate tumorigenesis into poorly differentiated carcinoma. Inhibition of AIB1 expression or function in the prostate epithelium may be a potential strategy to suppress prostate cancer initiation and progression.

Identification of membrane progestin receptors in human breast cancer cell lines and biopsies and their potential involvement in breast cancer

Novel membrane progestin receptors (mPRs) coupled to G proteins recently identified in several species, including humans, are potential intermediaries in rapid, nongenomic progestin actions observed in a wide variety of tissues. Here we demonstrate mPR mRNA and protein expression and specific membrane-associated progestin binding in MCF-7 and SK-BR-3 human breast cancer cells. Interestingly, human mPRalpha mRNA expression was higher in breast tumor biopsies than in normal tissue from the same breast. Recent studies indicate intracellular signaling pathways initiated by the mPRs are broadly similar to those induced during breast cancer growth and development. Taken together these results suggest a potential involvement of mPRs during the development or progression of breast cancer.