P/CAF associates with cyclin D1 and potentiates its activation of the estrogen receptor

DNA Damage Responses in Tumors Are Not Proliferative Stimuli, but Rather They Are DNA Repair Actions Requiring Supportive Medical Care

BACKGROUND

In tumors, somatic mutagenesis presumably drives the DNA damage response (DDR) via altered regulatory pathways, increasing genomic instability and proliferative activity. These considerations led to the standard therapeutic strategy against cancer: the disruption of mutation-activated DNA repair pathways of tumors.

PURPOSE

Justifying that cancer cells are not enemies to be killed, but rather that they are ill human cells which have the remnants of physiologic regulatory pathways.

RESULTS

1. Genomic instability and cancer development may be originated from a flaw in estrogen signaling rather than excessive estrogen signaling; 2. Healthy cells with genomic instability exhibit somatic mutations, helping DNA restitution; 3. Somatic mutations in tumor cells aim for the restoration of DNA damage, rather than further genomic derangement; 4. In tumors, estrogen signaling drives the pathways of DNA stabilization, leading to apoptotic death; 5. In peritumoral cellular infiltration, the genomic damage of the tumor induces inflammatory cytokine secretion and increased estrogen synthesis. In the inflammatory cells, an increased growth factor receptor (GFR) signaling confers the unliganded activation of estrogen receptors (ERs); 6. In breast cancer cells responsive to genotoxic therapy, constitutive mutations help the upregulation of estrogen signaling and consequential apoptosis. In breast tumors non-responsive to genotoxic therapy, the possibilities for ER activation via either liganded or unliganded pathways are exhausted, leading to farther genomic instability and unrestrained proliferation.

CONCLUSIONS

Understanding the real character and behavior of human tumors at the molecular level suggests that we should learn the genome repairing methods of tumors and follow them by supportive therapy, rather than provoking additional genomic damages.

Astrocyte DNA damage and response upon acute exposure to ethanol and corticosterone

Introduction: Astrocytes are the glial cells responsible for brain homeostasis, but if injured, they could damage neural cells even deadly. Genetic damage, DNA damage response (DDR), and its downstream cascades are dramatic events poorly studied in astrocytes. Hypothesis and methods: We propose that 1 h of 400 mmol/L ethanol and/or 1 μmol/L corticosterone exposure of cultured hippocampal astrocytes damages DNA, activating the DDR and eliciting functional changes. Immunolabeling against γH2AX (chromatin DNA damage sites), cyclin D1 (cell cycle control), nuclear (base excision repair, BER), and cytoplasmic (anti-inflammatory functions) APE1, ribosomal nucleolus proteins together with GFAP and S100β plus scanning electron microscopy studies of the astrocyte surface were carried out. Results: Data obtained indicate significant DNA damage, immediate cell cycle arrest, and BER activation. Changes in the cytoplasmic signals of cyclin D1 and APE1, nucleolus number, and membrane-attached vesicles strongly suggest a reactivity like astrocyte response without significant morphological changes. Discussion: Obtained results uncover astrocyte genome immediate vulnerability and DDR activation, plus a functional response that might in part, be signaled through extracellular vesicles, evidencing the complex influence that astrocytes may have on the CNS even upon short-term aggressions.

D-Type Cyclins in Development and Disease

D-type cyclins encode G1/S cell cycle checkpoint proteins, which play a crucial role in defining cell cycle exit and progression. Precise control of cell cycle exit is vital during embryonic development, with defects in the pathways regulating intracellular D-type cyclins resulting in abnormal initiation of stem cell differentiation in a variety of different organ systems. Furthermore, stabilisation of D-type cyclins is observed in a wide range of disorders characterized by cellular over-proliferation, including cancers and overgrowth disorders. In this review, we will summarize and compare the roles played by each D-type cyclin during development and provide examples of how their intracellular dysregulation can be an underlying cause of disease.

Targeting cyclin D1 as a therapeutic approach for papillary thyroid carcinoma

Cyclin D1 functions as a mitogenic sensor that specifically binds to CDK4/6, thereby integrating external mitogenic inputs and cell cycle progression. Cyclin D1 interacts with transcription factors and regulates various important cellular processes, including differentiation, proliferation, apoptosis, and DNA repair. Therefore, its dysregulation contributes to carcinogenesis. Cyclin D1 is highly expressed in papillary thyroid carcinoma (PTC). However, the particular cellular mechanisms through which abnormal cyclin D1 expression causes PTC are poorly understood. Unveiling the regulatory mechanisms of cyclin D1 and its function in PTC may help determine clinically effective strategies, and open up better opportunities for further research, leading to the development of novel PTC regimens that are clinically effective. This review explores the mechanisms underlying cyclin D1 overexpression in PTC. Furthermore, we discuss the role of cyclin D1 in PTC tumorigenesis via its interactions with other regulatory elements. Finally, recent progress in the development of therapeutic options targeting cyclin D1 in PTC is examined and summarized.

Blocking cell cycle progression through CDK4/6 protects against chronic kidney disease

Acute and chronic kidney injuries induce increased cell cycle progression in renal tubules. While increased cell cycle progression promotes repair after acute injury, the role of ongoing tubular cell cycle progression in chronic kidney disease is unknown. Two weeks after initiation of chronic kidney disease, we blocked cell cycle progression at G1/S phase by using an FDA-approved, selective inhibitor of CDK4/6. Blocking CDK4/6 improved renal function and reduced tubular injury and fibrosis in 2 murine models of chronic kidney disease. However, selective deletion of cyclin D1, which complexes with CDK4/6 to promote cell cycle progression, paradoxically increased tubular injury. Expression quantitative trait loci (eQTLs) for CCND1 (cyclin D1) and the CDK4/6 inhibitor CDKN2B were associated with eGFR in genome-wide association studies. Consistent with the preclinical studies, reduced expression of CDKN2B correlated with lower eGFR values, and higher levels of CCND1 correlated with higher eGFR values. CDK4/6 inhibition promoted tubular cell survival, in part, through a STAT3/IL-1β pathway and was dependent upon on its effects on the cell cycle. Our data challenge the paradigm that tubular cell cycle progression is beneficial in the context of chronic kidney injury. Unlike the reparative role of cell cycle progression following acute kidney injury, these data suggest that blocking cell cycle progression by inhibiting CDK4/6, but not cyclin D1, protects against chronic kidney injury.

