Ligand-independent recruitment of steroid receptor coactivators to estrogen receptor by cyclin D1

The Jensen Symposium

The Jensen Symposium was held at the University of Cincinnati in December 2003 to honor the pioneering contributions of Dr. Elwood Jensen to the field of nuclear hormone action. Those in attendance were treated to an outstanding scientific program that served as an update of recent progress and illustrated the breadth of activity in the nuclear receptor field. Here we highlight recent findings presented at the Symposium that provide new insights into the mechanisms of nuclear receptor action and the diverse roles of members of the nuclear receptor superfamily in development and homeostasis.

Cdc25B as a steroid receptor coactivator

The traditional role of the Cdc25 family of dual-specificity phosphatases is to activate cyclin-dependent kinases (CDKs) to enable progression through the cell cycle. This chapter reports that in addition to its cell cycle role, Cdc25B functions as a novel steroid receptor coactivator (SRC). When overexpressed in transgenic mammary glands, Cdc25B can up-regulate the expression of two estrogen receptor (ER)-target genes: cyclin D1 and Lactoferrin. In addition, when coexpressed with ER, Cdc25B can coactivate an ER-dependent reporter in the presence of estradiol. The coactivation of Cdc25B can be extended to the glucocorticoid receptor (GR), progesterone receptor (PR), and androgen receptor (AR). Because of the respective importance of ER and AR in breast and prostate cancer, this chapter focuses on the coactivation of both receptors by Cdc25B. We demonstrate that Cdc25B can interact directly with these nuclear receptors, recruit and enhance the activity of histone acetyltransferases (HATs), and potentiate cell-free transcription independent of its cell cycle regulatory function. Furthermore, because Cdc25B is up-regulated in highgrade and poorly differentiated prostate tumors, which are likely transiting from the hormone-dependent to hormone-independent state, we hypothesize that the coactivation of AR by Cdc25B may induce genes responsible for this progression. Taken together, it is highly conceivable that Cdc25B can promote neoplasia by its two disparate functions of (1) coactivation to induce higher levels of expression of steroid receptor target genes and (2) its role of activating CDKs to deregulate progression of the cell cycle, DNA replication, and mitosis.

Beta-catenin/Tcf-1-mediated transactivation of cyclin D1 promoter is negatively regulated by thyroid hormone

Cyclin D1 is an oncogenic cyclin frequently over-expressed in cancer. To examine the effect of thyroid hormone (T3) and its receptor (TR) on the transcription of cyclin D1 gene, we co-transfected the chloramphenicol acetyl-transferase (CAT) reporter plasmid containing cyclin D1 promoter together with the expression plasmids for TRbeta1 and wild-type or mutant beta-catenin (SA) into 293T cells. In the presence of T3, beta-catenin-dependent transactivation of cyclin D1 promoter was suppressed by co-transfection of TRbeta1. The suppression by T3/TRbeta1 was in a dose-dependent manner. The CAT reporter gene in which Tcf/Lef-1 sites were fused to heterologous promoter was also suppressed by T3/TRbeta1. Furthermore, inhibition of endogenous wild-type beta-catenin by T3/TRbeta1 was observed in SW480 colon carcinoma cells with mutation of the adenomatous polyposis coli gene. These results indicate that the T3-bound TR inhibits the transcription of cyclin D1 through the Tcf/Lef-1 site, which is positively regulated by the Wnt-signaling pathway.

Cyclin D1 repression of peroxisome proliferator-activated receptor gamma expression and transactivation

The cyclin D1 gene is overexpressed in human breast cancers and is required for oncogene-induced tumorigenesis. Peroxisome proliferator-activated receptor gamma (PPAR gamma) is a nuclear receptor selectively activated by ligands of the thiazolidinedione class. PPAR gamma induces hepatic steatosis, and liganded PPAR gamma promotes adipocyte differentiation. Herein, cyclin D1 inhibited ligand-induced PPAR gamma function, transactivation, expression, and promoter activity. PPAR gamma transactivation induced by the ligand BRL49653 was inhibited by cyclin D1 through a pRB- and cdk-independent mechanism, requiring a region predicted to form an helix-loop-helix (HLH) structure. The cyclin D1 HLH region was also required for repression of the PPAR gamma ligand-binding domain linked to a heterologous DNA binding domain. Adipocyte differentiation by PPAR gamma-specific ligands (BRL49653, troglitazone) was enhanced in cyclin D1(-/-) fibroblasts and reversed by retroviral expression of cyclin D1. Homozygous deletion of the cyclin D1 gene, enhanced expression by PPAR gamma ligands of PPAR gamma and PPAR gamma-responsive genes, and cyclin D1(-/-) mice exhibit hepatic steatosis. Finally, reduction of cyclin D1 abundance in vivo using ponasterone-inducible cyclin D1 antisense transgenic mice, increased expression of PPAR gamma in vivo. The inhibition of PPAR gamma function by cyclin D1 is a new mechanism of signal transduction cross talk between PPAR gamma ligands and mitogenic signals that induce cyclin D1.

Phenotypic changes in MCF-7 cells during prolonged exposure to tamoxifen

MCF-7 breast tumor cells form multicellular nodules (foci) over a confluent monolayer in an estradiol (E2)-dependent, antiestrogen-sensitive reaction. A cell line cloned from MCF-7 that displays these phenotypes was probed to determine the effects of long term exposure to tamoxifen on the growth of foci, estrogen receptor alpha (ERalpha) status, and gene responsiveness to E2. In one of two experiments, a heterogeneous cell population emerged (TMX2) that over-expressed estrogen receptor alpha wild type mRNA (ERalpha mRNA) (approximately 20-fold) missing exon 3 (ERDelta3 mRNA) and its corresponding protein (ERDelta3P). On a per mRNA to protein basis, ERDelta3P and wild-type ERalpha were equivalently expressed. Return of the TMX2 population to medium without tamoxifen eventually selected for a population that expressed predominately wild-type ERalpha, whereas TMX2 clones over expressing ERDelta3 mRNA and ERDelta3P retained this phenotype in tamoxifen-free media. In both experiments, expression of all ERalpha mRNAs and proteins declined to barely detectable levels during 6-12 months exposure, concomitant with a progressive increase in the ability of the cells to form foci independently of E2 or tamoxifen. Selection for these various populations suggests that tamoxifen can induce and/or support certain cellular changes that lead to altered ERalpha expression, E2-independent cell growth and resistance to antiestrogens.

Collagen-IV and laminin-1 regulate estrogen receptor alpha expression and function in mouse mammary epithelial cells

The expression level and functional activity of estrogen receptor alpha is an important determinant of breast physiology and breast cancer treatment. However, it has been difficult to identify the signals that regulate estrogen receptor because cultured mammary epithelial cells generally do not respond to estrogenic signals. Here, we use a combination of two- and three-dimensional culture systems to dissect the extracellular signals that control endogenous estrogen receptor alpha. Its expression was greatly reduced when primary mammary epithelial cells were placed on tissue culture plastic; however, the presence of a reconstituted basement membrane in combination with lactogenic hormones partially prevented this decrease. Estrogen receptor alpha expression in primary mammary fibroblasts was not altered by these culture conditions, indicating that its regulation is cell type specific. Moreover, estrogen receptor-dependent reporter gene expression, as well as estrogen receptor alpha levels, were increased threefold in a functionally normal mammary epithelial cell line when reconstituted basement membrane was added to the medium. This regulatory effect of reconstituted basement membrane was reproduced by two of its components, collagen-IV and laminin-1, and it was blocked by antibodies against alpha2, alpha6 and beta1 integrin subunits. Our results indicate that integrin-mediated response to specific basement membrane components, rather than cell rounding or cell growth arrest induced by reconstituted basement membrane, is critical in the regulation of estrogen receptor alpha expression and function in mammary epithelial cells.

