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

AIB1/SRC-3/NCOA3 function in estrogen receptor alpha positive breast cancer

The estrogen receptor alpha (ERα) is a steroid receptor that is pivotal in the initiation and progression of most breast cancers. ERα regulates gene transcription through recruitment of essential coregulators, including the steroid receptor coactivator AIB1 (Amplified in Breast Cancer 1). AIB1 itself is an oncogene that is overexpressed in a subset of breast cancers and is known to play a role in tumor progression and resistance to endocrine therapy through multiple mechanisms. Here we review the normal and pathological functions of AIB1 in regard to its ERα-dependent and ERα-independent actions, as well as its genomic conservation and protein evolution. We also outline the efforts to target AIB1 in the treatment of breast cancer.

Retracted: AIB1 induces epithelial-mesenchymal transition in gastric cancer via the PI3K/AKT signaling

Amplified in breast cancer 1 (AIB1) is overexpression in various cancers and promotes tumor cell proliferation, survival, and invasiveness. However, the role of AIB1 in the regulation of gastric cancer (GC) cell epithelial-mesenchymal transition (EMT) is still largely unclear. In the present study, immunohistochemistry showed that AIB1 was upregulated in our cohort of patients with GC and correlated with poor survival. Knockdown of AIB1 reduced the invasive ability of GC cells, downregulated the expression of epithelial cell marker E-cadherin, and upregulated mesenchymal cell marker vimentin. AIB1 overexpression elicited the opposite effect. PI-103, the inhibitor of the PI3K/AKT signaling, partially reversed AIB1 overexpression mediated a decrease in E-cadherin and an increase in vimentin. The present data demonstrated that AIB1 augmented the EMT via activation of PI3K/AKT signaling. In conclusion, our results suggested a novel role of AIB1 in GC invasion and EMT and raised the possibility of using this molecule as an indicator for GC treatment.

Depletion of the Transcriptional Coactivator Amplified in Breast Cancer 1 (AIB1) Uncovers Functionally Distinct Subpopulations in Triple-Negative Breast Cancer

The transcriptional coactivator Amplified in Breast Cancer 1 (AIB1) plays a major role in the progression of hormone and HER2-dependent breast cancers but its role in triple negative breast cancer (TNBC) is undefined. Here, we report that established TNBC cell lines, as well as cells from a TNBC patient-derived xenograft (PDX) that survive chemotherapy treatment in vitro express lower levels of AIB1 protein. The surviving cell population has an impaired tube-formation phenotype when cultured onto basement membrane, a property shared with TNBC cells that survive shRNA-mediated depletion of AIB1 (AIB1LOW cells). DNA analysis by exome sequencing revealed that AIB1LOW cells represent a distinct subpopulation. Consistent with their in vitro phenotype AIB1LOW cells implanted orthotopically generated slower growing tumors with less capacity for pulmonary metastases. Gene expression analysis of cultured cells and tumors revealed that AIB1LOW cells display a distinct expression signature of genes in pro-inflammatory pathways, cell adhesion, proteolysis and tissue remodeling. Interestingly, the presence of this AIB1LOW expression signature in breast cancer specimens is associated with shorter disease free survival of chemotherapy treated patients. We concluded that TNBC cell lines contain heterogeneous populations with differential dependence on AIB1 and that the gene expression pattern of AIB1LOW cells may represent a signature indicative of poor response to chemotherapy in TNBC patients.

Insulin-like growth factor binding protein-3 links obesity and breast cancer progression

Obesity is associated epidemiologically with poor breast cancer prognosis, but the mechanisms remain unclear. Since IGF binding protein-3 (IGFBP-3) influences both breast cancer growth and adipocyte maturation, it may impact on how obesity promotes breast oncogenesis. This study investigated the role of endogenous IGFBP-3 on the development of obesity and subsequently on breast tumor growth. Wild-type (WT) C57BL/6 or IGFBP-3-null (BP3KO) mice were fed a high-fat diet (HFD) or control chow-diet for 15 weeks before orthotopic injection with syngeneic EO771 murine breast cancer cells. When the largest tumor reached 1000 mm3, tissues and tumors were excised for analysis. Compared to WT, BP3KO mice showed significantly reduced weight gain and mammary fat pad mass (contralateral to tumor) in response to HFD, despite similar food intake. EO771 tumor weight and volume were increased by HFD and decreased by BP3KO. Despite differences in tumor size, tumors in BP3KO mice showed no differences from WT in the number of mitotically active (Ki67+) and apoptotic (cleaved caspase-3+) cells, but had greater infiltration of CD3+ T-cells. These data suggest that endogenous (circulating and/or stromal) IGFBP-3 is stimulatory to adipose tissue expansion and enhances mammary tumor growth in immune-competent mice, potentially by suppressing T-cell infiltration into tumors.

The PPARγ agonist efatutazone delays invasive progression and induces differentiation of ductal carcinoma in situ

PURPOSE

Ductal carcinoma in situ (DCIS) is a pre-invasive lesion of the breast considered a precursor of invasive ductal carcinoma. This study aimed to determine whether activated PPARγ acts as a tumor suppressor in human DCIS progression.

METHODS

We utilized the high-affinity PPARγ agonist, efatutazone, to activate endogenous PPARγ in a well-defined model for the progression of basal (triple negative) DCIS, MCFDCIS cells, cultured under 2D and 3D conditions. We studied the effects of activated PPARγ on DCIS progression in MCFDCIS xenograft and C3(1)/Tag transgenic mice treated with 30 mg/kg of efatutazone.

RESULTS

In vitro, efatutazone did not alter the MCFDCIS cell proliferation but induced phenotypic and gene expression changes, indicating that activated PPARγ is able to differentiate MCFDCIS cells into more luminal and lactational-like cells. In addition, MCFDCIS tumorsphere formation in 3D was reduced by PPARγ activation. In vivo, efatutazone-treated MCFDCIS tumors exhibited fat deposition along with upregulation of PPARγ responsive genes in both epithelial and stromal compartments, suggesting features of milk-producing mammary epithelial cell differentiation. The efatutazone-treated lesions were less invasive with fewer CD44+/p63+ basal progenitor cells. PPARγ activation downregulated Akt phosphorylation in these tumors, although the ERK pathway remained unchanged. Similar trends in gene expression changes consistent with lactational and luminal cell differentiation were observed in the C3(1)/Tag mouse model after efatutazone treatment.

CONCLUSIONS

Our data suggest that activation of the PPARγ pathway differentiates DCIS lesions and may be a useful approach to delay DCIS progression.