ERα promotes transcription of tumor suppressor gene ApoA-I by establishing H3K27ac-enriched chromatin microenvironment in breast cancer cells

Apolipoprotein A-I (ApoA-I), the main protein component of high-density lipoprotein (HDL), plays a pivotal role in reverse cholesterol transport (RCT). Previous studies indicated a reduction of serum ApoA-I levels in various types of cancer, suggesting ApoA-I as a potential cancer biomarker. Herein, ectopically overexpressed ApoA-I in MDA-MB-231 breast cancer cells was observed to have antitumor effects, inhibiting cell proliferation and migration. Subsequent studies on the mechanism of expression regulation revealed that estradiol (E2)/estrogen receptor α (ERα) signaling activates ApoA-I gene transcription in breast cancer cells. Mechanistically, our ChIP-seq data showed that ERα directly binds to the estrogen response element (ERE) site within the ApoA-I gene and establishes an acetylation of histone 3 lysine 27 (H3K27ac)‍-enriched chromatin microenvironment. Conversely, Fulvestrant (ICI 182780) treatment blocked ERα binding to ERE within the ApoA-I gene and downregulated the H3K27ac level on the ApoA-I gene. Treatment with p300 inhibitor also significantly decreased the ApoA-I messenger RNA (mRNA) level in MCF7 cells. Furthermore, the analysis of data from The Cancer Genome Atlas (TCGA) revealed a positive correlation between ERα and ApoA-I expression in breast cancer tissues. Taken together, our study not only revealed the antitumor potential of ApoA-I at the cellular level, but also found that ERα promotes the transcription of ApoA-I gene through direct genomic effects, and p300 may act as a co-activator of ERα in this process.

Steroid Hormone Receptors: Links With Cell Cycle Machinery and Breast Cancer Progression

Progression of cells through cell cycle consists of a series of events orchestrated in a regulated fashion. Such processes are influenced by cell cycle regulated expression of various proteins where multiple families of transcription factors take integral parts. Among these, the steroid hormone receptors (SHRs) represent a connection between the external hormone milieu and genes that control cellular proliferation. Therefore, understanding the molecular connection between the transcriptional role of steroid hormone receptors and cell cycle deserves importance in dissecting cellular proliferation in normal as well as malignant conditions. Deregulation of cell cycle promotes malignancies of various origins, including breast cancer. Indeed, SHR members play crucial role in breast cancer progression as well as management. This review focuses on SHR-driven cell cycle regulation and moving forward, attempts to discuss the role of SHR-driven crosstalk between cell cycle anomalies and breast cancer.

Cyclin D1 in Cancer: A Molecular Connection for Cell Cycle Control, Adhesion and Invasion in Tumor and Stroma

Cyclin D1, an important regulator of cell cycle, carries out a central role in the pathogenesis of cancer determining uncontrolled cellular proliferation. In normal cells, Cyclin D1 expression levels are strictly regulated, conversely, in cancer, its activity is intensified in various manners. Different studies demonstrate that CCDN1 gene is amplified in several tumor types considering it as a negative prognostic marker of this pathology. Cyclin D1 is known for its role in the nucleus, but recent clinical studies associate the amount located in the cytoplasmic membrane with tumor invasion and metastasis. Cyclin D1 has also other functions: it governs the expression of specific miRNAs and it plays a crucial role in the tumor-stroma interactions potentiating most of the cancer hallmarks. In the present review, we will summarize the current scientific evidences that highlight the involvement of Cyclin D1 in the pathogenesis of different types of cancer, best of all in breast cancer. We will also focus on recent insights regarding the Cyclin D1 as molecular bridge between cell cycle control, adhesion, invasion, and tumor/stroma/immune-system interplay in cancer.

Compensatory Estrogen Signal Is Capable of DNA Repair in Antiestrogen-Responsive Cancer Cells via Activating Mutations

Cancer cells are embarrassed human cells exhibiting the remnants of same mechanisms for DNA stabilization like patients have in their healthy cells. Antiestrogens target the liganded activation of ERs, which is the principal means of genomic regulation in both patients and their tumors. The artificial blockade of liganded ER activation is an emergency situation promoting strong compensatory actions even in cancer cells. When tumor cells are capable of an appropriate upregulation of ER signaling resulting in DNA repair, a tumor response may be detected. In contrast, when ER signaling is completely inhibited, tumor cells show unrestrained proliferation, and tumor growth may be observed. The laboratory investigations of genomic mechanisms in antiestrogen-responsive and antiestrogen-unresponsive tumor cells have considerably enhanced our knowledge regarding the principal regulatory capacity of estrogen signaling. In antiestrogen-responsive tumor cells, a compensatory increased expression and liganded activation of estrogen receptors (ERs) result in an apoptotic death. Conversely, in antiestrogen resistant tumors exhibiting a complete blockade of liganded ER activation, a compensatory effort for unliganded ER activation is characteristic, conferred by the increased expression and activity of growth factor receptors. However, even extreme unliganded ER activation is incapable of DNA restoration when the liganded ER activation is completely blocked. Researchers mistakenly suspect even today that in tumors growing under antiestrogen treatment, the increased unliganded activation of estrogen receptor via activating mutations is an aggressive survival technique, whilst it is a compensatory effort against the blockade of liganded ER activation. The capacity of liganded ERs for genome modification in emergency states provides possibilities for estrogen/ER use in medical practice including cancer cure.

Emerging roles of metazoan cell cycle regulators as coordinators of the cell cycle and differentiation

In multicellular organisms, cell proliferation must be tightly coordinated with other developmental processes to form functional tissues and organs. Despite significant advances in our understanding of how the cell cycle is controlled by conserved cell-cycle regulators (CCRs), how the cell cycle is coordinated with cell differentiation in metazoan organisms and how CCRs contribute to this process remain poorly understood. Here, we review the emerging roles of metazoan CCRs as intracellular proliferation-differentiation coordinators in multicellular organisms. We illustrate how major CCRs regulate cellular events that are required for cell fate acquisition and subsequent differentiation. To this end, CCRs employ diverse mechanisms, some of which are separable from those underpinning the conventional cell-cycle-regulatory functions of CCRs. By controlling cell-type-specific specification/differentiation processes alongside the progression of the cell cycle, CCRs enable spatiotemporal coupling between differentiation and cell proliferation in various developmental contexts in vivo. We discuss the significance and implications of this underappreciated role of metazoan CCRs for development, disease and evolution.