Alternative mammary oncogenic pathways are induced by D-type cyclins; MMTV-cyclin D3 transgenic mice develop squamous cell carcinoma

The three human D-type cyclins, cyclin D1, D2 and D3 share the ability to bind to and activate cdk4 and 6. MMTV-cyclin D1 transgenic mice develop mainly adenocarcinoma, while MMTV-cyclin D2 mice show a lack of alveologenesis during pregnancy and only develop carcinoma at low frequency. The effect of cyclin D3 overexpression in mammary glands remains hitherto unknown. We generated MMTV-cyclin D3 transgenic mice and report here that they develop exclusively squamous cell carcinoma. We show that although cyclin D3 transgene expression was detected early in puberty, postnatal development and mammary gland proliferation were normal in virgin animals. In contrast, multiparous mice develop multiple foci of abnormal growth that correspond to various stages of squamous metaplasia. Therefore, our results support a role for cyclin D3 in squamous differentiation. In addition, we found that p16 expression during involution is abolished, while p27 expression increased in MMTV-cyclin D3 mice, two modifications that have been reported in the other MMTV-D-type cyclin transgenic models. Our observations indicate that despite biochemical redundancy in vitro and in vivo, D-type cyclins promote distinct oncogenic pathways.

Cyclin D1 expression in pre-cancerous and cancerous lesions of the canine mammary gland

Overexpression of cyclin D1, the regulatory subunit of cyclin-dependent kinases (cdk4 and cdk6) involved in cell cycle control, has often been found in breast cancer and other types of human cancer. Increased expression, or stability, of cyclin D1 molecules may cause sufficient cdk4 activation to produce retinoblastoma protein phosphorylation independently of mitogenic signals; this results in commitment of cells to the G1 phase at mitosis. In the present study, cyclin D1 expression was investigated in pre-cancerous and cancerous lesions of the canine mammary gland by a complex experimental approach, which included Western blot and immunohistochemical analysis of cyclin D1 and the related molecular system. Furthermore, to define relationships between cell growth and expression of cyclin D1, proliferative activity was studied by the AgNOR technique. The study provided the following information. Cyclin D1 overexpression was largely independent of the type of proliferative anomaly. Indeed, cyclin D1 was expressed in 60% of the pre-cancerous lesions and in 44% of cancerous lesions. Mitotic activity and cyclin D1 expression were related: mammary lesions that expressed cyclin D1 showed a high proliferative ratio, the opposite being true of cyclin D1-negative cell populations. This study may contribute to the establishment of an animal model for anti-cancer research based on cyclin D1 suppression or cdk inactivation, or both.

Bidirectional signaling between the estrogen receptor and the epidermal growth factor receptor

Interactions between the estrogen receptor (ER) and the epidermal growth factor receptor (EGFR) contribute to the biological effects of these binding protein families. EGFR stimulates DNA synthesis and gene transcription in the uterus, related in part to estrogen-independent activation of the nuclear ER. This results from signal transduction enacted by the plasma membrane tyrosine kinase growth factor receptor, leading to 1) phosphorylation and activation of the nuclear ER, and 2) phosphorylation of coregulator proteins. More recently, it has been shown that a pool of ERalpha resides in or associates with the plasma membrane as a cytoplasmic protein. These ERs utilize the membrane EGFR to rapidly signal through various kinase cascades that influence both transcriptional and nontranscriptional actions of estrogen in breast cancer cells. This is congruent with a general theme of receptor signaling, where membrane G protein-coupled receptors activate tyrosine kinase growth factor receptors (EGFR, IGF-I receptor) that subsequently signal to MAPKs and other pathways. Overall, the bidirectional cross-talk between EGFR and cellular pools of ER contributes to reproductive organ physiology and pathophysiology.

Cyclin D3 is a cofactor of retinoic acid receptors, modulating their activity in the presence of cellular retinoic acid-binding protein II

Ligand-induced transcription activation of retinoic acid (RA) target genes by nuclear receptors (retinoic acid (RAR) and retinoid X (RXR) receptors) depends on the recruitment of coactivators. We have previously demonstrated that the small 15-kDa cellular RA-binding protein II (CRABPII) is a coactivator present in the RA-dependent nuclear complex. As identifying cell-specific partners of CRABPII might help to understand the novel control of RA signaling, we performed a yeast two-hybrid screen of a hematopoietic HL-60 cDNA library using human CRABPII as bait and have subsequently identified human cyclin D3 as a partner of CRABPII. Cyclin D3 interacted with CRABPII in a ligand-independent manner and equally bound RAR alpha, but not RXR alpha, and only in the presence of RA. We further show that cyclin D3 positively modulated RA-mediated transcription through CRABPII. Therefore, cyclin D3 may be part of a ternary complex with CRABPII and RAR. Finally, we show that cyclin D3 expression paralleled HL-60 differentiation and arrest of cell growth. These findings led us to speculate that control of cell proliferation during induction of differentiation may directly involve, at the transcriptional level, nuclear receptors, coactivators, and proteins of the cell cycle in a cell- and nuclear receptor-specific manner.

Aberrant ubiquitin-mediated proteolysis of cell cycle regulatory proteins and oncogenesis

The ubiquitin pathway plays a central role in the regulation of cell growth and cell proliferation by controlling the abundance of key cell cycle proteins. Increasing evidence indicates that unscheduled proteolysis of many cell cycle regulators contributes significantly to tumorigenesis and is indeed found in many types of human cancers. Aberrant proteolysis with oncogenic potential is elicited by two major mechanisms: defective degradation of positive cell cycle regulators (i.e., proto-oncoproteins) and enhanced degradation of negative cell cycle regulators (i.e., tumor suppressor proteins). In many cases, increased protein stability is a result of mutations in the substrate that prevent the recognition of the protein by the ubiquitin-mediated degradation machinery. Alternatively, the specific recognition proteins mediating ubiquitination (ubiquitin ligases) are not expressed or harbor mutations rendering them inactive. In contrast, the overexpression of a ubiquitin ligase may result in the enhanced degradation of a negative cell cycle regulator. This chapter aims to review the involvement of the ubiquitin pathway in the scheduled destruction of some important cell cycle regulators and to discuss the implications of their aberrant degradation for the development of cancer.

Selective modulation of postmenopausal women: cutting the Gordian knot of hormone replacement therapy with breast carcinoma

BACKGROUND

Hormone replacement therapy (HRT) has proven and presumable benefits for women desiring postmenopausal health preservation. Among HRT-associated risks, the fear of breast carcinoma intimidates women and physicians contemplating hormonal treatment and limits long-term compliance. Identifying effective alternative medications that are not associated with breast carcinoma or that even may prevent its development would be a major advance.

