Estrogen induces early and timed activation of cyclin-dependent kinases 4, 5, and 6 and increases cyclin messenger ribonucleic acid expression in rat uterus

Cyclin-Dependent Kinase 4/6 Inhibitors as Neoadjuvant Therapy of Hormone Receptor-Positive/HER2-Negative Early Breast Cancer: What do we Know so Far?

The introduction of cyclin-dependent kinase 4/6 (CDK4/6) inhibitors to the treatment of hormone receptor-positive and human epidermal growth factor receptor 2-negative (HR+/HER2-) metastatic breast cancer is regarded as one of the greatest achievements of the last decades in breast oncology. To date, palbociclib, abemaciclib and ribociclib are the 3 approved CDK4/6 inhibitors that combined with endocrine therapy are now considered as the standard first-line treatment of metastatic HR+/HER2- breast cancer. The great success of these drugs in the setting of metastatic disease and the need to combat the high risk of recurrence have paved the way for a number of clinical trials to explore the use of CDK4/6 inhibitors in the neoadjuvant treatment of early breast cancer. In this review, we summarize the main findings of clinical trials that examined the use of CDK4/6 inhibitors in combination with hormone therapy or chemotherapy as neoadjuvant treatment of hormone receptor-positive and HER2-negative breast cancer. Active clinical trials that investigate different treatment schemes are also briefly presented and current limitations and future goals are discussed.

The Therapeutic Potential of CDK4/6 Inhibitors, Novel Cancer Drugs, in Kidney Diseases

Inflammation is a crucial pathological feature in cancers and kidney diseases, playing a significant role in disease progression. Cyclin-dependent kinases CDK4 and CDK6 not only contribute to cell cycle progression but also participate in cell metabolism, immunogenicity and anti-tumor immune responses. Recently, CDK4/6 inhibitors have gained approval for investigational treatment of breast cancer and various other tumors. Kidney diseases and cancers commonly exhibit characteristic pathological features, such as the involvement of inflammatory cells and persistent chronic inflammation. Remarkably, CDK4/6 inhibitors have demonstrated impressive efficacy in treating non-cancerous conditions, including certain kidney diseases. Current studies have identified the renoprotective effect of CDK4/6 inhibitors, presenting a novel idea and potential direction for treating kidney diseases in the future. In this review, we briefly reviewed the cell cycle in mammals and the role of CDK4/6 in regulating it. We then provided an introduction to CDK4/6 inhibitors and their use in cancer treatment. Additionally, we emphasized the importance of these inhibitors in the treatment of kidney diseases. Collectively, growing evidence demonstrates that targeting CDK4 and CDK6 through CDK4/6 inhibitors might have therapeutic benefits in various cancers and kidney diseases and should be further explored in the future.

CDK4/6 inhibitor resistance mechanisms and treatment strategies (Review)

In recent years, the incidence rate of breast cancer has increased year by year, and it has become a major threat to the health of women globally. Among all breast cancer subtypes, the hormone receptor (HR)⁺/human epidermal growth factor receptor 2 (HER2)⁻ luminal subtype breast cancer is the most common form of breast cancer. Cyclin-dependent kinase 4 and 6 (CDK4/6) inhibitors, the hotspots in the field of targeted therapy for breast cancer, have proved to exhibit a good effect on patients with HR⁺/HER2⁻ breast cancer in a number of clinical trials, but the problem of drug resistance is inevitable. At present, three specific CDK4/6 inhibitors (palbociclib, ribociclib and abemaciclib) have been approved by the USA Food and Drug Administration for the first-line treatment of HR⁺/HER2⁻ breast cancer. The drug resistance mechanisms of CDK4/6 inhibitors can be divided into cell cycle-specific resistance and cell cycle non-specific resistance. With the discovery of the drug resistance mechanism of CDK4/6 inhibitors, various targeted strategies have been proposed. The present review mainly discusses the mechanism of CDK4/6 inhibitors, drug resistance mechanisms and treatment strategies after resistance.

Design, synthesis, molecular docking and ADME studies of novel indole-thiazolidinedione derivatives and their antineoplastic activity as CDK6 inhibitors

Several indole-thiazolidinedione derivatives (9–24) were designed and synthesized as CDK6 inhibitors, and their anticancer activity was probed on the MCF-7 cell line and the effects on gene expression profiles were elucidated.

CDK4/6 inhibitors in breast cancer: differences in toxicity profiles and impact on agent choice. A systematic review and meta-analysis

Introduction: CDK4/6 inhibitor approval for hormone-responsive breast tumors has significantly changed therapeutic algorithms, with three drugs currently approved.Areas covered: Here, we analyze the toxicity profiles of palbociclib, ribociclib, and abemaciclib through a systematic review and meta-analysis. Palbociclib and ribociclib showed high rates of hematological toxicity, primarily neutropenia, and were associated with a low rate of severe infections. Abemaciclib was associated with a high rate of gastrointestinal toxicities, primarily diarrhea, of grade 1-2 in most cases. Ribociclib was associated with a high rate of hepatic, and respiratory toxicity and with QTc prolongation. The toxicity rate of ribociclib was higher in metastatic patients than non-metastatic patients, with approximately 33% more grade 3-4 toxicities and 21% more grade 3-4 neutropenic events. A 5% higher risk of diarrhea was observed in postmenopausal patients. Pre-treated patients did not show a higher toxicity rate for palbociclib/ribociclib than previously untreated patients, while a 26% higher risk of any grade neutropenia and 6% higher risk of grade 3-4 diarrhea were observed with abemaciclib.Expert opinion: Considering the similar efficacies and indications of palbociclib, ribociclib, and abemaciclib, the evaluation of their toxicity profiles may facilitate treatment choice.

