Construction of a cyclin D1-Cdk2 fusion protein to model the biological functions of cyclin D1-Cdk2 complexes

Long-term breast cancer response to CDK4/6 inhibition defined by TP53-mediated geroconversion

Inhibition of CDK4/6 kinases has led to improved outcomes in breast cancer. Nevertheless, only a minority of patients experience long-term disease control. Using a large, clinically annotated cohort of patients with metastatic hormone receptor-positive (HR+) breast cancer, we identify TP53 loss (27.6%) and MDM2 amplification (6.4%) to be associated with lack of long-term disease control. Human breast cancer models reveal that p53 loss does not alter CDK4/6 activity or G1 blockade but instead promotes drug-insensitive p130 phosphorylation by CDK2. The persistence of phospho-p130 prevents DREAM complex assembly, enabling cell-cycle re-entry and tumor progression. Inhibitors of CDK2 can overcome p53 loss, leading to geroconversion and manifestation of senescence phenotypes. Complete inhibition of both CDK4/6 and CDK2 kinases appears to be necessary to facilitate long-term response across genomically diverse HR+ breast cancers.

Mode of action exploration for prostate epithelial cell injury caused by bisphenol A

Bisphenol A (BPA) is a typical food chemical contaminant with various detrimental effects, especially on reproductive system. Male prostate damage is also one of its major adverse health effects, of which mode of action (MOA) remains unclear. This study aims to explore the MOA for prostate toxicity of BPA using human normal prostate epithelial cell RWPE-1 for 28-day human-relevant-level exposure. A physiological based pharmacokinetic model was used to determine the concentration of BPA based on the actual oral exposure in China. The possible key events were identified by high-throughput transcriptome sequencing and validated by qPCR, Western blot and cell cycle assay, and the benchmark concentration analysis were conducted. The enriched KEGG pathways include the endocytosis, cell cycle, cellular senescence, MAPK and TNF signaling pathways. With increasing BPA concentrations, the increased mRNA and/or protein expressions of MAPKAPK2, c-JUN and c-fos in the MAPK signaling pathway, the increased mRNA expressions of CCND1 and CDKN1A, the decreased mRNA expression of CDC25C, the increased proportion of G0/G1 phase and S phase, as well as the decreased proportion of G2/M phase, were observed. The lowest value of benchmark concentration lower confidence limit (BMCL) was retrieved from G2/M phase ratio, with 110.580 and 175.862 nM for BMCL5 and BMCL10, respectively, much higher than the male gonad maximum concentration of 0.019 nM of BPA at the current exposure level of adult Chinese males. In conclusion, the MOA of BPA induced male prostatic toxicity at human-relevant levels may include: key event (KE)1-MAPK signaling pathway activation, KE2-disorder of cell cycle regulatory gene expression (increased expression of CCND1 and CDKN1A, decreased expression of CDC25C), and KE3-disturbance of cell cycle (increased proportion of G0/G1 and S phases, decreased proportion of G2/M phases). However, more studies are needed to validate and complete the MOA.

Biomarker potential of competing endogenous RNA networks in Polycystic Ovary Syndrome (PCOS)

Polycystic ovary syndrome (PCOS) is the most common condition affecting women of reproductive age globally. PCOS continues to be the largest contributing factor to female infertility despite significant progress in our knowledge of the molecular underpinnings and treatment of the condition. The fact that PCOS is a very diverse condition makes it one of the key reasons why we haven't been able to overcome it. Non-coding RNAs (ncRNAs) are implicated in the development of PCOS, according to growing evidence. However, it is unclear how the complex regulatory relationships between the many ncRNA types contribute to the growth of this malignancy. Competing endogenous RNA (ceRNA), a recently identified mechanism in the RNA world, suggests regulatory interactions between various RNAs, including long non-coding RNAs (lncRNAs), microRNAs (miRNAs), transcribed pseudogenes, and circular RNAs (circRNAs). Recent studies on PCOS have shown that dysregulation of multiple ceRNA networks (ceRNETs) between these ncRNAs plays crucial roles in developing the defining characteristics of PCOS development. And it is believed that such a finding may open a new door for a deeper comprehension of PCOS's unexplored facets. In addition, it may be able to provide fresh biomarkers and effective therapy targets for PCOS. This review will go over the body of information that exists about the primary roles of ceRNETs before highlighting the developing involvement of several newly found ceRNETs in a number of PCOS characteristics.

Reciprocal regulation of p21 and Chk1 controls the Cyclin D1-RB pathway to mediate senescence onset after G2 arrest

Senescence is an irreversible proliferation withdrawal that can be initiated after DNA damage-induced cell cycle arrest in G2 phase to prevent genomic instability. Senescence onset in G2 requires p53 and RB family tumour suppressors, but how they are regulated to convert a temporary cell cycle arrest into a permanent one remains unknown. Here, we show that a previously unrecognised balance between the CDK inhibitor p21 and Chk1 controls D-type cyclin-CDK activity during G2 arrest. In non-transformed cells, p21 activates RB in G2 by inhibiting Cyclin D1-CDK2/CDK4. The resulting G2 exit, which precedes appearance of senescence markers, is associated with a mitotic bypass, Chk1 downregulation and DNA damage foci reduction. In p53/RB-proficient cancer cells, compromised G2 exit correlates with sustained Chk1 activity, delayed p21 induction, untimely Cyclin E1 re-expression and genome reduplication. Conversely, Chk1 depletion promotes senescence by inducing p21 binding to Cyclin D1 and Cyclin E1-CDK complexes and down-regulating CDK6, whereas Chk2 knockdown enables RB phosphorylation and delays G2 exit. In conclusion, p21 and Chk2 oppose Chk1 to maintain RB activity, thus promoting DNA damage-induced senescence onset in G2.

Multiomic analysis identifies CPT1A as a potential therapeutic target in platinum-refractory, high-grade serous ovarian cancer

Resistance to platinum compounds is a major determinant of patient survival in high-grade serous ovarian cancer (HGSOC). To understand mechanisms of platinum resistance and identify potential therapeutic targets in resistant HGSOC, we generated a data resource composed of dynamic (±carboplatin) protein, post-translational modification, and RNA sequencing (RNA-seq) profiles from intra-patient cell line pairs derived from 3 HGSOC patients before and after acquiring platinum resistance. These profiles reveal extensive responses to carboplatin that differ between sensitive and resistant cells. Higher fatty acid oxidation (FAO) pathway expression is associated with platinum resistance, and both pharmacologic inhibition and CRISPR knockout of carnitine palmitoyltransferase 1A (CPT1A), which represents a rate limiting step of FAO, sensitize HGSOC cells to platinum. The results are further validated in patient-derived xenograft models, indicating that CPT1A is a candidate therapeutic target to overcome platinum resistance. All multiomic data can be queried via an intuitive gene-query user interface (https://sites.google.com/view/ptrc-cell-line).