METHODS

This article discusses the clinical perspective of HRT and selective estrogen receptor modulators (SERMs) in light of the molecular and cellular mechanisms of estrogen and progesterone action on the breast. Emphasis is placed on the potential of selective receptor modulation as the future of postmenopausal treatment.

RESULTS

Current epidemiologic evidence suggests that HRT is associated with a small but substantial increase in the risk of breast carcinoma, and combined estrogen-progesterone regimens further increase this hazard. Ample biologic data support this clinical association and propose multiple molecular mechanisms for the effects of estrogen and progesterone on breast cells. SERMs are a novel class of drugs that demonstrate estrogen agonistic and antagonistic actions in a tissue specific manner. SERMs act by binding the estrogen receptor and selectively modulating its effect on gene transcription at target tissues.

CONCLUSIONS

SERMs offer an alternative to HRT that can successfully circumvent the intimidating side-effect of breast carcinoma. Further insight into the molecular mechanisms of SERM action may enable the development of agents with improved target-tissue selectivity. Identifying selective modulators with unique pharmacologic properties would facilitate the creation of individualized treatment for the postmenopausal woman according to her particular predisposition for menopause-related morbidities and her overall clinical profile.

Nonnuclear actions of estrogen

Estrogen has long been observed to endow cardiovascular protective effects, as evidenced by sex-specific differences in the incidence of hypertensive and coronary artery disease, the development of atherosclerosis, and myocardial remodeling after infarction. To exert its tissue-specific effects, the classic estrogen receptor (ER) functions as a ligand-dependent transcription factor. However, there is growing evidence that in response to 17beta-estradiol and heterologous signals, the ER can also mediate signaling cascades at the membrane and in the cytoplasm via various second messengers, such as receptor-mediated protein kinases. This review summarizes the current understanding of nonnuclear ER signaling and discusses the relevance to eliciting the beneficial cardiovascular effects of estrogen. These include vasodilation, inhibition of response to vessel injury, limiting myocardial injury after infarction, and attenuating cardiac hypertrophy. Defining the full repertoire of ER function promises to expose novel, highly specific targets for pharmacological interventions and may ultimately lead to the primary and secondary prevention of cardiovascular diseases.

Involvement of G1/S cyclins in estrogen-independent proliferation of estrogen receptor-positive breast cancer cells

Estrogen receptor-mediated transcription is enhanced by overexpression of G1/S cyclins D1, E or A in the presence as well in the absence of estradiol. Excess of G1/S cyclins also prevents the inhibition of transactivation of estrogen receptor (ER) by the pure antiestrogen ICI 182780. Cyclin D1 mediates this transactivation independent of complex formation to its CDK4/6 partner. This raises the possibility that overexpression of G1/S cyclins renders growth of ER-positive breast cancer hormone-independent and resistant to treatment with antiestrogens. Transient transfection of ER-positive breast cancer cell lines T47D and MCF7 with G1/S cyclins could overcome the growth arrest induced by ICI 182780 treatment. The ability of various cyclin D1 mutants to overcome the ICI 182780 mediated growth arrest corresponded with their ability to stimulate cyclin A- and E2F- promoter based reporter activities in the presence of ICI 182780. Transfection of a mutant cyclin D1 (cyclin D1-KE) that was unable to bind CDK4 and was reported to transactivate ER in the presence of ICI 182780, could not stimulate proliferation in ICI 182780 treated cells. On the other hand, cyclin D1-LALA, which is unable to stimulate ERE transactivation, could overcome the ICI 182780 cell cycle arrest. Furthermore, transient transfection of T47D cells using cyclin D1 together with a catalytic inactive mutant of CDK4 (CDK4-DN) indicated that the observed effect is due to binding to CDK inhibitors. However, a moderate, sixfold overexpression of cyclin D1 in stably transfected MCF7 cells did not overcome the ICI 182780 mediated growth arrest. These results indicate that CDK-independent transactivation of the estrogen receptor by cyclin D1 is by itself, not sufficient to result in estradiol-independent growth of breast cancer cells, whereas a vast overexpression of G1/S cyclins is able to do so, most likely by capturing of CDK inhibitors.

Linking cyclins to transcriptional control

Cell cycle activation is coordinated by D-type cyclins which are rate limiting and essential for the progression through the G1 phase of the cell cycle. D-type cyclins bind to and activate the cyclin-dependent kinases Cdk4 and Cdk6, which in turn phosphorylate their downstream target, the retinoblastoma protein Rb. Upon Rb phosphorylation, the E2F transcription factors activate the expression of S-phase genes and thereby induce cell cycle progression. The raise of cyclin D levels in early G1 also serves to titrate Kip/Cip proteins away from cyclinE/Cdk2 complexes, further accelerating cell cycle progression. Therefore, cyclin D plays essential roles in the response to mitogens, transmitting their signal to the Rb/E2F pathway. Surprisingly, cyclin D1-deficient animals are viable and have developmental abnormalities limited to restricted tissues, such as retina, the nervous system and breast epithelium. This observation, combined with several other studies, have raised the possibility that cyclin D1 may have new activities that are unrelated to its function as a cdk regulatory subunit and as regulator of Rb. Effectively, cyclin D has been reported to have transcriptional functions since it interacts with several transcription factors to regulate their activity. Most often, this effect does not rely on the kinase function of Cdk4, indicating that this function is probably independent of cell cycle progression. Further extending its role in gene regulation, cyclin D interacts with histone acetylases such as P/CAF or NcoA/SRC1a but also with components of the transcriptional machinery such as TAF(II)250. Therefore, these studies suggest that the functions of cyclin D might need to be reevaluated. They have established a new cdk-independent role of cyclin D1 as a transcriptional regulator, indicating that cyclin D1 can act via two different mechanisms, as a cdk activator it regulates cell cycle progression and as a transcriptional regulator, it modulates the activity of transcription factors.

ErbB-2-induced mammary tumor growth: the role of cyclin D1 and p27Kip1

The neu (c-erbB-2, HER2) proto-oncogene encodes a receptor tyrosine kinase that is a member of an important growth factor receptor family which includes the epidermal growth factor receptor (EGFR, ErbB1), ErbB3 and ErbB4. The neu is found over-expressed in 20-30% of human breast tumors. The c-erbB-2 is sufficient for the induction of mammary tumorigenesis in transgenic mice and the pathology of these mammary tumors strongly resembles human breast cancer. Murine transgenic models engineered to recapitulate human breast cancer provide an excellent and straightforward approach to dissect the molecular mechanisms governing the onset and progression of this disease. The molecular mechanisms by which ErbB-2 transforms cells involves direct effects on components of the cell-cycle regulatory apparatus. Recent studies have demonstrated a key role for components of the cell-cycle, in particular cyclin D1 and p27Kip1 (p27) in the onset and progression of ErbB-2-induced murine mammary tumorigenesis. Such studies have provided further impetus to therapeutics targeting these cell-cycle proteins.