Treatment for the endocrine resistant breast cancer: Current options and future perspectives

Endocrine resistance remains a challenge and an unmet need for managing hormone receptor-positive breast cancer. The mechanisms of endocrine resistance are multifaceted and are likely to evolve over time following various single or combination therapies. The purpose of this review article is to provide general understanding of molecular basis of endocrine resistance of breast cancer and to offer comprehensive review on current treatment options and potential new treatment strategies for endocrine resistant breast cancers. Last but not the least, we discuss current challenges and future directions for management of endocrine resistant breast cancers.

Overexpression of SRC-3 promotes esophageal squamous cell carcinoma aggressiveness by enhancing cell growth and invasiveness

Steroid receptor coactivator‐3 (SRC‐3), a transcriptional coactivator for nuclear receptors and other transcription factors, plays an important role in the genesis and progression of several cancers. However, studies investigated the role of SRC‐3 in esophageal squamous cell carcinomas (ESCCs) are limited, and the role of SRC‐3 in tumor progression remains unclear. We examined the expression of SRC‐3 in 8 ESCC cell lines and 302 human ESCC tissues by qPCR, Western blot, and immunohistochemistry. In addition, ESCC cell lines were subjected to proliferation and invasion assays, tumorigenicity assay, flow cytometry assay, qPCR, Western blot, and Chromatin Immunoprecipitation assay to investigate the role of SRC‐3 in cancer progression. SRC‐3 was overexpressed in 48% of cases and correlated with poor overall (P = 0.0076) and progression‐free (P = 0.0069) survival of surgically resected ESCC patient. Cox regression analysis revealed that SRC‐3 is an independent prognostic marker. Furthermore, we found that activation of insulin‐like growth factor (IGF)/AKT) was involved in the SRC‐3 on the cell growth and invasiveness in two ESCC cell lines, Eca109 and EC18 cells. SRC‐3 overexpression is clinically and functionally relevant to the progression of human ESCC, and might be a useful molecular target for ESCC prognosis and treatment.

Steroid receptor coactivator-3 is a pivotal target of gambogic acid in B-cell Non-Hodgkin lymphoma and an inducer of histone H3 deacetylation

Gambogic acid (GA), the active ingredient from gamboges, has been verified as a potent anti-tumor agent in many cancer cells. Nevertheless, its function in lymphoma, especially in B-cell Non-Hodgkin lymphoma (NHL), remains unclear. Amplification and/or overexpression of steroid receptor coactivator-3 (SRC-3) have been detected in multiple tumors and have confirmed its critical roles in carcinogenesis, progression, metastasis and therapy resistance in these cancers. However, no clinical data have revealed the overexpression of SRC-3 and its role in B-cell NHL. In this study, we demonstrated the anti-tumor effects of GA, which included cell growth inhibition, G1/S phase cell cycle arrest and apoptosis in B-cell NHL. We also verified that SRC-3 was overexpressed in B-cell NHL in both cell lines and lymph node samples from patients. The overexpressed SRC-3 was a central drug target of GA, and its down-regulation subsequently modulated down-stream gene expression, ultimately contributing to apoptosis. Silencing SRC-3 decreased the expression of Bcl-2, Bcl-6 and cyclin D3, but not of NF-κB and IκB-α. GA treatment did not inhibit the activation of AKT signaling pathway, but induced the deacetylation of histone H3 at lysine 9 and lysine 27. Down-regulated SRC-3 was observed to interact with more HDAC1 to mediate the deacetylation of H3. As the component of E3 ligase, Cullin3 was up-regulated and mediated the degradation of SRC-3. Our results demonstrate that GA is a potent anti-tumor agent that can be used for therapy against B-cell NHL, especially against those with an abundance of SRC-3.

Insulin-like growth factor binding protein-3 inhibits cell adhesion via suppression of integrin β4 expression

We previously reported that IGF binding protein-3 (IGFBP-3), a major IGF-binding protein in human serum, regulates angiogenic activities of human head and neck squamous cell carcinoma (HNSCC) cells and human umbilical vein endothelial cells (HUVECs) through IGF-dependent and IGF-independent mechanisms. However, the role of IGFBP-3 in cell adhesion is largely unknown. We demonstrate here that IGFBP-3 inhibits the adhesion of HNSCC cells and HUVECs to the extracellular matrix (ECM). IGFBP-3 reduced transcription of a variety of integrins, especially integrin β₄, and suppressed phosphorylation of focal adhesion kinase (FAK) and Src in these cells through both IGF-dependent and IGF-independent pathways. IGFBP-3 was found to suppress the transcription of c-fos and c-jun and the activity of AP1 transcription factor. The regulatory effect of IGFBP-3 on integrin β₄ transcription was attenuated by blocking c-jun and c-fos gene expression via siRNA transfection. Taken together, our data show that IGFBP-3 has IGF-dependent and -independent inhibitory effects on intracellular adhesion signaling in HNSCC and HUVECs through its ability to block c-jun and c-fos transcription and thus AP-1-mediated integrin β₄ transcription. Collectively, our data suggest that IGFPB-3 may be an effective cancer therapeutic agent by blocking integrin-mediated adhesive activity of tumor and vascular endothelial cells.

Endocrine resistance in breast cancer--An overview and update

Tumors that express detectable levels of the product of the ESR1 gene (estrogen receptor-α; ERα) represent the single largest molecular subtype of breast cancer. More women eventually die from ERα+ breast cancer than from either HER2+ disease (almost half of which also express ERα) and/or from triple negative breast cancer (ERα-negative, progesterone receptor-negative, and HER2-negative). Antiestrogens and aromatase inhibitors are largely indistinguishable from each other in their abilities to improve overall survival and almost 50% of ERα+ breast cancers will eventually fail one or more of these endocrine interventions. The precise reasons why these therapies fail in ERα+ breast cancer remain largely unknown. Pharmacogenetic explanations for Tamoxifen resistance are controversial. The role of ERα mutations in endocrine resistance remains unclear. Targeting the growth factors and oncogenes most strongly correlated with endocrine resistance has proven mostly disappointing in their abilities to improve overall survival substantially, particularly in the metastatic setting. Nonetheless, there are new concepts in endocrine resistance that integrate molecular signaling, cellular metabolism, and stress responses including endoplasmic reticulum stress and the unfolded protein response (UPR) that provide novel insights and suggest innovative therapeutic targets. Encouraging evidence that drug combinations with CDK4/CDK6 inhibitors can extend recurrence free survival may yet translate to improvements in overall survival. Whether the improvements seen with immunotherapy in other cancers can be achieved in breast cancer remains to be determined, particularly for ERα+ breast cancers. This review explores the basic mechanisms of resistance to endocrine therapies, concluding with some new insights from systems biology approaches further implicating autophagy and the UPR in detail, and a brief discussion of exciting new avenues and future prospects.