Cyclin-dependent kinase 4 and 6 inhibitors for hormone receptor-positive breast cancer: past, present, and future

The development and approval of cyclin-dependent kinase (CDK) 4 and 6 inhibitors for hormone receptor-positive and human epidermal growth factor receptor 2 (HER2)-negative metastatic breast cancer represents a major milestone in cancer therapeutics. Three different oral CDK4/6 inhibitors, palbociclib, ribociclib, and abemaciclib, have significantly improved progression-free survival by a number of months when combined with endocrine therapy. More recently, improvement in overall survival has been reported with ribociclib and abemaciclib. The toxicity profile of all three drugs is well described and generally easily manageable with dose reductions when indicated. More myelotoxicity is observed with palbociclib and ribociclib, but more gastrointestinal toxicity is observed with abemaciclib. Emerging data is shedding light on the resistance mechanisms associated with CDK4/6 inhibitors, including cell cycle alterations and activation of upstream tyrosine kinase receptors. A number of clinical trials are exploring several important questions regarding treatment sequencing, combinatorial strategies, and the use of CDK4/6 inhibitors in the adjuvant and neoadjuvant settings, thereby further expanding and refining the clinical application of CDK4/6 inhibitors for patients with breast cancer.

Role of Cyclin-Dependent Kinase Inhibitors in Endometrial Cancer

Endometrial Cancer (EC) is an important cause of death in women worldwide. Despite early diagnosis and optimal treatment of localized disease, relapsed patients have few therapeutic options because after first line therapy, currently no standard of care exists. On the basis of endocrine positivity of most endometrioid ECs, Endocrine Therapy (ET) is a reasonable and widely accepted option. Better knowledge of molecular mechanisms involved in cancer highlighted the deregulated activity of Cyclin-Dependent Kinases (CDKs) in the cell cycle as a hallmark of carcinogenesis supporting the development of a new class of drugs: CDK inhibitors (CDKis). The aim of this review is to give an overview on CDKis preclinical, early clinical activity and future development in EC. Use of CDKis has a strong preclinical rationale but we have poor clinical data. Similar to breast cancer, most ongoing trials are investigating synergistic associations between CDKis and ET. These trials will probably help in defining the best clinical setting of CDKis in ECs, which are the best partner drugs, and how to manage CDKis toxicities with a focus on potential biomarkers of response.

CDK 4/6 Inhibitors in Breast Cancer: Current Controversies and Future Directions

PURPOSE OF REVIEW

To describe the clinical role of CDK 4/6 inhibitors in hormone receptor-positive (HR+) metastatic breast cancer (HR+ MBC) as well as current controversies and evolving areas of research.

RECENT FINDINGS

Palbociclib, ribociclib, and abemaciclib are each approved in combination with an aromatase inhibitor or fulvestrant for HR+ MBC. Abemaciclib is also approved as monotherapy for pre-treated patients. Key questions in the field include whether all patients with HR+ MBC should receive a CDK 4/6 inhibitor up front versus later line, impact on overall survival, role of continued CDK 4/6 blockade, mechanism of clinical resistance, and treatment sequencing. The development of CDK 4/6 inhibitors has changed the therapeutic management of HR+ MBC. Additional research is needed to determine optimal treatment sequencing, understand mechanisms governing resistance, and develop novel therapeutic strategies to circumvent or overcome clinical resistance and further improve the outcomes of patients with MBC.

Efficacy and safety of palbociclib in heavily pretreated patients with HR+/HER2- metastatic breast cancer

AIM

CDK4/6 inhibitors in the first and second treatment line in patients with HR+/HER2- metastatic breast cancer (mBC) in combination with hormonal therapy improve progression-free survival. Role of CDK4/6 inhibitors in further treatment lines remains unclear.

METHODS

Retrospective analysis of 24 HR+/HER2- heavily pretreated mBC patients is presented.

RESULTS

A total of 58.3% patients achieved stable disease. No objective response was observed. Median progression-free survival was 4.8 months; median overall survival was 11 months. Treatment was well tolerated.

CONCLUSION

Favorable toxicity profile and efficacy of palbociclib/aromatase inhibitors combination in heavily pretreated luminal mBC patients in this study emphasize the need for further investigation of such drugs in this population.

Clinical Management of Potential Toxicities and Drug Interactions Related to Cyclin-Dependent Kinase 4/6 Inhibitors in Breast Cancer: Practical Considerations and Recommendations

Aberrations of the cell cycle are pervasive in cancer, and selective cell cycle inhibition of cancer cells is a target of choice for a number of novel cancer therapeutics. Cyclin-dependent kinases (CDKs) are key regulatory enzymes that control cell cycle transitions and the commitment to cell division. Palbociclib and ribociclib are both orally active, highly selective reversible inhibitors of CDK4 and CDK6 that are approved by the U.S. Food and Drug Administration (FDA) for hormone receptor-positive metastatic breast cancer in combination with specific endocrine therapies. A third oral CDK4/6 inhibitor, abemaciclib, received Breakthrough Therapy designation status from the FDA and is also being developed in breast cancer. The most common adverse events associated with palbociclib and ribociclib are hematologic, particularly neutropenia. However, the neutropenia associated with CDK4/6 inhibitors is distinct from chemotherapy-induced neutropenia in that it is rapidly reversible, reflecting a cytostatic effect on neutrophil precursors in the bone marrow. Most hematologic abnormalities seen with CDK4/6 inhibitors are not complicated and are adequately managed with standard supportive care and dose adjustments when indicated. Cytopenias are less prevalent with abemaciclib, although fatigue and gastrointestinal toxicity is more common with this agent. This review focuses on the clinical management of potential toxicities and drug interactions seen with the use of CDK4/6 inhibitors in breast cancer, with a focus on palbociclib and ribociclib, and summarizes practical management strategies for an oncologist.

IMPLICATIONS FOR PRACTICE

The emergence of modern cyclin-dependent kinase (CDK) inhibitors has changed the treatment paradigm for metastatic hormone receptor (HR)-positive breast cancer. Palbociclib, ribociclib, and abemaciclib are highly selective reversible inhibitors of CDK4 and CDK6. Palbociclib is U.S. Food and Drug Administration (FDA)-approved in the first- and second-line settings in combination with endocrine therapy for HR-positive metastatic breast cancer. Ribociclib is FDA-approved in the first-line setting. Abemaciclib has received FDA Breakthrough Therapy designation status. This review focuses on the clinical management of potential toxicities and drug interactions seen with the use of CDK4/6 inhibitors in breast cancer.

