Cyclin D1 is required for transformation by activated Neu and is induced through an E2F-dependent signaling pathway

Gene regulatory network underlying the immortalization of epithelial cells

BACKGROUND

Tumorigenic transformation of human epithelial cells in vitro has been described experimentally as the potential result of spontaneous immortalization. This process is characterized by a series of cell-state transitions, in which normal epithelial cells acquire first a senescent state which is later surpassed to attain a mesenchymal stem-like phenotype with a potentially tumorigenic behavior. In this paper we aim to provide a system-level mechanistic explanation to the emergence of these cell types, and to the time-ordered transition patterns that are common to neoplasias of epithelial origin. To this end, we first integrate published functional and well-curated molecular data of the components and interactions that have been found to be involved in such cell states and transitions into a network of 41 molecular components. We then reduce this initial network by removing simple mediators (i.e., linear pathways), and formalize the resulting regulatory core into logical rules that govern the dynamics of each of the network components as a function of the states of its regulators.

RESULTS

Computational dynamic analysis shows that our proposed Gene Regulatory Network model recovers exactly three attractors, each of them defined by a specific gene expression profile that corresponds to the epithelial, senescent, and mesenchymal stem-like cellular phenotypes, respectively. We show that although a mesenchymal stem-like state can be attained even under unperturbed physiological conditions, the likelihood of converging to this state is increased when pro-inflammatory conditions are simulated, providing a systems-level mechanistic explanation for the carcinogenic role of chronic inflammatory conditions observed in the clinic. We also found that the regulatory core yields an epigenetic landscape that restricts temporal patterns of progression between the steady states, such that recovered patterns resemble the time-ordered transitions observed during the spontaneous immortalization of epithelial cells, both in vivo and in vitro.

CONCLUSION

Our study strongly suggests that the in vitro tumorigenic transformation of epithelial cells, which strongly correlates with the patterns observed during the pathological progression of epithelial carcinogenesis in vivo, emerges from underlying regulatory networks involved in epithelial trans-differentiation during development.

Periodic expression of Kv10.1 driven by pRb/E2F1 contributes to G2/M progression of cancer and non-transformed cells

Progression of cell cycle is associated with changes in K⁺ channel expression and activity. In this study, we report that Kv10.1, a K⁺ channel that increases cell proliferation and tumor growth, is regulated at the transcriptional level by the pRb/E2F1 pathway. De-repression of E2F1 by HPV-E7 oncoprotein leads to increased expression of Kv10.1. In proliferating cells, E2F1 transcription factor binds directly to the Kv10.1 promoter during (or close to) G2/M, resulting in transient expression of the channel. Importantly, this happens not only in cancer cells but also in non-transformed cells. Lack of Kv10.1 in both cancer and non-transformed cells resulted in prolonged G2/M phase, as indicated by phosphorylation of Cdk1 (Y15) and sustained pRb hyperphosphorylation. Our results strongly suggest that Kv10.1 expression is coupled to cell cycle progression and facilitates G2/M progression in both healthy and tumor cells.

Cyclin D1, cancer progression, and opportunities in cancer treatment

Mammalian cells encode three D cyclins (D1, D2, and D3) that coordinately function as allosteric regulators of cyclin-dependent kinase 4 (CDK4) and CDK6 to regulate cell cycle transition from G1 to S phase. Cyclin expression, accumulation, and degradation, as well as assembly and activation of CDK4/CDK6 are governed by growth factor stimulation. Cyclin D1 is more frequently dysregulated than cyclin D2 or D3 in human cancers, and as such, it has been more extensively characterized. Overexpression of cyclin D1 results in dysregulated CDK activity, rapid cell growth under conditions of restricted mitogenic signaling, bypass of key cellular checkpoints, and ultimately, neoplastic growth. This review discusses cyclin D1 transcriptional, translational, and post-translational regulations and its biological function with a particular focus on the mechanisms that result in its dysregulation in human cancers.

Evaluation of heterocyclic steroids and curcumin derivatives as anti-breast cancer agents: Studying the effect on apoptosis in MCF-7 breast cancer cells

Anticancer agents consisting of hybrid molecules are used to improve effectiveness and diminish drug resistance. The current study aimed to introduce newly synthesized hetero-steroids of promising anticancer effects. Besides, the pro-apoptotic effects of new compounds were investigated extensively. Several pyrimidino-, triazolopyrimidino-, pyridazino-, and curcumin-steroid derivatives were synthesized, elucidated and confirmed using the spectral and analytical data. The synthesized hetero-steroids, compounds 9, 10, 11, 12, 13, 14, 15, 18, 20, 21, 22 and 24, were tested for their cytotoxic effects versus human breast cancer cells (MCF-7) using neutral red supravital dye uptake assay. Compound 24 (IC50=18μM) showed more inhibitory influence on MCF-7 growth. Using QRT-PCR (Quantitative real time-polymerase chain reaction), CCND1, Survivin, BCL-2, CDC2, P21 and P53, genes expression levels were investigated. The study results disclose that compounds 4, 7, 18, 24 knocked down the expression levels of CCND1, Survivin, BCL-2 and CDC2. However, P21 and P53 were up-regulated by compounds 21, 22. This study introduced promising pro-apoptotic anticancer agents acting through the modulation of key regulators of apoptosis and cell cycle genes.

The Dipole Potential Modifies the Clustering and Ligand Binding Affinity of ErbB Proteins and Their Signaling Efficiency

Although activation of the ErbB family of receptor tyrosine kinases (ErbB1-4) is driven by oligomerization mediated by intermolecular interactions between the extracellular, the kinase and the transmembrane domains, the transmembrane domain has been largely neglected in this regard. The largest contributor to the intramembrane electric field, the dipole potential, alters the conformation of transmembrane peptides, but its effect on ErbB proteins is unknown. Here, we show by Förster resonance energy transfer (FRET) and number and brightness (N&B) experiments that the epidermal growth factor (EGF)-induced increase in the homoassociation of ErbB1 and ErbB2 and their heteroassociation are augmented by increasing the dipole potential. These effects were even more pronounced for ErbB2 harboring an activating Val → Glu mutation in the transmembrane domain (NeuT). The signaling capacity of ErbB1 and ErbB2 was also correlated with the dipole potential. Since the dipole potential decreased the affinity of EGF to ErbB1, the augmented growth factor-induced effects at an elevated dipole potential were actually induced at lower receptor occupancy. We conclude that the dipole potential plays a permissive role in the clustering of ErbB receptors and that the effects of lipid rafts on ligand binding and receptor signaling can be partially attributed to the dipole potential.

v-Src Oncogene Induces Trop2 Proteolytic Activation via Cyclin D1

Proteomic analysis of castration-resistant prostate cancer demonstrated the enrichment of Src tyrosine kinase activity in approximately 90% of patients. Src is known to induce cyclin D1, and a cyclin D1-regulated gene expression module predicts poor outcome in human prostate cancer. The tumor-associated calcium signal transducer 2 (TACSTD2/Trop2/M1S1) is enriched in the prostate, promoting prostate stem cell self-renewal upon proteolytic activation via a γ-secretase cleavage complex (PS1, PS2) and TACE (ADAM17), which releases the Trop2 intracellular domain (Trop2 ICD). Herein, v-Src transformation of primary murine prostate epithelial cells increased the proportion of prostate cancer stem cells as characterized by gene expression, epitope characteristics, and prostatosphere formation. Cyclin D1 was induced by v-Src, and Src kinase induction of Trop2 ICD nuclear accumulation required cyclin D1. Cyclin D1 induced abundance of the Trop2 proteolytic cleavage activation components (PS2, TACE) and restrained expression of the inhibitory component of the Trop2 proteolytic complex (Numb). Patients with prostate cancer with increased nuclear Trop2 ICD and cyclin D1, and reduced Numb, had reduced recurrence-free survival probability (HR = 4.35). Cyclin D1, therefore, serves as a transducer of v-Src-mediated induction of Trop2 ICD by enhancing abundance of the Trop2 proteolytic activation complex. Cancer Res; 76(22); 6723-34. ©2016 AACR.