Multiparameter analyses of cell cycle regulatory proteins in human breast cancer: a key to definition of separate pathways in tumorigenesis

Breast cancer is one of the most common cancer forms affecting many women. The disease nevertheless has widely varying behavior and therefore patient outcome, and an important undertaking is to define and understand the molecular mechanisms behind these actions. Defects in the G1/S transition in the cell cycle affect both tumor proliferation and the fidelity of check points responsible for chromosomal integrity and DNA damage response and has lately been shown to represent one of a rather limited set of key aberrations in the transformation process. Many cell cycle regulatory proteins are either oncogenes or suppressor genes or are closely associated to the transformation process. The types of aberrations in the G1/S transition seem to be different in various cancers but are nevertheless often linked to clinical behaviors. In this review the role of multiparameter analyses of cell cycle regulatory proteins in breast cancer will be outlined with special attention to pattern analyses as well as the definition of two contrasting pathways in tumorigenesis defined by either cyclin D1 or cyclin F overexpression.

Transcriptional activation by the oestrogen receptor alpha is modulated through inhibition of cyclin-dependent kinases

We have investigated the interaction between the expression of p21(WAF1/CIP1/SDI1), a stoichiometric inhibitor of Cdk, and the transcriptional activity of the oestrogen receptor alpha (ER(alpha). Transient transfection experiments demonstrated that the expression of p21(WAF1/CIP1/SDI1) amplified the transcriptional activation by ER(alpha). A dominant negative mutant of Cdk2 also enhanced the ER(alpha) transcriptional activity, indicating that the underlying mechanism relies on the inhibition of Cdk2 activity and cell cycle arrest. In agreement with this conclusion, experiments with p21(WAF1/CIP1/SDI1) mutants demonstrated that the domain involved in the binding of p21(WAF1/CIP1/SDI1) to Cdks was indispensable for the modulation of ER(alpha) activity. In addition, we show that expression of p21(WAF1/CIP1/SDI1) alleviates the block on CBP function mediated by Cdk2 and in turn stimulates transcriptional activation by ER(alpha) in a CBP-histone acetyltransferase (HAT)-dependent manner. These results suggest a novel mechanism by which p21(WAF1/CIP1/SDI1) functions as an enhancer of ER(alpha) activity through the modulation of CBP function.

Cyclin D1 Is a Ligand-independent Co-repressor for Thyroid Hormone Receptors

Thyroid hormone receptors (TRs) are critical regulators of growth, differentiation, and homeostasis. TRs function by regulating the expression of thyroid hormone (T3) target genes in both ligand-dependent and -independent pathways. Distinct classes of co-regulatory proteins modulate these two pathways. We show here a novel role of cyclin D1 as a T3-independent co-repressor for TRs. Cyclin D1 interacted with TR in vitro and in cells in a ligand-independent manner. Cyclin D1 acted to repress both the silencing activity of the unliganded TR and the transcriptional activity of the liganded TR. The repression was not due to the inhibition of the binding of TR to the thyroid hormone response element but by serving as a ligand-independent bridging factor to selectively recruit HDAC3 to form ternary complexes. The repression was augmented by increasing expression of HDAC3 but not by HDAC1 and was alleviated by trichostatin A. Thus, cyclin D1 is a novel ligand-independent co-repressor that opens a new paradigm to understand the molecular basis of the silencing action of TR.

Cyclin D-cdk4 activity modulates the subnuclear localization and interaction of MEF2 with SRC-family coactivators during skeletal muscle differentiation

Prior work has indicated that D-type cyclin-cdk4 complexes, which are only active in proliferating cells, can suppress the skeletal muscle differentiation program in proliferating myoblasts. In this study, we show that cyclin D-cdk activity can block the activity of the MEF2 family of transcriptional regulators, which are crucial regulators of skeletal muscle gene expression. We have found that cyclin D-cdk activity blocks the association of MEF2C with the coactivator protein GRIP-1 and thereby inhibits the activity of MEF2. During skeletal muscle differentiation, GRIP-1 is localized to punctate nuclear structures and can apparently tether MEF2 to such structures. Cotransfection of GRIP-1 can both potentiate the transcriptional activity of a Gal4-MEF2C construct and induce MEF2C localization to punctate nuclear structures. Consistent with the absence of punctate nuclear GRIP-1 in proliferating myoblasts, we have found that ectopic cyclin D-cdk4 expression disrupts the localization of both GRIP-1 and MEF2C to these punctate subnuclear structures. Our findings indicate that cyclin D-cdk4 activity represses skeletal muscle differentiation in proliferating cells by blocking the association of MEF2 with the coactivator GRIP-1 and concomitantly disrupts the association of these factors with punctate nuclear subdomains within the cell.

To ERR in the estrogen pathway

Estrogens control a variety of physiological and disease-linked processes, most notably reproduction, bone remodeling and breast cancer, and their effects are transduced through classic unclear receptors referred to as estrogen receptor-alpha (ER alpha) and ER beta. Recent results obtained using the estrogen-related receptors (ERR alpha, -beta and -gamma), a subfamily of orphan nuclear receptors closely related to the ERs, have shown that the ERRs share target genes, coregulatory proteins, ligands and sites of action with the ERs. In addition, the ERRs can actively influence the estrogenic response, suggesting that pharmacological modulation of ERR activity will be clinically useful to prevent and/or treat a variety of conditions related to women's health.

Acetylation in hormone signaling and the cell cycle

The last decade has seen a substantial change in thinking about the role of acetylation in regulating diverse cellular processes. The correlation between histone acetylation and gene transcription has been known for many years. The cloning and biochemical characterization of the enzymes that regulate this post-translational modification has led to an understanding of the diverse role histone acetyltransferases (HATs) play in cellular function. Histone acetylases modify histones, transcription factors, co-activators, nuclear transport proteins, structural proteins and components of the cell cycle. This review focuses on the role of histone acetylases in coordinating hormone signaling and the cell cycle. Transition through the cell cycle is regulated by a family of protein kinase holoenzymes, the cyclin-dependent kinases (Cdks) and their heterodimeric cyclin partners. Recent studies have identified important cross-talk between the cell cycle regulatory apparatus and proteins regulating histone acetylation. The evidence for a dynamic interplay between components regulating the cell cycle and acetylation of target substrates provides an important new level of complexity in the mechanisms governing hormone signaling.

Cyclin D1 represses the basic helix-loop-helix transcription factor, BETA2/NeuroD

Expression of the hormone secretin in enteroendocrine cells is restricted to the nondividing villus compartment of the intestine, implying that terminal differentiation is linked to cell cycle arrest and that differentiation is repressed in actively proliferating cells. We have shown previously that the basic helix-loop-helix protein, BETA2/NeuroD, induces cell cycle withdrawal in addition to increasing secretin gene expression. A number of transcription factors important for differentiation are repressed by D cyclins. Repression by D cyclins appears to be independent of its effects on the cell cycle. We show that cyclin D1 represses BETA2/NeuroD-dependent transcription of the secretin gene. Examination of cyclin box mutants shows that repression is unrelated to Cdk4 activation. Although cyclin D1 and BETA2 associate in vivo, they do not directly interact. Cyclin D1 may be recruited to BETA2 by binding to the C-terminal domain of the p300 coactivator, downstream from the BETA2-binding site. In the small intestine, cyclin D1 expression occurs only in the actively proliferating crypts of Lieberkuhn but not in villi. Thus repression by cyclin D1 may serve to prevent secretin gene transcription from occurring in relatively immature epithelial progenitor cells.