NeoPalAna: Neoadjuvant Palbociclib, a Cyclin-Dependent Kinase 4/6 Inhibitor, and Anastrozole for Clinical Stage 2 or 3 Estrogen Receptor-Positive Breast Cancer

Purpose: Cyclin-dependent kinase (CDK) 4/6 drives cell proliferation in estrogen receptor-positive (ER⁺) breast cancer. This single-arm phase II neoadjuvant trial (NeoPalAna) assessed the antiproliferative activity of the CDK4/6 inhibitor palbociclib in primary breast cancer as a prelude to adjuvant studies.Experimental Design: Eligible patients with clinical stage II/III ER⁺/HER2^- breast cancer received anastrozole 1 mg daily for 4 weeks (cycle 0; with goserelin if premenopausal), followed by adding palbociclib (125 mg daily on days 1-21) on cycle 1 day 1 (C1D1) for four 28-day cycles unless C1D15 Ki67 > 10%, in which case patients went off study due to inadequate response. Anastrozole was continued until surgery, which occurred 3 to 5 weeks after palbociclib exposure. Later patients received additional 10 to 12 days of palbociclib (Cycle 5) immediately before surgery. Serial biopsies at baseline, C1D1, C1D15, and surgery were analyzed for Ki67, gene expression, and mutation profiles. The primary endpoint was complete cell cycle arrest (CCCA: central Ki67 ≤ 2.7%).Results: Fifty patients enrolled. The CCCA rate was significantly higher after adding palbociclib to anastrozole (C1D15 87% vs. C1D1 26%, P < 0.001). Palbociclib enhanced cell-cycle control over anastrozole monotherapy regardless of luminal subtype (A vs. B) and PIK3CA status with activity observed across a broad range of clinicopathologic and mutation profiles. Ki67 recovery at surgery following palbociclib washout was suppressed by cycle 5 palbociclib. Resistance was associated with nonluminal subtypes and persistent E2F-target gene expression.Conclusions: Palbociclib is an active antiproliferative agent for early-stage breast cancer resistant to anastrozole; however, prolonged administration may be necessary to maintain its effect. Clin Cancer Res; 23(15); 4055-65. ©2017 AACR.

Improving Response to Hormone Therapy in Breast Cancer: New Targets, New Therapeutic Options

The majority of breast cancer expresses the estrogen and or progesterone receptors (ER and PR). In tumors without concomitant HER2 amplification, hormone therapy is a major treatment option for all disease stages. Resistance to hormonal therapy is associated with disease recurrence and progression. Recent studies have identified a number of resistance mechanisms leading to estrogen-independent growth of hormone receptor-positive (HR+) breast cancer as a result of genetic and epigenetic alterations, which could be exploited as novel therapeutic targets. These include acquired mutations in ER-alpha (ESR1) in response to endocrine deprivation; constitutive activation of cyclin-dependent kinases (CDK) 4 and 6; cross talk between ER and growth factor receptor signaling such as HER family members, fibroblast growth factor receptor (FGFR) pathways, intracellular growth, and survival signals PI3K/Akt/mTOR; and epigenetic modifications by histone deacetylase (HDAC) as well as interactions with tumor microenvironment and host immune response. Inhibitors of these pathways are being developed to improve efficacy of hormonal therapy for treatment of both metastatic and early-stage disease. Two agents are currently approved in the United States for the treatment of metastatic HR+ breast cancer, including the mTOR inhibitor everolimus and the CDK4/6 inhibitor palbociclib. Management of toxicity is a critical aspect of treatment; the primary toxicity of everolimus is stomatitis (treated with topical steroids) and of palbociclib is neutropenia (treated with dose reduction/delay). Many agents are in clinical trials, primarily in combination with hormone therapy; novel combinations are under active investigation.

Estrogen receptor alpha-36 (ER-α36): A new player in human breast cancer

Prevailing wisdom is that estrogen receptor (ER)-α mediated genomic estrogen signaling is responsible for estrogen-stimulated cell proliferation and development of ER-positive breast cancer. However, accumulating evidence indicates that another estrogen signaling pathway, non-genomic or rapid estrogen signaling, also plays an important role in mitogenic estrogen signaling. Previously, our laboratory cloned a 36 kDa variant of ER-α, ER-α36, and found that ER-α36 is mainly expressed in the cytoplasm and at the plasma membrane. ER-α36 mediates rapid estrogen signaling and inhibits genomic estrogen signaling. In this review, we review and update the biological function of ER-α36 in ER-positive and -negative breast cancer, breast cancer stem/progenitor cells and tamoxifen resistance, potential interaction and cross-talk of ER-α36 with other ERs and growth factor receptors, and intracellular pathways of ER-α36-mediated rapid estrogen signaling. The potential function and underlying mechanism of ER-α in development of ER-positive breast cancer will also be discussed.

The Role of CDK4/6 Inhibition in Breast Cancer

Imbalance of the cyclin D and cyclin-dependent kinase (CDK) pathway in cancer cells may result in diversion away from a pathway to senescence and toward a more proliferative phenotype. Cancer cells may increase cyclin D-dependent activity through a variety of mechanisms. Therapeutic inhibition of CDKs in tumors to negate their evasion of growth suppressors has been identified as a key anticancer strategy. In this review, we outline the development of CDK inhibitory therapy in breast cancer, including the initial experience with the pan-CDK inhibitor flavopiridol and the next generation of oral highly selective CDK4 and CDK6 inhibitors PD0332991 (palbociclib), LEE011 (ribociclib), and LY2835219 (abemaciclib). Data from phase I and II studies in estrogen receptor-positive (ER+) breast cancer demonstrate promising efficacy with manageable toxic effects, chiefly neutropenia. We discuss these studies and the phase III studies that are accruing or nearing completion. We describe the application of such therapy to other breast cancer settings, including HER2-positive breast cancer and the adjuvant treatment of early breast cancer. We also discuss potential concerns surrounding the combination of CDK inhibitors with chemotherapy and their effects on repair of double-strand DNA breaks in cancer cells. Oral highly selective CDK inhibitors show great promise in improving the outcomes of patients with ER+ breast cancer, although caution must apply to their combination with other agents and in the early breast cancer setting.