Less-well known functions of cyclin/CDK complexes

Cyclin-dependent kinases (CDKs) are activated by cyclins, which play important roles in dictating the actions of CDK/cyclin complexes. Cyclin binding influences the substrate specificity of these complexes in addition to their susceptibility to inhibition or degradation. CDK/cyclin complexes are best known to promote cell cycle progression in the mitotic cell cycle but are also crucial for important cellular processes not strictly associated with cellular division. This chapter primarily explores the understudied topic of CDK/cyclin complex functionality during the DNA damage response. We detail how CDK/cyclin complexes perform dual roles both as targets of DNA damage checkpoint signaling as well as effectors of DNA repair. Additionally, we discuss the potential CDK-independent roles of cyclins in these processes and the impact of such roles in human diseases such as cancer. Our goal is to place the spotlight on these important functions of cyclins either acting as independent entities or within CDK/cyclin complexes which have attracted less attention in the past. We consider that this will be important for a more complete understanding of the intricate functions of cell cycle proteins in the DNA damage response.

The double dealing of cyclin D1

The cell cycle is tightly regulated by cyclins and their catalytic moieties, the cyclin-dependent kinases (CDKs). Cyclin D1, in association with CDK4/6, acts as a mitogenic sensor and integrates extracellular mitogenic signals and cell cycle progression. When deregulated (overexpressed, accumulated, inappropriately located), cyclin D1 becomes an oncogene and is recognized as a driver of solid tumors and hemopathies. Recent studies on the oncogenic roles of cyclin D1 reported non-canonical functions dependent on the partners of cyclin D1 and its location within tumor cells or tissues. Support for these new functions was provided by various mouse models of oncogenesis. Finally, proteomic and transcriptomic data identified complex cyclin D1 networks. This review focuses on these aspects of cyclin D1 pathophysiology, which may be crucial for targeted therapy.Abbreviations: aa, amino acid; AR, androgen receptor; ATM, ataxia telangectasia mutant; ATR, ATM and Rad3-related; CDK, cyclin-dependent kinase; ChREBP, carbohydrate response element binding protein; CIP, CDK-interacting protein; CHK1/2, checkpoint kinase 1/2; CKI, CDK inhibitor; DDR, DNA damage response; DMP1, cyclin D-binding myb-like protein; DSB, double-strand DNA break; DNA-PK, DNA-dependent protein kinase; ER, estrogen receptor; FASN, fatty acid synthase; GSK3β, glycogen synthase-3β; HAT, histone acetyltransferase; HDAC, histone deacetylase; HK2, hexokinase 2; HNF4α, and hepatocyte nuclear factor 4α; HR, homologous recombination; IR, ionizing radiation; KIP, kinase inhibitory protein; MCL, mantle cell lymphoma; NHEJ, non-homologous end-joining; PCAF, p300/CREB binding-associated protein; PGC1α, PPARγ co-activator 1α; PEST, proline-glutamic acid-serine-threonine, PK, pyruvate kinase; PPAR, peroxisome proliferator-activated receptor; RB1, retinoblastoma protein; ROS, reactive oxygen species; SRC, steroid receptor coactivator; STAT, signal transducer and activator of transcription; TGFβ, transforming growth factor β; UPS, ubiquitin-proteasome system; USP22, ubiquitin-specific peptidase 22; XPO1 (or CRM1) exportin 1.

Structural insights into the functional diversity of the CDK-cyclin family

Since their characterization as conserved modules that regulate progression through the eukaryotic cell cycle, cyclin-dependent protein kinases (CDKs) in higher eukaryotic cells are now also emerging as significant regulators of transcription, metabolism and cell differentiation. The cyclins, though originally characterized as CDK partners, also have CDK-independent roles that include the regulation of DNA damage repair and transcriptional programmes that direct cell differentiation, apoptosis and metabolic flux. This review compares the structures of the members of the CDK and cyclin families determined by X-ray crystallography, and considers what mechanistic insights they provide to guide functional studies and distinguish CDK- and cyclin-specific activities. Aberrant CDK activity is a hallmark of a number of diseases, and structural studies can provide important insights to identify novel routes to therapy.

Cyclin C directly stimulates Drp1 GTP affinity to mediate stress-induced mitochondrial hyperfission

Mitochondria exist in an equilibrium between fragmented and fused states that shifts heavily toward fission in response to cellular damage. Nuclear-to-cytoplasmic cyclin C relocalization is essential for dynamin-related protein 1 (Drp1)–dependent mitochondrial fission in response to oxidative stress. This study finds that cyclin C directly interacts with the Drp1 GTPase domain, increases its affinity to GTP, and stimulates GTPase activity in vitro. In addition, the cyclin C domain that binds Drp1 is contained within the non–Cdk binding second cyclin box domain common to all cyclin family members. This interaction is important, as this domain is sufficient to induce mitochondrial fission when expressed in mouse embryonic fibroblasts in the absence of additional stress signals. Using gel filtration chromatography and negative stain electron microscopy, we found that cyclin C interaction changes the geometry of Drp1 oligomers in vitro. High–molecular weight low–GTPase activity oligomers in the form of short filaments and rings were diminished, while dimers and elongated filaments were observed. Our results support a model in which cyclin C binding stimulates the reduction of low–GTPase activity Drp1 oligomers into dimers capable of producing high–GTPase activity filaments.

Lack of cyclin D3 induces skeletal muscle fiber-type shifting, increased endurance performance and hypermetabolism

The mitogen-induced D-type cyclins (D1, D2 and D3) are regulatory subunits of the cyclin-dependent kinases CDK4 and CDK6 that drive progression through the G1 phase of the cell cycle. In skeletal muscle, cyclin D3 plays a unique function in controlling the proliferation/differentiation balance of myogenic progenitor cells. Here, we show that cyclin D3 also performs a novel function, regulating muscle fiber type-specific gene expression. Mice lacking cyclin D3 display an increased number of myofibers with higher oxidative capacity in fast-twitch muscle groups, primarily composed of myofibers that utilize glycolytic metabolism. The remodeling of myofibers toward a slower, more oxidative phenotype is accompanied by enhanced running endurance and increased energy expenditure and fatty acid oxidation. In addition, gene expression profiling of cyclin D3-/- muscle reveals the upregulation of genes encoding proteins involved in the regulation of contractile function and metabolic markers specifically expressed in slow-twitch and fast-oxidative myofibers, many of which are targets of MEF2 and/or NFAT transcription factors. Furthermore, cyclin D3 can repress the calcineurin- or MEF2-dependent activation of a slow fiber-specific promoter in cultured muscle cells. These data suggest that cyclin D3 regulates muscle fiber type phenotype, and consequently whole body metabolism, by antagonizing the activity of MEF2 and/or NFAT.