Functional interaction of STAT3 transcription factor with the coactivator NcoA/SRC1a

Signal transducer and activator of transcription 3 (STAT3) transcription factors are cytoplasmic proteins that induce gene activation in response to cytokine receptor stimulation. Following tyrosine phosphorylation, STAT3 proteins dimerize, translocate to the nucleus, and activate specific target genes. This transcriptional activation by STAT3 proteins has been shown to require the recruitment of coactivators such as CREB-binding protein (CBP)/p300. In the present study, we show that steroid receptor coactivator 1, NcoA/SRC1a, originally identified as a nuclear receptor coactivator, also functions as a coactivator of STAT3 proteins. In coimmunoprecipitations, NcoA/SRC1a was found to associate with STAT3 following IL-6 stimulation of HepG2 hepatoma cells. Pull-down experiments indicated that the N-terminal part of NcoA/SRC1a associates with the activation domain of STAT3. Overexpression of NcoA/SRC1a or its SRC1e isoform enhanced transcriptional activation by STAT3 proteins in transient transfection experiments. This ability of NcoA/SRC1a to enhance STAT3 activity is dependent upon the presence of the CBP-interacting domain, activation domain 1. Using chromatin immunoprecipitation assays, we found that STAT3, NcoA/SRC1a, and CBP/p300 are simultaneously recruited to the p21(waf1) promoter following interleukin-6 stimulation. Taken together, these data suggest that CBP/p300 and NcoA/SRC1a may function in a common pathway to regulate STAT3 transcriptional activity.

Prognostic value of CCND1 gene status in sporadic breast tumours, as determined by real-time quantitative PCR assays

The CCND1 gene, a key cell-cycle regulator, is often altered in breast cancer, but the mechanisms underlying CCND1 dysregulation and the clinical significance of CCND1 status are unclear. We used real-time quantitative PCR and RT-PCR assays based on fluorescent TaqMan methodology to quantify CCND1 gene amplification and expression in a large series of breast tumours. CCND1 overexpression was observed in 44 (32.8%) of 134 breast tumour RNAs, ranging from 3.3 to 43.7 times the level in normal breast tissues, and correlated significantly with positive oestrogen receptor status (P=0.0003). CCND1 overexpression requires oestrogen receptor integrity and is exacerbated by amplification at 11q13 (the site of the CCND1 gene), owing to an additional gene dosage effect. Our results challenge CCND1 gene as the main 11q13 amplicon selector. The relapse-free survival time of patients with CCND1-amplified tumours was shorter than that of patients without CCND1 alterations, while that of patients with CCND1-unamplified-overexpressed tumours was longer (P=0.011). Only the good prognostic significance of CCND1-unamplified-overexpression status persisted in Cox multivariate regression analysis. This study confirms that CCND1 is an ER-responsive or ER-coactivator gene in breast cancer, and points to the CCND1 gene as a putative molecular marker predictive of hormone responsiveness in breast cancer. Moreover, CCND1 amplification status dichotomizes the CCND1-overexpressing tumors into two groups with opposite outcomes.

Endocrine-responsive breast cancer and strategies for combating resistance

Deaths from breast cancer have fallen markedly over the past decade due, in part, to the use of endocrine agents that reduce the levels of circulating oestrogens or compete with oestrogen for binding to its receptor. However, many breast tumours either fail to respond or become resistant to endocrine therapies. By understanding the mechanisms that underlie this resistance, we might be able to develop strategies for overcoming or bypassing it.

Cyclin A is a prognostic indicator in early stage breast cancer with and without tamoxifen treatment

Overexpression of G1-S regulators cyclin D1 or cyclin A is frequently observed in breast cancer and is also to result in ligand-independent activation of oestrogen receptor in vitro. This might therefore, provide a mechanism for failure of tamoxifen treatment. We examined by immunohistochemical staining the effect of deregulation of these, and other cell cycle regulators on tamoxifen treatment in a group of 394 patients with early stage breast cancer. In univariate analysis, expression of cyclin A, Neu, Ki-67 index, and lack of OR expression were significantly associated with worse prognosis. When adjusted by the clinical model (for lymph node status, age, performance status, T-classification, grade, prior surgery, oestrogen receptor status and tamoxifen use), only overexpression of cyclin A and Neu were significantly associated with worse prognosis with hazard ratios of, respectively, 1.709 (P=0.0195) and 1.884 (P=0.0151). Overexpression of cyclin A was found in 86 out of the 201 OR-positive cases treated with tamoxifen, and was the only independent marker associated with worse prognosis (hazard ratio 2.024, P=0.0462). In conclusion, cyclin A is an independent predictor of recurrence of early stage breast cancer and is as such a marker for response in patients treated with tamoxifen.

Cyclin D1: mechanism and consequence of androgen receptor co-repressor activity

Androgen receptor regulation is pivotal for prostate growth and development. Activation of the receptor is dictated by association with androgen (ligand) and through interaction with co-activators and co-repressors. We have shown previously that cyclin D1 functions as a co-repressor to inhibit ligand-dependent androgen receptor activation. We demonstrate that cyclin D1 directly binds the N terminus of the androgen receptor and that this interaction is independent of ligand. Furthermore, we show that the interaction occurs in the nucleus and does not require the LXXLL motif of cyclin D1. Although two distinct transactivation domains exist in the N terminus (AF-1 and AF-5), the data shown support the hypothesis that cyclin D1 targets the AF-1 transactivation function. The constitutively active AF-5 domain was refractory to cyclin D1 inhibition. By contrast, cyclin D1 completely abolished androgen receptor activity, even in the presence of potent androgen receptor co-activators. This action of cyclin D1 at least partially required de-acetylase activity. Finally, we show that transient, ectopic expression of cyclin D1 results in reduced cell cycle progression in androgen-dependent LNCaP cells independent of CDK4 association. Collectively, our data support a model wherein cyclin D1 has a mitogenic (CDK4-dependent) function and an anti-mitogenic function (dependent on regulation of the AF-1 domain) that can collectively control the rate of androgen-dependent cellular proliferation. These findings provide insight into the non-cell cycle functions of cyclin D1 and provide the impetus to study its pleiotropic effects in androgen-dependent cells, especially prostatic adenocarcinomas.

Activation of retinoblastoma protein in mammary gland leads to ductal growth suppression, precocious differentiation, and adenocarcinoma

The retinoblastoma (Rb) tumor suppressor controls cellular proliferation, survival, and differentiation and is functionally inactivated by mutations or hyperphosphorylation in most human cancers. Although activation of endogenous Rb is thought to provide an effective approach to suppress cell proliferation, long-term inhibition of apoptosis by active Rb may have detrimental consequences in vivo. To directly test these paradigms, we targeted phosphorylation-resistant constitutively active Rb alleles, Rb Delta Ks, to the mouse mammary gland. Pubescent transgenic females displayed reduced ductal elongation and cell proliferation at the endbuds. Post-puberty transgenic mice exhibited precocious cellular differentiation and beta-casein expression and extended survival of the mammary epithelium with a moderate but specific effect on the expression of E2F1, IGF1R alpha, and phospho-protein kinase B/AKT. Remarkably, approximately 30% Rb Delta K transgenic females developed focal hyperplastic nodules, and approximately 7% exhibited full-blown mammary adenocarcinomas within 15 mo. Expression of the Rb Delta K transgene in these mammary tumors was reduced greatly. Our results suggest that transient activation of Rb induces cancer by extending cell survival and that the dual effects of Rb on cell proliferation and apoptosis impose an inherent caveat to the use of the Rb pathway for long-term cancer therapy.