Placental estrogen suppresses cyclin D1 expression in the nonhuman primate fetal adrenal cortex

We have previously shown that estrogen selectively suppresses growth of the fetal zone of the baboon fetal adrenal cortex, which produces the C19-steroid precursors, eg, dehydroepiandrosterone sulfate, which are aromatized to estrogen within the placenta. In the present study, we determined whether fetal adrenal expression of cell cycle regulators are altered by estrogen and thus provide a mechanism by which estrogen regulates fetal adrenocortical development. Cyclin D1 mRNA levels in the whole fetal adrenal were increased 50% (P < .05), and the number of cells in the fetal adrenal definitive zone expressing cyclin D1 protein was increased 2.5-fold (P < .05), whereas the total number of cells in the fetal zone and fetal serum dehydroepiandrosterone sulfate levels were elevated 2-fold (P < .05) near term in baboons in which fetal serum estradiol levels were decreased by 95% (P < .05) after maternal administration of the aromatase inhibitor letrozole and restored to normal by concomitant administration of letrozole plus estradiol throughout second half of gestation. However, fetal adrenocortical expression of cyclin D2, the cyclin-dependent kinase (Cdk)-2, Cdk4, and Cdk6, and Cdk regulatory proteins p27(Kip1) and p57(Kip2) were not changed by letrozole or letrozole plus estradiol administration. We suggest that estrogen controls the growth of the fetal zone of the fetal adrenal by down-regulating cyclin D1 expression and thus proliferation of progenitor cells within the definitive zone that migrate to the fetal zone. We propose that estrogen restrains growth and function of the fetal zone via cyclin D1 to maintain estrogen levels in a physiological range during primate pregnancy.

Identification of estrogen target genes during zebrafish embryonic development through transcriptomic analysis

Estrogen signaling is important for vertebrate embryonic development. Here we have used zebrafish (Danio rerio) as a vertebrate model to analyze estrogen signaling during development. Zebrafish embryos were exposed to 1 µM 17β-estradiol (E2) or vehicle from 3 hours to 4 days post fertilization (dpf), harvested at 1, 2, 3 and 4 dpf, and subjected to RNA extraction for transcriptome analysis using microarrays. Differentially expressed genes by E2-treatment were analyzed with hierarchical clustering followed by biological process and tissue enrichment analysis. Markedly distinct sets of genes were up and down-regulated by E2 at the four different time points. Among these genes, only the well-known estrogenic marker vtg1 was co-regulated at all time points. Despite this, the biological functional categories targeted by E2 were relatively similar throughout zebrafish development. According to knowledge-based tissue enrichment, estrogen responsive genes were clustered mainly in the liver, pancreas and brain. This was in line with the developmental dynamics of estrogen-target tissues that were visualized using transgenic zebrafish containing estrogen responsive elements driving the expression of GFP (Tg(5xERE:GFP)). Finally, the identified embryonic estrogen-responsive genes were compared to already published estrogen-responsive genes identified in male adult zebrafish (Gene Expression Omnibus database). The expressions of a few genes were co-regulated by E2 in both embryonic and adult zebrafish. These could potentially be used as estrogenic biomarkers for exposure to estrogens or estrogenic endocrine disruptors in zebrafish. In conclusion, our data suggests that estrogen effects on early embryonic zebrafish development are stage- and tissue- specific.

Endometrial tumorigenesis in Pten(+/-) mice is independent of coexistence of estrogen and estrogen receptor α

Numerous studies support the role for mutations in the phosphatase and tensin homologue (PTEN) tumor suppressor gene and unopposed estrogen stimulation in the pathogenesis of uterine endometrioid carcinoma. However, the relation between PTEN signaling and estrogen/estrogen receptor in endometrial tumorigenesis remains unresolved. We used genetically engineered mice as a model to address this relation. Mice with a single deleted Pten allele (Pten(+/-)) spontaneously develop complex atypical hyperplasia and ~20% develop endometrial cancer. To determine the effect of removing endogenous estrogen, we performed oophorectomies on Pten(+/-) mice. Although there was a reduction in the number and severity of hyperplastic lesions, the endometrial phenotype persisted, suggesting that Pten mutation, independent of estrogen, can initiate the development of complex atypical hyperplasia. To recapitulate the situation in women with unopposed estrogen, we implanted 17β-estradiol pellets in adult female Pten heterozygous mice, resulting in increased carcinoma incidence. Because studies have shown that estrogen largely acts on the endometrium via estrogen receptor ERα, we generated Pten(+/-)ERα(-/-) mice. Strikingly, 88.9% of Pten(+/-)ERα(-/-) mice developed endometrial hyperplasia/carcinoma. Furthermore, Pten(+/-)ERα(-/-) mice showed a higher incidence of in situ and invasive carcinoma, suggesting that endometrial tumorigenesis can progress in the absence of ERα. Thus, the relation between Pten alterations and estrogen signaling in the development of endometrial carcinoma is complex; the results presented herein have important implications for the treatment of endometrial hyperplasia and carcinoma in women.

Adenovirus mediated homozygous endometrial epithelial Pten deletion results in aggressive endometrial carcinoma

Pten is the most frequently mutated gene in uterine endometriod carcinoma (UEC) and its precursor complex atypical hyperplasia (CAH). Because the mutation frequency is similar in CAH and UEC, Pten mutations are thought to occur relatively early in endometrial tumorigenesis. Previous work from our laboratory using the Pten(+/-) mouse model has demonstrated somatic inactivation of the wild type allele of Pten in both CAH and UEC. In the present study, we injected adenoviruses expressing Cre into the uterine lumen of adult Pten floxed mice in an attempt to somatically delete both alleles of Pten specifically in the endometrium. Our results demonstrate that biallelic inactivation of Pten results in an increased incidence of carcinoma as compared to the Pten(+/-) mouse model. In addition, the carcinomas were more aggressive with extension beyond the uterus into adjacent tissues and were associated with decreased expression of nuclear ERα as compared to associated CAH. Primary cultures of epithelial and stromal cells were prepared from uteri of Pten floxed mice and Pten was deleted in vitro using Cre expressing adenovirus. Pten deletion was evident in both the epithelial and stromal cells and the treatment of the primary cultures with estrogen had different effects on Akt activation as well as Cyclin D3 expression in the two purified components. This study demonstrates that somatic biallelic inactivation of Pten in endometrial epithelium in vivo results in an increased incidence and aggressiveness of endometrial carcinoma compared to mice carrying a germline deletion of one allele and provides an important in vivo and in vitro model system for understanding the genetic underpinnings of endometrial carcinoma.