Tumor suppressor p53 promotes ferroptosis in oxidative stress conditions independent of modulation of ferroptosis by p21, CDKs, Rb and E2F

p53 is a well-established critical cell cycle regulator. By inducing transcription of the gene encoding p21, p53 inhibits cyclin-dependent kinase (CDK)-mediated phosphorylation of cell cycle inhibitor RB proteins. Phosphorylation of RB releases E2F transcription factor proteins that transactivate cell cycle-promoting genes. Here we sought to uncover the contribution of p53, p21, CDK, RB, and E2F to the regulation of ferroptosis, an oxidative form of cell death. Our studies have uncovered unexpected complexity in this regulation. First, we showed that elevated levels of p53 enhance ferroptosis in multiple inducible and isogenic systems. On the other hand, we found that p21 suppresses ferroptosis. Elevation of CDK activity also suppressed ferroptosis under conditions where p21 suppressed ferroptosis, suggesting that the impact of p21 must extend beyond CDK inhibition. Furthermore, we showed that overexpression of E2F suppresses ferroptosis in part via a p21-dependent mechanism, consistent with reports that this transcription factor can induce transcription of p21. Finally, deletion of RB genes enhanced ferroptosis. Taken together, these results show that signals affecting ferroptotic sensitivity emanate from multiple points within the p53 tumor suppressor pathway.

Interrogation of novel CDK2/9 inhibitor fadraciclib (CYC065) as a potential therapeutic approach for AML

Over the last 50 years, there has been a steady improvement in the treatment outcome of acute myeloid leukemia (AML). However, median survival in the elderly is still poor due to intolerance to intensive chemotherapy and higher numbers of patients with adverse cytogenetics. Fadraciclib (CYC065), a novel cyclin-dependent kinase (CDK) 2/9 inhibitor, has preclinical efficacy in AML. In AML cell lines, myeloid cell leukemia 1 (MCL-1) was downregulated following treatment with fadraciclib, resulting in a rapid induction of apoptosis. In addition, RNA polymerase II (RNAPII)-driven transcription was suppressed, rendering a global gene suppression. Rapid induction of apoptosis was observed in primary AML cells after treatment with fadraciclib for 6-8 h. Twenty-four hours continuous treatment further increased efficacy of fadraciclib. Although preliminary results showed that AML cell lines harboring KMT2A rearrangement (KMT2A-r) are more sensitive to fadraciclib, we found that the drug can induce apoptosis and decrease MCL-1 expression in primary AML cells, regardless of KMT2A status. Importantly, the diversity of genetic mutations observed in primary AML patient samples was associated with variable response to fadraciclib, confirming the need for patient stratification to enable a more effective and personalized treatment approach. Synergistic activity was demonstrated when fadraciclib was combined with the BCL-2 inhibitor venetoclax, or the conventional chemotherapy agents, cytarabine, or azacitidine, with the combination of fadraciclib and azacitidine having the most favorable therapeutic window. In summary, these results highlight the potential of fadraciclib as a novel therapeutic approach for AML.

CRL4AMBRA1 is a master regulator of D-type cyclins

D-type cyclins are central regulators of the cell division cycle and are among the most frequently deregulated therapeutic targets in human cancer¹, but the mechanisms that regulate their turnover are still being debated^2,3. Here, by combining biochemical and genetics studies in somatic cells, we identify CRL4^AMBRA1 (also known as CRL4^DCAF3) as the ubiquitin ligase that targets all three D-type cyclins for degradation. During development, loss of Ambra1 induces the accumulation of D-type cyclins and retinoblastoma (RB) hyperphosphorylation and hyperproliferation, and results in defects of the nervous system that are reduced by treating pregnant mice with the FDA-approved CDK4 and CDK6 (CDK4/6) inhibitor abemaciclib. Moreover, AMBRA1 acts as a tumour suppressor in mouse models and low AMBRA1 mRNA levels are predictive of poor survival in cancer patients. Cancer hotspot mutations in D-type cyclins abrogate their binding to AMBRA1 and induce their stabilization. Finally, a whole-genome, CRISPR-Cas9 screen identified AMBRA1 as a regulator of the response to CDK4/6 inhibition. Loss of AMBRA1 reduces sensitivity to CDK4/6 inhibitors by promoting the formation of complexes of D-type cyclins with CDK2. Collectively, our results reveal the molecular mechanism that controls the stability of D-type cyclins during cell-cycle progression, in development and in human cancer, and implicate AMBRA1 as a critical regulator of the RB pathway.

The AMBRA1 E3 ligase adaptor regulates the stability of cyclin D

The initiation of cell division integrates a large number of intra- and extracellular inputs. D-type cyclins (hereafter, cyclin D) couple these inputs to the initiation of DNA replication1. Increased levels of cyclin D promote cell division by activating cyclin-dependent kinases 4 and 6 (hereafter, CDK4/6), which in turn phosphorylate and inactivate the retinoblastoma tumour suppressor. Accordingly, increased levels and activity of cyclin D-CDK4/6 complexes are strongly linked to unchecked cell proliferation and cancer2,3. However, the mechanisms that regulate levels of cyclin D are incompletely understood4,5. Here we show that autophagy and beclin 1 regulator 1 (AMBRA1) is the main regulator of the degradation of cyclin D. We identified AMBRA1 in a genome-wide screen to investigate the genetic basis of  the response to CDK4/6 inhibition. Loss of AMBRA1 results in high levels of cyclin D in cells and in mice, which promotes proliferation and decreases sensitivity to CDK4/6 inhibition. Mechanistically, AMBRA1 mediates ubiquitylation and proteasomal degradation of cyclin D as a substrate receptor for the cullin 4 E3 ligase complex. Loss of AMBRA1 enhances the growth of lung adenocarcinoma in a mouse model, and low levels of AMBRA1 correlate with worse survival in patients with lung adenocarcinoma. Thus, AMBRA1 regulates cellular levels of cyclin D, and contributes to cancer development and the response of cancer cells to CDK4/6 inhibitors.

Cell cycle during neuronal migration and neocortical lamination

OBJECTIVES

In order to understand the relationships between neocortical lamination and cell cycle, various cells, such as neural stem cell, migrating postmitotic neuron, Cajal-Retzius (CR) cell, and mature pyramidal cell in various cell phases were investigated in mouse cortices.

METHODS

With mouse neocortex and hippocampus, the immunofluorescent labeling, BrdU assay, and DiI tracing technique were implemented in the study.

RESULTS

(1) During mouse development, the neocortex expressed different proteins, such as FOXP2, CDP, and Nestin, which could be used as the markers for cortical lamination. (2) The neural stem cells were mainly located in the subventricular zone, with the expressions of Nestin, Cyclin A2, Cyclin E1, and CDT1, suggesting that they were in the repeated cell cycle. Furthermore, the migrating neurons in the neocortex were Cyclin D1- (G1 phase-specific marker) positive, suggesting that they were in the G1 phase. However, Pyramidal cells that developed from postmitotic migrating neurons and settled in the cortical plate were Cyclin D1- negative, suggesting that they were in the G0 phase. (3) Reelin positive CR cells appeared in the molecular layer of the neocortex in early embryonic day (E10), which could express Cyclin A2, Cyclin E1, and CDT1 as pyramidal cells, but not Cyclin D1, suggesting that they may have exited the cell cycle and entered the G0 phase.

CONCLUSION

The neural migration, neural proliferation, and cell cycle alterations play an important role during cortical lamination. During the cortical development and lamination, the neural stem cells and migrating postmitotic neurons are in different cell cycle phases, but pyramidal cells and CR cells have exited the cell cycle.