Nuclear receptors in cell life and death

The balance between cell proliferation and programmed cell death (apoptosis) determines body patterns during animal development and controls compartment sizes, tissue architecture and remodeling. The removal of primordial structures by apoptosis allows the organism to develop sex specifically and to adapt for novel functions at later stages; apoptosis also limits the size of evolving structures. It is a ubiquitous function that is essential for all cells. Although inappropriate regulation or execution of apoptosis leads to disease, such as cancer, there is now evidence for its great therapeutic potential. This would be particularly true if apoptosis could be targeted at defined cell compartments, rather than acting ubiquitously like chemotherapy. Here, we discuss the potential of nuclear receptor ligands, many of which act through their cognate receptors in defined body compartments as modulators of cell life and death, with special emphasis on the molecular pathways by which these receptors affect cell-cycle progression, survival and apoptosis.

Role of transcription factors in the pathogenesis of pituitary adenomas: a review

The diversity inherent in every organ has its roots in gene-expression variation and is revealed through distinctions in the molecular profile and hence the identity of individual cell type. Study into the molecular mechanisms of the development of individual cell type within the pituitary, which is under the control of transcription factors, has provided a basis for a deeper insight into the molecular mechanisms underlying the pathogenesis of a variety of hormone-producing pituitary tumors. Identification of some of these transcription factors in pituitary adenomas further supports their role in the pathogenesis of pituitary adenomas. Understanding the molecular mechanisms of regulation of proliferation of pituitary cell types by transcription factors offers a basis for hope that rational genetic or pharmacologic therapies for pituitary tumors can be designed in the future.

Cdc25B functions as a novel coactivator for the steroid receptors

We have previously demonstrated that overexpression of Cdc25B in transgenic mice resulted in mammary gland hyperplasia and increased steroid hormone responsiveness. To address how Cdc25B enhances the hormone responsiveness in mammary glands, we showed that Cdc25B stimulates steroid receptor-dependent transcription in transient transfection assays and in a cell-free assay with chromatin templates. Surprisingly, the effect of Cdc25B on steroid receptors is independent of its protein phosphatase activity in vitro. The direct interactions of Cdc25B with steroid receptors, on the other hand, were evidenced in in vivo and in vitro assays, suggesting the potential direct contribution of Cdc25B on the steroid receptor-mediated transcription. In addition, p300/CBP-associated factor and CREB binding protein were shown to interact and synergize with Cdc25B and further enhance its coactivation activity. Thus, we have uncovered a novel function of Cdc25B that serves as a steroid receptor coactivator in addition to its role as a regulator for cell cycle progression. This dual function might likely contribute to its oncogenic action in breast cancer.

Expression of p21Waf1/Cip1 and cyclin D1 is increased in butyrate-resistant HeLa cells

Sodium butyrate induced cell cycle arrest in mammalian cells through an increase in p21Waf1/Cip1, although another study showed that this arrest is related to pRB signaling. We isolated variants of HeLa cells adapted to growth in 5 mm butyrate. One of these variants, clone 5.1, constitutively expressed elevated levels of p21Waf1/Cip1 when incubated in regular growth medium and in the presence of butyrate. Despite this elevated level of p21Waf1/Cip1, the cells continue to proliferate, albeit at a slower rate than parental HeLa cells. Western blot analyses showed that other cell cycle regulatory proteins were not up-regulated to compensate for the elevated expression of p21Waf1/Cip1. However, cyclin D1 was down-regulated by butyrate in HeLa cells but not in clone 5.1. We conclude that continued expression of cyclin D1 allowed clone 5.1 to grow in the presence of butyrate and elevated levels of p21Waf1/Cip1.

Estrogen regulation of cyclin D1 gene expression in ZR-75 breast cancer cells involves multiple enhancer elements

Cyclin D1 gene expression is induced by 17beta-estradiol (E2) in human breast cancer cells and is important for progression of cells through the G(1) phase of the cell cycle. The mechanism of activation of cyclin D1 is mitogen- and cell context-dependent, and this study describes the role of multiple promoter elements required for induction of cyclin D1 by E2 in estrogen receptor (ER)-positive ZR-75 breast cancer cells. Transcriptional activation of cyclin D1 by E2 was dependent, in part, on a proximal cAMP-response element at -66, and this was linked to induction of protein kinase A-dependent pathways. These results contrasted to a recent report showing that induction of cyclin D1 by E2 in ER-positive MCF-7 and HeLa cells was due to up-regulation of c-jun and subsequent interaction of c-Jun-ATF-2 with the CRE. Moreover, further examination of the proximal region of the cyclin D1 promoter showed that three GC-rich Sp1-binding sites at -143 to -110 were also E2-responsive, and interaction of ERalpha and Sp1 proteins at these sites was confirmed by electromobility shift and chromatin immunoprecipitation assays. Thus, induction of cyclin D1 by E2 in ZR-75 cells is regulated through nuclear ERalpha/Sp1 and epigenetic protein kinase A activation pathways, and our results suggest that this mechanism may be cell context-dependent even among ER-positive breast cancer cell lines.

Identification of an estrogen-inducible phosphatase (PP5) that converts MCF-7 human breast carcinoma cells into an estrogen-independent phenotype when expressed constitutively

The proliferation of many estrogen receptor (ER)-positive breast cancer cells depends on estradiol, and tumors arising from these cells are often responsive initially to treatment with selective ER modulators, which produce an antiestrogen effect. However, tumors that are refractory to the antiestrogenic effects of selective ER modulators often reemerge, and the prognosis for these patients is poor because of the lack of additional effective therapy. Accordingly, deciphering the cellular events associated with estrogen-dependent growth and the subsequent outgrowth of tumors with an estrogen-independent phenotype is of considerable interest. Here we show that the expression of PP5, an evolutionarily conserved Ser/Thr phosphatase that functions as an inhibitor of glucocorticoid- and p53-induced signaling cascades leading to growth suppression, is responsive to 17beta-estradiol (E(2)) in ER-positive human breast carcinoma cells (MCF-7). Northern analysis revealed that E(2)-induced PP5 expression is blocked by treatment with tamoxifen, and a consensus ER recognition element was identified in the PP5 promoter. The PP5-ER recognition element associates with human ERs and confers E(2)-induced transcriptional activation to reporter plasmids. The specific inhibition of PP5 expression ablates E(2)-mediated proliferation in MCF-7 cells without having an apparent effect on E(2)-induced expression of c-myc or cyclin D1. Thus, although critical for cell growth, PP5 likely acts either downstream or independently of c-Myc and Cyclin D1. To further characterize the role of PP5 in E(2)-regulated growth control, we constructed stable MCF-7 cell lines in which the expression of PP5 was placed under the control of tetracycline-regulated transactivator and operator plasmids. Studies with these cells revealed that the constitutive overexpression of PP5 affords E(2)-dependent MCF-7 cells with the ability to proliferate in E(2)-depleted media. Together, these studies indicate that E(2)-induced PP5 expression functions to enhance E(2)-initiated signaling cascades leading to cell division and that aberrant PP5 expression may contribute to the development of MCF-7 cells with an estrogen-independent phenotype.