Cell growth density modulates cancer cell vascular invasion via Hippo pathway activity and CXCR2 signaling

Metastasis of cancer cells involves multiple steps, including their dissociation from the primary tumor and invasion through the endothelial cell barrier to enter the circulation and finding their way to distant organ sites where they extravasate and establish metastatic lesions. Deficient contact inhibition is a hallmark of invasive cancer cells, yet surprisingly the vascular invasiveness of commonly studied cancer cell lines is regulated by the density at which cells are propagated in culture. Cells grown at high density were less effective at invading an endothelial monolayer than cells grown at low density. This phenotypic difference was also observed in a zebrafish model of vascular invasion of cancer cells after injection into the yolk sac and extravasation of cancer cells into tissues from the vasculature. The vascular invasive phenotypes were reversible. A kinome-wide RNAi screen was used to identify drivers of vascular invasion by panning shRNA library transduced non-invasive cancer cell populations on endothelial monolayers. The selection of invasive subpopulations showed enrichment of shRNAs targeting the LATS1 (large tumor suppressor 1) kinase that inhibits the activity of the transcriptional coactivator YAP in the Hippo pathway. Depletion of LATS1 from non-invasive cancer cells restored the invasive phenotype. Complementary to this, inhibition or depletion of YAP inhibited invasion in vitro and in vivo. The vascular invasive phenotype was associated with a YAP-dependent up-regulation of the cytokines IL6, IL8, and CXCL1, 2, and 3. Antibody blockade of cytokine receptors inhibited invasion and confirmed that they are rate-limiting drivers that promote cancer cell vascular invasiveness and could provide therapeutic targets.

Conditionally reprogrammed normal and transformed mouse mammary epithelial cells display a progenitor-cell-like phenotype

Mammary epithelial (ME) cells cultured under conventional conditions senesce after several passages. Here, we demonstrate that mouse ME cells isolated from normal mammary glands or from mouse mammary tumor virus (MMTV)-Neu–induced mammary tumors, can be cultured indefinitely as conditionally reprogrammed cells (CRCs) on irradiated fibroblasts in the presence of the Rho kinase inhibitor Y-27632. Cell surface progenitor-associated markers are rapidly induced in normal mouse ME-CRCs relative to ME cells. However, the expression of certain mammary progenitor subpopulations, such as CD49f⁺ ESA⁺ CD44⁺, drops significantly in later passages. Nevertheless, mouse ME-CRCs grown in a three-dimensional extracellular matrix gave rise to mammary acinar structures. ME-CRCs isolated from MMTV-Neu transgenic mouse mammary tumors express high levels of HER2/neu, as well as tumor-initiating cell markers, such as CD44⁺, CD49f⁺, and ESA⁺ (EpCam). These patterns of expression are sustained in later CRC passages. Early and late passage ME-CRCs from MMTV-Neu tumors that were implanted in the mammary fat pads of syngeneic or nude mice developed vascular tumors that metastasized within 6 weeks of transplantation. Importantly, the histopathology of these tumors was indistinguishable from that of the parental tumors that develop in the MMTV-Neu mice. Application of the CRC system to mouse mammary epithelial cells provides an attractive model system to study the genetics and phenotype of normal and transformed mouse epithelium in a defined culture environment and in vivo transplant studies.

Regulation of HER2 oncogene transcription by a multifunctional coactivator/corepressor complex

Transcription of the HER2 oncogene can be repressed by estrogen (E2). We now show that, a splice isoform of the nuclear receptor coactivator AIB1, AIB1-Δ4, is able to reverse E2 repression of HER2 gene expression in breast cancer cells. The first 224 amino acids of AIB1 that are absent in AIB1-Δ4, bind a co-repressor, ANCO1. Using chromatin immunoprecipitation assay approaches in MCF7 and BT474 cell lines, we demonstrate that AIB1 and AIB1-Δ4 can bind to the E2 regulatory site in the first intron of the HER2 gene, after E2 treatment, but only full-length AIB1 recruits ANCO1. Consistent with E2-induced chromatin repression, the AIB1-ANCO1 complex recruits HDAC3 and HDAC4 to the intronic estrogen response element and the proximal promoter acquires the repressive chromatin mark H3K9me3 and loses H3K4me1. In contrast, AIB1-Δ4 does not recruit ANCO 1, HDAC3, or HDAC4 and the proximal promoter retains activation marks of H3K4me1. In cell lines with low levels of ANCO1 (T47D), E2 does not repress HER2 gene transcription but the repressive response can be restored by overexpression of ANCO1. ANCO1 can also repress other E2-responsive genes, indicating that AIB1, AIB1-Δ4 and ANCO1 are important determinants of endocrine and growth factor responsiveness in breast 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.

The nuclear coactivator amplified in breast cancer 1 maintains tumor-initiating cells during development of ductal carcinoma in situ

The key molecular events required for the formation of Ductal Carcinoma in Situ (DCIS) and its progression to invasive breast carcinoma have not been defined. Here we show that the nuclear receptor coactivator Amplified In Breast cancer 1 (AIB1) is expressed at low levels in normal breast but is highly expressed in DCIS lesions. This is of significance since reduction of AIB1 in human MCFDCIS cells restored a more normal 3D mammary acinar structure. Reduction of AIB1 in MCFDCIS cells, both prior to DCIS development or in existing MCFDCIS lesions in vivo, inhibited tumor growth and led to smaller, necrotic lesions. AIB1 reduction in MCFDCIS cells was correlated with significant reduction in the CD24−/CD44+ Breast Cancer Initiating Cells (BCIC) population, and a decrease in myoepithelial progenitor cells in the DCIS lesions in vitro and in vivo. Loss of AIB1 in MCFDCIS cells was also accompanied by a loss of expression of NOTCH 2, 3 and 4, JAG2, HES1, GATA3, HER2 and HER3 in vivo. These signaling molecules have been associated with differentiation of breast epithelial progenitor cells. These data indicate that AIB1 plays a central role in the initiation and maintenance of DCIS and that reduction of AIB1 causes loss of BCIC, loss of components of the NOTCH, HER2 and HER3 signaling pathways and fewer DCIS myoepithelial progenitor cells in vivo. We propose that increased expression of AIB1, through maintenance of BCIC, facilitates formation of DCIS, a necessary step prior to development of invasive disease.

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.

Expression of steroid receptor coactivator 3 in ovarian epithelial cancer is a poor prognostic factor and a marker for platinum resistance

BACKGROUND

Steroid receptor coactivator 3 (SRC3) is an important coactivator of a number of transcription factors and is associated with a poor outcome in numerous tumours. Steroid receptor coactivator 3 is amplified in 25% of epithelial ovarian cancers (EOCs) and its expression is higher in EOCs compared with non-malignant tissue. No data is currently available with regard to the expression of SRC-3 in EOC and its influence on outcome or the efficacy of treatment.