Dynamic Phosphorylation and Dephosphorylation of Cyclase-Associated Protein 1 by Antagonistic Signaling through Cyclin-Dependent Kinase 5 and cAMP Are Critical for the Protein Functions in Actin Filament Disassembly and Cell Adhesion

Cyclase-associated protein 1 (CAP1) is a conserved actin-regulating protein that enhances actin filament dynamics and also regulates adhesion in mammalian cells. We previously found that phosphorylation at the Ser307/Ser309 tandem site controls its association with cofilin and actin and is important for CAP1 to regulate the actin cytoskeleton. Here, we report that transient Ser307/Ser309 phosphorylation is required for CAP1 function in both actin filament disassembly and cell adhesion. Both the phosphomimetic and the nonphosphorylatable CAP1 mutant, which resist transition between phosphorylated and dephosphorylated forms, had defects in rescuing the reduced rate of actin filament disassembly in the CAP1 knockdown HeLa cells. The phosphorylation mutants also had defects in alleviating the elevated focal adhesion kinase (FAK) activity and the enhanced focal adhesions in the knockdown cells. In dissecting further phosphoregulatory cell signals for CAP1, we found that cyclin-dependent kinase 5 (CDK5) phosphorylates both Ser307 and Ser309 residues, whereas cAMP signaling induces dephosphorylation at the tandem site, through its effectors protein kinase A (PKA) and exchange proteins directly activated by cAMP (Epac). No evidence supports an involvement of activated protein phosphatase in executing the dephosphorylation downstream from cAMP, whereas preventing CAP1 from accessing its kinase CDK5 appears to underlie CAP1 dephosphorylation induced by cAMP. Therefore, this study provides direct cellular evidence that transient phosphorylation is required for CAP1 functions in both actin filament turnover and adhesion, and the novel mechanistic insights substantially extend our knowledge of the cell signals that function in concert to regulate CAP1 by facilitating its transient phosphorylation.

Cyclin D-Cdk4,6 Drives Cell-Cycle Progression via the Retinoblastoma Protein's C-Terminal Helix

The cyclin-dependent kinases Cdk4 and Cdk6 form complexes with D-type cyclins to drive cell proliferation. A well-known target of cyclin D-Cdk4,6 is the retinoblastoma protein Rb, which inhibits cell-cycle progression until its inactivation by phosphorylation. However, the role of Rb phosphorylation by cyclin D-Cdk4,6 in cell-cycle progression is unclear because Rb can be phosphorylated by other cyclin-Cdks, and cyclin D-Cdk4,6 has other targets involved in cell division. Here, we show that cyclin D-Cdk4,6 docks one side of an alpha-helix in the Rb C terminus, which is not recognized by cyclins E, A, and B. This helix-based docking mechanism is shared by the p107 and p130 Rb-family members across metazoans. Mutation of the Rb C-terminal helix prevents its phosphorylation, promotes G1 arrest, and enhances Rb's tumor suppressive function. Our work conclusively demonstrates that the cyclin D-Rb interaction drives cell division and expands the diversity of known cyclin-based protein docking mechanisms.

Conditional control of fluorescent protein degradation by an auxin-dependent nanobody

The conditional and reversible depletion of proteins by auxin-mediated degradation is a powerful tool to investigate protein functions in cells and whole organisms. However, its wider applications require fusing the auxin-inducible degron (AID) to individual target proteins. Thus, establishing the auxin system for multiple proteins can be challenging. Another approach for directed protein degradation are anti-GFP nanobodies, which can be applied to GFP stock collections that are readily available in different experimental models. Here, we combine the advantages of auxin and nanobody-based degradation technologies creating an AID-nanobody to degrade GFP-tagged proteins at different cellular structures in a conditional and reversible manner in human cells. We demonstrate efficient and reversible inactivation of the anaphase promoting complex/cyclosome (APC/C) and thus provide new means to study the functions of this essential ubiquitin E3 ligase. Further, we establish auxin degradation in a vertebrate model organism by employing AID-nanobodies in zebrafish.

Current approaches to conditionally deplete target proteins require site-specific genetic engineering or have poor temporal control. Here the authors overcome these limitations by combining the AID system with nanobodies to reversibly degrade GFP-tagged proteins in living cells and zebrafish.

A Precise Cdk Activity Threshold Determines Passage through the Restriction Point

At the restriction point (R), mammalian cells irreversibly commit to divide. R has been viewed as a point in G1 that is passed when growth factor signaling initiates a positive feedback loop of Cdk activity. However, recent studies have cast doubt on this model by claiming R occurs prior to positive feedback activation in G1 or even before completion of the previous cell cycle. Here we reconcile these results and show that whereas many commonly used cell lines do not exhibit a G1 R, primary fibroblasts have a G1 R that is defined by a precise Cdk activity threshold and the activation of cell-cycle-dependent transcription. A simple threshold model, based solely on Cdk activity, predicted with more than 95% accuracy whether individual cells had passed R. That a single measurement accurately predicted cell fate shows that the state of complex regulatory networks can be assessed using a few critical protein activities.

CUB domain-containing protein 1 and the epidermal growth factor receptor cooperate to induce cell detachment

Background

While localized malignancies often respond to available therapies, most disseminated cancers are refractory. Novel approaches, therefore, are needed for the treatment of metastatic disease. CUB domain-containing protein1 (CDCP1) plays an important role in metastasis and drug resistance; the mechanism however, is poorly understood.

Methods

Breast cancer cell lines were engineered to stably express EGFR, CDCP1 or phosphorylation site mutants of CDCP1. These cell lines were used for immunoblot analysis or affinity purification followed by immunoblot analysis to assess protein phosphorylation and/or protein complex formation with CDCP1. Kinase activity was evaluated using phosphorylation site-specific antibodies and immunoblot analysis in in vitro kinase assays. Protein band excision and mass spectrometry was utilized to further identify proteins complexed with CDCP1 or ΔCDCP1, which is a mimetic of the cleaved form of CDCP1. Cell detachment was assessed using cell counting.

Results

This paper reports that CDCP1 forms ternary protein complexes with Src and EGFR, facilitating Src activation and Src-dependent EGFR transactivation. Importantly, we have discovered that a class of compounds termed Disulfide bond Disrupting Agents (DDAs) blocks CDCP1/EGFR/Src ternary complex formation and downstream signaling. CDCP1 and EGFR cooperate to induce detachment of breast cancer cells from the substratum and to disrupt adherens junctions. Analysis of CDCP1-containing complexes using proteomics techniques reveals that CDCP1 associates with several proteins involved in cell adhesion, including adherens junction and desmosomal cadherins, and cytoskeletal elements.

Conclusions

Together, these results suggest that CDCP1 may facilitate loss of adhesion by promoting activation of EGFR and Src at sites of cell-cell and cell-substratum contact.

Electronic supplementary material

The online version of this article (doi:10.1186/s13058-016-0741-1) contains supplementary material, which is available to authorized users.