Differential control of transcription by DNA-bound cyclins

Different cyclins mediate different cell-cycle transitions. Some cyclins, such as cyclin A and cyclin E, form stable complexes with proteins that bind directly or indirectly to DNA and thus might be recruited to certain regions of the genome at specific times in the cell cycle. Furthermore, cyclins contain structural motifs that are also present in known transcriptional modulators. We found that cyclin A is a potent transcriptional repressor and cyclin E is a potent transcriptional activator when bound to DNA via a heterologous DNA binding domain. The former activity was linked to the integrity of the cyclin A cyclin fold, whereas the latter activity related to the ability of cyclin E to activate cdk2 and recognize substrates. Furthermore, we found that cyclin E, but not cyclin A, activated transcription in a cell-cycle-dependent manner when present in physiological concentrations as an unfused protein. These results suggest that cyclin A and cyclin E intrinsically differ with respect to their ability to modulate transcription when tethered to DNA.

Nuclear hormone receptors and gene expression

The nuclear hormone receptor superfamily includes receptors for thyroid and steroid hormones, retinoids and vitamin D, as well as different "orphan" receptors of unknown ligand. Ligands for some of these receptors have been recently identified, showing that products of lipid metabolism such as fatty acids, prostaglandins, or cholesterol derivatives can regulate gene expression by binding to nuclear receptors. Nuclear receptors act as ligand-inducible transcription factors by directly interacting as monomers, homodimers, or heterodimers with the retinoid X receptor with DNA response elements of target genes, as well as by "cross-talking" to other signaling pathways. The effects of nuclear receptors on transcription are mediated through recruitment of coregulators. A subset of receptors binds corepressor factors and actively represses target gene expression in the absence of ligand. Corepressors are found within multicomponent complexes that contain histone deacetylase activity. Deacetylation leads to chromatin compactation and transcriptional repression. Upon ligand binding, the receptors undergo a conformational change that allows the recruitment of multiple coactivator complexes. Some of these proteins are chromatin remodeling factors or possess histone acetylase activity, whereas others may interact directly with the basic transcriptional machinery. Recruitment of coactivator complexes to the target promoter causes chromatin decompactation and transcriptional activation. The characterization of corepressor and coactivator complexes, in concert with the identification of the specific interaction motifs in the receptors, has demonstrated the existence of a general molecular mechanism by which different receptors elicit their transcriptional responses in target genes.

Role of estrogen receptor beta in estrogen action

There was a time when the classification of sex hormones was simple. Androgens were male and estrogens female. What remains true today is that in young adults androgen levels are higher in males and estrogen levels higher in females. More recently we have learned that estrogens are necessary in males for regulation of male sexual behavior, maintenance of the skeleton and the cardiovascular system, and for normal function of the testis and prostate. The importance of androgen in females was never in doubt, it is after all the precursor of estrogen as the substrate for aromatase, the enzyme that produces estrogen. In addition, the tissue distribution of androgen receptors suggests that androgens themselves are important in the ovary, uterus, breast, and brain. New information promises to clarify some of the complex issues of the physiological roles of estrogen and the contribution of estrogen to the development of neoplastic diseases in humans. The discovery of the second estrogen receptor, the creation of mutant mice defective in both estrogen receptors and in the aromatase gene, the solution of the structures of the ligand-binding domains of estrogen receptor alpha (ERalpha) and estrogen receptor beta (ERbeta), the finding of novel routes through which estrogen receptors can modulate transcription, and the identification of a man with a bi-allelic disruptive mutation of the ERalpha gene are but some of the milestones. This review focuses on the mechanistic aspects of signal transduction mediated by ERs and on the physiological consequences of deficiency of estrogen or estrogen receptor in the available mouse models.

Insulin-like growth factor 1 and oestradiol promote cell proliferation of MCF-7 breast cancer cells: new insights into their synergistic effects

In MCF-7 breast cancer cells, the insulin-like growth factor 1 receptor (IGF-1R) and the oestrogen receptor (ER) are coexpressed and the two signalling systems are engaged in a crosstalk that results in synergistic growth. However, coupling between the signalling cascades is poorly understood. Oestradiol enhances IGF-1R signalling by inducing the expression of insulin receptor substrate 1 (IRS-1), a substrate of the IGF-1R. Oestradiol induced expression of IRS-1 results in enhanced tyrosine phosphorylation of IRS-1 after IGF-1 stimulation, followed by enhanced mitogen activated protein kinase, phosphoinositide 3' kinase, and Akt activation. Oestradiol can also potentiate the effect of IGF-1 on the expression of cyclin D1 and cyclin E, and on the phosphorylation of the retinoblastoma protein (RB). These effects are greatly diminished in SX13 cells, which exhibit a 50% reduction in IGF-1R expression but few functional IGF-1Rs at the surface. Oestradiol and IGF-1 regulate the expression of two cyclin dependent kinase inhibitors, p21 and p27, differently. Whereas IGF-1 increases p21 expression and reduces p27 expression, oestradiol has no effect on p21. In summary, in MCF-7 cells, oestrogen potentiates the effect of IGF-1 on IGF-1R signalling and its effects on certain cell cycle components.

Cyclin D1 represses STAT3 activation through a Cdk4-independent mechanism

STAT3 transcription factors are cytoplasmic proteins that induce gene activation in response to cytokine receptor stimulation. Following tyrosine phosphorylation, STAT3 proteins dimerize, translocate into the nucleus, and activate specific target genes. Activation is transient, and down-regulation of STAT3 signaling occurs within a few hours. In this study, we show that cyclin D1 inhibits STAT3 activation. In co-immunoprecipitation and pull-down assays, cyclin D1 was found to associate with the activation domain of STAT3 upon interleukin-6 stimulation. Overexpression of cyclin D1 inhibited transcriptional activation by STAT3 proteins. This effect was not shared by cyclin E, was independent of association with Cdk4, and was unaffected by inhibitors of Cdk4. Whereas cyclin D1 had no effect on the steady-state level of STAT3 proteins in the cytoplasm, it was found to reduce the STAT3 nuclear level in HepG2 cells. These results suggest a model by which cyclin D1 is part of a feedback network controlling the down-regulation of STAT3 activity and highlight a new activity for this cell cycle regulatory protein.

Differential expression of p16/p21/p27 and cyclin D1/D3, and their relationships to cell proliferation, apoptosis, and tumour progression in invasive ductal carcinoma of the breast

In order to understand the intricate relationship of cell proliferation and apoptosis in tumour development, proliferation markers (Ki-67 and c-myc), apoptosis, cell-cycle inducers cyclin D1 and D3, and cell-cycle inhibitors p16(INK4), p21(CIP1), and p27(KIP1) were evaluated in ductal breast carcinoma. The heterogeneous nature of breast tumours provides a system by which the changes in cell-cycle genes can be explored under a wide range of proliferation and apoptotic indices. To address the above issues, immunohistochemical studies were conducted in 40 pairs of tumours and adjacent normal ductal tissues. The TUNEL method was used to identify apoptotic cells. Except for p27/KIP1, the proliferation (Ki-67, c-myc) and the apoptotic indexes together with levels of p16/INK4a, p21/CIP1, cyclin D1, and cyclin D3, were clearly elevated among tumour tissues, while absent in the adjacent normal tissues. Spearman correlation analysis indicated strong associations among apoptotic index, Ki-67, c-myc, and tumour grade. In addition, p21/CIP1 and cyclin D3 were positively correlated, while p16/INK4a, p27/KIP1, and cyclin D1 were negatively correlated with tumour grade. There was clear decoupling between p21 and p27, as well as decoupling between cyclin D1 and cyclin D3, in terms of their relationship to cell proliferation and apoptosis, indicating differential roles in tumour progression.