METHODS

Immunohistochemistry was performed for SRC3, oestrogen receptor-α, HER2, PAX2 and PAR6, and protein expression was quantified using automated quantitative immunofluorescence (AQUA) in 471 EOCs treated between 1991 and 2006 with cytoreductive surgery followed by first-line treatment platinum-based therapy, with or without a taxane.

RESULTS

Steroid receptor coactivator 3 expression was significantly associated with advanced stage and was an independent prognostic marker. High expression of SRC3 identified patients who have a significantly poorer survival with single-agent carboplatin chemotherapy, while with carboplatin/paclitaxel treatment such a difference was not seen.

CONCLUSION

Steroid receptor coactivator 3 is a poor prognostic factor in EOCs and appears to identify a population of patients who would benefit from the addition of taxanes to their chemotherapy regimen, due to intrinsic resistance to platinum therapy.

AIB1 predicts bladder cancer outcome and promotes bladder cancer cell proliferation through AKT and E2F1

Background:

We previously demonstrated that AIB1 overexpression is an independent molecular marker for shortened survival of bladder cancer (BC) patients. In this study, we characterised the role and molecular mechanisms of AIB1 in BC tumorigenicity.

Methods:

AIB1 expression was measured by immunohistochemistry in non-muscle-invasive BC tissue and adjacent normal bladder tissue. In addition, the tumorigenicity of AIB1 was assessed with in vitro and in vivo functional assays.

Results:

Overexpression of AIB1 was observed in tissues from 46 out of 146 patients with non-muscle-invasive BC and was an independent predictor for poor progression-free survival. Lentivirus-mediated AIB1 knockdown inhibited cell proliferation both in vitro and in vivo, whereas AIB1 overexpression promoted cell proliferation in vitro. The growth-inhibitory effect induced by AIB1 knockdown was mediated by G1 arrest, which was caused by reduced expression of key cell-cycle regulatory proteins through the AKT pathway and E2F1.

Conclusion:

Our results suggest that AIB1 promotes BC cell proliferation through the AKT pathway and E2F1. Furthermore, AIB1 overexpression predicts tumour progression in patients with non-muscle-invasive BC.

Estradiol receptors in breast cancer cells: associated co-factors as targets for new therapeutic approaches

Estrogen receptors α (ERα) and β (ERβ) are nuclear receptors which transduce estradiol (E2) response in many tissues including the mammary gland and breast cancers (BC). They activate or inhibit specific genes involved in cell cycle progression and cell survival through multiple enzyme activities leading to malignant transformation. Hormone therapy (antiestrogens (AEs) and aromatase inhibitors (AIs) have been widely used to block the mitogenic action of E2 in patients with ER-positive BC. ERs act in concert with numerous other proteins outside and inside the nucleus where co-activators such as histone modifying enzymes help reaching optimum gene activation. Moreover, E2-mediated gene regulation can occur through ERs located at the plasma membrane or G protein-coupled estrogen receptor (GPER), triggering protein kinase signaling cascades. Classical AEs as well as AIs are inefficient to block the cascades of events emanating from the membrane and from E2 binding to GPER, leading patients to escape anti-hormone treatments and hormone therapy resistance. Many pathways are involved in resistance, mostly resulting from over-expression of growth factor membrane receptors, in particular the HER2/ErbB2 which can be inhibited by specific antibodies or tyrosine kinases inhibitors. Together with the Hsp90 molecular chaperone machinery, a complex interplay between ERs, co-activators, co-repressors and growth factor-activated membrane pathways represents potent targets which warrant to be manipulated alone and in combination to designing novel therapies. The discovery of new potential targets arising from micro array studies gives the opportunity to activate or inhibit different new ER-modulating effectors for innovative therapeutic interventions.

The role of AIB1 in breast cancer

Amplified in breast cancer 1 (AIB1) is a member of the p160 steroid receptor coactivator family that mediates the transcriptional activities of nuclear receptors including estrogen receptor (ER) and progesterone receptor (PR), as well as certain other transcription factors, including E2F1 and p53. AIB1 is widely implicated in nuclear receptor-mediated diseases, particularly malignant diseases, including breast, prostate, gastric and pancreatic cancers. AIB1 was initially implicated in hormone-dependent breast cancer, where increasing levels of AIB1 mRNA and protein were detected in some of these specimens and the overexpression of AIB1 in mice led to an increased incidence of tumors. More recent studies revealed that AIB1 also affects the growth of hormone-independent breast cancer via signaling pathways such as those of E2F1, IGF-I, EGF and PI3K/Akt/mTOR. The pleiotropic effect of AIB1 and the roles it plays in both normal development and cancer have presented a great challenge to formulating an effective therapeutic strategy for breast cancer. In this review, we highlight the significant progress made with the recent findings and present an overview of the current understanding of the influence of AIB1 on breast cancer via hormone-dependent and -independent signaling pathways.

Role of the nuclear receptor coactivator AIB1/SRC-3 in angiogenesis and wound healing

The nuclear receptor coactivator amplified in breast cancer 1 (AIB1/SRC-3) has a well-defined role in steroid and growth factor signaling in cancer and normal epithelial cells. Less is known about its function in stromal cells, although AIB1/SRC-3 is up-regulated in tumor stroma and may, thus, contribute to tumor angiogenesis. Herein, we show that AIB1/SRC-3 depletion from cultured endothelial cells reduces their proliferation and motility in response to growth factors and prevents the formation of intact monolayers with tight junctions and of endothelial tubes. In AIB1/SRC-3(+/-) and (-/-) mice, the angiogenic responses to subcutaneous Matrigel implants was reduced by two-thirds, and exogenously added fibroblast growth factor (FGF) 2 did not overcome this deficiency. Furthermore, AIB1/SRC-3(+/-) and (-/-) mice showed similarly delayed healing of full-thickness excisional skin wounds, indicating that both alleles were required for proper tissue repair. Analysis of this defective wound healing showed reduced recruitment of inflammatory cells and macrophages, cytokine induction, and metalloprotease activity. Skin grafts from animals with different AIB1 genotypes and subsequent wounding of the grafts revealed that the defective healing was attributable to local factors and not to defective bone marrow responses. Indeed, wounds in AIB1(+/-) mice showed reduced expression of FGF10, FGFBP3, FGFR1, FGFR2b, and FGFR3, major local drivers of angiogenesis. We conclude that AIB1/SRC-3 modulates stromal cell responses via cross-talk with the FGF signaling pathway.