CCND1 mRNA Overexpression is Highly Related to Estrogen Receptor Positivity but not to Proliferative Markers in Primary Breast Cancer

To elucidate the role of CCND1 alterations in sporadic breast cancer we investigated the possible link between CCND1 mRNA levels versus estrogen-receptor (ER) status and a proliferation marker, S-phase fraction (SPF), measured by flow cytometry. CCND1 expression was quantified by means of real-time quantitative RT-PCR in a well-characterized series of 33 primary breast cancer patients. Eighteen tumors (54.5%) showed CCND1 overexpression ranging from 3.3 to 29.5 times the level observed in normal breast tissue. Seventeen (94.4%) of the 18 cases with CCND1 overexpression were ER-positive compared to seven (46.7%) of the 15 cases with normal CCND1 expression (p=0.0074). CCND1 overexpression was independent of SPF and DNA-ploidy status. These data suggest that the CCND1 gene does not act as an oncogene responsible for more rapid cell proliferation in breast cancer, but could be involved in the regulation of hormone sensitivity associated with ER.

Transcriptomic Profiling of Posterior Polymorphous Corneal Dystrophy

Purpose

To investigate the molecular basis of posterior polymorphous corneal dystrophy (PPCD) by examining the PPCD transcriptome and the effect of decreased ZEB1 expression on corneal endothelial cell (CEnC) gene expression.

Methods

Next-generation RNA sequencing (RNA-seq) analyses of corneal endothelium from two PPCD-affected individuals (one with PPCD3 and one of unknown genetic cause) compared with two age-matched controls, and primary human CEnC (pHCEnC) transfected with siRNA-mediated ZEB1 knockdown. The expression of selected differentially expressed genes was validated by quantitative polymerase chain reaction (qPCR) and/or assessed by in situ hybridization in the corneal endothelium of four independent cases of PPCD (one with PPCD3 and three of unknown genetic cause).

Results

Expression of 16% and 46% of the 104 protein-coding genes specific to ex vivo corneal endothelium was lost in the endothelium of two individuals with PPCD. Thirty-two genes associated with ZEB1 and 3 genes (BMP4, CCND1, ZEB1) associated with OVOL2 were differentially expressed in the same direction in both individuals with PPCD. Immunohistochemistry staining and RNA-seq analyses demonstrated variable expression of type IV collagens in PPCD corneas. Decreasing ZEB1 expression in pHCEnC altered expression of 711 protein-coding genes, many of which are associated with canonical pathways regulating various cellular processes.

Conclusions

Identification of the altered transcriptome in PPCD and in a cell-based model of PPCD provided insight into the molecular alterations characterizing PPCD. Further study of the differentially expressed genes associated with ZEB1 and OVOL2 is expected to identify candidate genes for individuals with PPCD and without a ZEB1 or OVOL2 mutation.

Magnolol pretreatment attenuates heat stress-induced IEC-6 cell injury

OBJECTIVE

Heat stress (HS) is an important environmental stressor that adversely influences livestock during the summer. The aim of this study was to investigate whether magnolol protects against HS-induced intestinal epithelial cell injury.

MATERIALS AND METHODS

An intestinal epithelial cell line (IEC-6) was subjected to HS at 42 °C, with and without magnolol pretreatment. Cell injury was detected by monitoring lactate dehydrogenase (LDH) release. MTS (3-(4,5-dimethylthiazol-2-yl)-5-(3-carboxymethoxyphenyl)-2-(4-sulfophenyl)-2H-tetrazolium) assay was used to assess cell proliferation and viability, including identifying effective concentrations of magnolol. Flow cytometry confirmed G1-phase cell-cycle arrest and its alleviation by magnolol. Active DNA synthesis was measured by incorporation of nucleic acid 5-ethynyl-2'-deoxyuridine (EdU). G1-phase cell-cycle-related gene expression was assessed by real-time reverse transcription polymerase chain reaction (RT-PCR) and levels of G1-phase-related proteins by Western blotting.

RESULTS

HS induced IEC-6 cell injury and decreased cell viability, as demonstrated by data from LDH and MTS assays, respectively. Based on a number of criteria, IEC-6 cells subjected to HS were arrested in the G1 phase of the cell cycle. Magnolol pretreatment decreased HS-induced cell injury through relief of this cell-cycle arrest.

CONCLUSIONS

Magnolol pretreatment attenuates HS-induced injury in IEC-6 cells. Magnolol is potentially promising as a protective strategy for HS in livestock.

Dissociable roles for histone acetyltransferases p300 and PCAF in hippocampus and perirhinal cortex-mediated object memory

The importance of histone acetylation for certain types of memory is now well established. However, the specific contributions of the various histone acetyltransferases to distinct memory functions remain to be determined; therefore, we employed selective histone acetyltransferase protein inhibitors and short-interference RNAs to evaluate the roles of CREB-binding protein (CBP), E1A-binding protein (p300) and p300/CBP-associated factor (PCAF) in hippocampus and perirhinal cortex (PRh)-mediated object memory. Rats were tested for short- (STM) and long-term memory (LTM) in the object-in-place task, which relies on the hippocampus and PRh for spatial memory and object identity processing, respectively. Selective inhibition of these histone acetyltransferases by small-interfering RNA and pharmacological inhibitors targeting the HAT domain produced dissociable effects. In the hippocampus, CBP or p300 inhibition impaired long-term but not short-term object memory, while inhibition of PCAF impaired memory at both delays. In PRh, HAT inhibition did not impair STM, and only CBP and PCAF inhibition disrupted LTM; p300 inhibition had no effects. Messenger RNA analyses revealed findings consistent with the pattern of behavioral effects, as all three enzymes were upregulated in the hippocampus (dentate gyrus) following learning, whereas only CBP and PCAF were upregulated in PRh. These results demonstrate, for the first time, the necessity of histone acetyltransferase activity for PRh-mediated object memory and indicate that the specific mnemonic roles of distinctive histone acetyltransferases can be dissociated according to specific brain regions and memory timeframe.

Cell cycle, cytoskeleton dynamics and beyond: the many functions of cyclins and CDK inhibitors

While targeting experiments carried out on the genes encoding many cell cycle regulators have challenged our views of cell cycle control, they also suggest that redundancy might not be the only explanation for the observed perplexing phenotypes. Indeed, several observations hint at functions of cyclins and CDK inhibitors that cannot be accounted for by their sole role as kinase regulators. They are found involved in many cellular transactions, depending or not on CDKs that are not directly linked to cell cycle control, but participating to general mechanisms such as transcription, DNA repair or cytoskeleton dynamics. In this review we discuss the roles that these alternative functions might have in cancer cell proliferation and migration that sometime even challenge their definition as proliferation markers.