Comprehensive and Holistic Analysis of HT-29 Colorectal Cancer Cells and Tumor-Bearing Nude Mouse Model: Interactions Among Fractions Derived From the Chinese Medicine Formula Tian Xian Liquid in Effects on Human Colorectal Carcinoma

The Chinese medicine formula Tian Xian Liquid (TXL) has been used clinically for cancer therapy in China for more than 25 years. However, the comprehensive and holistic effects of its bioactive fractions for various antitumor therapeutic effects have not been unraveled. This is the first study to scientifically elucidate the holistic effect of Chinese medicine formula for treating colon cancer, hence allowing a better understanding of the essence of Chinese medicine formula, through the comparison of the actions of TXL and its functional constituent fractions, including ethyl acetate (EA), butanol (BU), and aqueous (WA) fractions. Tissue-specific proliferative/antiproliferative effects of these fractions on human colorectal carcinoma HT-29 cells and splenocytes were studied by using the MTT assay. Their modulations on the expression of markers of antiproliferation, antimetastasis, reversion of multidrug resistance in treated HT-29 cells were examined with real-time polymerase chain reaction and Western blot analysis, and their modulations in a xenografted nude mouse model were examined by Western blot analysis. Results revealed that EA fraction slightly inhibited the proliferation of HT-29 cells, but tissue-specifically exerted the most potent antiproliferative effect on splenocytes. On the contrary, only TXL and BU fraction tissue-specifically contributed to the proliferation of splenocytes, but inhibited the proliferation of HT-29 cells. WA fraction exerted the most potent antiproliferative effect on HT-29 cells and also the strongest inhibitory action on tumor size in the nude mouse model in our previous study. In the HT-29 model, TXL and WA fraction exerted the most pronounced effect on upregulation of p21 mRNA and protein; TXL, and EA and WA fractions exerted the effect on downregulation of G1 phase cell cycle protein, cyclin D1 mRNA and protein; EA and BU fractions exerted the most prominent anti-invasive effect on anti-invasion via downregulation of MMP-1 mRNA; TXL potently reversed most multidrug resistance via downregulation of MDR-1 protein. In conclusion, the comprehensive and holistic effects of TXL were demonstrated with ( a) mutual accentuation and mutual enhancement, ( b) mutual counteraction and mutual suppression, and ( c) mutual antagonism among the 3 constituent fractions. Moreover, the design of the present study may lead to further development of more tissue-specific effective drugs with minimal side effects for clinical use in combating carcinoma.

Phosphatases and kinases regulating CDC25 activity in the cell cycle: clinical implications of CDC25 overexpression and potential treatment strategies

Alterations in the cell-cycle regulatory genes result in uncontrolled cell proliferation leading to several disease conditions. Cyclin-dependent kinases (CDK) and their regulatory subunit, cyclins, are essential proteins in cell-cycle progression. The activity of CDK is regulated by a series of phosphorylation and dephosphorylation at different amino acid residues. Cell Division Cycle-25 (CDC25) plays an important role in transitions between cell-cycle phases by dephosphorylating and activating CDKs. CDC25B and CDC25C play a major role in G2/M progression, whereas CDC25A assists in G1/S transition. Different isomers of CDC25 expressions are upregulated in various clinicopathological situations. Overexpression of CDC25A deregulates G1/S and G2/M events, including the G2 checkpoint. CDC25B has oncogenic properties. Binding to the 14-3-3 proteins regulates the activity and localization of CDC25B. CDC25C is predominantly a nuclear protein in mammalian cells. At the G2/M transition, mitotic activation of CDC25C protein occurs by its dissociation from 14-3-3 proteins along with its phosphorylation at multiple sites within its N-terminal domain. In this article, we critically reviewed the biology of the activation/deactivation of CDC25 by kinases/phosphatases to maintain the level of CDK-cyclin activities and thus the genomic stability, clinical implications due to dysregulation of CDC25, and potential role of CDC25 inhibitors in diseases.

Cell-Cycle Control of Bivalent Epigenetic Domains Regulates the Exit from Pluripotency

Here we show that bivalent domains and chromosome architecture for bivalent genes are dynamically regulated during the cell cycle in human pluripotent cells. Central to this is the transient increase in H3K4-trimethylation at developmental genes during G1, thereby creating a "window of opportunity" for cell-fate specification. This mechanism is controlled by CDK2-dependent phosphorylation of the MLL2 (KMT2B) histone methyl-transferase, which facilitates its recruitment to developmental genes in G1. MLL2 binding is required for changes in chromosome architecture around developmental genes and establishes promoter-enhancer looping interactions in a cell-cycle-dependent manner. These cell-cycle-regulated loops are shown to be essential for activation of bivalent genes and pluripotency exit. These findings demonstrate that bivalent domains are established to control the cell-cycle-dependent activation of developmental genes so that differentiation initiates from the G1 phase.

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.

Progesterone receptor-cyclin D1 complexes induce cell cycle-dependent transcriptional programs in breast cancer cells

The progesterone receptor (PR) and its coactivators are direct targets of activated cyclin-dependent kinases (CDKs) in response to peptide growth factors, progesterone, and deregulation of cell cycle inhibitors. Herein, using the T47D breast cancer model, we probed mechanisms of cell cycle-dependent PR action. In the absence of exogenous progestin, the PR is specifically phosphorylated during the G2/M phase. Accordingly, numerous PR target genes are cell cycle regulated, including HSPB8, a heat-shock protein whose high expression is associated with tamoxifen resistance. Progestin-induced HSPB8 expression required cyclin D1 and was insensitive to antiestrogens but blocked by antiprogestins or inhibition of specificity factor 1 (SP1). HSPB8 expression increased with or without ligand when cells were G2/M synchronized or contained high levels of cyclin D1. Knockdown of PRs abrogated ligand-independent HSPB8 expression in synchronized cells. Notably, PRs and cyclin D1 copurified in whole-cell lysates of transiently transfected COS-1 cells and in PR-positive T47D breast cancer cells expressing endogenous cyclin D1. PRs, cyclin D1, and SP1 were recruited to the HSPB8 promoter in progestin-treated T47D breast cancer cells. Mutation of PR Ser345 to Ala (S345A) or inhibition of CDK2 activity using roscovitine disrupted PR/cyclin D1 interactions with DNA and blocked HSPB8 mRNA expression. Interaction of phosphorylated PRs with SP1 and cyclin D1 provides a mechanism for targeting transcriptionally active PRs to selected gene promoters relevant to breast cancer progression. Understanding the functional linkage between PRs and cell cycle regulatory proteins will provide keys to targeting novel PR/cyclin D1 cross talk in both hormone-responsive disease and HSPB8-high refractory disease with high HSPB8 expression.

Recruitment of trimeric proliferating cell nuclear antigen by G1-phase cyclin-dependent kinases following DNA damage with platinum-based antitumour agents

BACKGROUND

In cycling tumour cells, the binary cyclin-dependent kinase Cdk4/cyclin D or Cdk2/cyclin E complex is inhibited by p21 following DNA damage to induce G1 cell-cycle arrest. However, it is not known whether other proteins are also recruited within Cdk complexes, or their role, and this was investigated.

METHODS

Ovarian A2780 tumour cells were exposed to the platinum-based antitumour agent 1R,2R-diaminocyclohexane(trans-diacetato)(dichloro)platinum(IV) (DAP), which preferentially induces G1 arrest in a p21-dependent manner. The Cdk complexes were analysed by gel filtration chromatography, immunoblot and mass spectrometry.