Treating cancer with selective CDK4/6 inhibitors

Uncontrolled cellular proliferation, mediated by dysregulation of the cell-cycle machinery and activation of cyclin-dependent kinases (CDKs) to promote cell-cycle progression, lies at the heart of cancer as a pathological process. Clinical implementation of first-generation, nonselective CDK inhibitors, designed to inhibit this proliferation, was originally hampered by the high risk of toxicity and lack of efficacy noted with these agents. The emergence of a new generation of selective CDK4/6 inhibitors, including ribociclib, abemaciclib and palbociclib, has enabled tumour types in which CDK4/6 has a pivotal role in the G1-to-S-phase cell-cycle transition to be targeted with improved effectiveness, and fewer adverse effects. Results of pivotal phase III trials investigating palbociclib in patients with advanced-stage oestrogen receptor (ER)-positive breast cancer have demonstrated a substantial improvement in progression-free survival, with a well-tolerated toxicity profile. Mechanisms of acquired resistance to CDK4/6 inhibitors are beginning to emerge that, although unwelcome, might enable rational post-CDK4/6 inhibitor therapeutic strategies to be identified. Extending the use of CDK4/6 inhibitors beyond ER-positive breast cancer is challenging, and will likely require biomarkers that are predictive of a response, and the use of combination therapies in order to optimize CDK4/6 targeting.

Kinase-independent role of cyclin D1 in chromosomal instability and mammary tumorigenesis

Cyclin D1 is an important molecular driver of human breast cancer but better understanding of its oncogenic mechanisms is needed, especially to enhance efforts in targeted therapeutics. Currently, pharmaceutical initiatives to inhibit cyclin D1 are focused on the catalytic component since the transforming capacity is thought to reside in the cyclin D1/CDK activity. We initiated the following study to directly test the oncogenic potential of catalytically inactive cyclin D1 in an in vivo mouse model that is relevant to breast cancer. Herein, transduction of cyclin D1^−/− mouse embryonic fibroblasts (MEFs) with the kinase dead KE mutant of cyclin D1 led to aneuploidy, abnormalities in mitotic spindle formation, autosome amplification, and chromosomal instability (CIN) by gene expression profiling. Acute transgenic expression of either cyclin D1^WT or cyclin D1^KE in the mammary gland was sufficient to induce a high CIN score within 7 days. Sustained expression of cyclin D1^KE induced mammary adenocarcinoma with similar kinetics to that of the wild-type cyclin D1. ChIP-Seq studies demonstrated recruitment of cyclin D1^WT and cyclin D1^KE to the genes governing CIN. We conclude that the CDK-activating function of cyclin D1 is not necessary to induce either chromosomal instability or mammary tumorigenesis.

Cyclin alterations in diverse cancers: Outcome and co-amplification network

Cyclin genes are key regulatory components of the cell cycle. With the development of new agents, cyclin-related genes are becoming increasingly important as they can be targeted. Yet, the biological implications of these alterations have not been fully studied. Clinical characteristics and outcome parameters were compared for patients harboring cyclin alterations versus not. CCN alterations were found in 13% of our population (50/392; all amplifications) and were associated with breast cancer (P < 0.0001), a higher median number of concomitant molecular alterations (P < 0.0001), and liver metastases (P = 0.046). Harboring a cyclin amplification was not associated with overall survival, the time to metastasis/recurrence, nor with the best progression-free survival. In a Cox regression model, gastrointestinal histology (P < 0.0001), PTEN (P < 0.0001), and CDK alterations (P = 0.041) had a significant association with poorer overall survival. CCN amplifications significantly correlated with alterations in FGF/FGFR family genes as well as in MET and ARFRP1. An extended correlation study shed light on a network of co-amplifications influenced in part by genes that were localized on the same amplicons. CCN amplifications are common across cancers and had distinctive biological associations. Customized combinations targeting the cyclin pathway as well as the extended co-amplification network may be necessary in order to address resistance mechanisms.

Cyclin D1 Promotes Androgen-Dependent DNA Damage Repair in Prostate Cancer Cells

Therapy resistance and poor outcome in prostate cancer is associated with increased expression of cyclin D1. Androgens promote DNA double-strand break repair to reduce DNA damage, and cyclin D1 was also shown to enhance DNA damage repair (DDR). In this study, we investigated the significance of cyclin D1 in androgen-induced DDR using established prostate cancer cells and prostate tissues from cyclin D1 knockout mice. We demonstrate that endogenous cyclin D1 further diminished the dihydrotestosterone (DHT)-dependent reduction of γH2AX foci in vitro. We also show that cyclin D1 was required for the androgen-dependent DNA damage response both in vitro and in vivo. Furthermore, cyclin D1 was required for androgen-enhanced DDR and radioresistance of prostate cancer cells. Moreover, microarray analysis of primary prostate epithelial cells from cyclin D1-deficient and wild-type mice demonstrated that most of the DHT-dependent gene expression changes are also cyclin D1 dependent. Collectively, our findings suggest that the hormone-mediated recruitment of cyclin D1 to sites of DDR may facilitate the resistance of prostate cancer cells to DNA damage therapies and highlight the need to explore other therapeutic approaches in prostate cancer to prevent or overcome drug resistance.

The role of microRNAs in the pathogenesis of HIV-related lymphomas

The incidence of HIV-related lymphomas (HRLs) is increased by 60-100 times in patients with HIV. When compared to the general population, patients with HRLs often present with extranodal lymphoid proliferation, most frequently of the gastrointestinal tract, central nervous system, liver and bone marrow. MicroRNAs (miRs) are non-coding double-stranded RNA molecules of 18-25 nucleotides that regulate post-translational gene expression by inhibiting translation or promoting degradation of messenger RNA complementary sequences. Before their discovery, tumorigenesis was thought to have been caused by the alteration of protein-coding oncogenes and tumor-suppressor genes, but once identified in B-cell chronic lymphocytic leukemia, miRs function as either oncogenes or tumor-suppressor genes was confirmed in different types of malignancies. Since miRs are clearly involved in tumorigenesis in many cancers, their role in HRLs is now receiving attention. A few studies have been conducted thus far in some HRLs on the involvement of miR in the pathogenesis of lymphoid malignancies. Since B-cell lymphomas arise from various stages of B-cell development in both HIV-infected and HIV-naïve patients, investigators have tried to determine the different miR signatures in B-cell development. As classic immunohistochemistry staining is sometimes not enough for the differential diagnosis of HRLs, in the present review, we have described the potential use of miRs in the prognosis and diagnosis of these diseases.

Single and Multiple Gene Manipulations in Mouse Models of Human Cancer

Mouse models of human cancer play a critical role in understanding the molecular and cellular mechanisms of tumorigenesis. Advances continue to be made in modeling human disease in a mouse, though the relevance of a mouse model often relies on how closely it is able to mimic the histologic, molecular, and physiologic characteristics of the respective human cancer. A classic use of a genetically engineered mouse in studying cancer is through the overexpression or deletion of a gene. However, the manipulation of a single gene often falls short of mimicking all the characteristics of the carcinoma in humans; thus a multiple gene approach is needed. Here we review genetic mouse models of cancers and their abilities to recapitulate human carcinoma with single versus combinatorial approaches with genes commonly involved in cancer.