Non-canonical functions of cell cycle cyclins and cyclin-dependent kinases

The roles of cyclins and their catalytic partners, the cyclin-dependent kinases (CDKs), as core components of the machinery that drives cell cycle progression are well established. Increasing evidence indicates that mammalian cyclins and CDKs also carry out important functions in other cellular processes, such as transcription, DNA damage repair, control of cell death, differentiation, the immune response and metabolism. Some of these non-canonical functions are performed by cyclins or CDKs, independently of their respective cell cycle partners, suggesting that there was a substantial divergence in the functions of these proteins during evolution.

Consequence of the tumor-associated conversion to cyclin D1b

Clinical evidence suggests that cyclin D1b, a variant of cyclin D1, is associated with tumor progression and poor outcome. However, the underlying molecular basis was unknown. Here, novel models were created to generate a genetic switch from cyclin D1 to cyclin D1b. Extensive analyses uncovered overlapping but non-redundant functions of cyclin D1b compared to cyclin D1 on developmental phenotypes, and illustrated the importance of the transcriptional regulatory functions of cyclin D1b in vivo. Data obtained identify cyclin D1b as an oncogene, wherein cyclin D1b expression under the endogenous promoter induced cellular transformation and further cooperated with known oncogenes to promote tumor growth in vivo. Further molecular interrogation uncovered unexpected links between cyclin D1b and the DNA damage/PARP1 regulatory networks, which could be exploited to suppress cyclin D1b-driven tumors. Collectively, these data are the first to define the consequence of cyclin D1b expression on normal cellular function, present evidence for cyclin D1b as an oncogene, and provide pre-clinical evidence of effective methods to thwart growth of cells dependent upon this oncogenic variant.

CDK4/6 inhibitors in breast cancer

Deregulation of the cyclin-dependent kinase (CDK) 4/6-retinoblastoma (RB) axis can occur through a number of mechanisms and contributes towards the unrestrained growth witnessed in a variety of cancers including breast cancers. Recent years have seen the development of selective CDK4/6 inhibitors, which have delivered promising preclinical and clinical results in breast cancer and other tumours. A number of trials assessing antitumour efficacy in various disease settings and combinations are ongoing. The cyclin D1-CDK-Rb axis and its role in the cell cycle of normal and cancer cells are delineated. The early pan-CDK inhibitor flavopiridol and subsequent preclinical and clinical development of selective CDK4/6 inhibitors are described. Ongoing studies in breast cancer with novel CDK4/6 inhibitors (palbociclib, abemaciclib and ribociclib) are explored. A literature search of these topics was performed through PubMed. Abstracts from major oncology meetings were also reviewed. Selective CDK4/6 inhibitors, as represented by the competing compounds currently in clinical development, comprise a novel, safe and, thus far, promisingly efficacious group of drugs. Considerable resources are being devoted towards exploring the efficacy of these drugs in combination with endocrine therapies, an approach that has yielded encouraging results and accelerated approval by the US Food and Drugs Administration for one of these agents (palbociclib). The results of confirmatory phase 3 trials are, however, awaited. We discuss further therapy combinations in development and highlight potential areas for caution including the potential for antagonistic interactions with cytotoxic chemotherapies.

Minireview: Tipping the balance: ligand-independent activation of steroid receptors

Steroid receptors are prototypical ligand-dependent transcription factors and a textbook example for allosteric regulation. According to this canonical model, binding of cognate steroid is an absolute requirement for transcriptional activation. Remarkably, the simple one ligand-one receptor model could not be farther from the truth. Steroid receptors, notably the sex steroid receptors, can receive multiple inputs. Activation of steroid receptors by other signals, working through their own signaling pathways, in the absence of the cognate steroids, represents the most extreme form of signaling cross talk. Compared with cognate steroids, ligand-independent activation pathways produce similar but not identical outputs. Here we review the phenomena and discuss what is known about the underlying molecular mechanisms and the biological significance. We hypothesize that steroid receptors may have evolved to be trigger happy. In addition to their cognate steroids, many posttranslational modifications and interactors, modulated by other signals, may be able to tip the balance.

Cyclin D2 is a GATA4 cofactor in cardiogenesis

The G1 cyclins play a pivotal role in regulation of cell differentiation and proliferation. The mechanisms underlying their cell-specific roles are incompletely understood. Here, we show that a G1 cyclin, cyclin D2 (CycD2), enhances the activity of transcription factor GATA4, a key regulator of cardiomyocyte growth and differentiation. GATA4 recruits CycD2 to its target promoters, and their interaction results in synergistic activation of GATA-dependent transcription. This effect is specific to CycD2 because CycD1 is unable to potentiate activity of GATA4 and is CDK-independent. GATA4 physically interacts with CycD2 through a discreet N-terminal activation domain that is essential for the cardiogenic activity of GATA4. Human mutations in this domain that are linked to congenital heart disease interfere with CycD2-GATA4 synergy. Cardiogenesis assays in Xenopus embryos indicate that CycD2 enhances the cardiogenic function of GATA4. Together, our data uncover a role for CycD2 as a cardiogenic coactivator of GATA4 and suggest a paradigm for cell-specific effects of cyclin Ds.

New roles of cyclin D1

Cyclins encode regulatory subunits of holoenzymes that phosphorylate a variety of cellular substrates. Although the classic role of cyclins in cell cycle progression and tumorigenesis has been well characterized, new functions have been identified, including the induction of cellular migration and invasion, enhancement of angiogenesis, inhibition of mitochondrial metabolism, regulation of transcription factor signaling via a DNA-bound form, the induction of chromosomal instability, enhancement of DNA damage sensing and DNA damage repair, and feedback governing expression of the noncoding genome. This review describes the mechanisms of these new functions of cyclin D1.

MKP1-dependent PTH modulation of bone matrix mineralization in female mice is osteoblast maturation stage specific and involves P-ERK and P-p38 MAPKs

Limited information is available on the role of MAPK phosphatase 1 (MKP1) signaling in osteoblasts. We have recently reported distinct roles for MKP1 during osteoblast proliferation, differentiation, and skeletal responsiveness to parathyroid hormone (PTH). As MKP1 regulates the phosphorylation status of MAPKs, we investigated the involvement of P-ERK and P-p38 MAPKs in MKP1 knockout (KO) early and mature osteoblasts with respect to mineralization and PTH response. Calvarial osteoblasts from 9-14-week-old WT and MKP1 KO male and female mice were examined. Western blot analysis revealed downregulation and sustained expressions of P-ERK and P-p38 with PTH treatment in differentiated osteoblasts derived from KO males and females respectively. Exposure of early osteoblasts to p38 inhibitor, SB203580 (S), markedly inhibited mineralization in WT and KO osteoblasts from both genders as determined by von Kossa assay. In osteoblasts from males, ERK inhibitor U0126 (U), not p38 inhibitor (S), prevented the inhibitory effects of PTH on mineralization in early or mature osteoblasts. In osteoblasts from KO females, PTH sustained mineralization in early osteoblasts and decreased mineralization in mature cells. This effect of PTH was attenuated by S in early osteoblasts and by U in mature KO cells. Changes in matrix Gla protein expression with PTH in KO osteoblasts did not correlate with mineralization, indicative of MKP1-dependent additional mechanisms essential for PTH action on osteoblast mineralization. We conclude that PTH regulation of osteoblast mineralization in female mice is maturation stage specific and involves MKP1 modulation of P-ERK and P-p38 MAPKs.