RESULTS

The active forms of Cdk4 and Cdk2 complexes in control tumour cells have a molecular size of ~140 kDa, which increased to ~290 kDa when inhibited following G1 checkpoint activation by DAP. Proteomic analysis identified Cdk, cyclin, p21 and proliferating cell nuclear antigen (PCNA) in the inhibited complex, and biochemical studies provided unequivocal evidence that the increase in ~150 kDa of the inhibited complex is consistent with p21-dependent recruitment of PCNA as a trimer, likely bound to three molecules of p21. Although p21 alone was sufficient to inhibit the Cdk complex, PCNA was critical for stabilising p21.

CONCLUSION

G1 Cdk complexes inhibited by p21 also recruit PCNA, which inhibits degradation and, thereby, prolongs activity of p21 within the complex.

Assembly, activation, and substrate specificity of cyclin D1/Cdk2 complexes

Previous studies have shown conflicting data regarding cyclin D1/cyclin-dependent kinase 2 (Cdk2) complexes, and considering the widespread overexpression of cyclin D1 in cancer, it is important to fully understand their relevance. While many have shown that cyclin D1 and Cdk2 form active complexes, others have failed to show activity or association. Here, using a novel p21-PCNA fusion protein as well as p21 mutant proteins, we show that p21 is a required scaffolding protein, with cyclin D1 and Cdk2 failing to complex in its absence. These p21/cyclin D1/Cdk2 complexes are active and also bind the trimeric PCNA complex, with each trimer capable of independently binding distinct cyclin/Cdk complexes. We also show that increased p21 levels due to treatment with chemotherapeutic agents result in increased formation and kinase activity of cyclin D1/Cdk2 complexes, and that cyclin D1/Cdk2 complexes are able to phosphorylate a number of substrates in addition to Rb. Nucleophosmin and Cdh1, two proteins important for centrosome replication and implicated in the chromosomal instability of cancer, are shown to be phosphorylated by cyclin D1/Cdk2 complexes. Additionally, polypyrimidine tract binding protein-associated splicing factor (PSF) is identified as a novel Cdk2 substrate, being phosphorylated by Cdk2 complexed with either cyclin E or cyclin D1, and given the many functions of PSF, it could have important implications on cellular activity.

Constitutive CCND1/CDK2 activity substitutes for p53 loss, or MYC or oncogenic RAS expression in the transformation of human mammary epithelial cells

Cancer develops following the accumulation of genetic and epigenetic alterations that inactivate tumor suppressor genes and activate proto-oncogenes. Dysregulated cyclin-dependent kinase (CDK) activity has oncogenic potential in breast cancer due to its ability to inactivate key tumor suppressor networks and drive aberrant proliferation. Accumulation or over-expression of cyclin D1 (CCND1) occurs in a majority of breast cancers and over-expression of CCND1 leads to accumulation of activated CCND1/CDK2 complexes in breast cancer cells. We describe here the role of constitutively active CCND1/CDK2 complexes in human mammary epithelial cell (HMEC) transformation. A genetically-defined, stepwise HMEC transformation model was generated by inhibiting p16 and p53 with shRNA, and expressing exogenous MYC and mutant RAS. By replacing components of this model, we demonstrate that constitutive CCND1/CDK2 activity effectively confers anchorage independent growth by inhibiting p53 or replacing MYC or oncogenic RAS expression. These findings are consistent with several clinical observations of luminal breast cancer sub-types that show elevated CCND1 typically occurs in specimens that retain wild-type p53, do not amplify MYC, and contain no RAS mutations. Taken together, these data suggest that targeted inhibition of constitutive CCND1/CDK2 activity may enhance the effectiveness of current treatments for luminal breast cancer.

Constitutive Cdk2 activity promotes aneuploidy while altering the spindle assembly and tetraploidy checkpoints

The cell has many mechanisms for protecting the integrity of its genome. These mechanisms are often weakened or absent in many cancers, leading to high rates of chromosomal instability in tumors. Control of the cell cycle is crucial for the function of these checkpoints, and is frequently lost in cancers as well. Overexpression of Cyclin D1 in a large number of breast cancers causes overactivation of the cyclin-dependent kinases, including Cdk2. Constitutive Cdk2 activation through Cyclin D1 generates tumors in mice that are aneuploid and have many characteristics indicative of chromosomal instability. Expression of these complexes in the MCF10A cell line leads to retinoblastoma protein (Rb) hyperphosphorylation, a subsequent increase in proliferation rate, and increased expression of the spindle assembly checkpoint protein Mad2. This results in a strengthening of the spindle assembly checkpoint and renders cells more sensitive to the spindle poison paclitaxel. Constitutive Rb phosphorylation also causes a weakening of the p53-dependent tetraploidy checkpoint. Cells with overactive Cdk2 fail to arrest after mitotic slippage in the presence of paclitaxel or cytokinesis failure during treatment with cytochalasin-B, generating 8N populations. This additional increase in DNA content appears to further intensify the tetraploidy checkpoint in a step-wise manner. These polyploid cells are not viable long-term, either failing to undergo division or creating daughter cells that are unable to undergo subsequent division. This study raises intriguing questions about the treatment of tumors with overactive Cdk2.

Glucocorticoids and histone deacetylase inhibitors cooperate to block the invasiveness of basal-like breast cancer cells through novel mechanisms

Aggressive cancers often express E-cadherin in cytoplasmic vesicles rather than on the plasma membrane and this may contribute to the invasive phenotype of these tumors. Therapeutic strategies are not currently available that restore the anti-invasive function of E-cadherin in cancers. MDA-MB-231 cells are a frequently used model of invasive triple-negative breast cancer, and these cells express low levels of E-cadherin that is mislocalized to cytoplasmic vesicles. MDA-MB-231 cell lines stably expressing wild-type E-cadherin or E-cadherin fused to glutathione S-transferase or green fluorescent protein were used as experimental systems to probe the mechanisms responsible for cytoplasmic E-cadherin localization in invasive cancers. Although E-cadherin expression partly reduced cell invasion in vitro, E-cadherin was largely localized to the cytoplasm and did not block the invasiveness of the corresponding orthotopic xenograft tumors. Further studies indicated that the glucocorticoid dexamethasone and the highly potent class I histone deacetylase (HDAC) inhibitor largazole cooperated to induce E-cadherin localization to the plasma membrane in triple-negative breast cancers, and to suppress cellular invasion in vitro. Dexamethasone blocked the production of the cleaved form of the CDCP1 (that is, CUB domain-containing protein 1) protein (cCDCP1) previously implicated in the pro-invasive activities of CDCP1 by upregulating the serine protease inhibitor plasminogen activator inhibitor-1. E-cadherin preferentially associated with cCDCP1 compared with the full-length form. In contrast, largazole did not influence CDCP1 cleavage, but increased the association of E-cadherin with γ-catenin. This effect on E-cadherin/γ-catenin complexes was shared with the nonisoform selective HDAC inhibitors trichostatin A (TSA) and vorinostat (suberoylanilide hydroxamic acid, SAHA), although largazole upregulated endogenous E-cadherin levels more strongly than TSA. These results demonstrate that glucocorticoids and HDAC inhibitors, both of which are currently in clinical use, cooperate to suppress the invasiveness of breast cancer cells through novel, complementary mechanisms that converge on E-cadherin.