Small molecule inhibition of the steroid receptor coactivators, SRC-3 and SRC-1

Overexpression of steroid receptor coactivator (SRC)-1 and SRC-3 is associated with cancer initiation, metastasis, advanced disease, and resistance to chemotherapy. In most of these cases, SRC-1 and SRC-3 have been shown to promote tumor cell growth by activating nuclear receptor and multiple growth factor signaling cascades that lead to uncontrolled tumor cell growth. Up until now, most targeted chemotherapeutic drugs have been designed largely to block a single pathway at a time, but cancers frequently acquire resistance by switching to alternative growth factor pathways. We reason that the development of chemotherapeutic agents against SRC coactivators that sit at the nexus of multiple cell growth signaling networks and transcriptional factors should be particularly effective therapeutics. To substantiate this hypothesis, we report the discovery of 2,2'-bis-(Formyl-1,6,7-trihydroxy-5-isopropyl-3-methylnaphthalene (gossypol) as a small molecule inhibitor of coactivator SRC-1 and SRC-3. Our data indicate that gossypol binds directly to SRC-3 in its receptor interacting domain. In MCF-7 breast cancer cells, gossypol selectively reduces the cellular protein concentrations of SRC-1 and SRC-3 without generally altering overall protein expression patterns, SRC-2, or other coactivators, such as p300 and coactivator-associated arginine methyltransferase 1. Gossypol reduces the concentration of SRC-3 in prostate, lung, and liver cancer cell lines. Gossypol inhibits cell viability in the same cancer cell lines where it promotes SRC-3 down-regulation. Additionally, gossypol sensitizes lung and breast cancer cell lines to the inhibitory effects of other chemotherapeutic agents. Importantly, gossypol is selectively cytotoxic to cancer cells, whereas normal cell viability is not affected. This data establish the proof-of-principle that, as a class, SRC-1 and SRC-3 coactivators are accessible chemotherapeutic targets. Given their function as integrators of multiple cell growth signaling systems, SRC-1/SRC-3 small molecule inhibitors comprise a new class of drugs that have potential as novel chemotherapeutics able to defeat aspects of acquired cancer cell resistance mechanisms.

Regulation of the BRCA1 gene by an SRC3/53BP1 complex

Background

Steroid Receptor coactivator 3(SRC3) is an oncogene and a member of the SRC family of nuclear receptor coactivator proteins that mediate the transcriptional effects of nuclear hormone receptors as well as other transcription factors.

Results

We have used protein purification and mass spectrometry to identify the 53BP1 tumour suppressor as a novel SRC3-associated protein. Copurification was demonstrated using multiple antibodies, and was not dependent on DNA damage suggesting that SRC3 is not directly involved in the DNA damage response. However using chromatin immunoprecipitation(ChIP) and siRNA knockdown, we have demonstrated that both SRC3 and 53BP1 co-occupy the same region of the BRCA1 promoter and both are required for BRCA1 expression in HeLa cells.

Conclusions

Our results suggest that both 53BP1 and SRC3 have a common function that converge at the BRCA1 promoter and possibly other genes important for DNA repair and genomic stability.

LIP expression is regulated by IGF-1R signaling and participates in suppression of anoikis

Background

The transcription factor, CCAAT enhancer binding protein-β (C/EBPβ), is expressed as several distinct protein isoforms (LAP1, LAP2 and LIP) that have opposing actions in cellular proliferation and differentiation. Increases in the ratio of LIP/LAP are associated with aggressive, metastatic breast cancer; however, little is known regarding the molecular mechanisms that regulate LIP expression or the biological actions of an increase in the LIP/LAP ratio. Metastasis is highly dependent upon the suppression of anoikis and the role of C/EBPβ and LIP in this anchorage-independent, survival process is currently not known in mammary epithelial cells. IGF-1R signaling is important for the survival of breast cancer cells and crosstalk between IGF-1R and EGFR signaling pathways have been implicated in the development of more aggressive disease. We therefore evaluated in mammary epithelial cells whether IGF-1R signaling regulates the LIP/LAP ratio, analyzed the potential interplay between EGFR and IGF-1R signaling and addressed the biological significance of increased LIP expression in cellular survival and suppression of anoikis.

Results

Our data provide the first evidence that IGF-1R signaling regulates LIP expression in an EGFR independent manner to increase the LIP/LAP ratio in mammary epithelial cells. Although crosstalk between IGF-1R signaling and EGFR signaling is detectable in MCF10A cells, this crosstalk is not required for the IGF-1 mediated regulation of LIP expression. Rather, the critical regulator of IGF-1 induced LIP expression appears to be EGFR-independent, Akt activity. Our data also demonstrate that increases in LIP expression promote cell survival via suppression of anoikis. Likewise, knockdown of total C/EBPβ leads to increased cell death and suggest that C/EBPβ expression is important for survival and resistance to anoikis. IGF-1 treatment can partially rescue vector control cells from anoikis; however, cells with reduced C/EBPβ expression do not survive anoikis.

Conclusions

Taken together, our data demonstrate that IGF-1R signaling regulates LIP expression in an EGFR independent manner to increase the LIP/LAP ratio in mammary epithelial cells. C/EBPβ expression and elevations in LIP play an important role in regulating cellular survival via suppression of anoikis, in an IGF-1R mediated context or in a manner independent of IGF-1R signaling.

MicroRNA: Biogenesis, Function and Role in Cancer

MicroRNAs are small, highly conserved non-coding RNA molecules involved in the regulation of gene expression. MicroRNAs are transcribed by RNA polymerases II and III, generating precursors that undergo a series of cleavage events to form mature microRNA. The conventional biogenesis pathway consists of two cleavage events, one nuclear and one cytoplasmic. However, alternative biogenesis pathways exist that differ in the number of cleavage events and enzymes responsible. How microRNA precursors are sorted to the different pathways is unclear but appears to be determined by the site of origin of the microRNA, its sequence and thermodynamic stability. The regulatory functions of microRNAs are accomplished through the RNA-induced silencing complex (RISC). MicroRNA assembles into RISC, activating the complex to target messenger RNA (mRNA) specified by the microRNA. Various RISC assembly models have been proposed and research continues to explore the mechanism(s) of RISC loading and activation. The degree and nature of the complementarity between the microRNA and target determine the gene silencing mechanism, slicer-dependent mRNA degradation or slicer-independent translation inhibition. Recent evidence indicates that P-bodies are essential for microRNA-mediated gene silencing and that RISC assembly and silencing occurs primarily within P-bodies. The P-body model outlines microRNA sorting and shuttling between specialized P-body compartments that house enzymes required for slicer –dependent and –independent silencing, addressing the reversibility of these silencing mechanisms. Detailed knowledge of the microRNA pathways is essential for understanding their physiological role and the implications associated with dysfunction and dysregulation.