Cyclin D1 in invasive breast carcinoma: favourable prognostic significance in unselected patients and within subgroups with an aggressive phenotype

AIMS

  To study the clinicopathological and prognostic value of cyclin D1 overexpression in patients with breast carcinoma.

METHODS AND RESULTS

  Immunohistochemistry was performed on paraffin-embedded tissue specimens from 290 invasive breast carcinomas to detect the proteins cyclin D1, oestrogen receptor (ER), progesterone receptor (PR), p53, c-erbB2, and topoisomerase IIα (topoIIα). Cyclin D1 staining was quantified using a computerized image analysis method. Cyclin D1 overexpression characterized smaller, ER-positive and PR-positive tumours (P = 0.017, P < 0.0001, and P < 0.0001, respectively), of a lower histological and nuclear grade (P = 0.011 and P < 0.0001, respectively), and with reduced expression of topoIIα (P = 0.001) and p53 (P < 0.001). Cyclin D1 was found to have an independent favourable impact on the overall survival of both the unselected cohort of patients (P = 0.011) and of patients with ER-negative and lymph node-positive tumours (P = 0.034 and P = 0.015, respectively). In triple-negative tumours, cyclin D1 overexpression was found to have independent favourable impacts on both overall and relapse-free survival (P = 0.002 for both).

CONCLUSIONS

  This is the first immunohistochemical study to dissociate the advantageous prognostic effect of cyclin D1 overexpression from its association with ER expression, and to provide evidence that cyclin D1 overexpression may be a marker of prolonged survival in patient subgroups with aggressive phenotypes.

17Beta-estradiol signaling and regulation of proliferation and apoptosis of rat Sertoli cells

The aim of the present study was to investigate the intracellular signaling events downstream of the classical estrogen receptors (ESRs) and G protein-coupled estrogen receptor 1 (GPER) involved in regulation of proliferation and apoptosis of rat Sertoli cells, in which we have previously described ESR1, ESR2, and GPER. ESRs play a role in Sertoli cell proliferation, and GPER, but not ESRs, plays a role modulating gene expression involved with apoptosis. The present study shows that 17beta-estradiol (E2) and the GPER-selective agonist G-1 rapidly activate phosphatidylinositol 3-kinase (PIK3)/serine threonine protein kinase (AKT) and cyclic AMP response element-binding (CREB) phosphorylation. E2 and the ESR1-selective agonist 4,4',4″-(4-propyl-(1H)-pyrazole-1,3,5-triyl)trisphenol (PPT) increase the expression of cyclin D1 (CCND1), whereas the ESR2-selective agonist 2,3-bis(4-hydroxyphenyl)-propionitrile (DPN) and G-1 do not change the expression of this protein, suggesting that ESR1 is the upstream receptor regulating Sertoli cell proliferation. E2- or PPT-ESR1, through activation of epidermal growth factor receptor (EGFR)/mitogen-activated protein kinase 3/1 (MAPK3/1) and PIK3 pathways, induces upregulation of CCND1. KG-501, the compound that disrupts the phospho-CREB/CREB binding protein (CBP) complex, does not change E2- or PPT-ESR1-mediated CCND1 expression, suggesting that phospho-CREB/cyclic AMP response element/CBP is not involved in the expression of this protein. E2- or G-1-GPER, through activation of EGFR/MAPK3/1 and PIK3 pathways, may be involved in the upregulation of antiapoptotic proteins BCL2 and BCL2L2. E2- or G-1-GPER/EGFR/MAPK3/1/phospho-CREB decreases BAX expression. Taken together, these results show a differential effect of E2-GPER on the CREB-mediated transcription of proapoptotic and antiapoptotic genes of the same BCL2 gene family. ESR1 and GPER can mediate the rapid E2 actions in the Sertoli cells, which in turn can modulate nuclear transcriptional events important for Sertoli cell function and maintenance of normal testis development and homeostasis. Our findings are important to clarify the role of estrogen in a critical period of testicular development, and to direct further studies, which may contribute to better understanding of the causes of male infertility.

High-Dimensional Sparse Factor Modeling: Applications in Gene Expression Genomics

We describe studies in molecular profiling and biological pathway analysis that use sparse latent factor and regression models for microarray gene expression data. We discuss breast cancer applications and key aspects of the modeling and computational methodology. Our case studies aim to investigate and characterize heterogeneity of structure related to specific oncogenic pathways, as well as links between aggregate patterns in gene expression profiles and clinical biomarkers. Based on the metaphor of statistically derived "factors" as representing biological "subpathway" structure, we explore the decomposition of fitted sparse factor models into pathway subcomponents and investigate how these components overlay multiple aspects of known biological activity. Our methodology is based on sparsity modeling of multivariate regression, ANOVA, and latent factor models, as well as a class of models that combines all components. Hierarchical sparsity priors address questions of dimension reduction and multiple comparisons, as well as scalability of the methodology. The models include practically relevant non-Gaussian/nonparametric components for latent structure, underlying often quite complex non-Gaussianity in multivariate expression patterns. Model search and fitting are addressed through stochastic simulation and evolutionary stochastic search methods that are exemplified in the oncogenic pathway studies. Supplementary supporting material provides more details of the applications, as well as examples of the use of freely available software tools for implementing the methodology.

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.

Estrogen carcinogenesis in breast cancer

Many studies have reported a correlation between elevated estrogen blood levels and breast cancer and this observation has raised controversy concerning the long-term use of hormonal replacement therapy. This review will not address further this controversial topic; but rather, this review focuses on the role of estrogen signaling in first, the normal development of the breast and second, how alterations of this signaling pathway contribute to breast cancer.

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?

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.