Targeting ErbB-2 nuclear localization and function inhibits breast cancer growth and overcomes trastuzumab resistance

Membrane overexpression of ErbB-2/HER2 receptor tyrosine kinase (membrane ErbB-2 (MErbB-2)) has a critical role in breast cancer (BC). We and others have also shown the role of nuclear ErbB-2 (NErbB-2) in BC, whose presence we identified as a poor prognostic factor in MErbB-2-positive tumors. Current anti-ErbB-2 therapies, as with the antibody trastuzumab (Ttzm), target only MErbB-2. Here, we found that blockade of NErbB-2 action abrogates growth of BC cells, sensitive and resistant to Ttzm, in a scenario in which ErbB-2, ErbB-3 and Akt are phosphorylated, and ErbB-2/ErbB-3 dimers are formed. Also, inhibition of NErbB-2 presence suppresses growth of a preclinical BC model resistant to Ttzm. We showed that at the cyclin D1 promoter, ErbB-2 assembles a transcriptional complex with Stat3 (signal transducer and activator of transcription 3) and ErbB-3, another member of the ErbB family, which reveals the first nuclear function of ErbB-2/ErbB-3 dimer. We identified NErbB-2 as the major proliferation driver in Ttzm-resistant BC, and demonstrated that Ttzm inability to disrupt the Stat3/ErbB-2/ErbB-3 complex underlies its failure to inhibit growth. Furthermore, our results in the clinic revealed that nuclear interaction between ErbB-2 and Stat3 correlates with poor overall survival in primary breast tumors. Our findings challenge the paradigm of anti-ErbB-2 drug design and highlight NErbB-2 as a novel target to overcome Ttzm resistance.

miR-17/20 sensitization of breast cancer cells to chemotherapy-induced apoptosis requires Akt1

The serine threonine kinase Akt1 has been implicated in the control of cellular metabolism, survival and growth. Herein, disruption of the ubiquitously expressed member of the Akt family of genes, Akt1, in the mouse, demonstrates a requirement for Akt1 in miRNA-mediated cellular apoptosis. The miR-17/20 cluster is known to inhibit breast cancer cellular proliferation through G1/S cell cycle arrest via binding to the cyclin D1 3'UTR. Here we show that miR-17/20 overexpression sensitizes cells to apoptosis induced by either Doxorubicin or UV irradiation in MCF-7 cells via Akt1. miR-17/20 mediates apoptosis via increased p53 expression which promotes Akt degradation. Akt1 −/− mammary epithelial cells which express Akt2 and Akt3 demonstrated increased apoptosis to DNA damaging agents. Akt1 deficiency abolished the miR-17/20-mediated apoptosis. These results demonstrated a novel pathway through which miR17/20 regulate p53 and Akt controlling breast cancer cell apoptosis.

E2F activators signal and maintain centrosome amplification in breast cancer cells

Centrosomes ensure accurate chromosome segregation by directing spindle bipolarity. Loss of centrosome regulation results in centrosome amplification, multipolar mitosis and aneuploidy. Since centrosome amplification is common in premalignant lesions and breast tumors, it is proposed to play a central role in breast tumorigenesis, a hypothesis that remains to be tested. The coordination between the cell and centrosome cycles is of paramount importance to maintain normal centrosome numbers, and the E2Fs may be responsible for regulating these cycles. However, the role of E2F activators in centrosome amplification is unclear. Because E2Fs are deregulated in Her2(+) cells displaying centrosome amplification, we addressed whether they signal this abnormal process. Knockdown of E2F1 or E2F3 in Her2(+) cells decreased centrosome amplification without significantly affecting cell cycle progression, whereas the overexpression of E2F1, E2F2, or E2F3 increased centrosome amplification in MCF10A mammary epithelial cells. Our results revealed that E2Fs affect the expression of proteins, including Nek2 and Plk4, known to influence the cell/centrosome cycles and mitosis. Downregulation of E2F3 resulted in cell death and delays/blocks in cytokinesis, which was reversed by Nek2 overexpression. Nek2 overexpression enhanced centrosome amplification in Her2(+) breast cancer cells silenced for E2F3, revealing a role for the E2F activators in maintaining centrosome amplification in part through Nek2.

Depression of testes-specific protease 50 (TSP50) inhibits cell proliferation and induces apoptosis in laryngocarcinoma

Testes-specific protease 50 (TSP50) is a potential cancer-associated gene that may be involved in human laryngocarcinoma. The present study aimed to investigate suppressive effects on the HEp2 human laryngocarcinoma cell line by transfection with TSP50-specific short hairpin RNA (shRNA). Western blot analysis was used to detect the expression levels of TSP50. MTT assay was used to evaluate cell proliferation. Wound healing was used in cell migration and invasion assays to evaluate the cell exercise capacity. Nuclear staining assay was used to evaluate cell apoptosis after knockdown of TSP50. Expression levels of TSP50 protein in the shRNA group were downregulated successfully by transfection, and the knockdown of endogenous TSP50 in HEp2 cells greatly inhibited nuclear factor κB (NF-κB) activity. MTT results showed that the cell proliferation in the shRNA group was significantly more depressed than that in the blank (P < 0.05) and negative control groups (P < 0.05). Additionally, a decreased number of migrated cells in the shRNA group was observed (P < 0.05) using a cell migration and invasion assay. Moreover, knockdown of endogenous TSP50 expression can induce apoptosis in HEp2 Cells. These data indicated that knockdown of TSP50 may cause inhibition of proliferation, migration, and invasion of HEp2 cells. This study provides a new perspective in understanding the molecular mechanisms underlying the progression of laryngocarcinoma and offers a potential therapeutic target for laryngocarcinoma.

Transcriptional regulation of oncogenic protein kinase Cϵ (PKCϵ) by STAT1 and Sp1 proteins

Overexpression of PKCϵ, a kinase associated with tumor aggressiveness and widely implicated in malignant transformation and metastasis, is a hallmark of multiple cancers, including mammary, prostate, and lung cancer. To characterize the mechanisms that control PKCϵ expression and its up-regulation in cancer, we cloned an ∼ 1.6-kb promoter segment of the human PKCϵ gene (PRKCE) that displays elevated transcriptional activity in cancer cells. A comprehensive deletional analysis established two regions rich in Sp1 and STAT1 sites located between -777 and -105 bp (region A) and -921 and -796 bp (region B), respectively, as responsible for the high transcriptional activity observed in cancer cells. A more detailed mutagenesis analysis followed by EMSA and ChIP identified Sp1 sites in positions -668/-659 and -269/-247 as well as STAT1 sites in positions -880/-869 and -793/-782 as the elements responsible for elevated promoter activity in breast cancer cells relative to normal mammary epithelial cells. RNAi silencing of Sp1 and STAT1 in breast cancer cells reduced PKCϵ mRNA and protein expression, as well as PRKCE promoter activity. Moreover, a strong correlation was found between PKCϵ and phospho-Ser-727 (active) STAT1 levels in breast cancer cells. Our results may have significant implications for the development of approaches to target PKCϵ and its effectors in cancer therapeutics.

Cyclin D1 integrates estrogen-mediated DNA damage repair signaling

The cyclin D1 gene encodes the regulatory subunit of a holoenyzme that phosphorylates the retinoblastoma protein (pRb) and nuclear respiratory factor (NRF1) proteins. The abundance of cyclin D1 determines estrogen-dependent gene expression in the mammary gland of mice. Using estradiol (E2) and an E2-dendrimer conjugate that is excluded from the nucleus, we demonstrate that E2 delays the DNA damage response (DDR) via an extranuclear mechanism. The E2-induced DDR required extranuclear cyclin D1, which bound ERα at the cytoplasmic membrane and augmented AKT phosphorylation (Ser473) and γH2AX foci formation. In the nucleus, E2 inhibited, whereas cyclin D1 enhanced homology-directed DNA repair. Cyclin D1 was recruited to γH2AX foci by E2 and induced Rad51 expression. Cyclin D1 governs an essential role in the E2-dependent DNA damage response via a novel extranuclear function. The dissociable cytoplasmic function to delay the E2-mediated DDR together with the nuclear enhancement of DNA repair uncovers a novel extranuclear function of cyclin D1 that may contribute to the role of E2 in breast tumorigenesis.