Extracellular taurine induces angiogenesis by activating ERK-, Akt-, and FAK-dependent signal pathways

Taurine, a non essential sulfur-containing amino acid, plays a critical role in cardiovascular functions. We here examined the effect of taurine on angiogenesis and its underlying signal pathway. Taurine treatment increased angiogenesis in vitro and in vivo, which was followed by activation of the phosphatidylinositol 3-kinase (PI3K)/Akt, MEK/ERK, and Src/FAK signaling pathways. Further, taurine promoted endothelial cell cycle progression to the S and G2/M phases by up-regulating the positive cell cycle proteins, particularly cyclins D1 and B, as well as down-regulating the negative cell cycle proteins, p53 and p21(WAF1/CIP1), resulting in Rb phosphorylation. This angiogenic event was inhibited by inhibitors of PI3K and MEK. In addition, a PI3K inhibitor blocked the activation of Akt and ERK, while Akt knockdown did not affect taurine-induced ERK activation, indicating that PI3K is an upstream mediator of both MEK and Akt. Taurine-induced endothelial cell migration was suppressed by Src inhibitor, but not by other inhibitors, suggesting that the increase in cell migration is regulated by Src-dependent pathway. Moreover, inhibition of cellular taurine uptake by β-alanine and taurine transporter knockdown promoted taurine-induced cell proliferation, ERK and Akt activation, and in vivo angiogenesis, suggesting that extracellular taurine induces angiogenesis. However, taurine did not induce vascular inflammation and permeability in vitro and in vivo. These data demonstrate that extracellular taurine promotes angiogenesis by Akt- and ERK-dependent cell cycle progression and Src/FAK-mediated cell migration without inducing vascular inflammation, indicating that it is potential use for the treatment of vascular dysfunction-associated human diseases.

Regulation of p21, MMP-1, and MDR-1 Expression in Human Colon Carcinoma HT29 Cells by Tian Xian Liquid, a Chinese Medicinal Formula, In Vitro and In Vivo

Ethnopharmacological relevance. Tian-Xian liquid (TXL), a commercially available Chinese medicine decoction, has been used as an anticancer dietary agent for more than 10 years without reported side effects. Aim of the study. The safety and quality consistency of TXL and its mechanisms of action on antiproliferation, antimetastasis, and reversion of multidrug resistance (MDR) regimens were explored. Materials and methods. In this study, an atomic absorption spectrophotometer and reversed phase high performance liquid chromatography with photodiode array detection (HPLC-DAD) were used to evaluate the main toxic elements and the quality consistency among different batches of TXL extracts, respectively. HT29 human colon cancer cell line and tumor-bearing nude mice were used. TXL was provided by China-Japan Feida Union Company Limited. The effect of TXL on in vitro proliferation of HT29 human colon cancer cell line was examined. The percentages of treated cells distributed in different phases of the cell cycles were analyzed by flow cytometry. Antiproliferative effect after treatment with TXL was assessed by determination of the protein levels of p21, cyclinD1, PCNA, and cdk-2, which are the key regulators for cell cycle progression. Meanwhile, the protein levels of MMP-1 and MDR-1 (multidrug resistance protein-1) were also determined to assess the effect of TXL on antimetastasis and reversion of MDR regimen, respectively. Results. The contents of main toxic elements were lower in TXL extract compared with the standard set by the Department of Health of the Government of Hong Kong Special Administrative Region (SAR). Our HPLC results showed that the relative standard deviations of the amount of the 5 standards were less than 5% in different batches of TXL. Immunoblotting analysis revealed a dramatic induction of cyclin kinase inhibitor p21 as well as an inhibition of cyclinD1, PCNA, and cdk-2 in the TXL-treated in vitro models, thereby, impeding cell progression from G1/S phase. Results obtained from the in vivo study also demonstrated that TXL upregulated the protein level of p21 and downregulated the protein levels of MMP-1 and MDR-1. Conclusions. Results obtained from the present investigation not only demonstrate the safety and quality of TXL extract but also demonstrate that TXL possesses antiproliferative and antimetastatic activities and brings about reversion of MDR on HT29 cell and on xenografted tissue in tumor-implanted nude mice.

Chk1-cyclin A/Cdk1 axis regulates origin firing programs in mammals

DNA replication is key to ensuring the complete duplication of genomic DNA prior to mitosis and is tightly regulated by both cell cycle machinery and checkpoint signals. Regulation of the S phase program occurs at several stages, affecting origin firing, replication fork elongation, fork velocity, and fork stability, all of which are dependent on S-phase-promoting kinase activity. Somatic mammalian cells use well-established origin programs by which specific regions of the genome are replicated at precise times. However, the mechanisms by which S phase kinases regulate origin firing in mammals are largely unknown. Here, we discuss recent advances in the understanding of how S phase programs are regulated in mammals at the correct regions and at the appropriate times.

Functional modules in the Arabidopsis core cell cycle binary protein-protein interaction network

As in other eukaryotes, cell division in plants is highly conserved and regulated by cyclin-dependent kinases (CDKs) that are themselves predominantly regulated at the posttranscriptional level by their association with proteins such as cyclins. Although over the last years the knowledge of the plant cell cycle has considerably increased, little is known on the assembly and regulation of the different CDK complexes. To map protein-protein interactions between core cell cycle proteins of Arabidopsis thaliana, a binary protein-protein interactome network was generated using two complementary high-throughput interaction assays, yeast two-hybrid and bimolecular fluorescence complementation. Pairwise interactions among 58 core cell cycle proteins were tested, resulting in 357 interactions, of which 293 have not been reported before. Integration of the binary interaction results with cell cycle phase-dependent expression information and localization data allowed the construction of a dynamic interaction network. The obtained interaction map constitutes a framework for further in-depth analysis of the cell cycle machinery.

A novel class of cyclin-dependent kinase inhibitors identified by molecular docking act through a unique mechanism

The cyclin-dependent kinase (Cdk) family is emerging as an important therapeutic target in the treatment of cancer. Cdks 1, 2, 4, and 6 are the key members that regulate the cell cycle, as opposed to Cdks that control processes such as transcription (Cdk7 and Cdk9). For this reason, Cdks 1, 2, 4, and 6 have been the subject of extensive cell cycle-related research, and consequently many inhibitors have been developed to target these proteins. However, the compounds that comprise the current list of Cdk inhibitors are largely ATP-competitive. Here we report the identification of a novel structural site on Cdk2, which is well conserved between the cell cycle Cdks. Small molecules identified by a high throughput in silico screen of this pocket exhibit cytostatic effects and act by reducing the apparent protein levels of cell cycle Cdks. Drug-induced cell cycle arrest is associated with decreased Rb phosphorylation and decreased expression of E2F-dependent genes. Multiple lines of evidence indicate that the primary mechanism of action of these compounds is the direct induction of Cdk1, Cdk2, and Cdk4 protein aggregation.