From translational research to improved local control and survival: the Gilbert Fletcher Award Lecture, Lugano, March 2000

Recent clinical trials have shown us that it is possible to improve local control and survival by the concomitant use of radiotherapy and chemotherapy in a large variety of solid tumors, such as head and neck, lung, cervical, and anal cancer. The selection of drugs for this combined treatment, however, has been based on a rather empiric approach. Further research combining the clinical and laboratory expertise now offers the possibility of predicting and improving treatment efficacy for radiotherapy and systemic treatment, given alone or in combination. New methods, such as chromosome and gene expression profiling of individual tumors, are now becoming available with the comparative genomic hybridization assay and microchip DNA arrays. These assays will hopefully be of help in selecting patients for their optimal treatment regimen in the near future. Detailed knowledge of the mechanisms of action of these two treatment modalities will also lead to the development of new and more effective drugs, to be used concomitantly. It is the challenge of translational research to implement its opportunities into daily clinical practice. In analogy to the results obtained with concomitant radiotherapy and chemotherapy, this will lead to improved local control and survival rates in cancer patients.

Multifaceted regulation of cell cycle progression by estrogen: regulation of Cdk inhibitors and Cdc25A independent of cyclin D1-Cdk4 function

Estrogens induce proliferation of estrogen receptor (ER)-positive MCF-7 breast cancer cells by stimulating G(1)/S transition associated with increased cyclin D1 expression, activation of cyclin-dependent kinases (Cdks), and phosphorylation of the retinoblastoma protein (pRb). We have utilized blockade of cyclin D1-Cdk4 complex formation through adenovirus-mediated expression of p16(INK4a) to demonstrate that estrogen regulates Cdk inhibitor expression and expression of the Cdk-activating phosphatase Cdc25A independent of cyclin D1-Cdk4 function and cell cycle progression. Expression of p16(INK4a) inhibited G(1)/S transition induced in MCF-7 cells by 17-beta-estradiol (E(2)) with associated inhibition of both Cdk4- and Cdk2-associated kinase activities. Inhibition of Cdk2 activity was associated with delayed removal of Cdk-inhibitory activity in early G(1) and decreased cyclin A expression. Cdk-inhibitory activity and expression of both p21(Cip1) and p27(Kip1) was decreased, however, in both control and p16(INK4a)-expressing cells 20 h after estrogen treatment. Expression of Cdc25A mRNA and protein was induced by E(2) in control and p16(INK4a)-expressing MCF-7 cells; however, functional activity of Cdc25A was inhibited in cells expressing p16(INK4a). Inhibition of Cdc25A activity in p16(INK4a)-expressing cells was associated with depressed Cdk2 activity and was reversed in vivo and in vitro by active Cdk2. Transfection of MCF-7 cells with a dominant-negative Cdk2 construct inhibited the E(2)-dependent activation of ectopic Cdc25A. Supporting a role for Cdc25A in estrogen action, antisense CDC25A oligonucleotides inhibited estrogen-induced Cdk2 activation and DNA synthesis. In addition, inactive cyclin E-Cdk2 complexes from p16(INK4a)-expressing, estrogen-treated cells were activated in vitro by treatment with recombinant Cdc25A and in vivo in cells overexpressing Cdc25A. The results demonstrate that functional association of cyclin D1-Cdk4 complexes is required for Cdk2 activation in MCF-7 cells and that Cdk2 activity is, in turn, required for the in vivo activation of Cdc25A. These studies establish Cdc25A as a growth-promoting target of estrogen action and further indicate that estrogens independently regulate multiple components of the cell cycle machinery, including expression of p21(Cip1) and p27(Kip1).

Bcl-2 induces cyclin D1 promoter activity in human breast epithelial cells independent of cell anchorage

Cyclin D1 expression is co-regulated by growth factor and cell adhesion signaling. Cell adhesion to the extracellular matrix activates focal adhesion kinase (FAK), which is essential for cyclin D1 expression. Upon the loss of cell adhesion, cyclin D1 expression is downregulated, followed by apoptosis in normal epithelial cells. Since bcl-2 prevents apoptosis induced by the loss of cell adhesion, we hypothesized that bcl-2 induces survival signaling complementary to cell adhesion-mediated gene regulation. In the present study, we investigated the role of bcl-2 on FAK activity and cyclin D1 expression. We found that bcl-2 overexpression induces cyclin D1 expression in human breast epithelial cell line MCF10A independent of cell anchorage. Increased cyclin D1 expression in stable bcl-2 transfectants is not related to bcl-2-increased G1 duration, but results from cyclin D1 promoter activation. Transient transfection studies confirmed anchorage-independent bcl-2 induction of cyclin D1 promoter activity in human breast epithelial cell lines (MCF10A, BT549, and MCF-7). We provide evidence that bcl-2 induction of cyclin D1 expression involves constitutive activation of focal adhesion kinase, regardless of cell adhesion. The present study suggests a potential oncogenic activity for bcl-2 through cyclin D1 induction, and provides an insight into the distinct proliferation-independent pathway leading to increased cyclin D1 expression in breast cancer.

Resistance to endocrine therapy of breast cancer: recent advances and tomorrow's challenges

The role of endocrine therapy in early as well as advanced breast cancer cannot be overrated. Long-term tamoxifen exposure (5 years) in the adjuvant setting has been shown to be effective not only in improving relapse-free and overall survival but also in reducing the incidence of contralateral cancers. Promising results have been achieved in breast cancer prevention with use of antiestrogens. Novel aromatase inhibitors and inactivators have been found superior to conventional treatment in metastatic disease and are currently being evaluated in the adjuvant setting to improve relapse-free and overall survival. If potential health hazards from estrogen deprivation with regard to cardiovascular disease as well as bone metabolism can be addressed, adjuvant endocrine therapy may include such drugs in the future. However, while endocrine therapy of breast cancer has become more and more important in the clinic, the major problems in hormonal therapy are primary and acquired resistance to endocrine manipulations. The causes for endocrine resistance and possible ways to delay or avoid this phenomenon are only allusively understood. Elucidation of the mechanisms underlying endocrine resistance in vivo represents the key to improve our treatment strategies. Due to intense use of in vitro models and animal systems, many potential mechanisms of endocrine resistance have been described; however, our understanding of the problem of drug resistance in vivo remains limited. Hopefully, ongoing programs on translational research in the neoadjuvant, adjuvant, and palliative settings will provide information that will improve our understanding of the biology of endocrine resistance in vivo and, thus, provide us with a better rationale to improve early as well as late endocrine therapy in breast cancer patients. The present publication summarizes the state of the art with respect to endocrine resistance.