Overview of cyclins D1 function in cancer and the CDK inhibitor landscape: past and present

INTRODUCTION

Intensive efforts, over the last decade, have been made to inhibit the kinase activity of cyclins that act as mediators during cell-cycle progression. Activation of the cyclin D1 oncogene, often by amplification or rearrangement, is a major driver of multiple types of human tumors including breast and squamous cell cancers, B-cell lymphoma, myeloma and parathyroid adenoma.

AREAS COVERED

In this review, the authors summarize the activity of cyclins and cyclin-dependent kinases in cell-cycle progression and transcription. They focus on cyclin D1/CDK4/CDK6, a central mediator in the transition from G1 to S phase. Furthermore, the authors discuss the first generation of pan-cyclin-dependent kinase inhibitors that failed to meet expectation and discuss, in detail, the second generation of highly specific cyclin D1/CDK4/CDK6 inhibitors that are proving to be more efficacious.

EXPERT OPINION

The mechanism by which cyclin D1 drives tumorigenesis may be dependent on kinase and kinase-independent functions. Further evidence is necessary to delineate the roles of cyclin D1 in early pre-neoplastic lesions where its overexpression may promote genomic instability in a kinase-independent manner.

When Good Turns Bad: Regulation of Invasion and Metastasis by ErbB2 Receptor Tyrosine Kinase

Overexpression and activation of ErbB2 receptor tyrosine kinase in breast cancer is strongly linked to an aggressive disease with high potential for invasion and metastasis. In addition to inducing very aggressive, metastatic cancer, ErbB2 activation mediates processes such as increased cancer cell proliferation and survival and is needed for normal physiological activities, such as heart function and development of the nervous system. How does ErbB2 activation make cancer cells invasive and when? Comprehensive understanding of the cellular mechanisms leading to ErbB2-induced malignant processes is necessary for answering these questions. Here we present current knowledge about the invasion-promoting function of ErbB2 and the mechanisms involved in it. Obtaining detailed information about the "bad" behavior of ErbB2 can facilitate development of novel treatments against ErbB2-positive cancers.

HER2/Neu tumorigenesis and metastasis is regulated by E2F activator transcription factors

HER2 / Neu is amplified and overexpressed in a large proportion of human breast cancers, but the signaling pathways that contribute to tumor development and metastatic progression are not completely understood. Using gene expression data and pathway signatures we predicted a role for activator E2F transcription factors in Neu induced tumors. This was genetically tested by interbreeding Neu transgenics with knockouts of the three activator E2Fs. Loss of any E2F delayed Neu induced tumor onset. E2F1 loss accelerated tumor growth while E2F2 and E2F3 loss did not. Strikingly, it was observed that loss of E2F1 or E2F2 significantly reduced the metastatic capacity of the tumor and this was associated with a reduction in circulating tumor cells in the E2F2 knockout. Gene expression analysis between the tumors in the various E2F mutant backgrounds revealed that there was extensive compensation by other E2F family members in the individual knockouts, underscoring the importance of the E2Fs in HER2 / Neu induced tumors. Extension to HER2 positive human breast cancer revealed a number of HER2+ subtypes based on E2F activity with differences in relapse free survival times. Taken together these data demonstrate that the E2F transcription factors are integral to HER2+ tumor development and progression.

Aryl hydrocarbon receptor ligands inhibit igf-ii and adipokine stimulated breast cancer cell proliferation

Obesity increases human cancer risk and the risk for cancer recurrence. Adipocytes secrete paracrine factors termed adipokines that stimulate signaling in cancer cells that induce proliferation. The aryl hydrocarbon receptor (AHR) is a ligand-activated transcription factor that plays roles in tumorigenesis, is regulated by exogenous lipophilic chemicals, and has been explored as a therapeutic target for cancer therapy. Whether exogenous AHR ligands modulate adipokine stimulated breast cancer cell proliferation has not been investigated. We provide evidence that adipocytes secrete insulin-like growth factor 2 (IGF-2) at levels that stimulate the proliferation of human estrogen receptor (ER) positive breast cancer cells. Using highly specific AHR ligands and AHR short interfering RNA (AHR-siRNA), we show that specific ligand-activated AHR inhibits adipocyte secretome and IGF-2-stimulated breast cancer cell proliferation. We also report that a highly specific AHR agonist significantly (P < 0.05) inhibits the expression of E2F1, CCND1 (known as Cyclin D1), MYB, SRC, JAK2, and JUND in breast cancer cells. Collectively, these data suggest that drugs that target the AHR may be useful for treating cancer in human obesity.

Cooperation between Dmp1 loss and cyclin D1 overexpression in breast cancer

Cyclin D1 is a component of the core cell-cycle machinery and is frequently overexpressed in breast cancer. It physically interacts with the tumor suppressor Dmp1 that attenuates the oncogenic signals from Ras and HER2 by inducing Arf/p53-dependent cell-cycle arrest. Currently, the biological significance of Dmp1-cyclin D1 interplay in breast cancer has not been determined. Here, we show that cyclin D1 bound to Dmp1 to activate both Arf and Ink4a promoters and, consequently, induced apoptosis or G2/M cell-cycle delay in normal cells to protect them from neoplastic transformation. The cyclin D1-induced Ink4a/Arf gene expression was dependent on Dmp1 because the induction was not detected in Dmp1-deficient or DMP1-depleted cells. Arf/Ink4a expression was increased in pre-malignant mammary glands from Dmp1(+/+);MMTV-cyclin D1 and Dmp1(+/+);MMTV-D1T286A mice but significantly down-regulated in those from Dmp1-deficient mice. Selective Dmp1 deletion was found in 21% of the MMTV-D1 and D1T286A mammary carcinomas, and the Dmp1 heterozygous status significantly accelerated mouse mammary tumorigenesis with reduced apoptosis and increased metastasis. Overall, our study reveals a pivotal role of combined Dmp1 loss and cyclin D1 overexpression in breast cancer.

Cyclin D1 determines estrogen signaling in the mammary gland in vivo

The CCND1 gene, which is frequently overexpressed in cancers, encodes the regulatory subunit of a holoenzyme that phosphorylates the retinoblastoma protein. Although it is known that cyclin D1 regulates estrogen receptor (ER)α transactivation using heterologous reporter systems, the in vivo biological significance of cyclin D1 to estrogen-dependent signaling, and the molecular mechanisms by which cyclin D1 is involved, are yet to be elucidated. Herein, genome-wide expression profiling conducted of 17β-estradiol-treated castrated virgin mice deleted of the Ccnd1 gene demonstrated that cyclin D1 determines estrogen-dependent gene expression for 88% of estrogen-responsive genes in vivo. In addition, expression profiling of 17β-estradiol-stimulated cyclin D1 small interfering RNA treated MCF7 cells shows cyclin D1 is required for estrogen-mediated gene expression in vitro. Genome-wide chromatin immunoprecipitation-Seq analysis revealed a cyclin D1-DNA bound form associated with genes that were regulated by estrogen in a cyclin D1-dependent manner. The cyclin D1-dependent estrogen signaling pathways identified in vivo were highly enriched for extracellular membrane-associated growth factor receptors (epidermal growth factor receptor, ErbB3, and EphB3) and their ligands (amphiregulin, encoded by AREG gene), and matrix metalloproteinase. The AREG protein, a pivotal ligand for epidermal growth factor receptors to promote cellular proliferation, was induced by cyclin D1 via the AREG promoter. Chromatin immunoprecipitation analysis demonstrated the recruitment of cyclin D1 to the breast cancer 1 (Brca1)/ERα binding site of the Areg gene. Cyclin D1 genetic deletion demonstrated the in vivo requirement for cyclin D1 in assembling the estrogen-dependent amplified in breast cancer 1-associated multiprotein complex. The current studies define a requirement for cyclin D1 in estrogen-dependent signaling modules governing growth factor receptor and ligand expression in vivo and reveal a noncanonical function of cyclin D1 at ERα target gene promoters. Cyclin D1 mediates the convergence of ERα and growth factor signaling at a common cis-element of growth factor genes.