Nuclear cyclin D1/CDK4 kinase regulates CUL4 expression and triggers neoplastic growth via activation of the PRMT5 methyltransferase

Cyclin D1 elicits transcriptional effects through inactivation of the retinoblastoma protein and direct association with transcriptional regulators. The current work reveals a molecular relationship between cyclin D1/CDK4 kinase and protein arginine methyltransferase 5 (PRMT5), an enzyme associated with histone methylation and transcriptional repression. Primary tumors of a mouse lymphoma model exhibit increased PRMT5 methyltransferase activity and histone arginine methylation. Analyses demonstrate that MEP50, a PRMT5 coregulatory factor, is a CDK4 substrate, and phosphorylation increases PRMT5/MEP50 activity. Increased PRMT5 activity mediates key events associated with cyclin D1-dependent neoplastic growth, including CUL4 repression, CDT1 overexpression, and DNA rereplication. Importantly, human cancers harboring mutations in Fbx4, the cyclin D1 E3 ligase, exhibit nuclear cyclin D1 accumulation and increased PRMT5 activity.

Nonhistone protein acetylation as cancer therapy targets

Acetylation and deacetylation are counteracting, post-translational modifications that affect a large number of histone and nonhistone proteins. The significance of histone acetylation in the modification of chromatin structure and dynamics, and thereby gene transcription regulation, has been well recognized. A steadily growing number of nonhistone proteins have been identified as acetylation targets and reversible lysine acetylation in these proteins plays an important role(s) in the regulation of mRNA stability, protein localization and degradation, and protein-protein and protein-DNA interactions. The recruitment of histone acetyltransferases (HATs) and histone deacetylases (HDACs) to the transcriptional machinery is a key element in the dynamic regulation of genes controlling cellular proliferation, differentiation and apoptosis. Many nonhistone proteins targeted by acetylation are the products of oncogenes or tumor-suppressor genes and are directly involved in tumorigenesis, tumor progression and metastasis. Aberrant activity of HDACs has been documented in several types of cancers and HDAC inhibitors (HDACi) have been employed for therapeutic purposes. Here we review the published literature in this field and provide updated information on the regulation and function of nonhistone protein acetylation. While concentrating on the molecular mechanism and pathways involved in the addition and removal of the acetyl moiety, therapeutic modalities of HDACi are also discussed.

Transcriptional role of cyclin D1 in development revealed by a genetic-proteomic screen

Cyclin D1 belongs to the core cell cycle machinery, and it is frequently overexpressed in human cancers. The full repertoire of cyclin D1 functions in normal development and oncogenesis is unclear at present. Here we developed Flag- and haemagglutinin-tagged cyclin D1 knock-in mouse strains that allowed a high-throughput mass spectrometry approach to search for cyclin D1-binding proteins in different mouse organs. In addition to cell cycle partners, we observed several proteins involved in transcription. Genome-wide location analyses (chromatin immunoprecipitation coupled to DNA microarray; ChIP-chip) showed that during mouse development cyclin D1 occupies promoters of abundantly expressed genes. In particular, we found that in developing mouse retinas-an organ that critically requires cyclin D1 function-cyclin D1 binds the upstream regulatory region of the Notch1 gene, where it serves to recruit CREB binding protein (CBP) histone acetyltransferase. Genetic ablation of cyclin D1 resulted in decreased CBP recruitment, decreased histone acetylation of the Notch1 promoter region, and led to decreased levels of the Notch1 transcript and protein in cyclin D1-null (Ccnd1(-/-)) retinas. Transduction of an activated allele of Notch1 into Ccnd1(-/-) retinas increased proliferation of retinal progenitor cells, indicating that upregulation of Notch1 signalling alleviates the phenotype of cyclin D1-deficiency. These studies show that in addition to its well-established cell cycle roles, cyclin D1 has an in vivo transcriptional function in mouse development. Our approach, which we term 'genetic-proteomic', can be used to study the in vivo function of essentially any protein.

Influence of cell cycle on ecdysteroid receptor in CHO-K1 cells

CHO-K1 cells are routinely used for characterization of ecdysone receptor (EcR) function, because these vertebrate cells are devoid of endogenous ecdysone receptor protein. Moreover, the endogenous expression of RXR, the vertebrate orthologue of Ultraspiracle (Usp), the most important heterodimerization partner, is neglectable. In contrast to insect cells, there is also no influence of moulting hormone on CHO-K1 cells on cell proliferation either in the absence or presence of transiently expressed EcR. In contrast to Usp, which is exclusively found in nuclei, EcR is heterogeneously distributed between cytoplasm and nuclei in non-synchronized cells. Synchronization of CHO-K1 cells by nocodazole revealed that the cell cycle influences receptor concentration with lowest amounts in late S-phase and G2/M phase and intracellular distribution of the receptor protein showing a minimum of receptors present in nuclei during S-phase. EcR, but not Usp reduces cyclin D1 expression and cyclin D1 concentration is impaired by cyclin D1. Coimmunoprecipitation studies reveal physical interaction of EcR and cyclin D1.

Integrated actions of progesterone receptor and cell cycle machinery regulate breast cancer cell proliferation

Multiple laboratories have investigated progesterone receptor (PR) involvement in breast cancer cell cycle progression. There is now a growing body of evidence demonstrating complex interactions between PR and cell cycle regulatory proteins. Here we review the current literature linking PR to cell cycle control and discuss gaps in the current knowledge. A more complete understanding of the relationships between PR and cell cycle regulatory molecules may reveal additional avenues for prevention and treatment of steroid receptor positive breast cancers.

Enhanced skin carcinogenesis and lack of thymus hyperplasia in transgenic mice expressing human cyclin D1b (CCND1b)

Cyclin D1b is an alternative transcript of the cyclin D1 gene (CCND1) expressed in human tumors. Its abundance is regulated by a single base pair polymorphism at the exon 4/intron 4 boundary (nucleotide 870). Epidemiological studies have shown a correlation between the presence of the G870A allele (that favors the splicing for cyclin D1b) with increased risk and less favorable outcome in several forms of cancer. More recently, it has been shown that, unlike cyclin D1a, the alternative transcript D1b by itself has the capacity to transform fibroblasts in vitro. In order to study the oncogenic potential of cyclin D1b, we developed transgenic mice expressing human cyclin D1b under the control of the bovine K5 promoter (K5D1b mice). Seven founders were obtained and none of them presented any significant phenotype or developed spontaneous tumors. Interestingly, K5D1b mice do not develop the fatal thymic hyperplasia, which is characteristic of the cyclin D1a transgenic mice (K5D1a). Susceptibility to skin carcinogenesis was tested in K5D1b mice using two-stage carcinogenesis protocols. In two independent experiments, K5D1b mice developed higher papilloma multiplicity as compared with wild-type littermates. However, when K5D1b mice were crossed with cyclin D1KO mice, the expression of cyclin D1b was unable to rescue the carcinogenesis-resistant phenotype of the cyclin D1 KO mice. To further explore the role of cyclin D1b in mouse models of carcinogenesis we carried out in silico analysis and in vitro experiments to evaluate the existence of a mouse homologous of the human cyclin D1b transcript. We were unable to find any evidence of an alternatively spliced transcript in mouse Ccnd1. These results show that human cyclin D1b has different biological functions than cyclin D1a and confirm its oncogenic properties.