Proteomic analysis of pathways involved in estrogen-induced growth and apoptosis of breast cancer cells

BACKGROUND

Estrogen is a known growth promoter for estrogen receptor (ER)-positive breast cancer cells. Paradoxically, in breast cancer cells that have been chronically deprived of estrogen stimulation, re-introduction of the hormone can induce apoptosis.

METHODOLOGY/PRINCIPAL FINDINGS

Here, we sought to identify signaling networks that are triggered by estradiol (E2) in isogenic MCF-7 breast cancer cells that undergo apoptosis (MCF-7:5C) versus cells that proliferate upon exposure to E2 (MCF-7). The nuclear receptor co-activator AIB1 (Amplified in Breast Cancer-1) is known to be rate-limiting for E2-induced cell survival responses in MCF-7 cells and was found here to also be required for the induction of apoptosis by E2 in the MCF-7:5C cells. Proteins that interact with AIB1 as well as complexes that contain tyrosine phosphorylated proteins were isolated by immunoprecipitation and identified by mass spectrometry (MS) at baseline and after a brief exposure to E2 for two hours. Bioinformatic network analyses of the identified protein interactions were then used to analyze E2 signaling pathways that trigger apoptosis versus survival. Comparison of MS data with a computationally-predicted AIB1 interaction network showed that 26 proteins identified in this study are within this network, and are involved in signal transduction, transcription, cell cycle regulation and protein degradation.

CONCLUSIONS

G-protein-coupled receptors, PI3 kinase, Wnt and Notch signaling pathways were most strongly associated with E2-induced proliferation or apoptosis and are integrated here into a global AIB1 signaling network that controls qualitatively distinct responses to estrogen.

Role of the nuclear receptor coactivator AIB1-Delta4 splice variant in the control of gene transcription

The oncogene amplified in breast cancer 1 (AIB1) is a nuclear receptor coactivator that plays a major role in the progression of various cancers. We previously identified a splice variant of AIB1 called AIB1-Δ4 that is overexpressed in breast cancer. Using mass spectrometry, we define the translation initiation of AIB1-Δ4 at Met(224) of the full-length AIB1 sequence and have raised an antibody to a peptide representing the acetylated N terminus. We show that AIB1-Δ4 is predominantly localized in the cytoplasm, although leptomycin B nuclear export inhibition demonstrates that AIB1-Δ4 can enter and traffic through the nucleus. Our data indicate an import mechanism enhanced by other coactivators such as p300/CBP. We report that the endogenously and exogenously expressed AIB1-Δ4 is recruited as efficiently as full-length AIB1 to estrogen-response elements of genes, and it enhances estrogen-dependent transcription more effectively than AIB1. Expression of an N-terminal AIB1 protein fragment, which is lost in the AIB1-Δ4 isoform, potentiates AIB1 as a coactivator. This suggests a model whereby the transcriptional activity of AIB1 is squelched by a repressive mechanism utilizing the N-terminal domain and that the increased coactivator function of AIB1-Δ4 is due to the loss of this inhibitory domain. Finally, we show, using Scorpion primer technology, that AIB1-Δ4 expression is correlated with metastatic capability of human cancer cell lines.

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.

Altered AIB1 or AIB1Δ3 expression impacts ERα effects on mammary gland stromal and epithelial content

Amplified in breast cancer 1 (AIB1) (also known as steroid receptor coactivator-3) is a nuclear receptor coactivator enhancing estrogen receptor (ER)α and progesterone receptor (PR)-dependent transcription in breast cancer. The splice variant AIB1Δ3 demonstrates increased ability to promote ERα and PR-dependent transcription. Both are implicated in breast cancer risk and antihormone resistance. Conditional transgenic mice tested the in vivo impact of AIB1Δ3 overexpression compared with AIB1 on histological features of increased breast cancer risk and growth response to estrogen and progesterone in the mammary gland. Combining expression of either AIB1 or AIB1Δ3 with ERα overexpression, we investigated in vivo cooperativity. AIB1 and AIB1Δ3 overexpression equivalently increased the prevalence of hyperplastic alveolar nodules but not ductal hyperplasia or collagen content. When AIB1 or AIB1Δ3 overexpression was combined with ERα, both stromal collagen content and ductal hyperplasia prevalence were significantly increased and adenocarcinomas appeared. Overexpression of AIB1Δ3, especially combined with overexpressed ERα, led to an abnormal response to estrogen and progesterone with significant increases in stromal collagen content and development of a multilayered mammary epithelium. AIB1Δ3 overexpression was associated with a significant increase in PR expression and PR downstream signaling genes. AIB1 overexpression produced less marked growth abnormalities and no significant change in PR expression. In summary, AIB1Δ3 overexpression was more potent than AIB1 overexpression in increasing stromal collagen content, inducing abnormal mammary epithelial growth, altering PR expression levels, and mediating the response to estrogen and progesterone. Combining ERα overexpression with either AIB1 or AIB1Δ3 overexpression augmented abnormal growth responses in both epithelial and stromal compartments.

Regulation of ERalpha-mediated transcription of Bcl-2 by PI3K-AKT crosstalk: implications for breast cancer cell survival

Both estrogen, through the estrogen receptor (ER), and growth factors, through the phosphatidylinositol-3-kinase (PI3K)-AKT pathway, have been shown to independently promote cell survival. Here, we investigated the role of ER/PI3K-AKT crosstalk in the regulation of cell survival in MCF-7 breast carcinoma cells. The ER inhibitor ICI 182,780 was used to determine the requirement of the ER for estrogen in the suppression of tumor necrosis factor-alpha (TNFalpha) induced apoptosis. Gene reporter assays and Western blot analyses were used to determine the involvement of the pro-survival factor Bcl-2 and the coactivator GRIP1 in this survival crosstalk. We demonstrated that an intact ER signaling pathway was required for estrogen to suppress apoptosis induced by TNFalpha. Our gene reporter assays revealed that ERalpha, not ERbeta, was targeted by AKT, resulting in transcriptional potentiation of the full-length Bcl-2 promoter, ultimately leading to increased Bcl-2 protein levels. AKT targeted both activation function (AF) domains of the ERalpha for maximal induction of Bcl-2 reporter activity, although the AF-II domain was predominately targeted. In addition, AKT also caused an upregulation of GRIP1 protein levels. Finally, AKT and GRIP1 cooperated to increase Bcl-2 protein expression to a greater level than either factor alone. Collectively, our study suggests a role for ER/PI3K-AKT crosstalk in cell survival and documents the ability of AKT to regulate Bcl-2 expression via differential activation of ERalpha and ERbeta as well as regulation of GRIP1.