Heterogeneity of genetic associations of CDKAL1 and HHEX with susceptibility of type 2 diabetes mellitus by gender

We examined the genetic associations of previously identified sequence variants with type 2 diabetes mellitus (T2DM) and its potentially genetic heterogeneity by gender in a large-scale cohort. A total of 613 T2DM patients and 8221 control subjects from the Korea Association REsource (KARE) cohort were included in the analysis of genetic association of T2DM with 33 nucleotide polymorphic markers identified by previous studies. The association analysis was further conducted with data partitioned by gender. The association analysis resulted in five nucleotide sequence variants associated with the susceptibility of T2DM after Bonferonni correction (P < 0.0015). One was located near the gene of hematopoietically expressed homeobox (HHEX), and the others were all in the gene of cyclin-dependent kinase 5 regulatory subunit-associated protein 1-like 1 (CDKAL1). Further analysis revealed that the sequence variant (rs5015480) near HHEX and two SNPs (rs7756992 and rs9465871) in CDKAL1 were associated with the susceptibility of T2DM in females (P<0.005), but not in males (P>0.005). We suggested heterogeneous genetic associations of the T2DM susceptibility with the CDKAL1 and HHEX genes by gender.

An IGF1/insulin receptor substrate-1 pathway stimulates a mitotic kinase (cdk1) in the uterine epithelium during the proliferative response to estradiol

Estrogens are potent mitogens for some target organs, such as the uterus, and cancers that develop in this organ might be linked to the proliferative action of these hormones. However, the mechanism by which estrogens influence the cell cycle machinery is not known. We found that a null mutation for the insulin receptor substrate (IRS)-1, a docking protein that is important for IGF1 signaling, compromised hormone-induced mitosis in the uterine epithelium; BrdU incorporation was not affected. This selective effect on mitosis was associated with a reduction in uterine cyclin B-associated kinase activity; cyclin A-associated kinase activity was not changed. The null mutation also reduced the extent of hormone-induced phosphorylation of endogenous uterine histone H1, as determined with phospho-specific antiserum. Uterine epithelial cyclin dependent kinase (cdk)1 was induced in response to hormone, but the level of the kinase protein, as determined by immunoblotting, was noticeably less in the irs1 null mutant than that in the wild-type (WT) mouse, especially around the time of peak mitosis (24 h). Since IRS-1 binds/activates phosphatidylinositol 3-kinase (PI3K), the absence of this docking protein could impair signaling of a known pathway downstream of AKT that stimulates translation of cell cycle components. Indeed, we found that phosphorylation of uterine AKT (Ser473) in irs1 null mutants was less than that in WTs following treatment. Based on earlier studies, it is also possible that an IGF1/IRS-1/PI3K/AKT pathway regulates posttranslational changes in cdk1. This model may provide insights as to how a growth factor pathway can mediate hormone action on cell proliferation.

Aromatase and 11beta-hydroxysteroid dehydrogenase 2 localisation in the testes of pigs from birth to puberty linked to changes of hormone pattern and testicular morphology

Oestrogens and glucocorticoids are important for spermatogenesis and are regulated via aromatase for oestradiol synthesis and 11beta-hydroxysteroid dehydrogenase 2 (11beta-HSD 2) as an inactivator of cortisol. In the present study postnatal changes of these two enzymes were monitored together with testicular development and hormone concentrations. Pigs were assigned to three periods: Weeks 0-5, Weeks 5-11 or Weeks 11-17. In Period 1, groups of four piglets were killed after each week. Blood plasma and testes were sampled immediately post mortem. For Periods 2 and 3, groups of six pigs were fitted with vein catheters for daily blood collection. Testes from all pigs were obtained after killing. Levels of testosterone, oestradiol, LH, FSH and cortisol were determined radioimmunologically. The 11beta-HSD 2- and aromatase-expressing cells were stained immunocytochemically. All hormones were maximal 2 weeks after birth. A rise of LH, testosterone and oestradiol occurred again at Week 17. FSH and cortisol remained basal. Parallel to the first postnatal rise, the presence of aromatase and 11beta-HSD 2 in Leydig cells increased, together with germ and Sertoli cell numbers. Expression was low from 3 to 5 weeks, was resumed after Week 5 and was maximal at Week 17. The amount of 11beta-HSD 2 in germ cells was greatest at birth, decreased thereafter and was absent after Week 3.

An unusual member of the Cdk family: Cdk5

The proline-directed serine threonine kinase, Cdk5, is an unusual molecule that belongs to the well-known large family of proteins, cyclin-dependent kinases (Cdks). While it has significant homology with the mammalian Cdk2 and yeast cdc2, unlike the other Cdks, it has little role to play in cell cycle regulation and is activated by non-cyclin proteins, p35 and p39. It phosphorylates a spectrum of proteins, most of them associated with cell morphology and motility. A majority of known substrates of Cdk5 are cytoskeletal elements, signalling molecules or regulatory proteins. It also appears to be an important player in cell-cell communication. Highly conserved, Cdk5 is most abundant in the nervous system and is of special interest to neuroscientists as it appears to be indispensable for normal neural development and function. In normal cells, transcription and activity of Cdk5 is tightly regulated. Present essentially in post-mitotic neurons, its normal activity is obligatory for migration and differentiation of neurons in developing brain. Deregulation of Cdk5 has been implicated in Alzheimer's disease, amyotrophic lateral sclerosis, Parkinson's disease, Huntington's disease and acute neuronal injury. Regulators of Cdk5 activity are considered as potential therapeutic molecules for degenerative diseases. This review focuses on the role of Cdk5 in neural cells as regulator of cytoskeletal elements, axonal guidance, membrane transport, synaptogenesis and cell survival in normal and pathological conditions.

Extraneuronal roles of cyclin-dependent kinase 5

Cyclin-dependent kinase 5 (Cdk5) is recognized as an essential molecule in the brain, where it regulates several neuronal activities, including cytoskeletal remodeling and synaptic transmission. While activity of Cdk5 has primarily been associated with neurons, there are now substantial data indicating that the kinase's activity and function are more general. An increasing body of evidence has established Cdk5 kinase activity, the presence of the Cdk5 activators, p35 and p39, and Cdk5 functions in non-neuronal cells, including myocytes, pancreatic beta-cells, monocytic and neutrophilic leucocytes, glial cells and germ cells. In this review, we present the diverse roles of Cdk5 in several extraneuronal paradigms. The unique properties of each of the different cell types appear to involve distinct means of Cdk5 regulation and function. The potential mechanisms through which Cdk5 regulates extraneuronal cell activities such as exocytosis, gene transcription, wound healing and senescence are discussed.