Mammary tumors initiated by constitutive Cdk2 activation contain an invasive basal-like component

The basal-like subtype of breast cancer is associated with invasiveness, high rates of postsurgical recurrence, and poor prognosis. Aside from inactivation of the BRCA1 tumor-suppressor gene, little is known concerning the mechanisms that cause basal breast cancer or the mechanisms responsible for its invasiveness. Here, we show that the heterogeneous mouse mammary tumor virus-cyclin D1-Cdk2 (MMTV-D1K2) transgenic mouse mammary tumors contain regions of spindle-shaped cells expressing both luminal and myoepithelial markers. Cell lines cultured from these tumors exhibit the same luminal/myoepithelial mixed-lineage phenotype that is associated with human basal-like breast cancer and express a number of myoepithelial markers including cytokeratin 14, P-cadherin, alpha smooth muscle actin, and nestin. The MMTV-D1K2 tumor-derived cell lines form highly invasive tumors when injected into mouse mammary glands. Invasion is associated with E-cadherin localization to the cytoplasm or loss of E-cadherin expression. Cytoplasmic E-cadherin correlates with lack of colony formation in vitro and beta-catenin and p120(ctn) localization to the cytoplasm. The data suggest that the invasiveness of these cell lines results from a combination of factors including mislocalization of E-cadherin, beta-catenin, and p120(ctn) to the cytoplasm. Nestin expression and E-cadherin mislocalization were also observed in human basal-like breast cancer cell lines, suggesting that these results are relevant to human tumors. Together, these results suggest that abnormal Cdk2 activation may contribute to the formation of basal-like breast cancers.

p53-mediated growth suppression in response to Nutlin-3 in cyclin D1 transformed cells occurs independently of p21

Interaction of cyclin D1 with cyclin-dependent kinases (CDK) results in the hyperphosphorylation of the RB family of proteins, thereby inactivating the tumor-suppressive function of RB. Our previous findings suggest that constitutive cyclin D1/CDK activity inhibits p53-mediated gene repression by preventing the appropriate regulation of CDK activity by the CDK inhibitor p21, a transcriptional target of p53. To study the role of cyclin D1 in driving human mammary cell transformation, we expressed a constitutively active cyclin D1-CDK fusion protein (D1/CDK) in immortalized human mammary epithelial cells. D1/CDK-expressing human mammary epithelial cells grew anchorage-independently in the presence of wild-type p53, consistent with the idea that D1/CDK disrupts downstream p53 signaling. Using this transformation model, we examined the sensitivity of the D1/CDK-expressing cells to Nutlin-3, an HDM2 antagonist that activates p53. Surprisingly, treatment of D1/CDK-transformed cells with Nutlin-3 prevented their anchorage-independent growth. The Nutlin-3-induced growth arrest was enforced in D1/CDK-expressing cells despite the presence of hyperphosphorylated RB implicating a p53-dependent, RB-independent mechanism for growth suppression. Further analysis identified that CDC2 and cyclin B1, key cell cycle regulators, were stably down-regulated following p53 stabilization by Nutlin-3, consistent with direct interaction between p53 and the CDC2 and cyclin B1 promoters, leading to the repression of transcription by methylation. In contrast to D1/CDK expression, direct inactivation of p53 resulted in no repression of CDC2 and no cell cycle arrest. We conclude that induction of p53 by Nutlin-3 is a viable therapeutic strategy in cancers with constitutive CDK signaling due to the direct repression of specific p53 target genes.

Nuclear factor-kappaB-dependent cyclin D1 induction and DNA replication associated with N-methyl-D-aspartate receptor-mediated apoptosis in rat striatum

Cell cycle reentry has been found during apoptosis of postmitotic neurons under certain pathological conditions. To evaluate whether nuclear factor-kappaB (NF-kappaB) activation promotes cell cycle entry and neuronal apoptosis, we studied the relation among NF-kappaB-mediated cyclin induction, bromodeoxyuridine (BrdU) incorporation, and apoptosis initiation in rat striatal neurons following excitotoxic insult. Intrastriatally injected N-methyl-D-aspartate receptor agonist quinolinic acid (QA, 60 nmol) elicited a rise in cyclin D1 mRNA and protein levels (P<0.05). QA-induced NF-kappaB activation occurred in striatal neurons and nonneuronal cells and partially colocalized with elevated cyclin D1 immunoreactivity and TUNEL-positive nuclei. QA triggered DNA replication as evidenced by BrdU incorporation; some striatal BrdU-positive cells were identified as neurons by colocalization with NeuN. Blockade of NF-kappaB nuclear translocation with the recombinant peptide NF-kappaB SN50 attenuated the QA-induced elevation in cyclin D1 and BrdU incorporation. QA-induced internucleosomal DNA fragmentation was blunted by G(1)/S-phase cell cycle inhibitors. These findings suggest that NF-kappaB activation stimulates cyclin D1 expression and triggers DNA replication in striatal neurons. Excitotoxin-induced neuronal apoptosis may thus result from, at least partially, a failed cell cycle attempt.

Borealin is repressed in response to p53/Rb signaling

Rb/E2F regulates many genes that encode proteins required for the cell cycle. Using affymetrix microarrays we previously identified genes regulated by the Rb proteins p130 and p107, many of which are involved in the cell cycle. Several genes with unknown functions were also repressed by p130 and p107, of which some have recently been found to have various roles in mitosis, the spindle checkpoint and cytokinesis. This study focuses on the regulation of borealin/dasra/cdca8, which encodes a recently discovered member of the chromosomal passenger complex. It is recorded that borealin is a cell cycle regulator, down-regulated in response to p53/Rb-signaling, and up-regulated in many types of cancerous tissues.

Tumors Initiated by Constitutive Cdk2 Activation Exhibit Transforming Growth Factor β Resistance and Acquire Paracrine Mitogenic Stimulation during Progression

Cyclin D1/cyclin-dependent kinase 2 (Cdk2) complexes are present at high frequency in human breast cancer cell lines, but the significance of this observation is unknown. This report shows that expression of a cyclin D1-Cdk2 fusion protein under the control of the mouse mammary tumor virus (MMTV) promoter results in mammary gland hyperplasia and fibrosis, and mammary tumors. Cell lines isolated from MMTV-cyclin D1-Cdk2 (MMTV-D1K2) tumors exhibit Rb and p130 hyperphosphorylation and up-regulation of the protein products of E2F-dependent genes. These results suggest that cyclin D1/Cdk2 complexes may mediate some of the transforming effects that result from cyclin D1 overexpression in human breast cancers. MMTV-D1K2 cancer cells express the hepatocyte growth factor (HGF) receptor, c-Met. MMTV-D1K2 cancer cells also secrete transforming growth factor beta (TGFbeta), but are relatively resistant to TGFbeta antiproliferative effects. Fibroblasts derived from MMTV-D1K2 tumors secrete factors that stimulate the proliferation of MMTV-D1K2 cancer cells, stimulate c-Met tyrosine phosphorylation, and stimulate the phosphorylation of the downstream signaling intermediates p70(s6k) and Akt on activating sites. Together, these results suggest that deregulation of the Cdk/Rb/E2F axis reprograms mammary epithelial cells to initiate a paracrine loop with tumor-associated fibroblasts involving TGFbeta and HGF, resulting in desmoplasia. The MMTV-D1K2 mice should provide a useful model system for the development of therapeutic approaches to block the stromal desmoplastic reaction that likely plays an important role in the progression of multiple types of human tumors.