Steroid receptor coactivator-3 expression in lung cancer and its role in the regulation of cancer cell survival and proliferation

Steroid receptor coactivator-3 (SRC-3) is a histone acetyltransferase and nuclear hormone receptor coactivator, located on 20q12, which is amplified in several epithelial cancers and well studied in breast cancer. However, its possible role in lung cancer pathogenesis is unknown. We found SRC-3 to be overexpressed in 27% of non-small cell lung cancer (NSCLC) patients (n = 311) by immunohistochemistry, which correlated with poor disease-free (P = 0.0015) and overall (P = 0.0008) survival. Twenty-seven percent of NSCLCs exhibited SRC-3 gene amplification, and we found that lung cancer cell lines expressed higher levels of SRC-3 than did immortalized human bronchial epithelial cells (HBEC), which in turn expressed higher levels of SRC-3 than did cultured primary human HBECs. Small interfering RNA-mediated downregulation of SRC-3 in high-expressing, but not in low-expressing, lung cancer cells significantly inhibited tumor cell growth and induced apoptosis. Finally, we found that SRC-3 expression is inversely correlated with gefitinib sensitivity and that SRC-3 knockdown results in epidermal growth factor receptor tyrosine kinase inhibitor-resistant lung cancers becoming more sensitive to gefitinib. Taken together, these data suggest that SRC-3 may be an important oncogene and therapeutic target for lung 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.

MicroRNA involvement in the pathogenesis and management of breast cancer

MicroRNAs (miRNAs) are a highly abundant class of endogenous small non-coding RNAs (18-25 nucleotides in length) that regulate gene expression by targeting protein-coding mRNAs post-transcriptionally. miRNAs have been implicated in cancer development and progression. As miRNAs and their regulatory functions are further revealed, the more the importance of miRNA-directed gene regulation is emphasised. In the human genome, 695 mature miRNAs have been identified, although computational calculation predicts that this may increase to >1000. Deregulation of miRNA expression profiles is thought to be implicated in the pathogenesis of many human cancers including breast tumours. Breast cancer subtypes are observed to have deranged miRNA expression signatures, which makes miRNAs important targets for developing a novel molecular classification of breast cancer and opening avenues for more individualised treatment strategies for patients with breast 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.

Involvement of SRC-3 in deguelin-induced apoptosis in Jurkat cells

The aim of the study was to investigate the anticancer effects and the molecular mechanisms of deguelin on Jurkat cells. Cell viability was assessed by MTT assay. Terminal deoxynucleotide transferase-mediated dUTP nick end labeling (TUNEL) assay and transmission electron microscopy were used to detect cell apoptosis. A propidium iodide method was used to study cell cycle distribution. RT-PCR and Western blotting were employed to assess the expression levels of steroid receptor coactivator-3 (SRC-3), nuclear factor-kappaB (NF-kappaB) and some apoptosis related genes, including Bcl-2 and Bcl-xL. Deguelin was able to inhibit cell proliferation by a cell-cycle arrest in the G(1)/G(0) phase and induce apoptosis in Jurkat cells in vitro, with a 24-h IC(50) value of 43.73 +/- 0.35 nmol/L. The antileukemia effect of deguelin might be correlated well with the downregulation of the expression of SRC-3 and its related transcription factor NF-kappaB, which thus influenced the expression of apoptosis related genes Bcl-2 and Bcl-xL. Deguelin presented potent effects on growth arrest and apoptosis induction in Jurkat cells in vitro via the interruption of SRC-3.

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.

Tyrosine phosphorylation of the nuclear receptor coactivator AIB1/SRC-3 is enhanced by Abl kinase and is required for its activity in cancer cells

Overexpression and activation of the steroid receptor coactivator amplified in breast cancer 1 (AIB1)/steroid receptor coactivator-3 (SRC-3) have been shown to have a critical role in oncogenesis and are required for both steroid and growth factor signaling in epithelial tumors. Here, we report a new mechanism for activation of SRC coactivators. We demonstrate regulated tyrosine phosphorylation of AIB1/SRC-3 at a C-terminal tyrosine residue (Y1357) that is phosphorylated after insulin-like growth factor 1, epidermal growth factor, or estrogen treatment of breast cancer cells. Phosphorylated Y1357 is increased in HER2/neu (v-erb-b2 erythroblastic leukemia viral oncogene homolog 2) mammary tumor epithelia and is required to modulate AIB1/SRC-3 coactivation of estrogen receptor alpha (ERalpha), progesterone receptor B, NF-kappaB, and AP-1-dependent promoters. c-Abl (v-Abl Abelson murine leukemia viral oncogene homolog 1) tyrosine kinase directly phosphorylates AIB1/SRC-3 at Y1357 and modulates the association of AIB1 with c-Abl, ERalpha, the transcriptional cofactor p300, and the methyltransferase coactivator-associated arginine methyltransferase 1, CARM1. AIB1/SRC-3-dependent transcription and phenotypic changes, such as cell growth and focus formation, can be reversed by an Abl kinase inhibitor, imatinib. Thus, the phosphorylation state of Y1357 can function as a molecular on/off switch and facilitates the cross talk between hormone, growth factor, and intracellular kinase signaling pathways in cancer.

Steroid receptor coactivator-3/AIB1 promotes cell migration and invasiveness through focal adhesion turnover and matrix metalloproteinase expression

Steroid receptor coactivator-3 (SRC-3)/AIB1 is a member of the p160 nuclear receptor coactivator family involved in development and cell cycle progression. We previously showed that SRC-3/AIB1 is required for prostate cancer cell proliferation and survival. Here, we reported that the elevated SRC-3/AIB1 expression is significantly correlated with human prostate cancer seminal vesicle invasion and lymph node metastasis. Furthermore, SRC-3/AIB1 is associated with increased prostate cancer cell migration and invasion. SRC-3/AIB1 is required for focal adhesion turnover and focal adhesion kinase activation. In addition, SRC-3/AIB1 directly regulates transcription of matrix metalloproteinase (MMP)-2 and MMP-13 through its coactivation of AP-1 and PEA3. Taken together, these data suggest that SRC-3/AIB1 plays an essential role in prostate cancer cell invasion and metastasis.

Phosphoinositide 3-kinase/AKT signaling can promote AIB1 stability independently of GSK3 phosphorylation

The transcriptional coactivator AIB1 is an oncogene overexpressed in different types of tumors, including breast cancer. Although the subcellular compartimentalization of AIB1 seems to be intimately linked to abnormal proliferation, the molecular mechanisms that regulate its subcellular distribution are not well defined. Here, we report that the nuclear accumulation and half-life of AIB1 vary between cancer cell lines. Using these differences as an experimental model, our results reveal that alterations to the Akt signaling pathway and nuclear export determine the stability of AIB1 and nuclear content of this coactivator. Moreover, our results show that AIB1 is degraded in the nucleus by the proteasome in an ubiquitin-dependent manner. However, this process does not require phosphorylation by GSK3, thereby revealing an alternative mechanism for regulating the turnover of AIB1. We define a new region at the carboxy terminus of AIB1 that is required for proteasome-dependent transcriptional activation and is preceded by a PEST domain that is required for adequate protein turnover. Based on differences in Akt signaling and the subcellular distribution of AIB1 between different cell lines, our results suggest that dysregulation of nuclear shuttling and proteasomal degradation may modulate the oncogenic potential of AIB1.