Stability of A+U-Rich Element Binding Factor 1 (AUF1)-Binding Messenger Ribonucleic Acid Correlates with the Subcellular Relocalization of AUF1 in the Rat Uterus upon Estrogen Treatment

The nucleocytoplasmic shuttling protein, A+U-rich element binding factor 1 (AUF1), is one of the RNA-binding proteins that specifically bind adenylate-uridylate rich elements (AREs) in mRNA 3'-untranslated regions (UTRs), and acts as a regulator of ARE-mediated mRNA degradation in the cytoplasm. We previously reported that in the female rat uterus, the levels of specific AUF1 isoform mRNAs (p40/p45) were increased by 17 beta-estradiol (E2) treatment. Therefore, we examined the role of AUF1 in the regulation of E2-mediated mRNA turnover in the rat uterus. We identified ABIN2 and Ier2/pip92 mRNAs as candidate targets of AUF1 in the rat uterus. We found that AUF1-binding elements were present in the 3'-UTR of both mRNAs and that the 3'-UTRs functioned as mRNA turnover regulatory elements. In the ovariectomized rat uterus, the nucleocytoplasmic localization of AUF1p40/p37 isoform proteins was regulated by E2. We also found that cytoplasmic AUF1-bound mRNA levels changed coincidentally with the cytoplasmic levels of AUF1p40/p37. Finally, we confirmed that the subcellular localization of AUF1p40 controlled the stability of target mRNAs in vitro, such that cytoplasmically localized AUF1p40 led to marked mRNA stabilization, whereas nuclear-localized AUF1p40 stabilized target mRNA only slightly. These results suggested that E2-inducible ARE-containing gene transcripts are regulated, at least in part, via mRNA stabilization through the nucleocytoplasmic relocalization of AUF1.

Molecular cues to implantation

Successful implantation is the result of reciprocal interactions between the implantation-competent blastocyst and receptive uterus. Although various cellular aspects and molecular pathways of this dialogue have been identified, a comprehensive understanding of the implantation process is still missing. The receptive state of the uterus, which lasts for a limited period, is defined as the time when the uterine environment is conducive to blastocyst acceptance and implantation. A better understanding of the molecular signals that regulate uterine receptivity and implantation competency of the blastocyst is of clinical relevance because unraveling the nature of these signals may lead to strategies to correct implantation failure and improve pregnancy rates. Gene expression studies and genetically engineered mouse models have provided valuable clues to the implantation process with respect to specific growth factors, cytokines, lipid mediators, adhesion molecules, and transcription factors. However, a staggering amount of information from microarray experiments is also being generated at a rapid pace. If properly annotated and explored, this information will expand our knowledge regarding yet-to-be-identified unique, complementary, and/or redundant molecular pathways in implantation. It is hoped that the forthcoming information will generate new ideas and concepts for a process that is essential for maintaining procreation and solving major reproductive health issues in women.

Comparative genomic hybridization of estrogen-induced ectopic uterine-like stem cell neoplasms in the hamster kidney: nonrandom chromosomal alterations

Karyotype and comparative genomic hybridization (CGH) analyses were performed to identify nonrandom/consistent chromosomal alterations in solely estrogen (E)-induced primary ectopic uterine-like stem cell tumors in the kidney (EULTK) of the Syrian hamster, using a criterion of >/=20% frequency for nonrandom occurrence. Employing this criterion, EULTK karyotype analysis showed consistent gains in chromosomes 3, 6, 11, 14, 16, 20, and 21. Consistent trisomies were seen in all of these nonrandomly gained chromosomes. Only chromosomes 3 and 6 exhibited appreciable tetrasomies. Chromosome losses were observed consistently in chromosomes 7, 12, 17, and 19. Employing the same criterion, CGH analysis of primary EULTKs showed nonrandom amplified sequences at 1pa1-a4, 2cen-pter, 3pa1-a4, 6qb2-b4, 20qa1-a4, 21qa1-a2, Xqa3-qter and regional consistent losses at 1qc1-qter, 2qb1-c1, 3qa2-a7, 11qb5-qter, 15qa2-a5, 18qa2-a4, and 21pa. Moreover, 88% of the EULTKs examined exhibited amplification of the 6qb2-b4 region, where c-myc resides. The data presented lend credence to the supposition that chromosomal instability (CIN) is elicited by the upstream overexpression and subsequent amplification of c-myc by Es in multipotential interstitial uterine stem cells present in the kidney, thus leading to neoplastic development.

Identification of temporal patterns of gene expression in the uteri of immature, ovariectomized mice following exposure to ethynylestradiol

Estrogen induction of uterine wet weight provides an excellent model to investigate relationships between changes in global gene expression and well-characterized physiological responses. In this study, time course microarray GeneChip data were analyzed using a novel approach to identify temporal changes in uterine gene expression following treatment of immature ovariectomized C57BL/6 mice with 0.1 mg/kg 17alpha-ethynylestradiol. Functional gene annotation information from public databases facilitated the association of changes in gene expression with physiological outcomes, which allowed detailed mechanistic inferences to be drawn regarding cell cycle control and proliferation, transcription and translation, structural tissue remodeling, and immunologic responses. These systematic approaches confirm previously established responses, identify novel estrogen-regulated transcriptional effects, and disclose the coordinated activation of multiple modes of action that support the uterotrophic response elicited by estrogen. In particular, it was possible to elucidate the physiological significance of the dramatic induction of arginase, a classic estrogenic response, by elucidating its mechanistic relevance and delineating the role of arginine and ornithine utilization in the estrogen-stimulated induction of uterine wet weight.

Role of cell-cycle regulatory proteins in gynecological cancer

Human malignant tumors are characterized by abnormal proliferation resulting from alterations in cell-cycle regulatory mechanisms. This review summarizes the current knowledge about these aberrations in malignant tumors of the ovary, endometrium, cervix uteri, and vulva. The data indicate that analysis of single cell cycle stimulating or inhibiting proteins partly produces unexpected, apparently paradoxical results, and cell-cycle regulatory pathways should be regarded as a whole in order to identify the molecular mechanisms leading to abnormal tumor cell proliferation. For the papillomavirus (HPV)- associated cervical and vulvar carcinomas, the manifold effects of the viral oncogenes E6 and E7 on cell-cycle control are described.