Cellular mechanism through which parathyroid hormone-related protein induces proliferation in arterial smooth muscle cells: definition of an arterial smooth muscle PTHrP/p27kip1 pathway

Parathyroid hormone-related protein (PTHrP) is present in vascular smooth muscle (VSM), is markedly upregulated in response to arterial injury, is essential for normal VSM proliferation, and also markedly accentuates neointima formation following rat carotid angioplasty. PTHrP contains a nuclear localization signal (NLS) through which it enters the nucleus and leads to marked increases in retinoblastoma protein (pRb) phosphorylation and cell cycle progression. Our goal was to define key cell cycle molecules upstream of pRb that mediate cell cycle acceleration induced by PTHrP. The cyclin D/cdk-4,-6 system and its upstream regulators, the inhibitory kinases (INKs), are not appreciably influenced by PTHrP. In striking contrast, cyclin E/cdk-2 kinase activity is markedly increased by PTHrP, and this is a result of a specific, marked, PTHrP-induced proteasomal degradation of p27(kip1). Adenoviral restoration of p27(kip1) fully reverses PTHrP-induced cell cycle progression, indicating that PTHrP mediates its cell cycle acceleration in VSM via p27(kip1). In confirmation, adenoviral delivery of PTHrP to murine primary vascular smooth muscle cells (VSMCs) significantly decreases p27(kip1) expression and accelerates cell cycle progression. p27(kip1) is well known to be a central cell cycle regulatory molecule involved in both normal and pathological VSM proliferation and is a target of widely used drug-eluting stents. The current observations define a novel "PTHrP/p27(kip1) pathway" in the arterial wall and suggest that this pathway is important in normal arterial biology and a potential target for therapeutic manipulation of the arterial response to injury.

Rapamycin Disrupts Cyclin/Cyclin-Dependent Kinase/p21/Proliferating Cell Nuclear Antigen Complexes and Cyclin D1 Reverses Rapamycin Action by Stabilizing These Complexes

Rapamycin and its derivatives are promising anticancer agents, but the exact mechanisms by which these drugs induce cell cycle arrest and inhibit tumor growth are unknown. A biochemical analysis of human mammary tumor cell lines indicated that rapamycin-induced antiproliferative effects correlated with down-regulation of cellular p21 levels and the levels of p21 in cyclin-dependent kinase (Cdk) 2 and 4 complexes. Cyclin D1 overexpression reversed rapamycin action and this reversal correlated with increased levels of cellular p21, higher levels of p21 associated with Cdk2, and stabilization of cyclin D1/Cdk2/p21/proliferating cell nuclear antigen (PCNA) complexes. Experiments using a novel cyclin D1-Cdk2 fusion protein or a kinase-dead mutant of the fusion protein indicated that reversal of rapamycin action required not only the formation of complexes with p21 and PCNA but also complex-associated kinase activity. Similar results were observed in vivo. The rapamycin derivative RAD001 (everolimus) inhibited the growth of mouse mammary tumors, which correlated with the disruption of cyclin D1/Cdk2 complexes. The potential implications of these results with respect to the use of rapamycin derivatives in breast cancer therapy are discussed.

Influence of thyroid hormone receptors on breast cancer cell proliferation

BACKGROUND

The involvement of thyroid hormones in the development and differentiation of normal breast tissue has been established. However, the association between breast cancer and these hormones is controversial. Therefore, the objective of the present study was to determine the protein expression pattern of thyroid hormone receptors in different human breast pathologies and to evaluate their possible relationship with cellular proliferation.

PATIENTS AND METHODS

The presence of thyroid hormone receptors was evaluated by immunohistochemistry and western blot analysis in 84 breast samples that included 12 cases of benign proliferative diseases, 20 carcinomas in situ and 52 infiltrative carcinomas.

RESULTS

TR-alpha was detected in the nuclei of epithelial cells from normal breast ducts and acini, while in any pathological type this receptor was located in the cytoplasm. However, TR-beta presented a nuclear location in benign proliferative diseases and carcinomas in situ and a cytoplasmatic location in normal breast and infiltrative carcinomas. The highest proliferation index was observed in carcinomas in situ, although in infiltrative carcinomas an inverse correlation between this index and the TR-alpha expression was encountered.

CONCLUSIONS

The results of this study reveal substantial changes in the expression profile of thyroid hormone receptors suggesting a possible deregulation that could trigger breast cancer development.

Modulation of Cell Cycle Components by Epigenetic and Genetic Events

Cell cycle progression is monitored by surveillance mechanisms, or cell cycle checkpoints, that ensure that initiation of a later event is coupled with the completion of an early cell cycle event. Deregulated proliferation is a characteristic feature of tumor cells. Moreover, defects in many of the molecules that regulate the cell cycle have been implicated in cancer formation and progression. Key among these are p53, the retinoblastoma protein (pRb) and its related proteins, p107 and pRb2/p130, and cdk inhibitors (p15, p16, p18, p19, p21, p27), all of which act to keep the cell cycle from progressing until all repairs to damaged DNA have been completed. The pRb (pRb/p16(INK4a)/cyclin D1) and p53 (p14(ARF)/mdm2/p53) pathways are the two main cell-cycle control pathways frequently targeted in tumorigenesis, and the alterations occurring in each pathway depend on the tumor type. Virtually all human tumors deregulate either the pRb or p53 pathway, and oftentimes both pathways simultaneously. This review focuses on the genetic and epigenetic alterations affecting the components of mechanisms regulating the progression of the cell cycle and leading to cancer formation and progression.

p130/p107/p105Rb-dependent transcriptional repression during DNA-damage-induced cell-cycle exit at G2

The progression of normal cells from G2 into mitosis is stably blocked when their DNA is damaged. Tumor cells lacking p53 arrest only transiently in G2, but eventually enter mitosis. We show that an important component of the stable G2 arrest in normal cells is the transcriptional repression of more than 20 genes encoding proteins needed to enter into and progress through mitosis. Studies from a number of labs including our own have shown that, by inducing p53 and p21/WAF1, DNA damage can trigger RB-family-dependent transcriptional repression. Our studies reported here show that p130 and p107 play a key role in transcriptional repression of genes required for G2 and M in response to DNA damage. For plk1, repression is partially abrogated by loss of p130 and p107, and is completely abrogated by loss of all three RB-family proteins. Mouse cells lacking RB-family proteins do not accumulate with a 4N content of DNA when exposed to adriamycin, suggesting that all three RB-family proteins contribute to G2 arrest in response to DNA damage. Stable arrest in the presence of functional p53-to-RB signaling is probably due to the ability of cells to exit the cell cycle from G2, a conclusion supported by our observation that KI67, a marker of cell-cycle entry, is downregulated in both G1 and G2 in a p53-dependent manner.

Flat epithelial atypia: concepts and controversies of an intraductal lesion of the breast

Image-guided biopsies of the breast play an integral role in the diagnostic evaluation of mammographically detected calcifications. Apart from cancer, on these biopsies we are increasingly recognising a hitherto poorly categorised group of benign to atypical entities collectively known as columnar cell lesions. A variant of this theme is flat epithelial atypia, a columnar lesion characterised by mildly atypical epithelial cells. There is emerging evidence to suggest that flat epithelial atypia may represent a precursor of or the earliest morphologically recognisable form of low-grade ductal carcinoma in situ. In addition, these lesions are often associated with tubular carcinomas and lobular neoplasia. This article reviews the current concepts and controversies surrounding flat epithelial atypia, with special emphasis on the biology, histological and clinical features. Potential diagnostic challenges faced by pathologists in the evaluation of these atypical columnar lesions are also discussed.