Pax6 exerts regional control of cortical progenitor proliferation via direct repression of Cdk6 and hypophosphorylation of pRb

The mechanisms by which early spatiotemporal expression patterns of transcription factors such as Pax6 regulate cortical progenitors in a region-specific manner are poorly understood. Pax6 is expressed in a gradient across the developing cortex and is essential for normal corticogenesis. We found that constitutive or conditional loss of Pax6 increases cortical progenitor proliferation by amounts that vary regionally with normal Pax6 levels. We compared the gene expression profiles of equivalent Pax6-expressing progenitors isolated from Pax6^+/+ and Pax6^−/− cortices and identified many negatively regulated cell-cycle genes, including Cyclins and Cdks. Biochemical assays indicated that Pax6 directly represses Cdk6 expression. Cyclin/Cdk repression inhibits retinoblastoma protein (pRb) phosphorylation, thereby limiting the transcription of genes that directly promote the mechanics of the cell cycle, and we found that Pax6 inhibits pRb phosphorylation and represses genes involved in DNA replication. Our results indicate that Pax6’s modulation of cortical progenitor cell cycles is regional and direct.

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Pax6 loss increases cortical progenitor proliferation by region-specific amounts

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The size of this effect correlates directly with normal Pax6 expression levels

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Expression of many key cell-cycle regulators is increased in the absence of Pax6

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Pax6 directly represses Cdk6 expression and controls pRb phosphorylation

Cyclin D1 cooperates with p21 to regulate TGFβ-mediated breast cancer cell migration and tumor local invasion

Introduction

Deregulation of the cell cycle machinery is often found in human cancers. Modulations in the cell cycle regulator function and expression result not only in proliferative advantages, but also lead to tumor progression and invasiveness of the cancer. In particular, cyclin D1 and p21 are often over-expressed in human cancers, correlating with high tumor grade, poor prognosis and increased metastasis. This prompted us to investigate the role of the cyclin D1/p21 signaling axis downstream of transforming growth factor beta (TGFβ) in breast cancer progression.

Methods

Cyclins mRNA and protein expressions were assessed by quantitative real-time PCR and Western blot in triple negative breast cancer cell lines. Co-localization and interaction between cyclin D1 and p21 were performed by immunocytochemistry and co-immunoprecipitation, respectively. Cell migration was assessed by wound healing and quantitative time-lapse imaging assays. In addition, the effects of cyclin D1 on cellular structure and actin organization were examined by staining with F-actin marker phalloidin and mesenchymal intermediate filament vimentin. Finally, a mammary fat pad xenograft mouse model was used to assess mammary tumor growth and local invasion.

Results

We found TGFβ to specifically up-regulate the expression of cyclin D1 in triple negative breast cancer cells. Induction of cyclin D1 is also required for TGFβ-mediated cell migration. Suppression of cyclin D1 expression not only resulted in a rounded and epithelial-like phenotype, but also prevented TGFβ-induced vimentin and F-actin co-localization at the cell edge as well as invadopodia formation. Furthermore, TGFβ promoted the nuclear co-localization and physical interaction between cyclin D1 and p21. The co-expression of cyclin D1 and p21 proteins are required for the initial steps of tumor development, as double knockdown of these two molecules prevented primary tumor formation in a Xenograft mouse model. Moreover, the in vivo studies indicated that locally advanced features of the invasive tumors, including skeletal muscle, mammary fat pad and lymphovascular invasion, as well as ulcerated skin, were attenuated in the absence of cyclin D1 and p21.

Conclusions

Thus, our findings highlight the cyclin D1/p21 signaling axis as a critical regulator of TGFβ-mediated tumor growth initiation and local tumor cell invasion, both in vitro and in vivo.

Cdk4 and nek2 signal binucleation and centrosome amplification in a her2+ breast cancer model

Centrosome amplification (CA) is a contributor to carcinogenesis, generating aneuploidy, and chromosome instability. Previous work shows that breast adenocarcinomas have a higher frequency of centrosome defects compared to normal breast tissues. Abnormal centrosome phenotypes are found in pre-malignant lesions, suggesting an early role in breast carcinogenesis. However, the role of CA in breast cancers remains elusive. Identification of pathways and regulatory molecules involved in the generation of CA is essential to understanding its role in breast tumorigenesis. We established a breast cancer model of CA using Her2-positive cells. Our goal was to identify centrosome cycle molecules that are deregulated by aberrant Her2 signaling and the mechanisms driving CA. Our results show some Her2+ breast cancer cell lines harbor both CA and binucleation. Abolishing the expression of Cdk4 abrogated both CA and binucleation in these cells. We also found the source of binucleation in these cells to be defective cytokinesis that is normalized by downregulation of Cdk4. Protein levels of Nek2 diminish upon Cdk4 knockdown and vice versa, suggesting a molecular connection between Cdk4 and Nek2. Knockdown of Nek2 reduces CA and binucleation in this model while its overexpression further enhances centrosome amplification. We conclude that CA is modulated through Cdk4 and Nek2 signaling and that binucleation is a likely source of CA in Her2+ breast cancer cells.

EYA1 phosphatase function is essential to drive breast cancer cell proliferation through cyclin D1

The Drosophila Eyes Absent Homologue 1 (EYA1) is a component of the retinal determination gene network and serves as an H2AX phosphatase. The cyclin D1 gene encodes the regulatory subunits of a holoenzyme that phosphorylates and inactivates the pRb protein. Herein, comparison with normal breast showed that EYA1 is overexpressed with cyclin D1 in luminal B breast cancer subtype. EYA1 enhanced breast tumor growth in mice in vivo, requiring the phosphatase domain. EYA1 enhanced cellular proliferation, inhibited apoptosis, and induced contact-independent growth and cyclin D1 abundance. The induction of cellular proliferation and cyclin D1 abundance, but not apoptosis, was dependent upon the EYA1 phosphatase domain. The EYA1-mediated transcriptional induction of cyclin D1 occurred via the AP-1-binding site at -953 and required the EYA1 phosphatase function. The AP-1 mutation did not affect SIX1-dependent activation of cyclin D1. EYA1 was recruited in the context of local chromatin to the cyclin D1 AP-1 site. The EYA1 phosphatase function determined the recruitment of CBP, RNA polymerase II, and acetylation of H3K9 at the cyclin D1 gene AP-1 site regulatory region in the context of local chromatin. The EYA1 phosphatase regulates cell-cycle control via transcriptional complex formation at the cyclin D1 promoter.

The oncoprotein HBXIP activates transcriptional coregulatory protein LMO4 via Sp1 to promote proliferation of breast cancer cells

Hepatitis B X-interacting protein (HBXIP) is an important oncoprotein that plays critical role in the development of cancer. In this study, we report that HBXIP activates LIM-only protein 4 (LMO4), a transcriptional coregulatory protein, in promotion of cell proliferation. We observed that the messenger RNA (mRNA) expression levels of HBXIP were positively associated with those of LMO4 in clinical breast cancer tissues. We further identified that HBXIP upregulated LMO4 at the levels of promoter, mRNA and protein in MCF-7 and LM-MCF-7 breast cancer cell lines. The expression of cyclin D1 and cyclin E, downstream effectors of LMO4, could be upregulated by HBXIP through LMO4. Then, chromatin immunoprecipitation (ChIP) assay revealed that HBXIP was able to interact with the promoter region of LMO4. Electrophoretic mobility shift assay showed that HBXIP occupied the -237/-206 region of LMO4 promoter containing Sp1 binding element. The mutant of Sp1 binding site in the LMO4 promoter impeded the interaction of HBXIP with the promoter. Co-immunoprecipitation, ChIP and luciferase reporter gene assays showed that HBXIP activated LMO4 promoter through binding to Sp1. In function, flow cytometry, 3-(4,5-dimethylthiazol-2-yl)-2,5-diphenyltetrazolium bromide, 5-ethynyl-2'-deoxyuridine (EdU) incorporation assays and animal transplantation assays demonstrated that HBXIP-enhanced cell proliferation of breast cancer through upregulating LMO4 in vitro and in vivo. Thus, we concluded that oncoprotein HBXIP is able to activate the transcriptional coregulatory protein LMO4 through transcription factor Sp1 in promotion of proliferation of breast cancer cells. HBXIP may serve as a driver gene to activate transcription in the development of cancer.