The nuclear receptor coactivator amplified in breast cancer-1 is required for Neu (ErbB2/HER2) activation, signaling, and mammary tumorigenesis in mice

Overexpression of the oncogene amplified in breast cancer 1 (AIB1)/steroid receptor coactivator-3 (SRC-3) induces mammary tumorigenesis in mice. In breast cancer, high levels of AIB1/SRC-3 and the growth factor receptor HER2/neu predict resistance to endocrine therapy and poor outcome. However, a mechanistic relationship between AIB1/SRC-3 and HER2/neu in the development of breast cancer has not been shown. Here, we show that deletion of one allele of SRC-3 significantly delays Neu-induced mammary tumor development in mice. Homozygous deletion of SRC-3 in mice completely prevents Neu-induced tumor formation. By ages 3 to 4 months, Neu/SRC-3(+/-) mice exhibit a noticeable reduction in lateral side-bud formation, accompanied by reduced cellular levels of phosphorylated Neu compared with Neu/SRC-3(wt) mice. In Neu-induced tumors, high levels of SRC-3, phosphorylated Neu, cyclin D1, cyclin E, and proliferating cell nuclear antigen expression are observed, accompanied by activation of the AKT and c-Jun NH(2) kinase (JNK) signaling pathways. In comparison, phosphorylated Neu, cyclin D1, and cyclin E are significantly decreased in Neu/SRC-3(+/-) tumors, proliferation is reduced, and AKT and JNK activation is barely detectable. Our data indicate that AIB1/SRC-3 is required for HER2/neu oncogenic activity and for the phosphorylation and activation of the HER2/neu receptor. We predict that reducing AIB1/SRC-3 levels or activity in the mammary epithelium could potentiate therapies aimed at inhibiting HER2/neu signaling in breast cancer.

Minireview: nuclear receptors and breast cancer

Until recently, the study of nuclear receptor (NR) function in breast cancer biology has been largely limited to estrogen and progesterone receptors. The development of reliable gene expression arrays, real-time quantitative RT-PCR, and immunohistochemical techniques for studying NR superfamily members in primary human breast cancers has now revealed the presence and potential importance of several additional NRs in the biology of breast cancer. These include receptors for steroid hormones (including androgens and corticosteroids), fat-soluble vitamins A and D, fatty acids, and xenobiotic lipids derived from diet. It is now clear that after NR activation, both genomic and nongenomic NR pathways can coordinately activate growth factor signaling pathways. Advances in our understanding of both NR functional networks and epithelial cell growth factor signaling pathways have revealed a frequent interplay between NR and epithelial cell growth factor family signaling that is clinically relevant to breast cancer. Understanding how growth factor receptors and their downstream kinases are activated by NRs (and vice-versa) is a central goal for maximizing treatment opportunities in breast cancer. In addition to the estrogen receptor, it is predicted that modulating the activity of other NRs will soon provide novel prevention and treatment approaches for breast cancer patients.

miRNAs: Little known mediators of oncogenesis

Cancer progression is mediated by overexpression of oncogenes and downregulation or loss of tumor suppressors. Proteins, which were traditionally categorized into these groups, have been recently joined by a species of RNA molecules known as microRNAs (miRNAs). miRNAs belong to a class of approximately 22-nt-long non-coding RNAs found in eukaryotes that hinder gene expression by inducing degradation or inhibiting translation of select mRNAs. A growing number of miRNAs have been implicated in promoting or suppressing tumorigenesis in a variety of tissues. The supporting evidence ranges from suggestive expression profiling data to direct functional validation using methods of forward and reverse genetics. We discuss the nature of published results, as well as the merits and pitfalls of various approaches aimed at identification of cancer-related miRNAs and their mRNA targets.

Proto-oncogene ACTR/AIB1 promotes cancer cell invasion by up-regulating specific matrix metalloproteinase expression

Overexpression of ACTR/AIB1 is frequently found in different cancers with distant metastasis. To address its possible involvement in tumor metastasis, we performed invasion assays to examine the effect of ACTR alteration on the invasiveness of breast cancer cells (MDA-MB-231 or T-47D) and found that high levels of ACTR are required for their strong invasiveness. Molecular analysis indicates that ACTR functions as a coactivator of AP-1 to up-regulate the expression of matrix metalloproteinases such as MMP-7 and MMP-10 and reduce cell adhesion to specific extracellular matrix proteins. These novel findings provide a mechanistic link between ACTR and MMPs, and suggest that ACTR may also play an important role in cancer progression by facilitating tumor invasion.

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.

Epidermal growth factor receptor tyrosine phosphorylation and signaling controlled by a nuclear receptor coactivator, amplified in breast cancer 1

The steroid receptor coactivator amplified in breast cancer 1 (AIB1) as well as epidermal growth factor receptor (EGFR) family members are frequently overexpressed in epithelial tumors, and their expression is associated with poor prognosis. However, a direct role of AIB1 in EGF signaling has not been determined. To address this, we reduced endogenous AIB1 levels using RNA interference in lung, breast, and pancreatic cancer cell lines. We found that a knockdown of AIB1 levels resulted in a loss of the growth response of these cell lines to EGF. Further analysis revealed that the depletion of AIB1 reduced tyrosine phosphorylation of EGFR at multiple residues both at autophosphorylation and Src kinase phosphorylation sites. AIB1 knockdown did not affect tyrosine phosphorylation of the receptor tyrosine kinases, platelet-derived growth factor receptor and HER3, or overall tyrosine phosphorylation of cellular proteins. However, EGF-dependent phosphorylation of HER2 was decreased. EGFR levels and membrane trafficking were not changed by AIB1 depletion, but there was less recruitment of Src homology 2 domain-containing proteins to the EGFR. This led to a substantial reduction in EGF-induced phosphorylation of signal transducers and activators of transcription 5 and c-Jun NH(2)-terminal kinase but no significant change in the activation of AKT. Vanadate treatment of cells revealed that the reduction in EGFR tyrosine phosphorylation is dependent in part on changes in cellular phosphatase activity. We propose that a portion of the oncogenic effect of AIB1 could be through control of EGFR and HER2 activity and subsequent modulation of cellular signaling pathways.