Estrogen stimulates expression of p21Waf1/Cip1 in mouse uterine luminal epithelium

p21Waf1/Cip1 was originally identified as an inhibitor of the cell cycle. Recent evidence suggests that it can act as a positive regulator of the cell cycle under the influence of some growth stimulators. We investigated the effects of ovarian steroids on the expression of p21, DNA synthesis, and mitosis in the uterus. Capsules containing 17beta-estradiol (E2) were subcutaneously implanted in ovariectomized mice that were sacrificed on different days. Their uteri were collected for p21 immunohistochemical staining. To study mitosis and DNA synthesis, colchicine and bromodeoxyuridine (BrdU) were injected into mice 3 or 5 h before sacrifice. The results showed that p21 expression, BrdU incorporation, and the mitotic index in uterine luminal epithelium increased 1 to 2 d after E2 stimulation and then declined to basal levels between d 3 and 6. Furthermore, cotreatment with progesterone (P4) and E2 suppressed both p21 expression and the DNA synthesis stimulated by E2 alone in uterine epithelial cells. Our results show that estrogen stimulates p21 expression and cell proliferation in uterine luminal epithelium and that cotreatment with P4 prevents both effects, suggesting that p21 may act as a positive cell-cycle regulator.

A+U-rich-element RNA-binding factor 1/heterogeneous nuclear ribonucleoprotein D gene expression is regulated by oestrogen in the rat uterus

Oestrogen-mediated gene expression is regulated at both the transcriptional and post-transcriptional levels. The molecular mechanism of transcriptional regulation has been well characterized. On the other hand, there is little understanding of the mechanism of post-transcriptional regulation. To clarify the mechanism of oestrogen-mediated post-transcriptional regulation, we focused on A+U-rich-element RNA-binding factor 1/heterogeneous nuclear ribonucleoprotein D (AUF1/hnRNP D), which is known as a regulator of cytosolic mRNA degradation and nuclear pre-mRNA maturation. However, little is known about the expression levels and the regulation of AUF1/hnRNP D mRNA in tissues. We further investigated the expression levels of AUF1/hnRNP D isoform mRNAs to determine whether AUF1/hnRNP D gene expression is regulated by oestrogen in the ovariectomized adult female rat uterus. Uterine AUF1/hnRNP D mRNA was induced by a single subcutaneous injection (1 microg/kg) of 17beta-oestradiol (E2), reaching a peak level within 6 h. Furthermore, we observed that the E2-induced AUF1/hnRNP D isoform mRNAs are p45 and p40 transcripts, and that E2-mediated induction is suppressed by the oestrogen receptor antagonist ICI 182,780. Finally, using the transcriptional inhibitor actinomycin D, we confirmed that the E2-mediated increase in AUF1/hnRNP D mRNA is caused by E2-dependent AUF1/hnRNP D mRNA stabilization.

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.

Molecular cloning and characterization of mouse EBAG9, homolog of a human cancer associated surface antigen: expression and regulation by estrogen

We previously identified a human estrogen-responsive gene, EBAG9 (ER-binding fragment-associated antigen9) (Watanabe, T. et al., Mol. Cell. Biol. 18, 442-449, 1998). It was later reported as RCAS1 (receptor-binding cancer antigen expressed on SiSo cells) that induced apoptosis and suppressed the growth of several cells such as activated T cells (Nakashima, M. et al., Nat. Med. 5, 938-942, 1999). Here, we have isolated both cDNA and genomic DNA of mouse EBAG9/RCAS1. Mouse EBAG9 gene spans about 30 kb in genomic DNA and consists of 7 exons. Mouse EBAG9 cDNA encodes a protein that contains the transmenbrane segment and coiled-coil domain. An alignment between the predicted mouse and human EBAG9 shows a high degree of homology at the amino acid level (98%). Northern and Western blot analyses demonstrate that EBAG9 is expressed in several tissues including the heart, brain, spleen, liver, kidney, and testis, and also in developing embryo. In the uterus, a target organ for estrogen, the EBAG9 was shown to be upregulated in vivo by 17beta-estradiol. To determine the biological action of mouse EBAG9, NIH3T3 fibroblastic cells were incubated with recombinant EBAG9 protein, resulting in suppression of cell growth. These findings suggest that EBAG9 is an in vivo estrogen-responsive gene that inhibits the cell growth.

Inhibition of human breast cancer cell growth by blockade of the mevalonate-protein prenylation pathway is not prevented by overexpression of cyclin D1

Overexpression of the cyclin D1 (CCND1) gene, encoding a downstream effector of mitogenic signals that plays a central role in G1 phase progression, is often found in cancerous cells. In sporadic breast cancer (BC), this is one of the most frequent and early genetic lesions identified so far, found in more than 50% of the tumors. Inhibitors of the mevalonate/protein prenylation pathway belong to a new family of cancer therapeutic agents that act by blocking intracellular mitogenic signal transduction pathways, thereby preventing expansion of pre-cancerous foci and inhibiting growth of transformed cells. It is not known at present whether constitutively high intracellular levels of cyclin D1 might interfere with the cytostatic actions of mevalonate/protein prenylation inhibitors. This possibility was investigated here by assessing the cell cycle effects of Simvastatin, a non-toxic upstream inhibitor of the mevalonate pathway, on human BC MCF-7 cells expressing either normal or enhanced levels of cyclin D1 from of a stably transfected, tet-inducible expression vector. Results show that constitutive overexpression of this protein, such as that found in sporadic BCs, does not influence the growth inhibitory effects of Simvastatin in vitro. In addition, D1-overexpressing embryo fibroblasts were also found to be responsive to the cell cycle effects of mevalonate/protein prenylation pathway blockade, further suggesting that high intracellular levels of cyclin D1 do not prevent the cytostatic actions of compounds targeting this metabolic pathway.

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).