DDX5 regulates DNA replication and is required for cell proliferation in a subset of breast cancer cells

Understanding factors required for DNA replication will enrich our knowledge of this important process and potentially identify vulnerabilities that can be exploited in cancer therapy. We applied an assay that measures the stability of maintenance of an episomal plasmid in human tissue culture cells to screen for new DNA replication factors. We identify an important role for DDX5 in G(1)-S-phase progression where it directly regulates DNA replication factor expression by promoting the recruitment of RNA polymerase II to E2F-regulated gene promoters. We find that the DDX5 locus is frequently amplified in breast cancer and that breast cancer-derived cells with amplification of DDX5 are much more sensitive to its depletion than breast cancer cells and a breast epithelial cell line that lacks DDX5 amplification. Our results show a novel role for DDX5 in cancer cell proliferation and suggest DDX5 as a therapeutic target in breast cancer treatment.

SIGNIFICANCE

DDX5 is required for cell proliferation by controlling the transcription of genes expressing DNA replication proteins in cancer cells in which the DDX5 locus is amplified, and this has uncovered a dependence on DDX5 for cell proliferation. Given the high frequency of DDX5 amplification in breast cancer, our results highlight DDX5 as a promising candidate for targeted therapy of breast tumors with DDX5 amplification, and indeed we show that DDX5 inhibition sensitizes a subset of breast cancer cells to trastuzumab.

Morinda citrifolia (Noni) Juice Augments Mammary Gland Differentiation and Reduces Mammary Tumor Growth in Mice Expressing the Unactivated c-erbB2 Transgene

Morinda citrifolia (noni) is reported to have many beneficial properties, including on immune, inflammatory, quality of life, and cancer endpoints, but little is known about its ability to prevent or treat breast cancer. To test its anticancer potential, the effects of Tahitian Noni Juice (TNJ) on mammary carcinogenesis were examined in MMTV-neu transgenic mice. Mammary tumor latency, incidence, multiplicity, and metastatic incidence were unaffected by TNJ treatment, which suggests that it would not increase or decrease breast cancer risk in women taking TNJ for its other benefits. However, noni may be useful to enhance treatment responses in women with existing HER2/neu breast cancer since TNJ resulted in significant reductions in tumor weight and volume and in longer tumor doubling times in mice. Remarkably, its ability to inhibit the growth of this aggressive form of cancer occurred with the mouse equivalent of a recommended dose for humans (<3 oz/day). A 30-day treatment with TNJ also induced significant changes in mammary secondary ductule branching and lobuloalveolar development, serum progesterone levels, and estrous cycling. Additional studies investigating TNJ-induced tumor growth suppression and modified reproductive responses are needed to characterize its potential as a CAM therapy for women with and without HER2⁺ breast cancer.

Small non-coding RNAs govern mammary gland tumorigenesis

Small non-coding RNAs include siRNA, miRNA, piRNA and snoRNA. The involvement of miRNAs in the regulation of mammary gland tumorigenesis has been widely studied while the role for other small non-coding RNAs remains unclear. Here we summarize the involvement of miRNA in breast cancer onset and progression through regulating the cell cycle and cellular proliferation. The regulation of breast cancer stem cells and tumor regeneration by miRNA is reviewed. In addition, the emerging evidence demonstrating the involvement of piRNA and snoRNA in breast cancer is briefly described.

A crossroad of microRNAs and immediate early genes (IEGs) encoding oncogenic transcription factors in breast cancer

Signaling networks are involved in development, as well as in malignancy of the mammary gland. Distinct external stimuli activate intricate signaling cascades, which culminate in the activation of specific transcriptional programs. These signal-specific transcriptional programs are instigated by transcription factors (TFs) encoded by the immediate early genes (IEGs), and they lead to diverse cellular outcomes, including oncogenesis. Hence, regulating the expression of IEGs is of great importance, and involves several complementary transcriptional and posttranscriptional mechanisms, the latter entails also microRNAs (miRNAs). miRNAs are a class of non-coding RNAs, which have been implicated in regulation of various aspects of signaling networks. Through examination of the basic characteristics of miRNA function, we highlight the benefits of using miRNAs as regulators of early TFs and signaling networks. We further focus on the role of miRNAs as regulators of IEGs, which shape the initial steps of signaling-induced transcription. We especially emphasize the role of miRNAs in buffering external noise and maintaining low basal activation of IEGs in the absence of proper stimuli.

RNA interference-mediated silencing of NANOG reduces cell proliferation and induces G0/G1 cell cycle arrest in breast cancer cells

Since the processes of normal embryogenesis and neoplasia share many of similar pathways, tumor development has been interpreted as an abnormal form of organogenesis. NANOG is a homeodomain-containing transcription factor that functions to maintain self-renewal and proliferation of embryonic stem cells (ESCs). Aberrant expression of NANOG has been observed in many types of human malignancies. However, its potential implication in tumorigenesis has not been fully clarified. In this study, we have employed small interference RNA (RNAi) technology to silence endogenous NANOG expression in breast cancer cells and successfully selected three independent clones with stably inhibited NANOG expression of MCF-7 cells. Functional analysis revealed that down-regulation of NANOG reduced cell proliferation, colony formation and migration ability of MCF-7 cells. Consistently, proliferation of breast cancer MDA-MB-231 cells was also significantly inhibited after the knockdown of NANOG expression. Interestingly, we found that the expression levels of cyclinD1 and c-myc were markedly down-regulated and the cell cycle were blocked at the G0/G1 phases after the knockdown of NANOG, while the expression of cyclinE and signal transducers and activators of transcription3 (STAT3) remained unaffected. In addition, the expression of NANOG and cyclinD1 can be rescued after the transfection of pcDNA3.1 (-)-NANOG expression vector into the three clones. Finally, our chromatin immunoprecipitation (ChIP) experiment showed that NANOG protein can bind to the promoter region of cyclinD1 and regulate cells cycle. Taken together, our findings may not only establish a molecular basis for the role of NANOG in modulating cell cycle progression of breast cancer cells but also suggest a potential target for the treatment of at least some subtypes of breast cancer.

K63-linked ubiquitination in kinase activation and cancer

Ubiquitination has been demonstrated to play a pivotal role in multiple biological functions, which include cell growth, proliferation, apoptosis, DNA damage response, innate immune response, and neuronal degeneration. Although the role of ubiquitination in targeting proteins for proteasome-dependent degradation have been extensively studied and well-characterized, the critical non-proteolytic functions of ubiquitination, such as protein trafficking and kinase activation, involved in cell survival and cancer development, just start to emerge, In this review, we will summarize recent progresses in elucidating the non-proteolytic function of ubiquitination signaling in protein kinase activation and its implications in human cancers. The advancement in the understanding of the novel functions of ubiquitination in signal transduction pathways downstream of growth factor receptors may provide novel paradigms for the treatment of human cancers.