Progesterone receptors act as sensors for mitogenic protein kinases in breast cancer models

Progesterone receptors (PR), members of the nuclear receptor superfamily, function as ligand-activated transcription factors and initiators of c-Src kinase and mitogen-activated protein kinase signaling. Bidirectional cross-talk between PR and mitogenic protein kinases results in changes in PR post-translational modification, leading to alterations in PR transcriptional activity and promoter selectivity. PR-induced rapid activation of cytoplasmic protein kinases insures precise regulatory input to downstream cellular processes that are dependent upon nuclear PR, such as cell-cycle progression, and pro-survival signaling. Here, we review interactions between PR and mitogenic protein kinases and discuss the consequences of specific post-translational modifications on PR action in breast cancer cell-line models.

Inhibition of tumor growth and metastasis by non-small cell lung cancer cells transfected with cyclin D1-targeted siRNA

To observe whether cyclin D1 siRNA-mediated inhibition of cyclin D1 represents a promising antigrowth and antimetastatic strategy for cancer gene therapy, particularly for non-small cell lung cancers. To stably transfect the A549 cell line with a cyclin D1-targeted siRNA to downregulate cyclin D1 expression and observe the effects on protein expression, and tumor growth in vitro and in vivo. Expression of cyclin D1-targeted siRNA resulted in a decrease in cyclin D1, MMP-2, RhoA, and Rac1 protein levels, as detected by Western blot and immunofluorescence studies. Transfected cells also exhibited a marked decrease in the rate of cell growth, and decreased invasive capacity, compared to cells transduced with a scrambled siRNA plasmid and untransduced A549 cells. siRNA-mediated inhibition of cyclin D1 expression represents a promising antigrowth and antimetastatic strategy for cancer gene therapy, particularly for non-small cell lung cancers. It is the reason for inhibiting tumor growth so that cyclin D1 siRNA can inhibit the cell cycle progression. In addition, the mechanism of inhibiting tumor metastasis was related to the decrease in the expression of MMP-2, RhoA, and Rac1 after cyclin D1 was decreased by cyclin D1 siRNA.

Hormonal prevention of breast cancer: significance of promotional environment

Early full-term pregnancy reduces the risk of mammary cancer in humans. Rats and mice also exhibit this phenomenon of parity protection. Short-term treatment with pregnancy levels of estradiol (E2) is also highly effective in preventing mammary carcinogenesis. Earlier it has been demonstrated that parous rats treated with carcinogen develop latent microscopic mammary tumors that do not progress further to form overt mammary cancers. In the current investigation, we wanted to find out if short-term treatment with pregnancy levels of E2 also prevents mammary carcinogenesis similar to parity. Rats were injected with N-methyl-N-nitrosourea at 7 weeks of age and treated with 20 microg, 100 microg, 200 microg, or 30mg of E2 in silastic capsules for 3 weeks. 100 microg (17%), 200 microg (17%), and 30mg (17%) doses of E2 resulted in levels of E2 equivalent to pregnancy level and were effective in preventing overt mammary cancer incidence compared with control (100%) or 20 microg (73%) E2 treatment, which did not result in pregnancy levels of E2 in the circulation. Although a significant reduction of overt cancers was observed in the pregnancy levels of E2 treated groups, there was no difference in the incidence of latent microscopic mammary cancers between the E2 treated and the controls. Proliferation of latent microscopic mammary cancers was examined using immunohistochemistry for cyclin D1 expression. Proliferation in the latent microscopic mammary cancers of the protected groups was significantly lower (approximately 2.0-3.0-fold) than the latent microscopic mammary cancers in the unprotected groups. These findings indicate that mammary cancer development can be blocked by inhibiting or blocking promotion and progression of carcinogen initiated cells.

GSK-3beta regulates cyclin D1 expression: a new target for chemotherapy

Cyclin D1 is known as a proto-oncogene whose gene amplification and protein overexpression are frequently observed in tumor cells. It acts as a mitogenic signal sensor and is expressed as a delayed-early response to many mitogenic signals. Cyclin-dependent kinases (CDKs) 4 and 6 are cyclin D1 binding partners, and activated cyclin D1/CDK4 and cyclin D1/CDK6 complex phosphorylate the retinoblastoma protein to induce the expression of target genes essential for S phase entry, resulting in facilitation of the progression from G1 to S phase. As well as acting as a positive regulator of the cell cycle, cyclin D1 is known to bind and modulate the actions of several transcription factors. Since the protein level of cyclin D1 reflects cell cycle progression, the rates of protein production and degradation are strictly regulated. Glycogen synthase kinase-3beta (GSK-3beta), a serine/threonine protein kinase, has been shown to play an important role in the determination of cyclin D1 expression level by regulating mRNA transcription and protein degradation. This review highlights the regulatory mechanisms of cyclin D1 expression level, with special attention to the involvement of GSK-3beta.

Cyclin D3 action in androgen receptor regulation and prostate cancer

Prostate cancer (PCa) cell proliferation is dependent on activation of the androgen receptor (AR), a ligand-dependent transcription factor. AR activation controls G1-S phase progression through fostering enhanced translation of the D-type cyclins, which promote cell cycle progression through activation of CDK4/6. However, the D-type cyclins harbor additional, CDK4/6 kinase-independent, functions through manipulation of transcription factors, including AR. It was previously established that cyclins D1 and D3 have the potential to modulate AR, and with regard to cyclin D1, disruption of this function occurs in human tumors. Therefore, it was essential to interrogate cyclin D3 function in this tumor type. Here, we show that cyclin D3 is found in association with AR in PCa cells, as mediated through a conserved motif. Cyclin D3 functions to attenuate AR activity through defined mechanisms that include modulation of ligand-dependent conformational changes and modulation of chromatin binding activity. Accumulated cyclin D3 slows cell proliferation in AR-dependent cells, thus suggesting that androgen-induced D-type cyclin production serves to temper the mitogenic response to androgen. Supporting this hypothesis, it is shown that cyclin D3 expression is reduced in primary PCas as a function of tumor grade, and inversely correlates with the proliferative index. In total, these data identify cyclin D3 as a critical modulator of the androgen response, whose deregulation may foster unchecked AR activity in PCa.