Early increase in cyclin-D1 expression and accelerated entry of mouse hepatocytes into S phase after administration of the mitogen 1, 4-Bis[2-(3,5-Dichloropyridyloxy)] benzene

We have previously demonstrated that hepatocyte proliferation induced by the mitogen 1,4-bis[2-(3,5-dichloropyridyloxy)] benzene (TCPOBOP) is independent of changes in cytokines, immediate early genes, and transcription factors that are considered to be necessary for regeneration of the liver after partial hepatectomy (PH) or necrosis. To further investigate the differences between mitogen-induced mouse hepatocyte proliferation and liver regeneration after PH, we have measured the expression of cyclin D1, cyclin D3, cyclin E, and cyclin A and of the cyclin-dependent kinases CDK2, CDK4, and CDK6. The involvement of the cyclin-dependent kinase inhibitors p21 and p27 and of the oncosuppressor gene p53 was also examined at different times after stimulation of hepatocyte proliferation. Results showed that a single administration of TCPOBOP caused a very rapid increase in the levels of cyclin D1, a G1 protein, when compared with two thirds PH (8 hours versus 30 hours). The early increase in cyclin D1 protein levels was associated with a faster onset of increased expression of S-phase-associated cyclin A (24 hours versus 36 hours with PH mice). Accordingly, measurement of bromodeoxyuridine (BrdU) incorporation revealed that, although approximately 8% of hepatocytes were BrdU-positive as early as 24 hours after TCPOBOP, no significant changes in BrdU incorporation were observed at the same time point after two thirds PH. The expression of other proteins involved in cell cycle control, such as cyclin-dependent kinases (CDK4, CDK2, CDK6), was also analyzed. Results showed that expression of CDK2 was induced much more rapidly in TCPOBOP-treated mice (2 hours) than in mice subjected to PH (36 hours). A different pattern of expression in the two models of hepatocyte proliferation, although less dramatic, was also observed for CDK4 and CDK6. Expression of the CDK inhibitors p21 and p27 and the oncosuppressor gene p53 variably increased after two thirds PH, whereas basically no change in protein levels was found in TCPOBOP-treated mice. The results demonstrate that profound differences in many cell cycle-regulatory proteins exist between direct hyperplasia and compensatory regeneration. Cyclin D1 induction is one of the earlier events in hepatocyte proliferation induced by the primary mitogen TCPOBOP and suggests that a direct effect of the mitogen on this cyclin may be responsible for the rapid onset of DNA synthesis observed in TCPOBOP-induced hyperplasia.

Underdeveloped uterus and reduced estrogen responsiveness in mice with disruption of the estrogen-responsive finger protein gene, which is a direct target of estrogen receptor alpha

The biological roles of estrogen-responsive finger protein (efp) in vivo were evaluated in mice carrying a loss-of-function mutation in efp by gene-targeted mutagenesis. Although efp homozygous mice were viable and fertile in both sexes, the uterus that expressed abundant estrogen receptor alpha exhibited significant underdevelopment. When the ovariectomized homozygotes were subjected to 17beta-estradiol treatment, they showed remarkably attenuated responses to estrogen, as exemplified by decreased interstitial water imbibition and retarded endometrial cell increase, at least, attributable to the lower ratio of G1 to S-phase progression in epithelial cells. These results suggest that efp is essential for the normal estrogen-induced cell proliferation and uterine swelling as one of the direct targets of estrogen receptor alpha.

Progesterone inhibits estrogen-induced cyclin D1 and cdk4 nuclear translocation, cyclin E- and cyclin A-cdk2 kinase activation, and cell proliferation in uterine epithelial cells in mice

The response of the uterine epithelium to female sex steroid hormones provides an excellent model to study cell proliferation in vivo since both stimulation and inhibition of cell proliferation can be studied. Thus, when administered to ovariectomized adult mice 17beta-estradiol (E2) stimulates a synchronized wave of DNA synthesis and cell division in the epithelial cells, while pretreatment with progesterone (P4) completely inhibits this E2-induced cell proliferation. Using a simple method to isolate the uterine epithelium with high purity, we have shown that E2 treatment induces a relocalization of cyclin D1 and, to a lesser extent, cdk4 from the cytoplasm into the nucleus and results in the orderly activation of cyclin E- and cyclin A-cdk2 kinases and hyperphosphorylation of pRb and p107. P4 pretreatment did not alter overall levels of cyclin D1, cdk4, or cdk6 nor their associated kinase activities but instead inhibited the E2-induced nuclear localization of cyclin D1 to below the control level and, to a lesser extent, nuclear cdk4 levels, with a consequent inhibition of pRb and p107 phosphorylation. In addition, it abrogated E2-induced cyclin E-cdk2 activation by dephosphorylation of cdk2, followed by inhibition of cyclin A expression and consequently of cyclin A-cdk2 kinase activity and further inhibition of phosphorylation of pRb and p107. P4 is used therapeutically to oppose the effect of E2 during hormone replacement therapy and in the treatment of uterine adenocarcinoma. This study showing a novel mechanism of cell cycle inhibition by P4 may provide the basis for the development of new antiestrogens.

Dose-response characteristics of uterine responses in rats exposed to estrogen agonists

Assays for uterine response have played major roles in developing an understanding of estrogen-mediated processes and for identifying compounds with hormonal activity. Data from assays measuring increases in uterine wet weight in rats were evaluated in terms of their dose-response characteristics. Analysis using a Hill equation found inconsistent estimates for the ED50 (concentration giving half-maximal response) and n (steepness of response) among the assays. This variability reflects disparate assay protocols and limitations of the dose-response data collected in the experiments. Although uterine wet weight is easily measured, it arises from several physiological processes (e.g., water retention, cell proliferation). This contributes to the assay variability with different protocols. The potential use of the Hill equation for dose-response analysis to estimate a benchmark dose was also considered using these data sets as surrogates for receptor-mediated toxicological effects. Strengths and weaknesses were identified, but overall the Hill equation should likely become a favored option for determining a benchmark dose, particularly when a data set demonstrates a maximal response. For screening purposes, empirical analysis using the Hill equation provides adequate information for classifying and prioritizing compounds. To develop an understanding of how incremental exposures to compounds with estrogen agonist activities would affect intact adult females, quantitative analyses are required that account for the pharmacokinetics of estradiol and subsequent interactions of the receptor complexes in regulating the responses.