Induction of cell proliferation and survival genes by estradiol-repressed microRNAs in breast cancer cells

Background

In estrogen responsive MCF-7 cells, estradiol (E₂) binding to ERα leads to transcriptional regulation of genes involved in the control of cell proliferation and survival. MicroRNAs (miRNAs) have emerged as key post-transcriptional regulators of gene expression. The aim of this study was to explore whether miRNAs were involved in hormonally regulated expression of estrogen responsive genes.

Methods

Western blot and QPCR were used to determine the expression of estrogen responsive genes and miRNAs respectively. Target gene expression regulated by miRNAs was validated by luciferase reporter assays and transfection of miRNA mimics or inhibitors. Cell proliferation was evaluated by MTS assay.

Results

E₂ significantly induced bcl-2, cyclin D1 and survivin expression by suppressing the levels of a panel of miRNAs (miR-16, miR-143, miR-203) in MCF-7 cells. MiRNA transfection and luciferase assay confirmed that bcl-2 was regulated by miR-16 and miR-143, cyclinD1 was modulated by miR-16. Importantly, survivin was found to be targeted by miR-16, miR-143, miR-203. The regulatory effect of E₂ can be either abrogated by anti-estrogen ICI 182, 780 and raloxifene pretreatment, or impaired by ERα siRNA, indicating the regulation is dependent on ERα. In order to investigate the functional significance of these miRNAs in estrogen responsive cells, miRNAs mimics were transfected into MCF-7 cells. It revealed that overexpression of these miRNAs significantly inhibited E₂-induced cell proliferation. Further study of the expression of the miRNAs indicated that miR-16, miR-143 and miR-203 were highly expressed in triple positive breast cancer tissues, suggesting a potential tumor suppressing effect of these miRNAs in ER positive breast cancer.

Conclusions

These results demonstrate that E₂ induces bcl-2, cyclin D1 and survivin by orchestrating the coordinate downregulation of a panel of miRNAs. In turn, the miRNAs manifest growth suppressive effects and control cell proliferation in response to E₂. This sheds a new insight into the integral post-transcriptional regulation of cell proliferation and survival genes by miRNAs, a potential therapeutic option for breast cancer.

Jumonji/ARID1 B (JARID1B) protein promotes breast tumor cell cycle progression through epigenetic repression of microRNA let-7e

MicroRNAs (miRs) function as tumor suppressors or oncogenes in multiple tumor types. Although miR expression is tightly regulated, the molecular basis of miR regulation is poorly understood. Here, we investigated the influence of the histone demethylase Jumonji/ARID1 B (JARID1B) on miR regulation in breast tumor cells. In MCF-7 cells with stable RNAi-mediated suppression of JARID1B expression we identified altered regulation of multiple miRs including let-7e, a member of the let-7 family of tumor suppressor miRs. Chromatin immunoprecipitation analysis demonstrated JARID1B binding to the let-7e promoter region as well as removal of the of H3K4me3 histone mark associated with active gene expression. These results suggest that JARID1B epigenetically represses let-7e expression. JARID1B stimulates tumor cell proliferation by promoting the G(1) to S transition. As predicted, suppression of JARID1B resulted in an accumulation of MCF-7 cells in G(1). We confirmed that cyclin D1, which also promotes G(1) progression, is a direct target of let-7e, and we show that cyclin D1 expression is suppressed in JARID1B knockdown cells. Cyclin D1 expression and cell cycle progression were restored following inhibition of let-7e, suggesting that JARID1B repression of let-7e contributes to cyclin D1 expression and JARID1B-mediated cell cycle progression. Our results indicate that the JARID1B demethylase contributes to tumor cell proliferation through the epigenetic repression of a tumor suppressor miR.

The role of caveolin-1 in human breast cancer

Caveolin-1 is the essential constituent protein of specialised plasma membrane invaginations called caveolae. The unique topology of caveolin-1 facilitates the role of caveolae as molecular hubs, integrating the activity of a multitude of signalling molecules. Despite improvements in our understanding of caveolin-1 interactions and the function of caveolae, the relationship between dysfunctional caveolin-1 and tumourigenesis remains contentious. Perhaps most intriguing has been the demonstration of both oncogenic and tumour suppressor function within particular tumour types, including breast cancer. In this review, the biological and clinical relevance of caveolin-1 in human breast cancer are considered. Evidence is systematically presented for the potential tumour suppressor and oncogenic functions of caveolin-1. Specific reference is made to interactions between caveolin-1 and signalling pathways in the clinical and biological subtypes of breast cancer. Areas of controversy are discussed and technical considerations are highlighted. Translational implications and potential for specific therapeutic manipulation of caveolin-1 are evaluated in the context of evidence from in vitro and in vivo studies.

Testes-specific protease 50 (TSP50) promotes cell proliferation through the activation of the nuclear factor κB (NF-κB) signalling pathway

TSP50 (testes-specific protease 50) is a testis-specific expression protein, which is expressed abnormally at high levels in breast cancer tissues. This makes it an attractive molecular marker and a potential target for diagnosis and therapy; however, the biological function of TSP50 is still unclear. In the present study, we show that overexpression of TSP50 in CHO (Chinese-hamster ovary) cells markedly increased cell proliferation and colony formation. Mechanistic studies have revealed that TSP50 can enhance the level of TNFα (tumour necrosis factor α)- and PMA-induced NF-κB (nuclear factor κB)-responsive reporter activity, IκB (inhibitor of NF-κB) α degradation and p65 nuclear translocation. In addition, the knockdown of endogenous TSP50 in MDA-MB-231 cells greatly inhibited NF-κB activity. Co-immunoprecipitation studies demonstrated an interaction of TSP50 with the NF-κB-IκBα complex, but not with the IKK (IκB kinase) α/β-IKKγ complex, which suggested that TSP50, as a novel type of protease, promoted the degradation of IκBα proteins by binding to the NF-κB-IκBα complex. Our results also revealed that TSP50 can enhance the expression of NF-κB target genes involved in cell proliferation. Furthermore, overexpression of a dominant-negative IκB mutant that is resistant to proteasome-mediated degradation significantly reversed TSP50-induced cell proliferation, colony formation and tumour formation in nude mice. Taken together, the results of the present study suggest that TSP50 promotes cell proliferation, at least partially, through activation of the NF-κB signalling pathway.

Cyclin D as a therapeutic target in cancer

Cyclin D1, and to a lesser extent the other D-type cyclins, is frequently deregulated in cancer and is a biomarker of cancer phenotype and disease progression. The ability of these cyclins to activate the cyclin-dependent kinases (CDKs) CDK4 and CDK6 is the most extensively documented mechanism for their oncogenic actions and provides an attractive therapeutic target. Is this an effective means of targeting the cyclin D oncogenes, and how might the patient subgroups that are most likely to benefit be identified?

Examining the role of cyclin D1 in breast cancer

Cyclin D1 overexpression is found in more than 50% of human breast cancers and causes mammary cancer in transgenic mice. Dysregulation of cyclin D1 gene expression or function contributes to the loss of normal cell cycle control during tumorigenesis. Recent studies have demonstrated that cyclin D1 conducts additional specific functions to regulate gene expression in the context of local chromatin, promote cellular migration and inhibit mitochondrial metabolism. It is anticipated that these additional functions contribute to the pathology associated with dysregulated cyclin D1 abundance. This article discusses evidence that examines the significance of cyclin D1 in breast cancer with emphasis on its role in breast cancer stem cell expansion.

Expression ratio of CCND1 to CDKN2A mRNA predicts RB1 status of cultured cancer cell lines and clinical tumor samples

Background

The retinoblastoma product (RB1) is frequently deregulated in various types of tumors by mutation, deletion, or inactivation through association with viral oncoproteins. The functional loss of RB1 is recognized to be one of the hallmarks that differentiate cancer cells from normal cells. Many researchers are attempting to develop anti-tumor agents that are preferentially effective against RB1-negative tumors. However, to identify patients with RB1-negative cancers, it is imperative to develop predictive biomarkers to classify RB1-positive and -negative tumors.

Results

Expression profiling of 30 cancer cell lines composed of 16 RB1-positive and 14 RB1-negative cancers was performed to find genes that are differentially expressed between the two groups, resulting in the identification of an RB1 signature with 194 genes. Among them, critical RB1 pathway components CDKN2A and CCND1 were included. We found that microarray data of the expression ratio of CCND1 and CDKN2A clearly distinguished the RB1 status of 30 cells lines. Measurement of the CCND1/CDKN2A mRNA expression ratio in additional cell lines by RT-PCR accurately predicted RB1 status (12/12 cells lines). The expression of CCND1/CDKN2A also correlated with RB1 status in xenograft tumors in vivo. Lastly, a CCND1/CDKN2A assay with clinical samples showed that uterine cervical and small cell lung cancers known to have a high prevalence of RB1-decifiency were predicted to be 100% RB1-negative, while uterine endometrial or gastric cancers were predicted to be 5-22% negative. All clinically normal tissues were 100% RB1-positive.

Conclusions

We report here that the CCND1/CDKN2A mRNA expression ratio predicts the RB1 status of cell lines in vitro and xenograft tumors and clinical tumor samples in vivo. Given the high predictive accuracy and quantitative nature of the CCND1/CDKN2A expression assay, the assay could be utilized to stratify patients for anti-tumor agents with preferential effects on either RB1-positive or -negative tumors.

The elements of human cyclin D1 promoter and regulation involved

Cyclin D1 is a cell cycle machine, a sensor of extracellular signals and plays an important role in G1-S phase progression. The human cyclin D1 promoter contains multiple transcription factor binding sites such as AP-1, NF-қB, E2F, Oct-1, and so on. The extracellular signals functions through the signal transduction pathways converging at the binding sites to active or inhibit the promoter activity and regulate the cell cycle progression. Different signal transduction pathways regulate the promoter at different time to get the correct cell cycle switch. Disorder regulation or special extracellular stimuli can result in cell cycle out of control through the promoter activity regulation. Epigenetic modifications such as DNA methylation and histone acetylation may involved in cyclin D1 transcriptional regulation.

The G1 phase Cdks regulate the centrosome cycle and mediate oncogene-dependent centrosome amplification

Because centrosome amplification generates aneuploidy and since centrosome amplification is ubiquitous in human tumors, a strong case is made for centrosome amplification being a major force in tumor biogenesis. Various evidence showing that oncogenes and altered tumor suppressors lead to centrosome amplification and aneuploidy suggests that oncogenes and altered tumor suppressors are a major source of genomic instability in tumors, and that they generate those abnormal processes to initiate and sustain tumorigenesis. We discuss how altered tumor suppressors and oncogenes utilize the cell cycle regulatory machinery to signal centrosome amplification and aneuploidy.

PACSIN 2 represses cellular migration through direct association with cyclin D1 but not its alternate splice form cyclin D1b

Cyclin D1 overexpression is a common feature of many human malignancies. Genomic deletion analysis has demonstrated a key role for cyclin D1 in cellular proliferation, angiogenesis, and cellular migration. To investigate the mechanisms contributing to cyclin D1 functions, we purified cyclin D1a-associated complexes by affinity chromatography and identified the PACSIN 2 (protein kinase C and casein kinase substrate in neurons 2) protein by mass spectrometry. The PACSIN 2, but not the related PACSIN 1 and 3, directly bound wild-type cyclin D1 (cyclin D1a) at the carboxyl terminus, and failed to bind cyclin D1b, the alternative splicing variant of cyclin D1. PACSIN 2 knockdown induced cellular migration and reduced cell spreading in LNCaP cells expressing cyclin D1a. In cyclin D1(-/-) mouse embryonic fibroblasts (MEFs), cyclin D1a, but not cyclin D1b, reduced the cell spreading to a polarized morphology. siPACSIN 2 had no effect on cellular migration of cyclin D1(-/-) MEFs. Cyclin D1a restored the migratory ability of cyclin D1(-/-) MEFs, which was further enhanced by knocking down PACSIN 2 with siRNA. The cyclin D1-associated protein, PACSIN 2, regulates cell spreading and migration, which are dependent on cyclin D1 expression.

Reviewing once more the c-myc and Ras collaboration: converging at the cyclin D1-CDK4 complex and challenging basic concepts of cancer biology

The c-myc is a proto-oncogene that manifests aberrant expression at high frequencies in most types of human cancer. C-myc gene amplifications are often observed in various cancers as well. Ample studies have also proved that c-myc has a potent oncogenicity, which can be further enhanced by collaborations with other oncogenes such as Bcl-2 and activated Ras. Studies on the collaborations of c-myc with Ras or other genes in oncogenicity have established several basic concepts and have disclosed their underlying mechanisms of tumor biology, including "immortalization" and "transformation". In many cases, these collaborations may converge at the cyclin D1-CDK4 complex. In the meantime, however, many results from studies on the c-myc, Ras and cyclin D1-CDK4 also challenge these basic concepts of tumor biology and suggest to us that the immortalized status of cells should be emphasized. Stricter criteria and definitions for a malignantly transformed status and a benign status of cells in culture also need to be established to facilitate our study of the mechanisms for tumor formation and to better link up in vitro data with animal results and eventually with human cancer pathology.

Identification of the Rac-GEF P-Rex1 as an essential mediator of ErbB signaling in breast cancer

While the small GTPase Rac1 and its effectors are well-established mediators of mitogenic and motile signaling by tyrosine kinase receptors and have been implicated in breast tumorigenesis, little is known regarding the exchange factors (Rac-GEFs) that mediate ErbB receptor responses. Here, we identify the PIP(3)-Gβγ-dependent Rac-GEF P-Rex1 as an essential mediator of Rac1 activation, motility, cell growth, and tumorigenesis driven by ErbB receptors in breast cancer cells. Notably, activation of P-Rex1 in breast cancer cells requires the convergence of inputs from ErbB receptors and a Gβγ- and PI3Kγ-dependent pathway. Moreover, we identified the GPCR CXCR4 as a crucial mediator of P-Rex1/Rac1 activation in response to ErbB ligands. P-Rex1 is highly overexpressed in human breast cancers and their derived cell lines, particularly those with high ErbB2 and ER expression. In addition to the prognostic and therapeutic implications, our findings reveal an ErbB effector pathway that is crucial for breast cancer progression.

The canonical NF-kappaB pathway governs mammary tumorigenesis in transgenic mice and tumor stem cell expansion

The role of mammary epithelial cell (MEC) NF-κB in tumor progression in vivo is unknown, as murine NF-κB components and kinases either are required for murine survival or interfere with normal mammary gland development. As NF-κB inhibitors block both tumor-associated macrophages (TAM) and MEC NF-κB, the importance of MEC NF-κB to tumor progression in vivo remained to be determined. Herein, an MEC-targeted inducible transgenic inhibitor of NF-κB (IκBαSR) was developed in ErbB2 mammary oncomice. Inducible suppression of NF-κB in the adult mammary epithelium delayed the onset and number of new tumors. Within similar sized breast tumors, TAM and tumor neoangiogenesis was reduced. Coculture experiments demonstrated MEC NF-κB enhanced TAM recruitment. Genome-wide expression and proteomic analysis showed that IκBαSR inhibited tumor stem cell pathways. IκBαSR inhibited breast tumor stem cell markers in transgenic tumors, reduced stem cell expansion in vitro, and repressed expression of Nanog and Sox2 in vivo and in vitro. MEC NF-κB contributes to mammary tumorigenesis. As we show that NF-κB contributes to expansion of breast tumor stem cells and heterotypic signals that enhance TAM and vasculogenesis, these processes may contribute to NF-κB-dependent mammary tumorigenesis.